TLRs are pivotal pattern recognition receptors in initiating innate immunity and triggering adaptive immunity. TLR pathways have been comprehensively investigated in mammals. However, the teleost-specific TLR19 pathway remains largely unknown. In this study, we identified TLR19 from grass carp (Ctenopharyngodon idella), and explored the ligand, adaptor, and signaling pathways. Pathogen-associated molecular pattern binding and luciferase activity assays indicate that TLR19 recognizes and responds to dsRNA analog (polyinosinic:polycytidylic acid). Confocal fluorescence microscopy demonstrates that TLR19 is synthesized in ribosomes not binding on endoplasmic reticulum, then transfers to early endosome post-polyinosinic:polycytidylic acid stimulation. Fluorescence colocalization and immunoprecipitation experiments confirm TLR19 interacts with adaptor TRIF, not MyD88, TIRAP, or SARM1. TLR19 facilitates protein and phosphorylation levels of IRF3, inhibits phosphorylation of IRF7. TLR19 enhances the promoter activities and mRNA expressions of major IFNs and NF-κBs; in contrast, grass carp TLR3 just significantly motivates IFN1 expression post-grass carp reovirus (GCRV) infection. Further investigations reveal that TLR19 inhibits GCRV replication by overexpression, knockdown, Western blotting techniques and virus titer assays, and protects cells from GCRV infection by flow cytometry and MTT method. Collectively, these results demonstrate that teleost-specific TLR19 recognizes dsRNA, recruits adaptor molecule TRIF, enhances IRF3 protein and phosphorylation levels, triggers both IFN and NF-κB pathways, and prevents viral proliferation. This is the first attempt to systematically clarify the TLR19 signaling pathway, which is the third TLR member recognizing dsRNA. The results will serve the antiviral immune mechanisms in teleost and evolutionary immunology.

Characterization of the transcriptomic response to infection is an effective approach to understanding the immune mechanisms. Herein we challenged grass carp (Ctenopharyngodon idella) with grass carp reovirus (GCRV) and sequenced four cDNA libraries obtained from head-kidney and spleen by using Illumina Miseq. As a result, we gained a total of 21.52 Gb clean data with 107.96 million reads and de novo assembled 55,199 unigenes with an average length of 1,470 bp. Comparative transcriptome analysis reveals that 217 unigenes are differentially expressed (fold-change of at least 4) between resistant and susceptible fish in both head-kidney and spleen and of which 36 unigenes were validated by RT-qPCR experiment. The expression profile of immune-related genes demonstrates that the immune response of spleen is more intense than that of head-kidney. Remarkably, 11,811 unigenes contain multiple transcripts, of which 322 unigenes possess notably differentially expressed transcripts between the four transcriptomic datasets. Furthermore, the splicing transcripts of IL-12p40 and IL-1R1 are firstly found to play diverse roles in the antiviral response of fishes. This study provides a complete transcriptome dataset of C. idella, which is valuable for the studies of immune complexity and, moreover, throws light on the regulatory role of AS in antiviral immunity.

Autophagy plays many physiological and pathophysiological roles. However, the roles and the regulatory mechanisms of autophagy in response to viral infections are poorly defined in teleost fishes, such as grass carp (Ctenopharyngodon idella), which is one of the most important aquaculture species in China. In the present study, we found that both grass carp reovirus (GCRV) infection and hydrogen peroxide (H₂O₂) treatment induced the accumulation of reactive oxygen species (ROS) in C. idella kidney (CIK) cells, and thereby stimulate autophagy. Suppressing ROS accumulation with N-acetyl-L-cysteine (NAC) significantly inhibited the GCRV-induced autophagy activation and enhanced GCRV replication. Whereas ROS-induced autophagy in turn restricted GCRV replication, further investigations revealed that the multifunctional cellular protein high-mobility group box 1b (HMGB1b) serves as a heat shock protein 70 (HSP70)-dependent pro-autophagic protein in grass carp. Upon H₂O₂ treatment, cytoplasmic HSP70 translocated to the nucleus where it interacted with HMGB1b, and promoted cytoplasmic translocation of HMGB1b. Overexpression and siRNA-mediated knockdown assays indicated that HSP70 and HMGB1b synergistically enhance ROS-induced autophagic activation in cytoplasm. Moreover, HSP70 reinforced an association of HMGB1b with the C. idella ortholog of Beclin 1 (a mammalian ortholog of the autophagy-associated yeast protein ATG6), by directly interacting with C. idella Beclin 1. In summary, this study highlights the antiviral function of ROS-induced autophagy in response to GCRV infection and reveals the positive role of HSP70 in HMGB1b-mediated autophagy initiation in teleost.