Prolonged reversal of diabetic state in NOD mice by xenografts of microencapsulated rat islets

Advances in single-cell nanoencapsulation and applications in diseases

Single-cell nanoencapsulation is a method of coating the surface of single cell with nanomaterials. In the early 20th century, with the introduction of various types of organic or inorganic nano-polymer materials, the selection of cell types, and the functional modification of the outer coating, this technology has gradually matured. Typical preparation methods include interfacial polycondensation, complex condensation, spray drying, microdroplet ejection, and layer-by-layer (LbL) self-assembly. The LbL assembly technology utilises nanomaterials with opposite charges deposited on cells by strong interaction (electrostatic interaction) or weak interaction (hydrogen bonding, hydrophobic interaction), which drives compounds to spontaneously form films with complete structure, stable performance and unique functions on cells. According to the needs of the disease, choosing appropriate cell types and biocompatible and biodegradable nanomaterials could achieve the purpose of promoting cell proliferation, immune isolation, reducing phagocytosis of the reticuloendothelial system, prolonging the circulation time in vivo, and avoiding repeated administration. Therefore, encapsulated cells could be utilised in various biomedical fields, such as cell catalysis, biotherapy, vaccine manufacturing and antitumor therapy. This article reviews cell nanoencapsulation therapies for diseases, including the various cell sources used, nanoencapsulation technology and the latest advances in preclinical and clinical research.

Mild formation of core-shell hydrogel microcapsules for cell encapsulation

Internal gelation has been an important sol-gel route for the preparation of spherical microgel for drug delivery, cell therapy, or tissue regeneration. Despite high homogeneity and permeability, the internal gelated microgels often result in weak mechanical stability, unregular interface morphology and low cell survival rate. In this work, we have extensively improved the existing internal gelation approach and core-shell hydrogel microcapsules (200-600 μm) with a smooth surface, high mechanical stability and cell survival rate, are successfully prepared by using internal gelation. A coaxial flow-focusing capillary-assembled microfluidic (CFCM) device was developed for the gelation. Rapid gelling behavior of alginate in the internal gelation makes it suitable for producing well-defined and homogenous alginate hydrogel microstructures that serve as the shell of the microcapsules. 2-[4-(2-hydroxyethyl)piperazin-1-yl]ethanesulfonic acid (HEPES) was used in the shell stream during the internal gelation. Thus, a high concentration of acid in the oil solution can be used for better crosslinking the alginate while maintaining high cell viability. We further demonstrated that the gelation conditions in our approach were mild enough for encapsulating HepG2 cells and 3T3 fibroblasts without losing their viability and functionality in a co-culture environment.

Bioprinting an Artificial Pancreas for Type 1 Diabetes

PURPOSE OF REVIEW

Pancreatic islet cell transplantation is currently the only curative cell therapy for type 1 diabetes mellitus. However, its potential to treat many more patients is limited by several challenges. The emergence of 3D bioprinting technology from recent advances in 3D printing, biomaterials, and cell biology has provided the means to overcome these challenges.

RECENT FINDINGS

3D bioprinting allows for the precise fabrication of complex 3D architectures containing spatially distributed cells, biomaterials (bioink), and bioactive factors. Different strategies to capitalize on this ability have been investigated for the 3D bioprinting of pancreatic islets. In particular, with co-axial bioprinting technology, the co-printability of islets with supporting cells such as endothelial progenitor cells and regulatory T cells, which have been shown to accelerate revascularization of islets and improve the outcome of various transplantations, respectively, has been achieved. 3D bioprinting of islets for generation of an artificial pancreas is a newly emerging field of study with a vast potential to improve islet transplantation.

Synthetic poly(ethylene glycol)-based microfluidic islet encapsulation reduces graft volume for delivery to highly vascularized and retrievable transplant site

Transplant of hydrogel-encapsulated allogeneic islets has been explored to reduce or eliminate the need for chronic systemic immunosuppression by creating a physical barrier that prevents direct antigen presentation. Although successful in rodents, translation of alginate microencapsulation to large animals and humans has been hindered by large capsule sizes (≥500 μm diameter) that result in suboptimal nutrient diffusion in the intraperitoneal space. We developed a microfluidic encapsulation system that generates synthetic poly(ethylene glycol)-based microgels with smaller diameters (310 ± 14 μm) that improve encapsulated islet insulin responsiveness over alginate capsules and allow transplant within vascularized tissue spaces, thereby reducing islet mass requirements and graft volumes. By delivering poly(ethylene glycol)-encapsulated islets to an isolated, retrievable, and highly vascularized site via a vasculogenic delivery vehicle, we demonstrate that a single pancreatic donor syngeneic islet mass exhibits improved long-term function over conventional alginate capsules and close integration with transplant site vasculature. In vivo tracking of bioluminescent allogeneic encapsulated islets in an autoimmune type 1 diabetes murine model showed enhanced cell survival over unencapsulated islets in the absence of chronic systemic immunosuppression. This method demonstrates a translatable alternative to intraperitoneal encapsulated islet transplant.

A severe atherosclerosis mouse model on the resistant NOD background

Atherosclerosis is a complex disease affecting arterial blood vessels and blood flow that could result in a variety of life-threatening consequences. Disease models with diverged genomes are necessary for understanding the genetic architecture of this complex disease. Non-obese diabetic (NOD) mice are highly polymorphic and widely used for studies of type 1 diabetes and autoimmunity. Understanding atherosclerosis development in the NOD strain is of particular interest as human atherosclerosis on the diabetic and autoimmune background has not been successfully modeled. In this study, we used CRISPR/Cas9 genome editing to genetically disrupt apolipoprotein E (ApoE) and low-density lipoprotein receptor (LDLR) expression on the pure NOD background, and compared phenotype between single-gene-deleted mice and double-knockout mutants with reference to ApoE-deficient C57BL/6 mice. We found that genetic ablation of Ldlr or Apoe in NOD mice was not sufficient to establish an atherosclerosis model, in contrast to ApoE-deficient C57BL/6 mice fed a high-fat diet (HFD) for over 12 weeks. We further obtained NOD mice deficient in both LDLR and ApoE, and assessed the severity of atherosclerosis and immune response to hyperlipidemia in comparison to ApoE-deficient C57BL/6 mice. Strikingly, the double-knockout NOD mice treated with a HFD developed severe atherosclerosis with aorta narrowed by over 60% by plaques, accompanied by destruction of pancreatic islets and an inflammatory response to hyperlipidemia. Therefore, we succeeded in obtaining a genetic model with severe atherosclerosis on the NOD background, which is highly resistant to the disease. This model is useful for the study of atherosclerosis in the setting of autoimmunity.

Tissue-engineering approaches in pancreatic islet transplantation

Pancreatic islet transplantation is a promising alternative to whole-pancreas transplantation as a treatment of type 1 diabetes mellitus. This technique has been extensively developed during the past few years, with the main purpose of minimizing the complications arising from the standard protocols used in organ transplantation. By using a variety of strategies used in tissue engineering and regenerative medicine, pancreatic islets have been successfully introduced in host patients with different outcomes in terms of islet survival and functionality, as well as the desired normoglycemic control. Here, we describe and discuss those strategies to transplant islets together with different scaffolds, in combination with various cell types and diffusible factors, and always with the aim of reducing host immune response and achieving islet survival, regardless of the site of transplantation.

Obstacles in the Application of Microencapsulation in Islet Transplantation

Several factors stand in the way of successful clinical transplantation of alginate-polylysine-alginate microencapsulated pancreatic islets. These obstacles can be classified into three categories. The first regards the technical aspects of the production process. Limiting factors are the insufficient ability to produce small capsules with an adequate production rate, and insufficient insight into the factors determining the optimal chemical and mechanical properties of the capsules. The second category regards the functional aspects of the microencapsulated islets, such as the limitations of the transplantation site and the absence of a physiologic insulin response of the encapsulated islets to elevated blood glucose levels. The third category regards the fact that survival times of encapsulated islet grafts are still limited to several weeks or months, which is mainly explained by a pericapsular fibrotic overgrowth reaction as a consequence of the bioincom-patibility of the capsule membrane. This study describes these obstacles, and thereby summarizes the requirements needed for successful clinical application of encapsulated islet transplantation.

Development of glucose-responsive 'smart' insulin systems

PURPOSE OF REVIEW

The complexity of modern insulin-based therapy for type I and type II diabetes mellitus and the risks associated with excursions in blood-glucose concentration (hyperglycemia and hypoglycemia) have motivated the development of 'smart insulin' technologies (glucose-responsive insulin, GRI). Such analogs or delivery systems are entities that provide insulin activity proportional to the glycemic state of the patient without external monitoring by the patient or healthcare provider. The present review describes the relevant historical background to modern GRI technologies and highlights three distinct approaches: coupling of continuous glucose monitoring (CGM) to deliver devices (algorithm-based 'closed-loop' systems), glucose-responsive polymer encapsulation of insulin, and molecular modification of insulin itself.

RECENT FINDINGS

Recent advances in GRI research utilizing each of the three approaches are illustrated; these include newly developed algorithms for CGM-based insulin delivery systems, glucose-sensitive modifications of existing clinical analogs, newly developed hypoxia-sensitive polymer matrices, and polymer-encapsulated, stem-cell-derived pancreatic β cells.

SUMMARY

Although GRI technologies have yet to be perfected, the recent advances across several scientific disciplines that are described in this review have provided a path towards their clinical implementation.

Long-term glycemic control using polymer-encapsulated human stem cell-derived beta cells in immune-competent mice

The transplantation of glucose-responsive, insulin-producing cells offers the potential for restoring glycemic control in individuals with diabetes. Pancreas transplantation and the infusion of cadaveric islets are currently implemented clinically, but these approaches are limited by the adverse effects of immunosuppressive therapy over the lifetime of the recipient and the limited supply of donor tissue. The latter concern may be addressed by recently described glucose-responsive mature beta cells that are derived from human embryonic stem cells (referred to as SC-β cells), which may represent an unlimited source of human cells for pancreas replacement therapy. Strategies to address the immunosuppression concerns include immunoisolation of insulin-producing cells with porous biomaterials that function as an immune barrier. However, clinical implementation has been challenging because of host immune responses to the implant materials. Here we report the first long-term glycemic correction of a diabetic, immunocompetent animal model using human SC-β cells. SC-β cells were encapsulated with alginate derivatives capable of mitigating foreign-body responses in vivo and implanted into the intraperitoneal space of C57BL/6J mice treated with streptozotocin, which is an animal model for chemically induced type 1 diabetes. These implants induced glycemic correction without any immunosuppression until their removal at 174 d after implantation. Human C-peptide concentrations and in vivo glucose responsiveness demonstrated therapeutically relevant glycemic control. Implants retrieved after 174 d contained viable insulin-producing cells.

Engineering of microscale three-dimensional pancreatic islet models in vitro and their biomedical applications

Diabetes now is the most common chronic disease in the world inducing heavy burden for the people's health. Based on this, diabetes research such as islet function has become a hot topic in medical institutes of the world. Today, in medical institutes, the conventional experiment platform in vitro is monolayer cell culture. However, with the development of micro- and nano-technologies, several microengineering methods have been developed to fabricate three-dimensional (3D) islet models in vitro which can better mimic the islet of pancreases in vivo. These in vitro islet models have shown better cell function than monolayer cells, indicating their great potential as better experimental platforms to elucidate islet behaviors under both physiological and pathological conditions, such as the molecular mechanisms of diabetes and clinical islet transplantation. In this review, we present the state-of-the-art advances in the microengineering methods for fabricating microscale islet models in vitro. We hope this will help researchers to better understand the progress in the engineering 3D islet models and their biomedical applications such as drug screening and islet transplantation.

Islet and stem cell encapsulation for clinical transplantation

Over the last decade, improvements in islet isolation techniques have made islet transplantation an option for a certain subset of patients with long-standing diabetes. Although islet transplants have shown improved graft function, adequate function beyond the second year has not yet been demonstrated, and patients still require immunosuppression to prevent rejection. Since allogeneic islet transplants have experienced some success, the next step is to improve graft function while eliminating the need for systemic immunosuppressive therapy. Biomaterial encapsulation offers a strategy to avoid the need for toxic immunosuppression while increasing the chances of graft function and survival. Encapsulation entails coating cells or tissue in a semipermeable biocompatible material that allows for the passage of nutrients, oxygen, and hormones while blocking immune cells and regulatory substances from recognizing and destroying the cell, thus avoiding the need for systemic immunosuppressive therapy. Despite advances in encapsulation technology, these developments have not yet been meaningfully translated into clinical islet transplantation, for which several factors are to blame, including graft hypoxia, host inflammatory response, fibrosis, improper choice of biomaterial type, lack of standard guidelines, and post-transplantation device failure. Several new approaches, such as the use of porcine islets, stem cells, development of prevascularized implants, islet nanocoating, and multilayer encapsulation, continue to generate intense scientific interest in this rapidly expanding field. This review provides a comprehensive update on islet and stem cell encapsulation as a treatment modality in type 1 diabetes, including a historical outlook as well as current and future research avenues.

Current status of islet encapsulation

Cell encapsulation is a method of encasing cells in a semipermeable matrix that provides a permeable gradient for the passage of oxygen and nutrients, but effectively blocks immune-regulating cells from reaching the graft, preventing rejection. This concept has been described as early as the 1930s, but it has exhibited substantial achievements over the last decade. Several advances in encapsulation engineering, chemical purification, applications, and cell viability promise to make this a revolutionary technology. Several obstacles still need to be overcome before this process becomes a reality, including developing a reliable source of islets or insulin-producing cells, determining the ideal biomaterial to promote graft function, reducing the host response to the encapsulation device, and ultimately a streamlined, scaled-up process for industry to be able to efficiently and safely produce encapsulated cells for clinical use. This article provides a comprehensive review of cell encapsulation of islets for the treatment of type 1 diabetes, including a historical perspective, current research findings, and future studies.

Mini-review: Islet transplantation to create a bioartificial pancreas

Donor scarcity precludes the use of pancreatic transplantation to treat type I diabetes. Xenogeneic islet transplantation offers the possibility of overcoming this problem; however, it entails the use of immunoisolation devices to prevent immune rejection of the transplanted islets. These devices consist of a semipermeable membrane, which surrounds the islets and isolates them from the host's immune system, while allowing the passage of insulin and essential nutrients, including glucose. Problems associated with proposed device designs include diffusion limitations, biocompatibility, device retrieval in the event of failure, and mechanical integrity. Microencapsulation appears to be the most promising system of immunoisolation, however, the design of a device suitable for human clinical use remains a challenge. (c) 1994 John Wiley & Sons, Inc.

The use of biomaterials in islet transplantation

Pancreatic islet transplantation is a therapeutic option to replace destroyed β cells in autoimmune diabetes. Islets are transplanted into the liver via the portal vein; however, inflammation, the required immunosuppression, and lack of vasculature decrease early islet viability and function. Therefore, the use of accessory therapy and biomaterials to protect islets and improve islet function has definite therapeutic potential. Here we review the application of niche accessory cells and factors, as well as the use of biomaterials as carriers or capsules, for pancreatic islet transplantation.

Chemoselective cross-linking and functionalization of alginate via Staudinger ligation

In this study, we demonstrate the applicability of functionalized alginate to serve as a platform for the covalent cross-linking or immobilization of complementary phosphine functionalized groups via the chemoselective Staudinger ligation scheme. Azide groups were covalently linked to alginate through a heterobifunctional polyethylene glycol (PEG) linker and carbodiimide. Degree of azide functionalization was varied as a function of carbodiimide concentration and determined by proton nuclear magnetic resonance ((1)H NMR) and infrared spectroscopy. Spontaneous and covalently cross-linked alginate-PEG gels were generated via the Staudinger ligation scheme upon incubation of the azide functionalized alginate with PEG chains having 1-methyl-2-diphenylphosphino-terephthalate (MDT) as end groups. Modulation of the MDT to N(3) ratio resulted in variability of gel characteristics. In addition, azide functionalized alginate retained its capacity to instantaneously form hydrogels via electrostatic interaction with multivalent cations such as Ca(2+) and Ba(2+). Subsequently, covalent linkage of phosphine functionalized agents postgelation of the alginate was feasible, as illustrated via linkage of MDT-PEG-carboxyfluorescein. Capitalization of the chemoselective and cell compatible Staudinger ligation scheme for covalent cross-linking of alginate hydrogels may enhance the utility of this polymer for the stable encapsulation of various cell types, in addition to their use in the immobilization of labeling agents, proteins, and other bioactive molecules.

Modification of islet of langerhans surfaces with immunoprotective poly(ethylene glycol) coatings via interfacial photopolymerization

Poly(ethylene glycol) (PEG) has been used previously to alter immune interactions and systemic clearance of therapeutic proteins. We present herein chemical approaches for the conceptually similar treatment of therapeutic cells and tissues whereby immune and cell adhesive interactions may be reduced or interrupted, in the context of the transplantation of xenogeneic islets of Langerhans for the treatment of insulin-dependent diabetes mellitus. Visible-light-initiated interfacial photopolymerization of multifunctional PEG-based macromers was performed directly upon the surface of rat islets of Langerhans to produce conformal barrier hydrogel coatings with thickness of order 10 microm. The islets continued to be normal in ultrastructure and function as reflected by response to a glucose challenge in vitro.

Challenges and emerging technologies in the immunoisolation of cells and tissues

Protection of transplanted cells from the host immune system using immunoisolation technology will be important in realizing the full potential of cell-based therapeutics. Microencapsulation of cells and cell aggregates has been the most widely explored immunoisolation strategy, but widespread clinical application of this technology has been limited, in part, by inadequate transport of nutrients, deleterious innate inflammatory responses, and immune recognition of encapsulated cells via indirect antigen presentation pathways. To reduce mass transport limitations and decrease void volume, recent efforts have focused on developing conformal coatings of micron and submicron scale on individual cells or cell aggregates. Additionally, anti-inflammatory and immunomodulatory capabilities are being integrated into immunoisolation devices to generate bioactive barriers that locally modulate host responses to encapsulated cells. Continued exploration of emerging paradigms governed by the inherent challenges associated with immunoisolation will be critical to actualizing the clinical potential of cell-based therapeutics.

Evaluation of alginate purification methods: effect on polyphenol, endotoxin, and protein contamination

Alginate, a polysaccharide extracted from brown seaweed, is widely used for the microencapsulation of islets of Langerhans, allowing their transplantation without immunosuppression. This natural polymer is known to be largely contaminated. The implantation of islets encapsulated using unpurified alginate leads to the development of fibrotic cell overgrowth around the microcapsules and normalization of the blood glucose is restricted to a very short period if it is achieved at all. Several research groups have developed their own purification method and obtained relatively good results. No comparative evaluation of the efficiencies of these methods has been published. We conducted an evaluative study of five different alginate preparations: a pharmaceutical-grade alginate in its raw state, the same alginate after purification according to three different published methods, and a commercially available purified alginate. The results showed that all purification methods reduced the amounts of known contaminants, that is, polyphenols, endotoxins, and proteins, although with varying efficiencies. Increased viscosity of alginate solutions was observed after purification of the alginates. Despite a general efficiency in decreasing contamination levels, all of the purified alginates contained relatively high residual amounts of protein contaminants. Because proteins may be immunogenic, these residual proteins may have a role in persisting microcapsule immunogenicity.

Pharmacologic transgene control systems for gene therapy

Pharmacologic transgene-expression dosing is considered essential for future gene therapy scenarios. Genetic interventions require precise transcription or translation fine-tuning of therapeutic transgenes to enable their titration into the therapeutic window, to adapt them to daily changing dosing regimes of the patient, to integrate them seamlessly into the patient's transcriptome orchestra, and to terminate their expression after successful therapy. In recent years, decisive progress has been achieved in designing high-precision trigger-inducible mammalian transgene control modalities responsive to clinically licensed and inert heterologous molecules or to endogenous physiologic signals. Availability of a portfolio of compatible transcription control systems has enabled assembly of higher-order control circuitries providing simultaneous or independent control of several transgenes and the design of (semi-)synthetic gene networks, which emulate digital expression switches, regulatory transcription cascades, epigenetic expression imprinting, and cellular transcription memories. This review provides an overview of cutting-edge developments in transgene control systems, of the design of synthetic gene networks, and of the delivery of such systems for the prototype treatment of prominent human disease phenotypes.

Insulin-like growth factor II allows prolonged blood glucose normalization with a reduced islet cell mass transplantation

IGF-II has been reported to decrease neonatal islet cell apoptosis and in vitro adult islet cell necrosis and apoptosis, but the usefulness of IGF-II in a transplantation setting is unknown. We evaluated the effect of in vitro IGF-II incubations on microencapsulated rat islet survival both in vitro and in minimal mass transplantations into diabetic mice. After 6 d in culture, fresh examinations, histology, fluorescence microscopy, sodium 3'-[1-(phenyl-amino-carbonyl)-3,4-tetrazolium]-bis (4-methoxy-6-nitro)-benzene sulfonic acid hydrate assay, and apoptosis studies all indicated that IGF-II significantly improves islet cell viability in a dose-dependent fashion. IGF-II 100 ng/ml and 500 ng/ml induced a 51% and 83% increase of viable islets (P = 0.052, P < 0.01). A 20%, 29%, and 33% reduction of the apoptotic index was observed with 50, 100, and 500 ng/ml incubations respectively (P < 0.05; P < 0.005; P < 0.001). Ten weeks after transplantation of 150 encapsulated rat islet equivalents incubated with IGF-II 500 ng/ml, 80% of diabetic mice were normoglycemic. Without IGF-II preincubation, only 8% of the recipients remained normoglycemic with the transplantation of 150 islets and 42% with 300 islets (P < 0.05). In conclusion, IGF-II promotes islet cell survival, and allows successful transplantation using a smaller number of islets.

Alginate microcapsules for pancreatic islet cell graft immunoprotection: struggle and progress towards the final cure for type 1 diabetes mellitus

Lights and shadows have been associated with the use of alginate/polyaminoacidic microcapsules for transplantation of pancreatic islet cells for the therapy of diabetes mellitus, with no recipient pharmacological immunosuppression. In fact, preliminary success in rodents has generally not matched the results achieved in diabetic higher mammals. The restricted availability of cadaveric human donor organs/tissue, coupled to regulatory hurdles in the use of microcapsules in patients, has significantly delayed the progress of microencapsulated islet grafts into pilot clinical trials. While the basic formulation of microcapsules from the author's laboratory, originally comprised of an alginate gel (AG) core, a double poly-L-ornithine (PLO) coat and an outer AG coat, has virtually remained unchanged, highly purified 'clinical grade' AG has been introduced in order to try to surmount regulatory restrictions. In parallel, novel insulin-producing cell types have been employed to fill the capsules, with particular regard to non-human tissue, such as adult and, more recently, neonatal porcine islets. In particular, using neonatal porcine islets enveloped in AG-PLO microcapsules, hyperglycaemia has been corrected in several diabetic animal models. Should standardisation and optimisation problems associated with both AG procurement and other membrane physical-chemical fabrication parameters be surmounted, microcapsules containing either human or, possibly, pig islets, could be close to approval for Phase I human clinical trials.

Tissue responses against immunoisolating alginate-PLL capsules in the immediate posttransplant period

Alginate-polylysine (PLL) capsules are commonly applied for immunoisolation of living cells for the treatment of a wide variety of diseases. Large-scale application of the technique, however, is hampered by insufficient biocompatibility of the capsules with failure of the grafts as a consequence. Most studies addressing biocompatibility issues of alginate-PLL capsules have focused on the degree of overgrowth on the capsules after graft failure and not on the reaction against the capsules in the immediate posttransplant period. Therefore, capsules were implanted in the peritoneal cavity of rats and retrieved 1, 5, and 7 days later for histological examination and X-ray photoelectron spectroscopy analysis for evaluation of chemical changes at the capsule surface. After implantation, the nitrogen signal increased from 5% on day 0, to 8.6% on day 7, illustrating protein adsorption on the capsule's surface. This increase in protein content of the membrane was accompanied by an increase in the percentage of overgrown capsules from 0.5 +/- 0.3% on day 1 to 3.3 +/- 1.6% on day 7. The cellular overgrowth was composed of monocytes/macrophages, granulocytes, fibroblasts, erythrocytes, multinucleated giant cells, and basophils. This overgrowth was not statical as generally assumed but rather dynamic as illustrated by our observation that at day 1 after implantation we mainly found monocytes/macrophages and granulocytes that on later time points were substituted by fibroblasts. As the inflammatory reaction predictably interfere with survival of encapsulated cells, efforts should be made to suppress activities or recruitment of inflammatory cells. These efforts may be temporary rather than permanent because most inflammatory cells have disappeared after 2 weeks of implantation.

Cellular support systems for alginate microcapsules containing islets, as composite bioartificial pancreas

To improve the functional performance of microencapsulated islets, we examined the effects of putative cellular support systems, consisting of rat purified Sertoli cells (SC) and astrocytes (AA), on coenveloped allogeneic islets. Coincubation of islets with SC but not AA, resulted in significant stimulation of beta cell mitogenesis, coupled with a significant increase in in vitro glucose-stimulated insulin release. Preliminarily, the xenotransplantation of coencapsulated rat islets and homologous SC significantly prolonged remission of hyperglycemia in diabetic mice.

Effects of micro-encapsulation on morphology and endocrine function of cryopreserved neonatal porcine islet-like cell clusters

BACKGROUND

For the success of clinical islets transplantation, the development of a long-term storage method is necessary. However, the structure of digested islets is scanty for culture and cryopreservation. In this study, the effect of micro-encapsulation to cryopreserved porcine islet-like cell clusters (ICCs) was investigated.

METHODS

The ICCs prepared from neonatal pigs by collagenase digestion and culture technique were cryopreserved and micro-encapsulated in 5% agarose membranes. After cryopreservation, ICC cultured without encapsulation (group A) and cultured with encapsulation (group B) were assessed by comparison with no cryopreserved ICC (control) both in vitro by static incubation test and in vivo in a xenotransplantation study.

RESULTS

Micro-encapsulation was able to maintain the fine morphology and the number of ICCs of group B after 7 days of culture. There were not significant differences in insulin secretion of group B and control on day 1 and 7 of culture (1 day:11+/-0.99, 7 days: 5.30+/-1.08 microU/ICC/hr NS versus control). On day 7 of culture, the retrieval rate of group B (105.2+/-9.8%) is obviously higher compared with group A (63.0+/-6.3%). In the xenotransplatation model, the ICCs of group B showed long survival time (7.9+/-0.4 weeks) and good transplantation effect.

CONCLUSION

Our study suggests that micro-encapsulation is one of the useful method for cryopreserved ICC to maintain the fine morphology and effectively recover the endocrine function.

Time course of transforming growth factor-beta(1) (TGF-beta(1)) mRNA expression in the host reaction to alginate-poly-L-lysine microcapsules following implantations into rat epididymal fat pads

Microencapsulation of islets of Langerhans within semipermeable membranes has been proposed to prevent their immune destruction after transplantation. However, the successful application of this method is impaired by a pericapsular reaction, which eventually induces graft failure. Our goal is to study the role of cytokines in the pathogenesis of this reaction, using the model of alginate-poly-L-lysine microcapsule implantation into Wistar rat epididymal fat pads (EFP). The specific objective of this study was to determine the time course of transforming growth factor (TGF)-beta(1) mRNA expression by semi-quantitative reverse transcriptase-polymerase chain reaction. Microcapsules induced an increase of TGF-beta(1) mRNA expression that reached a maximum 14 days after implantation. Seven, 14, 30, and 60 days after microcapsule implantation, the expression of TGF-beta(1) mRNA was significantly higher in pericapsular infiltrate cells than in nonimplanted EFP cells (p<0.05, p<0.0001, p<0.005, and p<0.01, respectively). Injection of physiological saline induced a small and gradual augmentation of TGF-beta(1) mRNA expression with a maximum 30 days after injection (p<0.01 vs. nonimplanted EFP cells). These results demonstrated that microcapsule implantation, in comparison with saline injection, induce an early, extended, and amplified TGF-beta(1) mRNA expression. This suggests that TGF-beta(1) plays a role in the pathogenesis of the pericapsular host reaction.

Evaluation of graft-host response for various tissue sources and animal models

The efficacy of pancreatic islet transplants in correcting hyperglycemia and slowing the progression of complications in diabetics has been confirmed by many experimental and clinical studies. Unfortunately, the availability of human islets is extremely limited and, therefore, treatment of large numbers of human diabetic patients will almost certainly require either the use of islets harvested from animals (xenografts) or the use of insulin-secreting genetically modified cells of either human or animal origin. There is currently no effective regimen which will allow long-term survival of xenogeneic islets from widely unrelated donor-recipient combinations, such as pig-to-rodent, pig-to-dog, or pig-to-primate. There is considerable interest in the development of immunoisolation techniques for protection of donor islets. However, most materials used in immunoisolation devices are relatively bio-incompatible. Poly-L-lysine-alginate microcapsules are biocompatible and provide an optimal geometry for transmembrane diffusion of insulin and nutrients. Microcapsules allow long-term survival of xenogeneic islets in diabetic rodents or dogs with induced diabetes. However, mice and rats with spontaneous diabetes destroy encapsulated islet grafts within 2 to 3 weeks. Biopsies reveal large numbers of macrophages, immunoglobulins and limited numbers of helper and cytotoxic T-cells in the peri-microcapsule environment of the peritoneal cavity. Cytokines have been identified in peritoneal fluid from mice with islet grafts and may play a role in encapsulated islet destruction. Targeted immunomodulation by treatment of recipients with either anti-helper T-cell antibodies, or fusion proteins which block costimulatory interactions between antigen presenting cells and host T-cells have demonstrated synergy in significant prolongation of encapsulated islet xenograft survival in NOD mice with spontaneous diabetes. Technical improvements in microcapsule design also have contributed to prolonged graft survival. "Double-wall" microencapsulation provides a more durable microcapsule and islet pretreatment prior to encapsulation reduces the frequency of defective capsules with islets entrapped in the membrane. Long-term durability of encapsulated islet grafts remains a concern and further improvements in microcapsule design are a prerequisite to clinical trials.

Transplantation of pancreatic islets contained in minimal volume microcapsules in diabetic high mammalians

To minimize technical problems relating to excessive size (600-800 mu in diameter) of standard alginate microcapsules (CSM) for pancreatic islet graft immunoisolation, we have developed two novel minimal volume, chemically identical, capsule prototypes (MVC): 1) coherent microcapsules (CM), and 2) medium-size microcapsules (300-400 mu, MSM). CM, which envelop each individual islet within a thin alginate hydrogel cast, are prepared by emulsification, whereas MSM are made by atomizing the islet-alginate suspension through a special microdroplet generator. Upon graft into diabetic rodents, CM have shown to immunoprotect both allo- and xenogeneic nondiscordant islets, and restored normoglycemia. In higher mammals, at subtherapeutic doses, CM fully immunoprotected islet allografts (pig-->pig), but only temporarily xenografts (dog-->pig). We then used MSM to immunoisolate canine islet allografts in the peritoneal cavity of dogs with spontaneous insulin-dependent diabetes. Of three grafted dogs, two showed full remission of hyperglycemia with insulin withdrawal. MSM could represent an intermediate solution between CSM and CM for peritoneal immunoisolated islet transplants.

Bioartificial pancreas: alternative supply of insulin-secreting cells

In this study, insulin secretion function of INS-1 cells immunoisolated in microcapsules was evaluated. Following encapsulation, the immunoisolated INS-1 cells continued to propagate and flourish within the microcapsules during the entire two-month in vitro incubation period. The insulin secretion from encapsulated INS-1 cells following seven days of in vitro culture increased from 1.6 +/- 0.2 ng/2h/10(6) cells in a glucose-free medium to 11.5 +/- 2.1 ng/2h/10(6) cells at 16.7 mM glucose. In vivo, transplants of 1.2 x 10(7) cells into each of six diabetic C57BL/6 mice resulted in the restoration of normoglycemia in all graft recipients for up to 60 days post transplantation. Most capsules recovered from two animals 30 days post transplantation were free of cell overgrowth and physically intact. Immunostaining for insulin of the cells within the recovered capsules clearly indicated the presence of insulin. The presented data demonstrate the potential use of an immunoisolated beta-cell line for the treatment of diabetes.

Lentivirus-mediated Bcl-2 expression in betaTC-tet cells improves resistance to hypoxia and cytokine-induced apoptosis while preserving in vitro and in vivo control of insulin secretion

betaTC-tet cells are conditionally immortalized pancreatic beta cells which can confer long-term correction of hyperglycemia when transplanted in syngeneic streptozocin diabetic mice. The use of these cells for control of type I diabetes in humans will require their encapsulation and transplantation in non-native sites where relative hypoxia and cytokines may threaten their survival. In this study we genetically engineered betaTC-tet cells with the anti-apoptotic gene Bcl-2 using new lentiviral vectors and showed that it protected this cell line against apoptosis induced by hypoxia, staurosporine and a mixture of cytokines (IL-1beta, IFN-gamma and TNF-alpha). We further demonstrated that Bcl-2 expression permitted growth at higher cell density and with shorter doubling time. Expression of Bcl-2, however, did not inter- fere either with the intrinsic mechanism of growth arrest present in the betaTC-tet cells or with their normal glucose dose-dependent insulin secretory activity. Furthermore, Bcl-2 expressing betaTC-tet cells retained their capacity to secrete insulin under mild hypoxia. Finally, transplantation of these cells under the kidney capsule of streptozocin diabetic C3H mice corrected hyperglycemia for several months. These results demonstrate that the murine betaTC-tet cell line can be genetically modified to improve its resistance against different stress-induced apoptosis while preserving its normal physiological function. These modified cells represent an improved source for cell transplantation therapy of type I diabetes.

Studies on smaller (approximately 315 microM) microcapsules: IV. Feasibility and safety of intrahepatic implantations of small alginate poly-L-lysine microcapsules

The most successful transplantation site of nonencapsulated islets of Langerhans is the liver. Because usual alginate poly-L-lysine microcapsules were too large (700-1200 microm diameter) for intravascular implantations and were almost exclusively implanted intraperitoneally, the question of the preferred implantation site of microencapsulated islets has received little attention. The feasibility of implanting smaller (approximately 315 microm) alginate poly-L-lysine microcapsules into the liver and the effect of such implantations on portal pressure and liver histology was evaluated in Wistar rats. A bolus of 10,000 microcapsules of 315 microm diameter was injected intraportally (group 1; n = 22). The portal pressure increased from 6.4 +/- 1.8 mmHg to a maximum of 19 mmHg, returned to basal levels within 2 h, and remained normal after 2 months. In group 2 (n = 3), following the injection of 10,000 larger microcapsules (420 microm), the portal pressure increased to > 60 mmHg and two out of the three rats died within 3 h. When 5,000 microcapsules of 420-microm diameter were injected (group 3; n = 5), the portal pressure peaked to 30 +/- 8 mmHg and remained elevated after 4 h (12 +/- 3 mmHg), but returned to normal (8 +/- 1 mmHg) after 2 weeks. Histological studies showed normal hepatic architecture without collagen deposition into portal tracts occupied by microcapsules.

CONCLUSION

intrahepatic implantations of approximately 315-microm alginate poly-L-lysine microcapsules are feasible and safe. These results justify further investigation of this potential implantation site for microencapsulated islets.

In vitro and in vivo performance of porcine islets encapsulated in interfacially photopolymerized poly(ethylene glycol) diacrylate membranes

The usefulness of interfacial photopolymerization of poly(ethylene glycol) (PEG) diacrylate at a variety of concentrations and molecular weights to form hydrogel membranes for encapsulating porcine islets of Langerhans was investigated. The results from this study show in vitro and in vivo function of PEG-encapsulated porcine islets and the ability of PEG membranes to prevent immune rejection in a discordant xenograft model. Encapsulated islets demonstrated an average viability of 85% during the first week after encapsulation, slightly but significantly lower than unencapsulated controls. Encapsulated porcine islets were shown to be glucose responsive using static glucose stimulation and perifusion assays. Higher rates of insulin release were observed for porcine islets encapsulated in lower concentrations of PEG diacrylate (10-13%), not significantly reduced relative to unencapsulated controls, than were observed in islets encapsulated in higher concentrations (25%) of PEG diacrylate. Perifusion results showed biphasic insulin release from encapsulated islets in response to glucose stimulation. Streptozotocin-induced diabetic athymic mice maintained normoglycemia for up to 110 days after the implantation of 5,000-8,000 encapsulated porcine islet equivalents into the peritoneal cavity. Normoglycemia was also confirmed in these animals using glucose tolerance tests. PEG diacrylate-encapsulated porcine islets were shown to be viable and contain insulin after 30 days in the peritoneal cavity of Sprague-Dawley rats, a discordant xenograft model. From these studies, we conclude that PEG diacrylate encapsulation of porcine islets by interfacial photopolymerization shows promise for use as a method of xenoprotection toward a bioartificial endocrine pancreas.

Lack of recurrence of insulin-dependent diabetes mellitus in syngeneic and allogeneic islet-transplanted diabetic biobreeding rats

BACKGROUND

Previous reports on experimental islet transplantation in animal models of human insulin-dependent diabetes mellitus show that islet grafts are susceptible to autoimmune destruction similar to that seen in native pancreatic islets. In this study, we demonstrated a lack of disease recurrence in diabetic BioBreeding (BB) rats after syngeneic and allogeneic islet transplantation.

METHODS

Four hundred to 1200 islets from BB (RT1u) and Lewis (RT1(1)) donors, isolated with stationary collagenase digestion and Ficoll density purification, were intraportally transplanted into spontaneously diabetic BB rats. The recipients received no immunologic manipulations before or after islet transplantation.

RESULTS

When more than 900 syngeneic islets or when more than 600 allogeneic islets were transplanted, BB recipients remained normoglycemic for over 280 days, irrespective of age at onset, duration of exogenous insulin treatment, or age at transplantation. When at least 500 islets were transplanted, the recipients survived for a long period with normoglycemia or in a noninsulin-dependent diabetic state. Upon histological examination, mononuclear cell infiltration was observed in every islet graft examined, but the severity of infiltration in most of the grafts was mild to moderate. These results indicate that the islet grafts in the BB recipients were destroyed extremely slowly.

CONCLUSIONS

It is conceivable that in our BB colony, a state of immunologically low responsiveness that allows diabetic animals to accept syngeneic or allogeneic islet grafts, occurs around the onset period and becomes more pronounced with aging. Our BB rat colony can be considered to be a novel substrain with unique immunological characteristics.

In vitro and in vivo evaluation of insulin-producing beta TC6-F7 cells in microcapsules

In the present study, the insulin secretory capacity of beta TC6-F7 cells in microcapsules was evaluated. The cell mass within capsules was found to expand in a three-dimensional fashion, in contrast to cells seeded on plates that grew as a monolayer. In in vitro studies, both free and encapsulated cells were found to secrete insulin in the absence of glucose, at 13.6 +/- 1.1 and 14.5 +/- 0.9 ng.10(6) cells-1.60 min-1, respectively, with the response rising to a maximum of 26.0 +/- 0.8 and 31 +/- 2.3 ng.10(6) cells-1.60 min-1 in the presence of 16.8 mM glucose. Encapsulated cells were able to produce Ca2+ responses in the presence of KCl (50 mM) and BAY K 8644 (100 microM). In in vivo studies, intraperitoneal transplantation of 3.0 x 10(6) microencapsulated cells into mice (n = 5) with streptozotocin-induced diabetes resulted in the restoration of normoglycemia up to 57 days. Insulin concentrations rose from 0.4 +/- 0.1 ng/ml before the graft administration to 2.2 +/- 0.8 ng/ml after the transplantation in the normoglycemic recipients. An oral glucose challenge in transplant recipients demonstrated a flat glucose response, suggesting extremely high glucose clearance rates. These data demonstrate the potential use of the immunoisolated beta-cell lines for the treatment of diabetes.

A sensitivity study of the key parameters in the interfacial photopolymerization of poly(ethylene glycol) diacrylate upon porcine islets

A method has been defined to interfacially photopolymerize poly(ethylene glycol) diacrylates (PEG diacrylates) to form a crosslinked hydrogel membrane upon the surfaces of porcine islets of Langerhans to serve as an immune barrier for allo- and xenotransplantation. A sensitivity study of six key parameters in the interfacial photopolymerization process was performed to aid in determination of the optimal encapsulation conditions, leading to the most uniform hydrogel membranes and viable islets. The key parameters included the concentrations of the components of the initiation scheme, namely eosin Y, triethanolamine, and 1-vinyl 2-pyrrolidinone. Other parameters investigated included the duration and flux of laser irradiation and the PEG diacrylate molecular weight. Each parameter was doubled and halved from the standard conditions used in the encapsulation process while holding all the remaining parameters at the standard conditions. The effects of changing each parameter on islet viability, encapsulation efficiency, and gel thickness were quantified. Islet viability was sensitive to the duration of laser illumination, viability significantly increasing as the duration was reduced. Encapsulation efficiency was sensitive to the concentrations of eosin Y, triethanolamine, and 1-vinyl 2-pyrrolidinone, to the laser flux, and to the PEG diacrylate molecular weight. Increasing the concentration of eosin Y significantly improved the encapsulation efficiency, while decreasing the concentration of 1-vinyl 2-pyrrolidinone and increasing the concentration of triethanolamine had the greatest effects in significantly reducing the encapsulation efficiency. Gel thickness was sensitive to the concentrations of triethanolamine and 1-vinyl 2-pyrrolidinone, to the duration of laser illumination, and to the PEG diacrylate molecular weight. Increasing the PEG diacrylate molecular weight significantly increased the gel thickness, while decreasing the concentration of 1-vinyl 2-pyrrolidinone and increasing the concentration of triethanolamine had the greatest effects in significantly reducing the gel thickness. From this sensitivity study, conditions were determined to encapsulate porcine islets, resulting in greater than 90% islet viability and greater than 90% encapsulation efficiency.

Alginate-based microcapsules for immunoprotected islet transplantation

Islet transplantation is a promising therapeutic approach for the treatment of insulin-dependent diabetes mellitus. Nevertheless, its broader clinical use is hampered by the shortage of human organ donors as well as the need for a permanent immunosuppressive drug therapy in order to avoid rejection. Microencapsulation shall help to overcome this problem by creating an immunoprotected transplantation site. Biocompatibility of the encapsulation material and the possible immuno-interaction of the grafted tissue and the host immune system need to be examined very carefully. In transplantation experiments, we could show that the long-term function of the graft is dependent on the species of the islet donor, indicating that there has to be a recognition of the encapsulated islet despite the encapsulation membrane. This could be confirmed by in vitro data in the mixed lymphocyte islet culture (MLIC). Moreover, morphological studies of the tissue reaction toward encapsulated syngeneic vs. allogeneic vs. xenogeneic encapsulated islets reveal that the greater the difference between donor and recipient species the greater the amount of fibrous tissue formation. Thus, for the outcome of transplantation experiments, not only the material-related biocompatibility but as well the reaction towards the whole device (consisting of the capsule plus the encapsulated tissue) are crucial. Therefore, immunoprotection does not only comprise the protection of the grafted tissue from the host immune effector mechanisms but as well the inhibition of the recognition of the graft by the host immune system.

An encapsulation system for the immunoisolation of pancreatic islets

Over a thousand combinations of polyanions and polycations were tested to search for new polymer candidates that would be suitable for encapsulation of living cells. The combination of sodium alginate, cellulose sulfate, poly (methylene-co-guanidine) hydrochloride, calcium chloride, and sodium chloride was most promising. In parallel, a novel multiloop chamber reactor was developed to control the time of complex formation and to negate gravitational effects such as pancreatic islet sedimentation and droplet deformation during the encapsulation process. Encapsulated rat islets demonstrated glucose-stimulated insulin secretion in vitro, and reversed diabetes in mice. This new capsule formulation and encapsulation system allows independent adjustments of capsule size, wall thickness, mechanical strength, and permeability, which may offer distinct advantages for immunoisolating cells.

Normalization of diabetes in spontaneously diabetic cynomologus monkeys by xenografts of microencapsulated porcine islets without immunosuppression

Porcine pancreatic islets were microencapsulated in alginate-polylysine-alginate capsules and transplanted intraperitoneally into nine spontaneously diabetic monkeys. After one, two, or three transplants of 3-7 x 10(4) islets per recipient, seven of the monkeys became insulin independent for periods ranging from 120 to 804 d with fasting blood glucose levels in the normoglycemic range. Glucose clearance rates in the transplant recipients were significantly higher than before the graft administration and the insulin secretion during glucose tolerance tests was significantly higher compared with pretransplant tests. Porcine C-peptide was detected in all transplant recipients throughout their period of normoglycemia while none was found before the graft administration. Hemoglobin A1C levels dropped significantly within 2 mo after transplantation. While ketones were detected in the urine of all recipients before the graft administration, all experimental animals became ketone free 2 wk after transplantation. Capsules recovered from two recipients 3 mo after the restoration of normoglycemia were found physically intact with enclosed islets clearly visible. The capsules were free of cellular overgrowth. Examination of internal organs of two of the animals involved in our transplantation studies for the duration of 2 yr revealed no untoward effect of the extended presence of the microcapsules.

Xenografts of porcine islets immunoprotected in hollow fibres reduce the incidence of diabetes in non-obese diabetic mice

Non-obese diabetic (NOD) mice develop an autoimmune disease with a long prodromal period and constitute a model for investigating the prevention of human insulin-dependent diabetes mellitus. Since insulin injected prophylactically has been shown to reduce incidence of diabetes in NOD mice, we tested a new strategy consisting of prophylactic xenografts of porcine pancreatic islets immunoprotected in semipermeable hollow fibres. Female NOD mice were transplanted twice (at 60 and 180 days of age) with islet-containing or empty fibres. Within the group grafted with protected islets, the incidence of diabetes was reduced (37 vs 75%; p < 0.01), the onset of disease was delayed (p < 0.02), and the severity of lymphocytic inflammation of endogenous islets was reduced (p < 0.02). When already diabetic mice were not taken into account for analysis, blood glucose level was slightly lower in those grafted with islet-containing fibres (p < 0.04). Graft function was also evidenced by HPLC separation of porcine insulin in NOD sera. Histological and perifusion studies of fibres retrieved from recipients confirmed immunoprotection. During co-transfer, T splenocytes from mice grafted with islet-containing fibres were able to reduce the capacity of T cells from diabetic donors to adoptively transfer the disease (p < 0.01). Antigens for islet-cell autoantibodies (ICA) in pancreata from both groups were compared by immunofluorescence with the same ICA-positive human sera to ensure that differences were due to antigen quantitative changes. These antigens, which could serve as an index of a possibly more extensive antigen beta-cell rest, were decreased (p < 0.01) in mice grafted with protected islets. Reduction of diabetes and insulitis following early islet transplantation may thus be due to generation of cellular mechanisms that actively suppress disease, and possibly in part to a decrease in antigens which make beta cells less vulnerable to autoimmune aggression. These effects can be obtained with xenogeneic islets protected in hollow fibres, thereby eliminating the need for immunosuppression. Based on the concept of prophylactic insulin therapy, this form of insulin administration offers a controlled means of delivering insulin to meet the physiological needs of recipients.

Normalization of diabetes by xenotransplantation of cryopreserved microencapsulated pancreatic islets. Application of a new strategy in islet banking

To develop a requisite islet bank for the clinical implementation of an injectable bioartificial endocrine pancreas, microencapsulated islets were cryopreserved and assessed both in vitro by static glucose challenge and in a transplantation study. The insulin response of cryopreserved encapsulated rat islets was comparable with fresh islets. Transplantation of 800-900 banked rat islets resulted in the normalization of the metabolic blood glucose perturbation, body weight, and general health characteristics in 8 out of 8 diabetic mice for the study duration of 90 days. Whereas free islets are easily fragmented and lost during the freezing process, the capsule protects the fragile islets from freezing damage, increasing the retrieval rate from 79.5 +/- 9.8% to 97.2 +/- 1.3.

The rat epididymal fat pad as an implantation site for the study of microcapsule biocompatibility: validation of the method

The study of microcapsule biocompatibility is hindered by their uneven distribution and low recovery when implanted into the peritoneum. We evaluated the use of the rat epididymal fat pad as a microcapsule implantation site for biocompatibility studies. The recovery rate of microcapsules containing 85Sr-labeled microspheres was 99.6 +/- 0.75%. Microcapsules made from the same batch of nonpurified alginate, were injected into both fat pads of male Lewis rats (n = 18) and retrieved 14 days later. A semiquantitative fibrosis score scaled from 0 to 3.0 showed that the pericapsular reaction was uniform throughout a fat pad, and that the results of the two fat pads were equivalent because the null hypothesis of inequivalence was rejected (P < .001). Thus, this method can be used to compare the biocompatibility of microcapsule of differing compositions.

Biomaterials in tissue engineering

Biomaterials play a pivotal role in field of tissue engineering. Biomimetic synthetic polymers have been created to elicit specific cellular functions and to direct cell-cell interactions both in implants that are initially cell-free, which may serve as matrices to conduct tissue regeneration, and in implants to support cell transplantation. Biomimetic approaches have been based on polymers endowed with bioadhesive receptor-binding peptides and mono- and oligosaccharides. These materials have been patterned in two- and three-dimensions to generate model multicellular tissue architectures, and this approach may be useful in future efforts to generate complex organizations of multiple cell types. Natural polymers have also played an important role in these efforts, and recombinant polymers that combine the beneficial aspects of natural polymers with many of the desirable features of synthetic polymers have been designed and produced. Biomaterials have been employed to conduct and accelerate otherwise naturally occurring phenomena, such as tissue regeneration in wound healing in the otherwise healthy subject; to induce cellular responses that might not be normally present, such as healing in a diseased subject or the generation of a new vascular bed to receive a subsequent cell transplant; and to block natural phenomena, such as the immune rejection of cell transplants from other species or the transmission of growth factor signals that stimulate scar formation. This review introduces the biomaterials and describes their application in the engineering of new tissues and the manipulation of tissue responses.

Complement activation by alginate-polylysine microcapsules used for islet transplantation

A foreign body reaction is frequently observed around implanted microcapsules of alginate-polylysine. Since complement activation can play a role in this reaction, we checked in vitro the ability of empty alginate-polylysine microcapsules to activate complement. Human serum was incubated with microcapsules, and complement activation was evaluated by two methods: the complement hemolytic activity (CH50) and the assay of the C3adesArg fragment. The occurrence of complement activation in the presence of microcapsules was suggested both by a CH50 decrease and by high C3adesArg levels despite C3adesArg adsorption to the capsule membrane. Capsule membrane protection against the cytotoxic effects of complement was also tested. No hemolysis occurred when microencapsulated sensitized sheep erythrocytes were incubated with activated complement. In conclusion, the microcapsule membrane can protect cells against activated complement fragments. Nevertheless, alginate-polylysine microcapsules do activate complement, and this effect must be considered for its use as an implant.