High concentrations of lipopolysaccharide-binding protein in serum of patients with severe sepsis or septic shock inhibit the lipopolysaccharide response in human monocytes

Abstract

Lipopolysaccharide-binding protein (LBP), an acute-phase protein recognizing lipopolysaccharide (LPS), catalyzes in low concentrations its transfer to the cellular LPS receptor consisting of CD14 and Toll-like receptor-4. It has recently been shown that high concentrations of recombinant LBP can protect mice in a peritonitis model from the lethal effects of LPS. To determine whether in humans the acute-phase rise of LBP concentrations can inhibit LPS binding to monocytes and induction of proinflammatory cytokines, LBP concentrations were analyzed in 63 patients meeting the American College of Chest Physicians/Society of Critical Care Medicine criteria of severe sepsis or septic shock and the ability of these sera to modulate LPS effects in vitro was assessed employing different assays. Transfer of fluorescein isothiocyanate-labeled LPS to human monocytes was assessed by a fluorescence-activated cell sorter-based method, and activation of monocytes was investigated by measuring LPS-induced tumor necrosis factor-alpha secretion in the presence of the sera. Anti-LBP antibodies and recombinant human LBP were instrumental for depletion and reconstitution of acute-phase sera and subsequent assessment of their modulating effects on LPS activity. Sera of patients with severe sepsis/septic shock exhibited a diminished LPS transfer activity and LPS-induced tumor necrosis factor-alpha secretion as compared with sera from healthy controls. LBP depletion of sepsis sera and addition of rhLBP resulting in concentrations found in severe sepsis confirmed that LBP was the major serum component responsible for the observed effects. In summary, the inhibition of LPS effects by high concentrations of LBP in acute-phase serum, as described here, may represent a novel defense mechanism of the host in severe sepsis and during bacterial infections.

Lipopolysaccharide (LPS)-binding Protein Inhibits Responses to Cell-bound LPS

“…Whereas LBP can promote LPS signaling at all LBP concentrations, the inhibitory mechanisms require relatively high LBP concentrations to have a significant impact. In the same experiments, high concentrations of LBP were inhibitory only when the LPS concentration was low (Յ1 ng/ml), whereas at higher LPS concentrations, much higher concentrations of LBP were required to inhibit responses to the LPS (27,28). LBP concentrations often increase to very high levels in the blood of septic patients, and progressive immunodepletion of LBP from the serum of these patients revealed that the LBP can have a strong inhibitory effect on the activity of LPS (28).…”

“…High acute phase concentrations of LBP in the plasma of septic humans decrease monocyte responses to LPS (28). The inhibitory mechanism of LBP can be explained in part by its ability to neutralize the bioactivity of LPS by transferring it to plasma lipoproteins (6,29).…”

“…The inhibitory mechanism of LBP can be explained in part by its ability to neutralize the bioactivity of LPS by transferring it to plasma lipoproteins (6,29). Other inhibitory mechanisms are suggested by the ability of LBP to inhibit cell responses to LPS in serum-free medium (27,28). Possible mechanisms might include the formation of large LPS⅐LBP complexes that are internalized by mCD14 but that contribute little to LPS signaling (30).…”

Circulating lipopolysaccharide-binding protein (LBP) as a marker of obesity-related insulin resistance

“…LBP could carry the potential of a diagnostic marker in sepsis and invasive infections. 23,24 Thus, the high circulating LBP concentration associated with insulin resistance and obesity could reflect the physiological response to LPS-associated metabolic disturbances. However, the decrease in circulating LBP concentration was associated with the decrease in fat mass after weight loss and not with the decrease in LPS.…”

“…At 20 min, regular insulin (Actrapid, Novo, Denmark; 0.03 U kg À1 ) was injected as a bolus. Additional samples were obtained from a contralateral antecubital vein at times 1,2,3,4,5,6,7,8,10,12,14,16,19,20,22,23,24,25,27,30,40,50,60,70,80,90, 100, 120, 140, 160 and 180 min. Samples were rapidly collected through a three-way stopcock connected to the butterfly needle.…”