Neutrophil extracellular traps

A scanning electron microscope image of NETs engulfing fungal cells (Candida albicans) in an infected mouse lung. (Click on image for more details).

Neutrophil extracellular traps (NETs) are networks of extracellular fibers, primarily composed of DNA from neutrophils, which bind pathogens.^[1]

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.^[1]

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.^[2] NETs have also been documented in association with Plasmodium falciparum infections in children.^[3] NETs disarm pathogens with antimicrobial proteins such as neutrophil elastase and histones that are bound to the DNA. 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. 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.^[4] 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.^[5] NETs also have been shown to be associated with the production of IgG antinuclear double stranded DNA antibodies in children infected with falciparum malaria.^[3]

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 ^[1]. However, under flow conditions, NETs can form much larger structures, hundreds of nanometers in length and width.^[6]

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) ^[6]

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

References

1. ^ ^{a b c} Brinkmann, Volker; Ulrike Reichard, Christian Goosmann, Beatrix Fauler, Yvonne Uhlemann, David S. Weiss, Yvette Weinrauch, Arturo Zychlinsky (2004-03-05). "Neutrophil Extracellular Traps Kill Bacteria" (HTML, PDF). Science (AAAS) 303 (5663): 1532–1535. doi:10.1126/science.1092385. PMID 15001782. http://www.sciencemag.org/cgi/content/full/303/5663/1532. Retrieved 2007-04-09. 
2. ^ Urban, CF; Reichard U, Brinkmann V, Zychlinsky A (April 2006). "Neutrophil extracellular traps capture and kill Candida albicans yeast and hyphal forms". Cell Microbiol. 8 (4): 668–76. doi:10.1111/j.1462-5822.2005.00659.x. PMID 16548892. 
3. ^ ^{a b} Baker, V; Godwin E. Imade, Norman B. Molta, Pallavi Tawde, Sunday D. Pam, Michael O. Obadofin, Soloman A. Sagay, Daniel Z. Egah, Daniel Iya,Bangmboye B. Afolabi, Murray Baker, Karen Ford, Robert Ford, Kenneth H. Roux, and Thomas C. S. Keller (February 2008). "Cytokine-associated neutrophil extracellular traps and antinuclear antibodies in Plasmodium falciparum infected children under six years of age". Malaria Journal. 7 (41): 41. doi:10.1186/1475-2875-7-41. PMC 2275287. PMID 18312656. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2275287. 
4. ^ Hakkim A, Fürnrohr BG, Amann K, Laube B, Abed UA, Brinkmann V, Herrmann M, Voll RE, Zychlinsky A. (2010). "Impairment of neutrophil extracellular trap degradation is associated with lupus nephritis". Proc Natl Acad Sci U S A 107 (21): 9813–8. doi:10.1073/pnas.0909927107. PMC 2906830. PMID 20439745. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2906830. 
5. ^ Gupta, AK; Hasler P, Holzgreve W, Gebhardt S, Hahn S. (November 2005). "Induction of neutrophil extracellular DNA lattices by placental microparticles and IL-8 and their presence in preeclampsia". Hum Immunol 66 (11): 1146–54. doi:10.1016/j.humimm.2005.11.003. PMID 16571415. 
6. ^ ^{a b} Clark, SR; Ma AC, Tavener AS, McDonald B, Goodarzi Z, Kelly MM, Patel KD, Chakrabarti S, McAvoy E, Sinclair GD, Keys EM, Allen-Vercoe E, DeVinney R, Doig CJ, Green FHY and Kubes P (2007). "Platelet Toll-Like Receptor-4 Activates Neutrophil Extracellular Traps to Ensnare Bacteria in Endotoxemic and Septic Blood". Nature Medicine 13 ((4)): 463–9. doi:10.1038/nm1565. PMID 17384648. http://www.nature.com/nm/journal/v13/n4/pdf/nm1565.pdf.