Potential and Distribution of Transplanted Hematopoietic Stem Cells in a Nonablated Mouse Model

Spatial mapping of human hematopoiesis at single-cell resolution reveals aging-associated topographic remodeling

ABSTRACT

The spatial anatomy of hematopoiesis in the bone marrow (BM) has been extensively studied in mice and other preclinical models, but technical challenges have precluded a commensurate exploration in humans. Institutional pathology archives contain thousands of paraffinized BM core biopsy tissue specimens, providing a rich resource for studying the intact human BM topography in a variety of physiologic states. Thus, we developed an end-to-end pipeline involving multiparameter whole tissue staining, in situ imaging at single-cell resolution, and artificial intelligence-based digital whole slide image analysis and then applied it to a cohort of disease-free samples to survey alterations in the hematopoietic topography associated with aging. Our data indicate heterogeneity in marrow adipose tissue (MAT) content within each age group and an inverse correlation between MAT content and proportions of early myeloid and erythroid precursors, irrespective of age. We identify consistent endosteal and perivascular positioning of hematopoietic stem and progenitor cells (HSPCs) with medullary localization of more differentiated elements and, importantly, uncover new evidence of aging-associated changes in cellular and vascular morphologies, microarchitectural alterations suggestive of foci with increased lymphocytes, and diminution of a potentially active megakaryocytic niche. Overall, our findings suggest that there is topographic remodeling of human hematopoiesis associated with aging. More generally, we demonstrate the potential to deeply unravel the spatial biology of normal and pathologic human BM states using intact archival tissue specimens.

Mobilization-based chemotherapy-free engraftment of gene-edited human hematopoietic stem cells

Hematopoietic stem/progenitor cell gene therapy (HSPC-GT) is proving successful to treat several genetic diseases. HSPCs are mobilized, harvested, genetically corrected ex vivo, and infused, after the administration of toxic myeloablative conditioning to deplete the bone marrow (BM) for the modified cells. We show that mobilizers create an opportunity for seamless engraftment of exogenous cells, which effectively outcompete those mobilized, to repopulate the depleted BM. The competitive advantage results from the rescue during ex vivo culture of a detrimental impact of mobilization on HSPCs and can be further enhanced by the transient overexpression of engraftment effectors exploiting optimized mRNA-based delivery. We show the therapeutic efficacy in a mouse model of hyper IgM syndrome and further developed it in human hematochimeric mice, showing its applicability and versatility when coupled with gene transfer and editing strategies. Overall, our findings provide a potentially valuable strategy paving the way to broader and safer use of HSPC-GT.

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HSPC mobilizers create an opportunity to engraft exogenous cells in depleted niches

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Ex vivo culture endows HSPCs with migration advantage by rescuing CXCR4 expression

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Cultured HSPCs outcompete mobilized HSPCs for engraftment in depleted BM niches

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Transient engraftment enhancers coupled with gene editing confer a competitive advantage

Insights Into Bone Marrow Niche Stability: An Adhesion and Metabolism Route

The bone marrow microenvironment provides critical cues for hematopoietic stem cell (HSC) self-renewal and differentiation and contributes to their malignant conversion. The microenvironment comprises a complex mixture of multiple cell types, soluble factors, and extracellular matrix in specialized regions termed 'niches.' Positioning of the various cellular players within these niches depends on their repertoire of adhesion molecules and chemotactic signaling, involving integrins and chemokine receptors and the corresponding intracellular players such as kinases and GTPases. The mechanical role of adhesion is to control the strength and morphology of the cell-cell and cell-extracellular matrix contacts and thereby the energy needed for the optimal localization of cells to their surroundings. While it is clear that biomechanical adhesive bonds are energetically expensive, the crosstalk between cell adhesion and metabolic pathways in the normal and malignant microenvironment is far from understood. The metabolic profile of the various cell types within the niche includes key molecules such as AMPK, glucose, mTOR, and HIF-1α. Here, we describe our most recent understanding of how the interplay between adhesion and these metabolic components is indispensable for bone marrow niche stability. In parallel, we compare the altered crosstalk of different cell types within the bone marrow niches in hematological malignancies and propose potential therapeutic associations.

Prostacyclin is an endosteal bone marrow niche component and its clinical analog iloprost protects hematopoietic stem cell potential during stress

Hematopoietic stem cells (HSCs) with superior reconstitution potential are reported to be enriched in the endosteal compared to central bone marrow (BM) region. To investigate whether specific factors at the endosteum may contribute to HSC potency, we screened for candidate HSC niche factors enriched in the endosteal compared to central BM regions. Together with key known HSC supporting factors Kitl and Cxcl12, we report that prostacyclin/prostaglandin I₂ (PGI₂ ) synthase (Ptgis) was one of the most highly enriched mRNAs (>10-fold) in endosteal compared to central BM. As PGI₂ signals through receptors distinct from prostaglandin E₂ (PGE₂ ), we investigated functional roles for PGI₂ at the endosteal niche using therapeutic PGI₂ analogs, iloprost, and cicaprost. We found PGI₂ analogs strongly reduced HSC differentiation in vitro. Ex vivo iloprost pulse treatment also significantly boosted long-term competitive repopulation (LT-CR) potential of HSCs upon transplantation. This was associated with increased tyrosine-phosphorylation of transducer and activator of transcription-3 (STAT3) signaling in HSCs but not altered cell cycling. In vivo, iloprost administration protected BM HSC potential from radiation or granulocyte colony-stimulating factor-induced exhaustion, and restored HSC homing potential with increased Kitl and Cxcl12 transcription in the BM. In conclusion, we propose that PGI₂ is a novel HSC regulator enriched in the endosteum that promotes HSC regenerative potential following stress.

Uncoupling key determinants of hematopoietic stem cell engraftment through cell-specific and temporally controlled recipient conditioning

Hematopoietic stem cells (HSCs) are typically characterized by transplantation into irradiated hosts in a highly perturbed microenvironment. Here, we show that selective and temporally controlled depletion of resident HSCs through genetic deletion of Gata2 constitutes efficient recipient conditioning for transplantation without irradiation. Strikingly, we achieved robust engraftment of donor HSCs even when delaying Gata2 deletion until 4 weeks after transplantation, allowing homing and early localization to occur in a completely non-perturbed environment. When HSCs from the congenic strains Ly5.1 and Ly5.2 were competitively transplanted, we found that the more proliferative state of Ly5.2 HSCs was associated with superior long-term engraftment when using conditioning by standard irradiation, while higher CXCR4 expression and a better homing ability of Ly5.1 HSCs strongly favored the outcome in our inducible HSC depletion model. Thus, the mode and timing of recipient conditioning challenges distinct functional features of transplanted HSCs.

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Inducible gene deletion of Gata2 rapidly and selectively depletes the HSC pool

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Gata2 deletion constitutes efficient recipient conditioning for HSC transplantation

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The model enables detection of HSC engraftment in a non-perturbed microenvironment

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Transplantation without irradiation uniquely challenges homing properties of HSCs

Murine models of clonal hematopoiesis to assess mechanisms of cardiovascular disease

Clonal hematopoiesis (CH) is a phenomenon whereby somatic mutations confer a fitness advantage to hematopoietic stem and progenitor cells (HSPC) and thus facilitate their aberrant clonal expansion. These mutations are carried into progeny leukocytes leading to a situation whereby a substantial fraction of an individual's blood cells originate from the HSPC mutant clone. Although this condition rarely progresses to a hematological malignancy, circulating blood cells bearing the mutation have the potential to affect other organ systems as they infiltrate into tissues under both homeostatic and disease conditions. Epidemiological and clinical studies have revealed that CH is highly prevalent in the elderly and is associated with an increased risk of cardiovascular disease and mortality. Recent experimental studies in murine models have assessed the most commonly mutated "driver" genes associated with CH, and have provided evidence for mechanistic connections between CH and cardiovascular disease. A deeper understanding of the mechanisms by which specific CH mutations promote disease pathogenesis is of importance, as it could pave the way for individualized therapeutic strategies targeting the pathogenic CH gene mutations in the future. Here, we review the epidemiology of CH and the mechanistic work from studies using murine disease models, with a particular focus on the strengths and limitations of these experimental systems. We intend for this review to help investigators select the most appropriate models to study CH in the setting of cardiovascular disease.

Role of ex vivo Expanded Mesenchymal Stromal Cells in Determining Hematopoietic Stem Cell Transplantation Outcome

Overall, the human organism requires the production of ∼1 trillion new blood cells per day. Such goal is achieved via hematopoiesis occurring within the bone marrow (BM) under the tight regulation of hematopoietic stem and progenitor cell (HSPC) homeostasis made by the BM microenvironment. The BM niche is defined by the close interactions of HSPCs and non-hematopoietic cells of different origin, which control the maintenance of HSPCs and orchestrate hematopoiesis in response to the body’s requirements. The activity of the BM niche is regulated by specific signaling pathways in physiological conditions and in case of stress, including the one induced by the HSPC transplantation (HSCT) procedures. HSCT is the curative option for several hematological and non-hematological diseases, despite being associated with early and late complications, mainly due to a low level of HSPC engraftment, impaired hematopoietic recovery, immune-mediated graft rejection, and graft-versus-host disease (GvHD) in case of allogenic transplant. Mesenchymal stromal cells (MSCs) are key elements of the BM niche, regulating HSPC homeostasis by direct contact and secreting several paracrine factors. In this review, we will explore the several mechanisms through which MSCs impact on the supportive activity of the BM niche and regulate HSPC homeostasis. We will further discuss how the growing understanding of such mechanisms have impacted, under a clinical point of view, on the transplantation field. In more recent years, these results have instructed the design of clinical trials to ameliorate the outcome of HSCT, especially in the allogenic setting, and when low doses of HSPCs were available for transplantation.

Human bone marrow-derived stromal cell behavior when injected directly into the bone marrow of NOD-scid-gamma mice pre-conditioned with sub-lethal irradiation

BACKGROUND

Direct bone marrow injection of cells into murine marrow cavities is used in a range of cell characterization assays and to develop disease models. While human bone marrow-derived stromal cells (hBMSC, also known as mesenchymal stem cells (MSC)) are frequently described in therapeutic applications, or disease modeling, their behavior following direct injection into murine bone marrow is poorly characterized. Herein, we characterized hBMSC engraftment and persistence within the bone marrow of NOD-scid interleukin (IL)-2γ^-/- (NSG) mice with or without prior 2 Gy total-body γ-irradiation of recipient mice.

METHODS

One day after conditioning NSG mice with sublethal irradiation, 5 × 10⁵ luciferase (Luc) and green fluorescent protein (GFP)-expressing hBMSC (hBMSC-Luc/GFP) were injected into the right femurs of animals. hBMSC-Luc/GFP were tracked in live animals using IVIS imaging, and histology was used to further characterize hBMSC location and behavior in tissues.

RESULTS

hBMSC-Luc/GFP number within injected marrow cavities declined rapidly over 4 weeks, but prior irradiation of animals delayed this decline. At 4 weeks, hBMSC-Luc/GFP colonized injected marrow cavities and distal marrow cavities at rates of 2.5 ± 2.2% and 1.7 ± 1.9% of total marrow nucleated cells, respectively in both irradiated and non-irradiated mice. In distal marrow cavities,  hBMSC were not uniformly distributed and appeared to be co-localized in clusters, with the majority found in the endosteal region.

CONCLUSIONS

While significant numbers of hBMSC-Luc/GFP could be deposited into the mouse bone marrow via direct bone marrow injection, IVIS imaging indicated that the number of hBMSC-Luc/GFP in that bone marrow cavity declined with time. Irradiation of mice prior to transplant only delayed the rate of hBMSC-Luc/GFP population decline in injected femurs. Clusters of hBMSC-Luc/GFP were observed in the histology of distal marrow cavities, suggesting that some transplanted cells actively homed to distal marrow cavities. Individual cell clusters may have arisen from discrete clones that homed to the marrow, and then underwent modest proliferation. The transient high-density population of hBMSC within the injected femur, or the longer-term low-density population of hBMSC in distal marrow cavities, offers useful models for studying disease or regenerative processes. Experimental designs should consider how relative hBMSC distribution and local hBMSC densities evolve over time.

Limited rejuvenation of aged hematopoietic stem cells in young bone marrow niche

Hematopoietic stem cells (HSCs) exhibit functional alterations, such as reduced regenerative capacity and myeloid-biased differentiation, with age. The HSC niche, which is essential for the maintenance of HSCs, also undergoes marked changes with aging. However, it has been technically challenging to directly evaluate the contribution of niche aging to age-associated HSC alterations without niche-damaging myeloablation in HSC transplantation assays. We herein transplanted an excess of aged HSCs into young mice without preconditioning. Although aged HSCs successfully engrafted in the intact young bone marrow niche, they poorly regenerated downstream progenitors and exhibited persistent myeloid-biased differentiation, resulting in no significant functional rejuvenation. Transcriptome and methylome analyses revealed that the young niche largely restored the transcriptional profile of aged HSCs, but not their DNA methylation profiles. Therefore, the restoration of the young niche is insufficient for rejuvenating HSC functions, highlighting a key role for age-associated cell-intrinsic defects in HSC aging.

New Insights on the Role of the Mesenchymal-Hematopoietic Stem Cell Axis in Autologous and Allogeneic Hematopoiesis

Cytoreductive protocols are integral both as conditioning regimens for bone marrow (BM) transplantation and as part of therapies for malignancies, but their associated comorbidities represent a long-standing clinical problem. In particular, they cause myeloablation that debilitates the physiological role of mesenchymal stem and precursor cells (MSPCs) in sustaining hematopoiesis. This review addresses the damaging impact of cytoreductive regimens on MSPCs. In addition, it discusses prospects for alleviating the resulting iatrogenic comorbidities. New insights into the structural and functional dynamics of hematopoietic stem cell (HSC) niches reveal the existence of "empty" niches and the ability of the donor-derived healthy HSCs to outcompete the defective HSCs in occupying these niches. These findings support the notion that conditioning regimens, conventionally used to ablate the recipient hematopoiesis to create space for engraftment of the donor-derived HSCs, may not be a necessity for allogeneic BM transplantation. In addition, the capacity of the MSPCs to cross-talk with HSCs, despite major histocompatibility complex disparity, and suppress graft versus host disease indicates the possibility for development of a conditioning-free, MSPCs-enhanced protocol for BM transplantation. The clinical advantage of supplementing cytoreductive protocols with MSPCs to improve autologous hematopoiesis reconstitution and alleviate cytopenia associated with chemo and radiation therapies for cancer is also discussed.

Low-Dose Busulfan Reduces Human CD34+ Cell Doses Required for Engraftment in c-kit Mutant Immunodeficient Mice

Humanized animal models are central to efforts aimed at improving hematopoietic stem cell (HSC) transplantation with or without genetic modification. Human cell engraftment is feasible in immunodeficient mice; however, high HSC doses and conditioning limit broad use of xenograft models. We assessed human CD45⁺ chimerism after transplanting varying doses of human CD34⁺ HSCs (2 × 10⁵ to 2 × 10⁶ cells/mouse) with or without busulfan (BU) pretransplant conditioning in c-kit mutant mice that do not require conditioning (non-obese diabetic [NOD]/B6/severe combined immunodeficiency [SCID]/ interleukin-2 receptor gamma chain null (IL-2rγ^-/-) Kit^W41/W41 [NBSGW]). We then tested a range of BU (5-37.5 mg/kg) using 2 × 10⁵ human CD34⁺ cells. Glycophorin-A erythrocyte chimerism was assessed after murine macrophage depletion using clodronate liposomes. We demonstrated successful long-term engraftment of human CD34⁺ cells at all cell doses in this model, and equivalent engraftment using 10-fold less CD34⁺ cells with the addition of BU conditioning. Low-dose BU (10 mg/kg) was sufficient to allow human engraftment using 2 × 10⁵ CD34⁺ cells, whereas higher doses (≥37.5 mg/kg) were toxic. NBSGW mice support human erythropoiesis in the bone marrow; however, murine macrophage depletion provided only minimal and transient increases in peripheral blood human erythrocytes. Our xenograft model is therefore useful in HSC gene therapy and genome-editing studies, especially for modeling in disorders, such as sickle cell disease, where access to HSCs is limited.

Antibody Conditioning Enables MHC-Mismatched Hematopoietic Stem Cell Transplants and Organ Graft Tolerance

Hematopoietic cell transplantation can correct hematological and immunological disorders by replacing a diseased blood system with a healthy one, but this currently requires depleting a patient's existing hematopoietic system with toxic and non-specific chemotherapy, radiation, or both. Here we report an antibody-based conditioning protocol with reduced toxicity and enhanced specificity for robust hematopoietic stem cell (HSC) transplantation and engraftment in recipient mice. Host pre-treatment with six monoclonal antibodies targeting CD47, T cells, NK cells, and HSCs followed by donor HSC transplantation enabled stable hematopoietic system reconstitution in recipients with mismatches at half (haploidentical) or all major histocompatibility complex (MHC) genes. This approach allowed tolerance to heart tissue from HSC donor strains in haploidentical recipients, showing potential applications for solid organ transplantation without immune suppression. Fully mismatched chimeric mice developed antibody responses to nominal antigens, showing preserved functional immunity. These findings suggest approaches for transplanting immunologically mismatched HSCs and solid organs with limited toxicity.

Successful ex vivo expansion of mouse hematopoietic stem cells

Ex vivo expansion of hematopoietic stem cells (HSCs) is considered the holy grail in stem cell biology and therapy, as it has long been difficult to make this procedure possible. Yamazaki's research team has established new, polyvinyl alcohol-based culture conditions and shown a significant expansion of mouse HSCs from a small number of cells after a month of culture. Surprisingly, expanded HSCs were able to reconstitute unconditioned normal mice. There is generally a technical concern in limiting dilution assay to estimate a fold-expansion of HSCs. But, this work paves the way toward expansion of human HSCs useful for transplantation medicine.

Blockade of plasminogen activator inhibitor-1 empties bone marrow niche sufficient for donor hematopoietic stem cell engraftment without myeloablative conditioning

Upon hematopoietic stem cell transplantation (HSCT), the availability of recipients' niches in the bone marrow (BM) is one of the factors that influence donor HSC engraftment and hematopoietic reconstitution. Therefore, myeloablative conditioning, such as irradiation and/or chemotherapy, which creates empty niches in the recipients' BM, is required for the success of HSCT. However, the conventional myeloablation causes extensive damages to the patients' BM, which results in the treatment-induced severe complications and even mortality. Thus, alternative and mild conditioning could fulfill the need for safer HSCT-based therapies for hematological and nonhematological disorders. Recently, we have demonstrated that pharmacological inhibition of plasminogen activator inhibitor-1 (PAI-1) activity increases cellular motility and cause detachment of HSCs from the niches. In this study, we performed HSCT using a PAI-1 inhibitor without any myeloablative conditioning. Donor HSCs were transplanted to recipient mice that were pretreated with saline or a PAI-1 inhibitor. Saline pretreated nonmyeloablative recipients showed no engraftment. In contrast, donor cell engraftment was detected in the PAI-1 inhibitor pretreated recipients. Multilineage differentiation, including lymphoid and myeloid cells, was observed in the PAI-1 inhibitor pretreated recipients. Donor-derived cells that exhibited multilineage reconstitution as well as the existence of stem/progenitor cells were detected in the secondary recipients, confirming the maintenance of donor HSCs in the BM of PAI-1 inhibitor pretreated primary recipients. The results indicate that the PAI-1 blockade vacates functional niches in the recipients' BM, which allows the engraftment of long-term multilineage HSCs without myeloablative conditioning.

Bone Metastasis: Find Your Niche and Fit in

Metastasis to bones is determined by both intrinsic traits of metastatic tumor cells and properties appertaining to the bone microenvironment. Bone marrow niches are critical for all major steps of metastasis, including the seeding of disseminated tumor cells (DTCs) to bone, the survival of DTCs and microscopic metastases under dormancy, and the eventual outgrowth of overt metastases. In this review, we discuss the role of bone marrow niches in bone colonization. The emphasis is on complicated and dynamic nature of cancer cells-niche interaction, which may underpin the long-standing mystery of metastasis dormancy, and represent a therapeutic target for elimination of minimal residue diseases and prevention of life-taking, overt metastases.

A revisionist history of adult marrow stem cell biology or 'they forgot about the discard'

The adult marrow hematopoietic stem cell biology has largely been based on studies of highly purified stem cells. This is unfortunate because during the stem cell purification the great bulk of stem cells are discarded. These cells are actively proliferating. The final purified stem cell is dormant and not representative of the whole stem cell compartment. Thus, a large number of studies on the cellular characteristics, regulators and molecular details of stem cells have been carried on out of non-represented cells. Niche studies have largely pursued using these purified stem cells and these are largely un-interpretable. Other considerations include the distinction between baseline and transplant stem cells and the modulation of stem cell phenotype by extracellular vesicles, to cite a non-inclusive list. Work needs to proceed on characterizing the true stem cell population.

Extrinsic regulation of hematopoietic stem cells in development, homeostasis and diseases

Lifelong generation of blood and immune cells depends on hematopoietic stem cells (HSCs). Their function is precisely regulated by complex molecular networks that integrate and respond to ever changing physiological demands of the body. Over the past several years, significant advances have been made in understanding the extrinsic regulation of HSCs during development and in homeostasis. Propelled by technical advances in the field, the cellular and molecular components of the microenvironment that support HSCs in vivo are emerging. In addition, the interaction of HSCs with their niches is appreciated as a critical contributor to the pathogenesis of a number of hematologic disorders. Here, we review these advances in detail and highlight the extrinsic regulation of HSCs in the context of development, homeostasis, and diseases. WIREs Dev Biol 2017, 6:e279. doi: 10.1002/wdev.279 For further resources related to this article, please visit the WIREs website.

In vivo engineering of bone tissues with hematopoietic functions and mixed chimerism

Synthetic biomimetic matrices with osteoconductivity and osteoinductivity have been developed to regenerate bone tissues. However, whether such systems harbor donor marrow in vivo and support mixed chimerism remains unknown. We devised a strategy to engineer bone tissues with a functional bone marrow (BM) compartment in vivo by using a synthetic biomaterial with spatially differing cues. Specifically, we have developed a synthetic matrix recapitulating the dual-compartment structures by modular assembly of mineralized and nonmineralized macroporous structures. Our results show that these matrices incorporated with BM cells or BM flush transplanted into recipient mice matured into functional bone displaying the cardinal features of both skeletal and hematopoietic compartments similar to native bone tissue. The hematopoietic function of bone tissues was demonstrated by its support for a higher percentage of mixed chimerism compared with i.v. injection and donor hematopoietic cell mobilization in the circulation of nonirradiated recipients. Furthermore, hematopoietic cells sorted from the engineered bone tissues reconstituted the hematopoietic system when transplanted into lethally irradiated secondary recipients. Such engineered bone tissues could potentially be used as ectopic BM surrogates for treatment of nonmalignant BM diseases and as a tool to study hematopoiesis, donor-host cell dynamics, tumor tropism, and hematopoietic cell transplantation.

Numerous niches for hematopoietic stem cells remain empty during homeostasis

Hematopoietic stem cells (HSCs) reside in and are maintained by special microenvironments, termed niches. It is assumed that the HSC niche space remains occupied by endogenous cells and that myelosuppressive conditioning is required to achieve high levels of HSC engraftment. We herein demonstrate that upon the transplantation of very large numbers of purified HSCs into normal mice not exposed to myeloablation, donor HSCs engrafted in niches distant from filled HSC niches without replacing host HSCs and subsequently proliferated and generated hematopoietic progenitors, leading to marked increases in the overall HSC numbers in bone marrow. Additionally, stem cell factor that is produced by CXC chemokine ligand 12-abundant reticular cells is involved in HSC engraftment. In contrast, host granulocyte/macrophage progenitors (GMPs) were replaced by the progeny of transplanted donor HSCs, and overall GMP numbers remained unchanged. Thus, inconsistent with the classical concept, numerous empty HSC niches are available for engraftment and proliferation in bone marrow.

Extended time-lapse in vivo imaging of tibia bone marrow to visualize dynamic hematopoietic stem cell engraftment

Homing, engraftment and proliferation of hematopoietic stem/progenitor cell (HSC/HPCs) are crucial steps required for success of a bone marrow transplant. Observation of these critical events is limited by the opaque nature of bone. Here we demonstrate how individual HSCs engraft in long bones by thinning one side of the tibia for direct and unbiased observation. Intravital imaging enabled detailed visualization of single Sca-1⁺, c-Kit⁺, Lineage⁻ (SKL) cell migration to bone marrow niches and subsequent proliferation to reconstitute hematopoiesis. This longitudinal study allowed direct observation of dynamic HSC/HPC activities during engraftment in full color for up to six days in live recipients. Individual SKL cells, but not mature or committed progenitor cells, preferentially homed to a limited number of niches near highly vascularized endosteal regions, and clonally expanded. Engraftment of SKL cells in P-selectin and osteopontin knockout mice showed abnormal homing and expansion of SKL cells. CD150⁺, CD48⁻ SKL populations initially engrafted in the central marrow region, utilizing only a subset of niches occupied by the parent SKL cells. Our study demonstrates that time-lapse imaging of tibia can be a valuable tool to understand the dynamic characteristics of functional HSC and niche components in various mouse models.

Alterations in the bone marrow microenvironment may elicit defective hematopoiesis: a comparison of aplastic anemia, chronic myeloid leukemia, and normal bone marrow

Hematopoiesis involves complex interactions between hematopoietic cells and the bone marrow (BM) microenvironment. The specific causes and mechanisms underlying dysregulated hematopoiesis are unknown. Here, BM biopsy specimens from patients with aplastic anemia (AA) and chronic myeloid leukemia (CML) and normal marrow were analyzed by semiquantitative immunohistochemistry to determine changes in the hematopoietic stem cell (HSC) compartment and BM microenvironment. HSC levels were lowest in AA and highest in CML. T and B lymphocytes were decreased in AA (p < 0.01) and CML (p < 0.01). Natural killer cells were observed in AA, but were absent in CML and healthy controls (p < 0.01). Macrophages and mast cells were absent in CML. There were significant differences between AA and CML stromal cell components. No nestin⁺ cells were observed in CML and the mean number of stromal cell-derived factor-1-positive cells was lowest in CML. Osteopontin⁺ cells were higher in AA than in CML (p < 0.01); osteonectin⁺ cells were higher in CML than in AA (p < 0.01). There was no significant difference in the expression of osteocalcin between AA and CML. The number of endothelial cells was highest in CML and lowest in AA (p < 0.01). Our findings suggest that changes in BM microenvironment components might be related to defective hematopoiesis leading to AA and/or CML.

Therapeutic targeting and rapid mobilization of endosteal HSC using a small molecule integrin antagonist

The inherent disadvantages of using granulocyte colony-stimulating factor (G-CSF) for hematopoietic stem cell (HSC) mobilization have driven efforts to identify alternate strategies based on single doses of small molecules. Here, we show targeting α₉β₁/α₄β₁ integrins with a single dose of a small molecule antagonist (BOP (N-(benzenesulfonyl)-L-prolyl-L-O-(1-pyrrolidinylcarbonyl)tyrosine)) rapidly mobilizes long-term multi-lineage reconstituting HSC. Synergistic engraftment augmentation is observed when BOP is co-administered with AMD3100. Impressively, HSC in equal volumes of peripheral blood (PB) mobilized with this combination effectively out-competes PB mobilized with G-CSF. The enhanced mobilization observed using BOP and AMD3100 is recapitulated in a humanized NODSCIDIL2Rγ^−/− model, demonstrated by a significant increase in PB CD34⁺ cells. Using a related fluorescent analogue of BOP (R-BC154), we show that this class of antagonists preferentially bind human and mouse HSC and progenitors via endogenously primed/activated α₉β₁/α₄β₁ within the endosteal niche. These results support using dual α₉β₁/α₄β₁ inhibitors as effective, rapid and transient mobilization agents with promising clinical applications.

Mobilizing haematopoietic stem cells to the peripheral blood has largely replaced bone marrow transplants as a strategy in the clinic. Here, Cao et al. report the use of an α₉β₁/α₄β₁ integrin antagonist to induce rapid mobilization of blood stem cells from the bone marrow in a humanized mouse model.

Concise reviews: A stem cell apostasy: a tale of four H words

The field of hematopoietic stem cell (HSC) biology has become increasingly dominated by the pursuit and study of highly purified populations of HSCs. Such HSCs are typically isolated based on their cell surface marker expression patterns and ultimately defined by their multipotency and capacity for self-generation. However, even with progressively more stringent stem cell separation techniques, the resultant HSC population remains heterogeneous with respect to both self-renewal and differentiation capacity. Critical studies on unseparated whole bone marrow have definitively shown that long-term engraftable HSCs are in active cell cycle and thus continually changing phenotype. Therefore, they cannot be purified by current approaches dependent on stable surface epitope expression because the surface markers are continually changing as well. These critical cycling cells are discarded with current stem cell purifications. Despite this, research defining such characteristics as self-renewal capacity, lineage-commitment, bone marrow niches, and proliferative state of HSCs continues to focus predominantly on this small subpopulation of purified marrow cells. This review discusses the research leading to the hierarchical model of hematopoiesis and questions the dogmas pertaining to HSC quiescence and purification.

The role of novel and known extracellular matrix and adhesion molecules in the homeostatic and regenerative bone marrow microenvironment

Maintenance of haematopoietic stem cells and differentiation of committed progenitors occurs in highly specialized niches. The interactions of haematopoietic stem and progenitor cells (HSPCs) with cells, growth factors and extracellular matrix (ECM) components of the bone marrow (BM) microenvironment control homeostasis of HSPCs. We only start to understand the complexity of the haematopoietic niche(s) that comprises endosteal, arterial, sinusoidal, mesenchymal and neuronal components. These distinct niches produce a broad range of soluble factors and adhesion molecules that modulate HSPC fate during normal hematopoiesis and BM regeneration. Adhesive interactions between HSPCs and the microenvironment will influence their localization and differentiation potential. In this review we highlight the current understanding of the functional role of ECM- and adhesion (regulating) molecules in the haematopoietic niche during homeostatic and regenerative hematopoiesis. This knowledge may lead to the improvement of current cellular therapies and more efficient development of future cellular products.

Safety and biodistribution study of bone marrow-derived mesenchymal stromal cells and mononuclear cells and the impact of the administration route in an intact porcine model

BACKGROUND AIMS

Bone marrow mononuclear cells (BM-MNCs) and bone marrow-derived mesenchymal stem stromal cells (BM-MSCs) could have therapeutic potential for numerous conditions, including ischemia-related injury. Cells transplanted intravascularly may become entrapped in the lungs, which potentially decreases their therapeutic effect and increases the risk for embolism.

METHODS

Twelve pigs were divided into groups of 3 and received (99m)Tc- hydroxymethyl-propylene-amine-oxime-labeled autologous BM-MNCs or allogeneic BM-MSCs by either intravenous (IV) or intra-arterial (IA) transplantation. A whole body scan and single photon emission computed tomography/computed tomography (SPECT/CT) were performed 8 h later, and tissue biopsies were collected for gamma counting. A helical CT scan was also performed on 4 pigs to detect possible pulmonary embolism, 2 after IV BM-MSC injection and 2 after saline injection.

RESULTS

The transplantation route had a greater impact on the biodistribution of the BM-MSCs than the BM-MNCs. The BM-MNCs accumulated in the spleen and bones, irrespective of the administration route. The BM-MSCs had relatively higher uptake in the kidneys. The IA transplantation decreased the deposition of BM-MSCs in the lungs and increased uptake in other organs, especially in the liver. Lung atelectases were frequent due to mechanical ventilation and attracted transplanted cells. CT did not reveal any pulmonary embolism.

CONCLUSIONS

Both administration routes were found to be safe, but iatrogenic atelectasis might be an issue when cells accumulate in the lungs. The IA administration is effective in avoiding pulmonary entrapment of BM-MSCs. The cell type and administration method both have a major impact on the acute homing.

Impact of aging on the regenerative properties of bone marrow-, muscle-, and adipose-derived mesenchymal stem/stromal cells

Mesenchymal stem/stromal cells (MSCs) are promising cell sources for regenerative therapies due to their multipotency and ready availability, but their application can be complicated by patient-specific factors like age or illness. MSCs have been investigated for the treatment of many musculoskeletal disorders, including osteoarthritis and osteoporosis. Due to the prevalence of these diseases in older populations, researchers have studied how aging affects MSC properties and have found that proliferation and differentiation potential are impaired. However, these effects have never been compared among MSCs isolated from multiple tissue sources in the same, healthy donor. Revealing differences in how MSCs are affected by age could help identify an optimal cell source for musculoskeletal therapies targeting older patients. MSCs were isolated from young and old rabbit bone marrow, muscle, and adipose tissue. Cell yield and viability were quantified after isolation procedures, and expansion properties were assessed using assays for proliferation, senescence, and colony formation. Multipotency was also examined using lineage-specific stains and spectrophotometry of metabolites. Results were compared between age groups and among MSC sources. Results showed that MSCs are differentially influenced by aging, with bone marrow-derived stem cells having impaired proliferation, senescence, and chondrogenic response, whereas muscle-derived stem cells and adipose-derived stem cells exhibited no negative effects. While age reduced overall cell yield and adipogenic potential of all MSC populations, osteogenesis and clonogenicity remained unchanged. These findings indicate the importance of age as a factor when designing cell-based therapies for older patients.

Characterization of mechanical and regenerative properties of human, adipose stromal cells

The stromal vascular fraction (SVF) of human adipose tissue is a heterogeneous population, with component cell types that may or may not contribute to its regenerative potential. Recent findings indicate that single-cell mechanical biomarkers are characteristic of cell type and can be used comparably to gene and protein expressions to identify cell populations. In this study, we characterized mechanical properties and differentiation potential of cell types present in the SVF. Fluorescence-activated cell sorting was used to isolate four distinct populations based on surface markers: endothelial cells (EC), adipose-derived stem cells (ASCs), pre-adipocytes, and smooth muscle cells (SMC). Atomic force microscopy was used to mechanically characterize sorted cell populations and unsorted SVF. Differentiation capabilities of sorted and unsorted populations were evaluated by quantifying lipid production and calcified matrix deposition. Cells populating the SVF exhibited a range of mechanical properties, with ECs, ASCs, pre-adipocytes, and unsorted SVF cells being significantly more compliant than SMCs. Lineage-specific metabolite production was most robust in SVF cells, followed by ASCs, with the other cell types showing little or no potential, suggesting the unsorted populations may benefit from a paracrine response that is lacking once the cells are sorted into more uniform cell populations.

Kit regulates HSC engraftment across the human-mouse species barrier

In-depth analysis of the cellular and molecular mechanisms regulating human HSC function will require a surrogate host that supports robust maintenance of transplanted human HSCs in vivo, but the currently available options are problematic. Previously we showed that mutations in the Kit receptor enhance engraftment of transplanted HSCs in the mouse. To generate an improved model for human HSC transplantation and analysis, we developed immune-deficient mouse strains containing Kit mutations. We found that mutation of the Kit receptor enables robust, uniform, sustained, and serially transplantable engraftment of human HSCs in adult mice without a requirement for irradiation conditioning. Using this model, we also showed that differential KIT expression identifies two functionally distinct subpopulations of human HSCs. Thus, we have found that the capacity of this Kit mutation to open up stem cell niches across species barriers has significant potential and broad applicability in human HSC research.

Marrow Hematopoietic Stem Cells Revisited: They Exist in a Continuum and are Not Defined by Standard Purification Approaches; Then There are the Microvesicles

Current concepts of hematopoiesis are encompassed in a hierarchical stem cell model. This developed initially from studies of colony-forming unit spleen and in vitro progenitors for different cell lineages, but then evolved into a comprehensive model of cells with different in vivo differentiative and proliferative potential. These cells were characterized and purified based largely on expression of a variety of lineage-specific and stem cell-specific surface epitopes. Monoclonal antibodies were bound to these epitopes and then used to physically and fluorescently separate different classes of these cells. The gold standard for the most primitive marrow stem cells was long-term multilineage repopulation and renewal in lethally irradiated mice. Progressive work seemed to have clonally defined a Lineage negative (Lin-), Sca-1+, c-kit+, CD150+ stem cell with great proliferative, differentiative, and renewal potential. This cell was stable and in the G0 phase of cell cycle. However, continued work in our laboratory indicated that the engraftment, differentiation, homing, and gene expression phenotype of the murine marrow stem cells continuously and reversibly changes with passage through cell cycle. Most recently, using cycle-defining supravital dyes and fluorescent-activated cell sorting and S-phase-specific tritiated thymidine suicide, we have established that the long-term repopulating hematopoietic stem cell is a rapidly proliferating, and thus a continually changing cell; as a corollary it cannot be purified or defined on a clonal single cell basis. Further in vivo studies employing injected and ingested 5-Bromodeoxyuridine (BrdU), showed that the G0 Lin-Sca-1, c-kit+ Flt3- cell was rapidly passing through cell cycle. These data are explained by considering the separative process: the proliferating stem cells are eliminated through the selective separations leaving non-representative dormant G0 stem cells. In other words, they throw out the real stem cells with the purification. This system, where the marrow stem cell continuously and reversibly changes with obligate cell cycle transit, is further complicated by the consideration of the impact of tissue microvesicles on the cell phenotypes. Tissue microvesicles have been found to alter the phenotype of marrow cells, possibly explaining the observations of "stem cell plasticity." These alterations, short-term, are due to transfer of originator cell mRNA and as yet undefined transcription factors. Long-term phenotype change is due to transcriptional modulation; a stable epigenetic change. Thus, the stem cell system is characterized by continuous cycle and microvesicle-related change. The challenge of the future is to define the stem cell population.

Circadian rhythms in leukocyte trafficking

A broad range of immunological processes oscillates over the course of a day. Recent findings have identified a molecular basis for the circadian clock in the regulation of the immune system. These rhythms manifest themselves in oscillatory behavior of immune cells and proinflammatory mediators, which causes a time-dependent sensitivity in the reaction to pathogens. This rhythmicity impacts disease manifestations and severity and provides an option for therapy that incorporates chronopharmacological considerations. This review will focus on the current knowledge and relevance of rhythmic immune cell trafficking. It will provide an overview of the molecular clock machinery and its interrelations with leukocyte migration and the immune response.

Analyzing hematopoietic stem cell homing, lodgment, and engraftment to better understand the bone marrow niche

The existence of a bone marrow (BM) niche--the location in which hematopoietic stem cells (HSCs) reside--was proposed more than 30 years ago. Recent data suggest that the interaction of HSCs with cellular and extracellular components within the BM is critical for HSC regulation. The tracking of immunofluorescently labeled, prospectively isolated HSCs to and within the BM cavity allows the assessment of the regulatory processes involved in (1) homing, which involves transendothelial migration into the BM; (2) lodgment, including transmarrow migration through the extravascular space; and (3) BM reconstitution. Together, such analyses provide a better understanding of the cellular and extracellular components involved in the regulation of HSC quiescence and differentiation. Homing and lodgment of transplanted HSCs, the first critical steps in engraftment, involve multiple interactions between HSCs and the BM microenvironment. Herein, we describe a refined method of analyzing homing efficiency and spatial distribution of HSCs harvested from endosteal and/or central BM regions; we also review alternate methods. Using these techniques, microenvironment modifications within the recipient or surface protein-expression modifications on donor HSCs in animal models provide insights into components influencing the homing, lodgment, and engraftment processes.

Cellular complexity of the bone marrow hematopoietic stem cell niche

The skeleton serves as the principal site for hematopoiesis in adult terrestrial vertebrates. The function of the hematopoietic system is to maintain homeostatic levels of all circulating blood cells, including myeloid cells, lymphoid cells, red blood cells, and platelets. This action requires the daily production of more than 500 billion blood cells. The vast majority of these cells are synthesized in the bone marrow, where they arise from a limited number of hematopoietic stem cells (HSCs) that are multipotent and capable of extensive self-renewal. These attributes of HSCs are best demonstrated by marrow transplantation, where even a single HSC can repopulate the entire hematopoietic system. HSCs are therefore adult stem cells capable of multilineage repopulation, poised between cell fate choices which include quiescence, self-renewal, differentiation, and apoptosis. While HSC fate choices are in part determined by multiple stochastic fluctuations of cell autonomous processes, according to the niche hypothesis, signals from the microenvironment are also likely to determine stem cell fate. While it had long been postulated that signals within the bone marrow could provide regulation of hematopoietic cells, it is only in the past decade that advances in flow cytometry and genetic models have allowed for a deeper understanding of the microenvironmental regulation of HSCs. In this review, we will highlight the cellular regulatory components of the HSC niche.

Megakaryocytes co-localise with hemopoietic stem cells and release cytokines that up-regulate stem cell proliferation

We report transplanted hemopoietic stem cells (HSC) preferentially lodge within two cells of mature megakaryocytes (MM). With both populations comprising ~0.2% of bone marrow cells, this strongly suggests a key functional interaction. HSC isolated from the endosteum (eLSKSLAM) showed significantly increased hemopoietic cell proliferation while in co-culture with MM. Furthermore, eLSKSLAM progeny retained HSC potential, maintaining long-term multi-lineage reconstitution capacity in lethally ablated recipients. Increased hemopoietic cell proliferation was not MM contact dependent and could be recapitulated with media supplemented with two factors identified in MM-conditioned media: insulin-like growth factor binding protein-3 (IGFBP-3) and insulin-like growth factor-1 (IGF-1). We demonstrate that HSC express the receptor for IGF-1 and that IGF-1/IGFBP-3 induced increased hemopoietic cell proliferation can be blocked by an anti-IGF-1 neutralising antibody. However, co-cultures of 8N, 16N or 32N MM with eLSKSLAM showed that MM of individual ploidy did not significantly increase hemopoietic cell proliferation. Our data suggests that MM are an important component of the HSC niche and regulate hemopoietic cell proliferation through cytokine release.

The prospective isolation of viable, high ploidy megakaryocytes from adult murine bone marrow by fluorescence activated cell sorting

Mature megakaryocytes (MM) can be up to 65 μM in diameter and due to their size, viable and pure MM populations have been difficult to isolate in large numbers. Here in, we report a fluorescence activated cell sorting (FACS) method by which viable and pure populations of 8 N, 16 N, 32 N, and 64 N MM can be isolated from murine bone marrow (BM).

Extramedullary hematopoiesis: a new look at the underlying stem cell niche, theories of development, and occurrence in animals

Extramedullary hematopoiesis (EMH) is the formation and development of blood cells outside the medullary spaces of the bone marrow. Although widely considered an epiphenomenon, secondary to underlying primary disease and lacking serious clinical or diagnostic implications, the presence of EMH is far from incidental on a molecular basis; rather, it reflects a well-choreographed suite of changes involving stem cells and their microenvironment (the stem cell niche). The goals of this review are to reconsider the molecular basis of EMH based on current knowledge of stem cell niches and to examine its role in the pathophysiologic mechanisms of EMH in animals. The ability of blood cells to home, proliferate, and mature in extramedullary tissues of adult animals reflects embryonic patterns of hematopoiesis and establishment or reactivation of a stem cell niche. This involves pathophysiologic alterations in hematopoietic stem cells, extracellular matrix, stromal cells, and local and systemic chemokines. Four major theories involving changes in stem cells and/or their microenvironment can explain the development of most occurrences of EMH: (1) severe bone marrow failure; (2) myelostimulation; (3) tissue inflammation, injury, and repair; and (4) abnormal chemokine production. EMH has also been reported within many types of neoplasms. Understanding the concepts and factors involved in stem cell niches enhances our understanding of the occurrence of EMH in animals and its relationship to underlying disease. In turn, a better understanding of the prevalence and distribution of EMH in animals and its molecular basis could further inform our understanding of the hematopoietic stem cell niche.

Altered matrix at the chondro-osseous junction leads to defects in lymphopoiesis

The collagen X transgenic and null (ColX-Tg/KO) mice have revealed a link between endochondral ossification (EO) and hematopoiesis, and thus serve as model systems to study hematopoietic niches. The altered collagen X function in ColX-Tg/KO mice resulted not only in skeletal defects, which included changes in growth plate ultrastructure, altered localization of heparan sulfate proteoglycans (HSPG), and reduced trabecular bone, but also in hematopoietic defects, which included reduced B lymphocyte numbers throughout life without associated increases in B cell apoptosis. Consequently, the ColX-Tg/KO mice exhibited diminished in vitro and in vivo immune responses. Moreover, reduced expression of several hematopoietic and B lymphopoietic cytokines were measured from ColX-KO-derived hypertrophic chondrocyte and trabecular osteoblast cultures. Together, these data expand the current hematopoietic niche model by including the EO-derived extracellular matrix, for example, the collagen X/HSPG network, as well as the EO-derived hypertrophic chondrocytes and trabecular osteoblasts as hematopoietic signal mediating cells.

The location and cellular composition of the hemopoietic stem cell niche

While it is accepted that hemopoietic stem cells (HSC) are located in a three-dimensional microenvironment, termed a niche, the cellular and extracellular composition, as well as the multifaceted effects the components of the niche have on HSC regulation, remains undefined. Over the past four decades numerous advances in the field have led to the identification of roles for some cell types and propositions of potentially a number of HSC niches. We present evidence supporting the roles of multiple cell types and extracellular matrix molecules in the HSC niche, as well as discuss the potential significant overlap and intertwining of previously proposed distinct HSC niches.

The niche as a target for hematopoietic manipulation and regeneration

Hematopoietic stem cells (HSCs), rare primitive cells capable of reconstituting all blood cell lineages, are the only stem cells currently routinely used for therapeutic purposes. Clinical experience has shown that HSC number is an important limiting factor in treatment success. Strategies to expand HSCs are of great clinical appeal, as they would improve therapeutic use of these cells in stem cell transplantation and in conditions of bone marrow failure. The microenvironment in which HSCs reside, known as the niche, has long been considered a critical regulator of HSCs. Data accumulated over the past decade strongly confirm the importance of the niche in HSC behavior. A number of niche components as well as signaling pathways, such as Notch, have been implicated in the interaction of the microenvironment with HSCs and continue to be genetically evaluated in the hope of defining the critical elements that are required and which, if modified, can initiate HSC behaviors. In this review, we highlight the known characteristics of HSCs, challenges in their expansion, the niche phenomenon, and explain why niche stimulated HSC expansion is of utmost interest in the field, while beginning to bring to the fore potential caveats of niche manipulation. Lastly, the potential pitfalls of avoiding malignancy and controlling self-renewal versus differentiation will be briefly reviewed.

Defining the hematopoietic stem cell niche: the chicken and the egg conundrum

Understanding the in vivo regulation of hematopoietic stem cells (HSCs) will be critical to identifying key factors involved in the regulation of HSC self-renewal and differentiation. The niche (microenvironment) in which HSCs reside has recently regained attention accompanied by a dramatic increase in the understanding of the cellular constituents of the bone marrow HSC niche. The use of sophisticated genetic models allowing modulation of specific lineages has demonstrated roles for mesenchymal-derived elements such as osteoblasts and adipocytes, vasculature, nerves, and a range of hematopoietic progeny of the HSC as being participants in the regulation of the bone marrow microenvironment. Whilst providing significant insight into the cellular composition of the niche, is it possible to manipulate any given cell lineage in vivo without impacting, knowingly or unknowingly, on those that remain?

The relationship between bone, hemopoietic stem cells, and vasculature

A large body of evidence suggests hemopoietic stem cells (HSCs) exist in an endosteal niche close to bone, whereas others suggest that the HSC niche is intimately associated with vasculature. In this study, we show that transplanted hemopoietic stem and progenitor cells (HSPCs) home preferentially to the trabecular-rich metaphysis of the femurs in nonablated mice at all time points from 15 minutes to 15 hours after transplantation. Within this region, they exist in an endosteal niche in close association with blood vessels. The preferential homing of HSPCs to the metaphysis occurs rapidly after transplantation, suggesting that blood vessels within this region may express a unique repertoire of endothelial adhesive molecules. One candidate is hyaluronan (HA), which is highly expressed on the blood vessel endothelium in the metaphysis. Analysis of the early stages of homing and the spatial dis-tribution of transplanted HSPCs at the single-cell level in mice devoid of Has3-synthesized HA, provides evidence for a previously undescribed role for HA expressed on endothelial cells in directing the homing of HSPCs to the metaphysis.

Hematopoietic stem cell lodgment in the adult bone marrow stem cell niche

Treatment of malignant blood disorders, such as leukemia, that can provide a better chance of long-term remission involves myeloablation followed by transplantation of matched donor hematopoietic stem cells (HSCs). For successful engraftment and re-establishment of hematopoiesis to occur in the recipient, the transplanted HSCs must first migrate from the blood circulation to the bone marrow (BM), a process known as homing, then localize and anchor in suitable microenvironments within the BM, a process known as lodgment. After lodgment, the specific fate of the transplanted HSCs is determined through complex, bidirectional interactions with various stromal cell components in the niche. Ultimately, these interactions dictate the clinical outcome of the transplantation. Through the use of transgenic mouse models, considerable evidence has been accumulated in an attempt to unveil the possible underlying mechanisms that govern these processes. Here, we will emphasize the major factors that are involved in the regulation of lodgment of transplanted HSCs. Specifically, we will first introduce early observations on the spatial distribution of hematopoietic progenitors within the BM, then we will discuss the soluble factors, chemokines, cell-cell interactions, and cell-matrix interactions that have been studied and known to influence the site of HSC lodgment within the BM following transplantation.

Stromal-derived factor-1/CXCR4 signaling: indispensable role in homing and engraftment of hematopoietic stem cells in bone marrow

Homing and engraftment of hematopoietic stem/progenitor cells (HSPCs) in bone marrow is the major determining factor in success of hematopoietic stem cell transplantation. This is a complex, multistep process orchestrated by the coordinated interplay between adhesion molecules, cytokines, growth factors, and regulatory cofactors, many of which remain to be defined. Recent studies have highlighted the pivotal role of unique stromal-derived factor-1 (SDF-1)/CXCR4 signaling in the regulation of HSPC homing and subsequent engraftment. In addition, studies suggest that SDF-1/CXCR4 signaling acts as an essential survival-promoting factor of transplanted HSPCs as well as maintenance of quiescent HSCs in bone marrow niche. These pleiotropic effects exerted by SDF-1/CXCR4 axis make this unique signaling initiator very promising, not only for optimal hematopoietic reconstitution but also for the development of innovative approaches to achieve restoration, regeneration, or repair of other damaged tissues potentially amendable to reversal by stem cell transplantation. This goal can only be achieved when the role of SDF-1/CXCR4 axis in hematopoietic transplantation is clearly defined. Hence, this review presents current knowledge of the mechanisms through which SDF-1/CXCR4 signaling promotes restoration of hematopoiesis by regulating the homing and engraftment of HSPCs.

Methods to analyze the homing efficiency and spatial distribution of hematopoietic stem and progenitor cells and their relationship to the bone marrow endosteum and vascular endothelium

The tracking of immunofluorescent labeled hematopoietic stem and progenitor cells (HSC/HPC) within the bone marrow (BM) cavity allows the assessment of the regulatory processes involved in transendothelial migration, trans-marrow migration, and finally lodgement into the HSC niche. This is of interest as the extracellular and cellular components involved in the regulation of HSC quiescence and differentiation are still not completely understood. Homing of transplanted HSC is the first critical step in the interaction between HSC and the microenvironment of the BM. As a consequence, murine models allowing the evaluation of the structural relationship between migrating HSC, the endosteal bone surface, and the vascular components of the BM enhance our understanding of hematopoietic regulation.

Annexin-2 is a regulator of stromal cell-derived factor-1/CXCL12 function in the hematopoietic stem cell endosteal niche

OBJECTIVE

Previously, we reported that annexin-2 (anxa2) plays an important role in hematopoietic stem cell (HSC) localization to the endosteal/osteoblastic marrow niche. This study explored the role that annexin-2 plays in presenting stromal cell-derived factor-1 (or CXCL12) to HSCs.

MATERIALS AND METHODS

Competitive long-term bone marrow transplant assays were used to determine if HSC engraftment is altered in annexin-2-deficient animals. Colony-forming cell assays, CXCL12 enzyme-linked immunosorbent assay, and real-time reverse transcription polymerase chain reaction analyses were used to determine stem or progenitor cell mobilization by granulocyte colony-stimulating factor. Immunohistochemistry, immunoprecipitation, binding assays, and chemotactic assays were employed to determine if annexin-2 is associated with CXCL12. Degradation assays were also used to determine if annexin-2 and CXCL12 protect each other from proteolytic degradation.

RESULTS

Anxa2(-/-) animals had fewer HSCs in their marrow, and the HSCs in anxa2(-/-) animals express less CXCR4 and CXCR7, suggesting a cell intrinsic defect. Transplantation studies of wild-type marrow into anxa2(-/-) animals demonstrated a cell-extrinsic defect in the anxa2(-/-) animals. CXCL12 binds directly to annexin-2, and this interaction facilitates presentation of CXCL12 to HSCs. Yet the binding of CXCL12 to annexin-2 did not protect CXCL12 from proteolytic cleavage after stem or progenitor cell mobilization by granulocyte colony-stimulating factor.

CONCLUSIONS

These results suggest that annexin-2 serves as an anchor for CXCL12 to help in the localization of HSCs to the niche.

The endosteal 'osteoblastic' niche and its role in hematopoietic stem cell homing and mobilization

The concept of hematopoietic stem cell (HSC) niche was formulated in 1978, but HSC niches remained unidentified for the following two decades largely owing to technical limitations. Sophisticated live microscopy techniques and genetic manipulations have identified the endosteal region of the bone marrow (BM) as a preferential site of residence for the most potent HSC - able to reconstitute in serial transplants - with osteoblasts and their progenitors as critical cellular elements of these endosteal niches. This article reviews the path to the discovery of these endosteal niches (often called 'osteoblastic' niches) for HSC, what cell types contribute to these niches with their known physical and biochemical features. In the past decade, a first wave of research uncovered many mechanisms responsible for HSC homing to, and mobilization from, the whole BM tissue. However, the recent discovery of endosteal HSC niches has initiated a second wave of research focusing on the mechanisms by which most primitive HSC lodge into and migrate out of their endosteal niches. The second part of this article reviews the current knowledge of the mechanisms of HSC lodgment into, retention in and mobilization from osteoblastic niches.

Phenotypically identical hemopoietic stem cells isolated from different regions of bone marrow have different biologic potential

Hemopoietic stem cells (HSCs) reside within a specified area of the bone marrow (BM) cavity called a "niche" that modulates HSC quiescence, proliferation, differentiation, and migration. Our previous studies have identified the endosteal BM region as the site for the HSC niche and demonstrated that hemopoietic stem and progenitor populations (HSPCs, LSK) isolated from different BM regions exhibit significantly different hemopoietic potential. In this study, we have analyzed subpopulations of LSK cells isolated from different regions of the BM and showed that CD150(+)CD48(-)LSK HSCs within the endosteal BM region have superior proliferative capacity and homing efficiency compared with CD150(+)CD48(-)LSK HSCs isolated from the central BM. Furthermore, we show, for the first time, that a subset of CD150(+)CD48(+)LSK progenitor cells, previously defined as B-lymphoid primed hemopoietic cells, are capable of multilineage reconstitution, however, only when isolated from the endosteal region. In addition, we provide evidence for an unrecognized role of CD48 in HSC homing. Together, our data provide strong evidence that highly purified HSCs show functional differences depending on their origin within the BM and that the most primitive HSCs reside within the endosteal BM region.