Evidence that the local effect of ultraviolet radiation on the growth of murine melanomas is immunologically mediated.

We had reported previously that the outgrowth of melanoma is enhanced when melanoma cells are injected into UV-irradiated skin of syngeneic mice. To determine whether this effect was specific for melanomas, we compared the growth of 13 different tumor cell lines (3 melanomas, 6 fibrosarcomas, 2 undifferentiated skin tumors, a squamous cell carcinoma, and a spontaneous hepatocarcinoma) in UV-irradiated and nonirradiated syngeneic mice. C3H/HeN(MTV-) mice were exposed to 4.8 kJ/m2 UV-B (280-320 nm) radiation twice a week for 3 weeks; the tumor cells were injected into the UV-irradiated pinna 24 h after the final UV irradiation. The growth of all the melanomas and 4 of the fibrosarcomas was enhanced in UV-irradiated mice, indicating that the effect of UV radiation was not specific for melanomas or tumors of a particular etiology. Using an in vivo immunization and challenge assay, we found that the 7 tumors exhibiting enhanced development in UV-irradiated skin were highly immunogenic, whereas the remaining 6 tumors were not. This suggested that enhanced tumor outgrowth resulted from an immunosuppressive effect of the UV radiation. When tested further, we found that UV-B radiation had no effect on melanoma outgrowth in congenitally athymic mice, sublethally X-irradiated mice, or mice depleted of Thy1+ cells in vivo. These results indicate that immunological mechanisms play a role in the effect of UV radiation on the growth of murine melanomas.