Many adjuvant-active analogs of muramyl dipeptide (MDP) have been described. Unfortunately, most have been found to induce pyrexia or other adverse side effects in several species. In the guinea pig, these side effects include the guinea pig distress syndrome, which lasts 24 to 48 h, and enhancement of endotoxin shock. A dose-related response to MDP or N-acetylmuramyl-L-alpha-aminobutyryl-D-isoglutamine [( Abu1]-MDP) was observed. However, we found several adjuvant-active analogs which were completely innocuous in guinea pigs. 2-(2-Acetamido-2-deoxy-D-glucose-3-O-yl)-D-hexanoyl-L-alanyl-D-isoglutam ine (3'-n-propyl-MDP) was one such compound. To understand the differences between the toxic and nontoxic compounds, macrophage stimulation in vitro was studied. Both compounds stimulated guinea pig peritoneal macrophages to produce lymphocyte-activating factor in vitro. However, the supernatant from 3'-n-propyl-MDP-stimulated macrophages had no observable effects on guinea pigs, whereas the dialyzed supernatant from [Abu1]-MDP-stimulated cells readily induced the guinea pig distress syndrome. Sephadex G-150 chromatography showed that both analogs stimulated production of a major (60,000) and minor (11,000) peak of lymphocyte-activating factor activity. The factor causing the guinea pig distress syndrome was found only in the low-molecular-weight fraction of [Abu1]-MDP-treated cell supernatants. Thus, one difference between the nontoxic and toxic compounds is their ability to produce lymphocyte-activating factor in the absence of the toxic factor. The lack of toxicity of 3'-n-propyl-MDP makes it an attractive compound for use in vaccines
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