Toll-like receptors (TLRs) are a class of proteins that play a key role in the innate immune system. They are single-pass membrane-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 breached 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. TLR1, TLR2, TLR4, TLR5, TLR6, and TLR10 are located on the cell membrane, whereas TLR3, TLR7, TLR8, and TLR9 are located in intracellular vesicles.
Pattern recognition receptors (PRRs) play a crucial role in the proper function of the innate immune system. PRRs are germline-encoded host sensors, which detect molecules typical for the pathogens. They are proteins expressed, mainly, by cells of the innate immune system, such as dendritic cells, macrophages, monocytes, neutrophils and epithelial cells, to identify two classes of molecules: pathogen-associated molecular patterns (PAMPs), which are associated with microbial pathogens, and damage-associated molecular patterns (DAMPs), which are associated with components of host's cells that are released during cell damage or death. They are also called primitive pattern recognition receptors because they evolved before other parts of the immune system, particularly before adaptive immunity. PRRs also mediate the initiation of antigen-specific adaptive immune response and release of inflammatory cytokines.
Toll-like receptor 3 (TLR3) also known as CD283 is a protein that in humans is encoded by the TLR3 gene. TLR3 is a member of the toll-like receptor family of pattern recognition receptors of the innate immune system. TLR3 recognizes double-stranded RNA in endosomes, which is a common feature of viral genomes internalised by macrophages and dendritic cells.
Imiquimod, sold under the brand name Aldara among others, is a medication that acts as an immune response modifier that is used to treat genital warts, superficial basal cell carcinoma, and actinic keratosis. Scientists at 3M's pharmaceuticals division discovered the drug and 3M obtained the first FDA approval in 1997. As of 2015, imiquimod is generic and is available worldwide under many brands.
Toll-like receptor 2 also known as TLR2 is a protein that in humans is encoded by the TLR2 gene. TLR2 has also been designated as CD282. TLR2 is one of the toll-like receptors and plays a role in the immune system. TLR2 is a membrane protein, a receptor, which is expressed on the surface of certain cells and recognizes foreign substances and passes on appropriate signals to the cells of the immune system.
Toll-like receptor 7, also known as TLR7, is a protein that in humans is encoded by the TLR7 gene. Orthologs are found in mammals and birds. It is a member of the toll-like receptor (TLR) family and detects single stranded RNA.
Toll-like receptor 5, also known as TLR5, is a protein which in humans is encoded by the TLR5 gene. It is a member of the toll-like receptor (TLR) family. TLR5 is known to recognize bacterial flagellin from invading mobile bacteria. It has been shown to be involved in the onset of many diseases, which includes Inflammatory bowel disease. Recent studies have also shown that malfunctioning of TLR5 is likely related to rheumatoid arthritis, osteoclastogenesis, and bone loss. Abnormal TLR5 functioning is related to the onset of gastric, cervical, endometrial and ovarian cancers.
Toll-like receptor 4 is a protein that in humans is encoded by the TLR4 gene. TLR4 is a transmembrane protein, member of the toll-like receptor family, which belongs to the pattern recognition receptor (PRR) family. Its activation leads to an intracellular signaling pathway NF-κB and inflammatory cytokine production which is responsible for activating the innate immune system.
Toll-like receptor 6 is a protein that in humans is encoded by the TLR6 gene. TLR6 is a transmembrane protein, member of toll-like receptor family, which belongs to the pattern recognition receptor (PRR) family. TLR6 acts in a heterodimer form with toll-like receptor 2 (TLR2). Its ligands include multiple diacyl lipopeptides derived from gram-positive bacteria and mycoplasma and several fungal cell wall saccharides. After dimerizing with TLR2, the NF-κB intracellular signalling pathway is activated, leading to an pro-inflammatory cytokine production and activation of innate immune response. TLR6 has also been designated as CD286.
Toll-like receptor 8 is a protein that in humans is encoded by the TLR8 gene. TLR8 has also been designated as CD288. It is a member of the toll-like receptor (TLR) family.
Toll-like receptor 9 is a protein that in humans is encoded by the TLR9 gene. TLR9 has also been designated as CD289. It is a member of the toll-like receptor (TLR) family. TLR9 is an important receptor expressed in immune system cells including dendritic cells, macrophages, natural killer cells, and other antigen presenting cells. TLR9 preferentially binds DNA present in bacteria and viruses, and triggers signaling cascades that lead to a pro-inflammatory cytokine response. Cancer, infection, and tissue damage can all modulate TLR9 expression and activation. TLR9 is also an important factor in autoimmune diseases, and there is active research into synthetic TLR9 agonists and antagonists that help regulate autoimmune inflammation.
Ibudilast is an anti-inflammatory drug used mainly in Japan, which acts as a phosphodiesterase inhibitor, inhibiting the PDE4 subtype to the greatest extent, but also showing significant inhibition of other PDE subtypes.
PPAR agonists are drugs which act upon the peroxisome proliferator-activated receptor. They are used for the treatment of symptoms of the metabolic syndrome, mainly for lowering triglycerides and blood sugar.
Entolimod (CBLB502) is being developed by Cleveland Biolabs, Inc. for dual indications under the U.S. Food & Drug Administration’s (FDA) animal efficacy rule as a pivotal-stage radiation countermeasure, and under the FDA’s traditional drug approval pathway as a cancer treatment.
Resiquimod (R-848) is a drug that acts as an immune response modifier, and has antiviral and antitumour activity. It is used as a topical gel in the treatment of skin lesions such as those caused by the herpes simplex virus and cutaneous T cell lymphoma, and as an adjuvant to increase the effectiveness of vaccines. In an animal disease model, systemic administration of resiquimod-loaded nanoparticles has been shown to improve response rates to cancer immunotherapy with a checkpoint inhibitor through stimulation of tumor-associated macrophages. It has several mechanisms of action, being both an agonist for toll-like receptor 7 and 8, and an upregulator of the opioid growth factor receptor. On 28 April 2016, orphan designation (EU/3/16/1653) was granted by the European Commission to Galderma R&D, France for resiquimod to be used in the treatment of cutaneous T-cell lymphoma.
(+)-Naloxone (dextro-naloxone) is a drug which is the opposite enantiomer of the opioid antagonist drug (−)-naloxone. Unlike (-)-naloxone, (+)-naloxone has no significant affinity for opioid receptors, but instead has been discovered to act as a selective antagonist of Toll-like receptor 4. This receptor is involved in immune system responses, and activation of TLR4 induces glial activation and release of inflammatory mediators such as TNF-α and Interleukin-1.
Motolimod (VTX-2337) is a drug which acts as a potent and selective agonist of toll-like receptor 8 (TLR8), a receptor involved in the regulation of the immune system. It is used to stimulate the immune system, and has potential application as an adjuvant therapy in cancer chemotherapy, although clinical trials have shown only modest benefits. It also worsens neuropathic pain in animal models and has been used to research the potential of targeting TLR8 in some kinds of chronic pain syndromes.
Vesatolimod (GS-9620) is an antiviral drug developed by Gilead Sciences, which acts as a potent and selective agonist of Toll-like receptor 7 (TLR7), a receptor involved in the regulation of the immune system. It is used to stimulate the immune system, which can increase its ability to combat chronic viral infections. Vesatolimod is in clinical trials to determine whether it is safe and effective in patients with Hepatitis B and HIV/AIDS, and has also shown activity against other viral diseases such as norovirus and enterovirus 71.
CU-CPT9a is a drug which acts as a potent and selective antagonist of Toll-like receptor 8 (TLR8), with an IC50 of 0.5nM. Activation of toll-like receptors triggers release of cytokines and other signalling factors, leading to inflammation. This is an essential part of the immune system's response to infection, but chronic activation of TLR signalling is thought to be involved in various inflammatory and autoimmune disorders. CU-CPT9a has immunosuppressant properties, and may have applications in the treatment of autoimmune disorders, as well as being used in scientific research into the function of TLR8.
(+)-Morphine also known as dextro-morphine is the "unnatural" enantiomer of the opioid drug (-)-morphine. Unlike "natural" levo-morphine, unnatural dextro-morphine is not present in Papaver somniferum and is the product of laboratory synthesis.
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