Hematopoietic stem cell gene therapy: towards clinically significant gene transfer efficiency.

Abstract

Early clinical gene therapy and gene marking trials using retroviral vectors to transduce hematopoietic stem cells (HSC) revealed two major shortcomings of this new treatment modality. One was insufficient expression or even silencing of the integrated vector sequences, and the second was the low gene transfer efficiency achieved to in vivo repopulating cells. It became clear that neither rodent models nor human in vitro surrogate assays for stem cells were predictive of in vivo transgene levels in human target cells. Using the rhesus monkey model we have focused on improving gene transfer efficiency into HSC. Immune rejection of trans duced cells has been shown to occur in mature peripheral target cells such as lymphocytes or myocytes. Our studies and those from other investiga tors suggest that transgenes introduced via HSC induces immunologic tolerance towards the foreign product. In vivo priming of target cells, i.e. mobilization of HSC into the circulation with granulocyte colony stimulating factor and stem cell factor, as well as optimization of the in vitro transduction conditions, now allow a stable in vivo gene transfer efficiency of up to 10-15% in both lymphoid and myeloid circulating cells in non-human primates, levels that would be adequate for many clinical applications.