A transgenic line that reports CSF1R protein expression provides a definitive marker for the mouse mononuclear phagocyte system

The proliferation, differentiation and survival of cells of the mononuclear phagocyte system (MPS, progenitors, monocytes, macrophages and classical dendritic cells) is controlled by signals from the macrophage colony-stimulating factor receptor (CSF1R). Cells of the MPS lineage have been identified using numerous surface markers and transgenic reporters but none is both universal and lineage-restricted. Here we report the development and characterization of a novel CSF1R reporter mouse. A Fusion Red (FRed) cassette was inserted in-frame with the C-terminus of CSF1R, separated by a T2A-cleavable linker. The insertion had no effect of CSF1R expression or function. CSF1R-FRed was expressed in monocytes and macrophages and absent from granulocytes and lymphocytes. In bone marrow, CSF1R-FRed was absent in lineage-negative hematopoietic stem cells (HSC), arguing against a direct role for CSF1R in myeloid lineage commitment. It was highly-expressed in marrow monocytes and common myeloid progenitors (CMP) but significantly lower in granulocyte-macrophage progenitors (GMP). In sections of bone marrow, CSF1R-FRed was also detected in osteoclasts, CD169 + resident macrophages and, consistent with previous mRNA analysis, in megakaryocytes. In lymphoid tissues, CSF1R-FRed highlighted diverse MPS populations including classical dendritic cells. Whole mount imaging of non-lymphoid tissues in mice with combined CSF1R-FRed/ Csf1r -EGFP confirmed the restriction of CSF1R expression to MPS cells. The two markers highlight the remarkable abundance and regular distribution of tissue MPS cells including novel macrophage populations within tendon and skeletal muscle and underlying the mesothelial/serosal/capsular surfaces of every major organ. The CSF1R-FRed mouse provides a novel reporter with exquisite specificity for cells of the MPS.