Synthetic sulpholipopolysaccharides: novel adjuvants for humoral immune responses

Referring to the strong immunostimulating activity of combinations of lipophilic agents and dextran sulphate, conjugates with chemical determinants of both types of adjuvants were synthesized and then examined for immunostimulatory capabilities in mice. Saturated fatty acids with varying chain lengths and sulphate groups were coupled covalently at defined ratios to the polysaccharide Ficoll (MW 400,000). Chemical analysis of 60 of the sulpholipopolysaccharides synthesized revealed that the number of sulphate groups per monosaccharide unit varied from 0 to 1.6, and the number of lipid groups from 0 to 0.8. Adjuvanticity of these conjugates for the humoral immune response was determined using sheep red blood cells (SRBC) and dinitrophenyl-haptenated bovine serum albumin (DNP-BSA) as antigens. Five days after intraperitoneal injection of adjuvant and antigen, the numbers of direct anti-SRBC plaque-forming cells (PFC) in the spleen were determined. Anti-DNP antibody titres were measured from 1 to 4 weeks after immunization. PFC responses to 2 X 10(6) SRBC were augmented up to a 100-fold by conjugates of Ficoll and sulphate (sulphopolysaccharides: SPs) or lipid groups (lipopolysaccharides: LPs). Introduction of low or moderate numbers of lipid groups in SPs reduced adjuvanticity. Adjuvant activity of sulpholipopolysaccharides (SLPs) with varying sulphate and high lipid content depended on the sulphate contents and the chain length of the lipids. Sulphate reduced adjuvanticity of the SLPs, and the number of sulphate groups required for complete annihilation increased with the chain length of the lipid. LPs and SLPs, including conjugates that did not enhance anti-SRBC PFC responses, augmented serum antibody responses to DNP-BSA while SPs were hardly effective.