Fig. 3.5 The microbicidal respiratory burst in phagocytes is initiated by activation-induced assembly of the phagocyte NADPH oxidase. First panel: neutrophils are highly specialized for the uptake and killing of pathogens, and contain several different kinds of cytoplasmic granules, such as the primary and secondary granules shown in the first panel. These granules contain antimicrobial peptides and enzymes. Second panel: in resting neutrophils, the cytochrome b558 subunits (gp91 and p22) of the NADPH oxidase are localized in the plasma membrane; the other oxidase components (p40, p47, and p67) are located in the cytosol. Signaling by phagocytic receptors and by fMLF or C5a receptors synergizes to activate Rac2 and induce the assembly of the complete, active NADPH oxidase in the membrane of the phagolysosome, which has formed by the fusion of the phagosome with lysosomes and primary and secondary granules. Third panel: active NADPH oxidase transfers an electron from its FAD cofactor to molecular oxygen, forming the superoxide ion O2– (blue) and other free oxygen radicals in the lumen of the phagolysosome. Potassium and hydrogen ions are then drawn into the phagolysosome to neutralize the charged superoxide ion, increasing acidification of the vesicle. Acidification dissociates granule enzymes such as cathepsin G and elastase (yellow) from their proteoglycan matrix, leading to their cleavage and activation by lysosomal proteases. O2– is converted by superoxide dismutase (SOD) to hydrogen peroxide (H2O2), which can kill microorganisms, and can be converted by myeloperoxidase, a heme-containing enzyme, to microbicidal hypochlorite (OCl–) and by chemical reaction with ferrous (Fe2+) ions to the hydroxyl (•OH) radical.
In addition to killing microbes engulfed by phagocytosis, neutrophils use another rather novel mechanism of destruction that is directed at extracellular pathogens. During infection, some activated neutrophils undergo a unique form of cell death in which the nuclear chromatin, rather than being degraded as occurs during apoptosis, is released into the extracellular space and forms a
MOVIE 3.4 fibril matrix known as neutrophil extracellular traps, or NETs (Fig. 3.6). NETs act to capture microorganisms, which may then be more efficiently phagocytosed by other neutrophils or macrophages. NET formation requires the generation of ROS, and patients with CGD have reduced NET formation, which may contribute to their susceptibility to microorganisms.
Macrophages can phagocytose pathogens and produce the respiratory burst immediately upon encountering an infecting microorganism, and this can be sufficient to prevent an infection from becoming established. In the nineteenth century, the immunologist Élie Metchnikoff believed that the innate response
Pattern recognition by cells of the innate immune system.
of macrophages encompassed all host defenses; indeed, invertebrates such as the sea star that he was studying rely entirely on innate immunity to overcome infection. Although this is not the case in humans and other vertebrates, the innate response of macrophages still provides an important front line of defense that must be overcome if a microorganism is to establish an infection that can be passed on to a new host.