The development of spatial distributions of CFU-S and in-vitro CFC in femora of mice of different ages

One more stem cell niche: how the sensitivity of chronic myeloid leukemia cells to imatinib mesylate is modulated within a "hypoxic" environment

This is a review (by no means comprehensive) of how the stem cell niche evolved from an abstract concept to a complex system, implemented with a number of experimental data at the cellular and molecular levels, including metabolic cues, on which we focused in particular. The concept was introduced in 1978 to model bone marrow sites suited to host hematopoietic stem cells (HSCs) and favor their self-renewal, while restraining clonal expansion and commitment to differentiation. Studies of the effects of low oxygen tension on HSC maintenance in vitro led us to hypothesize niches were located within bone marrow areas where oxygen tension is lower than elsewhere. We named these areas hypoxic stem cell niches, although a low oxygen tension is to be considered physiological for the environment where HSCs are maintained. HSCs were later shown to have the option of cycling in low oxygen, which steers this cycling to the maintenance of stem cell potential. Cell subsets capable of withstanding incubation in very low oxygen were also detected within leukemia cell populations, including chronic myeloid leukemia (CML). The oncogenetic Bcr/Abl protein is completely suppressed in these subsets, whereas Bcr/Abl messenger ribonucleic acid is not, indicating that CML cells resistant to low oxygen are independent of Bcr/Abl for persistence in culture but remain genetically leukemic. Accordingly, leukemia stem cells of CML selected in low oxygen are refractory to the Bcr/Abl inhibitor imatinib mesylate. Bcr/Abl protein suppression turned out to be actually determined when glucose shortage complicated the effects of low oxygen, indicating that ischemia-like conditions are the driving force of leukemia stem cell refractoriness to imatinib mesylate. These studies pointed to “ischemic” stem cell niches as a novel scenario for the maintenance of minimal residual disease of CML. A possible functional relationship of the “ischemic” with the “hypoxic” stem cell niche is discussed.

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.

Alpha-irradiation of Haemopoietic Tissue in Pre- and Postnatal Mice: 2. Effects of Mid-term Contamination with239Puin Utero

The distribution of 239Pu in various tissues of foetal and postnatal offspring of pregnant mice, injected i.v. at 13 days gestation with 30 kBq 239Pu/kg (in some cases with 10 or 100 kBq/kg), together with the numbers of haemopoietic progenitors in the bone marrow, spleen and liver, were measured through to 1 year post-partum. The quality of the haemopoietic microenvironment in these mice was also measured using the renal-capsule implant method. The largest radiation dose received by any haemopoietic organ was that in the liver, amounting to 10-14 mGy, as reported previously. In spite of normal numbers of haemopoietic spleen colony-forming cells (CFC-S) in the liver and seeding, at birth, into the bone marrow where the level of plutonium was minimal, a long-term deficit in their number rapidly developed. The development of the stromal microenvironment, however, was also deficient, suggesting that the dose of alpha-irradiation to the foetal liver was sufficient to cause sublethal damage in those cells destined to become the precursors of the supportive haemopoietic microenvironment in bone marrow and spleen. The results of this study suggest that although the placenta affords significant shielding to the tissues of the developing foetus from maternal contamination, the long-term effects on haemopoiesis are comparable to those in mice contaminated as adults. This further implies that the developing haemopoietic tissues are exquisitely sensitive to 239Pu contamination.

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.

Osteopontin: a bridge between bone and blood

The production of mature blood cells within the bone marrow (BM) is attributed to a pool of haemopoietic stem cells (HSC). It is now evident that HSC reside preferentially at the endosteal region within the BM where bone-lining osteoblasts are a key cellular component of the HSC niche that directly regulates HSC fate. Osteoblasts synthesise proteins that stimulate and inhibit HSC proliferation. In addition to angiopoietin 1 (Ang-1), osteoblasts synthesise and express the highly acidic glycoprotein, osteopontin (Opn), which, like Ang-1, acts as a potent constraining factor on HSC proliferation. Overexpression of Opn is a feature of haemopoietic malignancies, such as multiple myeloma and chronic myeloid leukaemia, although its exact role in the aetiology and progression of these diseases remains unclear. Through osteoblasts and their cell surface and expressed proteins including Opn, bone is able to regulate the tissue that resides within it. In doing so, Opn can be considered a bridge between bone and blood.

Parathyroid hormone: A novel tool for treating bone marrow depletion in cancer patients caused by chemotherapeutic drugs and ionizing radiation

Between 1958 and the late 1970s it was learned that PTH (the parathyroid hormone) could directly stimulate the initiation of DNA replication by murine CFU-S (colony-forming unit-spleen) cells via cyclic AMP, stimulate the proliferation of normal and X-irradiated murine and rat bone marrow cells, control hematopoiesis, and increase the survival of X-irradiated mice and rats when injected any time between 18h before and 3h after X-irradiation. Since then, it has been shown that the hematopoietic stem cell niche consists of PTH receptor-bearing, osteoblastic trabecular bone-lining cells that maintain the stem cells' (HSCs') proliferatively quiescent 'stemness' by various gene up-regulating and down-regulating signals caused by the tight adhesion of the HSCs to the osteoblastic niche-lining cells. Stimulating the osteoblastic lining cells with recombinant human PTH-(1-34) (Forteo) causes a cyclic AMP-mediated enlargement of the HSC pool and promotes bone marrow transplant engraftment and growth and the survival of lethally irradiated mice. But this is only the beginning of the exploitation of the PTHs for marrow engraftment. It must now be determined whether the marrow engraftment-enhancing action of this potent bone growth-stimulating PTH can be extended from mice to rats and monkeys. It must be determined whether two other PTH peptides, rhPTH-(1-84) [Preos]and [Leu(27)]cyclo(Glu(22)-Lys(26))hPTH-(1-31)NH(2) [Ostabolin-C]) are as effective as or better than rhPTH-(1-34)(Forteo). Since, all three peptides are on the market, or nearing the market, for safely and strongly stimulating bone growth and treating osteoporosis one or all of them may become valuable tools for safely promoting the engraftment of peripherally harvested HSCs in cancer patients whose bone marrows have been 'emptied' by chemotherapeutic drugs or ionizing radiation.

Low oxygen tension inhibits osteogenic differentiation and enhances stemness of human MIAMI cells

We recently reported the isolation of a unique subpopulation of human stromal cells from bone marrow (BM) termed marrow-isolated adult multilineage inducible (MIAMI) cells, capable of differentiating in vitro into mature-like cells from all three germ layers. The oxygen tension (pO2) in BM ranges from 1 to 7%, which prompted us to examine the role of pO2 in regulating the capacity of MIAMI cells both to self-renew and maintain their pluripotentiality (stemness) or to progress toward osteoblastic differentiation. MIAMI cells were grown under low-pO2 conditions (1, 3, 5, and 10% oxygen) or air (21% oxygen). The proliferation rate of cells exposed to 3% oxygen (3 days) increased, resulting in cell numbers more than threefold higher than those of cells exposed to air (at 7 days). In cells grown under osteoblastic differentiation conditions, the expression of the osteoblastic markers osteocalcin, bone sialoprotein, osterix, and Runx2 and alkaline phosphatase activity was upregulated when incubated in air; however, it was blocked at low (3%) pO2. Similarly, biomineralization of long-term cell cultures was high under osteoblastic differentiation conditions in air but was undetectable at low (3%) pO2. In contrast, low pO2 upregulated mRNAs for OCT-4, REX-1, telomerase reverse transcriptase, and hypoxia-inducible factor-1 alpha, and increased the expression of SSEA-4 compared to air. Moreover, the expression of embryonic stem cell markers was sustained even under osteogenic culture conditions. Similar results were obtained using commercially available marrow stromal cells. We hypothesize a physiological scenario in which primitive MIAMI cells self-renew while localized to areas of low pO2 in the bone marrow, but tend to differentiate toward osteoblasts when they are located closer to blood vessels and exposed to higher pO2. Our results strongly suggest that maintaining developmentally primitive human cells in vitro at low pO2 would be more physiological and favor stemness over differentiation.

Mechanisms of hematopoietic stem cell mobilization: When innate immunity assails the cells that make blood and bone

Mobilization is now used worldwide to collect large numbers of hematopoietic stem and progenitor cells (HSPCs) for transplantation. Although the first mobilizing agents were discovered largely by accident, discovery of more efficient mobilizing agents will require a better understanding of the molecular mechanisms responsible. During the past 5 years, a number of mechanisms have been identified, shedding new light on the dynamics of the hematopoietic system in vivo and on the intricate relationship between hematopoiesis, innate immunity, and bone. After briefly reviewing the mechanisms by which circulating HSPCs home into the bone marrow and what keeps them there, the current knowledge of mechanisms responsible for HSPC mobilization in response to hematopoietic growth factors such as granulocyte colony-stimulating factor, chemotherapy, chemokines, and polyanions will be discussed together with current strategies developed to further increase HSPC mobilization.

Sustained stromal stem cell self-renewal and osteoblastic differentiation during aging

We have reported the isolation of a unique subpopulation of human stromal cells from bone marrow termed marrow-isolated adult multilineage inducible (MIAMI) cells. The expression of embryonic stem cell markers SSEA-4, Oct-4, Rex-1, and telomerase reverse transcriptase indicates the developmentally immature status of these cells. They resemble primitive stem cells in their capacity to differentiate, at least in vitro, into mature-like cells from all three germ layers. MIAMI cells are characterized by a unique molecular profile that distinguishes them from other marrow stromal cell populations. Although the frequency of MIAMI cells, among all marrow nucleated cells, decreases from 0.01% at age 3 to 0.0018% at age 45, their numbers remain unchanged after age 45. The level of expression of the markers characteristic of MIAMI cells remains constant independent of age and gender. In long-term in vitro expansion experiments aging increased the population doubling time by about 30%, whereas specific in vitro differentiation of MIAMI cells toward osteoblastic cells was unaffected. Because the oxygen tension in bone marrow ranges from 1% to 7%, we examined the role of oxygen tension in regulating the capacity of MIAMI cells to self-renew and maintain their pluripotentiality during long-term culture. Low oxygen tension upregulated mRNAs for primitive embryonic stem cell markers. Our results suggest that maintaining developmentally primitive human cells in vitro at low oxygen tension is more physiologic and favors stemness. For osteoblastic differentiation, gap-junctional communication mediated by connexin43 is required. Its inhibition not only blocked osteoblastic differentiation but stimulated the adipocytic differentiation.

Kinetics of metastatic breast cancer cell trafficking in bone

PURPOSE

In vivo studies have focused on the latter stages of the bone metastatic process (osteolysis), whereas little is known about earlier events, e.g., arrival, localization, and initial colonization. Defining these initial steps may potentially identify the critical points susceptible to therapeutic intervention.

EXPERIMENTAL DESIGN

MDA-MB-435 human breast cancer cells engineered with green fluorescent protein were injected into the cardiac left ventricle of athymic mice. Femurs were analyzed by fluorescence microscopy, immunohistochemistry, real-time PCR, flow cytometry, and histomorphometry at times ranging from 1 hour to 6 weeks.

RESULTS

Single cells were found in distal metaphyses at 1 hour postinjection and remained as single cells up to 72 hours. Diaphyseal arrest occurred rarely and few cells remained there after 24 hours. At 1 week, numerous foci (2-10 cells) were observed, mostly adjacent to osteoblast-like cells. By 2 weeks, fewer but larger foci (> or =50 cells) were seen. Most bones had a single large mass at 4 weeks (originating from a colony or coalescing foci) which extended into the diaphysis by 4 to 6 weeks. Little change (<20%) in osteoblast or osteoclast numbers was observed at 2 weeks, but at 4 to 6 weeks, osteoblasts were dramatically reduced (8% of control), whereas osteoclasts were reduced modestly (to approximately 60% of control).

CONCLUSIONS

Early arrest in metaphysis and minimal retention in diaphysis highlight the importance of the local milieu in determining metastatic potential. These results extend the Seed and Soil hypothesis by demonstrating both intertissue and intratissue differences governing metastatic location. Ours is the first in vivo evidence that tumor cells influence not only osteoclasts, as widely believed, but also eliminate functional osteoblasts, thereby restructuring the bone microenvironment to favor osteolysis. The data may also explain why patients receiving bisphosphonates fail to heal bone despite inhibiting resorption, implying that concurrent strategies that restore osteoblast function are needed to effectively treat osteolytic bone metastases.

Perspective: fundamental and clinical concepts on stem cell homing and engraftment: a journey to niches and beyond

In many ways, the homing of hematopoietic stem cells to bone marrow and other tissues defines these cells and their immediate and long-term fates Once homed, an inevitable series of proliferative and differentiative events presumptively follows. These comments, of course, hold for both homing to marrow, or alternatively, to other nonmarrow tissues. In this review, we will specifically focus on homing and engraftment to bone marrow because this is the best-studied and clinically applicable system.

Parathyroid hormone (PTH) and hematopoiesis: New support for some old observations

Forty-seven years ago, the parathyroid hormone (PTH) in one injection of Lilly's old bovine parathyroid extract, PTE, was found to greatly increase the 30-day survival of heavily X-irradiated rats when given from 18 h before to as long as 3 h after irradiation but no later. This was the first indication that PTH might stimulate hematopoiesis. Recent studies have confirmed the relation between PTH and hematopoiesis by showing that hPTH-(1-34)OH increases the size of the hematopoietic stem cell pool in mice. The peptide operates through a cyclic AMP-mediated burst of Jagged 1 production in osteoblastic cells lining the stem cells' niches on trabecular bone surfaces. The osteoblastic cells' Jagged 1 increases the hematopoietic stem cell pool by activating Notch receptors on attached stem cells. PTH-triggered cyclic AMP signals also directly stimulate the proliferation of the hematopoietic stem cells. However, the single PTH injection in the early experiments using PTE probably increased the survival of irradiated rats mainly by preventing the damaged hematopoietic progenitors from irreversibly initiating self-destructive apoptogenesis during the first 5 h after irradiation. It has also been shown that several daily injections of hPTH-(1-34)OH enable lethally irradiated mice to survive by stimulating the growth of transplanted normal bone marrow cells. If the osteogenic PTHs currently entering or on the verge of entering the market for treating osteoporosis can also drive hematopoiesis in humans as well as rodents, they could be potent tools for reducing the damage inflicted on bone marrow by cytotoxic cancer chemotherapeutic drugs and ionizing radiation.

Membrane-bound stem cell factor is a key regulator in the initial lodgment of stem cells within the endosteal marrow region

OBJECTIVE

The transmembrane isoform of stem cell factor (tm-SCF) has been implicated in the adhesion of hemopoietic stem cells to the extracellular matrix within the bone marrow microenvironment in vitro. In addition, in vivo SCF has been shown to play a role in cell mobilization and migration. The aim of this study was to determine if SCF is an integral component of the hemopoietic "niche" of the bone marrow in situ.

MATERIALS AND METHODS

To analyze the role of tm-SCF in cell lodgment, purified populations of primitive progenitors and hemopoietic stem cells (HSC) were transplanted into a hemopoietic microenvironment devoid of tm-SCF, and the spatial distribution of engrafted cells was analyzed. In addition, populations of HSC were isolated using non-neutralizing and neutralizing antibodies to the SCF receptor c-kit, and their spatial distribution was analyzed post-transplant.

RESULTS

The data demonstrated a significant impairment in the lodgment of transplanted cells within the endosteal marrow region in mice lacking tm-SCF, with a reduction of almost 30% by 15 hours post-transplant. The role of tm-SCF was confirmed by analyzing the spatial distribution of HSC isolated using a neutralizing antibody to c-kit.

CONCLUSION

The data demonstrate that although tm-SCF does not appear to play a role in the homing of transplanted cells to the bone marrow, it is critical in the lodgment and detainment of HSC within their hemopoietic "niche."

The topologic and chronologic patterns of hematopoietic cell seeding in host femoral bone marrow after transplantation

The early stages of homing, seeding, and engraftment of hematopoietic stem and progenitor cells are poorly characterized. We have developed an optical technique that allows in vivo tracking of transplanted, fluorescent-tagged cells in the host femurs. In this study we used fluorescence microscopy to monitor the topologic and chronologic patterns of hematopoietic cell seeding in the femoral bone marrow (BM) of mice. PKH-labeled cells homed to the femur within minutes after injection into a peripheral vein. Most cells drifted within the marrow space and gradually seeded in clusters close to the endosteal surface of the epiphyseal cortex. Three days after transplantation 85% to 94% (14%) of PKH-labeled cells in the femoral marrow were located within 100 microm of the epiphyseal bone surface (P <.001 versus the more central cells), whereas labeled cells were absent in the femoral diaphysis. Primary seeding of juxtaendosteal, epiphyseal marrow occurred independently of recipient conditioning (myeloablated and nonconditioned hosts), donor-recipient antigen disparity, or the phenotype of the injected cells (whole BM and lineage-negative cells) and was consistently observed in secondary recipients of BM-homed cells. Seeding in regions close to the epiphyseal bone was also observed in freshly excised femurs perfused ex vivo and in femurs assessed without prior placement of optical windows, indicating that the site of primary seeding was not affected by surgical placement of optical windows. Four to 5 days after transplantation, cellular clusters appeared in the more central regions of the epiphyses and in the diaphyses. Centrally located cells showed decreased PKH fluorescence, suggesting that they were progeny of the seeding cells, and brightly fluorescent cells (quiescent first-generation seeding cells) were observed close to the bone surface for as long as 24 days after transplantation. These data indicate that the periphery of the femoral marrow hosts primary seeding and that quiescent cells continue to reside in the periphery for weeks and do not divide. The site of proliferation of transplanted cells is the center of the marrow space.

Transplanted hematopoietic cells seed in clusters in recipient bone marrow in vivo

The process of hematopoietic stem and progenitor cell (HSPC) seeding in recipient bone marrow (BM) early after transplantation is not fully characterized. In vivo tracking of HSPCs, labeled with PKH dyes, through an optical window surgically implanted on the mouse femur revealed that transplanted cells cluster in the recipient BM. Within the first day after intravenous injection, 86 +/- 6% of the cells seeded in clusters (p < 0.001 versus scattered cells) in the endosteal surfaces of the epiphyses. The primary clusters were formed by concomitant seeding of 6-10 cells over an area of approximately 70 microm, and secondarily injected cells did not join the already existing clusters but formed new clusters. Major antigen-disparate HSPCs participated in formation of the primary clusters, and T lymphocytes were also incorporated. After 4 to 5 days, some cellular clusters were observed in the more central regions of the BM, where the brightness of PKH fluorescence decreased, indicating cellular division. These later clusters were classified as secondary, assuming that the mechanisms of migration in the BM might be different from those of primary seeding. Some clusters remained in the periphery of the BM and retained bright fluorescence, indicating cellular quiescence. The number of brightly fluorescent cells in the clusters decreased exponentially to two to three cells after 24 days (p < 0.001). The data suggest that the hematopoietic niche is a functional unit of the BM stromal microenvironment that hosts seeding of a number of transplanted cells, which form a cluster. This may be the site where auxiliary non-HSPC cells, such as T lymphocytes, act in support of HSPC engraftment.

Spatial localization of transplanted hemopoietic stem cells: inferences for the localization of stem cell niches

The spatial distribution of subpopulations of hemopoietic progenitor cells following syngeneic transplantation was investigated at the single-cell level. The location of infused hemopoietic progenitor cells within the femoral bone marrow of nonablated recipients was determined by 5-(and-6)-carboxyfluorescein diacetate succinimidyl ester labeling of cells and in situ fixation by perfusion. Analysis performed over 15 hours after infusion demonstrated that the spatial distribution of transplanted marrow cells is not a random process. Although the majority of cells enter the bone marrow from the central marrow vessels, the subsequent localization within the bone marrow varied according to their phenotype. Candidate "stem cells" demonstrated selective redistribution and were significantly enriched within the endosteal region, whereas mature terminally differentiated and lineage-committed cells selectively redistributed away from the endosteal region and were predominantly in the central marrow region. Together, these data strongly support historical evidence of the presence of endosteal hemopoietic stem cell niches.

Radionuclide therapy with bone-seeking compounds: Monte Carlo calculations of dose-volume histograms for bone marrow in trabecular bone

The purpose of the present work was to investigate how haematopoietic stem cell survival is affected by the differences in the dose distribution that arise from different radionuclides contained in bone-seeking radiopharmaceuticals. This was carried out in three steps: (a) calculations of representative dose distributions in individual bone marrow cavities that are irradiated by sources of 89Sr, 186Re, 117mSn or 153Sm, uniformly distributed on the bone surfaces; (b) assessment of the corresponding haematopoietic stem cell survival and (c) a comparison of these results with results obtained using the assumption of a uniform dose distribution. Two different idealized models of the geometry of trabecular bone were formulated, each consisting of an infinite array of identical elements. Monte Carlo simulations were used to generate dose-volume histograms that were used to assess haematopoietic stem cell survival with two different assumptions about spatial cell distributions. Compared with a homogeneous dose distribution, the estimated cell survival was markedly higher for 117mSn and 153Sm, and only slightly different for 89Sr and 186Re. The quantitative results differed between the two geometric models and the assumptions about spatial cell distribution, but the trends were the same. The results imply that it is necessary to include dose distributions for individual bone marrow cavities in considerations concerning bone marrow toxicity.

The expansion of murine bone marrow cells preincubated in hypoxia as an in vitro indicator of their marrow-repopulating ability

In liquid cultures of murine bone marrow cells stimulated with interleukin-3 and granulocyte/macrophage colony-stimulating factor, hypoxia (1% oxygen) induced a reversible block of hematopoiesis, maintaining the progenitors' expansion potential unreduced. Progenitors repopulating day-14 hypoxic cultures with cells or granulocyte/macrophage colony-forming units (CFU-GM) were found, on the basis of their maintenance in hypoxia (12% and 76%, respectively), to belong to different subsets, the latter being much more efficiently maintained. The maintenance in hypoxic cultures of progenitors detectable by marrow-repopulating ability (MRA) assay was 18% for MRAcell progenitors and 69% for MRACFU progenitors. Thus, the repopulation of hypoxic cultures with cells or CFU-GM closely reflected the presence of progenitors capable of repopulating, with cells or CFU-GM, the bone marrow of lethally irradiated syngeneic animals. Progenitors repopulating hypoxic cultures were, like MRA progenitors, significantly resistant to 5-fluorouracil, progenitors repopulating cultures with CFU-GM being two-fold more resistant than those repopulating cultures with cells. We concluded that the repopulation of day-14 hypoxic cultures occurring after their transfer to air is to be considered an indicator of the maintenance of MRA progenitors in hypoxia. The relevance of these results to stem cell biology and their potential practical applications are discussed.

Pattern of localization of primitive hematopoietic cells in vivo using a novel mouse model

Bone marrow transplantation is increasingly used as a treatment for numerous immunologic, hematologic, and malignant disorders. However, the mechanism by which transplanted hematopoietic stem cells are engrafted is not completely understood. Many traditional techniques have been used to study the engraftment of transplanted stem cells. Most of these methods are ex vivo and, in some cases, donor cells must be modified to enable detection. We describe a novel alternative for identifying unmodified primitive donor cells in a murine host. This mouse model is based on the differential capacity of adenine phosphoribosyltransferase (APRT)-positive and APRT-negative cells to sequester and incorporate radiolabeled adenine. Aprt is the gene encoding the adenine phosphoribosyltransferase purine salvage enzyme and has been ablated in 129sv mice. Following the injection of APRT-positive c-kit-positive enriched hematopoietic cells into syngeneic, sublethally irradiated APRT-deficient mice, engrafted cells and their presumptive progeny were successfully tracked by polymerase chain reaction. Their presence also was visualized by autoradiography of paraffin-embedded tissue sections. APRT-positive c-kit-positive enriched cells were detected in the bone marrow, spleen, lung, and thymus of nonirradiated mice. Donor cells and their progeny were more widely distributed in tissues of sublethally irradiated mice than of their nonirradiated counterparts, demonstrating that the pattern of localization of c-kit-positive enriched cells differs between nonirradiated and sublethally irradiated syngeneic recipients. The Aprt mouse model provides a sensitive method for further studying the mechanism of engraftment of unmodified donor hematopoietic cells in relation to the tissue architecture of the recipient.

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

Increasingly, allogeneic and even more often autologous bone marrow transplants are being done to correct a wide variety of diseases. In addition, autologous marrow transplants potentially provide an opportune means of delivering genes in transfected, engrafting stem cells. However, despite its widespread clinical use and promising gene therapy applications, relatively little is known about the mechanisms of engraftment in marrow transplant recipients. This is especially so in the nonablated recipient setting. Our data show that purified lineage negative rhodamine 123/Hoechst 33342 dull transplanted hematopoietic stem cells engraft into the marrow of nonablated syngeneic recipients. These cells have multilineage potential, and maintain a distinct subpopulation with “stem cell” characteristics. The data also suggests a spatial localization of stem cell “niches” to the endosteal surface, with all donor cells having a high spatial affinity to this area. However, the level of stem cell engraftment observed following a transplant of “stem cells” was significantly lower than that expected following a transplant of the same number of unseparated marrow cells from which the purified cells were derived, suggesting the existence of a “nonstem cell facilitator population,” which is required in a nonablated syngeneic transplant setting.

The chemokine SDF-1 is a chemoattractant for human CD34+ hematopoietic progenitor cells and provides a new mechanism to explain the mobilization of CD34+ progenitors to peripheral blood

Hematopoietic progenitor cells migrate in vitro and in vivo towards a gradient of the chemotactic factor stromal cell-derived factor-1 (SDF-1) produced by stromal cells. This is the first chemoattractant reported for human CD34+ progenitor cells. Concentrations of SDF-1 that elicit chemotaxis also induce a transient elevation of cytoplasmic calcium in CD34+ cells. SDF-1-induced chemotaxis is inhibited by pertussis toxin, suggesting that its signaling in CD34+ cells is mediated by seven transmembrane receptors coupled to Gi proteins. CD34+ cells migrating to SDF-1 include cells with a more primitive (CD34+/CD38- or CD34+/DR-) phenotype as well as CD34+ cells phenotypically committed to the erythroid, lymphoid and myeloid lineages, including functional BFU-E, CFU-GM, and CFU-MIX progenitors. Chemotaxis of CD34+ cells in response to SDF-1 is increased by IL-3 in vitro and is lower in CD34+ progenitors from peripheral blood than in CD34+ progenitors from bone marrow, suggesting that an altered response to SDF-1 may be associated with CD34 progenitor mobilization.

Hematopoietic stem cells and myeloid precursor cells in nonobese diabetic (NOD) mice

Expression of diabetes susceptibility genes at the hemopoietic stem cell level is sufficient for the development of the disease in nonobese diabetic (NOD) mice. This work investigated whether the defects that reside within the stem cells have consequences on the homeostasis of the stem and progenitor cell compartments. The fraction of cyclically active stem cells, or spleen colony-forming units (CFU-s), is enlarged, and their differentiation toward megakaryocytopoiesis seems to be enhanced in NOD mice at 3 and 10 weeks of age as compared to C57BL/6 mice. Whereas colony-forming unit assay (CFU-A) numbers are normal in the bone marrow, they are significantly increased in the spleen of NOD mice. A strain-dependent difference in granulocyte-macrophage colony-forming cell (GM-CFC) numbers was observed; they were higher in NOD mice than in B6 mice of three and ten weeks of age. These results suggest that autoimmune type 1 diabetes mellitus of the NOD mouse is accompanied by disorders of myelopoiesis and megakaryopoiesis. This observation is in keeping with the role of macrophages in the development of diabetes and with the hyperactivity of diabetic platelets.

Abnormal localization of immature precursors (ALIP) in the bone marrow of myelodysplastic syndromes: current state of knowledge and future directions

It has been suggested that the occurrence of abnormal localization of immature precursors (ALIP) in the bone marrow biopsy (BMB) may be of diagnostic and prognostic significance in myelodysplastic syndromes (MDS). The recognition of ALIP has been based exclusively on bone marrow histological appearances. During the last decade technical advances have led to the widespread use of various immunophenotypic markers for the diagnostic and prognostic purposes which has contributed enormously in understanding the development of haemopoietic cells and the cellular origin of various haematological malignancies. In addition proliferation antigens, growth factors, oncogenes, anti-oncogenes and other biological discoveries have opened new vistas to our knowledge of the normal and neoplastic growth processes. Despite this, the precise nature of ALIP and their significance in relation to the aetiopathogenesis and evolution of MDS remains unclear. Indeed the diagnostic value of ALIP in MDS is debatable. Furthermore, the precise cell lineages which comprise ALIP are not defined. The purpose of this review is to address these issues and to incorporate our new findings on the histological and immunophenotypic characterization of immature cell aggregates.

The architecture of bone marrow cell populations

Marrow is a loosely bound tissue in which hemopoiesis has frequently been considered to be randomly distributed. The case is presented, however, for an organized and structured marrow in which close relationships exist between hemopoietic tissue and a regulatory microenvironment. Distributions of myeloid cells in the mouse femur are described, and a dynamic picture of their movement, with differentiation and maturation from the endosteal surface of the bone to their release via the central venous sinus, is painted. It is also shown that this structure is established within three weeks of birth. By contrast, mature lymphoid cells (but not their progenitors) are uniformly distributed. Regulatory stromal elements in the marrow are also structured and their localization is found to correspond closely to the properties of the progenitor populations. Such structure has potential practical importance, particularly in the field of medical, industrial or accidental radiation exposure where bone may introduce non-uniform dose distributions in the marrow.