Calcitonin gene-related peptide and nitric oxide are involved in ultraviolet radiation-induced immunosuppression

Contact hypersensitivity responsiveness to dinitrofluorobenzene is depressed in mice that are sensitized through skin sites exposed to ultraviolet (UV) radiation. Local impairment of contact hypersensitivity by UV has been associated with a reduction in antigen-presenting cell activity within UV-irradiated skin sites marked by a decrease in the density of Ia-positive epidermal Langerhans cells. Our recent studies have demonstrated that neurogenic mediators (e.g. calcitonin gene-related peptide (CGRP) and nitric oxide (NO) contribute to cutaneous inflammation following exposure of rats to high-dose UV radiation. Since CGRP and NO inhibit antigen presentation by dendritic cells in vitro, we have investigated the possible involvement of CGRP and NO in local immunosuppression in UV-irradiated rodents. Hindpaw skin of Sprague-Dawley rats and back skin of UV-susceptible C57BL/6 mice was exposed to acute UV radiation (2.0 J/cm² and 0.5 J/cm², respectively). Alterations in cutaneous CGRP content were analyzed by a specific radioimmunoassay (RIA). In separate experiments, the CGRP receptor antagonist CGRP-(8–37) (10⁻⁵ M) and the nitric oxide synthase inhibitor $\text{N}{}^{\mathrm{<mtext>G</mtext>}}\text{-}\text{nitro}\text{-}\text{L}\text{-}\text{arginine}$ methyl ester (L-NAME) (2 × 10⁻⁵ M) were topically applied to UV-exposed skin before induction of contact hypersensitivity with dinitrofluorobenzene. Finally, we examined the effects of UV irradiation and epicutaneous application of CGRP on Ia-positive Langerhans cells by immunohistochemical analysis of epidermal sheets. It was found that UV exposure lead to a decrease in skin CGRP levels starting already 2 h after irradiation and reaching a minimum (less than 40% of non-irradiated control skin) at 6–12 h. Contact hypersensitivity reactions were significantly suppressed by UV radiation in rat skin (by 51%) and murine skin (by 80%). Topical administration of both CGRP-(8–37) and L-NAME before sensitization restored the capacity to respond to haptens applied to UV-exposed skin. Both UV exposure and topical CGRP reduced the density of Ia-positive epidermal cells. Our data indicate that CGRP may be released from sensory neurons following cutaneous UV irradiation and that CGRP and NO contribute to UV-induced local immunosuppression. Moreover, topical administration of CGRP or its antagonist may be able to modulate epidermal Langerhans cell activity in vivo.