Efficacy of the bystander effect in the herpes simplex virus thymidine kinase-mediated gene therapy is influenced by the expression of connexin43 in the target cells

Tumoricidal bystander effect observed in the herpes simplex virus thymidine kinase (HSVtk)/ganciclovir (GCV) gene therapy was studied between different rat glioma cell lines (9L and C6 cells) under both in vitro and in vivo conditions. For that purpose, mixed populations of wild-type cells (9Lwt and C6wt) and respective HSVtk gene-transduced cells (9Ltk and C6tk) were examined for their sensitivity to GCV. A potent in vitro bystander effect was observed in 9Lwt/9Ltk and 9Lwt/C6tk combinations but not in C6wt/9Ltk and C6wt/C6tk combinations. In vivo bystander effect studied in a subcutaneous tumor model in athymic nude mice was also potent in 9Lwt/9Ltk and 9Lwt/C6tk combinations. Because the expression of connexin43, a major protein in the connexin family gene products, in 9L cells is much higher than that in C6 cells, the results suggest that the amount of connexin in target (wild-type) cells but not in effector (HSVtk gene-bearing) cells is important for the generation of the bystander effect. This hypothesis was further confirmed by the observation that in vitro bystander effect in C6wt/C6tk combination was potentiated by transduction of the connexin43 gene to the target cells.