Platelet CD40L transiently disturbs T-cell homeostasis. Flow cytometric analysis of T-cell distribution in blood from adult Apoe^−/− mice treated with activated Cd40l^+/+Apoe^−/− or Cd40l^−/−Apoe^−/− platelets, or vehicle 12 hours after injection. Percentage of CD4⁺ T cells (A), CD8⁺ T cells (B), and CD4⁺CD25⁺Foxp3⁺ regulatory T cells (C). (D) Forty-eight hours after injection of activated platelets, no differences in T-cell phenotype were observed. Anti-CD25 treatment increases atherosclerotic plaque area (E) on injection of Cd40l^+/+Apoe^−/− platelets and reverses the protective effect of platelet-CD40L deficiency on atherosclerosis. (E) Quantification and (F) representative figures of atherosclerotic plaques in the aortic root of Apoe^−/− mice treated with activated Cd40l^+/+Apoe^−/− or Cd40l^−/−Apoe^−/− platelets in the absence or presence of an anti-CD25 antibody (n = 30). *P < .05 vs Cd40l^+/+Apoe^−/−.