Sustained multilineage gene persistence and expression in dogs transplanted with CD34⁺ marrow cells transduced by RD114-pseudotype oncoretrovirus vectors

Previous studies have shown that the choice of envelope protein (pseudotype) can have a significant effect on the efficiency of retroviral gene transfer into hematopoietic stem cells. This study used a competitive repopulation assay in the dog model to evaluate oncoretroviral vectors carrying the envelope protein of the endogenous feline virus, RD114. CD34-enriched marrow cells were divided into equal aliquots and transduced with vectors produced by the RD114-pseudotype packaging cells FLYRD (LgGLSN and LNX) or by the gibbon ape leukemia virus (GALV)-pseudotype packaging cells PG13 (LNY). A total of 5 dogs were studied. One dog died because of infection before sustained engraftment could be achieved, and monitoring was discontinued after 9 months in another animal that had very low overall gene-marking levels. The 3 remaining animals are alive with follow-ups at 11, 22, and 23 months. Analyses of gene marking frequencies in peripheral blood and marrow by polymerase chain reaction revealed no significant differences between the RD114 and GALV-pseudotype vectors. The LgGLSN vector also contained the enhanced green fluorescent protein (GFP), enabling us to monitor proviral expression by flow cytometry. Up to 10% of peripheral blood cells expressed GFP shortly after transplantation and approximately 6% after the longest follow-up of 23 months. Flow cytometric analysis of hematopoietic subpopulations showed that most of the GFP-expressing cells were granulocytes, although GFP-positive lymphocytes and monocytes were also detected. In summary, these results show that RD114-pseudotype oncoretroviral vectors are able to transduce hematopoietic long-term repopulating cells and, thus, may be useful for human stem cell gene therapy.