Although most of our information about photoimmunology comes from studies of laboratory animals, evidence of immunological effects of sunlight and UV exposure on humans is beginning to accumulate. Exposing human skin to sunlight or artificial sources of UV radiation causes the same types of alterations to human Langerhans cells as previously described in mice. Furthermore, human keratinocytes secrete a variety of immune modulatory factors following UV exposure, including IL-1, IL-6, Il.-lO, TNFa and PGE2. Sensitization of humans with contact allergens through the UV-irradiated skin also suppresses the contact hypersensitivity reaction. Interestingly, although the UV-induced suppression of contact hypersensitivity was evident in approximately 40-50% of irradiated normal human volunteers, almost 100% of skin cancer patients were susceptible to the suppressive effects of UV radiation. These findings not only suggest that there is a genetic component that influences the sensitivity of humans to UV-induced immune suppression, similar to that in mice, but also that the UV-induced suppression of immune reactivity may be an additional risk factor in the induction of human skin cancer.
One question of considerable importance concerns the potential of UV-induced immune suppression to alter the immune response to pathogenic microorganisms. While animal studies have clearly shown that UV exposure can suppress the immune response to a variety of infectious agents and increase the incidence, severity and duration of infection, it is not clear whether photoimmunosuppression similarly affects the immune response to infectious agents in humans. Some evidence suggests that the ability of sunlight to trigger recrudescence of herpes simplex virus infection is due to the immunosuppressive activity of UV radiation. In addition, UV irradiation of the site of elicitation of a delayed hypersensitivity reaction to leprosy antigens in healthy, immune subjects reduced the granulomatous reaction and decreased the number of CD4 T cells in the reaction site. Thus, UV radiation seems to have the ability to modify the immune responses of humans to infectious agents, but whether this translates into an increase in the pathogenesis of infectious diseases remains unclear.
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