Do neutrophil extracellular traps contribute to the heightened risk of thrombosis in inflammatory diseases?

Abstract

Thrombotic events, both arterial and venous, are a major health concern worldwide. Further, autoimmune diseases, such as systemic lupus erythematosus, anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis, and antiphospholipid syndrome, predispose to thrombosis, and thereby push the risk for these morbid events even higher. In recent years, neutrophils have been identified as important players in both arterial and venous thrombosis. Specifically, chromatin-based structures called neutrophil extracellular traps (NETs) play a key role in activating the coagulation cascade, recruiting platelets, and serving as scaffolding upon which the thrombus can be assembled. At the same time, neutrophils and NETs are emerging as important mediators of pathogenic inflammation in the aforementioned autoimmune diseases. Here, we first review the general role of NETs in thrombosis. We then posit that exaggerated NET release contributes to the prothrombotic diatheses of systemic lupus erythematosus, ANCA-associated vasculitis, and antiphospholipid syndrome.

Keywords: Antiphospholipid syndrome; Lupus; Neutrophil extracellular traps; Thrombosis; Vasculitis.

Figure 1

Schematic representation of potential mechanisms by which neutrophil extracellular traps may promote thrombosis in systemic autoimmune diseases. First, a number of stimuli may promote NETosis in systemic autoimmune diseases including ribonucleoprotein (RNP)/anti-RNP complexes in systemic lupus erythematosus, anti-neutrophil cytoplasmic antibody (ANCA) engagement with surface proteinase 3 (PR3) in vasculitis, and the interaction of anti-beta-2 glycoprotein I (β₂GPI) with surface β₂GPI in antiphospholipid syndrome. The DNA component of NETs activates factor XII (FXII), initiating a cascade (along with factor XI) that ultimately leads to the formation of thrombin. Histones in NETs activate platelets and sequester certain anticoagulant molecules like thrombomodulin and protein C. Neutrophil serine proteases present in NETs, such as neutrophil elastase and cathespin G, cleave the anticoagulant molecules tissue factor pathway inhibitor (TFPI) and antithrombin, and also activate platelets through various pathways including protease-activated receptor 4. NETs also may present procoagulant tissue factor in some contexts. Finally, NETs serve as scaffolding for the assembly and aggregation of platelets and red blood cells (RBCs). NET: Neutrophil extracellular trap.

Figure 2

Antiphospholipid syndrome neutrophils are prone to “spontaneous” neutrophil extracellular trap release. Freshly-isolated neutrophils from a healthy control (A) or antiphospholipid syndrome (APS) patient (B) were seeded onto poly-lysine-coated coverslips and incubated in serum-free media for 2 h. Samples were then fixed with paraformaldehyde and stained with Hoechst 33342 (DNA = blue) and anti-neutrophil elastase (Abcam, green). Cells were not specifically permeabilized and neutrophil elastase staining is therefore primarily extracellular. These representative micrographs show more neutrophil extracellular trap release in the APS neutrophils, as determined by overlapping DNA and neutrophil elastase staining.