The liposome component was formed from the lipids egg phosphatidylcholine, egg phosphatidylethanolamine, and oleic acid, and from the membrane stabilizer cholesterol hemisuccinate.The treated sk in area subsequently was coveredwith aluminum foil to prevent photoreactivation at this time point.This dose was applied because it prev iously had been shown to induce intense ICAM expression on the sur face of most keratinocy tes haf ter application. At the indicated time points, mm punch biopsies were obtained, snap frozen immediately, and stored in liquidnitrogen.After staining, the slides were mounted with aqueous mounting gel. Positive staining was identified by light microscopy as brown reaction products.Irradiated and non irradiated sk in sites were always tested in parallel.The test chambers were removed haf ter application, and the test reaction was evaluated hlater and scored as positive if there were erythema, infiltration, and papules.From each test site, mm punch biopsies were obtained, fixed in formalin, embedded in paraf fin, stainedwith hematox ylineosin, and subsequently histologically evaluated.For semiquantitative assessment of lymphocy tes in the epidermis and dermis, three serial sections per specimen were analyzed, and the number of lymphocy tes in three highpower. Cyclobutane py rimidine dimer formation was studied in prev iously sunprotected human buttock sk in haf ter exposure to MED of UVB radiation. Thus, we did not detect endogenous photoreactivation under conditions in which exogenous photoreactivating enzyme was active in irradiated sk in.When the time of exposure to photoreactivating light was increased from to min, a timedependent increase in the removal of cyclobutane py rimidine dimers was obser ved. Therefore, a min photoreactivation period was used for all further experiments.The ef ficacy of photolyaseinduced dimer removal was essentially identical immediately af ter photoreactivation, indicating that the repair ef fect was already max imal at the end of the photoreactivating period.To further determine the specificity of the photolyase treatment, the number of dimers present in the epidermal compartment of these same sk in areas was simultaneously measured.In subjects hypersensitive tonickel sulfate, positive patch test reactions were obser ved if patch testing was per formed in un irradiated sk in areas. Histological examination of the same sk in area revealed spongiosis and the presence of an intraepidermal and intradermal inf lammatoryinfiltrate consistentwith a delayedtype hypersensitiv ity reaction. Elicitation of this hypersensitiv ity reaction was completely prevented in the same indiv iduals if patch tests were applied to a sk in area immediately af ter exposure to MED of UVB radiation.Data represent one of three essentially identical experiments.The major factor responsible for retention of lymphocy tes in the epidermis during inf lammatoryand immunological reactions, including allergic contact dermatitis, is ICAM expression on keratinocy tes.Histological pictures of patch test reactions to nickel sulfate in human skin.The recent clon ing of a human photolyase gene homologue and reports of photoreactivation in human white blood cells and fibroblasts have suggested that endogenous photoreactivation may of fer a natural means of photoprotection in human sk in. In addition, using dif ferent techn iques to measure dimers might give dif ferent results.In the present study, the only condition under which photoreactivation was obser ved was when exogenous photolyase in liposomes was applied before photoreactivating light.