TMEM119 marks a subset of microglia in the human brain

Microglia are resident myeloid cells of the central nervous system (CNS), activated in the brains of various neurological diseases. Microglia are ontogenetically and functionally distinct from monocyte‐derived macrophages that infiltrate the CNS under pathological conditions. However, a lack of specific markers that distinguish resident microglia from circulating blood‐derived macrophages in human brain tissues hampers accurate evaluation of microglial contributions to the human brain pathology. By comparative analysis of five comprehensive microglial transcriptome datasets, we identified an evolutionarily conserved protein TMEM119 as the most promising candidate for human microglial markers. TMEM119 was expressed on immortalized human microglia, in which the expression levels were not elevated by exposure to lipopolysaccharide, IFNγ, IL‐4, IL‐13 or TGFβ1. Notably, TMEM119 immunoreactivity was expressed exclusively on a subset of Iba1+ CD68+ microglia with ramified and amoeboid morphologies in the brains of neurodegenerative diseases, such as Alzheimer's disease (AD), whereas Iba1+ CD68+ infiltrating macrophages do not express TMEM119 in demyelinating lesions of multiple sclerosis and necrotic lesions of cerebral infarction. TMEM119 mRNA levels were elevated in AD brains, although the protein levels were not significantly different between AD and non‐AD cases by western blot and morphometric analyses. TMEM119‐positive microglia did not consistently express polarized markers for M1 (CD80) or M2 (CD163, CD209) in AD brains. These results suggest that TMEM119 serves as a reliable microglial marker that discriminates resident microglia from blood‐derived macrophages in the human brain.