RNA-Seq transcriptome analysis of the olive flounder (Paralichthys olivaceus) kidney response to vaccination with heat-inactivated viral hemorrhagic septicemia virus

Viral hemorrhagic septicemia (VHS) is a highly contagious disease of cultured flounder caused by VHS virus (VHSV). To develop effective VHSV vaccines, it is essential to understand the molecular mechanisms underlying the host's protective response against VHSV. The purpose of this study is to clarify which genes are involved in the protective response of olive flounder after VHSV vaccination. We first injected olive flounder intraperitoneally with 107 TCID50 heat-inactivated VHSV vaccine and evaluated the vaccine efficacy at 20 °C. Fish vaccinated with heat-inactivated VHSV were significantly protected compared to non-vaccinated fish, with a relative percentage survival of 83%. To analyze the vaccination-induced changes in the expression profiles of genes, kidneys were collected from control and vaccinated fish at days 1, 3, and 7 after vaccination and global gene expression profiling was carried out by RNA sequencing. The analysis revealed that 15,001 genes were differentially expressed by at least 2-fold between vaccinated fish and non-vaccinated controls. Of these, 58 genes clustered into the acute phase response, Toll-like receptor, interferon-inducible/regulatory proteins, and apoptosis pathways. These data provided insights into the molecular mechanisms underlying the protective immune response of olive flounder against heat-inactivated VHSV vaccine and might aid future studies to develop a highly immunogenic vaccine against VHSV in flounder.