Ex vivo culture of cord blood CD34(+) cells expands 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 im prove 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 eff ects of a 7-day period of ex vivo culture on the hematopoietic capacity of CB CD34(+) cells. Medium for the ex vivo cultures contained either serum an d six recombinant human hematopoietic growth factors (GFs), including Flt-3 ligand (FL), Kit ligand (KL = stem cell factor), thrombopoietin (Tpo), int erleukin 3 (IL-3), granulocyte colony-stimulating factor (G-CSF), and inter leukin 6 (IL-6), or a serum-free medium containing only FL, KL, and Tpo. Af ter culture under both ex vivo conditions, the total numbers of viable cell s, CD34(+) cells, colony-forming cells (CFCs), and long-term culture initia ting cells (LTC-ICs) were increased. In contrast, the severe combined immun odeficiency (SCID) mouse engrafting potential (SEP) of cultured cells was s lightly decreased, as compared with fresh cells. Nevertheless, cultured hum an: CB CD34(+) cells were able to generate engraftment, shown to persist fo r up to 20 weeks after transplantation. We next tested the efficacy of retr oviral transduction of cultured cells. Transduced cultured human cells were able to engraft in NOD/SCID mice, as tested 4 weeks after transplantation, and EGFP(+)CD34(+) cells and EGFP(+) 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 a ppear to allow gene transduction into hematopoietic SCID engrafting cells, while at least qualitatively preserving their in vivo engraftment potential .