Temperature-dependent viral hemorrhagic septicemia virus (VHSV) infection kinetics and immune response in primary olive flounder spleen cell culture and the host

The replication and mortality caused by the viral hemorrhagic septicemia virus (VHSV) in fish vary depending on temperature. VHSV causes mortality at the temperatures below 15 °C, while infection is not established in olive flounder at temperatures above 25 °C. However, how VHSV infection manifests at the cellular level under different temperature conditions is not understood. In this study, we aimed to elucidate the mechanism by which VHSV infection is controlled by comparing viral replication and immune responses in vitro and in vivo. In the in vitro experiment, viral mRNA levels on day 5 post-challenge differed more than 10-fold between temperatures, highest at 15 °C, followed by 20 °C, and lowest at 25 °C, with replication gradually increasing over 5 days. In vivo, replication peaked by day 2 and then declined at all temperatures. VHSV infection showed a controlled tendency in the fish, whereas the virus exhibited a continuously increasing infection pattern for up to 5 days in olive flounder spleen cell cultures at a temperature of 25 °C. The level of viral infection within the cells (25 °C) was lower compared to conditions below 20 °C; however, the expression levels of interferon-related genes and pro-inflammatory cytokine genes were relatively low. The expression of ISG15 and Mx genes in spleen (25 °C) was significantly high at 24 h post challenge in the in vivo experiment.