The role of antibodies in acquired thrombotic thrombocytopenic purpura

Acquired thrombotic thrombocytopenic purpura (TTP) is an autoimmune disease in which anti-ADAMTS13 autoantibodies cause severe enzyme deficiency. ADAMTS13 deficiency causes the loss of regulation of von Willebrand factor multimeric size and platelet-tethering function, which results in the formation of disseminated microvascular platelet microthrombi. The aims of my thesis were to develop novel assays to determine the domain specificity of anti-ADAMTS13 antibodies and use these to characterise the repertoire of antibodies in patients with acquired TTP. Functional analyses were also performed to explore the pathogenic mechanisms of these antibodies. 92 acquired TTP episodes were analysed at presentation, and through treatment and remission/relapse. Epitope mapping revealed 97% of episodes had autoantibodies that recognised the ADAMTS13 N-terminal domains. 41% episodes had antibodies recognising the N-terminal domains alone; 59% had antibodies against the C-terminal domains. Changes in autoantibody specificity were detected in 9/16 patients at relapse, suggesting a continued development of the disease. Functional analyses on IgG from 43 patients revealed inhibitory IgG were limited to anti-spacer domain antibodies. However, 15/43 patients had autoantibodies with no detectable inhibitory action, and as many as 32/43 patients had antibodies with inhibitory function insufficient to account for the severe deficiency state, suggesting that in many patients there is an alternative pathogenic mechanism. There were markedly reduced ADAMTS13 antigen levels in all presentation samples, median 6% normal (range 0-47%). ADAMTS13 antigen in the lowest quartile at first presentation was associated with increased mortality (odds ratio 5·7). This work has shown that anti-spacer domain autoantibodies are the major inhibitory antibodies in acquired TTP. However, depletion of ADAMTS13 antigen (rather than enzyme inhibition) is a dominant pathogenic mechanism. ADAMTS13 antigen levels at presentation have prognostic significance. Taken together, these results provide new insights into the pathophysiology of acquired TTP