Structural identification of an epitope of antigenic factor 5 in mannans of Candida albicans NIH B-792 (serotype B) and J-1012 (serotype A) as beta-1,2-linked oligomannosyl residues

Antibodies as Models and Tools to Decipher Candida albicans Pathogenic Development: Review about a Unique Monoclonal Antibody Reacting with Immunomodulatory Adhesins

Candidiasis, caused mainly by Candida albicans, a natural commensal of the human digestive tract and vagina, is the most common opportunistic fungal infection at the mucosal and systemic levels. Its high morbi-mortality rates have led to considerable research to identify the molecular mechanisms associated with the switch to pathogenic development and to diagnose this process as accurately as possible. Since the 1980s, the advent of monoclonal antibody (mAb) technology has led to significant progress in both interrelated fields. This linear review, intended to be didactic, was prompted by considering how, over several decades, a single mAb designated 5B2 contributed to the elucidation of the molecular mechanisms of pathogenesis based on β-1,2-linked oligomannoside expression in Candida species. These contributions starting from the structural identification of the minimal epitope as a di-mannoside from the β-1,2 series consisted then in the demonstration that it was shared by a large number of cell wall proteins differently anchored in the cell wall and the discovery of a cell wall glycoplipid shed by the yeast in contact of host cells, the phospholipomannan. Cytological analysis revealed an overall highly complex epitope expression at the cell surface concerning all growth phases and a patchy distribution resulting from the merging of cytoplasmic vesicles to plasmalema and further secretion through cell wall channels. On the host side, the mAb 5B2 led to identification of Galectin-3 as the human receptor dedicated to β-mannosides and signal transduction pathways leading to cytokine secretion directing host immune responses. Clinical applications concerned in vivo imaging of Candida infectious foci, direct examination of clinical samples and detection of circulating serum antigens that complement the Platelia Ag test for an increased sensitivity of diagnosis. Finally, the most interesting character of mAb 5B2 is probably its ability to reveal C. albicans pathogenic behaviour in reacting specifically with vaginal secretions from women infected versus colonized by this species as well as to display higher reactivity with strains isolated in pathogenic circumstances or even linked to an unfavourable prognosis for systemic candidiasis. Together with a detailed referenced description of these studies, the review provides a complementary reading frame by listing the wide range of technologies involving mAb 5B2 over time, evidencing a practical robustness and versatility unique so far in the Candida field. Finally, the basic and clinical perspectives opened up by these studies are briefly discussed with regard to prospects for future applications of mAb 5B2 in current research challenges.

The Mnn10/Anp1-dependent N-linked outer chain glycan is dispensable for Candida albicans cell wall integrity

Candida albicans cell wall glycoproteins, and in particular their mannose-rich glycans, are important for maintaining cellular integrity as well as host recognition, adhesion, and immunomodulation. The asparagine (N)-linked mannose outer chain of these glycoproteins is produced by Golgi mannosyltransferases (MTases). The outer chain is composed of a linear backbone of ∼50 α1,6-linked mannoses, which acts as a scaffold for addition of ∼150 or more mannoses in other linkages. Here, we describe the characterization of C. albicans OCH1, MNN9, VAN1, ANP1, MNN10, and MNN11, which encode the conserved Golgi MTases that sequentially catalyze the α1,6 mannose outer chain backbone. Candida albicans och1Δ/Δ, mnn9Δ/Δ, and van1Δ/Δ mutants block the earliest steps of backbone synthesis and like their Saccharomyces cerevisiae counterparts, have severe cell wall and growth phenotypes. Unexpectedly, and in stark contrast to S. cerevisiae, loss of Anp1, Mnn10, or Mnn11, which together synthesize most of the backbone, have no obvious deleterious phenotypes. These mutants were unaffected in cell morphology, growth, drug sensitivities, hyphal formation, and macrophage recognition. Analyses of secreted glycosylation reporters demonstrated that anp1Δ/Δ, mnn10Δ/Δ, and mnn11Δ/Δ strains accumulate glycoproteins with severely truncated N-glycan chains. This hypo-mannosylation did not elicit increased chitin deposition in the cell wall, which in other yeast and fungi is a key compensatory response to cell wall integrity breaches. Thus, C. albicans has evolved an alternate mechanism to adapt to cell wall weakness when N-linked mannan levels are reduced.

Differences in fungal immune recognition by monocytes and macrophages: N-mannan can be a shield or activator of immune recognition

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Cytokine response to N-mannan mutants was dependent on the immune cell type used.

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N-mannan mutants stimulated less cytokines from monocytes but more from macrophages.

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N-mannan can therefore act as both an immune agonist or an immune shield.

We designed experiments to assess whether fungal cell wall mannans function as an immune shield or an immune agonist. Fungal cell wall β-(1,3)-glucan normally plays a major and dominant role in immune activation. The outer mannan layer has been variously described as an immune shield, because it has the potential to mask the underlying β-(1,3)-glucan, or an immune activator, as it also has the potential to engage with a wide range of mannose detecting PRRs. To resolve this conundrum we examined species-specific differences in host immune recognition in the och1Δ N-mannosylation-deficient mutant background in four species of yeast-like fungi. Irrespective of the fungal species, the cytokine response (TNFα and IL-6) induced by the och1Δ mutants in human monocytes was reduced compared to that of the wild type. In contrast, TNFα production induced by och1Δ was increased, relative to wild type, due to increased β-glucan exposure, when mouse or human macrophages were used. These observations suggest that N-mannan is not a major PAMP for macrophages and that in these cells mannan does shield the fungus from recognition of the inner cell wall β-glucan. However, N-mannan is a significant inducer of cytokine for monocytes. Therefore the metaphor of the fungal “mannan shield” can only be applied to some, but not all, myeloid cells used in immune profiling experiments of fungal species.

Presence of O-glycosidically linked oligosaccharides in the cell wall mannan of Candida krusei purified with Benanomicin A

Cell wall mannan of the pathogenic yeast Candida krusei was prepared using the antibiotic Benanomicin A, which has a lectin-like function. The chemical structure of this molecule was found to be similar to that of mannan prepared from the same yeast by the conventional method using Fehling reagent. Only a few degradation products were detected when the mannan prepared using Fehling reagent was subjected to alkali treatment (β-elimination), but multiple α-1,2-linked oligosaccharides were detected when the mannan purified with Benanomicin A was treated with alkali. These results indicate that most of the O-linked sugar chains in mannan were lost under conventional conditions when exposed to the strongly alkaline Fehling reagent. In contrast, the O-glycosidic bond in mannan was not cleaved and the O-linked sugar chains were maintained and almost intact following treatment with the mild novel preparation method using Benanomicin A. Therefore, we argue that the new mannan preparation method using Benanomicin A is superior to conventional methods. In addition, our study suggests that some yeast mannans, whose overall structure has already been reported, may contain more O-linked sugar chains than previously recognized.

Candida albicans-derived mannoproteins activate NF-κB in reporter cells expressing TLR4, MD2 and CD14

The ability of soluble C. albicans 20A (serotype A) mannoprotein (CMP) to serve as a ligand for toll-like receptor 4 (TLR4) and its co-receptors was examined using commercially available and stably-transfected HEK293 cells that express human TLR4, MD2 and CD14, but not MR. These TLR4 reporter cells also express an NF-κB-dependent, secreted embryonic alkaline phosphatase (SEAP) reporter gene. TLR4-reporter cells exhibited a dose-dependent SEAP response to both LPS and CMP, wherein peak activation was achieved after stimulation with 40–50 μg/mL of CMP. Incubation on polymyxin B resin had no effect on CMP’s ligand activity, but neutralized LPS-spiked controls. HEK293 Null cells lacking TLR4 and possessing the same SEAP reporter failed to respond to LPS or CMP, but produced SEAP when activated with TNFα. Reporter cell NF-κB responses were accompanied by transcription of IL-8, TNFα, and COX-2 genes. Celecoxib inhibited LPS-, CMP-, and TNFα-dependent NF-κB responses; whereas, indomethacin had limited effect on LPS and CMP responses. SEAP production in response to C. albicans A9 mnn4Δ mutant CMP, lacking phosphomannosylations on N-linked glycans, was significantly greater (p ≤ 0.005) than SEAP responses to CMP derived from parental A9 (both serotype B). These data confirm that engineered human cells expressing TLR4, MD2 and CD14 can respond to CMP with NF-κB activation and the response can be influenced by variations in CMP-mannosylation. Future characterizations of CMPs from other sources and their application in this model may provide further insight into variations observed with TLR4 dependent innate immune responses targeting different C. albicans strains.

Hypolipidemic effect of mannans from C. albicans serotypes a and B in acute hyperlipidemia in mice

Mannans, which are biological macromolecules of polysaccharide origin and function as immunomodulators, have been shown to stimulate macrophages in vivo by interaction with the mannose receptor. Thus, they can be used to stimulate macrophages in order to effectively remove circulating atherogenic lipoproteins. Our primary aim was to evaluate the hypolipidemic potential of mannans from C. albicans serotype A (mannan A) and serotype B (mannan B) in a murine model of hyperlipidemia. Mannan A and mannan B were shown to significantly (p<0.05) stimulate both the proliferation (p <0.05) and nitric oxide production of murine peritoneal macrophages in vitro. Pre-treatment of CBA/Lac mice with mannan A prior to induction of hyperlipidemia significantly (p<0.001) reduced serum atherogenic LDL-cholesterol, total cholesterol, and triglycerides. Mannan B exhibited a similar, but more potent, hypolipidemic effect. Electron microscopic analysis of liver revealed a significant (p<0.001) decrease in the volume of lipid droplets when hyperlipidemic mice were pretreated by both mannans. In conclusion, our findings would suggest that both polysaccharide-based biological macromolecules evaluated in the present study, specifically, the natural immunomodulators (mannans A and B), appeared to function as effective lipid-lowering macromolecules, which could potentially serve as adjunct therapy to more conventional hypolipidemic medications such as a statin drug.

Application of novel analytical ultracentrifuge analysis to solutions of fungal mannans

Polysaccharides, the most abundant biopolymers, are required for a host of activities in lower organisms, animals, and plants. Their solution characterization is challenging due to their complex shape, heterogeneity, and size. Here, recently developed data analysis approaches were applied for traditional sedimentation equilibrium and velocity methods in order to investigate the molar mass distribution(s) of a subtype of polysaccharide, namely, mannans from four Candida spp. The molecular weight distributions of these mannans were studied using two recently developed equilibrium approaches: SEDFIT-MSTAR and MULTISIG, resulting in corroboratory distribution profiles. Additionally, sedimentation velocity data for all four mannans, analyzed using ls-g*(s) and Extended Fujita approaches, suggest that two of the fungal mannans (FM-1 and FM-3) have a unimodal distribution of molecular species whereas two others (FM-2 and FM-4) displayed bi-modal and broad distributions, respectively: this demonstrates considerable molecular heterogeneity in these polysaccharides, consistent with previous observations of mannans and polysaccharides in general. These methods not only have applications for the characterization of mannans but for other biopolymers such as polysaccharides, DNA, and proteins (including intrinsically disordered proteins).

Candida albicans β-1,2-mannosyltransferase Bmt3 prompts the elongation of the cell-wall phosphopeptidomannan

β-1,2-Linked mannosides are expressed on numerous cell-wall glycoconjugates of the opportunistic pathogen yeast Candida albicans. Several studies evidenced their implication in the host-pathogen interaction and virulence mechanisms. In the present study, we characterized the in vitro activity of CaBmt3, a β-1,2-mannosyltransferase involved in the elongation of β-1,2-oligomannosides oligomers onto the cell-wall polymannosylated N-glycans. A recombinant soluble enzyme Bmt3p was produced in Pichia pastoris and its enzyme activity was investigated using natural and synthetic oligomannosides as potential acceptor substrates. Bmt3p was shown to exhibit an exquisite enzymatic specificity by adding a single terminal β-mannosyl residue to α-1,2-linked oligomannosides capped by a Manβ1-2Man motif. Furthermore, we demonstrated that the previously identified CaBmt1 and CaBmt3 efficiently act together to generate Manβ1-2Manβ1-2[Manα1-2]n sequence from α-1,2-linked oligomannosides onto exogenous and endogenous substrates.

Dressed to impress: impact of environmental adaptation on the Candida albicans cell wall

Candida albicans is an opportunistic fungal pathogen of humans causing superficial mucosal infections and life‐threatening systemic disease. The fungal cell wall is the first point of contact between the invading pathogen and the host innate immune system. As a result, the polysaccharides that comprise the cell wall act as pathogen associated molecular patterns, which govern the host–pathogen interaction. The cell wall is dynamic and responsive to changes in the external environment. Therefore, the host environment plays a critical role in regulating the host–pathogen interaction through modulation of the fungal cell wall. This review focuses on how environmental adaptation modulates the cell wall structure and composition, and the subsequent impact this has on the innate immune recognition of C. albicans.

Comparative Analysis of Protein Glycosylation Pathways in Humans and the Fungal Pathogen Candida albicans

Protein glycosylation pathways are present in all kingdoms of life and are metabolic pathways found in all the life kingdoms. Despite sharing commonalities in their synthesis, glycans attached to glycoproteins have species-specific structures generated by the presence of different sets of enzymes and acceptor substrates in each organism. In this review, we present a comparative analysis of the main glycosylation pathways shared by humans and the fungal pathogen Candida albicans: N-linked glycosylation, O-linked mannosylation and glycosylphosphatidylinositol-anchorage. The knowledge of similarities and divergences between these metabolic pathways could help find new pharmacological targets for C. albicans infection.

The Mnn2 mannosyltransferase family modulates mannoprotein fibril length, immune recognition and virulence of Candida albicans

The fungal cell wall is the first point of interaction between an invading fungal pathogen and the host immune system. The outer layer of the cell wall is comprised of GPI anchored proteins, which are post-translationally modified by both N- and O-linked glycans. These glycans are important pathogen associated molecular patterns (PAMPs) recognised by the innate immune system. Glycan synthesis is mediated by a series of glycosyl transferases, located in the endoplasmic reticulum and Golgi apparatus. Mnn2 is responsible for the addition of the initial α1,2-mannose residue onto the α1,6-mannose backbone, forming the N-mannan outer chain branches. In Candida albicans, the MNN2 gene family is comprised of six members (MNN2, MNN21, MNN22, MNN23, MNN24 and MNN26). Using a series of single, double, triple, quintuple and sextuple mutants, we show, for the first time, that addition of α1,2-mannose is required for stabilisation of the α1,6-mannose backbone and hence regulates mannan fibril length. Sequential deletion of members of the MNN2 gene family resulted in the synthesis of lower molecular weight, less complex and more uniform N-glycans, with the sextuple mutant displaying only un-substituted α1,6-mannose. TEM images confirmed that the sextuple mutant was completely devoid of the outer mannan fibril layer, while deletion of two MNN2 orthologues resulted in short mannan fibrils. These changes in cell wall architecture correlated with decreased proinflammatory cytokine induction from monocytes and a decrease in fungal virulence in two animal models. Therefore, α1,2-mannose of N-mannan is important for both immune recognition and virulence of C. albicans.

Synthesis of a Candida albicans tetrasaccharide spanning the β1,2-mannan phosphodiester α-mannan junction

The cell wall phosphomannan of Candida species is a complex N-linked glycoprotein with a glycan chain containing predominantly an α-linked mannose backbone with α-mannose branches. A minor β-mannan component is attached to the branches either via a glycosidic bond (acid stable β-mannan) or a phosphodiester bond (acid-labile β-mannan). The α-mannan residues of the cell wall phosphomannan do not afford protective antibody, while the β-mannan portion is a protective antigen and has become an attractive target as the key epitope of a conjugate vaccine. We report the first synthesis of a tetrasaccharide 1 consisting of a β1,2-mannopyranosyl trisaccharide linked via a phosphodiester to methyl α-mannopyranoside. This encompasses the attachment site of the acid labile β-mannan to the α-mannan component of the cell wall phosphomannan. The trisaccharide was formed by an iterative process to first create a β-glucopyranoside linkage and then epimerize the C-2 center via an oxidation-reduction sequence. The phosphate diester linkage was accessed via an anomeric H-phosphonate. The binding of phosphomannan fragment 1 with the protective antibody C3.1 has been evaluated and compared with a β-mannotrioside in hapten inhibition experiments. The observed activities are rationalized with a model for docked in the binding site of C3.1.

The mannan of Candida albicans lacking β-1,2-linked oligomannosides increases the production of inflammatory cytokines by dendritic cells

Mannans are mannose polymers attached to cell wall proteins in all Candida species, including the pathogenic fungus Candida albicans. Mannans are sensed by pattern recognition receptors expressed on innate immune cells. However, the detailed structural patterns affecting immune sensing are not fully understood because mannans have a complex structure that includes α- and β-mannosyl linkages. In this study, we focused on the β-1,2-mannosides of N-linked mannan in C. albicans because this moiety is not present in the non-pathogenic yeast Saccharomyces cerevisiae. To investigate the impact of β-1,2-mannosides on immune sensing, we constructed a C. albicans ∆mnn4/∆bmt1 double deletant. Thin-layer chromatography and nuclear magnetic resonance analyses revealed that the deletant lacked β-1,2-mannosides in N-linked mannan. Mannans lacking the β-1,2-mannosides induced the production of higher levels of inflammatory cytokines, including IL-6, IL-12p40 and TNF-α, in mice dendritic cells compared to wild-type mannan. Our data show that β-1,2-mannosides in N-linked mannan reduce the production of inflammatory cytokines by dendritic cells.

Design of a Candida albicans disaccharide conjugate vaccine by reverse engineering a protective monoclonal antibody

A disaccharide-chicken serum albumin conjugate vaccine against Candida albicans infections has been developed by reverse engineering a protective monoclonal antibody, C3.1. The binding site of C3.1 binds short oligosaccharides of β1,2-linked mannopyranose residues present in the fungal cell wall phosphomannan. By delineating the fine detail of the molecular recognition of the cell wall β-mannan antigen, a disaccharide epitope was deduced to be the minimum size epitope that should induce the formation of protective antibody. Sequential functional group replacement of disaccharide hydroxyl groups to yield a series of monodeoxy and mono-O-methyl β1,2-linked mannobioside congeners established that three hydroxyl groups are essential for binding. Two of these, O-3 and O-4, are located on the internal mannose residue of the disaccharide, and a third, O-3', is located on the terminal mannose. Synthesis of a series of trisaccharides that mandate binding of either the reducing or nonreducing disaccharide epitopes provided the final indication that a disaccharide protein conjugate should have the potential to induce protective antibody. When disaccharide was conjugated to chicken serum albumin this vaccine produced antibodies in rabbits that recognized the native cell wall phosphomannan. In proof of concept protection experiments, three immunized rabbits showed a reduction in fungal burden when challenged with live C. albicans.

Immunochemistry of pathogenic yeast, Candida species, focusing on mannan

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.

Importance of α- and β/α-linked mannooligosaccharides in antibody response against C. dubliniensis

A conjugate of C. dubliniensis cell-wall mannan and human serum albumin (HSA) induced significant level of anti-mannan IgGs in sera of immunized rabbits, whereas mannan alone was not immunogenic. Binding affinities of anti-mannan IgGs induced by the conjugate were evaluated by inhibition ELISA (iELISA) using mannooligosaccharides (dimer-octamer), derived from the side chains of C. dubliniensis mannan, as the inhibitors. Inhibition power of the mannooligosaccharides increased exponentially with their size, with dimer being the weakest (IC(50) = 4 mmol/L) and heptamer/octamer the strongest inhibitors (IC(50) = 0.01 mmol/L). In addition, the mannooligosaccharides proved effective as inhibitors against antiserum obtained from rabbits immunized with C. dubliniensis heat-killed cells, demonstrating a high correlation in the IC(50) values with anti-conjugate serum (Pearson's correlation coefficient r = 0.98; P < 0.01). These findings suggest that a) the mannooligosaccharides comprising the side chains of C. dubliniensis mannan may represent relevant points of interaction with host immune system during infection and b) anti-mannan antibodies induced by the two antigens (the mannan conjugate and the yeast) are of similar specificities.

A multifunctional mannosyltransferase family in Candida albicans determines cell wall mannan structure and host-fungus interactions

The cell wall proteins of fungi are modified by N- and O-linked mannosylation and phosphomannosylation, resulting in changes to the physical and immunological properties of the cell. Glycosylation of cell wall proteins involves the activities of families of endoplasmic reticulum and Golgi-located glycosyl transferases whose activities are difficult to infer through bioinformatics. The Candida albicans MNT1/KRE2 mannosyl transferase family is represented by five members. We showed previously that Mnt1 and Mnt2 are involved in O-linked mannosylation and are required for virulence. Here, the role of C. albicans MNT3, MNT4, and MNT5 was determined by generating single and multiple MnTDelta null mutants and by functional complementation experiments in Saccharomyces cerevisiae. CaMnt3, CaMnt4, and CaMnt5 did not participate in O-linked mannosylation, but CaMnt3 and CaMnt5 had redundant activities in phosphomannosylation and were responsible for attachment of approximately half of the phosphomannan attached to N-linked mannans. CaMnt4 and CaMnt5 participated in N-mannan branching. Deletion of CaMNT3, CaMNT4, and CaMNT5 affected the growth rate and virulence of C. albicans, affected the recognition of the yeast by human monocytes and cytokine stimulation, and led to increased cell wall chitin content and exposure of beta-glucan at the cell wall surface. Therefore, the MNT1/KRE2 gene family participates in three types of protein mannosylation in C. albicans, and these modifications play vital roles in fungal cell wall structure and cell surface recognition by the innate immune system.

Protein glycosylation in Candida

Candidiasis is a significant cause of invasive human mycosis with associated mortality rates that are equivalent to, or worse than, those cited for most cases of bacterial septicemia. As a result, considerable efforts are being made to understand how the fungus invades host cells and to identify new targets for fungal chemotherapy. This has led to an increasing interest in Candida glycobiology, with an emphasis on the identification of enzymes essential for glycoprotein and adhesion metabolism, and the role of N- and O-linked glycans in host recognition and virulence. Here, we refer to studies dealing with the identification and characterization of enzymes such as dolichol phosphate mannose synthase, dolichol phosphate glucose synthase and processing glycosidases and synthesis, structure and recognition of mannans and discuss recent findings in the context of Candida albicans pathogenesis.

Some properties of beta-1,2-mannosyltransferases related to the biosynthesis of the acid-labile oligomannosyl side chains in Candida albicans NIH B-792 strain cells

We detected the beta-1,2-mannosyltransferases (beta-1,2-MTs), which participate in the biosynthesis of oligomannosyl side chains in the mannan acid-labile fraction, in a particulate insoluble fractions prepared from Candida albicans NIH B-792 strain cells grown at 27 degrees C and at 37 degrees C in a yeast extract-added Sabouraud liquid medium (YSLM). The beta-1,2-MT VI-6 prepared from the cells grown at 27 degrees C exhibited the maximum activity at pH 7.0 and at 30 degrees C. The beta-1,2-MT VI-6 activity was only slightly affected by Mn2+, Mg2+, Ca2+, and ethylenediaminetetraacetic acid, but completely inhibited by Zn2+ and Ni2+. The beta-1,2-MT activities from the cells grown at 37 degrees C were lower than that from the cells grown at 27 degrees C, especially on the longer beta-1,2-mannooligosaccharides than tetraose.

Synthesis of a heptasaccharide fragment of the mannan from Candida guilliermondii cell wall and its conjugate with BSA

The 3-aminopropyl glycoside of a heptasaccharide fragment of the cell wall mannan from Candida guilliermondii 18, which corresponds to the antigenic Factor 9, has been synthesized by a convergent approach based on glycosylation of a tetrasaccharide acceptor with a trisaccharide donor as the key step to give a protected heptasaccharide 17. Subsequent two-step deprotection of 17 afforded the heptamannoside 18, which was then conjugated with BSA using the squarate procedure.

Structural analysis of cell wall mannan of Candida sojae, a new yeast species isolated from defatted soybean flakes

We investigated the structural and immunochemical characteristics of cell wall mannan obtained from Candida sojae JCM 1644, which is a new yeast species isolated from defatted soybean flakes. The results of a slide-agglutination test and of an enzyme-linked immunosorbent assay using anti-factor sera to the pathogenic Candida species indicated that the cells and the C. sojae mannan were cross-reactive to the specific anti-factor sera against Candida albicans serotype A (FAb 6) and Candida guilliermondii (FAb 9). Two-dimensional homonuclear Hartmann-Hahn analysis indicated that the mannan consisted of various linked oligomannosyl side chains containing alpha-1,2-, alpha-1,3-, alpha-1,6- and beta-1,2-linked mannose residues. However, although the determinants of antigenic factors 6 and 9 could be not found in this mannan, branched side chains, Manbeta1-2Manalpha1-3[Manalpha1-6]Manalpha1-(2Manalpha1-)n2Man and a linear alpha-1,6-linked polymannosyl backbone, which are cross-reacted by FAbs 6 and 9, respectively, were identified. The mannan was subjected to acetolysis in order to determine the polymerization length of the alpha-1,2-linked oligomannosyl residue in the side chains. The result of (1)H-nuclear magnetic resonance analysis of the released oligosaccharides showed that the remarkable regularity in the length of alpha-1,2-linked oligomannosyl side chains, which were previously found in mannans of other Candida species, is not observed in this mannan.

Endoplasmic reticulum alpha-glycosidases of Candida albicans are required for N glycosylation, cell wall integrity, and normal host-fungus interaction

The cell surface of Candida albicans is enriched in highly glycosylated mannoproteins that are involved in the interaction with the host tissues. N glycosylation is a posttranslational modification that is initiated in the endoplasmic reticulum (ER), where the Glc(3)Man(9)GlcNAc(2) N-glycan is processed by alpha-glucosidases I and II and alpha1,2-mannosidase to generate Man(8)GlcNAc(2). This N-oligosaccharide is then elaborated in the Golgi to form N-glycans with highly branched outer chains rich in mannose. In Saccharomyces cerevisiae, CWH41, ROT2, and MNS1 encode for alpha-glucosidase I, alpha-glucosidase II catalytic subunit, and alpha1,2-mannosidase, respectively. We disrupted the C. albicans CWH41, ROT2, and MNS1 homologs to determine the importance of N-oligosaccharide processing on the N-glycan outer-chain elongation and the host-fungus interaction. Yeast cells of Cacwh41Delta, Carot2Delta, and Camns1Delta null mutants tended to aggregate, displayed reduced growth rates, had a lower content of cell wall phosphomannan and other changes in cell wall composition, underglycosylated beta-N-acetylhexosaminidase, and had a constitutively activated PKC-Mkc1 cell wall integrity pathway. They were also attenuated in virulence in a murine model of systemic infection and stimulated an altered pro- and anti-inflammatory cytokine profile from human monocytes. Therefore, N-oligosaccharide processing by ER glycosidases is required for cell wall integrity and for host-fungus interactions.

Influence of outer region mannosylphosphorylation on N-glycan formation by Candida albicans: normal acid-stable N-glycan formation requires acid-labile mannosylphosphate addition

The pathogenic yeast Candida albicans produces large N-glycans with outer regions containing only mannose residues. The outer region comprises a primary branch with multiple secondary and tertiary branches. Tertiary branches are linked to secondary branches by phosphodiester bridges. In the current model of outer chain elongation in the genetically related yeast Saccharomyces cerevisiae, synthesis of the branches occurs sequentially, primary to tertiary. Thus, disruption of mannosylphosphorylation, the initial step in tertiary branch formation, should not affect primary or secondary branch production. Compared to its wild-type parent, a C. albicans mutant defective in tertiary branch mannosylphosphorylation (mnn4Delta/mnn4Delta) made outer regions with reduced susceptibility to low acid acetolysis treatment, suggesting that the secondary or primary region had been modified. Higher acid acetolysis conditions were required to release the secondary branches from the primary branches. The released secondary branches constitute the subset of the wild-type secondary branches that lack a phosphate group. In contrast, the acid-stable region of both wild-type and mnn4Delta S. cerevisiae strains required high acid acetolysis conditions to release the secondary branches, despite having smaller and less complex secondary and tertiary branches. These results suggest that the complex and longer secondary and tertiary branches of C. albicans affect the conformation of the acid-stable region to render it more susceptible to acetolysis which implies secondary and tertiary branch formation in C. albicans are interdependent events and occur concurrently, rather than sequentially.

Galectin-3 induces death of Candida species expressing specific beta-1,2-linked mannans

Lectins play a critical role in host protection against infection. The galectin family of lectins recognizes saccharide ligands on a variety of microbial pathogens, including viruses, bacteria, and parasites. Galectin-3, a galectin expressed by macrophages, dendritic cells, and epithelial cells, binds bacterial and parasitic pathogens including Leishmania major, Trypanosoma cruzi, and Neisseria gonorrhoeae. However, there have been no reports of galectins having direct effects on microbial viability. We found that galectin-3 bound only to Candida albicans species that bear beta-1,2-linked oligomannans on the cell surface, but did not bind Saccharomyces cerevisiae that lacks beta-1,2-linked oligomannans. Surprisingly, binding directly induced death of Candida species containing specific beta-1,2-linked oligomannosides. Thus, galectin-3 can act as a pattern recognition receptor that recognizes a unique pathogen-specific oligosaccharide sequence. This is the first description of antimicrobial activity for a member of the galectin family of mammalian lectins; unlike other lectins of the innate immune system that promote opsonization and phagocytosis, galectin-3 has direct fungicidal activity against opportunistic fungal pathogens.

Hybridoma passage in vitro may result in reduced ability of antimannan antibody to protect against disseminated candidiasis

We previously reported the enhanced resistance of monoclonal antibodies B6.1 (an immunoglobulin M [IgM]) and C3.1 (an IgG3) against experimental candidiasis. Both MAbs recognize the same fungal epitope. We have since found that a highly passaged B6.1 hybridoma (hp-B6.1) resulted in antibody that has little protective potential. The potential clinical applicability of the antibody and our interest in understanding antibody protection against candidiasis led us to investigate an explanation for this phenomenon. Antibody genetic structure of hp-B6.1, the original hybridoma clone (ori-B6.1) stored frozen since 1995, a subclone of hp-B6.1 that produces protective antibody, the IgG3-producing hybridoma, and a nonprotective IgG1-producing hybridoma were compared. Variable region gene sequences of heavy (V(H)) and light chains showed genetic instability of V(H) chains with only the hp-B6.1; the V(H) sequences from ori-B6.1 and the subclone were, however, identical. Activation-induced cytidine deaminase levels were greatest in the B6.1 hybridomas, which may explain the instability. The constant region CH3 domain remained unchanged, implying normal N-glycation and complement-fixing potential, and antibody binding affinities appeared unchanged. Complement fixation assays surprisingly showed that ori-B6.1 antibody fixes C3 more rapidly than does hp-B6.1 antibody. The V(H) region primary structure may affect complement activation, which could explain our result. Indeed, antibody from the hp-B6.1 subclone fixed complement like antibody from ori-B6.1. These results show that the greatest protection occurs when antimannan antibodies possess the dual abilities of recognizing the appropriate carbohydrate epitope and rapidly fixing complement; loss of the latter property results in the loss of protective potential by the antibody.

Induction of salivary antibodies to inhibitCandida albicansadherence to human epithelial cells by tonsillar immunization in rabbits

To examine the possibility of a vaccine for Candida albicans infection in the oral cavity, we induced salivary antibodies by immunization of killed-C. albicans ATCC 18804 on the palatine tonsils of rabbits. The enzyme-linked immunosorbent assay reaction of salivary antibodies was high against C. albicans serotype A. The saliva antibodies greatly inhibited C. albicans adherence to cloned epithelial cells from human gingiva. Tonsillar immunizations of C. albicans ATCC 18804 induce salivary antibodies that prevent C. albicans adherence to epithelial cells, and thus should prove useful in the prevention of oral candidiasis caused by C. albicans serotype A.

Synthesis of glycoconjugate vaccines for Candida albicans using novel linker methodology

[reaction: see text] The cell wall phosphomannan of Candida species is a complex N-linked glycoprotein with a glycan chain that contains predominantly alpha-linked mannose residues. However, it is the minor beta-mannan component of the phosphomannan of clinically important Candida strains that provides immunological protection in animal models of fungal disease and hence holds promise as a component of conjugate vaccines. This important antigen occurs in different forms linked to the alpha-mannan backbone via a phosphodiester bond (acid-labile beta-mannan) or directly via a glycosidic bond. To reproducibly synthesize and evaluate conjugate vaccines, a robust method for the synthesis of the different oligosaccharide epitopes is required. Here, we report the gram-scale syntheses of both types of epitopes by an approach that utilizes glucosyl trichloroacetimidate donor 2 to first create a beta-glucopyranoside linkage and then epimerizes the C-2 center via an oxidation-reduction sequence that provides an efficient multigram scale route to the beta-mannopyranosides 5, 8, and 15. Reaction of glycosides 16-18 with homobifunctional adipic acid p-nitrophenyl diesters in dry DMF gave the corresponding half esters in good yields, and of sufficient stability to permit chromatographic purification. Subsequent conjugation with BSA and tetanus toxiod (TT) under mild conjugation conditions afforded the corresponding tri- and tetrasaccharide neoglycoproteins with good efficiency. The conjugation method is also applicable to the coupling of small amounts (mg) of larger oligosaccharides with different proteins.

Candida albicans serotype B strains synthesize a serotype-specific phospholipomannan overexpressing a beta-1,2-linked mannotriose

Candida albicans strains consist of serotypes A and B depending on the presence of terminal beta-1,2-linked mannose residues in the acid-stable part of serotype A phosphopeptidomannan (PPM). The distribution of C. albicans serotypes varies according to country and human host genetic and infectious backgrounds. However, these epidemiological traits have not yet been related to a phenotypically stable molecule as cell surface expression of the serotype A epitope depends on the growth conditions. We have shown that C. albicans serotype A associates beta-mannose residues with another molecule, phospholipomannan (PLM), which is a member of the mannoseinositolphosphoceramide family. In this study, PLM from serotype B strains was analysed in order to provide structural bases for the differences in molecular mass and antigenicity observed between PLMs from both serotypes. Through these analyses, carbon 10 was shown to be the location of a second hydroxylation of fatty acids previously unknown in fungal sphingolipids. Minor differences observed in the ceramide moiety appeared to be strain-dependent. More constant features of PLM from serotype B strains were the incorporation of greater amounts of phytosphingosine C20, a twofold reduced glycosylation of PLM and overexpression of a beta-1,2 mannotriose, the epitope of protective antibodies. This specific beta-mannosylation was observed even when growth conditions altered serotype A PPM-specific epitopes, confirming the potential of PLM as a phenotypically stable molecule for serotyping. This study also suggests that the regulation of beta-mannosyltransferases, which define specific immunomodulatory adhesins whose activity depends on the mannosyl chain length, are part of the genetic background that differentiates serotypes.

Antigenicity ofCandida tropicalisstrain cells cultured at 27 and 37°C

All cells of four Candida tropicalis strains IFO 0199 (Ct-0199), IFO 0587 (Ct-0587), IFO 1400 (Ct-1400), and IFO 1647 (Ct-1647), obtained by cultivation at 27 and 37 degrees C for 48 h in yeast extract-added Sabouraud liquid medium, showed the shapes of typical budding yeast and the same agglutination patterns against factor sera 1, 4, 5 and 6 in the commercially available kit 'Candida Check'. The cells of the C. tropicalis IFO 0589 strain display the same properties at 27 degrees C but formed hyphae at 37 degrees C. The cell wall mannan (Ct-0589-37-M) obtained from the strain cells cultured at 37 degrees C had lost most of its reactivity against factor sera 4, 5 and 6 in an enzyme-linked immunosorbent assay, in contrast to the mannan (Ct-0589-27-M) at 27 degrees C. The 1H-nuclear magnetic resonance patterns of the mannans obtained from the cells of the four C. tropicalis strains IFO 0199, IFO 0587, IFO 1400, and IFO 1647, obtained by cultivation at 37 degrees C, did not change compared to those at 27 degrees C. By contrast, the Ct-0589-37-M had significantly lost the beta-1,2-linked mannopyranose units, corresponding to the serum factors 5 and 6. These results show that the IFO 0589 strain is an unusual strain among the general C. tropicalis strains studied.

Disappearance of antigenic factor 6 in Candida glabrata IFO 0622 strain cells cultured at high temperature

The Candida glabrata IFO 0622 strain cells obtained after cultivation at 27 degrees C and at 37 degrees C and then at 27 degrees C (37-27 degrees C) for 48 h in yeast extract-added Sabouraud liquid medium (YSLM) showed the same agglutination patterns against factor sera 1, 4, 6, and 34 in the commercially available factor serum kit 'Candida Check'. On the other hand, the cells of the strain cultured at 37 degrees C had completely lost its reactivity against the factor serum 6. The enzyme-linked immunosorbent assay (ELISA) of the cell wall mannans obtained from the strain cells showed the same reactivity with the agglutination patterns against the factor sera. The 1H-nuclear magnetic resonance (NMR) pattern of the mannan obtained from the strain cells cultured at 37 degrees C showed that the mannan had completely lost the non-reducing beta-1,2-linked mannopyranose unit in the mannotetraose Manbeta1-2Manalpha1-2Manalpha1-2Man, corresponding to the serum factor 6.

Structural changes of cell wall mannans of Candida guilliermondii IFO 10279 strain cells cultured at high temperature

The morphology, structure, and antigenicity of the cells and the cell wall mannans of the Candida guilliermondii IFO 10279 strain cultivated at 33 and 34 degrees C for 48 h in yeast extract-added Sabouraud liquid medium (YSLM) were compared with those cultivated at 27 degrees C and 33 degrees C and then at 27 degrees C (33-27 degrees C). This strain showed little growth at higher than 35 degrees C. The density of the yeast formed cells decreased, with dry weights of about 50% at 33 and 34 degrees C, and only the cells at 34 degrees C revealed a failure of cytokinesis. The structure of the mannans revealed by (1)H-NMR analysis that the mannans obtained at both 33 and 34 degrees C had drastically decreased two consecutive beta-1,2-linked mannopyranose units at the nonreducing terminal of the alpha-linked oligosaccharides and increased one beta-1,2-linked mannopyranose unit at the nonreducing terminal attached to the alpha-1,3-linked mannose unit and the non-reducing terminal alpha-1,3- and alpha-1,2-linked mannopyranose units. The enzyme-linked immunosorbent assay (ELISA) showed that the mannans obtained at 33 and 34 degrees C had decreased reactivity against the factor serum 9 and increased its reactivity against the factor serum 4, in the commercially available factor serum kit "Candida Check".

Structure of Fungal Cell Wall Polysaccharides

Candida albicans mannan consists of the alpha-1,6-linked backbone moiety and the alpha-1,2- and alpha-1,3-linked side chains. It also contains alpha-1,6-branched mannose units, beta-1,2-linked mannose units, and phosphate groups. The cell wall mannans of the genus Candida possess three types of beta-1,2 linked mannose units. One is linked via the phosphodiester linkage, the second type is connected to an alpha-1,2-linked mannose unit, and the third type is attached to an alpha-1,3-linked mannose unit. These beta-1,2-linked mannose units showed a strong antigenicity and produce the characteristic NMR chemical shifts. Using two-dimensional NMR techniques, we will practically determine the structure of these polysaccharides in a nondestructive manner.

Outer chain N-glycans are required for cell wall integrity and virulence of Candida albicans

The outer layer of the Candida albicans cell wall is enriched in highly glycosylated mannoproteins that are the immediate point of contact with the host and strongly influence the host-fungal interaction. N-Glycans are the major form of mannoprotein modification and consist of a core structure, common to all eukaryotes, that is further elaborated in the Golgi to form the highly branched outer chain that is characteristic of fungi. In yeasts, outer chain branching is initiated by the action of the alpha1,6-mannosyltransferase Och1p; therefore, we disrupted the C. albicans OCH1 homolog to determine the importance of outer chain N-glycans on the host-fungal interaction. Loss of CaOCH1 resulted in a temperature-sensitive growth defect and cellular aggregation. Outer chain elongation of N-glycans was absent in the null mutant, demonstrated by the lack of the alpha1,6-linked polymannose backbone and the underglycosylation of N-acetylglucosaminidase. A null mutant lacking OCH1 was hypersensitive to a range of cell wall perturbing agents and had a constitutively activated cell wall integrity pathway. These mutants had near normal growth rates in vitro but were attenuated in virulence in a murine model of systemic infection. However, tissue burdens for the Caoch1delta null mutant were similar to control strains with normal N-glycosylation, suggesting the host-fungal interaction was altered such that high burdens were tolerated. This demonstrates the importance of N-glycan outer chain epitopes to the host-fungal interaction and virulence.

Candida albicans Pmr1p, a secretory pathway P-type Ca2+/Mn2+-ATPase, is required for glycosylation and virulence

The cell surface of Candida albicans is the immediate point of contact with the host. The outer layer of the cell wall is enriched in highly glycosylated mannoproteins that are implicated in many aspects of the host-fungus interaction. Glycosylation of cell wall proteins is initiated in the endoplasmic reticulum and then elaborated in the Golgi as the protein passes through the secretory pathway. Golgi-bound mannosyltransferases require Mn(2+) as an essential cofactor. In Saccharomyces cerevisiae, the P-type ATPase Pmr1p transports Ca(2+) and Mn(2+) ions into the Golgi. To determine the effect of a gross defect in glycosylation on host-fungus interactions of C. albicans, we disrupted the PMR1 homolog, CaPMR1. This mutation would simultaneously inhibit many Golgi-located, Mn(2+)-dependent mannosyltransferases. The Capmr1Delta null mutant was viable in vitro and had no growth defect even on media containing low Ca(2+)/Mn(2+) ion concentrations. However, cells grown in these media progressively lost viability upon entering stationary phase. Phosphomannan was almost completely absent, and O-mannan was severely truncated in the null mutant. A defect in N-linked outer chain glycosylation was also apparent, demonstrated by the underglycosylation of surface acid phosphatase. Consistent with the glycosylation defect, the null mutant had a weakened cell wall, exemplified by hypersensitivity to Calcofluor white, Congo red, and hygromycin B and constitutive activation of the cell integrity pathway. In a murine model of systemic infection, the null mutant was severely attenuated in virulence. These results demonstrate the importance of glycosylation for cell wall structure and virulence of C. albicans.

Direct chemical synthesis of the beta-D-mannans: the beta-(1-->2) and beta-(1-->4) series

The direct syntheses of a beta-(1-->2)-mannooctaose and of a beta-(1-->4)-mannohexaose are reported by means of 4,6-O-benzylidene-protected beta-mannosyl donors. The synthesis of the (1-->2)-mannan was achieved by means of the sulfoxide coupling protocol, whereas the (1-->4)-mannan was prepared using the analogous thioglycoside/sulfinamide methodology. In the synthesis of the (1-->4)-mannan, the glycosylation yields and stereoselectivities remain approximately constant with increasing chain length, whereas those for the (1-->2)-mannan consist of two groups with the formation of the tetra- and higher saccharides giving yields and selectivities consistently lower than those of the lower homologues. The decrease in yield after the trisaccharide in the (1-->2)-mannan synthesis is attributed to steric interference by the n-3 residue and is consistent with the collapsed, disordered structure predicted by early computational work. The consistently high yields and selectivities seen in the synthesis of the (1-->4)-mannan are congruent with the more open, ordered structure originally predicted for this polymer. The lack of order in the structure of the (1-->2)-mannan, as compared to the high degree of order in the (1-->4)-mannan, is also evident from a comparison of the NMR spectra of the two polymers and even from their physical nature: the (1-->2)-mannan is a gum and the (1-->4)-mannan is a high melting solid.

Loss of cell wall mannosylphosphate in Candida albicans does not influence macrophage recognition

The outer layer of the cell wall of the human pathogenic fungus Candida albicans is enriched with heavily mannosylated glycoproteins that are the immediate point of contact between the fungus and cells of the host, including phagocytes. Previous work had identified components of the acid-labile fraction of N-linked mannan, comprising beta-1,2-linked mannose residues attached via a phosphodiester bond, as potential ligands for macrophage receptors and modulators of macrophage function. We therefore isolated and disrupted the CaMNN4 gene, which is required for mannosyl phosphate transfer and hence the attachment of beta-1,2 mannose oligosaccharides to the acid-labile N-mannan side chains. With the mannosylphosphate eliminated, the mnn4Delta null mutant was unable to bind the charged cationic dye Alcian Blue and was devoid of acid-labile beta-1,2-linked oligomannosaccharides. The mnn4Delta mutant was unaffected in cell growth and morphogenesis in vitro and in virulence in a murine model of systemic C. albicans infection. The null mutant was also not affected in its interaction with macrophages. Mannosylphosphate is therefore not required for macrophage interactions or for virulence of C. albicans.

Surface glycans of Candida albicans and other pathogenic fungi: physiological roles, clinical uses, and experimental challenges

Although fungi have always been with us as commensals and pathogens, fungal infections have been increasing in frequency over the past few decades. There is a growing body of literature describing the involvement of carbohydrate groups in various aspects of fungal disease. Carbohydrates comprising the cell wall or capsule, or as a component of glycoproteins, are the fungal cell surface entities most likely to be exposed to the surrounding environment. Thus, the fungus-host interaction is likely to involve carbohydrates before DNA, RNA, or even protein. The interaction between fungal and host cells is also complex, and early studies using whole cells or crude cell fractions often produced seemingly conflicting results. What was needed, and what has been developing, is the ability to identify specific glycan structures and determine how they interact with immune system components. Carbohydrate analysis is complicated by the complexity of glycan structures and by the challenges of separating and detecting carbohydrates experimentally. Advances in carbohydrate chemistry have enabled us to move from the foundation of composition analysis to more rapid characterization of specific structures. This, in turn, will lead to a greater understanding of how fungi coexist with their hosts as commensals or exist in conflict as pathogens.