RETROVIRAL TRANSDUCTION OF HUMAN CD34(-CORD BLOOD PROGENITOR CELLS WITH A MUTATED DIHYDROFOLATE-REDUCTASE CDNA() UMBILICAL)

Umbilical cord blood cells (UCB) have become a major target population for experimental and clinical studies using transfer of genes involve d in inborn enzymatic diseases. Cord blood contains hematopoietic prog enitor cells at a high frequency, and expanding these cells ex vivo ge nerates sufficient numbers of hematopoietic precursors for transplanta tion into adults, e.g., as supportive treatment. As clinical reports a bout retroviral transduction into UCB cells have not been as encouragi ng as the first preclinical data, we have established a retroviral tra nsduction system that allows expansion and selection of hematopoietic progenitor cells from UCB. CD34-enriched UCB cells were transduced wit h a retroviral vector encoding a mutated dihydrofolate reductase cDNA that confers MTX resistance. We observed increased resistance to MTX i n transduced granulocyte macrophage-colony forming units (CFU-GM) afte r co-culture of CD34(+) UCB cells with the virus-producing cell line, or after incubation with virus-containing supernatant. The supernatant -based transduction protocol included a prestimulation with recombinan t interleukin-l (rhIL-1), rhkit-ligand, and rhIL-3 to increase the per centage of cells in S phase to greater than 50%. Using this protocol w e measured a 72-fold expansion of CFU-GM and a 2.5-fold selective adva ntage of transduced versus nontransduced progenitor cells after exposu re to low-dose methotrexate in liquid culture. Polymerase chain reacti on analysis revealed integration of proviral DNA into the majority of transduced colonies before and after ex vivo expansion. The retroviral vector and transduction protocol reported here provides an experiment al system for selection and expansion of retrovirally transduced proge nitor/stem cells from UCB that may help improve the efficiency of curr ent clinical gene therapy strategies.