Platelet-activating factor, a molecular sensor for cellular damage, activates systemic immune suppression

Ultraviolet (UV) radiation plays a critical role in the induction of nonmelanoma skin cancer. UV radiation is also immune suppressive, and the immune suppression induced by UV irradiation has been identified as a major risk factor for skin cancer induction. Previously, we showed that UV exposure activates a cytokine cascade involving prostaglandin (PG)E₂, interleukin (IL)-4, and IL-10 that induces immune suppression. However, the earliest molecular events that occur immediately after UV exposure, especially those upstream of PG₂, are not well defined. UV-irradiated keratinocytes secrete the inflammatory phospholipid mediator, platelet-activating factor (PAF). Because PAF upregulates the production of immunomodulatory compounds, including PGE₂, we tested the hypothesis that UV-induced PAF activates cytokine production and initiates UV-induced immune suppression. Both UV and PAF activated cyclooxygenase (COX)-2 and IL-10 reporter gene construct transcription. PAF mimicked the effects of UV in vivo and suppressed delayed-type hypersensitivity (DTH). Furthermore, immune suppression was blocked when UV-irradiated mice were injected with PAF receptor antagonists. In addition to the well-known role of PAF as a proinflammatory lipid mediator, we propose that the PAF receptor senses cellular damage through the recognition of PAF and/or PAF-like molecules, such as oxidized phosphatidylcholine, which activates cytokine transcription and induces systemic immune suppression.