Abrogation of epidermal antigen-presenting cell function by ultraviolet radiation administered in vivo

The exposure of animals to ultraviolet radiation (UVR) can result in dramatic alterations to their immunologic potential. Loss of ability to be contact-sensitized to skin-reactive chemicals applied to UVR-exposed surfaces, and an enhanced susceptibility to tumors induced by UVR represent two well-documented effects of such exposure. The skin Langerhans' cell (LC) is a cell type, having antigen presentation capabilities, which is functionally inactivated by UVR. The localized loss in the ability to induce a contact hypersensitivity (CS) response and the early events associated with the acquisition of a tumor-susceptible state have been hypothesized to involve LC inactivation by UVR. In this report we have investigated the relationship between the UVR-mediated loss of CS responsiveness and antigen-presenting cell (APC) function by LC following in vivo exposure of animals to UVR. The UVR-mediated loss of CS responsiveness was found to be both time-dependent and dosage-dependent, but the loss of epidermal APC function following the UVR exposure of dissociated epidermal cells or skin was immediate and dosage-dependent. The lack of time dependency associated with the loss of epidermal APC function may be due to the nature of the assay system being employed. Add-back experiments, using the proliferation of purified antigen-primed T-cells as a read-out system to evaluate APC function, require that the accessory cell be totally functional. Initiation of CS responses may still be possible, however, with LC that are partially UVR-inactivated. Lymph-node-associated and circulating accessory cells could be integrated into the response, providing any necessary components that the UV-inactivated LC lacks. We tested this possibility and conclude from these studies that this is indeed a viable possibility. Our results, therefore, provide a possible explanation for apparent differences in LC UVR susceptibility observed when in vitro and in vivo assays are employed.