HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY Streptokinase-induced platelet activation involves antistreptokinase antibodies and cleavage of protease-activated receptor-1

Streptokinase activates platelets, limiting its effectiveness as a thrombolytic agent. The role of antistreptokinase antibodies and proteases in streptokinase-induced platelet activation was investigated. Streptokinase induced localization of human IgG to the platelet surface, platelet aggregation, and thromboxane A2 production. These effects were inhibited by a monoclonal antibody to the platelet Fc receptor, IV.3. The platelet response to streptokinase was also blocked by an antibody directed against the cleavage site of the platelet thrombin receptor, proteaseactivated receptor-1 (PAR-1), but not by hirudin or an active site thrombin inhibitor, Ro46-6240. In plasma depleted of plasminogen, exogenous wild-type plasminogen, but not an inactive mutant protein, S741A plasminogen, supported platelet aggregation, suggesting that the protease cleaving PAR-1 was streptokinase-plasminogen. Streptokinase-plasminogen cleaved a synthetic peptide corresponding to PAR-1, resulting in generation of PAR-1 tethered ligand sequence and selectively reduced binding of a cleavage-sensitive PAR-1 antibody in intact cells. A combination of streptokinase, plasminogen, and antistreptokinase antibodies activated human erythroleukemic cells and was inhibited by pretreatment with IV.3 or pretreating the cells with the PAR-1 agonist SFLLRN, suggesting Fc receptor and PAR-1 interactions are necessary for cell activation in this system also. Streptokinase-induced platelet activation is dependent on both antistreptokinase-Fc receptor interactions and cleavage of PAR-1. (Blood. 2000;95:1301-1308)