B-Cell Tolerance Checkpoints in Healthy Humans and Patients with Systemic Lupus Erythematosus

B-Cell Receptor Repertoire: Recent Advances in Autoimmune Diseases

In the field of contemporary medicine, autoimmune diseases (AIDs) are a prevalent and debilitating group of illnesses. However, they present extensive and profound challenges in terms of etiology, pathogenesis, and treatment. A major reason for this is the elusive pathophysiological mechanisms driving disease onset. Increasing evidence suggests the indispensable role of B cells in the pathogenesis of autoimmune diseases. Interestingly, B-cell receptor (BCR) repertoires in autoimmune diseases display a distinct skewing that can provide insights into disease pathogenesis. Over the past few years, advances in high-throughput sequencing have provided powerful tools for analyzing B-cell repertoire to understand the mechanisms during the period of B-cell immune response. In this paper, we have provided an overview of the mechanisms and analytical methods for generating BCR repertoire diversity and summarize the latest research progress on BCR repertoire in autoimmune diseases, including systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), primary Sjögren's syndrome (pSS), multiple sclerosis (MS), and type 1 diabetes (T1D). Overall, B-cell repertoire analysis is a potent tool to understand the involvement of B cells in autoimmune diseases, facilitating the creation of innovative therapeutic strategies targeting specific B-cell clones or subsets.

Aberrant B Cell Signaling in Autoimmune Diseases

Aberrant B cell signaling plays a critical in role in various systemic and organ-specific autoimmune diseases. This is supported by genetic evidence by many functional studies in B cells from patients or specific animal models and by the observed efficacy of small-molecule inhibitors. In this review, we first discuss key signal transduction pathways downstream of the B cell receptor (BCR) that ensure that autoreactive B cells are removed from the repertoire or functionally silenced. We provide an overview of aberrant BCR signaling that is associated with inappropriate B cell repertoire selection and activation or survival of peripheral B cell populations and plasma cells, finally leading to autoantibody formation. Next to BCR signaling, abnormalities in other signal transduction pathways have been implicated in autoimmune disease. These include reduced activity of several phosphates that are downstream of co-inhibitory receptors on B cells and increased levels of BAFF and APRIL, which support survival of B cells and plasma cells. Importantly, pathogenic synergy of the BCR and Toll-like receptors (TLR), which can be activated by endogenous ligands, such as self-nucleic acids, has been shown to enhance autoimmunity. Finally, we will briefly discuss therapeutic strategies for autoimmune disease based on interfering with signal transduction in B cells.

Transforming mutations in the development of pathogenic B cell clones and autoantibodies

Autoimmune diseases are characterized by serum autoantibodies, some of which are pathogenic, causing severe manifestations and organ injury. However, autoantibodies of the same antigenic reactivity are also present in the serum of asymptomatic people years before they develop any clinical signs of autoimmunity. Autoantibodies can arise during multiple stages of B cell development, and various genetic and environmental factors drive their production. However, what drives the development of pathogenic autoantibodies is poorly understood. Advances in single-cell technology have enabled the deep analysis of rare B cell clones producing pathogenic autoantibodies responsible for vasculitis in patients with primary Sjögren's syndrome complicated by mixed cryoglobulinaemia. These findings demonstrated a cascade of genetic events involving stereotypic immunoglobulin V(D)J recombination and transforming somatic mutations in lymphoma genes and V(D)J regions that disrupted antibody quality control mechanisms and decreased autoantibody solubility. Most studies consider V(D)J mutations that enhance autoantibody affinity to drive pathology; however, V(D)J mutations that increase autoantibody propensity to form insoluble complexes could be a major contributor to autoantibody pathogenicity. Defining the molecular characteristics of pathogenic autoantibodies and failed tolerance checkpoints driving their formation will improve prognostication, enabling early treatment to prevent escalating organ damage and B cell malignancy.

Circulating autoreactive proteinase 3+ B cells and tolerance checkpoints in ANCA-associated vasculitis

BACKGROUND

Little is known about the autoreactive B cells in antineutrophil cytoplasmic antibody–associated (ANCA-associated) vasculitis (AAV). We aimed to investigate tolerance checkpoints of circulating antigen-specific proteinase 3–reactive (PR3⁺) B cells.

METHODS

Multicolor flow cytometry in combination with bioinformatics and functional in vitro studies were performed on baseline samples of PBMCs from 154 well-characterized participants of the RAVE trial (NCT00104299) with severely active PR3-AAV and myeloperoxidase-AAV (MPO-AAV) and 27 healthy controls (HCs). Clinical data and outcomes from the trial were correlated with PR3⁺ B cells (total and subsets).

RESULTS

The frequency of PR3⁺ B cells among circulating B cells was higher in participants with PR3-AAV (4.77% median [IQR, 3.98%–6.01%]) than in participants with MPO-AAV (3.16% median [IQR, 2.51%–5.22%]) and participants with AAV compared with HCs (1.67% median [IQR, 1.27%–2.16%], P < 0.001 for all comparisons), implying a defective central tolerance checkpoint in patients with AAV. Only PBMCs from participants with PR3-AAV contained PR3⁺ B cells capable of secreting PR3-ANCA IgG in vitro, proving they were functionally distinct from those of participants with MPO-AAV and HCs. Unsupervised clustering identified subtle subsets of atypical autoreactive PR3⁺ memory B cells accumulating through the maturation process in patients with PR3-AAV. PR3⁺ B cells were enriched in the memory B cell compartment of participants with PR3-AAV and were associated with higher serum CXCL13 levels, suggesting an increased germinal center activity. PR3⁺ B cells correlated with systemic inflammation (C-reactive protein and erythrocyte sedimentation rate, P < 0.05) and complete remission (P < 0.001).

CONCLUSION

This study suggests the presence of defective central antigen-independent and peripheral antigen-dependent checkpoints in patients with PR3-AAV, elucidating the selection process of autoreactive B cells.

Trial registration

ClinicalTrials.gov NCT00104299.

Funding

The Vasculitis Foundation, the National Institute of Allergy and Infectious Diseases of the NIH, and the Mayo Foundation for Education and Research.

Belimumab promotes negative selection of activated autoreactive B cells in systemic lupus erythematosus patients

Belimumab has therapeutic benefit in active systemic lupus erythematosus (SLE), especially in patients with high-titer anti-dsDNA antibodies. We asked whether the profound B cell loss in belimumab-treated SLE patients is accompanied by shifts in the immunoglobulin repertoire. We enrolled 15 patients who had been continuously treated with belimumab for more than 7 years, 17 matched controls, and 5 patients who were studied before and after drug initiation. VH genes of sort-purified mature B cells and plasmablasts were subjected to next-generation sequencing. We found that B cell-activating factor (BAFF) regulates the transitional B cell checkpoint, with conservation of transitional 1 (T1) cells and approximately 90% loss of T3 and naive B cells after chronic belimumab treatment. Class-switched memory B cells, B1 B cells, and plasmablasts were also substantially depleted. Next-generation sequencing revealed no redistribution of VH, DH, or JH family usage and no effect of belimumab on representation of the autoreactive VH4-34 gene or CDR3 composition in unmutated IgM sequences, suggesting a minimal effect on selection of the naive B cell repertoire. Interestingly, a significantly greater loss of VH4-34 was observed among mutated IgM and plasmablast sequences in chronic belimumab-treated subjects than in controls, suggesting that belimumab promotes negative selection of activated autoreactive B cells.

Checkpoints for Autoreactive B Cells in the Peripheral Blood of Lupus Patients Assessed by Flow Cytometry

OBJECTIVE

Antinuclear antibodies (ANAs) are diagnostic in several autoimmune disorders, yet the failure to achieve B cell tolerance in these diseases is still poorly understood. Although secreted ANAs detected by an indirect immunofluorescence assay are the gold standard for autoreactivity, there has been no convenient assay with which to measure the frequency of circulating B cells that recognize nuclear antigens (ANA+ B cells) in patients. The aim of this study was to generate an assay to easily identify these B cells and to examine its utility in a study of autoreactive B cells in systemic lupus erythematosus (SLE).

METHODS

We developed and validated a novel flow cytometry-based assay that identifies ANA+ B cells using biotinylated nuclear extracts, and utilized it to examine B cell tolerance checkpoints in peripheral blood mononuclear cells obtained from SLE patients and healthy controls.

RESULTS

We observed progressive selection against ANA+ B cells as they matured from transitional to naive to CD27+IgD- and CD27+IgD+ memory cells in both healthy subjects and SLE patients; however, ANA+ naive B cells in SLE patients were not anergized to the same extent as in healthy individuals. We also showed that anergy induction is restored in SLE patients treated with belimumab, an inhibitor of BAFF.

CONCLUSION

This assay will enable studies of large populations to identify potential genetic or environmental factors affecting B cell tolerance checkpoints in healthy subjects and patients with autoimmune disease and permit monitoring of the B cell response to therapeutic interventions.

Pathways leading to an immunological disease: systemic lupus erythematosus

SLE is a chronic autoimmune disease caused by perturbations of the immune system. The clinical presentation is heterogeneous, largely because of the multiple genetic and environmental factors that contribute to disease initiation and progression. Over the last 60 years, there have been a number of significant leaps in our understanding of the immunological mechanisms driving disease processes. We now know that multiple leucocyte subsets, together with inflammatory cytokines, chemokines and regulatory mediators that are normally involved in host protection from invading pathogens, contribute to the inflammatory events leading to tissue destruction and organ failure. In this broad overview, we discuss the main pathways involved in SLE and highlight new findings. We describe the immunological changes that characterize this form of autoimmunity. The major leucocytes that are essential for disease progression are discussed, together with key mediators that propagate the immune response and drive the inflammatory response in SLE.

Single-cell based high-throughput sequencing of full-length immunoglobulin heavy and light chain genes

Single-cell PCR and sequencing of full-length Ig heavy (Igh) and Igk and Igl light chain genes is a powerful tool to measure the diversity of antibody repertoires and allows the functional assessment of B-cell responses through direct Ig gene cloning and the generation of recombinant mAbs. However, the current methodology is not high-throughput compatible. Here we developed a two-dimensional bar-coded primer matrix to combine Igh and Igk/Igl chain gene single-cell PCR with next-generation sequencing for the parallel analysis of the antibody repertoire of over 46 000 individual B cells. Our approach provides full-length Igh and corresponding Igk/Igl chain gene-sequence information and permits the accurate correction of sequencing errors by consensus building. The use of indexed cell sorting for the isolation of single B cells enables the integration of flow cytometry and Ig gene sequence information. The strategy is fully compatible with established protocols for direct antibody gene cloning and expression and therefore advances over previously described high-throughput approaches to assess antibody repertoires at the single-cell level.

Immunopathogenic mechanisms of systemic autoimmune disease

Systemic lupus erythematosus, Sjögren's syndrome, and dermatomyositis are systemic autoimmune diseases that develop after environmental triggering of genetically susceptible individuals. The precise cellular and molecular mechanisms leading to autoimmune disease, and what factors determine which organs are involved, remain poorly understood. Recent insights into genetic susceptibility now make obvious that environmental triggers often act via cellular pathways containing disease-associated polymorphisms. In the breaking of tolerance, the initiating tissue--including dendritic cells--provides a decisive microenvironment that affects immune-cell differentiation, leading to activation of adaptive immunity. Type 1 interferon produced by innate immune cells has a central role in systemic autoimmunity and activates B cells and T cells. In turn, B-cell-derived autoantibodies stimulate dendritic cells to produce type 1 interferon; thus, a positive feedforward loop is formed that includes both the innate and adaptive systems. New treatments could simultaneously and specifically target several such vital pathways in autoimmunity.

A role for gut-associated lymphoid tissue in shaping the human B cell repertoire

Transitional 2 B cells home to gut-associated lymphoid tissue and present an activated phenotype in healthy subjects, but gut immune compartments are depleted in SLE.

We have tracked the fate of immature human B cells at a critical stage in their development when the mature B cell repertoire is shaped. We show that a major subset of bone marrow emigrant immature human B cells, the transitional 2 (T2) B cells, homes to gut-associated lymphoid tissue (GALT) and that most T2 B cells isolated from human GALT are activated. Activation in GALT is a previously unknown potential fate for immature human B cells. The process of maturation from immature transitional B cell through to mature naive B cell includes the removal of autoreactive cells from the developing repertoire, a process which is known to fail in systemic lupus erythematosus (SLE). We observe that immature B cells in SLE are poorly equipped to access the gut and that gut immune compartments are depleted in SLE. Thus, activation of immature B cells in GALT may function as a checkpoint that protects against autoimmunity. In healthy individuals, this pathway may be involved in generating the vast population of IgA plasma cells and also the enigmatic marginal zone B cell subset that is poorly understood in humans.

A fully synthetic human Fab antibody library based on fixed VH/VL framework pairings with favorable biophysical properties

This report describes the design, generation and testing of Ylanthia, a fully synthetic human Fab antibody library with 1.3E+11 clones. Ylanthia comprises 36 fixed immunoglobulin (Ig) variable heavy (VH)/variable light (VL) chain pairs, which cover a broad range of canonical complementarity-determining region (CDR) structures. The variable Ig heavy and Ig light (VH/VL) chain pairs were selected for biophysical characteristics favorable to manufacturing and development. The selection process included multiple parameters, e.g., assessment of protein expression yield, thermal stability and aggregation propensity in fragment antigen binding (Fab) and IgG1 formats, and relative Fab display rate on phage. The framework regions are fixed and the diversified CDRs were designed based on a systematic analysis of a large set of rearranged human antibody sequences. Care was taken to minimize the occurrence of potential posttranslational modification sites within the CDRs. Phage selection was performed against various antigens and unique antibodies with excellent biophysical properties were isolated. Our results confirm that quality can be built into an antibody library by prudent selection of unmodified, fully human VH/VL pairs as scaffolds.

Expansion of polyreactive B cells cross-reactive to HLA and self in the blood of a patient with kidney graft rejection

Antibody rejection is often accompanied by nondonor HLA specific antibodies (NDSA) and self-reactive antibodies that develop alongside donor-specific antibodies (DSA). To determine the source of these antibodies, we immortalized 107 B-cell clones from a kidney transplant recipient with humoral rejection. Two of these clones reacted to HLA class I or MICA. Both clones were also reactive to self-antigens and a lysate of a kidney cell line, hence revealing a pattern of polyreactivity. Monoclonality was verified by the identification of a single rearranged immunoglobulin heavy chain variable region (VH) sequence for each clone. By tracking their unique CDR3 sequence, we found that one such polyreactive clone was highly expanded in the patient blood, representing ~0.2% of circulating B cells. The VH sequence of this clone showed evidence of somatic mutations that were consistent with its memory phenotype and its expansion. Lastly, the reactivity of the expanded polyreactive B-cell clone was found in the patient serum at time of rejection. In conclusion, we provide here proof of principle at the clonal level that human antibodies can cross-react to HLA and self. Our findings strongly suggest that polyreactive antibodies contribute to DSA, NDSA as well as autoantibodies, in transplant recipients.

Multiparameter flow cytometry and bioanalytics for B cell profiling in systemic lupus erythematosus

B lymphocyte involvement in systemic lupus erythematosus has been recognized for several decades, mainly in the context of autoantibody production. Both mouse and human studies reveal that different types of antibody responses, as well as antibody-independent effector functions can be ascribed to distinct subpopulations (subsets) of circulating B cells. Characterizing human B cell subsets can advance the field of autoimmunity even further by establishing B cell signatures associated with disease severity, progression, and response-to-treatment. For this purpose, we have developed specialized B cell reagent panels for multiparameter flow cytometry, and combine their use with advanced bioinformatics strategies that together will likely be advantageous for improving the characterization, prognosis, and for possibly improving treatment regimens of chronic inflammatory diseases such as lupus.

Translational Mini-Review Series on B cell subsets in disease. Transitional B cells in systemic lupus erythematosus and Sjögren's syndrome: clinical implications and effects of B cell-targeted therapies

Systemic lupus erythematosus (SLE) and Sjögren's syndrome are autoimmune disorders which are characterized by a disturbed B cell homeostasis which leads ultimately to dysfunction of various organs. One of the B cell subsets that appear in abnormal numbers is the population of transitional B cells, which is increased in the blood of patients with SLE and Sjögren's syndrome. Transitional B cells are newly formed B cells. In mice, transitional B cells undergo selection checks for unwanted specificity in the bone marrow and the spleen in order to eliminate autoreactive B cells from the circulating naive B cell population. In humans, the exact anatomical compartments and mechanisms of the specificity check-points for transitional B cells remain unclear, but appear to be defective in SLE and Sjögren's syndrome. This review aims to highlight the current understanding of transitional B cells and their defects in the two disorders before and after B cell-targeted therapies.

BAFF and selection of autoreactive B cells

B cell activating factor (BAFF) is a crucial survival factor for transitional and mature B cells, and is a promising therapeutic target for systemic lupus erythematosus (SLE). A BAFF inhibitor, belimumab, is the first new drug in 50 years to be approved for the treatment of SLE. However, the mechanism of action of this drug is not entirely clear. In this review we will focus on the role of the BAFF-APRIL signaling pathway in the selection of autoreactive B cells, and discuss whether altered selection is the mechanism for the therapeutic efficacy of BAFF inhibition in SLE.

Single B cell antibody technologies

Monoclonal antibodies (mAbs) are arguably the most significant class of biologics for use as pharmaceuticals and diagnostics. Many technological concepts exist for the generation and identification of therapeutically relevant mAbs, including the isolation and cloning of immunoglobulin (Ig) encoding genes from single B-lineage cells. This review summarizes various single B cell approaches and describes their use for the discovery of mAbs with potential therapeutic values or in basic research.

Treatment of systemic lupus erythematosus in two patients with extreme B-cell lymphopenia: importance of immunomonitoring and avoidance of B-cell targeted therapy

INTRODUCTION

Because dysfunction of the B-cell compartment is thought to be important in the pathogenesis of systemic lupus erythematosus (SLE), there has been a recent focus on therapies that target humoral immunity via multiple mechanisms. The aim of this paper was to demonstrate the importance of immunomonitoring in two cases with class II lupus nephritis on steroids who presented with a flare-up of disease. After a thorough work-up for infectious triggers of disease activity, conversion to another histopathological class of lupus nephritis was suspected. Deterioration of the patients' clinical condition made kidney biopsy impossible, and as B-cell targeted therapy was considered, we decided to perform an immunophenotypic analysis and to tailor therapy to the results of the lymphocyte profile. As we incidentally found extremely low B-cell counts, any B-cell-targeted therapy was prohibited, and cyclophosphamide (Cy) was considered a viable therapeutic option.

METHODS

We performed flow-cytometric lymphocyte (Ly) phenotyping (CD19, CD3, CD3CD4, CD3CD8, CD56/16) on two patients with class II lupus nephritis before and after two intravenous (i.v.) Cy pulse administrations. During all this time, patients were on steroids.

RESULTS

 Both patients showed extreme B-cell lymphopenia, a marker of active SLE, which was not greatly impacted by the treatment over the follow-up period.

CONCLUSIONS

As current therapies are aimed at targeting the B cell, an important component of adaptive immunity, caution must be exercised before their use. In addition, monitoring of Ly subsets is essential due to the occurrence of extreme B-cell lymphopenia.

Expression and role of CR1 and CR2 on B and T lymphocytes under physiological and autoimmune conditions

The involvement of complement in the development and regulation of antibody responses under both healthy and pathological conditions is known for long. Unravelling the molecular mechanisms underlying the events however is still in progress. This review focuses on the role of complement receptors CR1 (CD35) and CR2 (CD21) expressed on T and B cells. Alteration in the expression and function of these receptors may contribute to the initiation and maintenance of immune complex mediated autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis. Recent data regarding complement receptor expression on T lymphocytes and on memory B cells are also discussed.

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.

Signalling of the BCR is regulated by a lipid rafts-localised transcription factor, Bright

Regulation of BCR signalling strength is crucial for B-cell development and function. Bright is a B-cell-restricted factor that complexes with Bruton's tyrosine kinase (Btk) and its substrate, transcription initiation factor-I (TFII-I), to activate immunoglobulin heavy chain gene transcription in the nucleus. Here we show that a palmitoylated pool of Bright is diverted to lipid rafts of resting B cells where it associates with signalosome components. After BCR ligation, Bright transiently interacts with sumoylation enzymes, blocks calcium flux and phosphorylation of Btk and TFII-I and is then discharged from lipid rafts as a Sumo-I-modified form. The resulting lipid raft concentration of Bright contributes to the signalling threshold of B cells, as their sensitivity to BCR stimulation decreases as the levels of Bright increase. Bright regulates signalling independent of its role in IgH transcription, as shown by specific dominant-negative titration of rafts-specific forms. This study identifies a BCR tuning mechanism in lipid rafts that is regulated by differential post-translational modification of a transcription factor with implications for B-cell tolerance and autoimmunity.

The BLyS family: toward a molecular understanding of B cell homeostasis

The B Lymphocyte Stimulator (BLyS) family of ligands and receptors regulates humoral immunity by controlling B lymphocyte survival and differentiation. Herein, we review the ligands and receptors of this family, their biological functions, and the biochemical processes through which they operate. Pre-immune B lymphocytes rely on BLyS signaling for their survival, whereas antigen experienced B lymphocytes generally interact more avidly with a homologous cytokine, A Proliferation Inducing Ligand (APRIL). The molecular basis for signaling via the three BLyS family receptors reveals complex interplay with other B lymphocyte signaling systems, affording the integration of selective and homeostatic processes. As our understanding of this system advances, molecular targets for manipulating humoral immunity in both health and disease should be revealed.

Carboxypeptidase D: A Novel TGF-β Target Gene Dysregulated in Patients with Lupus Erythematosus

Transforming growth factor-beta (TGF-beta) is a multifunctional cytokine that mainly acts as an inhibitor of immune functions. A lack of functional TGF-beta leads to autoimmune disease in animal models and dysregulated TGF-beta signaling is implicated in human autoimmune diseases. To define target genes that play a part in the inhibitory role of TGF-beta in the immune system, we have identified genes stimulated by TGF-beta in macrophages by gene-chip analysis. One of the TGF-beta regulated genes is carboxypeptidase D (CpD), a 180-kDa type I membrane protein. We have demonstrated that CpD is regulated by TGF-beta in various cell types of both, murine and human origin and, interestingly, is significantly downregulated in CD14 positive cells isolated from patients with lupus erythematosus (LE). Moreover, we show that downregulation of CpD leads to downmodulation of TGF-beta itself, suggesting a role for CpD in a positive feedback loop, providing further evidence for a role of this enzyme in LE. To our knowledge, this is the first report that demonstrates carboxypeptidase D as a TGF-beta target gene that is implicated in the pathogenesis of LE.

Anti-nuclear antibody production and autoimmunity in transgenic mice that overexpress the transcription factor Bright

The B cell-restricted transcription factor, B cell regulator of Ig(H) transcription (Bright), up-regulates Ig H chain transcription 3- to 7-fold in activated B cells in vitro. Bright function is dependent upon both active Bruton's tyrosine kinase and its substrate, the transcription factor, TFII-I. In mouse and human B lymphocytes, Bright transcription is down-regulated in mature B cells, and its expression is tightly regulated during B cell differentiation. To determine how Bright expression affects B cell development, transgenic mice were generated that express Bright constitutively in all B lineage cells. These mice exhibited increases in total B220(+) B lymphocyte lineage cells in the bone marrow, but the relative percentages of the individual subpopulations were not altered. Splenic immature transitional B cells were significantly expanded both in total cell numbers and as increased percentages of cells relative to other B cell subpopulations. Serum Ig levels, particularly IgG isotypes, were increased slightly in the Bright-transgenic mice compared with littermate controls. However, immunization studies suggest that responses to all foreign Ags were not increased globally. Moreover, 4-wk-old Bright-transgenic mice produced anti-nuclear Abs. Older animals developed Ab deposits in the kidney glomeruli, but did not succumb to further autoimmune sequelae. These data indicate that enhanced Bright expression results in failure to maintain B cell tolerance and suggest a previously unappreciated role for Bright regulation in immature B cells. Bright is the first B cell-restricted transcription factor demonstrated to induce autoimmunity. Therefore, the Bright transgenics provide a novel model system for future analyses of B cell autoreactivity.