FcγRIIA Activation Research Articles

Syk Is Regulated Downstream Of FcγRIIA In Platelets By Transient Tyrosine Phosphorylation and Ubiquitylation

Platelet FcγRIIA contributes to the pathophysiology of heparin-induced thrombocytopenia (HIT) and other immune-mediated thrombocytopenia and thrombosis syndromes. Activation of FcγRIIA results in tyrosine phosphorylation of Syk in human platelets, but little is known about ubiquitylation of Syk. Protein ubiquitylation has been shown to regulate physiological and pathological cellular processes by regulating signaling networks. In the Platelet RNA and Expression-1 (PRAX1) study, we studied platelets from 154 healthy subjects and identified increased expression of mRNAs encoding proteins involved in ubiquitylation in platelets that were highly reactive to FcγRIIA stimulation. Previously, we (CD, SPK) reported that platelet stimulation via GPVI/FcRγ results in transient Syk phosphorylation and ubiquitylation, involving c-cbl and TULA-2. Briefly, c-Cbl acts as an E3 ligase transferring ubiquitin to the lysine residue of the target protein in the ubiquitylation reaction and TULA-2 is a dephosphorylating enzyme that removes phosphate group from ubiquitinated syk. In this study, we investigated Syk tyrosine phosphorylation and ubiquitylation downstream of platelet FcγRIIA engagement. FcγRIIA on washed human platelets was activated by cross-linking with IV.3 antibody (10ug/ml) and goat anti-mouse antibody (GAM) (30ug/ml). At specific time intervals following activation, platelets were lysed and the lysates were immunoblotted for total Syk and for pY525/526 phospho-Syk. We observed phosphorylation and ubiquitylation of Syk within 15 sec, and peaking within 1 to 3 min. The MW of the Syk species was consistent with 1 to 3 ubiquitin (Ub) moieties per Syk molecule. Alternatively, platelet FcγRIIA was activated via anti-CD9; Ub-Syk was observed at 15 sec, while ubiqutinated and phosphorylated Syk was observed at 2 min. Additional ubiquitinated bands appeared by 2 min; all diminished after 5 min. To understand if the ubiquitinated Syk was degraded by the proteasomal system, human platelets were pre-treated with proteasomal inhibitors (MG132 or Epoxomicin) followed by activation of FcγRIIA. There was no accumulation of Ub-Syk observed for up to 5 minutes after FcγRIIA stimulation in the presence of proteasomal inhibitors compared to controls, indicating that Ub-Syk is not degraded by the proteasomes and inactivated. Activation of FcγRIIA by cross linking with IV.3/GAM in HEL cells, a cell line model, showed a transient increase in ubiquitylation and phosphorylation of Syk within 15 sec which decreased by 3 min. To understand if activation of Syk by upstream Src-family kinases is necessary for ubiquitylation in this model system, SFK inhibitor (PP2) (10uM) was pre-incubated with HEL cells followed by FcγRIIA activation. Western blot analysis showed elimination of phosphorylated and ubiquitinated Syk upon activation of FcγRIIA in HEL cells in presence of PP2. In contrast, ubiquitylation and phosphorylation of Syk was observed in control cells treated with PP3 or vehicle (DMSO) and as well as in untreated cells. HEL cells were also pretreated with proteasomal inhibitors (MG132 or Epoxomicin) followed by activation of FcγRIIA. There was no accumulation of phosphorylated and ubiquitinated Syk observed for up to 10 min of stimulation of FcγRIIA in HEL cells in presence of proteasomal inhibitors compared to controls indicating no degradation of Ub-syk by proteasomal complex. Finally, we directly compared the ubiquitylation and phosphorylation of Syk in GPVI/FcRγ-stimulated vs.FcγRIIA-stimulated platelets from the same donors. Of note, FcγRIIA-stimulated platelets demonstrated both different kinetics and extent of Syk ubiquitylation than did GPVI/FcRγ-stimulated platelets. In this study, we demonstrated that FcγRIIA signaling results in transient Syk tyrosine phosphorylation and ubiquitylation in platelets and HEL cells. The ubiquitinated Syk is not destined for proteasomal degradation. Limited ubiquitylation of proteins has been noted to modulate downstream signaling events and protein-protein interactions. Further studies are needed to decipher the molecular partners of ubiquitinated Syk following FcγRIIA activation, and to elucidate the differences between GPVI/FcRγ and FcγRIIA signaling via Syk. Disclosures: No relevant conflicts of interest to declare.

Influenza Virus-Containing Immune Complexes Activate Platelets Through FcγRIIa Resulting In Bioactive Lipid Synthesis and Microparticle Release

After red blood cells, platelets represent the most abundant cell lineage in blood, where they sentinel the vasculature and play crucial functions in haemostasis and the prevention of bleedings. Platelets also express a vast array of immune mediators and receptors, suggesting that they can also be regarded as tiny immune cells capable of the recognition of pathogens. Indeed, platelets express functional Toll-like receptors and are thought to actively participate to innate immunity. During severe cases of H1N1 influenza A virus infection, circulating platelets display markers of activation. The specific platelet activation triggers during Influenza infection remain however unknown. In this study, we incubated human platelets with H1N1 influenza A virus (IAV) and monitored platelet activation. We found that IAV is a highly potent inducer of de novo lipid mediators production and a trigger for the release of microparticles from platelets. This activation process takes place independently of the Toll-like receptor 4 and requires the presence of serum, pointing to the contribution of soluble factor(s) present in blood. We observed that the virus scaffolds with immunoglobulin G to form large immune-complexes (∼1micron in size) that activate platelets through the engagement of FcgRIIA. Accordingly, the serum of naïve mice housed in a pathogen-free facility is ineffective at initiating human platelet activation when incubated in the presence of IAV. Intrigued by the fact that all the tested human sera were capable of forming immune complexes with IAV, we hypothesized that significant antibody cross-reactivity between different influenza viruses was sufficient to promote the formation of virus-containing immune complexes. Using in vivo approaches, we found that the antibodies from H3N2 influenza virus-immunized mice generate immune complexes when put in presence of H1N1, activating human platelets and transgenic mouse platelets that express FcgRIIA. Taken together, our observations demonstrate that beyond their activities in haemostasis and innate immunity, platelets can also play a role during the active stage of adaptive immune responses against pathogens through FcγRIIa activation. Disclosures:No relevant conflicts of interest to declare.

β2-Glycoprotein 1 Antibodies Directly Activate the Platelet IgG Receptor, FcγRIIa, and Cause Thrombosis in FCGR2A Transgenic but Not in Wild Type Mice

Abstract 106Antibodies targeting β2-glycoprotein 1 (β2-GP1; β2-Abs) are of primary importance in antiphospholipid syndrome (APS), a thrombotic autoimmune disorder. The predominance of the IgG antibody isotype in APS is conspicuously associated with increased risk of thrombosis, raising the question whether the platelet IgG receptor, FcγRIIa, may play a role in thrombosis in APS, as is the case in heparin-induced thrombocytopenia (HIT). The hypothesis that platelet FcγRIIa may contribute to thrombosis in APS has received little attention, with research emphasis instead placed on several proposed alternative mechanisms of action. We have shown that, like HIT antibodies, antibodies targeting VEGF or CD154 are also potently thrombotic in mice transgenic for human FcγRIIa but have no activity in mice lacking FcγRIIa (i.e., wild type mice). We therefore asked whether antiphospholipid antibodies can activate platelets and cause thrombosis via FcγRIIa. To this end, we tested mouse monoclonal (mAb) and goat polyclonal anti-human β2-Abs alone or complexed to human β2-glycoprotein 1 (i.e., to form immune complexes, or ICs) by the serotonin release assay (SRA) and platelet aggregation methods using washed human platelets. We found that two of three commercially available polyclonal anti-β2-Abs (pAb1 and pAb3), both alone or in IC form, induced platelet granule release and aggregation, and that this activity was abolished by anti-FcγRIIa mAb, IV.3. pAb2 and mAb were inactive. Activity analysis (SRA) of preformed ICs using constant pAb1 concentration with varied β2-GP1 stoichiometries revealed a zone-of-equivalence pattern, with maximal activity near balanced stoichiometry (1:1). Because pAb2 did not activate platelets, we sought to determine its capacity to form higher order ICs (which are known to be required for FcγRIIa activation) by size exclusion chromatography (SEC). All antibodies tested in isolation were shown by HPLC-SEC to be free of aggregates or degradation products. Antibody-antigen complex size analysis revealed that, as expected, mAb+β2-GP1 in 1:1 stoichiometry failed to form higher order ICs (i.e., complexes having ≥2 IgGs/complex). pAb2 (inactive) did form higher order ICs at 1:1 and 4:1 (IgG:Ag) stoichiometries, but less extensively than pAb1 (active), which efficiently formed higher order ICs at both 1:1 and 4:1 stoichiometries. pAb1 alone (i.e., not in IC form) caused aggregation, while pAb2 did not. Furthermore, preincubation of washed platelets with pAb2 prevented pAb1-induced aggregation, suggesting that pAb1 activity is β2-GP1-specific. A single intravenous injection of anti-β2-GP1 ICs (20 μg β2-GP1 plus 120 μg pAb1, a 1:2 molar stoichiometry) induced severe thrombocytopenia (>90%) and caused thrombotic shock in FcγRIIa-transgenic mice but not in wild type mice. Symptoms of shock occurred within 10 minutes. Pervasive occlusive thrombi were observed in the lungs of all FcγRIIa-transgenic but not in any wild type mice (H&E microscopy). Injection of pAb2 ICs produced none of these effects in transgenic or wild type mice. Finally, injection of pAb1 or pAb2 alone (neither of which bind mouse β2-GP1) had no effects in transgenic mice. In summary, these findings confirm previous in vitro studies that β2-Abs can directly activate platelets in a manner wholly dependent on the platelet IgG receptor, FcγRIIa. Additionally, we have shown for the first time in vivo that β2-Abs can also cause FcγRIIa-dependent thrombosis. This mechanism may contribute to thrombosis in APS, suggesting that further studies on the importance of FcγRIIa in APS are warranted. Disclosures:No relevant conflicts of interest to declare.

Raft coalescence and FcγRIIA activation upon sphingomyelin clustering induced by lysenin

Activation of immunoreceptor FcγRIIA by cross-linking with antibodies is accompanied by coalescence of sphingolipid/cholesterol-rich membrane rafts leading to the formation of signaling platforms of the receptor. In this report we examined whether clustering of the raft lipid sphingomyelin can reciprocally induce partition of FcγRIIA to rafts. To induce sphingomyelin clustering, cells were exposed to non-lytic concentrations of GST-lysenin which specifically recognizes sphingomyelin. The lysenin/sphingomyelin complexes formed microscale assemblies composed of GST-lysenin oligomers engaging sphingomyelin of rafts. Upon sphingomyelin clustering, non-cross-linked FcγRIIA associated with raft-derived detergent-resistant membrane fractions as revealed by density gradient centrifugation. Pretreatment of cells with GST-lysenin also increased the size of detergent-insoluble molecular complexes of activated FcγRIIA. Sphingomyelin clustering triggered tyrosine phosphorylation of the receptor and its accompanying proteins, Cbl and NTAL, in the absence of receptor ligands and enhanced phosphorylation of these proteins in the ligand presence. These data indicate that clustering of plasma membrane sphingomyelin induces coalescence of rafts and triggers signaling events analogous to those caused by FcγRIIA activation.

Comparison of Platelet-Activating Potential of Bevacizumab+VEGF and Aflibercept+VEGF Complexes

AbstractAbstract 3200Anti-angiogenic therapy with the monoclonal antibody (mAb) bevacizumab (bev; Genentech) is associated with venous and arterial thrombosis in cancer patients. We recently showed that bev immune complexes (ICs) activate platelets in vitro and cause thrombotic thrombocytopenia in mice transgenic for the human platelet IgG receptor, FcγRIIa (FCGR2A mice; Jax Labs). Previous studies conducted in other labs using wild type mice injected with bevacizumab failed to identify thrombotic side effects; however, wild type mice do not have platelet FcγRIIa IgG receptors. Platelets from human donors or from FCGR2A mice are directly activated when FcγRIIa is clustered by the Fc domains of mAbs that have been assembled into higher order ICs by multivalent antigens, such as was shown for bev (Meyer, et al, J Thromb Haemost, 2009;7:171) and for anti-CD154 ICs (Robles-Carrillo, et al, J Immunol, 2010;185:1577). Aflibercept (aflib, Regeneron/sanofi-aventis, also known as VEGF Trap) is a soluble decoy receptor protein that binds all isoforms of VEGF-A including VEGF165 and inhibits VEGF165 activity. Aflib also binds the related angiogenic molecule PlGF, and like bev has anti-angiogenic activity. The aflib molecule contains the Fc domain of the IgG isotype antibody; however, although VEGF165 efficiently clusters bev into higher order ICs, VEGF165 does not cluster aflib into higher order complexes (Rudge JS, et al, PNAS, 2007;104:18363). This suggests that aflib+VEGF165 should not be able to trigger platelet FcγRIIa activation (because FcγRIIa requires clustering for activation). We therefore hypothesized that aflib+VEGF165 complexes would fail to activate platelet FcγRIIa in vitro or in FCGR2A transgenic mice. For in vitro studies, we chose the washed platelet aggregation method as a measure of IC-induced platelet activation. To assess platelet activation in vivo, we measured IC-induced reductions in the number of circulating platelets (thrombocytopenia) following intravenous injection of ICs into FCGR2A mice. Aflib+VEGF165 or bev+VEGF165 complexes were pre-formed at room temperature in the presence or absence of heparin. (Heparin can enhance bev IC-induced platelet activation. Many cancer patients are routinely exposed to heparin to maintain patency of chemotherapy ports.) In 5 donors, bev+VEGF165 complexes (500nM) rapidly induced full aggregation of human platelets in an FcγRIIa-dependent manner, whereas 500nM aflib+VEGF165 failed to induce aggregation under all conditions tested. Within 5 minutes following intravenous injection of bev+VEGF165, FCGR2A mice (n=10) experienced signs of distress consistent with thrombotic shock, including rapid shallow breathing, hunched posture, and decreased or dysfunctional locomotor activity. Animals receiving aflib complexes (n=10) did not exhibit these symptoms. Platelet counts 10 minutes following IC injection revealed thrombocytopenia in animals receiving bev+VEGF165 but not aflib+VEGF165 complexes. Specifically, animals injected with PBS (vehicle; n=5) had baseline platelet counts of (1173±179;mean±SD), whereas animals receiving bev+VEGF165 had mean platelet counts of 331±217 (with heparin; P < 0.001) and 725±303 (without heparin; P < 0.003). Animals receiving aflib+VEGF165 had mean platelet counts of 966±168 (with heparin) and 878±250 (without heparin), which were not statistically different from baseline animal counts. These results indicate that aflibercept lacks that platelet activating potential observed with bevacizumab, and also suggest that anti-angiogenic therapy with aflibercept may lack the capacity to induce the platelet associated thrombotic side effects observed in patients receiving bevacizumab. Disclosures:Meyer:Regeneron Pharmaceuticals, Inc.: Florida Hospital received research funding from Regeneron Pharmaceuticals in support of some of this work.

Differential signal transduction, membrane trafficking, and immune effector functions mediated by FcγRI versus FcγRIIa

AbstractReceptors for the fragment crystallizable region of immunoglobulin-G (FcγRs) play an important role in linking the humoral and cellular arms of the immune response. In this study, we present a comprehensive functional comparison of 2 human Fc-receptors, FcγRI and FcγRIIa. Activation of FcγRI results in a novel signaling cascade that links phospholipase D1 to sphingosine kinase-1 in U937 cells and primary human monocytes. This induces the expression of proinflammatory mediators and is associated with trafficking of immune complexes into human leukocyte antigen-DM positive antigen-processing compartments coupled with improved MHC class II–mediated antigen presentation to T lymphocytes. In contrast, activation of FcγRIIa elicits signaling through phospholipase Cγ1, resulting in increases in intracellular calcium, activation of nicotinamide adenine dinucleotide phosphate-oxidative burst, and differential membrane trafficking combined with impaired antigen presentation and proinflammatory cytokine expression. These data provide a mechanistic insight into the disparate activities associated with Fc receptors in immunity, namely, reinforcement of immune responses through stimulation of proinflammatory signaling and antigen presentation, versus the maintenance of immunologic homeostasis through the noninflammatory clearance of immune complexes.

Contribution of PIP-5 kinase Iα to raft-based FcγRIIA signaling

Receptor FcγIIA (FcγRIIA) associates with plasma membrane rafts upon activation to trigger signaling cascades leading to actin polymerization. We examined whether compartmentalization of PI(4,5)P 2 and PI(4,5)P 2-synthesizing PIP5-kinase Iα to rafts contributes to FcγRIIA signaling. A fraction of PIP5-kinase Iα was detected in raft-originating detergent-resistant membranes (DRM) isolated from U937 monocytes and other cells. The DRM of U937 monocytes contained also a major fraction of PI(4,5)P 2. PIP5-kinase Iα bound PI(4,5)P 2, and depletion of the lipid displaced PIP5-kinase Iα from the DRM. Activation of FcγRIIA in BHK transfectants led to recruitment of the kinase to the plasma membrane and enrichment of DRM in PI(4,5)P 2. Immunofluorescence studies revealed that in resting cells the kinase was associated with the plasma membrane, cytoplasmic vesicles and the nucleus. After FcγRIIA activation, PIP5-kinase Iα and PI(4,5)P 2 co-localized transiently with the activated receptor at distinct cellular locations. Immunoelectron microscopy studies revealed that PIP5-kinase Iα and PI(4,5)P 2 were present at the edges of electron-dense assemblies containing activated FcγRIIA in their core. The data suggest that activation of FcγRIIA leads to membrane rafts coalescing into signaling platforms containing PIP5-kinase Iα and PI(4,5)P 2.

The CD300a (IRp60) inhibitory receptor is rapidly up-regulated on human neutrophils in response to inflammatory stimuli and modulates CD32a (FcγRIIa) mediated signaling

To achieve an adequate response, cells of the immune system must be tightly regulated to avoid hypo or hyper responsiveness. One of the mechanisms used by the immune system to avoid excessive inflammation is the modulation of the response through inhibitory receptors containing immunoreceptor tyrosine based inhibitory motifs (ITIM). Here, we show that human neutrophils from peripheral blood express the ITIM containing CD300a (also known as IRp60 and CMRF-35H) receptor. By using the HL-60 differentiation model, we show that the expression of CD300a receptor is developmentally regulated. Stimulation of human neutrophils with LPS and GM-CSF increased the cell surface expression of CD300a as a result of the rapid translocation of an intracellular pool of the receptor to the cell surface. Co-ligation of CD300a with the immunoreceptor tyrosine based activating motif (ITAM) containing CD32a (FcγRIIa) activation receptor inhibited CD32a mediated signalling; whereas, it did not inhibit toll-like receptor (TLR)-4 mediated reactive oxygen species (ROS) production. Therefore, at least for human neutrophils, the inhibitory signals mediated by the CD300a receptor may be selective in their action.

A Novel Antibody-Dependent Cellular Cytotoxicity Mechanism Involved in Defense against Malaria Requires Costimulation of Monocytes FcγRII and FcγRIII

Clinical experiments have shown that the Ab-dependent cell-mediated inhibition of Plasmodium falciparum is a major mechanism controlling malaria parasitemia and thereby symptoms. In this study, we demonstrate that a single merozoite per monocyte (MN) is sufficient to trigger optimal antiparasitic activity. Using particulate Ag as pseudomerozoites, we show that only Ags, and no other parasite-derived factor, are required to trigger MN activation and that a single Ag is as potent as the complex combination of Ags constituting the merozoite surface. Moreover, we found that soluble Ags binding at least two Abs are as effective as the parasite at stimulating MN and that nonmalarial Ags are as efficient provided they are targeted by cytophilic Abs. Indeed, only cytophilic IgGs are potent and, in agreement with immunoepidemiological findings, IgG3 is superior to IgG1. Very low Ab concentrations (>700 pM), i.e., in the range of molecules having a hormonal effect, are effective, in contrast to Abs having a direct, neutralizing effect. Finally, Ab-dependent cell-mediated inhibition proved to require the synergistic activation of both FcgammaRIIa and FcgammaRIIIa which both distinguish it from other Ab-dependent cellular cytotoxicity and implies that all MN are not equally effective. These findings have both fundamental and practical implications, particularly for vaccine discovery.

Differential Enhancement of Dengue Virus Immune Complex Infectivity Mediated by Signaling-Competent and Signaling-Incompetent Human FcγRIA (CD64) or FcγRIIA (CD32)

Fcgamma receptor (FcgammaR)-mediated entry of infectious dengue virus immune complexes into monocytes/macrophages is hypothesized to be a key event in the pathogenesis of complicated dengue fever. FcgammaRIA (CD64) and FcgammaRIIA (CD32), which predominate on the surface of such dengue virus-permissive cells, were compared for their influence on the infectivity of dengue 2 virus immune complexes formed with human dengue virus antibodies. A signaling immunoreceptor tyrosine-based activation motif (ITAM) incorporated into the accessory gamma-chain subunit that associates with FcgammaRIA and constitutively in FcgammaRIIA is required for phagocytosis mediated by these receptors. To determine whether FcgammaRIA and FcgammaRIIA activation functions are also required for internalization of infectious dengue virus immune complexes, we generated native and signaling-incompetent versions of each receptor by site-directed mutagenesis of ITAM tyrosine residues. Plasmids designed to express these receptors were transfected into COS-7 cells, and dengue virus replication was measured by plaque assay and flow cytometry. We found that both receptors mediated enhanced dengue virus immune complex infectivity but that FcgammaRIIA appeared to do so far more effectively. Abrogation of FcgammaRIA signaling competency, either by expression without gamma-chain or by coexpression with gamma-chain mutants, was associated with significant impairment of phagocytosis and of dengue virus immune complex infectivity. Abrogation of FcgammaRIIA signaling competency was also associated with equally impaired phagocytosis but had no discernible effect on dengue virus immune complex infectivity. These findings point to fundamental differences between FcgammaRIA and FcgammaRIIA with respect to their immune-enhancing capabilities and suggest that different mechanisms of dengue virus immune complex internalization may operate between these FcgammaRs.

A New Role for FcγRIIA in the Potentiation of Human Platelet Activation Induced by Weak Stimulation.

The low affinity receptor for immunoglobulin G, FcγRIIA, is expressed in human platelets, mediates heparin-associated thrombocytopenia, and participates in platelet activation induced by von Willebrand factor. Activation of FcγRIIA occurs upon clustering of the receptor induced by immunocomplexes, and consists in the phosphorylation of two tyrosine residues within the ITAM, typically promoted by an associated Src kinase. The phosphorylated receptor acts as a docking site for SH2 domain-containing signaling proteins, including the tyrosine kinase Syk. This event initiates an intracellular tyrosine kinase-based signaling cascade that eventually leads to phosphorylation and activation of phospholipase C (PLC) γ2, and elicits cellular responses. To date, very little is known on the possible involvement of FcγRIIA in platelet activation induced by soluble agonists. We have found that stimulation of platelets with agonists acting on G-protein-coupled receptors resulted in Src-kinase-mediated tyrosine phosphorylation of FcγRIIA. Treatment of platelets with the blocking monoclonal antibody IV.3 against FcγRIIA, but not with control IgG, inhibited platelet aggregation induced by TRAP1, TRAP4, the thromboxane A2 analogue U46619, and low concentrations of thrombin. By contrast, platelet aggregation induced by high doses of thrombin was unaffected by blockade of FcγRIIA. We also found that the anti-FcγRIIA monoclonal antibody IV.3 inhibited pleckstrin phosphorylation and calcium mobilization induced by low, but not high, concentrations of thrombin. Thrombin- and U46619-induced tyrosine phosphorylation of Syk and PLCγ2, which represent substrates typically involved in FcγRIIA-mediated signaling, was clearly reduced by incubation with anti-FcγRIIA antibody IV.3. Morever, we were able to demonstrated that platelet stimulation by thrombin induced the association of FcγRIIA with Syk. Signaling through immunoreceptor typically takes places in characteristic membrane microdomains called lipid rafts. Upon stimulation with thrombin, FcγRIIA relocated in lipid rafts, and thrombin-induced tyrosine phosphorylation of FcγRIIA occurred within these membrane domains. Controlled disruption of lipid rafts by depleting membrane cholesterol prevented tyrosine phosphorylation of FcγRIIA, and impaired platelet aggregation induced by U46619 or by low, but not high, concentrations of thrombin. These results indicate that FcγRIIA can be activated in human platelets downstream G-protein-coupled receptors, and initiates a tyrosine kinase-based signaling pathway that significantly contributes to platelet activation and aggregation in response to weak stimulation.

Streptococcus sanguis-induced platelet activation involves two waves of tyrosine phosphorylation mediated by FcγRIIA and αIIbβ3

The low-affinity IgG receptor, FcgammaRIIA, has been implicated in Streptococcus sanguis-induced platelet aggregation. Therefore, it is likely that signal transduction is at least partly mediated by FcgammaRIIA activation and a tyrosine kinase-dependent pathway. In this study the signal transduction mechanisms associated with platelet activation in response to the oral bacterium, S. sanguis were characterised. In the presence of IgG, S. sanguis strain 2017-78 caused the tyrosine phosphorylation of FcgammaRIIA 30s following stimulation, which led to the phosphorylation of Syk, LAT, and PLCgamma2. These early events were dependent on Src family kinases but independent of either TxA(2) or the engagement of the alpha(IIb)beta(3) integrin. During the lag phase prior to platelet aggregation, FcgammaRIIA, Syk, LAT, and PLCgamma2 were each dephosphorylated, but were re-phosphorylated as aggregation occurred. Platelet stimulation by 2017-78 also induced the tyrosine phosphorylation of PECAM-1, an ITIM-containing receptor that recruits protein tyrosine phosphatases. PECAM-1 co-precipitated with the protein tyrosine phosphatase SHP-1 in the lag phase. SHP-1 was also maximally tyrosine phosphorylated during this phase, suggesting a possible role for SHP-1 in the observed dephosphorylation events. As aggregation occurred, SHP-1 was dephosphorylated, while FcgammaRIIA, Syk, LAT, and PLCgamma2 were rephosphorylated in an RGDS-sensitive, and therefore alpha(IIb)beta(3)-dependent, manner. Additionally, TxA(2) release, 5-hydroxytryptamine secretion and phosphatidic acid formation were all blocked by RGDS. Aspirin also abolished these events, but only partially inhibited alpha(IIb)beta(3) -mediated re-phosphorylation. Therefore, S. sanguis -bound IgG cross links FcgammaRIIA and initiates a signaling pathway that is down-regulated by PECAM-1-bound SHP-1. Subsequent engagement of alpha(IIb)beta(3) leads to SHP-1 dephosphorylation permiting a second wave of signaling leading to TxA(2) release and consequent platelet aggregation.

Early events in the activation of Fc gamma RIIA in human neutrophils: stimulated insolubilization, translocation to detergent-resistant domains, and degradation of Fc gamma RIIA.

The signal transduction mechanisms associated with the ligation of FcgammaRIIA in human neutrophils are as yet only incompletely characterized. In the present study, we have investigated the distribution and fate of FcgammaRIIA following its cross-linking. The results obtained indicate that cross-linking of FcgammaRIIA led, within a few seconds, to its translocation into a nonionic detergent-insoluble fraction. This was followed, within a couple of minutes, by a substantial loss of immunoreactive FcgammaRIIA in the cells. The stimulated degradation of FcgammaRIIA was blocked by the Src kinase inhibitor PP1 but not by wortmannin, ST-638, piceatannol, or cytochalasin B. Cross-linked FcgammaRIIA could be solubilized by saponin (in the presence of Nonidet P-40) and by beta-octylglucoside. Sucrose gradient analysis of the distribution of FcgammaRIIA revealed that its cross-linking led to its translocation into the pellets and not the light buoyant density fractions classically associated with lipid rafts. Disruption of cholesterol-containing membrane microdomains with filipin prevented the degradation of FcgammaRIIA but did not inhibit the stimulation of the pattern of tyrosine phosphorylation or the mobilization of calcium that followed FcgammaRIIA cross-linking. These data suggest that both cholesterol-rich domains and Src kinases are required for the degradation of the activated FcgammaRIIA and provide new insights into the early events following FcgammaRIIA cross-linking.