Transgenic mice that expressed the superantigen protein encoded in the C3H exogenous mouse mammary tumor virus long terminal repeat deleted their V beta 14+ T cells during the shaping of their immune repertoire and showed no evidence of virus production in their mammary glands after infection by milk-borne C3H exogenous virus. However, they developed mammary gland tumors that had newly integrated copies of C3H exogenous virus, although the latency of tumor formation was much longer than in their nontransgenic littermates that retained their V beta 14+ T cells. After four generations, infectious C3H virus was completely eliminated from the transgenic mouse pedigree. These data support the hypothesis that endogenous mouse mammary tumor proviruses are retained in the genome as protection against exogenous virus infection and subsequent tumorigenesis and show that there may be natural selection against the virus in vivo
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