Homozygous kinase-dead Csf1r mutation in outbred mice reveals essential and redundant functions of tissue resident macrophages

The proliferation, differentiation and survival of cells of the macrophage lineage depends on signals from the macrophage colony-stimulating factor receptor (CSF1R). On a C57BL/6J background homozygous kinase-dead Csf1r mutation ( Csf1r ^E631K/E631K - E631K ^m/m ) causes perinatal lethality. Here we demonstrate that E631K ^m/m mice on a mixed genetic background (C57 x BALB/c F2) are osteopetrotic and growth retarded but viable as adults with no other gross developmental deficits. They lack osteoclasts, microglia and most peripheral tissue resident macrophages and exhibit perturbed hematopoiesis. Although CD169 ⁺ tissue resident macrophages in bone marrow are considered an essential component of the hematopoietic niche, CD169 is undetectable in E631K ^m/m marrow and F4/80 ⁺ macrophages are depleted. These changes are associated with expansion of mature and immature granulocytes and reduced B cells, whereas monocytes and stem and progenitor populations are unaffected as a proportion of total cells. Erythropoiesis in bone marrow is maintained in E631K ^m/m mice, associated with a residual population of CSF1R-independent CD169 ^-ve /F4/80 ⁺ macrophages. Nevertheless, splenic extramedullary hematopoiesis in E631K ^m/m mice indicates a degree of bone marrow insufficiency. Red pulp macrophages are retained but CD169 ⁺ marginal metallophil macrophages are absent and CD209b ⁺ (SIGNR1) macrophages are present but disorganized. Circulating white blood cell count is unchanged in E631K ^m/m mice, but the proportion of neutrophils is greatly increased whilst B cells and monocytes are reduced. This novel model reveals the essential roles of CSF1R-dependent macrophages in hematopoiesis and demonstrates that many developmental and homeostatic functions attributed to murine resident tissue macrophages are redundant and/or specific to inbred mouse strains.