CCL2 upregulation triggers hypoxic preconditioning-induced protection from stroke

Background: a brief exposure to systemic hypoxia (i.e.,hypoxic preconditioning; hpc) prior to transient middle cerebral artery occlusion (tmcao) reduces infarct volume,blood-brain barrier disruption,and leukocyte migration. ccl2 (mcp-1),typically regarded as a leukocyte-derived pro-inflammatory chemokine,can also be directly upregulated by hypoxia-induced transcription. we hypothesized that such a hypoxia-induced upregulation of ccl2 is required for hpc-induced ischemic tolerance.methods: adult male sw/nd4,ccl2-null,and wild-type mice were used in these studies. cortical ccl2/ccr2 message,protein,and cell-type specific immunoreactivity were determined following hpc (4 h,8% o 2) or room air control (21% o 2) from 6 h through 2 weeks following hpc. circulating leukocyte subsets were determined by multi-parameter flow cytometry in naïve mice and 12 h after hpc. ccl2-null and wild-type mice were exposed to hpc 2 days prior to tmcao,with immunoneutralization of ccl2 during hpc achieved by a monoclonal ccl2 antibody.results: cortical ccl2 mrna and protein expression peaked at 12 h after hpc (both p < 0.01),predominantly in cortical neurons,and returned to baseline by 2 days. a delayed cerebral endothelial ccl2 message expression (p < 0.05) occurred 2 days after hpc. the levels of circulating monocytes (p < 0.0001),t lymphocytes (p < 0.0001),and granulocytes were decreased 12 h after hpc,and those of b lymphocytes were increased (p < 0.0001),but the magnitude of these respective changes did not differ between wild-type and ccl2-null mice. hpc did decrease the number of circulating ccr2 +monocytes (p < 0.0001) in a ccl2-dependent manner,but immunohistochemical analyses at this 12 h timepoint indicated that this leukocyte subpopulation did not move into the cns. while hpc reduced infarct volumes by 27% (p < 0.01) in wild-type mice,ccl2-null mice subjected to tmcao were not protected by hpc. moreover,administration of a ccl2 immunoneutralizing antibody prior to hpc completely blocked (p < 0.0001 vs. hpc-treated mice) the development of ischemic tolerance.conclusions: the early expression of ccl2 in neurons,the delayed expression of ccl2 in cerebral endothelial cells,and ccl2-mediated actions on circulating ccr2 +monocytes,appear to be required to establish ischemic tolerance to focal stroke in response to hpc,and thus represent a novel role for this chemokine in endogenous neurovascular protection. © 2012 stowe et al; licensee biomed central ltd.