The role of cytokines and adhesion molecules for mobilization of peripheral blood stem cells

Homing and mobilization of hematopoietic stem cells and hematopoietic cancer cells are mirror image processes, utilizing similar signaling pathways and occurring concurrently: circulating cancer cells constitute an ideal target for concurrent treatment with chemotherapy and antilineage-specific antibodies

Adhesion molecules and stromal cell-derived factor-1 (SDF-1)/CXCR4 signaling play key role in homing and mobilization of hematopoietic progenitor (HPC) and hematopoietic cancer clonogenic cells (HCC). High expression of VLA-4 is required for homing of HPC and HCC, whereas downregulation of these molecules is required for successful mobilization of HPC and HCC. Upregulation and activation of the SDF-1/CXCR4 signaling is required for homing of HPC and HCC, whereas disruption of the SDF-1 signaling is required for mobilization of HPC and HCC. Hence, mobilizations of HPC and HCC occur concurrently. It is proposed that drug resistance evolves as a result of repeated cycles of chemotherapy. Following each cycle of chemotherapy, HCC lose adhesion molecules and SDF-1 signaling. Surviving cells, released from tumor sites, circulate until re-expression of adhesion molecules and CXCR4 occurs, then homing to stroma of distal tissues occurs. Cytokines secreted by cells in the new microenvironment induce proliferation and drug resistance of HCC. This process is amplified in each cycle of chemotherapy resulting in disease progression. A novel model for treatment is proposed in which circulating HCC are the target for clinical intervention, and concurrent treatment with chemotherapy and antilineage-specific antibodies will result in abrogation of the 'vicious cycle' of conventional anticancer therapy.

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

OBJECTIVE

It is notable that there is significant inter-individual variability in humans and inter-strain variability in mice in the ability to mobilize hematopoietic stem and progenitor cells, suggesting that there is genetic regulation of mobilization. In the murine system, loci on chromosomes 2 and 11 have been linked to an inter-strain variation in granulocyte colonystimulating factor (G-CSF)-induced stem cell mobilization proficiency. The aim of this study was to verify this linkage and to gain insight into the function of these loci.

METHODS

Animals congenic for the loci on chromosomes 2 and 11 were generated by a speed-congenic approach and the function of the loci were further analyzed in doubly congenic animals and with a competitive transplantation/mobilization protocol.

RESULTS

The analysis of congenic animals verified that both loci are linked to mobilization proficiency. Analysis of mobilization in doubly congenic animals suggested that both loci act in the same regulatory pathway. Mobilization experiments conducted with mice that had previously been competitively repopulated with congenic and parental-strain BM revealed that the locus on chromosome 11 operates via a progenitor cell-intrinsic mechanism.

CONCLUSION

We confirmed linkage of loci on chromosomes 2 and 11 to G-CSF-induced mobilization and thus validated their role as regulators of hematopoietic progenitor cell mobilization in mice. These findings will be useful for further studies directed at identifying genes that regulate mobilization proficiency.

Molecular biology of hematopoietic stem cells

Human CD34+ hematopoietic stem and progenitor cells are capable of maintaining a life-long supply of the entire spectrum of blood cells dependent on systemic needs. Recent studies suggest that hematopoietic stem cells are, beyond their hematopoietic potential, able to differentiate into nonhematopoietic cell types, which could open novel avenues in the field of cellular therapy. Here, we concentrate on the molecular biology underlying basic features of hematopoietic stem cells. Immunofluorescence analyses, culture assays, and transplantation models permit an extensive immunological as well as functional characterization of human hematopoietic stem and progenitor cells. New methods such as cDNA array technology have demonstrated that distinct gene expression patterns of transcription factors and cell cycle genes molecularly control self-renewal, differentiation, and proliferation. Furthermore, several adhesion molecules have been shown to play an important role in the regulation of hematopoiesis and stem cell trafficking. Progress has also been made in elucidating molecular mechanisms of stem cell aging that limit replicative potential. Finally, more recent data provide the first molecular basis for a better understanding of transdifferentiation and developmental plasticity of hematopoietic stem cells. These findings could be helpful for non-hematopoietic cell therapeutic approaches.

Differential gene expression underlying the functional distinctions of primary human CD34+ hematopoietic stem and progenitor cells from peripheral blood and bone marrow

The restorative capacity of human CD34(+) hematopoietic cells is clinically used in the autologous and allogeneic transplant setting to support cytotoxic therapy. We examined gene expression patterns of highly enriched bone marrow CD34(+) (BM-CD34(+)) or G-CSF-mobilized peripheral blood CD34(+) (PB-CD34(+)) cells by cDNA array technology, quantitative real-time RT-PCR, and flow cytometry, to identify molecular causes underlying the functional differences between circulating and sedentary hematopoietic stem and progenitor cells. The greater cell cycle and DNA synthesis activity of BM-CD34(+) compared to PB-CD34(+) cells was reflected by the 2- to 5-fold higher expression of 9 genes involved in cell cycle, 11 genes regulating DNA synthesis, and the cell cycle-initiating transcription factor E2F-1. The 2- to 3-fold greater expression of 5 pro-apoptotic genes in PB-CD34(+) cells indicated a higher apoptotic activity, which could functionally be corroborated by apoptosis assays. Thrombin receptor (PAR1), known to play a role in trafficking of malignant cells, was 3.6-fold higher expressed in circulating CD34(+) cells than in BM-CD34(+) cells. Guidance via thrombin receptor might molecularly mediate stem cell migration. In summary, our study provides gene expression profiles of primary human CD34(+) hematopoietic cells of blood and marrow. Our data molecularly confirm and explain the finding that CD34(+) cells residing in the bone marrow are cycling more rapidly, whereas circulating CD34(+) cells consist of a higher number of quiescent stem and progenitor cells. Moreover, our data give novel molecular insights into stem cell migration and differentiation.

Spontaneous circulation of myeloid-lymphoid-initiating cells and SCID-repopulating cells in sickle cell crisis

The only curative therapy for sickle cell disease (SCD) is allogeneic hematopoietic stem cell (HSC) transplantation. Gene therapy approaches for autologous HSC transplantation are being developed. Although earlier engraftment is seen when cells from GCSF-mobilized blood are transplanted than when bone marrow is transplanted, administration of GCSF to patients with SCD can cause significant morbidity. We tested whether primitive hematopoietic progenitors are spontaneously mobilized in the blood of patients with SCD during acute crisis (AC-SCD patients). The frequency of myeloid-lymphoid-initiating cells (ML-ICs) and SCID-repopulating cells (SRCs) was significantly higher in blood from AC-SCD patients than in blood from patients with steady-state SCD or from normal donors. The presence of SRCs in peripheral blood was not associated with detection of long-term culture-initiating cells, consistent with the notion that SRCs are more primitive than long-term culture-initiating cells. As ML-ICs and SRCs were both detected in blood of AC-SCD patients only, these assays may both measure primitive progenitors. The frequency of ML-ICs also correlated with increases in stem cell factor, GCSF, and IL-8 levels in AC-SCD compared with steady-state SCD and normal-donor sera. Because significant numbers of ML-ICs and SRCs are mobilized in the blood without exogenous cytokine treatment during acute crisis of SCD, collection of peripheral blood progenitors during crisis may yield a source of autologous HSCs suitable for ex-vivo correction by gene therapy approaches and subsequent transplantation.

Current understanding of stem cell mobilization: the roles of chemokines, proteolytic enzymes, adhesion molecules, cytokines, and stromal cells

Mobilization of hematopoietic stem and progenitor cells from the bone marrow into the circulation by repetitive, daily stimulations with G-CSF alone, or in combination with cyclophosphamide, is increasingly used clinically; however, the mechanism is not fully understood. Moreover, following mobilization stem cells also home back to the bone marrow, suggesting that stem cell release/mobilization and homing are sequential events with physiological roles. Previously, a role for cytokines such as G-CSF and SCF, and adhesion molecules such as VLA-4 and P/E selectins, was determined for stem cell mobilization. Recent results using experimental animal models and samples from clinical mobilization protocols demonstrate major involvement of chemokines such as stromal derived factor-1 (SDF-1) and IL-8, as well as proteolytic enzymes such as elastase, cathepsin G, and various MMPs in the mobilization process. These results will be reviewed together with the central roles of SDF-1 and CXCR4 interactions in G-CSF or G-CSF in combination with cyclophosphamide-induced mobilization. Furthermore, the central role of this chemokine in stem cell homing to the bone marrow as well as retention of undifferentiated cells within this tissue will also be discussed.

In vivo induction of human immunodeficiency virus type 1 entry into nucleus-free cells by CD4 gene transfer to hematopoietic stem cells: a hypothetical possible strategy for therapeutic intervention

As a useful alternative to employing soluble CD4 to inhibit binding of human immunodeficiency virus type 1 (HIV-1) to target cells, the introduction of CD4-bearing erythrocyte has been proposed by two study groups (see Refs. (5,6)). Prominently, Nicolau and colleagues demonstrated that the electroinserted CD4 molecules in the membranes of erythrocytes are capable of mediating HIV-1 entry. The implications of the studies are that inactivation of the integration-dependent retrovirus by the facilitation of entry into the nucleus-free cells, referred to as 'fake host trap' or 'host cell decoy', may be a possible therapeutic approach. Here we expand this concept to include genetic modification of autologous hematopoietic stem cells and review the relevant theoretical basis. Effective application of molecular technologies to induce partial replacement of hematopoiesis may be critical for this strategy.

Comparison between granulocyte colony-stimulating factor and granulocyte-macrophage colony-stimulating factor in the mobilization of peripheral blood stem cells

Peripheral blood stem cells (PBSC) have become the preferred source of stem cells for autologous transplantation because of the technical advantage and the shorter time to engraftment. Mobilization of CD34+ into the peripheral blood can be achieved by the administration of granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), or both, either alone or in combination with chemotherapy. G-CSF and GM-CSF differ somewhat in the number and composition of PBSCs and effector cells mobilized to the peripheral blood. The purpose of this review is to give a recent update on the type and immunologic properties of CD34+ cells and CD34+ cell subsets mobilized by G-CSF or GM-CSF with emphasis on (1) relative efficacy of CD34+ cell mobilization; (2) relative toxicities of G-CSF and GM-CSF as mobilizing agents; (3) mobilization of dendritic cells and their subsets; (4) delineation of the role of adhesion molecules, CXC receptor 4, and stromal cell-derived factor-1 signaling pathway in the release of CD34+ cell to the peripheral blood after treatment with G-CSF or GM-CSF.

Gene expression profiling identifies significant differences between the molecular phenotypes of bone marrow-derived and circulating human CD34+ hematopoietic stem cells

CD34+ hematopoietic stem cells are used clinically to support cytotoxic therapy, and recent studies raised hope that they could even serve as a cellular source for nonhematopoietic tissue engineering. Here, we examined in 18 volunteers the gene expressions of 1185 genes in highly enriched bone marrow CD34+ (BM-CD34+) or granulocyte-colony-stimulating factor-mobilized peripheral blood CD34+ (PB-CD34+) cells by means of cDNA array technology to identify molecular causes underlying the functional differences between circulating and sedentary hematopoietic stem and progenitor cells. In total, 65 genes were significantly differentially expressed. Greater cell cycle and DNA synthesis activity of BM-CD34+ than PB-CD34+ cells were reflected by the 2- to 5-fold higher expression of 9 genes involved in cell cycle progression, 11 genes regulating DNA synthesis, and cell cycle-initiating transcription factor E2F-1. Conversely, 9 other transcription factors, including the differentiation blocking GATA2 and N-myc, were expressed 2 to 3 times higher in PB-CD34+ cells than in BM-CD34+ cells. Expression of 5 apoptosis driving genes was also 2 to 3 times greater in PB-CD34+ cells, reflecting a higher apoptotic activity. In summary, our study provides a gene expression profile of primary human CD34+ hematopoietic cells of the blood and marrow. Our data molecularly confirm and explain the finding that CD34+ cells residing in the bone marrow cycle more rapidly, whereas circulating CD34+ cells consist of a higher number of quiescent stem and progenitor cells. Moreover, our data provide novel molecular insight into stem cell physiology.

Mobilization of peripheral blood stem cells with high-dose cyclophosphamide or the DHAP regimen plus G-CSF in non-Hodgkin's lymphoma

Our study analyzes the mobilization of hematopoietic stem cells after two chemotherapeutic regimens in non-Hodgkin's lymphoma (NHL) patients. The study included 72 patients with NHL (42 follicular and 30 large cells). The mean age was 37 years (range 17-60). Sixty-four patients (88.9%) had stage III-IV disease. Forty-eight patients (66.7%) had bone marrow involvement. Systemic B symptoms were present in 42 patients (58.3%). Mobilization chemotherapy regimens were randomly assigned as DHAP in 38 patients (52.7%) or cyclophosphamide (CPM) (5 g/m(2)) in 34 (47.2%) and the results of 132 procedures were analyzed. At the time of PBSC mobilization, 46 patients (63.9%) were considered to be responsive (complete remission, partial remission or sensitive relapse) and 26 (36.1%) not responsive (refractory relapse or refractory to therapy). Pre-apheresis CD34+ blood cell count and number of previous chemotherapy treatments were used to predict the total number of CD34+ cells in the apheresis product. The mobilizing regimens (CPM or DHAP) were similar in achieving the threshold CD34+ cell yield, for optimal engraftment. Since DHAP was very effective as salvage treatment, we suggest using DHAP as a mobilizing regimen in patients with active residual lymphoma at the time of stem cell collection.

Changes in integrin expression are associated with altered homing properties of Lin(-/lo)Thy1.1(lo)Sca-1(+)c-kit(+) hematopoietic stem cells following mobilization by cyclophosphamide/granulocyte colony-stimulating factor

OBJECTIVE

Although migration of hematopoietic stem cells (HSC) is essential for normal hematopoiesis and successful hematopoietic cell transplantation, little is known about the mechanisms that underlie this movement. We have sought to characterize the factors that regulate HSC migration by analyzing changes in expression of particular adhesion receptors associated with cyclophosphamide/granulocyte colony-stimulating factor (Cy/G-CSF)-induced HSC mobilization.

METHODS

Expression by Lineage(-/lo)Thy1.1(lo)Sca-1(+)c-kit(+) HSC of members of the beta1 integrin family of adhesion molecules was assessed in untreated or Cy/G-CSF-treated mice by multiparameter flow cytometry. In parallel, the in vivo homing properties of normal and mobilized HSC were compared following intravenous transfer of fluorescently marked HSC.

RESULTS

Normal adult HSC express high levels of several beta1 integrin family members. Following Cy/G treatment, bone marrow HSC selectively downregulate alpha 2 integrin expression and upregulate alpha 5 expression. HSC found in the blood following Cy/G-CSF treatment express significantly lower levels of multiple integrins than their bone marrow and/or splenic counterparts. Changes in integrin expression by blood-borne HSC correlate with a 50% decrease in their ability to home to the bone marrow in short-term assays, and with previously observed defects in competitive engraftment by these HSC. Similar reductions in bone marrow (BM) homing are observed for BM HSC treated with alpha 4 integrin function blocking mAb prior to injection. Modulation of integrin expression induced by mobilization was not associated with cell-cycle progression.

CONCLUSION

Changes in integrin expression and function are associated with HSC mobilization and likely significantly affect the engraftment potential of hematopoietic stem cells.

Bone and bone-marrow interactions: haematological activity of osteoblastic growth peptide (OGP)-derived carboxy-terminal pentapeptide. Mobilizing properties on white blood cells and peripheral blood stem cells in mice

Osteogenic growth peptide (OGP) increases blood and bone marrow cellularity in mice, and enhances engraftment of bone marrow transplant. Carboxy-terminal pentapeptide (OGP10-14) holds several properties of full-length polypeptide. We evaluated whether synthetic OGP-derived pentapeptide (sOGP10-14) has some activity on peripheral blood cell recovery after cyclophosphamide-induced aplasia, and on stem cell mobilization. Peripheral blood stem cell (PBSC) mobilization was evaluated by administering granulocyte-colony stimulating factor (G-CSF) or sOGP10-14 after cyclophosphamide (CTX) injection. Haematological parameters and CD34/Sca-1 positive cells were sequentially evaluated. Colony-forming tests were performed in bone marrow cells from CTX-, G-CSF- and sOGP10-14-treated mice. sOGP10-14 was able to enhance band cells and monocyte recovery after cyclophosphamide administration. White blood cell (WBC) counts reached the maximum peak by day +10 but, on day +7, a significant recovery was already detected in sOGP10-14 treated mice. On day +10 the WBC increase in sOGP10-14-treated mice was comparable to that found in G-CSF treated ones. Moreover, CD34/Sca-1 positive early precursors were significantly mobilized by sOGP10-14 compared to the control group. In sOGP10-14-treated mice, the colony-forming unit-granulocyte-macrophage-megakaryocyte (GEMM-CFU) and burst-forming unit-erythroid (BFU-E) were significantly increased in bone marrow cells in comparison to mice treated with CTX only. These results suggest a central role of sOGP10-14 in bone and bone marrow interaction, and a possible role of sOGP10-14 as a mobilizing agent.

Quantitative expression of adhesion molecules on granulocyte colony-stimulating factor-mobilized peripheral blood, bone marrow, and cord blood CD34+ cells

The purpose of this study is to investigate the function of the main adhesion receptors on CD34(+) cells during hematopoietic stem cell transplantation. Expression was quantified by flow cytometry using calibration beads. CD34(+) cells were isolated from either bone marrow (BM), cord blood (CB), or peripheral blood (PB) from study patients and a control group after granulocyte colony-stimulating factor (G-CSF) administration. The study of the CD34(+) cell differentiation showed that CD34(+) cells are mainly CD38(+) and HLA-DR(+), whatever the type of harvest. However, quantitative analysis elicited a weaker expression of CD38 on PB and CB CD34(+) cells in comparison to BM CD34(+) cells. The proportions of CD34(+)/CD49d(+) and CD34(+)/CD49e(+) were smaller on PB cells, without quantitative expression variation. This phenotypic variation promotes CD34(+) cells to exit from BM into circulation, inducing the mobilization. The homing of the CD34(+) cells to the BM involves the CD62L receptor. The expression of this receptor was found to be more frequent and stronger on PB cells than on BM or CB cells. The CD11b, CD18, and CD54 receptors are implicated in CD34(+) cell adhesion to BM microenvironment. No significant variation in CD34(+)/CD11b(+) and CD34(+)/CD18(+) cell frequency was noted. Moreover, the CD54 receptor was more frequently expressed on CB and PB cells. Quantitative analysis revealed that CD18 was more strongly expressed on BM than on PB cells to promote progenitors adhesion by interacting with stromal cells. Finally, the quantitative expression of the main receptors on CD34(+) cells explained cellular functions during the different steps of hematopoietic stem cells transplantation.

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

In healthy adults, hematopoiesis takes place in the bone marrow, where the majority of hematopoietic progenitor cells (HPC) reside. In patients undergoing chemo- and/or radiotherapy, hematopoiesis is seriously disturbed. Reconstitution of bone-marrow function can be achieved by bone marrow transplantation or peripheral blood stem cell transplantation. The success of stem cell transplantation depends on the ability of intravenously infused stem cells to lodge in the bone marrow, a process referred to as homing. However, the molecular mechanisms that govern this process are poorly understood. It is hypothesized that homing is a multistep process, consisting of adhesion of the HPC to endothelial cells of the marrow sinusoids, followed by transendothelial migration directed by chemoattractants, and finally anchoring within the extravascular bone marrow spaces where proliferation and differentiation will occur. In this review, we discuss the factors that determine the engraftment potential of stem cells, and focus on various aspects of migration and homing of HPC, i.e., the role of the chemokine stromal cell-derived factor-1 (SDF-1) and its receptor CXCR-4, the involvement of adhesion molecules, and the induction of actin polymerization in the HPC. Defining the role of chemokines and adhesion molecules in human stem cell migration and engraftment will help us uncover the underlying mechanisms that regulate stem cell homing and will eventually advance clinical stem cell transplantation.

An in vitro model of hematopoietic stem cell homing demonstrates rapid homing and maintenance of engraftable stem cells

Hematopoietic stem cell (HSC) homing is believed to rely heavily on adhesion interactions between stem cells and stroma. An in vitro assay was developed for adhesion of engraftable HSCs in bone marrow suspensions to pre-established Dexter-type long-term bone marrow culture stromal layers. The cell numbers in the adherent layer and supernatant were examined, along with the engraftment capability of adherent layer cells to indicate the number of HSCs that homed to in vitro stroma. The cell number in the supernatant declined over the 24-hour period. The number of test cells adhering to the stromal layer increased during the first hour and then fell at 6 and 24 hours. The number of test HSCs adhering to the stromal layer was substantial at 20 minutes, increased during the first hour, and then remained constant at 1, 6, and 24 hours of adhesion. These data indicate that adhesion of engraftable HSCs occurs quickly and increases during the first hour of contact with pre-established stroma, that adhesion plateaus within 1 hour of contact, and that HSCs maintain their engraftment capability for at least 24 hours of stromal adhesion. Long-term engraftment from test cells at more than 1 hour of adhesion represents 70.7% of the predicted engraftment from equivalent numbers of unmanipulated marrow cells, indicating that 2 of 3 test engraftable HSCs adhered. These findings demonstrate the usefulness of this model system for studying stem-stromal adhesion, allowing further dissection of the mechanism of HSC homing and exploration of possible manipulations of the process. (Blood. 2001;98:1012-1018)

Recent Developments in the Regulation of Peripheral Blood Stem Cell Mobilization and Engraftment by Cytokines, Chemokines, and Adhesion Molecules

Peripheral blood stem cells (PBSC) have become the preferred source of stem cells for autologous transplantation because of the technical advantage and the shorter time to engraftment. Administration of hematopoietic growth factors such as granulocyte colony-stimulating factor (G-CSF) or granulocyte-macrophage colony-stimulating factor (GM-CSF) results in mobilization of PBSCs into the peripheral blood. G-CSF and GM-CSF differ somewhat in the number and composition of CD34(+) cells and effector cells mobilized to the peripheral blood; however, the molecular mechanism underlying the release and engraftment of CD34(+) cells by these growth factors is poorly understood. This review provides a recent update on the involvement of hematopoietic growth factors, chemokines, adhesion molecules, and chemokine receptors in the regulation of stem cell release and engraftment. The involvement of very late antigen-4 (VLA-4), VLA-5, leukocyte function associated-1 molecule (LFA-1), and L-selectin and their receptors CXCR4 and its ligand SDF-1 will be discussed, and cross talk between these factors will also be reviewed in the context of stem cell release and engraftment. Finally, PBSC mobilization by chemokines will be reviewed in relation to hematopoietic growth factors.