**6. Neutrophil extracellular traps in viral infections**

Viruses have an extraordinary ability to evade the immune system, and the innate immune system is regarded as the first line of defense. Innate immune cells recognize a wide variety of pathogens through their pattern-recognition receptors (PRRs) that include Toll-like receptors (TLRs), NOD-like receptors (NLRs), and RIG-like receptors (RLRs) that recognize pathogenassociated molecular patterns (PAMPs). Several PRRs recognize viral ligands such as TLR-3, TLR-7, TLR-8, RIG-1, and MDA5, and the activation of these PRRs induces the synthesis of antiviral interferons (types I and II), tumor necrosis factor α, interleukin-15, and interleukin-18 [53–55].

The role of NETs in the control of several bacterial infections has been broadly analyzed. However, research on their role in viral infections remains scarce. It has recently been shown that viral infections or virus-derived molecules may act as strong inducers of NETs. Several viruses that induce the formation of NETs have been identified. In some cases, NETs neutralize the viral particles by the MPO or the granule-derived defensins, associated to NETs. The α-defensin protein directly inhibits the influenza virus replication and protein synthesis [56]. Some viruses, such as those of the herpesvirus family, contain proteins with endonuclease activity, so they can degrade NETs and allow viral escape and dissemination. NETs antiviral activity consists in the sequestering of viral particles, thus preventing fusion of viruses with target cells and direct neutralization of virions. It is worth mentioning that viruses do not necessarily infect the neutrophils. However, neutrophils can sense viral particles through their PRRs or via secondary signals produced upon infection of other host cells. The use of secondary signals to induce the release of NETs has important advantages in the context of viral infections [56, 57].

Viruses that induce the release of NETs *in vitro* do so under a non-productive infection of neutrophils. In the case of HIV-1, neutrophils sense this virus by endosomal PRRs that detect viral nucleic acid via TLR-7 and TLR-8, and then undergo NETosis. The respiratory syncytial virus (RSV) induces NETosis through TLR-4. Hantaviruses induce NETs formation by signaling through β<sup>2</sup> integrins. Influenza virus A can stimulate neutrophils directly to release NETs. Viruses also produce NETs indirectly without engagement of the PRRs expressed by neutrophils. Interleukin-8 (IL-8) triggers NETosis. Although NETs formation by viruses is now well established, it is not so clear how NETs contribute to antiviral immunity. In some viruses, as in a mouse model of poxvirus infection, induction of NETs with LPS prior to infection strongly reduced the number of virus-infected liver cells, and this protective effect was reversed by DNAse treatment. Noroviruses can be reduced by their binding to histone H1. Some viral mechanisms counteract NETs formation, as for HIV-1 envelope glycoprotein which stimulates DCs to produce cellular IL-10 through dendritic cell-specific ICAM-grabbing non-integrin (DC-SIGN), IL-10 is an immunosuppressive cytokine that, among other functions, inhibits TLR-induced ROS production (54). IL-10 homologs have been found in the genome of large DNA viruses that include ubiquitous human virus, such as human cytomegalovirus (HCMV) and Epstein-Barr virus (EBV). Kaposi's sarcoma-associated herpesvirus (KSHV) impairs the release of NETs, and dengue virus serotype-2 can arrest NETs release by interfering with glucose uptake [6]. Taken together, these findings suggest that virus-induced release of NETs may help to control viral dissemination by direct and indirect mechanisms, whereas, at the same time, viral evasion mechanisms target the formation of NETs.

In 2015, Moreno-Altamirano et al. [16] demonstrated that dengue virus serotype-2 inhibits PMA-induced formation of NETs, arresting neutrophils at the chromatin de-condensation step which, based on a previous report [6], suggests that DENV-2 inhibits the formation of NETs by interfering with glucose uptake and glycolysis.
