Distribution of IL-18 and IL-18 receptor in human skin: various forms of IL-18 are produced in keratinocytes

Human interleukin-18 (IL-18) enhances IL-12-mediated IFN-gamma production b y lymphocytes and Fas/perforin-mediated cytolysis by NK cells. IL-18 is syn thesized as a 24 kDa proform, and the proform is processed in the cytoplasm into an 18 kDa mature form. Active and precursor forms of IL-18 have been detected in immunocompetent cells, and active IL-18 exerts its functions th rough its receptor. We sought to determine which human skin cells are respo nsible for production of IL-18 and which express its receptor. Monoclonal a ntibodies against human IL-18 and polyclonal antibody against IL-18 recepto r were provided for this analysis. Formalin-ein bedded and frozen sections of human epidermis were analyzed by immunoperoxidase and immunofluorescence staining. IL-18 was detected in all living cell layers of the epidermis, h air follicles, arrectores pilorum, eccrine ducts and endothelial cells. IL- 18 was localized in the cytoplasm of cells in living epidermal cell layers. In contrast, IL-18 receptor was mainly detected in keratinocytes and expre ssed in the cell periphery in living cell layers. Since keratinocytes were the main source of IL-18 and its receptor, cultured human keratinocytes wer e further analyzed by immunoblotting. IL-18 receptor was identified as an 8 0 kDa single band. The mature 18 kDa and precursor 24 kDa forms of IL-18 we re detected by our monoclonal antibody (mAb) 21 and mAb 132, respectively, while only the 18 kDa form was detected by a commercial mAb, 125-2H. Cultur ed keratinocytes showed positive granular staining for IL-18 in the cytopla sm and positive staining for IL-18 receptor mainly in the cell periphery. O ur findings indicate that mature IL-18, precursor IL-18 and IL-18 receptor are simultaneously expressed with different localizations by human epiderma l keratinocytes. Keratinocytes might be activated by their own IL-18 in an autocrine or paracrine fashion.