Protection of herpes simplex virus thymidine kinase-transduced cells from ganciclovir-mediated cytotoxicity by bystander cells: the Good Samaritan effect.

Although considerable attention has been directed in the field of gene therapy toward elucidating the mechanism by which a transduced cell could kill a bystander cell, little is known about how bystander cells may affect transduced cells. We hypothesized that bystander cells, particularly if they were capable of gap junctional communication, could protect cells transduced with the herpes simplex virus thymidine kinase (HSV-TK) from ganciclovir (GCV)-induced cytotoxicity. To test this hypothesis, we used a rat hepatocyte cell line (WB) that can carry out efficient gap junctional communication, a WB clone transduced with HSV-TK (WB-TK), and a communication-incompetent subclone of WB cells (aB1). We cocultured WB-TK cells with either WB or aB1 cells, treated them with GCV, and then plated the cells into selective media that permitted us to quantify independently the surviving fraction of WB-TK cells or bystander cells. We found that WB bystander cells conferred up to a 1000-fold protection on WB-TK cells treated with GCV. aB1 cells conferred detectable, but significantly less, protection. These findings demonstrate that herpes simplex virus thymidine kinase-transduced cells can be significantly protected by bystander cells, particularly those that can carry out gap junctional communication. Whether this "Good Samaritan" effect improves the overall efficacy of gene therapy, by prolonging the survival of the source of toxic metabolites, or decreases effectiveness by increasing the survival of transduced cells will need to be determined in vivo.