Molecular architecture of the Cryptococcus neoformans capsule

Many microbes are surrounded by phagocytosis-inhibiting capsules. We took advantage of the large size of the polysaccharide capsule of the pathogenic yeast Cryptococcus neoformans to examine capsular architecture and the relationship between molecular architecture and the interaction of the capsule with potentially opsonic serum proteins. Our experimental design used complementary approaches in which (i) assessment of permeability to macromolecules of different Stokes radii; (ii) determination of the binding of Fab fragments of anticapsular antibodies as a measure of matrix density; (iii) capsular deconstruction by treatment with dimethyl sulphoxide; and (iv) evaluation of capsule plasticity, were used to probe the molecular structure of the capsule. The results showed that the capsule is a matrix with a variable porosity that increases with distance from the cell wall. A high density of the matrix at the capsule interior prevents penetration of large macromolecules to sites near the cell wall. In contrast, the capsular edge that is the interface with phagocytes presents capsular polysaccharide in a very low density that exhibits considerable plasticity and permeability to macromolecules. Notably, the capsule of yeast cells harvested from infected tissue showed a greater matrix density than yeast cells grown in vitro under capsule induction conditions.