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Ultrastructural and Quantitative Studies of Mammary Tumor Virus Production in Cultured Mouse Mammary Tumor Cells¹

Institute for Medical Research, Camden, New Jersey 08103

A whole-cell-mounting technique was successfully applied to the study of mouse mammary tumor virus budding from cells in tissue culture. The technique permitted the visualization of ultrastructural detail, including the cores and the surface spikes of budding virions. The latter are difficult to discriminate in thin sections. Budding mammary tumor virus was compared with budding murine leukemia virus. The latter lacked the spikes and differed from mammary tumor virus with respect to the assembly of the cores. Binding of specific antibody to budding mammary tumor virions was directly visualized without the use of conjugated antibody. The antibody binding was found to be specific since the antibody was bound only to the virus bud and not to other regions of the cell membrane. Binding was not observed with control sera. The whole-cell-mounting technique was also used as an assay for virus production in infected cells. It was found that the MMT/R-5 cell line contained a higher ratio of producing cells than the MMT (Sykes) cell line from which it was derived by alternate in vivo-in vitro passage in Amsterdam rats. As the cultures aged, the productivity of MMT approached zero while that of the MMT/R-5 cells increased.

¹ Presented in part at the 68th Annual Meeting of the American Society for Microbiology, Detroit, Michigan, May 1968. This investigation was supported by USPHS Research Grants CA-08740 and CA-08515 and by a grant from The Lillia Babbitt Hyde Foundation.

Received 6/30/69. Accepted 7/18/69.