Neutrophil extracellular traps (NETs) have emerged as a key feature of cellular innate immunity mediated by polymorphonuclear neutrophils (PMNs), the primary leukocyte population in humans. Forming web-like structures composed of DNA, histones, and antimicrobial proteins, NETs trap and kill microbial invaders and thus enhance host defense. However, they have also been linked to inflammatory states, e.g. in atherosclerosis or psoriasis. Whilst DNA has been in focus as a primary structural component of NETs, we here characterize naRNA (NET-associated RNA), as a new canonical, abundant, and largely unexplored NET component. naRNA decorated all types of NETs in complex with the antimicrobial peptide LL37. In fact, naRNA was pre-associated with LL37 intracellularly as a ‘composite’ danger-associated molecular pattern (DAMP) prior to neutrophil activation. Externalized, naRNA propagated NET formation in naïve PMN, dependent on TLR8 in humans and Tlr13 in mice, in vitro and in vivo. naRNA-TLR8/Tlr13 signaling contributed significantly to the highly sensitive pro-inflammatory response of both tissue cells, like keratinocytes, and other immune cell types, such as macrophages. Those responses could be blocked by inhibition and genetic ablation of RNA receptors or RNase treatment. Importantly, in vivo naRNA strongly drove skin inflammation whereas genetic ablation of RNA sensing drastically ameliorated skin inflammation in the imiquimod psoriasis model. Our data highlight naRNA as a novel composite DAMP signaling and amplifying neutrophil activation. Moreover, naRNA emerges as the likely driver of inflammation in conditions previously linked to NETs and extracellular RNA, suggesting blockade of TLR-mediated RNA sensing as potential new intervention target.
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