Dose and timing requirements for immunogenicity of viral poultry vaccine antigen: investigations of emulsion-based depot function

The release requirements for vaccine antigens delivered by adjuvants with presumed depot function are poorly understood. Water-in-oil (W/O) emulsions are routinely used in many poultry vaccines. They strongly activate antibody production, and are regarded as a depot from which antigens are slowly released, resulting in prolonged antigen residence. However, from earlier studies we concluded that W/O adjuvant activity is partly based on the immunostimulatory activity of the oil phase. Here we assess the dose and regimen requirements for viral antigen in immunization experiments in chickens. Three-week-old to 4-week-old White Leghorn chickens were repeatedly injected with inactivated infectious bursal disease virus antigen over 48 days. Our aim was to compare the antibody responses in repeatedly injected animals, receiving fractioned doses of antigen, with the responses in animals receiving only one injection of the full dose of antigen formulated in either a W/O emulsion or in saline. We observed that repeated administration of small amounts of antigen results in a gradual increase of specific humoral immune responses during the immunization regimen. Immunization with a higher first dose evoked an early higher antibody response, which, however, reached a similar plateau level at the end of the regimen. When compared with lower first-dose regimens, a slow decline of serum antibody titre 2 weeks after the end of antigen injections indicated that repeated injection of small doses of antigen indeed mimics the efficacy of depot-forming adjuvants. All regimens of fractioned antigen in saline, however, proved less effective, when compared with a single-dose vaccination of the cumulative amount of antigen formulated in a W/O emulsion. From our data we confirm that W/O emulsions are very effective vaccine vehicles for improving antigen-specific humoral responses in chickens, owing to a combination of antigen residence-prolonging activity and direct immune stimulation.