BCR ligation induces receptor editing in IgM+IgD- bone marrow B cells in vitro

The role of Raptor in lymphocytes differentiation and function

Raptor, a key component of mTORC1, is required for recruiting substrates to mTORC1 and contributing to its subcellular localization. Raptor has a highly conserved N-terminus domain and seven WD40 repeats, which interact with mTOR and other mTORC1-related proteins. mTORC1 participates in various cellular events and mediates differentiation and metabolism. Directly or indirectly, many factors mediate the differentiation and function of lymphocytes that is essential for immunity. In this review, we summarize the role of Raptor in lymphocytes differentiation and function, whereby Raptor mediates the secretion of cytokines to induce early lymphocyte metabolism, development, proliferation and migration. Additionally, Raptor regulates the function of lymphocytes by regulating their steady-state maintenance and activation.

B cells in the balance: Offsetting self-reactivity avoidance with protection against foreign

Antibodies are theoretically limitless in their diversity and specificity to foreign antigens; however they are constrained by the need to avoid binding to self. Germinal centers (GC) allow diversification and maturation of the antibody response towards the foreign antigen. While self-tolerance mechanisms controlling self-reactivity during B cell maturation are well recognized, the mechanisms by which GCs balance self-tolerance and foreign binding especially in the face of cross-reactivity between self and foreign, remain much less well defined. In this review we explore the extent to which GC self-tolerance restricts affinity maturation. We present studies suggesting that the outcome is situationally dependent, affected by affinity and avidity to self-antigen, and the extent to which self-binding and foreign-binding are interdependent. While auto-reactive GC B cells can mutate away from self while maturing towards the foreign antigen, if no mutational trajectories allow for self-reactive redemption, self-tolerance prevails and GC responses to the foreign pathogen are restricted, except when self-tolerance checkpoints are relaxed. Finally, we consider whether polyreactivity is subject to the same level of restriction in GC responses, especially if polyreactivity is linked to an increase in foreign protection, as occurs in certain broadly neutralizing antibodies. Overall, the outcomes for GC B cells that bind self-antigen can range from redemption, transient relaxation in self-tolerance or restriction of the antibody response to the foreign pathogen.

Transforming mutations in the development of pathogenic B cell clones and autoantibodies

Autoimmune diseases are characterized by serum autoantibodies, some of which are pathogenic, causing severe manifestations and organ injury. However, autoantibodies of the same antigenic reactivity are also present in the serum of asymptomatic people years before they develop any clinical signs of autoimmunity. Autoantibodies can arise during multiple stages of B cell development, and various genetic and environmental factors drive their production. However, what drives the development of pathogenic autoantibodies is poorly understood. Advances in single-cell technology have enabled the deep analysis of rare B cell clones producing pathogenic autoantibodies responsible for vasculitis in patients with primary Sjögren's syndrome complicated by mixed cryoglobulinaemia. These findings demonstrated a cascade of genetic events involving stereotypic immunoglobulin V(D)J recombination and transforming somatic mutations in lymphoma genes and V(D)J regions that disrupted antibody quality control mechanisms and decreased autoantibody solubility. Most studies consider V(D)J mutations that enhance autoantibody affinity to drive pathology; however, V(D)J mutations that increase autoantibody propensity to form insoluble complexes could be a major contributor to autoantibody pathogenicity. Defining the molecular characteristics of pathogenic autoantibodies and failed tolerance checkpoints driving their formation will improve prognostication, enabling early treatment to prevent escalating organ damage and B cell malignancy.

[Expression of terminal deoxynucleotidyl transferase in the pharyngeal and palatine tonsils in local infectious and inflammatory diseases of the upper respiratory tract and pharynx in childhood]

Along with central immune organs, the peripheral lymphoepithelial organs of the pharynx are actively involved in protecting the body from infections. Adaptive, or induced, immunity occurs during the postnatal ontogenesis of immunocompetent lymphocytes, which includes the secondary somatic recombination of the V genes with the participation of recombination-activating gene (RAG) and terminal deoxynucleotidyl transferase (Tdt) proteins. This publication discusses the results of detection of Tdt-positive cells in the pharyngeal and palatine tonsils of children of different ages, who had been operated on for adenoid vegetations and chronic tonsillitis. Attention is drawn to the localization of Tdt⁺ cells, the level of Tdt expression, an attempt to clarify the phenotype, destination, and place in the diagnostic arrays of functional markers when an adaptive immunity is generated in children.

Regulation of Immune Functions by Non-Neuronal Acetylcholine (ACh) via Muscarinic and Nicotinic ACh Receptors

Acetylcholine (ACh) is the classical neurotransmitter in the cholinergic nervous system. However, ACh is now known to regulate various immune cell functions. In fact, T cells, B cells, and macrophages all express components of the cholinergic system, including ACh, muscarinic, and nicotinic ACh receptors (mAChRs and nAChRs), choline acetyltransferase, acetylcholinesterase, and choline transporters. In this review, we will discuss the actions of ACh in the immune system. We will first briefly describe the mechanisms by which ACh is stored in and released from immune cells. We will then address Ca²⁺ signaling pathways activated via mAChRs and nAChRs on T cells and B cells, highlighting the importance of ACh for the function of T cells, B cells, and macrophages, as well as its impact on innate and acquired (cellular and humoral) immunity. Lastly, we will discuss the effects of two peptide ligands, secreted lymphocyte antigen-6/urokinase-type plasminogen activator receptor-related peptide-1 (SLURP-1) and hippocampal cholinergic neurostimulating peptide (HCNP), on cholinergic activity in T cells. Overall, we stress the fact that ACh does not function only as a neurotransmitter; it impacts immunity by exerting diverse effects on immune cells via mAChRs and nAChRs.

Aged Gut Microbiota Contributes to Systemical Inflammaging after Transfer to Germ-Free Mice

Advanced age is associated with chronic low-grade inflammation, which is usually referred to as inflammaging. Elderly are also known to have an altered gut microbiota composition. However, whether inflammaging is a cause or consequence of an altered gut microbiota composition is not clear. In this study, gut microbiota from young or old conventional mice was transferred to young germ-free (GF) mice. Four weeks after gut microbiota transfer immune cell populations in spleen, Peyer’s patches, and mesenteric lymph nodes from conventionalized GF mice were analyzed by flow cytometry. In addition, whole-genome gene expression in the ileum was analyzed by microarray. Gut microbiota composition of donor and recipient mice was analyzed with 16S rDNA sequencing. Here, we show by transferring aged microbiota to young GF mice that certain bacterial species within the aged microbiota promote inflammaging. This effect was associated with lower levels of Akkermansia and higher levels of TM7 bacteria and Proteobacteria in the aged microbiota after transfer. The aged microbiota promoted inflammation in the small intestine in the GF mice and enhanced leakage of inflammatory bacterial components into the circulation was observed. Moreover, the aged microbiota promoted increased T cell activation in the systemic compartment. In conclusion, these data indicate that the gut microbiota from old mice contributes to inflammaging after transfer to young GF mice.

TAK1 maintains the survival of immunoglobulin λ-chain-positive B cells

TAK1 (MAP3K7) mediation of the IκB kinase (IKK) complex-nuclear factor-κB (NF-κB) pathway is crucial for the activation of immune response and to perpetuate inflammation. Although progress has been made to understand TAK1 function in the B-cell receptor (BCR) signaling, the physiological roles of TAK1 in B-cell development, particularly in the bone marrow (BM), remain elusive. Previous studies suggested that the IKK complex is required for the development of immunoglobulin light chain λ-positive B cells, but not for receptor editing. In contrast, NF-κB activity is suggested to be involved in the regulation of receptor editing. Thus, NF-κB signaling in early B-cell development is yet to be fully characterized. Therefore, we addressed the role of TAK1 in early B-cell development. TAK1-deficient mice showed significant reduction of BM Igλ-positive B-cell numbers without any alteration in the BCR editing. Furthermore, the expression of survival factor Bcl-2 was reduced in TAK1-deficient BM B cells as assessed by microarray and quantitative PCR analyses. Ex vivo over-expression of exogenous Bcl-2 enhanced the survival of TAK1-deficient Igλ-positive B cells. TAK1-IKK-NF-κB signaling contributes to the survival of λ-chain-positive B cells through NF-κB-dependent anti-apoptotic Bcl-2 expression.

Targeting Anti-Insulin B Cell Receptors Improves Receptor Editing in Type 1 Diabetes-Prone Mice

Autoreactive B lymphocytes that commonly arise in the developing repertoire can be salvaged by receptor editing, a central tolerance mechanism that alters BCR specificity through continued L chain rearrangement. It is unknown whether autoantigens with weak cross-linking potential, such as insulin, elicit receptor editing, or whether this process is dysregulated in related autoimmunity. To resolve these issues, we developed an editing-competent model in which anti-insulin Vκ125 was targeted to the Igκ locus and paired with anti-insulin VH125Tg. Physiologic, circulating insulin increased RAG-2 expression and was associated with BCR replacement that eliminated autoantigen recognition in a proportion of developing anti-insulin B lymphocytes. The proportion of anti-insulin B cells that underwent receptor editing was reduced in the type 1 diabetes-prone NOD strain relative to a nonautoimmune strain. Resistance to editing was associated with increased surface IgM expression on immature (but not transitional or mature) anti-insulin B cells in the NOD strain. The actions of mAb123 on central tolerance were also investigated, because selective targeting of insulin-occupied BCR by mAb123 eliminates anti-insulin B lymphocytes and prevents type 1 diabetes. Autoantigen targeting by mAb123 increased RAG-2 expression and dramatically enhanced BCR replacement in newly developed B lymphocytes. Administering F(ab')2123 induced IgM downregulation and reduced the frequency of anti-insulin B lymphocytes within the polyclonal repertoire of VH125Tg/NOD mice, suggesting enhanced central tolerance by direct BCR interaction. These findings indicate that weak or faulty checkpoints for central tolerance can be overcome by autoantigen-specific immunomodulatory therapy.

Microbes and B cell development

Animals and many of their chronic microbial inhabitants form relationships of symbiotic mutualism, which occurs when coexisting life-forms derive mutual benefit from stable associations. While microorganisms receive a secure habitat and constant food source from vertebrate hosts, they are required for optimal immune system development and occupy niches otherwise abused by pathogens. Microbes have also been shown to provide vertebrate hosts with metabolic capabilities that enhance energy and nutrient uptake from the diet. The immune system plays a central role in the establishment and maintenance of host-microbe homeostasis, and B lineage cells play a key role in this regulation. Here, I reviewed the structure and function of the microbiota and the known mechanisms of how nonpathogenic microbes influence B cell biology and immunoglobulin repertoire development early in life. I also discuss what is known about how B lineage cells contribute to the process of shaping the composition of commensal/mutualistic microbe membership.

Internalization of B cell receptors in human EU12 μHC⁺ immature B cells specifically alters downstream signaling events

It has been recognized for a long time that engagement of B cell antigen receptors (BCRs) on immature B cells or mature B cells leads to completely opposite cell fate decisions. The underlying mechanism remains unclear. Here, we show that crosslinking of BCRs on human EU12 μHC⁺ immature B cells resulted in complete internalization of cell surface BCRs. After loss of cell surface BCRs, restimulation of EU12 μHC⁺ cells showed impaired Ca²⁺ flux, delayed SYK phosphorylation, and decreased CD19 and FOXO1 phosphorylation, which differ from those in mature Daudi or Ramos B cells with partial internalization of BCRs. In contrast, sustained phosphorylation and reactivation of ERK upon restimulation were observed in the EU12 μHC⁺ cells after BCR internalization. Taken together, these results show that complete internalization of cell surface BCRs in EU12 μHC⁺ cells specifically alters the downstream signaling events, which may favor receptor editing versus cell activation.

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.

Microbial colonization influences early B-lineage development in the gut lamina propria

The RAG1/RAG2 endonuclease (RAG) initiates the V(D)J recombination reaction that assembles immunoglobulin heavy (IgH) and light (IgL) chain variable region exons from germline gene segments to generate primary antibody repertoires. IgH V(D)J assembly occurs in progenitor (pro-) B cells followed by that of IgL in precursor (pre-) B cells. Expression of IgH μ and IgL (Igκ or Igλ) chains generates IgM, which is expressed on immature B cells as the B-cell antigen-binding receptor (BCR). Rag expression can continue in immature B cells, allowing continued Igκ V(D)J recombination that replaces the initial VκJκ exon with one that generates a new specificity. This 'receptor editing' process, which can also lead to Igλ V(D)J recombination and expression, provides a mechanism whereby antigen encounter at the Rag-expressing immature B-cell stage helps shape pre-immune BCR repertoires. As the major site of postnatal B-cell development, the bone marrow is the principal location of primary immunoglobulin repertoire diversification in mice. Here we report that early B-cell development also occurs within the mouse intestinal lamina propria (LP), where the associated V(D)J recombination/receptor editing processes modulate primary LP immunoglobulin repertoires. At weanling age in normally housed mice, the LP contains a population of Rag-expressing B-lineage cells that harbour intermediates indicative of ongoing V(D)J recombination and which contain cells with pro-B, pre-B and editing phenotypes. Consistent with LP-specific receptor editing, Rag-expressing LP B-lineage cells have similar VH repertoires, but significantly different Vκ repertoires, compared to those of Rag2-expressing bone marrow counterparts. Moreover, colonization of germ-free mice leads to an increased ratio of Igλ-expressing versus Igκ-expressing B cells specifically in the LP. We conclude that B-cell development occurs in the intestinal mucosa, where it is regulated by extracellular signals from commensal microbes that influence gut immunoglobulin repertoires.

Type 1 diabetes: translating mechanistic observations into effective clinical outcomes

Type 1 diabetes (T1D) remains an important health problem, particularly in western countries, where the incidence has been increasing in younger children. In 1986, Eisenbarth described T1D as a chronic autoimmune disease. Work over the past three-and-a-half decades has identified many of the genetic, immunological and environmental factors that are involved in the disease and have led to hypotheses concerning its pathogenesis. Clinical trials have been conducted to test these hypotheses but have had mixed results. Here, we discuss the findings that have led to our current concepts of the disease mechanisms involved in T1D and the clinical studies promoted by these studies. The findings from preclinical and clinical studies support the original proposed model for how T1D develops but have also suggested that this disease is more complex than was originally thought and will require broader treatment approaches.

Role of recombination activating genes in the generation of antigen receptor diversity and beyond

V(D)J recombination is the process by which antibody and T-cell receptor diversity is attained. During this process, antigen receptor gene segments are cleaved and rejoined by non-homologous DNA end joining for the generation of combinatorial diversity. The major players of the initial process of cleavage are the proteins known as RAG1 (recombination activating gene 1) and RAG2. In this review, we discuss the physiological function of RAGs as a sequence-specific nuclease and its pathological role as a structure-specific nuclease. The first part of the review discusses the basic mechanism of V(D)J recombination, and the last part focuses on how the RAG complex functions as a sequence-specific and structure-specific nuclease. It also deals with the off-target cleavage of RAGs and its implications in genomic instability.

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.

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.

Activation-induced cytidine deaminase mediates central tolerance in B cells

The Aicda gene product, activation-induced cytidine deaminase (AID), initiates somatic hypermutation, class-switch recombination, and gene conversion of Ig genes by the deamination of deoxycytidine, followed by error-prone mismatch- or base-excision DNA repair. These processes are crucial for the generation of genetically diverse, high affinity antibody and robust humoral immunity, but exact significant genetic damage and promote cell death. In mice, physiologically significant AID expression was thought to be restricted to antigen-activated, mature B cells in germinal centers. We now demonstrate that low levels of AID in bone marrow immature and transitional B cells suppress the development of autoreactivity. Aicda(-/-) mice exhibit significantly increased serum autoantibody and reduced capacity to purge autoreactive immature and transitional B cells. In vitro, AID deficient immature/transitional B cells are significantly more resistant to anti-IgM-induced apoptosis than their normal counterparts. Thus, early AID expression plays a fundamental and unanticipated role in purging self-reactive immature and transitional B cells during their maturation in the bone marrow.

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

Clonal deletion of autoreactive B cells is crucial to prevent autoimmunity, but the signaling mechanisms that regulate this checkpoint remain undefined. Here we characterized a previously unrecognized Ca²⁺-driven Erk activation pathway, which was pro-apoptotic and biochemically distinct from DAG-induced Erk activation. This pathway required PKCδ and RasGRP proteins and depended on Stim1 concentrations, which control the magnitude of Ca²⁺ 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 PKCδ-deficient mice, which developed B cell autoimmunity. Conversely, Stim1 overexpression conferred a competitive disadvantage to developing B cells. These findings establish Ca²⁺-dependent Erk signaling as a critical pro-apoptotic pathway that mediates B cell negative selection.

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.

Tonic BCR signaling represses receptor editing via Raf- and calcium-dependent signaling pathways

Light chain receptor editing is an important mechanism that prevents B cell self-reactivity. We have previously shown that tonic signaling through the BCR represses RAG expression at the immature B cell stage, and that initiation of light chain rearrangements occurs in the absence of these tonic signals in an in vitro model of B cell development. To further test our hypothesis we studied the effect of itpkb deficiency (itpkb(-/-) mice) or Raf hyper-activation (Raf-CAAX transgenic mice), two mutations that enhance BCR signaling, on receptor editing in an in vivo model. This model relies on transferring bone marrow from wild-type or mutant mice into mice expressing an anti-kappa light chain transgene. The anti-kappa transgene induces receptor editing of all kappa light chain expressing B cells, leading to a high frequency of lambda light chain expressing B cells. Anti-κ transgenic recipients of bone marrow from itpkb(-/-) or Raf-CAAX mice showed lower levels of editing to λ light chain than did non-transgenic control recipients. These results provide evidence in an in vivo model that enhanced BCR signaling at the immature B cell stage of development suppresses light chain receptor editing.

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.

S1P3 confers differential S1P-induced migration by autoreactive and non-autoreactive immature B cells and is required for normal B-cell development

During B-cell development, immature B-cell fate is determined by whether the BCR is engaged in the bone marrow. Immature B cells that are non-autoreactive continue maturation and emigrate from the marrow, whereas autoreactive immature B cells remain and are tolerized. However, the microenvironment where these events occur and the chemoattractants responsible for immature B-cell trafficking within and out of the bone marrow remain largely undefined. Sphingosine 1-phosphate (S1P) is a chemoattractant that directs lymphocyte trafficking and thymocyte egress and in this study we investigated whether S1P contributes to B-cell development, egress and positioning within the bone marrow. Our findings show that immature B cells are chemotactic toward S1P but that this response is dependent on Ag receptor specificity: non-autoreactive, but not autoreactive, immature B cells migrate toward S1P and are shown to require S1P3 receptor for this response. Despite this response, S1P3 is shown not to facilitate immature B-cell egress but is required for normal B-cell development including the positioning of transitional B cells within bone marrow sinusoids. These data indicate that S1P3 signaling directs immature B cells to a bone marrow microenvironment important for both tolerance induction and maturation.

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.

Tolerance checkpoints in B-cell development: Johnny B good

B-cell development up to the immature B-cell stage takes place in the bone marrow, while final maturation into mature B cells occurs in the spleen. During differentiation, the precursor and immature B cells have to pass several checkpoints, including those in which they are censored for being auto-reactive, and therefore being potentially dangerous. Numerous studies have shown that the immature B-cell stage in the bone marrow and the transitional B-cell stages in the spleen comprise distinct checkpoints at which auto-reactivity is censored. Recently, evidence has been provided that the large pre-BII stage in the bone marrow, at which the pre-BCR is expressed, is yet another B-cell tolerance checkpoint. Here, we review these findings and speculate on directions for possible further experimentation.

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.

Mechanisms controlling expression of the RAG locus during lymphocyte development

Recombination activating genes (RAG)1 and RAG2 are expressed in developing B and T lymphocytes and are required for the rearrangement of antigen receptor genes. In turn, RAG expression is regulated by the products of these assembled immunoglobulin (Ig) and T cell receptor (TCR) genes. Upon successful assembly of Ig genes, the antigen receptor is expressed on the immature B cell surface and tested for autoreactivity leading to either maintenance or inactivation of RAG expression. Successful assembly of TCR genes is followed by surface TCR expression and testing for its ability to interact with self-MHC, which if appropriate leads to the inactivation of RAG expression. Recent studies in B and T lymphocytes demonstrate that the reduction in RAG expression at the immature B and double-positive (DP) T cell stages is mediated through tonic (foreign antigen independent) receptor signaling. In B cells, tonic signaling activates PI(3)K and Akt kinases, which phosphorylate and lead to the cytoplasmic sequestration of FoxO proteins, the key transcriptional activators of RAG expression. In T cells, tonic signaling activates Abl and Erk kinases, leading to the transcriptional inactivation of RAGs.

Signaling pathways regulating RAG expression in B lymphocytes

Development of B-cell lymphopoiesis is dependent on the presence of recombination activating genes RAG1 and RAG2 enzymes. They control the rearrangements of immunoglobulin variable, diversity and joining region segments, and allow progression of the cellular maturation. RAG1 and RAG2 are successively up- and down-regulated at each B-cell stage to progressively generate a B-cell receptor for which unforeseeable antigenic specificity results from a stochastic process. Therefore, in autoreactive immature B cells, new round of RAG re-expression can be observed to eliminate self-reactivity. In some circumstances, RAG up-regulation can also be found in peripheral mature B lymphocytes, specifically in autoimmune diseases. It is therefore of utmost importance to unravel signaling pathways that trigger RAG induction in normal and pathological conditions. Therapeutic modulation of cytokines or intracellular contacts involved in RAG expression might restrict the development of inappropriate autoimmune repertoire.

RS rearrangement frequency as a marker of receptor editing in lupus and type 1 diabetes

Continued antibody gene rearrangement, termed receptor editing, is an important mechanism of central B cell tolerance that may be defective in some autoimmune individuals. We describe a quantitative assay for recombining sequence (RS) rearrangement that we use to estimate levels of antibody light chain receptor editing in various B cell populations. RS rearrangement is a recombination of a noncoding gene segment in the κ antibody light chain locus. RS rearrangement levels are highest in the most highly edited B cells, and are inappropriately low in autoimmune mouse models of systemic lupus erythematosus (SLE) and type 1 diabetes (T1D), including those without overt disease. Low RS rearrangement levels are also observed in human subjects with SLE or T1D.

Antigen receptor editing in anti-DNA transitional B cells deficient for surface IgM

In response to encounter with self-Ag, autoreactive B cells may undergo secondary L chain gene rearrangement (receptor editing) and change the specificity of their Ag receptor. Knowing at what differentiative stage(s) developing B cells undergo receptor editing is important for understanding how self-reactive B cells are regulated. In this study, in mice with Ig transgenes coding for anti-self (DNA) Ab, we report dsDNA breaks indicative of ongoing secondary L chain rearrangement not only in bone marrow cells with a pre-B/B cell phenotype but also in immature/transitional splenic B cells with little or no surface IgM (sIgM(-/low)). L chain-edited transgenic B cells were detectable in spleen but not bone marrow and were still found to produce Ab specific for DNA (and apoptotic cells), albeit with lower affinity for DNA than the unedited transgenic Ab. We conclude that L chain editing in anti-DNA-transgenic B cells is not only ongoing in bone marrow but also in spleen. Indeed, transfer of sIgM(-/low) anti-DNA splenic B cells into SCID mice resulted in the appearance of a L chain editor (Vlambdax) in the serum of engrafted recipients. Finally, we also report evidence for ongoing L chain editing in sIgM(low) transitional splenic B cells of wild-type mice.

B cell receptor basal signaling regulates antigen-induced Ig light chain rearrangements

BCR editing in the bone marrow contributes to B cell tolerance by orchestrating secondary Ig rearrangements in self-reactive B cells. We have recently shown that loss of the BCR or a pharmacologic blockade of BCR proximal signaling pathways results in a global "back-differentiation" response in which immature B cells down-regulate genes important for the mature B cell program and up-regulate genes characteristic of earlier stages of B cell development. These observations led us to test the hypothesis that self-Ag-induced down-regulation of the BCR, and not self-Ag-induced positive signals, lead to Rag induction and hence receptor editing. Supporting this hypothesis, we found that immature B cells from xid (x-linked immunodeficiency) mice induce re-expression of a Rag2-GFP bacterial artificial chromosome reporter as well as wild-type immature B cells following Ag incubation. Incubation of immature B cells with self-Ag leads to a striking reversal in differentiation to the pro-/pre-B stage of development, consistent with the idea that back-differentiation results in the reinduction of genes required for L chain rearrangement and receptor editing. Importantly, Rag induction, the back-differentiation response to Ag, and editing in immature and pre-B cells are inhibited by a combination of phorbol ester and calcium ionophore, agents that bypass proximal signaling pathways and mimic BCR signaling. Thus, mimicking positive BCR signals actually inhibits receptor editing. These findings support a model whereby Ag-induced receptor editing is inhibited by BCR basal signaling on developing B cells; BCR down-regulation removes this basal signal, thereby initiating receptor editing.

IL-6 contributes to the expression of RAGs in human mature B cells

Mature B cells acquire the capacity to revise rearranged Ig V region genes in secondary lymphoid organs. In previous studies, we demonstrated that cross-linking the BCR and the CD40 induces the expression of the RAG1 and RAG2 enzymes and, thereby, secondary rearrangements. We examine herein the mechanism that underpins RAG1 and RAG2 expression in peripheral and tonsil B cells. Coordinated engagement of the BCR and CD40 promoted the synthesis of IL-6 and, thereby, up-regulation of its receptor on activated B lymphocytes. Furthermore, we provide evidence that IL-6 initiates the expression of RAGs in circulating B cells, and extends those in tonsil B cells. Thus, neutralization of IL-6 or blocking of its receptor inhibits RAG expression. Moreover, we demonstrate that IL-6 impedes BCR-mediated termination of RAG gene expression in both population of B cells. The recovered inhibition of RAG gene transcription by IL-6 receptor blockade supports the notion that once recombination is launched, its termination is also regulated by IL-6. Taken together, these studies provide new insight into the dual role of IL-6 in inducing and terminating expression of the recombinase machinery for secondary rearrangements in mature human B cells.

CpG DNA stimulates autoreactive immature B cells in the bone marrow

Polyclonal activation of developing B cells is an injurious process, because most of these cells are nontolerant and express autoreactive receptors. CpG DNA is a polyclonal activator of mature B cells, but its effect on developing B cells is unclear. We tested whether developing, nontolerant B cells are responsive to mitogenic stimulation by CpG DNA and whether such a stimulus can interfere with the establishment of central tolerance. We found that developing B cells express Toll-like receptor 9 and undergo a polyclonal response to CpG DNA stimulation, as revealed by proliferation and differentiation to antibody-producing cells. In vitro and ex vivo experiments revealed that stimulation with CpG DNA protects immature B cells from negative selection imposed by apoptosis and receptor editing and results in the production of autoantibodies. Finally, we found that in vivo administration of CpG DNA activates immature B cells in the bone marrow and suppresses the expression of recombination-activating genes in a mouse model of central tolerance and receptor editing. These results suggest that mitogenic signals provided by CpG DNA stimulate nontolerant immature B cells in the bone marrow and have the potential to interfere with central tolerance.

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.

Subversion of B lymphocyte tolerance by hydralazine, a potential mechanism for drug-induced lupus

Accumulating evidence indicates that epigenetic alterations contribute to exacerbated activation or deregulation of the mechanisms that maintain tolerance to self-antigens in patients with lupus, a systemic autoimmune disease that can be triggered by medications taken to treat a variety of conditions. Here, we tested the effect of hydralazine, an antihypertensive drug that triggers lupus, on receptor editing, a chief mechanism of B lymphocyte tolerance to self-antigens. Using mice expressing transgenic human Igs, we found that hydralazine impairs up-regulation of RAG-2 gene expression and reduces secondary Ig gene rearrangements. Receptor editing was also partially abolished in a dose-dependent manner by a specific inhibitor of MEK1/2. Adoptive transfer of bone marrow B cells pretreated with hydralazine or with a MEK inhibitor to naïve syngeneic mice resulted in autoantibody production. We conclude that, by disrupting receptor editing, hydralazine subverts B lymphocyte tolerance to self and contributes to generation of pathogenic autoreactivity. We also postulate that inhibition of the Erk signaling pathway contributes to the pathogenesis of hydralazine-induced lupus and idiopathic human lupus.

Immunoglobulin kappa enhancers are differentially regulated at the level of chromatin structure

The kappa intronic and the kappa 3' enhancers synergize to regulate recombination and transcription of the Ig kappa locus. Although these enhancers have overlapping functions, the kappa i enhancer appears to predominate during receptor editing, while the kappa 3' enhancer may be more important for initiating Ig kappa germline transcription to target locus recombination and, later in development, somatic hypermutation. Changes in chromatin structure appear to regulate both enhancers, and previous reports suggest that both enhancers are packaged into an accessible chromatin structure only in B lineage cells. Why these enhancers cannot activate the demethylated, accessible, protein-associated Ig kappa allele in pro-B cells is not known. Furthermore, how the enhancers function to reactivate the locus for receptor editing or to quantitatively promote hypermutation in B cells is vague. Quantitative analysis of Ig enhancer chromatin structure in murine pro-, pre-and splenic B cells demonstrated that the kappa i enhancer maintains a highly accessible chromatin structure under a variety of conditions. This stable chromatin structure mirrored the highly accessible structure characterizing the Ig mu intronic enhancer, despite the fact that Ig mu is activated prior to Ig kappa during B cell development. Surprisingly, parallel analysis of the kappa 3' enhancer demonstrated its accessible chromatin structure is markedly unstable, as characterized by sensitivity to changes in environmental conditions. These data unexpectedly suggest that kappa locus regulation is compartmentalized along the gene in B lineage cells. Furthermore, these findings raise the possibility that environmentally dependent regulation of kappa 3' enhancer structure underlies changes in kappa activation during B cell development.

B‐Cell Self‐Tolerance in Humans

Two mechanisms account for generation of the human antibody repertoire; V(D)J recombination during the early stages of B-cell development in the bone marrow and somatic mutation of immunoglobulin genes in mature B cells responding to antigen in the periphery. V(D)J recombination produces diversity by random joining of gene segments and somatic mutation by introducing random point mutations. Both are required to attain the degree of antigen receptor diversification that is necessary for immune protection: defects in either mechanism are associated with increased susceptibility to infection. However, the downside of producing enormous random diversity in the antibody repertoire is the generation of autoantibodies. To prevent autoimmunity B cells expressing autoantibodies are regulated by strict mechanisms that either modify the specificity of autoantibodies or the fate of cells expressing such antibodies. Abnormalities in B-cell self-tolerance are associated with a large number of autoimmune diseases, but the precise nature of the defects is less well defined. Here we summarize recent data on the self-reactive B-cell repertoire in healthy humans and in patients with autoimmunity.

Receptor editing in lymphocyte development and central tolerance

The specificities of lymphocytes for antigen are generated by a quasi-random process of gene rearrangement that often results in non-functional or autoreactive antigen receptors. Regulation of lymphocyte specificities involves not only the elimination of cells that display 'unsuitable' receptors for antigen but also the active genetic correction of these receptors by secondary recombination of the DNA. As I discuss here, an important mechanism for the genetic correction of antigen receptors is ongoing recombination, which leads to receptor editing. Receptor editing is probably an adaptation that is necessitated by the high probability of receptor autoreactivity. In both B cells and T cells, the genes that encode the two chains of the antigen receptor seem to be specialized to promote, on the one hand, the generation of diverse specificities and, on the other hand, the regulation of these specificities through efficient editing.

Generation of peripheral B cells occurs via two spatially and temporally distinct pathways

We have identified a population of newly formed bone marrow (BM) B cells that shares multiple characteristics with late transitional B cells in the spleen. Both late splenic transitional B cells and cells within this uncharacterized BM population expressed the cell-surface phenotype AA4(+) CD23(+), yet the developmental kinetics and the renewal rate of AA4(+) CD23(+) BM B cells mirrored recently formed BM B cells. Further, unlike the least mature B cells in the BM and spleen, AA4(+) CD23(+) BM B cells expressed the homing receptor CD62L, were dependent on the antiapoptotic cytokine receptor BR3 and the tec family kinase Btk, and proliferated in response to IL-4 plus CD40 stimulation. Finally, frequencies of lambda light chain-positive B cells declined among AA4(+) CD23(+) B cells in both the BM and spleen, suggesting that V-gene selection events correlate with CD23 expression in both compartments. These observations indicate that the first step in B-cell maturation occurs in both the BM and the periphery and suggest that recently formed B cells exit the BM as a heterogeneous pool of immature and semimature B cells.

Re-shaping the T cell repertoire: TCR editing and TCR revision for good and for bad

Protection against the universe of pathogens requires a functional, diverse T cell repertoire. However, the price that is paid for an evolved, effective immune system includes the potential danger of generating autoaggressive T cells. Autoimmune diseases result from inherent breach of tolerance to self-antigens leading to disruption of the regulatory to autoaggressive T cell homeostatic balance. The immune system has evolved mechanisms to control those processes. For T cells, positive and negative selection in the thymus assures that only fully functional, non-self-reactive T cells will populate the periphery. Failure of this central tolerance would result in autoaggressive T cells escaping into the periphery. However, other means of escaping negative selection can occur in the periphery, i.e., TCR revision, or the altering of TCR expression after thymic egress. Here the potential benefits, i.e., expansion and re-shaping of the T cell repertoire as potentiated by TCR editing and revision are considered. Furthermore, the potential to develop autoaggressive TCR and thus enhance autoimmunity is considered.

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.

Dual isotype expressing B cells [kappa(+)/lambda(+)] arise during the ontogeny of B cells in the bone marrow of normal nontransgenic mice

Central to the clonal selection theory is the tenet that a single B cell expresses a single receptor with a single specificity. Previously, based on our work in anti-phosphocholine transgenic mouse models, we suggested that B cells escaped clonal deletion by coexpression of more than one receptor on their cell surface. We argued that "receptor dilution" was necessary when: (i) the expressed immunoglobulin receptor is essential for immune protection against pathogens and (ii) this protective receptor is autoreactive and would be clonally deleted, leaving a hole in the B cell repertoire. Here, we demonstrate that dual isotype expressing B cells arise during the normal ontogeny of B cells in the bone marrow and populate both the spleen and peritoneal cavity of nontransgenic mice. Furthermore, single cell analysis of the expressed immunoglobulin light chains suggests that receptor editing may play a role in the generation of a significant fraction of dual isotype expressing B cells.

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.

2 BCR or NOT 2 BCR - receptor dilution: a unique mechanism for preventing the development of holes in the protective B cell repertoire

The clonal selection theory and the associated corollaries have had a major influence in shaping our thinking about lymphoid cell development as well as how these cells respond to antigenic challenges. Among these concepts are that a single B cell expresses a single receptor with a single antigen specificity. While these hypotheses have proven invaluable in expanding our understanding of immune response, over time numerous observations have been made that suggest that the single cell, single receptor, single specificity model is not absolute. In this manuscript, we review this literature as it pertains to B cells and provide a summary that supports the notion that in certain situations, the over-arching rules by which we consider development and response of immune cells may be compromised. The result of compromising allelic and isotype exclusion is a small but real population of dual receptor expressing B cells. A number of mechanisms that have been proposed for generating these dual expressing B cells are presented and discussed. We also consider the negative implications of dual receptor expression on regulating and controlling autoreactive B cell populations as well as its beneficial contributions to preserving essential receptor specificities and thereby preventing the development of holes in the immune repertoire. Previously, the dual receptor expressing population has received relatively little attention. Improvements in the tools available to examine individual B cell populations have resulted in our identification of and discrimination between novel populations of B cells, including novel dual receptor expressing populations. This combined with continuing increases in our understanding of how the immune repertoire relates to a protective immune response will strengthen and further define this novel aspect of immune cell development.

Potential Contribution of VH Gene Replacement in Immunity and Disease

VH replacement occurs through RAG-mediated recombination between a cryptic recombination signal sequence (cRSS) presented in a rearranged VH gene and a 23-bp RSS from an upstream VH gene. VH replacement renews the entire VH coding region and extends the immunoglobulin heavy-chain (IgH) CDR3 regions preferentially with charged amino acids. VH replacement occurs in bone marrow-immature B cells and contributes significantly to the primary B-cell repertoire in humans. However, the biological significance of VH replacement is not clear. Our recent studies revealed elevated frequencies of VH replacement products in different autoimmune diseases, including rheumatoid arthritis, systemic lupus erythematosus, and Sjögren's syndrome. Moreover, elevated frequencies of VH replacement products were also found in patients with human immunodeficiency virus or hepatitis C virus infections. These results provide the first clue that VH replacement contributes to autoimmune disease and antiviral immunity, and they also suggest a potential link between viral infection and autoimmune disease.

Expression of a Dromedary Heavy Chain-Only Antibody and B Cell Development in the Mouse

In mature B cells of mice and most mammals, cellular release of single H chain Abs without L chains is prevented by H chain association with Ig-specific chaperons in the endoplasmic reticulum. In precursor B cells, however, surface expression of mu-H chain in the absence of surrogate and conventional L chain has been identified. Despite this, Ag-specific single H chain Ig repertoires, using mu-, gamma-, epsilon-, or alpha-H chains found in conventional Abs, are not produced. Moreover, removal of H chain or, separately, L chain (kappa/lambda) locus core sequences by gene targeting has prevented B cell development. In contrast, H chain-only Abs are produced abundantly in Camelidae as H2 IgG without the C(H)1 domain. To test whether H chain Abs can be produced in mice, and to investigate how their expression affects B cell development, we introduced a rearranged dromedary gamma2a H chain into the mouse germline. The dromedary transgene was expressed as a naturally occurring Ag-specific disulphide-linked homodimer, which showed that B cell development can be instigated by expression of single H chains without L chains. Lymphocyte development and B cell proliferation was accomplished despite the absence of L chain from the BCR complex. Endogenous Ig could not be detected, although V(D)J recombination and IgH/L transcription was unaltered. Furthermore, crossing the dromedary H chain mice with mice devoid of all C genes demonstrated without a doubt that a H chain-only Ab can facilitate B cell development independent of endogenous Ig expression, such as mu- or delta-H chain, at early developmental stages.

Impaired early B cell tolerance in patients with rheumatoid arthritis

Autoantibody production is a characteristic of most autoimmune diseases including rheumatoid arthritis (RA). The role of these autoantibodies in the pathogenesis of RA remains elusive, but they appear in the serum many years before the onset of clinical disease suggesting an early break in B cell tolerance. The stage of B cell development at which B cell tolerance is broken in RA remains unknown. We previously established in healthy donors that most polyreactive developing B cells are silenced in the bone marrow, and additional autoreactive B cells are removed in the periphery. B cell tolerance in untreated active RA patients was analyzed by testing the specificity of recombinant antibodies cloned from single B cells. We find that autoreactive B cells fail to be removed in all six RA patients and represent 35-52% of the mature naive B cell compartment compared with 20% in healthy donors. In some patients, RA B cells express an increased proportion of polyreactive antibodies that can recognize immunoglobulins and cyclic citrullinated peptides, suggesting early defects in central B cell tolerance. Thus, RA patients exhibit defective B cell tolerance checkpoints that may favor the development of autoimmunity.