Peripheral human red blood cell development in human immune system mouse model with heme oxygenase-1 deficiency

• Immuno-deficient mice with murine heme oxygenase-1 deletion allow persistence of human red blood cells (hRBCs) in their peripheral blood.

• Immuno-deficient mice with murine heme oxygenase-1 deletion have reduced erythrophagocytic macrophages.

A challenge for human immune system (HIS) mouse models has been the lack of human red blood cells (hRBCs) survival after engraftment of these immune-deficient mice with human CD34+ hematopoietic stem cells (HSCs). This limits the use of HIS models for preclinical testing of targets directed at hRBCs-related diseases. Even though human white blood cells can develop in the peripheral blood of these human HSC-engrafted mice, peripheral hRBCs are quickly phagocytosed by murine macrophages upon egress from the bone marrow (BM). Genetic ablation of murine myeloid cells results in severe pathology in resulting mice, rendering such an approach to increase hRBC survival in HIS mice impractical. Heme oxygenase-1 (HMOX-1) deficient mice have reduced macrophages due to toxic build-up of intracellular heme upon engulfment of red blood cells, but do not have an overall loss of myeloid cells. We took advantage of this observation and generated a HMOX-1-/- on a humanized M-CSF/SIRPa/CD47 Rag2-/- IL-2Rg-/- background. These mice have reduced murine macrophages but comparable level of murine myeloid cells to HMOX-1+/+ control mice in the same background. Injected hRBCs survive longer in HMOX-1-/- mice than in HMOX-1+/+ controls. Additionally, upon human HSC-engraftment, hRBCs can be observed in the peripheral blood of HMOX-1-/- humanized M-CSF/SIRPa/CD47 Rag2-/- IL-2Rg-/- mice and hRBC levels can be increased by treatment with human erythropoietin. Since hRBC are present in the peripheral blood of engrafted HMOX-1-/- mice, these mice have the potential to be used for hematological disease modeling, and to test therapeutic treatments for hRBC diseases in vivo.