Epo Dependent Signaling In Macrophages Regulates Stress Erythropoiesis

Steady State erythropoiesis occurs in the bone marrow and is primarily homeostatic. In response to anemic stress the need for new erythrocytes quickly outpaces the erythropoietic capacity of steady state erythropoiesis. At these times, stress erythropoiesis predominates. Stress erythropoiesis is best understood in mice where this process is primarily extra-medullary occurring in the adult spleen and liver and in the fetal liver during development. Stress erythropoiesis utilizes progenitors and signals that are distinct from steady state erythropoiesis. Using a variety of experimental systems, we have developed a model for stress erythropoiesis during the recovery from anemic stress. This recovery can be divided into three stages. Amplification of progenitors that exhibit stem cell properties, the induction of a signal that promotes the switch from amplifying stress progenitors to differentiating stress progenitors and the final stage where stress progenitors rapidly differentiate into new erythrocytes. We have identified specific stress progenitor populations at each stage on this process as well as the signals that regulate the amplification, the switch to differentiation and differentiation of stress erythroid progenitors. Here we show that macrophage dependent signals play key roles at each stage of stress erythropoiesis. The transition from amplifying stress erythroid progenitors to differentiating stress erythroid progenitors is mediated by Epo dependent signaling in macrophages which changes the signals made by the macrophage microenvironment from those that promote amplification (Wnt family factors) to those that promote differentiation (PGE2). This paradigm is true for murine and human stress erythroid progenitors. This analysis reveals a dynamic interplay between progenitor cells, the macrophage microenvironment and hypoxic tissues in vivo during the recovery from anemic stress.