Influence of adjuvants on the immune response of sheep to a novel Staphylococcus aureus vaccine

Abstract

Sheep were immunized twice with two Staphylococcus aureus vaccines which contained either killed bacterial cells shrouded with pseudocapsules or toxoided β haemolysin, together with various adjuvants. Circulating antibody responses were monitored using an ELISA (anti-pseudo-capsule responses) and an anti-β haemolysin assay. Combining a killed cell/pseudocapsule/dextran sulphate (DXS) vaccine and the toxoid vaccine did not cause any diminution of antibody responses compared with separate injection of the two preparations. Addition of calcium phosphate to DXS as an adjuvant for the combined vaccine did not extend the duration of anti-pseudocapsule responses compared with those obtained with dextran sulphate alone. Nor was there any benefit in terms of durability of the response by increasing the amount of DXS: doses of DXS of 10–20 mg/kg liveweight provoked significantly higher peak responses but caused acute clinical reactions at the vaccination site and did not prolong the antibody response. In contrast, the combined vaccine given with DXS and emulsified in Freund's Incomplete Adjuvant (FIA) resulted in a large anti-pseudocapsule response with elevated levels of antibody being sustained for at least one year; there was a significant IgG 2 anti-pseudocapsule response in animals receiving the vaccine with DXS and FIA. In the above experiments, 10 10 pseudocapsule-shrouded bacterial cells were used in the vaccine. Reducing the concentration of cells to 10 9 caused a slightly reduced anti-pseudocapsule response (not significant) whereas increasing the concentration to 10 11 did not increase the response.