Why do microencapsulated islet grafts fail in the absence of fibrotic overgrowth?

The survival of microencapsulated islet grafts is limited, even if capsular overgrowth is restricted to a small percentage of the capsules. In search of processes other than overgrowth contributing to graft failure, we have studied the islets in non-overgrown capsules at several time points after allotransplantation in the rat. All recipients of islet allografts became normoglycemic. Grafts were retrieved at 4 and 8 weeks after implantation and at 15.3 +/- 2.3 weeks postimplant, 2 weeks after the mean time period at which graft failure occurred. Overgrowth of capsules was complete within 4 weeks postimplant, and it was usually restricted to <10% of the capsules. During the first 4 weeks of implantation, 40% of the initial number of islets was lost. Thereafter, we observed a decrease in function rather than in numbers of islets, as illustrated by a decline in the ex vivo glucose-induced insulin response. At 4 and 8 weeks postimplant, beta-cell replication was 10-fold higher in encapsulated islets than in islets in the normal pancreas, but these high replication rates were insufficient to prevent a progressive increase in the percentage of nonviable tissue in the islets. Necrosis and not apoptosis proved to be the major cause of cell death in the islets. The necrosis mainly occurred in the center of the islets, which indicates insufficient nutrition as a major causative factor. Our study demonstrates that not only capsular overgrowth but also an imbalance between beta-cell birth and beta-cell death contributes to the failure of encapsulated islet grafts. Our observations indicate that we should focus on finding or creating a transplantation site that, more than the unmodified peritoneal cavity, permits for close contact between the blood and the encapsulated islet tissue.