Johanna A. Kremer Hovinga

Rare immune-mediated thrombotic thrombocytopenic purpura (iTTP) is a life-threatening disease resulting from a severe autoantibody-mediated ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 motifs, member 13) deficiency. Acute iTTP episodes are medical emergencies, but when treated appropriately >95% of patients survive. However, at least half of survivors will eventually experience a relapse. How remission of an initial episode is achieved and factors contributing to reemergence of anti-ADAMTS13 Abs and a relapsing course are poorly understood. In acquired hemophilia and systemic lupus erythematosus, anti-idiotypic Abs counteracting and neutralizing pathogenic autoantibodies contribute to remission. We selected and amplified the splenic anti-idiotypic IgG1 Fab κ/λ repertoire of two relapsing iTTP patients on previously generated monoclonal inhibitory anti-ADAMTS13 Fabs by phage display to explore whether anti-idiotypic Abs have a role in iTTP. We obtained 27 single anti-idiotypic Fab clones, half of which had unique sequences, although both patients shared four H chain V region genes (VH1-69*01, VH3-15*01, VH3-23*01, and VH3-49*03). Anti-idiotypic Fab pools of both patients fully neutralized the inhibitor capacity of the monoclonal anti-ADAMTS13 Abs used for their selection. Preincubation of plasma samples of 22 unrelated iTTP patients stratified according to functional ADAMTS13 inhibitor titers (>2 Bethesda units/ml, or 1–2 Bethesda units/ml), with anti-idiotypic Fab pools neutralized functional ADAMTS13 inhibitors and restored ADAMTS13 activity in 18–45% of those cases. Taken together, we present evidence for the presence of an anti-idiotypic immune response in iTTP patients. The interindividual generalizability of this response is limited despite relatively uniform pathogenic anti-ADAMTS13 Abs recognizing a dominant epitope in the ADAMTS13 spacer domain.