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The active metabolite of vitamin A, retinoic acid, is a key mediator of balanced immune responses. The major nuclear receptor of retinoic acid, retinoic receptor alpha (RARα), functions as a transcriptional regulator, with both active and repressive effects on transcription depending on interactions with nuclear cofactors dictated by ligand-binding effects on protein confirmation. While significant advances have been made in understanding the combined effects of retinoic acid and RARα, the individual roles of each remain incompletely identified. Epigenetic effects of all-trans retinoic acid (At-RA) and RARα on the transcriptome of T helper cells were assessed using a novel transgenic mouse strain designed to overexpress RARα in T cells and a conditional knock out strain in which RARα was specifically deleted from T cells. At-RA and RARα had divergent roles in promoting Th17 and Treg differentiation, with RARα expression favoring Th17 differentiation over Treg differentiation, and At-RA promoting Treg differentiation over Th17 differentiation. Transcriptome analysis identified groups of At-RA and RARα differentially regulated genes (DEGs). Comparison of these genes to the H3k27 acetylated and tri-methylated epigenetic modifications demonstrated that RARα expression increased the overall level of these epigenetic modifications in all DEG groups, with enhanced control of transcriptional regulation mediated by higher RARα expression. Additionally, expression of transcriptional repressors was strongly regulated by At-RA in a RARα-dependent manner and had repressive effects on the differentiation of T helper cells. Immunometabolism was also enhanced by RARα expression, leading us to study potential non-genomic roles of RARα on signaling pathways. The major TCR signaling pathways were enhanced by RARα but suppressed by At-RA, suggesting a mechanism by which RARα regulates cellular metabolism upon T cell activation. In summary, we identified distinct epigenetic and non-genomic effects of RARα as novel regulatory mechanisms by which vitamin A and retinoic acid influence immune responses. Further research into these findings, notably RARα involvement in signal transduction pathways of immune cells, will define how this research can be translated into clinically-relevant applications.

The intestinal immune system is regulated by microbes and their metabolites. The roles of gut microbial metabolites in regulating intestinal inflammation and tumorigenesis are incompletely understood. We systematically studied the roles of short-chain fatty acids (SCFAs) and their receptors (GPR43 or GPR41) in regulating tissue bacterial load, acute versus chronic inflammatory responses, and intestinal cancer development. SCFA receptor-, particularly GPR43-, deficient mice were defective in mounting appropriate acute immune responses to promote barrier immunity, and developed uncontrolled chronic inflammatory responses following epithelial damage. Further, intestinal carcinogenesis was increased in GPR43-deficient mice. Dietary fiber and SCFA administration suppressed intestinal inflammation and cancer in both GPR43-dependent and independent manners. The beneficial effect of GPR43 was not mediated by altered microbiota but by host tissue cells and hematopoietic cells to a lesser degree. We found that inability to suppress commensal bacterial invasion into the colonic tissue is associated with the increased chronic Th17-driven inflammation and carcinogenesis in the intestine of GPR43-deficient mice. In sum, our results reveal the beneficial function of the SCFA-GPR43 axis in suppressing bacterial invasion and associated chronic inflammation and carcinogenesis in the colon.

The intestine relies upon T regulatory and effector cells to regulate immune response to multiple antigens. A full understanding of this phenomenon would be significant in the treatment of food intolerance and inflammatory bowel diseases (IBDs). The role of Retinoic Acid (RA) in T-cell migration to the gut is well documented. However, the distribution of tissues where this exposure to RA occurs has not been extensively mapped. In order to determine this, the cre-lox system was used to engineer a RA-responsive reporter gene that expresses the fluorescent protein tdTomato following RA exposure. The tissues were then imaged and analyzed using histo-cytometry to determine distribution of cells with RA exposure. RA exposure in various tissue microenvironments was characterized using flow cytometry, PCR, and confocal microscopy imaging to determine the changes in lymphoid expression of tdTomato during immune activation. It was found that intestinal and lymphoid tissues had greater concentrations of cells with prior RA exposure, particularly the Peyer’s Patch, MLN, and Spleen. The preliminary results of these experiments indicate that immune activation leads to a higher density of tdTomato expressing cells in the intestine and lymphoid tissues, but lower in peripheral organs. These results indicate that immunization causes T-cells to be drawn out of peripheral tissues and into gut-associated lymphoid tissues. It is worth looking into the composition of these T-cells as compared to the base population.