Mutual, reciprocal SDF-1/CXCR4 interactions between hematopoietic and bone marrow stromal cells regulate human stem cell migration and development in NOD/SCID chimeric mice

The chemokine SDF-1 (CXCL12) and its receptor CXCR4 are involved in regulation of migration, survival, and development of multiple cell types, including human hematopoietic CD34+/CD38-/low and stromal STRO-1+ stem cells. During steady-state homeostasis, CXCR4 is expressed by hematopoietic cells and also by stromal cells, which are the main source for SDF-1 in the bone marrow (BM). Stress-induced modulations in SDF-1 and CXCR4 levels participate in recruitment of immature and maturing leukocytes from the BM reservoir to damaged organs as part of host defense and repair mechanism. In addition, trafficking of SDF-1 is mediated by CXCR4, expressed by endothelial and various stromal cell types in the BM, spleen, and other organs, but not by hematopoietic cells. Transcytosis of functional SDF-1 to the BM takes place also in the stem cell-rich endothelium and endosteum regions, regulating hematopoietic and stromal interactions in the stem cell niche. Dynamic levels of SDF-1 and CXCR4 expression induce proliferation of hematopoietic and mesenchymal progenitors, recruitment of bone-resorbing osteoclasts, osteoblasts, neutrophils, and other myeloid cells, leading to leukocyte mobilization. These studies will be reviewed together with the mechanisms that regulate SDF-1 and CXCR4 physiologic function, inactivation, presentation, and availability. Moreover, the role and the dynamic modulations of this ligand and its receptor in alarm and pathologic conditions will be discussed as well.

Osteoclasts degrade endosteal components and promote mobilization of hematopoietic progenitor cells

Here we investigated the potential role of bone-resorbing osteoclasts in homeostasis and stress-induced mobilization of hematopoietic progenitors. Different stress situations induced activity of osteoclasts (OCLs) along the stem cell-rich endosteum region of bone, secretion of proteolytic enzymes and mobilization of progenitors. Specific stimulation of OCLs with RANKL recruited mainly immature progenitors to the circulation in a CXCR4- and MMP-9-dependent manner; however, RANKL did not induce mobilization in young female PTPepsilon-knockout mice with defective OCL bone adhesion and resorption. Inhibition of OCLs with calcitonin reduced progenitor egress in homeostasis, G-CSF mobilization and stress situations. RANKL-stimulated bone-resorbing OCLs also reduced the stem cell niche components SDF-1, stem cell factor (SCF) and osteopontin along the endosteum, which was associated with progenitor mobilization. Finally, the major bone-resorbing proteinase, cathepsin K, also cleaved SDF-1 and SCF. Our findings indicate involvement of OCLs in selective progenitor recruitment as part of homeostasis and host defense, linking bone remodeling with regulation of hematopoiesis.

Stress-induced cholinergic signaling promotes inflammation-associated thrombopoiesis

To study the role of the stress-induced “readthrough” acetylcholinesterase splice variant, AChE-R, in thrombopoiesis, we used transgenic mice overexpressing human AChE-R (TgR). Increased AChE hydrolytic activity in the peripheral blood of TgR mice was associated with increased thrombopoietin levels and platelet counts. Bone marrow (BM) progenitor cells from TgR mice presented an elevated capacity to produce mixed (GEMM) and megakaryocyte (Mk) colonies, which showed intensified labeling of AChE-R and its interacting proteins RACK1 and PKC. When injected with bacterial lipopolysaccharide (LPS), parent strain FVB/N mice, but not TgR mice, showed reduced platelet counts. Therefore, we primed human CD34+ cells with the synthetic ARP26 peptide, derived from the cleavable C-terminus of AChE-R prior to transplantation, into sublethally irradiated NOD/SCID mice. Engraftment of human cells (both CD45+ and CD41+ Mk) was significantly increased in mice that received ARP26-primed CD34+ human cells versus mice that received fresh nonprimed CD34+ human cells. Moreover, ARP26 induced polyploidization and proplatelet shedding in human MEG-01 promegakaryotic cells, and human platelet engraftment increased following ex vivo expansion of ARP26-treated CD34+ cells as compared to cells expanded with thrombopoietin and stem cell factor. Our findings implicate AChE-R in thrombopoietic recovery, suggesting new therapeutic modalities for supporting platelet production.

Mobilizing the older patient with myeloma

Although hematopoietic progenitor/stem cells (HPC) have been used for autologous transplants for approximately 25 years, it is only recently that we have begun to finally understand the factors which play important roles in causing these cells to leave their marrow niches and circulate in the blood. Still less is understood about factors important in homing of these cells from the blood to the marrow, and their re-engraftment there. Nonetheless, a significant amount of clinical information exists on how to make these cells leave the marrow in order to facilitate their collection from the blood for use as a transplant graft. This review provides an overview of what is currently known about the factors influencing mobilization of HPC from the marrow into the blood. Further, it suggests how this knowledge may be used to individually optimize collection of HPC. It is particularly important to optimize collection in the older myeloma patient, where it has traditionally been difficult to collect adequate numbers of cells for the tandem transplant now thought to provide the best hope for long-term survival in this disease.

Biology of Normal and Acute Myeloid Leukemia Stem Cells

The substantial understanding that has been gained over the past 5 decades of the biology of blood formation is largely due to the development of functional quantitative assays for cells at all stages of differentiation, from multipotential stem cells to mature cells. The majority of studies have involved the mouse because the ease with which repopulation studies can be carried out with this animal model allows the assay of complete lineage development from stem cells. In the past decade, advances in repopulation assays for human stem cells using xenotransplantation have greatly enhanced our understanding of human stem cell biology. Importantly, the xenotransplantation methodology has also been used to identify the cancer stem cell that initiates and sustains leukemic proliferation, providing key evidence for the cancer stem cell hypothesis. This hypothesis argues that cancer cells are functionally heterogeneous and hierarchically organized such that only specific cells are capable of sustaining tumor growth and continuously producing the cells that make up the bulk of the tumor. Recent studies have also brought into focus the importance of the intimate relationship between the stem cell (normal or leukemic) and its microenvironment. Coming into view are the molecular players involved in stem cell homing, migration, and adhesion, as well as the cellular components of the microenvironmental niche. Here we review recent studies that have begun, to elucidate the interplay between normal and leukemic human stem cells and their microenvironment.

cAMP-induced PKCzeta activation increases functional CXCR4 expression on human CD34+ hematopoietic progenitors

Chemokines are key regulators of hematopoiesis and host defense. We report here that functional expression of the chemokine receptor CXCR4 on human immature CD34+ hematopoietic progenitors was increased as a result of sustained elevation in cellular cAMP by dbcAMP and prostaglandin E2. This effect of cAMP was specifically mediated by PKCzeta activity. CXCR4 expression and PKCzeta activation by cAMP were decreased after the inhibition of cAMP effector-Rap1 by Spa1 overexpression. Interference with the activation of Rac1, a downstream target of Rap1, prevented the cAMP-induced increase in PKCzeta activity and CXCR4 levels. Functional manifestation of the effects of cAMP-elevating agents revealed an increased ability of human CD34+ cells to transmigrate the bone marrow (BM) endothelial layer and adhere to BM stroma in vitro, and it augmented the homing potential to the BM and spleens of immunodeficient mice in a Rac1- and a PKCzeta-dependent manner. cAMP- and TNFalpha-stimulated pathways converged in PKCzeta-activated CXCR4 expression and MMP-2/MMP-9 secretion. cAMP treatment had a beneficial effect on CD34+ cell survival in a PKCzeta-mediated fashion. Taken together, our data reveal major roles for cAMP-induced PKCzeta activation in signaling governing the motility and development of CD34+ cells.

Chemokine receptor CXCR4-dependent internalization and resecretion of functional chemokine SDF-1 by bone marrow endothelial and stromal cells

Regulation of the availability of chemokine SDF-1 (CXCL12) in bone marrow is still not fully understood. Here we describe a unique function for the chemokine receptor CXCR4 expressed on bone marrow endothelial cells, which efficiently internalize circulating SDF-1, resulting in its translocation into the bone marrow. Translocated SDF-1 increased the homing of transplanted human CD34(+) hematopoietic progenitors to the bone marrow. The chemokine transporter function of CXCR4 was a characteristic of endothelial and stromal cells but not of hematopoietic cells. Thus, chemokine translocation across the blood-bone marrow barrier allows effective transfer of functional SDF-1 from the periphery to the stem cell niche in the bone marrow during both homeostasis and 'alarm' situations.

Motility, proliferation, and egress to the circulation of human AML cells are elastase dependent in NOD/SCID chimeric mice

The role of the proteolytic enzyme elastase in motility and proliferation of leukemic human acute myeloblastic leukemia (AML) cells is currently unknown. We report a correlation between abnormally high levels of elastase in the blood of AML patients and the number of leukemic blast cells in the circulation. In AML cells, we observed expression of cell-surface elastase, which was regulated by the chemokine stromal cell-derived factor-1 (SDF-1). In vitro inhibition of elastase prevented SDF-1-induced cell polarization, podia formation, and reduced migration of human AML cells as well as their adhesion. Elastase inhibition also significantly impaired in vivo homing of most human AML cells to the bone marrow (BM) of nonobese diabetic-severe combined immunodeficient (NOD/SCID)/beta-2 microglobulin knock-out (B2mnull) mice that underwent transplantation. Moreover, in vitro proliferation of AML cells was elastase dependent. In contrast, treatment with elastase inhibitor enhanced the proliferation rate of human cord blood CD34+ cells, including primitive CD34+/CD38- cells, and their in vivo homing. Finally, NOD/SCID mice previously engrafted with human AML cells and treated with elastase inhibitor had significantly reduced egress of leukemic cells into the circulation. Taken together, our data demonstrate that human AML cells constitutively secrete and express SDF-1-dependent cell-surface elastase, which regulates their migration and proliferation. (Blood. 2005;106:2120-2127)

Heat Shock Protein 60 Activates Cytokine-Associated Negative Regulator Suppressor of Cytokine Signaling 3 in T Cells: Effects on Signaling, Chemotaxis, and Inflammation

Previously, we reported that treatment of T cells with the 60-kDa heat shock protein (HSP60) inhibits chemotaxis. We now report that treatment of purified human T cells with recombinant human HSP60 or its biologically active peptide p277 up-regulates suppressor of cytokine signaling (SOCS)3 expression via TLR2 and STAT3 activation. SOCS3, in turn, inhibits the downstream effects of stromal cell-derived-1alpha (CXCL12)-CXCR4 interaction in: 1) phosphorylation of ERK1/2, Pyk2, AKT, and myosin L chain, required for cell adhesion and migration; 2) formation of rear-front T cell polarity; and 3) migration into the bone marrow of NOD/SCID mice. HSP60 also activates SOCS3 in mouse lymphocytes and inhibits their chemotaxis toward stromal cell-derived factor-1alpha and their ability to adoptively transfer delayed-type hypersensitivity. These effects of HSP60 could not be attributed to LPS or LPS-associated lipoprotein contamination. Thus, HSP60 can regulate T cell-mediated inflammation via specific signal transduction and SOCS3 activation.

How do stem cells find their way home?

Migration of hematopoietic stem cells through the blood, across the endothelial vasculature to different organs and to their bone marrow (BM) niches, requires active navigation, a process termed homing. Homing is a rapid process and is the first and essential step in clinical stem cell transplantation. Similarly, homing is required for seeding of the fetal BM by hematopoietic progenitors during development. Homing has physiological roles in adult BM homeostasis, which are amplified during stress-induced recruitment of leukocytes from the BM reservoir and during stem cell mobilization, as part of host defense and repair. Homing is thought to be a coordinated, multistep process, which involves signaling by stromal-derived factor 1 (SDF-1) and stem cell factor (SCF), activation of lymphocyte function-associated antigen 1 (LFA-1), very late antigen 4/5 (VLA-4/5) and CD44, cytoskeleton rearrangement, membrane type 1 (MT1)-matrix metalloproteinase (MMP) activation and secretion of MMP2/9. Rolling and firm adhesion of progenitors to endothelial cells in small marrow sinusoids under blood flow is followed by trans-endothelial migration across the physical endothelium/extracellular matrix (ECM) barrier. Stem cells finalize their homing uniquely, by selective access and anchorage to their specialized niches in the extravascular space of the endosteum region and in periarterial sites. This review is focused on mechanisms and key regulators of human stem cell homing to the BM in experimental animal models and clinical transplantation protocols.

Elevated serum levels of stromal-derived factor-1alpha are associated with increased osteoclast activity and osteolytic bone disease in multiple myeloma patients

Multiple myeloma (MM) is an incurable plasma cell (PC) malignancy able to mediate massive destruction of the axial and craniofacial skeleton. The aim of this study was to investigate the role of the potent chemokine, stromal-derived factor-1alpha (SDF-1alpha) in the recruitment of osteoclast precursors to the bone marrow. Our studies show that MM PC produce significant levels of SDF-1alpha protein and exhibit elevated plasma levels of SDF-1alpha when compared with normal, age-matched subjects. The level of SDF-1alpha positively correlated with the presence of multiple radiological bone lesions in individuals with MM, suggesting a potential role for SDF-1alpha in osteoclast precursor recruitment and activation. To examine this further, peripheral blood-derived CD14+ osteoclast precursors were cultured in an in vitro osteoclast-potentiating culture system in the presence of recombinant human SDF-1alpha. Although failing to stimulate an increase in TRAP+, multinucleated osteoclast formation, our studies show that SDF-1alpha mediated a dramatic increase in both the number and the size of the resorption lacunae formed. The increased osteoclast motility and activation in response to SDF-1alpha was associated with an increase in the expression of a number of osteoclast activation-related genes, including RANKL, RANK, TRAP, MMP-9, CA-II, and Cathepsin K. Importantly, the small-molecule CXCR4-specific inhibitor, 4F-Benzoyl-TE14011 (T140), effectively blocked osteoclast formation stimulated by the myeloma cell line, RPMI-8226. Based on these findings, we believe that the synthesis of high levels of SDF-1alpha by MM PC may serve to recruit osteoclast precursors to local sites within the bone marrow and enhance their motility and bone-resorbing activity. Therefore, we propose that inhibition of the CXCR4-SDF-1alpha axis may provide an effective means of treatment for MM-induced osteolysis.

Cycling G1 CD34+/CD38+Cells Potentiate the Motility and Engraftment of Quiescent G0 CD34+/CD38−/lowSevere Combined Immunodeficiency Repopulating Cells

The mechanism of human stem cell expansion ex vivo is not fully understood. Furthermore, little is known about the mechanisms of human stem cell homing/repopulation and the role that differentiating progenitor cells may play in these processes. We report that 2- to 3-day in vitro cytokine stimulation of human cord blood CD34(+)-enriched cells induces the production of short-term repopulating, cycling G1 CD34(+)/CD38(+) cells with increased matrix metalloproteinase (MMP)-9 secretion as well as increased migration capacity to the chemokine stromal cell-derived factor-1 (SDF-1) and homing to the bone marrow of irradiated nonobese diabetic severe/combined immunodeficiency (NOD/SCID) mice. These cycling G1 cells enhance SDF-1-mediated in vitro migration and in vivo homing of quiescent G0 CD34(+) cells, which is partially abrogated after inhibition of MMP-2/-9 activity. Moreover, the engraftment potential of quiescent G0 SCID repopulating cells (SRCs) is also increased by the cycling G1 CD34(+)/CD38(+) cells. This effect is significantly abrogated after incubation of cycling G1 cells with a neutralizing anti-CXCR4 antibody. Our data suggest synergistic interactions between accessory cycling G1 CD34(+)/CD38(+) committed progenitor cells and quiescent, primitive G0 CD34(+)/CD38(-/low) SRC/stem cells, the former increasing the motility and engraftment potential of the latter, partly via secretion of MMP-9.

Tumor necrosis factor promotes human T-cell development in nonobese diabetic/severe combined immunodeficient mice

A major problem after clinical hematopoietic stem cell transplantations is poor T-cell reconstitution. Studying the mechanisms underlying this concern is hampered, because experimental transplantation of human stem and progenitor cells into nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice usually results in low T-lymphocyte reconstitution. Because tumor necrosis factor alpha (TNFalpha) has been proposed to play a role in T-lineage commitment and differentiation in vitro, we investigated its potential to augment human T-cell development in vivo. Administration of TNF to irradiated NOD/SCID mice before transplantation of human mononuclear cells from either cord blood or adult G-CSF-mobilized peripheral blood (MPBL) led 2-3 weeks after transplantation to the emergence of human immature CD4(+)CD8(+) double-positive T-cells in the bone marrow (BM), spleen, and thymus, and in this organ, the human cells also express CD1a marker. One to 2 weeks later, single-positive CD4(+) and CD8(+) cells expressing heterogenous T-cell receptor alpha beta were detected in all three organs. These cells were also capable of migrating through the blood circulation. Interestingly, human T-cell development in these mice was associated with a significant reduction in immature lymphoid human CD19(+) B cells and natural killer progenitors in the murine BM. The human T cells were mostly derived from the transplanted immature CD34(+) cells. This study demonstrates the potential of TNF to rapidly augment human T lymphopoiesis in vivo and also provides clinically relevant evidence for this process with adult MPBL progenitors.

Atypical PKC-zeta regulates SDF-1-mediated migration and development of human CD34+ progenitor cells

The chemokine stromal cell-derived factor-1 (SDF-1) and its receptor, CXCR4, play a major role in migration, retention, and development of hematopoietic progenitors in the bone marrow. We report the direct involvement of atypical PKC-zeta in SDF-1 signaling in immature human CD34(+)-enriched cells and in leukemic pre-B acute lymphocytic leukemia (ALL) G2 cells. Chemotaxis, cell polarization, and adhesion of CD34(+) cells to bone marrow stromal cells were found to be PKC-zeta dependent. Overexpression of PKC-zeta in G2 and U937 cells led to increased directional motility to SDF-1. Interestingly, impaired SDF-1-induced migration of the pre-B ALL cell line B1 correlated with reduced PKC-zeta expression. SDF-1 triggered PKC-zeta phosphorylation, translocation to the plasma membrane, and kinase activity. Furthermore we identified PI3K as an activator of PKC-zeta, and Pyk-2 and ERK1/2 as downstream targets of PKC-zeta. SDF-1-induced proliferation and MMP-9 secretion also required PKC-zeta activation. Finally, we showed that in vivo engraftment, but not homing, of human CD34(+)-enriched cells to the bone marrow of NOD/SCID mice was PKC-zeta dependent and that injection of mice with inhibitory PKC-zeta pseudosubstrate peptides resulted in mobilization of murine progenitors. Our results demonstrate a central role for PKC-zeta in SDF-1-dependent regulation of hematopoietic stem and progenitor cell motility and development.

Stromal-derived factor-1 promotes the growth, survival, and development of human bone marrow stromal stem cells

The maintenance of bone marrow stromal stem cells (BMSSCs) is tightly controlled by the local microenvironment and by autocrine regulatory factors secreted by BMSSCs. To identify such factors, a cDNA subtraction library was generated from purified BMSSCs, based on their high expression of the STRO-1 antigen. Stromal-derived factor-1 (SDF-1) was one differentially expressed gene highly expressed by purified BMSSCs prior to culture. In vitro, immature preosteogenic cells expressed greater levels of SDF-1 when compared with mature cell types representative of osteoblasts and osteocytes/bone lining cells. Furthermore, SDF-1 expression was rapidly down-regulated when BMSSCs were cultured under osteoinductive conditions. BMSSCs were also shown to express functional cell surface SDF-1 receptors (CXCR4). Transduced BMSSC lines, secreting high SDF-1 levels, displayed an enhanced ability to form ectopic bone in vivo, in comparison with control BMSSC lines. Moreover, high SDF-1-expressing BMSSCs displayed an increased capacity for cellular growth and protection against interleukin-4-induced apoptosis. Similarly, fibroblast colony-forming units (CFU-Fs) also displayed increased growth and resistance to alpha-interferon-2a-induced apoptosis, in synergy with platelet-derived growth factor BB (PDGF-BB) and SDF-1 in vitro. These studies indicate that the chemokine, SDF-1, may play a role in the maintenance, survival, and osteogenic capacity of immature BMSSC populations.

Caspase-8 serves both apoptotic and nonapoptotic roles

Knockout of caspase-8, a cysteine protease that participates in the signaling for cell death by receptors of the TNF/nerve growth factor family, is lethal to mice in utero. To explore tissue-specific roles of this enzyme, we established its conditional knockout using the Cre/loxP recombination system. Consistent with its role in cell death induction, deletion of caspase-8 in hepatocytes protected them from Fas-induced caspase activation and death. However, application of the conditional knockout approach to investigate the cause of death of caspase-8 knockout embryos revealed that this enzyme also serves cellular functions that are nonapoptotic. Its deletion in endothelial cells resulted in degeneration of the yolk sac vasculature and embryonal death due to circulatory failure. Caspase-8 deletion in bone-marrow cells resulted in arrest of hemopoietic progenitor functioning, and in cells of the myelomonocytic lineage, its deletion led to arrest of differentiation into macrophages and to cell death. Thus, besides participating in cell death induction by receptors of the TNF/nerve growth factor family, caspase-8, apparently independently of these receptors, also mediates nonapoptotic and perhaps even antiapoptotic activities.

CXCR4 regulates migration and development of human acute myelogenous leukemia stem cells in transplanted NOD/SCID mice

The chemokine stromal cell-derived factor-1 (SDF-1) and its receptor CXCR4 participate in the retention of normal hematopoietic stem cells within the bone marrow (BM) and their release into the circulation. Homing and engraftment of human stem cells in immunodeficient mice are dependent on cell surface CXCR4 expression and the production of BM SDF-1, which acts also as a survival factor for both human and murine stem cells. However, the role of SDF-1/CXCR4 interactions in the control of human acute myelogenous leukemia (AML) cell trafficking and disease progression is poorly understood. In this study, we report that although some AML cells do not express surface CXCR4, all AML cells tested express internal CXCR4 and SDF-1. Culture of AML cells with SDF-1 promoted their survival, whereas addition of neutralizing CXCR4 antibodies, SDF-1 antibodies, or AMD3100 significantly decreased it. Pretreatment of primary human AML cells with neutralizing CXCR4 antibodies blocked their homing into the BM and spleen of transplanted NOD/SCID/B2m(null) mice. Furthermore, weekly administrations of antihuman CXCR4 to mice previously engrafted with primary AML cells led to a dramatic decrease in the levels of human AML cells in the BM, blood, and spleen in a dose- and time-dependent manner. Interestingly, the same treatment did not affect significantly the levels of normal human progenitors engrafted into NOD/SCID mice. Taken together, our findings demonstrated the importance of the SDF-1/CXCR4 axis in the regulation of in vivo motility and development of human AML stem cells and identified CXCR4 neutralization as a potential treatment for AML.

Unique SDF-1–induced activation of human precursor-B ALL cells as a result of altered CXCR4 expression and signaling

The mechanisms governing migration and extramedullary dissemination of leukemic cells remain obscure. In this study the migration and in vivo homing to the bone marrow of nonobese diabetic severe combined immunodeficient (NOD/SCID) mice injected with human precursor-B acute lymphoblastic leukemia (ALL) cells in comparison to normal CD34+ progenitors (both cord blood and mobilized peripheral blood) was investigated. Although migration and homing of both cell populations was dependent on stromal cell-derived factor 1 (SDF-1)/CXCR4 interactions, major differences in receptor expression as well as the migratory capacity toward various concentrations of SDF-1 were found. Furthermore, unlike normal CD34+ progenitors, in vivo homing of the leukemic cells was superior when recipient NOD/SCID mice were not irradiated prior to transplantation. In addition, we report differences in the adhesion molecules activated following SDF-1 stimulation, documenting a major role for very late antigen 4 (VLA-4), but not VLA-5 and lymphocyte function-associated antigen-1 (LFA-1), in homing of precursor-B ALL cells. Interestingly, Toxin-B and pertussis toxin inhibited the homing of the leukemic cells but not that of normal CD34+ progenitors or normal CD10+/CD19+ precursor-B cells, revealing differences in CXCR4 signaling pathways that are based on changes that acquired by the leukemic cells. Altogether, our data provide new insights into different SDF-1–induced signaling, activation, and consequent motility between normal CD34+ and precursor-B ALL progenitors, which may lead to improved clinical protocols. (Blood. 2004;103: 2900-2907)

CD44 and hyaluronic acid cooperate with SDF-1 in the trafficking of human CD34+ stem/progenitor cells to bone marrow

Trafficking of human CD34+ stem/progenitor cells (HSCs/HPCs) is regulated by chemokines, cytokines, proteolytic enzymes, and adhesion molecules. We report that the adhesion receptor CD44 and its major ligand, hyaluronic acid (HA), are essential for homing into the bone marrow (BM) and spleen of nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice and engraftment by human HSCs. Homing was blocked by anti-CD44 monoclonal antibodies (mAbs) or by soluble HA, and it was significantly impaired after intravenous injection of hyaluronidase. Furthermore, stromal cell-derived factor-1 (SDF-1) was found to be a rapid and potent stimulator of progenitor adhesion to immobilized HA, leading to formation of actin-containing protrusions with CD44 located at their tips. HPCs migrating on HA toward a gradient of SDF-1 acquired spread and polarized morphology with CD44 concentrating at the pseudopodia at the leading edge. These morphologic alterations were not observed when the progenitors were first exposed to anti-CD44 mAbs, demonstrating a crosstalk between CD44 and CXCR4 signaling. Unexpectedly, we found that HA is expressed on human BM sinusoidal endothelium and endosteum, the regions where SDF-1 is also abundant. Taken together, our data suggest a key role for CD44 and HA in SDF-1-dependent transendothelial migration of HSCs/HPCs and their final anchorage within specific niches of the BM.

Overexpression of CXCR4 on human CD34+ progenitors increases their proliferation, migration, and NOD/SCID repopulation

A major limitation to clinical stem cell-mediated gene therapy protocols is the low levels of engraftment by transduced progenitors. We report that CXCR4 overexpression on human CD34+ progenitors using a lentiviral gene transfer technique helped navigate these cells to the murine bone marrow and spleen in response to stromal-derived factor 1 (SDF-1) signaling. Cells overexpressing CXCR4 exhibited significant increases in SDF-1-mediated chemotaxis and actin polymerization compared with control cells. A major advantage of CXCR4 overexpression was demonstrated by the ability of transduced CD34+ cells to respond to lower, physiologic levels of SDF-1 when compared to control cells, leading to improved SDF-1-induced migration and proliferation/survival, and finally resulting in significantly higher levels of in vivo repopulation of nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice including primitive CD34+/CD38(-/low) cells. Importantly, no cellular transformation was observed following transduction with the CXCR4 vector. Unexpectedly, we documented lack of receptor internalization in response to high levels of SDF-1, which can also contribute to increased migration and proliferation by the transduced CD34+ cells. Our results suggest CXCR4 overexpression for improved definitive human stem cell motility, retention, and multilineage repopulation, which could be beneficial for in vivo navigation and expansion of hematopoietic progenitors.

The onset of p53-dependent apoptosis plays a role in terminal differentiation of human normoblasts

The p53 tumor suppressor gene was found to play a role in the differentiation of several tissue types. We report here that p53-dependent apoptosis plays a role in the final stages of physiological differentiation of normoblasts, resulting in nuclear condensation and expulsion without cell death. Blood samples of healthy newborns, cord blood as well as bone marrow, were analysed for apoptosis by TUNEL and p53 expression by immunostaining. While some samples exhibited simultaneously several distinct patterns of apoptosis, such as perinuclear, diffused nuclear or nuclear apoptotic bodies, others presented a single defined pattern. Overexpression of p53 protein was detected in normoblasts exhibiting either perinuclear or diffused nuclear p53, corresponding to the nuclear apoptotic pattern in the same sample. Similar results were also evident with colonies cultivated for 12-14 days in culture. Differentiated erythroid colonies exhibited overexpression of p53 and positive TUNEL staining only in the normoblasts. We further examined the state of caspase 3/7 and observed a decrease of this activated enzyme during erythroid differentiation in culture. This study suggests a novel role for apoptosis in normoblast differentiation where nuclear degradation occurs with a delay in the actual cell death. A pivotal role for the p53-dependent apoptosis in the erythroid lineage development is implied. However, this apoptotic process is not fully executed because of the exhaustion in caspase 3/7 and thus cells are diverted towards final stages of differentiation.

HGF, SDF-1, and MMP-9 are involved in stress-induced human CD34+ stem cell recruitment to the liver

Hematopoietic stem cells rarely contribute to hepatic regeneration, however, the mechanisms governing their homing to the liver, which is a crucial first step, are poorly understood. The chemokine stromal cell-derived factor-1 (SDF-1), which attracts human and murine progenitors, is expressed by liver bile duct epithelium. Neutralization of the SDF-1 receptor CXCR4 abolished homing and engraftment of the murine liver by human CD34+ hematopoietic progenitors, while local injection of human SDF-1 increased their homing. Engrafted human cells were localized in clusters surrounding the bile ducts, in close proximity to SDF-1-expressing epithelial cells, and differentiated into albumin-producing cells. Irradiation or inflammation increased SDF-1 levels and hepatic injury induced MMP-9 activity, leading to both increased CXCR4 expression and SDF-1-mediated recruitment of hematopoietic progenitors to the liver. Unexpectedly, HGF, which is increased following liver injury, promoted protrusion formation, CXCR4 upregulation, and SDF-1-mediated directional migration by human CD34+ progenitors, and synergized with stem cell factor. Thus, stress-induced signals, such as increased expression of SDF-1, MMP-9, and HGF, recruit human CD34+ progenitors with hematopoietic and/or hepatic-like potential to the liver of NOD/SCID mice. Our results suggest the potential of hematopoietic CD34+/CXCR4+cells to respond to stress signals from nonhematopoietic injured organs as an important mechanism for tissue targeting and repair.

Expression profiles of acute lymphoblastic and myeloblastic leukemias with ALL-1 rearrangements

The ALL-1 gene is directly involved in 5-10% of acute lymphoblastic leukemias (ALLs) and acute myeloid leukemias (AMLs) by fusion to other genes or through internal rearrangements. DNA microarrays were used to determine expression profiles of ALLs and AMLs with ALL-1 rearrangements. These profiles distinguish those tumors from other ALLs and AMLs. The expression patterns of ALL-1-associated tumors, in particular ALLs, involve oncogenes, tumor suppressors, antiapoptotic genes, drug-resistance genes, etc., and correlate with the aggressive nature of the tumors. The genes whose expression differentiates between ALLs with and without ALL-1 rearrangement were further divided into several groups, enabling separation of ALL-1-associated ALLs into two subclasses. One of the groups included 43 genes that exhibited expression profiles closely linked to ALLs with ALL-1 rearrangements. Further, there were evident differences between the expression profiles of AMLs in which ALL-1 had undergone fusion to other genes and AMLs with partial duplication of ALL-1. The extensive analysis described here pinpointed genes that might have a direct role in pathogenesis.