BONE MARROW TFR2 DELETION COOPERATES WITH ACTIVIN LIGAND TRAP RAP-536 IN AMELIORATING β-THALASSEMIA

β-thalassemia, caused by mutations in the β-globin gene, is characterized by anemia, ineffective erythropoiesis (IE), and iron overload. Current management of the disease is still suboptimal, with bone marrow transplantation and gene therapy being the sole curative options. Luspatercept, an activin receptor-ligand trap, is a promising novel approach which stimulates erythroid differentiation inhibiting the TGF-β pathway. However, its exact mechanism of action and the possible connection with erythropoietin remain to be clarified. Moreover, Luspatercept does not correct all the features of the disease. For these reasons, we investigated the effect of combining Luspatercept with hematopoietic transferrin receptor 2 (TFR2) inactivation, which stimulates erythropoiesis enhancing erythropoietin signaling,

To this aim, Hbb^th3/+ mice with or without hematopoietic Tfr2 (Tfr2^BMKO/Hbb^th3/+) were treated with RAP-536 (the murine Luspatercept analogue) or vehicle. As expected, both Tfr2 deletion and RAP treatment improved anemia and IE in Hbb^th3/+ mice. Furthermore, hematopoietic Tfr2 deletion, but not RAP-536, reduced spleen size and erythropoietin levels, and corrected iron-overload..Interestingly, Tfr2 deletion and RAP-536 had an additive effect on improving anemia and IE, further reducing spleen size and liver inflammation,. Of note, RAP-536 efficacy in correcting anemia and IE was comparable in both Hbb^th3/+ and Tfr2^BMKO/Hbb^th3/+ animals, suggesting a TFR2 and erythropoietin-independent effect of the drug on erythroid differentiation.

Our study sheds light on Luspatercept's mechanism of action,and  suggests that strategies aimed at inhibiting TFR2 may enhance the therapeutic efficacy of activin receptor ligand-traps, ameliorating anemia, IE and iron overload in β-thalassemia, and potentially preventing severe complications.