The Important Role of Vaccine Adjuvants
For a broad perspective on adjuvants and the important role they play in making animal vaccines safer and more efficient, check out a scientific paper published in the May-June 2003 edition of the Journal of Veterinary Internal Medicine.
The paper, authored by Anna R. Spickler and James A. Roth of Iowa State University's College of Veterinary Medicine, profiles the history and development of adjuvants, as well as the materials used to make them and their mechanisms of action.
Adjuvants are agents designed to stimulate the immune response to vaccine antigens. Adjuvants, along with vaccine administration, have become increasingly scrutinized in recent years because of the emergence of injection-site sarcomas. The tumors are rare and normally associated with felines. It's also still unclear what role, if any, adjuvants or vaccine antigens play in the development of sarcomas.
The authors believe the benefits of adjuvants and vaccine administration outweigh whatever risks may be associated with them. Their theory is that forgoing all vaccinations simply to avoid the possibility of an injection-site sarcoma poses greater health risks to animals than the sarcoma risk itself.
Spickler and Roth also suggest that in the future it may become increasingly possible to design and select adjuvants tailored to the specific needs of the antigen, species and situation.
First developed in the 1920s, the purpose of adjuvants was to boost immunity and immune responses to administered antigens. Materials employed in adjuvants include chemicals, microbial components and mammalian proteins.
In animal health, adjuvants have become a common way of reducing dependence on modified live antigens, which have the potential in immunocompromised animals of inducing the disease they're designed to prevent. Used along with killed antigens, adjuvants provide artificial signals that stimulate the immune system and can boost the efficacy of vaccines.
This is particularly important in very young or old animals with compromised immunity. Another benefit of adjuvants is that they can also lengthen the shelf-life of vaccines and decrease the amount of antigen needed, making the vaccine more cost-effective. Finally, adjuvanted vaccines have been demonstrated to provide immunity for one year.
The adjuvant's mechanism of action is not completely understood, but they appear to enhance antigen presentation, improve antigen stability and/or act as immuno-modulators. A single adjuvant may have more than one mechanism of action, including preserving the structure of the antigen as well as increasing cell migration to the lymph nodes. Some adjuvants appear to trap the antigen at the injection site and thus provide a continuing supply to local antigen presenting cells (APCs).
Primary types of adjuvants include aluminum and calcium salts; oil emulsions, such as shark liver oil; liposomes and archeosomes, as well as nanoparticles and microparticles made from biodegradable polymers. Saponins, or complex chemical compounds extracted from plants and trees, also are used.
Attempts have been made recently to fit adjuvant mechanisms into more general hypotheses of immune function. According to some current theories, all APCs must first be activated for antigen presentation before they can initiate immune response. In one model, this activation is thought to occur when pattern recognition receptors on the APC bind to conserved motifs in bacterial lipopolysaccharides, sugars and other moieties.
If this idea is correct, adjuvants may act by mimicking these primitive bacterial signals. In fact, many adjuvants are derivatives of bacteria or resemble the motifs in bacterial proteins, carbohydrates or DNA. However, this model may not explain adjuvants such as oil emulsions, saponins or aluminum salts.
Adjuvants are associated with adverse side effects that can include fever, arthritis, anorexia, soreness and lethargy. Aluminum adjuvants can cause inflammation, and some authors suggest that they should be avoided in cats. This recommendation, however, is controversial. Not only have adjuvants other than aluminum been used in cases that result in injection-site sarcomas, but tumors also have occurred when vaccines without adjuvants are used.
At present, sarcomas don't appear to be associated with any one specific brand of vaccine, and it can be difficult to establish the incidence of rare adverse effects for any particular vaccine. It's important to understand and appreciate these potential hazards, but in most instances the adverse effects of adjuvants are mild, with their benefits outweighing the hazards of their use.
