Complement and tissue factor-enriched neutrophil extracellular traps are key drivers in COVID-19 immunothrombosis

Emerging data indicate that complement and neutrophils are involved in the maladaptive host immune response that fuels hyper-inflammation and thrombotic microangiopathy increasing the mortality rate in coronavirus disease 2019 (COVID-19). Here, we investigated the interaction between complement and the platelet/neutrophil extracellular traps (NETs)/thrombin axis, using COVID-19 clinical samples, cell-based inhibition studies and NETs/human aortic endothelial cell (HAEC) co-cultures. Increased plasma levels of NETs, TF activity and sC5b-9 were detected in patients. Neutrophils yielded high tissue factor (TF) expression and released NETs carrying functionally active TF. Confirming our ex vivo findings, treatment of control neutrophils with COVID-19 platelet-rich plasma generated TF-bearing NETs that induced thrombotic activity of HAEC. Thrombin or NETosis inhibition or C5aR1 blockade attenuated platelet-mediated NET-driven thrombogenicity. Serum isolated from COVID-19 patients induces complement activation in vitro, which is consistent with high complement activity in clinical samples. Complement inhibition at the level of C3 with compstatin Cp40 disrupted TF expression in neutrophils. In conclusion, we provide a mechanistic basis that reveals the pivotal role of complement and NETs in COVID-19 immmunothrombosis. This study supports emerging strategies against SARS-CoV-2 infection that exploit complement therapeutics or NETosis inhibition.