Potentiation of erythroid abnormalities following macrophage depletion in aged rats.

OBJECTIVES: The effects of prolonged macrophage depletion on haematological parameters were investigated in aged rats and compared with those in young ones. METHODS: Four weekly i.v. injections of dichloromethylene diphosphonate-containing liposomes (Cl2MDP-CL) were employed to achieve a prolonged depletion of bone marrow (BM) and spleen macrophages. The number of BM macrophages was then assessed by flow cytometry, whereas the spleen clearance function was judged by the elimination of oxidised red blood cells (RBC). Haematological parameters and signs of RBC ageing (reduced MCV, increased density and augmented 4.1a/4.1b membrane protein ratio) were determined. Finally, the recovery from phlebotomy-induced acute anaemia was investigated. RESULTS: Following the Cl2MDP-CL treatment, in comparison with young rats, the aged animals showed: (i) reduced numbers of BM macrophages; (ii) greater impairment of spleen clearance function; (iii) similar anaemic condition and signs of RBC ageing; (iv) greater increase in white blood cell (WBC) numbers (mainly neutrophils). In addition, whereas aged control rats showed a recovery from phlebotomy-induced acute anaemia which was similar to that of the untreated young animals, in the aged-treated rats, a significantly diminished/delayed restoration of RBC, Hb and reticulocyte to normal values was observed, accompanied by a significantly higher increase in WBC numbers than in the other groups of animals. CONCLUSION: Haematological abnormalities because of Cl2MDP-CL-induced macrophage depletion are potentiated in aged rats in which the BM regenerative potential of the erythroid lineage as well as the clearance function of the spleen appear compromised. Thus, in aged rats, macrophage dysfunction is likely to interfere with erythroid homeostasis particularly during haemopoietic stress.

Aging of red blood cells and impaired erythropoiesis following prolonged administration of dichloromethylene diphosphonate containing liposomes in rats.

OBJECTIVES: To investigate whether macrophage-depleted rats may serve as a model for studying red blood cell (RBC) aging. METHODS: Rats were macrophage-depleted by 4 weekly injections of dichloromethylene diphosphonate-containing liposomes (Cl2MDP-CL). The macrophage content of spleens and bone marrows (BMs) was investigated by immunohistochemistry and light microscopy and by flow cytometry, respectively, after staining with macrophage-specific monoclonal antibodies. In addition, the ultrastructure of residual BM macrophages and their ability to phagocytose zymosan was studied. BM was also studied for apoptosis (by the TUNEL reaction) and for erythroid progenitor cell content. Furthermore, RBC indices, morphology, life span (by 51Cr labeling) and aging features (MCV, density, 4.1a/4.1b membrane protein ratio, anti-spectrin IgG binding, microvesiculation) were investigated. Serum TNF-alpha, iron, total iron-binding capacity (TIBC) and ferritin were also determined. RESULTS: Prolonged treatment with Cl2MDP-CL caused an almost complete depletion of macrophages in the spleen and a 58% reduction of those in the BM; the residual BM macrophages were activated as judged by their ultrastructure and phagocytic capacity in vitro. These alterations were accompanied by an increase in RBC life span and age-related RBC changes, as well as by mild anemia associated with a reduced reticulocyte count, reduced BM erythroid progenitors, increased numbers of apoptotic cells in the BM, low serum iron, high TIBC and increased serum TNF-alpha levels. CONCLUSIONS: Rats subjected to prolonged macrophage depletion showed an increased prevalence of senescent RBC in the circulation due to their impaired clearance by macrophages. Hence, these animals provide a model system in which mechanisms of RBC aging can be delineated. They also showed impaired erythropoiesis, presumably related to a reduction in BM macrophages and increased production of proinflammatory cytokines by residual activated marrow macrophages and other cells.

Impaired phagocyte antibacterial effector functions in beta-thalassemia: a likely factor in the increased susceptibility to bacterial infections.

Bacterial diseases are serious complications of beta-thalassemia syndromes but the mechanisms underlying the increased susceptibility to these infections are not fully understood. Factors which are likely to be involved are anemia, splenectomy, iron-overload and alterations in innate/adaptive immune responses. There is substantial evidence that a defect in innate effector functions of phagocytes (neutrophils, monocytes/macrophages) plays an important role in the weakened resistance to pathogenic bacteria and is at least in part due to iron overload. There is substantial evidence of an iron-related defect in bacterial phagocytosis by neutrophils. Moreover, reduced chemotaxis by these phagocytes has been repeatedly demonstrated. Similarly, an impairment of monocyte bacterial phagocytosis and generation of anti-bacterial compounds have recently been delineated but any relation to iron overload needs to be established. Additional mechanisms of defective innate immune responses such as altered expression of pathogen recognising receptors and function seem possible and have to be explored. Further insight into innate phagocyte effector functions in beta-thalassemia is essential for understanding the increased susceptibility to bacterial infections and their management.