Most patients with severe infection with influenza A virus (IAV) progress to acute respiratory distress syndrome and even multiple organ dysfunction syndrome (MODS). Neutrophil extracellular traps (NETs) can be induced by pathogens and are responsible for immune tissue damage. We conducted a prospective study on the production and effects of NETs in H7N9 and H1N1 patients. We investigated NET production in plasma and supernatant of cultured neutrophils by measuring cell-free deoxyribonucleic acid (DNA) and myeloperoxidase (MPO)-DNA complexes with PicoGreen dye and enzyme-linked immunosorbent assay methods, respectively. We also observed NET structure by immunofluorescence staining. We found that patients with severe influenza showed elevated plasma NET level on the day of admission. Neutrophils from these patients showed higher capacity to release MPO-DNA complex in response to interleukin-8 or lipopolysaccharide stimulation. We also found that NETs from H7N9 and H1N1 patients increased the permeability of alveolar epithelial cells, and, consequently, NET production was positively correlated with acute physiology and chronic health evaluation (APACHE) II score and MODS. These data indicate that high level of NETs contributes to lung injury and is correlated with severity of disease. Thus, NETs might be a key factor to predict the poor prognosis in IAV patients.

Neutrophil extracellular traps (NETs) are intricate fibrous structures released by neutrophils in response to specific stimuli. These structures are composed of depolymerized chromatin adorned with histones, granule proteins, and cytosolic proteins. NETs are formed via two distinct pathways known as suicidal NETosis, which involves NADPH oxidase (NOX), and vital NETosis, which is independent of NOX. Certain proteins found within NETs exhibit strong cytotoxic effects against both pathogens and nearby host cells. While NETs play a defensive role against pathogens, they can also contribute to tissue damage and worsen inflammation. Despite extensive research on the pathophysiological role of NETs, less attention has been paid to their components, which form a unique structure containing various proteins that have significant implications in a wide range of diseases. This review aims to elucidate the components of NETs and provide an overview of their impact on host defense against invasive pathogens, autoimmune diseases, and cancer.

Summary: Activation of Toll‐like receptors (TLRs) triggers rapid inflammatory cytokine production in various cell types. The exogenous product of growth‐arrest‐specific gene 6 (Gas6) and Protein S (ProS) inhibit the TLR‐triggered inflammatory responses through the activation of Tyro3, Axl and Mer (TAM) receptors. However, regulation of the Gas6/ProS‐TAM system remains largely unknown. In the current study, mouse macrophages are shown to constitutively express Gas6 and ProS, which synergistically suppress the basal and TLR‐triggered production of inflammatory cytokines, including those of tumour necrosis factor‐α, interleukin‐6 and interleukin‐1β, by the macrophages in an autocrine manner. Notably, TLR signalling markedly decreases Gas6 and ProS expression in macrophages through the activation of the nuclear factor‐κB. Further, the down‐regulation of Gas6 and ProS by TLR signalling facilitates the TLR‐mediated inflammatory cytokine production in mouse macrophages. These results describe a self‐regulatory mechanism of TLR signalling through the suppression of Gas6 and ProS expression.

Background: The extensive geographical distribution and high mortality rate of severe fever with thrombocytopenia syndrome (SFTS) have made it an important threat to public health. Neutrophil extracellular traps (NETs) can be activated by a variety of pathogens and are associated with thrombocytopenia in viral infections. We aimed to identify NET production and its predictive value for disease progression and prognosis in patients with SFTS.Methods: A prospective study was performed with a multicenter cohort of patients with SFTS (n = 112) to quantify serum NET levels. Three markers of NETs—namely, cell-free DNA (cfDNA), myeloperoxidase-DNA complexes, and lactoferrin-DNA complexes—were measured with PicoGreen double-stranded DNA assays and enzyme-linked immunosorbent assays. Receiver operating characteristic curves and multivariate regression analyses were performed to calculate the predictive value of cfDNA levels.Results: SFTS was characterized by pronounced NET formation. The serum levels of NETs changed dynamically during disease progression, with an inverse pattern of the trends of platelet and neutrophil levels. High cfDNA levels were strongly associated with multiple pathological processes, including coagulopathy, myocardial damage, liver dysfunction, and the development of encephalopathy. A high level of cfDNA (>711.7 ng/mL) at the time of the initial diagnosis predicted severe illness in patients with SFTS (odds ratio, 8.285 [95% confidence interval, 2.049–33.503]; P = .003).Conclusions: This study has a high degree of clinical impact for identification of cfDNA as a useful predictive biomarker of clinical outcomes of SFTS.