IL-21 counteracts the ability of Tregs to inhibit T-cell proliferation and activation marker expression. CFSE-labeled BALB/c.Thy1.1⁺ CD4⁺CD25⁻ Tconv (2.5 × 10⁴) were cultured with anti-CD3 and CD19⁺ B cells (5 × 10⁴) with the indicated ratios of Thy1.2⁺CD4⁺CD25⁺ Tregs alone or in the presence of 100 ng/mL IL-21. At day 3, cells were analyzed by flow cytometry. The relative cell number of conventional T cells (A) and their expression profiles for CD25 and CD69 (B), and CFSE (C) are shown. The average Tconv cell number in the absence of cytokine or Tregs was 108 558 ± 19 694 and, with the addition of IL-21, was 109 311 ± 25 925. Data are representative of at least 5 independent experiments. *P < .05; **P < .01; ***P < .001.