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Home >> Animal Biotechnology >> Vaccine Production by Biotechnology Methods >> Vaccine Production by Biotechnology Methods Introduction
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Bio technological Approaches To Vaccine production
Introduction
At present, the majority of veterinary vaccines are produced by conventional methods similar to those implemented by Jenner or Pasteur. These include live, attenuated vaccines and killed or inactivated vaccines. Both of these types of vaccines have proven to be effective particularly in reducing the clinical manifestation following exposure to virulent filed strains of the pathogens.
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One of the important impediments in the case of live vaccines is to ensure that the organism is attenuated sufficiently not to cause the disease, but still replicate to a sufficient level to induce an appropriate immune response. However, only a limited number of viral disease can be prevented by live attenuated viral vaccines state and most DNA-containing viruses have the potential to establish persistent (or latent) infection. New viral strains may arise by recombination of the vaccine virus with other viral strains in animal populations; pregnant animals or their offspring may be adversely affected by the vaccine strain.
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Certain apparently avirulent viral strains can revert to virulence, either by host induced proteolysis or surface proteins of the phenotype, as in the activation of the HN and F0 proteins of influenza virus or by mutations in the genotype, as in the reversion of attenuated oral Sabin poliovirus vaccine (Kew et al., 1981). Similarly the so called inactivated foot and mouth disease (FMD) viral vaccines are found to be infectious far frequently. For example, at least 44% of the outbreaks of FMD in Europe from 1968 to 1961 were caused by incompletely inactivated vaccines or due to escape of virus from vaccine manufacturing facilities (FAO, 1981).
| | Similarly, outbreaks of FMD in three South American countries in 1979 and 1980 were caused by three different vaccines that contained infectious virus (Casus, 1981). In addition, conventional whole agent vaccines, particularly crude preparations, have been implicated in post vaccinal pyrogenic and allergic reactions, abortions, Guillain-Barre neurological syndromes and adverse sequelae.
Conventional killed or inactivated viral vaccines also have potential risks, for example, incompletely or improperly killed batches of virus could result in contamination of the vaccines with active wild, type virus or with contaminating virus in the vaccine. Therefore, elaborate safety testing is a crucial and costly part of the production process.
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Improvements in conventional biochemistry, recombinant DNA technology, peptide synthesis, molecular genetics and protein purification had laid the foundation for the development of new vaccines which should be more efficacious, cost effective and which have fewer side effects.
Animal vaccines must be cheap, easy to administer and effective. To produce a vaccine, genetic engineers move genes around in the infectious organisms to separate the pathogenic factors (components that cause the disease) from the antigens (components that stimulate an immune response).
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Five strategies are currently being applied to the generation of new types of vaccines:
1. Recombinant DNA cloning of immunogenic surface protein
2. Chemical synthesis of polypeptide vaccines
3. Construction of recombinant vaccines having guest genes for foreign surface proteins.
4. Genetic engineering of non pathogenic mutant agents
5. Production of monoclonal epitopes of the surface proteins of infectious agents
6. Nucleic acid vaccines
One approach is to remove virulent genes from the infectious organisms. Scours is a disease caused by E. coli that affects newborn calves and piglets. The resulting diarrhoea and severe dehydration can cause heavy mortality. Disease, causing strains of the bacteria produce one protein which allows the bacteria to adhere to the gut of the young animals and another which governs the water loss. If the gene for the water loss protein is removed, scours can be prevented. The organism sticks to the gut without causing diarrhoea and acts as a vaccine, stimulating an immune response against the adhering protein. The vaccine is often given to pregnant animals which pass immunity to their offspring.
Recently, recombinant DNA technology has helped to develop new generation vaccines, which are cheaper, safer and more effective. Some vaccines are made not by disarming the pathogen, but by transforming the genes coding for the antigens of pathogen into those coding for harmless characteristics. This method has been used to produce rabies vaccine. The gene Jor the surface glycoprotein of the rabies virus is inserted into the DNA of another virus vaccinia. Vaccinia virus causes cowpox, but is relatively harmless to dogs. It acts as a vector, transporting the piece of rabies virus RNA into a vaccinated individual and subsequently producing rabies antibodies. This stimulates a protective immune response against rabies. The steps involved in the production of recombinant vaccines are given in Figures 3.1a and b.
Subunit vaccines are produced by genetic engineering. They are purified single proteins from the surface of a pathogen which can be produced cheaply in fermenters. The great advantage of subunit vaccines is that they contain no live, potentially infectious organisms. The synthetic vaccines are advantageous because the immune system of the animal is challenged with only one antigen, thereby omitting other components of the virion that might adversely affect the immune response. The major drawback with subunit/peptide vaccines is that the antigenic mass cannot be greater than the amount injected. There is no amplification of the antigen. The conventional vaccine for FMD (killed vaccine) was responsible for nearly half of the outbreaks of the disease in Europe between 1968 and 1981, because it contained a low percentage of live organisms. It may now be replaced by a subunit vaccine.
Many important antigens are proteins and can be made in harmless organisms through genetic engineering. Genetic engineering also provides a way to produce vaccines even if the infectious organisms cannot be grown in animals or in artificial cultures. Genetic engineers have taken a gene that codes for a surface protein (hepatitis B surface antigen) and inserted it into E. coli or into yeast. When the proteins are produced by gene expression in new hosts, it is easy to purify the whole protein to obtain a genetically engineered vaccine. Only small regions of the proteins called epitopes are bound by antibodies.
Immunity of flu lasts for a year or so, not because the vaccines are no good, but because the virus keeps changing its protein coat. Scientists have investigated several strains of the flu virus, looking for regions that do not vary and were exposed sufficiently to stimulate an immune reaction. They found a peptide of
18 amino acids which they knew, from the protein structure studies, was located in an exposed part of a viral protein called haemagglutinin. This region was short enough to be produced chemically in a peptide synthesizer. Joining this synthetic peptide to a carrier protein provides a vaccine that protected mice from several different strains of the influenza virus.
Protein and peptide preparations are dead vaccines. Live vaccines can also be produced by genetic engineering. If genes from other organisms are inserted into the DNA of vaccinia, the virus used as a vaccine will produce the corresponding proteins as it grows inside human cells. Experimental vaccines to protect animals against infection from rabies, herpes, hepatitis B and influenza have been produced in this way. Up to 25 genes can be inserted into vaccinia DNA and there are plans to produce multiple vaccines to confer immunity to several diseases simultaneously.
Anti idiotype antibodies have been produced against very many antigens and are being used as an antigen. One of the biggest advantages of this is that the virulent inactivated microbe need not be used, instead anti-idiotypic antibodies could be used for development of immune response. Anti-idiotypic antibody mimics the structure of the original antigen. Anti-idiotypic MAbs have also been evaluated as immunogen.
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