The role of mitochondrial complex I in the proinflammatory response to polylactide implants

During the foreign body response, immune cells are metabolically rewired after exposure to breakdown products of various biomaterials, including polylactide (PLA) and polyethylene. Particles of polyethylene interact with Toll-like receptor 4 on macrophages, resulting in increased oxygen consumption that forms reactive oxygen species at complex I of the mitochondrial electron transport chain (mETC). However, PLA degradation products bind to monocarboxylate transporters for downstream signaling with elevated oxygen consumption rates, whose functional implication is unclear and remains inferred from cellular responses to polyethylene biomaterials. By chemically probing the function of the mETC, we show that proinflammatory macrophages activated by exposure to amorphous PLA (aPLA) breakdown products rely on mitochondrial respiration for ATP production, independent of oxygen consumption rates. In contrast, macrophages activated by semi-crystalline PLA (cPLA) breakdown products exhibit a metabolic phenotype wherein ATP levels are unaffected by changing oxygen consumption rates. In subcutaneous implants, the incorporation of metformin in aPLA or cPLA to chemically inhibit complex I did not effectively modulate the proinflammatory response to biomaterials, suggesting that PLA degradation products elicit a distinct metabolic program, thus providing an alternative perspective on the role of mitochondrial respiration in the inflammatory response to biomaterials.