COEXPRESSION OF EXOGENOUS AND ENDOGENOUS MOUSE MAMMARY-TUMOR VIRUS-RNA IN-VIVO RESULTS IN VIRAL RECOMBINATION AND BROADENS THE VIRUS HOST-RANGE

Mouse mammary tumor virus is a replication-competent B-type murine ret rovirus responsible for mammary gland tumorigenesis in some strains of laboratory mice. Mouse mammary tumor virus is transmitted horizontall y through the milk (exogenous or milk-borne virus) to susceptible offs pring or vertically through the germ line (endogenous provirus). Exoge nously acquired and some endogenous mouse mammary tumor viruses are ex pressed at high levels in lactating mammary glands. We show here that there is packaging of the endogenous Mtv-1 virus, which is expressed a t high levels in the lactating mammary glands of C3H/HeN mice, by the virions of exogenous C3H mouse mammary tumor virus [MMTV(C3H)]. The ma mmary tumors induced in C3H/HeN mice infected with exogenous MMTV(C3H) virus contained integrated copies of recombinant virus containing a r egion of the env gene from an endogenous virus. This finding indicates that there was copackaging of the Mtv-1 and MMTV(C3H) RNAs in the sam e virions. Moreover, because Mtv-1 encodes a superantigen protein with a V beta specificity different from that encoded by the exogenous vir us, the packaging of Mtv-1 results in an infectious virus with a broad er host range than MMTV(C3H).