In:
Science, American Association for the Advancement of Science (AAAS), Vol. 379, No. 6638 ( 2023-03-24), p. 1252-1264
Abstract:
Adjuvants are immunostimulatory substances that are added to vaccines to boost the immune response. Until recently, the only adjuvants available for clinical use were aluminum salts, either alone (alum) or in proprietary mixtures, and oil-in-water emulsions containing squalene, which have relatively low potency and substantial side effects, respectively. Natural surfactants (saponins) have been used as adjuvants in veterinary medicine for almost a century. The first saponin adjuvant, AS01, has recently been approved for use in the human shingles vaccine Shingrix and the malaria vaccine Mosquirix. It is also a promising adjuvant candidate for a tuberculosis vaccine. AS01 is a liposome-based formula that contains monophosphoryl lipid A and a saponin known as QS-21, which act synergistically to induce strong antibody and helper T cell responses. QS-21 is one of 〉 100 structurally related QS saponins produced by the Chilean soapbark tree ( Quillaja saponaria ). These include QS-7 and QS-17, which, together with QS-21, are included in the Novovax NVX-CoV2373 COVID-19 vaccine. Because of their chemical complexity, the only current commercial source of QS saponins is the bark of the soapbark tree. The increasing demand for these highly valuable molecules has led to growing interest in the possibility of developing environmentally sustainable, “free-from-tree” strategies to access these compounds in other ways. RATIONALE The biosynthetic pathway for QS saponins is not known. Elucidation of the genes and enzymes from the soapbark tree comprising the “instruction manual” for making QS saponins would provide insights into how these molecules are biosynthesized and diversified. It would also pave the way for new routes to access and engineer natural and new-to-nature immunostimulants. RESULTS We first sequenced the genome of the soapbark tree. Then, using genome mining and combinatorial transient expression in Nicotiana benthamiana , a wild relative of tobacco, we identified a total of 16 enzymes that together enable the production of advanced QS pathway intermediates that represent a bridgehead for adjuvant bioengineering. We also discovered a further three enzymes needed to make QS-7, a saponin with comparable adjuvant properties to QS-21 and low toxicity that is present in low abundance in Q. saponaria bark extract. Our work opens up the possibility of producing QS-7 and other related QS molecules in a heterologous expression system. CONCLUSION We have elucidated the pathway for the biosynthesis of advanced QS intermediates. Using our transient plant expression system, we were able to purify all of the intermediates at preparative scale, showing the power of this platform for rapid access to these molecules. We further demonstrate the production of the vaccine adjuvant QS-7. A number of plant natural products, including QS saponins and foxglove cardiac glycosides, have a d- fucose moiety, but the biosynthetic pathway to this sugar in plants was not previously known. In the present work, a route to d- fucosylation was discovered in which d- fucose is formed in situ from a 4-keto-6-deoxy- d- glucose attached to the saponin scaffold rather than being synthesized before transfer. Although the levels of QS-7 produced in N. benthamiana were low, they were comparable to those found in many tissues of Q. saponaria , with the exception of bark, which is about threefold higher. The availability of the complete gene sets for the biosynthesis of the heptasaccharide triterpene glycoside bridgehead QS saponins and QS-7 now present an opportunity for future endeavors to optimize production in heterologous expression systems with the ultimate aim of attaining commercial-scale yields. The availability of the complete genome sequence of Q. saponaria , coupled with our powerful and rapid transient expression platform, open up opportunities to access QS-21 and a diverse array of other saponins. This provides the potential to ultimately generate designer saponins with optimal immunostimulatory activity and low toxicity using metabolic engineering approaches. Genomics-driven elucidation of saponin biosynthesis in the soapbark tree. The soapbark tree ( Q. saponaria ) produces 〉 100 structurally related QS saponins, including the vaccine adjuvants QS-7 and QS-21. Transient combinatorial expression of a total of 16 enzymes in tobacco enabled the production of an advanced bridgehead for adjuvant bioengineering that can be used to access QS-7, paving the way for new routes to access and engineer natural and new-to-nature immunostimulants.
Type of Medium:
Online Resource
ISSN:
0036-8075
,
1095-9203
DOI:
10.1126/science.adf3727
Language:
English
Publisher:
American Association for the Advancement of Science (AAAS)
Publication Date:
2023
detail.hit.zdb_id:
128410-1
detail.hit.zdb_id:
2066996-3
detail.hit.zdb_id:
2060783-0
SSG:
11
