Retinoic acid and TGF-β orchestrate organ-specific programs of tissue residency

Tissue-resident memory T (T_RM) cells are integral to tissue immunity, persisting in diverse anatomical sites where they adhere to a common transcriptional framework. How these cells integrate distinct local cues to adopt the common T_RM cell fate remains poorly understood. Here, we show that whereas skin T_RM cells strictly require transforming growth factor β (TGF-β) for tissue residency, those in other locations utilize the metabolite retinoic acid (RA) to drive an alternative differentiation pathway, directing a TGF-β-independent tissue residency program in the liver and synergizing with TGF-β to drive T_RM cells in the small intestine. We found that RA was required for the long-term maintenance of intestinal T_RM populations, in part by impeding their retrograde migration. Moreover, enhanced RA signaling modulated T_RM cell phenotype and function, a phenomenon mirrored in mice with increased microbial diversity. Together, our findings reveal RA as a fundamental component of the host-environment interaction that directs immunosurveillance in tissues.