(107f) Eliminating Oxygen Supply Limitations for Transplanted Microencapsulated Islets in the Treatment of Type I Diabetes

Type I diabetes is a disease that results from a person's impaired ability to produce insulin, a protein that regulates the blood glucose concentration. Insulin is produced by β cells in the Islets of Langerhans, which are aggregates of cells averaging about 150μm in diameter and constituting about 1 to 2% of the pancreas volume. The efficacy of islet transplantation as a treatment for diabetes has been demonstrated in humans by the Edmonton Protocol, but obstacles remain for wide scale application. One major issue is that successful islet transplantation requires permanent use of multiple immunosuppressive agents. These agents may have serious side effects as well as a substantial financial burden. Microencapsulation is used for full or partial protection of transplanted islets from immune rejection. However, the microcapsule prevents islet revascularization and creates an additional mass transfer resistance for oxygen transport to islets. This reduced oxygen transfer can lead to a hypoxic core within the islet that results in tissue death and reduced function. We are currently studying two approaches to enhance microencapsulated islet survival and function by reducing oxygen transport limitations. The first method involves incorporating a perfluorocarbon emulsion into alginate microcapsules to enhance oxygen permeability in order to protect islets from hypoxia. The second method involves dispersing the islets into single cells and allowing them to reaggregate into cell clusters smaller than the original islet. The smaller aggregates will be less prone to the development of a necrotic core and should be able to function normally because of adequate oxygen supply and the presence of cell to cell contacts.

A theoretical reaction-diffusion model was developed to predict the oxygen partial pressure profile, extent of cell death, and rate of insulin secretion in alginate microcapsules containing an islet, islet cell aggregates, and dispersed single cells exposed to specified external pO₂ values, with or without PFC. Results show that hypoxic conditions are reduced, therefore enhancing islet viability and substantially maintaining insulin secretion in PFC capsules. Modeling results have predicted that a capsule containing aggregates with half the diameter of a 150μm islet, and a total tissue volume equivalent to one islet, can remain fully functional while the function of an intact islet has dropped to 20% of its normal level. Theoretical predictions have demonstrated that islet cell aggregates are extremely beneficial in maintaining beta cell function in low oxygen environments.

Methods have been developed to assess encapsulated tissue through nuclei counting, DNA quantification, and oxygen consumption rate measurements. Experiments with islets, islet cell aggregates, and INS-1 cells (a beta cell line) are underway to verify the predicted results from the reaction-diffusion model. After two days of culture in a limited oxygen environment comparisons will be made between islets, islet cell aggregates, and INS-1 cells within normal alginate and PFC alginate microcapsules in their viability by measurement of oxygen consumption rate, function by measuring insulin content, and total tissue content by measuring DNA or performing nuclei counts. We hope to experimentally demonstrate that enhanced capsule permeability and islet cell aggregates are beneficial to maintaining islet survival and function in low oxygen environments.