Improved patency of collagen-impregnated grafts after in vitro autogenous endothelial cell seeding

Cellularized small-caliber tissue-engineered vascular grafts: looking for the ultimate gold standard

Due to the lack of efficacy of synthetic vascular substitutes in the replacement of small-caliber arteries, vascular tissue engineering (VTE) has emerged as a promising solution to produce viable small-caliber tissue-engineered vascular grafts (TEVG). Previous studies have shown the importance of a cellular intimal layer at the luminal surface of TEVG to prevent thrombotic events. However, the cellularization of a TEVG seems to be a critical approach to consider in the development of a TEVG. To date, no standard cellularization method or cell type has been established to create the ideal TEVG by promoting its long-term patency and function. In this review, advances in VTE are described and discussed with a particular focus on the construction approaches of cellularized small-caliber TEVGs, the cell types used, as well as their preclinical and clinical applications.

In Vitro Endothelialization of Carbon-Coated Dacron Vascular Grafts

Physical vapour deposition is used to coat vascular prostheses with pyrolytic carbon. This coating may facilitate the development of an endothelial monolayer in grafts implanted in laboratory animals. This in vitro study compared the adherence and growth of cultured porcine aortic endothelial cells (EC) seeded in vitro on carbon-coated Dacron, to uncoated prostheses. The cells were incubated at 5 × 104 cells/cm2. Progress was monitored at different times (TO + 2 hours, D 1, D 4, D 8) by microscopic observation, when cell counts were made; and by scanning electronic microscopy (SEM) to evaluate morphological EC-graft interactions. Cell adherence was independent of the carbon coating but cellular growth occurred only on carbon-coated Dacron. The SEM observations showed both the shape of the adherent cells, which were rounded on uncoated Dacron and extensively spread on carbon-coated prostheses, and the morphology of the carbon coating.

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.

Phenotypic Reversion of Smooth Muscle Cells in Hybrid Vascular Prostheses

Our purpose was to evaluate whether or not and when phenotypic modulation of smooth muscle cells (SMCs) in hybrid vascular prostheses preincorporated with SMCs occurs upon implantation. Two types of hybrid vascular grafts incorporated with vascular cells derived from canine jugular veins were prepared: grafts containing a collagen gel layer covered with an endothelial monolayer at the luminal surface (Model I graft) and those containing an endothelial monolayer and SMC multilayer (Model II graft). They were bilaterally implanted into carotid arteries of the same dogs from which the cells had been harvested for 2 wk (n = 3) and 12 wk (n = 3). The time-dependent changes in populations of three SMC phenotypes (synthetic, intermediate, and contractile) in the neoarterial layers were quantified by morphometric evaluation using a transmission electron microscope in hybrid vascular grafts. Before implantation, all the SMCs were of the synthetic phenotype. In Model II grafts at 2 wk, synthetic and intermediate SMCs were dominant especially in the luminal layer. On the other hand, neoarterial layers at 12 wk were dominated by contractile SMCs, which were evenly distributed throughout the entire neoarterial tissues. A markedly delayed phenotypic reversion was noted for the Model I grafts at 12 wk. In the hybrid grafts, during about 3 mo of implantation, neoarterial SMCs transformed from the synthetic to the contractile phenotypes, which was promoted by SMC incorporation.

Allogenic, Multidonor In Vitro Endothelialization of Small Diameter PTFE Grafts in Baboons

The early effect of allogenic in vitro endothelialization was assessed in small- diameter vascular prostheses. Cryopreserved and pooled venous endothelial cells (ECs) from 13 baboons of similar blood group were used for confluent in vitro endothelialization of fibrin glue-coated polytetrafluoroethylene vascular grafts. Bilateral femoral interpositions (12 cm) of experimental and control grafts were subsequently implanted into a separate group of 18 male baboons with the same blood group as that of the pooled cells. After sixteen days of implantation the patency rate of grafts of the endothelialized group was similar to that of the control group (55.6% vs 61.1%, p>0.1). Scanning electron microscopy revealed that 44.4% of experimental grafts were completly free of endothelium, while the remaining grafts showed a moderate mean EC coverage of 34.4 17.1%. These remaining ECs were found mainly as small islands densely covered by leukocytes. The majority of these white cells resembled granulocytes, although adherent lymphocytes were also regularly observed. Since surface morphology does not explain the mechanism of EC loss, further experiments must elucidate this question.

If rejection is found to be the primary cause for this cell loss, the use of major histocompatibility complex (MHC) matched EC subpools might be considered.

A study of a three-dimensional PLGA sponge containing natural polymers co-cultured with endothelial and mesenchymal stem cells as a tissue engineering scaffold

The interaction between vascular endothelial cells (ECs) and vascular smooth muscle cells (VSMCs) in a complex hemodynamic and mechanical environment plays an important role in the control of blood vessel growth and function. Despite the importance of VSMCs, substitutes are needed for vascular therapies. A potential VSMC substitute is human adult bone marrow derived mesenchymal stem cells (hMSCs). In this study, the effect of poly(lactic-co-glycolic acid) (PLGA) scaffolds containing three natural polymers (demineralized bone particles, silk, and small intestine submucosa) on the phenotype of MSCs and SMCs cultured with or without ECs was investigated. The study objective was to create a media equivalent for a tissue engineered blood vessel using PLGA, natural polymers, and MSCs co-cultured with ECs. The PLGA containing the natural polymers silk and SIS showed increased proliferation and cell adhesion. The presence of silk and DBP promoted a MSC phenotype change into a SMC-like phenotype at the mRNA level; however these differences at the protein level were not seen. Additionally, PLGA containing SIS did not induce SMC gene or protein upregulation. Finally, the effect of ECs in combination with the natural polymers was tested. When co-cultured with ECs, the mRNA of SMC specific markers in MSCs and SMCs were increased when compared to SMCs or MSCs alone. However, MSCs, when co-cultured with ECs on PLGA containing silk, exhibited significantly increased α-SMA and calponin expression when compared to PLGA only scaffolds. These results indicate that the natural polymer silk in combination with the co-culture of endothelial cells was most effective at increasing cell viability and inducing a SMC-like phenotype at the mRNA and protein level in MSCs.

Implantation study of small-caliber "biotube" vascular grafts in a rat model

We developed autologous vascular grafts, called "biotubes," by simple and safe in-body tissue architecture technology, which is a practical concept of regenerative medicine, without using special sterile conditions or complicated in vitro cell treatment processes. In this study, biotubes of extremely small caliber were first auto-implanted to rat abdominal aortas. Biotubes were prepared by placing silicone rods (outer diameter 1.5 mm, length 30 mm) used as a mold into dorsal subcutaneous pouches in rats for 4 weeks. After argatroban coating, the obtained biotubes were auto-implanted to abdominal aortas (n = 6) by end-to-end anastomosis using a custom-designed sutureless vascular connecting system under microscopic guidance. Graft status was evaluated by contrast-free time-of-flight magnetic resonance angiography (TOF-MRA). All grafts were harvested at 12 weeks after implantation. The patency rate was 66.7 % (4/6). MRA showed little stenosis and no aneurysmal dilation in all biotubes. The original biotube had wall thickness of about 56.2 ± 26.5 μm at the middle portion and mainly random and sparse collagen fibers and fibroblasts. After implantation, the wall thickness was 235.8 ± 24.8 μm. In addition, native-like vascular structure was regenerated, which included (1) a completely endothelialized luminal surface, (2) a mesh-like elastin fiber network, and (3) regular circumferential orientation of collagen fibers and α-SMA positive cells. Biotubes could be used as small-caliber vascular prostheses that greatly facilitate the healing process and exhibit excellent biocompatibility in vascular regenerative medicine.

Arterial shear stress augments the differentiation of endothelial progenitor cells adhered to VEGF-bound surfaces

Our ongoing studies show that vascular endothelial cell growth factor (VEGF)-bound surfaces selectively capture endothelial progenitor cells (EPCs) in vitro and in vivo, and that surface-bound VEGF stimulates intracellular signal transduction pathways over prolonged culture periods, resulting in inductive differentiation of EPCs. In this article, we investigated whether simulated arterial shear stress augments the differentiation of EPCs adhered to a VEGF-bound surface. Human peripheral blood-derived mononuclear cells adhered to a VEGF-bound surface were exposed to 1 day of shear stress (15 dynes/cm(2), corresponding to shear load in arteries). Shear stress suppressed the expression of mRNAs encoding CD34 and CD133, which are markers for EPCs, and augmented the expression of mRNAs encoding CD31 and von Willebrand factor (vWF) as well as vWF protein, which are markers for endothelial cells (ECs). Shear stress enhanced expression of ephrinB2 mRNA, a marker for arterial ECs, but did not significantly change expression of EphB4 mRNA, a marker for venous ECs. Focused protein array analysis showed that mechanotransduction by shear stress activated the p38 and MAPK pathways in EPCs. Thus, arterial shear stress, in concert with surface-bound VEGF, augments the differentiation of EPCs. These results strongly support previous observation of rapid differentiation of EPCs captured on VEGF-bound stents in a porcine model.

Luminal surface design of electrospun small-diameter graft aiming at in situ capture of endothelial progenitor cell

If endothelial progenitor cells (EPCs), which circulate in blood flow, are captured on the luminal surface of an implanted artificial graft and sooner or later proliferate to form a fully endothelialized surface, such a small-diameter artificial graft must exhibit a high patency rate. This study aimed at designing a luminal surface of elastomeric electrospun mesh graft, which is capable of selective capture of EPCs under arterial flow and has a high antithrombogenic potential until full endothelization is achieved. The designed luminal surface layer is composed of a photopolymerized gelatin gel layer that enables the release of impregnated heparin and selective adhesion of circulating EPCs via complexation between surface-fixed vascular endothelial growth factor (VEGF) and cellular VEGF receptor. Human mononuclear cells seeded and cultured on such a gel layer expressed endothelial cell surface markers. Confocal laser scanning microscopy observation revealed that VEGF is highly surface-enriched, and heparin is homogeneously distributed in the gel layer. A continuously slow release of heparin was observed. Thus, a prototype luminal surface was fabricated on electrospun segmented polyurethane tubes for in vivo study.

Animal models for the assessment of novel vascular conduits

The development of an ideal small-diameter conduit for use in vascular bypass surgery has yet to be achieved. The ongoing innovation in biomaterial design generates novel conduits that require preclinical assessment in vivo, and a number of animal models have been used for this purpose. This article examines the rationale behind animal models used in the assessment of small-diameter vascular conduits encompassing the commonly used species: baboons, sheep, pigs, dogs, rabbits, and rodents. Studies on the comparative hematology for these species relative to humans are summarized, and the hydrodynamic values for common implant locations are also compared. The large- and small-animal models are then explored, highlighting the characteristics of each that determine their relative utility in the assessment of vascular conduits. Where possible, the performance of expanded polytetrafluoroethylene is given in each animal and in each location to allow direct comparisons between species. New challenges in animal modeling are outlined for the assessment of tissue-engineered graft designs. Finally, recommendations are given for the selection of animal models for the assessment of future vascular conduits.

Vascular prostheses: performance related to cell-shear responses

BACKGROUND

This work concerned the endothelialization of vascular prostheses and subsequent improvement of functionality with respect to tissue engineering. The aim of the study was to investigate the initial, pre-shear stress cellular behavior with respect to three vascular biomaterials to explain subsequent cellular responses to physiological shear stresses.

MATERIALS AND METHODS

Expanded polytetrafluoroethylene (ePTFE), polyethyleneterephthalate (polyester; Dacron; PET), and electrostatically spun polyurethane (PU) (all pre-impregnated with collagen I/III) were cell-seeded with L929 immortalized murine fibroblasts or human umbilical vein endothelial cells (HUVECs). Cytoskeletal involvement, cell height profiles, and immunohistochemistry were examined after 7 d static culture.

RESULTS

All three vascular biomaterials demonstrated different structures. Cell behavior varied both between the materials and the two cell types: cytoskeletal involvement was greater for the HUVECs and the more fibrous surfaces; height profiles were greater for the L929 and PET, and lowest on PU. Immunohistochemistry of HUVEC samples also showed differences: PU revealed the greatest expression of intercellular adhesion molecule-1 and E-selectin (PET and ePTFE the lowest, respectively); ePTFE produced the greatest for vascular cell adhesion molecule-1 (PET the lowest).

CONCLUSIONS

Material substrate influenced the cellular response. Cells demonstrating firm adhesion increased their cytoskeletal processes and expression of cell-substratum and inter-cellular adhesion markers, which may explain their ability to adapt more readily to shear stress. The fibrous PU structure appeared to be most suited to further shear stress exposure. This study demonstrated the potential of the underlying vascular material to affect the long-term cellular functionality of the prosthesis.

Proliferation of endothelial cell on polytetrafluoroethylene vascular graft materials carried VEGF gene plasmid

OBJECTIVE

To investigate whether vascular endothelial growth factor (VEGF) gene plasmid carried by polytetrafluoroethylene (PTFE) vascular graft materials could transfect endothelial cells (ECs) and promote their growth.

METHODS

PTFE vascular graft materials carried with pCDI-hVEGF(121), pCDI or pEGFP were incubated in Tris-buffer solution and the values of optical density of 260 nm at different time were plotted, then the DNA controlled release curve was made. ECs derived from human umbilical vein were seeded on the pCDI-hVEGF(121)/pCDI/pEGFP-PTFE materials or tissue culture plates, ECs numbers were counted and VEGF protein concentrations at different time were measured by enzyme-linked immunoadsorbent assay method. Green fluorescent protein (GFP) expression in ECs on pEGFP-PTFE materials was examined with fluorescence microscopy.

RESULTS

The controlled release curve showed that the gene released from PTFE materials was rapid within 8 h, then slowed down and that the gene released continuously even after 72 h. At 24, 72 and 120 h, ECs number and proliferation rate of pCDI-hVEGF(121)-PTFE materials were higher than those of pCDI or pEGFP-PTFE materials (P<0.05). VEGF protein concentration of pCDI-hVEGF(121)-PTFE materials was higher than that of pCDI or pEGFP-PTFE materials at 6, 24, 72 and 120 h (P<0.01). GFP expression in ECs on the pEGFP-PTFE materials could be detected by fluorescence microscopy.

CONCLUSION

PTFE graft can be used as a carrier of VEGF gene plasmid, VEGF gene carried by PTFE can transfect ECs and promote ECs growth.

Influence of various cell-detachment solutions on endothelial cells after catheter abrasion for prosthesis colonization prior to implantation

We (1) evaluated the effectiveness of different media on the detachment of endothelial cells (ECs) from a catheter brush (Cragg thrombolytic catheter system) that was previously used for endothelial cell abrasion in 10 cm human umbilical vein (HUV) segments; (2) tested the practicability of endovascular catheter brush abrasion followed by EC detachment from the catheter brush, in vitro culture of harvested ECs, and finally endothelialization of a prosthesis; and (3) analyzed the defect created by the catheter brush in HUV segments after endovascular catheter abrasion. Best results in detachment of ECs from the catheter brush were obtained with a mixture of phosphate-buffered saline + 1% human albumin. EC vitality was time-dependent in the collected HUV segments postdelivery. Harvested EC viability decreased from (26.28 +/- 5.76)% (0-3 h postdelivery) to (17.29 +/- 4.56)% (after 4-8 h). ECs were easily cultured ex vivo within 2-3 weeks; seeded on nitinol stents, they grew to confluency and formed a monolayer on the stent surface (determined by scanning electron microscopy - SEM). Histological and SEM analysis of HUV segments that had undergone previous catheter brush abrasion revealed slight disruption of the intima but intact subintimal layers. Our findings indicate an advantageous method of capturing and culturing primary ECs for gene therapy or for the analysis and diagnosis of certain blood vessel diseases, especially in cases in which endovascular intervention is performed anyway. Moreover, and of high relevance to the biomaterial field, theoretically the procedure could be used to endothelialze a prosthesis ex vivo for implantation into the patient from whom the ECs were harvested, to reduce the inherent thrombogenicity of the prosthesis.

Dual role of VEGF in pretreated experimental ePTFE arterial grafts

BACKGROUND

Lack of endothelialization and abnormal smooth muscle cell (SMC) growth adversely affect the outcome of vascular synthetic grafts. The aims of our study were to investigate how a coating of extracellular matrix (ECM) and vascular endothelial growth factor (VEGF) might affect the endothelialization rate, smooth muscle cells (SMC) proliferation, and myointimal hyperplasia in experimental arterial ePTFE grafts.

METHODS

In each of 30 male Lewis rats, a 1-cm-long ePTFE graft was inserted at the level of the abdominal aorta. Animals were randomized in five groups (six animals each): groups A and A1 received ePTFE grafts coated with a synthetic extracellular matrix (growth factor-reduced matrigel) containing VEGF; groups B and B1 received ePTFE grafts coated with synthetic ECM; and group C received ePTFE grafts alone. The grafts were explanted at 30 days from surgery for immunohistochemical analysis.

RESULTS

Both endothelialization rate and myointimal hyperplasia were augmented in group A versus groups B and C, and these findings were statistically significant. SMC density resulted significantly higher in group A versus groups B and C, and this was associated with an altered expression of bFGF and TGFbeta.

CONCLUSIONS

Pretreating ePTFE grafts with synthetic ECM and VEGF results in better endothelialization, but also in undesired higher SMC density and myointimal hyperplasia.

Neointimal hyperplasia on a cell-seeded polytetrafluoroethylene graft is promoted by transfer of tissue plasminogen activator gene and inhibited by transfer of nitric oxide synthase gene

OBJECTIVE

The objective of this study was to examine the effect of tissue plasminogen activator (tPA) and endothelial nitric oxide synthase (eNOS) on thrombosis and neointimal hyperplasia on a polytetrafluoroethylene (PTFE) graft seeded with smooth muscle cells (SMCs).

METHODS

SMCs retrovirally transduced with tPA and eNOS genes were seeded on PTFE grafts and then implanted into the infrarenal rabbit aorta. Thrombosis and neointimal hyperplasia on the grafts were examined after 30 and 100 days of implantation.

RESULTS

At 30 days of implantation, thrombus was observed on the luminal surface of both unseeded and SMC seeded control grafts, whereas grafts seeded with SMCs secreting tPA were nearly free of thrombus. At 100 days, the neointima on grafts seeded with tPA transduced SMCs was significantly thicker (925 +/- 150 microm, n = 5) than neointima on the other grafts (range, 132 to 374 microm; P < .001). Neointima thickness on grafts seeded with eNOS transduced SMCs (154 +/- 27 microm) was similar to that of unseeded grafts (132 +/- 16 microm, P > .05); both were thinner than those on grafts seeded with SMCs transduced with only lacZ gene (287 +/- 35 microm). The ratio of seeded cells in the neointima was significantly higher on SMC/tPA grafts (46% +/- 8%) than SMC/NOS grafts (21% +/- 6%, P < .05), indicating tPA transduced cells proliferated more than eNOS transduced cells.

CONCLUSIONS

Engineered tPA expression in seeded SMCs causes significantly more neointimal hyperplasia, despite the favorable inhibition of luminal thrombus. eNOS expression in the seeded cells inhibits neointimal hyperplasia.

Smooth muscle cells improve endothelial cell retention on polytetrafluoroethylene grafts in vivo

OBJECTIVE

We investigated the influence of smooth muscle cells (SMC) on endothelial cell (EC) retention on polytetrafluoroethylene (PTFE) grafts and the effect of SMC seeding on intimal hyperplasia in vivo in a rabbit model.

METHODS

Fibronectin-coated PTFE grafts (4 mm diameter) were seeded with either EC alone, SMC alone, or SMC followed 24 hours later by EC. The grafts were connected to an extracorporal aortic shunt for 1 hour or were individually implanted for 1, 30, and 100 days into the infrarenal aorta as an end-to-side bypass graft. The number of retained cells was compared at 1 hour and at 1 day after implantation. Neointimal thickness was measured 30 and 100 days after implantation.

RESULTS

After 1-hour exposure to blood flow, EC retention rate was greater (P <.005) if seeded on top of SMC (98% +/- 2%; n = 8) versus being seeded alone (65 +/- 11%; n = 8). SMC retention rate was 95 +/- 5% (n = 8) when seeded alone. Similar cell retention was obtained 1 day after implantation. After 30-day implantation the neointima was thicker in grafts seeded with EC and SMC (282 +/- 136 microm; n = 3) than with EC only (52 +/- 45 microm; n = 3; P <.001). However, the neointimal thickness for dual-cell-seeded grafts (126 +/- 60 microm; n = 3) was not significantly different (P =.09) from EC-seeded grafts (79 +/- 48 microm; n = 3) after 100-day implantation.

CONCLUSION

EC retention on PTFE grafts in vivo is improved if seeded over a layer of SMC. Further studies are needed to determine whether overlying EC modulate proliferation of underlying SMC.

A biologically active VEGF construct in vitro: implications for bioengineering-improved prosthetic vascular grafts

Prosthetic arterial grafts are unable to develop an intact endothelial lining after implantation, predisposing them to fail. Strategies have been sought to enhance endothelialization using growth factors and cytokines. This study assessed the biologic activity of vascular endothelial growth factor (VEGF) covalently linked to bovine serum albumin (BSA). Native and modified VEGF were assayed for endothelial cell migration and proliferation. Migration assays were performed comparing the effects of 2% fetal bovine serum (FBS), 50 ng/mL, 100 ng/mL, and 200 ng/mL of native VEGF and VEGF-BSA. Proliferation assays were performed by using Alamar Blue comparing cellular growth in 1% FBS, 10% FBS, 100 ng/mL unbound VEGF, and 100 ng/mL VEGF-BSA. VEGF is a potent chemotactic agent for endothelial cells in both unbound and bound states. Native VEGF solutions (50 ng/mL, 100 ng/mL, and 200 ng/mL) stimulated 23.9 cells/high power field (HPF), 35.3 cells/HPF, and 49.1 cells/HPF (p < 0.005). VEGF-BSA solutions stimulated 25.9 cells/HPF, 39.1 cells/HPF, and 69.0 cells/HPF (p < 0.001). VEGF-BSA and native VEGF supported similar increased cellular proliferation compared with 1% FBS media (p < 0.002). Modified VEGF retains its chemotactic and proliferative properties in vitro. These findings suggest that bare prosthetic surfaces lined with VEGF bound to a "basecoat" albumin may support endothelial cell proliferation and migration and thereby offer new strategies to improve graft patency.

Engineering of vascular ingrowth matrices: are protein domains an alternative to peptides?

Anastomotic intimal hyperplasia and surface thrombogenicity are the main reasons for the high failure rate of prosthetic small-diameter vascular grafts. While anastomotic intimal hyperplasia is a multifactorial event, ongoing surface thrombogenicity is primarily caused by the lack of an endothelium, even after years of clinical implantation. After decades of poorly performing synthetic artery-grafts, tissue engineering has emerged as a promising approach to generate biologically functional bio-synthetic hybrid grafts mimicking native arteries regarding the presence of an endothelial lining on the blood surface. "In vitro endothelialization" represented the first generation of such tissue-engineered vascular grafts, utilising cell culture techniques for the creation of a confluent autologous endothelium on ePTFE grafts. The clinical long-term results with this method in almost 200 patients are highly encouraging, showing patencies equal to vein grafts. Since "in vitro endothelialization" requires cell culture facilities, it will always be confined to large centres. Therefore, research of the 1990s turned to the development of spontaneously endothelializing implants, to make tissue-engineered grafts amenable to the entire vascular-surgical community. Apart from scaffold designs allowing transmural ingrowth, biological signalling through a facilitating ingrowth matrix holds a key to spontaneous endothelialization. In biological signalling, the increasingly deeper understanding of bio-active molecules and the discovery of domains and peptide sequences during the 1980s created the expectation in the 1990s that peptide signalling may be all that is needed. This present review highlights the possible problems associated with such a reductionist approach. Using the fibronectin molecule, we demonstrated that domains may be more suitable modules in tissue engineering than peptide sequences.

In vivo retention of endothelial cells adenovirally transduced with tissue-type plasminogen activator and seeded onto expanded polytetrafluoroethylene

PURPOSE

Seeding a prosthetic graft with genetically engineered vascular endothelial cells (ECs) has the potential to enhance the graft's antithrombotic properties. However, it has been reported that ECs transduced with tissue-type plasminogen activator (tPA) have very low levels of retention on grafts, probably because of increased proteolytic activity. We examined the retention of human tPA (htPA)-transduced ECs after the cells were seeded onto expanded polytetrafluoroethylene (ePTFE) grafts and implanted into dogs. We also examined the function of secreted htPA in this model.

METHODS AND RESULTS

Canine jugular venous ECs were transduced with adenoviral vectors encoding htPA (Adex1CAhtPA) and beta-galactosidase (Adex1CALacZ). There was a positive relationship between the percentage of X-gal ECs staining and the multiplicity of infection (MOI) of Adex1CALacZ. The level of htPA production in vitro increased with the increasing MOI of Adex1CAhtPA, but decreased gradually 4 days after infection. ECs coinfected with Adex1CAhtPA and Adex1CALacZ (htPAEC) or ECs infected with Adex1CALacZ alone (LacZEC) were seeded onto ePTFE grafts at densities equivalent to confluence to visualize retained ECs in an in vivo flow study. The grafts were implanted into canine carotid arteries and harvested after 5 hours of exposure to blood flow. The harvested grafts showed patchy defects in ECs, most of which were covered with mural thrombi. There was no significant difference in retention between htPAEC (29.3% +/- 8.7%) and LacZEC (19.5% +/- 3. 6%). There was a significant negative correlation between the in vivo EC retention on the grafts and the in vitro cellular passage level of ECs (P =.041; r = -.40). htPAEC produced 210.3 +/- 22.2 ng htPA antigen/10(6) cells per 6 hours in vitro and continued to secrete htPA on the harvested graft.

CONCLUSIONS

We demonstrated that a large amount of functional htPA was produced by adenovirally modified canine ECs. The results of the in vivo study may suggest that overexpression of tPA has little effect on the short-term retention of early passage ECs seeded onto ePTFE grafts.

Protein-like molecular architecture: biomaterial applications for inducing cellular receptor binding and signal transduction

The development of biomaterials with desirable biocompatibility has presented a difficult challenge for tissue engineering researchers. First and foremost, materials themselves tend to be hydrophobic and/or thrombogenic in nature, and face compatibility problems upon implantation. To mediate this problem, researchers have attempted to graft protein fragments onto biomaterial surfaces to promote endothelial cell attachment and minimize thrombosis. We envisioned a novel approach, based on the capability of biomolecules to self-assemble into well-defined and intricate structures, for creating biomimetic biomaterials that promote cell adhesion and proliferation. One of the most intriguing self-assembly processes is the folding of peptide chains into native protein structures. We have developed a method for building protein-like structural motifs that incorporate sequences of biological interest. A lipophilic moiety is attached onto a N alpha-amino group of peptide chain, resulting in a "peptide-amphiphile." The alignment of amphiphilic compounds at the lipid-solvent interface is used to facilitate peptide alignment and structure initiation and propagation, while the lipophilic region absorbs to hydrophobic surfaces. Peptide-amphiphiles containing potentially triple-helical or alpha-helical structural motifs have been synthesized. The resultant head group structures have been characterized by CD spectroscopy and found to be thermally stable over physiological temperature ranges. Triple-helical peptide-amphiphiles have been applied to studies of surface modification and cell receptor binding. Cell adhesion and spreading was promoted by triple-helical peptide-amphiphiles. Cellular interaction with the type IV collagen sequence alpha 1(IV) 1263-1277 increased signal transduction, with both the time and level of induction dependent upon triple-helical conformation. Collectively, these results suggest that peptide-amphiphiles may be used to form stable molecular structure on biomaterial surfaces that promote cellular activities and improve biocompatibility.

Impregnated polyester arterial prostheses: performance and prospects

Impregnated polyester arterial prostheses have gained wide acceptance by most vascular surgery teams, probably because these prostheses are easy to use, without any preclotting. We offer here a synthesis of the main studies that have appraised the experimental and clinical performance of these prostheses, and we delineate their major prospects.

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.

In vitro endothelialisation of arteriovenous loop grafts for haemodialysis

OBJECTIVES

To evaluate the feasibility in a pilot study of in vitro endothelialisation of PTFE grafts used as interposition arteriovenous fistulas in uraemic patients.

METHODS

Autologous saphenous vein endothelial cells were harvested and cultured on PTFE grafts in seven patients undergoing maintenance haemodialysis. The patients had several previous failures of vascular access sites. The patients were followed with duplex ultrasound, clinical examination and in one case an explanted graft was examined.

RESULTS

At the end of follow-up four of the seven patients had patent grafts. One patient occluded the graft immediately postoperatively and another after 3.5 months. The former patient received a second endothelialised graft. In two further patients revision of the outflow was performed. In two patients a functioning graft was excised, in one case because of bleeding of a venous aneurysm and in one case because of suspected infection. The former which was excised 5 weeks postoperatively revealed that 85% of the surface was covered by endothelial cells.

CONCLUSIONS

This pilot study shows that in vitro endothelialisation of PTFE grafts used for haemodialysis is possible in uraemic patients. In this highly problematic patient group the results are promising with endothelial cell coverage after 5 weeks of implantation.

Cell adhesion and short-term patency in human endothelium preseeded 1.5-mm polytetrafluoroethylene vascular grafts: an experimental study

It has been shown that endothelialization improves short-term patency of 1.5-mm expanded polytetrafluoroethylene vascular grafts. A model for endothelialization of 1.5-mm expanded polytetrafluoroethylene vascular grafts with human endothelial cells is described. In this model, the adherence of endothelial cells was increased significantly in grafts coated with serum proteins and collagen. By means of this protocol, graft patency was tested after implantation in two animal models: the rat aorta and the rabbit common carotid artery. Anastomosis was performed with a 3M Precise Microvascular Anastomotic System. In both animal models, no significant loss of endothelial cells in the precoated grafts (rat, n = 8) were noted 1 hour after blood flow restoration. All uncoated grafts showed significant endothelial cell loss. In the rabbit model, all nonendothelialized grafts (n = 8) clotted 5 to 25 minutes after flow restoration. Seven (n = 8) endothelialized grafts showed no clotting during 1 hour's observation: one clotted immediately for a patency rate of 87.5 percent. These results indicate that endothelialization of 1.5-mm grafts is practical. Furthermore, adhesion of endothelial cells to the graft walls is not affected by short-term, pulsatile, high-pressure blood flow.

Photoreactive analog of peptide FN-C/H-V from the carboxy-terminal heparin-binding domains of fibronectin supports endothelial cell adhesion and spreading on biomaterial surfaces

The extracellular matrix protein fibronectin (FN) plays an important role in cell adhesion, spreading, and motility. Several cell-adhesion promoting domains exist within fibronectin, and peptide sequences from these domains have been shown to play an important role in cell interactions with fibronectin. Recently, a peptide sequence (FN-C/H-V) from the 33/66 kD carboxy-terminal heparin-binding domains of fibronectin was shown to promote the adhesion and spreading of vascular endothelial cells in vitro. Endothelial cell spreading on this peptide was followed by cytoskeletal reorganization, focal contact formation, and, ultimately, cell migration. In the current study, a photoreactive analog of FN-C/H-V (ASD-V) was generated using a heterobifunctional photoreactive crosslinking agent, sulfosuccinimidyl 2-(pazidosalicylamido) ethyl-1,3'-dithio-propionate. ASD-V was then covalently coupled to polystyrene (PS) and polyethylene terephthalate film (PET) in order to assess the utility of ASD-V for preparing biomaterial surfaces with endothelial cell-adhesion promoting properties. The effects of pre-adsorption time and initial coating concentration on the efficiency of ASD-V coupling to PS and to PET were examined. Contact angle measurements and atomic force microscopy were used to characterize ASD-V-modified surfaces. Finally, the adhesion and spreading of vascular endothelial cells on ASD-V-modified surfaces was assessed. Our results suggest that photoreactive peptides are an effective and convenient means of modifying biomaterial surfaces to impart adhesion-promoting properties and that ASD-V, when coupled to PS and PET, promotes endothelial cell adhesion and spreading and may therefore be useful as a biomaterial surface modification in applications where re-endothelialization is desired (e.g., autologous endothelial seeding of vascular grafts, or transplantation of genetically engineered endothelial cells via polymer-coated stents.

A hybrid vascular model biomimicking the hierarchic structure of arterial wall: neointimal stability and neoarterial regeneration process under arterial circulation

A layered-structure hybrid vascular graft, mimicking the hierarchic structure of the intima and media of a natural artery, is expected to exhibit antithrombogenicity and to accelerate neoarterial tissue formation. Two models of hybrid vascular grafts were prepared on knitted Dacron fabric grafts (inner diameter 4 mm, length 6 cm). Model I grafts consisted of an endothelial cell monolayer formed on collagenous matrix, and model II grafts consisted of an endothelial cell monolayer that formed on hybrid collagenous medial tissue in which smooth muscle cells were incorporated. Both models (n = 17 for each model) were implanted bilaterally in carotid arteries and left in place for up to 26 weeks. Although all of the implanted grafts were patent, the two models significantly differed in the degree of maturity of the regenerated neoarterial wall especially at earlier implantation periods. At 2 weeks, model II grafts showed a much higher degree of neointimal integrity than model I grafts: a smooth and organized neointimal layer was formed, which was free from leukocyte adhesion. On further increase of implantation time, the formation of neomedia (subendothelial smooth muscle cell layers) and circumferential orientation of both smooth muscle cells and collagenous extracellular matrix were much more advanced in model II grafts than in model I grafts. At 26 weeks after implantation, layered elastic laminae regenerated along circumferentially oriented smooth muscle cells in neomedia were observed only in model II grafts. Irrespective of model, little excessive smooth muscle cell proliferation occurred. A hierarchically structured hybrid graft eventually provided a more integrated neointimal layer and accelerated neoarterial tissue formation much more than model I grafts. The significance of the incorporation of smooth muscle cells into hybrid grafts is discussed.

Shear stress-conditioned, endothelial cell-seeded vascular grafts: improved cell adherence in response to in vitro shear stress

BACKGROUND

Prosthetic vascular grafts with adherent endothelial cell monolayers may prove useful for small-caliber vessel bypass. However, endothelial cells adhere poorly to prosthetic graft material, and they are stripped when exposed to in vivo shear stress. This study sought to determine whether in vitro shear stress conditioning improves endothelial cell adhesion and decreases thrombogenicity of endothelial cell-seeded grafts.

METHODS

The lumens of 1.5 mm (inside diameter) spun polyurethane polymer vascular grafts were seeded with bovine aortic endothelial cells and cultured in vitro for 6 days with or without continuous laminar shear stress, first at 1 to 2 dynes/cm2 for 3 days, then at approximately 25 dynes/cm2 for 3 days. Grafts preconditioned by shear stress and the static control grafts were then exposed to arterial shear stress at 25 dynes/cm2 for 25 seconds. The number of dislodged cells was counted, and the grafts were examined by light and scanning electron microscopy. Whole blood clotting time in the grafts was also determined.

RESULTS

Exposure of grafts to acute shear stress dislodged 1.35 x 10(6) +/- 0.44 x 10(6) cells from static grafts compared with 1.05 x 10(4) +/- 0.16 x 10(4) cells from grafts preconditioned by shear stress. By light and electron microscopy an intact endothelial monolayer was observed to cover the lumen of shear stress-conditioned grafts, whereas few cells remained on the luminal surface of grafts not previously exposed to shear stress. The clotting time in shear stress-conditioned grafts was significantly prolonged in relation to grafts not exposed to shear stress.

CONCLUSIONS

These findings show that endothelial cell adhesion and retention on vascular grafts in vitro is markedly enhanced by preconditioning the seeded endothelial cell monolayer with long-term shear stress. Consequently, vascular grafts containing shear stress-conditioned endothelial monolayers are less thrombogenic in vitro than small-caliber vascular grafts without intact endothelial cell monolayers.

Endothelial seeding of compliant polyurethane vascular graft material

Compliance was measured in a new compliant polyurethane vascular graft material and in polytetrafluoroethylene (PTFE) graft material using an ultrasonic device; attachment of indium-111 oxine-labelled human endothelial cells to both surfaces with a range of surface coatings was assessed. Compliant polyurethane was six to eight times more compliant than PTFE (P < 0.01) at all pressures in the range 50-120 mmHg, and endothelial cell attachment to uncoated polyurethane was three times better than to uncoated PTFE at times up to 90 min (P < 0.01). Attachment to polyurethane was also better after blood clot, collagen and fibronectin treatment at times up to 30 min (P < 0.05). Endothelial seeding of compliant graft material may provide a prosthetic vascular substitute with characteristics similar to those of autologous vein.

An integrated approach to the design and engineering of hybrid arterial prostheses

PURPOSE

A newly devised hybrid small-caliber graft was developed. The graft consisted of three components: a microporous polyurethane graft (inside diameter 3 mm; length 5 cm) with compliance close to that of a natural artery; an artificial basement membrane composed of a complex gel of type I collagen and dermatan sulfate, which showed enhanced adhesion and growth of endothelial cells (ECs) and reduced adhesion of platelets in vitro; and an autogenous EC monolayer with high degrees of cell-substrate and cell-cell interactions, which was performed before implantation.

METHODS

Twenty EC-seeded grafts were implanted bilaterally into carotid arteries of dogs without anticoagulant. The implantation period was up to 26 weeks.

RESULTS

The overall patency rate for seeded grafts was 75%. The percentage of endothelial coverage of seeded grafts was 98% as implanted, 92% at 2 weeks, and 100% after 12 weeks. The mean intimal thickness of grafts was around 80 microns at 12 weeks. Little additional increase was observed at 26 weeks.

CONCLUSIONS

It appears that the complete endothelialization as implanted, high cell-to-substrate adhesive strength that resists hydrodynamic shear stress, and biomechanical compatibility of the polyurethane graft functioned cooperatively to provide a vascular graft with high antithrombogenicity and minimal hyperplasia. The integrated approach of combining biomechanical and cellular engineering designs leading to an important functional smaller-caliber graft is discussed.

Clinical in vitro endothelialization of femoropopliteal bypass grafts: an actuarial follow-up over three years

PURPOSE

The creation of an endothelial coverage on prosthetic vascular surfaces may improve the performance of synthetic small diameter vascular grafts. In vitro lining with cultured autologous endothelial cells offers a confluent endothelium at the time of implantation.

METHODS

Between June 1989 and December 1991, 49 patients who had no saphenous vein available entered the study. Indication for operation was disabling claudication in 37 patients and critical ischemia in 12 patients. With a random 1:2 assignment, 33 patients were admitted to the endothelialized group and 16 control patients received an untreated polytetrafluoroethylene prosthesis. Cultured autologous endothelial cells from the external jugular vein were confluently lined onto polytetrafluoroethylene grafts precoated with fibrinolytically inhibited fibrin glue. The follow-up was based on angiography, platelet labeling studies with indium 111-labeled oxine, assessment of the ankle-brachial index, and duplex sonography.

RESULTS

First-passage mass cultures of 16 million endothelial cells-required for the confluent lining of a 70 cm long 6 mm graft-were reached 25.1 +/- 11.2 days after vein excision. Growth failure occurred in 27.3%. After 32 months, the actuarial patency was 84.7% for endothelialized grafts and 55.4% for control grafts (p < 0.041 by Breslow test; p < 0.068 by Mantel-Cox test). The ankle-brachial index was continually diverging, reaching significantly lower values in the control group at 24 months (0.98 +/- 0.14 in the endothelialized group versus 0.70 +/- 0.12 in the control; p < 0.0023). The uptake of indium 111-labeled platelets--measured at 9 days, 3 months, 6 months, and 12 months--was significantly lower in the endothelialized group during the entire observation period.

In vitro endothelialization of a mesosystemic shunt: a clinical case report

The existence of a confluently covering endothelium that is free of any thrombotic appositions can be proved 30 days after clinical implantation of an in vitro endothelialized expanded polytetrafluoroethylene graft. The recipient of the mesosystemic H-graft was a 69-year-old man who had a thrombosed portal vein following pancreatitis. Autologous endothelial cells were obtained from the external jugular vein under local anesthesia, applying the in situ cannulation technique. After low-density plating, first-passage mass cultures of 1.22 x 10(6) endothelial cells were obtained 14 days after vein excision. After precoating was accomplished with fibrinolytically inhibited fibrin glue, a 10 mm expanded polytetrafluoroethylene graft was confluently lined with the autologous endothelial cells at a seeding density of 1.2 x 10(5) cells/cm2. After a maturation period of an additional 9 days and the microbiologic exclusion of a possible infection, an 11 cm graft segment was implanted between the superior mesenteric vein and the inferior vena cava. In spite of a patent shunt the patient had a repeat bleeding episode, needed parenteral nutrition, and died of sepsis on day 30. Immediately after the graft had been taken out, specimens were processed by scanning electron microscopy and light microscopy for the immunohistochemical proof of the endothelial nature of the surface-covering cell layer. The entire graft surface displayed a confluent cell lining that was free of any thrombotic appositions. A strongly positive stain result for both factor VIII-related antigen and the fixation-resistant CD34 molecule identified these cells as endothelial. No alpha-actin-positive cells could be detected. The underlying protein matrix was well preserved and unaltered in thickness and appearance, compared with preimplantation samples. None of the specimens showed any evidence of infection. This human demonstration of an intact endothelium on a patent venous prosthesis further establishes in vitro lining as a method that actually creates a persistent and functioning endothelium on a synthetic graft surface.

A new and simple technique to achieve a confluent and flow resistant endothelium on vascular ePTFE-grafts using human serum

Confluent endothelialisation of vascular grafts in vitro before implantation has been suggested as a way to improve patency. With the aim of creating a confluent endothelium or expanded polytetrafluoroethylene (ePTFE) vascular grafts, using a minimum of allo- or xenogenic material, we have investigated the use of human serum for endothelial cell (EC) growth and adhesion to the grafts. Loss of fibronectin from the serum was measured before and after coating. After serum- or collagen I-precoating, the grafts were seeded with cultured adult human endothelial cells from the great saphenous vein and then kept under rotational culture conditions for 2 days. To evaluate the endothelial resistance to detachment the grafts were mounted in parallel to a heart-lung machine, delivering a pulsatile flow of human blood for 1 hour. Evaluation was performed using scanning electron microscopy (SEM). By simply pressing serum through the graft wall, fibronectin corresponding to 19 micrograms/cm2 graft surface was consumed. It was possible to achieve a confluent endothelium on both serum- and collagen I-precoated grafts (n = 8). After being subjected to blood flow, SEM revealed a complete endothelial lining of the grafts (n = 12) except for two serum-precoated grafts which showed < or = 10% and between 10 and 20% denuded areas, and one collagen-coated graft that showed < or = 10% of denuded area. This method provides a way to achieve a confluent and flow resistant endothelial lining on ePTFE vascular grafts.

Endothelial seeding of vascular prostheses: a technique of in situ enzymatic retrieval of endothelial cells without vein sacrifice

The success of endothelial seeding of vascular prosthetic grafts is largely dependent on endothelial cell retrieval. An in situ cell harvesting technique using enzymatic dissolution in a temporarily excluded segment of the internal jugular vein is described. With this technique the cellular yield is adequate and the internal jugular vein remains patent. This technique allows for high-density seeding in a single stage. In two-stage seeding the excellent yield of cells reduces the duration of in vitro endothelial cell culture and allows for secondary massive and early seeding.

Enhanced patency of venous Dacron grafts by endothelial cell sodding

The effect of microvessel endothelial cell (EC) sodding on the patency of Dacron grafts interposed in canine inferior vena cava was studied. EC were harvested enzymatically from canine omentum and isolated by density gradient centrifugation. Preclotted, knitted Dacron grafts were sodded with > 10(6) EC/cm2 surface. The results demonstrate significant improvement in patency of sodded grafts placed in the inferior vena cava as compared with control grafts (p < 0.001 in grafts with a distal arteriovenous fistula and p < 0.05 in grafts without a distal fistula). The neointima of the sodded grafts were thinner and scanning electron microscopy confirmed the presence of a confluent layer of EC. In addition, the production of prostacyclin but not thromboxane A2 was significantly enhanced in the sodded grafts as compared with controls. We conclude that microvessel EC sodding of Dacron grafts significantly improves the patency rate and inhibits neointimal thickening of the prosthesis. The mechanism is unknown but may involve a more rapid endothelialization of the graft surface with the potential of producing more prostacyclin and less thromboxane A2.

A new device for endothelial cell seeding of a small-caliber vascular prosthesis

A device permitting homogeneous endothelial cell seeding of a small-caliber arterial prosthesis has been developed. The prosthesis is maintained firmly attached to a rotative scaffolding device. This device is activated by an electrical motor at constant and adjustable speed. The whole system is maintained at 37 degrees C in a cell culture incubator. The 4 mm internal diameter polytetrafluoroethylene (PTFE) prosthesis was coated with biological glue and seeded with human saphenous vein endothelial cells obtained by mechanical detachment. Cell seeding density was 2.10(4) cells/cm2 (Group A, n = 6) or 10(5) cells/cm2 (Group B, n = 6). Rotation speed was 8 revolutions per hour (rph) during 90 min. Analysis of the homogeneity of cell seeding was permitted by cell counts on five different segments of the prosthesis. Each longitudinal segment was analyzed at three different subsegments of the circumference. The average adhesion was 43 +/- 4% in Group A and 38% +/- in Group B of seeded cells. No difference could be observed between the different segments and subsegments. In the two groups, cells were spread, and in Group B, a complete endothelial cell layer was obtained on the graft surface. This study permits validation of the device to allow homogeneous cell seeding in an arterial prosthesis.

Prostacyclin production from seeded prosthetic vascular grafts

Endothelial cell seeding has been proposed as a method of improving patency rates in small-calibre prosthetic vascular grafts. In vivo, endothelial cells normally produce prostacyclin (PGI2), a potent antiplatelet agent. The aim of this study was to determine whether seeded grafts show significant PGI2 production after in vivo implantation. Grafts were seeded with either autologous canine venous endothelial cells or autologous microvascular endothelial cells. After 12 weeks, PGI2 production was assessed under basal and stimulated conditions. Seeded grafts were compared with non-seeded controls and the corresponding aorta. The overall patency rate in seeded grafts was 80 per cent compared with 10 per cent in non-seeded grafts (P < 0.01). Grafts seeded with cells from either source produced significantly more PGI2 than unseeded grafts in both basal and stimulated states (P < 0.05). The aorta produced significantly more PGI2 than seeded grafts under both conditions (P < 0.01). Endothelial cell seeding produces a functional graft and leads to an improved patency rate.

An artificial blood vessel with an endothelial-cell monolayer

An artificial blood vessel with an endothelial-cell monolayer was used as an arterial substitute in rats. Endothelial cells were isolated from the aorta of a Wistar rat by the digestion method. The cell identification was established by the cobblestone appearance of a confluent cell monolayer, by an expression of factor VIII-related antigen, and by the presence of Weibel-Palade bodies. The luminal surface of the thin-walled polytetrafluoroethylene (PTFE) graft (4 mm in diameter and 10 mm in length) was coated with an endothelial-cell monolayer for 7 days in vitro. An interpositional graft was placed using the endothelial cell-coated PTFE prosthesis on the right common carotid artery in seven rats. A total of 10 rats received an interpositional graft with the noncoated PTFE prosthesis as a control. The patency rate at 1 month after implantation was significantly higher in the coated group than in the control group. The vascular prosthesis with an endothelial-cell monolayer is a promising technique to inhibit the development of thrombosis.

Endothelial cell seeding: a review

The concept of endothelial cell seeding, designed to provide vascular grafts with a nonthrombogenic lining, has progressed from crude animal experiments during the past two decades to detailed in vitro functional studies using human cells. Although favorable results have been obtained in animal studies this has yet to be translated to humans, where current application of these techniques has been limited to a very few clinical trials. The history, current status and future directions are reviewed herein.

Endothelial cell seeding after carotid endarterectomy in a canine model reduces platelet uptake

Post-endarterectomy platelet deposition may play an important role not only in vessel wall healing, but also in the development of progressive stenosis. Using a canine model, we investigated the effect of endothelial cell seeding on platelet deposition on endarterectomised arteries. Thirteen dogs underwent bilateral carotid intimectomy (5 cm long) and one side was seeded with an average of 2 x 10(6) viable freshly harvested endothelial cells. Blood flow was restored 20 min after seeding. On the contralateral side, a sham-seeding was performed. Deposition of 111indium-labelled autologous platelets was studied with sequential gamma camera images 3-5 h, 1, 2, 3, 4 days and 4 weeks after surgery. Platelet uptake was statistically reduced on the seeded side. Animals were killed at 4 weeks (nine dogs) and 5 weeks (four dogs) after surgery. Seven arteries for each group were found to be occluded. We conclude that endothelial cell seeding on endarterectomised arteries is feasible and reduces platelet uptake. Improvement in the efficiency of seeding and reduction of endothelial cell loss might permit clinical application of this technique.

Effect of glow discharge surface modification of plasma TFE vascular graft material on fibronectin and laminin retention and endothelial cell adhesion

The effects of glow discharge surface modification of plasma TFE vascular graft material (plasma TFE, Atrium Medical Corp., Hollis, NH) on the binding and retention of fibronectin and laminin as well as endothelial adherence were examined. The binding of both fibronectin and laminin to plasma TFE increased as a function of protein concentration in the range of 0.02 to 200 micrograms/ml. Binding to plasma TFE was not saturable in this range of protein concentrations. Fibronectin and laminin binding to plasma TFE was time-dependent, reaching a maximal level (1.8 and 3.2 micrograms/cm2 of bound fibronectin and laminin, respectively) after 30 min (fibronectin) and 4 hr (laminin). Binding was not the result of an accumulation of unbound protein in graft interstices since graft prewetting did not alter the amount of either fibronectin or laminin associated with plasma TFE. In addition, binding was not the result of protein modification during radiolabeling, since unlabeled fibronectin and laminin could compete effectively with their iodinated counterparts for binding to plasma TFE. Dissociation of bound fibronectin and laminin from plasma TFE was rapid, and less than 12% remained bound 60 min after washing. Plasma TFE subjected to glow discharge in O2 and carrying negatively charged functional groups on graft surfaces, exhibited a fivefold increase in fibronectin and laminin binding. In addition, more than 85% of the bound fibronectin and laminin was retained 24 hr after washing. The enhanced retention of fibronectin or laminin by these grafts was associated with an increase in endothelial cell adhesion.(ABSTRACT TRUNCATED AT 250 WORDS)

Optimal seeding conditions for human endothelial cells

An in vitro model of endothelial cell seeding has been developed to individually evaluate the steps required for seeding arterial prostheses. Human saphenous vein endothelial cells are radiolabeled with tritiated thymidine and seeded onto 4 mm polytetrafluoroethylene grafts. Grafts are then placed into a perfusion circuit for determination of cellular retention. Using this model, the following variables were studied: (1) graft coating (fibronectin versus serum versus plasma); (2) time of incubation of cells with graft (0, 20, 90 minutes); (3) density of the initial seeding solution (4 x 10(3)-6 x 10(5) cells/cm2). The data suggest that incubation of a graft with plasma provides an adhesive surface that is as effective as fibronectin for enhancing cell retention. With this particular model, seeding densities between 1 and 2 x 10(5) cells/cm2 produce a confluent monolayer with optimal utilization of cells. A shorter 20 minute incubation period resulted in the retention of only half of the seeded cells, while postperfusion attachment increased significantly with a 90 minute incubation period. Data derived from this system can be used to construct a protocol that may be useful for clinical in vivo seeding trials.