Desensitization of rabbit skin by repeated exposure to UV-visible light of sites injected with Rose Bengal.

We have shown in a previous paper that irradiation of rabbit skin sites injected with Rose Bengal (RB) produces immediate increase in vascular permeability and accumulation of PMNs. Studies on the development of temporary tolerance and the biological parameters related to the development of such tolerant state by repeated exposure to light of RB-injected sites are reported here. The increase in VP and PMN migration induced by RB (10 nmol) are of an immediate nature, i.e., occur within the first 3 h of irradiation, and the reaction subsides gradually after 24 h. When such moderate insult is repeated, the skin becomes tolerant to subsequent exposure to light in the presence of RB. This tolerant state is temporary, i.e., the desensitized sites are fully recovered in 72 h. The loss of responsiveness of RB-injected sites previously exposed to light was not due to diffusion of the injected dye from the sites since reinjected sites also showed reduced response and the sites injected three days before but not irradiated showed normal response. The sites that were made tolerant to RB-induced phototoxic reactions, when injected with compound 48/80, an agent known to degranulate mast cells, did not show an increase in VP. This suggests that either the mast cells were depleted from the sites or the mast cells in the sites were rendered refractory by previous exposure to light. It was also found that the sites made tolerant to RB plus light were unresponsive to exogenously injected histamine. The sites tolerant to RB plus light when injected with zymosan-activated serum (ZAS) did not stimulate the migration of PMNs. This loss of chemotactic response to ZAS may have relevance to photodamage of vascular endothelium. These observations are discussed in relation to the development of the tolerant state by repeated exposures to subthreshold doses of light in solar urticaria.