Murine mammary tumor virus (MuMTV) infection of an epithelial cell line established from C57BL/6 mouse mammary glands.

An epithelioid cell line derived from the mammary glands of a C57BL/6 mouse and designated C57MG cell line was found to be susceptible to infection by routine mammary tumor virus (MuMTV). Although uninfected C57MG cells contain endogenous MuMTV-related DNA sequences, no RNA sequences homologous to MuMTV were detectable, even after treatment with the glucocorticoid, dexamethasone. However, after infection with MuMTV, these cells acquire additional MuMTV DNA, and viral RNA and proteins were readily detectable. Most, if not all, of the additional MuMTV DNA in infected C57MG cells appeared to be integrated. Synthesis of viral RNA and protein, and the release of virions into culture fluid by infected C57MG cells, was stimulated by incorporation of dexamethasone in the growth medium. The efficiency of infection by MuMTV in C57MG cells was similar to that in nonmurine cells. There was a direct relationship between multiplicity of infection (m.o.i.) and the average number of MuMTV RNA molecules detected per cell 5 weeks after infection. However, even at the highest m.o.i. used (4 × 10⁵ virions/cell), a plateau in the average number of viral RNA molecules per cell was not achieved. The origin of MuMTV synthesized by infected C57MG cells, whether it is the progeny of infecting MuMTV or of the endogenous C57BL/6 MuMTV or of both, remains undetermined. We have been unable to detect any morphological or growth pattern changes in C57MG cells following infection with MuMTV.