Uptake and processing of immunoglobulin-coated liposomes by subpopulations of rat liver macrophages

Abstract

In vivo uptake and processing by liver macrophages (Kupffer cells) of liposomes, covalently coated with rabbit immunoglobulin (Ig liposomes) was studied following intravenous injection in rats. Rabbit Ig liposomes were labeled with trace amounts of cholesteryl[ 14C]oleate and [ 3H]cholesteryl hexadecyl ether. 1 h after injection of the liposomes, the non-parenchymal cells were isolated and subjected to centrifugal elutriation with stepwise-increasing flow rates; thus, five sub-fractions of Kupffer cells were obtained ranging in size from 9 to 14 μm in diameter. The cells were assayed for peroxidase activity and protein content. Rabbit Ig liposomes were taken up preferentially by Kupffer cells with diameters larger than 11 μm, which constitute less than 25% of the total Kupffer cell population. The intralysosomal degradation of the ingested liposomes was monitored by measuring the 3H/ 14C ratio of the cells. Due to the rapid release from the cells of the [ 14C]oleate formed from the cholesteryl[ 14C]oleate and the virtually complete retention of the non-metabolizable [ 3H]cholesteryl hexadecyl ether the 3H/ 14C ratio of the cells increases with proceeding hydrolysis of the liposomes. Thus, we were able to show that, in vivo, the Kupffer cells of the larger size classes, are not only more active in liposome uptake, but are also substantially more active in liposome degradation than smaller cells. The maintenance of the observed heterogeneity of rat liver Kupffer cells, with respect to liposome uptake under in vitro culture conditions, was examined. Subfractions were maintained in monolayer culture for 2 days and incubated with rabbit Ig liposomes. Binding and uptake of liposomes by the cells was monitored by measuring cell-associated radioactivity at 4°D and 37°C, respectively. In contrast to our in vivo results, we observed maximal in vitro liposome binding and uptake in those subfractions containing small cells (10–11 μm diameter), while the fractions containing cells larger than 12 μm, which were more active in vivo, were substantially less active than the smaller cells. The maximum we observed was even more pronounced when the liposome concentration was increased. We conclude that liver macrophage subfractions that barely participate in liposome uptake from the bloodstream in vivo, possess the potential to develop the capacity in vitro to phagocytose rabbit Ig-coated liposomes to extents equal to or even higher than the cells belonging to those subfractions containing the phagocytically most active cells under in vivo conditions.