Thromboembolic complication is common in severe coronavirus disease (COVID-19), leading to an investigation into the presence of prothrombotic antibodies akin to those found in heparin-induced thrombocytopenia (HIT). In a study of samples from 130 hospitalized patients collected 3.6 days after COVID-19 diagnosis, 80% had IgG antibodies recognizing complexes of heparin and platelet factor 4 (PF4/H), and 41% had antibodies inducing PF4-dependent P-selectin expression in CpG-treated normal platelets. Unlike HIT, both PF4/H-reactive and platelet-activating antibodies were found in COVID-19 patients regardless of recent heparin exposure. Notably, PF4/H-reactive IgG antibodies correlated with those targeting the receptor-binding domain (RBD) of the SARS-CoV-2 spike protein. Moreover, introducing exogenous RBD to or removing RBD-reactive IgG from COVID-19 plasma or IgG purified from COVID-19 plasma significantly reduced their ability to activate platelets. RBD-specific antibodies capable of platelet activation were cloned from peripheral blood B cells of COVID-19 patients. These antibodies possessed sequence motifs in the heavy-chain complementarity-determining region 3 (HCDR3) resembling those identified in pathogenic HIT antibodies. Furthermore, IgG+ B cells having these HCDR3 signatures were markedly expanded in severe COVID-19 patients. Importantly, platelet-activating antibodies present in COVID-19 patients were associated with a specific elevation of platelet α-granule proteins in the plasma and showed a positive correlation with markers for inflammation and tissue damage, suggesting functionality of these antibodies in patients. The demonstration of functional and structural similarities between certain RBD-specific antibodies in COVID-19 patients and pathogenic antibodies typical of HIT suggests a novel mechanism whereby RBD-specific antibodies might contribute to thrombosis in COVID-19.
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