IL-21 down-regulates IL-2 and counteracts Treg suppression in vivo. (A) Thy1.1⁺ CD4⁺ DO11.10 T cells (1 × 10⁶) were adoptively transferred into Thy1.2⁺ BALB/c recipients. Mice were immunized with OVA/alum intraperitoneally on day 1 and received daily intraperitoneal injections of IL-21 (1 μg) or vehicle control (PBS). Plots represent secreted IL-2, and graph represents collated IL-2 mean fluorescence intensity for Thy1.1⁺CD4⁺ T cells harvested from the spleen at day 5. (B) Thy1.1⁺ CD4⁺ DO11.10 T cells (1 × 10⁶) were adoptively transferred into Thy1.2⁺ BALB/c recipients alone or with Thy1.2⁺CD4⁺CD25⁺ DO11.10 Tregs (2.5 × 10⁵). Treg populations were preincubated alone or with 20 ng/mL IL-2 as indicated. Mice were immunized with OVA/alum intraperitoneally on day 1 and received daily intraperitoneal injections of IL-21 (1 μg) or vehicle control. At day 5, absolute numbers of Thy1.1⁺CD4⁺DO11.10⁺ cells in the spleen were determined by flow cytometry. Data are representative of at least 2 independent experiments. *P < .05. ns indicates not significant.