The potential to treat type 1 diabetes using transplanted islet cells has long been recognized, but in reality this has proven challenging partly because the transplanted islet cells die if they don’t receive enough oxygen. Researchers at Massachusetts Institute of Technology (MIT) and Beta-O2 Technologies have now demonstrated how a miniaturized, implantable device that supplies encapsulated islet cell transplants with a renewable supply of oxygen can treat diabetic rats successfully for months, without the need for immunosuppression, and with minimal loss of islet cells. The in vivo tests, reported in Scientific Reports, showed that nearly 90% of the transplanted islets remained viable for months, and most of the recipient rats maintained normal blood glucose levels during the test period.
“Getting oxygen to these cells is a difficult problem,” comments Clark Colton, M.D., an MIT professor of chemical engineering, and senior author of the researchers’ published paper. “The benefits of this approach are: you keep the islets alive to perform their function, you don't need as much tissue, and you reduce the ability of the implants to provoke an immune response.” The researchers report their findings in a paper entitled “Long-Term Viability and Function of Transplanted Islets Macroencapsulated at High Density Are Achieved by Enhanced Oxygen Supply.”
Islet transplantation can effectively cure type 1 diabetes, but it requires immunosuppression, the authors explain. Encapsulating the islets can feasibly eliminate the need for immunosuppression, but this then limits oxygen getting to the islets, which is the primary cause of failure.
In healthy pancreatic tissue that is oxygenated through capillaries, all the islet cells receive oxygen-rich blood, at an oxygen partial pressure of about 100 mmHg. In contrast, transplanted islets, whether encapsulated or nonencapsulated, are not served by a native capillary network and rely solely on diffusion for their supply of oxygen and nutrients, and for hormone secretion and waste product removal. Encapsulating the islets to protect them from the host immune system then “aggravates the problem and further impairs islet viability and function,” the authors write. Breakdown of the dead donor islet cells into component parts only acts to exacerbate the immune response further.
A number of different approaches to boosting the oxygen supply to transplanted encapsulated islets and cells have been attempted. These include reducing the diffusion distances, increasing oxygen permeability of the encapsulating material, inducing neovascularization to bring blood flow closer to the tissue, and providing exogenous oxygen to the encapsulated tissue.
Previous work by Dr. Colton’s laboratory has demonstrated that the outer surface of islets needs to be exposed to at least 50 mmHg oxygen for the insulin-producing beta cells to remain viable and functional. The device in development by the Beta-O2 team is designed to provide a renewable supply of enough oxygen directly to encapsulated islet implants.
The MIT and Beta-O2 Technologies researchers first carried out a series of studies to determine the optimum conditions needed for encapsulated islets to remain viable and functional over long periods of time, and how to miniaturize the device—while still containing sufficient islet cells—into a format that is small enough to be implanted in human patients.
The resulting device comprises islets encapsulated in a slab of alginate about 600 μm thick. The slab is bounded on its outer face by a membrane that protects the cells from the recipient’s immune system, but allows the diffusion of insulin, nutrients, and oxygen. The slab's inner face is bounded by a gas-permeable membrane, underneath which is sited a gas chamber, about 5 mm thick, which carries atmospheric gases, including oxygen.
The oxygen diffuses from the chamber across the gas-permeable membrane and into the islet cells embedded in the alginate. As the oxygen is consumed by the cells, the oxygen partial pressure in the chamber drops, until it reaches the minimum 50 mmHg level required by the islet cells and needs replenishing. The researcher’s studies indicated that to keep the oxygen partial pressure in the chamber at 50 mmHg for 24 hours, the starting partial pressure of oxygen in the chamber would need to be at least 300 mmHg. Then, after 24 hours, when the oxygen partial pressure has dropped to 50 mmHg, the oxygen supply in the chamber is replenished through a port implanted under the skin.
For their in vivo tests, the team implanted devices containing about 2400 islet equivalent (IEQs), at densities of up to 4800 IEQ/cm2, into 137 diabetic rats. The animals' glucose levels were monitored, and the devices were removed selectively after differing periods of time. The results confirmed that nearly all of the animals remained normoglycemic for varying periods of time, with 66 demonstrating normal blood glucose levels for more than 8 weeks. Although one animal became hyperglycemic, “all other animals remained normoglycemic until device explantation from 78 to 228 days following implantation,” the authors write. “Normoglycemic periods of 6 months or more were observed in some animals with all surface densities.” The results of intravenous glucose tolerance tests carried out at day 20 and day 40 after implantation were also very similar to those of normal, nondiabetic animals, “demonstrating fast response of the device implant,” the team comments. As expected, when the devices were removed the animals reverted to becoming hyperglycemic.
Importantly, most of the implanted islets survived, the authors note. “Nearly all of the viable islets initially implanted in the device retained their viability throughout the duration of these experiments, even at the highest density of 4,800 IEQ/cm2, as demonstrated by high OCR [oxygen consumption rate], recovery at explantation.”
Normoglycemia time did vary significantly between different implantations under otherwise identical conditions, dependent upon islet quality, the authors note. “This variation correlated with islet quality parameters IEQ and OCR, which emphasizes the importance of initial islet quality in determining the outcome of each transplantation.”
The team says that one of the key findings of the study is that the technology can be scaled down in size enough to make it feasible for use in humans. “…the size of the device for implantation into humans can be substantially reduced,” they state. “Consequently, for example, a dose of 250,000 IEQ could be supported under these conditions in a device having about 50 cm2 surface area for supply of O2 from the gas chamber.” And by sandwiching a gas chamber in between two islet compartments back-to-back, the surface area could further be reduced to 25 cm2, which could be achieved with a device disk of just 5.6 cm diamter. “A dose of 500,000 IEQ would require two such devices or a single device with 8.0 cm diameter. Such size reduction would make implantation in humans more feasible,” the team states. “Consequently, for example, a dose of 250,000 IEQ macroencapsulated devices small enough for clinical use and provide a firm basis for design of devices for testing in large animals and humans.”
Prior studies of islet transplantation in mice and in humans have suggested that only a fraction of the islet dose survives implantation and early engraftment. In contrast, the team adds “…our results demonstrate that enhanced in situ supply of exogenous oxygen directly to islets encapsulated at high densities can maintain viability and function of the islets with only a small loss over long periods of time. By eliminating the substantial loss of viability and function currently experienced, the limited supply of islets can be used much more efficiently, and human preparations normally discarded because of insufficient numbers of islets might be used fruitfully.”
And while encapsulating the islet cells separates and protects the cells from the host immune system, reducing islet cell death will also further improve immunoprotection by reducing the likelihood of immune reaction against proteins and peptides released by dead transplanted islet cells. “By keeping the cells alive, you minimize the immune response,” Colton says.
Beta-O2 Technologies is currently developing a new version of the device in which the oxygen chamber is implanted below the skin, separate from the islets, and which may only need to be replenished once a week.