Development of an attenuated oral vaccine strain of tilapia Group B Streptococci serotype Ia by gene knockout technology

Characterization and evaluation of an oral vaccine via nano-carrier for surface immunogenic protein (Sip) delivery against Streptococcus agalactiae infection

Streptococcus agalactiae causes systemic disease in a variety of wild and farmed fish, resulting in high levels of morbidity and mortality, as well as serious economic losses to the Nile tilapia aquaculture industry. The development of economic and applicable oral vaccines is therefore urgently needed for the sustainable development of Nile tilapia aquaculture. In this study, mesoporous silica nanoparticles (MSNs) were fabricated using sol-gel synthesis technology, and the antigens of surface immunogenic protein (Sip) was loaded into MSNs to develop a nanovaccine MSNs-Sip@HP55. The results showed that the prepared nanovaccine exhibited pH-controlled release, which could survive in the simulated gastric environment (pH 1.5), and release antigens in the simulated intestinal environment at pH 7.4. The nanovaccine could induce innate and adaptive immune responses in Nile tilapia. When the challenge doses were 1.5 × 10⁶, 1.18 × 10⁶, and 0.88 × 10⁶ CFU/mL, the relative protection rates in immunized Nile tilapia were 63.33 %, 64.23 %, and 76.31 %, respectively. Taken together, the nanovaccine exhibited a high antigen utilization rate and was easily administered orally via feeding, which could protect Nile tilapia against challenge with S. agalactiae in large-scale farms. Oral vaccine based on MSNs carriers is a potentially promising strategy for the development of fish vaccines.

Construction of Streptococcus agalactiae sialic acid mutant and evaluation of its potential as a live attenuated vaccine in Nile tilapia (Oreochromis niloticus)

AIMS

This study aimed to develop a live attenuated vaccine as an effective approach to prevent streptococcosis in tilapia (Oreochromis niloticus).

METHODS AND RESULTS

We eliminated the virulence factor, sialic acid (Sia) encoded by the neuA-D gene cluster from the Group B Streptococcus (Streptococcus agalactiae, GBS) strain WC1535, to construct Sia-deficient S. agalactiae (ΔSia) mutant by homologous recombination. Results showed that the ΔSia mutant had higher adherence to HEp-2 cells and lower resistance to RAW264.7 cell phagocytosis than the wild-type S. agalactiae. The virulence of the ΔSia mutant to tilapia dramatically decreased with no virulence recovery. The relative percent survivals (RPSs) were 50.00% and 54.50% at 30 days when challenged at the wild-type WC1535 doses of 1.0 × 10⁷ and 5.0 × 10⁷ CFU fish^-1 , respectively, via intraperitoneal (IP) injection. The tilapia vaccinated via IP injection with the ΔSia mutant induced strong antibody agglutination titers. The expression of IL-1β, TNF-α, MHC-Iα, and MHC-IIβ could be enhanced in the intestine, spleen, and head kidney for tilapia administered with the ΔSia mutant.

CONCLUSIONS

GBS Sia plays a critical role in adherence to HEp-2 cells and resistance to the immune clearance of RAW264.7 cells. Moreover, the ΔSia mutant is a safe, stable, and immunogenic live attenuated vaccine candidate to protect tilapia against GBS infection.

SIGNIFICANCE AND IMPACT OF STUDY

The results offer more evidence of the importance of Sia in GBS and may be instructive in the control of tilapia streptococcosis.

Immuno-protective efficiency of feed-based whole-cell inactivated bivalent vaccine against Streptococcus and Aeromonas infections in red hybrid tilapia (Oreochromis niloticus × Oreochromis mossambicus)

Streptococcosis and motile aeromonad septicemia (MAS) are well-known diseases in tilapia culture, which cause mass mortality with significant economic losses. The development of feed-based bivalent vaccines in controlling these diseases has been initiated, however, the mechanisms of immunities and cross-protection in fish remain unclear. This study was conducted to assess the immuno-protective as well as the cross-protective efficacy of a newly developed feed-based bivalent vaccine against Streptococcus and Aeromonas infections in red hybrid tilapia. A total of five groups of fish were vaccinated orally through two different techniques; bivalent vaccine (inactivated Streptococcus iniae and Aeromonas hydrophila) sprayed on feed pellets (BS group); bivalent vaccine (inactivated S. iniae and A. hydrophila) incorporated in feed (BI group); monovalent inactivated S. iniae and A. hydrophila vaccine separately incorporated into feed as monovalent S. iniae (MS group) and monovalent A. hydrophila (MA group); and control group (without vaccine). The feed-based vaccine was delivered orally at 5% of body weight for five consecutive days. The booster doses were given in the same manner on weeks 2 and 6. Serum and skin mucus samples were collected to assess the IgM responses using indirect ELISA. The first administration of the feed-based vaccine stimulated the IgM levels that lasted until week 3, while the second booster ensured that the IgM levels remained high for a period of 16 weeks in the BI, MS and MA groups. The BI group developed a strong and significantly (P < 0.05) higher systemic and mucosal IgM responses against both S. iniae and A. hydrophila and also cross-protective antigen S. agalactiae and A. veronii compared to the BS and control groups. Quantitative real-time PCR results also showed that the relative expressions of IL-8, INF-γ and IgM in the BI immunized fish spleen, head kidney and hindgut exhibited various significant (P < 0.05) rising trends following both the vaccination and the challenge phase. On weeks 10, all fish were challenged through the intraperitoneal route, where relative percent survivals (RPS) of 82.22 ± 3.85% when challenged with S. iniae, 77.78 ± 3.85% when challenged with A. hydrophila and 77.78 ± 3.85% when co-challenged with both S. iniae and A. hydrophila were observed in the BI group, which were significantly higher (P < 0.05) compared to the other groups. The BI group also showed significantly (P < 0.05) higher partial cross-protections following challenges with S. agalactiae (RPS at 60.00 ± 6.67%) and A. veronii (RPS at 57.78 ± 7.70%). This study demonstrated that immunization with feed-based BI vaccine elicited immune responses that were capable of protecting red hybrid tilapia against streptococcosis and MAS.

An effective live attenuated vaccine against Streptococcus agalactiae infection in farmed Nile tilapia (Oreochromis niloticus)

Streptococcus agalactiae is an important pathogen associated with various aquatic animals, especially tilapia. Streptococcosis has greatly limited the healthy development of tilapia aquaculture in recent times. The development of novel effective vaccines is important for the prevention and control of streptococcosis in fish. We previously constructed a non-encapsulated S. agalactiae strain △cps by the in-frame deletion method. Here, we evaluated whether this mutant △cps is safe for tilapia and suitable for protection against streptococcosis. We observed that the △cps strain was non-pathogenic to tilapia, and there was no reversion of virulence when it was passaged in tilapia. Moreover, the △cps strain survived for at least 11 d in the main immune organs of tilapia. The tilapia vaccinated via intraperitoneal (IP) injection with △cps strain induced a high antibody titer, and the IgM antibody levels were significantly higher in the vaccinated group than in the control group. The vaccination protected tilapia against the S. agalactiae challenge with a relative percent survival of 90.47%. In addition, tilapia immunized with the △cps strain showed significantly higher expression level of IFN-γ, IL-1β, MyD88, IgM, and MHC-Iα in the head kidney than those in the control during the entire observation period. The expression of MHC-IIβ was inhibited during 1-7 d of immunization. These results revealed that the △cps strain is able to induce humoral and cell-mediated immune response in tilapia. Therefore, the strain △cps has a broad application prospect as a target for attenuation in vaccine development.