When Signaling Pathways Collide: Positive and Negative Regulation of Toll-like Receptor Signal Transduction

School of Biochemistry and Immunology, Trinity College Dublin, Dublin 2, Ireland.
Immunity (Impact Factor: 21.56). 07/2008; 29(1):12-20. DOI: 10.1016/j.immuni.2008.06.004
Source: PubMed

ABSTRACT Toll-like receptor (TLR) signaling is subjected to crosstalk from other signals, with a resulting positive or negative effect. There is complex crosstalk between the NLR family of immune-regulatory molecules and TLRs, and C-type lectin receptors such as Dectin-1 synergize with TLR2 via the tyrosine kinase Syk. Bruton's tyrosine kinase plays an important positive role in TLR signaling, whereas the TAM family of receptor tyrosine kinases is inhibitory. The tyrosine phosphatase SHP1 has been shown to positively regulate induction of interferon-beta, whereas SHP2 inhibits the kinase TBK1, limiting this response. K63-linked polyubiquination has also been shown to be critical for the initiation of TLR signaling. Finally, glucocorticoids affect TLR signaling by inducing the phosphatase MKP1 and inhibiting TBK1 activation. These recent findings emphasize the importance of considering TLR signaling in the context of other signaling pathways, as is likely to occur in vivo during infection and inflammation.

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    • "C.-Y. Lai et al. / Vaccine xxx (2014) xxx–xxx such as imiquimod, loxoribine, and CL264, selectively activate TLR7 but not TLR8 [7] [8] [9] [10] [11]. Activation of TLR7/8 triggers a MyD88- dependent signaling pathway to elicit production of inflammatory cytokines and type I interferons (IFNs) via activation of NF-␬B and IRF7, respectively [12] [13] [14]. Because of their efficiency in activating immune responses, the TLR7/8 agonists are being investigated for a broad variety of applications, including antiviral and antitumor therapies, and use as a vaccine adjuvant [7] [8] [9] [10] [11]. "
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    ABSTRACT: Toll-like receptors 7 (TLR7) and 8 (TLR8) recognize viral single-stranded RNA and small molecular weight agonists to activate anti-viral immune responses. TLR8s from different species have distinct ligand recognitions. For example, human TLR8 is responsive to ligand stimulation, but mouse and rat TLR8 are activated by small molecular weight agonists only in the presence of polyT-oligodeoxynucleotides. TLR7 and TLR8 have been reported to be absent and pseudogenized, respectively, in rabbit (Oryctolagus cuniculus). In this study, we detected the expression of rabbit (rab)TLR8 in immune-cell-associated tissues. Cell proliferation and cytokine expressions in rabbit splenocytes were induced by the TLR7/8 ligand but not by the TLR7 ligands, suggesting that rabTLR8 is functional but rabTLR7 is not. In rabbits, CL075, a TLR7/8 ligand, activated an antigen-specific antibody response, although one not as potent as aluminum salt or Freund's adjuvant. Nevertheless, CL075, alone or in combination with aluminum salt, generates fewer adverse effects than Freund's adjuvant at the injection sites. To further investigate the activation of rabTLR8, we cloned its cDNA. In cell-based assay, this rabTLR8 is activated by TLR7/8 ligand but not activated by TLR7 ligand. Upon stimulation the rabTLR8 had a lower activation compared to the activation of TLR8 from other species, except the mouse and rat TLR8s. Using different deletion and human-rabbit chimeric TLR8 expressing constructs, we showed that an extra peptide in the undefined region results in reduced activity of rabTLR8. These results provide a molecular basis for the mild activities of TLR7/8 ligands in rabbits, and suggest TLR7/8 agonists may provide safer immune stimuli in rabbits than in other non-rodent species.
    Vaccine 08/2014; 32(43). DOI:10.1016/j.vaccine.2014.07.104 · 3.62 Impact Factor
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    • "In this sense, there must be some intrinsic or adaptive mechanisms to protect against dysfunctional inflammation during infection. An increasing number of reports have indicated that the hosts have developed sophisticated negative mechanism to regulate the multiple layers of inflammatory response [20], [22], [23]. Indeed, some cell wall components of bacteria, such as lipoteichoic acids and lipopolysaccharides, were reported to activate the basic leucine zipper transcription factor NF-E2-related factor 2 (Nrf2), a key factor involved in antioxidant protein expression in human tracheal smooth muscle cells and monocytes [24]. "
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    ABSTRACT: Aims This study is to investigate the mechanisms by which macrophage-activating lipopeptide-2 (MALP-2) induces heme oxygenase (HO)-1, a cytoprotective enzyme that catalyzes the degradation of heme, in human monocytes. Methods Human monocytic THP-1 cells were cultured for transient transfection with plasmids and stimulation with MALP-2 for indicative time intervals. After incubation with MALP-2, cells were collected and disrupted, before being tested for promoter activity using luciferase assay. For analysis of proteins, immunoreactive bands were detected using an enhanced chemiluminescence Western blotting system, and the band intensity was measured by densitometryic analysis. For the detection of co-immunoprecipitation, SDS-PAGE was performed and the membranes were probed using respective antibodies. To investigate the cellular localization of NF-E2-related factor 2 (Nrf2), cells underwent immunofluorescence staining and confocal microscopy, and were analyzed using electrophoretic mobility shift assay. Results MALP-2-induced HO-1 expression and promoter activity were abrogated by transfection with dominant negative (DN) plasmids of TLR2 and TLR6, or their neutralizing antibodies. However, inhibition of MyD88 or transfection with the DN-MyD88 was insufficient to attenuate HO-1 expression. In contrast, mutation or silencing of MyD88 adapter-like (Mal) by DN-Mal or siRNA almost completely blocked HO-1 induction. Btk, c-Src and PI3K were also involved in MALP-2-induced HO-1 expression, as revealed by specific inhibitors LFM-A13, PP1 and LY294002, or by transfection with siRNA of c-Src. MALP-2-induced activation of PI3K was attenuated by transfection with DN mutant of Mal, and by pretreatment with LFM-A13 or PP1. Furthermore, MALP-2 stimulated the translocation of Nrf2 from the cytosol to the nucleus and Nrf2 binding to the ARE site in the HO-1 promoter, which could also be inhibited by pretreatment with a PI3K inhibitor, LY294002. Conclusions These results indicated that MALP-2 required TLR2/6, Btk, Mal and c-Src to activate PI3K, which in turn initiated the activation of Nrf2 for efficient HO-1 induction.
    PLoS ONE 07/2014; 9(7):e103433. DOI:10.1371/journal.pone.0103433 · 3.23 Impact Factor
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    • "Several mechanisms have been described which are involved in ET of NFκB -dependent readouts. These include the up-regulation of endogenous suppressors of NFκB activation such as SIGIRR, ST2, A20, Myd88 s and IRAK-M [9], [38]. NFκB is also important with respect to MΦ subset polarisation; IκBα over-expression resulted in M2 polarisation [19], whereas IKKβ deletion favoured M1 MΦs [20]. "
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    ABSTRACT: Macrophages (MΦs) determine oral mucosal responses; mediating tolerance to commensal microbes and food whilst maintaining the capacity to activate immune defences to pathogens. MΦ responses are determined by both differentiation and activation stimuli, giving rise to two distinct subsets; pro-inflammatory M1- and anti-inflammatory/regulatory M2- MΦs. M2-like subsets predominate tolerance induction whereas M1 MΦs predominate in inflammatory pathologies, mediating destructive inflammatory mechanisms, such as those in chronic P.gingivalis (PG) periodontal infection. MΦ responses can be suppressed to benefit either the host or the pathogen. Chronic stimulation by bacterial pathogen associated molecular patterns (PAMPs), such as LPS, is well established to induce tolerance. The aim of this study was to investigate the susceptibility of MΦ subsets to suppression by P. gingivalis. CD14(hi) and CD14(lo) M1- and M2-like MΦs were generated in vitro from the THP-1 monocyte cell line by differentiation with PMA and vitamin D3, respectively. MΦ subsets were pre-treated with heat-killed PG (HKPG) and PG-LPS prior to stimulation by bacterial PAMPs. Modulation of inflammation was measured by TNFα, IL-1β, IL-6, IL-10 ELISA and NFκB activation by reporter gene assay. HKPG and PG-LPS differentially suppress PAMP-induced TNFα, IL-6 and IL-10 but fail to suppress IL-1β expression in M1 and M2 MΦs. In addition, P.gingivalis suppressed NFκB activation in CD14(lo) and CD14(hi) M2 regulatory MΦs and CD14(lo) M1 MΦs whereas CD14(hi) M1 pro-inflammatory MΦs were refractory to suppression. In conclusion, P.gingivalis selectively tolerises regulatory M2 MΦs with little effect on pro-inflammatory CD14(hi) M1 MΦs; differential suppression facilitating immunopathology at the expense of immunity.
    PLoS ONE 07/2013; 8(7):e67955. DOI:10.1371/journal.pone.0067955 · 3.23 Impact Factor
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