Persistent expression of autoantibodies in SLE patients in remission

The autoimmune architecture of childhood idiopathic nephrotic syndrome

Idiopathic nephrotic syndrome, the most common glomerular disorder in children, has long been considered an immune-mediated disease based on the efficacy of glucocorticoids at inducing remission. Nevertheless, the immune processes leading to podocytopathy have largely remained elusive. The success of B-cell depletion with rituximab, descriptions of B-cell dysregulation during active disease, and the most recent discovery of autoantibodies targeting the major podocyte antigen nephrin point to an autoimmune humoral etiology for idiopathic nephrotic syndrome. Investigations of the immune factors involved in idiopathic nephrotic syndrome pathogenesis have uncovered common features with other autoimmune disorders that will aid in prognostication and in guiding the expansion of our glucocorticoid-sparing therapeutic arsenal. In this review, we discuss the emerging autoimmune architecture of idiopathic nephrotic syndrome, with a specific focus on pediatric steroid-sensitive disease, including the podocyte-reactive B-cell response that causes anti-podocyte antibodies, the predisposing genetic factors that shape the podocyte-reactive immune landscape, and the immune triggers driving active disease.

Opportunities and limitations of B cell depletion approaches in SLE

B cell depletion with rituximab, a chimeric monoclonal antibody that selectively targets B cells by binding CD20, has been used off label in severe and resistant systemic lupus erythematosus (SLE) for over two decades. Several biological mechanisms limit the efficacy of rituximab, including immunological reactions towards the chimeric molecule, increased numbers of residual B cells, including plasmablasts and plasma cells, and a post-treatment surge in B cell-activating factor (BAFF) levels. Consequently, rituximab induces remission in only a proportion of patients, and safety issues limit its use. However, the use of rituximab has established the value of B cell depletion strategies in SLE and has guided the development of several improved B cell depletion therapies for SLE. These include enhanced monoclonal antibodies, modalities that redirect the specificity of patient T cells using chimeric antigen receptor T cells or bispecific T cell engagers, and combination treatment that simultaneously inhibits the BAFF pathway. In this Review, we consider evidence gathered from over two decades of using rituximab in SLE and examine how B cell depletion therapies could be further optimized to achieve immunological and clinical efficacy. In addition, we discuss the prospects of B cell depletion strategies for personalized treatment in SLE based on genetic research and studies in pre-symptomatic individuals.

Association of hyperactivated transposon expression with exacerbated immune activation in systemic lupus erythematosus

BACKGROUND

Systemic Lupus Erythematosus (SLE) is a complex autoimmune disorder, and transposable elements (TEs) have been hypothesized to play a significant role in its development. However, limited research has explored this connection. Our study aimed to examine the relationship between TE expression and SLE pathogenesis.

METHODS

We analyzed whole blood RNA-seq datasets from 198 SLE patients and 84 healthy controls. The REdiscoverTE pipeline was employed to quantify TE and other gene expressions, identifying differentially expressed TEs. A TE score was calculated to measure overall TE expression for each sample. Gene ontology and gene set enrichment analyses were conducted to explore the functional implications of TE upregulation. Independent datasets were utilized to replicate the results and investigate cell type-specific TE expression.

RESULTS

Our analysis identified two distinct patient groups: one with high TE expression and another with TE expression comparable to controls. Patients with high TE expression exhibited upregulation of pathways involving nucleic acid sensors, and TE expression was strongly correlated with interferon (IFN) signatures. Furthermore, these patients displayed deregulated cell composition, including increased neutrophils and decreased regulatory T cells. Neutrophils were suggested as the primary source of TE expression, contributing to IFN production.

CONCLUSIONS

Our findings suggest that TE expression may serve as a crucial mediator in maintaining the activation of interferon pathways, acting as an endogenous source of nucleic acid stimulators in SLE patients.

B cell tolerance and autoimmunity: Lessons from repertoires

Adaptive immune cell function is regulated by a highly diverse receptor recombined from variable germline-encoded segments that can recognize an almost unlimited array of epitopes. While this diversity enables the recognition of any pathogen, it also poses a risk of self-recognition, leading to autoimmunity. Many layers of regulation are present during both the generation and activation of B cells to prevent this phenomenon, although they are evidently imperfect. In recent years, our ability to analyze immune repertoires at scale has drastically increased, both through advances in sequencing and single-cell analyses. Here, we review the current knowledge on B cell repertoire analyses, focusing on their implication for autoimmunity. These studies demonstrate that a failure of tolerance occurs at multiple independent checkpoints in different autoimmune contexts, particularly during B cell maturation, plasmablast differentiation, and within germinal centers. These failures are marked by distinct repertoire features that may be used to identify disease- or patient-specific therapeutic approaches.

Evaluation of a novel particle-based assay for detecting SLE-related autoantibodies

Anti-dsDNA, anti-Sm, and anti-ribosomal-P autoantibodies are hallmarks of systemic lupus erythematosus (SLE), being anti-dsDNA and anti-Sm included in 2019-ACR/EULAR SLE-Classification Criteria. Enzyme-linked (ELISA) and chemiluminescence assays (CIA) are widely established in immunology laboratories, but new technologies, such as particle-based multi-analyte technology (PMAT), are nowadays available. The present study aimed to compare the presence of anti-dsDNA and anti-Sm autoantibodies measured by CIA and PMAT and analyze diagnostic and clinical SLE activity performance. Anti-ribosomal-P autoantibodies by PMAT were also included. Consequently, anti-dsDNA and anti-Sm detected by CIA showed substantial agreement with PMAT (Cohen's kappa = 0.662 and 0.671, respectively). Anti-dsDNA autoantibodies measured by PMAT showed a positive correlation with clinical SLEDAI-2K (p < 0.001) and a negative correlation with complement consumption (p < 0.001). Anti-Sm and anti-ribosomal-P autoantibodies showed a positive correlation with SLEDAI-2K (p < 0.001 and p = 0.001, respectively) and a negative correlation with complement consumption (p < 0.001 and p = 0.001, respectively). Finally, anti-Sm autoantibodies were associated with renal involvement (p < 0.05).

Elevated Levels of Interferon-α Act Directly on B Cells to Breach Multiple Tolerance Mechanisms Promoting Autoantibody Production

OBJECTIVE

Elevated levels of serum interferon-α (IFNα) and the disruption of B cell tolerance are central to systemic lupus erythematosus (SLE) immunopathogenesis; however, the relationship between these 2 processes remains unclear. The purpose of this study was to investigate the impact of elevated IFNα levels on B cell tolerance mechanisms in vivo and determine whether any changes observed were due to the direct effect of IFNα on B cells.

METHODS

Two classical mouse models of B cell tolerance were used in conjunction with an adenoviral vector encoding IFNα to mimic the sustained elevations of IFNα seen in SLE. The role of B cell IFNα signaling, T cells, and Myd88 signaling was determined using B cell-specific IFNα receptor-knockout, CD4+ T cell-depleted, or Myd88-knockout mice, respectively. Flow cytometry, enzyme-linked immunosorbent assay, real-time quantitative polymerase chain reaction, and cell cultures were used to study the effects of elevated IFNα on the immunologic phenotype.

RESULTS

Elevation of serum IFNα disrupts multiple B cell tolerance mechanisms and leads to autoantibody production. This disruption was dependent upon B cell expression of IFNα receptor. Many of the IFNα-mediated alterations also required the presence of CD4+ T cells as well as Myd88, suggesting that IFNα acts directly on B cells to modify their response to Myd88 signaling and their ability to interact with T cells.

CONCLUSION

The results provide evidence that elevated IFNα levels act directly on B cells to facilitate autoantibody production and further highlight the importance of IFN signaling as a potential therapeutic target in SLE.

Anti-dsDNA B-Cell ELISpot as a Monitoring and Flare Prediction Tool in SLE Patients

Anti-dsDNA autoantibodies quantification and complement levels are widely used to monitor disease activity in systemic lupus erythematosus (SLE). However, better biomarkers are still needed. We hypothesised whether the dsDNA antibody-secreting B-cells could be a complementary biomarker in disease activity and prognosis of SLE patients. Fifty-two SLE patients were enrolled and followed for up to 12 months. Additionally, 39 controls were included. An activity cut-off (comparing active and non-active patients according to clinical SLEDAI-2K) was established for SLE-ELISpot, chemiluminescence and Crithidia luciliae indirect immunofluorescence tests (≥11.24, ≥374.1 and ≥1, respectively). Assays performances together with complement status were compared regarding major organ involvement at the inclusion and flare-up risk prediction after follow-up. SLE-ELISpot showed the best performance in identifying active patients. High SLE-ELISpot results were associated with haematological involvement and, after follow-up, with an increased hazard ratio for disease flare-up (3.4) and especially renal flare (6.5). Additionally, the combination of hypocomplementemia and high SLE-ELISpot results increased those risks up to 5.2 and 32.9, respectively. SLE-ELISpot offers complementary information to anti-dsDNA autoantibodies to evaluate the risk of a flare-up in the following year. In some cases, adding SLE-ELISpot to the current follow-up protocol for SLE patients can improve clinicians' personalised care decisions.

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.

The suppression of Brd4 inhibits peripheral plasma cell differentiation and exhibits therapeutic potential for systemic lupus erythematosus

The mechanisms that control B cell terminal differentiation remain undefined. Here, we investigate the role of bromodomain-containing protein 4 (Brd4) in regulating B cell differentiation and its therapeutic potential for B cell-mediated autoimmune diseases including systemic lupus erythematosus (SLE). We showed that Brd4 inhibitor PFI-1 suppressed plasmablast-mediated plasma cell differentiation in healthy human CD19⁺ B cells. PFI-1 reduced IgG and IgM secretion in costimulation-induced human B cells. We also observed a reduced percentage of plasma cells in mice with B cell-specific deletion of the Brd4 gene (Brd4^flox/floxCD19-cre⁺). Mechanistically, using the luciferase reporter assay and the chromatin immunoprecipitation, we explored that Brd4 regulates the expression of B lymphocyte-induced maturation protein 1 (BLIMP1), an important transcript factor that is involved in modulation of plasma cell differentiation. Interestingly, PFI-1 decreased the percentages of plasmablasts and plasma cells from patients with SLE. PFI-1 administration reduced the percentages of plasma cells, hypergammaglobulinemia, and attenuated nephritis in MRL/lpr lupus mice. Pristane-injected Brd4^flox/floxCD19-cre⁺ mice exhibited improved nephritis and reduced percentages of plasma cells. These findings suggest an essential factor of Brd4 in regulating plasma cell differentiation. Brd4 inhibition may be a potential strategy for the treatment of B cell-associated autoimmune disorders.

B cell subset composition segments clinically and serologically distinct groups in chronic cutaneous lupus erythematosus

OBJECTIVE

While the contribution of B-cells to SLE is well established, its role in chronic cutaneous lupus erythematosus (CCLE) remains unclear. Here, we compare B-cell and serum auto-antibody profiles between patients with systemic lupus erythematosus (SLE), CCLE, and overlap conditions.

METHODS

B-cells were compared by flow cytometry amongst healthy controls, CCLE without systemic lupus (CCLE+/SLE-) and SLE patients with (SLE+/CCLE+) or without CCLE (SLE+/CCLE-). Serum was analyed for autoreactive 9G4+, anti-double-stranded DNA, anti-chromatin and anti-RNA antibodies by ELISA and for anti-RNA binding proteins (RBP) by luciferase immunoprecipitation.

RESULTS

Patients with CCLE+/SLE- share B-cell abnormalities with SLE including decreased unswitched memory and increased effector B-cells albeit at a lower level than SLE patients. Similarly, both SLE and CCLE+/SLE- patients have elevated 9G4+ IgG autoantibodies despite lower levels of anti-nucleic acid and anti-RBP antibodies in CCLE+/SLE-. CCLE+/SLE- patients could be stratified into those with SLE-like B-cell profiles and a separate group with normal B-cell profiles. The former group was more serologically active and more likely to have disseminated skin lesions.

CONCLUSION

CCLE displays perturbations in B-cell homeostasis and partial B-cell tolerance breakdown. Our study demonstrates that this entity is immunologically heterogeneous and includes a disease segment whose B-cell compartment resembles SLE and is clinically associated with enhanced serological activity and more extensive skin disease. This picture suggests that SLE-like B-cell changes in primary CCLE may help identify patients at risk for subsequent development of SLE. B-cell profiling in CCLE might also indentify candidates who would benefit from B-cell targeted therapies.

Potential role of viral infection and B cells as a linker between innate and adaptive immune response in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease that involves several organ systems. Although B cells play a key role in SLE pathogenesis, the mechanisms behind B cell dysregulation in SLE development remained controversial. Finding the modules containing highly co-expressed genes in B cells could explain biological pathways involved in the pathogenesis of SLE, which may further support the reasons for the altered function of B cells in SLE disease. A total of three microarray gene expression datasets were downloaded from Gene Expression Omnibus. SLE samples were prepared from the purified B lymphocyte cells of the patients who have not received immunosuppressive drugs as well as high dose immunocytotoxic therapies or steroids. A weighted gene co-expression network was then constructed to find the relevant modules implicated in the SLE progression. Among 17 identified modules, 3 modules were selected through mapping to STRING and finding the ones that had highly connection at the protein level. These modules clearly indicate the involvement of several pathways in the pathogenesis of SLE including viral infection, adaptive immune response, and innate immune response in B lymphocytes. The WGCN analysis further revealed the co-expressed genes involved in both innate and adaptive immune systems. Mix infections and primary immunodeficiency might also dysregulate B lymphocytes, which may facilitate SLE development. As such, identifying novel biomarkers and pathways in lupus would be of importance.

Machine Learning to Quantify In Situ Humoral Selection in Human Lupus Tubulointerstitial Inflammation

In human lupus nephritis, tubulointerstitial inflammation (TII) is associated with in situ expansion of B cells expressing anti-vimentin antibodies (AVAs). The mechanism by which AVAs are selected is unclear. Herein, we demonstrate that AVA somatic hypermutation (SHM) and selection increase affinity for vimentin. Indeed, germline reversion of several antibodies demonstrated that higher affinity AVAs can be selected from both low affinity B cell germline clones and even those that are strongly reactive with other autoantigens. While we demonstrated affinity maturation, enzyme-linked immunosorbent assays (ELISAs) suggested that affinity maturation might be a consequence of increasing polyreactivity or even non-specific binding. Therefore, it was unclear if there was also selection for increased specificity. Subsequent multi-color confocal microscopy studies indicated that while TII AVAs often appeared polyreactive by ELISA, they bound selectively to vimentin fibrils in whole cells or inflamed renal tissue. Using a novel machine learning pipeline (CytoSkaler) to quantify the cellular distribution of antibody staining, we demonstrated that TII AVAs were selected for both enhanced binding and specificity in situ. Furthermore, reversion of single predicted amino acids in antibody variable regions indicated that we could use CytoSkaler to capture both negative and positive selection events. More broadly, our data suggest a new approach to assess and define antibody polyreactivity based on quantifying the distribution of binding to native and contextually relevant antigens.

Revisiting B cell tolerance and autoantibodies in seropositive and seronegative autoimmune rheumatic disease (AIRD)

Autoimmune rheumatic diseases (AIRD) are categorized seropositive or seronegative, dependent upon the presence or absence of specific autoreactive antibodies, including rheumatoid factor and anti-citrullinated protein antibodies. Autoantibody-based diagnostics have proved helpful in patient care, not only for diagnosis but also for monitoring of disease activity and prediction of therapy responsiveness. Recent work demonstrates that AIRD patients develop autoantibodies beyond those contained in the original categorization. In this study we discuss key mechanisms that underlie autoantibody development in AIRD: defects in early B cell development, genetic variants involved in regulating B cell and T cell tolerance, environmental triggers and antigen modification. We describe how autoantibodies can directly contribute to AIRD pathogenesis through innate and adaptive immune mechanisms, eventually culminating in systemic inflammation and localized tissue damage. We conclude by discussing recent insights that suggest distinct AIRD have incorrectly been denominated seronegative.

New insights into the role of antinuclear antibodies in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a prototypic autoimmune disease characterized by antinuclear antibodies (ANAs) that form immune complexes that mediate pathogenesis by tissue deposition or cytokine induction. Some ANAs bind DNA or associated nucleosome proteins, whereas other ANAs bind protein components of complexes of RNA and RNA-binding proteins (RBPs). Levels of anti-DNA antibodies can fluctuate widely, unlike those of anti-RBP antibodies, which tend to be stable. Because anti-DNA antibody levels can reflect disease activity, repeat testing is common; by contrast, a single anti-RBP antibody determination is thought to suffice for clinical purposes. Experience from clinical trials of novel therapies has provided a new perspective on ANA expression during disease, as many patients with SLE are ANA negative at screening despite previously testing positive. Because trial results suggest that patients who are ANA negative might not respond to certain agents, screening strategies now involve ANA and anti-DNA antibody testing to identify patients with so-called 'active, autoantibody-positive SLE'. Evidence suggests that ANA responses can decrease over time because of the natural history of disease or the effects of therapy. Together, these findings suggest that, during established disease, more regular serological testing could illuminate changes relevant to pathogenesis and disease status.

Evolving story of autoantibodies in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a prototypic autoimmune disease characterized by antinuclear antibody (ANA) production. ANAs bind to DNA, RNA and complexes of proteins and nucleic acids and are important markers for diagnosis and activity. According to current models, ANAs originate from antigen-driven processes; nevertheless, antibody responses to both DNA and RNA binding proteins display features unexpected in terms of current paradigms for antigenicity. These differences may reflect disturbances in both B and T cells critical for autoreactivity. Clinically, ANA testing has new uses for determining classification as well as assessing eligibility for clinical trials. Studies of patients with established disease show frequent seronegativity. In this setting, seronegativity may indicate a stage of disease called post-autoimmunity in which the natural history of disease or effects of immunosuppressive therapies modifies responses. The new uses of ANA testing highlight the importance of understanding autoantigenicity and developing sensitive and informative assays for clinical assessments.

Rare genetic variants in systemic autoimmunity

Autoimmune disease is a substantial cause of morbidity and is strongly influenced by genetic risk. Extensive efforts have characterized the overall genetic basis of many autoimmune diseases, typically by investigation of common variants. While these common variants have modest effects and may cumulatively predispose to disease, it is also increasingly apparent that rare variants have significantly greater effect on phenotype and are likely to contribute to autoimmune disease. Recent advances have illustrated the next fundamental step in elucidating the genetic basis of autoimmunity, moving beyond association to demonstrate the functional consequences of these variants.

Failure of B Cell Tolerance in CVID

Common variable immunodeficiency (CVID) comprises a group of related disorders defined by defects in B cell function and antibody production. Concurrent autoimmune features are common, but the underlying pathogenic mechanisms of autoimmunity in CVID are poorly understood. Overlap in some clinical and laboratory features suggests a shared pathogenesis, at least in part, with systemic lupus erythematosus (SLE). One important part of SLE pathogenesis is loss of B cell tolerance, an aspect that warrants further study in CVID. The study of inherently autoreactive 9G4⁺ B cells has led to a greater understanding of B cell tolerance defects in lupus. Study of these B cells in CVID has yielded conflicting results, largely due to differences in methodological approaches. In this study, we take a comprehensive look at 9G4⁺ B cells throughout B cell development in CVID patients and compare patients both with and without autoimmune features. Using flow cytometry to examine B cell subpopulations in detail, we show that only those CVID patients with autoimmune features demonstrate significant expansion of 9G4⁺ B cells, both in naïve and multiple memory populations. Examination of two autoreactive B cell subsets recently characterized in SLE, the activated naïve (aNAV) and double negative 2 (DN2) B cells, reveals an expanded 9G4⁺ DN2 population to be common among CVID patients. These results reveal that both multiple central and peripheral B cell tolerance defects are related to autoimmunity in CVID. Furthermore, these data suggest that the autoreactive DN2 B cell population, which has not previously been examined in CVID, may play an important role in the development of autoimmunity in patients with CVID.

B cells targeting therapy in the management of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease which affects the majority of organs and systems. Traditional therapies do not lead to complete remission of disease but only relieve symptoms and inflammation. B cells are the most important effector cell types in the pathogenesis of SLE. Therefore, therapies targeting B cells and their related cytokines are a very important milestone for SLE treatment. Several biologics that modulate B cells, either depleting B cells or blocking B cell functions, have been developed and evaluated in clinical trials. Belimumab, a fully humanized monoclonal antibody that specifically binds B cells activating factor (BAFF), was the first of these agents approved for SLE treatment. In this review, we explore the currently available evidence in B cell targeted therapies in SLE including agents that target B cell surface antigens (CD19, CD20, CD22), B cell survival factors (BAFF and a proliferation-inducing ligand, APRIL), cytokines (interleukin-1 and type 1 interferons) and co-stimulatory molecules (CD40 ligand). We highlighted the mechanisms of action and the individual characteristics of these biologics, and present an update on the clinical trials that have evaluated their efficacy and safety. Finally, we describe some of the emerging and promising therapies for SLE treatment.

The Plasma Interleukin (IL)-35 Level and Frequency of Circulating IL-35+ Regulatory B Cells are Decreased in a Cohort of Chinese Patients with New-onset Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a multisystemic autoimmune disease that is associated with the destruction of immune tolerance and activation of B cells. Interleukin (IL)-35 and IL-35-producing (IL-35⁺) regulatory B cells (Bregs) have been demonstrated to possess immunosuppressive functions, but their roles in the initiation and early development of SLE have not been explored. Here, we measured and compared the frequencies of blood regulatory B cell subsets and the concentrations of plasma IL-35, IL-10, IL-17A, tumor necrosis factor (TNF)-α, and interferon (IFN)-γ in 47 Chinese patients with newly diagnosed SLE and 20 matched healthy controls (HCs). The SLE patients had decreased percentages of IL-35⁺ B cells and IL-10⁺ B cells among the total blood B cells as well as decreased concentrations of plasma IL-35. In addition, higher levels of plasma IL-10, IFN-γ, TNF-α, and IL-17 along with higher frequencies of circulating plasma and memory B cells were observed in the SLE patients. The percentage of IL-35⁺ Bregs and the serum IL-35 level were inversely correlated with the SLE disease activity index and the erythrocyte sedimentation rate (ESR) levels. Our results indicate that IL-35⁺ Bregs and IL-35 may play protective roles in SLE initiation and progression.

Differences in Self-Recognition between Secreted Antibody and Membrane-Bound B Cell Antigen Receptor

The random gene segment rearrangement during B cell development ensures Ab repertoire diversity. Because this process might generate autoreactive specificities, it has been proposed that stringent selection mechanisms prevent the development of autoreactive B cells. However, conventional assays to identify autoreactive B cells usually employ in vitro-generated Abs, which differ from membrane-bound BCRs. In this study, we used a cell-based assay to investigate the autoreactivity of membrane-bound BCRs derived from different B cell developmental stages of human peripheral blood. Contrasted to soluble Ab counterparts, only a few of the tested BCRs were autoreactive, although the cell-based assay sensitively detects feeble Ag recognition of a germline-reverted murine BCR that was selected after OVA immunization of mice, whereas conventional assays failed to do so. Together, these data suggest that proper identification of autoreactive B cells requires the membrane-bound BCR, as the soluble Ab may largely differ from its BCR counterpart in Ag binding.

Activity and expression of E-NTPDase is altered in peripheral lymphocytes of systemic lupus erythematosus patients

BACKGROUND

Systemic lupus erythematosus (SLE) is an inflammatory autoimmune disease, where there is irreversible breakdown of immunological self-tolerance. Extracellular adenosine triphosphate (ATP) and adenosine are signaling molecules that play an important part in the immune response. During inflammation and the immune response, a group of enzymes control these molecules, including ectonucleoside triphosphate diphosphohydrolase (E-NTPDase), E-5'-nucleotidase, and ecto-adenosine deaminase (E-ADA). We determined the activity and expression of E-NTPDase, the expression of E-5'-nucleotidase, the activity of E-ADA in lymphocytes and serum of SLE patients.

METHODS

This study involved 35 patients with SLE and 30 healthy subjects as a control group. E-NTPDase activity and expression were increased in lymphocytes from SLE patients (31% and 37% for activity and expression, respectively) compared with the control group.

RESULTS

An approximately 42% increase in E-ADA activity in lymphocytes was observed in SLE patients compared with the control group, in serum the ADA activity was decreased by 57% in SLE patients. Expression of E-5'-nucleotidase was not changed in SLE patients.

CONCLUSIONS

E-NTPDase and E-ADA perform key functions in the modulation of the immune and inflammatory response in SLE.

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.

Antibody repertoire analysis in polygenic autoimmune diseases

High-throughput sequencing of the DNA/RNA encoding antibody heavy- and light-chains is rapidly transforming the field of adaptive immunity. It can address key questions, including: (i) how the B-cell repertoire differs in health and disease; and (ii) if it does differ, the point(s) in B-cell development at which this occurs. The advent of technologies, such as whole-genome sequencing, offers the chance to link abnormalities in the B-cell antibody repertoire to specific genomic variants and polymorphisms. Here, we discuss the current research using B-cell antibody repertoire sequencing in three polygenic autoimmune diseases where there is good evidence for a pathological role for B-cells, namely systemic lupus erythematosus, multiple sclerosis and rheumatoid arthritis. These autoimmune diseases exhibit significantly skewed B-cell receptor repertoires compared with healthy controls. Interestingly, some common repertoire defects are shared between diseases, such as elevated IGHV4-34 gene usage. B-cell clones have effectively been characterized and tracked between different tissues and blood in autoimmune disease. It has been hypothesized that these differences may signify differences in B-cell tolerance; however, the mechanisms and implications of these defects are not clear.

New insights into the role of renal resident cells in the pathogenesis of lupus nephritis

Systemic lupus erythematosus (SLE), an autoimmune disease of unknown etiology, is characterized by the production of autoantibodies and end-organ damage. Lupus nephritis affects up to 70% of patients with SLE and is the most critical predictor of morbidity and mortality. The immunopathogenesis of SLE is complex and most clinical trials of biologics targeting immune cells or their mediators have failed to show efficacy in SLE patients. It has therefore become increasingly clear that additional, local factors give rise to the inflammation and organ damage. In this review, we describe recent advances in the role of renal resident cells, including podocytes, mesangial cells, and epithelial cells, in the pathogenesis of lupus nephritis.

Altered frequencies of memory B cells in new-onset systemic lupus erythematosus patients

Systemic lupus erythematosus (SLE) is a chronic systemic autoimmune disease, characterized by B cell hyperactivity and pathogenic autoantibodies formation. The objective of this study is to evaluate the distribution of B cell subsets in patients with SLE. We included patients with SLE followed in First Affiliated Hospital of Xi'an JiaoTong University, Xi'an, China. Flow cytometry was used to measure frequencies of B cell subsets, including memory B cells, switched memory B cells, non-switched memory B cells, double-negative memory B cells, and naïve B cells in 130 patients with SLE and 55 healthy controls. The different distributions of B cell subsets were further evaluated by their associations with disease activity and clinical manifestation. SLE patients showed significant alteration of B cell subsets, including lower frequency of non-switched memory B cells and higher double-negative memory B cells. The frequencies of B cell subsets also varied between new-onset SLE patients and chronic SLE patients. Frequencies of total memory B cells, switched memory B cells, and non-switched memory B cells were lower in new-onset SLE patients and frequency of naïve B cells was higher compared with healthy controls. Chronic SLE patients showed increased switched memory B cells and double-negative memory B cells. In addition, switched memory B cells and double-negative B cells were higher in patients with lupus nephritis (LN) regardless of disease activity. Our findings suggested that abnormalities of the B cell subsets homeostasis might contribute to the pathogenesis of SLE.

Drivers of the immunopathogenesis in systemic lupus erythematosus

This review summarises a number of current insights into the pathogenesis of SLE. On the basis of the interaction of environmental factors within a predisposed host, a chronic autoimmune process gains function with a positive feed-forward loop between innate and adaptive immunity. A current focus of SLE pathogenesis is on the enhanced production of certain cytokines, such as type I interferons and BLyS/BAFF, suggesting continuous plasmacytoid dendritic and myeloid cell activity together with disturbances of B lineage cells (increased autoantibodies, including anti-dsDNA and plasmablasts, which correlate with SLE activity and memory B-cell abnormalities). Recent studies provided evidence that CD4⁺ and CD8⁺ T cells and B cells are hyporesponsive in SLE, likely reflecting their 'post-activation status'. Data of enhanced protein tyrosine phosphatase activity of lymphocytes in SLE require consideration if they represent a disease characteristic. Better understanding of the chronic autoimmune phase is needed in addition to those phases during flares and will permit improved treatment of SLE.

Altered expression of signalling lymphocyte activation molecule receptors in T-cells from lupus nephritis patients-a potential biomarker of disease activity

Objectives. The aim was to investigate whether the signalling lymphocyte activation molecule (SLAM) signalling pathways contribute to LN and whether SLAM receptors could be valuable biomarkers of disease activity.

Methods. Peripheral blood mononuclear cells from 30National Research Ethics Service SLE patients with biopsy-proven LN were analysed by flow cytometry. Clinical measures of disease activity were assessed. The expression of the SLAM family receptors on T-cell subpopulations [CD4, CD8 and double negative (DN) T cells] was measured and compared between lupus patients with active renal disease and those in remission.

Results. The frequency of CD8 T cells expressing SLAMF3, SLAMF5 and SLAMF7 was significantly lower in LN patients who were in remission. In contrast, these subsets were similar in patients with active renal disease and in healthy individuals. Patients with active nephritis had an increased percentage of circulating monocytes, consistent with a potential role played by these cells in glomerular inflammation. Changes in the frequency of DN T cells positive for SLAMF2, SLAMF4 and SLAMF7 were observed in lupus patients irrespective of the disease activity. We detected alterations in the cellular expression of the SLAM family receptors, but these changes were less obvious and did not reveal any specific pattern. The percentage of DN T cells expressing SLAMF6 could predict the clinical response to B-cell depletion in patients with LN.

Conclusion. Our study demonstrates altered expression of the SLAM family receptors in SLE T lymphocytes. This is consistent with the importance of the SLAM-associated pathways in lupus pathogenesis.

Decreased somatic hypermutation induces an impaired peripheral B cell tolerance checkpoint

Patients with mutations in AICDA, which encodes activation-induced cytidine deaminase (AID), display an impaired peripheral B cell tolerance. AID mediates class-switch recombination (CSR) and somatic hypermutation (SHM) in B cells, but the mechanism by which AID prevents the accumulation of autoreactive B cells in blood is unclear. Here, we analyzed B cell tolerance in AID-deficient patients, patients with autosomal dominant AID mutations (AD-AID), asymptomatic AICDA heterozygotes (AID+/-), and patients with uracil N-glycosylase (UNG) deficiency, which impairs CSR but not SHM. The low frequency of autoreactive mature naive B cells in UNG-deficient patients resembled that of healthy subjects, revealing that impaired CSR does not interfere with the peripheral B cell tolerance checkpoint. In contrast, we observed decreased frequencies of SHM in memory B cells from AD-AID patients and AID+/- subjects, who were unable to prevent the accumulation of autoreactive mature naive B cells. In addition, the individuals with AICDA mutations, but not UNG-deficient patients, displayed Tregs with defective suppressive capacity that correlated with increases in circulating T follicular helper cells and enhanced cytokine production. We conclude that SHM, but not CSR, regulates peripheral B cell tolerance through the production of mutated antibodies that clear antigens and prevent sustained interleukin secretions that interfere with Treg function.

Scarcity of autoreactive human blood IgA+ memory B cells

Class‐switched memory B cells are key components of the “reactive” humoral immunity, which ensures a fast and massive secretion of high‐affinity antigen‐specific antibodies upon antigenic challenge. In humans, IgA class‐switched (IgA⁺) memory B cells and IgA antibodies are abundant in the blood. Although circulating IgA⁺ memory B cells and their corresponding secreted immunoglobulins likely possess major protective and/or regulatory immune roles, little is known about their specificity and function. Here, we show that IgA⁺ and IgG⁺ memory B‐cell antibodies cloned from the same healthy humans share common immunoglobulin gene features. IgA and IgG memory antibodies have comparable lack of reactivity to vaccines, common mucosa‐tropic viruses and commensal bacteria. However, the IgA⁺ memory B‐cell compartment contains fewer polyreactive clones and importantly, only rare self‐reactive clones compared to IgG⁺ memory B cells. Self‐reactivity of IgAs is acquired following B‐cell affinity maturation but not antibody class switching. Together, our data suggest the existence of different regulatory mechanisms for removing autoreactive clones from the IgG⁺ and IgA⁺ memory B‐cell repertoires, and/or different maturation pathways potentially reflecting the distinct nature and localization of the cognate antigens recognized by individual B‐cell populations.

MEDI-551 Treatment Effectively Depletes B Cells and Reduces Serum Titers of Autoantibodies in Mice Transgenic for Sle1 and Human CD19

OBJECTIVE

To evaluate treatment with MEDI-551, a humanized anti-human CD19 monoclonal antibody, in a model of autoimmunity involving mice transgenic (Tg) for Sle1 and human CD19 (hCD19).

METHODS

Sle1.hCD19-Tg mice were given either a single intravenous dose of MEDI-551 or repeated doses of MEDI-551 biweekly for up to 12 weeks. The numbers of B cells in the blood, spleen, and bone marrow were determined by flow cytometry assay. In the spleen and bone marrow, the number of IgM- and IgG-specific antibody-secreting cells (ASCs) and the number of ASCs specific for anti-double-stranded DNA (anti-dsDNA) were determined by enzyme-linked immunospot assay. Serum autoantibody and total immunoglobulin levels were determined by enzyme-linked immunosorbent assay, and levels of inflammatory proteins were tested using a multianalyte profiling platform.

RESULTS

MEDI-551 treatment of Sle1.hCD19-Tg mice resulted in effective and sustained B cell depletion throughout the duration of the experiment. The frequency of IgM and IgG ASCs in the spleen was reduced by ≥90%, whereas in the bone marrow, the total ASC frequency was not changed. Levels of autoantibodies specific for dsDNA as well as antihistone and antinuclear antibodies were each reduced by 40-80%, but total serum immunoglobulin levels were largely unchanged at the end of 12 weeks of treatment.

CONCLUSION

These findings highlight the ability of MEDI-551 to deplete B cells and ASCs in autoimmune Sle1.hCD19-Tg mice. MEDI-551 treatment resulted in a robust reduction of autoantibodies but had minimal effect on total serum immunoglobulins. Thus, the novel ability of MEDI-551 to remove a broad range of B cells as well as to lower most disease-driving autoantibodies in an autoimmune disease mouse model warrants continued research. Several clinical studies to explore the safety and activity of MEDI-551 in autoantibody-associated autoimmune diseases are ongoing.

Pain in autoimmune disorders

Most autoimmune diseases are associated with pathological pain development. Autoimmune diseases with pathological pain include complex regional pain syndrome, rheumatoid arthritis, and Guillian-Barré syndrome to name a few. The present Review explores research linking the immune system to the development of pathological pain in autoimmune diseases. Pathological pain has been linked to T-cell activation and the release of cytokines from activated microglia in the dorsal horn of the spinal cord. New research on the role of autoantibodies in autoimmunity has generated insights into potential mechanisms of pain associated with autoimmune disease. Autoantibodies may act through various mechanisms in autoimmune disorders. These include the alteration of neuronal excitability via specific antigens such as the voltage-gated potassium channel complexes or by mediating bone destruction in rheumatoid arthritis. Although more research must be done to understand better the role of autoantibodies in autoimmune disease related pain, this may be a promising area of research for new analgesic therapeutic targets. © 2016 Wiley Periodicals, Inc.

Prolactin Rescues Immature B-Cells from Apoptosis Induced by B-Cell Receptor Cross-Linking

Prolactin has an immunomodulatory effect and has been associated with B-cell-triggered autoimmune diseases, such as systemic lupus erythematosus (SLE). In mice that develop SLE, the PRL receptor is expressed in early bone marrow B-cells, and increased levels of PRL hasten disease manifestations, which are correlated with a reduction in the absolute number of immature B-cells. The aim of this work was to determine the effect of PRL in an in vitro system of B-cell tolerance using WEHI-231 cells and immature B-cells from lupus prone MRL/lpr mice. WEHI-231 cells express the long isoform of the PRL receptor, and PRL rescued the cells from cell death by decreasing the apoptosis induced by the cross-linking of the B-cell antigen receptor (BCR) as measured by Annexin V and active caspase-3. This decrease in apoptosis may have been due to the PRL and receptor interaction, which increased the relative expression of antiapoptotic Bcl-xL and decreased the relative expression of proapoptotic Bad. In immature B-cells from MRL/lpr mice, PRL increased the viability and decreased the apoptosis induced by the cross-linking of BCR, which may favor the maturation of self-reactive B-cells and contribute to the onset of disease.

Autoregulatory function of interleukin-10-producing pre-naïve B cells is defective in systemic lupus erythematosus

INTRODUCTION

Pre-naïve B cells represent an intermediate stage in human B-cell development with some functions of mature cells, but their involvement in immune responses is unknown. The aim of this study was to determine the functional role of normal pre-naïve B cells during immune responses and possible abnormalities in systemic lupus erythematosus (SLE) that might contribute to disease pathogenesis.

METHODS

Pre-naïve, naïve, and memory B cells from healthy individuals and SLE patients were stimulated through CD40 and were analyzed for interleukin-10 (IL-10) production and co-stimulatory molecule expression and their regulation of T-cell activation. Autoreactivity of antibodies produced by pre-naïve B cells was tested by measuring immunoglobulin M (IgM) autoantibodies in culture supernatants after differentiation.

RESULTS

CD40-stimulated pre-naïve B cells produce larger amounts of IL-10 but did not suppress CD4(+) T-cell cytokine production. Activated pre-naïve B cells demonstrated IL-10-mediated ineffective promotion of CD4(+) T-cell proliferation and induction of CD4(+)FoxP3(+) T cells and IL-10 independent impairment of co-stimulatory molecule expression and tumor necrosis factor-alpha (TNF-α) and IL-6 production. IgM antibodies produced by differentiated pre-naïve B cells were reactive to single-stranded deoxyribonucleic acid. SLE pre-naïve B cells were defective in producing IL-10, and co-stimulatory molecule expression was enhanced, resulting in promotion of robust CD4(+) T-cell proliferation.

CONCLUSIONS

There is an inherent and IL-10-mediated mechanism that limits the capacity of normal pre-naïve B cells from participating in cellular immune response, but these cells can differentiate into autoantibody-secreting plasma cells. In SLE, defects in IL-10 secretion permit pre-naïve B cells to promote CD4(+) T-cell activation and may thereby enhance the development of autoimmunity.

Positive selection of natural poly-reactive B cells in the periphery occurs independent of heavy chain allelic inclusion

Natural autoreactive B cells are important mediators of autoimmune diseases. Receptor editing is known to play an important role in both central and peripheral B cell tolerance. However, the role of allelic inclusion in the development of natural autoreactive B cells is not clear. Previously, we generated μ chain (TgV_H3B4I) and μ/κ chains (TgV_H/L3B4) transgenic mice using transgene derived from the 3B4 hybridoma, which produce poly-reactive natural autoantibodies. In this study, we demonstrate that a considerable population of B cells edited their B cells receptors (BCRs) via light chain or heavy chain allelic inclusion during their development in TgV_H3B4I mice. Additionally, allelic inclusion occurred more frequently in the periphery and promoted the differentiation of B cells into marginal zone or B-1a cells in TgV_H3B4I mice. B cells from TgV_H/L3B4 mice expressing the intact transgenic 3B4 BCR without receptor editing secreted poly-reactive 3B4 antibody. Interestingly, however, B cell that underwent allelic inclusion in TgV_H3B4I mice also produced poly-reactive autoantibodies in vivo and in vitro. Our findings suggest that receptor editing plays a minor role in the positive selection of B cells expressing natural poly-reactive BCRs, which can be positively selected through heavy chain allelic inclusion to retain their poly-reactivity in the periphery.

High Affinity Antibodies against Influenza Characterize the Plasmablast Response in SLE Patients After Vaccination

Breakdown of B cell tolerance is a cardinal feature of systemic lupus erythematosus (SLE). Increased numbers of autoreactive mature naïve B cells have been described in SLE patients and autoantibodies have been shown to arise from autoreactive and non-autoreactive precursors. How these defects, in the regulation of B cell tolerance and selection, influence germinal center (GC) reactions that are directed towards foreign antigens has yet to be investigated. Here, we examined the characteristics of post-GC foreign antigen-specific B cells from SLE patients and healthy controls by analyzing monoclonal antibodies generated from plasmablasts induced specifically by influenza vaccination. We report that many of the SLE patients had anti-influenza antibodies with higher binding affinity and neutralization capacity than those from controls. Although overall frequencies of autoreactivity in the influenza-specific plasmablasts were similar for SLE patients and controls, the variable gene repertoire of influenza-specific plasmablasts from SLE patients was altered, with increased usage of JH6 and long heavy chain CDR3 segments. We found that high affinity anti-influenza antibodies generally characterize the plasmablast responses of SLE patients with low levels of autoreactivity; however, certain exceptions were noted. The high-avidity antibody responses in SLE patients may also be correlated with cytokines that are abnormally expressed in lupus. These findings provide insights into the effects of dysregulated immunity on the quality of antibody responses following influenza vaccination and further our understanding of the underlying abnormalities of lupus.

Pathophysiology of T follicular helper cells in humans and mice

Follicular helper T cells (TFH cells) compose a heterogeneous subset of CD4(+) T cells that induce the differentiation of B cells into plasma cells and memory cells. They are found within and in proximity to germinal centers in secondary lymphoid organs, and their memory compartment also circulates in the blood. Our knowledge on the biology of TFH cells has increased significantly during the past decade, largely as a result of mouse studies. However, recent studies on human TFH cells isolated from lymphoid organ and blood samples and recent observations on the developmental mechanism of human TFH cells have revealed both similarities and differences between human and mouse TFH cells. Here we present the similarities and differences between mouse and human lymphoid organ-resident TFH cells and discuss the role of TFH cells in response to vaccines and in disease pathogenesis.

Evaluation of epratuzumab as a biologic therapy in systemic lupus erythematosus

B cells play a key role in the pathogenesis of systemic lupus erythematosus. Some of the current biologic therapies target B cells or B-cell activating factors. Epratuzumab is a humanized monoclonal antibody, which targets CD22 on B cells. This review focuses on the safety and efficacy of epratuzumab in systemic lupus erythematosus based on the information from various published clinical trials and presentations at international meetings. Epratuzumab acts as a B-cell modulator through inhibition of B-cell receptor signaling. It has been shown to be efficacious in open-label and Phase I and Phase II randomized controlled trials. The drug has steroid-sparing properties and treatment is associated with significant improvements in Health Related Quality of Life and its safety profile is comparable to placebo.

Persistence of anti-desmoglein 3 IgG(+) B-cell clones in pemphigus patients over years

Pemphigus vulgaris (PV) is a prototypic tissue-specific autoantibody-mediated disease in which anti-desmoglein 3 (Dsg3) immunoglobulin G (IgG) autoantibodies cause life-threatening blistering. We characterized the autoimmune B-cell response over 14 patient-years in two patients with active and relapsing disease, then in one of these patients after long-term remission induced by multiple courses of rituximab (anti-CD20 antibody). Characterization of the anti-Dsg3 IgG⁺ repertoire by antibody phage display (APD) and PCR indicated that 6 clonal lines persisted in patient 1 (PV3) over 5.5 years, with only one new clone detected. Six clonal lines persisted in patient 2 (PV1) for 4 years, of which 5 persisted for another 4.5 years without any new clones detected. However, after long-term clinical and serologic remission, ~11 years after initial characterization, we could no longer detect any anti-Dsg3 clones in PV1 by APD. Similarly, in another PV patient, ~4.5 years after a course of rituximab that induced long-term remission, anti-Dsg3 B-cell clones were undetectable. These data suggest that in PV a given set of non-tolerant B-cell lineages causes autoimmune disease and that new sets do not frequently or continually escape tolerance. Therapy such as rituximab, aimed at eliminating these aberrant sets of lineages, may be effective for disease because new ones are unlikely to develop.

Clinical, paraclinical and serological findings in Susac syndrome: an international multicenter study

Background

Susac syndrome (SuS) is a rare disorder thought to be caused by autoimmune-mediated occlusions of microvessels in the brain, retina and inner ear leading to central nervous system (CNS) dysfunction, visual disturbances due to branch retinal artery occlusions (BRAO), and hearing deficits. Recently, a role for anti-endothelial cell antibodies (AECA) in SuS has been proposed.

Objectives

To report the clinical and paraclinical findings in the largest single series of patients so far and to investigate the frequency, titers, and clinical relevance of AECA in SuS.

Patients and methods

A total of 107 serum samples from 20 patients with definite SuS, 5 with abortive forms of SuS (all with BRAO), and 70 controls were tested for AECA by immunohistochemistry employing primate brain tissue sections.

Results

IgG-AECA >1:100 were detected in 25% (5/20) of patients with definite SuS and in 4.3% (3/70) of the controls. Median titers were significantly higher in SuS (1:3200, range 1:100 to 1:17500) than in controls (1:100, range 1:10 to 1:320); IgG-AECA titers >1:320 were exclusively present in patients with SuS; three controls had very low titers (1:10). Follow-up samples (n = 4) from a seropositive SuS patient obtained over a period of 29 months remained positive at high titers. In all seropositive cases, AECA belonged to the complement-activating IgG1 subclass. All but one of the IgG-AECA-positive samples were positive also for IgA-AECA and 45% for IgM-AECA. SuS took a severe and relapsing course in most patients and was associated with bilateral visual and hearing impairment, a broad panel of neurological and neuropsychological symptoms, and brain atrophy in the majority of cases. Seropositive and seronegative patients did not differ with regard to any of the clinical or paraclinical parameters analyzed.

Conclusions

SuS took a severe and protracted course in the present cohort, resulting in significant impairment. Our finding of high-titer IgG1 and IgM AECA in some patients suggest that humoral autoimmunity targeting the microvasculature may play a role in the pathogenesis of SuS, at least in a subset of patients. Further studies are warranted to define the exact target structures of AECA in SuS.

Protein kinase Cδ promotes transitional B cell-negative selection and limits proximal B cell receptor signaling to enforce tolerance

Protein kinase Cδ (PKCδ) deficiency causes autoimmune pathology in humans and mice and is crucial for the maintenance of B cell homeostasis. However, the mechanisms underlying autoimmune disease in PKCδ deficiency remain poorly defined. Here, we address the antigen-dependent and -independent roles of PKCδ in B cell development, repertoire selection, and antigen responsiveness. We demonstrate that PKCδ is rapidly phosphorylated downstream of both the B cell receptor (BCR) and the B cell-activating factor (BAFF) receptor. We found that PKCδ is essential for antigen-dependent negative selection of splenic transitional B cells and is required for activation of the proapoptotic Ca(2+)-Erk pathway that is selectively activated during B cell-negative selection. Unexpectedly, we also identified a previously unrecognized role for PKCδ as a proximal negative regulator of BCR signaling that substantially impacts survival and proliferation of mature follicular B cells. As a consequence of these distinct roles, PKCδ deficiency leads to the survival and development of a B cell repertoire that is not only aberrantly autoreactive but also hyperresponsive to antigen stimulation.