Transduction of Primate Cells with Feline Foamy Virus Envelope Pseudotyped Prototype Foamy Virus Vectors.

Because of their genetic and biological similarity to humans, non-human primates are the best pre-clinical models for testing the efficacy and safety of gene therapy systems. However, the presence of endogenous simian foamy virus infection in nearly all non-human primates kept in captivity complicates foamy virus (FV) vector stem cell transduction studies in these animals. A major concern is that repopulating cells exposed to FV vector stocks will elicit an immune response in non-human primate hosts. Though human serum does not inactivate prototype foamy virus (PFV) vectors, a one hour incubation of PFV vector stock in the presence of serum samples from Papio Cynophalus (baboon), Macaca Mulatta (rhesus macaque), or Macaca Fasicularis (long-tailed macaque) results in a 75–100% drop in titer. To overcome this serum mediated inactivation we sought to pseudotype PFV vectors in the feline foamy virus (FFV) envelope. The wild-type envelope from the FUV strain of FFV does not pseudotype our PFV vectors. Therefore we generated chimeras with regions of both the FFV and PFV envelope. By substituting portions of the FFV envelope leader peptide sequence and membrane spanning domain with corresponding PFV envelope regions we generated chimeric envelopes capable of high titer (10⁵–10⁶ FFU/ml) PFV vector production. Serum samples from Macaca Mulatta produced less inactivation of the FFV pseudotyped than the PFV pseudotyped vectors. Furthermore, both the PFV and FFV pseudotyped vectors demonstrated efficient transduction of baboon mesenchymal stem cells (27–43%) and baboon embryonic stem cells (37–40%). However, the FFV pseudotyped vectors transduced both human and baboon CD34⁺ cells less efficiently than the PFV pseudotyped vectors. We plan to test PFV vectors pseudotyped by other FV envelopes for inactivation by primate serum, and for their ability to transduce primate hematopoietic cells.