Anti-idiotypic small molecules neutralize pathogenic anti-ADAMTS13 autoantibodies in autoimmune Thrombotic Thrombocytopenic Purpura in vitro - new treatment tools in vivo? (P5173)

Abstract

Abstract A therapy aiming at neutralization of inhibitory autoantibodies (Abs) causing autoimmune thrombotic thrombocytopenic purpura (aTTP) would be highly desirable. We screened a large combinatorial small protein library of Designed-Ankryin-Repeat-Protein (DARPins, Molecular Partners AG, Switzerland; library size 1015-23) for anti-idiotypic binders to an equimolar pool of three spleen-derived inhibitory anti-ADAMTS13 monoclonal Abs of two aTTP patients containing 1 of 4 shared CDR3 motifs. Four unique anti-idiotypic DARPins were purified and their neutralization potential tested against 8 spleen-derived anti-ADAMTS13 Abs (200 nM), belonging to the same, shared CDR3 motif used for selection. Anti-idiotypic DARPins restored ADAMTS13 activity up to 100% (2000 nM). By ELISA we found Abs in plasma of 27 of 37 randomly selected aTTP patients that bound specifically to the anti-idiotypic DARPins. Pre-incubation with anti-idiotypic DARPins (960 nM) prevented binding of plasma anti-ADAMTS13 Ab to recombinant ADAMTS13 in a competition ELISA (n=5). Our findings of anti-idiotypic DARPins able to neutralize several, although similar (belonging to the same CDR3 motif) pathogenic anti-ADAMTS13 Abs and the fact that the DARPins are also specifically recognized by plasma-derived anti-ADAMTS13 Abs of different aTTP patients are promising and suggest that possibly only a limited number of the selected DARPins are required to develop a general treatment for aTTP.