PROJECT SUMMARY Many diseases of the central nervous system (CNS), including Alzheimer’s disease (AD), are associated with immune system dysfunction. A population of perivascular and leptomeningeal macrophages (which we now collectively refer to as parenchymal-border macrophages, or PBMs), resides at the borders of the CNS parenchyma and interact with the cerebrospinal fluid (CSF). Our preliminary data indicate that pharmacological or genetic ablation of PBMs leads to impaired vasomotion and disrupted CSF dynamics. Moreover, our findings show that depletion of PBMs results in buildup of perivascular extracellular matrix (ECM) proteins and shrinkage of the perivascular space. Aged mice are characterized by impaired CSF dynamics and this is associated with an altered PBM phenotype (reduced Lyve1 expression and induction of MHCII). Most surprisingly, injection of macrophage colony-stimulating factor (M-CSF) into the CSF reversed the age-associated effects on CSF flow, further linking PBM dysfunction to abnormal CSF dynamics. We have recently showed that the skull bone marrow supplies dural myeloid cells, and that after injury or during neuroinflammation these cells can invade the CNS parenchyma. Our preliminary results demonstrate that skull bone marrow-derived monocytes also contribute to PBM turnover. Given the strategic location of the PBMs and the contribution of skull bone marrow cells to their renewal, in this proposal we will focus on PBMs to understand their roles in AD. We hypothesize that aging promotes the activation of PBMs and their acquisition of a pro-inflammatory phenotype, resulting in the build-up of ECM, thus leading to perivascular spaces less permissive to flow and an overall impairment of CSF dynamics. This in turn contributes to impaired amyloid clearance from the brain parenchyma and deposition along the vasculature, aggravating parenchymal amyloidosis and precipitating cerebral amyloid angiopathy (CAA). We further hypothesize that therapeutic manipulation of the PBMs may improve CSF dynamics and alleviate, at least to some extent, the pathology associated with AD and CAA. In this project we will closely interact and collaborate with other projects of the program, as well as the surgery and imaging core. Proof of our hypothesis will introduce a new player, namely PBMs, in AD and CAA pathophysiology. Not less importantly, it may herald the discovery of new therapeutic targets and interventions that can delay the onset or even improve the ongoing pathophysiology of devastating aging-associated neurological diseases.