Inhibition of proteolytic activity of ADAMTS13 and variants by autoantibodies. Recombinant WT or variants M1 to M5 (final concentration of 0.2nM) was incubated without (−) or with (+) 35μM of mAb II-1 (A) or 5 to 10 μL of heat-inactivated normal human plasma (N) or plasma from TTP patient 1 (P; B) for 30 minutes. The residual activity was determined by the cleavage of multimeric VWF. EDTA (10mM) was included in the last lane as a negative control. The relative residual activity was determined by the ratio of product (P) to high molecular weight VWF (HMW) multimer using ImageJ Version 1.45m software and normalized to the activity in the presence of normal human plasma. (C) The percentage of inhibition (mean ± SD) by a panel of 12 TTP patient plasmas. **P < .001, statistically highly significant difference between WT and 3 variants (ie, M3, M4, and M5).