Eptifibatide-induced thrombocytopenia and thrombosis in humans require FcγRIIa and the integrin β3 cytoplasmic domain

Thrombocytopenia and thrombosis following treatment with the integrin alpha IIb beta 3 antagonist eptifibatide are rare complications caused by patient antibodies specific for ligand-occupied alpha IIb beta 3. 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 alpha IIb beta 3-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 beta 3 cytoplasmic domain, the platelet Fc gamma RIIa Fc receptor, the protein-tyrosine kinase Syk, and phospholipase C gamma 2. Each activation event was inhibited by preincubation of the platelets with Fab fragments of the Fc gamma RIIa-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 Fc gamma RIIa and the alpha IIb beta 3 complex but lacked most of the beta 3 cytoplasmic domain. Taken together, these data suggest a novel mechanism whereby eptifibatide-dependent antibodies engage the integrin beta 3 subunit such that Fc gamma RIIa and its downstream signaling components become activated, resulting in thrombocytopenia and a predisposition to thrombosis.