Specific blockade of VWF string cleavage by antibody to ADAMTS13. HUVECs were stimulated with 100μM histamine in buffer ± 20 μg/mL anti–ADAMTS-13 (BL156) for 2 minutes. After washing, the cells were incubated for 5, 10, and 20 minutes in Ca²⁺/Zn²⁺-containing buffer alone or buffer containing 20 μg/mL anti–ADAMTS-13. After incubation, the cells were fixed and stained with rabbit anti–VWF plus goat anti–rabbit IgG-488. The number and lengths of cell-anchored VWF strings were measured from microscope fields (201 μm × 150 μm; ×200) from 3 to 4 experiments at each time point and shown as means ± SD. (A) VWF strings with lengths exceeding 150 μm after 5-, 10-, and 20-minute incubations with Ca²⁺/Zn²⁺-containing buffer alone (□) or buffer containing 20 μg/mL anti–ADAMTS-13 (▩). (B) Lengths of long VWF strings (> 150 μm, 101-150 μm, and 51-100 μm) and lengths of cleaved VWF strings (< 20 μm) in the presence and absence of anti–ADAMTS-13 after each incubation time. A greater number of long VWF strings was quantified in the presence of anti–ADAMTS-13. (C) HUVECs were stimulated with 100μM histamine for 2 minutes, washed, and incubated in serum-free media ± 20 μg/mL anti–ADAMTS-13. Levels of VWF released into cell supernatants and collected into 10mM EDTA after 5 minutes (P = .05; n = 10) and 15 minutes (P = .01; n = 8) were significantly lower in supernatants from cells incubated with antibodies to ADAMTS-13.