Bone marrow spawns brain killers

The scavenger receptor CD36 contributes to the neurotoxicity of bone marrow-derived monocytes through peroxynitrite production

CD36, a class B scavenger receptor present in microglia, endothelium and leukocytes, plays a key role in ischemic brain injury by promoting the expression of inflammatory genes and production of reactive oxygen species (ROS). However, it is not known whether ischemic brain damage is mediated by CD36 activation in resident brain cells, i.e., microglia, or by blood-borne cells that infiltrate the brain. To address this question, we studied oxygen-glucose deprivation (OGD) in hippocampal slice cultures, a model of ischemic injury that does not involve cells extrinsic to the brain. We found that CD36 gene knockout does not afford protection of hippocampal slices to OGD-induced cytotoxicity. In contrast, immunoactivated bone marrow-derived monocytes-macrophages (BMM) from wild type (WT) mice trigger hippocampal damage when incubated with brain slices via a mechanism that is prevented in CD36-/- BMM. The neurotoxic activity of CD36+/+ BMM was attributed to reactive oxygen species (ROS) since it was concomitant with increased ROS production and could be prevented by treatment with a selective ROS scavenger, MnTBAP, or a peroxynitrite decomposition catalyst, FeTPPS. Importantly, ROS production and accumulation 3-nitrotyrosine in hippocampal proteins (a hallmark of peroxynitrite production) was significantly dampened in immunoactivated CD36-/- BMM, whereas production of NO-derived metabolites (nitrite and nitrate) was unaltered. We conclude that CD36 signaling may not contribute to injury induced by OGD in the brain itself but is involved in the neurotoxicity mediated by activated BMM. These findings are consistent with the hypothesis that CD36 in infiltrating inflammatory cells drives peroxynitrite-mediated ischemic brain damage. Accordingly, targeting CD36 in the vascular compartment may protect against neurotoxicity in the ischemic brain.

Inflammatory cell infiltration after endothelin-1-induced cerebral ischemia: histochemical and myeloperoxidase correlation with temporal changes in brain injury

Accumulation of neutrophils in brain after transient focal stroke remains controversial with some studies showing neutrophils to be deleterious, whereas others suggest neutrophils do not contribute to ischemic injury. Myeloperoxidase (MPO) has been used extensively as a marker for quantifying neutrophil accumulation, but is an indirect method and does not detect neutrophils alone. To elucidate the interaction of macrophages in the neutrophil inflammatory response, we conducted double-label immunofluorescence in brain sections at 0, 1, 2, 3, 7, and 15 days after ischemia. Each of these results was obtained from the same animal to determine correlations between neutrophil infiltration and ischemic damage. It was found that MPO activity increased up to 3 days after cerebral ischemia. Dual-staining revealed that macrophages engulf neutrophils in the brain and that this engulfment of neutrophils increased with time, with 50% of neutrophils in the brain engulfed at 3 days and approximately 85% at 15 days (N=5, P<0.05). Interestingly, at 7 days the amount of dual-staining was decreased to 20% (N=5, P<0.05). Neutrophil infiltration was positively correlated with ischemic damage in both the cortex and striatum (r(2)=0.86 and 0.80, respectively, P<0.01). The results of this study indicate that the MPO from neutrophils phagocytized by macrophages may continue to contribute to the overall MPO activity, and that previous assessments that have utilized this marker to measure neutrophil accumulation may have mis-calculated the number of neutrophils within the ischemic territory and hence their contribution to the evolution of the infarct at later time points. Thus any biphasic infiltration of neutrophils may have been masked by the accumulation of macrophages.

AM-36 modulates the neutrophil inflammatory response and reduces breakdown of the blood brain barrier after endothelin-1 induced focal brain ischaemia

BACKGROUND AND PURPOSE

Following transient focal stroke, rapid accumulation and activation of neutrophils in the ischaemic region is deleterious due to release of reactive oxygen species and myeloperoxidase (MPO). The purpose of this study was to examine whether AM-36, both a Na+ channel blocker and an antioxidant, afforded neuroprotection by modulating neutrophil accumulation into brain, following endothelin-1 (ET-1) induced middle cerebral artery occlusion (MCAo) in conscious rats.

EXPERIMENTAL APPROACH

AM-36 was administered at 3 and 24 h after ET-1-induced MCAo. Functional recovery was determined using grid-walking and cylinder tests. Image analysis of brain sections was used to determine infarct volume. The effect of AM-36 on neutrophil infiltration and their interaction with macrophages was examined in rats at 48 h following MCAo by both an MPO assay and double-label immunofluorescence. Blood brain barrier (BBB) breakdown was measured by the area stained by intravenous Evans Blue.

KEY RESULTS

AM-36 reduced functional deficits in both tests such that no difference existed from pre-ischaemic values at 48 h. Neutrophil infiltration, assessed by MPO activity, and infarct volume were significantly reduced following AM-36 administration by 54 and 60% respectively. Similarly, immunofluorescence revealed that AM-36 reduced neutrophil infiltration by approximately 50% in selected brain regions, when compared to controls, and also modulated macrophage phagocytosis of neutrophils. Breakdown of the BBB was significantly reduced by 60% following AM-36 treatment.

CONCLUSIONS AND IMPLICATIONS

These findings suggest that AM-36 can directly modulate the neutrophil inflammatory response and reduce BBB breakdown following MCAo.