An activin receptor IIA ligand trap corrects ineffective erythropoiesis in β-thalassemia

Michael Dussiot et al . show that an activin receptor IIA ligand trap ameliorates anemia in a mouse model of β-thalassemia by blocking the deleterious effects of GDF11. Mechanistically, GDF11 inactivation reversed ineffective erythropoiesis by promoting terminal erythroblast differentiation and by inducing apoptosis of immature erythroblasts. Also in this issue, Rajasekhar Suragani et al . show related findings using a modified activin receptor IIB ligand trap. The pathophysiology of ineffective erythropoiesis in β-thalassemia is poorly understood. We report that RAP-011, an activin receptor IIA (ActRIIA) ligand trap, improved ineffective erythropoiesis, corrected anemia and limited iron overload in a mouse model of β-thalassemia intermedia. Expression of growth differentiation factor 11 (GDF11), an ActRIIA ligand, was increased in splenic erythroblasts from thalassemic mice and in erythroblasts and sera from subjects with β-thalassemia. Inactivation of GDF11 decreased oxidative stress and the amount of α-globin membrane precipitates, resulting in increased terminal erythroid differentiation. Abnormal GDF11 expression was dependent on reactive oxygen species, suggesting the existence of an autocrine amplification loop in β-thalassemia. GDF11 inactivation also corrected the abnormal ratio of immature/mature erythroblasts by inducing apoptosis of immature erythroblasts through the Fas–Fas ligand pathway. Taken together, these observations suggest that ActRIIA ligand traps may have therapeutic relevance in β-thalassemia by suppressing the deleterious effects of GDF11, a cytokine which blocks terminal erythroid maturation through an autocrine amplification loop involving oxidative stress and α-globin precipitation.