The iC3b receptor on Candida albicans: subcellular localization and modulation of receptor expression by glucose

Patient isolates of Candida albicans from blood, urine, or mucosal sites express a surface receptor for C3 fragment iC3b that is recognized by monoclonal antibodies (MAb) directed against alpha-chain epitopes of the neutrophil iC3b receptor, also known as the type 3 complement receptor (CR3) or CD11b/CD18. Because 60% of these patients were hyperglycemic, the effect of glucose on receptor expression was investigated. As assessed by flow cytometry, yeasts grown in 20 mM D-glucose exhibited a four- to six-fold increase in receptor expression compared with yeasts grown in 20 mM L-glutamate. Receptor expression increased as glucose concentration rose from 0 to 20 mM (equivalent to plasma glucose concentrations of 0-360 mg/dl). Augmentation of receptor expression by growth in glucose led to significant inhibition of phagocytosis compared with that of organisms grown in equimolar L-glutamate. SDS-PAGE, Western blotting, and immunodetection of extracts of yeast cell wall, membrane, and cytosol disclosed a protein of 165 kDa in membrane and cytosolic extracts, consistent with the published Mr of the alpha-chain of neutrophil CR3. These studies provide a mechanism to explain the predilection of C. albicans to infect the hyperglycemic host.