The role of IL-22 in the pathogenesis of skin diseases

IL-22 is an IL-10 family cytokine that acts mainly on epithelial cells. It is produced by immune cell subsets, including CD4? T cells, natural killer cells, and natural killer T cells. In the skin, IL-22 mediates keratinocyte proliferation and epidermal hyperplasia, inhibits terminal differentiation of keratinocytes, and induces the production of antimicrobial proteins. Although IL-22 production was initially linked with IL-17 expression in Th17 cells, IL-22 production can also occur in an apparently unique subset of cells that lacks the production of IL-17 and IFN-γ (Th22). Interestingly, Th22 cells express skin homing chemokine receptors CCR4 and CCR10. Indeed, Th22 cells reside in the normal skin and are shown to be enriched in the lesional skin of inflammatory skin diseases, indicating the importance of IL-22 in skin homeostasis and pathogenesis of skin diseases. Although psoriasis is the first example of an organ-specific immune disorder for which the role of IL-22 has been comprehensively studied, a growing body of evidence indicates that this cytokine also plays a critical role in the pathogenesis of atopic dermatitis. In this review, we discuss the role of IL-22 in the pathogenesis of skin diseases, particularly focusing on psoriasis and atopic dermatitis. Targeting IL-22 may have promise as a potential therapeutic for various skin diseases.