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Updated: May 17, 2026
Rapid and Refined CD11b Magnetic Isolation of Primary Microglia with Enhanced Purity and Versatility
Published on: April 13, 2017
Microglial repopulation model reveals a robust homeostatic process for replacing CNS myeloid cells
Nicholas H Varvel1, Stefan A Grathwohl, Frank Baumann
1German Center for Neurodegenerative Diseases, DZNE, and Department of Cellular Neurology, Hertie Institute for Clinical Brain Research, University of Tuebingen, 72076 Tuebingen, Germany. nicholas.varvel@dzne.de
Abstract:
Under most physiological circumstances, monocytes are excluded from parenchymal CNS tissues. When widespread monocyte entry occurs, their numbers decrease shortly after engraftment in the presence of microglia. However, some disease processes lead to focal and selective loss, or dysfunction, of microglia, and microglial senescence typifies the aged brain. In this regard, the long-term engraftment of monocytes in the microglia-depleted brain remains unknown. Here, we report a model in which a niche for myeloid cells was created through microglia depletion. We show that microglia-depleted brain regions of CD11b-HSVTK transgenic mice are repopulated with new Iba-1-positive cells within 2 wk. The engrafted cells expressed high levels of CD45 and CCR2 and appeared in a wave-like pattern frequently associated with blood vessels, suggesting the engrafted cells were peripheral monocytes. Although two times more numerous and morphologically distinct from resident microglia up to 27 wk after initial engraftment, the overall distribution of the engrafted cells was remarkably similar to that of microglia. Two-photon in vivo imaging revealed that the engrafted myeloid cells extended their processes toward an ATP source and displayed intracellular calcium transients. Moreover, the engrafted cells migrated toward areas of kainic acid-induced neuronal death. These data provide evidence that circulating monocytes have the potential to occupy the adult CNS myeloid niche normally inhabited by microglia and identify a strong homeostatic drive to maintain the myeloid component in the mature brain.
Insights
Peripheral monocytes can repopulate the brain's myeloid niche when microglia are depleted. These engrafted monocytes function similarly to microglia, indicating a strong drive to maintain myeloid cells in the mature central nervous system (CNS).
Area of Science:
- Neuroimmunology
- Central Nervous System (CNS) research
- Cellular and Molecular Neuroscience
Background:
- Monocytes are typically excluded from the healthy central nervous system (CNS) parenchyma.
- Microglia, the resident immune cells of the CNS, normally limit monocyte infiltration.
- Microglial dysfunction or depletion, seen in aging and disease, raises questions about CNS myeloid cell replacement.
Purpose of the Study:
- To investigate the long-term engraftment and behavior of peripheral monocytes in a microglia-depleted adult CNS.
- To determine if circulating monocytes can occupy the CNS myeloid niche normally filled by microglia.
- To understand the CNS's homeostatic response to myeloid cell loss.
Main Methods:
- Utilized CD11b-HSVTK transgenic mice for inducible microglia depletion, creating a myeloid cell niche.
- Employed immunohistochemistry (Iba-1 staining) and flow cytometry (CD45, CCR2 expression) to characterize engrafted cells.
- Conducted two-photon in vivo imaging to observe myeloid cell behavior, process extension, calcium signaling, and migration patterns in response to ATP and neuronal injury.
Main Results:
- Microglia-depleted brain regions were repopulated by Iba-1-positive cells within two weeks, expressing monocyte markers (CD45, CCR2).
- Engrafted monocytes persisted for at least 27 weeks, exhibiting distinct morphology but distributing similarly to resident microglia.
- In vivo imaging showed engrafted myeloid cells responding to ATP, displaying calcium transients, and migrating towards sites of neuronal death.
Conclusions:
- Circulating monocytes can successfully engraft and occupy the adult CNS myeloid niche in the absence of microglia.
- Engrafted monocytes exhibit functional characteristics of microglia, including process extension and directed migration.
- The study demonstrates a significant homeostatic mechanism for maintaining myeloid cell presence in the mature brain.
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