A recent study conducted at the University of Pennsylvania and the Children’s Hospital of Philadelphia, USA, has revealed the mechanism of platelet hyperactivation in critically ill coronavirus disease 2019 (COVID-19) patients.
According to the study findings, aberrant platelet aggregation and cardiovascular complications observed in COVID-19 patients are primarily mediated via tyrosine kinase Syk-induced immunoreceptor signaling as well as C5a/C5aR complement pathway signaling. The study is currently available on the bioRxiv* preprint server.
Acute respiratory distress syndrome (ARDS) and thrombosis are two major characteristics of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Cardiovascular complications due to hypercoagulation are associated with significant morbidity and mortality in hospitalized COVID-19 patients. Studies investigating mechanistic details of thrombosis in COVID-19 have indicated that hyperactivation of platelet and subsequent hyperactivation of blood coagulation pathway are the major causes of blood clot formation inside blood vessels, which is further associated with pulmonary embolism.
Besides inducing blood clot formation, platelets play vital roles in priming immune cells and mediating inflammation. In severe COVID-19 patients, an increased formation of platelet-monocyte aggregates has been observed.
In the current study, the scientists have characterized the platelet activation pathways in critically ill COVID-19 patients to identify the factors/pathways responsible for cardiovascular complications.
The study was conducted on confirmed COVID-19 patients who were admitted to the hospital of the University of Pennsylvania. In addition, seropositive healthcare workers with COVID-19 negative status (COVID-19 recovered individuals) and healthy donors were also enrolled.
Peripheral blood samples collected from all participants were processed for platelet isolation. Afterward, in vitro platelet activation assays were carried out to examine the ability of patient-derived plasma samples to activate healthy donor-derived control platelets.
The scientists examined the surface expression of P-selectin as a marker of platelet activation. Among all study groups, the P-selectin expression was highest in COVID-19 patients compared to that in COVID-19 recovered individuals and healthy donors.
By experimentally stimulating platelet activation, the scientists observed that degranulation and upregulation of surface P-selectin were highest in healthy donors, followed by COVID-19 recovered individuals and COVID-19 patients. These observations indicate that although COVID-19 patients exhibit platelet hyperactivation at baseline, the platelets are functionally deprived of performing physiological activities.
Based on the clinical status of thrombosis and comorbidities, the COVID-19 patients included in the study were administered with different types of anticoagulation treatments, including full-dose, standard-of-care, and prophylactic dose treatments. The scientists utilized this difference in anticoagulation treatment to further investigate the surface expression of P-selection. Their analysis revealed that patients receiving full-dose treatment had the highest surface expression of P-selection.
Similarly, patients with risk factors for cardiovascular disease exhibited higher P-selectin expression than those without risk factors. These observations indicate that platelet hyperactivation in COVID-19 patients is associated with full-dose anticoagulation treatment and/or presence of cardiovascular disease risk factors.
Mechanism of platelet hyperactivation
To determine the mechanism of platelet hyperactivation, the scientists incubated patient-derived plasma samples with healthy donor-derived control platelets and examined the surface expression of P-selectin and LAMP-3, which a component of dense granules in platelet cytoplasm. They also examined the platelet surface expression of the immunoreceptor FcγRIIa and the Complement Receptor C3aR to evaluate the impacts of immune complexes on platelet activation.
Interestingly, the treatment with patient-derived plasma resulted in increased expressions of FcγRIIa and LAMP-3 on the surface of control platelets. However, no change in P-selectin expression was observed. These observations highlight the event of preferential platelet activation by COVID-19 patient plasma.
By comparing expression profiles of all tested platelet surface components with several clinical variables (blood cell count, levels of d-dimer, ferritin, and C-reactive protein, body mass index, and titers of anti-SARS-CoV-2 antibodies), the scientists identified the strongest positive correlation between blood ferritin level and platelet-activating ability of patient plasma.
By comparing LAMP-3 surface expression between patients with and without clinical thrombosis, the scientists revealed that the plasma collected from patients prior to a blood clotting event had the highest ability to induce control platelet activation.
By analyzing a number of analytes in blood, the scientists observed that circulating inflammatory mediators, neutrophil activity, and proteins associated with cardiovascular complications are primarily associated with the platelet-activating ability of patient plasma.
Because of the observed correlation of immunological and inflammatory pathways with platelet activation, the scientists further extended their analysis by blocking the FcγRIIa-Syk and C5a-C5aR signaling pathways on platelets. The experimental inhibition of these pathways by antibody-mediated neutralization, IgG depletion, or small molecule Syk inhibitor (fostamatinib) resulted in inhibition of platelet activation by patient plasma and prevention of platelet aggregation in the photochemically injured epithelium.
The study identifies FcγRIIa-Syk and C5a-C5aR signaling pathways as major mediators of platelet hyperactivation in critically ill COVID-19 patients. Therapeutic interventions targeting these pathways could be beneficial in preventing thrombotic events in COVID-19.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.