Microglia are the resident immune cells of the brain parenchyma and arise from the myeloid lineage. The role microglia play in maintenance of a healthy brain and decline of the brain in disease has become an intensively studied topic but much remains to be understood.
Microglial origins and function
Microglia are truly a fascinating cell type. While most immune cells in the body originate from the fetal liver and from continual postnatal hematopoiesis, microglia originate from the embryonic yolk sac and migrate to the brain prior to birth. Development of the blood-brain barrier then blocks any further entry of these cells such that the original migrant microglia become the resident immune cells of the brain for the life of the organism.
These long-lived immune cells constantly roam their environment using their repertoire of cell surface receptors to detect pathogens, debris, and sites of injury. Microglia perform a ‘housekeeping’ role by phagocytosing debris, dead or dying cells, and protein aggregates such as the amyloid beta plaques, tau neurofibrillary tangles, and α-synuclein aggregates common to various neurodegenerative diseases. Additionally, microglia mediate synaptic pruning in the brain where they help to refine the brain’s circuitry.
The ‘dark side’ of microglia
However, there is also a ‘dark side’ to microglia as they have become increasingly implicated in the progression of various neurodegenerative diseases. In the healthy brain and during early stages of neurodegenerative disease, microglia are thought to play a protective role by clearing damaged cells, debris, and protein aggregates such as amyloid beta. However, during more advanced stages of disease microglia seem to reduce their phagocytic activity and upregulate their release of inflammatory cytokines leading to a chronic inflammatory state involving neuronal cell death and neurodegeneration (Sevenich L 2018). This transition, from serving a homeostatic role to responding to more advanced disease requires the upregulation of TREM2 (triggering receptors expressed on myeloid cells 2). Of note, mutant variants of TREM2 strongly increase risk for AD, further highlighting the critical role that microglia play in the progression of neurodegenerative disease.
A growing array of reagents to study the role of microglia in both homeostasis and disease is being developed and critical among these are antibodies that can distinguish microglia from peripheral immune cells and other cell types present in brain.
Choosing a microglial marker:
TMEM119, TREM2, and AIF1 have all been employed as microglial markers in numerous studies and expression profiling on isolated microglia populations continues to expand and refine the list of markers and their specificity. It is worth noting however that the cellular identity of brain microglia and peripheral immune cells is not ‘fixed’ but has instead been shown to be quite plastic and dynamic as these cells respond to environmental cues. For example, when the brain is experiencing an inflammatory state, as occurs during neurodegenerative disease, peripheral macrophages have been shown to infiltrate the brain parenchyma and acquire microglia-specific markers (Grassivaro T et al 2020). Accordingly, because of the plasticity in the phenotype of peripheral macrophages and microglia it may not be possible to use a single marker at a fixed time point to define cellular origins. Rather, it may be the case that during neuroinflammation and disease peripheral macrophages that migrate to the brain and acquire microglia-specific markers may also begin to function as microglia. If that is the case, then the distinction between brain microglia and macrophages that have migrated into the brain from the periphery becomes less clear.
TMEM119 is a microglia-specific marker that can distinguish between brain microglia, peripheral macrophages, and other CNS cell types (Bennett ML et al. 2016).
Immunofluorescence of TMEM119-FLAG transfected COS-7 cells using mouse α-TMEM119 clone L128/43 (green) and rabbit α-flag tag (red). Yellow/orange staining shows 100% correspondence between the two antibodies for recognition of transfected cells. Blue staining is DAPI and stains nuclei of both transfected and untransfected cells.
AIF1, also known as ionized calcium-binding adapter molecule 1 (Iba1), is a cytoplasmic protein that is highly expressed in the monocytic lineage, including both microglia and macrophages. AIF1 is known to mediate phagocytosis and the formation of membrane ruffles in activated microglia.
Immunofluorescence of AIF1/IBA1-FLAG transfected COS-7 cells using chicken α-AIF1/IBA1 (green) and mouse α-flag tag (red). Yellow/orange staining shows 100% correspondence between the two antibodies for recognition of transfected cells. Blue staining is DAPI and stains nuclei of both transfected and untransfected cells.
In addition to TREM2 being a critical modulator of microglial activation it is also a well characterized microglial marker.
Immunofluorescence of TREM2-FLAG transfected COS-7 cells using chicken α-TREM2 (green) and mouse α-flag tag (red). Yellow/orange staining shows 100% correspondence between the two antibodies for recognition of transfected cells. Blue staining is DAPI and stains nuclei of both transfected and untransfected cells.
Bennett ML et al. New tools for studying microglia in the mouse and human CNS. Proc Natl Acad Sci USA. 2016 Mar 22;113(12):E1738-46. doi: 10.1073/pnas.1525528113
Lawson LJ et al. Heterogeneity in the distribution and morphology of microglia in the normal adult mouse brain. Neuroscience 1990;39(1):151-70. doi: 10.1016/0306-4522(90)90229-w
Sevenich L. Brain-Resident Microglia and Blood-Borne Macrophages Orchestrate Central Nervous System Inflammation in Neurodegenerative Disorders and Brain Cancer. Front Immunol. 2018 Apr 6;9:697. doi: 10.3389/fimmu.2018.00697
Grassivaro T et al. Convergence between Microglia and Peripheral Macrophages Phenotype during Development and Neuroinflammation. J Neurosci. 2020 Jan 22;40(4):784-795. doi: 10.1523/JNEUROSCI.1523-19.2019.
Koellhoffer EC et al. Old Maids: Aging and Its Impact on Microglia Function. Int J Mol Sci 2017 Apr 5;18(4):769. doi: 10.3390/ijms18040769.