The curved leucine-rich repeat region of Toll-like receptors, represented here by TLR3

Toll-like receptors (TLRs) are a class of single membrane-spanning non-catalytic receptors that recognize structurally conserved molecules derived from microbes once they have breached physical barriers such as the skin or intestinal tract mucosa, and activate immune cell responses. A leucine-rich repeat (LRR is a Protein Structural motif that forms an α/β horseshoe fold. In Biochemistry, a receptor is a Protein molecule embedded in either the Plasma membrane or Cytoplasm of a cell to which a mobile signaling A microorganism (also spelled micro organism or micro-organism and also called a microbe) is an Organism that is Microscopic (usually The skin is the outer covering of living tissue of an animal (or plant The mucous membranes (or mucosae; singular mucosa) are linings of mostly endodermal origin covered in Epithelium, which are involved in They are believed to play a key role in the innate immune system. Immune system|Adaptive immune systemThe innate immune system comprises the cells and mechanisms that defend the host from infection by other organisms in a non-specific manner

They receive their name from their similarity to the Toll gene identified in Drosophila in 1985 by Christiane Nüsslein-Volhard. The Toll genes encode members of the Toll-like receptor class of proteins Christiane Nüsslein-Volhard (born October 20, 1942 in Magdeburg) is a German Biologist who won the Albert Lasker Award ^[1]

Diversity

TLRs are a type of pattern recognition receptor (PRR) and recognize molecules that are broadly shared by pathogens but distinguishable from host molecules, collectively referred to as pathogen-associated molecular patterns (PAMPs). Pattern recognition receptors, or PRR s are Proteins expressed by cells of the Immune system to identify molecules associated with microbial Pathogens A pathogen (from Greek πάθος pathos "suffering passion" and γἰγνομαι (γεν- gignomai (gen- "I give birth to" infectious Pathogen-associated molecular patterns, or PAMP s are small molecular motifs consistently found on Pathogens They are recognized by Toll-like receptors and They form a receptor superfamily with the Interleukin-1 receptors (Interleukin-1 Receptor/Toll-Like Receptor Superfamily) that all have in common a so called TIR (Toll-IL-1 receptor) domain. Interleukin-1 receptor (IL-1R is a Cytokine receptor which binds Interleukin 1.

Three subgroups of TIR domains exist. Proteins with subgroup 1 TIR domains are receptors for interleukins that are produced by macrophages, monocytes and dendritic cells and all have extracellular Immunoglobulin (Ig) domains. Macrophages ( Greek: "big eaters" from makros "large" + phagein "eat" ( Mø) are cells within the tissues that Monocyte is a type of Leukocyte, part of the Human body 's Immune system. Dendritic cells (DCs are Immune cells and form part of the Mammalian Immune system. Antibodies (also known as immunoglobulins, abbreviated Ig) are Gamma globulin Proteins that are found in Blood or other Bodily 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 cytosolic and mediate signaling from proteins of subgroups 1 and 2. An adaptor protein is a Protein which is accessory to main proteins in a Signal transduction pathway

TLRs are present in vertebrates, as well as in invertebrates. Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes An invertebrate is an Animal lacking a Vertebral column. The group includes 98% of all animal Species — all animals except those in the Chordate Molecular building blocks of the TLRs are represented in bacteria and in plants, and in the latter kingdom, 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. An immune system is a collection of mechanisms within an Organism that protects against Disease by identifying and killing Pathogens and Tumor

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 that could provoke fever and shock in experimental animals. Heinrich Hermann Robert Koch ( December 11 1843 – May 27 1910) was a German Physician. Endotoxins (not to be confused with Enterotoxin) are potentially toxic, natural compounds found inside Pathogens such as Bacteria. Gram-negative bacteria are those Bacteria that do not retain Crystal violet dye in the Gram staining protocol Fever (also known as pyrexia, from the Greek pyretos meaning fire or a febrile response, from the Latin word Febris A model organism is a Species that is extensively studied to understand particular biological Phenomena, with the expectation that discoveries made In the decades that followed, endotoxin was chemically characterized and identified as a lipopolysaccharide (LPS) produced by most Gram-negative bacteria. Lipopolysaccharides ( LPS) are large Molecules consisting of a Lipid and a Polysaccharide joined by a Covalent bond; they are found Other molecules (bacterial lipopeptides, flagellin, and unmethylated DNA) were shown in turn to provoke host responses that are normally protective. A lipopeptide is a Molecule consisting of a Lipid connected to a Peptide. Flagellin is a Protein that arranges itself in a hollow cylinder to form the filament in Bacterial Flagellum. Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known 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 to the presence of microbial infections. An immune system is a collection of mechanisms within an Organism that protects against Disease by identifying and killing Pathogens and Tumor An infection is the detrimental Colonization of a host Organism by a foreign Species. They are named for their similarity to Toll, a receptor first identified in the fruit fly Drosophila melanogaster, and originally known for its developmental function in that organism. The Toll genes encode members of the Toll-like receptor class of proteins Drosophila melanogaster (from the Greek for black-bellied dew-lover) is a two-winged insect that belongs to the Diptera, the order 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^[2], which it achieved by activating the synthesis of antimicrobial peptides. Jules A Hoffmann, born in Echternach, Luxemburg on 2 August 1941, is a French citizen, Research Director and Member of the Board Mycosis (plural mycoses) is a condition in which Fungi pass the resistance barriers of the human or animal body and establish Infections Classification

The first reported human Toll-like receptor was described by Nomura and colleagues in 1994,^[3] mapped to a chromosome by Taguchi and colleagues in 1996,^[4]. 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 (IL-1) receptor, also had homology to drosophila Toll; the cytoplasmic portions of both molecules were similar. Interleukin-1 (IL-1 is one of the first Cytokines ever described ^[5]

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. Charles Alderson Janeway Jr (1943-2003 was a noted Immunologist. ^[6] However, the function of the TLRs remained unknown in the wake of this work, and in particular, no ligand had been identified for any mammalian TLR.

The function of the TLRs was discovered by Bruce A. Beutler and colleagues^[7]. Bruce A Beutler, an immunologist and geneticist was born in Chicago, Illinois on 29 December, 1957, and is a US citizen These workers used positional cloning to prove that mice that could not respond to LPS had mutations that abolished the function of TLR4. A genetic screen (often shortened to screen) is a procedure or test to identify and select individuals who possess a Phenotype of interest This identified TLR4 as a key component of the receptor for LPS, and strongly suggested that other Toll-like receptors might detect other signature molecules of microbes, such as those mentioned above.

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 of microbial origin, and to signal the presence of infections.

Extended family

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^[8][9][10]. 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, but appears to have been damaged at some point in the past by a retrovirus. The House Mouse ( Mus musculus) is one of the most numerous species of the genus Mus commonly termed a Mouse. A retrovirus is any Virus belonging to the viral family Retroviridae. On the other hand, mice express TLRs 11, 12, and 13, none of which is represented in humans. Other mammals may express TLRs which are not found in humans. This may complicate the process of using experimental animals as models of human innate immunity.

Ligands

Because the specificity of Toll-like receptors (and other innate immune receptors) 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). Pathogen associated molecules that meet this requirement are usually critical to the pathogen's function and cannot be eliminated or changed through mutation; they are said to be evolutionarily conserved. Well conserved features in pathogens include bacterial cell-surface lipopolysaccharides (LPS), lipoproteins, lipopeptides and lipoarabinomannan; proteins such as flagellin from bacterial flagella; double-stranded RNA of viruses or the unmethylated CpG islands of bacterial and viral DNA; and certain other RNA and DNA. The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have Lipopolysaccharides ( LPS) are large Molecules consisting of a Lipid and a Polysaccharide joined by a Covalent bond; they are found A lipoprotein is a biochemical assembly that contains both Proteins and Lipids The lipids or their derivatives may be covalently or non-covalently bound Lipoarabinomannan, also called LAM, is a Glycolipid, and a Virulence factor associated with Mycobacterium tuberculosis, the Bacteria Flagellin is a Protein that arranges itself in a hollow cylinder to form the filament in Bacterial Flagellum. A flagellum ( plural flagella) is a tail-like structure that projects from the Cell body of certain Prokaryotic and Eukaryotic cells and it Ribonucleic acid ( RNA) is a Nucleic acid that consists of a long chain of Nucleotide units A virus (from the Latin virus meaning Toxin or Poison) is a sub-microscopic infectious agent that is unable CpG sites are regions of DNA where a Cytosine Nucleotide occurs next to a Guanine nucleotide in the linear sequence of bases Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known For most of the TLRs, ligand 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. In Biochemistry, a ligand ( latin ligare = to bind is a substance that is able to bind to and form a complex with a Biomolecule ^[11][12]. See the table below for a summary of known TLR ligands.

Endogenous ligands

The stereotypic inflammatory response provoked by TLR activation has prompted speculation that endogenous activators of TLRs might participate in autoimmune diseases. TLRs have been suspected of binding to host molecules including fibrinogen (involved in blood clotting) and heat shock proteins (HSPs)and host DNA. Fibrin (also called Factor Ia) is a Protein involved in the clotting of blood Coagulation is a complex process by which Blood forms Clots It is an important part of Hemostasis (the cessation of blood loss from a damaged vessel whereby Heat shock proteins ( HSP) are a group of Proteins whose expression is increased when the cells are exposed to elevated temperatures or other stress

Signaling

TLRs are believed to function as dimers. A dimer is a Chemical or Biological entity consisting of two subunits called Monomers which are held together by either Intramolecular forces Though most TLRs appear to function as homodimers, TLR2 forms heterodimers with TLR1 or TLR6, each dimer having a different ligand specificity. A dimer is a Chemical or Biological entity consisting of two subunits called Monomers which are held together by either Intramolecular forces A dimer is a Chemical or Biological entity consisting of two subunits called Monomers which are held together by either Intramolecular forces In Chemistry, a ligand is either an Atom, Ion, or Molecule (see also Functional group) that bonds to a central metal generally 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 and LPS Binding Protein (LBP) are known to facilitate the presentation of LPS to MD-2. Cluster of differentiation 14 also known as CD14 is a human Gene. Lipopolysaccharide binding protein, also known as LBP, is a human Gene.

Signaling pathway of toll-like receptors. Dashed grey lines represent unknown associations

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. ENU, also known as N -ethyl- N -nitrosourea (chemical formula C3H7N3O2 is a highly potent Mutagen. When activated, TLRs recruit adapter molecules within the cytoplasm of cells in order to propagate a signal. Four adapter molecules are known to be involved in signaling. These proteins are known as MyD88, Tirap (also called Mal), Trif, and Tram^[13][14][15]. Myeloid differentiation primary response gene (88 (MYD88 is a universal Adapter protein as it is used by all TLRs (except TLR 3) to activate Transcription Tirap can refer to Tirap district, a district in Arunachal Pradesh bordering Myanmar, Nagaland and Assam TIR-domain-containing adapter-inducing interferon-β ( TRIF) is is an adapter in responding to activation of Toll-like receptors (TLRs A tram, tramcar, trolley, trolley car, or streetcar is a railborne vehicle, of lighter weight and construction than a Train The adapters activate other molecules within the cell, including certain protein kinases (IRAK1, IRAK4, TBK1, and IKKi) that amplify the signal, and ultimately lead to the induction or suppression of genes that orchestrate the inflammatory response. Interleukin-1 receptor-associated kinase 1, also known as IRAK1, is a human Gene. 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 TANK-binding kinase 1, also known as TBK1, is a human Gene. In all, thousands of genes are activated by TLR signaling, and collectively, the TLRs constitutes one of the most powerful and important gateways for gene modulation.

Summary of known mammalian toll-like receptors

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. Cellular compartments in Cell biology comprise all closed parts within a cell whose lumen is usually surrounded by a single or double lipid layer Furthermore, they are expressed by different types of leucocytes or other cell types:

Activation and effects

Following activation by ligands of microbial origin, several reactions are possible. A cell type is a distinct morphological or functional form of cell. TLR 1 is a member of the Toll-like receptor family (TLR of Pattern recognition receptors of the Innate immune system. A lipopeptide is a Molecule consisting of a Lipid connected to a Peptide. Myeloid differentiation primary response gene (88 (MYD88 is a universal Adapter protein as it is used by all TLRs (except TLR 3) to activate Transcription Monocyte is a type of Leukocyte, part of the Human body 's Immune system. Macrophages ( Greek: "big eaters" from makros "large" + phagein "eat" ( Mø) are cells within the tissues that Dendritic cells (DCs are Immune cells and form part of the Mammalian Immune system. B cells are Lymphocytes that play a large role in the humoral immune response (as opposed to the cell-mediated immune response, which is governed by TLR-2 is a Biomolecule, which plays a role in the human Immune system. Glycolipids are Carbohydrate -attached Lipids. Their role is to provide Energy and also serve as markers for cellular recognition Dendritic cells (DCs are Immune cells and form part of the Mammalian Immune system. A mast cell (or mastocyte) is a resident cell of several types of tissues and contains many granules rich in Histamine and Heparin A lipoprotein is a biochemical assembly that contains both Proteins and Lipids The lipids or their derivatives may be covalently or non-covalently bound Lipoteichoic acid (LTA is a surface-associated Adhesion Amphiphile from Gram-positive bacteria and regulator of Autolytic wall Enzymes Not to be confused with Glycoprotein. Peptidoglycan, also known as murein, is a Polymer consisting of sugars and amino Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. The 70 kilodalton heat shock proteins ( Hsp70 s are a family of ubiquitously expressed Heat shock proteins Proteins with similar structure exist in virtually In Biology, a host is an organism that harbors a Virus or Parasite, or a mutual or Commensal Symbiont, typically providing nourishment Zymosan is a Glucan. It binds to TLR 2. Zymosan is prepared from yeast cell wall and consists of protein-carbohydrate complexes A fungus (ˈfʌŋgəs is a eukaryotic Organism that is a member of the kingdom Fungi (ˈfʌndʒaɪ TLR 3 is a member of the Toll-like receptor family of Pattern recognition receptors of the Innate immune system. Ribonucleic acid ( RNA) is a Nucleic acid that consists of a long chain of Nucleotide units Poly IC (or polyinosinicpolycytidylic acid) is an Immunostimulant. A virus (from the Latin virus meaning Toxin or Poison) is a sub-microscopic infectious agent that is unable TIR-domain-containing adapter-inducing interferon-β ( TRIF) is is an adapter in responding to activation of Toll-like receptors (TLRs Toll-like receptor 4, also known as TLR4, is a human Gene. TLR 4 is a Toll-like receptor. Lipopolysaccharides ( LPS) are large Molecules consisting of a Lipid and a Polysaccharide joined by a Covalent bond; they are found Gram-negative bacteria are those Bacteria that do not retain Crystal violet dye in the Gram staining protocol TIR-domain-containing adapter-inducing interferon-β ( TRIF) is is an adapter in responding to activation of Toll-like receptors (TLRs The intestinal epithelium is the Epithelium that covers the small and Large intestine. Heat shock proteins ( HSP) are a group of Proteins whose expression is increased when the cells are exposed to elevated temperatures or other stress Fibrin (also called Factor Ia) is a Protein involved in the clotting of blood Heparan sulfate (HS is a linear Polysaccharide found in all animal tissues Hyaluronan (also called hyaluronic acid or hyaluronate) is a non-sulfated Glycosaminoglycan distributed widely throughout connective, epithelial Toll-like receptor 5, also known as TLR5, is a human Gene. Flagellin is a Protein that arranges itself in a hollow cylinder to form the filament in Bacterial Flagellum. Toll-like receptor 6, also known as TLR6, is a human Gene. TLR6 has also been designated as CD286 ( Cluster of differentiation 286 Mycoplasma is a Genus of bacteria which lack a Cell wall. Without a cell wall they are unaffected by many common Antibiotics such Toll-like receptor 7, also known as TLR7, is an immune Gene possessed by humans and other mammals and additionally in avian species Imidazoquinoline is a double cyclic organic molecule its derivatives and compounds are often used for antiviral and antiallergic creams Dendritic cells (DCs are Immune cells and form part of the Mammalian Immune system. Guanosine is a Nucleoside comprising Guanine attached to a Ribose ( Ribofuranose) ring via a β-N9- Glycosidic bond. Toll-like receptor 8, also known as TLR8, is a human Gene. TLR8 has also been designated as CD288 ( Cluster of differentiation 288 TLR 9 is a Toll-like receptor. It recognizes unmethylated CpG sites on DNA molecules CpG sites are regions of DNA where a Cytosine Nucleotide occurs next to a Guanine nucleotide in the linear sequence of bases Toll-like receptor 10, also known as TLR10, is a human Gene. TLR10 has also been designated as CD290 ( Cluster of differentiation 290 Profilin is an Actin-binding protein involved in the dynamic turnover and restructuring of the Actin Cytoskeleton. The liver is a vital organ in the human body and is present in Vertebrates and some other animals The kidneys are complicated organs that have numerous biological roles In Anatomy, the urinary bladder is a hollow muscular, and distensible (or elastic organ that sits on the Pelvic floor in Mammals It is the In biology and medicine epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body Immune cells can produce signalling factors called cytokines which trigger inflammation. Cytokines are a category of signalling Proteins and Glycoproteins that like Hormones and Neurotransmitters, are used extensively in cellular Inflammation ( Latin, inflamatio, to set on fire is the complex biological response of vascular tissues to harmful stimuli such as Pathogens In the case of a bacterial factor, the pathogen might be phagocytosed and digested, and its antigens presented to CD4+ T cells. Phagocytosis is the cellular process of engulfing solid particles by the Cell membrane to form an internal Phagosome, or "food vacuole An antigen (from antibody-generating) or immunogen is a substance that prompts the generation of Antibodies and can cause an immune response T helper cells (also known as effector T cells or Th cells) are a sub-group of Lymphocytes (a type of White blood cell or In the case of a viral factor, the infected cell may shut off its protein synthesis and may undergo programmed cell death (apoptosis). Immune cells that have detected a virus may also release anti-viral factors such as interferons. Interferons ( IFN s are natural Proteins produced by the cells of the Immune system of most Vertebrates in response to challenges by foreign agents

The discovery of the Toll-like receptors finally identified the innate immune receptors that were responsible for many of the innate immune functions that had been studied for many years. Interestingly, TLRs seem only to be involved in the cytokine production and cellular activation in response to microbes, and do not play a significant role in the adhesion and phagocytosis of microorganisms. Phagocytosis is the cellular process of engulfing solid particles by the Cell membrane to form an internal Phagosome, or "food vacuole

Drugs interactions

Imiquimod (cardinally used in dermatology), and its successor R848, are ligands for TLR7 and TLR8 ^[18]. Imiquimod ( INN) is a prescription medication that acts as an immune response modifier Dermatology (from Greek grc δέρμα derma, "skin" and grc -λογία -logia) is a branch of Medicine dealing with

References

External links

• MeSH Toll-Like+Receptors
• TollML: Toll-like receptors and ligands database at University of Munich