Anti-cysteine/spacer antibodies that open ADAMTS13 are a common feature in iTTP.

Immune-mediated thrombotic thrombocytopenic purpura (iTTP) is caused by an autoantibody-mediated deficiency of ADAMTS13 (A Disintegrin And Metalloprotease with ThromboSpondin type 1 repeats member 13). In healthy individuals, ADAMTS13 has a folded conformation where the central spacer domain interacts with the C-terminal CUB domains. We recently showed that ADAMTS13 adopts an open conformation in iTTP and that patient immunoglobulin G's (IgGs) can open ADAMTS13. Anti-ADAMTS13 autoantibodies in iTTP patients are directed against the different ADAMTS13 domains but almost all patients have autoantibodies binding to the cysteine/spacer (CS) domains. In this study, we investigated whether the autoantibodies against the CS and CUB domains can disrupt the S-CUB interaction of folded ADAMTS13 thereby opening ADAMTS13. Therefore, we purified anti-CS and anti-CUB autoantibodies from 13 acute iTTP patients by affinity chromatography. The successfully purified anti-CS (10/13 patients) and anti-CUB (4/13 patients) autoantibody fractions were further tested in our ADAMTS13 conformation ELISA to study whether they could open ADAMTS13. Interestingly, all purified anti-CS fractions (10/10 patients) were able to open ADAMTS13. On the other hand, only half of the purified anti-CUB fractions (2/4 patients) opened ADAMTS13. Our finding highlights that anti-CS autoantibodies that open ADAMTS13 are a common feature of the autoimmune response in iTTP.