Induced Innate Immunity

Early induced innate immunity begins 4 - 96 hours after exposure to an infectious agent and

involves the recruitment of defense cells as a result of pathogen-associated molecular patterns or

PAMPs binding to pattern-recognition receptors or PRRs. The induced responses of innate

immunity depends upon the cytokines and chemokines that are produced in response to pathogen

recognition. The macrophage-derived cytokines promote the phagocytic response through

recruitment and production of fresh phagocytes and opsonizing molecules. We also discuss the

role of the cytokines known as interferons, which are induced by viral infection, and at a class of

lymphoid cells, known as natural killer (NK) cells, that are activated by interferons to contribute

to innate host defense against viruses and other intracellular pathogens. The induced innate

responses either succeed in clearing the infection or contain it while an adaptive response

develops [1].

Cytokines

Cytokines are small proteins that are released by various cells in the body, usually in response to

an activating stimulus, and induce responses through binding to specific receptors.

Mode of action

1. Autocrine manner: they can act in autocrine manner, affecting the behavior of the cell

that releases the cytokine.

2. Paracrine manner: they can act in a paracrine manner, affecting the behavior of adjacent

cells.

3. Endocrine manner: Some cytokines can act in an endocrine manner, affecting the

behavior of distant cells.

Chemokines

Chemokines are a class of cytokines that have chemoattractant properties, inducing cells with

the appropriate receptors to migrate toward the source of the chemokine. The cytokines secreted

by macrophages in response to pathogens.

Examples

Interleukin-1 (IL-1),

Interleukin-6 (IL-6),

Interleukin-12 (IL-12),

TNF-α,

Chemokine interleukin-8 (IL-8).

Three major structural families: There are three major structural families.

1. The hematopoietin familyThis family includes growth hormones as well as many interleukins with roles in

both adaptive and innate immunity. For example IL-6

2. The TNF familyTNF-α is a part of this family which functions in both innate and adaptive immunity.

3. Chemokine family This family includes some membrane-bound members, for example IL-1 and IL-2.

All these three families have important local and systemic effects that contribute to both innate

and adaptive immunity [2, 3].

Function of chemokines

Chemokines function mainly as chemoattractant for leukocytes, recruiting monocytes,

neutrophils, and other effector cells from the blood to sites of infection. They can be released by

many different types of cell and serve to guide cells involved in innate immunity and also the

lymphocytes in adaptive immunity. Some chemokines also function in lymphocyte development,

migration, and angiogenesis (the growth of new blood vessels).

Classification of chemokines

Chemokines fall mainly into two related but distinct groups

1. The CC chemokinesThe CC chemokines in humans are mostly encoded in one region of chromosome 4, have

two adjacent cysteine residues in their amino-terminal region. CC chemokines bind to CC

chemokine receptors, of which there are nine so far, designated CCR1-9. . The CC

chemokines promote the migration of monocytes or other cell types. An example is

macrophage chemoattractant protein-1 (MCP-1).Other CC chemokines such as RANTES

may promote the infiltration into tissues of a range of leukocytes including effector T

cells. The only known C chemokine (with only one cysteine) is called lymphotactin and

is thought to attract T-cell precursors to the thymus.

2. The CXC chemokines CXC chemokines, the genes for which are mainly found in a cluster on chromosome 17,

have an amino acid residue between the equivalent two cysteines.CXC chemokines bind

to CXC receptors; there are five of these, CXCR1-5. These receptors are expressed on

different cell types; in general, CXC chemokines with a Glu-Leu-Arg (ELR) tripeptide

motif immediately before the first cysteine promote the migration of neutrophils. IL-8 is

an example of this type of chemokine. Other CXC chemokines that lack this motif, such

as the B lymphocyte chemokine (BLC), guide lymphocytes to their proper destination.

3. The CX3C chemokinesA newly discovered molecule called fractalkine is unusual in several ways: it has three

amino acid residues between the two cysteines, making it a CX3C chemokine; it is

multimodular; and it is tethered to the membrane of the cells that express it, where it

serves both as a chemoattractant and as an adhesion protein.

Role of chemokines in cell recruitment

The role of chemokines such as IL-8 and MCP-1 in cell recruitment is twofold.

First, they act on the leukocyte as it rolls along endothelial cells at sites of inflammation,

converting this rolling into stable binding by triggering a change of conformation in the adhesion

molecules known as leukocyte integrins. This allows the leukocyte to cross the blood vessel wall

by squeezing between the endothelial cells.

Second, the chemokines direct the migration of the leukocyte along a gradient of the chemokine

that increases in concentration toward the site of infection. This is achieved by the binding of the

small, soluble chemokines to proteoglycan molecules in the extracellular matrix and on

endothelial cell surfaces, thus displaying the chemokines on a solid substrate along which the

leukocytes can migrate.

Vasoactive mediators

Chemokines do not act alone in cell recruitment, which also requires the action of vasoactive

mediators to bring leukocytes close to the blood vessel endothelium and cytokines such as TNF-

α to induce the necessary adhesion molecules on the endothelial cells [4, 5]

Cell-adhesion molecules:

During inflammatory response, certain molecules called cell-adhesion molecules are induced on

the surface of local blood vessels endothelium, which act as a mediator in the recruitment of

activated phagocytes to site of infection. This recruitment is one of the most important function

of innate immunity.

Families of cell-adhesion molecules:

The adhesion molecules are grouped according to their molecular structure. Adhesion molecules

are grouped into three families which are important for leukocyte recruitment. They play a part

in leukocyte migration,directing many aspects of tissue and organ development, homing, and cell

cell interactions: the selectins, the integrins, and proteins of the immunoglobulin superfamily.

1.Selectins: Membrane glycoproteins with a distal lectinlike domain that binds specific

carbohydrate groups.Members of this family are induced on activated endothelium and initiate

endothelial leukocyte interactions by binding to fucosylated oligosaccharide ligands on passing

leukocytes. The three family members are E-selectin (endothelial), L-selectin (leukocyte), and P-

selectin (platelet).

2.Integrin: Phagocyte adhesion to vascular endothelium is mediated by integrins . Two of the

leukocyte integrins that function as complement receptors are CR3 and CR4.

3.immnologlobulin superfamily:

Various roles in cell adhesion. They are the ligands for integrins. Vascular endothelium, when it

is activated by inflammatory mediators, expresses two adhesion molecules ICAM-1 and ICAM-

2. These are ligands for integrins. The leukocyte integrins important for extravasation are LFA-1

and Mac-1 [6, 7]

Extravasation:

Under normal conditions, leukocytes flow is faster and are restricted to center of blood vessels

but monocytes may migrate continuously into the tissues, where they differentiate into

macrophage.

During the inflammatory response, the induction of adhesion molecules on the endothelial cells,

as well as induced changes in the adhesion molecules expressed on leukocytes, recruit large

numbers of circulating leukocytes, initially neutrophils and later monocytes, into the site of an

infection. The local blood vessels also dilate (slowing blood flow) which allow the leukocytes

movement towards endothelium cells.The migration of leukocytes out of blood vessels, a process

known as extravasation, is thought to occur in four steps.

Step 1 (Rolling adhesion) :

The first step involves family of selectins. The activation of endothelium is driven by interactions

with macrophage cytokines, particularly TNF-α, which induces rapid externalization of granules

(Weibel Palade bodies) containing P-selectin. After the appearance of P-selectin on the cell

surface, E-selectin is synthesized and both these proteins then interact with sulfatedsialyl-

Lewisx, which is present on the surface of neutrophils. This interaction cannot anchor the cells

against the shearing force of the flow of blood, and instead they roll along the endothelium,

continually making and breaking contact

Step 2 (Tight binding):

This second step depends upon interactions between the circulating monocytes and

polymorphonuclear leukocyte integrins LFA-1 and Mac-1 (which required for extravasation, and

for migration toward chemoattractants) with molecules on endothelium such as ICAM-1, which

is also induced on endothelial cells by TNF-α. LFA-1 and Mac-1 normally adhere only weakly,

but IL-8 or other chemokines, bound to proteoglycans on the surface of endothelial cells, trigger

a conformational change in LFA-1 and Mac-1 on the rolling leukocyte, which greatly increases

its adhesive properties. In consequence, the leukocyte attaches firmly to the endothelium and

rolling is arrested.

Step 3 (Diapedesis):

This step involve LFA-1 and Mac1, as well as a further adhesive interaction involving an

immunoglobulin-related molecule called PECAM or CD31 that are expressed both on leukocyte

and at the intercellular junctions of endothelial cells. Tight binding of these molecules arrests the

rolling and allows the leukocyte to squeeze between the endothelial cells.

It then penetrates the basement membrane (an extracellular matrix structure) with the aid of

proteolytic enzymes that break down the proteins of the basement membrane. The movement

through the vessel wall is known as diapedesis, and enables phagocytes to enter the subepithelial

tissues.

Step 4 (migration):

The fourth and final step in extravasation is the migration of leukocytes through the tissues under

the influence of chemokine. Chemokines such as IL-8 are produced at the site of infection and

bind to proteoglycans in the extracellular matrix. They form a matrix-associated concentration

gradient along which the leukocyte can migrate to the focus of infection. Neutrophils make up

the first wave of cells that cross the blood vessel wall to enter inflammatory sites while onocytes

can be recruited later [8-10].

Tumor necrosis factor-α (cytokine):

To prevent pathogen from entering into tissues or organs from blood, one of the role of

inflammatory mediators is to express protein on surface of endothelial cells that starts blood

clotting and thus blocking blood vessel by cutting off blood flow. In the early stage the lymph

that leaks out in tissue carries pathogen that is enclosed in phagocytic cell, especially dendritic

cells, and take to lymph node where adaptive immune response can be initiated.

Importance of TNF-α in control of local infection:

The importance of TNF-α is that it controls and respond to infection at localized area. It was

determined by experiments in which rabbits are infected locally with a bacterium. Normally, the

infection will be control at the site of the infection; if, however, an injection of anti-TNF-α

antibody is also given to block the action of TNF-α, the infection spreads via the blood to other

organs.

Release of TNF-α systematically:

Once an infection spreads to the bloodstream, however, the TNF-α which works so effectively in

local infection become catastrophic if release systematically.

Sepsis:

The presence of infection in the bloodstream which is complemented by the release of TNF-α by

macrophages in the lymphatic organs (liver, spleen, and other sites).

Causes of TNF-α f release systematically:

Vasodilation

Increased vascular permeability

This will results in septic shock, which triggers dispersed intravascular coagulation (blood

clotting) leading to the generation of clots in many small vessels and the massive consumption of

clotting proteins, which results in inability of patient to clot blood.

Consequences:

Fail to clot blood

Failure of vital organs (kidneys, liver, heart, and lungs)

Failure of normal perfusion of blood

Death occur

Mutant TNF-α receptor gene:

If mutation in TNF-α receptor gene the individual become resistant to septic shock but at a same

time don’t able to control infection at local area. Although the features of TNF-α that make it so

valuable in containing local infection are precisely those that give it a central role in the

pathogenesis of septic shock, it is clear from the evolutionary conservation of TNF-α that its

benefits in the former area far offset the overwhelming consequences of its systemic release [11-

14].

Cytokines released by phagocytes activate the acute-phase response.

The cytokines produced by macrophages has long-range effects. One of them is the elevation of

body temperature, which is mainly caused by TNF-α, IL- 1, and IL-6. These are termed

endogenous pyrogens because they cause fever and derive from an endogenous source rather

than from bacterial components. Fever is generally beneficial to host defense; most pathogens

grow better at lower temperatures and adaptive immune responses are more intense at elevated

temperatures. Host cells are also protected from the deleterious effects of TNF-α at raised

temperatures.

Acute-phase response:

This involves a shift in the proteins secreted by the liver into the blood plasma and results from

the action of IL-1, IL-6, and TNF-α on hepatocytes. In the acute-phase response, levels of some

plasma proteins go down, while levels of others increase markedly. The proteins whose synthesis

is induced by TNF-α, IL-1, and IL-6 are called acute-phase proteins. They mimic the action of

antibodies, but, unlike antibodies, these proteins have broad specificity for pathogen-associated

molecular patterns and depend only on the presence of cytokines for their production.

Examples: C-reactive protein and Mannan-binding lectin [15-18]

Interferons:

Infection of cells with viruses induces the production of proteins that are known as interferons

because they were found to interfere with viral replication in previously uninfected tissue culture

cells. They are believed to have a similar role in vivo, blocking the spread of viruses to

uninfected cells. These antiviral effector molecules, called interferon-α (IFN-α) and interferon-β

(IFN-β).

Defense Contribution by interferons:

Interferons make several contributions to defense against viral infection. An obvious and

important effect is the induction of a state of resistance to viral replication in all cells. . IFN-α

and IFN-β are secreted by the infected cell and then bind to a common cell-surface receptor,

known as the interferon receptor, on both the infected cell and nearby cells. This signaling

pathway, which we will describe in detail in Chapter 6, rapidly induces new gene transcription as

the Janus-family kinases directly phosphorylate signal-transducing activators of transcription

known as STATs, which translocate to the nucleus where they activate the transcription of

several different genes. In this way interferon induces the synthesis of several host cell proteins

that contribute to the inhibition of viral replication.

Functions of interferons: (IFN)-α and –β)

The interferons (IFN)-α and -β have three major functions:

1. They induce resistance to viral replication in uninfected cells by activating genes that

cause the destruction of mRNA and inhibit the translation of viral and some host proteins

2. They can induce MHC class I expression in most cell types in the body, thus enhancing

their resistance to NK cells

3. They activate NK cells, which then kill virus-infected cells selectively [19-23].

Natural killer cells and interferons; acting as early defense:

Natural killer cells (NK cells) develop in the bone marrow from the common lymphoid

progenitor cell and circulate in the blood. They are larger than T and B lymphocytes, have

distinctive cytoplasmic granules, and are functionally identified by their ability to kill certain

lymphoid tumor cell lines in vitro without the need for prior immunization or activation

Mechanism of killing:

The mechanism of NK cell killing is the same as that used by the cytotoxic T cells generated in

an adaptive immune response; cytotoxic granules are released onto the surface of the bound

target cell, and the effector proteins they contain penetrate the cell membrane and induce

programmed cell death

Activation by interferons:

NK cells are activated in response to interferons or macrophage-derived cytokines. Although NK

cells that can kill sensitive targets can be isolated from uninfected individuals, this activity is

increased by between twentyfold and one hundredfold when NK cells are exposed to IFN-α and

IFN-β or to the NK cell-activating factor IL-12, which is one of the cytokines produced early in

many infections.

IL-12, in synergy with TNF-α, can also elicit the production of large amounts of IFN-γ by NK

cells, and this secreted IFN-γ is crucial in controlling some infections before T cells have been

activated to produce this cytokine [24-26].

NK Cells

NK cells possess receptors for self-molecules that inhibit their activation against uninfected host

cells. If NK cells are to mediate host defense against infection with viruses and other pathogens,

they must have some mechanism for distinguishing infected from uninfected cells. For this

process recognition of “altered self” is involved.

NK cells surface receptor

NK cells have two types of surface receptor that control their cytotoxic activity.

1. One type is an 'activating receptor:' it triggers killing by the NK cell. Several types of

receptor

Provide this activation signal, including calcium-binding C-type lectins that recognize a wide

variety of carbohydrate ligands present on many cells.

2. A second set of receptors inhibit activation, and prevent NK cells from killing normal

Host cells. These 'inhibitory receptors' are specific for MHC class I alleles, which helps to

explain why NK cells selectively kill target cells bearing low levels of MHC class I molecules.

Methods of recognition by NK cells

1. Thus, one possible mechanism by which NK cells distinguish infected from uninfected

cells is by recognizing alterations in MHC class I expression (Fig. 2.42).

2. Another is that they recognize changes in cell-surface glycoproteins induced by viral or

bacterial infection.

Mechanism

A proposed mechanism of recognition is shown. NK cells can use several different receptors that

signal them to kill, including Lectin like activating receptors, or 'killer receptors,' that recognize

carbohydrate on self-cells. However, another set of receptors, killer inhibitory receptors (KIRs)

in the human, recognize MHC class I molecules and inhibit killing by NK cells by overruling the

actions of the killer receptors. This inhibitory signal is lost when cells do not express MHC class

I and perhaps also in cells infected with virus, which might inhibit MHC class I expression or

alter its conformation. In contrast, infected cells can fail to increase MHC class I expression,

making them targets for activated NK cells.

.Inhibitory NK receptors

. In humans, there are inhibitory receptors that recognize distinct HLA-B and HLA-C alleles

(these are MHC class I alleles encoded by the B and C loci of the human MHC or Human

Leukocyte Antigen gene complex). Being members of the immunoglobulin gene superfamily;

they are usually called p58 and p70, or killer inhibitory receptors (KIRs). In

Addition, human NK cells express a heterodimer of two C-type lectins, called CD94 and NKG2.

Other inhibitory NK receptors specific for the products of the MHC class I loci are rapidly being

defined, and all are members of either the immunoglobulin-like KIR family or the Ly49-

Like C-type lectins.

Mechanism of inhibitory receptors

Signaling by the inhibitory NK receptors suppresses the killing activity of NK cells. This means

that NK cells will not kill healthy genetically identical cells with normal expression of MHC

class I molecules, such as the other cells of the body. Virus-infected cells, however, can become

susceptible to killing by NK cells by a variety of mechanisms.

1. Inhibition of protein synthesis

First, some viruses inhibit all protein synthesis in their host cells, so synthesis of MHC class I

proteins would be blocked in infected cells, even while being augmented by interferon in

uninfected cells. The reduced level of MHC class I expression in infected cells would make them

correspondingly less able to inhibit NK cells through their MHC-specific receptors, and therefore

more susceptible to killing.

2. Prevention of export of MHC

Second, some viruses can selectively prevent the export of MHC class I molecules, which might

allow the infected cell to evade recognition by the cytotoxic T cells of the adaptive immune

response but would make it susceptible to killing by NK cells. Finally virus infection alters the

glycosylation of cellular proteins, perhaps allowing recognition by activating receptors to

dominate or removing the normal ligand for the inhibitory receptors. Either of these last two

mechanisms could allow infected cells to be detected even when the level of MHC class I

expression had not been altered [27-29].

Summary:

Innate immunity can use a variety of induced effector mechanisms to clear an infection or,

failing that, to hold it in check until the pathogen can be recognized by the adaptive immune

system. These effector mechanisms are all regulated by germ line-encoded receptor systems that

are able to discriminate between no infected self and infectious non self-ligands. Thus the

phagocytes' ability to discriminate between self and pathogen controls its release of pro

inflammatory chemokines and cytokines that act together to recruit more phagocytic cells,

especially neutrophils, which can also recognize pathogens, to the site of infection. Furthermore,

cytokines released by tissue phagocytic cells induce fever, the production of acute-phase

response proteins including the pathogen-binding mannan-bindinglectin and the C-reactive

proteins, and the mobilization of antigen-presenting cells that induce the adaptive immune

response. Viral pathogens are recognized by the cells in which they replicate, leading to the

production of interferon that serves to inhibit viral replication and to activate NK cells, which in

turn can distinguish infected from non-infected cells.

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