Competitive repopulation of retrovirally transduced haemopoietic stem cells

Gene transfer into haemopoietic stem cells (HSC) may be useful in gene ther apy for a variety of inherited and acquired human diseases. Cell division i s required for retroviral transduction, and cytokine stimulation is often u sed to increase mitosis of quiescent HSC, Exposure to cytokines has been sh own to have an unfavourable effect on the engraftment of these cells when c ompeted with unmanipulated BSC. We now show that a similar engraftment defe ct is present when HSC are manipulated and transduced with the human multip le drug resistance (MDR) gene. The extent of the unfavourable competition d epended on the relative numbers of cytokine-treated and fresh cells when th e two populations of cells were administered simultaneously into marrow-abl ated isogenic mice. When the manipulated transduced cells were given 2 or 4 d before the unmanipulated cells there was a much greater engraftment of th e manipulated cells. The data suggested that the manipulated cells were at a relative disadvantage for marrow engraftment as compared to fresh cells, presumably due to the more efficient homing and engraftment properties of t hese latter unmanipulated cells. However, the manipulated cells had no intr insic inability to engraft when they were the predominant donor fell popula tion. In all cases the percent of MDR transduced cells in the engrafting ma nipulated cells remained relatively constant at about 25-30%. These results have implications for the use of manipulated transduced stem cells in gene therapy, suggesting that administering them before adding fresh cells can overcome their engraftment defect.