Mucosal surface and skin protection by MHC class I-based immune regulation

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The exposure of the mucosal surfaces and the skin to the environment subjects these tissues to not only physical injuries and chemical hazards but also microbial damage and immune destruction. Tissue injuries lead to cascades of inflammatory responses involving both innate and adaptive immune cells systemically in the lymphoid system and locally in the nonlymphoid target tissues. A well-orchestrated innate and adaptive immune regulatory response in inflamed tissues is essential for damage control and wound healing. In the adaptive branch, the CD8 and CD4 lineages of αβ T cells develop in the thymus based on recognition of antigens presented by MHC class I (MHCI) and MHC class II (MHCII), respectively, yielding a repertoire that discriminates self from nonself. In the mucosal surfaces and the skin, the symbiosis with microbiota poses a challenge to the concept of self and nonself discrimination, as the mutualistic microbiota may not fit into the traditional definition of self or nonself. In a recently-published study of mucosal immune regulation at the interface with the gut microbiota, we reported the discovery of a novel subset of T cells, MHCI-restricted CD4+Foxp3+ regulatory T (CI-Treg) cells. They were generated through cross-differentiation from the CD8 lineage to CD4 T cells in a “selfless” mode and effectively controlled mucosal inflammation. CI-Treg cells recognize MHCI which is broadly expressed in all nucleated tissue cells and typically presents endogenous tissue antigens, unlike the standard MHCII-restricted CD4 Treg cells which recognize antigens processed through the exogenous pathway and presented by rare antigen-presenting cells. Microbiota plays a major role in the expansion / homeostasis of CI-Treg cells. In humans, MHCI-restricted CD4 T cells were identified long ago in health or disease conditions. However, how CI-Treg cells are generated and how they function in the mucosal surfaces and the skin remain largely unknown. Our study with a surgical approach in mice revealed that the mesenteric lymph nodes — which drain the gut mucosae, but not the thymus, were required for CI-Treg generation. Interestingly, the generation of CI-Treg cells required MHCII, but the cross-differentiated cells retained the MHCI-restricted specificity. We will use a combination of surgical, molecular and cellular approaches to investigate the role of microbial and host signals in generation and homeostatic expansion of CI-Treg cells. We will also examine the function of CI-Treg cells in protecting the mucosal surfaces and the skin from chemically-induced inflammatory damage, immune-mediated destruction, or inflammatory injury initiated by physical obstruction. The knowledge from this study will help design future interventions to control inflammatory tissue damage in the mucosal surfaces and the skin.