Fibrin stimulates platelets to increase factor VIIIa binding site expression

Factor (F)VIII functions as an enzymatic cofactor on the membranes of stimulated platelets. However, thrombin stimulates platelets to express only a small number of binding sites for FVIII. We wished to determine whether molecules that are likely to be present in a developing thrombus stimulate platelets to up-regulate FVIII binding site expression. Flow cytometry was utilized to measure binding of fluorescein-labeled FVIIIa to activated platelets and a FXase assay was utilized to measure platelet-dependent function. Various agonists as well as normal and mutant fibrinogens and fibrin were evaluated as co-stimuli. Thrombin-stimulated platelets expressed 214 +/- 67 binding sites for thrombin-activated FVIII (FVIIIa) and none of the established soluble agonists enhanced binding site exposure. However, the presence of 5 micro g mL(-1) fibrin increased the number of FVIIIa binding sites/platelet three- to eight-fold (1470 +/- 130, range 600-1800) with a parallel increase in platelet-based FXase assay. Binding site up-regulation was not stimulated by fibrinogen and was blocked by inhibitors of GPIIbIIIa. Mutant fibrin lacking the gamma-chain C-terminal four residues was ineffective while fibrin with altered RGD sequences did stimulate expression of FVIIIa binding sites indicating that co-stimulation is mediated by the fibrin gamma-chain termini. Fibrin-enhanced expression of FVIIIa binding sites was not supported by D364H fibrin, which does not aggregate normally, and was blocked by the GPRP peptide, which inhibits fibrin polymerization. Polymerized fibrin can function as a platelet co-stimulus, up-regulating expression of binding sites for FVIIIa.