The immunoglobulin (Ig)alpha and Igbeta cytoplasmic domains are independently sufficient to signal B cell maturation and activation in transgenic mice

Mechanical control of antigen detection and discrimination by T and B cell receptors

The adaptive immune response is orchestrated by just two cell types, T cells and B cells. Both cells possess the remarkable ability to recognize virtually any antigen through their respective antigen receptors-the T cell receptor (TCR) and B cell receptor (BCR). Despite extensive investigations into the biochemical signaling events triggered by antigen recognition in these cells, our ability to predict or control the outcome of T and B cell activation remains elusive. This challenge is compounded by the sensitivity of T and B cells to the biophysical properties of antigens and the cells presenting them-a phenomenon we are just beginning to understand. Recent insights underscore the central role of mechanical forces in this process, governing the conformation, signaling activity, and spatial organization of TCRs and BCRs within the cell membrane, ultimately eliciting distinct cellular responses. Traditionally, T cells and B cells have been studied independently, with researchers working in parallel to decipher the mechanisms of activation. While these investigations have unveiled many overlaps in how these cell types sense and respond to antigens, notable differences exist. To fully grasp their biology and harness it for therapeutic purposes, these distinctions must be considered. This review compares and contrasts the TCR and BCR, placing emphasis on the role of mechanical force in regulating the activity of both receptors to shape cellular and humoral adaptive immune responses.

IgIT-Mediated Signaling Inhibits the Antimicrobial Immune Response in Oyster Hemocytes

The long-term evolutionary interaction between the host and symbiotic microbes determines their cooperative relationship. It is well known that the symbiotic microbes have evolved various mechanisms to either benefit or exploit the mammalian host immune system to maintain homeostasis. However, the strategies employed by the symbiotic microbes to overcome host immune responses in invertebrates are still not clear. In the current study, the hemolymph microbes in oyster Crassostrea gigas were found to be able to directly bind an oyster Ig superfamily member (IgSF) (designated as CgIgIT) to inhibit the immune responses of hemocytes. The mRNA transcripts of CgIgIT in hemocytes increased significantly after the stimulation with hemolymph microbes. CgIgIT was found to be located on the hemocyte membrane and it was able to directly bind the hemolymph microbes and polysaccharides via its three Ig domains and recruited the protein tyrosine phosphatase CgSHP2 through its ITIM. The recruited CgSHP2 inhibited the activities of CgERK, CgP38 and CgJNK proteins to reduce the productions of dual oxidase 2 (CgDuox2) and defensin 2 (CgDef2), which eventually protected the hemolymph microbes from CgDuox2/CgDef2-mediated elimination. Collectively, the results suggest that the oyster IgIT-SHP2 signaling pathway can recognize bacteria capable of residing in oyster hemolymph and inhibit innate immune responses, which contributes to the maintenance, colonization, and survival of hemolymph microbes.

An Ancient BCR-like Signaling Promotes ICP Production and Hemocyte Phagocytosis in Oyster

BCR/TCR-based adaptive immune systems arise in the jawed vertebrates, and B cell receptors (BCRs) play an important role in the clonal selection of B cells and their differentiation into antibody-secreting plasma cells. The existence of BCR-like molecule and the activation mechanism of the downstream response are still not clear in invertebrates. In this study, an ancient BCR-like molecule (designated as CgIgR) with an immunoreceptor tyrosine-based activation motif (ITAM) in its cytoplasmic tail was identified from the Pacific oyster Crassostrea gigas to investigate its involvement in immune response. CgIgR could bind different bacteria through five extracellular Ig domains and formed dimers. The activated CgIgR recruited CgSyk to promote CgERK phosphorylation. The CgIgR-mediated signaling promoted the production of immunoglobulin domain-containing proteins (CgICP-2 and CgLRRIG-1) through inducing CgH3K4me2. The produced CgICPs eventually facilitated hemocytes to phagocytize and eliminate V. splendidus. This study proposed that there was an ancient BCR-like molecule and BCR-like signaling in molluscs.

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An ancient BCR-like molecule (defined as CgIgR) was identified from C. gigas

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We propose IgR-mediated signaling induces CgERK activity in oyster

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IgR-mediated signaling induced CgH3K4me2 to promote the production of CgICPs

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CgICPs facilitated the hemocytes to phagocytize and eliminate V. splendidus

Molecular Design, Optimization, and Genomic Integration of Chimeric B Cell Receptors in Murine B Cells

Immune cell therapies based on the integration of synthetic antigen receptors comprise a powerful strategy for the treatment of diverse diseases, most notably T cells engineered to express chimeric antigen receptors (CAR) for targeted cancer therapy. In addition to T lymphocytes, B lymphocytes may also represent valuable immune cells that can be engineered for therapeutic purposes such as protein replacement therapy or recombinant antibody production. In this article, we report a promising concept for the molecular design, optimization, and genomic integration of a novel class of synthetic antigen receptors, chimeric B cell receptors (CBCR). We initially optimized CBCR expression and detection by modifying the extracellular surface tag, the transmembrane regions and intracellular signaling domains. For this purpose, we stably integrated a series of CBCR variants using CRISPR-Cas9 into immortalized B cell hybridomas. Subsequently, we developed a reliable and consistent pipeline to precisely introduce cassettes of several kb size into the genome of primary murine B cells also using CRISPR-Cas9 induced HDR. Finally, we were able to show the robust surface expression and antigen recognition of a synthetic CBCR in primary B cells. We anticipate CBCRs and our approach for engineering primary B cells will be a valuable tool for the advancement of future B cell- based immune cell therapies.

Crosstalk between ROR1 and the Pre-B cell receptor promotes survival of t(1;19) acute lymphoblastic leukemia

We report that t(1;19) ALL cells universally exhibit expression of and dependence on the cell surface receptor ROR1. We further identify t(1;19) ALL cell sensitivity to the kinase inhibitor dasatinib due to its inhibition of the pre-B cell receptor (pre-BCR) signaling complex. These phenotypes are a consequence of developmental arrest at an intermediate/late stage of B-lineage maturation. Additionally, inhibition of pre-BCR signaling induces further ROR1 upregulation, and we identify distinct ROR1 and pre-BCR downstream signaling pathways that are modulated in a counterbalancing manner-both leading to AKT phosphorylation. Consistent with this, AKT phosphorylation is transiently eliminated after dasatinib treatment, but is partially restored following dasatinib potentiation of ROR1 expression. Consequently, ROR1 silencing accentuates dasatinib killing of t(1;19) ALL cells.

BCR-mediated uptake of antigen linked to TLR9 ligand stimulates B-cell proliferation and antigen-specific plasma cell formation

The activation of Toll-like receptor 9 (TLR9) expressed within B cells is associated with enhanced humoral immunity. However the role of TLR9 in the stimulation of B-cell responses, and more specifically in shaping the outcome of B-cell differentiation, remains unclear. Here, we observed that immunization with the TLR9 agonist CpG linked to protein antigen gave rise to enhanced production of antigen-specific class-switched antibodies in vivo. Unlike dendritic cells, B cells are unable to acquire these conjugates by macropinocytosis and instead depend on uptake through a signaling-competent B-cell receptor (BCR), provided the overall BCR-antigen avidity exceeds a defined threshold. The resultant stimulation of intrinsic TLR9 leads to enhanced antigen-specific B-cell proliferation and differentiation to form extrafollicular plasma cells. Thus, the direct conjugation of antigen and CpG reveals a mechanism that may operate during the initiation of primary immune responses, and may prove useful as a strategy for the design of adjuvants suitable for vaccinations.

Autosomal recessive agammaglobulinemia: novel insights from mutations in Ig-beta

Agammaglobulinemia is a rare primary immuno-deficiency characterized by an early block of B-cell development in the bone marrow resulting in the absence of peripheral B cells and low/absent immunoglobulin serum levels. Mutations in the Bruton tyrosine kinase and in components of the pre-B-cell receptor (pre-BCR), such as mu heavy chain, surrogate light chain, and Igalpha have been found in 85% to 90% of patients affected by this disease. Here we review the recent advances in the characterization of molecular defects underlying an early block in B-cell development, focusing on the novel finding of the first two patients with agammaglobulinemia caused by mutations in Igbeta, the transmembrane protein that associates with Igalpha as part of the pre-BCR complex. Characterization of novel genetic defects involving components of the pre-BCR is crucial for a better understanding of the biology of early B-cell development and may have therapeutic and prognostic implications.

EBV LMP2A provides a surrogate pre-B cell receptor signal through constitutive activation of the ERK/MAPK pathway

Latent membrane protein 2A (LMP2A) of Epstein-Barr virus (EBV) provides developmental and survival signals that mimic those of a B-cell receptor (BCR). Expression of LMP2A during B-cell development results in the ability of B cells to exit the bone marrow in the absence of a BCR and persist in the periphery, where they would normally undergo apoptosis. This study extends the current knowledge of LMP2A function by examining the growth properties of bone marrow B cells from TgE LMP2A mice. Despite the lack of pre-BCR expression, bone marrow B cells from TgE LMP2A mice proliferate and survive in low concentrations of interleukin 7, similar to wild-type cells. Constitutive phosphorylation of ERK/MAPK and PI3K/Akt in TgE LMP2A bone marrow B cells is also reminiscent of signalling through the pre-BCR, altogether demonstrating that LMP2A provides a pre-BCR-like signal to developing B cells.

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.

The Ig-alpha ITAM is required for efficient differentiation but not proliferation of pre-B cells

Signals from the pre-B cell receptor (pre-BCR) mediated by the cytoplasmic tails of Ig-alpha/Ig-beta are essential for developing B cells. To analyze the role of Ig-alpha ITAM and non-ITAM tyrosines in pre-BCR signaling, we reconstituted individual tyrosine mutants of Ig-alpha in src homology 2 domain-containing leukocyte protein of 65 kDa (SLP-65)/Ig-alpha double-deficient pre-B cells. We show that the Ig-alpha mutants led to comparable pre-BCR expression on the cell surface, while the pre-BCR-induced tyrosine phosphorylation was different. We further show that the reconstitution of Ig-alpha and the resulting pre-BCR expression led to enrichment of the pre-BCR-expressing cells in vitro irrespective of the introduced Ig-alpha mutation. We show that, even though the enrichment rate increased by lowering the IL-7 concentration, residual amounts of IL-7 were required for optimal enrichment. Our results indicate that surface IL-7 receptor expression is modulated by the pre-BCR, thereby increasing the IL-7 sensitivity of the respective cells. In contrast to the comparable pre-B cell proliferation, however, the Ig-alpha mutants differed in their capacity to induce calcium flux and activate efficient pre-B cell differentiation. Together, our data suggest that ITAM tyrosines and Y204 are required for efficient pre-B cell differentiation but not proliferation.

Analysis of the Individual Contributions of Igα (CD79a)- and Igβ (CD79b)-Mediated Tonic Signaling for Bone Marrow B Cell Development and Peripheral B Cell Maturation

The individual contribution of Igalpha and Igbeta for BCR-triggered fates is unclear. Prior evidence supports conflicting ideas concerning unique as well as redundant functions for these proteins in the context of BCR/pre-BCR signaling. Part of this ambiguity may reflect the recent appreciation that Igalpha and Igbeta participate in both Ag-independent (tonic) and Ag-dependent signaling. The present study undertook defining the individual requirement for Igalpha and Igbeta under conditions where only ligand-independent tonic signaling was operative. In this regard, we have constructed chimeric proteins containing one or two copies of the cytoplasmic domains of either Igalpha or Igbeta and Igalpha/Igbeta heterodimers with targeted Tyr-->Phe modifications. The ability of these proteins to act as surrogate receptors and trigger early bone marrow and peripheral B cell maturation was tested in RAG2(-/-) primary pro-B cell lines and in gene transfer experiments in the muMT mouse model. We considered that the threshold for a functional activity mediated by the pre-BCR/BCR might only be reached when two functional copies of the Igalpha/Igbeta ITAM domain are expressed together, and therefore the specificity conferred by these proteins can only be observed in these conditions. We found that the ligand-independent tonic signal is sufficient to drive development into mature follicular B cells and both Igalpha and Igbeta chains supported formation of this population. In contrast, neither marginal zone nor B1 mature B cell subsets develop from bone marrow precursors under conditions where only tonic signals are generated.

Ig beta tyrosine residues contribute to the control of B cell receptor signaling by regulating receptor internalization

Immunoglobulin (Ig)alpha and Igbeta initiate B cell receptor (BCR) signaling through immune receptor tyrosine activation motifs (ITAMs) that are targets of SH2 domain-containing kinases. To examine the function of Igbeta ITAM tyrosine resides in mature B cells in vivo, we exchanged these residues for alanine by gene targeting (Igbeta(AA)). Mutant mice showed normal development of all B cell subtypes with the exception of B1 cells that were reduced by fivefold. However, primary B cells purified from Igbeta(AA) mice showed significantly decreased steady-state and ligand-mediated BCR internalization and higher levels of cell surface IgM and IgD. BCR cross-linking resulted in decreased Src and Syk activation but paradoxically enhanced and prolonged BCR signaling, as measured by cellular tyrosine phosphorylation, Ca(++) flux, AKT, and ERK activation. In addition, B cells with the ITAM mutant receptor showed an enhanced response to a T-independent antigen. Thus, Igbeta ITAM tyrosines help set BCR signaling threshold by regulating receptor internalization.

ITAM-mediated tonic signalling through pre-BCR and BCR complexes

Studies carried out over the past few years provide strong support for the idea that Ig alpha-Ig beta-containing complexes such as the pre-B-cell receptor and the B-cell receptor can signal independently of ligand engagement, and this has been termed tonic signalling. In this Review, I discuss recent literature that is relevant to the potential mechanisms by which tonic signals are initiated and regulated, and discuss views on how tonic and ligand-dependent (aggregation-mediated) signalling differ. These mechanisms are relevant to the possibility that tonic signals generated through immunoreceptor tyrosine-based activation motif (ITAM)-containing proteins that are expressed by oncogenic viruses induce transformation in non-haematopoietic cells.

B cell ligand discrimination through a spreading and contraction response

B cells recognize foreign antigens by virtue of cell surface immunoglobulin receptors and are most effectively activated by membrane-bound ligands. Here, we show that in the early stages of this process, B cells exhibit a two-phase response in which they first spread over the antigen-bearing membrane and then contract, thereby collecting bound antigen into a central aggregate. The extent of this response, which is both signaling- and actin-dependent, determines the quantity of antigen accumulated and hence the degree of B cell activation. Brownian dynamic simulations reproduce essential features of the antigen collection process and suggest a possible basis for affinity discrimination. We propose that dynamic spreading is an important step of the immune response.

Basal immunoglobulin signaling actively maintains developmental stage in immature B cells

In developing B lymphocytes, a successful V(D)J heavy chain (HC) immunoglobulin (Ig) rearrangement establishes HC allelic exclusion and signals pro-B cells to advance in development to the pre-B stage. A subsequent functional light chain (LC) rearrangement then results in the surface expression of IgM at the immature B cell stage. Here we show that interruption of basal IgM signaling in immature B cells, either by the inducible deletion of surface Ig via Cre-mediated excision or by incubating cells with the tyrosine kinase inhibitor herbimycin A or the phosphatidylinositol 3-kinase inhibitor wortmannin, led to a striking “back-differentiation” of cells to an earlier stage in B cell development, characterized by the expression of pro-B cell genes. Cells undergoing this reversal in development also showed evidence of new LC gene rearrangements, suggesting an important role for basal Ig signaling in the maintenance of LC allelic exclusion. These studies identify a previously unappreciated level of plasticity in the B cell developmental program, and have important implications for our understanding of central tolerance mechanisms.

Gene rearrangement is a hallmark of B cell maturation. By interrupting basal cell signaling through the rearranged IgM receptor, immature B cells "back-differentiate" to an earlier stage in their development

Mechanisms of selection mediated by interleukin-7, the preBCR, and hemokinin-1 during B-cell development

Many of the stromal-derived signals and factors that regulate B lymphopoiesis have been identified. We review recent evidence from our laboratory that shows that there are at least three phases during B-cell development when cells direct their own maturation, independent of stromal cells. Following the expression of the preB-cell receptor (preBCR), cells acquire the ability to proliferate in low levels of interleukin-7 (IL-7), which acts as a self-selecting mechanism to expand cells that have successfully expressed a preBCR in environments that are non-permissive to preBCR- cells. Second, the preBCR is required for a contact-mediated event between B-cell progenitors. Disruption at this stage prevents the further maturation of progenitors to the lipopolysaccharide (LPS)-responsive stage. Finally, the transition from IL-7 receptor to mature antigen receptor-based signaling is enhanced by a novel member of the tachykinin family, hemokinin-1. This series of maturation, survival, and differentiation signals is generated by B-lineage cells as they progress through developmental checkpoints on the way to becoming functionally mature cells.

Positive and negative selection during B lymphocyte development

Our laboratory is interested in a variety of issues related to lymphocyte development. More specifically, we have focused on the processes that regulate the decision to commit to the B lymphocyte (B cell) lineage, then the subsequent signals that are involved in maintaining this commitment to the B cell lineage. These signals result in the positive selection of those B cells that properly execute the complex genetic changes associated with B cell development, then trigger the elimination of B cells that are responsive to self-antigens and, therefore, possess the potential to mediate autoimmune disease. Our general experimental approach has been to address these issues from the perspective of signal transduction. Our goal is to define the biochemical and genetic processes that are integrated in order to accomplish these selection processes. To do so, we employ in vivo animal models as well as more defined in vitro studies, using both primary and transformed cell lines. For the past several years, we have been primarily interested in the precise mechanisms by which the B cell antigen receptor (BCR), and intermediate forms of this receptor, regulate these complex developmental processes. We have used the ongoing studies described below as two representative examples of how we are approaching these issues and some of the insights that we have made. To place both of these studies in context, we will begin with a brief introduction into B cell development.

A B cell receptor with two Igalpha cytoplasmic domains supports development of mature but anergic B cells

B cell receptor (BCR) signaling is mediated through immunoglobulin (Ig)alpha and Igbeta a membrane-bound heterodimer. Igalpha and Igbeta are redundant in their ability to support early B cell development, but their roles in mature B cells have not been defined. To examine the function of Igalpha-Igbeta in mature B cells in vivo we exchanged the cytoplasmic domain of Igalpha for the cytoplasmic domain of Igbeta by gene targeting (Igbetac-->alphac mice). Igbetac-->alphac B cells had lower levels of surface IgM and higher levels of BCR internalization than wild-type B cells. The mutant B cells were able to complete all stages of development and were long lived, but failed to differentiate into B1a cells. In addition, Igbetac-->alphac B cells showed decreased proliferative and Ca2+ responses to BCR stimulation in vitro, and were anergic to T-independent and -dependent antigens in vivo.

B cell progenitors are arrested in maturation but have intact VDJ recombination in the absence of Ig-alpha and Ig-beta

Ig-alpha and Ig-beta mediate surface expression and signaling of diverse B cell receptor complexes on precursor, immature, and mature B cells. Their expression begins before that of the Ig chains in early progenitor B cells. In this study, we describe the generation of Ig-alpha-deficient mice and their comparative analysis to mice deficient for Ig-beta, the membrane-IgM, and recombination-activating gene 2 to determine the requirement of Ig-alpha and Ig-beta in survival and differentiation of pro-B cells. We find that in the absence of Ig-alpha, B cell development does not progress beyond the progenitor stage, similar to what is observed in humans lacking this molecule. However, neither in Ig-alpha- nor in Ig-beta-deficient mice are pro-B cells impaired in V(D)J recombination, in the expression of intracellular Ig micro-chains, or in surviving in the bone marrow microenvironment. Finally, Ig-alpha and Ig-beta are not redundant in their putative function, as pro-B cells from Ig-alpha and Ig-beta double-deficient mice are similar to those from single-deficient animals in every aspect analyzed.

Multitasking of Ig-alpha and Ig-beta to regulate B cell antigen receptor function

Since their discovery as signaling subunits of the B cell antigen receptor (BCR), Ig-alpha and Ig-beta are discussed to serve either a redundant or distinct function for B cell development, maintenance, and activation. Dependent upon the experimental system that has been used to address this issue, evidence could be provided to support both possibilities. Only recently has it become clear that Ig-alpha and Ig-beta possess a unique signaling identity but that both together are required to orchestrate proper B cell function in vivo. Here we discuss some of the underlying mechanisms that may involve direct coupling to discrete subsets of BCR effector proteins, such as protein tyrosine kinases or the intracellular adaptor SLP-65/BLNK.

E protein function in lymphocyte development

Lymphocytes arise from hematopoietic stem cells through the coordinated action of transcription factors. The E proteins (E12, E47, HEB and E2-2) have emerged as key regulators of both B and T lymphocyte differentiation. This review summarizes the current data and examines the various functions of E proteins and their antagonists, Id2 and Id3, throughout lymphoid maturation. Beyond an established role in B and T lineage commitment, E proteins continue to be essential at subsequent stages of development. E protein activity regulates the expression of surrogate and antigen receptor genes, promotes Ig and TCR rearrangements, and coordinates cell survival and proliferation with developmental progression in response to TCR signaling. Finally, this review also discusses the role of E47 as a tumor suppressor.

Ligand-independent signaling functions for the B lymphocyte antigen receptor and their role in positive selection during B lymphopoiesis

Signal transduction through the B cell antigen receptor (BCR) is determined by a balance of positive and negative regulators. This balance is shifted by aggregation that results from binding to extracellular ligand. Aggregation of the BCR is necessary for eliciting negative selection or activation by BCR-expressing B cells. However, ligand-independent signaling through intermediate and mature forms of the BCR has been postulated to regulate B cell development and peripheral homeostasis. To address the importance of ligand-independent BCR signaling functions and their regulation during B cell development, we have designed a model that allows us to isolate the basal signaling functions of immunoglobulin (Ig)alpha/Igbeta-containing BCR complexes from those that are dependent upon ligand-mediated aggregation. In vivo, we find that basal signaling is sufficient to facilitate pro-B --> pre-B cell transition and to generate immature/mature peripheral B cells. The ability to generate basal signals and to drive developmental progression were both dependent on plasma membrane association of Igalpha/Igbeta complexes and intact immunoregulatory tyrosine activation motifs (ITAM), thereby establishing a correlation between these processes. We believe that these studies are the first to directly demonstrate biologically relevant basal signaling through the BCR where the ability to interact with both conventional as well as nonconventional extracellular ligands is eliminated.

Interference with immunoglobulin (Ig)alpha immunoreceptor tyrosine-based activation motif (ITAM) phosphorylation modulates or blocks B cell development, depending on the availability of an Igbeta cytoplasmic tail

To determine the function of immunoglobulin (Ig)alpha immunoreceptor tyrosine-based activation motif (ITAM) phosphorylation, we generated mice in which Igalpha ITAM tyrosines were replaced by phenylalanines (Igalpha(FF/FF)). Igalpha(FF/FF) mice had a specific reduction of B1 and marginal zone B cells, whereas B2 cell development appeared to be normal, except that lambda1 light chain usage was increased. The mutants responded less efficiently to T cell-dependent antigens, whereas T cell-independent responses were unaffected. Upon B cell receptor ligation, the cells exhibited heightened calcium flux, weaker Lyn and Syk tyrosine phosphorylation, and phosphorylation of Igalpha non-ITAM tyrosines. Strikingly, when the Igalpha ITAM mutation was combined with a truncation of Igbeta, B cell development was completely blocked at the pro-B cell stage, indicating a crucial role of ITAM phosphorylation in B cell development.

Association of SLP-65/BLNK with the B cell antigen receptor through a non-ITAM tyrosine of Ig-alpha

The cytoplasmic adaptor protein SLP-65 (BLNK or BASH) is a critical downstream effector of the B cell antigen receptor (BCR). Tyrosine-phosphorylated SLP-65 assembles intracellular signaling complexes such as the Ca(2 +) initiation complex encompassing phospholipase C-gamma2 and Bruton's tyrosine kinase. It is, however, unclear how the SLP-65 signaling module can be recruited to the plasma membrane. Here we show that following B cell stimulation, SLP-65 associates directly with the BCR signaling subunit, the Ig-alpha / Ig-beta heterodimer. The interaction is mediated by the Src homology 2 domain of SLP-65 and the phosphorylated Ig-alpha tyrosine 204, which is located outside of the immunoreceptor tyrosine-based activation motif. Our data identify an unexpected BCR phosphorylation pattern and indicate that Ig-alpha has the capability to serve as transmembrane adaptor in BCR signaling.

Tyrosine kinase activation in the decision between growth, differentiation, and death responses initiated from the B cell antigen receptor

Immunoglobulin-containing receptors expressed on B lineage lymphocytes play critical roles in the development and function of the humoral arm of the immune system. The preB cell antigen receptor (preBCR) contains the immunoglobulin mu heavy chain (Ig mu) and signals to the preB cell that heavy chain rearrangement has been successful, a process termed heavy chain selection. The B cell antigen receptor (BCR) contains both Ig heavy and light chains and is expressed on immature and mature B cells before and after antigen encounter. Both receptor types from a complex with the Ig alpha and Ig beta proteins that link the predominantly extracellular Ig with intracellular signal transduction pathways. Signaling through the BCR induces different cellular responses depending on the nature of the signaling agent and the development stage of the target cell. These responses include clonal anergy and apoptotic deletion in immature B cells and survival, proliferation, and differentiation in mature B and preB cells. Several protein tyrosine kinases are activated rapidly following engagement of the BCR/preBCR complexes, including members of the Src family (Lyn and Blk), the Syk/ZAP70 family (Syk), and the Tec family (Btk). In this review, we discuss possible mechanisms by which engagement of these similar receptor complexes can give rise to different cellular responses and the role that these kinases play in this process.

B cell development is arrested at the immature B cell stage in mice carrying a mutation in the cytoplasmic domain of immunoglobulin beta

The B cell receptor (BCR) regulates B cell development and function through immunoglobulin (Ig)alpha and Ig beta, a pair of membrane-bound Ig superfamily proteins, each of which contains a single cytoplasmic immunoreceptor tyrosine activation motif (ITAM). To determine the function of Ig beta, we produced mice that carry a deletion of the cytoplasmic domain of Ig beta (Ig beta Delta C mice) and compared them to mice that carry a similar mutation in Ig alpha (MB1 Delta C, herein referred to as Ig alpha Delta C mice). Ig beta Delta C mice differ from Ig alpha Delta C mice in that they show little impairment in early B cell development and they produce immature B cells that respond normally to BCR cross-linking as determined by Ca(2+) flux. However, Ig beta Delta C B cells are arrested at the immature stage of B cell development in the bone marrow and die by apoptosis. We conclude that the cytoplasmic domain Ig beta is required for B cell development beyond the immature B cell stage and that Ig alpha and Ig beta have distinct biologic activities in vivo.

The reduced expression of surrogate light chains in B cell precursors from senescent BALB/c mice is associated with decreased E2A proteins

Senescent mice exhibit decreased numbers of pre-B cells in the bone marrow. Herein, we show that the molecules, lambda5 and VpreB, which comprise the surrogate light chain component of the pre-B cell receptor, are reduced in pro-B/early pre-B cells derived in vitro from the bone marrow of 18-27 months old BALB/c mice after stimulation with IL-7. Both lambda5 and VpreB expression were decreased at the mRNA level as indicated by semi-quantitative RT-PCR; this suggests that the reduced surrogate light chains seen in senescent B cell precursors result from dysfunctional transcriptional regulation. The transcription of surrogate light chains is regulated, in part, by E2A (E47) gene products. Levels of E2A proteins, including E47, were decreased in senescent B cell precursors by up to 90%. Reduced E2A (E47) expression and subsequent reduced transcription of the surrogate light chain components lambda5 and VpreB may, in part, explain the diminished production of B lineage cells observed in senescence.

Signal transduction elements of the B cell antigen receptor and their role in immunodeficiencies

The primary function of B lymphocytes is to contribute to the elimination of foreign antigens by producing large amounts of soluble antibodies. The activation of B cells through their antigen receptor triggers a dynamic network of intracellular signaling proteins. The recent identification of the cytoplasmic adaptor protein SLP-65 (also called BLNK or BASH) provided insight in how the antigen receptor-regulated protein tyrosine kinases couple to downstream signaling cascades, including the mobilization of Ca2+ ions, activation of mitogen-activated kinases and reorganization of the cytoskeleton architecture. While these events have been mostly studied in mature B cells, it is now clear that the components of the antigen receptor and its downstream effector elements play also a central role during early and late B cell development, and in the apoptotic elimination of B cells with reactivity to self-antigens. Thus, genetic defects affecting the expression of antigen receptor subunits or its intracellular signaling proteins can interfere with B cell development and activation, and can cause severe antibody deficiencies in mouse and man. In this article I summarize our current picture of the B cell antigen receptor, how the extracellular signal is transported into the cell interior, and how dysregulation of these processes contribute to immune defects.

Functional dissection of BCR signaling pathways

Signal transduction by the BCR is critical for progression through developmental checkpoints as well as for immune responses. Recent results obtained in mice deficient either in an adaptor molecule, BLNK (alternatively named SLP-65 or BASH), or in phosphatidylinositol 3-kinase have revealed similar - though not identical - phenotypes to those of Btk(-/-) mice, suggesting a functional link between BLNK, Btk and phosphatidylinositol 3-kinase.

Aberrant B cell receptor signaling from B29 (Igbeta, CD79b) gene mutations of chronic lymphocytic leukemia B cells

Chronic lymphocytic leukemia (CLL) B cells characteristically exhibit low or undetectable surface B cell receptor (BCR) and diminished responses to BCR-mediated signaling. These features suggest that CLL cells may have sustained mutations affecting one or more of the BCR proteins required for receptor surface assembly and signal transduction. Loss of expression and mutations in the critical BCR protein B29 (Igbeta, CD79b), are prevalent in CLL and could produce the hallmark features of these leukemic B cells. Because patient CLL cells are intractable to manipulation, we developed a model system to analyze B29 mutations. Jurkat T cells stably expressing micro, kappa, and mb1 efficiently assembled a functional BCR when infected with recombinant vaccinia virus bearing wild-type B29. In contrast, a B29 CLL mutant protein truncated in the transmembrane domain did not associate with mu or mb1 at the cell surface. Another B29 CLL mutant lacking the C-terminal immunoreceptor tyrosine activation motif tyrosine and distal residues brought the receptor to the surface as well as wild-type B29 but showed significant impairment in anti-IgM-stimulated signaling events including mitogen-activated protein kinase activation. These findings demonstrate that B29 mutations previously identified in CLL patients can affect BCR-dependent signaling and may contribute to the unresponsive B cell phenotype in CLL. Finally, the features of the B29 mutations in CLL predict that they may be generated by somatic hypermutation.

Expression of Ig-β (CD79b) by chronic lymphocytic leukemia B cells that lack immunoglobulin heavy-chain allelic exclusion

Because immunoglobulin (Ig)-β (CD79b) is required for immunoglobulin allelic exclusion, we examined the CD79b expressed by four chronic lymphocytic leukemia (CLL) samples that expressed more than one immunoglobulin heavy-chain allele and five samples that had normal immunoglobulin heavy-chain allelic exclusion. All leukemia cell samples stained poorly with monoclonal antibodies specific for extracellular epitopes of CD79b. However, all samples expressed functional CD79b genes, regardless of whether they did or did not express more than one immunoglobulin heavy-chain allele. We identified variant CD79b genes that had conservative base substitutions restricted to regions encoding the extracellular immunoglobulin-like domain of CD79b. However, these variants were not restricted to samples lacking immunoglobulin heavy-chain allelic exclusion and most likely reflect genetic polymorphism. Collectively, these data indicate that the unusual expression of more than one immunoglobulin heavy allele by CLL B cells is not associated with structural, nonconservative mutations in the signal-transduction domains of CD79b.

Expression of Ig-β (CD79b) by chronic lymphocytic leukemia B cells that lack immunoglobulin heavy-chain allelic exclusion

Because immunoglobulin (Ig)-β (CD79b) is required for immunoglobulin allelic exclusion, we examined the CD79b expressed by four chronic lymphocytic leukemia (CLL) samples that expressed more than one immunoglobulin heavy-chain allele and five samples that had normal immunoglobulin heavy-chain allelic exclusion. All leukemia cell samples stained poorly with monoclonal antibodies specific for extracellular epitopes of CD79b. However, all samples expressed functional CD79b genes, regardless of whether they did or did not express more than one immunoglobulin heavy-chain allele. We identified variant CD79b genes that had conservative base substitutions restricted to regions encoding the extracellular immunoglobulin-like domain of CD79b. However, these variants were not restricted to samples lacking immunoglobulin heavy-chain allelic exclusion and most likely reflect genetic polymorphism. Collectively, these data indicate that the unusual expression of more than one immunoglobulin heavy allele by CLL B cells is not associated with structural, nonconservative mutations in the signal-transduction domains of CD79b.

Role of Syk in B-cell development and antigen-receptor signaling

Antigen receptors (BCRs) on developing B lymphocytes play two opposing roles-promoting survival of cells that may later bind a foreign antigen and inhibiting survival of cells that bind too strongly to self-antigens. It is not known how these opposing outcomes are signaled by BCRs on immature B cells. Here we analyze the effect of a null mutation in the Syk tyrosine kinase on maturing B cells displaying a transgene-encoded BCR that binds hen egg lysozyme (HEL). In the absence of HEL antigen, HEL-specific BCRs are expressed normally on the surface of Syk-deficient immature B-lineage cells, but this fails to promote maturation beyond the earliest stages of B-lineage commitment. Binding of HEL antigen, nevertheless, triggers phosphorylation of CD79alpha/beta BCR subunits and modulation of receptors from the surface in Syk-deficient cells, but it cannot induce an intracellular calcium response. Continuous binding of low- or high-avidity forms of HEL, expressed as self-antigens, fails to restore the signal needed for maturation. Compared with the effects in the same system of null mutations in other BCR signaling elements, such as CD45 and Lyn kinase, these results indicate that Syk is essential for transmitting a signal that initiates the program of B-lymphocyte maturation.

A negative regulatory role for Ig-alpha during B cell development

The development of B cells requires the expression of an antigen receptor at distinct points during maturation. The Ig-alpha/beta heterodimer signals for these receptors, and mice harboring a truncation of the Ig-alpha intracellular domain (mb-1(delta(c)/delta(c)) have severely reduced peripheral B cell numbers. Here we report that immature mb-1(delta(c)/delta(c) B cells are activated despite lacking a critical Ig-alpha-positive signaling motif. As a consequence of abnormal activation, transitional immature IgMhighIgDlow B cells are largely absent in mb-1delta(c)/delta(c) mutants, accounting for the paucity of mature B cells. Thus, Ig-alpha cytoplasmic tail truncation yields an antigen receptor complex on immature B cells that signals constitutively. These data illustrate a role for Ig-alpha in negatively regulating antigen receptor signaling during B cell development.

Ig-alpha cytoplasmic truncation renders immature B cells more sensitive to antigen contact

To study the function of Ig-alpha in the selection of autoreactive B cells, we have analyzed mb-1 cytoplasmic truncation mutant mice (mb-1delta(c)/delta(c)), which coexpress transgenes encoding hen egg lysozyme (HEL) and HEL-specific immunoglobulin. We demonstrate that in the presence of soluble HEL (sHEL) and dependent on the mb-1delta(c) mutation, most immature B cells bearing the HEL-specific Ig transgene undergo rearrangements of endogenous kappa light chains, resulting in loss of HEL specificity. Moreover, immature B cells from Ig-alpha mutant mice respond to BCR cross-linking with an exaggerated and prolonged calcium response and induction of protein tyrosine phosphorylation. Our data imply a negative signaling role for Ig-alpha in immature B cells.

B-cell-receptor-dependent positive and negative selection in immature B cells

This review touches on only a small part of the complex biology of B cells, but serves to illustrate the point that the antigen receptor is the most important of many cell-surface receptors affecting cell-fate decisions. Receptor expression is necessary, but not sufficient, for cell survival. It is also essential that a B cell's antigen-receptor specificity be appropriate for its environment. The need to balance reactivity with self tolerance has resulted in an intricate feedback control (affected by both the recombinase and cell survival) that regulates independent selection events at the level of the receptor and the cell.

Mutations in Igalpha (CD79a) result in a complete block in B-cell development

Mutations in Btk, mu heavy chain, or the surrogate light chain account for 85-90% of patients with early onset hypogammaglobulinemia and absent B cells. The nature of the defect in the remaining patients is unknown. We screened 25 such patients for mutations in genes encoding components of the pre-B-cell receptor (pre-BCR) complex. A 2-year-old girl was found to have a homozygous splice defect in Igalpha, a transmembrane protein that forms part of the Igalpha/Igbeta signal-transduction module of the pre-BCR. Studies in mice suggest that the Igbeta component of the pre-BCR influences V-DJ rearrangement before cell-surface expression of mu heavy chain. To determine whether Igalpha plays a similar role, we compared B-cell development in an Igalpha-deficient patient with that seen in a mu heavy chain-deficient patient. By immunofluorescence, both patients had a complete block in B-cell development at the pro-B to pre-B transition; both patients also had an equivalent number and diversity of rearranged V-DJ sequences. These results indicate that mutations in Igalpha can be a cause of agammaglobulinemia. Furthermore, they suggest that Igalpha does not play a critical role in B-cell development until it is expressed, along with mu heavy chain, as part of the pre-BCR.

Expression levels of B cell surface immunoglobulin regulate efficiency of allelic exclusion and size of autoreactive B-1 cell compartment

Surface-expressed immunoglobulin (Ig) has been shown to have a critical role in allelic exclusion of Ig heavy (H) and light (L) chains. Although various degrees of suppression of endogenous Ig expression are observed in Ig transgenic (Tg) mice, it was not clear whether this difference is due to different onsets of Tg expression or to different levels of Tg expression, which are obviously affected by integration sites of the transgene. In this study we generated antierythrocyte antibody Tg mice that carry tandem joined H and L chain transgenes (H+L) and confirmed that homozygosity of the transgene loci enhances the level of transgene expression as compared with heterozygosity. Suppression of endogenous H and L chain gene expression was stronger in homozygous than in heterozygous Tg mice. Similar results were obtained in control Tg mice carrying the H chain only. These results suggest that there is a threshold of the B cell receptor expression level that induces allelic exclusion. In addition, despite the same B cell receptor specificity, the size of Tg autoreactive B-1 cell compartment in the peritoneal cavity is larger in homozygous than in heterozygous mice, although the number of the Tg B-2 cell subset decreased in the spleen and bone marrow of homozygous Tg mice as compared with heterozygous Tg mice. By contrast, homozygosity of the H chain alone Tg line, which does not recognize self-antigens, did not increase the size of the peritoneal B-1 subset. These results suggest that the size of the B-1 cell subset in the Tg mice may depend on strength of signals through B cell receptors triggered by self-antigens.

Genetic analysis of B cell antigen receptor signaling

In B lymphocytes, a signaling complex that contributes to cell fate decisions is the B cell antigen receptor (BCR). Data from knockout experiments in cell lines and mice have revealed distinct functions for the intracellular protein tyrosine kinases (Lyn, Syk, Btk) in BCR signaling and B cell development. Combinations of intracellular signaling pathways downstream of these PTKs determine the quality and quantity of BCR signaling. For example, concerted actions of the PLC-gamma 2 and PI3-K pathways are required for proper calcium responses. Similarly, the regulation of ERK and JNK responses involves both PLC-gamma 2 and GTPases pathways. Since the immune response in vivo is regulated by alteration of these signaling outcomes, achieving a precise understanding of intracellular molecular events leading to B lymphocyte proliferation, deletion, anergy, receptor editing, and survival still remains a challenge for the future.

B cell development: signal transduction by antigen receptors and their surrogates

Constitutive signal transduction by B cell antigen-receptors and/or their surrogates appears to be critical for progression through multiple developmental checkpoints and for survival of mature B cells in the periphery. Antigen-induced signaling via the B cell receptor can compensate for defects in constitutive signaling and initiates receptor editing, apoptosis and anergy in normal mice - purging the repertoire of autoreactive cells. Thus development and survival of mature B cells seem to require continuous receptor signaling of a defined amplitude.

Targeted gene disruption reveals a role for natural secretory IgM in the maturation of the primary immune response

Accelerated development of the secondary immune response may be attributable in part to the rapid delivery of antigen to lymphoid follicles by circulating antibody elicited on primary immunization. Here we provide evidence indicating that the nonspecific IgM present in naive mice (natural antibody) plays a role in the acceleration of the primary response. Targeted deletion of the Ig microseconds polyadenylation site by use of Cre recombinase allowed the creation of mice that, although harboring a normal number of B cells expressing surface IgM, completely lacked serum IgM while retaining the other Ig isotypes. These mice retained a broadly normal B lymphocyte distribution (although containing a somewhat expanded peritoneal B1a subset) but exhibited substantial delays in mounting affinity-matured IgG responses to T cell-dependent antigens. The T cell-independent response, however, was augmented. The data indicate that the IgM present before antigen challenge (as well, possibly, as that elicited immediately after immunization) accelerates maturation of the primary response, presumably by complexing with the antigen and facilitating lymphocyte activation and/or antigen trapping.

Early function of Pax5 (BSAP) before the pre-B cell receptor stage of B lymphopoiesis

The formation of the pre-B cell receptor (BCR) corresponds to an important checkpoint in B cell development that selects pro-B (pre-BI) cells expressing a functionally rearranged immunoglobulin mu (Igmu) heavy chain protein to undergo the transition to the pre-B (pre-BII) cell stage. The pre-BCR contains, in addition to Igmu, the surrogate light chains lambda5 and VpreB and the signal transducing proteins Igalpha and Igbeta. The absence of one of these pre-BCR components is known to arrest B cell development at the pre-BI cell stage. Disruption of the Pax5 gene, which codes for the B cell-specific activator protein (BSAP), also blocks adult B lymphopoiesis at the pre-BI cell stage. Moreover, expression of the mb-1 (Igalpha) gene and VH-to-DHJH recombination at the IgH locus are reduced in Pax5-deficient B lymphocytes approximately 10- and approximately 50-fold, respectively. Here we demonstrate that complementation of these deficiencies in pre-BCR components by expression of functionally rearranged Ig mu and chimeric Igmu-Igbeta transgenes fails to advance B cell development to the pre-BII cell stage in Pax5 (-/-) mice in contrast to RAG2 (-/-) mice. Furthermore, the pre-BCR is stably expressed on cultured pre-BI cells from Igmu transgenic, Pax5-deficient bone marrow, but is unable to elicit its normal signaling responses. In addition, the early developmental block is unlikely to be caused by the absence of a survival signal, as it could not be rescued by expression of a bcl2 transgene in Pax5-deficient pre-BI cells. Together, these data demonstrate that the absence of Pax5 arrests adult B lymphopoiesis at an early developmental stage that is unresponsive to pre-BCR signaling.

Affinity dependence of the B cell response to antigen: a threshold, a ceiling, and the importance of off-rate

Initiation and affinity maturation of the humoral immune response is driven by antigen interaction with BCR. To study how signaling and antigen presentation through BCR depend on antigen/BCR affinity, lysozyme-specific B cell transfectants were challenged with mutated lysozymes differing in their binding kinetics. For detectable triggering, the antigen/BCR complex needed a Ka > 10(6) M(-1) (dissociation half-life > approximately 1 s). Mutated lysozymes whose binding was below this threshold could nevertheless be presented if complexed with soluble antibody. Above the threshold, the concentration of antigen required to trigger a response decreased as the affinity (particularly dissociation half-life) increased. However, a plateau was reached at Kas > approximately 10(10) M(-1) (dissociation half-life > 0.5 hr), supporting the idea of a ceiling to affinity maturation.

Precursor B cells showing H chain allelic inclusion display allelic exclusion at the level of pre-B cell receptor surface expression

Within the pools of muH chain-producing precursor and mature B cells from normal and lambda5-defective mice, the frequency of cells in which both H chain alleles were productively VHDJH rearranged was determined. An equally high percentage (2%-4%) of cells carrying two productively VHDJH-rearranged H chain loci was found in precursor and mature B cell pools of both mouse strains. In all of these cells, one allele encodes a muH chain incapable of forming a surface-expressed pre-B cell receptor. Hence, allelic exclusion is maintained at the level of pre-B cell receptor expression. The surprising conservation of H chain allelic exclusion in lambda5-defective B cells suggests that an alternative form of pre-B cell receptor might function to ensure this allelic exclusion.