A novel triterpene saponin from Quillaja brasiliensis: Complete structural analysis and innunoadjuvant potential

Quillaja brasiliensis (Qb) is a South American endemic tree from the Quillajaceae family, which includes only one genus and two accepted species, the other being Quillaja saponaria (Qs). Saponins from Quillaja species share similar chemical and biological properties, particularly their immunoadjuvant and immunostimulant activities, making them valuable for vaccine development [ 1 ]. Qs saponins, such as Quil-A ®​ , are used in veterinary vaccines, but their high reactogenicity limits their application in humans. However, QS-21, derived from Quil-A ®​ , has been tested in human vaccines for various diseases. Qb emerges as a renewable source of saponins, as its leaves are rich in these compounds. Extracts and purified fractions from Qb have demonstrated immunoadjuvant activity comparable to Quil-A ®​ . This study reports the identification of a novel triterpene saponin, an isomer of QS-21, named Qb1, and its evaluation as an adjuvant. From a saponin-enriched fraction obtained from Qb leaves, Qb1 was isolated using silica MPLC and semi-preparative HPLC. Its structure was elucidated through NMR spectroscopy and mass spectrometry ([ Fig. 1 ]) [ 2 ]. Fig. 1 Chemical structures of saponin Qb1, isolated from Quillaja brasiliensis leaves, and the two constituent isomeric saponins QS-21, isolated from Quillaja saponaria . In a murine Hepatitis E vaccine model, BALB/c mice were immunized subcutaneously with recombinant ORF2 capsid protein produced in E. coli, either alone or in combination with saponins. The experimental groups included: (1) Qb1+ORF2, (2) QS-21+ORF2, and (3) ORF2 alone (control without adjuvant). Qb1 induced specific IgG responses and promoted strong cellular immunity, increasing CD45 + CD3 + CD4 + , CD45 + CD3 + CD8 + , and CD24 + B220 + cells, as well as IFN-γ, TNF-α, and IL-4 cytokine levels. These findings highlight Qb1 as a promising adjuvant. Publication History Article published online: 05 March 2026 © 2026. Thieme. All rights reserved. Georg Thieme Verlag KG Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany