ABSTRACT
BACKGROUND: The most primitive engrafting hematopoietic stem cell (HSC) resides mainly in a tumor growth factor-beta (TGF-beta)-dependent quiescent phase of the cell cycle. In this study, ex vivo expansion of UC blood (UCB) HSCs has been investigated, with the aim of showing whether quiescent HSCs can be recovered from expansion culture. METHODS: AC133(+) stem/progenitor cells from six full term-pregnancies UCB-samples were immunomagnetically selected, followed by ex vivo expansion culture in the presence of thrombopoietin (TPO), c-kit ligand (KL), flt-3 ligand (FL) and IL-6. Quiescent HSCs were detected by a clonogenic assay that allows the detection of multipotent and committed single- lineage quiescent stem/progenitor cells, named mHPP-Q and cHPP-Q, respectively, by means of a TGF-beta blocking Ab. RESULTS: Expansion culture of fresh selected AC133(+) cells for 1 week caused maintenance rather than expansion of mHPP-Q cells and a 1-fold increase in cHPP-Q cells. A further week culture initiated with 7-day expanded AC133(+) cells resulted in an additional 1.5-fold expansion of cHPP-Q while no mHPP-Q cells could be detected. Amplification of cHPP-Q cells in long-term expansion cultures initiated with 14-day expanded AC133(+) cells was observed for at least a further 4 weeks. DISCUSSION: A small proportion of HPP-Q cells recovered from 7-day expansion cultures retain their multilineage potential: longer culturing of these cells results in the loss of multilineage potential while they maintain quiescent behavior and high proliferative potential.
