Endothelial cell seeding: a review

Microenvironment influences vascular differentiation of murine cardiovascular progenitor cells

We examined the effects of the microenvironment on vascular differentiation of murine cardiovascular progenitor cells (CPCs). We isolated CPCs and seeded them in culture exposed to the various extracellular matrix (ECM) proteins in both two-dimensional (2D) and 3D culture systems. To better understand the contribution of the microenvironment to vascular differentiation, we analyzed endothelial and smooth muscle cell differentiation at both day 7 and day 14. We found that laminin and vitronectin enhanced vascular endothelial cell differentiation while fibronectin enhanced vascular smooth muscle cell differentiation. We also observed that the effects of the 3D electrospun scaffolds were delayed and not noticeable until the later time point (day 14), which may be due to the amount of time necessary for the cells to migrate to the interior of the scaffold. The study characterized the contributions of both ECM proteins and the addition of a 3D culture system to continued vascular differentiation. Additionally, we demonstrated the capability bioengineer a CPC-derived vascular graft.

Improved endothelialization and reduced thrombosis by coating a synthetic vascular graft with fibronectin and stem cell homing factor SDF-1α

Failure of synthetic small-diameter vascular grafts is determined mainly by the lack of endothelial cells, as these cells inhibit thrombosis and intimal hyperplasia. Coating of graft material with homing factors for circulating stem cells has the potential to improve endogenous endothelialization of these grafts and to reduce graft failure. Synthetic knitted polyester grafts (6mm diameter) were coated with FN and SDF-1α before surgical interposition in the carotid artery of sheep. Similar uncoated vascular grafts were implanted in the contralateral side as internal controls. To study the early attraction of stem cells, grafts were implanted in a first series of nine sheep and explanted after 1 or 3 days. In coated grafts, four times higher fractions of CD34(+) and three to four times higher fractions of CD117(+) cells adhering to the vessel walls were found than in control grafts (P<0.05). When such coated and non-coated grafts were implanted in 12 other sheep and explanted after 3 months, all coated grafts were patent, while one control graft was occluded. EcNOS staining revealed that FN-SDF-1α coating significantly increased coverage with endothelial cells from 27 ± 4% of the graft to 48 ± 4% compared with the controls (P=0.001). This was associated with a significant reduction of intimal hyperplasia (average thickness 1.03 ± 0.09 mm in controls vs. 0.69 ± 0.04 mm in coated grafts; P=0.009) and significantly less adhesion of thrombotic material in the middle part of the graft (P=0.029). FN-SDF-1α coating of synthetic small-caliber vascular grafts stimulated the attraction of stem cells and was associated with improved endothelialization and reduced intimal hyperplasia and thrombosis.

Endothelialization and flow conditioning of fibrin-based media-equivalents

It is generally accepted that endothelialization and subsequent development of a functional endothelium are of paramount importance to the success of any bioartificial artery. In this study, we aimed to assess the ability of smooth muscle cell-remodeled, fibrin-based media-equivalents (MEs) to be endothelialized, examine the morphological changes of endothelial cells (ECs) associated with exposure to physiologically-relevant shear stress in a custom-built bioreactor, and determine if adherent ECs are capable of withstanding average physiological shear stresses. It was found that MEs could be readily endothelialized with surface coverages of 98.8 +/- 0.9% after two days, and the ECs expressed von Willebrand factor. Furthermore, EC retention remained high (steady: 96.5 +/- 4.4%, pulsatile: 94.3 +/- 4.3%) under exposure to physiologically relevant shear stresses for 48 h. The results indicate that these MEs are conducive to generating an EC monolayer, with the ECs possessing adhesion strength sufficient to withstand physiological shear stress and maintain a normal phenotype.

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

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

Physical properties of artificial extracellular matrix protein films prepared by isocyanate crosslinking

Artificial extracellular matrix proteins, genetically engineered from elastin- and fibronectin-derived repeating units, were crosslinked with hexamethylene diisocyanate in dimethylsulfoxide. The resulting hydrogel films were transparent, uniform, and highly extensible. Their tensile moduli depended on crosslinker concentration and spanned the range characteristic of native elastin. The water content of the films was low ( approximately 27%), but the temperature-dependent swelling behavior of the crosslinked materials was reminiscent of the lower critical solution temperature property of the soluble polymers.

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.

Performance of a polyurethane vascular prosthesis carrying a dipyridamole (Persantin) coating on its lumenal surface

A porous polyurethane vascular prosthesis with an internal diameter of 5 mm was studied. The graft carries a coating of immobilized dipyridamole (Persantin(R)) on the surface of its lumen. Dipyridamole is a potent nontoxic inhibitor of platelet activation/aggregation, and also a strong inhibitor of vascular smooth muscle cell proliferation. The polyurethane material is also known as Chronoflex(R), and already finds use as a vascular access graft. The coated vascular graft was studied in vitro (hemocompatibility, interaction with blood platelets and cultured endothelial cells), as well as in two established in vivo models. In the first in vivo study, coated grafts were implanted in goats, as a bypass of the carotid artery (four animals, eight grafts, length of the graft was approximately 12 cm). Four uncoated grafts were used as controls in otherwise identical experiments. In the second in vivo experiment, eight sheep were used. Each animal received one coated and one uncoated prosthesis as an interposition graft in the carotid artery (length of the graft was 4 cm). The in vitro experiments revealed that the dipyridamole coating has three beneficial effects: reduced thrombogenicity, reduced adherence of blood platelets, and accommodation of a confluent monolayer of endothelial cells. The goat experiments showed patency of the coated grafts in three of the eight cases. The sheep experiments were not useful for the evaluation of the dipyridamole coating because deterioration of the polyurethane material was observed. The in vivo results indicate that the dipyridamole coating may positively influence the patency rate, probably because the coating promotes the growth of an endothelial cell lining. The sheep data show, however, that the limited stability of the Chronoflex(R) material precludes its issue for the construction of permanent small-bore vascular grafts.

Clinical experience with autologous endothelial cell-seeded polytetrafluoroethylene coronary artery bypass grafts

OBJECTIVE

Autologous endothelial cell seeding was used to improve the patency of 4-mm polytetrafluoroethylene vascular prostheses.

METHODS

Since 1995, 14 patients with coronary artery disease received 21 autologous endothelial cell-seeded polytetrafluoroethylene vascular bypass grafts for coronary artery revascularization. The polytetrafluoroethylene grafts were seeded with the endothelial cells in a multiple step procedure, including cell culture techniques before coronary bypass operation. With the use of extracorporal circulation and cardioplegic arrest, a bypass operation was performed by means of conventional surgical techniques.

RESULTS

After a mean postoperative follow-up of 27.7 months (range, 7.5-48 months), the graft patency rate is 90.5%. Follow-up angiograms of the aorta-coronary polytetrafluoroethylene bypass grafts showed patent bypasses in all cases except two. Angiograms of all 19 patent endothelial cell-seeded polytetrafluoroethylene bypass grafts showed a smooth luminal borderline without stenotic regions. The percutaneous transluminal angioscopic evaluation showed a glossy white and smooth endoluminal graft surface without any fibrin, platelet, or erythrocyte deposits. Intravascular ultrasonographic examinations confirmed the results.

CONCLUSION

Patency of autologous endothelial cell-seeded 4-mm polytetrafluoroethylene vascular prostheses as coronary artery bypass grafts was much better than that of unseeded polytetrafluoroethylene grafts. Further evaluations and a larger population of patients will prove whether the encouraging patency will last.

Accelerated healing of Dacron grafts seeded by preclotting with autologous bone marrow blood

> Studies have suggested that bone marrow-derived cells in the circulation may have the capacity and potential to endothelialize and heal vascular graft surfaces. We have investigated whether accelerated endothelialization could be achieved for Dacron grafts seeded by preclotting with bone marrow blood (BMB). Five 8 mm x 6 cm Dacron grafts seeded and preclotted with BMB and four controls preclotted with peripheral blood were implanted in the descending thoracic aorta (DTA) of mongrel dogs for 2 and 4 weeks. Two additional BMB DTA grafts were studied for 3 months. Five pairs of BMB and control grafts (4 mm x 6 cm) were bilaterally implanted into the carotids of dogs for 1 week and five pairs for 4 weeks. All grafts remained patent. BMB seeding/preclotting was a simple, effective method to accelerate early graft endothelialization without increasing thrombogenicity. Further studies are needed before clinical application can be recommended.

In vivo experiments with mesothelial cell seeded ePTFE vascular grafts

OBJECTIVES

To investigate the influence of mesothelial cell (MC) seeding on patency and neointimal formation of small diameter ePTFE grafts in a canine model.

MATERIALS AND METHODS

MC were isolated from the omentum, cultured, seeded on fibronectin-coated ePTFE grafts (4 cm, 4 mm ID), and implanted in the carotid artery of five Beagle dogs. Each dog also received a non-seeded control graft. Patency was assessed by palpation immediately after implantation, and non-invasively by magnetic resonance angiography (MRA) after 1 week and just prior to sacrifice (4 weeks). Intimal thickness was quantified on histological sections by use of computer-aided morphometry.

RESULTS

All grafts were patent after implantation. After 1 week, MRA showed the loss of lumen diameter in two seeded grafts. After 4 weeks, two seeded grafts were occluded, one seeded graft was severely stenosed, and all others were without angiographic lumen reduction. Histology and morphometry confirmed that two seeded grafts were occluded, and demonstrated that the other three seeded grafts showed significantly more intima formation (0.22-1.34 mm) than the control grafts (< 0.08 mm; p < 0.01).

CONCLUSIONS

The MC seeding process decreases patency and increases neointimal formation of small diameter ePTFE grafts in dogs and does not seem to be useful for reduction of graft thrombogenicity.

Effect of retroviral transduction on human endothelial cell phenotype and adhesion to Dacron vascular grafts

PURPOSE

Retroviral transduction for genetic enhancement of endothelial cell (EC) anti-thrombotic phenotype offers potential for improving the clinical success of vascular graft seeding; however, application of this technique may bring concomitant alteration in cell functionality.

METHODS

Human microvascular ECs were transduced with a retroviral vector encoding for the marker gene beta-galactosidase. Transduced endothelial cells (rtECs) and nontransduced endothelial cells (ntECs) were evaluated by flow cytometry for expression of intercellular adhesion molecule (ICAM)-1 and tissue factor (TF) on both smooth (coverslips) and graft (Dacron, 6 mm inside diameter) surfaces under static and shear exposed conditions. Graft EC retention was measured after 6-hour pulsatile perfusions. Platelet and neutrophil adherence was measured on perfused coverslips.

RESULTS

Lower levels of ICAM-1 were expressed by rtECs on coverslips under both static (p < 0.01 vs static ntECs) and shear exposed conditions (p < 0.01 vs static and shear ntECs). Accordingly, fewer polymorphonuclear leukocytes adhered to rtEC monolayers (p < 0.01 vs ntECs). No difference in ICAM-1 and TF expression by static graft seeded rtECs and ntECs was observed. However, graft-seeded rtECs that were exposed to wall shear stress displayed less TF than sheared ntECs (p < 0.05). Transduction did not affect EC retention to the sheared graft surface.

CONCLUSIONS

These data suggest that retroviral transduction does not elicit a prothrombotic/proinflammatory phenotype, rather indices of these states appear in some conditions to be reduced. Further, transduction does not adversely affect EC adherence to Dacron graft surfaces under arterial hemodynamics.

Tissue factor expression by cells used for sodding of prosthetic vascular grafts

Sodding of vascular grafts involves coating the biomaterial with cells prepared from collagenase-digested fat tissue after removal of the adipocytes by centrifugation. The goal of this study was to investigate the staining characteristics of the sodding cells as well as their ability to express the procoagulant protein tissue factor, and to compare these findings to those found with extensively purified microvascular endothelial cells (MEC) prepared from similar tissue. Sodding cells and MEC, isolated using immunomagnetic separation with anti-PECAM antibodies, were prepared from liposuction material and endothelial-specific staining was compared. The expression of tissue factor on these cells was examined using both an ELISA and a chromogenic assay to assess the rate of generation of factor Xa. Sodding cells expressed significantly more tissue factor than the unstimulated MEC in which the expression was undetectable (sodding cells 2466 +/- 830 pg/mL, P < 0.05). There was no further increase in tissue factor expression in the sodding cells with stimulation with lipopolysaccharide (LPS); however, purified MEC expressed significantly more tissue factor after exposure to LPS (1247 +/- 356 pg/mL, P < 0.05). These results were confirmed by the determination of procoagulant activity of the cells whereby the procoagulant activity on unstimulated MEC was significantly less than that found after stimulation of these cells, and it was also less than stimulated and unstimulated sodding cells (absorbance at 405 nm: 0.423 +/- 0.125, unstimulated MEC; 1.000 +/- 0.438, stimulated MEC; 1.129 +/- 0.396, unstimulated sodding cells; 1.171 +/- 0.254, stimulated sodding cells, P < 0.05). Staining of these two cells types also demonstrated significant uptake of acetylated LDL (Ac-LDL) in the purified MEC which was essentially absent in the sodding cells. Further, vWf staining was found to a greater degree in the purified MEC than in the sodding cells. These experiments demonstrated that the cells prepared for cell sodding express large amounts of tissue factor. The sodding cells do not stain for antigens known to be specific for endothelial cells, whereas MEC do and therefore the concentration of endothelial cells in the sodding cells is small. The significance of the tissue factor expression on the surface of sodded grafts is not yet known.

Production of plasminogen activator inhibitor-1 by human saphenous vein endothelial cells seeded onto endarterectomy surfaces in vitro

PURPOSE

Endothelial cells produce many biologically important factors that may be used as functional markers, including plasminogen activators and their inhibitors (PAI). PAI-1 is a common peptide with a central role in the balance of thrombosis and fibrinolysis in vivo, and its production by vascular endothelial cells has been demonstrated for many in vitro cell lines.

METHODS

The basal rate of PAI-1 release from cultured human adult endothelial cells was studied in both a well-plate-seeding model and after seeding onto human endarterectomy specimens. The effect of nonspecific stimulation with thrombin on PAI-1 release was examined in well-plate cultures. PAI-1 was measured by enzyme-linked immunosorbent assay.

RESULTS

Cultured human saphenous endothelial cells release PAI-1 constitutively at a steady rate, which can be increased in the short term by the addition of thrombin.

CONCLUSION

After seeding onto endarterectomy specimens, seeded endothelial cells release significant amounts of PAI-1, which suggests that they retain functional integrity and may potentially influence thrombosis in vivo after seeding.

Early attachment of leucocytes, platelets and fibrinogen in endothelial cell-seeded Dacron venous conduits

BACKGROUND

There is a need for prosthetic venous conduits in surgery for trauma, cancer and thrombotic disease. Such conduits in use today have a low patency rate, leaving room for much improvement.

METHODS

This experimental study investigated the dynamics of the early attachment of radiolabelled platelets, leucocytes and fibrinogen to endothelial cell-seeded Dacron venous conduits in sheep. Grafts were placed as jugular vein interposition grafts, seeded on one side, not seeded on the other, and followed for 4 h.

RESULTS

No difference could be demonstrated between the two graft types. Platelets showed an increasing attachment during the whole period, leucocytes an immediate attachment followed by an undulating pattern, and fibrinogen an immediate attachment with a tendency to decrease. Measurements on the vein itself showed a high attachment and, for platelets, an extremely high attachment when measured after the graft in the direction of flow.

CONCLUSION

The seeding process did not seem to affect early thrombogenicity. The carefully dissected vein wall showed highly thrombogenic properties, in many ways as high as in the prosthetic graft.

Seeding of vascular grafts with genetically modified endothelial cells. Secretion of recombinant TPA results in decreased seeded cell retention in vitro and in vivo

BACKGROUND

Seeding of small-diameter vascular grafts with endothelial cells (ECs) genetically engineered to secrete fibrinolytic or antithrombotic proteins offers the potential to improve graft patency rates.

METHODS AND RESULTS

Sheep venous ECs were transduced with a retroviral vector encoding human tissue plasminogen activator (TPA). The ECs were seeded onto 4-mm-ID synthetic (Dacron) grafts. Retention of the seeded ECs was measured 2 hours after placement of the seeded grafts both in vitro in a nonpulsatile flow system and in vivo (in sheep) as femoral and carotid interposition grafts. On exposure to flow in vitro, ECs transduced with TPA were retained at a significantly lower rate (median, 67%) than either untransduced ECs (81%) or ECs transduced with a control retroviral vector producing beta-galactosidase (beta-Gal) (80%) (P < .05 for TPA versus either control). On implantation in vivo, ECs transduced with TPA were retained at a very low rate (median, 0%), significantly less than the retention of ECs transduced with the beta-Gal vector (32%; P < .00001). Decreased in vivo retention of ECs transduced with TPA correlated modestly with increased in vitro cellular passage level (r2 = .48; P < .0001) but not with in vivo blood flow rate (P = .45). Addition of the protease inhibitor aprotinin to the cell culture and graft perfusion media resulted in a significant (P < .05) increase in in vitro retention of ECs transduced with TPA.

CONCLUSIONS

Increased TPA expression significantly decreases seeded EC adherence in vitro and in vivo. Gene therapy strategies for decreasing graft thrombosis may require expression of antithrombotic molecules that lack proteolytic activity.

Fibrinolysis profiles and platelet activation after endothelial cell seeding of prosthetic vascular grafts

There is no convincing evidence that endothelial cell seeding of prosthetic grafts in humans confers any of the advantages seen in animals. However, partial endothelial coverage might exert a subtle effect not detectable with indirect end points such as patency or scintigraphy. This study examined seeded cell function by measuring fibrinolytic and platelet activation markers in patients receiving seeded and control prosthetic grafts. Thirty-two patients were randomly assigned to seeded (n = 15) and control (n = 17) groups. Preoperatively and 3, 6, and 12 months postoperatively, plasma levels of fibrinopeptide A (FPA), B beta 1-42 fragment, cross-linked fibrin degradation products (XL-FDP), thromboxane A2 (TXA2), platelet factor 4 (PF4), and beta-thromboglobulin (beta TG) were measured. Patients with seeded grafts had significantly lower levels of FPA at 6 and 12 months (p < 0.05) and a significant overall group effect (p < 0.05). These patients also tended to have higher levels of XL-FDP (p < 0.1). No other significant differences were seen. The lower rate of conversion of fibrinogen to fibrin and the trend toward increased fibrinolysis seen in seeded grafts may be due to the metabolic effects of viable retained seeded cells. Although comparable platelet activation indicates that endothelial coverage remains limited, seeding may exert an antithrombotic influence at the graft surface.

Culture of human adult endothelial cells on endarterectomy surfaces

OBJECTIVES

Endothelial cell seeding of prosthetic grafts has not been as successful as initially hoped and the application of seeding technology to alternative reconstructive procedures such as endarterectomy and angioplasty has been increasingly considered. The success of such seeding depends on the ability of the seeded cells to attach to, and form a monolayer on the endarterectomised vessel wall which was the aim of this study.

METHODS

Using a seeding chamber model, heterologous human adult endothelial cells were seeded onto fresh human endarterectomy specimens and cultured. Studies of endothelial call adherence to endarterectomy specimens were performed using 111-Indium oxine labelled cells using methodology analogous to graft seeding.

RESULTS

Mean endothelial cell adherence of 70% (S.D. 10%) after 1 h incubation was achieved and the successful development of a monolayer of human adult venous endothelial cells on endarterectomised arteries was demonstrated in vitro.

CONCLUSIONS

These results indicate that closed endarterectomy appears to offer a surface with cell attachment that is superior to prosthetic grafts. Where femoral endarterectomy is appropriate, endothelial seeding potentially offers a method of reducing thrombogenicity and intimal hyperplasia, improving patency and avoiding a prosthetic graft whilst preserving collateral circulation and autologous vein.

Indium-111 platelet scintigraphy in vascular disease

Indium-111 labelled platelet scintigraphy permits simple, accurate evaluation of platelet kinetics and sites of deposition. The most meaningful results are obtained from serial scans, although these are clinically useful in only a few of the many applications that have been suggested, principally in studies of acute thrombosis, prosthetic graft thrombogenicity and antiplatelet medication. Key pitfalls are associated with selection of regions and patient variation.

Vascular prostheses for aortocoronary bypass grafting: a review

Alternative conduits must be chosen when autologous grafts are not available for coronary artery bypass grafting (CABG). Viable grafts do not always have perfect characteristics for CABG, and homologous venous conduits have been used with unsatisfactory results. Many small caliber vascular grafts used for coronary bypass conduits have been developed in the past, but most of them have failed except in rare instances. In this paper the current problems in available conduits, new technologies for improvement, animal models, and possibilities for the future for CABG conduits are discussed.