ATRA inhibits Th17 polarization and promotes FoxP3 expression. (A) CD4⁺ T cells isolated by MACS beads from OTII TCR transgenic mice were incubated with isolated CD11c⁺ cells pulsed with 1 μg OVA peptide and cultured for 3 days under neutral or Th17-favoring (IL-6, TGF-β1, anti–IFN-γ, and anti–IL-4) conditions with or without 1 μM ATRA. The figure depicts intracellular staining for IL-17 production and FoxP3 expression, and it is representative of 2 independent experiments. Numbers on plots are percentages of total cells. (B,C) Polyclonal CD4⁺CD62L⁺CD25⁻CD44⁻ cells isolated by flow cytometry were stimulated for 3 days with anti-CD3 and anti-CD28 under neutral or Th17-favoring conditions with or without ATRA. (B) Average IL-17 concentration in supernatants from 3 independent experiments as measured by ELISA. The P value was determined by a 2-tailed paired t test; a single asterisk denotes significance (P < .05). (C) FoxP3 expression normalized to β-actin levels and relative to Th-neutral conditions without ATRA; error bars represent standard deviation (n = 3); the data are representative of 2 independent experiments.