A cholesterol implant used as a delivery system to immunize mice with bovine serum albumin

Evolving importance of biologics and novel delivery systems in the face of microbial resistance

Methods to control infectious diseases in livestock are growing in importance. As the size of the average farm increases - for poultry, dairy and beef cattle, swine, and fish - the risk of rapid spread of infectious diseases increases as well. This increases the need for alternative methods of control of infectious agents. Improvements in specific immunogens, adjuvants, and delivery systems are needed to meet the demand for vaccines to ensure a healthy and safe meat supply. This article explores the challenges, trends, and recent advances in the control of infectious diseases through the use of biologics.

Manipulating the immune response; applications in livestock breeding

There are many opportunities for the use of immune modulation techniques in livestock that offer the potential to reduce the requirements for chemical usage and surgical intervention in standard management practices. While vaccination has been used for many years for disease control, there are areas in which vaccination has not been very successful, including the induction of mucosal responses, the induction of cellular responses, and the ability to induce extended duration of protection after a single administration of antigen. In addition, new areas of immunological intervention such as immunisation against reproductive hormones offer new opportunities to modify not only reproductive performance, but also growth, metabolism, carcass quality and behaviour in livestock. These new techniques bring increased need for enhanced efficacy and duration of response. While extensive studies in vaccination have shown that many of the desired immunological responses can be induced in experimental conditions, effective application in the field is dependent upon the development of vaccine delivery methods that are practical within the confines of an effective livestock management system. This paper outlines restrictions that may be imposed on vaccine delivery to livestock and introduces controlled antigen delivery as a potential method for single dose vaccination.

Vaccine delivery to animals

For many years vaccination of animals has been practiced to prevent infectious diseases using inactivated organisms or modified live organisms. The live vaccines were effective but lacked safety. The vaccines made with inactivated organisms required an adjuvant to induce an immune response that was not as effective as either the clinical disease or live vaccines. An 'ideal' vaccine would induce effective immunity specific for the type of infection, have long duration, require minimal or no boosters, have impeccable safety, would not induce adverse reactions, and be easy to administer. The desire to meet these criteria, and especially safety, has resulted in the development of vaccines that do not depend on the use of the viable disease agent. The emphasis on subunit or inactivated vaccines that meet the desired criteria of a perfect vaccine has resulted in a critical need for better adjuvants and delivery systems. This has resulted in a technological innovation revolution with development of a wide array of different technologies to generate effective vaccines. This review will describe the historical relevance of adjuvants used for parenterally administered inactivated/subunit vaccines as well as describe some of the exciting technological advances including adjuvants (ISCOMS), delivery systems (recombinant vectors, microparticles), and novel approaches (transgenic plants, naked DNA) that are currently being, or will be used in the future, in the search for better, more effective vaccines that meet the current and future needs of veterinary medicine.

Biodegradable implants for the delivery of veterinary vaccines: design, manufacture and antibody responses in sheep

Biodegradable implants made from cholesterol and lecithin (C:L) were used to deliver a recombinant antigen (recombinant Dichelobacter nodosus pili) and adjuvant (Quil A) to sheep. Implants (5.5- x 1.8-mm) were placed subcutaneously and compared to a conventional vaccination regime (2 injections, 4 weeks apart) for antibody responses and tissue compatibility. Release profiles of antigen and adjuvant were also studied in vitro and in vivo. The presence of Quil A in vaccine implants had a marked effect on the rate at which antigen was released with 29 and 44% being released in the first 24 h from implants containing pili alone and pili with Quil A, respectively. Sheep produced significant levels of antibody when immunized with implants, however the response was short-lived and of significantly lower intensity than the response stimulated by two injections of antigen with Quil A (P < 0.01). A second implant system was developed where implants coated with C:L to delay antigen release, were used in combination with uncoated implants to deliver a priming dose and boosting dose of antigen. Antibody titres stimulated by the 4 double implant system were equivalent to those stimulated by a conventional regime of two injections (four weeks apart) for the first six weeks of the experiment. After this time, antibody levels in the groups which received implants dropped significantly. In vitro studies revealed that some of the implant coatings had caused a delay in the release of antigen (the rate of release peaked at 72 h), however this was not long enough to provide a significant boosting effect. In all cases, implants were well tolerated by sheep and caused less local reaction than injected vaccines.

The effects of continuous and intermittent delivery of antigens of Boophilus microplus on the development of murine antibodies

Protective antigens solubilised from the membranes of the midgut (LI-GM) of the adult cattle tick Boophilus microplus were delivered to mice either continuously for up to 4 weeks from osmotic pumps implanted subcutaneously or intermittently in a pulsatile fashion by injections or by a combination of both modes of delivery. The effects of delivery profile on antigen specific antibody levels and avidity were compared. LI-GM delivered either by three injections (weeks 0, 2 and 4), or continuously from osmotic pumps (over 4 weeks) induced similar levels of antibodies in mice. The mode of delivery of LI-GM when in the presence of the adjuvant Quil A did not generally affect either the level or the avidity of the antibody response; indeed a single injection of LI-GM in the presence of Quil A stimulated an immune response similar to that induced by several combinations of pulsatile and of continuous delivery where mice were exposed to antigen and adjuvant for up to 4 weeks. LI-GM incubated at 37 degrees C in vitro and in vivo for periods from 4 h to 14 days was partly degraded into low molecular weight (less than 29 kDa) components. The immunogenicity of LI-GM incubated in vitro for 4 h was significantly decreased, although its antigenicity was not affected after incubation for up to 14 days. In conclusion, delivery of LI-GM continuously from osmotic pumps demonstrated that single-step immunisation of mice with tick antigens was feasible. However, it was also demonstrated that the continuous delivery of antigen was only advantageous (i.e. potential for a decrease in the number of times an animal must be handled) compared with delivery by injections when no adjuvant was used. Further work is necessary to establish the effect antigen degradation has in limiting the immune response resulting from continuous delivery of antigen from osmotic pumps.

Vaccines against blood-sucking arthropods

This paper provides selected personal insights on the development of vaccines against blood-sucking arthropods, with particular emphasis on vaccines against ticks. The emergence of novel or concealed antigens of haematophagous ectoparasites as candidate vaccine antigens is reviewed and the effect of feeding by the parasite on the expression of protective antigens is considered. The distribution of protective antigens through life cycle stages, the stage of the life cycle targeted by protective responses, and the nature of these responses, are commented on briefly. Concealed antigens of the gut, including the peritrophic membrane, and other internal organs, are evaluated for the role they play in induction of immunity artificially. Some of the work carried out to purify and characterise protective antigens of tick guts is described. A commentary is developed on vaccines that combine both "concealed" and "exposed" antigens. Some of the problems associated with the infestation and challenge of vaccinated hosts in the field are identified and the delivery of parasite antigens as vaccines that are both protective and "user-friendly" is emphasised as a major problem to be solved.