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.