Identification of Anergic B Cells within a Wild-Type Repertoire

Immune Tolerance Regulation Is Critical to Immune Homeostasis

The body's immune response plays a critical role in defending against external or foreign antigens while also preserving tolerance to self-antigens. This equilibrium, referred to as immune homeostasis, is paramount for overall health. The regulatory mechanisms governing the maintenance of this delicate immune balance are notably complex. It is currently accepted that immune tolerance is a dynamic outcome regulated by multiple factors, including central and peripheral mechanisms. Its induction or elimination plays a significant role in autoimmune diseases, organ transplantation, and cancer therapy, markedly impacting various major diseases in modern clinical practice. Overall, our current understanding of immune tolerance is still very limited. In this review article, we summarized the main mechanisms that have been known to mediate immune tolerance so far, including endogenous immune tolerance, adaptive immune tolerance, other immune tolerance mechanisms, and the homeostasis of immune tolerance, identified the key factors that regulate immune tolerance, and provided new clues for immune system recovery in many autoimmune diseases, organ transplantation, and tumor therapy.

Immunophenotyping and Therapeutic Insights from Chronic Mucocutaneous Candidiasis Cases with STAT1 Gain-of-Function Mutations

PURPOSE

Heterozygous STAT1 Gain-of-Function (GOF) mutations are the most common cause of chronic mucocutaneous candidiasis (CMC) among Inborn Errors of Immunity. Clinically, these mutations manifest as a broad spectrum of immune dysregulation, including autoimmune diseases, vascular disorders, and malignancies. The pathogenic mechanisms of immune dysregulation and its impact on immune cells are not yet fully understood. In treatment, JAK inhibitors have shown therapeutic effectiveness in some patients.

METHODS

We analyzed clinical presentations, cellular phenotypes, and functional impacts in five Taiwanese patients with STAT1 GOF.

RESULTS

We identified two novel GOF mutations in 5 patients from 2 Taiwanese families, presenting with symptoms of CMC, late-onset rosacea, and autoimmunity. The enhanced phosphorylation and delayed dephosphorylation were displayed by the patients' cells. There are alterations in both innate and adaptive immune cells, including expansion of CD38+HLADR +CD8+ T cells, a skewed activated Tfh cells toward Th1, reduction of memory, marginal zone and anergic B cells, all main functional dendritic cell lineages, and a reduction in classical monocyte. Baricitinib showed therapeutic effectiveness without side effects.

CONCLUSION

Our study provides the first comprehensive clinical and molecular characteristics in STAT1 GOF patient in Taiwan and highlights the dysregulated T and B cells subsets which may hinge the autoimmunity in STAT1 GOF patients. It also demonstrated the therapeutic safety and efficacy of baricitinib in pediatric patient. Further research is needed to delineate how the aberrant STAT1 signaling lead to the changes in cellular populations as well as to better link to the clinical manifestations of the disease.

Glycan-Reactive Innate-like B Cells and Developmental Checkpoints

Using an Ig H chain conferring specificity for N-acetyl-d-glucosamine (GlcNAc), we developed transgenic (VHHGAC39 TG) mice to study the role of self-antigens in GlcNAc-reactive B-1 B cell development. In VHHGAC39 TG mice, GlcNAc-reactive B-1 B cell development during ontogeny and in adult bone marrow was normal. However, adult TG mice exhibited a block at transitional-2 immature B cell stages, resulting in impaired allelic exclusion and accumulation of a B cell subset coexpressing endogenous Ig gene rearrangements. Similarly, VHHGAC39 B cell fitness was impeded compared with non-self-reactive VHJ558 B TG cells in competitive mixed bone marrow chimeras. Nonetheless, adult VHHGAC39 mice immunized with Streptococcus pyogenes produce anti-GlcNAc Abs. Peritoneal cavity B cells transferred from VHHGAC39 TG mice into RAG-/- mice also exhibited robust expansion and anti-GlcNAc Ab production. However, chronic treatment of young VHHGAC39 mice with GlcNAc-specific mAbs leads to lower GlcNAc-binding B cell frequencies while increasing the proportion of GlcNAc-binding B1-a cells, suggesting that Ag masking or clearance of GlcNAc Ags impedes maturation of newly formed GlcNAc-reactive B cells. Finally, BCR H chain editing promotes expression of endogenous nontransgenic BCR alleles, allowing potentially self-reactive TG B cells to escape anergy or deletion at the transitional stage of precursor B cell development. Collectively, these observations indicate that GlcNAc-reactive B cell development is sensitive to the access of autologous Ags.

Peripheral apoptosis and limited clonal deletion during physiologic murine B lymphocyte development

Self-reactive and polyreactive B cells generated during B cell development are silenced by either apoptosis, clonal deletion, receptor editing or anergy to avoid autoimmunity. The specific contribution of apoptosis to normal B cell development and self-tolerance is incompletely understood. Here, we quantify self-reactivity, polyreactivity and apoptosis during physiologic B lymphocyte development. Self-reactivity and polyreactivity are most abundant in early immature B cells and diminish significantly during maturation within the bone marrow. Minimal apoptosis still occurs at this site, however B cell receptors cloned from apoptotic B cells show comparable self-reactivity to that of viable cells. Apoptosis increases dramatically only following immature B cells leaving the bone marrow sinusoids, but above 90% of cloned apoptotic transitional B cells are not self-reactive/polyreactive. Our data suggests that an apoptosis-independent mechanism, such as receptor editing, removes most self-reactive B cells in the bone marrow. Mechanistically, lack of survival signaling rather than clonal deletion appears to be the underpinning cause of apoptosis in most transitional B cells in the periphery.

The Src-family kinase Lyn plays a critical role in establishing and maintaining B cell anergy by suppressing PI3K-dependent signaling

Although the Src family kinase (SFK) Lyn is known to be involved in induction and maintenance of peripheral B cell tolerance, the molecular basis of its action in this context remains unclear. This question has been approached using conventional as well as B cell-targeted knockouts of Lyn, with varied conclusions likely confused by collateral loss of Lyn functions in B cell and myeloid cell development and activation. Here we utilized a system in which Lyn gene deletion is tamoxifen inducible and B cell restricted. This system allows acute elimination of Lyn in B cells without off-target effects. This genetic tool was employed in conjunction with immunoglobulin transgenic mice in which peripheral B cells are autoreactive. DNA reactive Ars/A1 B cells require continuous inhibitory signaling, mediated by the inositol phosphatase SHIP-1 and the tyrosine phosphatase SHP-1, to maintain an unresponsive (anergic) state. Here we show that Ars/A1 B cells require Lyn to establish and maintain B cell unresponsiveness. Lyn primarily functions by restricting PI3K-dependent signaling pathways. This Lyn-dependent mechanism complements the impact of reduced mIgM BCR expression to restrict BCR signaling in Ars/A1 B cells. Our findings suggest that a subset of autoreactive B cells requires Lyn to become anergic and that the autoimmunity associated with dysregulated Lyn function may, in part, be due to an inability of these autoreactive B cells to become tolerized.

Failed Downregulation of PI3K Signaling Makes Autoreactive B Cells Receptive to Bystander T Cell Help

The role of T cell help in autoantibody responses is not well understood. Because tolerance mechanisms govern both T and B cell responses, one might predict that both T cell tolerance and B cell tolerance must be defeated in autoantibody responses requiring T cell help. To define whether autoreactive B cells depend on T cells to generate autoantibody responses, we studied the role of T cells in murine autoantibody responses resulting from acute B cell-specific deletion of regulatory phosphatases. Ars/A1 B cells are DNA reactive and require continuous inhibitory signaling by the tyrosine phosphatase SHP-1 and the inositol phosphatases SHIP-1 and PTEN to maintain unresponsiveness. Acute B cell-restricted deletion of any of these phosphatases results in an autoantibody response. In this study, we show that CD40-CD40L interactions are required to support autoantibody responses of B cells whose anergy has been compromised. If the B cell-intrinsic driver of loss of tolerance is failed negative regulation of PI3K signaling, bystander T cells provide sufficient CD40-mediated signal 2 to support an autoantibody response. However, although autoantibody responses driven by acute B cell-targeted deletion of SHP-1 also require T cells, bystander T cell help does not suffice. These results demonstrate that upregulation of PI3K signaling in autoreactive B cells, recapitulating the effect of multiple autoimmunity risk alleles, promotes autoantibody responses both by increasing B cells' cooperation with noncognate T cell help and by altering BCR signaling. Receptiveness to bystander T cell help enables autoreactive B cells to circumvent the fail-safe of T cell tolerance.

Interleukin 4-driven reversal of self-reactive B cell anergy contributes to the pathogenesis of systemic lupus erythematosus

OBJECTIVES

Reactivation of anergic autoreactive B cells (BND cells) is a key aetiological process in systemic lupus erythematosus (SLE), yet the underlying mechanism remains largely elusive. This study aimed to investigate how BND cells participate in the pathogenesis of SLE and the underlying mechanism.

METHODS

A combination of phenotypical, large-scale transcriptome and B cell receptor (BCR) repertoire profiling were employed at molecular and single cell level on samples from healthy donors and patients with SLE. Isolated naïve B cells from human periphery blood were treated with anti-CD79b mAb in vitro to induce anergy. IgM internalisation was tracked by confocal microscopy and was qualified by flow cytometer.

RESULTS

We characterised the decrease and disruption of BND cells in SLE patients and demonstrated IL-4 as an important cytokine to drive such pathological changes. We then elucidated that IL-4 reversed B cell anergy by promoting BCR recycling to the cell surface via STAT6 signalling.

CONCLUSIONS

We demonstrated the significance of IL-4 in reversing B cell anergy and established the scientific rationale to treat SLE via blocking IL-4 signalling, also providing diagnostic and prognostic biomarkers to identify patients who are most likely going to benefit from such treatments.

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.

An integrated proteome and transcriptome of B cell maturation defines poised activation states of transitional and mature B cells

During B cell maturation, transitional and mature B cells acquire cell-intrinsic features that determine their ability to exit quiescence and mount effective immune responses. Here we use label-free proteomics to quantify the proteome of B cell subsets from the mouse spleen and map the differential expression of environmental sensing, transcription, and translation initiation factors that define cellular identity and function. Cross-examination of the full-length transcriptome and proteome identifies mRNAs related to B cell activation and antibody secretion that are not accompanied by detection of the encoded proteins. In addition, proteomic data further suggests that the translational repressor PDCD4 restrains B cell responses, in particular those from marginal zone B cells, to a T-cell independent antigen. In summary, our molecular characterization of B cell maturation presents a valuable resource to further explore the mechanisms underpinning the specialized functions of B cell subsets, and suggest the presence of 'poised' mRNAs that enable expedited B cell responses.

Bruton's tyrosine kinase inhibition reduces disease severity in a model of secondary progressive autoimmune demyelination

Bruton's tyrosine kinase (BTK) is an emerging target in multiple sclerosis (MS). Alongside its role in B cell receptor signaling and B cell development, BTK regulates myeloid cell activation and inflammatory responses. Here we demonstrate efficacy of BTK inhibition in a model of secondary progressive autoimmune demyelination in Biozzi mice with experimental autoimmune encephalomyelitis (EAE). We show that late in the course of disease, EAE severity could not be reduced with a potent relapse inhibitor, FTY720 (fingolimod), indicating that disease was relapse-independent. During this same phase of disease, treatment with a BTK inhibitor reduced both EAE severity and demyelination compared to vehicle treatment. Compared to vehicle treatment, late therapeutic BTK inhibition resulted in fewer spinal cord-infiltrating myeloid cells, with lower expression of CD86, pro-IL-1β, CD206, and Iba1, and higher expression of Arg1, in both tissue-resident and infiltrating myeloid cells, suggesting a less inflammatory myeloid cell milieu. These changes were accompanied by decreased spinal cord axonal damage. We show similar efficacy with two small molecule inhibitors, including a novel, highly selective, central nervous system-penetrant BTK inhibitor, GB7208. These results suggest that through lymphoid and myeloid cell regulation, BTK inhibition reduced neurodegeneration and disease progression during secondary progressive EAE.

Peripheral death by neglect and limited clonal deletion during physiologic B lymphocyte development

Autoreactive B cells generated during B cell development are inactivated by clonal deletion, receptor editing or anergy. Up to 97% of immature B cells appear to die before completing maturation, but the anatomic sites and reasons underlying this massive cell loss are not fully understood. Here, we directly quantitated apoptosis and clonal deletion during physiologic B lymphocyte development using Rosa26INDIA apoptosis indicator mice. Immature B cells displayed low levels of apoptosis in the bone marrow but started dying at high levels in the periphery upon release from bone marrow sinusoids into the blood circulation. Clonal deletion of self-reactive B cells was neither a major contributor to apoptosis in the bone marrow nor the periphery. Instead, most peripheral transitional 1 B cells did not encounter the signals required for positive selection into the mature B cell compartments. This study sheds new light on B cell development and suggests that receptor editing and/or anergy efficiently control most primary autoreactivity in mice.

CARD19, a Novel Regulator of the TAK1/NF-κB Pathway in Self-Reactive B Cells

The caspase recruitment domain family member (CARD)11-Bcl10-Malt1 signalosome controls TGF-β-activated kinase 1 (TAK1) activation and regulates BCR-induced NF-κB activation. In this study, we discovered that CARD19 interacted with TAK1 and inhibited TAB2-mediated TAK1 ubiquitination and activation. Although CARD19 deficiency in mice did not affect B cell development, it enhanced clonal deletion, receptor editing, and anergy of self-reactive B cells, and it reduced autoantibody production. Mechanistically, CARD19 deficiency increased BCR/TAK1-mediated NF-κB activation, leading to increased expression of transcription factors Egr2/3, as well as the E3 ubiquitin ligases c-Cbl/Cbl-b, which are known inducers of B cell tolerance in self-reactive B cells. RNA sequencing analysis revealed that although CARD19 deficiency did not affect the overall Ag-induced gene expression in naive B cells, it suppressed BCR signaling and increased hyporesponsiveness of self-reactive B cells. As a result, CARD19 deficiency prevented Bm12-induced experimental systemic lupus erythematosus. In summary, CARD19 negatively regulates BCR/TAK1-induced NF-κB activation and its deficiency increases Egr2/3 and c-Cbl/Cbl-b expression in self-reactive B cells, thereby enhancing B cell tolerance.

Yin and yang roles of B lymphocytes in solid tumors: Balance between antitumor immunity and immune tolerance/immunosuppression in tumor-draining lymph nodes

The role of B cells in antitumor immunity has been reported to be either promotive or suppressive, but the specific mechanism remains to be comprehensively understood. However, this complicated situation likely depends on the temporal and spatial relationship between the developing tumor and B cells that recognize tumor antigens. Unlike responses against microbial or pathogenic infections, tumor cells are derived from autologous cells that have mutated and become aberrant; thus, elimination by the adaptive immune system is essentially inefficient. If tumor cells can evade immune attack at an early stage, non-destructive responses, such as tolerance and immunosuppression, are established over time. In tumor-draining lymph nodes (TDLNs), tumor antigen-reactive B cells potentially acquire immunoregulatory phenotypes and contribute to an immunosuppressive microenvironment. Therefore, triggering and enhancing antitumor responses by immunotherapies require selective control of these regulatory B cell subsets in TDLNs. In contrast, B cell infiltration and formation of tertiary lymphoid structures in tumors are positively correlated with therapeutic prognosis, suggesting that tumor antigen-specific activation of B cells and antibody production are advantageous for antitumor immunity in mid- to late-stage tumors. Given that the presence of B cells in tumor tissues may reflect the ongoing antitumor response in TDLNs, therapeutic induction and enhancement of these lymphocytes are expected to increase the overall effectiveness of immunotherapy. Therefore, B cells are promising targets, but the spatiotemporal balance of the subsets that exhibit opposite characteristics, that is, the protumor or antitumor state in TDLNs, should be understood, and strategies to separately control their functions should be developed to maximize the clinical outcome.

Failed down-regulation of PI3K signaling makes autoreactive B cells receptive to bystander T cell help

The role of T cell help in autoantibody responses is not well understood. Since tolerance mechanisms govern both T and B cell responses, one might predict that both T cell tolerance and B cell tolerance must be defeated in autoantibody responses requiring T cell help. To define whether autoreactive B cells depend on T cells to generate autoantibody responses, we studied the role of T cells in autoantibody responses resulting from acute cell-specific deletion of regulatory phosphatases. Ars/A1 B cells are DNA-reactive and require continuous inhibitory signaling by the tyrosine phosphatase SHP-1 and the inositol phosphatases SHIP-1 and PTEN to maintain unresponsiveness. Acute B cell-restricted deletion of any of these phosphatases results in an autoantibody response. Here we show that CD40-CD40L interactions are required to support autoantibody responses of B cells whose anergy has been compromised. If the B cell-intrinsic driver of loss of tolerance is failed negative regulation of PI3K signaling, bystander T cells provide sufficient CD40-mediated signal 2 to support an autoantibody response. However, while autoantibody responses driven by acute B cell-targeted deletion of SHP-1 also require T cells, bystander T cell help does not suffice. These results demonstrate that upregulation of PI3K signaling in autoreactive B cells, recapitulating the effect of multiple autoimmunity risk alleles, promotes autoantibody responses both by increasing B cells’ cooperation with non-cognate T cell help, as well as by altering BCR signaling. Receptiveness to bystander T cell help enables autoreactive B cells to circumvent the fail-safe of T cell tolerance.

Significance

Phosphatase suppression of PI3K signaling is an important mechanism by which peripheral autoreactive B cells are kept in an unresponsive/anergic state. Loss of this suppression, due to genetic alleles that confer risk of autoimmunity, often occurs in autoreactive B cells of individuals who develop autoimmune disease. Here we demonstrate that de-repression of PI3K signaling promotes autoantibody responses of a DNA-reactive B cell clone by relaxing dependence of autoantibody responses on T cell-derived helper signals. These results suggest that impaired regulation of PI3K signaling can promote autoantibody responses in two ways: by restoring antigen receptor signaling and by enabling autoreactive B cells to circumvent restrictions imposed by T cell tolerance mechanisms.

IgD shapes the pre-immune naïve B cell compartment in humans

B cell maturation and immunoglobulin (Ig) repertoire selection are governed by expression of a functional B cell receptor (BCR). Naïve B cells co-express their BCR as IgM and IgD isotype. However, the role of the additionally expressed IgD on naïve B cells is not known. Here we assessed the impact of IgD on naïve B cell maturation and Ig repertoire selection in 8 individuals from 3 different families with heterozygous loss-of-function or loss-of expression mutations in IGHD. Although naïve B cells from these individuals expressed IgM on their surface, the IGHD variant in heterozygous state entailed a chimeric situation by allelic exclusion with almost half of the naïve B cell population lacking surface IgD expression. Flow cytometric analyses revealed a distinct phenotype of IgD-negative naïve B cells with decreased expression of CD19, CD20 and CD21 as well as lower BAFF-R and integrin-β7 expression. IgD-negative B cells were less responsive in vitro after engaging the IgM-BCR, TLR7/9 or CD40 pathway. Additionally, a selective disadvantage of IgD-negative B cells within the T2 transitional and mature naïve B cell compartment as well as reduced frequencies of IgM^lo/- B cells within the mature naïve B cell compartment lacking IgD were evident. RNA-Ig-seq of bulk sorted B cell populations showed an altered selection of distinct V_H segments in the IgD-negative mature naïve B cell population. We conclude that IgD expression on human naïve B cells is redundant for generation of naïve B cells in general, but further shapes the naive B cell compartment starting from T2 transitional B cells. Our observations suggest an unexpected role of IgD expression to be critical for selection of distinct Ig V_H segments into the pre-immune Ig repertoire and for the survival of IgM^lo/- naïve B cells known to be enriched in poly-/autoreactive B cell clones.

NDRG1 is induced by antigen-receptor signaling but dispensable for B and T cell self-tolerance

Peripheral tolerance prevents the initiation of damaging immune responses by autoreactive lymphocytes. While tolerogenic mechanisms are tightly regulated by antigen-dependent and independent signals, downstream pathways are incompletely understood. N-myc downstream-regulated gene 1 (NDRG1), an anti-cancer therapeutic target, has previously been implicated as a CD4⁺ T cell clonal anergy factor. By RNA-sequencing, we identified Ndrg1 as the third most upregulated gene in anergic, compared to naïve follicular, B cells. Ndrg1 is upregulated by B cell receptor activation (signal one) and suppressed by co-stimulation (signal two), suggesting that NDRG1 may be important in B cell tolerance. However, though Ndrg1^-/- mice have a neurological defect mimicking NDRG1-associated Charcot-Marie-Tooth (CMT4d) disease, primary and secondary immune responses were normal. We find that B cell tolerance is maintained, and NDRG1 does not play a role in downstream responses during re-stimulation of in vivo antigen-experienced CD4⁺ T cells, demonstrating that NDGR1 is functionally redundant for lymphocyte anergy.

Immune regulatory effects of microRNA9-3

MicroRNAs are known to regulate cell proliferation, differentiation, and apoptosis. However, the immunological mechanism and role of microRNA9-3 (miR9-3) are unknown. This study used CRISPR/cas9 technology to knock out miR9-3 to modulate its expression level. FACS results showed that the absolute number of total B cells declined in miR9-3-deficiency in the spleen (Sp), bone marrow (BM), and lymph node (LN) to different levels compared to the wild-type. Also, the absolute numbers of Fo, T1, and T2 cells decreased both in Sp and LN. The absolute numbers of total T cells in Sp and LN declined sharply; CD4⁺ and CD8⁺ T cells showed a dramatic decrease in Sp, LN, and Th (thymus) of the miR9-3^- group. In BM, the cells number of immature B cells, pro-pre-B cells, pro-B cells, and pre-B cells reduced to different levels, while mature B cells were comparable to wild-type. These data illustrated that miR9-3-deficiency impaired the development of B cells in BM. Also, the development of T cells was severely impaired. In Th, the numbers of DN and DP cells were remarkably reduced in the miR9-3 mutant mice. Also, the numbers of DN-1, DN-3, and DN-4 cells decreased. The absolute number of cells in the hematopoietic stem cell (HSC) system such as LT-HSC (long-term HSC), ST-HSC (short-term HSC), MPP (multipotent progenitor), GMP (granulocyte-macrophage progenitor), CMP (common myeloid progenitors), MEP (megakaryocyte-erythroid progenitor), and CLP (common lymphoid progenitor) all were decreased in miR9-3 deficient mice. These results showed that miR9-3 deficiency initiated the damage to the entire hematopoietic system. Moreover, the absolute number of myeloid cells in both Sp and BM decreased in mutant mice. The cells number of NK cells showed a sharp reduction in Sp whereas the change was not significant in BM. The above results suggest that miR9-3 participates in the immune regulation of B cells, T cells, and the HSC system, highlighting its regulatory roles.

Low-Level Expression of CD138 Marks Naturally Arising Anergic B Cells

Autoreactive B cells are not entirely deleted, but some remain as immunocompetent or anergic B cells. Although the persistence of autoreactive B cells as anergic cells has been shown in transgenic mouse models with the expression of B cell receptor (BCR) reactive to engineered self-antigen, the characterization of naturally occurring anergic B cells is important to identify them and understand their contribution to immune regulation or autoimmune diseases. We report here that a low-level expression of CD138 in the splenic B cells marks naturally arising anergic B cells, not plasma cells. The CD138^int B cells consisted of IgM^lowIgD^high follicular (FO) B cells and transitional 3 B cells in homeostatic conditions. The CD138^int FO B cells showed an anergic gene expression profile shared with that of monoclonal anergic B cells expressing engineered BCRs and the gene expression profile was different from those of plasma cells, age-associated B cells, or germinal center B cells. The anergic state of the CD138^int FO B cells was confirmed by attenuated Ca²⁺ response and failure to upregulate CD69 upon BCR engagement with anti-IgM, anti-IgD, anti-Igκ, or anti-IgG. The BCR repertoire of the CD138^int FO B cells was distinct from that of the CD138^- FO B cells and included some class-switched B cells with low-level somatic mutations. These findings demonstrate the presence of polyclonal anergic B cells in the normal mice that are characterized by low-level expression of CD138, IgM downregulation, reduced Ca²⁺ and CD69 responses upon BCR engagement, and distinct BCR repertoire.

Comparative analysis of the repertoire of insulin-reactive B cells in type 1 diabetes-prone and resistant mice

Seropositivity for autoantibodies against multiple islet antigens is associated with development of autoimmune type 1 diabetes (T1D), suggesting a role for B cells in disease. The importance of B cells in T1D is indicated by the effectiveness of B cell-therapies in mouse models and patients. B cells contribute to T1D by presenting islet antigens, including insulin, to diabetogenic T cells that kill pancreatic beta cells. The role of B cell receptor (BCR) affinity in T1D development is unclear. Here, we employed single cell RNA sequencing to define the relationship between BCR affinity for insulin and B cell phenotype during disease development. We utilized immunoglobulin (Ig) heavy chain (VH125) mouse models in which high-affinity insulin-reactive B cells (IBCs) were previously shown to be anergic in diabetes-resistant VH125.C57BL/6-H2g7 and activated in VH125. NOD mice developing disease. Here, high-affinity IBCs were found in the spleen of prediabetic VH125. NOD mice and exhibited marginal zone or follicular phenotypes. Ig light chains expressed by these B cells are unmutated and biased toward Vκ4-74 and Vκ4-57 usage. Receptors expressed by anergic high-affinity IBCs of diabetes-resistant VH125.C57BL/6-H2g7 are also unmutated; however, in this genetic background light chains are polymorphic relative to those of NOD. Light chains derived from NOD and C57BL/6-H2g7 genetic backgrounds conferred divergent kinetics of binding to insulin when paired with the VH125 heavy chain. These findings suggest that relaxation of tolerance mechanisms in the NOD mouse leads to accumulation and partial activation of B cells expressing germline encoded high-affinity BCRs that support development of autoimmunity.

Improving naive B cell isolation by absence of CD45RB glycosylation and CD27 expression in combination with BCR isotype

In past years ex vivo and in vivo experimental approaches involving human naive B cells have proven fundamental for elucidation of mechanisms promoting B cell differentiation in both health and disease. For such studies, it is paramount that isolation strategies yield a population of bona fide naive B cells, i.e., B cells that are phenotypically and functionally naive, clonally non-expanded, and have non-mutated BCR variable regions. In this study different combinations of common as well as recently identified B cell markers were compared to isolate naive B cells from human peripheral blood. High-throughput BCR sequencing was performed to analyze levels of somatic hypermutation and clonal expansion. Additionally, contamination from mature mutated B cells intrinsic to each cell-sorting strategy was evaluated and how this impacts the purity of obtained populations. Our results show that current naive B cell isolation strategies harbor contamination from non-naive B cells, and use of CD27-IgD⁺ is adequate but can be improved by including markers for CD45RB glycosylation and IgM. The finetuning of naive B cell classification provided herein will harmonize research lines using naive B cells, and will improve B cell profiling during health and disease, e.g. during diagnosis, treatment, and vaccination strategies.

Insights Into the Host Contribution of Endocrine Associated Immune-Related Adverse Events to Immune Checkpoint Inhibition Therapy

Blockade of immune checkpoints transformed the paradigm of systemic cancer therapy, enabling substitution of a cytotoxic chemotherapy backbone to one of immunostimulation in many settings. Invigorating host immune cells against tumor neo-antigens, however, can induce severe autoimmune toxicity which in many cases requires ongoing management. Many immune-related adverse events (irAEs) are clinically and pathologically indistinguishable from inborn errors of immunity arising from genetic polymorphisms of immune checkpoint genes, suggesting a possible shared driver for both conditions. Many endocrine irAEs, for example, have analogous primary genetic conditions with varied penetrance and severity despite consistent genetic change. This is akin to onset of irAEs in response to immune checkpoint inhibitors (ICIs), which vary in timing, severity and nature despite a consistent drug target. Host contribution to ICI response and irAEs, particularly those of endocrine origin, such as thyroiditis, hypophysitis, adrenalitis and diabetes mellitus, remains poorly defined. Improved understanding of host factors contributing to ICI outcomes is essential for tailoring care to an individual’s unique genetic predisposition to response and toxicity, and are discussed in detail in this review.

SARS-CoV-2 infection relaxes peripheral B cell tolerance

Severe SARS-CoV-2 infection is associated with strong inflammation and autoantibody production against diverse self-antigens, suggesting a system-wide defect in B cell tolerance. BND cells are a B cell subset in healthy individuals harboring autoreactive but anergic B lymphocytes. In vitro evidence suggests inflammatory stimuli can breach peripheral B cell tolerance in this subset. We asked whether SARS-CoV-2-associated inflammation impairs BND cell peripheral tolerance. To address this, PBMCs and plasma were collected from healthy controls, individuals immunized against SARS-CoV-2, or subjects with convalescent or severe SARS-CoV-2 infection. We demonstrate that BND cells from severely infected individuals are significantly activated, display reduced inhibitory receptor expression, and restored BCR signaling, indicative of a breach in anergy during viral infection, supported by increased levels of autoreactive antibodies. The phenotypic and functional BND cell alterations significantly correlate with increased inflammation in severe SARS-CoV-2 infection. Thus, autoreactive BND cells are released from peripheral tolerance with SARS-CoV-2 infection, likely as a consequence of robust systemic inflammation.

CD45RB Glycosylation and Ig Isotype Define Maturation of Functionally Distinct B Cell Subsets in Human Peripheral Blood

Glycosylation of CD45RB (RB+) has recently been identified to mark antigen-experienced B cells, independent of their CD27 expression. By using a novel combination of markers including CD45RB glycosylation, CD27 and IgM/IgD isotype expression we segregated human peripheral blood B cell subsets and investigated their IGHV repertoire and in vitro functionality. We observed distinct maturation stages for CD27-RB+ cells, defined by differential expression of non-switched Ig isotypes. CD27-RB+ cells, which only express IgM, were more matured in terms of Ig gene mutation levels and function as compared to CD27-RB+ cells that express both IgM and IgD or cells that were CD27-RB-. Moreover, CD27-RB+IgM+ cells already showed remarkable rigidity in IgM isotype commitment, different from CD27-RB+IgMD+ and CD27-RB- cells that still demonstrated great plasticity in B cell fate decision. Thus, glycosylation of CD45RB is indicative for antigen-primed B cells, which are, dependent on the Ig isotype, functionally distinct.

Role of inhibitory signaling in peripheral B cell tolerance*

At least 20% of B cells in the periphery expresses an antigen receptor with a degree of self-reactivity. If activated, these autoreactive B cells pose a risk as they can contribute to the development of autoimmune diseases. To prevent their activation, both B cell-intrinsic and extrinsic tolerance mechanisms are in place in healthy individuals. In this review article, I will focus on B cell-intrinsic mechanisms that prevent the activation of autoreactive B cells in the periphery. I will discuss how inhibitory signaling circuits are established in autoreactive B cells, focusing on the Lyn-SHIP-1-SHP-1 axis, how they contribute to peripheral immune tolerance, and how disruptions of these circuits can contribute to the development of autoimmunity.

Adaptive tolerance: Protection through self-recognition

The random nature of immunoglobulin gene segment rearrangement inevitably leads to the generation of self-reactive B cells. Avoidance of destructive autoimmune reactions is necessary in order to maintain physiological homeostasis. However, current central and peripheral tolerance concepts fail to explain the massive number of autoantibody-borne autoimmune diseases. Moreover, recent studies have shown that in physiological mouse models autoreactive B cells were neither clonally deleted nor kept in an anergic state, but were instead able to mount autoantibody responses. We propose that activation of autoreactive B cells is induced by polyvalent autoantigen complexes that can occur under physiological conditions. Repeated encounter of autoantigen complexes leads to the production of affinity-matured autoreactive IgM that protects its respective self-targets from degradation. We refer to this novel mechanism as adaptive tolerance. This article discusses the discovery of adaptive tolerance and the unexpected role of high affinity IgM autoantibodies.

Btk Supports Autoreactive B Cell Development and Protects against Apoptosis but Is Expendable for Antigen Presentation

Bruton's tyrosine kinase (Btk) propagates B cell signaling, and BTK inhibitors are in clinical trials for autoimmune disease. Although autoreactive B cells fail to develop in the absence of Btk, its role in mature cells is unknown. To address this issue, a model of conditional removal (Btk flox/Cre-ERT2 ) was used to excise Btk from mature transgenic B cells that recognize the pathophysiologic autoantigen insulin. Anti-insulin B cells escape central tolerance and promote autoimmune diabetes, mimicking human autoreactive cells. Lifelong Btk deficiency was previously shown to eliminate 95% of anti-insulin B cells, but in this model, mature anti-insulin B cells survived for weeks after targeted Btk deletion, even when competing with a polyclonal repertoire. BCR-stimulated cells could still signal via Syk, PLCy2, and CD22, but failed to upregulate the antiapoptotic protein Bcl-xL, and proliferation was impaired. Surprisingly, Btk-depleted anti-insulin B cells could still present Ag and activate T cells, a critical function in promoting T cell-mediated islet cell destruction. Thus, pharmacologic targeting of Btk may be most effective by blocking expansion of established autoreactive cells, and preventing emergence of new ones.

Multiple tolerance checkpoints restrain affinity maturation of B cells expressing the germline precursor of a lupus patient-derived anti-dsDNA antibody in knock-in mice

Anti-dsDNA antibodies are a hallmark of systemic lupus erythematosus and are highly associated with its exacerbation. Cumulative evidence has suggested that somatic hypermutation contributes to the high-affinity reactivity of anti-dsDNA antibodies. Our previous study demonstrated that these antibodies are generated from germline precursors with low-affinity ssDNA reactivity through affinity maturation and clonal expansion in patients with acute lupus. This raised the question of whether such precursors could be subject to immune tolerance. To address this, we generated a site-directed knock-in (KI) mouse line, G9gl, which carries germline-reverted sequences of the VH-DH-JH and Vκ-Jκ regions of patient-derived, high-affinity anti-dsDNA antibodies. G9gl heterozygous mice had a reduced number of peripheral B cells, only 27% of which expressed G9gl B cell receptor (BCR). The remaining B cells harbored non-KI allele-derived immunoglobulin heavy (IgH) chains or fusion products of upstream mouse VH and the KI gene, suggesting that receptor editing through VH replacement occurred in a large proportion of B cells in the KI mice. G9gl BCR-expressing B cells responded to ssDNA but not dsDNA, and exhibited several anergic phenotypes, including reduced surface BCR and shortened life span. Further, G9gl B cells were excluded from germinal centers (GCs) induced by several conditions. In particular, following immunization with methylated bovine serum albumin-conjugated bacterial DNA, G9gl B cells occurred at a high frequency in memory B cells but not GC B cells or plasmablasts. Collectively, multiple tolerance checkpoints prevented low-affinity precursors of pathogenic anti-dsDNA B cells from undergoing clonal expansion and affinity maturation in GCs.

Single-cell analysis identifies dynamic gene expression networks that govern B cell development and transformation

Integration of external signals and B-lymphoid transcription factor activities organise B cell lineage commitment through alternating cycles of proliferation and differentiation, producing a diverse repertoire of mature B cells. We use single-cell transcriptomics/proteomics to identify differentially expressed gene networks across B cell development and correlate these networks with subtypes of B cell leukemia. Here we show unique transcriptional signatures that refine the pre-B cell expansion stages into pre-BCR-dependent and pre-BCR-independent proliferative phases. These changes correlate with reciprocal changes in expression of the transcription factor EBF1 and the RNA binding protein YBX3, that are defining features of the pre-BCR-dependent stage. Using pseudotime analysis, we further characterize the expression kinetics of different biological modalities across B cell development, including transcription factors, cytokines, chemokines, and their associated receptors. Our findings demonstrate the underlying heterogeneity of developing B cells and characterise developmental nodes linked to B cell transformation.

HIV-1 Env Does Not Enable the Development of Protective Broadly Neutralizing Antibodies in an Experimental Autoimmune Encephalomyelitis Mouse Model

Recent studies showed that immunological tolerance may restrict the development of Env-specific autoreactive broadly neutralizing antibodies. This evidence is consistent with the finding that Env immunization of a systemic lupus erythematosus (SLE) murine model produced antibodies that neutralize tier 2 HIV-1 strains. In this study, we address the possibility of eliciting neutralizing anti-Env antibodies in other autoimmune diseases such as multiple sclerosis (MS). While, as reported for SLE, we showed for the first time that a small number of HIV-1 negative, relapsing remitting MS patients exhibited antibodies with neutralizing properties, our attempts at inducing those antibodies in a EAE mouse model of MS failed. The success in eliciting Env-specific neutralizing antibodies might be related to the specific characteristics of the autoimmune disease, or it might rely in improving the vaccination design. Studies using mouse models are useful to gain insight in how HIV-specific neutralizing antibody responses are regulated in order to develop a protective HIV-1 vaccine.

Immunoglobulin D and its encoding genes: An updated review

Since the identification of a functional Cδ gene in ostriches, immunoglobulin (Ig) D has been considered to be an extremely evolutionarily conserved Ig isotype besides the IgM found in all classes of jawed vertebrates. However, in contrast to IgM (which remains stable over evolutionary time), IgD shows considerable structural plasticity among vertebrate species and, moreover, its functions are far from elucidated even in humans and mice. Recently, several studies have shown that high expression of the IgD-B-cell receptor (IgD-BCR) may help physiologically autoreactive B cells survive in peripheral lymphoid tissues thanks to unresponsiveness to self-antigens and help their entry into germinal centers to "redeem" autoreactivity via somatic hypermutation. Other studies have demonstrated that secreted IgD may enhance mucosal homeostasis and immunity by linking B cells with basophils to optimize T-helper-2 cell-mediated responses and to constrain IgE-mediated basophil degranulation. Herein, we review the new discoveries on IgD-encoding genes in jawed vertebrates in the past decade. We also highlight advances in the functions of the IgD-BCR and secreted IgD in humans and mice.

miRNA-Mediated Control of B Cell Responses in Immunity and SLE

Loss of B cell tolerance is central to autoimmune diseases such as systemic lupus erythematosus (SLE). As such, the mechanisms involved in B cell development, maturation, activation, and function that are aberrantly regulated in SLE are of interest in the design of targeted therapeutics. While many factors are involved in the generation and regulation of B cell responses, miRNAs have emerged as critical regulators of these responses within the last decade. To date, miRNA involvement in B cell responses has largely been studied in non-autoimmune, immunization-based systems. However, miRNA profiles have also been strongly associated with SLE in human patients and these molecules have proven critical in both the promotion and regulation of disease in mouse models and in the formation of autoreactive B cell responses. Functionally, miRNAs are small non-coding RNAs that bind to complementary sequences located in target mRNA transcripts to mediate transcript degradation or translational repression, invoking a post-transcriptional level of genetic regulation. Due to their capacity to target a diverse range of transcripts and pathways in different immune cell types and throughout the various stages of development and response, targeting miRNAs is an interesting potential therapeutic avenue. Herein, we focus on what is currently known about miRNA function in both normal and SLE B cell responses, primarily highlighting miRNAs with confirmed functions in mouse models. We also discuss areas that should be addressed in future studies and whether the development of miRNA-centric therapeutics may be a viable alternative for the treatment of SLE.

BAFF promotes heightened BCR responsiveness and manifestations of chronic GVHD after allogeneic stem cell transplantation

Patients with chronic graft-versus-host disease (cGVHD) have increased B cell-activating factor (BAFF) levels, but whether BAFF promotes disease after allogeneic bone marrow transplantation (allo-BMT) remains unknown. In a major histocompatibility complex-mismatched model with cGVHD-like manifestations, we first examined B-lymphopenic μMT allo-BMT recipients and found that increased BAFF levels in cGVHD mice were not merely a reflection of B-cell number. Mice that later developed cGVHD had significantly increased numbers of recipient fibroblastic reticular cells with higher BAFF transcript levels. Increased BAFF production by donor cells also likely contributed to cGVHD, because BAFF transcript in CD4+ T cells from diseased mice and patients was increased. cGVHD manifestations in mice were associated with high BAFF/B-cell ratios and persistence of B-cell receptor (BCR)-activated B cells in peripheral blood and lesional tissue. By employing BAFF transgenic (Tg) mice donor cells, we addressed whether high BAFF contributed to BCR activation in cGVHD. BAFF increased NOTCH2 expression on B cells, augmenting BCR responsiveness to surrogate antigen and NOTCH ligand. BAFF Tg B cells had significantly increased protein levels of the proximal BCR signaling molecule SYK, and high SYK protein was maintained by BAFF after in vitro BCR activation or when alloantigen was present in vivo. Using T cell-depleted (BM only) BAFF Tg donors, we found that BAFF promoted cGVHD manifestations, circulating GL7+ B cells, and alloantibody production. We demonstrate that pathologic production of BAFF promotes an altered B-cell compartment and augments BCR responsiveness. Our findings compel studies of therapeutic targeting of BAFF and BCR pathways in patients with cGVHD.

B cell tolerance and antibody production to the celiac disease autoantigen transglutaminase 2

Autoantibodies to transglutaminase 2 (TG2) are hallmarks of celiac disease. To address B cell tolerance and autoantibody formation to TG2, we generated immunoglobulin knock-in (Ig KI) mice that express a prototypical celiac patient-derived anti-TG2 B cell receptor equally reactive to human and mouse TG2. We studied B cell development in the presence/absence of autoantigen by crossing the Ig KI mice to Tgm2-/- mice. Autoreactive B cells in Tgm2+/+ mice were indistinguishable from their naive counterparts in Tgm2-/- mice with no signs of clonal deletion, receptor editing, or B cell anergy. The autoreactive B cells appeared ignorant to their antigen, and they produced autoantibodies when provided T cell help. The findings lend credence to a model of celiac disease where gluten-reactive T cells provide help to autoreactive TG2-specific B cells by involvement of gluten-TG2 complexes, and they outline a general mechanism of autoimmunity with autoantibodies being produced by ignorant B cells on provision of T cell help.

Genetic Loss of LCK Kinase Leads to Acceleration of Chronic Lymphocytic Leukemia

Most patients with chronic lymphocytic leukemia (CLL) exhibit an indolent disease course and unresponsive B cell receptors (BCRs) exemplified by an anergic phenotype of their leukemic cells. In up to 5% of patients, CLL transforms from an indolent subtype to an aggressive form of B cell lymphoma (Richter's syndrome), which is associated with worse disease outcome and severe downregulation of NFAT2. Here we show that ablation of the tyrosine kinase LCK, which has previously been characterized as a main NFAT2 target gene in CLL, leads to loss of the anergic phenotype, thereby restoring BCR signaling, which results in an acceleration of CLL. Our study identifies LCK as a main player in mediating BCR unresponsiveness and its role as a crucial regulator of anergy in CLL.

Endotypes in T1D: B lymphocytes and early onset

PURPOSE OF REVIEW

Although type 1 diabetes (T1D) is characterized by destruction of the pancreatic beta cells by self-reactive T cells, it has become increasingly evident that B cells also play a major role in disease development, likely functioning as antigen-presenting cells. Here we review the biology of islet antigen-reactive B cells and their participation in autoimmune diabetes.

RECENT FINDINGS

Relative to late onset, individuals who develop T1D at an early age display increased accumulation of insulin-reactive B cells in islets. This B-cell signature is also associated with rapid progression of disease and responsiveness to B-cell depletion therapy. Also suggestive of B-cell participation in disease is loss of anergy in high-affinity insulin-reactive B cells. Importantly, loss of anergy is seen in patient's healthy first-degree relatives carrying certain T1D risk alleles, suggesting a role early in disease development.

SUMMARY

Recent studies indicate that islet-reactive B cells may play a pathogenic role very early in T1D development in young patients, and suggest utility of therapies that target these cells.

Impaired B cell anergy is not sufficient to breach tolerance to nuclear antigen in Vκ8/3H9 lupus-prone mice

Background

Systemic lupus erythematosus (SLE) is a severe autoimmune disease in which immune tolerance defects drive production of pathogenic anti-nuclear autoantibodies. Anergic B cells are considered a potential source of these autoantibodies due to their autoreactivity and overrepresentation in SLE patients. Studies of lupus-prone mice have shown that genetic defects mediating autoimmunity can breach B cell anergy, but how this breach occurs with regards to endogenous nuclear antigen remains unclear. We investigated whether B and T cell defects in congenic mice (c1) derived from the lupus-prone New Zealand Black strain can breach tolerance to nuclear self-antigen in the presence of knock-in genes (Vκ8/3H9; dKI) that generate a ssDNA-reactive, anergic B cell population.

Methods

Flow cytometry was used to assess splenic B and T cells from 8-month-old c1 dKI mice and serum autoantibodies were measured by ELISA. dKI B cells stimulated in vitro with anti-IgM were assessed for proliferation and activation by examining CFSE decay and CD86. Cytokine-producing T cells were identified by flow cytometry following culture of dKI splenocytes with PMA and ionomycin. dKI B cells from 6-8-week-old mice were adoptively transferred into 4-month-old wild type recipients and assessed after 7 days via flow cytometry and immunofluorescence microscopy.

Results

c1 dKI mice exhibited B cell proliferation indicative of impaired anergy, but had attenuated autoantibodies and germinal centres compared to wild type littermates. This attenuation appeared to stem from a decrease in PD-1^hi T helper cells in the dKI strains, as c1 dKI B cells were recruited to germinal centres when adoptively transferred into c1 wild type mice.

Conclusion

Anergic, DNA-specific autoreactive B cells only seem to drive profound autoimmunity in the presence of concomitant defects in the T cell subsets that support high-affinity plasma cell production.

Histone H2A-Reactive B Cells Are Functionally Anergic in Healthy Mice With Potential to Provide Humoral Protection Against HIV-1

Peripheral tolerance is essential for silencing weakly autoreactive B cells that have escaped central tolerance, but it is unclear why these potentially pathogenic B cells are retained rather than being eliminated entirely. Release from peripheral tolerance restraint can occur under certain circumstances (i.e., strong TLR stimulus), that are present during infection. In this regard, we hypothesized that autoreactive B cells could function as a reserve population that can be activated to contribute to the humoral immune response, particularly with pathogens, such as HIV-1, that exploit immune tolerance to avoid host defense. In this study, we identify a population of autoreactive B cells with the potential to neutralize HIV-1 and experimentally release them from the functional restrictions of peripheral tolerance. We have previously identified murine monoclonal antibodies that displayed autoreactivity against histone H2A and neutralized HIV-1 in vitro. Here, we identify additional H2A-reactive IgM monoclonal antibodies and demonstrate that they are both autoreactive and polyreactive with self and foreign antigens and are able to neutralize multiple clades of tier 2 HIV-1. Flow cytometric analysis of H2A-reactive B cells in naïve wildtype mice revealed that these B cells are present in peripheral B cell populations and we further document that murine H2A-reactive B cells are restrained by peripheral tolerance mechanisms. Specifically, we show endogenous H2A-reactive B cells display increased expression of the inhibitory mediators CD5 and phosphatase and tensin homolog (PTEN) phosphatase and fail to mobilize calcium upon immunoreceptor stimulation; all characterized markers of anergy. Moreover, we show that toll-like receptor stimulation or provision of CD4 T cell help induces the in vitro production of H2A-reactive antibodies, breaking tolerance. Thus, we have identified a novel poly/autoreactive B cell population that has the potential to neutralize HIV-1 but is silenced by immune tolerance.

Tracing Self-Reactive B Cells in Normal Mice

BCR transgenic mice dominate studies of B cell tolerance; consequently, tolerance in normal mice expressing diverse sets of autoreactive B cells is poorly characterized. We have used single B cell cultures to trace self-reactivity in BCR repertoires across the first and second tolerance checkpoints and in tolerized B cell compartments of normal mice. This approach reveals affinity "setpoints" that define each checkpoint and a subset of tolerized, autoreactive B cells that is long-lived. In normal mice, the numbers of B cells avidly specific for DNA fall significantly as small pre-B become immature and transitional-1 B cells, revealing the first tolerance checkpoint. By contrast, DNA reactivity does not significantly change when immature and transitional-1 B cells become mature follicular B cells, showing that the second checkpoint does not reduce DNA reactivity. In the spleen, autoreactivity was high in transitional-3 (T3) B cells, CD93⁺IgM^-/loIgD^hi anergic B cells, and a CD93^- anergic subset. Whereas splenic T3 and CD93⁺ anergic B cells are short-lived, CD93^-IgM^-/loIgD^hi B cells have half-lives comparable to mature follicular B cells. B cell-specific deletion of proapoptotic genes, Bak and Bax, resulted in increased CD93^-IgM^-/loIgD^hi B cell numbers but not T3 B cell numbers, suggesting that apoptosis regulates differently persistent and ephemeral autoreactive B cells. The self-reactivity and longevity of CD93^-IgM^-/loIgD^hi B cells and their capacity to proliferate and differentiate into plasmacytes in response to CD40 activation in vitro lead us to propose that this persistent, self-reactive compartment may be the origin of systemic autoimmunity and a potential target for vaccines to elicit protective Abs cross-reactive with self-antigens.

Factors That Govern the Induction of Long-Lived Antibody Responses

The induction of long-lasting, high-titer antibody responses is critical to the efficacy of many vaccines. The ability to produce durable antibody responses is governed by the generation of the terminally differentiated antibody-secreting B cells known as long-lived plasma cells (LLPCs). Once induced, LLPCs likely persist for decades, providing long-term protection against infection. The factors that control the generation of this important class of B cells are beginning to emerge. In particular, antigens with highly dense, multivalent structures are especially effective. Here we describe some pathogens for which the induction of long-lived antibodies is particularly important, and discuss the basis for the extraordinary ability of multivalent antigens to drive differentiation of naïve B cells to LLPCs.

mTORC1 as a cell-intrinsic rheostat that shapes development, preimmune repertoire, and function of B lymphocytes

Ample evidence indicates that nutrient concentrations in extracellular milieux affect signaling mediated by environmental sensor proteins. For instance, the mechanistic target of rapamycin (mTOR) is reduced during protein malnutrition and is known to be modulated by concentrations of several amino acids when in a multiprotein signaling complex that contains regulatory-associated protein of mTOR. We hypothesized that a partial decrease in mTOR complex 1 (mTORC1) activity intrinsic to B-lineage cells would perturb lymphocyte development or function, or both. We show that a cell-intrinsic decrease in mTORC1 activity impacted developmental progression, antigen receptor repertoire, and function along the B lineage. Thus, preimmune repertoires of B-lineage cells were altered in the marrow and periphery in a genetic model of regulatory-associated protein of mTOR haplo-insufficiency. An additional role for mTORC1 was revealed when a B-cell antigen receptor transgene was found to circumvent the abnormal B-cell development: haploinsufficient B cells were profoundly impaired in responses to antigen in vivo. Collectively, our findings indicate that mTORC1 serves as a rheostat that shapes differentiation along the B lineage, the preimmune repertoire, and antigen-driven selection of mature B cells. The findings also reveal a range in the impact of this nutrient sensor on activity-response relationships for distinct endpoints.-Raybuck, A. L., Lee, K., Cho, S. H., Li, J., Thomas, J. W., Boothby, M. R. mTORC1 as a cell-intrinsic rheostat that shapes development, preimmune repertoire, and function of B lymphocytes.

Understanding and measuring human B-cell tolerance and its breakdown in autoimmune disease

The maintenance of immunological tolerance of B lymphocytes is a complex and critical process that must be implemented as to avoid the detrimental development of autoreactivity and possible autoimmunity. Murine models have been invaluable to elucidate many of the key components in B-cell tolerance; however, translation to human homeostatic and pathogenic immune states can be difficult to assess. Functional autoreactive, flow cytometric, and single-cell cloning assays have proven to be critical in deciphering breaks in B-cell tolerance within autoimmunity; however, newer approaches to assess human B-cell tolerance may prove to be vital in the further exploration of underlying tolerance defects. In this review, we supply a comprehensive overview of human immune tolerance checkpoints with associated mechanisms of enforcement, and highlight current and future methodologies which are likely to benefit future studies into the mechanisms that become defective in human autoimmune conditions.

Self-reactivity on a spectrum: A sliding scale of peripheral B cell tolerance

Efficient mechanisms of central tolerance, including receptor editing and deletion, prevent highly self-reactive B cell receptors (BCRs) from populating the periphery. Despite this, modest self-reactivity persists in (and may even be actively selected into) the mature B cell repertoire. In this review, we discuss new insights into mechanisms of peripheral B cell tolerance that restrain mature B cells from mounting inappropriate responses to endogenous antigens, and place recent work into historical context. In particular, we discuss new findings that have arisen from application of a novel in vivo reporter of BCR signaling, Nur77-eGFP, expression of which scales with the degree of self-reactivity in both monoclonal and polyclonal B cell repertoires. We discuss new and historical evidence that self-reactivity is not just tolerated, but actively selected into the peripheral repertoire. We review recent progress in understanding how dual expression of the IgM and IgD BCR isotypes on mature naive follicular B cells tunes responsiveness to endogenous antigen recognition, and discuss how this may be integrated with other features of clonal anergy. Finally, we discuss how expression of Nur77 itself couples chronic antigen stimulation with B cell tolerance.

Challenges and Opportunities for Consistent Classification of Human B Cell and Plasma Cell Populations

The increasingly recognized role of different types of B cells and plasma cells in protective and pathogenic immune responses combined with technological advances have generated a plethora of information regarding the heterogeneity of this human immune compartment. Unfortunately, the lack of a consistent classification of human B cells also creates significant imprecision on the adjudication of different phenotypes to well-defined populations. Additional confusion in the field stems from: the use of non-discriminatory, overlapping markers to define some populations, the extrapolation of mouse concepts to humans, and the assignation of functional significance to populations often defined by insufficient surface markers. In this review, we shall discuss the current understanding of human B cell heterogeneity and define major parental populations and associated subsets while discussing their functional significance. We shall also identify current challenges and opportunities. It stands to reason that a unified approach will not only permit comparison of separate studies but also improve our ability to define deviations from normative values and to create a clean framework for the identification, functional significance, and disease association with new populations.