Impaired splenic erythropoiesis in phlebotomized mice injected with CL2MDP-liposome: an experimental model for studying the role of stromal macrophages in erythropoiesis

Erythropoiesis occurs in the presence of erythropoietin (EPO) without macro phages in vitro, In hematopoietic tissues, however, erythroid cells associa te closely with stromal macrophages, forming erythroblastic islands via int eractions with adhesion molecules. To elucidate the role of macrophages in erythropoiesis, we selectively abrogated stromal macrophages of splenic red pulp of phlehotomized mice by injection with dichloromethylene diphosphona te encapsulated in multilamellar liposomes (CL2MDP-liposome). In the spleen , no erythropoietic activity occurred until 5 days after the treatment. Col ony assay revealed that the erythropoiesis was suppressed at the level of C FU-E. The splenic erythropoietic activity gradually developed front day 6 a fter the treatment, when F4/80(+) macrophages began to appear in the red pu lp. EPO mRNA was expressed in kidney but not in liver or spleen of phleboto mized mice injected with CL2MDP-liposome, and the serum EPO concentration i n these mice was higher than that in phlebotomized mice. These findings sug gest that abrogation of stromal macrophages by injection with CL2MDP-liposo me impairs the splenic microenvironment for erythropoiesis induced by hypox ic stress, and this may be an excellent experimental model for further char acterization of the in vivo role of splenic macrophages in erythropoiesis.