IgG Antibodies Directed towards a Single Antigenic Surface In the Spacer Domain of ADAMTS13 Predominate In Plasma of Patients with Acquired TTP.

The majority of patients diagnosed with thrombotic thrombocytopenic purpura (TTP) have primarily autoantibodies directed towards the spacer domain of ADAMTS13. Previously, we have shown that residues Arg660, Tyr661 and Tyr665 are involved in binding of anti-ADAMTS13 antibodies. Inspection of the three-dimensional structure of the spacer domain reveals that Arg658 and Phe592 are in close proximity to Arg660, Tyr661 and Tyr665.

In this study we explored whether Arg658 and Phe592 contribute to a previously identified antigenic surface in the spacer domain. We also addressed Ig class distribution as well as the contribution of antibodies directed towards the carboxy-terminal TSP2-8 and CUB1-2 domains in a large cohort of patients.

The epitope specificity of anti-spacer domain directed antibodies was examined using two recombinant anti-spacer domain directed antibodies I-9 and II-1 and plasma of 48 patients with acute acquired TTP by means of immunoprecipitation of ADAMTS13 variants containing single or multiple alanine substitutions. Reactivity of patient-derived antibodies directed towards the TSP2-8 and CUB1-2 domains was also addressed by immunoprecipitation. Quantitative Ig class analysis was performed using human monoclonal antibody II-1 as an internal reference.

Plasma from all 48 patients included in this study contained anti-ADAMTS13 antibodies of subclass IgG; plasma of 5 patients contained anti-ADAMTS13 IgM and in plasma of 9 patients had anti-ADAMTS13 antibodies of subclass IgA1. No anti-ADAMTS13 antibodies of subclass IgA2 were observed. Plasma of some patients contained anti-TSP2-8 (∼17%) and anti-CUB1-2 (∼35%) domain directed antibodies. Analysis of anti-spacer domain antibodies revealed that residues Arg568 and Phe592 contribute to an antigenic surface in the spacer domain. The majority of patients (90%) lost the reactivity towards the spacer domain of ADAMTS13 through the introduction of multiple alanine substitutions at positions Arg568, Phe592, Arg660, Tyr661 and Tyr665.

IgG antibodies directed towards a single antigenic surface in the spacer domain of ADAMTS13 predominate in plasma of patients with acquired TTP.

No relevant conflicts of interest to declare.