Proprotein convertase subtilisin/kexin type 6 (PCSK6) is a serine endoprotease involved in proprotein processing. Although PCSK6 is upregulated in idiopathic pulmonary fibrosis (IPF)  and correlates with disease severity, its precise mechanistic role remains unclear. This study aims to elucidate the regulatory mechanism of PCSK6 in pulmonary fibrosis and explore its potential as  a therapeutic target.

PCSK6 expression profiles were examined using public databases of IPF patients and bleomycin (BLM)-induced pulmonary fibrosis mice. AAV5-PCSK6 was delivered oropharyngeally to C57BL/6 mice, followed by BLM challenge. Lung tissues were analyzed at days 14 and 42 post-BLM administration via histology, flow cytometry and qPCR. Single-cell and bulk RNA sequencing were integrated to dissect cellular composition and interaction networks in fibrotic lungs. In vitro studies using fibroblasts, bone marrow-derived macrophages (BMDMs), and human lung organoids were conducted to validate PCSK6-mediated effects.

PCSK6 was predominantly expressed by inflammatory fibroblasts in both IPF and fibrotic mouse lungs. These fibroblasts exhibited pro-fibrotic microenvironment remodeling properties and showed strong interactions with SPP1+ macrophages. Compared to empty AAV controls, PCSK6-overexpressing mice displayed exacerbated pulmonary inflammation and fibrosis. Transcriptomics analyses revealed upregulation of inflammation-, fibrosis-, senescence-, and oxidative stress-related genes in AAV-PCSK6 mice, with enriched epithelial-mesenchymal transition and inflammatory pathways. Single-cell RNA sequencing demonstrated increased proportions of transitional epithelial cells, inflammatory fibroblasts, SPP1+ fibroblasts, and lymphatic endothelial cells in AAV-PCSK6 mice, along with enhanced inflammatory fibroblast-SPP1+ macrophage crosstalk. Fibroblasts overexpressing PCSK6 exhibited elevated expression of inflammation-, fibrosis-, and senescence-associated genes. PCSK6 recombinant protein polarized BMDMs toward an SPP1+ pro-fibrotic phenotype. In human lung organoids, PCSK6 recombinant protein treatment upregulated inflammatory and fibrotic genes while promoting fibroblast activation.

PCSK6 is primarily expressed by inflammatory fibroblastsand  drives pulmonary fibrosis progression by promting SPP1+ macrophage polarization and fibroblast-immune cell interactions. These findings hightlight PCSK6 as a potential early therapeutic target and underscore the importance of fibroblast–macrophage crosstalk  in fibrotic lung disease.