Active and passive immune responses and protection of Nile tilapia (Oreochromis niloticus) immunized with tilapia lake virus inactivated vaccines

Abstract

Tilapia tilapinevirus, also known as tilapia lake virus (TiLV), is a contagious viral pathogen, resulting in mass mortalities and economic losses for tilapia industry. Here, we developed two simple cell-culture, heat-killed and formalin-killed vaccines (HKV and FKV) aiming to prevent this disease. The vaccine efficacies were evaluated by intraperitoneal injection in juvenile tilapia with each vaccine containing 1.8 × 106 TCID50 inactivated virus, followed by a booster dose at 21-day post primary vaccination (dppv) in the same manner. At 28 dppv, the fish were challenged with a lethal dose of TiLV. Expression of five immune genes in head kidney and spleen of experimental fish was assessed at 14 and 21-dppv and again 7-day post booster. At the same time points, TiLV-specific IgM responses were evaluated by enzyme-linked immunosorbent assay (ELISA). The results showed that both vaccines conferred significant protection, with relative percentage survival of 71.3% and 79.6% for HKV and FKV, respectively. Significant up-regulation of IgM and IgT was observed in the head kidney of fish vaccinated with HKV at 21 dpv, while IgM, IgD and CD4 expression increased in the head kidney of fish receiving FKV at this time point. Both vaccines induced a specific IgM response in both serum and mucus. Then, we run the same vaccination regime on broodstock including four male and eight female fish per treatment with the double vaccine dose. Mating was performed one week later. Broodstock blood sera, fertilized eggs and larvae were collected from 6–14 week post primary vaccination for measurement of TiLV‐specific antibody levels. Meanwhile, passive immunization using sera from the immunized female broodstock was administered to naïve tilapia juvenile to assess if antibodies induced in immunized broodstock were protective. The results showed that TiLV‐specific antibodies were generated in majority of both male and female broodstock vaccinated with either the HKV or FKV and these antibodies were transferred to the fertilized eggs and 1–3-day-old larvae from vaccinated broodstock. Moreover, passive immunization proved that the antibodies elicited by TiLV vaccination were able to confer protection against TiLV challenge with RPS of 85%-90% in naïve juvenile tilapia. In conclusion, immunization of tilapia broodstock with HKV or FKV might be a potential strategy to reduce the risk of vertical transmission and protect the tilapia fertilized eggs and early stage of larvae from TiLV.