Effect of an Oral Bivalent Vaccine on Immune Response and Immune Gene Profiling in Vaccinated Red Tilapia (Oreochromis spp.) during Infections with Streptococcus iniae and Aeromonas hydrophila.

Abstract

Simple SummaryStreptococcosis and aeromonasis are major bacterial diseases in tilapia cultures, causing mass mortality with substantial economic losses to global aquaculture. The development of oral monovalent vaccines to prevent these diseases has been attempted; still, the mechanism of immunity of oral bivalent vaccines against streptococcosis and aeromonasis infections, as well as the protective ability of the host against these two diseases, remains understudied. To explore the immunological role of an oral bivalent vaccine, we compared the immune responses and immune gene profiling in red tilapia post-challenged with Streptococcus iniae and Aeromonas hydrophila. Our results showed that the degrees of expression of different innate and adaptive immune-associated genes in both mucosal and systemic immune organs were significantly higher in the bivalent vaccinated fish compared to the monovalent and control (unvaccinated) groups.Streptococcosis and aeromonasis inflicted by Streptococcus iniae and Aeromonas hydrophila, respectively, have affected tilapia industries worldwide. In this study, we investigated antibody responses and explored the mechanisms of protection rendered by an oral bivalent vaccine in red tilapia following challenges with S. iniae and A. hydrophila. The results of specific IgM antibody response revealed that the IgM titers against S. iniae and A. hydrophila in the bivalent incorporated (BI) vaccine group were significantly higher (p < 0.05) than those in the bivalent spray (BS) vaccine fish and unvaccinated control fish throughout the experiment. Real-time qPCR results also showed that the gene expression of CD4, MHC-I, MHC-II, IgT, C-type lysozyme, IL-1β, TNF-α, and TGF-β remained significantly higher (p < 0.05) than that of the controls between 24 and 72 h post-infection (hpi) in both mucosal (hindgut) and systemic (spleen and head–kidney) organs of BI vaccinated fish. Furthermore, the highest relative expression of the TGF-β, C-type lysozyme, and IgT genes in the BI vaccinated group was observed in the challenged fish’s spleen (8.8-fold), head kidney (4.4-fold), and hindgut (19.7-fold) tissues, respectively. The present study suggests that the bivalent incorporated (BI) vaccine could effectively improve the immune function and activate both humoral and cell-mediated immunities in vaccinated red tilapia following the bacterial challenges.