IRF-8/interferon (IFN) consensus sequence-binding protein is involved in toll-like receptor (TLR) signaling and contributes to the cross-talk between TLR and IFN-γ signaling pathways

Toll-like receptor (TLR) and interferon-γ (IFN-γ) signaling pathways are important for both innate and adaptive immune responses. However, the cross-talk between these two signaling pathways is incompletely understood. Here we show that IFN-γ and LPS synergistically induce the expression of proinflammatory factors, including interleukin-1 (IL-1), IL-6, IL-12, NO, and tumor necrosis factor-α (TNF-α). Comparable synergism was observed between IFN-γ and peptidoglycan (PGN; a TLR2 ligand) and poly(I:C) (a TLR3 ligand) in the induction of IL-12 promoter activity. IFN-γ enhanced lipopolysaccharide (LPS)-induced ERK and JNK phosphorylation but had no effect on LPS-induced NF-κB activation. Interestingly, we found that IRF-8-/- macrophages were impaired in the activation of LPS-induced ERK and JNK and the production of proinflammatory cytokines induced by LPS or IFN-γ plus LPS. Retroviral transduction of IRF-8 into IRF-8-/- macrophages rescued ERK and JNK activation. Furthermore, co-immunoprecipitation experiments show that IRF-8 physically interacts with TRAF6 at a binding site between amino acid residues 356 and 305 of IRF-8. Transfection of IRF-8 enhanced TRAF6 ubiquitination, which is consistent with a physical interaction of IRF-8 with TRAF6. Taken together, the results suggest that the interaction of IRF-8 with TRAF6 modulates TLR signaling and may contribute to the cross-talk between IFN-γ and TLR signal pathways.