Solar ultraviolet radiation (UVR) is well known for its immunosuppressive properties. UVR can suppress immune reactions both in a local and a systemic fashion. One of the major molecular mediators of photoimmunosuppression is UVR-induced DNA damage. In contrast to immunosuppressive drugs, UVR does not act in a general but antigen-specific fashion. This is due to the induction of regulatory T cells. Epidermal Langerhans cells harboring UVR-induced DNA damage appear to be essentially involved in the induction of these cells. Cytokines including interleukin (IL)-12, -18 and -23 exert the capacity to reduce UVR-induced DNA damage via induction of DNA repair. Accordingly, these cytokines prevent UVR-mediated immunosuppression. In contrast to IL-18, IL-12 and IL-23 can also inhibit the suppressive activity of regulatory T cells by a mechanism which still needs to be determined. Clarification of the molecular mechanisms underlying UVR-induced immunosuppression will help to develop new immunosuppressive therapeutic strategies by utilizing UVR-induced regulatory T cells for the treatment of immune-mediated diseases. In addition, these insights will contribute to a better understanding of photocarcinogenesis since suppression of the immune system by UVR essentially contributes to the induction of skin cancer.
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