Photoprotection: sunscreens and the immunomodulatory effects of UV irradiation

UV-B irradiation (UVR) of the host, in both humans and animal models, induc es dose-related acute and chronic changes in skin which include erythema an d photoageing, and induction of cancer. It can also induce modulation of im mune responses of the host to antigens presented following irradiation. Com mercially-available, broad-spectrum, high SPF (15, 15 +) sunscreens protect against most effects of UV irradiation. An exception is the effects of UVR on immune responsiveness, with varying degrees of protection having been r eported. We examined a system of UV-induced systemic suppression of contact hypersensitivity (CHS) responses in BALB/c mice. A range of commercially-a vailable, broad spectrum, high SPF (15 +) sunscreens demonstrated at best p artial protection against systemic immunosuppression, yet were able to prot ect against two hallmarks of acute UVR-induced damage: skin oedema and kera tinocyte proliferation. Two major models have been identified for the induc tion of immunosuppression following UVR, one identifying trans-urocanic aci d (trans-UCA; deaminated histidine, located in the stratum corneum) as the critical photoreceptor, the other featuring DNA. UVR of trans-UCA produces cis-UCA, which itself is immunomodulatory. There was some abrogation of tra ns to cis isomerisation of urocanic acid in UV-irradiated, sunscreen-protec ted mice. However, the majority of the immunomodulation seen in these mice was abrogated by pretreatment with a monoclonal antibody to cis-urocanic ac id. It is possible to induce formation of cis-urocanic acid in BALB/c skin in the absence of immunosuppression, using lower doses of UV radiation, ind icating that formation of cis-urocanic acid in the stratum corneum is not n ecessarily sufficient to induce immunosuppression in the UV-irradiated host . The mechanisms of induction of the immunomodulated state in the UV-irradi ated host are potentially diverse and the subject of ongoing debate. Our st udies maintain a role for cis-UCA, and form the basis for further studies o n its involvement in immunomodulation by UVR in sunscreen-protected hosts. (C) 1998 Elsevier Science B.V. All rights reserved.