CD40L enhances collagen-dependent thrombus formation in Apoe-deficient mice under flow. (A) Heparin/D-phenylalanyl-L-prolyl-L-arginine chloromethylketone-anticoagulated blood from Cd40l^+/+Apoe^−/− or Cd40l^−/−Apoe^−/− mice was passed over VWF or collagen at shear rates of 1700 or 1000 seconds, respectively. (B) Representative phase-contrast images for platelets perfused over collagen are shown after 4 minutes of flow. Arrows indicate platelet aggregates (n = 4-6). *P < .05. (C) Size distribution of multiplatelet thrombi on collagen, established from morphometric image analysis reported as numbers of platelets per agglomerate. *P < .001. (D) Role of CD40L and CD40 in the formation of platelet leukocyte-aggregate mice. Numbers of leukocytes (cells/mm²) on immobilized Cd40l^+/+Apoe^−/− or Cd40l^−/−Apoe^−/− platelets (n = 6). *P < .05. (E) After in vitro coincubation, flow cytometric analysis of frequencies of CD41⁺CD11b⁺ cell aggregates, demonstrating the contribution of both platelet CD40L and leukocyte CD40 (n = 6). *P < .05 vs WT/WT.