Human platelet-initiated formation and uptake of the C5-9 complex of human complement.

We have studied the interaction of radiolabeled complement components with normal human platelets, platelets from a patient with paroxysmal nocturnal hemoglobinuria, and rabbit platelets in the absence of known complement activators or in the presence of cobra venom factor (CVF). When unwashed platelets in platelet rich plasma, or washed platelets suspended in serum or autologous plasma, were incubated for 30 min, C3 and terminal components (C5, C8, and C9) were found to bind to them. The terminal components were shown to be bound as the C5–9 complex, rather than as individual proteins, by eluting them from the platelet membrane and examining their behavior on ultracentrifugation. They co-sedimented at a rate characteristic of the stable C5–9 complex (22S). The complex so formed did not differ by ultracentrifugational criteria from that binding to rabbit platelets after CVF activation of complement. Although C3 was not included in the complex, it did not appear to be bound by nonspecific absorption.