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

Generating Monocyte-Derived Endothelial-like Cells for Vascular Regeneration

A major limitation in engineering vascular grafts is the lack of proper endothelium to prevent thrombosis and subsequent graft failure. Obtaining endothelial cells from patients' vasculature is intrusive and requires extensive culture time. Here we present an alternative strategy wherein abundant and easily accessible monocytes from peripheral blood are cultured and differentiated towards an endothelial-like state capable of preventing thrombosis through production of nitric oxide and formation of endothelial adherens junctions. Considering the plethora of monocytes present within peripheral blood, this method provides a robust alternative to generating endothelial cells required for vascular graft production.

An In Vitro Co-Culture Model of Bone Marrow Mesenchymal Stromal Cells and Peripheral Blood Mononuclear Cells Promotes the Differentiation of Myeloid Angiogenic Cells and Pericyte-Like Cells

Mesenchymal stromal cells (MSCs) are known to stimulate the survival and growth of endothelial cells (ECs) by producing paracrine signals, as well as to differentiate into pericytes and thereby support blood vessel formation and stability. On the other hand, cells with an EC-like phenotype have been found within the CD14⁺ and CD34⁺ cell populations of peripheral blood (PB) mononuclear cells (MNCs). The aim of this study was to investigate the proangiogenic differentiation potential of human MSC-MNC co-cultures. Bone marrow-derived MSCs (2,500 cells/cm²) were co-cultured with MNCs (50,000 cells/cm²), which were isolated from the PB of healthy donors. MSCs and MNCs cultured alone at same cell densities were used as controls. Cells in MNC fraction and in co-cultures were isolated for CD14, CD34, and CD31 surface markers with magnetic-activated cell sorting. Co-cultures were analyzed for cell proliferation and morphology, as well as for the expression of various hematopoietic, endothelial, and pericyte markers by immunocytochemistry, quantitative PCR (qPCR), and flow cytometry. Vascular endothelial growth factor (VEGF) expression and secretion was measured with qPCR and enzyme-linked immunosorbent assay, respectively. Our results show that in co-cultures with MSCs, CD14⁺CD45⁺ MNCs differentiated into spindle-shaped, nonproliferative, EC-like, myeloid angiogenic cells (MACs) expressing CD31, but also into pericyte-like cells expressing neural/glial antigen 2 (NG2) and CD146. Functionality of the isolated MACs was demonstrated in co-cultures with human umbilical vein endothelial cells, where they supported the formation of tube-like structures. NG2⁺ cells of MNC-origin were found among both CD34^-CD14⁺ and CD34^-CD14^- cell populations, indicating the existence of different subtypes of pericyte-like cells. In addition, VEGF was shown to be secreted in MSC-MNC co-cultures, mainly by MSCs. In conclusion, MSCs were shown to possess proangiogenic capacity in MSC-MNC co-cultures as they supported the differentiation of functional MACs, as well as the differentiation of pericyte-like cells of MNC origin. This phenomenon was mediated at least partially via secreted VEGF.

Endothelialization of arterial vascular grafts by circulating monocytes

Recently our group demonstrated that acellular tissue engineered vessels (A-TEVs) comprised of small intestinal submucosa (SIS) immobilized with heparin and vascular endothelial growth factor (VEGF) could be implanted into the arterial system of a pre-clinical ovine animal model, where they endothelialized within one month and remained patent. Here we report that immobilized VEGF captures blood circulating monocytes (MC) with high specificity under a range of shear stresses. Adherent MC differentiate into a mixed endothelial (EC) and macrophage (Mφ) phenotype and further develop into mature EC that align in the direction of flow and produce nitric oxide under high shear stress. In-vivo, newly recruited cells on the vascular lumen express MC markers and at later times they co-express MC and EC-specific proteins and maintain graft patency. This novel finding indicates that the highly prevalent circulating MC contribute directly to the endothelialization of acellular vascular grafts under the right chemical and biomechanical cues.

Acellular tissue engineered vessels functionalised with VEGF are coated with a layer of endothelial cells after in vivo implantation, but the source of the cells are unknown. Here the authors provide evidence that monocytes expressing VEGF receptors can transdifferentiate into endothelial cells via a macrophage intermediate.

Implantation of VEGF-functionalized cell-free vascular grafts: regenerative and immunological response

Recently, our group demonstrated that immobilized VEGF can capture flowing endothelial cells (ECs) from the blood in vitro and promote endothelialization and patency of acellular tissue-engineered vessels (A-TEVs) into the arterial system of an ovine animal model. Here, we demonstrate implantability of submillimeter diameter heparin and VEGF-decorated A-TEVs in a mouse model and discuss the cellular and immunologic response. At 1 mo postimplantation, the graft lumen was fully endothelialized, as shown by expression of EC markers such as CD144, eNOS, CD31, and VEGFR2. Interestingly, the same cells coexpressed leukocyte/macrophage (Mϕ) markers CD14, CD16, VEGFR1, CD38, and EGR2. Notably, there was a stark difference in the cellular makeup between grafts containing VEGF and those containing heparin alone. In VEGF-containing grafts, infiltrating monocytes (MCs) converted into anti-inflammatory M2-Mϕs, and the grafts developed well-demarcated luminal and medial layers resembling those of native arteries. In contrast, in grafts containing only heparin, MCs converted primarily into M1-Mϕs, and the endothelial and smooth muscle layers were not well defined. Our results indicate that VEGF may play an important role in regulating A-TEV patency and regeneration, possibly by regulating the inflammatory response to the implants.-Smith, R. J., Jr., Yi, T., Nasiri, B., Breuer, C. K., Andreadis, S. T. Implantation of VEGF-functionalized cell-free vascular grafts: regenerative and immunological response.

Endothelial cells are able to synthesize and release catecholamines both in vitro and in vivo

Recently it has been demonstrated that catecholamines are produced and used by macrophages and mediate immune response. The aim of this study is to verify whether endothelial cells (ECs), which are of myeloid origin, can produce catecholamines. We demonstrated that genes coding for tyrosine hydroxylase, Dopa decarboxylase, dopamine β hydroxylase (DβH), and phenylethanolamine-N-methyl transferase, enzymes involved in the synthesis of catecholamines, are all expressed in basal conditions in bovine aorta ECs, and their expression is enhanced in response to hypoxia. Moreover, hypoxia enhances catecholamine release. To evaluate the signal transduction pathway that regulates catecholamine synthesis in ECs, we overexpressed in bovine aorta ECs either protein kinase A (PKA) or the transcription factor cAMP response element binding, because PKA/cAMP response element binding activation induces tyrosine hydroxylase transcription and activity in response to stress. Both cAMP response element binding and PKA overexpression enhance DβH and phenylethanolamine-N-methyl transferase gene expression and catecholamine release, whereas H89, inhibitor of PKA, exerts the opposite effect, evidencing the role of PKA/cAMP response element binding transduction pathway in the regulation of catecholamine release in bovine aorta ECs. We then evaluated by immunohistochemistry the expression of tyrosine hydroxylase, Dopa decarboxylase, DβH, and phenylethanolamine-N-methyl transferase in femoral arteries from hindlimbs of C57Bl/6 mice 3 days after removal of the common femoral artery to induce chronic ischemia. Ischemia evokes tyrosine hydroxylase, Dopa decarboxylase, DβH, and phenylethanolamine-N-methyl transferase expression in the endothelium. Finally, the pharmacological inhibition of catecholamine release by fusaric acid, an inhibitor of DβH, reduces the ability of ECs to form network-like structures on Matrigel matrix. In conclusion, our study demonstrates for the first time that ECs are able to synthesize and release catecholamines in response to ischemia.

Interaction between marrow-derived human mesenchymal stem cells and peripheral blood mononuclear cells in endothelial cell differentiation

BACKGROUND AND AIMS

In adult connective tissues, mesenchymal stem cells (MSCs) play a key role in normal tissue turnover and repair. MSCs can participate in these processes not only through proliferation and differentiation but also through paracrine/autocrine functions. These characteristics make MSCs the optimal target in the development of cell-based therapies. This study describes a novel interaction between human MSC and blood mononuclear cells (MNCs), resulting in formation of blood vessel-like structures.

MATERIALS AND METHODS

Human marrow-derived MSCs and peripheral blood MNCs were co-cultured in monolayer cultures as well as in bovine collagen sponge up to 20 days. No exogenously supplied growth factors were applied. Morphological changes and formations of three dimensional structures were detected by light microscopy. The process was further stu-died for the expression of different endothelial cell markers. The expression of PECAM-1 and endoglin was studied by immunohistochemistry and the expression of vascular endothelial growth factor receptors 1 and 2 using quantitative real time PCR.

RESULTS

In co-cultures of human MSCs and MNCs, the previously nonadherent cells attached and started to elongate and formed tube-like structures within one week. At day 10, elongated PECAM-1 and endoglin expressing cells were detected in co-cultures. At day 20, PECAM-1 and endoglin-positive vessel-like structures were observed. VEGFR1 was up-regulated in co-cultures after 10 days, and expression levels increased with time. No PECAM-1, endoglin or VEGFR1 expressing cells were discovered in MSC-cultures without MNCs at any time point.

CONCLUSIONS

This study demonstrates induction of endothelial differentiation in co-cultures of human MSCs and MNCs, indicating a mechanism by which local application of MSCs could induce angiogenesis in vivo.

Engineering blood vessels using stem cells: innovative approaches to treat vascular disorders

Vascular disease is the leading cause of mortality in the USA, providing the impetus for new treatments and technologies. Current therapies rely on the implantation of stents or grafts to treat injured blood vessels. However, these therapies may be immunogenic or may incompletely recover the functional integrity of the vasculature. In light of these shortcomings, cell-based therapies provide new treatment options to heal damaged areas with more suitable substitutes. Current clinical trials employing stem cell-based therapies involve the transfusion of harvested endothelial progenitor cells. While the results from these trials have been encouraging, utilizing tissue-engineered approaches could yield technologically advanced solutions. This article discusses engineered stem cell-based therapies from three angles: the differentiation of adult stem cells, such as mesenchymal stem cells and endothelial progenitor cells, into vascular lineages; investigation of human embryonic stem cells and induced pluripotent stem cells as inexhaustible sources of vascular cells; and tissue-engineering approaches, which incorporate these vascular progenitor cells into biomimetic scaffolds to guide regeneration. The optimal solution to vascular disease lies at the interface of these technologies--embedding differentiated cells into engineered scaffolds to impart precise control over vascular regeneration.

Both CD133+ Cells and Monocytes Provide Significant Improvement for Hindlimb Ischemia, Although They do not Transdifferentiate Into Endothelial Cells

To address a number of questions regarding the experimental use of bone marrow (BM) stem cells in hindlimb ischemia, including which is the best cell type (e.g., purified hematopoietic stem cell or monocytes), the best route of delivery [intramuscular (IM) or intravenous (IV)], and the mechanism of action (transdifferentiation or paracrine effects), we have compared the neovascularization capacities of CD133+ stem cells and monocytes (CD11b+) from the BM of Tie2-GFP mice either via IV or IM in a murine severe hindlimb ischemia model. To test the effect of cytokine administration, an extra group received BM conditioned medium. Peripheral blood flow as well as capillary density and GPF-positivity detection in ischemic muscles was evaluated 7, 14, and 21 days postinjection. In addition, CD133+ and CD11b+ cells from transgenic animals were cultured in vitro with angiogenic media for 7, 14, and 21 days to assess GFP expression. In all four cell-treated groups, blood flow and capillary density significantly recovered compared with the mice that received no cells or conditioned medium. There were no differences with respect to cell types or administration routes, with the exception of a faster flow recovery in the CD133+-treated cell group. We did not find GFP+ cells in the ischemic muscles and there was no GFP expression after in vitro proangiogenic culture. Our study shows that both purified CD133+ stem cells and myeloid mononuclear cells, either IM or IV administered, have similar neoangiogenic ability. Nevertheless, transdifferentiation into endothelial cells is not the mechanism responsible for their beneficial effect.

Infection of the endothelium by members of the order Rickettsiales

The vascular endothelium is the main target of a limited number of infectious agents, Rickettsia, Ehrlichia ruminantium, and Orientia tsutsugamushi are among them. These arthropod-transmitted obligately-intracellular bacteria cause serious systemic diseases that are not infrequently lethal. In this review, we discuss the bacterial biology, vector biology, and clinical aspects of these conditions with particular emphasis on the interactions of these bacteria with the vascular endothelium and how it responds to intracellular infection. The study of these bacteria in relevant in vivo models is likely to offer new insights into the physiology of the endothelium that have not been revealed by other models.

GM-CSF accelerates proliferation of endothelial progenitor cells from murine bone marrow mononuclear cells in vitro

OBJECTIVE

To test whether the GM-CSF accelerates the proliferation of bone marrow endothelial progenitor cells (BM EPCs).

METHODS

BM EPCs were induced by endothelial cell conditioned medium (EC-CM). The effect of different concentrations of GM-CSF on the proliferation of BM EPCs was evaluated by the formation of EC-cols, MTT assay, and cell cycle assay. The single progenitor cell growth curves were quantified.

RESULTS

The data indicated that GM-CSF accelerated the proliferation of BM EPCs both in colony numbers and colony size. MTT confirmed the effect of GM-CSF on accelerating the proliferation of BM EPCs. The single colony experiments showed that EC-cols expressed different proliferation capacity, suggesting that the EC-cols with different proliferation potentials might have been derived from different levels of immature progenitors. The cell cycle assay showed that the rate of cells entering into S phase was 9.3% in the group treated with GM-CSF and 2.1% in the controls. Furthermore, these cells displayed the specific endothelial cell markers and formed capillary-like structures.

CONCLUSIONS

GM-CSF accelerates proliferation of BM EPCs. The potential beneficial of GM-CSF in the application of treating vascular ischemic patients is promising.

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.

Rapid expansion of human adipose-derived stromal cells preserving multipotency

BACKGROUND

Adipose-derived stromal (stem) cells (ASC) have been shown to be of great therapeutic use in pre-clinical studies in diverse fields, but a standard expansion method has not been established. We investigated the effects of an endothelial growth medium (EGM-2) on ASC, focusing on proliferation and differentiation potentials.

METHODS

ASC were cultured in EGM-2 and DMEM. Doubling time and total cell number were compared between the two media. The proliferative effect of each growth factor supplemented in EGM-2 was also examined. Cultured cells in each medium were examined for surface marker expression using flow cytometry. Differentiation into the adipogenic, chondrogenic and osteogenic lineages was analyzed after culture in each medium.

RESULTS

ASC cultured with EGM-2 proliferated much more rapidly (10(5) times in 2 weeks) and reached the stationary phase earlier than those cultured with DMEM. Among the supplements contained in EGM-2, only fibroblast growth factor-2 (FGF-2) significantly promoted proliferation of ASC, although the proliferative effect of FGF-2 was much less than that of EGM-2, suggesting a synergism among other supplement factors. Flow cytometry and differentiation assays suggested that ASC cultured in EGM-2 preserved immunophenotype and differentiation capacity for at least three mesenchymal lineages (adipogenic, chondrogenic and osteogenic), similar to those cultured with DMEM.

DISCUSSION

The present expansion method markedly accelerates proliferation of ASC, preserving their multipotent differentiation capacities, and lays the groundwork for establishing a practical route to mega-expansion of ASC for clinical applications.

Early effects triggered by Larrea divaricata Cav. on murine macrophages at apoptotic concentrations

Decoction and infusion of Larrea divaricata were tested at apoptotic concentrations (1 and 4 mg/ml) on peritoneal murine macrophages. Consistent changes were observed after incubation with 4 mg/ml decoction. Phagocytosis of zymosan, lysosomal enzyme activity, nitric oxide production, TNF-alpha release, and expression of CD14, TLR4, and CR3 increased significantly. Decoction at 1 and 4 mg/ml increased the binding of LPS-FITC. Apoptosis triggered by L. divaricata decoction is consequence of cell activation. The effects are independent of nordihydroguaiaretic acid. This "activation and death" could be the mechanism of L. divaricata to exert the antituberculosis effect known in folk medicine.

Myeloid lineage progenitors give rise to vascular endothelium

Despite an important role in vascular development and repair, the origin of endothelial progenitors remains unknown. Accumulating evidence indicates that cells derived from the hematopoietic system participate in angiogenesis. However, the identity and functional role of these cells remain controversial. Here we show that vascular endothelial cells can differentiate from common myeloid progenitors and granulocyte/macrophage progenitors. Endothelial cells derived from transplanted bone marrow-derived myeloid lineage progenitors expressed CD31, von Willebrand factor, and Tie2 but did not express the hematopoietic markers CD45 and F4/80 or the pericyte markers desmin and smooth muscle actin. Lineage tracing analysis in combination with a Tie2-driven Cre/lox reporter system revealed that, in contrast to bone marrow-derived hepatocytes, bone marrow-derived endothelial cells are not the products of cell fusion. The establishment of both hematopoietic and endothelial cell chimerism after parabiosis demonstrates that circulating cells can give rise to vascular endothelium in the absence of acute radiation injury. Our findings indicate that endothelial cells are an intrinsic component of myeloid lineage differentiation and underscore the close functional relationship between the hematopoietic and vascular systems.