Use of lipophilic near-infrared dye in whole-body optical imaging of hematopoietic cell homing.

We develop an optical whole-body imaging technique for monitoring normal and leukemic hematopoietic cell homing in vivo. A recently developed near-infrared (NIR) lipophilic carbocyanine dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindotricarbocyanine iodide (DiR) is used to safely and directly label the membranes of human leukemic Pre-B ALL G2 cell lines as well as primary murine lymphocytes and erythrocytes. DiR has absorption and fluorescence maxima at 750 and 782 nm, respectively, which corresponds to low light absorption and autofluorescence in living tissues. This allows us to obtain a significant signal with very low background level. A charge-coupled device (CCD)-based imager is used for noninvasive whole-body imaging of DiR-labeled cell homing in intact animals. This powerful technique can potentially visualize any cell type without use of specific antibodies conjugated with NIR fluorescent tag or loading cells with transporter-delivered NIR fluorophores. Thus, in vivo imaging based on NIR lipophilic carbocyanine dyes in combination with advanced optical techniques may serve as a powerful alternative or complementation to other small animal imaging methods.