All blood cell lineages derive from a common hematopoietic stem cell (HSC). The current model implicates that the first lineage commitment step of adult pluripotent HSCs results in a strict separation into common lymphoid and common myeloid precursors. We present evidence for a population of cells which, although sustaining a high proliferative and combined lympho-myeloid differentiation potential, have lost the ability to adopt erythroid and megakaryocyte lineage fates. Cells in the Lin-Sca-1+c-kit+ HSC compartment coexpressing high levels of the tyrosine kinase receptor Flt3 sustain granulocyte, monocyte, and B and T cell potentials but in contrast to Lin-Sca-1+c-kit+Flt3- HSCs fail to produce significant erythroid and megakaryocytic progeny. This distinct lineage restriction site is accompanied by downregulation of genes for regulators of erythroid and megakaryocyte development. In agreement with representing a lymphoid primed progenitor, Lin-Sca-1+c-kit+CD34+Flt3+ cells display upregulated IL-7 receptor gene expression. Based on these observations, we propose a revised road map for adult blood lineage development.

Recent studies implicated the existence of adult lymphoid-primed multipotent progenitors (LMPPs) with little or no megakaryocyte-erythroid potential, questioning common myeloid and lymphoid progenitors as obligate intermediates in hematopoietic stem cell (HSC) lineage commitment. However, the existence of LMPPs remains contentious. Herein, global and single-cell analyses revealed a hierarchical organization of transcriptional lineage programs, with downregulation of megakaryocyte-erythroid genes from HSCs to LMPPs, sustained granulocyte-monocyte priming, and upregulation of common lymphoid (but not B and T cell-specific) genes. These biological and molecular relationships, implicating almost mutual exclusion of megakaryocyte-erythroid and lymphoid pathways, are established already in fetal hematopoiesis, as evidenced by existence of LMPPs in fetal liver. The identification of LMPPs and hierarchically ordered transcriptional activation and downregulation of distinct lineage programs is compatible with a model for HSC lineage commitment in which the probability for undergoing different lineage commitment fates changes gradually when progressing from HSCs to LMPPs.

Recent studies have suggested that regeneration of non-haematopoietic cell lineages can occur through heterotypic cell fusion with haematopoietic cells of the myeloid lineage. Here we show that lymphocytes also form heterotypic-fusion hybrids with cardiomyocytes, skeletal muscle, hepatocytes and Purkinje neurons. However, through lineage fate-mapping we demonstrate that such in vivo fusion of lymphoid and myeloid blood cells does not occur to an appreciable extent in steady-state adult tissues or during normal development. Rather, fusion of blood cells with different non-haematopoietic cell types is induced by organ-specific injuries or whole-body irradiation, which has been used in previous studies to condition recipients of bone marrow transplants. Our findings demonstrate that blood cells of the lymphoid and myeloid lineages contribute to various non-haematopoietic tissues by forming rare fusion hybrids, but almost exclusively in response to injuries or inflammation.

Hematopoietic stem cell (HSC) numbers are tightly regulated and maintained in postnatal hematopoiesis. Extensive studies have supported a role of the cytokine tyrosine kinase receptor Kit in sustaining cycling HSCs when competing with wild-type HSCs posttransplantation, but not in maintenance of quiescent HSCs in steady state adult bone marrow. In this study, we investigated HSC regulation in White Spotting 41 (KitW41/W41) mice, with a partial loss of function of Kit. Although the extensive fetal HSC expansion was Kit-independent, adult KitW41/W41 mice had an almost 2-fold reduction in long-term HSCs, reflecting a loss of roughly 10,000 Lin−Sca-1+Kithigh (LSK)CD34−Flt3− long-term HSCs by 12 wk of age, whereas LSKCD34+Flt3− short-term HSCs and LSKCD34+Flt3+ multipotent progenitors were less affected. Whereas homing and initial reconstitution of KitW41/W41 bone marrow cells in myeloablated recipients were close to normal, self-renewing KitW41/W41 HSCs were progressively depleted in not only competitive but also noncompetitive transplantation assays. Overexpression of the anti-apoptotic regulator BCL-2 partially rescued the posttransplantation KitW41/W41 HSC deficiency, suggesting that Kit might at least in the posttransplantation setting in part sustain HSC numbers by promoting HSC survival. Most notably, accelerated in vivo BrdU incorporation and cell cycle kinetics implicated a previously unrecognized role of Kit in maintaining quiescent HSCs in steady state adult hematopoiesis.

The cytokine tyrosine kinase receptors c-kit and flt3 are expressed and function in early mouse and human hematopoiesis. Through its ability to promote ex vivo expansion and oncoretroviral transduction of primitive human hematopoietic progenitors, the flt3 ligand (FL) has emerged as a key stimulator of candidate human hematopoietic stem cells (HSCs). However, recent studies in the mouse suggest that though it is present on shortterm repopulating cells, flt3 is not expressed on bone marrow long-term reconstituting HSCs, the ultimate target for the development of cell replacement and gene therapy. Herein we demonstrate that though only a fraction of human adult bone marrow and cord blood CD34 ؉ longterm culture-initiating cells (LTC-ICs) express flt3, most cord blood lymphomyeloid HSCs capable of in vivo reconstituting nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice are flt3 ؉ . The striking difference in flt3 and c-kit expression on mouse and candidate human HSCs translated into a corresponding difference in flt3 and c-kit function because FL was more efficient than SCF at supporting the survival of candidate human HSCs. In contrast, SCF is far superior to FL as a viability factor for mouse HSCs. Thus, the present data provide compelling evidence for a contrasting expression and response pattern of flt3 and c-kit on mouse and human HSCs. IntroductionAll blood cells arise from a common hematopoietic stem cell (HSC), 1 which, because of its self-renewal capacity, can maintain hematopoiesis throughout the lifespan of an individual. 2,3 The molecular mechanisms regulating HSC fate decisions between self-renewal and lineage commitment remain largely unknown. 2 However, evidence supports that blood-lineage maturation and HSC self-renewal are regulated in part by cytokines interacting with their specific surface receptors. Stem cell factor (SCF, or c-kit ligand) and thrombopoietin (TPO) have been shown to act directly on HSCs, 4-9 and gene-targeting studies have demonstrated that SCF and TPO play important roles in sustaining HSC number and function, in contrast to other cytokines. 10-13 Flt3 (or flk2) is, like c-kit, a cytokine tyrosine kinase receptor expressed primarily at very early stages of hematopoiesis. [14][15][16][17][18] Mice deficient in the expression of flk2 or flt3 ligand (FL) show deficient lymphopoiesis. 19,20 FL has also been implicated as a key and potent stimulator of candidate mouse and human HSCs and is frequently used to promote HSC ex vivo expansion and oncoretroviral-mediated gene transfer. [21][22][23][24][25] However, it has not been established whether this reflects a role of flt3 in HSC generation, maintenance, homing, engraftment, or expansion after transplantation.Surprisingly, recent studies revealed that mouse adult long-term reconstituting HSCs (LTR-HSCs) do not express flt3 during steady-state hematopoiesis and that the up regulation of flt3 expression within the HSC compartment is accompanied by loss of self-renewal capacity. 26,27 In addition, SCF, but not FL, eff...