Complement & platelets: Prothrombotic cell activation requires membrane attack complex induced release of danger signals.

Complement activation in the diseases paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic syndrome (aHUS) results in cytolysis and fatal thrombotic events which are largely refractory to anticoagulation and/or antiplatelet therapy. Anti-complement therapy, however, efficiently prevents thrombotic events in PNH and aHUS but the underlying mechanisms remained unresolved. We show that complement-mediated hemolysis in whole blood induces platelet activation similarly to activation by ADP. Blockage of C3 or C5 abolished platelet activation. We found that human platelets failed to respond functionally to the anaphylatoxins C3a and C5a. Instead, complement activation did lead to prothrombotic cell activation in whole blood when MAC-mediated cytolysis occurred. Consequently, we demonstrate that ADP receptor antagonists efficiently inhibited platelet activation although full complement activation, causing hemolysis, occurred. By employing an established model of mismatched erythrocyte transfusions in rats, we cross-validated the above findings in vivo utilizing the complement inhibitor OmCI and cobra venom factor (CVF). Consumptive complement activation in this animal model only led to a thrombotic phenotype when MAC-mediated cytolysis occurred. In conclusion, complement activation only induces substantial prothrombotic cell activation if terminal pathway activation culminates in MAC-mediated release of intracellular ADP. These results explain why anti-complement therapy efficiently prevents thromboembolisms without interfering negatively with hemostasis.