Role of the Milk Agent in Disappearance of Mammary Cancer in C3H/StWi Mice23

Abstract

In 1958, 11 years after the establishment of inbred C3H/StWi mice at Brown University, mammary carcinoma ceased to develop. Another subline, C3H/AnWi, maintained under identical conditions, continued to develop mammary tumors. Reciprocal foster-nursing experiments were carried out between the two sublines. The presumably tumor-free C3H/StWi females were found highly susceptible to the mammary tumor agent (MTA) from the C3H/An Wi subline. Furthermore, the infected C3H/StWi mothers subsequently passed the agent efficiently to their offspring. On the other hand, C3H/AnWi females fostered on C3H/StWi mothers developed a smaller percentage of tumors and only after a prolonged latent period. They did not readily pass the agent to their offspring. Samples from 28 of 63 tumors resulting from the reciprocal foster-nursing experiments were examined with the electron microscope. In every instance intracellular and extracellular particles representing the MTA were observed. The results from combined bioassay and correlative electron microscopy are interpreted to indicate that the C3H/StWi subline does not harbor an active MTA. The appearance of mammary tumors in the C3H/AnWi mice foster-nursed by C3H/StWi mothers could be explained by in utero infection by the male, directly or indirectly. Evidence for this hypothesis is the observation of virus-like particles identical to the MTA in thin sections of the cauda epididymis and seminal vesicles from C3H/AnWi males and from C3H/StWi males infected with MTA, but not from uninfected C3H/StWi males.