NET and NETosis – new phenomenon in immunology

Neutrophils are one of the first cells of the immune system recruited to the site of infection, representing the host’s most effective and numerous front-line defenders. Recently, a novel antimicrobial mechanism of neutrophils has been described: upon activation, they release DNA and a subset of their granule content, forming neutrophil extracellular traps (NETs). These extracellular, chromatin structures, which contain histones and neutrophil granule proteins, can trap and kill a broad spectrum of microbes, including Gram-positive and Gram-negative bacteria, fungi, protozoa and viruses. Some of the pathogens, which are trapped and exposed to high local concentrations of antimicrobial compounds, employ strategies against NET binding, including surface modification and/or degradation of NET by DNases. It has been suggested that NETs are formed during active cell death, recently named NETosis. New data indicate that this novel mechanism of cell death requires interaction between three processes – reactive oxygen species generation, histone citrullination and autophagy – and significantly differs from previously known types of cell death, including apoptosis and necrosis. Moreover, the release of nuclear chromatin was also described for other types of cells – mast cells and eosinophils. Mast cells, like neutrophils, under certain conditions release nuclear chromatin and may undergo a similar active cell death program, while eosinophils release only mitochondrial chromatin, and its release does not lead to the death of these cells.