Improved survival of macroencapsulated islets of Langerhans by preimplantation of the immunoisolating device: a morphometric study

Tissue engineering approaches and generation of insulin-producing cells to treat type 1 diabetes

Tissue engineering (TE) has become a new effective solution to a variety of medical problems, including diabetes. Mesenchymal stem cells (MSCs), which have the ability to differentiate into endodermal and mesodermal cells, appear to be appropriate for this function. The purpose of this review was to evaluate the outcomes of various researches on the insulin-producing cells (IPCs) generation from MSCs with TE approaches to increase efficacy of type 1 diabetes treatments. The search was performed in PubMed/Medline, Scopus and Embase databases until 2021. Studies revealed that MSCs could also differentiate into IPCs under certain conditions. Therefore, a wide range of protocols have been used for this differentiation, but their effectiveness is very different. Scaffolds can provide a microenvironment that enhances the MSCs to IPCs differentiation, improves their metabolic activity and up-regulate pancreatic-specific transcription factors. They also preserve IPCs architecture and enhance insulin production as well as protect against cell death. This systematic review offers a framework for prospective research based on data. In vitro and in vivo evidence suggests that scaffold-based TE can improve the viability and function of IPCs.

A therapeutic convection-enhanced macroencapsulation device for enhancing β cell viability and insulin secretion

Islet transplantation for type 1 diabetes treatment has been limited by the need for lifelong immunosuppression regimens. This challenge has prompted the development of macroencapsulation devices (MEDs) to immunoprotect the transplanted islets. While promising, conventional MEDs are faced with insufficient transport of oxygen, glucose, and insulin because of the reliance on passive diffusion. Hence, these devices are constrained to two-dimensional, wafer-like geometries with limited loading capacity to maintain cells within a distance of passive diffusion. We hypothesized that convective nutrient transport could extend the loading capacity while also promoting cell viability, rapid glucose equilibration, and the physiological levels of insulin secretion. Here, we showed that convective transport improves nutrient delivery throughout the device and affords a three-dimensional capsule geometry that encapsulates 9.7-fold-more cells than conventional MEDs. Transplantation of a convection-enhanced MED (ceMED) containing insulin-secreting β cells into immunocompetent, hyperglycemic rats demonstrated a rapid, vascular-independent, and glucose-stimulated insulin response, resulting in early amelioration of hyperglycemia, improved glucose tolerance, and reduced fibrosis. Finally, to address potential translational barriers, we outlined future steps necessary to optimize the ceMED design for long-term efficacy and clinical utility.

From Mesenchymal Stromal/Stem Cells to Insulin-Producing Cells: Progress and Challenges

Mesenchymal stromal cells (MSCs) are an attractive option for cell therapy for type 1 diabetes mellitus (DM). These cells can be obtained from many sources, but bone marrow and adipose tissue are the most studied. MSCs have distinct advantages since they are nonteratogenic, nonimmunogenic and have immunomodulatory functions. Insulin-producing cells (IPCs) can be generated from MSCs by gene transfection, gene editing or directed differentiation. For directed differentiation, MSCs are usually cultured in a glucose-rich medium with various growth and activation factors. The resulting IPCs can control chemically-induced diabetes in immune-deficient mice. These findings are comparable to those obtained from pluripotent cells. PD-L₁ and PD-L₂ expression by MSCs is upregulated under inflammatory conditions. Immunomodulation occurs due to the interaction between these ligands and PD-1 receptors on T lymphocytes. If this function is maintained after differentiation, life-long immunosuppression or encapsulation could be avoided. In the clinical setting, two sites can be used for transplantation of IPCs: the subcutaneous tissue and the omentum. A 2-stage procedure is required for the former and a laparoscopic procedure for the latter. For either site, cells should be transplanted within a scaffold, preferably one from fibrin. Several questions remain unanswered. Will the transplanted cells be affected by the antibodies involved in the pathogenesis of type 1 DM? What is the functional longevity of these cells following their transplantation? These issues have to be addressed before clinical translation is attempted.

Graphical Abstract

Nanotechnology in cell replacement therapies for type 1 diabetes

Islet transplantation is a promising long-term, compliance-free, complication-preventing treatment for type 1 diabetes. However, islet transplantation is currently limited to a narrow set of patients due to the shortage of donor islets and side effects from immunosuppression. Encapsulating cells in an immunoisolating membrane can allow for their transplantation without the need for immunosuppression. Alternatively, "open" systems may improve islet health and function by allowing vascular ingrowth at clinically attractive sites. Many processes that enable graft success in both approaches occur at the nanoscale level-in this review we thus consider nanotechnology in cell replacement therapies for type 1 diabetes. A variety of biomaterial-based strategies at the nanometer range have emerged to promote immune-isolation or modulation, proangiogenic, or insulinotropic effects. Additionally, coating islets with nano-thin polymer films has burgeoned as an islet protection modality. Materials approaches that utilize nanoscale features manipulate biology at the molecular scale, offering unique solutions to the enduring challenges of islet transplantation.

Transplantation of Macroencapsulated Insulin-Producing Cells

PURPOSE OF REVIEW

There is considerable interest in using macroencapsulation devices as a delivery strategy for transplanting insulin-producing cells. This review aims to summarize recent advances, to highlight remaining challenges, and to provide recommendations for the field.

RECENT FINDINGS

A variety of new device designs have been reported to improve biocompatibility and to provide protection for islet/beta cells from immune destruction while allowing continuous secretion of insulin. Some of these new approaches are in clinical trials, but more research is needed to determine how sufficient beta-cell mass can be transplanted in a clinically applicable device size, and that insulin is secreted with kinetics that will safely provide adequate controls of glucose levels. Macroencapsulation is a potential solution to transplant beta cells without immunosuppression in diabetes patients, but new strategies must be developed to show that this approach is feasible.

3D Printed porous polyamide macrocapsule combined with alginate microcapsules for safer cell-based therapies

Cell microencapsulation is an attractive strategy for cell-based therapies that allows the implantation of genetically engineered cells and the continuous delivery of de novo produced therapeutic products. However, the establishment of a way to retrieve the implanted encapsulated cells in case the treatment needs to be halted or when cells need to be renewed is still a big challenge. The combination of micro and macroencapsulation approaches could provide the requirements to achieve a proper immunoisolation, while maintaining the cells localized into the body. We present the development and characterization of a porous implantable macrocapsule device for the loading of microencapsulated cells. The device was fabricated in polyamide by selective laser sintering (SLS), with controlled porosity defined by the design and the sintering conditions. Two types of microencapsulated cells were tested in order to evaluate the suitability of this device; erythropoietin (EPO) producing C2C12 myoblasts and Vascular Endothelial Growth Factor (VEGF) producing BHK fibroblasts. Results showed that, even if the metabolic activity of these cells decreased over time, the levels of therapeutic protein that were produced and, importantly, released to the media were stable.

Improved Histological Evaluation of Vascularity around an Immunoisolation Device by Correlating Number of Vascular Profiles to Glucose Exchange

The aim of this study was to determine which vessels are important for the exchange of small molecules, such as glucose, from the microcirculation into an immunoisolation device. Reasonably, those vessels should be the ones of interest in histological evaluations. In a previous study, we examined the diffusion of glucose from the microcirculation into immunoisolation devices that had been implanted subcutaneously in rats for various times (i.e., 1, 2, and 4 weeks and 3 months). The glucose kinetic data were then correlated with the number of vascular profiles within 15 and 250 μm from the device. Significant correlations were found only at 250 μm. To examine the relation further between function and vascularization, we used the histological samples from the previous study and counted vascular profiles within various distances between 15 and 400 μm from the device. The number was then correlated with the already available glucose kinetic data. The highest correlations were found at 75 and 100 μm (p < 0.05). We therefore suggest that vascular profiles within 100 μm should be used when evaluating the vascularity of tissue surrounding an immunoisolation device. We also studied neovascularization asymmetries between the side of the membrane facing the skin and that facing the muscle. At 1 and 2 weeks about half of the devices were mainly vascularized on the side facing the skin, whereas the rest were equally vascularized on the two sides. At 3 months, all devices were well vascularized, and no striking vascularization asymmetries were seen.

The paracrine effects of adipose-derived stem cells on neovascularization and biocompatibility of a macroencapsulation device

The foreign-body response to biomaterials compromises the performance of many biomedical devices by severe fibrosis and limited neovascularization. Mesenchymal stem cells are known to secrete cytokines for treating inflammatory conditions. In this study, we aim to investigate whether the paracrine products of adipose-derived mesenchymal stem cells (ADSCs) can affect the microenvironment of biomaterials and improve tissue responses to biomaterial implants. A model system was built by loading ADSC spheroids into a macroencapsulation device composed of polytetrafluoroethylene (PTFE) filtration membranes. Soluble ADSC factors that diffused out of the device in vitro promoted the angiogenetic activity of endothelial cells and affected the secretion pattern of macrophages. In vivo study was carried out by subcutaneously embedding blank or ADSC-laden devices in rats. Following a 4 week implantation, the ADSC-laden devices were better vascularized and induced significantly less fibrotic tissue formation in comparison to the non-cellular controls. This study may facilitate our understanding of foreign-body responses and suggest new ways to improve the tissue reaction of biomedical devices for cell-based therapy.

Islet cell transplantation for the treatment of type 1 diabetes: recent advances and future challenges

Islet transplantation is a well-established therapeutic treatment for a subset of patients with complicated type I diabetes mellitus. Prior to the Edmonton Protocol, only 9% of the 267 islet transplant recipients since 1999 were insulin independent for >1 year. In 2000, the Edmonton group reported the achievement of insulin independence in seven consecutive patients, which in a collaborative team effort propagated expansion of clinical islet transplantation centers worldwide in an effort to ameliorate the consequences of this disease. To date, clinical islet transplantation has established improved success with insulin independence rates up to 5 years post-transplant with minimal complications. In spite of marked clinical success, donor availability and selection, engraftment, and side effects of immunosuppression remain as existing obstacles to be addressed to further improve this therapy. Clinical trials to improve engraftment, the availability of insulin-producing cell sources, as well as alternative transplant sites are currently under investigation to expand treatment. With ongoing experimental and clinical studies, islet transplantation continues to be an exciting and attractive therapy to treat type I diabetes mellitus with the prospect of shifting from a treatment for some to a cure for all.

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.

Oxygen supply to encapsulated therapeutic cells

Therapeutic cells encapsulated in immunobarrier devices have promise for treatment of a variety of human diseases without immunosuppression. The absence of sufficient oxygen supply to maintain viability and function of encapsulated tissue has been the most critical impediment to progress. Within the framework of oxygen supply limitations, we review the major issues related to development of these devices, primarily in the context of encapsulated islets of Langerhans for treating diabetes, including device designs and materials, supply of tissue, protection from immune rejection, and maintenance of cell viability and function. We describe various defensive measures investigated to enhance survival of transplanted tissue, and we review the diverse approaches to enhancement of oxygen transport to encapsulated tissue, including manipulation of diffusion distances and oxygen permeability of materials, induction of neovascularization with angiogenic factors and vascularizing membranes, and methods for increasing the oxygen concentration adjacent to encapsulated tissue so as to exceed that in the microvasculature. Recent developments, particularly in this latter area, suggest that the field is ready for clinical trials of encapsulated therapeutic cells to treat diabetes.

Human embryonic stem cell derived islet progenitors mature inside an encapsulation device without evidence of increased biomass or cell escape

There are several challenges to successful implementation of a cell therapy for insulin dependent diabetes derived from human embryonic stem cells (hESC). Among these are development of functional insulin producing cells, a clinical delivery method that eliminates the need for chronic immunosuppression, and assurance that hESC derived tumors do not form in the patient. We and others have shown that encapsulation of cells in a bilaminar device (TheraCyte) provides immunoprotection in rodents and primates. Here we monitored human insulin secretion and employed bioluminescent imaging (BLI) to evaluate the maturation, growth, and containment of encapsulated islet progenitors derived from CyT49 hESC, transplanted into mice. Human insulin was detectable by 7 weeks post-transplant and increased 17-fold over the course of 8 weeks, yet during this period the biomass of encapsulated cells remained constant. Remarkably, by 20 weeks post-transplant encapsulated cells secreted sufficient levels of human insulin to ameliorate alloxan induced diabetes. Further, bioluminescent imaging revealed for the first time that hESCs remained fully contained in encapsulation devices for up to 150 days, the longest period tested. Collectively, the data suggest that encapsulated hESC derived islet progenitors hold great promise as an effective and safe cell replacement therapy for insulin dependent diabetes.

Evaluation of a collagen-chitosan hydrogel for potential use as a pro-angiogenic site for islet transplantation

Islet transplantation to treat type 1 diabetes (T1D) has shown varied long-term success, due in part to insufficient blood supply to maintain the islets. In the current study, collagen and collagen:chitosan (10:1) hydrogels, +/- circulating angiogenic cells (CACs), were compared for their ability to produce a pro-angiogenic environment in a streptozotocin-induced mouse model of T1D. Initial characterization showed that collagen-chitosan gels were mechanically stronger than the collagen gels (0.7 kPa vs. 0.4 kPa elastic modulus, respectively), had more cross-links (9.2 vs. 7.4/µm(2)), and were degraded more slowly by collagenase. After gelation with CACs, live/dead staining showed greater CAC viability in the collagen-chitosan gels after 18 h compared to collagen (79% vs. 69%). In vivo, collagen-chitosan gels, subcutaneously implanted for up to 6 weeks in a T1D mouse, showed increased levels of pro-angiogenic cytokines over time. By 6 weeks, anti-islet cytokine levels were decreased in all matrix formulations ± CACs. The 6-week implants demonstrated increased expression of VCAM-1 in collagen-chitosan implants. Despite this, infiltrating vWF(+) and CXCR4(+) angiogenic cell numbers were not different between the implant types, which may be due to a delayed and reduced cytokine response in a T1D versus non-diabetic setting. The mechanical, degradation and cytokine data all suggest that the collagen-chitosan gel may be a suitable candidate for use as a pro-angiogenic ectopic islet transplant site.

Bioengineered sites for islet cell transplantation

Although islet transplantation has demonstrated its potential use in treating type 1 diabetes, this remains limited by the need for daily immunosuppression. Islet encapsulation was then proposed with a view to avoiding any immunosuppressive regimen and related side effects. In order to obtain a standard clinical procedure in terms of safety and reproducibility, two important factors have to be taken into account: the encapsulation design (which determines the graft volume) and the implantation site. Indeed, the implantation site should meet certain requirements: (1) its space must be large enough for the volume of transplanted tissues; (2) there must be proximity to abundant vascularization with a good oxygen supply; (3) there must be real-time access to physiologically representative blood glucose levels; (4) there must be easy access for implantation and the reversibility of the procedure (for safety); and finally, (5) the site should have minimal early inflammatory reaction and promote long-term survival. The aim of this article is to review possible preclinical/clinical implantation sites (in comparison with free islets) for encapsulated islet transplantation as a function of the encapsulation design: macro/microcapsules and conformal coating.

Mutual effect of subcutaneously transplanted human adipose-derived stem cells and pancreatic islets within fibrin gel

While subcutaneous tissue has been proposed as a potential site for pancreatic islet transplantation, concern remains that the microvasculature of subcutaneous tissue is too poor to support transplanted islets. In an effort to overcome this limitation, we evaluated whether fibrin gel with human adipose-derived stem cells (hADSCs) and rat pancreatic islets could cure diabetes mellitus when transplanted into the subcutaneous space of diabetic mice. Subcutaneously co-transplanted islets and hADSCs showed normalization of the diabetic recipient's blood glucose levels. The result was enhanced by co-treatment of fibroblast growth factor-2 (FGF2) in the fibrin gel. The hADSCs enhanced islet viability after transplantation by secreting various growth factors that can protect islets from hypoxic damage. Afterward, hADSCs could maintain islet viability by recruiting new microvasculature nearby the transplanted islets via overexpression of vascular endothelial growth factor (VEGF). The hADSCs did not directly differentiate into endothelial cells (no detection of biomarkers of human endothelial cells), but showed evidence of differentiation toward insulin-secreting cells (detection of human insulin). Mice receiving islet transplantation alone did not become normoglycemic. Collectively, co-transplantation of fibrin gel with islets and hADSCs will expand the indications for islet transplant therapy and the potential clinical application of cell-based therapy.

The TheraCyte™ device protects against islet allograft rejection in immunized hosts

Clinically, many candidates for islet transplantation are already immunized, which increases their risk of graft rejection. Encapsulation of pancreatic islets using the TheraCyte™ device has been shown to protect against allograft rejection in nonimmunized recipients. However, the capacity of the TheraCyte™ device to prevent rejection in immunized recipients has not yet been studied. In this study, the protective capacity of the TheraCyte™ device was evaluated in an allogeneic rat model. Lewis rats were used as islet donors, and nonimmunized (control) and alloimmunized, diabetic Wistar-Furth (WF) rats were used as recipients. Graft survival was shorter in immunized recipients than in nonimmunized recipients (mean survival, 5.3 ± 2.7 and 9.3 ± 1.6 days, respectively, p < 0.01) when nonencapsulated islets were transplanted under the kidney capsule. When islets were transplanted into the TheraCyte™ device, graft function was maintained during the 6-month study period in both immunized and nonimmunized rats. In oral glucose tolerance tests performed at 1 month after transplantation, both groups had similar insulin and blood glucose levels indicating similar metabolic functions. Volume densities and absolute volumes of tissue inside the devices 6 months after transplantation were also comparable between the two groups, indicating that both groups maintained similar amounts of endocrine tissue. A higher number of IFN-γ-producing CD8+ T-cells were detected in immunized WF rats compared to control WF rats transplanted with encapsulated islets. This suggests that donor-specific alloreactivity in recipient rats was sustained throughout the study period. This study suggests that the TheraCyte™ device protects islet allografts also in immunized recipients. Our results further highlight the potential for using macroencapsulation to avoid immunosuppressive therapy in clinical islet transplantation.

Exendin-4 increases the expression of hypoxia-inducible factor-1α in rat islets and preserves the endocrine cell volume of both free and macroencapsulated islet grafts

In this study, we evaluated the effects of exendin-4 on free and encapsulated islet grafts in a rodent model. We also investigated the role of a transcription factor, hypoxia-inducible factor-1 (HIF-1), in mediating the beneficial effects of exendin-4. Diabetic athymic mice were transplanted with free rat islets under the kidney capsule or with macroencapsulated rat islets SC with or without exendin-4, islet preculture (exendin-4 0.1 nM for 20 h), and/or recipient treatment (IP 100 ng/day, day 0-7). The mice were followed for 4 weeks and the graft function and β-cell volume were evaluated. The effects of exendin-4 on islet HIF-1α mRNA and protein expression and on ATP content in a rat insulinoma cell line (INS-1E) were also examined. Preculture with exendin-4 followed by recipient treatment improved the outcome of both free (73% graft function vs. 26% in controls, p = 0.03) and macroencapsulated islet grafts (100% vs. 25% in controls, p = 0.02). In macroencapsulated grafts, the exendin-4-treated group had significantly larger endocrine volume, less graft necrosis, and more blood vessels around the capsule. In rat islets cultured with exendin-4, HIF-1α mRNA and protein expression were significantly enhanced. ATP content was increased in exendin-4-treated INS-1E cells under hypoxic conditions. The improved functional outcome after transplantation of a marginal islet mass with a brief initial treatment with exendin-4 is related to a larger surviving endocrine cell volume. Exendin-4 may improve islet graft resistance to hypoxia during the peritransplant period by increasing the expression of HIF-1α.

Concise review: pancreas regeneration: recent advances and perspectives

The replacement of functional pancreatic β-cells is seen as an attractive potential therapy for diabetes, because diabetes results from an inadequate β-cell mass. Inducing replication of the remaining β-cells and new islet formation from progenitors within the pancreas (neogenesis) are the most direct ways to increase the β-cell mass. Stimulation of both replication and neogenesis have been reported in rodents, but their clinical significance must still be shown. Because human islet transplantation is limited by the scarcity of donors and graft failure within a few years, efforts have recently concentrated on the use of stem cells to replace the deficient β-cells. Currently, embryonic stem cells and induced pluripotent stem cells achieve high levels of β-cell differentiation, but their clinical use is still hampered by ethical issues and/or the risk of developing tumors after transplantation. Pancreatic epithelial cells (duct, acinar, or α-cells) represent an appealing alternative to stem cells because they demonstrate β-cell differentiation capacities. Yet translation of such capacity to human cells after significant in vitro expansion has yet to be achieved. Besides providing new β-cells, cell therapy also has to address the question on how to protect the transplanted cells from destruction by the immune system via either allo- or autoimmunity. Encouraging developments have been made in encapsulation and immunomodulation techniques, but many challenges still remain. Herein, we discuss recent advances in the search for β-cell replacement therapies, current strategies for circumventing the immune system, and mandatory steps for new techniques to be translated from bench to clinics.

Inconsistent formation and nonfunction of insulin-positive cells from pancreatic endoderm derived from human embryonic stem cells in athymic nude rats

Embryonic stem cell therapy has been proposed as a therapeutic strategy to restore β-cell mass and function in T1DM. Recently, a group from Novocell (now ViaCyte) reported successful development of glucose-responsive islet-like structures after implantation of pancreatic endoderm (PE) derived from human embryonic stem cells (hESC) into immune-deficient mice. Our objective was to determine whether implantation of hESC-derived pancreatic endoderm from Novocell into athymic nude rats results in development of viable glucose-responsive pancreatic endocrine tissue. Athymic nude rats were implanted with PE derived from hESC either via implantation into the epididymal fat pads or by subcutaneous implantation into TheraCyte encapsulation devices for 20 wk. Blood glucose, weight, and human insulin/C-peptide secretion were monitored by weekly blood draws. Graft β-cell function was assessed by a glucose tolerance test, and graft morphology was assessed by immunohistochemistry and immunofluorescence. At 20 wk postimplantation, epididymal fat-implanted PE progressed to develop islet-like structures in 50% of implants, with a mean β-cell fractional area of 0.8 ± 0.3%. Human C-peptide and insulin were detectable, but at very low levels (C-peptide = 50 ± 26 pmol/l and insulin = 15 ± 7 pmol/l); however, there was no increase in human C-peptide/insulin levels after glucose challenge. There was no development of viable pancreatic tissue or meaningful secretory function when human PE was implanted in the TheraCyte encapsulation devices. These data confirm that islet-like structures develop from hESC differentiated to PE by the protocol developed by NovoCell. However, the extent of endocrine cell formation and secretory function is not yet sufficient to be clinically relevant.

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.

Cell microencapsulation

In the past several decades, many attempts have been made to prevent the rejection of transplanted cells by the immune system. Cell encapsulation is primary machinery for cell transplantation and new materials and approaches were developed to encapsulate various types of cells to treat a wide range of diseases. This technology involves placing the transplanted cells within a biocompatible membrane in attempt to isolate the cells from the host immune attack and enhance or prolong their function in vivo. In this chapter, we will review the situation of cell microencapsulation field and discuss its potentials and challenges for cell therapy and regeneration of tissue function.

Focus on collagen: in vitro systems to study fibrogenesis and antifibrosis state of the art

Fibrosis represents a major global disease burden, yet a potent antifibrotic compound is still not in sight. Part of the explanation for this situation is the difficulties that both academic laboratories and research and development departments in the pharmaceutical industry have been facing in re-enacting the fibrotic process in vitro for screening procedures prior to animal testing. Effective in vitro characterization of antifibrotic compounds has been hampered by cell culture settings that are lacking crucial cofactors or are not holistic representations of the biosynthetic and depositional pathway leading to the formation of an insoluble pericellular collagen matrix. In order to appreciate the task which in vitro screening of antifibrotics is up against, we will first review the fibrotic process by categorizing it into events that are upstream of collagen biosynthesis and the actual biosynthetic and depositional cascade of collagen I. We point out oversights such as the omission of vitamin C, a vital cofactor for the production of stable procollagen molecules, as well as the little known in vitro tardy procollagen processing by collagen C-proteinase/BMP-1, another reason for minimal collagen deposition in cell culture. We review current methods of cell culture and collagen quantitation vis-à-vis the high content options and requirements for normalization against cell number for meaningful data retrieval. Only when collagen has formed a fibrillar matrix that becomes cross-linked, invested with ligands, and can be remodelled and resorbed, the complete picture of fibrogenesis can be reflected in vitro. We show here how this can be achieved. A well thought-out in vitro fibrogenesis system represents the missing link between brute force chemical library screens and rational animal experimentation, thus providing both cost-effectiveness and streamlined procedures towards the development of better antifibrotic drugs.

Human beta-cell precursors mature into functional insulin-producing cells in an immunoisolation device: implications for diabetes cell therapies

BACKGROUND

Islet transplantation is limited by the need for chronic immunosuppression and the paucity of donor tissue. As new sources of human beta-cells are developed (e.g., stem cell-derived tissue), transplanting them in a durable device could obviate the need for immunosuppression, while also protecting the patient from any risk of tumorigenicity. Here, we studied (1) the survival and function of encapsulated human beta-cells and their progenitors and (2) the engraftment of encapsulated murine beta-cells in allo- and autoimmune settings.

METHODS

Human islets and human fetal pancreatic islet-like cell clusters were encapsulated in polytetrafluorethylene devices (TheraCyte) and transplanted into immunodeficient mice. Graft survival and function was measured by immunohistochemistry, circulating human C-peptide levels, and blood glucose levels. Bioluminescent imaging was used to monitor encapsulated neonatal murine islets.

RESULTS

Encapsulated human islet-like cell clusters survived, replicated, and acquired a level of glucose responsive insulin secretion sufficient to ameliorate hyperglycemia in diabetic mice. Bioluminescent imaging of encapsulated murine neonatal islets revealed a dynamic process of cell death followed by regrowth, resulting in robust long-term allograft survival. Further, in the non-obese diabetic (NOD) mouse model of type I diabetes, encapsulated primary beta-cells ameliorated diabetes without stimulating a detectable T-cell response.

CONCLUSIONS

We demonstrate for the first time that human beta-cells function is compatible with encapsulation in a durable, immunoprotective device. Moreover, our study suggests that encapsulation of beta-cells before terminal differentiation will be a successful approach for new cell-based therapies for diabetes, such as those derived from stem cells.

Preimplantation of an immunoprotective device can lower the curative dose of islets to that of free islet transplantation: studies in a rodent model

Islet graft survival inside macroencapsulation devices is suboptimal. We hypothesized that induction of neovascularization by preimplantation of devices would improve the physiological conditions, thereby lowering the number of islets required for cure. Several rat islets were transplanted to TheraCyte immunoprotective devices implanted subcutaneously in diabetic athymic mice. Cure rates in the groups with preimplanted devices were significantly better than in those with freshly implanted devices (375 islets: 8/8 vs. 1/6, P=0.003; 125 islets: 6/6 vs. 0/7, P=0.001). Morphometric evaluations of the 125 islet groups showed higher fractional and absolute volumes of endocrine tissue in the group with preimplanted devices (P<0.001 and P=0.035, respectively). In the following dose titration study, using preimplanted devices, as low as 50 islets cured diabetic mice (100% cure, n=6). We conclude that preimplantation significantly lowers the curative dose of macroencapsulated islets to levels resembling those of free islets transplanted under the renal capsule.

Cell therapy and the safety of embryonic stem cell-derived grafts

Recent developments in the identification, in vitro culture and differentiation of stem cells point to the unprecedented potential of these cells, or their derivatives, to cure degenerative disorders. Human embryonic stem cells (hESC) offer the particular advantage of prolonged proliferative capacity and great versatility in the lineages that can be formed in culture. Translating these advantages into clinical benefits faces many challenges, including efficient differentiation into the desired cell type(s), maintaining genetic stability during long-term culture and, finally, ensuring the absence of potentially tumorigenic hESC from the final product. It is this final safety issue that will form the focus of this review.

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.

Xenotransplantation: an update on recent progress and future perspectives

Currently, the number of patients awaiting transplantation is continuously increasing, and shortage of available deceased organ donors is the major limitation for organ and cell allotransplantation. Research to develop alternative sources of tissues is ongoing and xenogeneic organs or cells represent an attractive solution. This review focuses on recent progress achieved in this field, including the development of newly genetically modified animal donors and new immunosuppressive approaches. As xenotransplantation is moving closer to clinical application, future perspectives must establish guidelines to ensure that future clinical trials are carried out ethically and safely.