単純ヘルペスウイルス由来チミジンキナーゼ遺伝子とガンシクロビルによる肝癌に対する遺伝子治療におけるbystander effectの検討

Abstract

Because it appears impossible to transfer a suicide gene to all the cells of a cancer, existence of a bystander effect caused by a suicide gene and prodrug system is critical to achieve effective antitumor effects. In the present study, possible mechanisms of the bystander effect caused by the herpes simplex virus thymidine kinase (HSV-tk) gene and ganciclovir (GCV) have been investigated not only in vilro but also in vivo. Murine and rat hepatocellular carcinoma (HCC) cells were retrovirally transduced with the HSV -tk gene. HSV-tk-transduced HCC cells were shown to be profoundly more sensitive to GCV compared to parental cells. HSV-tk-transduced cells exhibited marked cytotoxicity on parental cells in the presence of GCV. This in vitro bystander effect was shown to be substantially dependent on cell-to-cell contact. To investigate the in vivo bystander effect, parental HCC cells were mixed with HSV-tk-transduced counterparts and inoculated subcutaneously into syngeneic mice. When systemic GCV treatment was initiated 3 days after the inoculation, profound antitumor effects on preestablished tumors were observed, resulting in significant inhibition of tumor formation when only 5% of the inoculated cells were HSV-tk-transduced ones. Importantly, animals that did not develop tumors resisted subsequent rechallenge with parental HCC cells, indicating that tumor immunity was induced by the HSV-tk/GCV system. Furthermore, when GCV treatment was initiated after mice developed subcutaneous HCC tumors, profound antitumor effects were observed even on established tumors. When subcutaneous HCC tumors contained only 10% of HSV -tk-transduced cells, significant tumor eradication was induced by GCV treatment. Impor- tantly, when animals with subcutaneously established HCC were given an intratumoral infusion of retroviruses carrying the HSV-tk gene followed by GCV treatment, significant tumor regression and prolonged survival period were achieved. These results indicate that the HSV-tk/GCV system can induce potent antitumor effects in vivo by eliciting the host's tumor immunity.