Susceptibility of Mammary Tissues of “Genetically Resistant” Strains of Mice to Mammary Tumor Virus2

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Abstract
In mice of susceptible strains carrying the mammary tumor virus (MTV), mammary nodules develop after 12 weeks of treatment with estradiol plus deoxycorticosterone acetate (noduligenic treatment). Mammary tumors develop in a high percentage of such nodules within a relatively short period following transplantation into isologous hosts. With this previously established noduligenic test used as a criterion for susceptibility to the mammary tumor virus, a study was undertaken to determine whether the resistance of C57BL and Is (1/StBi) mice to mammary tumor development is due to resistance of their mammary tissues to MTV infection. Noduligenic treatment was administered to MTV-Free and MTV-carrying C57BL, Is, and (C57BL × Is)F1 hybrid mice, and to hybrid hosts carrying mammary tissue transplants from all three lines. Mammary nodules Failed to develop in MTV-Free and in MTV-carrying C57BL and Is mice, as well as in MTV-Free hybrids. In contrast, many nodules occurred in the mammary tissues of MTV-carrying hybrids, and also in Is, C57BL, and (C57BL × Is)F1 mammary tissues that had been transplanted into such hybrids. Mammary tumor virus-like particles were present in abundance in the nodule outgrowths and tumors, which subsequently developed in tissues from all three lines of mice, and histologically such tumors appeared to be types A and B. It is therefore suggested that resistance to mammary tumor development in C57BL and Is mice does not result From refractoriness of their mammary tissues to infection by MTV.