2. • The binding of microbes—bacteria, fungi, protozoan parasites, and
viruses—to phagocytes via pattern recognition receptors or
opsonins and opsonin receptors activates signaling pathways.
• These signaling pathways trigger actin polymerization, resulting in
membrane extensions around the microbe particles and their
internalization, forming phagosomes.
• The phagosomes then fuse with lysosomes and, in neutrophils,
with preformed primary and secondary granules .
• The resulting phagolysosomes contain an arsenal of antimicrobial
agents that then kill and degrade the internalized microbes.
• These agents include antimicrobial proteins and peptides (including
defensins and cathelicidins), low pH, acid-activated hydrolytic
enzymes (including lysozyme and proteases), and specialized
molecules that mediate oxidative attack.
3. • Oxidative attack on the phagocytosed microbes, which
occurs in neutrophils, macrophages, and dendritic cells,
employs highly toxic reactive oxygen species (ROS) and
reactive nitrogen species (RNS), which damage
intracellular components.
• The reactive oxygen species are generated by the
phagocytes’ unique NADPH oxidase enzyme complex (also
called phagosome oxidase), which is activated when
microbes bind to the phagocytic receptors.
• The oxygen consumed by phagocytes to support ROS
production by NADPH oxidase is provided by a metabolic
process known as the respiratory burst, during which
oxygen uptake by the cell increases several fold. NADPH
oxidase converts oxygen to superoxide ion (•O2 ); other
ROS generated by the action of additional enzymes are
hydrogen peroxide (H2O2), and hypochlorous acid (HClO),
the active component of household bleach.
4.
5. • The generation of RNS requires the transcriptional activation of the gene
for the enzyme inducible nitric oxide synthase (iNOS, or NOS2)—called
that to distinguish it from related NO synthases in other tissues.
• Expression of iNOS is activated by microbial components binding to
various PRRs. iNOS oxidizes L-arginine to yield L-citrulline and nitric oxide
(NO), a potent antimicrobial agent.
• In combinationwith superoxide ion (•O2 ) generated by NADPH oxidase,
NO produces an additional reactive nitrogen species, peroxynitrite
(ONOO) and toxic S-nitrosothiols.
• Collectively the ROS and RNS are highly toxic to phagocytosed microbes
due to the alteration of microbial molecules through oxidation,
hydroxylation, chlorination, nitration, and S-nitrosylation, along with
formation of sulfonic acids and destruction of iron-sulfur clusters in
proteins.
• ROS and RNS also can be released from activated neutrophils and
macrophages and kill extracellular pathogens.
6. • The importance to antimicrobial defense of phagosomal NADPH
oxidase and its products, ROS and RNS, is illustrated by chronic
granulomatous disease (CGD).
• Patients afflicted with this disease have dramatically increased
susceptibility to some fungal and bacterial infections, caused by
defects in subunits of NADPH oxidase that destroy its ability to
generate oxidizing species.
• In addition, studies with mice in which the genes encoding iNOS
were knocke out have shown that nitric oxide and substances
derived from it account for much of the antimicrobial activity of
macrophages against bacteria, fungi, and parasites.
• These mice lost much of their usual ability to control infections by
such intracellular pathogens as M. tuberculosis and Leishmania
major, the intracellular protozoan parasite that causes
leishmaniasis.
