Macrophages Prevent Human Red Blood Cell Reconstitution In NOD/SCID and NOD/SCID/γc^−/− Mice.

Abstract 3723

An animal model supporting human erythropoiesis will be highly valuable for assessing the biological function of human RBCs under physiological and disease settings, and for evaluating protocols of in vitro RBC differentiation from human embryonic stem cells. Although immunodeficient mice on the NOD background have been widely used to study human hematopoietic stem cell function in vivo, the successful use of these mice in the study of human erythropoiesis and RBC function has not been reported. We have previously shown that co-transplantation of human fetal thymic tissue (under renal capsule) and CD34⁺ fetal liver cells (FLCs; i.v.) in NOD/SCID or NOD/SCID/γc^−/− mice results in the development of multilineage human hematopoietic cells. Here, we analyzed human RBC reconstitution in these humanized mice. Although a large number of human erythrocytes, which consisted predominantly of immature nucleated erythrocytes, were detected in the bone marrow of human fetal thymus/CD34⁺ FLC-grafted mice, human RBCs were undetectable in blood of these mice, even in those with nearly full human chimerism in peripheral blood mononuclear cells (PBMCs). Recipient mouse macrophage-mediated rejection is, at least, one of the major mechanisms responsible for the lack of human RBCs in these mice, as human RBCs became detectable in blood following macrophage depletion and disappeared again after withdrawal of treatment. Furthermore, treatment with human erythropoietin (EPO) and human IL-3 significantly increased human RBC reconstitution in mice that were depleted of macrophages. Like the human RBCs developed in the humanized mice, exogenously injected normal human RBCs were also rapidly rejected by macrophages in NOD/SCID mice. Taken together, our data demonstrate that human RBCs are highly susceptible to rejection by macrophages in immunodeficient mice. Thus, strategies for preventing human RBC rejection by macrophages are required for using immunodeficient mice as an in vivo model to study human erythropoiesis and RBC function.