Extracellular acidity reprograms macrophage metabolism and innate responsiveness

Although organ hypofunction and immunosuppression are life-threatening features of severe sepsis, the hypofunctioning organs and immune cells usually regain normal functionality if patients survive. We tested the hypothesis that low extracellular pH (pHe) can induce reversible metabolic and functional changes in tissue macrophages. When compared with macrophages cultured at normal pHe, macrophages living in an acidic medium used less glucose and exogenous fatty acid to produce ATP. Lactate, glutamine, and de novo synthesized fatty acids supported ATP production by mitochondria that gained greater mass, maximal oxygen consumption rate, and spare respiratory capacity. The cells transitioned to a M2-like state, with altered immune responses to LPS and slightly decreased phagocytic ability, yet they regained basal energy production, normal mitochondrial function and pro-inflammatory responsiveness when neutral pHe was restored. Low pHe induces changes that support macrophage survival while rendering the cells less pro-inflammatory (more ‘tolerant’) and less able to phagocytose bacteria. Macrophage responses to low interstitial pH may contribute to the reversible organ hypofunction and immunoparalysis noted in many patients with sepsis. Resident peritoneal macrophages harvested from C57BL/6 mice were cultured in the basic DMEM medium at pH 6.8 or 7.4 (buffered with MOPS or HEPES) for 24 h. After the cells were untreated or treated with 10 ng/ml LPS for 2 h (n = 3/group), total RNA was isolated using Trizol.