Optical molecular imaging of stroke-induced brain inflammation in the mouse

Experimental and clinical evidence indicate that following stroke CD40 signalling is crucially involved in sustaining the inflammation and thereby contributes to the expansion of the lesion. Non-invasive imaging of CD40 receptor expression could provide a powerful tool to diagnose stroke-induced inflammation, to assess disease progression, to stratify patients for therapy, and to monitor response to therapeutic intervention. In the present study, we utilised non-invasive planar near-infrared fluorescence (NIRF) imaging to detect stroke-induced brain inflammation in a mouse stroke model. A monoclonal antibody against CD40 labelled with the NIRF dye Cy5.5 was injected intravenously 96 hours after transient middle cerebral artery occlusion (MCAO) in mice. NIRF imaging was performed 16 hours after injection of the compound. In MCAO-mice, high fluorescence intensities were detected through the skull and skin over the affected hemisphere. Corresponding ex-vivo NIRF images of the brain and brain sections confirmed localisation of the fluorescence in the ischemic territory. MCAO-mice receiving Cy5.5-labeled IgG as a control for non-specific accumulation did not show fluorescence enhancement over the ischemic hemisphere in-vivo. Single Plane Illumination Microscopy (SPIM) revealed the cellular localisation of the CD40 targeting contrast agent which was found to locate at activated microglia and endothelium after subsequent immunhistochemistry. Co-injection experiments with the green fluorescent cell tracker 6-carboxylfluorescein diacetate into the spleen of MCAO-mice revealed the presence of bloodderived mononuclear cells that were labelled with the CD40 targeting contrast agent. In conclusion, the results show that non-invasive NIRF imaging can be used to visualize stroke-induced brain inflammation in mice with high sensitivity and specificity.