Ultraviolet A radiation suppresses an established immune response: Implications for sunscreen design

The ultraviolet radiation present in sunlight is the primary cause of nonme lanoma skin cancer and has been implicated in the development of cutaneous malignant melanoma. In addition, ultraviolet is immune suppressive and the suppression induced by ultraviolet radiation has been identified as a risk factor for skin cancer induction. Ultraviolet also suppresses the immune re sponse to infectious agents. In most experimental models, ultraviolet is ap plied to immunologically naive animals prior to immunization. Of equal conc ern, however, is the ability of sunlight to suppress established immune rea ctions, such as the recall reaction in humans, which protects against micro bial infections. Here we demonstrate that solar-simulated ultraviolet radia tion, applied after immunization, suppresses immunologic memory and the eli citation of delayed-type hypersensitivity. Further, we found that wavelengt hs in the ultraviolet A region of the solar spectrum were critical for indu cing immune suppression. Ultraviolet A (320-400 nm) radiation was as effect ive as solar-simulated ultraviolet A + B (290-400 nm) in suppressing the el icitation of an established immune response. Irradiation with ultraviolet A l (340-400 nm) had no effect. Supporting a critical role for ultraviolet A in ultraviolet-induced immune suppression was the observation that applying a sunscreen that contained an ultraviolet B only filter had no protective effect, whereas, a sunscreen containing both ultraviolet A and ultraviolet B filters totally blocked ultraviolet-induced immune suppression. These dat a suggest that sunlight may depress the protective effect of prior vaccinat ion. In addition, the observation that ultraviolet A is immunosuppressive i ndicates the need for ultraviolet A protection when designing sun protectio n strategies.