The identification and functional significance of factor VIII components on normal platelets

Platelet von Willebrand factor

von Willebrand factor (vWF) circulates in the blood in two distinct compartments. One, plasma vWF, is synthesized and released from endothelial cells; the second, synthesized by megakaryocytes, circulates in platelets primarily stored in the alpha granules. Recent experimental and clinical studies of von Willebrand's disease (vWD) indicate that platelet vWF plays an important role in the bleeding time determination and the degree of clinical bleeding in vWD. Platelet vWF is released from the platelet alpha granules by various agonists and then rebinds to the glycoprotein IIb/IIIa complex. Fibrinogen or monoclonal antibodies against this complex inhibit 60 to 70% of the expression of platelet vWF. Aspirin inhibits 80% of the adenosine diphosphate-induced platelet vWF surface expression, and the platelet vWF surface expression that is not inhibited by aspirin can be almost totally inhibited by disruption of the platelet cytoskeleton. Platelet vWF may, in part, be expressed in the open canalicular system prebound to a receptor. Transfusion studies have shown that correction of the bleeding time in severe vWD requires both plasma and platelet vWF. On the basis of numerous studies, we hypothesize that platelet vWF plays an important role in platelet interaction with the subendothelial surfaces under conditions of high shear stress. After platelet contact, platelet vWF is released, binds to the glycoprotein IIb/IIIa complex, and forms a bridge between the subendothelial surface and the platelet, which initiates and supports platelet spreading. Platelet vWF also acts as an intercellular bridge between platelets and thereby promotes platelet aggregation. This process is important not only in the initial steps of hemostasis but also in the process of thrombosis.

The interaction between factor VIII clotting antigen (VIIICAg) and phospholipid

The interaction between factor VIII clotting antigen (VIIICAg) and phospholipid (PL) was studied using a two-site solid phase immunoradiometric assay (IRMA) for VIII CAg. Incubation (2 h, 37 degrees C) of normal plasma, cryoprecipitate or factor VIII concentrate with Diagen PL (0.5 mg/unit VIIICAg) resulted in 80-90% loss of IRMA-measurable VIIICAg. No loss of factor VIII related antigen (VIIIRAg) or factor VIII clotting activity (VIIIC) was seen. Treatment of factor VIII concentrate with purified PLs showed greatest VIIICAg reduction with phosphatidylserine, less with phosphatidylethanolamine, and very little with phosphatidylcholine. The action of phospholipase-C (PL-C) on VIIICAg-PL complexes was investigated, with enzyme activity being monitored by thin-layer chromatography. Treatment of normal plasma, cryoprecipitate or factor VIII concentrate with PL-C (5 u/unit VIIICAg) resulted in 25%, 25% and 30% increases in VIIICAg. No increase in VIIIC or VIIIRAg was seen. Preincubation of factor VIII concentrate with PL, followed by PL-C incubation, resulted in 70-80% recovery of measurable VIIICAg. Incubation of 'activated' prothrombin complex with PL-C increased VIIICAg by 44% (Autoplex) and 80% (FEIBA), indicating VIIICAg-PL complexes are present. Incubation of factor VIII concentrate with fresh platelet lysate led to a reduction in VIIICAg (100 u/dl to 21 u/dl). In fresh platelet lysate alone low VIIICAg levels were detectable (0.71 X 10(-3) u/10(9) plt). After PL-C incubation VIIICAg levels increased to 9.8 X 10(-3) u/10(9) plt) (14-fold increase). Thus VIIICAg in platelets may be hidden by VIIICAg-PL complexes.