Islet transplantation has long been seen as an effective alternative to regular insulin injections in type 1 diabetes. Some models have shown limited success in humans, but grafts eventually succumb to rejection and fail.
During the Friday afternoon symposium Novel Approaches to Advancing Islet Transplantation—Basic Science, eight researchers discussed new strategies that have shown promise in the lab.
One approach flips the usual strategy of encapsulating beta cells to protect them from host immune attack. Instead, researchers encapsulate hybridoma cells producing monoclonal antibodies that shift host T cell response from an immune response to immune acceptance of the graft. The new T cell response inhibits progression to clinical diabetes in NOD mice.
“The goal is to prevent the autoimmune destruction of beta cells resulting in overt diabetes in these animals,” said Riccardo Calafiore, MD, PhD, Director of Cardiovascular, Endocrine and Metabolic Clinical Physiology at the Laboratory for Cell Transplants at the University of Perugia in Perugia, Italy. “Blocking glucocorticoid-induced tumor necrosis factor receptor-related protein (GITR) expanded regulatory T cells over effector T cells. None of the treated animals developed diabetes over 180 days compared to half of the control animals that developed diabetes within 70 days.”
Jyuhn-Huang Juang, MD, Professor at Chang Gung University in Taoyuan, Taiwan, described a proof-of-concept study using a novel temperature-sensitive hydrogel to protect beta cells transplanted into the subdermal space. The study compared insulin secretion in beta cell culture to insulin secretion in beta cells transplanted subcutaneously into nude mice. Beta cell culture and beta cell transplantation showed similar levels of insulin secretion after two weeks, Dr. Juang reported, indicating graft protection and functionality.
Smaller versus bigger capsules
Encapsulated islet transplants have a built-in problem. Nutrients, oxygen, and glucose must diffuse into the capsules and insulin must diffuse out quickly enough to support appropriate biological activity.
“The larger the capsule, the more blunted the response,” said Peter Buchwald, PhD, Director of Drug Discovery at the Diabetes Research Institute and Associate Professor of Molecular and Cellular Pharmacology at the University of Miami Miller School of Medicine. “In capsules larger than a millimeter or so, the quick-release peak of insulin release is completely dampened by the diffusion distance. The effect is sustained-release insulin more than a quick-release insulin.”
Dr. Buchwald said successful encapsulation techniques must balance diffusion kinetics, favoring smaller capsules, and avoidance of immune response, favoring larger capsules.
Xenotransplants in nonhuman primates
Immunosuppression may help improve survival and function in encapsulated xenoislet grafts, according to findings from a study that transplanted encapsulated canine islets into nonhuman primates.
“The increase of daily insulin required after xenogeneic encapsulated islet transplantation was delayed with immunonsuppressants compared to those without immunonsuppressants, and we observed improved fibrosis of the islet transplants,” said Eun Young Lee, MD, PhD, Seoul St. Mary’s Hospital, College of Medicine, The Catholic University of Korea in Seoul, South Korea. “Considering the state of encapsulation, when we try human application of xenogeneic encapsulated islet transplantation, immunonsuppressants may be helpful.”
Antioxidants, not immunonsuppressants
Incorporating tannic acid, an antioxidant, into the microcapsule structure can suppress immune rejection of islet allo- and xenografts and restore euglycemia in diabetic NOD mice, according results of another study presented in the session.
“We are not using any immunosuppression strategies except this tannic acid encapsulation model,” said Jessie M. Barra, BS, a graduate trainee at the University of Alabama at Birmingham. “Using tannic acid-encapsulated neonatal porcine islets, we restored euglycemia in diabetic NOD mice and maintained beta cell function for more than 200 days. Future studies will help us determine the immunological mechanism of protection we see with these novel capsules.”
Embryonic thymic epithelial cells
Islet transplantation rejection is mediated largely by T cell response, which in turn is mediated by thymus gland activity to generate self-tolerant, pathogen-responsive T cells.
Injecting embryonic thymic epithelial cells directly into the thymus at the time of beta cell transplantation can promote the generation of alloantigen-specific T regulatory cells and down-regulate the generation of donor antigen-specific T cells in diabetic Balb/C mice, according to Yong Fan, PhD, Principal Investigator at the Allegheny Health Network Institute of Cellular Therapeutics.
“Our results demonstrate the possibility of immune modulation through intrathymic injection of donor thymic epithelial cells to induce donor-specific immune tolerance and improve the duration of transplanted islet function,” Dr. Fan said. “Modulating the T cell selection process in the thymus through allo-donor thymic epithelial cell therapy could be an exciting approach to prolong islet graft survival.”
Microparticles deliver TGF-β1
Delivering transforming growth factor beta-1 (TGF-β1) directly to the islet transplantation site using biodegradable microparticles promotes the differentiation of T regulatory cells from naïve CD4 cells at the graft site with minimal systemic exposure, according to another study presented during the session.
In the study, local generation of T regulatory cells promoted graft acceptance, insulin secretion, and survival in rodent models.
“Host T cells still recognize the islet transplant as foreign, but instead of attacking, they were induced to recognize islets in a more regulatory and tolerated way by TGF-β1,” said Ying Li, MEng, a graduate research assistant at the Stabler Diabetes Tissue Engineering Laboratory at the University of Florida College of Medicine. “We successfully engineered a localized release platform covering the duration of 14 days, the crucial timeframe for the body to promote acceptance of the graft. This could minimize islet graft rejection and our dependency on systemic immunosuppression.”
bFGF in white fat blocks graft rejection
According to another study, pretreating white adipose tissue with basic fibroblast growth factor (bFGF), then transplanting an islet graft into the pretreated area, is a novel and successful approach for the elusive goal of long-term graft survival without systemic immunosuppression.
“Our model prevents rejection in the mouse model using only bFGF in one location, no immunosuppressant drugs,” said Yuki Nakafusa, MD, from the Graduate School of Medical Sciences at Kyushu University in Fukuoka, Japan. “The graft was accepted without inducing systemic tolerance, so maybe local unresponsiveness is playing an important role for allograft acceptance in our model. We have not yet totally understood the mechanism that prevents rejection in this model.”