Conventional T cells are the major target for IL-21 during release from Treg-mediated suppression. (A) BALB/c lymph node Tconv (CD4⁺Foxp3⁻), Tregs (CD4⁺Foxp3⁺), and B cells (CD19⁺) were stained by flow cytometry for expression of IL-21R. (B) 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 in the presence of IL-21. Cell populations were deficient for the IL-21R as indicated. Tconv cell count at day 3 is shown. The average Tconv cell number in the absence of Tregs was 78 190 ± 2095. (C) IL-21R^−/−CD4⁺CD25⁻ Tconv (2.5 × 10⁴) were cultured with anti-CD3 and IL-21R^−/−CD19⁺ B cells (5 × 10⁴) with the indicated ratios of IL-21R^+/+CD4⁺CD25⁺ Tregs alone or in the presence of IL-21. Relative Tconv cell count at day 3 is shown. The average Tconv cell number in the absence of cytokine or Treg was 149 504 ± 9868. Data are representative of at least 3 independent experiments. *P < .05. **P < .01. ns indicates not significant.