toll-like receptor

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 (TLRs) 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 (epithelial and endothelial cells, and fibroblasts).{{cite journal | vauthors = Delneste Y, Beauvillain C, Jeannin P | title = [Innate immunity: structure and function of TLRs] | journal = Médecine/Sciences | volume = 23 | issue = 1 | pages = 67–73 | date = January 2007 | pmid = 17212934 | doi = 10.1051/medsci/200723167 | doi-access = free }}

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.{{cite journal | vauthors = Takeda K, Akira S | title = Toll-like receptors in innate immunity | journal = International Immunology | volume = 17 | issue = 1 | pages = 1–14 | date = January 2005 | pmid = 15585605 | doi = 10.1093/intimm/dxh186 | doi-access = }}{{cite journal | vauthors = Medzhitov R, Preston-Hurlburt P, Janeway CA | title = A human homologue of the Drosophila Toll protein signals activation of adaptive immunity | journal = Nature | volume = 388 | issue = 6640 | pages = 394–7 | date = July 1997 | pmid = 9237759 | doi = 10.1038/41131 | bibcode = 1997Natur.388..394M | doi-access = free }}

Upon activation, TLRs recruit adaptor proteins (proteins that mediate other protein-protein interactions) 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 (IKKi, IRAK1, IRAK4, and TBK1) 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 (apoptosis). 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 cells.{{cite journal | vauthors = Sharma N, Akhade AS, Qadri A | title = Sphingosine-1-phosphate suppresses TLR-induced CXCL8 secretion from human T cells | journal = Journal of Leukocyte Biology | volume = 93 | issue = 4 | pages = 521–8 | date = April 2013 | pmid = 23345392 | doi = 10.1189/jlb.0712328 | doi-access = }} Flagellin, a TLR5 ligand, induces cytokine secretion on interacting with TLR5 on human T cells.

File:TLR2.png

{{main|Toll-interleukin receptor}}

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). In addition to the recognition of exogenous PAMPs, TLRs can also bind to endogenous damage-associated molecular patterns (DAMPs) such as heat shock proteins (HSPs) or plasma membrane constituents.{{cite journal |vauthors=Sameer AS, Nissar S |title=Toll-Like Receptors (TLRs): Structure, Functions, Signaling, and Role of Their Polymorphisms in Colorectal Cancer Susceptibility |journal=Biomed Res Int |volume=2021 |pages=1157023 |date=2021 |pmid=34552981 |pmc=8452412 |doi=10.1155/2021/1157023 |doi-access=free}} 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 that are produced by macrophages, monocytes, and dendritic cells 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 cytosolic and mediate signaling from proteins of subgroups 1 and 2.

{{missing information|section|choanoflagellate TLR (pmid29848444) |date=December 2021}}

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.{{cite journal | vauthors = Rolls A, Shechter R, London A, Ziv Y, Ronen A, Levy R, Schwartz M | title = Toll-like receptors modulate adult hippocampal neurogenesis | journal = Nature Cell Biology | volume = 9 | issue = 9 | pages = 1081–8 | date = September 2007 | pmid = 17704767 | doi = 10.1038/ncb1629 | s2cid = 12517461 }}

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.{{cite journal | vauthors = Du X, Poltorak A, Wei Y, Beutler B | title = Three novel mammalian toll-like receptors: gene structure, expression, and evolution | journal = European Cytokine Network | volume = 11 | issue = 3 | pages = 362–71 | date = September 2000 | pmid = 11022119 | url = http://www.john-libbey-eurotext.fr/medline.md?issn=1148-5493&vol=11&iss=3&page=362 }}{{cite journal | vauthors = Chuang TH, Ulevitch RJ | title = Cloning and characterization of a sub-family of human toll-like receptors: hTLR7, hTLR8 and hTLR9 | journal = European Cytokine Network | volume = 11 | issue = 3 | pages = 372–8 | date = September 2000 | pmid = 11022120 | url = http://www.john-libbey-eurotext.fr/medline.md?issn=1148-5493&vol=11&iss=3&page=372 }}{{cite journal | vauthors = Tabeta K, Georgel P, Janssen E, Du X, Hoebe K, Crozat K, Mudd S, Shamel L, Sovath S, Goode J, Alexopoulou L, Flavell RA, Beutler B | display-authors = 6 | title = Toll-like receptors 9 and 3 as essential components of innate immune defense against mouse cytomegalovirus infection | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 101 | issue = 10 | pages = 3516–21 | date = March 2004 | pmid = 14993594 | pmc = 373494 | doi = 10.1073/pnas.0400525101 | bibcode = 2004PNAS..101.3516T | doi-access = free }} 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.{{cite journal | vauthors = Roach JC, Glusman G, Rowen L, Kaur A, Purcell MK, Smith KD, Hood LE, Aderem A | display-authors = 6 | title = The evolution of vertebrate Toll-like receptors | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 102 | issue = 27 | pages = 9577–82 | date = July 2005 | pmid = 15976025 | pmc = 1172252 | doi = 10.1073/pnas.0502272102 | bibcode = 2005PNAS..102.9577R | doi-access = free }} 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.

File:Toll Pathway of Drosophila melanogaster.jpg{{cite journal | vauthors = Lemaitre B, Hoffmann J | title = The host defense of Drosophila melanogaster | journal = Annual Review of Immunology | volume = 25 | pages = 697–743 | date = 2007 | pmid = 17201680 | doi = 10.1146/annurev.immunol.25.022106.141615 | url = http://infoscience.epfl.ch/record/151765 }}{{cite journal | vauthors = Valanne S, Wang JH, Rämet M | title = The Drosophila Toll signaling pathway | journal = Journal of Immunology | volume = 186 | issue = 2 | pages = 649–56 | date = January 2011 | pmid = 21209287 | doi = 10.4049/jimmunol.1002302 | doi-access = free }}{{cite journal | vauthors = Dudzic JP, Hanson MA, Iatsenko I, Kondo S, Lemaitre B | title = More Than Black or White: Melanization and Toll Share Regulatory Serine Proteases in Drosophila | journal = Cell Reports | volume = 27 | issue = 4 | pages = 1050–61 | date = April 2019 | pmid = 31018123 | doi = 10.1016/j.celrep.2019.03.101 | doi-access = free }}{{cite journal | vauthors = Hanson MA, Hamilton PT, Perlman SJ | title = Immune genes and divergent antimicrobial peptides in flies of the subgenus Drosophila | journal = BMC Evolutionary Biology | volume = 16 | issue = 1 | pages = 228 | date = October 2016 | pmid = 27776480 | doi = 10.1186/s12862-016-0805-y | pmc = 5078906 | doi-access = free | bibcode = 2016BMCEE..16..228H }}]]

The involvement of toll signalling in immunity was first demonstrated in the fruit fly, Drosophila melanogaster.{{cite journal | vauthors = Lemaitre B, Nicolas E, Michaut L, Reichhart JM, Hoffmann JA | title = The dorsoventral regulatory gene cassette spätzle/Toll/cactus controls the potent antifungal response in Drosophila adults | journal = Cell | volume = 86 | issue = 6 | pages = 973–83 | date = September 1996 | pmid = 8808632 | doi = 10.1016/s0092-8674(00)80172-5 | s2cid = 10736743 | url = http://infoscience.epfl.ch/record/151754 | doi-access = free }} 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 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.{{cite journal | vauthors = Issa N, Guillaumot N, Lauret E, Matt N, Schaeffer-Reiss C, Van Dorsselaer A, Reichhart JM, Veillard F | display-authors = 6 | title = The Circulating Protease Persephone Is an Immune Sensor for Microbial Proteolytic Activities Upstream of the Drosophila Toll Pathway | journal = Molecular Cell | volume = 69 | issue = 4 | pages = 539–550.e6 | date = February 2018 | pmid = 29452635 | doi = 10.1016/j.molcel.2018.01.029 | pmc = 5823974 | doi-access = free }} 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, 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) and induction of transcription of genes for antimicrobial peptides (AMPs) such as drosomycin.{{cite journal | vauthors = Ferrandon D, Imler JL, Hetru C, Hoffmann JA | title = The Drosophila systemic immune response: sensing and signalling during bacterial and fungal infections | journal = Nature Reviews. Immunology | volume = 7 | issue = 11 | pages = 862–74 | date = November 2007 | pmid = 17948019 | doi = 10.1038/nri2194 | s2cid = 11104900 }}

Drosophila have a total of 9 toll family and 6 spz family genes that interact with each other to differing degrees.{{cite journal | vauthors = Chowdhury M, Li CF, He Z, Lu Y, Liu XS, Wang YF, Ip YT, Strand MR, Yu XQ | display-authors = 6 | title = Drosophila | journal = The Journal of Biological Chemistry | volume = 294 | issue = 26 | pages = 10172–81 | date = June 2019 | pmid = 31088910 | pmc = 6664172 | doi = 10.1074/jbc.RA118.006804 | doi-access = free }}

{{main|TLR2}}

TLR2 has also been designated as CD282 (cluster of differentiation 282).

TLR3 does not use the MyD88 dependent pathway. Its ligand is retroviral double-stranded RNA (dsRNA), 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.{{cite journal | vauthors = Lee J, Sayed N, Hunter A, Au KF, Wong WH, Mocarski ES, Pera RR, Yakubov E, Cooke JP | display-authors = 6 | title = Activation of innate immunity is required for efficient nuclear reprogramming | journal = Cell | volume = 151 | issue = 3 | pages = 547–58 | date = October 2012 | pmid = 23101625 | pmc = 3506423 | doi = 10.1016/j.cell.2012.09.034 }}

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.{{cite journal | vauthors = Mathur R, Oh H, Zhang D, Park SG, Seo J, Koblansky A, Hayden MS, Ghosh S | display-authors = 6 | title = A mouse model of Salmonella typhi infection | journal = Cell | volume = 151 | issue = 3 | pages = 590–602 | date = October 2012 | pmid = 23101627 | pmc = 3500584 | doi = 10.1016/j.cell.2012.08.042 }}

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.{{Cite book |title=Paul's fundamental immunology |date=2023 |publisher=Wolters Kluwer/Lippincott Williams & Wilkins |isbn=978-1-9751-4251-3 |editor-last=Flajnik |editor-first=Martin F. |edition=8th |location=Philadelphia Baltimore New York London Buenos Aires Hong Kong Sydney Tokyo |pages= |chapter=Ch. 12. Pattern Recognition Receptors and the IL-1 Family |quote=Broadly, TLRs 1, 2, 4 to 6, and 10 are expressed on the plasma membrane, and TLRs 3, 7 to 9, and 11 to 13 are confined to the endosomes. |editor-last2=Singh |editor-first2=Nevil J. |editor-last3=Holland |editor-first3=Steven M.}} Furthermore, they are expressed by different types of leucocytes or other cell types:

File:Toll-Like Receptors (TLRs).png

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; proteins such as flagellin from bacterial flagella; double-stranded RNA of viruses; or the unmethylated CpG islands of bacterial and viral DNA; 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.{{Cite journal |last1=Schreibelt |first1=Gerty |last2=Benitez-Ribas |first2=Daniel |last3=Schuurhuis |first3=Danita |last4=Lambeck |first4=Annechien J. A. |last5=van Hout-Kuijer |first5=Maaike |last6=Schaft |first6=Niels |last7=Punt |first7=Cornelis J. A. |last8=Figdor |first8=Carl G. |last9=Adema |first9=Gosse J. |last10=de Vries |first10=I. Jolanda M. |date=2010-07-29 |title=Commonly used prophylactic vaccines as an alternative for synthetically produced TLR ligands to mature monocyte-derived dendritic cells |journal=Blood |volume=116 |issue=4 |pages=564–574 |doi=10.1182/blood-2009-11-251884 |issn=1528-0020 |pmid=20424184|doi-access=free |hdl=2066/89493 |hdl-access=free }}{{Cite journal |last1=Aleynick |first1=Mark |last2=Svensson-Arvelund |first2=Judit |last3=Pantsulaia |first3=Gvantsa |last4=Kim |first4=Kristy |last5=Rose |first5=Samuel A. |last6=Upadhyay |first6=Ranjan |last7=Yellin |first7=Michael |last8=Marsh |first8=Henry |last9=Oreper |first9=Daniel |last10=Jhunjhunwala |first10=Suchit |last11=Moussion |first11=Christine Carine |last12=Merad |first12=Miriam |last13=Brown |first13=Brian D. |last14=Brody |first14=Joshua D. |date=July 2023 |title=Pattern recognition receptor agonists in pathogen vaccines mediate antitumor T-cell cross-priming |journal=Journal for Immunotherapy of Cancer |volume=11 |issue=7 |pages=e007198 |doi=10.1136/jitc-2023-007198 |issn=2051-1426 |pmc=10373699 |pmid=37487664}} 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.{{cite journal | vauthors = Hoebe K, Du X, Georgel P, Janssen E, Tabeta K, Kim SO, Goode J, Lin P, Mann N, Mudd S, Crozat K, Sovath S, Han J, Beutler B | display-authors = 6 | title = Identification of Lps2 as a key transducer of MyD88-independent TIR signalling | journal = Nature | volume = 424 | issue = 6950 | pages = 743–8 | date = August 2003 | pmid = 12872135 | doi = 10.1038/nature01889 | bibcode = 2003Natur.424..743H | s2cid = 15608748 }}{{cite journal | vauthors = Hemmi H, Takeuchi O, Kawai T, Kaisho T, Sato S, Sanjo H, Matsumoto M, Hoshino K, Wagner H, Takeda K, Akira S | display-authors = 6 | title = A Toll-like receptor recognizes bacterial DNA | journal = Nature | volume = 408 | issue = 6813 | pages = 740–5 | date = December 2000 | pmid = 11130078 | doi = 10.1038/35047123 | bibcode = 2000Natur.408..740H | s2cid = 4405163 }} See the table above for a summary of known TLR 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), 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.

Image:Toll-like receptor pathways.svg

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 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 derived from viruses as well as endogenous nucleic acids in context of pathogenic events. Activation of these receptor leads to production of inflammatory cytokines 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, TIRAP (also called Mal), TRIF, and TRAM (TRIF-related adaptor molecule).{{cite journal | vauthors = Shigeoka AA, Holscher TD, King AJ, Hall FW, Kiosses WB, Tobias PS, Mackman N, McKay DB | display-authors = 6 | title = TLR2 is constitutively expressed within the kidney and participates in ischemic renal injury through both MyD88-dependent and -independent pathways | journal = Journal of Immunology | volume = 178 | issue = 10 | pages = 6252–8 | date = May 2007 | pmid = 17475853 | doi = 10.4049/jimmunol.178.10.6252 | doi-access = free }}{{cite journal | vauthors = Yamamoto M, Sato S, Hemmi H, Uematsu S, Hoshino K, Kaisho T, Takeuchi O, Takeda K, Akira S | display-authors = 6 | title = TRAM is specifically involved in the Toll-like receptor 4-mediated MyD88-independent signaling pathway | journal = Nature Immunology | volume = 4 | issue = 11 | pages = 1144–50 | date = November 2003 | pmid = 14556004 | doi = 10.1038/ni986 | s2cid = 13016860 }}{{cite journal | vauthors = Yamamoto M, Sato S, Hemmi H, Sanjo H, Uematsu S, Kaisho T, Hoshino K, Takeuchi O, Kobayashi M, Fujita T, Takeda K, Akira S | display-authors = 6 | title = Essential role for TIRAP in activation of the signalling cascade shared by TLR2 and TLR4 | journal = Nature | volume = 420 | issue = 6913 | pages = 324–9 | date = November 2002 | pmid = 12447441 | doi = 10.1038/nature01182 | bibcode = 2002Natur.420..324Y | s2cid = 16163262 }}

TLR signaling is divided into two distinct signaling pathways, the MyD88-dependent and TRIF-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, IRAK1 and IRAK2. IRAK kinases then phosphorylate and activate the protein TRAF6, 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.{{cite journal | vauthors = Kawai T, Akira S | title = The role of pattern-recognition receptors in innate immunity: update on Toll-like receptors | journal = Nature Immunology | volume = 11 | issue = 5 | pages = 373–84 | date = May 2010 | pmid = 20404851 | doi = 10.1038/ni.1863 | s2cid = 39414949 | doi-access = free }}

Both TLR3 and TLR4 use the TRIF-dependent pathway, which is triggered by dsRNA 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.

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Imiquimod (cardinally used in dermatology) is a TLR7 agonist, and its successor resiquimod, is a TLR7 and TLR8 agonist.{{cite book |author=Peter Fritsch |title=Dermatologie Venerologie : Grundlagen. Klinik. Atlas. |publisher=Springer |location=Berlin |year=2004 | language=de |isbn=3-540-00332-0 }} Recently, resiquimod has been explored as an agent for cancer immunotherapy,{{cite journal | vauthors = Rodell CB, Arlauckas SP, Cuccarese MF, Garris CS, Li R, Ahmed MS, Kohler RH, Pittet MJ, Weissleder R | display-authors = 6 | title = TLR7/8-agonist-loaded nanoparticles promote the polarization of tumour-associated macrophages to enhance cancer immunotherapy | language = En | journal = Nature Biomedical Engineering | volume = 2 | issue = 8 | pages = 578–588 | date = August 2018 | pmid = 31015631 | doi = 10.1038/s41551-018-0236-8 | pmc = 6192054 }} acting through stimulation of tumor-associated macrophages.

Several TLR ligands are in clinical development or being tested in animal models as vaccine adjuvants,{{cite journal | vauthors = Toussi DN, Massari P | title = Immune Adjuvant Effect of Molecularly-defined Toll-Like Receptor Ligands | journal = Vaccines | volume = 2 | issue = 2 | pages = 323–53 | date = April 2014 | pmid = 26344622 | pmc = 4494261 | doi = 10.3390/vaccines2020323 | doi-access = free }} 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.{{cite journal|url=https://ibic.lib.ku.ac.th/e-bulletin/IBBU201703005.pdf |archive-url=https://web.archive.org/web/20210428075843/https://ibic.lib.ku.ac.th/e-bulletin/IBBU201703005.pdf |archive-date=2021-04-28 |last1=Audarya|first1=S.D.|last2=Pattnaik|first2=B.|last3=Sanyal|first3=A.|last4=Mohapatra|first4=J.K.|title=Toll like Receptor 7 Messenger Ribonucleic Acid Expression Levels in Dairy Animals in an Outbreak of Foot-and-mouth disease|journal=Buffalo Bulletin|volume=36|issue=3 |pages=489–495 |date=2017}}

TLR4 has been shown to be important for the long-term side-effects of opioids. 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,{{cite journal | vauthors = Shavit Y, Wolf G, Goshen I, Livshits D, Yirmiya R | title = Interleukin-1 antagonizes morphine analgesia and underlies morphine tolerance | journal = Pain | volume = 115 | issue = 1–2 | pages = 50–9 | date = May 2005 | pmid = 15836969 | doi = 10.1016/j.pain.2005.02.003 | s2cid = 7286123 }}{{cite journal | vauthors = Mohan S, Davis RL, DeSilva U, Stevens CW | title = Dual regulation of mu opioid receptors in SK-N-SH neuroblastoma cells by morphine and interleukin-1β: evidence for opioid-immune crosstalk | journal = Journal of Neuroimmunology | volume = 227 | issue = 1–2 | pages = 26–34 | date = October 2010 | pmid = 20615556 | pmc = 2942958 | doi = 10.1016/j.jneuroim.2010.06.007 }} hyperalgesia and allodynia.{{cite book | vauthors = Komatsu T, Sakurada S, Katsuyama S, Sanai K, Sakurada T | title = Mechanism of allodynia evoked by intrathecal morphine-3-glucuronide in mice | volume = 85 | pages = 207–19 | year = 2009 | pmid = 19607972 | doi = 10.1016/S0074-7742(09)85016-2 | isbn = 978-0-12-374893-5 | series = International Review of Neurobiology }}{{cite journal | vauthors = Lewis SS, Hutchinson MR, Rezvani N, Loram LC, Zhang Y, Maier SF, Rice KC, Watkins LR | title = Evidence that intrathecal morphine-3-glucuronide may cause pain enhancement via toll-like receptor 4/MD-2 and interleukin-1beta | journal = Neuroscience | volume = 165 | issue = 2 | pages = 569–83 | date = January 2010 | pmid = 19833175 | pmc = 2795035 | doi = 10.1016/j.neuroscience.2009.10.011 }} Morphine induced TLR4 activation attenuates pain suppression by opioids and enhances the development of opioid tolerance and addiction, drug abuse, and other negative side effects such as respiratory depression and hyperalgesia.{{cite web | url = http://cen.acs.org/articles/90/web/2012/08/Small-Molecules-Target-Toll-Like.html | title = Small Molecules Target Toll-Like Receptors | work = Chemical & Engineering News | author = Drahl C | date = 22 August 2012 }} 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,{{cite journal | vauthors = Shen CH, Tsai RY, Shih MS, Lin SL, Tai YH, Chien CC, Wong CS | title = Etanercept restores the antinociceptive effect of morphine and suppresses spinal neuroinflammation in morphine-tolerant rats | journal = Anesthesia and Analgesia | volume = 112 | issue = 2 | pages = 454–9 | date = February 2011 | pmid = 21081778 | doi = 10.1213/ANE.0b013e3182025b15 | s2cid = 12295407 | doi-access = free }}{{cite journal | vauthors = Hook MA, Washburn SN, Moreno G, Woller SA, Puga D, Lee KH, Grau JW | title = An IL-1 receptor antagonist blocks a morphine-induced attenuation of locomotor recovery after spinal cord injury | journal = Brain, Behavior, and Immunity | volume = 25 | issue = 2 | pages = 349–59 | date = February 2011 | pmid = 20974246 | pmc = 3025088 | doi = 10.1016/j.bbi.2010.10.018 }} 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.{{cite journal | vauthors = Watkins LR, Hutchinson MR, Rice KC, Maier SF | title = The "toll" of opioid-induced glial activation: improving the clinical efficacy of opioids by targeting glia | journal = Trends in Pharmacological Sciences | volume = 30 | issue = 11 | pages = 581–91 | date = November 2009 | pmid = 19762094 | pmc = 2783351 | doi = 10.1016/j.tips.2009.08.002 }}{{cite journal | vauthors = Hutchinson MR, Zhang Y, Brown K, Coats BD, Shridhar M, Sholar PW, Patel SJ, Crysdale NY, Harrison JA, Maier SF, Rice KC, Watkins LR | title = Non-stereoselective reversal of neuropathic pain by naloxone and naltrexone: involvement of toll-like receptor 4 (TLR4) | journal = The European Journal of Neuroscience | volume = 28 | issue = 1 | pages = 20–9 | date = July 2008 | pmid = 18662331 | pmc = 2588470 | doi = 10.1111/j.1460-9568.2008.06321.x }} 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{{cite journal | vauthors = Hutchinson MR, Coats BD, Lewis SS, Zhang Y, Sprunger DB, Rezvani N, Baker EM, Jekich BM, Wieseler JL, Somogyi AA, Martin D, Poole S, Judd CM, Maier SF, Watkins LR | title = Proinflammatory cytokines oppose opioid-induced acute and chronic analgesia | journal = Brain, Behavior, and Immunity | volume = 22 | issue = 8 | pages = 1178–89 | date = November 2008 | pmid = 18599265 | pmc = 2783238 | doi = 10.1016/j.bbi.2008.05.004 }}{{cite journal | vauthors = Hutchinson MR, Lewis SS, Coats BD, Rezvani N, Zhang Y, Wieseler JL, Somogyi AA, Yin H, Maier SF, Rice KC, Watkins LR | title = Possible involvement of toll-like receptor 4/myeloid differentiation factor-2 activity of opioid inactive isomers causes spinal proinflammation and related behavioral consequences | journal = Neuroscience | volume = 167 | issue = 3 | pages = 880–93 | date = May 2010 | pmid = 20178837 | pmc = 2854318 | doi = 10.1016/j.neuroscience.2010.02.011 }}

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. 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 lipopeptides, flagellin, and unmethylated DNA) 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 to the presence of microbial infections.

The prototypic member of the family, the toll receptor ({{UniProt|P08953}}; Tl) in the fruit fly Drosophila melanogaster, was discovered in 1985 by 1995 Nobel Laureates Christiane Nüsslein-Volhard and Eric Wieschaus and colleagues. It was known for its developmental function in embryogenesis by establishing the dorsal-ventral axis. It was named after Christiane Nüsslein-Volhard's 1985 exclamation, "{{lang|de|Das ist ja toll!}}" ("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.{{cite journal | vauthors = Hashimoto C, Hudson KL, Anderson KV | title = The Toll gene of Drosophila, required for dorsal-ventral embryonic polarity, appears to encode a transmembrane protein | journal = Cell | volume = 52 | issue = 2 | pages = 269–79 | date = January 1988 | pmid = 2449285 | doi = 10.1016/0092-8674(88)90516-8 | s2cid = 19439405 }} 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,{{cite journal | vauthors = Nomura N, Miyajima N, Sazuka T, Tanaka A, Kawarabayasi Y, Sato S, Nagase T, Seki N, Ishikawa K, Tabata S | display-authors = 6 | title = Prediction of the coding sequences of unidentified human genes. I. The coding sequences of 40 new genes (KIAA0001-KIAA0040) deduced by analysis of randomly sampled cDNA clones from human immature myeloid cell line KG-1 | journal = DNA Research | volume = 1 | issue = 1 | pages = 27–35 | year = 1994 | pmid = 7584026 | doi = 10.1093/dnares/1.1.27 | doi-access = free }} mapped to a chromosome by Taguchi and colleagues in 1996.{{cite journal | vauthors = Taguchi T, Mitcham JL, Dower SK, Sims JE, Testa JR | title = Chromosomal localization of TIL, a gene encoding a protein related to the Drosophila transmembrane receptor Toll, to human chromosome 4p14 | journal = Genomics | volume = 32 | issue = 3 | pages = 486–8 | date = March 1996 | pmid = 8838819 | doi = 10.1006/geno.1996.0150 }} 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.{{cite journal | vauthors = Gay NJ, Keith FJ | title = Drosophila Toll and IL-1 receptor | journal = Nature | volume = 351 | issue = 6325 | pages = 355–6 | date = May 1991 | pmid = 1851964 | doi = 10.1038/351355b0 | bibcode = 1991Natur.351..355G | s2cid = 1700458 }}

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.{{cite journal | vauthors = Poltorak A, He X, Smirnova I, Liu MY, Van Huffel C, Du X, Birdwell D, Alejos E, Silva M, Galanos C, Freudenberg M, Ricciardi-Castagnoli P, Layton B, Beutler B | display-authors = 6 | title = Defective LPS signaling in C3H/HeJ and C57BL/10ScCr mice: mutations in Tlr4 gene | journal = Science | volume = 282 | issue = 5396 | pages = 2085–8 | date = December 1998 | pmid = 9851930 | doi = 10.1126/science.282.5396.2085 | bibcode = 1998Sci...282.2085P }} These workers used positional cloning 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.

Image:History_of_TLRs.jpg

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.{{cite journal | vauthors = Hoshino K, Takeuchi O, Kawai T, Sanjo H, Ogawa T, Takeda Y, Takeda K, Akira S | display-authors = 6 | title = Cutting edge: Toll-like receptor 4 (TLR4)-deficient mice are hyporesponsive to lipopolysaccharide: evidence for TLR4 as the Lps gene product | journal = Journal of Immunology | volume = 162 | issue = 7 | pages = 3749–52 | date = April 1999 | doi = 10.4049/jimmunol.162.7.3749 | pmid = 10201887 | s2cid = 7419784 | doi-access = free }}

Plant homologs of toll were discovered by Pamela Ronald in 1995 (rice XA21){{cite journal | vauthors = Song WY, Wang GL, Chen LL, Kim HS, Pi LY, Holsten T, Gardner J, Wang B, Zhai WX, Zhu LH, Fauquet C, Ronald P | display-authors = 6 | title = A receptor kinase-like protein encoded by the rice disease resistance gene, Xa21 | journal = Science | volume = 270 | issue = 5243 | pages = 1804–6 | date = December 1995 | pmid = 8525370 | doi = 10.1126/science.270.5243.1804 | url = http://www.escholarship.org/uc/item/4x0247kj | bibcode = 1995Sci...270.1804S | s2cid = 10548988 | url-access = subscription }} and Thomas Boller in 2000 (Arabidopsis FLS2).{{cite journal | vauthors = Gómez-Gómez L, Boller T | title = FLS2: an LRR receptor-like kinase involved in the perception of the bacterial elicitor flagellin in Arabidopsis | journal = Molecular Cell | volume = 5 | issue = 6 | pages = 1003–11 | date = June 2000 | pmid = 10911994 | doi = 10.1016/S1097-2765(00)80265-8 | doi-access = free }}

In 2011, Beutler and Hoffmann were awarded the Nobel Prize in Medicine or Physiology for their work.{{cite web | title = The Nobel Prize in Physiology or Medicine 2011 | url = https://www.nobelprize.org/nobel_prizes/medicine/laureates/2011/press.html | date = 3 October 2011 | work = Nobel Media AB }} Hoffmann and Akira received the Canada Gairdner International Award in 2011.{{cite news| url=https://www.thestar.com/news/canada/article/958934--b-c-doctor-wins-prestigious-medical-prize?bn=1 | work=The Star | first=Bob | last=Mitchell | name-list-style = vanc | title=B.C. doctor wins prestigious medical prize | date=23 March 2011}}

{{Reflist|30em}}

• NOD-like receptor
• Immunologic adjuvant
• RIG-I-like receptor

• {{MeshName|Toll-Like+Receptors}}
• {{MeshName|Toll+protein,+Drosophila}}
• [https://web.archive.org/web/20110719055538/http://tollml.lrz.de/ TollML: Toll-like receptors and ligands database] at University of Munich
• [https://web.archive.org/web/20110614152357/http://www.sinauer.com/pdf/nsp-immunity-3-10.pdf The Toll-Like Receptor Family of Innate Immune Receptors (pdf)]
• [https://web.archive.org/web/20110723135534/http://www.invivogen.com/sscat.php?ID=13&ID_cat=2 Toll-Like receptor Pathway]
• [http://www.picscience.net/animations/tollLikeReceptors.php BioScience Animations]

{{Immune receptors}}

{{TLR signaling pathway}}

{{Pattern recognition receptors}}

Category:Developmental genetics

Category:Insect immunity

Category:LRR proteins

Category:Signal transduction