Lipopolysaccharide Stimulates the MyD88-Independent Pathway and Results in Activation of IFN-Regulatory Factor 3 and the Expression of a Subset of Lipopolysaccharide-Inducible Genes

Abstract

Bacterial lipopolysaccharide (LPS) triggers innate immune responses through Toll-like receptor (TLR) 4, a member of the TLR family that participates in pathogen recognition. TLRs recruit a cytoplasmic protein, MyD88, upon pathogen recognition, mediating its function for immune responses. Two major pathways for LPS have been suggested in recent studies, which are referred to as MyD88-dependent and -independent pathways. We report in this study the characterization of the MyD88-independent pathway via TLR4. MyD88-deficient cells failed to produce inflammatory cytokines in response to LPS, whereas they responded to LPS by activating IFN-regulatory factor 3 as well as inducing the genes containing IFN-stimulated regulatory elements such as IP-10. In contrast, a lipopeptide that activates TLR2 had no ability to activate IFN-regulatory factor 3. The MyD88-independent pathway was also activated in cells lacking both MyD88 and TNFR-associated factor 6. Thus, TLR4 signaling is composed of at least two distinct pathways, a MyD88-dependent pathway that is critical to the induction of inflammatory cytokines and a MyD88/TNFR-associated factor 6-independent pathway that regulates induction of IP-10.

Heterogeneous requirement of IκB kinase 2 for inflammatory cytokine and matrix metalloproteinase production in rheumatoid arthritis: Implications for therapy

“…Two such molecules, MyD88 and Mal/TIRAP, have already been reported to play a role in TLR-4-induced TNF␣ and IL-6, but not IP-10, production (63,64). It has been suggested that IP-10 production requires the activation of interferon-regulatory factor 3 (65), and this may depend on the adapter molecule TRIF/TICAM-1, another recently identified MyD88 homolog (66,67). Thus, distinct adapter molecules of TLR-4 are likely to engage alternative signaling pathways and alternative kinases involved in I B␣ phosphorylation.…”

TLR signalling and activation of IRFs: revisiting old friends from the NF-κB pathway

“…Interestingly, the TLR4 ligand lipopolysaccharide (LPS) can still trigger activation of NF-kB in Myd88-deficient mice, albeit with delayed kinetics compared with wild-type cells, whereas most other TLRs are completely ineffective at activating NF-kB in these mice [13]. Although Myd88-deficient mice lose their ability to produce proinflammatory cytokines, such as TNF, IL-1, IL-6 and IL-12 p40, in response to LPS, they retain their capacity to produce IFN-inducible genes and co-stimulatory molecules and their dendritic cells (DCs) mature in response to LPS [14,15]. These studies define two arms of the LPS signalling pathway: a Myd88-dependent component that promotes fast activation of NF-kB and induction of proinflammatory molecules and a Myd88-independent pathway (also used by TLR3) that mediates slow activation of NF-kB and induction of IFN-inducible genes and costimulatory molecules (Figure 1).…”

“…The engagement of TLR3 and TLR4 by poly(I:C) and LPS, respectively, promotes phosphorylation of IRF3 and the induction of type I IFN genes [14,32,33]. An intense research effort culminated in key reports identifying two kinases responsible for this phosphorylation [34,35].…”

Identification of a TLR4‐ and TRIF‐dependent activation program of dendritic cells

“…In addition to this common pathway, TLR4 utilizes a MyD88-independent signaling pathway that results in the activation of interferon regulatory factor 3 (IRF-3), the induction of type I interferons, and the maturation of dendritic cells [9][10][11][12]. The adaptor protein TIR-domaincontaining adaptor inducing IFN-g ([TRIF]; also called TICAM-1) was recently demonstrated to activate IFN-g expression via IRF-3 and to associate with both TLR3 and IRF-3 [13,14].…”