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 patients in remission for increased risk of relapse. Nevertheless, little is known about how open the structure of ADAMTS13 is exactly in iTTP patients. In this study, we aimed to assess the uniformity of open ADAMTS13 across iTTP patients. To do this, we identified four monoclonal antibodies that recognize epitopes that are cryptic in closed ADAMTS13 from healthy donors but accessible upon antibody-mediated ADAMTS13 opening. Distributed across the 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, IgG from iTTP patients opened the conformation of ADAMTS13, corroborating the role of pathogenic autoantibodies in opening ADAMTS13 in iTTP. These new cryptic epitope-recognizing monoclonal antibodies hold promise to further enhance our understanding of compactly closed conformation of ADAMTS13 and may support the prediction of early relapses in the future.