Platelet CD40L promotes leukocyte adhesion to the atherosclerotic arterial wall. (A-D) Intravital microscopy of adhering, rhodamine-labeled leukocytes in carotid arteries of Apoe^−/− mice treated with activated Cd40l^+/+Apoe^−/−, Cd40l^−/−Apoe^−/− platelets, or vehicle every 5 days for 10 weeks (n = 4-6). *P < .05. (E) Plasma CCL2 levels after repeated injection of Cd40l^−/−Apoe^−/− platelets into Apoe^−/− mice compared with injection of Cd40l^+/+Apoe^−/− platelets and vehicle-treated mice (n = 9). *P < .05. (F) Double immunohistochemistry for FVIII (blue) and CCL2 (red) of a CD40l^+/+Apoe^−/− atherosclerotic lesion. (G) Double immunohistochemistry for Mac3 (blue) and CCL2 (red) of a CD40l^+/+Apoe^−/− atherosclerotic lesion. (H) Endothelial (luminal) deposition of CCL2, quantified by immunohistochemical staining, is decreased in the Cd40l^−/−Apoe^−/− treatment group compared with the Cd40l^+/+Apoe^−/− group (n = 6). *P < .05.