TIR domain-containing adaptors define the specificity of TLR signaling
Introduction
Toll-like receptors (TLRs) are germ-line-encoded receptors and play an important role in innate immunity from insects to mammals. TLRs recognize pathogen-associated molecular patterns (PAMPs) such as microbial components. To date, 10 TLRs (TLR1-TLR10) have been reportedly encoded in the human genome, and at least one ligand has been identified for each TLR, except for TLR8 and TLR10: triacylated lipoprotein for TLR1; peptidoglycan, the major gram-positive bacterial cell wall component, for TLR2; double-stranded RNA, which is considered to be generated in the lifecycle of RNA viruses, for TLR3; lipopolysaccharide (LPS), a gram-negative cell wall component, for TLR4; flagellin, a component of bacterial flagella, for TLR5; diacylated lipoprotein for TLR6; imidazoquinoline, an anti-viral drug, and its derivative R-848, for TLR7; and bacterial unmethylated CpG DNA for TLR9. Stimulation of TLRs by these ligands activates a signaling cascade, which culminates in proinflammatory cytokine production and subsequent immune responses. The signaling cascade downstream of TLR is generated from its Toll/interleukin-1 receptor (TIR)-domain (Takeda et al., 2003, Janeway and Medzhitov, 2002, Dunne and O’Neill, 2003, Imler and Hoffmann, 2001). Here, we describe an overview of the mechanisms of intracellular TLR signaling, with special emphasis on the events proximal to TLRs.
Section snippets
MyD88-dependent pathway
MyD88, a common adaptor protein also harboring a TIR domain, is associated with the TIR domain of TLRs, and leads to the activation of IRAK-1/4 and TRAF-6, culminating in the activation of NF-κB, a transcription factor crucial for the expression of pro-inflammatory cytokines and various mediators (Suzuki et al., 2002a, Suzuki et al., 2002b; Janssens and Beyaert, 2003, O’Neill, 2002a, Ghosh and Karin, 2002). Significant roles of MyD88 in TLR/interleukin-1 receptor signaling were demonstrated by
MyD88-independent pathway
Although the activation of NF-κB and MAPK through almost all of the TLRs is abolished in MyD88-deficient cells, the TLR4 ligand LPS stimulation still activates NF-κB and MAPK in MyD88-deficient cells, but with delayed kinetics compared with those of wild-type cells (Kawai et al., 1999). LPS-stimulated MyD88-deficient cells remain intact in their capacity to induce interferon (IFN)-inducible genes, such as IP-10, GARG-16, or IRG-1, and augment surface activation markers, including CD40, CD80, or
The second adaptor TIRAP: its physiological role
To examine the physiological in vivo role of TIRAP in the TLR signaling pathway, TIRAP knockout mice were generated by targeted gene disruption (Yamamoto et al., 2002a, Horng et al., 2002). Since TIRAP was identified as a molecule for the TLR4-specific signaling pathway, the growth rate of splenocytes was first compared between wild-type and TIRAP-deficient mice. Wild-type splenocytes proliferated in a dose-dependent manner in response to LPS, whereas TIRAP-deficient splenocytes were completely
The third adaptor TRIF
The analysis of TIRAP-deficient mice suggested that other molecules were responsible for the MyD88-independent pathway downstream of TLR3 or TLR4, and prompted us to search for other TIR domain-containing molecules. Database search screening resulted in the identification of some novel proteins harboring the TIR domain with unknown functions. One of these was named TRIF for TIR domain-containing adaptor inducing interferon-β (Yamamoto et al., 2002b, O’Neill, 2002b, Imler and Hoffmann, 2003,
The biological significance of TIR domain-containing adaptors
Analysis of MyD88-deficient mice clearly showed that both the TLR2- and TLR9-signaling pathways are completely dependent on MyD88 (Takeuchi et al., 2000). However, the production of type I interferon from certain cell types is stimulated by activation of TLR9 signaling but not by that of TLR2. If MyD88 was the only molecule connecting TLRs and downstream signaling molecules such as IRAK or TRAF6, the difference between the TLR2- and TLR9-mediated responses would be hard to explain. Analysis of
Other recent advances in understanding the MyD88-independent pathway
Recently, two groups reported that IKK-I/IKK-ε and TBK1/T2K act as molecules for mediating IRF3 phosphorylation (Fitzgerald et al., 2003, Sharma et al., 2003). IKK-I/IKK-ε and TBK1/T2K were primarily cloned as molecules possessing sequence homology with preexisting IKK such as IKKα and IKKβ, and therefore possibly having a role in NF-κB activation (Shimada et al., 1999, Peters et al., 2000, Pomerantz and Baltimore, 1999, Bonnard et al., 2000). Surprisingly, overexpression of both molecules led
Concluding remarks
Previously, TLR signaling has been considered to consist of MyD88-dependent and MyD88-independent pathways such that MyD88 and molecules for the MyD88-independent pathway were the only molecules which existed between TLR and downstream of the signal transducers. However, recent findings regarding the specific role of TIRAP in the MyD88-dependent pathway in TLR2 or TLR4 signaling change the hypothesis to a new paradigm that adaptor molecules harboring a TIR domain fractionate TLR signal
Acknowledgements
We thank the rest of Dr Akira’s laboratory members for helpful discussions, M. Hashimoto for secretarial assistance, and N. Okita and N. Iwami for technical assistance. This work was supported by grants from Special Coordination Funds, the Ministry of Education, Culture, Sports, Science and Technology, Research Fellowships from the Japan Society for the Promotion of Science for Young Scientists, The Uehara Memorial Foundation. The Naito Foundation, and the Junior Research Association from RIKEN.
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