SYSTEMIC GENE-THERAPY OF MURINE MELANOMA USING TISSUE-SPECIFIC EXPRESSION OF THE HSVTK GENE INVOLVES AN IMMUNE COMPONENT

Previously we have demonstrated safe and effective transfer of the HSV tk cytotoxic gene to primary murine melanoma tumors by direct injectio n of plasmid and retroviral vectors in which the HSVtk gene is driven by the tissue-specific tyrosinase promoter. However, for general clini cal application such forms of therapy should, ideally, be effective ag ainst disseminated metastases. We report here that the number of recen tly established lung metastases of B16 melanoma in C57BL mice treated with ganciclovir is reduced compared to controls after multiple i.v. a dministrations of high titer retroviral supernatant encoding the HSVrk gene, but not after administration of liposome-complexed plasmid DNA. Using polymerase chain reaction analysis, integration of the provirus was observed in metastasis-bearing lungs (4 of 6 mice) and in the spl eens of some ganciclovir-treated animals (2 of 6 mice) but not in the testes, brain, heart, liver, or kidney. The reduction in the number of experimental metastases in C57BL mice exceeded the anticipated extent of transduction of tumor cells, which is indicative of a marked bysta nder effect. This magnitude of reduction was not observed in immunodef icient athymic mice, suggesting that the immune system plays some part in the bystander effect. In support of these data, we show that, wher eas the parental tumor cells are only poorly immunogenic, an effective antitumor immune response is generated following the killing of neopl astic cells in vivo as a result of treatment with ganciclovir. These e ffects may be responsible for augmenting the efficacy of retroviral in fection. The combination of local cell. killing by the HSVtk/ganciclov ir system and the induction of antitumor immunity suggests new opportu nities for the design of vectors for the gene therapy of cancer.