Neonatal exposure to solar simulated ultraviolet radiation leads to deviation of immune development

Exposure to ultraviolet radiation (UVR) during childhood reduces the likelihood of developing auto-immune disease, but increases the risk of developing melanoma. As immune suppression is likely to contribute to both of these alterations, we propose that exposure to UVR during the neonatal period modifies development of the immune system. To test this hypothesis, neonatal mice were exposed to a single erythemal dose of solar simulated UVR at 3 days of age. At eight weeks postexposure the contact hypersensitivity response to oxazalone was significantly reduced in the neonatally irradiated mice. Analysis of the lymph nodes of such mice revealed an increase in the percentage of T regulatory cells. Furthermore, during the challenge phase of the contact hypersensitivity response, the number of T regulatory cells within the medullary cord region of lymph nodes was significantly increased, suggesting that these cells are responsible for the suppressed response in neonatally irradiated mice. When an equivalent number of T regulatory cells was isolated from irradiated or control mice, eight weeks after neonatal irradiation, the suppression was not transferred to naïve mice to a greater extent by the cells from the irradiated mice. As such, we propose that exposure to UVR during the neonatal period leads to a deviation of immune development, leading to suppressed immunity which is not due to the induction of T regulatory cells.