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 Quillaja saponaria 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 Q. saponaria 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 in vitro and in vivo 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.