Toll-like receptor
Toll-like receptors are a class of proteins that play a key role in the innate immune system. They are single-spanning receptors usually expressed on sentinel cells such as macrophages and dendritic cells, that recognize structurally conserved molecules derived from microbes. Once these microbes have reached physical barriers such as the skin or intestinal tract mucosa, they are recognized by TLRs, which activate immune cell 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, whereas TLR3, TLR7, TLR8, and TLR9 are located in intracellular vesicles.
TLRs received their name from their similarity to the protein coded by the .
Function
The ability of the immune system to recognize molecules that are broadly shared by pathogens is, in part, due to the presence of immune receptors called toll-like receptors that are expressed on the membranes of leukocytes including dendritic cells, macrophages, natural killer cells, cells of the adaptive immunity T cells, and B cells, and non-immune cells.The binding of ligands — either in the form of adjuvant used in vaccinations 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 within the cytosol of the immune cell to propagate the antigen-induced signal transduction pathway. These recruited proteins are then responsible for the subsequent activation of other downstream proteins, including protein kinases that further amplify the signal and ultimately lead to the upregulation or suppression of genes that orchestrate inflammatory responses and other transcriptional events. Some of these events lead to cytokine 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 antigens 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. 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 immune system and adaptive immune system through their presence in dendritic cells. Flagellin, a TLR5 ligand, induces cytokine secretion on interacting with TLR5 on human T cells.
Superfamily
TLRs are a type of pattern recognition receptor and recognize molecules that are broadly shared by pathogens but distinguishable from host molecules, collectively referred to as pathogen-associated molecular patterns. In addition to the recognition of exogenous PAMPs, TLRs can also bind to endogenous damage-associated molecular patterns such as heat shock proteins 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 domain.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 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 cytosolic and mediate signaling from proteins of subgroups 1 and 2.
Extended family
TLRs are present in vertebrates as well as invertebrates. 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.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 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, but appears to have been damaged at some point in the past by a retrovirus. 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. 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. Its receptor ectodomain recognizes the cleaved form of the cytokine spätzle, which is secreted in the haemolymph 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, fungi, and virulence factors. First, the Spätzle processing enzyme is activated in response to infection and cleaves spätzle. 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 is composed of MyD88, Tube, and Pelle. Signal from TICS is then transduced to Cactus, phosphorylated Cactus is polyubiquitylated and degraded, allowing nuclear translocation of DIF and induction of transcription of genes for antimicrobial peptides such as drosomycin.
Drosophila have a total of 9 toll family and 6 spz family genes that interact with each other to differing degrees.
TLR2
has also been designated as CD282.TLR3
does not use the MyD88 dependent pathway. Its ligand is retroviral double-stranded RNA, 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 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 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. 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 in humans. TLR11 deficient knockout mice, 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:| Receptor | Ligand | Ligand location | Adapter | Location | Cell types |
| TLR 1 | multiple triacyl lipopeptides | Bacterial lipoprotein | MyD88/MAL | cell surface |
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| TLR 2 | multiple glycolipids | Bacterial peptidoglycans | MyD88/MAL | cell surface | |
| TLR 2 | multiple lipopeptides and proteolipids | Bacterial peptidoglycans | MyD88/MAL | cell surface | |
| TLR 2 | lipoteichoic acid | Gram-positive bacteria | MyD88/MAL | cell surface | |
| TLR 2 | HSP70 | Host cells | MyD88/MAL | cell surface | |
| TLR 2 | zymosan | Fungi | MyD88/MAL | cell surface | |
| TLR 2 | Numerous others | MyD88/MAL | cell surface | ||
| TLR 3 | double-stranded RNA, poly I:C | viruses | TRIF | cell compartment | |
| TLR 4 | lipopolysaccharide | Gram-negative bacteria | MyD88/MAL/TRIF/TRAM | cell surface | |
| TLR 4 | several heat shock proteins | Bacteria and host cells | MyD88/MAL/TRIF/TRAM | cell surface | |
| TLR 4 | fibrinogen | host cells | MyD88/MAL/TRIF/TRAM | cell surface | |
| TLR 4 | heparan sulfate fragments | host cells | MyD88/MAL/TRIF/TRAM | cell surface | |
| TLR 4 | hyaluronic acid fragments | host cells | MyD88/MAL/TRIF/TRAM | cell surface | |
| TLR 4 | nickel | MyD88/MAL/TRIF/TRAM | cell surface | ||
| TLR 4 | Various opioid drugs | MyD88/MAL/TRIF/TRAM | cell surface | ||
| TLR 5 | Bacterial flagellin | Bacteria | MyD88 | cell surface | |
| TLR 5 | Profilin | Toxoplasma gondii | MyD88 | cell surface | |
| TLR 6 | multiple diacyl lipopeptides | Mycoplasma | MyD88/MAL | cell surface | |
| TLR 7 | imidazoquinoline | small synthetic compounds | MyD88 | cell compartment | |
| TLR 7 | loxoribine | small synthetic compounds | MyD88 | cell compartment | |
| TLR 7 | bropirimine | small synthetic compounds | MyD88 | cell compartment | |
| TLR 7 | resiquimod | small synthetic compounds | MyD88 | cell compartment | |
| TLR 7 | single-stranded RNA | RNA viruses | MyD88 | cell compartment | |
| TLR 8 | small synthetic compounds; single-stranded Viral RNA, phagocytized bacterial RNA | MyD88 | cell compartment | ||
| TLR 9 | unmethylated CpG Oligodeoxynucleotide DNA | Bacteria, DNA viruses | MyD88 | cell compartment | |
| TLR 10 | triacylated lipopeptides | unknown | cell surface | ||
| TLR 11 | Profilin | Toxoplasma gondii | MyD88 | cell compartment | |
| TLR 11 | Flagellin | Bacteria | MyD88 | cell compartment | |
| TLR 12 | Profilin | Toxoplasma gondii | MyD88 | cell compartment | |
| TLR 13 | bacterial ribosomal RNA sequence "CGGAAAGACC" | Virus, bacteria | MyD88, TAK-1 | cell compartment |