Amphotropic murine leukemia virus replication in human mammary epithelial cells and the formation of cytomegalovirus-promoter recombinants

Amphotropic murine leukemia virus (MLV) can replicate in human cells and is a potential contaminant in vector preparations for human gene transfer studies. We have recently shown that replication of amphotropic MLV in specific human sarcoma and lymphoma lines is possible in the absence of the viral 75-bp transcription enhancer elements. Here, we have tested the replication of an amphotropic MLV, MLV-(MOA), and an enhancer-deficient mutant of this virus in human breast carcinoma-derived cell lines. The proviral expression plasmids use a cytomegalovirus (CMV) promoter for the initial transcription of virus RNA. We found that all cells analyzed are permissive for replication of MLV-(MOA). Enhancer-deficient virus is unable to replicate. However, in two lines the replication defect can be rescued by the spontaneous insertion of a CMV promoter and enhancer into the U3 region. This recombinant virus MLV-(RCMV) replicates with kinetics similar to that of MLV-(MOA) but is restricted to specific cell lines. The potential formation of RCMV recombinants during MLV vector preparation must be considered.