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Reverse genetics systems are powerful tools for understanding the virulence mechanisms and gene functions of negative-sense RNA viruses. The reverse genetics systems commonly used for recombinant infectious hematopoietic necrosis virus (IHNV) are based on vaccinia virus infection. To avoid the potential biological safety risks associated with vaccinia virus, a recombinant IHNV virus strain Sn1203 (rIHNV-Sn1203) was rescued in this study using a mammalian cell line, BHK-21. The genome sequence authenticity of rIHNV-Sn1203 was confirmed using two silent genetic tags introduced by site-directed mutagenesis. Indirect immunofluorescence assays and transmission electron microscopy revealed that rIHNV-Sn1203 and wild-type IHNV-Sn1203 (wtIHNV-Sn1203) had identical immunogenicity and virion morphology. The virulence and pathogenicity of rIHNV-Sn1203 were assessed in vitro and in vivo. Although rIHNV-Sn1203 displayed trends toward delayed intracellular viral replication and lower virion yields compared with wtIHNV-Sn1203, statistical analyses revealed no significant differences between these two viruses. Moreover, rainbow trout challenged with rIHNV-Sn1203 and wtIHNV-Sn1203 showed indistinguishable mortality. Together, these results show that IHNV was successfully rescued using BHK-21 cells. This method is very convenient and may also be suitable for use in the recovery of other Novirhabdoviruses., (Copyright © 2019 Elsevier B.V. All rights reserved.)

• A recombinant IHNV virus expressing VP2 gene of IPNV was successfully constructed. • The recombinant virus showed significant protection efficacy against IHNV and IPNV. • The recombinant virus induced upregulation of innate and adaptive immune responses. • The recombinant virus could be a vaccine candidate protecting rainbow trout against two or more diseases. Infectious pancreatic necrosis virus (IPNV) and infectious hematopoietic necrosis virus (IHNV) are two common viral pathogens that cause severe economic losses in all salmonid species in culture, but especially in rainbow trout. Although vaccines against both diseases have been commercialized in some countries, no such vaccines are available for them in China. In this study, a recombinant virus was constructed using the IHNV U genogroup Blk94 virus as a backbone vector to express the antigenic gene, VP2, from IPNV via the reverse genetics system. The resulting recombinant virus (rBlk94-VP2) showed stable biological characteristics as confirmed by virus growth kinetic analyses, pathogenicity analyses, indirect immunofluorescence assays and western blotting. Rainbow trout were immunized with rBlk94-VP2 and then challenged with the IPNV ChRtm213 strain and the IHNV Sn1203 strain on day 45 post-vaccination. A significantly higher survival rate against IHNV was obtained in the rBlk94-VP2 group on day 45 post-vaccination (86%) compared with the PBS mock immunized group (2%). Additionally, IPNV loads decreased significantly in the rBlk94-VP2 immunized group in the liver (28.6-fold to 36.5-fold), anterior kidney (21.7-fold to 44.2-fold), and spleen (14.9-fold to 22.7-fold), as compared with the PBS mock control group. The mRNA transcripts for several innate and adaptive immune-related proteins (IFN-γ, IFN-1, Mx-1, CD4, CD8, IgM, and IgT) were also significantly upregulated after rBlk94-VP2 vaccination, and neutralizing antibodies against both IHNV and IPNV were induced on day 45 post-vaccination. Collectively, our results suggest that this recombinant virus could be developed as a vaccine vector to protect rainbow trout against two or more diseases, and our approach lays the foundations for developing live vaccines for rainbow trout. [ABSTRACT FROM AUTHOR]

Infectious hematopoietic necrosis virus (IHNV) was developed as a vector to aid the construction of vaccines against viral diseases such as viral hemorrhagic septicemia virus, spring viremia of carp virus, and influenza virus H1N1. However, the optimal site for foreign gene expression in the IHNV vector has not been determined. In the present study, five recombinant viruses with the green fluorescence protein (GFP) gene inserted into different genomic junction regions of the IHNV genomic sequence were generated using reverse genetics technology. Viral growth was severely delayed when the GFP gene was inserted into the intergenic region between the N and P genes. Real-time fluorescence quantitative PCR assays showed that the closer the GFP gene was inserted towards the 3ʹ end, the higher the GFP mRNA levels. Measurement of the GFP fluorescence intensity, which is the most direct method to determine the GFP protein expression level, showed that the highest GFP protein level was obtained when the gene was inserted into the intergenic region between the P and M genes. The results of this study suggest that the P and M gene junction region is the optimal site within the IHNV vector to express foreign genes, providing valuable information for the future development of live vector vaccines. [ABSTRACT FROM AUTHOR]