Dying for a cause: NETosis, mechanisms behind an antimicrobial cell death modality

REMIJSEM et al.
Cell Death Differ. 2011 April; 18(4): 581–588.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3131909/

Neutrophil extracellular traps (NETs) are composed of nuclear chromatin, associated mainly with nuclear histones and many granular antimicrobial proteins, as well as some cytoplasmic proteins.

NETs are formed in response to a variety of pro-inflammatory stimuli, such as LPS, IL-8 and TNF, as well as by various microorganisms and pathogens. Certain pathogens seem to have developed strategies to evade NETs, such as expression of DNases or modification of cell wall structures.

NETs are formed in the context of cell death. In contrast to apoptotic cells, NETotic cells do not display eat-me signals such as PS before plasma membrane disruption, preventing their pre-emptive clearance by phagocytes. In contrast to apoptosis or programmed necrosis (necroptosis), both the nuclear and granular membranes disintegrate during NETosis, but plasma integrity is maintained.

The presence of histones in NETs further indicates that nuclear—not mitochondrial—chromatin is the major constituent of NETs.

A word of caution about the in vivo analysis of NETosis is in order. It was recently reported that fibrin cannot be distinguished from NETs by scanning electron microscopy (SEM), that is, on the basis of morphological criteria. Moreover, both NETs and fibrin clots show DNAse-I breakdown, making it unlikely that they can be distinguished in this way. In this context, purity of neutrophil preparations is a serious challenge because contamination of neutrophil preparations with platelets can lead to misinterpretation of the process of NETosis. Therefore, in vivo data on NET formation obtained by EM and DNase-I digestion is unlikely to provide unequivocal evidence for NETosis. Consequently, fluorescence microscopy of NET markers and immunohistochemical analysis of TEM data have become almost indispensible.

Although the regulation of subcellular events during NETosis remains unclear, increasing evidence indicates that the collapse of the nuclear envelope during NETosis and concurrent intracellular chromatin decondensation are regulated by interplay between histone citrullination, superoxide production and autophagy.