TLR9 activation impairs phagocytosis-induced neutrophil apoptosis and prolongs E. coli-induced acute lung injury

• DOI: 10.4049/jimmunol.198.supp.129.1
• Publication Date: 2023-01-01T10:47:54Z
• Authors: Janos G Filep; Natalie Edner; Everton de Olivei...; Meriem Sekheri; Driss El Kebir
• Keywords: 0301 basic medicine; 03 medical and health sciences; 3. Good health

Abstract Neutrophil dysfunction, resulting in delayed apoptosis and inefficient bacterial clearance, is a characteristic feature of severe pathologies, including sepsis and cystic fibrosis. Human neutrophils detect and respond to bacterial DNA (CpG DNA) through TLR9. We investigated the impact of CpG DNA on phagocytosis, phagocytosis-induced neutrophil apoptosis and clearance of E coli. Culture of human neutrophils with CpG DNA (0.1–3.2 μg/ml) resulted in decreased phagocytosis of opsonized E. coli or yeast. CpG DNA upregulated C3R (CD11b) expression, downregulated C5aR (CD88) expression and induced release of neutrophil elastase. C5aR cleavage was prevented by a specific neutrophil elastase inhibitor and the broad-spectrum serine protease inhibitor PMSF. Consistently, CpG DNA reduced phagocytosis-induced NADPH oxidase-mediated activation of caspase8 and caspase-3. These actions of CpG DNA were blocked by the telomere-derived TLR9 inhibitory oligonucleotide 5′-TTT AGG GTT AGG GTT AGG G-3′. In mice, CpG DNA impaired pulmonary clearance of E coli, suppressed neutrophil apoptosis and delayed resolution of lung injury evoked by intratracheal instillation of live E. coli. Genetic deletion of TLR9 rendered mice unresponsive to CpG DNA. These results identify a novel mechanism, neutrophil elastase-mediated inactivation of C5aR-mediated phagocytosis, by which CpG DNA could contribute to neutrophil dysfunction and prolongation of tissue injury. Our findings also suggest a therapeutic potential for TLR9 antagonists or neutrophil elastase inhibitors for enhancing clearance of bacterial infections in an environment where bacterial DNA is abundantly present.