Stroma-conditioned medium and sufficient prestimulation improve fibronectin fragment-mediated retroviral gene transfer into human primitive mobilized peripheral blood stem cells through effects on their recovery and transduction efficiency

Mobilized peripheral blood stem cells (PBSC) are an attractive vehicle for cancer gene therapy. However these stem cells may have a reduced proliferative capacity due to previous cytotoxic chemotherapy treatment of the patient. In addition, primitive hematopoietic stem cells (HSC) from mobilized peripheral blood are almost exclusively quiescent, which makes it hard to induce proliferation in vitro and thus to improve stable transduction of introduced genes into a sufficiently large number of primitive stem cells. In this study CD34-selected mobilized PBSC from lymphoma and myeloma patients were used as target cells for retroviral-mediated gene transfer using a clinically relevant cell-and serum-free supernatant transduction protocol. We have investigated various parameters that may contribute to an improvement of the poor transduction efficiency of the primitive HSC, including prestimulation time, the use of the carboxy-terminal fibronectin fragment CH-296, as well as stromal cell line conditioned media. Retroviral supernatant transduction in combination with CH-296 increased significantly the gene transfer efficiency as compared to supernatant alone and made the use of polycations redundant. Gene transfer of primitive HSC (cobblestone area forming cell (CAFC) week 6) was specifically improved when this procedure was preceded by a 5-day pre-culture period as compared to a 2-day transduction procedure. However, irrespective of the numerical recovery, the CAFC week 6 after retroviral transduction produced less long-term culture colony-forming cells, suggesting a loss of individual stem cell quality. The addition of stroma-conditioned media during the pre-culture period did not affect the individual CAFC quality or transduction efficiency, but increased greatly the recovery of the total number of transduced and untransduced HSC leading to larger grafts containing higher numbers of transduced stem cells.