Stat5a/b are important for the in vitro induction of Foxp3. (A) Sorted splenic CD25⁻CD4⁺ T cells from Stat5^fl/fl and YFP⁺ CD25⁻CD4⁺ T cells from Stat5^fl/−, CD4Cre, Yfp⁺ mice were cultured with plate-bound anti-CD3 and anti-CD28 combined with or without TGF-β1, IL-2 (100 U/mL), and/or anti–IL-2 antibody as indicated. Representative of 3 experiments. Numbers in quadrants indicate FoxP3⁺ or FoxP3⁻ cells. (B) Sorted splenic CD25⁻CD4⁺ T cells from Stat3^fl/fl and Stat3^fl/fl, MMTVcre mice were cultured with plate-bound anti-CD3 and anti-CD28 combined with or without TGF-β1 and IL-6 as indicated. Numbers indicate the percentage of FoxP3⁺ cells delineated by the rectangles.