RARα is necessary for TGF-β1 induction of FoxP3. (A) Naive CD4⁺ cells were polyclonally stimulated for 3 days under Th17 conditions or TGF-β1 alone for 3 days with TTNBP (pan-RAR agonist) or LE540 (pan-RAR antagonist). IL-17 and FoxP3 expression were measured on fixed cells by intracellular staining. Numbers on plots are percentages of total cells. (B) Naive CD4⁺ T cells were polyclonally stimulated for 3 days with TGF-β1 and 1 μM ATRA, 1 μM A7980 (selective RARγ agonist), or 1 μM AM580 (selective RARα agonist). FoxP3 expression was measured on fixed cells by intracellular staining. Data are representative of 2 independent experiments. Numbers on graphs are percentages of cells that are FoxP3.⁺