Comparison of function of microencapsulated islets of Langerhans following intraperitoneal and subcutaneous transplantation in mice

Alan Kerby1, Sabina Checketts1, Sara Bohman2, James Bowe1, Peter Jones1, Aileen King1. 1King’s College, London, United Kingdom, 2Uppsala University, Uppsala, Sweden.

Background. Type 1 diabetes is characterised by the autoimmune destruction of β-cells in the islets of Langerhans. Islet transplantation is a current therapy for type 1 diabetes, but it requires the use of immunosuppressive drugs, which have serious adverse effects. To circumvent this, islets can be microencapsulated in alginate, which forms a physical barrier to immune cells thus preventing rejection, while still allowing the passage of nutrients and insulin. Microcapsules are commonly implanted into the peritoneal cavity due to the size of the graft. However this site is associated with low oxygen tension and high levels of mechanical stress. It has recently been suggested that the subcutaneous site may be suitable for implantation of microcapsules and we therefore investigated this site in comparison with the intraperitoneal site for transplantation of microencapsulated islets.

Aim. The aim of this study was to compare the function of microencapsulated islets recovered from intraperitoneal (i.p.) and subcutaneous (s.c.) sites 3 and 7 days after transplantation into mice. In addition, we investigated the efficacy of microencapsulated islets in reversing hyperglycaemia in both sites for a period of 28 days.

Methods. Normoglycaemic C57Bl/6J mice were transplanted s.c. and i.p. with 100-200 microencapsulated islets isolated from C57Bl/6J mice and glucose stimulated insulin release was measured following islet retrieval after 3 or 7 days. Also, alloxan-diabetic C57Bl/6J mice were implanted with 700 microencapsulated islets in either the s.c. or the i.p. site.

Results. After 3 days, there were no significant differences between glucose stimulated insulin release rates from microencapsulated islets retrieved from the i.p. and s.c. sites (506 ± 54 pg/islet/h vs 379 ± 69 pg/islet/h respectively, n=5 t-test, p=0.187). Seven days after transplantation, microencapsulated islets retrieved from the i.p. site showed higher glucose stimulated insulin release rates compared with islets retrieved from the s.c. site (median 117 pg/islet/h vs 60 pg/islet/h, i.p. and s.c. site respectively, n= 9, Mann Whitney Rank Sum Test). Diabetic mice (blood glucose 22.9 ± 1.2 mM, n=5) implanted with 700 microencapsulated islets in the i.p. site had normalised blood glucose levels the day after implantation (8.2 ± 2.4 mM, n=5 p<0.0001 vs pre-transplantation levels) and the blood glucose remained normalised 28 days after implantation (7.9 ± 2.5 mM, n=5). Mice implanted with microencapsulated islets in the s.c. site remained hyperglycaemic throughout the experiment (day 1: 24.1 ± 1.8 mM; day 28: >28mM, n=5).

Conclusion. These data suggest that the s.c. site is not suitable for the survival and function of microencapsulated islets, but islets function normally when transplanted i.p.