Evidence that Ultraviolet B radiation transiently inhibits emigration of Langerhans cells from exposed epidermis, thwarting contact hypersensitivity induction.

Langerhans cells (LC) play a critical role in the induction of contact hypersensitivity (CH), and ultraviolet B radiation (UVR) impairs CH induction in UVB-susceptible (UVB-S) mice via a TNF-α-dependent mechanism. A possible explanation of this effect is that UVR impairs CH in UVB-S mice by immobilizing LC transiently in the epidermis and upper dermis, thereby preventing their timely migration to draining lymph nodes. To test this hypothesis we examined in vitro and in vivo the effects of in vivo UVR on migration of la+ cells from skin of UVB-S and UVB-resistant (UVB-R) mice. Dorsal surfaces of ears of mice were irradiated with 400 J/cm2 UVB and either explanted in vitro or transplanted orthotopically to the thoracic wall of syngeneic recipients. After 24, 48, and 72 h the epidermis was recovered from these explants/grafts and the number of la+ cells determined by immunohistochemistry. Culture medium obtained after explants were removed was also evaluated for content of la+ cells. The density of la+-bearing cells in the epidermis of cultured untreated skin explants and of grafted skin fell progressively for both UVB-S and UVB-R skin during the observation period. The rate of decline in la+ cells density during this interval was greatly impaired if the skin was exposed to UVR prior to excision; this effect was seen equally in UVB-S and UVB-R skin. Recovery of la+ cells in the medium after removal of cultured untreated skin explants was maximum after 24 h and comparable in UVB-S and UVB-R skin. However, the number of la+ cells recovered in the medium from UVB-exposed skin was significantly reduced only if the skin donor was UVB-S. We conclude that the ability of UVR to impede LC migration from epidermis is significantly greater for UVB-S mice, accounting in part for the failure of these mice after UVR to acquire CH.