Adult endothelial progenitor cells from human peripheral blood maintain monocyte/macrophage function throughout in vitro culture

Maternal endothelial progenitor colony-forming units with macrophage characteristics are reduced in preeclampsia

BACKGROUND

Endothelial progenitor cells (EPCs) provide paracrine support to the vascular endothelium and may also replace damaged or senescent endothelial cells. Low numbers of endothelial progenitor colony-forming units (CFU-ECs) in culture are a predictive biomarker of vascular disease. We hypothesized that the number of CFU-ECs derived from maternal blood are decreased in women with preeclampsia compared to normal pregnancy.

METHODS

Primigravid women with singleton normal (n = 12) or preeclamptic (n = 12) pregnancies were studied during the third trimester. The culture assay was performed using a pre-plating step to eliminate mature endothelial cells and nonprogenitor cells; colonies per well were counted and further characterized.

RESULTS

Colony numbers were fourfold lower on average in preeclampsia compared to control samples (P < 0.005). A majority of the cells comprising individual colonies were positive for both endothelial (Ulex europaeus lectin staining and acetylated low-density lipoprotein (LDL) uptake) and monocyte/macrophage (CD45, CD14, CD115) characteristics. The SRY gene was detected in CFU-ECs derived from umbilical cord blood samples from male fetuses but not in CFU-ECs from peripheral blood of mothers with male fetuses. Maternal plasma concentrations of the antiangiogenic factor, soluble fms-like tyrosine kinase-1 (sFlt-1) were elevated (P < 0.0001) whereas placental growth factor (PlGF) was reduced (P < 0.01) in women with preeclampsia, but these factors did not correlate with CFU-EC counts.

CONCLUSIONS

CFU-ECs derived from culture of peripheral blood mononuclear cells, a correlate of cardiovascular risk in nonpregnancy populations, are rarified in women with preeclampsia compared to normal pregnancy. PCR analysis is consistent with a maternal origin of these cells.

The definition of EPCs and other bone marrow cells contributing to neoangiogenesis and tumor growth: is there common ground for understanding the roles of numerous marrow-derived cells in the neoangiogenic process?

Interest in the regulation of blood vessel formation as a mechanism to permit unregulated tumor cell growth was a prescient hypothesis of Dr. Judah Folkman nearly 3 decades ago. Understanding the cellular and molecular mechanisms that affect the recruitment, expansion, and turnover of the tumor microvasculature continues to evolve. While the fundamental paradigms for improving blood flow to growing, injured, diseased, or tumor infiltrated tissues are well known, the potential role of bone marrow derived circulating endothelial progenitor cells (EPCs) to function as postnatal vasculogenic precursors for tumor microvasculature has become a controversial premise. We will briefly review some recently published high profile papers that appear to derive polar interpretations for the role of EPCs in the angiogenic switch and discuss possible reasons for the disparate views in work conducted in both mouse and man.

Defining human endothelial progenitor cells

There is no specific marker to identify an endothelial progenitor cell (EPC) and this deficiency is restricting the ability of an entire field of research in defining these cells. We will review current methods to define EPC in the human system and suggest approaches to define better the cell populations involved in neoangiogenesis. PubMed was used to identify articles via the search term 'endothelial progenitor cell' and those articles focused on defining the term were evaluated. The only human cells expressing the characteristics of an EPC, as originally proposed, are endothelial colony forming cells. A variety of hematopoietic cells including stem and progenitors, participate in initiating and modulating neoangiogenesis. Future studies must focus on defining the specific hematopoietic subsets that are involved in activating, recruiting, and remodeling the vascular networks formed by the endothelial colony forming cells.

Endothelial progenitor cell-based neovascularization: implications for therapy

Ischemic cardiovascular events are a major cause of death globally. Endothelial progenitor cell (EPC)-based approaches can result in improvement of vascular perfusion and might offer clinical benefit. However, although functional improvement is observed, the lack of long-term engraftment of EPCs into neovessels has raised controversy regarding their mechanism of action. We and others have hypothesized that after ischemic injury, EPCs induce neovascularization through the secretion of cytokines and growth factors, which act in a paracrine fashion and induce sprouting angiogenesis by the surrounding endothelium. In this concise review, we discuss the (patho)physiology of EPC-induced neovascularization and focus on the paracrine signals secreted by EPCs and the effects they elicit. In future therapies, clinical administration of these paracrine modulators using slow-release depots might induce neovascularization and might therefore hold promise for vascular regenerative medicine.

Human peripheral blood endothelial progenitor cells synthesize and express functionally active tissue factor

INTRODUCTION

Endothelial progenitor cells are circulating cells able to home to sites of vascular damage and to contribute to the revascularization of ischemic areas. We evaluated whether endothelial progenitor cells synthesize tissue factor, a procoagulant protein also involved in angiogenesis.

MATERIALS AND METHODS

Endothelial progenitor cells were obtained from the peripheral blood mononuclear fraction of normal donors and cultured in endothelial medium supplemented with specific growth factors. The procoagulant activity expressed by cells disrupted by freeze-thaw cycles was assessed by a one stage clotting assay. Tissue factor mRNA expression was evaluated by RT-PCR.

RESULTS

Endothelial progenitor cells do not express procoagulant activity in baseline conditions. However, lipopolysaccharide induces the expression of procoagulant activity. The effect is dose-dependent and reaches statistical significance at 100 ng/mL lipopolysaccharide. Inhibition with an anti-tissue factor antibody and amplification of cDNA with primers based on the tissue factor sequence confirm the identity of this activity with tissue factor. The kinetics of tissue factor expression by endothelial progenitor cells is identical to that of human umbilical vein endothelial cells showing maximal activity within 4 hours, and then decreasing; in contrast, tissue factor expression by mononuclear cells lasts for longer times. Both 5,6-dichloro-beta D-ribofuranosyl-benzimidazole and cycloheximide prevented the expression of procoagulant activity. Stimulation of endothelial progenitor cells with tumor necrosis factor-alpha did not elicit any detectable procoagulant activity.

CONCLUSIONS

Endothelial progenitor cells can be stimulated by lipopolysaccharide to synthesize tissue factor. This protein might be involved in thrombotic phenomena and might contribute to endothelial progenitor cells related neovascularization.

Intraoperative cell transplantation for congestive heart failure: experience in China

Despite significant improvement in the management of congestive heart failure (CHF), it still is a major worldwide public health problem. Currently, cell-based regenerative medicine has been developed as a promising therapeutic option for patients with CHF. Considering the large and growing population, it is estimated that over 5 million patients in China may need such a cell-based therapy to replace or repair the damaged myocardium. Cardiac surgery has emerged as an important player in heart cell therapy in China in recent years. Here, we summarize our achievements in both preclinical and clinical studies of intraoperative cell transplantation, and present our understanding of future research in this attractive field.

Assessment of the potential of progenitor stem cells extracted from human peripheral blood for seeding a novel vascular graft material

OBJECTIVE

A novel nanocomposite has recently been developed based on polyhedral oligomeric silsesquioxane attached by direct reaction onto a urethane segment, as a potential vascular graft material; its trade name is UCL-Nano. The UCL-Nano has been demonstrated to have similar viscoelastic properties to the walls of a natural artery, to be resistant to degradation and to be able to sustain endothelial cell seeding. Human peripheral blood contains both circulating endothelial cells and endothelial progenitor cells, which may be suitable for conduit seeding. The aim of this study was to develop a system with the potential to deliver an endothelial cell-seeded bypass graft in a realistic time frame.

MATERIALS AND METHODS

Endothelial progenitor cells and circulating endothelial cells were isolated from human peripheral blood and were characterized by fluorescent-activated cell sorting, reverse transcriptase-polymerase chain reaction and immunohistochemistry. Isolated cells were seeded on nanocomposite and were maintained in culture for 35 days.

RESULTS

The UCL-Nano was successfully seeded with cells and a confluent cell layer was achieved after 14-day culture. Cells remained viable and confluent on the nanocomposite for 35 days.

CONCLUSION

In conclusion, these results suggest that this process has potential both for a realistic and achievable two-stage seeding process for vascular bypass grafts and for the potential development of a device, with the aim of achieving in situ seeding once implanted.

CD34+ cells augment endothelial cell differentiation of CD14+ endothelial progenitor cells in vitro

Neovascularization by endothelial progenitor cells (EPC) for the treatment of ischaemic diseases has been a topic of intense research. The CD34(+) cell is often designated as EPC, because it contributes to repair of ischaemic injuries through neovascularization. However, incorporation of CD34(+) cells into the neovasculature is limited, suggesting another role which could be paracrine. CD14(+) cells can also differentiate into endothelial cells and contribute to neovascularization. However, the low proliferative capacity of CD14(+) cell-derived endothelial cells hampers their use as therapeutic cells. We made the assumption that an interaction between CD34(+) and CD14(+) cells augments endothelial differentiation of the CD14(+) cells. In vitro, the influence of CD34(+) cells on the endothelial differentiation capacity of CD14(+) cells was investigated. Endothelial differentiation was analysed by expression of endothelial cell markers CD31, CD144, von Willebrand Factor and endothelial Nitric Oxide Synthase. Furthermore, we assessed proliferative capacity and endothelial cell function of the cells in culture. In monocultures, 63% of the CD14(+)-derived cells adopted an endothelial cell phenotype, whereas in CD34(+)/CD14(+) co-cultures 95% of the cells showed endothelial cell differentiation. Proliferation increased up to 12% in the CD34(+)/CD14(+) co-cultures compared to both monocultures. CD34-conditioned medium also increased endothelial differentiation of CD14(+) cells. This effect was abrogated by hepatocyte growth factor neutralizing antibodies, but not by interleukin-8 and monocyte chemoattractant protein-1 neutralizing antibodies. We show that co-culturing of CD34(+) and CD14(+) cells results in a proliferating population of functional endothelial cells, which may be suitable for treatment of ischaemic diseases such as myocardial infarction.

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.

RT-ABCDE strategy for management and prevention of human diseases

In this article, the authors summarized the RT-ABCDE strategy for the management and prevention of human diseases, which includes ReTro-ABCDE (Examination regularity, Disease and risk factor control, Changing lifestyle and reducing pathways of infection and spread, Biochemical and Antagonistic index control and therapeutic treatment as well as RT--Routine and Right Treatment). The RT-ABCDE strategy, a novel concept and an essential method, should be a routine strategy for disease control and prevention. It should be proposed and applied in both clinical and preventive medicine.

Endothelial progenitor cells for the treatment of diabetic vasculopathy: panacea or Pandora's box?

The discovery of endothelial progenitor cell (EPC) a decade ago has refuted the previous belief that vasculogenesis only occurs during embryogenesis. The reduced circulating concentration of EPCs is a surrogate marker of endothelial function and has been implicated in the pathogenesis of many vascular diseases. To date, the therapeutic benefit of neovascularization in ischaemic conditions in a non-diabetic setting has been demonstrated. This article aims to review the biology of EPCs in the diabetic setting with special emphasis on the effects of cardiovascular risk factor modification on EPC phenotype and methods to reverse or augment EPC dysfunction. The potential of the use of EPCs in the treatment of the diabetic vascular dysfunction will also be discussed.

Endothelial progenitor cell research in stroke: a potential shift in pathophysiological and therapeutical concepts

BACKGROUND AND PURPOSE

Stroke is the leading cause of disability in the Western world; however, few therapies are at hand to decrease this burden.

SUMMARY OF REVIEW

Endothelial progenitor cells (EPCs) have been introduced in cardiovascular medicine as factotums. EPCs can repair damaged endothelium and attenuate the development and progression of atherosclerosis. Also, EPCs can form new vessels in ischemic areas and thus promote recovery after ischemic events. In stroke, however, EPC research is limited. In our overview, we provide background information on EPC use as a risk marker and as a potential therapeutic agent.

CONCLUSIONS

In our opinion, the lack of EPC studies in stroke should instigate vascular neurologists to participate in EPC research, as EPCs could also change pathophysiological concepts and improve clinical treatments in vascular neurology.

Strikingly different angiogenic properties of endothelial progenitor cell subpopulations: insights from a novel human angiogenesis assay

OBJECTIVES

An endothelial cell (EC)-specific angiogenesis assay was developed to functionally characterize angiogenic properties of 2 distinct putative endothelial progenitor cells (EPCs): early EPCs and late outgrowth endothelial cells (OECs).

BACKGROUND

Endothelial progenitor cells promote revascularization of ischemic tissue. However, the nature of different EPCs and their role in angiogenesis remains debated.

METHODS

Tubulogenesis was assessed by immunohistochemistry in co-cultures of differentiated ECs (including human umbilical vein, coronary artery, and microvascular ECs) or non-ECs with monolayers of human fibroblasts (MRC5). Using adaptations of the co-culture assay, early EPCs and OECs, isolated from peripheral blood mononuclear cells, were assessed by 3-dimensional immunofluorescence microscopy for their capacity for: 1) independent tubulogenesis; 2) incorporation into pre-existing vascular networks; and 3) paracrine angiogenic effects using transwell cultures.

RESULTS

Branched interconnecting EC-specific tubules formed with all differentiated ECs after 72 h. Proangiogenic and antiangiogenic agents modulated tubulogenesis appropriately (vascular endothelial growth factor 10 ng: +142 +/- 13%, 1 microM anti-vascular endothelial growth factor: -44 +/- 7% vs. control, p < 0.001). In contrast, early EPCs, along with nonendothelial cell types, failed to independently form tubules or incorporate into differentiated EC tubules. Nevertheless, early EPCs indirectly augmented tubulogenesis by differentiated ECs even when physically separated by transwells (+115 +/- 4% vs. control; p < 0.001). By contrast, OECs independently formed tubules and incorporated into differentiated EC tubules but exerted no significant paracrine angiogenic effects.

CONCLUSIONS

A novel EC-specific tubulogenesis assay highlights strikingly different angiogenic properties of different EPCs: late OECs directly participate in tubulogenesis, whereas early EPCs augment angiogenesis in a paracrine fashion, with implications for optimizing cell therapies for neovascularization.

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.

Is it possible to obtain "true endothelial progenitor cells" by in vitro culture of bone marrow mononuclear cells?

In vitro-cultured bone marrow cells have been shown to contain some low-density lipoprotein (LDL) uptake-positive cells. Although a small portion of LDL uptake-positive cells had expression for endothelial markers, all of them demonstrated a phagocytosis function similar to monocyte/macrophages and expression of the panleukocyte surface marker CD45 and monocyte marker CD14. These LDL uptake-positive cells did not show significant proliferative capacity and died out gradually in long-term culture. In contrast, the bone marrow-derived LDL uptake-negative cells showed strong proliferation and expression of typical mesenchymal surface markers CD29 and CD44. Although cultured under endothelial promoting conditions, these mesenchymal stem cells (MSCs) did not show any sign of differentiation toward endothelial cells. In conclusion, adult bone marrow-derived LDL uptake-positive cells that have been reported so far actually are monocytes/macrophages that can express some endothelial markers but are not "true endothelial progenitor cells" (EPCs). MSCs, which are the only cell type that shows strong proliferation during long-term adherent culture for bone marrow cells, do not differentiate toward the endothelial lineage when grown under endothelial promoting conditions.

Expression of Tie-2 and Other Receptors for Endothelial Growth Factors in Acute Myeloid Leukemias Is Associated with Monocytic Features of Leukemic Blasts

We investigated the expression of Tie-2 in primary blasts from 111 patients with acute myeloid leukemia (AML) to evaluate a possible linkage between the expression of this receptor and the immunophenotypic and biologic properties of leukemic blasts. Tie-2 was expressed at moderate and high levels in 39 and 23 of 111 AMLs, respectively. The analysis of the immunophenotype clearly showed that Tie-2 expression in AML was associated with monocytic features. Interestingly, Tie-2 expression on AML blasts was associated with concomitant expression of other receptors for endothelial growth factors, such as vascular endothelial growth factor receptor 1 (VEGF-R1), -R2, and -R3. Tie-2(+) AMLs were characterized by high blast cell counts at diagnosis, a high frequency of Flt3 mutations, and increased Flt3 expression. The survival of Tie-2(+) AMLs is sustained through an autocrine pattern involving Angiopoietin-1 and Tie-2, as suggested by experiments showing induction of apoptosis in Tie-2(+) AMLs by agents preventing the binding of angiopoietins to Tie-2. Finally, the in vitro growth of Tie-2(+) AMLs in endothelial culture medium supplemented with VEGF and angiopoietins resulted in their partial endothelial differentiation. These observations suggest that Tie-2(+) AMLs pertain to a mixed monocytic/endothelial lineage, derived from the malignant transformation of the normal counterpart represented by monocytic cells expressing endothelial markers. The autocrine angiopoietin/Tie-2 axis may represent a promising therapeutic target to improve the outcome of patients with monocytic AML. Disclosure of potential conflicts of interest is found at the end of this article.

Working hypothesis to redefine endothelial progenitor cells

Since 1997, postnatal vasculogenesis has been purported to be an important mechanism for neoangiogenesis via bone marrow (BM)-derived circulating endothelial progenitor cells (EPCs). Based on this paradigm, EPCs have been extensively studied as biomarkers to assess severity of cardiovascular disease and as a cell-based therapy for several human cardiovascular disorders. In the majority of studies to date, EPCs were identified and enumerated by two primary methodologies; EPCs were obtained and quantified following in vitro cell culture, or EPCs were identified and enumerated by flow cytometry. Both methods have proven controversial. This review will attempt to outline the definition of EPCs from some of the most widely cited published reports in an effort to provide a framework for understanding subsequent studies in this rapidly evolving field. We will focus this review on studies that used cell culture techniques to define EPCs.

Redefining endothelial progenitor cells via clonal analysis and hematopoietic stem/progenitor cell principals

The limited vessel-forming capacity of infused endothelial progenitor cells (EPCs) into patients with cardiovascular dysfunction may be related to a misunderstanding of the biologic potential of the cells. EPCs are generally identified by cell surface antigen expression or counting in a commercially available kit that identifies "endothelial cell colony-forming units" (CFU-ECs). However, the origin, proliferative potential, and differentiation capacity of CFU-ECs is controversial. In contrast, other EPCs with blood vessel-forming ability, termed endothelial colony-forming cells (ECFCs), have been isolated from human peripheral blood. We compared the function of CFU-ECs and ECFCs and determined that CFU-ECs are derived from the hematopoietic system using progenitor assays, and analysis of donor cells from polycythemia vera patients harboring a Janus kinase 2 V617F mutation in hematopoietic stem cell clones. Further, CFU-ECs possess myeloid progenitor cell activity, differentiate into phagocytic macrophages, and fail to form perfused vessels in vivo. In contrast, ECFCs are clonally distinct from CFU-ECs, display robust proliferative potential, and form perfused vessels in vivo. Thus, these studies establish that CFU-ECs are not EPCs and the role of these cells in angiogenesis must be re-examined prior to further clinical trials, whereas ECFCs may serve as a potential therapy for vascular regeneration.