Platelet-associated anti–GPIIb-IIIa autoantibodies in chronic immune thrombocytopenic purpura recognizing epitopes close to the ligand-binding site of glycoprotein (GP) IIb

Localization of epitopes for platelet-associated (PA) anti–GPIIb-IIIa (αIIbβ3) autoantibodies in chronic immune thrombocytopenic purpura remains elusive. Previous studies suggest that PA antibodies recognize the tertiary structure of intact glycoprotein (GP) IIb-IIIa. To localize their epitopes using antigen-capture enzyme-linked immunosorbent assay (ELISA), the reactivity of 34 PA anti–GPIIb-IIIa antibodies was examined with recombinant GPIIb-IIIa having a defect in ligand-binding sites in either GPIIb or GPIIIa, and no major conformational change was induced: KO variant GPIIb-IIIa was attributed to a 2–amino acid insertion between residues 160 and 161 in the W3 4-1 loop in GPIIb, and CAM variant GPIIb-IIIa was attributed to D119Y in GPIIIa. In one third (11 of 34) of the patients, PA antibodies showed a marked decrease (less than 50%) in reactivity with KO compared with wild-type GPIIb-IIIa. Their reactivity was also impaired against GPIIbD163A-IIIa. In sharp contrast, they reacted normally with CAM GPIIb-IIIa. OP-G2, a ligand-mimetic monoclonal antibody, markedly inhibited their binding to GPIIb-IIIa in patients with impaired binding to KO GPIIb-IIIa, but small GPIIb-IIIa antagonists did not. In addition, a newly developed sensitive ELISA indicated that autoantibodies showing impaired binding to KO are more potent inhibitors for fibrinogen binding. The present data suggest that certain PA anti–GPIIb-IIIa autoantibodies recognize epitopes close to the ligand-binding site in GPIIb, but not in GPIIIa.