Qualitative and quantitative effects on epidermal Langerhans (Ia+) and Thy-1+ dendritic cells following topical application of phorbol diesters and mezerein.

Repeated twice-weekly applications of promoting doses of 12-O-tetradecanoylphorbol-13-acetate (TPA) to the dorsal skin of female adult SENCAR mice led to a reduction in numbers per unit area of epidermal Thy-1+ dendritic cells. Although no parallel effect was observed on Ia+ Langerhans cells, a concurrent reduction of dendritic morphology of both cell types was observed. Topical administration of TPA (2 micrograms) twice weekly for 4 or 8 weeks led to reductions in Thy-1+ cell numbers of 52 and 61%, respectively, whereas a single treatment was without effect. Similar effects were observed in animals initiated with 10 nmol 7,12-dimethylbenz[a]anthracene suggesting that the response was specific to the promotion, rather than the initiation, phase of two-stage tumorigenesis. All initiated animals bore tumors after 16 promoter treatments. In comparison with the potent promoter TPA, the weak overall, but stage-specific promoters 4-O-methylTPA and mezerein showed no effects on number or morphology of either dendritic cell type. These findings are therefore consistent with an important role for quantitative and qualitative alterations in epidermal non-keratinocytes of immune function in tumor promotion.