Production and purification of high-titer foamy virus vector for the treatment of leukocyte adhesion deficiency

Compared to other integrating viral vectors, foamy virus (FV) vectors have distinct advantages as a gene transfer tool, including their nonpathogenicity, the ability to carry larger transgene cassettes, and increased stability of virus particles due to DNA genome formation within the virions. Proof of principle of its therapeutic utility was provided with the correction of canine leukocyte adhesion deficiency using autologous CD34+ cells transduced with FV vector carrying the canine CD18 gene, demonstrating its long-term safety and efficacy. However, infectious titers of FV-human(h)CD18 were low and not suitable for manufacturing of clinical-grade product. Herein, we developed a scalable production and purification process that resulted in 60-fold higher FV-hCD18 titers from ~1.7 × 104 to 1.0 × 106 infectious units (IU)/ml. Process development improvements included use of polyethylenimine-based transfection, use of a codon-optimized gag, heparin affinity chromatography, tangential flow filtration, and ultracentrifugation, which reproducibly resulted in 5,000-fold concentrated and purified virus, an overall yield of 19 ± 3%, and final titers of 1-2 × 109 IU/ml. Highly concentrated vector allowed reduction of final dimethyl sulfoxide (DMSO) concentration, thereby avoiding DMSO-induced toxicity to CD34+ cells while maintaining high transduction efficiencies. This process development results in clinically relevant, high titer FV which can be scaled up for clinical grade production.