ATRA inhibits Th1 and Th2 polarization. Polyclonal CD4⁺CD62L⁺ cells isolated by MACS from C57Bl6J mice were polarized for 5 days under neutral, Th1 (IL-12 and anti–IL-4), or Th2 (IL-4 and anti–IFN-γ) favoring culture conditions with or without 1 μM ATRA. (A) Cytokine production at day 5 by intracellular staining. Numbers on plots are percentages of total cells. (B) Cells were washed thoroughly after 5 days in culture, then resuspended at 0.5 × 10⁶ cells/mL and restimulated overnight with plate-bound anti-CD3 and anti-CD28 in media without additional cytokines. Supernatants were collected and measured by ELISA; error bars represent standard deviations (n = 3). These experiments are representative of 3 independent experiments.