Neutrophil Extracellular Traps

Neutrophil extracellular traps (NETs) are networks of extracellular fibers, primarily composed of DNA from neutrophils, which bind pathogens.

It has long been known that neutrophils (our front-line of defence against infection) use two strategies to kill invading pathogens: engulfment of microbes and secretion of anti-microbials. In 2004, a novel third function was identified: formation of NETs, whereby neutrophils kill extracellular pathogens while minimizing damage to the host cells. Upon in vitro activation with the pharmacological agent phorbol myristate acetate (PMA), Interleukin 8 (IL-8) or lipopolysaccharide (LPS), neutrophils release granule proteins and chromatin to form an extracellular fibril matrix known as NETs through an active process.

NETs disarm pathogens with antimicrobial proteins such as neutrophil elastase and histones that are bound to the DNA. Analysis by immunofluorescence corroborated that NETs contained proteins from azurophilic granules (neutrophil elastase, cathepsin G and myeloperoxidase) as well as proteins from specific granules (lactoferrin) and tertiary granules (gelatinase), yet CD63, actin, tubulin and various other cytoplasmatic proteins were not. NETs provide for a high local concentration of antimicrobial components and bind, disarm, and kill microbes extracellularly independent of phagocytic uptake. In addition to their antimicrobial properties, NETs may serve as a physical barrier that prevents further spread of the pathogens. Furthermore, delivering the granule proteins into NETs may keep potentially injurious proteins like proteases from diffusing away and inducing damage in tissue adjacent to the site of inflammation.

High-resolution scanning electron microscopy has shown that NETs consist of stretches of DNA and globular protein domains with diameters of 15-17 nm and 25 nm, respectively. These aggregate into larger threads with a diameter of 50 nm. However, under flow conditions, NETs can form much larger structures, hundreds of nanometers in length and width.

More recently, it has also been shown that not only bacteria but also pathogenic fungi such as Candida albicans induces neutrophils to form NETs that capture and kill C. albicans hyphal as well as yeast-form cells. NETs have also been documented in association with Plasmodium falciparum infections in children. NETs might also have a deleterious effect on the host, because the exposure of extracellular histone complexes could play a role during the development of autoimmune diseases like lupus erythematosus. NETs could also play a role in inflammatory diseases, as NETs could be identified in preeclampsia, a pregnancy related inflammatory disorder in which neutrophils are known to be activated. NETs also have been shown to be associated with the production of IgG antinuclear double stranded DNA antibodies in children infected with falciparum malaria.

While it was originally proposed that NETs would be formed in tissues at a site of bacterial/yeast infection, NETs have also been shown to form within blood vessels during sepsis (specifically in the lung capillaries and liver sinusoids). Intra-vascular NET formation is tightly controlled and is regulated by platelets, which sense severe infection via platelet TLR4 and then bind to and activate neutrophils to form NETs. Platelet-induced NET formation occurs very rapidly (in minutes) and does not result in death of the neutrophils. NETs formed in blood vessels can catch circulating bacteria as they pass through the vessels. Trapping of bacteria under flow has been imaged directly in flow chambers in vitro and intravital microscopy demonstrated that bacterial trapping occurs in the liver sinusoids and lung capillaries (sites where platelets bind neutrophils).

These observations suggest that NETs might play an important role in the pathogenesis of infectious, inflammatory and thrombotic disorders.