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 VIII to platelets using a platelet captured
enzyme-linked
immunosorbent assay. Purified
factor VIII bound to activated normal platelets in a dose dependent manner.
Factor VIII also bound to platelets obtained from a patient with
Type 2N von Willebrand disease, although in this case the binding was reduced to approximately 50% of that seen with control platelets. Furthermore,
factor VIII bound to
Type 3 von Willebrand disease platelets in the absence of detectable
von Willebrand factor. In this instance the binding reaction appeared to be approximately 30% of that seen with the same number of normal platelets. An anti-A3 domain
monoclonal antibody, NMC-VIII/10, which recognizes the amino-terminal acidic region of the
factor VIII light chain, and an anti-C2 domain
monoclonal antibody, NMC-VIII/5, which also moderates the binding of
factor VIII to
phosphatidylserine, inhibited the association between
factor VIII and platelets. Inhibition was more remarkable with NMC-VIII/5 than with NMC-VIII/ 10 but not complete. The findings suggest that the binding of
factor VIII to activated platelets is not based on a single
ligand-receptor relationship, although a predominant role exists for the platelet
von Willebrand factor. Furthermore, both the amino-terminal acidic region of the A3 domain and the C2 domain participate in the binding of
factor VIII to activated platelets.