Positive selection of B cells expressing low densities of self-reactive BCRs

B cell tolerance and autoimmunity: Lessons from repertoires

Adaptive immune cell function is regulated by a highly diverse receptor recombined from variable germline-encoded segments that can recognize an almost unlimited array of epitopes. While this diversity enables the recognition of any pathogen, it also poses a risk of self-recognition, leading to autoimmunity. Many layers of regulation are present during both the generation and activation of B cells to prevent this phenomenon, although they are evidently imperfect. In recent years, our ability to analyze immune repertoires at scale has drastically increased, both through advances in sequencing and single-cell analyses. Here, we review the current knowledge on B cell repertoire analyses, focusing on their implication for autoimmunity. These studies demonstrate that a failure of tolerance occurs at multiple independent checkpoints in different autoimmune contexts, particularly during B cell maturation, plasmablast differentiation, and within germinal centers. These failures are marked by distinct repertoire features that may be used to identify disease- or patient-specific therapeutic approaches.

Peripheral apoptosis and limited clonal deletion during physiologic murine B lymphocyte development

Self-reactive and polyreactive B cells generated during B cell development are silenced by either apoptosis, clonal deletion, receptor editing or anergy to avoid autoimmunity. The specific contribution of apoptosis to normal B cell development and self-tolerance is incompletely understood. Here, we quantify self-reactivity, polyreactivity and apoptosis during physiologic B lymphocyte development. Self-reactivity and polyreactivity are most abundant in early immature B cells and diminish significantly during maturation within the bone marrow. Minimal apoptosis still occurs at this site, however B cell receptors cloned from apoptotic B cells show comparable self-reactivity to that of viable cells. Apoptosis increases dramatically only following immature B cells leaving the bone marrow sinusoids, but above 90% of cloned apoptotic transitional B cells are not self-reactive/polyreactive. Our data suggests that an apoptosis-independent mechanism, such as receptor editing, removes most self-reactive B cells in the bone marrow. Mechanistically, lack of survival signaling rather than clonal deletion appears to be the underpinning cause of apoptosis in most transitional B cells in the periphery.

Reactivity Graph Yields Interpretable IgM Repertoire Signatures as Potential Tumor Biomarkers

Combining adaptive and innate immunity induction modes, the repertoire of immunoglobulin M (IgM) can reflect changes in the internal environment including malignancies. Previously, it was shown that a mimotope library reflecting the public IgM repertoire of healthy donors (IgM IgOme) can be mined for efficient probes of tumor biomarker antibody reactivities. To better explore the interpretability of this approach for IgM, solid tumor-related profiles of IgM reactivities to linear epitopes of actual tumor antigens and viral epitopes were studied. The probes were designed as oriented planar microarrays of 4526 peptide sequences (as overlapping 15-mers) derived from 24 tumor-associated antigens and 209 cancer-related B cell epitopes from 30 viral antigens. The IgM reactivity in sera from 21 patients with glioblastoma multiforme, brain metastases of other tumors, and non-tumor-bearing neurosurgery patients was thus probed in a proof-of-principle study. A graph representation of the binding data was developed, which mapped the cross-reactivity of the mixture of IgM (poly)specificities, delineating different antibody footprints in the features of the graph-neighborhoods and cliques. The reactivity graph mapped the major features of the IgM repertoire such as the magnitude of the reactivity (titer) and major cross-reactivities, which correlated with blood group reactivity, non-self recognition, and even idiotypic specificities. A correlation between an aspect of this image of the IgM IgOme, namely, small cliques reflecting rare self-reactivities and the capacity of subsets of the epitopes to separate the diagnostic groups studied was found. In this way, the graph representation helped the feature selection in its filtering step and provided reduced feature sets, which, after recursive feature elimination, produced a classifier containing 51 peptide reactivities separating the three diagnostic groups with an unexpected efficiency. Thus, IgM IgOme approaches to repertoire studies is greatly augmented when self/viral antigens are used and the data are represented as a reactivity graph. This approach is most general, and if it is applicable to tumors in immunologically privileged sites, it can be applied to any solid tumors, for instance, breast or lung cancer.

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

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

Transfer of antigen-encoding bone marrow under immune-preserving conditions deletes mature antigen-specific B cells in recipients and inhibits antigen-specific antibody production

BACKGROUND AIMS

Pathological activation and collaboration of T and B cells underlies pathogenic autoantibody responses. Existing treatments for autoimmune disease cause non-specific immunosuppression, and induction of antigen-specific tolerance remains an elusive goal. Many immunotherapies aim to manipulate the T-cell component of T-B interplay, but few directly target B cells. One possible means to specifically target B cells is the transfer of gene-engineered BM that, once engrafted, gives rise to widespread specific and tolerogenic antigen expression within the hematopoietic system.

METHODS

Gene-engineered bone marrow encoding ubiquitous ovalbumin expression was transferred after low-dose (300-cGy) immune-preserving irradiation. B-cell responsiveness was monitored by analyzing ovalbumin-specific antibody production after immunization with ovalbumin/complete Freund's adjuvant. Ovalbumin-specific B cells and their response to immunization were analyzed using multi-tetramer staining. When antigen-encoding bone marrow was transferred under immune-preserving conditions, cognate antigen-specific B cells were purged from the recipient's preexisting B-cell repertoire and the repertoire that arose after bone marrow transfer.

RESULTS

OVA-specific B-cell deletion was apparent within the established host B-cell repertoire as well as that developing after gene-engineered bone marrow transfer. OVA-specific antibody production was substantially inhibited by transfer of OVA-encoding BM and activation of OVA-specific B cells, germinal center formation and subsequent OVA-specific plasmablast differentiation were all inhibited. Low levels of gene-engineered bone marrow chimerism were sufficient to limit antigen-specific antibody production.

RESULTS

These data show that antigen-specific B cells within an established B-cell repertoire are susceptible to de novo tolerance induction, and this can be achieved by transfer of gene-engineered bone marrow. This adds further dimensions to the utility of antigen-encoding bone marrow transfer as an immunotherapeutic tool.

Homeostasis and regulation of autoreactive B cells

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

Anti-Insulin B Cells Are Poised for Antigen Presentation in Type 1 Diabetes

Early breaches in B cell tolerance are central to type 1 diabetes progression in mouse and man. Conventional BCR transgenic mouse models (VH125.Tg NOD) reveal the power of B cell specificity to drive disease as APCs. However, in conventional fixed IgM models, comprehensive assessment of B cell development is limited. To provide more accurate insight into the developmental and functional fates of anti-insulin B cells, we generated a new NOD model (V_H125^SDNOD) in which anti-insulin VDJH125 is targeted to the IgH chain locus to generate a small (1-2%) population of class switch-competent insulin-binding B cells. Tracking of this rare population in a polyclonal repertoire reveals that anti-insulin B cells are preferentially skewed into marginal zone and late transitional subsets known to have increased sensitivity to proinflammatory signals. Additionally, IL-10 production, characteristic of regulatory B cell subsets, is increased. In contrast to conventional models, class switch-competent anti-insulin B cells proliferate normally in response to mitogenic stimuli but remain functionally silent for insulin autoantibody production. Diabetes development is accelerated, which demonstrates the power of anti-insulin B cells to exacerbate disease without differentiation into Ab-forming or plasma cells. Autoreactive T cell responses in V_H125^SDNOD mice are not restricted to insulin autoantigens, as evidenced by increased IFN-γ production to a broad array of diabetes-associated epitopes. Together, these results independently validate the pathogenic role of anti-insulin B cells in type 1 diabetes, underscore their diverse developmental fates, and demonstrate the pathologic potential of coupling a critical β cell specificity to predominantly proinflammatory Ag-presenting B cell subsets.

Microbial symbionts regulate the primary Ig repertoire

Symbiotic relationships help shape immune fitness. Chen et al. demonstrate that microbial symbionts influence host immunity by enriching frequencies of antibacterial specificities within the naive B cell receptor repertoire and that this may have consequences for mucosal and systemic immunity.

The ability of immunoglobulin (Ig) to recognize pathogens is critical for optimal immune fitness. Early events that shape preimmune Ig repertoires, expressed on IgM⁺ IgD⁺ B cells as B cell receptors (BCRs), are poorly defined. Here, we studied germ-free mice and conventionalized littermates to explore the hypothesis that symbiotic microbes help shape the preimmune Ig repertoire. Ig-binding assays showed that exposure to conventional microbial symbionts enriched frequencies of antibacterial IgM⁺ IgD⁺ B cells in intestine and spleen. This enrichment affected follicular B cells, involving a diverse set of Ig-variable region gene segments, and was T cell–independent. Functionally, enrichment of microbe reactivity primed basal levels of small intestinal T cell–independent, symbiont-reactive IgA and enhanced systemic IgG responses to bacterial immunization. These results demonstrate that microbial symbionts influence host immunity by enriching frequencies of antibacterial specificities within preimmune B cell repertoires and that this may have consequences for mucosal and systemic immunity.

Spontaneous germinal centers and autoimmunity

Germinal centers (GCs) are dynamic microenvironments that form in the secondary lymphoid organs and generate somatically mutated high-affinity antibodies necessary to establish an effective humoral immune response. Tight regulation of GC responses is critical for maintaining self-tolerance. GCs can arise in the absence of purposeful immunization or overt infection (called spontaneous GCs, Spt-GCs). In autoimmune-prone mice and patients with autoimmune disease, aberrant regulation of Spt-GCs is thought to promote the development of somatically mutated pathogenic autoantibodies and the subsequent development of autoimmunity. The mechanisms that control the formation of Spt-GCs and promote systemic autoimmune diseases remain an open question and the focus of ongoing studies. Here, we discuss the most current studies on the role of Spt-GCs in autoimmunity.

IgH isotype-specific B cell receptor expression influences B cell fate

Ig heavy chain (IgH) isotypes (e.g., IgM, IgG, and IgE) are generated as secreted/soluble antibodies (sIg) or as membrane-bound (mIg) B cell receptors (BCRs) through alternative RNA splicing. IgH isotype dictates soluble antibody function, but how mIg isotype influences B cell behavior is not well defined. We examined IgH isotype-specific BCR function by analyzing naturally switched B cells from wild-type mice, as well as by engineering polyclonal Ighγ1/γ1 and Ighε/ε mice, which initially produce IgG1 or IgE from their respective native genomic configurations. We found that B cells from wild-type mice, as well as Ighγ1/γ1 and Ighε/ε mice, produce transcripts that generate IgM, IgG1, and IgE in an alternative splice form bias hierarchy, regardless of cell stage. In this regard, we found that mIgμ > mIgγ1 > mIgε, and that these BCR expression differences influence respective developmental fitness. Restrained B cell development from Ighγ1/γ1 and Ighε/ε mice was proportional to sIg/mIg ratios and was rescued by enforced expression of the respective mIgs. In addition, artificially enhancing BCR signal strength permitted IgE⁺ memory B cells-which essentially do not exist under normal conditions-to provide long-lived memory function, suggesting that quantitative BCR signal weakness contributes to restraint of IgE B cell responses. Our results indicate that IgH isotype-specific mIg/BCR dosage may play a larger role in B cell fate than previously anticipated.

Altered B cell signalling in autoimmunity

Recent work has provided new insights into how altered B cell-intrinsic signals - through the B cell receptor (BCR) and key co-receptors - function together to promote the pathogenesis of autoimmunity. These combined signals affect B cells at two distinct stages: first, in the selection of the naive repertoire; and second, during extrafollicular or germinal centre activation responses. Thus, dysregulated signalling can lead to both an altered naive BCR repertoire and the generation of autoantibody-producing B cells. Strikingly, high-affinity autoantibodies predate and predict disease in several autoimmune disorders, including type 1 diabetes and systemic lupus erythematosus. This Review summarizes how, rather than being a downstream consequence of autoreactive T cell activation, dysregulated B cell signalling can function as a primary driver of many human autoimmune diseases.

The Global Self-Reactivity Profile of the Natural Antibody Repertoire Is Largely Independent of Germline DH Sequence

Natural antibodies (NAbs) are produced in the absence of exogenous antigenic stimulation and circulate in the blood of normal, healthy individuals. These antibodies have been shown to provide one of the first lines of defense against both bacterial and viral pathogens. Conservation of the NAb repertoire reactivity profile is observed both within and across species. One view holds that this conservation of NAb self-reactivities reflects the use of germline antibody sequence, whereas the opposing view holds that the self-reactivities reflect selection driven by key conserved self-antigens. In mice, B-1a B cells are a major source of NAbs. A significant fraction of the B-1a antibody repertoire is devoid of N nucleotides in H chain complementarity determining region 3 (CDR-H3) and, thus, completely germline encoded. To test the role of germline DH sequence on the self-reactivity profile of the NAb repertoire, we examined the composition and self-antigen specificity of NAbs produced by a panel of DH gene-targeted BALB/c mice, each strain of which expresses a polyclonal, altered CDR-H3 repertoire that differs from the wild-type norm. We found that in most cases the same key self-antigens were recognized by the NAbs created by each DH-altered strain. The differences in reactivity appeared to represent the genetic signature of the NAb repertoire of each mouse strain. These findings suggest that although germline CDR-H3 sequence may facilitate the production of certain NAbs, a core set of self-antigens are likely the main force driving the selection of Nab self-specificities.

Altered BCR and TLR signals promote enhanced positive selection of autoreactive transitional B cells in Wiskott-Aldrich syndrome

Wiskott-Aldrich syndrome (WAS) is an X-linked immunodeficiency disorder frequently associated with systemic autoimmunity, including autoantibody-mediated cytopenias. WAS protein (WASp)-deficient B cells have increased B cell receptor (BCR) and Toll-like receptor (TLR) signaling, suggesting that these pathways might impact establishment of the mature, naive BCR repertoire. To directly investigate this possibility, we evaluated naive B cell specificity and composition in WASp-deficient mice and WAS subjects (n = 12). High-throughput sequencing and single-cell cloning analysis of the BCR repertoire revealed altered heavy chain usage and enrichment for low-affinity self-reactive specificities in murine marginal zone and human naive B cells. Although negative selection mechanisms including deletion, anergy, and receptor editing were relatively unperturbed, WASp-deficient transitional B cells showed enhanced proliferation in vivo mediated by antigen- and Myd88-dependent signals. Finally, using both BCR sequencing and cell surface analysis with a monoclonal antibody recognizing an intrinsically autoreactive heavy chain, we show enrichment in self-reactive cells specifically at the transitional to naive mature B cell stage in WAS subjects. Our combined data support a model wherein modest alterations in B cell-intrinsic, BCR, and TLR signals in WAS, and likely other autoimmune disorders, are sufficient to alter B cell tolerance via positive selection of self-reactive transitional B cells.

Autoantibody signatures: progress and perspectives for early cancer detection

Becoming invasive is a crucial step in cancer development, and the early spread of tumour cells is usually undetected by current imaging technologies. In patients with cancer and no signs of overt metastases, sensitive methods have been developed to identify circulating autoantibodies and their antigen counterparts in several cancers. These technologies are often based on proteomic approaches, and recent advances in protein and antibody microarrays have greatly facilitated the discovery of new antibody biomarkers in sera from cancer patients. Interestingly, in a clinical application setting, combinations of multiple autoantibody reactivities into panel assays have recently been proposed as relevant screening tests and validated in several independent trials. In addition, autoantibody signatures seem to be particularly relevant for early detection of cancer in high-risk cancer patients. In this review, we highlight the concept that immunogenic epitopes associated with the humoural response and key pathogenic pathways elicit serum autoantibodies that can be considered as relevant cancer biomarkers. We outline the proteomic strategies employed to identify and validate their use in clinical practice for cancer screening and diagnosis. We particularly emphasize the clinical utility of autoantibody signatures in several cancers. Finally, we discuss the challenges remaining for clinical validation.

Immunology (1955-1975): the natural selection theory, the two signal hypothesis and positive repertoire selection

Observations suggesting the existence of natural antibody prior to exposure of an organism to the corresponding antigen, led to the natural selection theory of antibody formation of Jerne in 1955, and to the two signal hypothesis of Forsdyke in 1968. Aspects of these were not only first discoveries but also foundational discoveries in that they influenced contemporaries in a manner that, from our present vantage point, appears to have been constructive. Jerne's later hypothesis (1971, European Journal of Immunology 1: 1-9), that antibody-like receptors on lymphocytes were selected over evolutionary time for reactivity with the major histocompatibility complex (MHC) antigens of the species, was a first, but it was incorrect, and was foundational only to the extent that it emphasized the need to explain the Simonsen phenomenon. Although easily construed as derivative of Jerne (1971), the affinity/avidity model of Forsdyke (1975, Journal of Theoretical Biology 52: 187-198), which predicted that cell-surface components, including MHC antigens, would restrict antigen-reactivity by somatically shaping lymphocyte repertoires, was actually an extension of the two signal hypothesis. While presenting a mechanism for the positive selection of lymphocyte repertoires, and explaining the Simonsen phenomenon, the affinity/avidity model was not foundational in that it had to be independently rediscovered. For science to advance optimally we must seek to close temporal gaps so that first discoveries are also foundational. Listening to young scientists may be part of the solution.

Intraclonal cell expansion and selection driven by B cell receptor in chronic lymphocytic leukemia

The mutational status of the immunoglobulin heavy-chain variable region (IGHV) genes utilized by chronic lymphocytic leukemia (CLL) clones defines two disease subgroups. Patients with unmutated IGHV have a more aggressive disease and a worse outcome than patients with cells having somatic IGHV gene mutations. Moreover, up to 30% of the unmutated CLL clones exhibit very similar or identical B cell receptors (BcR), often encoded by the same IG genes. These "stereotyped" BcRs have been classified into defined subsets. The presence of an IGHV gene somatic mutation and the utilization of a skewed gene repertoire compared with normal B cells together with the expression of stereotyped receptors by unmutated CLL clones may indicate stimulation/selection by antigenic epitopes. This antigenic stimulation may occur prior to or during neoplastic transformation, but it is unknown whether this stimulation/selection continues after leukemogenesis has ceased. In this study, we focused on seven CLL cases with stereotyped BcR Subset #8 found among a cohort of 700 patients; in six, the cells expressed IgG and utilized IGHV4-39 and IGKV1-39/IGKV1D-39 genes, as reported for Subset #8 BcR. One case exhibited special features, including expression of IgM or IgG by different subclones consequent to an isotype switch, allelic inclusion at the IGH locus in the IgM-expressing cells and a particular pattern of cytogenetic lesions. Collectively, the data indicate a process of antigenic stimulation/selection of the fully transformed CLL cells leading to the expansion of the Subset #8 IgG-bearing subclone.

The importance of natural IgM: scavenger, protector and regulator

The existence of IgM has been known for more than a century, but its importance in immunity and autoimmunity continues to emerge. Studies of mice deficient in secreted IgM have provided unexpected insights into its role in several diverse processes, from B cell survival to atherosclerosis, as well as in autoimmunity and protection against infection. Among the various distinct properties that underlie the functions of IgM, two stand out: its polyreactivity and its ability to facilitate the removal of apoptotic cells. In addition, new B cell-targeted therapies for the treatment of autoimmunity have been shown to cause a reduction in serum IgM, potentially disrupting the functions of this immunoregulatory molecule and increasing susceptibility to infection.

B cell lymphoproliferation and organ-directed self-recognition to explain autoimmunity: back to the past

Autoimmune diseases are characterised by lymphoproliferation in target tissues with B and T lymphocytes often arranged in pseudofollicles, mimicking the structure of peripheral lymph nodes. Target organ tissue damage produces the clinical phenotype which may be diverse ranging from autoimmune endocrinopathies to malabsorption (coeliac disease) to structural damage within bones and joints (rheumatoid arthritis). Recently, B cell depletion has been shown to be effective in many autoimmune conditions suggesting a common pathological origin for these conditions which might be triggered by an autoimmune B cell that has escaped deletion. We postulate that a mutation in a transcription factor early in B cell development might allow persistence and foster proliferation of a clone of autoimmune B cells, capable of producing autoantibodies. A similar common mutation within the JAK2 tyrosine kinase gene has recently been described associated with the myeloproliferative disorders which are also characterised by diverse clinical disease phenotypes. There is considerable evidence that autoimmune diseases could be indolent lymphoproliferative disorders of B-cell origin, extending the forbidden clone hypothesis first proposed in the 1950s.

Identifying autoantibody signatures in cancer: a promising challenge

Biomarkers that show high sensitivity and specificity are needed for the early diagnosis and prognosis of cancer. An immune response to cancer is elicited in humans, as demonstrated, in part, by the identification of autoantibodies against a number of tumor-associated antigen (TAAs) in sera from patients with different types of cancer. Identification of TAAs and their cognate autoantibodies is a promising strategy for the discovery of relevant biomarkers. During the past few years, three proteomic approaches, including serological identification of antigens by recombinant expression cloning (SEREX), serological proteome analysis (SERPA) and, more recently, protein microarrays, have been the dominant strategies used to identify TAAs and their cognate autoantibodies. In this review, we aim to describe the advantages, drawbacks and recent improvements of these approaches for the study of humoral responses. Finally, we discuss the definition of autoantibody signatures to improve sensitivity for the development of clinically relevant tests.

Cracking the BAFF code

The tumour necrosis factor (TNF) family members B cell activating factor (BAFF) and APRIL (a proliferation-inducing ligand) are crucial survival factors for peripheral B cells. An excess of BAFF leads to the development of autoimmune disorders in animal models, and high levels of BAFF have been detected in the serum of patients with various autoimmune conditions. In this Review, we consider the possibility that in mice autoimmunity induced by BAFF is linked to T cell-independent B cell activation rather than to a severe breakdown of B cell tolerance. We also outline the mechanisms of BAFF signalling, the impact of ligand oligomerization on receptor activation and the progress of BAFF-depleting agents in the clinical setting.

The broad antibacterial activity of the natural antibody repertoire is due to polyreactive antibodies

Polyreactive antibodies bind to a variety of structurally unrelated antigens. The function of these antibodies, however, has remained an enigma, and because of their low binding affinity their biological relevance has been questioned. Using a panel of monoclonal polyreactive antibodies, we showed that these antibodies can bind to both Gram-negative and Gram-positive bacteria and acting through the classical complement pathway can inhibit bacterial growth by lysis, generate anaphylatoxin C5a, enhance phagocytosis, and neutralize the functional activity of endotoxin. Polyreactive antibody-enriched, but not polyreactive antibody-reduced, IgM prepared from normal human serum displays antibacterial activity similar to that of monoclonal polyreactive IgM. We conclude that polyreactive antibodies are a major contributor to the broad antibacterial activity of the natural antibody repertoire.

Characterization of a late transitional B cell population highly sensitive to BAFF-mediated homeostatic proliferation

We have characterized a distinct, late transitional B cell subset, CD21^int transitional 2 (T2) B cells. In contrast to early transitional B cells, CD21^int T2 B cells exhibit augmented responses to a range of potential microenvironmental stimuli. Adoptive transfer studies demonstrate that this subset is an immediate precursor of both follicular mature and marginal zone (MZ) B cells. In vivo, a large percentage of CD21^int T2 B cells has entered the cell cycle, and the cycling subpopulation exhibits further augmentation in mitogenic responses and B cell-activating factor of the TNF family (BAFF) receptor expression. Consistent with these features, CD21^int T2 cells exhibit preferential responses to BAFF-facilitated homeostatic signals in vivo. In addition, we demonstrate that M167 B cell receptor (BCR) idiotypic-specific B cells are first selected within the cycling CD21^int T2 population, ultimately leading to preferential enrichment of these cells within the MZ B cell compartment. These data, in association with the coordinate role for BAFF and microenvironmental cues in determining the mature BCR repertoire, imply that this subset functions as a unique selection point in peripheral B cell development.

Autoantigen-B cell antigen receptor interactions that regulate expression of B cell antigen receptor Loci

Levels of AgR (BCR) expression are regulated during B cell development, activation, and induction of tolerance. The mechanisms responsible for and consequences of this regulation are poorly understood. We have described a class of DNA-based autoantigen-reactive B cell that down-regulates BCR expression during development to mature follicular phenotype. In this study, we show that at immature stages of primary differentiation, individual B cells of this type can dynamically modulate levels of expression of BCR in inverse proportion to degree of autoantigen engagement and induced BCR signaling. These adjustments in BCR expression are not associated with cell death, BCR revision, or altered development, and do not require TLR 9. Strikingly, modulation of BCR subunit gene RNA levels and transcription parallels these changes in BCR expression, indicating a direct link between autoantigen-BCR interactions of this type and regulation of transcription of BCR-encoding loci. We propose that this adaptive process allows this class of autoreactive B cell to avoid conventional tolerance pathways and promotes development to mature phenotype.

B Cells Expressing a Natural Polyreactive Autoantibody Have a Distinct Phenotype and Are Overrepresented in Immunoglobulin Heavy Chain Transgenic Mice

Despite stringent regulation of disease-associated autoantibodies, a substantial proportion of circulating Abs in sera of healthy individuals exhibit self-reactivity. These Abs are referred to as naturally occurring or natural autoantibodies (NAAs). To understand the origin and function of NAAs, we have generated a new site-directed transgenic mouse model in which a prerearranged VDJ gene coding for the H chain of a typical polyreactive NAA, ppc1-5, is inserted into the IgH locus. This H chain, when combined with its original L chain, the lambda1 L chain, yields a NAA that characteristically binds a variety of self and non-self Ags including ssDNA, actin, ubiquitin, and nitrophenyl phosphocholine. Despite their autoreactivity, B cells expressing ppc1-5H/lambda1 NAA are not negatively selected, but rather are overrepresented in the transgenic mice. The shift toward lambda1 expression mainly occurs during the transition of immature to mature B cells in the spleen, suggesting a BCR selection process. The ppc1-5H/lambda1 B cells exhibit a phenotype that is different from those of the known mature B cell populations, and they are located predominantly in the lymphoid follicles of the spleen and the lymph nodes. These B cells are functionally active, producing high levels of Abs in vivo and responding well to BCR stimulation in vitro. The findings indicate that the ppc1-5/lambda1 natural autoantibodies originate from a distinct B cell subset that may be positively selected by virtue of its poly/autoreactivity.

Development and selection of edited B cells in B6.56R mice

Tolerance to dsDNA is broken in mice with a high-affinity anti-DNA H chain transgene, 56R, on the C57BL/6 background (B6.56R). B6.56R produce more anti-dsDNA Abs than BALBc.56R. To investigate how anti-DNA Abs are regulated on the B6 background, phenotypic and genetic studies were performed. B6.56R have reduced numbers of B cells and phenotypically altered B cell subsets, including relative increases in the proportions of IgM-negative bone marrow B cells, cells with a marginal zone phenotype, and cells with a transitional T3 phenotype. The peripheral B cell repertoire in B6.56R is restricted: most B cells express the 56R H chain and use a similar, limited subset of editor L chains. DNA binding is more common in B6.56R because the repertoire is shifted toward L chains that are more permissive for DNA binding. H chain editing is also observed and is increased in spontaneous as compared with LPS hybridomas. A subset of spontaneous hybridomas appears to lack H chain expression.

Signals from a self-antigen induce positive selection in early B cell ontogeny but are tolerogenic in adults

Positive and negative signals from self-Ags shape the B cell repertoire and the development of distinct B cell subsets, but little is known about what distinguishes these signals. To address this question, we have studied the development of anti-hen egg lysozyme MD4 Ig transgene B cells while systematically varying the level, distribution, and timing of exposure to different forms of hen egg lysozyme as a self-Ag. This process has allowed us to explore the effects of Ag independent of BCR specificity. Our findings show how the selection of autoreactive B cells is a competitive process involving immunogenic and tolerogenic forms of self-Ags. Due to a developmental switch during B cell ontogeny, autoreactive anti-hen egg lysozyme MD4 Ig transgene B cells are negatively selected by self-Ags in adult bone marrow but susceptible to positive selection by some of the same self-Ags in fetal and neonatal life. However, the persistence of B1 cells and IgM autoantibodies from early ontogeny enables autoreactive B cells from the adult bone marrow to escape negative selection. Our data suggest that this rescue may be due to the clearance or masking of self-Ag by IgM autoantibody. We discuss the implications of these findings in terms of B cell selection and the maintenance of self-tolerance during early and adult life.

An antibody VH gene that promotes marginal zone B cell development and heavy chain allelic inclusion

The Ig heavy (H) chain plays a pivotal role in the regulation of primary B cell development through its association with a variety of other proteins including Igalpha and Igbeta, the surrogate light chain components and bona fide L chains, to form transmembrane signaling complexes. Little is known about how alterations in the structure of the H chain variable region influence association with these proteins, or the signaling capacity of the complexes that form. Here we describe a line of VH 'knockin' mice in which the transgene-encoded VH region differs by eight amino acid residues from the VH region in a VH knockin line we previously constructed and characterized. The transgenic H chain locus in the line of mice we characterized earlier efficiently promotes H chain allelic exclusion and all phases of primary B cell development, resulting in the generation of mature B1, marginal zone (MZ) and follicular (FO) B cell compartments. In contrast, the transgenic H chain locus in the new line fails to enforce allelic exclusion, as evidenced by the majority of peripheral B cells expressing two H chains on their surfaces. Moreover, this locus inefficiently drives bone marrow B lymphopoiesis and FO B cell development. However, this H chain locus does promote MZ B cell development, from precursors that appear to be generated during fetal and neonatal life. We discuss these data in the context of previous findings on the influence of Ig H chain structure on primary B cell development.

“Altered-self” or “near-self” in the positive selection of lymphocyte repertoires?

Positive selection of lymphocyte repertoires is now recognized as applying to both B and T cells. However, much of the early literature on positive selection focussed on cell-mediated immunity (T cells), which biased consideration of its general biological role. The term "altered-self," which initially captured the idea of self (i.e. MHC) altered by the addition of what was later found to be a peptide fragment, has not proven robust and may now be clouding our understanding. It is recommended that the term "near-self" be reintroduced since it captures the essence of the probable underlying adaptive process-sub-threshold self-reactivity to countermand rapid pathogen mutation.

Role of TLR in B cell development: signaling through TLR4 promotes B cell maturation and is inhibited by TLR2

The role of TLR4 in mature B cell activation is well characterized. However, little is known about TLR4 role in B cell development. Here, we analyzed the effects of TLR4 and TLR2 agonists on B cell development using an in vitro model of B cell maturation. Highly purified B220(+)IgM(-) B cell precursors from normal C57BL/6 mouse were cultured for 72 h, and B cell maturation in the presence of the TLR agonists was evaluated by expression of IgM, IgD, CD23, and AA4. The addition of LPS or lipid A resulted in a marked increase in the percentage of CD23(+) B cells, while Pam3Cys had no effect alone, but inhibited the increase of CD23(+) B cell population induced by lipid A or LPS. The TLR4-induced expression of CD23 is not accompanied by full activation of the lymphocyte, as suggested by the absence of activation Ag CD69. Experiments with TLR2-knockout mice confirmed that the inhibitory effects of Pam3Cys depend on the expression of TLR2. We studied the effects of TLR-agonists on early steps of B cell differentiation by analyzing IL-7 responsiveness and phenotype of early B cell precursors: we found that both lipid A and Pam3Cys impaired IL-7-dependent proliferation; however, while lipid A up-regulates B220 surface marker, consistent with a more mature phenotype of the IgM(-) precursors, Pam3Cys keeps the precursors on a more immature stage. Taken together, our results suggest that TLR4 signaling favors B lymphocyte maturation, whereas TLR2 arrests/retards that process, ascribing new roles for TLRs in B cell physiology.

Antinuclear antigen B cells that down-regulate surface B cell receptor during development to mature, follicular phenotype do not display features of anergy in vitro

We previously demonstrated that B cells expressing a transgenic BCR with "dual reactivity" for the hapten arsonate and nuclear autoantigens efficiently complete development to follicular phenotype and stably reside in follicles in vivo. These B cells express very low levels of surface IgM and IgD, suggesting that they avoid central deletion and peripheral anergy by reducing their avidity for autoantigen via surface BCR (sBCR) down-regulation. Since a variety of states of B cell anergy have been previously described, a thorough examination of the functional capabilities of these B cells was required to test this hypothesis. In this study, we show that surface Ig cross-linking induces amounts of proximal BCR signaling in these B cells commensurate with their reduced sBCR levels. Functionally, however, they are comparable to nonautoreactive B cells in cell cycle progression, up-regulation of activation and costimulatory molecules, and Ab-forming cell differentiation when treated with a variety of stimuli in vitro. In addition, these B cells can efficiently process and present Ag and are capable of undergoing cognate interaction with naive TCR-transgenic T cells, resulting in robust IL-2 production. Together, these data reveal a lack of intrinsic anergy involving any known mechanism, supporting the idea that this type of antinuclear Ag B cell becomes indifferent to cognate autoantigen by down-regulating sBCR.

Receptor editing in positive and negative selection of B lymphopoiesis

In B lymphopoiesis, Ag receptor expression and signaling are critical to determine developmental progression, survival, and activation. Several positive and negative selection checkpoints to test this receptor have been described in B lymphopoiesis, aiming to ensure the generation of functionally competent, nonautoimmune repertoire. Secondary Ag receptor gene recombination allows B lymphocytes to replace an inappropriate receptor with a new receptor, a mechanism called receptor editing. This salvage mechanism uncouples the Ag receptor fate from that of the cell itself, suggesting that B cell repertoire is regulated by a process of receptor selection. Secondary rearrangements are stimulated in different stages of B cell development, where editing of the receptor is necessary to fulfill stage-specific requirements. In this study, we discuss the contribution of receptor editing in B lymphopoiesis and its regulation by positive and negative selection signals.

B Cell Positive Selection: Road Map to the Primary Repertoire?

Accumulating evidence indicates that positive selection events mediate differentiation, lineage commitment, and longevity of B lymphocytes. The BCR plays a central role, dictating the likelihood that newly formed cells will complete maturation, as well as whether cells persist within mature pools. Competition among B cells for limited, life span-promoting resources, which include self-ligands, lineage-specific cytokines, and innate receptor ligands, underlie these selective processes. Together, these observations suggest that positive selection is a critical feature in the establishment and maintenance of all lymphocyte pools, prompting re-evaluation of the underlying biological rationale for this process.