During viral and bacterial infections, pathogen-derived cytosolic nucleic acids are recognized by the intracellular RNA sensors retinoic acid-inducible gene I and melanoma-differentiated gene 5 and intracellular DNA sensors, including cyclic-di-GMP-AMP synthase, absent in melanoma 2, interferon (IFN)-gamma inducible protein 16, polymerase III, and so on. Binding of intracellular nucleic acids to these sensors activates downstream signaling cascades, resulting in the production of type I IFNs and pro-inflammatory cytokines to induce appropriate systematic immune responses. While these sensors also recognize endogenous nucleic acids and activate immune responses, they can discriminate between self- and non-self-nucleic acids. However, dysfunction of these sensors or failure of regulatory mechanisms causes aberrant activation of immune response and autoimmune disorders. In this review, we focus on how intracellular immune sensors recognize exogenous nucleic acids and activate the innate immune system, and furthermore, how autoimmune diseases result from dysfunction of these sensors.

TLRs recognize pathogen components and drive innate immune responses. They localize at either the plasma membrane or intracellular vesicles such as endosomes and lysosomes, and proper cellular localization is important for their ligand recognition and initiation of signaling. In this study, we disrupted ATP6V0D2, a component of vacuolar-type H adenosine triphosphatase (V-ATPase) that plays a central role in acidification of intracellular vesicles, in a macrophage cell line. ATP6V0D2-deficient cells exhibited reduced cytokine production in response to endosome-localized, nucleic acid-sensing TLR3, TLR7, and TLR9, but enhanced inflammatory cytokine production and NF-κB activation following stimulation with LPS, a TLR4 agonist. Moreover, they had defects in internalization of cell surface TLR4 and exhibited enhanced inflammatory cytokine production after repeated LPS stimulation, thereby failing to induce LPS tolerance. A component of the V-ATPase complex interacted with ARF6, the small GTPase known to regulate TLR4 internalization, and ARF6 deficiency resulted in prolonged TLR4 expression on the cell surface. Taken together, these findings suggest that ATP6V0D2-dependent intravesicular acidification is required for TLR4 internalization, which is associated with prevention from excessive LPS-triggered inflammation and induction of tolerance.

Backgrounds: Melanoma differentiation-associated gene 5 (MDA5)-mediated signaling contributes to antiviral innate immunity. Results: Overexpression of ADP-ribosylation factor-like protein 5B (Arl5B) repressed the MDA5-induced activation of the interferon ␤ promoter. Arl5B-deficient cells showed up-regulation of MDA5-mediated responses. Conclusion: Arl5B is a negative regulator of MDA5 signaling. Significance: Arl5B is an important suppressor for antiviral innate immune response.

Toll-like receptor 3 (TLR3) recognizes double-stranded RNA derived from virus and its synthetic analogue, polyinosinic–polycytidylic acid [poly(I:C)]. Upon poly(I:C) binding, TLR3 activates transcription factors to express inflammatory cytokines and type I interferon. TLR3 is located in the endosomes and its recognition of poly(I:C) and activation of downstream signaling is regulated by endosomal acidification. However, the mechanism of post-transcriptional regulation in TLR3-mediated innate responses remains unclear. Here, we focused on Human antigen R (HuR, also known as ELAVL1) that recognizes and binds to the 3′ untranslated regions (3′UTRs) of target mRNAs, thereby protecting them from mRNA degradation, and found that HuR-deficient murine macrophage cells showed significantly reduced Ifnb1 mRNA expression after poly(I:C) stimulation. HuR-deficient cells also showed a marked reduction in the expression of Atp6v0d2 mRNA, which encodes a subunit of vacuolar-type H+ ATPase (V-ATPase), and therefore reduced endosomal acidification. HuR associated with the 3′UTR of Atp6v0d2 mRNA and the stability of Atp6v0d2 mRNA was maintained by its association with HuR. Taken together, our results suggest that HuR stabilizes Atp6v0d2 mRNA, which is required for the TLR3-mediated innate immune responses.

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