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Classical or M1 activity of microglia/macrophages has been described in several neurodegenerative and brain inflammatory conditions and has also been linked to expansion of ischemic injury in post-stroke brain. While different pathways of M1 polarization have been suggested to occur in the post-stroke brain, the precise underlying mechanisms remain undefined. Using a transient middle cerebral artery occlusion (MCAO) rat model, we showed a progressive M2 to M1 polarization in the perilesional brain region with M1 cells becoming one of the dominant subsets by day 4 post-stroke. Comparing key receptors involved in M1 polarization (CD8, IFNγR, Clec4, FcγR, TLR3 and TLR4) and their signal transducers (Syk, Stat1, Irf3, and Traf6) at the day 4 time point, we showed a strong upregulation of CD8 along with SYK transducer in dissected perilesional brain tissue. We further showed that CD8 expression in the post-stroke brain was associated with activated (CD68+) macrophages and that progressive accumulation of CD8+CD68+ cells in the post-stroke brain coincided with increased iNOS (M1 marker) and reduced Arg1 (M2 marker) expression on these cells. In vitro ligand-based stimulation of the CD8 receptor caused increased iNOS expression and an enhanced capacity to phagocytose E. coli particles; and interestingly, CD8 stimulation was also able to repolarize IL4-treated M2 cells to an M1 phenotype. Our data suggest that increased CD8 signaling in the post-stroke brain is primarily associated with microglia/macrophages and can independently drive M1 polarization, and that modulation of CD8 signaling could be a potential target to limit secondary post-stroke brain damage. [ABSTRACT FROM AUTHOR]

Aquaporin-4 (AQP4) is the most abundant water channel in the brain, and its inhibition before inducing focal ischemia, using the AQP4 inhibitor TGN-020, has been showed to reduce oedema in imaging studies. Here, we aimed to evaluate, for the first time, the histopathological effects of a single dose of TGN-020 administered after the occlusion of the medial cerebral artery (MCAO). On a rat model of non-reperfusion ischemia, we have assessed vascular densities, albumin extravasation, gliosis, and apoptosis at 3 and 7 days after MCAO. TGN-020 significantly reduced oedema, glial scar, albumin effusion, and apoptosis, at both 3 and 7 days after MCAO. The area of GFAP-positive gliotic rim decreased, and 3D fractal analysis of astrocytic processes revealed a less complex architecture, possibly indicating water accumulating in the cytoplasm. Evaluation of the blood vessels revealed thicker basement membranes colocalizing with exudated albumin in the treated animals, suggesting that inhibition of AQP4 blocks fluid flow towards the parenchyma in the paravascular drainage pathways of the interstitial fluid. These findings suggest that a single dose of an AQP4 inhibitor can reduce brain oedema, even if administered after the onset of ischemia, and AQP4 agonists/antagonists might be effective modulators of the paravascular drainage flow. [ABSTRACT FROM AUTHOR]

Aquaporin-4 ( AQP4) and glutamate transporter-1 ( GLT-1) represent the major water and glutamate astrocyte buffering gateways in the brain. Utilizing perilesional ischemic human cortices, we have performed here for the first time an integrative assessment on both AQP4 and GLT-1, and on their proximity to blood vessels and neurons. Counting the relative number of AQP4±/ GLT-1±/glial fibrillary acidic protein± cells showed that double-positive variants were overall most frequent, and their number tended to decrease from organized and recent perilesional cortices to controls. AQP4/ GLT-1 colocalization showed higher coefficients for the perilesional cortices compared with controls, suggesting an increased water/glutamate-buffering capability. Distance frequency analysis of AQP4/ GLT-1 in relationship to neurons showed that both markers were concentrated at 20-40 μm around the perikarya; with AQP4 being more abundant in close proximity, these differences were not being driven by changes in neuropil density alone. Our study suggests a dual, simultaneous astrocytic function depending on the relative distance to neurons and vessels, with increased water and glutamate-buffering capability in the mid perineuronal space, and an increased water-buffering capability in the immediate perineuronal space, even higher than around vessels. Thus, adding specific AQP4/ GLT-1 modulator agents selectively depending on the acute/chronic phase of stroke might increase the efficacy of existing treatments. [ABSTRACT FROM AUTHOR]

Background: We previously identified increased tissue localization of monomeric C-reactive protein (mCRP) in the infarcted cortical brain tissue of patients following ischaemic stroke. Here, we investigated the relationship of mCRP expression in haemorrhagic stroke, and additionally examined the capacity of mCRP to travel to or appear at other locations within the brain that might account for later chronic neuroinflammatory or neurodegenerative effects. Methods: Immunohistochemistry was performed on Formalin-fixed, paraffin-embedded archived brain tissue blocks obtained at autopsy from stroke patients and age-matched controls. We modelled mCRP migration into the brain after haemorrhagic stroke by infusing mCRP (3.5 µg) into the hippocampus of mice and localized mCRP with histological and immunohistochemistry methods. Results: On human tissue in the early stages of haemorrhage, there was no staining of mCRP. However, with increasing post-stroke survival time, mCRP immunostaining was associated with some parenchymal brain cells, some stroke-affected neurons in the surrounding areas and the lumen of large blood vessels as well as brain capillaries. Further from the peri-haematoma region, however, mCRP was detected in the lumen of micro-vessels expressing aquaporin 4 (AQP4). In the hypothalamus, we detected clusters of neurons loaded with mCRP along with scattered lipofuscin-like deposits. In the peri-haematoma region of patients, mCRP was abundantly seen adjacent to AQP4 immunoreactivity. When we stereotactically injected mCRP into the hippocampus of mice, we also observed strong expression in distant neurones of the hypothalamus as well as cortical capillaries. Conclusions: mCRP is abundantly expressed in the brain after haemorrhagic stroke, directly impacting the pathophysiological development of the haematoma. In addition, it may have indirect effects, where the microcirculatory system appears to be able to carry it throughout the cortex as far as the hypothalamus, allowing for long-distance effects and damage through its capacity to induce inflammation and degenerate neuronal perivascular compartments. [ABSTRACT FROM AUTHOR]