Isolation of host-range variants of mouse mammary tumor viruses that efficiently infect cells in vitro

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Abstract
Host-range variants of mouse mammary tumor virus (MMTV) were isolated that have the ability to productively infect cells in vitro with high efficiency (at multiplicities of infection .ltoreq. 1) and with extremely short latent periods to the production of de novo virus (as short as 4 days after infection). These variants of the highly oncogenic MMTV of RIII, C3H, and GR mice were obtained by serial virus passage in feline cells. The resultant variant stocks react in group-specific radioimmunoassays for the MMTV major external glycoprotein (gp52) and major internal protein (p28), possess a protein profile similar to that of wild-type MMTV and contain a virion-associated DNA polymerase with a Mg2+ preference. Addition of dexamethasone and insulin to culture media enhances the titer of de novo MMTV to levels of approximately 1010 particles/75 cm2 flask (containing 5 .times. 106 cells) per 24 h. Variant stocks exhibit no evidence of contamination with either murine or feline type C retroviruses, as assayed by various techniques. The variants of MMTV derived from C3H and RIII mice exhibit differential host ranges that include the ability to productively infect feline, canine, bat, mink, murine and human cells. Use of these MMTV host-range variants now facilitates the study of the complete replicative cycle of MMTV and an elucidation of the interaction of MMTV with various hormones, physical or chemical carcinogens and tumor promoters in the initiation and promotion of mammary neoplasia.