Low cleavage of VWF-string by ADAMTS13 in the absence of platelets at all calcium concentrations. (A) HEPES buffer sometimes containing 1 U/mL ADAMTS13, with or without 1.5 mM calcium, was perfused over stimulated HUVECs at 2.5 dyn/cm2 for 5 minutes. Following a 2-minute wash, 108 platelets/mL were introduced under identical buffer and shear conditions. VWF-string length was measured in multiple fields of view 5 minutes after platelet perfusion. VWF-string length decreased by 30% upon inclusion of ADAMTS13, although the difference was not statistically significant. (B) Alexa-488–labeled mAb DD3.1 (DD3.1-Ax488) was perfused over stimulated HUVECs for 5 minutes at indicated wall shear stress (2.5, 10, 40 dyn/cm2) before introduction of 1 U/mL ADAMTS13 for additional 5 minutes. VWF strings (stretched green lines, sometimes denoted by white arrows) were typically no different before and after ADAMTS13 perfusion. Data are from the analysis of 17 to 71 strings in each independent run in different fields of view.
