Genetic analysis of progenitor cell mobilization by granulocyte colony-stimulating factor: verification and mechanisms for loci on murine chromosomes 2 and 11

Race and ethnicity influences collection of granulocyte colony-stimulating factor-mobilized peripheral blood progenitor cells from unrelated donors, a Center for International Blood and Marrow Transplant Research analysis

Little information exists on the effect of race and ethnicity on collection of peripheral blood stem cells (PBSC) for allogeneic transplantation. We studied 10,776 donors from the National Marrow Donor Program who underwent PBSC collection from 2006 to 2012. Self-reported donor race/ethnic information included Caucasian, Hispanic, Black/African American (AA), Asian/Pacific Islander (API), and Native American (NA). All donors were mobilized with subcutaneous filgrastim at an approximate dose of 10 μg/kg/day for 5 days. Overall, AA donors had the highest median yields of mononuclear cells per liter and CD34(+) cells per liter of blood processed (3.1 × 10(9) and 44 × 10(6), respectively), whereas Caucasians had the lowest median yields at 2.8 × 10(9) and 33.7 × 10(6), respectively. Multivariate analysis of CD34(+) per liter mobilization yields using Caucasians as the comparator and controlling for age, gender, body mass index, and year of apheresis revealed increased yields in overweight and obese AA and API donors. In Hispanic donors, only male obese donors had higher CD34(+) per liter mobilization yields compared with Caucasian donors. No differences in CD34(+) per liter yields were seen between Caucasian and NA donors. Characterization of these differences may allow optimization of mobilization regimens to allow enhancement of mobilization yields without compromising donor safety.

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.

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.

Pharmacological inhibition of EGFR signaling enhances G-CSF-induced hematopoietic stem cell mobilization

Mobilization of hematopoietic stem and progenitor cells (HSPCs) from bone marrow into peripheral blood by the cytokine granulocyte colony-stimulating factor (G-CSF) has become the preferred source of HSPCs for stem cell transplants. However, G-CSF fails to mobilize sufficient numbers of stem cells in up to 10% of donors, precluding autologous transplantation in those donors or substantially delaying transplant recovery time. Consequently, new regimens are needed to increase the number of stem cells in peripheral blood upon mobilization. Using a forward genetic approach in mice, we mapped the gene encoding the epidermal growth factor receptor (Egfr) to a genetic region modifying G-CSF-mediated HSPC mobilization. Amounts of EGFR in HSPCs inversely correlated with the cells' ability to be mobilized by G-CSF, implying a negative role for EGFR signaling in mobilization. In combination with G-CSF treatment, genetic reduction of EGFR activity in HSPCs (in waved-2 mutant mice) or treatment with the EGFR inhibitor erlotinib increased mobilization. Increased mobilization due to suppression of EGFR activity correlated with reduced activity of cell division control protein-42 (Cdc42), and genetic Cdc42 deficiency in vivo also enhanced G-CSF-induced mobilization. Our findings reveal a previously unknown signaling pathway regulating stem cell mobilization and provide a new pharmacological approach for improving HSPC mobilization and thereby transplantation outcomes.

Mobilization of hematopoietic stem cells in a thalassemic mouse model: implications for human gene therapy of thalassemia

Granulocyte colony-stimulating factor (G-CSF)-mobilized blood stem cells may become the preferable source of hematopoietic stem cells (HSCs) for gene therapy because of the higher yield of cells compared with conventional bone marrow harvesting. A G-CSF-associated risk of splenic rupture has been recognized in normal donors of HSCs, but limited information is available about the G-CSF effect in the presence of splenomegaly and extramedullary hematopoiesis. We investigated the G-CSF effect in a thalassemic mouse model (HBB(th-3)) as compared with a normal strain (C57BL/6), in terms of safety, mobilization efficacy, and distribution of stem cells among hematopoietic compartments. There was no death or clinical sequelae of splenic rupture in G-CSF-treated animals of either strain; however, hemorrhagic infarcts in the spleen were detected with low frequency in G-CSF-treated HBB(th-3) mice (12.5%). HBB(th-3) mice mobilized less effectively than C57BL/6 mice (Lin(-)Sca-1(+)c-Kit(+) cells/microl of peripheral blood mononuclear cells [PBMCs]: 90 +/- 55 vs. 255 +/- 174, respectively, p = 0.01; CFU-GM/ml PBMCs: 390 +/- 262 vs. 1131 +/- 875, p = 0.01) because of increased splenic trapping of hematopoietic stem and progenitor cells (Lin(-)Sca-1(+)c-Kit(+) cells per spleen (x10(5)): 487 +/- 35 vs. 109 +/- 19.6, p = 0.01; CFU-GM per spleen (x10(2)): 1470 +/- 347 vs. 530 +/- 425, p = 0.0006). Splenectomy restored the mobilization proficiency of thalassemic mice at comparable levels to normal mice and resulted in the development of a hematopoietic compensatory mechanism in the thalassemic liver that protected splenectomized mice from severe anemia. Our data imply that, in view of human gene therapy for thalassemia, either multiple cycles or alternative ways of mobilization may be required for a sufficient yield of transplantable HSCs. In addition, strategies to minimize the risk of G-CSF-induced splenic infarcts should be explored in a clinical setting.

A quantitative trait locus on chromosome 4 affects cycling of hematopoietic stem and progenitor cells through regulation of TGF-beta 2 responsiveness

The hematopoietic stem and progenitor cell (HSPC) compartment is subject to extensive quantitative genetic variation. We have previously shown that TGF-beta2 at low concentrations enhances flt3 ligand-induced growth of HSPCs, while it is potently antiproliferative at higher concentrations. This in vitro enhancing effect was subject to quantitative genetic variation, for which a quantitative trait locus (QTL) was tentatively mapped to chromosome 4 (chr.4). Tgfb2(+/-) mice have a smaller and more slowly cycling HSPC compartment, which has a decreased serial repopulation capacity, and are less susceptible to the lethal effect of high doses of 5-fluorouracil. To unequivocally demonstrate that these phenotypes can be attributed to the enhancing effect of TGF-beta2 on HSPC proliferation observed in vitro and are therefore subject to mouse strain-dependent variation as well, we generated congenic mice where the telomeric region of chr.4 was introgressed from DBA/2 into C57BL/6 mice. In these mice, the enhancing effect of TGF-beta2 on flt3 signaling, but not the generic antiproliferative effect of high concentrations of TGF-beta2, was abrogated, confirming the location of this QTL, which we named tb2r1, on chr.4. These mice shared a smaller and more slowly cycling HSPC compartment and increased 5-fluorouracil resistance but not a decreased serial repopulation capacity with Tgfb2(+/-) mice. The concordance of phenotypes between Tgfb2(+/-) and congenic mice indicates that HSPC frequency and cycling are regulated by tb2r1, while an additional QTL in the telomeric region of chr.4 may regulate the serial repopulation capacity of hematopoietic stem cells.

The use of experimental murine models to assess novel agents of hematopoietic stem and progenitor cell mobilization

The recent explosion in the understanding of the cellular and molecular mechanisms underlying hematopoietic stem and progenitor cell (HSPC) mobilization has facilitated development of novel therapeutic agents, targeted at improving mobilization kinetics as well as HSPC yield. With the development of new agents comes the challenge of choosing efficient and relevant preclinical studies for the testing of the HSPC mobilization efficacy of these agents. This article reviews the use of the mouse as a convenient small animal model of HSPC mobilization and transplantation, and outlines the range of murine assays that can be applied to assess novel HSPC mobilizing agents. Techniques to demonstrate murine HSPC mobilization are discussed, as well as the role of murine assays to confirm human HSPC mobilization, and techniques to investigate the biologic phenotype of HSPC mobilized by these novel agents. Technical aspects regarding mobilization regimens and control arms, and choice of experimental animals are also discussed.

Donor demographic and laboratory predictors of allogeneic peripheral blood stem cell mobilization in an ethnically diverse population

A reliable estimate of peripheral blood stem cell (PBSC) mobilization response to granulocyte colony-stimulating factor (G-CSF) may identify donors at risk for poor mobilization and help optimize transplantation approaches. We studied 639 allogeneic PBSC collections performed in 412 white, 75 black, 116 Hispanic, and 36 Asian/Pacific adult donors who were prescribed G-CSF dosed at either 10 or 16 microg/kg per day for 5 days followed by large-volume leukapheresis (LVL). Additional LVL (mean, 11 L) to collect lymphocytes for donor lymphocyte infusion (DLI) and other therapies was performed before G-CSF administration in 299 of these donors. Day 5 preapheresis blood CD34(+) cell counts after mobilization were significantly lower in whites compared with blacks, Hispanics, and Asian/Pacific donors (79 vs 104, 94, and 101 cells/microL, P < .001). In addition, donors who underwent lymphapheresis before mobilization had higher CD34(+) cell counts than donors who did not (94 vs 79 cells/microL, P < .001). In multivariate analysis, higher post-G-CSF CD34(+) cell counts were most strongly associated with the total amount of G-CSF received, followed by the pre-G-CSF platelet count, pre-G-CSF mononuclear count, and performance of prior LVL for DLI collection. Age, white ethnicity, and female gender were associated with significantly lower post-G-CSF CD34(+) cell counts.

Quantitative trait locus (QTL) mapping in aging systems

Understanding the genetic basis of the effects of aging on the decline in the immune response is an enormous undertaking. The most prominent age-related change in the immune system is thymic involution. This chapter will focus on the use of C57BL/6 J X DBA/2 J (BXD) recombinant inbred (RI) strains of mice to map genetic loci associated with age-related thymic involution in mice. Strategies to improve the power and precision in which complex traits such as the age-related decline in the immune response have been applied to the large set of BXD RI strains to detect quantitative trait loci (QTLs) that underlie thymic involution. More importantly, approaches have been developed to enable higher resolution mapping of these QTLs and, in some cases, may be adequate to carry out direct identification of candidate genes. It is likely that, given the complexity of the immune system development, the number of cells involved in an immune response, and especially the changes in the immune system with aging, multiple genetic loci and genes will contribute to the age-related changes in the immune response. This chapter outlines ongoing and general quantitative genetic linkage strategies that can be used for mapping and identification of the quantitative trait loci that may have a significant impact on age-related alteration of the immune system.

Peripheral blood stem cell yield in 400 normal donors mobilised with granulocyte colony-stimulating factor (G-CSF): impact of age, sex, donor weight and type of G-CSF used

Mobilised peripheral blood is now the main source of stem cells collected from normal donors. We report our experience of mobilising and collecting 400 normal healthy donors using standardised procedures and techniques. Target recipient doses were reached with one aphaeresis in 63% of donors and with two aphereses in 81% of donors. Approximately 2% of donors yielded such low progenitor values that they were termed 'poor mobilisers'. There were minor effects of donor age, weight and sex and where possible, larger male donors under the age of 55 years should be selected. Two forms of granulocyte colony-stimulating factor (G-CSF) were used at the same dose and no significant difference was seen in the yield of CD34+ cells collected/l of blood processed. However, a greater number of granulocyte-macrophage colony-forming cells were harvested using lenograstim (glycosylated G-CSF) compared with filgrastim (non-glycosylated G-CSF; P = 0.002). CD34+ cell yields were also measured halfway through the aphaeresis procedure. This was found to be highly predictive of final yield and facilitated distribution of the stem cell product to other centres. The observation that CD34+ yields did not decline in the second half compared with the first half of aphaeresis suggests that the circulating cell numbers are not static.

Increased hematopoietic stem cell mobilization in aged mice

Hematopoietic stem and progenitor cells (HSPCs) are located in the bone marrow in close association with a highly organized 3-dimensional structure formed by stroma cells, referred to as the niche. Mobilization of HSPCs from bone marrow to peripheral blood in response to granulocyte colony-stimulating factor (G-CSF) requires de-adhesion of HSPCs from the niche. The influence of aging of HSPCs on cell-stroma interactions has not been determined in detail. Using a mouse model of G-CSF-induced mobilization, we demonstrated that the ability to mobilize hematopoietic stem cells is approximately 5-fold greater in aged mice. Competitive mobilization experiments confirmed that enhanced mobilization ability was intrinsic to the stem cell. Enhanced mobilization efficiency of primitive hematopoietic cells from aged mice correlated with reduced adhesion of hematopoietic progenitor cells to stroma and with elevated levels of GTP-bound Cdc42. These results might indicate that stroma-stem cell interactions are dynamic over a lifetime and result in physiologically relevant changes in the biology of primitive hematopoietic cells with age.

Identification of quantitative trait loci regulating haematopoietic parameters in B6AKRF2 mice

The haematopoietic system is a complex organised tissue with a hierarchical structure. Identification of organisational pathways within the haematopoietic system is relevant for a better understanding of haematopoiesis in health and disease. We have analysed numerous haematopoietic parameters in two panels of a total of 157 genetically distinct B6AKRF2 mice, derived from an intercross between AKR and C57Bl/6 mice, strains known to differ in various stem cell traits. The major objective of our study was to assess the extent to which various haematopoietic parameters, such as stem cell numbers, progenitor cell cycling, progenitor cell mobilisation and neutrophil numbers in blood and bone marrow are coregulated. The genotypes of these mice were used to search for genetic loci that regulate these parameters. We found significant quantitative trait loci (QTL) associated with the number of stem cells (CAFC-35) in the bone marrow and the number of neutrophils in the blood. However, most haematopoietic parameters appeared to be controlled by non-heritable (epigenetic) factors, or by multiple QTLs. Our study reveals striking differences in structure of the haematopoietic hierarchy between individual mice. Surprisingly, stem and progenitor cell pool size and proliferation rate, as well as peripheral blood cell counts are all independently regulated.

Regulation of hematopoietic stem cell aging in vivo by a distinct genetic element

Until recently, stem cells were thought to be endowed with unlimited self-renewal capacity and, thus, assumed exempt from aging. But accumulating evidence over the past decade compellingly argues that a measurable and progressive replicative impairment in the hematopoietic, intestinal, and muscle stem cell activity exists from adulthood to old age, resulting in a decline in stem cell function and rendering stem cell aging as the possible link between cellular aging and organismal aging. By using a previously uncharacterized congenic animal model to study genetic regulation of hematopoietic stem cell aging, we have demonstrated definitively that a locus on murine chromosome 2 regulates hematopoietic stem cell aging. In addition to demonstrating that hematopoietic stem cell aging is regulated by a distinct genetic element, experimental evidence links the response of hematopoietic stem cells to DNA double-strand breaks to cellular aging, suggesting DNA integrity influences stem cell aging.

Senescence and functional failure in hematopoietic stem cells

Maintaining normal function of hematopoietic stem cells (HSCs) is critical to blood coagulation, oxygen transportation, and host defense against infection. A potential cause of HSC dysfunction is senescence, in which HSCs and progenitor cells suffer from proliferative arrest. Studies on humans and various animal models have indicated that HSCs can become senescent, showing a significant decline in functional ability with increasing age. There are genetic elements mapped to specific chromosomal sites that regulate HSC senescence. These elements may differ among species, strains, and even individuals. HSC senescence and related pathological effects may not be obvious during normal lifetime under most circumstances since individual primitive HSC clones can function long-term to produce progeny that sustain life-long mature blood cell production. Shortening of telomeres at the chromosomal ends could contribute to HSC senescence, especially when HSCs are stressed under certain pathological conditions. Future studies should define the molecular elements that are important in the regulation of HSC senescence.

Innovative strategies for PBPC mobilization

Stem cell transplantation, whether autologous or allogeneic, improves the outcome of patients with a number of hematologic malignancies or solid tumors. A relevant proportion of these patients are excluded from this treatment because sufficient numbers of hematopoietic stem cells cannot be obtained by standard cytokine-assisted mobilization. In this article we review the physiology of peripheral blood progenitor cell (PBPC) mobilization and discuss the role of adhesion molecules, such as integrins and selectins, chemokines and their ligands, such as SDF-1alpha and CXCR4, and proteolytic enzymes. Based on this knowledge, several innovative pharmacologic approaches have been proposed to boost the stem cell harvest. Some of them (CTCE, C3a receptor agonist and GrobetaT) are still subject of pre-clinical development, others, such as chemokine receptor ligand AMD3100, have recently been introduced in clinical trials and already deliver promising results. It appears possible to harvest PBPC successfully in poor mobilizers and to cut down the number of collections required in the remaining PBPC donors.

Stem cells, aging, and cancer: inevitabilities and outcomes

Given the unique abilities of a stem cell to self-renew, differentiate, and proliferate, it is no wonder that they are critically important to an organism during development and to maintain homeostasis. Likewise, when something goes awry within a stem cell, it is likely to have far-reaching effects, since stem cells persist throughout the lifetime of the individual. Two significant biological phenomena that involve stem cells are the inevitable process of aging and a major health issue whose incidence increases with aging: cancer. In this review, we summarize evidence and theories concerning these two stem cell processes. The inability of stem cells to be passaged indefinitely in mice and the data supporting regular replication of the quiescent stem cell pool are discussed. Further, the current evidence indicating a stem cell origin of acute myeloid leukemia, including examples from both experimental mouse models and human clinical samples, is evaluated. Finally, we propose a model in which aging of the stem cell population of the hematopoietic system in particular can create conditions that are permissive to leukemia development; in fact, we suggest that aging is a secondary event in leukemogenesis.

The biology of hematopoietic stem cells

Rarely has so much interest from the lay public, government, biotechnology industry, and special interest groups been focused on the biology and clinical applications of a single type of human cell as is today on stem cells, the founder cells that sustain many, if not all, tissues and organs in the body. Granting organizations have increasingly targeted stem cells as high priority for funding, and it appears clear that the evolving field of tissue engineering and regenerative medicine will require as its underpinning a thorough understanding of the molecular regulation of stem cell proliferation, differentiation, self-renewal, and aging. Despite evidence suggesting that embryonic stem (ES) cells might represent a more potent regenerative reservoir than stem cells collected from adult tissues, ethical considerations have redirected attention upon primitive cells residing in the bone marrow, blood, brain, liver, muscle, and skin, from where they can be harvested with relative sociological impunity. Among these, it is arguably the stem and progenitor cells of the mammalian hematopoietic system that we know most about today, and their intense study in rodents and humans over the past 50 years has culminated in the identification of phenotypic and molecular genetic markers of lineage commitment and the development of functional assays that facilitate their quantitation and prospective isolation. This review focuses exclusively on the biology of hematopoietic stem cells (HSCs) and their immediate progeny. Nevertheless, many of the concepts established from their study can be considered fundamental tenets of an evolving stem cell paradigm applicable to many regenerating cellular systems.