Strategies to improve macroencapsulated islet graft survival

Abstract

Chronic immunosuppressive therapy may have severe side-effects. In cell transplantation, the graft can be encapsulated within a membrane chamber, providing a physical barrier against the immune system. The cell graft then becomes dependent on the diffusion of nutrients and oxygen from the surrounding microcirculation. A major drawback has been the formation of avascular fibrotic tissue around the chamber. The immunoprotective device studied (TheraCyte ) has an outer membrane inducing neovascularization. However, major parts of the encapsulated graft are still lost soon after transplantation, probably because of relative hypoxia and malnutrition. The overall aim of this thesis was to assess various strategies to improve islet graft survival in the device, using rodent models.

The purpose of the first paper was to improve the method for histological evaluation of the vascularization around the device. Vascular profiles within various distances from the membrane surface were counted at different times and then correlated with glucose kinetics. We found that the vascular profiles within 100 ìm had the highest correlations with glucose kinetics and concluded that vessels within this distance are important for the exchange of small molecules between the circulation and the device s lumen. Therefore, we recommend that 100 ìm should be used in histological evaluations of the membrane vascularization.

In the second paper we hypothesized that preimplantation of the device should improve encapsulated islet graft survival. Previous studies have indicated that it takes up to 3 months for recovery of the microcirculation after membrane implantation. Therefore, we implanted empty devices and transplanted islets 3 months later in these chambers. This approach significantly improved the cure rates of diabetic animals, and the islet dose required for cure was reduced by about 10 times. Morphometry evaluations confirmed increased graft survival in preimplanted devices.

The third paper aimed at evaluating the effects of exendin-4 treatment on the metabolic outcome after islet transplantation. Exendin-4 inhibits islet apoptosis, stimulates islet differentiation and regeneration and has beneficial effects on peripheral tissues. We found that exendin-4 treatment significantly improved the metabolic outcome after free islet transplantation to the renal subcapsular site. The benefit lasted longer than the treatment, suggesting that exendin-4 had long-standing effects on the islet graft. This substance seems to be an interesting new approach to improve the survival also of encapsulated islet grafts.

In the last paper we evaluated the risk of recipient sensitization using macroencapsulated islets. A heterotopic heart graft was transplanted one month after free or encapsulated islet transplantation. The time-to-rejection was significantly shorter in the free islet group, while it did not differ between encapsulated islet graft recipients and naive animals. We therefore conclude that the device protects against sensitization, at least during the first month after transplantation.

Today, side-effects of the immunosuppressive therapy are one of the main limiting factors for the use of islet transplantation. If immunoprotection could be achieved by encapsulation of the islet graft, it should be possible to widen the indications. This thesis describes promising strategies to improve the survival of macroencapsulated islet grafts, which might contribute to make macroencapsulation a clinical reality.