Toll-like receptors (TLRs) are a class of
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
s that play a key role in the
innate immune system
The innate immune system or nonspecific immune system is one of the two main immunity strategies in vertebrates (the other being the adaptive immune system). The innate immune system is an alternate defense strategy and is the dominant immune s ...
. They are
single-spanning receptors usually expressed on
sentinel cells such as
macrophage
Macrophages (; abbreviated MPhi, φ, MΦ or MP) are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris and foreign substances, which do not have proteins that ...
s and
dendritic cell
A dendritic cell (DC) is an antigen-presenting cell (also known as an ''accessory cell'') of the mammalian immune system. A DC's main function is to process antigen material and present it on the cell surface to the T cells of the immune system ...
s, that recognize structurally conserved molecules derived from
microbes
A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells. The possible existence of unseen microbial life was suspected from antiquity, with an early attestation in ...
. Once these microbes have reached physical barriers such as the skin or
intestinal tract mucosa
A mucous membrane or mucosa is a membrane that lines various cavities in the body of an organism and covers the surface of internal organs. It consists of one or more layers of epithelial cells overlying a layer of loose connective tissue. It ...
, they are recognized by TLRs, which activate
immune cell
White blood cells (scientific name leukocytes), also called immune cells or immunocytes, are cell (biology), cells of the immune system that are involved in protecting the body against both infectious disease and foreign entities. White blood c ...
responses. The TLRs include
TLR1,
TLR2,
TLR3,
TLR4,
TLR5,
TLR6,
TLR7,
TLR8,
TLR9,
TLR10,
TLR11,
TLR12, and
TLR13. Humans lack genes for TLR11, TLR12 and TLR13 and mice lack a functional gene for TLR10. The receptors TLR1, TLR2, TLR4, TLR5, TLR6, and TLR10 are located on the
cell membrane
The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...
, whereas TLR3, TLR7, TLR8, and TLR9 are located in
intracellular
This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology, the study of life and of living organisms. It is intended as introductory material for novices; for more specific and technical definitions ...
vesicles (because they are sensors of
nucleic acid
Nucleic acids are large biomolecules that are crucial in all cells and viruses. They are composed of nucleotides, which are the monomer components: a pentose, 5-carbon sugar, a phosphate group and a nitrogenous base. The two main classes of nuclei ...
s).
TLRs received their name from their similarity to the protein coded by th
toll gene
Function
The ability of the immune system to recognize
molecule
A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
s that are broadly shared by
pathogen
In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory of d ...
s is, in part, due to the presence of
immune receptor
An immune receptor (or immunologic receptor) is a Receptor (biochemistry), receptor, usually on a cell membrane, which binds to a Ligand (biochemistry), ligand (usually another protein, such as cytokine) and causes a response in the immune system. ...
s called toll-like receptors (TLRs) that are expressed on the
membrane
A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...
s of
leukocyte
White blood cells (scientific name leukocytes), also called immune cells or immunocytes, are cells of the immune system that are involved in protecting the body against both infectious disease and foreign entities. White blood cells are genera ...
s including
dendritic cell
A dendritic cell (DC) is an antigen-presenting cell (also known as an ''accessory cell'') of the mammalian immune system. A DC's main function is to process antigen material and present it on the cell surface to the T cells of the immune system ...
s,
macrophages
Macrophages (; abbreviated MPhi, φ, MΦ or MP) are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris and foreign substances, which do not have proteins that ...
,
natural killer cell
Natural killer cells, also known as NK cells, are a type of cytotoxic lymphocyte critical to the innate immune system. They are a kind of large granular lymphocytes (LGL), and belong to the rapidly expanding family of known innate lymphoid cells ...
s, cells of the adaptive immunity
T cell
T cells (also known as T lymphocytes) are an important part of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell receptor (TCR) on their cell ...
s, and
B cell
B cells, also known as B lymphocytes, are a type of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted or inserted into the plasm ...
s, and non-immune cells (
epithelial
Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...
and
endothelial cells, and
fibroblasts).
The binding of
ligands
In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's ...
— either in the form of adjuvant used in
vaccination
Vaccination is the administration of a vaccine to help the immune system develop immunity from a disease. Vaccines contain a microorganism or virus in a weakened, live or killed state, or proteins or toxins from the organism. In stimulating ...
s or in the form of invasive moieties during times of natural infection — to the TLR marks the key
molecular events that ultimately lead to
innate immune responses and the development of antigen-specific acquired immunity.
Upon activation, TLRs recruit
adaptor proteins (proteins that mediate other protein-protein interactions) within the
cytosol
The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...
of the
immune cell
White blood cells (scientific name leukocytes), also called immune cells or immunocytes, are cell (biology), cells of the immune system that are involved in protecting the body against both infectious disease and foreign entities. White blood c ...
to propagate the antigen-induced
signal transduction pathway. These recruited
proteins
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, re ...
are then responsible for the subsequent activation of other
downstream proteins, including
protein kinases (IKKi,
IRAK1,
IRAK4
IRAK-4 (interleukin-1 receptor-associated kinase 4), in the IRAK family, is a protein kinase involved in signaling innate immune responses from Toll-like receptors. It also supports signaling from T-cell receptors. IRAK4 contains domain structure ...
, and
TBK1) that further amplify the signal and ultimately lead to the upregulation or suppression of
genes
In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
that orchestrate
inflammatory responses and other
transcriptional events. Some of these events lead to
cytokine
Cytokines () are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling.
Cytokines are produced by a broad range of cells, including immune cells like macrophages, B cell, B lymphocytes, T cell, T lymphocytes ...
production,
proliferation, and survival, while others lead to greater adaptive immunity.
If the ligand is a bacterial factor, the pathogen might be
phagocytosed and digested, and its
antigen
In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response.
...
s presented to
CD4+ T cells.
In the case of a viral factor, the infected cell may shut off its protein synthesis and may undergo programmed cell death (
apoptosis
Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...
). Immune cells that have detected a virus may also release anti-viral factors such as
interferons.
Toll-like receptors have also been shown to be an important link between innate and adaptive immunity through their presence in
dendritic cell
A dendritic cell (DC) is an antigen-presenting cell (also known as an ''accessory cell'') of the mammalian immune system. A DC's main function is to process antigen material and present it on the cell surface to the T cells of the immune system ...
s.
Flagellin, a TLR5 ligand, induces cytokine secretion on interacting with TLR5 on human T cells.
Superfamily

TLRs are a type of
pattern recognition receptor (PRR) and recognize molecules that are broadly shared by
pathogen
In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory of d ...
s but distinguishable from host molecules, collectively referred to as
pathogen-associated molecular patterns (PAMPs). In addition to the recognition of exogenous PAMPs, TLRs can also bind to endogenous
damage-associated molecular pattern
Damage-associated molecular patterns (DAMPs) are molecules within cells that are a component of the innate immune response released from damaged or dying cells due to trauma or an infection by a pathogen. They are also known as danger signals, an ...
s (DAMPs) such as
heat shock proteins (HSPs) or plasma membrane constituents. TLRs together with the
Interleukin-1 receptors form a receptor
superfamily, known as the "interleukin-1 receptor / toll-like receptor superfamily"; all members of this family have in common a so-called TIR (toll-IL-1 receptor) domain.
Three subgroups of TIR domains exist. Proteins with subgroup 1 TIR domains are receptors for
interleukins
Interleukins (ILs) are a group of cytokines (secreted proteins and signal molecules) that are expressed and secreted by white blood cells (leukocytes) as well as some other body cells. The human genome encodes more than 50 interleukins and related ...
that are produced by
macrophage
Macrophages (; abbreviated MPhi, φ, MΦ or MP) are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris and foreign substances, which do not have proteins that ...
s,
monocyte
Monocytes are a type of leukocyte or white blood cell. They are the largest type of leukocyte in blood and can differentiate into macrophages and monocyte-derived dendritic cells. As a part of the vertebrate innate immune system monocytes also ...
s, and
dendritic cell
A dendritic cell (DC) is an antigen-presenting cell (also known as an ''accessory cell'') of the mammalian immune system. A DC's main function is to process antigen material and present it on the cell surface to the T cells of the immune system ...
s and all have extracellular
Immunoglobulin (Ig) domains. Proteins with subgroup 2 TIR domains are classical TLRs, and bind directly or indirectly to molecules of microbial origin. A third subgroup of proteins containing TIR domains consists of
adaptor proteins that are exclusively
cytosol
The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...
ic and mediate signaling from proteins of subgroups 1 and 2.
Extended family
TLRs are present in
vertebrate
Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain.
The vertebrates make up the subphylum Vertebra ...
s as well as
invertebrate
Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...
s. Molecular building blocks of the TLRs are represented in bacteria and in plants, and
plant pattern recognition receptors are well known to be required for host defence against infection. The TLRs thus appear to be one of the most ancient, conserved components of the
immune system
The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...
.
In recent years TLRs were identified also in the mammalian nervous system. Members of the TLR family were detected on glia, neurons and on neural progenitor cells in which they regulate cell-fate decision.
It has been estimated that most mammalian species have between ten and fifteen types of toll-like receptors. Thirteen TLRs (named simply TLR1 to TLR13) have been identified in humans and mice together, and equivalent forms of many of these have been found in other mammalian species. However, equivalents of certain TLR found in humans are not present in all mammals. For example, a gene coding for a protein analogous to TLR10 in humans is present in
mice
A mouse (: mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus' ...
, but appears to have been damaged at some point in the past by a
retrovirus
A retrovirus is a type of virus that inserts a DNA copy of its RNA genome into the DNA of a host cell that it invades, thus changing the genome of that cell. After invading a host cell's cytoplasm, the virus uses its own reverse transcriptase e ...
. On the other hand, mice express TLRs 11, 12, and 13, none of which is represented in humans. Other mammals may express TLRs that are not found in humans. Other non-mammalian species may have TLRs distinct from mammals, as demonstrated by the anti-cell-wall
TLR14, which is found in the
Takifugu pufferfish.
This may complicate the process of using experimental animals as models of human innate immunity.
Vertebrate TLRs are divided by similarity into the families of TLR 1/2/6/10/14/15, TLR 3, TLR 4, TLR 5, TLR 7/8/9, and TLR 11/12/13/16/21/22/23.
[
]
TLRs in ''Drosophila'' immunity
The involvement of toll signalling in immunity was first demonstrated in the fruit fly, ''Drosophila melanogaster
''Drosophila melanogaster'' is a species of fly (an insect of the Order (biology), order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly", "pomace fly" ...
''. Fruit flies have only innate immune responses allowing studies to avoid interference of adaptive immune mechanisms on signal transduction. The fly response to fungal or bacterial infection occurs through two distinct signalling cascades, one of which is the toll pathway and the other is the immune deficiency pathway. The toll pathway is similar to mammalian TLR signalling, but unlike mammalian TLRs, toll is not activated directly by pathogen-associated molecular patterns ( PAMPs). Its receptor ectodomain recognizes the cleaved form of the cytokine spätzle, which is secreted in the haemolymph
Hemolymph, or haemolymph, is a fluid, similar to the blood in invertebrates, that circulates in the inside of the arthropod's body, remaining in direct contact with the animal's tissues. It is composed of a fluid plasma in which hemolymph ce ...
as an inactive dimeric precursor. The toll receptor shares the cytoplasmatic TIR domain with mammalian TLRs, but the ectodomain and intracytoplasmatic tail are different. This difference might reflect a function of these receptors as cytokine receptors rather than PRRs.
The toll pathway is activated by different stimuli, such as gram-positive bacteria
In bacteriology, gram-positive bacteria are bacteria that give a positive result in the Gram stain test, which is traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall.
The Gram stain ...
, fungi, and virulence factors. First, the Spätzle processing enzyme (SPE) is activated in response to infection and cleaves spätzle (''spz''). Cleaved spätzle then binds to the toll receptor and crosslinks its ectodomains. This triggers conformational changes in the receptor resulting in signalling through toll. From this point forward, the signalling cascade is very similar to mammalian signalling through TLRs. The toll-induced signalling complex (TICS) is composed of MyD88
Myeloid differentiation primary response 88 (MYD88) is a protein that, in humans, is encoded by the ''MYD88'' gene. originally discovered in the laboratory of Dan A. Liebermann (Lord et al. Oncogene 1990) as a Myeloid differentiation primary resp ...
, Tube, and Pelle (the orthologue of mammalian IRAK). Signal from TICS is then transduced to Cactus (homologue of mammalian IκB), phosphorylated Cactus is polyubiquitylated and degraded, allowing nuclear translocation of DIF (dorsal-related immunity factor; a homologue of mammalian NF-κB
Nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) is a family of transcription factor protein complexes that controls transcription (genetics), transcription of DNA, cytokine production and cell survival. NF-κB is found i ...
) and induction of transcription of genes for antimicrobial peptides
Antimicrobial peptides (AMPs), also called host defence peptides (HDPs) are part of the innate immune response found among all classes of life. Fundamental differences exist between Prokaryote, prokaryotic and eukaryota, eukaryotic cells that may ...
(AMPs) such as drosomycin
Drosomycin is an antifungal peptide from ''Drosophila melanogaster'' and was the first antifungal peptide isolated from insects. Drosomycin is induced by infection by the Toll signalling pathway, while expression in surface epithelia like the re ...
.
''Drosophila'' have a total of 9 ''toll'' family and 6 ''spz'' family genes that interact with each other to differing degrees.
TLR2
TLR2 has also been designated as CD282 (cluster of differentiation 282).
TLR3
TLR3 does not use the MyD88 dependent pathway. Its ligand is retroviral double-stranded RNA (dsRNA
Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself ( non-coding RNA) or by forming a template for the production of proteins ( messenger RNA). RNA and deoxy ...
), which activates the TRIF dependent signalling pathway. To explore the role of this pathway in retroviral reprograming, knock down techniques of TLR3 or TRIF were prepared, and results showed that only the TLR3 pathway is required for full induction of target gene expression by the retrovirus expression vector. This retroviral expression of four transcriptional factors ( Oct4, Sox2, Klf4 and c-Myc; OSKM) induces pluripotency in somatic cells. This is supported by study, which shows, that efficiency and amount of human iPSC generation, using retroviral vectors, is reduced by knockdown of the pathway with peptide inhibitors or shRNA knockdown of TLR3 or its adaptor protein TRIF. Taken together, stimulation of TLR3 causes great changes in chromatin remodeling and nuclear reprogramming, and activation of inflammatory pathways is required for these changes, induction of pluripotency genes and generation of human induced pluripotent stem cells (iPSC) colonies.
TLR11
As noted above, human cells do not express TLR11, but mice cells do. Mouse-specific TLR11 recognizes uropathogenic '' E.coli'' and the apicomplexan parasite ''Toxoplasma gondii
''Toxoplasma gondii'' () is a species of parasitic alveolate that causes toxoplasmosis. Found worldwide, ''T. gondii'' is capable of infecting virtually all warm-blooded animals, but members of the cat family (felidae) are the only known d ...
''. With ''Toxoplasma'' its ligand is the protein profilin and the ligand for ''E. coli'' is flagellin. The flagellin from the enteropathogen ''Salmonella'' is also recognized by TLR11.[
As mouse TLR11 is able to recognize ''Salmonella'' effectively, normal mice do not get infected by oral ''Salmonella'' Typhi, which causes food- and waterborne gastroenteritis and ]typhoid fever
Typhoid fever, also known simply as typhoid, is a disease caused by '' Salmonella enterica'' serotype Typhi bacteria, also called ''Salmonella'' Typhi. Symptoms vary from mild to severe, and usually begin six to 30 days after exposure. Often th ...
in humans. TLR11 deficient knockout mice
A knockout mouse, or knock-out mouse, is a genetically modified mouse (''Mus musculus'') in which researchers have inactivated, or " knocked out", an existing gene by replacing it or disrupting it with an artificial piece of DNA. They are importan ...
, on the other hand, are efficiently infected. As a result, this knockout mouse can act as a disease model of human typhoid fever.
Summary of known mammalian TLRs
Toll-like receptors bind and become activated by different ligands, which, in turn, are located on different types of organisms or structures. They also have different adapters to respond to activation and are located sometimes at the cell surface and sometimes to internal cell compartments. Furthermore, they are expressed by different types of leucocytes or other cell types:
Ligands
Because of the specificity of toll-like receptors (and other innate immune receptors) they cannot easily be changed in the course of evolution, these receptors recognize molecules that are constantly associated with threats (i.e., pathogen or cell stress) and are highly specific to these threats (i.e., cannot be mistaken for self molecules that are normally expressed under physiological conditions). Pathogen-associated molecules that meet this requirement are thought to be critical to the pathogen's function and difficult to change through mutation; they are said to be evolutionarily conserved. Somewhat conserved features in pathogens include bacterial cell-surface lipopolysaccharides (LPS), lipoproteins, lipopeptides, and lipoarabinomannan
Lipoarabinomannan, also called LAM, is a glycolipid, and a virulence factor associated with ''Mycobacterium tuberculosis'', the bacteria responsible for tuberculosis. Its primary function is to inactivate macrophages and scavenge oxidative radical ...
; proteins such as flagellin from bacterial flagella
A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...
; double-stranded RNA
Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...
of viruses; or the unmethylated CpG islands of bacterial and viral DNA
Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...
; and also of the CpG islands found in the promoters of eukaryotic DNA; as well as certain other RNA and DNA molecules. As TLR ligands are present in most pathogens, they may also be present in pathogen-derived vaccines (e.g. MMR, influenza, polio vaccines) most commercially available vaccines have been assessed for their inherent TLR ligands' capacity to activate distinct subsets of immune cells. For most of the TLRs, ligand
In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...
recognition specificity has now been established by gene targeting (also known as "gene knockout"): a technique by which individual genes may be selectively deleted in mice. See the table above for a summary of known TLR ligands.
Endogenous ligands
The stereotypic inflammatory response provoked by toll-like receptor activation has prompted speculation that endogenous activators of toll-like receptors might participate in autoimmune diseases. TLRs have been suspected of binding to host molecules including fibrinogen (involved in blood clotting
Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a thrombus, blood clot. It results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The process of co ...
), heat shock proteins (HSPs), HMGB1, extracellular matrix components and self DNA (it is normally degraded by nucleases, but under inflammatory and autoimmune conditions it can form a complex with endogenous proteins, become resistant to these nucleases and gain access to endosomal TLRs as TLR7 or TLR9). These endogenous ligands are usually produced as a result of non-physiological cell death.
Signaling
TLRs are believed to function as dimers. Though most TLRs appear to function as homodimers, TLR2 forms heterodimers with TLR1 or TLR6, each dimer having a different ligand specificity. TLRs may also depend on other co-receptors for full ligand sensitivity, such as in the case of TLR4's recognition of LPS, which requires MD-2. CD14
CD14 ( cluster of differentiation 14) is a human protein made mostly by macrophages as part of the innate immune system. It helps to detect bacteria in the body by binding lipopolysaccharide (LPS), a pathogen-associated molecular pattern (PAMP). ...
and LPS-Binding Protein ( LBP) are known to facilitate the presentation of LPS to MD-2.
A set of endosomal TLRs comprising TLR3, TLR7, TLR8 and TLR9 recognize nucleic acid
Nucleic acids are large biomolecules that are crucial in all cells and viruses. They are composed of nucleotides, which are the monomer components: a pentose, 5-carbon sugar, a phosphate group and a nitrogenous base. The two main classes of nuclei ...
derived from viruses as well as endogenous nucleic acids in context of pathogenic events. Activation of these receptor leads to production of inflammatory cytokines
Cytokines () are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling.
Cytokines are produced by a broad range of cells, including immune cells like macrophages, B cell, B lymphocytes, T cell, T lymphocytes ...
as well as type I interferons ( interferon type I) to help fight viral infection.
The adapter proteins and kinases that mediate TLR signaling have also been targeted. In addition, random germline mutagenesis with ENU has been used to decipher the TLR signaling pathways. When activated, TLRs recruit adapter molecules within the cytoplasm of cells to propagate a signal. Four adapter molecules are known to be involved in signaling. These proteins are known as MyD88
Myeloid differentiation primary response 88 (MYD88) is a protein that, in humans, is encoded by the ''MYD88'' gene. originally discovered in the laboratory of Dan A. Liebermann (Lord et al. Oncogene 1990) as a Myeloid differentiation primary resp ...
, TIRAP (also called Mal), TRIF, and TRAM (TRIF-related adaptor molecule).
TLR signaling is divided into two distinct signaling pathways, the MyD88-dependent and TRIF-dependent pathway.
MyD88-dependent pathway
The MyD88-dependent response occurs on dimerization of TLRs, and is used by every TLR except TLR3. Its primary effect is activation of NFκB and mitogen-activated protein kinase. Ligand binding and conformational change that occurs in the receptor recruits the adaptor protein MyD88, a member of the TIR family. MyD88 then recruits IRAK4
IRAK-4 (interleukin-1 receptor-associated kinase 4), in the IRAK family, is a protein kinase involved in signaling innate immune responses from Toll-like receptors. It also supports signaling from T-cell receptors. IRAK4 contains domain structure ...
, IRAK1 and IRAK2. IRAK kinases then phosphorylate and activate the protein TRAF6
TRAF6 is a TRAF human protein.
Function
The protein encoded by this gene is a member of the TNF receptor associated factor (TRAF) protein family. TRAF proteins are associated with, and mediate signal transduction from members of the TNF rece ...
, which in turn polyubiquinates the protein TAK1, as well as itself to facilitate binding to IKK-β. On binding, TAK1 phosphorylates IKK-β, which then phosphorylates IκB causing its degradation and allowing NFκB to diffuse into the cell nucleus and activate transcription and consequent induction of inflammatory cytokines.
TRIF-dependent pathway
Both TLR3 and TLR4 use the TRIF-dependent pathway, which is triggered by dsRNA
Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself ( non-coding RNA) or by forming a template for the production of proteins ( messenger RNA). RNA and deoxy ...
and LPS, respectively. For TLR3, dsRNA leads to activation of the receptor, recruiting the adaptor TRIF. TRIF activates the kinases TBK1 and RIPK1, which creates a branch in the signaling pathway. The TRIF/TBK1 signaling complex phosphorylates IRF3 allowing its translocation into the nucleus and production of interferon type I. Meanwhile, activation of RIPK1 causes the polyubiquitination and activation of TAK1 and NFκB transcription in the same manner as the MyD88-dependent pathway.
TLR signaling ultimately leads to the induction or suppression of genes that orchestrate the inflammatory response. In all, thousands of genes are activated by TLR signaling, and collectively, the TLRs constitute one of the most pleiotropic yet tightly regulated gateways for gene modulation.
TLR4 is the only TLR that uses all four adaptors. Complex consisting of TLR4, MD2 and LPS recruits TIR domain-containing adaptors TIRAP and MyD88 and thus initiates activation of NFκB (early phase) and MAPK. TLR4-MD2-LPS complex then undergoes endocytosis and in endosome it forms a signaling complex with TRAM and TRIF adaptors. This TRIF-dependent pathway again leads to IRF3 activation and production of type I interferons, but it also activates late-phase NFκB activation. Both late and early phase activation of NFκB is required for production of inflammatory cytokines.
Medical relevance
Imiquimod (cardinally used in dermatology
Dermatology is the branch of medicine dealing with the Human skin, skin.''Random House Webster's Unabridged Dictionary.'' Random House, Inc. 2001. Page 537. . It is a speciality with both medical and surgical aspects. A List of dermatologists, ...
) is a TLR7 agonist, and its successor resiquimod, is a TLR7 and TLR8 agonist. Recently, resiquimod has been explored as an agent for cancer immunotherapy, acting through stimulation of tumor-associated macrophages.
Several TLR ligands are in clinical development or being tested in animal models as vaccine adjuvants, with the first clinical use in humans in a recombinant herpes zoster vaccine in 2017, which contains a monophosphoryl lipid A component.
TLR7 messenger RNA expression levels in dairy animals in a natural outbreak of foot-and-mouth disease have been reported.
TLR4 has been shown to be important for the long-term side-effects of opioid
Opioids are a class of Drug, drugs that derive from, or mimic, natural substances found in the Papaver somniferum, opium poppy plant. Opioids work on opioid receptors in the brain and other organs to produce a variety of morphine-like effects, ...
s. Its activation leads to downstream release of inflammatory modulators including TNF-α and IL-1β, and constant low-level release of these modulators is thought to reduce the efficacy of opioid drug treatment with time, and is involved in opioid tolerance, hyperalgesia
Hyperalgesia ( or ; ''hyper'' from Greek ὑπέρ (''huper'') 'over' + ''-algesia'' from Greek ἄλγος (algos) 'pain') is an abnormally increased sensitivity to pain, which may be caused by damage to nociceptors or peripheral nerves and ...
and allodynia. Morphine induced TLR4 activation attenuates pain
Pain is a distressing feeling often caused by intense or damaging Stimulus (physiology), stimuli. The International Association for the Study of Pain defines pain as "an unpleasant sense, sensory and emotional experience associated with, or res ...
suppression by opioid
Opioids are a class of Drug, drugs that derive from, or mimic, natural substances found in the Papaver somniferum, opium poppy plant. Opioids work on opioid receptors in the brain and other organs to produce a variety of morphine-like effects, ...
s and enhances the development of opioid tolerance and addiction
Addiction is a neuropsychological disorder characterized by a persistent and intense urge to use a drug or engage in a behavior that produces natural reward, despite substantial harm and other negative consequences. Repetitive drug use can ...
, drug abuse
Substance misuse, also known as drug misuse or, in older vernacular, substance abuse, is the use of a drug in amounts or by methods that are harmful to the individual or others. It is a form of substance-related disorder, differing definitions ...
, and other negative side effects such as respiratory depression and hyperalgesia. Drugs that block the action of TNF-α or IL-1β have been shown to increase the analgesic effects of opioids and reduce the development of tolerance and other side-effects, and this has also been demonstrated with drugs that block TLR4 itself.
The "unnatural" enantiomers of opioid drugs such as (+)-morphine and (+)-naloxone lack affinity for opioid receptors, still produce the same activity at TLR4 as their "normal" enantiomers. So, "unnatural" entianomers of opioids such as (+)-naloxone, can be used to block the TLR4 activity of opioid analgesic drugs without having any affinity for μ-opioid receptor
Discovery
When microbes were first recognized as the cause of infectious diseases, it was immediately clear that multicellular organisms must be capable of recognizing them when infected and, hence, capable of recognizing molecules unique to microbes. A large body of literature, spanning most of the last century, attests to the search for the key molecules and their receptors. More than 100 years ago, Richard Pfeiffer, a student of Robert Koch, coined the term " endotoxin" to describe a substance produced by Gram-negative bacteria
Gram-negative bacteria are bacteria that, unlike gram-positive bacteria, do not retain the Crystal violet, crystal violet stain used in the Gram staining method of bacterial differentiation. Their defining characteristic is that their cell envelo ...
that could provoke fever and shock in experimental animals. In the decades that followed, endotoxin was chemically characterized and identified as a lipopolysaccharide (LPS) produced by most Gram-negative bacteria. This lipopolysaccharide is an integral part of the gram-negative membrane and is released upon destruction of the bacterium. Other molecules (bacterial lipopeptide
A lipopeptide is a molecule consisting of a lipid connected to a peptide. They are able to self-assemble into different structures. Many bacteria produce these molecules as a part of their metabolism, especially those of the genus ''Bacillus'', ...
s, flagellin, and unmethylated DNA
Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...
) were shown in turn to provoke host responses that are normally protective. However, these responses can be detrimental if they are excessively prolonged or intense. It followed logically that there must be receptors for such molecules, capable of alerting the host to the presence of infection, but these remained elusive for many years. Toll-like receptors are now counted among the key molecules that alert the immune system
The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...
to the presence of microbial infections.
The prototypic member of the family, the ''toll'' receptor (; Tl) in the fruit fly ''Drosophila melanogaster
''Drosophila melanogaster'' is a species of fly (an insect of the Order (biology), order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly", "pomace fly" ...
'', was discovered in 1985 by 1995 Nobel Laureates Christiane Nüsslein-Volhard and Eric Wieschaus
Eric Francis Wieschaus (born June 8, 1947 in South Bend, Indiana) is an American Evolutionary developmental biology, evolutionary developmental biologist and 1995 Nobel Prize-winner.
Early life
Born in South Bend, Indiana, he attended John Carro ...
and colleagues. It was known for its developmental function in embryogenesis
An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male ...
by establishing the dorsal
Dorsal (from Latin ''dorsum'' ‘back’) may refer to:
* Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism
* Dorsal, positioned on top of an aircraft's fuselage
The fus ...
-ventral
Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
axis. It was named after Christiane Nüsslein-Volhard's 1985 exclamation, "" ("That's amazing!"), in reference to the underdeveloped ventral portion of a fruit fly larva. It was cloned by the laboratory of Kathryn Anderson in 1988. In 1996, ''toll'' was found by Jules A. Hoffmann and his colleagues to have an essential role in the fly's immunity to fungal infection, which it achieved by activating the synthesis of antimicrobial peptides.
The first reported human toll-like receptor was described by Nomura and colleagues in 1994, mapped to a chromosome by Taguchi and colleagues in 1996. Because the immune function of toll in ''Drosophila'' was not then known, it was assumed that TIL (now known as TLR1) might participate in mammalian development. However, in 1991 (prior to the discovery of TIL) it was observed that a molecule with a clear role in immune function in mammals, the interleukin-1
The Interleukin-1 family (IL-1 family) is a group of 11 cytokines that plays a central role in the regulation of immune and inflammatory responses to infections or sterile insults.
Discovery
Discovery of these cytokines began with studies on t ...
(IL-1) receptor, also had homology to drosophila toll; the cytoplasmic portions of both molecules were similar.
In 1997, Charles Janeway and Ruslan Medzhitov showed that a toll-like receptor now known as TLR4 could, when artificially ligated using antibodies, induce the activation of certain genes necessary for initiating an adaptive immune response. TLR 4 function as an LPS sensing receptor was discovered by Bruce A. Beutler and colleagues. These workers used positional cloning
A genetic screen or mutagenesis screen is an experimental technique used to identify and select individuals who possess a phenotype of interest in a mutagenized population. Hence a genetic screen is a type of phenotypic screen. Genetic screens ...
to prove that mice that could not respond to LPS had mutations that abolished the function of TLR4. This identified TLR4 as one of the key components of the receptor for LPS.
In turn, the other TLR genes were ablated in mice by gene targeting, largely in the laboratory of Shizuo Akira and colleagues. Each TLR is now believed to detect a discrete collection of molecules — some of microbial origin, and some products of cell damage — and to signal the presence of infections.
Plant homologs of ''toll'' were discovered by Pamela Ronald in 1995 (rice XA21) and Thomas Boller in 2000 (''Arabidopsis'' FLS2).
In 2011, Beutler and Hoffmann were awarded the Nobel Prize in Medicine or Physiology for their work. Hoffmann and Akira received the Canada Gairdner International Award in 2011.
Notes and references
See also
* NOD-like receptor
* Immunologic adjuvant
In immunology, an adjuvant is a substance that increases or modulates the immune response to a vaccine. The word "adjuvant" comes from the Latin word , meaning to help or aid. "An immunologic adjuvant is defined as any substance that acts to acce ...
* RIG-I-like receptor
External links
*
*
TollML: Toll-like receptors and ligands database
at University of Munich
The Ludwig Maximilian University of Munich (simply University of Munich, LMU or LMU Munich; ) is a public university, public research university in Munich, Bavaria, Germany. Originally established as the University of Ingolstadt in 1472 by Duke ...
The Toll-Like Receptor Family of Innate Immune Receptors (pdf)
Toll-Like receptor Pathway
BioScience Animations
{{Pattern recognition receptors
Developmental genetics
Insect immunity
LRR proteins
Signal transduction