Tumor rejection mediated by an amphotropic murine leukemia virus

We studied the effects of murine leukemia virus infection on the growth of tumors in inbred strain 2 guinea pigs. Fibrosarcomas, induced by treatment of guinea pig fetal cells with chemical carcinogens, were exposed in vitro to the amphotropic murine leukemia virus, 4070A. Tumor cells exposed to murine leukemia virus 4070A in vitro expressed virus antigens, released both reverse transcriptase and infectious virus into supernatant fluids, and grew and regressed after injection into syngeneic animals. In contrast, uninfected tumor cells did not express virus antigens and grew progressively. Rejection of virus-infected tumor cells appeared to be a host-mediated event, since murine leukemia virus 4070A infection had no detectable cytopathic effect on fibrosarcoma cells. Rejection of virus-infected tumor cells occurred in guinea pigs unable to suppress the growth of the line 10 hepatoma at sites of injection of mycobacterial cell walls and unable to develop immunity to the line 10 hepatoma. Guinea pigs immunized with virus-infected tumor cells were unable to reject a challenge of uninfected tumor cells. The results are interpreted to mean that infection of guinea pig tumors in vitro by a murine leukemia virus led to synthesis of viral antigens by the tumor cells which in turn led to recognition of the tumor as foreign with consequent tumor destruction; tumor eradication occurred without development of immunity to cryptic, intrinsic tumor antigens.