The role of redox balance in pulmonary innate immunity

Introduction: Household air pollution (HAP) affects one half of the world population, comprises 90% of global particulate exposure, and is associated with mortality from respiratory infections. Particulates are known to alter cellular redox balance, including that of the alveolar macrophage (AM). We investigate redox balance as a mechanism to explain the link between particulate exposure and failure of innate immune function. Methods: Bronchoalveolar lavage was performed on healthy human volunteers in the UK (low HAP) and Malawi (high HAP). Antioxidant capacity was determined (concentrations of oxidised and reduced glutathione, and antioxidant capacities of glutathione peroxidase and superoxide dismutase). AM function was assessed by capacity to phagocytose, and to produce oxidative burst and proteolysis activity within the phagosome using reporter-fluorophore coated silica beads. AM cytokine responses to lipopolysaccharide and to respirable size wood smoke particles were measured before and after ex vivo treatment to reduce glutathione content. Personal exposure to particulates was estimated by the cytoplasmic content of black carbon within AM. Results: AM glutathione was similar in UK and Malawi non-smokers (of which 4.3% was in oxidised form). One quarter of Malawians exhibited a high oxidised glutathione phenotype (8.6% of total). Malawians exhibited higher levels of extracellular glutathione, and intracellular malondialdehyde and particulate content than UK volunteers. Nrf2 levels in Malawian AM were proportional to levels of particulate matter. We demonstrated no association between particulate matter content and AM cytokine responses to lipopolysaccharide or wood. Glutathione depletion did not consistently alter AM cytokine production. In vivo particulate exposure correlates with reduced AM phagosomal oxidative capacity, and increased cellular Nrf-2 concentration, but phagocytosis and phagosomal proteolysis are unaffected. Conclusion: Malawians have high levels of AM particulate exposure, as expected from HAP surveys. This is associated with evidence of oxidative stress (lipid peroxidation), antioxidant upregulation (raised extracellular glutathione, intracellular Nrf2) and a specific defect in AM function (reduced oxidative burst). Antibacterial function of the AM is key to preventing pulmonary infection: in a population subjected to high levels of household air pollution, particulate related defects in AM function could explain the high prevalence of infective respiratory illness