Working hypothesis to redefine endothelial progenitor cells

SDF1 gene variation is associated with circulating SDF1alpha level and endothelial progenitor cell number: the Bruneck Study

BACKGROUND

Stromal cell-derived factor-1 (SDF1) and its receptor CXC chemokine receptor 4 (CXCR4) play a critical role in progenitor cell homing, mobilization and differentiation. It would be interesting to assess the predictive value of SDF-1alpha level for EPC number, and to ascertain whether there is a relationship between SDF1 gene variation, plasma SDF-1alpha level, and the number and function of circulating EPCs. We also tested whether EPC number and function was related to CXCR4 gene variation.

METHODOLOGY AND PRINCIPAL FINDINGS

We genotyped a cohort of individuals who participated in the Bruneck Study for single nucleotide polymorphisms (SNPs) in the SDF1 and CXCR4 genes, and measured blood SDF1alpha level as well as EPC number and function. SDF1alpha levels were correlated with age, gender, alcohol consumption, circulating reticulocyte numbers, and concentrations of matrix metalloproteinase-9, C-reactive protein, cystatin C, fibrinogen and homocytein. In blood samples taken in 2005, EPC number was inversely associated with SDF1alpha level (p<0.001). EPC number in 2005 was also inversely associated with SDF1alpha level in 2000 (p = 0.009), suggesting a predictive value of plasma SDF1alpha level for EPC number. There was an association between the SDF1 gene rs2297630 SNP A/A genotype, increased SDF1alpha level (p = 0.002) and lower EPC number (p = 0.006).

CONCLUSIONS

Our data indicate that a SDF1 gene variation (rs2297630) has an influence on SDF1alpha level and circulating EPC number, and that plasma SDF1alpha level is a predictor of EPC number.

Influence of menstrual cycle on circulating endothelial progenitor cells

BACKGROUND

Endothelial progenitor cells (EPCs) are circulating mononuclear cells that participate in angiogenesis. The aim of this study was to determine the influence of the menstrual cycle on the number and function of EPCs, and to investigate their relationship with circulating concentrations of sex steroids and inflammatory mediators.

METHODS

Ten healthy nulliparous, premenopausal, non-smoking women with regular menses were studied over a single menstrual cycle. Venepuncture was performed in the menstrual, follicular, peri-ovulatory and luteal phases. EPCs were quantified by flow cytometry (CD133(+)CD34(+)KDR(+) phenotype) and the colony-forming unit (CFU-EPC) functional assay. Circulating concentrations of estradiol, progesterone and inflammatory mediators (TNF-alpha, IL-6, sICAM-1 and VEGF) were measured by immunoassays.

RESULTS

The numbers of CD133(+)CD34(+)KDR(+) cells were higher in the follicular phase (0.99 +/- 0.3 x 10(6) cells/l) compared with the peri-ovulatory phase (0.29 +/- 0.1 x 10(6) cells/l; P < 0.05). In contrast, the numbers of CFU-EPCs did not vary over the menstrual cycle. There were no correlations between EPCs and concentrations of either circulating sex steroids or inflammatory mediators.

CONCLUSIONS

CD133(+)CD34(+)KDR(+) cells but not CFU-EPCs vary during the menstrual cycle. Our findings suggest a potential role for circulating EPCs in the normal cycle of physiological angiogenesis and repair of the uterine endometrium that is independent of circulating sex steroids or inflammatory mediators.

Oxidative stress impairs endothelial progenitor cell function

Circulating endothelial progenitor cells (EPCs) in adult human peripheral blood were identified in 1997. Since their original identification, EPCs have been extensively studied as biomarkers to assess the risk of cardiovascular disease in human subjects and as a potential cell therapeutic for vascular regeneration. EPCs are exposed to oxidative stress during vascular injury as residents of blood vessel walls or as circulating cells homing to sites of neovascularization. Given the links between oxidative injury, endothelial cell dysfunction, and vascular disease, recent investigation has focused on the responses of EPCs to oxidant stress and the molecular mechanisms that control redox regulation in these specialized cells. In this review, we discuss the various cell and flow-cytometric techniques used to define and isolate EPCs from circulating blood and the current human and mouse genetic data, which offer insights into redox control in EPC biology and angiogenesis. Finally, we review how EPC responses to oxidant stress may be a critical determinant in maintaining the integrity and function of the cardiovascular system and how perturbations of redox control in EPCs may lead to various human diseases.

Isolation and characterization of endothelial progenitor cells from human blood

Circulating endothelial progenitor cells (EPCs) in adult human peripheral blood were originally identified in 1997 by Asahara et al., which challenged the paradigm that vasculogenesis is a process restricted to embryonic development. Since their original identification, EPCs have been extensively studied as biomarkers to assess the risk of cardiovascular disease in human subjects and as a potential cell therapeutic for vascular regeneration. Endothelial colony-forming cells (ECFCs), which are a subtype of EPCs, were recently identified from circulating adult and human umbilical cord blood. In contrast to other types of EPCs, which display various monocyte/macrophage phenotypes and functions, ECFCs are characterized by robust proliferative potential, secondary and tertiary colony formation upon replating, and de novo blood vessel formation in vivo when transplanted into immunodeficient mice. In this unit, we describe detailed methodologies for isolation and characterization of ECFCs from both human peripheral and umbilical cord blood.

Formation of STAT5/PPARgamma transcriptional complex modulates angiogenic cell bioavailability in diabetes

OBJECTIVE

Circulating angiogenic cells (CACs) expansion is a multistage process requiring sequential activation of transcriptional factors, including STAT5. STAT5, in concert with peroxisome proliferator-activated receptors (PPARs), seems to induce discrete biological responses in different tissues. In the present study we investigated the role of STAT5 and PPARgamma in regulating CAC expansion in normal and diabetic settings.

METHODS AND RESULTS

Normal and diabetic CACs were used. siRNA technology, EMSA, and chromatin immunoprecipitation (ChIP) assay as well as site-directed mutagenesis of the STAT5 response element in the PPARgamma promoter enabled us to demonstrate that STAT5 transcriptional activity controls PPARgamma expression. Moreover, FACS analysis, coimmunoprecipitation experiments, and ChIP assay revealed that a STAT5/PPARgamma transcriptional complex controls cyclin D1 expression and CAC progression into the cell-cycle. Conversely, PPARgamma agonists, by preventing the expression of STAT5 and the formation of the STAT5/PPARgamma heterodimeric complex failed to promote CAC expansion. Finally, we demonstrated that diabetic CAC functional capability can be recovered by molecules able to activate the STAT5/PPARgamma transcriptional complex.

CONCLUSIONS

Our data identify the STAT5/PPARgamma heterodimers as landmark of CAC expansion and provide evidences for a mechanism that partially rescues CAC bioavailability in diabetic setting.

Material-based deployment enhances efficacy of endothelial progenitor cells

Cell-based therapies are attractive for revascularizing and regenerating tissues and organs, but clinical trials of endothelial progenitor cell transplantation have not resulted in consistent benefit. We propose a different approach in which a material delivery system is used to create a depot of vascular progenitor cells in vivo that exit over time to repopulate the damaged tissue and participate in regeneration of a vascular network. Microenvironmental conditions sufficient to maintain the viability and outward migration of outgrowth endothelial cells (OECs) have been delineated, and a material incorporating these signals improved engraftment of transplanted cells in ischemic murine hindlimb musculature, and increased blood vessel densities from 260 to 670 vessels per mm(2), compared with direct cell injection. Further, material deployment dramatically improved the efficacy of these cells in salvaging ischemic murine limbs, whereas bolus OEC delivery was ineffective in preventing toe necrosis and foot loss. Finally, material deployment of a combination of OECs with another cell population commonly isolated from peripheral or cord blood, endothelial progenitor cells (EPCs) returned perfusion to normal levels in 40 days, and prevented toe and foot necrosis. Direct injection of an EPC/OEC combination was minimally effective in improving limb perfusion, and untreated limbs underwent autoamputation in 3 days. These results demonstrate that vascular progenitor cell utility is highly dependent on the mode of delivery, and suggest that one can create new vascular beds for a variety of applications with this material-controlled deployment of cells.

Circulating endothelial cells, bone marrow-derived endothelial progenitor cells and proangiogenic hematopoietic cells in cancer: From biology to therapy

Vascularization, a hallmark of tumorigenesis, is classically thought to occur exclusively through angiogenesis (i.e. endothelial sprouting). However, there is a growing body of evidence that endothelial progenitor cells (EPCs) and proangiogenic hematopoietic cells (HCs) are able to support the vascularization of tumors and may therefore play a synergistic role with angiogenesis. An additional cell type being studied in the field of tumor vascularization is the circulating endothelial cell (CEC), whose presence in elevated numbers reflects vascular injury. Levels of EPCs and CECs are reported to correlate with tumor stage and have been evaluated as biomarkers of the efficacy of anticancer/antiangiogenic treatments. Furthermore, because EPCs and subtypes of proangiogenic HCs are actively participating in capillary growth, these cells are attractive potential vehicles for delivering therapeutic molecules. The current paper provides an update on the biology of CECs, EPCs and proangiogenic HCs, and explores the utility of these cell populations for clinical oncology.

Foxc2 transcription factor: a newly described regulator of angiogenesis

Angiogenesis is a critical process to form new blood vessels from preexisting vessels under physiologic and pathologic conditions and involves cellular and morphologic changes such as endothelial cell proliferation, migration, and vascular tube formation. Despite evidence that angiogenic factors, including vascular endothelial growth factor and Notch, control various aspects of angiogenesis, the molecular mechanisms underlying gene regulation in blood vessels and surrounding tissues are not fully understood. Importantly, recent studies demonstrate that Forkhead transcription factor Foxc2 directly regulates expression of various genes involved in angiogenesis, CXCR4, integrin beta3, Delta-like 4 (Dll4), and angiopoietin 2, thereby controlling angiogenic processes. Thus, Foxc2 is now recognized as a novel regulator of vascular formation and remodeling. This review summarizes current knowledge about the function of Foxc2 in angiogenesis and discusses prospects for future research in Foxc2-mediated pathologic angiogenesis in cardiovascular disease.

Leptin enhances the recruitment of endothelial progenitor cells into neointimal lesions after vascular injury by promoting integrin-mediated adhesion

The adipocytokine leptin modulates vascular remodeling and neointima formation. Because endothelial progenitor cells (EPCs) participate in vascular repair, we analyzed the effects of leptin on human EPC function in vitro and in vivo. After 7 days in culture, EPCs expressed the leptin receptor and responded to leptin stimulation with increased STAT3 phosphorylation. Incubation of EPCs with leptin (at concentrations between 1 and 100 ng/mL) increased the number of EPCs adhering to vitronectin and fibronectin in a receptor-specific manner. It also enhanced the capacity of EPCs to incorporate into a monolayer of human endothelial cells and the adherence of these cells to activated platelets. Leptin upregulated alphavbeta5 and alpha4 integrin expression in EPCs, and the effects of leptin on EPC function could be prevented, at least in part, by RGD peptides and function-blocking antibodies. Intravenous injection of fluorescently labeled human EPCs into athymic nude mice shortly after vascular injury revealed that preincubation of EPCs with leptin augmented their accumulation within intimal lesions, accelerating reendothelialization and decreasing neointima formation in an alphavbeta5 and alpha4 integrin-dependent manner. Our findings suggest that leptin specifically modulates the adhesive properties and the homing potential of EPCs and may thus enhance their capacity to promote vascular regeneration in vivo.

New strategies for in vivo tissue engineering by mimicry of homing factors for self-endothelialisation of blood contacting materials

For years intensive research has been done to endothelialise vascular prostheses with autologous endothelial cells before implantation in patients. However, this procedure is extremely time-, labor- and cost-intensive and can be realized only in very few clinical cases. The discovery of circulating endothelial progenitor cells (EPCs) in 1997 brought new perspectives for the endothelialisation of blood contacting materials. Coating of synthetic graft surfaces with capture molecules for circulating EPCs mimics a pro-homing substrate for fishing out EPCs directly from the bloodstream after implantation. These cells with high proliferation potential can cover the graft with non-thrombogenic endothelium which maintains optimal haemostasis and minimize the risk of restenosis. In this review, different concepts are discussed to capture circulating EPCs on synthetic vascular grafts after implantation. We hypothesize that in vivo self-endothelialisation of blood contacting materials by homing factor-mimetic capture molecules for EPCs may bring revolutionary new perspectives towards future clinical application of stem cell and tissue engineering strategies.

Interleukin-3 promotes expansion of hemopoietic-derived CD45+ angiogenic cells and their arterial commitment via STAT5 activation

Interleukin-3 (IL-3) released by infiltrating inflammatory cells in different pathologic settings contributes to organ and tumor angiogenesis. Here we demonstrate that IL-3 expands a subset of CD45+ circulating angiogenic cells clonally derived from the hemopoietic progenitors. Moreover, CD45+ cells exposed to IL-3 acquire arterial specification and contribute to the formation of vessels in vivo. Depletion of signal transducer and activator of transcription 5 (STAT5) provides evidence that IL-3–mediated cell expansion and arterial morphogenesis rely on STAT5 activation. In addition, by means of Tie2-transgenic mice, we demonstrate that STAT5 also regulates IL-3–induced expansion and arterial specification of bonemarrow–derived CD45+ cells. Thus, our data provide the first evidence that, in inflammatory microenvironments containing IL-3, angiogenic cells derived from hemopoietic precursors can act as adult vasculogenic cells. Moreover, the characterization of the signaling pathway regulating these events provides the rationale for therapeutically targeting STAT5 in these pathologic settings.

Endothelial colony forming cells and mesenchymal stem cells are enriched at different gestational ages in human umbilical cord blood

Endothelial progenitor cells (EPCs) are used for angiogenic therapies and as biomarkers of cardiovascular disease. Human umbilical cord blood (UCB) is a rich source of endothelial colony forming cells (ECFCs), which are EPCs with robust proliferative potential that may be useful for clinical vascular regeneration. Previous studies show that hematopoietic progenitor cells are increased in premature UCB compared with term controls. Based on this paradigm, we hypothesized that premature UCB would be an enriched source of ECFCs. Thirty-nine UCB samples were obtained from premature infants (24-37 wk gestational age (GA)) and term controls. ECFC colonies were enumerated, clonally isolated, and identified by expression of endothelial cell surface antigens and functional analysis. GA of 33-36 wk UCB yielded predominantly ECFC colonies at equivalent numbers to term infants. UCB from 24 to 28 wk GA infants had significantly fewer ECFCs. Surprisingly, 24-28 wk GA UCB yielded predominantly mesenchymal stem cell (MSC) colonies, capable of differentiating into adipocytes, chondrocytes, and osteocytes. MSCs were rarely identified in 37-40 wk GA UCB. These studies demonstrate that circulating MSCs and ECFCs appear at different GA in the human UCB, and that 24-28 wk GA UCB may be a novel source of MSCs for therapeutic use in human diseases.

Endothelial stem cells and precursors for tissue engineering: cell source, differentiation, selection, and application

Endothelial cells are of great interest because of their potential in cell therapy for vascular diseases and ischemic tissue, tissue engineering for vascular grafts and vascularized tissue beds, and modeling for pharmaceutical transport across endothelial barriers. However, limited availability and proliferation capability of mature endothelial cells hampers development of these applications. Recent advances in stem cell technology have enabled researchers to derive endothelial or endothelial-like cells from stem cells or other precursor populations. The current state of these cell sources and their in vitro differentiation, selection, and applications are discussed in this review.

Circulating endothelial progenitor cells in patients with Eisenmenger syndrome and idiopathic pulmonary arterial hypertension

BACKGROUND

Impaired endothelial homeostasis underlies the pathophysiology of pulmonary arterial hypertension (PAH). We speculated that PAH patients are deficient in circulating endothelial progenitor cells (EPCs), potentially contributing to endothelial dysfunction and disease progression.

METHODS AND RESULTS

We recruited 41 patients with Eisenmenger syndrome (13 with Down syndrome), 55 with idiopathic PAH, and 47 healthy control subjects. Flow cytometry and in vitro assays were used to quantify EPCs and to assess cell function. The number of circulating CD34+, CD34+/AC133+, CD34+/KDR+, and CD34+/AC133+/KDR+ progenitor cells was low in Eisenmenger patients compared with healthy control subjects, and those with Down syndrome displayed even fewer EPCs. Reductions in EPC numbers correlated with New York Heart Association functional class, 6-minute walk distance, and plasma brain-type natriuretic peptide levels. The capacity of cultured peripheral blood mononuclear cells to form colonies and incorporate into tube-like structures was impaired in Eisenmenger patients. Idiopathic PAH patients had reduced numbers of EPCs, and the number of circulating EPCs correlated with invasive hemodynamic parameters in this cohort. Levels of immune inflammatory markers, cGMP, stable nitric oxide oxidation products, and asymmetric dimethylarginine were abnormal in patients with PAH and related to numbers of EPCs. Within the idiopathic PAH population, treatment with the phosphodiesterase inhibitor sildenafil was associated with a dose-dependent rise in EPC numbers, resulting in levels consistently above those found with other therapies.

CONCLUSIONS

Circulating EPC numbers are reduced in 2 well-characterized forms of PAH, which also exhibit raised levels of inflammatory mediators. Sildenafil treatment may represent a pharmacological means of increasing circulating EPC numbers long-term.

Monitoring of endothelial dysfunction in critically ill patients: the role of endothelial progenitor cells

PURPOSE OF REVIEW

This review provides an overview of sepsis as a prototypical critical illness and discusses the role of the endothelium in the pathophysiology of sepsis and sepsis-related organ dysfunction, the characterization and functions of endothelial progenitor cells, and investigates these cells both as a prognostic and therapeutic strategy in critically ill patients.

RECENT FINDINGS

Sepsis continues to be a major cause of morbidity and mortality worldwide. Preclinical and clinical sepsis studies have shown that the acute systemic inflammatory and procoagulant response results in structural and functional alterations in the endothelium, which may lead to organ failure and ultimately, death. In the last decade, the concept of postnatal vasculogenesis has been revolutionized to include angiogenesis by mature endothelial cells and vasculogenesis by endothelial progenitor cells. These cells are recruited from the bone marrow to areas of endothelial injury, at which point they differentiate and promote revascularization of the endothelium, which has been shown to have significant prognostic and therapeutic implications in a variety of ischemic vascular disorders.

SUMMARY

Circulating endothelial progenitor cells may be an important mechanism of vascular repair, and thus shows significant promise for prognostic and therapeutic strategies in critical illness, namely sepsis and sepsis-related organ dysfunction.

Circulating bone marrow-derived endothelial progenitor cells: characterization, mobilization, and therapeutic considerations in malignant disease

Until recently, tumor vascularization was thought to occur exclusively through angiogenesis. However, recent studies using different animal models of cancer suggested the importance of bone marrow-derived endothelial progenitor cells (EPCs) (i.e. postnatal vasculogenesis) in tumor vascularization and growth. EPCs are present in the peripheral blood, their levels are increased in response to certain signals/cytokines, and they home into the neovascular bed of malignant tissues. Furthermore, at the clinical level, evidence is emerging that changes in EPC levels might predict the efficacy of anticancer drug combinations that include antiangiogenic agents. On the basis of these observations, EPCs have attractive potential diagnostic and therapeutic applications for malignant diseases. In this paper, we review biological features of EPCs and speculate on the utility of these progenitor cells for medical oncology.

Technical notes on endothelial progenitor cells: ways to escape from the knowledge plateau

In the last 10 years an increasing interest has been devoted to the study of endothelial progenitor cells (EPCs), a subtype of immature cells involved in endothelial repair and neoangiogenesis. EPCs have been discovered as a novel integrated part of the cardiovascular system, which plays a comprehensive role in tissue homeostasis. Consistently, alterations and/or reduction of the circulating EPC pool have been associated with different manifestations of cardiovascular disorders and atherosclerosis. This is why, the extent of the EPC pool is now considered a mirror of vascular health, while EPC reduction has become a surrogate biomarker of cardiovascular risk and of the ongoing vascular damage. Unfortunately, the methods used to study EPCs still lack standardization, and this is significantly decelerating progress in the field. In this review, we focus on some aspects related to the two methods used to assess circulating EPCs: flow cytometry and cell culture. We uncover the many traps hidden in the choice of the right protocol, and suggest the best solutions on the basis of evidence and background theories.

Phosphatidylinositol-3-kinase-gamma is integral to homing functions of progenitor cells

Endothelial progenitor cells (EPCs) and hematopoietic progenitor cells are recruited to ischemic regions, improving neovascularization. beta1 and beta2 integrins play a crucial role for progenitor cell homing to ischemic tissues. Integrin activity is regulated by chemokines and their respective G protein-coupled receptors. The phosphatidylinositol-3-kinase catalytic subunit gamma (PI3Kgamma) is the PI3K isoform that selectively transduces signals from G protein-coupled receptors. Here, we investigated the role of PI3Kgamma as a signaling intermediate in the chemokine-induced integrin-dependent homing functions of progenitor cells. A pharmacological PI3Kgamma inhibitor significantly reduced chemokine-induced chemotaxis and stromal cell-derived factor (SDF)1alpha-induced transmigration of human EPCs. Moreover, the PI3Kgamma inhibitor significantly reduced SDF1alpha-induced adhesion of EPCs to intercellular adhesion molecule-1 and human umbilical vein endothelial cell monolayers. These findings were corroborated with Lin(-) bone marrow-derived progenitor cells from PI3Kgamma-deficient mice that displayed reduced SDF1alpha-induced migration and intercellular adhesion molecule-1 adhesion as compared with wild-type cells. Pharmacological inhibition or genetic ablation of PI3Kgamma reduced SDF1alpha-induced integrin activation in human EPCs and in murine Lin(-) BM-derived progenitor cells, respectively. In vivo, the homing of PI3Kgamma-deficient Lin(-) progenitor cells to ischemic muscles after intravenous infusion in the model of hindlimb ischemia and their neovascularization-promoting capacity was reduced as compared with wild-type cells. In conclusion, PI3Kgamma is integral to the integrin-dependent homing of progenitor cells.

Gender differences in endothelial progenitor cells and cardiovascular risk profile: the role of female estrogens

OBJECTIVE

Endothelial progenitor cells (EPCs) participate in vascular homeostasis and angiogenesis. The aim of the present study was to explore EPC number and function in relation to cardiovascular risk, gender, and reproductive state.

METHODS AND RESULTS

As measured by flow-cytometry in 210 healthy subjects, CD34(+)KDR(+) EPCs were higher in fertile women than in men, but were not different between postmenopausal women and age-matched men. These gender gradients mirrored differences in cardiovascular profile, carotid intima-media thickness, and brachial artery flow-mediated dilation. Moreover, EPCs and soluble c-kit ligand varied in phase with menstrual cycle in ovulatory women, suggesting cyclic bone marrow mobilization. Experimentally, hysterectomy in rats was followed by an increase in circulating EPCs. EPCs cultured from female healthy donors were more clonogenic and adherent than male EPCs. Treatment with 17beta-estradiol stimulated EPC proliferation and adhesion, via estrogen receptors. Finally, we show that the proangiogenic potential of female EPCs was higher than that of male EPCs in vivo.

CONCLUSIONS

EPCs are mobilized cyclically in fertile women, likely to provide a pool of cells for endometrial homeostasis. The resulting higher EPC levels in women than in men reflect the cardiovascular profile and could represent one mechanism of protection in the fertile female population.

Circulating angiogenic precursors in idiopathic pulmonary arterial hypertension

Vascular remodeling in idiopathic pulmonary arterial hypertension (IPAH) involves hyperproliferative and apoptosis-resistant pulmonary artery endothelial cells. In this study, we evaluated the relative contribution of bone marrow-derived proangiogenic precursors and tissue-resident endothelial progenitors to vascular remodeling in IPAH. Levels of circulating CD34+ CD133+ bone marrow-derived proangiogenic precursors were higher in peripheral blood from IPAH patients than in healthy controls and correlated with pulmonary artery pressure, whereas levels of resident endothelial progenitors in IPAH pulmonary arteries were comparable to those of healthy controls. Colony-forming units of endothelial-like cells (CFU-ECs) derived from CD34+ CD133+ bone marrow precursors of IPAH patients secreted high levels of matrix metalloproteinase-2, had greater affinity for angiogenic tubes, and spontaneously formed disorganized cell clusters that increased in size in the presence of transforming growth factor-beta or bone morphogenetic protein-2. Subcutaneous injection of NOD SCID mice with IPAH CFU-ECs within Matrigel plugs, but not with control CFU-ECs, produced cell clusters in the Matrigel and proliferative lesions in surrounding murine tissues. Thus, mobilization of high levels of proliferative bone marrow-derived proangiogenic precursors is a characteristic of IPAH and may participate in the pulmonary vascular remodeling process.

Chapter 13. An in vivo experimental model for postnatal vasculogenesis

Rapid and complete vascularization of ischemic tissues and thick engineered tissues is likely to require vasculogenesis. Therefore, the search for clinically relevant sources of vasculogenic cells and the subsequent development of experimental models of vasculogenesis is of utmost importance. Here, we describe a methodology adapted from the Matrigel plug assay to deliver human blood-derived endothelial progenitor cells (EPCs) and mature smooth muscle cells (SMCs) subcutaneously into immunodeficient mice. One week after implantation, an extensive microvascular network composed of the human EPCs and SMCs is formed within the Matrigel. The presence of human EPC-lined lumens containing host erythrocytes can be seen throughout the implants indicating not only the formation (de novo) of a vascular network, but also the development of functional anastomoses with the host circulatory system. This is a very versatile assay that allows (1) dialing the final microvessel density by varying either the total number of cells in the original cell suspension or the ratio between EPCs and SMCs, (2) studying the effect of substituting another type of perivascular cell for mature SMCs or another type of endothelial cell, (3) tracking each of the implanted cell types by labeling (e.g., GFP tagging) prior to implantation, and (4) studying the effect of genetically modifying the cells prior to implantation. Additionally, this assay is relatively simple to perform and it does not require an incision or surgical procedure. This murine model of human vasculogenesis is ideally suited for studies on the cellular and molecular components of microvessel development, pathologic neovascular responses, and for the development and investigation of strategies to enhance neovascularization of engineered human tissues and organs.

Stem cells and scaffolds for vascularizing engineered tissue constructs

The clinical impact of tissue engineering depends upon our ability to direct cells to form tissues with characteristic structural and mechanical properties from the molecular level up to organized tissue. Induction and creation of functional vascular networks has been one of the main goals of tissue engineering either in vitro, for the transplantation of prevascularized constructs, or in vivo, for cellular organization within the implantation site. In most cases, tissue engineering attempts to recapitulate certain aspects of normal development in order to stimulate cell differentiation and functional tissue assembly. The induction of tissue growth generally involves the use of biodegradable and bioactive materials designed, ideally, to provide a mechanical, physical, and biochemical template for tissue regeneration. Human embryonic stem cells (hESCs), derived from the inner cell mass of a developing blastocyst, are capable of differentiating into all cell types of the body. Specifically, hESCs have the capability to differentiate and form blood vessels de novo in a process called vasculogenesis. Human ESC-derived endothelial progenitor cells (EPCs) and endothelial cells have substantial potential for microvessel formation, in vitro and in vivo. Human adult EPCs are being isolated to understand the fundamental biology of how these cells are regulated as a population and to explore whether these cells can be differentiated and reimplanted as a cellular therapy in order to arrest or even reverse damaged vasculature. This chapter focuses on advances made toward the generation and engineering of functional vascular tissue, focusing on both the scaffolds - the synthetic and biopolymer materials - and the cell sources - hESCs and hEPCs.

Tie2-expressing monocytes: regulation of tumor angiogenesis and therapeutic implications

Tumor-infiltrating myeloid cells are involved in crucial processes during tumor development. A subset of monocytes that express the angiopoietin receptor Tie2 play an important role in tumor angiogenesis. Selective depletion of these Tie2-expressing monocytes (TEMs) in tumor-bearing mice inhibits tumor angiogenesis and growth, suggesting that they might regulate angiogenic processes in tumors by providing paracrine support to nascent blood vessels. TEMs have also been identified in human blood and tumors. We discuss here the therapeutic opportunities emanating from the discovery of TEMs, which include the identification of new antitumor targets, monitoring TEMs as surrogate markers for clinical responses in cancer patients, and the possible use of TEMs as cellular vehicles for gene delivery to tumors.