Quinidine purpura: evidence that glycoprotein V is a target platelet antigen

Quinidine-induced thrombocytopenia has been associated with both immune complex and autoantibody binding to platelets. In the present study, serum antibody from six of six patients with quindine purpura was shown by immunoblotting to bind to a single platelet membrane protein of mol wt 80,000. This target protein was absent from Bernard-Soulier (BSS) platelets. F(ab)2 prepared from one patient's serum also bound to this protein, indicating autoantibody rather than immune complex binding to the target antigen. Antibody binding to the 80-kd protein was preserved after treatment of platelets with concentrations of trypsin or chymotrypsin that completely removed glycoprotein Ib (GPIb). Preincubation of platelet proteins with one patient's serum blocked binding of a polyclonal rabbit antibody against glycoprotein V (GPV), indicating that these antibodies recognize the same antigen. By wheat germ affinity chromatography, GPV was shown to copurify with GPIb. Quinidine-induced antibody bound to the wheat germ-purified GPV but not to GPIb. We conclude that quinidine purpura is associated with autoantibody directed against platelet GPV.