Neutrophil extracellular traps (NETs) are extracellular structures composed of chromatin and granule proteins that bind and kill microorganisms. We show that upon stimulation, the nuclei of neutrophils lose their shape, and the eu- and heterochromatin homogenize. Later, the nuclear envelope and the granule membranes disintegrate, allowing the mixing of NET components. Finally, the NETs are released as the cell membrane breaks. This cell death process is distinct from apoptosis and necrosis and depends on the generation of reactive oxygen species (ROS) by NADPH oxidase. Patients with chronic granulomatous disease carry mutations in NADPH oxidase and cannot activate this cell-death pathway or make NETs. This novel ROS-dependent death allows neutrophils to fulfill their antimicrobial function, even beyond their lifespan.

/pdf/novel-cell-death-program-leads-to-neutrophil-extracellular-3y3jbyjspz.pdf

Novel cell death program leads to neutrophil extracellular traps

Neutrophils have long been viewed as the final effector cells of an acute inflammatory response, with a primary role in the clearance of extracellular pathogens. However, more recent evidence has extended the functions of these cells. The newly discovered repertoire of effector molecules in the neutrophil armamentarium includes a broad array of cytokines, extracellular traps and effector molecules of the humoral arm of the innate immune system. In addition, neutrophils are involved in the activation, regulation and effector functions of innate and adaptive immune cells. Accordingly, neutrophils have a crucial role in the pathogenesis of a broad range of diseases, including infections caused by intracellular pathogens, autoimmunity, chronic inflammation and cancer.

/pdf/neutrophils-in-the-activation-and-regulation-of-innate-and-2ccbalvc3n.pdf

Neutrophils in the activation and regulation of innate and adaptive immunity

It has been known for many years that neutrophils and platelets participate in the pathogenesis of severe sepsis, but the inter-relationship between these players is completely unknown We report several cellular events that led to enhanced trapping of bacteria in blood vessels: platelet TLR4 detected TLR4 ligands in blood and induced platelet binding to adherent neutrophils This led to robust neutrophil activation and formation of neutrophil extracellular traps (NETs) Plasma from severely septic humans also induced TLR4-dependent platelet-neutrophil interactions, leading to the production of NETs The NETs retained their integrity under flow conditions and ensnared bacteria within the vasculature The entire event occurred primarily in the liver sinusoids and pulmonary capillaries, where NETs have the greatest capacity for bacterial trapping We propose that platelet TLR4 is a threshold switch for this new bacterial trapping mechanism in severe sepsis

Platelet TLR4 activates neutrophil extracellular traps to ensnare bacteria in septic blood

Neutrophil extracellular traps (NETs) are part of the innate immune response to infections. NETs are a meshwork of DNA fibers comprising histones and antimicrobial proteins. Microbes are immobilized in NETs and encounter a locally high and lethal concentration of effector proteins. Recent studies show that NETs are formed inside the vasculature in infections and noninfectious diseases. Here we report that NETs provide a heretofore unrecognized scaffold and stimulus for thrombus formation. NETs perfused with blood caused platelet adhesion, activation, and aggregation. DNase or the anticoagulant heparin dismantled the NET scaffold and prevented thrombus formation. Stimulation of platelets with purified histones was sufficient for aggregation. NETs recruited red blood cells, promoted fibrin deposition, and induced a red thrombus, such as that found in veins. Markers of extracellular DNA traps were detected in a thrombus and plasma of baboons subjected to deep vein thrombosis, an example of inflammation-enhanced thrombosis. Our observations indicate that NETs are a previously unrecognized link between inflammation and thrombosis and may further explain the epidemiological association of infection with thrombosis.

https://www.pnas.org/content/pnas/107/36/15880.full.pdf

Extracellular DNA traps promote thrombosis

Neutrophils are innate immune phagocytes that have a central role in immune defence. Our understanding of the role of neutrophils in pathogen clearance, immune regulation and disease pathology has advanced dramatically in recent years. Web-like chromatin structures known as neutrophil extracellular traps (NETs) have been at the forefront of this renewed interest in neutrophil biology. The identification of molecules that modulate the release of NETs has helped to refine our view of the role of NETs in immune protection, inflammatory and autoimmune diseases and cancer. Here, I discuss the key findings and concepts that have thus far shaped the field of NET biology.

Neutrophil extracellular traps in immunity and disease

https://escholarship.org/content/qt08k2q0k0/qt08k2q0k0.pdf?t=lnpyjb

DNase expression allows the pathogen group A Streptococcus to escape killing in neutrophil extracellular traps.

Most invasive bacterial infections are caused by species that more commonly colonize the human host with minimal symptoms. Although phenotypic or genetic correlates underlying a bacterium's shift to enhanced virulence have been studied, the in vivo selection pressures governing such shifts are poorly understood. The globally disseminated M1T1 clone of group A Streptococcus (GAS) is linked with the rare but life-threatening syndromes of necrotizing fasciitis and toxic shock syndrome. Mutations in the GAS control of virulence regulatory sensor kinase (covRS) operon are associated with severe invasive disease, abolishing expression of a broad-spectrum cysteine protease (SpeB) and allowing the recruitment and activation of host plasminogen on the bacterial surface. Here we describe how bacteriophage-encoded GAS DNase (Sda1), which facilitates the pathogen's escape from neutrophil extracellular traps, serves as a selective force for covRS mutation. The results provide a paradigm whereby natural selection exerted by the innate immune system generates hypervirulent bacterial variants with increased risk of systemic dissemination.

/pdf/dnase-sda1-provides-selection-pressure-for-a-switch-to-27wqtzg8w4.pdf

DNase Sda1 provides selection pressure for a switch to invasive group A streptococcal infection

Intra-abdominal infection (IAI) is the second most common cause of severe sepsis in the intensive care unit (ICU). Even with optimal care, this disease process confers significant morbidity and mortality. The most common causes of IAI involve inflammation and perforation of the gastrointestinal tract including appendicitis, diverticulitis, and peptic ulcer disease. Other etiologies often more challenging to treat include postoperative complications, iatrogenic procedural complications, and traumatic injuries. Treatment is multimodal including, most importantly, source control in conjunction with timely systemic antimicrobial therapy, resuscitation, and supportive care. Given the wide spectrum of disease from focal isolated inflammation to diffuse peritonitis with septic shock and organ failure, the treatment is varied and complex. This chapter includes a review of clinical definitions and classification of the disease process as well as a basic overview of treatment.

Amelia J. Simpson

Papers

DNase expression allows the pathogen group A Streptococcus to escape killing in neutrophil extracellular traps.

DNase Sda1 provides selection pressure for a switch to invasive group A streptococcal infection

Classification and Principals of Treatment