Characterization of the Endocytosis of Dectin-1, an Innate Immune Receptor

Abstract

Fungal pathogens are recognized by Dectin-1, a pattern recognition receptor expressed on mammalian innate immune cells. Dectin-1 detects β-glucans, which are polymers of glucose that are a main component of the fungal cell wall. While purified, soluble β-glucans have been used in the clinic as immunostimulatory agents, their mechanism of action is unclear. Thus, we sought to elucidate the effects they have on the signaling and endocytosis of Dectin-1. We expressed Dectin-1 in murine macrophages, and developed immunofluorescence and reversible cell surface biotinylation assays to study its endocytosis. Three soluble β-glucans, laminarin, soluble Wellmune whole glucan particle, and phosphorylated curdlan, were used as ligands to the receptor and were found to trigger Dectin-1 signaling, albeit to various extents. We observed that the rate of Dectin-1 internalization increased upon stimulation with the ligands, and this uptake was determined to be dynamin- and likely clathrin-dependent. Upon internalization, Dectin-1 trafficked to early endosomes and the recycling compartment, but not to lysosomes, promoting the return of internalized receptor to the cell surface. Dectin-1 signaling was not required for its endocytosis, although endocytosis may regulate Dectin-1 signaling events. Our findings provide a molecular understanding to the therapeutic activity of purified, soluble β-glucans and offer considerations for the design of new vaccines.