Endothelial cell seeding of polytetrafluoroethylene vascular grafts in humans: a preliminary report

Progressive Reinvention or Destination Lost? Half a Century of Cardiovascular Tissue Engineering

The concept of tissue engineering evolved long before the phrase was forged, driven by the thromboembolic complications associated with the early total artificial heart programs of the 1960s. Yet more than half a century of dedicated research has not fulfilled the promise of successful broad clinical implementation. A historical account outlines reasons for this scientific impasse. For one, there was a disconnect between distinct eras each characterized by different clinical needs and different advocates. Initiated by the pioneers of cardiac surgery attempting to create neointimas on total artificial hearts, tissue engineering became fashionable when vascular surgeons pursued the endothelialisation of vascular grafts in the late 1970s. A decade later, it were cardiac surgeons again who strived to improve the longevity of tissue heart valves, and lastly, cardiologists entered the fray pursuing myocardial regeneration. Each of these disciplines and eras started with immense enthusiasm but were only remotely aware of the preceding efforts. Over the decades, the growing complexity of cellular and molecular biology as well as polymer sciences have led to surgeons gradually being replaced by scientists as the champions of tissue engineering. Together with a widening chasm between clinical purpose, human pathobiology and laboratory-based solutions, clinical implementation increasingly faded away as the singular endpoint of all strategies. Moreover, a loss of insight into the healing of cardiovascular prostheses in humans resulted in the acceptance of misleading animal models compromising the translation from laboratory to clinical reality. This was most evident in vascular graft healing, where the two main impediments to the in-situ generation of functional tissue in humans remained unheeded–the trans-anastomotic outgrowth stoppage of endothelium and the build-up of an impenetrable surface thrombus. To overcome this dead-lock, research focus needs to shift from a biologically possible tissue regeneration response to one that is feasible at the intended site and in the intended host environment of patients. Equipped with an impressive toolbox of modern biomaterials and deep insight into cues for facilitated healing, reconnecting to the “user needs” of patients would bring one of the most exciting concepts of cardiovascular medicine closer to clinical reality.

Improved Patency of ePTFE Grafts as a Hemodialysis Access Site by Seeding Autologous Endothelial Cells Expressing Fibulin-5 and VEGF

Small caliber synthetic vascular grafts used for dialysis access sites have high failure rates due to neointima formation and thrombosis. Seeding synthetic grafts with endothelial cells (ECs) provides a biocompatible surface that may prevent graft failure. We tested the use of ePTFE grafts seeded with autologous ECs expressing fibulin-5 and vascular endothelial growth factor (VEGF), as a dialysis access site in a porcine model. We connected the carotid arteries and jugular veins of 12 miniature pigs using 7-mm ePTFE grafts; five grafts were seeded with autologous venous ECs modified to express fibulin-5 and VEGF, and seven unseeded grafts were implanted at the same location and served as controls. At 6 months, after completion of angiography, the carotid arteries and jugular veins with the connecting grafts were excised and fixed. Autologous EC isolation and transduction and graft seeding were successful in all animals. At 3 months, 4 of 5 seeded grafts and 3 of 7 control grafts were patent. At 6 months, 4 of 5 (80%) seeded grafts and only 2 of 7 (29%) control grafts were patent. Seeding ePTFE vascular grafts with genetically modified ECs improved long term small caliber graft patency. The biosynthetic grafts offer a novel therapeutic modality for vascular access in hemodialysis.

Translational Challenges in Cardiovascular Tissue Engineering

Valvular heart disease and congenital heart defects represent a major cause of death around the globe. Although current therapy strategies have rapidly evolved over the decades and are nowadays safe, effective, and applicable to many affected patients, the currently used artificial prostheses are still suboptimal. They do not promote regeneration, physiological remodeling, or growth (particularly important aspects for children) as their native counterparts. This results in the continuous degeneration and subsequent failure of these prostheses which is often associated with an increased morbidity and mortality as well as the need for multiple re-interventions. To overcome this problem, the concept of tissue engineering (TE) has been repeatedly suggested as a potential technology to enable native-like cardiovascular replacements with regenerative and growth capacities, suitable for young adults and children. However, despite promising data from pre-clinical and first clinical pilot trials, the translation and clinical relevance of such TE technologies is still very limited. The reasons that currently limit broad clinical adoption are multifaceted and comprise of scientific, clinical, logistical, technical, and regulatory challenges which need to be overcome. The aim of this review is to provide an overview about the translational problems and challenges in current TE approaches. It further suggests directions and potential solutions on how these issues may be efficiently addressed in the future to accelerate clinical translation. In addition, a particular focus is put on the current regulatory guidelines and the associated challenges for these promising TE technologies.

Enabling non-invasive assessment of an engineered endothelium on ePTFE vascular grafts without increasing oxidative stress

Magnetic resonance imaging (MRI) in combination with contrast enhancement is a potentially powerful tool to non-invasively monitor cell distribution in tissue engineering and regenerative medicine. The most commonly used contrast agent for cell labeling is super paramagnetic iron oxide nanoparticles (SPIONs). However, uptake of SPIONs triggers the production of reactive oxygen species (ROS) in cells often leading to a pro-inflammatory phenotype. The objective of this study was to develop a labeling system to non-invasively visualize an engineered endothelium in vascular grafts without creating excessive oxidative stress. Specifically, we investigated: (1) chitosan-coated SPIONs (CSPIONs) as an antioxidant contrast agent for contrast enhancement, and (2) poly(1,8-octamethylene citrate) (POC) as an antioxidant interface to support cell adhesion and function of labeled cells on the vascular graft. While SPION-labeled endothelial cells (ECs) experienced elevated ROS formation and altered cell morphology, CSPION-labeled ECs cultured on POC-coated surfaces mitigated SPION-induced ROS formation and maintained EC morphology, phenotype, viability and functions. A monolayer of labeled ECs exhibited sufficient contrast with T2-weighed MR imaging. CSPION labeling of endothelial cells in combination with coating the graft wall with POC allows non-invasive monitoring of an engineered endothelium on ePTFE grafts without increasing oxidative stress.

Multifaceted prospects of nanocomposites for cardiovascular grafts and stents

Cardiovascular disease is the leading cause of death across the globe. The use of synthetic materials is indispensable in the treatment of cardiovascular disease. Major drawbacks related to the use of biomaterials are their mechanical properties and biocompatibility, and these have to be circumvented before promoting the material to the market or clinical setting. Revolutionary advancements in nanotechnology have introduced a novel class of materials called nanocomposites which have superior properties for biomedical applications. Recently, there has been a widespread recognition of the nanocomposites utilizing polyhedral oligomeric silsesquioxane, bacterial cellulose, silk fibroin, iron oxide magnetic nanoparticles, and carbon nanotubes in cardiovascular grafts and stents. The unique characteristics of these nanocomposites have led to the development of a wide range of nanostructured copolymers with appreciably enhanced properties, such as improved mechanical, chemical, and physical characteristics suitable for cardiovascular implants. The incorporation of advanced nanocomposite materials in cardiovascular grafts and stents improves hemocompatibility, enhances antithrombogenicity, improves mechanical and surface properties, and decreases the microbial response to the cardiovascular implants. A thorough attempt is made to summarize the various applications of nanocomposites for cardiovascular graft and stent applications. This review will highlight the recent advances in nanocomposites and also address the need of future research in promoting nanocomposites as plausible candidates in a campaign against cardiovascular disease.

Cardiovascular tissue engineering: where we come from and where are we now?

Tissue engineering was introduced by Vacanti and Langer in the 80’s, exploring the potential of this new technology starting with the well-known “human ear on the mouse back”. The goal is to create a substitute which supplies an individual therapy for patients with regeneration, remodeling and growth potential. The growth potential of these subjects is of special interest in congenital cardiac surgery, avoiding repeated interventions and surgery. Initial applications of tissue engineered created substitutes were relatively simple cardiovascular grafts seeded initially by end-differentiated autologous endothelial cells. Important data were collected from these initial clinical autologous endothelial cell seeded grafts in peripheral and coronary vessel disease. After these initial successfully implantation bone marrow cell were used to seed patches and pulmonary conduits were implanted in patients. Driven by the positive results of tissue engineered material implanted under low pressure circumstances, first tissue engineered patches were implanted in the systemic circulation followed by the implantation of tissue engineered aortic heart valves. Tissue engineering is an extreme dynamic technology with continuously modifications and improvements to optimize clinical products. New technologies are unified and so this has also be done with tissue engineering and new application features, so called transcatheter valve intervention. First studies are initiated to apply tissue engineered heart valves with this new transcatheter delivery system less invasive. Simultaneously studies have been started on tissue engineering of so-called whole organs since organ transplantation is restricted due to donor shortage and tissue engineering could overcome this problem. Initial studies of whole heart engineering in the rat model are promising and larger size models are initiated.

The evolution of vascular tissue engineering and current state of the art

Dacron® (polyethylene terephthalate) and Goretex® (expanded polytetrafluoroethylene) vascular grafts have been very successful in replacing obstructed blood vessels of large and medium diameters. However, as diameters decrease below 6 mm, these grafts are clearly outperformed by transposed autologous veins and, particularly, arteries. With approximately 8 million individuals with peripheral arterial disease, over 500,000 patients diagnosed with end-stage renal disease, and over 250,000 patients per year undergoing coronary bypass in the USA alone, there is a critical clinical need for a functional small-diameter conduit [Lloyd-Jones et al., Circulation 2010;121:e46-e215]. Over the last decade, we have witnessed a dramatic paradigm shift in cardiovascular tissue engineering that has driven the field away from biomaterial-focused approaches and towards more biology-driven strategies. In this article, we review the preclinical and clinical efforts in the quest for a tissue-engineered blood vessel that is free of permanent synthetic scaffolds but has the mechanical strength to become a successful arterial graft. Special emphasis is given to the tissue engineering by self-assembly (TESA) approach, which has been the only one to reach clinical trials for applications under arterial pressure.

Cardiovascular application of polyhedral oligomeric silsesquioxane nanomaterials: a glimpse into prospective horizons

Revolutionary advances in nanotechnology propose novel materials with superior properties for biomedical application. One of the most promising nanomaterials for biomedical application is polyhedral oligomeric silsesquioxane (POSS), an amazing nanocage consisting of an inner inorganic framework of silicon and oxygen atoms and an outer shell of organic groups. The unique properties of this nanoparticle has led to the development of a wide range of nanostructured copolymers with significantly enhanced properties including improved mechanical, chemical, and physical characteristics. Since POSS nanomaterials are highly biocompatible, biomedical application of POSS nanostructures has been intensely explored. One of the most promising areas of application of POSS nanomaterials is the development of cardiovascular implants. The incorporation of POSS into biocompatible polymers has resulted in advanced nanocomposite materials with improved hemocompatibility, antithrombogenicity, enhanced mechanical and surface properties, calcification resistance, and reduced inflammatory response, which make these materials the material of choice for cardiovascular implants. These highly versatile POSS derivatives have opened new horizons to the field of cardiovascular implant. Currently, application of POSS containing polymers in the development of new generation cardiovascular implants including heart valve prostheses, bypass grafts, and coronary stents is under intensive investigation, with encouraging outcomes.

Polymeric materials for tissue engineering of arterial substitutes

Cardiovascular disease is the leading cause of mortality in the United States. The limited availability of healthy autologous vessels for bypass grafting procedures has led to the fabrication of prosthetic vascular conduits. Synthetic polymeric materials, while providing the appropriate mechanical strength, lack the compliance and biocompatibility that bioresorbable and naturally occurring protein polymers offer. Vascular tissue engineering approaches have emerged in order to meet the challenges of designing a vascular graft with long-term patency. In vitro culture techniques that have been explored with vascular cell seeding of polymeric scaffolds and the use of bioactive polymers for in situ arterial regeneration have yielded promising results. This review describes the development of polymeric materials in various tissue engineering strategies for the improvement in the mechanical and biological performance of an arterial substitute.

Defining the potential for cell therapy for vascular disease using animal models

Cell-based therapeutics are currently being developed for a wide array of unmet medical needs. As obstructive vascular disease is the major cause of mortality in the world, cell-based strategies aimed at developing novel therapies or improving current therapies are currently under study. These studies are based on the evolving understanding of the biology of vascular progenitor cells, which has in turn led to the availability of well-defined sources of vascular cells for delivery. Crucial to the development of these approaches is the preclinical testing of cell delivery in animal models. This review highlights the crucial steps involved in the selection of cell sources and generation, delivery approaches, animal models to be used, and endpoints to be studied, in the context of cell delivery for vascular disease. Furthermore, the development of cell delivery to induce angiogenesis in ischemic limbs and to improve the response to large vessel injury will be discussed.

Enhanced Endothelial Cell Retention on Shear-Stressed Synthetic Vascular Grafts Precoated with RGD-Cross-Linked Fibrin

Clinical in vitro endothelialization has been shown to increase the patency of synthetic vascular grafts. The shear stress resistance of the cultured autologous endothelium represents a crucial cornerstone of the concept. We investigated whether an enrichment of the precoating matrix with adhesion sites can augment endothelial cell attachment. Adult human saphenous vein endothelial cells (AHSVECs) were seeded confluently ([58 +/- 11] x 10(3) AHSVECs/cm2) onto 10-cm-long ePTFE (expanded polytetrafluorethylene) vascular grafts (n = 24) precoated with commercial clinically approved fibrin gel (Tisseal) containing various concentrations of cross-linked RGD peptide (0.0, 4.0, 8.0, or 16.0 mg of RGD per milliliter of Tisseal fibrinogen component). Endothelialized grafts were postcultivated for 9 days before they were exposed to a pulsatile circulation model mimicking peak physiological shear stress conditions of the femoral artery (12 dyn/cm2; min/max, -60/+28 dyn/cm2). Cell loss after 24 h was quantitatively determined by image analysis of vital stains. Initial 24-h cell loss was 27.2 +/- 1.7% in grafts precoated with the non-RGD-enriched fibrin matrix. In contrast, cell loss was significantly less on fibrin containing 4.0 mg of RGD peptide per milliliter of Tisseal fibrinogen component (13.3 +/- 7.9%; p < 0.05). Cell loss on fibrin containing 8 and 16 mg of RGD per milliliter of Tisseal fibrinogen component was 41.0 +/- 27.4 and 43.0 +/- 23.2% (p > 0.05), respectively. We conclude that low concentrations of RGD peptide cross-linked into commercial fibrin matrices used for clinical in vitro lining of vascular grafts led to significantly increased endothelial cell retention. The failure of higher RGD concentrations to enhance endothelial cell attachment may be explained by competitive binding of endothelial cells to non-cross-linked RGD.

Engineering of bypass conduits to improve patency

For patients with severe coronary artery and distal peripheral vascular disease not amenable to angioplasty and lacking sufficient autologous vessels there is a pressing need for improvements to current surgical bypass options. It has been decades since any real progress in bypass material has reached mainstream surgical practice. This review looks at possible remedies to this situation. Options considered are methods to reduce prosthetic graft thrombogenicity, including endothelial cell seeding and developments of new prosthetic materials. The promise of tissue-engineered blood vessels is examined with a specific look at how peptides can improve cell adhesion to scaffolds.

Improving the clinical patency of prosthetic vascular and coronary bypass grafts: the role of seeding and tissue engineering

In patients requiring coronary or peripheral vascular bypass procedures, autogenous vein is currently the conduit of choice. If this is unavailable, then a prosthetic material is used. Prosthetic graft is liable to fail due to occlusion of the graft. To prevent graft occlusion, seeding of the graft lumen with endothelial cells is undertaken. Recent advances have also looked at developing a completely artificial biological graft engineered from the patient's cells with properties similar to autogenous vessels. This review encompasses the developments in the two principal technologies used in developing hybrid coronary and peripheral vascular bypass grafts, that is, seeding and tissue engineering.

Cellular engineering of vascular bypass grafts: role of chemical coatings for enhancing endothelial cell attachment

Surgical treatment of vascular disease has become common. The use of synthetic materials is limited to grafts larger than 5-6mm, because of the frequency of occlusion observed with small-diameter prosthetics. An alternative would be a hybrid or tissue-engineered graft with the surface coated with a monolayer of the patient's own cells. Currently, to be effective, high-density seeding regimens have to be undertaken. This is because endothelial cells (ECs) are washed off the graft lumen once exposed to physiological blood flow. EC attachment has been shown to be significantly improved by pre-coating with substances known to attach ECs selectively. The review examines the various types of coating and bonding technology used to date to enhance endothelial cell attachment onto the surface of prosthetic vascular bypass grafts.

Development and characterization of tissue-engineered aortic valves

Tissue-engineered aortic valves, known as recellularized heart valves, were developed by seeding human neonatal fibroblasts onto decellularized, porcine aortic valves. Recellularized heart valves were cultured up to 8 weeks in a novel bioreactor that imposed dynamic pulsatile fluid flow to expose the dermal fibroblasts to mechanical forces. Our data showed that, under static or dynamic flow conditions, dermal fibroblasts attached to and migrated into the decellularized, porcine valve scaffolding. The human cells remained viable as indicated by MTT viability staining. Gradual colonization of the decellularized porcine scaffolding by the human dermal fibroblasts was shown histologically by hematoxylin & eosin staining, immunocytochemically using a monoclonal antibody directed against prolyl-4-hydroxylase (an intracellular enzyme expressed by human fibroblasts synthesizing collagen), and quantitative digital image analyses. Thymidine and proline radiolabeled analog studies at 1, 2 and 4 weeks of individual leaflets cultured statically demonstrated that the human fibroblasts were mitotic and synthesized human extracellular matrix proteins, thereby supplementing the existing porcine matrix. The overall approach results in a heart valve populated with viable human cells. In the development of valves that perform in a similar manner as natural biological structures, this approach may present some unique benefits over current medical therapies.

Endothelial cell delivery for cardiovascular therapy

Obstructive atherosclerotic vascular disease stands as one of the greatest public health threats in the world. While a number of therapies have been developed to combat vascular disease, endothelial cell delivery has emerged as a distinct therapeutic modality. In this article, we will review the anatomy of the normal blood vessel and the biology of the intact endothelium, focusing upon its centrality in vascular biology and control over the components of the vascular response to injury so as to understand better the motivation for a cell-based form of therapy. Our discussion of cell delivery for cardiovascular therapy will be divided into surgical and interventional approaches. We will briefly recount the development of artificial grafts for surgical vascular bypass before turning our attention towards endothelial cell seeded vascular grafts, in which endothelial cells effectively provide local delivery of endogenous endothelial secretory products to maintain prosthetic integrity after surgical implantation. New techniques in tissue and genetic engineering of vascular grafts and whole blood vessels will be presented. Methods for percutaneous interventions will be examined as well. We will evaluate results of endoluminal endothelial cell seeding for treatment of restenosis and gene therapy approaches to enhance endogenous re-endothelialization. Finally, we will examine some innovations in endothelial cell delivery that may lead to the development of endothelial cell implants as a novel therapy for controlling proliferative vascular arteriopathy.

ENDOTHELIAL CELL CULTURE AND SEEDING OF PROSTHETIC VASCULAR GRAFTS: AN EXPERIMENTAL STUDY

Current synthetic vascular prostheses do not acquire lining of vascular endothelium in humans or dogs. Endothelial seeding of vascular grafts has been proposed as a means of reducing the thrombogenicity of these grafts. We examined feasibility of cultivating endothelial cells (EC) by tissue culture technique and their subsequent seeding onto small diameter polytetra fluoroethylene (PTFE) grafts. Twenty adult dogs underwent common carotid artery interposition with 4 mm PTFE grafts. Ten dogs received seeded and the remaining ten received unseeded grafts. Grafts were removed at 4 and 12 weeks and their gross/morphological features compared. Cumulative patency rates for seeded grafts were 70% as compared to unseeded ones 30%. Seeded grafts were completely surfaced with a mono-layer of endothelium by 4 weeks. Small graft patency appears to be related to the establishment of an endothelial surface, the development of which is clearly facilitated by seeding with autogenous endothelium.

The endothelium: a key to the future

The vascular endothelium is a complex modulator of a variety of biological systems and may well be the key to definitive success in the treatment of cardiovascular disorders. Surgically-induced endothelial injury may occur preoperatively during cardiac catheterization and intraoperatively from mechanical manipulation, ischemia, hypothermia, and exposure to cardioplegic solutions. The normal endothelium is antithrombogenic and yet promotes platelet aggregation and coagulation if injured. Vasospasm, occlusive intimal hyperplasia, and accelerated arteriosclerosis can also all occur as a result of endothelial injury. Furthermore, endothelial injury is harmful even in the absence of disruption of its monolayer integrity. Thus, preservation of the endothelium should be an additional objective for all cardiovascular surgeons. Synthetic vascular grafts, cardiac valves, and artificial ventricles do not spontaneously endothelialize and thus usually require some form of anticoagulation to maintain patency. Hence, endothelialization of prosthetic implants became an attractive concept. A number of different methods of obtaining an endothelial lining of prosthetic material has since been developed; these include facilitated endothelial cell migration, and endothelial cell seeding by using either venous or microvascular endothelial cells. Manipulating the endothelium might well provide the next major advancement for therapeutic and preventive measures for cardiovascular disease.

Quantification of vascular graft seeding by use of computer-assisted image analysis and genetically modified endothelial cells

Current methods for the evaluation of retention of endothelial cells seeded on vascular grafts are limited by the inability to specifically identify and quantitate seeded cells on a long-term basis. To address this problem we developed a method of quantification of graft surface coverage using genetic labeling of endothelial cells combined with computer-assisted image analysis. Rabbit aortic endothelial cells were transduced with a marker gene (lac-Z) and seeded on polytetrafluoroethylene grafts. After histochemical staining in which the genetically labeled cells turn blue, computer-assisted image analysis was used to measure the percentage of graft surface covered by the seeded cells. The utility of the method was evaluated by using it to assess the effect on graft coverage of seeded cell density and by precoating with fibronectin. Quantification of surface area coverage was automated and reproducible both between scans and between observers. Use of this method allowed the determination of a linear correlation between cell density in the seeding suspension and graft coverage (r2 = 0.93, p less than 0.0001). The method also permitted confirmation of the positive contribution of fibronectin coating to graft coverage by seeded cells: 73% coverage coated versus 8% coverage uncoated (p less than 0.0001). The ability of this method to specifically identify genetically marked endothelial cells and their progeny makes it attractive for use in studies targeted at optimization of graft coverage in vivo.

Cells derived from omental fat tissue and used for seeding vascular prostheses are not endothelial in origin. A study on the origin of epitheloid cells derived from omentum

The use of microvascular endothelial cells derived from omental tissue has been advocated to seed vascular grafts with autologous endothelial cells in high density. The purpose of our study was to evaluate the precise origin of these cells. Therefore we have compared cellular characteristics of these cells with those of endothelial cells isolated by collagenase treatment of human umbilical veins. The omental cells were isolated from from omental tissue from four different patients by incubation in a collagenase-dispase solution. Part of the material was processed by Percoll density gradient centrifugation in an attempt to purify the isolates. Cellular characteristics of both types of cells were determined by studying the morphologic features of the cells and by determining the presence of von Willebrand factor, antigens EN-4 and PAL-E specific for endothelial cells, cytokeratins 8 and 18, vimentin and desmin, and uptake of diI-acetylated low-density lipoprotein. Epitheloid cells from omental tissue, isolated after collagenase treatment and either purified or nonpurified by Percoll density gradient centrifugation, differed from human umbilical vein endothelial cells with respect to the presence of surface microvilli, the expression of von Willebrand factor, EN-4 and PAL-E, and the presence of cytokeratins 8 and 18 and desmin. von Willebrand factor (in a granular staining pattern) and the presence of EN-4 and PAL-E were only detected in human umbilical vein endothelial cells. Vimentin was present in both cell types, whereas cytokeratins 8 and 18 and desmin were only present in cells derived from omentum. From these data we conclude that the so called microvascular endothelial cells from omentum are not endothelial but mesothelial in nature.