Ultraviolet radiation modulates the immune system through the platelet -activating factor receptor

Ultraviolet radiation plays a critical role in the induction of non-melanoma skin cancer. UV radiation is also immune suppressive. Moreover, UV-induced systemic immune suppression is a major risk factor for skin cancer induction. Previous work had shown that UV exposure in vivo activates a cytokine cascade involving PGE2, IL-4, and IL-10 that induces immune suppression. However, the earliest molecular events that occur immediately after UV-exposure, especially those upstream of PGE2, were not well defined. To determine the initial events and mediators that lead to immune suppression after a pathological dose of UV, mouse keratinocytes were analyzed after sunlamp irradiation. It is known that UV-irradiated keratinocytes secrete the phospholipid mediator of inflammation, platelet-activating factor (PAF). Since PAF stimulates the production of immunomodulatory compounds, including PGE2, the hypothesis that UV-induced PAF activates cytokine production and initiates UV-induced immune suppression was tested. Both UV and PAF activated the transcription of cyclooxygenase (COX)-2 and IL-10 reporter gene constructs. A PAF receptor antagonist blocked UV-induced IL, 10 and COX-2 transcription. PAF mimicked the effects of UV in vivo and suppressed delayed-type hypersensitivity (DTH), and immune suppression was blocked when UV-irradiated mice were injected with a PAF receptor antagonist. This work shows that UV generates PAF-like oxidized lipids, that signal through the PAF receptor, activate cytokine transcription, and induce systemic immune suppression. ^