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Innate
immunity refers to antigen-nonspecific defense mechanisms that a host
uses immediately or within several hours after exposure to an antigen.
This is the immunity one is born with and is the initial response
by the body to eliminate microbes and prevent infection.
Unlike
adaptive immunity, innate immunity does not recognize every possible
antigen. Instead, it is designed to recognize a few highly
conserved structures present in many different microorganisms. The
structures recognized are called pathogen-associated molecular
patterns and include LPS from the gram-negative cell wall,
peptidoglycan, lipotechoic acids from the gram-positive cell wall, the
sugar mannose (common in microbial glycolipids and glycoproteins but
rare in those of humans), bacterial DNA, N-formylmethionine found in
bacterial proteins, double-stranded RNA from viruses, and glucans from
fungal cell walls. Most body defense cells have pattern-recognition
receptors for these common pathogen-associated molecular patterns
and so there is an immediate response against the invading
microorganism. Pathogen-associated molecular patterns can also be
recognized by a series of soluble pattern-recognition receptors in the
blood that function as opsonins and initiate the complement pathways.
In all, the innate immune system is thought to recognize approximately
103 molecular patterns. All of this will be discussed in
greater detail in upcoming sections.
The innate immune responses
involve:
-
phagocytic
cells (neutrophils, monocytes, and macrophages);
-
cells that
release inflammatory mediators (basophils, mast cells, and
eosinophils);
-
natural
killer cells (NK cells); and
-
molecules
such as complement proteins, acute phase proteins, and cytokines.
Examples of
innate immunity include anatomical barriers, mechanical removal,
bacterial antagonism, pattern-recognition receptors, antigen-nonspecific
defense chemicals, the complement pathways, phagocytosis, inflammation,
and fever. In the next several sections we will look at each of these
in greater detail.
We will now
take a closer look at pattern-recognition receptors, antigen-nonspecific
antimicrobial body chemicals, and cytokines.
Pattern-Recognition Receptors (Including Toll-Like
Receptors) and Cytokines
1.
Pattern-Recognition Receptors
In order to
protect against infection, one of the things the body must initially
do is detect the presence of microorganisms. The body does this by
recognizing molecules unique to microorganisms that are not associated
with human cells. These unique molecules are called pathogen-associated
molecular patterns. In all, the innate immune system is thought to
recognize approximately 103 molecular patterns.These
include:
a.
lipopolysaccharide (LPS) from the gram-negative cell wall;
b.
peptidoglycan found abundantly in the gram-positive cell wall
and to a lesser degree in the gram-negative cell wall ;
c.
lipoteichoic acids found in the gram-positive cell wall;
d.
mannose-rich glycans (common in microbial glycoproteins and
glycolipids but rare in those of humans);
e.
flagellin found in bacterial flagella;
f.
pilin from bacterial pili;
g.
bacterial and viral nucleic acid. Bacterial and viral genomes
contain a high frequency of unmethylated cytosine-guanine
dinucleotide sequences (a cytosine lacking a methyl or CH3
group and located adjacent to a guanine). Mammalian DNA has a low
frequency of cytosine-guanine dinucleotides and most are methylated;
h. N-formylmethionine,
an amino acid common to bacterial proteins;
i.
double-stranded RNA unique to most viruses;
j.
lipoteichoic acids, glycolipids, and zymosan from yeast cell
walls; and
k.
phosphorylcholine and other lipids common to microbial membranes.
To recognize
these microbial molecules, various body defense cells have on their
surface a variety of receptors called pattern-recognition receptors
capable of binding specifically to conserved portions of these
molecules.
There are
two functionally different classes of cell surface pattern-recognition
receptors: endocytic pattern-recognition receptors and signaling
pattern-recognition receptors.
1.
endocytic pattern-recognition receptors
Endocytic
pattern-recognition receptors are found on the surface of
phagocytes and promote the attachment of microorganisms to
phagocytes and their subsequent engulfment and destruction. They
include:
a.
mannose receptors
Mannose
receptors bind to terminal mannose and fucose groups on microbial
glycoproteins and glycolipids. (Human glycoproteins and
glycolipids typically have terminal N-acetylglucosamine and sialic
acid groups.)
b.
scavenger receptors
Scavenger receptors bind to bacterial cell wall components such as
LPS, peptidoglyan and teichoic acids. There are also scavenger
receptors for certain components of other types of microorganisms.
.
2.
signaling pattern-recognition receptors
Signaling
pattern-recognition receptors bind a number of microbial molecules:
LPS, peptidoglycan, teichoic acids, flagellin, pilin, and DNA from
bacteria; lipoteichoic acid, glycolipids, and zymosan from fungi;
and double-stranded RNA and certain proteins and glycoproteins from
viruses. These include toll-like receptors and CD14. Binding of
microbial molecules to their receptor promotes the synthesis and
secretion of intracellular regulatory molecules such as cytokines
that a crucial to initiating innate immunity and adaptive immunity.
a.
toll-like receptors (TLRs)
A series
of signaling pattern-recognition receptors known as toll-like
receptors (TLRs) play a major role in innate immunity and the
induction of adaptive immunity.
Different combinations of TLRs appear in different cell types and
seem to appear in pairs. Different TLRs directly or
indirectly bind different microbial molecules. For
example:
1.
TLRs found on cell surfaces:
a.
TLR-1/TLR-2 pairs bind uniquely bacterial lipopeptides and
glycosylphosphatidylinositol (GPI)-anchored proteins in
parasites;
b. TLR-2/TL6 pairs bind lipoteichoic acid from gram-positive
cell walls and zymosan from fungi;
c. TLR-2 plays a role in binding peptidoglycan fragments (glycopeptides);
d. TLR-4/TLR-4 pairs bind lipopolysaccharide from gram-negative
cell walls;
e. TLR-5* binds bacterial flagellin;
2.
TLRs found in the membranes of the endosomes used to degrade
pathogens:
a.
TLR-3* binds double-stranded viral RNA;
b. TLR-7* binds uracil-rich single-stranded viral RNA such as
in HIV;
c. TLR-8* binds single-stranded viral RNA;
d. TLR-9* binds unmethylated cytosine-guanine dinucleotide
sequences (CpG DNA) found in bacterial and viral genomes.
*other
member of pair is unknown
The
binding of a microbial molecule to its TLR transmits a signal to
the cell's nucleus inducing the expression of genes coding for the
synthesis of intracellular regulatory molecules called
cytokines. The cytokines, in turn, bind to cytokine receptors
on other defense cells.
Many of
the TLRs, especially thost that bind to bacterial and
fungal cell wall components, stimulate the transcription
and translation of cytokines such as interleukin-2
(IL-2), tumor necrosis factor-alpha (TNF-alpha), and interleukin-8
(IL-8) that trigger innate immune defenses
such as inflammation, fever, and phagocytosis in order to
provide an immediate response against the invading
microorganism. Most of the TLRs that bind to viral
components trigger the synthesis of cytokines
called interferons that block viral replication
within infected host cells. Cytokines such as interleukin-6
(IL-6) that promotes B-lymphocyte activity and
interleukin-12 that promotes T-lymphocyte activity
are also produced.
TLRs also
participate in adaptive immunity by triggering
various secondary signals needed for humoral immunity (the
production of antibodies) and cell-mediated immunity (the production
of cytotoxic T-lymphocytes and additional cytokines). Without innate
immune responses there could be no adaptive immunity.
a.
T-independent (TI) antigens allow B-lymphocytes to mount
an antibody without the requirement of interaction with T4-lymphocytes.
The resulting antibody molecules are generally of the IgM isotype
and do not give rise to a memory response. There are two basic
types of T-independent antigens: TI-1 and TI-2. TI-1
antigens are pathogen-associated molecular patterns such
as lipopolysaccharide (LPS) from the outer membrane of the gram-negative
cell wall and bacterial nucleic acid. These antigens
activate B-lymphocytes by binding to their specific toll-like
receptors rather than to B-cell receptors. Antibody
molecules generated against TI-1 antigens are often called "natural
antibodies" because they are always being made against bacteria
present in the body.
b. The
activation of naive T-lymphocytes requires co-stimulatory
signals involving the interaction of accessory molecules on
antigen-presenting cells (APCs) with their corresponding ligands
on T-lymphocytes. These co-stimulatory molecules
are only synthesized when toll-like receptors on
APCs bind to pathogen-associated molecular patterns of microbes
b. CD14
CD14
is found on monocytes, macrophages, and neutrophils and promotes the
ability of TLR-4 to respond to LPS and peptidoglycan. Interaction of
these molecules with CD14 and TLR-4 leads to
an elevated synthesis and secretion of proinflammatory cytokines
such as IL-1, IL-6, IL-8, TNF-alpha, and PAF. These cytokines then
bind to cytokine receptors on target cells and initiate inflammation
and activate both the complement pathways and the coagulation
pathway
c.
NODs
NOD (nucleotide-binding oligomerization domain)
proteins, including NOD1 and NOD2, are cytostolic proteins
that allow intracellular recognition of peptidoglycan
components.
1.
NOD1 recognizes peptidoglycan containing the
muramyl dipeptide NAG-NAM-gamma-D-glutamyl-meso
diaminopimelic acid, part of the peptidoglycan monomer in common
gram-negative bacteria and just a few gram-positive bacteria.
2.
NOD2 recognizes peptidoglycan containing the
muramyl dipeptide NAG-NAM-L-alanyl-isoglutamine
found in practically all bacteria.
As
macrophages phagocytose either whole bacteria or peptidoglycan
fragments released during bacterial growth, the peptidoglycan is
broken down into muramyl dipeptides. Binding of the muramyl
dipetides to NOD1 or NOD2 leads to the activation
of genes coding for proinflammatory cytokines in a manner
similar to the cell surface toll-like receptors.
d.
secreted pattern recognition receptors
In
addition to the cell surface pattern-recognition receptors there are
also secreted pattern-recognition receptors. These bind to
microbial cell walls and enable them to be recognized by the
complement pathways and phagocytes. For example, mannan-binding
lectin is synthesized by the liver and released into the bloodstream
where it can bind to the carbohydrates on bacteria, yeast, some
viruses, and some parasites. This, in turn, activates the lectin
complement pathway and results in the production of C3b, a molecule
that promotes the attachment of microorganisms to phagocytes.
2. Cytokines
Cytokines
are low molecular weight, soluble proteins that are produced in
response to an antigen and function as chemical messengers for
regulating the innate and adaptive immune systems. They are
produced by virtually all cells involved in innate and adaptive
immunity, but especially by T helper (Th) lymphocytes. The activation
of cytokine-producing cells triggers them to synthesize and secrete
their cytokines. The cytokines, in turn, are then able to bind to
specific cytokine receptors on other cells of the immune system and
influence their activity in some manner.
Cytokines
are pleiotropic, redundant, and multifunctional.
-
Pleiotropic means that a
particular cytokine can act on a number of different types of cells
rather than a single cell type.
-
Redundant refers to to the
ability of a number of different cytokines to carry out the same
function.
-
Multifunctional means the
same cytokine is able to regulate a number of
different functions.
Some
cytokines are antagonistic in that one cytokine stimulates a
particular defense function while another cytokine inhibits that
function. Other cytokines are synergistic wherein two different
cytokines have a greater effect in combination than either of the two
would by themselves.
There are
three functional categories of cytokines:
1.
cytokines that regulate innate immune responses,
2. cytokines that regulate adaptive Immune responses, and
3. cytokines that stimulate hematopoiesis.
Cytokines that regulate innate immunity are produced
primarily by mononuclear phagocytes such as macrophages and dendritic
cells although they can also be produced by T-lymphocytes, NK cells,
and other cells. They are produced primarily in response to
pathogen-associated molecular patterns such as LPS,
peptidoglycan monomers, teichoic acids, and double-stranded DNA. Most
act on leukocytes and the endothelial cells that form blood vessels in
order to promote and control early inflammatory responses.
Examples
include:
a.
Tumor necrosis factor-alpha (TNF-alpha)
TNF-alpha is the principle cytokine that
mediates acute inflammation. In excessive
amounts it also is the principal cause of systemic
complications such as the shock cascade.
Functions include acting on endothelial cells to stimulate
inflammation and the coagulation pathway; stimulating endothelial
cells to produce selectins and ligands for leukocyte integrins
during diapedesis; stimulating endothelial cells and macrophages to
produce chemokines that contribute to diapedesis, chemotaxis and the
recruitment of leukocytes; stimulating macrophages to secrete
interleukin-1 (IL-1) for redundancy; activating neutrophils and
promoting extracellular killing by neutrophils; stimulating the
liver to produce acute phase proteins, and acting on muscles and fat
to stimulate catabolism for energy conversion. In addition, TNF is
cytotoxic for some tumor cells; interacts with the hypothalamus to
induce fever and sleep; stimulates the synthesis of collagen and
collagenase for scar tissue formation; and activates macrophages.
TNF is produced by monocytes,macrophages, dendritic cells, Th1
cells, and other cells.
b.
Interleukin-1 (IL-1)
IL-1
function similarly to TNF in that it mediates acute inflammatory
responses. It also works synergistically with TNF to
enhance inflammation. Functions of IL-1 include promoting
inflammation; activating the coagulation pathway, stimulating the
liver to produce acute phase proteins, catabolism of fat for energy
conversion, inducing feverand sleep; stimulates the synthesis of
collagen and collagenase for scar tissue formation; stimulates the
synthesis of adhesion factors on endothelial cells and leukocytes
for diapedesis; and activates macrophages. IL-1 is produced by
monocytes, macrophages, dendritic cells, and a variety of other
cells in the body.
c. Chemokines
Chemokines
are a group of cytokines that enable the
migration of leukocytes from the blood to the tissues at the site of
inflammation. They increase the affinity of integrins on
leukocytes for ligands on the vascular wall during diapedesis,
regulate the polymerization and depolymerization of actin in
leukocytes for movement and migration, and function as
chemoattractants for leukocytes. In addition, they trigger some WBCs
to release their killing agents for extracellular killing and induce
some WBCs to ingest the remains of damaged tissue. Chemokines also
regulate the movement of B-lymphocytes, T-lymphocytes, and dendritic
cells through the lymph nodes and the spleen. Certain chemokines
have also been shown to suppress HIV, probably by binding to the
chemokine receptors serving as the second binding factor for HIV on
CD4+ cells. When produced in excess amounts, chemokines
can lead to damage of healthy tissue as seen in such disorders as
rheumatoid arthritis, pneumonia, asthma, adult respiratory distress
syndrome (ARDS), and septic shock. Examples of chemokines include IL-8,
MIP-1a, MIP-1b, MCP-1, MCP-2, MCP-3, GRO-a, GRO-b, GRO-g, RANTES,
and eotaxin. Chemokines are produced by many cells including
leukocytes, endothelial cells, epithelial cells, and fibroblasts.
d.
Interleukin-12 (IL-12)
IL-12
is a primary mediator of early innate immune responses to
intracellular microbes. It is also an inducer of
cell-mediated immunity. It functions to stimulate the
synthesis of interferon-gamma by T-lymphocytes and NK cells;
increases the killing activity of CTLs and NK cells; and stimulates
the differentiation of naive T4-lymphocytes into interferon-gamma
producing Th1 cells. It is produced mainly by macrophages
and dendritic cells.
e. Type I
Interferons
Interferons modulate the activity of virtually every component of
the immune system. Type I interferons include more than 20
types of interferon-alpha, interferon-beta, interferon omega, and
interferon tau. There is only one type II interferon,
interferon-gamma.
Type I
interferons, produced by virtually any virus-infected cell,
provides an early innate immune response against viruses.
Interferons induce uninfected cells to produce enzymes capable of
degrading mRNA. These enzymes remain inactive until the
uninfected cell becomes infected with a virus. At this point, the
enzymes are activated and begin to degrade both viral and
cellular mRNA. This not only blocks viral protein
synthesis, it also eventually kills the infected cell.
They also promote body defenses by enhancing the activities of CTLs,
macrophages, dendritic cells, NK cells, and antibody-producing cells.
Type I
interferons also induce MHC-I antigen expression needed for
recognition of antigens by CTLs; augment macrophage, NK cell, CTL,
and B-lymphocyte activity; and induce fever. Interferon-alpha is
produced by T-lymphocytes, B-lymphocytes, NK cells, monocytes/macrophages;
interferon-beta by virus-infected cells, fibroblasts, macrophages,
epithelial cells, and endothelial cells.
f.
Interleukin-6 (IL-6)
IL-6
functions to stimulate the liver to produce acute phase proteins;
stimulates the proliferation of B-lymphocytes; and increases
neutrophil production. IL-6 is produced by many cells
including T-lymphocytes, macrophages, monocytes, endothelial cells,
and fibroblasts.
g.
Interleukin-10 (IL-10)
IL-10
is an inhibitor of activated macrophages and dendritic cells and as
such, regulates innate immunity and cell-mediated immunity.
IL-10 inhibits their production of IL-12, co-stimulator molecules,
and MHC-II molecules, all of which are needed for cell-mediated
immunity. IL-10 is produced mainly by macrophages, and Th2
cells.
h.
Interleukin 15 (IL-15)
IL-15
stimulates NK cell proliferation and proliferation of T-lymphocytes.
IL-15 is produced by various cells including macrophages.
i.
Interleukin-18 (IL-18)
IL-18
stimulates the production of interferon-gamma by NK cells and T-lymphocytes
and thus induces cell-mediated immunity. It is produced
mainly by macrophages.
3. Harmful Effects Associated with Pattern-Recognition Receptors and
Cytokine Production
There are
a number of harmful effects that are known to occur as a result of
either an overactive or an underactive innate immune response. These
include:
a.
Sepsis (Systemic Inflammatory Response Syndrome or SIRS)
from an Overactive Innate Immune Response
Cytokines such as tumor necrosis factor-alpha (TNF-alpha),
interleukin-1 (IL-1), and interleukin-8 (IL-8) are known as
proinflammatory cytokines because they promote inflammation.
Some cytokines, such as IL-8, are also known as chemokines
.
They promote an inflammatory response by enabling white blood cells
to leave the blood vessels and enter the surrounding tissue, by
chemotactically attracting these white blood cells to the infection
site, and by triggering neutrophils to release killing agents for
extracellular killing. In addition to promoting an inflammatory
response, these same cytokines activate the complement pathways
as well as the coagulation pathway
.
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Inflammation is the first response to
infection and injury and is critical to body defense. Basically,
the inflammatory response is an attempt by the body to restore and
maintain homeostasis after injury. Most of the body defense
elements are located in the blood, and inflammation is the means
by which body defense cells and defense chemicals leave the blood
and enter the tissue around an injured or infected site. The
release of proinflammatory cytokines eventually leads to
vasodilation of blood vessels. Vasodilation
is a reversible opening of the junctional
zones between endothelial cells
of the blood vessels and results in increased blood vessel
permeability. This enables plasmathe liquid portion of the blood,
called plasma, to enter the surrounding tissue. The plasma
contains defense chemicals such as antibody molecules, complement
proteins, lysozyme, and defensins. Increased capillary
permeability also enables white blood cells to squeeze out of the
blood vessels and enter the tissue. As can be seen, inflammation
is necessary part of body defense. Excessive or prolonged
inflammation can, however, cause harm as will be discussed below.
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As mentioned in a previous section, products
of the complement pathways
lead to more inflammation, opsonization of bacteria, chemotaxis of
phagocytes to the infected site, and MAC lysis of gram-negative
bacteria.
The products of the coagulation pathway
lead to the clotting of blood to stop bleeding, more inflammation,
and localization of infection.
At
moderate levels, inflammation, products of the complement pathways,
and products of the coagulation pathway are essential to body
defense. However, these same processes and products when excessive,
can cause considerble harm to the body.
When
there is a minor infection with few bacteria present,
low levels of cell wall components are present. This leads to
moderate cytokine production with the results being primarily
beneficial. However, in the case of a severe infection
with very large numbers of bacteria present, high levels of
cell wall components are present. This leads to excessive
cytokine production with the results causing damage to the body.
This
excessive inflammatory response is referred to as Systemic
Inflammatory Response Syndrome or SIRS.
Death is a result of
what is called the shock cascade. The sequence of events is
as follows:
This is seen during septicemia
,
a condition where bacteria enter the blood and cause harm. There
are approximately 750,000 cases of septicemia per year in the U.S.
and the mortality rate is between 20% and 50%. Over 210,000 people
a year in the U.S. die from septic shock. Approximately 45%
of the cases of septicemia are due to gram-positive bacteria, 45%
are a result of gram-negative bacteria, and 10% are due to fungi (mainly
the yeast Candida).
b. People
with an underactive form of TLR-4, the toll-like
receptor for bacterial LPS, have been found to be five times
as likely to contract a severe bacterial infection over a
five year period than those with noemal TLR-4.
c. Most
people that die as a result of Legionnaire's disease
have been found to have a mutation in the gene coding for
TLR-5.
d. People
with the autoimmune disease systemic lupus erythematosis
have an altered form of TLR-9 that reacts with the body's
own DNA.
e.
Mutations in the gene coding for NOD2 that prevent
the NOD2 from recognizing muramyl dipeptide make a person
more susceptible to Crohn's disease, an
inflammatory disease of the large intestines.
4.
Therapeutic Possibilities
Researchers
are now looking at various ways to either artificially
activate TLRs in order to enhance immune responses or inactivate TLRs
to lessen inflammatory disorders. Examples of agents being
evaluated in clinical studies include:
a. TLR-4
and TLR-9 activators: as vaccine adjuvants to activate the immune
system.
b. TLR-7 activator: as an antiviral against hepatitis C.
c. TLR-4 inhibitor: as an antisepsis agent against SIRS.
d. General TLR inhibitors: to treat autoimmune disorders.
A number of
human cytokines produced by recombinant DNA
technologies are now being used to treat various infections or
immune disorders. These include:
1. recombinant
interferon alfa-2a (Roferon-A): a cytokene used to
treat Kaposi's sarcoma, chronic myelogenous leukemia, and hairy cell
leukemia.
2. peginterferon
alfa-2a (Pegasys) :
used to treat hepatitis C
(HCV).
3. recombinant
interferon-alpha 2b (Intron A): a cytokine produced by
recombinant DNA technology and used to treat Hepatitis B; malignant
melanoma, Kaposi's sarcoma, follicular lymphoma, hairy cell leukemia,
warts, and Hepatitis C.
4. peginterferon alfa-2b (PEG-Intron; PEG-Intron
Redipen): used
to treat hepatitis C (HCV).
5. recombinant Interferon alfa-2b plus
the antiviral drug ribavirin (Rebetron):
used to treat hepatitis C (HCV).
6. recombinant interferon-alpha n3 (Alferon N): used
to treat warts.
7. recombinant iInterferon alfacon-1 (Infergen)
: used to treat
hepatitis C (HCV).
8. G-CSF (granulocyte colony stimulating factor):
for reduction of infection in people after myelotoxic anticancer
therapy for solid tumors.
9. GM-CSF (granulocyte-macrophage colony
stimulating factor): for hematopoietic reconstruction after bone
marrow transplant in people with lymphoid cancers.
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