Macrophages support pathological erythropoiesis in Polycythemia Vera and Beta-Thalassemia

Regulation of erythropoiesis is achieved by integration of distinct signals. Among these, macrophages are emerging as erythropoietin-complementary regulators of erythroid development, particularly under stress conditions. We investigated the contribution of macrophages for physiological and pathological conditions of enhanced erythropoiesis. We utilized mouse models of induced anemia, Polycythemia vera and β-thalassemia in which macrophages were chemically depleted. Our data indicate that macrophages contribute decisively for recovery from induced anemia as well as the pathological progression of Polycythemia vera and β-thalassemia by modulating erythroid proliferation and differentiation. We validated these observations in primary human cultures, showing a critical direct impact of macrophages on proliferation and enucleation of erythroblasts from healthy individuals and Polycythemia vera or β-thalassemic patients. In summary, we identify a new mechanism that we named “Stress Erythropoiesis Macrophage-supporting Activity” (SEMA) that contributes to the pathophysiology of these disorders and will have critical scientific and therapeutic implications in the near future

Absence Of Interleukin-6 Protects Bone Marrow Erythroid Recovery Under Inflammation, A Process Inhibited By Iron Mediated Ros (Reactive Oxygen Species) Upregulation

Inflammatory states seen in infection and chronic disorders are often characterized by a condition called anemia of inflammation (AI). The iron deficiency in AI is predominantly due to an altered balance of the cytokine, Interleukin-6 (IL6) and the hormone hepcidin (Hamp). We have previously shown that lack of IL6 or Hamp in knockout mouse models (IL6-KO, Hamp-KO) injected with the heat-killed pathogen Brucella abortus (BA) results in improved recovery from anemia. However, BM erythroid recovery in IL6-KO mice was far more improved in comparison to the iron overloaded Hamp-KO mice. This prompted us to investigate cellular responses driving BM erythropoiesis under inflammation in IL6-KO mice and the effect of iron overloading conditions on erythroid recovery under condition of AI. To address these questions, we generated a double knock out for IL6 and Hamp (DKO) and investigated BM erythropoiesis in WT, IL6-KO, Hamp-KO and DKO mice. The erythroid recovery in the BM of these mice were characterized by two phases. During the first phase (in the first 72 hours following BA administration), all the mice showed impaired BM erythropoiesis. Analysis of WT and IL6-KO mice indicated that impaired BM was associated by a surge in inflammatory cytokines such as IFN and TNF and a concurrent increase in mitochondrial ROS. Following 72 hours BM erythropoiesis recovered in the BM: compared to WT and Hamp-KO, BM erythropoiesis was qualitatively and quantitatively better in IL6-KO animals, showing the best profile in DKO mice. During the second phase (10 to 14-day post BA administration), we observed a second surge of inflammatory cytokines. During this phase, we observed a severe regression of BM erythropoiesis in DKO mice, while IL6-KO animals continued to show an excellent profile. During the second phase, we also observed that, while WT mice upregulated mitochondrial ROS, IL6-KO animals did not. We showed that ROS upregulation is triggered by erythropoietic stress (as seen in WT animals recovering after phlebotomy). However, we also postulated that inflammation and iron overload can further increase oxidative stress in erythroid cells, and that excessive ROS upregulation impairs erythroid recovery. Hence, BM erythropoiesis in IL6-KO mice is improved by reduced ROS formation in presence of inflammatory cytokines. However, if our model is correct, administration of iron to IL6-KO animals should impair BM erythropoiesis during the second phase as seen in DKO. In fact, iron administration impaired BM erythropoiesis in IL6-KO mice and, concurrently, increased ROS in erythroid cells. In accordance with our model, during the second phase also DKO and Hamp-KO mice showed increased ROS in BM erythroid cells. We are currently investigating which form of iron is responsible for this mechanism (labile iron pool versus transferrin bound iron) and what the role of IL6 is in this process