**2. Understanding the process of NETs formation**

The neutrophil extracellular traps (NETs) structures were described as another type of neutrophil cell death, different from apoptosis and necrosis. The research field on NETs has steadily been growing since 2004, when Brinkmann et al. reported for the first time this new function of activated neutrophils, demonstrating, by electron microscopy, that, when neutrophils are in the presence of bacteria, fungi, protozoa, or viruses, they acquire the capacity to form fibrillary structures, resembling nets or webs. These structures are composed mainly of nuclear material, chromatin fibers with diameters of 15–17 nm containing DNA decorated with neutrophil elastase (NE), myeloperoxidase (MPO), cathepsin G, proteinase 3 (PR3), high-mobility group protein B1 (HMGB-1), tryptase or antimicrobial peptide LL37, histones, and cytoplasmic proteins such as histones H1, H2A, H2B, H3, H4, G, lactoferrin, and gelatinase, among others [7].

Two mechanisms for the formation of NETs have been described: the suicide or lytic and vital NETosis [8]. In the first case, NETs release results from the activation of PMN by IL-8 or chemical compounds, such as phorbol myristate acetate (PMA). PMA activates neutrophils through the protein kinase C (PKC) and follows the Raf-MEK-ERK mitogen-activated protein kinase signaling pathway; the enzyme nicotinamide adenine dinucleotide phosphate (NADPH) oxidase induces the translocation of elastase from the cytosolic granules to the inner nucleus, helping the rupture of the chromatin through histones. Induction of NETs with PMA by this mechanism can be observed from 30 min post-activation and, by 6–8 h postactivation, a high number of extracellular traps (ETs) are well formed (**Figure 1**).

In contrast, vital NETosis has been demonstrated following pathogen recognition by host pattern recognition receptors (PRRs). Gram-negative bacteria products, such as lipopolysaccharide Beneficial and Deleterious Effects of Neutrophil Extracellular Traps on Infection http://dx.doi.org/10.5772/intechopen.68634 29

phagocytic capacity and, more recently, on their role as neutrophil extracellular traps (NETs)

When neutrophils fail to kill invading pathogens by the classical phagocytosis mechanism, PMNs can accomplish this function by neutrophil extracellular traps (NETs), a process reported as a novel form of cell death called NETosis, which is dependent of the generation of reactive oxygen species [2–5]. Neutrophils forming NETs have been demonstrated by activating neutrophils with phorbol myristate acetate (PMA), interleukin 8 (IL-8), lipopolysaccharide (LPS), or under contact of neutrophils with Gram-negative and Gram-positive bacteria. NETosis induction has also been described for viral infections, and some of the signaling pathways involved have been analyzed, finding the involvement of pathogen-associated molecular patterns (PAMPs), TLR-4, TLR-7, and TLR-8. Rodríguez-Espinosa et al. have shown that NETs formation takes place in two separate metabolic steps: the first one involves chromatin decondensation, which is independent of external glucose and glycolysis, whereas the second, which involves the chromatin release, is a process that is dependent on external glu-

The neutrophil extracellular traps (NETs) structures were described as another type of neutrophil cell death, different from apoptosis and necrosis. The research field on NETs has steadily been growing since 2004, when Brinkmann et al. reported for the first time this new function of activated neutrophils, demonstrating, by electron microscopy, that, when neutrophils are in the presence of bacteria, fungi, protozoa, or viruses, they acquire the capacity to form fibrillary structures, resembling nets or webs. These structures are composed mainly of nuclear material, chromatin fibers with diameters of 15–17 nm containing DNA decorated with neutrophil elastase (NE), myeloperoxidase (MPO), cathepsin G, proteinase 3 (PR3), high-mobility group protein B1 (HMGB-1), tryptase or antimicrobial peptide LL37, histones, and cytoplasmic proteins such as histones H1, H2A, H2B, H3, H4, G, lactoferrin, and gelatinase, among

Two mechanisms for the formation of NETs have been described: the suicide or lytic and vital NETosis [8]. In the first case, NETs release results from the activation of PMN by IL-8 or chemical compounds, such as phorbol myristate acetate (PMA). PMA activates neutrophils through the protein kinase C (PKC) and follows the Raf-MEK-ERK mitogen-activated protein kinase signaling pathway; the enzyme nicotinamide adenine dinucleotide phosphate (NADPH) oxidase induces the translocation of elastase from the cytosolic granules to the inner nucleus, helping the rupture of the chromatin through histones. Induction of NETs with PMA by this mechanism can be observed from 30 min post-activation and, by 6–8 h post-

In contrast, vital NETosis has been demonstrated following pathogen recognition by host pattern recognition receptors (PRRs). Gram-negative bacteria products, such as lipopolysaccharide

activation, a high number of extracellular traps (ETs) are well formed (**Figure 1**).

forming cells, in innate and adaptive immunity.

28 Role of Neutrophils in Disease Pathogenesis

**2. Understanding the process of NETs formation**

cose and glycolysis [6].

others [7].

**Figure 1.** Human peripheral blood neutrophils non-activated and activated with PMA (100 ng/ml) for different lengths of time. Neutrophil extracellular traps formation starts by 30 min post-activation; extracellular traps are more extended by 6 h post-activation (photographs taken by Moreno-Altamirano).

(LPS), activate neutrophils, by the ligation of TLRs (TLR-4 in the case of LPS), inducing the liberation of NETs. In the case of Gram-positive bacteria, the complement receptor 3 (CR3) and TLR-2 are required to induce vital NETosis; platelets are also inducers of vital NETosis, through CD11a. This mechanism maintains the external membrane integrity and thus the function of neutrophils, until cells are devoid of nucleus [7, 8].

A third mechanism for the induction of NETs, recently reported, is through autophagy [9, 10]. It is worth mentioning that neutrophils are not the only cells that form extracellular traps (ETs), and other immune cells, such as mast cells, eosinophils, and macrophages, can also release ETs. Although the molecular principles underlying the formation of ETs by mast cells [11], eosinophils [12], and monocytes/macrophages [13] are similar to those observed in neutrophils, there are some notable disparities. The most remarkable mechanism of ET formation has been described in eosinophils. In these cells, ETs are formed by both nuclear and mitochondrial DNAs, in a reactive oxygen species (ROS)-dependent manner.

Neutrophil extracellular traps are able to capture microorganisms trap microorganisms, killing them or not, this much depends on the type of pathogen involved. NETs are produced by the neutrophils of mice, humans, and some other animals, and can be induced by chemical compounds, bacteria, fungi, protozoa, and viruses. The role of NETs in viral infections is not yet clear. However, some viruses induce the release of NETs [14, 15].

While some viruses are immobilized and inactivated by NETs, others such as HIV induce the production of an IL-10-like protein that inhibits the formation of NETs [15], and dengue virus inhibits PMA-induced formation of NETs. Interestingly, neutrophils seem to be arrested at the chromatin decondensation step, failing to liberate NETs, thus suggesting a metabolicrelated mechanism of NETs inhibition [16].

Controversy surrounding neutrophil extracellular traps as a host defense mechanism makes it necessary to analyze how NETs limit the growth of various infectious agents, whereas, apparently, they have no effect on others. On the other hand, how NETs may cause damage and autoimmune diseases also needs to be investigated.
