Structure and immunological activity of QS-21 variant from Quillaja saponaria aerial biomass

QS-21 is a triterpenoid saponin adjuvant component widely used in human vaccines, but its commercial supply is limited because it is sourced almost exclusively from the bark of mature <i>Quillaja saponaria</i> trees in Chile. In this study, we report the identification, isolation, as well as structural and immunological characterization of QS-21-Rhamnose (QS-21-Rha), a naturally occurring structural variant that predominates in the leaves and twigs of young <i>Q. saponaria</i> shrubs cultivated in California. Analytical profiling showed that QS-21-Rha represents more than 95% of QS-21 variants in the aerial biomass. High-resolution MS/MS and NMR spectroscopy confirmed that QS-21-Rha differs from QS-21 by the C3 terminal rhamnose substitution of xylose. Both <i>in vitro</i> and <i>in vivo</i> studies demonstrated that QS-21-Rha elicits strong adaptive immune responses, notably robust CD4⁺ T cell activation with immunostimulatory potency comparable to, and exceeding, that of traditionally bark-derived QS-21. From a production standpoint, aerial tissues provide a renewable and higher-yielding source where 1 kg of QS-21 can be obtained from pruning 200 young shrubs, compared with ~1,700 kg of bark, which is equivalent to debarking 100-120 mature trees. These results establish QS-21-Rha as a chemically defined, immunologically active, and more sustainable vaccine adjuvant candidate to potentially address both supply-chain resilience and global vaccine access.