Density and proportions of the epidermal T cell population in human sun-exposed skin differ from those in sun-protected skin: preliminary immunohistochemical study

Epidermal T cells, which are found in clinically normal human skin, show topographic differences in density and proportions; however, the mechanisms and the biological consequences of such differences are still unknown. In a previous work, we showed that epidermal T cells are altered in number and composition after a single exposure to solar-simulated radiation (SSR). The purposes of the present investigation were, first, to compare the density of epidermal T cells and the proportion of T cell subpopulations in habitually sun-exposed versus sun-protected sites; second, to determine the effects of repetitive exposures to SSR on the latter cell populations. Biopsies from habitually sun-exposed, sun-protected and solar-simulated-exposed skin of 28 healthy volunteers were taken and immunohistochemistry was performed on cryostat sections. Compared with sun-protected sites, epidermal CD3⁺ T cell numbers of habitually sun-exposed sites were significantly lower. Double staining showed that the number of CD3⁺CD8⁺ T cells was significantly lower in sun-exposed than in sun-protected skin, whereas the numbers of CD3⁺CD4⁺ T cells were similar in both sites. Therefore, the CD4/CD8 ratio was markedly higher in sun-exposed compared to sun-protected sites. Moreover, repeated exposures of sun-protected skin to SSR induced a significant reduction in number of epidermal CD3⁺ T cells. The mean number of epidermal CD3⁺CD8⁺ double stained cells significantly decreased after such exposures, while the epidermal CD3⁺CD4⁺ T cell subpopulation was not significantly changed. In conclusion, both chronically sun-exposed skin and repeatedly SSR-exposed skin show a decrease in density of epidermal CD3⁺ and CD3⁺CD8⁺ T cells. We hypothesize that such sun-induced changes may weaken the immunosurveillance capacity of the skin and therefore increase the occurrence of skin cancer.