Tissue-Resident Alveolar Macrophages Reduce Ozone-induced Inflammation via MerTK-mediated Efferocytosis.

Lung inflammation, caused by acute exposure to ozone (O), one of the six criteria air pollutants, is a significant source of morbidity in susceptible individuals. Alveolar macrophages (AMØs) are the most abundant immune cells in the normal lung, and their number increases after O exposure. However, the role of AMØs in promoting or limiting O-induced lung inflammation has not been clearly defined. In this study, we used a mouse model of acute O exposure, lineage tracing, genetic knockouts, and data from O-exposed human volunteers to define the role and ontogeny of AMØs during acute O exposure. Lineage-tracing experiments showed that 12, 24, and 72 hours after exposure to O (2 ppm) for 3 hours, all AMØs were of tissue-resident origin. Similarly, in humans exposed to filtered air and O (200 ppb) for 135 minutes, we did not observe at ∼21 hours postexposure an increase in monocyte-derived AMØs by flow cytometry. Highlighting a role for tissue-resident AMØs, we demonstrate that depletion of tissue-resident AMØs with clodronate-loaded liposomes led to persistence of neutrophils in the alveolar space after O exposure, suggesting that impaired neutrophil clearance (i.e., efferocytosis) leads to prolonged lung inflammation. Moreover, depletion of tissue-resident AMØs demonstrated reduced clearance of intratracheally instilled apoptotic Jurkat cells, consistent with reduced efferocytosis. Genetic ablation of MerTK (MER proto-oncogene, tyrosine kinase), a key receptor involved in efferocytosis, also resulted in impaired clearance of apoptotic neutrophils after O exposure. Overall, these findings underscore the pivotal role of tissue-resident AMØs in resolving O-induced inflammation via MerTK-mediated efferocytosis.