Developmental regulation of B lymphocyte immune tolerance compartmentalizes clonal selection from receptor selection

Aberrant B Cell Signaling in Autoimmune Diseases

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

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

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

B Cells and Microbiota in Autoimmunity

Trillions of microorganisms inhabit the mucosal membranes maintaining a symbiotic relationship with the host's immune system. B cells are key players in this relationship because activated and differentiated B cells produce secretory immunoglobulin A (sIgA), which binds commensals to preserve a healthy microbial ecosystem. Mounting evidence shows that changes in the function and composition of the gut microbiota are associated with several autoimmune diseases suggesting that an imbalanced or dysbiotic microbiota contributes to autoimmune inflammation. Bacteria within the gut mucosa may modulate autoimmune inflammation through different mechanisms from commensals ability to induce B-cell clones that cross-react with host antigens or through regulation of B-cell subsets' capacity to produce cytokines. Commensal signals in the gut instigate the differentiation of IL-10 producing B cells and IL-10 producing IgA+ plasma cells that recirculate and exert regulatory functions. While the origin of the dysbiosis in autoimmunity is unclear, compelling evidence shows that specific species have a remarkable influence in shaping the inflammatory immune response. Further insight is necessary to dissect the complex interaction between microorganisms, genes, and the immune system. In this review, we will discuss the bidirectional interaction between commensals and B-cell responses in the context of autoimmune inflammation.

Homeostasis and regulation of autoreactive B cells

In contrast to the previous belief that autoreactive B cells are eliminated from the normal repertoire of B cells, many autoreactive B cells actually escape clonal deletion and develop into mature B cells. These autoreactive B cells in healthy individuals perform some beneficial functions in the host and are homeostatically regulated by regulatory T and B cells or other mechanisms to prevent autoimmune diseases. Autoreactive B-1 cells constitutively produce polyreactive natural antibodies for tissue homeostasis. Recently, autoreactive follicular B cells were reported to participate actively in the germinal center reaction. Furthermore, the selection and usefulness of autoreactive marginal zone (MZ) B cells found in autoimmune diseases are not well understood, although the repertoire of MZ B-cell receptors (BCRs) is presumed to be biased to detect bacterial antigens. In this review, we discuss the autoreactive B-cell populations among all three major B-cell subsets and their regulation in immune responses and diseases.

The c-Myc/miR17-92/PTEN Axis Tunes PI3K Activity to Control Expression of Recombination Activating Genes in Early B Cell Development

Appropriate PI3K signals generated by the antigen receptor are essential to promote B cell development. Regulation of recombination activating gene (RAG)-1 and RAG-2 expression is one key process that is mediated by PI3K to ensure developmental progression and selection. When PI3K signals are too high or too low, expression of RAGs does not turn off and B cell development is impaired or blocked. Yet, the mechanism which tunes PI3K activity to control RAG expression during B cell development in the bone marrow is unknown. Recently we showed that a c-Myc/miR17-92/PTEN axis regulates PI3K activity for positive and negative selection of immature B cells. Here, we show that the c-Myc/miR17-92/PTEN axis tunes PI3K activity to control the expression of RAGs in proB cells. Using different genetically engineered mouse models we show that impaired function of the c-Myc/miR17-92/PTEN axis alters the PI3K/Akt/Foxo1 pathway to result in dis-regulated expression of RAG and a block in B cell development. Studies using 38c-13 B lymphoma cells, where RAGs are constitutively expressed, suggest that this regulatory effect is mediated post-translationally through Foxo1.

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.

Follicular Dendritic Cell Activation by TLR Ligands Promotes Autoreactive B Cell Responses

A hallmark of autoimmunity in murine models of lupus is the formation of germinal centers (GCs) in lymphoid tissues where self-reactive B cells expand and differentiate. In the host response to foreign antigens, follicular dendritic cells (FDCs) maintain GCs through the uptake and cycling of complement-opsonized immune complexes. Here, we examined whether FDCs retain self-antigens and the impact of this process in autoantibody secretion in lupus. We found that FDCs took up and retained self-immune complexes composed of ribonucleotide proteins, autoantibody, and complement. This uptake, mediated through CD21, triggered endosomal TLR7 and led to the secretion of interferon (IFN) α via an IRF5-dependent pathway. Blocking of FDC secretion of IFN-α restored B cell tolerance and reduced the amount of GCs and pathogenic autoantibody. Thus, FDCs are a critical source of the IFN-α driving autoimmunity in this lupus model. This pathway is conserved in humans, suggesting that it may be a viable therapeutic target in systemic lupus erythematosus.

Mechanisms of central tolerance for B cells

Immune tolerance hinders the potentially destructive responses of lymphocytes to host tissues. Tolerance is regulated at the stage of immature B cell development (central tolerance) by clonal deletion, involving apoptosis, and by receptor editing, which reprogrammes the specificity of B cells through secondary recombination of antibody genes. Recent mechanistic studies have begun to elucidate how these divergent mechanisms are controlled. Single-cell antibody cloning has revealed defects of B cell central tolerance in human autoimmune diseases and in several human immunodeficiency diseases caused by single gene mutations, which indicates the relevance of B cell tolerance to disease and suggests possible genetic pathways that regulate tolerance.

Receptor editing and genetic variability in human autoreactive B cells

Lang et al. show in a humanized mouse model that human B cells undergo central tolerance via a combination of receptor editing and clonal deletion.

The mechanisms by which B cells undergo tolerance, such as receptor editing, clonal deletion, and anergy, have been established in mice. However, corroborating these mechanisms in humans remains challenging. To study how autoreactive human B cells undergo tolerance, we developed a novel humanized mouse model. Mice expressing an anti–human Igκ membrane protein to serve as a ubiquitous neo self-antigen (Ag) were transplanted with a human immune system. By following the fate of self-reactive human κ⁺ B cells relative to nonautoreactive λ⁺ cells, we show that tolerance of human B cells occurs at the first site of self-Ag encounter, the bone marrow, via a combination of receptor editing and clonal deletion. Moreover, the amount of available self-Ag and the genetics of the cord blood donor dictate the levels of central tolerance and autoreactive B cells in the periphery. Thus, this model can be useful for studying specific mechanisms of human B cell tolerance and to reveal differences in the extent of this process among human populations.

Calcium channels in Fc receptor signaling

The calcium ion (Ca(2+)) is the main common second messenger involved in signaling transduction subsequent to immunoreceptor activation. Its rapid intracellular elevation induces multiple cellular responses, such as secretion, proliferation, mobility, and gene transcription. Intracellular levels of Ca(2+) need to reach a specific threshold to efficiently transduce the signal to activate transcription factors through the recruitment of Ca(2+)-binding molecules. However, since Ca(2+) cannot be metabolized, its intracellular concentration is tightly regulated to avoid the induction of programmed cell death. This highly controlled regulation of Ca(2+) homeostasis has recently been clarified by the uncovering of new ion channels. The regulation of these channels allows the role of Ca(2+) in Fc receptor transduction pathways to be more precisely defined.

Accumulation of VH Replacement Products in IgH Genes Derived from Autoimmune Diseases and Anti-Viral Responses in Human

V_H replacement refers to RAG-mediated secondary recombination of the IgH genes, which renews almost the entire V_H gene coding region but retains a short stretch of nucleotides as a V_H replacement footprint at the newly generated V_H–D_H junction. To explore the biological significance of V_H replacement to the antibody repertoire, we developed a Java-based V_H replacement footprint analyzer program and analyzed the distribution of V_H replacement products in 61,851 human IgH gene sequences downloaded from the NCBI database. The initial assignment of the V_H, D_H, and J_H gene segments provided a comprehensive view of the human IgH repertoire. To our interest, the overall frequency of V_H replacement products is 12.1%; the frequencies of V_H replacement products in IgH genes using different V_H germline genes vary significantly. Importantly, the frequencies of V_H replacement products are significantly elevated in IgH genes derived from different autoimmune diseases, including rheumatoid arthritis, systemic lupus erythematosus, and allergic rhinitis, and in IgH genes encoding various autoantibodies or anti-viral antibodies. The identified V_H replacement footprints preferentially encoded charged amino acids to elongate IgH CDR3 regions, which may contribute to their autoreactivities or anti-viral functions. Analyses of the mutation status of the identified V_H replacement products suggested that they had been actively involved in immune responses. These results provide a global view of the distribution of V_H replacement products in human IgH genes, especially in IgH genes derived from autoimmune diseases and anti-viral immune responses.

Activation of Ras overcomes B-cell tolerance to promote differentiation of autoreactive B cells and production of autoantibodies

Newly generated immature B cells are selected to enter the peripheral mature B-cell pool only if they do not bind (or bind limited amount of) self-antigen. We previously suggested that this selection relies on basal extracellular signal-regulated kinase (Erk) activation mediated by tonic B-cell antigen receptor (BCR) signaling and that this signal can be replaced by an active rat sarcoma (Ras), which are small GTPase proteins. In this study we compared the activity of Ras and Erk in nonautoreactive and autoreactive immature B cells and investigated whether activation of Ras can break tolerance. Our results demonstrate lower levels of active Erk and Ras in autoreactive immature B cells, although this is evident only when these cells display medium/high avidity for self-antigen. Basal activation of Erk in immature B cells is proportional to surface IgM and dependent on sarcoma family kinases, whereas it is independent of B-cell activating factor, IFN, and Toll-like receptor signaling. Ectopic expression of the constitutively active mutant Ras form N-RasD12 in autoreactive cells raises active Erk, halts receptor editing via PI3 kinase, and promotes differentiation via Erk, breaking central tolerance. Moreover, when B cells coexpress autoreactive and nonautoreactive BCRs, N-RasD12 leads also to a break in peripheral tolerance with the production of autoantibodies. Our findings indicate that in immature B cells, basal activation of Ras and Erk are controlled by tonic BCR signaling, and that positive changes in Ras activity can lead to a break in both central and peripheral B-cell tolerance.

Role of the Igh intronic enhancer Eμ in clonal selection at the pre-B to immature B cell transition

We previously described a checkpoint for allelic exclusion that occurs at the pre-B cell to immature B cell transition and is dependent upon the IgH intronic enhancer, Eμ. We now provide evidence that the breach in allelic exclusion associated with Eμ deletion results from decreased Igμ levels that make it difficult for emerging BCRs to reach the signaling threshold required for positive selection into the immature B cell compartment. We show that this compartment is smaller in mice carrying an Eμ-deficient, but functional, IgH allele (VHΔ(a)). Pre-B cells in such mice produce ≈ 50% wild-type levels of Igμ (mRNA and protein), and this is associated with diminished signals, as measured by phosphorylation of pre-BCR/BCR downstream signaling proteins. Providing Eμ-deficient mice with a preassembled VL gene led not only to a larger immature B cell compartment but also to a decrease in "double-producers," suggesting that H chain/L chain combinations with superior signaling properties can overcome the signaling defect associated with low Igμ-chain and can eliminate the selective advantage of "double-producers" that achieve higher Igμ-chain levels through expression of a second IgH allele. Finally, we found that "double-producers" in Eμ-deficient mice include a subpopulation with autoreactive BCRs. We infer that BCRs with IgH chain from the Eμ-deficient allele are ignored during negative selection owing to their comparatively low density. In summary, these studies show that Eμ's effect on IgH levels at the pre-B cell to immature B cell transition strongly influences allelic exclusion, the breadth of the mature BCR repertoire, and the emergence of autoimmune B cells.

RasGRP Ras guanine nucleotide exchange factors in cancer

RasGRP proteins are activators of Ras and other related small GTPases by the virtue of functioning as guanine nucleotide exchange factors (GEFs). In vertebrates, four RasGRP family members have been described. RasGRP-1 through -4 share many structural domains but there are also subtle differences between each of the different family members. Whereas SOS RasGEFs are ubiquitously expressed, RasGRP proteins are expressed in distinct patterns, such as in different cells of the hematopoietic system and in the brain. Most studies have concentrated on the role of RasGRP proteins in the development and function of immune cell types because of the predominant RasGRP expression profiles in these cells and the immune phenotypes of mice deficient for Rasgrp genes. However, more recent studies demonstrate that RasGRPs also play an important role in tumorigenesis. Examples are skin- and hematological-cancers but also solid malignancies such as melanoma or prostate cancer. These novel studies bring up many new and unanswered questions related to the molecular mechanism of RasGRP-driven oncogenesis, such as new receptor systems that RasGRP appears to respond to as well as regulatory mechanism for RasGRP expression that appear to be perturbed in these cancers. Here we will review some of the known aspects of RasGRP biology in lymphocytes and will discuss the exciting new notion that RasGRP Ras exchange factors play a role in oncogenesis downstream of various growth factor receptors.

Censoring of self-reactive B cells by follicular dendritic cell-displayed self-antigen

In the secondary lymphoid organs, intimate contact with follicular dendritic cells (FDCs) is required for B cell retention and Ag-driven selection during the germinal center response. However, selection of self-reactive B cells by Ag on FDCs has not been addressed. To this end, we generated a mouse model to conditionally express a membrane-bound self-antigen on FDCs and to monitor the fate of developing self-reactive B cells. In this article, we show that self-antigen displayed on FDCs mediates effective elimination of self-reactive B cells at the transitional stage. Notwithstanding, some self-reactive B cells persist beyond this checkpoint, showing evidence of Ag experience and intact proximal BCR signaling, but they are short-lived and unable to elicit T cell help. These results implicate FDCs as an important component of peripheral B cell tolerance that prevents the emergence of naive B cells capable of responding to sequestered self-antigens.

Antigen and cytokine receptor signals guide the development of the naïve mature B cell repertoire

Immature B cells are generated daily in the bone marrow tissue. More than half of the newly generated immature B cells are autoreactive and bind a self-antigen, while the others are nonautoreactive. A selection process has evolved on the one hand to thwart development of autoreactive immature B cells and, on the other hand, to promote further differentiation of nonautoreactive immature B cells into transitional and mature B cells. These negative and positive selection events are carefully regulated by signals that emanate from the antigen receptor, whether antigen-mediated or tonic, and are influenced by signals that are generated by receptors that bind cytokines, chemokines, and other factors produced in the bone marrow tissue. These signals, therefore, are the predominant driving forces for the generation of a B cell population that is capable of protecting the body from infections while maintaining self-tolerance. Here, we review recent findings from our group and others that describe how tonic antigen receptor signaling and bone marrow cytokines regulate the selection of immature B cells.

Regulation of VH replacement by B cell receptor-mediated signaling in human immature B cells

VH replacement provides a unique RAG-mediated recombination mechanism to edit nonfunctional IgH genes or IgH genes encoding self-reactive BCRs and contributes to the diversification of Ab repertoire in the mouse and human. Currently, it is not clear how VH replacement is regulated during early B lineage cell development. In this article, we show that cross-linking BCRs induces VH replacement in human EU12 μHC(+) cells and in the newly emigrated immature B cells purified from peripheral blood of healthy donors or tonsillar samples. BCR signaling-induced VH replacement is dependent on the activation of Syk and Src kinases but is inhibited by CD19 costimulation, presumably through activation of the PI3K pathway. These results show that VH replacement is regulated by BCR-mediated signaling in human immature B cells, which can be modulated by physiological and pharmacological treatments.

Contribution of V(H) replacement products in mouse antibody repertoire

V_H replacement occurs through RAG-mediated recombination between the cryptic recombination signal sequence (cRSS) near the 3′ end of a rearranged V_H gene and the 23-bp RSS from an upstream unrearranged V_H gene. Due to the location of the cRSS, V_H replacement leaves a short stretch of nucleotides from the previously rearranged V_H gene at the newly formed V-D junction, which can be used as a marker to identify V_H replacement products. To determine the contribution of V_H replacement products to mouse antibody repertoire, we developed a Java-based V_H Replacement Footprint Analyzer (V_HRFA) program and analyzed 17,179 mouse IgH gene sequences from the NCBI database to identify V_H replacement products. The overall frequency of V_H replacement products in these IgH genes is 5.29% based on the identification of pentameric V_H replacement footprints at their V-D junctions. The identified V_H replacement products are distributed similarly in IgH genes using most families of V_H genes, although different families of V_H genes are used differentially. The frequencies of V_H replacement products are significantly elevated in IgH genes derived from several strains of autoimmune prone mice and in IgH genes encoding autoantibodies. Moreover, the identified V_H replacement footprints in IgH genes from autoimmune prone mice or IgH genes encoding autoantibodies preferentially encode positively charged amino acids. These results revealed a significant contribution of V_H replacement products to the diversification of antibody repertoire and potentially, to the generation of autoantibodies in mice.

Aging and neoteny in the B lineage

Aging and the physiologic decline of tissues and cells were once thought to be irreversible. However, recent studies suggest that various tissues, especially parts of the hematopoietic system, can be rejuvenated. Here we review potential mechanisms for this process and how they may be used to reverse age-related disorders and aging in general. We propose the novel hypothesis that altering the homeostatic process during cellular depletion can reverse aging in the hematopoietic system.

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.

Self-recognition and clonal selection: autoreactivity drives the generation of B cells

The diversity of B cell receptor (BCR) specificities is generated by VDJ recombination of gene segments during early B cell development, a process which bears the risk of producing BCRs that recognize and lead to the destruction of self-structures. Traditional thoughts have mainly focused on how such putatively dangerous specificities are dealt with and in how they contribute to the development of autoimmune diseases. However, a positive or even necessary role of self-recognition during B cell development has rarely been taken into account. Now, considerable data reveal that the pre-B cell receptor (pre-BCR), which marks an important checkpoint during B cell development, acts as a surrogate autoreactive receptor. This review outlines how autoreactivity is necessary for efficient B cell development and how autoreactive receptors drive positive selection, leading to a diverse repertoire of receptor specificities in the mature B cell pool.

Immature B cells preferentially switch to IgE with increased direct Sμ to Sε recombination

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Immunoglobulin heavy chain (IgH) class-switch recombination (CSR) replaces initially expressed Cμ (IgM) constant regions (C_H) exons with downstream C_H exons. Stimulation of B cells with anti-CD40 plus interleukin-4 induces CSR from Cμ to Cγ1 (IgG1) and Cε (IgE), the latter of which contributes to the pathogenesis of atopic diseases. Although Cε CSR can occur directly from Cμ, most mature peripheral B cells undergo CSR to Cε indirectly, namely from Cμ to Cγ1, and subsequently to Cε. Physiological mechanisms that influence CSR to Cγ1 versus Cε are incompletely understood. In this study, we report a role for B cell developmental maturity in IgE CSR. Based in part on a novel flow cytometric IgE CSR assay, we show that immature B cells preferentially switch to IgE versus IgG1 through a mechanism involving increased direct CSR from Cμ to Cε. Our findings suggest that IgE dysregulation in certain immunodeficiencies may be related to impaired B cell maturation.

BAFF-R expression correlates with positive selection of immature B cells

The interaction between BAFF and BAFF-R is crucial for the development of mature B cells. Here, we report that the expression of BAFF-R is first detectable on a fraction of mouse CD19(+) CD93(+) IgM(+) CD23(-) and human CD19(+) CD10(+) IgM(+) BM B cells. This BAFF-R(+) BM B-cell population shows higher levels of surface IgM expression and decreased RAG-2 transcripts than BAFF-R(-) immature B cells. When cultured, mouse BAFF-R(-), but not BAFF-R(+) immature B cells spontaneously undergo B-cell receptor editing. However, BAFF-R(+) immature B cells cultured in the presence of an anti-κ light chain antibody are induced to undergo receptor editing. This receptor editing correlates with down-modulation of surface BAFF-R expression and the up-regulation of RAG-2 at the RNA level. B-cell receptor (BCR) cross-linking on splenic T1 B cells results in down-modulation of the BAFF-R, and receptor editing and RAG-2 up-regulation in a minor fraction of B cells. BCR cross-linking on splenic T2/3 B cells results in partly down and partly up-modulation of BAFF-R expression and no evidence for receptor editing. Overall, our data indicate that BAFF-R expression is tightly regulated during B-cell development in mouse and human and its expression is correlated with positive selection.

Enforced expression of the apoptosis inhibitor Bcl-2 ablates tolerance induction in DNA-reactive B cells through a novel mechanism

How self tolerance is maintained during B cell development in the bone marrow has been a focal area of study in immunology. Receptor editing, anergy and clonal deletion all play important roles in the regulation of autoimmunity in the immature population. The mechanisms of tolerance induction in the periphery, however, are less well characterized. Overexpression of the apoptosis inhibitor Bcl-2 rescues autoreactive B cells from deletion and can contribute to the development of autoimmune disease in certain genetic backgrounds. Using a peptide-induced autoimmunity model, we recently identified a peripheral tolerance checkpoint in antigen-activated B cells that have undergone class switching and somatic hypermutation. At this checkpoint, receptor editing, induced by antigen engagement, dampened the autoantibody response. In this study, we show that receptor editing fails to be induced in antigen-activated DNA-reactive B cells that overexpress Bcl-2 (Bcl-2 Tg). The failure to induce RAG and receptor editing is likely due, at least partially, to the lack of self antigen. First, the levels of circulating DNA and of apoptotic bodies in the spleen of Bcl-2 Tg mice are significantly lower than in control mice. Second, in Bcl-2 Tg mice, RAG can be induced in a population of antigen-activated B cells by providing exogenous soluble antigen. These data suggest that, in addition to its anti-apoptotic activity, Bcl-2 may indirectly inhibit tolerance induction in B cells acquiring anti-nuclear antigen reactivity after peripheral activation by limiting the availability of self antigen.

Somatic hypermutation targeting is influenced by location within the immunoglobulin V region

The observed mutation pattern in immunoglobulin (Ig) V genes from peripheral B cells is influenced by several mechanisms, including the targeting of AID to specific DNA motifs, negative selection of B cells unable to express Ig receptor, and positive selection of B cells that carry affinity-increasing mutations. These influences, combined with biased codon usage, produce the well-known pattern of increased replacement mutation frequency in the CDR regions, and decreased replacement frequency in the framework regions. Through the analysis of over 12,000 mutated sequences, we show that the specific location in the V gene also significantly influences mutation accumulation. While this position-specific effect is partially explained by selection, it appears independently of the CDR/FWR structure. To further explore the specific targeting of SHM, we propose a statistical formalism describing the mutation probability of a sequence through the multiplication of independent probabilities. Using this model, we show that C→G (or G→C) mutations are almost as frequent as C→T and G→A mutations, in contrast with C→A (or G→T) mutations, which are as any other mutation. The proposed statistical framework allows us to precisely quantify the effect of V gene position, mutation substitution type, and micro-sequence specificity on the observed mutation pattern.

T. brucei infection reduces B lymphopoiesis in bone marrow and truncates compensatory splenic lymphopoiesis through transitional B-cell apoptosis

African trypanosomes of the Trypanosoma brucei species are extracellular protozoan parasites that cause the deadly disease African trypanosomiasis in humans and contribute to the animal counterpart, Nagana. Trypanosome clearance from the bloodstream is mediated by antibodies specific for their Variant Surface Glycoprotein (VSG) coat antigens. However, T. brucei infection induces polyclonal B cell activation, B cell clonal exhaustion, sustained depletion of mature splenic Marginal Zone B (MZB) and Follicular B (FoB) cells, and destruction of the B-cell memory compartment. To determine how trypanosome infection compromises the humoral immune defense system we used a C57BL/6 T. brucei AnTat 1.1 mouse model and multicolor flow cytometry to document B cell development and maturation during infection. Our results show a more than 95% reduction in B cell precursor numbers from the CLP, pre-pro-B, pro-B, pre-B and immature B cell stages in the bone marrow. In the spleen, T. brucei induces extramedullary B lymphopoiesis as evidenced by significant increases in HSC-LMPP, CLP, pre-pro-B, pro-B and pre-B cell populations. However, final B cell maturation is abrogated by infection-induced apoptosis of transitional B cells of both the T1 and T2 populations which is not uniquely dependent on TNF-, Fas-, or prostaglandin-dependent death pathways. Results obtained from ex vivo co-cultures of living bloodstream form trypanosomes and splenocytes demonstrate that trypanosome surface coat-dependent contact with T1/2 B cells triggers their deletion. We conclude that infection-induced and possibly parasite-contact dependent deletion of transitional B cells prevents replenishment of mature B cell compartments during infection thus contributing to a loss of the host's capacity to sustain antibody responses against recurring parasitemic waves.

African trypanosomiasis caused by Trypanosoma brucei species is fatal in both humans and animals and cannot be combated by vaccination because of extensive parasite antigenic variation. Effective trypanosome control and clearance from the bloodstream involves the action of antibodies specific for the parasite's highly diverse variable surface glycoprotein antigens. However, experimental infections in mice have shown that trypanosomiasis elicits a rapid process of B cell exhaustion and loss of protective antibody responses. Indeed, both marginal zone B cells, the first line of defense against blood-borne pathogens like T. brucei parasites, and follicular B cells, which are the major source for developing high-affinity antibody-producing plasma cells and memory B cells, become depleted during infection. In addition, existing B-cell memory, both against parasite antigens and non related pathogens, is destroyed early on in infection. Here, we demonstrate that during infection, B cell development is decreased in the bone marrow and early B cell development is taken over by the spleen. However, full maturation of developing B cells is abrogated by the occurrence of transitional B cell apoptosis. This impairs the replenishment of the mature marginal zone and follicular B cell pools and prevents the buildup of protective immunity against successive parasitemic waves.

STIM1, PKC-δ and RasGRP set a threshold for proapoptotic Erk signaling during B cell development

Clonal deletion of autoreactive B cells is crucial for the prevention of autoimmunity, but the signaling mechanisms that regulate this checkpoint remain undefined. Here we characterize a previously unrecognized Ca(2+)-driven pathway for activation of the kinase Erk, which was proapoptotic and biochemically distinct from Erk activation induced by diacylglycerol (DAG). This pathway required protein kinase C-δ (PKC-δ) and the guanine nucleotide-exchange factor RasGRP and depended on the concentration of the Ca(2+) sensor STIM1, which controls the magnitude of Ca(2+) entry. Developmental regulation of these proteins was associated with selective activation of the pathway in B cells prone to negative selection. This checkpoint was impaired in PKC-δ-deficient mice, which developed B cell autoimmunity. Conversely, overexpression of STIM1 conferred a competitive disadvantage to developing B cells. Our findings establish Ca(2+)-dependent Erk signaling as a critical proapoptotic pathway that mediates the negative selection of B cells.

Trafficking of B cell antigen in lymph nodes

The clonal selection theory first proposed by Macfarlane Burnet is a cornerstone of immunology (1). At the time, it revolutionized the thinking of immunologists because it provided a simple explanation for lymphocyte specificity, immunological memory, and elimination of self-reactive clones (2). The experimental demonstration by Nossal & Lederberg (3) that B lymphocytes bear receptors for a single antigen raised the central question of where B lymphocytes encounter antigen. This question has remained mostly unanswered until recently. Advances in techniques such as multiphoton intravital microscopy (4, 5) have provided new insights into the trafficking of B cells and their antigen. In this review, we summarize these advances in the context of our current view of B cell circulation and activation.

B cell receptor editing in tolerance and autoimmunity

Receptor editing is the process of ongoing antibody gene rearrangement in a lymphocyte that already has a functional antigen receptor. The expression of a functional antigen receptor will normally terminate further rearrangement (allelic exclusion). However, lymphocytes with autoreactive receptors have a chance at escaping negative regulation by "editing" the specificities of their receptors with additional antibody gene rearrangements. As such, editing complicates the Clonal Selection Hypothesis because edited cells are not simply endowed for life with a single, invariant antigen receptor. Furthermore, if the initial immunoglobulin gene is not inactivated during the editing process, allelic exclusion is violated and the B cell can exhibit two specificities. Here, we describe the discovery of editing, the pathways of receptor editing at the heavy (H) and light (L) chain loci, and current evidence regarding how and where editing happens and what effects it has on the antibody repertoire.

Pre-B-cell leukemias in Btk/Slp65-deficient mice arise independently of ongoing V(D)J recombination activity

The adapter protein Slp65 and Bruton's tyrosine kinase (Btk) are key components of the precursor-B (pre-B) cell receptor (pre-BCR) signaling pathway. Slp65-deficient mice spontaneously develop pre-B-cell leukemia, expressing high levels of the pre-BCR on their cell surface. As leukemic Slp65-deficient pre-B cells express the recombination activating genes (Rag)1 and Rag2, and manifest ongoing immunoglobulin (Ig) light-chain rearrangement, it has been hypothesized that deregulated recombinase activity contributes to malignant transformation. In this report, we investigated whether Rag-induced DNA damage is involved in oncogenic transformation of Slp65-deficient B cells. We employed Btk/Slp65 double-deficient mice carrying an autoreactive 3-83μδ BCR transgene. When developing B cells in their bone marrow express this BCR, the V(D)J recombination machinery will be activated, allowing for secondary Ig light-chain gene rearrangements to occur. This phenomenon, called receptor editing, will rescue autoreactive B cells from apoptosis. We observed that 3-83μδ transgenic Btk/Slp65 double-deficient mice developed B-cell leukemias expressing both the 3-83μδ BCR and the pre-BCR components λ5/VpreB. Importantly, such leukemias were found at similar frequencies in mice concomitantly deficient for Rag1 or the non-homologous end-joining factor DNA-PKcs. We therefore conclude that malignant transformation of Btk/Slp65 double-deficient pre-B cells is independent of deregulated V(D)J recombination activity.

The MAPK/ERK and PI3K pathways additively coordinate the transcription of recombination-activating genes in B lineage cells

Rag-1 and Rag-2 are essential for the construction of the BCR repertoire. Regulation of Rag gene expression is tightly linked with BCR expression and signaling during B cell development. Earlier studies have shown a major role of the PI(3)K/Akt pathway in regulating the transcription of Rag genes. In this study, by using the 38c13 murine B cell lymphoma we show that transcription of Rag genes is also regulated by the MEK/ERK pathways, and that both pathways additively coordinate in this regulation. The additive effect is observed for both ligand-dependent (upon BCR ligation) and ligand independent (tonic) signals. However, whereas the PI(3)K/Akt regulation of Rag transcription is mediated by Foxo1, we show in this study that the MEK/ERK pathway coordinates with the regulation of Rag by controlling the phosphorylation and turnover of E47 and its consequential binding to the Rag enhancer regions. Our results suggest that the PI(3)K and MEK/ERK pathways additively coordinate in the regulation of Rag transcription in an independent manner.

Peripheral B cell tolerance and function in transgenic mice expressing an IgD superantigen

Transitional B cells turn over rapidly in vivo and are sensitive to apoptosis upon BCR ligation in vitro. However, little direct evidence addresses their tolerance sensitivity in vivo. A key marker used to distinguish these cells is IgD, which, through alternative RNA splicing of H chain transcripts, begins to be coexpressed with IgM at this stage. IgD is also expressed at high levels on naive follicular (B-2) and at lower levels on marginal zone and B-1 B cells. In this study, mice were generated to ubiquitously express a membrane-bound IgD-superantigen. These mice supported virtually no B-2 development, a greatly reduced marginal zone B cell population, but a relatively normal B-1 compartment. B cell development in the spleen abruptly halted at the transitional B cell population 1 to 2 stage, a block that could not be rescued by either Bcl-2 or BAFF overexpression. The developmentally arrested B cells appeared less mature and turned over more rapidly than nontransgenic T2 cells, exhibiting neither conventional features of anergy nor appreciable receptor editing. Paradoxically, type-2 T-independent responses were more robust in the transgenic mice, although T-dependent responses were reduced and had skewed IgL and IgH isotype usages. Nevertheless, an augmented memory response to secondary challenge was evident. The transgenic mice also had increased serum IgM, but diminished IgG, levels mirrored by the increased numbers of IgM(+) plasma cells. This model should facilitate studies of peripheral B cell tolerance, with the advantages of allowing analysis of polyclonal populations, and of B cells naturally lacking IgD.

Inositol 1,4,5-trisphosphate 3-kinase B controls survival and prevents anergy in B cells

Inositol 1,4,5-trisphosphate 3-kinase B (or Itpkb) and inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P4), its reaction product, play an important role in the control of B lymphocyte fate and function in vivo. In order to investigate the fine mechanisms of Itpkb and Ins(1,3,4,5)P4 action in B cells, we crossed Itpkb(-/-) mice with transgenic mice expressing a 3-83μδ B cell receptor (BCR) specific for membrane-bound MHC-I H2-K(b) and H2-K(k) molecules. On a non-deleting H2-K(d) genetic background, we show that Itpkb is important for the control of Bim protein expression and B cell survival rather than for the control of B cell development from one stage to another. Analyses of cell surface markers expression, proapoptotic Bim protein expression, in vitro survival and in vivo turnover demonstrated that BCR transgenic Itpkb(-/-) B cells exhibit an anergic phenotype with the notable exception of their enhanced antigen-induced calcium signalling. On a deleting H2-K(b) genetic background, we show that Itpkb is not essential for BCR editing or negative selection. These data establish Itpkb as an important regulator of B cell survival and anergy in vivo.

TLR4 promotes B cell maturation: independence and cooperation with B lymphocyte-activating factor

We have previously shown that TLR4 triggering promotes the generation of CD23(+)CD93(+) transitional T2-like cells in vitro from mouse B cell precursors, suggesting a possible role for this receptor in B cell maturation. In this study, we perform an extensive study of cell surface markers and functional properties of B cells matured in vitro with LPS, comparatively with the well-known B cell maturation factor B lymphocyte-activating factor (BAFF). LPS increased generation of CD23(+) transitional B cells in a TLR4-dependent way, upregulating IgD and CD21 and downregulating CD93, without inducing cell proliferation, in a manner essentially equivalent to BAFF. For both BAFF and LPS, functional maturation of the IgM(+)CD23(+)CD93(+) cells was confirmed by their higher proliferative response to anti-CD40 plus IL-4 compared with IgM(+)CD23(neg)CD93(+) cells. BAFF-R-Fc-mediated neutralization experiments showed that TLR4-induced B cell maturation was independent of BAFF. Distinct from BAFF, maturation by LPS relied on the activation of canonical NF-kappaB pathway, and the two factors together had complementary effects, leading to higher numbers of IgM(+)CD23(+)CD93(+) cells with their simultaneous addition. Importantly, BCR cross-linking abrogated the generation of CD23(+) B cells by LPS or BAFF, indicating that signals mimicking central tolerance act on both systems. Addition of cyclosporin A reverted BCR-mediated inhibition, both for BAFF and LPS, suggesting similar regulation of signaling pathways by calcineurin. Finally, LPS-injected mice showed a rapid increase of mature B cells in the bone marrow, suggesting that TLR4 signaling may effectively stimulate B cell maturation in vivo, acting as an accessory stimulus in B cell development, complementary to the BAFF physiological pathway.

Ras activation of Erk restores impaired tonic BCR signaling and rescues immature B cell differentiation

B cell receptors (BCRs) generate tonic signals critical for B cell survival and early B cell development. To determine whether these signals also mediate the development of transitional and mature B cells, we examined B cell development using a mouse strain in which nonautoreactive immunoglobulin heavy and light chain–targeted B cells express low surface BCR levels. We found that reduced BCR expression translated into diminished tonic BCR signals that strongly impaired the development of transitional and mature B cells. Constitutive expression of Bcl-2 did not rescue the differentiation of BCR-low B cells, suggesting that this defect was not related to decreased cell survival. In contrast, activation of the Ras pathway rescued the differentiation of BCR-low immature B cells both in vitro and in vivo, whereas extracellular signal-regulated kinase (Erk) inhibition impaired the differentiation of normal immature B cells. These results strongly suggest that tonic BCR signaling mediates the differentiation of immature into transitional and mature B cells via activation of Erk, likely through a pathway requiring Ras.

B cell receptor and BAFF receptor signaling regulation of B cell homeostasis

B lymphocyte homeostasis depends on tonic and induced BCR signaling and receptors sensitive to trophic factors, such as B cell-activating factor receptor (BAFF-R or BR3) during development and maintenance. This review will discuss growing evidence suggesting that the signaling mechanisms that maintain B cell survival and metabolic fitness during selection at transitional stages and survival after maturation rely on cross-talk between BCR and BR3 signaling. Recent findings have also begun to unravel the molecular mechanisms underlying this crosstalk. In this review I also propose a model for regulating the amplitude of BCR signaling by a signal amplification loop downstream of the BCR involving Btk and NF-kappaB that may facilitate BCR-dependent B cell survival as well as its functional coupling to BR3 for the growth and survival of B lymphocytes.

TLR4-mediated signaling induces MMP9-dependent cleavage of B cell surface CD23

IgE production is inversely regulated by circulating and B cell surface levels of the low affinity IgE receptor, CD23. To begin to understand physiologic determinants of CD23 expression, we analyzed effects of BCR and TLR stimulation on CD23 levels. BCR and TLR 2, 3, 4, 6, and 9 agonists induced CD23 down-modulation from the cell surface. However, among the ligands only TLR4 agonists induced transcriptional activation of CD23 and generation of significant soluble CD23. These responses were induced by LPS both in vitro and in vivo, and were seen in both murine and human B cells. LPS also induced expression of matrix metalloprotease 9 (MMP9) and failed to induce CD23 cleaving activity in MMP9(-/-) cells, thus implicating MMP9 in the LPS-induced release of CD23 from the cell surface. Finally, type 1 transitional B cells uniquely produce MMP9 in response to LPS, suggesting a mechanism wherein endotoxin induces T1 cell expression of MMP9, which mediates cleavage of CD23 on distinct, mature B cells.

Cell- and stage-specific chromatin structure across the Complement receptor 2 (CR2/CD21) promoter coincide with CBF1 and C/EBP-beta binding in B cells

Stringent developmental transcription requires multiple transcription factor (TF) binding sites, cell-specific expression of signaling molecules, TFs and co-regulators and appropriate chromatin structure. During B-lymphopoiesis, human Complement receptor 2 (CR2/CD21) is detected on immature and mature B cells but not on B cell precursors and plasma cells. We examined cell- and stage-specific human CR2 gene regulation using cell lines modeling B-lymphopoiesis. Chromatin accessibility assays revealed a region between -409 and -262 with enhanced accessibility in mature B cells and pre-B cells, compared to either non-lymphoid or plasma cell-types, however, accessibility near the transcription start site (TSS) was elevated only in CR2-expressing B cells. A correlation between histone acetylation and CR2 expression was observed, while histone H3K4 dimethylation was enriched near the TSS in both CR2-expressing B cells and non-expressing pre-B cells. Candidate sites within the CR2 promoter were identified which could regulate chromatin, including a matrix attachment region associated with CDP, SATB1/BRIGHT and CEBP-beta sites as well as two CBF1 sites. ChIP assays verified that both CBF1 and C/EBP-beta bind the CR2 promoter in B cells raising the possibility that these factors facilitate or respond to alterations in chromatin structure to control the timing and/or level of CR2 transcription.

Autoreactive B cell receptors mimic autonomous pre-B cell receptor signaling and induce proliferation of early B cells

The majority of early immature B cells express autoreactive B cell receptors (BCRs) that are, according to the current view, negatively selected to avoid the production of self-reactive antibodies. Here, we show that polyreactive BCRs, which recognize multiple self-antigens, induced autonomous signaling and selective expansion of B cell precursors in a manner comparable to the pre-BCR. We found that the pre-BCR was capable of recognizing multiple self-antigens and that a signaling-deficient pre-BCR lacking the non-Ig region of the surrogate-light-chain component lambda5 was rescued by the complementarity-determining region 3 derived from heavy chains of polyreactive receptors. Importantly, bone marrow B cells from mice carrying Ig transgenes for an autoreactive BCR showed increased cell-cycle activity, which could not be detected in cells lacking the transgenic BCR. Together, the pre-BCR has evolved to ensure self-recognition because autoreactivity is required for positive selection of B cell precursors.

TOLL-like receptor ligands stimulate aberrant class switch recombination in early B cell precursors

TOLL-like receptor (TLR) ligands stimulate class switch recombination (CSR) in mature B cells. We showed earlier that developing B cells in the bone marrow (BM) express TLR9 and are responsive to CpG DNA. Since CSR is a critical process for synthesis of effector antibodies, we studied the competence of precursor B cells to undergo CSR in response to TLR ligands, and the regulation of these cells. We found that CSR is induced throughout B lymphopoiesis in response to CpG and to LPS. However, sequencing analysis revealed aberrant joining of the switch junctions. In addition, we found that this CSR is independent of IgM expression and/or VDJ assembly and is directed to a specific isotype by cytokines. Finally, we found that activation of the switched precursor B cells is regulated by Fas. Thus, BM B cells can be activated by TLR ligands to undergo CSR and to secrete non-IgM antibodies. However, the effector potential of these cells is regulated by the Fas pathway.

Ig knock-in mice producing anti-carbohydrate antibodies: breakthrough of B cells producing low affinity anti-self antibodies

Natural Abs specific for the carbohydrate Ag Galalpha1-3Galbeta1-4GlcNAc-R (alphaGal) play an important role in providing protective host immunity to various pathogens; yet little is known about how production of these or other anti-carbohydrate natural Abs is regulated. In this study, we describe the generation of Ig knock-in mice carrying functionally rearranged H chain and L chain variable region genes isolated from a B cell hybridoma producing alphaGal-specific IgM Ab that make it possible to examine the development of B cells producing anti-carbohydrate natural Abs in the presence or absence of alphaGal as a self-Ag. Knock-in mice on a alphaGal-deficient background spontaneously developed alphaGal-specific IgM Abs of a sufficiently high titer to mediate rejection of alphaGal expressing cardiac transplants. In the spleen of these mice, B cells expressing alphaGal-specific IgM are located in the marginal zone. In knock-in mice that express alphaGal, B cells expressing the knocked in BCR undergo negative selection via receptor editing. Interestingly, production of low affinity alphaGal-specific Ab was observed in mice that express alphaGal that carry two copies of the knocked in H chain. We suggest that in these mice, receptor editing functioned to lower the affinity for self-Ag below a threshold that would result in overt pathology, while allowing development of low affinity anti-self Abs.

Reduced receptor editing in lupus-prone MRL/lpr mice

The initial B cell repertoire contains a considerable proportion of autoreactive specificities. The first major B cell tolerance checkpoint is at the stage of the immature B cell, where receptor editing is the primary mode of eliminating self-reactivity. The cells that emigrate from the bone marrow have a second tolerance checkpoint in the transitional compartment in the spleen. Although it is known that the second checkpoint is defective in lupus, it is not clear whether there is any breakdown in central B cell tolerance in the bone marrow. We demonstrate that receptor editing is less efficient in the lupus-prone strain MRL/lpr. In an in vitro system, when receptor-editing signals are given to bone marrow immature B cells by antiidiotype antibody or after in vivo exposure to membrane-bound self-antigen, MRL/lpr 3-83 transgenic immature B cells undergo less endogenous rearrangement and up-regulate recombination activating gene messenger RNA to a lesser extent than B10 transgenic cells. CD19, along with immunoglobulin M, is down-regulated in the bone marrow upon receptor editing, but the extent of down-regulation is fivefold less in MRL/lpr mice. Less efficient receptor editing could allow some autoreactive cells to escape from the bone marrow in lupus-prone mice, thus predisposing to autoimmunity.