Toll Like Receptors And Their Therapeutic Potential

Cellular Basis Of Disease: Why has the discovery of Toll-like receptors revolutionised our understanding of how the innate immune system works, and what is the therapeutic potential?

The body has two immune systems: the innate immune system and the adaptive immune system.

Adaptive, or acquired, immunity refers to antigen-specific defence mechanisms that take several days to become protective and are designed to react with and remove a specific antigen. This is immunity develops throughout life.

Innate immunity refers to antigen-nonspecific defence mechanisms that a host uses immediately or within several hours after exposure to an antigen. This is the immunity that you are born with, and is the initial response by the body to eliminate microbes and prevent infection.

It is in the innate immune system that Toll-like receptors are important in helping our understanding.

The most important role of the innate immune system is to react rapidly to infectious agents with the initiation an inflammatory response, and to shape the subsequent adaptive immune responses.

There are currently two different models for immune system induction. The first model predicts the recognition of non-self determinants on pathogens, and the other, more controvertial, model predicts that there is recognition of damage or danger to self-tissues.

In the first model, pathogens are recognised by either specific or general components of their structure. A system referring to the patterns that are recognised are the pathogen associated molecular patterns (PAMPs) and the receptors recognising them are pattern recognition receptors (PRRs).

The second model, put forward by Matzinger, is that it is the danger itself that is sensed. It is argued that it is tissue damage or cellular debris from necrotic cells that sends the signal for the immune system to initiate a response. The presence of DNA or RNA, that shouldn't be outside of the cell, may cause an alarm signal. Heat shock proteins released from the cell, or mannose that is normally cleaved off, may also serve as an alarm signal. It is suggested that the PRRs are there to recognise these endogenous signals from ruptured cells, and not to recognise pathogens as proposed in the first model.

It is the first model that is most widely accepted in the scientific community, and it is this model of events that I shall describe.

Activation of the innate immune system is mediated by pattern recognition receptors (PRRs) on dendritic cells, macrophages and polymorphonuclear granulocytes, that recognise pathogen-associated molecular proteins (PAMPs). PRRs recognise molecules that are not associated to human cells (PAMPs).

The best characterised signalling PRRs are the Toll-like receptors (TLRs). They are present in plants, invertebrates and vertebrates, and represent a primitive host defence mechanism against bacteria, fungi and viruses.

There are 13 TLRs that have been discovered in mammals so far, named TLR-1 to TLR-13.

TLRs are primary transmembrane proteins of immune cells, that contain leucine repeats in their extracellular domains and a cytoplasmic tail that contains a conserved region called the Toll / IL1 receptor (TIR) domain.

The Toll gene was first discovered in the fruit fly Drosophila melanogaster, but has close homologues in mammalian

immune cells. This is where the name Toll-like receptor is derived.

Different combinations of TLRs appear in pairs in different cell types. Different TLRs bind directly or indirectly to different microbial molecules.

TLRs found on cell surfaces:

a. TLR-1/TLR-2 pairs bind uniquely bacterial lipopeptides and GPI-anchored proteins in parasites

b. TLR-2/TL6 pairs bind lipoteichoic acid from gram-positive cell walls and zymosan from fung

c. TLR-4/TLR-4 pairs bind lipopolysaccharide from gram-negative cell walls;

d. TLR-5 binds bacterial flagellin

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.

Those TLRs shown above without a pair are those that have not yet had their pairing discovered.

The binding of a microbial molecule to its TLR sends a signal to the nucleus of the cell, therefore inducing the expression of genes that code for cytokines.

The cytokines then bind to cytokine receptors on other defence cells. The cytokines trigger innate immune defences such as inflammation, fever and phagocytosis to provide an immediate response against the invading microorganism.

Put pic here.

TLR signalling consists of two different pathways: a MyD88-dependent pathway that leads to the production of inflammatory cytokines, and a MyD88-independent pathway that is associated with the stimulation of Interferons (IFNs) and the maturation of dendritic cells. MyD88 is a cytoplasmic adaptator protein with a TIR domain similar to that of TLRs'.

The MyD88-dependent pathway is present in all TLRs.

TLRs induce the recruitment of MyD88 via its TIR domain which activates Interleukin-1 Receptor Kinase 1 (IRAK1) by phosphorylation.

IRAKs are serine/threonine kinases that act as signal transduction mediators for the TIR family. The IRAK family consists of two active kinases, IRAK1 and IRAK4, and two inactive kinases, IRAK2 and IRAK-M.

IRAK1 is phosphorylated by IRAK4 and leaves the MyD88-TLR receptor complex and associates with Tumor Necrosis Factor Receptor-associated factor 6 (TRAF 6) through an adaptor called TIFA. TRAF6, then induces downstream signalling, causing the activation of NFkB (a transcription factor) which, in turn induces the production of pro-inflammatory cytokines and effector cytokines that direct the adaptive immune response, such as IL1 and IL12. In the absence of IRAK1, IL1 signalling is reduced but not completely stopped.