Endothelial Cell–Activating Antibodies in COVID‐19

OBJECTIVE: While endothelial dysfunction has been implicated in the widespread thromboinflammatory complications of COVID‐19, the upstream mediators of endotheliopathy remain, for the most part, unknown. This study was undertaken to identify circulating factors contributing to endothelial cell activation and dysfunction in COVID‐19. METHODS: Human endothelial cells were cultured in the presence of serum or plasma from 244 patients hospitalized with COVID‐19 and plasma from 100 patients with non–COVID‐19–related sepsis. Cell adhesion molecules (E‐selectin, vascular cell adhesion molecule 1, and intercellular adhesion molecule 1 [ICAM‐1]) were quantified using in‐cell enzyme‐linked immunosorbent assay. RESULTS: Serum and plasma from COVID‐19 patients increased surface expression of cell adhesion molecules. Furthermore, levels of soluble ICAM‐1 and E‐selectin were elevated in patient serum and correlated with disease severity. The presence of circulating antiphospholipid antibodies was a strong marker of the ability of COVID‐19 serum to activate endothelium. Depletion of total IgG from antiphospholipid antibody–positive serum markedly reduced the up‐regulation of cell adhesion molecules. Conversely, supplementation of control serum with patient IgG was sufficient to trigger endothelial activation. CONCLUSION: These data are the first to indicate that some COVID‐19 patients have potentially diverse antibodies that drive endotheliopathy, providing important context regarding thromboinflammatory effects of autoantibodies in severe COVID‐19.