Improved Foamy Virus Vectors with Minimal Viral Sequences

Foamy virus (FV) vectors show promise for gene therapy applications. However, existing FV vectors either retain a significant portion of the wild-type virus genome or are produced at low titers. We describe a transient cotransfection system that produces high-titer FV vectors with minimal cis-acting regions. These vector genomes have deletions in the gag, pol, env, and bel1-3 accessory genes, as well as the LTR U3 region, but retain an essential 2.5-kb cis-acting region. In addition, stop codons were introduced into the remaining gag sequences to prevent expression of viral peptides and to eliminate dominant-negative effects of a Gag-Pol fusion protein. Although these deleted foamy (deltaphi) vectors were produced at relatively low titers with our prior packaging construct, we designed separate helper plasmids for Gag, Pol, and Env expression that allowed us to routinely produce helper-free, unconcentrated vector stocks with titers of over 10(5) transducing units/ml by four-plasmid transient transfection. The deltaphi vector stocks were then concentrated by ultracentrifugation to titers over 10(7) transducing units/ml. A deltaphi vector containing a 9.2-kb transgene cassette was produced at unconcentrated titers of over 10(5) transducing units/ml, demonstrating the utility of these deleted vectors for large therapeutic genes.