Fibronectin coating of collagen modules increases in vivo HUVEC survival and vessel formation in SCID mice.

Modular tissue engineering is a novel approach to creating scalable, self-assembling, three-dimensional tissue constructs with inherent vascularization. Under initial methods, the subcutaneous implantation of human umbilical vein endothelial cell (HUVEC)-covered collagen modules in immunocompromised mice resulted in significant host inflammation and limited HUVEC survival. A minimally invasive injection technique was used to minimize surgery-related inflammation, and cell death was attributed to extensive apoptosis within 72 h of implantation. Coating collagen modules with fibronectin (Fn) was shown in vivo to reduce short-term HUVEC TUNEL staining by nearly 40%, while increasing long-term HUVEC survival by 30-45%, relative to collagen modules without fibronectin. Consequently, a ∼100% increase in the number of HUVEC-lined vessels was observed with Fn-coated modules, as compared to collagen-only modules, at 7 and 14 days post-implantation. Furthermore, vessels appeared to be perfused with host erythrocytes by day 7, and vessel maturation and stabilization was evident by day 14.

Complement inhibition reduces material-induced leukocyte activation with PEG modified polystyrene beads (Tentagel) but not polystyrene beads.

With isolated leukocytes, inhibiting complement reduced material-induced leukocyte activation (CD11b) with polyethylene glycol modified polystyrene beads (PS-PEG), but not with polystyrene beads (PS). The PS-PEG beads (TentaGel) were complement activating as measured by SC5b-9 levels consistent with the sensitivity of these beads to leukocyte inhibition with complement inhibitors. Following contact with PS and PS-PEG beads, isolated leukocytes in plasma and in the absence in platelets were found to significantly upregulate CD11b, while TF expression and exposure of phosphatidylserine remained at background levels. Complement inhibition by means of sCR1 partially reduced CD11b upregulation on PS-PEG beads, but had no effect with PS beads. Pyridoxal-5-phosphate (P5P) was able to significantly reduce both CD11b upregulation and exposure of phosphatidylserine with PS-PEG beads, although it did not appear to inhibit SC5b-9 production. Pentamidine and NAAGA inhibited complement and were effective in reducing CD11b upregulation with both PS and PS-PEG. However, they also had an inhibitory effect on leukocyte signaling mechanisms, precluding their utility for further study in this context. Leukocyte adhesion occurred to similar extents on both PS and PS-PEG beads. While sCR1 and P5P blocked adhesion and activation (for adherent leukocytes) on PS-PEG beads, they had no effect on leukocytes adherent to PS beads. The role of complement in leukocyte activation and adhesion was found to be material-dependent. Thus, leukocyte-material compatibility may be resolved by complement inhibition in some but not all cases. For these other materials (example here was PS), other mechanisms, such as fibrinogen adsorption and direct leukocyte release, may need exploitation to minimize leukocyte activation and adhesion.

Material-induced tissue factor expression but not CD11b upregulation depends on the presence of platelets.

Biomaterials activate leukocytes as well as platelets when exposed to blood. One feature of leukocyte activation at least at times beyond a few hours is tissue factor expression, contributing to a procoagulant state. We show here that platelet activation and specifically platelet-monocyte aggregate formation appears to be a precondition for tissue factor expression. Material-induced Tissue Factor (TF) expression by isolated leukocytes (6 x 10(6) cells/mL) resuspended in increasing concentrations of platelets in plasma was elevated when the platelet concentration was 50 x 10(6) platelets/mL or more; at lower platelet concentrations (1-25 x 10(6). cells/mL) the TF expression remained at background levels. On the other hand, significant CD11b upregulation was observed on leukocytes, in bulk and adherent to beads, at all platelet concentrations. This platelet effect on material-induced TF expression appeared to be mediated by the formation of platelet-monocyte aggregates. Anti-P-selectin, which blocked the association between platelets and leukocytes, reduced monocyte adhesion and material-induced TF expression for bulk monocytes. Anti-GPIIb/IIIa, a GPIIb/IIIa platelet antagonist, also reduced monocyte adhesion and material-induced TF expression in the bulk, most likely due to its inhibiting effect on the formation of platelet-monocyte aggregates, secondary to platelet activation. However, the antibody-associated reductions for bulk leukocytes (mainly neutrophils) were small and incomplete. Similar levels of TF expression, in the bulk, were observed with both polystyrene (PS), a strong platelet activator, and polyethylene glycol-modified PEG (PS-PEG), a mild platelet activator. The role of platelets in material-induced TF expression appears to be mediated in part via the formation of platelet-monocyte aggregates, although other mechanisms are likely also involved.

Viability of hydroxyethyl methacrylate-methyl methacrylate-microencapsulated PC12 cells after omental pouch implantation within agarose gels.

Hydroxyethyl methacrylate-methyl methacrylate (HEMA-MMA, 75 mol% HEMA). Microcapsules containing viable PC12 cells (as an allogeneic transplant model) were implanted into omental pouches in Wistar rats. Two different capsule preparations were tested, based on differences in polymer solutions during extrusion: 10% HEMA-MMA in TEG, and 9% HEMA-MMA in TEG with 30% poly(vinyl pyrrolidone) (PVP). The omental pouch proved to be an ideal transplant site in terms of implantation, recovery, and blood vessel proximity (nutrient supply). To minimize the fibrous overgrowth and damaged capsules previously seen on implantation of individual capsules, agarose gels were used to embed the capsules before implantation. Cells proliferated within the microcapsule-agarose device during the first 7 days of implantation, but overall cell viability declined over the 3-week period, when compared with similar capsules maintained in vitro. Nonetheless, approximately 50% of the initial encapsulated cells were still viable after 3 weeks in vivo. This approach to HEMA-MMA microcapsule implantation improved cell viability and capsule integrity after 3 weeks in vivo, compared with capsules implanted without agarose.

Tissue engineering as a platform for controlled release of therapeutic agents: implantation of microencapsulated dopamine producing cells in the brains of rats.

Tissue engineering can lead to novel controlled release devices and controlled release strategies (e.g., of growth factors) can enhance the performance of tissue engineered constructs. There are however a number of technical challenges that must be overcome before these goals can be realized. The apparently 'simple' challenge of implanting the device (e.g., capsules) in the optimal site must be met. In addition, adequate nutrient supply to the capsules is required to maintain cell viability. To illustrate this problem we describe a guide and delivery cannula technique to provide reliable and reproducible delivery of up to 120 PC12 cell containing capsules into the caudate putamen (CPu). Microencapsulation of mammalian cells is potentially a powerful means of delivering therapeutically important molecules such as insulin. It can also have numerous applications as a platform for gene therapy. However, realizing this potential has been more difficult than first anticipated.

Leukocyte activation and leukocyte procoagulant activities after blood contact with polystyrene and polyethylene glycol-immobilized polystyrene beads.

Beads (45 microm) of polystyrene (PS) and polyethylene glycol modified PS (TentaGel) with an amino or hydroxyl terminal group were incubated with blood to assess the effect of surface area and material chemistry on leukocyte activation. After a 2-hour incubation, blood contact with beads activated leukocytes in the bulk (tissue factor expression, CD11b up-regulation, and association with platelets) independently of material surface chemistry. On the other hand, activation of adherent leukocytes was material dependent. After blood contact with PS, polyethylene glycol-immobilized PS (PS-PEG) and PS-PEG-NH2 beads, CD11b up-regulation in the bulk, platelet-leukocyte aggregates, and leukocyte adhesion were all dependent on surface area, whereas tissue factor (TF) expression was not. Material-induced leukocyte activation in the bulk was also independent of the beads' capacity to activate platelets. However, monocyte adhesion and TF expression on beads appeared to be related to the presence of platelets on the surface. Material-induced TF expression was able to initiate the extrinsic pathway of coagulation, resulting in significant fibrin formation. Although not all of our markers of leukocyte activation varied with material area or chemistry, it was clear that these materials activated leukocytes in a way that resulted in increased procoagulant activity. During blood-material interactions, material-induced leukocyte activation may then contribute to thrombogenesis.

Does surface chemistry affect thrombogenicity of surface modified polymers?

With some exceptions, surface chemistry had little effect on platelet and leukocyte activation, and cell deposition, by scanning electron microscopy after blood exposure and clotting times among a group of 12 unmodified and plasma modified tubings. All materials activated platelets and leukocytes to detectable levels, although some materials increased the value of one activation parameter but not another. Unmodified materials [polyethylene (PE), Pellethane (PEU), latex, nylon, and Silastic] and modified materials (H(2)O plasma treated PE and PEU, CF(4) plasma treated PE, fluorinated PEU, NH(4) plasma treated PEU, polyethylene imine treated PEU, and heparin treated PEU) were characterised by XPS and contact angle. The objective of this project was to define a series of assays for the evaluation of hemocompatibility of cardiovascular devices with a view to clarify the specific requirements of ISO-10993-4, and to define an appropriate screening program for new blood contacting biomaterials. PE, PE--CF(4), PE--H(2)0, PEU--F, latex, and PEU-heparin were the exceptions to the general observations, although each behaved differently. PE proved to be least reactive, whereas PE-CF(4) was most reactive by several assays. Platelet microparticle formation (determined by flow cytometry), PTT, postblood exposure SEM, total SC5b-9, C3a, and platelet and leukocyte loss (cell counts) were able to distinguish differences among these materials, and often, but not always, showed expected correlations.

Complement activation by PVA as measured by ELIFA (enzyme-linked immunoflow assay) for SC5b-9.

Polyvinyl alcohol (PVA) coated onto polyethylene (PE) tubes exposed to human serum for 1 hour at 37 degrees C resulted in the production of 1.03 +/- 0.04 microg/cm2 of the soluble form of the terminal membrane attack complex, SC5b-9. This was approximately 20 x that produced by the polyethylene. About one quarter of this total was found associated with the surface of PVA. SC5b-9 concentrations were determined by enzyme-linked immunoflow assay (ELIFA) a variant on ELISA that involved drawing the test sample, the antibodies and the chromogenic reagent through a nitrocellulose membrane filter. ELIFA enabled analysis of protein concentrations in the presence of SDS, so that SDS (0.05%) was used to desorb adsorbed SC5b-9 prior to analysis together with SC5b-9 in the bulk to get a more complete picture of PVA-associated complement activation.

Microencapsulation of normal and transfected L929 fibroblasts in a HEMA-MMA copolymer.

Mouse L929 fibroblasts transfected to express a secreted form of human alkaline phosphatase (SEAP) were encapsulated in approximately 400-microm poly(hydroxyethyl methacrylate-co-methyl methacrylate) (HEMA-MMA) microcapsules as a baseline for the use of genetically engineered cells in encapsulation therapy. Although incubation of microcapsules with serum-containing medium resulted in maintaining the number of live encapsulated cells with the passage of time, incubation in a serum-free medium resulted in a three-fold proliferation of the encapsulated cells within a 3-week observation period. Similar to the results for incubation with serum-containing medium, co-encapsulation with a bovine dermal type I collagen, i.e., the inclusion of a matrix in the core of the capsules, resulted in maintenance of the initial number of live cells with the passage of time. SEAP measurements indicated that the transfected cells not only continued to express the transgene product after encapsulation, but also adapted to the capsule microenvironment to secrete SEAP at progressively larger amounts with the passage of time. However, SEAP expression only occurred when the transfected cells (encapsulated or non-encapsulated) were cultivated in serum-containing medium.

Viability of HEMA-MMA microencapsulated model hepatoma cells in rats and the host response.

Small diameter hydroxyethyl methacrylate-methyl methacrylate (HEMA-MMA; 75% HEMA) microcapsules containing an aggregate of viable rat hepatoma H4IIEC3 cells, after implantation into an omental pouch in Wistar rats, contained viable cells at 7 days but not 14 days. A similar transplantation of microencapsulated aggregates of human hepatoma HepG2 cells did not result in viable cells even at 7 days. The loss of viability was attributed to the tissue reaction, because both encapsulated cell types remained viable in vitro. However, it is not clear if the cells lost their viability in vivo, leading to the aggressive tissue reaction or if the latter caused the cells to starve or otherwise die. The tissue reactions to microcapsules containing rat or human hepatoma cells at day 1 was one cell layer thick and avascular. At later times, tissue reactions were comprised of three regions: macrophages, fibroblasts, and some foreign body giant cells apposed to the polymer membrane, a dense region of fibroblasts and collagen, and a region of vascularized granulation tissue. Prompt vascularization of the tissue reactions occurred after 4 days and was maintained for up to 14 days. Even at 14 days, immune cells were observed, suggesting a continued immune response toward antigens shed from the encapsulated cells.

Hydroxyethyl methacrylate-methyl methacrylate (HEMA-MMA) copolymers for cell microencapsulation: effect of HEMA purity.

Thermoplastic copolymers of 2-hydroxyethyl methacrylate (HEMA) and methyl methacrylate (MMA) (molar ratio: 75/25 HEMA-MMA) were synthesized using HEMA containing different amounts of ethylene glycol dimethacrylate (EGDMA) to investigate their suitability for cell microencapsulation. Pure HEMA (0.0% EGDMA) was obtained with preparative chromatography to prepare a linear copolymer. Microcapsules (with a diameter of 300-400 microm) were readily made with the copolymers by interfacial precipitation. Smaller and more transparent capsules were obtained using the copolymer prepared from purer HEMA. Chinese hamster ovary (CHO) fibroblasts, as model cells, were microencapsulated in the linear copolymer. The CHO cells survived the microencapsulation process and the metabolic activity of the encapsulated cells increased within the 14 days observation period.

Effect of an immobilization matrix and capsule membrane permeability on the viability of encapsulated HEK cells.

The effect of inclusion of an immobilization matrix and the capsule membrane permeability on the viability, metabolic activity, and proliferation of encapsulated HEK cells was investigated in vitro. In the absence of a matrix, a particular transfected HEK cell line formed a single aggregate in the core of the poly(hydroxyethyl methacrylate-co-methyl methacrylate) (HEMA-MMA) capsule, and the number of live cells decreased significantly with the passage of time. In contrast, co-encapsulation with a 1% (w/v) ultralow gelling temperature agarose matrix promoted the proliferation of the encapsulated cells. The initial number of approximately 200 live cells/capsule doubled 14 d after encapsulation and reached a plateau of approximately 500 live cells/capsule 28 d after encapsulation. The agarose matrix provided uniform distribution of the cells within the capsule core giving rise to multiple aggregates upon proliferation. Reduction of the polymer solution concentration, and hence the increase of the permeability of the capsule membrane, did not have an effect on the extent or rate of proliferation of cells co-encapsulated with agarose, and did not improve the viability of cells that were encapsulated without a matrix. These cells (transfected with the cDNA for human hepatic lipase) served as a model as part of a program evaluating the use of encapsulated cells for gene therapy.

Making microencapsulation work: conformal coating, immobilization gels and in vivo performance.

Microencapsulation of cells as a means of insulin or other protein delivery (for example, for gene therapy) has not yet realized its potential. Three aspects of this problem are illustrated with reference to the use of poly(hydroxyethyl methacrylate-co-methyl methacrylate) (HEMA-MMA). Conformal coating was used to coat cell aggregates with a very thin layer of a water-insoluble HEMA-MMA membrane that conforms to the shape of the aggregate, and minimizes the polymer's contribution to the total transplant volume. Cell aggregates were coated at a liquid-liquid interface of a discontinuous density gradient composed of both aqueous and organic liquids. Aggregates of HepG2 cells were coated and remained viable. Immobilization matrices were co-encapsulated in order to control cell phenotype. Ultralow gelling temperature agarose promoted the proliferation of HEK293 cells, while the viability of transfected C2C12 cells was improved in microcapsules that contained Matrigel. Rat or human hepatoma cells in HEMA-MMA microcapsules lost viability within a week after implantation into an omental pouch in Wistar rats. The loss of viability was attributed to the tissue reaction, although it is not clear if the cells lost their viability in vivo leading to the aggressive tissue reaction or if the latter caused the cells to starve or otherwise die. On the other hand, intraperitoneal implantation of microcapsules containing L929 cells in 'syngeneic' C3H mice in a high-strength agarose gel resulted in maintenance of viability of approximately 50% of the encapsulated cells. While progress is being made on several fronts, this type of tissue engineering construct is still several years away from routine use in humans.

Agarose enhances the viability of intraperitoneally implanted microencapsulated L929 fibroblasts.

To achieve immunoisolation, mouse L929 fibroblasts were encapsulated in approximately 400 microm poly(hydroxyethyl methacrylate-co-methyl methacrylate) (HEMA-MMA) microcapsules and were subsequently implanted in the peritoneal cavity of syngeneic C3H mice. As a baseline for the use of genetically engineered cells in cell encapsulation therapy, the L929 cells were transfected to express a secreted form of human alkaline phosphatase (SEAP). Implantation of empty microcapsules in a PBS suspension resulted in deformation, aggregation, and poor retrievability of the microcapsules. Incubation of microcapsules with medium containing xenogeneic horse serum prior to implantation increased the thickness of the fibrous tissue surrounding the microcapsules. However, immobilization of the microcapsules in a 4% (w/v) SeaPlaque agarose gel prior to implantation allowed complete recovery of the microcapsules and prevented their aggregation and deformation. As a result, approximately 50% of the encapsulated cells remained viable 21 days postimplantation. Moreover, once the viable cells were released from retrieved microcapsules and regrown as monolayers, they expressed SEAP at a level similar to their encapsulated but nonimplanted counterparts.

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.

Conformal coating of small particles and cell aggregates at a liquid-liquid interface.

Polymer encapsulation of allogeneic or xenogeneic tissue is under active investigation as a means of isolating transplanted cells, such as pancreatic islets, from the immune system. We report here a method for coating small particles and cell aggregates with a very thin water insoluble hydroxyethyl methacrylate-methyl methacrylate (HEMA-MMA) membrane that conforms to the shape of the aggregate, and minimizes the polymer's contribution to the total transplant volume. Cell aggregates were coated at a liquid-liquid interface of a discontinuous density gradient composed of both aqueous and organic liquids. By increasing the viscosity difference and decreasing the density difference between the two liquids of the coating interface, coatings from approximately 1 to 15 microns thick were formed. Aggregates of HepG2 cells and pancreatic islets were coated and remained viable.

Tumor necrosis factor (TNFalpha) production by rat peritoneal macrophages is not polyacrylate surface-chemistry dependent.

Polyacrylate films in the absence of added endotoxin caused rat peritoneal macrophages to secrete a small amount of TNFalpha. There was little difference, if any, among the materials, which included various co- or ter-polymers of hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, methacrylic acid, methyl methacrylate, and butyl methacrylate. The materials were surface characterized and endotoxin cleaned prior to testing. Equivalent endotoxin levels associated with the material were <0.03 EU/mL for all materials but two; for polyHEMA, the most contaminated material, it was 0.23 EU/mL. Films of the materials were incubated with freshly isolated rat peritoneal macrophages for 6 to 24 h before the TNFalpha levels in the supernatant were analyzed for biological activity, using L929 cells as a target. When endotoxin was added, far greater quantities of TNFalpha were generated at 24 h compared to 6 h, but still there was little effect with regard to material chemistry. Such an in vitro assay proved not to be useful for the screening of potential microencapsulation materials for peritoneal biocompatibility.

Flow cytometric study of in vitro neutrophil activation by biomaterials.

Neutrophil activation for adherent and nonadherent cells, as measured by flow cytometry, was not strongly dependent on material surface chemistry. We had hypothesized that material-induced neutrophil activation was an important parameter associated with material failure. All materials tested [cellophane, an acrylonitrile copolymer (AN69), Pellethane, nylon, polyethylene terephthalate, low density polyethylene, and polydimethylsiloxane] activated isolated human neutrophils, which were resuspended in plasma or serum, to similar extents based on L-selectin shedding, CD11b upregulation, and stimulation of the oxidative burst after 30-min exposure. Inhibition of complement activation by sCR1 unexpectedly had little effect if any on nonadherent neutrophils. However, neutrophil adhesion, but not the level of activation of the adherent cells, was strongly dependent on complement activation. Pretreatment with albumin did not inhibit adhesion or reduce neutrophil activation, but plasma pretreatment resulted in increased activation for nonadherent and adherent cells. More adhesion and a higher level of activation of adherent cells was observed following pretreatment with fibrinogen, a ligand of CD11b. Taken together these results suggest that upon contact with a material, neutrophil activation may occur though mechanisms that are not mediated by complement. For example, the presence of plasma proteins such as fibrinogen at the interface may trigger activation and the release of other activating agents. Although the material differences are small, the extent of activation may be significant and warrant further study of the mechanism and consequences of that activation.

Characterization of transient platelet contacts on a polyvinyl alcohol hydrogel by video microscopy.

Acridine orange labelled, washed human platelets were counted and tracked on polyvinyl alcohol (PVA), heparin-PVA and polyethylene (PE)-coated coverslips with a view to understand why transient contact on the PVA hydrogels lead to elevated platelet activation and consumption relative to polyethylene. Over the 4 min of initial contact that was studied, platelet adhesion was higher on PE than on PVA or heparin-PVA at both 40 and 200 s(-1), as expected, regardless of whether the surfaces were pre-treated with albumin or fibrinogen. Not all platelets appearing to make contact with the surface, actually attached. For example, less than 2% of the platelets contacting albumin pre-treated PVA (at 40 s(-1)) remained adherent at the end of the initial 60 s observation time, while the corresponding number for PE was greater than 9%. A greater fraction of the platelets remained adherent at the higher shear rate or with fibrinogen pre-treatment, but the difference between PVA and PE remained similar: for example, with fibrinogen pre-treatment at 200 s(-1), approximately 25% of the platelet contacts resulted in adhesion on PVA while 66% did so on PE. While net platelet adhesion was less for the hydrogels, than for PE, the total number of contacts (adherents + non-adherents) were more comparable and unexpectedly higher for albumin pre-treatment than for fibrinogen. Net platelet adhesion is but one component of the total platelet interaction with a material surface. Fluorescent video microscopy has been shown to be a useful, albeit not unequivocal, method for assessing the platelets that make contact with but do not adhere to a surface. reserved