ABSTRACT Erythropoiesis occurs at the erythroblastic island where the central macrophage (EBI M) supports proliferation and maturation of the surrounding erythroid cells. Alterations in EBI M contribute to the disordered erythropoiesis in various hematological diseases including polycythemia vera (PV) which is characterized by erythrocytosis due to V617F mutation in JAK2 (JAK2V/F). In this application, we propose to define the mechanisms by which EBI M support normal erythropoiesis in Aim 1 and to define the mechanisms by which JAK2V/F in EBI M contributes to pathogenesis of PV in Aim 2. Our overall hypothesis is that EBI M support normal erythropoiesis via Epor-Stat5 signaling-mediated production of growth factors Igf1 and Scf as well as iron carrier transferrin and that JAK2V/F-induced constitutive activation of Epor signaling in the EBI M leads to upregulation of transferrin receptor CD71 which contributes to the hyperproliferation of EBI M along with increased production of Igf1 and Scf leading to overproduction of red blood cells that can be alleviated by interfering with CD71, Igf1-Igf1r axis or/and Scf-cKit axis. Our hypothesis is based on our findings that i) EBI M expressed EPOR; ii) EPOR+ M expressed Igf1 and Scf as well as iron carrier transferrin (Trf); iii) EPO stimulation increased the levels of Igf1, Scf and Trf by EBI M; iv) selective deletion of Epor in M led to anemia along with decreased levels of Igf1, Scf and Trf; v) selective deletion of Igf1 or Trf in M also led to anemia; vi) although knock-in of Jak2V/F in erythroid cell alone using Gypa-tdTomato-Cre or in M alone using CD169- tdTomato-Cre led to erythrocytosis in mice, knock-in of Jak2V/F in both erythroid cells and M were required to fully recapitulate characteristics of human PV; vii) knock-in of Jak2V/F in M led to increased numbers of BM EBI M and spleen red pulp M along with increased levels of transferrin receptor Trfc (CD71), Igf1 and Scf. In Aim 1, we will generate