Restriction of murine leukemia proviral gene expression in somatic mouse cell hybrids

Fusion of the N-ecotropic murine leukemia virus (L-MuLV) producing A9HT subline of the mouse L-cell strain with C57B1 lymph node cells gave rise to hybrids with a suppressed production of infectious virus. The hybrids did not release reverse transcriptase or p30 antigen to their media, indicating that there was no detectable production of non-infectious virus. Upon segregation of chromosome 4 of the C57B1 partner cell—as evidenced by the loss of the Gpd-1^a marker—the hybrids resumed production of L-MuLV. The restriction could also be overcome by treatment of the hybrid cells with iododeoxyuridine which induces production of L-MuLV. Intracellular levels of p30 antigen and virus-specific RNA were three- to sevenfold lower in the restricted hybrids than in the A9HT parental cells or in segregating hybrid cell populations that have resumed virus production. Restriction of virus production in hybrid cells is thus correlated with the presence of chromosome 4 from the C57B1 partner cell which appears to affect the levels of intracellular viral RNA and proteins.