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Allosteric regulation of ADAMTS13 (A Disintegrin And Metalloproteinase with ThromboSpondin type-1 motif, member 13) activity involves an interaction between its Spacer (S) and CUB1-2 domains to keep the enzyme in a closed, latent conformation. Monoclonal antibodies (mAb) uncouple the S-CUB interaction to open the ADAMTS13 conformation and thereby disrupt the global enzyme latency. The molecular mechanism behind this mAb-induced allostery remains poorly understood. To gain insights in the mAb-induced S-CUB uncoupling and global latency disruption, we combined hydrogen/deuterium exchange mass spectrometry (HDX MS) with structural analysis of ADAMTS13 CUB1-2 mutants. Thereby, the CUB1 L3 and L9 loops were fine-mapped as the 17G2 mAb binding epitope. Indirect S-CUB uncoupling was observed as mAb binding induced extensive structural dynamics within both CUB1-2 domains without directly targeting the contiguous CUB1 surface that engages with the ADAMTS13 S domain. HDX MS analysis revealed the short interdomain linker to structurally cover the central CUB1-2 domain interface, which also showed some protein regions that became more exposed upon mAb binding. Therefore, repositioning of the central CUB1-2 interface appears crucial to transfer structural dynamics between both domains. Nevertheless, mutagenesis of the short linker did not disrupt the ADAMTS13 global latency as its closed conformation was preserved. Presumably, allosteric disruption of the global latency requires a structural impact extending beyond the central interface repositioning. As anti-ADAMTS13 autoantibodies from immune-mediated thrombotic thrombocytopenic purpura (iTTP) patients also induce an open ADAMTS13 conformation, our novel insights in the antibody-mediated global latency disruption boost our understanding of the iTTP disease pathology., (Copyright © 2025 American Society of Hematology.)

Immune-mediated thrombotic thrombocytopenic purpura (iTTP) is an ultrarare thrombotic disease caused by autoantibody-induced ADAMTS13 deficiency. Open ADAMST13 conformation, induced by autoantibodies, was identified as a novel biomarker for iTTP. Determining immunoprofiles in patients with iTTP has been shown to guide the development of novel targeted therapies. However, these studies were done in mainly Caucasian iTTP cohorts. To validate those findings across other ethnic cohorts, we investigated 195 acute TTP plasma samples from the Japanese iTTP registry. Seventy-six of the 195 samples had detectable ADAMTS13 antigen levels, of which 94.7% were shown to have an open ADAMTS13 conformation. A positive correlation was observed between ADAMTS13 inhibitor titers (a diagnostic parameter in Japan) and anti-ADAMTS13 immunoglobulin G autoantibody titers. Studying anti-M, anti-DT, anti-CS, anti-T2-T5, anti-T6-T8, anti-CUB1-2 autoantibodies and the corresponding immunoprofile showed that 73% of the patients had anti-CS autoantibodies and 25.8% had anti-M autoantibodies, with the latter being higher than in Caucasians. Stratifying patients according to their immunoprofiles revealed that the profile with only anti-CS autoantibodies was the most common immunoprofile similar to that in Caucasians (28.9%). Although this profile did not affect the 1-year TTP-related mortality rate, patients with autoantibodies against all 6 ADAMTS13 fragments had a higher risk for TTP-related death than other patients (P = .02). We here validated open ADAMTS13 as a novel biomarker for acute iTTP and determined the dominant immunoprofiling in the Japanese cohort, contributing to setting up the diagnosis and managing guidelines across different ethnic cohorts and developing ADAMTS13 variants that do not bind to the anti-CS autoantibodies., (© 2023 by The American Society of Hematology. Licensed under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0), permitting only noncommercial, nonderivative use with attribution. All other rights reserved.)

Immune-mediated thrombotic thrombocytopenic purpura (iTTP) is caused by an autoantibody-mediated deficiency in ADAMTS13. In healthy individuals, ADAMTS13 has a folded conformation in which the central spacer (S) domain interacts with the C-terminal CUB domains. We recently showed that ADAMTS13 adopts an open conformation in iTTP and that patient immunoglobulin G antibodies (IgGs) can open ADAMTS13. Anti-ADAMTS13 autoantibodies in patients with iTTP 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. To this end, we purified anti-CS and anti-CUB autoantibodies from 13 patients with acute iTTP by affinity chromatography. The successfully purified anti-CS (10/13 patients) and anti-CUB (4/13 patients) autoantibody fractions were tested further in our ADAMTS13 conformation enzyme-linked immunosorbent assay 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., (© 2021 by The American Society of Hematology. Licensed under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0), permitting only noncommercial, nonderivative use with attribution. All other rights reserved.)