Changes in murine bone marrow macrophages and erythroid burst-forming cells following the intravenous injection of liposome-encapsulated dichloromethylene diphosphonate (Cl2MDP)

: In order to explore the effect on bone marrow macrophages of liposome‐encapsulated dichloromethylene diphosphonate (Cl2MDP), mice were injected intravenously with a preparation of such liposomes at a dose known to deplete spleen and liver macrophages. Two days later, the macrophages in the marrow of the femoral bones were quantified by flow cytometry using a macrophage‐specific monoclonal antibody (F4/80), and their ultrastructure and phagocytic activity towards zymosan particles was assessed. To determine the effect on erythropoiesis of liposome‐encapsulated Cl2MDP‐induced changes in bone marrow macrophages, red blood cell parameters and the formation of erythroid burst‐forming unit (BFU‐E)‐derived colonies in vitro were evaluated. In mice injected with liposome‐encapsulated Cl2MDP, there was a 54% and 67% decrease in the total number of bone marrow macrophages as compared to uninjected controls and mice treated with empty liposomes, respectively. Moreover, residual macrophages showed an abnormal ultrastructure, with reduced numbers of crystalloid inclusions and increased numbers of large myelin figures. However, the phagocytic activity of these cells was unimpaired or slightly enhanced. In mice injected with liposome‐encapsulated Cl2MDP there was an approximately 60% decrease in the percentage and total number of circulating reticulocytes and a 54% reduction in the BFU‐E number, demonstrating deregulation of erythropoiesis under conditions of macrophage loss and impairment. The results suggest that mice treated with liposome‐encapsulated Cl2MDP are a model for studying the role of macrophages in erythropoiesis.