Glycyrrhizic acid increased FMD vaccine potency in a mineral-based adjuvant, but not in an oil-based adjuvant

Inactivated FMD vaccine with aluminium hydroxide induces Th2 response. Glycyrrhizin is a triterpenoid saponin extracted from the licorice plant, which has immunomodulatory activities. In this study, two different formulations (oil- and alum-based adjuvants) of FMD vaccines were used to see if glycyrrhizic acid (GZA) has an enhancing effect on the adjuvant, and whether its impact depends on the nature of the co-administered adjuvant or not. Balb/c mice were immunised subcutaneously on days 0, 14, and 28 with 2 μg of inactivated FMD virus formulated in Alum or Montanide ISA206 adjuvant with or without GZA. Two weeks later, IFN-γ, IL-2 and IL-4 cytokines, specific total IgG antibodies, IgG1 and IgG2a isotypes, IgG avidity index and serum neutralisation activity were evaluated. The results indicate that GZA enhanced IFN-γ, IL-2 and IFN-γ/IL-4 levels and specific IgG titre compared to the FMD-Alum group. In addition, FMD-Montanide ISA206-GZA enhanced IFN-γ and IFN-γ/IL-4 ratio, whereas it suppressed the specific IgG titre and the avidity index. Furthermore, serum neutralisation tests showed a significant increase in FMD-Alum-GZA group, while GZA in the Montanide-based vaccine suppressed the serum neutralisation response. These findings suggest that the adjuvant effect of GZA is dependent on the co-administered adjuvant.