Identification of Anergic B Cells within a Wild-Type Repertoire

Detection and activation of HIV broadly neutralizing antibody precursor B cells using anti-idiotypes

Many tested vaccines fail to provide protection against disease despite the induction of antibodies that bind the pathogen of interest. In light of this, there is much interest in rationally designed subunit vaccines that direct the antibody response to protective epitopes. Here, we produced a panel of anti-idiotype antibodies able to specifically recognize the inferred germline version of the human immunodeficiency virus 1 (HIV-1) broadly neutralizing antibody b12 (iglb12). We determined the crystal structure of two anti-idiotypes in complex with iglb12 and used these anti-idiotypes to identify rare naive human B cells expressing B cell receptors with similarity to iglb12. Immunization with a multimerized version of this anti-idiotype induced the proliferation of transgenic murine B cells expressing the iglb12 heavy chain in vivo, despite the presence of deletion and anergy within this population. Together, our data indicate that anti-idiotypes are a valuable tool for the study and induction of potentially protective antibodies.

The E3 ubiquitin ligase Itch restricts antigen-driven B cell responses

The E3 ubiquitin ligase Itch regulates antibody levels and prevents autoimmune disease in humans and mice, yet how Itch regulates B cell fate or function is unknown. We now show that Itch directly limits B cell activity. While Itch-deficient mice displayed normal numbers of preimmune B cell populations, they showed elevated numbers of antigen-experienced B cells. Mixed bone marrow chimeras revealed that Itch acts within B cells to limit naive and, to a greater extent, germinal center (GC) B cell numbers. B cells lacking Itch exhibited increased proliferation, glycolytic capacity, and mTORC1 activation. Moreover, stimulation of these cells in vivo by WT T cells resulted in elevated numbers of GC B cells, PCs, and serum IgG. These results support a novel role for Itch in limiting B cell metabolism and proliferation to suppress antigen-driven B cell responses.

Phenotypic and functional alterations of regulatory B cell subsets in adult allergic asthma patients

BACKGROUND

IL-10-producing regulatory B cells (Bregs) are widely ascribed immune regulatory functions. However, Breg subsets in human asthma have not been fully investigated.

OBJECTIVE

We studied Breg subsets in adult allergic asthma patients by assessing two major parameters, frequency and IL-10 expression. We then investigated factors that affect these two parameters in patients.

METHODS

Peripheral blood mononuclear cells (PBMCs) of adult allergic asthma patients (N = 26) and non-asthmatic controls (N = 28) were used to assess the frequency of five subsets of transitional B cells (TBs), three subsets of CD24^high CD27⁺ B cells and B1 cells. In addition to clinical data, IL-10 expression by individual Breg subsets was assessed by flow cytometry.

RESULTS

Asthma patients had decreases of CD5⁺ and CD1d⁺ CD5⁺ , but an increase of CD27⁺ TBs which was significant in patients with moderate asthma (60 < FEV1 < 80). Regardless of asthma severity, there was no significant alteration in the frequencies of 6 other Breg subsets tested. However, we found that oral corticosteroid (OCS) significantly affected the frequency of Bregs in Breg subset-specific manners. OCS decreased CD5⁺ and CD1d⁺ CD5⁺ TBs, but increased CD27⁺ TBs and CD10⁺ CD24^high CD27⁺ cells. Furthermore, OCS decreased IL-10 expression by CD27⁺ TBs, all 3 CD24^high CD27⁺ B cell subsets (CD5⁺ , CD10⁺ and CD1d⁺ ) and B1 cells. OCS-mediated inhibition of IL-10 expression was not observed in the other Breg subsets tested.

CONCLUSION & CLINICAL RELEVANCE

Alterations in the frequency of Bregs and their ability to express IL-10 are Breg subset-specific. OCS treatment significantly affects the frequency as well as their ability to express IL-10 in Breg subset-specific manners.

Linked CD4 T Cell Help: Broadening Immune Attack Against Cancer by Vaccination

In the last decade, immunotherapy with monoclonal antibodies targeting immunological check points has become a breakthrough therapeutic modality for solid cancers. However, only up to 50 % of patients benefit from this powerful approach. For others vaccination might provide a plausible addition or alternative. For induction of effective anticancer immunity CD4+ T cell help is required, which is often difficult to induce to self cancer targets because of tolerogenic mechanisms. Our approach for cancer vaccines has been to incorporate into the vaccine design sequences able to activate foreign T cell help, through genetically linking cancer targets to microbial sequences (King et al. in Nat Med 4(11):1281-1286, 1998; Savelyeva et al. in Nat Biotechnol 19(8):760-764, 2001). This harnesses the non-tolerized CD4 T cell repertoire available in patients to help induction of effective immunity against fused cancer antigens. Multiple immune effector mechanisms including antibody, CD8+ T cells as well as CD4 effector T cells can be activated using this strategy. Delivery via DNA vaccines has already indicated clinical efficacy. The same principle of linked T cell help has now been transferred to other novel vaccine modalities to further potentiate immunity against cancer targets.

Ibrutinib Therapy Releases Leukemic Surface IgM from Antigen Drive in Chronic Lymphocytic Leukemia Patients

PURPOSE

In chronic lymphocytic leukemia (CLL), disease progression associates with surface IgM (sIgM) levels and signaling capacity. These are variably downmodulated in vivo and recover in vitro, suggesting a reversible influence of tissue-located antigen. Therapeutic targeting of sIgM function via ibrutinib, an inhibitor of Bruton tyrosine kinase (BTK), causes inhibition and tumor cell redistribution into the blood, with significant clinical benefit. Circulating CLL cells persist in an inhibited state, offering a tool to investigate the effects of drug on BTK-inhibited sIgM.

EXPERIMENTAL DESIGN

We investigated the consequences of ibrutinib therapy on levels and function of sIgM in circulating leukemic cells of patients with CLL.

RESULTS

At week 1, there was a significant increase of sIgM expression (64% increase from pretherapy) on CLL cells either recently released from tissue or persisting in blood. In contrast, surface IgD (sIgD) and a range of other receptors did not change. SIgM levels remained higher than pretherapy in the following 3 months despite gradual cell size reduction and ongoing autophagy and apoptotic activity. Conversely, IgD and other receptors did not increase and gradually declined. Recovered sIgM was fully N-glycosylated, another feature of escape from antigen, and expression did not increase further during culture in vitro. The sIgM was fully capable of mediating phosphorylation of SYK, which lies upstream of BTK in the B-cell receptor pathway.

CONCLUSIONS

This specific IgM increase in patients underpins the key role of tissue-based engagement with antigen in CLL, confirms the inhibitory action of ibrutinib, and reveals dynamic adaptability of CLL cells to precision monotherapy.See related commentary by Burger, p. 2372.

Nur77 Links Chronic Antigen Stimulation to B Cell Tolerance by Restricting the Survival of Self-Reactive B Cells in the Periphery

Nur77 (Nr4a1) belongs to a small family of orphan nuclear receptors that are rapidly induced by BCR stimulation, yet little is known about its function in B cells. We have previously characterized a reporter of Nr4a1 transcription, Nur77-eGFP, in which GFP expression faithfully detects Ag encounter by B cells in vitro and in vivo. In this study, we report that Nur77 expression correlates with the degree of self-reactivity, counterselection, and anergy among individual B cell clones from two distinct BCR transgenic mouse models but is dispensable for all of these tolerance mechanisms. However, we identify a role for Nur77 in restraining survival of self-reactive B cells in the periphery under conditions of competition for a limited supply of the survival factor BAFF. We find that Nur77 deficiency results in the progressive accumulation of self-reactive B cells in the mature repertoire with age and is sufficient to break B cell tolerance in V_H3H9 H chain transgenic mice. We thus propose that Nur77 is upregulated in self-reactive B cells in response to chronic Ag stimulation and selectively restricts the survival of these cells, gradually pruning self-reactivity from the mature repertoire to impose a novel layer of peripheral B cell tolerance.

Non-Antibody-Secreting Functions of B Cells and Their Contribution to Autoimmune Disease

B cells play multiple important roles in the pathophysiology of autoimmune disease. Beyond producing pathogenic autoantibodies, B cells can act as antigen-presenting cells and producers of cytokines, including both proinflammatory and anti-inflammatory cytokines. Here we review our current understanding of the non-antibody-secreting roles that B cells may play during development of autoimmunity, as learned primarily from reductionist preclinical models. Attention is also given to concepts emerging from clinical studies using B cell depletion therapy, which shed light on the roles of these mechanisms in human autoimmune disease.

Elevated PTEN expression maintains anergy in human B cells and reveals unexpectedly high repertoire autoreactivity

It has been reported that 2.5%-30% of human peripheral CD27- B cells are autoreactive and anergic based on unresponsiveness to antigen receptor (BCR) stimulation and autoreactivity of cloned and expressed BCR. The molecular mechanisms that maintain this unresponsiveness are unknown. Here, we showed that in humans anergy is maintained by elevated expression of PTEN, a phosphatidylinositol 3,4,5P-3-phosphatase. Upregulation of PTEN was associated with reduced expression of microRNAs that control its expression. Pharmacologic inhibition of PTEN lead to significant restoration of responsiveness. Consistent with a role in conferring risk of autoimmunity, B cells from type 1 diabetics and autoimmune thyroid disease patients expressed reduced PTEN. Unexpectedly, in healthy individuals PTEN expression was elevated in on average 40% of CD27- B cells, with levels gradually decreasing as IgM levels increase. Our findings suggest that a much higher proportion of the peripheral repertoire is autoreactive than previously thought and that B cells upregulate PTEN in a manner that is proportional to the recognition of autoantigens of increasing avidity, thus tuning BCR signaling to prevent development of autoimmunity while providing a reservoir of cells that can be readily activated to respond when needed.

Clonal anergy of CD117+chB6+ B cell progenitors induced by avian leukosis virus subgroup J is associated with immunological tolerance

BACKGROUND

The pathogenesis of immunological tolerance caused by avian leukosis virus subgroup J (ALV-J), an oncogenic retrovirus, is largely unknown.

RESULTS

In this study, the development, differentiation, and immunological capability of B cells and their progenitors infected with ALV-J were studied both morphologically and functionally by using a model of ALV-J congenital infection. Compared with posthatch infection, congenital infection of ALV-J resulted in severe immunological tolerance, which was identified as the absence of detectable specific antivirus antibodies. In congenitally infected chickens, immune organs, particularly the bursa of Fabricius, were poorly developed. Moreover, IgM-and IgG-positive cells and total immunoglobulin levels were significantly decreased in these chickens. Large numbers of bursa follicles with no differentiation into cortex and medulla indicated that B cell development was arrested at the early stage. Flow cytometry analysis further confirmed that ALV-J blocked the differentiation of CD117+chB6+ B cell progenitors in the bursa of Fabricius. Furthermore, both the humoral immunity and the immunological capability of B cells and their progenitors were significantly suppressed, as assessed by (a) the antibody titres against sheep red blood cells and the Marek's disease virus attenuated serotype 1 vaccine; (b) the proliferative response of B cells against thymus-independent antigen lipopolysaccharide (LPS) in the spleen germinal centres; and (c) the capacities for proliferation, differentiation and immunoglobulin gene class-switch recombination of B cell progenitors in response to LPS and interleukin-4(IL-4) in vitro.

CONCLUSIONS

These findings suggested that the anergy of B cells in congenitally infected chickens is caused by the developmental arrest and dysfunction of B cell progenitors, which is an important factor for the immunological tolerance induced by ALV-J.

Silencing of high-affinity insulin-reactive B lymphocytes by anergy and impact of the NOD genetic background in mice

AIMS/HYPOTHESIS

Previous studies have demonstrated that high-affinity insulin-binding B cells (IBCs) silenced by anergy in healthy humans lose their anergy in islet autoantibody-positive individuals with recent-onset type 1 diabetes, and in autoantibody-negative first-degree relatives carrying certain risk alleles. Here we explore the hypothesis that IBCs are found in the immune periphery of disease-resistant C57BL/6-H2g7 mice, where, as in healthy humans, they are anergic, but that in disease-prone genetic backgrounds (NOD) they become activated and migrate to the pancreas and pancreatic lymph nodes, where they participate in the development of type 1 diabetes.

METHODS

We compared the status of high-affinity IBCs in disease-resistant VH125.C57BL/6-H2g7 and disease-prone VH125.NOD mice.

RESULTS

Consistent with findings in healthy humans, high-affinity IBCs reach the periphery in disease-resistant mice and are anergic, as indicated by a reduced expression of membrane IgM, unresponsiveness to antigen and failure to become activated or accumulate in the pancreatic lymph nodes or pancreas. In NOD mice, high-affinity IBCs reach the periphery early in life and increase in number prior to the onset of hyperglycaemia. These cells are not anergic; they become activated, produce autoantibodies and accumulate in the pancreas and pancreatic lymph nodes prior to disease development.

CONCLUSIONS/INTERPRETATION

These findings are consistent with genetic determination of the escape of high-affinity IBCs from anergy and their early contribution to the development of type 1 diabetes.

Control of autoreactive B cells by IgM and IgD B cell receptors: maintaining a fine balance

A substantial fraction of mature naïve B cells recognize endogenous antigens, and this autoreactivity must be controlled to prevent autoantibody secretion. Selective downregulation of the IgM BCR on autoreactive B cells has long been appreciated, and recent findings illustrate how this might impose tolerance. The BCR isotype maintained on autoreactive B cells, IgD, is less sensitive to endogenous antigens than IgM. This reduced sensitivity may be conferred by structural properties of IgD and/or differential association with activating and inhibitory co-receptors. Once activated, autoreactive B cells are normally excluded from rapid plasma cell responses, but they can enter the germinal center and lose their autoreactivity through a mutation-selection process termed clonal redemption.

Systemic ST6Gal-1 Is a Pro-survival Factor for Murine Transitional B Cells

Humoral immunity depends on intrinsic B cell developmental programs guided by systemic signals that convey physiologic needs. Aberrant cues or their improper interpretation can lead to immune insufficiency or a failure of tolerance and autoimmunity. The means by which such systemic signals are conveyed remain poorly understood. Hence, further insight is essential to understanding and treating autoimmune diseases and to the development of improved vaccines. ST6Gal-1 is a sialyltransferase that constructs the α2,6-sialyl linkage on cell surface and extracellular glycans. The requirement for functional ST6Gal-1 in the development of humoral immunity is well documented. Canonically, ST6Gal-1 resides within the intracellular ER-Golgi secretory apparatus and participates in cell-autonomous glycosylation. However, a significant pool of extracellular ST6Gal-1 exists in circulation. Here, we segregate the contributions of B cell intrinsic and extrinsic ST6Gal-1 to B cell development. We observed that B cell-intrinsic ST6Gal-1 is required for marginal zone B cell development, while B cell non-autonomous ST6Gal-1 modulates B cell development and survival at the early transitional stages of the marrow and spleen. Exposure to extracellular ST6Gal-1 ex vivo enhanced the formation of IgM-high B cells from immature precursors, and increased CD23 and IgM expression. Extrinsic sialylation by extracellular ST6Gal-1 augmented BAFF-mediated activation of the non-canonical NF-kB, p38 MAPK, and PI3K/AKT pathways, and accelerated tyrosine phosphorylation after B cell receptor stimulation. in vivo, systemic ST6Gal-1 did not influence homing of B cells to the spleen but was critical for their long-term survival and systemic IgG levels. Circulatory ST6Gal-1 levels respond to inflammation, infection, and malignancy in mammals, including humans. In turn, we have shown previously that systemic ST6Gal-1 regulates inflammatory cell production by modifying bone marrow myeloid progenitors. Our data here point to an additional role of systemic ST6Gal-1 in guiding B cell development, which supports the concept that circulating ST6Gal-1 is a conveyor of systemic cues to guide the development of multiple branches of immune cells.

Soluble antigen arrays disarm antigen-specific B cells to promote lasting immune tolerance in experimental autoimmune encephalomyelitis

Autoreactive lymphocytes that escape central immune tolerance may be silenced via an endogenous peripheral tolerance mechanism known as anergy. Antigen-specific therapies capable of inducing anergy may restore patients with autoimmune diseases to a healthy phenotype while avoiding deleterious side effects associated with global immunosuppression. Inducing anergy in B cells may be a particularly potent intervention, as B cells can contribute to autoimmune diseases through multiple mechanisms and offer the potential for direct antigen-specific targeting through the B cell receptor (BCR). Our previous results suggested autoreactive B cells may be silenced by multivalent 'soluble antigen arrays' (SAgAs), which are polymer conjugates displaying multiple copies of autoantigen with or without a secondary peptide that blocks intracellular cell-adhesion molecule-1 (ICAM-1). Here, key therapeutic molecular properties of SAgAs were identified and linked to the immunological mechanism through comprehensive cellular and in vivo analyses. We determined non-hydrolyzable 'cSAgAs' displaying multivalent 'click'-conjugated antigen more potently suppressed experimental autoimmune encephalomyelitis (EAE) compared to hydrolyzable SAgAs capable of releasing conjugated antigen. cSAgAs restored a healthy phenotype in disease-specific antigen presenting cells (APCs) by inducing an anergic response in B cells and a subset of B cells called autoimmune-associated B cells (ABCs) that act as potent APCs in autoimmune disease. Accompanied by a cytokine response skewed towards a Th2/regulatory phenotype, this generated an environment of autoantigenic tolerance. By identifying key therapeutic molecular properties and an immunological mechanism that drives SAgA efficacy, this work guides the design of antigen-specific immunotherapies capable of inducing anergy.

Conditional Selection of B Cells in Mice With an Inducible B Cell Development

Developing B cells undergo defined maturation steps in the bone marrow and in the spleen. The timing and the factors that control these differentiation steps are not fully understood. By targeting the B cell-restricted mb-1 locus to generate an mb-1 allele that expresses a tamoxifen inducible Cre and another allele in which mb-1 expression can be controlled by Cre, we have established a mouse model with an inducible B cell compartment. With these mice, we studied in detail the kinetics of B cell development and the consequence of BCR activation at a defined B cell maturation stage. Contrary to expectations, transitional 1-B cells exposed to anti-IgM reagents in vivo did not die but instead developed into transitional 2 (T2)-B cells with upregulated Bcl-2 expression. We show, however, that these T2-B cells had an increased dependency on the B cell survival factor B cell activating factor when compared to non-stimulated B cells. Overall, our findings indicate that the inducible mb-1 mouse strain represents a useful model, which allows studying the signals that control the selection of B cells in greater detail.

Foxo3 Promotes Apoptosis of B Cell Receptor-Stimulated Immature B Cells, Thus Limiting the Window for Receptor Editing

Central tolerance checkpoints are critical for the elimination of autoreactive B cells and the prevention of autoimmunity. When autoreactive B cells encounter their Ag at the immature B cell stage, BCR cross-linking induces receptor editing, followed by apoptosis if edited cells remain autoreactive. Although the transcription factor Foxo1 is known to promote receptor editing, the role of the related factor Foxo3 in central B cell tolerance is poorly understood. We find that BCR-stimulated immature B cells from Foxo3-deficient mice demonstrate reduced apoptosis compared with wild type cells. Despite this, Foxo3^-/- mice do not develop increased autoantibodies. This suggests that the increased survival of Foxo3^-/- immature B cells allows additional rounds of receptor editing, resulting in more cells "redeeming" themselves by becoming nonautoreactive. Indeed, increased Igλ usage and increased recombining sequence recombination among Igλ-expressing cells were observed in Foxo3^-/- mice, indicative of increased receptor editing. We also observed that deletion of high-affinity autoreactive cells was intact in the absence of Foxo3 in the anti-hen egg lysozyme (HEL)/membrane-bound HEL model. However, Foxo3 levels in B cells from systemic lupus erythematosus (SLE) patients were inversely correlated with disease activity and reduced in patients with elevated anti-dsDNA Abs. Although this is likely due in part to increased B cell activation in these SLE patients, it is also possible that low-affinity B cells that remain autoreactive after editing may survive inappropriately in the absence of Foxo3 and become activated to secrete autoantibodies in the context of other SLE-associated defects.

Anti-HIV-1 B cell responses are dependent on B cell precursor frequency and antigen-binding affinity

The discovery that humans can produce potent broadly neutralizing antibodies (bNAbs) to several different epitopes on the HIV-1 spike has reinvigorated efforts to develop an antibody-based HIV-1 vaccine. Antibody cloning from single cells revealed that nearly all bNAbs show unusual features that could help explain why it has not been possible to elicit them by traditional vaccination and instead would require a sequence of different immunogens. This idea is supported by experiments with genetically modified immunoglobulin (Ig) knock-in mice. Sequential immunization with a series of specifically designed immunogens was required to shepherd the development of bNAbs. However, knock-in mice contain superphysiologic numbers of bNAb precursor-expressing B cells, and therefore how these results can be translated to a more physiologic setting remains to be determined. Here we make use of adoptive transfer experiments using knock-in B cells that carry a synthetic intermediate in the pathway to anti-HIV-1 bNAb development to examine how the relationship between B cell receptor affinity and precursor frequency affects germinal center (GC) B cell recruitment and clonal expansion. Immunization with soluble HIV-1 antigens can recruit bNAb precursor B cells to the GC when there are as few as 10 such cells per mouse. However, at low precursor frequencies, the extent of clonal expansion is directly proportional to the affinity of the antigen for the B cell receptor, and recruitment to GCs is variable and dependent on recirculation.

Putting on the Brakes: Regulatory Kinases and Phosphatases Maintaining B Cell Anergy

B cell antigen receptor (BCR) signaling is a tightly regulated process governed by both positive and negative mediators/regulators to ensure appropriate responses to exogenous and autologous antigens. Upon naïve B cell recognition of antigen CD79 [the immunoreceptor tyrosine-based activation motif (ITAM)-containing signaling subunit of the BCR] is phosphorylated and recruits Src and Syk family kinases that then phosphorylate proximal intermediaries linked to downstream activating signaling circuitry. This plasma membrane localized signalosome activates PI3K leading to generation of PIP3 critical for membrane localization and activation of plecktrin homology domain-containing effectors. Conversely, in anergic B cells, chronic antigen stimulation drives biased monophosphorylation of CD79 ITAMs leading to recruitment of Lyn, but not Syk, which docks only to bi-phosphorylated ITAMS. In this context, Lyn appears to function primarily as a driver of inhibitory signaling pathways promoting the inhibition of the PI3K pathway by inositol phosphatases, SHIP-1 and PTEN, which hydrolyze PIP3 to PIP2. Lyn may also exert negative regulation of signaling through recruitment of SHP-1, a tyrosine phosphatase that dephosphorylates activating signaling molecules. Alleles of genes that encode or regulate expression of components of this axis, including SHIP-1, SHP-1, Csk/PTPn22, and Lyn, have been shown to confer risk of autoimmunity. This review will discuss functional interplay of components of this pathway and the impact of risk alleles on its function.

Bruton's Tyrosine Kinase Is Not Essential for B Cell Survival beyond Early Developmental Stages

Bruton's tyrosine kinase (Btk) is a crucial regulator of B cell signaling and is a therapeutic target for lymphoma and autoimmune disease. BTK-deficient patients suffer from humoral immunodeficiency, as their B cells fail to progress beyond the bone marrow. However, the role of Btk in fully developed, mature peripheral B cells is not well understood. Analysis using BTK inhibitors is complicated by suboptimal inhibition, off-target effects, or failure to eliminate BTK's adaptor function. Therefore a Btk^flox/Cre-ER^T2 mouse model was developed and used to excise Btk after B cell populations were established. Mice lacking Btk from birth are known to have reduced follicular (FO) compartments, with expanded transitional populations, suggesting a block in development. In adult Btk^flox/Cre-ER^T2 mice, Btk excision did not reduce FO B cells, which persisted for weeks. Autoimmune-prone B1 cells also survived conditional Btk excision, contrasting their near absence in global Btk-deficient mice. Therefore, Btk supports BCR signaling during selection into the FO and B1 compartments, but is not needed to maintain these cell populations. B1-related natural IgM levels remained normal, contrasting global Btk deficiency, but B cell proliferation and T-independent type II immunization responses were blunted. Thus, B cells have nuanced signaling responses that are differentially regulated by Btk for development, survival, and function. These findings raise the possibility that Btk may also be expendable for survival of mature human B cells, therefore requiring prolonged dosing to be effective, and that success of BTK inhibitors may depend in part on off-target effects.

Association of Defective Regulation of Autoreactive Interleukin-6-Producing Transitional B Lymphocytes With Disease in Patients With Systemic Sclerosis

OBJECTIVE

Systemic sclerosis (SSc) has the highest case-specific mortality of any rheumatic disease, and no effective therapy is available. A clear manifestation of SSc is the presence of autoantibodies. However, the origin of autoantibody-producing B lymphocytes, their mechanisms of activation and autoantibody production, and their role remain unclear. This study was undertaken to identify mechanisms that contribute to pathogenic B cell generation and involvement in SSc and to assess the altered distribution and function of B cells in SSc patients.

METHODS

Multicolor flow cytometry was performed to determine B cell subset distribution, cytokine production, and tolerance induction in SSc patients and healthy controls. Cytokine production following stimulation of the cells ex vivo was determined by multiplex assay.

RESULTS

A range of defects in B lymphocyte tolerance and cytokine production in SSc were noted. There was evidence of altered distribution of transitional B cell subsets, increased production of interleukin-6 (IL-6) and IL-8, and defective tolerance induction in SSc B cells. In addition, B cells from SSc patients had a reduced ability to produce IL-10 when stimulated through innate immune pathways. In contrast to healthy individuals, tolerance checkpoints in SSc patients failed to suppress the emergence of B cells that produce autoantibodies with specificity to the Scl-70 antigen, which is strongly associated with SSc. These defects were paralleled by altered intracellular signaling and apoptosis following B cell receptor engagement.

CONCLUSION

Our findings provide new insights into mechanisms underlying defective B lymphocyte responses in patients with SSc and their contribution to disease.

Igβ ubiquitination activates PI3K signals required for endosomal sorting

Veselits et al. show that Igβ ubiquitination activates PI3K and the accumulation of PIP₃ on BCR-associated endosomal membranes, which is necessary and sufficient for sorting into classical antigen-processing compartments. Surprisingly, proper BCR sorting is critical for endosomal TLR activation yet dispensable for T-dependent humoral immunity.

A wealth of in vitro data has demonstrated a central role for receptor ubiquitination in endocytic sorting. However, how receptor ubiquitination functions in vivo is poorly understood. Herein, we report that ablation of B cell antigen receptor ubiquitination in vivo uncouples the receptor from CD19 phosphorylation and phosphatidylinositol 3-kinase (PI3K) signals. These signals are necessary and sufficient for accumulating phosphatidylinositol (3,4,5)-trisphosphate (PIP₃) on B cell receptor–containing early endosomes and proper sorting into the MHC class II antigen-presenting compartment (MIIC). Surprisingly, MIIC targeting is dispensable for T cell–dependent immunity. Rather, it is critical for activating endosomal toll-like receptors and antiviral humoral immunity. These findings demonstrate a novel mechanism of receptor endosomal signaling required for specific peripheral immune responses.

Innate B-1 B Cells Are Not Enriched in Red Blood Cell Autoimmune Mice: Importance of B Cell Receptor Transgenic Selection

Autoimmune hemolytic anemia (AIHA) results from breakdown of humoral tolerance to RBC antigens. Past analyses of B-cell receptor transgenic (BCR-Tg) mice that recognize RBC autoantigens led to a paradigm in which autoreactive conventional B-2 B cells are deleted whereas extramedullary B-1 B cells escape deletion due to lack of exposure to RBCs. However, BCR-Tg mice utilized to shape the current paradigm were unable to undergo receptor editing or class-switching. Given the importance of receptor editing as mechanism to tolerize autoreactive B cells during central tolerance, we hypothesized that expansion of autoreactive B-1 B cells is a consequence of the inability of the autoreactive BCR to receptor edit. To test this hypothesis, we crossed two separate strains of BCR-Tg mice with transgenic mice expressing the BCR target on RBCs. Both BCR-Tg mice express the same immunoglobulin and, thus, secrete antibodies with identical specificity, but one strain (SwHEL) has normal receptor editing, whereas the other (IgHEL) does not. Similar to other AIHA models, the autoreactive IgHEL strain showed decreased B-2 B cells, an enrichment of B-1 B cells, and detectable anti-RBC autoantibodies and decreased RBC hematocrit and hemoglobin values. However, autoreactive SwHEL mice had induction of tolerance in both B-2 and B-1 B cells with anti-RBC autoantibody production without anemia. These data generate new understanding and challenge the existing paradigm of B cell tolerance to RBC autoantigens. Furthermore, these findings demonstrate that immune responses vary when BCR-Tg do not retain BCR editing and class-switching functions.

B cells in type 1 diabetes mellitus and diabetic kidney disease

Type 1 diabetes mellitus (T1DM) is an autoimmune disorder that affects an estimated 30 million people worldwide. It is characterized by the destruction of pancreatic β cells by the immune system, which leads to lifelong dependency on exogenous insulin and imposes an enormous burden on patients and health-care resources. T1DM is also associated with an increased risk of comorbidities, such as cardiovascular disease, retinopathy, and diabetic kidney disease (DKD), further contributing to the burden of this disease. Although T cells are largely considered to be responsible for β-cell destruction in T1DM, increasing evidence points towards a role for B cells in disease pathogenesis. B cell-depletion, for example, delays disease progression in patients with newly diagnosed T1DM. Loss of tolerance of islet antigen-reactive B cells occurs early in disease and numbers of pancreatic CD20⁺ B cells correlate with β-cell loss. Although the importance of B cells in T1DM is increasingly apparent, exactly how these cells contribute to disease and its comorbidities, such as DKD, is not well understood. Here we discuss the role of B cells in the pathogenesis of T1DM and how these cells are activated during disease development. Finally, we speculate on how B cells might contribute to the development of DKD.

Phenotyping of autoreactive B cells with labeled nucleosomes in 56R transgenic mice

The phenotypic characterization of self-reactive B cells producing autoantibodies is one of the challenges to get further insight in the physiopathology of autoimmune diseases. We took advantage of our previously developed flow cytometry method, using labeled nucleosomes, prominent autoantigens in systemic lupus erythematosus, to analyze the phenotype of self-reactive B cells in the anti-DNA B6.56R mouse model. We showed that splenic anti-nucleosome B cells express mostly kappa light chains and harbor a marginal zone phenotype. Moreover, these autoreactive B cells fail to acquire a germinal center phenotype and are less abundant in the transitional T3 compartment. In conclusion, the direct detection of autoreactive B cells helped determine their phenotypic characteristics and provided a more direct insight into the B cell tolerance process in B6.56R mice. This method constitutes an interesting new tool to study the mechanisms of B cell tolerance breakdown in B6.56R mice crossed with autoimmune prone models.

Cutting Edge: Endogenous IFN-β Regulates Survival and Development of Transitional B Cells

The transitional stage of B cell development is a formative stage in the spleen where autoreactive specificities are censored as B cells gain immune competence, but the intrinsic and extrinsic factors regulating survival of transitional stage 1 (T1) B cells are unknown. We report that B cell expression of IFN-β is required for optimal survival and TLR7 responses of transitional B cells in the spleen and was overexpressed in T1 B cells from BXD2 lupus-prone mice. Single-cell gene expression analysis of B6 Ifnb^+/+ versus B6 Ifnb^-⁄- T1 B cells revealed heterogeneous expression of Ifnb in wild-type B cells and distinct gene expression patterns associated with endogenous IFN-β. Single-cell analysis of BXD2 T1 B cells revealed that Ifnb is expressed in early T1 B cell development with subsequent upregulation of Tlr7 and Ifna1 Together, these data suggest that T1 B cell expression of IFN-β plays a key role in regulating responsiveness to external factors.

Immunological tolerance as a barrier to protective HIV humoral immunity

HIV-1 infection typically eludes antibody control by our immune system and is not yet prevented by a vaccine. While many viral features contribute to this immune evasion, broadly neutralizing antibodies (bnAbs) against HIV-1 are often autoreactive and it has been suggested that immunological tolerance may restrict a neutralizing antibody response. Indeed, recent Ig knockin mouse studies have shown that bnAb-expressing B cells are largely censored by central tolerance in the bone marrow. However, the contribution of peripheral tolerance in limiting the HIV antibody response by anergic and potentially protective B cells is poorly understood. Studies using mouse models to elucidate how anergic B cells are regulated and can be recruited into HIV-specific neutralizing antibody responses may provide insight into the development of a protective HIV-1 vaccine.

Breaching peripheral tolerance promotes the production of HIV-1-neutralizing antibodies

Schroeder et al. demonstrate that when peripheral tolerance is relaxed, tier 2 HIV-1–neutralizing antibodies can be elicited and identify new autoreactive antibody specificities against histone H2A capable of neutralizing tier 2 HIV-1.

A subset of characterized HIV-1 broadly neutralizing antibodies (bnAbs) are polyreactive with additional specificities for self-antigens and it has been proposed immunological tolerance may present a barrier to their participation in protective humoral immunity. We address this hypothesis by immunizing autoimmune-prone mice with HIV-1 Envelope (Env) and characterizing the primary antibody response for HIV-1 neutralization. We find autoimmune mice generate neutralizing antibody responses to tier 2 HIV-1 strains with alum treatment alone in the absence of Env. Importantly, experimentally breaching immunological tolerance in wild-type mice also leads to the production of tier 2 HIV-1–neutralizing antibodies, which increase in breadth and potency following Env immunization. In both genetically prone and experimentally induced mouse models of autoimmunity, increased serum levels of IgM anti-histone H2A autoantibodies significantly correlated with tier 2 HIV-1 neutralization, and anti-H2A antibody clones were found to neutralize HIV-1. These data demonstrate that breaching peripheral tolerance permits a cross-reactive HIV-1 autoantibody response able to neutralize HIV-1.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Immunogenicity of Isogenic IgG in Aggregates and Immune Complexes

A paradox in monoclonal antibody (mAb) therapy is that despite the well-documented tolerogenic properties of deaggregated IgG, most therapeutic IgG mAb induce anti-mAb responses. To analyze CD4 T cell reactions against IgG in various physical states, we developed an adoptive transfer model using CD4+ T cells specific for a Vκ region-derived peptide in the hapten-specific IgG mAb 36–71. We found that heat-aggregated or immune complexes (IC) of mAb 36–71 elicited anti-idiotypic (anti-Id) antibodies, while the deaggregated form was tolerogenic. All 3 forms of mAb 36–71 induced proliferation of cognate CD4+ T cells, but the aggregated and immune complex forms drove more division cycles and induced T follicular helper cells (T_FH) development more effectively than did the deaggregated form. These responses occurred despite no adjuvant and no or only trace levels of endotoxin in the preparations. Physical analyses revealed large differences in micron- and nanometer-sized particles between the aggregated and IC forms. These differences may be functionally relevant, as CD4+ T cell proliferation to aggregated, but not IC mAb 36–71, was nearly ablated upon peritoneal injection of B cell-depleting antibody. Our results imply that, in addition to denatured aggregates, immune complexes formed in vivo between therapeutic mAb and their intended targets can be immunogenic.

IgD attenuates the IgM-induced anergy response in transitional and mature B cells

Self-tolerance by clonal anergy of B cells is marked by an increase in IgD and decrease in IgM antigen receptor surface expression, yet the function of IgD on anergic cells is obscure. Here we define the RNA landscape of the in vivo anergy response, comprising 220 induced sequences including a core set of 97. Failure to co-express IgD with IgM decreases overall expression of receptors for self-antigen, but paradoxically increases the core anergy response, exemplified by increased Sdc1 encoding the cell surface marker syndecan-1. IgD expressed on its own is nevertheless competent to induce calcium signalling and the core anergy mRNA response. Syndecan-1 induction correlates with reduction of surface IgM and is exaggerated without surface IgD in many transitional and mature B cells. These results show that IgD attenuates the response to self-antigen in anergic cells and promotes their accumulation. In this way, IgD minimizes tolerance-induced holes in the pre-immune antibody repertoire.

Self-reactive B cells that are anergic express mainly IgD, yet the function of IgD is not clear. Here the authors analyse primary B cells from mice to show that IgD signalling attenuates self-antigen induced gene expression and promotes survival of anergic B cells that might go on to reactivate to foreign antigens and mutate away from self-reactivity.

Improving the efficacy and safety of biologic drugs with tolerogenic nanoparticles

The development of antidrug antibodies (ADAs) is a common cause for the failure of biotherapeutic treatments and adverse hypersensitivity reactions. Here we demonstrate that poly(lactic-co-glycolic acid) (PLGA) nanoparticles carrying rapamycin, but not free rapamycin, are capable of inducing durable immunological tolerance to co-administered proteins that is characterized by the induction of tolerogenic dendritic cells, an increase in regulatory T cells, a reduction in B cell activation and germinal centre formation, and the inhibition of antigen-specific hypersensitivity reactions. Intravenous co-administration of tolerogenic nanoparticles with pegylated uricase inhibited the formation of ADAs in mice and non-human primates and normalized serum uric acid levels in uricase-deficient mice. Similarly, the subcutaneous co-administration of nanoparticles with adalimumab resulted in the durable inhibition of ADAs, leading to normalized pharmacokinetics of the anti-TNFα antibody and protection against arthritis in TNFα transgenic mice. Adjunct therapy with tolerogenic nanoparticles represents a novel and broadly applicable approach to prevent the formation of ADAs against biologic therapies.

SLE-associated risk factors affect DC function

Numerous risk alleles for systemic lupus erythematosus (SLE) have now been identified. Analysis of the expression of genes with risk alleles in cells of hematopoietic origin demonstrates them to be most abundantly expressed in B cells and dendritic cells (DCs), suggesting that these cell types may be the drivers of the inflammatory changes seen in SLE. DCs are of particular interest as they act to connect the innate and the adaptive immune response. Thus, DCs can transform inflammation into autoimmunity, and autoantibodies are the hallmark of SLE. In this review, we focus on mechanisms of tolerance that maintain DCs in a non‐activated, non‐immunogenic state. We demonstrate, using examples from our own studies, how alterations in DC function stemming from either DC‐intrinsic abnormalities or DC‐extrinsic regulators of function can predispose to autoimmunity.

Contribution of secondary Igkappa rearrangement to primary immunoglobulin repertoire diversification

Abs reactive to DNA and DNA/histone complexes are a distinguished characteristic of primary immunoglobulin repertoires in autoimmune B6.MRL-Fas^lpr and MRL/MpJ-Fas^lpr mice. These mice are defective in Fas receptor, which is critical for the apoptosis of autoreactive B cells by an extrinsic pathway. In the present study, we explored the possibility that bone marrow small pre-B and immature B cells from adult B6.MRL-Fas^lpr mice and MRL/MpJ-Fas^lpr mice respectively, which contain autoreactive B-cell antigen receptors (BCR) and manifest autoimmune syndromes, exhibit enhanced receptor editing patterns. Indeed, FAS^lpr pre B and immature B cells were shown to possess more ongoing replacements of non-productive (nP) than productive (P) primary VκJκ rearrangements. Significantly, the P vs nP ratios of these replaced primary rearrangements were 1:2, thus indicating that κ light-chain production appears not to inhibit secondary rearrangements. In addition, we identified multiple atypical rearrangements, such as Vκ cRS (cryptic recombination signals) cleavages. These results suggest that the onset of light chain secondary rearrangements persists similarly as a non-selected mode and independent of BCR autoreactivity during certain developmental windows of bone marrow B cells in lupus-prone mice and control, and leads us to propose the function of secondary, de novo Igκ rearrangements to increase BCR diversity.

Continuous inhibitory signaling by both SHP-1 and SHIP-1 pathways is required to maintain unresponsiveness of anergic B cells

Cambier et al. show that the tyrosine phosphatase SHP-1 and the inositol phosphatase SHIP-1 are required to maintain B cell anergy.

Many autoreactive B cells persist in the periphery in a state of unresponsiveness called anergy. This unresponsiveness is rapidly reversible, requiring continuous BCR interaction with self-antigen and resultant regulatory signaling for its maintenance. Using adoptive transfer of anergic B cells with subsequent acute induction of gene deletion or expression, we demonstrate that the continuous activities of independent inhibitory signaling pathways involving the tyrosine phosphatase SHP-1 and the inositol phosphatase SHIP-1 are required to maintain anergy. Acute breach of anergy by compromise of either of these pathways leads to rapid cell activation, proliferation, and generation of short-lived plasma cells that reside in extrafollicular foci. Results are consistent with predicted/observed reduction in the Lyn–SHIP-1–PTEN–SHP-1 axis function in B cells from systemic lupus erythematosus patients.

Presence of Rheumatoid Factor during Chronic HCV Infection Is Associated with Expansion of Mature Activated Memory B-Cells that Are Hypo-Responsive to B-Cell Receptor Stimulation and Persist during the Early Stage of IFN Free Therapy

Approximately half of those with chronic hepatitis C virus (HCV) infection have circulating rheumatoid factor (RF), and a portion of these individuals develop cryoglobulinemic vasculitis. B cell phenotype/function in relation to RF in serum has been unclear. We examined B cell subset distribution, activation state (CD86), cell cycle state (Ki67), and ex-vivo response to BCR, TLR9 and TLR7/8 stimulation, in chronic HCV-infected donors with or without RF, and uninfected donors. Mature-activated B-cells of HCV-infected donors had lower CD86 expression compared to uninfected donors, and in the presence of RF they also showed reduced CD86 expression in response to BCR and TLR9 stimulation. Additionally, mature activated memory B cells of HCV RF+ donors less commonly expressed Ki67+ than HCV RF- donors, and did not proliferate as well in response to BCR stimulation. Proportions of mature-activated B cells were enhanced, while naïve B-cells were lower in the peripheral blood of HCV-RF+ compared to RF- and uninfected donors. None of these parameters normalize by week 8 of IFN free direct acting antiviral (DAA) therapy in HCV RF+ donors, while in RF- donors, mature activated B cell proportions did normalize. These data indicate that while chronic HCV infection alone results in a lower state of activation in mature activated memory B cells, the presence of RF in serum is associated with a more pronounced state of unresponsiveness and an overrepresentation of these B cells in the blood. This phenotype persists at least during the early time window after removal of HCV from the host.

Requirement for Transcription Factor Ets1 in B Cell Tolerance to Self-Antigens

The differentiation and survival of autoreactive B cells is normally limited by a variety of self-tolerance mechanisms, including clonal deletion, anergy, and clonal ignorance. The transcription factor c-ets-1 (encoded by the Ets1 gene) has B cell-intrinsic roles in regulating formation of Ab-secreting cells by controlling the activity of Blimp1 and Pax5 and may be required for B cell tolerance to self-antigen. To test this, we crossed Ets1(-/-) mice to two different transgenic models of B cell self-reactivity, the anti-hen egg lysozyme BCR transgenic strain and the AM14 rheumatoid factor transgenic strain. BCR transgenic Ets1(-/-) mice were subsequently crossed to mice either carrying or lacking relevant autoantigens. We found that B cells lacking c-ets-1 are generally hyperresponsive in terms of Ab secretion and form large numbers of Ab-secreting cells even in the absence of cognate Ags. When in the presence of cognate Ag, different responses were noted depending on the physical characteristics of the Ag. We found that clonal deletion of highly autoreactive B cells in the bone marrow was intact in the absence of c-ets-1. However, peripheral B cells lacking c-ets-1 failed to become tolerant in response to stimuli that normally induce B cell anergy or B cell clonal ignorance. Interestingly, high-affinity soluble self-antigen did cause B cells to adopt many of the classical features of anergic B cells, although such cells still secreted Ab. Therefore, maintenance of appropriate c-ets-1 levels is essential to prevent loss of self-tolerance in the B cell compartment.

Salmonella induces PD-L1 expression in B cells

Salmonella persists for a long time in B cells; however, the mechanism(s) through which infected B cells avoid effector CD8 T cell responses has not been characterized. In this study, we show that Salmonella infects and survives within all B1 and B2 cell subpopulations. B cells are infected with a Salmonella typhimurium strain expressing an ovalbumin (OVA) peptide (SIINFEKL) to evaluate whether B cells process and present Salmonella antigens in the context of MHC-I molecules. Our data showed that OVA peptides are presented by MHC class I K(b)-restricted molecules and the presented antigen is generated through proteasomal degradation and vacuolar processing. In addition, Salmonella-infected B cells express co-stimulatory molecules such as CD40, CD80, and CD86 as well as inhibitory molecules such as PD-L1. Thus, the cross-presentation of Salmonella antigens and the expression of activation molecules suggest that infected B cells are able to prime and activate specific CD8(+) T cells. However, the Salmonella infection-stimulated expression of PD-L1 suggests that the PD-1/PD-L1 pathway may be involved in turning off the cytotoxic effector response during Salmonella persistent infection, thereby allowing B cells to become a reservoir for the bacteria.

Reversing Tolerance in Isotype Switch-Competent Anti-Insulin B Lymphocytes

Autoreactive B lymphocytes that escape central tolerance and mature in the periphery are a liability for developing autoimmunity. IgG insulin autoantibodies that predict type 1 diabetes and complicate insulin therapies indicate that mechanisms for tolerance to insulin are flawed. To examine peripheral tolerance in anti-insulin B cells, we generated C57BL/6 mice that harbor anti-insulin VDJH-125 site directed to the native IgH locus (VH125(SD)). Class switch-competent anti-insulin B cells fail to produce IgG Abs following T cell-dependent immunization of VH125(SD) mice with heterologous insulin, and they exhibit markedly impaired proliferation to anti-CD40 plus insulin in vitro. In contrast, costimulation with LPS plus insulin drives robust anti-insulin B cell proliferation. Furthermore, VH125(SD) mice produce both IgM and IgG2a anti-insulin Abs following immunization with insulin conjugated to type 1 T cell-independent Brucella abortus ring test Ag (BRT). Anti-insulin B cells undergo clonal expansion in vivo and emerge as IgM(+) and IgM(-) GL7(+)Fas(+) germinal center (GC) B cells following immunization with insulin-BRT, but not BRT alone. Analysis of Igκ genes in VH125(SD) mice immunized with insulin-BRT reveals that anti-insulin Vκ from the preimmune repertoire is selected into GCs. These data demonstrate that class switch-competent anti-insulin B cells remain functionally silent in T cell-dependent immune responses, yet these B cells are vulnerable to reversal of anergy following combined BCR/TLR engagement that promotes Ag-specific GC responses and Ab production. Environmental factors that lead to infection and inflammation could play a critical yet underappreciated role in driving loss of tolerance and promoting autoimmune disease.

VprBP Is Required for Efficient Editing and Selection of Igκ+ B Cells, but Is Dispensable for Igλ+ and Marginal Zone B Cell Maturation and Selection

B cell development past the pro-B cell stage in mice requires the Cul4-Roc1-DDB1 E3 ubiquitin ligase substrate recognition subunit VprBP. Enforced Bcl2 expression overcomes defects in distal VH-DJH and secondary Vκ-Jκ rearrangement associated with VprBP insufficiency in B cells and substantially rescues maturation of marginal zone and Igλ(+) B cells, but not Igκ(+) B cells. In this background, expression of a site-directed Igκ L chain transgene increases Igκ(+) B cell frequency, suggesting VprBP does not regulate L chain expression from a productively rearranged Igk allele. In site-directed anti-dsDNA H chain transgenic mice, loss of VprBP function in B cells impairs selection of Igκ editor L chains typically arising through secondary Igk rearrangement, but not selection of Igλ editor L chains. Both H and L chain site-directed transgenic mice show increased B cell anergy when VprBP is inactivated in B cells. Taken together, these data argue that VprBP is required for the efficient receptor editing and selection of Igκ(+) B cells, but is largely dispensable for Igλ(+) B cell development and selection, and that VprBP is necessary to rescue autoreactive B cells from anergy induction.

B cell expression of the SH2-containing inositol 5-phosphatase (SHIP-1) is required to establish anergy to high affinity, proteinacious autoantigens

Many self-reactive B cells exist in the periphery in a rapidly reversible state of unresponsiveness referred to as anergy. Reversibility of anergy indicates that chronically occupied BCR must transduce non-durable regulatory signals that maintain unresponsiveness. Consistent with such a mechanism, studies of immunoglobulin transgenic, as well as naturally occurring polyclonal autoreactive B cells demonstrate activation of the inositol 5-phosphatase SHIP-1 in anergic cells, and low affinity chromatin autoantigen-reactive B cells have been shown to require expression of this phosphatase to maintain anergy. However, it has been reported that anergy of B cells recognizing high affinity soluble antigen may not require SHIP-1, and is instead mediated by upregulation of the inositol 3-phosphatase PTEN. To further explore this apparent difference in mechanism we analyzed the effect of B cell-targeted SHIP-1 deletion on immune tolerance of high affinity anti-HEL B cells in mice expressing soluble HEL (MD4.ML-5). We report that SHIP-1 functions to dampen responses of naïve and low-dose antigen-primed B cells in vitro, and is required for induction of B cell tolerance. Thus, while anergy of B cells reactive with low affinity and likely polyvalent chromatin antigens is maintained by activation of inhibitory signaling circuitry involving SHIP-1, anergy of B cells recognizing soluble self antigen with high affinity also requires increased activity of SHIP-1.

B-cell acquisition of antigen: Sensing the surface

B-cell antigen receptor (BCR) recognition and acquisition of antigen by B cells is the essential first step in the generation of effective antibody responses. As B-cell-mediated antigen presentation is also believed to play a significant role in the activation of CD4(+) Th-cell responses, considerable effort has focused on clarifying the nature of antigen/BCR interactions. Following earlier descriptions of interactions of soluble antigens with the BCR, it is now clear that B cells also recognize, physically extract and present antigens that are tethered to, or integral components of, the surfaces or extracellular matrix of other cells. In this issue of the European Journal of Immunology, Zeng et al. [Eur. J. Immunol. 2015. 45: XXXX-XXXX] examine how the physical property or "stiffness" of the surface displaying antigens to B cells influences the B-cell response. This commentary reports that antigen tethered on "less stiff" surfaces induces increased B-cell activation and antibody responses. I then infer how "sensing the surface" by B cells may represent a new component of the immune system's ability to detect "damage," and how this understanding may influence approaches to clinical therapies where immune activity is either unwanted or desired.

General Approach for Tetramer-Based Identification of Autoantigen-Reactive B Cells: Characterization of La- and snRNP-Reactive B Cells in Autoimmune BXD2 Mice

Autoreactive B cells are associated with the development of several autoimmune diseases, including systemic lupus erythematosus and rheumatoid arthritis. The low frequency of these cells represents a major barrier to their analysis. Ag tetramers prepared from linear epitopes represent a promising strategy for the identification of small subsets of Ag-reactive immune cells. This is challenging given the requirement for identification and validation of linear epitopes and the complexity of autoantibody responses, including the broad spectrum of autoantibody specificities and the contribution of isotype to pathogenicity. Therefore, we tested a two-tiered peptide microarray approach, coupled with epitope mapping of known autoantigens, to identify and characterize autoepitopes using the BXD2 autoimmune mouse model. Microarray results were verified through comparison with established age-associated profiles of autoantigen specificities and autoantibody class switching in BXD2 and control (C57BL/6) mice and high-throughput ELISA and ELISPOT analyses of synthetic peptides. Tetramers were prepared from two linear peptides derived from two RNA-binding proteins (RBPs): lupus La and 70-kDa U1 small nuclear ribonucleoprotein. Flow cytometric analysis of tetramer-reactive B cell subsets revealed a significantly higher frequency and greater numbers of RBP-reactive marginal zone precursor, transitional T3, and PDL-2(+)CD80(+) memory B cells, with significantly elevated CD69 and CD86 observed in RBP(+) marginal zone precursor B cells in the spleens of BXD2 mice compared with C57BL/6 mice, suggesting a regulatory defect. This study establishes a feasible strategy for the characterization of autoantigen-specific B cell subsets in different models of autoimmunity and, potentially, in humans.

Interleukin-10+ regulatory B cells arise within antigen-experienced CD40+ B cells to maintain tolerance to islet autoantigens

Impaired regulatory B cell (Breg) responses are associated with several autoimmune diseases in humans; however, the role of Bregs in type 1 diabetes (T1D) remains unclear. We hypothesized that naturally occurring, interleukin-10 (IL-10)-producing Bregs maintain tolerance to islet autoantigens, and that hyperglycemic nonobese diabetic (NOD) mice and T1D patients lack these potent negative regulators. IgVH transcriptome analysis revealed that islet-infiltrating B cells in long-term normoglycemic (Lnglc) NOD, which are naturally protected from diabetes, are more antigen-experienced and possess more diverse B-cell receptor repertoires compared to those of hyperglycemic (Hglc) mice. Importantly, increased levels of Breg-promoting CD40(+) B cells and IL-10-producing B cells were found within islets of Lnglc compared to Hglc NOD. Likewise, healthy individuals showed increased frequencies of both CD40(+) and IL-10(+) B cells compared to T1D patients. Rituximab-mediated B-cell depletion followed by adoptive transfer of B cells from Hglc mice induced hyperglycemia in Lnglc human CD20 transgenic NOD mouse models. Importantly, both murine and human IL-10(+) B cells significantly abrogated T-cell-mediated responses to self- or islet-specific peptides ex vivo. Together, our data suggest that antigen-matured Bregs may maintain tolerance to islet autoantigens by selectively suppressing autoreactive T-cell responses, and that Hglc mice and individuals with T1D lack this population of Bregs.

Multiple factors influence the contribution of individual immunoglobulin light chain genes to the naïve antibody repertoire

Background

The naïve antibody repertoire is initially dependent upon the number of germline V(D)J genes and the ability of recombined heavy and light chains to pair. Individual VH and VL genes are not equally represented in naïve mature B cells, suggesting that positive and negative selection also shape the antibody repertoire. Among the three member murine Vκ10 L chain family, the Vκ10C gene is under-represented in the antibody repertoire. Although it is structurally functional and accessible to both transcriptional and recombination machinery, the Vκ10C promoter is inefficient in pre-B cell lines and productive Vκ10C rearrangements are lost as development progresses from pre-B cells through mature B cells. This study examined VH/Vκ10 pairing, promoter mutations, Vκ10 transcript levels and receptor editing as possible factors that are responsible for loss of productive Vκ10C rearrangements in developing B cells.

Results

We demonstrate that the loss of Vκ10C expression is not due to an inability to pair with H chains, but is likely due to a combination of other factors. Levels of mRNA are low in sorted pre-B cells and undetectable in B cells. Mutation of a single base in the three prime region of the Vκ10C promoter increases Vκ10C promoter function in pre-B cell lines. Pre-B and B cells harbor disproportionate levels of receptor-edited productive Vκ10C rearrangements.

Conclusions

Our findings suggest that the weak Vκ10C promoter initially limits the amount of available Vκ10C L chain for pairing with H chains, resulting in sub-threshold levels of cell surface B cell receptors, insufficient tonic signaling and subsequent receptor editing to limit the numbers of Vκ10C-expressing B cells emigrating from the bone marrow to the periphery.

Spinal cord injury impacts B cell production, homeostasis, and activation

Complex interactions govern the interplay of central nervous and immune systems, including the generation, homeostatic maintenance, and activation of B cells. Accordingly, spinal cord injury will likely impact all of these processes. Several laboratories have recently explored this possibility, and their observations in aggregate reveal both acute and chronic consequences that can vary based on the injury location. Acute effects include a transient cessation of bone marrow B lymphopoiesis, with a corresponding drop in the peripheral follicular and transitional B cell subsets, whereas the marginal zone subset is preserved. Despite recovery of B lymphopoiesis by 28 days post injury, follicular B cell numbers remain depressed; this may reflect reduced levels of the homeostatic cytokine BLyS. In general, the ability to mount T dependent antibody responses after injury are intact, as are pre-existing memory B cell pools and antibody levels. In contrast, T-independent responses are chronically compromised. Both glucocorticoid-dependent and -independent processes mediate these effects, but a detailed understanding of the mechanisms involved awaits further study. Nonetheless, these observations in toto strengthen the growing appreciation for bidirectional interactions between the CNS and immune system, highlighting the need for further basic and translational efforts.

B cell receptor signaling-based index as a biomarker for the loss of peripheral immune tolerance in autoreactive B cells in rheumatoid arthritis

This study examines the loss of peripherally induced B cell immune tolerance in Rheumatoid arthritis (RA) and establishes a novel signaling-based measure of activation in a subset of autoreactive B cells - the Induced tolerance status index (ITSI). Naturally occurring naïve autoreactive B cells can escape the “classical” tolerogenic mechanisms of clonal deletion and receptor editing, but remain peripherally tolerized through B cell receptor (BCR) signaling inhibition (postdevelopmental “receptor tuning” or anergy). ITSI is a statistical index that numerically determines the level of homology between activation patterns of BCR signaling intermediaries in B cells that are either tolerized or activated by auto antigen exposure, and thus quantifies the level of peripheral immune tolerance. The index is based on the logistic regression analysis of phosphorylation levels in a panel of BCR signaling proteins. Our results demonstrate a new approach to identifying autoreactive B cells based on their BCR signaling features.

Activation of rheumatoid factor-specific B cells is antigen dependent and occurs preferentially outside of germinal centers in the lupus-prone NZM2410 mouse model

AM14 rheumatoid factor (RF) B cells in the MRL/lpr mice are activated by dual BCR and TLR7/9 ligation and differentiate into plasmablasts via an extrafollicular (EF) route. It was not known whether this mechanism of activation of RF B cells applied to other lupus-prone mouse models. We investigated the mechanisms by which RF B cells break tolerance in the NZM2410-derived B6.Sle1.Sle2.Sle3 (TC) strain in comparison with C57BL/6 (B6) controls, each expressing the AM14 H chain transgene in the presence or absence of the IgG2a(a) autoantigen. The TC, but not B6, genetic background promotes the differentiation of RF B cells into Ab-forming cells (AFCs) in the presence of the autoantigen. Activated RF B cells preferentially differentiated into plasmablasts in EF zones. Contrary to the MRL/lpr strain, TC RF B cells were also located within germinal centers, but only the formation of EF foci was positively correlated with the production of RF AFCs. Immunization of young TC.AM14 H chain transgenic mice with IgG2a(a) anti-chromatin immune complexes (ICs) activated RF B cells in a BCR- and TLR9-dependent manner. However, these IC immunizations did not result in the production of RF AFCs. These results show that RF B cells break tolerance with the same general mechanisms in the TC and the MRL/lpr lupus-prone genetic backgrounds, namely the dual activation of the BCR and TLR9 pathways. There are also distinct differences, such as the presence of RF B cells in GCs and the requirement of chronic IgG2a(a) anti-chromatin ICs for full differentiation of RF AFCs.

Does B cell receptor signaling in chronic lymphocytic leukaemia cells differ from that in other B cell types?

Chronic lymphocytic leukaemia (CLL) is an incurable malignancy of mature B cells. CLL is important clinically in Western countries because of its commonality and because of the significant morbidity and mortality associated with the progressive form of this incurable disease. The B cell receptor (BCR) expressed on the malignant cells in CLL contributes to disease pathogenesis by providing signals for survival and proliferation, and the signal transduction pathway initiated by engagement of this receptor is now the target of several therapeutic strategies. The purpose of this review is to outline current understanding of the BCR signal cascade in normal B cells and then question whether this understanding applies to CLL cells. In particular, this review studies the phenomenon of anergy in CLL cells, and whether certain adaptations allow the cells to overcome anergy and allow full BCR signaling to take place. Finally, this review analyzes how BCR signals can be therapeutically targeted for the treatment of CLL.