An Interference Phenomenon Associated With Resistance to Infection With Mouse Mammary Tumor Virus2

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Abstract
A recently developed mammary noduligenesis test for the detection of the mammary tumor virus (MTV) was used to 1) determine the susceptibility of BALB/c and C3Hf mice of various ages and physiological states to infection with MTV, and 2) determine whether the age of the host or that of the mammary tissues is responsible for the resistance of older mice to MTV. MTV-containing extracts from A or C3H tissues were administered to BALB/c and C3Hf mice of various ages and, in a second experiment, mammary tissues from BALB/c and C3Hf animals of different ages were transplanted into mammary gland-free fat pads of 3-week-old MTV-carrying isologous hosts. MTV infection of intact hosts or of mammary transplants was judged on the basis of the noduligenic test. The results indicated that BALB/c mice of all ages are susceptible to MTV, but that the incidence of MTV infection decreases with increasing age. Transplanted mammary tissues from BALB/c mice of all ages were equally susceptible to infection, however. It was concluded that age resistance to MTV in this strain is systemically mediated. C3Hf mice under 18 days of age, and also their transplanted mammary tissues, were susceptible to MTV infection. However, C3Hf mice 18 days or older and their mammary transplants were refractory to MTV infection. It is suggested that resistance to MTV in C3Hf mice over 18 days of age is due to the presence of an MTV-interfering agent in their mammary tissues. The possibility is discussed that this interfering agent is equivalent to the recently postulated nodule-inducing virus (NIV), an agent immunologically related to MTV. Both NIV and the MTV-interfering agent occur in C3Hf animals, while neither activity is present in mice of the BALB/c strain.