Efficient Transduction of Common Marmoset (Callithrix jacchus) Hematopoietic and Embryonic Stem Cells Using Foamyvirus Vectors.

Preclinical animal models are important for evaluating the safety and therapeutic efficacy of new therapeutic modalities such as gene therapy. From the different large animal models, nonhuman primate models have emerged over the last decades as highly desirable experimental systems from both a pathophysiologic and pharmacokinetic viewpoint and the study of nonhuman primates has provided important information on the efficacy and safety of gene therapy systems in vivo prior to human trials. The common marmoset (Callithrix jacchus) has the advantage that it is a small, and thus relatively inexpensive nonhuman primate model. Currently, very little data on the transduction efficiency of foamyviral vectors for gene transfer into marmoset stem cells exists. We therefore performed a direct comparison using identically designed gammaretroviral, lentiviral and foamyviral vector constructs expressing the enhanced green fluorescent protein (EGFP) from the spleen focus forming virus (SFFV) promoter pseudotyped with either the modified human foamy virus (HFV) envelope EM140 or the G-protein of vesicular stomatitis virus (VSV-G) for the transduction of common marmoset embryonic stem cells (CMES) as well as marmoset CD34+ hematopoietic progenitor cells. Virus stocks of these vectors were prepared by polyethyleneimine-mediated transfection of 293T cells and concentrated approximately 10-fold by centrifugation for 4 hours at 10.000 g at 4°C. Three different target cell populations were transduced:

1. common marmoset embryonic stem cells (CMES) or cryopreserved CD34-enriched cells from bone marrow of a common marmoset either

2. after a two-day prestimulation in the presence of IL-6, FLT3L, cSCF and TPO at a concentration of 100 ng/mL each, or

3. after overnight incubation with 100 ng/mL SCF only.

Equal numbers of cells were exposed to the four different vector preparations for 14 hours in 12-well dishes coated with CH-296. The read-out was based on fluorescence microscopy of colonies plated in methyl cellulose as well as flow cytometry (FACS). Foamyviral vectors with the foamyviral envelope were the most efficient gene transfer tool for marmoset hematopoietic CD34-positive cells with stable transduction rates of over 80% as assessed by flow cytometry at both 2 or 7 days after the end of transduction and on average 88% transduction efficiency into colony forming cells (CFU-C). Transduction of CFU-C with all other vector preparations was below 60%. In CMES, initial gene transfer rates of over 80% were achieved with the VSV-G pseudotype lentiviral vector, however, expression decreased to 13% after 7 days. In contrast, the foamyviral vector pseudotyped with the foamyviral envelope decreased only from 49% to 24% after 7 days. In conclusion, we achieved stable viral gene transfer and expression in CMES cells as well as highly efficient gene transfer into common marmoset hematopoietic CD34 positive cells using foamyviral vectors. These results suggest that foamyviral vectors may be highly feasible vectors for stem cell gene transfer and thus set the stage for a more detailed analysis of this vector system in transplantation studies in this nonhuman primate model.