Evolution of Inflammation and White Matter Injury in a Model of Transient Focal Ischemia

After an ischemic stroke, there is a prolonged inflammatory response and secondary phase of injury that is more amenable to treatment than acute neurotoxicity. Surprisingly, little is known about temporal and spatial relationships between inflammation and white matter injury. Here, we quantified development of white matter damage, inflammation, and a glial limitans at 1, 3, and 7 days after transient ischemia in the rat striatum using immunohistochemistry. Quantitative analysis showed that decreased staining for myelin basic protein and increased staining for damaged myelin basic protein began in the core, coincided with neutrophil infiltration, and progressed outward over time. Axon damage (i.e. accumulation of amyloid precursor protein) began at the edge of the lesion, coinciding with substantial microglia/macrophage activation, and progressed into the core. During the 7 days, activated microglia/macrophages dramatically increased only in the core and edge of the lesion. Detailed spatial analyses revealed that activated microglia/macrophages that surrounded undamaged axon bundles did not express ED1, a marker of phagocytic cells, whereas those inside damaged bundles expressed ED1. These results imply different contributions of neutrophils and microglia/macrophages to white matter injury after ischemic stroke. The distinct localizations of activated microglia/macrophages imply complex signals that regulate their migration toward and infiltration of damaged white matter.