Neutrophil extracellular traps in the pathology of cancer and other inflammatory diseases

Abstract

Neutrophil extracellular trap (NET) formation, first described in 2004 as a previously unknown strategy of neutrophils to fight microbes, has attracted an increasing interest in the research community. NETs are formed when neutrophils externalize their decondensed chromatin together with content from their azurophilic granules. In addition to their role in defense against microbes, NETs have been implicated as mediators of pathology in sterile inflammation, such as cancer and autoimmunity, and their potential as therapeutic targets is actively explored. However, targeting of NETs is challenging since the beneficial effects of their removal need to be balanced against the potential harmful loss of their function in microbial defense. Moreover, depending on the stimuli or species, NETs can be formed via distinct mechanisms and are not always made up of the same components, making direct comparisons between various studies challenging. This review focuses on the role of NETs in cancer-associated pathology, such as thrombosis, organ dysfunction, and metastasis. Different strategies to target NETs, by either preventing their formation or degrading existing ones, are also discussed.

Keywords: cancer-associated pathology; inflammation; neutrophil extracellular traps; thrombosis.

Graphical abstract

FIGURE 1.

Publications on neutrophil extracellular traps (NETs) during 2004–2021. Illustration of the steep increase in the number of publications including NETs from the first publication in 2004 until December 2021. Key findings in research on NETs are highlighted. KO, knockout; PAD4, peptidylarginine deiminase 4; RA, rheumatoid arthritis.

FIGURE 2.

Structure of neutrophil extracellular traps (NETs). A: schematic illustration of NET components. Neutrophils externalize their decondensed chromatin together with content from their azurophilic granules, such as neutrophil elastase (NE) and myeloperoxidase (MPO). NETs have prothrombotic properties, and both activated platelets and fibrin can be found in this extracellular network. Coagulation factors such as tissue factor (TF) and factor XII have been described to interact with NETs. B: scanning electron microscope image showing a neutrophil (orange) capturing Helicobacter pylori bacteria (purple) in a NET. The image was kindly provided by Dr. Volker Brinkmann, Max Planck Institute for Infection Biology, Berlin, Germany. Image created with BioRender.com with permission.

FIGURE 3.

Neutrophil properties connected to neutrophil extracellular trap (NET) formation. Not all neutrophils form NETs, and proportions varying between 10% and 60% of the total neutrophil population have been reported. Enhanced NET formation has been shown in the population of low-density neutrophils (LDNs) and CD177⁺ neutrophils. Other factors that are believed to play a role are the neutrophil maturation stage, where neutrophils newly released from the bone marrow have a higher NET forming capacity, age of the host, where older individuals form fewer NETs than young individuals, and biological sex of the host, where females are more prone to form NETs than males. Image created with BioRender.com with permission.

FIGURE 4.

Mechanisms of neutrophil extracellular trap (NET) formation. Three mechanisms of NET formation have been described: lytic, viable, and mitochondrial NET formation, which differ in their kinetics, inducing stimuli, reactive oxygen species (ROS) dependence, and their cellular mechanisms. GM-CSF, granulocyte-macrophage colony-stimulating factor; LPS, lipopolysaccharides; ND, not determined; PMA, phorbol myristate acetate. Image created with BioRender.com with permission.

FIGURE 5.

The role of platelets in neutrophil extracellular trap (NET) formation. NET formation can be induced indirectly via platelets. Platelets are activated by, e.g., tumor-derived factors, sterile inflammation, or infections and can promote NET formation via soluble factors and direct platelet-neutrophil interactions. EDA, extradomain A; I/R, ischemia-reperfusion; LPS, lipopolysaccharides; TRALI, transfusion-related acute lung injury; vWF, von Willebrand factor. Image created with BioRender.com with permission.

FIGURE 6.

Neutrophil extracellular traps (NETs) in thrombosis. NETs promote thrombosis formation, stabilization, and coagulation in various ways: 1) NETs bind factors such as von Willebrand factor (vWF), fibronectin, and fibrinogen and provide a scaffold for platelet activation and aggregation. 2) NETs enhance thrombin generation and bind tissue factor (TF). 3) NETs stabilize thrombi with their histone-DNA backbone by increasing the mechanical stability of the fibrin network and via antifibrinolytic mechanisms. 4) NETs/cell-free DNA activate the intrinsic coagulation cascade by activating factor XII. PAD4, peptidylarginine deiminase 4. Image created with BioRender.com with permission.

FIGURE 7.

Neutrophil extracellular trap (NET) formation contributes to tumor-induced organ dysfunction. Tumor-secreted factors promote the formation of NETs in the vasculature of peripheral organs. Systemic NET formation leads to both impaired vascular perfusion and vessel damage due to the presence of cytotoxic NET components and obstructed blood flow, which can cause endothelial activation, immune cell infiltration into the tissue, inflammation, and organ dysfunction. Image created with BioRender.com with permission.

FIGURE 8.

Neutrophil extracellular traps (NETs) promote metastasis formation. NETs can promote metastasis via distinct mechanisms. 1) Sequestering of circulating tumor cells (CTCs) in the vasculature: NETs and CTCs have been shown to interact, e.g., via β1 integrins or carcinoembryonic Ag cell adhesion molecule (CEACAM)1. NETs promote tumor cell proliferation, migration, and invasion by acting on Toll-like receptor (TLR)9. In addition, platelets promote the binding of NETs to CTCs. 2) Premetastatic niche formation: NET-associated neutrophil elastase (NE) degrades the antitumorigenic factor thrombospondin-1 (TSP-1) in the extracellular matrix (ECM). Recruitment of neutrophils with increased potential for NET formation contributes to generation of a premetastatic niche in the lung. Furthermore, lung-resident mesenchymal stem cells (LMSCs) have been reported to stimulate NET formation in the lung in a complement component 3 (C3)-dependent manner. 3) Promoting recurrence by awakening of dormant tumor cells: This effect has been connected to NE- and matrix metalloproteinase 9 (MMP9)-dependent degradation of laminin, which induces tumor cell proliferation. Image created with BioRender.com with permission.

FIGURE 9.

Different options to therapeutically target neutrophil extracellular traps (NETs). 1) Inhibition of NET formation: Possible compounds are peptidylarginine deiminase (PAD) inhibitors, neutrophil elastase (NE) inhibitors, inhibitors of gasdermin D, and the use of anti-citrullinated protein antibodies. 2) Degradation and destabilization of already formed NETs: The use of DNase I, which cleaves the DNA backbone of NETs, or heparin, which dissociates histones from the NET chromatin backbone. LMWH, low-molecular weight heparin; tACPA, therapeutic anti-citrullinated protein antibody. Image created with BioRender.com with permission.

FIGURE 10.

Neutrophil extracellular trap (NET) formation and pathological consequences in individuals with cancer. NET formation in individuals with cancer can be induced by tumor-derived cytokines and further promoted by surgery and infections. NETs can promote pathological and fatal cancer-related effects such as thrombosis, metastasis, cancer recurrence, and organ damage. Image created with BioRender.com with permission.