Effect of IL-21 on Tconv surface marker expression and Foxp3 induction. (A) BALB/c CD4⁺CD25⁻ Tconv (2.5 × 10⁴) and CD19⁺ B cells (5 × 10⁴) were cultured alone, with anti-CD3, with IL-21, or with both for 15 to 18 hours. Surface marker expression on gated Tconv is shown. (B) BALB/c Thy1.1⁺CD4⁺CD25⁻ T conv (2.5 × 10⁴) were cultured with anti-CD3, CD19⁺ B cells (5 × 10⁴), and Thy1.2⁺CD4⁺CD25⁺ Tregs (2.5 × 10⁴) alone or in the presence of IL-21. Three days later, Foxp3 expression by Thy1.1⁺CD4⁺ cells and Thy1.2⁺CD4⁺ cells was determined. There was negligible Foxp3 induction in Tconv (Foxp3 expression in gated Tregs is shown as a positive control). Data are representative of at least 3 independent experiments.