Superior protective efficacy of genotype IVa-based single-cycle viral hemorrhagic septicemia virus (VHSV) vaccine compared to formalin-killed VHSV vaccine against VHSV genotype Ia in rainbow trout (Oncorhynchus mykiss).

Current viral vaccine strategies for aquaculture primarily target homologous viral infections through adaptive immune responses. However, the emergence of diverse genetic variants of viral hemorrhagic septicemia virus (VHSV) poses a significant challenge to aquaculture, necessitating vaccines capable of providing broad-spectrum cross-protection. This study aimed to compare the protective efficacy of a formalin-killed virus (FKC) vaccine and a single-cycle VHSV (rVHSV-ΔG) vaccine, both based on VHSV genotype IVa, against a heterologous VHSV genotype Ia challenge in rainbow trout (Oncorhynchus mykiss). Upon heterologous challenge with VHSV genotype Ia, the rVHSV-ΔG vaccine demonstrated superior protective efficacy relative to the FKC vaccine. Despite comparable levels of cross-reactive antibody titers and Type I interferon responses among the experimental groups, the rVHSV-ΔG group exhibited significantly elevated expression of pro-inflammatory cytokines (tnf-α, il-1β, and il-6) and the chemokine cxcl8 following challenge, suggesting an important role of innate immune mechanisms in mediating the enhanced protective effect. Notably, elevated levels of histone modifications (H3K4me3, H3K27ac) at the promoter regions of these cytokine genes suggest that rVHSV-ΔG may induce trained immunity. These results indicate that live VHSV vaccines may leverage innate immune memory to promote strong pro-inflammatory responses, representing a promising strategy for cross-protection against diverse VHSV variants.