Functional characteristics of the rat liver macrophage population after a single intravenous injection of liposome-encapsulated muramyl peptides.

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In this study, we investigated different functional characteristics of the rat liver macrophage population after a single i.v. injection of liposome-encapsulated muramyl dipeptide (MDP) or its lipophilic derivative muramyl tripeptide-phosphatidylethanolamine (MTP-PE). The in situ induced tumoricidal activity of the liver macrophage population was determined in vitro against C26 colon adenocarcinoma cells. For investigating which cells are responsible for the observed cytotoxic effects, subfractions of the liver macrophage population, differing in cell size, were isolated at different intervals after injection of the liposomal muramyl peptides. From these subfractions, the number of cells, degree of cytotoxicity, and the response to an additional activation with free MDP in vitro were determined. Maximal induction of tumoricidal activity of the liver macrophage population was reached between 12 and 24 hours after injection of liposomal MDP, while no significant differences between the subfractions were observed. Heterogeneity of tumor cytolytic capacity was observed in subfractions of macrophages isolated at 2 and 48 hours after injection. At these time points, highest cytolytic activity was observed for the small to intermediate-size macrophages. No significant cytotoxicity was detectable in any subfraction 72 hours after injection of liposomal MDP. An identical pattern of macrophage tumoricidal activity was observed after injection of liposomal MTP-PE, although slightly lower cytotoxicity levels were found. When isolated during the first 12 hours after injection of liposomal MDP, the macrophage population was unable to respond to a subsequent in vitro exposure to MDP, with respect to tumor cytotoxicity. Twenty-four and 48 hours after injection, the smallest cells could be slightly reactivated, whereas the larger cells still remained unresponsive.(ABSTRACT TRUNCATED AT 250 WORDS)