Carbohydrate oxidation acidifies endosomes, regulating antigen processing and TLR9 signaling.

Phagocytes kill encapsulated microbes through oxidative cleavage of surface carbohydrates, releasing glycan fragments and microbial contents that serve as ligands for immune receptors, which tailor the immune response against the offending pathogen. The glycan fragments serve as MHC class II (MHC II) ligands and innate receptor agonists, whereas microbial proteins serve as substrates for proteolytic cleavage and MHC II presentation, and released nucleic acids activate innate pattern-recognition receptors (e.g., TLR9). In the current study, confocal microscopy of live macrophages and dendritic cells revealed that endocytosis of carbohydrates lead to vesicular acidification independent of proton pump activity. Acidification was dependent on NO-mediated oxidation in the presence of the ingested carbohydrate and was sufficient to negatively regulate T cell-dependent polysaccharide Ag cleavage, promote acid-dependent protein Ag processing, and facilitate CpG-mediated TLR9 signaling. Our findings lead to a model in which oxidation of carbohydrates from encapsulated microbes facilitates adaptive immune responses against microbial protein and carbohydrate Ags through promoting Ag processing for MHC II-mediated presentation as well as innate responses against released microbial DNA via TLR9 signaling.