78 Vaccination
The process of stimulating protective immune responses in animals against pathogenic microorganisms by exposing them to non-pathogenic forms or components of microorganisms is referred to as vaccination. A successful vaccine induces an effective long-term adaptive immune response directed at appropriate target antigens on the pathogen without causing disease in the recipient. Among the types of vaccines currently in use or being developed are those composed of inactivated microorganisms, live attenuated microorganisms, microbial products, synthetic peptides and DNA of microbial origin. When feasible, effective and safe, vaccination is one of the most cost-effective measures for controlling infectious disease, not only in companion animals but also in food-producing animals. The duration of protection following vaccination is influenced by many host factors including age, immune competence and the presence of maternal antibodies in the animal's circulation. In common with many disease control measures, however, vaccination has defined limitations. Effective vaccines for controlling equine infectious anaemia and African swine fever are not available at present. Protective immunity against Staphylococcus aureus using vaccination cannot be induced in a predictable manner and prevention of fungal infections through vaccination has had limited success.
In a large population of animals of a comparable age and immune status, the response to vaccination is not uniform. The immune response is influenced by many genetic and environmental factors and the outcome of vaccination tends to follow a normal distribution. A small percentage of animals have a weak response to vaccination and if challenged might be susceptible to infection. The majority of the animal population respond adequately and a small percentage respond strongly to vaccination. The addition of appropriate adjuvants to vaccines can enhance and prolong the duration of the immune response, decrease the antigen concentration required for effective immunization and promote the development of cell-mediated immune responses.
Adjuvants
Substances with the ability to increase or modulate the intrinsic immunogenicity of an antigen are referred to as adjuvants. If mixed with antigens before administration, adjuvants boost the immune response to antigens of low immunogenicity. They also enhance the immune response to small amounts of antigenic material. Proposed modes of action of adjuvants include retention and slow release of antigenic material from the site of injection (depot effect), increased immunogenicity of small or antigenically weak synthetic peptides and improved speed of response and persistence of response to effective antigens. Adjuvants may stimulate dendritic cell and macrophage activity and promote T and B lymphocyte responses. A wide range of substances including mineral salts, bacterial derivatives, biodegradable particles, emulsions and cytokines are currently used as adjuvants.
Although aluminium salts have been used as adjuvants for almost 80 years, their modes of action are not clearly defined. Their adjuvant activity is attributed to activation of macrophages and increased uptake of antigen by antigen-presenting cells. The adjuvant effect of bacterial derivatives such as muramyl dipeptides is attributed to their ability to stimulate macrophages and dendritic cells, interferon-γ production and T-helper (TH) cell activity. Antigenic material can be either encapsulated into biodegradable particles or carried on their surfaces through adsorption or covalent linkages. Liposomes are biodegradable particles which are taken up by antigen-presenting cells and their contents are processed via MHC class II-dependent pathways. Cytokines and related substances can be combined with antigenic material and used to direct the immune reaction toward a humoral or cell-mediated response. Immunostimulating complexes (ISCOMs) are saponin-based adjuvants which augment TH1 and TH2 cell responses. Their activity is attributed to interactions with macrophages and dendritic cells and activation of CD4+ T cells.
Inactivated vaccines
Infectious agents can be killed without substantially altering the immunogenicity of their antigens which induce protective immunity. Although most inactivating chemicals alter the immunogenicity of infectious agents, some such as formaldehyde cause limited antigenic change. A major limitation of inactivated vaccines is that some protective antigens are not produced readily in vitro. Because they are processed as exogenous antigens in the body, many inactivated vaccines can induce high levels of circulating antibody but are less effective at stimulating cell-mediated and mucosal immunity. As inactivated vaccines do not contain agents which can replicate, a greater antigenic mass and a more frequent administration of vaccine (booster injections) are required to achieve results comparable to those obtained with live attenuated vaccines. Advantages of inactivated vaccines include stability at ambient temperatures, safety for recipients due to their inability to revert to a virulent state and a long shelf life.
Live attenuated vaccines
The virulence of living organisms can be reduced by attenuation, a process that involves adapting them to grow under conditions whereby they lose their affinity for their usual host and do not produce disease in susceptible animals. Viruses can be attenuated by growing them in monolayers prepared from species to which they are not naturally adapted. Chick embryo attenuation has been employed successfully for a number of viruses which infect animals and humans. Live attenuated vaccines have many potential advantages over inactivated vaccines. They can be administered by a number of different routes and present all the relevant antigens required for the induction of protective immunity since they multiply in the recipient. They usually induce a satisfactory level of cell-mediated and humoral immunity at sites where protection is required such as mucosal surfaces. Disadvantages of modified live vaccines include the possibility of reversion to virulence, contamination with infectious agents capable of causing disease in the recipient and neutralization by maternal antibodies in young animals acquired by ingestion of colostrum. A live attenuated viral vaccine has a limited shelf life and should be refrigerated during transportation and storage to ensure its viability.
Vaccines produced by recombinant nucleic acid technology
Recombinant vaccines are classified into three categories: vaccines composed of antigens produced by recombinant nucleic acid technology or genetic engineering, vaccines consisting of genetically attenuated microorganisms and vaccines composed of modified live viruses or bacteria into which DNA encoding protective antigens are introduced by cloning. Vaccines produced by recombinant nucleic acid technology are composed of subunit proteins produced by recombinant bacteria or other microorganisms. DNA encoding the required antigen is cloned in a suitable bacterium or yeast strain in which the recombinant antigen is expressed.
Virulent microorganisms can be rendered less virulent by gene deletion or site-directed mutagenesis. Virulent viruses and bacteria can be modified by deletion of appropriate genes and animals vaccinated with such vaccines can be differentiated from animals infected with a field strain of the pathogen. The failure of some vaccines used in veterinary medicine to induce a protective immune response can result from problems related to delivery. Vaccines composed of modified live organisms called vectors, into which a gene encoding an antigenic determinant is introduced, can be used as a delivery system. Currently a small number of viral vectored vaccines have been approved for use in animals. A vaccinia vaccine vector carrying the rabies G glycoprotein has been used successfully as an oral vaccine administered to wild carnivores in bait. The G glycoprotein induces virus-neutralizing antibodies in vaccinated animals which protect against rabies. Other examples include a canarypox virus-vectored vaccine against canine distemper virus in dogs and a fowlpox virus-vectored vaccine designed to protect against avian influenza in poultry.
Synthetic peptide vaccines
If the structure of epitopes that can induce a protective immune response is known, it is possible to chemically synthesize peptides corresponding to these antigenic determinants. Only a small portion of antigenic molecules interact with specific receptors on B cells and T cells. For B cells, an antibody interacts with up to five amino acids in its antigen-binding site. Epitopes for T-cell receptors can be composed of 12 to 15 amino acids.
The general approach with synthetic peptide vaccines is to identify appropriate epitopes in protein components of the infectious agent and to synthesize a series of peptides corresponding to the amino acid sequences. Limited progress has been made with synthetic peptides for the induction of protective immune responses against infectious agents.
DNA vaccines
One of the most important developments in vaccine production in recent years involves the use of DNA, encoding microbial antigens cloned in a bacterial plasmid, for immunization. The procedure involves injection of a plasmid containing the DNA sequence for a protective antigen whose expression is controlled by a strong mammalian promoter. For an infectious agent expressing that antigen, injection of this recombinant plasmid into the skin or muscles of animals may result in the production of protein inducing immunity against that infectious agent. This leads to the expression in host cells of encoded genes, with the development of a significant immune response to the gene products in the recipient. Unlike viral vectors, the recombinant plasmid cannot replicate in the mammalian cells, but transfected host cells express the vaccine antigen. Although transfection rates appear to be low, antigen production has been detected in animals vaccinated with DNA intramuscularly for up to six months after injection. Because DNA vaccination induces intracellular processing of antigen, it seems to mimic a natural infection and is, therefore, an effective method of inducing T cell responses. Humoral responses, however, may not be as high as those obtained by injection of a purified antigen. Although immune response may be delayed following DNA vaccination, a persistent response may occur. In contrast to live viral vaccines, maternal antibody does not appear to affect the immune response in young animals.
Questions related to the safety of DNA vaccines remain unresolved. The possibility that foreign DNA could integrate into the host chromosome and induce neoplastic changes or other cellular alterations has been suggested. It has also been suggested that DNA introduced into the body by this method of vaccination might induce anti-DNA antibodies to the recipient's own DNA.
Reverse vaccinology
The availability of genome sequences for many infectious agents offers the possibility of identifying target genes that can code for relevant proteins, thereby providing an opportunity for the rational selection of vaccine candidates. This novel approach, termed reverse vaccinology, can be combined with immunological procedures to optimize epitope prediction, leading to the development of DNA vaccines. A limitation of reverse vaccinology is that the strain of organism selected may not be representative of the genetic diversity of the species. Comparison of the genomes of several different strains of bacteria indicates that important differences exist among different strains of the same bacterial species. Accordingly, it may be necessary to evaluate genome sequences from multiple strains of a microbial pathogen to identify appropriate targets for vaccine production.
Adverse reactions following vaccination
Undesirable reactions to vaccination may occur as a consequence of contamination of the vaccine during manufacture, reconstitution or administration. These adverse reactions may include allergic responses to vaccine components, especially protein from tissue culture fluids or chick embryo sources. Reactions at the injection site may follow introduction of pyogenic bacteria into the tissues. Granulomas at the injection site may result from the type of adjuvant present in a vaccine. There may be a risk of immunosuppression following administration of some live vaccines to particular breeds of animals (Box 78.1).
Vaccination failure
The outcome of vaccination is determined by many factors, some related to vaccine composition and others related to characteristics of the animals receiving the vaccine. Vaccine-related factors that can contribute to vaccination failure include inherent characteristics of the vaccine and problems with reconstitution and administration. Animal-related factors include the possibility of animals incubating the disease at time of vaccination, neutralization of live viral vaccines by colostral antibodies and immunosuppression caused by drugs or infectious agents.