Inactivation of ADAMTS13 by plasmin as potential cause of thrombotic thrombocytopenic purpura

Summary Background: ADAMTS13 deficiency causes accumulation of unusually large von Willebrand factor (VWF) molecules which cross-link platelets in circulation or on the endothelial surface. This process of intravascular agglutination leads to the microangiopathy thrombotic thrombocytopenic purpura (TTP). Most TTP cases have acquired anti-ADAMTS13 autoantibodies that inhibit enzyme function and/or clear it from circulation. However the reason for ADAMTS13 deficiency is not always easily identified in a subset of patients. Objectives: To determine the origin of ADAMTS13 deficiency in a case of acquired TTP. Methods: Western blotting of ADAMTS13 in plasmas from acute and remission phase. Results: The ADAMTS13 deficiency was not caused by mutations or (detectable) autoantibodies, but an abnormal ADAMTS13 truncated fragment (100kDa) was found in acute but not remission phase plasma. This fragment resulted from enzymatic proteolysis since recombinant ADAMTS13 was equally cleaved when in the presence of acute but not remission phase plasma. Inhibitor screening showed that ADAMTS13 was cleaved by a serine protease that could be dose-dependently inhibited by addition of exogenous alpha(2)-antiplasmin. Examination of the endogenous alpha(2)-antiplasmin antigen and activity confirmed deficiency of alpha(2)-antiplasmin function in acute but not remission phase plasma. To investigate the possibility of ADAMTS13 cleavage by plasmin in plasma, urokinase-type plasminogen activator was added to an (unrelated) congenital alpha(2)-antiplasmin deficient plasma sample to activate plasminogen. This experiment confirmed cleavage of endogenous ADAMTS13 similar to that observed in our TTP patient. Conclusion: We report the first acquired TTP patient with cleaved ADAMTS13 and show that plasmin is involved.status: publishe