Prolonged Glucose Normalization of Streptozotocin-Induced Diabetic Mice by Transplantation of Rat Islets Coencapsulated with Crosslinked Hemoglobin

Nanotechnology in cell replacement therapies for type 1 diabetes

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

Islet encapsulation

Review of emerging advances and persisting challenges in the engineering and translation of islet encapsulation technologies.

Intrasplenic Transplantation of Encapsulated Genetically Engineered Mouse Insulinoma Cells Reverses Streptozotocin-Induced Diabetes in Rats

Pancreatic islet transplantation is limited by shortage of donor organs. Although β-cell lines could be used, their secretion of insulin is characteristically glucose independent and immunoisolation is required. Here we show that intrasplenic transplantation of encapsulated glucose-responsive mouse insulinoma cells reversed streptozotocin (STZ)-induced diabetes in rats. MIN-6 cells derived from a transgenic mouse expressing SV 40 large T antigen in pancreatic β-cells were transfected with minigene encoding for human glucagon-like-peptide-1 under the control of rat insulin promoter. The cells were encapsulated in alginate/poly-L-lysine and used for cell transplantation in STZ-diabetic rats. Rats with nonfasting blood glucose (n-FBG) greater than 350 mg/dl were used. In group I rats (n = 6) 20 million encapsulated cells were injected into the spleen. Group II rats (n = 6) received empty capsules. n-FBG was measured biweekly. After 4 and 8 weeks, an intraperitoneal glucose tolerance test (IPGTT) was performed in group I; normal rats served as controls. Plasma insulin level was measured every other week (RIA). After 8 weeks, spleens were removed 1 day before sacrifice. In rats transplanted with cells the n-FBG was 100—150 mg/dl until the end of the study. After splenectomy, all cell recipients became diabetic (glucose 400 ± 20 mg/dl). Transplanted rats showed increase in body weight and insulin production (3.3 ± 1.0 ng/ml versus 0.92 ± 0.3 ng/ml; p < 0.01) and had normal IPGTT. Spleens contained capsules with insulin-positive cells. Overall, data from this work indicate that intrasplenic transplantation of xenogeneic encapsulated insulin-producing cells without immunosuppression reversed diabetes in rats. Excellent survival and function of the transplanted cells was due to the fact that the cells were separated from the bloodstream by the immunoisolatory membrane only and insulin was delivered directly to the liver (i.e., in a physiological manner).

Advances in Local Drug Release and Scaffolding Design to Enhance Cell Therapy for Diabetes

Islet transplant is a curative treatment for insulin-dependent diabetes. However, challenges, including poor tissue survival and a lack of efficient engraftment, must be overcome. An encapsulating or scaffolding material can act as a vehicle for agents carefully chosen for the islet transplant application. From open porous scaffolds to spherical capsules and conformal coatings, greater immune protection is often accompanied by greater distances to microvasculature. Generating a local oxygen supply from the implant material or encouraging vessel growth through the release of local factors can create an oxygenated engraftment site. Intricately related to the vascularization response, inflammatory interaction with the cell supporting implant is a long-standing hurdle to material-based islet transplant. Modulation of the immune responses to the islets as well as the material itself must be considered. To match the post-transplant complexity, the release rate can be tuned to orchestrate temporal responses. Material degradation properties can be utilized in passive approaches or external stimuli and biological cues in active approaches. A combination of multiple carefully chosen factors delivered in an agent-specialized manner is considered by this review to improve the long-term function of islets transplanted in scaffolding and encapsulating materials.

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.

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.

Synthesis and characterization of hemoglobin conjugates with antioxidant enzymes via poly(ethylene glycol) cross-linker (Hb-SOD-CAT) for protection from free radical stress

Hemoglobin (Hb) conjugated with the antioxidant enzymes (SOD and CAT), by employing dicarboxymethylated poly(ethylene glycol), was designed for protection of hemoglobin against free radicals. In this study, the conjugation process was confirmed by employing SDS-PAGE and SEC techniques. The average molecular weight of the conjugates was estimated to be around 1000 kDa. The enzymatic activities of the SOD and CAT in the conjugates (Hb-SOD-CAT) after conjugation were found to retain greater than 70% and 90% of the original bioactivity. Results show that antioxidant enzymes helped minimize methemoglobin (non-carrier of oxygen) formation during the conjugation process and also during storage at 4°C over a period of 1 month. In summary, the optimized (1:10 Hb/PEG) crosslinked conjugates with antioxidant enzymes showed protective properties from severe free radical stresses when incubated with hydrogen peroxide (0.1 and 1 mM) and xanthine (1 mM)/xanthine oxidase (10 and 20 mU/ml) system.

Anti-inflammatory peptide-functionalized hydrogels for insulin-secreting cell encapsulation

Pancreatic islet encapsulation within semi-permeable materials has been proposed for transplantation therapy of type I diabetes mellitus. Polymer hydrogel networks used for this purpose have been shown to provide protection from islet destruction by immunoreactive cells and antibodies. However, one of the fundamental deficiencies with current encapsulation methods is that the permselective barriers cannot protect islets from cytotoxic molecules of low molecular weight that are diffusible into the capsule material, which subsequently results in beta-cell destruction. Use of materials that can locally inhibit the interaction between the permeable small cytotoxic factors and islet cells may prolong the viability and function of encapsulated islet grafts. Here we report the design of anti-inflammatory hydrogels supporting islet cell survival in the presence of diffusible pro-inflammatory cytokines. We demonstrated that a poly(ethylene glycol)-containing hydrogel network, formed by native chemical ligation and presenting an inhibitory peptide for islet cell surface IL-1 receptor, was able to maintain the viability of encapsulated islet cells in the presence of a combination of cytokines including IL-1 beta, TNF-alpha, and INF-gamma. In stark contrast, cells encapsulated in unmodified hydrogels were mostly destroyed by cytokines which diffused into the capsules. At the same time, these peptide-modified hydrogels were able to efficiently protect encapsulated cells against beta-cell specific T-lymphocytes and maintain glucose-stimulated insulin release by islet cells. With further development, the approach of encapsulating cells and tissues within hydrogels presenting anti-inflammatory agents may represent a new strategy to improve cell and tissue graft function in transplantation and tissue engineering applications.

Perfluorocarbons: new tool for islets preservation in vitro

Pancreatic islet transplantations to treat type 1 diabetes often fail to function because of hypoxia. Perfluorocarbons (PFCs) exhibit a high oxygen solubility coefficient and maintain high oxygen partial pressure for extended times. They also serve as oxygen "reservoirs" for harvested organs in pancreas organ transplantation. Previous studies have shown the PFCs display antiadhesive effects on beta cells. The aim of this study was to evaluate the effects of PFC on islet viability and functionality and on extracellular matrix (ECM) disruption of islets via inhibition of adhesion. Primary cultures of rat islets were incubated for 24 hours in the presence or absence of 3.5% (weight/volume) PFCs in culture media. We studied viability (FDA/PI), stimulation index linked to insulin secretion (ELISA), and expression of insulin and laminin messenger RNAs (mRNAs). Immunostaining was performed on insulin and laminin. Islet viability was similar in the presence or absence of PFCs (about 80%). Stimulation index showed preservation of islet functionality in the presence of PFC (4.9 +/- 0.7) as compared with controls (2.8 +/- 0.5). Moreover, laminin mRNA expression was lower compared with controls (55% of PFC incubated vs control islets). Immunohistochemistry studies showed preservation of ECM inside the islets in the presence of PFCs versus controls at 24 hours after islet isolation. In conclusion, PFCs preserved islet viability and functionality and prevented ECM disruption. PFCs may represent a new tool for islet preservation in vitro.

Challenges and emerging technologies in the immunoisolation of cells and tissues

Protection of transplanted cells from the host immune system using immunoisolation technology will be important in realizing the full potential of cell-based therapeutics. Microencapsulation of cells and cell aggregates has been the most widely explored immunoisolation strategy, but widespread clinical application of this technology has been limited, in part, by inadequate transport of nutrients, deleterious innate inflammatory responses, and immune recognition of encapsulated cells via indirect antigen presentation pathways. To reduce mass transport limitations and decrease void volume, recent efforts have focused on developing conformal coatings of micron and submicron scale on individual cells or cell aggregates. Additionally, anti-inflammatory and immunomodulatory capabilities are being integrated into immunoisolation devices to generate bioactive barriers that locally modulate host responses to encapsulated cells. Continued exploration of emerging paradigms governed by the inherent challenges associated with immunoisolation will be critical to actualizing the clinical potential of cell-based therapeutics.

Improved phenotype of rat islets in a macrocapsule by co-encapsulation with cross-linked Hb

A number of rat islets were co-encapsulated in a diffusion chamber-type device, i.e., macrocapsule, with a thermoreversible polymeric extracellular matrix (ECM) and bioactive ingredient of cross-linked hemoglobin (Hb-C). The ECM was formed from an aqueous solution of N-isopropyl-acrylamide co-polymers with a small amount of acrylic acid, which exhibited unique sol-gel transition in a temperature range of 30-34 degrees C, without noticeable hysteresis. The incorporation of Hb-C in the islet macrocapsule showed a concentration-dependent effect on insulin secretion and viability of the entrapped islets. Insulin secretion stimulation by glucose and cell viability were more than doubled when compared with a control group (without Hb-C), at an optimum Hb-C concentration of 0.25 mM due to its unique oxygen transporting capacity. Furthermore, 0.25 mM Hb-C in the macrocapsule was able to support islet density up to 1000 islets/device in a 154 microl total volume without negative effects on islet functionality and viability. Hb-C incorporation is an effective strategy for a macrocapsule-type biohybrid artificial pancreas for Type-I diabetes treatment, which can be further developed to a rechargeable system by employing the thermoreversible ECM and designing a proper macrocapsule.