Long-term biocompatibility, chemistry, and function of microencapsulated pancreatic islets

Abstract

Transplantation of encapsulated living cells is a promising approach for the treatment of a wide variety of diseases. Large-scale application of the technique, however, is hampered by insufficient biocompatibility of the capsules. In the present study, we have implemented new as well as previously reported technologies to test biocompatibility issues of immunoisolating microcapsules on the long term (i.e. 2 years) instead of usually reported short time periods. When transplanted empty, the capsules proved to be highly biocompatible not only for short periods (i.e. 1 month) but also on the long term as evidenced by the absence of any significant biological response up to 2 years after implantation in rats. The immunoprotective properties of the capsules were confirmed by prolonged survival of encapsulated islet allografts up to 200 days. The surface of the applied capsule was analyzed and provides new insight in the chemical structure of true biocompatible and immunoprotective capsules applicable for transplantation of encapsulated islets in type I diabetes.

Introduction

The recent advances in clinical islet transplantation [1] have increased the interest in transplantation of an endogenous insulin source for control of the diabetic blood sugar. A major obstacle to overcome the large-scale application is the requirement to apply effective immunosuppression, which is associated with serious side effects, especially when applied for a life-long period. Immunoisolation by microencapsulation does not require the use of immunosuppression and has therefore the potential to be developed into a widely applicable therapy for patients with insulin dependent diabetes.

A commonly used procedure for immunoprotection is microencapsulation of tissues in alginate-based capsules as originally described by Lim and Sun [2]. During recent years, important advances have been made with this technology. Human trials have been started and showed temporary but pertinent survival of human microencapsulated tissue after allotransplantation of encapsulated parathyroid cells [3] and islets [4]. Also, microencapsulation has been shown to allow for prolonged survival of xenotransplanted islet grafts in both chemically induced and autoimmune diabetic rodents [5], dogs [4], and monkeys [6]. Although this illustrates the principle applicability of the alginate-encapsulation technique, a fundamental barrier has to be overcome since graft survival varies considerably from several days to months [7]. This variation in success rate is usually attributed to differences in the biological responses (i.e. biocompatibility) against the applied capsules. In order to improve the capsule properties, many groups have introduced their own technical modifications. As a consequence there are many different procedures with specific chemical characteristics which, regretfully, are never documented.

In the foregoing years we have concentrated on the identification and deletion of factors inducing biological responses against encapsulated islets. Alginates are composed of mannuronic acid (M) and guluronic acid (G) and can be contaminated with inflammatory substances. We found that the biological response against capsules can be reduced by applying alginates with a G-content of 40–45% instead of more than 50% and by introducing alginates with a high purity degree [5], [8], [9], [10], [11]. Also, an inadequate mechanical integrity of the capsules [9], [12] can cause immunological reactions in the recipient. Finally, we found that grafts can fail as the consequence of incomplete encapsulation of islets [13], [14].

In the present study, we changed the capsule production process by applying modifications which have previously shown to influence the biological response against capsule grafts [5], [8], [9], [10], [11], [12]. Empty capsules produced according to our modified procedure were implanted in the peritoneal cavity and retrieved after varying time intervals for evaluation of the biological response against the capsules. Next, syngeneic and allogeneic rat islets were encapsulated and transplanted in diabetic rats in order to assess the immunoprotective properties of the capsules. Finally, we analyzed and present the chemical structure of the immunoprotective capsule, which remained free of any biological response up to 2 years after implantation in rats.