Chylomicrons alter the fate of endotoxin, decreasing tumor necrosis factor release and preventing death.

The hypertriglyceridemia of infection was traditionally thought to represent the mobilization of substrate to fuel the body's response to the infectious challenge. However, we have previously shown that triglyceride-rich lipoproteins can protect against endotoxin-induced lethality. The current studies examine the mechanism by which this protection occurs. Rats infused with a lethal dose of endotoxin preincubated with chylomicrons had a reduced mortality compared with rats infused with endotoxin alone (15 vs. 76%, P < 0.001). Preincubation with chylomicrons increased the rate of clearance of endotoxin from plasma and doubled the amount of endotoxin cleared by the liver (30 +/- 1 vs. 14 +/- 2% of the total infused radiolabel, P < 0.001). In addition, autoradiographic studies showed that chylomicrons directed more of the endotoxin to hepatocytes and away from hepatic macrophages. Rats infused with endotoxin plus chylomicrons also showed reduced peak serum levels of tumor necrosis factor as compared with controls (14.2 +/- 3.3 vs. 44.9 +/- 9.5 ng/ml, mean +/- SEM, P = 0.014). In separate experiments, chylomicrons (1,000 mg triglyceride/kg) or saline were infused 10 min before the infusion of endotoxin. Chylomicron pretreatment resulted in a reduced mortality compared with rats infused with endotoxin alone (22 vs. 78%, P < 0.005). Therefore, chylomicrons can protect against endotoxin-induced lethality with and without preincubation with endotoxin. The mechanism by which chylomicrons protect against endotoxin appears to involve the shunting of endotoxin to hepatocytes and away from macrophages, thereby decreasing macrophage activation and the secretion of cytokines.

Detection of endotoxin in triglyceride-rich lipoproteins in vitro.

Numerous investigations have been performed in which volunteers have received infusions of triglyceride-rich lipoproteins without apparent screening of the infusates for bacterial endotoxin. This study was designed to examine the capacity of triglyceride-rich lipoproteins to mask their endotoxin content in vitro as measured by a chromogenic modification of the standard Limulus assay. Lipoproteins and lipoprotein-deficient plasma were isolated from normal human plasma by sequential ultracentrifugation under apyrogenic conditions. Individual lipoproteins and a synthetic lipid emulsion were suspended in 10% lipoprotein-deficient plasma. Samples were then incubated at 37 degrees C for 4 hours with increasing concentrations of E. coli (055:B5) endotoxin and assayed for detectable endotoxin activity. The capacity to inhibit detection of endotoxin in 10% lipoprotein-deficient plasma was significantly increased (10 to 100 times) by the addition of VLDL (1.0 mg triglyceride/ml), chylomicrons (1.0 mg triglyceride/ml), or the synthetic lipid emulsion (2.5 mg triglycerides/ml). These data demonstrate that triglyceride-rich lipoproteins, and the synthetic lipid emulsion, can markedly inhibit the detection of endotoxin by the Limulus assay in vitro. In addition to the potential of harm to experimental subjects, infusion of endotoxin could vitiate kinetic studies by direct alteration of lipoprotein metabolism and by inducing changes in hepatic blood flow. Thus experimental protocols that involve the infusion of humans with triglyceride-rich lipoproteins should include detailed testing for the presence of endotoxin.

Chylomicrons can inhibit endotoxin activity in vitro.

Trauma, thermal injury, and nonlethal doses of endotoxin can promote the translocation of endotoxin across the mucosal barrier of the colon into the mesenteric lymphatics and systemic circulation. Bacterial endotoxemia induces changes in lipid metabolism, including an increase in circulating triglyceride-rich lipoproteins. Because cholesterol-rich lipoproteins can neutralize the toxic activity of endotoxin, both in vitro and in vivo, we asked whether triglyceride-rich chylomicrons can inhibit endotoxin activity in vitro as measured by a chromogenic Limulus assay. We tested the effect of intact versus heat-denatured chylomicrons on the in vitro activity of increasing concentrations of Escherichia coli (055:B5) endotoxin. Intact chylomicrons inhibited up to 12-fold the detection of as much as 1 microgram of endotoxin/mg of chylomicron triglyceride, compared to denatured chylomicrons (P less than 0.001). This study shows that chylomicrons are potent inhibitors of endotoxin activity in vitro. Because translocated endotoxin from the colon associates with gut-derived chylomicrons in the mesenteric lymphatics, this may represent a natural defensive mechanism against endotoxemia of enteric origin.