Anti-idiotypic network involved in restoring ADAMTS13 activity in immune-mediated thrombotic thrombocytopenic purpura (iTTP)

Abstract

Abstract The culprit of iTTP, a fatal autoimmune disease is a severe deficiency of the von Willebrand factor-cleaving protease ADAMTS13, caused by autoantibodies inhibiting its enzymatic activity. The pathophysiology upholding remission in certain patients but leading to relapse in others is unknown. We hypothesize that one mechanism keeping pathogenic autoantibodies in check is a network of anti-idiotypic antibodies, which we aimed to identify and characterize. From splenic mononuclear cells of two frequently relapsing iTTP patients (A and C), the anti-idiotypic IgG1 Fab κ/λ repertoire was amplified by phage display technology on previously generated selecting monoclonal anti-ADAMTS13 Fabs (Schaller et al., Blood 2014). Purified Fabs were pooled (pools A (n=4) and C (n=4)) and each pool was tested for its ability to neutralize anti-ADAMTS13 antibodies thereby restoring ADAMTS13 activity in plasma of 22 iTTP patients using the FRETS-vWF73 assay. Single anti-idiotypic clones revealed a highly diverse IGHV gene usage within and between the two iTTP patients, but four IGHV genes were shared among them. Partial neutralization of functional ADAMTS13 inhibitors of >2BU/ml in 3/11 (27%) and 5/11 (45%) patients and full neutralization in 2/11 (18%) and 3/11 (27%) patients with lower inhibitor titer (1–2BU/ml) were shown for pools A and C, respectively. To summarize, we have identified functional anti-ADAMTS13 anti-idiotypes in the splenic IgG repertoire of two patients underlining the role of an anti-idiotypic response involved in remission of iTTP. The contribution of single anti-idiotypic Fabs to the neutralization potential and plasma mixing studies investigating acute and remission phase antibody repertoires are underway.