LAUSR

Significance CD11c+ microglia enriched for osteopontin (OPN) expression appear at distinct stages of brain development, aging, and several neurodegenerative disorders. Whether coexpression of CD11c and OPN results from microglial activation or represents a part of a subset-specific genetic program is unknown. We find that this CD11c+ microglial subset is formed before birth upon uptake of apoptotic neurons. Our analysis also suggests that it represents a stable subset that requires OPN to mediate engulfment of synaptic proteins, proliferate, and develop a proinflammatory phenotype. Definition of OPN-producing CD11c+ microglia as a specialized microglial subset provides insight into the contribution of microglial differentiation to brain development and function in health and disease. Expression of Itgax (encoding the CD11c surface protein) and Spp1 (encoding osteopontin; OPN) has been associated with activated microglia that can develop in healthy brains and some neuroinflammatory disorders. However, whether CD11c and OPN expression is a consequence of microglial activation or represents a portion of the genetic program expressed by a stable microglial subset is unknown. Here, we show that OPN production in the brain is confined to a small CD11c+ microglial subset that differentiates from CD11c− precursors in perinatal life after uptake of apoptotic neurons. Our analysis suggests that coexpression of OPN and CD11c marks a microglial subset that is expressed at birth and persists into late adult life, independent of environmental activation stimuli. Analysis of the contribution of OPN to the intrinsic functions of this CD11c+ microglial subset indicates that OPN is required for subset stability and the execution of phagocytic and proinflammatory responses, in part through OPN-dependent engagement of the αVβ3-integrin receptor. Definition of OPN-producing CD11c+ microglia as a functional microglial subset provides insight into microglial differentiation in health and disease.