Upregulation of Flt3 expression within the bone marrow Lin(-)Sca1(+)c-kit(+) stem cell compartment is accompanied by loss of self-renewal capacity

Genome-wide lineage-specific transcriptional networks underscore Ikaros-dependent lymphoid priming in hematopoietic stem cells

The mechanisms regulating lineage potential during early hematopoiesis were investigated. First, a cascade of lineage-affiliated gene expression signatures, primed in hematopoietic stem cells (HSCs) and differentially propagated in lineage-restricted progenitors, was identified. Lymphoid transcripts were primed as early as the HSC, together with myeloid and erythroid transcripts. Although this multilineage priming was resolved upon subsequent lineage restrictions, an unexpected cosegregation of lymphoid and myeloid gene expression and potential past a nominal myeloid restriction point was identified. Finally, we demonstrated that whereas the zinc finger DNA-binding factor Ikaros was required for induction of lymphoid lineage priming in the HSC, it was also necessary for repression of genetic programs compatible with self-renewal and multipotency downstream of the HSC. Taken together, our studies provide new insight into the priming and restriction of lineage potentials during early hematopoiesis and identify Ikaros as a key bivalent regulator of this process.

IFNalpha activates dormant haematopoietic stem cells in vivo

Maintenance of the blood system is dependent on dormant haematopoietic stem cells (HSCs) with long-term self-renewal capacity. After injury these cells are induced to proliferate to quickly re-establish homeostasis. The signalling molecules promoting the exit of HSCs out of the dormant stage remain largely unknown. Here we show that in response to treatment of mice with interferon-alpha (IFNalpha), HSCs efficiently exit G(0) and enter an active cell cycle. HSCs respond to IFNalpha treatment by the increased phosphorylation of STAT1 and PKB/Akt (also known as AKT1), the expression of IFNalpha target genes, and the upregulation of stem cell antigen-1 (Sca-1, also known as LY6A). HSCs lacking the IFNalpha/beta receptor (IFNAR), STAT1 (ref. 3) or Sca-1 (ref. 4) are insensitive to IFNalpha stimulation, demonstrating that STAT1 and Sca-1 mediate IFNalpha-induced HSC proliferation. Although dormant HSCs are resistant to the anti-proliferative chemotherapeutic agent 5-fluoro-uracil, HSCs pre-treated (primed) with IFNalpha and thus induced to proliferate are efficiently eliminated by 5-fluoro-uracil exposure in vivo. Conversely, HSCs chronically activated by IFNalpha are functionally compromised and are rapidly out-competed by non-activatable Ifnar(-/-) cells in competitive repopulation assays. Whereas chronic activation of the IFNalpha pathway in HSCs impairs their function, acute IFNalpha treatment promotes the proliferation of dormant HSCs in vivo. These data may help to clarify the so far unexplained clinical effects of IFNalpha on leukaemic cells, and raise the possibility for new applications of type I interferons to target cancer stem cells.

FLT3 receptor and ligand are dispensable for maintenance and posttransplantation expansion of mouse hematopoietic stem cells

Originally cloned from hematopoietic stem cell (HSC) populations and its ligand being extensively used to promote ex vivo HSC expansion, the FMS-like tyrosine kinase 3 (FLT3; also called FLK2) receptor and its ligand (FL) were expected to emerge as an important physiologic regulator of HSC maintenance and expansion. However, the role of FLT3 receptor and ligand in HSC regulation remains unclear and disputed. Herein, using Fl-deficient mice, we establish for the first time that HSC expansion in fetal liver and after transplantation is FL independent. Because previous findings in Flk2(-/-) mice were compatible with an important role of FLT3 receptor in HSC regulation and because alternative ligands might potentially interact directly or indirectly with FLT3 receptor, we here also characterized HSCs in Flk2(-/-) mice. Advanced phenotypic as well as functional evaluation of Flk2(-/-) HSCs showed that the FLT3 receptor is dispensable for HSC steady-state maintenance and expansion after transplantation. Taken together, these studies show that the FLT3 receptor and ligand are not critical regulators of mouse HSCs, neither in steady state nor during fetal or posttransplantation expansion.

Expansion of peripheral naturally occurring T regulatory cells by Fms-like tyrosine kinase 3 ligand treatment

Fms-like tyrosine kinase 3 ligand (FLT3L) plays a major role in dendritic cell (DC) biology. Deficiency of FLT3L causes a dramatic decrease in DC numbers, whereas increasing its availability (by repetitive injections for 7-10 days) leads to a 10-fold increase in DC numbers. In this study, we show that FLT3L treatment indirectly leads to an expansion of peripheral naturally occurring T regulatory cells (NTregs). The FLT3L-induced increase in NTregs was still observed in thymectomized mice, ruling out the role of the thymus in this mechanism. Instead, the increased number of NTregs was due to proliferation of preexisting NTregs, most likely due to favored interactions with increased number of DCs. In vitro, we show that DCs induce regulatory T-cell (Treg) proliferation by direct cell contact and in an interleukin-2-dependent, T-cell receptor-independent manner. FLT3L could prevent death induced by acute graft-versus-host disease (GVHD). This study demonstrates unique aspects in the regulation of Treg homeostasis by DCs, which were unappreciated until now. It also reinforces the relevance of FLT3L treatment in GVHD by its ability to increase both the number of tolerizing DCs and NTregs.

E2A proteins maintain the hematopoietic stem cell pool and promote the maturation of myelolymphoid and myeloerythroid progenitors

Hematopoiesis is a tightly controlled process maintained by a small pool of hematopoietic stem cells (HSCs). Here, we demonstrate that the LT-HSC, MPP, premegakaryocytic/erythroid, Pre CFU-E, Pre GM, MkP, and granulocyte-macrophage compartments were all significantly reduced in E2A-deficient bone marrow. Despite a severe depletion of erythroid progenitors, the erythrocyte and megakaryocyte compartments were equivalent in E2A-deficient bone marrow as compared with wild-type mice. E2A-deficient HSCs also failed to efficiently maintain the HSC pool on serial transplantation, and we demonstrate that the E2A proteins regulate cell cycle progression of HSCs by regulating the expression of p21(Cip1), p27(Kip1), and the thrombopoietin receptor, known regulators of HSC self-renewal activity. Based on these observations, we propose that the E2A proteins promote the developmental progression of the entire spectrum of early hematopoietic progenitors and to suppress an erythroid specific program of gene expression in alternative cell lineages. Last, the data mechanistically link E2A, cell cycle regulators, and the maintenance of the HSC pool in a common pathway.

Concise Review: Dendritic Cell Development in the Context of the Spleen Microenvironment

The dendritic cell (DC) population in spleen comprises a mixture of cells including endogenous DC progenitors, DC precursors migrating in from blood and bone marrow, and DC in different states of differentiation and activation. A role for different microenvironments in supporting the dynamic development of murine DC of different types or lineages is considered here. Recent evidence for production of DC dependent on splenic stromal cells is reviewed in the light of evidence that cell production is dependent on cells comprising an endothelial niche in spleen. The possibility that self-renewing progenitors in spleen give rise to DC with tolerogenic or regulatory rather than immunostimulatory function is considered.

Identification and characterization of hematopoietic stem and progenitor cell populations in mouse bone marrow by flow cytometry

The study of key mechanisms and molecules involved in the regulation of hematopoiesis in mouse models has been greatly facilitated by multi-parameter flow cytometry. Subpopulations of hematopoietic stem and progenitor cells can be identified and characterized using this technique. Furthermore, fluorescence-activated cell sorting (FACS) can prospectively isolate functionally-defined subpopulations of hematopoietic cells for use in further in vitro or in vivo analysis. The chapter describes methodology for preparing samples from mouse bone marrow, staining cells with fluorochrome conjugated antibodies, enrichment of HSC and progenitors, and finally data analysis.

Mouse hematopoietic stem cell identification and analysis

Hematopoietic stem cells (HSCs) remain by far the most well-characterized adult stem cell population both in terms of markers for purification and assays to assess functional potential. However, despite over 40 years of research, working with HSCs in the mouse remains difficult because of the relative abundance (or lack thereof) of these cells in the bone marrow. The frequency of HSCs in bone marrow is about 0.01% of total nucleated cells and approximately 5,000 can be isolated from an individual mouse depending on the age, sex, and strain of mice as well as purification scheme utilized. This prohibits the study of processes in HSCs, which require large amounts of starting material. Adding to the challenge is the continual reporting of new markers for HSC purification, which makes it difficult for the uninitiated in the field to know which purification strategies yield the highest proportion of long-term, multilineage HSCs. This report will review different hematopoietic stem and progenitor purification strategies and compare flow cytometry profiles for HSC sorting and analysis on different instruments. We will also discuss methods for rapid flow cytometric analysis of peripheral blood cell types, and novel strategies for working with rare cell populations such as HSCs in the analysis of cell cycle status by BrdU, Ki-67, and Pyronin Y staining. The purpose of this review is to provide insight into some of the recent experimental and technical advances in mouse hematopoietic stem cell biology.

Stem cells and aging in the hematopoietic system

The effector cells of the blood have limited lifetimes and must be replenished continuously throughout life from a small reserve of hematopoietic stem cells (HSCs) in the bone marrow. Although serial bone marrow transplantation experiments in mice suggest that the replicative potential of HSCs is finite, there is little evidence that replicative senescence causes depletion of the stem cell pool during the normal lifespan of either mouse or man. Studies conducted in murine genetic models defective in DNA repair, intracellular ROS management, and telomere maintenance indicate that all these pathways are critical to the longevity and stress response of the stem cell pool. With age, HSCs show an increased propensity to differentiate towards myeloid rather than lymphoid lineages, which may contribute to the decline in lymphopoiesis that attends aging. Challenges for the future include assessing the significance of 'lineage skewing' to immune dysfunction, and investigating the role of epigenetic dysregulation in HSC aging.

Hematopoietic stem cells reversibly switch from dormancy to self-renewal during homeostasis and repair

Bone marrow hematopoietic stem cells (HSCs) are crucial to maintain lifelong production of all blood cells. Although HSCs divide infrequently, it is thought that the entire HSC pool turns over every few weeks, suggesting that HSCs regularly enter and exit cell cycle. Here, we combine flow cytometry with label-retaining assays (BrdU and histone H2B-GFP) to identify a population of dormant mouse HSCs (d-HSCs) within the lin(-)Sca1+cKit+CD150+CD48(-)CD34(-) population. Computational modeling suggests that d-HSCs divide about every 145 days, or five times per lifetime. d-HSCs harbor the vast majority of multilineage long-term self-renewal activity. While they form a silent reservoir of the most potent HSCs during homeostasis, they are efficiently activated to self-renew in response to bone marrow injury or G-CSF stimulation. After re-establishment of homeostasis, activated HSCs return to dormancy, suggesting that HSCs are not stochastically entering the cell cycle but reversibly switch from dormancy to self-renewal under conditions of hematopoietic stress.

Resolution of unique Sca-1highc-Kit- lymphoid-biased progenitors in adult bone marrow

We have identified a distinctive lymphoid-restricted progenitor population in adult mouse bone marrow based on a unique c-Kit(-)Sca-1(high)Flt3(+) AA4(+) surface phenotype. These cells are highly lymphoid biased and rapidly generate B and T cells after adoptive transfer. However, whereas previously described lymphoid progenitors such as common lymphoid progenitors express TdT and relatively high levels of RAG2, and are enriched for cells with an active V(D)J recombinase, Flt3(+) AA4(+) cells within the c-Kit(-)Sca-1(high) bone marrow fraction are TdT(-), are RAG2(low), and do not display evidence for ongoing or past recombinase activity. Furthermore, unlike common lymphoid progenitors that readily generate B cells upon stimulation with IL-7, c-Kit(-)Sca-1(high)Flt3(+) precursors do not express abundant levels of the IL-7R, and require costimulation with Flt3 ligand and IL-7 to generate B cells in vitro. Moreover, these findings suggest that hematopoietic stem cells in adults generate an array of lymphoid-biased progenitor populations characterized by distinct gene expression and cytokine response profiles.

Genetic deletion of JAM-C reveals a role in myeloid progenitor generation

Hematopoietic stem cells (HSCs) have the capacity to self-renew and continuously differentiate into all blood cell lineages throughout life. At each branching point during differentiation, interactions with the environment are key in the generation of daughter cells with distinct fates. Here, we examined the role of the cell adhesion molecule JAM-C, a protein known to mediate cellular polarity during spermatogenesis, in hematopoiesis. We show that murine JAM-C is highly expressed on HSCs in the bone marrow (BM). Expression correlates with self-renewal, the highest being on long-term repopulating HSCs, and decreases with differentiation, which is maintained longest among myeloid committed progenitors. Inclusion of JAM-C as a sole marker on lineage-negative BM cells yields HSC enrichments and long-term multilineage reconstitution when transferred to lethally irradiated mice. Analysis of Jam-C-deficient mice showed that two-thirds die within 48 hours after birth. In the surviving animals, loss of Jam-C leads to an increase in myeloid progenitors and granulocytes in the BM. Stem cells and myeloid cells from fetal liver are normal in number and homing to the BM. These results provide evidence that JAM-C defines HSCs in the BM and that JAM-C plays a role in controlling myeloid progenitor generation in the BM.

Lin-Sca1+kit- bone marrow cells contain early lymphoid-committed precursors that are distinct from common lymphoid progenitors

The significance of a population in mouse bone marrow of lineage-negative (Lin(-)), Sca1-positive, c-kit-negative (LSK(-)) cells, which is reported to be devoid of long-term repopulation capacity or myeloid potential, is unknown. In this study, we show that the LSK(-) population is composed of several subsets defined by the expression of flt3, CD25, and IL-7Ralpha. The first subset was CD25(-) and more than 90% expressed either flt3, IL-7Ralpha, or both. The CD25(-)LSK(-) population had T cell, B cell, and NK cell potential in vivo, and most of this activity was localized in the flt3(+) subset, irrespective of the expression of IL-7Ralpha. Although lymphoid potential of flt3(+)LSK(-) cells in vivo was 3-fold lower than that of lin(-)Sca1(low)kit(low)IL7Ralpha(+) common lymphoid progenitors (CLPs), their cloning efficiency in vitro was 10-fold lower than that of CLPs. Furthermore, although the myeloid potential of flt3(+)LSK(-) cells was 10-fold lower than that of CLPs in the absence of M-CSF, the relative myeloid potential of both populations was similar in its presence. These observations suggest differential growth factor requirements of both populations. The second subset of LSK(-) cells was homogeneously CD25(+)flt3(-)IL7Ralpha(+) and could be generated from both CD25(-)LSK(-) cells and from CLPs, but did not engraft in immunodeficient Rag1(-/-) or Rag1(-/-)gamma(c)(-/-) hosts. This population, of which the significance is unclear, was increased in Rag1(-/-) mice and in old mice. Thus, the LSK(-) population is phenotypically and functionally heterogeneous and contains early lymphoid-committed precursors. Our findings imply that the early stages of lymphoid commitment are more complex than was thus far assumed.

Two distinct types of murine blast colony-forming cells are multipotential hematopoietic precursors

Two types of blast colonies can be stimulated to develop in semisolid agar cultures of murine bone marrow cells. Typically, these are either multicentric colonies stimulated by stem cell factor (SCF) plus interleukin-6 (IL-6) or dispersed colonies stimulated by Flt3 ligand (FL) plus IL-6. Both types of blast colony-forming cells (BL-CFCs) can generate large numbers of lineage-committed granulocyte-macrophage progenitor cells and exhibit some capacity for self-generation and the formation of eosinophil and megakaryocyte progenitor cells. However, the two populations of BL-CFCs are largely distinct and partially separable by fluorescence-activated cell sorting and are distinguished by differing capacity to form granulocyte-committed progeny. Both types of BL-CFCs can generate dendritic cells and small numbers of lymphocytes but the FL-responsive BL-CFCs have a greater capacity to form both B and T lymphocytes. Both types of blast colonies offer remarkable opportunities to analyze multilineage commitment at a clonal level in vitro.

Flow-cytometric phosphoprotein analysis reveals agonist and temporal differences in responses of murine hematopoietic stem/progenitor cells

Hematopoietic stem cells (HSCs) are probably the best-studied adult tissue-restricted stem cells. Although methods for flow cytometric detection of phosphoproteins in hematopoeitic progenitors and mature cells are available, analogous protocols for HSC are lacking. We present a robust method to study intracellular signaling in immunophenotypically-defined murine HSC/progenitor cell (HPC)-enriched populations. Using this method, we uncover differences in the response dynamics of several phosphoproteins representative of the Ras/MAP-Kinase(K), PI3K, mTOR and Jak/STAT pathways in HSC/HPCs stimulated by Scf, Thpo, as well as several other important HSC/HPC agonists.

E47 controls the developmental integrity and cell cycle quiescence of multipotential hematopoietic progenitors

Little is known about the transcriptional regulators that control the proliferation of multipotent bone marrow progenitors. Understanding the mechanisms that restrict proliferation is of significant interest since the loss of cell cycle integrity can be associated with hematopoietic exhaustion, bone marrow failure, or even oncogenic transformation. Herein, we show that multipotent LSKs (lineage(-)Sca(high)c-kit(+)) from E47-deficient mice exhibit a striking hyperproliferation associated with a loss of cell cycle quiescence and increased susceptibility to in vivo challenge with a mitotoxic drug. Total LSKs contain long-term self-renewing hematopoietic stem cells and downstream multipotential progenitors (MPPs) that possess very limited or no self-renewal ability. Within total LSKs, we found specific developmental and functional deficits in the MPP subset. E47 knockout mice have grossly normal numbers of self-renewing hematopoietic stem cells but a 50-70% reduction in nonrenewing MPPs and downstream lineage-restricted populations. The residual MPPs in E47 knockout mice fail to fully up-regulate flk2 or initiate V(D)J recombination, hallmarks of normal lymphoid lineage progression. Consistent with the loss of normal cell cycle restraints, we show that E47-deficient LSKs have a 50% decrease in p21, a cell cycle inhibitor and known regulator of LSK proliferation. Moreover, enforced expression studies identify p21 as an E47 target gene in primary bone marrow LSKs. Thus, E47 appears to regulate the developmental and functional integrity of early hematopoietic subsets in part through effects on p21-mediated cell cycle quiescence.

High level in vitro expansion of murine hematopoietic stem cells

Development of strategies to extensively expand hematopoietic stem cells (HSCs) in vitro will be a major factor in enhancing the success of a range of transplant-based therapies for malignant and genetic disorders. In addition to potential clinical applications, the ability to increase the number of HSCs in culture will facilitate investigations into the mechanisms underlying self-renewal. In this unit, we describe a robust strategy for consistently achieving over 1000-fold net expansion of HSCs in short-term in vitro culture by using novel engineered fusions of the N-terminal domain of nucleoporin 98 (NUP98) and the homeodomain of the hox transcription factor, HOXA10 (so called NUP98-HOXA10hd fusion). We also provide a detailed protocol for monitoring the magnitude of HSC expansion in culture by limiting dilution assay of competitive lympho-myeloid repopulating units (CRU Assay). These procedures provide new possibilities for achieving significant numbers of HSCs in culture, as well as for studying HSCs biochemically and genetically.

Impaired function of primitive hematopoietic cells in mice lacking the Mixed-Lineage-Leukemia homolog MLL5

The human Mixed-Lineage-Leukemia-5 (MLL5) gene is located in a genomic region frequently deleted in patients with myeloid malignancies and encodes a widely expressed nuclear protein most closely related to MLL1, a Trithorax transcriptional regulator with established involvement in leukemogenesis. Although the physiologic function of MLL5 is completely unknown, domain structure and homology to transcriptional regulators with histone methyltransferase activity suggest a role in epigenetic gene regulation. To investigate physiologic functions of Mll5, we have generated a knockout mouse mutant using Cre/loxP technology. Adult homozygous Mll5-deficient mice are obtained at reduced frequency because of postnatal lethality. Surviving animals display a variety of abnormalities, including male infertility, retarded growth, and defects in multiple hematopoietic lineages. Interestingly, Mll5(-/-) mice die of sublethal whole-body irradiation but can be rescued with wild-type bone marrow grafts. Flow cytometric ana-lysis, bone marrow reconstitution, and in vivo BrdU-labeling experiments reveal numerical, functional, and cell-cycle defects in the lineage-negative Sca-1(+), Kit(+) (LSK) population, which contains short- and long-term hematopoietic stem cells. Together, these in vivo findings establish several nonredundant functions for Mll5, including an essential role in regulating proliferation and functional integrity of hematopoietic stem/progenitor cells.

The long road to the thymus: the generation, mobilization, and circulation of T-cell progenitors in mouse and man

The majority of T cells develop in the thymus. T-cell progenitors in the thymus do not self-renew and so progenitor cells must be continuously imported from the blood into the thymus to maintain T-cell production. Recent work has shed light on both the identity of the cells that home to the thymus and the molecular mechanisms involved. This review will discuss the cells in the bone marrow and blood that are involved in early thymopoiesis in mouse and man. Understanding the pre-thymic steps in T-cell development may translate into new therapeutics, especially in the field of hematopoietic stem cell transplantation.

Isolation and assessment of long-term reconstituting hematopoietic stem cells from adult mouse bone marrow

Suspensions of multipotent hematopoietic stem cells with long-term repopulating activity can now be routinely isolated from adult mouse bone marrow at purities of 30%. A robust method for obtaining these cells in a single step using multiparameter cell sorting to isolate the CD45(mid)lin(-)Rho(-)SP subset is described here, together with a detailed protocol for assessing their regenerative activity in mice transplanted with single cells. These procedures provide unprecedented power and precision for characterizing the molecular and biological properties of cells with hematopoietic stem cell activity at the single cell level.

Wnt3a deficiency irreversibly impairs hematopoietic stem cell self-renewal and leads to defects in progenitor cell differentiation

Canonical Wnt signaling has been implicated in various aspects of hematopoiesis. Its role is controversial due to different outcomes between various inducible Wnt-signaling loss-of-function models and also compared with gain-of-function systems. We therefore studied a mouse deficient for a Wnt gene that seemed to play a nonredundant role in hematopoiesis. Mice lacking Wnt3a die prenatally around embryonic day (E) 12.5, allowing fetal hematopoiesis to be studied using in vitro assays and transplantation into irradiated recipient mice. Here we show that Wnt3a deficiency leads to a reduction in the numbers of hematopoietic stem cells (HSCs) and progenitor cells in the fetal liver (FL) and to severely reduced reconstitution capacity as measured in secondary transplantation assays. This deficiency is irreversible and cannot be restored by transplantation into Wnt3a competent mice. The impaired long-term repopulation capacity of Wnt3a(-/-) HSCs could not be explained by altered cell cycle or survival of primitive progenitors. Moreover, Wnt3a deficiency affected myeloid but not B-lymphoid development at the progenitor level, and affected immature thymocyte differentiation. Our results show that Wnt3a signaling not only provides proliferative stimuli, such as for immature thymocytes, but also regulates cell fate decisions of HSC during hematopoiesis.

Regulation of FLT3 and its ligand in normal hematopoietic progenitor cells

FLT3 and its ligand (FL) are one of the regulators of normal hematopoiesis. Ligand-independent activation of FLT3 occurs in about 30% of acute myeloid leukemia cases and is one goal for selectively targeted therapies. However, the function of FLT3/FL in the regulation of non-malignant immature hematopoietic cells is poorly characterized. In order to elucidate the role of FLT3 in normal hematopoiesis, human adult CD34(+) hematopoietic progenitor cells were cultured in cytokine-supplemented liquid culture in the presence or absence of FLT3 inhibition by CEP-701 (lestaurtinib). Total cell number, lineage-committed, and primitive progenitors and apoptosis were assayed. FLT3 expression and FL secretion in various conditions were analyzed by fluorescent activated cell sorter and enzyme-linked immunosorbent assay. Effects of nonspecific targeting of FLT3 were evaluated with addition of imatinib (Gleevec) to cell cultures. It is demonstrated that FLT3 inhibition impaired cell and progenitor cell growth and increased the rate in apoptosis. Effects were observed independent of addition of FL. The dose-dependent growth inhibition was partially equalized by inhibiting FL with a neutralizing antibody. FLT3 inhibition resulted in markedly increased production of FL by cultured CD34(+) cells as well as upregulation of FLT3 expression. Imatinib mimicked effects of selective FLT3 inhibition. In conclusion, FLT3 and its ligand regulate proliferation of hematopoietic progenitor cells in an autocrine/paracrine manner Nonspecific inhibition of FLT3 may contribute to hematotoxicity caused by imatinib treatment.

Gene regulatory networks directing myeloid and lymphoid cell fates within the immune system

Considerable progress is being achieved in the analysis of gene regulatory networks that direct cell fate decisions within the hematopoietic system. In addition to transcription factors that are pivotal for cell fate specification and commitment, recent evidence suggests the involvement of microRNAs. In this review we attempt to integrate these two types of regulatory components into circuits that dictate cell fate choices leading to the generation of innate as well as adaptive immune cells. The developmental circuits are placed in the context of a revised scheme for hematopoiesis that suggests that both the innate (myeloid) and adaptive (lymphoid) lineages of the immune system arise from a common progenitor.

Lineage development of hematopoietic stem and progenitor cells

Hematopoietic stem cells have the potential to develop into multipotent and different lineage-restricted progenitor cells that subsequently generate all mature blood cell types. The classical model of hematopoietic lineage commitment proposes a first restriction point at which all multipotent hematopoietic progenitor cells become committed either to the lymphoid or to the myeloid development, respectively. Recently, this model has been challenged by the identification of murine as well as human hematopoietic progenitor cells with lymphoid differentiation capabilities that give rise to a restricted subset of the myeloid lineages. As the classical model does not include cells with such capacities, these findings suggest the existence of alternative developmental pathways that demand the existence of additional branches in the classical hematopoietic tree. Together with some phenotypic criteria that characterize different subsets of multipotent and lineage-restricted progenitor cells, we summarize these recent findings here.

Delineating the cellular pathways of hematopoietic lineage commitment

The prevailing model for adult hematopoiesis postulates that the first lineage commitment step results in a strict separation of common myeloid and common lymphoid pathways. However, the recent identification of granulocyte/monocyte (GM)-lymphoid restricted lymphoid-primed multipotent progenitors (LMPPs) and primitive common myeloid progenitors (CMPs) within the "HSC" compartment provide compelling support for establishment of independent GM-megakaryocyte/erythroid (GM-MkE) and GM-lymphoid commitment pathways as decisive early lineage fate decisions. These changes in lineage potentials are corroborated by corresponding changes in multilineage transcriptional priming, as LMPPs down-regulate MkE priming but become GM-lymphoid transcriptionally primed, whereas CMPs are GM-MkE primed. These distinct biological and molecular relationships are established already in the fetal liver.

E2A proteins promote development of lymphoid-primed multipotent progenitors

The first lymphoid-restricted progeny of hematopoietic stem cells (HSCs) are lymphoid-primed multipotent progenitors (LMPPs), which have little erythromyeloid potential but retain lymphoid, granulocyte, and macrophage differentiation capacity. Despite recent advances in the identification of LMPPs, the transcription factors essential for their generation remain to be identified. Here, we demonstrated that the E2A transcription factors were required for proper development of LMPPs. Within HSCs and LMPPs, E2A proteins primed expression of a subset of lymphoid-associated genes and prevented expression of genes that are not normally prevalent in these cells, including HSC-associated and nonlymphoid genes. E2A proteins also restricted proliferation of HSCs, MPPs, and LMPPs and antagonized differentiation of LMPPs toward the myeloid fate. Our results reveal that E2A proteins play a critical role in supporting lymphoid specification from HSCs and that the reduced generation of LMPPs underlies the severe lymphocyte deficiencies observed in E2A-deficient mice.

Acquired hematopoietic stem cell defects determine B-cell repertoire changes associated with aging

Aging is associated with an inability to mount protective antibody responses to vaccines and infectious agents. This decline is associated with acquisition of defects in multiple cellular compartments, including B cells. While peripheral B-cell numbers do not decline with aging, the composition of the compartment appears to change, with loss of naïve follicular B cells, accumulation of antigen-experienced cells, and alteration of the antibody repertoire. The underlying cause of this change is unknown. We tested the hypothesis that aging-associated repertoire changes can be attributed directly to decreased B lymphopoiesis. Using an Ig transgenic model to report changes in the B-cell repertoire, we show that the reduced B-cell generative capacity of "aged" long-term reconstituting hematopoietic stem cells (LT-HSCs) alters the representation of antigen specificities in the peripheral B-cell repertoire. Further, we show that reconstitution using suboptimal numbers of fully functional LT-HSCs results in the generation of a similarly altered B-cell repertoire. This may be an important factor to consider when deciding the number of bone marrow cells to transplant in the clinical setting. In conclusion, when B lymphopoiesis is limited peripheral B-cell homeostasis is altered. This is reflected in reduced diversity of the B-cell repertoire, which likely reduces the protective quality of the immune response.

Human Flt3 is expressed at the hematopoietic stem cell and the granulocyte/macrophage progenitor stages to maintain cell survival

FLT3/FLK2, a member of the receptor tyrosine kinase family, plays a critical role in maintenance of hematopoietic homeostasis, and the constitutively active form of the FLT3 mutation is one of the most common genetic abnormalities in acute myelogenous leukemia. In murine hematopoiesis, Flt3 is not expressed in self-renewing hematopoietic stem cells, but its expression is restricted to the multipotent and the lymphoid progenitor stages at which cells are incapable of self-renewal. We extensively analyzed the expression of Flt3 in human (h) hematopoiesis. Strikingly, in both the bone marrow and the cord blood, the human hematopoietic stem cell population capable of long-term reconstitution in xenogeneic hosts uniformly expressed Flt3. Furthermore, human Flt3 is expressed not only in early lymphoid progenitors, but also in progenitors continuously along the granulocyte/macrophage pathway, including the common myeloid progenitor and the granulocyte/macrophage progenitor. We further found that human Flt3 signaling prevents stem and progenitors from spontaneous apoptotic cell death at least through up-regulating Mcl-1, an indispensable survival factor for hematopoiesis. Thus, the distribution of Flt3 expression is considerably different in human and mouse hematopoiesis, and human FLT3 signaling might play an important role in cell survival, especially at stem and progenitor cells that are critical cellular targets for acute myelogenous leukemia transformation.

Extrinsic and intrinsic regulation of early natural killer cell development

Natural killer (NK) cells are lymphocytes that play a critical role in both adaptive and innate immune responses. These cells develop from multipotent progenitors in the embryonic thymus and neonatal or adult bone marrow and recent evidence suggests that a subset of these cells may develop in the thymus. Thymus- and bone marrow-derived NK cells have unique phenotypes and functional abilities supporting the hypothesis that the microenvironment dictates the outcome of NK cell development. A detailed understanding of the mechanisms controlling this developmental program will be required to determine how alterations in NK cell development lead to disease and to determine how to harness this developmental program for therapeutic purposes. In this review, we discuss some of the known extrinsic stromal-cell derived factors and cell intrinsic transcription factors that function in guiding NK cell development.

Long-term propagation of distinct hematopoietic differentiation programs in vivo

Heterogeneity in the differentiation behavior of hematopoietic stem cells is well documented but poorly understood. To investigate this question at a clonal level, we isolated a subpopulation of adult mouse bone marrow that is highly enriched for multilineage in vivo repopulating cells and transplanted these as single cells, or their short-term clonal progeny generated in vitro, into 352 recipients. Of the mice, 93 showed a donor-derived contribution to the circulating white blood cells for at least 4 months in one of four distinct patterns. Serial transplantation experiments indicated that two of the patterns were associated with extensive self-renewal of the original cell transplanted. However, within 4 days in vitro, the repopulation patterns subsequently obtained in vivo shifted in a clone-specific fashion to those with less myeloid contribution. Thus, primitive hematopoietic cells can maintain distinct repopulation properties upon serial transplantation in vivo, although these properties can also alter rapidly in vitro.

Stem cells remember their grade

The stem cell state is understood based on what cells do in performance assays, crude measures of a highly refined state. In this issue of Cell Stem Cell, Dykstra et al. (2007) reveal stem cell gradation and the extent to which that gradation is retained in stem cell daughters of hematopoietic stem cells.

The transcription factor EGR1 controls both the proliferation and localization of hematopoietic stem cells

EGR1 is a member of the immediate early response transcription factor family and functions in cell growth, development, and stress responses in many tissues. Here we report an additional role for EGR1 in regulating homeostasis of hematopoietic stem cells (HSCs). HSCs normally express Egr1 at high levels, but dramatically downregulate its expression when induced to divide and migrate. Consistent with this finding, mice lacking Egr1 exhibit significant increases in steady-state levels of dividing HSCs in the bone marrow (BM), and a striking spontaneous mobilization of HSCs into the peripheral blood. These data identify EGR1 as a transcriptional regulator of stem cell migration that normally functions to promote HSC quiescence and retention in the niche. The ability of this single factor to regulate both proliferation and mobilization of HSCs suggests that EGR1 commands a genetic program that coordinates stem cell division and migration to maintain appropriate HSC number and function.

Fms-related tyrosine kinase 3 expression discriminates hematopoietic stem cells subpopulations with differing engraftment-potential: identifying the most potent combination

BACKGROUND

Fms-related tyrosine kinase 3 (Flt3)-ligand (FL) promotes the proliferation, differentiation, development, and mobilization of hematopoietic cells. We previously found that FL-mobilized hematopoietic stem cells (HSC) engraft efficiently, whereas FL-expanded bone marrow HSC do not. The function of FL-mobilized c-Kit(+) Sca-1(+)Lin(-)(KSL) subpopulations has not been systematically evaluated. A precise definition of the repopulating ability is needed to define which HSC subpopulations are critical for long-term chimerism and tolerance induction. FL significantly mobilized c-Kit(hi) and c-Kit(lo) Sca-1(+)Lin(-) cells into peripheral blood (PB). Here, we evaluated the influence of Flt3 expression on long-term repopulating ability of HSC subpopulations.

METHODS

c-Kit(hi) or c-Kit(lo) KSL cells were sorted from PB of FL-treated green fluorescent protein-positive donors. The function of these cells was evaluated using competitive reconstitution assays, colony-forming units spleen, and colony forming cell assays. The function of c-Kit(hi) CD34(-)Flt3(-) KSL, c-Kit CD34(+)Flt3(-) KSL, c-Kit(hi) CD34(+)Flt3(+) KSL were investigated in an in vivo transplantation model.

RESULTS

Only FL-mobilized PB c-Kit(hi) KSL cells exhibited high spleen colony-forming unit activity, generated high numbers of both lymphoid and myeloid colonies in vitro, and rescued ablated recipients. FL-mobilization expanded both c-Kit(hi) CD34(+)Flt3(-) cells (short-term HSC) and c-Kit(hi) CD34(-)Flt3(-) KSL cells (long-term HSC). There was a significant decrease in c-Kit CD34Flt3 KSL late multipotent progenitors in PB. A combination of c-Kit(hi) CD34Flt3 and c-Kit CD34(+)Flt3(-) KSL cells offered the most effective rescue of ablated recipients.

CONCLUSIONS

These data suggest that engraftment of purified HSC is influenced by both short- and long-term repopulating populations and that Flt3 expression may be useful for selecting the most critical HSC subpopulations for transplantation.

Elucidation of the phenotypic, functional, and molecular topography of a myeloerythroid progenitor cell hierarchy

The major myeloid blood cell lineages are generated from hematopoietic stem cells by differentiation through a series of increasingly committed progenitor cells. Precise characterization of intermediate progenitors is important for understanding fundamental differentiation processes and a variety of disease states, including leukemia. Here, we evaluated the functional in vitro and in vivo potentials of a range of prospectively isolated myeloid precursors with differential expression of CD150, Endoglin, and CD41. Our studies revealed a hierarchy of myeloerythroid progenitors with distinct lineage potentials. The global gene expression signatures of these subsets were consistent with their functional capacities, and hierarchical clustering analysis suggested likely lineage relationships. These studies provide valuable tools for understanding myeloid lineage commitment, including isolation of an early erythroid-restricted precursor, and add to existing models of hematopoietic differentiation by suggesting that progenitors of the innate and adaptive immune system can separate late, following the divergence of megakaryocytic/erythroid potential.

Reciprocal activation of GATA-1 and PU.1 marks initial specification of hematopoietic stem cells into myeloerythroid and myelolymphoid lineages

A hierarchical hematopoietic development with myeloid versus lymphoid bifurcation has been proposed downstream of the multipotent progenitor (MPP) stage, based on prospective isolation of progenitors capable of generating only myeloerythroid cells (common myeloid progenitor, CMP) or only lymphocytes (common lymphoid progenitor, CLP). By utilizing GATA-1 and PU.1 transcription factor reporters, here we identified progenitor populations that are precursors for either CMPs or CLPs. Two independent populations expressing either GATA-1 or PU.1 resided within the CD34(+)Sca-1(+)c-Kit(+) MPP fraction. The GATA-1(+) MPP displayed potent myeloerythroid potential without giving rise to lymphocytes, whereas the PU.1(+) MPP showed granulocyte/monocyte/lymphoid-restricted progenitor activity without megakaryocyte/erythroid differentiation. Furthermore, GATA-1(+) and PU.1(+) MPPs possessed huge expansion potential and differentiated into the original CMPs and CLPs, respectively. Thus, the reciprocal activation of GATA-1 and PU.1 primarily organizes the hematopoietic lineage fate decision to form the earliest hematopoietic branchpoint that comprises isolatable myeloerythroid and myelolymphoid progenitor populations.

Flk2+ common lymphoid progenitors possess equivalent differentiation potential for the B and T lineages

Mature blood cells develop from multipotent hematopoietic stem cells through a series of sequential intermediates in which the developmental potential for particular blood lineages is progressively extinguished. We previously reported the identification of one of these developmental intermediates, the common lymphoid progenitor (CLP), which can give rise to T cells, B cells, dendritic cells (DCs), and natural killer cells (NKs), but lacks myeloid and erythroid potential. Recently, several studies have suggested that the T-cell and DC potential of CLP is limited or absent, and/or that CLP contains significant myeloid potential. Here, we show that the originally identified CLP population can be divided into functionally distinct subsets based on the expression of the tyrosine kinase receptor, Flk2. The Flk2(+) subset contains robust in vivo and in vitro T-cell, B-cell, DC, and NK potential, but lacks myeloid potential and, therefore, represents an oligopotent, lymphoid-restricted progenitor. This population of cells does not appear to be B cell-biased and robustly reconstitutes both B and T lineages in vivo, consistent with its being a physiologic progenitor of both of these subsets. Thus, Flk2 expression defines a homogeneous, readily obtainable subset of bone marrow CLP that is completely lymphoid-committed and can differentiate equivalently well into both B and T lineages.

Fine-tuning of hematopoietic stem cell homeostasis: novel role for ubiquitin ligase

Homeostasis of hematopoietic stem cells (HSCs) is a tightly regulated process, controlled by intrinsic and extrinsic signals. Although a variety of molecules involved in HSC maintenance and self-renewal are known, it remains unclear how robust HSC homeostasis is achieved. In this issue of Genes & Development, Rathinam and colleagues (992-997) report a new player in HSC homeostasis, c-Cbl ubiquitin ligase. They show that this E3 ubiquitin ligase acts as a negative regulator of cytokine signaling.

Canonical notch signaling is dispensable for the maintenance of adult hematopoietic stem cells

Gain-of-function experiments have demonstrated the potential of Notch signals to expand primitive hematopoietic progenitors, but whether Notch physiologically regulates hematopoietic stem cell (HSC) homeostasis in vivo is unclear. To answer this question, we evaluated the effect of global deficiencies of canonical Notch signaling in rigorous HSC assays. Hematopoietic progenitors expressing dominant-negative Mastermind-like1 (DNMAML), a potent inhibitor of Notch-mediated transcriptional activation, achieved stable long-term reconstitution of irradiated hosts and showed a normal frequency of progenitor fractions enriched for long-term HSCs. Similar results were observed with cells lacking CSL/RBPJ, a DNA-binding factor that is required for canonical Notch signaling. Notch-deprived progenitors provided normal long-term reconstitution after secondary competitive transplantation. Furthermore, Notch target genes were expressed at low levels in primitive hematopoietic progenitors. Taken together, these results rule out an essential physiological role for cell-autonomous canonical Notch signals in HSC maintenance.

Murine hematopoietic stem cells and multipotent progenitors express truncated intracellular form of c-kit receptor

The c-kit receptor plays a vital role in self-renewal and differentiation of hematopoietic stem cells (HSCs) and multipotent progenitors (MPPs). We have discovered that besides c-kit, the murine multipotent HSC/MPP-like cell line EML expresses the transcript and protein for a truncated intracellular form of c-kit receptor, called tr-kit. Notably, the tr-kit transcript and protein levels were down-regulated during cytokine-induced differentiation of the HSC/MPP-like cell line EML into myeloerythroid lineages. These findings prompted us to analyze tr-kit expression in purified murine fetal liver and bone marrow cell populations containing long-term repopulating (LTR) HSCs, short-term repopulating (STR) HSCs, MPPs, lineage-committed progenitors, and immature blood cells. Remarkably, these studies have revealed that in contrast to more widespread expression of c-kit, tr-kit is transcribed solely in cell populations enriched for LTR-HSCs, STR-HSCs, and MPPs. On the other hand, cell populations in which HSCs and MPPs are either present at a much lower frequency or are absent altogether, cells representing more advanced stages of differentiation into lymphoid and myeloid lineages do not express tr-kit. The observation that tr-kit is co-expressed with c-kit only in more primitive HSC- and MPP-enriched cell populations raises an exciting possibility that tr-kit functions either as a new component of the stem cell factor (SCF)/c-kit pathway or is involved in a novel signaling pathway, present exclusively in HSC and MPPs. Taken together, these findings necessitate functional characterization of tr-kit and analysis of its potential role in the self-renewal, proliferation, and/or differentiation of HSC and multipotent progenitors.

Vaccinia virus infection modulates the hematopoietic cell compartments in the bone marrow

Successful proliferation and differentiation of hematopoietic progenitor cells in bone marrow (BM) is essential to generate all mature blood cell types, including those involved in the immune response. Although vaccinia virus (VV) is known to induce a strong immune response, the effect of VV infection on hematopoiesis remains largely unknown. Here, we showed that in vivo VV infection results in the expansion of c-Kit(hi)Sca-1+Lin- (KSL) hematopoietic stem cells. The in vivo expansion of the KSL population requires MyD88 that is a critical adaptor for Toll-like receptor-mediated signaling. Moreover, in BM of VV-infected mice, common myeloid progenitors (CMP) was decreased because of the rapid differentiation of CMP to more mature cells. However, the CMP compartment was not affected by VV infection in the absence of MyD88. The common lymphoid progenitor (CLP) cell population was increased regardless of MyD88 status, suggesting the independent regulation of CMP and CLP compartments by VV infection. VV infection also enhanced the potential of progenitors that preferentially induce the programming of dendritic cell (DC) development toward plasmacytoid DC. Therefore, the host immune response is gearing toward antiviral responses as early as at the precursor level upon VV infection.

Down-regulation of Mpl marks the transition to lymphoid-primed multipotent progenitors with gradual loss of granulocyte-monocyte potential

Evidence for a novel route of adult hematopoietic stem-cell lineage commitment through Lin-Sca-1+Kit+Flt3hi (LSKFlt3hi) lymphoid-primed multipotent progenitors (LMPPs) with granulocyte/monocyte (GM) and lymphoid but little or no megakaryocyte/erythroid (MkE) potential was recently challenged, as LSKFlt3hi cells were reported to possess MkE potential. Herein, residual (1%-2%) MkE potential segregated almost entirely with LSKFlt3hi cells expressing the thrombopoietin receptor (Mpl), whereas LSKFlt3hiMpl- LMPPs lacked significant MkE potential in vitro and in vivo, but sustained combined GM and lymphoid potentials, and coexpressed GM and lymphoid but not MkE transcriptional lineage programs. Gradually increased transcriptional lymphoid priming in single LMPPs from Rag1GFP mice was shown to occur in the presence of maintained GM lineage priming, but gradually reduced GM lineage potential. These functional and molecular findings reinforce the existence of GM/lymphoid-restricted progenitors with dramatically down-regulated probability for committing toward MkE fates, and support that lineage restriction occurs through gradual rather than abrupt changes in specific lineage potentials.