The Use of Plants to Make Vaccines: Part-3 is the third and last of the 3-set series of articles discussing issues involved in making vaccines in plants. In this Part-3 article we will focus on research efforts in plant engineered vaccines; problems encountered in making plant-based vaccines; and the steps involved in making vaccines in plants.
More than 100 recombinant proteins including plant-derived vaccine antigens and antibodes have been expressed in different plant tissues.
Plant based systems for making vaccines are in effect a totally bio-contained sterile environment. Therefore there is no possibility of contamination with animal viruses and these vaccines are free of extraneous agents. Plant based systems are suitable for making non-replicating subunits which cannot change again to virulence. Plant based vaccines are stable and therefore safe from the storage and handling perspective. They are environmentally safe and bio-secure.
Hepatitis E vaccine derived from plants is a cost effective solution (from the delivery and administration point of view) for combating hepatitis E as the disease mainly occurs in poor countries. Enteric non-A, non-B (NANB) hepatitis is a scourge in poor countries (and even in rapidly developing countries like China) affecting adults in the 15 to 40 age group and the causative factor is HEV which is a difficult pathogen to culture. This HEV infection could lead to fatal hepatitis----and currently there is neither antiviral drug nor vaccine for this, mainly because, culturing the pathogen is difficult, and this is a constraining factor in getting live attenuated strains for making the vaccine. So the strategy has been to develop recombinant subunit vaccine.
Plant-based vaccines approved by regulatory authorities
Dow AgroSciences' ConcertTM vaccine to counter Newcastle disease virus is possibly the first plant-made vaccine to get US regulatory approval. Dow developed this product in a cultured tobacco cell line and it comes as a purified injectible product. That apart, Cuba has succeeded in making scFv mAb required for making recombinant HBV vaccine in transgenic tobacco plants, obviating the need for using large numbers of mice. These are probably the only two instances of plant made vaccines approved by regulatory authorities.
Other Research Efforts in Plant Engineered Vaccines
Over eighty percent of adults are prone to tooth decay. A combined effort of Guy's Dental Institute and others has resulted in a vaccine expressed in GM tobacco plant-------and this also happens to be the first GM plant vaccine that has reached the clinical trial stage. This tooth vaccine essentially gets rid of harmful bacteria and it stays relevant until the patient takes antibiotics---in which case the vaccine is no longer effective. Similarly, plant based HIV and rabies vaccine have reached the human clinical trial stage.
Problems encountered in making vaccines in plants
One of the problems in making vaccines in plants is the fear that these genetically modified plants would make their way into the normal food sources. For example, people who don't need the vaccine may end up eating it. Growing plants in captivity is one option, the other is choosing plants that do not breed through seeds or pollen --- and in this context the banana (for example vaccine for hepatitis B virus expressed in bananas) is the right candidate as the banana plant does not breed outside its immediate surroundings and children tend to like it as well. Besides, it is in the tropical areas that vaccines are most needed and the banana plant grows well thereabouts although on the negative side it takes about 12 months to bear fruit.
The key to efficacy of vaccines is the dose ---- overdose can elicit propensity for tolerance of disease, while insufficient quantity of vaccine will not serve the purpose of effective immunization. Currently when plants are used to make vaccines, getting the plant to make the right quantity of vaccine is one of the problems. The quantity of vaccine is especially important as oral proteins have a tendency to get absorbed by the digestive system before reaching the immune system------so the plant component that is proposed to be eaten as vaccine supplement must reckon with this factor too. Vaccines administered orally are also susceptible to immune tolerance.
During the making of plant-derived vaccines, what if genes that code for resistance to antibiotics or other substances get inadvertently transmitted? Although the likelihood of plant viruses infecting humans is nil, the risks of viruses from plant pests impinging the manufacturing process is a possibility.
Research Institutions/Companies Engaged in Plant-based Vaccine Research
The Dow Chemical Company has received the first ever regulatory approval for a vaccine that is made from plant cells and not from the whole plant ---- which means it obviates concerns regarding using whole crop plants for making vaccines. Their patented Concert(TM) Plant-Cell-Produced System is effectively a bio-contained production system that makes subunit vaccines using plant cells and not animal cell culture or fertilized eggs.
Today, even green plants that have not been genetically modified are used to produce vaccines on a commercial scale. An example is the first cGMP compliant plant based manufacturing facility of iBio, Inc. in Newark, Delaware for protein production (using their proprietory gene expression technology in green plants called iBioLaunch™ platform) where thousands of plants can be grown in one batch in multi-plant trays. This is a classic example of a biological process that has been extrapolated to an industrial process obviating culturing and fermentation that is required in the classical method of making vaccines. This plant-based vaccine factory has technological affiliation with Fraunhofer-Gesellschaft using the latter's plant viral technology to make proteins in quickly growing plant biomass. It works this way. The launch vectors made of a c DNA sequence that encodes the target protein are put into agrobacteria (plant viral gene expression vector).Using vacuum infiltration, these launch-vector loaded agrobacteria are impregnated in the aerial parts of green plants. This ensures full leaf coverage and enhances protein production trends. When the agrobacteria has infiltrated the plants, what happens is virus replication amplifies both viral sequences of launch vectors and the cloned target sequences. Then consequent to translation of these recombinant viral vectors mRNAs, the target protein gets expressed in fresh plant tissue especially leaf and stem tissue. This is called transient expression. The term "transient" conveys the meaning that the target gene sequence doesn't get fused with the plant chromosome and consequently doesn't involve the making of a transgenic plant. Thereafter the expressed plant's biomass is plucked, ground and an extract of the active protein substance is obtained for further purification.
Steps involved in making vaccines in plants
What are the steps involved in creating a plant that makes vaccines?
Making vaccines in plants entails controlling a plant's auto immune response mechanisms in order to facilitate the expression of vaccine antigens in those plants. First identify the target protein. In other words, the first step is to identify the antigenic protein that is required to counter a specific pathogen. Then identify and isolate the gene that codes for that protein and connect it to an appropriate regulatory sequence which can sustain good levels of gene expression and insert the whole thing into the plant's genome-- so that it becomes part of the plant cell's genetic make-up. This can be done as follows.
a) The gene is inserted into a plant vector so that new DNA can be transferred into plant cells.
b) Electrical discharge or biolistic particle bombardment is used to facilitate gene transfer into plant cells.
These transformed plant cells are made to grow on to callus and then to full plants. Now the plant can be reckoned to have been modified because it can produce the antigenic protein in whole of the plant ---so when fruits of the plant are eaten it is as good as eating the vaccine. The other option is to sow the seeds of these plants to grow other plants in greenhouses and the protein expressed in plant parts can be extracted and purified. The extracted antigens could be spray dried or by other low temperature and low impact drying methods. The extracted and dried antigen could be made available as a single dose food excipient that could be taken orally as a drink. Else the antigen could be made available as a microencapsulated product intended to work in the gut mucosal lining.
Protocols to be maintained while making vaccines in plants
• Several protocols would have to be maintained as regards plant-derived vaccine manufacture.
• Documentation of stages of manufacture
• Microbial limit tests
• How can date of manufacture be defined?
• DNA sequence of the gene in the plasmid construct
• Residual pesticide analysis reports
• How the transgenic plants would be disposed off after use.
• Test for presence genes inducing antibiotic resistance.
Plants intended for producing vaccines should preferably be grown in green houses as that would facilitate easy process validation and recording of key events such as when the cultivation was started and information pertaining to source description of plant material used. Another issue when making vaccine in plants is whether the insertion of the transgene results in expression of unwanted plant toxins or other plant proteins. In this regard, toxicity studies would be essential. That apart, risk of allergic reactions to plant components must be rigidly evaluated. Importantly, plants used for producing vaccines must be reckoned as medicinal plants.
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Part 1: http://www.biotecharticles.com/Agriculture-Article/The-Use-of-Plants-to-Make-Vaccines-Part-1-431.html
Part 2: http://www.biotecharticles.com/Agriculture-Article/The-Use-of-Plants-to-Make-Vaccines-Part-2-432.html