Bystander effect in suicide gene therapy using immortalized neural stem cells transduced with herpes simplex virus...

Medulloblastomas (MBs) are the most common malignant brain tumor in children. The current therapeutic strategies are ineffective against the infiltrative and disseminative nature of MBs in about one-third of patients. Based on studies which have revealed the tumor-tropic characteristic of neural stem cells (NSCs), we used an immortalized neural stem cell line C17.2 as a cellular therapeutic delivery system to evaluate the antitumor effect of herpes simplex virus thymidine kinase gene (HSVtk) on MBs. We first stably transfected the HSVtk gene into C17.2 cells to produce C17.2tk cells, and then mixed C17.2tk with the human MB cell line Daoy at various ratios supplemented with ganciclovir (GCV). Both in vitro and in vivo experiments yielded promising results. Even at a C17.2tk: Daoy ratio as low as 1:16, more than 25% cells were killed in vitro. In vivo co-implantation study showed that when C17.2tk: Daoy ratio was 1:8, tumor growth inhibition was still evident and the mice had significantly prolonged survival. These results might partially be explained by the inherent tumor-tropic properties of NSCs and the bystander effect coupled with expression of connexin-43 (Cx43) between C17.2tk and Daoy cells. Our study clearly showed for the first time that immortalized neural stem cells used as vectors to deliver HSVtk gene therapy have a strong tumoricidal effect on MBs.