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 framework for the design of autoantibody-resistant ADAMTS13 variants for further therapeutic development.
- ANTI-ADAMTS13 AUTOANTIBODIES
- SPACER DOMAIN