Ontologically distinct populations of macrophages differentially contribute to organ fibrosis through unknown mechanisms.We applied lineage tracing, single-cell RNA sequencing and single-molecule fluorescence in situ hybridisation to a spatially restricted model of asbestos-induced pulmonary fibrosis.We demonstrate that tissue-resident alveolar macrophages, tissue-resident peribronchial and perivascular interstitial macrophages, and monocyte-derived alveolar macrophages are present in the fibrotic niche. Deletion of monocyte-derived alveolar macrophages but not tissue-resident alveolar macrophages ameliorated asbestos-induced lung fibrosis. Monocyte-derived alveolar macrophages were specifically localised to fibrotic regions in the proximity of fibroblasts where they expressed molecules known to drive fibroblast proliferation, including platelet-derived growth factor subunit A. Using single-cell RNA sequencing and spatial transcriptomics in both humans and mice, we identified macrophage colony-stimulating factor receptor (M-CSFR) signalling as one of the novel druggable targets controlling self-maintenance and persistence of these pathogenic monocyte-derived alveolar macrophages. Pharmacological blockade of M-CSFR signalling led to the disappearance of monocyte-derived alveolar macrophages and ameliorated fibrosis.Our findings suggest that inhibition of M-CSFR signalling during fibrosis disrupts an essential fibrotic niche that includes monocyte-derived alveolar macrophages and fibroblasts during asbestos-induced fibrosis.

Convincing evidence has emerged demonstrating that impairment of mitochondrial function is critically important in regulating alveolar epithelial cell (AEC) programmed cell death (apoptosis) that may contribute to aging-related lung diseases, such as idiopathic pulmonary fibrosis (IPF) and asbestosis (pulmonary fibrosis following asbestos exposure). The mammalian mitochondrial DNA (mtDNA) encodes for 13 proteins, including several essential for oxidative phosphorylation. We review the evidence implicating that oxidative stress-induced mtDNA damage promotes AEC apoptosis and pulmonary fibrosis. We focus on the emerging role for AEC mtDNA damage repair by 8-oxoguanine DNA glycosylase (OGG1) and mitochondrial aconitase (ACO-2) in maintaining mtDNA integrity which is important in preventing AEC apoptosis and asbestos-induced pulmonary fibrosis in a murine model. We then review recent studies linking the sirtuin (SIRT) family members, especially SIRT3, to mitochondrial integrity and mtDNA damage repair and aging. We present a conceptual model of how SIRTs modulate reactive oxygen species (ROS)-driven mitochondrial metabolism that may be important for their tumor suppressor function. The emerging insights into the pathobiology underlying AEC mtDNA damage and apoptosis is suggesting novel therapeutic targets that may prove useful for the management of age-related diseases, including pulmonary fibrosis and lung cancer.

Alveolar epithelial cell (AEC) mitochondrial dysfunction and apoptosis are important in idiopathic pulmonary fibrosis and asbestosis. Sirtuin 3 (SIRT3) detoxifies mitochondrial reactive oxygen species, in part, by deacetylating manganese superoxide dismutase (MnSOD) and mitochondrial 8-oxoguanine DNA glycosylase. We reasoned that SIRT3 deficiency occurs in fibrotic lungs and thereby augments AEC mtDNA damage and apoptosis. Human lungs were assessed by using immunohistochemistry for SIRT3 activity acetylated MnSOD Murine AEC SIRT3 and cleaved caspase-9 (CC-9) expression were assayed by immunoblotting with or without SIRT3 enforced expression or silencing. mtDNA damage was measured by using quantitative PCR and apoptosis ELISA. Pulmonary fibrosis after asbestos or bleomycin exposure was evaluated in 129SJ/wild-type and SIRT3-knockout mice ( ) by using fibrosis scoring and lung collagen levels. Idiopathic pulmonary fibrosis lung alveolar type II cells have increased MnSOD acetylation compared with controls. Asbestos and HO diminished AEC SIRT3 protein expression and increased mitochondrial protein acetylation, including MnSOD SIRT3 enforced expression reduced oxidant-induced AEC OGG1 acetylation, mtDNA damage, and apoptosis, whereas SIRT3 silencing promoted these effects. Asbestos- or bleomycin-induced lung fibrosis, AEC mtDNA damage, and apoptosis in wild-type mice were amplified in animals. These data suggest a novel role for SIRT3 deficiency in mediating AEC mtDNA damage, apoptosis, and lung fibrosis.-Jablonski, R. P., Kim, S.-J., Cheresh, P., Williams, D. B., Morales-Nebreda, L., Cheng, Y., Yeldandi, A., Bhorade, S., Pardo, A., Selman, M., Ridge, K., Gius, D., Budinger, G. R. S., Kamp, D. W. SIRT3 deficiency promotes lung fibrosis by augmenting alveolar epithelial cell mitochondrial DNA damage and apoptosis.