Combination of stem cell factor and granulocyte colony-stimulating factor mobilizes the highest number of primitive haemopoietic progenitors as shown by pre-colony-forming unit (pre-CFU) assay

Harvesting, processing and inventory management of peripheral blood stem cells

By 2003, 97% autologous transplants and 65% of allogeneic transplants in Europe used mobilised peripheral blood stem cells (PBSC). Soon after their introduction in the early 1990's, PBSC were associated with faster haemopoietic recovery, fewer transfusions and antibiotic usage, and a shorter hospital stay. Furthermore, ease and convenience of PBSC collection made them more appealing than BM harvests. Improved survival has hitherto been demonstrated in patients with high risk AML and CML. However, the advantages of PBSC come at a price of a higher incidence of extensive chronic GVHD. In order to be present in the blood, stem cells undergo the process of “mobilisation” from their bone marrow habitat. Mobilisation, and its reciprocal process – homing – are regulated by a complex network of molecules on the surface of stem cells and stromal cells, and enzymes and cytokines released from granulocytes and osteoclasts. Knowledge of these mechanisms is beginning to be exploited for clinical purposes. In current practice, stem cell are mobilised by use of chemotherapy in conjunction with haemopoietic growth factors (HGF), or with HGF alone. Granulocyte colony stimulating factor has emerged as the single most important mobilising agent, due to its efficacy and a relative paucity of serious side effects. Over a decade of use in healthy donors has resulted in vast experience of optimal dosing and administration, and safety matters. PBSC harvesting can be performed on a variety of cell separators. Apheresis procedures are nowadays routine, but it is important to be well versed in the possible complications in order to avoid harm to the patient or donor. To ensure efficient collection, harvesting must begin when sufficient stem cells have been mobilised. A rapid, reliable, standardized blood test is essential to decide when to begin harvesting; currently, blood CD34+ cell counting by flow cytometry fulfils these criteria. Blood CD34+ cell counts strongly correlate with the apheresis yields. These are, in turn, predictive of the speed of haemopoietic recovery after transplantation, which has helped establish the adequate cell dose for transplantation. Following collection, PBSC may be transfused unmanipulated, processed to select specific cell subtypes, or stored for future use. Cryopreservation techniques allow long term storage of stem cells without significant loss of viability. Increasingly demanding calls for safety led to introduction of vapour phase storage, separate storage of infected material, and mandatory quality control measures at all stages of the cryopreservation process and subsequent thawing and transfusion. At the same time, safety of the personnel working in stem cell processing and storage laboratories is safeguarded by a set of regulations devised to minimize the risk of infection, injury or hypoxia. Requirements for quality and safety have been shaped into a number of documents and directives in Europe and USA, emphasising the importance of product traceability, reporting of adverse reactions, quality management systems (standard operating procedures, guidelines, training records, reporting mechanisms and records), requirements for cell reception, quarantine, process control, validation and storage. Establishments that collect, process and store stem cells must be accredited or licensed by appropriate national or international authorities on a regular basis. These regulatory measures have recently become law across the European Union.

Current status of stem cell mobilization

New advances in effective mobilization of peripheral blood stem cells have permitted a greater proportion of patients to benefit from autologous stem cell transplantation. In this review, the relative merits of peripheral blood and mobilized bone marrow are discussed. All available agents are reviewed. A critical assessment of the appropriate dosing and frequency of available growth factors is undertaken, and the most commonly used chemotherapy plus growth factor combinations are covered. Specific recommendations for patients who are poor mobilizers are dealt with including the role of plerixafor.

The effect of peptide stimulation on haematopoietic stem cell mobilisation including engraftment characteristics and a note on donor side effects

Aplasia or irreversible bone marrow failure and a variety of haematologic malignancies, as well as an increasing number of solid tumours, currently include various forms of marrow or equivalent transplantation in routine management. In both allogeneic and autologous procedures stable recipient immunohaematopoietic reconstitution depends upon infusing the requisite population harvested at a precise time following commencement of a stimulatory peptide. In a first step this prospective study documented the safety of apheresis, defined side effects and enumerated mononuclear, CD34+ and CD3+ cells obtained. In the second stage delivery of the graft, characterised in this way and with the additional measurement of in vitro growth in clonogenic assay, to the suitably conditioned patient was correlated with recovery of neutrophil and platelet numbers appearing in the circulation. In a third and ongoing analysis the influence of passenger T-lymphocytes is being evaluated for impact on infection and a potential anti-tumour effect. The conclusion is that this technology is reliable, has a high degree of patient acceptability without untoward complications, and that local results correspond to international experience thereby providing an important and relevant measure of quality control.

High-Level in Vivo Gene Marking after Gene-Modified Autologous Hematopoietic Stem Cell Transplantation without Marrow Conditioning in Nonhuman Primates

The successful engraftment of genetically modified hematopoietic stem cells (HSCs) without toxic conditioning is a desired goal for HSC gene therapy. To this end, we have examined the combination of intrabone marrow transplantation (iBMT) and in vivo expansion by a selective amplifier gene (SAG) in a nonhuman primate model. The SAG is a chimeric gene consisting of the erythropoietin (EPO) receptor gene (as a molecular switch) and c-Mpl gene (as a signal generator). Cynomolgus CD34+ cells were retrovirally transduced with or without SAG and returned into the femur and humerus following irrigation with saline without prior conditioning. After iBMT without SAG, 2-30% of colony-forming cells were gene marked over 1 year. The marking levels in the peripheral blood, however, remained low (<0.1%). These results indicate that transplanted cells can engraft without conditioning after iBMT, but in vivo expansion is limited. On the other hand, after iBMT with SAG, the peripheral marking levels increased more than 20-fold (up to 8-9%) in response to EPO even at 1 year posttransplant. The increase was EPO-dependent, multilineage, polyclonal, and repeatable. Our results suggest that the combination of iBMT and SAG allows efficient in vivo gene transduction without marrow conditioning.

New approaches to hematopoietic cell transplantation in oncology

The use of high-dose chemotherapy followed by autologous HCT and the use of allogeneic HCT in children and adolescents with high-risk ALL, AML, and NBL has successfully improved outcomes. For other diseases, however, the role of HCT in treatment remains a subject of further research. The availability of HCT was significantly expanded by developing alternative graft sources that currently include BM, peripheral blood, and UCB from autologous and allogeneic related or unrelated donors. Progress in autologous HCT has been achieved by the identification of more effective and less toxic preparative regimens and by ex vivo purging of stem cell products. In allogeneic HCT, graft-versus-leukemia or graft-versus-tumor effects are being exploited increasingly to lower relapse rates. In addition, immunomodulation to promote tolerance, as well as allogeneic antitumor reactions have been achieved by antibody therapy, cytokine therapy, or cell-based immunotherapy. Future improvements are likely, as evidenced by promising preliminary results in the development of stem cell collection techniques, in vitro stem cell expansion, and purging techniques of stem cell grafts. The development of less intensive or nonmyeloablative preparative regimens may further reduce regimen-related morbidity and mortality Specific immunotherapy may facilitate tolerance induction in mismatched allogeneic HCT and support allogeneic HCT in the setting of donor-host HLA disparity. Ultimately, advances in cytokine therapy, tumor-specific vaccines, and gene therapy may decrease or even eradicate recurrence of the malignant disease after HCT.

Murine hematopoietic stem cell distribution and proliferation in ablated and nonablated bone marrow transplantation

The engraftment of donor bone marrow (BM) cells in nonablated mice is inefficient. Niche availability has been thought to be the reason, and cytoablation with irradiation or cytotoxic agents is routinely used with the belief that this frees the preoccupied niches in recipients. In this study, donor cell redistribution and proliferation in ablated and nonablated mice were compared by implanting donor cells directly into the femur cavity of sedated mice. The redistribution of Lin(-) donor cells into BM was similar between ablated and nonablated mice. Poor engraftment in nonablated mice was shown to be the result of inefficient donor cell proliferation rather than because of a lack of space. Competitive repopulation assays demonstrated that the donor hematopoietic stem cells (HSCs) were present in nonirradiated recipients for at least 6 months after transplantation, but that they did not expand as did their counterparts in lethally irradiated mice. This study suggests that efficient bone marrow transplantation in nonablated recipients may be possible as a result of better understanding of HSC proliferative regulation and appropriate in vitro manipulation.

Introduction of the green fluorescent protein gene into hematopoietic stem cells results in prolonged discrepancy of in vivo transduction levels between bone marrow progenitors and peripheral blood cells in nonhuman primates

BACKGROUND

The green fluorescent protein (GFP) has proven a useful marker in retroviral gene transfer studies targeting hematopoietic stem cells (HSCs) in mice. However, several investigators have reported very low in vivo peripheral blood marking levels in nonhuman primates after transplantation of HSCs transduced with the GFP gene. We retrovirally marked cynomolgus monkey HSCs with the GFP gene, and tracked in vivo marking levels within both bone marrow progenitor cells and mature peripheral blood cells following autologous transplantation after myeloablative conditioning.

METHODS

Bone marrow cells were harvested from three cynomolgus macaques and enriched for the primitive fraction by CD34 selection. CD34(+) cells were transduced with one of three retroviral vectors all expressing the GFP gene and were infused after myeloablative total body irradiation (500 cGy x 2). Following transplantation, proviral levels and fluorescence were monitored among clonogenic bone marrow progenitors and mature peripheral blood cells.

RESULTS

Although 13-37% of transduced cells contained the GFP provirus and 11-13% fluoresced ex vivo, both provirus and fluorescence became almost undetectable in the peripheral blood within several months after transplantation regardless of the vectors used. However, on sampling of bone marrow at multiple time points, significant fractions (5-10%) of clonogenic progenitors contained the provirus and fluoresced ex vivo reflecting a significant discrepancy between GFP gene marking levels within bone marrow cells and their mature peripheral blood progeny. The discrepancy (at least one log) persisted for more than 1 year after transplantation. Since no cytotoxic T lymphocytes against GFP were detected in the animals, an immune response against GFP is an unlikely explanation for the low levels of transduced peripheral blood cells. Administration of granulocyte colony stimulating factor and stem cell factor resulted in mobilization of transduced bone marrow cells detectable as mature granulocyte progeny which expressed the GFP gene, suggesting that transduced progenitor cells in bone marrow could be mobilized into the peripheral blood and differentiated into granulocytes.

CONCLUSIONS

Low levels of GFP-transduced mature cells in the peripheral blood of nonhuman primates may reflect a block to differentiation associated with GFP; this block can be overcome in part by nonphysiological cytokine treatment ex vivo and in vivo.

Stem cells on the way to restorative medicine

Stem cells are defined by their unique properties of self-renewal and multilineage differentiation. Several decades ago, cells with such developmental plasticity have been identified in the embryo and in the bone marrow of the adult; in other organs, such cells could not be demonstrated. Here, recent findings are briefly summarized indicating that the elementary stem cell capabilities are retained by a limited number of cells present in many organs of the adult. Other data suggest that, on response to another microenvironment, "organ-specific" stem cells are able to acquire different fates. If confirmed these findings will have considerable impact on the future of clinical stem cell therapy.

The synergy between stem cell factor (SCF) and granulocyte colony-stimulating factor (G-CSF): molecular basis and clinical relevance

Stem cell factor (SCF), an essential growth factor in normal hematopoiesis, exerts potent effects when combined with cytokines. In particular, its synergy with granulocyte colony-stimulating factor (G-CSF) results in important biologic responses. These include enhancement of ex vivo long-term expansion of human primitive hematopoietic cells and increased mobilization of peripheral blood progenitor cells (PBPC) for transplantation. Despite the clinical importance of the interaction between SCF and G-CSF, the absence of a model system in which it could be studied at the cellular level had impaired the ability to understand the basis of their co-operation. To overcome this impediment, a system was recently generated which recapitulates the biologic synergy between SCF and G-CSF. MO7e-G cells have allowed the identification of key events in the synergistic actions of these cytokines on proliferation and gene expression. Among the biochemical and molecular events mediated by these cytokines are the down-regulation of p27kip1 and the independent phosphorylation of STAT3 on tyrosine and serine residues. Recent work has provided increasing evidence for the clinical importance of the combination of SCF and G-CSF. The elucidation of the intracellular events triggered by their receptors is now shedding light on key mediators of their synergistic effects. The identification of these pathways is of considerable importance for understanding fundamental aspects of hematopoiesis, and as potential targets for therapeutic intervention.

Cytokine control of developmental programs in normal hematopoiesis and leukemia

The establishment of a system for in vitro clonal development of hematopoietic cells made it possible to discover the cytokines that regulate hematopoiesis. These cytokines include colony stimulating factors and others, which interact in a network, and there is a cytokine cascade which couples growth and differentiation. A network allows considerable flexibility and a ready amplification of response to a particular stimulus. A network may also be necessary to stabilize the whole system. Cells called hematopoietic stem cells (HSC) can repopulate all hematopoietic lineages in lethally irradiated hosts, and under appropriate conditions give rise to neuronal, muscle, and epithelial cells. Granulocyte colony stimulating factor induces migration of both HSC and in vitro colony forming cells from the bone marrow to peripheral blood. Granulocyte colony stimulating factor is also used clinically to repair irradiation and chemotherapy associated suppression of normal hematopoiesis in cancer patients, and to stimulate normal granulocyte development in patients with infantile congenital agranulocytosis. It is suggested that there may also be appropriate conditions under which in vitro colony forming cells have a wider differentiation potential similar to that shown by HSC. An essential part of the developmental program is cytokine suppression of apoptosis by changing the balance in expression of apoptosis inducing and suppressing genes. Decreasing the level of cytokines that suppress therapeutic induction of apoptosis in malignant cells can improve cancer therapy. Cytokines and some other compounds can reprogram abnormal developmental programs in leukemia, so that the leukemic cells differentiate to mature non dividing cells, and this can also be used for therapy. There is considerable plasticity in the developmental programs of normal and malignant cells.

Stem cell factor: laboratory and clinical aspects

Stem cell factor is an essential haemopoietic progenitor cell growth factor with proliferative and anti-apoptotic functions. Molecular biologists have now dissected some of the various pathways through which this cytokine signals to the nucleus. At the same time, new molecules have become available which can inhibit SCF signalling. This provides an exciting prospect for the treatment of Kit+ malignancies such as acute myeloblastic leukaemia. The capacity of SCF to synergize with other cytokines has been exploited in the ex vivo expansion of haemopoietic progenitors and dendritic cells, which may also hold therapeutic promise. In this review the last 5 years' literature on these issues is reviewed and collated.

Assessment of rapid remobilization intervals with G-CSF and SCF in murine and rhesus macaque models

BACKGROUND

Defining the optimum regimen and time for repeat peripheral blood progenitor cell mobilization would have important clinical applications.

STUDY DESIGN AND METHODS

Remobilization with SCF and G-CSF at 2 weeks after an initial mobilization in mice and at 2 or 4 weeks after an initial mobilization in nonhuman primates was examined. In mice, competitive repopulation assays were used to measure long-term progenitor cell-repopulating activity. In monkeys, mobilization of hematopoietic progenitor CFUs was used as a surrogate marker for progenitor cell-repopulating ability.

RESULTS

Efficacy of progenitor cell remobilization differed in the two animal species. In mice, peripheral blood progenitor cell-repopulating ability with repeat mobilization at 2 weeks was 70 percent of that with the initial mobilization. In monkeys, there was no significant difference in peripheral blood progenitor cell mobilization between the initial and the repeat mobilizations at 2 weeks. In mobilizations separated by 4 weeks, however, peripheral blood progenitor cell mobilization was higher than that with initial mobilizations.

CONCLUSION

In animal models, mobilization of peripheral blood progenitor cells with remobilization after a 2-week interval is similar to or moderately decreased from that with the initial mobilization. Progenitor cell collection at this time point may be useful in certain clinical circumstances. A 4-week interval between remobilizations may be preferable. Clinical trials in humans would be useful to clarify these issues.

Il-17 mobilizes peripheral blood stem cells with short- and long-term repopulating ability in mice

Autologous and allogeneic bone marrow transplantations have evolved as important cancer therapy modalities. For both indications, peripheral blood has been shown to have distinct advantages over bone marrow as the stem cell source. Cytokine combinations for mobilization have enhanced stem cell yield and accelerated engraftment. However, novel mobilizing agents and strategies are needed to further improve clinical outcomes. Within the donor graft, the dynamic equilibrium between T cells and stem cells critically influences engraftment and transplantation results. IL-17 is a cytokine produced almost exclusively from activated T cells. IL-17 was expressed in vivo with adenovirus technology. Here, proof-of-principle studies demonstrate that IL-17 effectively mobilizes hemopoietic precursor cells (CFU-granulocyte-erythrocyte-macrophage-monocyte, CFU-high proliferative potential) and primitive hemopoietic stem cells (Lin(-/low)c-kit(+)Sca1(+)). Moreover, mouse IL-17 adenovirus-mobilized peripheral blood stem cells rescued lethally irradiated mice. Bone marrow was found to be 45-75% of donor origin at 1 year. In secondary recipients, donor-derived bone marrow cells ranged from 45 to 95%. These data show that IL-17 mobilizes stem cells in mice with short- and long-term reconstituting capacity. Additional comparative studies are needed as well as studies in tumor models to refine distinct potential clinical applications for IL-17-mobilized peripheral blood stem cells.