In vitro infectivity assay for mouse mammary tumor virus.

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Abstract
Studies of mouse mammary tumor virus (MMTV) were impeded by the lack of an in vitro infectivity assay. A rapid, quantitative in vitro assay for MMTV infectivity based on the detection of positively staining foci by immunoperoxidase was developed. This assay and a 50% end-point titration of MMTV infectivity gave identical virus titers. Infection of a rat hepatoma cell line, a feline kidney cell line, and a normal murine mammary gland cell line by virus from the mouse mammary tumor GR3A cell line was linear with respect to virus concentration. The infectious titers obtained in both homologous and heterologous cell lines were not significantly different, demonstrating a lack of host range specificity. Virus infectivity was inactivated by heating at 55.degree. C and by UV irradiation. Rabbit anti-MMTV serum neutralized the infectivity with a 50% neutralization end point of 1:5000. Applications of this assay to the study of the immunological, biological and biochemical characteristics of MMTV are discussed.