Clonal and systemic analysis of long-term hematopoiesis in the mouse

Predicting clonal self-renewal and extinction of hematopoietic stem cells

A single hematopoietic stem cell (HSC) can generate a clone, consisting of daughter HSCs and differentiated progeny, which can sustain the hematopoietic system of multiple hosts for a long time. At the same time, this massive expansion potential must be restrained to prevent abnormal, leukemic proliferation. We used an interdisciplinary approach, combining transplantation assays with mathematical and computational methods, to systematically analyze the proliferative potential of individual HSCs. We show that all HSC clones examined have an intrinsically limited life span. Daughter HSCs within a clone behaved synchronously in transplantation assays and eventually exhausted at the same time. These results indicate that each HSC is programmed to have a finite life span. This program and the memory of the life span of the mother HSC are inherited by all daughter HSCs. In contrast, there was extensive heterogeneity in life spans between individual HSC clones, ranging from 10 to almost 60 mo. We used model-based machine learning to develop a mathematical model that efficiently predicts the life spans of individual HSC clones on the basis of a few initial measurements of donor type cells in blood. Computer simulations predict that the probability of self-renewal decays with a logistic kinetic over the life span of a normal HSC clone. Other decay functions lead to either graft failure or leukemic proliferation. We propose that dynamical fate probabilities are a crucial condition that leads to self-limiting clonal proliferation.

Dynamic variation in cycling of hematopoietic stem cells in steady state and inflammation

Both fast-cycling and quiescent mouse hematopoietic stem cells (HSCs) can reconstitute lifelong hematopoiesis, and HSC cycling status can fluctuate over time in steady state and accelerate upon inflammation.

Hematopoietic stem cells (HSCs) maintain blood production. How often mouse HSCs divide and whether each HSC contributes simultaneously, sequentially, or repetitively to hematopoiesis remains to be determined. We track division of 5-(and-6)-carboxyfluorescein diacetate succinimidyl ester (CFSE)–labeled HSC in vivo. We found that, in steady-state mice, bone marrow cells capable of reconstituting lifelong hematopoiesis are found within both fast-cycling (undergoing five or more divisions in 7 wk) and quiescent (undergoing zero divisions in 12–14 wk) lineage marker–negative c-Kit⁺ Sca-1⁺ populations. The contribution of each population to hematopoiesis can fluctuate with time, and cells with extensive proliferative history are prone to return to quiescence. Furthermore, injection of the bacterial component lipopolysaccharide increased the proliferation and self-renewal capacity of HSCs. These findings suggest a model in which all HSCs undergo dynamic and demand-adapted entry into and exit out of the cell cycle over time. This may facilitate a similar degree of turnover of the entire HSC pool at the end of life.

Tracking adult stem cells

The maintenance of stem-cell-driven tissue homeostasis requires a balance between the generation and loss of cell mass. Adult stem cells have a close relationship with the surrounding tissue--known as their niche--and thus, stem-cell studies should preferably be performed in a physiological context, rather than outside their natural environment. The mouse is an attractive model in which to study adult mammalian stem cells, as numerous experimental systems and genetic tools are available. In this review, we describe strategies commonly used to identify and functionally characterize adult stem cells in mice and discuss their potential, limitations and interpretations, as well as how they have informed our understanding of adult stem-cell biology. An accurate interpretation of physiologically relevant stem-cell assays is crucial to identify adult stem cells and elucidate how they self-renew and give rise to differentiated progeny.

High-throughput, sensitive quantification of repopulating hematopoietic stem cell clones

Retroviral vector-mediated gene therapy has been successfully used to correct genetic diseases. However, a number of studies have shown a subsequent risk of cancer development or aberrant clonal growths due to vector insertion near or within proto-oncogenes. Recent advances in the sequencing technology enable high-throughput clonality analysis via vector integration site (VIS) sequencing, which is particularly useful for studying complex polyclonal hematopoietic progenitor/stem cell (HPSC) repopulation. However, clonal repopulation analysis using the current methods is typically semiquantitative. Here, we present a novel system and standards for accurate clonality analysis using 454 pyrosequencing. We developed a bidirectional VIS PCR method to improve VIS detection by concurrently analyzing both the 5' and the 3' vector-host junctions and optimized the conditions for the quantitative VIS sequencing. The assay was validated by quantifying the relative frequencies of hundreds of repopulating HPSC clones in a nonhuman primate. The reliability and sensitivity of the assay were assessed using clone-specific real-time PCR. The majority of tested clones showed a strong correlation between the two methods. This assay permits high-throughput and sensitive assessment of clonal populations and hence will be useful for a broad range of gene therapy, stem cell, and cancer research applications.

Genetic disruption of both Fancc and Fancg in mice recapitulates the hematopoietic manifestations of Fanconi anemia

Fanconi anemia (FA) is an inherited chromosomal instability syndrome characterized by bone marrow failure, myelodysplasia (MDS), and acute myeloid leukemia (AML). Eight FA proteins associate in a nuclear core complex to monoubiquitinate FANCD2/FANCI in response to DNA damage. Additional functions have been described for some of the core complex proteins; however, in vivo genetic proof has been lacking. Here we show that double-mutant Fancc(-/-);Fancg(-/-) mice develop spontaneous hematologic sequelae including bone marrow failure, AML, MDS and complex random chromosomal abnormalities that the single-mutant mice do not. This genetic model provides evidence for unique core complex protein function independent of their ability to monoubiquitinate FANCD2/FANCI. Importantly, this model closely recapitulates the phenotypes found in FA patients and may be useful as a preclinical platform to evaluate the molecular pathogenesis of spontaneous bone marrow failure, MDS and AML in FA.

Some hematopoietic stem cells are more equal than others

Hematopoietic stem cells (HSCs) save lives in routine clinical practice every day, as they are the key element in transplantation-based therapies for hematologic malignancies. The success of clinical stem cell transplantation critically relies on the ability of stem cells to reconstitute the hematopoietic system for many decades after the administration of the powerful chemotherapy and/or irradiation that is required to eradicate malignant cells, but also irreversibly ablates patients’ own blood forming capacity. Surprisingly, despite enormous efforts and continuous progress in the field, our understanding of the basic biology of HSCs is still rather incomplete. Several recent studies substantially refine our understanding of the cells at the very top of the hematopoietic hierarchy, and suggest that we may need to revise the criteria we typically use to identify and define HSCs.

Stem cell origins and animal models of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a common malignant tumor that almost always occurs within a preexisting background of chronic liver disease and cirrhosis. Currently, medical therapy is not effective in treating most HCC, and the only hope of cure is either resection or liver transplantation. A small minority of patients is eligible for these therapies, which entail major morbidity at the very least. In spite of immense scientific advances during the past 3 decades, patient survival has improved very little. In order to reduce morbidity and mortality from HCC, improvements in early diagnosis and development of novel local and systemic therapies for advanced disease are essential, in addition to efforts geared towards primary prevention. Studies with experimental animal models that closely mimic human disease are very valuable in understanding physiological, cellular and molecular mechanisms underlying the disease. Furthermore, appropriate animal models have the potential to increase our understanding of the effects of image-guided minimally invasive therapies and thereby help to improve such therapies. In this review, we examine the evidence for stem cell origins of such tumors, critically evaluate existing models and reflect on how to develop new models for minimally invasive, image-guided treatment of HCC.

Hematopoietic activity of human short-term repopulating cells in mobilized peripheral blood cell transplants is restricted to the first 5 months after transplantation

Kinetics of hematopoietic recovery driven by different types of human stem and progenitor cells after transplantation are not fully understood. Short-term repopulating cells (STRCs) dominate early hematopoiesis after transplantation. STRCs are highly enriched in adult mobilized peripheral blood compared with cord blood, but the length of their contribution to hematopoiesis remains unclear. To understand posttransplantation durability and lineage contribution of STRCs, we compared repopulation kinetics of mobilized peripheral blood (high STRC content) with cord blood transplants (low STRC content) in long-lived NOD.Cg-Prkdc(scid)Il2rg(tm1Wjl)/SzJ (IL2RG(-/-)) mice. This comparison demonstrates that quantitative contribution of human STRCs to hematopoiesis is restricted to the first 5 months after transplantation. The ratio of STRCs to long-term repopulating cells dramatically changes during ontogeny. This model enables to precisely determine early and late engraftment kinetics of defined human repopulating cell types and to preclinically assess the engraftment kinetics of engineered stem cell transplants.

Distinct hematopoietic stem cell subtypes are differentially regulated by TGF-beta1

The traditional view of hematopoiesis has been that all the cells of the peripheral blood are the progeny of a unitary homogeneous pool of hematopoietic stem cells (HSCs). Recent evidence suggests that the hematopoietic system is actually maintained by a consortium of HSC subtypes with distinct functional characteristics. We show here that myeloid-biased HSCs (My-HSCs) and lymphoid-biased HSCs (Ly-HSCs) can be purified according to their capacity for Hoechst dye efflux in combination with canonical HSC markers. These phenotypes are stable under natural (aging) or artificial (serial transplantation) stress and are exacerbated in the presence of competing HSCs. My- and Ly-HSCs respond differently to TGF-beta1, presenting a possible mechanism for differential regulation of HSC subtype activation. This study demonstrates definitive isolation of lineage-biased HSC subtypes and contributes to the fundamental change in view that the hematopoietic system is maintained by a continuum of HSC subtypes, rather than a functionally uniform pool.

Retroviral vector integration in post-transplant hematopoiesis in mice conditioned with either submyeloablative or ablative irradiation

X-linked chronic granulomatous disease (X-CGD) is an inherited immunodeficiency with absent phagocyte NADPH oxidase activity caused by defects in the gene encoding gp91^phox. Here we evaluated strategies for less intensive conditioning for gene therapy of genetic blood disorders without selective advantage for gene correction, such as might be used in a human X-CGD protocol. We compared submyeloablative with ablative irradiation as conditioning in murine X-CGD, examining engraftment, oxidase activity and vector integration in mice transplanted with marrow transduced with a gamma-retroviral vector for gp91^phox expression. The frequency of oxidase-positive neutrophils in the donor population was unexpectedly higher in many 300 cGy-conditioned mice compared to lethally irradiated recipients, as was the fraction of vector-marked donor secondary CFU-S12. Vector integration sites in marrow, spleen, and secondary CFU-S12 DNA from primary recipients were enriched for cancer-associated genes, including Evi1, and integrations in or near cancer-associated genes were more frequent in marrow and secondary CFU-S12 from 300 cGy-conditioned mice compared to fully ablated mice. These findings support the concept that vector integration can confer a selection bias, and suggest that intensity of the conditioning regimen may further influence the effects of vector integration on clonal selection in post-transplant engraftment and hematopoiesis.

Cellular barcoding tool for clonal analysis in the hematopoietic system

Clonal analysis is important for many areas of hematopoietic stem cell research, including in vitro cell expansion, gene therapy, and cancer progression and treatment. A common approach to measure clonality of retrovirally transduced cells is to perform integration site analysis using Southern blotting or polymerase chain reaction-based methods. Although these methods are useful in principle, they generally provide a low-resolution, biased, and incomplete assessment of clonality. To overcome those limitations, we labeled retroviral vectors with random sequence tags or "barcodes." On integration, each vector introduces a unique, identifiable, and heritable mark into the host cell genome, allowing the clonal progeny of each cell to be tracked over time. By coupling the barcoding method to a sequencing-based detection system, we could identify major and minor clones in 2 distinct cell culture systems in vitro and in a long-term transplantation setting. In addition, we demonstrate how clonal analysis can be complemented with transgene expression and integration site analysis. This cellular barcoding tool permits a simple, sensitive assessment of clonality and holds great promise for future gene therapy protocols in humans, and any other applications when clonal tracking is important.

Pig-a mutation: kinetics in rat erythrocytes following exposure to five prototypical mutagens

An in vivo mutation assay has been developed based on flow cytometric enumeration of glycosylphosphatidylinositol (GPI) anchor-deficient rat erythrocytes. With this method, blood is incubated with anti-CD59-PE and SYTO 13 dye, and flow cytometry is used to score the frequency of CD59-negative erythrocytes. The experiments described herein were designed to define the kinetics of mutant erythrocyte appearance and disappearance from peripheral blood to support appropriate treatment and sampling designs for the assay. Wistar Han rats were treated with one of five prototypical mutagens: N-ethyl-N-nitrosourea (ENU); 7,12-dimethyl-1,2-benz[a]anthracene (DMBA); 4-nitroquinoline-1-oxide; benzo[a]pyrene; and N-methyl-N-nitrosourea. ENU and DMBA were also evaluated in Sprague Dawley rats. Animals were treated on three consecutive days (days 1-3) via oral gavage, and blood specimens were obtained on days -1, 4, 15, 30, 45, and 90 (and day 180 for ENU). A second endpoint of genotoxicity, the frequency of peripheral blood micronucleated reticulocytes, was measured on day 4. Each chemical induced micronuclei and the GPI anchor-deficient phenotype. Increased mutant cell frequencies were evident at day 15. Mutant reticulocyte frequencies remained relatively stable for some chemicals, but others peaked and then dropped significantly. The differences in kinetics observed are presumably related to the degree to which mutation occurs in hematopoietic stem cells versus more committed cells with limited self-renewal capacity. Collectively, the results suggest that enumerating GPI anchor-deficient erythrocytes is an efficient means of evaluating the in vivo mutagenic potential of chemicals. The kinetics and ease of scoring this blood-based endpoint suggest that integration into routine toxicology studies will be feasible.

Heterogeneity of non-cycling and cycling synchronized murine hematopoietic stem/progenitor cells

Purified long-term multilineage repopulating marrow stem cells have been considered to be homogenous, but functionally these cells are heterogeneous. Many investigators urge clonal studies to define stem cells but, if stem cells are truly heterogeneous, clonal studies can only define heterogeneity. We have determined the colony growth and differentiation of individual lineage negative, rhodamine low, Hoechst low (LRH) stem cells at various times in cytokine culture, corresponding to specific cell cycle stages. These highly purified and cycle synchronized (98% in S phase at 40 h of culture) stem cells were exposed to two cytokine cocktails for 0, 18, 32, or 40 h and clonal differentiation assessed 14 days later. Total heterogeneity as to gross colony morphology and differentiation stage was demonstrated. This heterogeneity showed patterns of differentiation at different cycle times. These data hearken to previous suggestions that stem cells might be similar to radioactive isotopes; decay rate of a population of radioisotopes being highly predictable, while the decay of individual nuclei is heterogeneous and unpredictable (Till et al., 1964). Marrow stem cells may be most adequately defined on a population basis; stem cells existing in a continuum of reversible change rather than a hierarchy.

The 'stem cell' concept: is it holding us back?

Developmental biology, regenerative medicine and cancer biology are increasingly occupied with the molecular characterization of stem cells. Yet recent work adds to a growing body of literature suggesting that 'stemness' cannot be reduced to the molecular features of cell types, and is instead an emergent property of cell lineages under feedback control.

Cancer stem cells in solid tumors: an overview

It has long been appreciated that significant functional and morphologic heterogeneity can exist within the individual cells that comprise a tumor. Increasing evidence indicates that many solid tumors are organized in a hierarchical manner in which tumor growth is driven by a small subset of cancer stem cells (CSCs) or tumor-initiating cells. Although these cells represent a small percentage of the overall tumor population, they are the only cells capable of initiating and driving tumor growth. Emerging evidence indicates that these cells are also resistant to chemotherapy and radiation therapy, which has led to much speculation and interest surrounding the potential clinical applicability of CSCs.

MafB restricts M-CSF-dependent myeloid commitment divisions of hematopoietic stem cells

While hematopoietic stem cell (HSC) self-renewal is well studied, it remains unknown whether distinct control mechanisms enable HSC divisions that generate progeny cells with specific lineage bias. Here, we report that the monocytic transcription factor MafB specifically restricts the ability of M-CSF to instruct myeloid commitment divisions in HSCs. MafB deficiency specifically enhanced sensitivity to M-CSF and caused activation of the myeloid master-regulator PU.1 in HSCs in vivo. Single-cell analysis revealed that reduced MafB levels enabled M-CSF to instruct divisions producing asymmetric daughter pairs with one PU.1(+) cell. As a consequence, MafB(-/-) HSCs showed a PU.1 and M-CSF receptor-dependent competitive repopulation advantage specifically in the myelomonocytic, but not T lymphoid or erythroid, compartment. Lineage-biased repopulation advantage was progressive, maintained long term, and serially transplantable. Together, this indicates that an integrated transcription factor/cytokine circuit can control the rate of specific HSC commitment divisions without compromising other lineages or self-renewal.

Amelioration of murine beta-thalassemia through drug selection of hematopoietic stem cells transduced with a lentiviral vector encoding both gamma-globin and the MGMT drug-resistance gene

Correction of murine models of beta-thalassemia has been achieved through high-level globin lentiviral vector gene transfer into mouse hematopoietic stem cells (HSCs). However, transduction of human HSCs is less robust and may be inadequate to achieve therapeutic levels of genetically modified erythroid cells. We therefore developed a double gene lentiviral vector encoding both human gamma-globin under the transcriptional control of erythroid regulatory elements and methylguanine methyltransferase (MGMT), driven by a constitutive cellular promoter. MGMT expression provides cellular resistance to alkylator drugs, which can be administered to kill residual untransduced, diseased HSCs, whereas transduced cells are protected. Mice transplanted with beta-thalassemic HSCs transduced with a gamma-globin/MGMT vector initially had subtherapeutic levels of red cells expressing gamma-globin. To enrich gamma-globin-expressing cells, transplanted mice were treated with the alkylator agent 1,3-bis-chloroethyl-1-nitrosourea. This resulted in significant increases in the number of gamma-globin-expressing red cells and the amount of fetal hemoglobin, leading to resolution of anemia. Selection of transduced HSCs was also obtained when cells were drug-treated before transplantation. Mice that received these cells demonstrated reconstitution with therapeutic levels of gamma-globin-expressing cells. These data suggest that MGMT-based drug selection holds promise as a modality to improve gene therapy for beta-thalassemia.

Stem cell concepts renew cancer research

Although uncontrolled proliferation is a distinguishing property of a tumor as a whole, the individual cells that make up the tumor exhibit considerable variation in many properties, including morphology, proliferation kinetics, and the ability to initiate tumor growth in transplant assays. Understanding the molecular and cellular basis of this heterogeneity has important implications in the design of therapeutic strategies. The mechanistic basis of tumor heterogeneity has been uncertain; however, there is now strong evidence that cancer is a cellular hierarchy with cancer stem cells at the apex. This review provides a historical overview of the influence of hematology on the development of stem cell concepts and their linkage to cancer.

Stem cell aging: survival of the laziest?

The question whether stem cells age remains an enigma. Traditionally, aging was thought to change the properties of hematopoietic stem cells (HSC). We discuss here a new model of stem cell aging that challenges this view. It is now well-established that the HSC compartment is heterogeneous, consisting of epigenetically fixed subpopulations of HSC that differ in self-renewal and differentiation capacity. New data show that the representation of these HSC subsets changes during aging. HSC that generate lymphocyte-rich progeny are depleted, while myeloid-biased HSC are enriched in the aged HSC compartment. Myeloid-biased HSC, even when isolated from young donors, have most of the characteristics that had been attributed to aged HSC. Thus, the distinct behavior of the HSC isolated from aged hosts is due to the accumulation of myeloid-biased HSC. By extension this means that the properties of individual HSC are not substantially changed during the lifespan of the organism and that aged hosts do not contain many aged HSC. Myeloid-biased HSC give rise to mature cells slowly but contribute for a long time to peripheral hematopoiesis. We propose that such slow, "lazy" HSC are less likely to be transformed and therefore may safely sustain hematopoiesis for a long time.

Overnight transduction with foamyviral vectors restores the long-term repopulating activity of Fancc-/- stem cells

Fanconi anemia (FA) is a complex genetic disorder characterized by congenital abnormalities, bone marrow failure, and myeloid malignancies. Identification of 13 FA genes has been instrumental to explore gene transfer technologies aimed at correction of autologous FA-deficient stem cells. To date, 3 human FA stem cell gene therapy trials with standard 4-day transduction protocols using gammaretroviral vectors failed to provide clinical benefit. In addition, 2- to 4 day ex vivo manipulation of bone marrow from mice containing a disruption of the homologue of human FANCC (Fancc) results in a time-dependent increase in apoptosis and a risk for malignant transformation of hematopoietic cells. Here, we show that a 14-hour transduction period allows a foamyviral vector construct expressing the human FANCC cDNA to efficiently transduce murine FA stem cells with 1 to 2 proviral integrations per genome. Functionally, the repopulating activity of Fancc(-/-) stem cells from reconstituted mice expressing the recombinant FANCC transgene was comparable with wild-type controls. Collectively, these data provide evidence that short-term transduction of c-kit(+) cells with a foamyviral vector is sufficient for functional correction of a stem cell phenotype in a murine FA model. These data could have implications for future gene therapy trials for FA patients.

The intestinal stem cell

The epithelium of the adult mammalian intestine is in a constant dialog with its underlying mesenchyme to direct progenitor proliferation, lineage commitment, terminal differentiation, and, ultimately, cell death. The epithelium is shaped into spatially distinct compartments that are dedicated to each of these events. While the intestinal epithelium represents the most vigorously renewing adult tissue in mammals, the stem cells that fuel this self-renewal process have been identified only recently. The unique epithelial anatomy makes the intestinal crypt one of the most accessible models for the study of adult stem cell biology. This review attempts to provide a comprehensive overview of four decades of research on crypt stem cells.

The use of experimental murine models to assess novel agents of hematopoietic stem and progenitor cell mobilization

The recent explosion in the understanding of the cellular and molecular mechanisms underlying hematopoietic stem and progenitor cell (HSPC) mobilization has facilitated development of novel therapeutic agents, targeted at improving mobilization kinetics as well as HSPC yield. With the development of new agents comes the challenge of choosing efficient and relevant preclinical studies for the testing of the HSPC mobilization efficacy of these agents. This article reviews the use of the mouse as a convenient small animal model of HSPC mobilization and transplantation, and outlines the range of murine assays that can be applied to assess novel HSPC mobilizing agents. Techniques to demonstrate murine HSPC mobilization are discussed, as well as the role of murine assays to confirm human HSPC mobilization, and techniques to investigate the biologic phenotype of HSPC mobilized by these novel agents. Technical aspects regarding mobilization regimens and control arms, and choice of experimental animals are also discussed.

Unaltered repopulation properties of mouse hematopoietic stem cells transduced with lentiviral vectors

Recent studies of retroviral-mediated gene transfer have shown that retroviral integrations themselves may trigger nonmalignant clonal expansion of hematopoietic stem cells (HSCs) in transplant recipients. These observations suggested that previous conclusions of HSC dynamics based on gamma-retroviral gene marking should be confirmed with improved vectors having a more limited capacity to transactivate endogenous genes. Because of the low trans-activation activity of self-inactivating lentiviral vectors (LVs), we have investigated whether the LV marking of mouse HSCs induces a competitive repopulation advantage in recipients of serially transplants. As deduced from analyses conducted in primary and secondary recipients, we concluded that lentivirally transduced HSCs have no competitive repopulation advantages over untransduced HSCs. By linear amplification-mediated polymerase chain reaction (LAM-PCR) analysis, we characterized LV-targeted genes in HSC clones that engrafted up to quaternary recipients. Although 9 clones harbored integrations close to defined retroviral insertion sites, none was characterized as a common integration site, and none was present in HSC clones repopulating quaternary recipients. Taken together, our results show unaltered repopulation properties of HSCs transduced with LVs, and confirm early studies suggesting the natural capacity of a few HSC clones to generate a monoclonal or oligoclonal hematopoiesis in transplant recipients.

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.

Elevated expression of phospholipid transfer protein in bone marrow derived cells causes atherosclerosis

BACKGROUND

Phospholipid transfer protein (PLTP) is expressed by various cell types. In plasma, it is associated with high density lipoproteins (HDL). Elevated levels of PLTP in transgenic mice result in decreased HDL and increased atherosclerosis. PLTP is present in human atherosclerotic lesions, where it seems to be macrophage derived. The aim of the present study is to evaluate the atherogenic potential of macrophage derived PLTP.

METHODS AND FINDINGS

Here we show that macrophages from human PLTP transgenic mice secrete active PLTP. Subsequently, we performed bone marrow transplantations using either wild type mice (PLTPwt/wt), hemizygous PLTP transgenic mice (huPLTPtg/wt) or homozygous PLTP transgenic mice (huPLTPtg/tg) as donors and low density lipoprotein receptor deficient mice (LDLR-/-) as acceptors, in order to establish the role of PLTP expressed by bone marrow derived cells in diet-induced atherogenesis. Atherosclerosis was increased in the huPLTPtg/wt-->LDLR-/- mice (2.3-fold) and even further in the huPLTPtg/tg-->LDLR-/- mice (4.5-fold) compared with the control PLTPwt/wt-->LDLR-/- mice (both P<0.001). Plasma PLTP activity levels and non-HDL cholesterol were increased and HDL cholesterol decreased compared with controls (all P<0.01). PLTP was present in atherosclerotic plaques in the mice as demonstrated by immunohistochemistry and appears to co-localize with macrophages. Isolated macrophages from PLTP transgenic mice do not show differences in cholesterol efflux or in cytokine production. Lipopolysaccharide activation of macrophages results in increased production of PLTP. This effect was strongly amplified in PLTP transgenic macrophages.

CONCLUSIONS

We conclude that PLTP expression by bone marrow derived cells results in atherogenic effects on plasma lipids, increased PLTP activity, high local PLTP protein levels in the atherosclerotic lesions and increased atherosclerotic lesion size.

Assessment of lymphocyte development in radiation bone marrow chimeras

Transplantation of marrow between mouse strains congenic for CD45 after lethal irradiation establishes hematopoiesis driven by genetically marked cells in recipient animals. After several weeks, peripheral blood or primary and secondary lymphoid organs of transplant recipients can be evaluated for the presence of donor-derived cells. Two- or three-color flow cytometry can be used to identify the progeny of transplanted cells, to document their cell-surface phenotypes, and to follow development of T, B, and myeloid lineages in vivo.

Bioluminescence imaging of hematopoietic stem cell repopulation in murine models

Hematopoietic stem cells (HSCs) have been studied for decades in order to understand their stem cell biology and their potential as treatments in gene therapy, and those studies have resulted in tremendous advancement of understanding HSCs. However, most of the studies required the sacrifice of cohorts of the animals in order to obtain data for analysis, resulting in the use of large animal numbers along with difficult long-term studies. The dynamic engraftment and expansion of HSC are not fully observed and analyzed. Until recently, with the development of optical imaging, HSC repopulation can be continuously monitored in the same animal over a long period of time, reducing animal numbers and opening a new dimension for investigation. In this chapter, bioluminescence imaging of murine HSC is described for observing the dynamic repopulation process after transplantation. Photons emitted from transplanted murine HSCs expressing firefly luciferase within the mice can be visualized in light-sealed chamber with a highly sensitive digital camera after injection of substrate D-luciferin. Xenogen IVIS200 imaging system is used to record the process, and other similar imaging systems can also be used for this process.

Lhx2 expression promotes self-renewal of a distinct multipotential hematopoietic progenitor cell in embryonic stem cell-derived embryoid bodies

The molecular mechanisms regulating the expansion of the hematopoietic system including hematopoietic stem cells (HSCs) in the fetal liver during embryonic development are largely unknown. The LIM-homeobox gene Lhx2 is a candidate regulator of fetal hematopoiesis since it is expressed in the fetal liver and Lhx2^−/− mice die in utero due to severe anemia. Moreover, expression of Lhx2 in embryonic stem (ES) cell-derived embryoid bodies (EBs) can lead to the generation of HSC-like cell lines. To further define the role of this transcription factor in hematopoietic regulation, we generated ES cell lines that enabled tet-inducible expression of Lhx2. Using this approach we observed that Lhx2 expression synergises with specific signalling pathways, resulting in increased frequency of colony forming cells in developing EB cells. The increase in growth factor-responsive progenitor cells directly correlates to the efficiency in generating HSC-like cell lines, suggesting that Lhx2 expression induce self-renewal of a distinct multipotential hematopoietic progenitor cell in EBs. Signalling via the c-kit tyrosine kinase receptor and the gp130 signal transducer by IL-6 is necessary and sufficient for the Lhx2 induced self-renewal. While inducing self-renewal of multipotential progenitor cells, expression of Lhx2 inhibited proliferation of primitive erythroid precursor cells and interfered with early ES cell commitment, indicating striking lineage specificity of this effect.

A new mechanism for the aging of hematopoietic stem cells: aging changes the clonal composition of the stem cell compartment but not individual stem cells

Whether hematopoietic stem cells (HSCs) change with aging has been controversial. Previously, we showed that the HSC compartment in young mice consists of distinct subsets, each with predetermined self-renewal and differentiation behavior. Three classes of HSCs can be distinguished based on their differentiation programs: lymphoid biased, balanced, and myeloid biased. We now show that aging causes a marked shift in the representation of these HSC subsets. A clonal analysis of repopulating HSCs demonstrates that lymphoid-biased HSCs are lost and long-lived myeloid-biased HSCs accumulate in the aged. Myeloid-biased HSCs from young and aged sources behave similarly in all aspects tested. This indicates that aging does not change individual HSCs. Rather, aging changes the clonal composition of the HSC compartment. We show further that genetic factors contribute to the age-related changes of the HSC subsets. In comparison with B6 mice, aged D2 mice show a more pronounced shift toward myeloid-biased HSCs with a corresponding reduction in the number of both T- and B-cell precursors. This suggests that low levels of lymphocytes in the blood can be a marker for HSC aging. The loss of lymphoid-biased HSCs may contribute to the impaired immune response to infectious diseases and cancers in the aged.

CD61 enriches long-term repopulating hematopoietic stem cells

Among the subsets that define hematopoietic stem cells (HSCs), CD34- c-kit+ Sca-1+ lineage marker- (CD34-KSL) cells are regarded as one of the populations that have the highest enrichment of HSCs in adult mouse bone marrow. Here, we demonstrate that long-term repopulating hematopoietic stem cells (LTR-HSCs) have high expression of CD61 (integrin beta3) within the CD34-KSL population. Approximately 60% of CD34-KSL cells showed high expression of CD61. CD61HighCD34-KSL populations also exhibited significantly greater properties of HSC, such as expression of HSC markers, the side population (SP) phenotype, and ability for long-term repopulation. In both SP cells and non-SP (NSP) cells, CD61HighCD34-KSL cells also contained significantly more LTR-HSCs than CD61Low/-CD34-KSL cells. Our results indicate that CD61 is exploitable for HSC enrichment as a supportive positive cell surface marker.

Back to the beginning--the quest for thymic epithelial stem cells

Stem cell-based therapies hold much promise for the rejuvenation of aged or damaged tissues; however, before such cells can be used therapeutically, they must first be accurately identified. In this issue of the European Journal of Immunology it is reported that MTS24, a marker previously associated with progenitor cells of the thymic epithelium, fails to accurately identify epithelial cell populations with the ability to reconstitute a functional thymus. This finding demonstrates that much progress needs to be made before thymic epithelial stem cells can be harnessed for clinical benefit.

Stem cell fate analysis revisited: interpretation of individual clone dynamics in the light of a new paradigm of stem cell organization

Many experimental findings on heterogeneity, flexibility, and plasticity of tissue stem cells are currently challenging stem cell concepts that assume a cell intrinsically predefined, unidirectional differentiation program. In contrast to these classical concepts, nonhierarchical self-organizing systems provide an elegant and comprehensive alternative to explain the experimental data. Here we present the application of such a self-organizing concept to quantitatively describe the hematopoietic stem cell system. Focusing on the analysis of individual-stem-cell fates and clonal dynamics, we particularly discuss implications of the theoretical results on the interpretation of experimental findings. We demonstrate that it is possible to understand hematopoietic stem cell organization without assumptions on unidirectional developmental hierarchies, preprogrammed asymmetric division events or other assumptions implying the existence of a predetermined stem cell entity. The proposed perspective, therefore, changes the general paradigm of thinking about stem cells.

Epigenetic control of hematopoietic stem cell aging the case of Ezh2

Hematopoietic stem cells have potent, but not unlimited, selfrenewal potential. The mechanisms that restrict selfrenewal are likely to play a role during aging. Recent data suggest that the regulation of histone modifications by Polycomb group genes may be of crucial relevance to balance selfrenewal and aging. We provide evidence for the involvement of one of these Polycomb group genes, Ezh2, in aging of the hematopoietic stem cell system.

Stromal cell lines from the aorta-gonado-mesonephros region are potent supporters of murine and human hematopoiesis

OBJECTIVE

The hematopoietic system is nurtured by a supportive stroma environment allowing maintenance and differentiation of hematopoietic stem cells (HSC). However, only a limited number of these stromal cell clones support hematopoiesis in the absence of cytokine supplementation. So far, only two bone marrow-derived stromal cell lines (OP9 and S17) are capable of inducing hematopoietic differentiation of totipotent murine and human embryonic stem cells (ESC). Here, the potential of more than 100 stromal cell lines developed from the aorta-gonado-mesonephros (AGM) region was investigated in supporting adult and embryonic hematopoiesis. In addition, extensive phenotypic analysis should elucidate possible mechanisms involved in maintenance of hematopoietic stem cell function.

METHODS

More than 100 stromal cell clones derived from the AGM region of E10.5 mouse embryos were isolated. Hematopoietic stem cell support was tested for adult murine and human cord blood hematopoietic stem cells and hematopoietic cells derived from murine ESC. Genotypic and phenotypic characterization was performed including gene array analysis.

RESULTS

It was demonstrated that multiple clones showed high efficiency in supporting maintenance and expansion of primitive murine and human hematopoietic progenitors. In addition, we demonstrated for the first time that AGM stromal cell lines are also potent inducers of hematopoietic differentiation of murine ESC. Microarray analysis of AGM lines revealed a characteristic genotype with expression of genes involved in regulating hematopoiesis as well as mesodermal and early B cell development.

CONCLUSION

These AGM stromal cell lines may be of value in elucidating molecular mechanisms regulating early stem cell development and hematopoietic differentiation from ES-derived mesoderm.

Use of flt3 ligand to evaluate residual hematopoiesis after heterogeneous irradiation in mice

We evaluated the possibility of using plasma Flt3 ligand (FL) concentration as a biological indicator of bone marrow function after heterogeneous irradiation. Mice were irradiated with 4, 7.5 or 11 Gy with 25, 50, 75 or 100% of the bone marrow in the field of irradiation. This model of irradiation resulted in graded and controlled damage to the bone marrow. Mice exhibited a pancytopenia correlated with both the radiation dose and the percentage of bone marrow irradiated. The FL concentration in the blood increased with the severity of bone marrow aplasia. Nonlinear regression analysis showed that the FL concentration was strongly correlated with the total number of residual colony-forming cells 3 days after irradiation, allowing a precise estimate of residual hematopoiesis. Moreover, the FL concentration on day 3 postirradiation was correlated with the duration and severity of subsequent pancytopenia, suggesting that variations in FL concentrations might be used as a predictive indicator of bone marrow aplasia, especially by the use of linear regression equations describing these correlations. Our results provide a rationale for the use of FL concentration as a biological indicator of residual hematopoiesis after heterogeneous irradiation.

Individual stem cells with highly variable proliferation and self-renewal properties comprise the human hematopoietic stem cell compartment

Hematopoiesis requires tight regulation of the hematopoietic stem cell (HSC) population; however, the dynamics of HSC use at steady state are uncertain. Over 3-7 months, we evaluated the repopulation and self-renewal of more than 600 individual human 'severe combined immunodeficiency mouse-repopulating cells' (SRCs), tracked on the basis of lentiviral integration sites, in serially transplanted immune-deficient mice, as well as of SRC daughter cells that migrated to different marrow locations in a single mouse. Our data demonstrate maintenance by self-renewing SRCs after an initial period of clonal instability, a result inconsistent with the clonal succession model. We found wide variation in proliferation kinetics and self-renewal among SRCs, as well as between SRC daughter cells that repopulated equivalently, suggesting that SRC fate is unpredictable before SRCs enter more rigid 'downstream' developmental programs.

Evidence for diversity in transcriptional profiles of single hematopoietic stem cells

Hematopoietic stem cells replenish all the cells of the blood throughout the lifetime of an animal. Although thousands of stem cells reside in the bone marrow, only a few contribute to blood production at any given time. Nothing is known about the differences between individual stem cells that dictate their particular state of activation readiness. To examine such differences between individual stem cells, we determined the global gene expression profile of 12 single stem cells using microarrays. We showed that at least half of the genetic expression variability between 12 single cells profiled was due to biological variation in 44% of the genes analyzed. We also identified specific genes with high biological variance that are candidates for influencing the state of readiness of individual hematopoietic stem cells, and confirmed the variability of a subset of these genes using single-cell real-time PCR. Because apparent variation of some genes is likely due to technical factors, we estimated the degree of biological versus technical variation for each gene using identical RNA samples containing an RNA amount equivalent to that of single cells. This enabled us to identify a large cohort of genes with low technical variability whose expression can be reliably measured on the arrays at the single-cell level. These data have established that gene expression of individual stem cells varies widely, despite extremely high phenotypic homogeneity. Some of this variation is in key regulators of stem cell activity, which could account for the differential responses of particular stem cells to exogenous stimuli. The capacity to accurately interrogate individual cells for global gene expression will facilitate a systems approach to biological processes at a single-cell level.

The hematopoietic stem cell (HSC) has the remarkable property of being able to generate more stem cells or cells that are committed to undergo differentiation into specific blood lineages. Currently, very little is known about the specific mechanisms that underlie self-renewal or lineage commitment. Although it is possible that some of these mechanisms are influenced by the specific environment in which the HSC dwells, the ultimate fate decision has to occur at the single HSC level. The authors have developed a method that amplifies the messages from the majority of genes that are active in a single stem cell and combines it with large-scale genetic expression analysis through the use of nucleic acid microarrays. A significant fraction of these genes are found to be highly variable in an apparently very homogeneous stem cell population, which could be the substrate for differences in behavior of individual stem cells. Understanding the genetic expression events at the single-cell level would grant the ability to expand HSCs or to direct their differentiation into specific populations, both important from a therapeutic point of view. Furthermore, the same techniques can be applied to other stem cell systems to investigate their physiology.

Clonal diversity of the stem cell compartment

PURPOSE OF REVIEW

Hematopoietic stem cells are functionally heterogeneous even when isolated as phenotypically homogenous populations. How this heterogeneity is generated is incompletely understood. Several models have been formulated to explain the generation of diversity. All of these assume the existence of a single type of hematopoietic stem cell that generates heterogeneous daughter stem cells in response to extrinsic or intrinsic (stochastic) signals. This view has encouraged the idea that stem cells can be instructed to adapt their function. Newer data, however, challenge this concept. Here, we summarize these findings and discuss their implication for applications of stem cells.

RECENT FINDINGS

Hematopoietic stem cells that differ in function have been documented during development and within the adult stem cell compartment. The differences in function are stably inherited to daughter stem cells when these cells proliferate to self-renew. Collectively, the data show that the adult stem cell compartment consists of a limited number of distinct classes of stem cells.

SUMMARY

The most important stem cell functions, including self-renewal and differentiation capacity, are preprogrammed through epigenetic or genetic mechanisms. Thus, stem cells are much more predictable than previously thought. Changes in the stem cell compartment through disease or aging can be interpreted as shifts in its clonal composition, rather than a modification of individual hematopoietic stem cells.

Global gene expression analyses of hematopoietic stem cell-like cell lines with inducible Lhx2 expression

Background

Expression of the LIM-homeobox gene Lhx2 in murine hematopoietic cells allows for the generation of hematopoietic stem cell (HSC)-like cell lines. To address the molecular basis of Lhx2 function, we generated HSC-like cell lines where Lhx2 expression is regulated by a tet-on system and hence dependent on the presence of doxycyclin (dox). These cell lines efficiently down-regulate Lhx2 expression upon dox withdrawal leading to a rapid differentiation into various myeloid cell types.

Results

Global gene expression of these cell lines cultured in dox was compared to different time points after dox withdrawal using microarray technology. We identified 267 differentially expressed genes. The majority of the genes overlapping with HSC-specific databases were those down-regulated after turning off Lhx2 expression and a majority of the genes overlapping with those defined as late progenitor-specific genes were the up-regulated genes, suggesting that these cell lines represent a relevant model system for normal HSCs also at the level of global gene expression. Moreover, in situ hybridisations of several genes down-regulated after dox withdrawal showed overlapping expression patterns with Lhx2 in various tissues during embryonic development.

Conclusion

Global gene expression analysis of HSC-like cell lines with inducible Lhx2 expression has identified genes putatively linked to self-renewal / differentiation of HSCs, and function of Lhx2 in organ development and stem / progenitor cells of non-hematopoietic origin.

The mobilization of hematopoietic progenitors to peripheral blood is predictive of the hematopoietic syndrome after total or partial body irradiation of mice

PURPOSE

In previous studies we showed that administration of mobilizing growth factors (MGFs) to mice previously exposed to total body irradiation mobilizes to peripheral blood (PB) a number of progenitors that correlates with the total reserve of progenitors surviving the exposure. Now we have tested whether this finding is independent of the radiosensitivity of the mice and of the homogeneity of the radiation exposure. Also we have investigated whether numbers of mobilized progenitors predict the hematopoietic syndrome after irradiation.

METHODS AND MATERIALS

Mice were subjected to partial or total body irradiation and treated with MGFs. Thereafter, the number of colony-forming units granulocyte-macrophage progenitors in PB was correlated with the total reserve of surviving progenitors and with the nadir of leukocytes after the irradiation.

RESULTS

The number of progenitors mobilized to PB after irradiation of normal and radiosensitive mice showed the same correlation with respect to the reserve of bone marrow progenitors surviving the exposure. Additionally, the number of mobilized progenitors correlated with the leukocytes' nadir after the irradiation, regardless of homogeneous or inhomogeneous exposures.

CONCLUSIONS

In a mouse experimental model, the number of hematopoietic progenitors mobilized to PB by MGFs is a good predictor of the hematopoietic syndrome occurring after total or partial body irradiation.

Bone-marrow haematopoietic-stem-cell niches

Adult stem cells hold many promises for future clinical applications and regenerative medicine. The haematopoietic stem cell (HSC) is the best-characterized somatic stem cell so far, but in vitro expansion has been unsuccessful, limiting the future therapeutic potential of these cells. Here we review recent progress in characterizing the composition of the HSC bone-marrow microenvironment, known as the HSC niche. During homeostasis, HSCs, and therefore putative bone-marrow HSC niches, are located near bone surfaces or are associated with the sinusoidal endothelium. The molecular crosstalk between HSCs and the cellular constituents of these niches is thought to control the balance between HSC self-renewal and differentiation, indicating that future successful expansion of HSCs for therapeutic use will require three-dimensional reconstruction of a stem-cell-niche unit.

The GOD of hematopoietic stem cells: a clonal diversity model of the stem cell compartment

Hematopoietic stem cells (HSC) show heterogeneous behavior even when isolated as phenotypically homogeneous populations. The cellular and molecular mechanisms that control the generation of diversity (GOD) in the HSC compartment are not well understood, but have been the focus of much debate. There is increasing evidence that the most important HSC functions, self-renewal and differentiation, are epigenetically preprogrammed and therefore predictable. Indeed, recent data show that the adult HSC compartment consists of a limited number of functionally distinct subsets of HSC. This contradicts older models of HSC behavior, which postulated a single type of HSC that can be continuously molded into different subtypes of HSC. We propose a clonal diversity model where the adult HSC compartment consists of a fixed number of different types of HSC, each with epigenetically preprogrammed behavior. Aging or disease may change the overall function of the HSC population. The model predicts that these changes reflect the relative composition of the HSC subsets, rather than changes in individual HSC. This view has implications for using HSC in experimental and clinical settings. Selection for the appropriate subsets of HSC, rather than attempts to force HSC to adjust, should improve their utility in transplantation and gene transfer applications.

Senescence of hematopoietic stem cells and bone marrow failure

Functional failure in hematopoietic stem cells (HSCs) may bring fatal consequences because HSCs are the ultimate source of mature blood cells, which need continuous replenishment. One potential cause of HSC dysfunction is senescence, in which HSCs and progenitor cells enter a state of proliferative arrest. HSC senescence is genetically regulated and one particular regulator is the telomerase gene. Mutations in the telomerase gene complex have been found in patients with bone marrow failure syndromes. During a normal lifetime, HSC clones function over the long term and may not show any functional loss under normal circumstances. However, pathologic environments may limit HSC proliferation, accelerate HSC turnover, and shorten the functional life of HSCs, leading to HSC clonal exhaustion and senescence.

AMD3100 mobilizes hematopoietic stem cells with long-term repopulating capacity in nonhuman primates

AMD3100, a bicyclam antagonist of the chemokine receptor CXCR4, has been shown to induce rapid mobilization of CD34(+) hematopoietic cells in mice, dogs, and humans, offering an alternative to G-CSF mobilization of peripheral-blood hematopoietic stem cells. In this study, AMD3100-mobilized CD34(+) cells were phenotypically analyzed, marked with Neo(R)-containing retroviral vectors, and subsequently transplanted into myeloablated rhesus macaques. We show engraftment of transduced AMD3100-mobilized CD34(+) cells with Neo(R) gene marked myeloid and lymphoid cells up to 32 months after transplantation, demonstrating the ability of AMD3100 to mobilize true long-term repopulating hematopoietic stem cells. More AMD3100-mobilized CD34(+) cells are in the G(1) phase of the cell cycle and more cells express CXCR4 and VLA-4 compared with G-CSF-mobilized CD34(+) cells. In vivo gene marking levels obtained with AMD3100-mobilized CD34(+) cells were better than those obtained using CD34(+) cells mobilized with G-CSF alone. Overall, these results indicate that AMD3100 mobilizes a population of hematopoietic stem cells with intrinsic characteristics different from those of hematopoietic stem cells mobilized with G-CSF, suggesting fundamental differences in the mechanism of AMD3100-mediated and G-CSF-mediated hematopoietic stem cell mobilization. Thus, AMD3100-mobilized CD34(+) cells represent an alternative source of hematopoietic stem cells for clinical stem cell transplantation and genetic manipulation with integrating retroviral vectors.