Systemic suppression of contact hypersensitivity by UVB radiation is unrelated to the UVB-induced alterations in the morphology and number of Langerhans cells.

Exposure of mice to UVB (280-320 nm) ultraviolet radiation reduces contact hypersensitivity (CHS) reactions to chemicals that are applied subsequently to unirradiated skin. It also decreases the number and alters the morphology of Langerhans cells at the site of irradiation. We addressed the question of whether the systemic suppression of CHS was related to these modifications of Langerhans cells by UVB radiation. In mice treated on the dorsum with UVB radiation, the number and morphology of Langerhans cells in the unexposed areas of skin used for inducing and eliciting CHS appeared normal. Therefore, the depression of CHS could not be attributed to a depletion of Langerhans cells at the sites of application of the sensitizing agent. We also examined the correlation between alterations in Langerhans cells and systemic suppression of CHS after treatment with various types of nonionizing radiation. Treatment of mice with UVA (320-400 nm) radiation eliminated detectable Langerhans cells from the exposed skin, based on ATPase staining and electron microscopy, but did not reduce CHS; in fact, CHS was enhanced in these animals. Neither rose bengal nor eosin, in combination with visible (greater than 400 nm) radiation, affected the number or appearance of Langerhans cells, even though microscopic evidence of phototoxicity was present. However, rose bengal plus visible radiation depressed CHS reactions that were induced and elicited through unexposed skin. Depletion of Langerhans cells from dorsal skin by exposure to UVA radiation did not prevent suppression of CHS by subsequent exposure of the Langerhans cell-depleted skin to UVB radiation. We conclude that systemic suppression of CHS by UVB irradiation is not related to the numerical and morphological alterations in Langerhans cells that occur locally at the site of irradiation.