Neutrophil Extracellular Trap–Associated Protein Activation of the NLRP3 Inflammasome Is Enhanced in Lupus Macrophages

TLDR

Neutrophil extracellular traps (NETs) are a key antimicrobial defense, but their clearance is impaired and production is increased in systemic lupus erythematosus, contributing to endothelial toxicity and immune activation. To better understand the role of NETs in fostering dysregulated inflammation, we examined inflammasome activation in response to NETs or to LL‑37, an antibacterial protein externalized on NETs. LL‑37 activates the NLRP3 inflammasome via P2X7 receptor–mediated potassium efflux. NETs and LL‑37 trigger caspase‑1 activation and IL‑1β/IL‑18 release in macrophages, a response that is amplified in lupus macrophages and is further promoted by IL‑18–induced NETosis, establishing a feed‑forward inflammatory loop that may drive disease flares and organ damage.

Abstract

Neutrophil extracellular traps (NETs) represent an important defense mechanism against microorganisms. Clearance of NETs is impaired in a subset of patients with systemic lupus erythematosus, and NETosis is increased in neutrophils and, particularly, in low-density granulocytes derived from lupus patients. NETs are toxic to the endothelium, expose immunostimulatory molecules, activate plasmacytoid dendritic cells, and may participate in organ damage through incompletely characterized pathways. To better understand the role of NETs in fostering dysregulated inflammation, we examined inflammasome activation in response to NETs or to LL-37, an antibacterial protein externalized on NETs. Both NETs and LL-37 activate caspase-1, the central enzyme of the inflammasome, in both human and murine macrophages, resulting in release of active IL-1β and IL-18. LL-37 activation of the NLRP3 inflammasome utilizes P2X7 receptor-mediated potassium efflux. NET and LL-37-mediated activation of the inflammasome is enhanced in macrophages derived from lupus patients. In turn, IL-18 is able to stimulate NETosis in human neutrophils. These results suggest that enhanced formation of NETs in lupus patients can lead to increased inflammasome activation in adjacent macrophages. This leads to release of inflammatory cytokines that further stimulate NETosis, resulting in a feed-forward inflammatory loop that could potentially lead to disease flares and/or organ damage.