Factor VIII influences binding of factor IX and factor X to intact human platelets

To investigate the influence of factor VIII on the binding of factors IX and X to the surface of intact human platelets, washed collagen-stimulated platelets were incubated with factor IX and factor X in the presence or absence of factor VIII. Platelets were then lysed and IX:Ag and X:Ag were determined in the platelet lysate. Platelet bound IX:Ag was higher after incubation of platelets with factor IX in the presence of native or activated factor VIII than after incubation of platelets with factor IX alone. Thrombin degraded factor VIII did not support binding of factor IX to stimulated platelets. Preactivation of factor IX enhanced binding to platelets; influence by factor VIII on binding to platelets was greater with native factor IX than with preactivated factor IX. Presence of native or activated factor VIII also increased binding of factor X to platelets. In contrast to factor IX, preactivation of factor X did not influence binding to platelets, but in the presence of factor VIII preactivation of factor X increased binding of factor X to platelets. Our data suggest that mediation of binding of factors IX and X to the surface of platelets is one of the mechanisms by that factor VIII exerts its cofactor function in the activation of factor X.