A Role for Stimulator of Interferon Genes in Toll-Like Receptor 8 Signaling

Abstract

Abstract The innate immune system is equipped with multiple receptors to detect microbial nucleic acids and induce type I interferon (IFN) to restrict viral replication. When dysregulated these receptor pathways induce inflammation in response to host nucleic acids and promote development and persistence of autoimmune diseases like Systemic Lupus Erythematosus (SLE). IFN production is regulated by the Interferon Regulatory Factor (IRF) transcription factor family downstream of several innate immune receptors such as Toll-like receptors (TLRs) and Stimulator of Interferon Genes (STING). Both TLRs and STING activate TANK-binding kinase I (TBKI) and IkappaB kinase-ɛ (IKKɛ), which phosphorylate and activate IRFs. The pathway by which TLRs and STING activate IFN response are considered independent. Here we show that STING plays a previously undescribed role in human TLR8 signaling. Stimulation with TL8-506 (TL8), TLR8 ligand, induced IFN secretion in primary human monocytes and THP-1 cells and was reduced by inhibition of STING. Using THP-1 cells encoding reporters for IRF and NFkB activity, two major signaling pathways downstream of TLRs, we found that TL8 stimulation increased IRF and NFkB activity. When STING was inhibited, IRF, but not NFkB, activity was reduced. TL8-induced IRF activity was dependent on IKKɛ, not TBKI. Bulk RNA transcriptomic analysis show that TLR8 induces gene signatures associated with SLE which are downregulated by inhibition of STING. These data demonstrate that STING is required for full TLR8-IRF signaling. These studies build a new framework for crosstalk between cytosolic and endosomal innate immune receptors, which could be used to treat IFN driven autoimmune diseases.