B cell antigen receptor cross-linking induces tyrosine phosphorylation and membrane translocation of a multimeric Shc complex that is augmented by CD19 co-ligation

The B-lymphoid Grb2 interaction code

The growth factor receptor-bound protein 2 (Grb2) is a ubiquitously expressed and evolutionary conserved adapter protein possessing a plethora of described interaction partners for the regulation of signal transduction. In B lymphocytes, the Grb2-mediated scaffolding function controls the assembly and subcellular targeting of activating as well as inhibitory signalosomes in response to ligation of the antigen receptor. Also, integration of simultaneous signals from B-cell coreceptors that amplify or attenuate antigen receptor signal output relies on Grb2. Hence, Grb2 is an essential signal integrator. The key question remains, however, of how pathway specificity can be maintained during signal homeostasis critically required for the balance between immune cell activation and tolerance induction. Here, we summarize the molecular network of Grb2 in B cells and introduce a proteomic approach to elucidate the interactome of Grb2 in vivo.

The adaptor protein Shc plays a key role during early B cell development

The adaptor protein Shc is phosphorylated downstream of many cell surface receptors, including Ag and cytokine receptors. However, the role of Shc in B cell development has not been addressed. Here, through conditional expression of a dominant negative Shc mutant and conditional loss of Shc protein expression, we tested a role for Shc during early B lymphopoiesis. We identified a requirement for Shc beginning at the transition from the pre-pro-B to pro-B stage, with a strong reduction in the number of pre-B cells. This developmental defect is due to increased cell death rather than impaired proliferation or commitment to the B lineage. Additional studies suggest a role for Shc in IL-7-dependent signaling in pro-B cells. Shc is phosphorylated in response to IL-7 stimulation in pro-B cells, and pro-B cells from mice with impaired Shc signaling display increased apoptosis. Together, these data demonstrate a critical role for Shc in early B lymphopoiesis with a requirement in early B cell survival. In addition, we also identify Shc as a required player in signaling downstream of the IL-7R in early B cells.

Src-family kinases in B-cell development and signaling

The Src-family protein tyrosine kinases (SFKs) are known to play key roles in initiating signal transduction by the B-cell antigen receptor (BCR). In addition, numerous studies have shown that this family of molecules also contributes to signaling by BCR surrogates during B-lymphocyte lineage development and maturation. Paradoxically, ablation of SFKs not only results in obvious defects in B-cell development but also in the onset of autoimmunity. Thus SFKs, most notably Lyn, play both activating and inhibitory roles in B-cell function. Confounding analyses of SFK function in B cells is the varied coexpression of family members that mediate redundant as well as unique functions. In this review, we will focus mainly on the role of Lyn in mediating positive and negative roles in B-cell activation and how these affect immune signaling and disease progression.

CD19, CD21, and CD22: multifaceted response regulators of B lymphocyte signal transduction

B lymphocyte development and function depend upon the activity of intrinsic and B cell antigen receptor (BCR)-induced signals. These signals are interpreted, amplified, fine-tuned, or suppressed through the precise actions of specialized cell surface coreceptors, or "response regulators," that inform B cells of their extracellular environment. Important cell surface response regulators include the CD19/CD21 complex, CD22, and CD72. CD19 establishes a novel Src-family protein tyrosine kinase (PTK) amplification loop that regulates basal signaling thresholds and intensifies Src-family PTK activation following BCR ligation. In turn, CD22 limits the intensity of CD19-dependent, BCR-generated signals through the recruitment of potent phosphotyrosine and phosphoinositide phosphatases. Herein we discuss our current understanding of how CD19/CD21 and CD22 govern the emergence and intensity of BCR-mediated signals, and how alterations in these tightly controlled regulatory activities contribute to autoimmunity in mice and humans.

Convergence of signaling pathways on the activation of ERK in B cells

The B cell receptor (BCR) initiates three major signaling pathways: the Ras pathway, which leads to extracellular signal-regulated kinase (ERK) activation; the phospholipase C-gamma pathway, which causes calcium mobilization; and the phosphoinositide 3-kinase (PI 3-kinase) pathway. These combine to induce different biological responses depending on the context of the BCR signal. Both the Ras and PI 3-kinase pathways are important for B cell development and activation. Several model systems show evidence of cross-regulation between these pathways. Here we demonstrate through the use of PI 3-kinase inhibitors and a dominant-negative PI 3-kinase construct that the BCR-induced phosphorylation and activation of ERK is dependent on PI 3-kinase. PI 3-kinase feeds into the Ras signaling cascade at multiple points, both upstream and downstream of Ras. We also show that ERK activation is dependent on phospholipase C-gamma, in keeping with its dependence on calcium mobilization. Last, the activation of PI 3-kinase itself is completely dependent on Ras. We conclude that the PI 3-kinase and Ras signaling cascades are intimately connected in B cells and that the activation of ERK is a signal integration point, since it requires simultaneous input from all three major signaling pathways.

A filarial nematode-secreted phosphorylcholine-containing glycoprotein uncouples the B cell antigen receptor from extracellular signal-regulated kinase-mitogen-activated protein kinase by promoting the surface Ig-mediated recruitment of Src homology 2 domain-containing tyrosine phosphatase-1 and Pac-1 mitogen-activated kinase-phosphatase

Unraveling the molecular mechanisms by which filarial nematodes, major human pathogens in the tropics, evade the host immune system remains an elusive goal. We have previously shown that excretory-secretory product-62 (ES-62), a homologue of phosphorylcholine-containing molecules that are secreted by human parasites and which is active in rodent models of filarial infection, is able to polyclonally activate certain protein tyrosine kinase and mitogen-activating protein kinase signal transduction elements in B lymphocytes. Such activation mediates desensitization of subsequent B cell Ag receptor (BCR) ligation-induced activation of extracellular signal-regulated kinase-mitogen-activated protein (ErkMAP) kinase and ultimately B cell proliferation. We now show that the desensitization is due to ES-62 targeting two major regulatory sites of B cell activation. Firstly, pre-exposure to ES-62 primes subsequent BCR-mediated recruitment of SHP-1 tyrosine phosphatase to abolish recruitment of the RasErkMAP kinase cascade via the Igalphabeta-ShcGrb2Sos adaptor complex interactions. Secondly, any ongoing ErkMAP kinase signaling in ES-62-primed B cells is terminated by the MAP kinase phosphatase, Pac-1 that is activated consequently to challenge via the BCR.

The SH2 domain containing tyrosine phosphatase-1 down-regulates activation of Lyn and Lyn-induced tyrosine phosphorylation of the CD19 receptor in B cells

SHP-1 is a cytosolic tyrosine phosphatase implicated in down-regulation of B cell antigen receptor signaling. SHP-1 effects on the antigen receptor reflect its capacity to dephosphorylate this receptor as well as several inhibitory comodulators. In view of our observation that antigen receptor-induced CD19 tyrosine phosphorylation is constitutively increased in B cells from SHP-l-deficient motheaten mice, we investigated the possibility that CD19, a positive modulator of antigen receptor signaling, represents another substrate for SHP-1. However, analysis of CD19 coimmunoprecipitable tyrosine phosphatase activity in CD19 immunoprecipitates from SHP-1-deficient and wild-type B cells revealed that SHP-1 accounts for only a minor portion of CD19-associated tyrosine phosphatase activity. As CD19 tyrosine phosphorylation is modulated by the Lyn protein-tyrosine kinase, Lyn activity was evaluated in wild-type and motheaten B cells. The results revealed both Lyn as well as CD19-associated Lyn kinase activity to be constitutively and inducibly increased in SHP-1-deficient compared with wild-type B cells. The data also demonstrated SHP-1 to be associated with Lyn in stimulated but not in resting B cells and indicated this interaction to be mediated via Lyn binding to the SHP-1 N-terminal SH2 domain. These findings, together with cyanogen bromide cleavage data revealing that SHP-1 dephosphorylates the Lyn autophosphorylation site, identify Lyn deactivation/dephosphorylation as a likely mechanism whereby SHP-1 exerts its influence on CD19 tyrosine phosphorylation and, by extension, its inhibitory effect on B cell antigen receptor signaling.

Regulation of B lymphocyte responses to foreign and self-antigens by the CD19/CD21 complex

The membrane protein complex CD19/CD21 couples the innate immune recognition of microbial antigens by the complement system to the activation of B cells. CD21 binds the C3d fragment of activated C3 that becomes covalently attached to targets of complement activation, and CD19 co-stimulates signaling through the antigen receptor, membrane immunoglobulin. CD21 is also expressed by follicular dendritic cells and mediates the long-term retention of antigen that is required for the maintenance of memory B cells. Understanding of the biology of this receptor complex has been enriched by analyses of genetically modified mice; these analyses have uncovered roles not only in positive responses to foreign antigens, but also in the development of tolerance to self-antigens. Studies of signal transduction have begun to determine the basis for the coreceptor activities of CD19. The integration of innate and adaptive immune recognition at this molecular site on the B cell guides the appropriate selection of antigen by adaptive immunity and emphasizes the importance of this coreceptor complex.

Tyrosine Phosphorylation of Shc in Response to B Cell Antigen Receptor Engagement Depends on the SHIP Inositol Phosphatase

Tyrosine phosphorylation of Shc in response to B cell Ag receptor (BCR) engagement creates binding sites for the Src homology 2 (SH2) domain of Grb2. This facilitates the recruitment of both Grb2 · Sos complexes and Grb2 · SHIP complexes to the plasma membrane where Sos can activate Ras and SH2 domain-containing inositol phosphatase (SHIP) can dephosphorylate phosphatidylinositol 3,4,5-trisphosphate. Given the importance of Shc phosphorylation, we investigated the mechanism by which the BCR stimulates this response. We found that both the SH2 domain and phosphotyrosine-binding (PTB) domain of Shc are important for BCR-induced tyrosine phosphorylation of Shc and the subsequent binding of Grb2 to Shc. The unexpected finding that the PTB domain of Shc is required for Shc phosphorylation was investigated further. Because the major ligand for the Shc PTB domain is SHIP, we asked whether the interaction of Shc with SHIP was required for BCR-induced tyrosine phosphorylation of Shc. Using SHIP-deficient DT40 cells, we show that SHIP is necessary for the BCR to induce significant levels of Shc tyrosine phosphorylation. BCR-induced tyrosine phosphorylation of Shc could be restored in the these cells by expressing wild-type SHIP but not by expressing a mutant form of SHIP that cannot bind to Shc. This suggests that BCR-induced tyrosine phosphorylation of Shc may depend on the binding of SHIP to the Shc PTB domain. Thus, we have described a novel role for SHIP in BCR signaling, promoting the tyrosine phosphorylation of Shc.

CD19 Amplifies B Lymphocyte Signal Transduction by Regulating Src-Family Protein Tyrosine Kinase Activation

Ligation of the B cell Ag receptor (BCR) induces cellular activation by stimulating Src-family protein tyrosine kinases (PTKs) to phosphorylate members of the BCR complex. Subsequently, Src-family PTKs, particularly Lyn, are proposed to phosphorylate and bind CD19, a cell-surface costimulatory molecule that regulates mature B cell activation. Herein, we show that B cells from CD19-deficient mice have diminished Lyn kinase activity and BCR phosphorylation following BCR ligation. Tyrosine phosphorylation of other Src-family PTKs was also decreased in CD19-deficient B cells. In wild-type B cells, CD19 was constitutively complexed with Vav, Lyn, and other Src-family PTKs, with CD19 phosphorylation and its associations with Lyn and Vav increased after BCR ligation. Constitutive CD19/Lyn/Vav complex signaling may therefore be responsible for the establishment of baseline signaling thresholds in B cells before Ag receptor ligation, in addition to accelerating signaling following BCR engagement or other transmembrane signals. In vitro kinase assays using purified CD19 and purified Lyn revealed that the kinase activity of Lyn was significantly increased when coincubated with CD19. Thus, constitutive and induced CD19/Lyn complexes are likely to regulate basal signaling thresholds and BCR signaling by amplifying the kinase activity of Lyn and other Src-family PTKs. These in vivo and in vitro findings demonstrate a novel mechanism by which CD19 regulates signal transduction in B lymphocytes. The absence of this CD19/Src-family kinase amplification loop may account for the hyporesponsive phenotype of CD19-deficient B cells.

The src homology domain 2-containing inositol phosphatase SHIP forms a ternary complex with Shc and Grb2 in antigen receptor-stimulated B lymphocytes

The inositol phosphatase SHIP has been implicated in signaling events downstream of a variety of receptors and is thought to play an inhibitory role in stimulated B cells. We and others have reported that SHIP is rapidly tyrosine phosphorylated upon B cell antigen receptor (BCR) cross-linking and forms a complex with the adapter protein Shc. Here, we report that cross-linking of the BCR induces association between Grb2 and SHIP as well as association between Shc and SHIP. We made use of a Grb2-deficient B cell line to demonstrate both in vitro and in vivo that Grb2 expression is required for the efficient association between Shc and SHIP. The results indicate that SHIP, Shc, and Grb2 form a ternary complex in stimulated B cells, with Grb2 stabilizing the interaction between Shc and SHIP. The interactions between Shc, Grb2, and SHIP are therefore analogous to the interactions between Shc, Grb2, and SOS. Shc and Grb2 may help to localize SHIP to the cell membrane, regulating SHIP's inhibitory function following BCR stimulation.

PU.1 and Spi-B are required for normal B cell receptor-mediated signal transduction

PU.1 and Spi-B have previously been implicated in the regulation of genes encoding B cell receptor (BCR) signaling components. Spi-B-/- B lymphocytes respond poorly to BCR stimulation; PU.1-/- mice, however, lack B cells, precluding an analysis of BCR responses. We now show that PU.1+/- Spi-B-/- B cells exhibit more extensive defects than Spi-B-/- B cells, indicating that both PU.1 and Spi-B are required for normal BCR signaling. Strikingly, BCR cross-linking results in substantially reduced protein tyrosine phosphorylation in mutant B cells. Further analysis shows that Igalpha is phosphorylated and syk is recruited and becomes phosphorylated but that BLNK and PLCgamma phosphorylation are defective in mutant cells. Our data support the existence of a novel component coupling syk to downstream targets.

Translocation of ornithine decarboxylase to the surface membrane during cell activation and transformation

Ornithine decarboxylase (ODC) is highly up-regulated in proliferating and transforming cells. Here we show that upon induction, an initial cytosolic increase of ODC is followed by translocation of a fraction of the enzyme to the surface membrane. ODC membrane translocation is mediated by a p47(phox) membrane-targeting motif-related sequence, as indicated by reduced ODC activity in the membrane fraction of cells treated with a competing, ODC-derived (amino acids 165-172) peptide, RLSVKFGA, which is homologous to the p47(phox) membrane-targeting sequence. p47(phox) membrane translocation is known to be dependent on the phosphorylation of the targeting motif. Analogously, overexpressed ODC.S167A, a mutant ODC lacking the putative phosphorylation site Ser67, is unable to move to the surface membrane. Cells blocked with the RLSVKFGA peptide showed defective transformation, indicating that the motif-mediated translocation of ODC is prerequisite to its biological function. Constitutive targeting of ODC to the membrane using a plasmid encoding the chimeric protein, wild-type ODC with C-terminal linkage to the farnesylation motif of K-ras, caused impaired cytokinesis with an accumulation of polykaryotic cells. Impaired cytokinesis confirms that ODC is involved in mitotic cytoskeletal rearrangement events and pinpoints the importance of relevant membrane targeting to its physiological function.

Lymphocyte antigen receptor signal integration and regulation by the SHC adaptor

The Shc adaptor protein transduces signals from transmembrane receptors to the Ras pathway of cell activation by providing binding sites for the recruitment to the submembrane compartment of the Grb2/Sos G-nucleotide exchange complex. The need for Shc in this process is however unclear since Grb2 can be recruited directly to phosphotyrosine containing membrane receptors through its src-homology-2 domain. Evidence from studies in lymphocytes indicates that Shc is multifunctional and is involved in the integration of independent signals to the Ras pathway. Furthermore, Shc may be a key control point at which signaling can be modulated both by interfering signals and by feedback mechanisms. Here we review recent literature to support these functions for Shc.

Convergence of CD19 and B Cell Antigen Receptor Signals at MEK1 in the ERK2 Activation Cascade

CD19 plays a critical role in regulating B cell responses to Ag. We have studied the mechanism by which coligation of CD19 and the B cell Ag receptor, membrane Ig (mIg), augments signal transduction, including synergistic enhancement of release of intracellular Ca2+ and extracellular signal-regulated protein kinase 2 (ERK2) activation, in Daudi human B lymphoblastoid cells. The pathway leading to ERK2 activation was further dissected to determine how signals derived from CD19 and mIgM interact. The best-defined pathway, known to be activated by mIgM, consists of the sequential activation of the mitogen-activated protein kinase (MAPK) cascade that includes Ras, Raf, MAPK kinase 1 (MEK1), and ERK2. Ligation of CD19 alone had little effect on these. CD19-mIgM coligation did not increase activation of Ras or Raf beyond that induced by ligation of mIgM alone. In contrast, coligation resulted in synergistic activation of MEK1. Furthermore, synergistic activation of ERK2 occurred in the absence of changes in intracellular Ca2+, and was not blocked by inhibition of protein kinase C activity and represents a separate pathway by which CD19 regulates B cell function. Thus, the CD19-dependent signal after CD19-mIgM coligation converges with that generated by mIgM at MEK1. The intermediate kinases in the MAPK cascade leading to ERK2 integrate signals from lymphocyte coreceptors.

Involvement of guanosine triphosphatases and phospholipase C-gamma2 in extracellular signal-regulated kinase, c-Jun NH2-terminal kinase, and p38 mitogen-activated protein kinase activation by the B cell antigen receptor

Mitogen-activated protein (MAP) kinase family members, including extracellular signal-regulated kinase (ERK), c-Jun NH2-terminal kinase ( JNK), and p38 MAP kinase, have been implicated in coupling the B cell antigen receptor (BCR) to transcriptional responses. However, the mechanisms that lead to the activation of these MAP kinase family members have been poorly elucidated. Here we demonstrate that the BCR-induced ERK activation is reduced by loss of Grb2 or expression of a dominant-negative form of Ras, RasN17, whereas this response is not affected by loss of Shc. The inhibition of the ERK response was also observed in phospholipase C (PLC)-gamma2-deficient DT40 B cells, and expression of RasN17 in the PLC-gamma2-deficient cells completely abrogated the ERK activation. The PLC-gamma2 dependency of ERK activation was most likely due to protein kinase C (PKC) activation rather than calcium mobilization, since loss of inositol 1,4,5-trisphosphate receptors did not affect ERK activation. Similar to cooperation of Ras with PKC activation in ERK response, both PLC-gamma2-dependent signal and GTPase are required for BCR-induced JNK and p38 responses. JNK response is dependent on Rac1 and calcium mobilization, whereas p38 response requires Rac1 and PKC activation.

Tyrosine 474 of ZAP-70 is required for association with the Shc adaptor and for T-cell antigen receptor-dependent gene activation

The protein tyrosine kinase ZAP-70 plays a central role in T-cell activation. Following receptor engagement, ZAP-70 is recruited to the phosphorylated subunits of the T-cell antigen receptor (TCR). This event results in ZAP-70 activation and in association of ZAP-70 with a number of signaling proteins. Among these is the Shc adaptor, which couples the activated TCR to Ras. Shc interaction with ZAP-70 is mediated by the Shc PTB domain. The inhibitory effect of a Shc mutant containing the isolated PTB domain suggests that Shc interaction with ZAP-70 might be required for TCR signaling. Here, we show that a point mutation (Phe474) of the putative Shc binding site on ZAP-70, spanning tyrosine 474, prevented ZAP-70 interaction with Shc and the subsequent binding of Shc to phospho-zeta. Neither ZAP-70 catalytic activity nor the pattern of protein phosphorylation induced by TCR triggering was affected by this mutation. However expression of the Phe474 ZAP-70 mutant resulted in impaired TCR-dependent gene activation. ZAP-70 could effectively phosphorylate Shc in vitro. Only the CH domain, which contains the two Grb2 binding sites on Shc, was phosphorylated by ZAP-70. Both Grb2 binding sites were excellent substrates for ZAP-70. The data show that Tyr474 on ZAP-70 is required for TCR signaling and suggest that Shc association with ZAP-70 and the resulting phosphorylation of Shc might be an obligatory step in linking the activated TCR to the Ras pathway.

Qualitative regulation of B cell antigen receptor signaling by CD19: selective requirement for PI3-kinase activation, inositol-1,4,5-trisphosphate production and Ca2+ mobilization

Genetic ablation of the B cell surface glycoprotein CD19 severely impairs the humoral immune response. This requirement is thought to reflect a critical role of CD19 in signal transduction that occurs upon antigen C3dg coligation of antigen receptors with CD19 containing type 2 complement receptors (CR2). Here we show that CD19 plays a key accessory role in B cell antigen receptor signaling independent of CR2 coligation and define molecular circuitry by which this function is mediated. While CD19 is not required for antigen-mediated activation of receptor proximal tyrosines kinases, it is critical for activation of phosphatidylinositol 3-kinase (PI3-kinase). PI3-Kinase activation is dependent on phosphorylation of CD19 Y484 and Y515. Antigen-induced CD19-dependent PI3-kinase activation is required for normal phosphoinositide hydrolysis and Ca2+ mobilization responses. Thus, CD19 functions as a B cell antigen receptor accessory molecule that modifies antigen receptor signaling in a qualitative manner.

Identification of two tyrosine phosphoproteins, pp70 and pp68, which interact with phospholipase Cgamma, Grb2, and Vav after B cell antigen receptor activation

Tyrosine phosphorylation of cellular proteins mediates the assembly and localization of effector proteins through interactions facilitated by modular Src homology 2 (SH2) and phosphotyrosine binding domains. We describe here two tyrosine-phosphorylated proteins with Mr values of 70,000 and 68,000 that interact with Grb2, phospholipase C (PLCgamma1 and PLCgamma2), and Vav after B cell receptor cross-linking. The interaction of pp70 and pp68 with PLC and Vav is mediated by the carboxyl-terminal SH2 domain of PLC and the SH2 domain of Vav. In contrast, the interaction of pp70 and pp68 with Grb2 requires cooperative binding of the SH2 and SH3 domains of Grb2. Western blot analysis demonstrated that neither pp70 nor pp68 represented the recently described linker protein SLP-76, which binds Grb2, PLC, and Vav in T cells after T cell receptor activation. Moreover, SLP-76 protein was not detected in a number of B cell lines or in normal mouse B cells. Hence, we propose that pp70 and pp68 likely represent B cell homologs of SLP-76 which facilitate and coordinate B cell activation.

Conjugation of Blocked Ricin to an Anti-CD19 Monoclonal Antibody Increases Antibody-Induced Cell Calcium Mobilization and CD19 Internalization

CD19 (B4) is a signal transduction molecule restricted to the B-cell lineage and the target of the immunotoxin anti-B4–blocked ricin (anti-B4–bR), which is composed of the monoclonal antibody (MoAb) anti-B4 and the modified plant toxin blocked ricin. To explore the influence of conjugation of blocked ricin to anti-B4 on functional activation of CD19, we investigated the effects of anti-B4–bR, and that of unconjugated anti-B4, on intracellular calcium mobilization and ligand/receptor internalization. The data showed that anti-B4–bR was more potent than anti-B4 in triggering cell calcium mobilization. Two other immunotoxins that bind to the B-cell surface, anti-CD20–bR and anti-CD38–bR, were devoid of the calcium increasing effect of anti-B4–bR. Furthermore, anti-B4 conjugated to ricin A-chain was also without effect in Namalwa cells, indicating that the ricin B-chain component was required for anti-B4–bR effect. Anti-B4–bR-induced calcium mobilization was inhibited in the presence of lactose, yet the calcium response induced by cross-linking anti-B4–bR with a second step antibody was not affected. The extent of CD19 modulation induced by anti-B4–bR was higher than that induced by anti-B4, and lactose dampened the effect of the immunotoxin down to that of the MoAb. Moreover, the number of internalized immunotoxin molecules was higher than that of unconjugated MoAb. Although a mechanism involving dimerization of the immunotoxin cannot be excluded, our findings suggest that the residual binding activity of the blocked ricin B-chain to cell surface molecules plays an important role in the greater calcium fluxes and greater internalization rate of anti-B4–bR, and is of functional significance in the mechanism of intoxication of cells by the immunotoxin.

Conjugation of Blocked Ricin to an Anti-CD19 Monoclonal Antibody Increases Antibody-Induced Cell Calcium Mobilization and CD19 Internalization

CD19 (B4) is a signal transduction molecule restricted to the B-cell lineage and the target of the immunotoxin anti-B4–blocked ricin (anti-B4–bR), which is composed of the monoclonal antibody (MoAb) anti-B4 and the modified plant toxin blocked ricin. To explore the influence of conjugation of blocked ricin to anti-B4 on functional activation of CD19, we investigated the effects of anti-B4–bR, and that of unconjugated anti-B4, on intracellular calcium mobilization and ligand/receptor internalization. The data showed that anti-B4–bR was more potent than anti-B4 in triggering cell calcium mobilization. Two other immunotoxins that bind to the B-cell surface, anti-CD20–bR and anti-CD38–bR, were devoid of the calcium increasing effect of anti-B4–bR. Furthermore, anti-B4 conjugated to ricin A-chain was also without effect in Namalwa cells, indicating that the ricin B-chain component was required for anti-B4–bR effect. Anti-B4–bR-induced calcium mobilization was inhibited in the presence of lactose, yet the calcium response induced by cross-linking anti-B4–bR with a second step antibody was not affected. The extent of CD19 modulation induced by anti-B4–bR was higher than that induced by anti-B4, and lactose dampened the effect of the immunotoxin down to that of the MoAb. Moreover, the number of internalized immunotoxin molecules was higher than that of unconjugated MoAb. Although a mechanism involving dimerization of the immunotoxin cannot be excluded, our findings suggest that the residual binding activity of the blocked ricin B-chain to cell surface molecules plays an important role in the greater calcium fluxes and greater internalization rate of anti-B4–bR, and is of functional significance in the mechanism of intoxication of cells by the immunotoxin.

Shc contains two Grb2 binding sites needed for efficient formation of complexes with SOS in B lymphocytes

Cross-linking of the B-cell antigen receptor (BCR) induces tyrosine phosphorylation of Shc, which is believed to lead to the activation of Ras. Previous work has shown that tyrosine-phosphorylated Shc forms complexes with another adapter protein, Grb2, and the Ras guanine nucleotide exchange factor SOS. Here, we demonstrate that phosphorylation of Shc by the hematopoietic cell-specific tyrosine kinase Syk induces binding of Grb2 to Shc, suggesting that Syk phosphorylates Shc in stimulated B cells. Surprisingly, Syk-phosphorylated Shc possesses two Grb2 binding sites rather than the one site that has been previously reported. Both of these sites are required for efficient formation of Shc-Grb2-SOS complexes in vitro and in vivo. We suggest that two Grb2 proteins anchored by a single Shc protein bind simultaneously to one SOS molecule, resulting in a complex that is more stable than a complex containing only a single Grb2 protein bound to one SOS molecule. This model is consistent with our observation that BCR stimulation greatly increases the amount of SOS associated with Grb2.

The complexity of signaling pathways activated by the BCR

Cross-linking of the B cell antigen receptor (BCR) leads to the activation of three types of intracellular protein tyrosine kinases. These tyrosine kinases then phosphorylate signaling components to activate a variety of signaling reactions, including phosphatidylinositol 4,5-bisphosphate hydrolysis, Ras activation, and phosphatidylinositol 3-kinase activation. Each of these signaling reactions, and also the signaling molecules Vav and HS1, appears to be important for at least some of the many types of B cell responses to antigen. The complexity of BCR signaling reactions may be required to allow the B cell to respond in a number of distinct ways to antigen (proliferation, survival, apoptosis, maturational arrest, etc.) depending on the maturation state of the B cell, the location in the body, the physical nature of the antigen, and the possible presence of the antigen in complex with antibody or complement components.

Co-ligation of the antigen and Fc receptors gives rise to the selective modulation of intracellular signaling in B cells. Regulation of the association of phosphatidylinositol 3-kinase and inositol 5'-phosphatase with the antigen receptor complex

Cross-linking of the Fc receptor (FcR) to surface immunoglobulin (sIg) on B cells inhibits the influx of extracellular calcium and abrogates the proliferative signal. The mechanism by which this occurs is not well understood. In this report we show that co-cross-linking the FcR to the antigen receptor gives rise to very selective modulation of signal transduction in B cells. Co-cross-linking sIg and the FcR enhanced the phosphorylation of the FcR, the adapter protein, Shc, and the inositol 5'-phosphatase Ship. Furthermore, phosphorylation of the FcR induced its association with Ship. Cross-linking of the FcR and sIg decreased the tyrosine phosphorylation of CD19, which led to a reduction in the association of phosphatidylinositol 3-kinase. In addition, the phosphorylation of several other proteins of 73, 39, and 34 kDa was reduced. Activation of the cells with either F(ab')2 or intact anti-IgG induced very similar changes in levels of tyrosine phosphorylation of most other proteins, and no differences in the activation of several protein kinases were observed. These results indicate that the inhibitory signal that is transmitted through the FcR is not mediated by a global shutdown of tyrosine phosphorylation but is, rather, a selective mechanism involving localized changes in the interactions of adapter proteins and the enzymes Ship and phosphatidylinositol 3-kinase with the antigen receptor complex.

Initiation and processing of signals from the B cell antigen receptor

Current models of signal transduction from the antigen receptors on B and T cells still resemble equations with several unknown elements. Data from recent knockout experiments in cell lines and mice contradict the assumption that Src-family kinase and tyrosine kinases of the Syk/Zap-70 family are the transducer elements that set signaling from these receptors in motion. Using a functional definition of signaling elements, we discuss the current knowledge of signaling events from the BCR and suggest the existence of an as-yet-unknown BCR transducer complex.

Alteration of B-cell antigen receptor signaling by CD19 co-ligation. A study with bispecific antibodies

The activation of B-cell antigen receptor-associated protein tyrosine kinases is an early and crucial event in B-cell signaling. Apart from the B-cell antigen receptor (BCR), the B-cell-specific transmembrane glycoprotein CD19 has also been shown to directly activate intracellular signaling cascades. In addition, because CD19 and the BCR are associated on the surface of activated B-cells, it has been proposed that close approximation between these two entities is crucial for optimal B-cell triggering. To test this hypothesis, bispecific antibodies were generated that bind membrane IgM and CD19 simultaneously. Although CD19 bispecific antibodies strongly induced tyrosine phosphorylation, they were, in contrast to muF(ab)2 fragments, unable to induce a proliferative response. Detailed analysis of the early signaling events showed that compared with muF(ab)2 fragments CD19 bispecific antibodies potently raised the intracellular [Ca2+], which was correlated with an efficient tyrosine phosphorylation of syk. Strikingly, the assembly of Grb2 complexes that may couple the BCR to p21(ras) was clearly altered by the CD19 bispecific antibody. In addition to the reported Shc and 145-kDa phosphoproteins, a prominent 90-95-kDa phosphoprotein resembling CD19 was detected in the Grb2 complexes. Thus, studies with CD19 bispecific antibodies show that CD19 co-ligation both quantitatively and qualitatively alters BCR signaling.

Distinct mechanisms mediate SHC association with the activated and resting B cell antigen receptor

Ligation of the B cell antigen receptor (BCR) complex initiates tyrosine phosphorylation of the receptor's transducer components, Ig-alpha and Ig-beta and tyrosine kinase-dependent accumulation of GTP-bound, activated p21ras. The mechanism of receptor coupling to p21ras activation and the roles of Ig-alpha and Ig-beta are unknown. The results reported here indicate that the resting, nonphosphorylated BCR associates with the Grb-2/Sos-linker SHC via the Ig-alpha immunoreceptor-based tyrosine activation motif (ITAM). Ig-alpha specificity of this interaction is determined by the sequence DCSM found in Ig-alpha, but not Ig-beta. Tyrosine phosphorylation of Ig-alpha and Ig-beta ITAM allows recruitment of SHC, which now binds directly to both Ig-alpha and Ig-beta via a phosphotyrosine/SH2 interaction. In confirmation of recent studies by Saxton et al. (J. Immunol. 1994. 153: 623) receptor ligation leads to tyrosine phosphorylation of SHC and to the formation of a phospho-SHC/Grb2/Sos complex. In view of previous studies which demonstrated p21ras co-capping with ligated BCR, the data presented here suggest that Ig-alpha/beta- and SHC tyrosine phosphorylation-dependent recruitment of the Grb2/Sos complex to the receptor can occur and may provide a mechanism by which the nucleotide exchange activity of Sos could mediate activation of BCR-localized p21ras.

Evidence for a preformed transducer complex organized by the B cell antigen receptor

The B cell antigen receptor (BCR) consists of the membrane-bound immunoglobulin (mIg) molecule and the Ig-alpha/Ig-beta heterodimer, which functions as signaling subunit of the receptor. Stimulation of the BCR activates protein tyrosine kinases (PTKs) that phosphorylate a number of substrate proteins, including the Ig-alpha/Ig-beta heterodimer of the BCR itself. How the PTKs become activated after BCR engagement is not known at present. Here, we show that BCR-negative J558L cells treated with the protein tyrosine phosphatase inhibitor pervanadate/H2O2 display only a weak substrate phosphorylation. However, in BCR-positive transfectants of J558L, treatment with pervanadate/H2O2 induces a strong phosphorylation of several substrate proteins. Treatment with pervanadate/H2O2 does not result in receptor crosslinking, yet the pattern of protein phosphorylation is similar to that observed after BCR stimulation by antigen. The response requires cellular integrity because tyrosine phosphorylation of most substrates is not visible in cell lysates. Cells that express a BCR containing an Ig-alpha subunit with a mutated immunoreceptor tyrosine-based activation motif display a delayed response. The data suggest that, once expressed on the surface, the BCR organizes protein tyrosine phosphatases, PTKs, and their substrates into a transducer complex that can be activated by pervanadate/H202 in the absence of BCR crosslinking. Assembly of this preformed complex seems to be a prerequisite for BCR-mediated signal transduction.

Sos, Vav, and C3G participate in B cell receptor-induced signaling pathways and differentially associate with Shc-Grb2, Crk, and Crk-L adaptors

B cell antigen receptor (BCR)-mediated signal transduction controls B cell proliferation and differentiation. The BCR activates Ras, presumably by the formation of a Shc-Grb2 adaptor complex, which recruits the Grb2-associated guanine nucleotide exchange factor Sos to the plasma membrane. In order to reveal additional BCR-induced signaling events involving the Grb2 adaptor, we undertook the isolation of Grb2-binding proteins. Using the yeast two-hybrid system and bacterial fusion proteins, Vav and C3G were identified as Grb2 binders. Vav is a putative nucleotide exchange factor and a target for BCR-induced tyrosine phosphorylation. C3G exerts nucleotide exchange activity on the Ras-related Rap1 protein. While Sos binds to both Grb2 Src homology-3 (SH3) domains, Vav was found to associate selectively with the carboxyl-terminal SH3 domain, while C3G bound selectively to the amino-terminal SH3 domain of bacterially expressed Grb2. Despite the association of Vav with Grb2 in vitro, we could not demonstrate an interaction between endogenous Vav and Grb2 molecules in primary B cells. Instead, Vav was found to inducibly associate with the Grb2-related adaptor protein Crk upon BCR stimulation. C3G did not bind to either Grb2, Shc, or Crk in vivo. Instead, C3G was found in association with the Crk-L adaptor, both before and after BCR stimulation. We show that Crk-L also participates in BCR signaling, since it inducibly interacts with tyrosine-phosphorylated Cbl. We conclude that, in addition to Sos, Vav and C3G play a role in BCR-mediated signal transduction. These guanine nucleotide exchange factors selectively associate with Grb2, Crk, and Crk-L, respectively, which may serve to direct them to different target molecules. Since Cbl binds to Grb2, Crk, as well as Crk-L, we hypothesize that Cbl may affect the function of all three exchangers.

In vivo association of v-Abl with Shc mediated by a non-phosphotyrosine-dependent SH2 interaction

A necessary downstream element of Abelson murine leukemia virus (Ab-MLV)-mediated transformation is Ras, which can be activated by the phosphotyrosine-dependent association of Shc with the Grb2-mSos complex. Here we show that Shc is tyrosine-phosphorylated and associates with Grb2 in v-Abl-transformed cells, whereas Shc in NIH3T3 cells is phosphorylated solely on serine and is not Grb2-associated. In addition, Shc coprecipitates with P120 v-Abl and P70 v-Abl, which lacks the carboxyl terminus. Surprisingly, a kinase-defective mutant of P120 also binds Shc, demonstrating that Shc/v-Abl association is a phosphotyrosine-independent interaction. Glutathione S-transferase fusion proteins were used to map the interacting domains and showed that Shc from both NIH3T3 and v-Abl-transformed cells binds to the Abl SH2 domain and that P120 v-Abl binds to a region in the amino terminus of Shc. Consistent with these data, a v-Abl mutant encoding only the Gag and SH2 regions was able to bind Shc in vivo. The unique non-phosphotyrosine-mediated binding of Shc may allow direct tyrosine phosphorylation of Shc by v-Abl and subsequent activation of the Ras pathway through assembly of a signaling complex with Grb2-mSos.

Conformational changes in CD45 upon monoclonal antibody crosslinking

The CD45 protein tyrosine phosphatase is expressed in different isoforms that result from alternative splicing of three exons (A, B, and C) encoding regions near the N-terminus of the extracellular part of the molecule. We describe here a novel epitope on the N-terminal end of CD45 that is recognized by the MAb BL-TSub/2. Crossblocking studies showed that BL-TSub/2 and UCHL1 (CD45RO) binding sites are partially overlapping. However, in marked contrast to the CD45RO epitope, protease treatment of cells strongly diminished BL-TSub/2 binding. Similar to the UCHL1 epitope, the BL-TSub/2 binding site involves carbohydrate moieties, since neuraminidase treatment abrogated the reactivity of the MAb. Markedly, preincubation of cells with both CD45 common and CD45RA MAb induced a pronounced increase of BL-TSub/2 binding. This latter finding suggests that crosslinking of the CD45 molecule leads to conformational changes that could influence association of the molecule with putative ligands.

Activation-induced association of a 145-kDa tyrosine-phosphorylated protein with Shc and Syk in B lymphocytes and macrophages

Engagement of many cell surface receptors results in tyrosine phosphorylation of an overlapping set of protein substrates. Some proteins, such as the adaptor protein Shc, and a frequently observed Shc-associated protein, p145, are common substrates in a variety of receptor signaling pathways and are thus of special interest. Tyrosine-phosphorylated Shc and p145 coprecipitated with anti-Shc antibodies following B cell antigen receptor (BCR) cross-linking or interleukin-4 (IL-4) receptor activation in B cells, and after lipopolysaccharide (LPS) treatment or IgG Fc receptor (Fc gamma R) cross-linking in macrophages. In the case of BCR stimulation, we have shown that this represented the formation of an inducible complex. Furthermore, in response to LPS activation or Fc gamma R cross-linking of macrophages and BCR cross-linking (but not IL-4 treatment) of B cells, we observed a similar tyrosine-phosphorylated p145 protein associated with the tyrosine kinase Syk. We did not detect any Shc associated with Syk, indicating that a trimolecular complex of Shc, Syk, and p145 was not formed in significant amounts. By several criteria, the Syk-associated p145 was very likely the same protein as the previously identified Shc-associated p145. The Syk-associated p145 and the Shc-associated p145 exhibited identical mobility by SDS-polyacrylamide gel electrophoresis and identical patterns of induced tyrosine phosphorylation. The p145 protein that coprecipitated with either Shc or Syk bound to a GST-Shc fusion protein. In addition, a monoclonal antibody developed against Shc-associated p145 also immunoblotted the Syk-associated p145. The observations that p145 associated with both Shc and Syk proteins, in response to stimulation of a variety of receptors, suggest that it plays an important role in coordinating early signaling events.

CD16-mediated p21ras activation is associated with Shc and p36 tyrosine phosphorylation and their binding with Grb2 in human natural killer cells

The Src homology (SH) 2/SH3 domain-containing protein Grb2 and the oncoprotein Shc have been implicated in a highly conserved mechanism that regulates p21ras activation. We investigated the involvement of these adaptor proteins in the signaling pathway induced by CD16 or interleukin (IL) 2R triggering in human natural killer (NK) cells. Both p46 and p52 forms of Shc were rapidly and transiently tyrosine phosphorylated upon CD16 or IL-2 stimulation with different kinetics. Shc immunoprecipitates from lysates of CD16- or IL-2-stimulated NK cells contained Grb2 and an unidentified 145-kD tyrosine phosphoprotein. Grb2 immunoprecipitates from anti-CD16-stimulated NK cells contained not only Shc, but also a 36-kD tyrosine phosphoprotein (p36). The interaction between Grb2 and Shc or p36 occurred via the Grb2SH2 domain as indicated by in vitro binding assays using a bacteriologically synthesized glutathione S-transferase-Grb2SH2 fusion protein. We also present evidence that p21ras is activated by CD16 and IL-2R cross-linking. Accumulation of guanosine triphosphate-bound Ras was detected within 1 minute and occurred with kinetics similar to inductive protein tyrosine phosphorylation and Grb2 association of Shc and p36 adaptor proteins.

Specific binding of Fyn and phosphatidylinositol 3-kinase to the B cell surface glycoprotein CD19 through their src homology 2 domains

CD19 is a B cell surface protein capable of forming non-covalent molecular complexes with a number of other B cell surface proteins including the CD21/CD81/Leu-13 complex as well as with surface immunoglobulin. CD19 tyrosine phosphorylation increases after B cell activation, and is proposed to play a role in signal transduction through its cytoplasmic domain, which contains nine tyrosine residues. Several second messenger proteins have been shown to immunoprecipitate with CD19, including p59 Fyn (Fyn), p59 Lyn (Lyn) and phosphatidylinositol-3 kinase (PI-3 kinase). These associations are predicted to occur via the src-homology 2 (SH2) domains of the second messenger proteins. Two of the cytoplasmic tyrosines in the CD19 cytoplasmic region contain the consensus binding sequence for the PI-3 kinase SH2 domain (YPO4-X-X-M). However, the reported consensus binding sequence for the Fyn and Lyn SH2 domains (YPO4-X-X-I/L) is not found in CD19. We investigated the capacity of CD19 cytoplasmic tyrosines to bind both Fyn and PI-3 kinase SH2-domain fusion proteins. In activated B cells, both Fyn and PI-3 kinase SH2-domain fusion proteins precipitate CD19. Using synthetic tyrosine-phosphorylated peptides comprising each of the CD19 cytoplasmic tyrosines and surrounding amino acids, we investigated the ability of the Fyn SH2 and PI-3 kinase SH2 fusion proteins to bind to the different CD19 cytoplasmic phosphotyrosine peptides. ELISA revealed that the two CD19 cytoplasmic tyrosine residues contained within the Y-X-X-M sequences (Y484 and Y515) bound preferentially to the PI-3 kinase SH2-domain fusion proteins. Two different tyrosines (Y405 and Y445) bound preferentially to the Fyn SH2-domain fusion protein via a novel sequence, Y-E-N-D/E, different from that previously reported for the Fyn SH2 domain. In precipitation studies, peptide Y484 was able to compete with tyrosine phosphorylated CD19 specifically for binding to the PI-3 kinase SH2 domain fusion proteins, while peptides Y405 and Y445 were able to compete specifically for binding to the Fyn SH2 domain fusion proteins. These results indicate that CD19 may be capable of binding both Fyn and PI-3 kinase concurrently, suggesting a mechanism for CD19 signal transduction, in which binding of PI-3 kinase to the Fyn SH3 domain results in activation of PI-3 kinase.

Hematopoietic cell phosphatase is recruited to CD22 following B cell antigen receptor ligation

Hematopoietic cell phosphatase is a nonreceptor protein tyrosine phosphatase that is preferentially expressed in hematopoietic cell lineages. Motheaten mice, which are devoid of (functional) hematopoietic cell phosphatase, have severe disturbances in the regulation of B cell activation and differentiation. Because signals transduced via the B cell antigen receptor are known to guide these processes, we decided to analyze molecular interactions between the hematopoietic cell phosphatase and the B cell antigen receptor. Ligation of the B cell antigen receptor induces moderate tyrosine phosphorylation of hematopoietic cell phosphatase and the formation of a multi-molecular complex containing additional 68-70- and 135-kDa phosphoproteins. In resting B cells most of the hematopoietic cell phosphatase proteins reside in the cytosolic compartment, whereas after B cell antigen receptor cross-linking, a small fraction translocates toward the membrane where it specifically binds to the 135-kDa phosphoprotein. This 135-kDa glycoprotein was identified as CD22, a transmembrane associate of the B cell antigen receptor complex. Together these findings provide the first direct evidence that this cytoplasmic tyrosine phosphatase is involved in antigen receptor-mediated B cell activation, suggesting that in vivo B cell antigen receptor constituents or associated molecules may serve as substrate for its catalytic activity.