Adamts13^−/− mice show an increase in the expression of endothelial P-selectin, and higher soluble P-selectin and plasma VWF. Increased plasma VWF (A) and soluble P-selectin (B) concentrations were found by ELISA in the Adamts13^−/− compared with Adamts13^+/+ mice. (C) 1-μm fluorescent microspheres coupled to anti–P-selectin or control Ig were infused through the retroorbital venous plexus in the same mouse, and their binding to the unstimulated veins was analyzed. Representative photographs are shown. Lines delineate the blood vessel. Almost no control Ig beads bound to unstimulated veins, compared with several hundred anti–P-selectin beads. Bars, 100 μm. (D) Quantification of the number of anti–P-selectin beads binding per square millimeter. A significantly higher number of anti–P-selectin beads was observed binding to unstimulated mesenteric veins of Adamts13^−/− than to Adamts13^+/+ mice, suggesting increased spontaneous Weibel-Palade body release. Data represent the mean ± SEM.