The Granulocyte Colony-Stimulating Factor Receptor Is Required for the Mobilization of Murine Hematopoietic Progenitors Into Peripheral Blood by Cyclophosphamide or Interleukin-8 But Not Flt-3 Ligand

Outcomes of stem cell mobilization and engraftment in patients with multiple myeloma according to CD56 expression status

BACKGROUND

The molecular mechanism underlying the mobilization and engraftment of CD34+ cells is poorly understood. The most relevant factors in the regulation of stem cell release and engraftment include chemokines, adhesion molecules, and chemokine receptors. Previously, it was suggested that the absence of CD56 expression could be used as a predictive factor for mobilization failure at the time of diagnosis. Here, we investigated the effect of CD56 expression status on both mobilization and engraftment processes. Additionally, other factors affecting mobilization and engraftment efficacy were investigated.

METHODS

Data from 79 multiple myeloma patients undergoing autologous stem cell transplantation between 2015 and 2020 were analyzed for peripheral stem cell mobilization and posttransplant neutrophil and platelet engraftment according to CD56 expression on myeloma cells.

RESULTS

No difference in either the median number of CD34+ cells collected or time to engraftment was found between the CD56+ and CD56- groups. The age of the patients (p = 0.025) and peak number of circulating CD34+ cells in peripheral blood (p = 0.005) were important predictors for a higher number of collected CD34+ cells. The average time to recovery of leukocytes and platelets after transplantation was markedly correlated with the number of transplanted stem cells and peak number of circulating CD34+ cells in peripheral blood, respectively (p = 0.049 and p = 0.003).

CONCLUSIONS

Our results indicated no effect of CD56 expression status on the mobilization and engraftment of PBSCs. Our results also support the notion that the peak number of circulating CD34+ cells in peripheral blood is clinically important for rapid platelet engraftment following HPC transplantation.

Role of macrophages and phagocytes in orchestrating normal and pathologic hematopoietic niches

The bone marrow (BM) contains a mosaic of niches specialized in supporting different maturity stages of hematopoietic stem and progenitor cells such as hematopoietic stem cells and myeloid, lymphoid, and erythroid progenitors. Recent advances in BM imaging and conditional gene knockout mice have revealed that niches are a complex network of cells of mesenchymal, endothelial, neuronal, and hematopoietic origins, together with local physicochemical parameters. Within these complex structures, phagocytes, such as neutrophils, macrophages, and dendritic cells, all of which are of hematopoietic origin, have been found to be important in regulating several niches in the BM, including hematopoietic stem cell niches, erythropoietic niches, and niches involved in endosteal bone formation. There is also increasing evidence that these macrophages have an important role in adapting hematopoiesis, erythropoiesis, and bone formation in response to inflammatory stressors and play a key part in maintaining the integrity and function of these. Likewise, there is also accumulating evidence that subsets of monocytes, macrophages, and other phagocytes contribute to the progression and response to treatment of several lymphoid malignancies such as multiple myeloma, Hodgkin lymphoma, and non-Hodgkin lymphoma, as well as lymphoblastic leukemia, and may also play a role in myelodysplastic syndrome and myeloproliferative neoplasms associated with Noonan syndrome and aplastic anemia. In this review, the potential functions of macrophages and other phagocytes in normal and pathologic niches are discussed, as are the challenges in studying BM and other tissue-resident macrophages at the molecular level.

Macrophage Regulation of Granulopoiesis and Neutrophil Functions

Significance: Neutrophils are potent effector cells of innate immunity requiring precise regulation of their numbers and functions in blood and tissues. Recent Advances: Macrophages have emerged as modulators of neutrophil properties. In inflammatory conditions, tissue macrophages modulate neutrophil trafficking and activation. Further, macrophages govern granulopoiesis in the bone marrow hematopoietic niche. Interactions of macrophages and neutrophils can be induced by cytokines and damage-associated molecular patterns, and they are also regulated by oxidative signaling. Critical Issues: We review the impact of macrophages on neutrophil development and function, and its consequences in health and disease. Future Directions: Targeting the liaison between macrophages and neutrophils might provide an interesting therapeutic strategy to reduce tissue inflammation and promote immune tolerance. Antioxid. Redox Signal. 35, 182-191.

Neurogenic Heterotopic Ossifications Develop Independently of Granulocyte Colony-Stimulating Factor and Neutrophils

Neurogenic heterotopic ossifications (NHOs) are incapacitating heterotopic bones in periarticular muscles that frequently develop following traumatic brain or spinal cord injuries (SCI). Using our unique model of SCI-induced NHO, we have previously established that mononucleated phagocytes infiltrating injured muscles are required to trigger NHO via the persistent release of the pro-inflammatory cytokine oncostatin M (OSM). Because neutrophils are also a major source of OSM, we investigated whether neutrophils also play a role in NHO development after SCI. We now show that surgery transiently increased granulocyte colony-stimulating factor (G-CSF) levels in blood of operated mice, and that G-CSF receptor mRNA is expressed in the hamstrings of mice developing NHO. However, mice defective for the G-CSF receptor gene Csf3r, which are neutropenic, have unaltered NHO development after SCI compared to C57BL/6 control mice. Because the administration of recombinant human G-CSF (rhG-CSF) has been trialed after SCI to increase neuroprotection and neuronal regeneration and has been shown to suppress osteoblast function at the endosteum of skeletal bones in human and mice, we investigated the impact of a 7-day rhG-CSF treatment on NHO development. rhG-CSF treatment significantly increased neutrophils in the blood, bone marrow, and injured muscles. However, there was no change in NHO development compared to saline-treated controls. Overall, our results establish that unlike monocytes/macrophages, neutrophils are dispensable for NHO development following SCI, and rhG-CSF treatment post-SCI does not impact NHO development. Therefore, G-CSF treatment to promote neuroregeneration is unlikely to adversely promote or affect NHO development in SCI patients. © 2020 American Society for Bone and Mineral Research.

Development of a cyclophosphamide stress test to predict resilience to aging in mice

The concept of resilience, defined as the ability to recover from stress, is a potential platform to predict healthy aging. However, specific stress tests for resilience have not yet been fully established in humans so investigations in animal models are of interest. The chemotherapeutic drug cyclophosphamide (Cyp) was selected as a chemical stressor to investigate resilience response in C57Bl/6 male mice at 4, 15, and 28 months of age. Following a single intraperitoneal injection of Cyp (100 mg/kg), tail blood was collected for counting white blood cells (WBC) every other day for 25 days, and physiological performance tests performed. Cyp induced a consistent pattern in neutrophil count in all three age groups, with a nadir at day 5 and a rebound at day 7 with different rates in each group. The neutrophil to lymphocyte ratio (NLR) showed an age-dependent rebound response 7 days after Cyp injection, with a similar pattern of decline back toward baseline. Mice in the 15-month age group with high pre-injection Cyp NLR had significantly higher total WBC counts after Cyp injection compared with mice with low pre-injection Cyp NLR, indicating a correlation between NLR and Cyp-altered WBC counts. In addition, mice with high pre-injection Cyp NLR showed significant learning impairment compared with mice with low pre-injection Cyp NLR, suggesting low NRL intensity can predict resilience to age-related cognitive decline. These observations provide the rationale to translate findings from the mouse to humans in developing in vitro Cyp stress tests.

Albumin Modifies Responses to Hematopoietic Stem Cell Mobilizing Agents in Mice

Albumin, the most abundant plasma protein, not only controls osmotic blood pressure, but also serves as a carrier for various small molecules, including pharmaceuticals. Its impact on pharmacological properties of many drugs has been extensively studied over decades. Here, we focus on its interaction with the following mobilizing agents: Granulocyte-colony stimulating factor (G-CSF) and AMD3100, where such analyses are lacking. These compounds are widely used for hematopoietic stem cell mobilization of healthy donors or patients. Using albumin-deficient (Alb−/−) mice, we studied the contribution of albumin to mobilization outcomes. Mobilization with the bicyclam CXCR4 antagonist AMD3100 was attenuated in Alb−/− mice compared to wild-type littermates. By contrast, mobilization with recombinant human G-CSF (rhG-CSF), administered twice daily over a five-day course, was significantly increased in Alb−/− mice. In terms of a mechanism, we show that rhG-CSF bioavailability in the bone marrow is significantly improved in Alb−/− mice, compared to wild-type (WT) littermates, where rhG-CSF levels dramatically drop within a few hours of the injection. These observations likely explain the favorable mobilization outcomes with split-dose versus single-dose administration of rhG-CSF to healthy donors.

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.

Cellular players of hematopoietic stem cell mobilization in the bone marrow niche

Hematopoietic stem cells (HSC) reside in perivascular regions of the bone marrow (BM) embedded within a complex regulatory unit called the niche. Cellular components of HSC niches include vascular endothelial cells, mesenchymal stromal progenitor cells and a variety of mature hematopoietic cells such as macrophages, neutrophils, and megakaryocytes-further regulated by sympathetic nerves and complement components as described in this review. Three decades ago the discovery that cytokines induce a large number of HSC to mobilize from the BM into the blood where they are easily harvested, revolutionised the field of HSC transplantation-curative for immune-deficiencies and some malignancies. However, despite now routine use of granulocyte-colony stimulating factor (G-CSF) to mobilise HSC for transplant, only in last 15 years has research on the mechanisms behind why and how HSC can be induced to move into the blood began. These studies have revealed the complexity of the niche that retains HSC in the BM. This review describes how BM niches and HSC themselves change during administration of G-CSF-or in the recovery phase of chemotherapy-to facilitate movement of HSC into the blood, and research now leading to development of novel therapeutics to further boost HSC mobilization and transplant success.

Impaired Mobilization of Vascular Reparative Bone Marrow Cells in Streptozotocin-Induced Diabetes but not in Leptin Receptor-Deficient db/db Mice

Diabetes is associated with impaired mobilization of bone marrow stem/progenitor cells that accelerate vascularization of ischemic areas. This study characterized mobilization of vascular reparative bone marrow progenitor cells in mouse models of diabetes. Age-matched control or streptozotocin (STZ)-induced diabetic, and db/db mice with lean-controls were studied. Mobilization induced by G-CSF, AMD3100 or ischemia was evaluated by flow cytometric enumeration of circulating Lin(-)Sca-1(+)cKit(+) (LSK) cells, and by colony forming unit (CFU) assay. The circulating WBCs and LSKs, and CFUs were reduced in both models with a shorter duration (10-12 weeks) of diabetes compared to their respective controls. Longer duration of STZ-diabetes (≥20 weeks) induced impairment of G-CSF- or AMD3100-mobilization (P < 0.01, n = 8). In db/db mice, mobilization by G-CSF or AMD3100 was either increased or unaffected (P < 0.05, n = 6 to 8). Proliferation, migration, and ischemia-induced mobilization, of LSK cells were impaired in both models. Leptin receptor antagonist, PESLAN-1, increased G-CSF- or AMD3100-mobilization of WBCs and LSKs, compared to the untreated. Leptin increased basal WBCs, decreased basal and AMD3100-mobilized LSK cells, and had no effect on G-CSF. These results suggest that mobilopathy is apparent in STZ-diabetes but not in db/db mice. Leptin receptor antagonism would be a promising approach for reversing diabetic bone marrow mobilopathy.

Neutrophil-mediated inflammation in the pathogenesis of Clostridium difficile infections

Clostridium difficile is the most important cause of nosocomial infectious diarrhea in the western world. C. difficile infections are a major healthcare burden with approximately 500,000 new cases every year and an estimated annual cost of nearly $1 billion in the U.S. Furthermore, the infections are no longer restricted to health care facilities, and recent studies indicate spread of C. difficile infection to the community as well. The clinical spectrum of C. difficile infection ranges from asymptomatic colonization to severe diarrhea, fulminant colitis and death. This spectrum results from a complex interplay between bacterial virulence factors, the colonic microbiome and the host inflammatory response. The overall vigor of host inflammatory response is believed to be an important determinant of C. difficile disease severity, and a more robust immune response is associated with worse outcomes. Neutrophils are the primary cells that respond to C. difficile invasion and neutrophilic inflammation is the hallmark of C. difficile-associated disease. In this review, we will focus on the role of neutrophils (infiltration to infected tissue, pathogen clearance and resolution of inflammation) in the immuno-pathogenesis of C. difficile-associated disease (CDAD).

Concise Review: CXCR4/CXCL12 Signaling in Immature Hematopoiesis--Lessons From Pharmacological and Genetic Models

Dominant, although nonexclusive roles of CXCR4 and its chief ligand CXCL12 in bone marrow (BM) retention and preservation of the relative quiescence of hematopoietic stem/progenitor cells (HSPCs), along with their involvement in human immunodeficiency virus infection, in trafficking of mature hematopoietic cells to sites of inflammation and in orderly migration of nonhematopoietic cells during embryogenesis, explain the significant interest of the scientific community in the mode of action of this receptor-ligand pair. In this focused review, we seek to distil from the large body of information that has become available over the years some of the key findings about the role of CXCR4/CXCL12 in normal immature hematopoiesis. It is hoped that understanding the mechanistic insights gained there from will help generate hypotheses about potential avenues in which cancer/leukemia cell behavior can be modified by interference with this pathway.

New insights in the mobilization of hematopoietic stem cells in lymphoma and multiple myeloma patients

Following chemotherapy and/or the administration of growth factors, such as granulocyte-colony stimulated factor (G-CSF), hematopoietic stem cells (HSC) mobilize from bone marrow to peripheral blood. This review aims to systematically present the structure of the HSC "niche" and elucidate the mechanisms of their mobilization. However, this field is constantly evolving and new pathways and molecules have been shown to contribute to the mobilization process. Understanding the importance and the possible primary pathophysiologic role of each pathway is rather difficult, since they share various overlapping components. The primary initiating event for the mobilization of HSC is chemotherapy-induced endogenous G-CSF production or exogenous G-CSF administration. G-CSF induces proliferation and expansion of the myelomonocytic series, which leads to proteolytic enzyme activation. These enzymes result in disruption of various receptor-ligand bonds, which leads to the disanchorage of HSC from the bone marrow stroma. In everyday clinical practice, CXC chemokine receptor-4 (CXCR4) antagonists are now being used as mobilization agents in order to improve HSC collection. Furthermore, based on the proposed mechanisms of HSC mobilization, novel mobilizing agents have been developed and are currently evaluated in preclinical and clinical studies.

The novel CXCR4 antagonist POL5551 mobilizes hematopoietic stem and progenitor cells with greater efficiency than Plerixafor

Mobilized blood has supplanted bone marrow (BM) as the primary source of hematopoietic stem cells for autologous and allogeneic stem cell transplantation. Pharmacologically enforced egress of hematopoietic stem cells from BM, or mobilization, has been achieved by directly or indirectly targeting the CXCL12/CXCR4 axis. Shortcomings of the standard mobilizing agent, granulocyte colony-stimulating factor (G-CSF), administered alone or in combination with the only approved CXCR4 antagonist, Plerixafor, continue to fuel the quest for new mobilizing agents. Using Protein Epitope Mimetics technology, a novel peptidic CXCR4 antagonist, POL5551, was developed. In vitro data presented herein indicate high affinity to and specificity for CXCR4. POL5551 exhibited rapid mobilization kinetics and unprecedented efficiency in C57BL/6 mice, exceeding that of Plerixafor and at higher doses also of G-CSF. POL5551-mobilized stem cells demonstrated adequate transplantation properties. In contrast to G-CSF, POL5551 did not induce major morphological changes in the BM of mice. Moreover, we provide evidence of direct POL5551 binding to hematopoietic stem and progenitor cells (HSPCs) in vivo, strengthening the hypothesis that CXCR4 antagonists mediate mobilization by direct targeting of HSPCs. In summary, POL5551 is a potent mobilizing agent for HSPCs in mice with promising therapeutic potential if these data can be corroborated in humans.

Alternatively spliced, truncated GCSF receptor promotes leukemogenic properties and sensitivity to JAK inhibition

Granulocyte colony-stimulating factor (GCSF) drives the production of myeloid progenitor and precursor cells toward neutrophils via the GCSF receptor (GCSFR, gene name CSF3R). Children with severe congenital neutropenia chronically receive pharmacologic doses of GCSF, and ∼30% will develop myelodysplasia/acute myeloid leukemia (AML) associated with GCSFR truncation mutations. In addition to mutations, multiple isoforms of CSF3R have also been reported. We found elevated expression of the alternatively spliced isoform, class IV CSF3R in adult myelodysplastic syndrome/AML patients. Aside from its association with monosomy 7 and higher rates of relapse in pediatric AML patients, little is known about the biology of the class IV isoform. We found developmental regulation of CSF3R isoforms with the class IV expression more representative of a progenitor cell stage. Striking differences were found in phosphoprotein signaling involving Janus kinase (JAK)-signal transducer and activator of transcription (STAT) and cell cycle gene expression. Enhanced proliferation by class IV GCSFR was associated with diminished STAT3 and STAT5 activation, yet showed sensitivity to JAK2 inhibitors. Alterations in the C-terminal domain of the GCSFR result in leukemic properties of enhanced growth, impaired differentiation and resistance to apoptosis, suggesting that they can behave as oncogenic drivers, sensitive to JAK2 inhibition.

Chemokines and adult bone marrow stem cells

The adult bone contains a number of distinct populations of stem cells, including haematopoietic stem cells, mesenchymal stem cells, endothelial progenitor cells and fibrocytes. While haematopoietic stem cells are required to provide a lifelong supply of blood cells it is thought that the other populations of stem cells play a role in tissue regeneration and potentially disease. The chemokine CXCL12 is produced constitutively in the bone marrow and, acting via CXCR4, is critical in maintaining HSPCs in a quiescent state and retaining all subsets of stem and progenitor cells in the bone marrow environment. The cytokine G-CSF, used clinically to mobilize haematopoietic stem cells for bone marrow transplants, activates the sympathetic nervous system and bone marrow macrophages to reduce the expression of CXCL12 by bone marrow stromal cells, thereby promoting the exit of haematopoietic stem cells from the bone marrow. Understanding the molecular mechanisms underlying G-CSF stimulated mobilization has led to development of CXCR4 antagonists as fast acting mobilizing agents for haematopoietic stem cells. Evidence now suggests that CXCR4 antagonists can similarly mobilize distinct subsets of progenitor cells, namely the endothelial progenitor cells and mesenchymal stem cells, but this requires conditioning of the bone marrow with VEGF rather than G-CSF.

Hematopoietic stem cell mobilization: updated conceptual renditions

Despite its specific clinical relevance, the field of hematopoietic stem cell mobilization has received broad attention, owing mainly to the belief that pharmacologic stem cell mobilization might provide clues as to how stem cells are retained in their natural environment, the bone marrow 'niche'. Inherent to this knowledge is also the desire to optimally engineer stem cells to interact with their target niche (such as after transplantation), or to lure malignant stem cells out of their protective niches (in order to kill them), and in general to decipher the niche's structural components and its organization. Whereas, with the exception of the recent addition of CXCR4 antagonists to the armamentarium for mobilization of patients refractory to granulocyte colony-stimulating factor alone, clinical stem cell mobilization has not changed significantly over the last decade or so, much effort has been made trying to explain the complex mechanism(s) by which hematopoietic stem and progenitor cells leave the marrow. This brief review will report some of the more recent advances about mobilization, with an attempt to reconcile some of the seemingly inconsistent data in mobilization and to interject some commonalities among different mobilization regimes.

Mobilisation strategies for normal and malignant cells

This review evaluates the latest information on the mobilisation of haemopoietic stem cells for transplantation, with the focus on what is the current best practice and how new understanding of the bone marrow stem cell niche provides new insights into optimising mobilisation regimens. The review then looks at the mobilisation of mesenchymal stromal cells, immune cells as well as malignant cells and what clinical implications there are.

Mobilization of hematopoietic stem/progenitor cells: general principles and molecular mechanisms

Hematopoietic stem/progenitor cell mobilization can be achieved by a variety of bone marrow niche modifications, although efficient mobilization requires simultaneous expansion of the stem/progenitor cell pool and niche modification. Many of the mechanisms involved in G-CSF-induced mobilization have been described. With regard to mobilization of hematopoietic stem/progenitor cells, challenges for the future include the analysis of genetic factors responsible for the great variability in mobilization responses, and the identification of predictors of mobilization efficiency, as well as the development of mobilizing schemes for poor mobilizers. Moreover, improved regimens for enhanced or even preferential mobilization of nonhematopoietic stem/progenitor cell types, and their therapeutic potential for endogenous tissue repair will be questions to be vigorously pursued in the near future.

Mobilization of hematopoietic stem cells by depleting bone marrow macrophages

An important factor contributing to hematopoietic stem cell (HSC) mobilization is the ability of mobilizing cytokines and chemotherapy to disturb the cellular components of HSC niches, particularly osteoblasts and their progenitors, and to inhibit the production of HSC supportive cytokines and chemokines. Although the mechanisms by which niche cells are inhibited by mobilizing treatments is still incompletely understood, it has recently emerged that bone marrow macrophages play a critical role in maintaining osteoblasts, bone formation, and the expression of CXCL12, KIT ligand, and angiopoietin-1 necessary to HSC maintenance. In this chapter, we describe how to mobilize HSC into the blood in mice by depleting macrophages with clodronate-loaded liposomes and compare this mode of mobilization to mobilization induced by granulocyte colony-stimulating factor and cyclophosphamide. Detailed methods to analyze mobilization of phenotypic and functional reconstituting HSC are described with examples.

How I treat patients who mobilize hematopoietic stem cells poorly

Transplantation with 2-5 × 106 mobilized CD34+cells/kg body weight lowers transplantation costs and mortality. Mobilization is most commonly performed with recombinant human G-CSF with or without chemotherapy, but a proportion of patients/donors fail to mobilize sufficient cells. BM disease, prior treatment, and age are factors influencing mobilization, but genetics also contributes. Mobilization may fail because of the changes affecting the HSC/progenitor cell/BM niche integrity and chemotaxis. Poor mobilization affects patient outcome and increases resource use. Until recently increasing G-CSF dose and adding SCF have been used in poor mobilizers with limited success. However, plerixafor through its rapid direct blockage of the CXCR4/CXCL12 chemotaxis pathway and synergy with G-CSF and chemotherapy has become a new and important agent for mobilization. Its efficacy in upfront and failed mobilizers is well established. To maximize HSC harvest in poor mobilizers the clinician needs to optimize current mobilization protocols and to integrate novel agents such as plerixafor. These include when to mobilize in relation to chemotherapy, how to schedule and perform apheresis, how to identify poor mobilizers, and what are the criteria for preemptive and immediate salvage use of plerixafor.

G-CSF receptor activation of the Src kinase Lyn is mediated by Gab2 recruitment of the Shp2 phosphatase

Src activation involves the coordinated regulation of positive and negative tyrosine phosphorylation sites. The mechanism whereby receptor tyrosine kinases, cytokine receptors, and integrins activate Src is not known. Here, we demonstrate that granulocyte colony-stimulating factor (G-CSF) activates Lyn, the predominant Src kinase in myeloid cells, through Gab2-mediated recruitment of Shp2. After G-CSF stimulation, Lyn dynamically associates with Gab2 in a spatiotemporal manner. The dephosphorylation of phospho-Lyn Tyr507 was abrogated in Shp2-deficient cells transfected with the G-CSF receptor but intact in cells expressing phosphatase-defective Shp2. Auto-phosphorylation of Lyn Tyr396 was impaired in cells treated with Gab2 siRNA. The constitutively activated Shp2E76A directed the dephosphorylation of phospho-Lyn Tyr507 in vitro. Tyr507 did not undergo dephosphorylation in G-CSF-stimulated cells expressing a mutant Gab2 unable to bind Shp2. We propose that Gab2 forms a complex with Lyn and after G-CSF stimulation, Gab2 recruits Shp2, which dephosphorylates phospho-Lyn Tyr507, leading to Lyn activation.

Expression of the G-CSF receptor in monocytic cells is sufficient to mediate hematopoietic progenitor mobilization by G-CSF in mice

Granulocyte colony-stimulating factor (G-CSF), the prototypical mobilizing cytokine, induces hematopoietic stem and progenitor cell (HSPC) mobilization from the bone marrow in a cell-nonautonomous fashion. This process is mediated, in part, through suppression of osteoblasts and disruption of CXCR4/CXCL12 signaling. The cellular targets of G-CSF that initiate the mobilization cascade have not been identified. We use mixed G-CSF receptor (G-CSFR)-deficient bone marrow chimeras to show that G-CSF-induced mobilization of HSPCs correlates poorly with the number of wild-type neutrophils. We generated transgenic mice in which expression of the G-CSFR is restricted to cells of the monocytic lineage. G-CSF-induced HSPC mobilization, osteoblast suppression, and inhibition of CXCL12 expression in the bone marrow of these transgenic mice are intact, demonstrating that G-CSFR signals in monocytic cells are sufficient to induce HSPC mobilization. Moreover, G-CSF treatment of wild-type mice is associated with marked loss of monocytic cells in the bone marrow. Finally, we show that bone marrow macrophages produce factors that support the growth and/or survival of osteoblasts in vitro. Together, these data suggest a model in which G-CSFR signals in bone marrow monocytic cells inhibit the production of trophic factors required for osteoblast lineage cell maintenance, ultimately leading to HSPC mobilization.

Dendritic cell recovery post-lymphodepletion: a potential mechanism for anti-cancer adoptive T cell therapy and vaccination

Adoptive transfer of autologous tumor-reactive T cells holds promise as a cancer immunotherapy. In this approach, T cells are harvested from a tumor-bearing host, expanded in vitro and infused back to the same host. Conditioning of the recipient host with a lymphodepletion regimen of chemotherapy or radiotherapy before adoptive T cell transfer has been shown to substantially improve survival and anti-tumor responses of the transferred cells. These effects are further enhanced when the adoptive T cell transfer is followed by vaccination with tumor antigens in combination with a potent immune adjuvant. Although significant progress has been made toward an understanding of the reasons underlying the beneficial effects of lymphodepletion to T cell adoptive therapy, the precise mechanisms remain poorly understood. Recent studies, including ours, would indicate a more central role for antigen presenting cells, in particular dendritic cells. Unraveling the exact role of these important cells in mediation of the beneficial effects of lymphodepletion could provide novel pathways toward the rational design of more effective anti-cancer immunotherapy. This article focuses on how the frequency, phenotype, and functions of dendritic cells are altered during the lymphopenic and recovery phases post-induction of lymphodepletion, and how they affect the anti-tumor responses of adoptively transferred T cells.

Mathematical modeling as a tool for planning anticancer therapy

We review a large volume of literature concerning mathematical models of cancer therapy, oriented towards optimization of treatment protocols. The review, although partly idiosyncratic, covers such major areas of therapy optimization as phase-specific chemotherapy, antiangiogenic therapy and therapy under drug resistance. We start from early cell cycle progression models, very simple but admitting explicit mathematical solutions, based on methods of control theory. We continue with more complex models involving evolution of drug resistance and pharmacokinetic and pharmacodynamic effects. Then, we consider two more recent areas: angiogenesis of tumors and molecular signaling within and among cells. We discuss biological background and mathematical techniques of this field, which has a large although only partly realized potential for contributing to cancer treatment.

Insights into the biology of mobilized hematopoietic stem/progenitor cells through innovative treatment schedules of the CXCR4 antagonist AMD3100

OBJECTIVE

The CXCR4 antagonist AMD3100 mobilizes hematopoietic stem/progenitor cells (HSPC) in several species. Few data are available on the biology of HSPC mobilized with AMD3100 as single agent. To further study the kinetics and properties of AMD3100-mobilized HSPC, and to explore the size of mobilizable pools of HSPC targeted by AMD3100, we studied the effect of a continuous infusion scheme with saturating doses of AMD3100 [AMDi].

MATERIALS AND METHODS

Using established procedures, we evaluated mice mobilized with AMD3100, or those transplanted with AMD3100-mobilized HSPC.

RESULTS

Relative to single-bolus AMD3100 [AMDb], the number of circulating CFU-C or CRU was dramatically higher after [AMDi]. During [AMDi], circulating CFU-C accumulated slowly, but after its discontinuation, CFU-C disappeared rapidly. Compared to bone marrow (BM)-c-kit(+) cells, AMD3100-mobilized (AMDb or AMDi) c-kit(+) cells showed reduced expression of several cytoadhesion molecules, similar to granulocyte colony-stimulating factor-mobilized c-kit(+) cells. In contrast to the latter, expression of CXCR4 and CD26 were not reduced on AMD3100-mobilized c-kit(+) cells. BM homing of [AMDi]-mobilized CFU-C was >50% increased over normal BM-CFU-C. Hematopoietic recovery after transplantation of [AMDi]-mobilized peripheral blood was comparable to that of continuous infusion granulocyte colony-stimulating factor-mobilized peripheral blood. AMD3100-mobilized HSPC were predominantly in G(0), and partial bromodeoxyuridine-labeling experiments documented underrepresentation of labeled cells (<5%) among [AMDb]-mobilized c-kit(+) cells, suggesting that cycling cells in BM, or those that recently completed cell cycle, are not targeted for mobilization by AMD3100.

CONCLUSIONS

Our data demonstrate that [AMDi] is an efficacious mobilization scheme fully supporting transplantation demands and expands previous knowledge about properties and size of AMD3100-sensitive BM-HSPC pools.

The coordinated action of G-CSF and ELR + CXC chemokines in neutrophil mobilization during acute inflammation

In this study, we have identified a unique combinatorial effect of the chemokines KC/MIP-2 and the cytokine granulocyte colony-stimulating factor (G-CSF) with respect to the rapid mobilization of neutrophils from the bone marrow in a model of acute peritonitis. At 2 hours following an intraperitoneal injection of thioglycollate, there was a 4.5-fold increase in blood neutrophil numbers, which was inhibited 84% and 72% by prior administration of blocking mAbs against either the chemokines KC/MIP-2 or G-CSF, respectively. An intraperitoneal injection of G-CSF acted remotely to stimulate neutrophil mobilization, but did not elicit recruitment into the peritoneum. Further, in vitro G-CSF was neither chemotactic nor chemokinetic for murine neutrophils, and had no priming effect on chemotaxis stimulated by chemokines. Here, we show that, in vitro and in vivo, G-CSF induces neutrophil mobilization by disrupting their SDF-1alpha-mediated retention in the bone marrow. Using an in situ perfusion system of the mouse femoral bone marrow to directly assess mobilization, KC and G-CSF mobilized 6.8 x 10(6) and 5.4 x 10(6) neutrophils, respectively, while the infusion of KC and G-CSF together mobilized 19.5 x 10(6) neutrophils, indicating that these factors act cooperatively with respect to neutrophil mobilization.

Hematopoietic progenitor cell mobilization results in hypoxia with increased hypoxia-inducible transcription factor-1 alpha and vascular endothelial growth factor A in bone marrow

Despite the fact that many hypoxia-inducible genes are important in hematopoiesis, the spatial distribution of oxygen in the bone marrow (BM) has not previously been explored in vivo. Using the hypoxia bioprobe pimonidazole, we showed by confocal laser scanning microscopy that the endosteum at the bone-BM interface is hypoxic, with constitutive expression of hypoxia-inducible transcription factor-1alpha (HIF-1alpha) protein in steady-state mice. Interestingly, at the peak of hematopoietic stem and progenitor cell (HSPC) mobilization induced by either granulocyte colony-stimulating factor or cyclophosphamide, hypoxic areas expand through the central BM. Furthermore, we found that HSPC mobilization leads to increased levels of HIF-1alpha protein and increased expression of vascular endothelial growth factor A (VEGF-A) mRNA throughout the BM, with an accumulation of VEGF-A protein in BM endothelial sinuses. VEGF-A is a cytokine known to induce stem cell mobilization, vasodilatation, and vascular permeability in vivo. We therefore propose that the expansion in myeloid progenitors that occurs during mobilization depletes the BM hematopoietic microenvironment of O(2), leading to local hypoxia, stabilization of HIF-1alpha transcription factor in BM cells, increased transcription of VEGF-A, and accumulation of VEGF-A protein on BM sinuses that increases vascular permeability. Disclosure of potential conflicts of interest is found at the end of this article.

Granulocyte colony-stimulating factor and an RARalpha specific agonist, VTP195183, synergize to enhance the mobilization of hematopoietic progenitor cells

BACKGROUND

Failure to mobilize adequate numbers of hematopoietic stem and progenitor cells (HSPC) is an important clinical problem. Since bone marrow (BM) neutrophils play a central role in HSPC mobilization, we hypothesized that granulocyte colony-stimulating factor (G-CSF)-mediated mobilization would be enhanced by further expanding the size of the BM granulocyte pool.

METHODS

We tested the potential of the retinoic acid receptor alpha (RARalpha) specific agonist VTP195183, and the pan-RAR agonist all-trans retinoic acid (ATRA), to enhance G-CSF-mediated mobilization of HSPC, in two mouse strains.

RESULTS

Pretreatment of mice with VTP195183 significantly increased the number of leukocytes, colony-forming cells, and early engrafting hematopoietic stem cells (HSC) mobilized in the blood in response to G-CSF. In contrast, ATRA had only a marginal effect on G-CSF-induced mobilization. HSPC mobilization synergy between VTP195183 and G-CSF occurred only when mice were preconditioned with VTP195183 prior to G-CSF. This preconditioning was shown to increase the numbers of granulocyte/macrophage progenitors in the BM. Treatment with VTP195183 and G-CSF was accompanied by enhanced levels of active neutrophil proteases in the BM extracellular fluid compared to G-CSF treatment alone.

CONCLUSIONS

VTP195183 treatment increases the numbers of immature granulocyte progenitors in BM and subsequently synergizes to enhance G-CSF-mediated mobilization of HSPC. These data demonstrate a novel approach to improve G-CSF-induced mobilization by accelerating granulocyte maturation in the BM. These findings are currently being tested in a clinical trial of VTP195183 plus G-CSF for mobilization of HSPC in human patients.

Repeated hematopoietic stem and progenitor cell mobilization without depletion of the bone marrow stem and progenitor cell pool in mice after repeated administration of recombinant murine G-CSF

Administration of recombinant-human G-CSF (rhG-CSF) is highly efficient in mobilizing hematopoietic stem and progenitor cells (HSC/HPC) from the bone marrow (BM) toward the peripheral blood. This study was designed to investigate whether repeated G-CSF-induced HSC/HPC mobilization in mice could lead to a depletion of the bone marrow HSC/HPC pool with subsequent loss of mobilizing capacity. To test this hypothesis Balb/c mice were treated with a maximum of 12 repeated 5-day cycles of either 10 microg rhG-CSF/day or 0.25 microg rmG-CSF/day. Repeated administration of rhG-CSF lead to strong inhibition of HSC/HPC mobilization toward the peripheral blood and spleen after >4 cycles because of the induction of anti-rhG-CSF antibodies. In contrast, after repeated administration of rmG-CSF, HSC/HPC mobilizing capacity remained intact for up to 12 cycles. The number of CFU-GM per femur did not significantly change for up to 12 cycles. We conclude that repeated administration of G-CSF does not lead to depletion of the bone marrow HSC/HPC pool.

Mobilization of bone marrow-derived progenitors

Bone marrow (BM) is a source of various stem and progenitor cells in the adult, and it is able to regenerate a variety of tissues following transplantation. In the 1970s the first BM stem cells identified were hematopoietic stem cells (HSCs). HSCs have the potential to differentiate into all myeloid (including erythroid) and lymphoid cell lineages in vitro and reconstitute the entire hematopoietic and immune systems following transplantation in vivo. More recently, nonhematopoietic stem and progenitor cells have been identified that can differentiate into other cell types such as endothelial progenitor cells (EPCs), contributing to the neovascularization of tumors as well as ischemic tissues, and mesenchymal stem cells (MSCs), which are able to differentiate into many cells of ectodermal, endodermal, and mesodermal origins in vitro as well as in vivo. Following adequate stimulation, stem and progenitor cells can be forced out of the BM to circulate into the peripheral blood, a phenomenon called "mobilization." This chapter reviews the molecular mechanisms behind mobilization and how these have led to the various strategies employed to mobilize BM-derived stem and progenitor cells in experimental and clinical settings. Mobilization of HSCs will be reviewed first, as it has been best-explored--being used extensively in clinics to transplant large numbers of HSCs to rescue cancer patients requiring hematopoietic reconstitution--and provides a paradigm that can be generalized to the mobilization of other types of BM-derived stem and progenitor cells in order to repair other tissues.

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

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

Involvement of Proteases in Cytokine-Induced Hematopoietic Stem Cell Mobilization

The number of circulating stem cells and progenitor cells can be increased by physiological stress, such as exercise, stress, and infections. The process of shifting the stem cells from the bone marrow into the peripheral blood is referred to as "mobilization" or "egress." Cytokine-mobilized hematopoietic progenitor cells (HPCs) are currently used for autologous or allogeneic stem cell transplantation in a variety of malignant and nonmalignant diseases. In spite of the wide-spread use of mobilized peripheral blood stem cells for transplantation, the mechanisms underlying mobilization are still incompletely understood. Here we discuss the role of neutrophils and proteases as mediators of stem cell mobilization.

THE EFFECTS OF GRANULOCYTE COLONY-STIMULATING FACTOR IN PRECLINICAL MODELS OF INFECTION AND ACUTE INFLAMMATION

The cytokine granulocyte colony-stimulating factor (G-CSF) is a potent endogenous trigger for the release of neutrophils from bone marrow stores and for their activation for enhanced antimicrobial activity. G-CSF has been widely evaluated in preclinical models of acute illness, with generally promising though divergent results. A recombinant G-CSF molecule has recently undergone clinical trials to assess its efficacy as an adjuvant therapy in community-acquired and nosocomial pneumonia, however, these studies failed to provide convincing evidence of benefit. We undertook a systematic review of the published literature reporting the effects of modulation of G-CSF in preclinical in vivo models to determine whether evidence of differential efficacy might explain the disappointing results of human studies and point to disease states that might be more likely to benefit from G-CSF therapy. G-CSF has been evaluated in 86 such studies involving a variety of different models. The strongest evidence of benefit was seen in studies involving intraperitoneal challenge with live organisms; benefit was evident whether the agent was given before or after challenge. G-CSF demonstrates anti-inflammatory activity in models of systemic challenge with viable organisms or endotoxin, but only when the agent is given before challenge; evidence of benefit after challenge was minimal. Preclinical models of intrapulmonary challenge only show efficacy when the cytokine is administered before the infectious challenge, and suggested harm in gram-negative pneumonia resulting from challenge with Escherichia coli or Klebsiella. There is little evidence for therapeutic efficacy in noninfectious models of acute illness. We conclude that the most promising populations for evaluation of G-CSF are neutropenic patients with invasive infection and patients with intra-abdominal infection, particularly those with the syndrome of tertiary, or recurrent, peritonitis. Significant variability in the design and reporting of studies of preclinical models of acute illness precludes more sophisticated data synthesis.

Hematopoietic Progenitor Cell Mobilization by Granulocyte Colony-Stimulating Factor and Erythropoietin in the Absence of Matrix Metalloproteinase-9

The use of mobilized hematopoietic progenitor cells (HPC) has largely replaced the use of bone marrow HPC for autologous and allogeneic transplantation; however, the mechanisms of HPC mobilization remain unclear. A better understanding of these mechanisms, may allow the development of improved (potentially more rapid and/or higher yield) HPC mobilization strategies, especially for patients who mobilize poorly using current mobilization protocols. Clinically, granulocyte colony-stimulating factor (G-CSF) is widely used to induce HPC mobilization, and evidence suggests that metalloproteinase enzymes released by activated granulocytes play an important role in the G-CSF-induced HPC mobilization. These enzymes may act to disrupt putative cell-cell and/or cell-extracellular matrix interactions within the hematopoietic microenvironment thereby releasing HPC into the blood. Matrix metalloproteinase-9 (MMP-9) appears to be important for G-CSF-induced mobilization. Using an MMP-9 knock-out (KO) mouse model, we investigated the role of MMP-9 in G-CSF and erythropoietin (EPO)-based HPC mobilization at clinically relevant cytokine doses. There were few hematologic or hematopoietic differences between the wild-type and MMP-9KO mice during steady-state hematopoiesis. When treated subcutaneously with EPO (500 U/kg per day) and G-CSF (15 microg/kg per day) for 5 days and assayed on day 6, similarly increased extramedullary hematopoiesis and numbers of HPC in the spleen and blood were observed for both the wild-type and MMP-9KO mice. These data demonstrate that MMP-9 is not required for EPO + G-CSF mobilization and that alternative mobilization mechanisms must be active at clinically relevant cytokine concentrations.

The CXCL12-CXCR4 chemotactic pathway as a target of adjuvant breast cancer therapies

Dose-dense adjuvant breast cancer chemotherapy is a new treatment strategy that aims to improve tumour control by using more frequent cytotoxic dosing together with continuous granulocyte colony-stimulating factor (G-CSF) to minimize neutropaenia. In addition to stimulating neutrophil proliferation, G-CSF mobilizes neutrophils from the bone marrow through proteolytic disruption of the chemokine receptor CXCR4 and its chemotactic ligand CXCL12. As breast cancers also express CXCR4 and oestrogen induces CXCL12, the success of dose-dense treatment could partly reflect inhibition of CXCR4-dependent micrometastatic homing and/or paracrine survival, and suggests a benefit of adjuvant oestrogen suppression for patients with oestrogen-receptor-negative, CXCR4-positive disease.

Characterization of hematopoietic progenitor mobilization in protease-deficient mice

Recent evidence suggests that protease release by neutrophils in the bone marrow may contribute to hematopoietic progenitor cell (HPC) mobilization. Matrix metalloproteinase-9 (MMP-9), neutrophil elastase (NE), and cathepsin G (CG) accumulate in the bone marrow during granulocyte colony-stimulating factor (G-CSF) treatment, where they are thought to degrade key substrates including vascular cell adhesion molecule-1 (VCAM-1) and CXCL12. To test this hypothesis, HPC mobilization was characterized in transgenic mice deficient in one or more hematopoietic proteases. Surprisingly, HPC mobilization by G-CSF was normal in MMP-9–deficient mice, NE × CG-deficient mice, or mice lacking dipeptidyl peptidase I, an enzyme required for the functional activation of many hematopoietic serine proteases. Moreover, combined inhibition of neutrophil serine proteases and metalloproteinases had no significant effect on HPC mobilization. VCAM-1 expression on bone marrow stromal cells decreased during G-CSF treatment of wild-type mice but not NE × CG-deficient mice, indicating that VCAM-1 cleavage is not required for efficient HPC mobilization. G-CSF induced a significant decrease in CXCL12α protein expression in the bone marrow of Ne × CG-deficient mice, indicating that these proteases are not required to down-regulate CXCL12 expression. Collectively, these data suggest a complex model in which both protease-dependent and -independent pathways may contribute to HPC mobilization.

Potent receptor-mediated cytotoxicity of granulocyte colony-stimulating factor-Pseudomonas exotoxin, a fusion protein against myeloid leukemia cells

A chimeric toxin in which the cell-surface binding domain of Pseudomonas exotoxin A was replaced with mature human granulocyte colony-stimulating factor (G-CSF) was produced in Escherichia coli, purified and tested for its biological activity on the human G-CSF-responsive myeloid leukemia cell line, UT7/GR. This fusion protein, termed G-CSF-PE40, showed potent cytotoxicity in the cell line in a dose-dependent manner. G-CSF-PE40 displaced binding of biotinylated G-CSF to its receptor, and the cytotoxicity of G-CSF-PE40 was neutralized by an excess of wild-type G-CSF, indicating the receptor-mediated effects of this chimeric toxin. When G-CSF-PE40 was injected into normal mice, they showed transient neutropenia but no significant changes in the numbers of red blood cells or platelets. Furthermore, G-CSF-PE40 prolonged the survival of mice transplanted with syngeneic myeloid leukemia cells. These observations suggest that G-CSF-PE40 may be useful in targeted therapy of myeloid leukemia cells expressing G-CSF receptors.

The integrin alphaMbeta2 anchors hematopoietic progenitors in the bone marrow during enforced mobilization

The sulfated polysaccharide fucoidan can rapidly mobilize hematopoietic progenitor cells (HPCs) and long-term repopulating stem cells from the bone marrow (BM) to the circulation. While searching for mechanisms involved in this phenomenon we found that BM myeloid cells bound to fucoidan through the integrin alphaMbeta2 (macrophage antigen-1 [Mac-1]) and L-selectin resulting in alphaMbeta2-independent release of neutrophil elastase, but inhibition of elastase activity did not impair fucoidan-induced mobilization. Mobilization of HPCs by fucoidan was enhanced in animals deficient in alphaM (alphaM-/-) compared with wild-type (alphaM+/+) animals and higher plasma levels of the chemokine CXCL12/stromal cell-derived factor-1 (SDF-1) were achieved in alphaM-/- mice by fucoidan treatment. However, in chimeric animals harboring alphaM+/+ and alphaM-/- HPCs in the BM, alphaM-/- HPCs were preferentially mobilized by fucoidan, suggesting that the enhanced mobilization is cell intrinsic and does not result from altered microenvironment. Suboptimal doses of granulocyte colony-stimulating factor (G-CSF) or cyclophosphamide (CY) also resulted in enhanced HPC mobilization in alphaM-/- mice compared with alphaM+/+ controls, but this difference was overcome when standard doses of G-CSF or CY were administered. Taken together, these data suggest that the integrin alphaMbeta2 participates in the retention of HPCs in the BM.

Use of matrix metalloproteinase (MMP)-9 knockout mice demonstrates that MMP-9 activity is not absolutely required for G-CSF or Flt-3 ligand-induced hematopoietic progenitor cell mobilization or engraftment

Recombinant growth factors (GFs) are used to mobilize hematopoietic stem cells (HSCs) for autologous and allogeneic transplantation; however, little is known about the mechanism(s) critical to this process. Increased levels of serum matrix metalloproteinase (MMP)-9 are detected during mobilization by G-CSF in humans or interleukin (IL)-8 in primates and mice, suggesting a role for this molecule in mobilization. Further, antibodies to MMP-9 block IL-8-induced mobilization. To investigate the role of MMP-9, we compared G-CSF and Flt-3 ligand (Flt-3L)-induced mobilization in wild-type (WT) and MMP-9 knockout (KO) mice. The absence of MMP-9 in the KO mice was confirmed by zymography, which also revealed that serum MMP-9 levels were elevated in WT mice following G-CSF administration. We report that MMP-9 KO mice did not have impaired G-CSF- or Flt-3L-induced hematopoietic progenitor mobilization, suggesting that MMP-9 is not an absolute requirement for this process. In addition, MMPs produced by HSCs have been demonstrated to be important for their transmigration; however, we demonstrate that the engraftment of MMP-9-deficient bone marrow HSCs was not impaired in sublethally irradiated WT recipients. We conclude that while MMP-9 may play an important role in GF-induced hematopoietic progenitor mobilization and engraftment in WT animals, compensatory upregulation of enzymes with a similar activity profile to MMP-9 may obscure the impact of MMP-9 deficiency in the KO model.

Neutrophil-derived MMP-9 mediates synergistic mobilization of hematopoietic stem and progenitor cells by the combination of G-CSF and the chemokines GRObeta/CXCL2 and GRObetaT/CXCL2delta4

Mobilized peripheral blood stem cells (PBSCs) are widely used for transplantation, but mechanisms mediating their release from marrow are poorly understood. We previously demonstrated that the chemokines GRObeta/CXCL2 and GRObetaT/CXCL2Delta4 rapidly mobilize PBSC equivalent to granulocyte colony-stimulating factor (G-CSF) and are synergistic with G-CSF. We now show that mobilization by GRObeta/GRObetaT and G-CSF, alone or in combination, requires polymorphonuclear neutrophil (PMN)-derived proteases. Mobilization induced by GRObeta/GRObetaT is associated with elevated levels of plasma and marrow matrix metalloproteinase 9 (MMP-9) and mobilization and MMP-9 are absent in neutrophil-depleted mice. G-CSF mobilization correlates with elevated neutrophil elastase (NE), cathepsin G (CG), and MMP-9 levels within marrow and is partially blocked by either anti-MMP-9 or the NE inhibitor MeOSuc-Ala-Ala-Pro-Val-CMK. Mobilization and protease accumulation are absent in neutrophil-depleted mice. Synergistic PBSC mobilization observed when G-CSF and GRObeta/GRObetaT are combined correlates with a synergistic rise in the level of plasma MMP-9, reduction in marrow NE, CG, and MMP-9 levels, and a coincident increase in peripheral blood PMNs but decrease in marrow PMNs compared to G-CSF. Synergistic mobilization is completely blocked by anti-MMP-9 but not MeOSuc-Ala-Ala-Pro-Val-CMK and absent in MMP-9-deficient or PMN-depleted mice. Our results indicate that PMNs are a common target for G-CSF and GRObeta/GRObetaT-mediated PBSC mobilization and, importantly, that synergistic mobilization by G-CSF plus GRObeta/GRObetaT is mediated by PMN-derived plasma MMP-9.

The role of G-protein signaling in hematopoietic stem/progenitor cell mobilization

The directed migration of mature leukocytes to inflammatory sites and the lymphocyte trafficking in vivo are dependent on G protein-coupled receptors and delivered through pertussis toxin (Ptx)-sensitive Gi-protein signaling. In the present study, we explored the in vivo role of G-protein signaling on the redistribution or mobilization of hematopoietic stem/progenitor cells (HPCs). A single injection of Ptx in mice elicits a long-lasting leukocytosis and a progressive increase in circulating colony-forming unit-culture (CFU-C) and colony-forming unit spleen (CFU-S). We found that the prolonged effect is sustained by a continuous slow release of Ptx bound to red blood cells or other cells and is potentially enhanced by an indirect influence on cell proliferation. Plasma levels of certain cytokines (interleukin 6 [IL-6], granulocyte colony-stimulating factor [G-CSF]) increase days after Ptx treatment, but these are unlikely initiators of mobilization. In addition to normal mice, mice genetically deficient in monocyte chemotactic protein 1 (MCP-1), matrix metalloproteinase 9 (MMP-9), G-CSF receptor, beta2 integrins, or selectins responded to Ptx treatment, suggesting independence of Ptx-response from the expression of these molecules. Combined treatments of Ptx with anti-very late activation antigen (anti-VLA-4), uncovered potentially important insight in the interplay of chemokines/integrins, and the synergy of Ptx with G-CSF appeared to be dependent on MMP-9. As Ptx-mobilized kit+ cells display virtually no response to stromal-derived factor 1 (SDF-1) in vitro, our data suggest that disruption of CXCR4/SDF-1 signaling may be the underlying mechanism of Ptx-induced mobilization and indirectly reinforce the notion that active signaling through this pathway is required for continuous retention of cells within the bone marrow. Collectively, our data unveil a novel example of mobilization through pharmacologic modulation of signaling.

Peripheral blood stem cell mobilization. A role for CXC chemokines

Chemokines induce rapid hematopoietic stem and progenitor cell mobilization and synergize with hematopoietic cytokines in mobilizing stem and progenitor cells. These proteins alone and in combination offer new paradigms for autologous and allogeneic peripheral blood stem cell transplantation (PBSCT). The mechanisms responsible for hematopoietic stem cell (HSC) mobilization either with growth factors or chemokines are largely unknown, but a better understanding of these mechanisms will permit the development of novel, more rapid and efficacious regimens. Studies presented herein indicate that the CXCR2 chemokine receptor that interacts with selective chemokine ligands, particularly GRObeta/CXCL2 and GRObeta-T, may be the dominant receptor mediating hematopoietic cell mobilization, and that polymorphonuclear neutrophils may be the primary CXCR2 expressing target cell for stem and progenitor cell mobilization.

Stem cell mobilization

Successful blood and marrow transplant (BMT), both autologous and allogeneic, requires the infusion of a sufficient number of hematopoietic progenitor/stem cells (HPCs) capable of homing to the marrow cavity and regenerating a full array of hematopoietic cell lineages in a timely fashion. At present, the most commonly used surrogate marker for HPCs is the cell surface marker CD34, identified in the clinical laboratory by flow cytometry. Clinical studies have shown that infusion of at least 2 x 10(6) CD34(+) cells/kg recipient body weight results in reliable engraftment as measured by recovery of adequate neutrophil and platelet counts approximately 14 days after transplant. Recruitment of HPCs from the marrow into the blood is termed mobilization, or, more commonly, stem cell mobilization. In Section I, Dr. Tsvee Lapidot and colleagues review the wide range of factors influencing stem cell mobilization. Our current understanding focuses on chemokines, proteolytic enzymes, adhesion molecules, cytokines and stromal cell-stem cell interactions. On the basis of this understanding, new approaches to mobilization have been designed and are now starting to undergo clinical testing. In Section II, Dr. Michele Cottler-Fox describes factors predicting the ability to mobilize the older patient with myeloma. In addition, clinical approaches to improving collection by individualizing the timing of apheresis and adjusting the volume of blood processed to achieve a desired product are discussed. Key to this process is the daily enumeration of blood CD34(+) cells. Newer methods of enumerating and mobilizing autologous blood HPCs are discussed. In Section III, Dr. John DiPersio and colleagues provide data on clinical results of mobilizing allogeneic donors with G-CSF, GM-CSF and the combination of both as relates to the number and type of cells collected by apheresis. Newer methods of stem cell mobilization as well as the relationship of graft composition on immune reconstitution and GVHD are discussed.

Mice expressing a neutrophil elastase mutation derived from patients with severe congenital neutropenia have normal granulopoiesis

Severe congenital neutropenia (SCN) is a syndrome characterized by an isolated block in granulocytic differentiation and an increased risk of developing acute myeloid leukemia (AML). Recent studies have demonstrated that the majority of patients with SCN and cyclic neutropenia, a related disorder characterized by periodic oscillations in the number of circulating neutrophils, have heterozygous germline mutations in the ELA2 gene encoding neutrophil elastase (NE). To test the hypothesis that these mutations are causative for SCN, we generated transgenic mice carrying a targeted mutation of their Ela2 gene ("V72M") reproducing a mutation found in 2 unrelated patients with SCN, one of whom developed AML. Expression of mutant NE mRNA and enzymatically active protein was confirmed. Mice heterozygous and homozygous for the V72M allele have normal numbers of circulating neutrophils, and no accumulation of myeloid precursors in the bone marrow was observed. Serial blood analysis found no evidence of cycling in any of the major hematopoietic lineages. Rates of apoptosis following cytokine deprivation were similar in wild-type and mutant neutrophils, as were the frequency and cytokine responsiveness of myeloid progenitors. The stress granulopoiesis response, as measured by neutrophil recovery after cyclophosphamide-induced myelosuppression, was normal. To define the leukemogenic potential of V72M NE, a tumor watch was established. To date, no cases of leukemia have been detected. Collectively, these data suggest that expression of V72M NE is not sufficient to induce an SCN phenotype or leukemia in mice.

G-CSF is an essential regulator of neutrophil trafficking from the bone marrow to the blood

Neutrophils are released from the bone marrow in a regulated fashion to maintain homeostatic levels in the blood and to respond to physiological stresses, including infection. We show that under basal conditions granulocyte colony-stimulating factor (G-CSF) is an essential regulator of neutrophil release from the bone marrow. Nonredundant signals generated by the membrane-proximal 87 amino acids of the G-CSF receptor (G-CSFR) are sufficient to mediate this response. Surprisingly, G-CSFR expression on neutrophils is neither necessary nor sufficient for their mobilization from the bone marrow, suggesting that G-CSF induces neutrophil mobilization indirectly through the generation of trans-acting signals. Evidence is provided suggesting that downregulation of stromal cell-derived factor 1 expression in the bone marrow may represent such a signal.

G-CSF induces stem cell mobilization by decreasing bone marrow SDF-1 and up-regulating CXCR4

Granulocyte colony-stimulating factor (G-CSF) induced hematopoietic stem cell mobilization is widely used for clinical transplantation; however, the mechanism is poorly understood. We report here that G-CSF induced a reduction of the chemokine stromal cell derived factor 1 (SDF-1) and an increase in its receptor CXCR4 in the bone marrow (BM), whereas their protein expression in the blood was less affected. The gradual decrease of BM SDF-1, due mostly to its degradation by neutrophil elastase, correlated with stem cell mobilization. Elastase inhibition reduced both activities. Human and murine stem cell mobilization was inhibited by neutralizing CXCR4 or SDF-1 antibodies, demonstrating SDF-1 CXCR4 signaling in cell egress. We suggest that manipulation of SDF-1 CXCR4 interactions may be a means with which to control the navigation of progenitors between the BM and blood to improve the outcome of clinical stem cell transplantation.

Decreased stroma adhesion capacity of CD34+ progenitor cells from mobilized peripheral blood is not lineage- or stage-specific and is associated with low beta 1 and beta 2 integrin expression

Molecular mechanisms leading to mobilization of hematopoietic cells from bone marrow (BM) to peripheral blood (PB) involve modulation of adhesion molecule expression on these cells that probably result in changes in adhesion capacity to the microenvironment. However, it is not clear whether these changes involve different stages or lineages of progenitor cells. In this study, we compared the capacity of mature and immature clonogenic progenitor cells from granulocyte colony-stimulating factor (G-CSF)-mobilized PB and normal BM CD34+ cells to adhere to complete marrow stroma. This functional capacity was assessed concurrently with molecular expression on CD34+ cells of integrins VLA-4 (alpha 4/beta 1), VLA-5 (alpha 5/beta 1), and LFA-1 (alpha L/beta 2) by interindividual (between mobilized PB and normal BM) and intraindividual (between mobilized PB and steady-state BM and PB in the same patient) analysis. The proportion of adherent clonogenic progenitor cells was significantly lower in PB than in BM, not only for total progenitor cells but also for mature and immature progenitor cells, and the difference was found for granulocytic and particularly for erythroid lineages. The lower adhesion capacity of PB CD34+ cells to stroma was associated with decreased expression (signal/noise MFI ratio) of integrin alpha 4, beta 1, alpha L, and beta 2 chains whereas that of alpha 5 chain did not differ from BM cells with the lowest expression level. Similar differences in integrin expression levels were also found between mobilized PB and steady-state BM CD34+ cells in the same patient except for the alpha L chain. Moreover, we demonstrated for the first time a strong positive correlation between mobilizing capacity and expression levels on mobilized CD34+ cells for the LFA-1 alpha L chain but not for VLA-4 or VLA-5. In conclusion, the decreased adhesion capacity of mobilized PB progenitor cells to stroma involves different maturation stages and different lineages. This is associated with down-regulation of integrins VLA-4 and LFA-1, but mobilizing capacity appears positively correlated with LFA-1 levels.