Retinoic Acid Modulates Immune Differentiation in a Human Small Intestinal In Vitro Model

Retinoic acid (RA) plays a key role in mucosal immune regulation and tolerance, with implications for inflammatory bowel disease (IBD). However, its effects have not been extensively studied in humanized in vitro models that recapitulate epithelial-immune interactions. We established a 3D in vitro small intestinal model composed of three epithelial cell types, naïve CD4⁺ T cells, and monocyte/dendritic cell (M/DC) precursors derived from CD34⁺ umbilical cord blood hematopoietic stem/progenitor cells. The epithelial microenvironment strongly suppressed monocyte/DC differentiation and T cell activation, indicating a regulatory role of epithelial-derived signals. Retinoic acid (RA) priming of M/DC precursors induced CD103⁺CD11b⁺Sirp1α^- regulatory DCs and promoted a shift from naive to memory-type T cells. Upon addition of pro-inflammatory cytokines (TNF-α, IFN-γ, IL-1β), the model mimicked an inflamed intestinal state, resulting in CD14⁺CD16⁺ inflammatory monocytes and increased T cell activation (CD25⁺CD69⁺). RA-primed DCs modestly counterbalanced T cell activation and IBD-like responses, even under inflammatory conditions. Flow cytometry and clustering analysis revealed distinct immune cell phenotypes depending on RA exposure and cytokine context. This model provides a reproducible and physiologically relevant human system to study RA-mediated immune programming in the intestinal mucosa and may support the development of novel therapeutic strategies for IBD and related inflammatory conditions. Statistical differences were evaluated using ANOVA with Tukey's post-hoc test (<i>n</i> = 4; <i>p</i> < 0.05).