Unraveling the Systemic and Local Immune Response of Rainbow Trout (Oncorhynchus mykiss) to the Viral Hemorrhagic Septicemic Virus

Abstract

Viral outbreaks have caused significant mortality and economic losses in aquaculture, highlighting the urgent need for effective therapies and a deeper understanding of antiviral and immune mechanisms in key species. This study investigates the constitutive and virus-induced antiviral responses in juvenile rainbow trout (Oncorhynchus mykiss) following infection with viral hemorrhagic septicemia virus (VHSV). Trout (30 g) were infected by immersion with VHSV (TCID₅₀ = 10⁵ mL^-1) for two hours. Samples were collected at 24, 72, and 120 h post-infection to assess hematology, innate immunity, viral load, and transcriptomic response. At 24 h post-infection, no immune response or increase in viral load was detected, suggesting the host had not yet recognized the virus and was still in the incubation phase. By 72 h, viral replication peaked, with high viral loads observed in mucosal tissues (skin and gills) and immune organs (kidney, spleen, liver), alongside strong up-regulation of antiviral genes, such as viperin. This gene maintained high expression through the final sampling point, indicating its key role in the antiviral response. At this stage, reduced immune competence was observed, marked by elevated nitric oxide and circulating thrombocytes. At 120 h, modest increases in peripheral monocyte, plasma lysozyme, and peroxidase activity were detected; however, these responses were insufficient to reduce viral load, suggesting the resolution phase had not yet begun. In summary, while a limited immune response was observed by the end of the trial, the consistent antiviral activity of viperin from peak infection to 120 h post-infection underscores its importance in the defence against VHSV in rainbow trout.

Keywords: antiviral; immune response; transcriptomics; viperin; viral load.

Figure 1

Lysozyme concentration (A), peroxidase activity (B), and NO concentration (C) of rainbow trout challenged with VHSV and sampled 24, 72 and 120 h post-infection. Values represent means ± SD (n = 6). Different letters stand for significant differences attributed to time. Different symbols stand for significant differences attributed to infection (CTRL vs. VHSV). (Multifactorial ANOVA; Tukey post hoc test; p ≤ 0.05.) CTRL (control group) and VHSV (infected group).

Figure 2

Viral quantification in skin, gills, gut, liver, HK and spleen of rainbow trout challenged with VHSV and sampled at 24, 72 and 120 h post-infection. Different letters stand for significant differences attributed to time. Different symbols stand for significant differences attributed to infection (CTRL vs. VHSV). (Multifactorial ANOVA; Tukey post hoc test; p ≤ 0.05).

Figure 3

Diverted stacked bar chart showing differentially expressed genes (DEGs) up and down-regulated in gills, HK and spleen in rainbow trout at 24, 72 and 120 h post-infection with VHSV.

Figure 4

Bubble charts of the gene ontology (GO) enrichment analysis in gills, HK and spleen of rainbow trout at 24, 72 and 120 h post-infection with VHSV. (A) Gills up-regulated; (B) gills down-regulated; (C) HK up-regulated; (D) HK down-regulated; (E) spleen up-regulated and (F) spleen down-regulated. MF: molecular function; BP: biological process.

Figure 5

Venn diagrams showing the number of common and unique genes up-regulated in gills (A), HK (B) and spleen (C), up-regulated at 24, 72 and 120 h post-infection.

Figure 6

Venn diagram showing the number of common and unique genes down-regulated in spleen at 72 and 120 h post-infection.