Multiple signaling pathways contribute to synergistic TLR ligand-dependent cytokine gene expression in human monocyte-derived macrophages and dendritic cells

• Published in: Journal of leukocyte biology Vol. 85; no. 4; pp. 664 - 672
• Main Authors: Maekelae, S M, Strengell, M, Pietilae, TE, Oesterlund, P, Julkunen, I
• Format: Journal Article
• Language: English
• Published: United States Society for Leukocyte Biology 01-04-2009
• Subjects: Cytokines - biosynthesis; Cytokines - genetics; Dendritic Cells - metabolism; Humans; innate immunity; Interferon Regulatory Factors - metabolism; Ligands; Macrophages - metabolism; MAP Kinase Signaling System; NF-kappa B - metabolism; Phosphatidylinositol 3-Kinases - metabolism; Receptor Cross-Talk; regulation; RNA, Messenger - biosynthesis; Signal Transduction; STAT Transcription Factors - metabolism; Toll-Like Receptor 3; Toll-Like Receptor 4; Toll-Like Receptor 8; Toll-Like Receptors - metabolism; transcription factors
• DOI: 10.1189/jlb.0808503
• EISSN: 1938-3673
• ISSN: 0741-5400

TLRs are innate immune receptors that recognize pathogen‐associated structures. Binding of ligands to different TLRs can induce the production of proinflammatory cytokines in a synergistic manner. We have analyzed the molecular mechanisms of synergy in TLR ligand‐stimulated human monocyte‐derived macrophages and dendritic cells (moDCs). Stimulation of moDCs with the TLR8 ligand together with the TLR3 or TLR4 ligand led to synergistic IL‐6, IL‐10, IL‐12, and TNF‐α mRNA expression and cytokine production. DNA‐binding assays showed that TLR3 and TLR8 stimulation induced binding of multiple IFN regulatory factor (IRF) and STAT transcription factors to the IL‐12p35 gene promoter IFN‐stimulated response element in moDCs and macrophages but with different binding profiles and kinetics. We also demonstrate that NF‐κB, MAPKs and PI‐3K pathways have an important role in TLR‐induced cytokine gene expression, as pharmacological inhibitors of these signaling pathways inhibited TLR3, TLR4, and TLR8 ligand‐induced cytokine mRNA expression and protein production. Especially, synergistic IL‐12p70 production was abolished completely in NF‐κB, MAPK p38, and PI‐3K inhibitor‐treated moDCs. Our data suggest that TLR‐dependent, synergistic cytokine gene expression results from enhanced activation and cooperation among NF‐κB, IRF, MAPK, PI‐3K, and STAT signaling pathways.