Toll-like receptors (TLRs) and their signalling pathways have therapeutic potential for the treatment of inflammation, cancer, infection, allergy and autoimmunity.
The elucidation of the structure of several TLRs may aid medicinal chemistry in the rational design of small-molecule agonists and antagonists.
Agonists for TLR4, TLR7, TLR8 and TLR9 are being tested as adjuvants for infectious diseases and in certain cancers. There is also potential for some of these agents in the de-sensitization of airways in allergic asthma and allergy.
TLR antagonists are being investigated for the treatment of inflammatory diseases. Examples here include the inhibition of TLR7 and TLR9 in systemic lupus erythematosus, inhibition of TLR4 in sepsis and asthma or allergy, and inhibition of TLR2 in ischaemia and reperfusion injury.
Emerging findings on the underlying mechanism of TLR action might also lend themselves to therapeutic manipulation. Examples here include modulation of microRNAs, the exploitation of viral proteins to prevent nuclear factor-κB signalling and the development of compounds that interfere with the ubiquitination of signalling molecules.
There is a growing interest in the targeting of Toll-like receptors (TLRs) for the prevention and treatment of cancer, rheumatoid arthritis, inflammatory bowel disease and systemic lupus erythematosus (SLE). Several new compounds are now undergoing preclinical and clinical evaluation, with a particular focus on TLR7 and TLR9 activators as adjuvants in infection and cancer, and inhibitors of TLR2, TLR4, TLR7 and TLR9 for the treatment of sepsis and inflammatory diseases. Here, we focus on TLRs that hold the most promise for drug discovery research, highlighting agents that are in the discovery phase and in clinical trials, and on the emerging new aspects of TLR-mediated signalling — such as control by ubiquitination and regulation by microRNAs — that might offer further possibilities of therapeutic manipulation.
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E.H. would like to thank the Health Research Board and L.A.J.O. thanks Science Foundation Ireland for financial support.
L.A.J.O is a cofounder of Opsona Therapeutics, which uses immune system research to develop new drugs and vaccines.
- Tumour necrosis factor.
(TNF). A pro-inflammatory cytokine involved in immune response. It induces apoptosis and inflammation.
A cell surface molecule that is associated with TLR4. It is required for immune signalling in response to lipopolysaccharide.
- Toll–interleukin-1 resistance (TIR) domain
A conserved intracellular domain of Toll and IL-1 families of receptors, which is also present in the adaptor molecules MYD88, MAL, TRIF and TRAM.
- Interleukin-1 receptor-associated kinase 4
(IRAK4). A kinase that interacts with MYD88 and TRAF6 and is crucial for TLR signalling.
- Nuclear factor-κB
(NF-κB). A transcription factor that is translocated to the nucleus following the stimuation of cell surface receptors by microbe-associated molecular patterns.
- Type I interferons
A class of cytokines, produced by plasmacytoid dendritic cells, that includes interferon α and β.
- Monophosphoryl lipid-A
(MPL). A non-toxic derivative of lipid-A from Gram-negative lipopolysaccharide.
- Coley's toxin
A mixture of lipopolysaccharides and bacterial DNA products named after William Coley, who in 1893 first used dead bacteria to improve the survival of inoperable patients with cancer.
- Regulatory T cells
A class of T cells that suppresses the immune response and prevents the development of autoimmune disorders.
- Structure–activity relationship
A medicinal chemistry approach that involves the correlation of structural features with the activity of compounds in a given assay to improve potency and/or efficacy.
- T-helper 1 cell (Th1) response
An immune response involved in cell-mediated immunity that is produced when dendritic cells interact with pre-T-helper cells to produce IFNγ, IL-1, IL-2 and TNF-β, and leads to the activation of macrophages.
- CpG-based oligonucleotides
Oligonucleotides with cytosine–phosphate–guanosine-containing motifs that are found mainly in bacterial and viral DNA and that induce an immune response through activation of TLR9. There are three classes of CpG-oligonucleotides, CpG-A, CpG-B and CpG-C.
- Immune modulatory oligonucleotides
(IMOs). DNA or RNA based oligonucleotides of varying length that are used to target TLR7, TLR8 or TLR9.
- T-helper 2 cell (Th2) response
Part of the humoral immune response that helps B-cells produce IL-4, IL-5, IL-6, IL-9, IL-10 and IL-13, which evoke a strong antibody response and are required for the production of IgE. This leads to eosinophil and mast-cell accumulation and an allergic response.
The biologically active part of LPS, its hydrophobic nature anchors LPS to the membrane.
A protein constituent of a bacterium's flagellum that specifically activates TLR5.
An antiviral pro-drug that when metabolized resembles RNA purine molecules. It impedes DNA and RNA metabolism and interferes with viral replication. It is active against many viruses including influenza and hepatitis B.
A cell surface protein found on activated B-cells and monocytes that provides the necessary stimulation to prime T cells. When this complex binds to CTLA-4 expressed on the surface of T helper cells, the T-cell response is reduced.
- Ectodomain of matrix protein 2
(M2e). A highly conserved region of the influenza A virus that consists of 23 amino acids of the M2 protein (the third integral membrane protein of the virus).
A precursor in the biosynthetic pathway for lipopolysaccharide that acts as a TLR4 antagonist.
- Systematic evolution of ligands by exponential enrichment
(SELEX). Also known as in vitro evolution. This process is used to produce either RNA or DNA oligonucleotides that specifically bind to a ligand. The process starts with a large library of oligonucleotides that are then exposed to the ligand. The process is repeated and the oligonucleotide sequence amplified until an aptamer that has a high affinity for the ligand is produced.
(miRNAs). Small 20–23 nucleotide non-coding RNA molecules that fine-tune gene expression by targeting specific messanger RNA.
- Locked nucleic acid miRNA inhibitor
Nucleic acid analogues in which the ribose ring is 'locked' by a methylene bridge, resulting in increased thermal stability and discrimination with respect to target nucleic acids.
A post-translational modification involving 3 enzymes: E1 the ubiquitin-activating enzyme, E2 the ubiquitin-conjugating enzyme and E3 the ubiquitin protein ligase. Tagging a molecule with K48-linked ubiquitins targets it for degradation by the proteasome
A protein with E3 ubiquitin ligase activity that activates the p38 MAP kinase pathway to negatively regulate TLR signalling.
- NF-κB essential modifier
(NEMO). The regulatory subunit of the IκB kinase complex that phosphorylates inhibitors of NF-κB, leading to their degradation and the translocation of NF-κB into the nucleus.
- IκB kinase (IKK) complex
A complex that contains two catalytic subunits, IKKα and IKKβ, and the regulatory subunit NEMO that acts as a master regulator of NF-κB activation.
- Inhibitors of apoptosis
(IAPs). Endogenous inhibitors of programmed cell death or apoptosis, which bind caspases and inhibit their activation.
- Second mitochondria-derived activator of caspase
(SMAC). A protein that promotes cytochrome-c-dependent cell activation by eliminating IAP, which leads to cell apoptosis.
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Hennessy, E., Parker, A. & O'Neill, L. Targeting Toll-like receptors: emerging therapeutics?. Nat Rev Drug Discov 9, 293–307 (2010). https://doi.org/10.1038/nrd3203
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