Activation of Fc gamma RII induces tyrosine phosphorylation of multiple proteins including Fc gamma RII

Next generation of multispecific antibody engineering

Multispecific antibodies recognize two or more epitopes located on the same or distinct targets. This added capability through protein design allows these man-made molecules to address unmet medical needs that are no longer possible with single targeting such as with monoclonal antibodies or cytokines alone. However, the approach to the development of these multispecific molecules has been met with numerous road bumps, which suggests that a new workflow for multispecific molecules is required. The investigation of the molecular basis that mediates the successful assembly of the building blocks into non-native quaternary structures will lead to the writing of a playbook for multispecifics. This is a must do if we are to design workflows that we can control and in turn predict success. Here, we reflect on the current state-of-the-art of therapeutic biologics and look at the building blocks, in terms of proteins, and tools that can be used to build the foundations of such a next-generation workflow.

Material matters: exploring the interplay between natural biomaterials and host immune system

Biomaterials are widely used for various medical purposes, for instance, implants, tissue engineering, medical devices, and drug delivery systems. Natural biomaterials can be obtained from proteins, carbohydrates, and cell-specific sources. However, when these biomaterials are introduced into the body, they trigger an immune response which may lead to rejection and failure of the implanted device or tissue. The immune system recognizes natural biomaterials as foreign substances and triggers the activation of several immune cells, for instance, macrophages, dendritic cells, and T cells. These cells release pro-inflammatory cytokines and chemokines, which recruit other immune cells to the implantation site. The activation of the immune system can lead to an inflammatory response, which can be beneficial or detrimental, depending on the type of natural biomaterial and the extent of the immune response. These biomaterials can also influence the immune response by modulating the behavior of immune cells. For example, biomaterials with specific surface properties, such as charge and hydrophobicity, can affect the activation and differentiation of immune cells. Additionally, biomaterials can be engineered to release immunomodulatory factors, such as anti-inflammatory cytokines, to promote a tolerogenic immune response. In conclusion, the interaction between biomaterials and the body's immune system is an intricate procedure with potential consequences for the effectiveness of therapeutics and medical devices. A better understanding of this interplay can help to design biomaterials that promote favorable immune responses and minimize adverse reactions.

Tight nanoscale clustering of Fcγ receptors using DNA origami promotes phagocytosis

Macrophages destroy pathogens and diseased cells through Fcγ receptor (FcγR)-driven phagocytosis of antibody-opsonized targets. Phagocytosis requires activation of multiple FcγRs, but the mechanism controlling the threshold for response is unclear. We developed a DNA origami-based engulfment system that allows precise nanoscale control of the number and spacing of ligands. When the number of ligands remains constant, reducing ligand spacing from 17.5 nm to 7 nm potently enhances engulfment, primarily by increasing efficiency of the engulfment-initiation process. Tighter ligand clustering increases receptor phosphorylation, as well as proximal downstream signals. Increasing the number of signaling domains recruited to a single ligand-receptor complex was not sufficient to recapitulate this effect, indicating that clustering of multiple receptors is required. Our results suggest that macrophages use information about local ligand densities to make critical engulfment decisions, which has implications for the mechanism of antibody-mediated phagocytosis and the design of immunotherapies.

Microvascular thrombosis: experimental and clinical implications

A significant amount of clinical and research interest in thrombosis is focused on large vessels (eg, stroke, myocardial infarction, deep venous thrombosis, etc.); however, thrombosis is often present in the microcirculation in a variety of significant human diseases, such as disseminated intravascular coagulation, thrombotic microangiopathy, sickle cell disease, and others. Further, microvascular thrombosis has recently been demonstrated in patients with COVID-19, and has been proposed to mediate the pathogenesis of organ injury in this disease. In many of these conditions, microvascular thrombosis is accompanied by inflammation, an association referred to as thromboinflammation. In this review, we discuss endogenous regulatory mechanisms that prevent thrombosis in the microcirculation, experimental approaches to induce microvascular thrombi, and clinical conditions associated with microvascular thrombosis. A greater understanding of the links between inflammation and thrombosis in the microcirculation is anticipated to provide optimal therapeutic targets for patients with diseases accompanied by microvascular thrombosis.

Cleavage of anti-PF4/heparin IgG by a bacterial protease and potential benefit in heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is due to immunoglobulin G (IgG) antibodies, which bind platelet factor 4 (PF4) modified by polyanions, such as heparin (H). IgG/PF4/polyanion complexes directly activate platelets via Fc gamma type 2 receptor A (FcγRIIA) receptors. A bacterial protease, IgG-degrading enzyme of Streptococcus pyogenes (IdeS), cleaves the hinge region of heavy-chain IgG, abolishing its ability to bind FcγR, including FcγRIIA. We evaluated whether cleavage of anti-PF4/H IgG by IdeS could suppress the pathogenicity of HIT antibodies. IdeS quickly cleaved purified 5B9, a monoclonal chimeric anti-PF4/H IgG1, which led to the formation of single cleaved 5B9 (sc5B9), without any reduction in binding ability to the PF4/H complex. However, as compared with uncleaved 5B9, the affinity of sc5B9 for platelet FcγRIIA was greatly reduced, and sc5B9 was also unable to induce heparin-dependent platelet activation. In addition, incubating IdeS in whole blood containing 5B9 or HIT plasma samples led to cleavage of anti-PF4/H antibodies, which fully abolished the ability to induce heparin-dependent platelet aggregation and tissue factor messenger RNA synthesis by monocytes. Also, when whole blood was perfused in von Willebrand factor-coated microfluidic channels, platelet aggregation and fibrin formation induced by 5B9 with heparin was strongly reduced after IdeS treatment. Finally, IdeS prevented thrombocytopenia and hypercoagulability induced by 5B9 with heparin in transgenic mice expressing human PF4 and FcγRIIA receptors. In conclusion, cleavage of anti-PF4/H IgG by IdeS abolishes heparin-dependent cellular activation induced by HIT antibodies. IdeS injection could be a potential treatment of patients with severe HIT.

Intracellular antibody signalling is regulated by phosphorylation of the Fc receptor TRIM21

Cell surface Fc receptors activate inflammation and are tightly controlled to prevent autoimmunity. Antibodies also simulate potent immune signalling from inside the cell via the cytosolic antibody receptor TRIM21, but how this is regulated is unknown. Here we show that TRIM21 signalling is constitutively repressed by its B-Box domain and activated by phosphorylation. The B-Box occupies an E2 binding site on the catalytic RING domain by mimicking E2-E3 interactions, inhibiting TRIM21 ubiquitination and preventing immune activation. TRIM21 is derepressed by IKKβ and TBK1 phosphorylation of an LxxIS motif in the RING domain, at the interface with the B-Box. Incorporation of phosphoserine or a phosphomimetic within this motif relieves B-Box inhibition, promoting E2 binding, RING catalysis, NF-κB activation and cytokine transcription upon infection with DNA or RNA viruses. These data explain how intracellular antibody signalling is regulated and reveal that the B-Box is a critical regulator of RING E3 ligase activity.

Antibodies are molecules made by the immune system that protect us from infections. They were discovered over 100 years ago, and for most of that time scientists thought they only worked in the bloodstream. Yet recent research showed that when a virus infects our cells it also carries antibodies in with it. Once inside the cell, a protein called TRIM21 recognises the antibody-covered virus and – together with other proteins called ubiquitin enzymes – targets it for destruction via the cell’s waste disposal system. At the same time TRIM21 sends a signal to the cell’s nucleus to activate certain genes that protect cells across the body from subsequent infection.

The genes activated by TRIM21 have potent antiviral activity. Yet they can also damage the body’s own tissues if they are switched on at the wrong time, which may lead to autoimmune diseases like rheumatoid arthritis and multiple sclerosis. It is thus critical that TRIM21 is carefully regulated and only activated during an infection, but it was not clear how this is achieved.

Dickson, Fletcher et al. now show that although TRIM21 is made all the time and is always ready to detect an incoming virus it is made in an inactive state. This is because part of TRIM21, called a B-Box, inhibits the protein’s own activity. This was an unexpected discovery because, although the B-Box domain is found in around 100 other human proteins, it was unclear what it did. Dickson, Fletcher et al. then combined structural biology and biochemical approaches to show that the B-Box mimics specific enzymes that work with TRIM21, and blocks them from binding to it. This keeps TRIM21 in an inactive state.

Next, Dickson, Fletcher et al. discovered that TRIM21 becomes active when a chemical tag, specifically a phosphate group, is added to the protein. This modification displaces the B-Box, allowing ubiquitin enzymes to bind to TRIM21 and switch on its activity. Further experiments then showed that this process helps regulate TRIM21 during infections with different viruses, including rhinovirus – the virus behind the common cold – and adenovirus – a common cause of respiratory infection.

Antibodies are now used to treat many medical conditions, but present technologies are based on our understanding of how antibodies work outside cells. By revealing the basis of antibody immunity inside cells, these new findings may lead to new treatments for a range of conditions. Future studies could also explore how failures in the TRIM21 system contribute to autoimmune diseases.

CD32a antibodies induce thrombocytopenia and type II hypersensitivity reactions in FCGR2A mice

The CD32a immunoglobulin G (IgG) receptor (Fcγ receptor IIa) is a potential therapeutic target for diseases in which IgG immune complexes (ICs) mediate inflammation, such as heparin-induced thrombocytopenia, rheumatoid arthritis, and systemic lupus erythematosus. Monoclonal antibodies (mAbs) are a promising strategy for treating such diseases. However, IV.3, perhaps the best characterized CD32a-blocking mAb, was recently shown to induce anaphylaxis in immunocompromised "3KO" mice. This anaphylactic reaction required a human CD32a transgene because mice lack an equivalent of this gene. The finding that IV.3 induces anaphylaxis in CD32a-transgenic mice was surprising because IV.3 had long been thought to lack the intrinsic capacity to trigger cellular activation via CD32a. Such an anaphylactic reaction would also limit potential therapeutic applications of IV.3. In the present study, we examine the molecular mechanisms by which IV.3 induces anaphylaxis. We now report that IV.3 induces anaphylaxis in immunocompetent CD32a-transgenic "FCGR2A" mice, along with the novel finding that IV.3 and 2 other well-characterized CD32a-blocking mAbs, AT-10 and MDE-8, also induce severe thrombocytopenia in FCGR2A mice. Using recombinant variants of these same mAbs, we show that IgG "Fc" effector function is necessary for the induction of anaphylaxis and thrombocytopenia in FCGR2A mice. Variants of these mAbs lacking the capacity to activate mouse IgG receptors not only failed to induce anaphylaxis or thrombocytopenia, but also very potently protected FCGR2A mice from near lethal doses of IgG ICs. Our findings show that effector-deficient IV.3, AT-10, and MDE-8 are promising candidates for developing therapeutic mAbs to treat CD32a-mediated diseases.

Platelet Activation and Thrombus Formation over IgG Immune Complexes Requires Integrin αIIbβ3 and Lyn Kinase

IgG immune complexes contribute to the etiology and pathogenesis of numerous autoimmune disorders, including heparin-induced thrombocytopenia, systemic lupus erythematosus, rheumatoid- and collagen-induced arthritis, and chronic glomerulonephritis. Patients suffering from immune complex-related disorders are known to be susceptible to platelet-mediated thrombotic events. Though the role of the Fc receptor, FcγRIIa, in initiating platelet activation is well understood, the role of the major platelet adhesion receptor, integrin αIIbβ3, in amplifying platelet activation and mediating adhesion and aggregation downstream of encountering IgG immune complexes is poorly understood. The goal of this investigation was to gain a better understanding of the relative roles of these two receptor systems in immune complex-mediated thrombotic complications. Human platelets, and mouse platelets genetically engineered to differentially express FcγRIIa and αIIbβ3, were allowed to interact with IgG-coated surfaces under both static and flow conditions, and their ability to spread and form thrombi evaluated in the presence and absence of clinically-used fibrinogen receptor antagonists. Although binding of IgG immune complexes to FcγRIIa was sufficient for platelet adhesion and initial signal transduction events, platelet spreading and thrombus formation over IgG-coated surfaces showed an absolute requirement for αIIbβ3 and its ligands. Tyrosine kinases Lyn and Syk were found to play key roles in IgG-induced platelet activation events. Taken together, our data suggest a complex functional interplay between FcγRIIa, Lyn, and αIIbβ3 in immune complex-induced platelet activation. Future studies may be warranted to determine whether patients suffering from immune complex disorders might benefit from treatment with anti-αIIbβ3-directed therapeutics.

A PKC-SHP1 signaling axis desensitizes Fcγ receptor signaling by reducing the tyrosine phosphorylation of CBL and regulates FcγR mediated phagocytosis

BACKGROUND

Fcγ receptors mediate important biological signals in myeloid cells including the ingestion of microorganisms through a process of phagocytosis. It is well-known that Fcγ receptor (FcγR) crosslinking induces the tyrosine phosphorylation of CBL which is associated with FcγR mediated phagocytosis, however how signaling molecules coordinate to desensitize these receptors is unclear. An investigation of the mechanisms involved in receptor desensitization will provide new insight into potential mechanisms by which signaling molecules may downregulate tyrosine phosphorylation dependent signaling events to terminate important signaling processes.

RESULTS

Using the U937IF cell line, we observed that FcγR1 crosslinking induces the tyrosine phosphorylation of CBL, which is maximal at 5 min. followed by a kinetic pattern of dephosphorylation. An investigation of the mechanisms involved in receptor desensitization revealed that pretreatment of U937IF or J774 cells with PMA followed by Fcγ receptor crosslinking results in the reduced tyrosine phosphorylation of CBL and the abrogation of downstream signals, such as CBL-CRKL binding, Rac-GTP activation and the phagocytic response. Pretreatment of J774 cells with GF109203X, a PKC inhibitor was observed to block dephosphorylation of CBL and rescued the phagocytic response. We demonstrate that the PKC induced desensitization of FcγR/ phagocytosis is associated with the inactivation of Rac-GTP, which is deactivated in a hematopoietic specific phosphatase SHP1 dependent manner following ITAM stimulation. The effect of PKC on FcγR signaling is augmented by the transfection of catalytically active SHP1 and not by the transfection of catalytic dead SHP1 (C124S).

CONCLUSIONS

Our results suggest a functional model by which PKC interacts with SHP1 to affect the phosphorylation state of CBL, the activation state of Rac and the negative regulation of ITAM signaling i.e. Fcγ receptor mediated phagocytosis. These findings suggest a mechanism for Fcγ receptor desensitization by which a serine-threonine kinase e.g. PKC downregulates tyrosine phosphorylation dependent signaling events via the reduced tyrosine phosphorylation of the complex adapter protein, CBL.

Bacillus anthracis peptidoglycan activates human platelets through FcγRII and complement

Platelet activation frequently accompanies sepsis and contributes to the sepsis-associated vascular leakage and coagulation dysfunction. Our previous work has implicated peptidoglycan (PGN) as an agent causing systemic inflammation in gram-positive sepsis. We used flow cytometry and fluorescent microscopy to define the effects of PGN on the activation of human platelets. PGN induced platelet aggregation, expression of the activated form of integrin αIIbβ3, and exposure of phosphatidylserine (PS). These changes were dependent on immunoglobulin G and were attenuated by the Fcγ receptor IIa-blocking antibody IV.3, suggesting they are mediated by PGN-anti-PGN immune complexes signaling through Fcγ receptor IIa. PS exposure was not blocked by IV.3 but was sensitive to inhibitors of complement activation. PGN was a potent activator of the complement cascade in human plasma and caused deposition of C5b-9 on the platelet surface. Platelets with exposed PS had greatly accelerated prothrombinase activity. We conclude that PGN derived from gram-positive bacteria is a potent platelet agonist when complexed with anti-PGN antibody and could contribute to the coagulation dysfunction accompanying gram-positive infections.

FcγRIIa proteolysis as a diagnostic biomarker for heparin-induced thrombocytopenia

BACKGROUND

A significant challenge in the management of heparin-induced thrombocytopenia (HIT) patients is making a timely and accurate diagnosis. The readily available enzyme immunoassays (EIAs) have low specificities. In contrast, platelet activation assays have higher specificities, but they are technically demanding and not widely available. In addition, ~ 10% of samples referred for HIT testing are initially classified as indeterminate by the serotonin release assay (SRA), which further delays accurate diagnosis. HIT is characterized by platelet activation, which leads to FcγRIIa proteolysis. This raises the possibility that identification of the proteolytic fragment of FcγRIIa could serve as a surrogate marker for HIT.

OBJECTIVES

To determine the specificity of platelet FcγRIIa proteolysis induced by sera from patients with HIT, and to correlate the results with those of the SRA.

METHODS/PATIENTS

Sera from HIT patients and control patients with other thrombocytopenic/prothrombotic disorders were tested for their ability to proteolyse FcγRIIa. The results were correlated with anti-platelet factor 4 (PF4)/heparin antibodies (EIA), and heparin-dependent platelet activation (SRA).

RESULTS

Only HIT patient samples (20/20) caused heparin-dependent FcγRIIa proteolysis, similar to what was shown by the SRA. None of the samples from the other patient groups or hospital controls caused FcγRIIa proteolysis. Among nine additional samples that tested indeterminate in the SRA, FcγRIIa proteolysis resolved five samples that had a positive anti-PF4/heparin EIA result; three had no FcγRIIa proteolysis, and two were shown to have heparin-dependent FcγRIIa proteolysis

CONCLUSIONS

This study suggests that heparin-dependent FcγRIIa proteolysis is at least as specific as the SRA for the diagnosis of HIT.

The role of Src kinase in macrophage-mediated inflammatory responses

Src kinase (Src) is a tyrosine protein kinase that regulates cellular metabolism, survival, and proliferation. Many studies have shown that Src plays multiple roles in macrophage-mediated innate immunity, such as phagocytosis, the production of inflammatory cytokines/mediators, and the induction of cellular migration, which strongly implies that Src plays a pivotal role in the functional activation of macrophages. Macrophages are involved in a variety of immune responses and in inflammatory diseases including rheumatoid arthritis, atherosclerosis, diabetes, obesity, cancer, and osteoporosis. Previous studies have suggested roles for Src in macrophage-mediated inflammatory responses; however, recently, new functions for Src have been reported, implying that Src functions in macrophage-mediated inflammatory responses that have not been described. In this paper, we discuss recent studies regarding a number of these newly defined functions of Src in macrophage-mediated inflammatory responses. Moreover, we discuss the feasibility of Src as a target for the development of new pharmaceutical drugs to treat macrophage-mediated inflammatory diseases. We provide insights into recent reports regarding new functions for Src that are related to macrophage-related inflammatory responses and the development of novel Src inhibitors with strong immunosuppressive and anti-inflammatory properties, which could be applied to various macrophage-mediated inflammatory diseases.

Cross-presentation of IgG-containing immune complexes

IgG is a molecule that functionally combines facets of both innate and adaptive immunity and therefore bridges both arms of the immune system. On the one hand, IgG is created by adaptive immune cells, but can be generated by B cells independently of T cell help. On the other hand, once secreted, IgG can rapidly deliver antigens into intracellular processing pathways, which enable efficient priming of T cell responses towards epitopes from the cognate antigen initially bound by the IgG. While this process has long been known to participate in CD4(+) T cell activation, IgG-mediated delivery of exogenous antigens into a major histocompatibility complex (MHC) class I processing pathway has received less attention. The coordinated engagement of IgG with IgG receptors expressed on the cell-surface (FcγR) and within the endolysosomal system (FcRn) is a highly potent means to deliver antigen into processing pathways that promote cross-presentation of MHC class I and presentation of MHC class II-restricted epitopes within the same dendritic cell. This review focuses on the mechanisms by which IgG-containing immune complexes mediate such cross-presentation and the implications that this understanding has for manipulation of immune-mediated diseases that depend upon or are due to the activities of CD8(+) T cells.

ITAM receptor-mediated generation of reactive oxygen species in human platelets occurs via Syk-dependent and Syk-independent pathways

BACKGROUND

Ligation of the platelet-specific collagen receptor, GPVI/FcRγ, causes rapid, transient disulfide-dependent homodimerization, and the production of intracellular reactive oxygen species (ROS) generated by the NADPH oxidase, linked to GPVI via TRAF4.

OBJECTIVES

The aim of this study was to evaluate the role of early signaling events in ROS generation following engagement of either GPVI/FcRγ or a second immunoreceptor tyrosine-based activation motif (ITAM)-containing receptor on platelets, FcγRIIa.

METHODS AND RESULTS

Using an H(2) DCF-DA-based flow cytometric assay to measure intracellular ROS, we show that treatment of platelets with either the GPVI agonists, collagen-related peptide (CRP) or convulxin (Cvx), or the FcγRIIa agonist 14A2, increased intraplatelet ROS; other platelet agonists such as ADP and TRAP did not. Basal ROS in platelet-rich plasma from 14 healthy donors displayed little inter-individual variability. CRP, Cvx or 14A2 induced an initial burst of ROS within 2 min followed by additional ROS reaching a plateau after 15-20 min. The Syk inhibitor BAY61-3606, which blocks ITAM-dependent signaling, had no effect on the initial ROS burst, but completely inhibited the second phase.

CONCLUSIONS

Together, these results show for the first time that ROS generation downstream of GPVI or FcγRIIa consists of two distinct phases: an initial Syk-independent burst followed by additional Syk-dependent generation.

The Syk-kinase inhibitor R406 impairs platelet activation and monocyte tissue factor expression triggered by heparin-PF4 complex directed antibodies

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a rare but severe complication of heparin therapy in which immunoglobulin G (IgG) antibodies against the platelet factor 4-heparin complex activate platelets through the FcγRIIA receptor. Clustering of FcγRIIA initiates signaling cascades involving tyrosine kinases including the spleen tyrosine kinase (Syk). Moreover, besides the critical role of platelets, the expression of tissue factor (TF) by human monocytes triggered by HIT antibodies has been shown to contribute to the hypercoagulability and the thrombotic complications in HIT patients.

OBJECTIVES

We investigated the effect of R406, a small molecule inhibitor of Syk developed as a potential treatment of autoimmune diseases, allergic disorders and B-cell related hematological malignancies, on FcγRIIA-mediated platelet activation. To further assess the potential activity of Syk inhibitors in HIT treatment, the effect of R406 was also evaluated on HIT antibodies-induced expression of TF and procoagulant activity of monocytic cells.

RESULTS

We show that R406 is a potent inhibitor of platelet signaling and functions initiated by FcγRIIA cross-linking by specific antibodies or by sera from HIT patients. Syk inhibition efficiently prevents FcγRIIA-induced LAT phosphorylation and activation of phosphoinositide 3-kinase, Akt, phospholipase Cγ2 and p38 MAP-kinase. As a consequence, FcγRIIA-induced platelet aggregation, granule secretion and microparticles production are strongly inhibited by R406. Moreover, the Syk inhibitor efficiently impairs the expression of TF and the procoagulant activity of human monocytes triggered by HIT antibodies.

CONCLUSION

Syk inhibitors may be of therapeutic interest in the treatment of HIT by reducing HIT antibodies-mediated platelet activation and monocyte procoagulant activity.

C-reactive protein-derived peptide 201-206 inhibits neutrophil adhesion to endothelial cells and platelets through CD32

The role of CRP as a regulator of inflammation is not fully understood. Structural rearrangement in CRP results in expression of potent proinflammatory actions. Proteolysis of CRP yields the C-terminal peptide Lys(201)-Pro-Gln-Leu-Trp-Pro(206). Here, we investigated the impact of this peptide on neutrophil interactions with endothelial cells and platelets, critical inflammatory events triggering acute coronary artery disease. CRP peptide 201-206 induced L-selectin shedding from human neutrophils and inhibited L-selectin-mediated neutrophil adhesion to TNF-α-activated HCAECs under nonstatic conditions. CRP peptide 201-206 also attenuated shear-induced up-regulation of platelet P-selectin expression, platelet capture of neutrophils, and subsequent homotypic neutrophil adhesion in human whole blood. Anti-CD32 but not anti-CD16 or anti-CD64 mAb effectively prevented the inhibitory actions of CRP peptide 201-206. Substitution of Lys(201), Gln(203), or Trp(205) with Ala in CRP peptide 201-206 resulted in loss of the biological activities, whereas peptides in which Pro(202), Leu(204), or Pro(206) was substituted with Ala retained biological activity. We identified amino acid residues involved in CRP peptide 201-206-FcγRII (CD32) interactions, which mediate potent antineutrophil and antiplatelet adhesion actions, and these findings open up new perspectives for limiting inflammation and thrombosis underlying coronary artery disease.

Regulation of tyrosine phosphorylation in macrophage phagocytosis and chemotaxis

Macrophages display a large variety of surface receptors that are critical for their normal cellular functions in host defense, including finding sites of infection (chemotaxis) and removing foreign particles (phagocytosis). However, inappropriate regulation of these processes can lead to human diseases. Many of these receptors utilize tyrosine phosphorylation cascades to initiate and terminate signals leading to cell migration and clearance of infection. Actin remodeling dominates these processes and many regulators have been identified. This review focuses on how tyrosine kinases and phosphatases regulate actin dynamics leading to macrophage chemotaxis and phagocytosis.

Involvement of the Fc gamma receptor IIA cytoplasmic domain in antibody-dependent enhancement of dengue virus infection

Sub-neutralizing concentrations of antibody to dengue virus (DENV) enhance DENV infection of Fc gamma receptor-expressing cells. This phenomenon, referred to as antibody-dependent enhancement (ADE), has been hypothesized to be responsible for the severe form of DENV infection, including dengue haemorrhagic fever and dengue shock syndrome. To analyse further the mechanisms of ADE in vitro, this study introduced a series of cytoplasmic mutants into human Fc gammaRIIA. The mutated Fc gammaRIIA was then expressed on COS-7 cells to see whether these mutants could enhance DENV infection. Wild-type Fc gammaRIIA enhanced DENV infection, consistent with previous reports using Fc gammaR-positive monocytes. Disruption of the immune tyrosine activation motif (ITAM) in the cytoplasmic domain of Fc gammaRIIA or removing the sequences between the two ITAM regions eliminated ADE. These findings suggest that the specific structure of the Fc gammaRIIA cytoplasmic domain is essential for the ability of Fc gammaRIIA to mediate ADE.

The new tyrosine-kinase inhibitor and anticancer drug dasatinib reversibly affects platelet activation in vitro and in vivo

Dasatinib is an oral potent adenosine triphosphate (ATP)-competitive inhibitor of BCR-ABL, cKIT, platelet-derived growth factor receptor, and SRC family kinases (SFKs), which has demonstrated high efficiency in patients with imatinib-resistant chronic myelogenous leukemia. Here, we show that dasatinib weakly affects platelet activation by thrombin or adenosine diphosphate but is a potent inhibitor of platelet signaling and functions initiated by collagen or FcgammaRIIA cross-linking, which require immunoreceptor tyrosine-based activation motif phosphorylation by SFKs. Accordingly, dasatinib treatment rapidly decreases the volume of thrombi formed under arterial flow conditions in whole blood from patients or mice perfused over a matrix of collagen. Moreover, treatment of mice with dasatinib increases the tail bleeding time in a dose-dependent manner. Interestingly, these effects are rapidly reversible after interruption of the treatment. Our data clearly demonstrate that, in contrast to imatinib, dasatinib affects platelet functions in vitro and in vivo, which has important implications in clinic and could explain increased risks of bleeding observed in patients. Moreover, dasatinib efficiently prevents platelet activation mediated by FcgammaRIIA cross-linking and by sera from patients with heparin-induced thrombocytopenia, suggesting that reversible antiplatelet agents acting as ATP-competitive inhibitors of SFKs may be of therapeutic interest in the treatment of this pathology.

Eptifibatide-induced thrombocytopenia and thrombosis in humans require FcgammaRIIa and the integrin beta3 cytoplasmic domain

Thrombocytopenia and thrombosis following treatment with the integrin alphaIIbbeta3 antagonist eptifibatide are rare complications caused by patient antibodies specific for ligand-occupied alphaIIbbeta3. Whether such antibodies induce platelet clearance by simple opsonization, by inducing mild platelet activation, or both is poorly understood. To gain insight into the mechanism by which eptifibatide-dependent antibodies initiate platelet clearance, we incubated normal human platelets with patient serum containing an alphaIIbbeta3-specific, eptifibatide-dependent antibody. We observed that in the presence of eptifibatide, patient IgG induced platelet secretion and aggregation as well as tyrosine phosphorylation of the integrin beta3 cytoplasmic domain, the platelet FcgammaRIIa Fc receptor, the protein-tyrosine kinase Syk, and phospholipase Cgamma2. Each activation event was inhibited by preincubation of the platelets with Fab fragments of the FcgammaRIIa-specific mAb IV.3 or with the Src family kinase inhibitor PP2. Patient serum plus eptifibatide did not, however, activate platelets from a patient with a variant form of Glanzmann thrombasthenia that expressed normal levels of FcgammaRIIa and the alphaIIbbeta3 complex but lacked most of the beta3 cytoplasmic domain. Taken together, these data suggest a novel mechanism whereby eptifibatide-dependent antibodies engage the integrin beta3 subunit such that FcgammaRIIa and its downstream signaling components become activated, resulting in thrombocytopenia and a predisposition to thrombosis.

FcgammaRII mediates platelet aggregation caused by disintegrins and GPIIb/IIIa monoclonal antibody, AP2

OBJECTIVE

Disintegrins, snake venom-derived Arg-Gly-Asp (RGD)-containing polypeptides, and GPIIb/IIIa antagonist (AP2) block fibrinogen binding to GPIIb/IIIa of activated platelets, however, the combination of these two agents caused platelet aggregation. We hypothesize that disintegrin initially binds to specific epitope of GPIIb/IIIa, causing conformational change, and the recruitment of FcgammaRII, which can be bound by AP2, and finally triggering platelet aggregation.

MATERIALS AND METHODS

We prepared human platelet suspensions and measured platelet aggregation, Ca2+ mobilization, thromboxane B2 formation, and signal transduction.

RESULTS

Disintegrin (e.g., accutin) and AP2 (a monoclonal antibody [mAb]-raised against GPIIb/IIIa) individually inhibited human platelet aggregation caused by collagen. However, as both accutin and AP2 were sequentially added into platelet suspension, platelet aggregation occurred. Accutin/AP2 caused shape change, cytosolic Ca2+ mobilization, P-selectin expression, and thromboxane A2 formation. Tirofiban, FcgammaRII mAb, or indomethacin completely inhibited platelet aggregation caused by accutin/AP2. Accutin/AP2 also caused tyrosine phosphorylation of signal molecules. Disintegrins enhanced AP2 binding to platelets, and AP2 also promoted disintegrin binding to platelets. FcgammaRII mAb inhibited the enhanced fluorescein isothiocyanate-disintegrin binding to platelet caused by AP2. Immunoprecipitation of the lysates of disintegrin/AP2-treated platelets using FcgammaRII Ab showed complex formation of GPIIb/IIIa and FcgammaRII.

CONCLUSION

FcgammaRII mediates platelet aggregation caused by disintegrin and AP2, triggering a phospholipase C, phospholipase A2, Src-, Syk kinases, and Ca2+-dependent activation process. AP2 triggers platelet aggregation via binding to accessible FcgammaRII and the conformation-altered GPIIb/IIIa caused by disintegrin.

Inhibition of "self" engulfment through deactivation of myosin-II at the phagocytic synapse between human cells

Phagocytosis of foreign cells or particles by macrophages is a rapid process that is inefficient when faced with “self” cells that display CD47—although signaling mechanisms in self-recognition have remained largely unknown. With human macrophages, we show the phagocytic synapse at cell contacts involves a basal level of actin-driven phagocytosis that, in the absence of species-specific CD47 signaling, is made more efficient by phospho-activated myosin. We use “foreign” sheep red blood cells (RBCs) together with CD47-blocked, antibody-opsonized human RBCs in order to visualize synaptic accumulation of phosphotyrosine, paxillin, F-actin, and the major motor isoform, nonmuscle myosin-IIA. When CD47 is functional, the macrophage counter-receptor and phosphatase-activator SIRPα localizes to the synapse, suppressing accumulation of phosphotyrosine and myosin without affecting F-actin. On both RBCs and microbeads, human CD47 potently inhibits phagocytosis as does direct inhibition of myosin. CD47–SIRPα interaction initiates a dephosphorylation cascade directed in part at phosphotyrosine in myosin. A point mutation turns off this motor's contribution to phagocytosis, suggesting that self-recognition inhibits contractile engulfment.

The low-molecular-weight phosphotyrosine phosphatase is a negative regulator of FcγRIIA-mediated cell activation

Activation of human platelets by cross-linking of the low-affinity receptor for immunoglobulin G (FcγRIIA) is initiated by Src kinase–mediated phosphorylation of the immunoreceptor tyrosine–based activation motif (ITAM) within the receptor, but the identity of the enzyme responsible for its dephosphorylation and inactivation is unknown. Here we report that the 18-kDa low-molecular-weight phosphotyrosine phosphatase (LMW-PTP) is expressed in human platelets and undergoes subcellular redistribution upon FcγRIIA cross-linking. In vitro, LMW-PTP was found to efficiently dephosphorylate activated FcγRIIA and LAT, but not Syk or phospholipase Cγ2. In the megakaryocytic cell line DAMI, antibody-induced phosphorylation of FcγRIIA was rapid and transient. The late dephosphorylation of FcγRIIA was dramatically delayed upon reduction of LMW-PTP expression by siRNA. Strikingly, overexpression of LMW-PTP resulted in the inhibition of antibody-induced phosphorylation of FcγRIIA, and caused a more rapid dephosphorylation. In addition, overexpression of LMW-PTP inhibited activation of Syk downstream of FcγRIIA and reduced intracellular Ca2+ mobilization. These results demonstrate that LMW-PTP is responsible for FcγRIIA dephosphorylation, and is implicated in the down-regulation of cell activation mediated by this ITAM-bearing immunoreceptor.

Dual ITAM-mediated proteolytic pathways for irreversible inactivation of platelet receptors: de-ITAM-izing FcgammaRIIa

Collagen binding to glycoprotein VI (GPVI) induces signals critical for platelet activation in thrombosis. Both ligand-induced GPVI signaling through its coassociated Fc-receptor gamma-chain (FcRgamma) immunoreceptor tyrosine-activation motif (ITAM) and the calmodulin inhibitor, W7, dissociate calmodulin from GPVI and induce metalloproteinase-mediated GPVI ectodomain shedding. We investigated whether signaling by another ITAM-bearing receptor on platelets, FcgammaRIIa, also down-regulates GPVI expression. Agonists that signal through FcgammaRIIa, the mAbs VM58 or 14A2, potently induced GPVI shedding, inhibitable by the metalloproteinase inhibitor, GM6001. Unexpectedly, FcgammaRIIa also underwent rapid proteolysis in platelets treated with agonists for FcgammaRIIa (VM58/14A2) or GPVI/FcRgamma (the snake toxin, convulxin), generating an approximate 30-kDa fragment. Immunoprecipitation/pull-down experiments showed that FcgammaRIIa also bound calmodulin and W7 induced FcgammaRIIa cleavage. However, unlike GPVI, the approximate 30-kDa FcgammaRIIa fragment remained platelet associated, and proteolysis was unaffected by GM6001 but was inhibited by a membrane-permeable calpain inhibitor, E64d; consistent with this, micro-calpain cleaved an FcgammaRIIa tail-fusion protein at (222)Lys/(223)Ala and (230)Gly/(231)Arg, upstream of the ITAM domain. These findings suggest simultaneous activation of distinct extracellular (metalloproteinase-mediated) and intracellular (calpain-mediated) proteolytic pathways irreversibly inactivating platelet GPVI/FcRgamma and FcgammaRIIa, respectively. Activation of both pathways was observed with immunoglobulin from patients with heparin-induced thrombocytopenia (HIT), suggesting novel mechanisms for platelet dysfunction by FcgammaRIIa after immunologic insult.

Neuro-apoptogenic and blood platelet targeting toxins in benthic marine cyanobacteria from the Portuguese coast

Six strains of marine cyanobacteria, of which five benthic, were isolated from an area of the Portuguese coast with no known apparent toxic microbial bloom. Five strains were lethal for mice. Four of them produced lethargy and four lead to bleeding. One of the toxic strains was from a genus (Aphanothece) not previously associated with toxin production. Extracts from four isolates induced SH-SY5Y-neuroblastoma cell apoptosis without affecting the viability of hepatocytes, NRK kidney cells, or fibroblasts. Aqueous extract from four isolates inhibited thrombin-induced blood platelet activation, with decreased P-selectin expression, platelet aggregation and shedding of platelet-derived micro-vesicles. Curiously, platelets treated with organic extracts from two of the cyanobacterial strains formed platelet micro-vesicles, expressed P-selectin on the surface and showed a distinct phosphotyrosine protein pattern, but failed to aggregate. We conclude that low-abundance marine cyanobacteria growing at low rates may be an important source for novel toxins that may be useful to dissect mammalian signalling pathways of apoptosis and platelet function.

Binding of IgG-Opsonized Particles to FcγR Is an Active Stage of Phagocytosis That Involves Receptor Clustering and Phosphorylation

Fc gammaR mediate the phagocytosis of IgG-coated particles and the clearance of IgG immune complexes. By dissecting binding from internalization of the particles, we found that the binding stage, rather than particle internalization, triggered tyrosine phosphorylation of Fc gammaR and accompanying proteins. High amounts of Lyn kinase were found to associate with particles isolated at the binding stage from J774 cells. PP2 (4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine), an Src kinase inhibitor, but not piceatannol, an inhibitor of Syk kinase, reduced the amount of Lyn associated with the bound particles and simultaneously diminished the binding of IgG-coated particles. Studies of baby hamster kidney cells transfected with wild-type and mutant Fc gammaRIIA revealed that the ability of the receptor to bind particles was significantly reduced when phosphorylation of the receptor was abrogated by Y298F substitution in the receptor signaling motif. Under these conditions, binding of immune complexes of aggregated IgG was depressed to a lesser extent. A similar effect was exerted on the binding ability of wild-type Fc gammaRIIA by PP2. Moreover, expression of mutant kinase-inactive Lyn K275R inhibited both Fc gammaRIIA phosphorylation and IgG-opsonized particle binding. To gain insight into the mechanism by which protein tyrosine phosphorylation can control Fc gammaR-mediated binding, we investigated the efficiency of clustering of wild-type and Y298F-substituted Fc gammaRIIA upon binding of immune complexes. We found that a lack of Fc gammaRIIA phosphorylation led to an impairment of receptor clustering. The results indicate that phosphorylation of Fc gammaR and accompanying proteins, dependent on Src kinase activity, facilitates the clustering of activated receptors that is required for efficient particle binding.

Phagocytic signaling molecules in lipid rafts of COS-1 cells transfected with FcgammaRIIA

COS-1 cells bearing FcgammaRIIA were used as a model to demonstrate co-localization of several enzymes previously shown to regulate neutrophil phagocytosis. In COS-1 cells, phospholipase D (PLD) in the membrane fraction was activated during phagocytosis. PLD was found almost exclusively in lipid rafts, along with RhoA and ARF1. Protein kinase C-delta (PKCdelta) and Raf-1 translocated to lipid rafts. In neutrophils, ceramide levels increase during phagocytosis, indicating that FcgammaRIIA engagement initiates ceramide generation. Applying this model, we transfected COS-1 cells with FcgammaRIIA that had been mutated in the ITAM region, rendering them unable to ingest particles. When the mutant receptors were engaged, ceramide was generated and MAPK was activated normally, thus these processes did not require actual ingestion of particles. These results indicate that signaling proteins for phagocytosis are either constitutively present in, or are recruited to, lipid rafts where they are readily available to activate one another.

Activation of pp125FAK by type 2B recombinant von Willebrand factor binding to platelet GPIb at a high shear rate occurs independently of alpha IIb beta 3 engagement

Shear-induced platelet aggregation (SIPA) involves the sequential interaction of von Willebrand factor (VWF) with both glycoprotein Ib (GPIb) and alphaIIbbeta3 receptors. Type 2B recombinant VWF (2B-rVWF), characterized by an increased affinity for GPIb, induces strong SIPA at a high shear rate (4000 s-1). Despite the increased affinity of 2B-rVWF for GPIb, patients with type 2B von Willebrand disease have a paradoxical bleeding disorder, which is not well understood. The purpose of this study was to determine if SIPA induced by 2B-rVWF was associated with alphaIIbbeta3-dependent platelet activation. To this end, we have addressed the influence of 2B-rVWF (Val553Met substitution) on SIPA-dependent variations of tyrosine protein phosphorylation (P-Tyr) and the effect of alphaIIbbeta3 blockers. At a high shear rate, 2B-rVWF induced a strong SIPA, as shown by a 92.7% +/- 0.4% disappearance of single platelets (DSP) after 4.5 minutes. In these conditions, increased P-Tyr of proteins migrating at positions 64 kd, 72 kd, and 125 kd were observed. The band at 125 kd was identified as pp125FAK using anti-phospho-FAK antibody. This effect, which required a high level of SIPA (> 70% DSP), was observed at 4000 s-1 but not at 200 s-1. Monoclonal antibodies (MoAbs) 6D1 (anti-GPIb) and 328 (anti-VWF A1 domain), completely abolished SIPA and p125FAK phosphorylation mediated by 2B-rVWF. In contrast, neither RGDS peptide nor MoAb 7E3, both known to block alphaIIbbeta3 engagement, had any effect on SIPA and pp125FAK. The size of aggregates formed at a high shear rate in the presence of 2B-rVWF was decreased by genistein, demonstrating the biologic relevance of pp125FAK. These findings provide a unique mechanism whereby the enhanced interaction of 2B-rVWF with GPIb, without engagement of alphaIIbbeta3, is sufficient to induce SIPA but does not lead to stable thrombus formation.

Fcgamma receptor transmembrane domains: role in cell surface expression, gamma chain interaction, and phagocytosis

We constructed chimeric receptors to dissect the role of the transmembrane (TM) domain in cell surface expression of and phagocytosis by the gamma chain-dependent Fcgamma receptors FcgammaRIIIA and FcgammaRI. FcgammaR chimeras containing the TM and cytoplasmic (CY) domains of the gamma chain were expressed on the cell surface and mediated an efficient phagocytic signal. In contrast, chimeras containing the FcgammaRIIIA TM were poorly expressed. Receptors containing the FcgammaRI TM and the gamma chain CY but lacking the gamma chain TM also were expressed efficiently and mediated phagocytosis, suggesting that a gamma chain dimer induced by the gamma chain TM is not required for efficient phagocytosis. Cotransfection of FcgammaRI or FcgammaRIIIA with the chimera CD8-gamma-gamma (EC-TM-CY) resulted in FcgammaR cell surface expression and phagocytosis, whereas CD8-CD8-gamma, whose TM does not associate with FcgammaR, allowed cell surface expression of (but not phagocytosis by) FcgammaRI. CD8-CD8-gamma also did not allow surface expression of FcgammaRIIIA. Exchanging FcgammaRI and CD8 TMs indicated that the C-terminal 11 amino acids of the FcgammaRI TM are essential for association of FcgammaRI with the gamma chain and phagocytosis. The data indicate that specific sequences in the FcgammaRIIIA and FcgammaRI TMs govern their different interactions with the gamma chain in cell surface expression and phagocytosis and that gamma chain TM sequences are not required for gamma chain-mediated phagocytosis. The data identify a specific region of the FcgammaRI TM and its asparagine as important for FcgammaRI cell surface expression in the absence of the gamma chain and for distinguishing the FcgammaRI and FcgammaRIIIA phenotypes.

Enhancement of human platelet activation by the combination of low concentrations of collagen and rabbit anticardiolipin antibodies

Low concentrations of collagen and anticardiolipin antibodies (ACLA), which were raised in rabbits by immunization with cardiolipin (CL), co-operatively activated human gel-filtrated platelets (GFP). GFP activated by adding ACLA 5 min prior to collagen (ACLA + Col) showed strong responses in cytosolic Ca2+ mobilization and cell aggregation; the responses decreased after 1 min, however, when collagen was added prior to ACLA (Col + ACLA). Col + ACLA was 30% less effective than the ACLA + Col in: (1) the phosphorylation of pleckstrin and myosin light chain; and (2) the secretion of alpha- and dense granules. Indomethacin inhibited Ca2+ mobilization, pleckstrin phosphorylation and cell aggregation in platelets stimulated by ACLA + Col. The thromboxane B2 level in platelets induced by ACLA + Col was similar to that stimulated by low concentrations of collagen alone. ACLA + Col increased the activities of phospholipase C (PLC) as determined by formation of phosphatidic acid (PA), whereas indomethacin and adenosine 2',5'-diphosphate, an antagonist of the ADP P2Y1 receptor, inhibited PA formation. These results suggest that ACLA, thromboxane A2 derived from the collagen pathway and secreted ADP co-operatively augment PLC activity and lead to platelet aggregation.

Involvement of pp125FAK and p60SRC in the signaling through Fc gamma RII-Fc gamma RIII in murine macrophages

Cross-linking the FcgammaRs can activate a wide variety of biological responses in macrophages. Receptor stimulation induces activation of protein tyrosine kinase cascades that result in phagocytosis, a process known to involve cytoskeletal rearrangements. Therefore, an involvement of non-receptor tyrosine kinases such as pp125FAK, in FcgammaR signaling is likely. Using the murine macrophage cell line J774, we demonstrate that FcgammaRII-RIII cross-linking induces a time- and dose-dependent increase in tyrosine phosphorylation of the focal adhesion kinase pp125FAK that correlates with an increase in its catalytic activity. Interestingly enough, pp125FAK activation results in its association both to the FcgammaRII-III and to p60Src. The results presented here define a novel-signaling pathway likely to be important in low affinity FcgammaRII-III mediated phagocytosis.

Molecular mechanisms of drug-induced thrombocytopenia

A wide range of drugs can induce thrombocytopenia. Molecular mechanisms for the formation of specific epitopes for all the drug-dependent antibodies appear to be very similar. A restricted set of glycoproteins on the platelet surface interacts with the drugs to form neoepitopes, to which the drug-dependent antibodies bind. Molecular mapping of antigenic sites may help characterize genetic polymorphisms that predispose to the formation of the antibody binding sites. Identification of antibody binding sites will enhance our understanding of the pathogenesis of immune drug-induced thrombocytopenia.

The snake venom toxin alboaggregin-A activates glycoprotein VI

The glycoprotein (GP)-Ib-IX-V receptor complex has recently been reported to signal through a pathway similar to that used by the collagen receptor GPVI, with a critical role described for the Fc receptor gamma-chain. The evidence for this was based in part on studies with the GPIbalpha-selective snake venom toxin, alboaggregin-A. In the present study, it is reported that alboaggregin-A has activity at the collagen receptor GPVI in addition to GPIbalpha, and evidence is provided that this contributes to protein tyrosine phosphorylation, shape change, and GPIIb-IIIa-dependent aggregation. This may explain why responses to alboaggregin-A are distinct from those to von Willebrand factor-ristocetin. (Blood. 2001;97:3989-3991)

Signal transduction via both human low-affinity IgG Fc receptors, Fc gamma RIIa and Fc gamma RIIIb, depends on the activity of different families of intracellular kinases

In contrast to IgG Fc receptor II (Fc gamma RIIa), the function of Src-family kinases, Syk and phosphoinositide 3-kinase (PI3K) in signal transduction of glycosylphosphatidylinositol-anchored Fc gamma RIIIb has not been analyzed in detail. Therefore pharmacological inhibitors were used to define the role of specific kinases in signalling of Fc gamma RIIa and Fc gamma RIIIb in myeloid cells. We demonstrate that the broad tyrosine kinase inhibitor genistein, the Src-family kinase inhibitor PP2, and the Syk kinase inhibitor piceatannol inhibit [Ca2+]i rise induced by both low-affinity Fc gamma R in neutrophils. Genistein and PP2 additionally prevent Fc gamma R-triggered hydrogen peroxide generation. In contrast, wortmannin, a PI3K inhibitor, which blocks Fc gamma RIIIb activation completely, abolishes Fc gamma RIIa-mediated [Ca2+]i flux only in the beginning. In addition, herbimycin A, a further specific inhibitor of Src-family kinases leads to a delayed Fc gamma RIIa-induced [Ca2+]i rise in THP-1 cells. In summary, our data demonstrate differences between both low-affinity IgG Fc receptors, and provide evidence for an essential role of Src-family kinases, Syk and PI3K in Fc gamma RIIIb-mediated signalling.

The clustered Fcgamma receptor II is recruited to Lyn-containing membrane domains and undergoes phosphorylation in a cholesterol-dependent manner

Phosphorylation of clustered Fcgamma receptor II (FcgammaRII) by Src family tyrosine kinases is the earliest event in the receptor signaling cascade. However, the molecular mechanisms for the interaction between FcgammaRII and these kinases are not elucidated. To asses this problem we isolated high molecular weight complexes of cross-linked FcgammaRII from non-ionic detergent lysates of U937 monocytic cells. CD55, a glycosylphosphatidylinositol-anchored protein, a ganglioside GM1 and Lyn, a Src family tyrosine kinase, were also located in these complexes. Gradient centrifugation demonstrated that the complexes containing cross-linked FcgammaRII displayed a low buoyant density. The FcgammaRII present in the complexes underwent tyrosine phosphorylation. Cross-linked FcgammaRII and Lyn occupied common 100-200 nm detergent-resistant membrane fragments, as demonstrated by immunoprecipitation and microscopy studies. Pretreatment of the cells with beta-cyclodextrin, a cholesterol acceptor, depleted membrane cholesterol and released CD55, GM1 and Lyn from the detergent-resistant complexes. In parallel, the association of Lyn with cross-linked FcgammaRII was disrupted and phosphorylation of the receptor inhibited. Reincorporation of cholesterol evoked the relocation of Lyn into the detergent-resistant membrane fraction and restored both Lyn association with cross-linked FcgammaRII and tyrosine phosphorylation of the receptor. Our data demonstrate that cholesterol-enriched membrane rafts can facilitate tyrosine phosphorylation of clustered FcgammaRII by Lyn kinase.

Fc gamma receptors differ in their structural requirements for interaction with the tyrosine kinase Syk in the initial steps of signaling for phagocytosis

Receptors for the constant region of IgG, Fc gamma receptors, are expressed on the surface of hematopoietic cells, where they mediate signaling events, such as phagocytosis, essential for host defense. Fc gamma receptors also play a role in the pathophysiology of autoimmune diseases. We have demonstrated that members of each of the three classes of human Fc gamma receptors, Fc gamma RI, Fc gamma RII, and Fc gamma RIII, mediate phagocytosis, but that important differences exist in their requirements for phagocytic signaling. For example, the Fc gamma receptors Fc gamma RI and Fc gamma RIIIA induce signaling largely by association with a gamma subunit containing a conserved cytoplasmic motif (ITAM) whose tyrosines are phosphorylated following receptor stimulation. Fc gamma RIIA contains a similar motif in its own cytoplasmic domain and does not require the gamma chain for phagocytic signaling. The tyrosine kinase Syk associates with the cytoplasmic domain of both the Fc gamma receptor gamma chain and Fc gamma RIIA and is required for phagocytosis by both Fc gamma receptor systems. To elucidate the differences in phagocytic signaling by the gamma chain and Fc gamma RIIA, we investigated the requirements for Fc gamma receptor/Syk co-immunoprecipitation, tyrosine phosphorylation, and phagocytosis. Both Fc gamma RIIA and the human gamma chain contain a tyrosine seven amino acids upstream of the ITAM motif. We observed that the upstream tyrosine plays a role in Fc gamma RIIA phagocytic signaling but is not involved in phagocytic signaling by the human gamma chain. Our data also indicate that the two ITAM tyrosines of the human gamma chain and Fc gamma RIIA do not contribute equally to Fc gamma receptor association with Syk kinase and phagocytic signaling. The data indicate that the carboxy-terminal tyrosine of the receptor cytoplasmic domain is especially important both for the interaction with Syk kinase and for phagocytosis. Elucidating such differences in gamma chain and Fc gamma RIIA signaling may be valuable in designing strategies for therapeutic intervention in hematopoietic and immunological disorders.

GATA and NF-Y participate in transcriptional regulation of FcgammaRIIA in megakaryocytic cells

Human FcgammaRIIA, expressed on platelets, neutrophils, and macrophages, plays a major role in platelet activation and immune clearance. Clinical observations indicate that regulation of expression of this receptor is an important factor influencing the course of immune thrombocytopenia. We used both transient transfection with FcgammaRIIA promoter constructs and electrophoretic mobility shift assays (EMSA) to study the regulation of FcgammaRIIA transcription. In HEL (erythromegakaryocytic) cells, the 200 bp immediately 5' of the ATG start codon accounted for the majority of the activity of a 3.6-kb promoter fragment. Putative GATA (-161) and NF-Y (-119) sites are present. EMSA analyses demonstrate specific binding of both GATA-1 and GATA-2 to labeled oligonucleotides containing the putative GATA site with HEL but not U937 (myelomonocytic) nuclear extracts. Antibodies to NF-Y supershift the specific -119 NF-Y complex with HEL, U937, Jurkat (T-lymphocytic), and HeLa (nonhematopoietic) nuclear extracts. Comparison of the activity of GATA and NF-Y mutant constructs in HEL and U937 demonstrates that while either GATA or NF-Y mutation results in a large decrease in the promoter activity (2.2- and 2.3-fold, respectively) in HEL cells, neither mutation is effective in reducing activity in U937 cells. This is the first example of a promoter active in the megakaryocyte lineage in which NF-Y cooperates additively with GATA factors to regulate transcription. Identification of other factors that must be operational for FcgammaRIIA transcription in myelomonocytic cells which lack GATA factors will bolster our ongoing efforts to dissect the function of these Fc receptors in megakaryocytic and myelomonocytic cells in vivo.

FcγRIIA requires a Gi-dependent pathway for an efficient stimulation of phosphoinositide 3-kinase, calcium mobilization, and platelet aggregation

FcγRIIA, the only Fcγ receptor present in platelets, is involved in heparin-associated thrombocytopenia (HIT). Recently, adenosine diphosphate (ADP) has been shown to play a major role in platelet activation and aggregation induced by FcγRIIA cross-linking or by sera from HIT patients. Herein, we investigated the mechanism of action of ADP as a cofactor in FcγRIIA-dependent platelet activation, which is classically known to involve tyrosine kinases. We first got pharmacologic evidence that the ADP receptor coupled to Gi was required for HIT sera or FcγRIIA clustering-induced platelet secretion and aggregation. Interestingly, the signaling from this ADP receptor could be replaced by triggering another Gi-coupled receptor, the α2A-adrenergic receptor. ADP scavengers did not significantly affect the tyrosine phosphorylation cascade initiated by FcγRIIA cross-linking. Conversely, the Gi-dependent signaling pathway, initiated either by ADP or epinephrine, was required for FcγRIIA-mediated phospholipase C activation and calcium mobilization. Indeed, concomitant signaling from Gi and FcγRIIA itself was necessary for an efficient synthesis of phosphatidylinositol 3,4,5-trisphosphate, a second messenger playing a critical role in the process of phospholipase Cγ2 activation. Altogether, our data demonstrate that converging signaling pathways from Gi and tyrosine kinases are required for platelet secretion and aggregation induced by FcγRIIA.

FcγRIIA requires a Gi-dependent pathway for an efficient stimulation of phosphoinositide 3-kinase, calcium mobilization, and platelet aggregation

FcγRIIA, the only Fcγ receptor present in platelets, is involved in heparin-associated thrombocytopenia (HIT). Recently, adenosine diphosphate (ADP) has been shown to play a major role in platelet activation and aggregation induced by FcγRIIA cross-linking or by sera from HIT patients. Herein, we investigated the mechanism of action of ADP as a cofactor in FcγRIIA-dependent platelet activation, which is classically known to involve tyrosine kinases. We first got pharmacologic evidence that the ADP receptor coupled to Gi was required for HIT sera or FcγRIIA clustering-induced platelet secretion and aggregation. Interestingly, the signaling from this ADP receptor could be replaced by triggering another Gi-coupled receptor, the α2A-adrenergic receptor. ADP scavengers did not significantly affect the tyrosine phosphorylation cascade initiated by FcγRIIA cross-linking. Conversely, the Gi-dependent signaling pathway, initiated either by ADP or epinephrine, was required for FcγRIIA-mediated phospholipase C activation and calcium mobilization. Indeed, concomitant signaling from Gi and FcγRIIA itself was necessary for an efficient synthesis of phosphatidylinositol 3,4,5-trisphosphate, a second messenger playing a critical role in the process of phospholipase Cγ2 activation. Altogether, our data demonstrate that converging signaling pathways from Gi and tyrosine kinases are required for platelet secretion and aggregation induced by FcγRIIA.

Effects of the protein tyrosine phosphatase CD45 on FcgammaRIIa signaling and neutrophil function

OBJECTIVE

Neutrophil receptors for the Fc portion of IgG (FcgammaR) trigger immune responses following cross-linking by IgG-coated foreign particles or immune complexes. Membrane-associated CD45, a protein tyrosine phosphatase termed leukocyte common antigen, has been shown to be essential for antigen receptor kinase mediated signaling in lymphocytes, and we hypothesized that CD45 may play a similar role in FcgammaR-mediated signaling and immune function in human neutrophils.

METHODS

The experimental approach was that of cell surface molecule ligation via cross-linking with specific antibodies. Antibody dependent cellular cytotoxicity (ADCC) was assessed using a single-cell plaque assay and IL-6 production measured using ELISA. Tyrosine phosphorylation levels were assessed with anti-phospho-tyrosine blots and F-actin polymerization by flow cytometry and confocal microscopy.

RESULTS

Neutrophils pretreated with anti-CD45 had a reduced ability to perform ADCC compared to untreated neutrophils. FcgammaRIIa cross-linking resulted in significantly increased concentrations of secreted IL-6 compared to untreated neutrophils, and IL-6 production was further enhanced by cocross-linking CD45 with FcgammaRIIa. Cross-linking CD45 alone also induced IL-6 production. FcgammaRIIa cross-linking resulted in increased protein tyrosine phosphorylation and F-actin polymerization in neutrophils. Cocross-linking CD45 with FcgammaRIIa resulted in abrogation of FcgammaRIIa mediated tyrosine phosphorylation and F-actin polymerization.

CONCLUSIONS

These data provide evidence that CD45 can regulate or enhance the stimulation and function of human neutrophils mediated through FcgammaR(s). In addition, CD45 ligation may play an essential role in cytokine induction pathways that lead to inflammatory reactions in vivo.

Antineutrophil cytoplasmic antibodies (ANCA) and systemic vasculitis: update of assays, immunopathogenesis, controversies, and report of a novel de novo ANCA-associated vasculitis after kidney transplantation

OBJECTIVES

To characterize antineutrophil cytoplasmic antibodies (ANCA), their major autoantigens, disease associations, and pathophysiology in systemic vasculitides. To describe a patient with a novel de novo ANCA-associated vasculitis after kidney transplantation.

METHODS

We reviewed and compiled the literature on ANCA-related topics and systemic vasculitis. Laboratory and clinical data from a cadaveric kidney transplant patient who developed necrotizing vasculitis involving glomerular capillaries, with crescent formation associated with P-ANCA and myeloperoxidase, were analyzed.

RESULTS

Large-scale multi-center testing of patient and normal sera by the European ANCA Assay Standardization Project using immunofluorescence assays and enzyme immunoassays indicate the assays have good sensitivity and specificity, and diagnostic utility for ANCA-associated vasculitis. A few investigations covering basic and clinical research with ANCA remain controversial: whether endothelial cells do or do not express a 29-kd neutral serine protease termed proteinase-3 (PR-3), the target of ANCA in most individuals with Wegener's granulomatosis, and whether anti-myeloperoxidase (MPO) ANCAs recognize a restricted number of epitopes on MPO. This issue has relevance for using monoclonal antibodies to treat patients with vasculitis who have adverse effects from immunosuppressive drugs. The two allelic forms of FcgammaRIIa (H131/R131) and the two of FcgammaRIIlb (NA1/NA2) are discussed as possible inheritable genetic elements for vasculitic disorders and for signaling responses. Stimulatory and costimulatory molecules, and cytokine profiles of T lymphocytes are characterized to show that these cells are actively involved in the ANCA-associated vasculitides. The patient described had a de novo ANCA associated small vessel vasculitis which developed after renal transplantation.

CONCLUSIONS

There have been significant advances in the development of sensitive and specific ANCA assays. The immunopathogenetic mechanism of ANCA involves the constitutive FcgammaRs, ligands, and signaling responses to activate cytokine-primed neutrophils. This may lead to the generation of reactive oxygen intermediates, degranulation, and secretion of intracellular granule contents, and ultimately inflammation and vasculitis.

Tyrosine phosphorylation and association of FcgammaRII and p72(Syk) are not limited to the FcgammaRII signalling pathway

The tyrosine kinase p72(Syk) plays a critical role in platelet signal transduction. It associates with the platelet receptor for the Fc domain of IgGs, FcgammaRII, following stimulation by FcgammaRII cross-linking. Here, we show that p72(Syk) and FcgammaRII tyrosine phosphorylation and association occured following platelet stimulation by: (1) two monoclonal antibodies, which form a bridge between a target antigen and FcgammaRII, and (2) the G-protein-coupled receptor agonist thrombin. The kinetics of the p72(Syk)/FcgammaRII association depended on the signalling pathway (i.e., the antigen targeted or the thrombin receptor). We established a direct relationship between the level of FcgammaRII phosphorylation and the detection of its association with p72(Syk). Inhibition of p72(Syk) by piceatannol resulted in partial or total inhibition of FcgammaRII phosphorylation, after immunological activation or addition of thrombin, respectively, suggesting that p72(Syk) participates in FcgammaRII phosphorylation. The results provide evidence that p72(Syk)/FcgammaRII association is not restricted to immunological activation.

FcgammaRII tyrosine phosphorylation differs between FcgammaRII cross-linking and platelet-activating anti-platelet monoclonal antibodies

Using glutathione S-transferase Syk fusion proteins, we evaluated the mode of platelet FcgammaRII tyrosine phosphorylation induced by FcgammaRII cross-linking or anti-CD9 monoclonal antibodies (mAb). The N-terminal SH2 domain of Syk (Syk-N-SH2), the C-terminal SH2 domain of Syk (Syk-C-SH2), and the domain having both the N- and C-terminal SH2 of Syk (Syk-NC-SH2) all bound to tyrosine-phosphorylated FcgammaRII with FcgammaRII cross-linking. In the case of anti-CD9 mAb-induced platelet activation, only Syk-C-SH2 and Syk-NC-SH2 bound to tyrosine-phosphorylated FcgammaRII. Since the SH2 domain is specific for a particular structure containing phosphotyrosine, these findings suggest that only one tyrosine residue in the immunoreceptor tyrosine-based activation motif (ITAM) is phosphorylated with anti-CD9 mAb, and that both are phosphorylated with FcgammaRII cross-linking. Synthetic peptides corresponding to the ITAM of human platelet FcgammaRII with the N-terminal tyrosine residue phosphorylated (N-P) or the C-terminal tyrosine residue phosphorylated (C-P), were used. N-P more potently dissociated Syk-C-SH2 from tyrosine-phosphorylated FcgammaRII than C-P, suggesting that the N-terminal tyrosine residue is phosphorylated upon anti-CD9 mAb-induced activation. Furthermore, these findings imply that Syk-N-SH2 binds to the phosphorylated C-terminal tyrosine residue of ITAM, and Syk-C-SH2 to the N-terminal tyrosine. Taken together, our findings suggest that FcgammaRII-dependent platelet activation without FcgammaRII dimerization, such as with anti-CD9 mAb, is distinct from that induced by FcgammaRII cross-linking.

Reactive Oxygen Intermediates Enhance Fcγ Receptor Signaling and Amplify Phagocytic Capacity

Receptors for the Fc region of IgG (FcγR) mediate internalization of opsonized particles by human neutrophils (PMN) and mononuclear phagocytes. Cross-linking of FcγR leads to activation of protein tyrosine kinases and phosphorylation of immunoreceptor tyrosine-based activation motifs (ITAMs) within FcγR subunits, both obligatory early signals for phagocytosis. Human PMN constitutively express two structurally distinct FcγR, FcγRIIa and FcγRIIIb, and can be induced to express FcγRI by IFN-γ. We have previously shown that stimulation of PMN through FcγRIIIb results in enhanced FcγRIIa-mediated phagocytic activity that is inhibited by catalase. In the present study, we have tested the hypothesis that reactive oxygen intermediates (ROI) have the capacity to regulate FcγR responses and defined a mechanism for this effect. We show that H2O2 augmented phagocytosis mediated by FcγRIIa and FcγRI in PMN and amplified receptor-triggered tyrosine phosphorylation of FcγR-associated ITAMs and signaling elements. Generation of endogenous oxidants in PMN by cross-linking FcγRIIIb similarly enhanced phosphorylation of FcγRIIa and Syk, a tyrosine kinase required for phagocytic function, in a catalase-sensitive manner. Our results provide a mechanism for priming phagocytes for enhanced responses to receptor-driven effects. ROI generated in an inflammatory milieu may stimulate quiescent cells to rapidly increase the magnitude of their effector function. Indeed, human monocytes incubated in the presence of stimulated PMN showed oxidant-induced increases in FcγRIIa-mediated phagocytosis. Definition of the role of oxidants as amplifiers of FcγR signaling identifies a target for therapeutic intervention in immune complex-mediated tissue injury.

Low affinity Fc gamma receptors on murine macrophages: mitogen-activated protein kinase activation and AP-1 DNA binding activity

Mouse macrophage cell lines such as J774 express Fc receptors for IgG2a immune complexes, which upon binding of the proper ligand, trigger several signal transduction pathways. A surface to nucleus signaling through these receptors has been demonstrated. We describe here the activation of the mitogen-activated protein kinase (MAPK) and an increase in the binding of the activator protein 1 (AP-1) to DNA upon receptor stimulation. The described effects are only partially blocked by inhibitors of the Ca2+/diacylglycerol-dependent protein kinase (PKC), suggesting that differential signaling pathways are activated upon receptor cross-linking and that they converge at or above the MAPK level. These results pave the way to our understanding of Fc gammaR cross-linking induced gene expression regulation.

The Role of the Human Fc Receptor FcγRIIA in the Immune Clearance of Platelets: A Transgenic Mouse Model

In humans, the Fc receptor for IgG, FcγRIIA, is expressed on macrophages and platelets and may play an important role in the pathophysiology of immune-mediated thrombocytopenia. Mice lack the genetic equivalent of human FcγRIIA. To better understand the role of FcγRIIA in vivo, FcγRIIA transgenic mice were generated and characterized. One transgenic mouse line expressed FcγRIIA on platelets and macrophages at levels equivalent to human cells, and cross-linking FcγRIIA on these platelets induced platelet aggregation. Immune-mediated thrombocytopenia in this transgenic line was studied using i.v. and i.p. administration of anti-mouse platelet Ab. In comparison with matched wild-type littermates that are negative for the FcγRIIA transgene, Ab-mediated thrombocytopenia was significantly more severe in the FcγRIIA transgenic mice. In contrast, FcR γ-chain knockout mice that lack functional expression of the Fc receptors FcγRI and FcγRIII on splenic macrophages did not demonstrate Ab-mediated thrombocytopenia. We generated FcγRIIA transgenic × FcR γ-chain knockout mice to examine the role of FcγRIIA in immune clearance in the absence of functional FcγRI and FcγRIII. In FcγRIIA transgenic × FcR γ-chain knockout mice, severe immune thrombocytopenia mediated by FcγRIIA was observed. These results demonstrate that FcγRIIA does not require the FcR γ-chain for expression or function in vivo. Furthermore, taken together, the data suggest that the human Fc receptor FcγRIIA plays a significant role in the immune clearance of platelets in vivo.