Rev-independent expression of synthetic gag-pol genes of human immunodeficiency virus type 1 and simian immunodeficiency virus:: Implications for the safety of lentiviral vectors

The safety of lentiviral vectors for clinical applications is still a major concern. The gag-pol expression plasmids and the lentiviral vectors used in previous studies contain homologous regions, which constitute a risk for recombination events. Synthetic gag-pol genes of human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) were therefore constructed, in which the codon usage was optimized for expression in human cells without altering the amino acid sequences. The synthetic gag-pol genes allowed efficient expression of these genes in the absence of Rev and the 5' untranslated leader region. Both the HIV-1 and the SIV synthetic gag-pol expression plasmids could mediate transduction of an SIV vector into nondividing human cells with titers of about 10(6) transducing units/ml. Similar titers were obtained with a four-plasmid vector-packaging system based on HIV-1. Using a biological assay, homologous recombination events between the synthetic gag-pol expression plasmids and an SIV vector were undetectable and in comparison with a previously used gag-pol expression plasmid at least approximately 100-fold less frequent. By eliminating regions of homology and sequences involved in packaging, synthetic gag-pol genes should improve the safety profile of lentiviral vectors.