Anti-protease Treatments Targeting Plasmin(ogen) and Neutrophil Elastase May Be Beneficial in Fighting COVID-19

To the Editor: Ji et al. (6) have convincingly shown that elevated plasmin(ogen) is a common risk factor in COVID-19 susceptibility. They highlighted the clinical relevance of plasmin(ogen) in patients who are susceptible to SARS-CoV-2 because of underlying conditions, such as hypertension, diabetes, cardiovascular diseases, cerebrovascular diseases, and chronic renal illnesses. They also pointed to other evidence in the literature that various functions of plasmin (ogen) increase the pathogenicity of COVID-19 (6). We recently proposed a novel hypothesis (10) concerning COVID-19 pathogenesis which is linked to that of Ji et al. (6). We are convinced that neutrophil extracellular traps (NETs) formation and its byproducts play a key role in COVID-19 pathogenesis (10). NETs are extensive structures released extracellularly from activated neutrophils in response to infection. They are composed of cytosolic protein assembled on a scaffold of released chromatin (10). These structures prevent the dissemination of microorganisms in blood by trapping them mechanically and by exploiting the coagulant function to segregate them within the circulation (5). In addition to this, NET components [DNA, histone, and granule proteins such as myeloperoxidase and elastase (NE)] also contribute to the triggering of an inflammatory process. The dysregulation of NET formation and the consecutive release of NET byproducts is involved in thrombosis and fibrinolysis disorders in autoimmune diseases, as well as non-autoimmune diseases, in particular viral infection (1, 5, 10). The coagulation system and innate immunity are coordinately activatedandhighly integratedduringvenousandarterial thrombus formation and progression (10). Platelet-neutrophil interactions at the site of deep vein thrombosis formation were found to induce NETosis, and to be of substantial relevance for thrombogenesis in thecontextofdeepvein thrombosis ingeneral.Barbosa da Cruz et al. (1) revealed that NE/DNA complexes in NETs play a central role in a mechanism that results in severe fibrinolytic failure. NE forms a tight complex with DNA that strongly impairs its inhibition by the 1-proteinase inhibitor ( 1-PI) (1). In this way, NE highly degrades plasminogen without generating plasmin, which in turn leads to the production of antifibrinolytic plasminogen fragments. NETs can therefore serve as a platform for NE-mediated activation of intravascular coagulation in vivo (1, 10). With regard to the interaction of plasmin with NETs, the serine proteases, thrombin and plasmin, were also found to interact with DNA, and bound to NETs in vitro (1, 9). In this respect, it is interesting to note that thrombin and plasmin, like NE, belong to the vast family of S1 peptidases, which share an overall similar structure and folding (1). Thrombin can induce neutrophil chemotaxis and aggregation at submicromolar concentrations. Moreover, plasmin has been shown to cause neutrophil aggregation and adhesion to the endothelial surface in vitro at submicromolar concentrations. Lim et al. (8) observed reduced histone (H2B, H3, and H4) and neutrophil elastase levels with the addition of thrombin and plasmin. While Ryan et al. (9) did not observe neutrophil lysis after treatment with plasmin, they speculated that thrombin and plasmin displace NE from NETs, thus allowing NE to undergo auto-proteolysis. Alternatively, NE could be directly proteolyzed by thrombin and plasmin, as was observed for the histones. Lim et al. (8) showed that thrombin and plasmin alter the NET proteome and concluded that NETs and their proteic byproducts are regulated according to physiological conditions, which affects their roles in inflammation and host response during viral infection. Further investigations on NETome dynamics are needed to circumscribe these interactions. Elastase-mediated activation of SARS-CoV-2 was originally reported by Taguchi and co-workers (11), and the potentially significant implications of elastase for viral pathogenesis have been proposed (3, 11). NE is one of the most abundant NET byproduct proteins identified in a recent proteome analysis (8). Human NE is a granular serine protease with broad substrate specificity, expressed and stored in human neutrophils, released upon neutrophil activation, and involved primarily in host defense. Thus NE attacks proteins of invading microorganisms, but enables the hydrolyzation of proteins in the host extracellular matrix, such as collagen-IV and elastin. As a consequence, NE plays a role in degenerative and inflammatory diseases. During the phagocytosis of such foreign substances as pathogen-derived compounds, elastase as other proteins are also excreted into the surrounding extracellular environment, where the activity of elastase is regulated by inhibitors (i.e., 1-PI). NE could be responsible, in part, for the high multiplication of SARS-CoV-2-mediated infection, by means of multiple mechanisms (3, 11). First, like other proteases (trypsin, TMPRSS2, or plasmin), NE enables S protein cleavage and entry into cells directly from the cell surface (11). Wild-type human SARS-CoV-2 prefers cell-surface entry, in contrast to cathepsin-mediated endosomal cell entry (11). This multiplies its infectivity. Second, NE is a known activator of epithelial Na channel (ENaC). Intact furin activity is necessary for full elastase-induced ENaC activation (3). Caldwell et al. (4) showed that NE activates near silent ENaC and increases airway epithelial Na transport. NE, which is found in microgram quantities in the lungs of cystic fibrosis patients, activates ENaC via cleavage of the subunit, as observed with plasmin. This results in Na hyperabsorption, reduced airway surface liquid height, and dehyPhysiol Rev 100: 1597–1598, 2020 Letter to the Editor doi:10.1152/physrev.00019. 2020