Zebrafish TRIF, a Golgi-localized protein, participates in IFN induction and NF-kappaB activation.

The antiviral immune responses were triggered by the innate immune recognition of viral infection. The type I IFNs (IFN-beta and IFN-alpha) are the key cytokines produced upon viral infection and consequently link innate immunity with adaptive immunity. A main antiviral system in mammals is TRIF-dependent TLRs pathway, but the TRIF-independent RIG-I pathway, has also been discovered recently. In this manuscript, our study focuses on the functional characterization of zebrafish TRIF based on the comparison of its sequence and functional evolution from zebrafish to mammals. Our experimental results show that the full length cDNA of zebrafish TRIF cloned by RACE-PCR approach encodes a protein of 556 amino acids. Luciferase reporter assay confirms that zebrafish TRIF is able to induce the IFN promoter as well as activate NF-kappaB response promoter. The IFN induction function of zebrafish TRIF is abolished when Ala359 is mutated to Pro or His. Laser confocal microscopy shows that zebrafish TRIF is colocalized with a Golgi apparatus marker, implying its unique subcellular localization in Golgi apparatus. In zebrafish, the mRNA expression of molecules participating in RIG-I pathway are much more sensitive and specific to polyinosine-polycytidylic acid induction compared with those in TRIF-dependent antiviral pathway. The TRIF-dependent TLR4 IFN induction signaling appears not to be functional in zebrafish, since IFN expression cannot be up-regulated by LPS. These two striking findings from de novo ligand induction experiments suggest a novel antiviral mechanism in zebrafish.