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

Background and Purpose—Macrophage inflammatory protein (MIP)-1 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-1 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-1 ; double-labeling immunofluorescence assays were analyzed with confocal microscopy to identify cellular sources of MIP-1 . Immunocytochemistry assays were also used to detect 2 MIP-1 receptors, CCR1 and CCR5. Results—We found marked increases in tissue concentrations of MIP-1 in the HI cerebral hemisphere, peaking from 8 to 72 hours after lesioning. Immunocytochemistry assays revealed that MIP-1 was constitutively expressed in physiologically activated microglia; from 8 to 120 hours after lesioning, MIP-1 immunoreactive monocytes and microglia accumulated in the lesion territory. In immunoreactive cells, MIP-1 was diffusely distributed throughout the cytoplasm at early post-HI time intervals; by 72 hours, MIP-1 immunoreactivity was typically concentrated adjacent to the nucleus, a pattern indicative of active MIP-1 production. In P7 to P12 brain, many cells expressed MIP-1 receptors; both CCR1 and CCR5 immunoreactivity were localized to endothelium and ependyma; CCR1immunoreactive astrocytes and neurons and CCR5-immunoreactive microglia were also identified. Conclusions—These data implicate MIP-1 as a mediator of the complex and sustained inflammatory response initiated by perinatal HI brain injury. (Stroke. 2002;33:795-801.)