Requirement for B cell linker protein (BLNK) in B cell development

Regulation of signaling in B cells through the phosphorylation of Syk on linker region tyrosines. A mechanism for negative signaling by the Lyn tyrosine kinase

The B cell antigen receptor (BCR) is coupled to the mobilization of Ca(2+) by the protein-tyrosine kinase, Syk. Syk, recruited to the clustered BCR, becomes phosphorylated on three tyrosines (Tyr-317, Tyr-342, and Tyr-346) located within the linker region that separates the C-terminal catalytic domain from the N-terminal tandem Src homology 2 domains. Phosphorylation within the linker region can be either activating or inhibitory to Ca(2+) mobilization depending on the sites that are modified. Syk that is not phosphorylated on linker region tyrosines couples the BCR to Ca(2+) mobilization through a phosphoinositide 3-kinase-dependent pathway. The phosphorylation of Tyr-342 and -346 enhances the phosphorylation and activation of phospholipase C-gamma and the early phase of Ca(2+) mobilization via a phosphoinositide 3-kinase-independent pathway. The phosphorylation of Tyr-317 strongly dampens the Ca(2+) signal. In cells that lack the Src family kinase, Lyn, the phosphorylation of the inhibitory Tyr-317 is suppressed leading to elevated production of inositol 1,4,5-trisphosphate and an amplified Ca(2+) signal. This provides a novel mechanism by which Lyn functions as an inhibitor of BCR-stimulated signaling. Thus, Syk and Lyn combine to determine the pathway through which the BCR is coupled to Ca(2+) mobilization as well as the magnitude and duration of the Ca(2+) flux.

Src homology region 2 domain-containing phosphatase 1 positively regulates B cell receptor-induced apoptosis by modulating association of B cell linker protein with Nck and activation of c-Jun NH2-terminal kinase

Src homology region 2 domain-containing phosphatase 1 (SHP-1) is a key mediator in lymphocyte differentiation, proliferation, and activation. We previously showed that B cell linker protein (BLNK) is a physiological substrate of SHP-1 and that B cell receptor (BCR)-induced activation of c-Jun NH(2)-terminal kinase (JNK) is significantly enhanced in cells expressing a form of SHP-1 lacking phosphatase activity (SHP-1-C/S). In this study, we confirmed that SHP-1 also exerts negative regulatory effects on JNK activation in splenic B cells. To further clarify the role of SHP-1 in B cells, we examined how dephosphorylation of BLNK by SHP-1 affects downstream signaling events. When a BLNK mutant (BLNK Delta N) lacking the NH(2)-terminal region, which contains four tyrosine residues, was introduced in SHP-1-C/S-expressing WEHI-231 cells, the enhanced JNK activation was inhibited. Among candidate proteins likely to regulate JNK activation through BLNK, Nck adaptor protein was found to associate with tyrosine-phosphorylated BLNK and this association was more pronounced in SHP-1-C/S-expressing cells. Furthermore, expression of dominant-negative forms of Nck inhibited BCR-induced JNK activation. Finally, BCR-induced apoptosis was suppressed in SHP-1-C/S-expressing cells and coexpression of Nck SH2 mutants or a dominant-negative form of SEK1 reversed this phenotype. Collectively, these results suggest that SHP-1 acts on BLNK, modulating its association with Nck, which in turn negatively regulates JNK activation but exerts a positive effect on apoptosis.

Delayed cellular maturation and decreased immunoglobulin kappa light chain production in immature B lymphocytes lacking B cell linker protein

B cell linker (BLNK) protein is a component of the B cell receptor (BCR) signaling pathway and BLNK(-/-) mice have a block in B lymphopoiesis at the pro-B/pre-B cell stage. To study the effect of BLNK mutation at later stages of B cell development, we introduce an innocuous transgenic BCR into BLNK(-/-) mice and show that two populations of immature B cells distinguishable by their IgM(low (lo)) and IgM(high (hi)) phenotypes are found in the bone marrow of these mice in contrast to a single population of IgM(hi) cells found in control BCR-transgenic BLNK(+/+) mice. The mutant IgM(lo) and IgM(hi) cells are at an earlier developmental stage compared with the control IgM(hi) cells as indicated by their differential expression of CD43, B220, and major histocompatibility complex class II antigens and their timing of generation in culture. Thus, in the absence of BLNK the differentiation of immature B cells is delayed. Furthermore, mutant IgM(lo) cells produce equivalent level of immunoglobulin (Ig) mu but less Ig kappa proteins than control and mutant IgM(hi) cells and this defect is attributed to a decrease in the amount of kappa transcripts being generated. Finally, splenic B cells in BCR-transgenic BLNK(-/-) mice are predominantly of the transitional B cell phenotype and are rapidly lost from the peripheral B cell pool. Taken together, the data suggest a role for BLNK and perhaps BCR signaling, in the regulation of kappa light chain expression and continued immature B cell differentiation.

Regulation of phospholipase C-gamma2 and phosphoinositide 3-kinase pathways by adaptor proteins in B lymphocytes

The importance of phosphoinositide 3-kinase (PI3K) and phospholipase C (PLC)-gamma2 in B cell function and development has been highlighted by gene targeting experiments in mice. In fact, these knockout mice exhibit a profound inhibition of proliferative responses upon B cell receptor (BCR) engagement. The molecular connections between these effectors and upstream tyrosine kinases such as Syk have been studied intensively in the past few years. This mechanism involves the action of cytoplasmic adaptor molecules, which participate in forming multicomponent signaling complexes, thereby directing the appropriate subcellular localization of effector enzymes. In addition to these cytoplasmic adaptor proteins, cell surface coreceptors can be viewed as transmembrane adaptor proteins, because coreceptors can also change the localization of effector enzymes, which in turn modulates the BCR-initiated signals.

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.

Origins and functions of B-1 cells with notes on the role of CD5

Whether B-1a (CD5+) cells are a distinct lineage derived from committed fetal/neonatal precursors or arise from follicular B-2 cells in response to BCR ligation and other, unknown signals remains controversial. Recent evidence indicates that B-1a cells can derive from adult precursors expressing an appropriate specificity when the (self-) antigen is present. Antibody specificity determines whether a B cell expressing immunoglobulin transgenes has a B-2, B-1a or marginal zone (MZ) phenotype. MZ cells share many phenotypic characteristics of B-1 cells and, like them, appear to develop in response to T independent type 2 antigens. Because fetal-derived B cell progenitors fail to express terminal deoxynucleotidyl transferase (TdT) and for other reasons, they are likely to express a repertoire that allows selection into the B-1a population. As it is selected by self-antigen, the B-1 repertoire tends to be autoreactive. This potentially dangerous repertoire is also useful, as B-1 cells are essential for resistance to several pathogens and they play an important role in mucosal immunity. The CD5 molecule can function as a negative regulator of BCR signaling that may help prevent inappropriate activation of autoreactive B-1a cells.

Regulation of B cell fates by BCR signaling components

Recent results obtained in mice harboring cytoplasmic mutations of Igalpha and/or Igbeta have reinforced the concept that the strength of BCR signaling is important for ensuring appropriate developmental outcomes as well as antigen-specific responses. To establish the optimal signaling intensity and duration, the BCR utilizes positive and negative regulatory molecules. Studies are beginning to reveal how these molecules maintain immunological homeostasis and tolerance.

AP2 adaptor complex-dependent internalization of CD5: differential regulation in T and B cells

CD5 is a key regulator of Ag receptor-mediated activation, selection, and differentiation in both T and B cells. Accumulating evidence indicates that lymphocyte activation and selection are sensitive to variations in levels of CD5 on the cell surface. We now show that CD5 expression on the surface of B and T cells is regulated posttranslationally by direct interaction with the mu(2) subunit of the AP2 adaptor complex that links transmembrane proteins to clathrin-coated pits. CD5 is rapidly internalized from the cell surface in lymphoid cell lines, mature splenic T and B cells, and peritoneal CD5(+) B cells following monovalent or bivalent ligation of the receptor. We mapped the mu(2) subunit binding site on CD5 to Y(429) and determined that the integrity of this site was necessary for CD5 internalization. Cross-linking of the Ag receptor with intact Abs inhibited CD5 internalization in B cells, but had the opposite effect in T cells. However, if F(ab')(2) Abs were used to stimulate the Ag receptor in B cells, the effect on CD5 internalization was now similar to that observed in T cells, indicating that signals through the Ag receptor and FcR regulate CD5 endocytosis in B cells. This was confirmed using an FcgammaRIIB1-deficient B cell line. The ability to differentially alter posttranslational CD5 expression in T and B cells is likely to be key in regulation of Ag receptor signaling and generation of tolerance in T and B lymphocytes.

Mechanisms of signaling by the hematopoietic-specific adaptor proteins, SLP-76 and LAT and their B cell counterpart, BLNK/SLP-65

Adaptor proteins lack catalytic activity and contain only protein-protein interaction domains. They have been shown to interact with an ever-growing number of signaling proteins and to play essential roles in many signaling pathways. SLP-76 and LAT are cell-type-specific adaptor proteins expressed in T cells, NK cells, platelets, and mast cells. In these cell types, SLP-76 and LAT are required for signaling by immunoreceptor tyrosine-based activation motif(ITAM)-containing receptors, including the T cell receptor (TCR), the pre-TCR, the high-affinity Fc epsilon receptor, and the platelet GPVI collagen receptor. In B cells, an analogous adaptor, BLNK/SLP-65, is required for signaling by the ITAM-containing B cell receptor. This review summarizes recent research on SLP-76, LAT, and BLNK. A major challenge in understanding adaptor protein function has been to sort out the many interactions mediated by adaptor proteins and to define the mechanisms by which adaptors mediate critical signaling events. In the case of LAT, SLP-76, and BLNK, the availability of tractable genetic systems, deficient in expression of each of these adaptor proteins, has facilitated in-depth investigation of their signaling functions and mechanisms of action. The picture that has emerged is one in which multiple adaptor proteins cooperate to bring about the formation of a large signaling complex, localized to specialized lipid microdomains within the cell membrane and known as GEMs. Adaptors not only recruit signaling proteins, but also play an active role in regulating the conformation and activation of many of the proteins recruited to the complex. In particular, recent research has shed light on the mechanisms by which multiple adaptor proteins cooperate to bring about the recruitment and activation of phospholipase C gamma in response to the activation of ITAM-containing receptors.

Interaction of Btk and Akt in B cell signaling

Reactive oxygen species (ROS) or reactive oxygen intermediates (ROIs) mediate complex signaling involving multiple pathways. In this report, we demonstrate for the first time that endogenous Bruton's tyrosine kinase (Btk) and Akt can interact with each other in DT40 chicken B cells and human Nalm6 B cells and that this interaction is inducible following H2O2 stimulation. This interaction is supported by visualizing the co-localization of Btk and Akt in the perinuclear region and membrane ruffles in COS-7 cells. We have also shown the involvement of phosphatidylinositol 3-kinase (PI 3-K) and Btk in the phosphorylation of Akt following stimulation by hydrogen peroxide (H2O2). Interestingly, Akt phosphorylation was found in the presence of Btk even in the absence of oxidative stress. In addition, we have investigated the involvement of PI 3-K in the MAPKs and ERK and JNK phosphorylation, in the presence or absence of Btk. Phosphorylation of both ERK and JNK increased when the PI 3-K pathway was inhibited and both pathways were modulated positively by Btk. Taken together, based on the study of endogenous conditions, we show the novel interaction of Btk and Akt in H2O2 signaling in B cells.

Regulation of B-cell signal transduction by adaptor proteins

An important role has emerged for adaptor molecules in linking cell-surface receptors, such as the B-cell antigen receptor, with effector enzymes. Adaptor proteins direct the appropriate subcellular localization of effectors and regulate their activity by inducing conformational changes, both of which, in turn, contribute to the spatio-temporal precision of B-cell signal-transduction events. In addition, adaptor molecules participate in establishing negative- or positive-feedback regulatory loops in signalling networks, thereby fine-tuning the B-cell response.

Primary immunodeficiency disorders: antibody deficiency

As a group, antibody deficiencies represent the most common types of primary immune deficiencies in human subjects. Often symptoms do not appear until the latter part of the first year of life, as passively acquired IgG from the mother decreases to below protective levels. As with the T-cell immune deficiencies, the spectrum of antibody deficiencies is broad, ranging from the most severe type of antibody deficiency with totally absent B cells and serum Igs to patients who have a selective antibody deficiency with normal serum Ig. In addition to the increased susceptibility to infections, a number of other disease processes (eg, autoimmunity and malignancies) can be involved in the clinical presentation. Fortunately, the availability of intravenous immune serum globulin has made the management of these patients more complete. Recently, molecular immunology has led to identification of the gene or genes involved in many of these antibody deficiencies. As discussed in this review, this has led to a better elucidation of the B-cell development and differentiation pathways and a more complete understanding of the pathogenesis of many of these antibody deficiencies.

Transcriptional control of B-cell development

Significant progress has recently been made in our understanding of how transcription factors such as PU.1, Notch1, E2A, EBF, Pax5, Bcl6, Blimp1 and XBP1 control different developmental decisions during the onset and terminal phase of B-lymphopoiesis. One emerging theme is that negative regulatory networks play an important role in suppressing alternative gene programs and their corresponding cell fates throughout B-cell development.

The direct recruitment of BLNK to immunoglobulin alpha couples the B-cell antigen receptor to distal signaling pathways

Following B-cell antigen receptor (BCR) ligation, the cytoplasmic domains of immunoglobulin alpha (Ig alpha) and Ig beta recruit Syk to initiate signaling cascades. The coupling of Syk to several distal substrates requires linker protein BLNK. However, the mechanism by which BLNK is recruited to the BCR is unknown. Using chimeric receptors with wild-type and mutant Ig alpha cytoplasmic tails we show that the non-immunoreceptor tyrosine-based activation motif (ITAM) tyrosines, Y176 and Y204, are required to activate BLNK-dependent pathways. Subsequent analysis demonstrated that BLNK bound directly to phospho-Y204 and that fusing BLNK to mutated Ig alpha reconstituted downstream signaling events. Moreover, ligation of the endogenous BCR induced Y204 phosphorylation and BLNK recruitment. These data demonstrate that the non-ITAM tyrosines of Ig alpha couple Syk activation to BLNK-dependent pathways.

NFkappaB-dependent signaling pathways

The transcription factor NFkappaB is activated by numerous stimuli. Once NFkappaB is fully activated, it participates in the regulation of various target genes in different cells to exert its biological functions. NFkappaB has often been referred to as a central mediator of the immune response, since a large variety of bacteria and viruses can lead to the activation of NFkappaB, which in turn controls the expression of many inflammatory cytokines, chemokines, immune receptors, and cell surface adhesion molecules. Recent studies have shown that NFkappaB may function more generally as a central regulator of stress responses, since different stressful conditions, including physical stress, oxidative stress, and exposure to certain chemicals, also lead to NFkappaB activation. Furthermore, NFkappaB blocks cell apoptosis in several cell types. Taken together, these findings make it clear that NFkappaB plays an important role in cell proliferation and differentiation. It is the intention of this review to cover the various NFkappaB-dependent signaling pathways, thereby to achieve a better understanding of the mechanisms of NFkappaB activation and the physiological functions of activated NFkappaB.

Essential immunoregulatory role for BCAP in B cell development and function

BCAP was recently cloned as a binding molecule to phosphoinositide 3-kinase (PI3K). To investigate the role of BCAP, mutant mice deficient in BCAP were generated. While BCAP-deficient mice are viable, they have decreased numbers of mature B cells and B1 B cell deficiency. The mice produce lower titers of serum immunoglobulin (Ig)M and IgG3, and mount attenuated responses to T cell--independent type II antigen. Upon B cell receptor cross-linking, BCAP-deficient B cells exhibit reduced Ca(2+) mobilization and poor proliferative responses. These findings demonstrate that BCAP plays a pivotal immunoregulatory role in B cell development and humoral immune responses.

Receptor-facilitated antigen presentation requires the recruitment of B cell linker protein to Igalpha

Ags that cross-link the B cell Ag receptor are preferentially and rapidly delivered to the MHC class II-enriched compartment for processing into peptides and subsequent loading onto MHC class II. Proper sorting of Ag/receptor complexes requires the recruitment of Syk to the phosphorylated immunoreceptor tyrosine-based activation motif tyrosines of the B cell Ag receptor constituent Igalpha. We postulated that the Igalpha nonimmunoreceptor tyrosine-based activation motif tyrosines, Y(176) and Y(204), contributed to receptor trafficking. Igalpha(YDeltaF(176,204))/Igbeta receptors were targeted to late endosomes, but were excluded from the vesicle lumen and could not facilitate the presentation of Ag to T cells. Subsequent analysis demonstrated that phosphorylation of Y(176)/Y(204) recruited the B cell linker protein, Vav, and Grb2. Reconstitution of Igalpha(YDeltaF(176,204))/Igbeta with the B cell linker protein rescued both receptor-facilitated Ag presentation and entry into the MHC class II-enriched compartment. Thus, aggregation accelerates receptor trafficking by recruiting two separate signaling modules required for transit through sequential checkpoints.

The transcription factor, Bright, and immunoglobulin heavy chain expression

Bright, or B cell regulator of immunoglobulin heavy chain transcription, is a B lymphocyte-specific protein first discovered for its ability to increase immunoglobulin transcription three- to sevenfold in antigen-activated B cells. It interacts with DNA through an ARID, or A/T-rich interaction domain, and is the only member of a previously undescribed family of DNA-binding proteins for which target genes have been identified. The mechanism(s) by which Bright facilitates transcription are unknown. Several proteins that associate with Bright may shed light upon its function. These include the nuclear matrix proteins sp100 and LYSp100B, and suggest that Bright may affect chromatin configuration and nuclear sublocalization. Furthermore, Bruton's tyrosine kinase is required for Bright binding activity, suggesting links between Bright, cell signaling cascades, and X-linked immunodeficiency disease.

Cellular function of phosphoinositide 3-kinases: implications for development, homeostasis, and cancer

The phosphoinositide 3-kinase (PI3K) family of enzymes is recruited upon growth factor receptor activation and produces 3' phosphoinositide lipids. The lipid products of PI3K act as second messengers by binding to and activating diverse cellular target proteins. These events constitute the start of a complex signaling cascade, which ultimately results in the mediation of cellular activities such as proliferation, differentiation, chemotaxis, survival, trafficking, and glucose homeostasis. Therefore, PI3Ks play a central role in many cellular functions. The factors that determine which cellular function is mediated are complex and may be partly attributed to the diversity that exists at each level of the PI3K signaling cascade, such as the type of stimulus, the isoform of PI3K, or the nature of the second messenger lipids. Numerous studies have helped to elucidate some of the key factors that determine cell fate in the context of PI3K signaling. For example, the past two years has seen the publication of many transgenic and knockout mouse studies where either PI3K or its signaling components are deregulated. These models have helped to build a picture of the role of PI3K in physiology and indeed there have been a number of surprises. This review uses such models as a framework to build a profile of PI3K function within both the cell and the organism and focuses, in particular, on the role of PI3K in cell regulation, immunity, and development. The evidence for the role of deregulated PI3K signaling in diseases such as cancer and diabetes is reviewed.

Fc gamma RIIB1/SHIP-mediated inhibitory signaling in B cells involves lipid rafts

One type of membrane microdomain, enriched in glycosphingolipids and cholesterol and referred to as lipid rafts, has been implicated in the generation of activating signals triggered by a variety of stimuli. Several laboratories, including ours, have recently demonstrated that the B cell receptor (BCR) inducibly localizes to the rafts upon activation and that functional lipid rafts are important for BCR-mediated "positive" signaling. In the later phases of the immune response, coligation of the BCR and the inhibitory receptor Fc gamma RIIB1 leads to potent inhibition of BCR-induced positive signaling through the recruitment of the inositol phosphatase SHIP to Fc gamma RIIB1. One potential model is that the Fc gamma RIIB1 itself might be excluded from the rafts basally and that destabilization of raft-dependent BCR signaling might be part of the mechanism for the Fc gamma RIIB1-mediated negative regulation. We tested this hypothesis and observed that preventing BCR raft localization is not the mechanism for this inhibition. Surprisingly, a fraction of Fc gamma RIIB1 is constitutively localized in the rafts and increases further after BCR + FcR coligation. SHIP is actively recruited to lipid rafts under negative stimulation conditions, and the majority of Fc gamma RIIB1-SHIP complexes localize to lipid rafts compared with non-raft regions of the plasma membrane. This suggested that this negative feedback loop is also initiated in the lipid rafts. Despite its basal localization to the rafts, Fc gamma RIIB1 did not become phosphorylated after BCR alone cross-linking and did not colocalize with the BCR that moves to rafts upon BCR engagement alone (positive signaling conditions), perhaps suggesting the existence of different subsets of rafts. Taken together, these data suggest that lipid rafts play a role in both the positive signaling via the BCR as well as the inhibitory signaling through Fc gamma RIIB1/SHIP.

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.

Hematopoietic progenitor kinase 1 associates physically and functionally with the adaptor proteins B cell linker protein and SLP-76 in lymphocytes

B cell linker protein (BLNK) is a SLP-76-related adaptor protein essential for signal transduction from the BCR. To identify components of BLNK-associated signaling pathways, we performed a phosphorylation-dependent yeast two-hybrid analysis using BLNK probes. Here we report that the serine/threonine kinase hematopoietic progenitor kinase 1 (HPK1), which is activated upon antigen-receptor stimulation and which has been implicated in the regulation of MAP kinase pathways, interacts physically and functionally with BLNK in B cells and with SLP-76 in T cells. This interaction requires Tyr(379) of HPK1 and the Src homology 2 (SH2) domain of BLNK/SLP-76. Via homology modeling, we defined a consensus binding site within ligands for SLP family SH2 domains. We further demonstrate that the SH2 domain of SLP-76 participates in the regulation of AP-1 and NFAT activation in response to T cell receptor (TCR) stimulation and that HPK1 inhibits AP-1 activation in a manner partially dependent on its interaction with SLP-76. Our data are consistent with a model in which full activation of HPK1 requires its own phosphorylation on tyrosine and subsequent interaction with adaptors of the SLP family, providing a mechanistic basis for the integration of this kinase into antigen receptor signaling cascades.

HSH2: a novel SH2 domain-containing adapter protein involved in tyrosine kinase signaling in hematopoietic cells

We isolated a cDNA clone encoding a novel Src homology (SH)2 domain-containing protein of 47 kDa from a human cDNA library. As its transcript was predominantly expressed in hematopoietic cells, this gene was termed HSH2 for hematopoietic SH2 protein. This protein contains several putative protein-binding motifs, SH3-binding proline-rich regions, and phosphotyrosine sites, but lacks enzymatic motifs. In a yeast two-hybrid screen, we identified a cytokine-regulated tyrosine kinase c-FES and an activated Cdc42-associated tyrosine kinase ACK1 as HSH2 interactors. HSH2 bound c-FES via its C-terminal region as well as its N-terminal region including the SH2 domain, whereas it bound ACK1 via its N-terminal proline-rich region. Furthermore, these two kinases bound and tyrosine-phosphorylated HSH2 in mammalian cells. Hence, we postulate that HSH2 functions as an adapter protein involved in tyrosine kinase signaling, and possibly regulates cytokine signaling and cytoskeletal reorganization, in hematopoietic cells.

Functional cloning of Src-like adapter protein-2 (SLAP-2), a novel inhibitor of antigen receptor signaling

In an effort to identify novel therapeutic targets for autoimmunity and transplant rejection, we developed and performed a large-scale retroviral-based functional screen to select for proteins that inhibit antigen receptor-mediated activation of lymphocytes. In addition to known regulators of antigen receptor signaling, we identified a novel adaptor protein, SLAP-2 which shares 36% sequence similarity with the known Src-like adaptor protein, SLAP. Similar to SLAP, SLAP-2 is predominantly expressed in hematopoietic cells. Overexpression of SLAP-2 in B and T cell lines specifically impaired antigen receptor-mediated signaling events, including CD69 surface marker upregulation, nuclear factor of activated T cells (NFAT) promoter activation and calcium influx. Signaling induced by phorbol myristate acetate (PMA) and ionomycin was not significantly reduced, suggesting SLAP-2 functions proximally in the antigen receptor signaling cascade. The SLAP-2 protein contains an NH2-terminal myristoylation consensus sequence and SH3 and SH2 Src homology domains, but lacks a tyrosine kinase domain. In antigen receptor-stimulated cells, SLAP-2 associated with several tyrosine phosphorylated proteins, including the ubiquitin ligase Cbl. Deletion of the COOH terminus of SLAP-2 blocked function and abrogated its association with Cbl. Mutation of the putative myristoylation site of SLAP-2 compromised its inhibitory activity and impaired its localization to the membrane compartment. Our identification of the negative regulator SLAP-2 demonstrates that a retroviral-based screening strategy may be an efficient way to identify and characterize the function of key components of many signal transduction systems.

Positive and negative regulation of T-cell activation by adaptor proteins

Adaptor proteins, molecules that mediate intermolecular interactions, are now known to be as crucial for lymphocyte activation as are receptors and effectors. Extensive work from numerous laboratories has identified and characterized many of these adaptors, demonstrating their roles as both positive and negative regulators. Studies into the molecular basis for the actions of these molecules shows that they function in various ways, including: recruitment of positive or negative regulators into signalling networks, modulation of effector function by allosteric regulation of enzymatic activity, and by targeting other proteins for degradation. This review will focus on a number of adaptors that are important for lymphocyte function and emphasize the various ways in which these proteins carry out their essential roles.

Four tyrosine residues in phospholipase C-gamma 2, identified as Btk-dependent phosphorylation sites, are required for B cell antigen receptor-coupled calcium signaling

Activation of phospholipase C-gamma2 (PLCgamma2) is the critical step in B cell antigen receptor (BCR)-coupled calcium signaling. Although genetic dissection experiments on B cells have demonstrated that Bruton's tyrosine kinase (Btk) and Syk are required for activating PLCgamma2, the exact activation mechanism of PLCgamma2 by these kinases has not been established. We identify the tyrosine residues 753, 759, 1197, and 1217 in rat PLCgamma2 as Btk-dependent phosphorylation sites by using an in vitro kinase assay. To evaluate the role of these tyrosine residues in phosphorylation-dependent activation of PLCgamma2, PLCgamma2-deficient DT40 cells were reconstituted with a series of mutant PLCgamma2s in which the phenylalanine was substituted for tyrosine. Substitution of all four tyrosine residues almost completely eliminated the BCR-induced PLCgamma2 phosphorylation, indicating that these residues include the major phosphorylation sites upon BCR engagement. Cells expressing PLCgamma2 with a single substitution exhibited some extent of reduction in calcium mobilization, whereas those expressing quadruple mutant PLCgamma2 showed greatly reduced calcium response. These findings indicate that the phosphorylations of the tyrosine residues 753, 759, 1197, and 1217, which have been identified as Btk-dependent phosphorylation sites in vitro, coordinately contribute to BCR-induced activation of PLCgamma2.

MIST functions through distinct domains in immunoreceptor signaling in the presence and absence of LAT

MIST (also termed Clnk) is an adaptor protein structurally related to SLP-76 and BLNK/BASH/SLP-65 hematopoietic cell-specific adaptor proteins. By using the BLNK-deficient DT40 chicken B cell system, we demonstrated MIST functions through distinct intramolecular domains in immunoreceptor signaling depending on the availability of linker for activation of T cells (LAT). MIST can partially restore the B cell antigen receptor (BCR) signaling in the BLNK-deficient cells, which requires phosphorylation of the two N-terminal tyrosine residues. Co-expression of LAT with MIST fully restored the BCR signaling and dispenses with the requirement of the two tyrosines in MIST for BCR signaling. However, some other tyrosine(s), as well as the Src homology (SH) 2 domain and the two proline-rich regions in MIST, is still required for full reconstitution of the BCR signaling, in cooperation with LAT. The C-terminal proline-rich region of MIST is dispensable for the LAT-aided full restoration of MAP kinase activation, although it is responsible for the interaction with LAT and for the localization in glycolipid-enriched microdomains. On the other hand, the N-terminal proline-rich region, which is a binding site of the SH3 domain of phospholipase Cgamma, is essential for BCR signaling. These results revealed a marked plasticity of MIST function as an adaptor in the cell contexts with or without LAT.

Synergistic regulation of immunoreceptor signaling by SLP-76-related adaptor Clnk and serine/threonine protein kinase HPK-1

Recently, the identification of Clnk, a third member of the SLP-76 family of adaptors expressed exclusively in cytokine-stimulated hemopoietic cells, has been reported by us and by others. Like SLP-76 and Blnk, Clnk was shown to act as a positive regulator of immunoreceptor signaling. Interestingly, however, it did not detectably associate with known binding partners of SLP-76, including Vav, Nck, and GADS. In contrast, it became complexed in activated T cells and myeloid cells with an as yet unknown tyrosine-phosphorylated polypeptide of approximately 92 kDa (p92). In order to understand better the function of Clnk, we sought to identify the Clnk-associated p92. Using a yeast two-hybrid screen and cotransfection experiments with Cos-1 cells, evidence was adduced that p92 is HPK-1, a serine/threonine-specific protein kinase expressed in hemopoietic cells. Further studies showed that Clnk and HPK-1 were also associated in hemopoietic cells and that their interaction was augmented by immunoreceptor stimulation. A much weaker association was detected between HPK-1 and SLP-76. Transient transfections in Jurkat T cells revealed that Clnk and HPK-1 cooperated to increase immunoreceptor-mediated activation of the interleukin 2 (IL-2) promoter. Moreover, the ability of Clnk to stimulate IL-2 promoter activity could be blocked by expression of a kinase-defective version of HPK-1. Lastly we found that in spite of the differential ability of Clnk and SLP-76 to bind cellular proteins, Clnk was apt at rescuing immunoreceptor signaling in a Jurkat T-cell variant lacking SLP-76. Taken together, these results show that Clnk physically and functionally interacts with HPK-1 in hemopoietic cells. Moreover, they suggest that Clnk is capable of functionally substituting for SLP-76 in immunoreceptor signaling, albeit by using a distinct set of intracellular effectors.

B cell adaptor containing src homology 2 domain (BASH) links B cell receptor signaling to the activation of hematopoietic progenitor kinase 1

The B cell adaptor containing src homology 2 domain (BASH; also termed BLNK or SLP-65), is crucial for B cell antigen receptor (BCR)-mediated activation, proliferation, and differentiation of B cells. BCR-mediated tyrosine-phosphorylation of BASH creates binding sites for signaling effectors such as phospholipase Cgamma (PLCgamma)2 and Vav, while the function of its COOH-terminal src homology 2 domain is unknown. We have now identified hematopoietic progenitor kinase (HPK)1, a STE20-related serine/threonine kinase, as a protein that inducibly interacts with the BASH SH2 domain. BCR ligation induced rapid tyrosine-phosphorylation of HPK1 mainly by Syk and Lyn, resulting in its association with BASH and catalytic activation. BCR-mediated activation of HPK1 was impaired in Syk- or BASH-deficient B cells. The functional SH2 domain of BASH and Tyr-379 within HPK1 which we identified as a Syk-phosphorylation site were both necessary for interaction of both proteins and efficient HPK1 activation after BCR stimulation. Furthermore, HPK1 augmented, whereas its kinase-dead mutant inhibited IkappaB kinase beta (IKKbeta) activation by BCR engagement. These results reveal a novel BCR signaling pathway leading to the activation of HPK1 and subsequently IKKbeta, in which BASH recruits tyrosine-phosphorylated HPK1 into the BCR signaling complex.

Epstein-Barr virus latent membrane protein 2A (LMP2A) employs the SLP-65 signaling module

In latently infected B lymphocytes, the Epstein-Barr virus (EBV) suppresses signal transduction from the antigen receptor through expression of the integral latent membrane protein 2A (LMP2A). At the same time, LMP2A triggers B cell survival by a yet uncharacterized maintenance signal that is normally provided by the antigen receptor. The molecular mechanisms are unknown as LMP2A-regulated signaling cascades have not been described so far. Using a novel mouse model we have identified the intracellular adaptor protein Src homology 2 (SH2) domain-containing leukocyte protein (SLP)-65 as a critical downstream effector of LMP2A in vivo. Biochemical analysis of the underlying signaling pathways revealed that EBV infection causes constitutive tyrosine phosphorylation of one of the two SLP-65 isoforms and complex formation between SLP-65 and the protooncoprotein CrkL (CT10 regulator of kinase like). This leads to antigen receptor-independent phosphorylation of Cbl (Casitas B lineage lymphoma) and C3G. In contrast, phospholipase C-gamma2 (PLC-gamma2) activation is completely blocked. Our data show that in order to establish a latent EBV infection, LMP2A selectively activates or represses SLP-65-regulated signaling pathways.

Modelling the human immune response: can mice be trusted? Commentary

The mouse is now the animal of choice for laboratory-based medical research. Although its contribution to advancing understanding of our inner workings is indisputable, we should acknowledge that mice and humans are tangibly different. This article highlights, and attempts some rationale for, discrepancies between the two species' immune systems.

The absence of SLP65 and Btk blocks B cell development at the preB cell receptor-positive stage

Mice deficient for the adapter protein SLP65 (BLNK) show a partial block in early B cell development, reduced numbers of mature B cells in the periphery, an absence of B1 cells and a reduction of IgM and IgG3 serum immunoglobulin levels. A strikingly similar phenotype is observed in Btk-deficient mice. To investigate the consequences of mutations in both SLP65 and Btk, we generated SLP65/ Btk double-mutant mice by crossing the single-mutant mice. Analysis of the double-mutant mice reveals a much more severe defect in B cell development. B cells in the SLP65/Btk double-mutant mice are arrested at the preB cell stage and, surprisingly, express the preB cell receptor. Normally, preB cell receptor expression in wild-type mice is restricted to a very small fraction of B cells making it difficult to identify these cells in the bone marrow. Together, the data demonstrate the synergistic role of SLP65 and Btk in B cell development and describe a situation where large numbers of preB cell receptor-positive cells accumulate in the bone marrow and spleen.

SHP-1 Requires Inhibitory Co-receptors to Down-modulate B Cell Antigen Receptor-mediated Phosphorylation of Cellular Substrates

Signaling through the B cell antigen receptor (BCR) is negatively regulated by the SH2 domain-containing protein-tyrosine phosphatase SHP-1, which requires association with tyrosine-phosphorylated proteins for activation. Upon BCR ligation, SHP-1 has been shown to associate with the BCR, the cytoplasmic protein-tyrosine kinases Lyn and Syk, and the inhibitory co-receptors CD22 and CD72. How SHP-1 is activated by BCR ligation and regulates BCR signaling is, however, not fully understood. Here we demonstrate that, in the BCR-expressing myeloma line J558L mu 3, CD72 expression reduces the BCR ligation-induced phosphorylation of the BCR component Ig alpha/Ig beta and its cytoplasmic effectors Syk and SLP-65. Substrate phosphorylation was restored by expression of dominant negative mutants of SHP-1, whereas the SHP-1 mutants failed to enhance phosphorylation of the cellular substrates in the absence of CD72. This indicates that SHP-1 is efficiently activated by CD72 but not by other pathways in J558L mu m3 cells and that inhibition of SHP-1 specifically activated by CD72 reverses CD72-induced dephosphorylation of cellular substrates in these cells. Taken together, BCR-induced SHP-1 activation is likely to require inhibitory co-receptors such as CD72, and SHP-1 appears to mediate the negative regulatory effect of CD72 on BCR signaling by dephosphorylating Ig alpha/Ig beta and its downstream signaling molecules Syk and SLP-65.

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.

Compensation between Vav-1 and Vav-2 in B cell development and antigen receptor signaling

Vav-1 and Vav-2 are closely related Dbl-homology GTP exchange factors (GEFs) for Rho GTPases. Mutation of Vav-1 disrupts T cell development and T cell antigen receptor-induced activation, but has comparatively little effect on B cells. We found that combined deletion of both Vav-1 and Vav-2 in mice resulted in a marked reduction in mature B lymphocyte numbers. Vav-1(-/-)Vav-2(-/-) B cells were unresponsive to B cell antigen receptor (BCR)-driven proliferation in vitro and to thymus-independent antigen in vivo. BCR-stimulated intracellular calcium mobilization was greatly impaired in Vav-1(-/-)Vav-2(-/-) B cells. These findings establish a role for Vav-2 in BCR calcium signaling and reveal that the Vav family of GEFs is critical to B cell development and function.

The adaptor protein BLNK is required for b cell antigen receptor-induced activation of nuclear factor-kappa B and cell cycle entry and survival of B lymphocytes

B lymphocytes lacking the adaptor protein B cell linker (BLNK) do not proliferate in response to B cell antigen receptor (BCR) engagement. We demonstrate here that BCR-activated BLNK(-)/- B cells fail to enter the cell cycle, and this is due to their inability to induce the expression of the cell cycle regulatory proteins such as cyclin D2 and cyclin-dependent kinase 4. BCR-stimulated BLNK(-)/- B cells also do not up-regulate the cell survival protein Bcl-x(L), which may be necessary for the cells to complete the cell cycle. In addition, BLNK(-)/- B cells exhibit a high rate of spontaneous apoptosis in culture. Examination of the various BCR-activated signaling pathways in mouse BLNK(-)/- B cells reveals the intact activation of Akt and mitogen-activated protein kinases but the impaired activation of nuclear factor (NF)-kappaB that is known to regulate genes involved in cell proliferation and survival. The inability to activate NF-kappaB in BCR-stimulated BLNK(-)/- B cells is due to a failure to induce the degradation of the inhibitory kappaB protein. In all these aspects, BLNK(-)/- B cells resemble xid B cells that have a mutation in Bruton's tyrosine kinase (Btk). Recently, phospholipase C (PLC)-gamma2 has also been demonstrated to be essential for NF-kappaB activation. Since BLNK has been shown separately to interact with both Btk and PLC-gamma2, our finding of normal Btk but impaired PLC-gamma2 activation in BCR-stimulated BLNK(-)/- B cells strongly suggests that BLNK orchestrates the formation of a Btk-PLC-gamma2 signaling axis that regulates NF-kappaB activation. Taken together, the NF-kappaB activation defect may be sufficient to explain the similar defects in BCR-induced B cell proliferation and T cell-independent immune responses in BLNK(-)/-, Btk(-)/-, and PLC-gamma2(-)/- mice.

Immune system dysfunction and autoimmune disease in mice lacking Emk (Par-1) protein kinase

Emk is a serine/threonine protein kinase implicated in regulating polarity, cell cycle progression, and microtubule dynamics. To delineate the role of Emk in development and adult tissues, mice lacking Emk were generated by targeted gene disruption. Emk(-/-) mice displayed growth retardation and immune cell dysfunction. Although B- and T-cell development were normal, CD4(+)T cells lacking Emk exhibited a marked upregulation of the memory marker CD44/pgp-1 and produced more gamma interferon and interleukin-4 on stimulation through the T-cell receptor in vitro. In addition, B-cell responses to T-cell-dependent and -independent antigen challenge were altered in vivo. As Emk(-/-) animals aged, they developed splenomegaly, lymphadenopathy, membranoproliferative glomerulonephritis, and lymphocytic infiltrates in the lungs, parotid glands and kidneys. Taken together, these results demonstrate that the Emk protein kinase is essential for maintaining immune system homeostasis and that loss of Emk may contribute to autoimmune disease in mammals.

Selection events operating at various stages in B cell development

B cells have to progress through various checkpoints during their process of development. The three transcription factors E2A, EBF (early B cell factor) and Pax5 play essential roles in B cell commitment checkpoints. The various forms of the BCR and their downstream signaling molecules, which are expressed at different stages of B cell development, act as critical checkpoint guards allowing (positive selection) or preventing (negative selection) developmental progression. The recent advances on the molecular mechanisms operating at these various checkpoints are here summarized and discussed.

Leukocyte-specific adaptor protein Grap2 interacts with hematopoietic progenitor kinase 1 (HPK1) to activate JNK signaling pathway in T lymphocytes

Immune cell-specific adaptor proteins create various combinations of multiprotein complexes and integrate signals from cell surface receptors to the nucleus, modulating the specificity and selectivity of intracellular signal transduction. Grap2 is a newly identified adaptor protein specifically expressed in lymphoid tissues. This protein shares 40--50% sequence homology in the SH3 and the SH2 domain with Grb2 and Grap. However, the Grap2 protein has a unique 120-amino acid glutamine- and proline-rich domain between the SH2 and C-terminal SH3 domains. The expression of Grap2 is highly restricted to lymphoid organs and T lymphocytes. In order to understand the role of this specific adaptor protein in immune cell signaling and activation, we searched for the Grap2 interacting protein in T lymphocytes. We found that Grap2 interacted with the hematopoietic progenitor kinase 1 (HPK1) in vitro and in Jurkat T cells. The interaction was mediated by the carboxyl-terminal SH3 domain of Grap2 with the second proline-rich motif of HPK1. Coexpression of Grap2 with HPK1 not only increased the kinase activity of HPK1 in the cell, but also had an additive effect on HPK1 mediated JNK activation. Furthermore, over expression of Grap2 and HPK1 induced significant transcriptional activation of c-Jun in the JNK signaling pathway and IL-2 gene reporter activity in stimulated Jurkat T cells. Therefore, our data suggest that the hematopoietic specific proteins Grap2 and HPK1 form a signaling complex to mediate the c-Jun NH(2)-terminal kinase (JNK) signaling pathway in T cells.

IkappaB kinase alpha is essential for mature B cell development and function

IkappaB kinase (IKK) alpha and beta phosphorylate IkappaB proteins and activate the transcription factor, nuclear factor (NF)-kappaB. Although both are highly homologous kinases, gene targeting experiments revealed their differential roles in vivo. IKKalpha is involved in skin and limb morphogenesis, whereas IKKbeta is essential for cytokine signaling. To elucidate in vivo roles of IKKalpha in hematopoietic cells, we have generated bone marrow chimeras by transferring control and IKKalpha-deficient fetal liver cells. The mature B cell population was decreased in IKKalpha(-/-) chimeras. IKKalpha(-/-) chimeras also exhibited a decrease of serum immunoglobulin basal level and impaired antigen-specific immune responses. Histologically, they also manifested marked disruption of germinal center formation and splenic microarchitectures that depend on mature B cells. IKKalpha(-/-) B cells not only showed impairment of survival and mitogenic responses in vitro, accompanied by decreased, although inducible, NF-kappaB activity, but also increased turnover rate in vivo. In addition, transgene expression of bcl-2 could only partially rescue impaired B cell development in IKKalpha(-/-) chimeras. Taken together, these results demonstrate that IKKalpha is critically involved in the prevention of cell death and functional development of mature B cells.

Cutting edge commentary: origins of B-1 cells

The origin of B-1a cells, a minority population of B cells that express CD5, are abundant in coelomic cavities, and often produce autoantibodies, has been the subject of study for many years. Accumulating evidence demonstrates that the hypothesis that only B cells arising in fetal or neonatal tissues have the potential to become B-1a cells cannot be true. Rather, B cell receptor-mediated signaling initiated by ligation of autoantigen has now been shown to be required for induction of the B-1a phenotype. Furthermore, cells with a functional B-1a phenotype can be induced from adult precursors by appropriate Ag. At the same time, microenvironment-specific events may determine the likelihood that a given B cell, either adult or fetal derived, enters this pathway. CD5 expression and possibly localization to the peritoneum appear to provide some protection to autoreactive cells otherwise slated for elimination.

The effects of the Epstein-Barr virus latent membrane protein 2A on B cell function

Epstein-Barr Virus (EBV) infects B-lymphocytes circulating through the oral epithelium and establishes a lifelong latent infection in a subset of mature-memory B cells. In these latently infected B cells, EBV exhibits limited gene expression with the latent membrane protein 2A (LMP2A) being the most consistently detected transcript. This persistent expression, coupled with many studies ofthe function of LMP2A in vitro and invivo, indicates that LMP2A is functioning to control some aspect of viral latency. Establishment and maintenance of viral latency requires exquisite manipulation of normal B cell signaling and function. LMP2A is capable of blocking normal B cell signal transduction in vitro, suggesting that LMP2A may act to regulate lytic activation from latency in vivo. Furthermore, LMP2A is capable of providing B cells with survival signals in the absence of normal BCR signaling. These data show that LMP2A may help EBV-infected cells to persist in vivo. This review discusses the advances that have been made in our understanding of LMP2A and the effects it has on B cell development, activation, and viral latency.

Characterization of the CIN85 adaptor protein and identification of components involved in CIN85 complexes

CIN85 is an 85-kDa adaptor protein whose functions in signaling pathways are presently unknown. Using the yeast two-hybrid screen, the B cell linker protein (BLNK) was identified as a binding partner of CIN85. Coimmunoprecipitation experiments using mammalian cells revealed that CIN85 directly bound to BLNK through its SH3 domains. Immunostaining analysis showed that CIN85 and BLNK were colocalized in the cytoplasm. These results indicate a potential role of CIN85 in the B cell receptor-mediated signaling pathway. It was also found that Crk-I, Crk-II, p130(Cas), p85-PI3K, Grb2, and Sos1 were components of CIN85 complexes. CIN85 interacted with itself through its coiled-coil region, resulting in formation of a tetramer. Both the coiled-coil region and SH3 domains of CIN85 were responsible for its subcellular localization. Our data suggest that CIN85 may serve for regulation of various signaling events through formation of its diverse complexes.

Antibody regulation of B cell development

Antibodies on the surface of B lymphocytes trigger adaptive immune responses and control a series of antigen-independent checkpoints during B cell development. These physiologic processes are regulated by a complex of membrane immunoglobulin and two signal transducing proteins known as Ig alpha and Ig beta. Here we focus on the role of antibodies in governing the maturation of B cells from early antigen-independent through the final antigen-dependent stages.

Lymphocytes with a complex: adapter proteins in antigen receptor signaling

Adapters can be defined as proteins that mediate intermolecular interactions within a signal transduction pathway and that lack both intrinsic enzymatic and transcriptional activity. Their essential role in lymphocyte signaling was revealed by recent analyses of mice and cell lines deficient in LAT, SLP-76 and BLNK. These and other adapters nucleate signaling complexes and facilitate coupling of antigen receptor triggering to functional responses in lymphocytes.

Control of B cell production by the adaptor protein lnk. Definition Of a conserved family of signal-modulating proteins

Lnk is an SH2 domain-containing adaptor protein expressed preferentially in lymphocytes. To illuminate the importance of Lnk, we generated lnk(-/-) mice. Whereas T cell development was unaffected, pre-B and immature B cells accumulated in the spleens. In the bone marrow, B-lineage cells were proportionately increased, reflecting enhanced production of pro-B cells that resulted in part from hypersensitivity of precursors to SCF, the ligand for c-kit. Hence, Lnk ordinarily acts to regulate B cell production. Further characterization of lnk(-/-) mice also revealed that full-length Lnk is a 68 kDa protein containing a conserved proline-rich region and a PH domain. Lnk is a representative of a multigene adaptor protein family whose members act, by analogy with Lnk, to modulate intracellular signaling.