Effects of UV on the migration and function of epidermal antigen presenting cells

Depletion of epidermal Langerhans cells (LC) and the concomitant depression of the skin immune system after excessive exposure to ultraviolet B light (UVB) has been established in the international literature for some time. Our investigations were intended to determine whether or not these phenomena occurred as a direct result of increased LC migration being triggered by the UVB exposure. To test this hypothesis, a sheep model was established in which the lymphatic vessels draining a defined region of skin were cannulated and the cells migrating towards the regional lymph node continuously collected. Cell populations in these collections were identified and enumerated by indirect immunofluorescence and flow cytometry. These experiments showed there was a significant, dose-dependent increase in the rate of LC migration from sheep skin after exposure to doses of UVB light exceeding 1 minimal erythemal dose (MED). In a series of parallel experiments, the functional characteristics of dendritic cells (DC) migrating from normal or UVB irradiated sheep were studied. To assay them, enriched preparations of DC were collected via cannulated afferent lymphatic vessels and pulsed with antigen prior to incubation with autologous peripheral blood lymphocytes. The relative efficiency of antigen presentation was determined by the ability of DC to induce T cell proliferation. Our data clearly demonstrate that there is a profound loss of normal antigen-presenting cell function after exposure to UVB light. Various experiments were undertaken to determine the mechanism(s) associated with these changes in migration kinetics and cellular function. Electron microscopic examinations of LC migrating from normal or UVB irradiated skin have demonstrated a profound loss of dendritic processes after UVB exposure. This provides a possible explanation for the changes in skin immunity after UVB exposure.