Fig. 3.7 ). This is the final step in the elimination of infectious agents and necrotic cells. The killing and degradation of microbes and elimination of dead-cell debris within neutrophils and macrophages occur most efficiently after their activation. All these killing mechanisms are normally sequestered in lysosomes, to which phagocytosed materials are brought. Thus, potentially harmful substances are segregated from the cell’s cytoplasm and nucleus to avoid damage to the phagocyte while it is performing its normal function.
Reactive Oxygen Species. ROS are produced by the rapid assembly and activation of a multicomponent enzyme, phagocyte oxidase (also called NADPH oxidase), which oxidizes NADPH (reduced nicotinamide-adenine dinucleotide phosphate) and, in the process, reduces oxygen to the superoxide anion (O2 • ) (
Fig. 3.7B ). In neutrophils, this oxidative reaction is tightly linked to phagocytosis, and is called the respiratory burst. Phagocyte oxidase is an enzyme complex consisting of at least seven proteins. In resting neutrophils, different components of the enzyme are located in the plasma membrane and the cytoplasm. In response to activating stimuli, the cytosolic protein components translocate to the phagosomal membrane, where they assemble and form the functional enzyme complex. Thus, the ROS are produced within the phagolysosome, where they can act on ingested particles without damaging the host cell. O2 • so produced is then converted into hydrogen peroxide (H2O2), mostly by spontaneous dis-mutation, a process of simultaneous oxidation and reduction. H2O2 is not able to kill microbes efficiently by itself. However, the azurophilic granules of neutrophils contain the enzyme myeloperoxidase (MPO), which, in the presence of a halide such as Cl−, converts H2O2 to hypochlorite (OCl2 − , the active ingredient in household bleach). The latter is a potent anti-microbial agent that destroys microbes by halogenation (in which the halide is bound covalently to cellular constituents) or by oxidation of proteins and lipids (lipid peroxidation). The H2O2-MPO-halide system is the most efficient bactericidal system of neutrophils. Nevertheless, inherited deficiency of MPO only causes a modest increase in susceptibility to infection, emphasizing the redundancy of microbicidal mechanisms in leukocytes. H2O2 also is converted to hydroxyl radical (OH•), another powerful destructive agent. As discussed in Chapter 2, these oxygen-derived free radicals bind to and modify cellular lipids, proteins, and nucleic acids, and thus destroy cells such as microbes.
Oxygen-derived radicals may be released extracellularly from leukocytes after exposure to microbes, chemokines, and antigen–antibody complexes, or following a phagocytic challenge. These ROS are implicated in tissue damage accompanying inflammation.
Serum, tissue fluids, and host cells possess anti-oxidant mechanisms that protect against these potentially harmful oxygen-derived radicals. These anti-oxidants are discussed in Chapter 2; they include (1) the enzyme superoxide dismutase, which is found in or can be activated in a variety of cell types; (2) catalase, which detoxifies H2O2; and (3) glutathione peroxidase, another powerful H2O2 detoxifier. The role of oxygen-derived free radicals in any given inflammatory reaction depends on the balance between production and inactivation of these metabolites by cells and tissues.
Genetic defects in the generation of ROS are the cause of an immunodeficiency disease called chronic granulomatous disease, described in Chapter 5.