CONTINUING MECHANISTIC STUDIES OF UVA PHOTOIMMUNOPROTECTION

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The UVB portion of sunlight causes sunburn, tanning, skin cancer, and suppresses immune function. Longer wavelength UVA is significantly less damaging, may contribute to photoageing and damage to deeper skin layers, but has been much less well studied. UVB-induced immunosuppression appears to be a prerequisite for skin cancer, and experimental protection from the immunosuppression results also in reduced severity of the long-term skin cancer outcome. We have identified a protective effect by UVA radiation against UVB-immunosuppression when UVA is administered to mice at non-burning environmentally relevant doses. This was an important and unprecedented finding, and is supported by recent observations also in humans. The aim of the present study is to clarify the mechanisms by which this resistance to UVB-induced immunosuppression is achieved, according to 2 main hypotheses: 1. UVA interferes with the actions of cis-urocanic acid, a natural epidermal UV-photoproduct that appears to initiate the immunosuppression by interacting with histamine. 2. UVA alters the balance of immunological control and thus activates normal antioxidant defences of the skin such as metallothionein and haem oxygenase, which antagonise the apparent oxidative requirement for UVB-immunosuppression. These pathways lead to the prediction that increasing the UVA component of the incident radiation will reduce skin cancer development. Humans typically receive disproportionately large UVA doses sunbathing through a UVB-sunscreen, or in cosmetic sunparlours. The assumption that UVA contributes to UVB skin damage may not be true at moderate UV doses, and a potential for UVA to protect from UVB-suppressed immunity and risk of skin cancer would suggest that broad spectrum sunscreens are contraindicated, and that the UVA effects need to be exploited.