Contribution of the glycidic moieties to the haemolytic and adjuvant activity of platycodigenin-type saponins from the root of Platycodon grandiflorum

Platycodin D (PD), platycodin D3 (PD3), and platycoside E (PE) were the platycodigenin-type saponins isolated from the root of Platycodon grandiflorum. All shared a platycodigenin skeleton and the same sugar side chains attached to C-28 of the aglycone, only differ from one another by the number of glycosyl units in sugar moieties attached to C-3. To assess the potential contribution of the glycidic chains to the biological activities and elucidate the structure-activity relationships of the platycodigenin-type saponins, these three saponins were evaluated for their haemolytic activities and adjuvant potentials on the cellular and humoral immune responses of mice against ovalbumin (OVA). Among three saponins, the ranking of the haemolytic activity was PD>PD3>PE (P<0.001). PD and PD3 could significantly enhance mitogen- and OVA-induced splenocyte proliferation in the OVA-immunized mice (P<0.001). The order of increasing OVA-stimulated splenocyte proliferation was PD>PD3>PE (P<0.05, P<0.01, or P<0.001). The sera OVA-specific IgG, IgG1, IgG2A and IgG2B antibody levels in the OVA-immunized mice were significantly enhanced by PD and PD3. However, PE only significantly promoted the production of the sera OVA-specific IgG2A and IgG2B antibody in the OVA-immunized mice. Adjuvant potentials of PD on antibody responses were higher than those of PD3 and PE (P<0.05, P<0.01, or P<0.001). Meanwhile, PD also significantly enhanced the mRNA expression of cytokines IL-2, IFN-gamma, IL-4, and IL-10 and transcription factors T-bet and GATA-3 in mice splenocyte induced by Con A (P<0.05, P<0.01, or P<0.001). These results suggested that the number of sugar residues in the glycidic chains attached to C-3 of aglycone could affect the haemolytic and adjuvant activities of platycodigenin-type saponins, and that PD had immunological adjuvant activity, and simultaneously elicited a Th1 and Th2 immune response by regulating gene expression of Th1/Th2 cytokines and transcription factors.