Cerebral ischemic preconditioning or tolerance is a powerful neuroprotective phenomenon by which a sublethal injurious stimulus renders the brain resistant to a subsequent damaging ischemic insult. We used lipopolysaccharide (LPS) as a preconditioning stimulus in a mouse model of middle cerebral artery occlusion (MCAO) to examine whether improvements in cerebrovascular function contribute to the protective effect. Administration of LPS 24 h before MCAO reduced the infarct by 68% and improved ischemic cerebral blood flow (CBF) by 114% in brain areas spared from infarction. In addition, LPS prevented the dysfunction in cerebrovascular regulation induced by MCAO, as demonstrated by normalization of the increase in CBF produced by neural activity, hypercapnia, or by the endotheliumdependent vasodilator acetylcholine. These beneficial effects of LPS were not observed in mice lacking inducible nitric oxide synthase (iNOS) or the nox2 subunit of the superoxide-producing enzyme NADPH oxidase. LPS increased reactive oxygen species and the peroxynitrite marker 3-nitrotyrosine in wild-type mice but not in nox2 nulls. The peroxynitrite decomposition catalyst 5,10,15, 20-tetrakis(4-sulfonatophenyl)porphyrinato iron (III) attenuated LPS-induced nitration and counteracted the beneficial effects of LPS on infarct volume, ischemic CBF, and vascular reactivity. Thus, LPS preserves neurovascular function and ameliorates CBF in regions of the ischemic territory at risk for infarction. This effect is mediated by peroxynitrite formed from iNOS-derived NO and nox2-derived superoxide. The data indicate that preservation of cerebrovascular function is an essential component of ischemic tolerance and suggest that combining neuroprotection and vasoprotection may be a valuable strategy for treating ischemic brain injury.

The immune response to stroke is comprised of inflammatory and regulatory processes. One cell type involved in both innate and adaptive immunity is the dendritic cell (DC). A DC population residing in the healthy brain (bDC) was identified using a transgenic mouse expressing enhanced yellow fluorescent protein (EYFP) under the promoter for the DC marker, CD11c (CD11c/EYFP Tg). To determine if bDC are involved in the immune response to cerebral ischemia, transient (40min) middle cerebral artery occlusion (MCAO) followed by 6, 24, or 72hr reperfusion was conducted in CD11c/EYFP Tg mice. Our results demonstrated that DC accumulated in the ischemic hemisphere at 24hr post-MCAO-reperfusion, particularly in the border region of the infarct where T lymphocytes accrued. To distinguish resident bDC from the infiltrating peripheral DC, radiation chimeras [1. wild type (WT) hosts restored with CD11c/EYFP Tg bone marrow (BM) or 2. CD11c/EYFP Tg hosts restored with WT BM] were generated and examined by immunocytochemistry. These data confirmed that DC populating the core of the infarct at 72hr were of peripheral origin, whereas those in the border region were comprised primarily of resident bDC. The brain resident (CD45 intermediate) cells of CD11c/EYFP Tg mice were analyzed by flow cytometry. Compared to microglia, bDC displayed increased major histocompatibility class II (MHC II) and co-stimulatory molecules following MCAO-reperfusion. High levels of MHC II and the co-stimulatory molecule CD80 on bDC at 72hr corresponded to peak lymphocyte infiltration, and suggested a functional interaction between these two immune cell populations.

The glucose concentration in the culture medium may affect the energy metabolism of cultured cells. The oxidative metabolism of glucose in astrocytes might also be affected because the glucose concentration (25 mmol/L) of many culture media is higher than the physiological levels (B3 mmol/L). In the present study, we assessed the effects of reducing the glucose concentration in the culture medium on the oxidative metabolism of glucose in cultured rat astroglia by measuring the oxidation rates of L-[U- 22 and astroglia 2 was also increased by 94% and 76%, respectively. In contrast, an elevated extracellular K þ concentration (7.4 mmol/L) did not affect glucose and lactate oxidation, although it increased 2-deoxy-D-[1-14 C]glucose phosphorylation. Astroglia grown in the physiological glucose concentration are more dependent on the oxidative metabolism of glucose than that in high-glucose concentration. Glucose concentration in culture medium has a strong influence on astrocytic oxidative capacity in vitro.

CD36, a class-B scavenger receptor involved in multiple functions, including inflammatory signaling, may also contribute to ischemic brain injury through yet unidentified mechanisms. We investigated whether CD36 participates in the molecular events underlying the inflammatory reaction that accompanies cerebral ischemia and may contribute to the tissue damage. We found that activation of nuclear factor-B, a transcription factor that coordinates postischemic gene expression, is attenuated in CD36-null mice subjected to middle cerebral artery occlusion. The infiltration of neutrophils and the glial reaction induced by cerebral ischemia were suppressed. Treatment with an inhibitor of inducible nitric oxide synthase, an enzyme that contributes to the tissue damage, reduced ischemic brain injury in wild-type mice, but not in CD36 nulls. In contrast to cerebral ischemia, the molecular and cellular inflammatory changes induced by intracerebroventricular injection of interleukin-1␤ were not attenuated in CD36-null mice. The findings unveil a novel role of CD36 in early molecular events leading to nuclear factor-B activation and postischemic inflammation. Inhibition of CD36 signaling may be a valuable therapeutic approach to counteract the deleterious effects of postischemic inflammation.

Background and Purpose-Toll-like receptors (TLRs) and the scavenger receptor CD36 are key molecular sensors for the innate immune response to invading pathogens. However, these receptors may also recognize endogenous "danger signals" generated during brain injury, such as cerebral ischemia, and trigger a maladaptive inflammatory reaction. Indeed, CD36 and TLR2 and 4 are involved in the inflammation and related tissue damage caused by brain ischemia. Because CD36 may act as a coreceptor for TLR2 heterodimers (TLR2/1 or TLR2/6), we tested whether such interaction plays a role in ischemic brain injury. Methods-The TLR activators FSL-1 (TLR2/6), Pam3 (TLR2/1), or lipopolysaccharide (TLR4) were injected intracerebroventricularly into wild-type or CD36-null mice, and inflammatory gene expression was assessed in the brain. The effect of TLR activators on the infarct produced by transient middle cerebral artery occlusion was also studied. Results-The inflammatory response induced by TLR2/1 activation, but not TLR2/6 or TLR4 activation, was suppressed in CD36-null mice. Similarly, TLR2/1 activation failed to increase infarct volume in CD36-null mice, whereas TLR2/6 or TLR4 activation exacerbated postischemic inflammation and increased infarct volume. In contrast, the systemic inflammatory response evoked by TLR2/6 activation, but not by TLR2/1 activation, was suppressed in CD36-null mice. Conclusions-In the brain, TLR2/1 signaling requires CD36. The cooperative signaling of TLR2/1 and CD36 is a critical factor in the inflammatory response and tissue damage evoked by cerebral ischemia. Thus, suppression of CD36-TLR2/1 signaling could be a valuable approach to minimize postischemic inflammation and the attendant brain injury. (Stroke. 2010;41:898-904.)