Linda Benedict, Enright, Frederick M., Elzer, Philip H., Hagius, Sue D. | 1/16/2013 10:16:43 PM
Frederick M. Enright, Sue D. Hagius and Philip H. Elzer
The immune system is the body’s defense against foreign invaders such as bacteria, viruses and parasites. The innate immune response is the first line of defense to either stop invaders or to stimulate the body to send in more efficient cells to trigger the adaptive immune response. Components of innate immunity include barriers (skin, mucosa, normal flora), chemicals (the low pH of the gut and urinary tract) and white blood cells, which dump toxic granules on invaders. The adaptive immune response needs to be initiated by an infection or vaccination for the body to develop immune cell types and antibodies, which are proteins that circulate in the blood and neutralize offending microbes. In agriculture, proper animal husbandry includes vaccination to strengthen herd immunity, resulting in healthier and more productive livestock.
Vaccines trigger an immune response, enabling the body to respond to a foreign invader such as a diseasecausing microbe. As early as the 15th century, people purposely exposed others to material from smallpox pustules so that the individual would get a milder form of the disease. Edward Jenner refined this practice in 1794 when he intentionally infected people with cowpox, a virus that normally infects cattle and causes only a slight infection in people. He was able to induce protection against small pox, which is caused by a related, deadly virus. This was the first vaccination as we know it, and the word vaccine is derived from the Latin word for cow, vacca.
To be effective, good vaccines share several characteristics:
There are several vaccine types. They may be live but attenuated (weakened and do not cause severe infection), killed, subunit (only part of the organism is used), recombinant (molecularly modified to delete or add genes) or DNA vaccines (naked DNA). They are used to cause the host to produce protective proteins. The research to develop vaccines is arduous, costly and time-consuming. The offending organism and the immune response required to protect against it must all be understood before an effective vaccine can be developed.
Many of the early bacterial vaccines were made by isolating the pathogenic bacteria from pure culture on laboratory agar plates, killing the harvested organisms with either heat or a chemical agent, and mixing the killed bacteria with an additional component, called an adjuvant, that boosts the immune response to the killed pathogen. Killed viral vaccines have also been available for many years.
Within the past 40 years scientists have discovered how to genetically alter the disease-causing genes in bacteria and viruses, which allows for the development of attenuated bacterial and viral pathogen vaccines. In most cases one or several genes responsible for the diseasecausing function of the bacteria or viral pathogen are removed. The resulting mutated organisms are almost identical to the parent organism but can no longer cause disease. These attenuated vaccines are often patented.
In the early 1990s, the LSU AgCenter patented its first animal vaccine. Lewis Hart, William Todd and Gene Luther developed a novel method to produce an anaplasmosis vaccine. The vaccine effectively protected cattle from the bacteria that cause bovine anaplasmosis. The AgCenter scientists worked on the development of the vaccine for almost 15 years before it was patented. Because it was demonstrated to be effective and safe, the novel vaccine was licensed and marketed by the Mallinckrodt Company as Plasvax. When Mallinckrodt was purchased by another animal health pharmaceutical company, the vaccine was no longer produced, and the right to license the vaccine was returned to the AgCenter. This vaccine is no longer under patent protection but is still being produced as an experimental vaccine with U.S. Department of Agriculture approval by a Louisiana company, University Products LLC, formed by Luther. The company has permission to sell the vaccine to producers in 17 states and Puerto Rico.
In a similar time frame, Fred Enright’s laboratory at the AgCenter collaborated with a laboratory at Virginia Tech University to demonstrate that a laboratory-manipulated strain of Brucella abortus was unable to establish long-term infections in goats, sheep and cattle and did not result in abortions when injected into fetal goats. Further work with the new rough strain, now named RB51, demonstrated that it was not only nonpathogenic but also effective as a vaccine in protecting laboratory animals and cattle from infection with pathogenic B. abortus. The first vaccination safety and protection studies in cattle were carried out at the AgCenter’s farm under Enright’s and Philip Elzer’s direction. This vaccine, unlike the anaplasmosis vaccine, is a live attenuated bacterial vaccine. It is now the official brucellosis vaccine for cattle in the United States and many other countries.
In related research Enright and collaborators in other states developed an irradiated B. abortus RB51 vaccine. This vaccine was produced by gamma irradi irradiating live cultures of RB51. The irradiated bacteria underwent a stress response and remained metabolically active but failed to replicate and were sterile. The metabolic activity and stress response were thought to result in an enhanced immune response, which protected laboratory animals from challenge infections with pathogenic B. abortus. This vaccine is now patented but has not been commercialized.
Another LSU AgCenter scientist, Tom Klei, developed an irradiated larval vaccine for Strongylus vulgaris, a nematode that parasitizes the large intestines of horses. This research was supported by grants from the USDA, Louisiana Board of Regents and a small Colorado-based biotechnology company. The research was never patented because shortly after the development of this vaccine, a worldwide pharmaceutical company commercialized a chemical compound that treated the parasite. This is a case where the potential market for an effective vaccine was eliminated by a completely different technology.
Richard Corstvet and Enright in the LSU AgCenter and Ray McClure at the LSU School of Veterinary Medicine patented several bacterial strains of Mannheimia haemolytica, which were initially recovered from a case of bovine pneumonia in a symptomatic calf. These isolated bacterial strains were capable of producing a capsular antigen and several other bacterial products, which stimulated excellent immunity to experimental challenges with pathogenic Mannheimia haemolytica. After eight years of research, this vaccine has not yet been commercialized.
Another AgCenter scientist, Ron Thune, began working on fish vaccines in the late 1990s. His research involved a killed bacterial vaccine for use against enteric septicemia of catfish, caused by the bacteria Edwardsiella ictaluri. To be effective, the killed vaccine required multiple exposures and was not practical for field use. Further research demonstrated that the pathogen rapidly invaded the fish and was spread to multiple organs within 30 minutes. The killed vaccine was unable to do this. Therefore, the research team began development of a live attenuated vaccine that could invade the fish and persist in various organs for two to four days. This particular vaccine provided excellent immunity against experimental challenge in the laboratory, but moving the vaccine to the field for commercial application under a variety of environmental conditions proved difficult. This attenuated vaccine was patented but never commercialized.
Currently, research on the pathogenic mechanisms used by the bacteria to establish intracellular infections in fish host cells has led to a new live vaccine strain, which can persist much longer in tissues and stimulate an even stronger protective immune response. This new strain of attenuated Edwardsiella ictaluri is being patented at this time. Thune’s research to develop this vaccine is an excellent example of the difficulty and time required (more than 15 years) to get a useful vaccine.
Vaccine development is a team effort involving multiple disciplines to achieve the end product. Microbiologists, immunologists, pathologists, molecular biologists, laboratory staff and farm personnel, as well as livestock producers, are all involved in the process.
It begins with a problem disease causing economic loss in a commodity and producers seeking help as to how to control the losses. Next, scientists try to identify the agent causing the disease and then they try to grow the microbe in the laboratory. At this stage the scientists attempt to attenuate or engineer the organism to determine its make-up and if it can be modified and made into a vaccine. Successful completion of the process benefits clientele by achieving healthier and more productive animals, which result in improved human and animal health in Louisiana.
Frederick M. Enright is professor emeritus; Sue D. Hagius, research associate; and Philip H. Elzer, professor, School of Animal Sciences. Elzer is also assistant vice chancellor and assistant director of the Louisiana Agricultural Experiment Station, LSU AgCenter.
(This article was published in the fall 2012 issue of Louisiana Agriculture magazine.)