On the Significance of Virus-Like Particles in Mammary Tissues of C3Hf Mice2

Abstract

Experiments were designed to determine whether the virus-like particles in mammary hyperplasias and neoplasias of low-cancer C3Hf mice represent a transmissible factor with demonstrable biologic properties. Virgin test m ice (agent-free BALB/c, BALB/c foster-nursed on C3H, BALB/c foster-nursed on C3Hf, and offspring of reciprocal matings of BALB/c and C3Hf) were given hormone treatments to promote or accelerate the appearance of hyperplastic alveolar nodules and tumors. Mammary hyperplasias appearing in the test mice were sampled for electron microscopy and/or transplanted into gland-free inguinal fat pads to test their tumorigenic potential. C3H-fostered BALB/c mice behaved in all respects as expected of animals infected with the mammary tumor virus (MTV): They had many tumors, nodules with high tumorigenic potential, and abundant virus-like particles. BALB/c mice foster-nursed on C3Hf mothers resembled BALB/c in having very few nodules and tumors and in lacking detectable virus-like particles. All hybrids had the characteristics of C3Hf: low tumor incidence, many nodules with poor tumorigenic potential, and abundant virus-like particles. These observations are discussed in the light of current theories of mammary tumorigenesis in mice. It is suggested that the C3Hf particle may represent a second agent (morphologically indistinguishable but biologically distinct from MTV) normally transmitted to offspring by either parent at conception but not ordinarily passed through milk.