New trends in photobiology

Within the last decade it has been found that the keratinocyte is not only a mechanical barrier to the outside but is also a fully immunocompetent cell that can release immunomodulating cytokines such as interleukin (IL) 1, IL 3, IL 6 and colony-stimulating factors (CSF). The constitutive production of these mediators by keratinocytes both in vivo and in vitro is very low; however, it can be dramatically enhanced by various stimuli such as tumour promotors or endotoxin. In addition, UV light is one of the most potent inducers of cytokine release. Accordingly, UV exposure results in increased production of IL 1, IL 3, IL 6, tumour necrosis factor and granulocyte/macrophage-CSF by epidermal cells. The secretion of these cytokines causes local immunologic and inflammatory reactions following UV irradiation. These factors, however, may also enter the circulation and thus may be responsible for systemic effects. In addition, UV light causes keratinocytes to release immunosuppressive factors which block contact hypersensitivity reaction and IL 1 activity. The production of such immunoinhibitors may play an essential pathogenic role during systemic UV-induced immunosuppression. This review will focus on the biological effects of epidermal-cell-derived cytokines, whose release is induced by UV light, and their role in immunologic and inflammatory reactions following UV exposure will be discussed.