The phenotypic convergence between microglia and peripheral macrophages during development and neuroinflammation paves the way for new therapeutic perspectives.

Microglia, the tissue resident macrophages of the brain, are increasingly recognized as key players for central nervous system development and homeostasis. They are long-lived cells deriving from a transient wave of yolk-sac derived erythro-myeloid progenitors early in development. Their unique ontology has prompted the search for specific markers to be used for their selective investigation and manipulation. The first generation of genome-wide expression studies has provided a bundle of transcripts (such as Olfml3, Fcrls, Tmem119, P2ry12, Gpr34, and Siglech) useful to distinguish microglia from peripheral macrophages. However, more recent reports have revealed that microglial phenotype is constantly shaped by the microenvironment in a time-, and context-dependent manner. In this article, we review data that provide additional pieces to this complex scenario and show the existence of unexpected phenotypic convergence between microglia and peripheral macrophages at certain developmental stages and under pathological conditions. These observations suggest that the two cell types act synergically boosting their mutual activities depending on the microenvironment. This novel information about the biology of microglia and peripheral macrophages sheds new light about their therapeutic potential for neuroinflammatory and neurodegenerative diseases.