Isolation and chemical and biological characterization of antigenic mutants of Candida albicans serotype A.

Candida albicans serotype A possesses a specific antigen designated antigenic factor 6, that resides in the mannans on the cell surface. To define the molecular structure of antigenic factor 6, as well as to determine the role of cell wall mannan in the ability of C. albicans to adhere to epithelial cells, we isolated antigenic factor 6-deficient mutants by screening with agglutinating monoclonal antibody (MAb) CA4-2 which corresponds to polyclonal antibody (PAb) factor 6. 1H-NMR spectral analyses of the purified mannans from the parent and the mutants showed loss of the signal at 4.77 ppm, corresponding to the beta-linkage of the side chain, in the mutants. Although the parent whole mannan as well as its mannohexaose fragment (M6) inhibited the agglutination reaction between C. albicans serotype A cells and MAb CA4-2, the mutant mannan showed lower inhibitory activity and a decreased amount of M6 in its acetolyzed products. These results indicate that the mutant mannan is defective in the side chain of M6 which is bound, via beta-linkage, to a branch of the inner side chains, resulting in the loss of reactivity with MAb CA4-2. Experiments on the adherence of C. albicans showed that the antigenic mutants as well as C. albicans serotype B strains, which lack antigenic factor 6, had significantly (p less than 0.01) less adherence ability than the parent strain. These results suggest that the side chains of M6 specific for C. albicans serotype A mannan are heavily involved in the adherence mechanisms as ligands.