Fluid-Niche and Microglial Signatures Prime Robust Intraventricular Macrophage Response to Blood During Brain Development.

Intraventricular macrophages (IVMs) reside in cerebrospinal fluid (CSF) and are considered a border-associated macrophage (BAM) population in the brain. Although they represent the first line of defense against intraventricular challenges, their developmental roles and responses to injury are poorly understood. This knowledge is relevant for conditions including neonatal intraventricular hemorrhage (IVH), where blood extravasates into brain ventricles, leading to life-long negative sequelae including cerebral palsy and hydrocephalus. Here, we show that IVMs are first responders to blood in developing brain ventricles, phagocytosing red blood cells and upregulating iron-processing machinery. Live imaging of developing mouse ventricles and choroid plexus revealed that IVMs are dynamic and morphologically distinct from non-IVM macrophages. Their transcriptional profiles distinguish them from other BAMs as they also exhibit signatures of "youth-associated microglia" and characteristics of cavity macrophages found in fluid niches such as the peritoneum. Our findings provide insights into IVM development and function, highlighting their therapeutic potential.