Electron Microscopic and Bioassay Studies of Milk From Mice of High and Low Mammary-Cancer and High and Low Leukemia Strains23

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Abstract
High-speed centrifugal pellets were prepared from defatted and decaseinated milk obtained from mice of 33 substrains of 15 different strains, 3 stocks, 2 germfree strains, and 2 hybrid lines with high or low incidence of mammary cancer and high or low incidence of leukemia. Electron microscopic studies were carried out on sections of these pellets, some of which were tested for tumor-inducing and hyperplastic nodule-inducing activity. Type-B virus particles were found in the milk of mice of high and of some low cancer strains. In addition, type-C virus particles and “signet-ring” particles were observed in some strains. A high degree of correlation was noted between the presence of type-B particles and tumor-inducing as well as nodule-inducing activity in RIII and A, high mammary-cancer strains, and to a lesser extent in Af and C3Hf, low cancer strains, and (C57 × Af)F1mice. A high degree of correlation was found in the milk of BALB/c and CBA mice, low mammary-cancer strains, between the absence of type-B particles and lack of tumor-inducing and nodule-inducing activity. Although no type-B particles could be found in C57BL milk, it showed high nodule-inducing activity. Milk from a substrain of AKR mice with medium leukemia incidence showed presence of type-B particles and tumor-inducing activity. Structures resembling lactic dehydrogenase (LDH) virus were found in several low cancer strains. Bioassays of milk samples from 2 of these strains failed, however, to reveal LDH virus activity.