Dual Signaling of MyD88 and TRIF Is Critical for Maximal TLR4-Induced Dendritic Cell Maturation

Abstract

TLR4 is a unique TLR because downstream signaling occurs via two separate pathways, as follows: MyD88 and Toll IL-1 receptor (TIR) domain-containing adaptor-inducing IFN-β (TRIF). In this study, we compared and contrasted the interplay of these pathways between murine dendritic cells (DCs) and macrophages during LPS stimulation. During TLR4 activation, neither pathway on its own was critical for up-regulation of costimulatory molecules in DCs, whereas the up-regulation of costimulatory molecules was largely TRIF dependent in macrophages. LPS-induced secreted factors, of which type I IFNs were one of the active components, played a larger role in promoting the up-regulation of costimulatory molecules in macrophages than DCs. In both cell types, MyD88 and TRIF pathways together accounted for the inflammatory response to LPS activation. Furthermore, signaling of both adaptors allowed maximal T cell priming by LPS-matured DCs, with MyD88 playing a larger role than TRIF. In sum, in our experimental systems, TRIF signaling plays a more important role in LPS-induced macrophage activation than in DC activation.

IL-6 Trans-Signaling Modulates TLR4-Dependent Inflammatory Responses via STAT3

“…Our data demonstrating that genetically reducing the level of STAT3 activity in gp130 F/F mice reduced IL-6 expression in response to LPS further supports a role for STAT3 in promoting IL-6 gene transcription. In addition, our observations that genetic targeting of Mal, which primarily facilitates NF-kB activation via the LPS/TLR4/MyD88 signaling axis, also led to a reduction in the levels of IL-6 produced in response to LPS is consistent with previous studies invoking the involvement of MyD88 and NF-kB in the production of IL-6 in response to LPS (42,43). We do note, however, that LPS-induced IL-6 production was not completely ablated in gp130 F/F :Mal 2/2 mice, suggesting that other TLR4 signaling mediators also contribute to LPS-induced IL-6 production, such as TRIF, albeit in a cell type-specific manner (43).…”

“…In addition, our observations that genetic targeting of Mal, which primarily facilitates NF-kB activation via the LPS/TLR4/MyD88 signaling axis, also led to a reduction in the levels of IL-6 produced in response to LPS is consistent with previous studies invoking the involvement of MyD88 and NF-kB in the production of IL-6 in response to LPS (42,43). We do note, however, that LPS-induced IL-6 production was not completely ablated in gp130 F/F :Mal 2/2 mice, suggesting that other TLR4 signaling mediators also contribute to LPS-induced IL-6 production, such as TRIF, albeit in a cell type-specific manner (43). Importantly, our gp130 F/F mice build on the current paucity of genetically defined mouse models to directly investigate the mechanistic basis by which overactivated endogenous IL-6/ STAT3 signaling promotes chronic inflammation.…”

Ischemic Injury Enhances Dendritic Cell Immunogenicity via TLR4 and NF-κB Activation

“…2B-D, IL-1b production was also elevated in DCs at 12 h post-isc treatment (p = 0.014); at 24 h, IL-12(p40) and TNF-a were released at significantly higher levels by isc DCs as compared with ctrl DCs (p , 0.0006); it should also be noted that TNF-a production has been shown to be subject to negative feedback inhibition via PGE upregulation as well as posttranscriptional mRNA destabilization, which may account for the decrease in TNF-a levels observed at 24 h compared with 12 h (38-41). IL-6 production is known to be increased following activation of TLR4, thereby leading to an inflammatory response (19,(42)(43)(44). This increase in IL-6 may thus serve to enhance the immune response initiated by DCs following isc injury.…”

“…Signaling transduction in response to TLR4 and TLR2 binding occurs through both MyD88-dependent and -independent pathways; that is, stimulation of TLR4 and TLR2 can trigger the association of MyD88 with the IL-1R-associated kinase 1 effector of the NF-kB pathway, whereas activation of the adaptor proteins TICAM1/TIR domain-containing adaptor-inducing IFN-b (TRIF) and TICAM2/TRIF-related adaptor molecule proceeds via a MyD88-independent signaling pathway associated with TLR4/TLR2 ligation and resulting in transcription of IFN-inducible genes (12). It has also been demonstrated that in order for complete DC maturation to occur following TLR4 activation, signaling through both pathways is necessary (44). We first examined MyD88 protein expression by Western blot analysis; a 2.16-fold increase in MyD88 was evident in isc DCs as compared with ctrl DCs (Fig.…”

Dendritic cell vaccines: A review of recent developments and their potential pediatric application

“…DC rely in part on signaling through toll like receptors (TLR) for maturation, resulting in expression of MHC Class I and II molecules and the secretion of pro-inflammatory cytokines. [36][37][38] Stimulation of TLR on DC can also facilitate the induction of Th1 immune responses. 39,40 In a review of several vaccine studies for malignant melanoma, Engel-Noerregaard reported significantly higher response rates in patients who received adjuvants as a part of their vaccine regimen.…”