Changes in murine bone marrow macrophages and erythroid burst-forming cells following the intravenous injection of liposome-encapsulated dichloromethylene diphosphonate (Cl 2 MDP) Giuliani AL, Wiener E, Lee MJ, Brown IN, Berti G, Wickramasinghe SN. Changes in murine bone marrow macrophages and erythroid burst-forming cells following the intravenous injection of liposome- encapsulated dichloromethylene diphosphonate (Cl 2 MDP). Eur J Haematol 2001: 66: 221–229. # Munksgaard 2001. Abstract: In order to explore the effect on bone marrow macrophages of liposome-encapsulated dichloromethylene diphosphonate (Cl 2 MDP), 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 Cl 2 MDP-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 Cl 2 MDP, 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 Cl 2 MDP 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 Cl 2 MDP are a model for studying the role of macrophages in erythropoiesis. A. L. Giuliani 1 , E. Wiener 2 , M. J. Lee 2 , I. N. Brown 2 , G. Berti 1 , S. N. Wickramasinghe 2 1 Dipartimento di Medicina Sperimentale e Diagnostica, Sezione di Patologia Generale, Universita ` degli Studi di Ferrara, Ferrara, Italy and 2 Division of Investigative Science, Imperial College School of Medicine, St Mary’s Campus, London, UK Key words: macrophages; erythropoiesis; bone marrow; dichloromethylene diphosphonate; liposomes Correspondence: Dr E Wiener, Department of Haematology, Imperial College School of Medicine, St Mary’s Campus, Norfolk Place, London W2 1PG Tel: +020 7594 3993 Fax: + 020 262 5418 e-mail: e.wiener@ic.ac.uk Accepted for publication 29 December 2000 Liposome-encapsulated dichloromethylene diphos- phonate (Cl 2 MDP) has been used successfully to achieve mononuclear phagocyte (monocyte, macro- phage) depletion in experimental animals. The Cl 2 MDP-containing liposomes are ingested by the phagocytes, their phospholipid bilayers disrupted by lysosomal lipophosphatases and the drug Abbreviations: Cl 2 MDP, dichloromethylene diphosphonate; PBS, phosphate-buffered saline; FCS, fetal calf serum; mAb, monoclonal antibody; BSA, bovine serum albumin; HBSS, Hanks’ balanced salt solution; MGG, May–Gru¨nwald Giemsa; FACS, fluorescence-activated cell sorter; TBS, Tris-buffered saline; APAAP, alkaline phosphatase–anti-alkaline phospha- tase; FITC, fluorescein isothiocyanate; BFU-E, erythroid burst- forming unit. Eur J Haematol 2001: 66: 221–229 Printed in UK. All rights reserved Copyright # Munksgaard 2001 EUROPEAN JOURNAL OF HAEMATOLOGY ISSN 0902-4441 221