Effects of Moderate Hypothermia on IL-1β–Induced Leukocyte Rolling and Adhesion in Pial Microcirculation of Mice and on Proinflammatory Gene Expression in Human Cerebral Endothelial Cells

Although the neuroprotective effects of hypothermia have been known for a long time, the molecular correlates of this neuroprotection are poorly understood. In this study, the authors investigated how hypothermia affects inflammatory responses in the brain elicited by systemic injection of IL-1β. Leukocyte rolling and adhesion were quantified in pial venules (20 to 50 μm) of C57/Bl6 mice 4 hours after intraperitoneal injection of IL-1β (5 μg/kg) using an open cranial window and intravital microscopy. Animals were subjected to moderate hypothermia (32°C) or normothermia (37°C) for 1 or 4 hours after IL-1β injection. Significant increases in leukocyte rolling and adhesion were observed in IL-1β–injected animals as compared with sham controls. Whereas 1-hour hypothermia did not affect IL-1β–induced leukocyte rolling and adhesion, 4-hour hypothermia caused a reduction in both rolling and adhesion. Molecular mechanisms of hypothermic effects were investigated in cultured human cerebral endothelial cells exposed to IL-1β (50 U/mL) for 4 hours at 37°C or 32°C followed by 18 hours at 37°C. Human cerebral endothelial cells exposed to IL-1β at 32°C showed attenuated NF-κB activation determined by the Luciferase yellow reporter gene assay and reduced expression of IL-8 and IL-1β measured by reverse transcriptase–polymerase chain reaction and enzyme-linked immunosorbent assay. Intracellular adhesion molecule-1 was induced to similar levels (threefold over control) at both temperatures. The expression of CD18 on neutrophils in vitro was not affected by either IL-1β or hypothermia. These findings suggest that mechanisms by which hypothermia reduces leukocyte rolling and adhesion include suppression of inflammatory gene transcription in brain endothelial cells.