Retrovirus-Mediated Gene Expression in Hematopoietic Cells Correlates Inversely with Growth Factor Stimulation

Cells of the hematopoetic system, especially hematopoietic progenitor and stem cells, are perceived as ideal targets for human gene therapy. In this report, the stability of retrovirus-mediated gene expression driven by three different potent promoters has been examined in purified human CD34⁺ cells. The promoters, murine stem cell virus (MSCV) long terminal repeat (LTR) and pgk, show gene expression in 10 times more hematopoietic colonies derived from CD34⁺ cells than the commonly used Moloney murine leukemia virus (Mo-MLV) LTR. Quantitative reverse transcriptase polymerase chain reaction (RT-PCR) analysis, however, demonstrates that the levels of gene expression in retrovirus-transduced cells decrease with time in long-term bone marrow cultures and in suspension cultures containing hematopoietic growth factors. Removal of hematopoietic growth factors from the suspension culture medium was associated with a decrease in cell proliferation and differentiation, but with stable gene expression. Retrovirus-mediated gene expression is, therefore, inversely related to proliferation and differentiation of the transduced CD34⁺ cells. These observations may have implications in future design and implementation of human gene therapy protocols.