Increased platelet expression of FcGammaRIIa and its potential impact on platelet reactivity in patients with end stage renal disease

Platelet FcγRIIa as a Marker of Cardiovascular Risk After Myocardial Infarction

BACKGROUND

A previous single-center study of patients with myocardial infarction (MI) showed that platelet FcγRIIa (pFCG) can distinguish patients at higher and lower risk of subsequent MI, stroke, and death.

OBJECTIVES

The authors performed an 800-patient 25-center study to validate the prognostic implications of pFCG.

METHODS

Patients with type 1 MI (ST-segment elevation and non-ST-segment elevation) were enrolled in a prospective noninterventional trial during their index hospitalization. Enrolled patients had at least 2 of the following characteristics: age ≥65 years, multivessel coronary artery disease, previous MI, chronic kidney disease, or diabetes mellitus. Flow cytometry was used to quantify pFCG at a core laboratory. A predefined threshold was used to identify high and low pFCG. Patients were queried every 6 months by telephone with a standardized questionnaire. Events were confirmed by review of medical records.

RESULTS

Treatment with antithrombotic therapy (aspirin, P2Y12 inhibitors, and anticoagulants) was similar in patients with high and low pFCG. The primary composite endpoint (MI, stroke, death) occurred more frequently in patients with high pFCG (HR: 2.09; 95% CI: 1.34-3.26; P = 0.001). Among individual components of the composite, both death (HR: 2.57; 95% CI: 1.50-4.40; P = 0.001) and MI (HR: 3.24; 95% CI: 1.64-6.37; P = 0.001) were more frequent in patients with high pFCG.

CONCLUSIONS

Quantifying pFCG identifies patients at higher and lower risk of subsequent cardiovascular events. This prognostic information will be useful in clinical decisions regarding the intensity and duration of antiplatelet therapy. (Assessment of Individual Risk of Cardiovascular Events by Platelet FcγRIIa; NCT05175261).

Assessing prognosis by quantifying FcγRIIa on fixed platelets

Introduction: FcγRIIa amplifies platelet activation and higher platelet FcγRIIa identifies patients at greater risk of subsequent cardiovascular events. We report the accuracy and precision of a modified test to quantify FcγRIIa on previously fixed platelets (pFCG test).Methods & results: An antibody clone (5G1) was developed after exposure of mice to formaldehyde treated FcγRIIa. Accuracy and precision of the modified test was evaluated with biologic specimens (platelets) and engineered synthetic cells conjugated with FcγRIIa (Slingshot Biosciences). The modified pFCG test on fixed platelets (using 5G1) consistently identified modestly more (∼300 molecules) of FcγRIIa on platelets compared with the pFCG test on nonfixed platelets (using clone FL18.26). With biologic specimens, the intra-assay coefficient of variation (CV) was 2.1 ± 0.1% (standard error of the mean, n = 750). The interassay CV was assessed intraday (4.5 ± 1%) and interday (up to 5 days after fixation, 6.5 ± 0.4%, n = 50). The pFCG test performed on Slingshot Synthetic cells conjugated with FcγRIIa demonstrated accuracy, linearity (R2 = 0.984) and similar interassay CV both intraday (2% ± 0.6%) and interday (20 nonconsecutive days, 9.9% ± 2.1%).Conclusion: In summary, modification of the pFCG test to be performed on fixed platelets allows accurate quantification of pFCG with high precision.

Endothelial Shear Stress and Platelet FcγRIIa Expression in Intracranial Atherosclerotic Disease

Intracranial atherosclerotic disease (ICAD) has been characterized by the degree of arterial stenosis and downstream hypoperfusion, yet microscopic derangements of endothelial shear stress at the luminal wall may be key determinants of plaque growth, vascular remodeling and thrombosis that culminate in recurrent stroke. Platelet interactions have similarly been a principal focus of treatment, however, the mechanistic basis of anti-platelet strategies is largely extrapolated rather than directly investigated in ICAD. Platelet FcγRIIa expression has been identified as a potent risk factor in cardiovascular disease, as elevated expression markedly increases the risk of recurrent events. Differential activation of the platelet FcγRIIa receptor may also explain the variable response of individual patients to anti-platelet medications. We review existing data on endothelial shear stress and potential interactions with the platelet FcγRIIa receptor that may alter the evolving impact of ICAD, based on local pathophysiology at the site of arterial stenosis. Current methods for quantification of endothelial shear stress and platelet activation are described, including tools that may be readily adapted to the clinical realm for further understanding of ICAD.

Human platelet IgG Fc receptor FcγRIIA in immunity and thrombosis

Beyond their prominent role in hemostasis and thrombosis, platelets are increasingly recognized as having immunologic functions. Supporting this, human platelets express FcγRIIA (CD32a), a low-affinity Fc receptor (FcR) for the constant region of IgG that recognizes immune complexes (ICs) and IgG-opsonized cells with high avidity. In leukocytes, FcγRIIA engagement initiates strong effector functions that are key for immune and inflammatory responses, including cytokine release, antibody-dependent cell-mediated killing of pathogens, and internalization of ICs. However, the physiologic relevance of platelet-expressed FcγRIIA has received little attention in previous reviews on FcRs. This article summarizes and discusses the available information on human platelet FcγRIIA. The importance of this receptor in heparin-induced thrombocytopenia, a prothrombotic adverse drug effect, is well documented. However, studies demonstrating platelet activation by IgG-opsonized bacteria point to the physiologic relevance of platelet FcγRIIA in immunity. In this context, platelet activation and secretion may facilitate both a direct antimicrobial function of platelets and crosstalk with other immune cells. Additionally, a role for platelet FcγRIIA in IgG-independent hemostasis and physiologic thrombosis, by means of amplifying integrin αII b β3 outside-in signaling, has also been proposed. Nonetheless, the thrombotic complications found in some infective and autoimmune diseases may result from unbalanced FcγRIIA-mediated platelet aggregation. Moreover, FcγRIIA is not expressed in mice, and thrombocytopenia and/or thrombotic events found after drug administration can only be recapitulated by the use of human FcγRIIA-transgenic mice. Altogether, the available data support a functional role for platelet FcγRIIA in health and disease, and emphasize the need for further investigation of this receptor.

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.