Homing of human hematopoietic stem and progenitor cells: new insights, new challenges?

Hypoxia-Induced Alpha-Globin Expression in Syncytiotrophoblasts Mimics the Pattern Observed in Preeclamptic Placentas

Preeclampsia (PE) is a pregnancy disorder associated with placental dysfunction and elevated fetal hemoglobin (HbF). Early in pregnancy the placenta harbors hematopoietic stem and progenitor cells (HSPCs) and is an extramedullary source of erythropoiesis. However, globin expression is not unique to erythroid cells and can be triggered by hypoxia. To investigate the role of the placenta in increasing globin levels previously reported in PE, flow cytometry, histological and immunostaining and in situ analyses were used on placenta samples and ex vivo explant cultures. Our results indicated that in PE pregnancies, placental HSPC homing and erythropoiesis were not affected. Non-erythroid alpha-globin mRNA and protein, but not gamma-globin, were detected in syncytiotrophoblasts and stroma of PE placenta samples. Similarly, alpha-globin protein and mRNA were upregulated in normal placenta explants cultured in hypoxia. The upregulation was independent of HIF1 and NRF2, the two main candidates of globin transcription in non-erythroid cells. Our study is the first to demonstrate alpha-globin mRNA expression in syncytiotrophoblasts in PE, induced by hypoxia. However, gamma-globin was only expressed in erythrocytes. We conclude that alpha-globin, but not HbF, is expressed in placental syncytiotrophoblasts in PE and may contribute to the pathology of the disease.

Hematopoietic stem and progenitor cells use podosomes to transcellularly cross the bone marrow endothelium

Bone marrow endothelium plays an important role in the homing of hematopoietic stem and progenitor cells upon transplantation, but surprisingly little is known on how the bone marrow endothelial cells regulate local permeability and hematopoietic stem and progenitor cells transmigration. We show that temporal loss of vascular endothelial-cadherin function promotes vascular permeability in BM, even upon low-dose irradiation. Loss of vascular endothelial-cadherin function also enhances homing of transplanted hematopoietic stem and progenitor cells to the bone marrow of irradiated mice although engraftment is not increased. Intriguingly, stabilizing junctional vascular endothelial-cadherin in vivo reduced bone marrow permeability, but did not prevent hematopoietic stem and progenitor cells migration into the bone marrow, suggesting that hematopoietic stem and progenitor cells use the transcellular migration route to enter the bone marrow. Indeed, using an in vitro migration assay, we show that human hematopoietic stem and progenitor cells predominantly cross bone marrow endothelium in a transcellular manner in homeostasis by inducing podosome-like structures. Taken together, vascular endothelial-cadherin is crucial for BM vascular homeostasis but dispensable for the homing of hematopoietic stem and progenitor cells. These findings are important in the development of potential therapeutic targets to improve hematopoietic stem and progenitor cell homing strategies.

Preeclampsia is Associated with Sex-Specific Transcriptional and Proteomic Changes in Fetal Erythroid Cells

Preeclampsia (PE) has been associated with placental dysfunction, resulting in fetal hypoxia, accelerated erythropoiesis, and increased erythroblast count in the umbilical cord blood (UCB). Although the detailed effects remain unknown, placental dysfunction can also cause inflammation, nutritional, and oxidative stress in the fetus that can affect erythropoiesis. Here, we compared the expression of surface adhesion molecules and the erythroid differentiation capacity of UCB hematopoietic stem/progenitor cells (HSPCs), UCB erythroid profiles along with the transcriptome and proteome of these cells between male and female fetuses from PE and normotensive pregnancies. While no significant differences were observed in UCB HSPC migration/homing and in vitro erythroid colony differentiation, the UCB HSPC transcriptome and the proteomic profile of the in vitro differentiated erythroid cells differed between PE vs. normotensive samples. Accordingly, despite the absence of significant differences in the UCB erythroid populations in male or female fetuses from PE or normotensive pregnancies, transcriptional changes were observed during erythropoiesis, particularly affecting male fetuses. Pathway analysis suggested deregulation in the mammalian target of rapamycin complex 1/AMP-activated protein kinase (mTORC1/AMPK) signaling pathways controlling cell cycle, differentiation, and protein synthesis. These results associate PE with transcriptional and proteomic changes in fetal HSPCs and erythroid cells that may underlie the higher erythroblast count in the UCB in PE.

Recent insights into endothelial control of leukocyte extravasation

In the process of leukocyte migration from the circulation across the vascular wall, the crosstalk with endothelial cells that line the blood vessels is essential. It is now firmly established that in endothelial cells important signaling events are initiated upon leukocyte adhesion that impinge on the regulation of cell-cell contact and control the efficiency of transendothelial migration. In addition, several external factors such as shear force and vascular stiffness were recently identified as important regulators of endothelial signaling and, consequently, leukocyte transmigration. Here, I review recent insights into endothelial signaling events that are linked to leukocyte migration across the vessel wall. In this field, protein phosphorylation and Rho-mediated cytoskeletal dynamics are still widely studied using increasingly sophisticated mouse models. In addition, activation of tyrosine phosphatases, changes in endothelial cell stiffness as well as different vascular beds have all been established as important factors in endothelial signaling and leukocyte transmigration. Finally, I address less-well-studied but interesting components in the endothelium that also control transendothelial migration, such as the ephrins and their Eph receptors, that provide novel insights in the complexity associated with this process.

Proteomic analysis of murine bone marrow niche microenvironment identifies thioredoxin as a novel agent for radioprotection and for enhancing donor cell reconstitution

Hematopoiesis is regulated by the bone marrow (BM) niche microenvironment. We recently found that posttransplant administration of AMD3100 (a specific and reversible CXCR4 antagonist) enhanced donor cell engraftment and promoted recovery of all donor cell lineages in a congeneic mouse transplant model. We hypothesized that AMD3100 enhances donor cell reconstitution in part by modulating the levels and constitution of soluble factors in the niche microenvironment. In the current study, the effects of the BM extracellular fluid (supernatant) from AMD3100-treated transplant recipient mice on colony-forming units (CFUs) were examined. A semiquantitative, mass spectrometry-based proteomics approach was used to screen for differentially expressed proteins between the BM supernatants of PBS-treated transplant mice and AMD3100-treated transplant mice. A total of 178 proteins were identified in the BM supernatants. Thioredoxin was among the 32 proteins that displayed greater than a twofold increase in spectral counts in the BM supernatant of AMD3100-treated transplant mice. We found that thioredoxin increased CFUs in a dose-dependent manner. Thioredoxin improved hematopoiesis in irradiated mice and protected mice from radiation-related death. Furthermore, ex vivo exposure to thioredoxin for 24 hours enhanced the long-term repopulation of hematopoietic stem cells. Additionally, combined posttransplant administration of thioredoxin and AMD3100 improved hematologic recovery in primary and secondary transplant recipient mice. Our studies demonstrated that factors in the BM niche microenvironment play a critical role in hematopoiesis. Identifying these factors provides clues on potential novel targets that can be used to enhance hematologic recovery in hematopoietic stem cell transplan`tation.

Impact of storage temperature and processing delays on cord blood quality: discrepancy between functional in vitro and in vivo assays

BACKGROUND

Optimal conditions of cord blood (CB) storage, processing, cryopreservation, and thawing are critical for banking and transplantation. Nevertheless, standardized procedures are still awaited.

STUDY DESIGN AND METHODS

We evaluated the impact of preprocessing storage and temperature on recovery, viability, and functional differentiation capacities of hematopoietic progenitor cells. We compared units stored at room temperature (RT) or at 4 °C for 72 hours before cryopreservation to units processed shortly after collection (<12 hr).

RESULTS

Postthaw results showed similar in vitro characteristics between immediate processing and 4 °C storage for cell recovery and viability, both significantly higher than RT storage. Surprisingly, we demonstrated that storage of CB units at RT before processing and cryopreservation profoundly altered in vivo hematopoietic reconstitution in mice, although in vitro hematopoietic colony-forming unit potential was unaltered.

CONCLUSION

Our findings challenge current CB storage practices and suggest standard in vitro quality assessments may not always be indicative of CB engraftment potential.

Splenectomy influences homing of transplanted stem cells in bone marrow-ablated mice

Cell mobilization, a process that influences circulation, margination, and finally, homing play key roles in the regeneration processes mediated by stem cells. Recent studies as well as prior studies from our group indicate an important role of the spleen in hematopoietic reconstitution, but to date the role of the spleen in hematopoietic reconstitution has been unclear and it has not been precisely documented in ablated animals. Therefore, we undertook the present study to define more closely the role of the spleen in hematopoietic reconstitution in lethally irradiated mice. After transplantation of irradiated mice with lacZ+ -marked lin- / CD117+ bone marrow cells, we compared splenectomized mice (T(S), splenectomy performed prior to irradiation) to nonsplenectomized, irradiated mice (T(N)) as well as to normal (unirradiated) mice. Impaired hematopoietic reconstitution was observed in T(S) mice. Splenectomy markedly altered the distribution of hematopoietic stem cells, as demonstrated by fluorescence-activated cell sorting analysis of endogenous CD117+ cells in the thymus and bone marrow of recipients. Cell engraftment was demonstrated by histochemical and polymerase chain reaction analyses of recipient tissues. These experiments demonstrated that in T(S) animals, transplanted hematopoietic stem cells mobilized to extravascular tissues, particularly the gastrointestinal tract. The number of donor cells in recipient tissues continued to increase for 30 days after transplantation with the highest numbers observed in the T(S) group. DNA marking analysis led to the conclusion that engrafted cells were not only integrated into recipient tissues but were also capable of performing complex cellular processes, including proliferation and repair. Our results are consistent with the novel possibility that cellular repair markedly affects stem cell regenerative functions and that repair is markedly influenced by the integrity and presence of organs not directly involved in specific tissue regeneration processes, particularly the spleen.

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.

Preservation of differentiation and clonogenic potential of human hematopoietic stem and progenitor cells during lyophilization and ambient storage

Progenitor cell therapies show great promise, but their potential for clinical applications requires improved storage and transportation. Desiccated cells stored at ambient temperature would provide economic and practical advantages over approaches employing cell freezing and subzero temperature storage. The objectives of this study were to assess a method for loading the stabilizing sugar, trehalose, into hematopoietic stem and progenitor cells (HPC) and to evaluate the effects of subsequent freeze-drying and storage at ambient temperature on differentiation and clonogenic potential. HPC were isolated from human umbilical cord blood and loaded with trehalose using an endogenous cell surface receptor, termed P2Z. Solution containing trehalose-loaded HPC was placed into vials, which were transferred to a tray freeze-dryer and removed during each step of the freeze-drying process to assess differentiation and clonogenic potential. Control groups for these experiments were freshly isolated HPC. Control cells formed 1450+/-230 CFU-GM, 430+/-140 BFU-E, and 50+/-40 CFU-GEMM per 50 microL. Compared to the values for the control cells, there was no statistical difference observed for cells removed at the end of the freezing step or at the end of primary drying. There was a gradual decrease in the number of CFU-GM and BFU-E for cells removed at different temperatures during secondary drying; however, there were no significant differences in the number of CFU-GEMM. To determine storage stability of lyophilized HPC, cells were stored for 4 weeks at 25 degrees C in the dark. Cells reconstituted immediately after lyophilization produced 580+/-90 CFU-GM ( approximately 40%, relative to unprocessed controls p<0.0001), 170+/-70 BFU-E (approximately 40%, p<0.0001), and 41+/-22 CFU-GEMM (approximately 82%, p = 0.4171), and cells reconstituted after 28 days at room temperature produced 513+/-170 CFU-GM (approximately 35%, relative to unprocessed controls, p<0.0001), 112+/-68 BFU-E (approximately 26%, p<0.0001), and 36+/-17 CFU-GEMM ( approximately 82%, p = 0.2164) These studies are the first to document high level retention of CFU-GEMM following lyophilization and storage for 4 weeks at 25 degrees C. This type of flexible storage stability would potentially permit the ability to ship and store HPC without the need for refrigeration.

Mesenchymal stem cells: emerging therapy for Duchenne muscular dystrophy

Multipotent cells that can give rise to bone, cartilage, fat, connective tissue, and skeletal and cardiac muscle are termed mesenchymal stem cells. These cells were first identified in the bone marrow, distinct from blood-forming stem cells. Based on the embryologic derivation, availability, and various pro-regenerative characteristics, research exploring their use in cell therapy shows great promise for patients with degenerative muscle diseases and a number of other conditions. In this review, the authors explore the potential for mesenchymal stem cell therapy in the emerging field of regenerative medicine with a focus on treatment for Duchenne muscular dystrophy.

Expression and function of cannabinoid receptors CB1 and CB2 and their cognate cannabinoid ligands in murine embryonic stem cells

BACKGROUND

Characterization of intrinsic and extrinsic factors regulating the self-renewal/division and differentiation of stem cells is crucial in determining embryonic stem (ES) cell fate. ES cells differentiate into multiple hematopoietic lineages during embryoid body (EB) formation in vitro, which provides an experimental platform to define the molecular mechanisms controlling germ layer fate determination and tissue formation.

METHODS AND FINDINGS

The cannabinoid receptor type 1 (CB1) and cannabinoid receptor type 2 (CB2) are members of the G-protein coupled receptor (GPCR) family, that are activated by endogenous ligands, the endocannabinoids. CB1 receptor expression is abundant in brain while CB2 receptors are mostly expressed in hematopoietic cells. However, the expression and the precise roles of CB1 and CB2 and their cognate ligands in ES cells are not known. We observed significant induction of CB1 and CB2 cannabinoid receptors during the hematopoietic differentiation of murine ES (mES)-derived embryoid bodies. Furthermore, mES cells as well as ES-derived embryoid bodies at days 7 and 14, expressed endocannabinoids, the ligands for both CB1 and CB2. The CB1 and CB2 antagonists (AM251 and AM630, respectively) induced mES cell death, strongly suggesting that endocannabinoids are involved in the survival of mES cells. Treatment of mES cells with the exogenous cannabinoid ligand Delta(9)-THC resulted in the increased hematopoietic differentiation of mES cells, while addition of AM251 or AM630 blocked embryoid body formation derived from the mES cells. In addition, cannabinoid agonists induced the chemotaxis of ES-derived embryoid bodies, which was specifically inhibited by the CB1 and CB2 antagonists.

CONCLUSIONS

This work has not been addressed previously and yields new information on the function of cannabinoid receptors, CB1 and CB2, as components of a novel pathway regulating murine ES cell differentiation. This study provides insights into cannabinoid system involvement in ES cell survival and hematopoietic differentiation.

Role of glycosylphosphatidylinositol-specific phospholipase D in the homing of umbilical cord blood, mobilized peripheral blood and bone marrow-derived hematopoietic stem/progenitor cells

Recent studies suggested that glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) correlated with tumor malignancy and prognosis of certain tumors. As hematopoietic stem/progenitor cells (HS/PC) homing was similar to tumor invasion and metastasis in some mechanisms, which arose our interests in whether GPI-PLD contribution to the homing of HS/PC. In this study, CD34(+) cells from umbilical cord blood (UCB), mobilized peripheral blood (MPB), and bone marrow (BM) were assayed for their differences in adhesion, migration, respectively. The expression of GPI-anchored proteins (CD48, CD90) on the cells were analyzed by flow cytometry. Semi-quantitive RT-PCR was used to detect GPI-PLD expression in the three different CD34(+) cells. The results showed that GPI-PLD had no effect on the adhesion of CD34(+) cells. While, spontaneous and SDF-1 induced migration of UCB and MPB, but not BM CD34(+) cells were decreased after 1,10-phenanthroline (an inhibitor of GPI-PLD) pretreatment. Furthermore, we found little difference in GPI-anchored adhesion molecules (CD48, CD90) expression between untreated and pretreated CD34(+) cells. GPI-PLD mRNA was low expressed in MPB and undetected in UCB and BM CD34(+) cells. Our results suggested that GPI-PLD probably had no contribution to HS/PC homing, which may due to its low or no expression in UCB, BM and MPB CD34(+) cells.

How Glycosylphosphatidylinositol-phospholipase D acts in homing of hematopoietic stem/progenitor cells?

Homing of hematopoietic stem/progenitor cells (HS/PC) to the bone marrow (BM) microenvironment is the first and essential step in HS/PC engraftment and initiation of the marrow reconstitution during clinical hematopoietic stem cell transplantation (HSCT). How to improve the homing efficiencies and make full use of HS/PC resources, especially umbilical cord blood (UCB), are of great importance in clinical practice. However, the cellular and molecular mechanisms that govern this process are poorly understood. Glycosylphosphatidylinositol-phospholipase D (GPI-PLD) is an enzyme which can regulate the expression of Glycosylphosphatidylinositol (GPI)-anchored proteins and modulate their correspondent functions by releasing GPI-anchored proteins from cell membrane. Recent studies suggested that the mechanisms of the malignancy and prognosis of certain tumors were correlated with GPI-PLD. HS/PC homing was similar to tumor invasion and metastasis in some process. Here we proposed the hypothesis that GPI-PLD might also has played a role in the homing of HS/PC by modulating the adhesion and migration of these cells. If GPI-PLD did participate in HS/PC homing, maybe the mechanisms of homing can herefrom be partly elucidated, which would benefit transplantation in clinical practice.

The tale of early hematopoietic cell seeding in the bone marrow niche

Since introduction of the notion of a "niche" that hosts engraftment and activity of hematopoietic cells, there is a massive effort to discover its structure and decipher its function. Our understanding of the niche is continuously changing with reinterpretation of traditional concepts and apprehension of new insights into the biology of hematopoietic cell homing, seeding, and engraftment. Here we discuss some of the early events in hematopoietic stem cell seeding and engraftment and propose a perspective based on visualization of labeled bone marrow cells in real time in vivo. Primary seeding of hematopoietic cells in the bone marrow niches evolves as a complex and dynamic process; however, it follows discrete topological and chronological patterns. Initial seeding occurs on the endosteal surface of the marrow, which includes heterogeneous niches for primary seeding. Several days after transplantation the endosteal niches become more restrictive, hosting primarily mitotically quiescent cells, and gradual centripetal migration is accompanied by engagement in proliferation and differentiation. The hematopoietic niches evolve as heterogeneous three-dimensional microenvironments that are continuously changing over time.

The potential of stem cell therapies for neurological diseases

As a novel neurotherapeutic strategy, stem cell transplantation has received considerable attention, yet little of this attention has been devoted to the probabilities of success of stem cell therapies for specific neurological disorders. Given the complexities of the cellular organization of the nervous system and the manner in which it is assembled during development, it is unlikely that a cellular replacement strategy will succeed for any but the simplest of neurological disorders in the near future. A general strategy for stem cell transplantation to prevent or minimize neurological disorders is much more likely to succeed. Two broad categories of neurological disease, inherited metabolic disorders and invasive brain tumors, are among the most likely candidates.

CD34+ cell adhesion molecule profiles differ between patients mobilized with granulocyte-colony-stimulating factor alone and chemotherapy followed by granulocyte-colony-stimulating factor

BACKGROUND

High-dose therapy with autologous peripheral blood progenitor cell support is widely utilized but requires successful CD34+ cell mobilization and collection. Chemotherapy plus growth factors appear to mobilize more CD34+ cells than growth factors alone. Because alterations in expression of adhesion molecules are important in the trafficking of hematopoietic progenitors, the possibility was explored that the mechanism of this superior mobilization may be greater down regulation of adhesion molecules.

STUDY DESIGN AND METHODS

The expression of eight adhesion molecules (CD11a, b, and c; 15s; 49d and e; 54; and 62L) on the collected CD34+ cells from 15 patients undergoing mobilization with chemotherapy plus granulocyte-colony-stimulating factor (G-CSF) was compared with those of 14 concomitant patients receiving G-CSF alone.

RESULTS

Patients receiving chemotherapy plus G-CSF mobilized more CD34+ cells and did not differ in prior chemotherapy or radiation. There were no significant differences in the percentage of CD34+ cells expressing any of the adhesion molecules examined between the two groups. The chemotherapy plus G-CSF-mobilized cells consistently showed higher expression intensity, and this showed significance or a strong trend for CD11a and c, CD15s, and CD54. Despite these higher expression levels, there were no differences in engraftment kinetics.

CONCLUSIONS

CD34+ cells mobilized by chemotherapy plus growth factors appear to have higher intensities of expression of several adhesion molecules. The significance of this observation will require further study.

Human microvascular endothelial cell activation by IL-1 and TNF-alpha stimulates the adhesion and transendothelial migration of circulating human CD14+ monocytes that develop with RANKL into functional osteoclasts

Circulating pre-OCs may be recruited to locally inflamed sites through specific interactions with activated microvasculature. We found that HMVECs stimulated the adhesion and TEM of circulating pre-OCs, in an ICAM-1- and CD44-dependent manner, leading to greater RANKL-induced OC formation and bone pit resorption.

INTRODUCTION

Inflammation is critical for healing processes but causes severe tissue destruction when chronic. Local osteoclast (OC) formation and bone resorption may increase at inflammatory sites through multiple mechanisms, including direct stimulation by inflamed microvasculature of circulating OC precursor (pre-OC) migration through a blood vessel barrier into bone or joint tissue. How this might occur is not yet well understood.

MATERIALS AND METHODS

Cytokine-activated human microvascular endothelial cell (HMVEC) monolayers, with or without IL-1 and TNF-alpha preactivation (24 h), were incubated in adhesion (1-3 h) or porous transwell transendothelial migration (TEM; 3 h) assays with human peripheral blood mononuclear cells (hPBMCs) or CD14+ monocyte or CD14- lymphocyte subsets. The number of cells that adhered or transmigrated, and their ability to thereafter develop with macrophage-colony stimulating factor (M-CSF) + RANKL into bone pit-resorbing OCs, were analyzed. Immunostaining and neutralizing antibodies to key cell adhesion molecules were used to determine their potential involvement in stimulated CD14+ monocyte TEM.

RESULTS

M-CSF + RANKL caused OC and bone pit formation only from hPBMCs and CD14+ cells but not CD14- cells. Adhesion of hPBMCs or CD14+ cells but not CD14- cells was stimulated by cytokine preactivation of HMVECs and led to the full capture of all circulating pre-OCs capable of developing into OCs. Cytokine-preactivated HMVECs also promoted the postadhesion TEM of hPBMCs and CD14+ populations, resulting in markedly greater OC formation and bone pit resorption by transmigrated cells. Immunodetectable vascular cell adhesion molecule (VCAM-1), intercellular adhesion molecule (ICAM-1), and CD44 levels increased on cytokine-treated HMVEC surfaces, and neutralizing antibodies to ICAM-1 or CD44, but not VCAM-1 or platelet endothelial cell adhesion molecule (PECAM-1), inhibited stimulated CD14+ cell TEM through activated HMVECs.

CONCLUSIONS

This is the first demonstration that cytokine-activated HMVECs efficiently capture and promote the TEM of circulating pre-OCs capable of differentiating into bone-resorbing OCs. Thus, direct pre-OC recruitment by activated microvasculature at inflammatory sites may significantly contribute to normal OC bone remodeling during fracture healing or exacerbate pathological bone loss in various chronic inflammatory disorders.

Therapeutic myocardial angiogenesis: past, present and future

One of the main goals in the treatment of myocardial ischemia is the development of effective therapy for angiogenesis and neovascularization. The first evidence demonstrating alleviation of myocardial ischemia and increased number of collateral blood vessels was reported in the early 90s following intra-coronary administration of basic fibroblast growth factor protein in canine. This study established the ground for extensive investigations to demonstrate the use of other angiogenic growth factor proteins, genes administered directly or incorporated in viruses, and more recently, endothelial progenitor stem cells (embryonic and adults). The positive results observed in animals failed, in most cases, to repeat themselves in clinical-trials in human patients. Therefore, additional experiments are warranted to allow full understanding of the mechanism underlying new blood vessel formation before further clinical studies are undertaken. This review will explore the milestones of angiogenic investigations and their clinical application.

Phenotypic and functional characteristics of CD34+cells are related to their anatomical environment: is their versatility a prerequisite for their bio-availability"

Human CD34+ hematopoietic progenitors (HP) are mainly resident in adult bone marrow (BM). However, their recent revelation in nonhematopoietic tissues implies their circulation through peripheral blood (PB). The intimate mechanisms of this physiological process are not yet understood. Our results showed that steady-state CD34+ HP exhibit a differential phenotypic profile according to their BM versus PB localization. We demonstrated that this phenotype could be modulated by incubation in the presence of their counterpart mononuclear cells (MNC) through cell interactions and cytokine production. Such a modulation mainly concerns migration-mediated cytokine and chemokine receptors as well as some adhesion molecules and partly results from MNC specificity. These phenotypic profiles are associated with distinct cell-cycle position, cloning efficiency, and migration capacity of CD34+ cells from the different anatomical sources. We therefore propose a definition for a circulating versus resident CD34+ cell profile, which mostly depends on their cellular environment. We suggest that blood would represent a supply of cells for which phenotypic and functional characteristics would be a prerequisite for their bio-availability.

PTX-sensitive signals in bone marrow homing of fetal and adult hematopoietic progenitor cells

Several examples suggest a relationship between in vitro migratory capacity and bone marrow (BM) homing. Pertussis toxin (PTX) is a potent inhibitor of serpentine receptor–associated inhibitory trimeric guanidine nucleotide binding (Gi) protein signals. As such, it blocks hematopoietic progenitor cell migration in vitro, but contrary to expectation, no effects on BM homing were observed in previous studies. We therefore re-examined the effect of PTX on homing of murine BM and fetal liver (FL). We found that BM homing of PTX-incubated progenitor cells (colony-forming cells in culture [CFU-Cs]) from BM or FL in irradiated and nonirradiated recipients was reduced by more than 75%, with a concomitant increase in circulating CFU-Cs in peripheral blood. Additional studies confirmed the functional significance of this reduction in homing: PTX-treated cells did not provide radioprotection, and their short-term engraftment in BM and spleen was drastically reduced. Furthermore, several approaches show that cell-intrinsic rather than host-derived mechanisms are responsible for the PTX-induced homing defect. In summary, we show that Gi protein signals are required for BM homing and, as such, provide a new example of the association between BM homing and in vitro migration. Moreover, our data suggest that the behavior of hematopoietic progenitors in obeying Gi signaling does not diverge from that of mature leukocytes.

Platelet characteristic antigens of CD34+ cells in cryopreserved cord blood: a study of platelet-derived microparticles in transplant processing

BACKGROUND AND OBJECTIVES

In previous studies, we found that platelet microparticles (PMPs) bind to cord blood (CB) CD34+ cells and transfer adhesion molecules to them, which enhances their engraftment. Before applying this phenomenon in actual transplants, we investigated the effect of PMPs on cryopreserved CD34+ cells in CB.

MATERIALS AND METHODS

We cryopreserved 18 CB units, then evaluated the binding of PMPs to CD34+ cells after thawing, by varying the expression of platelet characteristic antigens (CD41a, CD61, CD62P and CXCR4) on these cells. Adherence of the CD34+ cells, coated with freeze/thaw-induced PMPs, to endothelium and fibronectin was also studied, as were the effects of thrombin-induced PMPs from both fresh and preserved CB platelets.

RESULTS

PMPs induced by freezing and thawing adhered less well to CD34+ cells than did those from fresh CB, and cells coated with these PMPs had poor adherence. However, thrombin-induced PMPs from both fresh and preserved CB platelets bound equally well to cryopreserved CD34+ cells and improved their adhesion properties.

CONCLUSIONS

PMPs could be a useful tool for enhancing engraftment after CB transplantation.

Homing-associated cell adhesion molecules and cell cycle status on the nucleated cells in the bone marrow, mobilized peripheral blood and cord blood

Homing-associated cell adhesion molecules (H-CAM) on the CD34+ cells play an important role for the engraftment process following hematopoietic stem cell transplantation (HSCT). However, it seems that not only CD34+ cells but also other nucleated cells (NCs) with H-CAM could be implicated in the engraftment process and the proliferation of hematopoietic stem cells. We investigated the differences of HCAM and cell cycle status on the NCs in cord blood (CB), bone marrow (BM), and mobilized peripheral blood (PB). The proportions of CXCR4+ cells within the NC populations were greater in CB than in PB or BM (p=0.0493), although the proportions of CXCR4+, CD44+, and CD49d+ cells within the CB CD34+ cell populations were same within BM or PB. A lower proportion of CD34+CD49d+ cells within the CD34+ cell populations was more noted in CB than in PB or BM (p=0.0085). There were no differences in cell cycle status between CB and BM or PB. Our results suggest that the migrating potential of CB would be enhanced with increased CXCR4 expression on the NCs, but the adhesion potential of CB CD34+ cells would be less than that of PB and BM. These findings may help explain why the lower cell dose is required and engraftment is delayed in cord blood stem cell transplantation.

Disease- or therapy-related bone marrow damage cannot be overcome by changes in stem cell source or dose in allogeneic transplantation

OBJECTIVE

To test whether the functional impairment of the host bone marrow (BM) microenvironment pre-existing at the time of transplantation could be overcome by the increased content of immature cells in allogeneic peripheral blood stem cell transplantation (PBSCT) when compared with bone marrow transplantation (BMT).

METHODS

Cobble stone area forming cells (CAFC) were assayed in normal BM and BM after allogeneic BMT and PBSCT after stable engraftment. Groups were compared by two-tailed t-test.

RESULTS

While BM from 11 normal controls contained an average of 778.8 CAFC-d35 per 10(6) low density bone marrow cells (LDBMC, range 453-1231 per 10(6) LDBMC), BM from patients after BMT contained an average of 123.7 CAFC-d35 per 10(6) LDBMC (range 38-257) per 10(6) LDBMC. BM from patients transplanted with PBSC after myeloablative conditioning contained 128.3 (range 46-305) CAFC-d35 per 10(6) LDBMC (P = 0.89 compared with BMT). Similar results were obtained when patients after PBSCT with non-myeloablative conditioning were included (P = 0.62 compared with BMT). CAFC numbers in patients transplanted in early stages of myeloid leukaemia (acute myeloid leukaemia first remission, chronic myeloid leukaemia first chronic phase) were significantly higher than CAFC numbers in patients transplanted in more advanced stages (P = 0.008) or myelodysplastic syndrome (P = 0.023). The lowest CAFC numbers were found in two cases of retransplantation.

CONCLUSION

Our findings indicate that the functional state of the BM microenvironment rather than stem cell dose or source is limiting for the homing and engraftment of immature haemopoietic cells in clinical transplantation.

Cryopreservation of Stem Cells Using Trehalose: Evaluation of the Method Using a Human Hematopoietic Cell Line

While stem cell cryopreservation methods have been optimized using dimethylsulfoxide (DMSO), the established techniques are not optimal when applied to unfertilized human embryonic cells. In addition, important questions remain regarding the toxicity and characteristics of DMSO for treatment of stem cells for clinical use. The objective of this study was to establish an optimal method for cryopreservation of stem cells using low concentrations (0.2 M) of trehalose, a nontoxic disaccharide of glucose, which possesses excellent protective characteristics, in place of current methods utilizing high concentrations (1-2 M) of DMSO. A human hematopoietic cell line was used in this investigation as a surrogate for human stem cells. Trehalose was loaded into cells using a genetically engineered mutant of the pore-forming protein alpha-hemolysin from Staphylococcus aureus. This method results in a nonselective pore equipped with a metal-actuated switch that is sensitive to extracellular zinc concentrations, thus permitting controlled loading of trehalose. Preliminary experiments characterized the effects of poration on TF-1 cells and established optimal conditions for trehalose loading and cell survival. TF-1 cells were frozen at 1 degrees C/min to -80 degrees C with and without intra- and extracellular trehalose. Following storage at -80 degrees C for 1 week, cells were thawed and evaluated for viability, differentiation capacity, and clonogenic activity in comparison to cells frozen with DMSO. Predictably, cells frozen without any protective agent did not survive freezing. Colony-forming units (CFU) generated from cells frozen with intra- and extracellular trehalose, however, were comparable in size, morphology, and number to those generated by cells frozen in DMSO. There was no observable alteration in phenotypic markers of differentiation in either trehalose- or DMSO-treated cells. These data demonstrate that low concentrations of trehalose can protect hematopoietic progenitors from freezing injury and support the concept that trehalose may be useful for freezing embryonic stem cells and other primitive stem cells for therapeutic and investigational use.

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

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

Significance of increasing adhesion of cord blood hematopoietic cells and a new method: platelet microparticles

Hematopoietic recovery after transplantation with cord blood (CB) is slower than with bone marrow (BM) and mobilized peripheral blood. Adhesion molecules (AMs) on hematopoietic cells are involved in hematopoietic cells' homing. It may be possible to enhance CB CD34+ cells engraftment by increasing their expressions of AM. Twenty-three patients with childhood acute leukemia treated with unrelated CBT were studied. It was found that the time to neutrophil recovery correlated with CXCR4 and the time to platelet recovery correlated with both CD62L and CXCR4. Platelet microparticles (PMPs) carry some AMs such as aIIb b (CD41), P-selectin (CD62P), and CXCR4, CD34+ cells express platelet-binding antigens (CD162 and CD11b). It was found that AMs were increased dramatically on CD34+ cells surface in the presence of PMPs, and CD34+ cells covered with PMPs adhered better to human umbilical vein endothelial cells and fibronectin. These findings suggested that PMPs could increase adhesion of donor's cells to host BM in CBT.

CXCR-4 expression on bone marrow CD34+ cells prior to mobilization can predict mobilization adequacy in patients with hematologic malignancies

To investigate the mechanisms of mobilization and of the factors implicated in the homing of progenitors and possibly understand the reasons for unpredicted mobilization failure, we analyzed CXCR-4 (CD184) expression on bone marrow (BM) CD34+ cells prior to peripheral blood stem cell (PBSC) mobilization in 24 patients affected by hematologic malignancies (non-Hodgkin lymphoma, multiple myeloma, and acute myeloid leukemia). We wanted to determine whether the level of CXCR-4 expressed by hematopoietic stem cells could influence mobilization process and therefore could be considered a predictive factor for mobilization adequacy. These data were also compared with stromal cell function as assessed by colony forming unit-fibroblast (CFU-F) and CFU endothelial cells (CFU-En) assays and stromal layer confluence capacity exhibited by patients' BM cells. In this study, we also compared CXCR-4 expression on CD34+ cells from different sources and at different migration stages specifically bone marrow (BM), steady state peripheral blood (SSPB), fetal cord blood (FCB), cord blood (CB), and mobilized PBSC. Seven (29%) of the 24 patients undergoing mobilization failed to achieve an adequate number of CD34+ stem cells (5 x 10(6)/kg CD34+ cells) and showed a very high expression frequency of CXCR-4 on BM CD34(+) stem cells (mean number of positive cells, 97%) investigated before the mobilization regimen. We also found that high expression intensity per cell for CXCR-4 was associated with lower amounts of mobilized CD34+ cells whereas those patients (17 out of 24 patients, 71%) with lower expression intensity per cell of CD184 on BM CD34+ cells prior to mobilization harvested at least 5 x 10(6)/kg CD34+ cells. Setting a cut off of 5 x 10(6)/kg CD34+ cells harvested, patients mobilizing less had a mean value of 97% CD34+ cells expressing CXCR-4 with a relative mean channel fluorescence of 458 whereas patients mobilizing more than 5 x 10(6)/kg CD34+ progenitors showed a mean value of 59.8% CD34+/CXCR4+ cells with a relative mean channel fluorescence value of 305. Interestingly, in the poor mobilizers group, the marrow stromal microenvironment was found to be more severely damaged in comparison with that of good mobilizers. The comparative analysis of CXCR-4 expression showed no difference in percentage values between steady-state PB (87.4%) and BM (85.1%) stem cells whereas mobilized CD34+ stem cells have a lower expression frequency of CXCR-4 (71.6%) compared to that of progenitors from other sources. Fetal blood CD34+ stem cells had the lowest mean expression frequency of CD184 antigen (36.3%), while CB cells had the highest (94.8%). In conclusion, this study provides evidence that monitoring CXCR-4 CD34 double positive cells before mobilization can be regarded as a predictive factor for mobilization outcome, giving us directional cues for the choice of the best stem cell mobilization regimens.

Splenic marginal zone lymphoma

Splenic marginal zone lymphoma (SMZL) is a specific low-grade small B-cell lymphoma that is incorporated in the World Health Organization classification. Characteristic features are splenomegaly, moderate lymphocytosis with villous morphology, intrasinusoidal pattern of involvement of various organs, especially bone marrow, and relative indolent course. Tumor progression with increase of blastic forms and aggressive behavior are observed in a minority of patients. Molecular and cytogenetic studies have shown heterogeneous results probably because of the lack of standardized diagnostic criteria. To date, no definitive therapy has been established. Therapeutic options include treatment abstention, splenectomy, splenic irradiation, and chemotherapy.

Bone marrow to liver: the blood of Prometheus

The existence of hepatic stem or progenitor cells has been controversial for decades, though it was presumed that if such cells existed, they would lie within the liver. There is now consensus, however, that not only do facultative hepatic stem cells exist within the liver, but also that cells from extra-hepatic sites, in particular the bone marrow, can contribute to hepatocyte and cholangiocyte regeneration. Despite confidence that engraftment of marrow cells in the liver occurs, the mechanistic details of this process remain poorly understood. Moreover, the physiological importance and therapeutic utility of this phenomenon remains controversial.