Chylomicrons (CM) can bind endotoxin (lipopolysaccharide [LPS]
), forming CM-LPS complexes, and protect against endotoxic shock and death in rodent models of gramnegative sepsis. The liver appears to play a central role in this process, as demonstrated by the increased uptake of LPS by this organ. We examined the effect of CM on the uptake and cellular response to injected 125 I-LPS by hepatocytes and hepatic nonparenchymal cells. Whereas CM increased the uptake of LPS by both hepatocytes and Kupffer cells, the increase was proportionately greater in hepatocytes than Kupffer cells. Importantly, CM-LPS complexes inhibited inducible nitric oxide synthase (iNOS) mRNA expression and NO production in Kupffer cells and endothelial cells, reducing mRNA levels by 45% to 50% as compared with LPS alone. CM-bound LPS also reduced NO production by hepatocytes in response to cytokine stimulation. Lastly, CM-LPS complexes yielded a concentrationdependent inhibition of LPS-induced tumor necrosis factor ␣ (TNF-␣) production by Kupffer cells in vitro. These data indicate that the mechanism by which CM protect against endotoxicity may involve an increased uptake of LPS by hepatocytes. Moreover, uptake of CM-bound LPS by liver cells attenuates the capacity of these cells to respond to proinflammatory stimulation. These results highlight important anti-inflammatory properties of CM. (HEPATOLOGY 1998; 27:1341-1348.)
Sepsis caused by gram-negative bacterial infection is an important and frequently devastating problem, annually affecting approximately 500,000 patients 1 and causing over 150,000 deaths. [2][3][4][5][6] While the incidence of sepsis has more than doubled during the last quarter century, 7 there has not been a substantial improvement in the survival rate since the 1970s. 8,9 Even though there are many effective antibiotics, they are often unsuccessful as the sole mode of therapy.
Advances in cellular and molecular biology, however, have increased our understanding of the pathophysiology of sepsis and the acute inflammatory response. Gram-negative bacterial sepsis is initiated by host exposure to endotoxin (lipopolysaccharide [LPS]), an amphipathic macromolecule that activates a variety of cells, including neutrophils, macrophages, and endothelial cells. Once stimulated, these cells release cytokines (tumor necrosis factor [TNF], interleukin-1 [IL-1]), nitric oxide (NO), and other soluble mediators, thus contributing to a cascade of proinflammatory events. 10-17 Simultaneously, the infection also activates the acute-phase response, wherein hepatocytes secrete substances designed to limit the extent and severity of the inflammatory reaction. 18,19 The balance between the bactericidal proinflammatory responses and the regulatory anti-inflammatory responses functionally defines the difference between a healthy, protective reaction to LPS versus a pathological, potentially fatal one. While the acute inflammatory response is critical for the host to combat bacterial infections, when this defensive reaction is dysregulated and overly exuberant, it can culminate in shock, multiple organ failure, and death.
An important feature of the acute-phase response to infection is the resultant alteration in hepatic lipoprotein metabolism, the so-called ''lipemia of sepsis.'' It has long been observed that when animals or humans are challenged with infectious agents or LPS, their plasma becomes cloudy because of an increased concentration of circulating triglyceride (TG)-rich lipoproteins produced by the liver. [20][21][22][23][24][25][26][27] The hyperlipoproteinemia is now known to be mediated by cytokines, in particular, TNF-␣. [28][29][30][31] This response has been intuitively thought to represent the mobilization of energy stores to fuel the body' s response to the infection. But the observation that the cholesterol-rich lipoproteins can bind and neutralize LPS [32][33][34] prompted us to postulate that TG-rich lipoproteins (e.g., chylomicrons [CM]) could as well, and thus function as components of an innate, host homeostatic and defensive response to infection. [35][36][37][38][39] In theory, host exposure to LPS would trigger a cytokine-mediated release of TG-rich lipoproteins that would act to ''scavenge'' circulating LPS. Thereafter, the resultant lipoprotein-bound LPS would be selectively taken up by the liver, rapidly removing the toxin from the circulation, and rendering it harmless.
Efforts in our laboratory have focused on an alternative strategy to treat gram-negative sepsis, one based on the theory that TG-rich lipoproteins can function as components of an innate, nonadaptive host immune response. In our previous work, we demonstrated that TG-rich lipoproteins, i.e., CM and very-low-density lipoproteins, can neutralize LPS and thus protect against endotoxic shock and lethality in rodent models of sepsis. [35][36][37] We also found that CM