Novel cryptic ADAMTS13 epitopes uncover a distinct open ADAMTS13 conformation in immune-mediated TTP

Open ADAMTS13 conformation is gaining clinical interest as a biomarker for diagnosing immune-mediated thrombotic thrombocytopenic purpura (iTTP) and monitoring remission patients for increased relapse risks. Yet, little is known on how open the structure of ADAMTS13 exactly is in iTTP patients. In this study, we aimed to assess the uniformity of open ADAMTS13 across iTTP patients. Hereto, we identified four monoclonal antibodies (mAbs) that recognize epitopes cryptic in closed ADAMTS13 from healthy donors but accessible upon antibody-mediated ADAMTS13 opening. Distributed across its D, T7, T8 and CUB1 domains, these cryptic epitopes indicate ADAMTS13 closure through multiple interdomain contacts extending beyond the well-described S-CUB interaction. Interestingly, all acute iTTP patients consistently present one distinct open ADAMTS13 in which all novel cryptic epitopes are accessible. During remission, closed ADAMTS13 with all epitopes being cryptic, is predominantly found in patients with restored activity, whereas distinct open ADAMTS13 is present in patients with subclinical disease. Furthermore, IgGs from iTTP patients opened the conformation of ADAMTS13, corroborating the role of pathogenic autoantibodies in opening ADAMTS13 in iTTP. These new cryptic epitope-recognizing mAbs hold the promise to further enhance our understanding of ADAMTS13's compactly closed conformation and may support the prediction of early relapses in the future.