Evaluation of a tilapia epitopes vaccine against Streptococcus agalactiae based on phage display technology

Tilapia ranks among the world's leading aquaculture species, but Streptococcus agalactiae (S. agalactiae) severely limits the growth of the global tilapia industry. Vaccination has been proven to be a promising strategy for controlling this disease. In this study, a Ph.D.-12 phage display library was screened using serum from S. agalactiae survivors, leading to the identification and synthesis of two peptides (P1 and P2). Subsequently, an epitope vaccine was prepared by combining the peptide with adjuvant, and tilapia were immunized with a prime-boost strategy to evaluate its protective effect. Serum and tissue samples of immunized fish were collected periodically after the primary immunization, and the immunization effect was evaluated by combining enzyme-linked immunosorbent assay (ELISA), enzyme activity assays and quantitative real-time PCR (qRT-PCR). The results showed that the vaccine significantly enhanced host immune responses, including elevated specific antibody titers and upregulation of CD8, a marker of T cell activation. Notably, the epitope vaccine conferred protection against S. agalactiae challenge within 14 days post-challenge with a survival rate of up to 54.29 % in the group treated with Adjuvant-P1 (20 μg/g). Unlike traditional reverse vaccinology approaches, this method employed phage display to directly identify conformational epitopes, leading to the discovery of new conserved epitopes and suggesting that P1 and P2 peptide mimotopes hold potential as S. agalactiae candidate antigens. However, while this study provided preliminary evidence of the vaccine's efficacy against S. agalactiae over a 35-day period, further validation of the vaccine's durability and generalizability by extending the observation window and using different S. agalactiae strains is necessary. In conclusion, two mimotopes against S. agalactiae were directly identified by phage display in this study, which lays the foundation for the development of highly efficient and chemically synthesizable S. agalactiae vaccines for tilapia, and contributes to the promotion of the sustainable development of the tilapia industry.