Research progress on intestinal tissue-resident memory T cells in inflammatory bowel disease

Tissue-resident memory T (T_RM) cells are a recently discovered subpopulation of memory T cells that reside in non-lymphoid tissues such as the intestine and skin and do not enter the bloodstream. The intestine encounters numerous pathogens daily. Intestinal mucosal immunity requires a balance between immune responses to pathogens and tolerance to food antigens and symbiotic microbiota. Therefore, intestinal T_RM cells exhibit unique characteristics. In healthy intestines, T_RM cells induce necessary inflammation to strengthen the intestinal barrier and inhibit bacterial translocation. During intestinal infections, T_RM cells rapidly eliminate pathogens by proliferating, releasing cytokines, and recruiting other immune cells. Moreover, certain T_RM cell subsets may have regulatory functions. The involvement of T_RM cells in inflammatory bowel disease (IBD) is increasingly recognized as a critical factor. In IBD, the number of pro-inflammatory T_RM cells increases, whereas the number of regulatory subgroups decreases. Additionally, the classic markers, CD69 and CD103, are not ideal for intestinal T_RM cells. Here, we review the phenotype, development, maintenance, and function of intestinal T_RM cells, as well as the latest findings in the context of IBD. Further understanding of the function of intestinal T_RM cells and distinguishing their subgroups is crucial for developing therapeutic strategies to target these cells.