CD8⁺ Tregs from immunized β3^−/− mice inhibit CD8⁺ cytotoxic activity against platelets. Platelets (5 × 10⁶) were cultured with purified 1 × 10⁵ cytotoxic CD8⁺ T cells (CD8⁺CD178⁺) with or without 5 × 10⁵ CD8⁺ Treg cells (according to the original ratio) for 24 hours. Platelet cytotoxicity was measured with (A) remaining platelet number in supernatant, determined as number of platelets per second counted by flow cytometry⁶⁹  and (B) caspase 3/7 expression. N = 4. PLT, platelet; CTL, cytotoxic CD8⁺ T cells. *P < .05, **P < .01. Mean ± SEM.