Improving Animal Health Through Monoclonal Antibodies

David B. Snyder, assistant professor, Virginia-Maryland Regional College of Veterinary Medicine, University of Maryland, College Park.

Wouldn't it be nice if cattle producers could choose the sex of their next calf? Wouldn't it be nice if poultry producers could instantaneously identify the exact cause of the disease that was affecting their flock? Wouldn't it be nice if the horse industry could determine in minutes whether a mare was ready to breed or that she was already in the early stages of pregnancy?

Wouldn't it be nice if milk producers could within minutes quickly assure that milk from each cow was free of contaminants? And, what if a veterinarian could vaccinate an animal without using a vaccine?

Better yet, what if a veterinarian could successfully locate and destroy tumors in valuable animals without using surgery, irradiation, or chemotherapy? These are only a few of the probable applications that monoclonal antibodies will soon bestow on the animal world.

What Are Monoclonal Antibodies?

Antibodies are proteins produced by white blood cells in response to the presence of a foreign substance in the body, such as viruses and bacteria. Antibodies can bind to and inactivate cells of the foreign substance but will not harm other kinds of cells.

Until recently, the primary source of antibodies was blood serum from animals. Now, to produce large quantities of a single antibody, scientists use a technique called monoclonal antibody production. By fusing a cancerous cell with a cell that produces an antibody, scientists create a hybridoma, which produces large quantities of identical or monoclonal antibodies in a pure, highly concentrated form. The use of these new reagents has become widespread in both applied and research settings.

New Field Created

The 1984 Nobel Prize in Medicine was awarded to Kohler and Milstein, the scientists who originally described the method for generating immortal hybrid cells which secrete the almost magical monoclonal antibodies. The method they described showed researchers how to dissect a complex antibody response against a foreign substance or antigen into its individual component parts. The monoclonal antibodies which result from this proportioning have many unique and highly useful properties that sharply separate them from the group from which they were derived.

Since the original discovery of monoclonal antibodies in 1975, a new field has been created. This new field inspired by the utility of these highly useful probes has exploded, revolutionizing nearly every area of the concurrent biotechnological. movement. Monoclonal antibodies have been prepared for nearly every imaginable purpose, bringing about major advances in many diverse fields where conventional, mixed antibody reagents often have limited the scope of what could be achieved. (See previous article by Richard A. Goldsby for additional details about monoclonal antibodies.)

When cultured in an artificial environment, spleen cells consisting of various white blood cell groups and antibody-producing B lymphocytes normally have a short life span. However, a biotechnological process known as somatic cell hybridization Permits the fusion of antibody-secreting B cells with nonantibody secreting myeloma cells that have an infinite life span. The hybrid cell types produced by the process are called hybridomas. They combine the most desirable characteristics of both the parental cell types: antibody production and immortality. Individual hybridoma cells can be selected and cultured to yield a group of cloned cells that all produce and secrete large and unending quantities of identical monoclonal antibodies.

Antibodies themselves are serum proteins produced by individual B lymphocytes. Normal animals respond to infection by an organism or to injection of a foreign substance (antigen) by producing a mixed group of specific antibodies that are then secreted into the host's serum. Traditionally, the antibody containing serum (antiserum) from animals which have been deliberately exposed to various antigens by vaccination has been used in many diverse applications, most notably in veterinary diagnostics.

Since each antigen has many unique binding sites, different antibodies are produced by individual B cell clones that are specific for each of the attachment sites (epitopes). Hence, antibodies that are found in serum and were formed against each separate binding site, or epitope, on an antigen may be considered custom blends of different monoclonal antibodies. These blends of antibodies are produced by as many as one million different B cell clones. But they may be singled out and immortalized into monoclonal antibody-secreting clones by somatic cell hybridization.

just as the right key will turn the correct lock, antibodies are able to bind specifically and firmly their inducing antigen. Like guided missiles, antibodies find their targets, bind to them, and neutralize or kill their activity. In this way, antibodies serve as a highly selective defense mechanism for animals and thereby ease the removal of foreign substances from the body.

Uses of Monoclonal Antibodies

Monoclonal antibodies are not used extensively in the animal world. They are employed in the process of genetically engineering an animal vaccine. In this process, several fundamental things must be known before an attempt to produce a recombinant vaccine is made.

Producing Vaccines. Viruses, such as foot-and-mouth disease, are composed of large and complex proteins, all of which have many antibody binding sites. Since only antibodies specific to a few of these binding sites are able to kill or neutralize the virus, monoclonal antibodies are important for identifying which sites are involved in virus neutralization and on which protein they reside. With that fundamental information, the gene which encodes this protein may be isolated, and a recombinant protein having the amino acid sequence for the neutralization site may be produced.

While monoclonal antibodies identify gene products (proteins) that are important in the genetic engineering process, their utility does not stop at the first stage. After the potential recombinant vaccine has finally been expressed, the same monclonal antibodies are often used in quality assurance procedures. These procedures seek to assure that the pertinent neutralization sites on the new recombinant protein have not been altered.

Finally, the monoclonal antibodies can be used to purify selectively the recombinant protein for use as a subunit vaccine. It is doubtful that genetically engineered or certain kinds of synthetic vaccines would be as possible without monoclonal antibodies.

Therapeutic Role. Another use of monoclonal antibodies is a potential therapeutic role in preventing certain infectious diseases. One example is the use of monoclonal antibodies that bind to the antigen of certain strains of intestinal bacteria, such as Escherichia coli, or E. coli. This passively immunizes calves and pigs against neonatal diarrhea.

In this particular scheme, monoclonal antibodies against the antigen with hairlike binding sites are fed to newborn calves or pigs. In the gut, the antibodies attach to the binding sites, or pili, of the toxic strain of E. coli. But the toxic bacteria need these hairlike structures to attach to the gut wall. Even as these "bad" bacteria are unable to colonize because the monoclonal antibodies have blocked their attaching ability, other less pathogenic strains of E. coli then colonize the gut. This reduces the severity of the disease.

This process is favored over costly vaccination of pregnant cows that must be vaccinated annually or semiannually to provide similar natural protection through their colostrum and milk.

"Magic Bullet" Concept. Monoclonal antibodies specific for some tumor antigen or viral antigen can selectively kill or neutralize when they are administered to an ailing animal. The antibodies are administered alone or coupled to some toxic compound such as arsenic. The result of such a passive therapy is that the antibodies specifically seek out and destroy the tumor or virus anywhere in the body that that particular antigen is found.

This process makes tumor or viral therapy economically feasible in the animal world. This approach has the added advantage of producing only local reaction to the therapy, rather than throughout the system as may be caused by alternative therapies such as surgery, chemotherapy, or irradiation.