Endotoxin Binding and Elimination by Monocytes: Secretion of Soluble CD14 Represents an Inducible Mechanism Counteracting Reduced Expression of Membrane CD14 in Patients with Sepsis and in a Patient with Paroxysmal Nocturnal Hemoglobinuria

Abstract

Little is known about the role of peripheral blood mononuclear cells (PBMCs) in lipopolysaccharide (LPS) elimination. We studied the endotoxin elimination capacities (EEC) of PBMCs of 15 healthy volunteers, 13 patients with sepsis, and 1 patient suffering from paroxysmal nocturnal hemoglobinuria (PNH). Although expression of CD14, the best-characterized receptor for LPS to date, was reduced from 93.6% ± 0.8% in healthy subjects to 50.5% ± 6.5% in patients with sepsis and was 0.3% in a patient with septic PNH, EEC were found to be unchanged. There was no difference in the amount of tumor necrosis factor alpha (TNF-α) released by PBMCs of healthy donors and patients with sepsis. Anti-CD14 antibodies (MEM-18) completely suppressed EEC, binding of fluorescein isothiocyanate-labeled LPS to monocytes as determined by FACScan analysis, and TNF-α release in all three groups studied. The concentrations of soluble CD14 (sCD14) secreted by endotoxin-stimulated PBMCs from healthy donors and patients with sepsis amounted to 4.5 ± 0.4 and 20.1 ± 1.8 ng/ml, respectively. Based on our results, we suggest that PBMCs eliminate LPS by at least two different mechanisms; in healthy subjects, the membrane CD14 (mCD14) receptor is the most important factor for LPS elimination, while in patients with sepsis (including the septic state of PNH), increased sCD14 participates in LPS elimination. Secretion of sCD14 is strongly enhanced under conditions of low expression of mCD14 in order to counteract the reduction of mCD14 and maintain the function of monocytes. This sCD14 may substitute the role of mCD14 in LPS elimination during sepsis. The elimination of LPS by PBMCs correlates with the binding reaction and the secretion of TNF-α.

Association between serum soluble CD14 and IL‐8 levels and clinical outcome in primary biliary cholangitis

“…LPS activation causes the shedding and secretion of sCD14, thereby making sCD14 an important marker of LPS bioactivity. [38][39][40] Yokoyama et al 41 reported that human intrahepatic biliary epithelial cells constitutively expressed transcripts encoding TLR 1-6, TLR 9…”

Evaluation of CD14 in host defence

“…This form is stored intracellularly and is released spontaneously upon a short temperature shift at 37 ЊC; a process that is not altered by the presence of protease inhibitors [19,44]. Elevated levels of this 55-kDa sCD14 are detected exclusively in the sera of patients suffering from Paroxysmal Nocturnal Hemoglobinuria (PNH), whose monocytes do not express mCD14 as a result of a defect in GPI synthesis [44,46]. The enhanced release of sCD14 by monocytes of PNH patients might counteract the reduced mCD14 expression, as in vitro PNH monocytes are able to bind and respond to LPS in the presence of sCD14 [46].…”

“…Elevated levels of this 55-kDa sCD14 are detected exclusively in the sera of patients suffering from Paroxysmal Nocturnal Hemoglobinuria (PNH), whose monocytes do not express mCD14 as a result of a defect in GPI synthesis [44,46]. The enhanced release of sCD14 by monocytes of PNH patients might counteract the reduced mCD14 expression, as in vitro PNH monocytes are able to bind and respond to LPS in the presence of sCD14 [46]. The biological differences between these two forms of sCD14 molecules are not clear yet.…”

Chronic immune activation in common variable immunodeficiency (CVID) is associated with elevated serum levels of soluble CD14 and CD25 but not endotoxaemia

“…Complex interactions between LPS, CD14, myeloid differentiation-2 (MD-2) and Toll-like receptor (TLR)-4 results in an activation of the nuclear factor kappa-lightchain-enhancer of activated B cells (NFkB) signalling pathway and production of multiple inflammatory factors [24][25][26][27]. The MD-2/TLR-4/LPS complex induces shedding and secretion of soluble CD14 (sCD14) from myeloid cells [28][29][30]. Plasma or serum levels of sCD14 are generally accepted as reliable markers of bacterial translocation and endotoxaemia [24,[31][32][33][34].…”