Alveolar macrophages play a key role in tolerance to ozone

ABSTRACT Acute exposure to ozone (O 3 ) causes pulmonary inflammation and injury in humans and animal models. In rodents, acute O 3 -induced inflammation and injury can be mitigated by pre-exposure to relatively low concentration O 3 , a phenomenon referred to as tolerance. While tolerance was described long ago, the underlying mechanisms are not known, though upregulation of antioxidants has been proposed. To identify new mechanisms for O 3 tolerance, we generated a mouse model in which female C57BL6/NJ mice were pre-exposed to filtered air (FA) or 0.8 ppm O 3 for four days (4 hours/day), then challenged with 2 ppm O 3 (3 hours) 2 days later, and phenotyped for airway inflammation and injury 6 or 24 hours thereafter. As expected, pre-exposure to O 3 resulted in significantly reduced airway inflammation and injury at 24 hours, as well as reduced induction of antioxidant genes. Like previous studies in rats, tolerance was associated with changes in the frequency and proliferation of alveolar epithelial cells, but was not associated with upregulation of antioxidants, CCSP (SCGB1A1), or mucus. We found that alveolar macrophages (AMs) play a critical role in tolerance, as depletion of AMs using clodronate in mice pre-exposed to O 3 restored many responses to acute O 3 challenge. Further, AMs of O 3 tolerized mice exhibited decreased expression of genes involved in cellular signaling via Toll-like receptors, MYD88, and NF-kB, and proinflammatory cytokine production. We conclude that O 3 tolerance is highly, but not exclusively, dependent on AMs, and that further studies investigating how repeated O 3 exposure induces hypo-responsiveness in AMs are warranted.