Capsule Structural Heterogeneity and Antigenic Variation in Cryptococcus neoformans

Abstract

Cryptococcus neoformans is a human pathogenic fungus with a capsule composed primarily of glucuronoxylomannan (GXM) that is important for virulence. Current views of GXM structure postulate a polymer composed of repeating mannose trisaccharide motifs bearing a single ␤(1,2) glucuronic acid with variable xylose and O-acetyl substitutions to form six triads. GXM from different strains is notoriously variable in triad composition, but it is not known if the polymer consists of one or more motif-repeating units. We investigated the polymeric organization of GXM by using mass spectrometry to determine if its compositional motif arrangement was similar to that of bacterial capsular polysaccharides, namely, a polymer of a single repeating unit. The results were consistent with, and confirmatory for, the current view that the basic unit of GXM is a repeating mannose trisaccharide motif, but we also found evidence for the copolymerization of different GXM repeating units in one polysaccharide molecule. Analysis of GXM from isogenic phenotypic switch variants suggested structural differences caused by glucuronic acid positional effects, which implied flexibility in the synthetic pathway. Our results suggest that cryptococcal capsule synthesis is fundamentally different from that observed in prokaryotes and employs a unique eukaryotic approach, which theoretically could synthesize an infinite number of structural combinations. The biological significance of this capsule construction scheme is that it is likely to confer a powerful avoidance strategy for interactions with the immune system and phagocytic environmental predators. Consistent with this premise, the antigenic variation of a capsular epitope recognized by a nonprotective antibody was observed under different growth conditions.

Characterization of Phenotypic Switching in Cryptococcus neoformans Biofilms

“…Consistent with this view are the findings that Oacetylation of alginate in Pseudomonas aeruginosa contributed to biofilm formation [19] and the C. neoformans strain-related differences [10]. Previous work on the polysaccharide capsule of switch variants has shown that phenotypic switching can alter not only capsule size but also both the biochemical as well as the biophysical properties of the capsular polysaccharide [14,15,17]. These changes can involve the position of the glucuronic acid on the mannose backbone as well as the distribution of charged residues over the mannose backbone.…”

“…Therefore, the characteristics of C. neoformans phenotypic switching during biofilm formation were investigated. These phenotypes show variations in their capsular polysaccharide [14][15][16].…”

Role of Complement in Protection againstCryptococcus gattiiInfection

“…Interestingly, after the initial rapid clearance in wild-type and factor B-deficient animals, the remainder cleared at approximately the rate seen in C3-deficient mice, suggesting that these organisms represented a subpopulation that was not opsonized. Heterogeneity in capsule composition is known to occur in Cryptococcus (17) and may explain the second clearance phase.…”

Phenotypic Switching of Cryptococcus neoformans and Cryptococcus gattii

“…neoformans isolates (serotypes A and D). In RC2, phenotypic switching alters the biophysical and biochemical properties of GXM [37]. Viscosity data in solutions of different ionic strength, suggest that the spacing of GlcpA along the Mannose backbone differs between the GXM of the RC2-SM and that of the RC2-MC strain.…”