Can plant-based vaccines help in the fight against COVID-19?
As potential COVID-19 vaccines enter human clinical trials, people have started to consider another problem: Once a vaccine is available, how can it be produced in sufficient quantities to meet the needs of billions of people around the world?
Plant-based vaccines use genetic engineering technologies to introduce the antigen genes of pathogenic microorganisms into the plant genome, allowing the antigen protein to grow steadily within the edible part of the plant. This then creates an edible vaccine that can be taken orally.
Since the birth of the world's first genetically engineered plant in 1983, more than 100 pathogenic microorganism antigen genes have been integrated into the plant genome, and some genetic products have been confirmed as immunogenic via animal experiments. Reports show that several dozen vaccines have now been produced in plants using genetic engineering technology. These plant-derived vaccines can be produced to target such contagions as the hepatitis B virus, foot-and-mouth disease virus, rabies virus glycoprotein, tubercle bacillus and rotavirus.
Studies have shown that genes encoding hepatitis B virus surface antigens can be successfully expressed in tobacco, tomato, potato, lupin and lettuce plants. In recent years, Chinese researchers have introduced surface antigen genes for the hepatitis B virus into cherries, apples, potatoes and tomatoes to create genetically modified plants and express active hepatitis B virus surface antigen proteins. At present, the highest expression of the surface antigen is in potatoes.
Luo Wen and Liu Ruiqi with the School of Biotechnology at Xi'an University introduced the main steps in developing plant vaccines. These involve cloning the target gene and connecting it to a vector that can be expressed in plants to construct an efficient plant expression vector; introducing foreign genes into plants, allowing the plants to carry specific antigen-encoding genes, achieving genetic transformation of plant cells, and culturing tissue of recipient cells and plant regeneration via genetic engineering technologies; detecting the expression of antigen protein genes in plants; and isolating, purifying and conducting purity identification of the target products; as well as testing the antigenicity and immunogenicity of plant expression products.
Guan Zhengjun at the Key Laboratory of Resource Biology and Biotechnology in Western China, a lab jointly-built by China's Ministry of Education and Northwest University, told Science and Technology Daily that an edible vaccine based on genetically modified plants is one of the most promising directions for producing a new vaccine. Compared with traditional vaccines, plant-based vaccines are not only cheap, but the antigenic proteins expressed in them do not require extraction, purification and refrigeration, which greatly reduces the cost of production and transportation, and they are also relatively safe and effective.
Plant-based genetic engineering technologies have gradually matured and brought broad prospects for the development of oral vaccines. For example, tomatoes — which are delicious, inexpensive and nutritious — are one of the most commonly used receptor materials for genetic engineering technologies. The ability for them to be eaten raw also prevents possible damage to foreign proteins during the heating process.
However, at the same time, the safety and reliability of vaccines are always foremost concerns. In recent years, people have seen that there are still some uncertainties surrounding plant-derived oral vaccines, such as low expression of foreign proteins, possible digestion and degradation when taken orally, and safety issues. As such, anticipation for the technology has somewhat declined. How to solve these problems has become a major test for the development of future plant-based oral vaccines.
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