Fluorescent liposomes for intravital staining of Kupffer cells to aid in vivo microscopy in rats.

The potential of newly formulated fluorescent-labeled liposomes for the intravital staining of Kupffer cells was evaluated in rats. Fluorescently labeled phosphatidylcholine (PC) was incorporated into liposomes consisting of PC and phosphatidylserine. After intravenous injection, Kupffer cells in the rat liver were intravitally stained and were clearly delineated under the fluorescence image of both confocal laser scanning microscopy and in vivo microscopy. Specificity of the staining was confirmed by immunohistochemistry using the anti-rat macrophage antibody Ki-M2R, which suggested that the liposomes were selectively entrapped by the hepatic reticuloendothelial system. A time-course study revealed that the suitable observation window was between 16 and 24 h after the injection. Phagocytic activity of Kupffer cells after the administration of liposomes was examined by measuring the amount of hepatic uptake of intravenously administered fluorescent microspheres; no detrimental influence of the liposomes on the phagocytic activity was observed. Additionally, no histopathologic changes were found in the livers from liposome-treated rats. Therefore, the fluorescent-labeled liposomes appear to be a useful research tool for labeling Kupffer cells for in vivo microscopic observation of the liver.