No abstract

Microencapsulation refers to a technique of immunoisolation by coating single cells or tissue with a semipermeable membrane. By combining microencapsulation with a specific tissue culturing method, iso-, allo-, and xenotransplantation of parathyroid tissue has been achieved without immunosuppression in a long-term animal model. Prior to its clinical use, continued analyses of the alginate, used as a coating substance, determined its mitogenic properties. Purification of the commercially available alginate was achieved using patented electrophoretic procedures, resulting in an amitogenic alginate suitable for use in humans. However, this alginate exhibited entirely different physical properties. We have recently shown that isotransplanted parathyroid tissue remains vital and functioning in vivo over long periods of time using the novel amitogenic alginate. It is essential to document, whether the alginate is able to maintain immunoisolation. We have therefore assessed its in vivo function compared to the mitogenic alginate in a transgenic animal model. Altogether 600 parathyroid glands from 300 Lewis rats (donor animals) were excised and subjected to tissue culture. Thereafter they were allotransplanted to 30 parathyroidectomized Dark-Auita rats, microencapsulated with the amitogenic or the mitogenic alginate or naked, with 10 recipient animals in each group. Total serum calcium and parathyroid hormone levels were monitored continuously at weekly intervals for 30 weeks. After 26 weeks the transplant beds were excised and subjected to histologic examination. More than 6 months after allotransplantation 9 of 10 animals that had received amitogenic transplants, compared to 7 of 10 animals in the group with mitogenic microcapsules were normocalcemic. Animals that had received naked parathyroid tissue were hypocalcemic as soon as 2 weeks after allotransplantation. Correspondingly, normocalcemic animals showed vital parathyroid tissue inside the microcapsules, which were surrounded by a significantly smaller rim of fibroblasts when amitogenic alginate had been used. In addition to confirming physiologic long-term function, we were able to document for the first time that immunoisolation can also be achieved with the novel amitogenic alginate, which is suitable for clinical use.

Owing to the complexity of the parathyroid hormone's metabolic interactions, clinical hypoparathyroidism is one of the most difficult of all endocrine disorders to treat. Therefore, causative treatment of this disorder by transplantation of parathyroid glands is highly desirable. We have recently documented the long-term in vivo function of iso- and allotransplanted rat parathyroid tissue without systemic immunosuppression in an animal model. In view of the potential clinical use of this method, human parathyroid tissue has been microencapsulated and transplanted over the highest immunological barrier. In a controlled, long-term animal study in the parathyroidectomized rat, the effect of microencapsulation on xenotransplanted human parathyroid tissue was evaluated over 30 weeks (native and microencapsulated parathyroid tissue = 40 rats respectively). Functionally, human parathyroid tissue was able to replace that of the rat. All animals that had received microencapsulated parathyroid tissue were normocalcemic for 16 weeks; 27/40 at the end of the study. In contrast, serum calcium concentrations dropped to post-parathyroidectomy levels within 4 weeks in those animals that had received native tissue only. Histologic evaluation of the explanted, functionally successful xenografts showed vital parathyroid tissue inside intact microcapsules surrounded by a small rim of fibroblasts. Avital fibrotic remnants were demonstrated in animals with non-encapsulated parathyroid tissue. Thus, we have established the feasibility of microencapsulation of human parathyroid tissue, preserving its viability over long periods in vivo even if xenotransplanted. In combination with an improved tissue culture method, transplantation of human parathyroid tissue and maintenance of its physiological function is reproducibly achieved without postoperative systemic immunosuppression over the highest transplantation barrier. This may be a crucial step towards the first clinical application of this method.

Microencapsulation of tissues is an alternative to postoperative immunosuppression in transplantation. In 1994 iso-, allo- and xenotransplantation of microencapsulated parathyroid tissue was achieved in vivo. However, continued analysis of the coating substance (an alginate) determined mitogenic properties. Here, we report on the in vitro and in vivo function of parathyroid tissue microencapsulated with a novel amitogenic alginate suitable for use in humans. To assess in vitro function, parathyroid tissue encapsulated with mitogenic and amitogenic alginate was exposed to rising concentrations of calcium. For in vivo experiments, it was isotransplanted into parathyroidectomized rats. PTH release into medium and PTH serum levels as well as calcium levels of recipient rats were analyzed and compared to native (non-microencapsulated) tissue and empty capsules, respectively. In vivo, transplants were excised and subjected to histologic examination six months after trans-plantation. In vitro, parathyroid tissue encapsulated with amitogenic alginate releases approximately half of the PTH of the native tissue, not different from tissue encapsulated with the mitogenic alginate. In vivo, the novel alginate preserved parathyroid function similar to that of native tissue over the six month period resulting in complete reversal of hypoparathyroidism. Correspondingly, histologic examination revealed vital parathyroid tissue in intact microcapsules. By establishing in vitro function and successful long-term transplantation, we have documented the principle of microencapsulation of parathyroid tissue to be effective also with the novel amitogenic alginate, which is suitable for clinical use.

Permanent hypoparathyroidism is one of the most difficult of all endocrine disorders to treat medically. While autotransplantation of parathyroid tissue is clinically established, allotransplantation without immunosuppression is still at the level of animal experiments. Although persons affected by hypoparathyroidism are facing a clearly reduced quality of life, hypoparathyroidism rarely is a life threatening condition. Therefore, systemic immunosuppression for recipients of allotransplants is not justified. A conceptional alternative would be protecting the tissue to be transplanted from the immunologic response by coating it with a semipermeable membrane (microencapsulation). In 1994, we succeeded in iso-, allo- and xenotransplantation of microencapsulated parathyroid tissue in an animal model. Unfortunately, prior to the first clinical use, further analysis of the coating substance (alginate) demonstrated that it has mitogenic properties. Here, we report on the first successful transplantation of microencapsulated parathyroid tissue using a purified, non-mitogenic alginate which is suitable for clinical use.