In vitro activation of tumoricidal properties in rat alveolar macrophages by synthetic muramyl dipeptide encapsulated in liposomes

Alveolar macrophages (AM) lavaged from lungs of F344 rats were rendered tumoricidal by incubation in vitro with muramyl dipeptide (MDP). AM activated by this procedure were cytotoxic against syngeneic, allogeneic, and xenogeneic tumor cells but left cultures of normal (nontumorigenic) cells unharmed. MDP was encapsulated within multilamellar liposomes composed of phosphatidylserine-phosphatidylcholine. Dose-response experiments established that liposome-encapsulated MDP rendered AM tumoricidal at concentrations of approximately 4000 times lower than free MDP present in the media. These data demonstrate that AM can respond in vitro to MDP and that MDP encapsulated within liposomes is more efficient in rendering AM tumoricidal than unencapsulated MDP.