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Category: Microbial Genetics and Molecular Biology
Murine Monocytes: Origins, Subsets, Fates, and Functions, Page 1 of 2
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Monocytes are a conserved population of leukocytes that are present in all vertebrates, with some evidence of a parallel cell population in fly hemolymph ( 1 ). Monocytes are defined by their location in the bloodstream, their phenotype and nuclear morphology, as well as by their characteristic gene and microRNA expression signatures ( 2 – 5 ). In mice, monocytes represent 4% of the nucleated cells in the blood, with considerable marginal pools in the spleen and lungs that can be mobilized on demand ( 6 , 7 ). Within the blood, monocytes, and in particular the classical Ly6C+ mouse subset, exhibit a characteristically short half-life of 20 h ( 8 ), akin to that of similar ephemer neutrophils ( 9 ).
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Schematic of murine monocyte development and monocyte fates. Ly6C+ monocytes are continuously generated in the adult BM in steady state from hematopoietic stem cells (HSCs) via a sequence comprising common myeloid precursors (CMPs), GMPs, MDPs ( 11 ), and cMoPs ( 14 ). The BM also harbors a rare population of Ly6C– monocytes that could derive from Ly6C+ monocytes or cMoPs, but the function of this subset is unknown. Ly6C+ monocytes egress to the circulation, a process that requires CCR2 ( 18 ). Ly6C+ blood monocytes have a half-life of 20 h and several potential fates: (i) differentiation into Ly6C– monocytes that patrol the endothelial surface of the vasculature; (ii) extravasation to selected tissues whose steady-state macrophage compartment requires maintenance by monocyte recruitment, such as the intestine ( 6 , 7 ); and (iii) recruitment to sites of tissue injury, infection, wounds, tumors, and inflammation, where the cells differentiate depending on local cues and give rise to cells with macrophage or DC features. Ly6C+ monocytes can also return from the circulation to the BM cavities ( 12 ), where their fate remains unclear. Of note, the scheme refers to monocyte development in steady state, while under challenge alternative developmental routes could be activated that might, for instance, bypass the MDP stage.
Schematic of murine monocyte development and monocyte fates. Ly6C+ monocytes are continuously generated in the adult BM in steady state from hematopoietic stem cells (HSCs) via a sequence comprising common myeloid precursors (CMPs), GMPs, MDPs ( 11 ), and cMoPs ( 14 ). The BM also harbors a rare population of Ly6C– monocytes that could derive from Ly6C+ monocytes or cMoPs, but the function of this subset is unknown. Ly6C+ monocytes egress to the circulation, a process that requires CCR2 ( 18 ). Ly6C+ blood monocytes have a half-life of 20 h and several potential fates: (i) differentiation into Ly6C– monocytes that patrol the endothelial surface of the vasculature; (ii) extravasation to selected tissues whose steady-state macrophage compartment requires maintenance by monocyte recruitment, such as the intestine ( 6 , 7 ); and (iii) recruitment to sites of tissue injury, infection, wounds, tumors, and inflammation, where the cells differentiate depending on local cues and give rise to cells with macrophage or DC features. Ly6C+ monocytes can also return from the circulation to the BM cavities ( 12 ), where their fate remains unclear. Of note, the scheme refers to monocyte development in steady state, while under challenge alternative developmental routes could be activated that might, for instance, bypass the MDP stage.
Example flow cytometric analysis of BM-resident monocyte precursors and blood monocytes. (A) Analysis of myeloid BM compartment of C57BL/6 and CX3CR1gfp C57BL/6 mouse ( 9 ). Blood cells were lysed with BD lysis buffer (#349202). Cells are gated for scatter, singlets, and Lin (Ter-119, B220, Ly6G, NK1.1, TCRγδ, CD4, CD8)-negative cells expressing CD115. This gate comprises CD135+CD117+ MDPs and CD135–CD117+ cMoPs and CD135–CD117– monocytes. MDPs are CD11b–Ly6C–, while cMoPs are CD11bintLy6C+. BM monocytes are CD11b+ and can be subdivided according to Ly6C expression into major Ly6C+ and minor Ly6Clo-neg populations. Histogram shows CX3CR1-GFP reporter gene expression of MDPs, cMoPs, and monocytes, as compared to wild-type (WT) BM (gray). Note upregulation of reporter in Ly6Cint-neg BM monocytes. (B) Analysis of blood monocyte compartment of C57BL/6 and CX3CR1gfp C57BL/6 mouse ( 9 ). Blood cells were lysed with BD lysis buffer (#349202). Cells are gated for scatter (excluding sideward scatted [SSC] high neutrophils), singlets, and lineage negative according to Lin marker expression (B220, CD19, CD3, CD4, CD8, Ly6G, NK1.1, TCRγδ). Lin– cells comprise largely CD115+CD11b+ monocytes. “Ly6C+ monocytes” (red) can be defined as Ly6C+CD11c–CCR2+CD62L+CX3CR1int and “Ly6C– monocytes” (green) can be defined as Ly6C–CD11cintCCR2–CD62L–CX3CR1hi cells. Note that “Ly6Cint monocytes” (blue) also show intermediate expression of the other markers, as seen in the respective histograms. This supports the notion that these cells are a transitional population. FSC, forward scatter.
Example flow cytometric analysis of BM-resident monocyte precursors and blood monocytes. (A) Analysis of myeloid BM compartment of C57BL/6 and CX3CR1gfp C57BL/6 mouse ( 9 ). Blood cells were lysed with BD lysis buffer (#349202). Cells are gated for scatter, singlets, and Lin (Ter-119, B220, Ly6G, NK1.1, TCRγδ, CD4, CD8)-negative cells expressing CD115. This gate comprises CD135+CD117+ MDPs and CD135–CD117+ cMoPs and CD135–CD117– monocytes. MDPs are CD11b–Ly6C–, while cMoPs are CD11bintLy6C+. BM monocytes are CD11b+ and can be subdivided according to Ly6C expression into major Ly6C+ and minor Ly6Clo-neg populations. Histogram shows CX3CR1-GFP reporter gene expression of MDPs, cMoPs, and monocytes, as compared to wild-type (WT) BM (gray). Note upregulation of reporter in Ly6Cint-neg BM monocytes. (B) Analysis of blood monocyte compartment of C57BL/6 and CX3CR1gfp C57BL/6 mouse ( 9 ). Blood cells were lysed with BD lysis buffer (#349202). Cells are gated for scatter (excluding sideward scatted [SSC] high neutrophils), singlets, and lineage negative according to Lin marker expression (B220, CD19, CD3, CD4, CD8, Ly6G, NK1.1, TCRγδ). Lin– cells comprise largely CD115+CD11b+ monocytes. “Ly6C+ monocytes” (red) can be defined as Ly6C+CD11c–CCR2+CD62L+CX3CR1int and “Ly6C– monocytes” (green) can be defined as Ly6C–CD11cintCCR2–CD62L–CX3CR1hi cells. Note that “Ly6Cint monocytes” (blue) also show intermediate expression of the other markers, as seen in the respective histograms. This supports the notion that these cells are a transitional population. FSC, forward scatter.