C29-20 Context-Specific Macrophage Programs in Silica and Bleomycin-Induced Models of Pulmonary Fibrosis

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.