Role of NADPH oxidase in the mechanism of lung neutrophil sequestration and microvessel injury induced by gram-negative sepsis: Studies in p47^phox-/- and gp91^phox-/- mice

We addressed the role of O₂^.- generated by the NADPH oxidase complex in the mechanism of polymorphonuclear leukocyte (PMN) accumulation and transalveolar migration and lung microvascular injury. Studies were made in mice lacking the p47^phox and gp91^phox subunits of NADPH oxidase (p47^phox-/- and gp91^phox-/-) in which PMN are incapable of the respiratory burst. The mice were challenged i.p. with live Escherichia coli to induce sepsis. We observed time-dependent increases in PMN sequestration and migration from 1 to 6 h after challenge with 2 x 10⁸ E. coli. The responses in knockout mice were greater post-E. coli challenge compared with control mice; i.e., transalveolar PMN migration post-E. coli challenge increased by ∼50% in the null mice above values in wild type. The increased PMN infiltration was associated with decreased lung bacterial clearance. The generation of the chemoattractant macrophage-inflammatory protein-2 in lung tissue was greater in NADPH oxidase-defective mice after E. coli challenge than control mice; moreover, macrophage-inflammatory protein-2 Ab pretreatment prevented the PMN infiltration. We also observed that E. coli failed to increase lung microvascular permeability in p47^phox-/- and gp91^phox-/- mice despite the greater lung PMN sequestration. Thus, O₂^.- production is required for the induction of sepsis-induced lung microvascular injury. We conclude that NADPH oxidase-derived O₂^.- generation has an important bactericidal role, such that an impairment in bacterial clearance in NADPH oxidase-defective mice results in increased chemokine generation and lung tissue PMN infiltration.