Plasma-fibroblast gel as scaffold for islet transplantation

RNA aptamers specific for transmembrane p24 trafficking protein 6 and Clusterin for the targeted delivery of imaging reagents and RNA therapeutics to human β cells

The ability to detect and target β cells in vivo can substantially refine how diabetes is studied and treated. However, the lack of specific probes still hampers a precise characterization of human β cell mass and the delivery of therapeutics in clinical settings. Here, we report the identification of two RNA aptamers that specifically and selectively recognize mouse and human β cells. The putative targets of the two aptamers are transmembrane p24 trafficking protein 6 (TMED6) and clusterin (CLUS). When given systemically in immune deficient mice, these aptamers recognize the human islet graft producing a fluorescent signal proportional to the number of human islets transplanted. These aptamers cross-react with endogenous mouse β cells and allow monitoring the rejection of mouse islet allografts. Finally, once conjugated to saRNA specific for X-linked inhibitor of apoptosis (XIAP), they can efficiently transfect non-dissociated human islets, prevent early graft loss, and improve the efficacy of human islet transplantation in immunodeficient in mice.

Tissue Engineering Strategies for Improving Beta Cell Transplantation Outcome

Purpose of Review

Beta cell replacement therapy as a form of islet transplantation is a promising alternative therapy with the possibility to make selected patients with type 1 diabetes (T1D) insulin independent. However, this technique faces challenges such as extensive activation of the host immune system post-transplantation, lifelong need for immunosuppression, and the scarcity of islet donor pancreas. Advancement in tissue engineering strategies can improve these challenges and allow for a more widespread application of this therapy. This review will discuss the recent development and clinical translation of tissue engineering strategies in beta cell replacement therapy.

Recent Findings

Tissue engineering offers innovative solutions for producing unlimited glucose responsive cells and fabrication of appropriate devices/scaffolds for transplantation applications. Generation of pancreatic organoids with supporting cells in biocompatible biomaterials is a powerful technique to improve the function of insulin-producing cell clusters. Fabrication of physical barriers such as encapsulation strategies can protect the cells from the host immune system and allow for graft retrieval, although this strategy still faces major challenges to fully restore physiological glucose regulation.

Summary

The three main components of tissue engineering strategies including the generation of stem cell-derived insulin-producing cells and organoids and the possibilities for therapeutic delivery of cell-seeded devices to extra-hepatic sites need to come together in order to provide safe and functional insulin-producing devices for clinical beta cell replacement therapy.

Microvessel Network Formation and Interactions with Pancreatic Islets in Three-Dimensional Chip Cultures

The pancreatic islet is a highly vascularized micro-organ, and rapid revascularization postislet transplantation is important for islet survival and function. However, the various mechanisms involved in islet revascularization are not fully understood, and we currently lack good in vitro platforms to explore this. Our aim for this study was to generate perfusable microvascular networks in a microfluidic chip device, in which islets could be easily integrated, to establish an in vitro platform for investigations on islet-microvasculature interactions. We compared the ability of mesenchymal stem cells (MSCs) and fibroblasts to support microvascular network formation by human umbilical vein endothelial cells (HUVECs) and human induced pluripotent stem cell-derived endothelial colony-forming cell in two-dimensional and three-dimensional models of angiogenesis, and tested the effect of different culture media on microvessel formation. HUVECs that were supported by MSCs formed patent and perfusable networks in a fibrin gel, whereas networks supported by fibroblasts rapidly regressed. Network morphology could be controlled by adjusting relative cell numbers and densities. Incorporation of isolated rat islets demonstrated that islets recruit local microvasculature in vitro, but that the microvessels did not invade islets, at least during the course of these studies. This in vitro microvascularization platform can provide a useful tool to study how various parameters affect islet integration with microvascular networks and could also be utilized for studies of vascularization of other organ systems. Impact statement To improve pancreatic islet graft survival and function posttransplantation, rapid and adequate revascularization is critical. Efforts to improve islet revascularization are demanding due to an insufficient understanding of the mechanisms involved in the process. We have applied a microfluidics platform to generate microvascular networks, and by incorporating pancreatic islets, we were able to study microvasculature-islet interactions in real time. This platform can provide a useful tool to study islet integration with microvascular networks, and could be utilized for studies of vascularization of other organ systems. Moreover, this work may be adapted toward developing a prevascularized islet construct for transplantation.

Engineering the vasculature for islet transplantation

The microvasculature in the pancreatic islet is highly specialized for glucose sensing and insulin secretion. Although pancreatic islet transplantation is a potentially life-changing treatment for patients with insulin-dependent diabetes, a lack of blood perfusion reduces viability and function of newly transplanted tissues. Functional vasculature around an implant is not only necessary for the supply of oxygen and nutrients but also required for rapid insulin release kinetics and removal of metabolic waste. Inadequate vascularization is particularly a challenge in islet encapsulation. Selectively permeable membranes increase the barrier to diffusion and often elicit a foreign body reaction including a fibrotic capsule that is not well vascularized. Therefore, approaches that aid in the rapid formation of a mature and robust vasculature in close proximity to the transplanted cells are crucial for successful islet transplantation or other cellular therapies. In this paper, we review various strategies to engineer vasculature for islet transplantation. We consider properties of materials (both synthetic and naturally derived), prevascularization, local release of proangiogenic factors, and co-transplantation of vascular cells that have all been harnessed to increase vasculature. We then discuss the various other challenges in engineering mature, long-term functional and clinically viable vasculature as well as some emerging technologies developed to address them. The benefits of physiological glucose control for patients and the healthcare system demand vigorous pursuit of solutions to cell transplant challenges. STATEMENT OF SIGNIFICANCE: Insulin-dependent diabetes affects more than 1.25 million people in the United States alone. Pancreatic islets secrete insulin and other endocrine hormones that control glucose to normal levels. During preparation for transplantation, the specialized islet blood vessel supply is lost. Furthermore, in the case of cell encapsulation, cells are protected within a device, further limiting delivery of nutrients and absorption of hormones. To overcome these issues, this review considers methods to rapidly vascularize sites and implants through material properties, pre-vascularization, delivery of growth factors, or co-transplantation of vessel supporting cells. Other challenges and emerging technologies are also discussed. Proper vascular growth is a significant component of successful islet transplantation, a treatment that can provide life-changing benefits to patients.

The emerging field of pancreatic tissue engineering: A systematic review and evidence map of scaffold materials and scaffolding techniques for insulin-secreting cells

A bioartificial endocrine pancreas is proposed as a future alternative to current treatment options. Patients with insulin-secretion deficiency might benefit. This is the first systematic review that provides an overview of scaffold materials and techniques for insulin-secreting cells or cells to be differentiated into insulin-secreting cells. An electronic literature survey was conducted in PubMed/MEDLINE and Web of Science, limited to the past 10 years. A total of 197 articles investigating 60 different materials met the inclusion criteria. The extracted data on materials, cell types, study design, and transplantation sites were plotted into two evidence gap maps. Integral parts of the tissue engineering network such as fabrication technique, extracellular matrix, vascularization, immunoprotection, suitable transplantation sites, and the use of stem cells are highlighted. This systematic review provides an evidence-based structure for future studies. Accumulating evidence shows that scaffold-based tissue engineering can enhance the viability and function or differentiation of insulin-secreting cells both in vitro and in vivo.

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.

Selection of Implantation Sites for Transplantation of Encapsulated Pancreatic Islets

Pancreatic islet transplantation has been validated as a valuable therapy for type 1 diabetes mellitus patients with exhausted insulin treatment. However, this therapy remains limited by the shortage of donor and the requirement of lifelong immunosuppression. Islet encapsulation, as an available bioartificial pancreas (BAP), represents a promising approach to enable protecting islet grafts without or with minimal immunosuppression and possibly expanding the donor pool. To develop a clinically implantable BAP, some key aspects need to be taken into account: encapsulation material, capsule design, and implant site. Among them, the implant site exerts an important influence on the engraftment, stability, and biocompatibility of implanted BAP. Currently, an optimal site for encapsulated islet transplantation may include sufficient capacity to host large graft volumes, portal drainage, ease of access using safe and reproducible procedure, adequate blood/oxygen supply, minimal immune/inflammatory reaction, pliable for noninvasive imaging and biopsy, and potential of local microenvironment manipulation or bioengineering. Varying degrees of success have been confirmed with the utilization of liver or extrahepatic sites in an experimental or preclinical setting. However, the ideal implant site remains to be further engineered or selected for the widespread application of encapsulated islet transplantation.

Modular tissue engineering for the vascularization of subcutaneously transplanted pancreatic islets

The transplantation of pancreatic islets, following the Edmonton Protocol, is a promising treatment for type I diabetics. However, the need for multiple donors to achieve insulin independence reflects the large loss of islets that occurs when islets are infused into the portal vein. Finding a less hostile transplantation site that is both minimally invasive and able to support a large transplant volume is necessary to advance this approach. Although the s.c. site satisfies both these criteria, the site is poorly vascularized, precluding its utility. To address this problem, we demonstrate that modular tissue engineering results in an s.c. vascularized bed that enables the transplantation of pancreatic islets. In streptozotocin-induced diabetic SCID/beige mice, the injection of 750 rat islet equivalents embedded in endothelialized collagen modules was sufficient to restore and maintain normoglycemia for 21 days; the same number of free islets was unable to affect glucose levels. Furthermore, using CLARITY, we showed that embedded islets became revascularized and integrated with the host's vasculature, a feature not seen in other s.c.

STUDIES

Collagen-embedded islets drove a small (albeit not significant) shift toward a proangiogenic CD206⁺MHCII^-(M2-like) macrophage response, which was a feature of module-associated vascularization. While these results open the potential for using s.c. islet delivery as a treatment option for type I diabetes, the more immediate benefit may be for the exploration of revascularized islet biology.

Fibroblasts accelerate islet revascularization and improve long-term graft survival in a mouse model of subcutaneous islet transplantation

Pancreatic islet transplantation has been considered for many years a promising therapy for beta-cell replacement in patients with type-1 diabetes despite that long-term clinical results are not as satisfactory. This fact points to the necessity of designing strategies to improve and accelerate islets engraftment, paying special attention to events assuring their revascularization. Fibroblasts constitute a cell population that collaborates on tissue homeostasis, keeping the equilibrium between production and degradation of structural components as well as maintaining the required amount of survival factors. Our group has developed a model for subcutaneous islet transplantation using a plasma-based scaffold containing fibroblasts as accessory cells that allowed achieving glycemic control in diabetic mice. Transplanted tissue engraftment is critical during the first days after transplantation, thus we have gone in depth into the graft-supporting role of fibroblasts during the first ten days after islet transplantation. All mice transplanted with islets embedded in the plasma-based scaffold reversed hyperglycemia, although long-term glycemic control was maintained only in the group transplanted with the fibroblasts-containing scaffold. By gene expression analysis and histology examination during the first days we could conclude that these differences might be explained by overexpression of genes involved in vessel development as well as in β-cell regeneration that were detected when fibroblasts were present in the graft. Furthermore, fibroblasts presence correlated with a faster graft re-vascularization, a higher insulin-positive area and a lower cell death. Therefore, this work underlines the importance of fibroblasts as accessory cells in islet transplantation, and suggests its possible use in other graft-supporting strategies.

Harnessing the Foreign Body Reaction in Marginal Mass Device-less Subcutaneous Islet Transplantation in Mice

BACKGROUND

Islet transplantation is a successful β-cell replacement therapy for selected patients with type 1 diabetes mellitus. However, despite early insulin independence, long-term graft attrition gradually reverts recipients to exogenous insulin dependency. Undoubtedly, as insulin producing stem cell therapies progress, a transplant site that is retrievable is desirable. This prerequisite is currently incompatible with intrahepatic islet transplantation. Herein, we evaluate the functional capacity of a prevascularized subcutaneous site to accommodate marginal islet mass transplantation in mice.

METHODS

Syngeneic mouse islets (150) were transplanted either under the kidney capsule (KC), into a prevascularized subcutaneous device-less (DL) site, or into the unmodified subcutaneous (SC) tissue. The DL site was created 4 weeks before diabetes induction and islet transplantation through the transient placement of a 5-Fr vascular catheter. Recipient mice were monitored for glycemic control and intraperitoneal glucose tolerance.

RESULTS

A marginal islet mass transplanted into the DL site routinely reversed diabetes (n = 13 of 18) whereas all SC islet recipients failed to restore glycemic control (n = 0 of 10, P < 0.01, log-rank). As anticipated, nearly all islet-KC mice (n = 15 of 16) became euglycemic posttransplant. The DL recipients' glucose profiles were comparable to KC islet grafts, postintrapertioneal glucose tolerance testing, whereas SC recipients remained hyperglycemic postglucose challenge. All normoglycemic mice maintained graft function for 100 days until graft retrieval. DL and KC islet grafts stained positively for insulin, microvessels, and a collagen scaffold.

CONCLUSIONS

The device-less prevascularized approach supports marginal mass islet engraftment in mice.

Long-term function and optimization of mouse and human islet transplantation in the subcutaneous device-less site

Clinical islet transplantation has routinely been demonstrated to be an efficacious means of restoring glycemic control in select patients with autoimmune diabetes. Notwithstanding marked progress and improvements, the broad-spectrum application of this treatment option is restricted by the complications associated with intrahepatic portal cellular infusion and the scarcity of human donor pancreata. Recent progress in stem cell biology has demonstrated that the potential to expand new β cells for clinical transplantation is now a reality. As such, research focus is being directed toward optimizing safe extrahepatic transplant sites to house future alternative β cell sources for clinical use. The present study expands on our previous development of a prevascularized subcutaneous device-less (DL) technique for cellular transplantation, by demonstrating long-term (>365 d) durable syngeneic murine islet graft function. Furthermore, histological analysis of tissue specimens collected immediately post-DL site creation and acutely post-human islet transplantation demonstrates that this technique results in close apposition of the neovascularized collagen to the transplanted cells without dead space, thereby avoiding hypoxic luminal dead-space. Murine islets transplanted into the DL site created by a larger luminal diameter (6-Fr.) (n = 11), reversed diabetes to the similar capacity as our standard DL method (5-Fr.)(n = 9). Furthermore, glucose tolerance testing did not differ between these 2 transplant groups (p > 0 .05). Taken together, this further refinement of the DL transplant approach facilitates a simplistic means of islet infusion, increases the transplant volume capacity and may provide an effective microenvironment to house future alternative β cell sources.

Magnetoencapsulated human islets xenotransplanted into swine: a comparison of different transplantation sites

BACKGROUND

The fate of magnetically labeled, barium-gelled alginate/protamine sulfate/alginate microcapsules (APSA magnetocapsules) following xenotransplantation was assessed by magnetic resonance imaging (MRI) and histopathology.

METHODS

Magnetocapsules with and without human islets were transplanted into five different clinically accessible sites: portal vein, subcutaneous tissue, skeletal muscle, the liver and the kidney subcapsular space. The surface of APSA magnetocapsules was modified using clinical-grade heparin to mitigate an instant blood-mediated inflammatory reaction.

RESULTS

The accuracy of site-specific delivery was confirmed using a clinical 1.5T MRI setup, where the magnetocapsules appeared as distinct hypointense entities after transplantation. As proven by the Lee-White blood coagulation test, heparin-treated APSA magnetocapsules did not induce blood clotting for more than 48 h in vitro. Heparinized magnetocapsules induced innate and adaptive immune responses in vivo regardless of the transplantation sites.

CONCLUSION

We have demonstrated the feasibility of using a clinical 1.5T MRI to non-invasively detect the accuracy of APSA magnetocapsule injection into various clinically accessible transplantation sites. Among the investigated transplantation sites, the liver and kidney subcapsular space were found to be the least immuno-responsive toward xenografted magneto-encapsulated human islets.

Diabetes Is Reversed in a Murine Model by Marginal Mass Syngeneic Islet Transplantation Using a Subcutaneous Cell Pouch Device

Islet transplantation is a successful β-cell replacement therapy for selected patients with type 1 diabetes mellitus. Although high rates of early insulin independence are achieved routinely, long-term function wanes over time. Intraportal transplantation is associated with procedural risks, requires multiple donors, and does not afford routine biopsy. Stem cell technologies may require potential for retrievability, and graft removal by hepatectomy is impractical. There is a clear clinical need for an alternative, optimized transplantation site. The subcutaneous space is a potential substitute, but transplantation of islets into this site has routinely failed to reverse diabetes. However, an implanted device, which becomes prevascularized before transplantation, may alter this equation.

The authors apply a new implanted subcutaneous cell pouch (CP) device in a mouse diabetes model. Mouse islets transplanted into the CP restore glycemic control with well respond to glucose challenge. CP may serve as a potential alternative to clinical intraportal islet transplantation.

Bioengineered stem cells as an alternative for islet cell transplantation

Type 1 diabetes is an autoimmune and increasingly prevalent condition caused by immunological destruction of beta cells. Insulin remains the mainstay of therapy. Endeavours in islet transplantation have clearly demonstrated that type 1 diabetes is treatable by cellular replacement. Many challenges remain with this approach. The opportunity to use bioengineered embryonic or adult pluripotential stem cells, or islets derived from porcine xenograft sources could address future demands, but are still associated with considerable challenges. This detailed review outlines current progress in clinical islet transplantation, and places this in perspective for the remarkable scientific advances now occurring in stem cell and regenerative medicine approaches in the treatment of future curative treatment of diabetes.

Long-term allogeneic islet graft survival in prevascularized subcutaneous sites without immunosuppressive treatment

Establishment of noninvasive and efficient islet transplantation site together with the avoidance of immunosuppressive drugs for islet engraftment is currently the two major tasks for islet transplantation approach to treat patients with type 1 diabetes. Here, we proposed a method to achieve long-term allogeneic islet graft function without immunosuppression after transplantation in subcutaneous sites. Two agarose rods with basic fibroblast growth factor and heparin were implanted for 1 week in dorsal subcutaneous sites in diabetic rats. After rod removal, 1500 islets were transplanted into the prevascularized pockets. Islets transplanted in prevascularized but not nontreated subcutaneous sites rapidly reverted hyperglycemia in all streptozotocin-induced diabetic rats. In contrast to transient normalization of blood glucose when allogeneic islets were transplanted into liver, allogeneic islets transplanted into this prevascularized subcutaneous site demonstrated long-term graft survival and function in all three rat strain combinations (Fisher 344 to ACI, Lewis to ACI and Fisher 344 to Wistar), evidenced by nonfasting blood glucose level, plasma insulin concentration, intraperitoneal glucose tolerance test and immunohistochemistry. These results indicated that a subcutaneous site prevascularized by this method is potentially a suitable site for successful allogeneic islet transplantation without immunosuppression.

Therapeutic cell encapsulation techniques and applications in diabetes

The encapsulation of therapeutic cells permits the implantation of allogeneic and xenogeneic cells for the regulation of certain physiological processes damaged by the death or senescence of host tissues. The encapsulation of pancreatic cells for the treatment of diabetes is emphasized; however, many of the techniques are applicable to a wide array of mammalian cell applications. The summary of both established and novel encapsulation techniques, clinical trials, and commercial product developments highlights the metered but steady pace of therapeutic cell encapsulation towards implementation.

Strategy for clinical setting in intramuscular and subcutaneous islet transplantation

Intraportal islet transplantation has a long history as a procedure for clinical islet transplantation. However, many recent studies revealed that the intraportal procedure has some disadvantages in transplant efficiency and safety. Many candidates as an optimal transplant site for islets have been assessed, but further studies and clinical trials are still necessary. Intramuscular and subcutaneous spaces are important candidates, because the transplant and biopsy procedures are simple approaches with minimal invasion and few complications. Although they are sites with hypovascularity and hypoxia, which contribute to the poor transplant efficiency, many experimental trials for improving the outcome in intramuscular and subcutaneous islet transplantations have been performed, focusing on early angiogenesis and scaffolds for engrafting transplanted islets. We review current progress in intramuscular and subcutaneous islet transplantations and discuss ways to develop them as optimal transplant sites for islets.

Characterization of a nanogland for the autotransplantation of human pancreatic islets

Despite the clinical success of pancreatic islet transplantation, graft function is frequently lost over time due to islet dispersion, lack of neovascularization, and loss of physiological architecture. To address these problems, islet encapsulation strategies including scaffolds and devices have been developed, which produced encouraging results in preclinical models. However, islet loss from such architectures could represent a significant limitation to clinical use. Here, we developed and characterized a novel islet encapsulation silicon device, the NanoGland, to overcome islet loss, while providing a physiological-like environment for long-term islet viability and revascularization. NanoGlands, microfabricated with a channel size ranging from 3.6 nm to 60 μm, were mathematically modeled to predict the kinetics of the response of encapsulated islets to glucose stimuli, based on different channel sizes, and to rationally select membranes for further testing. The model was validated in vitro using static and perifusion testing, during which insulin secretion and functionality were demonstrated for over 30-days. In vitro testing also showed 70-83% enhanced islet retention as compared to porous scaffolds, here simulated through a 200 μm channel membrane. Finally, evidence of in vivo viability of human islets subcutaneously transplanted within NanoGlands was shown in mice for over 120 days. In this context, mouse endothelial cell infiltration suggesting neovascularization from the host were identified in the retrieved grafts. The NanoGland represents a novel, promising approach for the autotransplantation of human islets.

Cooperation by fibroblasts and bone marrow-mesenchymal stem cells to improve pancreatic rat-to-mouse islet xenotransplantation

Experimental and clinical experiences highlight the need to review some aspects of islet transplantation, especially with regard to site of grafting and control of the immune response. The subcutaneous space could be a good alternative to liver but its sparse vasculature is its main limitation. Induction of graft tolerance by using cells with immunoregulatory properties is a promising approach to avoid graft rejection. Both Fibroblasts and Mesenchymal Stem Cells (MSCs) have shown pro-angiogenic and immunomodulatory properties. Transplantation of islets into the subcutaneous space using plasma as scaffold and supplemented with fibroblasts and/or Bone Marrow-MSCs could be a promising strategy to achieve a functional extra-hepatic islet graft, without using immunosuppressive drugs. Xenogenic rat islets, autologous fibroblasts and/or allogenic BM-MSCs, were mixed with plasma, and coagulation was induced to constitute a Plasma-based Scaffold containing Islets (PSI), which was transplanted subcutaneously both in immunodeficient and immunocompetent diabetic mice. In immunodeficient diabetic mice, PSI itself allowed hyperglycemia reversion temporarily, but the presence of pro-angiogenic cells (fibroblasts or BM-MSCs) within PSI was necessary to improve graft re-vascularization and, thus, consistently maintain normoglycemia. In immunocompetent diabetic mice, only PSI containing BM-MSCs, but not those containing fibroblasts, normalized glycemia lasting up to one week after transplantation. Interestingly, when PSI contained both fibroblasts and BM-MSCs, the normoglycemia period showed an increase of 4-times with a physiological-like response in functional tests. Histology of immunocompetent mice showed an attenuation of the immune response in those grafts with BM-MSCs, which was improved by co-transplantation with fibroblasts, since they increased BM-MSC survival. In summary, fibroblasts and BM-MSCs showed similar pro-angiogenic properties in this model of islet xenotransplantation, whereas only BM-MSCs exerted an immunomodulatory effect, which was improved by the presence of fibroblasts. These results suggest that cooperation of different cell types with islets will be required to achieve a long-term functional graft.

Microwell scaffolds for the extrahepatic transplantation of islets of Langerhans

Allogeneic islet transplantation into the liver has the potential to restore normoglycemia in patients with type 1 diabetes. However, the suboptimal microenvironment for islets in the liver is likely to be involved in the progressive islet dysfunction that is often observed post-transplantation. This study validates a novel microwell scaffold platform to be used for the extrahepatic transplantation of islet of Langerhans. Scaffolds were fabricated from either a thin polymer film or an electrospun mesh of poly(ethylene oxide terephthalate)-poly(butylene terephthalate) (PEOT/PBT) block copolymer (composition: 4000PEOT30PBT70) and were imprinted with microwells, ∼400 µm in diameter and ∼350 µm in depth. The water contact angle and water uptake were 39±2° and 52.1±4.0 wt%, respectively. The glucose flux through electrospun scaffolds was three times higher than for thin film scaffolds, indicating enhanced nutrient diffusion. Human islets cultured in microwell scaffolds for seven days showed insulin release and insulin content comparable to those of free-floating control islets. Islet morphology and insulin and glucagon expression were maintained during culture in the microwell scaffolds. Our results indicate that the microwell scaffold platform prevents islet aggregation by confinement of individual islets in separate microwells, preserves the islet’s native rounded morphology, and provides a protective environment without impairing islet functionality, making it a promising platform for use in extrahepatic islet transplantation.

Advancing islet transplantation: from engraftment to the immune response

The promise and progress of islet transplantation for treating type 1 diabetes has been challenged by obstacles to patient accessibility and long-term graft function that may be overcome by integrating emerging technologies in biomaterials, drug delivery and immunomodulation. The hepatic microenvironment and traditional systemic immunosuppression stress the vulnerable islets and contribute to the limited success of transplantation. Locally delivering extracellular matrix proteins and trophic factors can enhance transplantation at extrahepatic sites by promoting islet engraftment, revascularisation and long-term function while avoiding unintended systemic effects. Cell- and cytokine-based therapies for immune cell recruitment and reprogramming can inhibit local and systemic immune system activation that normally attacks transplanted islets. Combined with antigen-specific immunotherapies, states of operational tolerance may be achievable, reducing or eliminating the long-term pharmaceutical burden. Integration of these technologies to enhance engraftment and combat rejection may help to advance the therapeutic efficacy and availability of islet transplantation.

Extrahepatic islet transplantation with microporous polymer scaffolds in syngeneic mouse and allogeneic porcine models

Intraportal transplantation of islets has successfully treated select patients with type 1 diabetes. However, intravascular infusion and the intrahepatic site contribute to significant early and late islet loss, yet a clinical alternative has remained elusive. We investigated non-encapsulating, porous, biodegradable polymer scaffolds as a vehicle for islet transplantation into extrahepatic sites, using syngeneic mouse and allogeneic porcine models. Scaffold architecture was modified to enhance cell infiltration leading to revascularization of the islets with minimal inflammatory response. In the diabetic mouse model, 125 islets seeded on scaffolds implanted into the epididymal fat pad restored normoglycemia within an average of 1.95 days and transplantation of only 75 islets required 12.1 days. Increasing the pore size to increase islet-islet interactions did not significantly impact islet function. The porcine model was used to investigate early islet engraftment. Increasing the islet seeding density led to a greater mass of engrafted islets, though the efficiency of islet survival decreased. Transplantation into the porcine omentum provided greater islet engraftment than the gastric submucosa. These results demonstrate scaffolds support murine islet transplantation with high efficiency, and feasibility studies in large animals support continued pre-clinical studies with scaffolds as a platform to control the transplant microenvironment.

Pancreatic Islet Culture and Preservation Strategies: Advances, Challenges, and Future Outlook

Postisolation islet survival is a critical step for achieving successful and efficient islet transplantation. This involves the optimization of islet culture in order to prolong survival and functionality in vitro. Many studies have focused on different strategies to culture pancreatic islets in vitro through manipulation of culture media, surface modified substrates, and the use of various techniques such as encapsulation, embedding, scaffold, and bioreactor culture strategies. This review aims to present and discuss the different methodologies employed to optimize pancreatic islet culture in vitro as well as address their respective advantages and drawbacks.

The cytoprotection of chitosan based hydrogels in xenogeneic islet transplantation: An in vivo study in streptozotocin-induced diabetic mouse

Immune rejection and scarcity of donor tissues are the restrictions of islets transplantation. In this study, the cytoprotection of chitosan hydrogels in xenogeneic islet transplantation was demonstrated. Wistar rat islets encapsulated in chitosan hydrogels were performed glucose challenge test and live/dead cell staining in vitro. Islets/chitosan hydrogels were transplanted into the renal subcapsular space of diabetic C57BL/6 mice. Non-fasting blood glucose level (NFBG), body weight, intraperitoneal glucose tolerance test (IPGTT), and glucose disappearance rate were determined perioperatively. The serum insulin level was analyzed, and the kidney transplanted with islets/chitosan hydrogels were retrieved for histological examination after sacrifice. The present results showed that islets encapsulated in chitosan hydrogels secreted insulin in response to the glucose stimulation as naked islets with higher cell survival. The NFBG of diabetic mice transplanted with islets/chitosan hydrogels decreased from 487+/-46 to 148+/-32 at one day postoperation and maintained in the range of 201+/-36 mg/dl for four weeks with an increase in body weight. IPGTT showed the glucose disappearance rate of mice transplanted with islets/chitosan hydrogels was significant faster than that of mice transplanted with naked islets; the serum insulin level increased from 0.29+/-0.06 to 1.69+/-0.65 microg/dl postoperatively. Histological examination revealed that the islets successfully engrafted at renal subcapsular space with positive insulin staining. The immunostain was negative for neither the T-cell lineages nor the monocyte/macrophages. This study indicates that the chitosan hydrogels deliver and protect encapsulated islets successfully in xenotransplantation.