Serological response of sheep to live and killed Staphylococcus aureus vaccines

Adult sheep were immunized intramuscularly with a killed, in vivo-grown Staphylococcus aureus vaccine combined either with dextran sulphate (group DD) or with Freund's incomplete adjuvant (group FF). Another group (LL) received an attenuated live S. aureus vaccine intracutaneously. The animals were given a primary vaccination followed two weeks later by a booster vaccination. A fourth group of sheep (group LD) was primed with the live vaccine and given a booster vaccination with the killed vaccine combined with dextran sulphate. ELISA was used to quantify blood serum levels of IgM, IgG1 and IgG2 antibody directed against the pseudocapsular antigens of S. aureus grown under in vivo conditions. Groups LD, DD, and FF had sharp increases in mean levels of IgM antibody in the first few weeks after vaccination with another large increase in mean values for group FF at 12 weeks after primary vaccination. Group LL showed virtually no increase in levels of IgG1 antibody; the other three groups had maximum mean values for IgG1 antibody at 5 weeks (FF) and 8 weeks (LD and DD) after primary vaccination. All groups had large IgG2 antibody responses (the largest for group LD), but the response for this isotype had waned by 14 weeks after primary vaccination. Examination of the ratios of IgG2 antibody to IgG1 antibody suggested that dextran sulphate may be a useful adjuvant for preferentially stimulating synthesis of IgG2 antibody against staphylococcal pseudocapsular antigens.