Page 165 - Veterinary Immunology, 10th Edition

chemoattractant protein-1 (CCL2) under the influence of IL-6.
VetBooks.ir Neutrophils are the martyrs of the immune system: they reach and

attack foreign material first, and in undergoing apoptosis, they
attract macrophages to the site of invasion. They also release

defensins that augment the antimicrobial activities of macrophages.
Phagocytosis by macrophages differs from the process in
neutrophils in that macrophages have few cytoplasmic granules.
Instead, they generate oxidants using a respiratory burst or

inducing nitric oxide synthesis. They also synthesize many new
proteins including pro- and antiinflammatory cytokines,
antimicrobial peptides and proteins, and enzymes. Macrophages
destroy bacteria by both oxidative and nonoxidative mechanisms.

In contrast to neutrophils, however, macrophages undertake
sustained, repeated phagocytic activity. In addition, macrophages
produce proteases such as collagenases and elastases that destroy
nearby connective tissue. They produce plasminogen activator that

generates plasmin, another potent protease. Thus macrophages can
“soften up” the local connective tissue matrix and permit more
effective penetration of the damaged tissue. Macrophages
phagocytose both apoptotic neutrophils and their exosomes. The

contents of neutrophil granules are not always destroyed but may
be carried to macrophage endosomes where they can continue to
inhibit the growth of bacteria. Thus neutrophils can enhance the
effectiveness of macrophages in host defense. Macrophages can also

release nuclear DNA and histones to form macrophage extracellular
traps (METs) in response to bacterial pathogens and their exotoxins.
Bovine monocyte-derived and alveolar macrophages exposed to
Mannheimia. haemolytica have also been shown to produce METs.

These METs, together with any captured bacteria, can then be
endocytosed and destroyed by other macrophages.

Generation of Reactive Nitrogen Species

Major species differences emerge between macrophages in relation
to the production of reactive nitrogen species (RNS), especially

nitric oxide. Thus in some mammals, especially laboratory rodents,
cattle, sheep, and horses (but not in humans, pigs, goats, or rabbits),
microbial PAMPs trigger macrophages to express type 2 nitric

oxide synthase (NOS2). (The overall gene expression profile of pig

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