Vascular graft-associated complement activation and leukocyte adhesion in an artificial circulation

Protein Adsorption and Cellular Adhesion and Activation on Biomedical Polymers

The design and development of new biomedical polymers for clinical application in devices, prostheses, and artificial organs requires a basic and fundamental understanding of biological interactions with biomedical polymers. Efforts in our laboratory have been directed towards appreciating the humoral and cellular interactions which govern protein adsorption and cellular adhesion and activation on biomedical polymers. Information and data are presented on protein adsorption from whole human blood, complement activation and receptors, and monocyte/macrophage adhesion and activation with growth factor release. Supported by experimental evidence, concepts regarding protein/polymer, cell/ polymer, cell/protein/polymer, and cell/cell interactions as they are related to in vivo events are presented.

Metabolic Response of Blood Cells to Synthetic Graft-Materials with Special Reference to a Fluoromer Passivated Dacron® Graft. An in Vitro Study Using Microcalorimetry

Microcalorimetry was used to study in vitro the metabolic response from human platelets and leukocytes when incubated with three different synthetic graft-materials. The graft to be studied primarily was Fluoromer Passivated Dacron (FPD) which was compared with ePTFE and with a knitted Teflon graft. A rapid increase in the metabolic activity of platelets was observed, followed by a steady-state for more than one hour, while the platelet metabolism did not differ among the various graft-materials. Leukocytes incubated with FPD showed a high initial metabolism, with a peak after about 15 minutes. After 60 minutes the metabolic response had reached control values. ePTFE and Teflon grafts differed significantly from FPD, without causing any peak metabolic activity. It may be concluded that FPD and ePTFE grafts, as evaluated in vitro, activate platelets to the same extent, while FPD causes a more extensive leukocyte activation. Whether these findings can be interpreted as differences in thrombogenicity and inflammatory responses has not been proven, but seems probable. This in vitro method should make it possible to further study human responses to synthetic materials a method possibly more reliable than animal experiments.

Perspectives on In Vivo Testing of Biomaterials, Prostheses, and Artificial Organs

The goal of in vivo testing of a medical device is to determine the safety or biocompatibility of the device in a biological environment. Biocompatibility is the ability of a medical device to perform with an appropriate host response in a specific application. Biocompatibility assessment is considered to be a measure of the magnitude and duration of adverse alterations in homeostatic mechanisms that determine the host response. Perspectives are provided on the role of injury, tissue responses to medical devices, and blood responses to medical devices. The concept of the normal foreign body reaction is presented. The potential importance of the macrophage, an important component of the foreign body reaction, in controlling the biocompatibility in the in vivo environment is discussed.

Interaction of a blood coagulation factor on electrically polarized hydroxyapatite surfaces

Although the polarization treatment of hydroxyapatite (HA) remarkably enhances the osteoconductivity, the mechanisms have not yet been completely understood. The interaction of proteins in blood and tissue fluids with biomaterials are reportedly triggers for later cellular responses and played a major role in osteoconductive processes. Considering this, we disclosed the interaction of polarized HA surface with a coagulation factor, fibrin stabilizing factor XIII (FXIII). The HA activated FXIII even in Ca2+ free buffer, based on the SDS-PAGE detections of alpha-polymer and gamma-dimer bands assigned to stabilized fibrin. The Ca2+ ions, possibly released from the HA surfaces, were examined whether they initiate the activation of the FXIII. It was experimentally proved by ICP analysis that the induced large negative charges on the electrically polarized HA significantly increased the released Ca2+ concentration for the short pre-incubation time of 3 min. The more Ca2+ ions released from the negatively charged HA (N-HA) surfaces were more effective in the activation of the FXIII, resulting in the rapider disappearance of the gamma-chain bands in fibrin. The slightly lower Ca2+ concentration in the positively charged HA, compared to the nonpolarized HA activated the FXIII at an almost equal rate. The accelerated activation contributed to the stabilization of fibrin scaffold. Therefore, the polarity difference of the induced charges of the polarized HA surface altered the rate of the FXIII activation. The early stage interaction of the HA surfaces with blood proteins was considered to be an essential process of the accelerated new bone formation near implanted N-HA surface.

Mass spectrometric mapping of fibrinogen conformations at poly(ethylene terephthalate) interfaces

We have characterized the adsorption of bovine fibrinogen onto the biomedical polymer polyethylene terephthalate (PET) by performing mass spectrometric mapping with a lysine-reactive biotin label. After digestion with trypsin, MALDI-TOF mass spectrometry was used to detect peptides from biotinylated bovine fibrinogen, with the goal of identifying lysines that were more accessible for reaction with the chemical label after adsorption. Peptides within domains that are believed to contribute to heparin binding, leukocyte activation, and platelet adhesion were found to be biotin labeled only after bovine fibrinogen adsorbed to the PET surface. Additionally, the accessibility of lysine residues throughout the entire molecule was observed to increase as the concentration of the adsorbing bovine fibrinogen solution decreased, suggesting that the proximity of biologically active motifs to hydrophilic residues leads to their exposure. The surface area per adsorbed bovine fibrinogen molecule was quantified on PET using optical waveguide lightmode spectroscopy (OWLS), which revealed higher surface densities for bovine fibrinogen adsorbed from higher concentration solutions. By measuring changes in both the identity and conformation of proteins that adsorb from complex mixtures such as blood or plasma, this technique may have applications in fundamental studies of protein adsorption and may allow for more accurate predictions of the biocompatibility of materials.

Autologous fibrin-coated small-caliber vascular prostheses improve antithrombogenicity by reducing immunologic response

OBJECTIVE

We have recently developed a thrombin-free fibrin-coated vascular prosthesis that has a high performance rate in producing graft antithrombogenicity. We hypothesized that autologous, compared with xenologous, fibrin coatings could improve the antithrombogenicity of grafts by reducing immunologic response.

METHODS

Autologous fibrin-coated vascular prostheses and/or xenologous fibrin-coated vascular prostheses (internal diameter, 2 mm; length, 2.5 cm) were implanted in the bilateral carotid arteries of 50 Japanese white rabbits. They were classified into 2 groups by the selection of grafts in the individual: group I (autologous fibrin-coated vascular prosthesis and xenologous fibrin-coated vascular prosthesis); and group II (group IIa: both autologous fibrin-coated vascular prostheses, or group IIx: both xenologous fibrin-coated vascular prostheses). During a maximum of 180 days after implantation, we evaluated the thrombotic, inflammatory, and immunologic responses associated with both types of graft.

RESULTS

All grafts were patent at each end point. In group I, both platelet deposition and anti-graft antibodies in autologous fibrin-coated vascular prostheses were significantly less than those in xenologous fibrin-coated vascular prostheses until postoperative day 30. At postoperative day 10, there were significantly fewer CD45-positive infiltrating cells in autologous fibrin-coated vascular prostheses, and intercellular adhesion molecule-1, vascular cell adhesion molecule-1, and nuclear factor-kappa B expression in autologous fibrin-coated vascular prostheses were less than those in xenologous fibrin-coated vascular prostheses. The neointimal hyperplasia in autologous fibrin-coated vascular prostheses was significantly decreased at postoperative day 180. In group II, serial changes of serum levels of immunoglobulin M, immunoglobulin G, interleukin-1beta, and tissue-type plasminogen activator/plasminogen activator inhibitor-1 ratio in autologous fibrin-coated vascular prostheses were significantly less than those in xenologous fibrin-coated vascular prostheses. In both grafts, platelet deposition significantly correlated with serum immunoglobulin G level and tissue-type plasminogen activator/plasminogen activator inhibitor-1 ratio.

CONCLUSION

These findings suggest that autologous fibrin coating in thrombin-free fibrin-coated vascular prostheses improve antithrombogenicity by reducing immunologic response and have a potential for clinical use in hybrid small-caliber vascular grafts.

Flow cytometric measurement of phagocytosis reveals a role for C3b in metal particle uptake by phagocytes

A methodology for the quick and efficient study of phagocytosis has been developed. It uses the flow cytometer to exploit the change in size and granularity that occurs in cells upon the ingestion of particulate material. The numbers of cells that have phagocytosed particles can be calculated from the distinct shift in regions that occurs. The method also allows the factors governing phagocytosis to be studied in detail through the use of blocking agents or antibodies. Blood-derived monocytes were studied to investigate the role of complement in metal particle phagocytosis to further understand aseptic loosening. Factor C3b was found to be fundamental to the opsonization and phagocytosis of metal particles by monocytes.

Biomaterial-associated thrombosis: roles of coagulation factors, complement, platelets and leukocytes

Our failure to produce truly non-thrombogenic materials may reflect a failure to fully understand the mechanisms of biomaterial-associated thrombosis. The community has focused on minimizing coagulation or minimizing platelet adhesion and activation. We have infrequently considered the interactions between the two although we are generally familiar with these interactions. However, we have rarely considered in the context of biomaterial-associated thrombosis the other major players in blood: complement and leukocytes. Biomaterials are known agonists of complement and leukocyte activation, but this is frequently studied only in the context of inflammation. For us, thrombosis is a special case of inflammation. Here we summarize current perspectives on all four of these components in thrombosis and with biomaterials and cardiovascular devices. We also briefly highlight a few features of biomaterial-associated thrombosis that are not often considered in the biomaterials literature: The importance of tissue factor and the extrinsic coagulation system. Complement activation as a prelude to platelet activation and its role in thrombosis. The role of leukocytes in thrombin formation. The differing time scales of these contributions.

Management of Aneurysm Sac Hygroma

PURPOSE

To document the management strategies and outcome of patients diagnosed with sac hygroma following open or endovascular abdominal aortic aneurysm (AAA) repair.

METHODS

Seven men (median 68 years, range 43-79) with previous open (n=3) or endovascular (n=4) AAA repairs and increasing aneurysm diameters documented on spiral computed tomography (CT) were diagnosed with sac hygroma based on the lack of a demonstrable endoleak on CT imaging; the presence of a gelatinous, clear fluid in the sac; and a nonpulsatile sac pressure that was about one third of the systemic blood pressure. The patients were followed at regular intervals with spiral CT and percutaneous CT-guided translumbar intrasac pressure measurements. Surgical interventions were performed for sac diameter increase >5 mm or expansion-related pain. Blood samples and fluid aspirated from the sac were analyzed to detect activation of the coagulation and fibrinolytic systems.

RESULTS

Over a median 21.5-month follow-up, open fenestration with resection of the aneurysm wall or open tight wrapping of the wall around the graft in 4 patients did not prevent hygroma reoccurrence, nor did repeated punctures with aspiration of fluid in the other 3 patients. Aneurysm diameters remained unchanged during the observation period.

CONCLUSIONS

Only symptomatic hygromas need be treated, but the treatment of choice remains to be established, since puncture, fenestration, or resection of the sac do not seem to be adequate.

What really is blood compatibility?

The criteria for nonthrombogenicity are classically defined as long clotting times and minimal platelet deposition. The inability to point to unequivocal progress in the development of truly nonthrombogenic materials, highlights the inadequacy if not actually invalidity of these criteria. Our approach is to define nonthrombogenicity in terms of: (1) a thrombin production rate constant, kp < 10(-4) cm s(-1); (2) low platelet consumption and low degree of platelet activation (e.g., microparticle formation); (3) perhaps some platelet spreading; and (4) low complement and leukocyte activation. Only when the target becomes clear, will it be possible to identify clear strategies for producing the materials we need.

Flow cytometric evaluation of material-induced platelet and complement activation

Flow cytometry is used to characterize the activation state of platelets and leukocytes within whole blood after contact for 4 h at 37 degrees C with various materials under conditions of low shear. The contact involved adding heparinized whole blood to small diameter tubes that were connected to two arms extending from a rocking platform. For all surfaces (polyethylene, polypropylene, Silastic, PVA hydrogel) tested there was strong evidence of platelet activation in the bulk blood: platelet-derived microparticles. P-selectin expression and platelet-leukocyte aggregates. Only contact with PVA hydrogel surfaces led to dramatic increases in CD11b up-regulation on monocytes and neutrophils that was inhibited by complement inhibition (sCRI). Flow cytometry was also used to evaluate the effectiveness of various agents to inhibit material-induced complement activation. The assay involved incubating 10 microm polystyrene beads for 1 h with serum at 37 degrees C before isolating the beads so as to label them with a monoclonal antibody against a neoantigen on SC5b-9. The beads were then identified by flow cytometry and the fluorescence associated with their SC5b-9 level recorded. The ability of C1-INH, pentamidine and benzamidine to moderately inhibit SC5b-9 levels suggests a role for classical complement activation in material-induced complement activation.

Specific inhibition of C3 to facilitate general complement inhibition on endotoxin affinity sorbents for apheresis applications

Complement activation, as a result of human blood exposure to biomaterial surfaces, continues to be a concern in medical applications. The purpose of this study was to identify sorbent(s) and surface modification(s) that allow specific removal of endotoxin, while minimizing complement activation. Maleic anhydride (MA) modification of Sepharose CL-4B, cellulose, and Toyopearl HW-65F resulted in reduced generation of C3a, a marker of complement activation, by two orders of magnitude over unmodified surfaces. Surfaces modified with both MA and polymyxin B (PMB), utilized for binding endotoxin, reduced complement activation in a similar manner. Western Blot analysis of the larger C3 cleavage product C3dg showed a similar reduction, for all MA-modified sorbents, as observed for C3a by ELISA. C3alpha43 levels (constituent of iC3b and C3c) were also reduced, although only MA-Sepharose CL-4B levels were similar to C3a. Activation of C5, measured as the SC5b-9 complex, was also reduced by two orders of magnitude after MA modification of Sepharose CL-4B; the decrease was similar to all chemical modifications tested. PMB immobilized via CNBr on MA-modified cellulose maintained its endotoxin-binding capacity, while the latter was eliminated when PMB was immobilized via CNBr to MA-modified Sepharose CL-4B and Toyopearl HW-65F.

Evaluation of a system for the perfusion of isolated, rodent organs

Perfusion of isolated organs is a common experimental approach. However, the surfaces of the perfusion system might alter the components of the blood and thereby negatively affect organ function. The aim of this study was to minimize the influence of the perfusion system on the blood components and to evaluate the system. Pressure and flow in the perfusion system consisting of a roller-pump, reservoir, oxygenator, hemo-filter and bubble-trap with a total tubing length of 4.5 m are controlled by a computer software (DASYLAB, Datalog, Moenchengladbach, Germany) via a transducer connected to the system. The organ to be perfused is positioned under a microscope (Orthoplan, Leica, Bensheim, Germany), allowing the investigation of microcirculatory parameters. The images raised are recorded on video tapes. To evaluate the system it was perfused with human blood (Hct 28 to 30%) for 90 min. Heparin (n = 6) or citrate (n = 6) served as anti-coagulants. The disappearance of cells from the blood was determined at time points 0, 1, 5, 10, 15, 20, 30, 45, 60, 75 and 90 min by means of a cell counter (AC T8, Coulter Beckmann, Krefeld, Germany). Cell activation was assessed by analysis of the expression of L- and P-selectin and CD11b. The activation of the complement system was examined by measuring the serum levels of the complement factors C3c and C4. There was no significant loss or activation of the blood cells at any of the above given time points. The serum levels of the complement factors remained within the physiological range and showed no changes throughout the whole experiments. Thus, the perfusion system does not have a negative influence on the blood and its individual components, and is therefore a reliable tool for perfusion experiments.

Endothelial cell delivery for cardiovascular therapy

Obstructive atherosclerotic vascular disease stands as one of the greatest public health threats in the world. While a number of therapies have been developed to combat vascular disease, endothelial cell delivery has emerged as a distinct therapeutic modality. In this article, we will review the anatomy of the normal blood vessel and the biology of the intact endothelium, focusing upon its centrality in vascular biology and control over the components of the vascular response to injury so as to understand better the motivation for a cell-based form of therapy. Our discussion of cell delivery for cardiovascular therapy will be divided into surgical and interventional approaches. We will briefly recount the development of artificial grafts for surgical vascular bypass before turning our attention towards endothelial cell seeded vascular grafts, in which endothelial cells effectively provide local delivery of endogenous endothelial secretory products to maintain prosthetic integrity after surgical implantation. New techniques in tissue and genetic engineering of vascular grafts and whole blood vessels will be presented. Methods for percutaneous interventions will be examined as well. We will evaluate results of endoluminal endothelial cell seeding for treatment of restenosis and gene therapy approaches to enhance endogenous re-endothelialization. Finally, we will examine some innovations in endothelial cell delivery that may lead to the development of endothelial cell implants as a novel therapy for controlling proliferative vascular arteriopathy.

Early inflammatory response after elective abdominal aortic aneurysm repair: a comparison between endovascular procedure and conventional surgery

OBJECTIVE

To determine the nature of and to compare the inflammatory responses induced by (1) endovascular and (2) conventional abdominal aortic aneurysm (AAA) repair.

MATERIAL AND METHODS

Twelve consecutive patients undergoing elective infrarenal AAA repair were prospectively studied. Seven patients were selected for endovascular procedures (the EAAA group); five patients underwent open surgery (the OAAA group). Three control patients undergoing carotid thromboendarterectomy were also included. Serial peripheral venous blood samples were collected preoperatively, immediately after declamping or placement of the endograft, and at hours 1, 3, 6, 12, 24, 48, and 72. Acute phase response expression of peripheral T lymphocyte and monocyte activation markers and adhesion molecules (flow cytometry), soluble levels of cell adhesion molecules (enzyme-linked immunosorbent assay), cytokine (tumor necrosis factor alpha, interleukin-6, and interleukin-8) release (enzyme-linked immunosorbent assay), and liberation of complement products (nephelometry) were measured.

RESULTS

Regarding acute phase response, the EAAA and OAAA groups showed significant increases in C-reactive protein (P <.001 and P =.001), body temperature (P =.035 and P =.048), and leukocyte count (P <.001 and P <.001). Similar time course patterns were observed with respect to body temperature (P =.372). Statistically significant different patterns were demonstrated for C-reactive protein (P =.032) and leukocyte count (P =.002). Regarding leukocyte activation, a significant upregulation of peripheral T lymphocyte CD38 expression was observed in the OAAA group only (P =.001). Analysis of markers such as CD69, CD40L, CD25, and CD54 revealed no perioperative fluctuations in any group. Regarding circulating cell adhesion molecules, the EAAA and OAAA groups displayed significant increases in soluble intercellular adhesion molecule-1 (P =.003 and P =.001); there was no intergroup difference (P =.193). All groups demonstrated high soluble von Willebrand factor levels (P =.018, P =. 007, and P =.027), there being no differences in the patterns (P =. 772). Otherwise, soluble vascular cell adhesion molecule-1, soluble E-selectin, and soluble P-selectin did not appear to vary in any group. Regarding cytokine release, although a tendency toward high tumor necrosis factor alpha and interleukin-8 levels was noticed in the EAAA group, global time course effects failed to reach statistical significance (P =.543 and P =.080). In contrast, interleukin-6 showed elevations in all groups (P =.058, P <.001, and P =.004). Time course patterns did not differ between the EAAA and OAAA groups (P =.840). Regarding complement activation, the C3d/C3 ratio disclosed significant postoperative elevations in the EAAA and OAAA groups (P =.013 and P =.009). This complement product release was reduced in the EAAA group (P <.001).

CONCLUSIONS

The current study indicated that both endovascular and coventional AAA repair induced significant inflammatory responses. Our findings showed that there were no large differences between the procedures with respect to circulating cell adhesion molecule and cytokine release. Moreover, the endoluminal approach produced a limited response in terms of acute phase reaction, T lymphocyte activation, and complement product liberation. This might support the concept that endovascular AAA repair represents an attractive alternative to open surgery. Given the relatively small sample size, further larger studies are required for confirmation of our observations.

Immunohistochemical analysis of vascular prostheses implanted with the left ventricular assist system

BACKGROUND

Dacron vascular prostheses are associated with thromboembolic complications and inflammatory responses; impregnation with bovine collagen reportedly stimulates additional inflammatory/immunologic complications. The Novacor (Baxter Healthcare Corp., Oakland, CA, USA) left ventricular assist system uses Dacron inflow and collagen-impregnated Dacron outflow prostheses.

METHODS

Explanted inflow and outflow prostheses were evaluated for inflammatory/immunologic, hemostatic, anticoagulant, and fibrinolytic pathways. Non-implanted prostheses immersed in whole blood or plasma were used as controls.

RESULTS

Immunoglobulins and complement components were observed in all prostheses with activated macrophages being present only in implanted prostheses. Antithrombin III was observed in all prostheses whereas fibrin, tissue plasminogen activator, and alpha-2 plasmin inhibitor were present only in implanted prostheses. Endothelial and smooth muscle cells associated with vascular structures containing collagen type IV and laminin were observed solely in implanted prostheses.

CONCLUSION

An inflammatory response occurs and key components of hemostatic, anticoagulant, and fibrinolytic pathways are present within implanted prostheses. These processes are accompanied by endothelial and smooth muscle cell infiltration which appear to lay the foundation for neovessel development.

Fibrinogen adsorption and host tissue responses to plasma functionalized surfaces

The physical and chemical characteristics of material surfaces are thought to play important roles in biomaterial-mediated tissue responses. To understand the importance of discrete biomaterial chemical characteristics in modifying host tissue responses, we constructed surfaces bearing different functional groups using radio frequency glow discharge plasma polymerization. Surfaces evaluated included those having high concentrations of -OH, -NH2, -CF3, and siloxyl groups. These surfaces and polyethylene terephthalate controls were used to assess the importance of particular physicochemical characteristics in surface:protein:cell interactions both in vitro and in vivo. The results obtained show that surface functionalities do significantly affect both the adsorption and "denaturation" of adsorbed fibrinogen (which is an important mediator of inflammatory responses to biomaterial implants). In addition, these surfaces provoke different degrees of acute inflammatory responses. Interestingly, the amounts of "denatured" fibrinogen that spontaneously accumulate on the individual surfaces correlate closely with the extent of biomaterial-mediated inflammation. These results suggest that surfaces that tend to "irreversibly" bind fibrinogen prompt greater acute inflammatory responses. Unexpectedly, all test surfaces except those bearing a siloxyl group engender relatively similar biomaterial-mediated fibrotic responses. Thus surface functionalities alone may not be sufficient to affect subsequent fibrotic responses.

A flow cytometric immunoassay to quantify adsorption of complement activation products (iC3b, C3d, SC5b-9) on artificial surfaces

Crosslinked agarose microspheres and various polystyrene microspheres were analyzed for complement components after incubation with serum at 37 degrees C for times up to 2 h. Quantification involved direct flow cytometric analysis of the beads after the bound complement proteins were indirectly fluorescently tagged by use of a monoclonal antibody against a complement protein: C5b-9, iC3b, C3d, C4d, Bb, C3a, and C1q. Calibration with fluorescein microbead standards demonstrated that the membrane attack complex (SC5b-9) was surface bound on all surfaces and that the surface concentration gradually increased to levels as high as 0.5 micrograms/cm2. Further, the surface bound represented a substantial percentage of the total generated. The iC3b level on polystyrene beads rapidly reached 0.09 micrograms/cm2 and the C3d levels were an order of magnitude less. On agarose beads the iC3b levels continually rose to 0.17 micrograms/cm2 and, as before, the C3d levels were substantially lower. The surface concentration of C4d and Bb on both surfaces were significant but less than 1.0 ng/cm2. There was minimal evidence of C3a and C1q adsorption for any surface. Use of amino-polystyrene beads moderately reduced the level of bound iC3b, C3d, and SC5b-9, whereas carboxylated beads reduced the levels by almost a factor of two. The appreciable amounts of iC3b and SC5b-9 consistently noted on the artificial surfaces tested in this paper suggests that for these two activation products in vitro analysis of material induced complement activation should also include surface analysis.

Acute thrombogenicity of intact and injured natural blood conduits versus synthetic conduits: neutrophil, platelet, and fibrin(ogen) adsorption under various shear-rate conditions

We investigated the acute thrombogenicity of synthetic arterial prostheses compared to biological arterial surfaces in contact with flowing nonanticoagulated blood. The acute events following blood/surface interactions were quantified using 51Cr-platelet deposition, 111In-neutrophil adhesion, and 125I-fibrin(ogen) adsorption on expanded polytetrafluoroethylene (ePTFE) synthetic arterial surfaces (Goretex and Impra) and on intact and injured biological arterial surfaces in ex vivo superfusion flow chambers at low (424/sec) and high (3397/sec) shear rates for 5 min at 37 degrees C. The hematological parameters were determined, and surface analysis was assessed by scanning electron microscopy. At low shear rate, the retention on intact arterial surfaces averaged 3.7 +/- 0.7 x 10(6) platelets/cm2, 26.5 +/- 4.2 x 10(3) neutrophils/cm2, and 10.7 +/- 2.2 cpm of fibrin(ogen)/cm2; retention remained statistically similar at the high shear rate on both Goretex and Impra ePTFE surfaces. In contrast, the deposition of platelets and neutrophils on injured arterial surfaces was significantly higher and increased with shear rate, although the significant increase in fibrin(ogen) adsorption was not influenced by the shear rate. At shear rates characterized by patent and stenosed arteries, ePTFE arterial prostheses demonstrated a low level of thrombogenicity compared to injured arteries. This favorable comparison can be considered as the first requirement for their successful use in arterial substitution.

Metabolic response of granulocytes and platelets to synthetic vascular grafts: preliminary results with an in vitro technique

The metabolism of granulocytes as well as platelets evoked by incubation with different synthetic vascular grafts was monitored during 6-h batch experiments using microcalorimetry. Standard knitted Dacron grafts, ePTFE-grafts, knitted Dacron grafts with collagen impregnation, and knitted Dacron grafts with external collagen-coating were used. The heat production per cell was calculated. A rapid increase of metabolic activity followed by a gradual decrease was demonstrated with both granulocyte suspension and platelet concentrate. Significant differences were obtained between the materials with a maximum response of Dacron grafts with collagen impregnation for both granulocyte and platelet response. The materials had different surface morphologies regarding cell adhesion after incubation as demonstrated with scanning electron microscopy with more pronounced adhesion on the collagen-impregnated grafts. The results suggest that microcalorimetry may be useful for the evaluation of cellular reactions on different biomaterials. However, further studies have to reveal the specificity of the reactions.

Mechanisms of biomaterial-induced superoxide release by neutrophils

Biomaterial-centered infection is an important cause of the failure of prosthetic implants and organs. Because neutrophils mediate host defense against infection, the effect of biomaterials on neutrophil superoxide release and the mechanism of that effect were investigated using three materials commonly employed in surgical practice. The graft materials were expanded polytetrafluorethylene (PTFE), polyurethane and woven dacron. Polystyrene, a commonly used laboratory support vessel, was also studied. Both polystyrene and polyurethane were activating, but serum inhibitable, whereas PTFE was nonactivating, and woven dacron was not activating unless serum was present. The signaling mechanisms used by these materials demonstrated time and material dependency. Pertussis toxin inhibition of G protein-dependent activation had little or no effect on biomaterial induced activation, whereas FMLP-induced activation of the same biomaterial-associated cells was inhibited. Protein kinase C inhibition with staurosporine greatly inhibited polystyrene-induced activation, but had only a partial effect with polyurethane and even less effect with the activation associated with serum-treated woven dacron. These studies demonstrated that biomaterial contact-induced neutrophil activation differed from that described for cells in suspension, and showed that activation mechanisms on one material cannot be extrapolated to mechanisms on other materials.

Surface modification of the biomedical polymer poly(ethylene terephthalate)

X-ray photoelectron spectroscopy was used to characterize modified surfaces of a biomedically important polymer, poly(ethylene terephthalate). Several modification schemes were investigated and direct silanization with 3-aminopropyltriethoxysilane was found to be the optimum procedure, resulting in an aminated surface. Surface coverage of up to 100% was achieved with retention of the polymeric structural integrity. Further activation of the silanized surface was accomplished with two cross-linkers, glutaraldehyde and sebacoyl chloride. A simple biomolecule, L-cysteine, was successfully immobilized onto a surface pre-treated with 3-aminopropyltriethoxysilane and glutaraldehyde, with a coverage of 42%.

The adhesion of labelled neutrophils on synthetic vascular grafts. An experimental porcine study

The adhesion of neutrophils onto different vascular grafts was studied in vivo in a pig model. In acute experiments autologous 111In-labelled neutrophils were reinfused after end-to-side implantation of 5 cm, 6 mm internal diameter grafts. The dynamic deposition on each graft was determined for 300 min in vivo. Static measurements in vitro concluded the study. The adhesion was greater in Dacron and collagen coated Dacron grafts compared to expanded polytetrafluoroethylene (ePTFE) and to Dacron grafts coated with a polymer. The segmental activity along all the grafts increased towards the distal anastomosis. The results suggest different inflammatory response to various graft materials.

TEM immunogold staining of C3 from plasma onto titanium oxides

Immunogold staining in conjunction with TEM was used to observe C3 adsorption from plasma in relation to the underlying titanium structure of thermal, anodic, and electropolished oxides. Heat treatments and oxide thickness were found to have no significant effect on the adsorption behavior of C3, while surface oxide type possibly has. Surface concentration of C3 was found to be time- and plasma concentration-dependent. Evidence is given for the possible involvement of C3 in protein exchange, i.e., the Vroman effect. Diluted plasma resulted in a random distribution of gold colloids, whereas clustering occurred with undiluted plasma. Although C3 concentrations present on grain boundaries followed the same trend as that found on the surface, C3 was found to have a higher grain boundary than bulk concentration for 0.1% plasma.

Improved biocompatibility of silicone rubber by removal of surface entrapped air nuclei

Biomaterials activate the complement system which is important since C3a promotes platelet aggregation and release, and C5a activates neutrophils that may augment coagulation. Tiny air nuclei (microbubbles) are found in the surface roughness of biomaterials on exposure to a liquid, therefore two interfaces exist: (a) a blood/biomaterial, and (b) a blood/air interface. Experiments were carried out that documented that air bubbles activate complement and augment in vitro platelet aggregation in human plasma. The air nuclei were removed from the surface of silicone rubber by a technique termed denucleation to determine if complement activation and platelet aggregation could be reduced. We observed a significant reduction in C3a and C5a in the plasma samples incubated with denucleated silicone rubber as compared to the control samples (p less than 0.001, ANOVA). The plasma incubated with the denucleated silicone caused reduced platelet aggregation as compared to the plasma incubated with the control silicone when added to a platelet suspension (p less than 0.001, ANOVA). Surface chemical analysis by x-ray photo-electron spectroscopy (XPS) showed no change in the silicone rubber surface after the denucleation procedure.

A hydrophilic plasma polymerized film composite with potential application as an interface for biomaterials

A hydrophilic polymer composite film (approx. 420 nm thick), with potential application as an interface for biomaterials has been prepared on nonorganic substrates, which include glass, silicon, and aluminum foil, using a glow discharge plasma polymerization technique. A thin film (110 nm thick) polymerized from hexane provided an adherent protective coating for the substrate material, and covalent bonding sites for the outer layer polymerized from N-vinyl-2-pyrrolidone. This outer layer provided the hydrophilic surface or interface. The two layers were copolymerized for a short period during transition between monomers to provide an intimate covalently bonded diffuse interphase. Preliminary in vitro and in vivo biocompatibility studies indicate that the hydrophilic film is non-cytotoxic, and does not increase the inflammatory response when compared with negative controls.

In vivo thrombus formation induced by complement activation on polymer surfaces

To clarify involvement of complement activation in thrombus formation on polymer surfaces, in vitro complement activation was evaluated for polyethylene (PE) tubes radiation-graft copolymerized with acrylamide (AAm), acrylic acid (AC), 2-hydroxyethyl methacrylate (HEMA), N-vinylpyrrolidone (NVP), and vinyl alcohol (VOH), and compared to their in vivo antithrombogenicity and cell adherence in canine peripheral veins. The complement-activating surfaces (NVP and VOH) cause preferential adhesion of leukocytes and were more thrombogenic than the low complement-activating surfaces (AAm, PE, and HEMA). Infusion of naja haje cobra venom factor depressed leukocyte adhesion, followed by a marked decrease in thrombogenesis, for the strong classical-pathway-activating surface (NVP). Although estimation of in vitro activation for AC was inconclusive because of a large effect of adsorption, AC behaved like VOH in vivo. These results suggest that C5a(des Arg) mediated activation of leukocytes may play a role in thrombus formation by complement activation on polymer surfaces.

A micromechanical technique for monitoring cell-substrate adhesiveness: measurements of the strength of red blood cell adhesion to glass and polymer test surfaces

We report a novel method for rapid comparison of the relative strength of adhesion of cells to different solid surfaces. A vertically oscillating micropipette is brought above an individual cell in such a manner that it makes contact with the cell at the lower limit of its travel. The pressure within the micropipette is gradually reduced until the cell attaches to the micropipette by suction and is lifted from the solid surface. The reduction in pressure required to detach a cell depends on the specific cell/substrate combination and serves as a relative measure of the strength of cell adhesion. A particular advantage of this approach over conventional methods is the ability to select particular cells from a population. As a test of the reproducibility of the method and its ability to distinguish the strength of adhesion of cells to different solid surfaces, we have used it to measure the adhesiveness of human red blood cells to hydrophilic glass, tissue culture grade polystyrene, polyethylene terephthalate, and polymethyl methacrylate. We find that results for the same surface are highly reproducible and that the method is capable of distinguishing small differences in the adhesiveness of red blood cells to the above surfaces.

Effect of albumin coating on the in vitro blood compatibility of Dacron arterial prostheses

A recirculating in vitro perfusion system was used to assess the effect of albumin precoating on the thrombogenicity of Dacron vascular grafts. A complete analysis of platelet activation was carried out, involving platelet count, release, adhesion and aggregation. Fibrin formation was assessed by measuring fibrinogen levels and fibrinopeptide A production; leucocyte interaction was analysed by measuring total leucocyte count as well as an analysis of cell adhesion to the surface by scanning electron microscopy. The platelet count decreased progressively with perfusion time for Dacron until by 30 min, it had declined to 69% +/- 2% of baseline. The platelet count did not, however, change significantly from baseline when albumin-coated Dacron was tested. Release of platelet factor 4 and beta-thromboglobulin at 180 min for Dacron was 37.8 +/- 29.8 times and 66.9 +/- 18.2 times baseline, respectively, while albumin coating caused significantly less (P less than 0.03) platelet release. Albumin coating diminished coagulation activation and fibrinopeptide A formation. The total leucocyte concentration decreased significantly for Dacron by 180 min, while that for albumin-coated Dacron did not change significantly from baseline levels. Albumin coating produced a film-like covering over the Dacron. For Dacron, there were numerous leucocytes and platelets adherent to the surface, whilst cellular deposition was minimal upon the albumin-coated surface. Thus, albumin coating improved the short-term blood compatibility of Dacron by all of the methods employed in this study.

Human monocyte/macrophage activation and interleukin 1 generation by biomedical polymers

In vitro cell culture techniques were used to evaluate the effect of several clinically significant biomedical polymers on monocyte activation and Interleukin 1 (IL1) production. Isolated human peripheral blood monocytes were cultured in the presence of a panel of five biomedical polymers routinely used in a variety of clinical applications: Polyethylene (PE), silica-free poly-dimethylsiloxane (PDMS), woven Dacron fabric, expanded polytetrafluoroethylene (ePTFE) and the segmented polyurethane, Biomer. Monocytes generated IL1 in the presence of all five materials. Maximal levels of IL1 were generated at 24 h in monocyte-polymer cultures supplemented with serum and additionally stimulated with lipopolysaccharide (LPS). No difference was observed due to serum source. Results from cultures supplemented with fetal bovine serum were not significantly different from those obtained with human serum supplemented cultures. The thymocyte proliferative activity generated by monocytes in the presence of these biomedical polymers was neutralized by a specific polyclonal anti-IL1 antiserum. Statistically significant differences in IL1 production were observed between polymers, allowing their classification according to reactivity into high (Dacron, PE), intermediate (ePTFE) and low (Biomer, PDMS) reactive groups.