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The Role of Cytokines and Adhesion Molecules for Mobilization of Peripheral Blood Stem Cells

Klinik für Hämatologie, Onkologie und klinische Immunologie, Heinrich-Heine-Universität Düsseldorf, Germany

Key Words. Mobilization • CD34⁺ cells • Transplantation • G-CSF • Adhesion molecules • VLA-4 • Antisense oligonucleotides • Integrin antagonists

Ralf Kronenwett, M.D., Klinik für Hämatologie, Onkologie und klinische Immunologie, Heinrich-Heine-Universität Düsseldorf, Moorenstr. 5, 40225 Düsseldorf, Germany. Telephone: 49-211-8117760; Fax: 49-211-8118853.

	ABSTRACT

Top Abstract Introduction Role of Cytokines in... Functional and Immunological... Transplantation of Mobilized... Role of Adhesion Molecules... Mobilization of CD34+ Cells... Conclusions References

 
CD34⁺ hematopoietic stem cells from peripheral blood are commonly used for autologous or allogeneic transplantation following high-dose therapy in malignant diseases. The introduction of hematopoietic growth factors such as G-CSF has greatly facilitated the mobilization of CD34⁺ cells. The mechanism of stem cell mobilization is not yet clear. It seems to be a multistep process with a crosstalk between cytokines and adhesion molecules. In this review, the role of hematopoietic growth factors, chemokines, and adhesion molecules for mobilization and homing of CD34⁺ cells is summarized. In addition, factors influencing the cytokine-induced mobilization in patients and healthy donors are described. The review closes with an overview of new classes of mobilizing drugs such as monoclonal antibodies, specific peptides, or antisense oligonucleotides targeting adhesion molecules.

	INTRODUCTION

Top Abstract Introduction Role of Cytokines in... Functional and Immunological... Transplantation of Mobilized... Role of Adhesion Molecules... Mobilization of CD34+ Cells... Conclusions References

 
CD34⁺ peripheral blood stem cells (PBSC) are widely used to support high-dose therapy in patients with hematological malignancies and solid tumors. Different methods have been used to mobilize hematopoietic stem and progenitor cells into the peripheral blood (PB). The preferred modality of mobilization is the use of hematopoietic growth factors such as G-CSF, which can be given either during steady-state hematopoiesis or after cytotoxic chemotherapy to enhance the rebound of circulating progenitor cells [1, 2]. The mechanisms involved in PBSC mobilization are not yet clear. It is apparently a multistep process including enhanced proliferation of early progenitor cells with subsequent migration and egress from the bone marrow (BM). Adhesive interactions between the CD34⁺ hematopoietic stem cells and their progenitors with cellular and matrix components of the BM environment are involved in mobilization and homing. Trafficking of stem cells is not only relevant for cytokine-supported BM recovery following cytotoxic chemotherapy, but also for normal hematopoiesis. In adults, small amounts of CD34⁺ cells are present in the PB during steady-state hematopoiesis, suggesting a continuous migration and exchange of hematopoietic stem cells between BM and other organs such as liver or spleen.

It is the objective of this review to give an overview of the role of cytokines and adhesion molecules for mobilization and homing of CD34⁺ cells. Particular emphasis is laid on new modalities of stem cell mobilization such as the targeting of integrins with monoclonal antibodies or antisense oligonucleotides.

	ROLE OF CYTOKINES IN THE MOBILIZATION OF PBSC

Top Abstract Introduction Role of Cytokines in... Functional and Immunological... Transplantation of Mobilized... Role of Adhesion Molecules... Mobilization of CD34+ Cells... Conclusions References

 
The most frequently used hematopoietic growth factors for PBSC mobilization are G-CSF and GM-CSF. Both factors are preferentially involved in the differentiation of myeloid progenitors into mature granulocytes and monocytes, respectively. The expression level of the receptors of G-CSF and GM-CSF is very low on the more primitive progenitor cells and increases during myeloid or myelomonocytic differentiation [3-5]. The G-CSF receptor is also expressed on stromal cells of the BM. Recently, it could be shown in a mouse model that the expression of the G-CSF receptor on CD34⁺ cells is not necessary for their mobilization by G-CSF, suggesting that G-CSF acts not directly on a primitive hematopoietic progenitor cell but via receptors on cells of the BM stroma [6].

In the first study using GM-CSF for PBSC mobilization in humans, an approximate 18-fold increase of circulating colony-forming units granulocyte-macrophage (CFU-GM) was found in patients with cancer who received the cytokine during steady-state hematopoiesis, while an additional fivefold greater increase was observed when GM-CSF was given following cytotoxic chemotherapy [7]. In another study, a dose-dependent increase of CFU-GM was described when G-CSF was given during steady-state hematopoiesis for four days [1]. The maximum increase of CFU-GM observed was 100-fold greater in comparison to baseline level. Similar findings were reported by other groups using G-CSF or GM-CSF in patients with cancer or healthy volunteers [8-13]. Comparing G-CSF and GM-CSF, no difference was found between both cytokines in their ability to mobilize PBSC [14, 15]. In our study, an 8.5-fold increase of CFU-GM in the PB over baseline levels was observed following continuous intravenous GM-CSF infusion (250 µg/m²/day). The patients were extensively pretreated and not eligible for BM harvesting [16]. The modest mobilization that we observed might be explained by the substantial amount of previous cytotoxic chemotherapy. In the following studies PBSC mobilization was combined with the administration of cytotoxic chemotherapy, which is associated with a lower probability of harvesting tumor cells, provided that the malignant cells are chemosensitive [2, 17-19]. In an intraindividual comparison, we observed a sevenfold greater yield of CD34⁺ cells per leukapheresis after G-CSF-supported chemotherapy compared with steady-state administration of G-CSF at a dose of 5 µg/kg/day [20]. This might be related to greater endogenous G-CSF serum levels observed during chemotherapy-induced neutropenia [21, 22]. It has been shown that myeloid precursor cells and mature neutrophils bind G-CSF via high-affinity receptors and therefore lead to a reduction of bioavailable G-CSF [23]. This mechanism may be particularly relevant for administration of G-CSF during steady-state hematopoiesis, when BM cellularity and the number of neutrophils with binding sites for G-CSF are greater than post-chemotherapy.

Still, there is no generally accepted dose and schedule for the administration of G-CSF or GM-CSF. In steady-state mobilization, significantly higher peak levels of circulating CD34⁺ cells in normal donors were found when the daily dose of G-CSF (filgrastim) was 10 µg instead of 3 or 5 µg/kg [24]. Similarly, an increase in the dose of glycosylated G-CSF (lenograstim) from 3, 5, 7.5 to 10 µg/kg/day improved PBSC mobilization in healthy volunteers in a dose-dependent manner [25]. In contrast, for administration of G-CSF post-chemotherapy, there was no relationship between the dose of G-CSF and the peak level of circulating CD34⁺ cells or the median CD34⁺ cell counts recorded over the entire collection period [26]. Even an increase of the G-CSF dose to 23 µg/kg was not superior in comparison with a dose of 2.8 µg/kg. An explanation could be the increased endogenous serum concentrations of G-CSF post-chemotherapy which reach a saturation level when the relatively low dose of 2.8 µg/kg is given in addition.

Independent of the administration schedule, a wide variation in the mobilization efficacy was observed between normal volunteers as well as patients [27]. In normal volunteers, an inverse correlation between age and yield of CD34⁺ cells collected following G-CSF administration was found [12]. Factors which influence PBSC mobilization in patients are primarily the dose of the cytotoxic chemotherapy used for mobilization, the underlying disease and the cumulative amount of previous cytotoxic treatment including radiotherapy. For example, administration of 7 g/m² in comparison with 4 g/m² cyclophosphamide resulted in significantly greater peak levels of CD34⁺ cells in PB of patients with multiple myeloma [28]. In another study, patient-associated factors that may influence the yield of CD34⁺ cells following G-CSF-supported cytotoxic chemotherapy were evaluated in 61 patients with lymphoma [18]. Previous cytotoxic chemotherapy and irradiation adversely affected the yield of CD34⁺ cells, suggesting that PBSC collection should be performed as early as possible during the course of the disease. Moreover, patients with Hodgkin's disease had a significantly smaller collection efficacy in comparison with patients with non-Hodgkin's lymphoma. Since the patients with Hodgkin's disease had been irradiated, diagnosis could not be evaluated as an independent variable. Patients' characteristics which were not related to the collection efficacy were age, sex, and disease status, but the observations made in normal donors still argue for an age-associated decrease of the progenitor cell pool. BM involvement at the time of PBSC collection was also not associated with the yield of CD34⁺ cells.

In addition to G-CSF and GM-CSF, other cytokines such as interleukin 3 (IL-3), IL-8, IL-11, stem cell factor (SCF), flt-3 ligand, or macrophage inflammatory protein-1 (MIP-1) might be used for mobilization of CD34⁺ cells [29-36]. The ability of these cytokines to mobilize PBSC when given as single agents is apparently not better than that of G-CSF or GM-CSF. Sequential administration of IL-3 and GM-CSF in patients with high-grade non-Hodgkin's lymphoma following salvage chemotherapy resulted in a similar mobilization efficacy in comparison with a historical control group which received G-CSF [31]. Whether other combinations and schedules might be beneficial is currently evaluated in different studies.

	FUNCTIONAL AND IMMUNOLOGICAL CHARACTERIZATION OF PBSC

Top Abstract Introduction Role of Cytokines in... Functional and Immunological... Transplantation of Mobilized... Role of Adhesion Molecules... Mobilization of CD34+ Cells... Conclusions References

 
There are several studies assessing the differences between CD34⁺ cells from BM and PB during G-CSF-enhanced marrow recovery. In a group of patients with cancer, we found a 3.7-fold greater peak concentration of CD34⁺ cells in the PB during G-CSF-supported recovery in comparison with BM samples from steady-state hematopoiesis [37] (Fig. 1). Interestingly, even during G-CSF administration and marrow recovery, the vast majority of circulating CD34⁺ cells are in the G₀/G₁ phase of the cell cycle suggesting that contact with the BM stroma is necessary for proliferation of hematopoietic progenitor cells [38]. Several studies also show that PB during G-CSF-treatment contains a greater proportion of more primitive CD34⁺ cells than BM as shown by functional assays for the enumeration of long-term culture-initiating cells and pre-CFU-GM [39]. This could be confirmed by immunophenotypical data showing that a greater proportion of mobilized PBSC expressed the early stem cell-associated antigen Thy-1 in comparison with BM [37] (Fig. 1). This provides a strong argument for the use of blood-derived progenitor cells rather than BM for transplantation, since early hematopoietic progenitor cells are particularly relevant for long-term hematopoiesis following PBSC transplantation. Mobilized CD34⁺ cells also differ from BM CD34⁺ cells with regard to their lower expression of c-kit and CD45RA [37, 40]. Adhesion molecules are also differentially expressed on CD34⁺ cells from PB and BM. This aspect will be adressed in detail in a separate paragraph of this review.

View larger version (29K): [in this window] [in a new window]   Figure 1. Intraindividual comparison between CD34+ cells from PB and BM. Left: Concentration of CD34+ cells in BM samples before the start of cytotoxic chemotherapy and in PB (peak level) obtained during cytokine-enhanced marrow recovery. The mean concentration of CD34+ cells was 2.3-fold greater in PB compared to BM samples. Right: Proportion of CD34+/Thy-1+ cells in BM samples from 20 patients before mobilization and 48 leukapheresis products (LP) collected during G-CSF-enhanced marrow recovery post-chemotherapy. A representative two-color dot blot analysis of an LP product is shown in the insert.

	TRANSPLANTATION OF MOBILIZED PBSC

Top Abstract Introduction Role of Cytokines in... Functional and Immunological... Transplantation of Mobilized... Role of Adhesion Molecules... Mobilization of CD34+ Cells... Conclusions References

 
The ability of PBSC to reconstitute hematopoiesis was shown by several groups in 1986 [42-45]. Since cytokines were not available at that time, PBSC were either collected during steady-state hematopoiesis or following cytotoxic chemotherapy. Transplantion of cytokine-mobilized PBSC was first reported in 1989 [8]. Several studies confirmed that cytokine-mobilized PBSC with or without cytotoxic chemotherapy can be used for autografting [2, 16, 17, 19, 46-48]. These studies showed that the time needed for hematological recovery and hospitalization was significantly shorter in patients autografted using PBSC when compared with BM grafting. Initially, the number of mononuclear cells or clonogenic progenitors was used for the characterization of the PBSC grafts. The introduction of immunofluorescence analysis with directly labeled anti-CD34 antibodies permitted a standardized characterization of PBSC. Data from several groups suggested that a minimum number of CD34⁺ cells is necessary for rapid and sustained engraftment. This threshold quantity of CD34⁺ cells needed for transplantation is generally thought to lie between 2.5 and 5.0 x 10⁶/kg body weight [2, 48, 49]. There is also a relationship between the number of CD34⁺ cells transplanted and the time required for hematological reconstitution. Patients who received a greater number of CD34⁺ cells/kg needed shorter recovery times than patients grafted with a smaller number of CD34⁺ progenitor cells [2, 50-52].

PBSC have also been used for allogeneic transplantation [12, 53-55]. Rapid and complete engraftment was observed with recovery times which were apparently shorter than following allogeneic BM transplantation. Randomized studies will follow to compare the incidence of acute and chronic graft-versus-host disease following allogeneic PBSC or BM transplantation. There is no doubt that long-term engraftment of donor cells is achieved following allogeneic PBSC transplantation as cytogenetic and molecular biological examinations have shown.

	ROLE OF ADHESION MOLECULES FOR MOBILIZATION AND HOMING OF CD34+ CELLS

Top Abstract Introduction Role of Cytokines in... Functional and Immunological... Transplantation of Mobilized... Role of Adhesion Molecules... Mobilization of CD34+ Cells... Conclusions References

 
Adhesive interactions between hematopoietic stem cells and components of the BM microenvironment play a central role for migration, circulation, and proliferation of hematopoietic stem cells [56-58]. The molecules involved are members of the ß1 and ß2 integrin, selectin and superimmunoglobulin families, which were first described in mature leukocytes in the context of inflammation [59-64] (Fig. 2). Corresponding ligands are expressed on endothelial cells and accessory marrow cells or form parts of the extracellular matrix of the BM microenvironment. For instance, L-selectin (CD62L), which recognizes carbohydrate residues on endothelial cells and mediates the initial contact of leukocytes with endothelium, is also highly expressed on circulating CD34⁺ progenitor cells suggesting its role for homing of stem cells following transplantation [65]. Several studies show a central role of the ß1 integrins very late antigen 4 ([VLA-4] CD29/CD49d) and VLA-5 (CD29/CD49e) for adhesion of hematopietic progenitor cells to components of the BM stroma. In particular, the VLA-4-mediated interaction between hematopoietic stem cells and BM stroma is of functional relevance for hematopoiesis as well as for mobilization and homing of CD34⁺ cells [57, 66-68]. VLA-4 is expressed on the majority of mononuclear hematopoietic cells and binds to the vascular cell adhesion molecule-1 (VCAM-1) as well as to the extracellular matrix protein fibronectin. Circulating CD34⁺ cells express VLA-4 at a lower level when compared with CD34⁺ cells residing in the BM. This finding suggests that the release of CD34⁺ cells and the ability to circulate is related to the presence and expression level of VLA-4 [57, 65, 69-72]. This view is supported by the finding that systemic treatment of primates and mice with monoclonal antibodies directed against VLA-4 resulted in a significant increase of circulating hematopoietic progenitor cells [73-76]. In addition, antibody treatment of mice was associated with inhibition of engraftment of hematopoietic stem cells in mice [77]. The egress of CD34⁺ cells into the PB might not only depend on the expression level but also on the functional state of the integrins. A rapid modulation of the functional state of integrins has been suggested for other physiological processes such as platelet aggregation [78, 79], cell migration [80], and embryonic development [81]. Recently, we examined the activation state of VLA-4 on CD34⁺ cells from BM and PB by flow cytometry using a VCAM-1/immunoglobulin fusion protein as soluble ligand [72] (Fig. 3). A significantly reduced functional state of the VLA-4 receptor was found on CD34⁺ cells from PB during G-CSF-enhanced marrow recovery in comparison to CD34⁺ cells from steady-state BM. Interestingly, the amount of circulating CD34⁺ cells during marrow recovery was inversely related to the activation state but not expression level of VLA-4, suggesting that a modulation of the functional state of VLA-4 is involved in the mobilization of CD34⁺ cells. The functional state of a receptor can be changed more rapidly than the level of protein expression on the surface. The mechanisms involved in the inactivation of VLA-4 on circulating CD34⁺ cells are not clear. We found that Mg2⁺ ions or contact with endothelial cells resulted in an activation of the VLA-4 receptor, suggesting that changes in the Mg2⁺ concentration in the vicinity of the VLA-4 receptor alters its activity on CD34⁺ cells located along the endothelial cell lining in vivo.

View larger version (34K): [in this window] [in a new window]   Figure 2. Role of adhesion molecules and their ligands for mobilization of CD34+ hematopoietic stem cells.

View larger version (23K): [in this window] [in a new window]   Figure 3. Examination of the functional state of VLA-4 on CD34+ cells using a VCAM-1/Immunoglobulin (VCAM-Ig) fusion protein as soluble ligand. (A) Schematic depiction of the binding of a bivalent VCAM-Ig fusion protein to two active VLA-4 molecules on the cell surface. Bound VCAM-Ig can be detected by flow cytometry using a secondary phycoerythrin (PE)-conjugated donkey antihuman IgG monoclonal antibody. (B) Immunofluorescence analysis of VCAM-Ig binding and 4 expression of CD34+ cells from steady-state BM and PB during G-CSF-supported hematopoietic recovery. Control staining is indicated in white; specific staining in grey histograms.

There is also an important interaction between cell adhesion and signal transduction pathways activated by cytokines [83]. For instance, SCF, GM-CSF, or IL-3 can increase transiently the adhesiveness of CD34⁺ cells to stroma through an activation of the integrins VLA-4 and VLA-5 [84, 85]. The contact between hematopoietic stem cells and stroma enhances the proliferative stimulus induced by cytokines [86]. The necessity of integrin activation via adhesion could explain why circulating CD34⁺ cells without contact to BM stroma are mainly quiescent cells in G₀/G₁ phase of the cell cycle.

Other adhesion molecules involved in the various processes of hematopoiesis during steady-state and mobilization are the platelet endothelial cell adhesion molecule-1 and CD44. The latter one is a highly glycosylated surface molecule with different isoforms arising from differences in glycosylation and alternative splicing. It is highly expressed on hematopoietic progenitor cells. The ligands of CD44, hyaluronic acid and fibronectin, are secreted by stromal cells suggesting an involvment of CD44 in binding of CD34⁺ cells to BM stroma. This is supported by the finding that monoclonal antibodies against CD44 inhibit adhesion to BM stroma, mobilize progenitor cells in mice and abolish hematopoiesis in long-term BM cultures [87-91].

Beside growth factors and adhesion molecules, the alpha chemokine stromal-derived factor 1 (SDF-1) plays a prominent role in stem cell migration [92]. The cellular receptor of SDF-1 is CXCR-4, which also serves as coreceptor for T cell-tropic HIV-1 strains [93]. CXCR-4 is expressed in CD34⁺ cells dependent on the differentiation state as the subset of CD34⁺/CD38^low and CD34⁺/HLA-DR^low cells representing a population of more immature progenitor cells were brightly positive for CXCR-4. In contrast, a lower level of CXCR-4 expression was found in the population of CD34⁺/CD38^bright and CD34⁺/HLA-DR^bright cells [94, 95]. SDF-1 seems to be a general chemoattractant for hematopoietic stem cells [92, 96, 97] and the SDF-1/CXCR-4 interaction is relevant during embryonic development, hematopoiesis, and migration of CD34⁺ hematopoietic stem cells.

In summary, mobilization of hematopoietic stem cells is a complex process which is only partially understood. It is likely that a proliferation of hematopoietic stem cells, as a consequence of cytotoxic chemotherapy, and the use of cytokines is associated with a loosening of adhesive interactions between stem cells and stromal elements. In addition, adhesion molecules and chemokines are required for the migration through the endothelial cell barrier. The mechanisms involved are similar to the mechanisms by which mature leukocytes migrate into sites of tissue injury or inflammation.

	MOBILIZATION OF CD34+ CELLS BY ANTIADHESIVE APPROACHES

Top Abstract Introduction Role of Cytokines in... Functional and Immunological... Transplantation of Mobilized... Role of Adhesion Molecules... Mobilization of CD34+ Cells... Conclusions References

 
Downregulation and functional inactivation of adhesion molecules are relevant for the mobilization of CD34⁺ cells during G-CSF-enhanced recovery. Therefore, a pharmacological modification of the expression, or the functional state of adhesion receptors such as integrins in vivo, could improve PBSC collection particularly in patients with ineffective peripheralization of CD34⁺ cells. It is conceivable to block interactions between hematopoietic progenitor cells and BM stroma by monoclonal antibodies against adhesion molecules or competitive inhibition of receptor-ligand interactions using peptides with specific binding motifs. Anti-4- or anti-ß2-integrin monoclonal antibodies have already been used for stem cell mobilization in primates [73]. Anti-4 treatment resulted in a significant increase of circulating hematopoietic progenitor cells with peak levels which were 200-fold over baseline levels within 24 h after injection. In contrast, anti-ß2 antibodies had no mobilizing effect. Similar observations were made in mice [74, 75]. The combination of anti-4 antibodies with either G-CSF or SCF treatment led to a five- to eightfold greater mobilization efficacy in comparison with anti-4 or cytokine treatment alone [74]. These findings indicate a synergistic effect of both mobilizing modalities. Recently, the authors showed that peripheralization of progenitor cells using anti-4 antibodies requires a functional c-kit/c-kit ligand pathway [76]. In addition, the increase of hematopoietic progenitors in the blood was associated with a downregulation of c-kit expression on the cells, suggesting an integrin/cytokine crosstalk during mobilization. In another study, hematopoietic progenitor cells could be mobilized in mice using CD44-directed monoclonal antibodies [98]. Whether a combination of monoclonal antibodies against adhesion molecules or their additional use with cytokines have synergistic effects has to be assessed in further studies.

Another possibility to diminish adhesive interactions is the use of peptides with specific binding motifs for the surface molecules in question. For instance, a small molecule inhibitor of VLA-4, which mimics the CS-1 ligand and prevents VLA-4-mediated binding to fibronectin, abrogated the airway hyper-responsiveness in allergic sheep when given by aerosol [99]. These observations indicate that leukocyte migration can be abolished by competitive inhibition of integrin receptor binding. The therapeutic use of small peptides is hampered by their low stability in serum. Therefore, cyclic peptides were designed or chemical modifications were made to prevent rapid degradation as a result of enzymatic cleavage [100-102]. One chemically modified small molecule inhibitor of VLA-4 was 106-fold more efficacious than the original peptide, showing that highly selective receptor antagonists to integrins are potent modulators of leukocyte migration [102].

Adhesive interactions can also be modulated by downregulating the expression of adhesion molecules. Gene expression can be switched off specifically by the use of antisense oligonucleotides which hybridize to the complementary sequence of a given target mRNA [103]. Antisense oligonucleotides which inhibit the expression of disease-related genes are evaluated in clinical trials for the treatment of viral infections, cancer, and inflammation [104]. Promising targets for antisense inhibition are also adhesion molecules. Recently, we found that inhibiton of ICAM-1 expression on monocytes using antisense oligonucleotides resulted in reduced adhesion to an endothelial cell layer and decreased transendothelial migration [105]. Since ICAM-1 with its ligand LFA-1 is not only involved in leukocyte migration but also in trafficking of hematopoietic progenitor cells [65], mobilization of CD34⁺ cells to the PB could be facilitated by an antisense-mediated inhibition of ICAM-1 expression. Another attempt to improve the mobilization of CD34⁺ hematopoietic progenitor cells is the use of antisense oligonucleotides directed against the mRNA of the 4 chain of VLA-4. Transfection of antisense oligonucleotides into enriched CD34⁺ cells resulted in a significant downregulation of VLA-4 surface expression and decreased adhesion to endothelial cells [106]. Using Dexter-type long-term BM cultures as a model for stem cell mobilization, an increase of hematopoietic progenitor cells was found in the nonadherent fraction of antisense-treated cultures, indicating a release of CD34⁺ cells from the stromal layer in vitro. These results could provide the basis for a novel clinical use of antisense oligonucleotides to improve progenitor mobilization for autologous or allogeneic transplantation. Interestingly, the transfection of VLA-4-directed antisense oligonucleotides in leukemic cell lines was associated with an inhibition of proliferation in the VLA-4-positive cell lines LAMA84, HL60 and BV173. In contrast, the growth of the VLA-4-negative cell line K562 was not affected [106]. The antiproliferative effect of the VLA-4 antisense oligonucleotides could therefore be beneficial for the treatment of patients with VLA-4-positive leukemias.

Still, there are several problems in the development of new drugs for stem cell mobilization. These are based on the use of mouse-derived monoclonal antibodies which can induce human antimouse antibodies following administration in man [107]. This problem might be overcome by the design of recombinant humanized or single-chain antibodies. Moreover, preclinical testing in suitable animal models is hampered because of species-specific antibody epitopes or binding motifs. The same holds true for the development of antisense-based drugs as their sequence is also species-specific and they are associated with a variety of unspecific side effects in mice which are not observed in man [108, 109]. Finally, facilities are necessary which provide the new drugs in sufficient amounts according to the guidelines of good manufacturing practice.

	CONCLUSIONS

Top Abstract Introduction Role of Cytokines in... Functional and Immunological... Transplantation of Mobilized... Role of Adhesion Molecules... Mobilization of CD34+ Cells... Conclusions References

 
The use of the hematopoietic growth factors G-CSF or GM-CSF has greatly facilitated the mobilization of PBSC and was an essential prerequisite for the use of blood-derived progenitor cells for transplantation. The introduction of other cytokines such as SCF, IL-11, IL-8, or flt-3 ligand has not unequivocally proved to be superior to the "classical" mobilization factors administered so far. Several studies have shown that peripheralization of PBSC is a multistep process with a crosstalk between hematopoietic growth factors and adhesion molecules. Therefore, new classes of mobilizing drugs such as monoclonal antibodies, specific peptides, or antisense oligonucleotides directed against integrins are subjects of current research. Whether inhibition of receptor-ligand interactions may provide a synergistic approach to improve cytokine-based PBSC mobilization has to be addressed in clinical studies.

	ACKNOWLEDGMENTS

Top Abstract Introduction Role of Cytokines in... Functional and Immunological... Transplantation of Mobilized... Role of Adhesion Molecules... Mobilization of CD34+ Cells... Conclusions References

 
Supported by a research grant from Deutsche Krebshilfe.

	References

Top Abstract Introduction Role of Cytokines in... Functional and Immunological... Transplantation of Mobilized... Role of Adhesion Molecules... Mobilization of CD34+ Cells... Conclusions References

 

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