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Engineered Oilbodies Produce Vaccines and Adjuvants
Recent problems with the US supply of flu vaccinehave increased interest in improving vaccine production methods.SemBioSys Genetics, Inc., (Calgary, Alberta, Canada,www.sembiosys.com), has justpatented an rDNA vaccine manufacturingmethod using plant-seed oilbodies to produce a single structure thatcontains both a vaccine antigen and an adjuvant.
The US patent (number 6,761,914) covers two approaches for attachingantigens to the surfaces of oilbodies—the natural oil-storageorganelles found in the seeds of oil-producing plants such as safflowerand sunflower. Wheninjected into mice, the antigen-coated oilbodies produced a higherimmunological response than either antigen alone or a simple emulsionof oilbodies and antigen.
“It’s really that discovery—that this assembly seems to give apotentiation of the immunological response—that is the basis of thispatent,” says Maurice Moloney, PhD, the chief scientific officer andscientific founder of SemBioSys. “If you think about the chemicalcomposition of these oilbodies, they are things that the human body isperfectly used to seeing, such as triglycerides and phospholipids,” heexplains. “So what we’re really doing is introducing a structure thatappears to fool the immune system into thinking that a whole organismhas gotten in. The particle also happens to be about the same size asmost pathogenic bacteria.”
Adjuvants generally are combined with antigens in an emulsion, toincrease the immune response to vaccines. Moloney says initial studiesindicate that the oilbodies show efficacy similar to alum adjuvants(the only type approved by the US Food and Drug Administration forhuman use) and Freund’s complete, a more rigorous adjuvant used inveterinary vaccines.
Producing the structures
SemBioSys uses two basic methods to produce the oilbody–antigenstructures. In one method, recombinant DNA inserted into plants isexpressed to produce a recombinant protein that binds to oleosin, aprotein that naturally occurs on the surface of oilbodies.“The protein basically has two domains,” explains Moloney. “One domainis responsible for ensuring that the protein ends up on the oilbody,and the other domain is the protein of interest.” The company isalready using this method on an experimental basis to manufacturerecombinant proteins such as insulin and apolipoprotein A, a potentialtreatment for cardiovascular disease. Those nonvaccine proteins arethen cleaved from the oilbodies, harvested via centrifugation, andpurified using conventional downstream processing.
In some cases, however, the plant will not attach the recombinantproteins to the oilbodies in vivo,and they are expressed into the cellcytoplasm instead. The company has developed a method for attaching theproteins to the oilbodies after harvesting. When the cell is disruptedand the cellular compartments are broken up, the targeting sequence onthe protein causes it to attach to the oleosin. If necessary, therecombinant protein can be produced in another organism, as long as theprotein contains the required targeting sequences.
The nature of the oilbodies simplifies the downstream purificationsteps. Because the extraction uses a water-based system, the oilbodiesnaturally float to the top when centrifuged. “The equipment we use isvery similar to what you find in a dairy for removing cream from milk,”notes Moloney. Keeping the oilbodies intact during harvesting doespresent a challenge, but SemBioSys has already developed a system fordoing so with ~90% efficiency.
For the additional purification to remove undesired proteins from theoilbodies, the costly urea-based laboratory systems can replaced atscale-up with several rounds of washing with inexpensive sodiumcarbonate and phosphate buffers.
Comparison to other subunit vaccines
SemBioSys’s methods may offer an alternative method for producingsubunit vaccines. Although subunit vaccine are more expensive tomanufacture than vaccines that contain the whole pathogen, they aredesirable because they reduce the likelihood of adverse reactions.
Because plants are grown in the field, plant-based subunit vaccineseliminate the start-up capital costs of fermentation vessels andcontrol systems needed for bacterial- and yeast-based systems. Althoughthe downstream purification of plant systems are more complex, the easeof separating and purifying the oilbodies may keep those costs down. Inaddition, plant systems may be able to handle certainpost-translational protein modifications and folding that neitherbacterial or yeast systems can maintain.
The main weakness of using plant systems to produce antigens is speed.It takes 18 months to two years to scale up these production systems,so they are not practical for producing flu vaccines, the strains forwhich change every year. The company is working on methods foraddressing this limitation, however, and plans a new patent filingsoon. In the meantime, SemBioSys signed an agreement with DowAgroSciences in May 2004 to apply its technology for the development ofan animal vaccine.