The fate of self-reactive B cells depends primarily on the degree of antigen receptor engagement and availability of T cell help

Factors Governing B Cell Recognition of Autoantigen and Function in Type 1 Diabetes

Islet autoantibodies predict type 1 diabetes (T1D) but can be transient in murine and human T1D and are not thought to be directly pathogenic. Rather, these autoantibodies signal B cell activity as antigen-presenting cells (APCs) that present islet autoantigen to diabetogenic T cells to promote T1D pathogenesis. Disrupting B cell APC function prevents T1D in mouse models and has shown promise in clinical trials. Autoantigen-specific B cells thus hold potential as sophisticated T1D biomarkers and therapeutic targets. B cell receptor (BCR) somatic hypermutation is a mechanism by which B cells increase affinity for islet autoantigen. High-affinity B and T cell responses are selected in protective immune responses, but immune tolerance mechanisms are known to censor highly autoreactive clones in autoimmunity, including T1D. Thus, different selection rules often apply to autoimmune disease settings (as opposed to protective host immunity), where different autoantigen affinity ceilings are tolerated based on variations in host genetics and environment. This review will explore what is currently known regarding B cell signaling, selection, and interaction with T cells to promote T1D pathogenesis.

MHCII restriction demonstrates B cells have very limited capacity to activate tumour-specific CD4+ T cells in vivo

There has been growing interest in the role of B cells in antitumour immunity and potential use in adoptive cellular therapies. To date, the success of such therapies is limited. The intrinsic capacity of B cells to specifically activate tumour-specific CD4+ T cells in vivo via TCR-dependent interactions remains poorly defined. We have developed an in vivo tumour model that utilizes MHCII I-E restriction which limits antigen presentation to tumour-specific CD4 T cells to either tumour-specific B cells or host myeloid antigen presenting cells (APCs) in lymphopenic RAG-/-mice. We have previously shown that these naive tumour-specific CD4+ T cells can successfully eradicate established tumours in this model when activated by host APCs. When naïve tumour-specific B cells are the only source of I-E+ APC, very limited proliferation of naïve CD4+ T cells is observed, whereas host I-E+ APCs are potent T cell activators. B cells pre-activated with an anti-CD40 agonistic antibody in vivo support increased T cell proliferation, although far less than host APCs. CD4+ T cells that have already differentiated to an effector/central memory phenotype proliferate more readily in response to naïve B cells, although still 100-fold less than in response to host APCs. This study demonstrates that even in a significantly lymphopenic environment, myeloid APCs are the dominant primary activators of tumour-specific T cells, in contrast to the very limited capacity of tumour-specific B cells. This suggests that future anti-tumour therapies that incorporate activated B cells should also include mechanisms that activate host APCs.

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.

Balancing B cell responses to the allograft: implications for vaccination

Balancing enough immunosuppression to prevent allograft rejection and yet maintaining an intact immune system to respond to vaccinations, eliminate invading pathogens or cancer cells is an ongoing challenge to transplant physicians. Antibody mediated allograft rejection remains problematic in kidney transplantation and is the most common cause of graft loss despite current immunosuppressive therapies. The goal of immunosuppressive therapies is to prevent graft rejection; however, they prevent optimal vaccine responses as well. At the center of acute and chronic antibody mediated rejection and vaccine responses is the B lymphocyte. This review will highlight the role of B cells in alloimmune responses including the dependency on T cells for antibody production. We will discuss the need to improve vaccination rates in transplant recipients and present data on B cell populations and SARS-CoV-2 vaccine response rates in pediatric kidney transplant recipients.

T Cell Help in the Autoreactive Germinal Center

The germinal center serves as a site of B cell selection and affinity maturation, critical processes for productive adaptive immunity. In autoimmune disease tolerance is broken in the germinal center reaction, leading to production of autoreactive B cells that may propagate disease. Follicular T cells are crucial regulators of this process, providing signals necessary for B cell survival in the germinal center. Here we review the emerging roles of follicular T cells in the autoreactive germinal center. Recent advances in immunological techniques have allowed study of the gene expression profiles and repertoire of follicular T cells at unprecedented resolution. These studies provide insight into the potential role follicular T cells play in preventing or facilitating germinal center loss of tolerance. Improved understanding of the mechanisms of T cell help in autoreactive germinal centers provides novel therapeutic targets for diseases of germinal center dysfunction.

Generation of memory B cells and their reactivation

The successful establishment of humoral memory response depends on at least two layers of defense. Pre-existing protective antibodies secreted by long-lived plasma cells act as a first line of defense against reinfection ("constitutive humoral memory"). Previously, a second line of defense in which pathogen-experienced memory B cells are rapidly reactivated to produce antibodies ("reactive humoral memory"), was considered as simply a back-up system for the first line (particularly for re-infection with homologous viruses). However, in the case of re-infection with similar but different strains of viruses, or in response to viral escape mutants, the reactive humoral memory plays a crucial role. Here, we review recent progress in our understanding of how memory B cells are generated in the pre-GC stage and during the GC reaction, and how these memory B cells are robustly reactivated with the help of memory Tfh cells to generate the secondary antibody response. In addition, we discuss how these advances may be relevant to the quest for a vaccine that can induce broadly reactive antibodies against influenza and HIV.

Memory B cells are reactivated in subcapsular proliferative foci of lymph nodes

Vaccine-induced immunity depends on the generation of memory B cells (MBC). However, where and how MBCs are reactivated to make neutralising antibodies remain unknown. Here we show that MBCs are prepositioned in a subcapsular niche in lymph nodes where, upon reactivation by antigen, they rapidly proliferate and differentiate into antibody-secreting plasma cells in the subcapsular proliferative foci (SPF). This novel structure is enriched for signals provided by T follicular helper cells and antigen-presenting subcapsular sinus macrophages. Compared with contemporaneous secondary germinal centres, SPF have distinct single-cell molecular signature, cell migration pattern and plasma cell output. Moreover, SPF are found both in human and mouse lymph nodes, suggesting that they are conserved throughout mammalian evolution. Our data thus reveal that SPF is a seat of immunological memory that may be exploited to rapidly mobilise secondary antibody responses and improve vaccine efficacy.

Transiently antigen-primed B cells return to naive-like state in absence of T-cell help

The perspective that naive B-cell recognition of antigen in the absence of T-cell help causes cell death or anergy is supported by in vivo studies of B cells that are continuously exposed to self-antigens. However, intravital imaging suggests that early B-cell recognition of large foreign antigens may be transient. Whether B cells are tolerized or can be recruited into humoural immune responses following such encounters is not clear. Here we show that in the presence of T-cell help, single transient antigen acquisition is sufficient to recruit B cells into the germinal centre and induce memory and plasma cell responses. In the absence of T-cell help, transiently antigen-primed B cells do not undergo apoptosis in vivo; they return to quiescence and are recruited efficiently into humoural responses upon reacquisition of antigen and T-cell help.

Cocapture of cognate and bystander antigens can activate autoreactive B cells

Autoantibodies against myelin oligodendrocyte glycoprotein (MOG) are associated with autoimmune central nervous system diseases like acute disseminated encephalomyelitis (ADEM). For ADEM, it is speculated that a preceding infection is the trigger of the autoimmune response, but the mechanism connecting the infection to the production of MOG antibodies remains a mystery. We reasoned that the ability of B cells to capture cognate antigen from cell membranes, along with small quantities of coexpressed "bystander" antigens, might enable B-cell escape from tolerance. We tested this hypothesis using influenza hemagglutinin as a model viral antigen and transgenic, MOG-specific B cells. Using flow cytometry and live and fixed cell microscopy, we show that MOG-specific B cells take up large amounts of MOG from cell membranes. Uptake of the antigen from the membrane leads to a strong activation of the capturing B cell. When influenza hemagglutinin is also present in the membrane of the target cell, it can be cocaptured with MOG by MOG-specific B cells via the B-cell receptor. Hemagglutinin and MOG are both presented to T cells, which in turn are activated and proliferate. As a consequence, MOG-specific B cells get help from hemagglutinin-specific T cells to produce anti-MOG antibodies. In vivo, the transfer of MOG-specific B cells into recipient mice after the cocapture of MOG and hemagglutinin leads to the production of class-switched anti-MOG antibodies, dependent on the presence of hemagglutinin-specific T cells. This mechanism offers a link between infection and autoimmunity.

The N-Terminal Fraction of Desmoglein 3 Encompassing its Immunodominant Domain is Present in Human Serum: Implications for Pemphigus Vulgaris Autoimmunity

Pemphigus vulgaris (PV) is considered as an autoimmune disease against a tissue-restricted antigen, desmoglein 3, a 130 kDa glycoprotein expressed by keratinocytes of skin and mucous membranes. Therefore, a breakdown of peripheral tolerance is generally invoked to explain this horror autotoxicus. The availability of a self-antigen and the strength of antigenic stimulation represent critical points in the regulation of immune system homeostasis. Our study shows for the first time that the immunodominant fraction of the PV self-antigen is present in sera of healthy individuals and patients as a circulating 30 kDa fragment (sDsg3). These findings provide a good explanation for the N-terminal specificity of antibody production and peptide recognition in PV patients by B and T cell, respectively. Moreover, the presence of the sDsg3 in human sera could allow to reconsider pemphigus as a disease against a circulating antigen; once produced, PV-autoantibodies also recognize the 130 kDa epidermal antigen desmoglein 3 on keratinocyte surface (kDsg3), thus triggering the acantholysis and the clinical manifestations of pemphigus.

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.

Transient CD4+ T Cell Depletion Results in Delayed Development of Functional Vaccine-Elicited Antibody Responses

We have recently demonstrated that CD4(+)T cell help is required at the time of adenovirus (Ad) vector immunization for the development of functional CD8(+)T cell responses, but the temporal requirement for CD4(+)T cell help for the induction of antibody responses remains unclear. Here we demonstrate that induction of antibody responses in C57BL/6 mice can occur at a time displaced from the time of Ad vector immunization by depletion of CD4(+)T cells. Transient depletion of CD4(+)T cells at the time of immunization delays the development of antigen-specific antibody responses but does not permanently impair their development or induce tolerance against the transgene. Upon CD4(+)T cell recovery, transgene-specific serum IgG antibody titers develop and reach a concentration equivalent to that in undepleted control animals. These delayed antibody responses exhibit no functional defects with regard to isotype, functional avidity, expansion after boosting immunization, or the capacity to neutralize a simian immunodeficiency virus (SIV) Env-expressing pseudovirus. The development of this delayed transgene-specific antibody response is temporally linked to the expansion of de novo antigen-specific CD4(+)T cell responses, which develop after transient depletion of CD4(+)T cells. These data demonstrate that functional vaccine-elicited antibody responses can be induced even if CD4(+)T cell help is provided at a time markedly separated from the time of vaccination.

IMPORTANCE

CD4(+)T cells have a critical role in providing positive help signals to B cells, which promote robust antibody responses. The paradigm is that helper signals must be provided immediately upon antigen exposure, and their absence results in tolerance against the antigen. Here we demonstrate that, in contrast to the current model that the absence of CD4(+)T cell help at priming results in long-term antibody nonresponsiveness, antibody responses can be induced by adenovirus vector immunization or alum-adjuvanted protein immunization even if CD4(+)T cell help is not provided until >1 month after immunization. These data demonstrate that the time when CD4(+)T cell help signals must be provided is more dynamic and flexible than previously appreciated. These data suggest that augmentation of CD4(+)T cell helper function even after the time of vaccination can enhance vaccine-elicited antibody responses and thereby potentially enhance the immunogenicity of vaccines in immunocompromised individuals.

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.

Systemic Lupus Erythematosus: Molecular Mimicry between Anti-dsDNA CDR3 Idiotype, Microbial and Self Peptides-As Antigens for Th Cells

Systemic lupus erythematosus (SLE) is marked by a T helper (Th) cell-dependent B cell hyperresponsiveness, with frequent germinal center reactions, and gammaglobulinemia. A feature of SLE is the finding of IgG autoantibodies specific for dsDNA. The specificity of the Th cells that drive the expansion of anti-dsDNA B cells is unresolved. However, anti-microbial, anti-histone, and anti-idiotype Th cell responses have been hypothesized to play a role. It has been entirely unclear if these seemingly disparate Th cell responses and hypotheses could be related or unified. Here, we describe that H chain CDR3 idiotypes from IgG⁺ B cells of lupus mice have sequence similarities with both microbial and self peptides. Matched sequences were more frequent within the mutated CDR3 repertoire and when sequences were derived from lupus mice with expanded anti-dsDNA B cells. Analyses of histone sequences showed that particular histone peptides were similar to VDJ junctions. Moreover, lupus mice had Th cell responses toward histone peptides similar to anti-dsDNA CDR3 sequences. The results suggest that Th cells in lupus may have multiple cross-reactive specificities linked to the IgVH CDR3 Id-peptide sequences as well as similar DNA-associated protein motifs.

Humoral immunity. Apoptosis and antigen affinity limit effector cell differentiation of a single naïve B cell

When exposed to antigens, naïve B cells differentiate into different types of effector cells: antibody-producing plasma cells, germinal center cells, or memory cells. Whether an individual naïve B cell can produce all of these different cell fates remains unclear. Using a limiting dilution approach, we found that many individual naïve B cells produced only one type of effector cell subset, whereas others produced all subsets. The capacity to differentiate into multiple subsets was a characteristic of clonal populations that divided many times and resisted apoptosis, but was independent of isotype switching. Antigen receptor affinity also influenced effector cell differentiation. These findings suggest that diverse effector cell types arise in the primary immune response as a result of heterogeneity in responses by individual naïve B cells.

Chronic lymphocytic leukemia cells express CD38 in response to Th1 cell-derived IFN-γ by a T-bet-dependent mechanism

Chronic lymphocytic leukemia (CLL) is a B cell malignancy associated with increased levels of inflammatory cytokines. Similarly, expression of CD38 on CLL cells correlates with CLL cell survival and proliferation, but the mechanisms that regulate CD38 expression and inflammatory cytokines remain unclear. We have recently demonstrated that patients have CLL-specific Th cells that support CLL proliferation. In this article, we show that CLL cells attract such Th cells, thereby establishing an Ag-dependent collaboration. Blocking experiments performed in vitro as wells as in vivo, using a xenograft model, revealed that secretion of IFN-γ was a major mechanism by which CLL-specific Th cells increased CD38 on CLL cells. The expression of the transcription factor T-bet in peripheral blood CLL cells significantly correlated with CD38 expression, and transient transfection of CLL cells with T-bet resulted in T-bet(hi)CD38(hi) cells. Finally, chromatin immunoprecipitation experiments revealed that T-bet can bind to regulatory regions of the CD38 gene. These data suggest that CLL cells attract CLL-specific Th cells and initiate a positive feedback loop with upregulation of T-bet, CD38, and type 1 chemokines allowing further recruitment of Th cells and increased type 1 cytokine secretion. This insight provides a cellular and molecular mechanism that links the inflammatory signature observed in CLL pathogenesis with CD38 expression and aggressive disease and suggests that targeting the IFN-γ/IFN-γR/JAK/STAT/T-bet/CD38 pathway could play a role in the therapy of CLL.

Idiotype-specific Th cells support oligoclonal expansion of anti-dsDNA B cells in mice with lupus

Systemic lupus erythematosus (SLE) is marked by a Th cell-dependent B cell hyperresponsiveness, with frequent germinal center reactions and hypergammaglobulinemia. The specificity of Th cells in lupus remains unclear, but B cell Ids have been suggested. A hallmark is the presence of anti-dsDNA, mutated IgG autoantibodies with a preponderance of arginines in CDR3 of the Ig variable H chain (IgVH). B cells can present V region-derived Id peptides on their MHC class II molecules to Id-specific Th cells. We show that Id-specific Th cells support the proliferation of anti-dsDNA Id(+) B cells in mice suffering from systemic autoimmune disease with SLE-like features. Mice developed marked clonal expansions of B cells; half of the IgVH sequences were clonally related. Anti-dsDNA B cells made up 40% of B cells in end-stage disease. The B cells expressed mutated IgVH with multiple arginines in CDR3. Hence, Id-driven T cell-B cell collaboration supported the production of classical anti-dsDNA Abs, recapitulating the characteristics of such Abs in SLE. The results support the concept that Id-specific Th cells may trigger the development of SLE and suggest that manipulation of the Id-specific T cell repertoire could play a role in treatment.

Chronic lymphocytic leukemia cells are activated and proliferate in response to specific T helper cells

There is increasing interest in the chronic lymphocytic leukemia (CLL) microenvironment and the mechanisms that may promote CLL cell survival and proliferation. A role for T helper (Th) cells has been suggested, but current evidence is only circumstantial. Here we show that CLL patients had memory Th cells that were specific for endogenous CLL antigens. These Th cells activated autologous CLL cell proliferation in vitro and in human → mouse xenograft experiments. Moreover, CLL cells were efficient antigen-presenting cells that could endocytose and process complex proteins through antigen uptake pathways, including the B cell receptor. Activation of CLL cells by Th cells was contact and CD40L dependent. The results suggest that CLL is driven by ongoing immune responses related to Th cell-CLL cell interaction. We propose that Th cells support malignant B cells and that they could be targeted in the treatment of CLL.

Flow cytometric analysis of cell division by dilution of CFSE and related dyes

The technique described in this unit uses the intracellular fluorescent label carboxyfluorescein diacetate succinimidyl ester (CFSE) to track proliferating cells. Covalently bound CFSE is divided equally between daughter cells, allowing discrimination of successive rounds of cell division. The technique is applicable to in vitro cell division, as well as to in vivo division of adoptively transferred cells and can resolve eight or more successive generations. CFSE is long lived, permitting analysis for several months after cell transfer, and has the same spectral characteristics as fluorescein, so monoclonal antibodies conjugated to phycoerythrin or other compatible fluorochromes may be used to immunophenotype the dividing cells. In addition, information is given on a second-generation dye, Cell Trace Violet (CTV), excited by 405-nm blue laser light. CTV is chemically related to CFSE, but allows the 488-nm line of the Argon laser to be used for other probes.

Global analysis of B cell selection using an immunoglobulin light chain-mediated model of autoreactivity

The nature of the immunoglobulin light chain affects peripheral B cell tolerance and autoreactivity.

The important subtleties of B cell tolerance are best understood in a diverse immunoglobulin (Ig) repertoire context encoding a full spectrum of autoreactivity. To achieve this, we used mice expressing Igκ transgenes that confer varying degrees of autoreactivity within a diverse heavy chain (HC) repertoire. These transgenes, coupled with a biomarker to identify receptor-edited cells and combined with expression cloning of B cell receptors, allowed us to analyze tolerance throughout B cell development. We found that both the nature of the autoantigen and the Ig HC versus light chain (LC) contribution to autoreactivity dictate the developmental stage and mechanism of tolerance. Furthermore, although selection begins in the bone marrow, over one third of primary tolerance occurs in the periphery at the late transitional developmental stage. Notably, we demonstrate that the LC has profound effects on tolerance and can lead to exacerbated autoantibody production.

Antigen-specific suppression of humoral immunity by anergic Ars/A1 B cells

Autoreactive anergic B lymphocytes are considered to be dangerous because of their potential for activation and recruitment into autoimmune responses. However, they persist for days and constitute ∼5% of the B cell pool. We assessed their functional potential in the Ars/A1 transgene model, where anergic B cells express a dual-reactive Ag receptor that binds, in addition to a self-Ag, the hapten p-azophenylarsonate (Ars). When Ars/A1 B cells were transferred into adoptive recipients that were immunized with foreign proteins covalently conjugated with Ars, endogenous IgG immune responses to both were selectively and severely diminished, and the development of T helper cells was impaired. Approximately 95% inhibition of the anti-Ars response was attained with ∼4000 transferred Ars/A1 B cells through redundant mechanisms, one of which depended on their expression of MHC class II but not upon secretion of IL-10 or IgM. This Ag-specific suppressive activity implicates the autoreactive anergic B cell as an enforcer of immunological tolerance to self-Ags.

Immunological aspects in chronic lymphocytic leukemia (CLL) development

Chronic lymphocytic leukemia (CLL) is unique among B cell malignancies in that the malignant clones can be featured either somatically mutated or unmutated IGVH genes. CLL cells that express unmutated immunoglobulin variable domains likely underwent final development prior to their entry into the germinal center, whereas those that express mutated variable domains likely transited through the germinal center and then underwent final development. Regardless, the cellular origin of CLL remains unknown. The aim of this review is to summarize immunological aspects involved in this process and to provide insights about the complex biology and pathogenesis of this disease. We propose a mechanistic hypothesis to explain the origin of B-CLL clones into our current picture of normal B cell development. In particular, we suggest that unmutated CLL arises from normal B cells with self-reactivity for apoptotic bodies that have undergone receptor editing, CD5 expression, and anergic processes in the bone marrow. Similarly, mutated CLL would arise from cells that, while acquiring self-reactivity for autoantigens-including apoptotic bodies-in germinal centers, are also still subject to tolerization mechanisms, including receptor editing and anergy. We believe that CLL is a proliferation of B lymphocytes selected during clonal expansion through multiple encounters with (auto)antigens, despite the fact that they differ in their state of activation and maturation. Autoantigens and microbial pathogens activate BCR signaling and promote tolerogenic mechanisms such as receptor editing/revision, anergy, CD5+ expression, and somatic hypermutation in CLL B cells. The result of these tolerogenic mechanisms is the survival of CLL B cell clones with similar surface markers and homogeneous gene expression signatures. We suggest that both immunophenotypic surface markers and homogenous gene expression might represent the evidence of several attempts to re-educate self-reactive B cells.

Liver-expressed Igkappa superantigen induces tolerance of polyclonal B cells by clonal deletion not kappa to lambda receptor editing

Analysis of tolerance in a polyclonal wild-type B cell population demonstrates apoptosis of cells reactive to antigen expressed on liver membrane.

Little is know about the nature of peripheral B cell tolerance or how it may vary in distinct lineages. Although autoantibody transgenic studies indicate that anergy and apoptosis are involved, some studies claim that receptor editing occurs. To model peripheral B cell tolerance in a normal, polyclonal immune system, we generated transgenic mice expressing an Igκ–light chain–reactive superantigen targeted to the plasma membrane of hepatocytes (pAlb mice). In contrast to mice expressing κ superantigen ubiquitously, in which κ cells edit efficiently to λ, in pAlb mice, κ B cells underwent clonal deletion. Their κ cells failed to populate lymph nodes, and the remaining splenic κ cells were anergic, arrested at a semi-mature stage without undergoing receptor editing. In the liver, κ cells recognized superantigen, down-regulated surface Ig, and expressed active caspase 3, suggesting ongoing apoptosis at the site of B cell receptor ligand expression. Some, apparently mature, κ B1 and follicular B cells persisted in the peritoneum. BAFF (B cell–activating factor belonging to the tumor necrosis factor family) overexpression rescued splenic κ B cell maturation and allowed κ cells to populate lymph nodes. Our model facilitates analysis of tissue-specific autoimmunity, tolerance, and apoptosis in a polyclonal B cell population. The results suggest that deletion, not editing, is the major irreversible pathway of tolerance induction among peripheral B cells.

Molecular underpinning of B-cell anergy

A byproduct of the largely stochastic generation of a diverse B-cell specificity repertoire is production of cells that recognize autoantigens. Indeed, recent studies indicate that more than half of the primary repertoire consists of autoreactive B cells that must be silenced to prevent autoimmunity. While this silencing can occur by multiple mechanisms, it appears that most autoreactive B cells are silenced by anergy, wherein they populate peripheral lymphoid organs and continue to express unoccupied antigen receptors yet are unresponsive to antigen stimulation. Here we review molecular mechanisms that appear operative in maintaining the antigen unresponsiveness of anergic B cells. In addition, we present new data indicating that the failure of anergic B cells to mobilize calcium in response to antigen stimulation is not mediated by inactivation of stromal interacting molecule 1, a critical intermediary in intracellular store depletion-induced calcium influx.

Acquisition of humoral transplantation tolerance upon de novo emergence of B lymphocytes

A major obstacle to transplantation tolerance is humoral immunity. In this paper, we demonstrate that the intrinsic developmental propensity of the B lymphocyte compartment for acquisition of self-tolerance can be harnessed to induce humoral unresponsiveness to transplanted alloantigens. In the current study, when transitional B cells developed in the presence of donor lymphoid cells, the mature B lymphocyte compartment failed to mount a donor-specific alloantibody response to an organ transplant--despite unrestrained acute T cell-mediated allograft rejection. Specifically, we generated an experimental system wherein a B6 strain B cell compartment developed de novo in the presence of F1 (B6xBALB/c) lymphoid cells and in a T cell-deficient setting. Following establishment of a steady-state B cell compartment, these B6 mice were transplanted with heterotopic cardiac allografts from allogeneic BALB/c donors. The mice were then inoculated with purified syngeneic B6 T cells. As expected, all cardiac allografts were acutely rejected. However, the B lymphocyte compartment of these mice was completely inert in its capacity to form a BALB/c-specific alloantibody response. Using an alloantigen-specific Ig transgenic system, we demonstrated that this profound degree of humoral tolerance was caused by clonal deletion of alloreactive specificities from the primary B cell repertoire. Thus, de novo B cell compartment development at the time of transplantation is of critical importance in recipient repertoire "remodeling" to a humoral tolerant state.

Humoral autoimmunity and transplant vasculopathy: when allo is not enough

For several decades, allograft rejection was believed to be mediated almost exclusively by cellular immune responses, but it is now realized that humoral responses also play a major role. Although directed typically against donor human leukocyte antigen, it is becoming increasingly evident that the antibody response can also target autoantigens that are shared between donor and recipient and that this autoantibody may contribute to graft rejection. Many aspects of transplant-induced humoral autoimmunity remain poorly understood and key questions persist; not least what triggers the response and how autoantibody causes graft damage. Here, we collate results from recent clinical and experimental studies in transplantation and autoimmune diseases to propose answers to these questions.

B cell activating factor (BAFF) and T cells cooperate to breach B cell tolerance in lupus-prone New Zealand Black (NZB) mice

The presence of autoantibodies in New Zealand Black (NZB) mice suggests a B cell tolerance defect however the nature of this defect is unknown. To determine whether defects in B cell anergy contribute to the autoimmune phenotype in NZB mice, soluble hen egg lysozyme (sHEL) and anti-HEL Ig transgenes were bred onto the NZB background to generate double transgenic (dTg) mice. NZB dTg mice had elevated levels of anti-HEL antibodies, despite apparently normal B cell functional anergy in-vitro. NZB dTg B cells also demonstrated increased survival and abnormal entry into the follicular compartment following transfer into sHEL mice. Since this process is dependent on BAFF, BAFF serum and mRNA levels were assessed and were found to be significantly elevated in NZB dTg mice. Treatment of NZB sHEL recipient mice with TACI-Ig reduced NZB dTg B cell survival following adoptive transfer, confirming the role of BAFF in this process. Although NZB mice had modestly elevated BAFF, the enhanced NZB B cell survival response appeared to result from an altered response to BAFF. In contrast, T cell blockade had a minimal effect on B cell survival, but inhibited anti-HEL antibody production. The findings suggest that the modest BAFF elevations in NZB mice are sufficient to perturb B cell tolerance, particularly when acting in concert with B cell functional abnormalities and T cell help.

Altered B cell development and anergy in the absence of Foxp3

The importance of regulatory T cells in immune tolerance is illustrated by the human immune dysregulatory disorder IPEX (immune dysregulation, polyendocrinopathy, enteropathy, X-linked), caused by a lack of regulatory T cells due to decreased or absent expression of Foxp3. Although the majority of work on regulatory T cells has focused on their ability to suppress T cell responses, the development of significant autoantibody titers in patients with IPEX suggests that regulatory T cells also contribute to the suppression of autoreactive B cells. Using a murine model, deficient in the expression of Foxp3, we show that B cell development is significantly altered in the absence of regulatory T cells. Furthermore, we identify a loss of B cell anergy as a likely mechanism to explain the production of autoantibodies that occurs in the absence of regulatory T cells. Our results suggest that regulatory T cells, by either direct or indirect mechanisms, modulate B cell development and anergy.

Kinetic modeling reveals a common death niche for newly formed and mature B cells

Background

B lymphocytes are subject to elimination following strong BCR ligation in the absence of appropriate second signals, and this mechanism mediates substantial cell losses during late differentiation steps in the bone marrow and periphery. Mature B cells may also be eliminated through this mechanism as well as through normal turnover, but the population containing mature cells destined for elimination has not been identified. Herein, we asked whether the transitional 3 (T3) subset, which contains most newly formed cells undergoing anergic death, could also include mature B cells destined for elimination.

Methodology/Principal Findings

To interrogate this hypothesis and its implications, we applied mathematical models to previously generated in vivo labeling data. Our analyses reveal that the death rate of T3 B cells is far higher than the death rates of all other splenic B cell subpopulations. Further, the model, in which the T3 pool includes both newly formed and mature primary B cells destined for apoptotic death, shows that this cell loss may account for nearly all mature B cell turnover.

Conclusions/Significance

This finding has implications for the mechanism of normal mature B cell turnover.

Dysregulation of germinal centres in autoimmune disease

In germinal centres, somatic hypermutation and B cell selection increase antibody affinity and specificity for the immunizing antigen, but the generation of autoreactive B cells is an inevitable by-product of this process. Here, we review the evidence that aberrant selection of these autoreactive B cells can arise from abnormalities in each of the germinal centre cellular constituents--B cells, T follicular helper cells, follicular dendritic cells and tingible body macrophages--or in the supply of antigen. As the progeny of germinal centre B cells includes long-lived plasma cells, selection of autoreactive B cells can propagate long-lived autoantibody responses and cause autoimmune diseases. Elucidation of crucial molecular signals in germinal centres has led to the identification of novel therapeutic targets.

Suppression of phosphatidylinositol 3,4,5-trisphosphate production is a key determinant of B cell anergy

Anergy is a critical physiologic mechanism to sensor self-reactive B cells. However, a biochemical understanding of how anergy is achieved and maintained is lacking. Herein, we investigated the role of the phosphoinositide 3-kinase (PI3K) lipid product PI(3,4,5)P(3) in B cell anergy. We found reduced generation of PI(3,4,5)P(3) in anergic B cells, which was attributable to reduced phosphorylation of the PI3K membrane adaptor CD19, as well as increased expression of the inositol phosphatase PTEN. Sustained production of PI(3,4,5)P(3) in B cells, achieved through conditional deletion of Pten, resulted in failed tolerance induction and abundant autoantibody production. In contrast to wild-type immature B cells, B cell receptor engagement of PTEN-deficient immature B cells resulted in activation and proliferation, indicating a central defect in early B cell responsiveness. These findings establish repression of the PI3K signaling pathway as a necessary condition to avert the generation, activation, and persistence of self-reactive B cells.

CD25+ T(reg) specifically suppress auto-Ab generation against pancreatic tissue autoantigens

To study B-cell tolerance against non-lymphoid tissue autoantigens, we generated transgenic rat insulin promoter (RIP)-OVA/hen egg lysozyme (HEL) mice expressing the model antigens, OVA and HEL, in pancreatic islets. Their vaccination with OVA or HEL induced far less auto-Ab titers compared with non-transgenic controls. Depletion of CD25(+) cells during immunization completely restored auto-Ab production, but did not affect antibodies against a foreign control antigen. Depletion at later time-points was not effective. OVA-specific CD25(+) FoxP3(+) T(reg) were more frequent in the autoantigen-draining pancreatic LN than in other secondary lymphatics of RIP-OVA/HEL mice. Consistently, B cells were suppressed in that LN and also in the spleen, which is known to concentrate circulating antigen, such as the antigens used for vaccination. Suppression involved preventing expansion of autoreactive B cells in response to autoantigen, reducing antibody production per B-cell and isotype changes. These findings demonstrate that CD25(+) T(reg) suppress auto-Ab production against non-lymphoid tissue antigens in an antigen-specific manner.

Central tolerance regulates B cells reactive with Goodpasture antigen alpha3(IV)NC1 collagen

Patients and rodents with Goodpasture's syndrome (GPS) develop severe autoimmune crescentic glomerulonephritis, kidney failure, and lung hemorrhage due to binding of pathogenic autoantibodies to the NC1 domain of the alpha3 chain of type IV collagen. Target epitopes are cryptic, normally hidden from circulating Abs by protein-protein interactions and the highly tissue-restricted expression of the alpha3(IV) collagen chain. Based on this limited Ag exposure, it has been suggested that target epitopes are not available as B cell tolerogens. To determine how pathogenic anti-GPS autoantibody responses are regulated, we generated an Ig transgenic (Tg) mouse model that expresses an Ig that binds alpha3(IV)NC1 collagen epitopes recognized by serum IgG of patients with GPS. Phenotypic analysis reveals B cell depletion and L chain editing in Tg mice. To determine the default tolerance phenotype in the absence of receptor editing and endogenous lymphocyte populations, we crossed Tg mice two generations with mice deficient in Rag. Resulting Tg Rag-deficient mice have central B cell deletion. Thus, development of Tg anti-alpha3(IV)NC1 collagen B cells is halted in the bone marrow, at which point the cells are deleted unless rescued by a Rag enzyme-dependent process, such as editing. The central tolerance phenotype implies that tolerizing self-Ag is expressed in bone marrow.

Flow cytometric analysis of cell division by dye dilution

The technique described in this unit uses the intracellular fluorescent label carboxyfluorescein diacetate succinimidyl ester (CFSE) to tag proliferating cells. Covalently bound CFSE is divided equally between daughter cells, allowing discrimination of successive rounds of cell division. The technique is applicable to in vitro cell division as well as to in vivo division of adoptively transferred cells and can resolve up to eight successive generations. CFSE is long lived, permitting analysis for several months after transfer, and has the same spectral characteristics as fluorescein, so monoclonal antibodies conjugated to phycoerythrin or other compatible fluorochromes may be used to immunophenotype the dividing cells.

Manipulating B cell homeostasis: a key component in the advancement of targeted strategies

Understanding the homeostatic mechanisms governing lymphocyte pools achieves critical importance as lymphocyte-targeted therapies expand in use and scope. The primacy of B lymphocyte stimulator (BLyS) family ligands and receptors in governing B lymphocyte homeostasis has become increasingly clear in recent years, affording insight into novel opportunities and potential pitfalls for targeted B cell therapeutics. Interclonal competition for BLyS-BR3 interactions determines the size of naïve B cell pools and can regulate the stringency of selection applied as cells complete maturation. Thus one of the predicted consequences of ablative therapies targeting primary pools is relaxed negative selection. This suggests that BLyS levels and B cell reconstitution rates may serve useful prognostic roles and that BLyS itself might be targeted to circumvent relapse. Alternatively, manipulations that allow rare, minimally autoreactive specificities to survive and mature may lead to opportunities in cases where antibody-based vaccine development has heretofore been unsuccessful. BLyS family ligands and receptors also play a role in activated and memory B cell pools, suggesting they might likewise be targeted to promote or delete particular antigen-experienced subpopulations in a similar way.

Sites and stages of autoreactive B cell activation and regulation

B cells are essential for the development and pathogenesis of both systemic and organ-specific autoimmune diseases. Autoreactive B cells are typically thought of as sources of autoantibody, but their most important pathogenetic roles may be to present autoantigens to T cells and to secrete proinflammatory cytokines. A rate-limiting step in the genesis of autoimmunity then is the activation of autoreactive B cells. Here, mechanisms are discussed that normally prevent such activation and how they break down during disease. Integrating classic work with recent insights, emphasis is placed on efforts to pinpoint the precursor cells for autoantibody-secreting cells and the unique stimuli and pathways by which they are activated.

Multiple paths to loss of anergy and gain of autoimmunity

B cells and autoimmunity: cells of the immune system have the capacity to recognize/neutralize a myriad array of disease-causing pathogens, while simultaneously minimizing damage to self tissue. Obvious breakdowns in this ability to distinguish between self and non-self are evident in multiple forms of autoimmune disease, where B and T cells mount damaging attacks on cells and organs. B cells may directly damage tissue by producing pathogenic antibodies that bind self antigen, fix complement or form immune complexes. Recent evidence also suggests B cells indirectly induce autoimmunity by concentrating low avidity self antigen through the B cell receptor and presenting self-peptides to autoreactive T cells. B cells may also initiate autoimmunity when provided sufficient help from autoreactive T cells that have escaped deletion in the thymus. Here, we will review the role of anergy in maintenance of tolerance and how alterations in the normal balance of positive and negative signals may contribute to the development of autoimmune disease in mouse models and humans.

Peripheral deletion of mature alloreactive B cells induced by costimulation blockade

Alloreactive B cells can contribute to graft rejection. Anti-CD154 treatment together with donor-specific transfusion (DST) results in the long-term survival of MHC-mismatched mouse heart grafts and inhibition of alloantibody production. To characterize the mechanism of B cell tolerance induced by the anti-CD154 and DST, we used 3-83Igi mice, on BALB/c (H-2K(d)) background, that express a B cell receptor that reacts with MHC class I antigens H-2K(b). Transplanting C57BL/6 (H-2K(b)) hearts into 3-83Igi mice, followed by tolerance induction, resulted in the peripheral deletion of mature but not immature 3-83 B cells. The sustained deletion of mature alloreactive B cells required the presence of the allograft and can be explained by the absence of T cell help.