Platelet CD40L promotes advanced plaque development. Silastic collars were placed in 14-week-old Apoe^−/− mice fed a 0.21% cholesterol diet. Activated Cd40l^+/+Apoe^−/− or Cd40l^−/−Apoe^−/− platelets, or vehicle were injected once every 5 days for 6 weeks. (A) Plaque volume was calculated from the plaque areas measured in 6 slices. (B) Macrophage infiltration: representative staining and percentage of Mac3⁺ cells of all plaque cells. (C) Cleaved caspase-3 was determined by immunohistochemistry and graded from 0 (not present) to 3 (highly present). (D) Iron deposits (percentage of mice containing lesions with iron depositions) were determined by Perl iron staining (n = 9/group); quantification (left) and representative figures (right) are shown. *P < .05 vs Cd40l^+/+Apoe^−/−. Scale bars represent 100 μm. N.D. indicates not detected.