Oral immunization with surface immunogenic protein from Streptococcus agalactiae expressed in Lactococcus lactis induces protective immune responses of tilapia (Oreochromis niloticus)

•We constructed recombinant Lactococcus lactis expressing Sip protein of Streptococcus agalactiae.•Oral immunization of tilapia using the recombinant L. lactis could induce humoral and cellular responses.•The highest relative percentage survival (RPS) of tilapia reached 61.6% by oral immunization. Streptococcus agalactiae infection has become a serious bacterial disease of tilapia for its damage. In order to effectively prevent the outbreak of streptococcicosis, a recombinant Lactococcus lactis expressing surface immunogenicity protein (Sip) of S. agalactiae was constructed as probiotic vector oral vaccine in this study. The immunogenicity of the recombinant L. lactis was evaluated through detecting the level of specific serum IgM antibody, expression level of immune-related genes and relative percent survival (RPS). The results showed that the recombinant L. lactis could express a 45.5 kDa protein consistent with the expected size of Sip. The recombinant Sip mainly expressed in the form of inclusion body and the concentration of purified Sip could reach 7.65 mg / mL. Nile tilapia were orally immunized with different concentrations (2.24 × 109, 2.24 × 1010 and 2.24 × 1011 CFU / mL) of recombinant L. lactis (NZ9000-pNZ8148-sip). The analysis results of specific serum antibody showed that the level of antibody of tilapia which were vaccinated with medium and high concentration of recombinant L. lactis (2.24 × 1010 CFU / mL and 2.24 × 1011 CFU / mL) were significantly higher than that of control group. The relative percentage of survival (RPS) of tilapia vaccinated with medium concentration of NZ9000-pNZ8148-sip reached 61.6 % at 21 days post-immunization. Quantitative real-time PCR (qRT-PCR) results revealed that the immune-related genes of IgT, IgM, CD8a and C3 were significantly upregulated expression in thymus, liver, spleen and hindgut. In conclusion, the recombinant L. lactis vaccine induced both humoral and cellular responses of tilapia. This study demonstrates the potential of using L. lactis as a delivery system to develop an oral vaccine against S. agalactiae.