Demonstration of muramyl dipeptide (MDP)-induced T suppressor cells responsible for MDP immunosuppressive activity.

Muramyl dipeptide (MDP), a synthetic immunostimulant, has been previously shown to increase or decrease the humoral and cellular immune responses, depending upon the experimental conditions used. In the present study we have investigated the mechanisms of MDP-induced immunosuppression. After repeated injections of high dosages of MDP in vivo, both adherent and B cell-enriched cell populations from MDP-treated mice were able to collaborate with normal complementary populations. In contrast, T cell-enriched populations exhibited suppressive activity which could be removed by treatment with anti-Thy-1.2 antiserum and complement. These results clearly indicate that MDP-induced immunosuppression is mediated by T cells.