Abstract
Viral hemorrhagic septicemia virus (VHSV) is a rhabdovirus that causes high mortality in cultured flounder. Naturally occurring VHSV strains vary greatly in virulence. Until now, little has been known about genetic alterations that affect the virulence of VHSV in flounder. We recently reported the full-genome sequences of 18 VHSV strains. In this study, we determined the virulence of these 18 VHSV strains in flounder and then the assessed relationships between differences in the amino acid sequences of the 18 VHSV strains and their virulence to flounder. We identified one amino acid substitution in the phosphoprotein (P) (Pro55-to-Leu substitution in the P protein; PP55L) that is specific to highly virulent strains. This PP55L substitution was maintained stably after 30 cell passages. To investigate the effects of the PP55L substitution on VHSV virulence in flounder, we generated a recombinant VHSV carrying PP55L (rVHSV-P) from rVHSV carrying P55 in the P protein (rVHSV-wild). The rVHSV-P produced high level of viral RNA in cells and showed increased growth in cultured cells and virulence in flounder compared to the rVHSV-wild. In addition, rVHSV-P significantly inhibited the induction of the IFN1 gene in both cells and fish at 6 h post-infection. An RNA-seq analysis confirmed that rVHSV-P infection blocked the induction of several IFN-related genes in virus-infected cells at 6 h post-infection compared to rVHSV-wild. Ectopic expression of PP55L protein resulted in a decrease in IFN induction and an increase in viral RNA synthesis in rVHSV-wild-infected cells. Taken together, our results are the first to identify that the P55L substitution in the P protein enhances VHSV virulence in flounder. The data from this study add to the knowledge of VHSV virulence in flounder and could benefit VHSV surveillance efforts and the generation of a VHSV vaccine.
Highlights
Viral hemorrhagic septicemia virus (VHSV) is the etiological agent of viral hemorrhagic septicemia (VHS), which causes huge economic losses to the fish culture industry
Recombinant VHSV containing this amino acid substitution caused increased mortality in flounder compared with the wild type
This is the first study to identify a naturally occurring amino acid substitution in VHSV that determines its virulence in flounder
Summary
Viral hemorrhagic septicemia virus (VHSV) is the etiological agent of viral hemorrhagic septicemia (VHS), which causes huge economic losses to the fish culture industry. The VHSV genome consists of approximately 11,200 nucleotides and contains six genes that encode the nucleocapsid- (N), phospho- (P), matrix- (M), glyco-(G), non-virion (NV)- and RNA polymerase (L) protein [2]. Phylogenetic analyses of the N- and G-encoding genes of VHSV have identified four main genotypes (I–IV), with several subgroups within genotypes I (minimum Ia-Ie) and IV (IVa–IVb) [3,4,5]. All VHSV strains far identified in cultured flounder in Asia belong to the genotype IVa clade. VHSV strains show a wide spectrum of virulence in cultured flounder that ranges from non-apparent infection to severe systemic disease causing high mortality. Our understanding of the viral factors responsible for VHSV virulence in flounder is limited
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
