Expression of interleukin (IL)-22, a member of the IL-10 cytokine family, has recently been reported in a number of human diseases, including mucosal-associated infections and inflammatory disorders of the intestine, skin, and joints. Both T cells and an emerging category of innate lymphoid cells are sources of IL-22, while the IL-22 receptor complex is reported to be restricted to cells of nonhematopoietic origin. The ligand-receptor distribution of IL-22-IL-22R permits immune cells to regulate responses of epithelial cells, endothelial cells, fibroblasts, and other tissue-resident stromal cells. This pathway appears to be critically important at barrier surfaces where epithelial cells play an active role in the initiation, regulation, and resolution of immune responses. Functional studies in murine model systems indicate that IL-22 has immunoregulatory properties in infection, inflammation, autoimmunity, and cancer. In these models, the functional consequences of IL-22 expression can be either pathologic or protective, depending on the context in which it is expressed. Therefore, advancing our understanding of the biology of IL-22-IL-22R may yield novel therapeutic targets in multiple human diseases. In this review, we discuss recent findings on the expression, regulation, and function of IL-22 at barrier surfaces, and offer insights into the next frontiers to be studied in this complex cytokine pathway.
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