Phagocytosis of apoptotic bodies by liver endothelial cells

Using electron microscopy and cytofluorimetry we studied the role of carbohydrate-specific recognition systems in the interaction of apoptotic bodies with normal and interleukin 1-activated sinusoidal endothelial cells. Microfluorimetric observation of liver tissue sections revealed octadecylrhodamine B-labelled apoptotic body binding to the sinusoidal wall of mouse liver, when they were injected intraportally. Plate-scanning cytofluorimetry demonstrated that about 20-25% of Acridine Orange-labelled apoptotic bodies could adhere specifically to cultured endothelial cells after 15 minutes of incubation. Adhesion increased to 30% when the cells were incubated for 60 minutes. Using a mixture of galactose/N-acetylglucosamine/mannose as competition solution apoptotic body adhesion was significantly reduced especially after longer times of incubation, when the percentage of inhibition reached 50%. Following 4 hours exposure of liver endothelial cells to 1 ng/ml human recombinant interleukin-1 beta adhesion markedly increased after 60 minutes of incubation, whereas the co-incubation of interleukin-1 beta with the inhibitors brings down the adhesion to basal values obtained in controls. Electron microscopic observation of the adhesion process showed that the number of endothelial cells binding apoptotic bodies gradually increased from low to high values with time. After 60 minutes of incubation, the majority of apoptotic bodies were seen inside phagosomes and only a few remained at the cell surface. Liver endothelial cells bound and endocytosed apoptotic bodies through carbohydrate-specific receptors. Moreover, this scavenger action was interleukin-1 enhanced, thus suggesting its possible activation during inflammatory and immune processes.