Enhancing clinical islet transplantation through tissue engineering strategies

Translation of cell therapies to treat autoimmune disorders

Autoimmune diseases are a diverse and complex set of chronic disorders with a substantial impact on patient quality of life and a significant global healthcare burden. Current approaches to autoimmune disease treatment comprise broadly acting immunosuppressive drugs that lack disease specificity, possess limited efficacy, and confer undesirable side effects. Additionally, there are limited treatments available to restore organs and tissues damaged during the course of autoimmune disease progression. Cell therapies are an emergent area of therapeutics with the potential to address both autoimmune disease immune dysfunction as well as autoimmune disease-damaged tissue and organ systems. In this review, we discuss the pathogenesis of common autoimmune disorders and the state-of-the-art in cell therapy approaches to (1) regenerate or replace autoimmune disease-damaged tissue and (2) eliminate pathological immune responses in autoimmunity. Finally, we discuss critical considerations for the translation of cell products to the clinic.

Novel Approaches to Immunomodulation for Solid Organ Transplantation

Despite significant advances in the field of transplantation in the past two decades, current clinically available therapeutic options for immunomodulation remain fairly limited. The advent of calcineurin inhibitor-based immunosuppression has led to significant success in improving short-term graft survival; however, improvements in long-term graft survival have stalled. Solid organ transplantation provides a unique opportunity for immunomodulation of both the donor organ prior to implantation and the recipient post transplantation. Furthermore, therapies beyond targeting the adaptive immune system have the potential to ameliorate ischemic injury to the allograft and halt its aging process, augment its repair, and promote recipient immune tolerance. Other recent advances include expanding the donor pool by reducing organ discard, and bioengineering and genetically modifying organs from other species to generate transplantable organs. Therapies discussed here will likely be most impactful if individualized on the basis of specific donor and recipient considerations.

Engineering Islets From Stem Cells: The Optimal Solution for the Treatment of Diabetes?

Diabetes is a metabolic disease characterized by insulin deficiency. Bioengineering of stem cells with the aim to restore insulin production and glucose regulation has the potential to cure diabetic patients. In this review, we focus on the recent developments for bioengineering of induced pluripotent stem cells (iPSCs), mesenchymal stem cells (MSCs), embryonic stem cells (ESCs), and pancreatic progenitor cells in view of generating insulin producing and glucose regulating cells for β-cell replacement therapies. Recent clinical trials using islet cells derived from stem cells have been initiated for the transplantation into diabetic patients, with crucial bottlenecks of tumorigenesis, post-transplant survival, genetic instability, and immunogenicity that should be further optimized. As a new approach given high expectations, bioengineered islets from stem cells occupies considerable potential for the future clinical application and addressing the treatment dilemma of diabetes.

Towards the Development of a Female Animal Model of T1DM Using Hyaluronic Acid Nanocoated Cell Transplantation: Refinements and Considerations for Future Protocols

Female mice (Black 6 strain) (C57BL/6) aged 6 weeks were subject to low dose streptozotocin (STZ) treatment for five consecutive days to mimic type 1 diabetes mellitus (T1DM) with insulitis. At two weeks after STZ injections, evaluation of the elevated glucose levels was used to confirm diabetes. The diabetic mice were then subject to the transplantation of pancreatic β-cells (MIN-6 line). Four groups of mice were studied. The first group was injected with saline-only acting as the placebo surgery control, also known as SHAM group, the second and third groups were injected with MIN-6 single cells and polyethylene glycol-modified dipalmitoyl-glycerol-phosphatidyl ethanolamine (PEG-DPPE) modified MIN-6 single cells (500 µg per 1.10⁶ cells), respectively, while the fourth group was injected with hyaluronic acid (HA)-coated MIN-6 single cells (5 bilayers). At seven- and fourteen-days following transplantation, the mice were euthanised. The renal and pancreatic tissues were then collected and histologically analysed. The induction of diabetes in female mice, through five-consecutive daily STZ injections resulted in inconsistent glycaemic levels. Interestingly, this shows an incomplete diabetes induction in female mice, of which we attribute to sex dimorphism and hormonal interferences. Transplantation failure of free-floating encapsulated cells was unable to decrease blood glucose hyperglycaemia to physiological ranges. The result is attributed to deprived cell–cell interactions, leading to decreased β-cells functionality. Overall, we highlight the necessity of refining T1DM disease models in female subjects when using multiple low-dose STZ injections together with transplantation protocols. Considerations need to be made regarding the different developmental stages of female mice and oestrogen load interfering with pancreatic β-cells susceptibility to STZ. The use of pseudo islets, cell aggregates and spheroids are sought to improve transplantation outcome in comparison to free-floating single cells.

Biomaterial-based approaches to engineering immune tolerance

The development of biomaterial-based therapeutics to induce immune tolerance holds great promise for the treatment of autoimmune diseases, allergy, and graft rejection in transplantation. Historical approaches to treat these immunological challenges have primarily relied on systemic delivery of broadly-acting immunosuppressive agents that confer undesirable, off-target effects. The evolution and expansion of biomaterial platforms has proven to be a powerful tool in engineering immunotherapeutics and enabled a great diversity of novel and targeted approaches in engineering immune tolerance, with the potential to eliminate side effects associated with systemic, non-specific immunosuppressive approaches. In this review, we summarize the technological advances within three broad biomaterials-based strategies to engineering immune tolerance: nonspecific tolerogenic agent delivery, antigen-specific tolerogenic therapy, and the emergent area of tolerogenic cell therapy.

Applying Immunomodulation to Promote Longevity of Immunoisolated Pancreatic Islet Grafts

Islet transplantation is a promising therapy for insulin-dependent diabetes, but large-scale application is hampered by the lack of a consistent source of insulin-producing cells and need for lifelong administration of immunosuppressive drugs, which are associated with severe side effects. To avoid chronic immunosuppression, islet grafts can be enveloped in immunoisolating polymeric membranes. These immunoisolating polymeric membranes protect islet grafts from cell-mediated rejection while allowing diffusion of oxygen, nutrients, and insulin. Although clinical trials have shown the safety and feasibility of encapsulated islets to control glucose homeostasis, the strategy does up till now not support long-term graft survival. This partly can be explained by a significant loss of insulin-producing cells in the immediate period after implantation. The loss can be prevented by combining immunoisolation with immunomodulation, such as combined administration of immunomodulating cytokines or coencapsulation of immunomodulating cell types such as regulatory T cells, mesenchymal stem cells, or Sertoli cells. Also, administration of specific antibodies or apoptotic donor leucocytes is considered to create a tolerant microenvironment around immunoisolated grafts. In this review, we describe the outcomes and limitations of these approaches, as well as the recent progress in immunoisolating devices. Impact statement Immunoisolation by enveloping islets in semipermeable membranes allows for successful transplantation of islet grafts in the absence of chronic immunosuppression, but the duration of graft survival is still not permanent. The reasons for long-term final graft failure is not fully understood, but combining immunoisolation with immunomodulation of tissues or host immune system has been proposed to enhance the longevity of grafts. This article reviews the recent progress and challenges of immunoisolation, as well as the benefits and feasibility of combining encapsulation approaches with immunomodulation to promote longevity of encapsulated grafts.

Three-dimensional cell-culture platform based on hydrogel with tunable microenvironmental properties to improve insulin-secreting function of MIN6 cells

Pancreatic β-cells have been reported to be mechanosensitive to cellular microenvironments, and subjecting the cells to more physiologically relevant microenvironments can produce better results than when subjecting them to the conventional two-dimensional (2D) cell-culture conditions. In this work, we propose a novel three-dimensional (3D) strategy for inducing multicellular spheroid formation based on hydrogels with tunable mechanical and interfacial properties. The results indicate that MIN6 cells can sense the substrates and form tightly clustered monolayers or multicellular spheroids on hydrogels with tunable physical properties. Compared to the conventional 2D cell-culture system, the glucose sensitivities of the MIN6 cells cultured in the 3D culture model is enhanced greatly and their insulin content (relative to the amount of protein) is increased 7.3-7.9 folds. Moreover, the relative gene and protein expression levels of some key factors such as Pdx1, NeuroD1, Piezo1, and Rac1 in the MIN6 cells are significantly higher on the 3D platform, compared to the 2D control group. We believe that this 3D cell-culture system developed for the generation of multicellular spheroids will be a promising platform for diabetes treatment in clinical islet transplantation.

Oxidative stress in pancreatic alpha and beta cells as a selection criterion for biocompatible biomaterials

The clinical success rate of islet transplantation, namely independence from insulin injections, is limited by factors that lead to graft failure, including inflammation, acute ischemia, acute phase response, and insufficient vascularization. The ischemia and insufficient vascularization both lead to high levels of oxidative stress, which are further aggravated by islet encapsulation, inflammation, and undesirable cell-biomaterial interactions. To identify biomaterials that would not further increase damaging oxidative stress levels and that are also suitable for manufacturing a beta cell encapsulation device, we studied five clinically approved polymers for their effect on oxidative stress and islet (alpha and beta cell) function. We found that 300 poly(ethylene oxide terephthalate) 55/poly(butylene terephthalate) 45 (PEOT/PBT300) was more resistant to breakage and more elastic than other biomaterials, which is important for its immunoprotective function. In addition, it did not induce oxidative stress or reduce viability in the MIN6 beta cell line, and even promoted protective endogenous antioxidant expression over 7 days. Importantly, PEOT/PBT300 is one of the biomaterials we studied that did not interfere with insulin secretion in human islets.

Functional zwitterionic biomaterials for administration of insulin

This review summarizes the structures and biomedical applications of zwitterionic biomaterials in the administration of insulin.

Lung Based Engineered Micro-Pancreas Sustains Human Beta Cell Survival and Functionality

The whole world has been affected by a dramatically increasing prevalence of diabetes. Today, the etiology of both type 1 and type 2 diabetes is thought to revolve around the dysfunction of β-cells, the insulin producing cells of the body. Within the pharmaceutical industry, the evaluation of new drugs for diabetes treatment is mostly done using cell lines or rodent islets and depends solely on the assessment of static insulin secretion. However, the use of cell lines or rodent islets is limiting lack of similarity of the human islet cells, leading to a constrain of the predictive value regarding the clinical potential of newly developed drugs. To overcome this issue, we developed an Engineered Micro-Pancreas as a unique platform for drug discovery. The Engineered Micro Pancreas is composed of (i) an organ-derived micro-scaffold, specifically a decellularized porcine lung-derived micro-scaffold and (ii) cadaveric islets seeded thereon. The Engineered Micro Pancreas remained viable and maintained insulin secretion in vitro for up to three months. The quantities of insulin were comparable to those secreted by freshly isolated human islets and therefore hold the potential for real-time and metabolic physiology mimicking drug screening.

Design of a vascularized synthetic poly(ethylene glycol) macroencapsulation device for islet transplantation

The use of immunoisolating macrodevices in islet transplantation confers the benefit of safety and translatability by containing transplanted cells within a single retrievable device. To date, there has been limited development and characterization of synthetic poly(ethylene glycol) (PEG)-based hydrogel macrodevices for islet encapsulation and transplantation. Herein, we describe a two-component synthetic PEG hydrogel macrodevice system, designed for islet delivery to an extrahepatic islet transplant site, consisting of a hydrogel core cross-linked with a non-degradable PEG dithiol and a vasculogenic outer layer cross-linked with a proteolytically sensitive peptide to promote degradation and enhance localized vascularization. Synthetic PEG macrodevices exhibited equivalent passive molecular transport to traditional microencapsulation materials (e.g., alginate) and long-term stability in the presence of proteases in vitro and in vivo, out to 14 weeks in rats. Encapsulated islets demonstrated high viability within the device in vitro and the incorporation of RGD adhesive peptides within the islet encapsulating PEG hydrogel improved insulin responsiveness to a glucose challenge. In vivo, the implementation of a vasculogenic, degradable hydrogel layer at the outer interface of the macrodevice enhanced vascular density within the rat omentum transplant site, resulting in improved encapsulated islet viability in a syngeneic diabetic rat model. These results highlight the benefits of the facile PEG platform to provide controlled presentation of islet-supportive ligands, as well as degradable interfaces for the promotion of engraftment and overall graft efficacy.

Layered PEGDA hydrogel for islet of Langerhans encapsulation and improvement of vascularization

Islets of Langerhans need to maintain their round morphology and to be fast revascularized after transplantation to preserve functional insulin secretion in response to glucose stimulation. For this purpose, a non-cell-adhesive environment is preferable for their embedding. Conversely, nutrient and oxygen supply to islets is guaranteed by capillary ingrowth within the construct and this can only be achieved in a matrix that provides adhesion cues for cells. In this study, two different approaches are explored, which are both based on a layered architecture, in order to combine these two opposite requirements. A non-adhesive islet encapsulation layer is based on polyethyleneglycole diacrylate (PEGDA). This first layer is combined with a second hydrogel based on thiolated-gelatin, thiolated-heparin and thiolated-hyaluronic acid providing cues for endothelial cell adhesion and acting as a growth factor releasing matrix. In an alternative approach, a conformal PEGDA coating is covalently applied on the surface of the islets. The coated islets are subsequently embedded in the previously mentioned hydrogel containing thiolated glycosaminoglycans. The suitability of this approach as a matrix for controlled growth factor release has been demonstrated by studying the controlled release of VEGF and bFGF for 14 days. Preliminary tube formation has been quantified on the growth factor loaded hydrogels. This approach should facilitate blood vessel ingrowth towards the embedded islets and maintain islet round morphology and functionality upon implantation.

Pancreatic islet macroencapsulation using microwell porous membranes

Allogeneic islet transplantation into the liver in combination with immune suppressive drug therapy is widely regarded as a potential cure for type 1 diabetes. However, the intrahepatic system is suboptimal as the concentration of drugs and nutrients there is higher compared to pancreas, which negatively affects islet function. Islet encapsulation within semipermeable membranes is a promising strategy that allows for the islet transplantation outside the suboptimal liver portal system and provides environment, where islets can perform their endocrine function. In this study, we develop a macroencapsulation device based on thin microwell membranes. The islets are seeded in separate microwells to avoid aggregation, whereas the membrane porosity is tailored to achieve sufficient transport of nutrients, glucose and insulin. The non-degradable, microwell membranes are composed of poly (ether sulfone)/polyvinylpyrrolidone and manufactured via phase separation micro molding. Our results show that the device prevents aggregation and preserves the islet’s native morphology. Moreover, the encapsulated islets maintain their glucose responsiveness and function after 7 days of culture (stimulation index above 2 for high glucose stimulation), demonstrating the potential of this novel device for islet transplantation.

The impact of cell surface PEGylation and short-course immunotherapy on islet graft survival in an allogeneic murine model

Islet transplantation is a promising therapy for Type 1 diabetes mellitus; however, host inflammatory and immune responses lead to islet dysfunction and destruction, despite potent systemic immunosuppression. Grafting of poly(ethylene glycol) (PEG) to the periphery of cells or tissues can mitigate inflammation and immune recognition via generation of a steric barrier. Herein, we sought to evaluate the complementary impact of islet PEGylation with a short-course immunotherapy on the survival of fully-MHC mismatched islet allografts (DBA/2 islets into diabetic C57BL/6J recipients). Anti-Lymphocyte Function-associated Antigen 1 (LFA-1) antibody was selected as a complementary, transient, systemic immune monotherapy. Islets were PEGylated via an optimized protocol, with resulting islets exhibiting robust cell viability and function. Following transplantation, a significant subset of diabetic animals receiving PEGylated islets (60%) or anti-LFA-1 antibody (50%) exhibited long-term (>100d) normoglycemia. The combinatorial approach proved synergistic, with 78% of the grafts exhibiting euglycemia long-term. Additional studies examining graft cellular infiltrates at early time points characterized the local impact of the transplant protocol on graft survival. Results illustrate the capacity of a simple polymer grafting approach to impart significant immunoprotective effects via modulation of the local transplant environment, while short-term immunotherapy serves to complement this effect.

STATEMENT OF SIGNIFICANCE

We believe this study is important and of interest to the biomaterials and transplant community for several reasons: 1) it provides an optimized protocol for the PEGylation of islets, with minimal impact on the coated islets, which can be easily translated for clinical applications; 2) this optimized protocol demonstrates the benefits of islet PEGylation in providing modest immunosuppression in a murine model; 3) this work demonstrates the combinatory impact of PEGylation with short-course immunotherapy (via LFA-1 blockage), illustrating the capacity of PEGylation to complement existing immunotherapy; and 4) it suggests macrophage phenotype shifting as the potential mechanism for this observed benefit.

Progress and challenges of the bioartificial pancreas

Pancreatic islet transplantation has been validated as a treatment for type 1 diabetes since it maintains consistent and sustained type 1 diabetes reversal. However, one of the major challenges in pancreatic islet transplantation is the body's natural immune response to the implanted islets. Immunosuppressive drug treatment is the most popular immunomodulatory approach for islet graft survival. However, administration of immunosuppressive drugs gives rise to negative side effects, and long-term effects are not clearly understood. A bioartificial pancreas is a therapeutic approach to enable pancreatic islet transplantation without or with minimal immune suppression. The bioartificial pancreas encapsulates the pancreatic islets in a semi-permeable environment which protects islets from the body's immune responses, while allowing the permeation of insulin, oxygen, nutrients, and waste. Many groups have developed various types of the bioartificial pancreas and tested their efficacy in animal models. However, the clinical application of the bioartificial pancreas still requires further investigation. In this review, we discuss several types of bioartificial pancreases and address their advantages and limitations. We also discuss recent advances in bioartificial pancreas applications with microfluidic or micropatterning technology.

Beta Cells Secrete Significant and Regulated Levels of Insulin for Long Periods when Seeded onto Acellular Micro-Scaffolds

The aim of this work is to obtain significant and regulated insulin secretion from human beta cells ex vivo. Long-term culture of human pancreatic islets and attempts at expanding human islet cells normally result in loss of beta-cell phenotype. We propose that to obtain proper ex vivo beta cell function, there is a need to develop three-dimensional structures that mimic the natural islet tissue microenvironment. We here describe the preparation of endocrine micro-pancreata (EMPs) that are made up of acellular organ-derived micro-scaffolds seeded with human intact or enzymatically dissociated islets. We show that EMPs constructed by seeding whole islets, freshly enzymatically-dissociated islets or even dissociated islets grown first in standard monolayer cultures express high levels of key beta-cell specific genes and secrete quantities of insulin per cell similar to freshly isolated human islets in a glucose-regulated manner for more than 3 months in vitro.

Hybrid Polycaprolactone/Alginate Scaffolds Functionalized with VEGF to Promote de Novo Vessel Formation for the Transplantation of Islets of Langerhans

Although regarded as a promising treatment for type 1 diabetes, clinical islet transplantation in the portal vein is still hindered by a low transplantation outcome. Alternative transplantation sites have been proposed, but the survival of extra-hepatically transplanted islets of Langerhans critically depends on quick revascularization after engraftment. This study aims at developing a new 3D scaffold platform that can actively boost vascularization and may find an application for extra-hepatic islet transplantation. The construct consists of a 3D ring-shaped polycaprolactone (PCL) scaffold with heparinized surface to electrostatically bind vascular endothelial growth factor (VEGF), surrounding a hydrogel core for islets encapsulation. Heparin immobilization improves the amount of VEGF retained by the construct, up to 3.6 fold, compared to untreated PCL scaffolds. In a chicken chorioallanthoic membrane model, VEGF immobilized on the construct enhances angiogenesis in close proximity and on the surface of the scaffolds. After 7 days, islets encapsulated in the alginate core show functional response to glucose stimuli comparable to free-floating islets. Thus, the developed platform has the potential to support rapid vascularization and islet endocrine function.

Repurposed biological scaffolds: kidney to pancreas

Advances in organ regeneration have been facilitated by gentle decellularization protocols that maintain distinct tissue compartments, and thereby allow seeding of blood vessels with endothelial lineages separate from populations of the parenchyma with tissue-specific cells. We hypothesized that a reconstituted vasculature could serve as a novel platform for perfusing cells derived from a different organ: thus discordance of origin between the vascular and functional cells, leading to a hybrid repurposed organ. The need for a highly vascular bed is highlighted by tissue engineering approaches that involve transplantation of just cells, as attempted for insulin production to treat human diabetes. Those pancreatic islet cells present unique challenges since large numbers are needed to allow the cell-to-cell signaling required for viability and proper function; however, increasing their number is limited by inadequate perfusion and hypoxia. As proof of principle of the repurposed organ methodology we harnessed the vasculature of a kidney scaffold while seeding the collecting system with insulin-producing cells. Pig kidneys were decellularized by sequential detergent, enzymatic and rinsing steps. Maintenance of distinct vascular and collecting system compartments was demonstrated by both fluorescent 10 micron polystyrene microspheres and cell distributions in tissue sections. Sterilized acellular scaffolds underwent seeding separately via the artery (fibroblasts or endothelioma cells) and retrograde (murine βTC-tet cells) up the ureter. After three-day bioreactor incubation, histology confirmed separation of cells in the vasculature from those in the collecting system. βTC-tet clusters survived in tubules, glomerular Bowman's space, demonstrated insulin immunolabeling, and thereby supported the feasibility of kidney-to-pancreas repurposing.

Engineering an "infectious" T(reg) biomimetic through chemoselective tethering of TGF-β1 to PEG brush surfaces

Modulation of immunological responses to allografts following transplantation is of pivotal importance to improving graft outcome and duration. Of the many approaches, harnessing the dominant tolerance induced by regulatory T cells (Treg) holds tremendous promise. Recent studies have highlighted the unique potency of cell surface-bound TGF-β1 on Treg for promoting infectious tolerance, i.e. to confer suppressive capacity from one cell to another. To mimic this characteristic, TGF-β1 was chemoselectively tethered to inert and viable polymer grafting platforms using Staudinger ligation. We report the synthesis and functional characterization of these engineered TGF-β1 surfaces. Inert beads tethered with TGF-β1 were capable of efficiently converting naïve CD4(+) CD62L(hi) T cells to functional Treg. Concordantly, translation of conjugation scheme from inert surfaces to viable cells also led to efficient generation of functional Treg. Further, the capacity of these platforms to generate antigen-specific Treg was demonstrated. These findings illustrate the unique faculty of tethered TGF-β1 biomaterial platforms to function as an "infectious" Treg and provide a compelling approach for generating tolerogenic microenvironments for allograft transplantation.

Engineering β-cell islets or islet-like structures for type 1 diabetes treatment

Type 1 diabetes mellitus is a disease characterized by the destruction of the β-cells in the pancreatic islets of Langerhans. The current primary treatment for type 1 diabetes is insulin injections administered multiple times throughout the day. However, this treatment cannot provide sustained physiological release of insulin and the insulin amount is not finely tuned to the glycemia condition. Pancreatic transplantation or islet transplantation would be the preferred treatment strategy but the lack of donor tissue and immunoincompatibility has been shown to be a roadblock to their widespread use. Bioengineering strategies are poised to combat these challenges. Islet encapsulation is expected to offer both immunoisolation and immunomodulation effects by: (1) physically protecting islets from the attacks of immunoglobulins, complements, and host immune cells, and (2) delivering immune regulatory and immunomodulatory factors locally to the islets to protect those islets from immune rejection. Semi-permeable coatings using biocompatible biomaterials can be used for immunoisolating islets away from the host immune systems. Immunoisolation technology also provides an opportunity to use other cell sources for cell therapy to treat type 1 diabetes. Recently, some studies reported that co-transplantation of islets with mesenchymal stem cells (MSCs) can control graft inflammation. MSCs have immunomodulatory property. They are able to secrete anti-inflammatory factors and repress the activity of various immune cells. Growth factors like interleukin 10 (IL-10) and leukemia inhibitory factor (LIF) also have immune regulatory properties. Therefore immunoisolation and immunomodulation technologies can be integrated and applied to β-cell encapsulation for the treatment of type 1 diabetes. Through engineering β-cell islets or islet-like microtissues, the viability and function of transplanted β-cells may be improved. In the meantime, the survival of transplanted β-cells can be further improved by promoting vascular network formation surrounding the transplanted islets or microtissues.

Mesenchymal Stem Cells: Rising Concerns over Their Application in Treatment of Type One Diabetes Mellitus

Type 1 diabetes mellitus (T1DM) is an autoimmune disorder that leads to beta cell destruction and lowered insulin production. In recent years, stem cell therapies have opened up new horizons to treatment of diabetes mellitus. Among all kinds of stem cells, mesenchymal stem cells (MSCs) have been shown to be an interesting therapeutic option based on their immunomodulatory properties and differentiation potentials confirmed in various experimental and clinical trial studies. In this review, we discuss MSCs differential potentials in differentiation into insulin-producing cells (IPCs) from various sources and also have an overview on currently understood mechanisms through which MSCs exhibit their immunomodulatory effects. Other important issues that are provided in this review, due to their importance in the field of cell therapy, are genetic manipulations (as a new biotechnological method), routes of transplantation, combination of MSCs with other cell types, frequency of transplantation, and special considerations regarding diabetic patients' autologous MSCs transplantation. At the end, utilization of biomaterials either as encapsulation tools or as scaffolds to prevent immune rejection, preparation of tridimensional vascularized microenvironment, and completed or ongoing clinical trials using MSCs are discussed. Despite all unresolved concerns about clinical applications of MSCs, this group of stem cells still remains a promising therapeutic modality for treatment of diabetes.

Bioluminescence tracking of alginate micro-encapsulated cell transplants

Cell-based therapies to treat loss-of-function hormonal disorders such as diabetes and Parkinson's disease are routinely coupled with encapsulation strategies, but an understanding of when and why grafts fail in vivo is lacking. Consequently, investigators cannot clearly define the key factors that influence graft success. Although bioluminescence is a popular method to track the survival of free cells transplanted in preclinical models, little is known of the ability to use bioluminescence for real-time tracking of microencapsulated cells. Furthermore, the impact that dynamic imaging distances may have, due to freely-floating microcapsules in vivo, on cell survival monitoring is unknown. This work addresses these questions by applying bioluminescence to a pancreatic substitute based on microencapsulated cells. Recombinant insulin-secreting cells were transduced with a luciferase lentivirus and microencapsulated in Ba²⁺ crosslinked alginate for in vitro and in vivo studies. In vitro quantitative bioluminescence monitoring was possible and viable microencapsulated cells were followed in real time under both normoxic and anoxic conditions. Although in vivo dispersion of freely-floating microcapsules in the peritoneal cavity limited the analysis to a qualitative bioluminescence evaluation, signals consistently four orders of magnitude above background were clear indicators of temporal cell survival. Strong agreement between in vivo and in vitro cell proliferation over time was discovered by making direct bioluminescence comparisons between explanted microcapsules and parallel in vitro cultures. Broader application of this bioluminescence approach to retrievable transplants, in supplement to currently used end-point physiological tests, could improve understanding and accelerate development of cell-based therapies for critical clinical applications. Copyright © 2014 John Wiley & Sons, Ltd.

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.

Encapsulated islets for diabetes therapy: history, current progress, and critical issues requiring solution

Insulin therapy became a reality in 1921 dramatically saving lives of people with diabetes, but not protecting them from long-term complications. Clinically successful free islet implants began in 1989 but require life long immunosuppression. Several encapsulated islet approaches have been ongoing for over 30 years without defining a clinically relevant product. Macro-devices encapsulating islet mass in a single device have shown long-term success in large animals but human trials have been limited by critical challenges. Micro-capsules using alginate or similar hydrogels encapsulate individual islets with many hundreds of promising rodent results published, but a low incidence of successful translation to large animal and human results. Reduction of encapsulated islet mass for clinical transplantation is in progress. This review covers the status of both early and current studies including the presentation of corporate efforts involved. It concludes by defining the critical items requiring solution to enable a successful clinical diabetes therapy.

Advances in biocompatibility and physico-chemical characterization of microspheres for cell encapsulation

Cell encapsulation has already shown its high potential and holds the promise for future cell therapies to enter the clinics as a large scale treatment option for various types of diseases. The advancement in cell biology towards this goal has to be complemented with functional biomaterials suitable for cell encapsulation. This cannot be achieved without understanding the close correlation between cell performance and properties of microspheres. The ongoing challenges in the field of cell encapsulation require a critical view on techniques and approaches currently utilized to characterize microspheres. This review deals with both principal subjects of microspheres characterization in the cell encapsulation field: physico-chemical characterization and biocompatibility. The up-to-day knowledge is summarized and discussed with the focus to identify missing knowledge and uncertainties, and to propose the mandatory next steps in characterization of microspheres for cell encapsulation. The primary conclusion of this review is that further success in development of microspheres for cell therapies cannot be accomplished without careful selection of characterization techniques, which are employed in conjunction with biological tests.

Microchip-based engineering of super-pancreatic islets supported by adipose-derived stem cells

Type 1 diabetes mellitus (T1DM) is a chronic disorder characterized by targeted autoimmune-mediated destruction of the β cells of Langerhans within pancreatic islets. Currently, islet transplantation is the only curative therapy; however, donor shortages and cellular damage during the isolation process critically limit the use of this approach. Here, we describe a method for creating viable and functionally potent islets for successful transplantation by co-culturing single primary islet cells with adipose-derived stem cells (ADSCs) in concave microwells. We observed that the ADSCs segregated from the islet cells, eventually yielding purified islet spheroids in the three-dimensional environment. Thereafter, the ADSC-exposed islet spheroids showed significantly different ultrastructural morphologies, higher viability, and enhanced insulin secretion compared to mono-cultured islet spheroids. This suggests that ADSCs may have a significant potential to protect islet cells from damage during culture, and may be employed to improve islet cell survival and function prior to transplantation. In vivo experiments involving xenotransplantation of microfiber-encapsulated spheroids into a mouse model of diabetes revealed that co-culture-transplanted mice maintained their blood glucose levels longer than mono-culture-transplanted mice, and required less islet mass to reverse diabetes. This method for culturing islet spheroids could potentially help overcome the cell shortages that have limited clinical applications and could possibly be developed into a bioartificial pancreas.

Survival of free and encapsulated human and rat islet xenografts transplanted into the mouse bone marrow

Bone marrow was recently proposed as an alternative and potentially immune-privileged site for pancreatic islet transplantation. The aim of the present study was to assess the survival and rejection mechanisms of free and encapsulated xenogeneic islets transplanted into the medullary cavity of the femur, or under the kidney capsule of streptozotocin-induced diabetic C57BL/6 mice. The median survival of free rat islets transplanted into the bone marrow or under the kidney capsule was 9 and 14 days, respectively, whereas that of free human islets was shorter, 7 days (bone marrow) and 10 days (kidney capsule). Infiltrating CD8⁺ T cells and redistributed CD4⁺ T cells, and macrophages were detected around the transplanted islets in bone sections. Recipient mouse splenocytes proliferated in response to donor rat stimulator cells. One month after transplantation under both kidney capsule or into bone marrow, encapsulated rat islets had induced a similar degree of fibrotic reaction and still contained insulin positive cells. In conclusion, we successfully established a small animal model for xenogeneic islet transplantation into the bone marrow. The rejection of xenogeneic islets was associated with local and systemic T cell responses and macrophage recruitment. Although there was no evidence for immune-privilege, the bone marrow may represent a feasible site for encapsulated xenogeneic islet transplantation.

Covalent layer-by-layer assembly of hyperbranched polymers on alginate microcapsulesto impart stability and permselectivity

The microencapsulation of cells has shown promise as a therapeutic vehicle for the treatment of a wide variety of diseases. While alginate microcapsules provide an ideal cell encapsulation material, polycations coatings are commonly employed to enhance stability and impart permselectivity. In this study, functionalized hyperbranched alginate and dendrimer polymers were used to generate discreet nanoscale coatings onto alginate microbeads via covalent layer-by-layer assembly. The bioorthogonal Staudinger ligation scheme was used to chemoselectively crosslink azide functionalized hyperbranched alginate (alginate-hN₃) to methyl-2-diphenylphosphino-terephthalate (MDT) linked PAMAM dendrimer (PAMAM-MDT). Covalent layer-by-layer deposition of PAMAM-MDT/alginate-hN₃ coatings onto alginate microbeads resulted in highly stable coatings, even after the inner alginate gel was liquefied to form microcapsules. The permselectivity of the coated microcapsules could be manipulated via the charge density of the PAMAM, the number of layers deposited, and the length of the functional arms. The cytocompatibility of the resulting PAMAM-MDT/alginate-hN₃ coating was evaluated using a beta cell line, with no significant detrimental response observed. The biocompatibility of the coatings in vivo was also found comparable to uncoated alginate beads. The remarkable stability and versatile nature of these coatings provides an appealing option for bioencapsulation and the release of therapeutic agents.

Bioorthogonal layer-by-layer encapsulation of pancreatic islets via hyperbranched polymers

Encapsulation of viable tissues via layer-by-layer polymer assembly provides a versatile platform for cell surface engineering, with nanoscale control over the capsule properties. Herein, we report the development of a hyperbranched polymer-based, ultrathin capsule architecture expressing bioorthogonal functionality and tailored physiochemical properties. Random carbodiimide-based condensation of 3,5-dicarboxyphenyl glycineamide on alginate yielded a highly branched polysaccharide with multiple, spatially restricted, and readily functionalizable terminal carboxylate moieties. Poly(ethylene glycol) (PEG) was utilized to link azido end groups to the structured alginate. Together with a phosphine-functionalized poly(amidoamine) dendrimer, nanoscale layer-by-layer coatings, covalently stabilized via Staudinger ligation, were assembled onto solid surfaces and pancreatic islets. The effects of electrostatic and/or bioorthogonal covalent interlayer interactions on the resulting coating efficiency and stability, as well as pancreatic islet viability and function, were studied. These hyperbranched polymers provide a flexible platform for the formation of covalently stabilized, ultrathin coatings on viable cells and tissues. In addition, the hyperbranched nature of the polymers presents a highly functionalized surface capable of bioorthogonal conjugation of additional bioactive or labeling motifs.

Proangiogenic hydrogels within macroporous scaffolds enhance islet engraftment in an extrahepatic site

The transplantation of allogeneic islets in recent clinical trials has shown substantial promise as a therapy for type 1 diabetes; however, long-term insulin independence remains inadequate. This has been largely attributed to the current intravascular, hepatic transplant site, which exposes islets to mechanical and inflammatory stresses. A highly macroporous scaffold, housed within an alternative transplant site, can support an ideal environment for islet transplantation by providing three-dimensional distribution of islets, while permitting the infiltration of host vasculature. In the present study, we sought to evaluate the synergistic effect of a proangiogenic hydrogel loaded within the void space of a macroporous poly(dimethylsiloxane) (PDMS) scaffold on islet engraftment. The fibrin-based proangiogenic hydrogel tested presents platelet derived growth factor (PDGF-BB), via a fibronectin (FN) fragment containing growth factor and major integrin binding sites in close proximity. The combination of the proangiogenic hydrogel with PDMS scaffolds resulted in a significant decrease in the time to normoglycemia for syngeneic mouse islet transplants. This benefit was associated with an observed increase in competent vessel branching, as well as mature intraislet vessels. Overall, the addition of the proangiogenic factor PDGF-BB, delivered via the FN fragment-functionalized hydrogel, positively influenced the efficiency of engraftment. These characteristics, along with its ease of retrieval, make this combination of a biostable macroporous scaffold and a degradable proangiogenic hydrogel a supportive structure for insulin-producing cells implanted in extrahepatic sites.

Vasculogenic bio-synthetic hydrogel for enhancement of pancreatic islet engraftment and function in type 1 diabetes

Type 1 diabetes (T1DM) affects one in every 400 children and adolescents in the US. Due to the limitations of exogenous insulin therapy and whole pancreas transplantation, pancreatic islet transplantation has emerged as a promising therapy for T1DM. However, this therapy is severely limited by donor islet availability and poor islet engraftment and function. We engineered an injectable bio-synthetic, polyethylene glycol-maleimide hydrogel to enhance vascularization and engraftment of transplanted pancreatic islets in a mouse model of T1DM. Controlled presentation of VEGF-A and cell-adhesive peptides within this engineered material significantly improved the vascularization and function of islets delivered to the small bowel mesentery, a metabolically relevant site for insulin release. Diabetic mice receiving islets transplanted in proteolytically degradable hydrogels incorporating VEGF-A exhibited complete reversal of diabetic hyperglycemia with a 40% reduction in the number of islets required. Furthermore, hydrogel-delivered islets significantly improved weight gain, regulation of a glucose challenge, and intra-islet vascularization and engraftment compared to the clinical standard of islet infusion through the hepatic portal vein. This study establishes a simple biomaterial strategy for islet transplantation to promote enhanced islet engraftment and function.

3D co-culturing model of primary pancreatic islets and hepatocytes in hybrid spheroid to overcome pancreatic cell shortage

Here, a spheroidal 3D co-culture model of primary (rat) pancreatic islets and hepatocytes with uniform size and shape was developed using hemispheric concave microwell arrays. We conducted morphological and functional analyses of hybrid spheroids versus mono-cultures of islets or hepatocytes (controls). For the establishment of a 3D hybrid model, a broad range of cell ratios - 1:1, 1:3, 1:5, 1:7, 3:1, 5:1 and 7:1 mixture - of hepatocytes and pancreatic islets were used. As control, each hepatocyte and pancreatic islet were mono-cultured forming 3D spheroids. The transient morphology of spheroid formation in 9 culture models was observed using optical microscopy. Cell viability under these culture environments was assessed, and the morphologies of the outer and inner porous cell-spheroid structures were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and imaging of stained spheroid sections. The pancreatic islet-specific function of hybrid spheroids was evaluated by measuring insulin secretion and in vivo test by xenotransplantation of encapsulated spheroids in microfibers with a consistent maintenance of normal blood glucose levels over 4 weeks, while liver-specific functions were measured in terms of albumin secretion, urea secretion and cytochrome P450 activity. These diverse observations and evaluations validated the positive and bidirectional effects of co-cultured 3D spheroids. The proposed 3D co-culture model demonstrated that both cells appeared to support each other's functions strongly in spheroids, even though smaller proportions of each cell type was evaluated compared to mono-culture models, suggesting that the proposed model could help overcome the problem of cell shortages in clinical applications.

Immuno-isolation of pancreatic islet allografts using pegylated nanotherapy leads to long-term normoglycemia in full MHC mismatch recipient mice

Two major hurdles need to be surmounted for cell therapy for diabetes: (i) allo-immune rejection of grafted pancreatic islets, or stem/precursor cell-derived insulin-secreting cells; and (ii) continuing auto-immunity against the diabetogenic endogenous target antigen. Nanotherapeutics offer a novel approach to overcome these problems and here we ask if creation of “stealth” islets encapsulated within a thin cage of pegylated material of 100–200 nanometers thick provides a viable option for islet transplantation. The aims of this study were to test islet viability and functionality following encapsulation within the pegylated cage, and functional efficacy in vivo in terms of graft-derived control of normoglycemia in diabetic mice. We first demonstrated that pegylation of the islet surface, plus or minus nanoparticles, improved long-term islet viability in vitro compared to non-pegylated (naked) control islets. Moreover, pegylation of the islets with nanoparticles was compatible with glucose-stimulated insulin secretion and insulin biogenesis. We next looked for functionality of the created “stealth” DBA/2 (H-2^d) islets in vivo by comparing glycemic profiles across 4 groups of streptozotozin-induced diabetic C57BL/6 (H-2^b) recipients of (i) naked islets; (ii) pegylated islets; (iii) pegylated islets with nanoparticles (empty); and (iv) pegylated islets with nanoparticles loaded with a cargo of leukemia inhibitory factor (LIF), a factor both promotes adaptive immune tolerance and regulates pancreatic β cell mass. Without any other treatment, normoglycemia was lost after 17 d (+/−7.5 d) in control group. In striking contrast, recipients in groups (ii), (iii), and (iv) showed long-term (>100 d) normoglycemia involving 30%; 43%, and 57% of the recipients in each respective group. In conclusion, construction of “stealth” islets by pegylation-based nanotherapeutics not only supports islet structure and function, but also effectively isolates the islets from immune-mediated destruction. The added value of nanoparticles to deliver immune modulators plus growth factors such as LIF expands the potential of this novel therapeutic approach to cell therapy for diabetes.

Emerging technologies in medical applications of minimum volume vitrification

Cell/tissue biopreservation has broad public health and socio-economic impact affecting millions of lives. Cryopreservation technologies provide an efficient way to preserve cells and tissues targeting the clinic for applications including reproductive medicine and organ transplantation. Among these technologies, vitrification has displayed significant improvement in post-thaw cell viability and function by eliminating harmful effects of ice crystal formation compared to the traditional slow freezing methods. However, high cryoprotectant agent concentrations are required, which induces toxicity and osmotic stress to cells and tissues. It has been shown that vitrification using small sample volumes (i.e., <1 µl) significantly increases cooling rates and hence reduces the required cryoprotectant agent levels. Recently, emerging nano- and micro-scale technologies have shown potential to manipulate picoliter to nanoliter sample sizes. Therefore, the synergistic integration of nanoscale technologies with cryogenics has the potential to improve biopreservation methods.

Poly(ethylene glycol) cross-linked hemoglobin with antioxidant enzymes protects pancreatic islets from hypoxic and free radical stress and extends islet functionality

The objective of this study was to investigate the efficiency of multifunctional poly(ethylene glycol)-based hemoglobin conjugates crosslinked with antioxidant enzymes for their ability to protect an oxygen carrier (hemoglobin) and insulin secreting islets from the combination of hypoxic and free radical stress under simulated transplantation conditions. In this study, RINm5F cells and isolated pancreatic islets were challenged with oxidants (H(2)O(2) or xanthine and xanthine oxidase) and incubated with conjugates (hemoglobin-hemoglobin or superoxide dismutase-catalase-hemoglobin) in normoxia (21% oxygen) or hypoxia (6% or 1% oxygen). Hemoglobin protection, intracellular free radical activity and cell viability in RINm5F cells measured by methemoglobin, dichlorofluorescein-diacetate, and (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) assay, respectively, showed that cells were better protected by conjugates containing antioxidant enzymes. Insulin secretion from islets and qualitative confocal evaluation of viability showed beta cells were protected by conjugates containing antioxidant enzymes when exposed to induced stress. Our study suggested that antioxidant enzymes play a significant role in hemoglobin protection and thus extended cell protection.