Generation of a high-titer packaging cell line for the production of retroviral vectors in suspension and serum-free media

Several patients with severe combined immunodeficiency-X1 disease and adenosine deaminase deficiency have been cured by retroviral-mediated gene therapy. Despite the earlier success, the production of retroviral vectors for clinical gene therapy is cumbersome, costly and lacks safety features because of the adherent nature of packaging cells and the necessity to supplement the culture media with bovine serum. The aim of this study was to generate a retrovirus packaging cell line that could be used for the production of large clinical batch vectors. Bicistronic vectors containing an internal ribosomal entry site followed by a selection gene were used to express Moloney murine leukemia gag-pol and amphotropic envelope viral proteins in HEK293 cells. The candidate clone (293GP-A2) that was selected as the packaging cell line could release recombinant green fluorescent protein retroviruses at 4 × 107 infectious viral particles per ml. Similar titers were achieved after these cells were adapted to grow in suspension and serum-free media. Furthermore, using the same culture conditions viral titers proved to be stable for a 3-month culture period. The 293GP-A2 packaging cell line has the potential to be cultured in bioreactors, opening the possibility for large-scale use of retroviral vectors in late stage clinical trials.