Late-Outgrowth Endothelial Cells Attenuate Intimal Hyperplasia Contributed by Mesenchymal Stem Cells After Vascular Injury
Objectives— Mesenchymal stem cells (MSCs) are one of a number of cell types undergoing extensive investigation for cardiac regeneration therapy. It has not yet been determined whether this cell therapy also substantially contributes to vascular remodeling of diseased vessels.
Methods and Results— Human MSCs and a variety of progenitor and vascular cells were used for in vitro and in vivo experiments. Wire-induced vascular injury mobilized MSCs into the circulation. Compared with human aortic smooth muscle cells, MSCs exhibited a 2.8-fold increase in the adhesion capacity in vitro (P<0.001) and a 6.3-fold increase in vivo (P<0.001). In all animal models, a significant amount of MSCs contributed to intimal hyperplasia after vascular injury. MSCs were able to differentiate into cells of endothelial or smooth muscle lineage. Coculture experiments demonstrated that late-outgrowth endothelial cells (OECs) guided MSCs to differentiate toward an endothelial lineage through a paracrine effects. In vivo, cell therapy with OECs significantly attenuated the thickness of the neointima contributed by MSCs (intima/media ratio, from 3.2±0.4 to 0.4±0.1, P<0.001).
Conclusions— Tissue regeneration therapy with MSCs or cell populations containing MSCs requires a strategy to attenuate the high potential of MSCs to develop intimal hyperplasia on diseased vessels.
Stem cell therapy has a promising future for the treatment of a variety of end-stage cardiovascular diseases such as heart failure, ischemic heart,1–3 and peripheral artery diseases.4 To date, bone marrow (BM) cells or mobilized peripheral mononuclear cells are among the most often used cell populations in clinical settings because of their convenience and autologous properties.4,5 Recently, the MAGIC cell randomized clinical trial, however, showed that intracoronary infusion of peripheral blood stem cells mobilized with granulocyte colony stimulating factor (G-CSF) aggravated restenosis after coronary stenting in myocardial infarction.6 Based on the heterogeneous populations of these cells and the differentiation capacity of stem cells, uncontrolled differentiation is a critical concern when applying these cells to humans.
Mesenchymal stem cells (MSCs) reside in the BM and can also be mobilized in response to G-CSF stimulation.7 MSCs from the BM are multipotent and have the capacity to differentiate into cardiomyocytes,8 endothelial cells (ECs),9,10 and smooth muscle cells (SMCs).11 Currently, MSCs are one of the cell types being used in clinical trials for postmyocardial infarction cardiac regeneration therapy.3,12 It has been shown that an intramyocardial injection of autologous MSCs or intravenous administration of MSCs can increase vasculogenesis and improve cardiac function after myocardial infarction in animal experiments and clinical trials.3,12 However, abundant evidence suggests that BM-derived circulating precursors can give rise to ECs and SMCs that contribute to vascular repair, remodeling, and lesion formation under physiological and pathological conditions.13,14 There is a very high possibility that MSCs residing in therapeutic cell populations can adhere to diseased, angioplastic, or stented vessels with cell administration, causing intimal hyperplasia and eventual restenosis. Herein, we sought to determine the contribution of MSCs to neointimal formation after vascular injury, and to investigate possible therapeutic strategies to modulate this contribution.
An expanded Materials and Methods section is available in the online data supplement #1 at http://atvb.ahajournals.org.
Male FVB wild-type, athymic nude (Jackson Laboratory, Bar Harbor, Me) and eGFP transgenic mice (FVB background) that ubiquitously express enhanced green fluorescent (GFP) (Level Biotechnology)15 were bred and maintained in the Laboratory of Animal Experiments at Chang Gung Memorial Hospital. The study protocol was reviewed and approved by the Institutional Animal Care and Use Committee, Chang Gung Memorial Hospital Faculty of Medicine, and the experiments were conducted according to the Guidelines of the American Physiological Society.
Mouse Femoral Wire-Injury Model
Femoral artery injury was induced by inserting a straight spring wire (0.38 mm in diameter, No. C-SF-15-15, Cook) for more than 5 mm toward the iliac artery.14
Mesenchymal Stem Cells Cultured From Peripheral Blood
To culture mouse MSCs in peripheral blood, pooled whole blood was harvested by cardiac puncture from 20 mice for each experiment. MSCs were cultured in 2 groups of mice, including the study group (48 hours after the femoral artery wire injury) and a control (sham-operated) group. After Ficoll density centrifugation (Histopaque-1083, density 1.083 g/mL), mononuclear cells (3×106) were plated on 6-cm dishes to culture the MSCs as described above.
Dye-Transfer Assay for Gap Junction Intercellular Communication
Lucifer yellow (LY; Mr: 457 Da; 0.2%) and rhodamine dextran (RD; Mr:10 kDa) were microinjected into cells. LY can penetrate through gap junction channels; however, RD is too large to pass through gap junction channels and therefore served as a tracer dye for cells originally receiving the dye. Transfer of the dye was visualized using a phase-contrast fluorescence microscope (Axioscope; Carl Zeiss, Jena, Germany), in which LY and RD were respectively detected as green and red. DAPI was used to label nuclei (blue). Five minutes after the donor cell was injected, the presence of dye transfer to other cells was estimated. On average, 25 microinjections were tried for each experiment.
MSCs Have a High Capacity for Adhering to the Matrix or the Remodeled Vessel Wall After Injury
The capacity for adhesion was investigated in a variety of human progenitor and vascular cells both in vitro and in vivo. In vitro, compared with other progenitor and vascular cells, MSCs had the strongest adhesion capacity to both fibronectin and collagen (Figure 1A, left and middle panels). Furthermore, mixing OECs with MSCs significantly increased the adhesion of OECs to the coated matrix (Figure 1A, right panel). In vivo, progenitor or vascular cells were directly injected via the abdominal aorta of mice 7 days after the right femoral artery wire injury. MSCs had the best adherence to the injured vessel wall compared with all other types of cells (Figure 1B). No adherence of injected cells was found in either small arterioles or the uninjured left femoral artery, suggesting that the injected cells did not cause cell embolism and were unable to adhere to intact and healthy vascular surfaces. The injected MSCs were also unable to adhere to the vessels 1 day after wire injury (data not shown), suggesting that MSC adhesion required matrix formation on the remodeled injured vessels, or the adherent platelets covered on the injured vascular surface interfered with MSC attachment to the underlying matrix.
Evidence of MSC Mobilization Into the Circulation
Blood G-CSF, VEGF, SDF-1α, and SCF concentrations were measured at serial time points in mice after femoral artery wire injury (supplemental Figure IA). The levels of these blood cytokines, especially G-CSF and VEGF, significantly increased within 24 hours. Blood G-CSF concentrations increased up to 8-fold 12 hours after vascular injury. Then, standard MSC cultivation was performed on mononuclear cells purified from pooled peripheral blood of mice 48 hours after wire injury. From the same amount of peripheral blood mononuclear cells, colonies of MSCs in the wire-injury group could be subcultured and maintained for long-term culture (at least 10 passages), whereas colonies from the controls could not be maintained for longer than 2 passages (supplemental Figure IB). MSCs cultured from the wire-injury group had homogeneous morphologies, were Lin−CD45−Sca-1+CD31− (supplemental Figure II), and were multipotent, as they were capable of differentiating into osteocytes (positive for Von Kossa staining, alkaline phosphatase, and bone morphogenetic protein-2) or adipocytes (positive for Oil Red O) in response to different differentiation media (supplemental Figure IC), and into ECs and SMCs (described as follows).
Contribution of MSCs to Intimal Hyperplasia
To investigate whether MSCs contribute to neointimal formation in vivo, studies were performed using 2 models, “cell therapy” and “physiological” models. In the cell therapy model, BM-derived MSCs from eGFP mice were injected via the tail vein into wild-type littermates 7 days after femoral artery wire injury (Figure 2). One day after the injection, scattered eGFP MSCs were found on the surface of injured vessels (Figure 2A). Four weeks after cell injection, clusters of eGFP MSCs were identified in the neointima (Figure 2B). Most of the eGFP MSCs were also α-SMA–positive. On average, 31%±15% of cells in the neointima were from the injected eGFP MSCs.
In the physiological model, eGFP MSCs and radioprotective whole eGFP-negative BM cells were transplanted into the tibia and the via tail vein, respectively, of wild-type littermates after 1000 cGy of whole-body irradiation (Figure 3A). Two months after MSC BM transplantation, femoral artery wire injury was performed. Four weeks later, the injured vessels had developed remarkable intimal hyperplasia, which contained a significant amount of eGFP-positive cells indicating the contribution of BM MSCs (39%±17%) (Figure 3B, lower panel, D28). In addition, flow cytometry and fluorescent microscopy proved successful engraftment of eGFP MSCs in the BM (Figure 3A).
Differentiation of MSCs Into Neointimal Cells
In mice after intra-BM eGFP MSC transplantation, the eGFP+ MSCs in neointima differentiated into a few different cell types. Some of the eGFP+ cells were α-SMA+, some were von Willebrand factor (vWF)+ or triple positive, and the others were pure eGFP+ on day 21 after the wire injury (Figure 3B, upper panel, D21). However, when using CD31 as a marker of highly differentiated ECs, only cells over the surface of the neointima were eGFP+CD31+ (Figure 3B, middle panel, D21). These findings suggested that mobilized BM MSCs underwent a differentiation process into either SMC- or EC-lineage cells. When injured vessels were investigated at late time points such as 4 weeks after wire injury, immunostaining of vWF and α-SMA was located only on the surface and the body of the neointima, respectively, suggesting that the differentiation processes had ceased (Figure 3B, lower panel, D28).
Modulation of the Contribution of MSCs to the Neointima by Cell Therapy
The findings described above raise the possibility of manipulating the differentiation of MSCs, which had adhered to the injured vascular surface, into the endothelial lineage to achieve early reendothelialization. Thus, we attempted to use cell therapy to modulate MSC differentiation by mixing eGFP mouse MSCs with human EPCs or OECs. Using the tail vein injection model, cell therapy with OECs significantly attenuated the contribution of eGFP MSCs and non MSC-derived SMCs to intimal hyperplasia (intima/media ratio, from 3.2±0.4 to 0.4±0.1, P<0.001) (Figure 4A, upper panel and Figure 4C). eGFP cells were either purely αSMA-positive or purely vWF-positive in the neointima 4 weeks after femoral artery wire injury. A subset of ECs was also positive for HLA-ABC, a marker specific for humans (Figure 4A, lower panel). These findings suggested that OEC therapy causes early re-endothelialization either by OECs themselves or by guiding the injected eGFP MSCs to differentiate into ECs. However, EPC therapy or OEC alone was unable to significantly attenuate the thickness of intimal hyperplasia (Figure 4B and 4C). To investigate the speed of reendothelialization, en face immunostaining was performed on the entire femoral artery 10 days after wire injury. There was more extensive endothelial covering of the injured femoral arteries in the “MSC+OEC” group, compared with the controls (Figure 4D).
Coculture Experiment: Cell Therapy Guides the Fate of MSCs
To clarify the effect of cell therapy on MSC differentiation, coculture experiments were performed by coculturing eGFP mouse MSCs with human OECs. MSCs did not express the endothelial marker, vWF, when cultured alone. However, when eGFP MSCs were cocultured with OECs, MSCs began expressing vWF by D14 (supplemental Figure IIIA). Further experiments were performed to address whether the effect of coculturing occurs through direct cell–cell interactions or via a paracrine effect. When the culture medium was enriched with endothelial growth factors (EGM-2), MSCs in both the coculture group and the MSC-only group transcribed a variety of EC-specific mRNAs (supplemental Figure IIIB). However, when culture medium without any growth factor was used (EBM-2), only MSCs in the coculture group transcribed KDR mRNA on D7 and D14 (supplemental Figure IIIC). A dye-transfer assay for gap junction intercellular communication was performed on the cocultured cells. Microinjection to either MSCs or OECs demonstrated only OEC-OEC and MSC-MSC dye transmission with no intercellular communication between OECs and MSCs (supplemental Figure IIID). Immunostaining demonstrated no connexin-43 formation between OECs and MSCs (supplemental Figure IIIE).
We have demonstrated that BM-derived MSCs can be mobilized after vascular injury and that they have a high potential to participate in the remodeling processes of the injured vasculature. In vitro and, particularly, in vivo, MSCs exhibited a strong capacity for adhesion to either the coated matrix or remodeled vessel wall after injury. On adhesion, a substantial proportion of MSCs proliferated and differentiated into SMCs and ECs in the neointima. Cell therapy with OECs modulated the differentiation of MSCs toward an endothelial-like lineage, leading to early reendothelialization and attenuation of intimal hyperplasia.
Over the past few years, we have witnessed a paradigm shift in our understanding of the underlying principles governing intimal hyperplasia in response to vascular injury. The process, formerly ascribed to a local medial vascular smooth muscle response, appears to be partially and systemically governed by cells from the BM.13,14 The MAGIC clinical trial performed G-CSF mobilization of BM stem cells in patients with acute myocardial infarction who underwent coronary stenting.6 In fact, the study ended prematurely because patients receiving G-CSF experienced an unexpectedly high rate of restenosis at the stent site. A close correlation between the gain in neointimal volume and improvements in systolic function was noted in the cell infusion group, suggesting that stem-cell therapy accelerated neointimal growth in proportion to the efficacy of cardiac regeneration. Our study highlights the possible role of MSCs in this adverse effect.
To date, a variety of cell populations have been applied to stem cell therapeutic trials, including BM-derived mononuclear cells,4 G-CSF-mobilized peripheral blood stem cells,6 skeletal myoblasts,16 and MSCs.3 We speculate that most of these cell populations recruit MSCs as well. In addition, to delete MSCs from the applied cell populations is technically difficult and may also influence the potentially beneficial contribution of MSCs to cardiac regeneration or angiogenesis. MSCs are highly proliferative and can differentiate into cardiomyocytes,8 ECs,9,10 or SMCs.11 In contrast to their hematopoietic counterparts, our data demonstrated that MSCs have a strikingly enhanced ability to adhere to matrix-coated tissue-culture surfaces and to remodeled vessel wall after injury, as compared with a variety of other progenitor/vascular cells. This phenomenon is supported by data showing that MSCs express several receptors associated with matrix- and cell-to-cell adhesive interactions.11 Furthermore, our study, for the first time, linked MSCs to the excessive repair processes of vessels in response to injury.
Human and a few mammalian MSCs were reported to be mobilized into the systemic circulation in response to G-CSF treatment,7 leading to engraftment at peripheral organ sites and differentiation according to the niche and repair of injured tissues.7,17–19 Evidence showed that stem cells with the capacity to differentiate into cardiomyocytes are actually mesenchymal rather than hematopoietic stem cells,8 suggesting that MSCs have a higher potential to differentiate into cells with a muscular phenotype, compared with HSCs. As shown in our study, vascular injury caused an approximately 8-fold increase in blood G-CSF concentrations along with modest elevations in other cytokines such as SCF, SDF-1α, and VEGF, providing an environment potentially optimal for MSC mobilization. Being able to culture MSCs from the peripheral blood of the wire-injured mice lends further support to the evidence for MSC mobilization.
The origins of cells contributing to intimal hyperplasia are diverse including local SMCs, BM-derived vascular progenitors, stem cells in the adventitia, as well as additional cell types.20 Because a tremendous amount of medial vascular SMCs underwent apoptosis in the animal model adopted in this study,21 a significant portion of vascular repair depends on systemically mobilized cells.13 Using the entire BM transplantation model with an irradiation dose of 1000 cGy, most of the reconstituted BM cells were hematopoietic cells. However, the estimated D0 (the radiation dose that reduces survival to 37%) of MSCs is 1.3 to 1.4 Gy, and 1 Gy already induces death in a significant portion of MSCs.22 Previously, Fukuda et al showed that reconstituted BM cells contained non-hematopoietic cells when an irradiation dose of 1050 cGy was used.8 Using an intra-BM MSC transplantation model, they also demonstrated that MSCs, but not HSCs, in the BM contributed to the regeneration of myocardial tissue after myocardial infarction. All of these findings support the hypothesis that MSCs can participate in the process of post wire-injury vascular remodeling. In line with this, our data showed that MSCs seeded onto the surface of injured vessel wall 7 days after wire injury and proliferated in a nodular or clustered pattern. These adherent MSCs proliferated and differentiated into both SMC- and EC-like cells. Because only a small portion of MSCs were reconstituted in our intra-BM transplantation model, the proportion of intimal hyperplasia contributed to by MSCs was probably more than that estimated by our study.
In an undifferentiated state, MSCs do not express EC surface markers such as CD31 or CD34.9 However, recent work has shown that altering culture conditions can render MSCs capable of differentiating into ECs.9,10 All these attributes make MSCs an interesting cell phenotype for investigation in light of their potential to differentiate into mesoderm-derived ECs23 and their ability to differentiate in vivo into ECs.24,25 As shown in our study, although coculture with OECs may help MSCs express EC phenotypes, VEGF-enriched medium alone can achieve this effect as well, in line with findings by Oswald et al.26 Because both our dye-transfer assay and connexin-43 immunostaining suggested no direct communication between OECs and MSCs, the influence of OECs on MSC differentiation is suggested to occur through a paracrine effect.
Regarding approaches to attenuate neointimal formation, numerous medical therapeutic strategies, including cell therapy, have been investigated. Current concepts support the hypothesis that the earlier that reendothelialization is achieved, the less neointima that forms. Although OECs alone have an inadequate capacity for adhesion, mixing OECs with MSCs is herein suggested to increase the adhesion of OECs to injured vessel walls via the mediation of MSCs between OECs and matrix formed on the injured vessels. It has been reported that early EPCs secrete more VEGF than do OECs.27 However, the influence of EPCs on MSC differentiation appears to be much less than that of OECs. One of the possible reasons is that the life-span of early EPCs is much shorter than that of the late OECs (A brief introduction to early EPCs and OECs is given in the online supplement #2.).27 Our data suggest that a coinfusion of OECs and MSCs may help achieve early reendothelialization. These findings can also explain the discrepancy among reports regarding the adverse effects of stem cell therapy on atherosclerosis,6,28 because there is a wide variation in the amount of endothelial progenitors in harvested BM cells and in mobilized cells after G-CSF treatment in different individuals.
In summary, our data clearly demonstrated the potential of stem cell therapy to contribute to atherosclerosis or poststenting restenosis. MSCs, either spontaneously mobilized from BM or delivered directly by cell therapy, are 2 possible sources of cells in intimal hyperplasia. The amount of MSC mobilization depends on the effect of cytokines released in response to the severity of the vascular injury. However, cell therapy containing an MSC population has a direct impact on neointimal formation. Although the contribution of MSCs and the repair processes carried out by local SMCs after vascular injury are individualized in different subjects, interactions between OECs and MSCs were demonstrated herein to accelerate reendothelialization with beneficial effects on regulating local SMCs and MSCs, leading to remarkable attenuation in intimal hyperplasia. In a clinical setting, it is suggested that before cells are applied to the target organs, cell manipulation should be attempted either by combined-cell therapy or by medical intervention to raise the content of endothelial progenitors.
We thank Christine Holmes and Yi-Chun Lin for technical assistance with the cell culture and microinjection.
Sources of Funding
This work was supported in part by National Science Council of Taiwan (NSC 95-2314-B-182A-046 and NSC 94-2314-B-182A-192).
Original received May 3, 2007; final version accepted October 17, 2007.
Britten MB, Abolmaali ND, Assmus B, Lehmann R, Honold J, Schmitt J, Vogl TJ, Martin H, Schachinger V, Dimmeler S, Zeiher AM. Infarct Remodeling After Intracoronary Progenitor Cell Treatment in Patients With Acute Myocardial Infarction (TOPCARE-AMI): mechanistic insights from serial contrast-enhanced magnetic resonance imaging. Circulation. 2003; 108: 2212–2218.
Zhang S, Wang D, Estrov Z, Raj S, Willerson JT, Yeh ETH. Both cell fusion and transdifferentiation account for the transformation of human peripheral blood CD34-positive cells into cardiomyocytes in vivo. Circulation. 2004; 110: 3803–3807.
Chen Sl, Fang Ww, Ye F, Liu YH, Qian J, Shan Sj, Zhang Jj, Chunhua RZ, Liao Lm, Lin S, Sun Jp. Effect on left ventricular function of intracoronary transplantation of autologous bone marrow mesenchymal stem cell in patients with acute myocardial infarction. Am J Cardiol. 2004; 94: 92–95.
Tateishi-Yuyama E, Matsubara H, Murohara T, Ikeda U, Shintani S, Masaki H, Amano K, Kishimoto Y, Yoshimoto K, Akashi H, Shimada K, Iwasaka T, Imaizumi T. Therapeutic angiogenesis for patients with limb ischaemia by autologous transplantation of bone-marrow cells: a pilot study and a randomised controlled trial. Lancet. 2002; 360: 427–435.
Tateno K, Minamino T, Toko H, Akazawa H, Shimizu N, Takeda S, Kunieda T, Miyauchi H, Oyama T, Matsuura K, Nishi Ji, Kobayashi Y, Nagai T, Kuwabara Y, Iwakura Y, Nomura F, Saito Y, Komuro I. Critical roles of muscle-secreted angiogenic factors in therapeutic neovascularization. Circ Res. 2006; 98: 1194–1202.
Kang HJ, Kim HS, Zhang SY, Park KW, Cho HJ, Koo BK, Kim YJ, Lee DS, Sohn DW, Han KS. Effects of intracoronary infusion of peripheral blood stem-cells mobilised with granulocyte-colony stimulating factor on left ventricular systolic function and restenosis after coronary stenting in myocardial infarction: the MAGIC cell randomised clinical trial. The Lancet. 2004; 363: 751–756.
Kocher AA, Schuster MD, Szabolcs MJ, Takuma S, Burkhoff D, Wang J, Homma S, Edwards NM, Itescu S. Neovascularization of ischemic myocardium by human bone-marrow-derived angioblasts prevents cardiomyocyte apoptosis, reduces remodeling and improves cardiac function. Nat Med. 2001; 7: 430–436.
Kawada H, Fujita J, Kinjo K, Matsuzaki Y, Tsuma M, Miyatake H, Muguruma Y, Tsuboi K, Itabashi Y, Ikeda Y, Ogawa S, Okano H, Hotta T, Ando K, Fukuda K. Nonhematopoietic mesenchymal stem cells can be mobilized and differentiate into cardiomyocytes after myocardial infarction. Blood. 2004; 104: 3581–3587.
Minguell JJ, Erices A, Conget P. Mesenchymal Stem Cells. Expe Biol Med. 2001; 226: 507–520.
Wang CH, Ciliberti N, Li SH, Szmitko PE, Weisel RD, Fedak PW, Al Omran M, Cherng WJ, Li RK, Stanford WL, Verma S. Rosiglitazone facilitates angiogenic progenitor cell differentiation toward endothelial lineage: a new paradigm in glitazone pleiotropy. Circulation. 2004; 109: 1392–1400.
Herreros J, Prosper F, Perez A, Gavira JJ, Garcia-Velloso MJ, Barba J, Sanchez PL, Canizo C, Rabago G, Marti-Climent JM, Hernandez M, Lopez-Holgado N, Gonzalez-Santos JM, Martin-Luengo C, Alegria E. Autologous intramyocardial injection of cultured skeletal muscle-derived stem cells in patients with non-acute myocardial infarction. Eur Heart J. 2003; 24: 2012–2020.
Barbash IM, Chouraqui P, Baron J, Feinberg MS, Etzion S, Tessone A, Miller L, Guetta E, Zipori D, Kedes LH, Kloner RA, Leor J. Systemic delivery of bone marrow-derived mesenchymal stem cells to the infarcted myocardium: feasibility, cell migration, and body distribution. Circulation. 2003; 108: 863–868.
Prockop DJ. Marrow stromal cells as stem cells for nonhematopoietic tissues. Science. 1997; 276: 71–74.
Bittira B, Shum-Tim D, Al-Khaldi A, Chiu RCJ. Mobilization and homing of bone marrow stromal cells in myocardial infarction. Eur J Cardiothorac Surg. 2003; 24: 393–398.
Gulati R, Jevremovic D, Peterson TE, Chatterjee S, Shah V, Vile RG, Simari RD. Diverse origin and function of cells with endothelial phenotype obtained from adult human blood. Circ Res. 2003; 93: 1023–1025.
Sata M, Maejima Y, Adachi F, Fukino K, Saiura A, Sugiura S, Aoyagi T, Imai Y, Kurihara H, Kimura K, Omata M, Makuuchi M, Hirata Y, Nagai R. A mouse model of vascular injury that induces rapid onset of medial cell apoptosis followed by reproducible neointimal hyperplasia. J Mol Cell Cardiol. 2000; 32: 2097–2104.
Reyes M, Lund T, Lenvik T, Aguiar D, Koodie L, Verfaillie CM. Purification and ex vivo expansion of postnatal human marrow mesodermal progenitor cells. Blood. 2001; 98: 2615–2625.
Yoon CH, Hur J, Park KW, Kim JH, Lee CS, Oh IY, Kim TY, Cho HJ, Kang HJ, Chae IH, Yang HK, Oh BH, Park YB, Kim HS. Synergistic neovascularization by mixed transplantation of early endothelial progenitor cells and late outgrowth endothelial cells: the role of angiogenic cytokines and matrix metalloproteinases. Circulation. 2005; 112: 1618–1627.
Ripa RS, Jorgensen E, Wang Y, Thune JJ, Nilsson JC, Sondergaard L, Johnsen HE, Kober L, Grande P, Kastrup J. Stem cell mobilization induced by subcutaneous granulocyte-colony stimulating factor to improve cardiac regeneration after acute ST-elevation myocardial infarction: result of the double-blind, randomized, placebo-controlled Stem Cells in Myocardial Infarction (STEMMI) Trial. Circulation. 2006; 113: 1983–1992.