CD8⁺ Tregs from immunized β3^−/− mice inhibit the adaptive antiplatelet immune response in vitro. For proliferation assays, purified splenic CD8⁺ T cells or CD8⁺ Tregs from WT, naïve β3^−/−, or WT platelet immunized β3^−/− mice were added to CD8⁺ T cell–depleted splenocytes in the presence of 1 × 10⁷ WT platelets per well to induce T- or B-cell proliferation. Control cells were CD8⁺-depleted splenocytes with PBS or platelets (PLT) as indicated. Both immunized β3^−/− CD8⁺ T cells and CD8⁺ Tregs inhibited (A,E) CD4⁺ T-cell proliferation, (B,F) CD19⁺ B-cell proliferation, and (C,G) platelet-associated IgG production. (D) Immunized β3^−/− CD8⁺ T cells also inhibited IL-10 cytokine production dose dependently. N = 8. Imm, immunized. *P < .05, **P < .01. Mean ± SEM.