In vitro cultivation of mouse mammary tumor virus: detection of MTV production by radioisotope labeling and identification by immune precipitation.

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Abstract
Explants of MTV-infected normal or neoplastic mammary tissue were maintained in organ culture in the presence of H3-uridine. H3-labeled virus particles were detected in both the organ culture medium and the cell disassociation supernatant. The particles were concentrated by standard methods of density centrifugation, and they were identified as MTV particles by 3 separate procedures. After buoyant density centrifugation, a radioactive peak of RNase-resistant material was found at that density previously shown to be typical for MTV. Using rabbit antiserum against MTV-infected and MTV-free mammary tissue, the radioactive peak found after the buoyant density centrifugation was precipitated by pretreatment with antiserum against MTV but not by pretreatment with antiserum against mouse tissue antigens. Finally, a high-molecular-weight RNA, characteristic of this virus and of other RNA tumor viruses of this type, was isolated from those density gradient fractions which contain the radioactive peak. MTV can be produced in detectable quantities in vitro and radioisotope labeling combined with immune precipitation is a rapid and convenient procedure for the detection and identification of such virus.