mAb-induced exposure of a cryptic epitope in the MP domain of ADAMTS13. In the open conformation, ADAMTS13 exposes a cryptic epitope in the MP domain, which is specifically detected by the anti-MP domain mAb 6A6. The binding of plasma ADAMTS13, after preincubation in the absence of mAbs (no Ab, orange) or in the presence (1.25 μg/mL) of the mAbs 15D1 (light green), 3E4 (red), and 17G2 (blue), to immobilized anti-MP domain mAb 6A6 was assessed by ELISA. Bound ADAMTS13 was detected using the biotinylated anti-T8 domain mAb 19H4 and HRP-labeled high-sensitivity streptavidin. Binding was calculated relative to binding of plasma ADAMTS13 in the presence of the activating anti-CUB1 mAb 17G2⁵  (2.5 µg/mL), which was set to 1. Differences (mean ± SD; n = 3) were compared using analysis of variance with multiple comparison. **P < .01; ***P < .005; ****P < .001.