SARS-CoV-2 SPIKE BINDS FIBRINOGEN-INDUCING ABNORMAL INFLAMMATORY BLOOD CLOTS

ABSTRACT

Background: Life threatening thrombotic events involving both the arterial and venous systems are prominently present in SARS-CoV-2 infected individuals presenting with severe COVID-19. Abnormal clotting also occurs in asymptomatically or mildly infected individuals and in people experiencing post-acute sequelae of SARS-CoV-2 infection (PASC). Clinical management of this clotting disorder has proven difficult in part because these fibrin clots are highly resistant to plasmin-mediated fibrinolysis. Methods: An array of different binding, biochemical, microscopic, and in vivo assays were performed in these studies. All experiments were performed at least three times in triplicate and reported differences were shown to be statistically significant. Results: We find that SARS-CoV-2 Spike directly binds to the terminal clotting factors, fibrinogen and fibrin (Kd of 5.3 μ M and 0.4 μ M respectively). Mixing Spike and plasma accelerates fibrin polymerization. Scanning electron microscopy reveals an abnormal clot structure with finer, denser, and roughened fibrin fibers. Scanning peptide competition assays indicate Spike binds fibrin at three sites 1) the plasmin cleavage site needed for fibrinolysis;2) a site involved in innate immune signaling via fibrin binding to Complement Receptor 3 (CR3);and 3) a site with no known function. Examination of mice injected 24h earlier with Spike pseudotyped HIV-ΔEnv virions reveals extensive intra-and extravascular fibrin deposition in the lung accompanied by endothelial activation, loss of tight junctions, increased influx of macrophages, and the generation of high levels of reactive oxygen species. This thromboinflammatory response is not observed when Bald virions are injected or when Spike pseudotyped virions are injected into mice lacking fibrinogen. Intriguingly, these Spike-induced proinflammatory effects are blocked by an anti-fibrin monoclonal antibody, 5B8, which interferes with fibrin binding to CR3. Conclusion: Our findings reveal that the SARS-CoV-2 Spike protein binding to fibrinogen/fibrin results in the formation of structurally abnormal, fibrinolysis-resistant blood clots whose inflammatory effects are effectively neutralized by a specific fibrin-targeting monoclonal antibody. While COVID-19 clotting was thought to occur as a result of systemic inflammation, our findings suggest clotting during SARS-CoV-2 infection in fact is a driver of inflammation. Targeting fibrin could lead to novel therapeutic approaches for patients with acute COVID-19 and PASC.