Characterization of neutrophil extracellular trap-associated RNA, a novel DAMP acting in concert with the antimicrobial peptide LL37 as intentional but self-limiting drivers of inflammation
Neutrophil extracellular traps (NETs) are a key antimicrobial feature of cellular innate immunity mediated by polymorphonuclear neutrophils (PMNs). NETs trap and kill microbes but have also been linked to inflammation in atherosclerosis, arthritis, or psoriasis by unknown mechanisms. We here report that NET-associated RNA (naRNA, which was RNA-sequenced from human primary neutrophil NETs here) stimulated further NET formation in naïve PMNs via a unique TLR8-NLRP3-caspase-1-gasdermin D-dependent inflammasome pathway. Keratinocytes also responded to naRNA with expression of psoriasis-related genes (e.g. IL17, IL36) via atypical NOD2-RIPK signaling. In vivo naRNA drove skin inflammation, which was drastically ameliorated by genetic ablation of RNA sensing. The naRNA-LL37 'composite DAMP' was pre-stored in resting neutrophil granules, defining sterile NETs as intentionally inflammatory webs that amplify neutrophil activation. However, the activity of the naRNA-LL37 DAMP was transient and hence supposedly self-limiting under physiological conditions. Only upon dysregulated NET release like psoriasis, TLR-NLRP3-mediated naRNA sensing may represent both potential cause of disease and new intervention target. Overall design: Briefly, NETs were prepared by 600 nM PMA treatment for 4 h at 37°C and 5% CO2, or cells were left untreated during the incubation as the mock control.