Neutrophils, which are key players in the initial response against invading pathogens, kill microbes by several mechanisms. Well known for engulfing microorganisms in phagosomes, where they are subjected to high concentrations of reactive oxygen species (ROS) and antimicrobial peptides discharged from granules, activated neutrophils also snare and kill microbes in extracellular traps. Fuchs et al., the researchers who initially described neutrophil extracellular traps (NETs, which consist of chromatin and granular proteins), used live-cell imaging to simultaneously monitor the morphology and viability of individual neutrophils and the appearance of NETs. Following activation with the phorbol ester phorbol 12-myristate 13-acetate (PMA), neutrophils underwent a series of morphological changes; eventually cells ruptured (indicated by the simultaneous loss of the dye calcein blue and exposure of phosphatidylserine). NETs only appeared with cell death. Morphological analysis of neutrophils fixed at various times after stimulation with PMA or Staphyloccus aureus indicated that the process of cell death that led to NET formation, which involved dissolution of the nuclear envelope and granular membranes so that chromatin could mix with granule contents, was distinct from apoptosis and necrosis. Moreover, cells activated to produce NET did not undergo DNA fragmentation as detected by TUNEL (terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling). NET production was enhanced by catalase inhibition and did not occur in the neutrophils of individuals with chronic granulomatous disease (associated with mutations in NADPH oxidase) and was blocked by pharmacological inhibition of NADPH. In both cases, NET production was rescued by hydrogen peroxide. Thus, the authors conclude that NET production involves a distinct form of active cell death that requires ROS generation and enables even dead neutrophils to retain antimicrobial activity.
T. A. Fuchs, U. Abed, C. Goosmann, R. Hurwitz, I. Schulze, V. Wahn, Y. Weinrauch, V. Brinkmann, A. Zychlinsky, Novel cell death program leads to neutrophil extracellular traps. J. Cell. Biol. 176, 231-241 (2007). [Abstract] [Full Text]