Endothelial cells and macrophages as allies in the healthy and diseased brain

CNS-associated macrophages contribute to intracerebral aneurysm pathophysiology

Intracerebral aneurysms (IAs) are pathological dilatations of cerebral arteries whose rupture leads to subarachnoid hemorrhage, a significant cause of disability and death. Inflammation is recognized as a critical contributor to the formation, growth, and rupture of IAs; however, its precise actors have not yet been fully elucidated. Here, we report CNS-associated macrophages (CAMs), also known as border-associated macrophages, as one of the key players in IA pathogenesis, acting as critical mediators of inflammatory processes related to IA ruptures. Using a new mouse model of middle cerebral artery (MCA) aneurysms we show that CAMs accumulate in the IA walls. This finding was confirmed in a human MCA aneurysm obtained after surgical clipping, together with other pathological characteristics found in the experimental model including morphological changes and inflammatory cell infiltration. In addition, in vivo longitudinal molecular MRI studies revealed vascular inflammation strongly associated with the aneurysm area, i.e., high expression of VCAM-1 and P-selectin adhesion molecules, which precedes and predicts the bleeding extent in the case of IA rupture. Specific CAM depletion by intracerebroventricular injection of clodronate liposomes prior to IA induction reduced IA formation and rupture rate. Moreover, the absence of CAMs ameliorated the outcome severity of IA ruptures resulting in smaller hemorrhages, accompanied by reduced neutrophil infiltration. Our data shed light on the unexplored role of CAMs as main actors orchestrating the progression of IAs towards a rupture-prone state.

The contribution of β-amyloid, Tau and α-synuclein to blood-brain barrier damage in neurodegenerative disorders

Central nervous system (CNS) accumulation of fibrillary deposits made of Amyloid β (Aβ), hyperphosphorylated Tau or α-synuclein (α-syn), present either alone or in the form of mixed pathology, characterizes the most common neurodegenerative diseases (NDDs) as well as the aging brain. Compelling evidence supports that acute neurological disorders, such as traumatic brain injury (TBI) and stroke, are also accompanied by increased deposition of toxic Aβ, Tau and α-syn species. While the contribution of these pathological proteins to neurodegeneration has been experimentally ascertained, the cellular and molecular mechanisms driving Aβ, Tau and α-syn-related brain damage remain to be fully clarified. In the last few years, studies have shown that Aβ, Tau and α-syn may contribute to neurodegeneration also by inducing and/or promoting blood-brain barrier (BBB) disruption. These pathological proteins can affect BBB integrity either directly by affecting key BBB components such as pericytes and endothelial cells (ECs) or indirectly, by promoting brain macrophages activation and dysfunction. Here, we summarize and critically discuss key findings showing how Aβ, Tau and α-syn can contribute to BBB damage in most common NDDs, TBI and stroke. We also highlight the need for a deeper characterization of the role of these pathological proteins in the activation and dysfunction of brain macrophages, pericytes and ECs to improve diagnosis and treatment of acute and chronic neurological disorders.

The niche matters: origin, function and fate of CNS-associated macrophages during health and disease

There are several cellular and acellular structural barriers associated with the brain interfaces, which include the dura, the leptomeninges, the perivascular space and the choroid plexus epithelium. Each structure is enriched by distinct myeloid populations, which mainly originate from erythromyeloid precursors (EMP) in the embryonic yolk sac and seed the CNS during embryogenesis. However, depending on the precise microanatomical environment, resident myeloid cells differ in their marker profile, turnover and the extent to which they can be replenished by blood-derived cells. While some EMP-derived cells seed the parenchyma to become microglia, others engraft the meninges and become CNS-associated macrophages (CAMs), also referred to as border-associated macrophages (BAMs), e.g., leptomeningeal macrophages (MnMΦ). Recent data revealed that MnMΦ migrate into perivascular spaces postnatally where they differentiate into perivascular macrophages (PvMΦ). Under homeostatic conditions in pathogen-free mice, there is virtually no contribution of bone marrow-derived cells to MnMΦ and PvMΦ, but rather to macrophages of the choroid plexus and dura. In neuropathological conditions in which the blood-brain barrier is compromised, however, an influx of bone marrow-derived cells into the CNS can occur, potentially contributing to the pool of CNS myeloid cells. Simultaneously, resident CAMs may also proliferate and undergo transcriptional and proteomic changes, thereby, contributing to the disease outcome. Thus, both resident and infiltrating myeloid cells together act within their microenvironmental niche, but both populations play crucial roles in the overall disease course. Here, we summarize the current understanding of the sources and fates of resident CAMs in health and disease, and the role of the microenvironment in influencing their maintenance and function.