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Interleukin (IL)-22 is a T cell-derived cytokine that has been reported recently to induce cutaneous inflammation in an experimental murine model of psoriasis, and to induce in vitro an inflammatory-like phenotype. In the present study, we assessed the presence of IL-22 and the IL-22 receptor 1 (IL-22R1) in skin lesions, skin-derived T cells, as well as IL-22 levels in sera from patients with psoriasis. IL-22R1 and IL-10R2 transcripts are expressed at a similar level in psoriatic and healthy skin. In contrast, IL-22 mRNA expression was up-regulated in psoriatic skin lesions compared to normal skin, whereas IL-22 mRNA levels in peripheral blood mononuclear cells from psoriatic patients and normal subjects were similar. Circulating IL-22 levels were significantly higher in psoriatic patients than in normal subjects. T cells isolated from psoriatic skin produced higher levels of IL-22 in comparison to peripheral T cells isolated from the same patients. IL-10 was expressed at similar levels in skin biopsies and peripheral blood mononuclear cells of psoriatic patients and normal subjects. Finally, we show here that supernatants of lesional psoriatic skin-infiltrating T cells induce an inflammatory response by normal human epidermal keratinocytes, resembling that observed in psoriatic lesions. Taken together, the results reported in this study indicate that IL-22 is a cytokine produced by skin-infiltrating lymphocytes that is potentially involved in initiation and/or maintenance of the pathogenesis of psoriasis.

Cutaneous inflammatory diseases such as psoriasis vulgaris and atopic dermatitis are associated with altered keratinocyte function, as well as with a particular cytokine production profile of skin-infiltrating T lymphocytes. In this study we show that normal human epidermal keratinocytes express a functional type II oncostatin-M (OSM) receptor (OSMR) consisting of the gp130 and OSMRβ components, but not the type I OSMR. The type II OSMR is expressed in skin lesions from both psoriatic patients and those with atopic dermatitis. Its ligand, OSM, induces via the recruitment of the STAT3 and MAP kinase pathways a gene expression profile in primary keratinocytes and in a reconstituted epidermis that is characteristic of proinflammatory and innate immune responses. Moreover, OSM is a potent stimulator of keratinocyte migration in vitro and increases the thickness of a reconstituted epidermis. OSM transcripts are enhanced in both psoriatic and atopic dermatitic skin as compared with healthy skin and mirror the enhanced production of OSM by T cells isolated from diseased lesions. Results from a microarray analysis comparing the gene-modulating effects of OSM with those of 33 different cytokines indicate that OSM is a potent keratinocyte activator similar to TNF-α, IL-1, IL-17, and IL-22 and that it acts in synergy with the latter cytokines in the induction of S100A7 and β-defensin 2 expression, characteristic of psoriatic skin. Taken together, these results demonstrate that OSM and its receptor play an important role in cutaneous inflammatory responses in general and that the specific effects of OSM are associated with distinct inflammatory diseases depending on the cytokine environment.

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