title: The "bystander effect": tumor regression when a fraction of the tumor mass is genetically modified.
abstract: 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.
authors.affiliation: Clinical Immunology Unit, University of Rochester Medical School, New York 14642.
authors.forename: S M
authors.initials: SM
authors.lastname: Freeman
authors.forename: C N
authors.initials: CN
authors.lastname: Abboud
authors.forename: K A
authors.initials: KA
authors.lastname: Whartenby
authors.forename: C H
authors.initials: CH
authors.lastname: Packman
authors.forename: D S
authors.initials: DS
authors.lastname: Koeplin
authors.forename: F L
authors.initials: FL
authors.lastname: Moolten
authors.forename: G N
authors.initials: GN
authors.lastname: Abraham
mesh_terms: D000818:Animals; D017209:Apoptosis; D002461:Cell Line, Transformed; D005260:Female; D015774:Ganciclovir / Q000633:toxicity*; D007708:Kirsten murine sarcoma virus; D051379:Mice; D008807:Mice, Inbred BALB C; D008854:Microscopy, Electron; D012192:Retroviridae Infections / Q000235:genetics / Q000473:pathology*; D012513:Sarcoma, Experimental / Q000235:genetics / Q000473:pathology* / Q000648:ultrastructure; D018139:Simplexvirus / Q000201:enzymology / Q000235:genetics; D013937:Thymidine Kinase / Q000235:genetics*; D014412:Tumor Virus Infections / Q000235:genetics / Q000473:pathology*
chemical_list: D013937:Thymidine Kinase; D015774:Ganciclovir