Letter | Published:

Essential role for TIRAP in activation of the signalling cascade shared by TLR2 and TLR4


Signal transduction through Toll-like receptors (TLRs) originates from their intracellular Toll/interleukin-1 receptor (TIR) domain, which binds to MyD88, a common adaptor protein containing a TIR domain1,2,3,4. Although cytokine production is completely abolished in MyD88-deficient mice, some responses to lipopolysaccharide (LPS), including the induction of interferon-inducible genes and the maturation of dendritic cells, are still observed5,6,7. Another adaptor, TIRAP (also known as Mal), has been cloned as a molecule that specifically associates with TLR4 and thus may be responsible for the MyD88-independent response8,9. Here we report that LPS-induced splenocyte proliferation and cytokine production are abolished in mice lacking TIRAP. As in MyD88-deficient mice, LPS activation of the nuclear factor NF-κB and mitogen-activated protein kinases is induced with delayed kinetics in TIRAP-deficient mice5. Expression of interferon-inducible genes and the maturation of dendritic cells is observed in these mice; they also show defective response to TLR2 ligands, but not to stimuli that activate TLR3, TLR7 or TLR9. In contrast to previous suggestions, our results show that TIRAP is not specific to TLR4 signalling and does not participate in the MyD88-independent pathway. Instead, TIRAP has a crucial role in the MyD88-dependent signalling pathway shared by TLR2 and TLR4.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.


  1. 1

    Akira, S., Takeda, K. & Kaisho, T. Toll-like receptors: critical proteins linking innate and acquired immunity. Nature Immunol. 2, 675–680 (2001)

  2. 2

    Imer, J. L. & Hoffmann, J. A. Toll-like receptors in innate immunity. Trends Cell Biol. 11, 304–311 (2001)

  3. 3

    Medzhitov, R. Toll-like receptors and innate immunity. Nature Rev. Immunol. 1, 135–145 (2002)

  4. 4

    Janeway, C. A. Jr & Medzhitov, R. Innate immune recognition. Annu. Rev. Immunol. 20, 197–216 (2002)

  5. 5

    Kawai, T., Adachi, O., Ogawa, T., Takeda, K. & Akira, S. Unresponsiveness of MyD88-deficient mice to endotoxin. Immunity 11, 115–122 (1999)

  6. 6

    Kaisho, T., Takeuchi, O., Kawai, T., Hoshino, K. & Akira, S. Endotoxin-induced maturation of MyD88-deficient dendritic cells. J. Immunol. 166, 5688–5694 (2001)

  7. 7

    Kawai, T. et al. Lipopolysaccharide stimulates the MyD88-independent pathway and results in activation of IRF-3 and the expression of a subset of LPS-inducible genes. J. Immunol. 167, 5887–5894 (2001)

  8. 8

    Horng, T., Barton, G. M. & Medzhitov, R. TIRAP: an adapter molecule in the Toll signaling pathway. Nature Immunol. 2, 835–841 (2001)

  9. 9

    Fitzgerald, K. A. et al. Mal (MyD88-adaptor-like) is required for Toll-like receptor-4 signal transduction. Nature 413, 78–83 (2001)

  10. 10

    Adachi, O. et al. Targeted disruption of the MyD88 gene results in loss of IL-1- and IL-18-mediated function. Immunity 9, 143–150 (1998)

  11. 11

    Hemmi, H. et al. A Toll-like receptor recognizes bacterial DNA. Nature 408, 740–745 (2000)

  12. 12

    Hemmi, H. et al. Small anti-viral compounds activate immune cells via the TLR7-MyD88-dependent signaling pathway. Nature Immunol. 3, 196–200 (2002)

  13. 13

    Alexopoulou, L., Holt, A. C., Medzhitov, R. & Flavell, R. A. Recognition of double-stranded RNA and activation of NF-κB by Toll-like receptor 3. Nature 413, 732–738 (2001)

  14. 14

    Takeuchi, O. et al. Cutting edge: preferentially the R-stereoisomer of the mycoplasmal lipopeptide macrophage-activating lipopeptide-2 activates immune cells through a toll-like receptor 2- and MyD88-dependent signaling pathway. J. Immunol. 164, 554–557 (2000)

  15. 15

    Takeuchi, O. et al. Differential roles of TLR2 and TLR4 in recognition of Gram-negative and Gram-positive bacterial cell wall components. Immunity 11, 443–451 (1999)

  16. 16

    Toshchakov, V. et al. TLR4, but not TLR2, mediates IFN-β-induced STAT1α/β-dependent gene expression in macrophages. Nature Immunol. 3, 392–398 (2002)

  17. 17

    O'Neill, L. A. Toll-like receptor signal transduction and the tailoring of innate immunity: a role for Mal? Trends Immunol. 23, 296–300 (2002)

  18. 18

    Sato, S. et al. A variety of microbial components induce tolerance to lipopolysaccharide by differentially affecting MyD88-dependent and -independent pathways. Int. Immunol. 14, 783–791 (2002)

  19. 19

    Iwamura, T. et al. Induction of IRF-3/-7 kinase and NF-κB in response to double-stranded RNA and virus infection: common and unique pathways. Genes Cells 6, 375–388 (2001)

Download references


We thank P. F. Mühlradt and H. Tomizawa for MALP-2 and R-848, respectively; E. Horita 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, and Research Fellowships from the Japan Society for the Promotion of Science for Young Scientists.

Author information

Correspondence to Shizuo Akira.

Ethics declarations

Competing interests

The authors declare that they have no competing financial interests.

Supplementary information

Supplementary Figure 1 (JPG 62 kb)

Supplementary Figure Legend (DOC 19 kb)

Rights and permissions

Reprints and Permissions

About this article

Further reading

Figure 1: Targeted disruption of the murine Tirap gene.
Figure 2: Impaired response to LPS in TIRAP-deficient splenocytes.
Figure 3: Cytokine production in peritoneal macrophages in response to TLR ligands.
Figure 4: Activation of signalling cascades in response to TLR2, TLR4 and TLR7 ligands in peritoneal macrophages.
Figure 5: LPS-induced activation of the MyD88-independent signalling pathway in TIRAP-deficient mice.


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.