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TOLL-LIKE RECEPTORS (TLRs)

TLR3

Double stranded RNALipoproteinZymosan

GPI Anchors

TLR2 TLR6

CpG DNA

TLR9

Imidazoquinolinesss RNA

TLR7TLR4

CD 14

LPS

MD2

TLR2TLR1

Triacylated LipoproteinFlagellin

TLR5

TOLL-LIKE RECEPTORS (TLRs)

Recognition of pathogen-associated molecular signatures is critically important in proper

activation of the immune system. The Toll-like receptor (TLR) signaling network is responsible

for innate immune response. In human, 10 TLRs recognize a variety of ligands from pathogens

to trigger immunological responses. (Medzhitov et al., 1997; Chaudhary et al., 1998; Rock et

al., 1998; Takeuchi et al., 1999; Du et al., 2000; Chuang et al., 2001) TLRs activate NK- B

and other signaling pathways, which results in the secretion of various inflammatory cytokines.

From Toll Receptors to Toll-Like Receptors

The first member of the TLR family identified was a Drosophila protein implicated in

dorsoventral patterning during embryonal development (Hashimoto et al., 1988). Gay and

Keith were the first to realize that the intracellular domain of Drosophila Toll showed striking

similarities to the intracellular domain of the mammalian interleukin-1 (IL-1) receptor, and

Lemaitre et al. demonstrated that Drosophila Toll was also involved in the immune response of

the adult fly. Different human homologues of Drosophila Toll were identified and shown to

induce activation of the transcription factor nuclear factor- B (NF- B) upon overexpression,

revealing that TLRs and IL-1 receptors trigger similar signal transduction cascades (Medzhitov

et al., 1997; Rock et al., 1998).

In 1997, Janeway et al. discovered the first human homologue of the drosophila Toll receptor,

now known as Toll-like receptor 4. It contained the Toll-like receptor/IL-1 receptor

intracytoplastic domain (Slack et al ., 2000 ), but instead of an immunoglobulin (Ig) extracellular

domain like the IL-1 receptor, it showed a structure similar to that of the fly receptor, composed

of leucine-rich repeats. This similarity represented the ancient strategy of pattern recognition

receptors conserved throughout evolution and utilized by both human beings and insects. In

1998, Poltorak et al. discovered by positional cloning that the lps gene in the lipopolysaccharide

(LPS)-nonresponsive mouse strain CH3/HeJ encoded a murine member of the TLR family,

providing the first clue of a function as pattern recognition receptors for mammalian TLRs. Once

Toll-like receptors were discovered to recognize pathogen-associated molecular patterns, they

became the most important group of pattern recognition receptors in the innate immune system.

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Structural importance of Toll-IL-1 receptor (TIR) homology domain

The structure of the Toll-like receptor has now been well characterized and has provided useful

information about the downstream cellular signaling that occurs after ligand binding (Kang et

al., 1996) (Figure 2.1). Toll-like receptors are transmembrane proteins with a series of leucine-

rich repeats in the N-terminal extracellular domain and a cytoplasmic portion greatly similar in

structure to that of IL-1 receptor (Bowie and O’Neill, 2000). This intracellular region is hence

referred to as the Toll-IL-1 receptor homology domain (Slack et al., 2000). The Toll-IL-1

receptor motif is also found in a number of important adaptor proteins that recruit downstream

kinases and transcription factors, such as myeloid differentiation factor 88, Toll-IL-1 receptor

domain containing adaptor protein, and Toll-IL-1 receptor domain containing adaptor inducing

interferon-β (IFN-β) (Kang et al., 1996).

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Figure 1. Toll-Like Receptors: Structure and Heterodimerisation

TLR Family Structure

Extracellular

TIR domain(Highly conserved cytoplasmic domain)

Transmembrane Domain

Leucine rich repeats

Intracellular

Functional Interaction between Receptors

Fungi

TLR2 TLR6

SIGNALING PATHWAY

It is the physical interaction between the Toll-IL-1 receptor domains of the Toll-like receptor and

the adaptor proteins that form a structural platform on which other downstream signaling

molecules dock to initiate the signaling cascade. This area is so crucial that even single point

mutations in the Toll-IL-1 receptor domain have been shown to abolish the host immune

response to pathogen-associated molecular pattern stimulation of some Toll-like receptors (Kang

et al., 1996).

Although the exact nature of the physical interaction between the Toll-like receptor Toll-IL-1

receptor domain and adaptor proteins has not been fully clarified, structure and function studies

have provided important information regarding the molecular basis of Toll-IL-1 receptor

signaling. A large, conserved area containing a special configuration of amino acids called the

‘‘BB loop’’ was shown to protrude away from the rest of the Toll-IL-1 receptor domain that may

mediate interactions with downstream adaptor molecules (Xu et al., 2000).

Also, evidence suggests that multiple signaling pathways are dependent on common Toll-IL-1

receptor residues, and the differential outcomes of Toll-like receptor activation probably reflect

diverging signaling pathways downstream of the Toll-IL-1 receptor domain (Ronni et al., 2003).

It is uncertain whether these structurally critical areas participate in the oligomerization of the

Toll-like receptors, in the interactions with adaptor proteins, or with the subsequently recruited

kinases. Attempts to isolate the critical amino acid residues within the Toll-IL-1 receptor domain

involved in Toll-like receptor signaling continue.

Characteristics of Toll-Like Receptor

The toll-like receptor (TLR) signaling pathway is the front-line subsystem against invasive

microorganisms for both innate and adaptive immunity (Iwasaki and Medzhitov , 2004 ). To

sense innumerable and various pathogenic threats, TLRs have evolved to recognize pathogen-

associated molecular patterns (PAMPs), which represent molecular features on the surface of

pathogens. The TLR gene family and their pathways have been evolutionarily well conserved in

both invertebrates and vertebrates (Hoffmann and Reichhart , 2002 ; Roach et al. , 2005 ).

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Each TLR binds to a variety of PAMPs that work as molecular markers of potential pathogens

that the host shall be defended against. For example, TLR4 was found to be a receptor for

lipolysaccharide (LPS) and essential to generate responses to Gram-negative bacteria in which

LPS is a part of the outer membrane (Poltorak et al ., 1998 ), TLR9 responds to DNA-containing

unmethylated CpG motifs (Hemmi et al ., 2000 ), TLR3 is activated by double-stranded RNA

(Alexopoulou et al ., 2001 ), and bacteria flagellin activates TLR5 (Hayashi et al. , 2001 ).

TLRs and interleukin 1 receptors (IL-1Rs) have a conserved region of amino acids, which is

known as the toll/IL-1R (TIR) domain (Slack et al ., 2000 ). Signaling of the TLR/IL-1R

superfamily is mediated through myeloid differentiation primary response gene 88 (MyD88), IL-

1R-associated kinases (IRAKs), transforming growth factor beta-activated kinase 1 (TAK1),

TAK1-binding protein 1 (TAB1), TAB2, tumor necrosis factor (TNF) receptor-associated factor

6 (TRAF6), etc. (Akira and Takeda , 2004 ).

It should be mentioned that TLR1, TLR2, TLR6, TLR4, and TLR5 are located on the plasma

membrane, whereas TLR3, TLR7, and TLR9 are not located on the cell surface (Akira and

Takeda , 2004 ). While ligands for each TLR and interactions downstream of receptors are now

being identified at a dramatic pace, doubt is now being cast on the global logic behind all TLR

pathways. It was argued that the TLR pathway forms an hourglass structure (Beutler , 2004 ), but

the precise shape of the global TLR signaling network and its functional implications has not

been elucidated. Since TLRs activate innate immunity and influence the nature of adaptive

immunity (Hoebe et al. , 2004 ), understanding the logic behind TLR signaling is the most

important topic in immunology.

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TLR3

Double stranded RNALipoproteinZymosan

GPI Anchors

TLR2 TLR6

CpG DNA

TLR9

Imidazoquinolinesss RNA

TLR7TLR4

CD 14

LPS

MD2

TLR2TLR1

Triacylated LipoproteinFlagellin

TLR5

Figure TLRs and their Ligands

Table 2.1 Toll-like receptors and their characteristics(Kang et al., 2006)

Toll-like receptor

Endogenous ligands

Exogenousligands

Cytokines and effector

molecules induced

Proposed effector functions

TLR1 None identified Tri-acylated LPMycobacterial 19-kd LP(TLR2/TLR1)

TNF-αIL-12

Defense against Mycobacteria and other organisms expressing tri-acylated LP

TLR2 HSPgp96HSP60HSP70

PG of gram-positive bacteriaAcylated Lipoprotein Zymosan of yeast Lipoteichoic acidGPI anchors (Trypanosoma cruzi)Outer membrane protein A (Klebsiella pneumoniae)LAM (Mycobacteria)Mycobacterial 19-kd LP

TNF-α, IL-1bIL-6IL-8IL-10IL-12NO12IL-4, IL-5,IL-6, IL-13(mast cells)

Defense against various gram-positive bacteria, Mycobacteria, Mycoplasma, protozoa, and fungi Activation of respiratory burstInduction of apoptosis. Mast cell activation and degranulation

TLR3 None identified Double-stranded RNA IFN-β Antiviral defense

TLR4 HSPgp96HSP60HSP70 β-defensin 2 Fibrinogen

LPSF protein of RSV1 Escherichia coli P fimbriaeMouse mammary tumor virus envelope proteins

TNF-α, IFN-βIL-1, IL-6,IL-10,IL-13, MacrophageInflammatory protein-1a/b

Defense against various gram-negative bacteria, fungi, and viruses Induction of apoptosis

TLR5 None identified Flagellin TNF-α,IL-1b,IL-6IL-10, IFN-γ

Defense against flagellated bacteriaDC maturation

TLR6 None identified Di-acylated LP (Mycoplasma)Zymosan of yeast GPI anchors (T cruzi)

TNF-α Defense against bacteria, fungi, mycoplasma, and protozoa

TLR7 Single-stranded RNA (influenza VirusSingle-stranded RNA (HIV-1)

Midazoquinolines (Imiquimod, Resiquimod) Loxoribine, Bropirimine

IFN-α (plasmacytoid DCs)IFN-γ (T cells)IFN-β, TNF-α,IL-1,IL-6, IL-8 IL-12, IL-18

Antiviral and antitumor defenseDC maturationActivation and migration of Langerhans cells from skin to lymph nodes TH1 development

TLR8 Single-strandedRNA (HIV-1)

Imidazoquinolines (Imiquimod, Resiquimod)

Similar to TLR7 Similar to TLR7

TLR9 Chromatin-IgG complexes

Unmethylated Cytidine-guanine DNALive or inactivated Herpes simplex virus

IFN-α (plasmacytoid DCs)IFN-β ,IFN-γ (NK cells), IL-6,IL-12

Antibacterial and antiviral defense TH1 development B cell proliferationDC maturation

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TLR10 None identified None identified Unknown Unknown

2.4 Receptors of the Adaptive versus those of the Innate Immune System

To appreciate the significance of Toll-like receptors and their pivotal role in immunology, it is

helpful to review the differences between adaptive and innate immunity (Kang et al., 1996). The

innate immune system is charged with curbing the proliferation of an invading pathogen during

the initial stages of an infection, before the lymphocyte expansion that characterizes the adaptive

immune response. The hallmark of the innate immune system is the rapidity with which it

responds to microbes and orchestrates the appropriate cellular response to defend the host. For

this response to occur, a foreign organism must be quickly recognized and identified as a threat.

At the heart of innate immunity is professional antigen presenting cells (APCs), such as

macrophages and dendritic cells (DCs) that provide continual surveillance of the environment

and are prepared to rapidly alert other vital components of the immune system to perilous

substances.

Instead of differentiating the countless potential microorganisms, APCs recognize patterns that

are common and indispensable among pathogen classes, termed ‘‘pathogen-associated molecular

patterns.’’ A classic example of a PAMP is the lipopolysaccharide (LPS) of gram-negative (GN)

bacteria, which serves as a vital structural component of the cell wall and is found across all GN

bacterial species. The receptors that recognize PAMPs are known as pattern-recognition

receptors and are found both on cellular membranes and as circulating plasma proteins. These

receptors are germ-line encoded, that is, they rely on inherited genetic material to provide the

different receptor specificities. It is estimated that the receptors of the innate immune system

number only in the hundreds and each type of receptor is identical for each individual

(Medzhitov and Janeway, 2000). Thus, the recognition of molecular patterns that are vital and

common to large groups of pathogenic organisms confers an evolutionary advantage to offspring

and is an efficient use of a finite genome.

During maturation, random combinations of genetic material create an incredibly large repertoire

of receptors, endowing each lymphocyte with a unique receptor. The differences in the way the

receptors of the adaptive and innate immune response are genetically engineered are

complementary: the variability of the adaptive immune response is much more significant, on the 7 | P a g e R A G H V E N D R A S A C H A N

order of 1018 potential lymphocyte receptors per individual, yet subsequent generations must

reinvent their own defense (Kang et al., 1996). The innate immune system has a limited variety

of highly evolved receptors that have been retained through generations. Also contributing to the

complementary functions are the differences in onset of action. In the adaptive immune response,

time is required for the selected cell to clonally proliferate and mature into a fully functional

effector cell, while cells of the innate immune system are able immediately to mount an effective

immune response. Toll-like receptors represent a class of membrane bound pattern recognition

receptors that not only recognize common pathogens but also, upon ligand binding, initiate a

cascade of cellular signaling that directs the subsequent immune responses.

2.5 TLRs Distribution on Dendritic Cells

DC maturation, which is mediated by TLR family signaling, is a critical link between innate and adaptive immunity (Iwasaki et al., 2004).

Figure 2.3 TLRs Distribution on Dendritic Cells (Iwasaki et al., 2004)

2.6 Toll-Like Receptor Signal Transduction

The TLR signaling through different intracellular molecules, such as MAP kinases and IκB

kinases which are conserved signaling elements for many receptors, leads to a distinct set of

proinflammatory gene expressions (Jayalakshmi et al., 2007).

2.6.1 TLR mediated MyD88-dependent and independent cellular signaling

The signaling pathways activated by TLRs are broadly classified into MyD88-dependent and

independent pathways (Takeda and Akira, 2005) as MyD88 is the universal adapter protein

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recruited by all TLRs except TLR3. The major pathways activated by TLR engagement are

passed through I B kinase (IKK), MAPK and phosphatidylinositol 3-kinase (PI3K)/Akt

pathways. These pathways regulate the balance between cell viability and inflammation. The

signaling pathways activated by a specific TLR are largely dictated by the adapter proteins

recruited to the intracellular domain of the TLR upon ligand binding (Akira and Takeda, 2004).

There are currently four cytosolic adaptor proteins that are thought to play a crucial role in

specificity of individual TLR-mediated signaling pathways. Amongst them, TLR4 signaling

involves all four adapter proteins, MyD88 (myeloid differentiation primary response gene 88),

MyD88 adapter like [MAL; also known as TIRAP (TIR domain-containing adapter protein)],

TIR domain-containing adapter protein inducing IFN- β [TRIF; also known as TICAM1 (TIR

domain-containing adapter molecule 1)], and TRIF-related adapter molecule [TRAM; also

known as TICAM2 (TIR domain-containing adapter molecule 2)] (McGettrick and O'Neill,

2004). The differential recruitment of these adapter proteins by different TLRs form the basis for

the specificity in the signaling process activated by them.

2.6.1.1 MyD88 is the primary adapter for microbial signaling

Every TLR member differentially utilizes adapters, but MyD88 (296 amino acid protein) seems

to be the widely used adapter molecule. MyD88 harbors a TIR domain as well as a death

domain. The carboxy terminal of TIR domain interacts with the cognate domains in the

cytoplasmic tails of the TLRs, and the amino terminal death domain mediates the interaction

with the corresponding domain of interleukin 1 receptor-associated kinase 4 (IRAK4) (Wesche

et al., 1997; Li et al., 2002). MyD88 was originally isolated as a myeloid differentiation primary

response gene that is rapidly induced upon IL-6 stimulated differentiation of M1 myloleukemic

cells into macrophages (Lord et al., 1990).

2.6.1.2 Adapters mediating MyD88-independent signaling

Most of the TLRs seem to be absolutely dependent on the expression of MyD88 for all of their

functions. MyD88-independent signaling events are controlled by TRIF/TRAM (for TLR4 and

TLR 2,6) and induce IRF3-dependent type I interferon production (Fitzgerald et al., 2003;

Hoebe et al., 2003; Oshiumi et al., 2003; Yamamoto et al., 2003).

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2.6.2 Kinases involved in signaling from adapters to transcription factors

2.6.2.1 Downstream of TLR signaling by adapters are mediated by IRAK family

The next component of downstream TLR signaling is the IRAK family members. IRAKs are

important mediators in the signal transduction of the TLR family as they may act to potentiate

the downstream signaling. So far, four IRAKs have been identified, such as IRAK1, IRAK2,

IRAK4 and IRAKM. IRAK1 and IRAK4 possess intrinsic serine/threonine protein kinase

activities, whereas IRAK2 and IRAKM lack this activity, that may negatively regulate TLR

mediated signaling. IRAK1 has three TRAF6 (tumor necrosis factor receptor associated factor 6)

binding motifs to mediate the interaction with TRAF6 (Ye et al., 2002) and undergoes

autophosphorylation. IRAK4 and IRAK1 are sequentially phosphorylated and dissociated from

MyD88, which results in activation of TRAF6.

2.6.2.2 TRAF6 is the central activator of MAPK during microbial infection

TRAF6 belongs to an E3 ubiquitin ligase family, which facilitates the synthesis of lysine 63

linked polyubiquitin chains (Chen, 2005). TRAF6 is the activator of canonical NF- B pathway

(Hayden and Ghosh, 2004). TRAF6 is ubiquitinated at K63 chains and this K63

polyubiquitinated TRAF6 mediates activation of the next component in the pathway, which is

most likely to be TGF-β activated kinase-1 (TAK1) (Sun et al., 2004). In fact, the TAK1

associated proteins, TAB2 and TAB3, contain a domain that interacts specifically with K63-

ubiquitin chains. This model for TLR signaling predicts that the TAK1-TAB complex associates

with K63-ubiquitinated TRAF6 to activate TAK1 kinase, which then activates the IKK complex

as well as the JNK kinases. Sato et al., 2003 reported that TRAF6 is involved in TRIF mediated

IRF3 activation and NF- B activation during TLR signaling. However, a recent paper delineated

the involvement of TRAF6 in TLR signaling, where TRAF6 is involved in MyD88 mediated

NF- B activation but not TRIF mediated NF- B activation (Gohda et al., 2004).

2.6.3 Transcription factors activated by TLR engagement

PAMPs stimulation through TLR-dependent and independent pathways converges at the

activation of transcription factors NF- B, IRF3/7/5, and/or AP-1. These transcription factors

collaborate with each other to produce a large number of cytokines, which are barely detectable

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in resting cells. The multi-transcription factor binding sites in the promoter of a given gene lead

to this highly specific activation (Jayalakshmi et al., 2007). The multistage gene regulation by

this interaction and the specific transcription factors activated is discussed below.

2.6.3.1 NF- B as double edged sword

The continued research on TLRs has led to the delineation of specificity in the regulation and

interaction of transcription factors upon stimulation leading to a highly specific gene expression.

NF- B is the major transcription factor, which functions on TLR signaling to control/elicit

inflammation. NF- B was first described as a B cell specific transcription factor that binds the B

site in the Ig light chain enhancer (Sen and Baltimore, 1986). Viral promoters contain NF- B

binding sites making it advantageous for its replication. So it is not exaggerating to say that cells

which have NF- B as a sword against the viral infection turn back against to them. NF- B has

often been called a ‘central mediator of the immune response’. MAL-MyD88 and TRAM-TRIF

pathways stimulate NF- B activation albeit with different kinetics (Selvarajoo, 2006). NF- B

activity was found to be inducible in all cell types and it is now known that members of the NF-

B/Rel family regulate many genes involved in immune and inflammatory responses (Pahl, 1999;

Hayden and Ghosh, 2004).

2.6.3.2 Activating protein-1 (AP1)

The JNK and p38 cascades are activated first and foremost in response to inflammatory

cytokines, bacterial products, and various stress factors. Activation of TAK1 during TLR

signaling results in the activation of MAPKs, including JNK/p38, leading to the activation of

AP-1 (Ninomiya et al., 1999; Akira and Takeda, 2004; Sato et al., 2005), which together with

NF- B governs the production of inflammatory cytokines and chemokines (Kawai and Akira,

2006). Activation of these JNK/p38 cascades is associated with selective activation of different

AP-1 subunits and transcription factors interacting with AP-1 (Johnson and Lapadat, 2002).

This activation via p38 is necessary for the full induction of TNF-α and IL-12 as inhibition of

p38 abrogates this biological response. All these studies together indicate that it is the differential

activation and binding of AP-1 subunits, which contribute to the inflammation.

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BACTERIAL DEATH

DIRECT ANTIMICROBIAL RESPONSEAPOPTOSIS OF HOST CELL

CELL MEDIATED IMMUNITY

INFLUENCE ADAPTIVE IMMUNE RESPONSE

T CELL

Myd88 Dependent Pathway

Myd88 Independent Pathway

PLASMA MEMBRANE

LPSLIPOPOLYSACCHARIDES

TIR

CD14MD2TLR4

DEGRADATION(Ubiquitin Mediated Proteolysis)

P

P

P

P

TRIF

AP1

ERK

MEK2MEK1

RIP1

IKKα IKKβ

IKKγ

P

NF

-кB

IкB

P

IкB

MKK7MKK4

JNK

MKK6MKK3

P38

IRF5

NF-кB

AP1

IRF5

INFLAMMATORY CYTOKINES(IL-12, IL-1, IL-6, TNF-α)

NUCLEUS

P10

5T

PL

2

P

IRF5

TAB1

TAK1

TAB2

TR

AF

6

IRAK1

IRAK4

TIRAP

TLR2 TLR6

MyD88TOLLIP

MAPK Signaling Pathway

TRAM

LIPOPROTEIN, ZYMOSAN

2.7 TLRs bridge the gap between innate and adaptive immunity

Identification of the different ligands of the Toll like receptors has allowed the study of the

cellular signaling that occurs after ligand engagement. The downstream signaling has revealed

the previously unrecognized role of the innate immune system as a regulator of the adaptive

immune response at several steps along the path from Toll-like receptor engagement to the

resultant inflammatory response. The first example of the impact of Toll-like receptors in

controlling the adaptive immune system is illustrated by the events that take place during the

physical interaction between APCs and T cells in the lymphoid organs (Kang et al., 1996).

The ‘‘2-signal hypothesis’’ states that when a circulating T cell encounters a captured antigen on

the surface of an APC in the context of a major histocompatibility complex, a second co-

stimulatory signal must be seen by the T cell for it to become activated and to clonally expand.

These essential co-stimulatory molecules include B7-1 (CD80) and B7-2 (CD86) on the surfaces

of APCs that engage their cognate receptors on T cells (CD28 and CD152) at the time of antigen

presentation. Engagement of Toll-like receptors by microbial products initiates the expression of

these second signals. If this critical communication between the T cell and the APC does not

occur, the T cell will invariably meet a fate of apoptosis or permanent anergy to the antigen

stimulus. This phenomenon constitutes a valuable safety mechanism to prevent an inadvertent

expansion of a T-cell clone; it requires that a pathogen must be recognized by the Toll-like

receptors of the innate immune system before a fully developed adaptive immunologic reaction

can occur (Kang et al., 1996).

Secondly, the innate immune system directs the type of adaptive immune response that is waged

against a stimulus. Naїve T cells have the potential to differentiate toward one of the two

mutually antagonistic poles of helper T (TH) cell types termed TH1 or TH2 subsets. The principal

function of the TH1 subset is to stimulate phagocyte-mediated defense against intracellular

organisms, whereas the TH2 cells promote IgE, eosinophil, and mast-cell-mediated immune

responses against extracellular pathogens (Barton et al., 2002; Trinchieri, 2003, Iwasaki and

Medzhitov, 2004). The APCs produce cytokines that instruct the expanding clone of T cells to

differentiate toward either a TH1 or a TH2 profile. It is becoming increasingly clear that the nature

of the antigen and the Toll-like receptor to which it binds can determine the specific cytokine

12 | P a g e R A G H V E N D R A S A C H A NFigure 2.4 TLR 2, 6 Signaling Pathway

milieu that the APC will produce to influence the polarity of the TH response (Kang et al.,

1996).

Lastly, APCs regulate a specialized subset of T cells known as regulatory T cells. Ordinarily the

peripheral effector T cells remain in a quiescent state owing to their suppression by regulatory T

cells. This mechanism of peripheral tolerance is important in protecting the host against the

development of potentially auto reactive cells, but the presence of these regulatory cells also

means that the concomitant, bystander suppression of the T cell bearing a useful receptor specific

for the pathogen-associated molecular patterns of an invading organism could result in

detrimental consequences to the host in the setting of an infection. There is now evidence that

IL- 6 secreted from Toll-like receptor activated DCs can relieve this suppression, allowing the

activation of the antigen-specific T cell during antigen presentation (Pasare and Medzhitov,

2003). Therefore, the Toll-like receptor expressing APCs not only provide the necessary co-

stimulatory signals, while presenting antigens to the naïve T cells and cytokines that instruct TH1

or TH2 differentiation but also suppress the inhibitory regulators of the T cells in the appropriate

setting, thereby permitting the mounting of an effective adaptive immune response.

Figure 2.5 TH Cell Proliferation activated by TLR (Akira et al., 2001)

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During infection, CD4+ TH cell responses polarize to become primarily TH1 or TH2. TH1 cells,

which make IFN- , are crucial for immunity to many bacterial and protozoal infections, whereas

TH2 cells, which make IL-4, IL-5, and IL-13, are important for resistance to Helminth infections.

Polarized TH1 responses are induced by dendritic cells (DCs), which respond to pathogen-derived

TLR ligands to produce IL-12 and related cytokines that are instrumental in TH1 cell outgrowth

and coordinately process and present Ag in the context of MHC class II to activate naїve TH cells.

It has become clear recently that TLR-activated DCs generally favor the development of TH1

responses due in large part to the fact that TLR ligation usually induces the production of IL-12,

a cytokine that plays a pivotal role in TH1 cell differentiation (Barton et al., 2002; Trinchieri,

2003, Iwasaki and Medzhitov, 2004). TLR ligands can activate dendritic cells to provide a

MyD88-dependent negative signal for TH2 cell development (Jie, et al., 2005).

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