Review

Endosomal Toll-like receptors in autoimmunity: mechanisms for clinical diversity

The endosomal Toll-like receptors (TLR3, TLR7 and TLR9) have been implicated in the pathogenesis of autoimmune diseases. Their signaling pathways show remarkable similarities and yet the outcomes following activation of each of these TLRs lead to clinically distinct autoimmune disease phenotypes. This review discusses how differences may arise at a molecular and cellular level to account for this diversity of responses. Understanding the roles of individual TLR pathways and the relationships between them and non-TLR innate immune pathways in the pathogenesis of diseases such as systemic lupus erythematosis highlights potential treatment targets for this spectrum of autoimmune diseases.

1 KEYWORDS: dendritic cells n innate immunity n interferon regulatory factors Sapna Trivedi & n kidney n lupus n Toll-like receptors Eric L Greidinger2† †Author for correspondence The innate immune system, as opposed to adap- for this purpose. However, despite overlapping 1Division of Nephrology & tive immune B and T cells, uses genetically activation pathways, the endosomal TLRs have Hypertension, University of preprogrammed pattern recognition receptors at times markedly different clinical effects on Miami Miller School of (PRRs) to recognize ‘danger signals’ that emerge autoimmune disease. Medicine, FL, USA when a potential pathogen is present. Innate The focus of this paper is to discuss the dif- 2Miami VA Medical Center and immune receptors, including Toll-like receptors ferences in the clinical effects of endosomal Division of Rheumatology & (TLRs), RIG-like receptors (RLRs), Nod-like TLR activation in autoimmunity, and to offer Immunology, 1120 NW 14th St., receptors and others, are typically expressed on possible explanations for these differences. The Clinical Research Building, macrophages, dendritic cells, epithelial cells and proposed explanations can broadly be divided Room 973 (D4–10), University endothelial cells, where they provide rapid early into two categories: differences in cell type of Miami Miller School of responses to microbial danger signals, includ- function and TLR expression, and differences Medicine, Miami, FL 33136, ing the induction of proinflammatory cytokine in the cascade of activation signals induced by USA secretion that recruits and activates additional particular innate immune receptors. Tel.: +1 305 243 6862 immune responses. Unlike other TLRs that Fax: +1 305 243 7414 are typically present on the surface of cells and Common features of endosomal TLRs [email protected] recognize bacterial danger signals, a group of In order to appreciate the differences between TLRs including TLR3, TLR7 and TLR9 local- the endosomal TLRs, the commonalities ize to cell endosomes and recognize viral danger between TLR3, TLR7, and TLR9 must first signals (dsRNA, ssRNA and hypomethylated be recognized. They are each expressed widely dsDNA, respectively). This group of endosomal across mammalian species, with conserved TLRs has been particularly implicated in the structure recognition and functional effects pathogenesis of autoimmune diseases. Human- from mice to humans. In each case, trafficking derived RNAs and DNAs that are targets of of these TLRs from the ER to the endosomal autoimmune responses in systemic lupus ery- compartment requires the functional form of the thematosus (SLE) and related conditions have UNC93B1 chaperone protein, without which been found to induce activation of these recep- agonists of these TLRs fail to induce activation tors [1]. Altered expression and function of these signals [7]. The endosomal TLRs are all prima- receptors has been linked to clinical manifes- rily expressed in dendritic cells [8], where their tations of lupus-like autoimmunity in animal activation induces secretion of type I interfer- model [2–5]. Moreover, inhibition of activation ons (IFN‑1), such as IFN‑a and IFN‑b, plus of the endosomal TLRs has been proposed to additional cytokines including IL‑6, IL‑12 be a mechanism of action of hydroxychloro- and TNF‑a [9,10]. Thus, not surprisingly, each quine and related compounds, mainstays of have also been shown to lead to upregulation autoimmunity therapy [6], and pharmaceutical of IFN‑inducible genes [11,12], to recruit helper firms have publicized their interest in develop- T cells, and to promote B‑cell activation and ing additional inhibitors of endosomal TLRs antibody production [13]. Plasmacytoid dendritic

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cells (PDCs) are frequently seen as the primary of previously inactivated cytoplasmic NF-KB producers of IFN‑1, but non-PDC subsets, transcription factors, and phosphorylates IFN which we will refer to as myeloid dendritic cells, regulatory factors (IRFs), including IRF3, are also capable of IFN‑1 p­roduction [14]. IRF5 and IRF7, which subsequently dimerize The intracellular signaling pathways of the and translocate to the nucleus to also modulate endosomal TLRs are homologous (Figure 1). gene expression [15–17]. With all three receptors, agonist ligation leads to conformational changes in the recep- Distinct clinical features reported with tor’s cytoplasmic tail, allowing recruitment of endosomal TLRs MyD88-family adapter molecules (TRIF in the The upregulation of IFN‑1 and IFN‑inducible case of TLR3 and MyD88 itself for both TLR7 genes is well recognized in systemic autoim- and TLR9). These bind to and induce phos- mune disease, where it is believed to play an phorylation of IRAK-family molecules, which important pathogenic role [18]. Although ini- in turn recruit additional proteins including tially observed in SLE [19], IFN‑1 secretion the TRAF6 E3 ubiquitin ligases and TANK and upregulation of IFN-inducible genes have binding kinase 1 (TBK1) or homologous also been found in other autoimmune condi- proteins to a signaling complex that induces tions including mixed connective-tissue dis- activating phosphorylations of the MAPK cas- ease, dermatomyositis, Sjögren’s syndrome and cade, induces release and nuclear translocation rheumatoid arthritis [20]. Since a number of

Membrane

TLR7 dsRNA Endosome ~ ssRNA dsRNA ~ ~ MyD88 ~ ~ MDA-5 RIG-1 ~ dsDNA TLR9 TLR3 IRAK Cytoplasm

IPS-1 STING TRIF TRAF6

TBKi IKK MAPK

IRF7 NF-κB IRF3 IRF5

Inflammatory Type 1 IFN cytokines

Nucleus

Figure 1. Endosomal TLR and non-TLR signaling pathways. Binding of endosomal or cytoplasmic nucleic acid ligands leads to the activation of TLR7 or TLR9 (via MyD88, IRAK and TRAF6), TLR3 (via TRIF and TRAF6) and MDA5 or RIG-I (via IPS-1 and STING). The pathways converge at the level of TBK1/IKK leading to activation of the IRF family of transcription factors, the MAPK cascade, and the NF‑kB family of transcription factors, cumulatively inducing IFN‑1 and inflammatory cytokine production.

434 Therapy (2009) 6(3) future science group Review Trivedi & Greidinger Endosomal Toll-like receptors in autoimmunity: mechanisms for clinical diversity Review

these conditions can share autoantigenic deter- tissue tropisms, the optimal responses generated minants with SLE but differ in their clinical against those pathogens would be expected to tissue targeting, studies have begun to inves- differ. For example, TLR7 can be activated tigate whether differences in innate immune by and induces protection against influenza responses could lead to d­ifferences in clinical virus [26]. The protective response against influ- disease manifestations. enza mediated by TLR7 appears to include Cases in which identical antigenic stimula- activating antiviral immune responses systemi- tion has led to differing patterns of clinical dis- cally (including induction of protective antibody ease expression have been reported. Mice with responses from activated B cells [27], while avoid- induced anti-RNP autoimmunity after stimula- ing excess inflammation in the lungs that could tion with the TLR3 and TLR7 agonist U1‑RNA cause life-threatening hypoxia. By contrast, were found to develop lung disease in TLR3- TLR3 knockout mice survive influenza infec- intact mice but renal disease in TLR3-null ani- tion better than TLR3-intact mice, due to the mals [4]. Likewise, mice treated with Y RNAs propensity of TLR3 activation to induce pneu- had different outcomes with regard to induction monitis [28]. However, with a different pathogen, of renal disease and sialoadenitis based on the respiratory syncyctial virus, the ability of TLR3 presence or absence of TLR3 expression of the to mount a more efficient immune response in test mice and on the endosomal TLR stimula- the lungs leads to a less severe histological pres- tory patterns of the individual Y RNAs [21]. In entation [29], despite the fact that respiratory syn- a spontaneous lupus model, differing effects of cyctial virus isolates are able to inhibit TLR7 TLR7 and TLR9 have been observed on tissue- and TLR9-induced responses [30]. Note that in specific disease manifestations: TLR7 knockouts both influenza virus and respiratory syncyctial had less severe nephritis than wild-type mice, virus, TLR3 stimulation leads to more aggressive while TLR9 knockouts had more severe nephri- immune responses, though the clinical outcomes tis and skin disease than wild-type mice (also of these aggressive TLR3 responses are opposite. supported by the finding of higher serum IFN‑a The association of TLR3 with proinflammatory levels and increased PDC activation) [22]. effects in the lung has also been observed with TLR7 has frequently been associated with rhinovirus [31] and in asthma [32]. the development of SLE in animal models. So the question arises: if differences bet­ TLR7 knockout mice are likely to develop a ween these TLRs really exist, how are these milder form of SLE [22], whereas overexpression m­echanistic differences defined? of TLR7 renders them more susceptible [23]. Antagonism of TLR7 can also prevent autoim- Differences in cell expression of mune lung and kidney disease [1,22]. By contrast, endosomal TLRs the effects on TLR9 knockout mice are not as While the endosomal TLRs can be frequently clear cut. Christensen et al. reported a protec- found on cells expressing other nonendosomal tive effect of TLR9 in MRL/lpr lupus mice [22], TLRs, TLR7 and 9 are seldom found in the but in the ‘chronic graft versus host’ disease same cells as TLR3. Expression of endosomal model, TLR9 knockout resulted in mice show- TLRs on different cell types may lead to differ- ing less severe nephritis [24]. In a recent study ent functional effects (Figure 2). The endosomal by Pawar and colleagues, combined TLR7 and TLRs are prominently expressed on dendritic 9 did not have additive or opposing effects on cells, but they are not all present on the same autoimmune lung and kidney injury [25]. In dendritic cells. TLR3 is expressed on myeloid the presence of TLR7 activation, TLR9, when dendritic cells (MDCs) whereas TLR7 and 9 stimulated, loses its protective ability but does are coexpressed on PDCs. Thus, to the extent not exacerbate disease. that activated MDCs preferentially traffic to the From an evolutionary point of view, it is lung and induce autoimmune interstitial lung highly plausible that selection pressure would disease [33], these effects are consistent with the exist on endosomal TLRs that would lead to biology of a TLR3-restricted cell type [34,35]. distinguishable immunologic and tissue-specific Likewise, trafficking of different inflammatory effects. The primary function of these PRRs is cell subsets to the kidney to induce lupus-like (presumably) to recognize microbial hazards lesions is more consistent with the biology of and to orchestrate antimicrobial responses that TLR7/9 expressing cells [36]. optimize fitness of the host. To the extent that TLR7 and TLR9 but not TLR3 are also endosomal TLRs recognize different microbial expressed on B cells [37,38]. Thus, TLR7 or 9 pathogens with different life cycles and different stimulation could be more likely to support

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TLR3 TLR7 and 9

Fibroblast Myeloid dendritic Plasmacytoid Naive B cell Memory B cell cell dendritic cell TLR9 TLR7

+ IFN 1

TLR9 TLR7 Fibrosis B cell

Lungs Therapyy © Future Science Group 2009 Kidneys Antibodies

Figure 2. Variation in the expression of endosomal TLRs depending on cell type. TLR3 expression on fibroblasts leading to fibrosis, and myeloid dendritic cells preferentially targeting the lung, while TLR7 and TLR9 expression on plasmacytoid dendritic cells preferentially targeting the kidney and on B cells with or without IFN‑1 exposure leading to antibody production. TLR: Toll-like receptor.

conditions characterized by B cell activation and Within the kidney, TLR3 is constitutively diversification, such as SLE. Differential activa- expressed on tubular epithelial cells, glomeru- tion of B cells between TLR7 or TLR9-driven lar mesangial cells and vascular smooth muscle lupus and TLR3-driven MCTD could help cells [44]. IFN‑g (which is not a type 1 IFN) explain the tendency for lupus patients to develop induces TLR3 in mesangial cells but downregu- a broader spectrum of autoantibodies than lates all TLR mRNA in macrophages [44]. In MCTD patients [39]. The ability of Fc receptor lupus-prone mice, viral dsRNA induces mesang- coligation to amplify TLR7 and TLR9 signals ial lysis without affecting dsDNA autoantibody could lead to a further shift of dendritic cell phe- production, consistent with the expression of notype [16], which may be distinct from the phe- TLR3 on mesangial cells but not on B cells [45]. notype induced by interactions between TLR3 TLR7 and TLR9 ligands are not taken up by signals and Fc receptor-associated signals [40]. intrinsic renal cells but do activate antibody pro- Unlike their typically coordinated expression duction consistent with effects on B cells [46]. in PDCs, in B cells TLR7 and TLR9 differ with TLR7 and TLR9 expression are observed in regard to the subsets in which they are expressed active glomerulonephritis, where they (but not and signal. Naive B cells express TLR9 but not TLR3) are observed on infiltrating immune TLR7, unless first stimulated with IFN‑1 [41]. cells [46]. TLR9 is also strongly expressed in By contrast, memory B cells respond to TLR7 proximal tubular cells in murine models of ligands in the absence of IFN‑1 [41]. In addition, lupus nephritis [47]. induction of a mature monoclonal rheumatoid Thus, differences exist in the cell expression factor antibody response in a murine system has profiles of the individual endosomal TLRs. In been found to have partially nonoverlapping animal models, such differences have been iden- dependence on both TLR7 and 9 [42]. tified even in the kidney itself. It is thus plausible Conversely to the situation with B cells, TLR3 to argue that different outcomes may be expected expression has been identified on fibroblasts, between stimulation of TLR3, 7 and 9 owing to which do not express TLR 7 or 9 [43]. This could differences in the target cells activated, even if potentially account for more prominent clini- the cellular activation program initiated by these cal manifestations of fibrosis seen in putatively receptors were otherwise identical. Differential TLR3-associated conditions, such as MCTD, as TLR-induced tissue trafficking is also possible. opposed to TLR7/9-associated conditions such Studies are needed to assess whether such dif- as lupus. ferences in cell expression profiles account for

436 Therapy (2009) 6(3) future science group Review Trivedi & Greidinger Endosomal Toll-like receptors in autoimmunity: mechanisms for clinical diversity Review

any of the variation in clinical phenotypes pro- and colleagues have observed that different posed to occur after selective activation of sets of epitopes are targeted on the hnRNP A2/B1 anti- these TLRs. gen, a known ARE-binding protein, between patients with SLE and those with MCTD [54]. Anti-inflammatory effects of Likewise, Jimenez-Boi recently reported the pres- endosomal TLRs ence in autoimmune and inflammatory condi- Each of the endosomal TLRs have also been tions of antibodies to two additional proteins observed in some circumstances to induce anti- involved in miRNA/ARE regulation of mRNA inflammatory effects, such as prevention of renal transcripts: T cell intracytoplasmic antigen 1 disease with TLR3 and TLR9 [21,22] and preven- (TIA‑1) and TIA‑1-related protein (TIAR) [55]. tion of sialadenitis with TLR7 [21]. Potentially Interestingly, anti-TIAR antibodies were asso- anti-inflammatory pathways reported to be ciated with lupus nephritis whereas in systemic induced by these TLRs under some circum- sclerosis anti-TIA‑1 was associated with lung stances include induction of suppressor of involvement. Anti-TIA‑1 antibodies could also cytokine signaling proteins [48,49] and enhance- be found in SLE patients and were generally asso- ment of proteosome destruction of intracellular ciated with more severe disease activity compared proinflammatory mediators[50] . In order to selec- with patients negative for anti-TIA‑1. This raises tively eliminate only the proinflammatory seque- the question whether differences in miRNA/ARE lae of endosomal TLR activation, a more optimal biology form the underpinnings of clinical differ- strategy might involve specific inhibition of only ences in autoimmune targeting between renal- the proinflammatory pathways downstream of targeted and lung-targeted syndromes of systemic TLR activation, or cell-targeting therapies that disease. In addition, there have been reports of selectively eliminate TLR-expressing cell types miRNA 146a/b (miR-146a/b) targeting TRAF6 particularly implicated in the pathogenesis of and IRAK1, and thereby potentially interacting tissue-specific immunophenotypes. Thus, B‑cell with TLR signaling [56]. targeting may make sense for TLR7-associated lupus nephritis, while anti-MDC therapy might Signaling differences between be more helpful for (putatively) TLR3-associated endosomal TLRs autoimmune pneumonitis. TLR3 signaling can be immediately appreci- ated to be different from that of TLR7 and 9, miRNAs in cell differentiation since TLR3 uses TRIF rather than MyD88 as The fact that various cell types respond differ- its primary signaling adapter molecule. However, ently to particular innate immune signals lead- TRIF is highly homologous to MyD88, and the ing to distinct patterns of immunopathology downstream events in both signaling systems, begs an additional question: what is it about the including NF-kB activation, MAP kinase activa- differentiation of one cell type as opposed to tion and induction of IFN‑1 inducible genes can another that accounts for such differences? In this be difficult to distinguish. In a study of MRL/ context, it is relevant to consider cellular path- lpr mice susceptible to developing SLE, dele- ways that are implicated in cell differentiation. tion of the MyD88 adaptor protein resulted in miRNA are short noncoding RNA molecules amelioration of disease, but treatment of these that inhibit gene expression [51]. RNA binding mice with poly-I:C, a TLR3/TRIF agonist that proteins have high affinity for the AU-rich ele- can also activate RLR [57], reconstituted SLE- ments (ARE) usually present in the 3´-UTR of like autoimmunity with nephritis as if MyD88 the mRNA [52]. This decreases mRNA stability signaling had been intact [58]. This suggests that or inhibits translation [40]. Originally studied in in mice developing in the absence of MyD88- the pathogenesis of various cancers, their role as mediated signals, activation of a combination regulators of cellular differentiation programs has of RLR and TLR3 signals can substitute for a been highlighted [53]. Recent studies have intro- MyD88-mediated response, and suggests that duced the possibility that regulation of cellular additional (potentially developmental) condi- differentiation at this level can also be associated tions must exist for TLR3-induced responses with the clinical expression of autoimmune dis- to support protection against, as opposed to ease. Autoantibodies generated in autoimmune induction of, lupus nephritis. For example, the syndromes may reflect differences in the biology absence of MyD88 pathways might lead SLE- of target tissues themselves or differences in the inducing PDCs to respond to a TLR3/RLR inflammatory infiltrates that traffic to different agonist instead of the usual TLR7 or 9. Given tissues. Regarding the latter possibility, Skriner that TLR7 and TLR9 are often expressed on the

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same cell types and have only been shown to Signaling differences between signal through the identical MyD88 pathway, non-TLR immune pathways one might speculate that no signaling differences Non-TLR innate immune pathways, includ- could exist between these two TLRs. However, ing some that have overlapping danger signal chimeric TLR receptors created with the extra- recognition with the endosomal TLRs, also cellular region of TLR4 fused with the trans- induce similar patterns of IFN‑1 activation, membrane and cytoplasmic regions of TLR3, 7 as well as induction of clinical auto­immunity. and 9 showed these TLRs to localize to the endo- Cytoplasmic DNA can stimulate receptors somal compartment, and to exhibit at least subtle including DNA-dependent activator of IFN differences in the levels of expression of a panel regulatory factors (DAI) [60]. In mice lacking of inflammatory cytokines [59]. The functional DNase II, DNA from engulfed apoptotic cells differences observed with TLR7 versus TLR9 and erythroid precursor cell nuclei accumulates stimulation and knockouts suggests that differ- in macrophages, leading to TLR-independent, ential signaling may be possible. It thus appears IFN-mediated autoimmune pathology [61]. that the cytoplasmic domains of the endosomal Depending upon their specific physiochemical TLRs define distinctive signaling properties, and properties, cytoplasmic RNAs could activate that additional studies with chimeric TLRs (such the cytoplasmic RLRs RIG‑I or MDA5 [62]. as swapping the extracellular and intracellular RIG-I and MDA5 each also induce IFN‑1 domains of TLR7 and 9) may provide for charac- expression, with RIG‑I and cytoplasmic terization of differences in the signaling pathways DNA recognition signals mediated by the of endosomal TLRs in future work. Moreover, a TLR-independent STING pathway [57], and schema can be proposed whereby even different both RIG-I and MDA5 signaling through stimuli of the same TLR could potentially induce IPS-1 [63]. distinct immune responses (Figure 3). Downstream signaling events inducing IRFs Specific TLR agonist A key common feature in all of these IFN‑1- acti­vating pathways, both TLR-dependent Cross-reactivity and non-TLR dependent exists, which may also account for receptor-to-receptor signaling diversity: these pathways converge downstream RLR, DAI at TBK1 or the homologous IKK-i, which TLR3, TLR7, TLR9 phosphorylate IkB to induce NF‑kB activa- tion, and also phosphorylate IRFs to activate TBK1 IFN‑1-associated and other proinflamma- tory responses after either cytoplasmic innate immune receptor or TLR-associated activation. Notably, phosphorylation of specific IRFs by Specific IRF phosphorylation TBK1 appears to depend on the formation of a complex including upstream elements of the innate immune receptor pathway in addi- tion to TBK1 – the entire complex appears to dictate which IRFs are allowed into proximity Stimulus-specific signal of activated TBK1 to receive their activating ? Including microRNA expression phosphory­lation. Thus, signal transduc­tion by specific innate immune receptors is linked to induction of specific IRFs: RLR signaling Figure 3. Stimulus-specific tuning of innate immune signaling. Individual depends on IRF7 [64], while DAI signaling innate immune stimuli ligate specific TLRs. The specific TLR(s) ligated directly depends on IRF3 and is IRF7 independent [60]. interact with the TBK1 phosphorylation complex to help dictate the specific IRFs Likewise, TLR9 and TLR7 activation lead to that are activated, leading to a stimulus-specific response. Cross-reactivity of a complex including IRF7 (which is consti- innate immune stimuli with other innate immune receptors such as RLRs or DAI can also impact the specificity of the IRFs activated by TBK1. Expression of tutively expressed in PDCs) [65]. TLR7 and regulatory molecules including miRNAs may occur within stimulus-specific TLR9 in PDCs stimulate IRF7 to a far greater responses, further feeding back on the pathway. extent than IRF3 [64]. By contrast, the acti- DAI: DNA-dependent activator of IFN regulatory factors; IRF: Interferon regulatory vated TLR3 complex is able to bind and acti- factor; RLR: RIG-like receptor; TBK: TANK binding kinase 1; TLR: Toll-like receptor. vate both IRF3 and IRF7 relati­vely equally [66].

438 Therapy (2009) 6(3) future science group Review Trivedi & Greidinger Endosomal Toll-like receptors in autoimmunity: mechanisms for clinical diversity Review

IRF3 is expressed in all cell types whereas pathway genes, such as TNF-b, the IRF5 gene IRF7 (as well as IRF5) are primar­ily expressed encodes an mRNA 3´‑UTR with an ARE ele- in B cells and dendritic cells. Glucocorticoid ment [75]. Thus, cell-type-specific differences in drugs, which have demonstrated effectiveness inflammatory responses could also be mediated for lupus-like autoimmune diseases irrespec- by effects at the level of IRF5 mRNA regula- tive of specific immunophenotype, are able tion. Future therapeutic agents the modulate to inhibit the activated form of TBK1 itself regulation of the IRF5 transcript could thus and the a­ctivation of IRF3 [67,68]. More potent also have relevance for treatment of lupus-like inhibitors of TBK1-like activities would thus be a­utoimmune syndromes. anticipated to have potent but potentially non- specific anti-inflammatory effects, and could Conclusion have other serious side effects, since TBK1 The systemic rheumatic diseases have long has also been implicated in oncogenesis and been characterized as conditions with the angiogenesis [69,70]. Therefore, it appears that potential to affect many different target tis- IRFs are involved in TLR and non-TLR sig- sues, but clinicians have had few tools avail- naling, with differences observed in d­ifferent able to either predict the likelihood of specific cell types. end-organ involvement or to therapeutically influence the expression of end-organ involve- IRFs & autoimmune disease ment. Recent data suggest that innate immune While IRF3 and 7 have been most consistently signals may participate in determining the tis- implicated in proinflammatory signaling, to sue targets of immune responses, and hence date polymorphisms in the IRF5 gene have contribute to the clinical disease patterning been most strongly linked to susceptibility for in individual patients. Additional study of lupus-like autoimmunity [71]. Like IRF3 and this area may lead to improved tools for the IRF7, IRF5 is activated by phosphorylation, assessment of risk of target organ involve- whereupon it forms a homodimer, translo- ment, new therapeutic modalities to modify cates to the nucleus, and binds to regulatory this risk, and new appreciation for mechanisms motifs in DNA that modulate gene expression, by which current t­herapies may impact on notably in the promoter regions of cytokine tissue targeting. genes [72]. Interestingly, IRF5 shows variation in its actions depending on very fine-grained Future perspective differences in cell specificity. IRF5-deficient Innate immune regulation is capable of dra- PDCs have normal IFN‑1 secretion while IRF5- matically influencing the phenotype of sys- deficient MDCs show impaired cytokine pro- temic auto­im­mune diseases with regard to organ duction [73]. In mice, IRF5 deficiency prevents involvement and severity. Measurement and dendritic cell induction of TNF-a by TLR3, 4 modification of these innate immune pathways or 9, but the same deficiency prevents dendritic provides new oppor­tunities to screen patients at cell induction of IFN‑1 only by TLR9, with- risk of particular organ involvement, to tailor out affecting TLR3 or TLR4-induced IFN‑1 therapy toward the organ systems specifically at production [74]. As with many inflammatory risk in each i­ndividual patient.

Executive summary Innate immune activation leading to IFN-1 production ƒƒ Following stimulation by nucleic acid ligands, innate immune system proteins (endosomal Toll-like receptors [TLRs] and cytoplasmic RIG-like receptors) signal through generally homologous pathways involving a series of mediators that ultimately result in the production of IFN‑1. Clinical diversity in autoimmune disease ƒƒ A diverse set of systemic autoimmune syndromes including lupus are characterized by IFN-1 activation. ƒƒ Phenotypic variability in the autoimmune diseases sharing IFN-1 activation may be mediated in part by which specific innate immune pathway(s) become activated. Differences in endosomal TLR signaling ƒƒ Differences in innate immune pathways that may contribute to different clinical outcomes exist at the cellular, transcriptional and translational levels. Therapeutic goals ƒƒ Putative therapeutic targets exist that may target specific differences in innate immune activation pathways, and have relevance to particular clinical subsets of autoimmune disease.

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Financial & competing interests disclosure involvement with any organization or entity with a finan- Dr Greidinger’s work was supported by the US Department cial interest in or financial conflict with the subject matter of Veterans Affairs (Merit Review grant), the NIH (grants or materials discussed in the manuscript apart from AI-1842 and AR-48805) and the Lupus Research Institute. those disclosed. No writing assistance was utilized in the The authors have no other relevant affiliations or financial production of this manuscript.

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