@article {Apostolidis2021.05.01.442279, author = {Sokratis A. Apostolidis and Amrita Sarkar and Heather M. Giannini and Rishi R. Goel and Divij Mathew and Aae Suzuki and Amy E. Baxter and Allison R. Greenplate and C{\'e}cile Alanio and Mohamed Abdel-Hakeem and Derek A. Oldridge and Josephine Giles and Jennifer E. Wu and Zeyu Chen and Yinghui Jane Huang and Ajinkya Pattekar and Sasikanth Manne and Oliva Kuthuru and Jeanette Dougherty and Brittany Weiderhold and Ariel R. Weisman and Caroline A. G. Ittner and Sigrid Gouma and Debora Dunbar and Ian Frank and Alexander C. Huang and Laura A. Vella and The UPenn COVID Processing Unit and John P. Reilly and Scott E. Hensley and Lubica Rauova and Liang Zhao and Nuala J. Meyer and Mortimer Poncz and Charles S. Abrams and E. John Wherry}, title = {Signaling through FcγRIIA and the C5a-C5aR pathway mediates platelet hyperactivation in COVID-19}, elocation-id = {2021.05.01.442279}, year = {2021}, doi = {10.1101/2021.05.01.442279}, publisher = {Cold Spring Harbor Laboratory}, abstract = {Patients with COVID-19 present with a wide variety of clinical manifestations. Thromboembolic events constitute a significant cause of morbidity and mortality in patients infected with SARS-CoV-2. Severe COVID-19 has been associated with hyperinflammation and pre-existing cardiovascular disease. Platelets are important mediators and sensors of inflammation and are directly affected by cardiovascular stressors. In this report, we found that platelets from severely ill, hospitalized COVID-19 patients exhibit higher basal levels of activation measured by P-selectin surface expression, and have a poor functional reserve upon in vitro stimulation. Correlating clinical features to the ability of plasma from COVID-19 patients to stimulate control platelets identified ferritin as a pivotal clinical marker associated with platelet hyperactivation. The COVID-19 plasma-mediated effect on control platelets was highest for patients that subsequently developed inpatient thrombotic events. Proteomic analysis of plasma from COVID-19 patients identified key mediators of inflammation and cardiovascular disease that positively correlated with in vitro platelet activation. Mechanistically, blocking the signaling of the FcγRIIa-Syk and C5a-C5aR pathways on platelets, using antibody-mediated neutralization, IgG depletion or the Syk inhibitor fostamatinib, reversed this hyperactivity driven by COVID-19 plasma and prevented platelet aggregation in endothelial microfluidic chamber conditions, thus identifying these potentially actionable pathways as central for platelet activation and/or vascular complications in COVID-19 patients. In conclusion, we reveal a key role of platelet-mediated immunothrombosis in COVID-19 and identify distinct, clinically relevant, targetable signaling pathways that mediate this effect. These studies have implications for the role of platelet hyperactivation in complications associated with SARS-CoV-2 infection.One-sentence summary The FcγRIIA and C5a-C5aR pathways mediate platelet hyperactivation in COVID-19Competing Interest StatementScEH has received consultancy fees from Sanofi Pasteur, Lumen, Novavax, and Merck for work unrelated to this report. ACH is a consultant for Immunai. EJW is consulting or is an advisor for Merck, Elstar, Janssen, Related Sciences, Synthekine and Surface Oncology. EJW is a founder of Surface Oncology and Arsenal Biosciences. EJW is an inventor on a patent (US Patent number 10,370,446) submitted by Emory University that covers the use of PD-1 blockade to treat infections and cancer.}, URL = {https://www.biorxiv.org/content/early/2021/05/03/2021.05.01.442279}, eprint = {https://www.biorxiv.org/content/early/2021/05/03/2021.05.01.442279.full.pdf}, journal = {bioRxiv} }