FUNCTIONAL-CHARACTERISTICS OF THE RAT-LIVER MACROPHAGE POPULATION AFTER A SINGLE INTRAVENOUS-INJECTION OF LIPOSOME-ENCAPSULATED MURAMYL PEPTIDES

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 deriva tive muramyl tripeptide-phosphatidylethanolamine (MTP-PE). The in situ induced tumoricidal activity of the liver macrophage population was d etermined in vitro against C26 colon adenocarcinoma cells. For investi gating which cells are responsible for the observed cytotoxic effects, subfractions of the liver macrophage population, differing in cell si ze, were isolated at different intervals after injection of the liposo mal muramyl peptides. From these subfractions, the number of cells, de gree of cytotoxicity, and the response to an additional activation wit h 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 diffe rences between the subfractions were observed. Heterogeneity of tumor cytolytic capacity was observed in subfractions of macrophages isolate d at 2 and 48 hours after injection. At these time points, highest cyt olytic activity was observed for the small to intermediate-size macrop hages. No significant cytotoxicity was detectable in any subfraction 7 2 hours after injection of liposomal MDP. An identical pattern of macr ophage tumoricidal activity was observed after injection of liposomal MTP-PE, although slightly lower cytotoxicity levels were found. When i solated during the first 12 hours after injection of liposomal MDP, th e 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 reactiv ated, whereas the larger cells still remained unresponsive. Seventy-tw o hours after injection, reactivating potential was only slightly decr eased, compared with macrophages isolated from control animals. We obs erved an approximately two-fold increase in the number of isolated cel ls from 2 up to 24 hours after injection of liposomal MDP, mainly as a result of an increase in the number of cells in the small-cell fracti ons. At 2 hours after injection, this increase is mainly due to an inc rease in the number of granulocytes in these fractions, which coincide s with an increase in the percentage of granulocytes in the blood at t his time point. At 12 and 24 hours after injection, no granulocytes we re observed in the different subfractions. The increase in the number of small-size cells at 12 and 24 hours after injection is accompanied by an increase in the percentage of monocytes in the blood at these ti me points. We conclude that it is likely that these cells play an impo rtant role in the cytotoxic potential of the liver, induced by liposom al muramyl peptides.