Noninvasive evaluation of the vascular response to transplantation of alginate encapsulated islets using the dorsal skin-fold model

An overview of current advancements in pancreatic islet transplantation into the omentum

Pancreatic islet transplantation to restore insulin production in Type 1 Diabetes Mellitus patients is commonly performed by infusion of islets into the hepatic portal system. However, the risk of portal vein thrombosis or elevation of portal pressure after transplantation introduces challenges to this procedure. Thus, alternative sites have been investigated, among which the omentum represents an ideal candidate. The surgical site is easily accessible, and the tissue is highly vascularized with a large surface area for metabolic exchange. Furthermore, the ability of the omentum to host large volumes of islets represents an intriguing if not ideal site for encapsulated islet transplantation. Research on the safety and efficacy of the omentum as a transplant site focuses on the utilization of biologic scaffolds or encapsulation of islets in a biocompatible semi-permeable membrane. Currently, more clinical trials are required to better characterize the safety and efficacy of islet transplantation into the omentum.

A nanofibrous encapsulation device for safe delivery of insulin-producing cells to treat type 1 diabetes

Transplantation of stem cell-derived β (SC-β) cells represents a promising therapy for type 1 diabetes (T1D). However, the delivery, maintenance, and retrieval of these cells remain a challenge. Here, we report the design of a safe and functional device composed of a highly porous, durable nanofibrous skin and an immunoprotective hydrogel core. The device consists of electrospun medical-grade thermoplastic silicone-polycarbonate-urethane and is soft but tough (~15 megapascal at a rupture strain of >2). Tuning the nanofiber size to less than ~500 nanometers prevented cell penetration while maintaining maximum mass transfer and decreased cellular overgrowth on blank (cell-free) devices to as low as a single-cell layer (~3 micrometers thick) when implanted in the peritoneal cavity of mice. We confirmed device safety, indicated as continuous containment of proliferative cells within the device for 5 months. Encapsulating syngeneic, allogeneic, or xenogeneic rodent islets within the device corrected chemically induced diabetes in mice and cells remained functional for up to 200 days. The function of human SC-β cells was supported by the device, and it reversed diabetes within 1 week of implantation in immunodeficient and immunocompetent mice, for up to 120 and 60 days, respectively. We demonstrated the scalability and retrievability of the device in dogs and observed viable human SC-β cells despite xenogeneic immune responses. The nanofibrous device design may therefore provide a translatable solution to the balance between safety and functionality in developing stem cell-based therapies for T1D.

Radio-metal cross-linking of alginate hydrogels for non-invasive in vivo imaging

Alginate hydrogels are cross-linked polymers with high water content, tuneable chemical and material properties, and a range of biomedical applications including drug delivery, tissue engineering, and cell therapy. However, their similarity to soft tissue often renders them undetectable within the body using conventional bio-medical imaging techniques. This leaves much unknown about their behaviour in vivo, posing a challenge to therapy development and validation. To address this, we report a novel, fast, and simple method of incorporating the nuclear imaging radio-metal 111In into the structure of alginate hydrogels by utilising its previously-undescribed capacity as an ionic cross-linking agent. This enabled non-invasive in vivo nuclear imaging of hydrogel delivery and retention across the whole body, over time, and across a range of model therapies including: nasal and oral drug delivery, stem cell transplantation, and cardiac tissue engineering. This information will facilitate the development of novel therapeutic hydrogel formulations, encompassing alginate, across disease categories.

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.

Zwitterionically modified alginates mitigate cellular overgrowth for cell encapsulation

Foreign body reaction (FBR) to implanted biomaterials and medical devices is common and can compromise the function of implants or cause complications. For example, in cell encapsulation, cellular overgrowth (CO) and fibrosis around the cellular constructs can reduce the mass transfer of oxygen, nutrients and metabolic wastes, undermining cell function and leading to transplant failure. Therefore, materials that mitigate FBR or CO will have broad applications in biomedicine. Here we report a group of zwitterionic, sulfobetaine (SB) and carboxybetaine (CB) modifications of alginates that reproducibly mitigate the CO of implanted alginate microcapsules in mice, dogs and pigs. Using the modified alginates (SB-alginates), we also demonstrate improved outcome of islet encapsulation in a chemically-induced diabetic mouse model. These zwitterion-modified alginates may contribute to the development of cell encapsulation therapies for type 1 diabetes and other hormone-deficient diseases.

Cryopreserved Alginate-Encapsulated Islets Can Restore Euglycemia in a Diabetic Animal Model Better than Cryopreserved Non-encapsulated Islets

Islet transplantation has been shown to restore normoglycemia clinically. One of the current limitations to the widespread clinical use of islet transplantation is culturing and preserving more than 1 million islet equivalents in preparation for transplant. One possible solution is to bank frozen islets and use them when needed. Although promising, the standard islet freezing protocol introduces stress and cell death, resulting in high variability of islet quality post thawing. This study aimed to develop an improved cryopreservation protocol using alginate-encapsulated islets to improve islet survival and function for future transplants. Our data showed that encapsulation improved islet survival and function after thawing the frozen islets. Frozen encapsulated islets have an islet yield recovery of 84% when compared to non-encapsulated islets at 72% after thawing. Post-thaw viability was 78% for non-encapsulated islets compared to 88% for encapsulated islets. The stimulation index values after a static glucose test following thawing were 1.9 ± 0.5, 2.9 ± 0.1, and 3.3 ± 0.3 for the non-encapsulated, 1.75% alginate, and 2.5% alginate groups, respectively. In a transplant study, the mice that received 1.75% alginate-encapsulated cryopreserved islets achieved normoglycemia on average 5 days after transplant. In comparison, control mice that received fresh islets took 4 days, while those receiving unencapsulated cryopreserved islets took 18 days. In conclusion, encapsulating islets in 1.75% alginate prior to freezing was shown to improve islet survival, function post thawing, and graft response significantly when compared to islets frozen without encapsulation.

In vivo imaging of type 1 diabetes immunopathology using eye-transplanted islets in NOD mice

AIMS/HYPOTHESIS

Autoimmune attack against the insulin-producing beta cells in the pancreatic islets results in type 1 diabetes. However, despite considerable research, details of the type 1 diabetes immunopathology in situ are not fully understood mainly because of difficult access to the pancreatic islets in vivo.

METHODS

Here, we used direct non-invasive confocal imaging of islets transplanted in the anterior chamber of the eye (ACE) to investigate the anti-islet autoimmunity in NOD mice before, during and after diabetes onset. ACE-transplanted islets allowed longitudinal studies of the autoimmune attack against islets and revealed the infiltration kinetics and in situ motility dynamics of fluorescence-labelled autoreactive T cells during diabetes development. Ex vivo immunostaining was also used to compare immune cell infiltrations into islet grafts in the eye and kidney as well as in pancreatic islets of the same diabetic NOD mice.

RESULTS

We found similar immune infiltration in native pancreatic and ACE-transplanted islets, which established the ACE-transplanted islets as reliable reporters of the autoimmune response. Longitudinal studies in ACE-transplanted islets identified in vivo hallmarks of islet inflammation that concurred with early immune infiltration of the islets and preceded their collapse and hyperglycaemia onset. A model incorporating data on ACE-transplanted islet degranulation and swelling allowed early prediction of the autoimmune attack in the pancreas and prompted treatments to intercept type 1 diabetes.

CONCLUSIONS/INTERPRETATION

The current findings highlight the value of ACE-transplanted islets in studying early type 1 diabetes pathogenesis in vivo and underscore the need for timely intervention to halt disease progression.

Improved cryopreservation yield of pancreatic islets using combination of lower dose permeable cryoprotective agents

Islet transplantation has been shown to restore normoglycemia in animal models and for type 1 diabetic patients in clinical trials. One method of storing islets intended for transplantation is via cryobanking at very low temperatures (-196 °C). Cryobanking islets without the use of cryoprotecting agents (CPAs) contributes to cellular shear stress and cell death. Although current CPA protocols vary, high concentrations of these agents are toxic to islets cells. This study tested the effects of the permeating CPA dimethyl sulfoxide (Me₂SO) with the addition of ethylene glycol (EG), both at reduced concentrations, on rat and human islet cell yield, viability, and glucose stimulated insulin release (GSIR). To test this, islets were treated using three combinations of CPAs (2M ME₂SO, 1M ME₂SO + 1M EG, and 1M ME₂SO + 0.5M EG). Next, fresh islets, 2M ME₂SO islets, and 1M ME₂SO + 0.5M EG isolated rat islets were transplanted into streptozotocin-induced (STZ) diabetic mice. Our data showed that cryopreservation with a reduced concentration of ME₂SO (1M ME₂SO + multimolar EG) achieved a higher percent yield and viability when compared to the current standard 2M ME₂SO treatment for both rat and human islets. Furthermore, STZ-induced diabetic mice achieved normoglycemia after transplantation with 1000 islet equivalents (IE), an average 12 days sooner, with islets cryopreserved with reduced-concentration (ME₂SO + 0.5M EG), compared to islets preserved with 2M ME₂SO. In conclusion, reduced concentration of penetrating CPAs during islet cryopreservation increases islet yield and viability in vitro and reduces delay before normoglycemia in diabetic mice.

Feasibility of Induced Pluripotent Stem Cell Therapies for Treatment of Type 1 Diabetes

IMPACT STATEMENT

This review of iPSCs to treat T1D provides a current assessment of the challenges and potential for this proposed new therapy.

Regenerative medicine and cell-based approaches to restore pancreatic function

The pancreas is a complex organ with exocrine and endocrine components. Many pathologies impair exocrine function, including chronic pancreatitis, cystic fibrosis and pancreatic ductal adenocarcinoma. Conversely, when the endocrine pancreas fails to secrete sufficient insulin, patients develop diabetes mellitus. Pathology in either the endocrine or exocrine pancreas results in devastating economic and personal consequences. The current standard therapy for treating patients with type 1 diabetes mellitus is daily exogenous insulin injections, but cell sources of insulin provide superior glycaemic regulation and research is now focused on the goal of regenerating or replacing β cells. Stem-cell-based models might be useful to study exocrine pancreatic disorders, and mesenchymal stem cells or secreted factors might delay disease progression. Although the standards that bioengineered cells must meet before being considered as a viable therapy are not yet established, any potential therapy must be acceptably safe and functionally superior to current therapies. Here, we describe progress and challenges in cell-based methods to restore pancreatic function, with a focus on optimizing the site for cell delivery and decreasing requirements for immunosuppression through encapsulation. We also discuss the tools and strategies being used to generate exocrine pancreas and insulin-producing β-cell surrogates in situ and highlight obstacles to clinical application.

Impact of donor age and weaning status on pancreatic exocrine and endocrine tissue maturation in pigs

BACKGROUND

During the process of islet isolation, pancreatic enzymes are activated and released, adversely affecting islet survival and function. We hypothesize that the exocrine component of pancreases harvested from pre-weaned juvenile pigs is immature and hence pancreatic tissue from these donors is protected from injury during isolation and prolonged tissue culture.

METHODS

Biopsy specimens taken from pancreases harvested from neonatal (5-10 days), pre-weaned juvenile (18-22 days), weaned juvenile (45-60 days), and young adult pigs (>90 days) were fixed and stained with hematoxylin and eosin. Sections were examined under a fluorescent microscope to evaluate exocrine zymogen fluorescence intensity (ZFI) and under an electron microscope to evaluate exocrine zymogen granule density (ZGD).

RESULTS

Exocrine content estimation showed significantly lower ZFI and ZGD in juvenile pig pancreases (1.5 ± 0.04 U/μm(2) , ZFI; 1.03 ± 0.07 × 10(3) /100 μm(2) , ZGD) compared to young adult pigs (2.4 ± 0.05U/μm(2) , ZFI; 1.53 ± 0.08 × 10(3) /100 μm(2) ZGD). Islets in juvenile pig pancreases were on average smaller (105.2 ± 11.2 μm) than islets in young adult pigs (192 ± 7.7 μm), but their insulin content was comparable (80.9 ± 2.2% juvenile; 84.2 ± 0.3% young adult, P > 0.05). All data expressed as mean ± SEM.

CONCLUSION

Porcine islet xenotransplantation continues to make strides toward utilization in clinical trials of type 1 diabetes. Porcine donor age and weaning status influence the extent of exocrine maturation of the pancreas. Juvenile porcine pancreases may represent an alternative donor source for islet xenotransplantation as their exocrine component is relatively immature; this preserves islet viability during extended tissue culture following isolation.

Pancreatic islet blood flow and its measurement

Pancreatic islets are richly vascularized, and islet blood vessels are uniquely adapted to maintain and support the internal milieu of the islets favoring normal endocrine function. Islet blood flow is normally very high compared with that to the exocrine pancreas and is autonomously regulated through complex interactions between the nervous system, metabolites from insulin secreting β-cells, endothelium-derived mediators, and hormones. The islet blood flow is normally coupled to the needs for insulin release and is usually disturbed during glucose intolerance and overt diabetes. The present review provides a brief background on islet vascular function and especially focuses on available techniques to measure islet blood perfusion. The gold standard for islet blood flow measurements in experimental animals is the microsphere technique, and its advantages and disadvantages will be discussed. In humans there are still no methods to measure islet blood flow selectively, but new developments in radiological techniques hold great hopes for the future.

Current status of encapsulated islet transplantation

Islet transplantation is a treatment modality for diabetes mellitus that can maintain insulin levels within a physiologically appropriate range. However, wider clinical application is limited by insufficient donor numbers and a need for lifelong immunosuppression. Despite various clinical and preclinical trials, there is no single standard immunosuppressive regimen that can suppress acute and chronic immune reactions with lower toxicity to grafted islets. One of the strategies for overcoming lifelong immunosuppression is the incorporation of encapsulation technology, which can provide a physical immune barrier by keeping out high molecular weight immune system components, while still allowing low molecular weight oxygen, insulin and nutrients to pass through. Encapsulated islet transplantation approaches that have been studied so far include macroencapsulation, microencapsulation, conformal coating and nanoencapsulation. Herein we will review the basic concepts of islet encapsulation technique, earlier works to recent progress related to clinical studies and corporate investigations on encapsulated islet transplantation.

Re-engineering islet cell transplantation

We are living exciting times in the field of beta cell replacement therapies for the treatment of diabetes. While steady progress has been recorded thus far in clinical islet transplantation, novel approaches are needed to make cell-based therapies more reproducible and leading to long-lasting success. The multiple facets of diabetes impose the need for a transdisciplinary approach to attain this goal, by targeting immunity, promoting engraftment and sustained functional potency. We discuss herein the emerging technologies applied to this rapidly evolving field.

A new in vivo magnetic resonance imaging method to noninvasively monitor and quantify the perfusion capacity of three-dimensional biomaterials grown on the chorioallantoic membrane of chick embryos

Adequate vascularization in biomaterials is essential for tissue regeneration and repair. Current models do not allow easy analysis of vascularization of implants in vivo, leaving it a highly desirable goal. A tool that allows monitoring of perfusion capacity of such biomaterials noninvasively in a cheap, efficient, and reliable in vivo model would hence add great benefit to research in this field. We established, for the first time, an in vivo magnetic resonance imaging (MRI) method to quantify the perfusion capacity of a model biomaterial, DegraPol(®) foam scaffold, placed on the embryonic avian chorioallantoic membrane (CAM) in ovo. Perfusion capacity was assessed through changes in the longitudinal relaxation rate before and after injection of a paramagnetic MRI contrast agent, Gd-DOTA (Dotarem(®); Guerbet S.A.). Relaxation rate changes were compared in three different regions of the scaffold, that is, at the interface to the CAM, in the middle and on the surface of the scaffold (p<0.05). The highest relaxation rate changes, and hence perfusion capacities, were measured in the interface region where the scaffold was attached to the CAM, whereas the surface of the scaffold showed the lowest relaxation rate changes. A strong positive correlation was obtained between relaxation rate changes and histologically determined vessel density (R(2) = 0.983), which corroborates our MRI findings. As a proof-of-principle, we measured the perfusion capacity in different scaffold materials, silk fibroin either with or without human dental pulp stem cells. For these, three to four times larger perfusion capacities were obtained compared to DegraPol; demonstrating that our method is sensitive to reveal such differences. In summary, we present a novel in vivo method for analyzing the perfusion capacity in three-dimensional-biomaterials grown on the CAM, enabling the determination of the perfusion capacity of a large variety of bioengineered materials.