Extracellular Chromatin Traps Interconnect Cell Biology, Microbiology, and Immunology

appealing target for inhibition (or enhancement) of NETosis because the reaction pathway of this enzyme is understood in molecular detail.Leshner et al. (2012) provide an elegant illustration of PAD4’s potential to induce NETs. They demonstrate that overexpression of PAD4 in heterologous cells leads to chromatin decondensation and unfolding of NET-like chromatin. Induced PAD4 expression leads to histone deimination and the release of NET-like webs from U2OS and NIH 3T3 cells. Specifically, the authors show that deimination of arginines in the amino terminus of histone H3 reduces the bind-ing of the heterochromatin protein HP1β to the adjacent lysine 9 in H3. The disruption of higher order chromatin packing may provide a molecular switch that regulates NET release in NETosis.Neeli and Radic (2013) observed that two ancient regulatory enzymes from the protein kinase C (PKC) family exhibit opposite effects on PAD4 activity. The authors used PMA and ionophore, two compounds that induce NETosis, along with inhibitors of PKC activity, to identify PKCα as an inhibitor of PAD4 and PKCζ as a facilitator of PAD4-mediated histone deimination. The authors conclude that evolutionary pressure ensured precise regulation of histone deimination because NETs make important, yet potentially dangerous contributions to innate immunity.The important question of whether extracellular chromatin traps are involved in autoimmune disease is critically reviewed by two groups of scientists. Darrah and Andrade (2013) present an analytical comparison of different modes of cell death and contrast the potential contributions of apoptosis, necrosis, and NETosis to the release of nuclear autoantigens. The authors survey different possibilities that may link NETosis to the stimulation of the adaptive immune system in systemic autoimmune disorders. The large num-ber of autoantigens that are integral components of NETs provide a compelling argument for NETosis’ role in the pathogenesis of autoimmune disorders and the loss of immune tolerance.Knight et al. (2012) take a broad view to summarize evidence linking NETosis with specific aspects of autoimmune disorders. By highlighting recent findings from studies of small vessel vasculitis, psoriasis, gout, Felty’s syndrome, and systemic lupus, the authors suggest two mechanisms for how NETs could participate in disease pathogenesis. One mechanism involves the contribution of NETs to organ damage; the other sees NET proteins as the trigger or exac-erbating factor in autoimmunity. In particular, post-translational modifications of NET proteins are highlighted as features that make them into “altered self,” autoantigens that may break tolerance and initiate autoimmunity.In a scientist’s career, the often elusive goal is to discover a biological process that is both unexpected and, once revealed, of obvious impor -tance. In that case, the new observation will attract the attention of others and prove to be of crucial significance in several overlapping areas of science. Nearly a decade after the original discovery of neu-trophil extracellular traps (NETs) by Brinkmann et al. (2004) , NETs have been firmly established as a fundamental biological mechanism used by neutrophils to respond to infections. Scores of additional reports confirmed and extended the initial observations to provide insights into the contributions of NETs to bacterial, fungal, viral, and protozoan infections (Brinkmann and Zychlinsky, 2012). NET release was shown to be the regulated outcome of a programed cell death process designated NETosis, and NETs were linked to autoim-munity and cardiovascular disease. As Editors, we are pleased by the enthusiastic response of 20 groups of scientists from 4 continents who participated in our effort to summarize, evaluate, and extend understanding of NETosis. This booklet captures some of the excite-ment that was shared with us by the authors.Goldmann and Medina (2013) introduce the topic of NETosis by describing aspects that differentiate NETosis from other forms of cell death and by reminding the readers that extracellular traps are produced by eosinophils, mast cells, and even monocytes/macrophages in addition to neutrophils. Thus, NETosis is more accurately called ETosis. In alternative manner, DNA for ETs can be derived from the nucleus or from mitochondria and the DNA can be released without completely inactivating all functions of the cell releasing the DNA. The authors provide a careful summary of steps in ETosis and illustrate the process with electron micrographs, while reminding us that many open questions still remain.Brinkmann et al. (2013) present a semi-automated method for enumerating cells that traverse sequential stages in NETosis. This useful and easily adaptable method by the original discov-erers of NETs relies on dual channel fluorescence and compares binding of anti-chromatin antibodies relative to staining with a DNA-intercalating dye. Image analysis computes the percentage of NETting neutrophils. The authors give examples of Toll-like receptor stimuli, crystals, and cytokines that induce NETosis. The new method should be ideally suited to high throughput screening for drugs that affect the efficiency of NET release.Additional contributors discuss mechanistic features of NETosis. Rohrbach et al. (2012) focus on the role of peptidylarginine deimi-nase 4 (PAD4) in the regulation of NETosis. By reviewing PAD4 structure and function, the authors discuss inhibitors of PAD4 and their potential use in suppressing NETosis. PAD4 is a particularly