Local and remote interactions between macrophages and microglia in neurological conditions.

In the central nervous system (CNS) parenchymal macrophages are called microglial cells and have a distinct developmental origin and can self-renew. However, during pathological conditions, when the blood-brain-barrier becomes leaky, including after injury, in multiple sclerosis or with glioblastoma, monocyte-derived macrophages (MDM) infiltrate the CNS and cohabit with microglia. In neurodegenerative diseases such as Alzheimer's disease or ALS, MDM mostly do not enter the CNS, and instead microglia take several identities. In the specific case of ALS, the affected motor neurons are even surrounded locally by microglia, while along the peripheral nerves, by MDM-derived macrophages. The specific functions and interactions of these different myeloid cells are only starting to be recognized, but hold high promise for more targeted therapies.