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 may or may 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). NETosis NET activation and release, or NETosis, is a dynamic process that can come in two forms, suicidal and vital NETosis. Overall, many of the key components of the process are similar for both types of NETosis, however, there are key differences in stimuli, timing, and ultimate result. Activation pathway The full NETosis activation pathway is still under investigation but a few key proteins have been identified and slowly a full picture of the pathway is emerging. The process is thought to begin with NADPH oxidase activation of protein-arginine deiminase 4 (PAD4) via reactive oxygen species (ROS) intermediaries. PAD4 is responsible for the citrullination of histones in the neutrophil, resulting in decondensation of chromatin. A NADPH oxidase–independent form of NETosis, relying solely on mitochondrial-derived ROS, has been also been described. Azurophilic granule proteins such as myeloperoxidase (MPO) and neutrophil elastase (NE) then enter the nucleus and further the decondensation process, resulting in the rupture of the nuclear envelope. The uncondensed chromatin enters the cytoplasm where additional granule and cytoplasmic proteins are added to the early-stage NET. The result of the process then depends on which NETosis pathway is activated. Suicidal NETosis Suicidal NETosis was first described in a 2007 study that noted that the release of NETs resulted in neutrophil death through a different pathway than apoptosis or necrosis. In suicidal NETosis, the intracellular NET formation is followed by the rupture of the plasma membrane, releasing it into the extracellular space. This NETosis pathway can be initiated through activation of toll-like receptors (TLRs), Fc receptors, and complement receptors with various ligands such as antibodies, PMA, and so on. The current understanding is that upon activation of these receptors, downstream signaling results in the release of calcium from the endoplasmic reticulum. This intracellular influx of calcium in turn activates NADPH oxidase,