A subunit vaccine of largemouth bass (Micropterus salmoides) with baculovirus-expressed largemouth bass ranavirus (LMBRaV) major capsid protein induces protective immunity against LMBRaV infection

Abstract

Largemouth Bass Ranavirus (LMBRaV) is associated with large mortalities of farmed Largemouth Bass (Micropterus salmoides), which has caused huge economic loss to its aquaculture in China. The major capsid protein (MCP), an effective antigen to induce a specific immune response in iridovirus, has been widely used as immunogen in vaccination experiments. In this study, we constructed the recombinant baculovirus autographa californica multiple nucleopolyhedrovirus-MCP (AcMNPV-MCP) through Bac-to-Bac baculovirus expression system and expressed MCP as a subunit vaccine in insect cells. Recombinant baculovirus was verified using PCR and the virus titer was calculated as 50 % tissue culture infectious dose (TCID50/mL) using the Reed-Muench method. After confirmed by western blot, the recombinant MCP was purified from baculovirus-infected cell lysates and injected intraperitoneally (12 µg/fish) into M. salmoides. Liver, spleen and head kidney were collected at indicated times to analyze the expression of anti-viral genes. The result showed that the mRNA expression levels of immune-related genes like interferon related factors and interleukins were significantly up-regulated in AcMNPV-MCP immunized group. Meanwhile, the number of leukocytes in immunized group were significantly higher than that in control group. After immunized for 35 days, M. salmoides were challenged with LMBRaV injection. After infection with LMBRaV, Non-vaccinated groups exhibited a mortality of 100 %, while the survival rate of vaccinated group was up to 63.3 %. qPCR found that the virus load of vaccinated group was significantly lower than that of Non-vaccinated groups. This finding will potentially allow for the development of more target subunit vaccine development against largemouth bass ranavirus.