Pathogenesis of thrombotic thrombocytopenic purpura: ADAMTS13 deficiency and beyond

Thrombotic thrombocytopenic purpura (TTP) is characterized by the systemic deposition of platelet thrombi with abundance of von Willebrand factor (vWF) in the arterioles and capillaries. Recently, vWF protease (ADAMTS13) activity was found to be severely deficient in hereditary TTP as well as acquired idiopathic TTP. Homozygous or compound heterozygous mutations of ADAMTS13 gene were demonstrated in hereditary TTP. Autoantibodies against ADAMTS13 were present in majority of patients with idiopathic TTP and ticlopidine- and clopidogrel-associated TTP. The deficiency of ADAMTS13 leaves unchecked the hyperadhesive vWF unfolded under high shear stress in the microvessels, resulting in the formation of platelet thrombi, which in turn causes TTP. ADAMTS13 activity is usually normal in hemolytic uremic syndrome. Approximately 0 to 67% of idiopathic TTP patients reported may not be severely deficient of ADAMTS13. Therefore, acquired TTP is not caused by ADAMTS13 deficiency alone and may be triggered by certain stimuli that possibly cause autoimmune reactivity to ADAMTS13, and also induce platelet aggregation either dependent on or independent of vWF, and/or vascular injury, to account for variable clinical and laboratory presentations. Plasma samples from TTP patients have been shown to induce endothelial cell apoptosis and activation. Platelet aggregating factors independent of vWF purified from the plasma of a subset of TTP patients have been reported and shown to be inhibited by normal plasma and immunoglobulin G purified from normal plasma. Defective fibrinolysis and abnormal natural coagulation inhibitors may enhance the thrombi formation in the microcirculation.