Adjuvants enhance the effectiveness of vaccines, and one of the most effective adjuvants is a costly extract from the soap bark tree. In order to reduce the cost and eliminate the difficult extraction process from the bark, synthetic biologists have introduced 38 different genes into yeast to replicate the production of the active molecule, QS-21. This complex chemical has a terpene core and multiple sugars, making it a challenging biosynthetic pathway to insert into yeast.
UC Berkeley and Berkeley Lab synthetic biologists utilized 38 separate genes to introduce into yeast in order to replicate the production of the active molecule QS-21. This molecule is a complex chemical with a terpene core and numerous sugars, making it possibly the longest biosynthetic pathway ever inserted into yeast.
Vaccines have been proven to save lives, especially during the recent pandemic. However, one key component of most vaccines, including the Novavax COVID-19 vaccine, often goes unnoticed: a molecule or compound that prepares the immune system to respond more effectively to infections. These components, known as adjuvants, are added in small quantities.
Having a strong immune system can be particularly important for infants and older individuals, as it can offer a significant level of protection.
However, one of the most powerful adjuvants, which is derived from the Chilean soap bark plant, is extremely challenging and costly to produce. In fact, it can cost several hundred million dollars per kilogram (2.2 pounds).
Researchers from the University of California, Berkeley, and Lawrence Berkeley National Laboratory (Berkeley Lab) have utilized synthetic biology to create the active ingredient of soap bark, known as QS-21, in yeast. This method of production not only reduces costs, but also has less of an impact on the environment.Friendly and non-toxic methods are being used to extract the compound from plants, reducing the need for harmful chemicals.
Although the amount produced from the yeast-based process is currently small, it shows potential to increase the availability of one of the most effective adjuvants and decrease the overall cost of vaccines.
“During the pandemic, there was a concern about the availability of the QS-21 adjuvant because it only comes from one tree,” stated Jay Keasling, UC Berkeley professor of chemical and biomolecular engineering and senior faculty scientist at Berkeley Lab. “This new method could have a significant impact on global health.”Perspective, there is a significant demand for an alternative source of this adjuvant.”
The process of creating QS-21 involved incorporating 38 different genes from six organisms into yeast — creating one of the longest biosynthetic pathways to ever be transferred into any organism, according to Keasling.
“The production of the powerful vaccine adjuvant QS-21 in yeast demonstrates the capability of synthetic biology to tackle major environmental and human health challenges,” said former UC Berkeley postdoctoral fellow Yuzhong Liu, who was the first author of the paper and is now an assistant professor at Scripps Research inLa Jolla, California.
The findings will appear in the May 8 issue of the Nature journal.
Expanding on previous malaria research
The advantage of including an adjuvant in a vaccine was first recognized in the 1920s, when alum, an aluminum salt, was found to enhance the efficacy of a diphtheria vaccine. Since then, alum has been incorporated into many vaccines that utilize a portion of a pathogen (although not the infectious part) to stimulate immunity. Adjuvants, by making vaccines more effective, also enable doctors to administer smaller doses of the active ingredient, known as an antigen.
Shortly after alum was identified to boost the efficacy of a diphtheria vaccine, it was found to have similar effects when used in a vaccine against a different disease.The effectiveness of vaccines has been highlighted by the discovery of soap-like molecules that have a similar impact. In the 1960s, researchers honed in on an extract from the Chilean soapbark tree, known as Quillaja saponaria, which activates various components of the immune system to enhance the effects of a vaccine antigen. For the past 25 years, one specific component of this extract, QS-21, has been a key non-aluminum adjuvant in vaccines, and has been tested in over 120 clinical trials. It is present in the shingles vaccine (Shingrix) for older adults, as well as in a malaria vaccine (Mosquirix) used in children for protection against malaria.Parasite Plasmodium falciparum, and the Novavax SARS-COVID-19 vaccine.
QS-21 is currently produced by removing bark from the tree and chemically extracting and separating its various compounds, some of which are harmful. Although QS-21 is a complex molecule that contains a terpene core and eight sugar molecules, it has been artificially created in the laboratory. However, this involves a 79-step process, starting from an intermediate chemical that also needs to be synthesized.
Keasling, who serves as the CEO of the U.S. Department of Energy-funded Joint BioEnergy Institute (JBEI) in Emeryville, Calif., wasThe researcher was asked to replicate the synthesis process in yeast, as he has experience in adding genes to yeast to produce terpene compounds, including artemisinin, a drug used to treat malaria, as well as various scents and flavors. Terpene compounds, such as those found in pine trees, are often known for their pleasant aroma.
“This project is an extension of our previous work on malaria,” he explained. “We initially focused on malaria treatment, but now this could potentially assist in the development of malaria vaccines in the future.”
<p.Adding the eight sugars presented a challenge, as did managing unexpected interactions between enzymes in yeast. All of this had to be achieved without disrupting the overall balance.critical metabolic pathways are essential for yeast growth. According to Keasling, the artemisinin biosynthetic pathway pales in comparison to the complexity of the eight sugars and terpenoid found in the QS-21 molecule. He expressed satisfaction in the progress of synthetic biology, which has now advanced enough to create a pathway for producing QS-21. This achievement reflects the significant progress made in the field over the last two decades. Keasling and his team collaborated with plant researcher Anne Osbourn at the John Innes Center in the United Kingdom, who had previously identified the enzymatic steps involved in the production of QS-21.The production of natural QS-21 by the pbark tree has been a focus of research for Osbourn and Keasling’s lab for the past five years. As Osbourn made new discoveries in the process and tested them in tobacco plants, Keasling’s lab incorporated these new genes into yeast to mimic the synthetic steps.
Keasling highlighted the successful collaboration, stating that as soon as Osbourn identified a new gene in the pathway, it was sent to their lab to be inserted into yeast. This collaboration also provided Osbourn with a way to verify the accuracy of her tobacco assay.
‘Everything from a single sugar’
In a recent publication, Osbourn and Keasling outlined the complete 20-step process.The soapbark tree produces QS-21, which can be reconstituted in tobacco, but tobacco is not a practical way to mass-produce this chemical compound due to its limitations as a test bed for plant chemistry. The new study describes a process to reconstitute QS-21 in yeast, with additional steps needed because yeast lacks certain enzymes found in plants. Currently, a liter of bioengineered yeast can produce about 100 micrograms of QS-21 in three days, with a market value of about $200. This method is more scalable than using plants. According to Keasling, even at current production levels, it is cheaper than producing QS-21 from the plant.The scientist developed yeast that can survive on sugar alone, which he sees as a major advantage. He explained, “My goal is to create everything from a single sugar. I just want to give yeast glucose, because we ultimately want this process to be scalable. If you give them a variety of complex compounds, it will lead to a process that cannot be scaled.” Keasling added, “Ultimately, I prefer to start with glucose, so that when production is carried out in large tanks, they can produce QS-21 as easily and inexpensively as possible.”
Keasling intends to leave the fine-tuning of the process for large-scale production to others, as he focuses on the initial development.The scientist hopes to modify the enzymatic processes he has implemented in yeast to create different versions of QS-21 that may have greater effectiveness. The use of yeast biosynthesis allows him to experiment with altering the QS-21 molecule to determine which parts can be removed without impacting its effectiveness.
This study was funded by a grant from the industry.
Â
