Depletion of CD8⁺ T cells results in more severe thrombocytopenia and impairs responsiveness to steroid therapy in vivo. For the active model, WT mice were transplanted with immunized β3^−/− splenocytes with or without depletion of CD8⁺ T cells. DEX treatment began at day 6. (A) Thrombocytopenia was more severe in mice given CD8⁺ T cell–depleted splenocytes compared with those transplanted with nondepleted splenocytes as indicated by ^&&&P < .001. Mice transplanted with CD8⁺ T cell–depleted immunized β3^−/− splenocytes were less responsive to oral DEX compared with mice transplanted with nondepleted splenocytes as indicated by ^###P < .001. ***P < .001, nondepleted splenocytes vs nondepleted splenocytes + DEX. N = 8. In our passive mouse model, CD8⁺ T cells were depleted from WT mice by injection of anti-CD8 mAb (400 μg intravenously) before inducing passive ITP with anti-β3 mAb (9D2, 1 μg intraperitoneally). Platelet counts were not significantly affected by anti-CD8 mAb depletion of CD8+ T cells. DEX was administered (10 mg/kg/day, intraperitoneally, daily) beginning at 4 hours after anti-β3 mAb injection. (B) Thrombocytopenia was more severe in CD8⁺ T cell–depleted mice compared with those given the anti-β3 integrin mAb alone. N = 6. (C) CD8⁺ T cell–depleted thrombocytopenic mice were less responsive to DEX compared with mice with normal levels of CD8⁺ T cells. N = 6. ***P < .001. Mean ± SD.