Efficacy of a formalin-inactivated vaccine against Streptococcus iniae infection in the farmed grouper Epinephelus coioides by intraperitoneal immunization

Vaccination is the most effective means of preventing infectious diseases; however, few vaccines are effective against Streptococcus iniae (S. iniae) in grouper. This work presents an efficacious and safe vaccine against S. iniae infections in the grouper Epinephelus coioides. The vaccine candidate was the S. iniae GSI-310 strain. The vaccination was administered by intraperitoneal injection, and consisted of formalin-inactivated antigens combined with an AS-F or ISA763A adjuvant. Peripheral blood samples were collected for RT-qPCR and phagocytosis and agglutination assays. Our results indicated that immunoglobulin M (igm) was maximally expressed in the two vaccinated groups at 3 months post-secondary vaccination (PSV). A significant upregulation of mRNA expression for interleukin-1β (il-1β) and tumor necrosis factor-α (tnf-α) was also observed in fish treated with antigens combined with ISA763A, which peaked at 3 months PSV. In fish treated with antigens combined with AS-F, il-1β and tnf-α expression peaked at 14 days post-primary vaccination (PPV). Phagocytic activity and index increased significantly in the two vaccinated groups. Furthermore, fish in the two vaccinated groups exhibited significantly elevated agglutination titers compared to fish in the control group, in which almost no agglutination reaction was detected. In the efficacy test, the vaccinated and control groupers were treated with S. iniae at 1, 3, and 6 months PSV. The relative percentage survival (RPS) values of antigens with AS-F and antigens with ISA763A were both 100% at 1 and 3 months PSV; at 6 months PSV, the RPS values for these groups were 100% and 97.7%, respectively. Furthermore, the level of protection observed in the field trial closely resembled that achieved on a laboratory scale. Therefore, the proposed vaccine mixed with AS-F or ISA763A improved immune responses and provided safe and long-lasting protection in farmed groupers.