Abstract: BACKGROUND: The gut–lung axis has emerged as a critical factor in the development of various pulmonary diseases. However, its role in idiopathic pulmonary fibrosis remains insufficiently investigated, and the underlying causal relationships are yet to be clarified. This study integrates genome-wide association studies, expression quantitative trait loci analysis, and colocalization-based molecular docking to comprehensively assess how gut microbiota may influence idiopathic pulmonary fibrosis through immune regulation, inflammatory mediators, and metabolic pathways. The research aims to provide mechanistic insights from genetic, transcriptional, and pharmacological perspectives.
OBJECTIVE: To explore the causal relationship between gut microbiota and idiopathic pulmonary fibrosis, to elucidate the mediating effects of immune cells, inflammatory proteins, and circulating metabolites, to screen key microbial taxa and potential target genes, and to predict candidate therapeutic agents that may contribute to early diagnosis and drug development for idiopathic pulmonary fibrosis.
METHODS: The analysis was based on publicly available summary statistics from genome-wide association studies involving 473 gut microbial species, sourced from the Finnish biobank and the expression Quantitative Trait Loci Gen consortium. Additional data included 731 immune cell traits, 91 inflammatory proteins, 233 blood metabolites, and idiopathic pulmonary fibrosis genome-wide association results. Univariable Mendelian randomization using the inverse-variance weighted approach and sensitivity analyses were performed to explore the causal relationship between gut microbiota and idiopathic pulmonary fibrosis. Two-step mediation Mendelian randomization was conducted to determine whether immune and inflammatory factors mediate the relationship between gut microbiota and idiopathic pulmonary fibrosis. Furthermore, causal relationships between microbial taxa and gene expression were evaluated using Mendelian randomization and summary-data-based Mendelian randomization methods. Colocalization and druggability analyses were carried out to prioritize candidate compounds, followed by molecular docking validation. 
RESULTS AND CONCLUSION: The abundance of Bacteroides faecis, Megasphaera, and Pandoraea was causally associated with idiopathic pulmonary fibrosis. Bacteroides faecis appeared to mediate disease risk through the linoleic acid (18:2) ratio and several CD4-positive T cell subsets. Pandoraea risk was similarly influenced by different CD4+ T cell subsets, whereas Megasphaera may exert protective effects via interleukin-33, low-density lipoprotein-related metabolites, and CD4-CD8- T cell subsets. Further analyses identified GNF-Pf-2272, compound 5155877, and protoporphyrin IX as candidate agents targeting lysine-specific demethylase 4C, carbonyl reductase 3, and 14-3-3 protein gamma isoform, respectively. Although the genome-wide association study data for pulmonary fibrosis used in this study were primarily derived from European populations, given the commonalities in multiple core pathways across the human genetic background, the findings still hold significant reference value for exploring ways to improve the risk of pulmonary fibrosis in the Chinese population by regulating the gut microbiota.

Key words: idiopathic pulmonary fibrosis, gut microbiota, immune cells, inflammatory protein, blood metabolites, multiomics