Airway Macrophages Encompass Transcriptionally and Functionally Distinct Subsets Altered by Smoking

Abstract

Alveolar macrophages (AMs) are functionally important innate cells involved in lung homeostasis and immunity and whose diversity in health and disease is a subject of intense investigations. Yet, it remains unclear to what extent conditions like smoking or chronic obstructive pulmonary disease (COPD) trigger changes in the AM compartment. Here, we aimed to explore heterogeneity of human AMs isolated from healthy nonsmokers, smokers without COPD, and smokers with COPD by analyzing BAL fluid cells by flow cytometry and bulk and single-cell RNA sequencing. We found that subpopulations of BAL fluid CD206 + macrophages could be distinguished based on their degree of autofluorescence in each subject analyzed. CD206 + autofluorescent high AMs were identified as classical, self-proliferative AM, whereas autofluorescent low AMs were expressing both monocyte and classical AM-related genes, supportive of a monocytic origin. Of note, monocyte-derived autofluorescent low AMs exhibited a functionally distinct immunoregulatory profile, including the ability to secrete the immunosuppressive cytokine IL-10. Interestingly, single-cell RNA-sequencing analyses showed that transcriptionally distinct clusters of classical and monocyte-derived AM were uniquely enriched in smokers with and without COPD as compared with healthy nonsmokers. Of note, such smoking-associated clusters exhibited gene signatures enriched in detoxification, oxidative stress, and proinflammatory responses. Our study independently confirms previous reports supporting that monocyte-derived macrophages coexist with classical AM in the airways of healthy subjects and patients with COPD and identifies smoking-associated changes in the AM compartment that may favor COPD initiation or progression.

Alveolar Macrophage Heterogeneity Goes up in Smoke?

“…On the basis of prior work in mice, the prevailing concept has been that AMs self-maintain with minimal contribution from blood monocytes during homeostasis ( 3 – 5 ). However, recent studies, including this one, suggest that monocytes contribute to the diversity of the AM pool ( 1 , 2 , 6 , 7 ). For example, Cluster 3 expressed both classical macrophage-associated genes in addition to monocyte-associated gene signatures.…”

“…In this issue of the Journal , Liégeois and colleagues (pp. 241–252 ) report on their examinations of the identity and heterogeneity of AMs comparing healthy nonsmokers, non-COPD smokers, and COPD smokers using a combination of approaches that include flow cytometry and bulk and single-cell RNA sequencing (scRNA-seq) of BAL cells ( 2 ) ( Figure 1 ).…”

“… Overview of the experimental approach and findings in the alveolar macrophage (AM) pool ( 2 ). The identity and heterogeneity of alveolar macrophages were assessed among healthy nonsmokers, non-COPD smokers, and COPD smokers using a combination of approaches, including flow cytometry and bulk and single-cell RNA-seq of BAL cells.…”

Matrix Metalloproteinases in Chronic Obstructive Pulmonary Disease

“…Nonetheless, in long-term inflammatory conditions, such as the causatives of COPD, macrophages promote persistent ECM degradation. For example, macrophages in bronchoalveolar lavage (BAL) of COPD smokers are more prone to degrade elastin than macrophages from healthy individuals due to increased MMP activity [ 56 , 64 ]. Accordingly, in several studies in humans and mice, MMPs, particularly macrophage-derived MMPs, have been associated with the pathogenesis of COPD.…”

Alveolar macrophages and airway hyperresponsiveness associated with respiratory syncytial virus infection

“…In the acute phase of infection, IMs will be chemotactic to the alveoli cavity and recruited to become RecAMs ( 37 ). In addition, RNA gene sequencing showed that the immunoprogramming of RecAMs was dynamic ( 32 , 35 ) and could develop into the same phenotype and provide the same functionality as RAMs during the peak inflammatory periods ( 38 , 39 ), including the production of pro-inflammatory cytokines and elimination of pathogens. RecAMs release anti-inflammatory factors to repair pathologically damaged tissues when the inflammation is subsiding.…”