Comparative studies of endotoxin uptake by isolated rat Kupffer and peritoneal cells

The process of uptake of endotoxin by cells of the reticuloendothelial system is of current interest. Rabbit peritoneal macrophages have been used to study macrophage-endotoxin interactions and have suggested a receptor-mediated process. It is generally believed that the site of in vivo endotoxin clearance is the liver and that this clearance involves the Kupffer cell population. In the current report, the uptake characteristics of iodine-125-labeled Salmonella minnesota lipopolysaccharide (LPS) were compared in both isolated rat Kupffer cells and elicited rat peritoneal cells. Both types of cells were isolated from male Sprague-Dawley rats fed a semisynthetic AIN-76 5% saturated-fat diet either by peritoneal lavage for peritoneal cells or by collagenase perfusion followed by purification on a 17.5% metrizamide gradient for Kupffer cells. Hot phenol water-extracted S. minnesota LPS was labeled with iodine by the chloramine-T method following a reaction with methyl-p-hydroxybenzimidate. The in vitro uptake of [125I]LPS by Kupffer cells was unsaturable up to concentrations of 33.33 micrograms/ml, while peritoneal cells became saturated at between 16.67 and 25 micrograms of LPS per ml. Uptake by both types of cells could be inhibited by a 10-fold excess of unlabeled LPS. Kinetic experiments demonstrated that Kupffer cells were unsaturable after 60 min of incubation, while peritoneal cells were saturable after 40 min of incubation. Pretreatment with 75 mM colchicine inhibited uptake by peritoneal cells but not Kupffer cells, while pretreatment with 12 mM 2-deoxyglucose inhibited uptake by Kupffer cells but not peritoneal cells. These results are consistent with a process of receptor-mediated endocytosis for peritoneal cells, while Kupffer cells may internalize endotoxins by absorptive pinocytosis. These results suggest that studies of peritoneal cell-endotoxin interactions do not accurately describe the physiologic process within the liver, the major site for the clearance of gut-derived endotoxins.