Immune Defense against Bacterial Pathogens: Innate Immunity (page 6)
(This chapter has 6 pages)
© Kenneth Todar, PhD
Toll-Like Receptors
Macrophages, dendritic cells,
and epithelial cells have a set of transmembrane receptors that
recognize different types of molecular determinants associated with
both pathogenic and non pathogenic bacteria. Foremost among
these are Toll-like
receptors (TLRs).
In macrophages and dendritic cells, a pathogen is exposed to a TLR
when it is engulfed within the phagosome membrane. Depending on
which TLR it binds to will
determine what the response will be. In this way, the TLRs identify the
nature of the pathogen and turn on a response appropriate for
dealing with it, generally by expression of
various cytokines. Humans have 12 different TLRs, each of which
specializes in a slightly different response to a pathogen (be it a
bacterium, virus or protozoa).
For example TLR-2 binds to the peptidoglycan of Gram-positive
bacteria such as streptococci and staphylococci; TLR-3 binds to
double-stranded RNA; TLR-4 is activated by the lipopolysaccharide
(endotoxin) in the outer membrane of Gram-negative such as Salmonella and E. coli; TLR-5
binds to the flagellin of motile bacteria like Listeria; TLR-6 forms a heterodimer
with TLR-2 and responds to peptidoglycan and certain bacterial
lipoproteins.
TLR-7 binds to the single-stranded RNA genomes of viruses such as
as influenza, mumps and measles.
In all these cases, binding of the pathogen to the TLR initiates a
signaling pathway that leads to the activation of a transcription
factor that turns on cytokine genes such as those for tumor necrosis
factor-alpha (TNF-α), Interleukin-1 (IL-1), and chemotactic attractants
that attract white blood cells to the site. These effector molecules
lead to inflammation at the site. Even before these late events
occur, the binding of Gram-positive bacteria to TLR-2 and Gram-negative
bacteria to TLR-4
enhances phagocytosis and the fusion of the phagosomes with lysosomes.
Formation of the phagolysosome
The phagosome migrates into the cytoplasm and collides with lysosomal
granules
which explosively discharge their contents into the membrane-enclosed
vesicle
(phagosome). Membranes of the phagosome and lysosome actually fuse
resulting
in a digestive vacuole called the phagolysosome. Other
lysosomes
will fuse with the phagolysosome. It is within the phagolysosome that
killing
and digestion of the engulfed microbe take place. Some of the
microbicidal
constituents of the lysosomes of neutrophils and macrophages include
lysozyme,
cationic proteins, various proteases and hydrolyases and
peroxidases.
The killing processes are confined to the phagolysosome, such that none
of the toxic substances
and
lethal activities of the phagocytes are turned against themselves.
Intracellular killing of organisms
After phagolysosome formation the first detectable effect on bacterial
physiology, occurring within a few minutes after engulfment, is loss of
viability (ability to reproduce). The exact mechanism is unknown.
Inhibition
of macromolecular synthesis occurs later. By 10 to 30 minutes after
ingestion
many pathogenic and nonpathogenic bacteria are killed followed by lysis
and digestion of the bacteria by lysosomal enzymes. The microbicidal
activities
of phagocytes are complex and multifarious. Metabolic products, as well
as lysosomal constituents, are responsible. These activities differ to
some extent in neutrophils, monocytes and macrophages.
The microbicidal activities of phagocytes are usually divided into oxygen-dependent and oxygen-independent events.
Oxygen-independent activity
Lysosomal granules contain a variety of extremely basic proteins that
strongly
inhibit bacteria, yeasts and even some viruses. A few molecules of any
one of these cationic proteins appear able to inactivate a bacterial
cell
by damage to their permeability barriers, but their exact modes of
action
are not known. The lysosomal granules of neutrophils contain
lactoferrin,
an extremely powerful iron-chelating protein, which withholds potential
iron
needed for bacterial growth. The pH of the phagolysosome may be as low
as 4.0 due to accumulation of lactic acid, which is sufficiently acidic
to prevent the growth of most pathogens. This acidic environment
apparently
optimizes the activity of many degradative lysosomal enzymes including
lysozyme, glycosylases, phospholipases, and nucleases.
Oxygen-dependent activity
Liganding of Fc receptors (on neutrophils, monocytes or macrophages)
and
mannose receptors (on macrophages) increases their O2
uptake,
called the respiratory burst. These receptors activate a
membrane-bound
NADPH
oxidase that reduces O2 to O2-
(superoxide).
Superoxide can be reduced to OH. (hydroxyl radical) or
dismutated
to H2O2 (hydrogen peroxide) by superoxide
dismutase.
O2-, OH., and H2O2
are activated oxygen species that are potent oxidizing agents in
biological
systems which adversely affect a number of cellular structures
including
membranes and nucleic acids. Furthermore, at least in the case of
neutrophils,
these reactive oxygen intermediates can act in concert with a lysosomal
enzyme called myeloperoxidase to function as the
myeloperoxidase
system, or MPO.
Myeloperoxidase is one of the lysosomal enzymes of neutrophils which
is released into the phagocytic vacuole during fusion to form the
phagolysosome.
Myeloperoxidase uses H2O2 generated during the
respiratory
burst to catalyze halogenation (mainly chlorination) of phagocytosed
microbes.
Such halogenations are a potent mechanism for killing cells.
When the NADPH oxidase and myeloperoxidase systems are operating in
concert, a series of reactions leading to lethal oxygenation and
halogenation
of engulfed microbes occurs.
Intracellular digestion
Dead microbes are rapidly degraded in phagolysosomes to low
molecular-weight
components. Various hydrolytic enzymes are involved including lysozyme,
proteases, lipases, nucleases, and glycosylases. Neutrophils die and
lyse
after extended phagocytosis, killing, and digestion of bacterial cells.
This makes up the characteristic properties of pus.
Macrophages egest digested debris and allow insertion of microbial
antigenic
components into the plasma membrane for presentation to lymphocytes in
the immunological response.
Figure 7. Phagocytosis of
Streptococcus
pyogenes by a macrophage. CELLS
alive!
Bacterial Defense Against Phagocytosis
Pathogenic bacteria have a variety of defenses against phagocytes.
In fact, most successful pathogens have some mechanism(s) to contend
with
the phagocytic defenses of the host. These mechanisms will be discussed
in detail later as part of the determinants of virulence of pathogens.
However, in general, pathogens may resist phagocytosis by:
Evading phagocytes by growing in regions of the body which
are
not accessible to them
Avoiding engulfment by phagocytes after contact
Being able to kill phagocytes either before or after
engulfment
Being able to survive inside of phagocytes (or other types
of
cells) and to persist as intracellular parasites
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