Adjuvant activity of 6-O-acyl-muramyldipeptides to enhance primary cellular and humoral immune responses in guinea pigs: adaptability to various vehicles and pyrogenicity.

Thirteen 6-O-acyl-N-acetylmuramyl-L-alanyl-D-isoglutamines (6-O-acyl-MDPs), including four inactive D-isoasparagine and L-isoglutamine analogs, were tested for their pyrogenicity and immunopotentiating activity to stimulate primary humoral and cellular immune responses in guinea pigs to a model protein antigen, ovalbumin, when administered in various vehicles. Among them, derivatives whose muramic acid residue was substituted by alpha-branched (and beta-hydroxylated) higher fatty acids at the carbon-6 position, especially 6-O-(2-tetradecylhexadecanoyl)-MDP (B3O-MDP) and, to a lesser extent, 6-O-(3-hydroxy-2-docosylhexacosanoyl)-MDP (BH48-MDP) and its L-serine analog [BH48-MDP(L-Ser)], were found to exert strong adjuvant activity in both the induction of delayed-type hypersensitivity and the stimulation of circulating precipitating antibody levels when combined with nonirritating vehicles (liposomes, squalene-in-water emulsion, and phosphate-buffered saline). These vehicles did not efficiently support the adjuvant activity of MDP, the parent molecule of the above lipophilic derivatives. Pyrogenicity tests showed that introduction of alpha-branched higher fatty acid groups but not of straight, long-chain fatty acids at the 6-position of the muramic acid residue resulted in marked decrease of the pyrogenicity inherent to MDP via intravenous administration.