Lack of superinfection interference in retroviral vector producer cells.

Vesicular stomatitis virus G protein (VSV-G)-pseudotyped retroviral vectors have become more feasible for clinical gene transfer protocols since stable tetracycline (tet)-regulated packaging cell lines have become available. Here, we analyzed superinfection interference in VSV-G-pseudotyped and classic amphotropic packaging cell lines. No superinfection interference was observed in VSV-G-pseudotyped packaging cell lines. Thus, integrated retroviral vector genomes accumulated during culture. Similar results were obtained with the amphotropic packaging cells, but to a lesser degree. In addition, VSV-G packaging cells were susceptible to infection with vector particles devoid of envelope proteins, which are produced by these cells in high titers when VSV-G expression is suppressed by tetracycline. For both packaging systems, superinfection could be blocked by azidothymidine (AZT). With regard to safety, this study suggests that in clinical protocols amphotropic producer clones should be tested for superinfection interference and VSV-G packaging cells should always be cultured in the presence of AZT.