Introduction: Lipids play a regulatory role in immune responses and inflammation. Sphingolipids can influence the expression of surface markers on dendritic cells (DCs) and affect their capacity to phagocytose the fungal pathogen Candida albicans. Farnesol, a quorum-sensing molecule produced by C. albicans, alters the expression of antigen-presentation markers on human monocyte-derived DCs (moDCs) resulting in less potent T cell priming. Therefore, the aim of this study is to investigate the effects of farnesol on the sphingolipid metabolism in human moDCs and explore the underlying molecular mechanism and functional consequences of farnesol-induced changes in sphingolipid composition.

Methods: Primary human monocytes were differentiated to moDCs in the presence of farnesol or solvent control. Quantification of sphingolipid metabolites was achieved by HPLC-MS/MS. The activity of specific enzymes involved in the de novo synthesis of sphingolipids was detected using a cell-free sphingolipid de novo synthesis assay and deuterated metabolites. The molecular mechanism underlying the changes in lipid composition was investigated by detecting reactive oxygen species (ROS) using DCF and MitoSox dyes. Mitochondrial respiration was measured using an Extracellular Flux Seahorse Analyzer.

Results: We found that farnesol increases the intracellular content of dihydrosphingolipid species in moDcs. The activity of the first enzyme in the sphingolipid de novo synthesis pathway is significantly enhanced upon treatment with farnesol, causing the accumulation of downstream metabolites. In addition, farnesol inhibits dihydroceramide desaturase (Des) activity, the last enzyme in the de novo synthesis pathway. We could link the indirect inhibition of Des by farnesol to the generation of ROS originating from the mitochondria. Further, the associated dihydroceramide accumulation may most likely account for the deficient mitochondrial function observed in our model.

Summary: In summary, our data reveals novel effects of farnesol on sphingolipid metabolism and mitochondrial function in DCs, indicating that C. albicans can manipulate host cell metabolism via farnesol secretion.