The “Bystander Effect”: Tumor Regression When a Fraction of the Tumor Mass Is Genetically Modified

Tumor cells expressing the herpes simplex virus thymidine kinase ( HSV-TK ) gene are sensitive to the drug ganciclovir (GCV). We demonstrate here that HSV-TK -positive cells exposed to GCV were lethal to HSV-TK -negative cells as a result of a “bystander effect.” HSV-TK -negative cells were killed in vitro when the population of cultured cells contained only 10% HSV-TK -positive cells. The mechanism of this “bystander effect” on HSV-TK -negative cells appeared to be related to the process of apoptotic cell death when HSV-TK -positive cells were exposed to GCV. Flow cytometric and electron microscopic analyses suggested that apoptotic vesicles generated from the dying gene-modified cells were phagocytized by nearby, unmodified tumor cells. Prevention of apoptotic vesicle transfer prevented the bystander effect. The toxic effect of HSV-TK -positive cells on HSV-TK -negative cells was reproduced in an in vivo model. A mixed population of tumor cells consisting of HSV-TK -positive and HSV-TK -negative cells was inoculated s.c. into mice. Regression of the tumor mass occurred when the inoculum consisted of as few as 10% HSV-TK -expressing tumor cells. The bystander effect was also demonstrated in i.p. tumor studies. Initial experiments demonstrated that prolonged survival (>70 days) occurred when a mixture containing 50% HSV-TK -positive and 50% HSV-TK -negative cells was injected i.p. followed by GCV treatment. Further, survival was prolonged for mice with a preexisting HSV-TK -negative i.p. tumor burden by injecting HSV-TK -positive cells and GCV. These results suggest that genetic modification of tumor cells may be useful for developing cancer therapies.