Abstract Introduction Myeloid cells orchestrate both tissue repair and fibrosis, yet how their distinct activation states contribute to progressive fibrotic remodeling remains unclear. Traditionally, fibrosis has been attributed to monocyte-derived macrophages releasing TGF-β, which drives fibroblast activation and matrix production. In support of this paradigm, we have shown that fibroblast-specific deletion of Tgfbr2 limits fibrosis in the bleomycin model of epithelial injury-induced lung fibrosis. However, this strategy had no effect in silica-induced fibrosis, suggesting that alternative, TGF-β-independent macrophage-fibroblast circuits may drive disease progression in silicosis. Methods We performed single-cell RNA sequencing of lung cells from untreated and silica-exposed mice across time points and integrated this dataset with a publicly available bleomycin-injury myeloid cell atlas. To define proliferative niches, we pulse-labeled silica-exposed mice with 5-ethynyl-2’-deoxyuridine (EdU) for confocal imaging. Results In uninjured lungs, we identified alveolar macrophages and three subsets of previously described tissue-resident interstitial macrophages. In both the bleomycin and silica models we observed Spp1+ macrophages, but with distinct transcriptional signatures. In bleomycin injury, Spp1+ macrophages upregulated Trem2, Cd63, and Cd9, consistent with the scar-associated macrophage (SAM) state described across multiple fibrotic tissues. However, in silica injury, Spp1+ macrophages strongly expressed Met and type I interferon response genes (Ifi205, Ifi209, Rsad2) along with Inhba, which encodes Activin A—a ligand known to stimulate Smad2/3 in fibroblasts via the Acvr1b (ALK4) receptor, independently of Tgfbr2. In parallel, both Spp1+ and Spp1⁻ interstitial macrophages expressed Pdgfb and Csf1r. In response to silica, activated fibroblasts expressed Pdgfrb, Csf1, Il34, and Hgf, forming a triad of reciprocal signaling axes—PDGF-PDGFRB, CSF1/IL34-CSF1R, and HGF-MET. Imaging of EdU-labeled lungs revealed co-localized proliferating fibroblasts and Spp1+/Spp1⁻ macrophages within silicotic granulomas, supporting the hypothesis that crosstalk between macrophages and fibroblasts might be sustaining both lineages. Conclusions These findings define a TGF-β-independent macrophage-fibroblast circuit that may support granulomatous fibrosis via context-specific signaling. While bleomycin injury favors SAM-like programs, silica activates interferon-driven macrophages that express Activin A, implicating alternative routes to Smad2/3 activation in fibroblasts. In parallel, PDGFB, CSF1/IL34, and HGF signaling form a reciprocal growth-factor network that may coordinate the mutual survival and expansion of macrophages and fibroblasts within fibrotic granulomas. This work reframes the fibrotic niche as an etiology-specific outcome of myeloid functional heterogeneity and identifies testable therapeutic targets—including Activin A-ALK4, PDGFB-PDGFRB, and CSF1R-ligand interactions—that could disrupt persistent profibrotic feedback in chronic granulomatous diseases like silicosis. This abstract is funded by: NIH R01HL171159
Molina et al. (Fri,) studied this question.