Abstract
BACKGROUND: If alginate microcapsules are to be used clinically for therapeutic cell transplants, capsule formulations must be designed to enhance optimal biocompatibility and immune acceptance. METHODS: Microcapsules were generated using highly purified, endotoxin-free, ultra-low viscosity, high mannuronic acid alginate. The capsules differed with respect to gelling cation (50 mM barium or 100 mM calcium), alginate concentration (2.0% or 3.3%), alginate density (homogeneous or inhomogeneous), and the presence or absence poly-L-lysine (PLL) coating. Four types of empty capsules were implanted intraperitoneally (i.p.) in normal NOD mice, and their biocompatibility was evaluated after various time periods in vivo. Encapsulated adult porcine islets (APIs) were transplanted i.p. in diabetic NOD mice, and immune acceptance was evaluated by graft survival times, host cell adherence to capsule surfaces, and flow cytometric analysis of peritoneal host cells. RESULTS: All empty alginate capsules were biocompatible in vivo, but barium-gelled alginate capsules without PLL were clearly the most biocompatible, since 99% of these empty capsules had no host cell adherence up to 9 months in vivo. In diabetic NOD mice, APIs functioned significantly longer in barium-alginate capsules without PLL than in calcium-alginate capsules with PLL and had strikingly less host cell adherence, although large numbers of host cells (predominantly macrophages and eosinophils) infiltrated the peritoneal cavities of recipients with APIs in both types of capsules. Addition of PLL coatings to barium-alginate capsules dramatically decreased graft survival. CONCLUSIONS: Inhomogeneous barium-gelled alginate capsules without PLL are the optimal candidates for clinical trials, based on their enhanced biocompatibility and immune acceptance in vivo.