Optimization of the microencapsulated islet for transplantation

Background. Microencapsulation of isolated islets is a good method for providing protection against immunologic reactions to the cells in both allogeneic and xenogenic grafts. Current methods of microencapsulation require chelation of the alginate-calcium core, which solubilizes the structural support of the capsules and may adversely affect durability. The purpose of the present study was to determine the in vitro response to glucose stimulation, of microencapsulated islets that have not been subjected to chelation. Materials and methods. Using an air-jet-syringe-pump droplet generator, islets isolated from male Sprague-Dawley rats were encapsulated in alginate, followed by washes with poly-L-lysine, saline, and a second coat of alginate. Different groups of the microencapsulated islets were then tested for response to high glucose perifusion, before or after chelation, and the responses were compared with those of unencapsulated islets. Results. Chelated microencapsulated islets showed a normal biphasic insulin response to stimulation with 16.7 mM (300 mg%). Thus, insulin secretion increased from a mean ± SEM basal rate of 3005 ± 645 to a stimulated rate of 5165 ± 1030 pg/min (P < 0.05, n = 6) in the first phase, comparable to results obtained with the unencapsulated islets. In contrast, unchelated microencapsulated islets did not respond to stimulation with 16.7 mM glucose. However, after a 24-h culture of these unchelated microcapsules, a small but significant response was observed. Conclusions. Although culturing unchelated microcapsules of islets may enhance their function, chelation of the microcapsular core is essential for optimal function of the enclosed islets.