Fetal liver macrophage development and function require Xpr1

Erythroblastic island macrophages (EBI-Macs) support the development of red blood cells by recycling iron, producing growth factors,and clearing nuclei expelled from erythroblasts. In the developing embryo, the liver is the primary site of hematopoiesis,and fetal liver macrophages perform the function of EBI-Macs.Here, we show that the phosphate exporter Xpr1 is critical for the development of fetal macrophages in the liver and the spleen.Single-cell RNA-seq and flow cytometry analyses in conditional mice lacking Xpr1in hematopoietic and/or CD206+cells revealed loss of the Kupffer cell transcriptional program and a shift in the development of fetal liver monocytes towards an interferon-activated monocyte/macrophage state.Functionally, this led to the failure to clear pyrenocytes. In adulthood,splenic red pulp and bone marrow macrophages were also absent upon loss of intrinsicXpr1.Collectively,these findings reveal that Xpr1 is required for the development, identity, and function of EBI-Macs. E15.5 embryonic livers were dissociated and cells isolated. Three Xpr1fl/fl and three Vav1iCre Xpr1fl/fl were pooled and CD45+CD19-CD3-B220-Ter119-CD49b-CD90.2- cells sorted from each pooled sample and subjected to scRNA-seq.