Possible role of genetic cell variants in the viral induction of mouse mammary tumors.

Out of three attempts to induce neoplasia in normal C57Bl mammary epithelial cells with the mouse mammary tumor virus (MuMTV) only one presented signs of tumorigenicity. Immunofluorescence showed that virus synthesis took place in all three sublines but tumorigenicity as detected by cell aggregation viability (CAV) and transplantation into syngeneic mice failed to occur in two of them. By comparison, cells from a BALB/c spontaneous mammary tumor that do not express MuMTV were 100% tumorigenic, whereas cells from a BALB/cfC3H tumor with a 95% virus-producing cell population had a normal CAV and were tumorigenic only in 60% of the test animals. This lack of correlation suggested that many of the virus-producing cell were not neoplastic and that neoplasia might occur under virus stimulation only if a restricted population of genetic cell variants existed. Accelerated tissue culture passages of virus-free C57Bl and BALB/c normal mammary cells resulted in their spontaneous neoplasia at Passages 23 and 50, respectively; when duplicated cells cryopreserved in early passages were revived and cultivated in the same manner, neoplasia occurred at Passages 27 and 58. The similarity of the passage numbers appears to confirm the existence of genetic cell variants among the normal cell population.

In vitro susceptibility of mink lung cells to the mouse mammary tumor virus.

Lung cells from mink embryos were infected in vitro with a purified mammary tumor virus isolated from RIII mouse milk. Specific virus antigen at the cell surface was detected by membrane immunofluorescence; B-type virions budding from the cell membrane were seen by electron microscopy. Nucleic acid hybridization confirmed replication and specificity of the virus produced.

Murine mammary tumor virus: characterization of infection of nonmurine cells.

Murine mammary tumor virus (MuMTV) was used to productively infect feline and mink cells. MuMTV "proviral" DNA could be detected in the infected cells by molecular hybridization using radioactive MuMTV complementary DNA as a probe. Kinetic analysis of MuMTV proviral DNA synthesis after infection showed that maximum MuMTV DNA synthesis was achieved by 8 h; however, this was followed by a decline in detectable proviral DNA and eventual stabilization at a lower level. MuMTV synthesis in feline cells was greatly stimulated by the synthetic glucocorticoid, dexamehtasone. On the other hand, MuMTV synthesis in mink cells was relatively at a much higher level in absence of dexamethasone and the stimulation with dexamethasone was not as marked as in the case with infected feline cells. Thermal denaturation of hybrids between MuMTV complementary DNA and infected mink cell RNA revealed no difference from homologous hybrids.

Experimental infection of a cat kidney cell line with the mouse mammary tumor virus.

A cat kidney cell line, CRFK-F2, was successfully inoculated in suspension and in monolayer culture with a purified mouse mammary tumor virus derived from RIII milk. The virus produced by the infected cells was identified by immunogluorescence, electron microscopy, and RNA-directed DNA polymerase assays; it was a B-type virion that did not cross-react with mouse or feline leukemia-sarcoma viruses, had spikes on its envelope, and had a RNA-directed DNA polymerase reaction that was typical of mouse mammary tumor virus. The producing cells were identified as cat cells by chromosome number, cytotoxic assays, and isoenzyme migratory patterns. A standardized method for the in vitro inoculation of cat cells is described that presently permits highly reproducible results. For the first time, the mouse mammary tumor virus is seen replicating in cells from another species, thus offering an opportunity to study the kinetics of infection of that virus.