Generation and validation of small ADAMTS13 fragments for epitope mapping of anti-ADAMTS13 autoantibodies in immune-mediated thrombotic thrombocytopenic purpura

Background: In immune-mediated thrombotic thrombocytopenic purpura (iTTP), patients develop an immune response against the multidomain enzyme ADAMTS13. ADAMTS13 consists of a metalloprotease (M) and disintegrin-like (D) domain, 8 thrombospondin type 1 repeats (T1-T8), a cysteine-rich (C), a spacer (S), and 2 CUB domains (CUB1-2). Previous epitope mapping studies have used relatively large overlapping ADAMTS13 fragments. Objectives: We aimed at developing small nonoverlapping ADAMTS13 fragments to fine map anti-ADAMTS13 autoantibodies in iTTP patients. Methods: A library of 16 ADAMTS13 fragments, comprising several small (M, DT, C, S, T2-T5, T6-T8, CUB1, CUB2), and some larger fragments with overlapping domains (MDT, MDTC, DTC, CS, T2-T8, CUB1-2, MDTCS, T2-C2), were generated. All fragments, and ADAMTS13, were expressed as a fusion protein with albumin domain 1, and purified. The folding of the fragments was tested using 17 anti-ADAMTS13 monoclonal antibodies with known epitopes. An epitope mapping assay using small ADAMTS13 fragments was set up, and validated by analyzing 18 iTTP patient samples. Results: Validation with the monoclonal antibodies demonstrated that single S and CUB1 were not correctly folded, and therefore CS and CUB1-2 fragments were selected instead of single C, S, CUB1, and CUB2 fragments. Epitope mapping of antibodies of patients with iTTP confirmed that 6 nonoverlapping ADAMTS13 fragments M, DT, CS, T2-T5, T6-T8, and CUB1-2 were sufficient to accurately determine the antibody-binding sites. Conclusion: We have developed a tool to profile patients with iTTP according to their anti-ADAMTS13 antibodies for a better insight in their immune response.