Regulation of B-cell development by antibody specificity

Ig-seq: Deep sequencing of the variable region of Atlantic salmon IgM heavy chain transcripts

Immunoglobulin M plays a key role in systemic protection of Atlantic salmon against pathogens. Until recent, studies have focused on antigen-specific antibodies and little is known about the IgM repertoire: its size, developmental changes and responses to antigens. We report the development of deep sequencing protocol to characterize the repertoire of IgM heavy chain variable region. Its structure and changes were examined at the early stages of life and after infection with virus of cardiac myopathy. Clonotypes are identified by the V and J gene segments and amino acid sequences of CDR3, which determine the contribution of the heavy chain to the antigen binding properties. A major fraction of transcripts are functional while the rest are either sterile (transcribed from noncoding parts of Ig loci) or include stop codons. Despite marked difference in frequencies of combinations of V and J genes, the size of repertoire is large. The IgM diversity steadily increases after hatch followed with temporal reduction during smoltification and recovery after seawater transfer. Most clonotypes are present only in one fish. However multiple transcripts in uninfected fish are produced exclusively from a small fraction of shared clonotypes. While only 4.7% of clonotypes are detected in three and more fish, they comprise 35% of transcripts. Increased frequencies of most abundant clonotypes were detected in the head kidney and blood at ten weeks after viral infection and all were shared. Occurrence of the same clonotypes in multiple individuals can be explained with either their simple structure or exposure to common antigens. Complexity of CDR3 assessed by contents of non complementary nucleotides is slightly lower in shared clonotypes but difference is small. High nucleotide diversity of CDR3 with identical amino acid sequences suggests selection.

Characteristics of natural antibody-secreting cells

Natural IgM plays a critical role in protection from pathogens and the prevention of autoimmunity. While its importance has been shown in many different settings, its origins are incompletely understood. This review focuses on the properties of the natural IgM antibody-secreting cells (ASCs), which arise mainly from the B-1 cell lineage. B-1 cells are generated in multiple waves during development, mostly in the fetal and early postfetal periods. The developmental time points can affect their repertoire: prenatal B-1 cells express a mainly germ line-encoded repertoire, while postnatally developing B-1 cells can express Ig with a greater degree of variation. Spleen and bone marrow, but not the body cavities, are primary sites of natural IgM secretion. Within these tissues heterogeneous populations of IgM ASCs can be found. While some ASCs express classical markers of B-1 lymphocytes, others express those of terminally differentiated plasma cells. A better understanding of the properties of these different natural IgM ASCs could aid their future therapeutic exploitation.

CD4+ T cells and CD40 participate in selection and homeostasis of peripheral B cells

Control of peripheral B cell development and homeostasis depends critically on coordinate signals received through the BAFFRs and BCRs. The extent to which other signals contribute to this process, however, remains undefined. We present data indicating that CD4(+) T cells directly influence naive B cell development via CD40 signaling. Loss of CD4(+) T cells or CD40-CD40L interaction leads to reduced B cell homeostatic proliferation and hindered B cell reconstitution posttransplantation. Furthermore, we demonstrate that in the absence of CD40 signals, these events are modulated by BCR self-reactivity. Strikingly, murine models lacking CD40 reveal a broadly altered BCR specificity and limited diversity by both single-cell cloning and high-throughput sequencing techniques. Collectively, our results imply that any setting of T cell lymphopenia or reduced CD40 function, including B cell recovery following transplantation, will impact the naive B cell repertoire.

Changes of human B and B-1a peripheral blood lymphocytes with age

In 2057 consecutive subjects admitted to the Department of Pathology, Section of Immunology of the Verona University Hospital, CD19+ and CD5/CD19 double positive cells were determined to assess the behaviour of total peripheral B-lymphocytes and B-1a (CD5+) compartments in humans during aging. We show that the absolute number of total B lymphocytes increases about three-fold from the baseline conditions in the first year of life and progressively decreases until adult age. A slower decrease was detected from the adult age onwards. A similar behaviour has been observed within the B-1a subset of B-lymphocytes, although the decrease after the adult age seems more pronounced. Possible physiological explanations and/or implications for the disease states are taken into account.

Notch-RBP-J-independent marginal zone B cell development in IgH transgenic mice with VH derived from a natural polyreactive antibody

Both the B cell antigen receptor (BCR) signaling and Notch signaling pathway play important roles in marginal zone (MZ) B cell development; however, if and how these two signaling pathways engage in crosstalk with each other remain unclear. In the present study, IgH transgenic mice (TgV(H)3B4) were crossed with mice with Notch downstream transcription factor RBP-J floxed alleles (RBP-J(f/f)) and Mx-Cre transgene. Subsequently, MZ B cell development was analyzed in 3B4/Cre/RBP-J(f/f) mice that expressed the transgenic 3B4 IgH and exhibited a deficiency in Notch signaling in B cells upon poly (I:C) injection. We observed that MZ B cell numbers were severely reduced, but still detectable in 3B4/Cre/RBP-J(f/f) mice, in contrast to increased numbers of MZ B cells in TgV(H)3B4 mice and almost no MZ B cells in Cre/RBP-J(f/f) mice. The majority of the MZ B cells in the 3B4/Cre/RBP-J(f/f) mice had the same antigen specificity with that of 3B4 antibody, indicating that a particular BCR specificity might direct MZ B cell development in the absence of Notch signaling. The number of MZ B precursor (MZP) cells was reduced sharply in 3B4/Cre/RBP-J(f/f) mice, and the number of transitional stage 1 and transitional stage 2 cells did not change that much, indicating that the interaction between BCR and Notch signaling likely occurred during the T2-MZP stage. Based on the transgenic mouse model, our data indicate that MZ B cells with certain BCR specificity can develop in a Notch-RBP-J independent manner.

Experimental models of B cell tolerance in transplantation

The use of conventional immunosuppression has successfully improved short-term allograft survival, however, long-term allograft survival has remained static and is complicated by serious side effects secondary to the long-term use of immunosuppressive agents. Immunological tolerance is the ultimate goal of organ transplantation, however it is an infrequent event in humans. Accordingly, over the past several decades, there has been a push to fully understand both the cellular and molecular mechanisms that play a role in the induction and maintenance of tolerance, with recent data implicating B cells and donor specific alloantibody as a barrier to and potential mediator of allograft tolerance. The study of B cells and alloantibody in transplant tolerance has evolved over recent years from using rodent models to non-human primate models. This review will discuss the role of B cells and alloantibody as antagonists and facilitators of transplantation tolerance, and highlight the experimental models developed for elucidating the mechanisms of B cell tolerance to alloantigen.

A CD25⁻ positive population of activated B1 cells expresses LIFR and responds to LIF

B1 B cells defend against infectious microorganisms by spontaneous secretion of broadly reactive “natural” immunoglobulin that appears in the absence of immunization. Among many distinguishing characteristics, B1 B cells display evidence of activation that includes phosphorylated STAT3. In order to identify the origin of pSTAT3 we examined interleukin-2 receptor (IL-2R) expression on B1 cells. We found that some (about 1/5) B1a cells express the IL-2R α chain, CD25. Although lacking CD122 and unresponsive to IL-2, B1a cells marked by CD25 express increased levels of activated signaling intermediates, interruption of which results in diminished CD25. Further, CD25⁺ B1a cells contain most of the pSTAT3 found in the B1a population as a whole. Moreover, CD25⁺ B1a cells express leukemia inhibitory factor receptor (LIFR), and respond to LIF by upregulating pSTAT3. Together, these results define a new subset of B1a cells that is marked by activation-dependent CD25 expression, expresses substantial amounts of activated STAT3, and contains a functional LIFR.

Bridging innate and adaptive antitumor immunity targeting glycans

Effective immunotherapy for cancer depends on cellular responses to tumor antigens. The role of major histocompatibility complex (MHC) in T-cell recognition and T-cell receptor repertoire selection has become a central tenet in immunology. Structurally, this does not contradict earlier findings that T-cells can differentiate between small hapten structures like simple glycans. Understanding T-cell recognition of antigens as defined genetically by MHC and combinatorially by T cell receptors led to the “altered self” hypothesis. This notion reflects a more fundamental principle underlying immune surveillance and integrating evolutionarily and mechanistically diverse elements of the immune system. Danger associated molecular patterns, including those generated by glycan remodeling, represent an instance of altered self. A prominent example is the modification of the tumor-associated antigen MUC1. Similar examples emphasize glycan reactivity patterns of antigen receptors as a phenomenon bridging innate and adaptive but also humoral and cellular immunity and providing templates for immunotherapies.

Alterations of peripheral blood CD5+ B cells in inflammatory bowel disease

OBJECTIVE

CD5(+) B cells comprise a unique subset of B cells that modulates innate as well as autoimmune systems. The aim of this study was to investigate alterations of the circulating CD5(+) B-cell subset in patients with inflammatory bowel disease (IBD) by evaluating various clinical parameters, including therapeutic regimens.

MATERIAL AND METHODS

Thirty-four patients with ulcerative colitis (UC), 19 patients with Crohn's disease (CD), and 46 healthy control subjects were enrolled in this study. CD5(+) B cells in peripheral blood collected from each subject were analyzed by flow cytometry. Multiple regression analysis was carried out to evaluate the factors related to the circulating CD5(+) B-cell subset in the IBD patients. In an in vitro examination, dexamethasone-induced apoptosis in peripheral blood B cells was examined by detecting cell surface binding of the annexin-V antibody.

RESULTS

Age and gender in the control subjects did not influence the circulating CD5(+) B-cell subset. Multiple regression analysis showed that the presence of UC, corticosteroid therapy, and number of white blood cells in peripheral blood each had a significant influence in decreasing the number of circulating CD5(+) B cells in the IBD patients. Furthermore, in vitro results showed that dexamethasone treatment significantly induced apoptosis in CD5(+) B cells, though apoptosis was similarly observed in CD5(-) B cells.

CONCLUSIONS

CD5(+) B cells may be involved in the pathogenesis of UC, and modulation of this subset by corticosteroid therapy may play a role in the treatment of IBD patients.

Preferential use of DH reading frame 2 alters B cell development and antigen-specific antibody production

All jawed vertebrates limit use of D(H) reading frames (RFs) that are enriched for hydrophobic amino acids. In BALB/c mice, DFL16.1 RF2 encodes valine and isoleucine. To test whether increased use of RF2 affects B cell function, we examined B cell development and Ab production in mice with an IgH allele (DeltaD-DmicroFS) limited to use of a single, frameshifted DFL61.1 gene segment. We compared the results of these studies to wild-type mice, as well as those previously obtained in mice limited to use of either a single normal D(H) or a single inverted D(H) that forces use of arginine in CDR-H3. All three of the mouse strains limited to a single D(H) produced fewer immature B cells than wild type. However, whereas mice limited to a single normal D(H) achieved normal B cell numbers in the periphery, mice forced to preferentially use RF2 had reduced numbers of mature B cells in the spleen and bone marrow, mirroring the pattern previously observed in mice enriched for charged CDR-H3s. There were two exceptions. B cells in the mice using RF2 normally populated the marginal zone and peritoneal cavity, whereas mice using inverted RF1 had increased numbers of marginal zone B cells and decreased numbers of B1a cells. When challenged with several T-dependent or T-independent Ags, Ag-specific Ab titers in the mice forced to use RF2 were altered. These findings indicate that B cell development and Ag-specific Ab production can be heavily influenced by the global amino acid content of the CDR-H3 repertoire.

B-1 B lymphocytes require Blimp-1 for immunoglobulin secretion

B-1 B cells produce circulating natural antibodies that provide “innate-like” protection against bacterial and viral pathogens. They also provide adaptive responses to blood and air-borne pathogens. B lymphocyte–induced maturation protein 1 (Blimp-1) is a transcriptional repressor that is required for the formation of B-2–derived antibody-secreting plasma cells. In this study, we used mice lacking Blimp-1 in the B cell lineage to show that Blimp-1 is not necessary for the formation or self-renewal of B-1 B cells but that Blimp-1 is required for normal immunoglobulin (Ig) secretion by B-1 cells. B-1 cells lacking Blimp-1 do not repress Pax5 mRNA and do not induce X-box binding protein 1, and μ secreted mRNA normally, showing that B-1 and B-2 cells both use a common pathway for Ig secretion. Blimp-1–deficient B-1 B cells are also defective in providing early protection against influenza infection.

New perspectives in B-1 B cell development and function

In contrast to the predominant population of B-2 B cells produced in the bone marrow, B-1 B cells are a minor population of B lymphocytes that are found in multiple tissues, including the pleural and peritoneal cavities in mice. Although the role of B-1 B cells as effectors of innate-like immunity is widely accepted, their developmental origin has been controversial. This review highlights recent experimental data from murine studies supporting the hypothesis that B-1 B cells belong to a developmental lineage distinct from B-2 B cells, and draws attention to recent studies that have defined new roles for the B-1a and B-1b B-cell subsets in the response to bacteria and self-antigens.

B cell receptors in TCL1 transgenic mice resemble those of aggressive, treatment-resistant human chronic lymphocytic leukemia

B cell chronic lymphocytic leukemia (B-CLL) is a clonal overgrowth of CD5(+) B lymphocytes. In this disease, the B cell antigen receptor (BCR) is intimately linked to disease severity, because patients with BCRs, comprised of unmutated V(H) genes, follow a much more aggressive course. This and related observations suggest that B-CLL derives from a B cell subset comprised of restricted BCR structural diversity and that antigen-selection and drive are major factors promoting the disease. Nevertheless, the initiating event(s) that lead to the development of B-CLL are still unclear, in part because of the lack of an animal model that spontaneously evolves the molecular abnormalities that occur in the human disease. Because overexpression of the TCL1 gene in murine B cells leads to a CD5(+) B cell lymphoproliferative disorder with many of the features of human B-CLL, we studied leukemias emerging in these mice to examine the extent to which their BCRs resemble those in B-CLL. Our data indicate that the immunoglobulin heavy and light chain rearrangements in TCL1 mice display minimal levels of somatic mutations and exhibit several molecular features found in the human disease. Like human B-CLL, TCL1 leukemic rearrangements from different mice can be very similar structurally and closely resemble autoantibodies and antibodies reactive with microbial antigens. Antigen-binding analyses confirm that selected TCL1 clones react with glycerophospholipid, lipoprotein, and polysaccharides that can be autoantigens and be expressed by microbes. This (auto)antigen-driven mouse model reliably captures the BCR characteristics of aggressive, treatment-resistant human B-CLL.

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.

Induction of tolerance in arthritogenic B cells with receptors of differing affinity for self-antigen

Multiple mechanisms of tolerance induction limit autoimmunity, but their relative contribution for lymphocytes recognizing self-antigens of differing availability is incompletely understood. The mechanisms applied to arthritogenic B cells expressing antigen-specific B cell receptors (BCRs) with different affinities for glucose-6-phosphate isomerase (GPI) were examined in the corresponding Ig gene knock-in mice. This ubiquitously expressed and blood-borne enzyme is the target autoantigen in the K/BxN model of inflammatory arthritis and perhaps in some humans with arthritis. Negative selection of B cells expressing high-affinity anti-GPI specificities, whose surface receptors were occupied by GPI, operated mainly at the transitional B cell stages in the spleen, preventing their final differentiation and entry into follicular areas. Receptor editing contributed to the purging of cells displaying anti-GPI BCRs, and significant numbers of autoreactive cells escaped through expression of an additional Ig light (L) chain, accumulating gradually in lymphoid organs. In contrast, low-affinity anti-GPI B cells, whose surface receptors did not carry GPI, matured normally. The "escaped" dual-L-chain cells and the "ignored" low-affinity cells are the likely precursors of cells that produce pathogenic autoantibodies once T cell help is provided. These studies portray, in a single system, the range of tolerance mechanisms applied to potentially pathogenic B cells, and serve as a base for dissecting where T cell help intervenes and where therapeutic agents impinge.

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.

Differential signalling during B-cell maturation

The molecular mechanism by which the antigen receptors (BCR) on B cells can elicit differential maturation state-specific responses is one of the central problems in B-cell differentiation yet to be resolved. Indeed, many of the early signalling events detected following BCR ligation, such as activation of protein tyrosine kinases (PTK), phospholipase C (PLC), phosphoinositide-3-kinase (PI 3K), protein kinase C (PKC) and the RasMAPK (mitogen activating protein kinase) signalling cascades are observed throughout B-cell maturation. However, it is becoming clear that the differential functional responses of these BCR-coupled signals observed during B-cell maturation are dependent on a number of parameters including signal strength and duration, subcellular localisation of the signal, maturation-restricted expression of downstream signalling effector elements/isoforms and modulation of signal by co-receptors. Thus, the combined signature of BCR signalling is likely to dictate the functional response and act as a developmental checkpoint for B-cell maturation.

Genetic aspects of immune-mediated adverse drug effects

Adverse drug effects (ADEs) are of great importance in medicine and account for up to 5% of all hospital admissions. ADEs can arise from several mechanisms and a wide range of drugs can cause immune-mediated ADEs (IMADEs). For a drug to elicit an IMADE, it must be both immunogenic (that is, able to sensitize the immune system) and antigenic (that is, able to evoke a response from a sensitized immune system). Unlike protein therapeutics, small-molecule drugs (or xenobiotics) are usually neither immunogenic nor antigenic. IMADEs are therefore the result of complex interactions between drug-metabolizing enzymes, immune sensitization and immune effectors. The genetic aspects of this interplay are discussed in this review.