Activation and binding of C3 by Candida albicans

Interaction with components of the complement system is an important aspect of the pathogenesis of infection by Candida albicans. The key role of C3 as an opsonic ligand and as an element in amplification of complement activation led us to examine several factors that influence the activation and binding of C3 cleavage fragments to the yeast. Activation and binding of C3 were determined by use of normal human serum containing 125I-labeled C3. Incubation of yeast-phase cells in 20% serum led to deposition of 2.5 X 10(5) to 3.0 X 10(5) molecules of C3 per yeast cell. Binding of C3 was absent in serum that was heat inactivated for 30 min at 37 degrees C or in serum that was chelated with EDTA. Chelation of serum with EGTA [ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid] reduced binding of C3 fragments by 31%. These results suggest that the alternative complement pathway is the primary mechanism for activation and binding of C3 fragments to C. albicans. Bound C3 could be partially removed (50%) by treatment with 1.0 M hydroxylamine. In contrast, 1.0 M hydroxylamine removed 98% of the C3 fragments bound to encapsulated Cryptococcus neoformans. These results suggest that ester bonds are the primary mechanism for binding C3 to C. neoformans, whereas binding of C3 to C. albicans involves both ester and amide bonds. Monoclonal antibodies specific for C3c and an iC3b neoantigen were used to identify the fragment of C3 that was bound to C. albicans. The results showed that the primary fragment bound to the yeast was C3b. An examination of the kinetics of activation and binding of C3 fragments showed that activation and binding were very rapid. Near-maximal binding occurred after a 2.5- to 5-min incubation period. In contrast, activation and binding of C3 fragments to C. neoformans proceeded at a slower rate, with maximal binding requiring 10 to 20 min. These results indicate that activation and binding of C3 fragments by the yeasts C. albicans and C. neoformans differ in several important characteristics.