Neutrophils infiltrating ultraviolet B-irradiated normal human skin display high IL-10 expression

Exposure to an erythemal dose of ultraviolet B (UVB) is known to induce interleukin (IL-10) expression in human skin. It is generally believed that this IL-10 is predominantly expressed by CD11b(+) HLA-DR+ macrophages that infiltrate the UVB-exposed skin. This cytokine is presumed to contribute to the immunosuppressive effects of UVB by inhibiting cell-mediated immune responses. We recently demonstrated that neutrophils, which also invade UVB-irradiated skin, express CD11b and HLA-DR as well. In addition, we showed that the presence of these neutrophils affects T-cell responses in primary T-cell cultures derived from UVB-exposed skin. Since neutrophils invade UVB-exposed skin and, like macrophages, express CD11b and HLA-DR, we sought to determine whether neutrophils represent another source of IL-10. Skin biopsies were obtained from four healthy volunteers before and 2 days after exposure to four minimal erythema doses of UVB. A series of immunohistochemical double-staining procedures using the following markers was performed: IL-10, CD11b, HLA-DR, CD36, neutrophil elastase, and CD66b. As expected IL-10 could be detected in CD11b(+) HLA-DR+ CD36(+) macrophages in the epidermis and dermis of UVB-exposed skin. Surprisingly, the majority of the abundant IL-10 expression was found in CD11b(+) HLA-DR+ elastase(+) CD66b(+) neutrophils. Cytospin preparations from dermal cell suspensions confirmed the IL-10 expression by neutrophils displaying characteristic multilobular nuclei. Thus, neutrophils in UVB-exposed skin express IL-10 and should be recognized as active coplayers in the creation of the UVB-induced immunosuppressive micro-environment.