Enhanced vaccine efficacy against Streptococcus agalactiae by molecular adjuvants for tilapia aquaculture in Low and Middle-Income Countries (LMICs)

Summary 

Aquaculture is the fastest growing animal food production sector globally and very important to food security. Tilapia is a fast growing fish and a very important aquaculture species for many low to middle income countries (LIMCs), since they provide an important source of protein and essential revenue for many low-income families. Intensification of tilapia farming has promoted severe disease outbreaks, resulting in high mortalities and economic hardship for tilapia farmers. Animal husbandry in LMICs is threatened by the increase in AMA resistance and if not properly managed would drive the increase in zoonotic infections.

Streptococcosis is one of the major bacterial diseases resulting in severe economic losses for tilapia farmers.  There is no commercial vaccine against Streptococcus infection till date, and farmers resort to using antibiotics to control this disease. To solve this problem, a novel bacterin vaccine, using molecular adjuvants flagellin and tilapia interferon-γ, will be developed and tested in this research project, using tilapia streptococcosis as a disease model.  Flagellin is a potent immune activator in fish. It induces the expression of the Th17 cytokine IL-17A/F1 and Th2 cytokine IL-4/13, but not the Th1 cytokine interferon-γ in fish. IFN-γ is a pleiotropic cytokine with immunostimulatory and immunomodulatory effects. A balanced Th1/Th2 and Th17 immune response is important for promoting protective immunity and eliminating tissue injury (pathological inflammation).

Therefore, novel injection vaccine formulation by incorporation of flagellin and interferon-γ as molecular adjuvants may enhance the immune response to the vaccine leading to long-lasting protection. This work will lead to the development of a cost-effective, easily administered vaccine for use in a large scale tilapia-production in Low and Middle-Income Countries (LMICs).

Project Outcomes

Streptococcus agalactiae causative agent of Streptococcosis result in massive mortalities and thereby huge economic loss in the tilapia aquaculture industry. The current study focussed to develop a novel bacterial vaccine used molecular adjuvants- flagellin and tilapia interferon gamma (IFNγ), against S. agalactiae infections in Nile tilapia. The fish were vaccinated with 100 μl formalin-killed S.agalactiae together with either flagellin or IFNγ and both together by intraperitoneal injection. A booster vaccination was carried out at 22 days post vaccination using the same procedure. The vaccinated fish were challenged with a virulent strain of S. agalactiae on day 36 and monitored three weeks for cumulative mortality and survival rates. Samples were collected at set time points during the trial to assess specific IgM titres, histopathology, and immune gene expression using RT-qPCR and several immune parameters. The vaccine conferred significant protection with relative percentage survival (RPS) of 59.37%, 71.87% and 81.25% was observed for bacterin vaccine adjuvanted with flagellin, IFNγ and both respectively with an RPS of 15.62% for the unadjuvanted bacterin control group after challenge with S. agalactiae. Similarly, the levels of specific IgM antibodies of fish vaccinated with molecular adjuvants were significantly enhanced post-vaccination. An upregulation of the immune related gene expression was observed in head kidney and spleen.  Furthermore, the measured immune parameters were significantly enhanced in fish administered with vaccine along with molecular adjuvants. The fish vaccinated with both the adjuvants showed significant upregulation in Lysozyme, Catalase, superoxide dismutase and myeloperoxidase activities when compared to the unvaccinated control group which indicated the efficacy of the booster vaccine increasing the antioxidant profiling in fishes. The highest bactericidal percentage of 76% was noticed in vaccinated fishes compared to the control group. A decreased melanomacrophage cells with decreased inflammation was observed in the spleen tissues by histopathological examinations. In conclusion, this study showed that both flagellin and IFNγ are promising candidates as molecular adjuvants to enhance the efficacy of fish vaccines against S.agalactiae infections in tilapia aquaculture. The findings emphasize safe potential approach for vaccination strategies employing novel vaccines incorporating molecular adjuvants, providing protection to fishes and inducing protective immunity against infectious bacterial diseases.