An iC3b receptor on Candida albicans: structure, function, and correlates for pathogenicity

Heretofore, the existence of membrane receptors for biologically active fragments of mammalian complement proteins has been confined to mammalian cells, where these receptors serve to protect the host by triggering multiple aspects of the phagocytic response to microbial invasion. In this study, we show that surface receptors for the C3 fragment iC3b are present on the yeast Candida albicans, where they promote the pathogenicity of this organism by inhibiting phagocytosis. These receptors share homology with the alpha-chain, but not with the beta-chain, of neutrophil receptors for iC3b, as determined by the binding of monoclonal antibodies, and are induced by mycelial transformation of the yeast and by high concentrations of glucose. Blockade of these receptors by monoclonal antibodies significantly augments phagocytosis for those strains studied. By binding iC3b noncovalently, these receptors impair phagocytic uptake of C. albicans by human polymorphonuclear leukocytes.