Determination of an action spectrum for ultraviolet radiation-induced immunosuppression in humans

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Ultraviolet (UV) radiation causes mutations in the skin which can develop into cancers many years later. The skin's immune system is an important defence that prevents most potentially cancerous cells from developing into skin tumours. UV also suppresses skin immunity, allowing cancer cells to proliferate unchecked. There is also evidence that this UV-induced immunosuppression may have important effects on infectious diseases such as herpes virus, leprosy and tropical ulcers. Sunlight contains UVB, which causes sunburn, and UVA. Until recently, UVA was thought to have little effect on skin immunity. We have previously shown that both broadband UVB and UVA are immunosuppressive, even after single exposures equivalent to 8 minutes of sunlight. It is not known which UVA wavelengths are most immunosuppressive. These studies will use a xenon arc solar simulator and a series of interference filters to produce narrow UVB and UVA wavebands. The effects of these wavebands on humans can be studied by deliberately eliciting immune responses to substances which volunteers are already sensitive to. One model of skin immunity in these studies is allergy to nickel (in earrings and costume jewellery), which affects 10% of women. The volunteers' backs are exposed to different UV wavelengths, and then nickel patches are taped to both the UV-exposed areas and adjacent, unexposed areas. By comparing the intensity (redness and thickness) of these nickel reactions, immunosuppression caused by each waveband can be measured. Similarly, the Mantoux reaction (in people vaccinated against tuberculosis) provides another, complementary model of skin immunity. Knowledge of the immune effects of different UV wavelengths (an action spectrum for UV immunosuppression) would improve understanding of skin cancer and enable development of better sunscreens, which currently offer only partial immune protection.