Enforced Bcl-2 expression inhibits antigen-mediated clonal elimination of peripheral B cells in an antigen dose-dependent manner and promotes receptor editing in autoreactive, immature B cells

Kidins220 regulates the development of B cells bearing the λ light chain

The ratio between κ and λ light chain (LC)-expressing B cells varies considerably between species. We recently identified Kinase D-interacting substrate of 220 kDa (Kidins220) as an interaction partner of the BCR. In vivo ablation of Kidins220 in B cells resulted in a marked reduction of λLC-expressing B cells. Kidins220 knockout B cells fail to open and recombine the genes of the Igl locus, even in genetic scenarios where the Igk genes cannot be rearranged or where the κLC confers autoreactivity. Igk gene recombination and expression in Kidins220-deficient B cells is normal. Kidins220 regulates the development of λLC B cells by enhancing the survival of developing B cells and thereby extending the time-window in which the Igl locus opens and the genes are rearranged and transcribed. Further, our data suggest that Kidins220 guarantees optimal pre-BCR and BCR signaling to induce Igl locus opening and gene recombination during B cell development and receptor editing.

LAPTM5 mediates immature B cell apoptosis and B cell tolerance by regulating the WWP2-PTEN-AKT pathway

Elimination of autoreactive developing B cells is an important mechanism to prevent autoantibody production. However, how B cell receptor (BCR) signaling triggers apoptosis of immature B cells remains poorly understood. We show that BCR stimulation up-regulates the expression of the lysosomal-associated transmembrane protein 5 (LAPTM5), which in turn triggers apoptosis of immature B cells through two pathways. LAPTM5 causes BCR internalization, resulting in decreased phosphorylation of SYK and ERK. In addition, LAPTM5 targets the E3 ubiquitin ligase WWP2 for lysosomal degradation, resulting in the accumulation of its substrate PTEN. Elevated PTEN levels suppress AKT phosphorylation, leading to increased FOXO1 expression and up-regulation of the cell cycle inhibitor p27Kip1 and the proapoptotic molecule BIM. In vivo, LAPTM5 is involved in the elimination of autoreactive B cells and its deficiency exacerbates autoantibody production. Our results reveal a previously unidentified mechanism that contributes to immature B cell apoptosis and B cell tolerance.

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

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

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.

miRNAs Are Essential for the Regulation of the PI3K/AKT/FOXO Pathway and Receptor Editing during B Cell Maturation

B cell development is a tightly regulated process dependent on sequential rearrangements of immunoglobulin loci that encode the antigen receptor. To elucidate the role of microRNAs (miRNAs) in the orchestration of B cell development, we ablated all miRNAs at the earliest stage of B cell development by conditionally targeting the enzymes critical for RNAi in early B cell precursors. Absence of any one of these enzymes led to a block at the pro- to pre-B cell transition due to increased apoptosis and a failure of pre-B cells to proliferate. Expression of a Bcl2 transgene allowed for partial rescue of B cell development, however, the majority of the rescued B cells had low surface immunoglobulin expression with evidence of ongoing light chain editing. Our analysis revealed that miRNAs are critical for the regulation of the PTEN-AKT-FOXO1 pathway that in turn controls Rag expression during B cell development.

Regulation of B-cell development and tolerance by different members of the miR-17∼92 family microRNAs

The molecular mechanisms that regulate B-cell development and tolerance remain incompletely understood. In this study, we identify a critical role for the miR-17∼92 microRNA cluster in regulating B-cell central tolerance and demonstrate that these miRNAs control early B-cell development in a cell-intrinsic manner. While the cluster member miR-19 suppresses the expression of Pten and plays a key role in regulating B-cell tolerance, miR-17 controls early B-cell development through other molecular pathways. These findings demonstrate differential control of two closely linked B-cell developmental stages by different members of a single microRNA cluster through distinct molecular pathways.

MicroRNA cluster 17∼92 plays a critical role in B-cell differentiation. Here the authors show that miR-19 regulates B-cell tolerance via suppressing the expression of PTEN, whereas miR- 17 is essential for early B-cell development independently of Pten, Phlpp2, or Bim.

Immune thrombocytopenia and B-cell-activating factor/a proliferation-inducing ligand

Primary immune thrombocytopenia (ITP) is an organ-specific autoimmune disorder characterized by autoantibody-mediated enhanced platelet destruction and dysmegakaryocytopoiesis. B cells have been demonstrated to play critical roles in the pathophysiology of ITP. B-cell-activating factor (BAFF) and a proliferation-inducing ligand (APRIL) are crucial cytokines supporting survival and differentiation of B cells, and dysregulation of BAFF/APRIL is involved in the pathogenesis of B-cell related autoimmune diseases including ITP. Currently ongoing clinical trials using BAFF and/or APRIL-blocking agents have yielded positive results in human systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA), further confirming the pathological role of BAFF/APRIL in autoimmunity. This review will describe the function of BAFF/APRIL and address the feasibility of BAFF/APRIL inhibition in the management of ITP.

A gammaherpesvirus Bcl-2 ortholog blocks B cell receptor-mediated apoptosis and promotes the survival of developing B cells in vivo

Gammaherpesviruses such as Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV, HHV-8) establish lifelong latency in their hosts and are associated with the development of several types of malignancies, including a subset of B cell lymphomas. These viruses are thought to co-opt the process of B cell differentiation to latently infect a fraction of circulating memory B cells, resulting in the establishment of a stable latency setpoint. However, little is known about how this infected memory B cell compartment is maintained throughout the life of the host. We have previously demonstrated that immature and transitional B cells are long-term latency reservoirs for murine gammaherpesvirus 68 (MHV68), suggesting that infection of developing B cells contributes to the maintenance of lifelong latency. During hematopoiesis, immature and transitional B cells are subject to B cell receptor (BCR)-mediated negative selection, which results in the clonal deletion of autoreactive B cells. Interestingly, numerous gammaherpesviruses encode homologs of the anti-apoptotic protein Bcl-2, suggesting that virus inhibition of apoptosis could subvert clonal deletion. To test this, we quantified latency establishment in mice inoculated with MHV68 vBcl-2 mutants. vBcl-2 mutant viruses displayed a marked decrease in the frequency of immature and transitional B cells harboring viral genome, but this attenuation could be rescued by increased host Bcl-2 expression. Conversely, vBcl-2 mutant virus latency in early B cells and mature B cells, which are not targets of negative selection, was remarkably similar to wild-type virus. Finally, in vivo depletion of developing B cells during chronic infection resulted in decreased mature B cell latency, demonstrating a key role for developing B cells in the maintenance of lifelong latency. Collectively, these findings support a model in which gammaherpesvirus latency in circulating mature B cells is sustained in part through the recurrent infection and vBcl-2-mediated survival of developing B cells.

Characterization of antigen-specific B cells using nominal antigen-coated flow-beads

In order to characterize the reactivity of B cells against nominal antigens, a method based on the coupling of antigens onto the surface of fluorescent core polystyrene beads was developed. We first demonstrate that murine B cells with a human MOG-specific BCR are able to interact with MOG-coated beads and do not recognize beads coated with human albumin or pp65. B cells purified from human healthy volunteer blood or immunized individuals were tested for their ability to interact with various nominal antigens, including viral, vaccine, self and alloantigens, chosen for their usefulness in studying a variety of pathological processes. A substantial amount of B cells binding self-antigen MOG-coated beads can be detected in normal blood. Furthermore, greater frequencies of B cell against anti-Tetanic Toxin or anti-EBNA1 were observed in primed individuals. This method can reveal increased frequencies of anti-HLA committed B cells in patients with circulating anti-HLA antibodies compared to unsensitized patients and normal individuals. Of interest, those specific CD19 cells were preferentially identified within CD27(-)IgD(+) (i-e naïve) subset. These observations suggest that a broad range of medical situations could benefit from a tool that allows the detection, the quantification and the characterization of antigen-specific blood B cells.

B cell reductive therapy in the treatment of autoimmune diseases: A focus on monoclonal antibody treatment of rheumatoid arthritis

The therapeutic approach to patients with autoimmune disorders is in the midst of a dramatic change. Monoclonal antibody technology has allowed us to dissect and now manipulate the human immune system with greater precision. It is now widely recognized that B lymphocytes play a role in the pathogenesis of many autoimmune diseases, though the extent and contribution is a matter of debate and active investigation. There is emerging data to suggest that both antibody-dependent and independent mechanisms contribute to disease pathogenesis. However, given the heterogeneous nature of autoimmune diseases, and the varied responses to B lymphocyte reduction, the role of B lymphocytes is likely disease-specific. The two clinical trials discussed in this review demonstrate remarkable consistency in the ability of B cell reduction to ameliorate the clinical manifestations of rheumatoid arthritis with minimal toxicity. B lymphocyte targeted approaches to autoimmune disease in general, and RA specifically, will not only provide an effective and potentially less toxic alternative treatment option, but also allow for a better understanding of the pathogenesis of these complex and morbid diseases.

A mouse variable gene fragment binds to DNA independently of the BCR context: a possible role for immature B-cell repertoire establishment

B-cell maturation occurs in several steps and requires constant stimulus for its continuing development. From the emergence of the pre-B-cell receptor, signal transduction stimulates and supports B-cell development. Current viewpoints indicate that both positive selection pressure for autoantigens and tonic signaling constitutively stimulate B-cell maturation. In this work, we tested for the presence of a putative DNA binding site in a variable gene segment in a germline configuration, independently of VDJ recombination. After a survey of the public antibody databases, we chose a single mouse heavy variable gene segment that is highly represented in anti-nucleic acid antibodies and tested it for ssDNA binding. A phage display approach was used to search for intrinsic binding to oligo deoxythymidine. The results revealed that binding to an antigen can be influenced by the use of a specific DNA binding V[Formula: see text] gene segment. Our data support the idea that some variable genes have intrinsic reactivity towards specific types of endogenous autoantigens, and this property may contribute to the establishment of the immature B-cell repertoire.

Regulatory T cells and other lymphocyte subpopulations in patients with melanoma developing interferon-induced thyroiditis during high-dose interferon-α2b treatment

CONTEXT

One of the side effects of interferon-alpha therapy is interferon-induced thyroiditis (IIT). The role of lymphocyte subpopulations in IIT melanoma patients remains to be defined.

OBJECTIVE

Our objective was to assess different peripheral blood lymphocyte subpopulations, mainly regulatory T cells (Tregs), in melanoma patients who developed IIT.

DESIGN, PATIENTS AND METHODS

From 30 melanoma patients receiving high-dose interferon (HDI)-alpha 2b (IFN-α2b) treatment, those who developed IIT (IIT patients) were selected and compared with patients who did not develop IIT (Co-MM) and healthy controls (Co-H). Peripheral blood mononuclear cells were obtained before treatment (BT), mid-treatment (MT), end of treatment (ET), 24 weeks post-treatment and at appearance of IIT (TT).

RESULTS

Nine patients developed IIT (30%): four Hashimoto's thyroiditis and five destructive thyroiditis. An increase in Tregs was observed in both melanoma groups during HDI treatment. A decrease in CD3(+) , NKT lymphocyte subpopulations and Bcl2 expression on B cells was also observed in both groups. However, no changes were observed in the percentage of CD4(+) , CD8(+) , CD3(+) γδ(+) , CD19(+) , transitional B cells (CD24(high) CD38(high) CD19(+) CD27(-) ), natural killer (NK), invariant NKT (iNKT) lymphocytes and Th1/Th2 balance when BT was compared with ET. At TT, IIT patients had a higher Tregs percentage than Co-MM (P = 0·012) and Co-H (P = 0·004), a higher iNKT percentage than Co-MM (P = 0·011), a higher transitional B cells percentage than Co-H (P = 0·015), a lower CD3(+) percentage than Co-H (P = 0·001) and a lower Bcl2 expression on B cells than Co-H (P < 0·001).

CONCLUSIONS

Our results point to the immunomodulatory effects of IFN-α on different lymphocyte subpopulations and a possible role of Tregs in melanoma patients who developed IIT.

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.

Endogenous antigen tunes the responsiveness of naive B cells but not T cells

In humans, up to 75% of newly generated B cells and about 30% of mature B cells show some degree of autoreactivity. Yet, how B cells establish and maintain tolerance in the face of autoantigen exposure during and after development is not certain. Studies of model B-cell antigen receptor (BCR) transgenic systems have highlighted the critical role of functional unresponsiveness or ‘anergy’. Unlike T cells, evidence suggests that receptor editing and anergy, rather than deletion, account for much of B-cell tolerance. However, it remains unclear whether the mature diverse B-cell repertoire of mice contains anergic autoreactive B cells, and if so, whether antigen was encountered during or after their development. By taking advantage of a reporter mouse in which BCR signalling rapidly and robustly induces green fluorescent protein expression under the control of the Nur77 regulatory region, antigen-dependent and antigen-independent BCR signalling events in vivo during B-cell maturation were visualized. Here we show that B cells encounter antigen during development in the spleen, and that this antigen exposure, in turn, tunes the responsiveness of BCR signalling in B cells at least partly by downmodulating expression of surface IgM but not IgD BCRs, and by modifying basal calcium levels. By contrast, no analogous process occurs in naive mature T cells. Our data demonstrate not only that autoreactive B cells persist in the mature repertoire, but that functional unresponsiveness or anergy exists in the mature B-cell repertoire along a continuum, a fact that has long been suspected, but never yet shown. These results have important implications for understanding how tolerance in T and B cells is differently imposed, and how these processes might go awry in disease.

A p53 axis regulates B cell receptor-triggered, innate immune system-driven B cell clonal expansion

Resting mature human B cells undergo a dynamic process of clonal expansion, followed by clonal contraction, during an in vitro response to surrogate C3d-coated Ag and innate immune system cytokines, IL-4 and BAFF. In this study, we explore the mechanism for clonal contraction through following the time- and division-influenced expression of several pro- and anti-apoptotic proteins within CFSE-labeled cultures. Several findings, involving both human and mouse B cells, show that a mitochondria-dependent apoptotic pathway involving p53 contributes to the high activation-induced cell death (AICD) susceptibility of replicating blasts. Activated B cell clones exhibit elevated p53 protein and elevated mRNA/protein of proapoptotic molecules known to be under direct p53 transcriptional control, Bax, Bad, Puma, Bid, and procaspase 6, accompanied by reduced anti-apoptotic Bcl-2. Under these conditions, Bim levels were not increased. The finding that full-length Bid protein significantly declines in AICD-susceptible replicating blasts, whereas Bid mRNA does not, suggests that Bid is actively cleaved to short-lived, proapoptotic truncated Bid. AICD was diminished, albeit not eliminated, by p53 small interfering RNA transfection, genetic deletion of p53, or Bcl-2 overexpression. DNA damage is a likely trigger for p53-dependent AICD because susceptible lymphoblasts expressed significantly elevated levels of both phosphorylated ataxia telangiectasia mutated-Ser(1980) and phospho-H2AX-Ser(139). Deficiency in activation-induced cytosine deaminase diminishes but does not ablate murine B cell AICD, indicating that activation-induced cytosine deaminase-induced DNA damage is only in part responsible. Evidence for p53-influenced AICD during this route of T cell-independent clonal expansion raises the possibility that progeny bearing p53 mutations might undergo positive selection in peripherally inflamed tissues with elevated levels of IL-4 and BAFF.

Negative selection by IgM superantigen defines a B cell central tolerance compartment and reveals mutations allowing escape

To analyze B lymphocyte central tolerance in a polyclonal immune system, mice were engineered to express a superantigen reactive to IgM of allotype b (IgM(b)). IgM(b/b) mice carrying superantigen were severely B cell lymphopenic, but small numbers of B cells matured. Their sera contained low levels of IgG and occasionally high levels of IgA. In bone marrow, immature B cells were normal in number, but internalized IgM and had a unique gene expression profile, compared with those expressing high levels of surface IgM, including elevated recombinase activator gene expression. A comparable B cell population was defined in wild-type bone marrows, with an abundance suggesting that at steady state ∼20% of normal developing B cells are constantly encountering autoantigens in situ. In superantigen-expressing mice, as well as in mice carrying the 3H9 anti-DNA IgH transgene, or 3H9 H along with mutation in the murine κ-deleting element RS, IgM internalization was correlated with CD19 downmodulation. CD19(low) bone marrow cells from 3H9;RS(-/-) mice were enriched in L chains that promote DNA binding. Our results suggest that central tolerance and attendant L chain receptor editing affect a large fraction of normal developing B cells. IgH(a/b) mice carrying the superantigen had a ∼50% loss in follicular B cell numbers, suggesting that escape from central tolerance by receptor editing from one IgH allele to another was not a major mechanism. IgM(b) superantigen hosts reconstituted with experimental bone marrow were demonstrated to be useful in revealing pathways involved in central tolerance.

The transcription factor Bright plays a role in marginal zone B lymphocyte development and autoantibody production

Previous data suggested that constitutive expression of the transcription factor Bright (B cell regulator of immunoglobulin heavy chain transcription), normally tightly regulated during B cell differentiation, was associated with autoantibody production. Here we show that constitutive Bright expression results in skewing of mature B lineage subpopulations toward marginal zone cells at the expense of the follicular subpopulation. C57Bl/6 transgenic mice constitutively expressing Bright in B lineage cells generated autoantibodies that were not the result of global increases in immunoglobulin or of breaches in key tolerance checkpoints typically defective in other autoimmune mouse models. Rather, autoimmunity correlated with increased numbers of marginal zone B cells and alterations in the phenotype and gene expression profiles of lymphocytes within the follicular B cell compartment. These data suggest a novel role for Bright in the normal development of mature B cell subsets and in autoantibody production.

Rescue of HIV-1 broad neutralizing antibody-expressing B cells in 2F5 VH x VL knockin mice reveals multiple tolerance controls

The HIV-1 broadly neutralizing Ab (bnAb) 2F5 has been shown to be poly-/self-reactive in vitro, and we previously demonstrated that targeted expression of its VDJ rearrangement alone was sufficient to trigger a profound B cell developmental blockade in 2F5 V(H) knockin (KI) mice, consistent with central deletion of 2F5 H chain-expressing B cells. In this study, we generate a strain expressing the entire 2F5 bnAb specificity, 2F5 V(H) × V(L) KI mice, and find an even higher degree of tolerance control than observed in the 2F5 V(H) KI strain. Although B cell development was severely impaired in 2F5 V(H) × V(L) KI animals, we demonstrate rescue of their B cells when cultured in IL-7/BAFF. Intriguingly, even under these conditions, most rescued B cell hybridomas produced mAbs that lacked HIV-1 Envelope (Env) reactivity due to editing of the 2F5 L chain, and the majority of rescued B cells retained an anergic phenotype. Thus, when clonal deletion is circumvented, κ editing and anergy are additional safeguards preventing 2F5 V(H)/V(L) expression by immature/transitional B cells. Importantly, 7% of rescued B cells retained 2F5 V(H)/V(L) expression and secreted Env-specific mAbs with HIV-1-neutralizing activity. This partial rescue was further corroborated in vivo, as reflected by the anergic phenotype of most rescued B cells in 2F5 V(H) × V(L) KI × Eμ-Bcl-2 transgenic mice and significant (yet modest) enrichment of Env-specific B cells and serum Igs. The rescued 2F5 mAb-producing B cell clones in this study are the first examples, to our knowledge, of in vivo-derived bone marrow precursors specifying HIV-1 bnAbs and provide a starting point for design of strategies aimed at rescuing such B cells.

A prospective study of lymphocyte subpopulations and regulatory T cells in patients with chronic hepatitis C virus infection developing interferon-induced thyroiditis

OBJECTIVE

One of the side effects of interferon-alpha (IFN-α) therapy is interferon-induced thyroiditis (IIT). The role of lymphocyte subpopulations in IIT remains to be defined. The aim of this study was to assess different peripheral blood lymphocyte subpopulations, mainly CD4(+) CD25(+) CD127low/-FoxP3(+) regulatory T cells (Tregs), in patients with chronic hepatitis C virus (HCV) infection who developed IIT.

DESIGN, PATIENTS AND METHODS

From 120 patients with chronic HCV who started antiviral treatment, those who developed IIT (IIT patients) were selected and compared with patients who did not develop IIT (Co-HCV). Peripheral blood mononuclear cells were obtained before treatment (BT), mid-treatment (MT), end of treatment (ET), 24 weeks post-treatment (PT) and at appearance of IIT (TT).

RESULTS

Eleven patients developed IIT: three Hashimoto's thyroiditis, one Graves'disease, one positive antithyroidal antibodies, one nonautoimmune hypothyroidism and five destructive thyroiditis. During antiviral treatment, an increase in CD8(+) and in Tregs was observed in both groups. A decrease in CD3(+) , CD19(+) and NKT lymphocyte subpopulations was also observed (all P < 0·05). However, no changes were observed in the percentage of CD4(+) , CD3(+) γδ(+) and iNKT lymphocytes, Th1/Th2 balance and Bcl2 expression on B cells when BT was compared with ET. At the appearance of IIT (TT), IIT patients had a higher Th1 response (CCR5(+) CCR7(-) ) (P < 0·01) and a higher Tregs percentage (P < 0·05) than Co-HCV.

CONCLUSIONS

Our results point to the immunomodulatory effects of IFN-α on different lymphocyte subpopulations and a possible role of Th1 response and Tregs in patients with HCV who developed IIT.

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.

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.

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.

Aberrant reactive oxygen and nitrogen species generation in rheumatoid arthritis (RA): causes and consequences for immune function, cell survival, and therapeutic intervention

The infiltration and persistence of hematopoietic immune cells within the rheumatoid arthritis (RA) joint results in elevated levels of pro-inflammatory cytokines, increased reactive oxygen (ROS) and -nitrogen (RNS) species generation, that feeds a continuous self-perpetuating cycle of inflammation and destruction. Meanwhile, the controlled production of ROS is required for signaling within the normal physiological reaction to perceived "foreign matter" and for effective apoptosis. This review focuses on the signaling pathways responsible for the induction of the normal immune response and the contribution of ROS to this process. Evidence for defects in the ability of immune cells in RA to regulate the generation of ROS and the consequence for their immune function and for RA progression is considered. As the hypercellularity of the rheumatoid joint and the associated persistence of hematopoietic cells within the rheumatoid joint are symptomatic of unresponsiveness to apoptotic stimuli, the role of apoptotic signaling proteins (specifically Bcl-2 family members and the tumor suppressor p53) as regulators of ROS generation and apoptosis are considered, evaluating evidence for their aberrant expression and function in RA. We postulate that ROS generation is required for effective therapeutic intervention.

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.

Stromal cell independent B cell development in vitro: generation and recovery of autoreactive clones

We describe and characterize a stromal cell independent culture system that efficiently supports pro-B cell to IgM+ B cell development with near normal levels of IgH and Igkappa diversity. Pro-B cells present in non-adherent bone marrow cells proliferate in the presence of IL-7 and subsequent to the removal of IL-7 and addition of BAFF, differentiate normally into IgM+ B cells. B cell development in vitro closely follows the patterns of development in vivo with culture-derived (CD) B cells demonstrating characteristic patterns of surface antigen expression and gene activation. IgM+ CD B cells respond to TLR stimulation by proliferation and differentiation into antibody-secreting cells. Self-reactive IgM+ B cell development is blocked in 3H9 IgH knockin mice; however, cultures of 3H9 IgH knockin pro-B cells yields high frequencies of "forbidden", autoreactive IgM+ B cells. Furthermore, serum IgG autoantibody exceeded that present in autoimmune, C4(-/-) animals following the reconstitution of RAG1(-/-) mice with IgM+ CD cells derived from BL/6 mice.

Decoration of T-independent antigen with ligands for CD22 and Siglec-G can suppress immunity and induce B cell tolerance in vivo

Autoreactive B lymphocytes first encountering self-antigens in peripheral tissues are normally regulated by induction of anergy or apoptosis. According to the “two-signal” model, antigen recognition alone should render B cells tolerant unless T cell help or inflammatory signals such as lipopolysaccharide are provided. However, no such signals seem necessary for responses to T-independent type 2 (TI-2) antigens, which are multimeric antigens lacking T cell epitopes and Toll-like receptor ligands. How then do mature B cells avoid making a TI-2–like response to multimeric self-antigens? We present evidence that TI-2 antigens decorated with ligands of inhibitory sialic acid–binding Ig-like lectins (siglecs) are poorly immunogenic and can induce tolerance to subsequent challenge with immunogenic antigen. Two siglecs, CD22 and Siglec-G, contributed to tolerance induction, preventing plasma cell differentiation or survival. Although mutations in CD22 and its signaling machinery have been associated with dysregulated B cell development and autoantibody production, previous analyses failed to identify a tolerance defect in antigen-specific mutant B cells. Our results support a role for siglecs in B cell self-/nonself-discrimination, namely suppressing responses to self-associated antigens while permitting rapid “missing self”–responses to unsialylated multimeric antigens. The results suggest use of siglec ligand antigen constructs as an approach for inducing tolerance.

NF-kappaB activity marks cells engaged in receptor editing

Because of the extreme diversity in immunoglobulin genes, tolerance mechanisms are necessary to ensure that B cells do not respond to self-antigens. One such tolerance mechanism is called receptor editing. If the B cell receptor (BCR) on an immature B cell recognizes self-antigen, it is down-regulated from the cell surface, and light chain gene rearrangement continues in an attempt to edit the autoreactive specificity. Analysis of a heterozygous mutant mouse in which the NF-κB–dependent IκBα gene was replaced with a lacZ (β-gal) reporter complementary DNA (cDNA; IκBα^+/lacZ) suggests a potential role for NF-κB in receptor editing. Sorted β-gal⁺ pre–B cells showed increased levels of various markers of receptor editing. In IκBα^+/lacZ reporter mice expressing either innocuous or self-specific knocked in BCRs, β-gal was preferentially expressed in pre–B cells from the mice with self-specific BCRs. Retroviral-mediated expression of a cDNA encoding an IκBα superrepressor in primary bone marrow cultures resulted in diminished germline κ and rearranged λ transcripts but similar levels of RAG expression as compared with controls. We found that IRF4 transcripts were up-regulated in β-gal⁺ pre–B cells. Because IRF4 is a target of NF-κB and is required for receptor editing, we suggest that NF-κB could be acting through IRF4 to regulate receptor editing.

Development of immunoglobulin lambda-chain-positive B cells, but not editing of immunoglobulin kappa-chain, depends on NF-kappaB signals

By genetically ablating IκB kinase (IKK)-mediated NF-κB activation in the B cell lineage, and by analyzing a mouse mutant in which Igλ⁺ B cells are generated in the absence of rearrangements in Igk, we define two distinct, consecutive phases of early B cell development that differ in their dependence on IKK-mediated NF-κB signaling. During the first phase, in which NF-κB signaling is dispensable, predominantly Igκ⁺ B cells are generated and undergo efficient receptor editing. In the second phase, predominantly Igλ⁺ B cells are generated, whose development is ontogenetically timed to occur after Igk rearrangements. This second phase of development is dependent on NF-κB signals, which can be substituted by transgenic expression of the pro-survival factor Bcl2.

Control of apoptosis in autoimmunity

Apoptosis and the subsequent removal of apoptotic cells underpin a healthy immune system. They are crucial for both the maintenance of self-tolerance and the contraction of clonally expanded lymphocytes at the conclusion of immune responses. Aberrant apoptosis and the disposal of apoptotic cells is implicated in the development of both systemic and organ-specific autoimmune disease and is a major contributing factor in disease susceptibility. Dissection of the molecular mechanisms involved in dysregulated apoptosis may reveal pathways which can be targeted for more effective therapeutic intervention. This review highlights the molecular events underlying programmed cell death and apoptotic cell uptake, and summarizes recent studies that link impaired apoptotic death to autoimmunity.

A role for interferon regulatory factor 4 in receptor editing

Receptor editing is the primary means through which B cells revise antigen receptors and maintain central tolerance. Previous studies have demonstrated that interferon regulatory factor 4 (IRF-4) and IRF-8 promote immunoglobulin light-chain rearrangement and transcription at the pre-B stage. Here, the roles of IRF-4 and -8 in receptor editing were analyzed. Our results show that secondary rearrangement was impaired in IRF-4 but not IRF-8 mutant mice, suggesting that receptor editing is defective in the absence of IRF-4. The role of IRF-4 in receptor editing was further examined in B-cell-receptor (BCR) transgenic mice. Our results show that secondary rearrangement triggered by membrane-bound antigen was defective in the IRF-4-deficient mice. Our results further reveal that the defect in secondary rearrangement is more severe at the immunoglobulin lambda locus than at the kappa locus, indicating that IRF-4 is more critical for the lambda rearrangement. We provide evidence demonstrating that the expression of IRF-4 in immature B cells is rapidly induced by self-antigen and that the reconstitution of IRF-4 expression in the IRF-4 mutant immature B cells promotes secondary rearrangement. Thus, our studies identify IRF-4 as a nuclear effector of a BCR signaling pathway that promotes secondary rearrangement at the immature B-cell stage.

Rearrangement of mouse immunoglobulin kappa deleting element recombining sequence promotes immune tolerance and lambda B cell production

The recombining sequence (RS) of mouse and its human equivalent, the immunoglobulin (Ig) kappa deleting element (IGKDE), are sequences found at the 3' end of the Ig kappa locus (Igk) that rearrange to inactivate Igk in developing B cells. RS recombination correlates with Ig lambda (Iglambda) light (L) chain expression and likely plays a role in receptor editing by eliminating Igk genes encoding autoantibodies. A mouse strain was generated in which the recombination signal of RS was removed, blocking RS-mediated Igk inactivation. In RS mutant mice, receptor editing and self-tolerance were impaired, in some cases leading to autoantibody formation. Surprisingly, mutant mice also made fewer B cells expressing lambda chain, whereas lambda versus kappa isotype exclusion was only modestly affected. These results provide insight into the mechanism of L chain isotype exclusion and indicate that RS has a physiological role in promoting the formation of lambda L chain-expressing B cells.

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.

What do we know about the mechanisms of elimination of autoreactive T and B cells and what challenges remain

Tolerance to self-antigens within the adaptive immune system is safeguarded, at least in part, through deletion of autoreactive T and B lymphocytes. This deletion can occur during the development of these cells in primary lymphoid organs, the thymus or bone marrow, respectively, or at the mature stage in peripheral lymphoid tissues. Deletion of autoreactive lymphocytes is achieved to a large extent through apoptotic cell death. This review describes current understanding of the mechanisms that mediate apoptosis of autoreactive lymphocytes during their development in primary lymphoid organs and during their activation in the periphery. In particular, we discuss the roles of the proapoptotic Bcl-2 family member Bim and the small family of Nur77-related transcriptional regulators in lymphocyte negative selection. Finally, we speculate on the processes that may lead to the activation of Bim when antigen receptors are activated on autoreactive T or B cells.

Surrogate-light-chain silencing is not critical for the limitation of pre-B cell expansion but is for the termination of constitutive signaling

The pre-B cell receptor (pre-BCR), composed of immunoglobulin mu heavy chain and the surrogate light chain (SLC) proteins lambda5 and Vpreb, signals for proliferation and maturation of developing pre-B cells. It has been assumed that pre-B cells stop cycling by the pre-BCR-mediated downregulation of SLC transcription. We generated transgenic mice expressing SLC throughout B cell development and, remarkably, found that enforced SLC expression had no effect on pre-B cell proliferation or differentiation. However, in the presence of conventional immunoglobulin light chains, SLC components had the capacity to induce constitutive BCR internalization, secondary immunoglobulin light-chain rearrangement, and a severe developmental arrest of immature B cells, dependent on the adaptor protein Slp65. Residual B cells in the spleen showed increased expression of surface CD5, which is a negative regulator of BCR signaling, and differentiated spontaneously into IgM+ plasma cells. Thus, the silencing of SLC genes is not essential for the limitation of pre-B cell proliferation, but is required for the prevention of constitutive activation of B cells.

Phospholipase Cgamma2 contributes to light-chain gene activation and receptor editing

Phospholipase Cgamma2 (PLCgamma2) is critical for pre-B-cell receptor (pre-BCR) and BCR signaling. Current studies discovered that PLCgamma2-deficient mice had reduced immunoglobulin lambda (Iglambda) light-chain usage throughout B-cell maturation stages, including transitional type 1 (T1), transitional type 2 (T2), and mature follicular B cells. The reduction of Iglambda rearrangement by PLCgamma2 deficiency was not due to specifically increased apoptosis or decreased proliferation of mutant Iglambda+ B cells, as lack of PLCgamma2 exerted a similar effect on apoptosis and proliferation of both Iglambda+ and Igkappa+ B cells. Moreover, PLCgamma2-deficient IgHEL transgenic B cells exhibited an impairment of antigen-induced receptor editing among both the endogenous lambda and kappa loci in vitro and in vivo. Importantly, PLCgamma2 deficiency impaired BCR-induced expression of IRF-4 and IRF-8, the two transcription factors critical for lambda and kappa light-chain rearrangements. Taken together, these data demonstrate that the PLCgamma2 signaling pathway plays a role in activation of light-chain loci and contributes to receptor editing.

Basal B cell receptor-directed phosphatidylinositol 3-kinase signaling turns off RAGs and promotes B cell-positive selection

PI3K plays key roles in cell growth, differentiation, and survival by generating the second messenger phosphatidylinositol-(3,4,5)-trisphosphate (PIP3). PIP3 activates numerous enzymes, in part by recruiting them from the cytosol to the plasma membrane. We find that in immature B lymphocytes carrying a nonautoreactive Ag receptor, PI3K signaling suppresses RAG expression and promotes developmental progression. Inhibitors of PI3K signaling abrogate this positive selection. Furthermore, immature primary B cells from mice lacking the p85alpha regulatory subunit of PI3K suppress poorly RAG expression, undergo an exaggerated receptor editing response, and, as in BCR-ligated cells, fail to progress into the G1 phase of cell cycle. Moreover, immature B cells carrying an innocuous receptor have sustained elevation of PIP3 levels and activation of the downstream effectors phospholipase C (PLC)gamma2, Akt, and Bruton's tyrosine kinase. Of these, PLCgamma2 appears to play the most significant role in down-regulating RAG expression. It therefore appears that when the BCR of an immature B cell is ligated, PIP3 levels are reduced, PLCgamma2 activation is diminished, and receptor editing is promoted by sustained RAG expression. Taken together, our results provide evidence that PI3K signaling is an important cue required for fostering development of B cells carrying a useful BCR.

Identification of Anergic B Cells within a Wild-Type Repertoire

The contribution of anergy to silencing of autoreactive B cells in physiologic settings is unknown. By comparing anergic and nonanergic immunoglobulin-transgenic mouse strains, we defined a set of surface markers that were used for presumptive identification of an anergic B cell cohort within a normal repertoire. Like anergic transgenic B cells, these physiologic anergic cells exhibited high basal intracellular free calcium and did not mobilize calcium, initiate tyrosine phosphorylation, proliferate, upregulate activation markers, or mount an immune response upon antigen-receptor stimulation. Autoreactive B cells were overrepresented in this cohort. On the basis of the frequency and lifespan of these cells, it appears that as many as 50% of newly produced B cells are destined to become anergic. In conclusion, our findings indicate that anergy is probably the primary mechanism by which autoreactive B cells are silenced. Thus maintenance of the unresponsiveness of anergic cells is critical for prevention of autoimmunity.

Bruton's tyrosine kinase and SLP-65 regulate pre-B cell differentiation and the induction of Ig light chain gene rearrangement

Bruton's tyrosine kinase (Btk) and the adapter protein SLP-65 (Src homology 2 domain-containing leukocyte-specific phosphoprotein of 65 kDa) transmit precursor BCR (pre-BCR) signals that are essential for efficient developmental progression of large cycling into small resting pre-B cells. We show that Btk- and SLP-65-deficient pre-B cells have a specific defect in Ig lambda L chain germline transcription. In Btk/SLP-65 double-deficient pre-B cells, both kappa and lambda germline transcripts are severely reduced. Although these observations point to an important role for Btk and SLP-65 in the initiation of L chain gene rearrangement, the possibility remained that these signaling molecules are only required for termination of pre-B cell proliferation or for pre-B cell survival, whereby differentiation and L chain rearrangement is subsequently initiated in a Btk/SLP-65-independent fashion. Because transgenic expression of the antiapoptotic protein Bcl-2 did not rescue the developmental arrest of Btk/SLP-65 double-deficient pre-B cells, we conclude that defective L chain opening in Btk/SLP-65-deficient small resting pre-B cells is not due to their reduced survival. Next, we analyzed transgenic mice expressing the constitutively active Btk mutant E41K. The expression of E41K-Btk in Ig H chain-negative pro-B cells induced 1) surface marker changes that signify cellular differentiation, including down-regulation of surrogate L chain and up-regulation of CD2, CD25, and MHC class II; and 2) premature rearrangement and expression of kappa and lambda light chains. These findings demonstrate that Btk and SLP-65 transmit signals that induce cellular maturation and Ig L chain rearrangement independently of their role in termination of pre-B cell expansion.

B-cell tolerance

Autoreactive B cells are actively tolerized to more abundant self-antigens by a series of checkpoints involving receptor editing, deletion, anergy and competition for growth factors. In contrast, B cells reactive against rare, sequestered or tissue specific self-antigens remain functionally naïve. During an immune response, the autoimmune danger from these cells is countered by a variety of mechanisms comprising control of self-antigen presentation, limitation of immunogenic and tolerogenic costimuli including T cell help, homeostatic control of growth and strict regulation of germinal centre reactions. In this overview we consider how knowledge of these checkpoints may be used to gain a better understanding of transplant tolerance and the generation of alloantibodies.

Receptor editing for better or for worse

Receptor editing has emerged from its original identification as a minor secondary mechanism of B cell tolerance to be considered as a dominant mechanism by which autoreactive immature B cells are rendered tolerant. Clonal deletion, previously regarded as the major mechanism of central B cell tolerance, has been shown by recent studies to operate secondarily and only when receptor editing is unable to provide a non-autoreactive specificity. Receptor editing has also been shown to operate during the development of wild-type B lymphocytes, and ongoing investigations demonstrate the influence of particular signaling molecules in the induction and/or inhibition of receptor editing. Together, these studies begin to map the signaling pathways that regulate receptor editing in autoreactive and non-autoreactive immature B cells.

Loss of the proapoptotic protein, Bim, breaks B cell anergy

Although B cells that respond with high avidity to self-antigen are eliminated early in their development, many autoreactive B cells escape elimination and are tolerized later in their lives via anergy. Anergic B cells are unresponsive to antigen and die prematurely. It has been suggested that the proapoptotic protein, Bim, controls the fate of anergic B cells. To test this idea, mice lacking Bim were crossed with mice that express soluble hen egg lysozyme and whose B cells bear receptors specific for the protein. In Bim^+/+ mice these B cells are anergic and die rapidly. If the mice lack Bim, however, the B cells live longer, are more mature, respond to antigen, and secrete anti–hen egg lysozyme antibodies. This break of tolerance is not due to expression of endogenous B cell receptors, nor is it dependent on T cells. Rather, it appears to be due to a reduced requirement for the cytokine BAFF. Normal B cells require BAFF both for differentiation and survival. Bim^−/− B cells, on the other hand, require BAFF only for differentiation. Therefore, autoreactive B cells are allowed to survive if they lack Bim and thus accumulate sufficient signals from differentiating factors to drive their maturation and production of autoantibodies.

Split Tolerance in Peripheral B Cell Subsets in Mice Expressing a Low Level of Igκ-Reactive Ligand

Peripheral B cell tolerance differs from central tolerance in anatomic location, in the stage of B cell development, and in the diversity of Ag-responsive cells. B cells in secondary lymphoid organs are heterogeneous, including numerous subtypes such as B-1, marginal zone, transitional, and follicular B cells, which likely respond differently from one another to ligand encounter. We showed recently that central B cell tolerance mediated by receptor editing was induced in mice carrying high levels of a ubiquitously expressed kappa-macroself Ag, a synthetic superantigen reactive to Igkappa. In this study, we characterize a new transgenic line that has a distinctly lower expression pattern from those described previously; the B cell tolerance phenotype of these mice is characterized by the presence of significant numbers of immature kappa+ B cells in the spleen, the loss of mature follicular and marginal zone B cells, the persistence of kappa+ B-1 cells in the peritoneal cavity, and significant levels of serum IgM,kappa. These findings suggest distinct signaling thresholds for tolerance among peripheral B cell subsets reactive with an identical ligand.

Receptor editing can lead to allelic inclusion and development of B cells that retain antibodies reacting with high avidity autoantigens

Receptor editing is a major B cell tolerance mechanism that operates by secondary Ig gene rearrangements to change the specificity of autoreactive developing B cells. In the 3-83Igi mouse model, receptor editing operates in every autoreactive anti-H-2K(b) B cell, providing a novel receptor without additional cell loss. Despite the efficiency of receptor editing in generating nonautoreactive Ag receptors, we show in this study that this process does not inactivate the autoantibody-encoding gene(s) in every autoreactive B cell. In fact, receptor editing can generate allelically and isotypically included B cells that simultaneously express the original autoreactive and a novel nonautoreactive Ag receptors. Such dual Ab-expressing B cells differentiate into transitional and mature B cells retaining the expression of the autoantibody despite the high avidity interaction between the autoantibody and the self-Ag in this system. Moreover, we find that these high avidity autoreactive B cells retain the autoreactive Ag receptor within the cell as a consequence of autoantigen engagement and through a Src family kinase-dependent process. Finally, anti-H-2K(b) IgM autoantibodies are found in the sera of older 3-83Igi mice, indicating that dual Ab-expressing autoreactive B cells are potentially functional and capable of differentiating into IgM autoantibody-secreting plasma cells under certain circumstances. These results demonstrate that autoreactive B cells reacting with ubiquitous membrane bound autoantigens can bypass mechanisms of central tolerance by coexpressing nonautoreactive Abs. These dual Ab-expressing autoreactive B cells conceal their autoantibodies within the cell manifesting a superficially tolerant phenotype that can be partially overcome to secrete IgM autoantibodies.

Hormonal modulation of B cell development and repertoire selection

Systemic lupus erythematosus is an autoimmune disorder characterized by the production of pathogenic autoantibodies, primarily to nuclear antigens. The etiology of SLE is not entirely understood, but it is well-appreciated that multiple factors such as genetics and environment contribute to disease progression and pathogenesis. There is also convincing evidence that gender plays an import role in SLE since the incidence of disease occurs with a female to male ratio of 9:1. While it is plausible that some sex-linked genes may contribute to the genetic predisposition for the disease, other likely culprits for this gender bias are the sex hormones estrogen and prolactin. The data implicating estrogen and prolactin in SLE, until recently, were largely circumstantial. However, within the last few years, data collected from both human and mouse studies have provided compelling evidence that alterations in sex hormone levels can alter tolerance of autoreactive B cells and exacerbate disease. In this review, we will discuss recent data demonstrating a role for estrogen and prolactin in SLE and the effect of these hormones on B cell maturation, selection and activation.

E2F1−/−C57BL/6 Mice Overexpressing a Human Bcl-2 Transgene in B Cells Develop a Mild Autoimmune Syndrome

Bcl-2, besides having an anti-apoptotic function, delays cell cycle progression at G1 to S. Overexpression of Bcl-2 in B cells induces an autoimmune syndrome (AIS) in pro-autoimmune genetic backgrounds. E2F1, a member of the E2F transcription factors, controls cell cycle, but it also induces cell death. E2F1(-/-) mice show an altered negative thymic selection but a conserved peripheral tolerance. As a consequence, these mice do not develop autoimmunity. Our aim was to evaluate whether deregulation of both apoptosis and cell cycle alters the mechanisms of tolerance and induces an AIS. C57BL/6 E2F1(-/-) mice were crossed with C57BL/6 mice overexpressing a human Bcl-2 transgene in B cells to obtain E2F1(-/-) hbcl-2 Tg mice. These mice were followed for up to 15 months of age with bleedings every three months to obtain serum and whole blood. The production of an AIS was assessed by quantitation of serum anti-DNA antibodies, renal light microscopy, and direct immunofluorescence in search of immunoglobulin deposits. E2F1(-/-) hbcl-2 Tg mice developed an AIS characterized by anti-DNA autoantibody production with renal damage observed after the 9th month of age. The lesions consisted mainly on cellular proliferation and mesangial deposits, compatible with a mesangial glomerulonephritis. The composition of deposits was predominantly of IgA, followed by IgM and IgG. Despite the development of renal damage, the AIS observed did not induce an accelerated mortality. The coexistence of an altered B cell apoptosis, together with the lack of E2F1, induces a mild AIS in the non-autoimmune background of C57BL/6 mice.