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Immunization of crossbred and F1 mice with combined killed and live Gross leukemia virus AKR type-C viral vaccines suppressed endogenous N-type AKR virus up to 10,000-fold for significant periods during early life. Since several previous studies in the same Assay of Virus in Tails of Mice Given Vaccine or Passive Antibodies. The tail virus assay procedure was essentially that described by Lilly et al. (9). Thirty to forty days after immunization of the F1 mice with MSV(GLV) (see next section) and at specified later periods 2 cm terminal segments of the tail were cut off each test and control mouse with a sharp scissors. Segments of tails were rinsed with ether and following evaporation they were ground with Alundum using chilled mortar and pestle and suspended in cold Eagle's basal medium containing 20% veal infusion broth. The extracts (approximately 2% by wt/vol) were lightly centrifuged and sonicated and tested for infectious murine leukemia virus by the XC cell method (18) using a continuous passage line of SC-1 wild mouse cells (19) or secondary NIH Swiss embryo cells. Titers were expressed as the number of plaque-forming units per 0.4 ml of 2% extract.Live MSV(GLV) Vaccines. MSV(GLV), a Gross leukemia virus (GLV) pseudotype murine sarcoma virus (MSV) maintained since 1967 in our laboratories by serial transmission in NIH Swiss mice, was produced in quantity by transmission to large numbers of newborn NIH Swiss mice. Ten percent (wt/vol) virus pools were made into lOX concentrated Moloney procedure concentrates (20) from subcutaneous sarcomas. High titered (103-104) sarcoma virus preparations were used as a live vaccine (see design of experiment below).

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.

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