Role of adjuvants in the modulation of antibody isotype, specificity, and induction of protection by whole blood-stage Plasmodium yoelii vaccines.

Mice were immunized with whole killed blood stage Plasmodium yoelii parasites in 15 adjuvant formulations then boosted and challenged with parasitized blood. Five of six groups immunized with the Ag in oil-in-water emulsions or formulations without oil were protected. Formulations that induced protection contained saponin, pertussis, copolymer P1004, and detoxified RaLPS. In contrast, none of nine groups of animals immunized with Ag in water-in-oil emulsions were protected. Ineffective adjuvants included CFA and water-in-squalene emulsions with copolymer L141 plus detoxified RaLPS, dimethyldioctadecyl ammonium bromide, and mycobacterial cell wall skeletons. Antibody was measured by ELISA against disrupted parasites and by indirect fluorescent antibody (immunofluorescence) using intact parasites. Protection was associated with antibody of the IgG2a isotype detected by immunofluorescence but not with other isotypes detected by immunofluorescence or any type antibody detected by ELISA. The water-in-oil adjuvants induced high titers by ELISA but low titers by immunofluorescence. These results, together with Western blot analyses, suggested that adjuvant vehicles control the specificity of antibody and that this, in turn, is essential for induction of protective immune responses in this model.