. 2018 Apr 6;9:697. doi: 10.3389/fimmu.2018.00697

Table 1.

Lineage tracing models and marker to distinguish microglia and monocyte-derived macrophages in the brain.

Approach	Cell type specificity	Principle	Advantages	Limitations	Reference
(a) Transplantation models

BMT; TBI	BMDM	HSC source of blood monocytes is replaced with modified/labeled HSCs	High chimerism No time consuming crossing into genetic disease models	Variability in myeloablation and reconstitution Artificial engraftment of BM cells in the CNS	(113)

BMT; HPI	BMDM	HSC source of blood monocytes is replaced with modified/labeled HSCs	High chimerism No time consuming crossing into genetic disease models	Variability in myelo-ablation and reconstitution	(37, 113)

BMT; Busulfan	BMDM	HSC source of blood monocytes is replaced with modified/labeled HSCs	High chimerism No time consuming crossing into genetic disease modelsChemical myeloablation No irradiation	Variability in myeloablation and reconstitution	(112)

Parabiosis	BMDM	HSC source of blood monocytes is replaced with modified/labeled HSCs	Constant influx of donor cells No myeloablationrequired No time consuming crossing into genetic disease models	Technically challengingLow chimerism	(39)

(b) Genetic lineage tracing models

Ccr2^RFP/wt; Cx3cr1^GFP/wt	Monocytes (red) MG (green)	Differential labeling of Cx3cr1^hi: Ccr2^neg MG (green) and of Cx3cr1^lo:Ccr2^hi monocytes (red)	MG and monocytes contain reporter for labeling	Recruitment to the brain leads to increased Cx3cr1 levels in monocyte-derived macrophages Ccr2 expression is downregulated in monocyte-derived macrophage upon differentiation	(114, 115)

Flt3-Cre	Monocytes and HSC-derived monocyte precursors	Label/modification induced in Flt3⁺ monocyte precursors	Useful for lineage tracing of myeloid precursors Useful complementary approach to MG restricted lineage tracing	Cre expression or transmittance restricted to male mice	(118)

Cx3cr1-Cre^ER	MG	Recombination is induced in all Cx3cr1⁺ cells upon tamoxifen pulse. Long-living MG retain the label/modification. While monocytes vanish and are replenished from precursors that were generated after Cre recombination in response to the tamoxifen pulse	Long-term labeling/modification is restricted to MG	Spontaneous modification reported in one model Low recombination in mMF (40–50%)	(42)

Sall1-Cre^ER	MG	Label/modification induced in Sall1⁺ MG	Sall1 expression is stable also in response to different stimuli	Targeting of non-hematopoietic cells in the liver, heart and kidney	(119)

(c) Cell-type restricted marker expression

CD45	MG^lo BMDM^hi	MG display lower surface expression	No requirement for combination of several markers	Activated in MG upregulate CD45 BMDM in brain malignancies downregulate CD45	(112, 113, 120)

Tmem119P2ry12Siglech	MG^hi BMDM^lo	MG show high expression	Applicable for mouse and human	Downregulation in MG during activation BMDM in GBM show increased expression	(58, 94, 112)

Sall1	MG^hi BMDM^lo	MG show high expression	Applicable for mouse and human Stable expression level at different activation levels	Low expression found on non-leukocytes in the liver, heart, and kidney	(119)

Itga4/Cd49d	MG^lo BMDM^hi	BMDM show high expression	Applicable for mouse and human Stable expression level at different activation levels	Expression found on T cells	(112)

BMT, bone marrow transplantation; TBI, total body irradiation; HPI, head protected irradiation; BMDM, bone marrow-derived macrophage; HSC, hematopoietic stem cell; MG, microglia; BM, bone marrow; CNS, central nervous system.