The identities and roles of cerebrospinal fluid macrophages during development and in the setting of neonatal intraventricular hemorrhage

Abstract

The choroid plexus (ChP), a complex epithelial structure in the brain’s ventricles known for the secretion of cerebrospinal fluid (CSF), is being increasingly recognized for its role as an immune system hub. The largest population of immune cells are macrophages, which exist as two distinct populations inside the ChP (on the blood side of the blood-CSF barrier) and on the epiplexus surface of the ChP (on the CSF side of the blood-CSF barrier). The CSF population of cells includes not only the cells on the surface of the ChP, but those on the ventricle walls and free-floating in the CSF—together, these are the intraventricular macrophages, the first immune cells that encounter the CSF milieu. One such condition in which the CSF milieu changes dramatically is in neonatal intraventricular hemorrhage, a common disease of preterm infants in which the fragile blood vessels of the adjacent germinal matrix (known as the lateral ganglionic eminence, or LGE, in mice) burst and causes bleeding directly into the brain ventricles. Macrophages are well-known for clearance of waste products, but how these immature macrophages interact with the harmful iron-containing red blood cells (RBCs) is currently unknown. This study investigated the developmental appearance and activity of the brain macrophages ex vivo, their expression profile, and how they responded to hemorrhage in the embryonic mouse brain. We describe this elusive population in the ventricle over embryonic brain development and devise a method of imaging the intraventricular macrophages ex vivo. We use this technology in E12.5 embryos to examine the interactions between microglia of the LGE and the developing blood vessels in this region. In E15.5 embryos, we visualized the ChP, and compared the motility of stromal and epiplexus cells. Sequencing of these populations revealed that the embryonic intraventricular macrophages share a transcriptional identity with a previously described developmental microglial type. Using a mouse model of neonatal IVH, we show that embryonic intraventricular macrophages robustly respond to RBCs in the ventricle, and that in their absence, there is an increased number of RBCs present in the brain ventricles 3 days after hemorrhage. Altogether, the work presented here provides compelling evidence that intraventricular macrophages contribute to the removal of RBCs from the ventricle. Future work will explore targeting this population pharmacologically to ameliorate symptoms from neonatal IVH.