Table 2.

Features of Newly-described microglial phenotypes

	Distinctive Features	unique phenotypic properties	phenotypic markers	Function	Incentives	Formation	Content	Distribution	Immunoreactivity	Animal models
Typical microglia	The resident non-activated microglia present ramified morphology with long extension; however, upon activation, they retract the extensions and become amoeboid [18]	--	TMEM119, CD11B, and P2RY12/P2RY13 expression, with low CD45 expression [78]	Regulate synaptic plasticity, learning, and memory mechanisms [79]	No mention	Erythromyeloid progenitor cells in the embryonic yolk sac [80], with the same origin as that of peripheral macrophages (myeloid progenitors)	Comprise 5–10% of all CNS cells [81]	in all brain regions, mainly in the gray matter [82]	High immunoreactivity for homeostatic markers (GFP in CX3CR1-GFP mice, P2RY12, IBA1, etc.)	---
LDAM	Containing large inclusions, i.e., lipofuscin granules [83]	Exhibit a unique transcriptional signature, show phagocytosis deficits, and produce increased levels of ROS and pro-inflammatory cytokines [51]	GRN, solute carrier family 33 Member 1 (SLC33A), SNX17, vacuolar protein sorting retromer complex component (VPS35), NPC2, and CLN3 [54]	Represent a dysfunctional and pro-inflammatory microglia state in the aging brain [51]	Increased extracellular lipid levels, inflammatory events, increased ROS levels, and intracellular metabolic changes [84]	Age-related neuroinflammation [51]	Approximately 50% of microglia in the aging brain accumulate lipid droplets [51]	Frequently found in the hippocampus and thalamus [51]	Immunoreactive for Iba1, Plin2, Plin3	Five human familial AD mutations (5XFAD) [85]
DAM	Rounded enlarged bodies, with 45% of DAMs being dystrophic [86]	upregulated expression of genes related to lipid metabolism, phagocytosis, and AD pathology, downregulated expression of homeostatic markers, without changes in expression of inflammatory cytokines [67]	anti-inflammatory DAM genes (e.g., Kcnj2, Nceh1, Timp2, CXCR4), pro-inflammatory DAM-specific gene (e.g.: Ptgs2/Cox2 or Tlr2, CD14, CD44)	Compared with normal microglia, DAMs show progressively increased lipid metabolism and expression of phagocytic genes, and therefore protect against AD and clear Aβ [67]	Accumulation of danger molecules present on apoptotic bodies of dying neural cells, lipid degradation products, and myelin debris [87]	Dependent on TREM2 signaling [67], also partially by APOE	A proportion of DAM increases with aging, accounting for 3% of all microglial cells in 20-month-old mice [67].	Within the cortex, but not the cerebellum, of AD mice [40]	Immunoreactive for IBA1 and HLA-DR	PS19 tau transgenic, SOD1-G93A transgenic, and aged mice; five human familial AD mutations (5XFAD) and CK-p25; PS2APP and APP/PS1 [67, 88]
MGnD	A phagocytic microglia phenotype with reduced ramifications and cell volume [89]	The signature genes associated with MGnD regulate lipid metabolism and phagocytosis [54]	Strong downregulated expression of homeostatic genes, with upregulated expression of selective genes including Spp1, Itgax, Axl, Clec7a, Lgals3, Apoe, and Grn [59]	Protective and represent an initial response to neuronal injury	Induced by neuronal apoptosis or Aβ plaques [59]	The switch from homeostatic microglia to MGnD is regulated by the TREM2-APOE pathway [59]	No mention	In APP-PS1 mice, MGnD primarily encircled amyloid plaques and dystrophic neurites, which are blanketed by homeostatic microglia in the periphery.	Low or no immunoreactivity for P2RY12	APP-PS1 mice, APP-PS1 Trem2−/− mice [59, 67]
Dark microglia	Electron microscopy reveals condensed cytoplasm and nucleoplasm, increased projections to synapses, and increased encircling of axon terminals and dendritic spines [90]	a downregulated expression of homeostatic markers, CX3CR1, IBA1 and P2RY12, but strongly expressed the microglia-specific 4D4 in their processes [62]	Strongly express CD11b, Trem246, and 4D4, with downregulated expression of IBA1, CX3CR1, and P2RY12 [64]	Extensively engulfing dendritic spines, axon terminals, and entire synapses	Chronic stress, aging, fractalkine signaling deficiency (CX3 CR1 knockout mice), and Alzheimer’s disease pathology (APP-PS1 mice) [64]	Derived from yolk sac, brain progenitors, or bone marrow-derived cells recruited to the brain in a CCR2‐independent manner [91]	In age-matched APP/PS1 littermates, the number of dark microglia corresponded to almost two‐thirds of the typical microglial population [64]	The hippocampal CA1 region (strata lacunosum-molecular and radiatum), subgranular layers of the cerebral cortex, basolateral nucleus of the amygdala, and hypothalamic median eminence [64]	Low immunoreactivity for homeostatic markers (GFP in CX3CR1-GFP mice, IBA1, CD11b, 4D4, TREM2); no ALDH1L1, OLIG2, P2RY12, 4C12, MHCII, CD206, CD11c expression [64]	APPSwe-PS1ΔE9、CX3CR1 knockout mice [64, 92]
PAM	Compared with typical microglia, PAM are amoeboid with thicker primary branches and larger cell bodies [93]	enrich many metabolic genes including almost the entire molecular machineries for oxidative phosphorylation, glycolysis and beta oxidation	Characterized by expression of numerous trophic factors including Igf1, Spp1, Lgals1, and Lgals3 [63]; contrastingly, many genes (Apoe, Igf1, Lilrb4, Lyz2, Colec12, Msr1, Map1lc3b) have upregulated expression [94]	Phagocytose newly formed oligodendrocytes and possibly new-born astrocytes during development	No mention	Independent of the TREM2-APOE axis [65]	Comprise one-third of the normal microglial population in APP-PS1 mice [64]	Developing cerebellar white matter and corpus callosum	Immunoreactive for CLEC7A	Trem2−/− or Apoe−/− [65]
WAM	Cluster in nodules	upregulated genes linked to atherosclerosis, cytokine signaling, and apoptosis [68], downregulation of genes expressed in homeostatic microglia, such as checkpoint genes	Increased expression of the NF-κB pathway and adhesion family GPCR GPR56 (ADGRG1) [95]	May represent a potentially protective response [68]	Aging and cerebral hypoperfusion	Dependent on age and TREM2 but not APOE signaling [68]; APOE dependent [96]	~ 20% in 21-month-old AD model mice	White matter tracts from the corpus callosum [68]	Immunoreactive for CD68 [96]	MTX rats (MTX through intraperitoneal injection every week at a dose of 200 mg/kg/week for a total of 4 weeks, and a final dose of 800 mg/kg) (97); Apoe−/− mice; APP/PS1 mouse model [68, 98]
ARM [70, 97]	No mention	MHC-II presentation (Cd74, Ctsb, Cstd), inflammatory processes (Cst7, Clec7a, Itgax) putative tissue repair genes (Dkk 1, Spp1, Gpnmb) and AD risk genes like APOE	Increased expression of histocompatibility complex class II genes (Cd74, H2-Ab1, and H2-Aa) and pro-inflammatory genes Cst7, Clec7a, and Itgax (encoding CD11c)	No mention	No mention	No mention	Increased from approximately ~ 3% in 3-month-old mice to ~ 12% of the total microglia in 21-month-old mice	No mention	No mention
IRM	Adopts a reactive morphology after rIFN-β exposure, with reduced dendrite length, branch, and terminal points [99]	the expression of thousands of interferon-stimulated genes [100]	Increased expression of Ifit3, Ifitm3, Irf7, and Oasl2 [100]	Contributing to the inflammatory tone of the aged and AD brain [71]	Aging, viral infection, cuprizone-induced demyelination	can be triggered by nucleic acid (NA)-containing plaques	15% of all APP/PS1 microglia [97]	No mention	Immunoreactive for IBA1	5XFAD mice [96], CK-p25 mice [75], APP-PS1, tauopathy (P301S和P301L)
NM	Chromosome fragmentation, membrane blebbing, and cell volume shrinkage	Release various pro-inflammatory cytokines and chemokines [101], including tumor necrosis factor-α and chemokine (C-C motif) ligand 2	--	Highly pro-inflammatory and immunogenic [72]	Different cellular stimuli, including TNF-α, FAS ligand, TRAIL, IFNγ, ischemia-reperfusion injury, and double-stranded RNA (dsRNA)	Through TLR4 activation	No mention	No mention	Immunoreactive for IBA1
Dystrophic microglia	Characterized by a dystrophic morphology, including process deramification, shortening, gnarling and beading, spheroid formation, and cytoplasmic fragmentation [77], 102]	Significant changes in genes controlling inflammation, including the NF-κB signaling pathway, and upregulation of complement genes C3 and complement factor B	--	Impaired neuronal activity, iron storage, reduced phagocytosis, and increased ROS production	Aging, iron-fed [76]	Through iron accumulation	There was an increase in the proportion of dystrophic microglia with age. In the case of neurodegenerative pathology, approximately 45% of the microglia were found to be dystrophic.	Near sites of tau pathology and amyloid plaques, in the neocortical gray matter of layers II–III in aged chimpanzees [30, 103]	Immunoreactive for IBA1（104）	aged Tupaia belangeri (mean age 7.5 years), Binge Alcohol Model [104],aged marmosets [105], LPS (0.33 mg/kg) administration to adult rats [106]