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Thrombosis |
From the Institute for Transfusion Medicine and Immunohematology (C.H., T.T., S.B., E.S.), Red Cross Blood Donor Service Baden-WürttembergHessen, Frankfurt am Main; Georg-Speyer-Haus (C.H., M.G.), Institute for Biomedical Research, Frankfurt am Main; and III Medizinische Klinik (R.O., U.K., C.P.), Klinikum Rechts der Isar, Technical University Munich, Germany.
Correspondence to Dr Torsten Tonn, Institute for Transfusion Medicine and Immunohematology, Red Cross Blood Donor Service Baden-WürttembergHessen, Sandhofstr. 1, 60528 Frankfurt am Main, Germany. E-mail ttonn{at}bsdhessen.de
| Abstract |
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Methods and Results CD34+ endothelial progenitor cells (EPCs) from cord blood were differentiated into CBECs. Endothelial phenotype was characterized, and lentiviral transduction of early-passage CBECs with a vector encoding FVIII and EGFP did not alter their functional properties and proliferative potential. CBEC could be expanded by 5 to 9 orders of magnitude, thus allowing the expansion of up to 1015 FVIII-secreting CBECs, starting from as little as 106 CD34+ cells. CBECs proved to be highly suitable for FVIII secretion, with 0.35 to 0.39 IU FVIII:C/5x104 cells per 48 hours (7.0 to 7.8 IU FVIII:C/106 cells per 48 hours), which remained stable over the expansion period.
Conclusions Our data indicate that CBECs are attractive target cells for inherited coagulation disorders such as hemophilia A, which on lentiviral transduction can be readily expanded to large numbers of transplantable gene-modified cells in vitro.
Key Words: hemophilia factor VIII cord blood endothelial progenitor cells lentiviral gene therapy
| Introduction |
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See page 2117
Under physiological conditions, FVIII synthesis mainly occurs in hepatocytes and liver sinusoidal endothelial cells.47 Because no FVIII-expressing cell lines derived from these cell types exist, recombinant FVIII protein for clinical use is presently produced at large scale in Chinese hamster ovary cells and baby hamster kidney cells.2 Most studies for the gene therapy of hemophilia A focused on the in vivo transduction of liver cells with viral vectors. Several in vivo studies yielded promising results.8,9 Long-lasting therapeutic FVIII plasma levels in hemophilia A mice could be achieved by systemic application of adenoviral, adeno-associated virus, retroviral, and lentiviral vectors. However, there remain concerns over the safety of these approaches. Potential side effects include adverse immunological reactions, vector-mediated cytotoxicity, and germ-line transmission.10,11 Extrahepatic cells such as bone marrow stromal cells, fibroblasts, and keratinocytes (see the study by VandenDriessche et al8 and the references therein) may also be useful in gene therapeutic approaches. Indeed, the implantation of ex vivo modified fibroblasts that secreted FVIII was shown to be well tolerated and led to detectable FVIII plasma levels in patients with severe hemophilia A.12
Although endothelial progenitor cells (EPCs) have been shown to be of interest for use in vascular regeneration, they also seem interesting as alternate target cells for ectopic FVIII expression. They are easily accessible from bone marrow, peripheral blood, or cord blood (CB),1319 and they can be differentiated into long-lived progeny. However, although most of the studies have been conducted with bone marrow or peripheral bloodderived endothelial cell populations, only little is known about the capacity of CB-derived EPCs. Nonetheless, for inherited coagulation disorders such as hemophilia, CB may be a suitable source that is easily obtainable at the time of birth. The amount of EPCs in CB seems to be limited because of the restricted blood volume. Therefore, therapeutic applications, especially those using gene-modified EPCs, may require additional expansion and enrichment of gene-modified EPCs. In addition, whereas peripheral blood outgrowth endothelial cells transfected with a FVIII expression plasmid have been shown to efficiently cure hemophilia in a murine knockout model,20 the impact of lentiviral transduction on the phenotype and proliferative potential of CB-derived EPCs (CBECs) has thus far not been addressed.
We therefore aimed to establish a lentiviral transduction protocol that allows highly efficient and stable transduction of CBECs using the enhanced green fluorescence protein (EGFP) gene as a marker and human B-domaindeleted FVIII, respectively. The deletion of the B domain increases the expression level of recombinant FVIII and does neither impair the procoagulant activity nor result in enhanced immunogenicity of the FVIII protein (see the study by Saenko et al18 and the references therein). We show that lentiviral transduction yields CBEC populations that have retained phenotypic characteristics of endothelial cells and that are readily expandable to large numbers of transplantable EPCs secreting high amounts of the FVIII transgene protein.
| Methods |
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Differentiation of Endothelial Cells From EPCs
CD34+ cells were cultured in gelatin-coated plates in endothelial differentiation medium consisting of 80% basal Iscove medium (Biochrom), 10% horse serum (PAN Biotech), 10% heat-inactivated FCS (Biochrom), 2 mmol/L L-alanyl-L-glutamine, 100 U/mL penicillin, and 100 µg/mL streptomycin (all from BioWhittaker). The medium contained the recombinant human cytokines vascular endothelial growth factor (VEGF) 50 ng/mL, fibroblast growth factor-2 (FGF-2) 20 ng/mL, stem cell factor (SCF) 50 ng/mL (all from R&D Systems), and stem cell growth factor-ß (SCGF-ß) 20 ng/mL (PeproTech). Before reaching confluency (usually after 2 to 3 weeks), CBECs were transferred to gelatin-coated tissue culture flasks in endothelial cell basal medium-2 (EGM-2) supplemented with 2% FCS, VEGF, epidermal growth factor, FGF-2, R3-IGF-1 (a pharmacologically more available isoform of insulin-like growth factor), hydrocortisone, ascorbic acid, heparin, gentamicin, and amphotericin B (BioWhittaker). CBECs could be expanded most efficiently when grown at low density (1x103/cm2).
Immunofluorescence
The following monoclonal antibodies were used: anti-CD14-PE, anti-CD34-PE, anti-CD45-FITC, anti-HLA-DR-FITC (all from BD Biosciences), anti-CD133-PE (Miltenyi Biotec), anti-CD146, anti-CD146-FITC, anti-
E-cadherin (all from Chemicon), anti-34-PC5 (Immunotech), anti-CD31-FITC, and anti-KDR (both from Sigma). Unconjugated primary monoclonal antibodies were detected by RPE-conjugated F(ab')2 fragment of rabbit anti-mouse immunoglobulin (Dako). Samples of cells were also stained with isotype-matched control antibodies (purchased from BD Biosciences and Immunotech). FACS analyses were performed using a FACScan flow cytometer (BD Biosciences) and Cell Quest software (BD Biosciences).
DiI-Ac-LDL Labeling
Fluorescent labeling of endothelial cells by uptake of acetylated (Ac)-LDL was performed by incubating CBECs with 2 µg/mL DiI-Ac-LDL (Harbor Bio-Products) in EGM-2 for 60 minutes at 37°C.
Lentiviral Constructs
We used 2 HIV-1derived self-inactivating lentiviral gene transfer constructs. The EGFP-encoding vector pHR'SIN.cPPT-SEW (kindly provided by Adrian Thrasher, London, UK) has been described elsewhere.21 The vector cPPT-C(FVIII
B)IGWS is a derivative of the bicistronic vector C(FVIII
B)IGWS22 coding for human B-domain deleted FVIII (BDD FVIII) and EGFP. cPPT-C(FVIII
B)IGWS additionally contains the central polypurine tract and central termination sequence (cPPT/CTS)23 from HIV-1 that was inserted 5' of the internal CMV promoter.
Production of Lentiviral Supernatants
Vector DNA was transiently introduced into 293T cells by triple cotransfection (calcium phosphate coprecipitation) with the packaging construct pCMV
R8.9124 and the pseudotyping construct pMD2.VSVG (kindly provided by Luigi Naldini, Turino, Italy)25 coding for the vesicular stomatitis virus glycoprotein. Viral supernatants were concentrated by ultracentrifugation. Virus titers were determined as 293T-transducing units (transducing units per milliliter).
Transduction of CBEC
CBECs were transduced in the presence of 4 µg/mL protamine sulfate (Sigma) and 50 µmol/L dNTPs (New England Biolabs). After spinoculation (1250g, 90 minutes, 32°C), the cells were incubated for an additional 16 hours at 37°C with the virus. EGFP expression was analyzed by FACS analysis and fluorescence microscopy at various time points.
FVIII Quantification
FVIII antigen (FVIII:Ag) and FVIII activity (FVIII:C) were determined with the Immunozym FVIII:Ag ELISA and the Immunochrom FVIII:C chromogenic assay, respectively (Immuno). FVIII levels are given as international units (IU) per milliliter, with 150 ng/mL corresponding to 1.0 IU/mL. FVIII standards in both assays were calibrated against WHO plasma standards by the manufacturer.
FVIII Western Blot
Confluent layers of CBECs were cultured with EGM-2 without FCS for 15 to 24 hours, because the serum content would not have allowed efficient FVIII concentration. Supernatants were filtered (0.22 µm) and concentrated up to 400-fold by ultrafiltration in Vivaspin 20 concentrators (Sartorius) with a molecular weight cutoff of 30 kDa. After SDS-PAGE and blotting, FVIII protein was visualized by incubation with polyclonal sheep anti-human factor VIII:C antibody (Enzyme Research Laboratories) and peroxidase-conjugated donkey anti-sheep IgG secondary antibody (Sigma) followed by enhanced chemiluminescence (Pierce). As positive controls, we used recombinant B-domaindeleted (BDD) human FVIII ReFacto (Pharmacia and Upjohn) and plasma-derived human FVIII (Octanate; kindly provided by Lothar Biesert, Octapharma, Frankfurt, Germany).
In Vitro Matrigel Assay
Prechilled 24-well plates were coated with 500 µL Matrigel basement membrane matrix (BD Biosciences) per well and incubated for 1 hour at 37°C. CBECs were seeded on top of the gelled Matrigel at 6x104 to 1x105 cells in 400 µL. Cultures were incubated at 37°C. After 8 to 10 hours, the cultures were checked for tube formation by phase contrast and fluorescence microscopy (Nikon Eclipse TE300).
Statistical Analysis
Data are presented as mean±SD. The means of transduction efficiencies and FVIII secretion levels of cells transduced at different multiplicities of infection (MOIs) were compared with the use of a 2-tailed paired Students t test. Statistical analysis was performed using the GraphPad Prism 3.0 software.
| Results |
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30 to 35 hours (denoted as WT).
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Cultures with CB mononuclear cells before CD34 isolation and with the CD34-depleted fraction either did not yield any detectable CBEC differentiation under otherwise identical conditions or yielded a considerably lower cell number (<1%). This strongly indicates that the isolation of CD34-expressing cells was important to enrich for CBECs or that CD34-negative cells may have inhibited endothelial cell differentiation. Although we did not attempt to prove this on a single-cell level, our data suggest that the CBECs were derived from CD34+ progenitor cells.
Phenotypic Characterization of CBECs
The adherent cells grew as monolayers of spindle-shaped flat cells (Figure 2). The cells were able to incorporate DiI-Ac-LDL (Figure 2) and showed formation of tubules in the Matrigel assay (see below). The cells were found to be uniformly positive for
E-cadherin (CD144), CD146, and CD31, which are typical of endothelial cells. The cells were uniformly negative for the hematopoietic markers CD45, CD14, CD133, and HLA-DR (Figure 2). FACS analysis of 5 batches of endothelial cells showed that the cells obtained with our protocol were heterogeneous concerning the expression of CD34 and KDR/VEGF receptor (Figure 2). CD34 expression was detected on 5% to 45% of cells, whereas KDR was weakly expressed by less than 5% of CBECs, as estimated by dot blot analysis (data not shown). The expression of the aforementioned cell-surface markers seemed to be unchanged throughout cell expansion (data not shown), suggesting that a population of endothelial progenitor cells was maintained throughout the expansion, which gave rise to the CBECs. Additionally, the expression of the endothelial marker von Willebrand factor and binding of Ulex europaeus agglutinin could be demonstrated by immunohistochemical analysis (data not shown).
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Transduction Efficiency
CBECs from 4 different pools of donors were transduced with the constructs pHR'SIN.cPPT-SEW encoding EGFP and cPPT-C(FVIII
B)IGWS encoding human BDD FVIII and EGFP. MOIs of 10 yielded transduction efficiencies of 88.2%±7.6% and 76.9%±5.0% with pHR'SIN.cPPT-SEW and cPPT-C(FVIII
B)IGWS, respectively. When the MOIs were raised to 100, there was no statistically significant increase in transduction efficiencies (90.4%±9.5% and 90.6%±9.7%, respectively; P>0.05 for both vectors). The percentage of EGFP-expressing cells as well as the EGFP expression level remained relatively unchanged throughout the expansion, suggesting that endothelial progenitor cells have been successfully transduced. Figure 3 shows the EGFP expression of transduced versus nontransduced cells 30 days after transduction with MOIs of 10 in a representative experiment.
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Influence of Lentiviral Transduction on CBEC Phenotype and Proliferation
We observed some cell death during and after transduction (below 10% with the lower MOI and 20% to 50% with the higher MOI) and a lag phase before the cells recovered to take up again the proliferation rate of untransduced control cells, as illustrated in Figure 1. To investigate whether delayed proliferation may be accompanied by alterations of the cell phenotype, we analyzed the expression of several surface markers of transduced cells several weeks after transduction. The expression of CD146, CD34, and KDR was unchanged in comparison to untransduced cells (data not shown), as was the uptake of DiI-Ac-LDL (Figures 4A and 4B). In addition, transduced CBECs retained their ability to form tubes in the Matrigel assay (Figures 4C and 4D), which is characteristic of functional endothelial cells.
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Quantification of Secreted FVIII Protein and Coagulation Activity
To investigate whether the transduction of CBECs with the BDD FVIII cDNA has also included endothelial progenitor cells, which would be necessary to allow expansion of these cells, the capacity of CBECs to express BDD FVIII was investigated at 5 time points throughout the expansion period. Supernatants of cells transduced with construct cPPT-C(FVIII
B)IGWS were analyzed for FVIII coagulation activity (FVIII:C) using a chromogenic assay. It seemed that the expression of FVIII was relatively constant over the expansion period, with high amounts of FVIII secreted. During the first 4 weeks after transduction of cells from 3 pools of donors, mean FVIII:C levels were 0.35 to 0.39 IU/5x104 cells per 48 hours at an MOI of 10, corresponding to 7.0 to 7.8 IU/106 cells per 48 hours (Figure 5A). FVIII:C secretion then decreased in all cultures, which was accompanied by reduced proliferation and finally senescence of CBECs.
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In a second set of experiments, it was analyzed whether higher MOIs additionally increase the amount of FVIII protein secreted. However, raising the MOI from 10 to 100 led only to a slight increase in FVIII:C secretion (P>0.05, n=4). Cells that were transduced with the control construct pHR'SIN.cPPT-SEW and untransduced cells did not secrete detectable amounts of FVIII:C (all <0.01 IU/5x104 cells per 48 hours; n=7).
To compare the amount of coagulation-active FVIII protein (FVIII:C) to the total amount of FVIII antigen secreted (FVIII:Ag), the supernatants were additionally analyzed using an FVIII:Ag-specific ELISA. In fact, the amount of FVIII protein secreted exceeded that of the functionally active FVIII. FVIII:Ag levels in the same CBEC supernatants were 0.45 to 0.66 IU/5x104 cells per 48 hours at an MOI of 10 compared with FVIII:C levels of 0.35 to 0.39 IU/5x104 cells per 48 hours (n=3). This resulted in mean ratios of FVIII:C/FVIII:Ag of 0.54 to 0.83 (n=3).
Characterization of Secreted FVIII by Western Blot Analysis
To additionally characterize the recombinant coagulation FVIII released by CBECs, Western blot analysis of the FVIII protein was performed. FVIII was enriched from CBEC supernatants by ultrafiltration and analyzed by SDS-PAGE and immunoblotting. As reference, we used plasma-derived FVIII (Octanate) and recombinant BDD FVIII (ReFacto). Octanate consists of a doublet of light chains of
80 kDa and heavy chains of various sizes attributable to multiple proteolytic steps involving the B domain in vivo. ReFacto is composed of an 80-kDa doublet of light chains and a 90-kDa heavy chain. The deletion of the B domain used to express ReFacto is almost identical to the deletion in construct cPPT-C(FVIII
B)IGWS. cPPT-C(FVIII
B)IGWS-transduced CBECs secreted FVIII protein correctly cleaved into heavy chain (90 kDa) and light chain (80 kDa) with only a small fraction of uncleaved FVIII protein (170 kDa) (Figure 5B), indicating that the intracellular cleavage of FVIII into heavy chain and light chain might not have been complete. No FVIII protein could be detected in concentrated supernatants of untransduced cells.
| Discussion |
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The amount of FVIII secreted was even higher when the total FVIII protein was analyzed by ELISA, which showed that only 54% to 83% of the secreted FVIII protein was active. This reduction in specific activity, which was not observed in HUVECs, could be caused by incomplete intracellular proteolysis, because Western blotting of concentrated cell supernatant demonstrated that a small part of the FVIII protein was secreted as a 170-kDa precursor whereas the major part of the FVIII protein was already cleaved into the active heavy and light chains of 90 and 80 kDa, respectively. It can be assumed that the FVIII protein secreted as a precursor protein will be additionally cleaved in the plasma of hemophilia patients when clinically applied. The high levels of FVIII secretion by CBECs might reflect the fact that at least 1 subtype of endothelial cells, the liver sinusoidal endothelial cells, is one of the physiological sites of FVIII expression in vivo.6,7 The data presented here (together with the study by Lin et al20) suggest that endothelial cells from extrahepatic origin possess the complex set of proteins necessary for the posttranslational processing of FVIII, which includes folding, glycosylation, sulfation, and proteolytic cleavage. Hence, this study indicates that CBECs seem to be particularly suited for the recombinant expression of FVIII and that the lentiviral vector allows for considerably higher and more efficient recombinant FVIII expression than previously used viral or nonviral expression vectors.
In addition, we propose CBECs as alternate cell source for therapeutic applications in inherited coagulation disorders, such as hemophilia A gene therapy. The establishment of a successful ex vivo gene therapeutic approach to treat hemophilia A patients depends on the identification of target cells that meet most or all of the following demands. Apart from the crucial requirement to secrete procoagulant FVIII protein, the target cells have to be easily accessible and easy to transfect or transduce. They also need to be expandable in vitro or in vivo under controlled conditions, and they or their progeny should have direct access to the circulation. The use of CBECs for patients implies the use of large, constantly replenished storage banks because of the finite proliferative potential of cells. However, a finite proliferative potential may be a prerequisite for vector-mediated gene transfer as issues of insertional mutagenesis resurface.28 Although CBECs seem to be suitable for autologous applications, they may also be valuable in allogeneic settings using immunoisolation devices or alginate encapsulation.29 An important question in the context of gene therapy regards the immunogenicity of the transduced cells. The use of a self-inactivating lentiviral gene transfer vector with a deletion in the 3'-long terminal repeat entails that the vector cannot replicate within the transduced cells and that no viral proteins are expressed from vectors that are comparable to the ones used in this study. Therefore, the only immunologically relevant proteins are the FVIII protein and the EGFP protein, the latter of which would not be required for therapeutic applications. Whether FVIII-transduced CBECs can trigger FVIII-specific immune responses in vivo has not been investigated yet.
In extension of a previous study using blood outgrowth endothelial cells,20 we have identified CB-derived EPCs and their progeny as potential novel target cells for gene therapy of hemophilia. The properties of EPCs isolated from different origins, such as peripheral blood, CB, or bone marrow, have not yet been thoroughly characterized and compared. Circulating CD34+ cells of all of these sources contribute to the formation of endothelium in vivo. However, the stimuli that induce mobilization of these cells and those that determine their migration into specific tissues are poorly understood. These stimuli seem to be released under pathological conditions, such as ischemia in myocardium or retina, after exposition to ionizing radiation or during tumor angiogenesis.3032 Although the development of therapeutic strategies using EPCs is very appealing, engraftment will depend on factors that favor the persistence of ex vivomodified EPCs. The factors allowing optimal local engraftment of EPCs are presently unknown, as are the advantages or disadvantages of EPCs from different origins, such as CB and peripheral blood.
In recent studies, gene therapy using gene-modified endothelial cells was usually discussed in the context of cardiovascular diseases and the support of neovascularization in ischemic tissues33 or the context of cancer therapy to inhibit angiogenesis in solid tumors.34 This study presents a novel cell type that can readily be transduced with lentiviral vectors and that might therefore prove valuable for vascular regeneration and cancer gene therapy using gene-modified EPCs. However, the study emphasizes a wider field of application for gene-modified endothelial cells. In particular, we propose the use of CBECs for gene therapy of hemophilia A and other congenital disorders that are characterized by the absence of plasma proteins such as factor IX, von Willebrand factor, or
1-antitrypsin.
| Acknowledgments |
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This work was supported by a grant of the Stiftung Hämotherapie-Forschung to T.T. and a grant from the Biotest AG (Dreieich, Germany). The authors thank the doctors and midwives of the Kreiskrankenhaus Neuperlach (Munich, Germany) for their support of the research and cord blood.
| Footnotes |
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Received August 15, 2003; accepted September 23, 2003.
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J. D. Pearson Using Endothelial Progenitor Cells for Gene Therapy Arterioscler Thromb Vasc Biol, December 1, 2003; 23(12): 2117 - 2118. [Full Text] [PDF] |
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