Background: Silicosis is an incurable fibrotic lung disease caused by inhalation of respirable crystalline silica. Excessive silica dust exposure is thought to alter the phenotype of alveolar macrophages, although the nature of this phenotypic switch is not well defined. 

Methods: Bronchoalveolar (BAL) cells were isolated from a cohort of stone benchtop industry workers with radiologically classified simple (n=6) or complicated silicosis (n=8) and compared to a control cohort (n=5). RNA sequencing (RNA-seq) transcriptomic analysis was performed to identify genes and biological pathways that are enriched in BAL cells from patients with silicosis.

Results: In silicosis patients, gene expression analysis showed 55 and 131 differentially expressed genes (DEGs) in simple and complicated cases, respectively, with 39 common to both. Extracellular matrix organization and degradation were the most significantly altered biological processes in both simple and complex disease. The analysis revealed an IL-4-driven profibrotic alveolar macrophage population (MERTK-hi, SPP1-hi, STAB1-hi, FABP4-low, MMP9-hi, MMP12-hi, and COL6A1-hi), and an influx of Langerhans cells (CD207 and CD1A). Additionally, ferroptosis-related genes (SAT1, SLC7A11, MGST1, AIFM2) were upregulated in silicosis BAL cells and positively correlated with macrophage iron and lipid content.

Conclusions: This study provides the first transcriptomic analysis of human BAL cells isolated from patients with silicosis, which identifies a distinct profibrotic alveolar macrophage phenotype that was under significant cellular stress (ferroptosis) and likely contributes to ECM remodelling underlying the pathogenesis of silicosis.