Ultraviolet radiation-induced damage to human Langerhans cells in vivo is not reversed by ultraviolet A or visible light

Exposure of human skin in vivo to UVB radiation induces pyrimidine dimers in DNA and alters the morphology and function of epidermal Langerhans cells. Cells in human skin have been reported to contain a photoreactivation repair mechanism that, following exposure to UVA or visible light, repairs UVB-induced pyrimidine dimers. The purpose of this study was to determine whether exposure to photoreactivating light would also reverse the UVB-induced morphologic alterations in human Langerhans cells. The skin of eight healthy volunteers was exposed to a low dose of UVB radiation (between 0.75 and 1.5 times the minimal erythema dose), and immediately thereafter exposed to photoreactivating light from either BLB fluorescent lamps (UVA radiation) or incandescent bulbs (visible light). After exposure to UVB radiation, the number of ATPase⁺ epidermal Langerhans cells was reduced in all subjects to between 21% and 65% of that in unirradiated skin, and the majority of the remaining cells exhibited morphologic alterations. Exposure of the UVB-irradiated skin to photoreactivating light did not reverse or reduce these effects. We conclude that UVB-induced morphologic alterations of human Langerhans cells are not subject to photoreactivation. These results imply either that pyrimidine dimers are not involved in these effects of UVB irradiation, or that photoreactivation does not occur in human Langerhans cells in situ.