Contribution of Epitope Specificity to the Binding of Monoclonal Antibodies to the Capsule of Cryptococcus neoformans and the Soluble Form of Its Major Polysaccharide, Glucuronoxylomannan

Incubation of encapsulated cryptococci with monoclonal antibodies (MAbs) specific for glucuronoxylomannan (GXM), the major capsular polysaccharide of Cryptococcus neoformans, produces two distinct capsular quellung-type reactions termed rim and puffy. The type of capsular reaction that occurs is determined by the epitope specificity of the MAb and the serotype of the yeast cell. Several biological activities, including opsonic activity, complement activation, and protective efficacy, are associated with the type of capsular reaction produced by a MAb. The goal of this study was to examine the reactivities of two families of anti-GXM MAbs with serotype A and D capsular polysaccharides in several immunochemical assays, including agglutination, immunofluorescence, quantitative precipitation, and enzyme-linked immunosorbent assay, in an effort to identify serological assays that are predictive of the capsular quellung reaction. The results showed that the type of capsular reaction (rim versus puffy) is a qualitative assessment of antibody-capsule interaction that cannot be predicted on the basis of a serological assay. The results further showed that antibody reactivity demonstrated in one serological assay is not necessarily predictive of results in another assay, particularly in cases where one assay examines antibody-capsule interactions, e.g., agglutination, and another assay examines interaction of antibody with soluble GXM. Taken together, the results suggest caution in interpretation of immunochemical assays for anti-GXM antibodies and recommend the use of multiple assays formats when studying anticryptococcal antibodies.