Uptake and deacylation of bacterial lipopolysaccharides by macrophages from normal and endotoxin-hyporesponsive mice

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.

The immunogenicity and antigenicity of lipid A are influenced by its physicochemical state and environment

We investigated the immunogenicity and antigenicity of synthetic lipid A and partial structures thereof. Included in the study were compounds which varied in the position of phosphate (1-mono-, 4'-mono-, and 1,4'-bisphosphates) and in the acylation (type, number, and distribution of fatty acids) and, in the case of monosaccharide compounds, the nature of the backbone sugar (D-glucosamine, D-glucose, 3-amino-3-deoxy-D-glucose, and 2,3-diamino-2,3-dideoxy-D-glucose). With the aid of the passive-hemolysis and passive-hemolysis-inhibition assays and by absorption experiments, five distinct antibody specificities were detected in polyclonal rabbit antisera raised against sheep erythrocyte-coated lipid A and lipid A incorporated into the membrane of liposomes (liposome-incorporated immunogens). Three antibody specificities reacted with disaccharide antigens specific for a 1-mono-, 4'-mono-, and 1,4'-bisphosphorylated beta-1,6-linked D-glucosamine disaccharide. Two antibodies reacted with either 1- or 4-phosphates of acylated D-gluco-configured monosaccharides and exhibited no cross-reaction with each other. However, they cross-reacted with disaccharide antigens with phosphate groups in the appropriate positions. We found that the physicochemical state and the environment of lipid A modulated its immunoreactivity. The immunogenicity was best expressed by erythrocyte-coated and liposome-incorporated immunogens. The antigenicity of lipid A was also greatly influenced by its physical surroundings. The reaction pattern of the above antibodies was highly specific in the hemolysis assay and in absorption experiments (the antibody reacted with antigen embedded in a cell membrane), whereas some cross-reactivities were observed in inhibition studies (the antibody reacts with antigen in aqueous solution). By using liposome-incorporated antigens as inhibitors, nonspecific reactions were avoided and specific ones were enhanced. Thus the antibodies described above against lipid A recognize epitopes in the hydrophilic backbone, the exposure of which depends on the intrinsic physicochemical properties of lipid A on the one hand and the physical environment on the other.

Deacylated lipopolysaccharide inhibits neutrophil adherence to endothelium induced by lipopolysaccharide in vitro

Selective deacylation of the nonhydroxylated fatty acids from S. typhimurium LPS by an acyloxyacyl hydrolase isolated from leukocytes reduces toxic activity of LPS in vivo. We examined the effect of deacylated LPS on neutrophil adherence to human umbilical vein endothelial cells (HUVE). Pretreatment of HUVE with LPS (13 ng/ml for 4 h) produced a marked increase in the adherence of subsequently added neutrophils. In contrast, there was no increase in the adherence of neutrophils to HUVE pretreated with deacylated LPS (up to 260 ng/ml for 4 h). Neutrophil adherence to HUVE pretreated with LPS decreased as the degree of LPS deacylation increased. Deacylated LPS was not only itself inactive, but it inhibited neutrophil-endothelial interactions induced by LPS. Neutrophil adherence to HUVE pretreated with LPS was inhibited by deacylated LPS in a dose-dependent manner. Complete inhibition of adherence was observed at a 20:1 ratio (wt/wt) of deacylated LPS to LPS. Significantly, inhibition of neutrophil adherence to HUVE pretreated with LPS was observed even when deacylated LPS was added to HUVE up to 60 min after LPS. Deacylated LPS, however, did not inhibit neutrophil adherence induced by pretreatment of HUVE with IL-1 or TNF-alpha. We conclude that enzymatic deacylation of the nonhydroxylated fatty acids of LPS abolishes the ability of LPS to induce surface expression of a neutrophil adherence promoting activity in HUVE. Furthermore, deacylated LPS inhibits neutrophil adherence to HUVE induced by LPS, perhaps by preventing the interaction of LPS with a specific cell-surface or intracellular target.

Dephosphorylation of the lipid A moiety of Escherichia coli lipopolysaccharide by mouse macrophages

An Escherichia coli deep rough lipopolysaccharide (LPS), biosynthetically labeled with 32PO4 and [3H]glucosamine, was used to study dephosphorylation of the lipid A moiety by murine macrophages. Over a 48-h incubation period, the macrophages removed approximately two-thirds of the 32P from [3H32P]LPS that was added to the culture medium. The LPS-derived phosphate was incorporated into cell components (e.g., phospholipids), as well as released from the cells. Cell lysates were also able to remove phosphate from [3H32P]LPS. The phosphatase activity was optimal at acidic pH and was greatly reduced by 10 mM sodium fluoride or heating at 80 degrees C. There was no evident difference in the LPS-dephosphorylating ability of macrophages from LPS-responsive and -hyporesponsive mice. The results indicate that murine macrophages dephosphorylate the lipid A moiety of deep rough E. coli LPS and raise the possibility that enzymatic dephosphorylation may modify LPS bioactivity.

Antibody- and complement-dependent cell injury assayed by 51Cr release from human peripheral blood mononuclear cells pretreated with lipopolysaccharide

Exposure of human peripheral blood mononuclear (MN) cells to deesterified (alkali-treated) lipopolysaccharide (LPS-OH) and then to 51Cr rendered the cells susceptible to 51Cr release in the presence of specific antibody and complement. The assay was optimized by using rough (Rb2 or Re) LPS. 51Cr release did not occur from cells preexposed to untreated or electrodialyzed LPS. Studies of isolated monocytes and lymphocytes revealed that the majority of the 51Cr released was derived from monocytes. The optimum concentration of LPS-OH was 10 micrograms/ml. Antiyersinia agglutinin-positive serum, but not a negative serum, obtained from patients with reactive yersinia arthritis caused 51Cr release from MN cells pretreated with yersinia LPS-OH. This implies that during yersinia infection antibodies are generated that can attack the cell membrane--LPS-OH complex. We conclude that the method provides a tool to demonstrate binding of LPS to MN cells in a manner that leads to cell injury in an immune host.

Detoxification of bacterial lipopolysaccharides (endotoxins) by a human neutrophil enzyme

Lipopolysaccharides in the cell walls of Gram-negative bacteria elicit toxic as well as potentially beneficial inflammatory responses in animals. It is now reported that tissue toxicity caused by lipopolysaccharides is preferentially reduced by an enzymatic activity in human neutrophils. Acyloxyacyl hydrolysis removes fatty acyl chains that are linked to the hydroxyl groups of 3-hydroxytetradecanoyl residues in the bioactive lipid A moiety of the lipopolysaccharides. Maximal acyloxyacyl hydrolysis reduced lipopolysaccharide tissue toxicity, as measured in the dermal Shwartzman reaction, by a factor of 100 or more. In contrast, the ability of the deacylated lipopolysaccharides to stimulate B lymphocytes to divide was decreased only by a factor of 12. It is suggested that during tissue invasion by Gram-negative bacteria acyloxyacyl hydrolysis may be a defense mechanism that reduces the toxicity of lipopolysaccharides while preserving some of their potentially beneficial inflammatory and immune stimuli.