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

Abstract

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