No abstract

Grafting of encapsulated living cells has the potential to cure a wide variety of diseases. Large-scale application of the technique, however, is hampered by insufficient biocompatibility of the capsules. A major factor in the biocompatibility of capsules is inadequate covering of the inflammatory poly-L-lysine (PLL) on the capsules' surface. In the present study, we investigate whether tissue responses against alginate-PLL capsules can be reduced by crosslinking the surface of the capsules with heparin or polyacrylic acid. Our transplant study in rats shows a tissue response composed of fibroblasts and macrophages on alginate-PLL-alginate and alginate-PLL-heparin capsules that was completely absent on alginate-PLL-polyacrylic acid capsules. Atomic force microscopy analyses of the capsules demonstrates that the improved biocompatibility of alginate-PLL-capsules by polyacrylic acid coating should not only be explained by a more adequate binding of PLL but also by the induction of a smoother surface. This study shows for the first time that biologic responses against capsules can be successfully deleted by chemically crosslinking biocompatible molecules on the surface of alginate-PLL capsules.

Adequate regulation of glucose levels by a microencapsulated pancreatic islet graft requires a minute-tominute regulation of blood glucose. To design such a transplant, it is mandatory to have sufficient insight in factors influencing the kinetics of insulin secretion by encapsulated islets. The present study investigates factors influencing the glucose-induced insulin response of encapsulated islets in vitro. We applied static incubations and did the following observations. (i) Small islets (90-120 µm) showed a similar instead of a lower glucose-induced insulin response, suggesting that inclusion of only small islets, which are associated with lower protrusion and failing rates, has no consequences for the functional performance of the graft. (ii) A capsule diameter of 800 µm showed identical rather than lower glucose-induced insulin responses as smaller, 500-µm capsules. (iii) Capsule membranes constructed with a conventional permeability interfered with diffusion of insulin, as illustrated by a lower response of islets in capsules with a 10-min poly-L-lysine (PLL) membrane than islets in capsules with a 5-min PLL membrane. (iv) Irrespective of the tested porosity, the capsules provided sufficient immunoprotection because the 10-min PLL membranes did block diffusion of the cytokines IL-1β (17 kDa) and TNF-α (70 kDa) while the 5-min PLL membranes interfered with the diffusion of the vast majority of the cytokines. We conclude that capsules containing small islets (90-120 µm) and a membrane with a lower permeability than routinely applied is preferred in order to obtain a graft with adequate glucose-induced insulin responses.

No abstract

Intraperitoneal transplantation of encapsulated islets can restore normoglycemia in diabetic recipients but not normal glucose tolerance nor normal insulin responses to a physiological stimulus. This study investigates whether the intraperitoneal implantation site as such contributes to the interference with optimal transport kinetics between the islets and the bloodstream. Insulin was infused into the peritoneal cavity of conscious and freely moving rats in doses of 20, 40, and 80 pmol.l-1.min-1 during 15 min, to mimic the gradual release of insulin from an encapsulated, i.e., a nonvascularized, islet graft. With 20 pmol.l-1.min-1, we observed virtually no rise of insulin levels, and it took 30 min until glucose levels had dropped significantly. With 40 and 80 pmol.l-1.min-1 insulin infusions, there was a dose-dependent rise of insulin and decrease of glucose levels. When compared with intraportal infusions with the same insulin dosages, however, they were strongly delayed and reduced as well as prolonged. Similar results were obtained when inulin instead of insulin was intraperitoneally infused, with indicates that the transport of insulin from the peritoneal cavity to the bloodstream is mainly by passive diffusion. With a view on the clinical efficacy of the bioartificial pancreas, our findings indicate that we should focus on finding or creating a transplantation site that, more than the unmodified peritoneal cavity, permits close contact between the bloodstream and the encapsulated islet tissue.