Platelet Activating Immune Complexes Identified in COVID-19 Associated Coagulopathy

Thrombosis is a prominent feature of coronavirus disease 2019 (COVID-19) and often occurs in patients who are critically ill; however, the mechanism is unclear. This COVID-19 associated coagulopathy (CAC) shares features with heparin-induced thrombocytopenia (HIT), including mild thrombocytopenia and thrombosis. We thus tested 10 CAC patients for anti-PF4/heparin antibodies and functional platelet activation. HIT was excluded in all samples based on anti-PF4/heparin antibody and serotonin release assay results. Of note, 6 CAC patients demonstrated platelet activation by the serotonin release assay that was inhibited by FcγRIIA receptor blockade, confirming an IgG-specific immune complex (IC)-mediated reaction. Platelet activation was independent of heparin, but inhibitable by both therapeutic and high dose heparin. All 6 samples were positive for IgG-specific antibodies targeting the receptor binding domain (RBD) or the spike protein of the SARS-CoV-2 virus. These samples were additionally characterized by significant endothelial activation, shown by increased von Willebrand factor antigen and activity. ADAMTS13 activity was not severely reduced, and ADAMTS13 inhibitors were not present, ruling out thrombotic thrombocytopenic purpura. Our study thus identifies platelet-activating ICs as a mechanism that contributes to CAC thrombosis. Scientific Category Thrombosis and Hemostasis Key Points Patients with COVID-19 thrombosis have immune complexes that activate platelets through FcγRIIA signalling Patients with COVID-19 thrombosis demonstrate increased VWF antigen and activity that is not related to severe ADAMTS13 reduction