Solar-simulated ultraviolet irradiation induces selective influx of CD4+ T lymphocytes in normal human skin.

The proportion and composition of the human cutaneous CD3+ T lymphocyte population was determined in situ following a single exposure to physiological, erythema-inducing doses of simulated solar radiation, mainly consisting of UV radiation. Biopsies were taken 1, 2 and 7 days after local irradiation of normal volunteers with 1, 2, and 4 MED by a Xenon-arc lamp and immunohistochemistry was performed on cryostat sections. UV radiation caused an initial decrease of intraepidermal CD3+ T cell numbers or even could lead to T-cell depletion 24 and 48 hours post-irradiation, and this was followed by an infiltration of T cells in the epidermis as determined 1 week after UV exposure. The number of dermal CD3+ T cells was increased 24 hours after irradiation, reached a maximum at 48 hours and subsequently declined at day 7, though remained significantly higher than the unirradiated control. Double staining demonstrated that the CD3+ T cells, which immigrated the (epi)dermis upon UV exposure, co-expressed CD4 but not CD8. Therefore the CD4/CD8 ratio in skin was markedly increased during the first week upon UV exposure. Our time course study shows that UV radiation affects the T cell population within the human skin by depleting the majority of epidermal T cells and initiating a selective influx of CD4+ T cells.