Inhibition of COVID-19 associated acute fibrin deposition in lung

National Institute Of Allergy And Infectious Diseases

Investigators

Abstract

COVID-induced lung fibrosis is a major confounding factor associated with the current pandemic related death. Understanding the mechanism of COVID-associated lung fibrosis and developing a therapeutic treatment are an urgent public health need to reduce the current pandemic-associated death. Through proteomic analyses of bronchoalveolar lavage fluid (BALF) from COVID patients, we found dramatically elevated levels of fibrinogen and prothrombin in the acute COVID lung fluid compared to the healthy donors. Using primary human bronchoalveolar epithelial cells as models, we discovered that SARS-CoV-2 infected primary lung cells but not other SARS-CoV-2 susceptible cells, including Vero and 293T cells, induced fibrin to clot in the absence of many plasma coagulation factors. Many fibrin fibers induced by SARS-CoV-2 infection originated from lung cells. These infected lung cell induced fibrin clotting occurred with all strains of SARS-CoV-2 pseudoviruses, including omicron, as well as the circulating replication competent SARS-CoV-2 strains tested. They are indistinguishable from those thrombin clots and can be inhibited by pharmacological direct thrombin inhibitors. Importantly, infected lung cells triggered fibrin clotting in 3 of 4 acute but not recovered COVID nor healthy BALF. We suggest the viral infection activated members of cell surface expressed transmembrane serine proteases to directly activate prothrombin for fibrin clotting, as evidenced from the shedding of one of the transmembrane proteases, ST14, in response to the infection, and both recombinant catalytic matriptase and human airway trypsin-like protease activated prothrombin for fibrin clot formation. Our findings revealed a lung cell-mediated fibrosis triggered by SARS-CoV-2 infection that is independent of plasma coagulation, and suggest the need to focus therapeutic treatments to respiratory airway rather than blood vessels to mitigate COVID-induced lung fibrosis. As COVID-19 evolves from pandemic to reoccurring seasonal infections, the primary health care measures to counter SARS-CoV-2 infections include vaccines and antiviral drugs. While COVID vaccines failed to provide sterile immunity against SARS-CoV-2 infections, they reduced severe COVID diseases. Understanding the mechanism of this protection is key to developing therapeutic treatment against severe COVID. Early publications suggested COVID vaccines reduced overt immune responses associated with SARS-CoV-2 infections, thus lessened pulmonary immunopathology. However, vaccination also benefited immune compromised population. Our work focused the protective mechanisms of SARS-CoV-2 vaccines against severe COVID-19 diseases. Early autopsy data showed the presence of extensive fibrosis in diseased lungs. However, clinical use of anticoagulant, including low molecular weight heparin, failed to mitigate severe COVID-associated mortality. We recently introduced a model for severe COVID-associated pulmonary hypercoagulation based on SARS-CoV-2 infection-induced fibrin deposition and showed it correlated with the disease severity. Unsing this model, we investigated the influence of vaccination to the viral-induced fibrin deposition. In contrary to our belief, plasma coagulation indices including prothrombin time (PT), partial thromboplastin time (PTT) as well as plasma concentrations of fibrinogen and prothrombin remained similar between COVID and healthy, or vaccinated and non-vaccinated COVID groups. Interestingly, vaccination reduced pulmonary inflammation and plasma infiltrations, resulted in lower pulmonary fibrinogen, prothrombin and D-dimer concentrations. Importantly, vaccinations protected against the viral-induced fibrin formation, suggesting a protective mechanism of SARS-Co-2 vaccine is to reduce the risk pulmonary fibrin deposition. The benefit of vaccine appears two-fold. First, vaccine reduces the viral load and, consequently, less viral-induced prothrombin activation. Second, vaccine reduces infiltration of coagulation components into infected lungs.