Influence of FcγRIIA H131R Polymorphism on the Risk of Thrombosis in HIT and Variable Effect of Normal IgG on Platelet Activation
Authors: Rollin Jerôme , Pouplard Claire , Leroux Dorothee , Saada Armand , Behier Céline , Gouilleux Gruart Valérie , Thibault Gilles , Gruel Yves 
Published: November 2019

Abstract
Introduction. Heparin-induced thrombocytopenia (HIT) results from an atypical immune response to platelet factor 4/heparin complexes (PF4/H), with rapid synthesis of IgG antibodies that activate platelets via FcγRIIA receptors. HIT is associated with thrombotic complications occurring in about 50% of affected patients.Moreover, the response of platelets to anti-PF4/H IgG varies from one subject to another. This variability, partly explained by gene polymorphisms affecting FcγRIIA signaling (Rollin et al, Blood 2012), might also depend on variations in the binding of HIT antibodies to FcγRIIA receptors.
Objective. The influence of FcγRIIA H131R polymorphism on the risk of HIT and thrombosis was investigated in parallel with the effect of normal IgG on the platelet response to HIT antibodies.
Patients and Methods. A cohort of 89 patients with definite HIT (PF4-specific ELISA and serotonin release assay both positive) and two control groups with (Abpos ; n=160) or without (Abneg ; n=179) significant levels of PF4-specific antibodies after cardiac surgery were studied. Platelet aggregation tests (PAT) were performed in 59 healthy donors using HIT plasma samples or 5B9, a humanized chimeric monoclonal antibody specific to PF4/H complexes that we have recently developed which fully mimics the effects of human HIT antibodies. The influence of normal IgG and IgG subclasses on platelet aggregation was also evaluated according to the FcγRIIA H131R polymorphism. Finally, tissue factor mRNA levels and plasma phospholipid-dependent procoagulant activity were measured after addition of HIT antibodies and heparin in the whole blood collected from healthy donors with different FCGR2A genotypes.
Results. Genotypes and allele frequencies were similar in the HIT patients and both Abneg and Abpos controls. However, interestingly, homozygous FcγRIIA 131RR subjects were more frequent among the HIT patients who had developed thrombotic complications compared to those without thrombosis (34.5% vs. 8.5%, respectively, p=0.008). The FcγRIIA 131RR genotype therefore appeared to increase the risk of thrombosis in HIT significantly (OR = 5.9; 95% CI 1.7-20). This association was not found in two other cohorts of patients who had developed ischemic stroke or venous thromboembolism in a medical context other than HIT, thus indicating that the impact of FcγRIIA H131R polymorphism on the risk of thrombosis was specific to HIT. PAT performed with platelet-rich plasma (PRP) showed that the lag time measured in response to 5B9 with heparin (5B9/H) was significantly longer with FcγRIIA 131HH platelets compared to RR platelets (p=0.03). Importantly, this variable platelet response to 5B9/H related to the FcγRIIA polymorphism was no longer detectable when PAT were performed with washed platelets, when the IgG fraction had been removed from the plasma, and when collagen was used as inducer. On the other hand, the addition of human polyvalent IgG to IgG-depleted PRP inhibited the aggregation induced by 5B9/H, but lower concentrations of IgG were required to achieve this effect in FcγRIIA 131HH subjects compared to RR donors. Importantly, this variable effect of normal IgG on platelet aggregation induced by HIT antibodies was shown to depend mainly on the IgG2 subclass, which binds to the H131 isoform more effectively, whereas IgG1 exerted a similar inhibitory effect in RR and HH homozygous donors. Finally, a stronger increase in TF mRNA levels was also evidenced in homozygous FcγRIIARR donors compared to homozygous FcγRIIAHH donors after incubation of 5B9 and heparin in whole blood. Moreover, this effect was associated with a greater relative shortening of plasma clotting time indicating that the release of procoagulant phospholipid microparticles was more pronounced in RR subjects in response to HIT antibodies (p=0.04).
Conclusion. These results demonstrate that HIT patients homozygous for the FcγRIIA 131R allele have a higher risk of thrombosis, probably due to an increased cell activation induced by antibodies to PF4/H with a lower inhibitory effect of endogenous IgG, especially of the IgG2subclass.

Disclosures
No relevant conflicts of interest to declare.