In vivo selection of biocompatible alginates for islet encapsulation and subcutaneous transplantation.

Islet encapsulation requires several properties, including (1) biocompatibility, (2) immunoprotection, and (3) oxygen diffusion, for islet survival and diabetes correction. New chemical alginates were tested in vivo and compared to traditional high-mannuronate and -guluronate alginates. New alginates with coupled peptide sequence (SLM-RGD3% and SLG-RGD3%), to improve encapsulated cell adherence in the matrix, and alginates with a very low density (VLDM7% and VLDG7%), to reduce implant size by loading a higher number of islets per volume of polymer, were implanted subcutaneously in 70 Wistar rats for comparison with alginates of high viscosity and high content in mannuronic (SLM3%) or guluronic acids (SLG3%). Permeability of alginates to lectins of 36-, 75-, and 150-kDa coupled to fluorescein isothiocynate was quantified before implantation and 2, 4, and 12 weeks after implantation. Biocompatibility (fibrosis, graft stability, immunologic infiltration by CD3/CD68 cells, and neovascularization) was assessed at each explantation time. Permeability to small molecules was found for all alginates. Impermeability to 150-kDa molecules, such as IgG, was observed only for SLM3% before implantation and was maintained up to 12 weeks after implantation. SLM3% and SLG3% demonstrated better graft stability with lower CD3/CD68 recruitment and fibrosis than the other alginates. SLM3% induced a significantly higher angiogenesis and maintained a pO2 at ~40 mm Hg up to 4 weeks after implantation as measured by in vivo EPR oximetry. SLM-encapsulated pig islets implanted subcutaneously in rats demonstrated no inflammatory/immunologic reactions and islet function for up to 60 days without immunosuppression. A traditional alginate made of high-mannuronic content (SLM3%) is an adapted material to immunoprotect islets in subcutaneous tissue. No improvement was found with a lower viscosity and RGD-peptide sequence