Glucose metabolism is upregulated in the mononuclear cell proteome during sepsis and supports endotoxin-tolerant cell function.

Metabolic adaptations shape immune cell function. In the acute response, a metabolic switch towards glycolysis is necessary for mounting a proinflammatory response. During the clinical course of sepsis, both suppression and activation of immune responses take place simultaneously. Leukocytes from septic patients present inhibition of cytokine production while other functions such as phagocytosis and production of reactive oxygen species (ROS) are preserved, similarly to the in vitro endotoxin tolerance model, where a first stimulation with lipopolysaccharide (LPS) affects the response to a second stimulus. Here, we sought to investigate how cellular metabolism is related to the modulation of immune responses in sepsis and endotoxin tolerance. Proteomic analysis in peripheral blood mononuclear cells (PBMCs) from septic patients obtained at intensive care unit admission showed an upregulation of proteins related to glycolysis, the pentose phosphate pathway (PPP), production of ROS and nitric oxide, and downregulation of proteins in the tricarboxylic acid cycle and oxidative phosphorylation compared to healthy volunteers. Using the endotoxin-tolerance model in PBMCs from healthy subjects, we observed increased lactate production in control cells upon LPS stimulation, while endotoxin-tolerant cells presented inhibited tumor necrosis factor-α and lactate production along with preserved phagocytic capacity. Inhibition of glycolysis and PPP led to impairment of phagocytosis and cytokine production both in control and in endotoxin-tolerant cells. These data indicate that glucose metabolism supports leukocyte functions even in a condition of endotoxin tolerance.

Human endotoxin tolerance is associated with enrichment of the CD14+ CD16+ monocyte subset.

Prior exposure to lipopolysaccharides (LPS) induces a state of cell resistance to subsequent LPS restimulation, known as endotoxin tolerance, mainly by repressing the expression of pro-inflammatory cytokines. We established an endotoxin tolerance model in human monocytes Endotoxin-tolerant cells showed a decrease in IκBα degradation and diminished expression of Tumor necrosis factor (TNF) (both messenger RNA [mRNA] and protein content). The myeloid differentiation factor 88 (MyD88)/MyD88 splice variant (MyD88s) ratio, an indirect way to test the Toll-like receptor 4 (TLR4) MyD88-dependent signaling cascade, did not change in endotoxin-tolerant cells when compared to LPS-stimulated or -unstimulated ones. Remarkably, cell population analysis indicated a significant increase of the CD14+ CD16+ subset only under the endotoxin-tolerant condition. Furthermore, endotoxin-tolerant cells produced higher amounts of C-X-C motif chemokine 10 (CXCL10), a typical MyD88-independent cytokine.