Ultraviolet B radiation up-regulates the expression of IL-15 in human skin

Ultraviolet B (UVB) radiation is a potent modulator of skin-related immune responses, particularly those involving the synthesis and the secretion of cytokines. The discovery of a new T cell mitogen, IL-15, prompted use to investigate its expression in skin and to examine the effects of UVB radiation on such expression. RNA from unirradiated and UVB-irradiated epidermal and dermal sheets derived from human foreskin as well as from unirradiated and UVB-irradiated skin cell populations were assayed for IL-15 expression by semiquantitative RT-PCR. Constitutive levels of IL-15 mRNA were detected in dermal sheets, but not in epidermal sheets. Following UVB treatment, IL-15 mRNA was induced in epidermal sheets and enhanced in dermal sheets. UVB-inducible epidermal expression of IL-15 mRNA was traced to HLA-DR- cells (presumably keratinocytes) and not to HLA-DR+ cells (Langerhans cells). Cultured keratinocytes and dermal fibroblasts displayed basal levels of IL-15 mRNA that were also up-regulated following UVB exposure. Immunoblot analysis revealed secretion of IL-15 protein by keratinocytes that was enhanced following UVB treatment. These results constitute the first report of IL-15 mRNA expression and protein production in human skin. In addition to expanding the known influence of UVB radiation on the capacity of keratinocytes and dermal fibroblasts to express immunomodulatory cytokines, these findings suggest a new mechanism by which UVB can promote Ag-independent T cell responses via elaboration of IL-15.