The ability of recombinant murine interferon-gamma (rMuIFN-gamma) to activate murine macrophages with or without several biological response modifiers (BRM), including synthetic muramyl dipeptide derivatives (MDPs), was investigated. Mouse peritoneal macrophages were activated by rMuIFN-gamma alone to the cytostatic state, but not the cytolytic state. Other BRM as well as bacterial lipopolysaccharide (LPS), including a lyophilized preparation of an attenuated strain of Streptococcus hemolyticus, a cell wall skeleton of bacillus Calmette-Guerin and synthetic MDPs, were highly active in generating the synergism with rMuIFN-gamma. Macrophages were endowed with the cytolytic activities by combinations of rMuIFN-gamma and MDP-Lys(L18); the combination of 100 U/ml of rMuIFN-gamma with 10 ng/ml of MDP-Lys(L18) was sufficient to induce cytolytic activities in macrophages. The synergism was observed when the macrophages primed with rMuIFN-gamma were treated with LPS or MDP-Lys(L18), but not when the sequence of treatment was reversed. The cytotoxicity of macrophages induced by rMuIFN-gamma with MDP-Lys(L18) was suppressed by priming with MDP-Lys(L18). The suppressive effect was also observed by priming with LPS in combinations of rMUIFN-gamma and LPS. The reason for the suppression of macrophage activation by priming with LPS and MDP-Lys(L18) is at present unknown.
