# Cryptococcus: History, Epidemiology and Immune Evasion
## 7 Polysaccharide Capsule  
The most important and best-studied virulence factor of _Cryptococcus_ spp. is its polysaccharide capsule, the outermost structure of yeast, which is chemically associated with the fungal cell wall and also permeates the yeast microenvironment [99,100]. Although there are other capsulated fungal species, none are considered pathogenic, only _C. neoformans_ and _C. gatti_ present such morphology associated with their pathogenicity [76,101]. Its components are constantly exuded into the extracellular medium, which facilitates their collection for purification assays by chromatography and structural characterization by magnetic resonance [102]. Its composition comprises mainly the polysaccharides glucuronoxylomanan (GXM) and glucuronoxylomanogalactan (GXMGa) [103,104]. GXM corresponds to about 90 to 95% of the polysaccharide capsule, having a molecular weight of 1700 to 7000 kDa, depending on the variant of _Cryptococcus_ spp. [105]. The basic unit of GXM is a mannose trisaccharide joined by \(\alpha\)-(1-3)-mannose bonds and with \(\beta\)-(1-2)- and \(\beta\)-(1-4)-glucuronosidic and \(\beta\)-(1-2)-xylosidic substitutions, having 6-\(O\)-acetylations in mannose residues, mainly in those where there are no bonds with the radical groups of glucuronic acid and xylose [106,107]. About two out of three mannoses are 6-\(O\)-acetylated, with the possibility for glucuronic acid-linked mannoses to also be linked [107,108]. Differences in the proportions  
Figure 1: Cell wall and the polysaccharide capsule structure of _Cryptococcus_ spp. with description of the molecular structure of the capsule polysaccharides glucuronoxylomanan (GXM) and glucuronoxylomanogalactan (GXMGa). of xylose, mannose and glucuronic acid of GXM separate the different serotypes found in _Cryptococcus_ spp. [104; 108], suggesting: 1:3:1 and 2:3:1 for serotypes D and A and 3:3:1 and 4:3:1 for serotypes B and C, respectively [108; 109]. However, these proportions are still not well established and further research is needed for this purpose. Something that differs from the polysaccharide capsule of _Cryptococcus_ spp. of bacterial capsular species is the existence of Structure Reported Groups (SRG), which are seven structures, all different from each other, in the configuration of substitutions in the mannose polymer in the case of GXM, ranging from M1 to M6 [102], with the last called Hexosaccharide 1, described by [PERSON] and authors [110] (Figure 1). GXMGal is a polysaccharide of lower molecular weight, about 100 kDa, and corresponds to about 5 to 10% of the total capsule [108]. The structural characterization of GXMGal is usually conducted from the purification of capsular variants of _C. neoformans_, such as CAP67, but has only one layer of GXMGal, with the absence of GXM, which facilitates the separation of GXMGal [111; 112]. GXMGal is a more complex polysaccharide than GXM, having a backbone composed of galactose polymer with \(\alpha\)-(1-6)-galactan bonds, linked by \(\beta\)-(1-3)-galacto-\(\alpha\)-(1-4)-mann-\(\alpha\)-(1-3)-mannan galactoman chains, with galactomannan side chain substitutions with \(\beta\)-(1-2)- and \(\beta\)-(1-3)-xylosyl and \(\beta\)-(1-3)-glucuronyl groups [111; 113], and rare double galactofuranose residues, associated with the galactan backbone by \(\beta\)-(1-2)- and \(\beta\)-(1-3)- bounds at the same galactose residue [114; 115] (Figure 1). GXMGal is the most abundant polysaccharide in the capsule of _C. neoformans_ due to its lower molecular weight compared to GXM. Nonetheless, attempts to visualize GXMGal by microscopy fail since this polysaccharide covers a thin layer on the surface of non-encapsulated variants [105]. There are indications that GXMGal recovers the innermost parts of the capsule of _C. neoformans_, since the removal of the outermost layers by radiation indicates the absence of galactose in these samples [116].