Sensitivity of insulin production from encapsulated islets to endotoxin-stimulated macrophage inflammatory mediators

Abstract

Chronic inflammation may compromise function of implanted encapsulated islets. Increased purity of alginate used for encapsulation prolongs encapsulated graft function, correlating with decreased presence of impurities like bacterial endotoxin. Limits for endotoxin contamination in biomaterials based on indirect inhibition of function of embedded cells have yet to be established. In a coculture system with RAW 264.7 monocyte/macrophage cells in the presence of 50 ng/mL murine recombinant gamma-interferon (mrIFN-gamma), the insulin response to glucose challenge of both rat and pig unencapsulated islets was prevented by endotoxin (LPS) in the medium down to 0.3 EU/mL (LOEL), but not 0.06 EU/mL (NOEL). Evaluation of nitrite concentrations in supernatants revealed that pig islets were more resistant to LPS-stimulated macrophage mediators than rat islets. Encapsulation in highly purified alginate produced little change in observed inhibitory effects of macrophage-generated nitric oxide (NO) toward islet function. Chemically released NO was much less effective in inhibiting insulin responsiveness to glucose challenge than was coculture of islets with LPS and mrIFN-gamma-stimulated RAW 264.7. These results taken together with other data suggest that an upper limit of 0.3 EU/mL LPS within the encapsulating alginate will not impair the function of implanted encapsulated islets by toxic concentrations of macrophage-mediated inflammatory agents.