Repeated irradiation with suberythemal ultraviolet B reduces the number of epidermal Langerhans cells

The influence of repeated low-dose ultraviolet B (UVB) radiation, to which we are exposed in daily life, has not been fully clarified, although the damage caused by exposure to high-dose UVB radiation has been well-studied in recent years. To investigate skin damage caused by repeated low-dose exposure, we evaluated the extent of injury to the Langerhans cells which are known to be involved in the cutaneous immune system. The backs of hairless mice were exposed to the following doses of UVB radiation: 100 mJ/cm² once, 50 mJ/cm² twice, 25 mJ/cm² four times or 10 mJ/cm² ten times. Skin specimens were taken for histochemical and electron microscopic examination 24 h after the final irradiation. Epidermis exposed to UVB radiation demonstrated a decrease in the number of Langerhans cells which showed less dendricity. The population of these cells in specimens exposed to repeated suberythemal doses was reduced to 40%, whereas exposure to a single high dose of UVB with the same energy resulted in a reduction of only 33%. These results indicate that repeated suberythemal doses injure Langerhans cells more than a single high-dose exposure. Furthermore, Birbeck granules in Langerhans cells of UVB-irradiated epidermis were reduced and tended to show shortening of their rod portion. The present study suggested that repeated challenge with suberythemal UVB radiation, to which we are all exposed in daily life, can cause substantial damage to Langerhans cells.