Expression of Interleukin‐17 in Ischemic Brain Tissue

Ischemic brain injury is acute local inflammation, leading to accumulation of pro-inflammatory cytokines. Cytokines influence the recruitment of leucocytes and play a key role in the inflammatory injury processes. Recently, a number of studies have demonstrated a close relationship between brain ischemia and cytokines. Interleukin-17 (IL-17) is a newly identified T-cell-specific cytokine. In this study, we evaluated the source and the action of IL-17 over the course of cerebral ischemia in rats (Sprague-Dawley) and humans. The levels of IL-17 in the ischemic hemisphere of the human brain, which was removed at necropsy, were assayed immunohistochemically. In rats, permanent middle cerebral artery occlusion (pMCAO) was obtained by inserting nylon monofilament into the right external carotid artery, occluding the right middle cerebral artery. The expression of IL-17 mRNA in rat was assayed using oligoprobe in situ hybridization. IL-17 production by neuroglial cells was assayed by double-staining using antibody glial fibrillary acidic protein (GFAP) and antibody IL-17. Levels of IL-17 were elevated in the ischemic hemispheres of human brain compared with the opposite normal hemispheres and peaked at days 3-5 after brain ischemia. The IL-17-positive cells were found in the ischemic lesion region. IL-17 mRNA was also elevated in ischemic hemispheres of pMCAO-operated rats, which were slightly elevated after 1 h and peaked at 6 days. IL-17 and GFAP double-stained were extensive in rat ischemic hemisphere. The ischemia-induced IL-17 expression in human brain reported here for the first time was very similar to that in rat model except that the peak was slightly earlier. We found for the first time that IL-17 was involved in an intense inflammatory reaction of brain ischemic injury in human. In pMCAO-operated rats, our findings suggest that IL-17 is produced by the neuroglial cells in the brain region undergoing ischemic insult. We suggest that in additional to T cells the neuroglial cell may be another cellular origin of IL-17 in later progression of brain ischemia.