Characterization of perivascular macrophages (pvMΦ) and their contributions to post-stroke pathology

Abstract

Brain perivascular macrophages (pvMΦ) are resident macrophages whose role in physiological and pathological conditions is still ambiguous. In part, the limited information about pvMΦ is due to a lack of specific markers that distinguish these cells from other border-associated macrophages (BAM). Therefore, our first objective was to characterize pvMΦ and further understand their role in the brain. Utilising bulk-RNA sequencing and histological techniques, we identified brain pvMΦ from naïve mice and showed that they are characterized by two transcriptionally distinct subsets based on the expression of Tim-4. To gain a better understanding of the function of pvMΦ, we examined the specific site where these cells are located. PvMΦ are located at the interfase between brain parenchyma and circulation. This suggests that they receive signals from both the central nervous system (CNS) and the periphery, indicating that these cells might be a unique conduit of body-to-brain communication. Using an in vivo model of systemic inflammation by I.P. injection of LPS, we showed that pvMΦ, especially the Tim-4+ subset, are activated following a peripheral inflammatory stimulus. This finding suggests that such activation may initiate a response in the CNS. In addition, due to their proximity to the blood-brain barrier (BBB), pvMΦ are believed to have a significant impact on various disorders related to BBB disruption, including stroke. Stroke is a pathological condition characterized by deleterious increases in oxidative stress. PvMΦ have been reported to participate in the pathophysiological mechanisms of subarachnoid haemorrhage and ischaemic stroke and, although these cells can be recognized through the high expression of a specific receptor CD163, which is responsible for scavenging haemoglobin-haptoglobin complexes, there is currently no data regarding the role of pvMΦ following intracerebral haemorrhage (ICH). As such, our second objective was to investigate and clarify the role of these cells in the pathophysiology of ICH. Using an in vivo model of ICH, we showed that, although pvMΦ are one of the main producers of several antioxidants, especially the selenoprotein GPx3, the depletion of these cells in a mouse model of ICH seems to be beneficial in the acute phase of stroke and detrimental in the resolution phase. GPx3 is the only secreted antioxidant selenoprotein able to neutralize H2O2 and fatty acid hydroperoxides in the extracellular space, interrupting the deleterious propagation of oxidative stress-induced damage during stroke. To be locally produced, GPx3 needs selenium, which can be obtained by dietary intake. To date, we lack knowledge of the main cells in the brain able to uptake and supply selenoproteins to neurons. Based on the close proximity to the circulation and the significant expression of GPx3, we hypothesized that pvMΦ might be the main supplier of selenoproteins. Furthermore, we suggested that stroke might affect selenium absorption, which could consequently influence the synthesis of selenoproteins from pvMΦ. Hence, our final objective was to gain insight into how stroke affects the concentration of circulating selonoproteins, and their activity, in patients, by performing a systematic review and meta-analysis. We showed that there is a significant reduction in the circulating GPx activity within 3 days from ischaemic stroke, whereas no consistent information is available regarding alterations to selenoproteins following haemorrhagic stroke.

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Stuart Allan (Supervisor), Craig Smith (Supervisor) & John Grainger (Supervisor)