Neutrophils are the most abundant leukocytes in human circulation and play fundamental roles against invading pathogens, trauma, and other types of tissue stress. For many years, their antimicrobial properties were considered primarily driven by degranulation, phagocytosis, synthesis of reactive oxygen species, and cytokine production. During the last several years, it has become apparent that neutrophils are significantly more versatile than originally thought and play important roles in shaping the immunologic landscape under homeostasis and during acute and chronic inflammatory diseases. A fundamental discovery that initiated this paradigm shift was published in 2004 by Arturo Zychlinsky and colleagues (1) and is the current Pillars of Immunology article. In this article, the authors identified an additional mechanism by which neutrophils fight microorganisms: the release of neutrophil extracellular traps (NETs), a meshwork of decondensed chromatin bound through electrical charge to a variety of neutrophil granule proteins that are endowed with antimicrobial properties, to the extracellular space.

Zychlinsky and colleagues found that NETs were devoid of cell membrane and had characteristic cargo that included proteins present in primary and secondary granules, histones, and DNA but not other specific neutrophil cytoplasmic proteins. Their original description reported IL-8, PMA, and LPS as NET inducers, as well as Gram-positive and -negative bacteria such as Staphylococcus aureus, Salmonella typhimurium, and Shigella flexneri. These bacteria were trapped in the NETs and their virulence factors were then “disarmed,” at least in part, by NET-bound proteases, including neutrophil elastase. The authors described not only trapping of the bacteria but also a decrease in their viability, which was driven by histones contained in the NETs. The formation of NETs was operational in vivo, as these structures were detected in infected and inflamed tissues from various species. The identification of these structures and their interaction with microbes suggested that they were involved in the sequestration and killing of bacteria through the delivery of high concentrations of antimicrobial molecules. After proposing that this mechanism would prevent infections from spreading and by ensuring a high concentration of antimicrobial factors in the tissue, the authors published additional manuscripts elegantly describing the molecular mechanisms involved in this intriguing and dramatic cellular event (2, 3). Since the original description of NETs, various groups have demonstrated that the mechanisms involved in NET formation are more heterogeneous and context-dependent than initially thought (4, 5), underscoring the previously unrecognized versatility of neutrophil biology.

Indeed, it is now recognized that a variety of microbial and sterile inflammatory stimuli can induce these lattices, including autoantibodies, immune complexes, cytokines, crystals, and chemical agonists (4, 69). Regarding the response to infectious agents, NETs may have a particularly important role in immobilizing large microbes, such as Candida albicans, and have microbicidal activity against a variety of viruses, parasites, bacteria, and other fungi (1014). In addition to their ability to trap and kill various microorganisms, NETs can neutralize virulence factors, promote functional opsonization of microbes, and inhibit microbial growth. Conversely, microbes have acquired elegant evolutionary strategies to evade the function of NETs. At the same time, NETs have been associated with inflammatory pathogenesis that can ensue after some infections, including COVID-19, such as immunothrombosis and end-organ damage (15, 16).

In addition to their role in infection control, the other physiologic effect of NETs appears to be their involvement in the coagulation pathway. Multiple studies have recently documented critical pleiotropic functions for neutrophils and NETs in clotting (17). Once again, this phenomenon can go awry, and NETs have been implicated in pathogenic immunothrombosis in a variety of infections as well as noninfectious diseases (16, 18).

Dysregulation in NET formation and degradation may play important roles in the pathogenesis of a variety of acute and chronic conditions, including systemic autoimmune diseases, such as systemic lupus erythematosus (19) and rheumatoid arthritis (8), autoinflammatory syndromes (20), various cancers and their ability to metastasize (21), atherosclerosis and other inflammatory vasculopathies (7), and fibrosing diseases (22) to name but a few. NETs also are an important source of modified autoantigens (4), and they can trigger type I IFN responses (4), activate the inflammasome machinery (6), and induce autoantibody formation and a variety of other proinflammatory responses. NET components can be internalized by immune and stromal cells through various pathways, modulating their phenotype and function (23, 24). Genetic or acquired dysregulation in the clearance of NETs has been described in various inflammatory diseases and is considered a phenomenon that may predispose to initiation and perpetuation of these conditions. There is now significant interest in targeting aberrant NET formation and/or impaired NET degradation in various diseases through the modulation of signaling pathways that are important in the generation and stability of these traps (25, 26). The next few years may be very relevant in determining whether clinical targeting of dysregulated NET formation may become part of the armamentarium to treat various diseases and whether this strategy is safe in patient populations.

In summary, the study by Zychlinsky and colleagues first defined the ability of neutrophils to form NETs and pioneered new insights into the fascinating and ever-expanding role of these innate immune cells in homeostasis and disease. Since its publication, the significance of this Pillars of Immunology article endures, highlighting the multifaceted role of neutrophils and how they may be targeted for the treatment of a variety of acute and chronic diseases.

This work was supported by the Intramural Research Program at the National Institute of Arthritis and Musculoskeletal and Skin Diseases/National Institutes of Health under Grant ZIAAR041199.

Abbreviations used in this article:

     
  • NET

    neutrophil extracellular trap.

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The National Institute of Arthritis and Musculoskeletal and Skin Diseases has collaborative research agreements with Astra Zeneca, Pfizer, and Bristol Myers Squib, and M.J.K. is on the scientific advisory board for Neutrolis and Citryll.