Failure to process chromatin on apoptotic microparticles in the absence of deoxyribonuclease 1 like 3 drives the development of systemic lupus erythematosus
Benjamin Sally
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Graduate School of Arts and Sciences
COLUMBIA UNIVERSITY
2017
© 2017 Benjamin Sally All rights reserved
ABSTRACT
Failure to process chromatin on apoptotic microparticles in the absence of deoxyribonuclease 1 like 3 drives the development of systemic lupus erythematosus
Benjamin Sally
Systemic lupus erythematosus is an autoinflammatory disorder driven by the development of autoantibodies to self-nucleic acids, in particular to DNA and to chromatin. Loss-of-function mutations of the secreted deoxyribonuclease
DNASE1L3 have been implicated in the development of aggressive familial lupus.
In addition, recent genome-wide association studies have linked a hypomorphic variant of DNASE1L3 to sporadic lupus. Studies in the lab determined that
Dnase1l3-deficient mice develop rapid autoantibody responses against dsDNA and chromatin, and at older ages this leads to a lupus-like inflammatory disease.
These disease manifestations were completely independent of the intracellular
DNA sensor STING, which has been implicated in other examples of self-DNA driven autoinflammatory diseases. My project focused on developing assays to track the activity of DNASE1L3, as well as identifying the endogenous source of self-DNA normally processed by DNASE1L3. Using mouse models that allow the depletion of specific cell populations, we found that circulating DNASE1L3 is produced by hematopoietic cells, in particular by CD11c+ dendritic cells and by tissue macrophages. Taking into account the unique properties of DNASE1L3, we discovered that this enzyme is uniquely able to digest chromatin contained within and on the surface of apoptotic microparticles. Loss of DNASE1L3 activity
in circulation results in elevated levels of DNA in plasma, in particular within microparticles. Microparticles are extensively bound by anti-chromatin autoantibodies isolated from both murine models of lupus as well as prototypical human clones. In addition, Dnase1l3-deficient mice have high levels of circulating
IgG that bind to microparticles from young ages, and these titers increased as disease progressed in aged animals. Pretreatment of microparticles with
DNASE1L3 largely abrogated this binding, demonstrating that DNASE1L3 directly reduces the immunogenicity of microparticles. We also studied two human patients with null mutations in DNASE1L3, and observed increased DNA circulating in plasma and, in particular, in their microparticles, demonstrating a conserved role for DNASE1L3 in mice and humans. Finally, we obtained plasma samples from a cohort of patients with sporadic SLE, and found that roughly 80% had circulating IgG that avidly bound microparticles. Roughly half of this group failed to bind to microparticles that had been pretreated with DNASE1L3, and this
DNASE1L3-sensitive group also presented with lower levels of DNASE1L3 activity. We conclude that extracellular chromatin associated with microparticles acts as a potential self-antigen capable of causing loss of tolerance to self-DNA and inflammatory disease in both mice and humans. The secretion of a DNA- processing enzyme thus represents a novel, conserved tolerogenic mechanism by which dendritic cells restrict autoimmunity.
Table of Contents
List of figures ii
Introduction Systemic lupus erythematosus in humans and mice 1 Processing of self-DNA as a mechanism to prevent autoimmunity 14 Mice lacking Dnase1l3 develop autoimmunity 21
Chapter 1: Development of a functional assay to identify sources of DNASE1L3 Background 26 Results 29 Conclusions 44
Chapter 2: Microparticles as the physiological target of DNASE1L3 Background 46 Results 48 Conclusions 60
Chapter 3: Microparticles as autoantigens Background 62 Results 63 Conclusions 74
Chapter 4: DNASE1L3 and microparticles in human lupus patients Background 76 Results 77 Conclusions 89
Discussion and future directions 91
Materials and methods 107
References 121
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List of figures
Figure 1 26 Phylogenetic tree of human and mouse deoxyribonucleases
Figure 2 28 Representation of the key features of DNASE1L3
Figure 3 29 Standard curve for detection of DNASE1L3 activity by qPCR
Figure 4 31 The C terminal domain of DNASE1L3 allows its activity to be specifically tracked as it confers the ability to digest coated DNA
Figure 5 33 The catalytic domain of DNASE1 and DNASE1L3 share extensive homology
Figure 6 35 Perturbations of the C terminal domain of DNASE1L3 dramatically affect its function
Figure 7 36 Dnase1l3-deficient animals do not process liposome-coated DNA ex vivo or in vivo
Figure 8 37 Active DNASE1L3 in circulation is produced by hematopoietic cells
Figure 9 39 The production of autoantibodies in Dnase1l3-deficient mice is driven by hematopoietic cells
Figure 10 40 Restoration of circulating DNASE1L3 into Dnase1l3-deficient animals delays the development of autoimmunity
Figure 11 42 DNASE1L3 is expressed primarily by dendritic cells and macrophages
Figure 12 44 DNASE1L3 in circulation is produced primarily by dendritic cells and macrophages
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Figure 13 49 Treatment with staurosporine induces rapid cell death and microparticle release
Figure 14 52 DNASE1L3 is uniquely able to process microparticle DNA
Figure 15 54 A membrane permeable DNA dye extensively stains microparticles only in the absence of DNASE1L3
Figure 16 55 DNASE1L3 treatment of microparticles alters their surface composition
Figure 17 57 Dnase1l3-deficient mice have higher levels of DNA circulating in plasma
Figure 18 58 Microparticles from Dnase1l3-deficient mice carry increased amounts of DNA compared to those of WT mice
Figure 19 59 Dnase1l3-deficient mice fail to process exogenous microparticles in vivo
Figure 20 65 DNASE1L3-sensitive chromatin on the surface of microparticles is antigenic in Dnase1l3-deficient mice
Figure 21 66 Microparticles from Dnase1l3-deficient mice at young ages are more coated by IgG and have higher levels of exposed self-antigens
Figure 22 68 Immunization with microparticles drives anti-chromatin antibody responses in inflammatory contexts
Figure 23 69 Treatment of microparticles with DNASE1L3 but not DNASE1 prevents binding by DNA-specific autoantibodies
Figure 24 71 The C terminal domain of DNASE1L3 is required for the processing of microparticle chromatin
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Figure 25 73 A subset of human 9G4+ antibodies bind to microparticles only in the absence of DNASE1L3
Figure 26 78 Analysis of human microparticles
Figure 27 80 DNASE1L3 null mutations in human patients result in defective DNA processing that parallels Dnase1l3-deficient mice
Figure 28 82 DNASE1L3 null mutations in humans result in defective processing of endogenous microparticles in circulation
Figure 29 83 DNASE1L3-sensitive binding of patient IgG to exogenous microparticles
Figure 30 85 Extensive binding of microparticles by the plasma of patients with sporadic SLE
Figure 31 86 Classification of sporadic SLE patients based on DNASE1L3 sensitivity of IgG binding
Figure 32 88 The activity of DNASE1L3 in the circulation of sporadic SLE patients differs according to the observed pattern of staining
Figure 33 89 Changes in DNASE1L3 activity in sporadic patients impacts the DNA cargo of microparticles
Figure 34 97 Proposed mechanism by which DNASE1L3 prevents the development of autoimmunity
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Acknowledgements
I would be remiss to not begin by acknowledging my PI, Dr. Boris Reizis.
He is not only a brilliant scientist and educator, but a phenomenal mentor as well.
Special mention must also go to Dr. Vanja Sisirak, a postdoctoral researcher in the lab with whom I worked very closely on this project. Indeed, Vanja was instrumental in my training after I joined the lab, and his input and guidance have proved vital many times over. I have been fortunate enough to have worked with two wonderful departments; the Department of Microbiology and Immunology of
Columbia and the Department of Pathology of NYU. In addition, I would like to thank all of my fellow lab members. The environment is truly wonderful; everyone is always willing to assist with troublesome experiments or give advice.
Furthermore, I would like to acknowledge all of our collaborators, who have provided invaluable reagents, materials, and discussions. In particular, I’d like to acknowledge Dr. Jill Buyon and Dr. Robert Clancy, who have graciously allowed us access to all of the human samples analyzed. In addition, they have been tremendously generous with their time and expertise. I would also like to thank all of the patients for providing material for this study; without such contributions our work would not be possible.
Finally, my profound thanks to my advisory committee at Columbia: Dr.
Donna Farber, Dr. Uttiya Basu, Dr. Kang Liu, and the non-affiliated evaluator Dr.
Robert Clancy. Your advice and support has been paramount during my time as a PhD student, and I appreciate your willingness to be available for this whole process.
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To my family, friends, and Mariana: Your love and support has been the cornerstone for anything and everything I’ve accomplished
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Introduction
Systemic lupus erythematosus: prevalence and therapeutic options
Systemic lupus erythematosus (SLE or lupus) is an autoinflammatory disease wherein pathogenesis is driven by the production of antibodies (Abs) to self-nuclear antigens. Targets may include ribonucleoproteins and/or nucleic acids. Following the initial loss of tolerance, auto-Ab production amplifies further inflammatory responses via the innate immune system, resulting in the activation of myeloid pathways and the production of type 1 interferon (interferon /, IFN)
(1). In turn, the production of auto-Abs is dramatically enhanced by these inflammatory molecules, resulting in a feed forward pathogenic loop (1). Immune complexes are then formed by these auto-Abs and nucleic acids, and upon their deposition in various tissues cause chronic inflammation, including arthritis, vasculitis, and glomerulonephritis (1). An additional aspect of SLE pathogenesis is its unpredictable flaring, the causes of which vary considerably between patients (1, 2). Known triggers include but are not limited to pregnancy (3), viral or bacterial infection (4), ultraviolet radiation (5), stress (6), and exposure to various environmental agents including pharmaceuticals, chemicals, or food products (7).
Current estimates as to the prevalence of SLE range from 20-150 cases per 100,000 persons (2). Like most complex inflammatory disorders, there are numerous genetic and environmental factors that have been linked to SLE development. Indeed, studies have shown that the concordance for SLE among
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monozygotic twins is approximately 24% (8). In addition, there is strong evidence for involvement of the endocrine system in SLE, as roughly 90% of patients are female (9). Furthermore, rates of SLE dramatically differ depending on ethnicity, with African Americans being a particularly vulnerable population for reasons unknown. Consequently, among African American women the rate is over 400 cases per 100,000 persons (10).
Because the presentation of SLE can vary wildly between patients, or, indeed, between flares of a single individual, its identification and assessment can be challenging. Currently, there are two main scoring systems for SLE patients: the Systemic Lupus Erythematosus Disease Activity Index (SLEDAI) and the American College of Rheumatology (ACR) criteria (11, 12). Both ultimately evaluate patients based on the presence or absence of a wide variety of symptoms, ranging from neurological (psychosis, seizures, sensory or motor neuropathy, etc.) to hematological (anemia, leukopenia, thrombocytopenia) to immunological (presence of auto-antibodies). Importantly, not every symptom has to be present to result in a diagnosis of SLE; of the 11 ACR criteria the presence of at 4 yields a sensitivity of 85% and a specificity of 95% for SLE (12).
SLEDAI scores, alternatively, are used to monitor patients longitudinally, as fluctuations in score play a major role in determining changes to the course of treatment.
Despite the high (and increasing) prevalence of SLE, therapeutic options remain inadequate. Because little is known regarding the mechanisms driving loss of tolerance in SLE, current treatments are limited to the treatment of
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symptoms rather than the underlying pathology (13). Widely used strategies include over the counter or prescription nonsteroidal anti-inflammatory drugs
(NSAIDs), the anti-malarial drug hydroxychloroquine, prednisone or other corticosteroids, and, in severe cases, immunosuppression with chemotherapeutics such as methotrexate (13, 14). These therapies all represent broad anti-inflammatory interventions that fail to address the specific dysfunction of normal immune pathways driving SLE. Furthermore, there are few effective therapies in development; indeed, there has been only one new drug successfully developed and approved by the FDA for the treatment of SLE in the past 60 years (15).
Pathogenesis of SLE: cells and cytokines
At the macro level, both branches of the immune system play essential roles in the development of SLE. The innate immune system contributes both in the early and late stages of the disease. Initially, myeloid populations such as dendritic cells activate T cells and produce B-cell activating factor (BAFF), which drives the adaptive response (16). Activated T lymphocytes, specifically TH1 and
TH17 helper populations then drive the systemic and renal activation of B cells
(17-19). B cell activation results in the production of anti-nuclear antigen (ANA) autoantibodies, which form immune complexes and are deposited throughout the body in various tissues, causing inflammation and pathogenesis via the activation of localized innate pathways (19, 20). Deposition of these autoantibody complexes at different sites in the body account for the variation in symptoms
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between individuals and/or flares (21). Thus, the activation of autoreactive B cells and their subsequent release of autoantibodies represents the central hallmark of
SLE.
While the primary cause of SLE is the deposition of immune complexes, deregulation of inflammatory cytokines is a major contributor to immune dysfunction and ultimately tissue injury. Inflammatory cytokines, such as type I and type II interferons (IFNs), interleukin (IL)-6, IL-1, and tumor necrosis factor- alpha (TNF), have all been implicated in the pathogenesis of SLE (22-27).
Furthermore, immunomodulatory cytokines have also shown to play roles in preventing SLE, including IL-10 (28) and transforming growth factor-beta (TGF)
(29). Finally and more broadly, regulatory T cells play an important role in limiting deleterious autoinflammatory conditions such as lupus, as shown by recent studies linking IL-21, IL-17, and IL-2 to SLE (30).
Work from the group of Virginia Pascual has shown that SLE patients frequently present with an enhanced type I IFN signature (31). Type I IFNs can have extensive systemic effects at multiple levels that contribute to SLE pathogenesis. Firstly, IFN promotes positive feedback loops, enhancing the synthesis of additional type I IFN via the induction of Toll-like receptor (TLR) 7
(32). Secondly, IFNs drive the maturation of dendritic cells (DCs), shifting the balance of immature versus mature DCs (33). Crucially, immature DCs are important in maintaining regulatory T cells in the periphery, and also help promote deletion of autoreactive T cells by presenting self-antigens in the absence of costimulatory molecules (33). Thirdly, mature DCs produce BAFF,
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which directly enhances the selection and survival of autoreactive B cells (34).
Finally, IFN both enhances the cytotoxic capabilities of CD8 T cells (35) and also increases the number of autoreactive CD4 T cells by driving the upregulated expression of CD80 and CD86 on APCs (33).
IL-6 is a proinflammatory cytokine that, in SLE, acts primarily at the level of B cells and T cells. It has been shown to play a key role in driving the hyperactivation of B cells (36) and in promoting autoreactive T cell responses
(37). In mouse models of rheumatoid arthritis, IL-6 has been shown to be vital for the differentiation of TH17 helper cells (38), a proinflammatory population that has also been implicated in the pathogenesis of SLE (18). Finally, IL-6 has been shown to play important roles in mediating local inflammatory responses in lupus patients. Elevated IL-6 is found in the kidneys of patients presenting with lupus nephritis (39), as well as in the cerebrospinal fluid of patients presenting with neuropsychiatric complications (40).
IFN-gamma (IFN) is the primary cytokine that drives TH1 responses. In other contexts, TH1 responses have been shown to counteract aberrant autoinflammatory reactions, largely thought to be because they inhibit the development of autoreactive TH17 cells (41). However, in the context of lupus it has been shown to induce production of IgG2a isotype antibodies, which can contribute to autoimmunity by the activation of the complement system (42).
Lupus-prone mice deficient for IFN present with an ameliorated form of the disease (43), and in human patients it has been suggested that an imbalance in favor of TH1 responses can drive disease (44). However, additional clinical
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studies have proved inconclusive, with some suggesting that levels of IFN are tied closely to the severity of lupus nephritis (45), while others observe no correlation (46). Although these data are not clear cut, what is apparent is that if
IFN plays a role in SLE pathogenesis, it is likely in exacerbating disease after the initial loss of tolerance.
In addition to the overproduction of proinflammatory cytokines, SLE can be exacerbated by disruption of regulatory cytokine production. Lupus-prone mice that also lack IL-10 develop a more severe form of the disease (28). IL-10 acts to dampen proinflammatory responses, including but not limited to TH1 helper cells, TH17 helper cells, and macrophages, making it a key molecule in preventing autoimmunity (47). Along these lines, TGF- is strikingly reduced in lupus patients undergoing active disease (29). TGF- also plays an immunosuppressive role, particularly via modulation of lymphocyte activation and differentiation (48). Reductions in TGF- lead to a loss of regulatory T cells in the periphery and an overall increase in inflammatory profile, providing a permissive environment for autoreactive responses (48).
IL-23 and IL-17 act synergistically to drive the differentiation of TH17 helper cells, which as previously discussed are a potent proinflammatory subtype.
These cells are generally balanced by the presence of regulatory T cells, which produce anti-inflammatory cytokines and prevent autoimmunity. IL-2, produced by other T cells, is a critical growth factor for regulatory T cells in particular.
Lower levels of IL-2 lead to a striking reduction in their population (49), and a corresponding increase in TH17 cells (50). The T cells of active SLE patients
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produce less IL-2 compared to controls (51), and patients also present with a reduced population of regulatory T cells (52). Thus, perturbing the balance of proinflammatory versus regulatory T cells correlates with SLE severity, indicating complex and opposing roles for these populations.
Mouse models of SLE and their applicability to human disease
One major factor that has drastically hampered understanding of human
SLE pathogenesis and consequently drug development is the lack of representative mouse models. The oldest and most widely used model is the
New Zealand Black/New Zealand White (NZB/NZW) F1 hybrid, which spontaneously develops lupus-like symptoms such as lymphadenopathy, splenomegaly, and ANA autoantibodies whose repertoire includes anti-DNA (53,
54). These anti-double stranded DNA (dsDNA) antibodies typically lead to glomerulonephritis and eventually fatal kidney failure (53, 54). Additional study of these mice led to the identification of three genetic loci that, when all were present in combination, drove the development of lupus-like disease in these mice, dubbed Sle1-3 (55). Unfortunately, due to the requirement for all three loci to be present, extensive genetic studies using this model are extremely challenging and, generally, unfeasible. Furthermore, in stark contrast to many human patients where anti-DNA antibodies are detectable prior to the development of overt symptoms (56), NZB/NZW mice develop autoantibodies against DNA slowly, and typically only after extensive immune activation (57).
Ultimately, this suggests that while anti-dsDNA antibodies may play a role in the
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ongoing pathogenesis of these animals, loss of tolerance to DNA is unlikely to be the initial insult driving SLE development.
An alternative model was subsequently discovered. A substrain of the inbred MRL line was characterized that developed a sex-independent SLE-like disease characterized by lymphadenopathy driven by double negative immature
T cells (58). These mice also presented with high levels of circulating immune complexes driven by ANA IgG, including high anti-dsDNA (58). Ultimately, this phenotype was attributed to an autosomal recessive mutation in the Fas receptor, dubbed lymphoproliferation (lpr). Mice harboring this mutation are deficient for
Fas activity, which dramatically impairs apoptosis in lymphocytes (59). This defect in negative selection of autoreactive cells is sufficient to drive the development of lupus-like disease. Crucially however, subsequent studies demonstrated that mice positive for the lpr mutation from other strains were asymptomatic, indicating vital roles for genes specific for the MRL background
(60). Thus, the requirement for a specific background represents a significant obstacle for in depth genetic analysis. In addition, the lpr model suffers from the same issue of specificity as the NZB/NZW mice, namely that loss of tolerance to
DNA is a consequence of inappropriate immune activation and is not driving pathogenesis.
Further efforts to pinpoint a monogenic model of disease led to the identification of the BXSB mouse strain, derived from the F1 intercross between
C57BL6 and SB/Le strains and subsequent backcrossing onto SB/Le (54). These mice developed lupus-like disease that, curiously, was much more severe in
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males, and further study identified the presence of the Y-linked autoimmune accelerator (Yaa) genetic element (61, 62). It was subsequently discovered that this element is due to a translocation of a portion of the X chromosome to the Y chromosome, resulting in the duplication of several genes and a 2-fold increase in expression (63, 64). Crucially, one of these duplicated genes is TLR7, which encodes an innate pattern recognition receptor specific for viral ssRNA (63-66).
Activation of TLR7 results in signaling through myeloid differentiation primary response gene 88 (MyD88), leading to the activation of proinflammatory pathways in DCs and B cells (67, 68). Importantly, signaling through TLR7 has been shown to act as a second signal in B cells, allowing for activation of autoreactive cells upon interactions with endogenous RNA ligands (69, 70).
Consequently, TLR7-transgenic mice have become a widely-used model to study
SLE (32, 63, 66, 67, 71, 72) and further analysis has revealed that polymorphisms in TLR7 are linked to human disease in certain populations (65,
73).
However, this model does have drawbacks. Disease in TLR7-transgenic animals is primarily driven by loss of tolerance to various forms of self-RNA (32,
63, 67), but in human disease anti-DNA antibodies are more pathogenic and predictive of disease outcomes (56, 74). An additional complication stems from the mechanism by which TLR7 is regulated. The endosomal TLRs (TLR7, TLR9, and TLR3) are produced in the endoplasmic reticulum and are dependent on the chaperone protein Unc-93 homolog B1 (UNC93B1) for trafficking to endosomes
(75). Consequently, changes in the expression level of one endosomal TLR also
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affect the availability of UNC93B1, meaning that TLR7 transgenic mice also present with reduced levels of endosomal TLR9 (71). Importantly, loss of TLR9 has been linked to exacerbation of autoimmunity (76), and it is unclear if this is due to increases in TLR7 or if TLR9 has additional immunomodulatory functions.
A final downside of this model is that TLR7 is sufficient to drive disease only at very high doses, as the two-fold increase caused by Yaa translocation requires the presence of additional susceptibility loci for overt autoimmune disease (77).
Thus, the requirement for high levels of TLR7 complicates genetic analysis and the extent to which it mimics human disease broadly is unclear.
In addition to genetic models of SLE, exposure to environmental triggers has been used to induce the disease independently of mouse background. Chief among these is the pristane-induced lupus model, which is driven by intraperitoneal injections of pristane (2,6,10,14-tetramethylpentadecane) into mice on a BALB/c background (78, 79). These mice develop autoantibodies characteristic of lupus, including anti-histone, anti-ribonucleoprotein, and anti-
DNA variants, generally presenting with similar repertoires and disease progression as MRL/lpr mice (78, 79). Additional mouse backgrounds have subsequently been tested, and develop autoantibodies in response to pristane injection to varying extents (80). Although this model is again not driven by loss of tolerance to DNA specifically, it has proved useful in determining whether various inflammatory pathways contribute to lupus pathogenesis. For instance, injection of pristane into mice deficient for IL-6 demonstrated the importance of this cytokine for the development of anti-DNA autoantibodies (23). Additionally,
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pristane injection into mice deficient for IFN-I receptor (IFNAR) led to extensive but not total ablation of the disease (81, 82), which corresponds with studies linking overexpression of interferon with disease severity in human patients (83-
85). One key difference, however, is that whereas in humans plasmacytoid dendritic cells (pDCs) are the key producers of interferon (86), in the pristane- induced mouse model it is produced by Ly6Chi monocytes that aggregate in the peritoneal cavity (87). Furthermore, interferon production by these cells requires the presence of TLR7, again suggesting that there is no loss of tolerance to DNA specifically in this model (88).
Thus, there is a critical need for the development of a monogenic mouse model that presents with extensive anti-DNA responses before proceeding to disease. While some of the aforementioned mouse models feature these autoantibodies to varying extents, none of these models recapitulate loss of tolerance to self-DNA. Modeling anti-DNA responses is important because high affinity IgG specific for dsDNA are especially pathogenic and have been demonstrated to correlate with the severity of symptoms in human disease, in particular glomerulonephritis and kidney injury (56, 74, 83, 89). Furthermore, loss of tolerance to self-DNA seems to be a key event leading to the development of sporadic disease, as many patients present with high levels of anti-DNA autoantibodies despite being otherwise asymptomatic (56).
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Genetics of SLE
SLE in humans typically falls into one of two categories: sporadic disease and familial (or monogenic) disease. Inherited SLE is very rare, and in humans has been definitively linked to only a small number of genes, all of which are involved in cell death pathways or in the clearance of apoptotic debris (90). In the study of sporadic disease, numerous genome wide association studies (GWAS) have identified more than 50 common risk loci for SLE susceptibility.
Unsurprisingly, the strongest association signal among common SLE variants is obtained from the HLA region (90). Other genes implicated generally fall into four different categories: activation of the adaptive immune system, innate immune signaling, renal-specific factors, and clearance of self-nucleic acids (91). Genes affecting activation of the adaptive system include those that modulate B cell receptor signaling (92), T and B cell crosstalk via MHC (93), and the differentiation of T cells into various effector phenotypes (94). Innate signaling pathways known to be dysregulated include the type I IFNs (95), immunoglobulin
Fc receptors (96), and various molecules upstream of nuclear factor kappa-light- chain-enhancer of activated B cells (NF-B) (97). Renal-specific factors include localized activation of myeloid cells or lymphocytes (98, 99), clearance of immune complexes from glomeruli (100), and potential disease pathways active in resident renal cells, such as angiotensin-converting enzyme (ACE) (101). The last category of molecules implicated by GWAS are predominantly involved in the processing of self-antigens, and includes the complement system (102) and the clearance of apoptotic cells (103).
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Identifying the antigenic form of self-DNA driving pathogenesis in SLE
In addition, a major outstanding question in the field concerns the nature of antigenic self-DNA that drives loss of tolerance. Despite extensive efforts, it remains unclear which specific form of self-DNA is recognized by autoreactive B cells, although different types of self-DNA have been implicated in SLE pathogenesis such as oxidized mitochondrial DNA and neutrophil extracellular traps (104-107). Mitochondria are organelles present in all nucleated cells, and contain their own DNA, packaged into structures called nucleoids (108). Because mitochondria are the site of oxidative phosphorylation in the body, they accumulate large quantities of reactive oxygen species, leading to a high rate of mutations due to oxidation of their DNA and the lack of proofreading DNA repair machinery (108). In lupus patients, neutrophils accumulate this oxidized mitochondrial DNA as a consequence of impaired nucleoid digestion, and this
DNA is instead extruded into the extracellular space (104). Oxidized mitochondrial DNA is both a potent stimulator of interferon and a target of autoantibodies in these individuals, as it is recognized by TLR9 in pDCs (104,
107).
In addition, oxidized mitochondrial DNA can be incorporated into neutrophil extracellular traps (NETs) (107). NETs are composed of processed self-DNA bound to various cytoplasmic proteins that are then organized into large filamentous structures (109). NET generation occurs via a specific form of cell death called NETosis, wherein fragmentation of the nucleus results in the availability of chromatin to be integrated and released in NETs (109). These
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structures play key antimicrobial roles, both via direct binding to pathogens (110) and by delivery of high local concentrations of antimicrobial peptides (111).
Importantly, they also play a role in the inactivation of virulence factors via the delivery of intracellular enzymes, such as elastase (109). Furthermore, they act to contain microbes at the initial site of infection, preventing dissemination (109).
However, because NETs contain abundant self-DNA, they have been linked to the development of autoimmunity, in particular through activation of TLR9 (105,
106). Thus, although both oxidized mitochondrial DNA and NETs may act as second signals, it is unclear whether they are capable of functioning as antigens for DNA-specific autoreactive B cells.
Vital roles for deoxyribonucleases in the prevention of autoimmunity
The existence of nucleic acid-sensing innate immune receptors presents a fundamental problem for the body, given the abundance of self-DNA and the limited capacity for structural differences been self and non-self nucleic acids.
This paradigm is addressed by the vitally important processing of self-DNA so as to render it non-immunogenic. This function is performed by a class of enzymes called deoxyribonucleases (DNASEs), which catalyze the hydrolytic cleavage of
DNA phosphodiester linkages. These fall into three broad categories, DNASE1 and the DNASE1-like family members, DNASE2, and DNASE3, which is also known as three-prime repair exonuclease 1 (TREX1).
DNASE2 is expressed at high levels in phagocytic cells, in particular macrophages, and is responsible for the digestion of DNA accumulated in
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lysosomes (112). Its potent activity is regulated by the requirement for activation by low pH and cleavage by lysosomal proteases (113). DNASE2 plays important roles in the digestion of DNA from apoptotic cells and also nuclei extruded from erythrocytes, both of which are normally engulfed by phagocytes (114). This function is absolutely vital, as mice deficient for DNASE2 die during embryonic development due to massive production of interferon triggered by undigested self-DNA (115, 116). Mice doubly deficient for both DNASE2 and IFNAR were viable, though did develop chronic polyarthritis that resembled human rheumatoid arthritis, driven in part by the leaking of DNA from lysosomes into the cytoplasm (116). It was subsequently discovered that recognition of accumulated self-DNA by the cytosolic DNA sensor and adaptor protein Stimulator of interferon genes (STING) was responsible for driving autoimmunity in this model, as mice lacking both DNASE2 and STING were both viable and completely failed to develop arthritis (117). STING activates the transcription factor IFN regulatory factor 3 (IRF3), leading to the production of IFN and other inflammatory chemokines (118). It remains unclear what upstream mechanisms are responsible for activation of STING in this context. STING is commonly triggered by the presence of cyclic 2’-5’ 3’-5’ GMP-AMP dinucleotides (cGAMP) produced by cyclic GMP-AMP synthase (cGAS) (119). However, double deficiency of both cGAS and DNASE2 failed to rescue the inflammatory phenotype despite the accumulation of cGAMP in DNASE2 knockout mice (120), suggesting that there are additional cytosolic DNA sensors upstream of STING activated by self-DNA.
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TREX1 is the most abundant 3’ to 5’ exonuclease, and is localized to the endoplasmic reticulum (112, 121). It functions to digest DNA that has been reverse transcribed from endogenous retroelements, which are present throughout the genome and are capable of driving the production of IFN if unprocessed (122). In TREX1 deficient cells, ssDNA accumulates and is recognized by cGAS, leading to activation of STING and aberrant production of
IFN via IRF3 (122, 123). Mice deficient for TREX1 are viable, but succumb to
IFN-driven inflammatory disease postweaning, with a median survival of less than 6 months (122, 124). Cause of death is typically cardiomyopathy leading to circulatory failure (124), but knockout mice also present with increased macrophage activation and neural inflammation (125, 126). Furthermore, mutations in TREX1 have been implicated in human disease. Discovered in 1984,
Aicardi-Goutières syndrome (AGS) is a rare inflammatory disorder that most commonly affects the brain and skin, causing encephalopathy and ultimately brain atrophy (127-129). The disease is driven by extremely high levels of IFN in the cerebrospinal fluid in the absence of infection (130, 131). Subsequent work revealed that mutations in any of seven genes causes AGS: the three genes encoding the RNase H2 endonuclease complex (132), the deoxynucleoside triphosphate triphosphohydrolase SAMHD1 (133), the dsRNA deaminase
ADAR1 (134), the cytosolic dsRNA receptor MDA5 (135), and TREX1 (136).
Notably, these enzymes are all involved in the processing or sensing of nucleic acids, implicating the accumulation of self-nucleic acids in the IFN-driven pathogenesis of AGS.
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Additionally, a separate mutation in TREX1 has been shown to drive the development of familial chilblain lupus erythematosus (CHLE) (137). CHLE is a poorly understood form of lupus wherein lesions are triggered by exposure to cold and/or damp climates, typically distributed symmetrically in the extremities
(138). These patients also often develop antibodies against Sjögren’s-syndrome- related antigen A (SSA), commonly referred to as anti-SSA/Ro (139). Anti-
SSA/Ro antibodies recognize nuclear antigens and are one of the most prominent specificities of autoantibodies in sporadic SLE (140), and have been strongly implicated in the development of neonatal lupus and/or congenital heart defects in utero (141). More broadly, genome-wide association studies have revealed strong associations between TREX1 and sporadic disease severity, suggesting a potential contribution to the exacerbation of symptoms (142).
However, although DNASE2 and TREX1 deficiencies lead to autoimmunity, these responses are driven by aberrant interferon production rather than loss of tolerance to DNA and are thus distinct from classical SLE. Nevertheless, these findings emphasize the importance of self-DNA processing in the prevention of autoimmunity.
While DNASE2 and TREX1 both digest intracellular forms of self-DNA,
DNASE1 is instead secreted extracellularly. DNASE1 was originally characterized in the context of pancreatic exocrine function, as it is released extensively into the digestive tract in addition to being present in circulation (143).
Given the importance of TLR signaling in the pathogenesis of lupus which is suggestive of an extracellular form of DNA being key, there was considerable
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interest in investigating potential roles for DNASE1 in SLE. Initial reports suggested that DNASE1 knockout animals developed mild anti-DNA and ANA
(144). However, the autoimmunity displayed by these mice was underwhelming compared to other models, and these results were never replicated. There were also tenuous links between DNASE1 and human disease; reports of minor reductions in DNASE1 activity (144) or DNASE1 mutations in small cohorts of
SLE patients (145). However, these studies were subsequently expanded upon and it is now accepted that if a role for DNASE1 in promoting autoimmunity exists, it is at best a minor contributor (146, 147). In addition, clinical trials wherein
DNASE1 was administered to lupus patients failed to demonstrate any therapeutic benefit whatsoever (148).
Strong links between DNASE1L3 and multiple forms of SLE
However, DNASE1 is just one member of the DNASE1 family, which also includes DNASE1-like 1 (DNASE1L1), DNASE1L2, and DNASE1L3. All family members share very similar catalytic domains and depend on the presence of
Ca2+ and Mg2+ (149). DNASE1L3 came to prominence when it was identified in a small cohort of consanguineous Saudi Arabian patients with an aggressive form of inherited SLE affecting children at extremely young ages characterized by very high titers of anti-DNA antibodies (150). Further study of these patients revealed that they had a frameshift mutation in DNASE1L3 that led to total loss of the enzyme’s function (150). A second study of similarly consanguineous patients in
Turkey with hypocomplementemic urticarial vasculitis syndrome (HUVS) also
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identified two separate inactivating mutations (one frameshift, one insertion leading to exon skipping) in DNASE1L3 (151). HUVS is a recurring urticaria characterized by glomerulonephritis and loss of complement components, in particular C1q (151). This seems to be driven by the development of high titers of anti-C1q autoantibodies, leading to C1q depletion (151). Indeed, anti-C1q antibodies have been extensively linked to the development of SLE, suggesting that complement plays a key role in the clearance of potentially immunogenic molecules from circulation (152-155). Consequently, HUVS is very strongly associated with SLE, as the majority of patients with HUVS progress to develop
SLE at some point in life (151). Finally and most recently, an additional family of patients was identified that also presented with extremely aggressive, early onset disease in Italy (156). Featuring the same mutation as the Turkish family, one extensively-studied Italian patient first presented with HUVS before proceeding to extensive autoinflammatory disease, including SLE, polyarthritis, and intestinal vasculitis, all of which were largely resistant to treatment with anti-inflammatory therapeutics (156). Thus, these studies demonstrated that very rare cases where
DNASE1L3 function is completely lost lead to extremely aggressive anti-DNA and anti-C1q responses, leading to rapid onset of SLE.
However, to date fewer than 10 patients lacking DNASE1L3 entirely have been identified. It was unclear whether DNASE1L3 was relevant to sporadic disease, as it had never been identified as having potential involvement in sporadic SLE by GWAS. Curiously, one locus linked to SLE by GWAS extensively is PXK (157, 158). PXK encodes for a kinase of unknown function,
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and efforts to link PXK to SLE have yet to be conclusive. However, PXK is in very close proximity to DNASE1L3 on chromosome 3, and closer examination of this genetic region led to the association between SLE and PXK to be reassigned to a single nucleotide polymorphism (SNP) in DNASE1L3 (159, 160). This particular
SNP results in an amino acid substitution wherein arginine is replaced by cysteine at position 206 of DNASE1L3, and studies have shown that this R206C
DNASE1L3 is hypomorphic (161, 162). Thus, reinterpretation of GWAS have now linked the function of DNASE1L3 with the development of sporadic SLE.
Unique features of DNASE1L3
Given these results, we decided to look more closely at DNASE1L3, also known as DNASE. Like the other DNASE1 family members, it has the same critical active site residues, an essential disulfide bridge, a calcium-binding domain, and an N terminal signaling peptide (163, 164). Unlike DNASE1, however, which is bound and inhibited by glomerular actin (165), DNASE1L3 lacks the residues that facilitate this interaction and is thus actin independent
(163). In addition, DNASE1L3 contains a greater number of positively charged residues, resulting in a much more basic isoelectric point compared to DNASE1
(9.5 vs 4.8) (164). Notably, efforts to enhance the activity of DNASE1 to improve therapeutic applications led researchers to replace residues at six positions with basic substitutes, resulting in a hyperactive DNASE1 (166). At four of these six positions DNASE1L3 already features basic residues, suggesting that it likely more potent than DNASE1.
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Another important difference is the C terminus of these enzymes.
DNASE1L3 has a unique and highly basic domain at its C terminus that is very strongly conserved across species (167). One of the earliest described applications for DNASE1L3 was its ability to block liposomal transfection, a function not possessed by DNASE1 (167). This activity was dependent on the C terminal domain, as a truncated form of DNASE1L3 was able to process uncoated but not coated DNA (167). In addition, this domain is sufficient to confer this activity, as fusing it to the C terminus of DNASE1 allows for the resulting chimeric protein to block transfection (167). In addition to digesting liposome- coated DNA, DNASE1L3 is also able to digest chromatin even in the absence of a helper protease, unlike DNASE1 (168, 169). This finding has led to suggestions that DNASE1L3 plays a role in nuclear fragmentation during apoptosis (170, 171) and necrosis (172). However, DNASE1L3 is secreted into the extracellular space and requires cleavage of its N-terminal signal peptide by the secretory pathway
(164), pointing to a likely function for DNASE1L3 in processing extracellular DNA.
In addition, DNASE1L3 is not upregulated in apoptotic cells (164), so any role during cell death may be a secondary function.
Generation and characterization of Dnase1l3-deficient mice
Given the strong links between DNASE1L3 and both inherited and sporadic SLE, our laboratory decided to investigate whether mice lacking
DNASE1L3 would develop the disease. Essential coding exons of Dnase1l3 was targeted via gene-trapping, resulting in the introduction of a LacZ cassette in its
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place (173). Dnase1l3LacZ/LacZ knockout (KO) mice were viable, fertile, and were born at Mendelian frequencies. Given that a major shortcoming of other lupus models was the reliance on a particular background, Dnase1l3 KO mice were crossed onto two common inbred strains, C57BL/6 (B6) and 129SvEv (129). In addition, mice on mixed and F1 backgrounds were analyzed.
All KO mice on both pure backgrounds and on mixed backgrounds developed ANA that stained HEp-2 cells perinuclearly, a pattern consistent with severe human SLE (173). Both male and female mice as young as 5 weeks of age presented with elevated levels of autoantibodies against both dsDNA and chromatin as measured by enzyme linked immunosorbent assay (ELISA) (173).
Notably, there was no increase in total IgG in anti-RNA IgG at any time point tested (173), demonstrating that loss of DNASE1L3 causes loss of tolerance against DNA specifically rather than broad immune activation. The rapid and specific responses to dsDNA and chromatin in these animals thus strongly suggest endogenous genomic DNA represents the primary autoantigen.
Having observed loss of tolerance to self-DNA at very early ages in these knockout mice, our laboratory next wanted to determine whether they went on to develop other symptoms of SLE. As these mice aged, there was an expansion of the CD11c+ MHC class II- CD11b+ Ly-6C- inflammatory monocyte population
(173), which have been described in other models as being key for clearance of immune complexes (174). KO mice also developed splenomegaly at 50 weeks of age, and within these spleens presented with spontaneous germinal center (GC) formation and a corresponding increase in GC B cells (173). All KO mice also
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displayed overt kidney pathology by 50 weeks, with significantly higher IgG deposition in kidney glomeruli compared to WT mice (173). In addition, KO mice on the 129 background had additional kidney irregularities, presenting with extensive glomerulonephritis (173). These studies in the lab thus conclude that the initial loss of tolerance to self-DNA in KO mice precedes extensive immune activation, deposition of immune complexes, and glomerulonephritis, all hallmarks of sporadic SLE in humans.
Acceleration of disease in Dnase1l3-deficient mice after treatment with IFN
Our laboratory was intrigued by the observation that despite extremely early anti-DNA responses, extensive immune activation was not present in KO mice until roughly 30 weeks of age. Previous studies have shown that artificially increasing the production of IFN can dramatically accelerate the development of
SLE (175). To test whether this also applied to Dnase1l3-deficient animals, WT and KO mice at young ages were injected with an adenoviral vector encoding
IFN-5. The efficacy of the vector was determined by both direct measurement of serum IFN and of the IFN-inducible marker Sca-1 (173). Notably, KO mice treated with IFN rapidly developed anti-dsDNA IgG, and in addition presented with novel anti-RNA IgG responses (173). In contrast to previous experiments using KO mice, animals that received IFN had expanded compartments of inflammatory monocytes and T cells one-week post-injection (173). Furthermore, roughly two thirds of KO mice who received IFN perished after 35 weeks (173).
These studies from the lab thus conclude that increases in IFN expression
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dramatically accelerate the immune activation observed in Dnase1l3-deficient animals, which is reflective of severe SLE in DNASE1L3-deficient human patients and sporadic patients where IFN is upregulated.
Disease in Dnase1l3 KO animals is independent of STING but not MyD88
Previous studies of DNASE1L3 have led to suggestions that it processes intracellular DNA (170-172), in a manner similar to DNASE2 or TREX1 (90, 176).
If this were the case, one would predict that autoreactivity in Dnase1l3-deficient animals would be dependent on STING, as is the case in DNASE2 and TREX1 knockout animals. To test the potential involvement of STING in driving anti-DNA responses in our model, mice doubly deficient in STING and DNASE1L3 or
MyD88 and DNASE1L3 were generated. Characterization of these double knockouts revealed that ablation of STING did not reduce the levels of ANA, anti- dsDNA IgG, anti-dsDNA-specific antibody secreting cells (ASCs) as measured by
ELISPOT, kidney deposition of IgG, or splenomegaly (173). In stark contrast, when MyD88 was deleted in Dnase1l3-deficient animals every parameter tested returned to WT levels (173). Because MyD88 is responsible for transducing signals through TLRs and the IL-1 receptor, these data strongly suggest that
DNASE1L3 targets extracellular DNA, which is in keeping with its secreted nature. In addition, these results clearly distinguish loss of DNASE1L3 from other examples of DNASE deficiency, as the autoinflammation caused by loss of
DNASE2 and TREX1 are both rescued by the additional loss of STING.
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Thus, these studies from the lab strongly suggest that Dnase1l3-deficient animals represent a novel model of SLE, wherein the initial loss of tolerance is against an extracellular form of genomic DNA. These mice then go on to develop overt disease much later in life, presenting with classical hallmarks of SLE: expansion of inflammatory monocytes, splenomegaly, increased germinal center reactions, and kidney IgG deposition resulting in overt glomerulonephritis.
Furthermore, the dispensability of STING for disease demonstrates that
DNASE1L3 functions differently than TREX1 or DNASE2 in that it is not processing intracellular DNA. My project has focused on addressing outstanding questions concerning the mechanism by which DNASE1L3 prevents autoimmunity. Chiefly, we have attempted to identify the form or forms of extracellular DNA processed by DNASE1L3, and also sought to understand the cell type or types responsible for the production of DNASE1L3 in the steady state.
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Chapter 1
Development of a functional assay to identify sources of DNASE1L3
Evolutionary relationships between the DNASE1 family members
Because DNASE1L3 is more potent, actin-independent, and has the unique C terminal domain, we wanted to determine the evolutionary relationship between the various deoxyribonucleases. To wit, using software from www.phylogeny.fr (177, 178), we generated a phylogenetic tree using human and mouse DNASE1, DNASE1L1, DNASE1L2, DNASE1L3, DNASE2, and
TREX1. Our analysis revealed that DNASE1L3 appears to have been the first
DNASE1 family member that evolved from DNASE2 (Fig. 1). Indeed, DNASE1 appears to have evolved significantly later in comparison, even after DNASE1L1, which potentially explains why it lacks some of the features of DNASE1L3. The very high degree of homology between human and mouse DNASEs is also notable, demonstrating that these enzymes are very strongly conserved between species.
Figure 1: Phylogenetic tree of human and mouse deoxyribonucleases. Branch length is proportional to the number of amino acid substitutions per site. Generated with one-click analysis from www.phylogeny.fr: alignment by MUSCLE, curation by Gblocks, tree construction by PhyML, visualization by TreeDyn.
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Technical challenges presented by DNASE1L3
Having observed the development of a lupus-like disease in mice deficient for Dnase1l3, we sought to explore the mechanism by which DNASE1L3 contributes to the enforcement of tolerance against self-DNA. The first priority was to establish a means of tracking DNASE1L3. Given the secreted nature of
DNASE1L3 (168), we anticipated assessment of DNASE1L3 to consist of conventional methods, such western blotting, intracellular staining and flow cytometry, and ELISA. Unfortunately, due to the extensive homology between
DNASE1L3 and DNASE1 (163), we were unable to identify a commercially available antibody that functioned adequately in any of our experiments (data not shown).
We then decided to investigate the possibility of generating a custom antibody within the lab by immunizing our knockout animals. This approach presented additional problems surrounding protein purification, however.
DNASE1L3 has a signal peptide at its N terminus (169), rendering a hexahistidine (His) tag ineffective (Fig. 2). Furthermore and as we later confirmed, the critical nature of the C terminal domain for the function of DNASE1L3 (167) suggested that a His tag at the end of this region would dramatically affect its stability and/or function. Lacking the tools to purify DNASE1L3 to the degree required for antibody generation, an alternative approach was required.
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Figure 2: Representation of the key features of DNASE1L3. Asterisks denote vital catalytic residues; bracket denotes a disulfide bridge that is required for function. The N terminal signal pepdide is cleaved during secretion.
Instead, we sought to develop a functional assay that would allow us to track the activity of DNASE1L3. We opted to take advantage of the unique C terminal domain of the enzyme. It had previously been noted that unlike DNASE1,
DNASE1L3 has the unique ability to block liposomally-mediated transfection
(167). This activity depended on the presence of the C terminal domain, as truncated forms of DNASE1L3 lost the ability to digest liposomally coated DNA
(167). Thus, since coated DNA is only able to be processed by DNASE1L3, we can use its digestion as a specific readout.
qPCR as a functional readout for the activity of DNASE1L3
We initially attempted to assess digestion of coated DNA by gel electrophoresis, but were unable to do so due to the nullification of the negative charge of DNA upon coating with positively-charged liposomal transfection reagents such as N-[1-(2,3-Dioleoyloxy)propyl]-N,N,N-trimethylammonium methyl-sulfate (DOTAP) or TransIT. Instead, we decided to use quantitative polymerase chain reaction (qPCR) to determine the extent to which DNA had been digested after incubation with enzyme. For each qPCR reaction, a standard curve was generated using serial dilutions of the target plasmid. Importantly, 2
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M of Mg2+ and Ca2+ were added to the reaction, since DNASE1L3 is dependent on these ions (169). Notably, this technique was extremely sensitive, as plasmid could be consistently detected at the femtogram level. Using the standard curve, the amount of DNA remaining after each digestion was extrapolated and the percent of input (i.e. 1 ng) was calculated (Fig. 3).
Figure 3: Standard curve for detection of DNASE1L3 activity by qPCR. Serial dilutions of template DNA were used to generate a standard curve. DNA remaining after digestion with various DNASE-containing supernatants or sera was assessed by qPCR and the standard used to calculate the percentage of DNA remaining. A unique standard curve was generated for each individual experiment.
DNASE1L3 is uniquely able to digest liposome-coated and nucleosomal
DNA via its C terminus
To test our assay, we generated recombinant DNASE1L3 or DNASE1 via transfection of HEK293 cells, followed by harvest of the supernatant. Importantly, we used KnockOut serum replacement from Thermo Fisher in lieu of using fetal bovine serum (FBS) in our cell cultures, as the latter contains abundant DNASE1 and DNASE1L3. We noted that recombinantly-generated DNASE1L3 was able to digest coated plasmid, whereas DNASE1 or an empty vector control did not (Fig.
4A). In contrast, both DNASE1 and DNASE1L3 digested naked DNA with similar efficiency. Furthermore, we generated a truncated form of DNASE1L3 lacking the
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positively charged C terminal domain, and noted that it also completely failed to process coated DNA, but was still able to digest uncoated DNA (Fig. 4A).
Encouraged by our ability to replicate previously published results with our assay, we next sought to test the mutated form of DNASE1L3 linked to sporadic lupus. The SNP rs35677470 is a nonsynonymous mutation of C686 to
T686, and results in the substitution of a cysteine residue in place of an arginine at position 206 of DNASE1L3 (159, 160). Notably, this substitution results in the loss of a potentially crucial hydrogen bond (159). The R206C variant had previously been reported to be inactive (179), so we decided to generate this variant using site-directed mutagenesis to test its activity in our assay. We noted that although the R206C variant is not completely bereft of activity, it was roughly
10 fold less active than wild type DNASE1L3 (Fig. 4A). Crucially, this reduced activity was observed in the digestion of both coated and uncoated DNA, suggesting that this variant is hypomorphic but can still target liposome-coated
DNA.
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Figure 4: The C terminal domain of DNASE1L3 allows its activity to be specifically tracked as it confers the ability to digest coated DNA. A. Digestion of native or liposome- coated plasmid DNA by recombinant DNASEs as assessed by qPCR. Supernatants containing the indicated DNASE were incubated with uncoated or coated plasmid DNA for one hour at 37oC, and the DNA remaining after digestion assessed by qPCR using GFP-specific primers. Data are expressed as a percentage of input DNA (means +/- SD of three independent experiments). B. Digestion of purified human genomic DNA (gDNA) or purified human nucleosomes (nDNA). DNASEs were incubated with the DNA substrates for 10 minutes at 37oC, and the DNA remaining was assessed by qPCR using primers specific for the human Alu genomic repeat. Data are expressed as a percentage of input DNA (means +/- SD of three independent experiments).
Finally, we were intrigued by previous studies of DNASE1L3 where it was shown to digest nucleosomal DNA from isolated nuclei much more efficiently than DNASE1 (169). Furthermore, DNASE1L3 was demonstrated to digest chromatin independently of helper proteases, whereas DNASE1 required supplementary proteolysis (168). Additional findings hypothesized a role for
DNASE1L3 in the processing of genomic DNA during cell death via both apoptosis (170) and necrosis (172). To test the efficacy of our DNASE1L3 against chromatin, we designed qPCR primers specific for the endogenous
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retroelement Alu, of which roughly 106 copies are present in all primate genomes.
We then incubated recombinant DNASEs with commercially purchased human nucleosomes (EpiCypher) and again performed qPCR to assess the extent to which the DNA had been digested. We allowed the incubation to proceed for only
10 minutes, since at longer timepoints DNASE1 was able to digest nucleosomal
DNA (data not shown). Overall though, we noted that DNASE1L3 was much more efficient than DNASE1 at digesting nucleosomal DNA, and that this activity was dependent on the presence of the basic C terminal domain (Fig. 4B). In addition, the hypomorphic R206C variant was again roughly 10 fold less efficient in processing nucleosomal DNA (Fig. 4B). Notably, all recombinant DNASEs had comparable activity against control genomic DNA (Fig. 4B), suggesting that the activity of DNASE1L3 against nucleosomal DNA is a unique property and is not due to differences in its production or release as opposed to other DNASEs.
Minor perturbations of the C terminus caused by mutations dramatically affect the activity of DNASE1L3
Having established a means of testing DNASE1L3 activity specifically, we once again decided to pursue the generation of a tagged form of the enzyme.
Initially, we began by placing a His tag immediately behind the signal peptide at the N terminus (DNASE1L3 His-N). Although this variant could digest uncoated
DNA normally, it was unable to process coated DNA (data not shown). As there is no crystal structure for DNASE1L3, we examined that of the well-characterized
DNASE1 (149, 165). The tertiary structure of DNASE1 is such that the N
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terminus and C terminus are placed in close proximity, and the high degree of homology between DNASE1L3 and DNASE1 (Fig. 5A) suggests that this is likely the case for DNASE1L3 as well. Additionally, the key residues forming the interface between DNASE1 and DNA are very closely replicated in DNASE1L3
(Fig. 5B). Thus, it seems likely that our His tag disrupts the C terminal domain, preventing this form of DNASE1L3 from digesting coated DNA despite it being active catalytically.
Figure 5: The catalytic domains of DNASE1 and DNASE1L3 share extensive homology. A. Global sequence alignment of bovine DNASE1 (PDB:2DNJ.a; top) and human DNASE1L3 (UniprotQ:Q13609). Red boxes represent amino acids of DNASE1 that contact DNA and orange lines represent selected residues contacting DNA. B. 3D structure of the complex of DNASE1 with DNA. Selected residues contacting DNA are colored orange.
Given the challenges DNASE1L3 presented, we decided to collaborate with structural biologists in order to more thoroughly understand the enzyme and to understand the importance of the C terminus. Molecular modeling revealed that the C terminus of DNASE1L3 forms an extremely rigid alpha helical structure
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when isolated (Fig. 6A) and also in the context of the entire protein (Fig. 6B). To test the importance of positioning of the C terminus, we created versions of
DNASE1L3 with a His tag immediately preceding the C terminal domain (His- preCT) and at the C terminus itself (His-CT). Modeling of these variants revealed that although the alpha helical nature of the basic domain was preserved, the tertiary structure was affected in the case of the His-preCT variant and the domain was masked in the His-CT form. (Fig 6C). Indeed, analysis of the activity of the His-CT DNASE1L3 revealed that while digestion of naked DNA was preserved, it was unable to process coated DNA or nucleosomal DNA, suggesting that the C terminus must be exposed to function (Fig. 6D). In contrast, the His-preCT DNASE1L3 retained its activity against coated DNA but was deficient in digestion of nucleosomal DNA (Fig. 6D). Overall, this suggests two distinct roles for the C terminal domain: it both allows for the penetration of
DNASE1L3 through liposomes and, likely, lipid bilayers, and also allows it to digest nucleosomal DNA.
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Figure 6: Perturbations of the C terminal domain of DNASE1L3 dramatically affect its function. A. ab initio structure prediction of the C terminus of DNASE1L3 (aa 282-305). Shown is the 3D structure of the lowest energy conformation (left) and the energy spectrum of the folding simulation (right). Importantly, there is a roughly 5 kcal gap between the lowest energy conformation and the first non-helical conformation, strongly suggesting an extremely rigid helical structure. B. Homology model of DNASE1L3 based on PDB:2NDJ, with ab initio structure prediction of the C terminus. The rigid -helical conformation of the C-terminal domain is unperturbed in the context of the whole protein. C. Homology modeling of hexahistidine-tagged versions of DNASE1L3. Although the -helical conformation of the C terminus is not affected, it is shielded in the His-CT variant and its packing against the protein is altered in the His-preCT mutant. D. Digestion by His-tagged DNASE1L3 variants, as in Figures 7A and 7B. Data are presented as percentage of input digestion, and are means +/- SD of three independent experiments.
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Dnase1l3-deficient mice are unable to process liposome-coated DNA
In addition to testing recombinant DNASE1L3, we wanted to validate our assay using our Dnase1l3-deficient animals. We began by isolating serum from wild type or knockout animals and assessing its capacity to digest coated or uncoated DNA. Indeed, although knockout mouse serum could readily process uncoated DNA, we observed a defect in the digestion of coated DNA (Fig. 7A).
To further confirm this phenotype, either uncoated or coated plasmid DNA was injected intravenously into wild type or knockout mice, with serum being collected
3 hours post-injection and assessed by qPCR using GFP-specific primers. Again, while both wild type and knockout animals cleared uncoated DNA, the knockout animals were unable to process DNA after liposome coating (Fig. 7B).
Figure 7: Dnase1l3-deficient animals do not process liposome-coated DNA ex vivo or in vivo. A. Sera from Dnase1l3-deficient animals does not process coated plasmid DNA as assessed by qPCR. Although robust digestion of naked DNA was observed by sera of both WT and KO mice, loss of DNASE1L3 resulted in an inability to digest liposome-coated DNA. Data are presented as % of input DNA, displaying individual mice as well as the median value. B. Dnase1l3-deficient animals fail to process circulating liposome-coated plasmid DNA. WT or KO mice were injected intravenously with 1 g of native or liposome-coated plasmid DNA, then bled 3 hours later. After isolation of serum, the presence of plasmid DNA was determined by qPCR for GFP. Individual mice are shown, with bars representing the median.
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Circulating DNASE1L3 produced by hematopoietic cells prevents autoimmunity
In order to test whether circulating DNASE1L3 was produced by hematopoietic cells or a radioresistant cell type, we performed reciprocal bone marrow (BM) transfers between WT and KO mice. We then tracked DNASE1L3 activity over time in these mice by making a slight modification to our qPCR assay in order to compare small changes in circulating DNASE1L3. Whereas previously the standard curve was generated using serial dilutions of DNA in order to assess the percentage of DNA digested, we now diluted wild type serum to different percentages and incubated it with the same amount of coated
DNA as our test reactions for a shorter amount Figure 8: Active DNASE1L3 in circulation is produced by of time (10 minutes vs. 1 hour). This allows for hematopoietic cells. Reciprocal bone marrow chimeras were set up smaller changes in the amount of DNASE1L3 to and circulating DNASE1L3 activity tracked over time by qPCR. Data are expressed as percentage of WT be resolved. Using this technique, we observed activity and are shown as means +/- SD of 6-8 mice per group. progressive loss of systemic DNASE1L3 in WT mice that received KO bone marrow, and progressive gain in KO mice that received WT bone marrow (Fig. 8). Intrigued at these results, we decided to set up an additional set of long-term chimeras, wherein WT or KO bone marrow was injected into lethally irradiated WT mice. Consistent with our previous results, we observed progressive loss of DNASE1L3 activity in mice receiving KO bone
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marrow (Fig. 9A). In the group receiving KO BM, circulating DNASE1L3 activity was lost by 40 weeks post-transfer, which coincided exactly with the development of anti-dsDNA auto-Ab (Fig. 9A). At the experimental endpoint, we observed that these mice had eventually developed the hallmark symptoms of
SLE: ANA, kidney deposition of IgG, and glomerulonephritis (Fig. 9B-D). We conclude that autoreactivity to self-DNA inversely correlates with circulating
DNASE1L3 produced by hematopoietic cells.
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Figure 9: The production of autoantibodies in Dnase1l3-deficient mice is driven by hematopoietic cells. A-D: The development of autoantibodies in WT mice lethally irradiated and reconstituted with bone marrow from WT or KO mice. A. Recipient mice were simultaneously assessed for circulating DNASE1L3 activity (left panel) and for the presence of anti-dsDNA IgG by ELISA (right panel) post reconstitution. The left panel is presented as percentage of WT activity (means +/- SD of six animals per group). The right panel shows individual mice, with bars representing the median. B. ANA in the sera of recipient mice at the 55 week experimental endpoint, as in Figure 3A. Images are representative of six animals per group. C. Deposition of IgG in the kidneys of recipient mice, as in Figure 4G. Kidney sections stained for the presence of IgG by immunofluorescence are shown (left panels, representative of six animals per group), as well as the percentage of glomeruli with IgG deposition (right panel, out of 40 glomeruli per kidney, individual mice plus bars representing median). D. Assessment of kidney pathology in recipient mice, as in Figure 4I. Kidney sections were stained by H&E and histopathology scores were generated by a blinded pathologist. Shown is the percentage of the kidney cortex affected by inflammation (left panel, individual mice and median) and the median cumulative score of glomerulonephritis (right panel, individual mice and median).
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In order to directly test the effects of circulating DNASE1L3 on the development of autoreactivity, we injected young Dnase1l3 deficient mice with an adenoviral vector encoding human DNASE1L3 (Ad-DNASE1L3). Adenoviral injection in vivo results in the transduction of hepatocytes, allowing for the production of protein for several weeks until cleared. Indeed, after four weeks we observed total restoration of WT levels of DNASE1L3 in the knockout mice injected with Ad-DNASE1L3 compared to a control adenovirus (Fig. 10A).
Crucially, we observed a significant delay in the development of anti-dsDNA IgG in these mice compared to controls (Fig. 10B). Overall, these results suggest that circulating DNASE1L3 produced by hematopoietic cells prevents the development of autoimmunity by digesting an extracellular source of self-DNA.
Figure 10: Restoration of circulating DNASE1L3 into Dnase1l3-deficient animals delays the development of autoimmunity. Young 4-week-old Dnase1l3-deficient mice were injected with adenoviruses encoding human DNASE1L3 (Ad-DNASE1L3) or GFP (Ad-GFP). A. Serum DNASE1L3 activity after adenoviral administration at the indicated time points, along with age- matched WT controls. Data are presented as percentage of WT activity, and individual animals are shown with bars as the median. B. Serum titers of anti-dsDNA IgG in the same mice as measured by ELISA. Data are shown as the median +/- the range of four (Ad-GFP or WT) and nine (Ad-DNASE1L3) animals per group.
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Identifying the cellular sources of circulating DNASE1L3
We then wanted to understand which cell type was responsible for the production of DNASE1L3 in systemic circulation. Because our Dnase1l3-deficient animals were generated through the use of a gene trap insertion of a LacZ cassette, we were able to use the fluorogenic substrate fluorescein di-V- galactoside (FDG) to quantify -galactosidase activity in cells expressing
Dnase1l3 by flow cytometry. We found that Dnase1l3 expression is largely restricted to myeloid cells, in particular CD11c+ classical dendritic cells (cDCs) and, to a lesser extent, macrophages (Fig 11A). Additionally, while it seems to not be expressed broadly in lymphocytes, innate-like B cell populations such as marginal zone (MZB) and CD5+ B-1a cells do have low levels of Dnase1l3 (Fig.
11A). Furthermore, this limited expression we observed was confirmed by previously published microarray data in both humans (Fig. 11B) (180) and mice
(Fig. 11C) (181). In addition, we performed qPCR on sorted cell populations from primary mouse splenocytes, and consistent with these previous results we noted that classical DCs had the highest levels of Dnase1l3 expression (Fig. 11D).
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Figure 11: DNASE1L3 is expressed primarily by dendritic cells and macrophages. A. Flow cytometry analysis of Dnase1l3 expression in immune cells. Splenocytes from Dnase1l3LacZ/LacZ KO or WT mice were stained for the presence of LacZ as a proxy for DNASE1L3 using the fluorogenic substrate FDG. Histograms of FDG in the indicated gated cell populations are shown. Data are representative of three independent experiments. B. The expression of DNASE1L3 in human tissues and cell types. The expression profile of DNASE1L3 (probe 205554_s_at) in the Primary Cell Atlas microarray expression database as visualized in the BioGPS browser (biogps.org) is shown, with relevant cell types indicated. C. The expression of Dnase1l3 in murine immune cell populations. The expression profile of Dnase1l3 in the Immgen microarray expression database of key populations (top panel) and monocytes/macrophages (bottom panel) are shown. D. The expression of Dnase1l3 in sorted murine splenocyte populations, as determined by qRT-PCR. Relative expression levels in the indicated cell types are shown, normalized to Actb (mean +/- SD of triplicate PCR reactions). Data are representative of five experiments.
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We went on to utilize the alternative protocol to test DNASE1L3 activity in our mice lacking specific populations to understand the extent to which each population contributes to normal WT activity in circulation. In Rag1-deficient animals that lack all lymphocytes, circulating DNASE1L3 was not reduced compared to wild type controls, ruling out potential contributions from innate-like
B cells (Fig. 12A). However, upon transient depletion of cDCs and intestinal macrophages via injection of diphtheria toxin (DTX) into CD11c-diphtheria toxin receptor (DTR) mice, systemic DNASE1L3 levels were reduced by more than
75% (Fig. 12B). To rule out any effects of the transgene, we injected WT mice with chlodronate liposomes, depleting tissue macrophages and reducing cDC numbers. These mice lost roughly 50% of DNASE1L3 activity compared to mice injected with control liposomes (Fig. 12C). And finally, depletion of intestinal macrophages via a single injection of anti-macrophage colony stimulating factor
1 receptor (anti-Csf1r) blocking antibody resulted in a loss of roughly 15% of systemic DNASE1L3 (Fig. 12D).
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Figure 12: DNASE1L3 in circulation is produced primarily by dendritic cells and macrophages. A. DNASE1L3 activity in the sera of Rag1-deficient mice (individual animals and median). B. DNASE1L3 activity in the sera of DC-depleted mice. Animals with Cre- inducible diphtheria toxin receptor (DTR) with or without DC-specific Cre deletion (Cd11c- Cre) were injected with diphtheria toxin (DTX) intraperitoneally every other day for 2 weeks, and their sera was analyzed for DNASE1L3 activity (individual animals and median). C. DNASE1L3 activity in the sera of WT animals treated with liposomes containing either PBS or chlodronate to facilitate macrophage depletion on the indicated days after treatment (individual animals and median). D. DNASE1L3 activity in the sera of WT animals 12 days after intraperitoneal injection of control IgG or anti-Csf1r blocking antibody (individual animals and median).
Conclusions
Overall, we have designed an assay allowing the tracking of systemic
DNASE1L3 activity based on its unique C terminal domain and resultant ability to digest liposome-coated DNA. Our data emphasize the extent to which this C terminal domain is crucial for the enzyme’s full activity, as the function of
DNASE1L3 is majorly impaired by small adjustments to the stability or angle of the alpha helix. Importantly, this C terminal domain appears to have two roles for
DNASE1L3; it both allows for the penetration of lipid bilayers and also for the digestion of nucleosomal DNA. We hypothesize that the former activity may be
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due to its extremely basic properties while the latter may be due to the very stable and rigid structure this domain adopts, though further experiments are necessary to confirm.
Furthermore, while dendritic cells and macrophages have been previously described to play key roles in the maintenance of tolerance and prevention of autoimmunity (182, 183), our data suggest a novel mechanism by which this takes place: the secretion of a systemic enzyme that processes extracellular self-
DNA. Its affinity for both membrane-encapsulated DNA and for nucleosomal DNA suggest that it processes some form of extracellular chromatin, preventing the activation of autoreactive B cells. The inverse correlation between DNASE1L3 levels and anti-dsDNA autoAb indicates that DNASE1L3 is constantly required for the prevention of autoimmunity against a ubiquitous source of potentially antigenic self-DNA.
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Chapter 2
Microparticles as the physiological target of DNASE1L3
Microparticles as potential targets of DNASE1L3
We next wanted to determine the physiological form of self-DNA targeted by DNASE1L3. Taking into account the known properties of DNASE1L3 and our initial results, we inferred several likely characteristics of its target. Firstly, the self-DNA would probably be extracellular in nature, given the secreted nature of
DNASE1L3 and its detectable presence in systemic circulation. Also pointing to an extracellular antigen is the dispensability of the intracellular DNA sensor
STING in driving autoimmunity in knockout animals and the subsequent lack of a pronounced interferon (IFN) signature. Secondly, this DNA must be more or less ubiquitous, due to the constant requirement for the presence of DNASE1L3 to prevent autoimmunity. Thirdly, given the efficiency with which DNASE1L3 processes coated DNA and nucleosomal DNA, it is likely membrane associated and chromatin based. Given these parameters, we hypothesized that apoptotic microparticles might be a potential target of DNASE1L3. Although microparticles have been primarily investigated in the context of thrombosis (184, 185) and cancer (186, 187), recent work has linked them to autoimmune disorders such as systemic sclerosis (188), rheumatoid arthritis (189), and other rheumatic diseases such as vasculitis (190).
Also known as microvesicles, microparticles are released from cells during both cell death and activation (191). They are small, membrane-bound vesicles
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that are known to contain a variety of cytoplasmic and nuclear cargo, including proteins, messenger RNAs, micro RNAs, and other signaling molecules (192).
Consequently, microparticles have been implicated in intercellular crosstalk and regulation, both locally and systemically (190, 193, 194). Their biological functions include modulation of the immune system, regulation of coagulation, and transfer of signaling molecules (191, 195-198). Finally, on top of potential roles in disease pathogenesis, microparticles can be used as biomarkers, since they reflect the surface markers and intracellular contents of their parent cells
(191, 199-202). In addition, microparticles arising from apoptotic cells display phosphatidylserine on their surfaces, allowing for the binding of annexin V (191).
The extent to which microparticle DNA is reflective of or represents total DNA in human plasma, however, remains unclear (203, 204).
Links between microparticles and disease
Several recent studies have linked microparticles to systemic lupus erythematosus (189, 205-210). A particularly interesting study showed that although lupus patients did not have an increase in the numbers of microparticles, microparticles isolated from their bloodstreams had higher levels of autoantibody coating compared to controls (205, 207). In addition, it was shown that microparticles can be bound by DNA-specific autoantibodies (210). Furthermore, it has been established that apoptotic cells incorporate genomic DNA into released microparticles (211). Finally, these microparticles have been shown to
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expose chromatin on their surfaces (210, 212-214), thus representing a potentially novel source of self-DNA for DNA-reactive B cells.
Previous therapeutic efforts focused on microparticles were conducted using DNASE1, which upon its discovery was tenuously linked to SLE (144, 145,
215). Although no role for DNASE1 could ever be conclusively demonstrated, we hypothesized that microparticles may instead represent a potentially novel target of DNASE1L3 for several reasons. They are released extracellularly into circulation and are stable enough to have systemic effects (193), are present normally in the plasma of both healthy subjects and patients with SLE (205, 207,
216), and are loaded with chromatin incorporated into cell membranes (190, 210,
211). Given that our previous results suggested that the natural substrate of
DNASE1L3 was likely to be ubiquitously produced, present in the blood, and chromatin based and/or membrane encapsulated (see Chapter 1), microparticles thus met all criteria as a physiological target. To test the potential effects of
DNASE1L3 on microparticle DNA, we began by inducing the release of microparticles from the Jurkat T cell leukemia line.
Induction and isolation of microparticles
Staurosporine (STS) is a molecule originally isolated from the bacterium
Streptomyces staurosporeus (217). STS functions as a competitive inhibitor of
ATP, as it binds with very high affinity to many protein kinases (218). Previous studies have shown that Jurkat cells readily release microparticles upon STS treatment (191). We sought to confirm these findings, and indeed observed rapid
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release of microparticles by Jurkat cells treated with STS (Fig. 13A) accompanied by comprehensive apoptosis (Fig. 13B). To quantify microparticles, we employed a combination of rapid centrifugation and flow cytometry to determine purity (Fig. 13C).
Figure 13: Treatment with staurosporine induces rapid cell death and microparticle release. A. Extensive production of microparticles from Jurkat cells after treatment with staurosporine. Jurkat cells were treated with 1 mM staurosporine (STS) for the indicated amount of time. Cultures were harvested and stained with Annexin V and propidium iodide (PI) to assess cell death. Cell death was accompanied by a concordant increase in microparticle release. B. Near total induction of apoptosis in Jurkat cells treated with staurosporine. Jurkat cells were stained with Annexin V and PI after treatment with DMSO control (top panels) or STS (bottom panels) for 24 hours. C. Preparation of microparticles yields significant enrichment. STS-treated Jurkat cell cultures were first centrifuged to pellet cells, and supernatants were then spun at high speeds to pellet microparticles. After resuspension in sterile PBS, microparticles are assessed for purity by flow cytometry.
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DNASE1L3 is uniquely able to digest microparticle DNA
In order to test the digestion of human microparticle DNA, we designed qPCR primers specific for the Alu retroelement. The Alu element is a primate- specific short interspersed element (SINE) that comprises roughly 11% of the human genetic code (219). Because more than 106 copies are dispersed throughout the genome, PCR typing between adjacent copies has been widely used both in forensic science and in the study of population genetics (220). In lieu of using PCR to assess the distances between individual elements, we designed primers to amplify Alu itself, reasoning that its abundance and wide distribution meant it was likely to be incorporated into microparticles at a high level.
We began by assessing the capacity of recombinantly generated DNASEs to digest microparticle DNA by qPCR. As hypothesized, DNASE1L3 proved to be extremely efficient at eliminating microparticle DNA (Fig. 14A). This was in stark contrast to empty vector controls as well as DNASE1 (Fig. 14A), in concordance with previous studies that demonstrated an inability of DNASE1 to digest microparticle DNA (221). Wanting to understand whether this activity required the
C terminal basic domain of DNASE1L3, we also assessed the capacity of our truncated DNASE1L3 to digest microparticle DNA. Confirming the importance of the C terminus, this modified DNASE1L3 was completely deficient in its ability to process microparticle DNA (Fig. 14A). Finally, we also assessed the variant of
DNASE1L3 associated with sporadic lupus, R206C. As with uncoated and
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coated plasmid DNA, DNASE1L3-R206C was hypomorphic, processing microparticle DNA with roughly 10-fold reduced efficiency (Fig. 14A).
One potential caveat was our reliance on microparticles produced from the
Jurkat cell line, which after extensive culture may not be reflective of cellular behavior in physiological settings. We decided to validate our results using microparticles produced ex vivo from mouse splenocytes. Because STS induces apoptosis and microparticle release much more extensively in proliferating cells, we stimulated the splenocytes with phorbol 12-myristate 13-acetate (PMA) and ionomycin. These two compounds are widely used in combination to drive proliferation, cytokine production, and cell activation via activation of protein kinase C (PKC) (222, 223). After 72 hours of stimulation with PMA/ionomycin, cells were treated with STS to induce microparticle release and microparticles were harvested as previously described for Jurkat cells.
In order to measure digestion of mouse DNA by qPCR, we designed primers specific for the B1 family of retroelements. B1 repetitive elements are
SINEs analogous to Alu in humans, and are present in similar abundance in the mouse genome (224). Consequently, we could perform qPCR using these primers and previously isolated mouse DNA to generate a standard curve. After digestion with recombinant DNASEs, we observed that again only DNASE1L3 was capable of digesting DNA from splenocyte microparticles (Fig. 14B and data not shown), suggesting that the activity of DNASE1L3 is not limited to microparticles produced from a particular type of cell.
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Finally, we also wanted to assess whether the sera of Dnase1l3-deficient animals would be able to digest microparticle DNA, or if loss of DNASE1L3 resulted in a comparable phenotype to that observed for liposome-coated DNA.
Jurkat microparticles were isolated and incubated with sera from WT or
Dnase1l3-deficient mice. qPCR for the remaining human DNA revealed that sera from KO animals was largely unable to process microparticle DNA, whereas sera from WT animals efficiently digested this substrate extensively (Fig. 14C). These data emphasize the importance of DNASE1L3 in the digestion of circulating microparticle DNA, as other serum DNASEs had minimal effect on microparticle chromatin.
Figure 14: DNASE1L3 is uniquely able to process microparticle DNA. A. Digestion of Jurkat microparticle DNA by recombinant DNASEs, as measured by qPCR. Data presented as means +/- SD of three independent experiments. B. Digestion of mouse splenocyte MPs by recombinant DNASE1L3. Four independent experiments are shown. C. Digestion of Jurkat MP DNA by sera from WT or KO mice. Individual animals and median are shown.
In addition to our qPCR-based assay, we wanted to assess digestion of microparticle DNA by an alternative readout. We ultimately opted to utilize flow
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cytometry in conjunction with a DNA-specific fluorescent dye to assess DNA integrity. Previous studies had shown limited effectiveness of propidium iodide
(PI) in staining microparticles (221), likely due to their intact membranes. As an alternative, we decided to use a membrane-permeable dye used primarily for cell-cycle analysis, Vybrant DyeCycle Green (VG; ThermoFisher). We found that this dye extensively stained microparticles produced from Jurkat cells at very low concentrations (1:200,000) (Fig. 15). When microparticles were treated with recombinant DNASE1L3, there was dramatically reduced VG staining compared to microparticles treated with empty vector controls (Fig. 15). In stark contrast, treatment with DNASE1 had minimal effect on the VG staining of microparticles
(Fig. 15). We also assessed the digestion of microparticles produced from splenocytes by flow cytometry, and confirmed that DNASE1L3 but not DNASE1 dramatically reduces VG staining (data not shown).
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Figure 15: A membrane permeable DNA dye extensively stains microparticles only in the absence of DNASE1L3. Jurkat microparticles were isolated and treated with recombinant DNASEs, then stained with Vybrant Green membrane permeable DNA dye at a dilution of 1 to 200,000 for 20 minutes at room temperature. MPs were then analyzed on an Attune NxT flow cytometer. Data are representative of six independent experiments.
The surface composition of microparticles is altered by DNASE1L3
We also wanted to test for the presence of high mobility group box protein
1 (HMGB1). HMGB1 is a non-structural nuclear protein that is known to associate with nucleosomes, helping to facilitate transcription (225). However,
HMGB1 also functions as a key proinflammatory cytokine extracellularly. Upon stimulation of macrophages with lipopolysaccharide (LPS), HMGB1 is released, which activates NF-B via TLR4 (226). In addition to being produced by activated cells, HMGB1 functions as an alarmin, as it is also commonly released during cell death (227). HMGB1 has been described to be released concomitantly with microparticles, and may indeed associate with them due to its affinity for
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nucleosomal DNA (226-228). Furthermore, HMGB1 has been linked to the pathogenesis of SLE as both a proinflammatory signal that contributes to cell activation and as a potential target of autoantibody development (229-231).
Consequently, we wanted to investigate whether DNASE1L3 was capable of reducing HMGB1 levels on microparticles through the processing of nucleosomal
DNA using flow cytometry. Upon digestion of microparticle DNA by DNASE1L3, there was comprehensive loss of HMGB1 binding to their surfaces (Fig. 16A).
Furthermore, we noted that HMGB1 staining was restricted to DNA-positive MPs as labeled by Vybrant green, and we further confirmed that DNASE1L3 has dramatic effects on microparticle DNA (Fig. 16B). We thus conclude that
DNASE1L3 processes microparticle DNA in vitro, likely reducing their immunogenicity and surface composition.
Figure 16: DNASE1L3 treatment of microparticles alters their surface composition. A. Jurkat MPs were treated with rDNASEs and then stained for the presence of HMGB1. Mean fluorescence intensity is indicated. B. Costaining of pretreated Jurkat MPs with anti-HMGB1 and Vybrant green. Notably, there are very few HMGB1 single positive MPs, indicated strong associations between HMGB1 and MP DNA.
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Loss of DNASE1L3 results in a failure to process endogenous circulating microparticles in vivo
We next sought to assess potential changes to endogenous microparticles present in DNASE1L3 knockout animals. Previous work in the lab had examined circulating DNA in the sera of knockout versus wild type mice using the fluorescent DNA-intercalating dye PicoGreen. We had observed no differences in
DNA in the sera between wild type and knockout animals (Fig. 17A). In light of our findings regarding microparticles, however, we decided to return to these experiments. Importantly, the process by which sera is collected from mice involves extended centrifugation at 22,000 x g, meaning that our serum samples were likely free of microparticles. Furthermore, during serum collection the coagulation process is allowed to proceed, which could result in the release of
DNA not normally present in steady state circulation.
Thus, in lieu of sera, we decided to isolate plasma using less rapid centrifugation. In order to collect plasma, we required an anticoagulating substance. Because we intended to perform qPCR on our isolates, we could not use the most common anticoagulant, ethylenediaminetetraacetic acid (EDTA).
Instead, we used commercially purchased heparin to isolate plasma from WT and Dnase1l3-deficient animals. When whole plasma was assessed by qPCR, we could now observe a significant difference between DNA present in the plasma of wild type and knockout animals (Fig. 17B).
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Figure 17: Dnase1l3-deficient mice have higher levels of DNA circulating in plasma. A. No difference in the amount of DNA in the sera of WT and KO mice, as determined by staining with the DNA intercalator PicoGreen. Individual mice and median are shown. B. qPCR on whole murine plasma from WT or KO animals using primers specific for the B1 genomic repeat. Individual mice and median are shown.
We then wanted to assess the endogenous microparticles of these mice more specifically. To that end, we isolated plasma as above, and after isolation spun the plasma at 22,000 x g for 30 minutes to pellet microparticles. These were then resuspended and stained with anti-CD41 and anti-Ter119, which are specific for platelets and red blood cells, respectively. This was necessary because platelets and debris from erythrocytes are approximately the same size as microparticles and are also pelleted by these centrifugation steps. The negative fraction was then counted on an Accuri tabletop flow cytometer in order to precisely determine the numbers of relevant apoptotic microparticles (Fig.
18A). Notably, although microparticle count varied considerably from mouse to mouse, we observed no difference between wild type and knockout animals (Fig.
18B). We then performed qPCR using B1-specific primers on this material and in conjunction with the flow cytometry data calculated the amount of DNA per microparticle. We noted that microparticles isolated from knockout animals contained on average four orders of magnitude more DNA compared to those from wild type animals (Fig. 18C). To further show the specificity of the defect to
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microparticle DNA, we also performed qPCR on the non-microparticle fraction of plasma (i.e. the supernatant after the rapid spin to pellet microparticles). We noted that there was virtually no DNA present in plasma outside of microparticles, and indeed were unable to detect DNA reliably despite our qPCR amplifying standards at the femtogram level (data not shown).
Figure 18: Microparticles from Dnase1l3-deficient mice carry increased amounts of DNA compared to those of WT mice. A. Identification of the relevant MP fraction from the plasma of mice. Only events within the MP gate that were negative for CD41 and Ter119 were counted as bona fide MPs. No differences between WT and KO mice were observed. Representative of nine mice per group. B. No difference in the total numbers of MPs isolated from WT or KO mice was observed. Individual animals and median are shown. C. Increased DNA in MPs isolated from KO mice compared to WTs. qPCR was performed on isolated MPs using B1 primers, and the total amount of DNA in the reaction was then divided by the number of MPs included. Individual mice and median are shown.
Finally, we also decided to inject exogenous microparticles generated from Jurkat cells into wild type and knockout animals to assess their ability to cope with a sudden burden of microparticle DNA. 6 x 106 Jurkat microparticles were injected intravenously into 6 wild type and 6 knockout animals, and the mice were then bled 3 hours later. After isolation of plasma, qPCR using Alu- specific primers was then performed. We again observed a defect in knockout
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animals in the processing of exogenous microparticles, noting a significant increase in microparticle DNA equivalents per L of plasma (Fig. 19). Knockout animals were hugely deficient in clearing microparticle DNA, as they made little impact on overall amounts for 24 hours (data not shown). We thus conclude that
DNASE1L3 processes DNA in circulating microparticles in vivo.
Figure 19: Dnase1l3-deficient mice fail to process exogenous microparticles in vivo. 6 x 106 Jurkat microparticles were injected intravenously into WT or KO mice, and plasma samples taken 3 hours post administration. qPCR for human Alu repeats was then performed. Individual mice and median are shown.
Conclusions
These data suggest that DNASE1L3 is responsible for the processing of nucleosomal DNA contained within circulating microparticles. Our initial results in
Dnase1l3-deficient animals suggested that its endogenous substrate was likely to be nucleosomal and/or membrane associated in nature. Microparticles represent an underappreciated source of extracellular genomic DNA (192, 202, 211, 221), and by definition have intact plasma membranes (221). Because chromatin is potentially immunogenic, its persistence in circulation on microparticles may compromise tolerance to self-DNA. DNASE1L3 rapidly processes the DNA load of microparticles, as we have demonstrated by two independent readouts.
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Furthermore, treatment of microparticles with DNASE1L3 reduces the binding of
HMGB1 to their surfaces, which represents a potent proinflammatory molecule that has been extensively linked to the pathogenesis of SLE (225, 229-232). This finding thus demonstrates that DNASE1L3 can fundamentally affect the profile of circulating microparticles even beyond their DNA content.
The sheer scale of granulocyte turnover alone (>109 per kg per day) (233) ensures that microparticles are ubiquitously present throughout circulation at the steady state. Thus, the previously observed requirement for the presence of
DNASE1L3 in order to prevent the development of anti-dsDNA autoreactivity is in keeping with the context of continuous microparticle release. Indeed, a recent paper that focused on deep sequencing of cell-free DNA from human plasma found that the majority is hematopoietic in origin, with an especially large amount coming from granulocytes (203). We believe that the vast majority of DNA analyzed by these authors originated from microparticles, since they used whole plasma and because we found virtually no DNA in the microparticle-free plasma fraction in mice.
Overall our results indicate that microparticles represent a novel source of antigenic self-DNA, in particular in the absence of DNASE1L3 where it is allowed to persist in circulation for extended periods of time. DNASE1L3 is uniquely capable of processing microparticle DNA due to its basic C terminal domain.
DNASE1 in contrast had minimal impact on microparticle DNA, confirming previous studies (191) and again emphasizing the importance of DNASE1L3 as opposed to DNASE1 in circulation. Importantly, our data demonstrate that there
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is no defect in microparticle release or production, but rather that produced microparticles are not rendered immunologically benign in circulation.
Additionally, the production of microparticles as a consequence of cell death may be relevant for human disease, which typically features flares that can be triggered by a huge variety of stimuli, including but not limited to exposure to UV radiation, viral infection, pregnancy, and physical injury (1, 12, 234-236). Even in individuals with no defect in DNASE1L3, overwhelming the system with a large quantity of DNA-loaded microparticles may allow for increased persistence and the opportunity for microparticles to trigger anti-DNA autoimmunity.
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Chapter 3
Microparticles as autoantigens
Roles for microparticles in human disease
Having established that microparticles are not processed fully in the absence of DNASE1L3, we next wanted to assess their potential as antigen- loaded targets of autoimmune responses. While the rapid development of autoantibodies against dsDNA and, in particular, chromatin in our knockout animals suggest a loss of tolerance to these molecules that may be caused by their increased presence in microparticles, we sought to determine whether microparticles themselves represent suitable B cell antigens. Understanding whether microparticle-driven autoimmunity is based on direct interaction with B cells is potentially key for further elucidation of the mechanisms underlying pathogenesis. Importantly, DNA may behave as an unconventional antigen due to its ability to stimulate the production of cytokines via TLR activation in addition to its intrinsic immunological properties.
Numerous studies have offered potential explanations for the systemic effects attributed to microparticles, including but not limited to priming of dendritic cell and neutrophil subsets (216), metalloprotease and cytokine production by fibroblasts (196), induction of soluble selectin release (188), crosslinking of receptors on endothelial cells (198), interactions with galectin-3 binding protein
(206), activation of interferon production by plasmacytoid DCs (237), and direct delivery of cytokines or other signaling molecules (185, 193, 208). Alternatively,
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microparticles have been proposed to be targets of autoantibodies, directly contributing to the formation of immune complexes in rheumatic diseases such as SLE (192, 205, 207, 209, 210, 214, 238). Furthermore, other mouse models of
SLE, including the MRL-lpr/lpr spontaneous lymphoproliferative mutants as well as the NZB/NZW F1 generation present with anti-microparticle autoantibodies
(209, 214). Given the rapid development of anti-dsDNA and anti-chromatin autoantibodies in our mice and the strong association between anti-DNA autoantibodies and disease severity in lupus patients (56, 74, 83, 89), we wanted to investigate the potential binding of autoantibodies to microparticles and the effects of DNASE1L3 on the repertoire of antigens present on microparticle surfaces.
Microparticles are bound by circulating IgG from Dnase1l3-deficint animals
We began by investigating the extent to which microparticles were bound by autoreactive antibodies from Dnase1l3-deficient mice compared to controls using flow cytometry. Microparticles isolated from Jurkat cells were incubated with sera from wild type or knockout mice, followed by a fluorescent secondary antibody. We observed dramatically increased binding of the sera of knockout animals compared to that of controls to microparticles (Fig. 20A), suggesting that microparticles represent viable B cell autoantigens. In addition, we tested sera from mice at different ages to establish whether binding to microparticles worsened during lupus pathogenesis. Strikingly, we observed clear increases in microparticle binding over time in knockout mice but not in wild type animals (Fig.
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20B), indicating that microparticles are indeed bound by pathogenic autoantibodies that increase in prevalence over time and correlate with disease severity in Dnase1l3-deficient mice.
We next wanted to investigate the potential effects of treatment with
DNASE1L3 on autoantibody binding to microparticles. To this end, we treated
Jurkat microparticles with an empty vector control, recombinant DNASE1, or recombinant DNASE1L3. From there, we stained each group of microparticles with the sera from five different knockout mice at 20 weeks of age. While
DNASE1 had minimal effects on the binding of mouse sera to autoantibodies compared to the empty vector control, treatment with DNASE1L3 completely ablated all binding (Fig. 20C). Overall these results demonstrate that microparticles represent viable B cell antigens, and that treatment of microparticles with DNASE1L3 dramatically reduces their immunogenicity.
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Figure 20: DNASE1L3-sensitive chromatin on the surface of microparticles is antigenic in Dnase1l3-deficient mice. A-C: Binding of circulating murine IgG to the surface of human apoptotic microparticles. Jurkat MPs were incubated with sera from Dnase1l3 KO or control WT animals, followed by secondary anti-mouse IgG fluorescent antibody. A. Representative histograms of mouse IgG fluorescence on the surface of Jurkat MPs. Percentage of positive MPs is indicated. B. Percentage of mouse IgG positive Jurkat MPs bound by WT or KO mice at the indicated time points. Data are presented as median +/- range of five animals per group. C. Jurkat microparticles were incubated with supernatants containing recombinant human DNASE1L3, DNASE1, or an empty vector control prior to staining with serum from Dnase1l3- deficient animals. Representative histograms are shown in the left panel, with the percentage of IgG positive MPs indicated. The right panel shows individual mice, with the bars representing the median.
Circulating microparticles from knockout animals are coated with IgG
Next, we investigated the coating of endogenous circulating microparticles by antibodies in wild type or knockout mice. As previously, we exsanguinated young mice at 10 weeks of age into heparin in order to isolate plasma. We found
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that even at 10 weeks of age, knockout animals had more extensive binding of
IgG to endogenous microparticle surfaces (Fig. 21A). Additionally, we stained these endogenous microparticles with PR1-3, a monoclonal antibody specific for anti-DNA/histone 2a/2b complexes (210). We noted significantly higher levels of
PR1-3 binding to microparticles isolated from knockout mice compared to those from wild type mice (Fig. 21B), suggesting that in addition to being more coated, endogenous microparticles in young Dnase1l3-deficient animals have higher levels of exposed chromatin and are thus more immunogenic.
Figure 21: Microparticles from Dnase1l3-deficient mice at young ages are more coated by IgG and have higher levels of exposed self-antigens. A. Staining of MPs isolated from mouse plasma from WT and KO mice for mouse IgG. The left panel shows representative histograms with the MFI indicated, and the right shows MFIs of individual mice with bars representing the median. B. MPs isolated from young WT or KO mice were stained using the anti-DNA/histone antibody PR1-3. Representative histograms are shown in the left panel with the percentage of PR1-3 positive MPs indicated, and the right shows percentage of PR1-3 positive MPs from individual mice with bars representing the median.
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Microparticles drive loss of tolerance to self-DNA in inflamed conditions
To further examine the ability of microparticles to drive anti-nucleic acid autoimmunity, we decided to inject them into mice and assess the subsequent autoantibody levels. Our initial experiments used microparticles isolated from
Jurkat cells, but unfortunately these elicited massive germinal center and inflammatory responses in all settings. In retrospect, this is unsurprising due to the fact that Jurkat cells constitutively release murine gammaretroviruses picked up during extensive passaging in culture (239) in addition to the presence of potent cross-species and anti-tumor host responses. Thus, we instead opted to produce syngeneic microparticles from mouse splenocytes treated with PMA and ionomycin, as previously (see Chapter 2). In order to assess de novo anti-DNA autoimmunity, we could not use Dnase1l3-deficient animals, as they have autoantibodies from extremely early ages. Instead, we injected wild type mice with an adenovirus encoding IFN-5, delivery of which has been shown to accelerate the development of SLE in experimental models (175) as well as in
Dnase1l3-deficient mice (173). These mice rapidly presented elevated levels of both IFN and the IFN-inducible gene Sca-1, confirming adenoviral efficacy (data not shown). Microparticle injections into these mice but not naïve mice produced very high titers of anti-nucleosome IgG (Fig. 22). These data demonstrate that
MPs can function as antigens that elicit chromatin-specific B cell autoimmunity in the context of elevated interferon.
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Figure 22: Immunization with microparticles drives anti- chromatin antibody responses in inflammatory contexts. WT mice were administered IFN5 adenovirus (IFN), MPs from syngeneic apoptotic splenocytes (MP), or both. Serum titers of anti- nucleosome IgG was measured one week post final injection by ELISA. Titers from individual animals and the median are shown.
DNASE1L3-sensitive chromatin on microparticles is accessible to autoantibodies
We next wanted to establish whether microparticles were bound by a variety of anti-chromatin monoclonal antibodies, and whether this binding was affected by treatment of microparticles with DNASE1L3. To this end, we obtained four hybridomas from our collaborator Dr. Mark Shlomchik that produce antibodies specific for different DNA/histone complexes implicated in SLE: PR1-3
(DNA/histone 2a/2b), PR2-8 (DNA/histone 2a/2b, DNA/histone 3/4), PR9-11
(naked DNA), and 29A-3H9 (isolated from 3H9 transgenic mouse that presents with very high anti-dsDNA and anti-ssDNA).
To test binding of these anti-chromatin antibodies and the potential effects of DNASE1L3, Jurkat microparticles were treated with recombinant DNASEs
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prior to staining with our hybridoma antibodies. We noted that all antibodies bound to microparticles treated with the empty vector control extensively, further confirming that microparticles represent viable antigens for anti-chromatin B cell responses (Fig. 23). Furthermore, treatment with DNASE1 had a minimal impact on autoantibody binding (Fig. 23). In contrast, treatment with DNASE1L3 significantly reducing binding of all autoantibodies, providing further evidence that
DNASE1L3 is key for modulating the immunogenicity of microparticles, limiting the avidity of anti-chromatin immune responses (Fig. 23).
Figure 23: Treatment of microparticles with DNASE1L3 but not DNASE1 prevents binding by DNA-specific autoantibodies. Microparticles isolated from Jurkat cells were treated with supernatants containing recombinant DNASE1, DNASE1L3, or an empty vector control. They were then stained with murine antibodies against DNA and/or chromatin originally isolated from mouse SLE models, followed by a fluorescent anti-mouse secondary. Shown are representative histograms from four experiments, with the percent of IgG positive MPs indicated.
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The C terminus of DNASE1L3 is required to prevent autoantibody binding to microparticles
We also wanted to test the extent to which the C terminus of DNASE1L3 is required for this activity, as well as the extent to which the DNASE1L3-R206C hypomorphic form retains its function. To that end, we again isolated Jurkat microparticles, and treated them with a vector control, DNASE1, DNASE1L3, truncated DNASE1L3, or DNASE1L3-R206C. From there, we stained these treated microparticles with purified PR1-3, which was provided by our collaborator Dr. Keith Elkon. Washing and staining with our secondary antibody was unchanged from previous experiments. Strikingly, we observed that truncated DNASE1L3 had a minimal effect on PR1-3 binding to microparticles, in stark contrast to wild type DNASE1L3 (Fig. 24A). In contrast, DNASE1L3-R206C was capable of preventing PR1-3 binding to similar levels, but given that our previous qPCR experiments suggested that it still retained considerable activity this was not unanticipated (Fig. 24A). We also went on to test the activity of our self-generated His-tagged DNASE1L3 variants. Although we had previously seen that the his-preCT variant, which has a his tag immediately preceding the C terminal -helix retained its activity against coated DNA. When tested for its ability to process microparticle DNA, however, we found that this variant was unable to reduce binding of PR1-3 (Fig. 24B). The his-CT variant of DNASE1L3, which has a His tag at the end of the C terminus, was previously unable to process coated DNA and was similarly deficient here (Fig. 24B). These data demonstrate that in the absence of DNASE1L3, microparticles are extensively
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bound by anti-chromatin autoantibodies and that incubation of microparticles with
DNASE1L3 is sufficient to render them non-immunogenic. In addition, the spatial integrity of the C terminal domain is vital for this function of DNASE1L3, as even slight perturbations of the enzyme’s tertiary structure can dramatically impact its ability to process microparticles.
Figure 24: The C terminal domain of DNASE1L3 is required for the processing of microparticle chromatin. A. Microparticles from Jurkat cells were incubated with recombinant DNASEs and subsequently stained with the purified anti-nucleosome antibody PR1-3 followed by a fluorescent secondary antibody. Histograms are shown with the percentage of IgG positive MPs indicated. Data are representative of three experiments. B. As in A, Jurkat MPs were treated with the indicated mutant forms of DNASE1L3 and then stained with PR1-3. Histograms are shown with the percentage of IgG positive MPs indicated. Data are representative of three experiments.
Microparticles are bound by prototypical autoantibodies from human SLE
Finally, we wanted to assess the relevance of microparticles in human disease. Human antibodies that are positive for the 9G4 idiotype are greatly
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enriched among SLE-associated autoantibody repertoires (240, 241). They have been shown to bind to a variety of antigens, including DNA, nuclear components, and apoptotic cell nuclei (240, 241). We obtained a panel of 22 chimeric antibodies consisting of Fab fragments isolated from patients fused with the mouse IgG Fc region from our collaborator, Dr. Iñaki Sanz. We wanted to assess binding of these antibodies to Jurkat microparticles that had been treated with an empty vector or with DNASE1L3 by flow cytometry, as we had done previously.
Given the minimal effects we had observed of DNASE1 on antibody binding and the limited quantities of primary antibody available, we elected to omit it from our analysis. We observed three different binding patterns: some antibodies did not bind to MPs at all, some bound to all MPs, and some bound to MPs treated with the vector control only (Fig. 25A). Of our panel of 22 antibodies, 9 bound to microparticles treated with the vector control, and of these, 6 then failed to bind to microparticles after DNASE1L3 treatment (Fig. 25B). These DNASE1L3- sensitive antibodies were reactive against apoptotic cell membranes, and were positive for anti-nuclear antigen staining (241). These findings suggest that microparticles may represent novel sources of antigen in sporadic SLE in addition to rare cases where there is DNASE1L3 deficiency.
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Figure 25: A subset of human 9G4+ antibodies bind to microparticles only in the absence of DNASE1L3. A. Jurkat MPs were treated with an empty vector control or with DNASE1L3, and were then incubated with 9G4+ mAbs. Three patterns were observed: no binding to any MPs, binding to all MPs, and binding to MPs treated with vector control only. Representative histograms of each group are shown from left to right, respectively. Data are reflective of three independent experiments. MFIs of IgG bound to MPs are indicated. B. Classification of the 22 different 9G4+ clones tested based on binding pattern.
Conclusions
Overall these results demonstrate that microparticles represent sources of self-antigen that are accessible to B cells. Furthermore, data from our knockout mice suggest that antibody responses to microparticles are enhanced over time, indicating that they play a role in disease pathogenesis. Additional evidence for this stems from the observation of increased coating of IgG of endogenous microparticles from knockout animals compared to wild type controls. On top of that, our experiments injecting microparticles into permissive mice show that they also likely play a role in the loss of tolerance observed in SLE. We hypothesize that failure to process microparticle DNA represents the primary insult driving the development of anti-dsDNA autoimmunity in Dnase1l3-deficient mice.
Identifying the specific form or forms of DNA recognized by B cells during the pathogenesis of SLE remains a major challenge of the field. While multiple
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types of DNA have been linked to the pathogenesis of SLE, including neutrophil extracellular traps, self-DNA complexed with anti-microbial peptides, and oxidized mitochondrial DNA (104-107, 242), their potential as B cell antigens remains unresolved. In contrast, our results demonstrate that microparticles enriched in genomic DNA in the absence of DNASE1L3 are extensively bound by autoreactive antibodies, many of which have been implicated in the pathogenesis of SLE (210, 214, 240, 241). Furthermore, the reductions in autoantibody binding observed specifically after treatment of microparticles with DNASE1L3 but not
DNASE1 indicate a fundamental role for DNASE1L3 in preventing the development of autoimmunity.
Importantly, our data emphasize that the defect in DNASE1L3-deficient animals stems from a failure to process microparticles rather than an issue in their production. After digestion with DNASE1L3, microparticles appear to be rendered inert with respect to anti-DNA and anti-chromatin immune responses, though the presence of additional autoantigens on their surface is also likely as evidenced by the binding of three 9G4+ antibodies to DNASE1L3-treated microparticles. Furthermore, our results using 9G4+ antibodies indicate a potential role for microparticles in sporadic lupus. Although it seems unlikely that there may be overt defects in microparticle DNA processing at the steady state, microparticles may represent novel sources of self-antigen in sporadic patients.
Finally, given the widespread links between cell death and/or stress and the pathogenesis of SLE (235, 236), the resulting temporary increase in microparticle burden may contribute to disease and/or flare onset.
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Overall we believe our Dnase1l3-deficient mouse represents a novel, comprehensive, monogenic model of SLE. This lupus is driven by a failure to process the DNA contained within apoptotic microparticles in circulation, which are normally produced primarily from granulocytes. Chromatin-containing microparticles function as potent self-antigens for autoreactive B cells, and the unique properties of DNASE1L3 allow it to process this chromatin and render microparticles to be nonimmunogenic. Finally, we believe that our model describes a novel cell-extrinsic mechanism of self-tolerance to DNA mediated by
DCs and macrophages that may allow for therapeutic intervention.
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Chapter 4
DNASE1L3 and microparticles in human lupus patients
Circulating cell-free DNA in humans is contained within microparticles
Given our comprehensive studies of Dnase1l3-deficient mice, our findings that a subset of 9G4+ antibodies are reactive against microparticles in a
DNASE1L3-sensitive manner, and the DNASE1L3-sensitive nature of microparticle chromatin, we believed that microparticles may represent unappreciated sources of antigen in human disease. In particular, recent work has suggested that loss of tolerance in human SLE is initially to chromatin, with subsequent epitope spreading to DNA specifically and other nuclear autoantigens (152, 243). Furthermore, it is increasingly accepted among the field that DNA-specific autoantibodies are among the most pathogenic variants, and their prevalence has been strongly linked to poorer outcomes for patients (56, 74,
83, 89). Because DNASE1L3 is able to dramatically reduce the efficiency with which DNA and chromatin-specific antibodies bind to microparticles in other contexts, we wanted to directly investigate human patients.
Thanks to a collaboration with the Department of Rheumatology at NYU and Dr. Jill P. Buyon and Dr. Robert M. Clancy, we were able to obtain samples from a large cohort of patients with SLE. We were initially interested in assessing the surface markers present on microparticles to determine the cells from which they are primarily derived. These studies were far more feasible to perform in human settings compared to mice simply due to the major differences in
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obtainable plasma volume. A recent study of cell-free DNA in human circulation focused on deep sequencing as a means of determining nucleosome positioning and, hence, cell type of origin (203). They concluded that the vast majority of cell- free DNA comes from hematopoietic cells, with a particularly large amount stemming from granulocytes (203). This was in line with a previous study where the authors examined a cohort of patients that received sex-mismatched bone marrow transplants, then examined cell-free DNA (244). They noted that the
DNA present in plasma was predominantly of donor origin, demonstrating that hematopoietic cells contribute the majority of cell-free DNA (244).
Circulating microparticles in humans are primarily of hematopoietic origin
We performed flow cytometry on microparticles isolated from human plasma, and observed positive staining of CD45 and CD66 on Annexin V positive microparticles, confirming that circulating microparticles originate largely from granulocytes (Fig. 26A). In addition, we wanted to determine the extent to which microparticle DNA comprises total plasma cell-free DNA. To assess this, after rapid plasma centrifugation of human samples, qPCR was run in parallel between the pelleted microparticles, the remaining soluble fraction, and unfractionated plasma. We found that there was virtually no DNA present in the non-microparticle fraction of plasma, and correspondingly microparticle DNA made up nearly all detectable DNA in total plasma (Fig. 26B). We thus conclude that microparticles released from granulocytes and other hematopoietic cells
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make up the bulk of circulating DNA, and therefore are likely an important source
of self-antigen in humans.
Figure 26: Analysis of human microparticles. A. Phenotype of circulating microparticles in human plasma. MPs were isolated and stained for CD42b and CD235a to eliminate platelets and RBCs. We defined MPs as being FSClo SSClo CD42bneg CD235aneg. Representative stainings are shown comparing Annexin V versus the endothelial marker CD31, the leukocyte marker CD45, and the granulocyte marker CD66b. B. Distribution of human genomic DNA between the microparticle fraction and the soluble fraction of human plasma. DNA was measured by qPCR in unfractioned plasma, MPs, and the soluble fraction. DNA amounts per L of the original unfractioned volume are shown in samples from SLE patients (n=8) or healthy controls (n=2).
Mutations in DNASE1L3 result in defective digestion of coated and
nucleosomal DNA in humans
Via our collaboration with Drs. Buyon and Clancy, we were extremely
fortunate to obtain plasma samples from DNASE1L3-deficient human patients
previously characterized with hypocomplementemic urticarial vasculitis syndrome
(HUVS) (151). Patient 1 had presented with SLE in addition to HUVS but is now
in remission, while patient 2 presented with HUVS only. Thus, neither patient was
affected by active SLE at the time of blood sampling, ruling out potential
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secondary effects of the disease. We also obtained blood samples from their haplodeficient parents, neither of whom ever manifested with effects of SLE nor
HUVS (151).
We began by assessing the activity of DNASE1L3 in the plasma of these patients. Using the digestion of liposome-coated plasmid DNA, we confirmed the total lack of DNASE1L3 activity in the plasma of these patients, while their haplodeficient parents had roughly 50% activity (Fig. 27A). In addition, we analyzed 3 patients who were heterozygous for the hypomorphic R206C variant of DNASE1L3, and observed that these patients had roughly 60% of DNASE1L3 activity (Fig. 27A), indicating that this variant is approximately 5 fold less active than the wild-type form in vivo.
In addition, we assessed the extent to which patient plasma could digest commercially-purchased nucleosomal DNA over short incubations, as we previously showed in mice (see Chapter 1). As in Dnase1l3-deficient mice,
DNASE1L3-null human patients were totally unable to process human nucleosomal DNA compared to normal controls (Fig. 27B). Additionally, while the haplodeficient parents could digest nucleosomes to a greater extent than the patients, they were also broadly deficient compared to controls (Fig. 27B). Finally, we determined whether null and haplodeficient patients were capable of processing microparticle DNA. We induced and isolated microparticles from
Jurkat cells, then incubated them with plasma from these null patients. As with digestion of nucleosomal DNA, null patients were greatly deficient in their
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capacity to digest microparticle DNA, while the haplodeficient parents were partially deficient (Fig. 27C).
Figure 27: DNASE1L3 null mutations in human patients result in defective DNA processing that parallels Dnase1l3-deficient mice. A-C. Plasma from 2 DNASE1L3- deficient patients, their haplodeficient parents, individuals with the hypomorphic R206C polymorphism (n=3), and normal control subjects (n=2-5) were analyzed. A. DNASE1L3 activity in patient plasma, measured by qPCR of DOTAP-coated plasmid DNA after incubation. Data are expressed as a percentage of the activity in reference control plasma. Individual human subjects and median are shown. B. Digestion of human nucleosomal DNA by human plasma. Data are expressed as a percentage of the input DNA. Individual human subjects and median are shown. C. Digestion of chromatin in microparticles by the soluble fraction of patient plasma. Individual subjects and median are shown.
Patients lacking DNASE1L3 present with increased microparticle DNA
Having confirmed that these patients were indeed deficient for DNASE1L3 and that our assay developed in mice could be applied to these rare human patients, we wanted to analyze the DNA content of circulating microparticles ex vivo. As we were working with human plasma, we utilized the same protocol optimized in mice in order to pellet microparticles from plasma. These pellets
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were resuspended and stained with anti-CD42b and anti-CD235a in order to eliminate platelets and erythrocyte debris respectively, which are commonly included after microparticle isolation due to their similar size. The stained microparticles were then assessed by flow cytometry, and the negative fraction counted precisely. In parallel, qPCR on these isolated microparticles was performed along with a human DNA standard curve, and, in conjunction with the precise quantification of starting material as determined by flow cytometry, the mass of DNA contained in each microparticle was calculated. Because platelets and erythrocytes do not contain DNA, we were unconcerned about potential false reads stemming from their inclusion in the qPCR reaction. Strikingly, we found that DNASE1L3-null patients had hugely elevated levels of DNA per microparticle compared to controls, with haplodeficient parents again at intermediate levels
(Fig. 28A). Additionally, we assessed three patients heterozygous for the R206C hypomorphic variant and found that they also had elevated levels of DNA in circulating microparticles (Fig. 28A).
As we previously showed that microparticle DNA represents the vast majority of circulating DNA in human plasma, we also assessed the levels of
DNA in unfractionated plasma from these patients by qPCR. Indeed, we observed increased DNA in total plasma of DNASE1L3-null patients compared to controls, with haplodeficient parents and R206C heterozygous patients intermediate (Fig. 28B). Collectively, we believe that these genetic data in both mice and humans demonstrate that DNASE1L3 is required for normal digestion of DNA in plasma, in particular in microparticle DNA.
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Figure 28: DNASE1L3 null mutations in humans result in defective processing of endogenous microparticles in circulation. A. The amount of genomic DNA in MPs isolated from patient plasma. Results are shown as the amount of DNA per MP as determined by qPCR. Individual subjects and median values are shown. B. The amount of genomic DNA per volume of total unfractionated patient plasma, as determined by qPCR. Individual subjects and median values are shown.
DNASE1L3-sensitive binding of patient IgG to microparticles
We next wanted to assess the extent of autoreactivity against microparticles in these null patients. To do this, we employed the same protocol used previously in Dnase1l3-deficient mice. Briefly, microparticles were isolated from Jurkat cells, then treated with an empty vector control or with recombinant
DNASE1L3. After incubation these microparticles were stained with plasma from patients, then washed and stained again with an anti-human IgG secondary antibody. Flow cytometry was employed to determine the extent of binding. We found that only plasma from the patient who had previously developed SLE contained microparticle-specific autoantibodies (Fig 29). Notably, this binding was completely absent when microparticles were treated with DNASE1L3 (Fig.
29). The patient who did not develop full blown SLE and the healthy haplodeficient parents did not have microparticle-specific autoantibodies (Fig. 29).
It is also notable that the first patient retains detectable levels of these
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autoantibodies despite being in disease remission after treatment with rituximab, a potent therapeutic antibody that blocks B cell activation. Given the very high levels of DNA in the microparticles of both patients and the lack of an immune response in the second patient, these data strongly suggest that accumulation of
DNA in microparticles is the primary insult that precedes loss of tolerance.
Figure 29: DNASE1L3-sensitive binding of patient IgG to exogenous microparticles. Jurkat microparticles were treated with an empty vector control or with DNASE1L3, then incubated with plasma from human subjects followed by a secondary anti-human IgG fluorescent antibody. Histograms from DNASE1L3 haplodeficient parents are shown as well as the fully deficient patients. Percentage of IgG positive MPs is indicated for each condition.
Binding of circulating IgG from sporadic SLE patients to microparticles
While studies of these DNASE1L3-null patients provide compelling genetic evidence validating many of our experiments using mice, the applicability of our findings to the much more prevalent sporadic disease remain unclear.
Furthermore, the rarity of patients that fully lack DNASE1L3 make detailed analysis unfeasible. Consequently, we sought to determine whether these findings were of relevance to SLE more generally. To accomplish this, we again
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took advantage of our collaboration with Drs. Buyon and Clancy of the NYU
Department of Rheumatology, obtaining plasma samples from 136 patients with
SLE and 12 controls. We began by assessing the prevalence of anti- microparticle autoantibodies in all patients and controls, again using microparticles isolated from Jurkat cells that were treated with a vector control or with DNASE1L3.
We observed major variability in the extent to which both treated and untreated microparticles were bound by patient plasma. We began by examining binding to untreated microparticles. As expected, there was a very strong correlation between the percentage of microparticles bound by IgG and the mean fluorescence intensity of stained microparticles (R2=0.82) (Fig. 30). We decided to define patients as being positive for anti-microparticle IgG if we observed binding to at least 5% of microparticles, calling them “Binders”. The roughly 20% of patients that failed to meet this threshold (28/136) we deemed to be “Non- binders” (Fig. 30). Notably, none of the control plasma tested bound more than
5% of microparticles (Fig. 30). Patients with anti-microparticle antibodies bound to 30.59 +/- 16.50% of control treated microparticles (Fig. 30).
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Binders Non binders Controls
90
80
70
60
50
40
30 % of IgG positive MPs IgG %positive of
20
10
0 100 1000 10000 100000 1000000 IgG total MFI
Figure 30: Extensive binding of microparticles by the plasma of patients with sporadic SLE. Microparticles from Jurkat cells were stained with the soluble fraction of patient plasma, followed by a fluorescent anti-human secondary antibody. Shown is the MFI of IgG fluorescence versus the percentage of positively stained microparticles for each human subject. Positive binding was defined as > 5% IgG positive MPs (blue circles). Patients who bound to fewer than 5% were classified as non-binders (red circles). Healthy control subjects all failed to bind (green circles).
We then compared the binding of each patient to microparticles treated with DNASE1L3, and noted that patients fell into three classifications: one group bound to microparticles only in the absence of DNASE1L3 (DNASE1L3- sensitive); one group bound to microparticles regardless of DNASE1L3 treatment
(DNASE1L3-insensitive); and one group failed to bind to microparticles regardless of treatment (non-binder) (Fig. 31A). To mathematically categorize patients as sensitive or insensitive, we compared the differences in both
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percentages of microparticles bound and the total mean fluorescence intensity
(MFI) between incubation with native or DNASE1L3-treated microparticles.
Patients were considered insensitive to DNASE1L3 treatment if there were no changes of at least 2 fold in either percent of microparticles bound or in MFI. Our calculations showed that 35.3% of patients bound to microparticles in a
DNASE1L3-sensitive manner (48/136), while 44.1% (60/136) bound regardless of treatment with DNASE1L3 (Fig. 31B). These data demonstrate that microparticles represent an important source of B cell self-antigens in sporadic
SLE and that treatment of microparticles with DNASE1L3 is sufficient to modulate their immunogenicity in some patients.
Figure 31: Classification of sporadic SLE patients based on DNASE1L3 sensitivity of IgG binding. A. Patient plasma was used to stain MPs treated with an empty vector control or with DNASE1L3. Representative histograms of IgG staining for each reactivity pattern are shown. Percentages of IgG positive MPs are indicated. B. The distribution of sporadic SLE patients into categories based on their staining pattern (n=136).
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DNASE1L3 activity varies between sporadic patients
We also wanted to assess the activity of circulating DNASE1L3 in these patients. To do this, we employed the same assay we had developed to look at depletion mouse models (see Chapter 1). In brief, different amounts of wild type serum were titrated and used to digest 1 ng of coated plasmid DNA in order to determine a standard. Digestion with patient plasma could thus be calculated as a percentage of normal activity. We began by assessing the activity of controls versus SLE patients, noting a much greater range of activity levels across patients (Fig. 32A). However, when we separated patients by their categorized binding to microparticles, we noted that patients that bound to microparticles in a
DNASE1L3-sensitive manner had significantly lower levels of DNASE1L3 activity compared to controls, non-binding patients, and DNASE1L3-insensitive binding patients (Fig. 32B). Because DNASE1L3 is an extracellular protein whose activity in circulation is key to protect against the development of autoimmunity, this cohort may be the most amenable to therapeutic intervention via the delivery of
DNASE1L3. Further investigation is required to determine whether the decline in
DNASE1L3 activity in these patients is potentially contributing to loss of tolerance or if it is a secondary effect due to ongoing disease. In addition, understanding exactly what is causing these differences could yield valuable insights into the regulation of DNASE1L3 both in inflamed conditions and at the steady state.
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Figure 32: The activity of DNASE1L3 in the circulation of sporadic SLE patients differs according to the observed pattern of staining. A. DNASE1L3 activity assays of all sporadic patients and healthy controls, as measured by the digestion of DOTAP- coated plasmid DNA, expressed as a percentage of the activity in reference control plasma. Individual subjects and median are shown. B. DNASE1L3 activity assays of SLE patients only, classified based on their previously determined pattern of binding to native and DNASE1L3- treated MPs. Individual subjects and median are shown.
Finally, we wanted to assess whether the variability we observed in
DNASE1L3 activity was reflected in differences in endogenous microparticle
DNA in patients. We isolated microparticles from human plasma by centrifugation, then counted microparticles by flow cytometry as previously, again only considering the fraction negative for the platelet and erythrocyte markers CD41b and CD235a. Due to limitations of detection, we only considered patients from whom we were able to isolate at least 1000 microparticles, leaving us with 36
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patients. We performed qPCR using Alu-specific primers to quantify the amount of DNA present in each sample, then divided by the number of microparticles used in the test to yield DNA per microparticle. Notably, there was a strong inverse correlation (R2=0.492) between circulating DNASE1L3 activity and DNA per microparticle in sporadic patients (Fig. 33). These data suggest that even small changes in DNASE1L3 activity can impact the amount of DNA present in circulating microparticles in sporadic patients.
Figure 33: Changes in DNASE1L3 activity in sporadic patients impacts the DNA cargo of microparticles. DNA load of MPs isolated from the plasma of sporadic patients was determined by qPCR and flow cytometry. The soluble fraction was used to assess DNASE1L3 activity. Individual patients and best fit line are shown.
Conclusions
Overall these data demonstrate that chromatin on the surface of circulating apoptotic microparticles represents an antigen for autoreactive B cells and immune complexes in sporadic and inherited human SLE. In patients lacking active DNASE1L3, we observed strikingly higher levels of DNA in circulating microparticles, confirming our studies using Dnase1l3-deficient mice.
Furthermore, the presence of high levels of microparticle DNA in the absence of
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active SLE in the second patient indicate that this elevation may be the primary defect that leads to loss of tolerance upon interaction of autoreactive B cells with
DNA-laden microparticles. Also intriguing is the consistent observation of anti-
C1q autoimmunity in DNASE1L3-deficient patients; further studies are required to determine the potential links between microparticles and complement. One possibility is that increased chromatin in microparticles due to loss of DNASE1L3 results in C1q binding with greater avidity, as previous studies have demonstrated that DNA is a substrate for C1q (153, 245, 246). This increased binding could lead to enhanced clearance of C1q and also potential activation of autoreactive clones, resulting in the high levels of anti-C1q autoantibodies observed in these patients and ultimately HUVS (151).
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Discussion and Future Directions
Dnase1l3-deficient mice represent a novel, monogenic model for the study of inherited SLE
Loss of DNASE1L3 in humans results in extremely aggressive, early onset
SLE driven by anti-dsDNA IgG that lacks a sex bias (150, 151). Upon knockout of
Dnase1l3 in mice, we observed the same features on two independent pure strains as well as on mixed backgrounds: loss of tolerance to self-DNA leading to elevated titers of anti-DNA and anti-chromatin antibodies at very early ages, followed by late-onset immune activation driven by immune complex deposition.
Importantly, this model provides the means to study inherited SLE in mice; previous efforts have been unsuccessful. As previously discussed, defects in the complement component C1Q have been tied to severe SLE, and in fact null mutations in C1Q cause SLE in human patients (90, 176). In mice, however,
C1Q is dispensable, as its knockdown fails to cause any discernable pathology
(247). Consequently, to our knowledge Dnase1l3-deficient mice represent the only mouse model of hereditary SLE reflective of human disease. In addition, these data provide an explanation for the extensive conservation observed between mouse and human DNASE1L3.
An outstanding question regarding this model concerns the delayed immune activation observed, which is in contrast to other mouse models and to
DNASE1L3-deficient patients. The rapid development of autoreactivity against dsDNA and chromatin only led to comprehensive immune activation in old
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animals, and furthermore these mice failed to lose tolerance to other self- antigens (e.g. RNA). One possible explanation for this is the lack of supplementary mutations prominent in other mouse models. For instance, the
Yaa mouse features enhanced sensing of RNA due to duplication of TLR7, while the Faslpr model is driven by aberrant lymphoproliferation and defective negative selection. Another important consideration is the lack of baseline immune stimulation in mice housed under specific pathogen-free conditions compared to human patients, especially considering the extensive literature linking viral infections and the resulting antiviral immune responses to SLE flaring in patients
(4, 24, 84). Strong evidence for this hypothesis stems from our observation that artificially introducing IFN into Dnase1l3-deficient animals dramatically accelerated autoimmunity and induced novel anti-RNA responses, and furthermore led to substantial mortality. Further studies could similarly take advantage of this model to examine the effects of other triggers linked to flares in human disease. In particular, induction of apoptosis in vivo by pharmaceutical means may yield insights into environmental effects on pathogenesis in humans.
We thus conclude that anti-DNA reactivity directed against chromatin in these mice causes the observed pathologies, and furthermore primes for severe SLE- like disease upon inflammatory stimulus.
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Autoimmunity in Dnase1l3-deficient mice is distinct from that seen in
Dnase2- or Trex1-deficient animals
Previous studies of DNASE1L3 pointed towards a potential role in the processing of intracellular DNA molecules, in particular during apoptosis or necrosis (170-172). However, cytoplasmic sensing of DNA requires the intracellular adaptor molecule STING, as evidenced by the ablation of disease when STING is deleted in conjunction with either DNASE2 or TREX1 (126, 248).
Disease in Dnase1l3-deficient animals, in contrast, was totally independent of
STING, suggesting that DNASE1L3 processes an extracellular form of DNA. In addition, our lab observed that disease was completely dependent on MyD88, which may point to a role for TLR or IL-1 signaling. Importantly, aberrant activation or any developmental defects in either DCs or B cells in very young
Dnase1l3-deficient animals was not detected. We therefore hypothesize that the observed primary response to DNA may be driven by direct recognition by and subsequent expansion of DNA-reactive B cells, in all probability by extrafollicular activation. These primary signals are then amplified by a MyD88-dependent pathway or pathways in B cells or possibly other cell types. Thus, a critical future direction remains identifying the specific MyD88-dependent signals involved, which would provide additional insight into the mechanisms underlying loss of tolerance to genomic DNA which drives autoimmunity in Dnase1l3-deficient animals. More broadly, the extrafollicular production of autoantibodies may be a unique property of self-DNA as an antigen, as it has recently been shown that
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activation of TLR7 or TLR9 in conjunction with BCR signaling results in robust affinity maturation in the absence of germinal centers (249).
The presence of DNASE1L3 in circulation is required constitutively to prevent the development of anti-dsDNA autoimmunity
We observed that the levels of DNASE1L3 inversely correlated with the development of anti-dsDNA Abs. This suggests that the presence of DNASE1L3 is constitutively required in circulation in order to prevent interactions between extracellular DNA and autoreactive B cells. We also demonstrate that the majority of serum DNASE1L3 is produced by DCs and subsets of tissue macrophages, in concordance with previously published microarray data. In particular, CD11c+ DCs produced roughly 70-75% of circulating DNASE1L3.
While DCs and macrophages have been previously shown to play key roles in mediating self-tolerance and thus preventing autoimmunity (182, 183), we believe our data present a novel mechanism by which this occurs. While it is established that phagocytosis of apoptotic cells, induction of tolerogenic T cell programs, and the production of anti-inflammatory molecules and cytokines are all means by which self-tolerance is maintained by DCs and macrophages, here we demonstrate that the release of a DNA-digesting enzyme into circulation is required to prevent the activation of autoreactive B cells.
One confounding observation we made concerned the kinetics of
DNASE1L3 activity after hematopoietic reconstitution. In wild-type mice receiving
Dnase1l3-deficient bone marrow, activity persisted in these animals until roughly
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30 weeks, despite typical splenic dendritic cell turnover being on the order of days (250). This may be due to the production of DNASE1L3 into circulation in response to lethal irradiation by a non-hematopoietic radio-resistant cell population such as hepatocytes, which normally express DNASE1L3 although they seemingly do not contribute to circulating levels at the steady state.
Alternatively, long-lived resident macrophage populations may persist and upregulate DNASE1L3 in response to the inflammation induced by total body irradiation, allowing for some degree of compensation. Further studies examining potential contributions of hepatocytes or tissue resident cells to systemic
DNASE1L3 under duress may provide context for these findings.
Microparticles are an endogenous substrate of DNASE1L3
The unique features of DNASE1L3 described here and by others led us to hypothesize that its endogenous substrate is chromatin-loaded circulating microparticles, for several reasons. Firstly, the requirement for DNASE1L3 systemically in order to prevent autoimmunity indicates that its substrate must be present in circulation ubiquitously. Microparticles containing DNA are present normally in circulation (205, 216) as a consequence of the constant and rapid turnover of various blood cell types, in particular granulocytes. Secondly,
DNASE1L3 is uniquely able to digest membrane-encapsulated DNA. This activity is dependent on its highly basic C terminus, the importance of which is made clear by the unusual degree to which it is conserved evolutionarily. Microparticles are intact structures that sequester DNA, as evidenced from the requirement to
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disrupt microparticle membrane integrity with detergents in order to robustly stain with propidium iodide. Thirdly, the efficiency with which DNASE1L3 digests DNA packaged into nucleosomes suggests its target is likely chromatin-based.
Microparticles contain the vast majority of detectable genomic DNA in human plasma, which is comprised of primarily nucleosomal chromatin fragments derived from leukocytes (203, 204, 251). Consequently, mice deficient for
Dnase1l3 had significantly higher levels of DNA within their circulating microparticles as well as in their total plasma, which was corroborated in both
DNASE1L3-deficient human patients and their haplodeficient parents. Thus, although it is possible that DNASE1L3 may process other types of self-DNA, our data demonstrate that genomic DNA contained within microparticles is a relevant endogenous target (Fig. 34).
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Figure 34: Proposed mechanism by which DNASE1L3 prevents the development of autoimmunity. As microparticles are released from cells undergoing constant turnover, DNASE1L3 released into circulation by dendritic cells and macrophages digests their chromatin contents. This prevents interactions between DNA-specific B cells and this antigenic self-DNA, ultimately resulting in tolerance to self-DNA. In the absence of DNASE1L3, chromatin-loaded microparticles persist in circulation and are able to be presented to DNA-specific T cells, ultimately resulting in the activation of autoreactive B cells and the release of anti-dsDNA antibodies.
Microparticles are relevant sources of B cell antigens
Previous studies have also shown that chromatin on circulating microparticles is exposed and can be bound by autoantibodies (210, 212, 213), demonstrating that microparticle DNA represents a viable antigen for B cells.
DNASE1L3 digests this exposed chromatin on microparticles, preventing the binding of autoantibodies from Dnase1l3-deficient animals and from the
DNASE1L3-deficient patient who had previously developed SLE. In addition, our
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experiments using murine hybridomas and 9G4+ monoclonals demonstrate that this chromatin is recognized by numerous archetypal autoantibodies found in both mouse and human SLE in a DNASE1L3-sensitive manner. Finally and most broadly, we observed binding by IgG from roughly 40% of sporadic patients to
DNASE1L3-sensitive chromatin on microparticles. Endogenous microparticles from lupus patients have been previously described to be extensively bound by complement and IgG (207, 210), demonstrating their relevance as SLE autoantigens in vivo. Furthermore, the ability of microparticles to drive autoimmunity in sporadic patients may explain the flaring aspect of SLE, as temporal increases in microparticle production and/or release could overwhelm the available pool of DNASE1L3, allowing for persistence of unprocessed chromatin in circulation.
In addition to the patients whose antibodies bound to microparticles only in the absence of DNASE1L3, a further 40% of patients had IgG that bound to microparticles both before and after DNASE1L3 treatment. Because they contain a myriad of potentially immunogenic proteins, lipids, and nucleic acids, extensive characterization of the exposed surface antigens available to be bound by patient antibodies is required. One such molecule that is present on microparticle surfaces and is unaffected by DNASE1L3 is Sjögren’s-syndrome-related antigen
A (SSA), antibodies against which are very strongly associated with both SLE and with disease-related complications (139-141). Furthermore, the apoptotic mechanisms that drive the export of chromatin from nuclei to microparticle surfaces remain uncharacterized, and additional in depth analysis of their
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contents may yield insights to this somewhat enigmatic pathway. Notably, the
R206C hypomorphic variant of DNASE1L3 has also been linked by GWAS to the development of scleroderma (160), which is often characterized by the presence of anti-centromere or anti-kinetochore autoantibodies (252). As evidenced by its ability to reduce the levels of the DNA-associated protein HMGB1 on microparticle surfaces, DNASE1L3 may also be capable of processing other potentially immunogenic molecules to prevent loss of tolerance in systemic scleroderma.
Beyond SLE: DNASE1L3 and other diseases
Another outstanding question concerns the strong associations between human DNASE1L3 deficiency and the development of anti-C1q antibodies, which eventually results in hypocomplemetemia and likely contributes to the severity of
SLE. C1q is normally involved in the clearance of apoptotic or other cellular debris, and strikingly has been shown to bind with increased avidity to such structures when they contain chromatin (153, 245, 246). Thus, we would hypothesize that in the absence of DNASE1L3, microparticles are more extensively bound by C1q. This may both deplete the pool of free C1q in circulation and may also contribute to activation of C1q-specific B cells, since
C1q is now presented in conjunction with immunogenic molecules contained within microparticles. As these cells become activated and produce anti-C1q, the pool of C1q is further reduced, leading to impaired clearance of chromatin-loaded, unprocessed microparticles. Persistence of these microparticles would allow for
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presentation to DNA-specific T and B cells, resulting in loss of tolerance and progression to SLE. Future experiments will investigate the potential impact of treatment with DNASE1L3 on the binding of complement to microparticles. In addition, determining whether anti-C1q antibodies are elevated in Dnase1l3 knockout mice is key, albeit with the caveat that these antibodies do not lead to
SLE development in mice as in humans (247). Finally, assessment of potential anti-C1q antibodies in sporadic patients will also be an important parameter, especially in the context of anti-microparticle immunity.
In addition, we are keen to investigate potential roles for DNASE1L3 in the regulation of cancer microparticles. As previously mentioned, microparticles have been implicated in the pathogenesis of cancer, both by enhancing metastasis and transformation through the delivery of various types of nucleic acids (184,
186, 187, 200). Thus, DNASE1L3 may be an important host factor in limiting tumorigenesis, which will be tested in both spontaneous and injected mouse models. An alternative possibility is that the presence of DNASE1L3 prevents host interactions with oncogenic DNA or oncoproteins, which may impair the development of specific immune responses. More broadly, we would like to explore the potential usage of microparticles as a diagnostic surveillance tool.
Because they contain mRNA in addition to DNA, we would like to perform RNA- seq on microparticles isolated from various disease models, in particular cancer.
These data can then be used to determine the cells of origin, so deviations from the steady state may be indicative of transformation. Such a liquid biopsy could
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be easily applied to humans, and may be a way to improve early detection of and screening for cancer.
DNASE1L3 activity as a marker for sporadic SLE
An intriguing finding in our sporadic cohort was the observation that diminished levels of circulating DNASE1L3 correlated with larger reductions in binding to microparticles after treatment with exogenous DNASE1L3. Although statistically significant, the biological impact is unclear, since individuals carrying the hypomorphic R206C variant have much greater reductions in activity without necessarily progressing to overt disease. In addition, the mechanisms driving differences in the activity of DNASE1L3 remain undiscovered. One possibility is the development of autoantibodies against DNASE1L3 itself, which will be tested in sporadic patients. Alternatively, preliminary data in the lab has suggested that
DNASE1L3 expression is induced in macrophages in response to inflammatory stimuli, which may also explain variability between not only patients but also samples from the same patient. With the help of a collaboration with experts in the field of biostatistics, we are in the process of closely examining our data in conjunction with all measured parameters available. We are keen to determine whether DNASE1L3-sensitive chromatin binding correlates with data obtained in the clinic, which may lead to translational opportunities. In parallel, we are beginning to investigate patients longitudinally, which may allow for in depth assessment of the impact of disease flares, therapeutic interventions, and/or
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inflammatory status on IgG binding to microparticles as well as the activity of
DNASE1L3.
Our initial studies of sporadic patients were deliberately limited to those of
European descent, with the aim being to specifically examine the impact of the
R206C variant. We subsequently expanded our studies to include African
American and Hispanic patients. Among the first group, we observed a greater fraction of patients who had no circulating anti-microparticle IgG compared to the cohort overall (~35% vs 20%). Given that disease in the latter populations tends to be more severe, anti-microparticle responses may represent an important and predictive disease parameter. Indeed, such a diagnostic test would be relatively straightforward to implement, and may lead to substantial quality of life improvements for patients who may be able to receive therapeutics prophylactically to prevent flaring as opposed to reactively.
Therapeutic applications of DNASE1L3
One therapeutic option that we are eager to explore further concerns the administration of exogenous DNASE1L3, since we were able to restore normal levels of DNASE1L3 activity and consequently significantly delay the onset of anti-dsDNA autoantibodies by the use of an adenoviral vector in Dnase1l3- deficient mice. A major drawback of this system, however, is that adenoviral vectors persist in hepatocytes for only 6-8 weeks, after which they are cleared by the immune system and subsequent administrations are ineffective due to memory responses. One solution is to take advantage of adeno-associated
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viruses (AAVs), which do not trigger immune responses and can thus be used for prolonged expression in hosts (253, 254). Our lab has obtained two such AAVs encoding DNASE1L3, and both have proved effective at restoring DNASE1L3 levels to those comparable to wild-type animals. It remains to be seen whether these AAVs are effective at ameliorating disease development, but if so such a treatment could be beneficial in sporadic SLE patients, especially since the cohort with DNASE1L3-sensitive antibodies also presented with lower
DNASE1L3 activity.
In addition, our laboratory is working on the development of hyperactive
DNASE1L3. As previously discussed, DNASE1 was modified to be actin- independent and more potent catalytically by the substitution of six amino acids, four of which are already present in native DNASE1L3 (166). Using site-directed mutagenesis, the additional two substitutions have been introduced, and we are in the process of testing its efficacy. Although it may prove challenging due to the importance of the tertiary structure of the C terminus, we are also investigating potential modifications of DNASE1L3 that may allow it to persist for longer in circulation. Ultimately, we envision such a molecule as being a valuable therapeutic tool for the treatment of both inherited and sporadic SLE: a potent, long-acting injectable enzyme, the dosage of which can be modified based on well-defined longitudinal parameters.
We are also intrigued at potential roles for DNASE1L3 outside of circulation. As mentioned, it is expressed by hepatocytes, despite our observations that hematopoietic cells are responsible for the pool of DNASE1L3
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in circulation. Consequently, it is possible that DNASE1L3 is also released into the digestive tract, as is the case for DNASE1. Given its potent activity and ability to process coated DNA, DNASE1L3 may play an important role in regulating the intestinal microbiome, potentially by limiting bacterial horizontal gene transfer. In addition, we have observed much more efficient adeno-associated viral transduction in Dnase1l3-deficient mice compared to wild-type animals, which may suggest that DNASE1L3 also plays a role in defense against DNA viruses.
Characterization of the microbiome and viriome of Dnase1l3-deficient animals may prove illuminating. If DNASE1L3 does indeed have a role as an anti-viral molecule in particular, its administration may prove effective for the treatment of enveloped DNA viruses, such as herpesviruses or poxviruses.
Conclusion
SLE is a very challenging disease to study for numerous reasons, chief among them being the heterogeneity of presentation between individual patients and/or flares. Indeed, it seems likely that what is collectively referred to as SLE in fact represents a collection of closely related disorders, wherein loss of tolerance occurs to a self-nuclear antigen. This can be followed by epitope spreading and the development of broader autoimmunity. Our studies have focused on
DNASE1L3, since it is strongly tied to the very early and robust development of anti-dsDNA autoantibodies. These antibodies are predictive of both disease flaring and of disease severity between patients, underlying the urgent need to
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understand more about how and why they are generated (255). Our data provide an example of one potential mechanism by which tolerance to self-DNA is lost.
In addition, it is possible that microparticles may be fundamentally involved in loss of tolerance to self-antigens that are not DNA-based. Given the large quantities of RNA and nuclear proteins they contain, their persistence in the bloodstream may also allow for delivery of additional autoantigens to reactive T or B cells. Further work is required to understand the extent of their contents and in what contexts they can contribute to the initial steps of autoimmunity. The complement system seems to be involved; preliminary work in the lab has found that microparticles are actively bound by multiple forms of complement.
Furthermore, C1q deficiency represents an additional monogenic form of SLE in humans (155), suggesting that microparticle clearance is fundamentally required to prevent aberrant self-reactivity.
In conclusion, our data demonstrate that chromatin in microparticles is a potential self-antigen for autoreactive B cells, and that it is processed by circulating DNASE1L3 in both mice and humans. This digestion is both restricted to DNASE1L3 and is absolutely required to prevent the development of rapid anti-DNA responses that progress over time to full blown SLE. In addition, these results provide mechanistic insight to the observed associations between null and hypomorphic mutations in DNASE1L3 with inherited and sporadic SLE, respectively. More broadly, we have discovered a novel, cell-extrinsic means by which tolerance to a self-nucleic acid is enforced by DCs and, to a lesser extent, macrophages. The secretion of an enzyme that protects against autoimmune
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development allows for unique opportunities for therapeutic intervention, as evidenced by our observation of delayed anti-DNA reactivity after re-expression of DNASE1L3 using an adenoviral vector. In particular, such a therapy may prove efficacious in the roughly 40% of patients who presented with extensive
IgG binding to DNASE1L3-sensitive chromatin, especially as this same cohort also featured lower levels of DNASE1L3 activity. In addition, mutations in
DNASE1L3 have been linked to other autoimmune diseases such as systemic scleroderma and HUVS, so delivery of exogenous DNASE1L3 may not be limited as a therapeutic tool to SLE alone.
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Materials and Methods
Dnase1l3-deficient animals
All experiments were performed according to the investigator’s protocol, which was approved by the Institutional Animal Care and Use Committees of
Columbia University and New York University. Mice with a targeted germline replacement of Dnase1l3 (Dnase1l3LacZ) were purchased from Taconic Knockout
Repository (model TF2732). Backcrossing onto 129SvEvTac and C57BL/6 backgrounds was performed for >10 generations, and pure strains were then intercrossed to yield Dnase1l3LacZ/LacZ knockout animals. Age-matched wild-type mice of the same backgrounds were bred in the same colony for use as controls.
Deletion of STING or MyD88 in Dnase1l3-deficient mice
STING-deficient mice (Tmem173gt/J) (256) and MyD88-deficient mice
(Myd88tm1.1Defr/J) (257) were obtained from Jackson Laboratories, both on pure
B6 backgrounds. Both strains were crossed to Dnase1l3LacZ B6 mice to produce double-heterozygous F1 litters, which were then backcrossed to Dnase1l3LacZ mice, thus generating Dnase1l3LacZ/LacZ mice that were either wild-type or homozygous null for Tmem173 or Myd88 (double knockout, dKO). To allow for infection-free survival of MyD88-deficient animals, breeder cages were administered trimethoprim and sulfadiazine-supplemented animal chow
(Uniprim®, Harlan Teklad). Upon weaning, mice were switched to a normal diet.
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Importantly, this diet had no effect on the development of autoimmunity in the littermate controls of dKO mice who lacked only DNASE1L3.
Human subjects
DNASE1L3-deficient HUVS patients 1 and 2 correspond to patients IV-4 and IV-5 from family 1 described in (151). Patient 1 (14-year-old female) developed SLE with ANA and high anti-dsDNA 6 years ago. She was treated with rituximab (most recent dose in May 2012) and has been in clinical remission since (limited ANA, normal anti-dsDNA), although patient still presents with active uveitis. Patient 2 (12-year-old female) has not developed SLE (negative ANA, no anti-dsDNA) but presents with ongoing HUVS with moderate to severe vasculitis of the kidney. Their study was approved by the Ethics Committees of Ankara
University (Turkey) and the Institutional Review Board of the University of Miami.
Informed consents were obtained from the parents.
Blood from patients with sporadic SLE and healthy controls was obtained from the NYU IRB-approved Rheumatology SAMPLE (Specimen and Matched
Phenotype Linked Evaluation) Biorepository. All patients signed IRB-approved informed consent forms. All patients with sporadic SLE met at least 4 revised
American College of Rheumatology criteria (12). Subjects were genotyped for
DNASE1L3 rs35677470 using the TaqMan SNP genotyping kit (ThermoFisher).
Three subjects (one healthy control and two SLE patients) were found to be heterozygous for the rare R206C hypomorphic allele.
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Adenoviruses
The adenoviral vector encoding IFN-5 has previously been described
(175). Human DNASE1L3 was cloned into an adenoviral vector and constructed by Welgen. Viral particles were produced and purified at Welgen, and were injected into Dnase1l3LacZ/LacZ mice intravenously at 0.5 or 1 x 1010 particles per mouse. No discernable differences were observed between the two doses. As a control, Dnase1l3LacZ/LacZ mice were injected with equivalent doses of adenoviral particles encoding GFP.
ELISA
Anti-dsDNA and anti-RNA IgG titers were determined as previously described (72, 258). Plates were pre-coated with poly-L-lysine (0.05 mg/mL) for 2 hours at room temperature, followed by coating with 0.1 mg/mL calf thymus DNA or yeast RNA (Sigma-Aldrich) as antigens. Anti-nucleosome IgG titers were assessed using plates coated with 1 g/mL of purified HeLa polynucleosomes
(Epicypher). After coating, sera were added and incubated overnight. The amount of bound IgG was measured with an alkaline phosphatase (AP)- conjugated goat anti-mouse IgG antibody (1:5000, Jackson ImmunoResearch).
Antigen-specific IgG titers were determined using serial dilution of serum from a positive animal as a standard, and thus expressed as arbitrary units per volume.
Anti-dsDNA IgG subtypes were measured as above using AP-conjugated antibodies to the indicated IgG subtypes (Southern Biotech). Total
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immunoglobulin isotype levels were also determined as above, using AP- conjugated antibodies to IgM, IgA, and IgG (Southern Biotech).
ELISPOT
To detect antibody-secreting cells (ASC) specific to dsDNA, 96 well multiscreen plates (Millipore) were pretreated with 35% ethanol for 1 minute, coated with 100 g/mL calf thymus dsDNA (Sigma-Aldrich), then washed and blocked with a solution of 3% fetal calf serum (FCS) and 3% bovine serum albumin (BSA) in PBS. Single-cell suspensions of total splenocytes were plated in duplicate in six two-fold serial dilutions starting at 106 cells per well. Cells were incubated on the plate for 5 hours at 37oC. Plates were then washed and incubated with AP-conjugated goat anti-mouse IgG (1:5000, Jackson
ImmunoResearch) overnight at 4oC. Spots were developed using the NBT/BCIT
(Sigma-Aldrich) system and counted on an ImmunoSpot Series 1 ELISPOT analyzer (Cellular Technology Ltd).
Anti-nuclear antibody staining (ANA)
Prefixed HEp-2 cells on glass slides were purchased from MBL Bion, and were incubated with mouse serum (1:100 dilution). This was followed by PE- labeled goat anti-mouse IgG and DAPI. Images were captured on a confocal fluorescent microscope (LSM 710 NLO) and were processed by Zen software
(Carl Zeiss).
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Kidney IgG deposition and histopathology
Both kidneys were collected and fixed in 4% paraformaldehyde, dehydrated in 30% sucrose, then frozen in OCT (TissueTek). Frozen sections (5
m) were stained with DAPI and PE-labeled goat anti-mouse IgG (eBioscience), then visualized by confocal microscopy as above.
Sections of formalin-fixed kidneys (2 m) were stained with hematoxylin and eosin, then evaluated by a pathologist (V. D’Agati) blinded to sample identity.
Mesangial and endocapillary proliferation, leukocyte infiltration, glomerular deposition, and apoptosis were scored individually on a scale from 0 (none) to 4
(severe), then added to produce a cumulative score. The percentage of cortical parenchyma with interstitial inflammation was also determined. Images were captured on a Zeiss AxioImager upright microscope and processed by AxioVision software (Carl Zeiss).
Germinal center immunohistochemistry
To visualize germinal center reactions, frozen spleen sections (5 m) were stained with PE-labeled anti-mouse B220 (eBioscience) and biotin-conjugated peanut agglutinin (PNA; Vector Laboratories) followed by FITC-conjugated
Streptavidin (eBioscience). Sections were then visualized by confocal microscopy as above.
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Flow cytometry
Suspensions of peripheral blood leukocytes or splenocytes were subjected to red blood cell lysis and were subsequently washed and stained with directly conjugated fluorescent antibodies to the relevant surface markers
(eBioscience). LacZ expression was analyzed as in (259). Single cell suspensions were washed and resuspended in HBSS containing 1 mM HEPES and 2% FCS. These cells were incubated for 20 minutes at 37oC, then were mixed with pre-warmed 2 mM fluorescein di--D-galactopyranoside (FDG,
Sigma-Aldrich) in a hypotonic solution for 1 minute. Immediately thereafter, cells were placed on ice and washed with ice-cold HBSS buffer. Endogenous lysosomal -gal activity was blocked by 0.3 mM chloroquine diphosphate. After loading with FDG, cells were stained for surface markers as above, then analyzed by flow cytometry. Data was collected on either LSR II (BD) or Attune
NxT (ThermoFisher) flow cytometers and was analyzed using FlowJo software
(Tree Star).
Recombinant deoxyribonucleases
Cloned open reading frames (ORFs) of human DNASE1 (NCBI:
NP_005214.2) and DNASE1L3 (NCBI: NP_004935.1) were subcloned into the pMSCV-IRES-GFP (pMIG) retroviral expression vector. The constructs for
DNASE1L3 variants were generated using the Q5 site-directed mutagenesis kit
(NEB), including the R206 to C substitution, the C-terminal truncation (amino acids 282-305), and the insertions of hexahistidine tags between aa 282-283
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(His-preCT) or between aa 305 and the stop codon (His-CT). The resulting constructs or the empty pMIG vector were used as plasmids for the transient transfection of HEK293 cells using the TransIT-293 transfection reagent (Mirrus).
Equal transfection efficiency was determined by GFP expression, as measured on an Accuri C6 tabletop flow cytometer (BD Biosciences). To avoid contamination with bovine DNASEs, transfection was performed in medium containing 15% KnockOUT serum supplement (Thermo Fisher) in lieu of FCS.
Transfected cells were cultured for 48 hours, and the supernatants were collected, filtered, supplemented with 4 mM CaCl2 and 4 mM MgCl2, then frozen in aliquots.
Analysis of DNASE1L3 activity
To measure the digestion of liposome-coated DNA, pMIG plasmid DNA was pre-incubated with N-[1-(2,3-Dioleoyloxy)propyl]-N,N,N-trimethylammonium methyl-sulfate (DOTAP) (Roche) in HBSS according to the manufacturer’s protocol. 1 ng of naked or DOTAP-coated plasmid per reaction was incubated with an equal volume of DNASE-containing supernatants or sera for 60 minutes at 37oC, for a final reaction volume of 2 L. The amount of DNA remaining after digestion was measured by qPCR with GFP-specific primers and expressed as a percentage of input DNA using a calibration curve generated from serial plasmid dilutions.
For the measurement of relative DNASE1L3 activity in vivo, the digestion was performed for 10 minutes using 1 L of mouse or human serum or plasma,
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again in a final reaction volume of 2 L. After qPCR, the amount of DNA remaining post-digestion was converted to a percentage of DNASE1L3 activity using a calibration curve generated from serial dilutions of multiple wild-type serum or plasma samples.
Purified human polynucleosomes from HeLa cells (Epicypher) or purified
Jurkat cell DNA (2 ng/reaction) were incubated with an equal volume of DNASE- containing supernatants for 15 minutes at 37oC in a total volume of 2 L. The amount of remaining DNA was measured by qPCR with primers specific for human genomic Alu repeats and expressed as a percentage of input DNA using a calibration curve generated from serial DNA dilutions.
Hematopoietic reconstitution
To reconstitute the hematopoietic system of recipient mice, 2 x 106 total
BM cells were isolated from WT or KO mice on the 129 background. Cells were transferred into lethally irradiated WT and KO recipients on the B6 background, which has the same MHC haplotype. Reconstitution was tracked by staining for the 129 donor strain specific CD229.1 marker. An additional set of experiments was performed using WT recipients only. BM cells from WT or KO mice on the
(129xB6) F1 background were transferred into lethally irradiated congenic
(129xB6.SJL) F1 recipients. Reconstitution was tracked by staining for the
B6.SJL recipient strain specific CD45.1 leukocyte marker. In all analyzed chimeras, the donor contribution was greater than 95%.
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Specific depletion of cell populations
Rag1-deficient mice were purchased from Jackson Laboratories. To deplete dendritic cells (DCs), animals with a Cre-inducible diphtheria toxin receptor (iDTR) allele (260) were crossed with the DC-specific Cre deletion strain
Cd11c-Cre (261). The resulting Cd11c-Cre/iDTR mice were administered diphtheria toxin (DTX) at a dose of 20 ng per gram of body weight every other day for two weeks as described (262). Efficient DC depletion was confirmed by flow cytometry at the endpoint, and minimal effects of DTX were observed on DC populations in mice lacking iDTR. To deplete macrophages, B6 mice were injected intraperitoneally with 150 g per gram of body weight of macrophage colony-stimulating factor 1 receptor (Csf1r) blocking antibody (clone AFS98,
(263)) purified from a hybridoma as previously described (264). Depletion was confirmed by flow cytometry. For macrophage depletion by clodronate, mice were injected intravenously with a single dose (200 L) of clodronate liposomes
(5 mg/mL) or control liposomes containing PBS (obtained from
ClondronateLiposomes.com). Depletion was again confirmed by flow cytometry.
Molecular modeling
ZEGA global sequence alignment of DNASE1 (PDB:2DNJ, chain a) and
DNASE1L3 (Uniprot:Q13609) was performed as previously described (265). 3D structures were visualized and analyzed with Internal Coordinate Mechanics
Software (ICM-Pro Molsoft LLC, La Jolla, CA). Amino acids in the DNASE1 crystal structure 2DNJ, chain a contacting DNA were identified by selecting
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amino acids exhibiting any atom center within a 5 Å radius of any atom center in the DNA. Prediction of the structure of the isolated DNASE1L3 C-terminus (282-
SSRAFTNSKKSVTLRKKTKSKRS-305) and the C-terminus and His mutants in the context of the whole DNASE1L3 protein was performed using the Biased-
Probability Monte Carlo algorithm, which was shown to be as accurate as experimental structure determination for short peptides (266). Homology modeling of DNASE1L3 was performed as previously described (267).
Generation, isolation, and analysis of microparticles
Microparticles from Jurkat cells were generated as previously described
(210). In brief, 1 mM staurosporine (STS, Sigma-Aldrich) was added overnight to
Jurkat cell cultures. Cells were harvested and collected by centrifugation for 5 minutes at 300 x g. The supernatants were collected and centrifuged at 22,000 x g for 30 minutes to pellet microparticles. Pellets were resuspended in sterile PBS and then analyzed on an Accuri C6 tabletop flow cytometer (BD Biosciences) to determine absolute numbers and ensure >95% enrichment. Where indicated, microparticles (105 per L) were incubated with an equal volume of DNASE- containing transfection supernatants for 1 hour at 37oC.
To generate mouse microparticles, primary splenocytes were cultured for
2-3 days with 50 ng/mL phorbol 12-myristate 13-acetate (PMA) and 2 g/mL ionomycin (Sigma-Aldrich). The resulting cultures of activated, proliferating splenocytes were treated with 1 mM STS overnight, and microparticles were collected and analyzed as for Jurkat cells. Weekly, 107 of these microparticles
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were injected intravenously into wild-type female 129 mice who had previously been administered either PBS or 5 x 109 IFN5 adenoviral particles. After four instances of microparticle injection, the mice were analyzed one week later.
To isolate microparticles from human plasma, blood was collected in tubes containing ethylenediaminetetraacetic acid (EDTA), and blood cells were removed by centrifugation at 2,000 x g for 10 minutes at 4oC. In some experiments, a second centrifugation step at 3,000 x g for 10 minutes at 4oC was used to deplete platelets as previously described (205). The resulting plasma
(either fresh or stored at -80oC) was centrifuged at 22,000 x g for 30-60 minutes to pellet microparticles, and the supernatant was used to measure DNASE1L3 activity and antibody binding specificities. Microparticles from murine plasma were isolated as well. Mice were euthanized and immediately exsanguinated by cardiac puncture into tubes containing heparin. Plasma was then isolated by centrifugation at 2,000 x g for 10 minutes at 4oC, and then centrifuged at 22,000 x g for 30 minutes to pellet microparticles. Plasma microparticles were resuspended in sterile PBS and stained for CD42b and CD235a (human) or
CD41 and Ter119 (mouse) to exclude platelets and erythrocyte debris, respectively. Stained microparticles were assessed on an Accuri C6 tabletop flow cytometer (BD Biosciences) to count absolute numbers of the unstained fraction.
To test the ability of mouse serum to digest microparticle DNA, Jurkat microparticles (5 x 105 per L) were incubated with an equal volume of serum for
1 hour at 37oC in a final reaction volume of 10 L. Human plasma was tested using the same protocol, with the exception that microparticles (5 x 105 per L)
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derived from mouse splenocytes were used instead. The DNA content of Jurkat- derived microparticles, human plasma microparticles, and total human plasma was determined by qPCR for human genomic Alu repeats. The DNA content of mouse splenocyte-derived microparticles, murine plasma microparticles, and total murine plasma was determined by qPCR for mouse genomic B1 repeats. In both cases, raw qPCR data was converted into the amount of genomic DNA remaining after digestion using calibration curves derived from serial dilutions of the respective genomic DNA. Data were then expressed either as the percent of input DNA or, in conjunction with our counting of absolute microparticle numbers through the use of the Accuri, as amount of DNA per microparticle.
Persistence of microparticles in vivo
6 x 106 Jurkat cell-derived microparticles were injected intravenously per mouse into wild-type mice. Recipients were euthanized at the indicated time points, and their tissues were analyzed by qPCR for the presence of human DNA using primers specific for the Alu human genomic repeat.
Staining of microparticles
For surface staining, 2.5 x 105 native or DNASE-treated Jurkat cell-derived microparticles were incubated with either purified anti-DNA/histone 2a/2b mAb
PR1-3 (10 g/mL), mouse sera (1:10 dilution), purified 9G4-positive Fab fragments derived from human patients fused to the Fc region of mouse IgG (240,
241, 268), or hybridoma supernatants (PR1-3, PL2-8 (anti-DNA/histone), PL9-11
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(anti-DNA) (269, 270), and 3H9 (anti-DNA) (271)) for 30 minutes at 4oC.
Hybridomas were kindly provided by M. Shlomchik (University of Pittsburgh), while 9G4-positive antibodies were generously provided by G. Silverman (New
York University) and I. Sanz (Emory University). Stained microparticles were washed by centrifugation at 22,000 x g for 30 minutes, and were then incubated with PD-labeled goat anti-mouse IgG secondary antibody (eBioscience) at 1:200 for 30 minutes at 4oC. Microparticles were then analyzed by flow cytometry without further washing on an Attune NxT flow cytometer (ThermoFisher).
Staining with human plasma or sera was performed as above at a 1:20 dilution, with a PE-labeled goat anti-human IgG secondary antibody (eBioscience).
Statistical analysis
In the interest of robust statistical analysis, normal distribution of values was not assumed (272). Unless otherwise indicated in figure legends, data were displayed as medians with a range of indicated values. Statistical significance was calculated by the nonparametric Mann-Whitney test. Significance is indicated throughout as follows: *=p ≤ 0.05; **=p ≤ 0.01; ***=p ≤ 0.001.
PCR primer sequences
GFP:
FW: AAGTTCATCTGCACCACCGG; RV: GCGCTCCTGGACGTAGCCTT
Human Alu repeats:
FW: TCACGCCTGTAATCCCAGCA; RV: AGCTGGGACTACAGGCGCCC
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Mouse B1 repeats:
FW: GGGCATGGTGGCGCACGCCT; RV: GAGACAGGGTTTCTCTGTGT
Disclosures
Neither myself nor anyone in my laboratory have any financial conflicts of interest to disclose.
This project was supported by NIH grants AR064460 and AI072571, the Lupus
Research Institute, and the Judith and Stewart Colton Center for Autoimmunity.
The majority of this work was published on June 9th, 2016 (173).
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