Retinol levels are higher in the small intestine (SI) than in the colon and correlate with enhanced RA signaling in the small intestinal lamina propria (SI-LP) CD103+ DCs. (a) RA receptor (RAR)α and RARγ mRNA levels in sorted SI and colonic CD103+ DCs. Mean (s.d.) of three to five experiments with sorted cells from one to two pooled mice per experiment. (b) Retinol levels in tissues. Mean (s.d.) of retinol determinations from tissues of eight individual mice. (c) RA (filled boxes) or carrier (filled circles, filled triangles) was administered intra-rectally into mice as described in the “Methods” section, and aldehyde dehydrogenase activity in SI and colonic LP CD11b+ CD103+ DCs was assessed by flow cytometry. Each symbol represents an individual mouse. DC, dendritic cell; MLN, mesenteric lymph node; PLN, peripheral lymph node; RA, retinoic acid; SPL, spleen.