Prevention of spontaneous leukemia in mice by oncornavirus immunization.

C57L, NIH, and SWR mice were immunized with inactivated Gross leukemia virus (GLV) and then mated with AKR males. Their F1 offspring were then immunized with the murine sarcoma virus pseudotype of GLV, MSV(GLV). The concentrations of infectious ecotropic AKR virus in tail extracts of immunized mice were 100- to 1,000-fold lower than in non-immunized controls when tested at 30--40 days of age. Although viral titers increased slightly with time, the titers remained at least one log10 lower in the immunized mice than in non-immunized F1 control mice at all times tested. The reduction in the level of expression of endogenous ecotropic virus showed a highly significant positive correlation with the reduction in incidence of spontaneous leukemia in these mice. These data thus show that successful immunoprevention of leukemia in mice can be achieved with viral vaccines.