Reevaluation of the Effect of Mouse Mammary Tumor Virus Infection on the BALB/c Mouse Hyperplastic Outgrowth23

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Abstract
The BALB/c (C−) mouse hyperplastic outgrowth line (D1) was used to study murine mammary tumor virus (MuMTV) expression in both D1 and tumors derived from D1. D1 was transplanted into virus-infected BALB/cfC3H (C+) and virus-uninfected C− animals. In duplicate studies, tumor incidence was the same in both groups. However, the tumor latency period was longer for D1 transplanted into C+ mice (D1/C+) than for D1 transplanted into C− mice (D1/C−). MuMTV was detected in 85% of D1/C+ outgrowths and in 29% of D1/C+ tumors but was never detected in D1/C− outgrowths or tumors. D1/C− outgrowth and tumors and most of the D1/C+ tissues expressed little or no MuMTV RNA. Some D1/C+ tumors expressed substantial levels of MuMTV RNA. These same tumors also had MuMTV antigen and contained the exogenously acquired C3H-MuMTV provirus in the cellular DNA as shown by DNA fragment patterns following cleavage with Pst I and Eco RI endonucleases. D1/C+ tumors containing no viral RNA were also antigen-negative and lacked the acquired C3H provirus. These studies indicate that D1 has substantially changed in its incidence and in its response to MuMTV. MuMTV infection was not tumorigenic in the traditional sense, a finding that has led to a reevaluation of our current models of virus-host relationships and the biology of precancerous conditions.