COVID-19: Role of neutrophil extracellular traps in acute lung injury

An unexplained pneumonia broke out in Wuhan, China, in December 2019. Clinical analyses of these and subsequent similar cases attributed the pathology to a novel β-coronavirus. The coronavirus was closely related to the severe acute respiratory syndrome coronavirus (SARS-CoV) and was eventually named SARS-CoV-2; the resultant disease is now termed COVID-19 [1]. As of this writing, there are over 6.6 million confirmed cases globally associated with nearly 400,000 deaths (11% mortality). Since the incidence of deaths lags reports of new cases of SARS-CoV-2 infections, the mortality rate is expected to increase.

In most people, COVID-19 presents as mild flu-like symptoms such as fever, cough, and sore throat [1]. However, in approximately 10% of individuals, SARS-CoV-2 infection can result in lethal respiratory failure, reminiscent of acute respiratory distress syndrome (ARDS) [1]. Many respiratory viruses can cause flu-like symptoms, and some may even cause uncomplicated pneumonia. However, the ability of SARS-CoV-2 to cause severe pneumonia, extensive lung inflammation and injury, sepsis, septic shock, coagulopathy, and multiorgan failure is an enigma. A major factor in the pathogenesis of severe COVID-19 is the host's excessive generation of cytokines (“cytokine storm”) and associated sequelae [2]. Specifically, cytokines have been proposed to promote the generation of extracellular antimicrobial material by innate immune cells, which, in turn, generate more cytokines, creating a detrimental feed-forward loop [3].

Neutrophils can extrude a web of DNA-rich material covered with antimicrobials that entrap and subsequently kill microbes appropriately, which are called neutrophil extracellular traps (NETs) [4]. Since the process of NET formation is often associated with neutrophil demise, the process is referred to as “NETosis” [4]. Consensus suggests that during an inflammatory response, cytokines such as interleukin (IL)-1β induce two major intracellular events in neutrophils: discharge of antimicrobials from granules as well as de-condensation and discharge of chromatin from the nucleus [4]. The antimicrobials become enmeshed in the expanding DNA network, which is ejected from the neutrophil to cover an extensive portion of the extracellular milieu. These antimicrobial-bearing DNA nets can trap and kill the inciting microbes [4]. Although the beneficial role of NETs in innate immune defense is rather obvious, excessive formation of NETs can also be detrimental to the host [4]. Notably, NETs can activate neighboring macrophages to generate additional cytokines, thereby eliciting a feed-forward immune response, that is, an IL-1β-NET loop [3].

The IL-1β-NET loop may be an oversimplification. While NETs can be induced by viral infections, they are not the only innate immune cells capable of generating extracellular traps. Macrophages can also extrude extracellular traps (METs) that are remarkably similar to those derived from neutrophils, released in response to similar stimuli, and capable of bacterial entrapment [5]. Macrophages are generally subdivided into pro-inflammatory (M1) or pro-resolving (M2) phenotypes, although this division is an oversimplification. M1, but not M2, macrophages can generate extracellular traps in response to “netting” neutrophils [5]. Thus, the current NET-IL-1β loop hypothesis should be extended to include an additional loop component, specifically, METs. The relevance of the feed-forward inflammatory loops described previously [3] and expanded herein to COVID-19 is as follows. As the moniker SARS-CoV-2 implies, the major lethal pathology of COVID-19 is respiratory inflammation and injury. Alveolar macrophages serve as sentinel cells and respond to airborne pathogens by secreting cytokines/chemokines to recruit neutrophils to aid in elimination of the threat. This innate immune response is facilitated by the presence of a concentrated reservoir of neutrophils in the lungs (marginated pool). If the lung pathology of COVID-19 spills over into the systemic circulation, thrombosis and septic shock can result. Viral infections lead to platelet activation, aggregation with neutrophils, and enhanced extracellular trap formation [3,6].

From a clinical perspective, a large proportion of patients exhibiting severe COVID-19 are elderly. Based on the available data on NETs, there are several caveats worth noting when treating these patients. NET formation in the elderly is defective; thus, their ability to respond to the initial threat is blunted [4]. In addition, efferocytosis of NETs and necrotic remnants is also ineffective, limiting resolution of the inflammatory response [4]. With respect to systemic complications, the platelets of older individuals are more prone to activation, facilitating thrombogenesis and exacerbating NET formation [7]. Caution should also be exercised when attempting to increase system oxygenation using ventilators, since this approach can also increase lung injury and NET formation [8,9]. Strikingly, it has been shown recently that the sera of COVID-19 patients exhibit higher levels of NETs [3,10].This study demonstrated a higher level of NETs in mechanically ventilated hospitalized COVID-19 patients compared to patients managed on room air [10]. As a final note, the increasing use of statins by the elderly as well as younger patients should be taken into consideration, since statins enhance NET formation.

Current efforts to address the COVID-19 pandemic are focused on the development of rapid diagnostic tests for testing individuals for SARS-CoV-19 infection as well as screening of various antiviral drugs and development of vaccines for therapeutic use. There has been no emphasis on the development of biomarkers for disease severity and disease outcome. We propose that patients presenting with relevant symptoms be screened for the presence of circulating extracellular DNA, histones, or both. In addition, a much more accurate approach would entail assessment of extracellular DNA/histones in bronchoalveolar lavage (BAL) fluid, although this would be limited to patients requiring intubation/ventilation. Enzyme assays to relate the activity of NE and myeloperoxidase (MPO) can also be employed as a prognostic strategy to evaluate NET-induced injury. If this screening identifies patients in whom macrophage or NETs may contribute to their symptoms, several approaches can be used to interfere with their formation [3]. Some of the known and safe therapeutic options to block NETs-IL1β axes include the use of recombinant DNase-1 (dornase alfa), sivelestat (NE inhibitor), and anakinra (IL-1β inhibitor) [3].

Conflict of Interest

There is no conflict of interest disclosed by authors.