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

Syndecan-4 and stromal cell-derived factor-1 alpha functionalized endovascular scaffold facilitates adhesion, spreading and differentiation of endothelial colony forming cells and functions under flow and shear stress conditions

Acellular vascular scaffolds with capture molecules have shown great promise in recruiting circulating endothelial colony forming cells (ECFCs) to promote in vivo endothelialization. A microenvironment conducive to cell spreading and differentiation following initial cell capture are key to the eventual formation of a functional endothelium. In this study, syndecan-4 and stromal cell-derived factor-1 alpha were used to functionalize an elastomeric biomaterial composed of poly(glycerol sebacate), Silk Fibroin and Type I Collagen, termed PFC, to enhance ECFC-material interaction. Functionalized PFC (fPFC) showed significantly greater ECFCs capture capability under physiological flow. Individual cell spreading area on fPFC (1474 ± 63 μm² ) was significantly greater than on PFC (1187 ± 54 μm² ) as early as 2 h, indicating enhanced cell-material interaction. Moreover, fPFC significantly upregulated the expression of endothelial cell specific markers such as platelet endothelial cell adhesion molecule (24-fold) and Von Willebrand Factor (11-fold) compared with tissue culture plastic after 7 days, demonstrating differentiation of ECFCs into endothelial cells. fPFC fabricated as small diameter conduits and tested using a pulsatile blood flow bioreactor were stable and maintained function. The findings suggest that the new surface functionalization strategy proposed here results in an endovascular material with enhanced endothelialization.

Differential sensitivity of assays for determining vein endothelial cell senescence

In vivo endothelialization of polymer-based cardiovascular implant materials is a promising strategy to reduce the risk of platelet adherence and the subsequent thrombus formation and implant failure. However, endothelial cells from elderly patients are likely to exhibit a senescent phenotype that may counteract endothelialization. The senescence status of cells should therefore be investigated prior to implantation of devices designed to be integrated in the blood vessel wall. Here, human umbilical vein endothelial cells (HUVEC) were cultivated up to passage (P) 4, 10 and 26/27 to determine the population doubling time and the senescence status by four different methods. Determination of the senescence-associated β-galactosidase activity (SA-β-Gal) was carried out by colorimetric staining and microscopy (i), as well as by photometric quantification (ii), and the expression of senescence-associated nuclear proteins p16 and p21 as well as the proliferation marker Ki67 was assessed by immunostaining (iii), and by flow cytometry (iv). The population doubling time of P27-cells was remarkably greater (103±65 h) compared to P4-cells (24±3 h) and P10-cell (37±15 h). Among the four different methods tested, the photometric SA-β-Gal activity assay and the flow cytometric determination of p16 and Ki67 were most effective in discriminating P27-cells from P4- and P10-cells. These methods combined with functional endothelial cell analyses might aid predictions on the performance of implant endothelialization in vivo.

Autologous endothelialized vein allografts in coronary artery bypass surgery - Long term results

BACKGROUND

Lack of autologous graft material restricts the ability to treat patients requiring coronary artery bypass surgery (CABG). An off the shelf tissue engineered small diameter vascular graft is the holy grail of cardiovascular surgery.

METHODS

Allograft saphenous veins were harvested from organ donors, cryopreserved, deendothelialized and then seeded with autologous endothelial cells prior to implantation during coronary artery bypass surgery. All patients treated were followed-up until death and angiographic results were collected. Grafts were explanted during autopsy and immunohistochemistry was performed.

RESULTS

Twelve patients received 15 engineered grafts. Mean patient survival was 9.1 ± 1.8 years. Six month graft patency was 80 (95% CI: 59-100) and 9 month graft patency was 50 (95% CI: 27-93) - graft patency detected up to 32 months after surgery. Immunohistochemistry in grafts explanted showed a presence of CD31 and CD68 positive cells in the luminal region of the vessel walls and layers of Collagen Type I in the abluminal vessel walls.

CONCLUSIONS

Our small diameter tissue engineered vascular graft shows openness up to 32 months after implantation. Immunohistochemistry suggests that monocyte activation may lead to vessel remodeling with thickening of the vessel wall. Research should concentrate on a manipulation of remodeling processes.

Endothelial Cell Seeding on PTFE Vascular Prostheses Using a Standardized Seeding Technique

A standardized method was developed for seeding endothelial cells (EC) in tubular vascular grafts. A rotational cell seeding device for tubular prostheses is presented and parameters influencing the kinetics of cell adhesion (rotation speed, graft diameter, cell suspension level, inoculated cell number) are reported. Seeding EC in 14 mm ID PTFE vascular grafts with rotation rate of 10 rph gave an adhesion rate of 80% in a homogeneous monolayer.

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.

Induction of Neointima Formation by Platelet Derived Angiogenesis Fraction in a Small Diameter, Wide Pore, PTFE Graft

Enhancement of endothelialization and patency of a small diameter (2 mm), wide pore, PTFE graft was attempted by coating the luminal surface with a platelet derived angiogenesis fraction (PDAF) and implanting it in a rat model. PDAF was delivered to the grafts by combining it with a carrier polymer. PDAF-treated grafts were initially implanted in the retroperitoneum for 21 days followed by removal of one for histology and in situ end to side bypass to the infrarenal aorta for the other. Vascularized grafts were examined at 14 days for patency and 100 days for patency and histology. Significant differences were noted in transmural ingrowth of capillaries and tissue at 21 days post implantation in PDAF-treated verses untreated grafts. Similarly, near significance was noted in capillary ingrowth and significance was noted in tissue ingrowth at 100 days in PDAF-treated grafts. Despite favorable trends particularly early in the time course, no significant differences in graft patency, endothelialization, or hydroxyproline content was demonstrated between PDAF-treated and untreated grafts. Results of this preliminary study are encouraging for further study of PDAF-treated PTFE grafts and the potential that rapid vascularized neointima formation results improved in graft patency rates.

Endothelial Cell Transplantation

Endothelial cells line the lumenal surface of al) elements of the vascular system. These cells exhibit numerous metabolic functions necessary for the maintenance of homeostasis. The critical role of endothelium in maintaining normal blood vessel function is exemplified by the poor clinical performance of small diameter polymeric vascular grafts which fail due, in part, to the lack of a functional endothelium on the lumenal surface. Extensive research has explored the potentiality of transplanting endothelial cells onto polymeric vascular grafts to improve graft function. Several critical issues have been explored including the source of endothelial cells for transplantation, the interaction of endothelium with polymers and the healing process of endothelial cell transplanted grafts. The future of endothelial cell transplantation will also include the use of these cells as vehicles for genetic engineering.

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.

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.

Regenerative implants for cardiovascular tissue engineering

A fundamental problem that affects the field of cardiovascular surgery is the paucity of autologous tissue available for surgical reconstructive procedures. Although the best results are obtained when an individual's own tissues are used for surgical repair, this is often not possible as a result of pathology of autologous tissues or lack of a compatible replacement source from the body. The use of prosthetics is a popular solution to overcome shortage of autologous tissue, but implantation of these devices comes with an array of additional problems and complications related to biocompatibility. Transplantation offers another option that is widely used but complicated by problems related to rejection and donor organ scarcity. The field of tissue engineering represents a promising new option for replacement surgical procedures. Throughout the years, intensive interdisciplinary, translational research into cardiovascular regenerative implants has been undertaken in an effort to improve surgical outcome and better quality of life for patients with cardiovascular defects. Vascular, valvular, and heart tissue repair are the focus of these efforts. Implants for these neotissues can be divided into 2 groups: biologic and synthetic. These materials are used to facilitate the delivery of cells or drugs to diseased, damaged, or absent tissue. Furthermore, they can function as a tissue-forming device used to enhance the body's own repair mechanisms. Various preclinical studies and clinical trials using these advances have shown that tissue-engineered materials are a viable option for surgical repair, but require refinement if they are going to reach their clinical potential. With the growth and accomplishments this field has already achieved, meeting those goals in the future should be attainable.

First human use of an allogeneic tissue-engineered vascular graft for hemodialysis access

An arteriovenous fistula is the current gold standard for chronic hemodialysis access. Tunneled catheters or synthetic grafts have poorer outcomes and much higher risks of infection. This report presents the first clinical use of a completely biological, allogeneic, nonliving, and human tissue-engineered vascular graft. Tissue-engineered vascular grafts built from allogeneic fibroblasts were implanted as shunts in three hemodialysis patients. The tissue-engineered vascular graft was stored for 9 months, without loss of mechanical strength. Implanted grafts showed no signs of degradation or dilation, with time points up to 11 months. Results of panel-reactive antibody and cross-reactivity tests showed no evidence of immune responses.

Evaluation of a novel vascular graft with a distal bifurcation designed to reduce the development of intimal hyperplasia. Experimental study in a porcine aorta model

OBJECTIVE

Abnormal haemodynamics is commonly agreed to be a major contributor to the development of distal anastomotic intimal hyperplasia. A new vascular graft design proposed by computational studies was used to demonstrate its surgical feasibility and to compare it with the conventional graft in a porcine model.

METHOD

The device was used in 12 eight-month-old pigs, six received the new graft and six had a conventional graft. The proximal graft end was implanted into the aorta, the distal graft end was implanted into the iliac artery. The host artery was ligated in order to simulate occlusion. At 20 weeks after surgery the pigs were killed and the device was excised for histological and morphometric analysis.

RESULTS

In five experimental grafts the reconstruction was occluded due to thrombosis; only one prosthesis was patent showing a minimum of neointimal hyperplasia. In the control group too only three of the six grafts were patent. A histological analysis revealed, as the cause of occlusion, fibrous tissue overgrowth corresponding in structure to neointimal hyperplasia. Differences in the number of obliterations and in occlusion rates between the profiles of the two groups were evaluated using the median test (P<0.05). The results were not statistically significant.

CONCLUSION

Although mathematical modelling had shown significant haemodynamic benefits of a naturally bifurcated graft, our study did not confirm its superiority over conventionally used prostheses.

Biological grafts for hemodialysis access: historical lessons, state-of-the-art and future directions

The vast majority of arteriovenous grafts (AVG) have been constructed using expanded polytetrafluoroethylene (ePTFE). While ePTFE grafts have the advantage of being relatively inexpensive and easy to manufacture, distribute, ship, and store, their primary patency rates are disappointing when compared with the native AVF. Though use of arteriovenous fistulas (AVF) in the United States has increased substantially, approximately 25% of hemodialysis patients continue to use AVG as their vascular access. We present here a comprehensive review of biological grafts and their use in hemodialysis vascular access. In this review, we discuss the use of synthetics and then explore the evolution of biological grafts over the past 20 years, their clinical impact, and future challenges in widespread clinical use in hemodialysis patients. Provided are in depth descriptions of currently used nonbiological arteriovenous grafts and the recent approaches in increasing the patency of synthetic grafts. Recent technological advances using tissue-engineered AVGs have shown promise for patients receiving hemodialysis and their potential to provide an attractive, viable option for vascular access have been discussed.

New biological solutions for hemodialysis access

Since Scribner described the first prosthetic chronic dialysis shunt in 1961, the surgical techniques and strategies to maintain vascular access have improved dramatically. Today, hundreds of thousands of patients worldwide are treated with some combination of native vein fistula, synthetic vascular graft, or synthetic semipermanent catheter. Despite significantly lower efficacy compared with autologous fistulae, the basic materials used for synthetic shunts and catheters have evolved surprisingly slowly. The disparity between efficacy rates and concomitant maintenance costs has driven a strong campaign to decrease the use of synthetic grafts and catheters in favor of native fistulae. Whether arguing the benefits of Fistula First or "Catheter Last," the fact that clinicians are in need of an alternative to expanded polytetrafluoroethylene (ePTFE) is irrefutable. The poor performance of synthetic materials has a significant economic impact as well. End-stage renal disease (ESRD) accounts for approximately 6% of Medicare's overall budget, despite a prevalence of about 0.17%. Of that, 15%-25% is spent on access maintenance, making hemodialysis access a critical priority for Medicare. This clinical and economic situation has spawned an aggressive effort to improve clinical care strategies to reduce overall cost and complications. While the bulk of this effort has historically focused on developing new synthetic biomaterials, more recently, investigators have developed a variety of cell-based strategies to create tissue-engineered vascular grafts. In this article, we review the evolution of the field of cardiovascular tissue engineering. We also present an update on the Lifeline™ vascular graft, an autologous, biological, and tissue-engineered vascular graft, which was the first tissue-engineered graft to be used clinically in dialysis patients.

Vascular Grafts and the Endothelium

This article discusses the importance of the endothelium for successful vascular grafts derived from both native arteries and synthetic materials. It also discusses the fundamental strategies to endothelialize synthetic grafts in animal experiments and in the clinic, as well as the use of endothelial progenitor cells (EPCs), bone marrow-derived cells, and mesothelium as endothelial substitutes.

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.

Modified prosthetic vascular conduits

Atherosclerosis in the form of peripheral arterial disease results in significant morbidity. Surgical treatment options for peripheral arterial disease include angioplasty, endarterectomy, and bypass grafting. For bypass grafting, vein remains the conduit of choice; however, poor quality and limited availability have led to the use of prosthetic materials. Unfortunately, because of a lack of endothelium and compliance mismatch, neointimal hyperplasia develops aggressively, resulting in high failure rates. To improve graft patency, investigators have developed surgical, chemical, and biological graft modifications. This review describes common prosthetic materials, as well as approaches currently in use and under investigation to modify and improve prosthetic conduits for bypass grafting in an effort to improve graft patency rates.

Magnetic forces enable rapid endothelialization of synthetic vascular grafts

BACKGROUND

Synthetic vascular grafts cannot be used in small vessels because of graft failure caused by thrombosis and neointima formation. Rapid endothelialization may overcome this limitation. We hypothesized that a magnetic graft would be able to capture and retain endothelial cells labeled with paramagnetic particles.

METHODS AND RESULTS

Porcine blood derived endothelial cells were allowed to endocytose superparamagnetic iron oxide microspheres. Cell survival was assessed by trypan blue exclusion and demonstrated a dose-dependent cell survival of 75% to 95%. A flexible magnetic sheet was annealed to the external surface of a knitted Dacron graft. Labeled cells (10(6)/mL) were placed within the graft for 5 minutes. Confocal and electron microscopy confirmed uniform cell capture at the magnetized surface. The effect of shear forces on the adherent cells was evaluated in a flow chamber. The cells remained attached at rates up to 300 mL/min, with cell loss commencing at 400 mL/min. Prototype magnetic grafts were implanted in porcine carotid arteries. Labeled cells were placed within the graft for 10 minutes at the time of implantation. The grafts were evaluated after one day and uniform cell coverage was noted on the magnetized surface. In comparison, relatively few labeled cells were seen attached to a nonmagnetized surface.

CONCLUSIONS

Magnetic forces can be used to rapidly cover a vascular graft with paramagnetically labeled cells. This biophysical interaction is sufficient to retain cells in the presence of blood flow. Applications of this technique may include rapid endothelialization of synthetic vascular grafts and dialysis fistulas.

Cardiovascular tissue engineering: state of the art

In patients requiring coronary or peripheral vascular bypass procedures, autogenous arterial or vein grafts remain as the conduit of choice even in the case of redo patients. It is in this class of redo patients that often natural tissue of suitable quality becomes unavailable; so that prosthetic material is then used. Prosthetic grafts are liable to fail due to graft occlusion caused by surface thrombogenicity and lack of elasticity. To prevent this, seeding of the graft lumen with endothelial cells has been undertaken and recent clinical studies have evidenced patency rates approaching reasonable vein grafts. Recent advances have also looked at developing a completely artificial biological graft engineered from the patient's cells with surface and viscoelastic properties similar to autogenous vessels. This review encompasses both endothelialisation of grafts and the construction of biological cardiovascular conduits.

Therapeutic uses of autologous endothelial cells for vascular disease

Endothelial cells play important structural and functional roles in vascular homoeostasis. Perturbations in endothelial cell number and function are directly involved with the initiation and progression of multiple cardiovascular diseases, including atherosclerosis, hypertension and congestive heart failure. Attempts to modify these disorders have included pharmacological strategies to improve vascular and thus endothelial function. A goal of biological approaches to these disorders is the delivery of endothelial cells that might act to provide beneficial endothelial-derived factors. However, this approach has generally been limited by the lack of readily available autologous endothelial cells for delivery. The isolation of circulation-derived endothelial progenitor cells allows for direct access to autologous endothelial cells for preclinical and clinical studies. Preclinical studies using autologous endothelial cells have demonstrated beneficial effects when delivered in animal models of vascular injury and grafting. These effects are related to the endothelial nature of the cells and may be paracrine in nature. Ongoing studies are aimed at defining the nature of these effects and optimizing delivery strategies cognizant of these mechanisms.

Improving Small-Diameter Vascular Grafts: From the Application of an Endothelial Cell Lining to the Construction of a Tissue-Engineered Blood Vessel

One of the main reasons why vascular reconstruction with synthetic small-diameter grafts has limited success is the absence of endothelial cells. To improve the outcome of nonvenous vascular bypass surgery, cell seeding of vascular grafts and other tissue-engineering techniques were developed. In this article, an overview is given of the artificial blood vessel as an alternative for venous vascular bypass surgery.

Endothelial and Smooth Muscle Cell Seeding onto Processed Ex Vivo Arterial Scaffolds Using 3D Vascular Bioreactors

Biomaterials derived from ex vivo tissues offer a viable alternative to synthetic materials for organ replacement therapies. In this study, we describe the use of a tissue engineering scaffold derived from ex vivo arterial tissue to assess vascular cell adhesion within a three-dimensional perfusion bioreactor. With the aim of maximizing seeding efficiency, five methods for endothelial cell (EC) and three independent methods for vascular smooth muscle cell (VSMC) adhesion were explored. Seeded constructs were maintained in vascular bioreactors under pulsatile flow conditions, culminating at 165 ml/min at 1.33 Hz to validate cell attachment and retention over time. Progressive modification of the seeding and flow regime protocols resulted in an increased of EC retention from 5.1 to 634 cells/mm2. Seeding VSMCs as sheets rather than cell suspensions bound and stabilized surface EC matrix fibers, resulting in multiple cell layers adhered to the scaffold with cells migrating to the medial/adventitial boundary. In conjunction with the bioscaffold, the vascular perfusion system serves as a useful tool to analyze cell adhesion and retention by allowing controlled manipulation of seeding and perfusion conditions.

Tissue engineering therapy for cardiovascular disease

The present treatments for the loss or failure of cardiovascular function include organ transplantation, surgical reconstruction, mechanical or synthetic devices, or the administration of metabolic products. Although routinely used, these treatments are not without constraints and complications. The emerging and interdisciplinary field of tissue engineering has evolved to provide solutions to tissue creation and repair. Tissue engineering applies the principles of engineering, material science, and biology toward the development of biological substitutes that restore, maintain, or improve tissue function. Progress has been made in engineering the various components of the cardiovascular system, including blood vessels, heart valves, and cardiac muscle. Many pivotal studies have been performed in recent years that may support the move toward the widespread application of tissue-engineered therapy for cardiovascular diseases. The studies discussed include endothelial cell seeding of vascular grafts, tissue-engineered vascular conduits, generation of heart valve leaflets, cardiomyoplasty, genetic manipulation, and in vitro conditions for optimizing tissue-engineered cardiovascular constructs.

Rapid endothelialization of PhotoFix natural biomaterial vascular grafts

To date, no off-the-shelf graft has performed better than the autologous vessel in applications requiring small-bore (< 6-mm diameter) vascular grafts. Much research has been devoted to seeding endothelial cells on synthetic grafts to improve their long-term clinical performance. One key challenge is the ability to retain the endothelium on the graft lumen for extended times. The goal of this research was to develop a process to seed endothelial cells inside a vascular graft and to quickly condition the cells so as to minimize their damage or removal under physiological flow. In addition, the use of PhotoFix(R) natural biomaterial grafts as an improved substrate for human umbilical vein endothelial cells has been evaluated. A motorized system that provides uniform cell seeding of a small-diameter graft (4-mm inner diameter, 10-cm length) by automated radial rotation has been developed. The same system is subsequently adapted for gradual increases in flow rates to strengthen the endothelium, which ultimately was exposed to a final flow rate of 300 ml/min. This process is accomplished without graft transfer, decreasing risks of contamination and physical damage. Cell coverage and cell morphology were evaluated with the use of fluorescence microscopy and scanning-electron microscopy to determine the effectiveness of the flow conditioning process. It was found that endothelial cells exhibit roughly 20-50% improved adhesion to PhotoFix vessels compared to fibrin-treated polytetrafluoroethylene (PTFE) synthetic grafts. Flow conditioning for 6 h enhanced in vitro cell retention by 24% and 40% on PhotoFix and PTFE grafts, respectively.

Reduction of non-endothelial cell contamination of microvascular endothelial cell seeded grafts decreases thrombogenicity and intimal hyperplasia

INTRODUCTION

fat derived microvascular endothelial cells (MVEC) seeded on prosthetic vascular grafts, improve patency in animals. Results in humans were disappointing, due to thrombogenicity and progressive intimal hyperplasia. Also in animals intimal hyperplasia was found. We postulate that contaminating cells present in the transplant are involved in the intimal hyperplasia. We developed a method to further purify human MVEC from 40-90%. Here we tested the effects of enrichment upon thrombogenicity and seeding-related intimal hyperplasia.

METHODS

liposuction fat was enzymatically digested and centrifuged. To enrich MVEC, contaminating macrophages and fibroblasts were removed with dynabeads coated with macrophage- and fibroblast-specific antibodies. Thrombogenicity was assessed by measuring tissue factor and thrombomodulin activity, presence of endothelial nitric oxide synthase and via perfusion of the cells with whole blood. To investigate seeding-related intimal hyperplasia, PTFE grafts were seeded with the cells and cultured for 3 weeks.

RESULTS

tissue factor activity of purified cells was reduced compared to nonpurified cells. Purified cells showed thrombomodulin activity and eNOS expression. Fragment 1+2 and Fibrinopeptide A generation after perfusion of purified cells were significantly lower than after perfusion of nonpurified cells, and only nonpurified cells were covered with platelets and fibrin. Prostheses seeded with nonpurified cells showed an EC monolayer above a multilayer of myofibroblasts, prostheses seeded with purified cells only showed a single EC monolayer. Mixing experiments with human umbilical cord EC (HUVEC) and fibroblasts showed that when more than 25% HUVEC were present a confluent EC layer was formed. When the amount of fibroblasts was 25% or less, no development of a subendothelial multilayer of myofibroblasts was found within 3 weeks.

CONCLUSION

reduction of non-endothelial cell contamination of microvascular endothelial cell seeded grafts decreases thrombogenicity and might prevent seeding-related intimal hyperplasia.

Small-caliber mesothelial cell-layered polytetraflouroethylene vascular grafts in New Zealand white rabbits

Reduction in the thrombogenicity of small-caliber synthetic vascular grafts by lining them with mesothelial cell has been suggested as a method to reduce thrombosis. The purpose of this research is to determine whether creation of a mesothelial lining on the inner surface of a synthetic vascular graft would improve the patency rate of a small-caliber vascular grafts. Carotid interposition grafting was performed using mesothelial-lined grafts (MLG) in 30 New Zealand rabbits and compared with similar carotid interposition grafts using non-mesothelial-lined grafts (NLG) on the contralateral side. The mesothelial lining was created by suturing a piece of harvested peritoneum with the visceral surface toward the lumen onto a 2-mm polytetraflouroethylene (PTFE) graft. Graft patency was studied by in vivo Dopler. In vitro evaluations were done with hematoxylin-eosin stains, broadband cytokeratin staining, and monoclonal antibodies for macrophages. Explanation of the grafts was done in terminal operation at 7, 14, and 21 days. The MLG showed progressive fibroblastic proliferation in direct proportion to the age of the graft, but this did not lead to graft occlusion. However, a significant number of NLG were not patent at each time period studied. We concluded that mesothelial cell lining of smallcaliber PTFE grafts could enhance the short-term patency more than using the PTFE without the mesothelial lining. The use of such hybrid small-caliber grafts has a potential for improving the patency of these artificial vascular graft substitutes.

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.

Tissue engineering of vascular bypass grafts: role of endothelial cell extraction

Surgical treatment of vascular disease has become common. The use of synthetic materials is limited to grafts larger than 5-6 mm, 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 patients' own endothelial cells. This review examines the various techniques and technologies used to date in order to extract endothelial cells for such graft engineering.

Acutely sodded expanded polytetrafluoroethylene grafts produce only prostacyclin: a qualitative difference from saphenous veins

Select subsets of patients require prosthetic graft material for revascularization. Although arterial prosthetic grafts of large caliber perform acceptably, grafts of <6 mm exhibit a high attrition rate. Microvessel endothelial sodding, a method resulting in the lining of prosthetic grafts with autologous endothelium, improves graft patency; however, aggressive antiplatelet therapy is still required, because terminating an antiplatelet regimen accelerates graft attrition. The present investigation was designed to address the acute production of vasoactive substances in microvessel endothelial cell sodded expanded polytetrafluoroethylene (ePTFE) grafts in an attempt to delineate a possible mechanism behind the continued requirement for antiplatelet therapy. Equal lengths of acutely sodded ePTFE grafts (canine falciform ligament source) and saphenous veins (SV) (canine source) were evaluated by superfusion bioassay. Basal secretion from ePTFE grafts relaxed the biodetector ring 1 +/- 3%, whereas SV relaxed the ring 10 +/- 3% (p < 0.05, ePTFE vs. SV). Relaxation with acetylcholine stimulation was 49 +/- 7% in grafts and 50 +/- 10% in veins (p = NS). Calcium ionophore stimulation produced relaxation of 37 +/- 9% from ePTFE grafts and 100 +/- 23% from SV (p < 0.05). Indomethacin added to perfusate reduced relaxations from sodded ePTFE grafts to 20.2 +/- 9.2% with acetylcholine stimulation and 12.5 +/- 4.3% with calcium ionophore (p < 0.05 vs. control); addition of N(G)-monomethyl-L-arginine (L-NMMA) had no effect on the release of vasoactive substances from ePTFE grafts. In contrast, relaxations of effluent from SV stimulated by acetylcholine and calcium ionophore were significantly attenuated with indomethacin and L-NMMA (p < 0.05 vs. control). Scanning electron microscopy demonstrated confluent endothelium in SV and a nonconfluent endothelial cell layer in grafts. Acutely sodded ePTFE grafts produce vasoactive substances that quantitatively and qualitatively differ from those produced by canine SV. The ePTFE grafts produce mainly prostanoids, whereas SV produce both nitric oxide and prostanoids. The endothelial cell isolation procedure and absence of immediate graft luminal confluence may contribute to the observed differences.

Optimal endothelialisation of a new compliant poly(carbonate-urea)urethane vascular graft with effect of physiological shear stress

OBJECTIVE

to define the optimal seeding conditions of a new stress free poly(carbonate-urea)urethane (CPU) graft with compliance similar to that of human artery with honeycomb structure engineered during the manufacturing process to enhance adhesion and growth of endothelial cells.

METHODS

(111)Indium-oxine radiolabeled human umbilical vein endothelial cells (HUVEC) were seeded onto CPU grafts at (a) concentrations from 2-24x10(5)cells/cm(2)and (b) incubated for 0.5, 1, 2, 4 and 6 h. Following incubation, graft segments were subjected to three washing/gamma counting procedures and scanning electron microscopy (SEM). Cell viability was measured using a modified Alamar blue(TM)assay. To test physiological retention a pulsatile flow phantom was used to subject optimally seeded (16x10(5), 4 h) CPU grafts to arterial shear stress for 6 h with real time acquisition of scintigraphic images of seeded grafts using a nuclear medicine gamma camera system.

RESULTS

the seeding efficiency of 54+/-13% post three washes was achieved using 16x10(5)cells/cm(2). Similarly in SEM micrographs a seeding density of 16x10(5)cells/cm(2)resulted in a confluent monolayer. Seeded CPU segments incubated for 4 h exhibited significantly higher resistance to wash-off than segments incubated for 30 min (p <0.05). Exposure of seeded grafts to pulsatile shear stress resulted in some cell loss with 67+/-3% of cells adherent following 6 h of perfusion with ongoing metabolic activity. Thus, optimal conditions were 16x10(5)cells/cm(2)at 4 h.

CONCLUSIONS

the optimal seeding conditions have been defined for "tissue-engineered" vascular graft which allow complete endothelialisation and high cell-to-substrate strength that resists hydrodynamic stress.

Clinical performance of vascular grafts lined with endothelial cells

The replacement of arteries with purely synthetic vascular prostheses often leads to the failure of such reconstructions when small-diameter or low-flow locations are concerned, due in part to the thrombogenicity of the internal graft surface. In order to improve long-term patency of these grafts, the concept of endothelial cell seeding has been suggested because this metabolically active endothelial surface plays major roles in preventing in vivo blood thrombosis and because vascular grafts placed in humans do not spontaneously form an endothelial monolayer whereas they do in animal models. The composite structure resulting from the combination of biologically active cells to prosthetic materials thus creates more biocompatible vascular substitutes. To achieve endothelialization of synthetic vascular grafts, previous efforts aimed at "one-stage" procedure (adding autologous endothelial cells to the graft at the time of implantation) in the 1980's seemed clinically feasible but results of reported clinical trials were controversial and mostly disappointing. An alternative method is an in vitro complete and preformed endothelial lining at the time of implantation: the "two-stage" procedure which implies harvest and culture of autologous endothelial cells. Up to date, the latter approach demonstrated its superiority in terms of significantly increased patency of the grafts that underwent endothelialization eight years earlier.

New concepts and materials in microvascular grafting: prosthetic graft endothelial cell seeding and gene therapy

Microsurgical free tissue transfer is currently associated with very high success rates and few complications. While interposition vein grafting is clearly an important adjunct to the microsurgeon's armamentarium, it has been associated with higher free flap/replantation failures and complication rates. With appropriate flap planning and surgical techniques, the need for interposition vein grafting should be quite infrequent and hopefully avoided if at all possible. Nevertheless, when necessary, the vein graft remains the gold standard, with virtually all alternative interposition grafts demonstrating lower patency rates. One of the more promising areas of research is the concept of genetic manipulation of the endothelial cell via molecular biological techniques. It is likely that in the near future this may become a clinical reality, not only improving the patency of microsurgical anastomoses and interposition vein grafts, but quite possibly altering the target organ functionally as well.

Complete in vitro prosthesis endothelialization induced by artificial extracellular matrix

This report presents our research on the conditions necessary to substain optimal in vitro prosthetic endothelialization using human endothelium cultures. Human vein endothelial cells were seeded at a concentration of 3 x 10(5)/cm2 in a gelatinized Dacron patch graft coated with a commercial collagen film, using a solution of fibrin glue. Endothelium adhesion, proliferation, and survival were measured by [3H]thymidine incorporation, after 7 days of incubation. Finally, the morphology of prosthetic endothelialization was analyzed by scanning electron microscopy. We observed that the Dacron patch grafts coated with collagen film were able to promote endothelialization better than the prostheses coated with highly concentrated collagen solution or gelatin. We therefore concluded that the collagen film that supports endothelial cell adhesion and proliferation uniformly covers the entire synthetic endoluminal surface of the Dacron graft, thus preventing endothelial cell alterations induced by direct contact with the synthetic prosthetic surface.

In vitro endothelialization of PTFE vascular grafts: A comparison of various substrates, cell densities, and incubation times

To establish the optimal conditions for achieving endothelial cell coverage of the luminal surfaces of small-caliber vascular grafts in vitro, the attachment of endothelial cells (ECs) cultured from human umbilical veins to polytetrafluoroethylene (PTFE) grafts was studied. Cell attachment and spreading were compared after PTFE grafts were (a) precoated with fibronectin (HFN), type I collagen, type IV collagen, plasma and fibrin with or without thrombin, singly or in combination; (b) seeded with cell densities varying from 0.5 x 10(5) to 6 x 10(5) cells/cm2; and (c) incubated at 30, 60, or 90 min. Cell coverage and spreading were assessed by means of scanning electron microscopy. Quantification of graft surface coverage was performed with computer-assisted image analysis. To determine optimal conditions of endothelialization among the 189 treatment combinations, analysis of variance was used. We conclude that a virtually confluent cell monolayer can be established on small-caliber PTFE grafts when precoated with fibrin glue or plasma, seeded with cell densities >/=4 x 10(5) cells/cm2, and incubated for 60 min. These parameters are compatible with an operating room vascular procedure.

Improving endothelial cell adhesion to vascular graft surfaces: clinical need and strategies

Synthetic vascular grafts do not spontaneously endothelialize in humans and require some form of anticoagulation to maintain patency. Preseeding synthetic graft materials such as expanded polytetrafluoroethylene (ePTFE) and polyethylene terephthalate (PET) with endothelial cells (EC) has been examined in various in vitro and in vivo models. Although various studies provide encouraging results, clinical trials for EC seeding on synthetic grafts have not been equally successful. This paper provides a brief review of the various reports on EC seeding in animal and clinical studies. We discuss the inefficiencies associated with the EC seeding process and examine plasma protein treatment of the graft surfaces as a viable option for improving EC attachment, retention and spreading. As an alternative to existing therapies we present data on a heterogeneous ligand treatment of fibronectin (Fn) and avidin-biotin for enhanced human umbilical vein endothelial cell (HUVEC) adhesion to ePTFE graft surfaces. Control consisted of HUVECs seeded on Fn treated ePTFE graft surfaces. Functionality of HUVECs was assessed by measuring prostacyclin production of cells on both homogeneous and heterogeneous ligand treated surfaces. Laminar flow studies with a variable width flow chamber and scanning electron microscopy were used to measure initial cell retention and observe initial cell spreading on ePTFE surfaces, respectively. HUVEC retention on heterogeneous ligand treated graft surface was significantly (p < 0.001) higher compared to homogeneous ligand treated surfaces for shear stress in the range of 10-30 dyn cm(-2). HUVEC showed more cellular spreading on the heterogeneous ligand treated surface after seeding for 1-2 h. In vivo experimentation was performed in immune deficient (nude) rats by replacing a section of both the femoral arteries with 8 mnm long, 1 mm internal diameter denucleated ePTFE grafts treated with homogeneous and heterogeneous ligands respectively. Both grafts were seeded with similar cell density for 15 min prior to implantation. EC attachment and retention was measured by staining EC with hematoxylin and counting the cells before and after flow using light microscopy. The results indicate that a heterogeneous ligand treatment of graft surfaces using avidin-biotin and Fn-integrin attachment mechanisms increase cell seeding efficiency, initial cell retention and cellular spreading.

Time-course for in vitro development of basement membrane, gap junctions, and repair by adult endothelial cells seeded on precoated ePTFE

OBJECTIVES

To establish some functional characteristics after in vitro endothelialisation of precoated ePTFE grafts.

MATERIALS AND METHODS

Saphenous vein endothelial cells were confluently seeded on human serum-precoated ePTFE. Collagen I-precoated ePTFE served as control. Time-courses for development of laminin, collagen IV and connexin43 was followed-up with immunofluorescence. Endothelial migration and proliferation was studied with cresyl violet staining and tritiated thymidine-labelling.

RESULTS

Six hours post-seeding, basement membrane components were visualised subendothelially as thin fibrils. After 24 h, there was a distinct fibril network, which did not seem to be further enhanced. Connexin43 was detected 6 h post-seeding, and the number of intercellular connections appeared to be similar up to day 7. The time-courses were similar on serum- and collagen I-precoated ePTFE. Endothelialisation of a 4 mm wide gap showed migrating fronts largely composed of proliferating cells. The process was completed within 10 days on collagen I-precoated ePTFE, and 15 days on serum-precoated ePTFE.

CONCLUSIONS

These findings suggest human endothelial cells, seeded on precoated ePTFE, maintain some differentiated functions seen in vivo. This is of importance for cell retention, functionality and endothelial repair of in vitro endothelialised ePTFE-grafts after implantation in man.

Using avidin-mediated binding to enhance initial endothelial cell attachment and spreading

Binding between the protein avidin and the vitamin biotin was used as an extrinsic, high affinity receptor-ligand system to augment the intrinsic integrin-dependent cellular adhesion mechanism. Glass substrates were coupled with avidin receptors through an adsorbed film of biotinylated bovine serum albumin (b-BSA). The avidin-treated slides then were seeded with biotinylated bovine aortic endothelial cells (BAEC). A 3:1 ratio of BSA:b-BSA provided the best results in terms of specific cellular attachment, growth, and spreading. Control surfaces consisted of bare glass or glass with adsorbed BSA. Attachment of unmodified BAEC to glass decreased in the presence of anti-beta 1 integrin antibody. Adhesion of biotinylated BAEC to avidin-treated slides was not affected by anti-beta 1 integrin antibody, consistent with integrin-independent avidin-mediated adhesion. The initial rate of cell spreading was greatest for avidin-biotin-mediated adhesion (80.0 +/- 25.6 microns2/h), followed by integrin-dependent cellular adhesion on plain glass (35.7 +/- 7.7 microns2/h) and, finally, by adhesion on BSA-coated protein surfaces (10.2 +/- 0.3 microns2/h). Biotinylated and unmodified BAEC, cultured for 1 h in serum-containing media, were subjected to laminar flow in a variable-height flow chamber that provided a range of shear stresses from 0.2 to 75 dynes/cm2. The critical shear stress required to detach 50% of the cells in serum-containing media increased from 4.6 +/- 0.8 dynes/cm2 for integrin-dependent adhesion to 12.6 +/- 1.2 dynes/cm2 for avidin-biotin-mediated adhesion. Avidin-mediated attachment for biotinylated BAEC increased initial cellular spreading rates and strength of attachment (i.e., at 1 h) by a factor of two and three, respectively. These results support the hypothesis that integrin-mediated cell attachment and spreading can be enhanced using high affinity integrin-independent binding.