IL-21 suppresses IL-2 and IFN-γ production by conventional T cells. BALB/c CD4⁺CD25⁻ Tconv (2.5 × 10⁴) were cultured with anti-CD3 and CD19⁺ B cells (5 × 10⁴) alone or in the presence of IL-21. Intracellular cytokine staining was performed at day 3. The proportion of T cells expressing the indicated cytokine (A), and representative contour plots for IL-2 and IFN-γ expression (B) are shown. (C) BALB/c CD4⁺CD25⁻ Tconv (2.5 × 10⁴) were cultured with anti-CD3 and CD19⁺ B cells (5 × 10⁴) with the indicated ratios of CD4⁺CD25⁺ Tregs alone or in the presence of either anti–IFN-γ antibody or IFN-γ. IL-21 was added where indicated. Relative Tconv cell counts at day 3 are shown. The average Tconv cell number in the absence of cytokine, blocking antibody, or Tregs was 91 011 ± 5998. Data are representative of at least 3 independent experiments. **P < .01; ***P < .001.