IL-21 partially reverses T_reg-mediated inhibition of CD8 T-cell proliferation. (A) Sorted T_regs (5 × 10⁴/well) were cultured with IL-2 (12.5 U/mL), IL-7 (10 ng/mL), IL-15 (30 ng/mL), or IL-21 (30 ng/mL) in the presence or absence of anti-CD3 and autologous dendritic cells. (B) Sorted CD8⁺CD25⁻ effector T cells (5 × 10⁴/well) were cultured with T_regs at ratio of 1:1 (CD8⁺/T_regs) in the presence of anti-CD3 and autologous dendritic cells. IL-21 (30 ng/mL) was added as indicated. Proliferation was measured by [³H]thymidine incorporation pulsed on day 3 and harvested 16 to 20 hours later. The results of CPM were calculated from triplicate wells with standard error of the mean. The P value was obtained by applying paired sample t test to evaluate the influence of T_regs on the CD8 proliferation with and without IL-21. Error bars represent plus or minus standard error calculated from triplicate results for each data point.