Resumo

Cryptococcosis is a fungal disease frequently associated with immunosuppression. The disease is caused by the yeast-like pathogens Cryptococcus neoformans and C. gattii, which secrete massive amounts of a polysaccharide called glucuronoxylomannan (GXM), the major constituent of the fungal capsule. GXM is an alpha-1,3 linked mannan laterally substituted with glucuronyl and xylosyl groups. The variability in the substitution groups determines species serotype of the species. GXM is both secreted and capsule-associated. In fact, this polysaccharide constitutes 90% of the capsular mass. The Cryptococcus genus also comprises environmental, non-pathogenic species, including C. flavescens, C. liquefaciens and C. albidus. These species supposedly produce capsule-associated GXM. However, little is known about the properties of the polysaccharide in non-neoformans isolates. In this study, we evaluated some of the properties of capsular polysaccharides of environmental isolates of C. gattii, C. flavescens, C. liquefaciens and C. albidus. Counter staining with India ink indicated that all four strains were encapsulated. This observation was confirmed by scanning electron microscopy, which clearly demonstrated the presence of capsular fibers associated to the cell surface. Analysis of GXM by fluorescence microscopy and flow cytometry with different monoclonal antibodies (mAbs 18B7, 12A1, 13F1 and 2D10) revealed positive serologic reactions with C. gattii and C. liquefaciens with three mAbs (18B7, 12A1, and 2D10). Similar microscopic analyses with C. flavescens and C. albidus revealed discrete binding of the four antibodies used in this study to the fungal cell surface. These results were confirmed by flow cytometry using the same antibodies. Our study reveals a considerable serologic diversity of GXM within the Cryptococcus genus, which likely reflects structural differences and still unknown functions of the polysaccharide.