Microglial repopulation model reveals a robust homeostatic process for replacing CNS myeloid cells

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.