Human progenitor cells rapidly mobilized by AMD3100 repopulate NOD/SCID mice with increased frequency in comparison to cells from the same donor mobilized by granulocyte colony stimulating factor

Kidney Immune Cell Characterization of Humanized Mouse Models

Experimental studies often fail to translate to clinical practice. Humanized mouse models are an important tool to close this gap. We immunophenotyped the kidneys of NOG (EXL) and NSG mouse strains engrafted with human CD34 + hematopoietic stem cells or PBMCs and compared with immune cell composition of normal human kidney. Human CD34 + hematopoietic stem cell engraftment results in steady renal immune cell populations in mouse kidney with key similarities in composition compared with human kidney. Successful translation of experimental mouse data to human diseases is limited because of biological differences and imperfect disease models. Humanized mouse models are being used to bring murine models closer to humans. However, data for application in renal immune cell-mediated diseases are rare. We therefore studied immune cell composition of three different humanized mouse kidneys and compared them with human kidney. NOG and NOGEXL mice engrafted with human CD34 + hematopoietic stem cells were compared with NSG mice engrafted with human PBMCs. Engraftment was confirmed with flow cytometry, and immune cell composition in kidney, blood, spleen, and bone marrow was analyzed in different models. The results from immunophenotyping of kidneys from different humanized mouse strains were compared with normal portions of human kidneys. We found significant engraftment of human immune cells in blood and kidney of all tested models. huNSG mice showed highest frequencies of hTCR + cells compared with huNOG and huNOGEXL in blood. huNOGEXL was found to have the highest hCD4 + frequency among all tested models. Non-T cells such as hCD20 + and hCD11c + cells were decreased in huNSG mice compared with huNOG and huNOGEXL. Compared with normal human kidney, huNOG and huNOGEXL mice showed representative immune cell composition, rather than huNSG mice. In summary, humanization results in immune cell infiltration in the kidney with variable immune cell composition of tested humanized mouse models and partially reflects normal human kidneys, suggesting potential use for translational studies.

Hematopoietic stem cell mobilization

Hematopoietic stem cell (HSC) transplantation has been used to treat hematopoietic diseases for over 50 years. HSCs can be isolated from bone marrow (BM), umbilical cord blood, or peripheral blood. Because of lower costs, shorter hospitalization, and faster engraftment, peripheral blood has become the predominant source of HSCs for transplantation. The major factors determining the rate of successful HSC transplantation include the degree of human leukocyte antigen matching between the donor and recipient and the number of HSCs for transplantation. Administration of granulocyte colony-stimulating factor (G-CSF) alone or combined with plerixafor (AMD3100) are clinical used methods to promote HSC mobilization from BM to the peripheral blood for HSC transplantations. However, a significant portion of healthy donors or patients may be poor mobilizers of G-CSF, resulting in an insufficient number of HSCs for the transplantation and necessitating alternative strategies to increase the apheresis yield. The detailed mechanisms underlying G-CSF-mediated HSC mobilization remain to be elucidated. This review summarizes the current research on deciphering the mechanism of HSC mobilization.

Advances in Model Systems for Human Cytomegalovirus Latency and Reactivation

Human cytomegalovirus (HCMV) is a highly prevalent beta-herpesvirus and a significant cause of morbidity and mortality following hematopoietic and solid organ transplant, as well as the leading viral cause of congenital abnormalities. A key feature of the pathogenesis of HCMV is the ability of the virus to establish a latent infection in hematopoietic progenitor and myeloid lineage cells. The study of HCMV latency has been hampered by difficulties in obtaining and culturing primary cells, as well as an inability to quantitatively measure reactivating virus, but recent advances in both in vitro and in vivo models of HCMV latency and reactivation have led to a greater understanding of the interplay between host and virus. Key differences in established model systems have also led to controversy surrounding the role of viral gene products in latency establishment, maintenance, and reactivation. This review will discuss the details and challenges of various models including hematopoietic progenitor cells, monocytes, cell lines, and humanized mice. We highlight the utility and functional differences between these models and the necessary experimental design required to define latency and reactivation, which will help to generate a more complete picture of HCMV infection of myeloid-lineage cells.

Role of CXCR4 in the progression and therapy of acute leukaemia

CXCR4 is expressed on leukaemia cells and haematopoietic stem cells (HSCs), and its ligand stromal-derived factor 1 (SDF-1) is produced abundantly by stromal cells in the bone marrow (BM). The SDF-1/CXCR4 axis plays important roles in homing to and retention in the protective BM microenvironment of malignant leukaemia cells and normal HSCs. CXCR4 expression is regulated by multiple mechanisms and the level of CXCR4 expression on leukaemia cells has prognostic indications in patients with acute leukaemia. CXCR4 antagonists can mobilize leukaemia cells from BM to circulation, which render them effectively eradicated by chemotherapeutic agents, small molecular inhibitors or hypomethylating agents. Therefore, such combinational therapies have been tested in clinical trials. However, new evidence emerged that drug-resistant leukaemia cells were not affected by CXCR4 antagonists, and the migration of certain leukaemia cells to the leukaemia niche was independent of SDF-1/CXCR4 axis. In this review, we summarize the role of CXCR4 in progression and treatment of acute leukaemia, with a focus on the potential of CXCR4 as a therapeutic target for acute leukaemia. We also discuss the potential value of using CXCR4 antagonists as chemosensitizer for conditioning regimens and immunosensitizer for graft-vs-leukaemia effects of allogeneic haematopoietic stem cell transplantation.

Decellularized adipose tissue scaffolds guide hematopoietic differentiation and stimulate vascular regeneration in a hindlimb ischemia model

Cellular therapies to stimulate therapeutic angiogenesis in individuals with critical limb ischemia (CLI) remain under intense investigation. In this context, the efficacy of cell therapy is dependent on the survival, biodistribution, and pro-angiogenic paracrine signaling of the cells transplanted. Hematopoietic progenitor cells (HPC) purified from human umbilical cord blood using high aldehyde dehydrogenase-activity (ALDH^hi cells) and expanded ex vivo, represent a heterogeneous mixture of progenitor cells previously shown to support limb revascularization in mouse models of CLI. The objectives of this study were to investigate the utility of bioscaffolds derived from human decellularized adipose tissue (DAT) to guide the differentiation of seeded HPC in vitro and harness the pro-angiogenic capacity of HPC at the site of ischemia after implantation in vivo. Probing whether the DAT scaffolds altered HPC differentiation, label-free quantitative mass spectrometry and flow cytometric phenotype analyses indicated that culturing the HPC on the DAT scaffolds supported their differentiation towards the pro-angiogenic monocyte/macrophage lineage at the expense of megakaryopoiesis. Moreover, implantation of HPC in DAT scaffolds within a unilateral hindlimb ischemia model in NOD/SCID mice increased cell retention at the site of ischemia relative to intramuscular injection, and accelerated the recovery of limb perfusion, improved functional limb use and augmented CD31⁺ capillary density when compared to DAT implantation alone or saline-injected controls. Collectively, these data indicate that cell-instructive DAT scaffolds can direct therapeutic HPC differentiation towards the monocyte/macrophage lineage and represent a promising delivery platform for improving the efficacy of cell therapies for CLI.

Characteristics of cells with engraftment capacity within CD34+ cell population upon G-CSF and Plerixafor mobilization

In the context of hematopoietic cell transplantation, hematopoietic stem cells and progenitor cells (HSC and HPC) are usually collected by apheresis following their mobilization by G-CSF alone or in combination with Plerixafor® when patients fail to respond to G-CSF alone. In medical practice, the quality of the hematopoietic graft is based on CD34⁺ cell content that is used to define "Good Mobilizer (GM)" or "Poor Mobilizer (PM)" patients but does not report the real HSC content of grafts. In this study, we assessed the HSC content within the CD34⁺ fraction of graft samples from 3 groups of patients: 1-GM patients receiving G-CSF only (GM^G-CSF), 2-PM patients receiving G-CSF only (PM^G-CSF), 3-PM patients receiving G-CSF + Plerixafor (PM^G-CSF+P). Although HSC from the 3 groups of patients displayed very similar phenotypic profiles, expression of "stemness" genes and metabolic characteristics, their capacity to engraft NSG mice differed revealing differences in terms of HSC between groups. Indeed according to mobilization regimen, we observed differences in migration capacity of HSC, as well as differences in engraftment intensity depending on the initial pathology (myeloma versus lymphoma) of patients. This suggests that mobilization regimen could strongly influence the long term engraftment efficiency of hematopoietic grafts.

The Role of the Human Cytomegalovirus UL133-UL138 Gene Locus in Latency and Reactivation

Human cytomegalovirus (HCMV) latency, the means by which the virus persists indefinitely in an infected individual, is a major frontier of current research efforts in the field. Towards developing a comprehensive understanding of HCMV latency and its reactivation from latency, viral determinants of latency and reactivation and their host interactions that govern the latent state and reactivation from latency have been identified. The polycistronic UL133-UL138 locus encodes determinants of both latency and reactivation. In this review, we survey the model systems used to investigate latency and new findings from these systems. Particular focus is given to the roles of the UL133, UL135, UL136 and UL138 proteins in regulating viral latency and how their known host interactions contribute to regulating host signaling pathways towards the establishment of or exit from latency. Understanding the mechanisms underlying viral latency and reactivation is important in developing strategies to block reactivation and prevent CMV disease in immunocompromised individuals, such as transplant patients.

The Biological and Clinical Relevance of G Protein-Coupled Receptors to the Outcomes of Hematopoietic Stem Cell Transplantation: A Systematized Review

Hematopoietic stem cell transplantation (HSCT) remains the only curative treatment for several malignant and non-malignant diseases at the cost of serious treatment-related toxicities (TRTs). Recent research on extending the benefits of HSCT to more patients and indications has focused on limiting TRTs and improving immunological effects following proper mobilization and engraftment. Increasing numbers of studies report associations between HSCT outcomes and the expression or the manipulation of G protein-coupled receptors (GPCRs). This large family of cell surface receptors is involved in various human diseases. With ever-better knowledge of their crystal structures and signaling dynamics, GPCRs are already the targets for one third of the current therapeutic arsenal. The present paper assesses the current status of animal and human research on GPCRs in the context of selected HSCT outcomes via a systematized survey and analysis of the literature.

Cytokine-induced hematopoietic stem and progenitor cell mobilization: unraveling interactions between stem cells and their niche

Peripheral blood hematopoietic stem and progenitor cells (HSPCs), mobilized by granulocyte colony‐stimulating factor, are widely used as a source for both autologous and allogeneic stem cell transplantation. The use of mobilized HSPCs has several advantages over traditional bone marrow–derived HSPCs, including a less invasive harvesting process for the donor, higher HSPC yields, and faster hematopoietic reconstitution in the recipient. For years, the mechanisms by which cytokines and other agents mobilize HSPCs from the bone marrow were not fully understood. The field of stem cell mobilization research has advanced significantly over the past decade, with major breakthroughs in the elucidation of the complex mechanisms that underlie stem cell mobilization. In this review, we provide an overview of the events that underlie HSPC mobilization and address the relevant cellular and molecular components of the bone marrow niche. Furthermore, current and future mobilizing agents will be discussed.

The potential of non-myeloablative heterochronous autologous hematopoietic stem cell transplantation for extending a healthy life span

Aging is a complex multifactorial process, a prominent component being the senescence of the immune system. Consequently, immune-related diseases develop, including atherosclerosis, cancer, and life-threatening infections, which impact on health and longevity. Rejuvenating the aged immune system could mitigate these diseases, thereby contributing to longevity and health. Currently, an appealing option for rejuvenating the immune system is heterochronous autologous hematopoietic stem cell transplantation (haHSCT), where healthy autologous bone marrow/peripheral blood stem cells are collected during the youth of an individual, cryopreserved, and re-infused when he or she has reached an older age. After infusion, young hematopoietic stem cells can reconstitute the compromised immune system and improve immune function. Several studies using animal models have achieved substantial extension of the life span of animals treated with haHSCT. Therefore, haHSCT could be regarded as a potential procedure for preventing age-related immune defects and extending healthy longevity. In this review, the pros, cons, and future feasibility of this approach are discussed.

Development of Stem-Cell-Mobilizing Agents Targeting CXCR4 Receptor for Peripheral Blood Stem Cell Transplantation and Beyond

The function of the CXCR4/CXCL12 axis accounts for many disease indications, including tissue/nerve regeneration, cancer metastasis, and inflammation. Blocking CXCR4 signaling with its antagonists may lead to moving out CXCR4⁺ cell types from bone marrow to peripheral circulation. We have discovered a novel series of pyrimidine-based CXCR4 antagonists, a representative (i.e., 16) of which was tolerated at a higher dose and showed better HSC-mobilizing ability at the maximal response dose relative to the approved drug 1 (AMD3100), and thus considered a potential drug candidate for PBSCT indication. Docking compound 16 into the X-ray crystal structure of CXCR4 receptor revealed that it adopted a spider-like conformation striding over both major and minor subpockets. This putative binding mode provides a new insight into CXCR4 receptor-ligand interactions for further structural modifications.

Circulating fibrocyte mobilization in negative pressure wound therapy

Non‐healing diabetic wounds are difficult to treat. They also create heavy financial burdens for both patients and society. Negative pressure wound therapy (NPWT) has been adopted to treat intractable wounds and has proved to be effective. However, the mechanisms that underlie the effects of this treatment are not entirely understood. Circulating fibrocytes are unique haematopoietic‐derived stem cells that have been reported to play a pivotal role in wound healing. Here, we have investigated the effect of NPWT on fibrocyte mobilization and the role of fibrocyte mobilization in the healing of diabetic wounds during NPWT. We show that the NPWT group exhibited 2.6‐fold to 12.1‐fold greater numbers of tail vein‐injected PKH‐26‐labelled fibrocytes in the diabetic wound sites compared with the control group. We also demonstrate that the full‐thickness skin wounds treated with NPWT exhibit significantly reduced mRNA and protein expression, blood vessel density and proliferating cells when exogenous fibrocyte mobilization is inhibited. We speculate that systemic mobilization of fibrocytes during NPWT may be a mechanism for healing intractable wounds in a diabetic rat model experiment and that enhancement of cell mobilization may represent a potential treatment idea for intractable wound healing across all fields of surgery.

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

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

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

Dipeptidyl Peptidase-4 Regulation of SDF-1/CXCR4 Axis: Implications for Cardiovascular Disease

Dipeptidyl peptidase-4 (DPP4) is a ubiquitously expressed protease that regulates diverse number of physiological functions. As a dipeptidase, it exerts its catalytic effects on proteins/peptides with proline, alanine, or serine in the penultimate (P1) amino acid residue from the amino terminus. The evidence to date supports an important effect of DPP4 in catalytic cleavage of incretin peptides and this perhaps represents the main mechanism by which DPP4 inhibition improves glycemic control. DPP4 also plays an important role in the degradation of multiple chemokines of which stromal cell-derived factor-1 (SDF-1, also known as CXCL12) is perhaps an increasingly recognized target, given its importance in processes, such as hematopoiesis, angiogenesis, and stem cell homing. In the current review, we will summarize the importance of DPP4-mediated enzymatic processing of cytokines/chemokines with an emphasis on SDF-1 and resultant implications for cardiovascular physiology and disease.

Humanized mouse models of human cytomegalovirus infection

The generation of humanized mouse models in which immune deficient mice are engrafted with human tissues allows for the direct in vivo investigation of human-restricted viruses. These humanized mouse models have been developed and improved over the past 30 years. It is now possible to achieve high levels of human cell engraftment producing human myeloid and lymphoid lineage cells. Humanized mouse models have been increasingly utilized in the study of human cytomegalovirus (HCMV), a human-specific beta-herpesvirus that infects myeloprogenitor cells and establishes a life-long latency in the infected host. This review focuses on the strengths and limitations of the current humanized mouse models used to study HCMV replication, pathogenesis and treatment.

Hematopoietic stem cell niche maintenance during homeostasis and regeneration

The bone marrow niche has mystified scientists for many years, leading to widespread investigation to shed light into its molecular and cellular composition. Considerable efforts have been devoted toward uncovering the regulatory mechanisms of hematopoietic stem cell (HSC) niche maintenance. Recent advances in imaging and genetic manipulation of mouse models have allowed the identification of distinct vascular niches that have been shown to orchestrate the balance between quiescence, proliferation and regeneration of the bone marrow after injury. Here we highlight the recently discovered intrinsic mechanisms, microenvironmental interactions and communication with surrounding cells involved in HSC regulation, during homeostasis and in regeneration after injury and discuss their implications for regenerative therapy.

Aldehyde dehydrogenase 1 activity in the developing human pancreas modulates retinoic acid signalling in mediating islet differentiation and survival

AIMS/HYPOTHESIS

Aldehyde dehydrogenase 1 (ALDH1), a human stem-cell marker, is an enzyme responsible for converting retinaldehydes to retinoic acids (RAs) to modulate cell differentiation. However, data on expression levels and functional roles of ALDH1 during human fetal pancreatic development are limited. The focus of this study was to characterise ALDH1 expression patterns and to determine its functional role in islet cell differentiation.

METHODS

The presence of ALDH1 in the human fetal pancreas (8-22 weeks) was characterised by microarray, quantitative RT-PCR, western blotting and immunohistological approaches. Isolated human fetal islet-epithelial cell clusters were treated with ALDH1 inhibitors, retinoic acid receptor (RAR) agonists and ALDH1A1 small interfering (si)RNA.

RESULTS

In the developing human pancreatic cells, high ALDH1 activity frequently co-localised with key stem-cell markers as well as endocrine transcription factors. A high level of ALDH1 was expressed in newly differentiated insulin(+) cells and this decreased as development progressed. Pharmacological inhibition of ALDH1 activity in human fetal islet-epithelial cell clusters resulted in reduced endocrine cell differentiation and increased cell apoptosis, and was reversed with co-treatment of RAR/RXR agonists. Furthermore, siRNA knockdown of ALDH1A1 significantly decreased RAR expression and induced cell apoptosis via suppression of the phosphoinositide 3-kinase (PI3K) pathway and activation of caspase signals.

CONCLUSIONS/INTERPRETATION

Our findings indicate that ALDH1(+) cells represent a pool of endocrine precursors in the developing human pancreas and that ALDH1 activity is required during endocrine cell differentiation. Inhibition of ALDH1-mediated retinoid signalling impairs human fetal islet cell differentiation and survival.

Optimizing autologous stem cell mobilization strategies to improve patient outcomes: consensus guidelines and recommendations

Autologous hematopoietic stem cell transplantation (aHSCT) is a well-established treatment for malignancies such as multiple myeloma (MM) and lymphomas. Various changes in the field over the past decade, including the frequent use of tandem aHSCT in MM, the advent of novel therapies for the treatment of MM and lymphoma, and the addition of new stem cell mobilization techniques, have led to the need to reassess current stem cell mobilization strategies. Mobilization failures with traditional strategies are common and result in delays in treatment and increased cost and resource utilization. Recently, plerixafor-containing strategies have been shown to significantly reduce mobilization failure rates, but the ideal method to maximize stem cell yields and minimize costs associated with collection has not yet been determined. A panel of experts convened to discuss the currently available data on autologous hematopoietic stem cell mobilization and transplantation and to devise guidelines to optimize mobilization strategies. Herein is a summary of their discussion and consensus.

Physiology and pharmacology of plerixafor

Autologous hematopoietic stem cell (HSC) transplantation is an important therapeutic option for patients with non-Hodgkin's lymphoma and multiple myeloma. The primary source of HSC is from the peripheral blood which requires mobilization from the bone marrow. Current mobilization regimens include cytokines such as G-CSF and/or chemotherapy. However not all patients mobilize enough HSC to proceed to transplant. The chemokine receptor CXCR4 and its ligand CXCL12 are an integral part of the mechanism of HSC retention in the bone marrow niche. The discovery of plerixafor, a selective inhibitor of CXCR4, has provided a new additional means of mobilizing HSC for autologous transplantation. Plerixafor consists of two cyclam rings with a phenylenebis(methylene) linker. It inhibits CXCL12 binding to CXCR4 and subsequent downstream events including chemotaxis. The molecular interactions of plerixafor have been defined indicating a unique binding mode to CXCR4. Plerixafor rapidly mobilizes HSC within hours compared with the multi-day treatment required by G-CSF in mouse, dog and non-human primate. The mobilized cells once transplanted are capable of timely and endurable engraftment. Additionally CXCR4 has been implicated in the pathology of HIV, inflammatory disease and cancer and the pharmacology of plerixafor in various disease models is described.

Risk factors for invasive fungal disease after allogeneic hematopoietic stem cell transplantation: a single center experience

Invasive fungal disease (IFD) is a major cause of morbidity and mortality after hematopoietic stem cell transplantation (HCT). We performed a retrospective review of 271 adults with a hematologic malignancy undergoing allogeneic HCT to determine the incidence of and risk factors for IFD and to examine the impact of IFD on nonrelapse mortality and overall survival. We defined IFD using standard criteria and selected proven and probable cases for analysis. Diagnoses in the study group included acute leukemia (42%), non-Hodgkin lymphoma (24%), myelodysplastic syndrome (15%), chronic lymphocytic leukemia (5%), and other hematologic disorders (14%). Conditioning included reduced-intensity (64%) and myeloablative (36%) regimens. Donor sources were HLA-matched sibling (60%), matched unrelated (20%), haploidentical (12%), and cord blood (8%). A total of 51 episodes of IFD were observed in 42 subjects (15%). Aspergillus spp (47%) was the most frequent causative organism, followed by Candida spp (43%). The majority of IFD cases (67%) were reported after day +100 post-HCT. In multivariate analysis, haploidentical donor transplantation (hazard ratio [HR], 3.82; 95% confidence interval [CI], 1.49-9.77; P = .005) and grade II-IV acute graft-versus-host disease (HR, 2.55; 95% CI, 1.07-6.10; P = .03) were risk factors for the development of IFD. Conversely, higher infused CD34(+) cell dose was associated with a lower risk of IFD (HR, 0.80; 95% CI, 0.68-0.94; P = .006, per 1 × 10(6) cells/kg increase in CD34(+) cell infusion). IFD-related mortality was 33.3%. Nonrelapse mortality was significantly higher in patients who developed IFD compared with those without IFD (P < .001, log-rank test). Patients with IFD had lower overall survival (5.8 months versus 76.1 months; P < .001, log-rank test). Further studies exploring strategies to increase the infused cell dose and determine adequate prophylaxis, especially against aspergillus, beyond day +100 are needed.

Getting blood from bone: an emerging understanding of the role that osteoblasts play in regulating hematopoietic stem cells within their niche

Blood and bone are dynamic tissues that are continuously renewed throughout life. Early observations based upon the proximity of bone and hematopoietic progenitor populations in marrow suggested that interactions between skeletal and hematopoietic elements are likely to be crucial in the development and function of each system. As a result of these morphologic observations, several groups have demonstrated that the osteoblasts play an important role in hematopoiesis by serving as a specific local microenvironment, or niche, for hematopoietic stem cells. Significant new developments in this area of active investigation have emerged since our last examination of this area in 2005. Here we discuss these new insights into the function and morphology of the hematopoietic stem cell niche, with a particular focus on cells of the osteoblastic lineage.

Mobilization of hematopoietic stem and progenitor cells using inhibitors of CXCR4 and VLA-4

Successful hematopoietic stem cell transplant requires the infusion of a sufficient number of hematopoietic stem/progenitor cells (HSPCs) that are capable of homing to the bone marrow cavity and regenerating durable trilineage hematopoiesis in a timely manner. Stem cells harvested from peripheral blood are the most commonly used graft source in HSCT. Although granulocyte colony-stimulating factor (G-CSF) is the most frequently used agent for stem cell mobilization, the use of G-CSF alone results in suboptimal stem cell yields in a significant proportion of patients. Both the chemokine receptor CXCR4 and the integrin α(4)β(1) (very late antigen 4 (VLA-4)) have important roles in the homing and retention of HSPCs within the bone marrow microenvironment. Preclinical and/or clinical studies have shown that targeted disruption of the interaction of CXCR4 or VLA-4 with their ligands results in the rapid and reversible mobilization of hematopoietic stem cells into the peripheral circulation and is synergistic when combined with G-CSF. In this review, we discuss the development of small-molecule CXCR4 and VLA-4 inhibitors and how they may improve the utility and convenience of peripheral blood stem cell transplantation.

[Hematopoietic stem cells mobilization: state of the art in 2011 and perspectives]

High-dose chemotherapy with stem cells support has largely improved in terms of hematopoietic stem and progenitor cells harvest procedures as well as in those, which target or manipulate the cellular composition of autologous graft. Optimal preparative regimens and supportive care had lead to better use of autologous transplantation procedure. For other patients assigned to hematopoietic transplantation, availability of allogeneic donors appears to be an interesting alternative source of hematopoietic stem cells. Since three decades, hematopoietic growth factors development has allowed mobilization optimization and collection of peripheral hematopoietic stem cells leading to reduced days of hospitalization and less blood products requirements, being more cost-effective for patients in autologous transplantation settings and for stem cell collection facilities in allogeneic ones. New perspectives include, besides ex vivo manipulation of graft, development of mobilizing drugs in order to perform transplantation even in poor mobilizers patients. An important goal is achieved with the description of genetic polymorphisms related to optimal mobilization of stem cells. New approach using more promising and selective agents called chemokines, such as plerixafor the main leader among these agents are now available and appear complementary for alternative approach using cytokines alone (G-CSF, GM-CSF, SCF). The aim of this review is to assess the evolution of theses biotechnologies and their role in different steps of autologous transplantation and allogeneic stem cells collection.

Mobilization of hematopoietic stem and leukemia cells

HSC mobilization is an essential homeostatic process during inflammation and for the maintenance of hematopoietic progenitors. It has been exploited for the therapeutic application of HSC transplantation. Recent evidence suggests that leukemic cells share surface molecules in common with stem cells and may be mobilized under similar conditions. This effect could be used for therapeutic interventions. In this review, we will provide evidence showing that leukemia cells and stem cells traffic similarly and may share a common niche. Studies are discussed comparing and contrasting the mechanism of normal stem cells and leukemic cell mobilization through the CXCR4/CXCL12 axis and other key intermediaries.

Hematopoietic stem cell mobilization for gene therapy of adult patients with severe β-thalassemia: results of clinical trials using G-CSF or plerixafor in splenectomized and nonsplenectomized subjects

The safety and efficacy of hematopoietic stem cell (HSC) mobilization was investigated in adult splenectomized (SPL) and non-SPL patients with thalassemia major, in two clinical trials, using different mobilization modes: granulocyte-colony-stimulating factor (G-CSF)-alone, G-CSF following pretreatment with hydroxyurea (HU), plerixafor-alone. G-CSF-mobilization was both safe and effective in non-SPL patients. However, in SPL patients the procedure resulted in excessive response to G-CSF, expressed as early hyperleukocytosis necessitating significant dose reduction, and suboptimal CD34(+) cells yields. One-month HU-pretreatment prevented hyperleukocytosis and allowed successful CD34(+) cell collections when an optimal washout period was maintained, but it significantly prolonged the mobilization procedure. Plerixafor resulted in rapid and effective mobilization in both SPL and non-SPL patients and was well-tolerated. For gene therapy of thalassemia, G-CSF or Plerixafor could be used as mobilization agents in non-SPL patients whereas Plerixafor appears to be the mobilization agent of choice in SPL adult thalassemics in terms of safety and efficacy.

Rapid mobilization of hematopoietic progenitors by AMD3100 and catecholamines is mediated by CXCR4-dependent SDF-1 release from bone marrow stromal cells

Steady-state egress of hematopoietic progenitor cells can be rapidly amplified by mobilizing agents such as AMD3100, the mechanism, however, is poorly understood. We report that AMD3100 increased the homeostatic release of the chemokine stromal cell derived factor-1 (SDF-1) to the circulation in mice and non-human primates. Neutralizing antibodies against CXCR4 or SDF-1 inhibited both steady state and AMD3100-induced SDF-1 release and reduced egress of murine progenitor cells over mature leukocytes. Intra-bone injection of biotinylated SDF-1 also enhanced release of this chemokine and murine progenitor cell mobilization. AMD3100 directly induced SDF-1 release from CXCR4(+) human bone marrow osteoblasts and endothelial cells and activated uPA in a CXCR4/JNK-dependent manner. Additionally, ROS inhibition reduced AMD3100-induced SDF-1 release, activation of circulating uPA and mobilization of progenitor cells. Norepinephrine treatment, mimicking acute stress, rapidly increased SDF-1 release and progenitor cell mobilization, whereas β2-adrenergic antagonist inhibited both steady state and AMD3100-induced SDF-1 release and progenitor cell mobilization in mice. In conclusion, this study reveals that SDF-1 release from bone marrow stromal cells to the circulation emerges as a pivotal mechanism essential for steady-state egress and rapid mobilization of hematopoietic progenitor cells, but not mature leukocytes.

Bone marrow as a source of hematopoietic stem cells for human gene therapy of β-thalassemia

β-Thalassemia is a severe inherited anemia caused by insufficient production of β-globin chains. Allogeneic hematopoietic stem cell (HSC) transplantation is currently the only cure, and is limited by donor availability and regimen-related toxicity and mortality. Gene therapy is a promising therapeutic tool for all thalassemic patients lacking a compatible donor and potentially provides transfusion independence in the absence of transplant-related complications, such as graft rejection and graft-versus-host disease. The issue of HSC procurement is critical in this setting because of the specific features of thalassemic syndromes, which include bone marrow (BM) expansion, ineffective erythropoiesis, and splenomegaly. Little is known about the efficiency of CD34(+) cell yield from steady-state BM harvests from thalassemic patients. We have collected data on safety and cell yield from 20 pediatric patients with β-thalassemia who underwent autologous BM harvest before allogeneic HSC transplantation, and from 49 age-matched sibling donors who also underwent BM harvest. The procedure was safe, as no significant adverse events occurred. In terms of cell yield, no difference was found between patients and normal donors in the number of CD34(+) cells and total nucleated cells harvested. Most importantly, no difference was found in the proportion of myeloid and erythroid progenitors, suggesting a similar repopulating capacity. On the basis of these results, we conclude that steady-state BM can be used as a safe and efficient source of HSC for gene therapy of β-thalassemia.

Advances in mobilization for the optimization of autologous stem cell transplantation

In autologous stem cell transplantation, mobilized peripheral blood has replaced the bone marrow as the preferred source of hematopoietic stem cells (HSCs). Because HSCs normally exist in the blood in very low numbers, the use of agents to "mobilize" HSCs from the marrow niche to the peripheral blood is essential for successful transplantation. Until recently, granulocyte colony-stimulating factor (G-CSF) and granulocyte-macrophage colony-stimulating factor were the only approved agents by the US Food and Drug Administration for use as peripheral blood stem cell (PBSC)-mobilizing agents in the United States, but G-CSF has become the gold standard. Unfortunately, some patients fail to mobilize sufficient numbers of PBSCs for transplantation in response to G-CSF with or without chemotherapy. Recently, a new agent, plerixafor (AMD3100) added to G-CSF has been approved to enhance PBSC mobilization. This review will discuss the current methodologies to improve hematopoietic stem cell mobilization.

Plerixafor for autologous CD34 cell mobilization

High-dose chemotherapy and autologous transplantation of hematopoietic cells is a crucial treatment option for hematologic malignancy patients. Current mobilization regimes often do not provide adequate numbers of CD34(+) cells. The chemokine receptor CXCR4 and ligand SDF-1 are integrally involved in homing and mobilization of hematopoietic progenitor cells. Disruption of the CXCR4/SDF-1 axis by the CXCR4 antagonist, plerixafor, has been demonstrated in Phase II and Phase III trials to improve mobilization when used in conjunction with granulocyte colony-stimulating factor (G-CSF). This approach is safe with few adverse events and produces significantly greater numbers of CD34(+) cells when compared to G-CSF alone. New plerixafor initiatives include use in volunteer donors for allogeneic hematopoietic cell transplant and in other disease targets.

Migratory strategies of normal and malignant stem cells

The regulated migration of stem cells is critical for organogenesis during development and for tissue -homeostasis and repair during adulthood. Human bone marrow (BM) represents an accessible reservoir containing regenerative cell types from hematopoietic, endothelial, and mesenchymal-stromal lineages that together coordinate hematopoiesis and promote the repair of damaged vasculature and tissues throughout the body. Thus, a detailed understanding of lineage-specific stem cell mobilization, homing, and subsequent engraftment in areas of injury or disease is of critical importance to the rational development of novel cell-mediated regenerative therapies. Stem cell trafficking via the circulation from site of origin to peripheral tissues requires fundamental molecular pathways governing (1) niche-specific deadhesion of progenitor cells; (2) chemoattraction to guide progenitor cell homing; and (3) interstitial navigation and adhesion/retention of recruited progenitor cells. This overview chapter summarizes the diversity of migratory strategies employed by hematopoietic, endothelial, and mesenchymal-stromal progenitor cells during repair and regeneration after tissue damage. Further elucidation of stem cell homing and migration pathways will allow greater application of stem cells for targeted cell therapy and/or drug delivery for tissue repair. Strikingly similar migratory mechanisms appear to govern the in vivo migration of recently characterized cancer stem cells (CSC) in leukemias and solid tumors, indicating that conserved principles of stem cell migration and niche specificity will provide new information to target CSC in anticancer therapy.

Cannabinoid receptor 2 and its agonists mediate hematopoiesis and hematopoietic stem and progenitor cell mobilization

Endocannabinoids are arachidonic acid derivatives and part of a novel bioactive lipid signaling system, along with their G-coupled cannabinoid receptors (CB₁ and CB₂) and the enzymes involved in their biosynthesis and degradation. However, their roles in hematopoiesis and hematopoietic stem and progenitor cell (HSPC) functions are not well characterized. Here, we show that bone marrow stromal cells express endocannabinoids (anandamide and 2-arachidonylglycerol), whereas CB₂ receptors are expressed in human and murine HSPCs. On ligand stimulation with CB₂ agonists, CB₂ receptors induced chemotaxis, migration, and enhanced colony formation of bone marrow cells, which were mediated via ERK, PI3-kinase, and Gαi-Rac1 pathways. In vivo, the CB₂ agonist AM1241 induced mobilization of murine HSPCs with short- and long-term repopulating abilities. In addition, granulocyte colony-stimulating factor -induced mobilization of HSPCs was significantly decreased by specific CB₂ antagonists and was impaired in Cnr2(-/-) cannabinoid type 2 receptor knockout mice. Taken together, these results demonstrate that the endocannabinoid system is involved in hematopoiesis and that CB₂/CB₂ agonist axis mediates repopulation of hematopoiesis and mobilization of HSPCs. Thus, CB₂ agonists may be therapeutically applied in clinical conditions, such as bone marrow transplantation.

Update on clinical experience with AMD3100, an SDF-1/CXCL12-CXCR4 inhibitor, in mobilization of hematopoietic stem and progenitor cells

PURPOSE OF REVIEW

Mobilized peripheral blood stem cells are increasingly used for the reconstitution of hematopoiesis in autologous and allogeneic transplants. New agents and approaches are emerging to improve mobilization efficacy while reducing duration and toxicity of mobilization. The purpose of this review is to overview clinical experience with AMD3100 (plerixafor) and its role in stem cell mobilization.

RECENT FINDINGS

AMD3100 is a bicyclam molecule that selectively and reversibly antagonizes the binding of stromal cell-derived factor-1 (SDF-1) to its receptor CXC motif receptor-4 (CXCR4) with subsequent egress of hematopoietic stem cells to the peripheral blood. AMD3100 safely and rapidly mobilizes stem cells in patients with lymphoma, myeloma and healthy donors, and is synergistic in combination with granulocyte-colony stimulating factor. In addition, AMD3100 disrupts the interaction between mouse and human leukemic blasts and the bone marrow stroma, mobilizing blasts to the peripheral blood and sensitizing them to chemotherapy.

SUMMARY

AMD3100 was recently FDA-approved for stem cell mobilization in combination with granulocyte-colony stimulating factor in patients with non-Hodgkin's lymphoma and multiple myeloma. Studies are underway testing AMD3100 as an adjunct to chemotherapy in patients with refractory acute myelogenous leukemia (and other hematologic malignancies), as a strategy to sensitize leukemic cells to chemotherapy and improve clinical outcomes.

Comparison of unmobilized and mobilized graft characteristics and the implications of cell subsets on autologous and allogeneic transplantation outcomes

Autologous and allogeneic hematopoietic stem cell transplantation (HSCT) are considered the standard of care for many malignancies, including lymphoma, myeloma, and some leukemias. In many cases, mobilized peripheral blood has become the preferred source of hematopoietic stem cells. The efficacy of different mobilization regimens and transplantation outcomes based on cell doses has been well studied; however, the characteristics of the stem cell graft may be of equal importance with respect to patient outcomes following autologous or allogeneic HSCT. This review summarizes available preclinical and clinical data for bone marrow and mobilized peripheral blood HSCT characteristics, defined as the cell types found in the graft as well as their gene expression profiles. It also explores how graft characteristics can affect bone marrow homing, engraftment, immune reconstitution, and other posttransplantation outcomes in both the allogeneic and autologous HSCT settings.

A combination of granulocyte-colony-stimulating factor (G-CSF) and plerixafor mobilizes more primitive peripheral blood progenitor cells than G-CSF alone: results of a European phase II study

BACKGROUND AIMS

Previous studies in xenograft models have shown that human peripheral blood progenitor cells (PBPC) mobilized with the CXCR4 antagonist plerixafor (AMD3100) have a higher bone marrow (BM) reconstitution potential than granulocyte-colony-stimulating factor (G-CSF)-mobilized PBPC.

METHODS

PBPC obtained during G-CSF-supported mobilization before and after a supplementary administration of AMD3100 from patients with multiple myeloma and non-Hodgkin's lymphoma (n=15; phase II study) were investigated for co-expression of primitive and lineage-associated markers, their proliferative activity in vitro and repopulation potential after clinical transplantation.

RESULTS

A significant increase in primitive CD34+ CD38(-) cells was observed in intraindividual comparisons of all patients after administration of G-CSF+AMD3100 (peripheral blood: median 8-fold, range 2,4-fold - 39-fold) compared with G-CSF alone. Using a long-term culture-initiating cell assay, this increase was confirmed. After transplantation of G-CSF+AMD3100-mobilized PBPC, the time to leukocyte reconstitution > 1 x 10(3)/microL and platelet reconstitution > 2 x 10(4)/microL was 14 (10-19 days) and 13 days (10-15 days), respectively. A complete and stable hematologic reconstitution (platelets > 1.5 x 10(5)/microL) was observed in 91% of all patients within 35 days.

CONCLUSIONS

An additional application of AMD3100 to a standard G-CSF mobilization regimen leads to a significant increase in primitive PBPC with high repopulation capacity.

CXCR4 in clinical hematology

Pharmacological manipulation of CXCR4 has proven clinically useful for mobilization of stem and progenitor cells and in several preclinical models of disease. It is a key component in the localization of leukocytes and stem cells. For patients with multiple myeloma and non-Hodgkin's Lymphoma, treatment with plerixafor, an inhibitor of CXCL12 binding to CXCR4, plus G-CSF mobilizes stem cells for autologous transplantation to a greater degree than the treatment with G-CSF alone, and in some cases when patients could not be mobilized with cytokines, chemotherapy, or the combination. Stem cells from healthy donors mobilized with single agent plerixafor have been used for allogeneic transplantation in acute myelogenous leukemia (AML) patients, although this is still in the early phase of clinical development. Plerixafor is also undergoing evaluation to mobilize tumor cells in patients with AML and chronic lymphocytic leukemia (CLL) to enhance the effectiveness of chemotherapy regimens. Plerixafor's effect on neutrophils may also restore circulating neutrophil counts to normal levels in patients with chronic neutropenias such as in WHIMs syndrome. Other areas where inhibition of CXCR4 may be useful based upon preclinical or clinical data include peripheral vascular disease, autoimmune diseases such as rheumatoid arthritis, pulmonary inflammation, and HIV.

Plerixafor for stem cell mobilization in patients with non-Hodgkin's lymphoma and multiple myeloma

OBJECTIVE

To evaluate the literature characterizing the mechanism of action, pharmacokinetics, pharmacodynamics, and therapeutic efficacy of plerixafor for hematopoietic stem cell (HSC) mobilization for autologous transplantation in patients with non-Hodgkin's lymphoma (NHL) or multiple myeloma.

DATA SOURCES

A PubMed search (1966-September 2009) was conducted using the key words plerixafor and AMD3100. Manufacturer's prescribing information was also used.

STUDY SELECTION AND DATA EXTRACTION

English-language articles were selected and data were extracted with a focus on clinical studies of HSC mobilization in patients with NHL or multiple myeloma.

DATA SYNTHESIS

Plerixafor exerts its effect by reversibly blocking the ability of HSC to bind to the bone marrow matrix. When used with granulocyte colony-stimulating factor (G-CSF), plerixafor helps increase the number of HSCs in the peripheral blood, where they can be collected for use in autologous transplantation. In clinical studies, plerixafor was rapidly absorbed after subcutaneous injection, reaching a maximum plasma concentration at approximately 0.5 hours. Plerixafor is renally excreted as the parent drug, with an elimination half-life ranging from 3 to 5 hours. Plerixafor increases circulating CD34+ cells in the peripheral blood, with a peak effect about 6-9 hours after subcutaneous administration. An approximate 2- to 3-fold increase in the CD34+ cell count is seen by the first dose of plerixafor after 4 consecutive days of G-CSF treatment. In 2 Phase 3 studies in patients with NHL or multiple myeloma, addition of plerixafor to G-CSF resulted in a higher CD34+ cell collection with fewer apheresis days, but failed to show better graft durability or overall patient survival for up to 12 months of follow-up.

CONCLUSIONS

Clinical trials have demonstrated that the addition of plerixafor to G-CSF was beneficial for HSC mobilization to peripheral blood for collection and subsequent transplantation in patients with NHL or multiple myeloma. Further studies should assess the benefit of the additive use of plerixafor on clinical outcomes.

Global transcriptional analysis for biomarker discovery and validation in cellular therapies

Potency testing is an important part of the evaluation of cellular therapy products. Potency assays are quantitative measures of a product-specific biologic activity that is linked to a relevant biologic property and, ideally, a product's in vivo mechanism of action. Both in vivo and in vitro assays can be used for potency testing. Since there is often a limited period of time between the completion of production and the release from the laboratory for administration to the patient, in vitro assays such as flow cytometry, ELISA, and cytotoxicity are typically used. Better potency assays are needed to assess the complex and multiple functions of cellular therapy products, some of which are not well understood. Gene expression profiling using microarray technology has been widely and effectively used to assess changes of cells in response to stimuli and to classify cancers. Preliminary studies have shown that the expression of non-coding microRNA (miRNA), which plays an important role in cellular development, differentiation, metabolism, and signal transduction, can distinguish between different types of stem cells and leukocytes. Both gene and miRNA expression profiling have the potential to be important tools for testing the potency of cellular therapies. Potency testing, the complexities associated with potency testing of cellular therapies, and the potential role of gene and miRNA expression microarrays in potency testing of cellular therapies are discussed.

New agents for mobilizing peripheral blood stem cells

Transplantation with bone marrow (BM) hematopoietic stem cells (HSC) has been used for curative therapy of hematologic diseases and inborn errors of metabolism for decades. More recently, alternative sources of HSC, particularly those induced to exit marrow and traffic to peripheral blood in response to external stimuli, have become the most widely used hematopoietic graft and show significant superiority to marrow HSC. Although a variety of agents can mobilize stem cells with different kinetics and efficiencies and these agents can be additive or synergistic when used in combination, currently G-CSF is the predominant stem cell mobilizer used clinically based upon potency, predictability and safety. Recent studies have demonstrated that the interaction between the chemokine stromal-derived factor 1 (SDF-1/CXCL12) and its receptor CXCR4 serves as a key regulator of HSC trafficking. AMD3100, a novel bicyclam CXCR4 antagonist, induces the rapid mobilization of HSC with both short- and long-term repopulation capacity. Mobilization with G-CSF and AMD3100 in clinical trials resulted in more patients achieving sufficient PBSC for transplantation than with G-CSF alone. Thus, chemokine axis-mobilization could allow rapid PBSC harvests with increased cell yields in difficult-to mobilize patients. Studies of autologous and allogeneic transplantation of AMD3100 mobilized grafts demonstrated prompt and stable engraftment. Enhanced homing properties of chemokine axis-mobilized PBSC suggest that these cells may have greater therapeutic utility in other areas including tissue repair and regeneration.

Plerixafor (AMD3100) and granulocyte colony-stimulating factor (G-CSF) mobilize different CD34+ cell populations based on global gene and microRNA expression signatures

Plerixafor (AMD3100) and granulocyte colony-stimulating factor (G-CSF) mobilize peripheral blood stem cells by different mechanisms. A rhesus macaque model was used to compare plerixafor and G-CSF-mobilized CD34(+) cells. Three peripheral blood stem cell concentrates were collected from 3 macaques treated with G-CSF, plerixafor, or plerixafor plus G-CSF. CD34(+) cells were isolated by immunoselection and were analyzed by global gene and microRNA (miR) expression microarrays. Unsupervised hierarchical clustering of the gene expression data separated the CD34(+) cells into 3 groups based on mobilization regimen. Plerixafor-mobilized cells were enriched for B cells, T cells, and mast cell genes, and G-CSF-mobilized cells were enriched for neutrophils and mononuclear phagocyte genes. Genes up-regulated in plerixafor plus G-CSF-mobilized CD34(+) cells included many that were not up-regulated by either agent alone. Two hematopoietic progenitor cell miR, miR-10 and miR-126, and a dendritic cell miR, miR-155, were up-regulated in G-CSF-mobilized CD34(+) cells. A pre-B-cell acute lymphocytic leukemia miR, miR-143-3p, and a T-cell miR, miR-143-5p, were up-regulated in plerixafor plus G-CSF-mobilized cells. The composition of CD34(+) cells is dependent on the mobilization protocol. Plerixafor-mobilized CD34(+) cells include more B-, T-, and mast cell precursors, whereas G-CSF-mobilized cells have more neutrophil and mononuclear phagocyte precursors.

Plerixafor, a CXCR4 antagonist for the mobilization of hematopoietic stem cells

Stem cells harvested from peripheral blood are the most commonly used graft source in hematopoietic stem cell transplantation. While G-CSF is the most frequently used agent for stem cell mobilization, the use of G-CSF alone results in suboptimal stem cell yields in a significant proportion of patients undergoing autologous transplantation. Plerixafor (AMD3100, Genzyme Corporation) is a bicyclam molecule that antagonizes the binding of the chemokine stromal cell-derived factor-1 (SDF-1) to its cognate receptor CXCR4. Plerixafor results in the rapid and reversible mobilization of hematopoietic stem cells into the peripheral circulation and is synergistic when combined with G-CSF. In clinical studies of autologous stem cell transplantation, the combination of plerixafor and G-CSF allows the collection of large numbers of stem cells in fewer apheresis sessions and can salvage those who fail G-CSF mobilization alone.

Mobilization of hematopoietic stem cells: state of the art

PURPOSE OF REVIEW

Hematopoietic stem cells (HSCs) normally reside in the bone marrow but can be forced into the blood, a process termed mobilization used clinically to harvest large numbers of HSCs for transplantation. Currently the mobilizing agent of choice is granulocyte colony-stimulating factor; however, not all patients mobilize well. This article reviews recent advances in understanding the molecular mechanisms responsible for the retention of HSCs in the bone marrow, which are perturbed during HSC mobilization, and the clinical application of these findings.

RECENT FINDINGS

The interaction between the chemokine SDF-1/CXCL12 and its receptor CXCR4 is critical to retain HSCs within the bone marrow, leading to the discovery that small synthetic CXCR4 antagonists are potent mobilizing agents that synergize with granulocyte colony-stimulating factor. Separate research has shown that HSC numbers in the bone marrow can be boosted by increasing the number of osteoblasts that support HSCs.

SUMMARY

HSC mobilization induced by granulocyte colony-stimulating factor may be enhanced by directly targeting the chemotactic interaction between HSCs and bone marrow stroma with CXCR4 antagonists. When the primary problem is reduced, however, HSC numbers in the bone marrow, due to repeated chemotherapy/radiotherapy treatments, an alternative is to enhance HSC content by enhancing bone formation prior to mobilization.