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 h ere that HSV-TK-positive cells exposed to GCV were lethal to HSV-TK-ne gative cells as a result of a ''bystander effect.'' HSV-TK-negative ce lls were killed in vitro when the population of cultured cells contain ed only 10% HSV-TK-positive cells. The mechanism of this ''bystander e ffect'' on HSV-TK-negative cells appeared to be related to the process of apoptotic cell death when HSV-TK-positive cells were exposed to GC V. Flow cytometric and electron microscopic analyses suggested that ap optotic vesicles generated from the dying gene-modified cells were pha gocytized by nearby, unmodified tumor cells. Prevention of apoptotic v esicle transfer prevented the bystander effect. The toxic effect of HS V-TK-positive cells on HSV-TK-negative cells was reproduced in an in v ivo model. A mixed population of tumor cells consisting of HSV-TK-posi tive and HSV-TK-negative cells was inoculated s.c. into mice. Regressi on of the tumor mass occurred when the inoculum consisted of as few as 10% HSV-TK-expressing tumor cells. The bystander effect was also demo nstrated in i.p. tumor studies. Initial experiments demonstrated that prolonged survival (>70 days) occurred when a mixture containing 50% H SV-TK-positive and 50% HSV-TK-negative cells was injected i.p. followe d by GCV treatment. Further, survival was prolonged for mice with a pr eexisting HSV-TK-negative i.p. tumor burden by injecting HSV-TK-positi ve cells and GCV. These results suggest that genetic modification of t umor cells may be useful for developing cancer therapies.