New vaccine technologies: Destined for cattle vaccines? (Proceedings)

Vaccine types, delivery methods and adjuvants in use in veterinary medicine have been relatively static for many years and limited to modified live organism and killed organism (KV) vaccines, needle injection and alum-type and water –in-oil emulsion adjuvants. These systems are safe, reliable and inexpensive to produce. The major infectious diseases in cattle have been well controlled through the use of these technologies.

The new technologies in vaccines, adjuvants and delivery systems are being developed and introduced into veterinary medicine and some will be available to food animal practitioners. The primary factor slowing the introduction of these technologies into food animal practice is cost of production and the presumed limit to what producers are willing to pay; however, better immunization with fewer adverse effects will ultimately result in their usage in food animals.

Needleless delivery systems include the air driven vaccine gun and the intradermal injector. Air gun usage has been minimally applied chiefly because of the interference by the hair coat of animals. Newer fine stream injectors are showing promise however cost remains a major deterrent. Intradermal injectors currently being utilized in small animal medicine and could be readily adapted to food animal medicine. A significant benefit to these technologies would be the minimization of introducing secondary infections through the repetitive use of needles. The air injection method primarily is an intramuscular injection method and has been reported to cause significant injections site reaction although it is usually limited to the dermis and skin surface. Intradermal injectors inject particulate antigens directly into the dendritic cells enhancing immune responses and will be the technologic choice for administration of DNA vaccines.

Adjuvants enhance the immune response for poorly stimulatory antigens like small protein and glycoprotein antigens by presenting the antigen as particulate, localizing the antigen to the injection site or by direct stimulation of the immune innate response (inducing local inflammatory reactions and stimulating the nonspecific proliferation of lymphocytes). A major concern with adjuvant usage is the potential for inducing vaccine hypersensitivities.

The common veterinary adjuvants are called depot adjuvants and include the Incomplete Freund's water-in-oil and aluminum salt adjuvants. Antigen depots also increase the exposure time of the antigen to the immune system. Emulsions of various oil-in-water/water-in-oil combinations surround the specified antigen while the aluminum salts for precipitates the antigen resulting in depots of more immunogenic particulate antigens. Aluminum salt adjuvants remain the standard in food animal practice because they are inexpensive, stable and have minimal adverse effects. Water and oil emulsions induce strongest and longest acting immune response of the commonly used adjuvants. However, this response also requires that vaccine with water/oil emulsion adjuvants have prolonged withdrawal times when used in food animals. Aluminum salts and water-in-oil adjuvants can also induce the formation of granulomas at the injection site.

Microsphere or particulate adjuvants are in limited veterinary usage in companion animal vaccines. Such adjuvants made of biodegradable polymers present the antigen bound to small particulate substrates with slow release to the antigen to the immune system. The particulate presentation stimulates interaction and uptake by the antigen processing cells.

ISCOMS, immunostimulatory complexes, are being developed and some have been introduced into companion animal vaccines. ISCOMS consist of a complex matrix of saponins, phospholipids and cholesterol incorporating the desired antigen. ISCOMS also are particulate in structure which further enhances their interactions with antigen processing cells. ISCOMS tend to localize in lymph nodes draining the injection site prolonging the immune response. ISCOMS can be administered at mucosal surfaces enhancing local antibody responses.

Block copolymers are adjuvants made up of plastic-like hydrophobic and hydrophilic molecules and can be bound to several viral and microorganism antigens. These non-biodegradable adjuvants are often included in water-oil emulsions and can cause intense local reactions. Block copolymers have been used in BRSV vaccine. Block copolymers induce an antibody response with a single injection as well as stimulate the cell mediated response.

Saponins, glycoside products called Quill A from the Chilean soap bark tree, are used in some companion animal vaccines. These adjuvants are quite toxic and require extensive purification to minimize the toxic effects. Saponins interact with cell membranes and incorporate into the cell membranes enhancing antigen processing stimulating both antibody and cell mediated responses.

Liposomes, lipid bi-layer vesicles of phospholipids and cholesterol, have shown value in enhancing immune responses, both antibody and CMI, to highly hydrophobic or hydrophilic antigens. These complexes are resistant to common mucosal and GI secretions and may be useful in local immunization programs.

Carrier proteins derived from clostridial toxoids or bovine serum albumin have been used in some veterinary products. These compounds link to small proteins or carbohydrate antigens increasing the antigenicity. A disadvantage is the production of endotoxin –like reactions in vaccine recipients. Toxoids and BSA have been associated with allergic reactions.

Bacterial products like clostridial toxins and lipopolysaccharides and whole bacteria, such as mycobacterium as in Freund's complete adjuvant, can act as adjuvants. These products stimulate both antibody formation and various cell mediated responses. Certain compounds have shown efficacy in stimulation of local immunity in mucosal surfaces and the GI tract in cats and mice. Toxic side effects have been demonstrated crude extracts of such products, but purification has reduced toxicity for many of the compounds.

DNA and cytokines also exhibit adjuvant properties. Certain cytokines act by enhancing specific immune functions while other cytokines function as general immune activators. DNA oligonucleotides act by induction of cytokines which then enhance various immune functions. Cytokine disadvantages include immunogenicity, species specificity and toxicity which must be overcome via production and selection of appropriate recipient animal species.

While modified live and killed microorganism vaccines remain the standard in food animal vaccines, sub-unit, vectored vaccines are being used in veterinary products. DNA vaccines are being developed for veterinary purposes. Newer technologies are addressing the problems of maternal interference, multiple dosages and duration of immunity. In addition, newer vaccines are likely to further reduce the concern for adverse vaccine reactions.

Sub-unit vaccines utilize only the antigenic moieties of an agent against which an immune response would protect the recipient from infection or disease by the whole organism. For viruses this is usually surface proteins that are responsible to mediating attachment to and invasion of susceptible cell. In the case of bacteria, the antigens may be derived from flagella or pili or other cell wall proteins. Sub-unit vaccines routinely require an adjuvant to enhance the immune response and often require multiple injections. Subunit vaccines generally are poor inducers of cell mediated immunity, but antibody is more often the immune response necessary to inhibit infection. Sub-unit vaccines include FMD and FELV vaccines

Vectored vaccines are genetically modified organisms that have the genes responsible for encoding the desired antigens incorporated into the genetic code of a "carrier" organism. The vector is non-infectious to the recipient and transmits the desired immunizing DNA/gene to susceptible cell where the antigens are produced and presented to immune cells. The vector with the hybridized DNA is also called a chimera—having genes of two or more unrelated agents. The common vectors are capripox and canarypox viruses, adenoviruses and flaviviruses. These vaccines stimulate both antibody and cell mediated antibody and, coincidentally, immunize with one dose. A concern is that repeated vaccinations may result in immunity to the vector virus eliminating its ability to infect/transmit the desired genes to the immune system. Currently, several vectored vaccines are used in companion animals.

DNA vaccines consist of gene segments of infectious organisms injected directly into cells for the production of the desired immunizing antigens. The current systems utilize intradermal injectors which deliver the DNA directly to the dendritic cells of the dermis. This system has shown promise in inducing antibody and cell mediated immunity with a single injection and with prolonged immunity. A DNA vaccine is being tested for FELV.

Vaccine companies are defining the duration of immunity for their products which will facilitate herd health program by permitting selection of those vaccines which provide the longest immunity for the disease of concern.