Erythropoietin-Associated Increase in the Proportion of Kupffer Cells

Abstract 3266

Erythropoietin (EPO), the hormone produced mainly by the kidney, promotes the survival, proliferation and differentiation of erythroid precursor cells. Hence, recombinant human EPO (rHuEPO) is one of the key therapies for the treatment of anemia in patients with end-stage renal disease and in cancer patients, suffering from chemotherapy-induced anemia. The past decade has revealed extra-hematopoietic sites of EPO production along with abundance of EPO receptors (EPO-Rs) in various tissues and cell lines, suggesting that this hormone may have pleiotropic activities. Indeed, previous studies in our laboratory have shown that murine bone marrow-derived macrophages express EPO-R and EPO affects their cellular phenotype.

Kupffer cells are liver macrophages that play a key role in the body's defense system against invading pathogens. These cells affect normal and pathological conditions of the liver either directly or through their influence on other cells, such as stellate cells and are important components in iron metabolism. In this study, we found that transgenic mice, overexpressing human EPO (tg6 mice), have a larger population of F4/80 positive liver macrophages compared to wild type (wt) mice. In addition, C57bl/6 wt mice, injected 3 times a week with 180 U/ml of rHuEPO, displayed a 2 fold increase in the percentage of F4/80 expressing cells compared to their untreated littermates. This increase seems to stem from EPO's effect on a subpopulation of cells expressing both F4/80 and CD11b. As for pathological conditions, we demonstrate the same effect of EPO in an experimental murine model of 5T33 multiple myeloma (MM). 5T33 MM mice that were treated with rHuEPO (10 consecutive days of 30 U/ml followed by 3 weekly injections of 30 U/ml for 2 weeks), presented a 3 fold increase in the percentage of cells double positive for F4/80 and CD11b. In addition, these mice displayed a 20% increase in the mean fluorescence intensity of CD11b, suggesting that EPO causes not only an increase in the proportion of Kupffer cells but also induces the activation of CD11b positive cells, as reflected by the elevated expression of this marker per cell.

Our findings thus contribute to the understanding of EPO's effects on the immune system in general, and on Kupffer cells in particular. These findings may have clinical significance in those cases where activation of liver macrophages is desired, and EPO treatment for anemia is recommended.