Phenotypic heterogeneity in expression of epitopes in the Cryptococcus neoformans capsule

The opportunistic yeast Cryptococcus neoformans is surrounded by a polysaccharide capsule comprised primarily of glucuronoxylomannan (GXM). GXM is a key component of the antigenic character of the capsule. Expression of the epitope that allows for binding of mAbs that require O-acetylation of GXM for mAb recognition was greatly influenced by cell age, growth conditions and serotype. Yeast cells of serotype A grown in vitro under capsule induction conditions showed considerable cell-to-cell variability in binding of two O-acetyl-dependent mAbs, and such mAbs uniformly failed to bind to GXM that covers yeast buds. Expression of the O-acetyl-dependent epitope increased with cell age. In contrast, all serotype A cells harvested from brain tissue bound the same O-acetyl-dependent mAbs. The ability of the cryptococcal capsule to activate the complement cascade and bind C3 occurred uniformly over the surface of all yeast cells, including the bud. Finally, the cell-to-cell variability in binding of O-acetyl-dependent mAbs with strains of serotype A was not found with strains of serotype D; almost all cells of serotype D showed homogeneous binding of O-acetyl-dependent mAbs. These results indicate that variability in expression of antigenic epitopes by GXM should be considered in selection of mAbs used for immunodiagnosis or immunotherapy.