Immersion prime and oral boost vaccination with an inactivated Vibrio harveyi vaccine confers a specific immune response and protection in Asian seabass (Lates calcarifer).

Asian seabass (Lates calcarifer) holds significant economic value in fish farming in the Asia-Pacific region. Vibriosis caused by Vibrio harveyi (Vh) is a severe infectious disease affecting intensive farming of this species, for which prevention strategies by vaccination have been developed. This study investigated an alternative approach to injectable vaccination to prevent vibriosis in Asian seabass juveniles. The strategy begins with an immersion prime vaccination with a heat-inactivated Vh vaccine, followed by two oral booster doses administered at 14- and 28-days post-vaccination (dpv). Expression of five immune genes TNFα, IL1ß, CD4, CD8, and IgM in the head kidney and spleen, along with investigation of anti-Vh antibody response (IgM) in both systemic and mucosal systems, was conducted on a weekly basis. The efficacy of the vaccines was assessed by a laboratory challenge test at 43 dpv. The results showed that the immunized fish displayed higher levels of mRNA transcripts of the immune genes after the immersion prime and the first oral booster dose compared to the control group. The expression levels peaked at 14 and 28 dpv and then declined to baseline at 35 and 42 dpv. Serum specific IgM antibodies were detected as early as 7 dpv (the first time point investigated) and exhibited a steady increase, reaching the first peak at 21 dpv, and a second peak at 35 dpv. Although the antibody levels gradually declined over subsequent weeks, they remained significantly higher than the control group throughout the experiment. A similar antibody response pattern was also observed in the mucosal compartment. The laboratory challenge test demonstrated high protection by injection with 1.65 × 104 CFU/fish, with a relative percent of survival (RPS) of 72.22 ± 7.86 %. In conclusion, our findings highlight the potential of an immersion prime-oral booster vaccination strategy as a promising approach for preventing vibriosis in Asian seabass.

Plant lyophilisate carrying S-HBsAg as an oral booster vaccine against HBV.

A formulation of a defined antigen dosage together with an appropriate and convenient immunisation regime are the main problems of a plant-derived oral vaccine against HBV. Both factors have to be mutually adjusted to ensure efficacious vaccination and to minimise the risk of oral tolerance acquisition. As based on previous research, solely oral immunisation appears to be of limited effectiveness, but a combined immunisation scheme via injection priming and oral boosting can be proposed. Thus, the previously optimised plant lyophilisate with accumulated S-HBsAg was orally administered in a series of mouse immunisation trials. The impact of various S-HBsAg oral dosages and concentrations on the induction of an active immune response was studied. Immunisation via i.m. priming followed by double oral boosting in 6-week intervals was comparably efficient as standard i.m. vaccination. Among the tested dosages, 2× 5 or 200 ng triggered effective and exclusively systemic responses. Mucosal adjuvants CTB or quillaja bark saponins as well as alhydrogel either had no or a negative effect on immune response, which indicates that a low-dosed plant lyophilisate as an oral booster vaccine may not require exogenous adjuvants. Additionally, a positive effect of S-HBsAg encapsulation in lyophilised cells on immune response stimulation was confirmed by comparison with the antigen released in the plant extract. The low-dosed plant lyophilisate with concentrated S-HBsAg was proven as an effective and convenient oral booster vaccine. Results of immunisation via a mixed injection-oral route provide the foundation for research on further standardisation of a plant-derived oral vaccine against HBV and details of immune response together with effects for health condition.