In ALI, NETs can promote the activation and transformation of lung macrophages into pro-inflammatory M1 types, while M1 type macrophages can promote the infiltration of neutrophils in the lungs and exacerbate the production of NETs. The two cooperate to cause the disorder of immune cells and aggravate tissue injury [
]. During influenza virus infections, the recruitment and activation of innate immune cells such as neutrophils and macrophages are out of control, and a large number of neutrophils accumulate and release NETs in the lungs, causing damage and leading to ARDS [
]. Similarly, the ratio of neutrophils to lymphocytes increased in patients with severe COVID-19, and the neutrophil subsets of patients with severe COVID-19 were more active in producing NETs, which would aggravate the inflammation of ARDS [
]. In line with this, multiple studies have shown that immunomodulatory therapy is effective against COVID-19 [
], and targeting innate immune-related factor Bruton’s tyrosine kinase can reduce centrocyte infiltration and NET secretion, and alleviate lung injury [
]. These findings once again confirm the contribution of inflammation and immune disorders in the formation and development of ARDS, in which NETs are an important factor.
Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is a global pandemic infectious disease that can affect multiple systems, mainly the respiratory system. Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are its most severe pulmonary manifestations [
111]. Elevated levels of NETs have been found in patients with COVID-19 ARDS [
109]. Serum from patients with COVID-19 triggered NET release from control neutrocytes in vitro, and NETs released from SARS-CoV-2-activated neutrocytes promoted lung epithelial cell death in vitro, suggesting that NETs may mediate SARS-CoV-2-induced ARDS [
99,
112,
113]. Angiotensin-converting enzyme 2, serine protease, viral replication, PAD4, and ROS were found to be associated with the activation of neutrophil-releasing NETs by SARS-CoV-2 [
99,
114]. Meanwhile, NETs can be used as a prognostic indicator for COVID-19. Multiple clinical studies have shown that circulating NET levels are associated with disease severity and affect clinical outcomes in patients with COVID-19, while NET levels are not associated with thrombosis development in patients, even though an association between NET formation and immunothrombosis is a well-established fact [
113,
115,
116]. Similar to lung injury caused by other factors, NETs and their components contribute to the development of COVID-19 ARDS by activating downstream inflammatory pathways, damaging endothelial and pulmonary epithelial cells, and disrupting barrier function [
77]. There is a higher rate of venous thromboembolism in severe COVID-19 ARDS than in other forms of ARDS [
117]. NETs interact with platelets to cause a thrombose-inflammatory cascade, forming an immune thrombus that promotes lung damage in COVID-19, and NETs released from recruited neutrophils against SARS-CoV-2 do more harm than good in the lungs of patients with COVID-19 [
118]. Neutrophils, which form NETs, also participate in the cytokine storm of COVID-19 [
119,
120]. The formation of NETs can lead to the production of excessive cytokines and chemokines, such as IL1β, IL6, IL8, IL10, TNF-α, and IFN-γ, which may trigger a cytokine storm leading to ALI, ARDS, and death [
98,
104,
121]. At the same time, IL1β and IL8 also mediate the production of NETs, and thus may lead to uncontrolled progressive inflammation during cytokine storms [
122,
123,
124]. The release of a large number of proinflammatory cytokines and chemokines can amplify the coagulation response and promote the formation of immune thrombosis [
120].
8. Clinical Research
At present, most clinical trials have been conducted on COVID-19 ARDS. Dornase alfa can act as a mucolytic agent and reduce NET levels in the lungs, thereby improving oxygenation and ventilation [
126]. Andrew G. Weber et al. applied nebulized dornase alfa combined with salbutamol in five patients with ARDS who required mechanical ventilation due to SARS-CoV-2. After 7 days of treatment, FiO2 demand decreased, the condition improved in all five patients, and there were no deaths. The therapeutic effect of dornase alfa on inhibiting NETs and improving lung function was confirmed [
126]. Similarly, the non-randomized trial of dornase alfa by Zachary M. Holliday’s group in the treatment of COVID-19 secondary ARDS also demonstrated the benefits of inhaled α-dornase, although the positive effect was limited to the time of administration [
127]. In addition, the results of a clinical trial involving 20 patients with ARDS from COVID-19 treated with invasive mechanical ventilation suggest that metoprolol can also reduce neutrophil extracellular trap content and other markers of lung inflammation, reduce aggravated lung inflammation, and improve oxygenation without adverse effects, and is therefore a possible treatment strategy [
128].
9. Treatment
Treatments targeting NETs have been validated by several studies and are considered to be a feasible treatment for ARDS, capable of alleviating pulmonary symptoms. Therapies targeting NETs include degrading already-formed NETs and inhibiting the formation of NETs. The most common treatment for NET degradation is DNase Ⅰ, which degrades DNA components in NETs, thereby reducing the deposition of NETs, and is currently approved for clinical use without toxicity [
131].
Since PAD4 is the key to the formation of NETs, the inhibition of PAD4 can reduce histone citrullination and block NETosis [
130,
134]. PAD4 inhibitors Cl-amidine and GSK484, and selective inhibitors such as streptomycin, can reduce the levels of citrulline histone H3 (CitH3) and inflammatory factors, and reduce the formation of NETs, thereby reducing lung injury [
9,
135]. Treatment with the PAD2/PAD4 inhibitor YW356 has also been shown to reduce PAD activation and alleviate LPS-induced acute lung injury [
79].
As mentioned above, NE plays an important role in the formation of NETs, so inhibiting NE is another way to reduce the production of NETs. Using NE inhibitors can reduce the severity of lung injury [
26,
31]. The selective NE inhibitor GW311616A and the nanoparticle-mediated small molecule NE inhibitor Sivelestat have been shown to effectively inhibit the formation of NETs [
9,
62].
The inhibitors of some signaling pathways can also affect the formation of NETs; for example, inhibitors of p38 MAPK kinase can reduce NETs in ALI [
95], and high-mobility histone B1 (HMGB1) can further drive NETs, so blocking it can prevent NET formation [
36]. The hypoglycemic agent metformin can also reduce NETosis and lung inflammation by specifically inhibiting HMGB1, activating AMPK, and inhibiting the mTOR pathway [
9,
16]. Anti-PD-L1 antibodies affect the release levels of NETs by regulating neutrophil autophagy via inhibiting the PI3K/Akt/mTOR pathway [
94].
Other therapies that target NETs, including the use of Protectin D1 (PD1) [
139], iron-chelating agent Deferasirox [
140], mesenchymal stem cells (MSCs) [
141], osteopontin (OPN) [
142], colchicine [
143], disulfiram [
144], Methoxyeugenol [
145], anti-CLEC5A [
146], Selinexor [
147], etc., can inhibit or reduce the production of NETs. At the same time, the stimulation of the ear vagus nerve [
148] and intravenous vitamin C [
129] also block NETosis.
11. Conclusions
In the process of ARDS, neutrophils accumulate in and infiltrate the lungs to generate NETosis, and the NETs produced destroy the lung endothelial cell barrier, causing tissue damage, forming immune thrombosis, and aggravating the condition of ARDS. The production of NETs and the release of contents that affect the surrounding tissues constantly stimulate each other, forming a vicious circle, leading to the continuous formation of NETs. At the same time, NETs can also cause cytokine storms and affect the body’s immune balance, further promoting the development of the disease. However, although there has been much research on the formation of NETs in ARDS, the mechanism affecting disease progression, and the corresponding treatment methods, specifically exploration and interpretation, still need further study.