Protective and pathogenic roles for B cells during systemic autoimmunity in NZB/W F1 mice

B-lymphocyte effector functions in health and disease

B-lymphocytes have traditionally been thought to contribute to immunity and autoimmune disease through terminal differentiation into plasma cells that secrete antibody. However, studies in mice and recent clinical studies have demonstrated that genetically altered B-cell function and B-cell-targeted therapies can significantly affect autoimmune diseases that were predominantly thought to be T-cell-mediated. B-cell depletion in mouse models of disease has also led to the identification of alternative B-cell effector functions that regulate normal immune responses and autoimmune disease. This review highlights multiple B-cell effector mechanisms, including the promotion of cellular immunity, the negative regulation of immune responses, and the production of pathogenic antibodies.

B cell-derived IL-10 suppresses inflammatory disease in Lyn-deficient mice

Lyn kinase deficient mice represent a well established genetic model of autoimmune/autoinflammatory disease that resembles systemic lupus erythematosus. We report that IL-10 plays a crucial immunosuppressive role in this model, modulating the inflammatory component of the disease caused by myeloid and T-cell activation. Double-mutant lyn(-/-)IL-10(-/-) mice manifested severe splenomegaly and lymphadenopathy, dramatically increased proinflammatory cytokine production, and severe tissue inflammation. Single-mutant lyn(-/-)mice showed expansion of IL-10-producing B cells. Interestingly, WT B cells adoptively transferred into lyn(-/-) mice showed increased differentiation into IL-10-producing B cells that assumed a similar phenotype to endogenous lyn(-/-) IL-10-producing B cells, suggesting that the inflammatory environment present in lyn(-/-) mice induces IL-10-producing B-cell differentiation. B cells, but not T or myeloid cells, were the critical source of IL-10 able to reduce inflammation and autoimmunity in double mutant lyn(-/-)IL-10(-/-) mice. IL-10 secretion by B cells was also crucial to sustain transcription factor Forkhead Box P3 (Foxp3) expression in regulatory T cells during disease development. These data reveal a dominant immunosuppressive function of B-cell-derived IL-10 in the Lyn-deficient model of autoimmunity, extending our current understanding of the role of IL-10 and IL-10-producing B cells in systemic lupus erythematosus.

Amplified B lymphocyte CD40 signaling drives regulatory B10 cell expansion in mice

BACKGROUND

Aberrant CD40 ligand (CD154) expression occurs on both T cells and B cells in human lupus patients, which is suggested to enhance B cell CD40 signaling and play a role in disease pathogenesis. Transgenic mice expressing CD154 by their B cells (CD154(TG)) have an expanded spleen B cell pool and produce autoantibodies (autoAbs). CD22 deficient (CD22(-/-)) mice also produce autoAbs, and importantly, their B cells are hyper-proliferative following CD40 stimulation ex vivo. Combining these 2 genetic alterations in CD154(TG)CD22(-/-) mice was thereby predicted to intensify CD40 signaling and autoimmune disease due to autoreactive B cell expansion and/or activation.

METHODOLOGY/PRINCIPAL FINDINGS

CD154(TG)CD22(-/-) mice were assessed for their humoral immune responses and for changes in their endogenous lymphocyte subsets. Remarkably, CD154(TG)CD22(-/-) mice were not autoimmune, but instead generated minimal IgG responses against both self and foreign antigens. This paucity in IgG isotype switching occurred despite an expanded spleen B cell pool, higher serum IgM levels, and augmented ex vivo B cell proliferation. Impaired IgG responses in CD154(TG)CD22(-/-) mice were explained by a 16-fold expansion of functional, mature IL-10-competent regulatory spleen B cells (B10 cells: 26.7×10(6)±6 in CD154(TG)CD22(-/-) mice; 1.7×10(6)±0.4 in wild type mice, p<0.01), and an 11-fold expansion of B10 cells combined with their ex vivo-matured progenitors (B10+B10pro cells: 66×10(6)±3 in CD154(TG)CD22(-/-) mice; 6.1×10(6)±2 in wild type mice, p<0.01) that represented 39% of all spleen B cells.

CONCLUSIONS/SIGNIFICANCE

These results demonstrate for the first time that the IL-10-producing B10 B cell subset has the capacity to suppress IgG humoral immune responses against both foreign and self antigens. Thereby, therapeutic agents that drive regulatory B10 cell expansion in vivo may inhibit pathogenic IgG autoAb production in humans.

Autoimmunity in 2010

There is now growing evidence that autoimmunity is the common trait connecting multiple clinical phenotypes albeit differences in tissue specificity, pathogenetic mechanisms, and therapeutic approaches cannot be overlooked. Over the past years we witnessed a constant growth of the number of publications related to autoimmune diseases in peer-reviewed journals of the immunology area. Original data referred to factors from common injury pathways (i.e. T helper 17 cells, serum autoantibodies, or vitamin D) and specific diseases such as multiple sclerosis, systemic lupus erythematosus, and rheumatoid arthritis. As an example, the issue of a latitudinal gradient in the prevalence and incidence rates has been proposed for all autoimmune diseases and was recently coined as geoepidemiology to suggest new environmental triggers for tolerance breakdown. The present article is aimed at reviewing the articles that were published over the past year in the major autoimmunity and immunology journals.

Interleukin-10 receptor expression and signalling were down-regulated in CD4⁺ T cells of lupus nephritis patients

Studies have indicated that interleukin (IL)-10 has a pathogenic role in systemic lupus erythematosus (SLE); however, a protective effect of IL-10 in SLE was also observed. Because the exact mechanism of IL-10 signalling in the pathogenesis of SLE is unclear, this study sought to assess the expression and signalling of interleukin-10 receptor (IL-10R) in peripheral leucocytes from patients with SLE. We used flow cytometry to examine the expression of IL-10R1 on different peripheral leucocytes from 28 SLE patients, of whom 14 had lupus nephritis (LN) and 14 were healthy controls. We also examined the effects of IL-10 on phosphorylation of signal transducer and activator of transcription (STAT)-3 and STAT-1 in peripheral blood mononuclear cells (PBMCs) obtained from 13 SLE patients and seven healthy controls. Plasma cytokines were detected by flow cytometric bead array (CBA) techniques. Although IL-10R1 expression levels on each peripheral leucocyte subset from 28 SLE patients and 14 healthy controls were similar, the expression levels on CD4(+) T cells from LN patients were significantly lower than on CD4(+) T cells from controls and SLE patients without nephritis (P < 0·01). IL-10R1 expression levels on CD4(+) and CD8(+) T cells were correlated negatively with the SLE disease activity index (P < 0·01). Additionally, the phosphorylation of STAT-3 was delayed and reduced in PBMCs from LN patients and active SLE patients. Plasma IL-10 levels were significantly higher in LN patients than controls. IL-10R1 expression on CD4(+) T cells and signalling in PBMCs were down-regulated in LN patients, indicating that IL-10 and its receptor may have a special role in LN pathogenesis.

Regulatory B cells and allergic diseases

B cells are generally considered to positively regulate immune responses by producing antigen-specific antibodies. B cells are classified into classical CD5^- conventional B cells and CD5⁺ B1 cells. The latter produce multi-specific autoantibodies and are thought to be involved in autoimmune diseases. However, evidence supporting a B cell negative regulatory function has accumulated over the past 30 years. Multiple reports have suggested that absence, or loss, of regulatory B cells exacerbates symptoms of both allergic (including contact hypersensitivity and anaphylaxis) and autoimmune (such as experimental autoimmune encephalomyelitis, chronic colitis, and collagen-induced arthritis) diseases, and in lupus-like models of autoimmunity. Regulatory B cells are characterized by production of the negative regulatory cytokines, IL-10 and TGF-β. IL-10-producing B cells were the first regulatory B cells to be recognized and were termed 'B10' cells. IL-10-producing regulatory B cells are of the CD19⁺CD5⁺IgM^hiIgD^loCD1d^hi type. Recently, a TGF-β-producing regulatory B cell subset, Br3, has been shown to be related to immune tolerance in food allergies. Moreover, forkhead box P3 (Foxp3)-expressing B cells have also been identified in humans and may act as regulatory B cells (Bregs). The functional image of regulatory B cells is similar to that of regulatory T cells. Because of the proliferative and apoptotic responses of Br1 and Br3 cells in immune tolerance in non-IgE-mediated food allergy, reciprocal roles and counter-regulatory mechanisms of Br1 and Br3 responses are also suspected. Additionally, different roles for regulatory B and T cells at different time points during initiation and progression of autoimmune disease are described.

Estrogen-induced protection against experimental autoimmune encephalomyelitis is abrogated in the absence of B cells

Increased remissions in multiple sclerosis (MS) during pregnancy suggest that elevated levels of sex steroids exert immunoregulatory activity. Estrogen (E2=17β-estradiol) protects against experimental autoimmune encephalomyelitis (EAE), but the cellular basis for E2-induced protection remains unclear. Studies demonstrate that depletion of B cells prior to induction of EAE exacerbates disease severity, implicating regulatory B cells. We thus evaluated pathogenic and E2-induced protective mechanisms in B-cell-deficient (μMT(-/-)) mice. EAE-protective effects of E2 were abrogated in μMT(-/-) mice, with no reduction in disease severity, cellular infiltration or pro-inflammatory factors in the central nervous system compared to untreated controls. E2 treatment of WT mice selectively upregulated expression of PD-L1 on B cells and increased the percentage of IL-10-producing CD1d(high) CD5(+) regulatory B cells. Upregulation of PD-L1 was critical for E2-mediated protection since E2 did not inhibit EAE in PD-L1(-/-) mice. Direct treatment of B cells with E2 significantly reduced proliferation of MOG(35-55)-specific T cells that required estrogen receptor-α (ERα). These results demonstrate, for the first time, a requirement for B cells in E2-mediated protection against EAE involving direct E2 effects on regulatory B cells mediated through ERα and the PD-1/PD-L1 negative co-stimulatory pathway. E2-primed B cells may represent an important regulatory mechanism in MS and have strong implications for women receiving current MS therapies that cause B-cell depletion.

Murine models of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a multifactorial autoimmune disorder. The study of diverse mouse models of lupus has provided clues to the etiology of SLE. Spontaneous mouse models of lupus have led to identification of numerous susceptibility loci from which several candidate genes have emerged. Meanwhile, induced models of lupus have provided insight into the role of environmental factors in lupus pathogenesis as well as provided a better understanding of cellular mechanisms involved in the onset and progression of disease. The SLE-like phenotypes present in these models have also served to screen numerous potential SLE therapies. Due to the complex nature of SLE, it is necessary to understand the effect specific targeted therapies have on immune homeostasis. Furthermore, knowledge gained from mouse models will provide novel therapy targets for the treatment of SLE.

HDAC inhibition in lupus models

Systemic lupus erythematosus (SLE) is a prototypic autoimmune inflammatory disease characterized by the production of autoantibodies directed against nuclear antigens such as nucleosomes, DNA and histone proteins found within the body's cells and plasma. Autoantibodies may induce disease by forming immune complexes that lodge in target organs or by crossreacting with targeted antigens and damaging tissue. In addition to autoantibody production, apoptotic defects and impaired removal of apoptotic cells contribute to an overload of autoantigens that initiate an autoimmune response. Besides the well-recognized genetic susceptibility to SLE, environmental and epigenetic factors play a crucial role in disease pathogenesis as evidenced by monozygotic twins typically being discordant for disease. Changes in DNA methylation and histone acetylation alter gene expression and are thought to contribute to the epigenetic deregulation in disease. In SLE, global and gene-specific DNA methylation changes have been demonstrated to occur. Additionally, aberrant histone acetylation is evident in individuals with SLE. Moreover, histone deacetylase inhibitors (HDACi) have been shown to reverse the skewed expression of multiple genes involved in SLE. In this review, we discuss the implications of epigenetic alterations in the development and progression of SLE, and how therapeutics designed to alter histone acetylation status may constitute a promising avenue to target disease.

B cells promote hepatic inflammation, biliary cyst formation, and salivary gland inflammation in the NOD.c3c4 model of autoimmune cholangitis

There are now several murine models of autoimmune cholangitis that have features both similar and distinct from human PBC. One such model, the NOD.c3c4 mouse, manifests portal cell infiltrates, anti-mitochondrial antibodies but also biliary cysts. The biliary cysts are not a component of PBC and not found in the other murine models. To address the immunopathology in these mice, we generated genetically B cell deficient Igμ(-/-) NOD.c3c4 mice and compared the immunopathology of these animals to control B cell sufficient NOD.c3c4 mice. B cell deficient mice demonstrated decreased number of non-B cells in the liver accompanied by reduced numbers of activated natural killer cells. The degree of granuloma formation and bile duct damage were comparable to NOD.c3c4 mice. In contrast, liver inflammation, biliary cyst formation and salivary gland inflammation was significantly attenuated in these B cell deficient mice. In conclusion, B cells play a critical role in promoting liver inflammation and also contribute to cyst formation as well as salivary gland pathology in autoimmune NOD.c3c4 mice, illustrating a critical role of B cells in modulating specific organ pathology and, in particular, in exacerbating both the biliary disease and the sialadenitis.

Therapeutic targeting of B cells for rheumatic autoimmune diseases

Autoreactive B cells are characterized by their ability to secrete autoantibodies directed against self-peptides. During the last decade, it has become increasingly apparent that B lymphocytes not only produce autoantibodies but also exert important regulatory roles independent of their function as antibody-producing cells. This is especially relevant in the context of autoimmunity, because autoreactive B cells have been shown to possess the ability to activate pathogenic T cells, to produce pro-inflammatory cytokines, and to promote the formation of tertiary lymphoid tissue in target organs. The production of monoclonal antibodies against B-cell-surface molecules has facilitated the characterization of several distinct B lymphocyte subsets. These cell-surface molecules have not only served as useful cell differentiation markers but have also helped to unravel the important biological functions of these cells. Some of these molecules, all of which are expressed on the cell surface, have proven to be effective therapeutic targets. In both animal models and in clinical assays, the efficient elimination of B lymphocytes has been shown to be useful in the treatment of rheumatoid arthritis and other autoimmune diseases. The treatment of most rheumatic autoimmune diseases relies mainly on the use of cytotoxic immunosuppressants and corticosteroids. Although this has resulted in improved disease survival, patients may nonetheless suffer severe adverse events and, in some cases, their relapse rate remains high. The increasing need for safer and more effective drugs along with burgeoning new insights into the pathogenesis of these disorders has fueled interest in biological agents; clinical trials involving the B-cell depletion agent rituximab have been especially promising. This article reviews the current knowledge of B-cell biology and pathogenesis as well as the modern therapeutic approaches for rheumatic autoimmune diseases focusing in particular on the targeting of B-cell-specific surface molecules and on the blocking of B-cell activation and survival.

B-cell pathology in juvenile idiopathic arthritis

Juvenile Idiopathic Arthritis (JIA) is the most common cause of chronic arthritis in childhood and adolescents and encompasses a heterogeneous group of different diseases. Due to the promising results of B-cell depleting therapies in rheumatoid arthritis the role of B-cells in autoimmune diseases has to be discussed in a new context. Additionally, experiments in mouse models have shed new light on the antibody-independent role of B-cells in the development of autoimmune diseases. In this review we will discuss the importance of B-cells in the pathogenesis of JIA appraising the question for an immunological basis of B-cell targeted therapy in JIA.

Targeting B cells for the treatment of SLE: the beginning of the end or the end of the beginning?

Systemic Lupus Erythematosus (SLE) is a systemic autoimmune disease for which therapeutic advances in immunosuppressive and support therapy have significantly improved survival over the last 5 decades. Unfortunately, SLE still carries substantially increased rates of mortality and end stage renal disease which are even more elevated in younger patients. No new drugs have been approved for SLE in over 50 years. Hence, a lot of hope and excitement has been generated by the development of biological agents designed to eliminate B cells either through direct killing (anti-B cell antibodies such as rituximab) or attrition by inhibition of survival (anti-BLyS/BAFF agents such as belimumab). Indeed a strong rationale for targeting B cells in SLE is supported by the major pathogenic roles they play in SLE through both autoantibody production and multiple antibody-independent functions. These hopes, however, have been darted by the failure of two different phase III randomized placebo-controlled trials of rituximab. Yet, clinicians continue to use rituximab off-label with the belief that it provides significant benefit and can rescue patients with disease that is refractory to current modalities. Moreover, recent positive results of two large controlled trials of belimumab have restored confidence that B cell targeting may after all be of benefit in SLE. In this review we discuss the background and rationale for the use of anti-B cell agents in SLE, review the available results, and provide models that could help reconcile the opposing results observed in different studies. These models could also help frame the design and evaluation of current and future B cell therapies.

Characterization of a rare IL-10-competent B-cell subset in humans that parallels mouse regulatory B10 cells

Regulatory B cells control inflammation and autoimmunity in mice, including the recently identified IL-10-competent B10 cell subset that represents 1% to 3% of spleen B cells. In this study, a comparable IL-10-competent B10 cell subset was characterized in human blood. B10 cells were functionally identified by their ability to express cytoplasmic IL-10 after 5 hours of ex vivo stimulation, whereas progenitor B10 (B10pro) cells required 48 hours of in vitro stimulation before they acquired the ability to express IL-10. B10 and B10pro cells represented 0.6% and approximately 5% of blood B cells, respectively. Ex vivo B10 and B10pro cells were predominantly found within the CD24(hi)CD27(+) B-cell subpopulation that was able to negatively regulate monocyte cytokine production through IL-10-dependent pathways during in vitro functional assays. Blood B10 cells were present in 91 patients with rheumatoid arthritis, systemic lupus erythematosus, primary Sjögren syndrome, autoimmune vesiculobullous skin disease, or multiple sclerosis, and were expanded in some cases as occurs in mice with autoimmune disease. Mean B10 + B10pro-cell frequencies were also significantly higher in patients with autoimmune disease compared with healthy controls. The characterization of human B10 cells will facilitate their identification and the study of their regulatory activities during human disease.

B-cell tolerance: mechanisms and implications

Advances in our knowledge of the spectrum of B-cell activities combined with the remarkable clinical efficacy of B-cell inhibitors in autoimmunity and transplantation settings serve to re-emphasise the importance of tolerance to self and foreign antigens in the B-cell repertoire. In particular, new information is emerging about the molecular mechanisms involved in B-cell tolerance induction and identification of B-cell selective defects that contribute to the pathogenesis of autoimmune/inflammatory diseases.

Regulatory B cells (B10 cells) and regulatory T cells have independent roles in controlling experimental autoimmune encephalomyelitis initiation and late-phase immunopathogenesis

Experimental autoimmune encephalomyelitis (EAE) is a T lymphocyte-mediated autoimmune disease of the CNS. Significant roles for B cells and a rare IL-10-producing CD1d(high)CD5(+) regulatory B cell subset (B10 cells) have been identified during the initiation and progression of EAE. Whether and how the regulatory functions of B10 cells and FoxP3(+) T regulatory cells (Tregs) overlap or influence EAE immunopathogenesis independently has remained unanswered. This study demonstrates that the number of endogenous or adoptively transferred B10 cells directly influenced EAE pathogenesis through their production of IL-10. B10 cell numbers expanded quickly within the spleen, but not CNS following myelin oligodendrocyte glycoprotein(35-55) immunization, which paralleled B10 cell regulation of disease initiation. The adoptive transfer of myelin oligodendrocyte glycoprotein(33-35)-sensitized B10 cells into wild-type mice reduced EAE initiation dramatically. However, B10 cells did not suppress ongoing EAE disease. Rather, Treg numbers expanded significantly within the CNS during disease progression, which paralleled their negative regulation of late-phase disease. Likewise, the preferential depletion of B10 cells in vivo during disease initiation enhanced EAE pathogenesis, whereas Treg depletion enhanced late-phase disease. B10 cells did not regulate T cell proliferation during in vitro assays, but significantly altered CD4(+) T cell IFN-gamma and TNF-alpha production. Furthermore, B10 cells downregulated the ability of dendritic cells to act as APCs and thereby indirectly modulated T cell proliferation. Thus, B10 cells predominantly control disease initiation, whereas Tregs reciprocally inhibit late-phase disease, with overlapping B10 cell and Treg functions shaping the normal course of EAE immunopathogenesis.

Regulatory B cells (B10 cells) have a suppressive role in murine lupus: CD19 and B10 cell deficiency exacerbates systemic autoimmunity

B cells play critical roles in the pathogenesis of lupus. To examine the influence of B cells on disease pathogenesis in a murine lupus model, New Zealand Black and New Zealand White F(1) hybrid (NZB/W) mice were generated that were deficient for CD19 (CD19(-/-) NZB/W mice), a B cell-specific cell surface molecule that is essential for optimal B cell signal transduction. The emergence of anti-nuclear Abs was significantly delayed in CD19(-/-) NZB/W mice compared with wild type NZB/W mice. However, the pathologic manifestations of nephritis appeared significantly earlier, and survival was significantly reduced in CD19(-/-) NZB/W mice compared with wild type mice. These results demonstrate both disease-promoting and protective roles for B cells in lupus pathogenesis. Recent studies have identified a potent regulatory B cell subset (B10 cells) within the rare CD1d(hi)CD5(+) B cell subset of the spleen that regulates acute inflammation and autoimmunity through the production of IL-10. In wild type NZB/W mice, the CD1d(hi)CD5(+)B220(+) B cell subset that includes B10 cells was increased by 2.5-fold during the disease course, whereas CD19(-/-) NZB/W mice lacked this CD1d(hi)CD5(+) regulatory B cell subset. However, the transfer of splenic CD1d(hi)CD5(+) B cells from wild type NZB/W mice into CD19(-/-) NZB/W recipients significantly prolonged their survival. Furthermore, regulatory T cells were significantly decreased in CD19(-/-) NZB/W mice, but the transfer of wild type CD1d(hi)CD5(+) B cells induced T regulatory cell expansion in CD19(-/-) NZB/W mice. These results demonstrate an important protective role for regulatory B10 cells in this systemic autoimmune disease.