Indoleamine 2,3-dioxygenase 2 deficiency exacerbates imiquimod-induced psoriasis-like skin inflammation

Indoleamine 2,3-dioxygenase 1 (IDO1) is an enzyme known to suppress immune responses, and several reports have showed that it is associated with psoriasis. IDO2 is an isoform of IDO1, recently identified as a catalytic enzyme in the tryptophan-kynurenine pathway, which is expressed in dendritic cells and monocytes. The expression of IDO2 in immune cells suggests that IDO2 may contribute to immune functions. However, the role of IDO2 in the pathogenesis of psoriasis remains unclear. In this study, to elucidate the role of IDO2 in psoriasis, we assessed imiquimod (IMQ)-induced psoriasis-like dermatitis in IDO2 knockout (KO) mice. Skin inflammation, evaluated by scoring erythema, scaling, and ear thickness, was significantly worse in the IDO2 KO mice than in the wild-type (WT) mice. The mRNA expression levels of TNF-α, IL-23p19, and IL-17A, key cytokines involved in the development of psoriasis, were also increased in the IDO2 KO mice. Furthermore, immunohistochemistry revealed that the number of Ki67-positive cells in the epidermis and CD4-, CD8-, and IL-17-positive lymphocytes infiltrating the dermis were significantly increased in the IDO2 KO mice. These results suggest that IDO2 might decrease IL-17 expression, thereby resulting in the suppression of skin inflammation in IMQ-induced psoriasis-like dermatitis.