Project Summary/abstract Polycythemia vera (PV) is a clonal hematologic myeloproliferative disorder (MPD) characterized by substantial symptomatic systemic inflammation, thromboembolic complications, and impaired survival. Most patients with PV are iron deficient at diagnosis, which is exacerbated by repeated therapeutic phlebotomies, the mainstay of treatment. As a result, PV patients often suffer from iron deficiency related symptoms without treatment options. How iron deficiency develops in PV patients prior to the initiation of phlebotomy is not well understood and whether and how it contributes to the inflammatory state in PV is unexplored. Furthermore, a recent renaissance in iron metabolism research and a more clear understanding of how anemia occurs in iron deficiency has enabled us to ask an overarching question: does iron deficiency play a causal role in the inflammation observed in PV patients? To address these questions, we generated preliminary data demonstrating that iron deficiency specifically within macrophages induces the production of inflammatory cytokines relative to iron replete macrophages. Consistently, we have previously shown that hepcidin-mimetic agents sequester iron in splenic macrophages in PV mice. Furthermore, we now confirm in our current phase II clinical trial using hepcidin mimetic PTG-300 that increased iron sequestration in macrophages simultaneously enables hematocrit control, reverses systemic iron deficiency, and reduces inflammation-associated symptoms in PV patients. These findings raise the strong possibility that iron depleted macrophages in PV patients may be responsible for the insufficiently suppressed hepcidin and in part result in the inability of return to steady state iron absorption, demonstrating a novel mechanism by which iron deficiency induces a negative feedback to increase hepcidin. In light of the important role of macrophages in inflammation, we hypothesize that iron deficiency specifically in macrophages is critical in the pathophysiology of PV. To test this hypothesis, we propose to evaluate markers of iron metabolism, markers of inflammation, and functional endpoints in splenic macrophage from PV mice on iron replete and iron deficient diet as well as treated with hepcidin-mimetics. Taken together, we anticipate that these studies will provide additional conceptual and technical support for the studies proposed in our revised resubmitted R01 application.