• Macrophage colony-stimulating factor (CSF1) receptor (CSF1R) signaling controls the proliferation, differentiation, and survival of macrophages. Germ-line deletion of Csf1r causes a global deficit in most tissue-resident macrophage populations.
• The Fms-intronic regulatory element (FIRE), a super-enhancer located in intron 2 of the mouse Csf1r locus, is selectively deleted in B-Csf1r KO (CBA) mice.
• Mouse CSF1R protein was undetectable on blood Ly6C⁺ and Ly6Clow monocytes, as well as on both large and small peritoneal macrophages recovered from B-Csf1r KO (CBA) mice. Moreover, CD45low and CD11b+ microglia were absent only in brain from homozygous B-Csf1r KO mice(CBA), but not wild-type control mice.
• This mouse model provides a powerful tool for dissecting the functions of tissue-specific macrophages and for microglia-transfection studies.

Gene targeting strategy for B-Csf1r KO mice(CBA). The Fms-intronic regulatory element (FIRE), a super-enhancer located in intron 2 of the mouse Csf1r locus, is selectively deleted in B-Csf1r KO (CBA) mice.

CSF1R expression analysis in wild-type CBA mice and homozygous B-Csf1r KO mice(CBA) by flow cytometry. Blood were collected from wild-type CBA mice (+/+) and homozygous B-Csf1r KO mice(CBA) (-/-). Protein expression was analyzed with anti-mouse CSF1R antibody (Biolegend, 135509) by flow cytometry in Ly6Clow and Ly6C+ monocyte populations. Mouse CSF1R was only detectable in wild-type CBA mice, but not in homozygous B-Csf1r KO mice(CBA).

CSF1R expression analysis in wild-type CBA mice and homozygous B-Csf1r KO mice(CBA) by flow cytometry. Peritoneal lavage cells were collected from wild-type CBA mice (+/+) and homozygous B-Csf1r KO mice(CBA) (-/-). Protein expression was analyzed with anti-mouse CSF1R antibody (Biolegend, 135509) by flow cytometry. There was two macrophages populations in the peritoneal lavage cells: small peritoneal macrophages (SPM) identified as F4/80lowCD11b+, and large peritoneal macrophages (LPM) identified as F4/80highCD11b+. Mouse CSF1R was only detectable in wild-type CBA mice, but not in homozygous B-Csf1r KO mice(CBA). Besides, the LPM population was almost lost in B-Csf1r KO mice(CBA).

Loss of microglia in homozygous B-Csf1r KO mice(CBA) by flow cytometry. Brains were collected from wild-type CBA mice (+/+) and homozygous B-Csf1r KO mice(CBA) (-/-, 6-7-week-old, female). Myelin-depleted single cells obtained by using Myelin Removal Beads II kit from Miltenyi Biotec (130-096-433) were analyzed by flow cytometry for CD45 and CD11b expression. Microglia=CD45lowCD11b+. Microphages=CD45+CD11b+. CD45lowCD11b+ microglia were almost absent only in the homozygous B-Csf1r KO mice(CBA) but not in wild-type control mice.