In vivo and in vitro properties of CD34+ and CD14+ endothelial cell precursors

Role of IL-4 in bone marrow driven dysregulated angiogenesis and age-related macular degeneration

Age-associated sterile inflammation can cause dysregulated choroidal neovascularization (CNV) as age-related macular degeneration (AMD). Intraocular fluid screening of 234 AMD patients identified high levels of IL-4. The purpose of this study was to determine the functional role of IL-4 in CNV formation using murine CNV model. Our results indicate that the IL-4/IL-4 receptors (IL4Rs) controlled tube formation and global proangiogenic responses of bone marrow cells. CCR2⁺ bone marrow cells were recruited to form very early CNV lesions. IL-4 rapidly induces CCL2, which enhances recruitment of CCR2⁺ bone marrow cells. This in vivo communication, like quorum-sensing, was followed by the induction of IL-4 by the bone marrow cells during the formation of mature CNVs. For CNV development, IL-4 in bone marrow cells are critically required, and IL-4 directly promotes CNV formation mainly by IL-4R. The IL-4/IL-4Rα axis contributes to pathological angiogenesis through communications with bone marrow cells leading to retinal degeneration.

Adipose-derived stem cell differentiation as a basic tool for vascularized adipose tissue engineering

The development of in vitro adipose tissue constructs is highly desired to cope with the increased demand for substitutes to replace damaged soft tissue after high graded burns, deformities or tumor removal. To achieve clinically relevant dimensions, vascularization of soft tissue constructs becomes inevitable but still poses a challenge. Adipose-derived stem cells (ASCs) represent a promising cell source for the setup of vascularized fatty tissue constructs as they can be differentiated into adipocytes and endothelial cells in vitro and are thereby available in sufficiently high cell numbers. This review summarizes the currently known characteristics of ASCs and achievements in adipogenic and endothelial differentiation in vitro. Further, the interdependency of adipogenesis and angiogenesis based on the crosstalk of endothelial cells, stem cells and adipocytes is addressed at the molecular level. Finally, achievements and limitations of current co-culture conditions for the construction of vascularized adipose tissue are evaluated.

The real face of endothelial progenitor cells - Circulating angiogenic cells as endothelial prognostic marker?

Endothelial progenitor cells (EPCs) have been extensively studied for almost 19 years now and were considered as a potential marker for endothelial regeneration ability. On the other hand, circulating endothelial cells (CEC) were studied as biomarker for endothelial injury. Yet, in the literature, there is also huge incoherency in regards to terminology and protocols used. This results in misleading conclusions on the role of so called "EPCs", especially in the clinical field. The discrepancies are mainly due to strong phenotypic overlap between EPCs and circulating angiogenic cells (CAC), therefore changes in "EPC" terminology have been suggested. Other factors leading to inconsistent results are varied definitions of the studied populations and the lack of universal data reporting, which could strongly affect data interpretation. The current review is focused on controversies concerning the use of "EPCs"/CAC and CEC as putative endothelial diagnostic markers.

Production of Human Endothelial Cells Free from Soluble Xenogeneic Antigens for Bioartificial Small Diameter Vascular Graft Endothelization

Arterial bypass graft implantation remains the primary therapy for patients with advanced cardiovascular disease, but most lack adequate saphenous vein or other conduits for bypass procedures and would benefit from a bioartificial conduit. This study aimed to produce human endothelial cells (hECs) in large scale, free from xenogeneic antigens, to develop a small diameter, compatible vessel for potential use as a vascular graft. Human adipose-derived stromal cells (hASCs) were isolated, cultured, and differentiated in the presence of human serum and used for the reendothelization of a decellularized rat aorta. hASC derived ECs (hASC-ECs) expressed VEGFR2, vWf and CD31 endothelial cell markers, the latter in higher levels than hASCs and HUVECs, and were shown to be functional. Decellularization protocol yielded aortas devoid of cell nuclei, with preserved structure, including a preserved basement membrane. When seeded with hASC-ECs, the decellularized aorta was completely reendothelized, and the hASC-ECs maintained their phenotype in this new condition. hASCs can be differentiated into functional hECs without the use of animal supplements and are capable of reendothelizing a decellularized rat aorta while maintaining their phenotype. The preservation of the basement membrane following decellularization supported the complete reendothelization of the scaffold with no cell migration towards other layers. This approach is potentially useful for rapid obtention of compatible, xenogeneic-free conduit.

Endothelial differentiation of mesenchymal stromal cells

Human mesenchymal stromal cells (hMSCs) are increasingly used in regenerative medicine for restoring worn-out or damaged tissue. Newly engineered tissues need to be properly vascularized and current candidates for in vitro tissue pre-vascularization are endothelial cells and endothelial progenitor cells. However, their use in therapy is hampered by their limited expansion capacity and lack of autologous sources. Our approach to engineering large grafts is to use hMSCs both as a source of cells for regeneration of targeted tissue and at the same time as the source of endothelial cells. Here we investigate how different stimuli influence endothelial differentiation of hMSCs. Although growth supplements together with shear force were not sufficient to differentiate hMSCs with respect to expression of endothelial markers such as CD31 and KDR, these conditions did prime the cells to differentiate into cells with an endothelial gene expression profile and morphology when seeded on Matrigel. In addition, we show that endothelial-like hMSCs are able to create a capillary network in 3D culture both in vitro and in vivo conditions. The expansion phase in the presence of growth supplements was crucial for the stability of the capillaries formed in vitro. To conclude, we established a robust protocol for endothelial differentiation of hMSCs, including an immortalized MSC line (iMSCs) which allows for reproducible in vitro analysis in further studies.

Prior endurance exercise prevents postprandial lipaemia-induced increases in reactive oxygen species in circulating CD31+ cells

We hypothesized that prior exercise would prevent postprandial lipaemia (PPL)-induced increases in intracellular reactive oxygen species (ROS) in three distinct circulating angiogenic cell (CAC) subpopulations. CD34(+), CD31(+)/CD14(-)/CD34(-), and CD31(+)/CD14(+)/CD34(-) CACs were isolated from blood samples obtained from 10 healthy men before and 4 h after ingesting a high fat meal with or without ∼50 min of prior endurance exercise. Significant PPL-induced increases in ROS production in both sets of CD31(+) cells were abolished by prior exercise. Experimental ex vivo inhibition of NADPH oxidase activity and mitochondrial ROS production indicated that mitochondria were the primary source of PPL-induced oxidative stress. The attenuated increases in ROS with prior exercise were associated with increased antioxidant gene expression in CD31(+)/CD14(-)/CD34(-) cells and reduced intracellular lipid uptake in CD31(+)/CD14(+)/CD34(-) cells. These findings were associated with systemic cardiovascular benefits of exercise, as serum triglyceride, oxidized low density lipoprotein-cholesterol, and plasma endothelial microparticle concentrations were lower in the prior exercise trial than the control trial. In conclusion, prior exercise completely prevents PPL-induced increases in ROS in CD31(+)/CD14(-)/CD34(-) and CD31(+)/CD14(+)/CD34(-) cells. The mechanisms underlying the effects of exercise on CAC function appear to vary among specific CAC types.

The promise of cell-based therapies for diabetic complications: challenges and solutions

The discovery of endothelial progenitor cells (EPCs) in human peripheral blood advanced the field of cell-based therapeutics for many pathological conditions. Despite the lack of agreement about the existence and characteristics of EPCs, autologous EPC populations represent a novel treatment option for complications requiring therapeutic revascularization and vascular repair. Patients with diabetic complications represent a population of patients that may benefit from cellular therapy yet their broadly dysfunctional cells may limit the feasibility of this approach. Diabetic EPCs have decreased migratory prowess and reduced proliferative capacity and an altered cytokine/growth factor secretory profile that can accelerate deleterious repair mechanisms rather than support proper vascular repair. Furthermore, the diabetic environment poses additional challenges for the autologous transplantation of cells. The present review is focused on correcting diabetic EPC dysfunction and the challenges involved in the application of cell-based therapies for treatment of diabetic vascular complications. In addition, ex vivo and in vivo functional manipulation(s) of EPCs to overcome these hurdles are discussed.

Revascularization of ischemic limbs after transplantation of human bone marrow cells with high aldehyde dehydrogenase activity

The development of cell therapies to treat peripheral vascular disease has proven difficult because of the contribution of multiple cell types that coordinate revascularization. We characterized the vascular regenerative potential of transplanted human bone marrow (BM) cells purified by high aldehyde dehydrogenase (ALDH(hi)) activity, a progenitor cell function conserved between several lineages. BM ALDH(hi) cells were enriched for myelo-erythroid progenitors that produced multipotent hematopoietic reconstitution after transplantation and contained nonhematopoietic precursors that established colonies in mesenchymal-stromal and endothelial culture conditions. The regenerative capacity of human ALDH(hi) cells was assessed by intravenous transplantation into immune-deficient mice with limb ischemia induced by femoral artery ligation/transection. Compared with recipients injected with unpurified nucleated cells containing the equivalent of 2- to 4-fold more ALDH(hi) cells, mice transplanted with purified ALDH(hi) cells showed augmented recovery of perfusion and increased blood vessel density in ischemic limbs. ALDH(hi) cells transiently recruited to ischemic regions but did not significantly integrate into ischemic tissue, suggesting that transient ALDH(hi) cell engraftment stimulated endogenous revascularization. Thus, human BM ALDH(hi) cells represent a progenitor-enriched population of several cell lineages that improves perfusion in ischemic limbs after transplantation. These clinically relevant cells may prove useful in the treatment of critical ischemia in humans.

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.

Conversion of cultured monocytes/macrophages into endothelial-like cells through direct contact with endothelial cells

When culturing human umbilical vein endothelial cells in a culture medium containing 4% human serum albumin, it was possible to maintain the epithelioid morphology and function for several months without subculturing. When coculturing endothelial cells and labeled monocytes/macrophages (Mo/Phi) that were collected from peripheral blood and allowed to engulf fluorescent latex beads, some Mo/Phi changed their shapes and became epithelioid cells that were indistinguishable from vascular endothelial cells. This transformation started within several hours of coculturing. At 7 days after the start of coculturing, more than half of the labeled cells were identified as endothelial-like cells morphologically. Furthermore, morphologically altered Mo/Phi did not express Mo/Phi-specific antigens, ie, the MHC Class II molecule and CD68, but expressed VE cadherin and vWF, which are specific antigens for endothelial cells, and labeled cells that changed into endothelial-like cells no longer engulfed fluorescent latex beads. This strongly suggests that peripheral blood monocytes differentiate into endothelial-like cells.

Biology of bone marrow-derived endothelial cell precursors

Over the past decade, the old idea that the bone marrow contains endothelial cell precursors has become an area of renewed interest. While some still believe that there are no endothelial precursors in the blood, even among those who do, there is no consensus as to what they are or what they do. In this review, we describe the problems in identifying endothelial cells and conclude that expression of endothelial nitric oxide synthase may be the most reliable antigenic indicator of the phenotype. The evidence for two different classes of endothelial precursors is also presented. We suggest that, though there is no single endothelial cell precursor, we may be able to use these phenotypic variations to our advantage in better understanding their biology. We also discuss how a variety of genetic, epigenetic, and methodological differences can account for the seemingly contradictory findings on the physiological relevance of bone marrow-derived precursors in normal vascular maintenance and in response to injury. Data on the impact of tumor type and location on the contribution of bone marrow-derived cells to the tumor vasculature are also presented. These data provide hope that we may ultimately be able to predict those tumors in which bone marrow-derived cells will have a significant contribution and design therapies accordingly. Finally, factors that regulate bone marrow cell recruitment to and function in the endothelium are beginning to be identified, and several of these, including stromal derived factor 1, monocyte chemoattractant factor-1, and vascular endothelial growth factor are discussed.

Angiogenic murine endothelial progenitor cells are derived from a myeloid bone marrow fraction and can be identified by endothelial NO synthase expression

OBJECTIVE

Endothelial progenitor cells (EPCs) contribute to postnatal neovascularization and are therefore of great interest for autologous cell therapies to treat ischemic vascular disease. However, the origin and functional properties of these EPCs are still in debate.

METHODS AND RESULTS

Here, ex vivo expanded murine EPCs were characterized in terms of phenotype, lineage potential, differentiation from bone marrow (BM) precursors, and their functional properties using endothelial NO synthase (eNOS)-green fluorescent protein transgenic mice. Despite high phenotypic overlap with macrophages and dendritic cells, EPCs displayed unique eNOS expression, endothelial lineage potential in colony assays, and angiogenic characteristics, but also immunologic properties such as interleukin-12p70 production and low levels of T-cell stimulation. The majority of EPCs developed from an immature, CD31(+)Ly6C+ myeloid progenitor fraction in the BM. Addition of myeloid growth factors such as macrophage-colony-stimulating factor (M-CSF) and granulocyte/macrophage (GM)-CSF stimulated the expansion of spleen-derived EPCs but not BM-derived EPCs.

CONCLUSIONS

The close relationship between EPCs and other myeloid lineages may add to the complexity of using them in cell therapy. Our mouse model could be a highly useful tool to characterize EPCs functionally and phenotypically, to explore the origin and optimize the isolation of EPC fractions for therapeutic neovascularization.

Monocytes/macrophages cooperate with progenitor cells during neovascularization and tissue repair: conversion of cell columns into fibrovascular bundles

The potential of monocytes/macrophages (MC/Mph) to contribute to neovascularization has recently become a topic of intense scrutiny. Here, we characterized the behavior of MC/Mph in cellular infiltrates, with emphasis on their spatial organization and localization in newly formed microvessels. To this end, we studied MC/Mph migration and assembly in basic fibroblast growth factor-supplemented Matrigel plugs placed in transgenic Tie2-beta-galactosidase mice for up to 4 weeks. In these plugs, along with Nile Red-positive adipocytes, we found MC/Mph distributed in cell cords, also containing various mature and progenitor tissue cells; and functional Tie2-positive or -negative microvessels embedded in bundles of fibrillar collagen surrounded by F4/80-positive MC/Mph. At earlier stages of infiltration, we found tubular destruction of the matrix (tunnels) and MC/Mph-lined capillary-like structures occasionally containing erythrocytes, indicating their propensity for endothelial trans-differentiation. We also analyzed in vitro the MCP-1-induced chemotactic migration of fluorescently labeled peritoneal MC/Mph incorporated in Matrigel-containing fluorescent protease substrates. Many of these MC/Mph produced MMP-12- and TIMP-1-dependent tunnels coupled with acquisition of a lumen. In conclusion, long-term implantation of Matrigel plugs qualifies as a novel experimental model of tissue regeneration, in which neovascularization intimately couples with fibrosis and organogenesis and in which cells of MC/Mph phenotype play a key structural role.

Acceleration of endothelial-like cell differentiation from CD14+ monocytes in vitro

OBJECTIVE

In vitro differentiation of endothelial cells has potential applications in vascular tissue engineering and cell-based therapy for many diseases. The objective of this study was to develop a new strategy that utilizes cytokines and lipopolysaccharide (LPS) to accelerate endothelial-like cell differentiation from peripheral blood CD14(+) monocytes.

METHODS

Peripheral blood CD14(+) monocytes were purified with immunobeads and cultured with an angiogenic growth factor-rich growth medium (EGM-2) with or without initial treatment of LPS in combination of granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4) for 4 days (the day 4 cultures). The cells were then continuously cultured in EGM-2 medium for an additional 4 or 10 days (the day 8 or day 14 cultures). Cell markers were determined by flow cytometry analysis and immunofluorescence staining. Cytokine/chemokine profile was studied by Bio-Plex immunoassay.

RESULTS

In the group of initial treatment of LPS in combination with GM-CSF, IL-4, and EGM-2, the majority of suspended CD14(+) monocytes were attached and changed their morphology to endothelial-like cells, which expressed high levels of endothelial cell markers CD31, von Willebrand factor, and vascular endothelial growth factor receptor-1 as well as two major endothelial tight junction proteins zonula occludens -1 and occludin in the day 8 cultures. Endothelial nitric oxide synthase expression was substantially increased. Endothelial-like cells were also able to uptake acetylated low-density lipoprotein and bind to Ulex europeus lectin. In addition, endothelial-like cells showed a unique cytokine/chemokine profile with substantial increases of macrophage inflammatory protein-1beta, IL-6, granulocyte colony-stimulating factor, and IL-8.

CONCLUSION

Initial treatment of LPS in combination with GM-CSF, IL-4, and EGM-2 is an effective strategy for acceleration of endothelial-like cell differentiation from peripheral blood CD14(+) monocytes in vitro.

Progenitor cells in vascular disease

Stem cell research has the potential to provide solutions to many chronic diseases via the field of regeneration therapy. In vascular biology, endothelial progenitor cells (EPCs) have been identified as contributing to angiogenesis and hence have therapeutic potential to revascularise ischaemic tissues. EPCs have also been shown to endothelialise vascular grafts and therefore may contribute to endothelial maintenance. EPC number has been shown to be reduced in patients with cardiovascular disease, leading to speculation that atherosclerosis may be caused by a consumptive loss of endothelial repair capacity. Animal experiments have shown that EPCs reendothelialise injured vessels and that this reduces neointimal formation, confirming that EPCs have an atheroprotective effect. Smooth muscle cell accumulation in the neointimal space is characteristic of many forms of atherosclerosis, however the source of these cells is now thought to be from smooth muscle progenitor cells (SMPCs) rather than the adjacent media. There is evidence for the presence of SMPCs in the adventitia of animals and that SMPCs circulate in human blood. There is also data to support SMPCs contributing to neointimal formation but their origin remains unknown. This article will review the roles of EPCs and SMPCs in the development of vascular disease by examining experimental data from in vitro studies, animal models of atherosclerosis and clinical studies.

The role of circulating precursors in vascular repair and lesion formation

The accumulation of smooth muscle cells (SMCs) plays a principal role in atherogenesis, post-angioplasty restenosis and transplantation-associated vasculopathy. Therefore, much effort has been expended in targeting the migration and proliferation of medial smooth muscle cells to prevent occlusive vascular remodeling. Recent evidence suggests that bone marrow-derived circulating precursors can also give rise to endothelial cells and smooth muscle cells that contribute to vascular repair, remodeling, and lesion formation under physiological and pathological conditions. This article overviews recent findings on circulating vascular progenitor cells and describes potential therapeutic strategies that target these cells to treat occlusive vascular diseases.

Functional adaptation: the key to plasticity of cardiovascular "stem" cells?

There is increasing evidence that cells of disparate phenotypes displaying various degrees of proliferative capacity engraft and function heterotopically in adult organisms. Efforts were made to reconcile these findings with the embryologic notions of pluripotent stem or progenitor cell, although the nature of the 'stemness' remained elusive. This topic is particularly important for the cardiovascular system, in which cytotrophoblasts, certain tumor cells, monocytes/macrophages, peritoneal mesothelial cells, and others acquire endothelial properties and/or perform endothelial functions. Here we suggest that this pluripotency reflects a fundamental characteristic of cellular diversity, which is manifested as the adaptive response to a functional pressure exerted by the cell's biochemical and biophysical microenvironments that would drive their differentiation. In this model, differentiation is a dynamic, reversible, and open-ended process where the cells would maintain the flexibility to respond to changing environmental clues with a fine tuning of their structure, a property that was previously called cellular plasticity. Pluripotent adult stem cells that display this property in culture, and, perhaps upon in vivo administration, were described. Therefore, we also suggest that differentiation of stem cells is a form of cellular plasticity within the larger context of functional adaptation, whereas their stemness remains associated with self-renewal.

Monocytes and macrophages form branched cell columns in matrigel: implications for a role in neovascularization

Linear arrays of cells, or cell columns, have been observed in the extracellular matrix prior to neovascularization, but their nature and significance remains elusive. Based on the emerging evidence implicating a role for monocytes and macrophages (MC/MPH) in vasculogenesis, we hypothesized that MC/MPH also can form linear or branched columns, facilitating the co-migration and the spatial arrangement of other cell types. To test this hypothesis, we studied the distribution of MC/MPH effected by chemotactic migration in novel in vitro and in vivo models of development. We induced transversal and lateral migration of THP-1 monocytoid cells in Matrigel in vitro. The effect of this process on co-localization of other micro-objects was assessed using erythrocytes and micron-sized plastic beads. In vivo, we analyzed MC/MPH infiltration in subcutaneously implanted Matrigel plugs containing angiogenic factors and across a microporous filter comprising the wall of a chamber filled with Matrigel, also placed subcutaneously in mice. In vitro, we found that migrating THP-1 cells induced the lasting degradation of Matrigel and produced cell columns, a process amplified by monocyte chemoattractant protein-1 (MCP-1). We also report the co-localization of erythrocytes with THP-1 cells in cell columns. Endothelium-free tunnels containing MC/MPH, neutrophils, or erythrocytes were also observed in the Matrigel-filled chambers. In free subcutaneous Matrigel plugs, we found MC/MPH-based columns harboring isolated Tie-2+ cells (a marker of endothelial progenitor phenotype), as well as fibroblasts, dendritic cells, and adypocytes. Many of these cell columns displayed conspicuous branching. Our data demonstrate formation of branched MC/MPH cell columns in vitro and in vivo, a previously unrecognized pattern of penetration of extracellular matrices by inflammatory cells. Thus, monocytes and macrophages influence the distribution of neovessels as well as their branching points. These cells are the "architects of development," assisting organogenesis, tumorigenesis, and wound healing by patterning the tissular space.