TLR Signaling Pathway

Toll-like receptor (TLR) signaling plays an essential role in the innate immune response. Activation of TLR signaling through recognition of pathogen-associated molecular patterns leads to the transcriptional activation of genes encoding for pro-inflammatory cytokines, chemokines and co-stimulatory molecules. These molecules subsequently control the activation of antigen-specific adaptive immune response. TLRs have been pursued as potential therapeutic targets for various inflammatory diseases and cancer.

Human TLRs were first identified as homologs to the toll receptor in Drosophila ^[1]. To date, ten proteins have been identified that belong to the human TLR family ^[2]. All TLRs are transmembrane proteins that consist of a leucine-rich repeat extracellular domain (for recognizing specific pathogens), a transmembrane region and a Toll-IL-1R domain (for initiating intracellular signaling events).

Variations in TLR ligands initiate specific immunological responses:

• TLR2 recognizes bacterial LAM, BLP, and PGN following their initial interaction with CD14 ^[3].
• TLR4 forms a homodimer complex with the MD-2 protein after the initial binding of bacterial LPS to CD14 ^[4].
• TLR5 activation occurs following interaction with bacterial flagellin ^[5].
• TLR1 and TLR6 function as co-receptors to TLR2 to promote unique signaling mechanisms based on specific pathogen binding ^[6].
• TLR1, TLR2, TLR4, TLR5, and TLR6 are all receptors located on the cell surface; other TLR receptors located within the endosome recognize additional pathogens.
• TLR3, TLR7, TLR8 and TLR9 are activated by viral dsDNA, viral ssRNA, and bacterial CpG, respectively ^{[7, 8, 9]}.
• TLR7 and TLR8 can be activated by the imidazoquinoline compounds imiquimod and R-848.

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TNF interacts with many pathway targets, including:

Activation of most TLRs upon ligand binding results the activation of NF-kB and MAPK pathways to elicit regulatory responses. The signaling events initiated by TLR activation are mediated by unique interaction between TIR domain–containing cytosolic adapters, including: myeloid differentiation primary-response protein-88 (MyD88), TIR domain–containing adapter protein (TIRAP), TIR domain–containing adapter–inducing IFNb (TRIF) and TRIF-related adapter molecule (TRAM) ^[10].

MyD88 serves as the central adapter protein associating with IRAK4, which recruits and phosphorylates IRAK1. Following interaction with TRAF6, the activated IRAK complex phosphorylates TAB1 and TAK1, which in turn activate the NF-κB and MAPK pathways ^[11] leading the expression of interleukins (IL-1B, IL-6, IL-8, and IL-12), macrophage inflammatory proteins, (MIP-1a and MIP1b), and cytokines (RANTES and TNFα) [13]. TLR3 and TLR7/TLR8 can mediate the activation of IRF3 and IRF7.TLR3 functions through a MyD88-independent pathway by interacting with TRIF, which activates a complex of IKKe, TRAF3, and TBK1 that phosphorylates IRF3 and IRF7 ^[12]. Activation of IRF3 results in the induction of genes that stimulate T cell immunogenic responses. IRF7 promotes an antiviral immune response by the induction of IFNα and IFNβ gene expression.

1. Valanne, S. et al. (2011) The drosophila toll signaling pathway. J of Immunol 186: 649-656. 
2. Oda, K. and Kitano, H. (2006) A comprehensive map of the toll-like receptor signaling network. Molecular Systems Biology 2: 2006.0015. 
3. Ray, A. et al. (2013) Bacterial cell wall macroamphiphiles: pathogen-/microbe-associated molecular patterns detected by mammalian innate immune system. Biochimie 95: 33-42.
4. O’Neill, L. et al. (2013) The history of toll-like receptors – redefining innate immunity. Nat Rev Immunol 13: 453-460.
5. Hayashi, F. et al. (2001) The innate immune response to bacterial flagellin is mediate by toll-like receptor 5.  Nature 410: 1099–1103. 
6. Takeuchi, O. et al. (2002) Cutting edge: role toll-like receptor 1 in mediating immune response to microbial lipoproteins.  J Immunol 169: 10–14. 
7. Alexopoulou, L. et al. (2001) Recognition of double-stranded RNA and activation of NF-kappaB by toll-like receptor 3.  Nature 413: 732–738. 
8. Diebold, S. et al. (2004) Innate antiviral responses by means of TLR7-mediated recognition of single-stranded RNA. Science 303: 1529–1531. 
9. Hemmi, H. et al. (2000) A toll-like receptor recognizes bacterial DNA. Nature 408: 740–745. 
10. Akira, S. and Takeda, K. (2004) Toll-like receptor signalling. Nat Rev Immunol 4: 499–511. 
11. Kawai, T. and Akira, S. (2006) TLR signaling. Cell Death and Differentiation 13: 816–825. 
12. Fitzgerald, KA. et al. (2003) IKK-epsilon and TBK1 are essential components of the IRF3 signaling pathway. Nat Immunol 4: 491–496. 
13. Moynagh, P. (2005) TLR signalling and activation of IRFs: revisiting old friends from the NF-kappaB pathway. Trends in Immunol 26: 469–476.