Engineered stromal layers and continuous flow culture enhance multidrug resistance gene transfer in hematopoietic progenitors.

We recently reported that in stroma-free cultures 11-33% of clonogenic cells derived from a bulk long-term culture [long-term culture-clonogenic cells (LTC-CC)] could be transduced by supernatant exposure or coculture of human CD34+ progenitors with MDR retroviral producer line A12M1. We reasoned that a stromal cell layer may generate niches in which LTC-CC could enter in the S-phase, thus becoming a more accessible target for gene delivery. In static culture studies in flasks, human engineered stromal cell line L87/4 or stromal murine M2-10B4 cells were used as feeder after irradiation, and CD34+ cells from either cord blood or peripheral blood of mobilized cancer patients were exposed to MDR supernatant for 7 consecutive days before 5-week culture for LTC-CC evaluation. In continuous flow perfusion culture studies, CD34+ cells were seeded over irradiated stromal murine M2-1OB4 cells and exposed to MDR supernatant for 7 days before LTC-CC evaluation. In mock-transduced controls, <5% of LTC-CC were found to he viable after exposure to 10 ng/ml Taxol. In cells exposed to MDR supernatant in static stroma cultures, 68 +/- 4% of seeded LTC-CC were found to be drug resistant and express MDR mRNA as evaluated by reverse transcription-PCR analysis of single colonies. The addition of cytokines did not further enhance transfer efficiency. After MDR retroviral exposure in continuous flow cultures, 88 +/- 5% of LTC-CC were found to be drug resistant (P < 0.01 versus static stroma culture). P-glycoprotein expression in CD34+ cells was evaluated using flow cytometry and found to he higher after continuous flow versus static cultures. Finally, very high levels of P-glycoprotein expression after MDR supernatant exposure in the presence of stroma were confirmed by APAAP staining of cultured cells. We conclude that engineered stromal cell layers and continuous flow culture conditions can significantly enhance retroviral-mediated gene transfer into human hematopoietic progenitor cells.