Platelet activation induced by an antiplatelet autoantibody against CD9 antigen and its inhibition by another autoantibody in immune thrombocytopenic purpura

Summary. In a patient with immune thrombocytopenic purpura (ITP), we found a novel platelet-activating IgG (act-IgG) and an inhibitory IgG (inhi-IgG) that prevented activation induced by both CD9 monoclonal antibody (mAb) and the act-IgG. Purified IgG from the patient plasma caused a rise in [Ca²⁺]_i and the aggregation of normal platelets, and bound to a 24 kD membrane protein. This aggregation was inhibited by aspirin, staurosporine, an inhibitor of protein kinase C, and F(ab')₂ fragments of MALL13, a CD9 mAb. When the platelet count of this patient rose to normal range, the act-IgG disappeared. About 2 weeks later, the relapse of thrombocytopenia was observed. The purified IgG obtained in this period did not activate platelets but inhibited both the rise in [Ca²⁺]_i and platelet aggregation stimulated by NNKY 1–19, a CD9 mAb, as well as the act-IgG, and bound to a 40 kD membrane protein. The inhi-IgG prevented the binding of IV-3, a mAb against Fcγ receptor II (FcγRII), but did not prevent the binding of NNKY 1–19 to its antigen. We suggest that the activating autoantibody recognized CD9 antigen and activated both the thromboxane- and phospholipase C-dependent pathways, while the inhibitory autoantibody recognized the FcγRII and inhibited CD9 antibody-induced platelet activation mediated via this receptor.