Construction of a retroviral vector production system with the minimum possibility of a homologous recombination.
A recombination between the short homologous regions of nucleotide sequences in the retroviral vector and packaging cell line has been thought to be a major cause of the production of replication-competent retrovirus (RCR). Therefore, the removal of overlapping sequences between the vector and the packaging constructs is crucial for minimizing the possibility of homologous recombination, and therefore, the production of RCR. We have recently constructed a series of retroviral vectors that contain no viral coding sequences, but still produce high viral titer and high-level gene expression. However, many previously constructed murine leukemia virus (MLV)-based packaging constructs contained significantly long 5' and/or 3' untranslated regions of MLV, which are also present in the retroviral vector, and as such could possibly lead to homologous recombination. To make a retroviral production system that is free from homologous recombination, we constructed expression plasmids for gag-pol and env, precisely starting from the start codon and ending at the stop codon of respective open reading frames. When the packaging function was provided from one plasmid, a vector containing bits of all three viral coding sequences produced RCR at a significant frequency, while our vector remained free of any RCR. Our retrovirus production system is anticipated to have the minimum possible frequency of RCR production due to the elimination of potential sites for homologous recombination. Based on these results, a highly efficient new packaging line Vamp that contains no overlapping sequences with our retroviral vector was also developed.