Abstract
The mechanism of adjuvant activity of the synthetic glycopeptide N‐acetylmuramyl‐L‐alanyl‐D‐isoglutamine or muramyl dipeptide (MDP) was studied using in vitro plaque‐forming cell (PFC) response to sheep erythrocytes (SRBC). Addition of MDP to DBA/2 mouse spleen cell cultures resulted regularly in a 2 to 3‐fold increase of PFC numbers/10^6^ recovered cells (p < 0.01). Supernates (SPN) from MDP‐stimulated cultures added to standard spleen cell + SRBC cultures brought about even more important increases of PFC numbers (p < 0.01 to p < 0.001). SPN from cultures supplemented with MDP alone (without SRBC) were more active than those of cell + MDP + SRBC cultures, and SPN removed on day 3 of culture were more active than those of day 5. This activity of SPN was maintained accross an H‐2 histocompatibility barrier. Although pretreatment of spleen cells with anti‐θ antigen serum entirely suppressed the anti‐SRBC PFC response in spite of the presence of MDP, SPN from these cultures were as active as SPN from normal spleen cell MDP‐stimulated cultures. In contrast, pretreatment of spleen cells with specific rabbit anti‐mouse macrophage serum entirely suppressed both anti‐SRBC response and SPN activity. It was concluded that the target cell for MDP is the macrophage which releases factors ultimately acting on B cells through T cell mediation.