Regulation of hematopoietic and leukemic stem cells [electronic resource]
Keywords: Immunology.

Hematopoiesis is organized as a cellular hierarchy in which long-term self-renewing hematopoietic stem cells (HSC) give rise to all lineages of blood cells for the lifetime of an organism. Following the prospective isolation of mouse HSC in the late 1980s, research over the past two decades has focused on refining the phenotypic compartment where HSC reside, and identifying the numerous cell intrinsic and extrinsic mechanisms important for their maintenance. Many of these pathways are misappropriated in hematopoietic malignancies such as leukemia. It has been proposed that like normal hematopoiesis, leukemia is organized as a cellular hierarchy in which a leukemic stem cell population (LSC) is uniquely able to self-renew and gives rise to the heterogeneous lineages of cells that compose the tumor. This thesis will cover the mechanisms regulating the behavior of normal hematopoietic stem cells and leukemic stem cells. First, phospho-specific flow cytometry was utilized to define the responsiveness of human HSC to cytokine stimulation. This direct biochemical analysis demonstrated that HSC respond directly and rapidly to a broader number of direct stimuli in vitro and in vivo than previously appreciated. This included Granulocyte Colony Stimulating Factor (G-CSF), a cytokine used clinically to mobilize HSC to the peripheral blood prior to bone marrow transplantation. The direct HSC response to G-CSF is shown to affect the behavior of the HSC pool in vitro (causing cell cycle entry and increased proliferation), and in vivo (negatively regulating active hematopoiesis in the short-term). Thus, it is proposed that cytokines, long known to impact the function of mature lineage cells, also are important for the steady-state regulation of HSC activity. Next, two mouse models of AML were utilized to determine the mechanisms important for regulation of LSC activity. It is demonstrated that the catalytic p110[lower case delta] subunit of Phosphoinositide 3-kinase (PI3K), which is constitutively activated and drives proliferation in human AML blasts, is dispensable for de novo leukemogenesis and LSC activity in AML driven by retroviral transfer of MLL-AF9 or HoxA9-Meis1 (H9M) oncogenes into early hematopoietic progenitors. Finally, the LSC in H9M AML are identified. Surprisingly, multiple phenotypic compartments were equally enriched for LSC activity. This was also the case when LSC activity in MLL-AF9 driven AML was assessed. Putative genetic LSC maintenance signatures for each model were also identified. There was both overlap and stark contrast in cell surface phenotypes of the LSC-enriched compartments and the genetic LSC maintenance signatures in these closely related leukemias. These data suggest that like normal hematopoiesis, a hierarchy does exist in which self-renewing cells give rise to terminally differentiated progeny. However, unlike HSC activity, which can be isolated to a single compartment by cell surface phenotype that is commonly regulated between individuals, LSC activity is a cell state that can be independent of surface phenotype and with context-specific regulatory mechanisms.