Genetic Engineering Can Help Control Disease

James L. Bittle, adjunct member, Department of Molecular Biology, Scripps Clinic and Research Foundation, La Jolla, CA.

Infectious diseases are still the main cause of illness and death in domestic livestock. The estimated annual loss incurred from infectious disease in cattle and swine exceeds 1 billion dollars and for all livestock species exceeds 2 billion dollars.

The widescale use of therapeutic drugs and biologics, such as antibiotics and vaccines, also has added to the cost of raising livestock. Yet they have reduced the losses only marginally because of the changes in agricultural production methods which often promote the occurrence of infectious disease. The concentration of animals in feed lots, dry lot dairies, integrated swine operations, and broiler production units are examples of husbandry that increase the spread of infectious agents as compared with less confined types of animal raising.

Methods of Control

Three major methods are used to reduce losses from infectious disease. One is to eliminate the infectious agent from the environment so that animals will not be exposed. This requires destroying any animal infected with the organism, whether it be the host animal or an intermediate host, such as an insect that carries the organism. This has been effective in controlling infectious agents that are not highly transmissible and that may not survive long periods outside the host. Examples are the eradication in the United States of hog cholera in swine and smallpox in humans.

The low-level feeding of antimicrobial drugs is another way to control infectious agents, especially in newborn animals. The development of resistant strains of many micro-organisms to a number of antimicrobials has caused concern although this method is still widely used.

A third method of control is to immunize animals with a form of the organism that will induce an immune response. This requires the use of vaccines containing an immunogen that induces persisting protection against the invading organism.

Actually, all three methods may be used in some form, but the third method, immunization, offers the greatest promise. It is simple, inexpensive, requires few administrations, and, most importantly, prevents infection and, therefore, minimizes damage that often accompanies infection. Greater safety and effectiveness in newer biological products will enhance their use.

Immune System

To understand how vaccines protect, it is important to understand how the immune system defends an animal against an infectious agent. (Editor's note: To conserve space, the author's explanation of the body's immune system was eliminated, and the reader's attention is directed to an explanation of that system in Gary A. Splitter's article.)

Vaccines in Current Use

Instead of allowing infections to occur naturally, it has long been the practice to expose animals to infectious agents artificially so that they will develop antibodies and be protected against the common disease-causing organisms. Thus, many vaccines have been developed and used in animals to control the more serious infectious diseases. The vaccines in use now are made from either attenuated living organisms or inactivated organisms.

The attenuated living organisms have reduced virulence, are required only in small amounts, and, in general, induce long-lasting immunity. They elicit a controlled subclinical infection and, in general, are very effective. Occasionally, however, they produce side effects that may be as severe as the natural infection.

Inactivated vaccines are safe in that they do not contain any infectious material, but they are weak in terms of stimulating an immune response. They usually require multiple injections over several weeks to induce an immune response comparable to that induced by living organisms. They also may cause undesirable side effects evident both at the site of inoculation and sometimes as a general side reaction as the animal responds adversely to the many antigenic components in the vaccine.

In other words, the use of whole organisms in either the living or inactivated form may cause adverse reactions. These reactions are due to certain components of the whole organism, that is, proteins, lipids or carbohydrates that may not be necessary for immunization.

Synthetic Vaccines

Biosynthetic Process. The objective of vaccine development over the years has been to identify the important antigens responsible for protection and to produce them in the purest form. But only recently has recombinant DNA technology, (rDNA, genetic engineering) become available to help produce defined antigens or antigenic determinants, on a large scale and in a cost-effective manner. The isolation of these antigenic determinants on the surface of infectious agents represents the first step in trying to produce a more specific antigen. Since these determinants occur in repeating subunits and their production is controlled by specific genes in the nucleus of the organism, these genes may be used to produce antigenic determinants.

By isolating the specific gene (DNA) that encodes for the surface antigenic determinant, and by using a plasmid (a piece of DNA that occurs naturally in bacteria and yeast) to insert this gene as a bacteria, yeast, or mammalian cell, the gene recombines with the cell's own genes to produce the antigenic determinant along with other cellular products. The antigenic determinant may be isolated and used as an immunogen. This immunogen will be recognized by the immune system as being foreign and will stimulate the production of antibodies or a cellular response that will protect the animal or prepare the animal's immune system for future infection with the infectious agent.

Antigenic determinants can be produced by growing the cells on a large scale and collecting and purifying the antigen as it is expressed. The antigen may have improved characteristics compared to the antigen derived from the whole organism. These characteristics are purity, safety, and stability. Also, the risk of having the vaccine contaminated with infectious material used in production of the whole organism is reduced. All of these characteristics help in developing improved vaccines.

Chemical Synthesis. Another method of producing antigenic determinants is chemical synthesis. Most antigenic determinants are proteins composed of chains of amino acids. Individual amino acids may be linked together in a linear form to mimic antigenic determinants. So, if the amino acid sequence of the native antigenic determinant is known, it can be made synthetically.

One way to determine the amino acid sequence of an antigenic site is to isolate the gene that encodes for it. The gene is composed of DNA that contains the genetic code in its nucleotide sequence. This nucleotide sequence can be determined, and it will translate into an amino acid sequence (the bases, adenine, thymine, guanine and cytosine code in triplet combination for each amino acid). Thus, an amino acid sequence for a surface protein may be derived from the nucleotide sequence of its gene. Only a small part of this surface protein may be required to produce an immunogen.

The peptide can be made by sequentially adding amino acids. Forty or 50 amino acids may be joined together in a linear sequence forming a peptide by using an amino acid synthesizer controlled by a computer program. The peptide is removed from the resin and may be coupled to a carrier protein or polymerized to increase its size. These forms of the antigenic determinant have been found to be active in inducing humoral and cellular immune responses.