Nervous Necrosis Virus (NNV) Booster Vaccination Increases Senegalese Sole Survival and Enhances Immunoprotection

SIMPLE SUMMARY: Viral encephalopathy and retinopathy (VER), caused by nervous necrosis virus (NNV), is a serious threat to Senegalese sole farming. We have previously demonstrated that immunization with an inactivated vaccine confers partial protection against the infection. However, a vaccination program must be finely adjusted to achieve the best results in terms of immune system stimulation and protection. In this study we show that a booster injection 30 days after prime vaccination increases sole survival and reduces NNV replication in brain (viral target organ). The analysis of immune-related genes expression indicated that T CD4+ lymphocytes and the proteins Mx and HERC4 may play an important role in the protection. These findings increase our understanding of sole immune response against NNV and may contribute to the development of effective protection measures. ABSTRACT: A re-immunization programme has been tested to improve the protective response elicited in sole by a previously developed BEI-inactivated betanodavirus vaccine. The vaccine was prepared using a reassortant RGNNV/SJNNV strain which is highly pathogenic for sole, and vaccination assays were performed by intraperitoneal injection. Experimental design included a prime- and a booster-vaccination group, which consisted of individuals that received a second vaccine injection at 30 days post vaccination), and their respective controls. A month after prime/booster vaccination, fish were challenged by intramuscular injection with the homologous NNV strain. Samples were collected at different times post vaccination and post challenge to assess the immune response and viral replication. Booster dose enhanced the protection against NNV infection because a significant increase in survival was recorded when compared with prime-vaccinated individuals (relative percent survival 77 vs. 55). In addition, a clear decrease in viral replication in the brain of challenged sole was observed. During the immune induction period, no differences in IgM production were observed between prime- and booster-vaccinated fish, and the expression of the antigen presenting cells (APC)-related molecule MHC class II antigen was the only differential stimulation recorded in the re-immunized individuals. However, a significant upregulation of mhcII and the lymphocytes T helper (Th) marker cd4 was observed after the challenge in the booster-vaccinated group, suggesting these cells play a role in the protection conferred by the booster injection. In addition, after viral infection, re-immunized fish showed specific and neutralizing antibody production and overexpression of other immune-related genes putatively involved in the control of NNV replication.