Effect of exogenous RA administration on effector and regulatory T-cell infiltration into the colon. Lethally irradiated Balb/c mice were transplanted with 10 × 10⁶ B6 BM together with B6 spleen cells (adjusted to yield a T-cell dose of 0.6 × 10⁶). Cohorts of mice were then treated with either DMSO (n = 6 to 8/group, black bars) or RA (n = 6 to 8/group, white bars). (A) Absolute cell number of CD4⁺IFN-γ⁺ or (B) CD4⁺IL-17⁺ T cells in the spleen, liver, lung, and colon of animals treated with either DMSO or RA and then killed 19 to 21 days after transplantation. (C) Absolute cell number of CD8⁺CD103⁺ cells in the colon of recipient mice. Data are presented as the mean (± standard error of the mean [SEM]) and are the cumulative results from 2 independent experiments. (D) Lethally irradiated Balb/c mice were transplanted with 10 × 10⁶ Foxp3^EGFP BM and spleen cells (adjusted to yield a T-cell dose of 0.6 × 10⁶). Cohorts of mice were then treated with either DMSO (n = 8) or RA (n = 8). Mice in both groups were killed 19 to 21 days after transplantation. The absolute cell number of Foxp3⁺ (GFP⁺) Tregs in the spleen, liver, and colon of mice is depicted. Data are presented as the mean (± SEM) and are the cumulative results from 2 independent experiments. (E) Percentage of CD4⁺ and CD8⁺ T cells that expressed either α4β7 in the colon or CCR9 in the small intestines of transplant recipients that were treated with either DMSO or RA. Animals were killed 19 to 21 days post transplantation. Data are presented as the mean (± SEM) and are the cumulative results from 2 independent experiments. Statistics: *P ≤ .05, **P < .01, ***P < .001.