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Antibodies that develop in patients with immune thrombotic thrombocytopenic purpura commonly target the spacer epitope R568/F592/R660/Y661/Y665 (RFRYY) of ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13). In this study we present a detailed contribution of each residue in this epitope for autoantibody binding. Different panels of mutations were introduced to create a large collection of full-length ADAMTS13 variants comprising conservative (Y F), semi-conservative (Y/F -> L), non-conservative (Y/F -> N) or alanine (Y/F/R -> A) substitutions. Previously reported gain-of-function (KYKFF) and truncated `MDTCS' variants were also included. Sera from 18 patients were screened against all variants. Conservative mutations of the aromatic residues did not reduce the binding of autoantibodies. Moderate resistance was achieved by replacing R568 and R660 by lysines or alanines. Semi-conservative mutations of aromatic residues showed a moderate effectiveness in autoantibody resistance. Non-conservative asparagine or alanine mutations of aromatic residues were the most effective. In the mixtures of autoantibodies from the majority (89%) of patients screened, autoantibodies targeting the spacer RFRYY epitope were preponderant compared to other epitopes. Reductions in ADAMTS13 proteolytic activity were observed for all full-length mutant variants, in varying degrees. The greatest reductions in activity were observed in the most autoanti-bodyresistant variants (15-35% residual activity in a FRETS-VWF73 assay). Among these, a triple-alanine mutant - RARAA - showed activity in a von Willebrand factor multimer assay. This study shows that non-conservative and alanine modifications of residues within the exosite-3 spacer RFRYY epitope in full-length ADAMTS13 resist the binding of autoantibodies from patients with immune thrombotic thrombocytopenic purpura, while retaining residual proteolytic activity. Our study provides a framewo

ADAMTS13 (a disintegrin and metalloprotease with a thrombospondin type-1 motif, member 13) and von Willebrand factor (VWF) can be considered as scale weights which control platelet adhesion during primary haemostasis. In a very uncommon condition designated thrombotic thrombocytopenic purpura (TTP), functional absence of ADAMTS13 tips the balance toward VWF-mediated platelet adhesion in the microcirculation. TTP is associated with a high mortality and arises from either a congenital or acquired autoimmune deficiency of the plasma enzyme ADAMTS13. In case of acquired ADAMTS13 deficiency, autoantibodies bind to and inhibit the function of ADAMTS13. Currently available treatments of TTP aim to supply ADAMTS13 through plasma exchange or are aimed at B-cell depletion with rituximab. None of the available therapeutics, however, aims at protection of ADAMTS13 from circulating autoantibodies. In this review, our aim is to describe the structure-function relationship of ADAMTS13 employing homology models and previously published crystal structures. Structural bioinformatics investigation of ADAMTS13 reveals many insights and explains how mutations and autoantibodies may lead to the pathophysiology of TTP. The results of these studies provide a roadmap for the further development of rationally designed therapeutics for the treatment of patients with acquired TTP. In addition, we share our opinion on the state of the art of the open-closed conformations of ADAMTS13 which regulate the activity of this highly specific VWF cleaving protease.

Immune-mediated thrombotic thrombocytopenic purpura (iTTP) is characterized by severe ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 repeats, member 13) deficiency, the presence of anti-ADAMTS13 autoantibodies and an open ADAMTS13 conformation with a cryptic epitope in the spacer domain exposed. A detailed knowledge of anti-ADAMTS13 autoantibodies will help identifying pathogenic antibodies and elucidating the cause of ADAMTS13 deficiency. We aimed at cloning anti-ADAMTS13 autoantibodies from iTTP patients to study their epitopes and inhibitory characteristics. We sorted anti-ADAMTS13 autoantibody expressing B cells from peripheral blood mononuclear cells of 13 iTTP patients to isolate anti-ADAMTS13 autoantibody sequences. Ninety-six B cell clones producing anti-ADAMTS13 autoantibodies were identified from which 30 immunoglobulin M (IgM) and 5 IgG sequences were obtained. For this study, we only cloned, expressed and purified the five IgG antibodies. In vitro characterization revealed that three of the five cloned IgG antibodies, TTP73-1, ELH2-1 and TR8C11, indeed recognize ADAMTS13. Epitope mapping showed that antibodies TTP73-1 and TR8C11 bind to the cysteine-spacer domains, while the antibody ELH2-1 recognizes the T2-T3 domains in ADAMTS13. None of the antibodies inhibited ADAMTS13 activity. Given the recent findings regarding the open ADAMTS13 conformation during acute iTTP, we studied if the cloned antibodies could recognize cryptic epitopes in ADAMTS13. Interestingly, all three antibodies recognize cryptic epitopes. In conclusion, we cloned three anti-ADAMTS13 autoantibodies from iTTP patients that recognize cryptic epitopes. Hence, these data nicely fit our recent finding that the conformation of ADAMTS13 is open during acute iTTP., SCOPUS: ar.j, info:eu-repo/semantics/published