Lipid-binding antiphospholipid antibodies trigger autoimmune signaling in severe COVID-19

Background: Antiphospholipid antibodies (aPL) are causally involved in the pathogenesis of the antiphospholipid syndrome (APS), but lipid-binding aPL are also frequently found in acute infections. While lipid-binding aPL are considered irrelevant for APS we have shown that they are prothrombotic in vivo and have delineated the underlying signaling pathway. COVID-19 associated coagulopathy has many similarities to severe and catastrophic APS e.g. stroke, venous and pulmonary clots, as well as microvascular thrombosis and aPL have been described in COVID-19. However, their role in COVID-19 is still poorly understood. Aims: In this study, we aimed to elucidate the role of lipid-binding aPL in severe COVID-19. Methods: B cells were analyzed using flow cytometry. aPL were measured by immunoassays including a home-made ELISA for lipidbinding aPL. Cultured cells were stimulated with immunoglobulins isolated from COVID-19 patients. Gene expression was measured by qRT-PCR, cellular procoagulant activity was determined by a clotting assay. In vivo thrombus formation was determined in the flowrestricted inferior vena cava mouse model. Results: We identified a distinct population of circulating B1a cells producing lipid-reactive aPL of the IgG isotype in COVID-19 patients. 43 of 53 COVID-19 patients tested positive for lipid-binding aPL. Positive patients had more often a critical clinical course and a higher mortality. In vitro, COVID-19 aPL induced inflammatory and prothrombotic pathways in monocytes and endothelial cells and rapidly decrypted cell surface tissue factor. These effects were dependent on a recently described signaling pathway involving lysobisphosphatidic acid (LBPA) presented by the endothelial protein C receptor (EPCR) on the cell surface. In vivo, COVID-19 aPL induced thrombus formation by this EPCR-LBPA-dependent signaling pathway. Conclusions: COVID-19 patients develop lipid-binding aPL produced by circulating B1a cells. These aPL are associated with a more severe course and mortality and show prothrombotic and inflammatory properties by targeting the previously delineated aPL autoimmune signaling pathway dependent on EPCR-LBPA.