Macrophages on the margin: choroid plexus immune responses

The choroid plexus (ChP) provides a critical brain barrier that is also home to diverse and dynamic immune cell populations throughout life, as revealed by recent transcriptomic studies. ChP macrophages represent the largest class of ChP immune cells and show heterogeneity. Advances in in vivo imaging provide unobstructed views of ChP immune cells in their physiological setting in the brain’s ventricles. Environmental cues and tightly knit receptor–ligand networks at the ChP govern immune cell recruitment and infiltration from the periphery into the central nervous system (CNS). The relevance of immune cell trafficking and dynamics at the ChP is increasingly appreciated in the context of neurological conditions involving neuroinflammation. The choroid plexus (ChP), an epithelial bilayer containing a network of mesenchymal, immune, and neuronal cells, forms the blood–cerebrospinal fluid (CSF) barrier (BCSFB). While best recognized for secreting CSF, the ChP is also a hotbed of immune cell activity and can provide circulating peripheral immune cells with passage into the central nervous system (CNS). Here, we review recent studies on ChP immune cells, with a focus on the ontogeny, development, and behaviors of ChP macrophages, the principal resident immune cells of the ChP. We highlight the implications of immune cells for ChP barrier function, CSF cytokines and volume regulation, and their contribution to neurodevelopmental disorders, with possible age-specific features to be elucidated in the future. The choroid plexus (ChP), an epithelial bilayer containing a network of mesenchymal, immune, and neuronal cells, forms the blood–cerebrospinal fluid (CSF) barrier (BCSFB). While best recognized for secreting CSF, the ChP is also a hotbed of immune cell activity and can provide circulating peripheral immune cells with passage into the central nervous system (CNS). Here, we review recent studies on ChP immune cells, with a focus on the ontogeny, development, and behaviors of ChP macrophages, the principal resident immune cells of the ChP. We highlight the implications of immune cells for ChP barrier function, CSF cytokines and volume regulation, and their contribution to neurodevelopmental disorders, with possible age-specific features to be elucidated in the future. a barrier formed by endothelial cells, astrocytic end feet, and pericytes that restricts the passage of blood-borne factors into the brain. a barrier formed primarily by ChP epithelial cells that restricts the passage of blood-borne factors into the CSF. macrophages that reside in the CNS barrier tissues, including the meninges, the ChP, and the perivascular space. They are also referred to as CNS-associated macrophages (CAMs). a principal part of the nervous system that comprises the brain and spinal cord. a body fluid that fills the brain ventricles and surrounds the brain and spinal cord. It contains ions and biomolecules including growth factors and neurotransmitters. a family of cytokines that induce chemotaxis in responsive cells. a vascularized epithelial tissue located in each brain ventricle. It is a key source of CSF and forms the BCSFB. secreted signaling proteins that regulate the functions of cells and/or interactions between cells. cells that are located on the apical, CSF-contacting surface of the ChP. mononuclear phagocytes found in all tissues and organs. They are scavengers of cellular debris and harmful substances. resident macrophages in the brain parenchyma with key roles in brain development and function. the total trajectory length that a cell body migrates. the rate at which a cell moves its processes. macrophages located in the stromal space between the ChP epithelial and endothelial cell layers.