Hypoxic-Ischemic Injury Induces Macrophage Inflammatory Protein-1α Expression in Immature Rat Brain

BACKGROUND AND PURPOSE: Macrophage inflammatory protein (MIP)-1alpha is a well-characterized monocyte chemoattractant; its role in regulating monocyte and microglial recruitment and activation in the injured neonatal brain is unknown. We evaluated the impact of acute hypoxic-ischemic (HI) brain injury on the expression of MIP-1alpha in neonatal rat brain. METHODS: To elicit forebrain ischemic injury, 7-day-old (P7) rats underwent right carotid ligation, followed by 2.5 hours of 8% oxygen exposure. We used an enzyme-linked immunosorbent assay and immunohistochemistry to detect MIP-1alpha; double-labeling immunofluorescence assays were analyzed with confocal microscopy to identify cellular sources of MIP-1alpha. Immunocytochemistry assays were also used to detect 2 MIP-1alpha receptors, CCR1 and CCR5. RESULTS: We found marked increases in tissue concentrations of MIP-1alpha in the HI cerebral hemisphere, peaking from 8 to 72 hours after lesioning. Immunocytochemistry assays revealed that MIP-1alpha was constitutively expressed in physiologically activated microglia; from 8 to 120 hours after lesioning, MIP-1alpha immunoreactive monocytes and microglia accumulated in the lesion territory. In immunoreactive cells, MIP-1alpha was diffusely distributed throughout the cytoplasm at early post-HI time intervals; by 72 hours, MIP-1alpha immunoreactivity was typically concentrated adjacent to the nucleus, a pattern indicative of active MIP-1alpha production. In P7 to P12 brain, many cells expressed MIP-1alpha receptors; both CCR1 and CCR5 immunoreactivity were localized to endothelium and ependyma; CCR1-immunoreactive astrocytes and neurons and CCR5-immunoreactive microglia were also identified. CONCLUSIONS: These data implicate MIP-1alpha as a mediator of the complex and sustained inflammatory response initiated by perinatal HI braininjury.