Kinetics of viral hemorrhagic septicemia virus (VHSV) and immune response in olive flounder (Paralichthys olivaceus) at altered temperature

Viral hemorrhagic septicemia virus (VHSV) is a pathogen that causes hemorrhagic septicemia in olive flounder at water temperatures below 15 °C, leading to symptoms such as abdominal swelling due to ascites and muscle hemorrhaging, and in severe cases, mortality. In this study, we investigated the proliferation of NCCs, the transcriptional analysis of CD4 and CD8, and the expression of pro-inflammatory cytokines (IL-1β, IL6, TNFα) and the cytokines (IL12, IL15, IFN-1β, IFNγ) involved in cytotoxic cell activation in the kidney of olive flounder during VHSV infection at suboptimal temperature (17 °C) and following a shift to optimal temperature (10 °C). Following viral infection, the population of NCCs and CD8 gene expression steadily increased. Meanwhile, IL12, IL15, and IFN-1β levels exhibited an early surge, reaching their peak within 1–2 days post-challenge at 17 °C. Notably, in olive flounder that were re-challenged with VHSV at 10 °C after an initial challenge at 17 °C, there was a rapid and robust expansion of NCCs and increased CD4 and CD8 gene expression, demonstrating the mounting of immune memory by NCCs and an efficient adaptive immune response from CD4⁺ and CD8⁺ T cells against VHSV infection.