Platelet CD40L contributes to the progression of established plaques. Seventeen-week-old Apoe^−/− mice were injected with activated Cd40l^+/+Apoe^−/− or Cd40l^−/−Apoe^−/− platelets, or vehicle once every 5 days for 12 weeks. (A) Plaque area was quantified in the aortic arch and main branch points at 29 weeks and compared with the baseline lesion size of 17-week-old Apoe^−/− mice. (B-E) Representative longitudinal sections (hematoxylin and eosin staining; n = 8/group). *P < .05 vs Cd40l^+/+Apoe^−/−. Scale bars represent 200 μm.