ICAM-1 Expression, Neutrophil & Monocyte Infiltration and Microglia Activation Reduced by Mild Hypothermia in a Rat Model of Transient Focal Cerebral Ischemia
BACKGROUND: Inflammation potentiates ischemic injury especially with reperfusion, while mild hypothermia is an effective neuroprotectant. We studied whether mild hypothermia’s protective effect may be due to blunting the inflammatory response. METHODS: We compared endothelial intracellular adhesion molecule (ICAM-1) expression, neutrophil & monocyte infiltration, and microglia activation under normothermic (38C n=24) and hypothermic (intraischemic brain cooling to 33C for 2h n=24) conditions 1,3, and 7 days following transient (2h) focal cerebral ischemia in rats. ICAM-1, ED-1 (to detect cells of monocyte lineage including activated microglia) and myeloperoxidase (MPO, to identify neutrophils) were detected using immunohistochemistry on brain sections. We measured cell densities of ICAM-1, ED-1, and MPO in the peri-infarct region. Infarct size was also measured from histology derived sizes of the ipsilateral hemisphere. RESULTS: Mild hypothermia reduced infarct size 1–7 days after stroke onset. 1d: 38C: 28±12%, 33C 8±6%; 3d: 38C: 39±23%, 33C: 11±11%; 7d: 38C: 37.3±5.8% vs 33C: 19.4±5.7% (p<0.05). The number of ICAM-1 positive vessels per high power field (HPF) decreased under hypothermia 1–7 days later. 1d: 38C: 35.8±1.7 vs 33C: 24.4±1.4 (p<0.01); 3d: 38C: 36.6±5.0 vs 33C: 23.5 1.3 (p=0.07); 7d: 38C: 69.9±2.7, 33C: 43±5.2 (p<0.001). In addition, neutrophil density (cells/12 HPF) decreased under hypothermic conditions at 1 and 3 days. 1d: 38C: 48±3.0 vs 33C: 1.3±0.6 (p<0.001), 3d: 38C: 75±3.5 vs 33C: 20.3±8.1 (p<0.001). Monocyte and microglial density (cells/HPF) was decreased by mild hypothermia at 3 and 7, but not 1 day. 1d: 38C: 17.5±0.9, 33C: 16.6±0.8, NS; 3d: 38C: 45.5±1.7, 33C: 19.2±0.9, (p<0.0001); 7d: 38C: 75.0 ±2.6, 33C: 24.8±3.0, (p<0.0001). CONCLUSIONS: Mild hypothermia reduces adhesion molecule expression, acute (neutrophil) and subacute (monocyte) leukocyte infiltration and microglial activation. These changes are present even days after hypothermic treatment, and suggest that hypothermia significantly attenuates the inflammatory response.