Understanding COVID-19-associated coagulopathy.

COVID-19-associated coagulopathy (CAC) is a life-threatening complication of SARS-CoV-2 infection. However, the underlying cellular and molecular mechanisms driving this condition are unclear. Evidence supports the concept that CAC involves complex interactions between the innate immune response, the coagulation and fibrinolytic pathways, and the vascular endothelium, resulting in a procoagulant condition. Understanding of the pathogenesis of this condition at the genomic, molecular and cellular levels is needed in order to mitigate thrombosis formation in at-risk patients. In this Perspective, we categorize our current understanding of CAC into three main pathological mechanisms: first, vascular endothelial cell dysfunction; second, a hyper-inflammatory immune response; and last, hypercoagulability. Furthermore, we pose key questions and identify research gaps that need to be addressed to better understand CAC, facilitate improved diagnostics and aid in therapeutic development. Finally, we consider the suitability of different animal models to study CAC.

Endothelium-protective, histone-neutralizing properties of the polyanionic agent defibrotide.

Neutrophil-mediated activation and injury of the endothelium play roles in the pathogenesis of diverse disease states ranging from autoimmunity to cancer to COVID-19. Neutralization of cationic proteins (such as neutrophil extracellular trap-derived [NET-derived] histones) with polyanionic compounds has been suggested as a potential strategy for protecting the endothelium from such insults. Here, we report that the US Food and Drug Administration-approved polyanionic agent defibrotide (a pleiotropic mixture of oligonucleotides) directly engages histones and thereby blocks their pathological effects on endothelium. In vitro, defibrotide counteracted endothelial cell activation and pyroptosis-mediated cell death, whether triggered by purified NETs or recombinant histone H4. In vivo, defibrotide stabilized the endothelium and protected against histone-accelerated inferior vena cava thrombosis in mice. Mechanistically, defibrotide demonstrated direct and tight binding to histone H4 as detected by both electrophoretic mobility shift assay and surface plasmon resonance. Taken together, these data provide insights into the potential role of polyanionic compounds in protecting the endothelium from thromboinflammation with potential implications for myriad NET- and histone-accelerated disease states.

Prothrombotic antiphospholipid antibodies in COVID-19.

Patients with coronavirus disease 19 (COVID-19) are at high risk for thrombotic arterial and venous occlusions. At the same time, lung histopathology often reveals fibrin-based occlusion in the small vessels of patients who succumb to the disease. Antiphospholipid syndrome (APS) is an acquired and potentially life-threatening thrombophilia in which patients develop pathogenic autoantibodies (aPL) targeting phospholipids and phospholipid-binding proteins. Case series have recently detected aPL in patients with COVID-19. Here, we measured eight types of aPL [anticardiolipin IgG/IgM/IgA, anti-beta-2 glycoprotein I IgG/IgM/IgA, and anti- phosphatidylserine/prothrombin (aPS/PT) IgG/IgM] in the sera of 172 patients hospitalized with COVID-19. We detected aPS/PT IgG in 24%, anticardiolipin IgM in 23%, and aPS/PT IgM in 18%. Any aPL was present in 52% of patients using the manufacturer's threshold and in 30% using a more stringent cutoff (≥40 units). Higher levels of aPL were associated with neutrophil hyperactivity (including the release of neutrophil extracellular traps/NETs), higher platelet count, more severe respiratory disease, and lower glomerular filtration rate. Similar to patients with longstanding APS, IgG fractions isolated from patients with COVID-19 promoted NET release from control neutrophils. Furthermore, injection of these COVID-19 IgG fractions into mice accelerated venous thrombosis. Taken together, these studies suggest that a significant percentage of patients with COVID-19 become at least transiently positive for aPL and that these aPL are potentially pathogenic.

Prothrombotic autoantibodies in serum from patients hospitalized with COVID-19.

Patients with COVID-19 are at high risk for thrombotic arterial and venous occlusions. Lung histopathology often reveals fibrin-based blockages in the small blood vessels of patients who succumb to the disease. Antiphospholipid syndrome is an acquired and potentially life-threatening thrombophilia in which patients develop pathogenic autoantibodies targeting phospholipids and phospholipid-binding proteins (aPL antibodies). Case series have recently detected aPL antibodies in patients with COVID-19. Here, we measured eight types of aPL antibodies in serum samples from 172 patients hospitalized with COVID-19. These aPL antibodies included anticardiolipin IgG, IgM, and IgA; anti-ß2 glycoprotein I IgG, IgM, and IgA; and anti-phosphatidylserine/prothrombin (aPS/PT) IgG and IgM. We detected aPS/PT IgG in 24% of serum samples, anticardiolipin IgM in 23% of samples, and aPS/PT IgM in 18% of samples. Antiphospholipid autoantibodies were present in 52% of serum samples using the manufacturer's threshold and in 30% using a more stringent cutoff (≥40 ELISA-specific units). Higher titers of aPL antibodies were associated with neutrophil hyperactivity, including the release of neutrophil extracellular traps (NETs), higher platelet counts, more severe respiratory disease, and lower clinical estimated glomerular filtration rate. Similar to IgG from patients with antiphospholipid syndrome, IgG fractions isolated from patients with COVID-19 promoted NET release from neutrophils isolated from healthy individuals. Furthermore, injection of IgG purified from COVID-19 patient serum into mice accelerated venous thrombosis in two mouse models. These findings suggest that half of patients hospitalized with COVID-19 become at least transiently positive for aPL antibodies and that these autoantibodies are potentially pathogenic.