Marked enhancement of macrophage activation induced by synthetic muramyl dipeptide (MDP) conjugate using monoclonal anti-MDP antibodies.

Activation of peritoneal exudate macrophages of mice to inhibit the in vitro proliferation of tumor target cells was achieved with low concentrations of N-acetyl-L-alanyl-D-isoglutamine (MDP for muramyl dipeptide) conjugated to a synthetic carrier. Addition to the cultures of monoclonal anti-MDP or anti-carrier antibodies renders a thousandfold-smaller concentration of the conjugate highly effective in activating macrophages. This synergistic effect was observed neither with a control monoclonal antibody of different specificity nor with an F(ab)2 fragment of the monoclonal anti-MDP antibody. Other controls, such as addition to the cultures of the carrier alone with its specific monoclonal antibodies, also demonstrated that there exists a requirement for the presence of MDP in the conjugate. The possible uses of such a system as well as the underlying mechanisms are discussed.