These mice possess normal HSCs, CMPs, MEPs, and GMP numbers, but have reduced numbers of lymphoid progenitors, which was functionally confirmed by worse lymphoid contribution on whole BM transplantation

These mice possess normal HSCs, CMPs, MEPs, and GMP numbers, but have reduced numbers of lymphoid progenitors, which was functionally confirmed by worse lymphoid contribution on whole BM transplantation. stromal cells can maintain granulopoiesis (23). Later, Whitlock and Witte isolated stromal BM cultures that initiated and maintained B lymphopoiesis (60, 104). Whitlock and Weissman cloned several stromal lines from these cultures and found several that supported normal populations of BM cells that formed cobblestones in culture, with each colony giving rise to both myeloid cells (early) and B-lineage cells (late) (103). The line AC6.2.1 derived from them Corticotropin-releasing factor (CRF) was used in the screening of fluorescence-activated cell sorting selected mouse and human BM subsets for the initial 2000-fold enrichment of cells containing HSCs (103). Finally, the OP9 stroma cell lines, which are deficient in macrophage colony-stimulating factor, were shown to trigger the differentiation of embryonic stem cells to blood cells (61). Although these initial experiments proved the importance of the niche for HSC activity, the mechanisms underlying the HSC-niche interactions remained unknown. With the development of new mice models and imaging techniques, we can now more precisely identify the niche components and prospectively investigate the role of the HSC niche. However, many questions still need further clarification: What kinds of cells are essential in the HSC niche? Is the HSC niche unique only to HSCs? What are the mechanisms of competition within the niche? How is the leukemic niche different? How does the niche protect HSCs from exhaustion? In this review, we will summarize recent discoveries addressing these questions and outline the next emerging challenges. Changing Candidate Cells Making Up the HSC Niche BM is a complex tissue with a high cell density. Hematopoietic cells constitute a major fraction of the BM, but there are Rabbit polyclonal to ACPT also several other kinds of cells, including mesenchymal stromal cells (MSCs), osteoblasts, Corticotropin-releasing factor (CRF) endothelial cells, and neuronal cells. A fundamental question was: What are the cells that constitute the HSC niche (94, 102)? Using the early cell surface markers that could enrich for HSCs, but not isolate pure HSCs, immunohistological evidence implied that the HSCs were in an endosteal region of the BM (109). In engineered mouse strains, in which osteoblasts and osteo-lineage cells were targeted by overexpression of parathyroid hormone (PTH) (11) or by deletion of the gene promoter driving thymidine kinase (Col2.3TK) markedly decreased the number of HSCs (95, 113). This suggested that it is the osteoblastic Corticotropin-releasing factor (CRF) lineage in the bone endosteum that governs the stem cell potential of the HSCs. However, these observations were only correlative and it has not yet been shown as to whether the observed effect is due to a direct interaction between osteoblasts and HSCs or an indirect mechanism may be involved. It also cannot be ruled out that there exist other cells besides osteoblasts in the BM that express PTH- or PTH-related protein receptor, BMPR1, and COLL1. The most definitive way to demonstrate that a cell directly neighbors the HSCs would be by imaging. However, the imaging of the HSC niche has for a long time been impaired by the rarity of HSCs and the necessity to use several markers to identify them. Progress came with the discovery of more specific markers and HSC-reporter mice. First, Kiel demonstrated that functional HSCs could be enriched by using SLAM family markers: HSCs are negative for CD48 and positive for CD150 (45). They showed that the CD150+CD48? fraction of HSCs resides near perivascular sinusoidal cells (20 out of 35 cells) rather than in the endosteal osteoblastic niche (5 out of 35 cells). Although now more studies confirm the perivascular niche of HSCs, there are inconsistencies regarding the type of endothelial cells that HSCs attach to and their role in regulating HSC biology. Kunisaki underline the importance of Sca-1+ small arterioles for the maintenance of quiescent HSCs (49). These Sca-1+ arterioles express also VEGFR2, VEGFR3, and Tie2 antigens (49). The whole-mount imaging of BM revealed that cells expressing markers of quiescent HSC preferentially localize to the Sca-1+ arterioles (49), whereas the cycling Ki67+ HSC localize more closely to sinusoids. Next, Itkin proposed that arterioles and sinusoids differentially regulate the metabolism and reactive oxygen production in neighboring HSCs (35). On an intraperitoneal injection of mice with hydroethidinea marker of reactive oxygen species (ROS) productiononly.