The Cathelicidin LL-37 and microbial dysbiosis in COPD patients receiving inhaled corticosteroids

Abstract

Introduction: Recent studies have suggested that suppression of the antimicrobial peptide LL-37 results in increased bacterial load and expansion of Streptococcus in murine models of COPD. We investigated the relationship between LL-37, bacterial load and airway inflammation in patients with COPD.

Methods: Stable patients with moderate to severe COPD, a blood eosinophil count <300cell/ul and receiving inhaled corticosteroid (ICS) therapy were enrolled. The microbiome was characterised by 16S rRNA sequencing and qPCR in sputum. Sputum LL-37 levels was measured by ELISA.

Results: 76 patients with COPD were enrolled, mean age 69.3, 51.2% male, 32.5% current smokers, mean CAT score of 20.6. The microbiome was heterogenous, with the majority of sequences identified as Proteobacteria or Firmicutes phyla. LL-37 concentrations ranged from 0.01-290.0ng/mL. Higher LL-37 was associated with elevated sputum bacterial load (16S copies/ml, r=0.34 95% CI 0.12-0.54,p=0.0024) and was negatively associated with microbiome diversity (Shannon diversity index p<0.0001 and Chao index p<0.0001). Patients were divided into tertiles of LL-37 concentrations, with the highest tertile showing altered beta-diversity (PERMANOVA p<0.0001). Random Forest analysis showed high sputum LL-37 levels associated with Haemophilus, while low LL-37 levels were associated with Streptococcus and commensal taxa such as Rothia. LL-37 showed a strong linear correlation with neutrophil extracellular traps, elastase and CXCL8 suggesting neutrophils as a likely source.

Conclusion: Elevated LL-37 is associated with higher bacterial load, neutrophilic inflammation and microbial dysbiosis in patients with COPD using regular ICS.