Stromal cell-dependent ex vivo expansion of human cord blood progenitors and augmentation of transplantable stem cell activity. 
In vitro maintenance and expansion of human hematopoietic stem cells is crucial for many clinical applications.  Thrombopoietin (TPO) and flt3/flk2 ligand (FL) have been suggested to support the proliferation of primitive hematopoietic progenitors and the expansion of transplantable stem cells in culture.  In this study, we examined the synergistic effects of the murine stromal cell line MS-5 and a combination of the two cytokines, TPO and FL, on the ex vivo expansion of human cord blood primitive progenitors and transplantable stem cells.  A monolayer of MS-5 cells with TPO/FL synergistically supported a more than 600-fold expansion of human cord blood CD34+ cells and CD34+CD38- cells in 2 weeks of culture.  Colony-forming unit in culture (CFU-C) and 5-week and 8-week cobblestone area-forming cells (CAFC) were also expanded approximately 300-, 4- and 13-fold, respectively.  When MS-5 cells were physically separated from progenitors by a Transwell filter, the synergy was reduced to a quarter of the control, suggesting that direct cell-cell contact between MS-5 cells and progenitors is required for maximum expansion.  The severe-combined immunodeficient (scid) mouse-reconstituting cell (SRC) assay demonstrated the slight augmentation of transplantable stem cell activity in culture.  These results indicated that MS-5 cells provide a milieu that stimulates the proliferation of primitive progenitors including transplantable stem cells.