Blood Cell Homeostasis Is Maintained in the Absence of Resident Mononuclear Phagocytes in Fetal Liver and Bone Marrow in CSF1R Knockout Rats

Erythrocytes are produced through a highly regulated process that requires an adequate supply of iron to develop properly. The constant production of red blood cells occurs in erythroblastic island (EBI) niches, which exist in the fetal liver during embryonic development and move to the bone marrow. The macrophages within this niche are thought to be essential regulators of erythropoiesis via recycling iron for haemoglobin synthesis and phagocytosing the extruded nuclei generated during terminal erythroid maturation. The proliferation, differentiation and survival of many resident tissue macrophages depends upon signals from the macrophage colony-stimulating factor receptor (CSF1R), including macrophages within haematopoietic tissues. Homozygous loss of function mutation in Csf1r in the rat produces a near complete loss of tissue resident macrophages, which is associated with severe postnatal growth retardation and osteopetrosis.

CSF1R signalling is also implicated in maintaining erythropoiesis in a model of anaemia. We aimed to determine the role of CSF1R-dependent macrophages in steady-state erythropoiesis and iron homeostasis. RNA-Sequencing of fetal liver from late gestation embryos showed no dysregulation of erythropoiesis or iron homeostasis in CSF1R knockout rats, despite loss of macrophage associated genes and reduced staining of the key iron recycling receptor CD163. Reduced CD163 was also observed in the liver and spleen in 3-week-old knockout rats. At three weeks of age, RNA-Sequencing of bone marrow revealed a significant reduction tissue resident macrophage associated genes. Despite the loss of bone marrow, liver and spleen macrophages and altered phenotype of the few remaining macrophages, CSF1R knockout rats have normal haematological parameters and show no indication of iron deficiency or overload. Altogether these results indicate that CSF1R-dependent macrophages are not absolutely required for early-life steady-state erythropoiesis and iron homeostasis in rats. Ongoing studies focus on the mechanism of erythrocyte development, erythrocyte recycling and iron homeostasis in the absence of CSF1R-dependent macrophages.