The role of interleukin-6 signalling in pleural infection: observational and genetic analyses.

Background: Pleural infection is associated with marked local and systemic inflammation leading to significant morbidity. It may be possible to therapeutically augment this response and interleukin-6 is a key signalling cascade in inflammatory pathologies.

Methods: We performed a prospective observational study recruiting patients with pleural effusions secondary to infection and measured interleukin-6 in matched pleural fluid and serum (n = 76). We subsequently performed a large-scale, two sample Mendelian Randomisation study (1601 cases and 830,709 controls), using genetic variation at IL6R to proxy the effect of interleukin-6 inhibition on pleural infection and overcome confounding inherent in observational analyses.

Findings: Pleural interleukin-6 levels in infection were 5000-fold higher than matched serum levels (median 72,752 pg/ml vs. 15 pg/ml). Pleural interleukin-6 predicted systemic inflammation (neutrophil count, C- reactive protein), correlated with clinical markers of disease severity (effusion size, pH, glucose), and was strongly associated with length of hospital stay. In Mendelian randomisation analyses, interleukin-6 inhibition was predicted to have a large protective effect on the incidence of infection (OR 0.23; 95% CI 0.14-0.39 per standard deviation decrease in C- reactive protein). The effect size was larger than that seen in COVID-19 and coronary artery disease, where interleukin-6 inhibition has been successful in trials.

Interpretation: Multiple lines of evidence suggest pleural interleukin-6 drives pathology in pleural infection. Targeting interleukin-6 may hold promise and should be considered in randomised trials.

Funding: This study has been funded by the National Institutes of Health and Care Research Bristol Biomedical Research Centre.