HUVEC-released ADAMTS13 attached to HUVEC-secreted VWF strings. HUVECs were stimulated with histamine for 2 minutes, washed, and incubated for 20 minutes (without fixative) with polyclonal goat antibodies made against different domains of human ADAMTS-13: (A) BL156 (Tsp1-4 domain); (B) BL152 (metalloprotease domain); and (C) BL153 (distinct peptide within metalloprotease domain); and secondary anti-goat IgG-594 (red). The cells were rewashed before staining with rabbit anti–VWF plus anti-rabbit IgG-488 (green) to visualize VWF strings. Cell nuclei were stained blue with DAPI. The images magnified ×600, represent 6 to 12 experiments, and demonstrate that blocking the activity of HUVEC-released ADAMTS-13 enables the ADAMTS-13 bound only to uncleaved VWF strings to be detected.