Neutrophil extracellular traps (NETs) are webs of DNA covered with antimicrobial molecules that constitute a newly described killing mechanism in innate immune defense. Previous publications reported that NETs take up to 3–4 h to form via an oxidant-dependent event that requires lytic death of neutrophils. In this study, we describe neutrophils responding uniquely to Staphylococcus aureus via a novel process of NET formation that did not require neutrophil lysis or even breach of the plasma membrane. The multilobular nucleus rapidly became rounded and condensed. During this process, we observed the separation of the inner and outer nuclear membranes and budding of vesicles, and the separated membranes and vesicles were filled with nuclear DNA. The vesicles were extruded intact into the extracellular space where they ruptured, and the chromatin was released. This entire process occurred via a unique, very rapid (5–60 min), oxidant-independent mechanism. Mitochondrial DNA constituted very little if any of these NETs. They did have a limited amount of proteolytic activity and were able to kill S. aureus. With time, the nuclear envelope ruptured, and DNA filled the cytoplasm presumably for later lytic NET production, but this was distinct from the vesicular release mechanism. Panton–Valentine leukocidin, autolysin, and a lipase were identified in supernatants with NET-inducing activity, but Panton–Valentine leukocidin was the dominant NET inducer. We describe a new mechanism of NET release that is very rapid and contributes to trapping and killing of S. aureus.

Neutrophil extracellular traps (NETs) are released, as neutrophils die in vitro, in a process requiring hours, leaving a temporal gap for invasive microbes to exploit. Functional neutrophils undergoing NETosis have not been documented. During Gram-positive skin infections, we directly visualized live PMN in vivo rapidly releasing NETs, which prevented bacterial dissemination. NETosis occurred during crawling thereby casting large areas of NETs. NET-releasing PMN developed diffuse decondensed nuclei ultimately becoming devoid of DNA. Cells with abnormal nuclei displayed unusual crawling behavior highlighted by erratic pseudopods and hyperpolarization consistent with the nucleus being a fulcrum for crawling. A combined requirement of Tlr2 and complement mediated opsonization tightly regulated NET release. Additionally live human PMN developed decondensed nuclei and formed NETS in vivo and intact anuclear neutrophils were abundant in Gram-positive human abscesses. Therefore early in infection, non-cell death NETosis occurs in vivo during Gram-positive infection in mice and humans.

In this review, we examine the evidence that neutrophil extracellular traps (NETs) play a critical role in innate immunity. We summarize how NETs are formed in response to various stimuli and provide evidence that NETosis is not universally a cell death pathway. Here we describe at least 2 different mechanisms by which NETs are formed, including a suicide lytic NETosis and a live cell or vital NETosis. We also evaluate the evidence for NETs in catching and killing pathogens. Finally, we examine how infections are related to the development of autoimmune and vasculitic diseases through unintended but detrimental bystander damage resulting from NET release.

During the systemic inflammatory response of severe sepsis, neutrophils accumulate in the liver microcirculation, but their functional significance is largely unknown. We show that neutrophils migrate to liver sinusoids during endotoxemia and sepsis where they exert protective effects by releasing neutrophil extracellular traps (NETs), which are DNA-based structures that capture and eliminate microbes. NETs released into the vasculature ensnare bacteria from the bloodstream and prevent dissemination. NET production requires platelet-neutrophil interactions and can be inhibited by platelet depletion or disruption of integrin-mediated platelet-neutrophil binding. During sepsis, NET release increases bacterial trapping by 4-fold (beyond the basal level provided by resident intravascular macrophages). Blocking NET formation reduces the capture of circulating bacteria during sepsis, resulting in increased dissemination to distant organs. Thus, NETs ensnare circulating bacteria and provide intravascular immunity that protects against bacterial dissemination during septic infections.

Lung-innervating nociceptor sensory neurons detect noxious or harmful stimuli and consequently protect organisms by mediating coughing, pain, and bronchoconstriction. However, the role of sensory neurons in pulmonary host defense is unclear. Here, we found that TRPV1+ nociceptors suppressed protective immunity against lethal Staphylococcus aureus pneumonia. Targeted TRPV1+-neuron ablation increased survival, cytokine induction, and lung bacterial clearance. Nociceptors suppressed the recruitment and surveillance of neutrophils, and altered lung ᵞδ T cell numbers, which are necessary for immunity. Vagal ganglia TRPV1+ afferents mediated immunosuppression through release of the neuropeptide calcitonin gene–related peptide (CGRP). Targeting neuroimmunological signaling may be an effective approach to treat lung infections and bacterial pneumonia.