Role and Regulation of CD1d in Normal and Pathological B Cells Mohammed S. Chaudhry and Anastasios Karadimitris This information is current as J Immunol 2014; 193:4761-4768; ; of September 28, 2021. doi: 10.4049/jimmunol.1401805 http://www.jimmunol.org/content/193/10/4761 Downloaded from References This article cites 69 articles, 34 of which you can access for free at: http://www.jimmunol.org/content/193/10/4761.full#ref-list-1

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The Journal of Immunology is published twice each month by The American Association of Immunologists, Inc., 1451 Rockville Pike, Suite 650, Rockville, MD 20852 Copyright © 2014 by The American Association of Immunologists, Inc. All rights reserved. Print ISSN: 0022-1767 Online ISSN: 1550-6606. Th eJournal of Brief Reviews Immunology

Role and Regulation of CD1d in Normal and Pathological B Cells Mohammed S. Chaudhry and Anastasios Karadimitris CD1d is a nonpolymorphic, MHC class I–like molecule a key role in antimicrobial, antitumor, and autoimmune re- that presents phospholipid and glycosphingolipid Ags sponses (3). iNKT cells are activated in response to a range of to a subset of CD1d-restricted T cells called invariant endogenous and exogenous lipids, with the glycosphingolipid NKT (iNKT) cells. This CD1d–iNKT cell axis reg- a-galactosylceramide (a-GalCer) being the prototypical and ulates nearly all aspects of both the innate and adaptive one of the most powerful, although not physiological (i.e., not immune responses. Expression of CD1d on B cells is synthesized in mammalian tissues), stimulating agonists (4). suggestive of the ability of these cells to present Ag to, Transcriptional regulation of CD1d and form cognate interactions with, iNKT cells. In this Downloaded from article, we summarize key evidence regarding the role CD1d is expressed on cells of both myeloid (monocytes, and regulation of CD1d in normal B cells and in hu- macrophages, dendritic cells) and lymphoid (B lymphocytes, moral immunity. We then extend the discussion to thymocytes but not mature T cells) lineage (5, 6); it is also expressed outside of the hematopoietic system (e.g., on epi- B cell disorders, with emphasis on autoimmune dis- thelial and vascular smooth muscle cells) (7). Expression of ease, viral infection, and neoplastic transformation of CD1d on B cells, the focus of this review, points to the po- http://www.jimmunol.org/ B lineage cells, in which CD1d expression can be al- tential of these cells to present lipid Ag to, and engage in tered as a mechanism of immune evasion and can have cross-talk with, iNKT cells. both diagnostic and prognostic importance. Finally, we Expression of CD1d is regulated by multiple transcription highlight current and future therapeutic strategies that factors (TFs). In , the ubiquitous TF SP1 activates aim to target the CD1d–iNKT cell axis in B cells. The transcription by binding to the proximal promoter (8, 9), Journal of Immunology, 2014, 193: 4761–4768. whereas LEF-1 represses CD1d transcription by binding to the distal promoter (10). In mice, a minimal proximal pro-

D1d is a nonpolymorphic, MHC class I–like, b2- moter region has been identified, which is regulated by various by guest on September 28, 2021 microglobulin-associated molecule that presents phos- members of the ETS family of TFs, including Elf-1 in murine C pholipid and glycosphingolipid Ags to a subset of B cells and PU.1 in cells of myeloid lineage (11). and murine CD1d share a retinoic acid response element in immunoregulatory T cells called type I (or invariant) and type  II NKT cells (1). Although CD1d is the only lipid-presenting the distal promoter ( 1.5 kb from ATG) (12), and retinoic molecule in rodents, there are four other CD1 molecules acid was shown to increase CD1d expression in myeloid and (CD1a, b, c, and e) in humans that interact with lipid-specific B cells in vitro (13–15). It is of interest that single nucleotide subsets distinct to NKT cells. polymorphisms in the proximal promoter of PWD inbred A hallmark of invariant NKT (iNKT) cells is their use of mice drastically reduce CD1d expression, with a consequent severe reduction in iNKT cell frequency (16). a semi-invariant ab TCR. In humans, it comprises an invariant TCRVa24–Ja18 chain paired nearly always with a noninvariant Lipid presentation by CD1d TCRVb11 chain; in mice, the homologous invariant TCRVa14– Central to its ability to function as an Ag-presenting molecule, a b b J 18 chain pairs with a limited set of TCRV -chains (TCRV 2, surface CD1d undergoes internalization and trafficking from 7, and 8). iNKT cells are the best-studied subset of CD1d- the cell surface to endosomal and lysosomal compartments restricted T cells and can be described as a type of innate- in the cytosol. In these compartments, CD1d exchanges its like lymphocyte that can bridge the innate and adaptive arms ligands with glycolipids, either endogenous to the cell or ac- of the (2). Following activation, iNKT cells quired from exogenous sources, before returning to the cell assume a Th1, Th2, or Th17 functional immune profile and surface to present these lipids (6). can exhibit direct cytotoxicity. This diverse range of functions Specifically, B cells may capture and internalize foreign lipid underpins the ability of the CD1d–iNKT cell axis to play Ag directly through the BCR, a concept that may be used in the

Centre for Haematology, Imperial College London, Hammersmith Hospital, London Abbreviations used in this article: B-CLPD, B cell chronic lymphoproliferative disorder; W12 0NN, United Kingdom CLL, chronic lymphocytic leukemia; a-GalCer, a-galactosylceramide; GC, germinal center; iNKT, invariant NKT; MM, multiple myeloma; MZ, marginal zone; PB, pe- Received for publication July 17, 2014. Accepted for publication August 28, 2014. ripheral blood; SAP, signaling lymphocyte activation molecule–associated ; SLE, This work was supported by the Wellcome Trust. systemic lupus erythematosus TD, T cell dependent; TF, transcription factor; TFH, T follicular helper; TI, T cell independent. Address correspondence and reprint requests to Prof. Anastasios Karadimitris, Imperial College London, Du Cane Road, London W12 0NN, U.K. E-mail address: a.karadimitris@ Ó imperial.ac.uk Copyright 2014 by The American Association of Immunologists, Inc. 0022-1767/14/$16.00

www.jimmunol.org/cgi/doi/10.4049/jimmunol.1401805 4762 BRIEF REVIEWS: CD1d IN NORMAL AND PATHOLOGICAL B CELLS

2 2 design of novel lipid-bound immunogens (17–19). Alterna- TCRJa18 / mice, the dependency of IgE responses on tively, B cells may, like dendritic cells, be able to capture and iNKT cells subsequently was shown in models of allergy, present ApoE/lipid complexes via the low-density lipoprotein such as OVA-induced asthma (27). Similarly, the use of 2 2 receptor in a BCR-independent manner (20). CD1d / mice revealed impaired Ab responses to , In the ensuing discussion, we highlight key studies that including Borrelia species and Streptococcus pneumoniae have helped to elucidate the potential role and regulation of polysaccharides (28, 29). More recently, it was shown CD1d expression on B cells in health and in disease. We aim that NKT cells play a critical role in the production of to show the importance of CD1d expression on B cells for Abs against the blood group A Ag (30). In comparison efficient humoral immune responses against pathogens and with wild-type mice, NKT cell–deficient mice did not develop in response to vaccines. In addition, we examine how CD1d, increased anti-A levels (IgM and IgG) on immunization with by marking the development of mature B cells, might provide blood group A RBCs. Furthermore, CD1d blockade by novel insights into the biology of autoimmune disease, EBV administration of a mAb prior to immunization also abrogated infection of B cells, and B lineage malignancies. We suggest the anti-A response in both wild-type and humanized mice. that a greater understanding of these processes will allow The precise nature of the interaction between iNKT and their exploitation for diagnostic, prognostic, and therapeutic B cells can vary and does not always depend on CD1d ex- benefit. pression specifically by B cells. To further elucidate this in- teraction, investigators took advantage of the adjuvant func- CD1d in normal B cells tion of lipid Ags, primarily a-GalCer. Downloaded from CD1d is expressed in mature, naive, and memory B cells, Cognate and noncognate interactions between iNKT cells and B cells. plasma cells, and B regulatory cells (14, 21). The last are a The help provided by iNKT cells to B cells may occur subset of immunoregulatory, IL-10–secreting B cells, which indirectly through noncognate mechanisms. This principle immunophenotypically correspond to recent bone marrow was demonstrated by the generation of a series of murine emigrants called transitional B cells, and are implicated in the bone marrow radiation chimeras lacking CD1d or CD40 pathogenesis of autoimmune disorders. Their role in immune on B lymphocytes or expressing CD1d or MHC class II http://www.jimmunol.org/ regulation was reviewed recently (21) and will not be dis- disjointly on APCs (31). These experiments showed that cussed further. B cell responses against nominal protein Ags could be enhanced by a-GalCer. This could occur in the absence of The humoral immune response CD1d, but not CD40, on B cells; furthermore, it required The B cell immune response can be categorized as T cell coexpression of CD1d and MHC class II on APCs. Taken independent (TI) or T cell dependent (TD). TI responses together, the findings suggested a noncognate mechanism for do not require direct interaction with Th cells. They can be iNKT cell help to B cells through licensing of APCs, increased further classified as type 1, in which the B cell is stimulated by by guest on September 28, 2021 TFH cell activation, and, thus, improved conventional T cell– activating ligands (e.g., CPG) that do not engage the BCR, or B cell interactions. In this role, a-GalCer is acting as a classical type 2, in which the BCR is engaged by multivalent epitopes, immunological adjuvant, stimulating early innate immune such as polysaccharides. TI responses generally lead to extra- responses to aid the establishment of protective adaptive follicular Ab-producing cells rather than germinal center (GC) responses. This is underlined by preclinical studies in which formation, do not generate high-affinity Abs, and produce few vaccines containing a-GalCer mixed with protein Ags induced plasma cells and atypical memory cells (22). The result is a more effective humoral and memory responses against several rapid, yet transient, innate-like response that does not lead to viral, parasitic, and bacterial pathogens (32). enhanced recall responses. However, there is now firm evidence that B cells, through TD responses occur through BCR activation in the presence + CD1d expression, are indeed also able to form cognate of cognate help from a specialized subset of CD4 Th cells, interactions with iNKT cells, driving a novel form of humoral termed T follicular helper (TFH) cells (23). TFH cells are regulated by the transcriptional repressor Bcl-6, use the che- immune response. First, it was demonstrated that iNKT cells mokine receptor CXCR5 to home toward the B cell follicles, in vitro promoted proliferation of autologous B lymphocytes and express key molecules important for T cell–B cell inter- and induced Ig production in a CD1d-dependent manner actions, including IL-21, PD-1, signaling lymphocyte activa- (33). iNKT cell help to B cells could occur both in the tion molecule–associated protein (SAP), ICOS, and CD40L presence and absence of a-GalCer, suggesting that B cells (24). B cells consequently differentiate within the follicles, expressing CD1d can present exogenous or endogenous lipid 2/2 giving rise to GCs, the hallmark of the TD response. Within to iNKT cells. Further work in MHC class II mice (which GCs, class-switch recombination, somatic hypermutation, and lack all class II–restricted T cells) provided support for this affinity maturation lead to generation of high-affinity Ig- concept in vivo. Coadministration of a-GalCer with TD Ag was secreting, long-lived plasma cells and memory cells, thus en- able to induce a limited Ab response in mice lacking Th cells, suring a strong anamnestic response (25). implying that iNKT cells could act as an alternative, albeit less Regulation of the humoral immune response by CD1d and efficient, cognate partner for B cells (34, 35). The CD1d- iNKT cells. Similar to conventional T cells, iNKT cells also restricted nature of these cognate iNKT cell–B cell interactions can regulate, enhance, and sustain humoral immune responses. also was shown to occur in vivo. Specifically, in B cell–deficient Early studies revealed that administration of a-GalCer to mice mMT mice reconstituted with B cells from wild-type or 2 2 induced iNKT cell activation, led to an IL-4–dependent CD1d / donors, a-GalCer enhanced Ab responses against activation of B cells, and, in some cases, resulted in an increase NP–keyhole limpet hemocyanin, dependent on CD1d ex- in total serum IgE levels (26). Using iNKT cell–deficient pression by the reconstituting B cells (36). The Journal of Immunology 4763

As a caveat, several of the studies up to this point had used are responsible for interacting with the Ag-presenting B cells a similar immunization strategy, using a combination of protein and driving the immune response. Ags mixed with uncoupled a-GalCer. Such a combination is Recent in situ imaging studies revealed that activated well suited to demonstrating the adjuvant properties of iNKT cells preferentially localize in the marginal zone (MZ) of a-GalCer but does not ensure efficient delivery to individual the spleen, a specialized region in continuous contact with B cells of both the nominal Ag and the adjuvant lipid. blood-borne Ags (41, 42). The MZ contains several innate This was addressed by novel immunization methodologies and innate-like lymphocytes, including MZ B cells, which that aimed to target glycolipid to the BCR and generate lipid- may play a prominent role in responding rapidly to TI Ags specific B cell responses, either by using NP hapten directly (43). It is of note that MZ B cells express high amounts of conjugated to a-GalCer (18) or protein Ag (hen egg lyso- CD1d and primarily secrete IgM and IgG3, the main Ab zyme) and a-GalCer both linked to bead particles (19). In isotypes produced in response to lipid-conjugated Ag (44). both cases, B cells activated by Ag can internalize both the Thus, it is plausible that these MZ B cells may be one of the Ag and the physically linked a-GalCer molecule through predominant recipients of cognate iNKTFH help. the BCR. Lipid can then bind to CD1d and subsequently be The physiological role of this type 2 TD response is not presented by the B cells to iNKT cells. Immunization of mice clear, because it may appear counterintuitive that there is in this way was shown to produce rapid increases in the titers a rapid immune response with affinity maturation and, yet, of hapten- or protein-specific IgM and IgG (18, 19). These this does not persist. It is possible that exposure in real life immunogenic formulations are not able to generate MHC to pathogen lipid ligands may be more prolonged than that of Downloaded from class II–restricted epitopes, ruling out a role for a cognate Th soluble a-GalCer and, thereby, could sustain the presence cell effect. However, humoral responses were shown to be of iNKTFH cell–dependent immune responses. Alterna- dependent on the presence of iNKT cells, CD1d expression tively, pathogens may simultaneously stimulate both lipid by B cells, CD40–CD40L signaling, CD80/CD86 costimu- and peptide B cell responses. Therefore, the role of iNKTFH lation, and IFN-g production, strongly supporting a role for cell–dependent GC formation may be to provide a rapid http://www.jimmunol.org/ a cognate iNKT cell–B cell interaction. response by lipid-specific B cells, which, although deliber- This work was followed up by studies using the same im- ately abrogated, can provide a platform of primed GCs that munization techniques to establish the longer-term outcome can be reused by newly activated peptide-specific B cells. for B cells that have received cognate help from iNKT cells. A more recent study added further complexity to this issue When NP–a-GalCer was used to ensure lipid delivery spe- (45). By immunizing mice with liposomal nanoparticles cifically to B cells, the response was characterized by extra- containing both S. pneumoniae polysaccharides and a-GalCer, follicular plasmablasts, GC formation, affinity maturation, the investigators demonstrated extrafollicular B cell prolifera- and a strong primary IgG response that were dependent on tion, leading to a prolonged, high affinity, class-switched Ab IL-21 production by iNKT cells (37). However, there was response and a strong anamnestic response. This was dependent by guest on September 28, 2021 impaired development of long-lived plasma cells and memory on CD1d expression by dendritic cells and B cells, indicating B cells, commensurate with a lack of an enhanced humoral a two-step cognate process. This outcome, incongruous with the memory response. Similar work involving the use of hen egg previous studies, may be explained by the use of a different lysozyme–a-GalCer to induce stable iNKT cell–B cell cognate antigenic formulation, because polysaccharides are able to act as interactions also found evidence of IL-21–dependent GC type 2 TI Ags; nevertheless, it demonstrates the potential to reactions but a lack of long-lived plasma cells, memory B cells, target cognate iNKT cell help to B cells in vaccination strategies. and enhanced recall responses (38). Finally, the outcomes of 2 2 Downregulation of CD1d in GC B cells: mechanisms and purpose. B cells in MHC class II / mice immunized with protein Ag In of the concept that the help provided by iNKTFH and a-GalCer were explored (39). In this different immuni- cells may be purposely halted, it is notable that CD1d ex- zation model, entailing the use of mixed radiation chimeras pression is lost from the surface of GC B cells (5). However, and B cell–transfer experiments, Ag-specific Ab responses the in vivo biological mechanism and significance of this occurred in a CD1d- and CD40-dependent manner, sug- phenomenon remain to be determined. It is of interest that, gestive of a cognate iNKT cell–B cell interaction. Further- in EBV-infected lymphoblasts, ENCODE analysis reveals that more, this again was accompanied by rapid, but short-lived, CD1d transcriptional downregulation (see below) is associated GC formation and transient, rather than long-lived, Ab with binding of the Polycomb component EZH2 and responses. Taken together, these studies suggest that the bivalent chromatin status (i.e., coexistence of H3K4me3 and cognate interactions between CD1d-expressing B cells and H3K27me3 marks) at the promoter of CD1d, suggestive of iNKT cells generate a hybrid immune signature, termed the Polycomb-mediated transcriptional repression of CD1d in type 2 TD immune response, encompassing features of both proliferating B cells (M.S. Chaudhry and A. Karadimitris, the classic TD and TI responses. unpublished observations). Intriguingly, these studies revealed a subset of iNKT cells Indeed, transcriptional downregulation of CD1d can be in- recapitulating features of TFH cells, including expression of duced in proliferating B cells activated in vitro (14). CD1d CXCR5, IL-21, PD-1, and CD28, all under control of the Bcl-6 downregulation was found to occur at both the cell surface and transcriptional program (38, 39). Using inducible knockout the mRNA level in response to various activating ligands, in- mice, it was further shown that SAP, which is essential for cluding CD40L. Mechanistically, CD1d downregulation was iNKT cell development (see below), is also critical for this associated with a decrease in RARa signaling, and could be cognate B cell interaction (40). Termed iNKTFH cells, these are reversed by agonists of this pathway, including all-trans retinoic the specialized NKT cells, which, like their T cell counterparts, acid and the specific RARa agonist AM580. Following in vitro 4764 BRIEF REVIEWS: CD1d IN NORMAL AND PATHOLOGICAL B CELLS activation, CD1d downregulation occurred at 24 h, reaching associated with restoration of the frequency and function of maximum effect at 5 d. iNKT cells and expression of CD1d on B cells. Upregulation of CD1d by AM580 predictably re-enabled Although these findings provide important insights into the lipid presentation. Following CD1d upregulation, lower doses mechanisms that underpin the quantitative and qualitative of a-GalCer presented by B cells led to increased stimulation by iNKT cell defects in SLE, the role of dysfunctional iNKT cells iNKT cells and B cell proliferation. In contrast, higher doses of in the pathogenesis of SLE and, indeed, of autoimmune a-GalCer induced an iNKT cell–cytotoxic effect, leading to disease, in general, remains unclear. In preclinical models, decreased numbers of B cells (14). It is tempting to speculate activation of the CD1d–iNKT cell axis can prevent or ame- that these findings can be extended to the in vivo setting. A liorate established autoimmune disease (48), providing im- B cell CD1d-iNKTFH cell interaction in the GC, if too per- petus for development of clinical protocols that will aim to sistent, may similarly have the potential to induce iNKT cell restore iNKT cell frequency and function in patients with cytotoxicity toward B cells. If appropriately timed, the down- autoimmune disorders, including SLE. regulation of CD1d observed in the GC may indeed exist to CD1d and virally infected B cells. Immune evasion through terminate such an interaction before it becomes disadvanta- downregulation of classical MHC molecules is a recognized geous. Given this temporal change in the pattern of CD1d strategy used by viruses to withstand elimination by host expression during mature B cell ontogeny, it would be of great defense mechanisms and to help ensure their propagation (49). interest to analyze the endogenous lipid repertoire generated by Highlighting the importance of CD1d in antiviral immune naive, GC, and memory B cells and presented by CD1d in the responses, several viruses, including herpes viruses, were shown Downloaded from context of a TD B cell response. Further insights on the sig- to downregulate expression of CD1d in APCs through post- nificance of CD1d downregulation selectively in GC B cells transcriptional mechanisms (50, 51). This raised the prospect would require a genetic approach that would ensure persis- that EBV, a ubiquitous human herpes virus that targets B cells tence of CD1d expression during the GC reaction. (52), could use a similar strategy. EBV has initial tropism for pharyngeal epithelial cells but

CD1d in pathological B cells http://www.jimmunol.org/ then spreads to local naive B cells (via surface CD21 and HLA Mature B cells are implicated in the pathogenesis of auto- class II molecules) within tonsillar lymphoid tissue. Currently immune disease, such as systemic lupus erythematosus (SLE). accepted models of chronic EBV infection propose that, to In addition, they are targets for viral infection, especially by persist in the host, it infects naive B cells and drives B cell B cell–tropic viruses, such as EBV, and for malignant trans- maturation through a GC-type reaction to produce memory formation, leading to a variety of B lineage malignancies, B cells in which the virus can reside latently long term (52). including B cell lymphomas and multiple myeloma (MM). EBV achieves this through the expression of different genetic Accumulating evidence, reviewed below, suggests an impor- latency programs (0–III) in different B cell subsets, which is

tant role for the CD1d–iNKT cell axis in these processes, with associated with the transcription of different viral , by guest on September 28, 2021 potential therapeutic repercussions. notably LMP1 and LMP2a (53, 54). CD1d and B cells in SLE. B cell dysfunction is central to the It is possible that the imitation of the process of GC for- pathogenesis of SLE, a systemic autoimmune disorder. This mation, as well as the consequent drastic downregulation of is highlighted by the ability of the anti-CD20 mAb rituximab CD1d in EBV-infected cells, confers protection to the virus- to induce clinical remissions in a substantial fraction of propagating B cells from iNKT cell attack. Indeed, evidence patients (46). from the study of patients with X-linked lymphoproliferative Recent work demonstrated a critical intersection of the disorder, which is caused by mutations in SH2D1A/SAP or CD1d–iNKT cell axis with B cells in healthy subjects and in XIAP, further suggests that the CD1d–iNKT cell axis might patients with SLE (47). Specifically, the presence of CD1d- be important in restraining EBV into a latent state. Individ- expressing B cells and their direct interaction with peripheral uals with X-linked lymphoproliferative disorder, in addition blood (PB) iNKT cells appears to be indispensable for the to other immune function defects, have profound defects in a-GalCer–dependent in vitro expansion of iNKT cells. Much iNKT cell development associated with active EBV replica- of this effect is mediated by transitional B cells (i.e., CD19+ tion and EBV-associated lymphoproliferation (55, 56). CD38hiCD24hi), which express the highest levels of CD1d Following on from these observations, the nature of the among PB B cell subsets. interaction between EBV-infected B cells and NKT cells was Compared with healthy controls, patients with SLE have examined in vitro (15). It was demonstrated that EBV in- a significantly lower frequency of PB iNKT cells, which secrete fection of resting B cells can be mitigated by the presence of abnormally low levels of IFN-g but high levels of IL-10. This iNKT cells, indicating that EBV-infected B cells are indeed finding correlates with considerably lower surface CD1d ex- a target for NKT cells. However, in a parallel to the physio- pression in patient versus control B cells (but not monocytes) logical activation of B cells, EBV infection of B cells rapidly and, in particular, in B regulatory cells. Like GC B cells, loss leads to loss of expression of CD1d, abrogating further in- of CD1d surface expression in SLE B cells is due, at least in teraction with NKT cells and, thus, providing a form of part, to BCR signaling, which is increased in the disease in immune evasion. Similarly, like their physiologically activated combination with other inflammatory stimuli, such as IFN-a. counterparts, CD1d expression in EBV-infected B cells in However, unlike the transcriptional loss observed in GC vitro could be restored by an RARa agonist, re-enabling the B cells, loss of CD1d expression in SLE is the result of an stimulation and cytotoxic effects of NKT cells, even in the enhanced rate of CD1d internalization in patient B cells. In absence of exogenous Ag (15). patients who had a clinical response to rituximab, restoration Downregulation of CD1d expression by EBV occurs at of the PB B cell pool following their initial depletion was the transcriptional level as opposed to the posttranscriptional The Journal of Immunology 4765 mechanisms that are used by other viruses. Using chromatin behavior of mature B lineage malignancies, in particular MM immunoprecipitation, it was shown that transcriptional re- and chronic lymphocytic leukemia (CLL), two of the most pression of CD1d in B cells occurs through binding of the common hematological malignancies. LEF-1/b-catenin complex (members of the Wnt pathway) to Multiple myeloma. MM is a malignant monoclonal plasma cell the CD1d distal promoter upon EBV infection and that disorder characterized by end-organ damage, including ane- treatment with AM580 restores CD1d expression by inhib- mia, hypercalcemia, renal insufficiency, and bone lesions (60). iting binding of LEF-1 in this region. CD1d is dynamically downregulated on myeloma plasma CD1d and malignant B cells. Numerous preclinical studies cells during disease progression (61). Specifically, in primary demonstrated the power of the CD1d–iNKT cell axis to myeloma samples, CD1d was highly expressed in premalignant enhance (or in some cases inhibit) antitumor immunity (57). and early-stage disease, but it was decreased, and subsequently iNKT cells can target CD1d-expressing tumors (mostly of lost, in advanced-stage MM. Similarly, loss of CD1d expres- hematological, including lymphoid origin) directly (58, 59) sion was observed in most myeloma cell lines, in terms of both or indirectly, by activating adaptive and innate immune surface and mRNA expression, thus implicating this molecule 2 responses that can secondarily target CD1d tumors (mostly as an important antisurvival factor in MM. These observations of epithelial origin). We restrict our discussion to the evidence suggest that loss of CD1d is advantageous for the propagation linking the CD1d–iNKT cell axis to the biology and clinical of the malignant clone and permits evasion of myeloma cells Downloaded from http://www.jimmunol.org/ by guest on September 28, 2021

FIGURE 1. Dynamic expression and role of CD1d on B cells in health and disease. Expression of CD1d on normal naive B cells permits their cognate interaction with iNKT cells. This leads to the development of the type 2 TD immune response, followed by CD1d downregulation in the GC. Pathological B cells in SLE, EBV infection, or B lineage malignancy display abnormal CD1d expression. Transcriptional regulation of CD1d in these cells also may involve use of physiological transcriptional pathways. Yellow highlighted boxes describe areas that warrant further research, and blue highlighted boxes describe potential therapeutic strategies. 4766 BRIEF REVIEWS: CD1d IN NORMAL AND PATHOLOGICAL B CELLS from iNKT cell–mediated immune surveillance, a process that proliferation in vitro (33), as well as with the finding that, would be further undermined by the defective (but reversible) unlike in MM, iNKT cells in CLL are fully functional (70). production of IFN-g by iNKT cells in patients with progressive In contrast to CLL, CD1d is highly expressed in other B- disease (62). CLPDs, such as splenic MZ lymphoma, lymphoplasmacytic However, in addition to immune escape, CD1d may act as lymphoma, mantle cell lymphoma, and variant hairy cell a signaling conduit in myeloma cells. In myeloma cell lines leukemia (66). With the exception of hairy cell leukemia, with restored expression of CD1d, ligation of surface CD1d by these are largely incurable B cell malignancies, and their high mAb induced caspase-independent apoptosis (an effect de- expression of surface CD1d would make them suitable targets pendent on the cytoplasmic tail of CD1d but not the Tyr for iNKT cell–based immunotherapeutic approaches. residue required for endosomal trafficking of CD1d) (61, 63). Thus, loss of CD1d also may be important because it allows Conclusions myeloma cells to avoid the triggering of CD1d-dependent, CD1d exhibits dynamic expression on B cells in both health downstream antisurvival signaling pathways. and disease, and a greater understanding of these processes The therapeutic potential of the CD1d–iNKT cell axis was will provide a platform for therapeutic opportunities (Fig. 1). explored in a phase I trial in which autologous myeloid Although current experimental evidence suggests an impor- dendritic cells loaded with a-GalCer were infused into tant role for CD1d and iNKT cells in the development of patients with indolent myeloma (64). The procedure was safe protective humoral immunity against pathogens, more work is and was associated with a significant reduction in paraprotein required to better define their role in this context. By contrast, Downloaded from level, indicating antimyeloma activity. Notably, this small the preclinical data demonstrating B cells as the direct or cohort of patients also was treated with lenalidomide, which indirect effectors of the potent adjuvant activity of the CD1d– previously was found to enhance iNKT cell function in iNKT cell axis are sufficiently convincing to justify develop- patients with myeloma. ment of protocols aiming to enhance vaccine efficacy in the B cell chronic lymphoproliferative disorders. B cell chronic clinical setting. lymphoproliferative disorders (B-CLPDs) are a heterogeneous High levels of CD1d expression by certain types of B lineage http://www.jimmunol.org/ group of diseases arising from the clonal expansion of mature malignancies make them suitable targets for iNKT cell–based B cells at different stages of differentiation (65). Immuno- immunotherapeutic approaches, including adoptive transfer phenotypic analysis of surface markers plays a key role in both of in vitro–expanded autologous iNKT cells or even chimeric the diagnosis and prognostication of B-CLPDs, especially AgR-modified iNKT cells targeting the B cell–specific CD19 CLL; thus, several studies examined the potential of CD1d to Ag. Furthermore, the emerging evidence suggests that regu- be used in such a manner. lation of CD1d expression during viral infection and malig- Several studies, each involving .100 patients, found that nant transformation reflects its regulation during late B cell development. The reversible nature of CD1d expression reg- surface expression of CD1d is significantly, although variably, by guest on September 28, 2021 downregulated in CLL compared with other B-CLPDs, and it ulated by the differential activity of the retinoic acid pathway is lower than in normal B cells; CD1d expression levels below points to therapeutic approaches in B lineage malignancies a set threshold are strongly predictive of CLL, and higher (including those driven by EBV) that could combine iNKT cell– CD1d expression levels are associated with reduced time to based cellular therapies with biological agents, such as all- treatment and overall survival (66–68). trans retinoic acid. CD1d expression also was associated with several other known poor prognostic markers in CLL, including expression Acknowledgments We thank the members of our laboratory for critical reading of this manuscript. of ZAP-70, CD38, and CD49d, unmutated IgVH status, and cytogenetic abnormalities (67–69). 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