Mechanisms involved in ultraviolet light-induced immunosuppression

Ultraviolet radiation (UV) represents one of the most important environmental factors affecting human health, especially with regard to its hazardous effects on the generation of skin cancer, suppression of the immune system and premature skin aging. At molecular level, various chromophores have been identified, and DNA remains the major chromophore in the skin. Epidermal Langerhans cells (LC) are considered as the main targets of UV, as UV inhibits their antigen-presenting activity and their capacity to stimulate allogeneic type 1 T cells. Keratinocytes are also a target of UV light and they produce and release numerous soluble and immunosuppressive mediators. In human skin, IL-10 is mainly produced by dermis CD11b + macrophages and neutrophils that infiltrate epidermis after intense UV. UV-induced immunosuppression is transferable with suppressor T cells whose phenotype is still debated (Natural Killer T cells and T regulatory type 1 cells). Although the mechanisms by which immune regulatory suppressor T cells act still remain unclear, there is increasing evidence that apoptosis of epidermal LC or reactive T cells may play an important role through the Fas/FasL system.