Resumo

Cryptococcus neoformans and C. gattii are pathogenic fungi that use a polysaccharide capsule as their principal virulence factor. The structural variability of glucuronoxylomannan (GXM), the major component of this surface coating, defines five fungal serotypes.The properties of serotype A GXM are well known, but other serotypes still require functional clarification. In this study, we isolated GXM from culture supernatants of five Cryptococcus strains belonging to five different serotypes and analyzed molecular dimensions, zeta potential, serologic properties, ability to interact with the cell wall of acapsular cells and antiphagocytic potential. Dynamic light scattering analysis revealed that GXM samples varied in dimensions and were distributed in two diameter populations in all cases. GXM fractions obtained from serotypes A and D strains of C. neoformans (H99 and B3501) ranged from 1 to 700 nm and 1500 to 5000 nm. Similar fractions from serotypes C and AD strains of C. gattii and C. neoformans (WM779 and HEC4152), respectively, ranged from 1 to 700 nm and 1000 to 3000 nm. The GXM sample from a serotype B strain of C. gattii(R265) presented diameter distribution ranging from 1 to 400nm and 700 to 1100nm. Zeta potential determination demonstrated that the GXM sample from the serotype C strain of C. gattii presented a lower negative charge, in comparison to polysaccharide fractions obtained from other cryptococcal serotypes. The same serotype C GXM fraction presented decreased serological reactivity with different monoclonal antibodies raised to the polysaccharide (18B7, 2D10 and 13F1). All the polysaccharide fractions efficiently coated the cell surface of the Cap67 acapsular mutant of C. neoformans, although different morphologic patterns (punctuated or annular) were observed. The GXM-coated yeast cells were incubated with macrophages for fluorimetric determination of phagocytic indices and fungal survival. Once again, the serotype C polysaccharide was more effective than other GXM fractions in promoting protection against phagocytosis and fungal survival after interaction with macrophages. These results indicate functional variability in different GXM serotypes, which might explain the pathogenic differences observed in individuals with cryptococcosis caused by distinct strains of C. neoformans and C. gattii.