Neutrophil extracellular traps induce IL-1β production by macrophages in combination with lipopolysaccharide

Upon exposure to invading microorganisms, neutrophils undergo NETosis, a recently identified type of programmed cell death, and release neutrophil extracellular traps (NETs). NETs are described as an antimicrobial mechanism, based on the fact that NETs can trap microorganisms and exhibit bactericidal activity through the action of NET-associated components. In contrast, the components of NETs have been recognized as damage-associated molecular pattern molecules (DAMPs), which trigger inflammatory signals to induce cell death, inflammation and organ failure. In the present study, to clarify the effect of NETs on cytokine production by macrophages, mouse macrophage-like J774 cells were treated with NETs in combination with lipopolysaccharide (LPS) as a constituent of pathogen-associated molecular patterns. The results revealed that NETs significantly induced the production of interleukin (IL)-1β by J774 cells in the presence of LPS. Notably, the NET/LPS-induced IL-1β production was inhibited by both caspase-1 and caspase-8 inhibitors. Furthermore, nucleases and serine protease inhibitors but not anti-histone antibodies significantly inhibited the NET/LPS-induced IL-1β production. Moreover, we confirmed that caspase-1 and caspase-8 were activated by NETs/LPS, and the combination of LPS, DNA and neutrophil elastase induced IL-1β production in reconstitution experiments. These observations indicate that NETs induce the production of IL-1β by J774 macrophages in combination with LPS via the caspase-1 and caspase-8 pathways, and NET-associated DNA and serine proteases are involved in NET/LPS-induced IL-1β production as essential components.