Fig. 2

Neutrophil Extracellular Trap (NET) production by a neutrophil. Neutrophils are stimulated to form NETs by several microbial, inflammatory and sterile endogenous triggers. These bind onto cell surface receptors including Toll-like Receptor 4, cytokine and complement receptors. Receptor binding leads to increased calcium release from the endoplasmic reticululm, activating Protein Kinase C (PKC). This leads to activation of NADPH Oxidase on the cell membrane and lysosomes, forming superoxide which reacts with water and chloride to form hypochlorite. Hypochlorite activates Protein Arginine Deiminase 4 (PAD4) which translocates to the nucleus where it catalyses hypercitrullination of histones 3 and 4 [26]. This causes the histones to lose their positive charge and in doing so weakens their binding to DNA, leading to decondensation of chromatin. There is loss of plasma membrane integrity then decondensed chromatin and histones are expelled into the extracellular space where they form complexes with granule/cytosolic proteins such as myeloperoxidase, neutrophil elastase and calprotectin. Recent research suggests NET production is an end-point of numerous cell signalling pathways – not all of which require each of the above steps – dependent upon the stimulant used to induce NETosis [25].