Effect of priming on subsequent response to inactivated influenza vaccine

Although shown to be a potent stimulator of serum antibody responses in animal models, the adjuvant immuno-stimulating complexes (ISCOMs) showed little adjuvant effect for inactivated influenza vaccines in a volunteer study. The result may be the non-comparability of the studies: animal studies were carried out chiefly in unprimed mice, while volunteers are mostly primed by previous infection and/or immunization. To test this, Balb/C mice were infected with influenza viruses or immunized with inactivated influenza vaccine, and subsequently given inactivated vaccine in saline or incorporated into ISCOMs. The serum in antibody responses was measured 1 month after immunization. The results confirm the adjuvant activity of ISCOM in unprimed mice, and show a marked reduction in adjuvant activity for primed mice. We argue that ISCOMs are important to prime the T cell response necessary for the serum antibody response to saline vaccine, but largely unnecessary where priming has been accomplished by prior exposure to influenza antigens. Further, the value of ISCOMs may lie in promoting antibody responses in unprimed subjects, and not in enhancing antibody titres.