Combinations of two synthetic adjuvants: synergistic effects of a surfactant and a polyanion on the humoral immune response.

Synergistic effects of two synthetic adjuvants, dimethyldioctadecylammonium bromide (DDA) and dextran sulfate (DXS) on the humoral response to sheep red blood cells (SRBC) were investigated. Mice received intraperitoneal (ip) injections of adjuvant and antigen simultaneously. The number of plaque-forming cells (PFC) in the spleen were determined 5 days later and circulating anti-SRBC antibodies were measured till 16 weeks after immunization. Although combinations of DDA and DXS were very effective in enhancing the PFC response to both moderate (2 X 10(7] and low (2 X 10(6] doses of SRBC, synergy between the adjuvants was only observed at the low dose of SRBC. Optimal augmentation of the primary response to the low antigen dose was evoked by the combination of the highest dose tested of either adjuvant (1 mumol DDA and 1 nmol DXS) resulting in a 560-fold increase of the number of PFC in the spleen as compared to controls. Even combinations of relatively small amounts of both adjuvants were very effective in augmenting the response to SRBC. Mice receiving half the amounts of both adjuvants with 2 X 10(6) SRBC displayed increased numbers of PFC in the spleen at Day 5 as well as increased titers of total anti-SRBC antibodies at Week 1 and Week 2 and 2-mercaptoethanol-resistant antibodies from Week 4 till Week 16 as compared to the calculated sum of responses in mice which received either DDA (0.05 mumol per mouse) or DXS (0.05 nmol per mouse). The mechanism behind the synergy between these adjuvants is discussed and the possibility of discerning adjuvants on their modes of action is suggested.