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Review
. 2012;88(6):250-65.
doi: 10.2183/pjab.88.250.

Immunochemistry of pathogenic yeast, Candida species, focusing on mannan

Nobuyuki Shibata  1 , Hidemitsu Kobayashi, Shigeo Suzuki
Affiliations
Review

Immunochemistry of pathogenic yeast, Candida species, focusing on mannan

Nobuyuki Shibata et al. Proc Jpn Acad Ser B Phys Biol Sci. 2012.

Abstract

This review describes recent findings based on structural and immunochemical analyses of the cell wall mannan of Candida albicans, and other medically important Candida species. Mannan has been shown to consist of α-1,2-, α-1,3-, α-1,6-, and β-1,2-linked mannopyranose units with few phosphate groups. Each Candida species has a unique mannan structure biosynthesized by sequential collaboration between species-specific mannosyltransferases. In particular, the β-1,2-linked mannose units have been shown to comprise a characteristic oligomannosyl side chain that is strongly antigenic. For these pathogenic Candida species, cell-surface mannan was also found to participate in the adhesion to the epithelial cells, recognition by innate immune receptors and development of pathogenicity. Therefore, clarification of the precise chemical structure of Candida mannan is indispensable for understanding the mechanism of pathogenicity, and for development of new antifungal drugs and immunotherapeutic procedures.

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Figures

Figure 1.
Figure 1.
Structure of the cell-wall mannan of S. cerevisiae. M denotes an α-D-mannopyranose unit. The acetolysis cleaves the α-1,6-linkage of the mannan to produce side chain oligosaccharides.
Figure 2.
Figure 2.
Sequential H-1-H-2′ connectivities of mannose units of the β-1,2-linked mannooligosaccharide (Ref. 27). (A) Mannotriose; (B) Mannotetraose; (C) Mannopentaose; (D) Mannohexaose. The right side of the diagonal shows COSY, and the left side of the diagonal shows NOESY. Primed letters indicate inter-unit H-1-H-2′ NOE cross-peaks and unprimed letters, the H-1-H-2-correlated cross-peaks due to J-coupling. Arrows indicate the direction of the sequential connectivity from the reducing terminal unit to the nonreducing terminal unit.
Figure 3.
Figure 3.
1H NMR spectra of the β-1,2-linked mannooligosaccharides (A) and their corresponding alcohols (B) (Ref. 27). (1) Mannobiose; (2) Mannotriose; (3) Mannotetraose; (4) Mannopentaose; (5) Mannohexaose; (6) Mannoheptaose. The capital letters from A to G and the small letters from a to c refer to the mannose units from the reducing terminal α-anomer unit and β-anomer unit of each oligosaccharide, respectively.
Figure 4.
Figure 4.
Conformation analysis of the β-1,2-linked mannooligosaccharides. (A) Relaxed-residue steric energy map of the β-1,2-linked mannobiose as a function of the Φ (H-1-C-1-O-1-C-2′) and Ψ (C-1-O-1-C-2′-H-2′) torsion angles. (B) Lowest energy conformers of the β-1,2-linked mannohexaose obtained by simulated annealing from 900 K molecular dynamics.
Figure 5.
Figure 5.
Structure of the cell wall mannan of Candida species. (A) C. albicans serotype A, (B) C. guilliermondii, (C) C. lusitaniae, (D) C. glabrata. M set in outlined type indicates β-D-mannopyranose unit. These mannan contain the β-1,2-linked mannose units at the nonreducing terminal side of the side chains as well as at the phosphodiesterified oligosaccharide moiety.
Figure 6.
Figure 6.
Additivity rule of the 1H-NMR chemical shift for mannose units. (A) TOCSY spectrum of the mannan of C. albicans serotype A (Ref. 38). The boxed regions in the spectrum indicate the H-1-H-2-correlated cross-peaks of the α- and β-mannose units in the acid-stable polysaccharide moiety. The circled cross-peaks correspond to the β-1,2-linked mannose units of the phosphodiesterified acid-labile oligosaccharide side chains. The broken- and continuous-line arrows indicate the shifts in the cross-peaks caused by the addition of β-1,2-linked mannose units and α-1,2-linked mannose units, respectively. (B) Structural diagram of the mannose units in the mannan and the shift effect of the cross-peaks caused by the addition of mannose units. The numbers on the mannose units correspond to the cross-peaks in panel A.
Figure 7.
Figure 7.
Structure of the cell wall mannan of C. albicans serotype B. This mannan contains 3,6-branched mannose units at the side chains. β-1,2-linked mannose units are present only on the phosphodiesterified oligosaccharide moiety.
Figure 8.
Figure 8.
Hypothetical substrates of β-1,2-mannosyltransferase I of three Candida species.
Figure 9.
Figure 9.
Biosynthesis pathway of N-linked mannan of C. albicans. The left side of the biosynthesis pathway is similar to that of S. cerevisiae and the right side is characteristic of C. albicans, the pathway of which involves the introduction of the α-1,6-branched mannose units and the β-1,2-linked mannose units in the side chains.
Figure 10.
Figure 10.
Pattern recognition receptors sensing C. albicans. The C. albicans mannan induces IL-1β and IL-23 secretion in a Dectin-2-dependent manner and induces Th17 cell differentiation. IL-17A from Th17 cells recruits neutrophils to inflammatory sites.
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