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

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-alpha 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-kappaB, 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-kappaB, 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-kappaB, IRF, MAPK, PI-3K, and STAT signaling pathways.