A critical role for IL-21 receptor signaling in the pathogenesis of systemic lupus erythematosus in BXSB-Yaa mice

IFN-α confers resistance of systemic lupus erythematosus nephritis to therapy in NZB/W F1 mice

The critical role of IFN-α in the pathogenesis of human systemic lupus erythematosus has been highlighted in recent years. Exposure of young lupus-prone NZB/W F1 mice to IFN-α in vivo leads to an accelerated lupus phenotype that is dependent on T cells and is associated with elevated serum levels of BAFF, IL-6, and TNF-α, increased splenic expression of IL-6 and IL-21, formation of large germinal centers, and the generation of large numbers of short-lived plasma cells that produce IgG2a and IgG3 autoantibodies. In this study, we show that both IgG2a and IgG3 autoantibodies are pathogenic in IFN-α-accelerated lupus, and their production can be dissociated by using low-dose CTLA4-Ig. Only high-dose CTLA4-Ig attenuates both IgG2a and IgG3 autoantibody production and significantly delays death from lupus nephritis. In contrast, BAFF/APRIL blockade has no effect on germinal centers or the production of IgG anti-dsDNA Abs but, if given at the time of IFN-α challenge, delays the progression of lupus by attenuating systemic and renal inflammation. Temporary remission of nephritis induced by combination therapy with cyclophosphamide, anti-CD40L Ab, and CTLA4-Ig is associated with the abrogation of germinal centers and depletion of short-lived plasma cells, but relapse occurs more rapidly than in conventional NZB/W F1 mice. This study demonstrates that IFN-α renders NZB/W F1 relatively resistant to therapeutic intervention and suggests that the IFN signature should be considered when randomizing patients into groups and analyzing the results of human clinical trials in systemic lupus erythematosus.

Unc93B1 restricts systemic lethal inflammation by orchestrating Toll-like receptor 7 and 9 trafficking

Toll-like receptor-7 (TLR7) and 9, innate immune sensors for microbial RNA or DNA, have been implicated in autoimmunity. Upon activation, TLR7 and 9 are transported from the endoplasmic reticulum (ER) to endolysosomes for nucleic acid sensing by an ER-resident protein, Unc93B1. Little is known, however, about a role for sensor transportation in controlling autoimmunity. TLR9 competes with TLR7 for Unc93B1-dependent trafficking and predominates over TLR7. TLR9 skewing is actively maintained by Unc93B1 and reversed to TLR7 if Unc93B1 loses preferential binding via a D34A mutation. We here demonstrate that mice harboring a D34A mutation showed TLR7-dependent, systemic lethal inflammation. CD4(+) T cells showed marked differentiation toward T helper 1 (Th1) or Th17 cell subsets. B cell depletion abolished T cell differentiation and systemic inflammation. Thus, Unc93B1 controls homeostatic TLR7 activation by balancing TLR9 to TLR7 trafficking.

Fine-mapping and transethnic genotyping establish IL2/IL21 genetic association with lupus and localize this genetic effect to IL21

OBJECTIVE

Genetic association of the IL2/IL21 region at chromosome 4q27 has previously been reported in lupus and a number of autoimmune and inflammatory diseases. This study was undertaken to determine whether this genetic effect could be localized, using a very large cohort of lupus patients and controls.

METHODS

We genotyped 45 tag single-nucleotide polymorphisms (SNPs) across the IL2/IL21 locus in 2 large independent lupus sample sets. We studied a set of subjects of European descent consisting of 4,248 lupus patients and 3,818 healthy controls, and an African American set of 1,569 patients and 1,893 healthy controls. Imputation in 3,004 additional controls from the Wellcome Trust Case Control Consortium was also performed. Genetic association between the genotyped markers was determined, and pairwise conditional analysis was performed to localize the independent genetic effect in the IL2/IL21 locus in lupus.

RESULTS

We established and confirmed the genetic association between IL2/IL21 and lupus. Using conditional analysis and transethnic mapping, we localized the genetic effect in this locus to 2 SNPs in high linkage disequilibrium: rs907715 located within IL21 (odds ratio 1.16 [95% confidence interval 1.10-1.22], P=2.17×10(-8)) and rs6835457 located in the 3'-untranslated flanking region of IL21 (odds ratio 1.11 [95% confidence interval 1.05-1.17], P=9.35×10(-5)).

CONCLUSION

Our findings establish the genetic association between lupus and IL2/IL21 with a genome-wide level of significance. Further, our findings indicate that this genetic association within the IL2/IL21 linkage disequilibrium block is localized to IL21. If other autoimmune IL2/IL21 genetic associations are similarly localized, then the IL21 risk alleles would be predicted to operate by a fundamental mechanism that influences the course of a number of autoimmune disease processes.

Key role for IL-21 in experimental autoimmune uveitis

IL-21 is a pleiotropic type 1 cytokine that shares the common cytokine receptor γ-chain, γ(c), with IL-2, IL-4, IL-7, IL-9, and IL-15. IL-21 is most homologous to IL-2. These cytokines are encoded by adjacent genes, but they are functionally distinct. Whereas IL-2 promotes development of regulatory T cells and confers protection from autoimmune disease, IL-21 promotes differentiation of Th17 cells and is implicated in several autoimmune diseases, including type 1 diabetes and systemic lupus erythematosus. However, the roles of IL-21 and IL-2 in CNS autoimmune diseases such as multiple sclerosis and uveitis have been controversial. Here, we generated Il21-mCherry/Il2-emGFP dual-reporter transgenic mice and showed that development of experimental autoimmune uveitis (EAU) correlated with the presence of T cells coexpressing IL-21 and IL-2 into the retina. Furthermore, Il21r(-/-) mice were more resistant to EAU development than wild-type mice, and adoptive transfer of Il21r(-/-) T cells induced much less severe EAU, underscoring the need for IL-21 in the development of this disease and suggesting that blocking IL-21/γ(c)-signaling pathways may provide a means for controlling CNS auto-inflammatory diseases.

Facultative role for T cells in extrafollicular Toll-like receptor-dependent autoreactive B-cell responses in vivo

Extrafollicular (EF) B-cell responses are increasingly being recognized as an alternative pathway of B-cell activation, particularly in autoimmunity. Critical cellular interactions required for the EF B-cell response are unclear. A key question in autoimmunity, in which Toll-like receptor (TLR) signals are costimulatory and could be sufficient for B-cell activation, is whether T cells are required for the response. This is pivotal, because autoreactive B cells are considered antigen-presenting cells for autoreactive T cells, but where such interactions occur has not been identified. Here, using AM14 site-directed transgenic rheumatoid factor (RF) mice, we report that B cells can be activated, differentiate, and isotype-switch independent of antigen-specific T-cell help, αβ T cells, CD40L signaling, and IL-21 signaling to B cells. However, T cells do dramatically enhance the response, and this occurs via CD40L and IL-21 signals. Surprisingly, the response is completely inducible T-cell costimulator ligand independent. These results establish that, although not required, T cells substantially amplify EF autoantibody production and thereby implicate T-independent autoreactive B cells as a potential vector for breaking T-cell tolerance. We suggest that these findings explain why autoreactivity first focuses on self-components for which B cells carry TLR ligands, because these will uniquely be able to activate B cells independently of T cells, with subsequent T-B interactions activating autoreactive T cells, resulting in chronic autoimmunity.

Analysis of the role of IL-21 in development of murine B cell progenitors in the bone marrow

IL-21 plays a key role in the late stage of B cell development, where it has been shown to induce growth and differentiation of mature B cells into Ig-secreting plasma cells. Because IL-21R has also been reported on bone marrow (BM) B cell progenitors, we investigated whether IL-21R influenced earlier stages of B cell development. IL-21R is functional as early as the pro-B cell stage, and the strength of receptor-mediated signaling increases as cells mature. The addition of IL-21 to B cell progenitors in cell culture resulted in the accelerated appearance of mature B cell markers and was associated with the induction of Aid, Blimp1, and germline transcripts. We also found that stimulation of both IL-21R and CD40 was sufficient to induce the maturation of early B cell progenitors into IgM- and IgG-secreting cells. Consistent with a role for IL-21 in promoting B cell differentiation, the number of B220(+)CD43(+)IgM(-) pro-B cells was increased, and the number of mature IgM(hi)IgD(hi) cells was decreased in BM of IL-21R-deficient mice. We also report in this paper that IL-21 is expressed by BM CD4(+) T cells. These results provide evidence that IL-21R is functional in B cell progenitors and indicate that IL-21 regulates B cell development.

Targeting interleukin-21 in inflammatory diseases

INTRODUCTION

IL-21, a new member of the type 1 cytokine superfamily, is produced by various subsets of CD4(+) T cells and binds to a composite receptor that consists of a specific receptor, termed IL-21 receptor and the common γ-chain subunit. Initially considered to be a critical regulator of T and B cell function, IL-21 is now known to regulate the activity of many other cell types, including both immune and non-immune cells.

AREAS COVERED

In this review, we discuss the biological features of IL-21 and summarize recent advances in the pathogenic role of IL-21 in chronic inflammatory diseases. Moreover, we discuss why IL-21 blockers can have a place in the therapeutic armamentarium for patients with immune-mediated diseases and the potential risks of such treatments.

EXPERT OPINION

Data emerging from studies in human and experimental models of autoimmunity suggest that IL-21 is critically involved in the initiation and/or progression of inflammatory reactions where self-reactive immune cells or antibodies cause damage in tissue. Thus, theoretically, targeting IL-21 could help attenuate the activation of inflammatory pathways and facilitate the resolution of tissue damaging immune responses. However, one should also take into consideration some potential risks that could derive from the blockade of IL-21.

CD8+ T regulatory cells express the Ly49 Class I MHC receptor and are defective in autoimmune prone B6-Yaa mice

The immune system includes a subpopulation of CD8(+) T cells equipped to inhibit the expansion of follicular T helper (T(FH)) cells, resulting in suppression of autoantibody production and associated lupus-like disease. These CD8(+) T regulatory (Treg) cells recognize Qa-1/peptide complexes on target T(FH) cells and depend on the IL-15 cytokine for development and function. Here we show that these CD8(+) Treg cells express a triad of surface receptors--CD44, CD122, and the class I MHC receptor Ly49--and account for <5% of CD8(+) T cells. Moreover, the development of systemic lupus erythematosus-like disease in B6-Yaa mutant mice is associated with a pronounced defect in CD8(+) Treg cell activity, suggesting that this regulatory subset may represent an effective therapeutic approach to systemic lupus erythematosus-like autoimmune disease.

Interleukin (IL)-21-independent pathogen-specific CD8+ T-cell expansion, and IL-21-dependent suppression of CD4+ T-cell IL-17 production

Although interleukin-21 (IL-21) potently activates and controls the differentiation of immune cells after stimulation in vitro, the role for this pleiotropic cytokine during in vivo infection remains poorly defined. Herein, the requirement for IL-21 in innate and adaptive host defence after Listeria monocytogenes infection was examined. In the innate phase, IL-21 deficiency did not cause significant defects in infection susceptibility, or in the early activation of natural killer and T cells. In the adaptive phase, L. monocytogenes-specific CD8(+) T cells expand to a similar magnitude in IL-21-deficient mice compared with control mice. Interestingly, the IL-21-independent expansion of L. monocytogenes-specific CD8(+) T cells was maintained even in the combined absence of IL-12 and type I interferon (IFN) receptor. Similarly, L. monocytogenes-specific CD4(+) T cells expanded and produced similar levels of IFN-γ regardless of IL-21 deficiency. Unexpectedly however, IL-21 deficiency caused significantly increased CD4(+) T-cell IL-17 production, and this effect became even more pronounced after L. monocytogenes infection in mice with combined defects in both IL-12 and type I IFN receptor that develop a T helper type 17-dominated CD4(+) T-cell response. Despite increased CD4(+) T-cell IL-17 production, L. monocytogenes-specific T cells re-expanded and conferred protection against secondary challenge with virulent L. monocytogenes regardless of IL-21 deficiency, or combined defects in IL-21, IL-12, and type I IFN receptor. Together, these results demonstrate non-essential individual and combined roles for IL-21, IL-12 and type I IFNs in priming pathogen-specific CD8(+) T cells, and reveal IL-21-dependent suppression of IL-17 production by CD4(+) T cells during in vivo infection.

Review: T helper 17 cells: their role in glomerulonephritis

T helper (Th) cells are an integral part of the host's immune response to eliminate invading pathogens. However, autoimmune or 'autoinflammatory' diseases can develop if Th cell responses are not effectively regulated. Several subsets of Th cells exist, including the Th17 subset that produces interleukin-17A, important in experimental models of organ-specific autoimmune inflammation. Its discovery has explained paradoxical observations in model systems thought to be Th1 mediated but were exacerbated in the absence of interferon-gamma, the prototypic Th1 effector cytokine. Th17 cells express unique transcription factors and secrete a unique pattern of cytokines. Interleukin-17A induces pro-inflammatory cytokines and chemokines and mediates neutrophil recruitment. Th17 cells have a reciprocal relationship with T regulatory cells and can also mediate suppression of Th1 responses. Recent studies also suggest that Th17 cells are not terminally differentiated but can switch into Th1 cells. Th17 cells have themselves been recently shown to induce antigen-specific cell-mediated proliferative glomerulonephritis. There is increasing evidence implicating Th17 cells in anti-glomerular basement membrane disease, lupus nephritis and pauci-immune glomerulonephritis. This review will review the discovery of the Th17 subset, its properties, its relationship with other Th subsets and assess the current evidence implicating Th17 cells in glomerulonephritis.

CD5+ B cells from individuals with systemic lupus erythematosus express granzyme B

Recently, we reported that IL-21 induces granzyme B (GzmB) and GzmB-dependent apoptosis in malignant CD5(+) B cells from patients with chronic lymphocytic leukemia. Several autoimmune diseases (AD) are associated with enhanced frequencies of CD5(+) B cells. Since AD are also associated with elevated IL-21 and GzmB levels, we postulated a link between CD5(+) B cells, IL-21 and GzmB. Here, we demonstrate that IL-21 and GzmB serum levels are highly correlated in subjects with systemic lupus erythematosus (SLE) and that freshly isolated CD5(+) SLE B cells constitutively express GzmB. IL-21 directly induced GzmB expression and secretion by CD5(+) B cells from several AD and from cord blood in vitro, and the simultaneous presence of BCR stimulation strongly enhanced this process. Furthermore, IL-21 suppressed both viability and expansion of CD5(+) B cells from SLE individuals. In summary, our study may explain the elevated levels of IL-21 and GzmB in SLE and other AD. Moreover, our data suggest that IL-21 may have disease-modifying characteristics by inducing GzmB in CD5(+) B cells and by suppressing their expansion. Our results provide the rationale for further evaluation of the therapeutic potential of IL-21 in certain AD such as SLE.

Inhibition of follicular T-helper cells by CD8(+) regulatory T cells is essential for self tolerance

The ability to produce vigorous immune responses that spare self tissues and organs depends on elimination of autoreactive T and B cells. However, purging of immature and mature self-reactive T and B cells is incomplete and may require additional censorship by cells programmed to suppress immune responses ¹. Regulatory T cells belonging to the CD4⁺ T cell subset may play a role in preventing untoward inflammatory responses, but T cell subsets programmed to inhibit the development of autoantibody formation and SLE-like disease have not been defined ². Here we delineate a CD8⁺ regulatory T cell lineage that is essential for maintenance of self tolerance and prevention of autoimmune disease. Genetic disruption of the inhibitory interaction between these CD44⁺ ICOSL⁺ CD8⁺ T cells and their target Qa-1⁺ follicular T helper cells results in the development of a lethal SLE-like autoimmune disease. These findings define a sublineage of CD8 T cells programmed to suppress rather than activate immunity that represents an essential regulatory element of the immune response and a guarantor of self tolerance.

Anticytokine therapy in systemic lupus erythematosus

Systemic lupus erythematosus is a prototype of heterogeneous autoimmune disease. There have been few newly approved therapeutic agents in lupus treatment for many reasons. Several animal studies and human data have shown that many potential cytokines are related to the pathogenesis and disease activity of systemic lupus erythematosus. Cytokines are produced by many immune cell types and have variable functions in the immune system. Following the success of biological agents in the treatment of inflammatory arthritis, inflammatory bowel disease, and psoriasis, biological targeting to specific cytokines or receptor molecules is now promising in the treatment of systemic lupus erythematosus. In addition to B-cell deleting modalities, clinical trials targeting potential cytokines associated with disease pathogenesis are underway at various clinical stages. Among potential cytokines, targeting agents against B-cell activating factor and interferon-alpha are in the most advanced stage, and belimumab (anti-B-cell activating factor antibody) could be the first biological agent approved in the treatment of systemic lupus erythematosus. Anti-tumor necrosis factor was tried with some success, but with a potential risk of infection in a small number of patients. In this review, we discuss the rationale for anticytokine therapies and review agents currently in clinical trials, and those that could be developed in the near future for systemic lupus erythematosus. We present the results mostly from published trials and data from http://clinicaltrials.gov/ct2/

Interleukin-21: a new mediator of inflammation in systemic lupus erythematosus

Systemic Lupus Erythematosus (SLE) is an autoimmune disorder characterized by excessive production of a variety of autoantibodies and a wide range of clinical manifestations. Pathogenesis of SLE is complex and not fully understood. There is however evidence that B and T cells are critical to the development of disease, and that T cell-derived cytokines are involved in the SLE-associated inflammatory response. One such cytokine seems to be interleukin (IL)-21, the latest identified member of the gamma-chain-related cytokine family. IL-21 has an important role in the control of the growth, survival, differentiation, and function of both T and B cells, and excessive production of IL-21 has been associated with the development of multiple immune-mediated diseases. Here we review data supporting the involvement of IL-21 in the pathogenesis of SLE.

Altered effector CD4+ T cell function in IL-21R-/- CD4+ T cell-mediated graft-versus-host disease

We previously showed that transplantation with IL-21R gene-deficient splenocytes resulted in less severe graft-versus-host disease (GVHD) than was observed with wild type splenocytes. In this study, we sought to find mechanism(s) explaining this observation. Recipients of donor CD4(+) T cells lacking IL-21R exhibited diminished GVHD symptoms, with reduced inflammatory cell infiltration into the liver and intestine, leading to prolonged survival. After transplantation, CD4(+) T cell numbers in the spleen were reduced, and MLR and cytokine production by CD4(+) T cells were impaired. These results suggest that IL-21 might promote GVHD through enhanced production of effector CD4(+) T cells. Moreover, we found that CD25 depletion altered neither the impaired MLR in vitro nor the ameliorated GVHD symptoms in vivo. Thus, the attenuated GVHD might be caused by an impairment of effector T cell differentiation itself, rather than by an increase in regulatory T cells and suppression of effector T cells.

The IL-2 defect in systemic lupus erythematosus disease has an expansive effect on host immunity

IL-2 production is decreased in systemic lupus erythematosus (SLE) patients and affects T cell function and other aspects of host immunity. Transcription factors regulating IL-2 production behave aberrantly in SLE T cells. In addition to IL-2 dysregulation, other IL-2 family members (IL-15 and IL-21) are abnormally expressed in SLE. Decreased IL-2 production in SLE patients leads to many immune defects such as decreased T_reg production, decreased activation-induced cell death (AICD), and decreased cytotoxicity. IL-2 deficiency results in systemic dysregulation of host immune responses in patients suffering from SLE disease.

T cells as therapeutic targets in SLE

T cells contribute to the initiation and perpetuation of autoimmunity in systemic lupus erythematosus (SLE), and seem to be directly involved in the development of related organ pathology. Defects associated with CD8(+) and T-regulatory (T(REG)) cell function manifest in parallel with the expanded CD3(+)CD4(-)CD8(-) T cell lineage. The cytokine expression pattern is uniquely characterized by decreased expression of interleukin (IL)-2 and increased production of IL-17 and related cytokines. Therapeutic approaches that limit the cognate interaction between T cells and B cells, prevent inappropriate tissue homing and restore T(REG) cell function and the normal cytokine milieu have been entertained. Biochemical characterization of SLE T cells has revealed distinct early and late signaling aberrations, and has enabled the identification of novel molecular targets that can be corrected with small molecules, and biomarkers that may foretell disease activity and predict organ damage.

Cytokines as therapeutic targets in SLE

Systemic lupus erythematosus (SLE) is a heterogeneous autoimmune disease involving most immune cells. Studies in both experimental animal models of lupus and patients with SLE have revealed a number of cytokine pathways that are important in the disease process. Among these are B-cell activating factor, which promotes B-cell survival and autoantibody production, interferon-alpha, which acts as an immune adjuvant, and tumor necrosis factor, which contributes to organ inflammation. This knowledge, in combination with the successful use of anti-TNF treatment in rheumatoid arthritis, has spurred the development of several biologic agents targeting different cytokines or their receptors in SLE. Consequently, many trials of anticytokine therapies for SLE are underway. Although most of these trials are small or in early phases, the results of some large studies have also been reported. In this Review, we discuss the rationale for anticytokine therapies in SLE and review agents currently in use, and those being developed and tested experimentally. We present the results from published trials and discuss the tentative conclusions that can be drawn regarding the efficacy of the new agents. Finally, we provide suggestions for the future of treatment for SLE, including new therapeutic strategies.

Understanding the development and function of T follicular helper cells

A fundamental function of T helper (Th) cells is to regulate B-cell proliferation and immunoglobulin class switching, especially in the germinal centers. Th1 and Th2 lineages of CD4(+) T cells have long been considered to play an essential role in helping B cells by promoting the production immunoglobulin G2a (IgG2a) and IgG1/IgE, respectively. Recently, it has become clear that a subset CD4(+) T cells, named T follicular helper (Tfh) cells, is critical to B-cell response induction. In this review, we summarize the latest advances in our understanding of the regulation of Tfh cell differentiation, the relationship of Tfh cells to other CD4(+) T-cell lineages, and the role of Tfh cells in health and disease.

The role of dendritic cells in the pathogenesis of systemic lupus erythematosus

The etiology of the autoimmune disease systemic lupus erythematosus is not known, but aberrant apoptosis and/or insufficient clearance of apoptotic material have been assigned a pivotal role. During apoptosis, nucleosomes and several endogenous danger-associated molecular patterns are incorporated in blebs. Recent data indicate that apoptotic blebs induce maturation of myeloid dendritic cells, resulting in IL-17 production by T cells. In this review we summarize current knowledge on the role of dendritic cells in the pathogenesis of systemic lupus erythematosus with special emphasis on the uptake of apoptotic blebs by dendritic cells, and the subsequent induction of Th17 cells.

Multiple checkpoints keep follicular helper T cells under control to prevent autoimmunity

Follicular helper T (Tfh) cells select mutated B cells in germinal centres, which can then differentiate into long-lived high affinity memory B cells and plasma cells. Tfh cells are regulated by a unique molecular programme orchestrated by the transcriptional repressor Bcl6. This transcription factor turns down expression of multiple genes, including transcriptional regulators of other T helper lineages and a vast amount of microRNAs. This enables Tfh cells to express a suite of chemokine receptors, stimulatory ligands and cytokines that enable migration into B-cell follicles, and provision of effective help to B cells. Not surprisingly, dysregulation of this powerful helper subset can lead to a range of autoantibody-mediated diseases; indeed, aberrant accumulation of Tfh cells has been linked with systemic lupus erythematosus, Sjogren's disease and autoimmune arthritis. Here we dissect multiple checkpoints that operate throughout Tfh cell development and maturation to maintain immunological tolerance while mounting robust and long-lasting antibody responses.

IL-21 is critical for GVHD in a mouse model

Immunological effects of IL-21 on T, B and natural killer (NK) cells have been reported, but the role of IL-21 in GVHD remains obscure. Here, we demonstrate that morbidity and mortality of GVHD was significantly reduced after BMT with splenocytes from IL-21R(-/-) mice compared with those from wild type mice. To further confirm our observation, we generated a decoy receptor for IL-21. GVHD was again less severe in mice receiving BM cells transduced with the IL-21 decoy receptor than control mice These results suggest that IL-21 critically regulates GVHD, and that blockade of the IL-21 signal may represent a novel strategy for the prophylaxis for GVHD.

A novel subpopulation of B-1 cells is enriched with autoreactivity in normal and lupus-prone mice

OBJECTIVE

B-1 cells have long been suggested to play an important role in lupus. However, reports to date have been controversial regarding their pathogenic or protective roles in different animal models. We undertook this study to investigate a novel subpopulation of B-1 cells and its roles in murine lupus.

METHODS

Lymphocyte phenotypes were assessed by flow cytometry. Autoantibody secretion was analyzed by enzyme-linked immunosorbent assay, autoantigen proteome array, and antinuclear antibody assay. Cell proliferation was measured by thymidine incorporation and 5,6-carboxyfluorescein succinimidyl ester dilution. B cell Ig isotype switching was measured by enzyme-linked immunospot assay.

RESULTS

Anti-double-stranded DNA (anti-dsDNA) autoantibodies were preferentially secreted by a subpopulation of CD5+ B-1 cells that expressed programmed death ligand 2 (termed L2pB1 cells). A substantial proportion of hybridoma clones generated from L2pB1 cells reacted to dsDNA. Moreover, these clones were highly cross-reactive with other lupus-related autoantigens. L2pB1 cells were potent antigen-presenting cells and promoted Th17 cell differentiation in vitro. A dramatic increase of circulating L2pB1 cells in lupus-prone BXSB mice was correlated with elevated serum titers of anti-dsDNA antibodies. A significant number of L2pB1 cells preferentially switched to IgG1 and IgG2b when stimulated with interleukin-21.

CONCLUSION

Our findings identify a novel subpopulation of B-1 cells that is enriched for autoreactive specificities, undergoes isotype switch, manifests enhanced antigen presentation, promotes Th17 cell differentiation, and is preferentially associated with the development of lupus in a murine model. Together, these findings suggest that L2pB1 cells have the potential to initiate autoimmunity through serologic and T cell-mediated mechanisms.

Evidence that Yaa-induced loss of marginal zone B cells is a result of dendritic cell-mediated enhanced activation

The development of systemic lupus is accelerated by the Yaa (Y-linked autoimmune acceleration) mutation, which is the consequence of a translocation of the telomeric end containing the Tlr7 gene from the X chromosome onto the Y chromosome. However, the loss of marginal zone (MZ) B cells, one of the Yaa-linked cellular abnormalities, has previously been shown to be unrelated to the Tlr7 gene duplication, and the present study therefore aimed to investigate the mechanism responsible for MZ B-cell loss. Analyses of Yaa and non-Yaa C57BL/6 male mice expressing an MD4 anti-HEL IgM transgene or those deficient in fms-like tyrosine kinase 3 ligand (FL) revealed that the proportion of MZ B cells in these Yaa mice was comparable to that of the respective non-Yaa control mice. Notably, the activation of MZ B cells was compromised in both of these transgenic model systems, due to the absence of cognate antigens or the impaired development of dendritic cells, respectively. These results contrasted with the loss of MZ B cells in non-Yaa mice treated with FL and the lack of accumulation of MZ B cells in Yaa mice treated with a B-cell survival factor, BAFF. Taken together, our results suggest that the persistent and enhanced activation of Yaa-bearing hyperactive MZ B cells by dendritic cells is responsible for the loss of this B-cell subset in Yaa mice.

Pathogenesis of human systemic lupus erythematosus: recent advances

Systemic lupus erythematosus (SLE) is an autoimmune disease with manifestations derived from the involvement of multiple organs including the kidneys, joints, nervous system and hematopoietic organs. Immune system aberrations, as well as heritable, hormonal and environmental factors interplay in the expression of organ damage. Recent contributions from different fields have developed our understanding of SLE and reshaped current pathogenic models. Here, we review recent findings that deal with (i) genes associated with disease expression; (ii) immune cell molecular abnormalities that lead to autoimmune pathology; (iii) the role of hormones and sex chromosomes in the development of disease; and (iv) environmental and epigenetic factors thought to contribute to the expression of SLE. Finally, we highlight molecular defects intimately associated with the disease process of SLE that might represent ideal therapeutic targets and disease biomarkers.

Signals that influence T follicular helper cell differentiation and function

Follicular helper T cells have recently emerged as a separate CD4(+) T helper lineage specialised in provision of help to B cells. They develop independently from Th1, Th2 and Th17 cells and are critical for humoral immunity, including the generation of long-lived and high affinity plasma cells and memory cells crucial for long-term protection against infections. A stepwise differentiation programme has emerged in which T cell receptor (TCR) signalling strength, CD28-mediated costimulation, B cell-derived inducible costimulator ligand signals, induction of c-maf and actions of cytokines, including interleukin (IL)-6 and IL-21, lead to upregulation of the transcriptional repressor B cell lymphoma 6 (Bcl-6) that drives T follicular helper (Tfh) cell differentiation. Bcl-6 turns on a repression programme that targets Blimp-1, transcriptional regulators of other helper lineages and microRNAs. Their concerted actions modulate expression of chemokine receptors, surface molecules and cytokines critical for follicular homing and B cell helper functions. Here, we review the nature of Tfh cells providing help to B cells during the two phases of B cell activation that occur in the outer T zone and, for some B cells, in germinal centres (GC). Recent insights into the signalling events that drive terminal differentiation of Tfh cells critical for selecting somatically mutated GC B cells and the consequences of Tfh dysregulation for immunodeficiency and autoimmune pathology are discussed.

T follicular helper cells during immunity and tolerance

Helper T cells are required for the generation of a potent immune response to foreign antigens. Amongst them, T follicular helper (Tfh) cells are specialized in promoting protective, long-lived antibody responses that arise from germinal centers. Within these structures, the specificity of B cell receptors may change, due to the process of random somatic hypermutation aimed at increasing the overall affinity of the antibody response. The danger of emerging self-reactive specificities is offset by a stringent selection mechanism delegated in great part to Tfh cells. Only those B cells receiving survival signals from Tfh cells can exit the germinal centers to join the long-lived pools of memory B cells and bone marrow-homing plasma cells. Thus, a crucial immune tolerance checkpoint to prevent long-term autoantibody production lies in the ability to tolerize Tfh cells and to control positive and negative selection signals delivered by this subset. This review tackles the known mechanisms that ensure Tfh tolerance, many of them shared by other T helper subsets during thymic development and priming, but others unique to Tfh cells. Amongst the latter are checkpoints at the stages of Tfh differentiation, follicular migration, growth, longevity, and quality control of selection signals. Finally, we also discuss the consequences of a breakdown in Tfh tolerance.

Dysregulation of germinal centres in autoimmune disease

In germinal centres, somatic hypermutation and B cell selection increase antibody affinity and specificity for the immunizing antigen, but the generation of autoreactive B cells is an inevitable by-product of this process. Here, we review the evidence that aberrant selection of these autoreactive B cells can arise from abnormalities in each of the germinal centre cellular constituents--B cells, T follicular helper cells, follicular dendritic cells and tingible body macrophages--or in the supply of antigen. As the progeny of germinal centre B cells includes long-lived plasma cells, selection of autoreactive B cells can propagate long-lived autoantibody responses and cause autoimmune diseases. Elucidation of crucial molecular signals in germinal centres has led to the identification of novel therapeutic targets.

Lupus nephritis: lessons from murine models

Lupus nephritis is a challenging clinical condition for which current therapies are unsatisfactory with respect to both remission induction and unwanted toxic effects. Despite intervention, the rates of end-stage renal disease seem to be increasing in the USA. Discoveries over the past decade have greatly improved our understanding of immune activation and effector inflammatory pathways in lupus nephritis; however, this increased understanding has not yet translated into the approval of an effective new therapeutic agent. An analysis of the mechanisms of action of novel immunomodulatory drugs in multiple models of murine lupus clearly shows that interacting networks of immune and effector pathways are recruited as the disease progresses. Reversing established disease by targeting a single cell population or inflammatory pathway is, therefore, difficult once long-lived autoreactive lymphocyte populations are present and peripheral organs are inflamed. Data from murine models of lupus suggest that we need to consider new paradigms for the management of systemic lupus erythematosus that include earlier immune intervention, long-term maintenance therapies and protection of target organs.

IL-21 and T follicular helper cells

Upon encounter with antigen, CD4(+) T cells differentiate into effector T(h) subsets with distinctive functions that are related to their unique cytokine profiles and anatomical locations. One of the most important T(h) functions is to provide signals to developing B cells that induce specific and appropriate antibody responses. The major CD4(+) T cell subset that helps B cells is the T follicular helper (T(FH)) cell, whose expression of the chemokine receptor CXCR5 [chemokine (C-X-C motif) receptor 5] serves to localize this cell to developing germinal centers (GCs) where it provides instructive signals leading to Ig class switching and somatic mutation. T(FH) cells produce high levels of IL-21, a cytokine that is critical for GC formation and also for the generation of T(FH) cells. Although T(FH) cells have been found to produce cytokines characteristic of other T(h) subsets, they represent a distinct lineage whose development is driven by the transcription factor B-cell CLL lymphoma-6 (BCL6). Consistent with their critical role in the generation of antibody responses, dysregulated T(FH) function has been associated with the development of systemic autoimmunity. Here, we review the role of IL-21 in the regulation of normal T(FH) development and function as well as in progression of autoimmune responses.

Endogenous IL-21 restricts CD8+ T cell expansion and is not required for tumor immunity

IL-21 has antitumor activity through actions on NK cells and CD8(+) T cells, and is currently in clinical development for the treatment of cancer. However, no studies have addressed the role of endogenous IL-21 in tumor immunity. In this study, we have studied both primary and secondary immune responses in IL-21(-/-) and IL-21R(-/-) mice against several experimental tumors. We found intact immune surveillance toward methylcholanthrene-induced sarcomas in IL-21(-/-) and IL-21R(-/-) mice compared with wild-type mice and B16 melanomas showed equal growth kinetics and development of lung metastases. IL-21R(-/-) mice showed competent NK cell-mediated rejection of NKG2D ligand (Rae1beta) expressing H-2b(-) RMAS lymphomas and sustained transition to CD8(+) T cell-dependent memory against H-2b(+) RMA lymphomas. alpha-Galactosylceramide stimulation showed equal expansion and activation of NKT and NK cells and mounted a powerful antitumor response in the absence of IL-21 signaling, despite reduced expression of granzyme B in NKT, NK, and CD8(+) T cells. Surprisingly, host IL-21 significantly restricted the expansion of Ag-specific CD8(+) T cells and inhibited primary CD8(+) T cell immunity against OVA-expressing EG7 lymphomas, as well as the secondary expansion of memory CD8(+) T cells. However, host IL-21 did not alter the growth of less immunogenic MC38 colon carcinomas with dim OVA expression. Overall, our results show that endogenous IL-21/IL-21R is not required for NK, NKT, and CD8(+) T cell-mediated tumor immunity, but restricts Ag-specific CD8(+) T cell expansion and rejection of immunogenic tumors, indicating novel immunosuppressive actions of this cytokine.

Lineage specification and heterogeneity of T follicular helper cells

T follicular helper (Tfh) cells were originally described as a non-polarized CD4(+) T cell subset with follicular homing capacity and a potent ability to induce antibody production from B cells. However, a number of studies published in the past year have revealed a degree of heterogeneity within the germinal center CD4(+) T cell population, which suggests additional complexity. The overzealous activities of Tfh cells, or inappropriate expression of certain cytokines, represent new pathways for the development of autoimmune diseases. This review focuses on current progress in unraveling the biology of Tfh cells in health and disease, and understanding the relationship of Tfh cells to other CD4(+) T cell lineages.

Pathogenesis of kidney disease in systemic lupus erythematosus

PURPOSE OF REVIEW

A combination of systemic autoimmunity and tissue response to immune injury underlie renal involvement in lupus erythematosus. In this review, we discuss recent literature investigating pathogenetic mechanisms of lupus glomerulonephritis.

RECENT FINDINGS

In lupus glomerulonephritis, glomerular immune complexes were believed to be the primary mediators of renal disease. Recent studies make it apparent that autoantibodies of multiple specificities participate in the formation of immune complexes, deposited in the kidneys. Renal infiltration by T cells, macrophages, and dendritic cells have a dominant role in the progression of lupus glomerulonephritis leading to renal failure. Activation of Toll-like receptors modulates autoantibody production and systemic interferon responses. However, glomerular cell responses to immune injury influence disease outcome. In addition, new insights on the genetics of susceptibility to end-organ damage in lupus glomerulonephritis have been discovered. Differential glomerular responses reflected in gene expression profiles during disease progression provide potential markers for diagnosis of lupus glomerulonephritis progression and flares. In addition, studies of end-organ responses provide new targets for therapeutic interventions.

SUMMARY

Lupus glomerulonephritis is a prototype of immune complex disease mediated by autoantibodies of multiple specificities, one of which is anti-DNA. Murine models of spontaneous systemic lupus erythematosus have been critical for understanding the underlying disease. Recent studies demonstrate that in addition to systemic autoimmunity, end-organ responses, and end-organ resistance to damage are also critical in determining disease outcome. This understanding should influence design of novel therapeutic approaches in systemic lupus erythematosus.

IL-21: roles in immunopathology and cancer therapy

Cytokines are secreted signalling molecules with decisive effects on haematopoiesis, innate and adaptive immunity, and immunopathology. Interleukin (IL)-21 is a novel cytokine produced by activated CD4(+) T cells and natural killer T (NKT) cells. IL-21 is part of a family of cytokines which include IL-2, -4, -7, -9 and -15 that all share the common IL-2 receptor gamma chain (gamma(c)) in their individual receptor complexes. IL-21 receptor (IL-21R) is widely expressed on both myeloid and lymphoid cell lineages and IL-21 actions include co-stimulation of B cell differentiation and immunoglobulin (Ig) production, co-mitogen of T cells, and stimulation of NK and CD8(+) T cell cytotoxic function. Initially, IL-21 was recognized for its anti-tumour effects in several preclinical tumour models, warranting its currently ongoing clinical development as a cancer immunotherapeutic. More recently, IL-21 has been associated with the development of a panel of autoimmune and inflammatory diseases, where neutralization of IL-21 has been suggested as a potential new therapy. In this review, we will cover the latest discoveries of IL-21 as a cancer therapy and its implications in immunopathologies.

Interleukin-17 and systemic lupus erythematosus: current concepts

The emerging role of interleukin (IL)-17 as a hallmark proinflammatory cytokine of the adaptive immune system, produced primarily by a new T helper cell subset termed 'Th17', has received considerable attention. Differentiation of Th17 cells is driven by the simultaneous presence of transforming growth factor-beta and certain inflammatory cytokines (e.g. IL-6, IL-21), and recent studies have shown that inflammation instigated by IL-17-producing cells is central to the development and pathogenesis of several human autoimmune diseases and animal models of autoimmunity. In this review, we focus on the information regarding IL-17 and systemic lupus erythematosus (SLE), a chronic autoimmune disease. The work that has explored the development and behaviour of IL-17-producing cells in SLE is discussed, and different mechanisms by which IL-17 could potentially augment inflammation and autoantibody production in the context of SLE are proposed.

New insights into the regulation of T cells by gamma(c) family cytokines

Common cytokine receptor gamma-chain (gamma(c)) family cytokines have crucial roles in the development, proliferation, survival and differentiation of multiple cell lineages of both the innate and adaptive immune systems. In this Review, we focus on our current understanding of the distinct and overlapping effects of interleukin-2 (IL-2), IL-7, IL-9, IL-15 and IL-21, as well as the IL-7-related cytokine thymic stromal lymphopoietin (TSLP), on the survival and proliferation of conventional alphabeta T cells, gammadelta T cells and regulatory T cells. This knowledge potentially allows for the therapeutic manipulation of immune responses for the treatment of cancer, autoimmunity, allergic diseases and immunodeficiency, as well as for vaccine development.

Interleukin-21 as a new therapeutic target for immune-mediated diseases

Cytokines have a decisive role in initiating and shaping pathologic responses in patients with various immune-inflammatory diseases. Recent studies indicate that interleukin (IL)-21, a cytokine produced mostly by activated CD4+ T cells, participates in the tissue damage in various tissues, owing to its ability to regulate the function of immune and non-immune cells. For instance, IL-21 controls the differentiation and functional activity of T cells, B cells and NK cells, limits the differentiation of inducible regulatory T cells (Tregs), and makes T cells resistant to the Treg-mediated immunesuppression. It also stimulates epithelial cells and fibroblasts to produce inflammatory mediators. Here, we focus on data supporting the pathogenic role of IL-21 in human inflammatory diseases and discuss pre-clinical studies that suggest that neutralization of IL-21 in vivo could be a new biological therapy to combat immune-mediated pathologies, such as inflammatory bowel diseases, diabetes, rheumatoid arthritis and systemic lupus erythematosus.

Interleukin 17 acts in synergy with B cell-activating factor to influence B cell biology and the pathophysiology of systemic lupus erythematosus

Studies have suggested involvement of interleukin 17 (IL-17) in autoimmune diseases, although its effect on B cell biology has not been clearly established. Here we demonstrate that IL-17 alone or in combination with B cell-activating factor controlled the survival and proliferation of human B cells and their differentiation into immunoglobulin-secreting cells. This effect was mediated mainly through the nuclear factor-kappaB-regulated transcription factor Twist-1. In support of the relevance of our observations and the potential involvement of IL-17 in B cell biology, we found that the serum of patients with systemic lupus erythematosus had higher concentrations of IL-17 than did the serum of healthy people and that IL-17 abundance correlated with the disease severity of systemic lupus erythematosus.