Endothelial cell labeling with indium-111-oxine as a marker of cell attachment to bioprosthetic surfaces

Radiolabeling of cell membrane-based nano-vesicles with 14C-linoleic acid for robust and sensitive quantification of their biodistribution

The rapid development of biomimetic cell membrane-based nanoparticles is still overshadowed by many practical challenges, one of which is the difficulty to precisely measure the biodistribution of such nanoparticles. Currently, this challenge is mostly addressed using fluorescent techniques with limited sensitivity, or radioactive labeling methods, which rarely account for the nanoparticles themselves, but their payloads instead. Here we report the development of a robust method for the innate radioactive labeling of cells and membrane-based nanoparticles and their consequent sensitive detection and biodistribution measurements. The preclinical potential of this method was demonstrated with Nano-Ghosts (NGs), manufactured from the cytoplasmic membranes of mesenchymal stem cells cultured with radioactively-labeled linoleic acid and achieving a cell labeling efficiency of 36%. Radiolabeling did not affect the physiochemical properties of the NGs, which stably retained their radiolabels. Using radioactivity measurements, we are now able to determine precisely the amount of NGs uptaken by tissues and cells, thereby providing further support to our presumed active NG targeting mechanisms. Biodistribution studies comparing radiolabeled NGs to fluorescently-labeled ones have validated our method and revealed new information, which could not be obtained otherwise, regarding the NGs' unique kinetics and rapid clearance, supporting their excellent safety profiles. The reported approach may be expanded to other membrane-based entities to facilitate and hasten their preclinical development and be used in parallel with other labeling methods to provide different and additional information.

Limitations of In-Vitro Labeling of Endothelial Cells with Indium-111 Oxine

Indium-111 oxine labeling is widely used as a marker of endothelial cell attachment to vascular prostheses. The long term effect of labeling human adult endothelial cells (HAECs) with this isotope has not been determined. In this study the viability of labeled HAECs, leakage of isotope from labeled cells and adherence of circulating isotope to fibronectin coated prostheses were investigated over 24 h. The effect of incubation time on labeling efficiency was also assessed. There were significant differences in cell viability between the labeled and unlabeled groups beyond 4 h (p < 0.005, 2-tailed, unpaired t-test). In the control group cell numbers increased by 42% while in the labeled group this had decreased by 20% at 24 h. Spontaneous leakage increased with time but was maximal in the first 2 h. Adherence of circulating isotope to fibronectin coated expanded polytetrafluoroethylene (ePTFE) grafts was minimal but was significantly greater to gelatin impregnated Dacron (GEL-SEAL) beyond 1 hour (p < 0.05). Incubation times greater than 5 minutes during labeling do not significantly improve labeling efficiency, and may contribute to toxicity by prolonging exposure to oxine. Indium-111 oxine labeling of HAECs is a suitable technique for acute studies of endothelial cell kinetics up to 4 h, but its use in chronic studies may lead to significant underestimations of cell retention.

Influence of substratum surface chemistry/energy and topography on the human fetal osteoblastic cell line hFOB 1.19: Phenotypic and genotypic responses observed in vitro

Time-dependent phenotypic response of a model osteoblast cell line (hFOB 1.19, ATCC, and CRL-11372) to substrata with varying surface chemistry and topography is reviewed within the context of extant cell-adhesion theory. Cell-attachment and proliferation kinetics are compared using morphology as a leading indicator of cell phenotype. Expression of (alpha2, alpha3, alpha4, alpha5, alphav, beta1, and beta3) integrins, vinculin, as well as secretion of osteopontin (OP) and type I collagen (Col I) supplement this visual assessment of hFOB growth. It is concluded that significant cell-adhesion events-contact, attachment, spreading, and proliferation-are similar on all surfaces, independent of substratum surface chemistry/energy. However, this sequence of events is significantly delayed and attenuated on hydrophobic (poorly water-wettable) surfaces exhibiting characteristically low-attachment efficiency and long induction periods before cells engage in an exponential-growth phase. Results suggest that a 'time-cell-substratum-compatibility-superposition principle' is at work wherein similar bioadhesive outcomes can be ultimately achieved on all surface types with varying hydrophilicity, but the time required to arrive at this outcome increases with decreasing cell-substratum-compatibility. Genomic and proteomic tools offer unprecedented opportunity to directly measure changes in the cellular machinery that lead to observed cell responses to different materials. But for the purpose of measuring structure-property relationships that can guide biomaterial development, genomic/proteomic tools should be applied early in the adhesion/spreading process before cells have an opportunity to significantly remodel the cell-substratum interface, effectively erasing cause and effect relationships between cell-substratum-compatibility and substratum properties. IMPACT STATEMENT: This review quantifies relationships among cell phenotype, substratum surface chemistry/energy, topography, and cell-substratum contact time for the model osteoblast cell line hFOB 1.19, revealing that genomic/proteomic tools are most useful in the pursuit of understanding cell adhesion if applied early in the adhesion/spreading process.

Indium-111 oxine labelling affects the cellular integrity of haematopoietic progenitor cells

PURPOSE

Cell-based therapy by transplantation of progenitor cells has emerged as a promising development for organ repair, but non-invasive imaging approaches are required to monitor the fate of transplanted cells. Radioactive labelling with (111)In-oxine has been used in preclinical trials. This study aimed to validate (111)In-oxine labelling and subsequent in vivo and ex vivo detection of haematopoietic progenitor cells.

METHODS

Murine haematopoietic progenitor cells (10(6), FDCPmix) were labelled with 0.1 MBq (low dose) or 1.0 MBq (high dose) (111)In-oxine and compared with unlabelled controls. Cellular retention of (111)In, viability and proliferation were determined up to 48 h after labelling. Labelled cells were injected into the cavity of the left or right cardiac ventricle in mice. Scintigraphic images were acquired 24 h later. Organ samples were harvested to determine the tissue-specific activity.

RESULTS

Labelling efficiency was 75 +/- 14%. Cellular retention of incorporated (111)In after 48 h was 18 +/- 4%. Percentage viability after 48 h was 90 +/- 1% (control), 58 +/- 7% (low dose) and 48 +/- 8% (high dose) (p<0.0001). Numbers of viable cells after 48 h (normalised to 0 h) were 249 +/- 51% (control), 42 +/- 8% (low dose) and 32 +/- 5% (high dose) (p<0.0001). Cells accumulated in the spleen (86.6 +/- 27.0% ID/g), bone marrow (59.1 +/- 16.1% ID/g) and liver (30.3 +/- 9.5% ID/g) after left ventricular injection, whereas most of the cells were detected in the lungs (42.4 +/- 21.8% ID/g) after right ventricular injection.

CONCLUSION

Radiolabelling of haematopoietic progenitor cells with (111)In-oxine is feasible, with high labelling efficiency but restricted stability. The integrity of labelled cells is significantly affected, with substantially reduced viability and proliferation and limited migration after systemic transfusion.

Radiolabeled Cell Distribution After Intramyocardial, Intracoronary, and Interstitial Retrograde Coronary Venous Delivery

Background— Several clinical studies are evaluating the therapeutic potential of delivery of various progenitor cells for treatment of injured hearts. However, the actual fate of delivered cells has not been thoroughly assessed for any cell type. We evaluated the short-term fate of peripheral blood mononuclear cells (PBMNCs) after intramyocardial (IM), intracoronary (IC), and interstitial retrograde coronary venous (IRV) delivery in an ischemic swine model.

Methods and Results— Myocardial ischemia was created by 45 minutes of balloon occlusion of the left anterior descending coronary artery. Six days later, 10 7 111 indium-oxine–labeled human PBMNCs were delivered by IC (n=5), IM (n=6), or IRV (n=5) injection. The distribution of injected cells was assessed by γ-emission counting of harvested organs. For each delivery method, a significant fraction of delivered cells exited the heart into the pulmonary circulation, with 26±3% (IM), 47±1% (IC), and 43±3% (IRV) of cells found localized in the lungs. Within the myocardium, significantly more cells were retained after IM injection (11±3%) compared with IC (2.6±0.3%) ( P <0.05) delivery. IRV delivery efficiency (3.2±1%) trended lower than IM infusion for PBMNCs, but this difference did not reach significance. The IM technique displayed the greatest variability in delivery efficiency by comparison with the other techniques.

Conclusions— The majority of delivered cells is not retained in the heart for each delivery modality. The clinical implications of these findings are potentially significant, because cells with proangiogenic or other therapeutic effects could conceivably have effects in other organs to which they are not primarily targeted but to which they are distributed. Also, we found that although IM injection was more efficient, it was less consistent in the delivery of PBMNCs compared with IC and IRV techniques.

Evaluation of an in vitro endothelialized vascular graft under pulsatile shear stress with a novel radiolabeling procedure

OBJECTIVE

To improve the hemocompatibility of vascular grafts, endothelial cell (EC) seeding of biomaterials prior to implantation is critical. The current in vitro study was designed to investigate such a feasibility on a collagen-coated heparin-bonded graft and to evaluate cell detachment upon pulsatile shear stress.

MATERIALS AND METHODS

Endothelial cells (EA-hy-926) were seeded onto grafts. The endothelialization of the grafts was evaluated by the [3H]-thymidine incorporation, scanning electron microscopy (SEM) and histological examinations. After in situ EC radiolabeling with a novel 99mTc technique, the prostheses were exposed to pulsatile shear stress (0.27 N/m2), mimicking the shear rate occurring in a superficial femoral artery, for 3 h in a flow circuit and EC loss quantified by gamma camera detection.

RESULTS

Complete EC coverage was achieved after 5 days. Three hours of artificial perfusion resulted in a low EC loss (12.9+/-0.8%, n = 7). SEM shows EC withstanding shear stress in valleys of prosthesis circumvolutions.

CONCLUSIONS

These satisfactory results could be explained by the high affinity of EC for heparinized surfaces in addition to cell surface receptors involved in adhesion to collagen.

Effect of prolonged pulsatile shear stress in vitro on endothelial cell seeded PTFE and compliant polyurethane vascular grafts

OBJECTIVES

Compliance mismatch between graft and native artery, and failure of the graft to develop an endothelial lining are the two main factors in graft failure. The objective of this study was to assess a new compliant graft for effective cell attachment and cell retention at physiological levels of pulsatile shear stress over a 6-hour period of physiological pulsatile flow.

DESIGN

Laboratory haemodynamic study.

MATERIALS AND METHODS

Human umbilical vein endothelial cells labelled with 111In-oxine were seeded on compliant polyurethane (CPU) and polytetrafluoroethylene (PTFE) vascular grafts. These were then exposed to varying shear stresses of up to 13.8 +/- 0.6 dyn/cm2 using a pulsatile flow model. Dynamic scintigraphy images were acquired using a gamma camera linked to an on-line computer during 6 h of perfusion and data presented as mean +/- standard error of mean.

RESULTS

Mean seeding efficiencies were significantly different at 4,316 +/- 505 and 825 +/- 504 CPM/cm2 on the CPU and PTFE grafts, respectively (p = 0.018). The flow experiment showed a higher percentage of cells retained on the CPU graft after exposure to shear stress caused by pulsatile flow compared to PTFE with respect to time. After 6 h pulsatile perfusion there was a significantly higher proportion of initial cells attached to CPU graft compared to PTFE graft (73 +/- 8% vs 42 +/- 8%, p = 0.018). The areas under the time activity curves over the 6-hour period were 280 +/- 26.4 for CPU and 176.0 +/- 30.0 for PTFE, confirming a significant greater total cell loss from PTFE compared with CPU grafts (51 +/- 7.0% vs 23 +/- 8.3%, p = 0.018, Wilcoxon matched-pairs signed-ranks test).

CONCLUSIONS

This flow model provides an effective method of assessing cell retention on graft materials under physiological conditions over a 6-hour period; CPU combines both excellent compliance and endothelial cell attachment rates after 6 h exposure to shear stress.

Coating PTFE vascular prostheses with a fibroblastic matrix improves cell retention when subjected to blood flow

An investigation was made into the effect of blood flow on endothelial cells (EC) and mesothelial cells (MC) seeded on a vascular expanded polytetrafluoroethylene (ePTFE) prosthesis coated with a fibroblastic matrix. Endothelial cells were obtained from the external jugular vein and MC from the omentum. To test the performance of prostheses, a custom designed, femoral "ex vivo" circuit was developed in mongrel dogs. Four study groups were established: a control group, A1, where prostheses were uncoated and seeded with EC; a second control group, A2, where prostheses were uncoated and seeded with MC; group B1 where prostheses were coated with a fibroblastic matrix and seeded with EC; and group B2 where coated prostheses were seeded with MC. All cells were labeled with 111Indium oxine (10 microCi/mL) before seeding. After the seeded cells had formed a monolayer on the ePTFE prostheses (which took approximately 24 h) the prostheses were placed in the "ex vivo" circuit. The rates of blood flow to which prostheses were exposed were measured at the point of inflow (117.5 +/- 12.50 mL/min, mean +/- SD) and outflow (72.6 +/- 14.3 mL/min). MC showed a greater baseline radionuclide uptake than did EC. The cells of groups B1 and B2 adhered sufficiently to the fibroblastic matrix and covered enough of the prosthetic surface to be positioned in the "ex vivo" circuit (76.90 +/- 8.24% surface covered in EC-seeded prostheses and 71.65 +/- 6.23% in MC-seeded prostheses). After exposure to blood flow the quantity of radionuclide-labeled cells and the prosthetic surface covered by them were greatly reduced though the fibroblast-coated prostheses showed greater cell retention.

Rapid postincubation endothelial retention by Dacron grafts

Defining the most appropriate conditions for strengthening the retention of endothelial cells (ECs) by small-diameter prosthetic endothelialized grafts is indispensable to their clinical application. The incubation time after seeding is one of the most important factors in EC retention. The effects of different postincubation times (0, 2, 4, 8, 16, 24, and 36 hr) on EC monolayers on two different types of graft, fibronectin-coated expanded polytetrafluoroethylene (ePTFE) and collagen-coated knitted Dacron grafts (4 mm x 5 cm) were examined. In situ counting of ECs on the grafts was performed by light microscopy. The percentage cell retention was calculated by dividing the cell counts for grafts exposed to pulsatile flow for 90 min by those for control grafts. To characterize the EC coverage of the grafts, scanning electron microscopy was also performed. The average cell density of control grafts ranged from 5.59 +/- 1.1 to 6.69 +/- 1.5 x 10(4) cells/cm2 and did not differ according to the kind of graft or incubation time. The knitted Dacron grafts showed the maximal cell retention (88 +/- 5%) after incubation for 8 hr, whereas ePTFE grafts did so after 24 hr (83 +/- 6%). Scanning electron microscopic examination after incubation for 8 hr revealed that the density of human ECs on the surfaces of ePTFE and Dacron grafts differed, although there was no morphological difference between the ECs on the two types of graft. Knitted Dacron grafts achieved a high percentage retention in a shorter time than ePTFE grafts.

Mesothelial versus endothelial cell seeding: evaluation of cell adherence to a fibroblastic matrix using 111In oxine

OBJECTIVES

The aim of this study was to compare the behaviour of mesothelial cells (MC) to that of endothelial cells (EC) when seeded onto a PTFE, prosthesis coated with a fibroblastic matrix.

DESIGN, MATERIAL AND METHODS

Three study groups were examined: a control group (Control) of PTFE prostheses with a fibroblast matrix (n = 8); Group EC, PTFE prostheses seeded with EC on a fibroblastic matrix (n = 8); and Group MC, PTFE, prostheses seeded with MC on a fibroblastic matrix (n = 8). All cell types were labelled with 111In (100 microCi/ml) 24 h after seeding, when the cells had formed a monolayer on the prosthetic surface. Radioactive levels were measured at 2, 4, 6, and 24 h.

RESULTS

Both EC and MC showed optimal adherence. The MC had a better radioactive uptake and retention than the EC. The number of EC and MC cells that remained adherent to the matrix was large enough to ensure complete covering of the prosthetic surface.

CONCLUSION

The use of MC is therefore feasible as an optimal alternative for achieving a natural covering on vascular prostheses prepared with a fibroblastic matrix.

Hydrophilic surface modification of metallic endoluminal stents

PURPOSE

Stainless steel endovascular stents are inherently thrombogenic so that thrombus accumulates on these devices, leading to acute vessel occlusion. A potential solution to this problem is stent surface modification with hydrophilic polymers, which might limit platelet adhesion and reactivity.

METHODS

N-vinylpyrrolidone (NVP) and potassium sulfopropyl acrylate (KSPA) hydrophilic monomers were gamma graft polymerized onto 1 cm2 stainless steel slabs and 4 mm Palmaz stainless steel stents. Surface characteristics of modified and plain stainless steel stents were then investigated with contact angle and x-ray photoelectron spectroscopy measurements, and in vitro and in vivo platelet reactivity was assessed as 111Indium platelet accumulation expressed as counts/min/cm2.

RESULTS

Surface modification of stainless steel slabs and stents with both NVP and KSPA hydrophilic polymers significantly reduced in vitro platelet adhesion (plain = 2249 +/- 723 counts/min/cm2, NVP = 428 +/- 156 counts/min/cm2, KSPA = 958 +/- 223 counts/min/cm2) and in vivo platelet accumulation after 1 hour of blood flow exposure (plain = 1407 +/- 796 counts/min/cm2, NVP = 426 +/- 175 counts/min/cm2, KSPA = 399 +/- 124 counts/min/cm2. In addition, platelet accumulation on modified stents indexed to plain stents was lowest in KSPA-modified stents (NVP = 79.3% +/- 31.7% of plain, KSPA = 51.2% +/- 36.2% of plain). Surface analysis confirmed surface grafting with both monomers, and SEM documented smoothing of the irregular surfaces of the stainless steel stents after grafting.

CONCLUSION

Hydrophilic polymer surface modification of stainless steel stents decreases initial stent surface platelet accumulation, which may decrease the risk of vessel thrombosis associated with the use of these devices.

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.

Loss of seeded endothelial cells in vivo. A study of Dacron grafts under different flow conditions

OBJECTIVES

To assess, in vivo, the loss of endothelial cells seeded in a Dacron graft at low, restricted arterial flow and at normal unrestricted flow.

DESIGN AND SETTING

Laboratory animal study.

MATERIALS

Indium-111-oxine labelled endothelial cells were seeded in externally supported, preclotted Dacron grafts, inserted as interposition grafts in the carotid arteries of nine sheep. Activity (radioactivity) was measured, flow established, on one side unrestricted 120-180 ml/minute, and on the other restricted to 50 ml/minute by a distal clamp.

OUTCOME MEASURES

The reduction in activity over the grafts was measured for 2 hours.

RESULTS

There was an immediate loss of activity to around 80% and thereafter a lower rate of cell loss to around 50%. No difference could be demonstrated in the loss from low flow compared to high flow grafts. All grafts were patent. There was no difference in thrombus weights.

CONCLUSION

No difference could be demonstrated in seeded endothelial cell loss from a Dacron graft in vivo at low flow (50 ml/minute) compared with unrestricted flow (120-180 ml/minute).

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.

Mesothelial seeding of knitted Dacron

Bilateral superficial femoral artery replacement using knitted Dacron was performed in 38 dogs. One side was seeded with omental mesothelium and the other acted as an unseeded control. 111In-labelled platelet accumulation on grafts was measured at 5 days and 2 months and the thrombogenicity index of seeded and unseeded grafts calculated. Patency was monitored for 2 months, at which time grafts were removed and luminal thrombus, ultrastructural cell cover and prostacyclin release were measured. Cell seeding did not influence the mean(s.e.m.) thrombogenicity index of 0.95(0.25) and 0.88(0.24) at 5 days in control and seeded grafts respectively; nor was there any difference between the groups at 2 months. Occlusion occurred in six control and four seeded grafts. Seeding did not significantly improve the percentage thrombus-free area or luminal cell cover. Neither did it enhance mean(s.e.m.) luminal 6-keto-prostaglandin F1 alpha release of 2.58(0.80) pg cm-2 in controls and 2.63(0.78) pg cm-2 in seeded grafts. Further studies demonstrated that only a mean(s.e.m.) of 4.4(1.9) per cent of the seeded inoculum was present on grafts 48 h after implantation, providing too few cells to achieve confluent cover. Mesothelial cell seeding might be useful in promoting a healed graft surface but critical levels of seeding density must be achieved before the technique can be properly evaluated.

Effect of seeding time and density on endothelial cell attachment to damaged vascular surfaces

An in vitro model to facilitate the study of endothelial cell seeding of damaged vascular surfaces has been developed. This may have applications in the study of endothelial seeding of angioplasty and endarterectomy sites. Using this model, the optimum endothelial seeding time for attachment to damaged vascular surfaces should not exceed 30 min and, to achieve confluent cell attachment, a seeding density > 5 x 10(5) cells/cm2 should be used.

111Indium labeling of microorganisms to facilitate the investigation of bacterial adhesion

The ability of bacteria to adhere to polymeric interfaces has attracted considerable attention in recent years. Metabolic labeling of microorganisms with 35S-methionine or other beta-emitters is commonly utilized for quantification of bacterial adhesion to biopolymers. Since the use of these isotopes is cumbersome, the possibility of labeling the microorganisms with 111Indium, a strong gamma-emitter, was explored. This report demonstrates that bacteria can be easily labeled with 111Indium. Staphylococcus aureus, Staphylococcus epidermiids, and Pseudomonas aeruginosa were labeled with either 111Indium-oxine or 35S-methionine; and labeling efficiency, retention of incorporated labels, and growth kinetics of labeled bacteria were compared under identical experimental conditions. Bacteria labeled with 111In-oxine incorporated approximately 90% of radioactivity within 10 min, whereas 35S-methionine incorporation required many hours of incubation. Both the incorporated isotopes were gradually released by rapidly growing bacteria into the suspension medium. Of the total incorporated labels, approximately 20% 111In and 15% 35S were released in the surrounding medium every 24 h. No release of incorporated labels occurred when cells were fixed with 2.5% buffered glutaraldehyde. Growth kinetics and scanning or transmission electron microscopic analysis showed no detectable differences among control (nonlabeled), 111In-, or 35S-labeled bacteria. Labeling of bacteria with 111In-oxine does not interfere with bacterial adherence. These observations suggest that 111In incorporation provides a simple and rapid method of labeling of microorganisms. Compared to currently available techniques, the use of 111In-labeled bacteria will facilitate the quantitation of adherent bacteria to interfaces.

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.

New evidence and new hope concerning endothelial seeding of vascular grafts

Endothelial cell (EC) seeding of prosthetic bypass grafts has been promoted as a method of improving graft patency. However, an efficient and reliable method of seeding vascular prostheses with ECs is lacking due to inefficient harvesting of ECs and poor attachment and proliferation of cells on the prosthetic surfaces. To investigate the effect of a commonly used prosthetic surface on EC attachment and proliferation, we measured the attachment and proliferation of ECs on polytetrafluoroethylene (PTFE) grafts uncoated or coated with gelatin, laminin, fibronectin, collagen type I and/or III, or RGD (arginine-glycine-aspartate)-containing peptide. EC attachment and proliferation were both significantly decreased on the untreated PTFE graft surface. Conversely, coating of PTFE with fibronectin, RGD, laminin, or gelatin significantly (p less than 0.05) improved the attachment of ECs, with the most striking increases occurring with laminin and gelatin. Similarly, all matrix components in this study improved EC proliferation compared with untreated PTFE, with RGD and gelatin producing the most significant improvement. PTFE adversely effects EC attachment and proliferation. These properties can be improved by treating PTFE graft surfaces with extracellular matrix components in relatively low concentrations. Future investigations are needed to determine whether there are combinations and concentrations of matrix components that will optimize these cellular functions on vascular prostheses.

Seeding of human microvascular endothelial cells onto polytetrafluoroethylene graft material

Microvascular endothelial cells from human omental samples were isolated and grown. Adhesion to polytetrafluoroethylene vascular graft material was then studied using scanning electron microscopy and adhesion of cells labelled with indium-111. Grafts coated with fibronectin and type I collagen were found to promote the best adhesion of cells at times up to 90 min. A coating of blood clot matrix was less effective but still resulted in a threefold increase in cell adhesion compared with controls.

111Indium is an unreliable in vivo label for vascular endothelial cells

We studied the retention of 111Indium-labeled canine endothelial cells on 32 grafts (16 dogs). Canine endothelial cells were harvested from the external jugular veins, grown in culture, and labeled with 111Indium oxine; 10(6) factor VIII positive cells were inoculated on fibronectin-coated, 4 mmID Hytrel grafts and cultured 18 hours to reach confluence. An autologous seeded graft was interposed in each of the common carotid arteries and exposed to flow for six hours. 111Indium label was measured pre- and postperfusion and corrected for decay. Twenty-five grafts from 13 dogs were available for study. Scanning electron microscopic planimetry was used to determine percent surface coverage by six mutually exclusive surface characteristics: endothelial cells, bare graft, white blood cells on graft, white blood cells on endothelium, white blood cells under endothelium, and thrombus. 111Indium retention was compared with percent coverage by scanning electron microscopy using regression analysis. 111Indium labeling projected an erroneous retention of 41% at zero percent coverage (r = 0.67; p less than 0.01). Multiple regression analysis revealed an equivalent distribution of 111Indium label over nonendothelial portions of the flow surface and indicated a leak rate into the circulation of 25.6% of the initial 111Indium label over six hours. We conclude that: 1) 111Indium labeling data usually overestimates endothelial cell retention; 2) an average of 4.67%/hour is lost into the general circulation; 3) 111Indium label can be found equally on surfaces of thrombus, white blood cells, and hydrophilic Hytrel graft; and 4) 111Indium labeling is not a reliable method for in vivo studies of endothelial cell retention.

Delayed exposure to pulsatile shear stress improves retention of human saphenous vein endothelial cells on seeded ePTFE grafts

Since significant loss of endothelial cells (ECs) from the surface of a seeded prosthetic graft occurs after implantation, improved cell retention following exposure to flow should increase the likelihood of long-term success with this technology. An in vitro pulsatile flow circuit was developed to study the effects of two variables on cell retention: cell density at the time of seeding and postseeding incubation time. Fibronectin-coated ePTFE grafts (4 mm x 5 cm) were seeded with human saphenous vein ECs at two densities, confluent (1 x 10(5) cells/cm2) or subconfluent (2 x 10(4)), and incubated in vitro for varying time intervals (90 min, 1, 3, or 7 days). Test grafts were exposed to 90 min of pulsatile flow in an in vitro flow circuit, then fixed, and stained, and in situ cell counts (cells/cm2) were determined for nine representative fields per graft. Paired control grafts were treated identically but were not exposed to flow. Cell retention was calculated using the formula: % retention = cells/cm2 perfused graft divided by cells/cm2 control graft. Grafts exposed to flow 90 min after seeding demonstrated significantly lower cell retention when compared to later time points. When cells were seeded at confluent density, maximal retention (92 +/- 3%) occurred 24 hr after seeding. Prolonged culture of cells seeded on ePTFE grafts at confluent density resulted in increased cell loss. In contrast, on grafts seeded at subconfluent density, retention improved as cells grew to confluence (16 +/- 4.5% initially to 82 +/- 7% at 7 days).(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of different vascular prostheses and matrices in relation to endothelial seeding

Thin-walled expanded polytetrafluoroethylene (ePTFE), woven Dacron and gelatin-impregnated Dacron (Gelseal) vascular grafts were compared, the grafts being coated with three different matrices: collagen IV, fibronectin and preclot matrix. In addition, untreated ePTFE and Gelseal were examined. The graft segments, coated with these matrices, were incubated with radiolabelled adult human endothelial cells for 30, 60 and 90 min. Endothelial cell adherence was calculated from the ratio of radioactive counts in the grafts to counts in grafts plus supernatants. Endothelial cell attachment to untreated grafts was poor, but a suitable matrix significantly improved adherence. All three matrices tested gave good results, although preclot was best; 30-60 min incubation was sufficient for optimum cell attachment. Cell adherence to both Dacron and ePTFE was significantly better than to Gelseal. The type of prosthetic polymer and the substrate protein coating used to promote endothelial cell adherence are two important factors which may determine the ultimate success of endothelial seeding in the operating room.

In vitro adherence and kinetics studies of adult human endothelial cell seeded polytetrafluoroethylene and gelatin impregnated Dacron grafts

Lining the luminal surface of small diameter vascular prostheses with living endothelial cells reduces thrombogenicity, decreases infection and improves patency. In vitro adherence and kinetics studies of adult human endothelial cell seeded Polytetrafluoroethylene (ePTFE) and Gelatin impregnated Dacron (Gelseal) were performed. Endothelial cell adherence on ePTFE and Gelseal coated with collagen IV, fibronectin and preclot matrices was compared. Untreated ePTFE and Gelseal were also used. Ten graft segments in each group coated with these matrices were incubated with radio-labelled adult human endothelial cells for 30, 60 and 90 min. Labelled endothelial cells seeded in supra-confluent densities on fibronectin coated ePTFE and Gelseal grafts were used for kinetic studies. Resultant endothelial cell monolayers were then exposed to varying shear stress at flow rates of 200 and 300 ml/min in an artificial flow circuit. Endothelial cell attachment to untreated grafts was poor and a suitable matrix significantly improved adherence with fibronectin and preclot but less so with collagen. A 30 min incubation was sufficient for optimum cell attachment. Cell adherence to ePTFE was significantly better than Gelseal. Scanning electron micrographs (SEM) of ePTFE showed preferential attachment to the nodes whilst on Gelseal, cells conformed to Dacron fibres at different levels and directions. Rapidly formed endothelial cell monolayers on ePTFE and Gelseal grafts resisted shear stress of flow with significant cell retention at 2 h. There was patchy coverage of both grafts with evidence of bridging of gaps between individual fibres in Gelseal.

Simultaneous measurement of endothelial cell damage, elastase release and chemiluminescence response during interaction between polymorphonuclear leukocytes and endothelial cells

Using cultured human umbilical cord vein endothelial cells and human blood neutrophils, the interaction between neutrophils and endothelial cells, in vitro, was studied. The aim of the study was to examine whether a respiratory burst stimulation by neutrophils would be observed by neutrophil/endothelial cell interaction and whether the respiratory burst stimulation of neutrophils by endothelial cells could be enhanced by lipopolysaccharide stimulation of neutrophils. The second aim was whether such an effect, or secretion of elastase, could cause an endothelial cell damage in vitro. Chemiluminescence as an indicator of oxygen-derived metabolites produced by neutrophils, elastase release by neutrophils, and endothelial cell damage, based on 111 In-oxine release from labelled endothelial cells, were measured simultaneously. The present investigation demonstrates that neutrophils can be directly stimulated by endothelial cells. A further amplification of this process following lipopolysaccharide priming up to 10 ng/ml blood could be demonstrated. A slight endothelial cell damage occurs following neutrophil stimulation, although elastase secretion does not increase during interaction between neutrophils and endothelial cells. These results raise the possibility that oxygen-derived metabolites rather than elastase contribute to an endothelial cell damage which might occur in conditions such as endotoxin-induced adult respiratory distress syndrome.

Fibronectin coating of expanded polytetrafluoroethylene (ePTFE) grafts and its role in endothelial seeding

Although fibronectin's role as a matrix to improve endothelial seeding has been demonstrated by other workers, the optimum concentration for use has never been described. Attachment of fibronectin to expanded polytetrafluoroethylene (ePTFE) was measured, using 125I-radiolabeled protein, at different concentrations and for different time periods. The absolute amount of fibronectin bound to the graft increased with the concentrations used in coating (p less than 0.001) and also with time (p less than 0.01); e.g., at 50 micrograms/ml, 90 min of incubation produced a molecular attachment of 4.0 x 10(11)/cm2 of graft. However, its percentage attachment decreased with a rise in concentration (p less than 0.001). After an initial loss of 22% in 30 min, the fibronectin-graft bond was found to be stable when exposed to a shear stress produced by flow at 200 ml/min. No significant difference in the cell adherence could be found in grafts coated with fibronectin concentrations of 50, 150, and 250 micrograms/ml, although it was significantly less at 10 and 25 micrograms/ml (p less than 0.05).

Effects of shear stress on endothelial cell monolayers on expanded polytetrafluoroethylene (ePTFE) grafts using preclot and fibronectin matrices

Animal studies have shown that endothelial seeding of vascular prosthetic grafts reduces thrombogenicity and improves their patency. However, for endothelial seeding to be of clinical benefit in humans, it must withstand shear stress of blood flow. Endothelial cells labelled with Indium-111-oxine were seeded in supra-confluent densities on preclot or fibronectin coated ePTFE graft segments over a period of 90 min. These grafts with rapidly formed endothelial cell monolayers were then exposed to varying shear stresses up to a flow rate of 300 ml/min, using tissue culture medium in an artificial flow circuit. Grafts coated with preclot matrix showed 2 h cell retentions of 82.4 +/- 6.8% at 25 ml/min, 79.9 +/- 8.2% at 100 ml/min, 75.4 +/- 9.5% at 200 ml/min and 58.3 +/- 15.5% at 300 ml/min whilst those for the fibronectin matrix were 57.8 +/- 9.9%, 55.2 +/- 13.3%, 55.4 +/- 12.9% and 56.5 +/- 15.2% respectively. Overall the preclot matrix was found to be better than fibronectin (P less than 0.001). Light and scanning electron microscopy revealed well-formed endothelial cell monolayers retained on preclot matrix up to a flow rate of 200 ml/min whereas uncovered patches were seen at 300 ml/min and at all flow rates on fibronectin matrix.

Attachment of indium-111 labelled endothelial cells to pretreated polytetrafluoroethylene vascular grafts

This study investigated the effect of different surface coatings on endothelial cell attachment to polytetrafluoroethylene (PTFE) vascular grafts. Small segments of PTFE vascular grafts were precoated with one of a number of substances: gelatin, poly-l-lysin, fibronectin, or collagen type I, III or IV. Indium-111 labelled endothelial cells were then seeded on to the grafts and left for either 10, 30, 60 or 120 min. The unattached cells were removed and the degree of cell attachment was calculated. All coatings were significantly better at enhancing endothelial cell attachment at all times compared with controls, and fibronectin was significantly better as a coating material than any other material used. By pretreating with a substance such as fibronectin, the number of endothelial cells attaching to PTFE vascular grafts can be greatly increased, thereby enhancing the cell seeding process.

An experimental collagen-impregnated Dacron graft: potential for endothelial seeding

This study evaluates the potential for endothelial seeding of a collagen-impregnated Dacron graft with or without surface modifiers (fibronectin, heparin) to attach and retain these cells during flow. Human umbilical endothelial cells were harvested, cultured, labeled with Indium111-oxine and seeded onto 30 mm X 4 mm diameter grafts. Six graft surfaces were studied: 1) a collagen-impregnated Dacron graft, HemashieldR (C); 2) C + fibronectin (C + F); 3) C + heparin (C + H); 4) C + F + H; 5) HytrelR + F (Hyt + F); and 6) Hyt + F + H. Radioactive loss determined the percentage attachment and then percentage retention of labeled inoculum after a one-hour in vitro perfusion. Scanning electron and light microscopy demonstrated the endothelium on the graft surface following perfusion. Fibronectin-coated grafts had a significantly higher percentage attachment than those without fibronectin (ANOVA, P less than 0.05). However, the percentage retention following perfusion was similar for all Dacron grafts and statistically inferior to the HytrelR grafts studied (ANOVA, P less than 0.05). SEM evaluation of the C + F + H graft surface was qualitatively the most impressive Dacron surface for seeding, yet was inferior to the HytrelR graft. We conclude that fibronectin benefits the initial attachment of endothelium to collagen-coated Dacron rivaling the HytrelR surface. Fibronectin does not improve percentage retention of the HemashieldR surface during perfusion, therefore, some of its initial benefit is lost.

Attachment and proliferation of fibroblast cells on polyetherurethane urea derivatives

Interactions of novel polyetherurethane urea derivatives with fibroblast cells as well as with plasma proteins were investigated. Fibronectin, which is a cell adhesion protein, was found to be very active in attaching fibroblast cells onto a heparinized polyetherurethane urea: its activity was found to be strongly dependent on the surface properties of the material. Fibronectin was easily adsorbed by the heparinized polyetherurethane urea, but the degree of its adsorption to the material in competition with other proteins was so low that cell attachment to polyetherurethane ureas was decreased by heparinization. Different degrees of cell attachment onto different materials due to different adsorptivities of plasma proteins were considered. Proliferation of fibroblast cells was suppressed on cationic polyetherurethane urea but unaffected on other derivatives of polyetherurethane urea. Since specific suppression of cell proliferation was not observed on the heparinized polyetherurethane urea, the latter material was expected to be useful as a long-term antithrombogenic material in vivo.

Alternative techniques of seeding cultured endothelial cells to ePTFE grafts of different diameters, porosities, and surfaces

Attachment of 111Indium-oxine labeled cultured canine venous endothelial cells to expanded polytetrafluorethylene (ePTFE) grafts was evaluated in vitro. Three alternative seeding techniques were studied in grafts having different diameters, porosities, and surfaces, including: (I) manual milking of blood containing endothelial cells within the graft; (II) a two-step procedure of incubating grafts initially with blood and then with an endothelial cell suspension; and (III) mechanical spinning of grafts filled with blood containing endothelium. Method II had significantly higher cell attachment to the graft (11.6%) than did Method I (1.5%) or III (4.7%). A somewhat higher seeding efficiency was noted in 10-mm-I.D. grafts (11.6%) compared to 6-mm-I.D. grafts (6.3%). Different graft porosity, created by altering internodal distances, did not cause significant changes in cell attachment (10 microns, 13.4%; 30 microns, 6.3%; 90 microns, 16.0%). Fibronectin-coated surfaces, which should have enhanced cell adhesion, demonstrated a 6.0% cell attachment, a lower efficiency than the 11.6% observed with a blood coating alone. Acetone-soaked surfaces, which should have predictably exhibited less hydrophobicity, produced quite variable attachments (range 3.4 to 59.7%, mean 23.4%). In the present investigation the best seeding technique was method II, the two-step incubation procedure. Consistent differences were not noted with various ePTFE graft configurations or surfaces.

Reduction in deposition of indium 111-labeled platelets after autologous endothelial cell seeding of Dacron aortic bifurcation grafts in humans: a preliminary report

Autologous endothelial seeding (AES) of vascular prostheses in dogs increases thrombus-free surface and improves prosthetic prostacyclin production, patency, and the ability to withstand hematogenous challenge with bacteria. No such information is available in human subjects. In the present study one limb of an aortic Dacron bifurcation prosthesis was seeded with autologous endothelial cells (ECs) harvested from the distal portion of the saphenous vein by enzymatic treatment. The deposition of indium 111-labeled platelets on the vascular prostheses was studied 1 and 4 months after operation. In seven of nine patients seeding resulted in decreased accumulation of radiolabeled platelets compared with sham-seeded control limbs (p less than 0.04), when studied 1 month after surgery. A decrease in platelet accumulation occurred over the whole prosthesis between 1 and 4 months, and no significant difference was noted at 4 months between seeded and nonseeded graft limbs. Although the seeding density was very low (440 ECs/cm2), the observed difference in platelet accumulation for AES-treated graft limbs in the early postoperative course merits further investigation of this technique in human beings.

Short-term in vivo stability of endothelial-lined polyester elastomer and polytetrafluoroethylene grafts

A fibronectin substrate will significantly enhance the strength of endothelial cell attachment on grafts constructed of polyester elastomer (PE) and polytetrafluoroethylene (e-PTFE). This experiment was undertaken to determine the short-term in vivo stability of endothelium on these fibronectin coated surfaces. Eight mongrel dogs underwent bilateral carotid artery replacement with both graft materials. All grafts were inoculated with 2,000 cells/mm2 using cultured autogenous venous endothelium labelled with Indium-111-oxine. The Indium-111 label in the grafts was measured immediately prior to implantation, after 1 hour of in vivo perfusion, and at explantation after 24 hours. The percentage of inoculated cells attached to the grafts before perfusion was similar for both materials, 93.3 +/- 3.0% versus 92.2 +/- 7.2%, for PE and e-PTFE respectively. All grafts were patent at one hour after implantation. PE grafts were found to have 93.8 +/- 3.9% of the attached cells present at one hour while e-PTFE grafts had only 54.5 +/- 10.8% remaining, p less than .001. After 24 hours, 5/8 (62.5%) e-PTFE grafts and 2/8 (25.0%) PE grafts remained patent, p = .13. Of the patent grafts however, endothelial cell retention was still superior on the PE grafts with 78.0 +/- 0.6% of the attached cells remaining compared to only 24.5 +/- 6.1% on e-PTFE, p less than .001. Occluded PE grafts had fewer cells remaining at 24 hours than patent ones, 78.0 +/- 0.6% versus 31.1 +/- 32.8%, respectively, p = .13. Histologically, patent PE grafts demonstrated nearly confluent endothelial monolayers while e-PTFE had patches of endothelial cells surrounded by a platelet-fibrin carpet.(ABSTRACT TRUNCATED AT 250 WORDS)

A method for toxicological evaluation of biomaterials based on colony formation of V79 cells

This report describes a method for cytotoxicity screening of biomaterials based on colony formation of V79 cells. For this test, two metals (titanium and nickel), two ceramics (alumina ceramic and tricalcium phosphate), and two types of polymeric material [high density polyethylene (HDP) and polyvinylchloride (PVC)] were used. Each metal and ceramic was cast into a disk and semidisk 49 mm in diameter and 1 to 2 mm thick. The HDP was molded into a petri dish and PVC was used as a thin film. The materials were sterilized by heating or with ethylene oxide and placed in plastic petri dishes, after which 8 ml cell suspension containing 100 cells were added to each dish. After 1 week, the colonies formed on the materials were fixed, stained, and then the number of colonies was counted. Titanium, alumina ceramic, and HDP showed no differences from the controls in terms of colonies. On the disks and the semidisks of nickel and tricalcium phosphate and on the thin disks of PVC, however, no colonies were detected. The V79 cells used in this experiment showed a rapid and logarithmically stable growth curve and such a high rate of colony formation as to form visible noticeable colonies, and were therefore suitable cells for screening test the cytotoxicity of biomaterials. Unlike other previously reported methods of in vitro cytotoxicity testing, this method permits assay of colonies formed from a single cell after proliferation directly on the materials. Moreover, the test with semidisks permits simple screening to assess the cytotoxicity is caused by either the chemical substances or the physical properties of the materials.(ABSTRACT TRUNCATED AT 250 WORDS)

Improved endothelial cell seeding with cultured cells and fibronectin-coated grafts

A possible approach to the low seeding efficiency of endothelial cells into prosthetic grafts is to increase the number of cells to be seeded in cell culture and improve seeding efficiency by graft precoating with fibronectin. The effect of cell culture on cell adhesion is unknown, however, and fibronectin also binds fibrin, which may increase the thrombogenicity of the graft luminal surface. To investigate these questions, freshly harvested canine jugular vein endothelial cells from six animals and similar cells harvested from six primary and eight secondary cell cultures were labeled with 111Indium and seeded into 5 cm, 4 mm PTFE grafts coated with fibronectin, using similar uncoated PTFE grafts as controls. Platelet accumulation and distribution on six similar coated and uncoated grafts placed in canine carotid, external jugular arterial venous shunts for 2 hr were also determined using autogenous 111Indium-labeled platelets. Significant differences between group means were determined using the paired Student's t test. Results reveal that seeding efficiency is significantly better in all groups of coated grafts compared to uncoated grafts (P less than 0.01). Cells derived from cell culture also had significantly higher seeding efficiencies than freshly harvested cells when seeded into coated grafts (P less than 0.05) and tended to have higher seeding efficiencies than harvested cells when seeded into uncoated grafts (P = 0.53). Fibronectin coating increased mean platelet accumulation on the entire graft luminal surface, but not to a statistically significant degree (P greater than 0.1). Whether this increased seeding efficiency will improve graft endothelialization remains to be investigated.

History of (micro) vascular surgery and the development of small-caliber blood vessel prostheses (with some notes on patency rates and re-endothelialization)

The historical development of vascular surgery is reviewed from ancient times (Ruphus of Ephesus, Aëtius of Amida) to recent developments (sutured anastomosis by Carrel). Attempts to anastomose blood vessels by means of nonsuturing technique, using a ring or short tube of diverse materials called prostheses, were undertaken at the start of this century and continued until shortly after World War II. With the advent of modern polymeric materials, prostheses of different types, sizes, structures, and fabrics have been used to substitute for blood vessels, both experimentally and clinically. Recently, blood vessel prostheses with small (1-1.5 mm) internal diameters became available and have been implanted experimentally. Patency rates, biophysical and structural properties, the re-endothelialization and the neointima formation of several types of microvascular prostheses are briefly reviewed.