Endogenous RA production and RAR signaling on donor T cells are enhanced after allogeneic BMT. (A) Lethally irradiated Balb/c mice were transplanted with Balb/c BM (10 × 10⁶) and spleen cells (0.4–0.5 × 10⁶, syngeneic) or B6 BM and an equivalent number of B6 spleen cells (allogeneic). Cohorts of animals were killed weekly, and colon tissue was analyzed for RA levels by mass spectrometric analysis as described in the section titled “Quantification of RA in tissues by mass spectrometry.” Data are derived from 9 to 10 mice per group and are cumulative results from 2 independent experiments. Statistics: *P ≤ .05. (B) Lethally irradiated Balb/c mice were transplanted with B6 BM and spleen cells to induce GVHD. RNA was extracted from sort-purified CD4⁺ and CD8⁺ T cells isolated from spleens of allogeneic transplant recipients at the indicated time points after BMT, and gene expression of RAR-α was analyzed by real-time q-PCR as described in the section titled “Real-time q-PCR.” Data are normalized for 18S ribosomal RNA and presented as fold increase over RAR-α expression in sort-purified original naive CD4⁺ or CD8⁺ donor T cells used for transplantation. Data are derived from 2 independent experiments and are presented as the mean ± standard error of the mean.