Long-term Efficacy and Biocompatibility of Encapsulated Islet Transplantation With Chitosan-Coated Alginate Capsules in Mice and Canine Models of Diabetes

Bioprinting and synthetic biology approaches to engineer functional endocrine pancreatic constructs

Diabetes is a complex disease affecting over 500 million people worldwide. Traditional approaches, such as insulin delivery, are mainstay treatments, but do not cure the disease. Recent advances in biofabrication and synthetic biology offer new hope for the development of tissue constructs recapitulating salient organ functions. Here, we discuss recent progress in bioprinting a functional endocrine pancreas, ranging from cell sources to main advances in biomaterials. We review innovative areas for the development of this field, with a particular focus on the convergence of synthetic biology and cell engineering with bioprinting, which opens new avenues for developing advanced in vitro models and regenerative, transplantable grafts, with the potential to provide independence from exogenous insulin administration.

Implantation of Islets Co-Seeded with Tregs in a Novel Biomaterial Reverses Diabetes in the NOD Mouse Model

BACKGROUND

Type 1 diabetes (T1D) results in autoreactive T cells chronically destroying pancreatic islets. This often results in irreplaceable loss of insulin-producing beta cells. To reverse course, a combinatorial strategy of employing glucose-responsive insulin restoration coupled with inhibiting autoreactive immune responses is required.

METHODS

Non-obese diabetic mice received a single intraperitoneal implantation of a novel biomaterial co-seeded with insulin-producing islets and T regulatory cells (Tregs). Controls included biomaterial seeded solely with islets, or biomaterial only groups. Mice were interrogated for changes in inflammation and diabetes progression via blood glucose monitoring, multiplex serum cytokine profiling, flow cytometry and immunohistochemistry assessments.

RESULTS

Islet and Tregs co-seeded biomaterial recipients had increased longevity, insulin secretion, and normoglycemia through 180 days post-implantation compared to controls. Serum profile revealed reduced TNFα, IFNγ, IL-1β and increased IL-10, insulin, C-Peptide, PP and PPY in recipients receiving co-seeded biomaterial. Evaluation of the resected co-seeded biomaterial revealed reduced infiltrating autoreactive CD8 + and CD4 + T cells concomitant with sustained presence of Foxp3 + Tregs; further analysis revealed that the few infiltrated resident effector CD4+ or CD8+ T cells were anergic, as measured by low levels of IFNγ and Granzyme-B upon stimulation when compared to controls. Interestingly, studies also revealed increased Tregs in the pancreas. However, there was no restoration of the pancreas beta cell compartment, suggesting normoglycemia and production of insulin levels were largely supported by the implanted co-seeded biomaterial.

CONCLUSION

These studies show the efficacy of a combinatorial approach seeding Tregs with pancreatic islets in a novel self-assembling organoid for reversing T1D.

Advancements and Challenges in Immune Protection Strategies for Islet Transplantation

Pancreatic islet transplantation is a crucial treatment for managing type 1 diabetes (T1D) in clinical settings. However, the limited availability of human cadaveric islet donors and the need for ongoing administration of immunosuppressive agents post-transplantation hinder the widespread use of this treatment. Stem cell-derived islet organoids have emerged as an effective alternative to primary human islets. Nevertheless, implementing this cell replacement therapy still requires chronic immune suppression, which may result in life-long side effects. To address these challenges, innovations such as encapsulation devices, universal stem cells, and immunomodulatory strategies are being developed to mitigate immune rejection and prolong the function of the transplant. This review outlines the contemporary challenges in pancreatic β cell therapy, particularly immune rejection, and recent progress in immune-isolation devices, hypoimmunogenic stem cells, and immune regulation of transplants. A comprehensive evaluation of the advantages and limitations of these approaches will contribute to improved future clinical investigations. With these promising advancements, the application of pancreatic β cell therapy holds the potential to effectively treat T1D and benefit a larger population of T1D patients.

Synthetic and Natural Biomaterials in Veterinary Medicine and Ophthalmology: A Review of Clinical Cases and Experimental Studies

In recent years, there has been a growing interest in 3D printing technology within the field of bioengineering. This technology offers the ability to create devices with intricate macro- and micro-geometries, as well as specific models. It has particularly gained attention for its potential in personalized medicine, allowing for the production of organ or tissue models tailored to individual patient needs. Further, 3D printing has opened up possibilities to manufacture structures that can substitute, complement, or enhance damaged or dysfunctional organic parts. To apply 3D printing in the medical field, researchers have studied various materials known as biomaterials, each with distinct chemical and physical characteristics. These materials fall into two main categories: hard and soft materials. Each biomaterial needs to possess specific characteristics that are compatible with biological systems, ensuring long-term stability and biocompatibility. In this paper, we aim to review some of the materials used in the biomedical field, with a particular focus on those utilized in veterinary medicine and ophthalmology. We will discuss the significant findings from recent scientific research, focusing on the biocompatibility, structure, applicability, and in vitro and in vivo biological characteristics of two hard and four soft materials. Additionally, we will present the current state and prospects of veterinary ophthalmology.

Challenges and Perspectives for Future Considerations in the Bioengineering of a Bioartificial Pancreas

There is an unrelenting interest in the development of a reliable bioartificial pancreas construct since the first description of this technology of encapsulated islets by Lim and Sun in 1980 because it promised to be a curative treatment for Type 1 Diabetes Mellitus (T1DM). Despite the promise of the concept of encapsulated islets, there are still some challenges that impede the full realization of the clinical potential of the technology. In this review, we will first present the justification for continued research and development of this technology. Next, we will review key barriers that impede progress in this field and discuss strategies that can be used to design a reliable construct capable of effective long-term performance after transplantation in diabetic patients. Finally, we will share our perspectives on areas of additional work for future research and development of the technology.

Long-term efficacy of encapsulated xenogeneic islet transplantation: Impact of encapsulation techniques and donor genetic traits

AIMS/INTRODUCTION

To investigate the long-term efficacy of various encapsulated xenogeneic islet transplantation, and to explore the impact of different donor porcine genetic traits on islet transplantation outcomes.

MATERIALS AND METHODS

Donor porcine islets were obtained from wild-type, α1,3-galactosyltransferase knockout (GTKO) and GTKO with overexpression of membrane cofactor protein genotype. Naked, alginate, alginate-chitosan (AC), alginate-perfluorodecalin (A-PFD) and AC-perfluorodecalin (AC-PFD) encapsulated porcine islets were transplanted into diabetic mice.

RESULTS

In vitro assessments showed no differences in the viability and function of islets across encapsulation types and donor porcine islet genotypes. Xenogeneic encapsulated islet transplantation with AC-PFD capsules showed the most favorable long-term outcomes, maintaining normal blood glucose levels for 180 days. A-PFD capsules showed comparable results to AC-PFD capsules, followed by AC capsules and alginate capsules. Conversely, blood glucose levels in naked islet transplantation increased to >300 mg/dL within a week after transplantation. Naked islet transplantation outcomes showed no improvement based on donor islet genotype. However, alginate or AC capsules showed delayed increases in blood glucose levels for GTKO and GTKO with overexpression of membrane cofactor protein porcine islets compared with wild-type porcine islets.

CONCLUSION

The AC-PFD capsule, designed to ameliorate both hypoxia and inflammation, showed the highest long-term efficacy in xenogeneic islet transplantation. Genetic modifications of porcine islets with GTKO or GTKO with overexpression of membrane cofactor protein did not influence naked islet transplantation outcomes, but did delay graft failure when encapsulated.

Encapsulation and immune protection for type 1 diabetes cell therapy

Type 1 Diabetes (T1D) involves the autoimmune destruction of insulin-producing β-cells in the pancreas. Exogenous insulin injections are the current therapy but are user-dependent and cannot fully recapitulate physiological insulin secretion dynamics. Since the emergence of allogeneic cell therapy for T1D, the Edmonton Protocol has been the most promising immunosuppression protocol for cadaveric islet transplantation, but the lack of donor islets, poor cell engraftment, and required chronic immunosuppression have limited its application as a therapy for T1D. Encapsulation in biomaterials on the nano-, micro-, and macro-scale offers the potential to integrate islets with the host and protect them from immune responses. This method can be applied to different cell types, including cadaveric, porcine, and stem cell-derived islets, mitigating the issue of a lack of donor cells. This review covers progress in the efforts to integrate insulin-producing cells from multiple sources to T1D patients as a form of cell therapy.

Hypoxia within subcutaneously implanted macroencapsulation devices limits the viability and functionality of densely loaded islets

Introduction

Subcutaneous macroencapsulation devices circumvent disadvantages of intraportal islet therapy. However, a curative dose of islets within reasonably sized devices requires dense cell packing. We measured internal PO2 of implanted devices, mathematically modeled oxygen availability within devices and tested the predictions with implanted devices containing densely packed human islets.

Methods

Partial pressure of oxygen (PO2) within implanted empty devices was measured by noninvasive 19F-MRS. A mathematical model was constructed, predicting internal PO2, viability and functionality of densely packed islets as a function of external PO2. Finally, viability was measured by oxygen consumption rate (OCR) in day 7 explants loaded at various islet densities.

Results

In empty devices, PO2 was 12 mmHg or lower, despite successful external vascularization. Devices loaded with human islets implanted for 7 days, then explanted and assessed by OCR confirmed trends proffered by the model but viability was substantially lower than predicted. Co-localization of insulin and caspase-3 immunostaining suggested that apoptosis contributed to loss of beta cells.

Discussion

Measured PO2 within empty devices declined during the first few days post-transplant then modestly increased with neovascularization around the device. Viability of islets is inversely related to islet density within devices.

A Case for Material Stiffness as a Design Parameter in Encapsulated Islet Transplantation

Diabetes is a disease that plagues over 463 million people globally. Approximately 40 million of these patients have type 1 diabetes mellitus (T1DM), and the global incidence is increasing by up to 5% per year. T1DM is where the body's immune system attacks the pancreas, specifically the pancreatic beta cells, with antibodies to prevent insulin production. Although current treatments such as exogenous insulin injections have been successful, exorbitant insulin costs and meticulous administration present the need for alternative long-term solutions to glucose dysregulation caused by diabetes. Encapsulated islet transplantation (EIT) is a tissue-engineered solution to diabetes. Donor islets are encapsulated in a semipermeable hydrogel, allowing the diffusion of oxygen, glucose, and insulin but preventing leukocyte infiltration and antibody access to the transplanted cells. Although successful in small animal models, EIT is still far from commercial use owing to necessary long-term systemic immunosuppressants and consistent immune rejection. Most published research has focused on tailoring the characteristics of the capsule material to promote clinical viability. However, most studies have been limited in scope to biochemical changes. Current mechanobiology studies on the effect of substrate stiffness on the function of leukocytes, especially macrophages-primary foreign body response (FBR) orchestrators, show promise in tailoring a favorable response to tissue-engineered therapies such as EIT. In this review, we explore strategies to improve the clinical viability of EIT. A brief overview of the immune system, the FBR, and current biochemical approaches will be elucidated throughout this exploration. Furthermore, an argument for using substrate stiffness as a capsule design parameter to increase EIT efficacy and clinical viability will be posed.

Recent advances in endocrine organoids for therapeutic application

The endocrine system, consisting of the hypothalamus, pituitary, endocrine glands, and hormones, plays a critical role in hormone metabolic interactions. The complexity of the endocrine system is a significant obstacle to understanding and treating endocrine disorders. Notably, advances in endocrine organoid generation allow a deeper understanding of the endocrine system by providing better comprehension of molecular mechanisms of pathogenesis. Here, we highlight recent advances in endocrine organoids for a wide range of therapeutic applications, from cell transplantation therapy to drug toxicity screening, combined with development in stem cell differentiation and gene editing technologies. In particular, we provide insights into the transplantation of endocrine organoids to reverse endocrine dysfunctions and progress in developing strategies for better engraftments. We also discuss the gap between preclinical and clinical research. Finally, we provide future perspectives for research on endocrine organoids for the development of more effective treatments for endocrine disorders.

Type 1 diabetes and engineering enhanced islet transplantation

The development of new therapeutic approaches to treat type 1 diabetes mellitus (T1D) relies on the precise understanding and deciphering of insulin-secreting β-cell biology, as well as the mechanisms responsible for their autoimmune destruction. β-cell or islet transplantation is viewed as a potential long-term therapy for the millions of patients with diabetes. To advance the field of insulin-secreting cell transplantation, two main research areas are currently investigated by the scientific community: (1) the identification of the developmental pathways that drive the differentiation of stem cells into insulin-producing cells, providing an inexhaustible source of cells; and (2) transplantation strategies and engineered transplants to provide protection and enhance the functionality of transplanted cells. In this review, we discuss the biology of pancreatic β-cells, pathology of T1D and current state of β-cell differentiation. We give a comprehensive view and discuss the different possibilities to engineer enhanced insulin-secreting cell/islet transplantation from a translational perspective.

Differentiation of Microencapsulated Neonatal Porcine Pancreatic Cell Clusters in Vitro Improves Transplant Efficacy in Type 1 Diabetes Mellitus Mice

BACKGROUND

Neonatal porcine pancreatic cell clusters (NPCCs) have been proposed as an alternative source of β cells for islet transplantation because of their low cost and growth potential after transplantation. However, the delayed glucose lowering effect due to the immaturity of NPCCs and immunologic rejection remain as a barrier to NPCC's clinical application. Here, we demonstrate accelerated differentiation and immune-tolerant NPCCs by in vitro chemical treatment and microencapsulation.

METHODS

NPCCs isolated from 3-day-old piglets were cultured in F-10 media and then microencapsulated with alginate on day 5. Differentiation of NPCCs is facilitated by media supplemented with activin receptor-like kinase 5 inhibitor II, triiodothyronine and exendin-4 for 2 weeks. Marginal number of microencapsulated NPCCs to cure diabetes with and without differentiation were transplanted into diabetic mice and observed for 8 weeks.

RESULTS

The proportion of insulin-positive cells and insulin mRNA levels of NPCCs were significantly increased in vitro in the differentiated group compared with the undifferentiated group. Blood glucose levels decreased eventually after transplantation of microencapsulated NPCCs in diabetic mice and normalized after 7 weeks in the differentiated group. In addition, the differentiated group showed nearly normal glucose tolerance at 8 weeks after transplantation. In contrast, neither blood glucose levels nor glucose tolerance were improved in the undifferentiated group. Retrieved graft in the differentiated group showed greater insulin response to high glucose compared with the undifferentiated group.

CONCLUSION

in vitro differentiation of microencapsulated immature NPCCs increased the proportion of insulin-positive cells and improved transplant efficacy in diabetic mice without immune rejection.

Advanced strategies to thwart foreign body response to implantable devices

Mitigating the foreign body response (FBR) to implantable medical devices (IMDs) is critical for successful long-term clinical deployment. The FBR is an inevitable immunological reaction to IMDs, resulting in inflammation and subsequent fibrotic encapsulation. Excessive fibrosis may impair IMDs function, eventually necessitating retrieval or replacement for continued therapy. Therefore, understanding the implant design parameters and their degree of influence on FBR is pivotal to effective and long lasting IMDs. This review gives an overview of FBR as well as anti-FBR strategies. Furthermore, we highlight recent advances in biomimetic approaches to resist FBR, focusing on their characteristics and potential biomedical applications.

Core-Shell Microcapsules: Biofabrication and Potential Applications in Tissue Engineering and Regenerative Medicine

The construction of biomaterial scaffolds that accurately recreate the architecture of living tissues in vitro is a major challenge in the field of tissue engineering and regenerative medicine. Core-shell microcapsules...

Modulating the foreign body response of implants for diabetes treatment

Diabetes Mellitus is a group of diseases characterized by high blood glucose levels due to patients' inability to produce sufficient insulin. Current interventions often require implants that can detect and correct high blood glucose levels with minimal patient intervention. However, these implantable technologies have not reached their full potential in vivo due to the foreign body response and subsequent development of fibrosis. Therefore, for long-term function of implants, modulating the initial immune response is crucial in preventing the activation and progression of the immune cascade. This review discusses the different molecular mechanisms and cellular interactions involved in the activation and progression of foreign body response (FBR) and fibrosis, specifically for implants used in diabetes. We also highlight the various strategies and techniques that have been used for immunomodulation and prevention of fibrosis. We investigate how these general strategies have been applied to implants used for the treatment of diabetes, offering insights on how these devices can be further modified to circumvent FBR and fibrosis.

Cell microencapsulation technologies for sustained drug delivery: Latest advances in efficacy and biosafety

The development of cell microencapsulation systems began several decades ago. However, today few systems have been tested in clinical trials. For this reason, in the last years, researchers have directed efforts towards trying to solve some of the key aspects that still limit efficacy and biosafety, the two major criteria that must be satisfied to reach the clinical practice. Regarding the efficacy, which is closely related to biocompatibility, substantial improvements have been made, such as the purification or chemical modification of the alginates that normally form the microspheres. Each of the components that make up the microcapsules has been carefully selected to avoid toxicities that can damage the encapsulated cells or generate an immune response leading to pericapsular fibrosis. As for the biosafety, researchers have developed biological circuits capable of actively responding to the needs of the patients to precisely and accurately release the demanded drug dose. Furthermore, the structure of the devices has been subject of study to adequately protect the encapsulated cells and prevent their spread in the body. The objective of this review is to describe the latest advances made by scientist to improve the efficacy and biosafety of cell microencapsulation systems for sustained drug delivery, also highlighting those points that still need to be optimized.

Alginate microgels as delivery vehicles for cell-based therapies in tissue engineering and regenerative medicine

Conventional stem cell delivery typically utilize administration of directly injection of allogenic cells or domesticated autogenic cells. It may lead to immune clearance of these cells by the host immune systems. Alginate microgels have been demonstrated to improve the survival of encapsulated cells and overcome rapid immune clearance after transplantation. Moreover, alginate microgels can serve as three-dimensional extracellular matrix to support cell growth and protect allogenic cells from rapid immune clearance, with functions as delivery vehicles to achieve sustained release of therapeutic proteins and growth factors from the encapsulated cells. Besides, cell-loaded alginate microgels can potentially be applied in regenerative medicine by serving as injectable engineered scaffolds to support tissue regrowth. In this review, the properties of alginate and different methods to produce alginate microgels are introduced firstly. Then, we focus on diverse applications of alginate microgels for cell delivery in tissue engineering and regenerative medicine.

Safety and function of a new pre-vascularized bioartificial pancreas in an allogeneic rat model

Cell encapsulation could overcome limitations of free islets transplantation but is currently limited by inefficient cells immune protection and hypoxia. As a response to these challenges, we tested in vitro and in vivo the safety and efficacy of a new macroencapsulation device named MailPan^®. Membranes of MailPan^® device were tested in vitro in static conditions. Its bio-integration and level of oxygenation was assessed after implantation in non-diabetic rats. Immune protection properties were also assessed in rat with injection in the device of allogeneic islets with incompatible Major Histocompatibility Complex. Finally, function was assessed in diabetic rats with a Beta cell line injected in MailPan^®. In vitro, membranes of the device showed high permeability to glucose, insulin, and rejected IgG. In rat, the device displayed good bio-integration, efficient vascularization, and satisfactory oxygenation (>5%), while positron emission tomography (PET)-scan and angiography also highlighted rapid exchanges between blood circulation and the MailPan^®. The device showed its immune protection properties by preventing formation, by the rat recipient, of antibodies against encapsulated allogenic islets. Injection of a rat beta cell line into the device normalized fasting glycemia of diabetic rat with retrieval of viable cell clusters after 2 months. These data suggest that MailPan^® constitutes a promising encapsulation device for widespread use of cell therapy for type 1 diabetes.

Engineering Strategies to Improve Islet Transplantation for Type 1 Diabetes Therapy

Type 1 diabetes is an autoimmune disease in which the immune system attacks insulin-producing beta cells of pancreatic islets. Type 1 diabetes can be treated with islet transplantation; however, patients must be administered immunosuppressants to prevent immune rejection of the transplanted islets if they are not autologous or not engineered with immune protection/isolation. To overcome biological barriers of islet transplantation, encapsulation strategies have been developed and robustly investigated. While islet encapsulation can prevent the need for immunosuppressants, these approaches have not shown much success in clinical trials due to a lack of long-term insulin production. Multiple engineering strategies have been used to improve encapsulation and post-transplantation islet survival. In addition, more efficient islet cryopreservation methods have been designed to facilitate the scaling-up of islet transplantation. Other islet sources have been identified including porcine islets and stem cell-derived islet-like aggregates. Overall, islet-laden capsule transplantation has greatly improved over the past 30 years and is moving towards becoming a clinically feasible treatment for type 1 diabetes.

Polymeric Approaches to Reduce Tissue Responses Against Devices Applied for Islet-Cell Encapsulation

Immunoisolation of pancreatic islets is a technology in which islets are encapsulated in semipermeable but immunoprotective polymeric membranes. The technology allows for successful transplantation of insulin-producing cells in the absence of immunosuppression. Different approaches of immunoisolation are currently under development. These approaches involve intravascular devices that are connected to the bloodstream and extravascular devices that can be distinguished in micro- and macrocapsules and are usually implanted in the peritoneal cavity or under the skin. The technology has been subject of intense fundamental research in the past decade. It has co-evolved with novel replenishable cell sources for cure of diseases such as Type 1 Diabetes Mellitus that need to be protected for the host immune system. Although the devices have shown significant success in animal models and even in human safety studies most technologies still suffer from undesired tissue responses in the host. Here we review the past and current approaches to modulate and reduce tissue responses against extravascular cell-containing micro- and macrocapsules with a focus on rational choices for polymer (combinations). Choices for polymers but also choices for crosslinking agents that induce more stable and biocompatible capsules are discussed. Combining beneficial properties of molecules in diblock polymers or application of these molecules or other anti-biofouling molecules have been reviewed. Emerging are also the principles of polymer brushes that prevent protein and cell-adhesion. Recently also immunomodulating biomaterials that bind to specific immune receptors have entered the field. Several natural and synthetic polymers and even combinations of these polymers have demonstrated significant improvement in outcomes of encapsulated grafts. Adequate polymeric surface properties have been shown to be essential but how the surface should be composed to avoid host responses remains to be identified. Current insight is that optimal biocompatible devices can be created which raises optimism that immunoisolating devices can be created that allows for long term survival of encapsulated replenishable insulin-producing cell sources for treatment of Type 1 Diabetes Mellitus.

Mathematical predictions of oxygen availability in micro- and macro-encapsulated human and porcine pancreatic islets

Optimal function of immunoisolated islets requires adequate supply of oxygen to metabolically active insulin producing beta-cells. Using mathematical modeling, we investigated the influence of the pO₂ on islet insulin secretory capacity and evaluated conditions that could lead to the development of tissue anoxia, modeled for a 300 μm islet in a 500 μm microcapsule or a 500 μm planar, slab-shaped macrocapsule. The pO₂ was used to assess the part of islets that contributed to insulin secretion. Assuming a 500 μm macrocapsule with a 300 μm islet, with oxygen consumption rate (OCR) of 100-300 nmol min^-1 mg^-1 DNA, islets did not develop any necrotic core. The nonfunctional zone (with no insulin secretion if pO₂  < 0.1 mmHg) was 0.3% for human islets (OCR ~100 nmol/min/mg DNA) and 35% for porcine islets (OCR ~300 nmol/min/mg DNA). The OCR of the islet preparation is profoundly affected by islet size, with optimal size of <250 μm in diameter (human) or <150 μm (porcine). Our data suggest that microcapsules afford superior oxygen delivery to encapsulated islets than macrocapsules, and optimal islet function can be achieved by encapsulating multiple, small (<150 μm) islets with OCR of ~100 nmol min^-1 mg^-1 DNA (human islets) or ~200 nmol min^-1 mg^-1 DNA (porcine islets).

Hydrogel materials for the application of islet transplantation

Type 1 diabetes mellitus is a serious disease comprising approximately 10% of all diabetes cases, and the global incidence of type 1 diabetes mellitus is steadily rising without any promise of a cure in the near future. Although islet transplantation has proven to be an effective means of treating type 1 diabetes mellitus and promoting insulin independence in patients, its widespread implementation has been severely constrained by instances of post-transplantation islet cell death, rejection, and severe adverse immune responses. Islet encapsulation is an active area of research aimed at shielding implanted islets from immunological rejection and inflammation while still allowing for effective insulin and nutrient exchange with donor cells. Given their promising physical and chemical properties, hydrogels have been a major subject of focus in the field of islet transplantation and encapsulation technology, offering promising advances towards immunologically privileged islet implants. The present review therefore summarizes the current state of research regarding the use of hydrogels in the context of islet transplantation, including both natural molecular hydrogels and artificial polymer hydrogels, with the goal of understanding the current strengths and weaknesses of this treatment strategy.

Paving the way for successful islet encapsulation

Type 1 diabetes mellitus (T1DM) is a disorder that decimates pancreatic β-cells which produce insulin. Direct pancreatic islet transplantation cannot serve as a widespread therapeutic modality owing to the need for lifelong immunosuppression and donor shortage. Therefore, several encapsulation techniques have been developed to enclose the islets in semipermeable vehicles that will allow oxygen and nutrient input as well as insulin, other metabolites and waste output, while accomplishing immunoisolation. Although encapsulation technology continues to face significant obstacles, recent advances in material science, stem cell biology and immunology potentially serve as pathways to success. This review summarizes the accomplishments of the past 5 years.

Concise Review: Canine Diabetes Mellitus as a Translational Model for Innovative Regenerative Medicine Approaches

Diabetes mellitus (DM) is a common spontaneous endocrine disorder in dogs, which is defined by persistent hyperglycemia and insulin deficiency. Like type 1 diabetes (T1D) in people, canine DM is a complex and multifactorial disease in which genomic and epigenomic factors interact with environmental cues to induce pancreatic β‐cell loss and insulin deficiency, although the pathogenesis of canine DM is poorly defined and the role of autoimmunity is further controversial. Both diseases are incurable and require life‐long exogenous insulin therapy to maintain glucose homeostasis. Human pancreatic islet physiology, size, and cellular composition is further mirrored by canine islets. Although pancreatic or isolated islets transplantation are the only clinically validated methods to achieve long‐term normoglycemia and insulin independence, their availability does not meet the clinical need; they target a small portion of patients and have significant potential adverse effects. Therefore, providing a new source for β‐cell replacement is an unmet need. Naturally occurring DM in pet dogs, as a translational platform, is an untapped resource for various regenerative medicine applications that may offer some unique advantages given dogs' large size, longevity, heterogenic genetic background, similarity to human physiology and pathology, and long‐term clinical management. In this review, we outline different strategies for curative approaches, animal models used, and consider the value of canine DM as a translational animal/disease model for T1D in people. stem cells translational medicine2019;8:450–455

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.

Low-adhesive ethylene vinyl alcohol-based packaging to xenogeneic islet encapsulation for type 1 diabetes treatment

Transplantation of encapsulated porcine islets is proposed to treat type 1 diabetes. However, the envelopment of fibrous tissue and the infiltration of immune cells impair islet function and eventually cause implant failure. It is known that hemodialysis using an ethylene vinyl alcohol (EVOH) membrane results in minor tissue responses. Therefore, we hypothesized that using a low-adhesive EVOH membrane for encapsulation may prevent host cell accumulation and fibrous capsule formation. In this study, rat islets suspended in chitosan gel were encapsulated in bags made from highly porous EVOH membranes, and their in vitro insulin secretion function as well as in vivo performance was evaluated. The results showed that the EVOH bag did not affect islet survival or glucose-stimulated insulin secretion. Whereas naked islets were dysfunctional after 7 days of culture in vitro, islets within the EVOH bag produced insulin continuously for 30 days. Streptozotocin-induced diabetic mice were given islets-chitosan gel-EVOH implants intraperitoneally (650-800 islets equivalent) and exhibited lower blood glucose levels and regained body weight during a 4-week observation period. The transplanted mice had higher levels of serum insulin and C-peptide, with an improved blood glucose disappearance rate. Retrieved implants had minor tissue adhesion, and histology showed a limited number of mononuclear cells and fibroblasts surrounding the implants. No invasion of host cells into the EVOH bags was noticed, and the encapsulated islets were intact and positive for insulin-glucagon immunostaining. In conclusion, an EVOH bag can protect encapsulated islets, limit fibrous capsule formation, and extend graft function.

Polymeric nano-shielded islets with heparin-polyethylene glycol in a non-human primate model

Intraportal pancreatic islet transplantation incurs huge cell losses during its early stages due to instant blood-mediated inflammatory reactions (IBMIRs), which may also drive regulation of the adaptive immune system. Therefore, a method that evades IBMIR will improve clinical islet transplantation. We used a layer-by-layer approach to shield non-human primate (NHP) islets with polyethylene glycol (nano-shielded islets, NSIs) and polyethylene glycol plus heparin (heparin nano-shielded islets; HNSIs). Islets ranging from 10,000 to 20,000 IEQ/kg body weight were transplanted into 19 cynomolgus monkeys (n = 4, control; n = 5, NSI; and n = 10, HNSI). The mean C-peptide positive graft survival times were 68.5, 64 and 108 days for the control, NSI and HNSI groups, respectively (P = 0.012). HNSI also reduced the factors responsible for IBMIR in vitro. Based on these data, HNSIs in conjunction with clinically established immunosuppressive drug regimens will result in superior outcomes compared to those achieved with the current protocol for clinical islet transplantation.

Improvement of islet function and survival by integration of perfluorodecalin into microcapsules in vivo and in vitro

Hypoxic injury of islets is a major obstacle for encapsulated islet transplantation into the peritoneal cavity. To improve oxygen delivery to encapsulated islets, we integrated 20% of the oxygen carrier material, perfluorodecalin (PFD), in alginate capsules mixed with islets (PFD-alginate). Integration of PFD clearly improved islet viability and decreased reactive oxygen species production compared to islets encapsulated with alginate only (alginate) and naked islets exposed to hypoxia in vitro. In PFD-alginate capsules, HIF-1α expression was minimal, and insulin expression was well maintained. Furthermore, the best islet function represented by glucose-stimulated insulin secretion was observed for the PFD-alginate capsules in hypoxic condition. For the in vivo study, the same number of naked islets and encapsulated islets (alginate and PFD-alginate) was transplanted into streptozotocin-induced diabetic mice. Nonfasting blood glucose levels and the area under the curve for glucose based on intraperitoneal glucose tolerance tests in the PFD-alginate group were lower than in the alginate group. The harvested islets stained positive for insulin in all groups, but the ratio of dead cell area was 4 times higher in the alginate group than in the PFD-alginate group. In conclusion, integration of PFD in alginate microcapsules improved islet function and survival by minimizing the hypoxic damage of islets after intraperitoneal transplantation.

Effects of Composition of Alginate-Polyethylene Glycol Microcapsules and Transplant Site on Encapsulated Islet Graft Outcomes in Mice

Background

Understanding the effects of capsule composition and transplantation site on graft outcomes of encapsulated islets will aid in the development of more effective strategies for islet transplantation without immunosuppression.

Methods

Here, we evaluated the effects of transplanting alginate (ALG)-based microcapsules (Micro) in the confined and well-vascularized epididymal fat pad (EFP) site, a model of the human omentum, as opposed to free-floating in the intraperitoneal cavity (IP) in mice. We also examined the effects of reinforcing ALG with polyethylene glycol (PEG). To allow transplantation in the EFP site, we minimized capsule size to 500 ± 17 μm. Unlike ALG, PEG resists osmotic stress, hence we generated hybrid microcapsules by mixing PEG and ALG (MicroMix) or by coating ALG capsules with a 15 ± 2 μm PEG layer (Double).

Results

We found improved engraftment of fully allogeneic BALB/c islets in Micro capsules transplanted in the EFP (median reversal time [MRT], 1 day) versus the IP site (MRT, 5 days; P < 0.01) in diabetic C57BL/6 mice and of Micro encapsulated (MRT, 8 days) versus naked (MRT, 36 days; P < 0.01) baboon islets transplanted in the EFP site. Although in vitro viability and functionality of islets within MicroMix and Double capsules were comparable to Micro, addition of PEG to ALG in MicroMix capsules improved engraftment of allogeneic islets in the IP site, but resulted deleterious in the EFP site, probably due to lower biocompatibility.

Conclusions

Our results suggest that capsule composition and transplant site affect graft outcomes through their effects on nutrient availability, capsule stability, and biocompatibility.

By evaluating the effects of the encapsulated islet grafts with different capsule compositions and transplant sites, the authors suggest that the islet grafts with micro capsules and implanted in vascularized sites may increase clinical efficacy. Supplemental digital content is available in the text.

Encapsulated Islet Transplantation: Where Do We Stand?

Transplantation of pancreatic islets encapsulated within immuno-protective microcapsules is a strategy that has the potential to overcome graft rejection without the need for toxic immunosuppressive medication. However, despite promising preclinical studies, clinical trials using encapsulated islets have lacked long-term efficacy, and although generally considered clinically safe, have not been encouraging overall. One of the major factors limiting the long-term function of encapsulated islets is the host's immunological reaction to the transplanted graft which is often manifested as pericapsular fibrotic overgrowth (PFO). PFO forms a barrier on the capsule surface that prevents the ingress of oxygen and nutrients leading to islet cell starvation, hypoxia and death. The mechanism of PFO formation is still not elucidated fully and studies using a pig model have tried to understand the host immune response to empty alginate microcapsules. In this review, the varied strategies to overcome or reduce PFO are discussed, including alginate purification, altering microcapsule geometry, modifying alginate chemical composition, co-encapsulation with immunomodulatory cells, administration of pharmacological agents, and alternative transplantation sites. Nanoencapsulation technologies, such as conformal and layer-by-layer coating technologies, as well as nanofiber, thin-film nanoporous devices, and silicone based NanoGland devices are also addressed. Finally, this review outlines recent progress in imaging technologies to track encapsulated cells, as well as promising perspectives concerning the production of insulin-producing cells from stem cells for encapsulation.

Developing a Rapid Algorithm to Enable Rapid Characterization of Alginate Microcapsules

The islets of Langerhans are endocrine tissue clusters that secrete hormones that regulate the body's glucose, carbohydrate, and fat metabolism, the most important of which is insulin, a hormone secreted by β-cells within the islets. In certain instances, a person's own immune system attacks and destroys them, leading to the development of type 1 diabetes (T1D), a life-long condition that needs daily insulin administration to maintain health and prolong survival. Islet transplantation is a surgical procedure that has demonstrated the ability to normalize blood sugar levels for up to a few years, but the need for chronic immunosuppression relegates it to a last resort that is often only used sparingly and in seriously ill patients. Islet microencapsulation is a biomedical innovation designed to protect islets from the immune system by coating them with a biocompatible polymer, and this new technology has demonstrated various degrees of success in small- and large-animal studies. This success is significantly impacted by microcapsule morphology and encapsulation efficiency. Since hundreds of thousands of microcapsules are generated during the process, characterization of encapsulated islets without the help of some degree of automation would be difficult, time-consuming, and error prone due to inherent observer bias. We have developed an image analysis algorithm that can analyze hundreds of microencapsulated islets and characterize their size, shape, circularity, and distortion with minimal observer bias. This algorithm can be easily adapted to similar nano- or microencapsulation technologies to implement stricter quality control and improve biomaterial device design and success.