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Endocytic and tumoricidal heterogeneity of rat liver macrophage populations
T Daemen1, A Veninga, F H Roerdink
1Laboratory of Physiological Chemistry, University of Groningen, The Netherlands.
Abstract:
The macrophage population of the liver has been reported to be heterogeneous with respect to endocytic and lysosomal enzyme activity. Yet we demonstrate that all liver macrophages in the rat can be activated to a tumoricidal state by the i.v. injection of liposomal muramyl dipeptide (MDP). After isolation, liver macrophages were fractionated according to size into five subfractions by means of elutriation centrifugation. Tumoricidal activity of liver macrophages, activated in vivo, was determined by an in vitro radioactivity release assay using B16 melanoma and C26 adenocarcinoma cells, labeled with [methyl-3H]thymidine, as target cells. Endocytic activity of the subpopulations both in vitro and in vivo was determined using [3H]-labeled liposome preparations. Finally, the extent to which the subpopulations become cytotoxic as a result of in vitro uptake of muramyl dipeptide-(MDP)-containing liposomes was studied employing the cytotoxicity assay described above. No significant differences in cytotoxicity between the macrophage subfractions were observed after i.v. injection of liposomal MDP, although endocytic uptake of liposomes per cell increased proportionally to cell size, both in vitro and in vivo. We found that in vitro uptake of MDP-containing liposomes by the subfractions produced the highest cytolytic activity in the small to intermediate-size macrophages. When taking into consideration the different extents of liposome uptake it can be concluded that the smaller liver macrophages are significantly more susceptible to activation than the larger cells. In vivo, low activation potential is balanced by high liposome uptake capacity thus allowing the whole macrophage population in the liver to become involved in the eradication of metastatic tumor growth.
Insights
All rat liver macrophages can be activated to a tumoricidal state using liposomal muramyl dipeptide (MDP). Smaller macrophages show higher susceptibility to activation, balancing their lower uptake capacity with greater cytotoxic potential for tumor eradication.
Area of Science:
- Immunology
- Cell Biology
- Hepatology
Background:
- Liver macrophages exhibit heterogeneous endocytic and lysosomal enzyme activity.
- Previous studies suggest functional differences within liver macrophage populations.
Purpose of the Study:
- To investigate the activation potential of different liver macrophage subpopulations to a tumoricidal state.
- To determine if liver macrophage heterogeneity impacts their response to liposomal muramyl dipeptide (MDP) activation.
Main Methods:
- Isolation and size-based fractionation of rat liver macrophages using elutriation centrifugation.
- In vivo and in vitro assessment of macrophage tumoricidal activity using a radioactivity release assay with labeled tumor cells.
- Evaluation of liposome uptake and cytotoxic activation of macrophage subpopulations following in vitro incubation with liposomal MDP.
Main Results:
- Intravenous injection of liposomal MDP activated all liver macrophage subfractions to a similar tumoricidal state.
- Endocytic uptake of liposomes per cell increased with cell size, both in vitro and in vivo.
- In vitro activation with liposomal MDP demonstrated higher cytolytic activity in smaller to intermediate-sized macrophages, indicating greater susceptibility.
Conclusions:
- Liver macrophage heterogeneity does not prevent uniform in vivo activation by liposomal MDP.
- Smaller liver macrophages are more susceptible to MDP-induced activation, with higher liposome uptake balancing this in larger cells.
- The entire liver macrophage population can participate in eradicating metastatic tumor growth through differential activation and uptake mechanisms.