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Vaccine production
Besides production of hormones and pharmaceutically important proteins, recombinant vaccine production rapidly has become an important target in biomedicine. Until about two dozen years ago, all vaccines were inactivated or innocuous virus strains. Upon injection of such inactivated viruses into a human, antibodies against this virus are produced, thus making this person resistant to the real virus if it strikes. However, occasionally viral inactivation upon vaccine production was incomplete, and vaccination therefore carries some risk. Genetic engineering, however, makes it possible to have single proteins of the virus produced in microorganisms; injection with these proteins also leads to the production of antibodies and, thus, to immunity to the virus. The obvious advantage of this approach is greatly improved safety of the vaccine. Much progress is being made in recombinant vaccine development (for example, see recent articles on http://www.dnavaccine.com), but as it takes several years for clinical trials and approval, to date the number of products on the market) is quite limited. Success stories in the fight against virus-related diseases in humans include development of a recombinant vaccine against hepatitis B. However, the recombinant vaccine field also has encountered a number of difficulties related to, for example, correct presention of recombinant antigens to the immune system, the limited lifetime of early engineered protein in the body, etc. Some examples of the recombinant vaccines have been described below.
A new generation of rabies vaccine
Rabies remains a major problem in the world today. The extensive reserve of the rabies virus in wild animals such as foxes and raccoons renders the control of the disease extremely difficult. Existing vaccines for humans are extremely expensive, and this has always prevented any large-scale preventive use, especially in the countries where rabies remains a major health problem.
After overexpression of a rabies glycoprotein (which is on the outside of the virus) in a microorganism, the highly expressed protein was used as antigen to raise antibodies. However, it was ineffective as a vaccine. The next idea was to incorporate the gene for the glycoprotein into a recombinant vaccinia virus. The vaccinia virus, leading to pox in cows, and immunologically related to the virus leading to smallpox in humans, has been shown in 1796 (by nurses working with Jenner) to be a wonderful vaccine for smallpox, and perhaps such a virus would also work to raise an immunoresponse against the protein from another virus when it is present in the vaccinia coat. This idea appears to have worked well, and after introduction of the rabies glycoprotein gene into the vaccinia virus, laboratory animals (mice and rabbits) were infected with a little bit of the live recombinant vaccinia virus. The animals remained healthy and became immune against the rabies virus. The rabies glycoprotein gene can also be engineered into different viruses, in some cases leading to multifunctional vaccines (for example, see http://www.scienceinafrica.co.za/2001/nov/caprirab.htm).
Vaccine production in plants
As a new direction in the development of affordable new vaccines, transgenic plants have been developed that express surface proteins of viruses that are pathogenic to animals or humans. For example, Agricultural Genetics in England has genetically engineered the cowpea mosaic virus to include a surface antigen from the foot-and-mouth disease virus; this virus affects lifestock. This genetically engineered virus was used to infect its natural host, black-eyed pea, and the introduced gene from the foot-and-mouth disease virus was expressed handsomely in the plant. The cowpea mosaic virus eventually kills the plant, and therefore the plant needs to be sacrificed a few weeks after infection. One leaf from the infected pea plant produces enough surface antigen to serve as vaccine for 200 doses.
To further improve the ease of vaccine production as well as vaccination, research teams have been trying to develop edible vaccines that could make vaccination programs as common as snacking on a banana or munching on alfalfa sprouts. One of the pioneers in this area is Charles Arntzen, who is on the faculty at ASU. A PBS interview with him regarding edible vaccines and plant biotechnology in general can be found at http://www.pbs.org/wgbh/harvest/interviews/arntzen.html. The September 2000 issue of Scientific American contains a nice review article on edible vaccines (http://www.sciam.com/article.cfm?articleID=000A9962-D220-1C73-9B81809EC588EF21&catID=2). Because edible vaccines would not need the purification, strict refrigeration, and injections that make conventional vaccines expensive to use, the edible vaccines would encourage preventive medicine in the Third World. However, there are lots of regulatory and safety (dosage) issues associated with “being vaccinated via a banana”, and current research is directed more toward oral (or otherwise simple) vaccination with more or less pure compounds that were produced inexpensively. However, oral vaccines, such as the polio vaccine, are difficult to develop, as one needs antigens that pass various challenges in the midgut, and only certain antigens can trigger a normal immune response in the mucosal gut before the enzymes and acids in the gastrointestinal system destroy them. Tomatoes and lettuce have now been transformed to produce hepatitis B surface antigens, and these appear antigenically similar to hepatitis B proteins isolated from infected humans. Also, the gene for the highly antigenic, but non-toxic, beta chain of the cholera toxin has been expressed in alfalfa, and mice fed with this alfalfa have been found to have an immune response against cholera toxin. If successful, this approach will yield low-cost vaccines against cholera and other devastating diseases. Cholera epidemics that still occur in Africa illustrate the dire need for vaccination in developing countries against diseases that are much less common in the US and Europe.
AIDS
In spite of the tremendous effort now in progress all over the world to develop a vaccine against HIV, the virus group causing AIDS, the results so far have been quite disappointing. The genetic instability of HIV viruses, as well as the risk of recombination with wild-type strains, will forbid the use of attenuated HIV viruses as vaccines. Genetically engineered vaccines are under way: the major surface antigen of HIV, coded for by the env gene, has been expressed in a variety of cells (yeast, E. coli, mammalian cell culture). However, the gene product has a poor immunogenicity. Moreover, the gene has been introduced into vaccinia virus. This has not led to high antibody production either. Unfortunately, it looks like the development of a HIV vaccine will have to follow a long and complex road before an effective vaccine will have emerged. The dramatically increased research efforts in this area in both industry and universities may contribute to a minimization of the time involved in getting there, but unfortunately that does not help the patients now suffering of AIDS (most of them in the developing world), and they will need to continue to depend on drugs such as nucleoside analogs and particularly protease inhibitors (which are getting increasingly effective, by the way) that are not affordable for most of the patients (except those in the developed world).
In May 1997, President Clinton pledged that there will be an AIDS vaccine in 10 years, i.e., by the year 2007. This pledge was probably politically motivated rather than based on scientific progress, as signs thus far are not at all promising that indeed a safe vaccine will have been developed by 2007. Updates on AIDS vaccines can be found at http://www.iavi.org/ (follow the "vaccine science" link).
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Center for Bioenergy & Photosynthesis Arizona State University Box 871604 Room PSD 209 Tempe, AZ 85287-1604
13 February 2006 |
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