CD14⁺CD16⁺ monocyte-derived TREM2 macrophages promote lung fibrosis in systemic sclerosis-interstitial lung disease

Scar-associated macrophages (SAMs), defined by triggering receptor expressed on myeloid cells 2 (TREM2) and/or glycoprotein-NMB (GPNMB) expression, have been implicated in lung fibrosis. This study investigated the characteristics and functional roles of SAMs in lung tissues from patients with systemic sclerosis-interstitial lung disease (SSc-ILD) and healthy controls (HCs).

SAM-related transcriptional profiles of lung tissues from patients with SSc-ILD and HCs were analyzed using single-cell RNA sequencing (scRNA-seq). Immunofluorescence staining of lung tissues was performed to detect CD68-positive SAMs expressing TREM2 or CCL2. Monocytes from patients with SSc-ILD and HCs were differentiated with granulocyte-macrophage colony-stimulating factor (GM-CSF) or M-CSF. The population of TREM2- and GPNMB-expressing cells was assessed by flow cytometry, and the levels of CCL2 and TGF-β1 in culture supernatants were measured in an ELISA. The fibrotic effects of macrophages on fibroblasts were examined in co-cultures. TREM2 inhibition was used to determine functional relevance.

scRNA-seq revealed the expansion of TREM2 macrophages highly expressing SAM-associated genes in the lower lung lobes of SSc-ILD patients. Immunofluorescence showed increased numbers of CD68⁺TREM2⁺ and CD68⁺CCL2⁺ macrophages in SSc-ILD lung tissues compared to HCs. In GM-CSF-, but not M-CSF-induced CD14⁺CD16⁺ monocyte-derived macrophages from patients with SSc-ILD, the upregulated expression of TREM2, GPNMB, CCL2, and TGF-β1 recapitulated the transcriptional features of SAM. When these macrophages were co-cultured with fibroblasts, the production of collagen, α-SMA and TGF-β was enhanced. By contrast, TREM2 inhibition significantly reduced these responses, as determined at the bulk RNA-seq and protein levels.

Our study identified a GM-CSF-TREM2-TGF-β1 axis driving monocytes-derived profibrotic macrophage-fibroblast crosstalk in SSc-ILD. It also showed that, in the lungs of SSc-ILD patients, CD14⁺CD16⁺ monocyte-derived SAMs drive fibrosis through a TREM2-mediated pathway, which may provide new therapeutic targets.