CD8⁺ T cells suppressed platelet apoptosis and clearance in vitro. (A-B) CD8⁺ T cells from immunized β3^−/− mice were added to CD8⁺-depleted splenocytes and then cocultured with WT platelets. Control has no CD8⁺ T cells added. (A) After 72 hours, platelets remaining in culture were identified by forward and side scatter characteristics and gated on 7-AAD⁻ and assessed for apoptosis with caspase 3/7 and Annexin V. N = 4. (B) CD8⁺ T cells from immunized β3^−/− mice were cocultured with platelets for 72 hours, and then the remaining platelets in solution were counted, with flow cytometry as events per second. More platelets remained in the coculture system after incubation with CD8⁺ T cells from immunized β3^−/− mice compared with control or naïve β3^−/− CD8⁺ T cells. N = 4. (C) Platelets were labeled with 9D2 primary antibody and FITC anti-mouse IgG. Platelets were then cocultured with macrophages with or without CD8⁺ T cells. After 24 hours, FITC-positive macrophages were detected by flow cytometry. CD8⁺, CD8⁺CD25⁺, CD8⁺CD122⁺, and CD8⁺CD103⁺ T cells from immunized β3^−/− and WT mice significantly suppressed phagocytosis of platelets by macrophages. Furthermore, the CD8⁺ T cells from immunized β3^−/− mice suppressed phagocytosis more effectively (P < .05). This inhibitory function was also dose dependent (supplemental Figure 4) *P < .05, **P < .01. Mean ± SEM.