Analysis of the efficacy of two molecular adjuvants, flagellin and IFN-γ, on the immune response against Streptococcus agalactiae in Nile tilapia (Oreochromis niloticus).

Aquaculture industry frequently encounters disease outbreaks, high mortalities, as well as emergence of new pathogens due to its intensification. Streptococcus agalactiae (Lancefield's group B Streptococcus) is an important pathogen extensively causing infectious disease in tilapia resulting in huge economic loss and mortality. To date, vaccination has proved to be successful in defending against infectious diseases prevailing among farmed fish species. This study aimed to develop an S. agalactiae inactivated vaccine (SAIV) using molecular adjuvants, flagellin and tilapia interferon gamma (IFN-γ), and to assess the generated immune response and protective efficacy of the adjuvant incorporated vaccine against S. agalactiae infection in Nile tilapia. The fish were vaccinated with SAIV together with either flagellin or IFN-γ and both together by intraperitoneal injection. The vaccinated fish were challenged with a virulent strain of S. agalactiae on day 36 and monitored for 3 weeks to assess cumulative mortality. The results showed that the vaccine offered significant protection with relative percentage survival (RPS) of 59.37%, 71.87%, and 81.25% observed for bacterin vaccine adjuvanted with flagellin, IFN-γ and both, respectively, with an RPS of 15.62% for the unadjuvanted bacterin control group after challenge with S. agalactiae. The vaccine induced specific IgM antibodies against S. agalactiae in the vaccinated groups, and the antibody response was significantly increased following booster vaccination in the fishes administered with vaccine adjuvanted with flagellin, IFN-γ and both. Furthermore, after vaccination, MHC-II and IgM gene expression was found significantly upregulated in head kidney and spleen, in line with an elevated specific IgM titer. Innate immune parameters including catalase, lysozyme, superoxide dismutase, myeloperoxidase, and bactericidal activities were significantly increased in fishes immunized when compared with the unvaccinated controls (P < 0.05). Histopathological examinations of tissue sections of the head kidney, spleen, liver, kidney, gills, and brain were performed on vaccinated and non-vaccinated fish which showed mild infiltrations. In conclusion, flagellin and IFN-γ have shown potential for use as molecular adjuvants to enhance the efficacy of fish vaccines against S. agalactiae infections.