C15-06 Gut Derived Metabolites Increase Type-17 Signaling in the Lungs Leading to More Severe Bacterial Pneumonia

Introduction Every year, over 2 million people die of pneumonia and Streptococcus pneumoniae (Spn) is a leading cause. Type-17 immunity plays an important role in host defense against bacterial infections. However, IL-17A has also been implicated in immune pathology associated with acute respiratory distress syndrome. Antibiotics, which alter the composition of the gut microbiome, and microbial metabolites, decrease levels of type-17 immunity in the gut and lungs. We hypothesized that gut derived metabolites can alter type-17 signaling in the lungs altering host susceptibility to acute bacterial infections. Methods Wild type C57BL/6J mice were given amoxicillin ad libitum in drinking water (0.26 mg/ml) starting at 8 weeks of age for 7 days, followed by a 7-day break, then 7 additional days of treatment. For indole supplementation, mice were gavaged with 10 µM indole every other day during amoxicillin administration. Mice were euthanized or challenged with Spn 3 days after cessation of amoxicillin. For infections, mice were challenged with 1-5x100000 CFU Spn via oropharyngeal aspiration and harvested 24 hours later. Right lung was used for CFU counts and gene expression. Left lung was digested in dispase and stained for flow cytometry. Statistical significance was assessed using Mann-Whitney tests. Results Amoxicillin treatment led to decreased Type-17 signaling in the terminal ileum (Panel A) and the lungs (Panel B) as well as increased severity of Spn pneumonia, as measured by systemic dissemination of bacteria (p = 0.037, panel C). Indole supplementation during amoxicillin treatment restored IL17A expressing immune cell populations (Panel D) and IL-17A expression in the lungs (Panel E) but did not improve disease severity and trended toward more severe disease (Panels F-H). IL-17A expression in the lungs correlated significantly with increased lung bacterial burdens (CFUs, p = 0.0002), alveolar permeability (total protein in BAL, p = 0.007), and decreased temperatures 24 hours after infection (p = 0.004). IFN-g gene expression in the lungs did not correlate with any measures of disease severity. Conclusions Our results show that amoxicillin treatment caused a decrease in type-17 signaling in the gut and lungs and this resulted in increased disease severity in Spn pneumonia infections. Supplementation with indole, a gut bacterial metabolite previously shown to induce type-17 signaling, rescued immune cell populations and IL-17A expression in the lungs when administered during amoxicillin treatment, but it did not improve disease severity. Further studies are ongoing to understand if too much IL-17A induction by the gut can result in more severe pneumonia outcomes. This abstract is funded by: NIH