Alloimmunity to human endothelial cells derived from cord blood progenitors

3D bioprinting of an implantable xeno-free vascularized human skin graft

Bioengineered tissues or organs produced using matrix proteins or components derived from xenogeneic sources pose risks of allergic responses, immune rejection, or even autoimmunity. Here, we report successful xeno-free isolation, expansion, and cryopreservation of human endothelial cells (EC), fibroblasts (FBs), pericytes (PCs), and keratinocytes (KCs). We further demonstrate the bioprinting of a human skin substitute with a dermal layer containing xeno-free cultured human EC, FBs, and PCs in a xeno-free bioink containing human collagen type I and fibronectin layered in a biocompatible polyglycolic acid mesh and subsequently seeded with xeno-free human KCs to form an epidermal layer. Following implantation of such bilayered skin grafts on the dorsum of immunodeficient mice, KCs form a mature stratified epidermis with rete ridge-like structures. The ECs and PCs form human EC-lined perfused microvessels within 2 weeks after implantation, preventing graft necrosis, and eliciting further perfusion of the graft by angiogenic host microvessels. As proof-of-concept, we generated 12 individual grafts using a single donor of all four cell types. In summary, we describe the fabrication of a bioprinted vascularized bilayered skin substitute under completely xeno-free culture conditions demonstrating feasibility of a xeno-free approach to complex tissue engineering.

Advanced in vitro Research Models to Study the Role of Endothelial Cells in Solid Organ Transplantation

The endothelium plays a key role in acute and chronic rejection of solid organ transplants. During both processes the endothelium is damaged often with major consequences for organ function. Also, endothelial cells (EC) have antigen-presenting properties and can in this manner initiate and enhance alloreactive immune responses. For decades, knowledge about these roles of EC have been obtained by studying both in vitro and in vivo models. These experimental models poorly imitate the immune response in patients and might explain why the discovery and development of agents that control EC responses is hampered. In recent years, various innovative human 3D in vitro models mimicking in vivo organ structure and function have been developed. These models will extend the knowledge about the diverse roles of EC in allograft rejection and will hopefully lead to discoveries of new targets that are involved in the interactions between the donor organ EC and the recipient's immune system. Moreover, these models can be used to gain a better insight in the mode of action of the currently prescribed immunosuppression and will enhance the development of novel therapeutics aiming to reduce allograft rejection and prolong graft survival.

Therapeutic Potential of Endothelial Colony-Forming Cells in Ischemic Disease: Strategies to Improve their Regenerative Efficacy

Cardiovascular disease (CVD) comprises a range of major clinical cardiac and circulatory diseases, which produce immense health and economic burdens worldwide. Currently, vascular regenerative surgery represents the most employed therapeutic option to treat ischemic disorders, even though not all the patients are amenable to surgical revascularization. Therefore, more efficient therapeutic approaches are urgently required to promote neovascularization. Therapeutic angiogenesis represents an emerging strategy that aims at reconstructing the damaged vascular network by stimulating local angiogenesis and/or promoting de novo blood vessel formation according to a process known as vasculogenesis. In turn, circulating endothelial colony-forming cells (ECFCs) represent truly endothelial precursors, which display high clonogenic potential and have the documented ability to originate de novo blood vessels in vivo. Therefore, ECFCs are regarded as the most promising cellular candidate to promote therapeutic angiogenesis in patients suffering from CVD. The current briefly summarizes the available information about the origin and characterization of ECFCs and then widely illustrates the preclinical studies that assessed their regenerative efficacy in a variety of ischemic disorders, including acute myocardial infarction, peripheral artery disease, ischemic brain disease, and retinopathy. Then, we describe the most common pharmacological, genetic, and epigenetic strategies employed to enhance the vasoreparative potential of autologous ECFCs by manipulating crucial pro-angiogenic signaling pathways, e.g., extracellular-signal regulated kinase/Akt, phosphoinositide 3-kinase, and Ca²⁺ signaling. We conclude by discussing the possibility of targeting circulating ECFCs to rescue their dysfunctional phenotype and promote neovascularization in the presence of CVD.

Cord blood-endothelial colony forming cells are immunotolerated and participate at post-ischemic angiogenesis in an original dorsal chamber immunocompetent mouse model

BACKGROUND

Cardiovascular diseases are the main cause of morbidity and mortality worldwide. Restoring blood supply to ischemic tissues is an essential goal for the successful treatment of these diseases. Growth factor or gene therapy efficacy remains controversial, but stem cell transplantation is emerging as an interesting approach to stimulate angiogenesis. Among the different stem cell populations, cord blood-endothelial progenitor cells (CB-EPCs) and more particularly cord blood-endothelial progenitor cell-derived endothelial colony forming cells (CB-ECFCs) have a great proliferative potential without exhibiting signs of senescence. Even if it was already described that CB-ECFCs were able to restore blood perfusion in hind-limb ischemia in an immunodeficient mouse model, until now, the immunogenic potential of allogenic CB-ECFCs remains controversial. Therefore, our objectives were to evaluate the immune tolerance potency of CB-ECFCs and their capacity to restore a functional vascular network under ischemic condition in immunocompetent mice.

METHODS

In vitro, the expression and secretion of immunoregulatory markers (HLA-G, IL-10, and TGF-β1) were evaluated on CB-ECFCs. Moreover, CB-ECFCs were co-cultured with activated peripheral blood mononuclear cells (PBMCs) for 6 days. PBMC proliferation was evaluated by [3H]-thymidine incorporation on the last 18 h. In vivo, CB-ECFCs were administered in the spleen and muscle of immunocompetent mice. Tissues were collected at day 14 after surgery. Finally, CB-ECFCs were injected intradermally in C57BL/6JRj mice close to ischemic macrovessel induced by thermal cauterization. Mice recovered until day 5 and were imaged, twice a week until day 30.

RESULTS

Firstly, we demonstrated that CB-ECFCs expressed HLA-G, IL-10, and TGF-β1 and secreted IL-10 and TGF-β1 and that they could display immunosuppressive properties in vitro. Secondly, we showed that CB-ECFCs could be tolerated until 14 days in immunocompetent mice. Thirdly, we revealed in an original ischemic model of dorsal chamber that CB-ECFCs were integrated in a new functional vascular network.

CONCLUSION

These results open up new perspectives about using CB-ECFCs as an allogeneic cell therapy product and gives new impulse to the treatment of cardiovascular diseases.

Immune Consequences of Endothelial Cells' Activation and Dysfunction During Sepsis

The vascular endothelium provides a direct interface between circulating blood cells and parenchymal cells. Thus, it has a key role in vasomotor tone regulation, primary hemostasis, vascular barrier, and immunity. In the case of systemic inflammation, endothelial cell (EC) activation initiates a powerful innate immune response to eliminate the pathogen. In some specific conditions, ECs may also contribute to the activation of adaptive immunity and the recruitment of antigen-specific lymphocytes. However, the loss of EC functions or an exaggerated activation of ECs during sepsis can lead to multiorgan failure.

Three Dimensional Bioprinting of a Vascularized and Perfusable Skin Graft Using Human Keratinocytes, Fibroblasts, Pericytes, and Endothelial Cells

Multilayered skin substitutes comprising allogeneic cells have been tested for the treatment of nonhealing cutaneous ulcers. However, such nonnative skin grafts fail to permanently engraft because they lack dermal vascular networks important for integration with the host tissue. In this study, we describe the fabrication of an implantable multilayered vascularized bioengineered skin graft using 3D bioprinting. The graft is formed using one bioink containing human foreskin dermal fibroblasts (FBs), human endothelial cells (ECs) derived from cord blood human endothelial colony-forming cells (HECFCs), and human placental pericytes (PCs) suspended in rat tail type I collagen to form a dermis followed by printing with a second bioink containing human foreskin keratinocytes (KCs) to form an epidermis. In vitro, KCs replicate and mature to form a multilayered barrier, while the ECs and PCs self-assemble into interconnected microvascular networks. The PCs in the dermal bioink associate with EC-lined vascular structures and appear to improve KC maturation. When these 3D printed grafts are implanted on the dorsum of immunodeficient mice, the human EC-lined structures inosculate with mouse microvessels arising from the wound bed and become perfused within 4 weeks after implantation. The presence of PCs in the printed dermis enhances the invasion of the graft by host microvessels and the formation of an epidermal rete. Impact Statement Three Dimensional printing can be used to generate multilayered vascularized human skin grafts that can potentially overcome the limitations of graft survival observed in current avascular skin substitutes. Inclusion of human pericytes in the dermal bioink appears to improve both dermal and epidermal maturation.

Progenitor-derived human endothelial cells evade alloimmunity by CRISPR/Cas9-mediated complete ablation of MHC expression

Tissue engineering may address organ shortages currently limiting clinical transplantation. Off-the-shelf engineered vascularized organs will likely use allogeneic endothelial cells (ECs) to construct microvessels required for graft perfusion. Vasculogenic ECs can be differentiated from committed progenitors (human endothelial colony-forming cells or HECFCs) without risk of mutation or teratoma formation associated with reprogrammed stem cells. Like other ECs, these cells can express both class I and class II major histocompatibility complex (MHC) molecules, bind donor-specific antibody (DSA), activate alloreactive T effector memory cells, and initiate rejection in the absence of donor leukocytes. CRISPR/Cas9-mediated dual ablation of β2-microglobulin and class II transactivator (CIITA) in HECFC-derived ECs eliminates both class I and II MHC expression while retaining EC functions and vasculogenic potential. Importantly, dually ablated ECs no longer bind human DSA or activate allogeneic CD4+ effector memory T cells and are resistant to killing by CD8+ alloreactive cytotoxic T lymphocytes in vitro and in vivo. Despite absent class I MHC molecules, these ECs do not activate or elicit cytotoxic activity from allogeneic natural killer cells. These data suggest that HECFC-derived ECs lacking MHC molecule expression can be utilized for engineering vascularized grafts that evade allorejection.

The effect of nacre extract on cord blood-derived endothelial progenitor cells: A natural stimulus to promote angiogenesis?

Angiogenesis is a critical parameter to consider for the development of tissue-engineered bone substitutes. The challenge is to promote sufficient vascularization in the bone substitute to prevent cell death and to allow its efficient integration. The capacity of nacre extract to restore the osteogenic activity of osteoarthritis osteoblasts has already been demonstrated. However, their angiogenic potential on endothelial progenitor cells (EPCs) was not yet explored. Therefore, the current study aimed at investigating if nacreous molecules affect EPC behavior. The gene and protein expression levels of endothelial cell (EC)-specific markers were determined in EPCs cultivated in presence of a nacre extract (ethanol soluble matrix [ESM] at two concentrations: 100 μg/mL and 200 μg/mL (respectively abbreviated ESM100 and ESM200)). Cell functionality was explored by proangiogenic factors production and in vitro tube formation assay. ESM200 increased the expression of some EC-specific genes. The in vitro tube formation assay demonstrated that ESM200 stimulated tubulogenesis affecting angiogenic parameters. We demonstrated that a stimulation with 200 μg/mL of ESM increased angiogenesis key elements. This in vitro study strongly highlights the proangiogenic effect of ESM. Due to its osteogenic properties, previously demonstrated, ESM could constitute the key element to develop an ideal prevascularized bone substitute. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A, 2019.

Gene editing advance re-ignites debate on the merits and risks of animal to human transplantation

In Australia, and internationally, the shortage of organ and tissue donors significantly limits the number of patients with critical organ or tissue failure who are able to receive a transplant each year. The rationale for xenotransplantation - the transplantation of living cells, tissues or organs from one species to another - is to meet this shortfall in human donor material. While early clinical trials showed promise, particularly in patients with type I diabetes whose insulin dependence could be temporarily reversed by the transplantation of porcine islet cells, these benefits have been balanced with scientific, clinical and ethical concerns revolving around the risks of immune rejection and the potential transmission of porcine endogenous retroviruses or other infectious agents from porcine grafts to human recipients. However, the advent of CRISPR/Cas9, a revolutionary gene editing technology, has reignited interest in the field with the possibility of genetically engineering porcine organs and tissues that are less immunogenic and have virtually no risk of transmission of porcine endogenous retroviruses. At the same time, CRISPR/Cas9 may also open up a myriad of possibilities for tissue engineering and stem cell research, which may complement xenotransplantation research by providing an additional source of donor cells, tissues and organs for transplantation into patients. The recent international symposium on gene editing, organised by the US National Academy of Sciences, highlights both the enormous therapeutic potential of CRISPR/Cas9 and the raft of ethical and regulatory challenges that may follow its utilisation in transplantation and in medicine more generally.

Endothelial cells: From innocent bystanders to active participants in immune responses

The endothelium is crucially important for the delivery of oxygen and nutrients throughout the body under homeostatic conditions. However, it also contributes to pathology, including the initiation and perpetuation of inflammation. Understanding the function of endothelial cells (ECs) in inflammatory diseases and molecular mechanisms involved may lead to novel approaches to dampen inflammation and restore homeostasis. In this article, we discuss the various functions of ECs in inflammation with a focus on pathological angiogenesis, attraction of immune cells, antigen presentation, immunoregulatory properties and endothelial-to-mesenchymal transition (EndMT). We also review the current literature on approaches to target these processes in ECs to modulate immune responses and advance anti-inflammatory therapies.

Impact of adipose tissue or umbilical cord derived mesenchymal stem cells on the immunogenicity of human cord blood derived endothelial progenitor cells

The application of autologous endothelial progenitor cell (EPC) transplantation is a promising approach in therapeutic cardiovascular diseases and ischemic diseases. In this study, we compared the immunogenicity of EPCs, adipose tissue (AD)-derived mesenchymal stem cells (MSCs) and umbilical cord (UC)-derived MSCs by flow cytometry and the mixed lymphocyte reaction. The impact of AD-MSCs and UC-MSCs on the immunogenicity of EPCs was analyzed by the mixed lymphocyte reaction and cytokine secretion in vitro and was further tested by allogenic peripheral blood mononuclear cell (PBMC) induced immuno-rejection on a cell/matrigel graft in an SCID mouse model. EPCs and AD-MSCs express higher levels of MHC class I than UC-MSCs. All three kinds of cells are negative for MHC class II. UC-MSCs also express lower levels of IFN-γ receptor mRNA when compared with EPCs and AD-MSCs. EPCs can stimulate higher rates of proliferation of lymphocytes than AD-MSCs and UC-MSCs. Furthermore, AD-MSCs and UC-MSCs can modulate immune response and inhibit lymphocyte proliferation induced by EPCs, mainly through inhibition of the proliferation of CD8+ T cells. Compared with UC-MSCs, AD-MSCs can significantly improve vessel formation and maintain the integrity of neovascular structure in an EPC+MSC/matrigel graft in SCID mice, especially under allo-PBMC induced immuno-rejection. In conclusion, our study shows that AD-MSC is a powerful candidate to minimize immunological rejection and improve vessel formation in EPC transplantation treatment.

Stromal Cells Act as Guardians for Endothelial Progenitors by Reducing Their Immunogenicity After Co-Transplantation

Regeneration of injured tissues requires effective therapeutic strategies supporting vasculogenesis. The lack of instantly available autologous cell sources and immunogenicity of allogeneic endothelial (progenitor) cells limits clinical progress. Based on the immunosuppressive potency of mesenchymal stem/progenitor cells (MSCs), we investigated whether crosstalk between endothelial colony-forming progenitor cells (ECFCs) and MSCs during vasculogenesis could lower allogeneic T cell responses against ECFCs allowing long-term engraftment in vivo. Immunodeficient mice received subcutaneous grafts containing human ECFCs alone, or pairs of human ECFCs/MSCs from the same umbilical cord (UC) to study vasculogenesis in the presence of human leukocyte antigen (HLA)-mismatched human peripheral blood mononuclear cells (PBMCs). In vitro, cell surface marker changes due to interferon gamma (IFNγ) stimulation during ECFC/MSC coculture were determined and further effects on allostimulated T cell proliferation and cytotoxic lysis were measured. IFNγ-induced HLA-DR expression on ECFCs and MSCs, but both cell types had significantly less HLA-DR in cocultures. ECFC-induced T cell proliferation was abolished after MSC coculture as a result of HLA-DR downregulation and indolamin-2,3-dioxygenase activation. Additionally, allospecific CD8⁺ T cell-mediated lysis of ECFCs was reduced in cocultures. ECFC/MSC coapplication in immunodeficient mice not only promoted the generation of improved blood vessel architecture after 6 weeks, but also reduced intragraft immune cell infiltration and endothelial HLA-DR expression following PBMC reconstitution. Crosstalk between UC-derived ECFCs and MSCs after combined transplantation can lower the risk of ECFC rejection, thus enabling their coapplication for therapeutic vasculogenesis. Stem Cells 2017;35:1233-1245.

Blocking MHC class II on human endothelium mitigates acute rejection

Acute allograft rejection is mediated by host CD8⁺ cytotoxic T lymphocytes (CTL) targeting graft class I major histocompatibility complex (MHC) molecules. In experimental rodent models, rejection requires differentiation of naive CD8⁺ T cells into alloreactive CTL within secondary lymphoid organs, whereas in humans, CTL may alternatively develop within the graft from circulating CD8⁺ effector memory T cells (T_EM) that recognize class I MHC molecules on graft endothelial cells (EC). This latter pathway is poorly understood. Here, we show that host CD4⁺ T_EM, activated by EC class II MHC molecules, provide critical help for this process. First, blocking HLA-DR on EC lining human artery grafts in immunodeficient mice reduces CD8⁺ CTL development within and acute rejection of the artery by adoptively transferred allogeneic human lymphocytes. Second, siRNA knockdown or CRISPR/Cas9 ablation of class II MHC molecules on EC prevents CD4⁺ T_EM from helping CD8⁺ T_EM to develop into CTL in vitro. Finally, implanted synthetic microvessels, formed from CRISPR/Cas9-modified EC lacking class II MHC molecules, are significantly protected from CD8⁺ T cell-mediated destruction in vivo. We conclude that human CD8⁺ T_EM-mediated rejection targeting graft EC class I MHC molecules requires help from CD4⁺ T_EM cells activated by recognition of class II MHC molecules.

Efficient gene disruption in cultured primary human endothelial cells by CRISPR/Cas9

RATIONALE

The participation of endothelial cells (EC) in many physiological and pathological processes is widely modeled using human EC cultures, but genetic manipulation of these untransformed cells has been technically challenging. Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 nuclease (Cas9) technology offers a promising new approach. However, mutagenized cultured cells require cloning to yield homogeneous populations, and the limited replicative lifespan of well-differentiated human EC presents a barrier for doing so.

OBJECTIVE

To create a simple but highly efficient method using CRISPR/Cas9 to generate biallelic gene disruption in untransformed human EC.

METHODS AND RESULTS

To demonstrate proof-of-principle, we used CRISPR/Cas9 to disrupt the gene for the class II transactivator. We used endothelial colony forming cell-derived EC and lentiviral vectors to deliver CRISPR/Cas9 elements to ablate EC expression of class II major histocompatibility complex molecules and with it, the capacity to activate allogeneic CD4(+) T cells. We show the observed loss-of-function arises from biallelic gene disruption in class II transactivator that leaves other essential properties of the cells intact, including self-assembly into blood vessels in vivo, and that the altered phenotype can be rescued by reintroduction of class II transactivator expression.

CONCLUSIONS

CRISPR/Cas9-modified human EC provides a powerful platform for vascular research and for regenerative medicine/tissue engineering.

Late-outgrowth endothelial progenitors from patients with coronary artery disease: endothelialization of confluent stromal cell layers

Patients with coronary artery disease (CAD) are the primary candidates to receive small-diameter tissue-engineered blood vessels (TEBVs). Peripheral blood derived endothelial progenitor cells (EPCs) from CAD patients (CAD EPCs) represent a minimally invasive source of autologous cells for TEBV endothelialization. We have previously shown that human CAD EPCs are highly proliferative and express many of the hallmarks of mature and healthy endothelial cells; however, their behavior on stromal cells that comprise the media of TEBVs has not yet been evaluated. Primary CAD EPCs or control human aortic endothelial cells (HAECs) were seeded over confluent, quiescent layers of human smooth muscle cells (SMCs) using a direct co-culture model. The percent coverage, adhesion strength, alignment under flow and generation of flow-induced nitric oxide of the seeded CAD EPCs were compared to that of HAECs. The integrin-binding profile of CAD EPCs was also evaluated over a layer of confluent, quiescent SMCs. Direct comparison of our CAD EPC results to analogous co-culture studies with cord blood EPCs show that both types of blood-derived EPCs are viable options for endothelialization of TEBVs.

Diabetes-associated macrovascular complications: cell-based therapy a new tool?

Diabetes mellitus and its ongoing macrovascular complications represent one of the major health problems around the world. Rise in obesity and population ages correlate with the increased incidence of diabetes. This highlights the need for novel approaches to prevent and treat this pandemic. The discovery of a reservoir of stem/progenitors in bone marrow and in mesenchymal tissue has attracted interest of both biologists and clinicians. A number of preclinical and clinical trials were developed to explore their potential clinical impact, as target or vehicle, in different clinical settings, including diabetes complications. Currently, bone marrow, peripheral blood, mesenchymal, and adipose tissues have been used as stem/progenitor cell sources. However, evidences have been provided that both bone marrow and circulating progenitor cells are dysfunctional in diabetes. These observations along with the growing advantages in genetic manipulation have spurred researchers to exploit ex vivo manipulated cells to overcome these hurdles. In this article, we provide an overview of data relevant to stem-progenitors potential clinical application in revascularization and/or vascular repair. Moreover, the hurdles at using progenitor cells in diabetic patients will be also discussed.

Development of a xeno-free autologous culture system for endothelial progenitor cells derived from human umbilical cord blood

Despite promising preclinical outcomes in animal models, a number of challenges remain for human clinical use. In particular, expanding a large number of endothelial progenitor cells (EPCs) in vitro in the absence of animal-derived products is the most critical hurdle remaining to be overcome to ensure the safety and efficiency of human therapy. To develop in vitro culture conditions for EPCs derived from human cord blood (hCB-EPCs), we isolated extracts (UCE) and collagen (UC-collagen) from umbilical cord tissue to replace their animal-derived counterparts. UC-collagen and UCE efficiently supported the attachment and proliferation of hCB-EPCs in a manner comparable to that of animal-derived collagen in the conventional culture system. Our developed autologous culture system maintained the typical characteristics of hCB-EPCs, as represented by the expression of EPC-associated surface markers. In addition, the therapeutic potential of hCB-EPCs was confirmed when the transplantation of hCB-EPCs cultured in this autologous culture system promoted limb salvage in a mouse model of hindlimb ischemia and was shown to contribute to attenuating muscle degeneration and fibrosis. We suggest that the umbilical cord represents a source for autologous biomaterials for the in vitro culture of hCB-EPCs. The main characteristics and therapeutic potential of hCB-EPCs were not compromised in developed autologous culture system. The absence of animal-derived products in our newly developed in vitro culture removes concerns associated with secondary contamination. Thus, we hope that this culture system accelerates the realization of therapeutic applications of autologous hCB-EPCs for human vascular diseases.

Characterization of a distinct population of circulating human non-adherent endothelial forming cells and their recruitment via intercellular adhesion molecule-3

Circulating vascular progenitor cells contribute to the pathological vasculogenesis of cancer whilst on the other hand offer much promise in therapeutic revascularization in post-occlusion intervention in cardiovascular disease. However, their characterization has been hampered by the many variables to produce them as well as their described phenotypic and functional heterogeneity. Herein we have isolated, enriched for and then characterized a human umbilical cord blood derived CD133⁺ population of non-adherent endothelial forming cells (naEFCs) which expressed the hematopoietic progenitor cell markers (CD133, CD34, CD117, CD90 and CD38) together with mature endothelial cell markers (VEGFR2, CD144 and CD31). These cells also expressed low levels of CD45 but did not express the lymphoid markers (CD3, CD4, CD8) or myeloid markers (CD11b and CD14) which distinguishes them from ‘early’ endothelial progenitor cells (EPCs). Functional studies demonstrated that these naEFCs (i) bound Ulex europaeus lectin, (ii) demonstrated acetylated-low density lipoprotein uptake, (iii) increased vascular cell adhesion molecule (VCAM-1) surface expression in response to tumor necrosis factor and (iv) in co-culture with mature endothelial cells increased the number of tubes, tubule branching and loops in a 3-dimensional in vitro matrix. More importantly, naEFCs placed in vivo generated new lumen containing vasculature lined by CD144 expressing human endothelial cells (ECs). Extensive genomic and proteomic analyses of the naEFCs showed that intercellular adhesion molecule (ICAM)-3 is expressed on their cell surface but not on mature endothelial cells. Furthermore, functional analysis demonstrated that ICAM-3 mediated the rolling and adhesive events of the naEFCs under shear stress. We suggest that the distinct population of naEFCs identified and characterized here represents a new valuable therapeutic target to control aberrant vasculogenesis.

Functional late outgrowth endothelial progenitors isolated from peripheral blood of burned patients

BACKGROUND

Bioengineered skin substitutes are increasingly considered as a useful option for the treatment of full thickness burn injury. Their viability following grafting can be enhanced by seeding the skin substitute with late outgrowth endothelial progenitor cells (EPCs). However, it is not known whether autologous EPCs can be obtained from burned patients shortly after injury.

METHODS

Late outgrowth EPCs were isolated from peripheral blood sampled obtained from 10 burned patients (extent 19.6±10.3% TBSA) within the first 24h of hospital admission, and from 7 healthy subjects. Late outgrowth EPCs were phenotyped in vitro.

RESULTS

In comparison with similar cells obtained from healthy subjects, growing colonies from burned patients yielded a higher percentage of EPC clones (46 versus 17%, p=0.013). Furthermore, EPCs from burned patients secreted more vascular endothelial growth factor (VEGF) into the culture medium than did their counterparts from healthy subjects (85.8±56.2 versus 17.6±14pg/mg protein, p=0.018). When injected to athymic nude mice 6h after unilateral ligation of the femoral artery, EPCs from both groups of subjects greatly accelerated the reperfusion of the ischaemic hindlimb and increased the number of vascular smooth muscle cells.

CONCLUSIONS

The present study supports that, in patients with burns of moderate extension, it is feasible to obtain functional autologous late outgrowth EPCs from peripheral blood. These results constitute a strong incentive to pursue approaches based on using autotransplantation of these cells to improve the therapy of full thickness burns.

Over-expression of sphingosine kinase-1 enhances a progenitor phenotype in human endothelial cells

OBJECTIVES

The use of endothelial progenitor cells in vascular therapies has been limited due to their low numbers present in the bone marrow and peripheral blood. The aim of this study was to investigate the effect of sphingosine kinase on the de-differentiation of mature human endothelial cells toward a progenitor phenotype.

METHODS

The lipid enzyme sphingosine kinase-1 was lentivirally over-expressed in human umbilical vein endothelial cells and cells were analyzed for progenitor phenotype and function.

RESULTS

Sphingosine kinase-1 mRNA expression was induced approximately 150-fold with a resultant 20-fold increase in sphingosine kinase-1 enzymatic activity. The mRNA expression of the progenitor cell markers CD34, CD133, and CD117 and transcription factor NANOG increased, while the endothelial cell markers analyzed were largely unchanged. The protein level of mature endothelial cell surface markers CD31, CD144, and von Willebrand factor significantly decreased compared to controls. In addition, functional assays provided further evidence for a de-differentiated phenotype with increased viability, reduced uptake of acetylated low-density lipoprotein and decreased tube formation in Matrigel in the cells over-expressing sphingosine kinase-1.

CONCLUSIONS

These findings suggest that over-expression of sphingosine kinase-1 in human endothelial cells promotes, in part, their de-differentiation to a progenitor cell phenotype, and is thus a potential tool for the generation of a large population of vascular progenitor cells for therapeutic use.

MicroRNA-16 and microRNA-424 regulate cell-autonomous angiogenic functions in endothelial cells via targeting vascular endothelial growth factor receptor-2 and fibroblast growth factor receptor-1

OBJECTIVE

MicroRNAs play key roles in modulating a variety of cellular processes by posttranscriptional regulation of their target genes. Vascular endothelial growth factor (VEGF), VEGF receptor-2 (VEGFR2), and fibroblast growth factor receptor-1 (FGFR1) were identified by bioinformatic approaches and subsequently validated as targets of microRNA (miR)-16 and miR-424 in endothelial cells (ECs).

METHODS AND RESULTS

Mimetics of these microRNAs reduced VEGF, VEGFR2, and FGFR1 expression, whereas specific antagonists enhanced their expression. Expression of mature miR-16 and miR-424 was upregulated on VEGF or basic fibroblast growth factor (bFGF) treatment. This upregulation was accompanied by a parallel increase in primary transcript (pri-miR)-16-1 and pri-miR-16-2 but not in pri-miR-424 levels, indicating a VEGF/bFGF-dependent transcriptional and posttranscriptional regulation of miR-16 and miR-424, respectively. Reduced expression of VEGFR2 and FGFR1 by miR-16 or miR-424 overexpression regulated VEGF and bFGF signaling through these receptors, thereby affecting the activity of downstream components of the pathways. Functionally, miR-16 or miR-424 overexpression reduced proliferation, migration, and cord formation of ECs in vitro, and lentiviral overexpression of miR-16 reduced the ability of ECs to form blood vessels in vivo.

CONCLUSION

We conclude that these miRNAs fine-tune the expression of selected endothelial angiogenic mediators in response to these growth factors. Altogether, these findings suggest that miR-16 and miR-424 play important roles in regulating cell-intrinsic angiogenic activity of ECs.

Participation of blood vessel cells in human adaptive immune responses

Circulating T cells contact blood vessels either when they extravasate across the walls of microvessels into inflamed tissues or when they enter into the walls of larger vessels in inflammatory diseases such as atherosclerosis. The blood vessel wall is largely composed of three cell types: endothelial cells lining the entire vascular tree; pericytes supporting the endothelium of microvessels; and smooth muscle cells forming the bulk of large vessel walls. Each of these cell types interacts with and alters the behavior of infiltrating T cells in different ways, making these cells active participants in the processes of immune-mediated inflammation. In this review, we compare and contrast what is known about the nature of these interactions in humans.

Cord blood-circulating endothelial progenitors for treatment of vascular diseases

Adult peripheral blood (PB) endothelial progenitor cells (EPC) are produced in the bone marrow and are able to integrate vascular structures in sites of neoangiogenesis. EPCs thus represent a potential therapeutic tool for ischaemic diseases. However, use of autologous EPCs in cell therapy is limited by their rarity in adult PB. Cord blood (CB) contains more EPCs than PB, and they are functional after expansion. They form primary colonies that give rise to secondary colonies, each yielding more than 10(7) cells after few passages. The number of endothelial cells obtained from one unit of CB is compatible with potential clinical application. EPC colonies can be securely produced, expanded and cryopreserved in close culture devices and endothelial cells produced in these conditions are functional as shown in different in vitro and in vivo assays. As CB EPC-derived endothelial cells would be allogeneic to patients, it would be of interest to prepare them from ready-existing CB banks. We show that not all frozen CB units from a CB bank are able to generate EPC colonies in culture, and when they do so, number of colonies is lower than that obtained with fresh CB units. However, endothelial cells derived from frozen CB have the same phenotypical and functional properties than those derived from fresh CB. This indicates that CB cryopreservation should be improved to preserve integrity of stem cells other than haematopoietic ones. Feasibility of using CB for clinical applications will be validated in porcine models of ischaemia.

Human umbilical cord blood-derived endothelial cells reendothelialize vein grafts and prevent thrombosis

OBJECTIVE

To accelerate vein graft reendothelialization and reduce vein graft thrombosis by infusing human umbilical cord blood-derived endothelial cells (hCB-ECs) because loss of endothelium contributes to vein graft thrombosis and neointimal hyperplasia.

METHODS AND RESULTS

Under steady flow conditions in vitro, hCB-ECs adhered to smooth muscle cells 2.5 to 13 times more than ECs derived from peripheral blood or human aorta (P<0.05). Compared with peripheral blood and human aorta ECs, hCB-ECs had 1.4-fold more cell surface α(5)β(1) integrin heterodimers per cell (P<0.05) and proliferated on fibronectin 4- to 10-fold more rapidly (P<0.05). Therefore, we used hCB-ECs to enhance reendothelialization of carotid interposition vein grafts implanted in NOD.CB17-Prkdc(scid)/J mice. Two weeks postoperatively, vein grafts from hCB-EC-treated mice demonstrated approximately 55% reendothelialization and no luminal thrombosis. In contrast, vein grafts from sham-treated mice demonstrated luminal thrombosis in 75% of specimens (P<0.05) and only approximately 14% reendothelialization. In vein grafts from hCB-EC-treated mice, 33±10% of the endothelium was of human origin, as judged by human major histocompatibility class I expression.

CONCLUSIONS

The hCB-ECs adhere to smooth muscle cells under flow conditions in vitro, accelerate vein graft reendothelialization in vivo, and prevent vein graft thrombosis. Thus, hCB-ECs offer novel therapeutic possibilities for vein graft disease.

Cross-reactivity of herpesvirus-specific CD8 T cell lines toward allogeneic class I MHC molecules

Although association between persistent viral infection and allograft rejection is well characterized, few examples of T-cell cross-reactivity between self-MHC/viral and allogeneic HLA molecules have been documented so far. We appraised in this study the alloreactivity of CD8 T cell lines specific for immunodominant epitopes from human cytomegalovirus (HCMV) and Epstein-Barr virus (EBV). CD8 T cell lines were generated after sorting with immunomagnetic beads coated with either pp65_495–503/A*0201, BMLF1_259–267/A*0201, or BZLF1_54–64/B*3501 multimeric complexes. Alloreactivity of the CD8 T cell lines against allogeneic class I MHC alleles was assessed by screening of (i) TNF-α production against COS-7 cells transfected with as many as 39 individual HLA class I-encoding cDNA, and (ii) cytotoxicity activity toward a large panel of HLA-typed EBV-transformed B lymphoblastoid cell lines. We identified several cross-reactive pp65/A*0201-specific T cell lines toward allogeneic HLA-A*3001, A*3101, or A*3201. Moreover, we described here cross-recognition of HLA-Cw*0602 by BZLF1/B*3501-specific T cells. It is noteworthy that these alloreactive CD8 T cell lines showed efficient recognition of endothelial cells expressing the relevant HLA class I allele, with high level TNF-α production and cytotoxicity activity. Taken together, our data support the notion that herpes virus-specific T cells recognizing allo-HLA alleles may promote solid organ rejection.

Immune privilege of endothelial cells differentiated from endothelial progenitor cells

AIMS

The application of autologous endothelial progenitor cells (EPC) is a promising approach in cardiovascular regeneration, but the availability of cells in appropriate numbers is the limiting factor. Allogeneic EPC would be an alternative, and we therefore analysed the immunogenicity of EPC-derived endothelial cells (EC) to evaluate their potential usefulness.

METHODS AND RESULTS

Circulating EPC from rat were differentiated into EC and characterized phenotypically and functionally. Major histocompatibility complex (MHC) expression in response to interferon-gamma was determined compared with rat aortic EC, and in vitro humoral and cellular allogeneic responses were analysed. To determine the in vivo effects, acellular aortic grafts were endothelialized in vitro with EPC-derived EC and transplanted in a complete allogeneic mismatch rat aortic interposition model. EPC-derived EC expressed endothelial-specific markers and low levels of MHC class I (MHC I), but no constitutive MHC class II (MHC II). When stimulated with interferon-gamma, they upregulated MHC I and moderately upregulated MHC II. They were protected against alloantibody/complement-mediated lysis and allospecific cytotoxic T lymphocyte activity. They were less potent in allogeneic stimulation of CD4 T cells than aortic EC. Seeding of EPC-derived EC into acellular grafts led to excellent endothelialization, and allogeneic aortic transplantation induced only mild inflammatory responses without signs of rejection.

CONCLUSION

EPC-derived EC are protected against allospecific cellular immune responses and humoral-mediated attacks in vitro. When transplanted in vivo as a component of vascular grafts, these cells are not rejected, which makes them useful in therapeutic applications, especially vascular reconstruction.

Dual delivery of VEGF and MCP-1 to support endothelial cell transplantation for therapeutic vascularization

Transplantation of endothelial cells (EC) for therapeutic vascularization is a promising approach in tissue engineering but has yet to be proven effective in clinical trials. This cell-based therapy is hindered by significant apoptosis of EC upon transplantation as well as poor recruitment of host mural cells to stabilize nascent vessels. Here, we address these deficiencies by augmenting endothelial cell transplantation with dual delivery of vascular endothelial growth factor (VEGF) - to improve survival of transplanted EC - and monocyte chemotactic protein-1 (MCP-1) - to induce mural cell recruitment. We produced alginate microparticles that deliver VEGF and MCP-1 with distinct release kinetics and that can be integrated into a collagen/fibronectin (protein) gel construct for delivery of EC. Combined delivery of VEGF and MCP-1 increased functional vessel formation from transplanted EC and also led to a higher number of smooth muscle cell-invested vessels than did EC therapy alone. Despite the well-known role of MCP-1 in inflammation, these beneficial effects were accomplished without a long-term increase in monocyte/macrophage recruitment or a shift to a pro-inflammatory (M1) macrophage phenotype. Overall, these data suggest a potential benefit of combined delivery of MCP-1 and VEGF from EC-containing hydrogels as a strategy for therapeutic vascularization.

Characterization of umbilical cord blood-derived late outgrowth endothelial progenitor cells exposed to laminar shear stress

Endothelial progenitor cells isolated from umbilical cord blood (CB-EPCs) represent a promising source of endothelial cells for synthetic vascular grafts and tissue-engineered blood vessels since they are readily attainable, can be easily isolated, and possess a high proliferation potential. The objective of this study was to compare the functional behavior of late outgrowth CB-EPCs with human aortic endothelial cells (HAECs). CB-EPCs and HAECs were cultured on either smooth muscle cells in a coculture model of a tissue-engineered blood vessels or fibronectin adsorbed to Teflon-AF-coated glass slides. Late outgrowth CB-EPCs expressed endothelial cell-specific markers and were negative for the monocytic marker CD14. CB-EPCs have higher proliferation rates than HAECs, but are slightly smaller in size. CB-EPCs remained adherent under supraphysiological shear stresses, oriented and elongated in the direction of flow, and expressed similar numbers of alpha(5)beta(1) and alpha(v)beta(3) integrins and antithrombotic genes compared to HAECs. There were some differences in mRNA levels of E-selectin and vascular cell adhesion molecule 1 between CB-EPCs and HAECs; however, protein levels were similar on the two cell types, and CB-EPCs did not support adhesion of monocytes in the absence of tumor necrosis factor-alpha stimulation. Although CB-EPCs expressed significantly less endothelial nitric oxide synthase protein after exposure to flow than HAECs, nitric oxide levels induced by flow were not significantly different. These results suggest that late outgrowth CB-EPCs are functionally similar to HAECs under flow conditions and are a promising cell source for cardiovascular therapies.

Endothelial cells in allograft rejection

In organ transplantation, blood borne cells and macromolecules (e.g., antibodies) of the host immune system are brought into direct contact with the endothelial cell lining of graft vessels. In this location, graft endothelial cells play several roles in allograft rejection, including the initiation of rejection responses by presentation of alloantigen to circulating T cells; the development of inflammation and thrombosis; and as targets of injury and agents of repair.

Endothelial cells in allograft rejection

In organ transplantation, blood borne cells and macromolecules (e.g., antibodies) of the host immune system are brought into direct contact with the endothelial cell lining of graft vessels. In this location, graft endothelial cells play several roles in allograft rejection, including the initiation of rejection responses by presentation of alloantigen to circulating T cells; the development of inflammation and thrombosis; and as targets of injury and agents of repair.

Dicer-dependent endothelial microRNAs are necessary for postnatal angiogenesis

Posttranscriptional gene regulation by microRNAs (miRNAs) is important for many aspects of development, homeostasis, and disease. Here, we show that reduction of endothelial miRNAs by cell-specific inactivation of Dicer, the terminal endonuclease responsible for the generation of miRNAs, reduces postnatal angiogenic response to a variety of stimuli, including exogenous VEGF, tumors, limb ischemia, and wound healing. Furthermore, VEGF regulated the expression of several miRNAs, including the up-regulation of components of the c-Myc oncogenic cluster miR-17-92. Transfection of endothelial cells with components of the miR-17-92 cluster, induced by VEGF treatment, rescued the induced expression of thrombospondin-1 and the defect in endothelial cell proliferation and morphogenesis initiated by the loss of Dicer. Thus, endothelial miRNAs regulate postnatal angiogenesis and VEGF induces the expression of miRNAs implicated in the regulation of an integrated angiogenic response.

IFN-alpha induces transcription of hypoxia-inducible factor-1alpha to inhibit proliferation of human endothelial cells

Expression of hypoxia-inducible factor (HIF)-1alpha, a transcription factor subunit increased by protein stabilization in response to hypoxia, is increased in human endothelial cells (ECs) by IFN-alpha under normoxic conditions. IFN-alpha increases HIF-1alpha transcript levels within 2 h by up to 50% and doubles HIF-1alpha protein expression. Based on pharmacological inhibition studies, the increase in HIF-1alpha mRNA involves new transcription, is independent of new protein synthesis, and requires JAK signaling. Protein knockdown by small interfering RNA confirms the involvement of JAK1 and TYK2, as well of IFN-stimulated gene factor 3 (ISGF3). IFN-gamma does not significantly induce HIF-1alpha mRNA, but increases the magnitude and duration of the IFN-alpha effect. IFN-alpha-induced HIF-1alpha protein translocates to the nucleus and can bind to hypoxia response elements in DNA. However, IFN-alpha treatment fails to induce transcription of several prototypic HIF-responsive genes (VEGF-A, PPARgamma, and prostacyclin synthase) due to an insufficient increase in HIF-1alpha protein levels. Although certain other HIF-responsive genes (PHD3 and VEGF-C) are induced following IFN-alpha and/or IFN-gamma treatment, these responses are not inhibited by siRNA knockdown of HIF-1alpha. Additionally, IFN-alpha induction of ISGF3-dependent genes involved in innate immunity (viperin, OAS2, and CXCL10) are also unaffected by knockdown of HIF-1alpha. Interestingly, knockdown of HIF-1alpha significantly reduces the capacity of IFN-alpha to inhibit endothelial cell proliferation. We conclude that IFN-alpha induces the transcription of HIF-1alpha in human endothelial cells though a JAK-ISGF3 pathway under normoxic conditions, and that this response contributes to the antiproliferative activity of this cytokine.