Negative signaling via FcgammaRIIB1 in B cells blocks phospholipase Cgamma2 tyrosine phosphorylation but not Syk or Lyn activation

FcɣRIIB regulates autoantibody responses by limiting marginal zone B cell activation

FcɣRIIB is an inhibitory receptor expressed throughout B cell development. Diminished expression or function is associated with lupus in mice and humans, in particular through an effect on autoantibody production and plasma cell differentiation. Here, we analysed the effect of B cell-intrinsic FcɣRIIB expression on B cell activation and plasma cell differentiation. Loss of FcɣRIIB on B cells (Fcgr2b cKO mice) led to a spontaneous increase in autoantibody titers. This increase was most striking for IgG3, suggestive of increased extrafollicular responses. Marginal zone (MZ) B cells had the highest expression of FcɣRIIB in both mouse and human. This high expression of FcɣRIIB was linked to increased MZ B cell activation, Erk phosphorylation, and calcium fluxin the absence of FcɣRIIB triggering. Marked increases in IgG3+ plasma cells and B cells were observed during extrafollicular plasma cell responses in Fcgr2b cKO mice. The increased IgG3 response following immunization of Fcgr2b cKO mice was lost in MZ-deficient Notch2/Fcgr2b cKO mice. Importantly, SLE patients exhibited decreased expression of FcɣRIIB, most strongly in MZ B cells. Thus, we present a model where high FcɣRIIB expression in MZ B cells prevents their hyperactivation and ensuing autoimmunity.

Syk and Lyn phosphorylation induced by FcγRI and FγRII crosslinking is determined by the differentiation state of U-937 monocytic cells

Fcgamma receptor (FcgammaR)-mediated phagocytosis by mononuclear phagocytes is an essential function in host defense. This process is initiated by crosslinking of membrane FcgammaRs, which induces phosphorylation and activation of Src and Syk tyrosine kinases. Activation of these enzymes is essential for initiating the biochemical cascade that results in the cytoskeletal and membrane changes involved in phagocytosis. Phagocytic capacity and other effector functions of mononuclear phagocytes change during differentiation/maturation of these cells. This is a complex process governed by different soluble and micro-environmental factors, giving rise to populations of cells with distinct phenotypic characteristics. Several agents, including calcitriol, have been shown to induce in vitro differentiation-related phenotypic changes in monocytic cell lines. In this paper, we characterized the changes in the initial biochemical signals associated with the increase in FcgammaR-mediated phagocytosis induced by calcitriol in monocytic U-937 cells. The 10-fold increase in phagocytic capacity is not accompanied by an increase in FcgammaR expression. However, the phosphorylation levels of Lyn and Syk after FcgammaRI or FcgammaRII crosslinking are increased after calcitriol treatment. Our results suggest that signaling induced by FcgammaR in mononuclear phagocytes is not only dependent on the quantity of FcgammaRs aggregated by a stimulus, but it is highly dependent on the cell's differentiation state.

Decreased transcription of the human FCGR2B gene mediated by the -343 G/C promoter polymorphism and association with systemic lupus erythematosus

The role for inhibitory Fc gamma receptors class IIb (FcgammaRIIb) in the onset, progression and severity of several animal models of autoimmune diseases is well established. By contrast, the pathogenic potential of FcgammaRIIb in human autoimmune diseases remains largely unknown. Here we report the identification of a polymorphism in the human FCGR2B promoter (dbSNP no. rs3219018) that is associated in homozygosity with systemic lupus erythematosus (SLE) phenotype in European-Americans (OR=11.1, P=0.003). Experimental evidence correlates the polymorphism (a G-C substitution at position -343 relative to the start of transcription) with altered FcgammaRIIb expression and function. The G-C substitution correlated with decreased transcription of the FCGR2B promoter, and resulted in decreased binding of the AP1 transcription complex to the mutant promoter sequence. The surface expression of FcgammaRIIb receptors was significantly reduced in activated B cells from (-343 C/C) SLE patients. These findings suggest that genetic defects may lead to deregulated expression of the FCGR2B gene in -343 C/C homozygous subjects, and may play a role in the pathogenesis of human SLE.

Visualization of negative signaling in B cells by quantitative confocal microscopy

Numerous biochemical experiments have invoked a model in which B-cell antigen receptor (BCR)-Fc receptor for immunoglobulin (Ig) G (FcgammaRII) coclustering provides a dominant negative signal that blocks B-cell activation. Here, we tested this model using quantitative confocal microscopic techniques applied to ex vivo splenic B cells. We found that FcgammaRII and BCR colocalized with intact anti-Ig and that the SH2 domain-containing inositol 5'-phosphatase (SHIP) was recruited to the same site. Colocalization of BCR and SHIP was inefficient in FcgammaRII-/- but not gamma chain-/- splenic B cells. We also examined the subcellular location of a variety of enzymes and adapter proteins involved in signal transduction. Several proteins (CD19, CD22, SHP-1, and Dok) and a lipid raft marker were co-recruited to the BCR, regardless of the presence or absence of FcgammaRII and SHIP. Other proteins (Btk, Vav, Rac, and F-actin) displayed reduced colocalization with BCR in the presence of FcgammaRII and SHIP. Colocalization of BCR and F-actin required phosphatidylinositol (PtdIns) 3-kinase and was inhibited by SHIP, because the block in BCR/F-actin colocalization was not seen in B cells of SHIP-/- animals. Furthermore, BCR internalization was inhibited with intact anti-Ig stimulation or by expression of a dominant-negative mutant form of Rac. From these results, we propose that SHIP recruitment to BCR/FcgammaRII and the resulting hydrolysis of PtdIns-3,4,5-trisphosphate prevents the appropriate spatial redistribution and activation of enzymes distal to PtdIns 3-kinase, including those that promote Rac activation, actin polymerization, and receptor internalization.

Signal transduction by human-restricted Fc gamma RIIa involves three distinct cytoplasmic kinase families leading to phagocytosis

Recent experiments indicate an important role for Src family and Syk protein tyrosine kinases and phosphatidylinositol 3-kinase in the signal transduction process initiated by mouse receptors for IgG and leading to phagocytosis. Considerably less is known regarding signal transduction by the human-restricted IgG receptor, FcgammaRIIa. Furthermore, the relationship among the Src family, Syk, and phosphatidylinositol 3-kinase in phagocytosis is not understood. Here, we show that FcgammaRIIa is phosphorylated by an Src family member, which results in recruitment and concomitant activation of the distal enzymes Syk and phosphatidylinositol 3-kinase. Using a FcgammaRI-p85 receptor chimera cotransfected with kinase-inactive mutants of Syk or application of a pharmacological inhibitor of Syk, we show that Syk acts in parallel with phosphatidylinositol 3-kinase. Our results indicate that FcgammaRIIa-initiated monocyte or neutrophil phagocytosis proceeds from the clustered IgG receptor to Src to phosphatidylinositol 3-kinase and Syk.

Enzymatic activity of the Src homology 2 domain-containing inositol phosphatase is regulated by a plasma membrane location

The negative regulatory role of the Src homology 2 domain-containing inositol 5-phosphatase (SHIP) has been invoked in a variety of receptor-mediated signaling pathways. In B lymphocytes, co-clustering of antigen receptor surface immunoglobulin with FcgammaRIIb promotes the negative effects of SHIP, but how SHIP activity is regulated is unknown. To explore this issue, we investigated the effect of SHIP phosphorylation, receptor tyrosine engagement by its Src homology 2 domain, and membrane recruitment of SHIP on its enzymatic activity. We examined two SHIP phosphorylation kinase candidates, Lyn and Syk, and observed that the Src protein-tyrosine kinase, Lyn is far superior to Syk in its ability to phosphorylate SHIP both in vitro and in vivo. However, we found a minimal effect of phosphorylation or receptor tyrosine engagement of SHIP on its enzymatic activity, whereas membrane localization of SHIP significantly reduced cellular phosphatidylinositol 3,4, 5-triphosphate levels. Based on our results, we propose that a membrane localization of SHIP is the crucial event in the induction of its phosphatase effects.

FcgammaRIIb modulation of surface immunoglobulin-induced Akt activation in murine B cells

We examined activation of the serine/threonine kinase Akt in the murine B cell line A20. Akt is activated in a phosphoinositide 3-kinase (PtdIns 3-kinase)-dependent manner upon stimulation of the antigen receptor, surface immunoglobulin (sIg). In contrast, Akt induction is reduced upon co-clustering of sIg with the B cell IgG receptor, FcgammaRIIb. Co-clustering of sIg-FcgammaRIIb transmits a dominant negative signal and is associated with reduced accumulation of the PtdIns 3-kinase product phosphatidylinositol 3,4,5-trisphosphate (PtdIns 3,4,5-P3), known to be a potent activator of Akt. PtdIns 3-kinase is activated to the same extent with and without FcgammaRIIb co-ligation, indicating conditions supporting the generation of PtdIns 3,4,5-P3. We hypothesized that the decreased Akt activity arises from the consumption of PtdIns 3,4,5-P3 by the inositol-5-phosphatase Src homology 2-containing inositol 5-phosphatase (SHIP), which has been shown by us to be tyrosine-phosphorylated and associated with FcgammaRIIb when the latter is co-ligated. In direct support of this hypothesis, we report here that Akt induction is greatly reduced in fibroblasts expressing catalytically active but not inactive SHIP. Likewise, the reduction in Akt activity upon sIg-FcgammaRIIb co-clustering is absent from avian B cells lacking expression of SHIP. These findings indicate that SHIP acts as a negative regulator of Akt activation.

Superclustering of B Cell Receptor and FcγRIIB1 Activates Src Homology 2-Containing Protein Tyrosine Phosphatase-1

FcγRIIB1 (CD32) is a receptor that binds the Fc domain of Ag-complexed IgG. Coaggregation of B cell receptor (BCR) and FcγRIIB1 generates a dominant negative signal that inhibits B cell activation. In Ag-specific Id-positive B cells, the co-cross-linking of BCR and FcγRIIB1 by anti-Id Ab resulted in the association of both Src homology 2-containing protein tyrosine phosphatase (SHP-1) and Src homology 2-containing inositol phosphatase (SHIP) with the FcγRIIB1; however, only SHIP activity was detected. “Superclustering” of the BCR and FcγRIIB1 complex induced by stimulation with anti-Id Ab plus polyvalent Ag synergistically activated SHP-1. The degree of co-cross-linking between BCR and FcγRIIB1 may determine the activation status of SHP-1 and SHIP.

Intravenous immunoglobulin inhibits IgE production in human B lymphocytes

BACKGROUND

Intravenous immunoglobulin (IVIG) is commonly used as both an immune-enhancing and immune-modulating agent. Treatment with high doses of IVIG diminishes IgE secretion in patients with severe steroid-dependent asthma.

OBJECTIVE

We studied the action of IVIG on IgE production in highly purified B lymphocytes stimulated without additional T cells to determine the action of IVIG on B lymphocytes.

METHODS

Human B cells were purified from tonsils, and T lymphocytes were removed by E-rosetting. B cells were cultured with IL-4 (400 U/mL) and anti-CD40 antibodies (1 microg/mL¿, with or without additional IVIG. Cell proliferation was determined by 3[H]-thymidine uptake, and supernatant IgE was determined by ELISA. Cell cycle analysis was performed by flow cytometry, and IgE transcripts were measured by in situ hybridization.

RESULTS

IVIG (5 mg/mL) decreased B-cell proliferation in IL4/anti-CD40-stimulated B cells by an average of 74% (+/-6%). Addition of IVIG up to 48 hours after initiation of cell culture led to significant diminution of cell proliferation at 96 to 120 hours. This effect was dose dependent, with 10 mg/mL being the most effective and doses under 0.1 mg/mL having minimal effect. IVIG diminished the number of stimulated cells progressing in the cell cycle by 30%, and there was no difference in cell viability between IVIG-treated and IVIG-untreated cells. The production of IgE in culture by anti-CD40/IL4-stimulated B lymphocytes was curtailed by greater than 80% after addition of 5 mg/mL IVIG. This was associated with a decrease in IgE (epsilon) transcripts in IVIG-treated cultures.

CONCLUSION

These data indicate that diminution of IgE production in anti-CD40/IL-4-stimulated B cells by IVIG is due to inhibition of early events related to proliferation and progression in the cell cycle.

B Cell Antigen Receptor (BCR)-Mediated Formation of a SHP-2-pp120 Complex and Its Inhibition by FeγRIIB1-BCR Coligation

Accumulating evidence indicates that the Src homology 2-containing tyrosine phosphatase 2 (SHP-2) plays an important role in signal transduction through receptor tyrosine kinase and cytokine receptors. In most models, SHP-2 appears to be a positive mediator of signaling. However, coligation of FcγRIIB1 with B cell Ag receptors (BCR) inhibits BCR-mediated signaling by a mechanism that may involve recruitment of phosphatases SHP-1, SHP-2, and the SH2 containing inositol 5′phosphatase (SHIP) to the phosphorylated FcγRIIB1 immunoreceptor tyrosine-based inhibitory motif. The role of SHP-2 in BCR-mediated cell activation and in FcγRIIB1-mediated inhibitory signaling is unclear. In this study we assessed the association of SHP-2 with phosphotyrosine-containing cellular protein(s) before and after stimulation through these receptors. BCR stimulation induced the association of SHP-2 with a single major tyrosyl-phosphorylated molecule (pp120) that had an apparent molecular mass of 120 kDa. Coligation of FcγRIIB1 with BCR led to a rapid decrease in SHP-2 association with pp120. Analysis of the subcellular localization of pp120 showed that the complex of SHP-2 and tyrosyl-phosphorylated p120 occurs predominantly in the cytosol. Furthermore, the binding of the two molecules was mediated by the interaction of tyrosyl-phosphorylated p120 with the SHP-2 N-terminal SH2 domain. These findings indicate that SHP-2 and pp120 function in BCR signaling, and this function may be inhibited by FcγRIIB1 signaling.

Inhibitory signaling by B cell Fc gamma RIIb

The fact that B cells undergo feedback suppression, or negative signaling, through the interaction of secreted antibody with specific antigen has been extensively documented but the mechanisms involved in the process have been elusive. Experiments over the past year using B cell deletion mutants and dominant-negative enzymes have firmly established an important role for SH2-domain-containing inositol 5-phosphatase (SHIP) in negative signaling. Negative signaling through SHIP appears to inhibit the Ras pathway through SH2 domain competition with Grb2 and Shc and may involve consumption of intracellular lipid mediators that act as allosteric enzyme activators or that promote entry of extracellular Ca2+.

Activation-induced bi-dentate interaction of SHIP and Shc in B lymphocytes

SHIP is a SH2 domain-containing inositol polyphosphatase that is selectively tyrosine phosphorylated and associated with the adapter protein Shc in B lymphocytes upon co-crosslinking surface immunoglobulin and Fc gamma RIIB1. We previously observed that this stimulation condition is associated with a reduction in the interaction of Grb2 with phosphorylated Shc, an enhanced interaction of Shc with SHIP, and a block in the Ras signaling pathway. We proposed that the SH2 domain of SHIP competes with Grb2 in binding to phospho-Shc, resulting in a block in Ras signaling. To test this model, we examined the mode of SHIP-Shc interaction. Using recombinant Shc and SHIP interaction domains and purified Shc and SHIP phosphopeptides, we show that the interaction is bi-dentate such that the SH2 domain of SHIP recognizes phosphorylated Y317 and doubly-phosphorylated Y239/Y240 of Shc and the Shc PTB domain recognizes phosphorylated NPxpY motifs within SHIP. We observed no role for the Shc SH2 domain in the interaction. These findings are consistent with our earlier model that SHIP and Grb2 compete for binding to phospho-Shc and support the notion that, in addition to the hydrolysis of inositol phosphates and phospholipids, SHIP contributes to anti-proliferative biochemistry by blocking protein-protein interactions.

Recruitment and phosphorylation of SH2-containing inositol phosphatase and Shc to the B-cell Fc gamma immunoreceptor tyrosine-based inhibition motif peptide motif

Recently, we and others have demonstrated that negative signaling in B cells selectively induces the tyrosine phosphorylation of a novel inositol polyphosphate phosphatase, p145SHIP. In this study, we present data indicating that p145SHIP binds directly a phosphorylated motif, immunoreceptor tyrosine-based inhibition motif (ITIM), present in the cytoplasmic domain of Fc gammaRIIB1. Using recombinant SH2 domains, we show that binding is mediated via the Src homology region 2 (SH2)-containing inositol phosphatase (SHIP) SH2 domain. SHIP also bound to a phosphopeptide derived from CD22, raising the possibility that SHIP contributes to negative signaling by this receptor as well as Fc gammaRIIB1. The association of SHIP with the ITIM phosphopeptide was activation independent, while coassociation with Shc was activation dependent. Furthermore, experiments with Fc gammaRIIB1-deficient B cells demonstrated a genetic requirement for expression of Fc gammaRIIB1 in the induction of SHIP phosphorylation and its interaction with Shc. Based on these results, we propose a model of negative signaling in which co-cross-linking of surface immunoglobulin and Fc gammaRIIB1 results in sequential tyrosine phosphorylation of the ITIM, recruitment and phosphorylation of p145SHIP, and subsequent binding of Shc.

Differential effects of B cell receptor and B cell receptor-FcgammaRIIB1 engagement on docking of Csk to GTPase-activating protein (GAP)-associated p62

The stimulatory and inhibitory pathways initiated by engagement of stimulatory receptors such as the B cell receptor for antigen (BCR) and inhibitory receptors such as Fcgamma receptors of the IIB1 type (FcgammaRIIB1) intersect in ways that are poorly understood at the molecular level. Because the tyrosine kinase Csk is a potential negative regulator of lymphocyte activation, we examined the effects of BCR and FcgammaRIIB1 engagement on the binding of Csk to phosphotyrosine-containing proteins. Stimulation of a B lymphoma cell line, A20, with intact anti-IgG antibody induced a direct, SH2-mediated association between Csk and a 62-kD phosphotyrosine-containing protein that was identified as RasGTPase-activating protein-associated p62 (GAP-A.p62). In contrast, stimulation of A20 cells with anti-IgG F(ab')2 resulted in little increase in the association of Csk with GAP-A.p62. The effect of FcgammaRIIB1 engagement on this association was abolished by blockade of FcgammaRIIB1 with the monoclonal antibody 2.4G2. Furthermore, the increased association between Csk and GAP-A.p62 seen upon stimulation with intact anti-Ig was abrogated in the FcgammaRIIB1-deficient cell line IIA1.6 and recovered when FcgammaRIIB1 expression was restored by transfection. The differential effects of BCR and BCR-FcgammaRIIB1-mediated signaling on the phosphorylation of GAP-A.p62 and its association with Csk suggest that docking of Csk to GAP-A.p62 may function in the negative regulation of antigen receptor-mediated signals in B cells.

Co-receptors of B lymphocytes

The past year has seen advances in our understanding of accessory membrane proteins that modulate the B cell response to antigen-receptor stimulation. The generation of complement receptor deficient mice has reinforced our appreciation of the importance of complement receptors in the B cell response to antigen. The association of inositol polyphosphate 5-phosphatase with FcgammaRIIB suggests another mechanism, in addition to recruitment of the phosphotyrosine phosphatase SHP-1, by which secreted immunoglobulin can limit further response to antigen. The in vivo function of CD22 in regulating the threshold of antigen-receptor signalling has been shown using CD22-deficient mice. Lastly, B cell receptor signalling in the B-1 subset of B lymphocytes has been demonstrated to be negatively regulated by CD5.

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.

Protein tyrosine phosphatases in signal transduction

Protein-tyrosyl phosphorylation, regulated by protein tyrosine kinases and protein tyrosine phosphatases (PTPs), is a key cellular control mechanism. Until recently, little was known about PTPs. However, the past two years have witnessed an explosion of information about PTP structure, regulation and function. Crystal structures of several PTPs have provided insights into enzymatic mechanisms and regulation and suggested the design of 'substrate-trapping' mutants. Candidate homophilic and heterophilic ligands for transmembrane PTPs have been identified, and roles for transmembrane PTPs in regulating cell-cell interactions have been suggested. Finally, progress has been made in understanding signaling by Src homology 2 domain containing PTPs and PTPs controlling yeast osmoregulatory pathways.

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.