Evaluation of immune responses and protection in Asian seabass (Lates calcarifer Bloch, 1790) against Vibrio vulnificus using immersion and oral nanoemulsion vaccines.

Vaccination is an important strategy in aquaculture for enhancing disease resistance and ensuring the health and productivity of cultured fish. This study investigated the distinct systemic and mucosal immune responses elicited by immersion (ImV) and oral (OrV) nanoemulsion vaccines encapsulating Vibrio vulnificus in Asian seabass (Lates calcarifer). Two vaccination methods, ImV and OrV, were administered to separate groups of Asian seabass, followed by challenges with V. vulnificus via immersion and intraperitoneal (i.p.) injection administration. Immune responses were assessed by measuring specific IgM antibodies, lysozyme activity, bactericidal activity, and immune-related gene expression across multiple organs, including serum, head kidney, liver, gills, skin, and intestine. The results demonstrate that the OrV group had a significantly increased level of specific IgM antibody in their serum, head kidney and liver compared to the immersion vaccine group and control, with the OrV group maintaining higher antibody levels post-immersion and injection challenge. In mucosal tissues, the ImV group elicited stronger IgM responses in the gills and skin, whereas the OrV group enhanced antibody responses in the liver and intestine. Lysozyme and bactericidal activities varied by organ and vaccination route, indicating differential activation of innate immune defenses. Additionally, immune-related gene expression was upregulated in a tissue-specific manner, reflecting the targeted activation of adaptive and innate immune mechanisms by each vaccine type. Both vaccination strategies significantly improved survival and relative percent survival (RPS), with ImV offering stronger protection against immersion challenges, while OrV provided strong protection in systemic (injection) challenges. These findings suggest that ImV and OrV groups induced distinct immune responses in Asian seabass, with immersion vaccination primarily enhancing mucosal immunity at barrier surfaces and oral vaccination promoting systemic and intestinal mucosal immunity. The integration of both vaccination strategies could provide synergistic protection, enhancing vaccine efficacy against V. vulnificus and other pathogens, thereby optimizing disease prevention and fish health management in aquaculture settings.