Regulation of Hematopoietic Stem Cells

Abstract: This thesis is about the regulation of hematopoietic stem cells (HSCs) that represent rare cells residing in the bone marrow (BM) of adults. They are multipotent cells that have the capacity to differentiate to all mature cells of the blood system and have the capacity to self-renew, i.e. generate new HSCs. These cells have tremendous therapeutic potential to treat a variety of hematopoietic disorders through blood and marrow transplantation. Today, peripheral blood (PB) is the most commonly used source of HSCs since HSCs can be mobilized from BM to peripheral blood. A third more convenient source for harvesting HSCs is cord blood (CB). However, the yield of HSCs in CB is too low for successful transplantation to most adult patients. Thus, ideally HSCs from CB needs to be increased in number (expanded) ex vivo before transplantation. Expansion of HSCs holds great promise but has been met with limited success due to incomplete knowledge regarding regulation of HSCs. Thus, deeper understanding of the regulatory mechanisms that govern HSCs fate is critical to allow expansion of HSC ex vivo and improve HSC-based therapies. The studies presented in this thesis have identified factors involved in the regulation of HSC fate decisions. In summary, our results demonstrate that increased Smad4 expression, a key component in the transforming growth factor-β (TGF-β) signaling pathway, sensitizes human CB HSPCs to TGF-β. This leads to growth arrest and apoptosis in vitro and reduced HSPC reconstitution capacity in vivo with no effect on lineage distribution. Together, these findings demonstrate an important role for TGF-β signaling in the regulation of human HSCs in vivo (Article I). Furthermore, with the purpose to investigate and characterize a novel regulator, we have studied the role of pigment epithelium-derived factor (PEDF) in murine HSCs. Absence of PEDF leads to reduced HSC numbers and impaired engraftment following transplantation. Here, we report for the first time that PEDF is a critical regulatory factor for HSC function (Article II). Last, we have identified the cell surface antigen CD9 as a positive marker for HSCs that provides a simple alternative for stem cell isolation at high purity. Using CD9 as a tool we have dissected heterogeneity within the HSC pool as defined by CD9 expression (Article III). Taken together, we have identified several molecules of human and murine HSCs that are important for stem cell function and fate outcome.