The systemic activation of macrophages by liposomes containing immunomodulators

The demonstration that appropriately activated macrophages can destroy microorganisms and cancer cells has prompted an intense search to identify agents which can render these cells active in vivo. Several natural products, e. g., lymphokines or synthetic molecules, e. g., MDP can produce the tumoricidal state in macrophages. The in vivo use of these agents has been limited, since they have a very short half life. Liposomes offer a most useful carrier system to transport agents to phagocytic cells in vivo. Once in the circulation, liposomes are cleared by phagocytic cells and this passive localization provides an effective mechanism for targeting liposome-entrapped materials to macrophages. In this review we have described the exploitation of this mechanism to deliver lymphokines or other synthetic molecules to macrophages in situ. Since not all liposomes home equally to macrophages, there is still a great need to identify vesicles with ideal properties for this task. The potential application of liposome encapsulated agents to activate macrophages is tremendous. Only future studies will determine the effectiveness and limitations for activated macrophages in enhancing host defense against infections and cancer.