The screening of albumin as a key serum component in preventing release of neutrophil extracellular traps by selectively inhibiting mitochondrial ROS generation.

Neutrophil extracellular traps (NETs) are extracellular DNA webs released from neutrophils to mediate the host antimicrobial defense. As NETs could also induce thrombosis and cause organ injury, their release should be strictly controlled; however, the intrinsic mechanisms that prevent unfavorable NETs are not well understood. Herein, an accidental finding of NET release from human peripheral neutrophils was first described in a serum-free culture, which was later determined to be a conserved NET prevention effect of serum. In contrast to canonical NETs induced by phorbol-12-myristate-13-acetate (PMA), NET formation by serum-free culture was rapid and without prevalent NETosis. Next, albumin was screened out as a key serum component that mediated the suppression of NETs. Moreover, NETs induced upon serum or albumin deficiency were independent of the canonical pathway that involves NADPH oxidase 2 (NOX2) activation and cytosol reactive oxygen species (ROS) production. Instead, the generation of mitochondrial ROS (mtROS) was upregulated to promote NET release. Albumin exhibited mtROS scavenging activity and thus inhibited NETs. Serum-free culture also induced the release of NET-bound oxidized mtDNA, which stimulated interferon-β (IFN-β) production. Overall, our research provides new evidence that characterizes the NET production in serum-free culture and determines the mechanisms by which serum albumin inhibits NETs.