LL-37 modulates IL-17A/F-mediated airway inflammation by selectively suppressing Lipocalin-2.

BackgroundLevels of the human cationic host defence peptide (CHDP) LL-37 is enhanced in the lungs during neutrophilic airway inflammation. LL-37 drives Th17 differentiation, and Th17 cells produce IL-17A and IL-17F which forms the biologically active heterodimer IL-17A/F. While IL-17 is a critical mediator of neutrophilic airway inflammation, LL-37 exhibits contradictory functions; LL-37 can both promote and mitigate neutrophil recruitment depending on the inflammatory milieu. The impact of LL-37 on IL-17-induced responses in the context of airway inflammation remains largely unknown. Therefore, we examined signaling intermediates and downstream responses mediated by the interplay of IL-17A/F and LL-37, in human bronchial epithelial cells (HBEC). As LL-37 can get citrullinated during airway inflammation, we also examined LL-37-mediated downstream response compared to that with citrullinated LL-37 (citLL- 37), in HBEC. ResultsUsing an aptamer-based proteomics approach, we identified proteins that are altered in response to IL-17A/F in HBEC. Proteins enhanced in response to IL-17A/F were primarily neutrophil chemoattractants, including chemokines and proteins associated with neutrophil migration such as lipocalin-2 (LCN-2) and Elafin. We showed that selective depletion of LCN-2 mitigated neutrophil migration, thus functionally demonstrating LCN-2 as a critical neutrophil chemoattractant. We further demonstrated that LL-37 and citLL-37 selectively suppresses IL- 17A/F-induced LCN-2 production, in bronchial epithelial cells. Mechanistic studies revealed that LL-37 and citLL-37 suppressed IL-17A/F-mediated C/EBP{beta}, a transcription factor required for LCN-2 production. In contrast, LL-37 and citLL-37 enhanced the ribonuclease Regnase-1, which is a negative regulator of IL-17 and LCN-2. In an animal model of neutrophilic airway inflammation with elevated IL-17A/F in the lungs, we demonstrated that CRAMP (mouse ortholog of LL-37) negatively correlates with LCN-2. ConclusionsOverall, our findings showed that LL-37 and citLL-37 can selectively suppress the abundance of IL-17A/F-mediated LCN-2, a protein that is critical for neutrophil migration, in bronchial epithelial cells. These results suggest that LL-37, and its modified citrullinated form, has the potential to negatively regulate IL-17-mediated neutrophil migration to control airway inflammation. To our knowledge, this is the first study to report that the immunomodulatory function of LL-37 engages an RNA binding protein, Regnase-1, indicating post-transcriptional regulation of airway inflammation by the peptide.