THE FLUID-PHASE SC5B-9 TERMINAL COMPLEMENT COMPLEX BINDS TO THE GPIIBIIIA COMPLEX OF THROMBIN-STIMULATED HUMAN BLOOD-PLATELETS INHIBITING PLATELET-AGGREGATION/

We have investigated interactions between human blood platelets and th e vitronectin-containing fluid-phase terminal complement complex, the SC5b-9, which is a non-cytolytic variant of the membrane attack comple x C5b-9(m). SC5b-9 affinity for the platelet membrane glycoprotein IIb /IIIa (GPIIb/IIIa) complex was demonstrated by crossed radio-immunoele ctrophoresis of solubilized, washed platelets followed by incubation o f the immunoplates with I-125-labelled SC5b-9 and exposure to X-ray fi lms. Platelet adhesion to surfaces of polystyrene coated with the SC5b -9 complex was studied under static conditions in an enzyme immunoassa y (EIA). Thrombin-stimulated platelets but not non-stimulated platelet s adhered to the SC5b-9-coated surface, and platelet adherence was inh ibited in a dose-dependent manner by the tetrapeptide RGDS (Arg-Gly-As p-Ser). This indicates an interaction between platelet GPIIb/IIIa and an RGD sequence in SC5b-9, possibly in its vitronectin moiety. The eff ect of the SC5b-9 complex on platelet aggregation was examined in a du al-channel aggregometer, Here the SC5b-9 complex inhibited both ADP-an d thrombin-induced platelet aggregation in a dose-dependent manner, Th ese results were confirmed by electron microscopical examination of th e contents of the aggregometer cuvettes. Platelets which had been thro mbin-stimulated in the presence of SC5b-9 appeared activated and had u ndergone secretion, but showed no aggregation. By contrast, platelets from the control experiments which had been thrombin-stimulated in the absence of SC5b-9 were aggregated. To the best of our knowledge, this is the first report on a biological role of the SC5b-9 complex in pla telet function.