Factor H Binds to Von Willebrand Factor (VWF) and Regulates Its Cleavage by ADAMTS-13

Factor H is a plasma protein that regulates activation of the alternative complement system. Mutations in the factor H gene are associated with a rare form of thrombotic microangiopathy, known as atypical hemolytic uremic syndrome. Hemolytic uremic syndrome (HUS) and thrombotic thrombocytopenic purpura (TTP) are characterized by systemic (TTP) or renal (HUS) microvascular thrombosis. The clinical presentations of TTP and HUS have some common clinical features, including the presence of thrombocytopenia, intravascular hemolysis, and mechanical injury to red cells. We investigated whether factor H has any interaction with proteins involved in the pathogenesis of thrombotic thrombocytopenic purpura, namely von Willebrand Factor (VWF) and ADAMTS-13. We found that factor H binds to the A1 and A2 domains of VWF, and inhibits cleavage of VWF by ADAMTS-13 under static and flowing conditions. Factor H deficient mice had a delayed thrombous formation after vessel wall injury, and showed a lower mortality rate and a higher platelet count after Shiga toxin/lipopolysaccharide challenge compared to their wild-type littermates. We concluded that factor H, in addition to its complement regulatory activity, might regulates cleavage of VWF by ADAMTS-13. Presence of an abnormal factor H that is unable to control complement activity on cell surfaces but still capable of inhibiting cleavage of VWF might contribute to the pathogenesis of thrombotic microangiopathies.