Platelet activation by SARS-CoV-2 implicates the release of active tissue factor by infected cells

Abstract

Platelets are hyper-activated in COVID-19. However, the mechanisms promoting platelet activation by SARS-CoV-2 are not well understood. This may be due to inherent challenges at discriminating the contribution of viral versus host components produced by infected cells. This is particularly true for enveloped viruses and extracellular vesicles, as they are concomitantly released during infection and share biophysical properties. To study this, we evaluated whether SARS-CoV-2 itself, or components derived from SARS-CoV-2-infected human lung epithelial cells, could activate isolated platelets from healthy donors. Activation was measured by the surface expression of P-selectin and the activated conformation of integrin αIIbβ3, degranulation, aggregation under flow conditions and the release of extracellular vesicles. We find that neither SARS-CoV-2 nor purified Spike activate platelets. In contrast, TF produced by infected cells was highly potent at activating platelets. This required trace amounts of plasma containing the coagulation factors FX, FII and FVII. Robust platelet activation involved thrombin and the activation of protease-activated receptor (PAR)-1 and -4 expressed by platelets. Virions and extracellular vesicles were identified by electron microscopy. Through size-exclusion chromatography, TF activity was found to be associated with virus or extracellular vesicles, which were indistinguishable. Increased TF mRNA expression and activity were also found in lungs in a murine model of COVID-19 and in plasma of severe COVID-19 patients, respectively. In summary, TF activity from SARS-CoV-2-infected cells activates thrombin, which signals to PARs on platelets. Blockade of molecules in this pathway may interfere in platelet activation and coagulation that is characteristic of COVID-19.

Cytokines and Lipid Mediators of Inflammation in Lungs of SARS-CoV-2 Infected Mice

“…Considering that endothelial cells can be infected by SARS-CoV-2 (39) and that multiple inflammatory cytokines are produced in response to infection, the observation that TF expression is induced during SARS-CoV-2 infection is not without surprise. In fact, a recent report further indicates that TF is released during SARS-CoV-2 infection, triggering platelet activation in the presence of residual amounts of plasma (40). D-dimer concentration in blood was maximal on day 1 post-infection suggesting that clotting issues were occurring very soon after infection.…”

Antiplatelet therapy for patients with COVID-19: Systematic review and meta-analysis of observational studies and randomized controlled trials

“…However, thrombin is not only a central enzyme involved in coagulation cascades but also a potent inducer of platelet activation ( 78 , 79 ). In a more recent study, the TF/thrombin pathway was found to be pivotal for platelet activation in an ex vivo SARS-CoV-2 infection model ( 80 ). The authors concluded that TF activity from SARS-CoV-2-infected cells activates thrombin, which signals to protease-activated receptors (PARs) on platelets ( 80 ).…”

“…In a more recent study, the TF/thrombin pathway was found to be pivotal for platelet activation in an ex vivo SARS-CoV-2 infection model ( 80 ). The authors concluded that TF activity from SARS-CoV-2-infected cells activates thrombin, which signals to protease-activated receptors (PARs) on platelets ( 80 ). Taken together, it is plausible to speculate that the key upstream pathway that promotes platelet activation during SARS-CoV-2 infection is inhibited by heparin through disturbing thrombin ligation to platelet Glycoprotein Ib (GP Ib) and PARs ( 78 , 80 ), whereby the anticipated antithrombotic effects of aspirin and P2Y12 inhibitors in the above trials were diluted.…”

Good practice statements for antithrombotic therapy in the management of COVID‐19: Guidance from the SSC of the ISTH

“…Despite many in vitro studies showing that platelets are activated in hospitalized patients with COVID‐19 pneumonia, 81 multiple trials in moderate COVID‐19 patients showed no benefit of add‐on antiplatelet therapy to anticoagulants in improving clinical outcomes and conversely increased bleeding risk 74,75 . The REMAP‐CAP trial in critically ill COVID‐19 patients compared the use of add‐on therapy with either low‐dose aspirin or clopidogrel or other P2Y 12 inhibitors and found no benefit in improving the primary outcome of survival and freedom from organ support at 21 days with the different classes of antiplatelet agents 82 .…”