# Urokinase System in Pathogenesis of Pulmonary Fibrosis: A Hidden Threat of COVID-19
## 3 Discussion  
The post-COVID-19 sequuela in the form of pulmonary fibrosis is a ticking time bomb. Despite the apparent successful recovery from COVID-19, the development of pulmonary fibrosis is one of the most common complications and a serious mortality risk factor in post-COVID-19 syndrome. The early evidence suggests that even in asymptomatic individuals, pulmonary fibrosis may become one of the major long-term complications of COVID-19 [7]. Understanding the underlying mechanisms and searching for treatment options for pulmonary fibrosis is an urgent need in the post-COVID-19 era. In the current study, we found that uPAR expression was decreased in the lung tissue of COVID-19 patients, which could have led to the pro-fibrogenic accumulation of inactive uPA followed by IL-6 and ACE2 upregulation and EMT induction in lung epithelial cells. Consistently, uPAR deficiency in mice contributed to more severe pulmonary fibrosis, weight loss and poor survival, which was resolved by plasminogen treatment that substantially decreased lung fibrosis in the absence of uPAR. Malfunction of the fibrinolytic system during COVID-19 is accompanied by the development of disseminated intravascular coagulation and increased fibrin D-dimer levels [27,45]. Plasmin is a crucial enzyme in fibrinolysis, activated by other proteolytic enzymes, including uPA. uPA was first discovered in the late 1940s in urine, so the term urokinase was coined [46]. Active uPA generates fibrinolytic plasmin from plasminogen, and this reaction is amplified by uPAR on the cell surface since the latter stimulates the conversion of pro-uPA into uPA [21,22,47]. Moreover, uPA activation is also catalyzed by plasmin itself [24,25,26]. uPAR is a mobile GPI-anchored membrane receptor that assembles the so-called uPAR-interactome, orchestrating multiple ligand-receptor interactions on the cell surface [48]. Recently, uPAR has been shown to form dimers interacting simultaneously with uPA and vitronectin [49]. This finding deepens our understanding of the partners of the urokinase system that determine its cellular effects and functions in the body. uPA and uPAR participate in the pathogenesis of lung injury and pulmonary fibrosis [17,50], and their expression and function are subject to change in COVID-19. Bulk RNA-seq analysis of normal human bronchial epithelial cells infected with SARS-CoV2 in vitro demonstrated significant upregulation of _PLAUI_ and _PLAUIR_[51]. [PERSON] et al. found that both _PLAUI_ and _PLAUIR_ are significantly (\(-40\)-fold) downregulated in bronchoalveolar lavage fluid (BALF) of COVID-19 patients by bulk-transcriptomic data [52]. Another analysis of bulk BALF transcriptomes also demonstrated _PLAUIR_ downregulation in COVID-19 patients [51]. A more in-depth single-cell transcriptomic study of BALF by [PERSON] et al., however, revealed that _PLAU_ expression is strongly upregulated in severe COVID-19 cases as compared to healthy controls [53]. To resolve these controversies, here, we compared single-cell transcriptomic data obtained from warm postmortem lung biopsies of patients diseased from COVID-19-associated pneumonia and lung biopsies from individuals with pulmonary fibrosis and control samples from healthy donors [35,36]. COVID-19 and pulmonary fibrosis displayed important similarities in _PLAUI/PLAUR_ gene expression across epithelial and monocyte/macrophage cell lineages. Our findings demonstrate that the lung monocytes/macrophages serve as a main source of _PLAUR_ expression, which was significantly decreased in COVID-19 and PF (Figure 3A,C). In contrast, _PLAU_ expression was overall low in all monocyte/macrophage cell types in control lungs, but under the PF condition, specific alveolar and monocyte-derived macrophagesappeared to express _PLAUI_ (Figure 3B). Among all epithelial cells, only airway basal cells were detected to persistently express _PLAUI_ and _PLAUR_, and their expression was significantly decreased in COVID-19 and PF patients (Figure 1).