Modulation of Toll-interleukin 1 receptor mediated signaling.

Toll-like receptors (TLRs) belong to the Toll-interleukin 1 receptor superfamily, which is defined by a common intracellular Toll-IL-1 receptor (TIR) domain. A group of TIR domain containing adaptors (MyD88, TIRAP, TRIF and TRAM), are differentially recruited to the Toll-IL-1 receptors, contributing to the specificity of signaling. The IL-1 mediated signaling pathway serves as a "prototype" for other family members. Genetic and biochemical studies reveal that IL-1R uses adaptor molecule MyD88 to mediate a very complex pathway, involving a cascade of kinases organized by multiple adapter molecules into signaling complexes, leading to activation of the transcription factor NFkappaB. Several Toll-like receptors utilize variations of the "prototype" pathway by employing different adaptor molecules. Double-stranded RNA triggered, TLR3-mediated signaling is independent of MyD88, IRAK4, and IRAK. The adapter molecule TRIF is utilized by TLR3 to mediate the activation of NFkappaB and IRF3. LPS-induced, TLR4-mediated signaling employs multiple TIR-domain containing adaptors, MyD88/TIRAP to mediate NFkappaB activation, TRIF/TRAM for IRF3 activation. Recent studies have also begun to unravel how these pathways are negatively regulated. SIGIRR (also known as TIR8), a member of TIR superfamily that does not activate the transcription factors NFkappaB and IRF3, instead negatively modulates responses. Cells from SIGIRR-null mice show enhanced activation in response to either IL-1 or certain Toll ligands. In addition to SIGIRR, several other negative regulators have been shown to inhibit the TIR signaling, including ST2, IRAKM, MyD88s, SOCS1, and Triad3A. The coordinated positive and negative regulation of the TIR signaling ensures the appropriate modulation of the innate and inflammatory responses.