All known have been shown to signal through Toll/IL-1R homology (TIR) domain-containing adaptor molecules, MyD88, and/or TIR domain-containing adaptor-inducing IFN-? (TRIF). Mice deficient in both MyD88 and TRIF are unresponsive to stimulation with all known TLR ligands.
TNF-alpha:
TNF blockage is very effective in the treatment of rheumatoid arthritis. The reason that TNF is so important is not entirely clear. However, TNF signals through an evolutionarily conserved pathway and is a point of connection between two arms of the mammalian inante immune response. It is a second wave activator of NF-kB, capable of spreading the alarm sounded by TLRs to virtually all cell types in the body.
Upon TNF-alpha binding to the TNF receptor, the receptors aggregate and bind adaptor proteins, leading to activation of the IkB kinase (IKK) complex. Phosphorylation of IkB by IKK leads to ubiquitination and degradation of IkB and allows free NF-kB to bind target genes. one such target is IkB?, and its production results in a negative feedback loop.
LPS:
LPS signals through TLR4. TLR4 activates two downstream pathways. a MyD88-dependent pathway that leads to the production of proinflammatory cytokines such as IL-6, TNF and IL-12 with quick activation of NF-kB and MAPK and a MyD88-independent pathway associated with activation of IRF-3, subsequent induction of IFN-beta, and maturation of DCs, with delayed activation of NF-kB and MAPK. Thus each pathway is thought to directly activate NF-kB. The MyD88 dependent pathways involves the early phase of NFkB activation which leads to the production of inflammatory cytokines. The MyD88 independent pathway activates interferon (IFN) regulatory factor (IRF3) and involves the late phase of NFkB activation, both of which lead to the produciton of IFNb and the expression of IFN inducible genes.
1. The MyD88-dependent pathway recruits the kinases interleukin-1 receptor-associated kinase 1 (IRAK1) and IRAK4, which phosphorylate TNF receptor associated factor 6 (TRAF6), leading to the activation of the IKK complex. NF-kB activation through the MyD88 dependent pathway occurs earlier than through the MyD88 independent pathway. MyD88 intereacts with TLR4 thourgh a TIR domain and recruits IRAK to the receptor cojmplex through a death domain, and then IRAK is sequentially activated. Activated IRAK subsequently activates NFkB via TNFR-associated factor (TRAF).
2. MyD88-independent pathway: is not completely understood. The pathway is dependent on the TIR domain-containing adaptor inducing interferon-?(Trif) adaptor molecule, and Trif-related adaptor molecule (Tram), receptor-interactor protein 1 (RIP1), and RIP3 have been identified as important factors in the pathway. In addition to inducing expression of IFN-inducible genes, the MyD88 independent pathway leads to the LPS mediated maturation of DCs. When cultured with LPS, MyD88 deficient BMDCs upregulate the cell surface expression of co-stimulatory molecules, such as CD40, CD80 and CD86 and induce the proliferation of T cells. By contrast, TLR4 defiicent DCs fail to mature in response to LPS, indicating that DC maturation proceeds in a MyD88 independent manner.
Interferon-regulatory factor 3 (IRF3) is a MyD88 independent pathway specific factor that directly regulates early response genes (for example, those encoding interferon-? and IP-10) and is active within 30 min of LPS stimulation.
The end result of these two pathways is the same as with the TNF-alpha activated pathway: degradation of IkB, which is followed by activation of IkB?gene transcription.