Stages of TLR Signaling
Authors: Gough Nancy R. 
Published: June 2008

Toll-like receptors (TLRs) recognize products derived from pathogens to initiate immune and inflammatory responses. Signaling through this large family of receptors involves several different adaptor proteins that show specificity for particular TLRs, as well as kinases of the IRAK (interleukin-1 receptor-associated kinase) family that are downstream of most members of the TLR family. There are four members of the IRAK family in mammals: IRAKM is an inhibitor of TLR signaling, whereas IRAK1, IRAK2, and IRAK4 are positive mediators in the pathway. In the absence of IRAK4, TLR signaling is severely impaired, whereas lack of IRAK1 activity only partially compromises TLR signaling. Kawagoe
et al
. characterize the TLR responses of
Irak2
–/–
mice and show that IRAK2 appears to be important for mediating a sustained TLR response, whereas IRAK1 appears to mediate a transient early response (see commentary by Meylan and Tschopp). The
Irak2
–/–
mice had increased survival rates when exposed to the TLR4 or TLR9 ligands, lipopolysaccharide (LPS) or CpG DNA, respectively. Additionally, cytokine production by
Irak2
–/–
macrophages or dendritic cells was decreased after exposure to ligands for TLR2 or TLR6 (MALP-2), TLR4 (LPS), TLR7 (resiquimod), or TLR9 (CpG DNA), but not for the TLR3 ligand polyinosinic-polycytidylic acid. Transcriptional analysis of genes activated in response to TLR2 signaling suggested that expression was the same as that in wild-type cells at early times (1 hour after MALP-2 addition) but was reduced at later times (after 4 hours). Activation of the downstream mitogen-activated protein kinases (MAPKs) JNK, p38, and ERK was the same in
Irak2
–/–
macrophages. Early-stage activation of the transcription factor nuclear factor κB (NF-κB) was present, but the sustained (beyond 4 hours) DNA-binding activity of NF-κB was lost. The kinase activity of IRAK2 appeared to be required for its function, because a kinase-inactive mutant failed to restore TLR2-mediated responses in
Irak2
–/–
macrophages. The abundance of IRAK1 was decreased after TLR2 stimulation in wild-type and
Irak2
–/–
macrophages; however, the abundance of IRAK2 and IRAK4 did not change. The kinase activity of IRAK1 was highest 30 minutes after TLR2 activation, whereas IRAK2 activity was highest 4 to 8 hours after TLR2 activation. Experiments with double-knockout cells (
Irak1
–/Y
,
Irak2
–/–
) suggested that these two kinases are redundant in TLR signaling. The double-knockout mice were completely resistant to septic shock induced by LPS or cpG DNA. Furthermore, the double-knockout macrophages completely lacked activation of downstream MAPKs, and NF-κB activation was much lower. Thus, IRAK1 and IRAK2 both contribute to TLR signaling, although the time course of their contributions appears to be sequential--IRAK1 first, then IRAK2.

T. Kawagoe, S. Sato, K. Matsushita, H. Kato, K. Matsui, Y. Kumagai, T. Saitoh, T. Kawai, O. Takeuchi, S. Akira, Sequential control of Toll-like receptor-dependent responses by IRAK1 and IRAK2.
Nat. Immunol.
9
, 684-691 (2008).
[PubMed]

E. Meylan, J. Tschopp, IRAK2 takes its place in TLR signaling.
Nat. Immunol.
9
, 581-582 (2008).
[PubMed]