The vaccine adjuvant QS-21 is a key component in many newer vaccines, and supplies are stressed. To ease that concern, researchers recently produced QS-21 in a heterologous expression system in what is thought to be a first. This opens the possibility of tuning the adjuvant as well as making it more accessible.

“Access to environmentally sustainable sources of QS-21 is a key driver,” Anne Osbourn, PhD, senior author, professor at John Innes, an international center of excellence in plant science, genetics, and microbiology in Norwich, U.K., tells GEN.

Currently, QS-21 is extracted from the bark of wild, 10 year or older Chilean soapbark trees (Quillaja saponaria), although suppliers are working to develop plantation- or lab-grown sources.

Acyl chain pathway identified

To produce QS-21 in a heterologous expression system, the team last year identified the steps to make the triterpene glycoside scaffold. Now, Osbourn and colleagues have identified the pathway for the acyl chain, which stimulates T cell proliferation in patients, reconstituting the entire QS-21 pathway in Nicotiana benthamiana, a close relative of tobacco. This opens a path to bioengineer bespoke saponin-based adjuvants.

Specifically, as the Osbourn group described in a recent paper, they identified CCL1, which generates the branched short-chain acyl-CoA molecule 2-MB-CoA, as the starting substrate to biosynthesize the acyl chain of QS-21. They also identified the specific C9 acyl chain monomer used to biosynthesize the QS-21 acyl chain.

Furthermore, they determined that CCL1, PKSs, and five additional enzymes are needed to biosynthesize the QS-21 acyl chain: the ketoreductases KR1 and KR2; the BAHD acyltransferases ACT2 and ACT3; and the glycosyltransferase UGT73CZ2, which catalyzes the final step in the QS-21 pathway.

To increase yield in the host, the team generated a proline-to-leucine substitution at position 519 of threonine deaminase, which the literature reports results in a 140+-fold increase in free isoleucines. In the leaves of N. benthamiana, the increase in L-isoleucine varied from seven- to 18-fold.

“The [resulting] molecule is the same as the molecule from Quillaja,” Osbourn says, based upon multiple tests. Consequently, in the future with optimization, the biopharma industry will be able to produce “free-from-tree” QS-21 and, importantly, engineer saponins to exhibit specific features, such as enhanced immunostimulation or low toxicity.

“QS-21 is one of the most potent adjuvants known,” the paper notes, based on its ability to cause cytotoxic T cells to proliferate. As a saponin-based adjuvant, it works synergistically with monophosphoryl lipid A (a component of ASO1 formula used in GSK’s shingles vaccine, Shingrix, and its malaria vaccine, Matrix-M, and other vaccines) “to create strong antibody and helper T cell responses.”

Osbourn is forming a company based on this work.

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