Complete biosynthesis of the potent vaccine adjuvant QS-21.

QS-21 is a potent vaccine adjuvant currently sourced by extraction from the Chilean soapbark tree. It is a key component of human vaccines for shingles, malaria, coronavirus disease 2019 and others under development. The structure of QS-21 consists of a glycosylated triterpene scaffold coupled to a complex glycosylated 18-carbon acyl chain that is critical for immunostimulant activity. We previously identified the early pathway steps needed to make the triterpene glycoside scaffold; however, the biosynthetic route to the acyl chain, which is needed for stimulation of T cell proliferation, was unknown. Here, we report the biogenic origin of the acyl chain, characterize the series of enzymes required for its synthesis and addition and reconstitute the entire 20-step pathway in tobacco, thereby demonstrating the production of QS-21 in a heterologous expression system. This advance opens up unprecedented opportunities for bioengineering of vaccine adjuvants, investigating structure-activity relationships and understanding the mechanisms by which these compounds promote the human immune response.

Elucidation of the pathway for biosynthesis of saponin adjuvants from the soapbark tree.

The Chilean soapbark tree (Quillaja saponaria) produces soap-like molecules called QS saponins that are important vaccine adjuvants. These highly valuable compounds are sourced by extraction from the bark, and their biosynthetic pathway is unknown. Here, we sequenced the Q. saponaria genome. Through genome mining and combinatorial expression in tobacco, we identified 16 pathway enzymes that together enable the production of advanced QS pathway intermediates that represent a bridgehead for adjuvant bioengineering. We further identified the enzymes needed to make QS-7, a saponin with excellent therapeutic properties and low toxicity that is present in low abundance in Q. saponaria bark extract. Our results enable the production of Q. saponaria vaccine adjuvants in tobacco and open the way for new routes to access and engineer natural and new-to-nature immunostimulants.