Progenitor Cell Numbers, Preserves Engraftment Capacity in Nonobese Diabetic/Severe Combined Immunodeficient Mice, and Enhances Retroviral Transduction Efficiency

Ex vivo culture of hematopoietic stem/progenitor cells could potentially improve the efficacy of human placental/umbilical cord blood (CB) in clinical hematopoietic stem cell (HSC) transplantation and allow gene transduction using conventional retroviral vectors. Therefore, we first examined the effects of a 7-day period of ex vivo culture on the hematopoietic capacity of CB CD34 1 cells. Medium for the ex vivo cultures contained either serum and six recombinant human hematopoietic growth factors (GFs), including Flt-3 ligand (FL), Kit ligand (KL 5 stem cell factor), thrombopoietin (Tpo), interleukin 3 (IL-3), granulocyte colony-stimulating factor (G-CSF), and interleukin 6 (IL-6), or a serum-free medium containing only FL, KL, and Tpo. After culture under both ex vivo conditions, the total numbers of viable cells, CD34 1 cells, colony-forming cells (CFCs), and long-term culture initiating cells (LTC-ICs) were increased. In contrast, the severe combined immunodeficiency (SCID) mouse engrafting potential (SEP) of cultured cells was slightly decreased, as compared with fresh cells. Nevertheless, cultured human CB CD34 1 cells were able to generate engraftment, shown to persist for up to 20 weeks after transplantation. We next tested the efficacy of retroviral transduction of cultured cells. Transduced cultured human cells were able to engraft in NOD/SCID mice, as tested 4 weeks after transplantation, and EGFP 1 CD34 1 cells and EGFP 1 CFCs were isolated from the chimeras. Thus, although additional improvements in ex vivo culture are still needed to expand the numbers and function of human HSCs, the current conditions appear to allow gene transduction into hematopoietic SCID engrafting cells, while at least qualitatively preserving their in vivo engraftment potential. 2927 OVERVIEW SUMMARY This study investigated improving ex vivo culture conditions for retroviral transduction of human hematopoietic stem cells (HSCs). Since successful hematopoietic gene therapy requires the transduced stem cells to engraft in vivo , we first evaluated the effects of ex vivo culture on the in vivo SCID engrafting potential (SEP) of cord blood (CB) CD34 1 cells. We found that the numbers of hematopoietic progenitor cells were increased after a 7-day period of ex vivo culture, and SEP was only slightly decreased. We then observed that human CB CD34 1 cells transduced during ex vivo culture in serum-free medium containing Flt3 ligand (FL), Kit ligand (KL 5 stem cell factor), and thrombopoietin (Tpo), generated EGFP 1 CD34 1 cells and EGFP 1 CFCs in NOD/SCID chimeras. Thus, ex vivo culture allowed efficient gene transfer into human hematopoietic stem/progenitor cells, while preserving their in vivo engraftm ent potential.