German physician and psychiatrist Pío del Río-Hortega first discovered microglia in 1919. Initially thought to be static and supportive cells in the central nervous system (CNS), subsequent research revealed their dynamic nature and immune functions. Over time, scientists have made significant advancements in understanding microglia, uncovering their roles in immune surveillance, inflammation, brain development, and neurological disorders.
The identification and characterization of microglia markers have played a crucial role in advancing our knowledge of microglial cells. These markers serve as invaluable tools for elucidating microglial activation, phenotype characterization, and functional roles in health and disease.
This article will describe microglia function, microglia phenotypes, and microglia markers, including those selectively discriminating microglia from peripheral macrophages, as well as indicators of activation and of particular phenotypes.
Microglia are a type of neuroglia throughout the brain and spinal cord, accounting for about 10% to 20% of the total number of adult glia [1][2]. Different from peripheral macrophages, which arise from the hematopoietic stem cells and maturate in bone marrow, microglia originate from primitive myeloid progenitor cells that arise in the yolk sac during early embryonic development and subsequently migrate to the developing CNS [3].
Microglia exhibit a characteristic ramified morphology in the resting state (M0 Microglia). Their cell bodies have fine and ramified processes that constantly extend and retract to monitor the microenvironment. They have small, dark, rod-shaped nuclei with condensed chromatin.
When brain tissue injury or pathogen invades the brain, static ramified microglia retract their branches and transform into activated "brain macrophages," also known as reactive microglia [4][5]. Activated microglia adopt an amoeboid or hypertrophic appearance with enlarged cell bodies and contraction of processes. They acquire phagocytosis and migrate to the injured nerve tissue area, engulfing and destroying microorganisms and cell debris [6-8].
Figure 1: The origin of microglia [9]
Microglia display remarkable plasticity and can adopt different phenotypes in response to various stimuli and microenvironmental cues. The two major phenotypes described for microglia are the pro-inflammatory M1 phenotype and the anti-inflammatory M2 phenotype.
M1 microglia are classically activated and often associated with pro-inflammatory responses. When encountering pathogens, injury, or inflammatory signals, resting microglia undergo a phenotypic switch to the M1 state. M1 microglia release pro-inflammatory cytokines and toxic substances, such as CD16, CD32, CD86, IL-1β, TNF-α, IL-6, and iNOS, thus killing pathogens.
M1 microglia play a critical role in host defense, clearance of pathogens, and initiation of the immune response. However, excessive activation of M1 microglia can lead to chronic neuroinflammation and tissue damage, contributing to the pathogenesis of various diseases, including cerebrovascular diseases, neurodegenerative diseases, neurodevelopmental disorders, and mental disorders.
M2 microglia are alternatively activated and associated with anti-inflammatory and tissue repair functions. M2 microglia are involved in dampening the immune response, promoting tissue healing, and maintaining tissue homeostasis. M2 microglia are characterized by the upregulation of specific markers, including CD163, CD206, and Arg1. These markers are often used to identify and characterize M2-phenotype microglia in experimental studies.
There are different subsets of M2 microglia, each with distinct functional characteristics. The M2a subtype is induced by IL-4 and IL-13 and is involved in tissue repair and extracellular matrix remodeling. M2b microglia are activated by immune complexes and TLR ligands, contributing to immunoregulation and the resolution of inflammation. The M2c subtype is induced by IL-10, TGF-β, and glucocorticoids and is associated with anti-inflammatory and immunosuppressive functions.
Figure 2: Activated microglia markers of the M1 and M2 polarization spectrum—cellular and released [9]
Microglia, as resident macrophages in the brain, contribute to the development of brain tissue structure, the maintenance of CNS physiological functions, and the restoration of impaired immune responses for subsequent repair[10]. In physiological conditions, microglia serve as immune sentinels in the microenvironment of nerve cells to provide immune surveillance and defense.
Active microglia are highly phagocytic, clearing cellular debris, dead neurons, and protein aggregates. They contribute to the inflammatory response by releasing pro-inflammatory cytokines, but can also switch to an anti-inflammatory phenotype to resolve inflammation and promote tissue repair.
Microglia interact with neurons and synaptic elements, participating in synaptic pruning and remodeling. They secrete neurotrophic factors, support neurogenesis, and maintain the integrity of the blood-brain barrier. Additionally, microglia communicate with other brain cells, influencing neuronal function and overall brain homeostasis.
However, chronic microglial activation is also associated with many neurodegenerative diseases, including Alzheimer's Disease [11-13] multiple sclerosis[14], and delayed neuronal death following ischemia [15][16].
Some proteins have been identified as markers of microglia. Microglia-specific markers help to accurately characterize and distinguish microglia from other cells in the CNS. These markers also enable researchers to quantify microglia in different brain regions and at different stages of development or disease. Additionally, they aid in studying microglial functions and interactions with other cells in the CNS. Microglia-specific markers can serve as biomarkers for neurological disorders and provide potential targets for therapeutic interventions.
CUSABIO provides many antibodies and ELISA kits used to identify and characterize microglia markers in flow cytometry (FC) and immunohistochemistry (IHC) assays.
Classification | Expression | Antibodies | ELISA Kits | |
---|---|---|---|---|
General Microglial Markers | Iba1 | Selectively expressed in microglia and macrophages[17]. Often used for IHC and IF staining to visualize and identify microglia in brain tissue. | Iba1 Antibody | Human Allograft inflammatory factor 1(AIF1) ELISA kit |
CD11b | Expressed on microglia as well as other myeloid cells. Upregulated during microglial activation. Commonly used in conjunction with other markers to identify and quantify microglia in FC and IHC. | CD11b Antibody | Human Integrin αM, ITG/CD11b ELISA Kit | |
CD68 | A common marker for macrophage lineage cells, primarily localized to microglia within the brain parenchyma, and perivascular macrophages in the cerebral blood vessels and, occasionally, parenchyma[18]. | CD68 Antibody | Human CD68 ELISA kit | |
CD45 | Intermediate expression levels of CD45 combined with expression of CD11b is often used to distinguish microglia from peripheral myeloid cells. | CD45 Monoclonal Antibody | / | |
CX3CR1 | Used to detect microglia in naive tissues. | CX3CR1 Antibody | Human CX3CR1 ELISA Kit | |
ferritin | ferritin IHC combined with morphological identification can be used to visualize microglia in the CNS, and increases in ferritin levels with inflammation are thought to be caused by increases in microglial number and activation. | / | Mouse ferritin ELISA Kit | |
vimentin | During early brain development, microglia can express vimentin as they migrate and colonize the developing brain tissue. Vimentin expression in microglia is also observed in certain neuroinflammatory and neurodegenerative conditions, such as in response to brain injury, stroke, or neurodegenerative diseases like Alzheimer's disease. | VIM Antibody | Human vimentin ELISA Kit | |
FCRLS | Highly expressed gene specific to murine microglia, but there is no ortholog in humans. | / | / | |
Siglec-H | A marker for microglia in mice, absent from CNS-associated macrophages and CNS-infiltrating monocytes except for a minor subset of cells | / | / | |
F4/80 | Expressed on the macrophages and microglia. | / | / | |
P2RY12 | Exclusively expressed by microglia in the murine CNS and consistently expressed by human microglia throughout development. Involved in microglial process surveillance, chemotaxis, and neuronal interaction. Specific for homeostatic microglia and downregulated upon microglial activation. | / | / | |
TMEM119 | Specific to microglia that does not stain infiltrating peripheral immune cells. | / | Human TMEM119 ELISA kit | |
Discriminating Markers of Microglia | CD16 | Involved in ADCC and pro-inflammatory responses. | Fcgr3 Antibody | / |
|
Upregulated during M1 polarization and commonly used in FC to identify and quantify M1 microglia. | FCGR2B Antibody | Human Low affinity immunoglobulin gamma Fc region receptor II-b (FCGR2B) ELISA kit | |
CD86 | Upregulated on M1-phenotype microglia and associated with pro-inflammatory cytokine production. Frequently used as a marker for M1 polarization in experimental studies. | CD86 Antibody | Human soluble CD86 ELISA Kit | |
iNOS | Upregulated in M1-phenotype microglia. A key marker for M1 microglial activation and neuroinflammation. | NOS2 Antibody | Human iNOS ELISA KIT | |
IL-1β | Involved in the amplification of neuroinflammation and a hallmark of M1 polarization. Commonly used as a functional marker to assess M1 microglial activation. | IL1B Antibody | Human IL-1β ELISA Kit | |
CD206 | Upregulated in M2-phenotype microglia and associated with anti-inflammatory and tissue repair functions. Widely used as a marker for M2 polarization in experimental studies. | MRC1 Antibody | Human mannose receptor (MR) ELISA Kit | |
Arg1 | Highly expressed in M2-phenotype microglia and associated with the production of anti-inflammatory factors[19]. A key marker for M2 polarization and tissue healing processes. | Arg1 Antibody | Human Arg1 ELISA kit | |
Ym1 | Highly expressed in M2-phenotype microglia and commonly used as markers for M2 polarization. Involved in tissue repair, immunomodulation, and anti-inflammatory responses. | / | / | |
Ym2 | Involved in tissue remodeling, immunomodulation, and anti-inflammatory responses. | / | / | |
CD163 | Upregulated in M2-phenotype microglia and associated with the clearance of pro-inflammatory molecules. A valuable marker for M2 polarization and is often used in IHC and FC analyses[20]. | CD163 Recombinant Monoclonal Antibody | Human soluble CD163 ELISA Kit |
It is important to consider the heterogeneity of microglia and the influence of various factors on marker expression, necessitating the use of multiple markers and functional assays for a comprehensive understanding of microglial phenotypes and functions.
References
[1] Vaughan DW, Peters A (1974).Neuroglial cells in the cerebral cortex of rats from young adult to old age: an electron microscopy study [J]. J. Neurocytol.3, 405-429.
[2] Banati R (2003). Neuropathological imaging: in vivo detection of glial activation as a measure of disease and adaptive change in the brain [J]. Brit. Med. Bul.65, 121-131.
[3] Gomez Perdiguero E., Schulz C., Geissmann F. (2013). Development and homeostasis of “resident” myeloid cells: the case of the microglia [J]. Glia 61, 112–120.
[4] Kreutzberg G. W. (1996) Microglia: a sensor for pathological events in the CNS [J]. Trends Neurosci.19, 312-318.
[5] Stence N., Waite M., Dailey E. (2001) Dynamics of microglial-activation: a confocal time-lapse analysis in hippocampal slices [J]. Glia.33, 256-266.
[6] Giordana MT, Attanasio A, et al. (1994) Reactive cell proliferation and microglia following injury to the rat brain [J]. Neuropathol.Appl. Neurobiol.20, 163-174.
[7] Dihne M, Block F, Korr H, Topper R (2001). Time course of glial proliferation and glial apoptosis following excitotoxic CNS injury [J]. Brain Res. 902, 178-189.
[8] Eugenin EA, Eckardt D, et al. (2001) Microglia at brain stab wounds express connexin 43 and in vitro form functional gap junctions after treatment with interferon gamma and tumour necrosis factor alpha [J]. Proc. Natl. Acad. Sci USA.98, 4190-4195.
[9] Jurga AM, Paleczna M, Kuter KZ. Overview of General and Discriminating Markers of Differential Microglia Phenotypes [J]. Front Cell Neurosci. 2020 Aug 6;14:198.
[10] Wang J, He W, Zhang J. A richer and more diverse future for microglia phenotypes [J]. Heliyon. 2023 Mar 21;9(4):e14713.
[11] McGeer PL, McGeer EG (1995). The inflammatory response system of brain: implications for therapy of Alzheimer and other neurodegenerative diseases [J]. Brain Res. Rev.21, 195-218.
[12] McGeer PL, McGeer EG (1996). Anti-inflammatory drugs in the fight against Alzheimer’s disease.Ann.N. Y. Acad. Sci.777, 213-220.
[13] Barger SW, Harmon AD (1997).Microglial activation by Alzheimer amyloid precursor protein and modulation by apolipoprotein E [J]. Nature 388, 878-881.
[14] Diemel LT, Copelman CA, Cuzner ML (1998). Macrophages in CNS remyelination: friend or foe [J]? Neurochem.Res. 23, 341-347.
[15] Lees GJ (1993).The possible contribution of microglia and macrophages to delayed neuronal death after ischaemia [J]. J. Neurol.Sci. 114, 119-122.
[16] Tikka TM, Koistinaho JE (2001). Minocycline provides neuroprotection against N-methyl-D-aspartate neurotoxicity by inhibiting microglia [J]. J. Immunol. 166, 7527-7533.
[17] Yun S. P., Kam T.-I., et al. (2018). Block of A1 astrocyte conversion by microglia is neuroprotective in models of Parkinson’s disease [J]. Nat. Med. 24, 931–938.
[18] Fiala M, Liu QN, et al. Cyclooxygenase-2-positive macrophages infiltrate the Alzheimer's disease brain and damage the blood-brain barrier [J]. Eur J Clin Invest 2002; 32: 360–371.
[19] Munder (2009). Arginase: An emerging key player in the mammalian immune system [J]. British Journal of Pharmacology, 158(3), 638-651.
[20] Fabriek et al. (2005). CD163-positive macrophages in inflammatory multiple sclerosis lesions: Distribution and impact on leukocyte recruitment [J]. Journal of Neuroimmunology, 164(1-2), 106-114.
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