Purified <i>Astragalus</i> Polysaccharide Combined with Inactivated Vaccine Markedly Prevents Infectious Haematopoietic Necrosis Virus Infection in Rainbow Trout (<i>Oncorhynchus mykiss</i>)

Rainbow trout (<i>Oncorhynchus mykiss</i>) is experiencing a catastrophic pandemic. In recent years, infectious hematopoietic necrosis virus (IHNV) has spread nationwide, resulting in significant mortality. Currently, there are no available treatments or vaccines for IHNV in China. Here, the <i>Astragalus</i> extract was purified and characterized. Then, we developed an inactivated IHNV vaccine with purified <i>Astragalus</i> polysaccharide (P-APS) as an adjuvant. Safety assays showed that IHNV was successfully inactivated. After a serious IHNV challenge, the cumulative mortality rates were 76.0, 38.0, and 22.1% in control, vaccine, and P-APS + vaccine groups, respectively. P-APS + vaccine was effective at reducing head kidney damage and apoptosis after IHNV challenge by histopathological and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) analyses. The P-APS + vaccine group showed better results in enhancing specific antibodies (IgM) and immune enzyme activities (C3, LZM, GOT, and GPT). RNA-seq revealed that many immune-related pathways were significantly enriched. <i>TLR2</i>, <i>TLR7</i>, <i>C3</i>, <i>IFN-γ</i>, <i>IgM</i>, <i>MHC1</i>, <i>MHC2</i>, <i>MX1</i>, and <i>VIG1</i> were identified as core genes based on RNA-seq and PPI networks. Mechanistic investigations showed that P-APS + vaccine activates the immune pathway by upregulating the expression of these genes. P-ASP+vaccine induced effective innate and adaptive immune responses that were stronger than single vaccines after vaccination and IHNV challenged. Our findings will provide a promising vaccine candidate against IHNV.