The cytosolic sensor, DDX41, activates antiviral and inflammatory immunity in response to stimulation with double-stranded DNA adherent cells of the olive flounder, Paralichthys olivaceus.

DDX41, a receptor belonging to the DExD family, functions as a DNA sensor in the mammalian cytoplasm and mediates the antiviral response in host cells. Here, the olive flounder DDX41 was found to have 2267-bp long and encodes a putative protein of 614 amino acid residues. The olive flounder DDX41 mRNA was presented in all tested tissues, and was distinctly expressed in fish naturally infected with LCDV. High expression levels were observed in the heart, liver, kidney and stomach. Furthermore, the olive flounder DDX41 mRNA expression increased significantly in adherent (monocyte-like) cells following stimulation with a DNA virus. Reporter assays showed that the transcriptional activity of the IFN-I promoter was enhanced in DDX41-overexpressing HINAE cells treated with C-di-GMP (dinucleotides). Overexpression of DDX41 also induced the antiviral and inflammatory cytokine gene expression through cytoplasmic C-di-GMP treatment. These results suggest that DDX41 functions as a cytosolic DNA sensor that is capable of inducing antiviral activity and inflammatory responses in the olive flounder.

ERdj5 in Innate Immune Cells Is a Crucial Factor for the Mucosal Adjuvanticity of Cholera Toxin.

Cholera toxin (CT) is one of most strong mucosal adjuvants, but it cannot be clinically used owing to its toxicity. The cytosolic A1 subunit of CT (CTA1) is the molecule responsible for its immunostimulatory activity, which increases the concentration of cyclic AMP and causes the induction of pro-inflammatory cytokines in innate immune cells. However, the importance of endoplasmic reticulum (ER) molecules involved in CTA1 retro-translocation to induce immune responses remained to be investigated. ERdj5 is an ER protein which is expected to transfer CTA1 to the Hrd1 complex for the retro-translocation of CTA1. In this study, we investigated the physiological relevance of ERdj5 in immune stimulation by CT. ERdj5-knockout (ERdj5 KO) mice had decreased production of antigen-specific IgG in the serum and IgA in the mucosal secretion after intranasal immunization with Ag and CT. Especially, IgG2c isotypes were specifically reduced in the absence of ERdj5. ERdj5 KO dendritic cells (DCs) failed to full activation with decreased expression of costimulatory molecules, such as MHC class II, CD80, and CD 86. In ERdj5 KO DCs, secretion of pro-inflammatory cytokines, such as IL-1ß, TNF-α, and IL-6, was reduced. The cytokine signatures of several helper T cells were reduced in ERdj5 KO mice following intranasal CT immunization. The absence of ERdj5 affects the immunostimulatory properties of CT but does not affect the response to the CTB pentamer, the response to alum, total antibody production, or cytokine release from DCs exposed to CpG. Interestingly, CT enhanced the expression of ER stress proteins in ERdj5 KO innate immune cells. These results suggested that ERdj5 contributed as a decisive factor to the immunostimulatory capacity of CT via CTA1 retro-translocation.