THE ROLE OF PLATELET VON-WILLEBRAND-FACTOR IN THE BINDING OF FACTOR-VIII TO ACTIVATED PLATELETS

Factor VIII binds to activated platelets and contributes to the tenase complex assembled on the platelet membrane surface. We have examined the role of platelet von Willebrand factor in the binding of factor VI II to platelets using a platelet captured enzyme-linked immunosorbent assay. Purified factor VIII bound to activated normal platelets in a d ose dependent manner. Factor VIII also bound to platelets obtained fro m a patient with Type 2N von Willebrand disease, although in this case the binding was reduced to approximately 50% of that seen with contro l platelets. Furthermore,factor VIII bound to Type 3 von Willebrand di sease platelets in the absence of detectable von Willebrand factor. In this instance the binding reaction appeared to be approximately 30% o f that seen with the same number of normal platelets. An anti-A3 domai n monoclonal antibody, NMC-VIII/10, which recognizes the amino-termina l acidic region of the factor VIII light chain, and an anti-C2 domain monoclonal antibody, NMC-VIII/5, which also moderates the binding of f actor VIII to phosphatidylserine, inhibited the association between fa ctor VIII and platelets. Inhibition was more remarkable with NMC-VIII/ 5 than with NMC-VIII/10 but not complete. The findings suggest that th e binding of factor VIII to activated platelets is not based on a sing le ligand-receptor relationship, although a predominant role exists fo r the platelet von Willebrand factor. Furthermore, both the amino-term inal acidic region of the A3 domain and the C2 domain participate in t he binding of factor VIII to activated platelets. (C) 1998 Elsevier Sc ience Ltd.