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Chapter 4 : The Architecture and Antigenic Composition of the Polysaccharide Capsule
Category: Clinical Microbiology; Fungi and Fungal Pathogenesis
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This chapter focuses on chemical, physical, antigenic, architectural, and dynamical properties of the polysaccharide capsule. It examines how changes in these parameters can influence the interaction with the host and the virulence of the yeast. The capsule is composed mainly of polysaccharide. The chemical composition of the capsule is responsible for several physical characteristics. The chapter reviews the antigenic properties of the capsule and the structural properties that could have important consequences during infection. Many different monoclonal antibodies (MAbs) that specifically recognize the Cryptococcus neoformans capsule have been obtained. The study of these antibodies has contributed to the identification of multiple structural features of the capsule. Further, the chapter discusses other important processes involved in the physical organization of the capsule, such as the polysaccharide transport mechanisms and anchoring of the polysaccharide fibers to the cell wall. Anchoring of capsular components to the cell wall is crucial for capsule assembly. However, additional interpolysaccharide interactions are expected to occur at the capsular microenvironment. The current literature indicates that at least four types of polysaccharide-polysaccharide interaction occur at the cryptococcal capsule-cell wall interface, including glucuronoxylomannan (GXM)-GXM, GXM-galactoxylomannan (GalXM), GXM-glucans, and GXM-chitin.
Key Concept Ranking
- Nuclear Magnetic Resonance Spectroscopy
Capsule features shown by scanning electron microscopy.(A) Cells with enlarged capsule, where differences in polysaccharide density are clearly observed. (B) Detailed magnification of the three defined capsular regions based on the differential density. The region observed corresponds to the inset highlighted in panel A. (C) Detail of the inner region of the capsule. After growth in capsule-inducing medium, the outer regions were removed by γ-irradiation treatment, making visible the high-polysaccharide-density region close to the cell wall. (D) Cell grown in non-capsule-inducing conditions. Note how in this cell, the density of polysaccharide in the regions close to the cell wall is much lower than in cells with enlarged capsules (panels A, B, and C).
Basic composition of GXM and GalXM.(A–F) Structure of the six basic motifs described in GXM. (G) GalXM structure.
Proposed model for capsule growth. (Left) Cells with small capsules. After capsule enlargement, new fibers with larger molecular weight are intercalated between the old fibers, yielding a high-density region in the inner part of the capsule (middle). Finally, the capsule grows by apical addition of longer fibers (right), which yields an intermediatedensity region (R2) and a low-density region in the outer layer (R3). (Bottom right) Magnification showing the three density regions.
Model proposed for polysaccharide export in C. neoformans. According to the current literature, GXM is packaged in post-Golgi vesicles ( 1 ) that are targeted to the cell surface. The vesicles move outside the plasma membrane, then cross the cell wall ( 2 ) and the existing capsular network ( 3 ) by still unknown mechanisms to be released into the extracellular space ( 4 ), where they are supposedly lysed ( 5 ) for polysaccharide incorporation into the growing capsule.