Leukocytes roll more slowly in histamine-stimulated Adamts13^−/− veins. Histamine produced during inflammation is a secretagogue of Weibel-Palade bodies. Histamine was injected i.p., and the stimulated veins were observed 15 min later by intravital microscopy. Endogenous platelets and leukocytes were labeled with Rhodamine 6G. Representative photographs are shown. Lines delineate the blood vessel. White arrows indicate platelet-VWF strings. Platelet-VWF strings anchoring to endothelium were observed only in Adamts13^−/− mice. (A) Black arrows indicate leukocytes interacting with platelet-VWF strings in the Adamts13^−/− mouse. (B) The cumulative histogram allows direct comparison of rolling velocities of the leukocytes. Leukocyte rolling velocity was significantly lower in Adamts13^−/− compared with Adamts13^+/+ mice veins (P < 0.001). (C) Stimulated veins of Adamts13^−/− mice infused with either control Ig (left) or anti–P-selectin Ig (right). Platelet-VWF strings anchored to endothelium do not support leukocyte rolling if P-selectin is inhibited. The single leukocyte seen in the photograph on the right is firmly adherent. Bars, 50 μm. Videos 2 and 3 are available.