Retinoic Acid Is Elevated in the Mucosa of Patients With Active Ulcerative Colitis and Displays a Proinflammatory Role by Augmenting IL-17 and IFNγ Production

BACKGROUND: All-trans retinoic acid (RA) plays a crucial role in promoting Foxp3+ Treg generation while reciprocally inhibiting Th1/Th17 generation. Our previous research highlighted that in the face of inflammatory conditions, RA plays a contrary role where it aggravates intestinal inflammation by promoting interferon (IFN) γ and interleukin (IL)-17 differentiation in vitro. METHODS: In this study we translated our in vitro results into a clinical setting where we estimated mucosal and serum RA levels along with the immunophenotypic profile (IL-17, IFNγ, Foxp3, IL-10) in adaptive (CD4, CD8) and innate-like T cells (mucosal associated invariant T cells and γδ T cells) in patients with ulcerative colitis in remission or with active inflammation. RESULTS: This is the first study to estimate RA levels in the human gut and shows that patients with active disease had increased mucosal RA levels as compared with patients in remission (4.0 vs 2.5 ng/mL; P < 0.01) and control patients (3.4 vs 0.8 ng/mL; P < 0.0001). This effect was accompanied by significantly elevated IL-17 and IFNγ in tissue CD4+, CD8+, mucosal associated invariant T+ cells, and γδ + T cells. Moreover, the raised RA levels in patients with active disease showed a positive correlation with proinflammatory cytokines (IL-17, IFNγ) and a negative correlation with IL-10. We also found that RA negatively correlated with IL-9, thereby reinstating our previous finding that RA inhibits Th9 differentiation. CONCLUSIONS: These data confirm our previous in vitro results that in the presence of inflammation, RA plays a crucial role in maintaining gut inflammation by upregulating proinflammatory markers.