Genetic modulation of TLR8 response following bacterial phagocytosis

Abstract

Human Toll-like receptors (TLRs) TLR7, TLR8, and TLR9 are important immune sensors of foreign nucleic acids encountered by phagocytes. Although there is growing evidence implicating TLR7 and TLR9 in the detection of intracellular pathogenic bacteria, characterization of such a role for TLR8 is currently lacking. A recent genetic study has correlated the presence of a TLR8 single nucleotide polymorphism (SNP) (rs3764880:A>G; p.Met1Val) with the development of active tuberculosis, suggesting a role for TLR8 in the detection of phagosomal bacteria. Here we provide the first direct evidence that TLR8 sensing is activated in human monocytic cells following Helicobacter pylori phagocytosis. In addition, we show that rs3764880 fine tunes translation of the two TLR8 main isoforms, without affecting protein function. Although we show that TLR8 variant 2 (TLR8v2) is the prevalent form of TLR8 contributing to TLR8 function, we also uncover a role for the TLR8 long isoform (TLR8v1) in the positive regulation of TLR8 function in CD16(+)CD14(+) differentiated monocytes. Thus, TLR8 sensing can be activated following bacterial phagocytosis, and rs3764880 may play a role in the modulation of TLR8-dependent microbicidal response of infected macrophages.

Bacterial RNA: An Underestimated Stimulus for Innate Immune Responses

“…Along the same lines, B. burgdorferi extracts depleted enzymatically for nucleic acids showed dependency on TLR7 with respect to IFNs, yet they also induced NF-kB-dependent genes (37). Using murine DCs, Helicobacter pylori RNA induced proinflammatory cytokines (71), and phagocytosis of H. pylori activated TLR8 in human monocytes (72). Mycobacterium bovis bacillus Calmette-Guérin upregulated TLR8 in differentiated macrophages, and TLR8 gene variants were associated with susceptibility to pulmonary tuberculosis, arguing for a contribution of TLR8 signaling in mycobacteria infection (73).…”

Phagosomal signaling byBorrelia burgdorferiin human monocytes involves Toll-like receptor (TLR) 2 and TLR8 cooperativity and TLR8-mediated induction of IFN-β

“…1A). In concert with the role of TLR8 in the production of NF-κB-dependent cytokines (33)(34)(35), IRS957 also caused a significant decrease in secretion of TNF-α, IL-6, and IL-10 by Bb-infected monocytes (Fig. 1B).…”

“…Human TLR8 can be activated by nucleic acids derived from a variety of RNA viruses (45)(46)(47)(48) as well as Escherichia coli RNA in genetically complemented HEK293 cells (49). To date, TLR8 has been linked to the recognition of two different intracellular bacteria, Mycobacterium bovis bacille Calmette-Guérin (bacillus Calmette-Guérin) (35) and Helicobacter pylori (50). In the case of bacillus Calmette-Guérin, TLR8 protein expression levels markedly increased in infected macrophages derived from a human acute monocytic leukemia cell line (THP-1 cells).…”

“…Interestingly, a significant boost in TLR8 expression levels was also seen in the blood of patients with untreated Mycobacterium tuberculosis infection (50). Using activated THP-1 cells, Gantier et al (35) demonstrated that expression of the two main human TLR8 variant mRNAs significantly increased following H. pylori phagocytosis. In this study, we reveal a previously unappreciated direct role for human TLR8 in transcription of IFN-β in Bbinfected monocytes and cooperativity with TLR2 in the production of several NF-κB-dependent cytokines.…”

Campylobacter jejuni Outer Membrane Vesicles Play an Important Role in Bacterial Interactions with Human Intestinal Epithelial Cells

“…The detailed mechanisms and host cell receptor(s) leading to the signaling cascade(s) that result in the host response to coincubation with bacterial OMVs have not yet been identified. OMV-induced signaling has been shown to occur via the specific glycolipid and cholesterol-enriched domains in the cell plasma membrane termed "lipid rafts" (10,17,23,25,32,37). To investigate the role of lipid rafts in the host cellular response to C. jejuni OMVs, T84 IECs were treated with 10 mM methyl-beta-cyclodextrin (M␤CD), a lipid-raft-disrupting agent, for 1 h before coincubation with C. jejuni 11168H OMVs for 24 h. This pretreatment of T84 cells with M␤CD resulted in a significant (P Ͻ 0.05) inhibition of the OMV-stimulated induction of IL-8 (Fig.…”