Transplantation of encapsulated cells and tissues

Maintenance of Neovascularization at the Implantation Site of an Artificial Device by bFGF and Endothelial Cell Transplant

Development of a subcutaneously implantable bioartificial pancreas (BAP) with immunoisolatory function could have a great impact on the treatment of diabetes mellitus. We have developed an implantable BAP device with an ethylene vinyl alcohol (EVAL) membrane. In the present study, we used basic fibroblast growth factors (bFGF), which was incorporated in a carrier for sustained release, in order to induce neovascularization when the device was implanted subcutaneously. To maintain the vasculature thus formed, a cell infusion port was attached to the BAP device, through which the device was filled with human liver vascular endothelial cell line TMNK-1, and the vasculature could be adequately maintained. Mice were divided into the following three groups. In group 1, a bFGF-free BAP device was implanted subcutaneously. In group 2, a sustained-release bFGF-impregnated BAP device was implanted. In group 3, a sustained-release bFGF-impregnated BAP device was implanted, and 3 × 106 TMNK-1 cells were infused into the implanted device every week. Neovascularization induced in the subcutaneous tissue around the implanted BAP device was macroscopically examined and histologically evaluated. In addition, the tissue blood flow was measured using a laser blood flow meter. In mice in group 3, neovascularization was significantly induced and maintained until week 8 postimplantation. It was confirmed by scanning electron microscopy that infused TMNK-1 cells adhered to the inner polyethylene surface of the device. It was demonstrated that the use of bFGF and vascular endothelial TMNK-1 cells induced and maintained adequate vasculature and tissue blood flow surrounding the implantable bag-type BAP device. We believe that the present study will contribute to BAP development for the treatment of diabetes.

Core/Shell Macrobeads for the Protection of Islets from Immune System Rejection

Core/shell macrobeads, composed of a hydrogel core and a biodegradable polymeric shell, were designed and evaluated for the protection of islets from both fibrous tissue development as well as the modulation of the immune system rejection response. The core/shell macrobeads (2,000μm diameter) were prepared with approximately 150μm thick polymeric shells. Islets isolated from pancreas of C57/BL6 mice were encapsulated in these core/shell macrobeads. The islets-loaded core/shell macrobeads were planted in the peritoneal cavity of diabetes mellitus-induced C57/BL6 mice and the blood glucose concentrations were measured.

Cytokine loaded layer-by-layer ultrathin matrices to deliver single dermal papilla cells for spot-by-spot hair follicle regeneration

Polymer nanocoated dermal papilla cells promoting hair regeneration.

A three-dimensional microfluidic approach to scaling up microencapsulation of cells

Current applications of the microencapsulation technique include the use of encapsulated islet cells to treat Type 1 diabetes, and encapsulated hepatocytes for providing temporary but adequate metabolic support to allow spontaneous liver regeneration, or as a bridge to liver transplantation for patients with chronic liver disease. Also, microcapsules can be used for controlled delivery of therapeutic drugs. The two most widely used devices for microencapsulation are the air-syringe pump droplet generator and the electrostatic bead generator, each of which is fitted with a single needle through which droplets of cells suspended in alginate solution are produced and cross-linked into microbeads. A major drawback in the design of these instruments is that they are incapable of producing sufficient numbers of microcapsules in a short-time period to permit mass production of encapsulated and viable cells for transplantation in large animals and humans. We present in this paper a microfluidic approach to scaling up cell and protein encapsulations. The microfluidic chip consists of a 3D air supply and multi-nozzle outlet for microcapsule generation. It has one alginate inlet and one compressed air intlet. The outlet has 8 nozzles, each having 380 micrometers inner diameter, which produce hydrogel microspheres ranging from 500 to 700 μm in diameter. These nozzles are concentrically surrounded by air nozzles with 2 mm inner diameter. There are two tubes connected at the top to allow the air to escape as the alginate solution fills up the chamber. A variable flow pump 115 V is used to pump alginate solution and Tygon® tubing is used to connect in-house air supply to the air channel and peristaltic/syringe pump to the alginate chamber. A pressure regulator is used to control the flow rate of air. We have encapsulated islets and proteins with this high throughput device, which is expected to improve product quality control in microencapsulation of cells, and hence the outcome of their transplantation.

The reconstruction of lung alveolus-like structure in collagen-matrigel/microcapsules scaffolds in vitro

This study attempted to use collagen–Matrigel as extracellular matrix (ECM) to supply cells with three-dimensional (3D) culture condition and employ alginate-poly-l-lysine-alginate (APA) microcapsules to control the formation of alveolus-like structure in vitro. We tested mice foetal pulmonary cells (FPCs) by immunohistochemistry after 2D culture. The alveolus-like structure was reconstructed by seeding FPCs in collagen–Matrigel mixed with APA microcapsules 1.5 ml. A self-made mould was used to keep the structure from contraction. Meanwhile, it provided static stretch to the structure. After 7, 14 and 21 days of culture, the alveolus-like structure was analysed histologically and immunohistochemically, or by scanning transmission electron microscopy (TEM). We also observed these structures under inverted phase contrast microscope. The expression of pro-surfactant protein C (SpC) was detected by reverse transcription-polymerase chain reaction (RT-PCR). We obtained fibroblasts, epithelial cells and alveolar type II (AE2) cells in FPCs. In the reconstructed structure, seeding cells surrounding the APA microcapsules constructed alveolus-like structures, the size of them ranges from 200 to 300 μm. In each reconstructed lung tissue sheet, microcapsules had integrity. Pan-cytokeratin, vimentin and SpC positive cells were observed in 7- and 14-day cultured structures. TEM showed lamellar bodies of AE2 cells in the reconstructed tissues whereas RT-PCR expressed SpC gene. Primary mice FPCs could form alveolus-like structures in collagen–Matrigel/APA microcapsules engineered scaffolds, which could maintain a differentiated state of AE2 cells.

Microencapsulation of islets within alginate/poly(ethylene glycol) gels cross-linked via Staudinger ligation

Functionalized alginate and poly(ethylene glycol) (PEG) polymers were used to generate covalently linked alginate-PEG (XAlgPEG) microbeads of high stability. The cell-compatible Staudinger ligation scheme was used to cross-link phosphine-terminated PEG chemoselectively to azide-functionalized alginate, resulting in XAlgPEG hydrogels. XAlgPEG microbeads were formed by co-incubation of the two polymers, followed by ionic cross-linking of the alginate using barium ions. The enhanced stability and gel properties of the resulting XAlgPEG microbeads, as well as the compatibility of these polymers for the encapsulation of islets and beta cells lines, were investigated. The data show that XAlgPEG microbeads exhibit superior resistance to osmotic swelling compared with traditional barium cross-linked alginate (Ba-Alg) beads, with a five-fold reduction in observed swelling, as well as resistance to dissolution via chelation solution. Diffusion and porosity studies found XAlgPEG beads to exhibit properties comparable with standard Ba-Alg. XAlgPEG microbeads were found to be highly cell compatible with insulinoma cell lines, as well as rat and human pancreatic islets, where the viability and functional assessment of cells within XAlgPEG are comparable with Ba-Alg controls. The remarkable improved stability, as well as demonstrated cellular compatibility, of XAlgPEG hydrogels makes them an appealing option for a wide variety of tissue engineering applications.

Nanoencapsulating living biological cells using electrostatic layer-by-layer self-assembly: platelets as a model

In the literature, a few biological cells have been used as templates to form microcapsules of a variety of shapes and sizes. In this study, we proved the concept that living cells like platelets can be encapsulated with polyelectrolytes using electrostatic layer-by-layer self-assembly (LBL), and, most importantly, the encapsulation process did not induce activation of the platelets. Glycol-chitosan and poly-L-glutamic acid were electrostatically deposited onto platelets, and the encapsulation was confirmed using confocal laser scanning microscopy and scanning electron microscopy. Transmission electron microscopy observation further confirmed that the encapsulation process was mild and the activation of platelets was negligible. The encapsulation of living biological cells like platelets can serve as a model system in a wide range of biomedical applications including local and sustained drug delivery, immune protection of artificial tissues, and versatile artificial blood.

Transplantation of microencapsulated cells expressing VEGF improves angiogenesis in implanted xenogeneic acellular dermis on wound

BACKGROUND

Cell-based gene therapy using cells that express angiogenic factors is an alternative technique for therapeutic angiogenesis in transplantation of xenogeneic acellular dermis matrix (ADM). However, immune rejection is a substantial obstacle to implantation of genetically engineered allogeneic or xenogeneic cells.

OBJECTIVE

To evaluate application of microencapsulated cells that express vascular endothelial growth factor (VEGF) in xenogeneic ADM transplants to improve wound angiogenesis and healing.

MATERIALS AND METHODS

NIH3T3 cells were genetically modified to secrete VEGF and enveloped in semipermeable microcapsules. Microencapsulated VEGF-NIH3T3 cells were implanted in defects on the dorsa of guinea pigs with xenogeneic ADM and autologous split-thickness skin grafts. Cell structure and microencapsulation were observed at microscopy, and expression of VEGF was detected using an enzyme-linked immunosorbent assay (ELISA) and immunochemistry. Extent of angiogenesis in the ADM and the survival rate of the composite skin were evaluated after 2 weeks. In addition, expression of human VEGF and CD31 in the implanted acellular dermis was assessed, and microvessel density was calculated.

RESULTS

Microencapsulated VEGF-expressing NIH3T3 cells were prepared successfully, and demonstrated proliferation and viability, and expressed VEGF both in vitro and in vivo. Extent of angiogenesis and survival rate of the composite skin containing the microencapsulated VEGF-expressing cells were significantly greater than in controls. Microencapsulated VEGF-expressing NIH3T3 cells augmented early angiogenesis in ADM implanted on wound and improved healing.

CONCLUSION

Microencapsulated xenogeneic cell-based gene therapy may be a novel approach to therapeutic angiogenesis in transplantation of xenogeneic ADM skin.

Treatment of diabetes with encapsulated islets

Cell encapsulation has been proposed for the treatment of a wide variety of diseases since it allows for transplantation of cells in the absence of undesired immunosuppression. The technology has been proposed to be a solution for the treatment of diabetes since it potentially allows a mandatory minute-to-minute regulation of glucose levels without side-effects. Encapsulation is based on the principle that transplanted tissue is protected for the host immune system by a semipermeable capsule. Many different concepts of capsules have been tested. During the past two decades three major approaches of encapsulation have been studied. These include (i) intravascular macrocapsules, which are anastomosed to the vascular system as AV shunt, (ii) extravascular macrocapsules, which are mostly diffusion chambers transplanted at different sites and (iii) extravascular microcapsules transplanted in the peritoneal cavity. The advantages and pitfalls of the three approaches are discussed and compared in view of applicability in clinical islet transplantation.

Construction of a recombinant eukaryotic expression plasmid containing human calcitonin gene and its expression in NIH3T3 cells

AIM

To construct a recombinant eukaryotic expression plasmid containing human calcitonin (hCT) gene and express the gene in murine fibroblast NIH3T3 cells.

MATERIALS AND METHODS

A murine Igkappa-chain leader sequence and hCT gene were synthesized and cloned into pCDNA3.0 to form the pCDNA3.0-Igkappa-hCT eukaryotic expression vector, which was transfected into NIH3T3 cells. The mRNA and protein expressions and secretion of hCT were detected. Primarily cultured osteoclasts were incubated with the supernatant of pCDNA3.0-Igk-hCT-transfected NIH3T3 cells, and their numbers were counted and morphology observed.

RESULTS

The expression and secretion of hCT were successfully detected in pCDNA3.0-Igk-hCT-transfected NIH3T3 cells. The number of osteoclasts was decreased and the cells became crumpled when they were incubated with the supernatant of pCDNA3.0-Igk-hCT-transfected NIH3T3 cells.

CONCLUSION

A recombinant eukaryotic expression vector containing hCT gene was successfully constructed and expressed in NIH3T3 cells. The secreted recombinant hCT inhibited the growth and morphology of osteoclasts.

Bioartificial Pancreas for the Treatment of Diabetes

Recent advances in islet transplantation using highly purified islets and effective immunosuppression strategies have resulted in substantial improvement in achieving insulin independence in type 1 diabetes patients. However, there are side effects from long-term immunosuppression, and transplant rejection and/or the recurrence of autoimmune attack of the transplanted islets cannot be completely prevented, even with immunosuppressive treatment. Therefore, construction of a safe and functional bioartificial pancreas (BAP) that provides an adequate environment for islet cells may be an important approach to treat diabetic patients. Various types of BAP devices have been developed and examined in animals. In this review, I introduce the previous BAP studies and our approach of BAP development.

Non-invasive evaluation of alginate/poly-l-lysine/alginate microcapsules by magnetic resonance microscopy

In this report, we present data to demonstrate the utility of (1)H MR microscopy to non-invasively examine alginate/poly-l-lysine/alginate (APA) microcapsules. Specifically, high-resolution images were used to visualize and quantify the poly-l-lysine (PLL) layer, and monitor temporal changes in the alginate gel microstructure during a month long in vitro culture. The thickness of the alginate/PLL layer was quantified to be 40.6+/-6.2 microm regardless of the alginate composition used to generate the beads or the time of alginate/PLL interaction (2, 6, or 20 min). However, there was a notable difference in the contrast of the PLL layer that depended upon the guluronic content of the alginate and the alginate/PLL interaction time. The T(2) relaxation time and the apparent diffusion coefficient (ADC) of the alginate matrix were measured periodically throughout the month long culture period. Alginate beads generated with a high guluronic content alginate demonstrated a temporal decrease in T(2) over the duration of the experiment, while ADC was unaffected. This decrease in T(2) is attributed to a reorganization of the alginate microstructure due to periodic media exchanges that mimicked a regular feeding regiment for cultured cells. In beads coated with a PLL layer, this temporal decrease in T(2) was less pronounced suggesting that the PLL layer helped maintain the integrity of the initial alginate microstructure. Conversely, alginate beads generated with a high mannuronic content alginate (with or without a PLL layer) did not display temporal changes in either T(2) or ADC. This observation suggests that the microstructure of high mannuronic content alginate beads is less susceptible to culture conditions.

Vitreous Cryopreservation of Cell–Biomaterial Constructs Involving Encapsulated Hepatocytes

We put forward a new strategy for cryopreservation, namely vitrification or ice-free preservation, of cell-biomaterial constructs for tissue-engineering applications. In this study, for a period of 6 days, we tested vitrified and control hepatocytes entrapped at 2 different cell densities (1.5 x 10(6) and 5 x 10(6) cells/mL) in 2 types of engineered collagen matrices (M- and G-collagen) as models to evaluate efficacy and universality of the developed vitrification method. The nature of collagens caused differences in capsule sizes (100-200 microm versus 350-450 microm). The developed method included rapid step-wise introduction of microencapsulated hepatocytes to vitrification solution (40v/v% ethylene glycol 0.6 M sucrose in medium) and their direct immersion in liquid nitrogen. Vitrification did not affect viability and functions of the microencapsulated hepatocytes, which exhibited trends similar to those of untreated controls in the decline of their functions and the rate of cell death during continuous culture, irrespective of physical and chemical properties of the biomaterial and cell density. For control and vitrification, the percentage of live cells varied from 80.3% +/- 0.9% to 82.3% +/- 1.4% in capsules formed by M-collagen, from 82.8% +/- 1.1% to 85.0% +/- 3.3% in capsules formed by G-collagen with cells entrapped at low density, and from 84.4% +/- 1.3% to 86.8% +/- 0.6% in capsules formed by G-collagen with cells entrapped at high density (p > 0.05). Within the same day, the maximum relative change in cell viability and functions between control and vitrification was 4% and 16%, respectively. The developed vitrification approach, which is an alternative to freezing, can be applied to other tissue-engineered constructs with comparable sizes, various cell numbers, and various properties of the biomaterials involved.

Cryopreservation of Insulin-Producing Cells Microencapsulated in Sodium Cellulose Sulfate

INTRODUCTION

Diabetes mellitus may be treated with pancreatic islet cell transplantation. The use of xenogenic islet cells may overcome the shortage of human donor organs. Microencapsulation seems to be a promising method for immunoprotection. Since isolation, purification, encapsulation, and transplantation of islet cells are labor-intensive, cryopreservation has emerged as an attractive system for islet banking. In this study sodium cellulose sulfate (NaCS), a novel method for microencapsulation of islet cells, was tested for its capability to protect cells during cryopreservation.

METHODS

HIT-T15 cells were microencapsulated in NaCS. Cells were frozen and thawed using three different media containing varying amounts of dimethylsulfoxide (DMSO) and glycerol. Cell viability and cell growth were monitored using 3-(-4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide before freezing and 1 week after thawing.

RESULTS

NaCS did not show any negative impact on the growth rates of encapsulated HIT-T15 cells compared with nonencapsulated controls. Nonencapsulated cells were adequately cryopreserved by both DMSO- and glycerol-containing freezing media. DMSO was not suitable for cryopreservation of encapsulated HIT-T15 cells, whereas glycerol seemed to produce no considerable cell loss during freezing and thawing.

DISCUSSION

Islet banking of cells encapsulated in NaCS was feasible. Microencapsulation did not harm islet cell recovery. As NaCS is less immunogenic and more biocompatible than other materials used for microencapsulation, it may be a promising method for immunoisolation of islet cells to replace the endocrine pancreas in a physiological way.

Application of self-assembled ultra-thin film coatings to stabilize macromolecule encapsulation in alginate microspheres

Alginate-based hydrogels have several unique properties that have enabled them to be used as a matrix for the entrapment of a variety of enzymes, proteins and cells for applications in bioprocessing, drug delivery and chemical sensing. However, control over release rates or, in some cases, stable encapsulation remains a difficult goal, especially for small particles with high surface-area-to-volume ratios. In this work, the potential to limit diffusion of macromolecules embedded in alginate spheres with nanofilm coatings was assessed. Alginate microspheres were fabricated using an emulsification process with high surfactant concentration to form beads in the size range of 2-10 microm. Using calcium chloride for ionotropic gelation, dextran was encapsulated in the gel phase by mixing with the alginate in solution. The exterior surface was then modified with polyelectrolyte coatings using the layer-by-layer self assembly technique. Leaching studies to assess retention of dextran with varying molecular weights confirmed that the application of multi-layer thin films to the alginate microspheres was effective in reducing leaching rate and total loss of the encapsulated material from the microspheres. For the best case, the rate of release for dextran of 2,000,000 Dalton molecular weight decreased from 1% h(-1) in bare microspheres to 0.1% h(-1) in polyelectrolyte-coated microspheres. The effectiveness of nanofilms reducing loss of the encapsulated macromolecules was found to vary between different polycation materials used. These studies support the feasibility of using these microsystems for development of long-term stable encapsulated systems, such as implantable biosensors.

Stable encapsulation of active enzyme by application of multilayer nanofilm coatings to alginate microspheres

In an effort to improve the stability for long-term biosensor use, layer-by-layer self-assembly was explored as a potential technique to provide a diffusion barrier to encapsulated glucose oxidase inside alginate microspheres (<5 microm), fabricated using an emulsification technique. The total loss of encapsulated enzyme was reduced to less than 25 and 15% with the application of single PAH/PSS and crosslinked PAH/PAA coatings, respectively, in comparison to at least a 45% loss observed with uncoated and PDDA/PSS-coated microspheres. Furthermore, it was found that enzyme within PAH/PSS- and crosslinked PAH/PAA-coated spheres retained more than 84 and 60% of initial activity, respectively, after three months, whereas uncoated and PDDA/PSS-coated microspheres retained less than 20%.

The bioartificial pancreas: progress and challenges

Diabetes remains a devastating disease, with tremendous cost in terms of human suffering and healthcare expenditures. A bioartificial pancreas has the potential as a promising approach to preventing or reversing complications associated with this disease. Bioartificial pancreatic constructs are based on encapsulation of islet cells with a semipermeable membrane so that cells can be protected from the host's immune system. Encapsulation of islet cells eliminates the requirement of immunosuppressive drugs, and offers a possible solution to the shortage of donors as it may allow the use of animal islets or insulin-producing cells engineered from stem cells. During the past 2 decades, several major approaches for immunoprotection of islets have been studied. The microencapsulation approach is quite promising because of its improved diffusion capacity, and technical ease of transplantation. It has the potential for providing an effective long-term treatment or cure of Type 1 diabetes.

Combined physical and chemical immobilization of glucose oxidase in alginate microspheres improves stability of encapsulation and activity

Chemical sensors utilizing immobilized enzymes and proteins are important for monitoring chemical processes and biological systems. In this study, calcium-cross-linked alginate hydrogel microspheres were fabricated as enzyme carriers by an emulsification technique. Glucose oxidase (GOx) was encapsulated in alginate microspheres using three different methods: physical entrapment (emulsion), chemical conjugation (conjugation), and a combination of physical entrapment and chemical conjugation (emulsion-conjugation). Nano-organized coatings were applied on alginate/GOx microspheres using the layer-by-layer self-assembly technique in order to stabilize the hydrogel/enzyme system under biological environment. The encapsulation of GOx and formation of nanofilm coating on alginate microspheres were verified with FTIR spectral analysis, zeta-potential analysis, and confocal laser scanning microscopy. To compare both the immobilization properties of enzyme encapsulation techniques and the influence of nanofilms with uncoated microspheres, the relationship between enzyme loading, release, and effective GOx activity (enzyme activity per unit protein loading) were studied over a period of four weeks. The results produced four key findings: (1) the emulsion-conjugation technique improved the stability of GOx in alginate microspheres compared to the emulsion technique, reducing the GOx leaching from microsphere from 50% to 17%; (2) the polyelectrolyte nanofilm coatings increased the GOx stability over time, but also reduced the effective GOx activity; (3) the effective GOx activity for the emulsion-conjugation technique (about 3.5 x 10(-)(5) AU microg(-)(1) s(-)(1)) was higher than that for other methods, and did not change significantly over four weeks; and (4) the GOx concentration, when compared after one week for microspheres with three bilayers of poly(allylamine hydrochloride)/sodium poly(styrene sulfonate) ({PAH/PSS}) coating, was highest for the emulsion-conjugation technique. As a result, the comparison of these three techniques showed the emulsion-conjugation technique to be a potentially effective and practical way to fabricate alginate/GOx microspheres for implantable glucose biosensor application.

Stabilization of glucose oxidase in alginate microspheres with photoreactive diazoresin nanofilm coatings

The nanoassembly and photo-crosslinking of diazo-resin (DAR) coatings on small alginate microspheres for stable enzyme entrapment is described. Multilayer nanofilms of DAR with poly(styrene sulfonate) (PSS) were used in an effort to stabilize the encapsulation of glucose oxidase enzyme for biosensor applications. The activity and physical encapsulation of the trapped enzyme were measured over 24 weeks to compare the effectiveness of nanofilm coatings and crosslinking for stabilization. Uncoated spheres exhibited rapid loss of activity, retaining only 20% of initial activity after one week, and a dramatic reduction in effective activity over 24 weeks, whereas the uncrosslinked and crosslinked {DAR/PSS}-coated spheres retained more than 50% of their initial activity after 4 weeks, which remained stable even after 24 weeks for the two and three bilayer films. Nanofilms comprising more polyelectrolyte layers maintained higher overall activity compared to films of the same composition but fewer layers, and crosslinking the films increased retention of activity over uncrosslinked films after 24 weeks. These findings demonstrate that enzyme immobilization and stabilization can be achieved by using simple modifications to the layer-by-layer self-assembly technique.

Spontaneous Loading of Positively Charged Macromolecules into Alginate-Templated Polyelectrolyte Multilayer Microcapsules

A simple and high-efficiency approach to loading macromolecules into microscale carriers is presented. Calcium-cross-linked alginate hydrogel microspheres were fabricated by an emulsification technique and then used as negatively charged templates to form polyelectrolyte multilayer coatings. A calcium ion chelator, EDTA, was used to free the Ca(2+)-cross-linked alginate hydrogel within {poly(allylamine hydrochloride)/poly (styrene sulfonate)}(4) ({PAH/PSS}(4)) coating, allowing partial release of alginate. The retention of alginate in {PAH/PSS}(4) microcapsule was confirmed by FTIR spectroscopy and confocal microscopy. Real-time confocal microscopy was used to investigate the loading process of positively charged macromolecules (dextran-amino, and peroxidase) into alginate-templated microcapsules, which showed the loading occurred in <2 min for dextran-amino and <10 min for peroxidase, respectively. A high loading efficiency of 25 mug peroxidase in approximately 1.0 x 10(7) microcapsules (2.5 pg POx/capsule) was achieved with a low concentration of peroxidase loading solution (10 mug/mL). This spontaneous loading technique for encapsulating positively charged molecules in alginate-templated polyelectrolyte microcapsules shows strong potential for biosensor and drug delivery applications.

Alginate assessment by NMR microscopy

Alginate hydrogels have long been used to encapsulate cells for the purpose of cell transplantation. However, they also have been criticized because they fail to consistently maintain their integrity for extended periods of time. Two issues of critical importance that have yet to be thoroughly addressed concerning the long-term integrity of alginate/poly-L-lysine/alginate microcapsules are: (i) are there temporal changes in the alginate/poly-L-lysine interaction and (ii) are there temporal changes in the alginate gel structure. NMR microscopy is a non-invasive analytical technique that can address these issues. in this report, we present data to demonstrate the utility of (1)H NMR microscopy to (i) visualize the poly-L-lysine layer in an effort to address the first question, and (ii) to observe temporal changes in the alginate matrix that may represent changes in the gel structure.

Neovascularization of the amniotic membrane as a biological immune barrier

The clinical application of islet transplantation is limited due to the limited source and the morbidity of systemic immunosuppression to prevent rejection. The two problems can be solved by using encapsulated islets. We have used amniotic membranes as biocompatible natural immune barriers. The objective of this study was to assess the revascularization of the membrane, which is necessary to ensure islet viability when the membrane is used for islet encapsulation. The amniotic membranes, obtained from full-term pregnant female dogs, were molded to form macrocapsules, which were implanted in the peritoneal cavity. The capsules were removed after 3, 10, 15, and 30 days and examined histopathologically using hematoxylin and eosin and by immunohistochemistry for neovascularization using factor VIII to detect angiogenesis. Upon histopathological examination, all specimens showed localized, moderate inflammation and congested blood vessels with no thrombosis or rejection. There was a mild degree of fibroblast proliferation starting from day 10 to day 30. Immunohistochemical staining revealed that the number of blood vessels was 7, 11, 13, 10 per high-power microscopic field on days 3, 10, 15, and 30, respectively. We concluded from this study that implanted amniotic sac capsules were vascularized within the omental tissue from day 10 on with significant blood vessel formation starting on day 3 by immunohistochemical study.

In vitro-generation of surrogate islets from adult stem cells

Type 1 diabetes is one of the more costly chronic diseases of children and adolescents throughout North America and Europe, exhibiting an average estimated prevalence rate of nearly 0.2%. It occurs in genetically predisposed individuals when the immune system attacks and destroys specifically the insulin-producing beta cells of the pancreatic islets of Langerhans. While routine insulin therapy can provide diabetic patients with their daily insulin requirements, non-compliance and undetected hyperglycemic excursions often lead to subsequent long-term microvascular and macrovascular complications. The only real cure for type 1 diabetes is replacement of the beta cell mass, currently being accomplished through ecto-pancreatic transplantation and islet implantation. Both of these procedures suffer from a chronic shortage of available donor tissue in comparison to the number of potential recipients. To circumvent this need, three alternative approaches are being intensively investigated: (1) the production of surrogate cells by genetically modifying non-endocrine cells to secrete insulin in response to glucose challenge; (2) the trans-differentiation of non-endocrine stem/progenitor cells or mature cells to glucose-responsive adult tissue; and (3) the regulated differentiation of islet stem/progenitor cells to produce large numbers of mature, functional islets. In recent years, each of these approaches has made impressive advances, leading to the most important question, 'how soon will this new science be available to the patient?' In the present review, we discuss some of the recent advances, focusing primarily on the differentiation of islet stem cells to functional endocrine pancreas that may form the basis for future treatment.

Survival of microencapsulated adult pig islets in mice in spite of an antibody response

The aim of this study was to assess the capacity of simple alginate capsules to protect adult pig islets in a model of xenotransplantation. Adult pig islets were microencapsulated in alginate, with either single alginate coats (SAC) or double alginate coats (DAC), and transplanted into the streptozotocin-induced diabetic B6AF1 mice. Normalization of glucose levels was associated with an improvement of the glucose clearance during intravenous glucose tolerance tests. After explantation, all mice became hyperglycemic, demonstrating the efficacy of the encapsulated pig islets. Explanted capsules were mainly free of fibrotic reaction and encapsulated islets were still functional, responding to glucose stimulation with a 10-fold increase in insulin secretion. However, a significant decrease in the insulin content and insulin responses to glucose was observed for encapsulated islets explanted from hyperglycemic mice. An immune response of both IgG and IgM subtypes was detectable after transplantation. Interestingly, there were more newly formed antibodies in the serum of mice transplanted with SAC capsules than in the serum of mice transplanted with DAC capsules. In conclusion, alginate capsules can prolong the survival of adult pig islets transplanted into diabetic mice for up to 190 days, even in the presence of an antibody response.

NMR properties of alginate microbeads

Alginates are a family of unbranched polysaccharides with properties that vary widely depending on their composition. In the presence of multivalent cations (frequently Ca2+), alginates form a gel. Consequently, alginates have been used to encapsulate a variety of biological materials, including cells. In this study, we present NMR relaxation and diffusion data from alginate microbeads with similar size and properties to those used in the development of a bioartificial pancreas. Our data demonstrate that the transverse relaxation time (T2) of water within the gel depends on the guluronic acid content of the alginate, whereas the longitudinal relaxation time (T1) and the apparent diffusion coefficient of water do not. Our data further suggest that the diffusion of Ca2+ ions is hindered by the presence of a poly-L-lysine layer, a layer commonly added to provide mechanical support to the beads and immunoprotection to the encapsulated cells in the event of implantation. The impact of these data on our understanding of the role of alginate gels in the development of a bioartificial pancreas is discussed.

Immunoisolation techniques for islet cell transplantation

Diabetes remains a devastating disease, with tremendous cost in terms of human suffering and healthcare expenditures. The burden of diabetes is primarily related to the multiple complications, including retinopathy, nephropathy, neuropathy and cardiovascular disease that can develop as the disease progresses. It has been shown that these complications can be prevented, and in some cases, reversed by islet cell transplantation, which, until recently, had remained elusive as a viable routine treatment modality. In recent studies, islet cell transplantation has shown great promise as a viable alternative to solid pancreas transplantation. However, severe shortage of human pancreases and the need to use immunosuppressive drugs to prevent transplant rejection, remain major obstacles to routine use of islet cell transplants for the treatment of patients with Type 1 diabetes. In the attempt to overcome these barriers, many procedures have been designed to immunoisolate islet cells for transplantation. The ultimate goal in islet cell transplantation is the availability of unlimited supply of cells to be transplanted in a simple procedure performed with little or no use of immunosuppressive drugs. The development of reliable procedures to immunoisolate islets by microencapsulation prior to transplantation has a great deal of potential to accomplish this objective.

Use of in vitro-generated, stem cell-derived islets to cure type 1 diabetes: how close are we?

Recent successes in treating type 1 diabetic patients with islet transplantation portends a future need for an increase in available islets. Ductal structures of the adult pancreas contain multipotent stem cells that, under the proper in vitro conditions, can both self-renew and differentiate into functional islets of Langerhans. In vitro-generated islets exhibit temporal changes in mRNA transcripts for islet-associated markers as well as regulated insulin responses following glucose challenge. When implanted into diabetic mice, in vitro-generated islets induce neovascularization and reverse insulin-dependent diabetes. The possibility of growing functional endocrine pancreas from stem cells provides new opportunities to produce large numbers of islets, even autologous islets, for use as implants.

Non-Invasive monitoring of a bioartificial pancreas in vitro and in vivo

Monitoring biochemical processes relevant to the function, survival, and longevity of tissue-engineered pancreatic constructs is important for the development of an optimum construct design as well as patient care management after implantation. In this report we demonstrate the ability of nuclear magnetic resonance (NMR) techniques to monitor aspects of intracellular metabolism, overall morphology, and distribution of a microencapsulation based bioartificial pancreas in vitro and in vivo.

Prevention of morphological changes in alginate microcapsules for islet xenotransplantation

BACKGROUND

Alginate microcapsule swelling, which occurs as a result of increased hydrophilicity owing to the Ca++ that remains after rapid chelation of the inner alginate core, is a problem in encapsulation. We have previously shown that exchange of the residual divalent Ca++ with the monovalent Na+ through the use of 6 mmol/L Na2SO4 decreases swelling in chelated alginate-polylysine-alginate microcapsules, and this process enhances their durability. The purpose of the present study was to examine the morphology of Na2SO4-treated microcapsules in long-term incubation with the use of serum-supplemented culture medium.

METHODS

Spherical beads of purified alginate (3%) that were gelled with 1.1% CaCl2 were first coated with polylysine, and then with 0.24% alginate. After rapid chelation of the inner alginate core with 55 mmol/L sodium citrate, the capsules were either incubated for 30 minutes in 6 mmol/L Na2SO4 or left untreated (control). Each group of capsules was then placed in a flask containing Ham's culture medium supplemented with 20% porcine serum and incubated at 37 degrees C.

RESULTS

The diameters of Na2SO4-treated capsules only increased modestly from a mean +/- SD of 635 +/- 22.08 to 684.53 +/- 17.86 microm (P<0.0001) by day 7, with no further increases thereafter. In contrast, control capsules showed a steady increase in their mean diameters, which changed from 639.55 +/- 21.44 to 735.48 +/- 108.85 microm (P < 0.0001) by day 66. In addition, whereas treated capsules remained spherical, control capsules showed progressive polymorphism.

CONCLUSION

We have developed a new method of making more durable and stable microcapsules that can be used for islet cell xenotransplantation.

Complete protection of islets against allorejection and autoimmunity by a simple barium-alginate membrane

We describe a new technique for microencapsulation with high-mannuronic acid (high-M) alginate crosslinked with BaCl(2) without a traditional permselective component, which allows the production of biocompatible capsules that allow prolonged survival of syngeneic and allogeneic transplanted islets in diabetic BALB/c and NOD mice for >350 days. The normalization of the glycemia in the transplanted mice was associated with normal glucose profiles in response to intravenous glucose tolerance tests. After explantation of the capsules, all mice became hyperglycemic, demonstrating the efficacy of the encapsulated islets. The retrieved capsules were free of cellular overgrowth and islets responded to glucose stimulation with a 5- to 10-fold increase of insulin secretion. Transfer of splenocytes isolated from transplanted NOD mice to NOD/SCID mice adoptively transferred diabetes, indicating that NOD recipients maintained islet-specific autoimmunity. In conclusion, we have developed a simple technique for microencapsulation that prolongs islet survival without immunosuppression, providing complete protection against allorejection and the recurrence of autoimmune diabetes.

The effects of alginate composition on encapsulated betaTC3 cells

The effects of alginate composition on the growth of murine insulinoma betaTC3 cells encapsulated in alginate/poly-L-lysine/alginate (APA) beads, and on the overall metabolic and secretory characteristics of the encapsulated cell system, were investigated for four different types of alginate. Two of the alginates used had a high guluronic acid content (73% in guluronic acid residues) with varying molecular weight, while the other two had a high mannuronic acid content (68% in mannuronic acid residues) with varying molecular weight. Each composition was tested using two different polymer concentrations. Our data show that betaTC3 cells encapsulated in alginates with a high guluronic acid content experienced a transient hindrance in their metabolic and secretory activity because of growth inhibition. Conversely, betaTC3 cells encapsulated in alginates with a high mannuronic acid content experienced a rapid increase in metabolic and secretory activity as a result of rapid cell growth. Our data also demonstrate that an increase in either molecular weight or concentration of high mannuronic acid alginates did not alter the behavior of the encapsulated betaTC3 cells. Conversely, an increase in molecular weight and concentration of high guluronic acid alginates prolonged the hindrance of glucose metabolism, insulin secretion and cell growth. These observations can be best interpreted by changes in the microstructure of the alginate matrix, i.e., interaction between the contiguous guluronic acid residues and the Ca2+ ions, as a result of the different compositions.

Pancreatic stem cells: building blocks for a better surrogate islet to treat type 1 diabetes

Type 1, insulin-dependent, diabetes is one of the more costly chronic diseases of children, adolescents and adults in Europe and North America. While routine insulin injections currently provide diabetic patients with their daily insulin requirements, blood glucose excursions are common, leading eventually to microvascular and macrovascular complications and early death. A 'cure' for Type 1 diabetes relies on replacement of the beta-cell mass which, today, is accomplished by pancreas transplants or islets of Langerhans implants. Recent advances in the isolation of stem cells that possess the capacity to differentiate to functional endocrine pancreas provide new opportunities to produce large numbers of islets, even autologous islets, that can be used as implants. We discuss briefly this new technology and its meaning for diabetes.

Xenogeneic transplantation

This review summarizes the clinical history and rationale for xenotransplantation; recent progress in understanding the physiologic, immunologic, and infectious obstacles to the procedure's success; and some of the strategies being pursued to overcome these obstacles. The problems of xenotransplantation are complex, and a combination of approaches is required. The earliest and most striking immunologic obstacle, that of hyperacute rejection, appears to be the closest to being solved. This phenomenon depends on the binding of natural antibody to the vascular endothelium, fixation of complement by that antibody, and finally, activation of the endothelium and initiation of coagulation. Therefore, these three pathways have been targeted as sites for intervention in the process. The mechanisms responsible for the next immunologic barrier, that of delayed xenograft/acute vascular rejection, remain to be fully elucidated. They probably also involve multiple pathways, including antibody and/or immune cell binding and endothelial cell activation. The final immunologic barrier, that of the cellular immune response, involves mechanisms that are similar to those involved in allograft rejection. However, the strength of the cellular immune response to xenografts is so great that it is unlikely to be controlled by the types of nonspecific immunosuppression used routinely to prevent allograft rejection. For this reason, it may be essential to induce specific immunologic unresponsiveness to at least some of the most antigenic xenogeneic molecules.

Durability of sodium sulfate-treated polylysine-alginate microcapsules

Chelated hollow microcapsules are unstable under in vitro conditions because of their hygroscopic nature. Nongel inducing cations, such as Na+, stabilize the outer membrane of the alginate-polylysine-alginate microcapsules leading to more stable beads. We made different batches of empty capsules with a mean +/- SEM diameter of 607+/-11 microns, and found that within 1 week of incubating these capsules in normal saline at 37 degrees C, they increased to 718+/-10 microns (p < 0.05, n = 5). In initial experiments, we made different batches of capsules and divided them into two groups. One group was left untreated (control) whereas the other was treated with 6 mM Na2SO4 for 30 min, before incubation in saline at 37 degrees C. Control capsules increased in weight and size, before beginning to melt in less than 1 week. In contrast, treated capsules rapidly lost weight and remained intact during 1 month of follow-up. In perifusion experiments, we found no deleterious effect of sodium sulfate treatment on the function of islets enclosed in the capsules.

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.

Immunocompatibility and biocompatibility of cell delivery systems

Immunoisolation therapy overcomes important disadvantages of implanting free cells. By mechanically blocking immune attacks, synthetic membranes around grafted cells should obviate the need for immunosuppression. The membrane used for encapsulation must be biocompatible and immunocompatible to the recipient and also to the encapsulated graft. The ability of the host to accept the implanted graft depends not only on the material used for encapsulation, but also on the defense reaction of the recipient, which is very individual. Such a reaction usually starts as absorption of cell-adhesive proteins, immunoglobulins, complement components, growth factors and some other proteins on the surface of the device. The absorption of proteins is difficult to avoid, but the amount and specificity of absorbed proteins can be controlled to some extent by selection and modification of the device material. If the adsorption of proteins to the surface of the implanted material is reduced, the overgrowth of the device with fibroblast-like and macrophage-like cells is also reduced. Cell adhesion at the surface of the implanted device is, in addition to the selected polymeric material, greatly influenced by the device content. Xenografts trigger a more vigorous inflammatory reaction than allografts, most probably due to the release of antigenic products from encapsulated deteriorated and dying cells which diffuse through the membrane and activate adhering immune cells. There is an evident effect of autoimmune status on the fate of the encapsulated graft. While encapsulated xenogeneic islets readily reverse streptozotocin-induced diabetes in mice, the same xenografts are short-functioning in NOD autoimmune diabetes-prone mice. Autoantibodies, to which most devices are impermeable, are not involved. Among the cytotoxic factors which are responsible for the limited survival of the encapsulated graft the most important are cytokines and perhaps some other low-molecular-weight factors released by activated macrophages at the surface of the encapsulating membrane.

Technology of mammalian cell encapsulation

Entrapment of mammalian cells in physical membranes has been practiced since the early 1950s when it was originally introduced as a basic research tool. The method has since been developed based on the promise of its therapeutic usefulness in tissue transplantation. Encapsulation physically isolates a cell mass from an outside environment and aims to maintain normal cellular physiology within a desired permeability barrier. Numerous encapsulation techniques have been developed over the years. These techniques are generally classified as microencapsulation (involving small spherical vehicles and conformally coated tissues) and macroencapsulation (involving larger flat-sheet and hollow-fiber membranes). This review is intended to summarize techniques of cell encapsulation as well as methods for evaluating the performance of encapsulated cells. The techniques reviewed include microencapsulation with polyelectrolyte complexation emphasizing alginate-polylysine capsules, thermoreversible gelation with agarose as a prototype system, interfacial precipitation and interfacial polymerization, as well as the technology of flat sheet and hollow fiber-based macroencapsulation. Four aspects of encapsulated cells that are critical for the success of the technology, namely the capsule permeability, mechanical properties, immune protection and biocompatibility, have been singled out and methods to evaluate these properties were summarized. Finally, speculations regarding future directions of cell encapsulation research and device development are included from the authors' perspective.

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.

Tissue engineering: confronting the transplantation crisis

Tissue engineering is the development of biological substitutes and/or the fostering of tissue regeneration/remodelling. It is emerging as a technology which has the potential to confront the crisis in transplantation caused by the shortage of donor tissues and organs. With the development of this technology, ther is emerging a new industry which is at the interface of biotechnology and the traditional medical implant field. For this technology and the associated industry to realize their full potential, there are core, enabling technologies that need to be developed. This is the focus of the Georgia Tech/Emory Center for the Engineering of Living Tissues, newly established in the United States, with an Engineering Research Center Award from the National Science Foundation. With the development of these core technologies, tissue engineering will evolve from an art form to a technology based on science and engineering.