Canonical Notch2 signaling regulates development and maintenance of iron-recycling macrophages and iron homeostasis.

Red pulp macrophages (RPM) and bone marrow macrophages (BMM) are iron-recycling cells, involved in iron homeostasis and erythropoiesis. Here we show, by conditional deletion strategies using Cx3cr1Cre-mediated targeting to inactivate Notch signalling components in mice, that canonical Notch2 signalling regulates the development of RPM and BMM. Loss of functional Notch2, or its nuclear mediator Rbpj, impair RPM and BMM development and cause iron overload in the spleen and bone marrow. In the absence of Notch2, prototypic RPM genes are downregulated, which is accompanied by splenic extramedullary haematopoiesis and changes in splenic microarchitecture. Treatment of mice with an anti-Notch2 blocking antibody recapitulates the impaired erythrophagocyte and de novo extramedullary hematopoiesis phenotypes characteristic to the genetic model. Furthermore, early postnatal transfer of bone marrow and fetal liver progenitors rescues the defects in RPM and BMM in a Notch2-dependent manner, demonstrating the potential to restore defective tissue resident macrophage niches by Notch-competent progenitors. Thus, our study demonstrates that canonical Notch2 signalling is required for the development, maintenance, and function of iron-recycling macrophages.