Identification of Siglec-1-negative alveolar macrophages with proinflammatory phenotypes in chronic obstructive pulmonary disease

Alveolar macrophages (AMs) in patients with chronic obstructive pulmonary disease (COPD) orchestrate persistent inflammation in the airway. However, subpopulations of AMs participating in chronic inflammation have been poorly characterized. We previously reported that Siglec-1 expression on AMs, which is important for bacteria engulfment, was decreased in COPD. Here, we show that Siglec-1-negative AMs isolated from COPD lung tissues exhibit a proinflammatory phenotype and are associated with poor clinical outcomes in patients with COPD. Using flow cytometry, we segregated three subsets of AMs based on the expression of Siglec-1 and their side scattergram (SSC) and forward scattergram (FSC) properties: Siglec-1^þSSC^hiFSC^hi, Siglec-1^-SSC^hiFSC^hi, and Siglec-1^-SSC^loFSC^lo subsets. The Siglec-1^-SSC^loFSC^lo subset number was increased in COPD. RNA sequencing revealed upregulation of multiple proinflammatory signaling pathways and emphysema-associated matrix metalloproteases in the Siglec-1^-SSC^loFSC^lo subset. Gene set enrichment analysis indicated that the Siglec-1^-SSC^loFSC^lo subset adopted intermediate phenotypes between monocytes and mature alveolar macrophages. Functionally, these cells produced TNF-a, IL-6, and IL-8 at baseline, and these cytokines were significantly increased in response to viral RNA. The increase in Siglec-1-negative AMs in induced sputum is associated with future exacerbation risk and lung function decline in patients with COPD. Collectively, the novel Siglec-1^-SSC^loFSC^lo subset of AMs displays proinflammatory properties, and their emergence in COPD airways may be associated with poor clinical outcomes. NEW & NOTEWORTHY Alveolar macrophages (AMs) in patients with chronic obstructive pulmonary disease (COPD) orchestrate persistent inflammation in the airway. We find that Siglec-1-negative alveolar macrophages have a wide range of proinflammatory landscapes and a protease-expressing phenotype. Moreover, this subset is associated with the pathogenesis of COPD and responds to viral stimuli.