Abstract

BackgroundOur original demonstration of immunomodulatory effects of erythropoietin in multiple myeloma, led us to the search of the cells in the immune system that are direct targets to erythropoietin. The finding that lymphocytes do not express erythropoietin receptors, has led to the hypothesis that other cells act as direct targets and thus mediate the erythropoietin effects. Having found erythropoietin effects on dendritic cells thus led to the question of whether macrophages act as target cells to erythropoietin. Design and Methods EPO effects on macrophages were investigated both in-vivo and in-vitro. The in-vivo studies were performed on splenic macrophages and inflammatory peritoneal macrophages, in recombinant human erythropoietin -treated, compared to untreated mice, as well as in transgenic mice over-expressing human erythropoietin (tg6), compared to their control wild type counterparts. The in-vitro effects of erythropoietin on macrophage surface markers and function were investigated in murine bone marrow-derived macrophages treated with recombinant human erythropoietin. Results Erythropoietin effects on macrophages were found under both the in-vivo and in-vitro experiments. In-vivo treatment led to increased numbers of splenic macrophages, and of the splenic macrophages expressing CD11b, CD80 and major histocompatibility complex class II. The peritoneal inflammatory macrophages obtained from erythropoietin -treated mice displayed increased expression of F4/80, CD11b, CD80 and major histocompatibility complex class II, and augmented phagocytic activity. The macrophages derived in-vitro from bone marrow cells expressed erythropoietin receptor transcripts, and in-vitro stimulation with erythropoietin activated multiple signaling pathways, including signal transducer and activator of transcription (STAT)1 and 5, mitogen-activated protein kinase, phosphatidylinositol 3-kinase and nuclear factor kappa B. Erythropoietin treatment in-vitro to these cells up-regulated the cell surface expression of CD11b, F4/80 and CD80, it enhanced their phagocytic activity and nitric oxide secretion, and also displayed augmented interleukin 12 secretion and decreased interleukin 10 secretion in response to Lipopolysaccharide. ConclusionsOur results show that macrophages are direct targets of erythropoietin and that erythropoietin treatment enhances their pro-inflammatory activity and function. These findings point to a multifunctional role of erythropoietin and its potential clinical applications as an immunomodulating agent.