AIM: To establish a novel coculture system for ex vivo expansion of umbilical cord blood(UCB) hematopoietic progenitors using thrombopoietin (TPO)/Flt-3 ligand(FL)-transduced human marrow-derived mesenchymal stem cells (tfhMSCs) as feeder.METHODS: UCB CD34+ cells were isolated and cultured using four culture systems in serum-containing or serumfree medium. Suitable aliquots of cultured cells were used to monitor cell production, clonogenic activity,and long-term culture-initiating culture (LTC-IC) output.Finally, the severe-combined immunodeficient (SCID)mouse-repopulating cell (SRC) assay was performed to confirm ability of the cultured cells to reconstitute longterm hematopoiesis.RESULTS: There were no significant differences in the number of total nucleated cells among different culture systems in serum-containing medium during 21-d culture. However, on d 14, the outputs of CD34+ cells,CFU-C and CFU-GEMM in tfhMSCs coculture system were significantly enhanced. LTC-IC assay demonstrated that the tfhMSCs coculture system had the most powerful activity. The severe-combined immunodeficient (SCID)mouse repopulating cell (SRC) assay confirmed extensive ability of the expanded cells to reconstitute long-term hematopoiesis. Furthermore, PCR analysis demonstrated the presence of human hematopoietic cells in the bone marrow and peripheral blood cells of NOD/SCID mice.CONCLUSION: The TPO/FL-transduced hMSCs, in combination with additive cytokines, can effectively expand hematopoietic progenitors from UCB in vitro and the tfhMSCs coculture system may be a suitable system for ex vivo manipulation of primitive progenitor cells under contact culture conditions.