Increased expression of platelet IgG Fc receptors in immune heparin-induced thrombocytopenia

Our previous finding that heparin-dependent antibodies in heparin-induced thrombocytopenia (HIT) bind to platelets via platelet IgG Fc receptors (FcRs) prompted this study. Platelet FcRs in 16 patients with HIT, 23 control patients, and 42 normal subjects were studied. Patients with HIT had substantially increased platelet FcRs during the acute illness. Those who suffered serious thrombotic complications or died shortly after diagnosis had significantly more FcRs per platelet than those with milder disease. Consistent with their increased FcRs, platelets of patients with HIT showed increased aggregation reactivity to aggregated IgG and heparin-dependent antibodies. Platelet FcRs in patients with HIT remained elevated for 1 to 3 months after the acute illness then stabilized to a mean value not significantly different from either control group. The increased expression of FcRs on HIT platelets and their increased reactivity to heparin-dependent antibodies may contribute to the pathogenesis of thrombocytopenia and thrombosis in HIT.