Cellular Sentinels: Toll like receptors ring alarms as key players in immunity

The Toll-like receptors (TLRs) are a family of evolutionarily ancient Patten Recognition Receptors (PRRs) that rapidly detect microbial infection and stimulate the production of pro-inflammatory and antiviral cytokines and chemokines, as well as initiating significant metabolic shifts within the cell. Acting as cellular sentinels for infectious threats, signalling downstream of TLRs sets the stage for successful development of adaptive immunity ¹.

The repertoire of these type I transmembrane proteins varies by organism, with the human genome encoding ten TLRs. Human TLRs are localised to the either the plasma membrane or to endosomes and interact directly with conserved microbial structures known as Pathogen Associated Molecular Patterns (PAMPs) ^1,2. They are expressed on a wide variety of cell types including monocytes, macrophages, neutrophils, eosinophils, T cells, B cells, NK cells, dendritic cells, mast cells and epithelial cells ³. Table 1 summarizes the cellular localization and ligand specificity of each receptor. Extensive structural information from mammalian TLRs shows that ligand binding involves direct molecular interactions between the appropriate PAMP and TLR homo- or heterodimers. These TLR-PAMP interactions involve the N-terminal TLR ectodomains, which consist of multiple leucine-rich-repeats (LRRs) that adopt a characteristic horseshoe-shaped conformation.

Upon PAMP-mediated TLR dimerization, the cytosolic Toll/interleuckin-1 receptor (TIR) domains of each receptor are brought into proximity. This initiates a signalling cascade leading to a variety of cellular responses. The adapter protein MAL/TIRAP is a peripheral membrane protein that associates with dimerised TIR domains and initiates the assembly of a large signalling complex known as the myddosome ⁴. Multiple copies of MyD88 are recruited via TIR domain interactions, which then engage and activate molecules of the IRAK4, IRAK2 and IRAK1 serine/threonine kinases via death domain-death domain interactions. TRAF6, an E3 ubiquitin ligase, is then recruited to the myddosome, leading to a range of downstream signalling events. Activation of NF-κB and AP-1 transcription factors proceeds through the IKK and MAPK kinases, respectively and drives the expression of inflammatory genes. TRAF6 also acts via TBK1 kinase to swiftly initiate glycolysis following TLR engagement. ¹

MyD88 is required for signalling by all TLRs except for TLR3 and TLR4. TLR4 signals through the peripheral membrane adaptor protein TRAM via TRIF and the postulated triffosome signalling complex in addition to MyD88. TLR3 signals exclusively through TRIF in a manner that does not require the TRAM adapter protein. The triffosome, like the myddosome, can activate TRAF6, but additionally activates TRAF3, leading to IRF3 activation and the induction of type I interferons ¹.

As potent mediators of immune responses, TLR signalling pathways are relevant in many areas of human disease ². There is considerable interest in TLR agonist molecules as candidates for novel vaccine adjuvants, key vaccine components that enhance immune responses when co-administered with antigens ^2,5. Rare mutations in components of the TLR signalling pathways can lead to greater vulnerability to certain infectious diseases. Patients with inactivating mutations in IRAK4 and MyD88 are highly susceptible to severe bacterial infections in childhood, particularly Streptococcus pneumoniae, Pseudomonas aeruginosa and Staphylococcus aureus ^6,7. Polymorphisms in various TLR genes are also associated with susceptibility to various autoimmune diseases including Type 1 Diabetes, Graves’ Disease, Systemic Lupus Erythematosus, Rheumatoid Arthritis and Multiple Sclerosis although the mechanistic role of these polymorphisms is currently unclear ⁸. Multiple lines of evidence support a role for TLR signalling in the pathogenesis of sepsis, but clinical trials of molecules targeting TLRs as treatments for sepsis, such as the TLR4 antagonist eritoran, have proved disappointing so far ⁹. TLR expression is increased in many tumour types, and activation of these receptors may contribute to the inflammatory tumour microenvironment and promote tumour progression. However, activation of these same pathways on immune cells can enhance anti-tumor activities and presents a promising therapeutic avenue in cancer immunotherapy ^2,10.

Using Kegg Pathway Assessment, we compared known TLR signalling pathway genes with our catalogue of HAP1 and Cancer cell lines. Of the known 102 TLR signalling pathway genes identified in the Kegg Assessment, we have 57 cell lines available. Check out the list below for gene coverage and links to product pages.