Tissue-engineered vascular grafts demonstrate evidence of growth and development when implanted in a juvenile animal model

Comparison of a closed system to a standard open technique for preparing tissue-engineered vascular grafts

We developed a prototype for a closed apparatus for assembling tissue-engineered vascular grafts (TEVGs) with the goal of creating a simple operator-independent method for making TEVGs to optimize safety and enable widespread application of this technology. The TEVG is made by seeding autologous bone marrow-derived mononuclear cells onto a biodegradable tubular scaffold and is the first man-made vascular graft to be successfully used in humans. A critical barrier, which has prevented the widespread clinical adoption of the TEVG, is that cell isolation, scaffold seeding, and incubation are performed using an open method. To reduce the risk of contamination, the TEVG is assembled in a clean room. Clean rooms are expensive to build, complex to operate, and are not available in most hospitals. In this investigation, we used an ovine model to compare the safety and efficacy of TEVGs created using either a standard density centrifugation-based open method or the new filter-based closed system. We demonstrated no graft-related complications and maintenance of growth capacity in TEVGs created using the closed apparatus. In addition, the use of the closed system reduced the amount of time needed to assemble the TEVG by ∼ 50%. Adaptation of similar methodologies may facilitate the safe translation and the widespread use of other tissue engineering technologies.

Pediatric tubular pulmonary heart valve from decellularized engineered tissue tubes

Pediatric patients account for a small portion of the heart valve replacements performed, but a pediatric pulmonary valve replacement with growth potential remains an unmet clinical need. Herein we report the first tubular heart valve made from two decellularized, engineered tissue tubes attached with absorbable sutures, which can meet this need, in principle. Engineered tissue tubes were fabricated by allowing ovine dermal fibroblasts to replace a sacrificial fibrin gel with an aligned, cell-produced collagenous matrix, which was subsequently decellularized. Previously, these engineered tubes became extensively recellularized following implantation into the sheep femoral artery. Thus, a tubular valve made from these tubes may be amenable to recellularization and, ideally, somatic growth. The suture line pattern generated three equi-spaced leaflets in the inner tube, which collapsed inward when exposed to back pressure, per tubular valve design. Valve testing was performed in a pulse duplicator system equipped with a secondary flow loop to allow for root distention. All tissue-engineered valves exhibited full leaflet opening and closing, minimal regurgitation (<5%), and low systolic pressure gradients (<2.5 mmHg) under pulmonary conditions. Valve performance was maintained under various trans-root pressure gradients and no tissue damage was evident after 2 million cycles of fatigue testing.

Stem cell therapy and tissue engineering for correction of congenital heart disease

This review article reports on the new field of stem cell therapy and tissue engineering and its potential on the management of congenital heart disease. To date, stem cell therapy has mainly focused on treatment of ischemic heart disease and heart failure, with initial indication of safety and mild-to-moderate efficacy. Preclinical studies and initial clinical trials suggest that the approach could be uniquely suited for the correction of congenital defects of the heart. The basic concept is to create living material made by cellularized grafts that, once implanted into the heart, grows and remodels in parallel with the recipient organ. This would make a substantial improvement in current clinical management, which often requires repeated surgical corrections for failure of implanted grafts. Different types of stem cells have been considered and the identification of specific cardiac stem cells within the heterogeneous population of mesenchymal and stromal cells offers opportunities for de novo cardiomyogenesis. In addition, endothelial cells and vascular progenitors, including cells with pericyte characteristics, may be necessary to generate efficiently perfused grafts. The implementation of current surgical grafts by stem cell engineering could address the unmet clinical needs of patients with congenital heart defects.

Bio-artificial heart as ultimate treatment of end-stage heart failure

End-stage heart failure is a major health problem, but implementation of guidelines and optimizing medical therapy for this devastating disease should decrease mortality. If optimal conservative therapy is no longer sufficient, a mechanical support system may be required as final destination therapy or as bridge-to-transplant. Since the first heart transplantation in 1967, this therapy has become the criterion standard for end-stage heart failure, but is limited due to organ shortage. Tissue engineering could help overcome this limitation and provide regeneration, remodeling, and growth potential. This so-called bio-artificial heart would be available, created by a decellularized extracellular matrix and seeded with in vitro proliferated autologous cardiovascular cells. Results of the first experimental studies have been promising, but numerous challenges must be met before this procedure will be available.

Platform technologies for decellularization, tunic-specific cell seeding, and in vitro conditioning of extended length, small diameter vascular grafts

The aim of this study was to generate extended length, small diameter vascular scaffolds that could serve as potential grafts for treatment of acute ischemia. Biological tissues are considered excellent scaffolds, which exhibit adequate biological, mechanical, and handling properties; however, they tend to degenerate, dilate, and calcify after implantation. We hypothesized that chemically stabilized acellular arteries would be ideal scaffolds for development of vascular grafts for peripheral surgery applications. Based on promising historical data from our laboratory and others, we chose to decellularize bovine mammary and femoral arteries and test them as scaffolds for vascular grafting. Decellularization of such long structures required development of a novel "bioprocessing" system and a sequence of detergents and enzymes that generated completely acellular, galactose-(α1,3)-galactose (α-Gal) xenoantigen-free scaffolds with preserved collagen, elastin, and basement membrane components. Acellular arteries exhibited excellent mechanical properties, including burst pressure, suture holding strength, and elastic recoil. To reduce elastin degeneration, we treated the scaffolds with penta-galloyl glucose and then revitalized them in vitro using a tunic-specific cell approach. A novel atraumatic endothelialization protocol using an external stent was also developed for the long grafts and cell-seeded constructs were conditioned in a flow bioreactor. Both decellularization and revitalization are feasible but cell retention in vitro continues to pose challenges. These studies support further efforts toward clinical use of small diameter acellular arteries as vascular grafts.

A cautionary tale for autologous vascular tissue engineering: impact of human demographics on the ability of adipose-derived mesenchymal stem cells to recruit and differentiate into smooth muscle cells

Autologous tissue-engineered blood vessels (TEBVs) generated using adult stem cells have shown promising results, but many preclinical evaluations do not test the efficacy of stem cells from patient populations likely to need therapy (i.e., elderly and diabetic humans). Two critical functions of these cells will be (i) secreting factors that induce the migration of host cells into the graft and (ii) differentiating into functional vascular cells themselves. The purpose of this study was to analyze whether adipose-derived mesenchymal stem cells (AD-MSCs) sourced from diabetic and elderly patients have a reduced ability to promote human smooth muscle cell (SMC) migration and differentiation potential toward SMCs, two important processes in stem cell-based tissue engineering of vascular grafts. SMC monolayers were disrupted in vitro by a scratch wound and were induced to close the wound by exposure to media conditioned by AD-MSCs from healthy, elderly, and diabetic patients. Media conditioned by AD-MSCs from healthy patients promoted the migration of SMCs and did so in a dose-dependent manner; heating the media to 56°C eliminated the media's potency. AD-MSCs from diabetic and elderly patients had a decreased ability to differentiate into SMCs under angiotensin II stimulation; however, only AD-MSCs from elderly donors were unable to promote SMC migration. Gender and body-mass index of the patients showed no effect on either critical function of AD-MSCs. In conclusion, AD-MSCs from elderly patients may not be suitable for autologous TEBVs due to inadequate promotion of SMC migration and differentiation.

Implantation of inferior vena cava interposition graft in mouse model

Biodegradable scaffolds seeded with bone marrow mononuclear cells (BMCs) are often used for reconstructive surgery to treat congenital cardiac anomalies. The long-term clinical results showed excellent patency rates, however, with significant incidence of stenosis. To investigate the cellular and molecular mechanisms of vascular neotissue formation and prevent stenosis development in tissue engineered vascular grafts (TEVGs), we developed a mouse model of the graft with approximately 1 mm internal diameter. First, the TEVGs were assembled from biodegradable tubular scaffolds fabricated from a polyglycolic acid nonwoven felt mesh coated with ε-caprolactone and L-lactide copolymer. The scaffolds were then placed in a lyophilizer, vacuumed for 24 hr, and stored in a desiccator until cell seeding. Second, bone marrow was collected from donor mice and mononuclear cells were isolated by density gradient centrifugation. Third, approximately one million cells were seeded on a scaffold and incubated O/N. Finally, the seeded scaffolds were then implanted as infrarenal vena cava interposition grafts in C57BL/6 mice. The implanted grafts demonstrated excellent patency (>90%) without evidence of thromboembolic complications or aneurysmal formation. This murine model will aid us in understanding and quantifying the cellular and molecular mechanisms of neotissue formation in the TEVG.

Targeted imaging of matrix metalloproteinase activity in the evaluation of remodeling tissue-engineered vascular grafts implanted in a growing lamb model

OBJECTIVES

The clinical translation of tissue-engineered vascular grafts has been demonstrated in children. The remodeling of biodegradable, cell-seeded scaffolds to functional neovessels has been partially attributed to matrix metalloproteinases. Noninvasive assessment of matrix metalloproteinase activity can indicate graft remodeling and elucidate the progression of neovessel formation. Therefore, matrix metalloproteinase activity was evaluated in grafts implanted in lambs using in vivo and ex vivo hybrid imaging. Graft growth and remodeling was quantified using in vivo x-ray computed tomography angiography.

METHODS

Cell-seeded and unseeded scaffolds were implanted in 5 lambs as inferior vena cava interposition grafts. At 2 and 6 months after implantation, in vivo angiography was used to assess graft morphology. In vivo and ex vivo single photon emission tomography/computed tomography imaging was performed with a radiolabeled compound targeting matrix metalloproteinase activity at 6 months. The neotissue was examined at 6 months using qualitative histologic and immunohistochemical staining and quantitative biochemical analysis.

RESULTS

The seeded grafts demonstrated significant luminal and longitudinal growth from 2 to 6 months. In vivo imaging revealed subjectively greater matrix metalloproteinase activity in grafts versus native tissue. Ex vivo imaging confirmed a quantitative increase in matrix metalloproteinase activity and demonstrated greater activity in unseeded versus seeded grafts. The glycosaminoglycan content was increased in seeded grafts versus unseeded grafts, without significant differences in collagen content.

CONCLUSIONS

Matrix metalloproteinase activity remained elevated in tissue-engineered grafts 6 months after implantation and could indicate remodeling. Optimization of in vivo imaging to noninvasively evaluate matrix metalloproteinase activity could assist in the serial assessment of vascular graft remodeling.

The performance of cross-linked acellular arterial scaffolds as vascular grafts; pre-clinical testing in direct and isolation loop circulatory models

There is a significant need for small diameter vascular grafts to be used in peripheral vascular surgery; however autologous grafts are not always available, synthetic grafts perform poorly and allografts and xenografts degenerate, dilate and calcify after implantation. We hypothesized that chemical stabilization of acellular xenogenic arteries would generate off-the-shelf grafts resistant to thrombosis, dilatation and calcification. To test this hypothesis, we decellularized porcine renal arteries, stabilized elastin with penta-galloyl glucose and collagen with carbodiimide/activated heparin and implanted them as transposition grafts in the abdominal aorta of rats as direct implants and separately as indirect, isolation-loop implants. All implants resulted in high patency and animal survival rates, ubiquitous encapsulation within a vascularized collagenous capsule, and exhibited lack of lumen thrombogenicity and no graft wall calcification. Peri-anastomotic neo-intimal tissue overgrowth was a normal occurrence in direct implants; however this reaction was circumvented in indirect implants. Notably, implantation of non-treated control scaffolds exhibited marked graft dilatation and elastin degeneration; however PGG significantly reduced elastin degradation and prevented aneurismal dilatation of vascular grafts. Overall these results point to the outstanding potential of crosslinked arterial scaffolds as small diameter vascular grafts.

Nerve regeneration and elastin formation within poly(glycerol sebacate)-based synthetic arterial grafts one-year post-implantation in a rat model

The objective of this study was to evaluate the long-term performance of cell-free vascular grafts made from a fast-degrading elastic polymer. We fabricated small arterial grafts from microporous tubes of poly(glycerol sebacate) (PGS) reinforced with polycaprolactone (PCL) nanofibers on the outer surface. Grafts were interpositioned in rat abdominal aortas and characterized at 1 year post-implant. Grafts remodeled into "neoarteries" (regenerated arteries) with similar gross appearance to native rat aortas. Neoarteries mimic arterial tissue architecture with a confluent endothelium and media and adventita-like layers. Patent vessels (80%) showed no significant stenosis, dilation, or calcification. Neoarteries contain nerves and have the same amount of mature elastin as native arteries. Despite some differences in matrix organization, regenerated arteries had similar dynamic mechanical compliance to native arteries in vivo. Neoarteries responded to vasomotor agents, albeit with different magnitude than native aortas. These data suggest that an elastic vascular graft that resorbs quickly has potential to improve the performance of vascular grafts used in small arteries. This design may also promote constructive remodeling in other soft tissues.

Transcatheter tissue engineered heart valves

Valvular heart disease represents a leading cause of mortality worldwide. Transcatheter heart valve replacement techniques have been recently introduced into the clinical routine expanding the treatment options for affected patients. However, despite this technical progress toward minimally invasive, transcatheter strategies, the available heart valve prostheses for these techniques are bioprosthetic and associated with progressive degeneration. To overcome such limitations, the concept of heart valve tissue engineering has been repeatedly suggested for future therapy concepts. Ideally, a clinically relevant heart valve tissue engineering concept would combine minimally invasive strategies for both, living autologous valve generation as well as valve implantation. Therefore, merging transcatheter techniques with living tissue engineered heart valves into a trascatheter tissue engineered heart valve concept could significantly improve current treatment options for patients suffering from valvular heart disease. This report provides an overview on transcatheter tissue engineered heart valves and summarizes available pre-clinical data.

Mucin covalently bonded to microfibers improves the patency of vascular grafts

Due to high incidence of vascular bypass procedures, an unmet need for suitable vessel replacements exists, especially for small-diameter (<6 mm) vascular grafts. Here, we developed a novel, bilayered, synthetic vascular graft of 1-mm diameter that consisted of a microfibrous luminal layer and a nanofibrous outer layer, which was tailored to possess the same mechanical property as native arteries. We then chemically modified the scaffold with mucin, a glycoprotein lubricant on the surface of epithelial tissues, by either passive adsorption or covalent bonding using the di-amino-poly(ethylene glycol) linker to microfibers. Under static and physiological flow conditions, conjugated mucin was more stable than adsorbed mucin on the surfaces. Mucin could slightly inhibit blood clotting, and mucin coating suppressed platelet adhesion on microfibrous scaffolds. In the rat common carotid artery anastomosis model, grafts with conjugated mucin, but not adsorbed mucin, exhibited excellent patency and higher cell infiltration into the graft walls. Mucin, which can be easily obtained from autologous sources, offers a novel method for improving the hemocompatibility and surface lubrication of vascular grafts and many other implants.

Off-the-shelf human decellularized tissue-engineered heart valves in a non-human primate model

Heart valve tissue engineering based on decellularized xenogenic or allogenic starter matrices has shown promising first clinical results. However, the availability of healthy homologous donor valves is limited and xenogenic materials are associated with infectious and immunologic risks. To address such limitations, biodegradable synthetic materials have been successfully used for the creation of living autologous tissue-engineered heart valves (TEHVs) in vitro. Since these classical tissue engineering technologies necessitate substantial infrastructure and logistics, we recently introduced decellularized TEHVs (dTEHVs), based on biodegradable synthetic materials and vascular-derived cells, and successfully created a potential off-the-shelf starter matrix for guided tissue regeneration. Here, we investigate the host repopulation capacity of such dTEHVs in a non-human primate model with up to 8 weeks follow-up. After minimally invasive delivery into the orthotopic pulmonary position, dTEHVs revealed mobile and thin leaflets after 8 weeks of follow-up. Furthermore, mild-moderate valvular insufficiency and relative leaflet shortening were detected. However, in comparison to the decellularized human native heart valve control - representing currently used homografts - dTEHVs showed remarkable rapid cellular repopulation. Given this substantial in situ remodeling capacity, these results suggest that human cell-derived bioengineered decellularized materials represent a promising and clinically relevant starter matrix for heart valve tissue engineering. These biomaterials may ultimately overcome the limitations of currently used valve replacements by providing homologous, non-immunogenic, off-the-shelf replacement constructs.

Animal models for vascular tissue-engineering

Because of rise in cardiovascular disease throughout the world, there is increasing demand for small diameter blood vessels as replacement grafts. The present review focuses on the animal models that have been used to test small-diameter TEVs with emphasis on the attributes of each model. Small animal models are used to test short-term patency and address mechanistic hypotheses; and large, preclinical animal models are employed to test long-term patency, remodeling and function in an environment mimicking human physiology. We also discuss recent clinical trials that employed laboratory fabricated TEVs and showed very promising results. Ultimately, animal models provide a testing platform for optimizing vascular grafts before clinical use in patients without suitable autologous vessels.

Transcatheter aortic valve implantation using anatomically oriented, marrow stromal cell-based, stented, tissue-engineered heart valves: technical considerations and implications for translational cell-based heart valve concepts

OBJECTIVES

While transcatheter aortic valve implantation (TAVI) has rapidly evolved for the treatment of aortic valve disease, the currently used bioprostheses are prone to continuous calcific degeneration. Thus, autologous, cell-based, living, tissue-engineered heart valves (TEHVs) with regeneration potential have been suggested to overcome these limitations. We investigate the technical feasibility of combining the concept of TEHV with transapical implantation technology using a state-of-the-art transcatheter delivery system facilitating the exact anatomical position in the systemic circulation.

METHODS

Trileaflet TEHVs fabricated from biodegradable synthetic scaffolds were sewn onto self-expanding Nitinol stents seeded with autologous marrow stromal cells, crimped and transapically delivered into the orthotopic aortic valve position of adult sheep (n = 4) using the JenaValve transapical TAVI System (JenaValve, Munich, Germany). Delivery, positioning and functionality were assessed by angiography and echocardiography before the TEHV underwent post-mortem gross examination. For three-dimensional reconstruction of the stent position of the anatomically oriented system, a computed tomography analysis was performed post-mortem.

RESULTS

Anatomically oriented, transapical delivery of marrow stromal cell-based TEHV into the orthotopic aortic valve position was successful in all animals (n = 4), with a duration from cell harvest to TEHV implantation of 101 ± 6 min. Fluoroscopy and echocardiography displayed sufficient positioning, thereby entirely excluding the native leaflets. There were no signs of coronary obstruction. All TEHV tolerated the loading pressure of the systemic circulation and no acute ruptures occurred. Animals displayed intact and mobile leaflets with an adequate functionality. The mean transvalvular gradient was 7.8 ± 0.9 mmHg, and the mean effective orifice area was 1.73 ± 0.02 cm(2). Paravalvular leakage was present in two animals, and central aortic regurgitation due to a single-leaflet prolapse was detected in two, which was primarily related to the leaflet design. No stent dislocation, migration or affection of the mitral valve was observed.

CONCLUSIONS

For the first time, we demonstrate the technical feasibility of a transapical TEHV delivery into the aortic valve position using a commercially available and clinically applied transapical implantation system that allows for exact anatomical positioning. Our data indicate that the combination of TEHV and a state-of-the-art transapical delivery system is feasible, representing an important step towards translational, transcatheter-based TEHV concepts.

Elastic, double-layered poly (l-lactide-co-ϵ-caprolactone) scaffold for long-term vascular reconstruction

Synthetic vessel grafts have been used as vascular substitutes for cardiovascular bypass procedures. In this study, we developed a novel tubular double-layered poly(l-lactide-co-ϵ-caprolactone) scaffold that did not require pretreatment with cell seeding by promoting autologous tissue regeneration by inducing the proliferation and differentiation of endothelial and smooth muscle progenitor cells after implantation. The patency and mechanical properties were maintained for one year after implantation, although 95% of the poly(l-lactide-co-ϵ-caprolactone) scaffolds had degraded. After this period, there was a lining of endothelial cells, an accumulation of collagen and elastin, and the development of neovascularization inside the poly(l-lactide-co-ϵ-caprolactone).

Tissue-engineered vascular grafts for use in the treatment of congenital heart disease: from the bench to the clinic and back again

Since the first tissue-engineered vascular graft (TEVG) was implanted in a child over a decade ago, growth in the field of vascular tissue engineering has been driven by clinical demand for improved vascular prostheses with performance and durability similar to an autologous blood vessel. Great strides were made in pediatric congenital heart surgery using the classical tissue engineering paradigm, and cell seeding of scaffolds in vitro remained the cornerstone of neotissue formation. Our second-generation bone marrow cell-seeded TEVG diverged from tissue engineering dogma with a design that induces the recipient to regenerate vascular tissue in situ. New insights suggest that neovessel development is guided by cell signals derived from both seeded cells and host inflammatory cells that infiltrate the graft. The identification of these signals and the regulatory interactions that influence cell migration, phenotype and extracellular matrix deposition during TEVG remodeling are yielding a next-generation TEVG engineered to guide neotissue regeneration without the use of seeded cells. These developments represent steady progress towards our goal of an off-the-shelf tissue-engineered vascular conduit for pediatric congenital heart surgery.

Functionality, growth and accelerated aging of tissue engineered living autologous vascular grafts

Living autologous tissue engineered vascular-grafts (TEVGs) with growth-capacity may overcome the limitations of contemporary artificial-prostheses. However, the multi-step in vitro production of TEVGs requires extensive ex vivo cell-manipulations with unknown effects on functionality and quality of TEVGs due to an accelerated biological age of the cells. Here, the impact of biological cell-age and tissue-remodeling capacity of TEVGs in relation to their clinical long-term functionality are investigated. TEVGs were implanted as pulmonary-artery (PA) replacements in juvenile sheep and followed for up to 240 weeks (∼4.5years). Telomere length and telomerase activity were compared amongst TEVGs and adjacent native tissue. Telomerase-activity of in vitro expanded autologous vascular-cells prior to seeding was <5% as compared to a leukemic cell line, indicating biological-aging associated with decreasing telomere-length with each cellular-doubling. Up to 100 weeks, the cells in the TEVGs had consistently shorter telomeres compared to the native counterpart, whereas no significant differences were detectable at 240 weeks. Computed tomography (CT) analysis demonstrated physiological wall-pressures, shear-stresses, and flow-pattern comparable to the native PA. There were no signs of degeneration detectable and continuous native-analogous growth was confirmed by vessel-volumetry. TEVGs exhibit a higher biological age compared to their native counterparts. However, despite of this tissue engineering technology related accelerated biological-aging, growth-capacity and long-term functionality was not compromised. To the contrary, extensive in-vivo remodeling processes with substantial endogenous cellular turnover appears to result in "TEVG rejuvenation" and excellent clinical performance. As these large-animal results can be extrapolated to approximately 20 human years, this study suggests long-term clinical-safety of cardiovascular in vitro tissue engineering and may contribute to safety-criteria as to first-in-man clinical-trials.

Stem cell sources for vascular tissue engineering and regeneration

This review focuses on the stem cell sources with the potential to be used in vascular tissue engineering and to promote vascular regeneration. The first clinical studies using tissue-engineered vascular grafts are already under way, supporting the potential of this technology in the treatment of cardiovascular and other diseases. Despite progress in engineering biomaterials with the appropriate mechanical properties and biological cues as well as bioreactors for generating the correct tissue microenvironment, the source of cells that make up the vascular tissues remains a major challenge for tissue engineers and physicians. Mature cells from the tissue of origin may be difficult to obtain and suffer from limited proliferative capacity, which may further decline as a function of donor age. On the other hand, multipotent and pluripotent stem cells have great potential to provide large numbers of autologous cells with a great differentiation capacity. Here, we discuss the adult multipotent as well as embryonic and induced pluripotent stem cells, their differentiation potential toward vascular lineages, and their use in engineering functional and implantable vascular tissues. We also discuss the associated challenges that need to be addressed in order to facilitate the transition of this technology from the bench to the bedside.

Concise review: tissue-engineered vascular grafts for cardiac surgery: past, present, and future

In surgical repair for heart or vascular disease, it is often necessary to implant conduits or correct tissue defects. The most commonly used graft materials to date are (a) artificial grafts; (b) autologous tissues, such as pericardium and saphenous vein; (c) allografts; and (d) xenografts. However, none of these four options offer growth potential, and all are associated with varying levels of thrombogenicity and susceptibility to infection. The lack of growth potential of these four options is particularly important in pediatric cardiac surgery, where patients will often outgrow their vascular grafts and require additional operations. Thus, developing a material with sufficient durability and growth potential that will function as the child grows older will eliminate the need for reoperation and significantly reduce morbidity and mortality of some types of congenital heart defects. Vascular tissue engineering is a relatively new field that has undergone enormous growth over the last decade. The goal of vascular tissue engineering is to produce neovessels and neo-organ tissue from autologous cells using a biodegradable polymer as a scaffold. The most important advantage of tissue-engineered implants is that these tissues can grow, remodel, rebuild, and respond to injury. Once the seeded autologous cells have deposited an extracellular matrix and the original scaffold is biodegraded, the tissue resembles and behaves as native tissue. When tissue-engineered vascular grafts are eventually put to use in the clinical arena, the quality of life in patients after surgery will be drastically improved.

Current advances in the translation of vascular tissue engineering to the treatment of pediatric congenital heart disease

Tissue-engineered vascular grafts (TEVGs) hold great promise for the improvement of outcomes in pediatric patients with congenital cardiac anomalies. Currently used synthetic grafts have several limitations, including thrombogenicity, increased risk of infection, and lack of growth potential. The first pilot clinical trial of TEVGs demonstrated the feasibility of this new technology and revealed an excellent safety profile. However, long-term follow-up from this trial revealed the primary graft-related complication to be stenosis, affecting 16 percent of grafts within 7 years post-implantation. In order to determine the mechanism behind TEVG stenosis and ultimately to create improved second generation TEVGs, our group has returned to the bench to study vascular neotissue formation in a variety of large and small animal models. The purpose of this report is to review the recent advances in the understanding of neotissue formation and vascular tissue engineering.

Prenatally engineered autologous amniotic fluid stem cell-based heart valves in the fetal circulation

Prenatal heart valve interventions aiming at the early and systematic correction of congenital cardiac malformations represent a promising treatment option in maternal-fetal care. However, definite fetal valve replacements require growing implants adaptive to fetal and postnatal development. The presented study investigates the fetal implantation of prenatally engineered living autologous cell-based heart valves. Autologous amniotic fluid cells (AFCs) were isolated from pregnant sheep between 122 and 128 days of gestation via transuterine sonographic sampling. Stented trileaflet heart valves were fabricated from biodegradable PGA-P4HB composite matrices (n = 9) and seeded with AFCs in vitro. Within the same intervention, tissue engineered heart valves (TEHVs) and unseeded controls were implanted orthotopically into the pulmonary position using an in-utero closed-heart hybrid approach. The transapical valve deployments were successful in all animals with acute survival of 77.8% of fetuses. TEHV in-vivo functionality was assessed using echocardiography as well as angiography. Fetuses were harvested up to 1 week after implantation representing a birth-relevant gestational age. TEHVs showed in vivo functionality with intact valvular integrity and absence of thrombus formation. The presented approach may serve as an experimental basis for future human prenatal cardiac interventions using fully biodegradable autologous cell-based living materials.

Evaluation of the use of an induced puripotent stem cell sheet for the construction of tissue-engineered vascular grafts

OBJECTIVE

The development of a living, tissue-engineered vascular graft (TEVG) holds great promise for advancing the field of cardiovascular surgery. However, the ultimate source and time needed to procure these cells remain problematic. Induced puripotent stem (iPS) cells have recently been developed and have the potential for creating a pluripotent cell line from a patient's own somatic cells. In the present study, we evaluated the use of a sheet created from iPS cell-derived vascular cells as a potential source for the construction of TEVG.

METHODS

Male mouse iPS cells were differentiated into embryoid bodies using the hanging-drop method. Cell differentiation was confirmed by a decrease in the proportion of SSEA-1-positive cells over time using fluorescence-activated cell sorting. The expression of endothelial cell and smooth muscle cell markers was detected using real-time polymerase chain reaction (PCR). The differentiated iPS cell sheet was made using temperature-responsive dishes and then seeded onto a biodegradable scaffold composed of polyglycolic acid-poly-l-lactide and poly(l-lactide-co-ε-caprolactone) with a diameter of 0.8 mm. These scaffolds were implanted as interposition grafts in the inferior vena cava of female severe combined immunodeficiency/beige mice (n = 15). Graft function was serially monitored using ultrasonography. The grafts were analyzed at 1, 4, and 10 weeks with histologic examination and immunohistochemistry. The behavior of seeded differentiated iPS cells was tracked using Y-chromosome fluorescent in situ hybridization and SRY real-time PCR.

RESULTS

All mice survived without thrombosis, aneurysm formation, graft rupture, or calcification. PCR evaluation of iPS cell sheets in vitro demonstrated increased expression of endothelial cell markers. Histologic evaluation of the grafts demonstrated endothelialization with von Willebrand factor and an inner layer with smooth muscle actin- and calponin-positive cells at 10 weeks. The number of seeded differentiated iPS cells was found to decrease over time using real-time PCR (42.2% at 1 week, 10.4% at 4 weeks, 9.8% at 10 weeks). A fraction of the iPS cells were found to be Y-chromosome fluorescent positive at 1 week. No iPS cells were found to co-localize with von Willebrand factor or smooth muscle actin-positive cells at 10 weeks.

CONCLUSIONS

Differentiated iPS cells offer an alternative cell source for constructing TEVG. Seeded iPS cells exerted a paracrine effect to induce neotissue formation in the acute phase and were reduced in number by apoptosis at later time points. Sheet seeding of our TEVG represents a viable mode of iPS cell delivery over time.

Mechanical property characterization of electrospun recombinant human tropoelastin for vascular graft biomaterials

The development of vascular grafts has focused on finding a biomaterial that is non-thrombogenic, minimizes intimal hyperplasia, matches the mechanical properties of native vessels and allows for regeneration of arterial tissue. In this study, the structural and mechanical properties and the vascular cell compatibility of electrospun recombinant human tropoelastin (rTE) were evaluated as a potential vascular graft support matrix. Disuccinimidyl suberate (DSS) was used to cross-link electrospun rTE fibers to produce a polymeric recombinant tropoelastin (prTE) matrix that is stable in aqueous environments. Tubular 1cm diameter prTE samples were constructed for uniaxial tensile testing and 4mm small-diameter prTE tubular scaffolds were produced for burst pressure and cell compatibility evaluations from 15 wt.% rTE solutions. Uniaxial tensile tests demonstrated an average ultimate tensile strength (UTS) of 0.36±0.05 MPa and elastic moduli of 0.15±0.04 and 0.91±0.16 MPa, which were comparable to extracted native elastin. Burst pressures of 485±25 mm Hg were obtained from 4mm internal diameter scaffolds with 453±74 μm average wall thickness. prTE supported endothelial cell growth with typical endothelial cell cobblestone morphology after 48 h in culture. Cross-linked electrospun rTE has promising properties for utilization as a vascular graft biomaterial with customizable dimensions, a compliant matrix and vascular cell compatibility.

Challenges in translating vascular tissue engineering to the pediatric clinic

The development of tissue-engineered vascular grafts for use in cardiovascular surgery holds great promise for improving outcomes in pediatric patients with complex congenital cardiac anomalies. Currently used synthetic grafts have a number of shortcomings in this setting but a tissue engineering approach has emerged in the past decade as a way to address these limitations. The first clinical trial of this technology showed that it is safe and effective but the primary mode of graft failure is stenosis. A variety of murine and large animal models have been developed to study and improve tissue engineering approaches with the hope of translating this technology into routine clinical use, but challenges remain. The purpose of this report is to address the clinical problem and review recent advances in vascular tissue engineering for pediatric applications. A deeper understanding of the mechanisms of neovessel formation and stenosis will enable rational design of improved tissue-engineered vascular grafts.

Characterization of the natural history of extracellular matrix production in tissue-engineered vascular grafts during neovessel formation

BACKGROUND

The extracellular matrix (ECM) is a critical determinant of neovessel integrity.

MATERIALS AND METHODS

Thirty-six (polyglycolic acid + polycaprolactone and poly lactic acid) tissue-engineered vascular grafts seeded with syngeneic bone marrow mononuclear cells were implanted as inferior vena cava interposition grafts in C57BL/6 mice. Specimens were characterized using immunohistochemical staining and qPCR for representative ECM components in addition to matrix metalloproteinases (MMPs). Total collagen, elastin, and glycosaminoglycan (GAG) contents were determined. MMP activity was measured using zymography.

RESULTS

Collagen production on histology demonstrated an initial increase in type III at 1 week followed by type I production at 2 weeks and type IV at 4 weeks. Gene expression of both type I and type III peaked at 2 weeks, whereas type IV continued to increase over the 4-week period. Histology demonstrated fibrillin-1 deposition at 1 week followed by elastin production at 4 weeks. Elastin gene expression significantly increased at 4 weeks, whereas fibrillin-1 decreased at 4 weeks. GAG demonstrated abundant production at each time point on histology. Gene expression of decorin significantly increased at 4 weeks, whereas versican decreased over time. Biochemical analysis showed that total collagen production was greatest at 2 weeks, and there was a significant increase in elastin and GAG production at 4 weeks. Histological characterization of MMPs showed abundant production of MMP-2 at each time point, while MMP-9 decreased over the 4-week period. Gene expression of MMP-2 significantly increased at 4 weeks, whereas MMP-9 significantly decreased at 4 weeks.

CONCLUSIONS

ECM production during neovessel formation is characterized by early ECM deposition followed by extensive remodeling.

A critical role for macrophages in neovessel formation and the development of stenosis in tissue-engineered vascular grafts

The primary graft-related complication during the first clinical trial evaluating the use of tissue-engineered vascular grafts (TEVGs) was stenosis. We investigated the role of macrophages in the formation of TEVG stenosis in a murine model. We analyzed the natural history of TEVG macrophage infiltration at critical time points and evaluated the role of cell seeding on neovessel formation. To assess the function of infiltrating macrophages, we implanted TEVGs into mice that had been macrophage depleted using clodronate liposomes. To confirm this, we used a CD11b-diphtheria toxin-receptor (DTR) transgenic mouse model. Monocytes infiltrated the scaffold within the first few days and initially transformed into M1 macrophages. As the scaffold degraded, the macrophage infiltrate disappeared. Cell seeding decreased the incidence of stenosis (32% seeded, 64% unseeded, P=0.024) and the degree of macrophage infiltration at 2 wk. Unseeded TEVGs demonstrated conversion from M1 to M2 phenotype, whereas seeded grafts did not. Clodronate and DTR inhibited macrophage infiltration and decreased stenosis but blocked formation of vascular neotissue, evidenced by the absence of endothelial and smooth muscle cells and collagen. These findings suggest that macrophage infiltration is critical for neovessel formation and provides a strategy for predicting, detecting, and inhibiting stenosis in TEVGs.

Determining the fate of seeded cells in venous tissue-engineered vascular grafts using serial MRI

A major limitation of tissue engineering research is the lack of noninvasive monitoring techniques for observations of dynamic changes in single tissue-engineered constructs. We use cellular magnetic resonance imaging (MRI) to track the fate of cells seeded onto functional tissue-engineered vascular grafts (TEVGs) through serial imaging. After in vitro optimization, murine macrophages were labeled with ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles and seeded onto scaffolds that were surgically implanted as inferior vena cava interposition grafts in SCID/bg mice. Serial MRI showed the transverse relaxation times (T(2)) were significantly lower immediately following implantation of USPIO-labeled scaffolds (T(2) = 44 ± 6.8 vs. 71 ± 10.2 ms) but increased rapidly at 2 h to values identical to control implants seeded with unlabeled macrophages (T(2) = 63 ± 12 vs. 63 ± 14 ms). This strongly indicates the rapid loss of seeded cells from the scaffolds, a finding verified using Prussian blue staining for iron containing macrophages on explanted TEVGs. Our results support a novel paradigm where seeded cells are rapidly lost from implanted scaffolds instead of developing into cells of the neovessel, as traditionally thought. Our findings confirm and validate this paradigm shift while demonstrating the first successful application of noninvasive MRI for serial study of cellular-level processes in tissue engineering.

Comparison of human bone marrow mononuclear cell isolation methods for creating tissue-engineered vascular grafts: novel filter system versus traditional density centrifugation method

INTRODUCTION

We created the first tissue-engineered vascular graft (TEVG) to be successfully used in humans. The TEVG is made by seeding autologous bone marrow-derived mononuclear cells (BM-MNCs) onto a biodegradable tubular scaffold fabricated from polyglycolic-acid mesh coated with a 50:50 copolymer of poly-L-lactide and-ɛ-caprolactone. In the initial clinical study, the BM-MNCs were isolated using a Ficoll density centrifugation method. Use of this cell isolation technique is problematic in that it is performed using an open system and therefore is susceptible to contamination. As a first step toward creating a closed system for assembling a TEVG, we evaluated the use of a filter-based method for isolating BM-MNCs and compared it to density centrifugation in Ficoll.

METHODS

BM-MNCs were isolated from human BM using density centrifugation in Ficoll or a filter-based method. BM-MNCs were seeded onto biodegradable tubular scaffold and incubated for 24 h before implantation. The TEVG were implanted as inferior vena cava interposition grafts in SCID/bg mice (n=24) using microsurgical technique. Grafts were followed with ultrasonography and computed tomography-angiography. Ten weeks after implantation the TEVG were explanted and examined using histology and immunohistochemistry.

RESULTS

Both methods isolated similar number of cells (Ficoll: 8.5±6.6×10(6)/mL, Filter: 6.6±3.5×10(6)/mL; p=0.686) with similar viability as assayed using fluorescence-activated cell sorting (FACS) (Ficoll: 97.0%±1.5%, Filter: 95.9%±3.0%; p=0.339). FACS analysis demonstrated that the fraction of lymphocytes and monocytes to total cells was lower in the filter group (CD4 in Ficoll: 8.9%±1.1%, CD4 in Filter: 3.5%±0.8%; p=0.002, CD8 in Ficoll: 9.4%±2.1%, CD8 in Filter: 3.9%±1.4%; p=0.021, Monocyte in Ficoll: 6.9%±1.0%, Monocyte in Filter: 2.7%±1.0%; p=0.008), consistent with granulocyte contamination (Ficoll: 46.6±2.7×10(6)/mL, Filter: 58.1±5.2×10(6)/mL; p<0.001). The ratio of stem cells to BM-MNCs was comparable between groups. There were no statistically significant differences with regard to TEVG patency and morphology between groups. Both methods of cell isolation produced neovessels with similar histology.

CONCLUSION

Filter-based BM-MNC isolation is comparable to BM-MNC isolation using density centrifugation in Ficoll for TEVG assembly. The filter-based cell isolation technique has the added advantage of the potential to create a closed disposable system.

Tissue-engineered vascular grafts form neovessels that arise from regeneration of the adjacent blood vessel

We developed a tissue-engineered vascular graft composed of biodegradable scaffold seeded with autologous bone marrow-derived mononuclear cells (BMMCs) that is currently in clinical trial and developed analogous mouse models to study mechanisms of neovessel formation. We previously reported that seeded human BMMCs were rapidly lost after implantation into immunodeficient mice as host macrophages invaded the graft. As a consequence, the resulting neovessel was entirely of host cell origin. Here, we investigate the source of neotissue cells in syngeneic BMMC-seeded grafts, implanted into immunocompetent mouse recipients. We again find that seeded BMMCs are lost, declining to 0.02% at 14 d, concomitant with host macrophage invasion. In addition, we demonstrate using sex-mismatched chimeric hosts that bone marrow is not a significant source of endothelial or smooth muscle cells that comprise the neovessel. Furthermore, using composite grafts formed from seeded scaffold anastomosed to sex-mismatched natural vessel segments, we demonstrate that the adjacent vessel wall is the principal source of these endothelial and smooth muscle cells, forming 93% of proximal neotissue. These findings have important implications regarding fundamental mechanisms underlying neotissue formation; in this setting, the tissue-engineered construct functions by mobilizing the body's innate healing capabilities to "regenerate" neotissue from preexisting committed tissue cells.

Substrates for cardiovascular tissue engineering

Cardiovascular tissue engineering aims to find solutions for the suboptimal regeneration of heart valves, arteries and myocardium by creating 'living' tissue replacements outside (in vitro) or inside (in situ) the human body. A combination of cells, biomaterials and environmental cues of tissue development is employed to obtain tissues with targeted structure and functional properties that can survive and develop within the harsh hemodynamic environment of the cardiovascular system. This paper reviews the up-to-date status of cardiovascular tissue engineering with special emphasis on the development and use of biomaterial substrates. Key requirements and properties of these substrates, as well as methods and readout parameters to test their efficacy in the human body, are described in detail and discussed in the light of current trends toward designing biologically inspired microenviroments for in situ tissue engineering purposes.

Injectable living marrow stromal cell-based autologous tissue engineered heart valves: first experiences with a one-step intervention in primates

AIMS

A living heart valve with regeneration capacity based on autologous cells and minimally invasive implantation technology would represent a substantial improvement upon contemporary heart valve prostheses. This study investigates the feasibility of injectable, marrow stromal cell-based, autologous, living tissue engineered heart valves (TEHV) generated and implanted in a one-step intervention in non-human primates.

METHODS AND RESULTS

Trileaflet heart valves were fabricated from non-woven biodegradable synthetic composite scaffolds and integrated into self-expanding nitinol stents. During the same intervention autologous bone marrow-derived mononuclear cells were harvested, seeded onto the scaffold matrix, and implanted transapically as pulmonary valve replacements into non-human primates (n = 6). The transapical implantations were successful in all animals and the overall procedure time from cell harvest to TEHV implantation was 118 ± 17 min. In vivo functionality assessed by echocardiography revealed preserved valvular structures and adequate functionality up to 4 weeks post implantation. Substantial cellular remodelling and in-growth into the scaffold materials resulted in layered, endothelialized tissues as visualized by histology and immunohistochemistry. Biomechanical analysis showed non-linear stress-strain curves of the leaflets, indicating replacement of the initial biodegradable matrix by living tissue.

CONCLUSION

Here, we provide a novel concept demonstrating that heart valve tissue engineering based on a minimally invasive technique for both cell harvest and valve delivery as a one-step intervention is feasible in non-human primates. This innovative approach may overcome the limitations of contemporary surgical and interventional bioprosthetic heart valve prostheses.

Tissue engineering on matrix: future of autologous tissue replacement

Tissue engineering aims at the creation of living neo-tissues identical or close to their native human counterparts. As basis of this approach, temporary biodegradable supporter matrices are fabricated in the shape of a desired construct, which promote tissue strength and provide functionality until sufficient neo-tissue is formed. Besides fully synthetic polymer-based scaffolds, decellularized biological tissue of xenogenic or homogenic origin can be used. In a second step, these scaffolds are seeded with autologous cells attaching to the scaffold microstructure. In order to promote neo-tissue formation and maturation, the seeded scaffolds are exposed to different forms of stimulation. In cardiovascular tissue engineering, this "conditioning" can be achieved via culture media and biomimetic in vitro exposure, e.g., using flow bioreactors. This aims at adequate cellular differentiation, proliferation, and extracellular matrix production to form a living tissue called the construct. These living autologous constructs, such as heart valves or vascular grafts, are created in vitro, comprising a viable interstitium with repair and remodeling capabilities already prior to implantation. In situ further in vivo remodeling is intended to recapitulate physiological vascular architecture and function. The remodeling mechanisms were shown to be dominated by monocytic infiltration and chemotactic host-cell attraction leading into a multifaceted inflammatory process and neo-tissue formation. Key molecules of these processes can be integrated into the scaffold matrix to direct cell and tissue fate in vivo.

Scaffolds in tissue engineering of blood vessels

Tissue engineering of small diameter (<5 mm) blood vessels is a promising approach for developing viable alternatives to autologous vascular grafts. It involves in vitro seeding of cells onto a scaffold on which the cells attach, proliferate, and differentiate while secreting the components of extracellular matrix that are required for creating the tissue. The scaffold should provide the initial requisite mechanical strength to withstand in vivo hemodynamic forces until vascular smooth muscle cells and fibroblasts reinforce the extracellular matrix of the vessel wall. Hence, the choice of scaffold is crucial for providing guidance cues to the cells to behave in the required manner to produce tissues and organs of the desired shape and size. Several types of scaffolds have been used for the reconstruction of blood vessels. They can be broadly classified as biological scaffolds, decellularized matrices, and polymeric biodegradable scaffolds. This review focuses on the different types of scaffolds that have been designed, developed, and tested for tissue engineering of blood vessels, including use of stem cells in vascular tissue engineering.

Crosslinked urethane doped polyester biphasic scaffolds: Potential for in vivo vascular tissue engineering

In vivo tissue engineering uses the body as a bioreactor for tissue regeneration, thus placing stringent requirements on tissue scaffolds, which should be mechanically robust for immediate implantation without a long in vitro cell culture time. In addition to mechanical strength, vascular grafts fabricated for in vivo tissue engineering approach must have matching mechanical properties to the target tissues to avoid compliance mismatch, which is one of the reasons for graft failure. We recently synthesized a new generation of strong and elastic biodegradable crosslinked urethane-doped polyesters (CUPE) to address the challenge of developing soft, elastic yet strong biodegradable polymers. This study evaluated the tensile strength, burst pressure, and suture retention of CUPE biphasic scaffolds to determine if the scaffolds met the requirements for immediate implantation in an in vivo tissue engineering approach. In addition, we also examined the hemocompatibility and inflammatory potential of CUPE to demonstrate its potential in serving as a blood-contacting vascular graft material. Tensile strength of CUPE biphasic scaffolds (5.02 ± 0.70 MPa) was greater than native vessels (1.43 ± 0.60 MPa). CUPE scaffolds exhibited tunable burst pressure ranging from 1500 mmHg to 2600 mmHg, and adequate suture retention values (2.45 ± 0.23 N). CUPE showed comparable leukocyte activation and whole blood clotting kinetics to poly(L-lactic acid) PLLA. However, CUPE incited a lesser release of inflammatory cytokines and was found to be non hemolytic. Combined with the mechanical properties and previously demonstrated anti-thrombogenic nature, CUPE may serve as a viable graft material for in vivo blood vessel tissue engineering.

The development and translation of the tissue-engineered vascular graft

This lecture will define the classic tissue engineering paradigm, describe cell trafficking with regard to neotissue formation, and explain the role of the host in neotissue formation.

Hyaluronan-based scaffold for in vivo regeneration of the rat vena cava: Preliminary results in an animal model

The aim of this study was to develop a prosthetic graft that could perform as a small-diameter vascular conduit for vein regeneration. The difficulty of obtaining significant long-term patency and good wall mechanical strength in vivo has been a significant obstacle in achieving small-diameter vein prostheses. Fifteen Male Wistar rats weighing 250-350 g were used. Tubular structures of hyaluronan (HYAFF-11 tubules, 2 mm diameter, and 1.5 cm length) were implanted in the vena cava of rats as temporary absorbable guides to promote regeneration of veins. Performance was assessed at 30, 60, and 90 days after surgery by histology (hematoxylin-eosin and Weighert solution) and immunohistochemistry (antibodies to von Willebrand factor and to Myosin Light-Chain Kinase). These experiments resulted in two novel findings: (1) sequential regeneration of vascular components led to complete vein wall regeneration 30 days after surgery; (2) the biomaterial used created the ideal environment for the delicate regeneration process during the critical initial phases, yet its biodegradability allowed for complete degradation of the construct 4 months after implantation, at which time, a new vein remained to connect the vein stumps. This work demonstrates the complete vena cava regeneration inside the hyaluronic acid-based prosthesis, opening new perspective of microsurgical applications, like replantation of the upper limb, elongation of vascular pedicle of free flaps, cardiovascular surgery, and pediatric microvascular surgery.