Indoxyl sulfate (IS) mediates pro-inflammatory responses in severe pneumonia in patients with rheumatoid arthritis associated interstitial lung disease.

OBJECT: Patients with rheumatoid arthritis-associated interstitial lung disease (RA-ILD) have a high risk of serious infection, in particular severe pneumonia. This study aimed to investigate the transcriptional landscape, lower respiratory tract (LRT) microbiome and metabolomic profiles in the lung of RA-ILD patients with pneumonia.

METHOD: A total of 10 RA-ILD with pneumonia were enrolled in this study. In addition, 11 patients with COVID-19-associated pneumonia and 6 patients with non-autoimmune and non-COVID-19-related ILD with pneumonia were included as controls. Bronchoalveolar lavage fluid (BALF) was collected and prepared for metagenomic next-generation sequencing (mNGS), non-targeted metabolomics and bulk RNA-seq.

RESULT: Neutrophil-related genes were shared in the BALF cells of RA-ILD patients with pneumonia and patients with COVID-19-associated pneumonia. Carnobacterium, Wujia, Intestinimonas, Apibacter, Anaerotignum and Parvimonas were enriched in the LRT microbiome of RA-ILD, while Wujia, Apibacter, Pseudocitrobacter, and Thermobacillus were enriched in the LRT microbiome of COVID-19. Metabolomics analysis of BALF revealed significant elevation of indoxyl sulfate (IS) in the BALF of RA-ILD patients in comparison to COVID-19. Mechanistically, IS exerts an pro-inflammatory effect on macrophages and bronchial epithelial cells for pro-inflammatory cytokine production and potentiated neutrophils for neutrophil extracellular traps (NETs) formation.

CONCLUSIONS

Our results demonstrated a significant differences in the LRT microbiome and BALF metabolites between RA-ILD and COVID-19 patients with pneumonia, although they displayed similar local immune responses against lung infection. Alterations of LRT microbiome and related metabolites may be implicated in the pathogenesis of pneumonia in RA-ILD.