Studies of the long-term effect of Fv-1 gene restriction in clones of NIH/3T3 cells infected with murine leukemia virus

Long-term culture of MuLV-infected Fv-1-resistant cells progressively leads to virus production comparable to levels found in permissive cells. We have studied N- and B-tropic MuLV-infected NIH/3T3 cells cloned early after infection to determine the role of reinfection and of late virus production in overcoming Fv-1 restriction in resistant cells. We found that in absence of reinfection a very low percentage of B-tropic MuLV-infected resistant clones (0.25%) became virus producers. Among several B-tropic MuLV-infected resistant non-producer clones analyzed, none had acquired newly integrated viral sequences. These results confirm, by an independent method, that the Fv-1 gene restriction prevents the integration of viral DNA and indicate that it is a permanent effect. No virus production seems to occur in resistant cells late after infection.