Efficient retroviral-mediated gene transfer to human cord blood stem cells with in vivo repopulating potential

Recent studies have shown efficient gene transfer to primitive progenitors in human cord blood (CB) when the cells are incubated in retrovirus-containing supernatants on fibronectin-coated dishes. We have now used this approach to achieve efficient gene transfer to human CB cells with the capacity to regenerate lymphoid and myeloid progeny in nonobese diabetic (NOD)/severe combined immunodeficiency (SCID) mice. CD34(+) cell-enriched populations were first cultured for 3 days in serum-free medium containing interleukin-3 (IL-3), IL-6, granulocyte colony-stimulating factor, Flt3-ligand, and Steel factor followed by two 24-hour incubations with a MSCV-NEO virus-containing medium obtained under either serum-free or serum-replete conditions. The presence of serum during the latter 2 days made no consistent difference to the total number of cells, colony-forming cells (CFC), or long-term culture-initiating cells (LTC-IC) recovered at the end of the 5-day culture period, and the cells infected under either condition regenerated similar numbers of human CD34(+) (myeloid) CFC and human CD19(+) (B lymphoid) cells for up to 20 weeks in NOD/SCID recipients. However, the presence of serum increased the viral titer in the producer cell-conditioned medium and this was correlated with a twofold to threefold higher efficiency of gene transfer to all progenitor types. With the higher titer viral supernatant, 17% +/- 3% and 17% +/- 8%, G418-resistant in vivo repopulating cells and LTC-IC were obtained. As expected, the proportion of NEO + repopulating cells determined by polymerase chain reaction analysis of in vivo generated CFC was even higher (32% +/- 10%). There was no correlation between the frequency of gene transfer to LTC-IC and colony-forming unit-granulocyte-macrophage (CFU-GM), or to NOD/SCID repopulating cells and CFU-GM (r2 = 0.16 and 0.17, respectively), whereas values for LTC-IC and NOD/SCID repopulating cells were highly and significantly correlated (r2 = 0.85). These findings provide further evidence of a close relationship between human LTC-IC and NOD/SCID repopulating cells (assessed using a >/= 6-week CFC output endpoint) and indicate the predictive value of gene transfer measurements to such LTC-IC for the design of clinical gene therapy protocols.