The marrow homing efficiency of murine hematopoietic stem cells remains constant during ontogeny

A novel conditioning-free hematopoietic stem cell transplantation model in zebrafish

Transplantation is the most common assay for measuring the in vivo functionality of hematopoietic stem cells (HSCs). Although various HSC transplantation strategies have been developed in zebrafish, they are underutilized because of challenges related to immune matching and preconditioning toxicity. To circumvent these limitations, we developed a simple and robust transplantation model using HSC-deficient hosts. Homozygous runx1W84X mutants are devoid of definitive hematopoietic cells, including HSCs and adaptive immune cells; thus, they require no preconditioning regimen for transplantation. Marrow cell transplantation into runx1-mutant zebrafish 2 days after fertilization significantly improved their survival to adulthood and resulted in robust, multilineage, long-lasting, serially repopulating engraftment. Furthermore, we demonstrated that engraftment into runx1 homozygous mutants was significantly higher than into runx1 heterozygotes, demonstrating that the improved transplantation success is attributable to the empty HSC niche in mutants and not just the embryonic environment. Competitive transplantation of marrow cells into runx1 mutants revealed a stem cell frequency similar to that of murine marrow cells, which demonstrates the utility of this model for quantifying HSC function. The streamlined approach and robustness of this assay will help broaden its feasibility for future high-throughput transplantation experiments in zebrafish and will enable further novel discoveries in the biology of HSCs.

Excessive R-loops trigger an inflammatory cascade leading to increased HSPC production

Hematopoietic stem and progenitor cells (HSPCs) arise during embryonic development and are essential for sustaining the blood and immune systems throughout life. Tight regulation of HSPC numbers is critical for hematopoietic homeostasis. Here, we identified DEAD-box helicase 41 (Ddx41) as a gatekeeper of HSPC production. Using zebrafish ddx41 mutants, we unveiled a critical role for this helicase in regulating HSPC production at the endothelial-to-hematopoietic transition. We determined that Ddx41 suppresses the accumulation of R-loops, nucleic acid structures consisting of RNA:DNA hybrids and ssDNAs whose equilibrium is essential for cellular fitness. Excess R-loop levels in ddx41 mutants triggered the cGAS-STING inflammatory pathway leading to increased numbers of hemogenic endothelium and HSPCs. Elevated R-loop accumulation and inflammatory signaling were observed in human cells with decreased DDX41, suggesting possible conservation of mechanism. These findings delineate that precise regulation of R-loop levels during development is critical for limiting cGAS-STING activity and HSPC numbers.

Engineering Hematopoietic Stem Cells: Lessons from Development

Cell engineering has brought us tantalizingly close to the goal of deriving patient-specific hematopoietic stem cells (HSCs). While directed differentiation and transcription factor-mediated conversion strategies have generated progenitor cells with multilineage potential, to date, therapy-grade engineered HSCs remain elusive due to insufficient long-term self-renewal and inadequate differentiated progeny functionality. A cross-species approach involving zebrafish and mammalian systems offers complementary methodologies to improve understanding of native HSCs. Here, we discuss the role of conserved developmental timing processes in vertebrate hematopoiesis, highlighting how identification and manipulation of stage-specific factors that specify HSC developmental state must be harnessed to engineer HSCs for therapy.

IGF-1-mediated osteoblastic niche expansion enhances long-term hematopoietic stem cell engraftment after murine bone marrow transplantation

The efficiency of hematopoietic stem cell (HSC) engraftment after bone marrow (BM) transplantation depends largely on the capacity of the marrow microenvironment to accept the transplanted cells. While radioablation of BM damages osteoblastic stem cell niches, little is known about their restoration and mechanisms governing their receptivity to engraft transplanted HSCs. We previously reported rapid restoration and profound expansion of the marrow endosteal microenvironment in response to marrow radioablation. Here, we show that this reorganization represents proliferation of mature endosteal osteoblasts which seem to arise from a small subset of high-proliferative, relatively radio-resistant endosteal cells. Multiple layers of osteoblasts form along the endosteal surface within 48 hours after total body irradiation, concomitant with a peak in marrow cytokine expression. This niche reorganization fosters homing of the transplanted hematopoietic cells to the host marrow space and engraftment of long-term-HSC. Inhibition of insulin-like growth factor (IGF)-1-receptor tyrosine kinase signaling abrogates endosteal osteoblast proliferation and donor HSC engraftment, suggesting that the cytokine IGF-1 is a crucial mediator of endosteal niche reorganization and consequently donor HSC engraftment. Further understanding of this novel mechanism of IGF-1-dependent osteoblastic niche expansion and HSC engraftment may yield clinical applications for improving engraftment efficiency after clinical HSC transplantation.

Post-natal oogenesis: a concept for controversy that intensified during the last decade

For decades, scientists have considered that female mammals are born with a lifetime reserve of oocytes in the ovary, irrevocably fated to decline after birth. However, controversy in the matter of the possible presence of oocytes and granulosa cells that originate from stem cells in the adult mammalian ovaries has been expanded. The restricted supply of oocytes in adult female mammals has been disputed in recent years by supporters of neo-oogenesis, who claim that germline stem cells (GSCs) exist in the ovarian surface epithelium (OSE) or the bone marrow (BM). Differentiation of ovarian stem cells (OSCs) into oocytes, fibroblast-like cells, granulosa phenotype, neural and mesenchymal type cells and generation of germ cells from OSCs under the contribution of an OSC niche that consists of immune system-related cells and hormonal signalling has been claimed. Although these arguments have met with intense suspicion, their confirmation would necessitate the revision of the current classic knowledge of female reproductive biology.

Developmental changes in hematopoietic stem cell properties

Hematopoietic stem cells (HSCs) comprise a rare population of cells that can regenerate and maintain lifelong blood cell production. This functionality is achieved through their ability to undergo many divisions without activating a poised, but latent, capacity for differentiation into multiple blood cell types. Throughout life, HSCs undergo sequential changes in several key properties. These affect mechanisms that regulate the self-renewal, turnover and differentiation of HSCs as well as the properties of the committed progenitors and terminally differentiated cells derived from them. Recent findings point to the Lin28b-let-7 pathway as a master regulator of many of these changes with important implications for the clinical use of HSCs for marrow rescue and gene therapy, as well as furthering our understanding of the different pathogenesis of childhood and adult-onset leukemia.

Megakaryocytes promote murine osteoblastic HSC niche expansion and stem cell engraftment after radioablative conditioning

Successful hematopoietic stem cell (HSC) transplantation requires donor HSC engraftment within specialized bone marrow microenvironments known as HSC niches. We have previously reported a profound remodeling of the endosteal osteoblastic HSC niche after total body irradiation (TBI), defined as relocalization of surviving megakaryocytes to the niche site and marked expansion of endosteal osteoblasts. We now demonstrate that host megakaryocytes function critically in expansion of the endosteal niche after preparative radioablation and in the engraftment of donor HSC. We show that TBI-induced migration of megakaryocytes to the endosteal niche depends on thrombopoietin signaling through the c-MPL receptor on megakaryocytes, as well as CD41 integrin-mediated adhesion. Moreover, niche osteoblast proliferation post-TBI required megakaryocyte-secreted platelet-derived growth factor-BB. Furthermore, blockade of c-MPL-dependent megakaryocyte migration and function after TBI resulted in a significant decrease in donor HSC engraftment in primary and competitive secondary transplantation assays. Finally, we administered thrombopoietin to mice beginning 5 days before marrow radioablation and ending 24 hours before transplant to enhance megakaryocyte function post-TBI, and found that this strategy significantly enhanced donor HSC engraftment, providing a rationale for improving hematopoietic recovery and perhaps overall outcome after clinical HSC transplantation.

All hematopoietic stem cells engraft in submyeloablatively irradiated mice

Significant controversy exists regarding the impact of hematopoietic stroma damage by irradiation on the efficiency of engraftment of intravenously transplanted stem cells. It was previously demonstrated that in normal syngenic mice, all intravenously transplanted donor stem cells, present in the bone marrow, compete equally with those of the host. In this study, we comprehensively compared the blood cell production derived from transplanted donor stem cells with that from the host stem cells surviving various doses of submyeloablative irradiation. We compared the partial chimerism resulting from transplantation with theoretical estimates that assumed transplantation efficiencies ranging from 100% to 20%. The highest level of consensus between the experimental and the theoretical results was 100% for homing and engraftment (ie, the utilization of all transplanted stem cells). These results point to a very potent mechanism through which intravenously administered hematopoietic stem cells are captured from circulation, engraft in the hematopoietic tissue, and contribute to blood cell production in irradiated recipients. The damage done to hematopoietic stroma and to the trabecular bone by submyeloablative doses of ionizing radiation does not negatively affect the homing and engraftment mechanisms of intravenously transplanted hematopoietic progenitor and stem cells.

Hematopoietic stem cell subtypes expand differentially during development and display distinct lymphopoietic programs

Adult hematopoietic stem cells (HSCs) with serially transplantable activity comprise two subtypes. One shows a balanced output of mature lymphoid and myeloid cells; the other appears selectively lymphoid deficient. We now show that both of these HSC subtypes are present in the fetal liver (at a 1:10 ratio) with the rarer, lymphoid-deficient HSCs immediately gaining an increased representation in the fetal bone marrow, suggesting that the marrow niche plays a key role in regulating their ensuing preferential amplification. Clonal analysis of HSC expansion posttransplant showed that both subtypes display an extensive but variable self-renewal activity with occasional interconversion. Clonal analysis of their differentiation programs demonstrated functional and molecular as well as quantitative HSC subtype-specific differences in the lymphoid progenitors they generate but an indistinguishable production of multipotent and myeloid-restricted progenitors. These findings establish a level of heterogeneity in HSC differentiation and expansion control that may have relevance to stem cell populations in other hierarchically organized tissues.

Congenic interval of CD45/Ly-5 congenic mice contains multiple genes that may influence hematopoietic stem cell engraftment

The B6.SJL-Ptprc(d)Pep3(b)/BoyJ (B6.SJL) congenic mouse strain, a valuable and widely used tool in murine bone marrow transplantation studies, has long been considered equivalent to the parental C57B/L6 (B6) strain with the exception of a small congenic interval on chromosome 1 harboring an alternative CD45/Ly-5 alloantigen (Ly-5.1). In this study we compared functional properties of stem and stromal cells between the strains, and delineated the boundary of the B6.SJL congenic interval. We identified a 25% reduction in homing efficiency, 3.8-fold reduction in transplantable long-term hematopoietic stem cells (LT-HSCs), a 5-fold reduction in LT-HSCs capable of 24-hour homing, and a cell-intrinsic engraftment defect of 30% to 50% in B6.SJL-derived bone marrow cells relative to B6-derived cells. These functional differences were independent of stem cell number, cycling, or apoptosis. Genotypic analysis revealed a 42.1-mbp congenic interval in B6.SJL including 306 genes, and at least 124 genetic polymorphisms. Moreover, expression profiling revealed 288 genes differentially expressed between nonhematopoietic stromal cells of the 2 strains. These results indicate that polymorphisms between the B6 and SJL genotype within the B6.SJL congenic interval influence HSC engraftment and result in transcriptional variation within bone marrow stroma.

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.

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.

Steel factor coordinately regulates the molecular signature and biologic function of hematopoietic stem cells

Hematopoietic stem cells (HSCs) regenerated in vivo display sustained differences in their self-renewal and differentiation activities. Variations in Steel factor (SF) signaling are known to affect these functions in vitro, but the cellular and molecular mechanisms involved are not understood. To address these issues, we evaluated highly purified HSCs maintained in single-cell serum-free cultures containing 20 ng/mL IL-11 plus 1, 10, or 300 ng/mL SF. Under all conditions, more than 99% of the cells traversed a first cell cycle with similar kinetics. After 8 hours in the 10 or 300 ng/mL SF conditions, the frequency of HSCs remained unchanged. However, in the next 8 hours (ie, 6 hours before any cell divided), HSC integrity was sustained only in the 300 ng/mL SF cultures. The cells in these cultures also contained significantly higher levels of Bmi1, Lnk, and Ezh2 transcripts but not of several other regulators. Assessment of 21 first division progeny pairs further showed that only those generated in 300 ng/mL SF cultures contained HSCs and pairs of progeny with similar differentiation programs were not observed. Thus, SF signaling intensity can directly and coordinately alter the transcription factor profile and long-term repopulating ability of quiescent HSCs before their first division.

Hemopoietic stem cells with higher hemopoietic potential reside at the bone marrow endosteum

It is now evident that hemopoietic stem cells (HSC) are located in close proximity to bone lining cells within the endosteum. Accordingly, it is unlikely that the traditional method for harvesting bone marrow (BM) from mice by simply flushing long bones would result in optimal recovery of HSC. With this in mind, we have developed improved methodologies based on sequential grinding and enzymatic digestion of murine bone tissue to harvest higher numbers of BM cells and HSC from the endosteal and central marrow regions. This methodology resulted in up to a sixfold greater recovery of primitive hemopoietic cells (lineage(-)Sca(+)Kit(+) [LSK] cells) and HSC as shown by transplant studies. HSC from different anatomical regions of the marrow exhibited important functional differences. Compared with their central marrow counterparts, HSC isolated from the endosteal region (a) had 1.8-fold greater proliferative potential, (b) exhibited almost twofold greater ability to home to the BM following tail vein injection and to lodge in the endosteal region, and (c) demonstrated significantly greater long-term hemopoietic reconstitution potential as shown using limiting dilution competitive transplant assays.

Distinct roles of integrins alpha6 and alpha4 in homing of fetal liver hematopoietic stem and progenitor cells

Homing of hematopoietic stem cells (HSCs) into the bone marrow (BM) is a prerequisite for establishment of hematopoiesis during development and following transplantation. However, the molecular interactions that control homing of HSCs, in particular, of fetal HSCs, are not well understood. Herein, we studied the role of the alpha6 and alpha4 integrin receptors for homing and engraftment of fetal liver (FL) HSCs and hematopoietic progenitor cells (HPCs) to adult BM by using integrin alpha6 gene-deleted mice and function-blocking antibodies. Both integrins were ubiquitously expressed in FL Lin(-)Sca-1(+)Kit(+) (LSK) cells. Deletion of integrin alpha6 receptor or inhibition by a function-blocking antibody inhibited FL LSK cell adhesion to its extracellular ligands, laminins-411 and -511 in vitro, and significantly reduced homing of HPCs to BM. In contrast, the anti-integrin alpha6 antibody did not inhibit BM homing of HSCs. In agreement with this, integrin alpha6 gene-deleted FL HSCs did not display any homing or engraftment defect compared with wild-type littermates. In contrast, inhibition of integrin alpha4 receptor by a function-blocking antibody virtually abrogated homing of both FL HSCs and HPCs to BM, indicating distinct functions for integrin alpha6 and alpha4 receptors during homing of fetal HSCs and HPCs.

Male Germ Line Stem Cells Have an Altered Potential to Proliferate and Differentiate During Postnatal Development in Mice1

Spermatogonial stem cells (SSCs) continuously support spermatogenesis after puberty. However, accumulating evidence suggests that SSCs differ functionally during postnatal development. For example, mutant mice exist in which SSCs support spermatogenesis in the first wave after birth but cease to do so thereafter, resulting in infertility in adults. Studies using a retroviral vector have shown that the vector transduces pup SSCs more efficiently than adult SSCs, which suggests that pup SSCs divide more frequently. Thus, it is hypothesized that the SSCs in pup and adult testes have different characteristics. As an approach to testing this hypothesis in the present study, we investigated the proliferation kinetics of pup SSCs (6-9 days old) and their self-renewal/differentiation patterns for the first 2 mo after transplantation, and compared them to those of adult SSCs. Using serial transplantation, we found that the number of pup SSCs declined over the first week after transplantation. Thereafter, it increased ~4-fold by 1 mo and ~9-fold by 2 mo after transplantation, which indicates that pup SSCs continuously proliferate from 1 wk to 2 mo after transplantation. Compared to the proliferation of SSCs derived from adult intact testes, that of pup SSCs was lower at 1 mo but similar at 2 mo, indicating the delayed proliferation of pup SSCs. However, the pup SSCs regenerated spermatogenic colonies at 1 mo that were similar in length to those of SSCs from adult intact testes. Therefore, these results suggest that some functional differences exist in SSCs during postnatal development, and that these differences may affect the abilities of SSCs to self-renew and differentiate.

Identification of a new intrinsically timed developmental checkpoint that reprograms key hematopoietic stem cell properties

Hematopoietic stem cells (HSCs) execute self-renewal divisions throughout fetal and adult life, although some of their properties do alter. Here we analyzed the magnitude and timing of changes in the self-renewal properties and differentiated cell outputs of transplanted HSCs obtained from different sources during development. We also assessed the expression of several "stem cell" genes in corresponding populations of highly purified HSCs. Fetal and adult HSCs displayed marked differences in their self-renewal, differentiated cell output, and gene expression properties, with persistence of a fetal phenotype until 3 weeks after birth. Then, 1 week later, the HSCs became functionally indistinguishable from adult HSCs. The same schedule of changes in HSC properties occurred when HSCs from fetal or 3-week-old donors were transplanted into adult recipients. These findings point to the existence of a previously unrecognized, intrinsically regulated master switch that effects a developmental change in key HSC properties.

Hematopoietic stem cells proliferate until after birth and show a reversible phase-specific engraftment defect

The regulation of HSC proliferation and engraftment of the BM is an important but poorly understood process, particularly during ontogeny. Here we show that in mice, all HSCs are cycling until 3 weeks after birth. Then, within 1 week, most became quiescent. Prior to 4 weeks of age, the proliferating HSCs with long-term multilineage repopulating activity displayed an engraftment defect when transiting S/G2/M. During these cell cycle phases, their expression of CXC chemokine ligand 12 (CXCL12; also referred to as stromal cell-derived factor 1 [SDF-1]) transiently increased. The defective engrafting activity of HSCs in S/G2/M was reversed when cells were allowed to progress into G1 prior to injection or when the hosts (but not the cells) were pretreated with a CXCL12 antagonist. Interestingly, the enhancing effect of CXCL12 antagonist pretreatment was exclusive to transplants of long-term multilineage repopulating HSCs in S/G2/M. These results demonstrate what we believe to be a new HSC regulatory checkpoint during development. They also suggest an ability of HSCs to express CXCL12 in a fashion that changes with cell cycle progression and is associated with a defective engraftment that can be overcome by in vivo administration of a CXCL12 antagonist.

Impact of sex and age on bone marrow immune responses in a murine model of trauma-hemorrhage

Although studies have demonstrated that trauma markedly alters the bone marrow immune responses, sex and age are crucial determinants under such conditions and have not been extensively examined. To study this, 21- to 27-day-old (premature), 6- to 8-wk-old (mature), and 20- to 24-mo-old (aged) male and female (proestrus) C3H/HeN mice were sham operated or subjected to trauma (i.e., midline laparotomy) and hemorrhagic shock (30 ± 5 mmHg for 90 min) followed by fluid resuscitation. Twenty-four hours after resuscitation, bone marrow cells were harvested. Trauma-hemorrhage induced an increased number of the early pluripotent stem cell-associated bone marrow cell subsets (Sca1+CD34−CD117+/−lin+/−) in young mice. The CD117+proportion of these cell subsets increased in mature proestrus females, but not in males. Aged males displayed significant lower numbers of Sca1+CD34−CD117+/−lin+/−cells compared with young male mice. Trauma-hemorrhage also increased development of granulocyte/macrophage progenitor cells (CD11b+Gr-1+). Proliferative responses to granulocyte macrophage colony-stimulating factor were maintained in mature and aged proestrus females, but decreased in young mice and mature males. Augmented differentiation into monocyte/macrophage lineage in mature and aged proestrus females was observed and associated with the maintained release of TNF-α and IL-6. Conversely, increased IL-10 and PGE2production was observed in the male trauma-hemorrhage groups. Thus, sex- and age-specific effects in bone marrow differentiation and immune responses after trauma-hemorrhage occur, which are likely to contribute to the sex- and age-related differences in the systemic immune responses under such conditions.

Identification and characterization of leukemia stem cells in murine MLL-AF9 acute myeloid leukemia

Using a mouse model of human acute myeloid leukemia (AML) induced by the MLL-AF9 oncogene, we demonstrate that colony-forming cells (CFCs) in the bone marrow and spleen of leukemic mice are also leukemia stem cells (LSCs). These self-renewing cells (1) are frequent, accounting for 25%-30% of myeloid lineage cells at late-stage disease; (2) generate a phenotypic, morphologic, and functional leukemia cell hierarchy; (3) express mature myeloid lineage-specific antigens; and (4) exhibit altered microenvironmental interactions by comparison with the oncogene-immortalized CFCs that initiated the disease. Therefore, the LSCs responsible for sustaining, expanding, and regenerating MLL-AF9 AML are downstream myeloid lineage cells, which have acquired an aberrant Hox-associated self-renewal program as well as other biologic features of hematopoietic stem cells.

Murine serum obtained from bone marrow-transplanted mice promotes the proliferation of hematopoietic stem cells by co-culture with MS-5 murine stromal cells

To examine whether serum obtained from bone marrow-transplanted mice can selectively expand hematopoietic stem cells (HSCs) among whole bone marrow cells in vitro, whole bone marrow cells were cultured with or without MS-5 murine stromal cells in the presence of serum obtained from transplanted mice on day 3 (day 3 serum) or serum from normal mice for 7 days. When whole bone marrow cells and MS-5 cells were co-cultured in day 3 serum for 7 days, the c-kit-positive, Sca-1-positive, lineage marker-negative cells (KSL cells) expanded approximately 25 times; however, when they were co-cultured in normal serum for 7 days, the KSL cells expanded approximately 1.3 times. Direct contact between the whole bone marrow cells and MS-5 cells was essential for expansion of KSL cells in the co-culture, and it upregulated the expression of some cytokines in MS-5. Above all, the day 3 serum specifically upregulated the expression of SCF, SDF-1 alpha, G-CSF, IL-11 and IL-6 in MS-5. The level of testosterone in the day 3 serum was higher than normal serum and the addition of the testosterone in the culture expanded the KSL cells among whole bone marrow cells on MS-5 cells and also upregulated the expression of SDF-1 alpha, IL-11 and IL-6 in MS-5. These data indicates that the serum of bone marrow-transplanted mice contains a factor(s) that induced changes in the expression levels of various cytokines in MS-5 stromal cells and enabled the MS-5 cells to expand the KSL cells among whole bone marrow cells.

Long-term bovine hematopoietic engraftment with clone-derived stem cells

Therapeutic cloning by somatic cell nuclear transfer offers potential for treatment of a wide range of degenerative disease. Nuclear transplantation with neo (r)-marked somatic nuclei from 10-13-year-old cows was used to generate cloned bovine fetuses. Clone fetal liver (FL) hematopoietic stem cells (HSC) were transplanted into two busulfan-treated and one untreated nuclear donor cows. Hematopoiesis was monitored over 13-16 months by in vitro progenitor and HSC assays. Chimerism was demonstrated by PCR in blood, marrow, lymph nodes, and endothelium, peaking at levels of 9-17% in blood granulocytes but at lower levels in lymphocyte subsets (0.1-0.01%). Circulating progenitors showed high levels of chimerism (up to 60% neo (r+)) with persisting fetal features. At sacrifice, the animal that had no pre-transplant myelosupression showed persisting donor cells in blood and lymph nodes, and in marrow 0.25% of progenitor cells and a detectable fraction of stem cells were neo (r+). The fetal HSC showed a 10-fold competition advantage over adult HSC. Cloning generated histocompatible HSC capable of long-term multilineage engraftment in a large animal model.

Effects of aging on the homing and engraftment of murine hematopoietic stem and progenitor cells

To test the hypothesis that aging has negative effects on stem-cell homing and engraftment, young or old C57BL/6 bone marrow (BM) cells were injected, using a limiting-dilution, competitive transplantation method, into old or young Ly5 congenic mice. Numbers of hematopoietic stem cells (HSCs) and progenitor cells (HPCs) recovered from BM or spleen were measured and compared with the numbers initially transplanted. Although the frequency of marrow competitive repopulation units (CRUs) increased approximately 2-fold from 2 months to 2 years of age, the BM homing efficiency of old CRUs was approximately 3-fold lower than that of young CRUs. Surprisingly, the overall size of individual stem-cell clones generated in recipients receiving a single CRU was not affected by donor age. However, the increased ages of HSC donors and HSC transplant recipients caused marked skewing of the pattern of engraftment toward the myeloid lineage, indicating that HSC-intrinsic and HSC-extrinsic (microenvironmental) age-related changes favor myelopoiesis. This correlated with changes after transplantation in the rate of recovery of circulating leukocytes, erythrocytes, and platelets. Recovery of the latter was especially blunted in aged recipients. Collectively, these findings may have implications for clinical HSC transplantation in which older persons increasingly serve as donors for elderly patients.

Degeneration of stroma reduces retention of homed cells in bone marrow of lethally irradiated mice

Cytotoxic drugs or irradiation are generally administered before bone marrow (BM) transplantation because of the idea that host bone marrow 'niches' become available to the donor cells for engraftment. How BM stromal cells respond to the radiation, which ultimately modulates grafting of donor cells, is poorly understood. In this study, we examined homing and marrow retention of PKH26+ donor cells in BM of age-matched C57BL/6J mice conditioned at different doses of irradiation. When we injected donor cells into mice that received 900 cGy, the percent homing was highest (15.8 +/- 1.5%) as compared to the lower doses of radiation. Despite the highest levels of homing of donor cells in these mice, about 70% (p < 0.005) homed cells were detached from the marrow within 72 h of transplantation. In contrast, a 2- to 2.5-fold (p < 0.03) multiplication of homed PKH-26+ Sca-1+ cells was observed in sublethally irradiated mice. While determining that CD45- CD106+ cells in BM of the mice received 900 cGy, we found that more than 80% of cells were depleted. It was also revealed from this investigation that grafted cells conferred partial protection to the endogenous myeloid colony-forming cells from radiation injury. Collectively, the present study implicates radiation-induced degeneration of stroma as a cause of poor retention of donor cells in BM of lethally irradiated mice. These results may have important clinical implications in designing conditioning regimens for BM transplantation.

Identification of hematopoietic stem/progenitor cells: strength and drawbacks of functional assays

A major challenge in hematopoiesis is to conceive assays that could bring useful insights into experimental and clinical hematology. This means identifying separately the various classes of hematopoietic progenitors that are produced sequentially during the progression from stem cells to differentiated functional cells. Standardized short-term colony assays easily quantify lineage-committed myeloid precursors, but identification of primitive cells, which have both the ability to repopulate durably myeloid and lymphoid lineages and perhaps to self-renew, still depends on in vivo assays. Whatever the assay, two important requisites have to be solved: one is the definition of appropriate read-outs that will depend solely on the function of these cells, and the second is to evaluate precisely their numbers and proliferative potential in quantitative assays. When evaluating hematopoiesis, three parameters have to be taken into account: (1) the lack of reliable correlation between the phenotype of a given cell and its function. This is especially problematic in post-transplantation situations where cells from transplanted animals are analysed; (2) functionally heterogeneous cells are identified in a single assay; and (3) ontogeny-related changes in hematopoietic cell proliferation and self-renewal that, in human beings, hampers the exploration of adult stem cells. Nevertheless, years of progress in the manipulation of hematopoietic stem cells have recently resulted in the purification of a cell subset that repopulates irradiated recipients with absolute efficiency.

Identification and isolation of hematopoietic stem cells

Hematopoietic stem cells (HSCs) are defined by their ability to repopulate all of the hematopoietic lineages in vivo and sustain the production of these cells for the life span of the individual. In the absence of reliable direct markers for HSCs, their identification and enumeration depends on functional long-term, multilineage, in vivo repopulation assays. The extremely low frequency of HSCs in any tissue and the absence of a specific HSC phenotype have made their purification and characterization a highly challenging goal. HSCs and primitive hematopoietic cells can be distinguished from mature blood cells by their lack of lineage-specific markers and presence of certain other cell-surface antigens, such as CD133 (for human cells) and c-kit and Sca-1 (for murine cells). Functional analyses of purified subpopulations of primitive hematopoietic cells have led to the development of several procedures for isolating cell populations that are highly enriched in cells with in vivo stem cell activity. Simplified methods for obtaining these cells at high yield have been important to the practical exploitation of such advances. This article reviews recent progress in identifying human and mouse HSCs and current techniques for their purification.

The biology of hematopoietic stem cells

Rarely has so much interest from the lay public, government, biotechnology industry, and special interest groups been focused on the biology and clinical applications of a single type of human cell as is today on stem cells, the founder cells that sustain many, if not all, tissues and organs in the body. Granting organizations have increasingly targeted stem cells as high priority for funding, and it appears clear that the evolving field of tissue engineering and regenerative medicine will require as its underpinning a thorough understanding of the molecular regulation of stem cell proliferation, differentiation, self-renewal, and aging. Despite evidence suggesting that embryonic stem (ES) cells might represent a more potent regenerative reservoir than stem cells collected from adult tissues, ethical considerations have redirected attention upon primitive cells residing in the bone marrow, blood, brain, liver, muscle, and skin, from where they can be harvested with relative sociological impunity. Among these, it is arguably the stem and progenitor cells of the mammalian hematopoietic system that we know most about today, and their intense study in rodents and humans over the past 50 years has culminated in the identification of phenotypic and molecular genetic markers of lineage commitment and the development of functional assays that facilitate their quantitation and prospective isolation. This review focuses exclusively on the biology of hematopoietic stem cells (HSCs) and their immediate progeny. Nevertheless, many of the concepts established from their study can be considered fundamental tenets of an evolving stem cell paradigm applicable to many regenerating cellular systems.

A mouse model to study organ homing behaviour of haemopoietic progenitor cells reveals high selectivity but low efficiency of multipotent progenitors to home into haemopoietic organs

To study the homing behaviour of an enriched multipotent primitive haemopoietic progenitor cell (HPC) population in mice, undifferentiated murine factor-dependent multipotent HPCs (FDCP-mix), stably transfected with the green fluorescence protein gene, were intravenously injected into congenic mice. After 2 or 24 h, cell suspensions were prepared from bone marrow, spleen, lung, liver, muscle, colon, kidney, brain or blood of the mice and analysed by flow cytometry. Using direct quantifiable determination of total HPC numbers homed per organ and a method to estimate the degree of organ contamination by HPC that were present in blood vessels within the organs before preparation, the highest absolute numbers of HPC were detected in the liver and lungs at 2 h but this was sharply decreased at 24 h, whereas HPC selectively accumulated in the bone marrow and spleen at 24 h after transplantation. Only a few HPC were detected in other organs. The seeding efficiency of homed FDCP-mix HPC to the bone marrow and spleen was approximately 1.5% and ranged between that of primary whole bone marrow cells and lineage-depleted freshly isolated bone marrow cells. Pretreatment of HPC with inhibitors of signal transduction indicated that short-term homing of multipotent HPC into haemopoietic organs is an active process requiring co-ordinated intracellular signalling through Rho family small GTPases and protein kinases. Thus, short-term homing of FDCP-mix HPC into haemopoietic organs is of low efficiency but high selectivity, and provides a system to analyse the mechanisms and manipulation of primitive HPC which saves large numbers of donor animals.

Germline stem cells and follicular renewal in the postnatal mammalian ovary

A basic doctrine of reproductive biology is that most mammalian females lose the capacity for germ-cell renewal during fetal life, such that a fixed reserve of germ cells (oocytes) enclosed within follicles is endowed at birth. Here we show that juvenile and adult mouse ovaries possess mitotically active germ cells that, based on rates of oocyte degeneration (atresia) and clearance, are needed to continuously replenish the follicle pool. Consistent with this, treatment of prepubertal female mice with the mitotic germ-cell toxicant busulphan eliminates the primordial follicle reserve by early adulthood without inducing atresia. Furthermore, we demonstrate cells expressing the meiotic entry marker synaptonemal complex protein 3 in juvenile and adult mouse ovaries. Wild-type ovaries grafted into transgenic female mice with ubiquitous expression of green fluorescent protein (GFP) become infiltrated with GFP-positive germ cells that form follicles. Collectively, these data establish the existence of proliferative germ cells that sustain oocyte and follicle production in the postnatal mammalian ovary.

Transplantable stem cells: home to specific niches

PURPOSE OF REVIEW

Although the concept of engraftment and clinical reconstitution of the bone marrow was described several decades ago, the analysis of individual steps within this process remains a major focus of much current research in stem cell biology. In particular, this extends to the identification and characterization of the specific stem cell niche first proposed by Schofield in 1978. It is appropriate, therefore, that on the 25th anniversary of this publication, that we review recent progress in our understanding of the location and composition of the bone marrow stem cell niche and of the mechanisms involved in the initial phases of hematopoietic stem cell engraftment.

RECENT FINDINGS

During the past 12 months there have been significant advancements in our understanding of the interplay of molecules involved in the homing of hematopoietic stem cells to the bone marrow. In addition, innovative methodologies have become available for the visualization of hematopoietic stem cells within the bone marrow in situ. In an important development in this area, studies our now focusing on events after transendothelial migration into the marrow cords, including mechanisms involved in hematopoietic stem cell migration to and lodgment within the hematopoietic stem cell niche. Furthermore, there have been numerous new reports analyzing the molecular regulation of hematopoietic stem cells within the bone marrow niche in situ.

SUMMARY

Overall, recent advancements in our understanding of hematopoietic stem cell biology and, in particular, the interaction of hematopoietic stem cells with the hematopoietic microenvironment paves the way for expanded use in regenerative medicine.

Different in vivo repopulating activities of purified hematopoietic stem cells before and after being stimulated to divide in vitro with the same kinetics

OBJECTIVE

The Hoechst 33342-effluxing side population (SP) of adult mouse bone marrow (BM) contains most of the hematopoietic stem cells (HSCs). Here we measured the HSC content of specific subsets of SP cells and then used a highly HSC-enriched fraction to investigate the effect of different growth factors on the initial rate of HSC proliferation in vitro and the accompanying maintenance (or loss) of HSCs in the first-division progeny.

MATERIALS AND METHODS

Staining with Rhodamine-123 (Rho) was used to subfractionate lineage marker-negative (lin-) SP cells. Cells were assayed for HSCs by examining their ability to generate sustained (>4 months) multi-lineage lympho-myeloid clones in irradiated hosts. Cultures of single lin- Rho- SP cells were used to monitor growth factor effects on HSC proliferation and function.

RESULTS

More than 40% of mice injected with single lin- Rho- SP cells showed long-term lympho-myeloid reconstitution. Some clones peaked within 8 weeks but others developed more slowly apparently unrelated to the pattern of lineage representation. 3/3 clones tested repopulated secondary mice. Either Steel factor+interleukin-11 (+/- flt3-ligand) or Steel factor+thrombopoietin stimulated at least 75% of single lin- Rho- SP cells to divide in vitro with the same synchronous kinetics. However, in the first cocktail, the frequency of HSCs among the first-division doublets was preserved but in the latter it was greatly diminished.

CONCLUSION

Exogenous growth factors can differentially affect the ability of HSCs to execute a self-renewal division within a single cell cycle even when the kinetics of proliferation are the same.

The EndoglinPositive Sca-1Positive RhodamineLow Phenotype Defines a Near-Homogeneous Population of Long-Term Repopulating Hematopoietic Stem Cells

Endoglin, an ancillary TGF-beta receptor, is differentially expressed in long-term repopulating hematopoietic stem cells (LTR-HSC). Here, we describe simple and highly efficient purification schemes for mouse bone marrow LTR-HSCs using Endoglin as a marker. The Endoglin positive and Sca-1 positive (Endo(Pos) Sca-1(Pos)) population, which contains about 36% of "Side Population" (SP) cells, is highly enriched for LTR-HSCs. In long-term competitive reconstitution assays, 100 such cells reconstituted all lethally irradiated recipients. Interestingly, the Endo(Pos) Sca-1(Pos) population contains comparable LTR-HSC activity in both SP and non-SP fractions, indicating that many HSCs are not captured by the SP phenotype. Furthermore, LTR-HSCs are exclusively found in the Endo(Pos) Sca-1(Pos) Lin(Neg/Low) (lineage negative/low), but not in the Endo(Neg) Sca-1(Pos) Lin(Neg/Low) population, suggesting that the Endo(Pos) population may contain all LTR-HSCs in mouse bone marrow. Finally, we demonstrated that the Endo(Pos) Sca-1(Pos) Rh(Low) (Rhodamine-123 low) phenotype, without using CD34, c-Kit, or Lineage markers, defines a nearly homogenous population of LTR-HSCs.