RNA Externalized by Neutrophil Extracellular Traps Promotes Inflammatory Pathways in Endothelial Cells.

OBJECTIVE: Neutrophil extracellular traps (NETs) are extracellular lattices composed of nucleic material bound to neutrophil granule proteins. NETs may play pathogenic roles in the development and severity of autoimmune diseases such as systemic lupus erythematosus (SLE), at least in part, through induction of type I interferon (IFN) responses via externalization of oxidized immunostimulatory DNA. A distinct subset of SLE proinflammatory neutrophils (low-density granulocytes [LDGs]) displays enhanced ability to form proinflammatory NETs that damage the vasculature. We undertook this study to assess whether NET-bound RNA can contribute to inflammatory responses in endothelial cells (ECs) and the pathways that mediate this effect. METHODS: Expression of newly synthesized and total RNA was quantified in NETs from healthy controls and lupus patients. The ability of ECs to take up NET-bound RNA and downstream induction of type I IFN responses were quantified. RNAs present in NETs were sequenced and specific small RNAs were tested for induction of endothelial type I IFN pathways. RESULTS: NETs extruded RNA that was internalized by ECs, and this was enhanced when NET-bound nucleic acids were oxidized, particularly in lupus LDG-derived NETs. Internalization of NET-bound RNA by ECs was dependent on endosomal Toll-like receptors (TLRs) and the actin cytoskeleton and induced type I IFN-stimulated genes (ISGs). This ISG induction was dependent on NET-associated microRNA let-7b, a small RNA expressed at higher levels in LDG-derived NETs, which acted as a TLR-7 agonist. CONCLUSION: These findings highlight underappreciated roles for small RNAs externalized in NETs in the induction of proinflammatory responses in vascular cells, with implications for lupus vasculopathy.