Phagocytosed bacteria can also be killed by a diverse arsenal of oxygen-independent killing mechanisms utilizing bacteriolytic enzymes and antimicrobial peptides.
In addition to the oxygen-dependent respiratory burst reactions, microorganisms can be killed by bacteriolytic enzymes and cationic antimicrobial peptides that are stored within the granules of the neutrophil’s cytoplasm. These oxygen-independent killing mechanisms are directed toward the bacterial cell membrane, causing its breakdown and leakage. Neutrophils contain particularly large amounts of cationic antimicrobial proteins such as defensins and antimicrobial peptides called cathelicidins. Similar to lysozymes and cathepsins stored in the specific granules, these cationic antimicrobial proteins break down the bacterial wall. In addition, lysosomal hydrolytic enzymes that digest bacterial proteins and lactoferrins that chelate iron from nutritional bacterial pathways contribute to the destruction of the invading bacteria. These mechanisms are not as efficient as oxygendependent killing pathways. Neutrophils from patients with defects in oxygen-dependent pathways, such as those with chronic granulomatous disease (see Folder 10.4), are still able to destroy phagocytosed bacteria to some degree. However, because of the low efficiency of these processes, individuals with these defects are more susceptible to serious infections.
After intracellular digestion by the neutrophil, the remnants of degraded material are stored in residual bodies or exocytosed. Most neutrophils die in this process; the accumulation of dead bacteria and dead neutrophils constitutes the thick exudate called pus. The yellow–green color of the pus and of mucus secretions (e.g., from infected lungs) comes from the heme pigment of MPO enzyme in azurophilic granules of neutrophils.
Inflammation and wound healing also involve monocytes, lymphocytes, eosinophils, basophils, and fibroblasts.
Monocytes also enter the connective tissue as a secondary response to tissue injury. At the site of tissue injury, they transform into macrophages that phagocytose cell and tissue debris, fibrin, remaining bacteria, and dead neutrophils. Normal wound healing depends on the participation of
FIGURE 10.8 • Pathways leading to synthesis of reactive oxygen intermediates during neutrophil’s respiratory burst reactions.
This schematic diagram shows a phagolysosome that contains already pagocytosed bacterium. Two oxygen-dependent killing mechanisms are depicted in this drawing. The first mechanism depends on a phagocyte oxidase (phox) system that utilizes the NADPH oxidase complex (contains five subunits). This complex transports excess electrons across the membrane of the phagolysosome, where they interact with molecular oxygen to generate free superoxide anions. These anions are converted into reactive oxygen intermediates. Another enzyme superoxide dismutase converts superoxide anions to singlet oxygen and hydrogen peroxide, which further reacts with superoxide anions to produce bactericidal hydroxyl radicals and more singlet oxygen molecules. The second mechanism involves lysosomal enzyme myeloperoxidase (MPO) found in the azurophilic granules of neutrophils. MPO catalizes the production of hypochlorous acids from hydrogen peroxide and chloride anions. Hypochlorous acid is further metabolized to a highly toxic hypochlorite (bleach) and chlorine. Some of the hypochloride may spontaneously break down to yield toxic singlet oxygen and chloride ions. All molecules produced during oxygen bursts in neutrophils (associated with red arrows) are highly effective in killing ingested bacteria.