Beta-1,2-linked oligomannosides inhibit Candida albicans binding to murine macrophage

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.

Simple Carbohydrate Derivatives Diminish the Formation of Biofilm of the Pathogenic Yeast Candida albicans

The opportunistic human fungal pathogen Candida albicans relies on cell morphological transitions to develop biofilm and invade the host. In the current study, we developed new regulatory molecules, which inhibit the morphological transition of C. albicans from yeast-form cells to cells forming hyphae. These compounds, benzyl α-l-fucopyranoside and benzyl β-d-xylopyranoside, inhibit the hyphae formation and adhesion of C. albicans to a polystyrene surface, resulting in a reduced biofilm formation. The addition of cAMP to cells treated with α-l-fucopyranoside restored the yeast-hyphae switch and the biofilm level to that of the untreated control. In the β-d-xylopyranoside treated cells, the biofilm level was only partially restored by the addition of cAMP, and these cells remained mainly as yeast-form cells.

N-Oxy lipid-based click chemistry for orthogonal coupling of mannan onto nanoliposomes prepared by microfluidic mixing: Synthesis of lipids, characterisation of mannan-coated nanoliposomes and in vitro stimulation of dendritic cells

New synthetic aminooxy lipid was designed and synthesized as a building block for the formulation of functionalised nanoliposomes (presenting onto the outer surface of aminooxy groups) by microfluidic mixing. Orthogonal binding of cellular mannan (Candida glabrata (CCY 26-20-1) onto the outer surface of functionalised nanoliposomes was modified by orthogonal binding of reducing termini of mannans to oxime lipids via a click chemistry reaction based on aminooxy coupling (oxime ligation). The aminooxy lipid was proved as a suitable active component for preparation of functionalised nanoliposomes by the microfluidic mixing method performed with the instrument NanoAssemblr™. This "on-chip technology" can be easily scaled-up. The structure of mannan-liposomes was visualized by transmission and scanning electron microscopy, including immunogold staining of recombinant mannan receptor bound onto mannosylated-liposomes. The observed structures are in a good correlation with data obtained by DLS, NTA, and TPRS methods. In vitro experiments on human and mouse dendritic cells demonstrate selective internalisation of fluorochrome-labelled mannan-liposomes and their ability to stimulate DC comparable to lipopolysaccharide. We describe a potentially new drug delivery platform for mannan receptor-targeted antimicrobial drugs as well as for immunotherapeutics. Furthermore, the platform based on mannans bound orthogonally onto the surface of nanoliposomes represents a self-adjuvanted carrier for construction of liposome-based recombinant vaccines for both systemic and mucosal routes of administration.

The Cek1‑mediated MAP kinase pathway regulates exposure of α‑1,2 and β‑1,2‑mannosides in the cell wall of Candida albicans modulating immune recognition

The Cek1 MAP kinase (MAPK) mediates vegetative growth and cell wall biogenesis in the fungal pathogen Candida albicans. Alterations in the fungal cell wall caused by a defective Cek1‑mediated signaling pathway leads to increased β‑1,3‑glucan exposure influencing dectin‑1 fungal recognition by immune cells. We show here that cek1 cells also display an increased exposure of α‑1,2 and β‑1,2‑mannosides (α‑M and β‑M), a phenotype shared by strains defective in the activating MAPKK Hst7, suggesting a general defect in cell wall assembly. cek1 cells display walls with loosely bound material as revealed by transmission electron microscopy and are sensitive to tunicamycin, an inhibitor of N‑glycosylation. Transcriptomal analysis of tunicamycin treated cells revealed a differential pattern between cek1 and wild type cells which involved mainly cell wall and stress related genes. Mapping α‑M and β‑M epitopes in the mannoproteins of different cell wall fractions (CWMP) revealed an important shift in the molecular weight of the mannan derived from mutants defective in this MAPK pathway. We have also assessed the role of galectin‑3, a member of a β‑galactoside‑binding protein family shown to bind to and kill C. albicans through β‑M recognition, in the infection caused by cek1 mutants. Increased binding of cek1 to murine macrophages was shown to be partially blocked by lactose. Galectin-3(-/-) mice showed increased resistance to fungal infection, although galectin-3 did not account for the reduced virulence of cek1 mutants in a mouse model of systemic infection. All these data support a role for the Cek1‑mediated pathway in fungal cell wall maintenance, virulence and antifungal discovery.

Cell Wall Changes in Amphotericin B-Resistant Strains from Candida tropicalis and Relationship with the Immune Responses Elicited by the Host

We have morphologically characterizedCandida tropicalisisolates resistant to amphotericin B (AmB). These isolates present an enlarged cell wall compared to isolates of regular susceptibility. This correlated with higher levels of β-1,3-glucan in the cell wall but not with detectable changes in chitin content. In line with this, AmB-resistant strains showed reduced susceptibility to Congo red. Moreover, mitogen-activated protein kinases (MAPKs) involved in cell integrity were already activated during regular growth in these strains. Finally, we investigated the response elicited by human blood cells and found that AmB-resistant strains induced a stronger proinflammatory response than susceptible strains. In agreement, AmB-resistant strains also induced stronger melanization ofGalleria mellonellalarvae, indicating that the effect of alterations of the cell wall on the immune response is conserved in different types of hosts. Our results suggest that resistance to AmB is associated with pleiotropic mechanisms that might have important consequences, not only for the efficacy of the treatment but also for the immune response elicited by the host.

Candida albicans β-1,2 mannosyl transferase Bmt3: Preparation and evaluation of a β (1,2), α (1,2)-tetramannosyl fluorescent substrate

We describe for the first time the chemical synthesis of a tetramannoside, containing both α (1→2) and β (1→2) linkages. Dodecylthio (lauryl) glycosides were prepared from odorless dodecyl thiol and used as donors for the glycosylation steps. This tetramannoside, was coupled to a mantyl group, and revealed to be a perfect substrate of β-mannosyltransferase Bmt3, confirming the proposed specificity and allowing the preparation of a pentamannoside sequence (β Man (1,2) β Man (1,2) α Man (1,2) α Man (1,2) α Man) usable as a novel substrate for further elongation studies.

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.

β-1,2-Mannosyltransferases 1 and 3 Participate in Yeast and Hyphae O- and N-Linked Mannosylation and Alter Candida albicans Fitness During Infection

β-1,2-mannosylation of Candida albicans glycoconjugates has been investigated through the identification of enzymes involved in the addition of β-1,2-oligomannosides (β-Mans) to phosphopeptidomannan and phospholipomannan. β-1,2-oligomannosides are supposed to have virulence properties that they confer to these glycoconjugates. In a previous study, we showed that cell wall mannoproteins (CWMPs) harbor β-Mans in their O-mannosides; therefore, we analyzed their biosynthesis and impact on virulence. In this study, we demonstrate that O-mannans are heterogeneous and that α-mannosylated O-mannosides, which are biosynthesized by Mnt1 and Mnt2 α-1,2-mannosyltransferases, can be modified with β-Mans but only at the nonreducing end of α-1,2-mannotriose. β-1,2-mannosylation of this O-mannotriose depends on growth conditions, and it involves 2 β-1,2-mannosyltransferases, Bmt1 and Bmt3. These Bmts are essential for β-1,2-mannosylation of CWMPs and expression of β-Mans on germ tubes. A bmt1Δ mutant and a mutant expressing no β-Mans unexpectedly disseminated more in BALB/c mice, whereas they had neither attenuated nor enhanced virulence in C57BL/6 mice. In galectin (Gal)3 knockout mice, the reference strain was more virulent than in C57BL/6 mice, suggesting that the β-Mans innate receptor Gal3 is involved in C. albicans fitness during infection.

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.

Single-molecule analysis of the major glycopolymers of pathogenic and non-pathogenic yeast cells

Most microbes are coated with carbohydrates that show remarkable structural variability and play a crucial role in mediating microbial-host interactions. Understanding the functions of cell wall glycoconjugates requires detailed knowledge of their molecular organization, diversity and heterogeneity. Here we use atomic force microscopy (AFM) with tips bearing specific probes (lectins, antibodies) to analyze the major glycopolymers of pathogenic and non-pathogenic yeast cells at molecular resolution. We show that non-ubiquitous β-1,2-mannans are largely exposed on the surface of native cells from pathogenic Candida albicans and C. glabrata, the former species displaying the highest glycopolymer density and extensions. We also find that chitin, a major component of the inner layer of the yeast cell wall, is much more abundant in C. albicans. These differences in molecular properties, further supported by flow cytometry measurements, may play an important role in strengthening cell wall mechanics and immune interactions. This study demonstrates that single-molecule AFM, combined with immunological and fluorescence methods, is a powerful platform in fungal glycobiology for probing the density, distribution and extension of specific cell wall glycoconjugates. In nanomedicine, we anticipate that this new form of AFM-based nanoglycobiology will contribute to the development of sugar-based drugs, immunotherapeutics, vaccines and diagnostics.

Evaluation of immunostimulatory activities of synthetic mannose-containing structures mimicking the β-(1->2)-linked cell wall mannans of Candida albicans

Immunostimulatory properties of synthetic structures mimicking the β-(1→2)-linked mannans of Candida albicans were evaluated in vitro. Contrary to earlier observations, tumor necrosis factor (TNF) production was not detected after stimulation with mannotetraose in mouse macrophages. Divalent disaccharide 1,4-bis(α-D-mannopyranosyloxy)butane induced TNF and some molecules induced low levels of gamma interferon (IFN-γ) in human peripheral blood mononuclear cells (PBMC).

Members 5 and 6 of the Candida albicans BMT family encode enzymes acting specifically on β-mannosylation of the phospholipomannan cell-wall glycosphingolipid

A family of nine genes encoding proteins involved in the synthesis of β-1,2 mannose adhesins of Candida albicans has been identified. Four of these genes, BMT1-4, encode enzymes acting stepwise to add β-mannoses on to cell-wall phosphopeptidomannan (PPM). None of these acts on phospholipomannan (PLM), a glycosphingolipid member of the mannose-inositol-phosphoceramide family, which contributes with PPM to β-mannose surface expression. We show that deletion of BMT5 and BMT6 led to a dramatic reduction of PLM glycosylation and accumulation of PLM with a truncated β-oligomannoside chain, respectively. Disruptions had no effect on sphingolipid biosynthesis and on PPM β-mannosylation. β-Mannose surface expression was not affected, confirming that β-mannosylation is a process based on specificity of acceptor molecules, but liable to global regulation.

Ability of Candida albicans mutants to induce Staphylococcus aureus vancomycin resistance during polymicrobial biofilm formation

Candida albicans and Staphylococcus aureus form vigorous polymicrobial biofilms in serum, which may serve as the source of coinfection in patients. More importantly, S. aureus is highly resistant to vancomycin during polymicrobial biofilm formation, with no decreases in bacterial viability observed with up to 1,600 microg/ml drug. In these mixed-species biofilms, S. aureus preferentially associates with C. albicans hyphae, which express a variety of unique adhesins. We tested C. albicans mutants deficient in transcriptional regulators of morphogenesis (CPH1 and EFG1) and biofilm formation (BCR1) to investigate the role of hyphae in mediating polymicrobial biofilm formation. These mutants also have reduced expression of hypha-specific adhesins. The ability to form polymicrobial biofilms correlated with the ability to form hyphae in these mutants. However, only mutants that could adhere to the abiotic surface could induce S. aureus vancomycin resistance, regardless of the presence of hyphae. In examining factors that may mediate interspecies adhesion, we found that the C. albicans ALS family of adhesins (Als1 to Als7 and Als9) was not involved, and neither was the hypha-specific adhesin Hwp1. Therefore, polymicrobial biofilm formation and subsequent antibiotic resistance is a multifactorial process that may require a unique combination of fungal and/or bacterial adhesins.

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.

Galectins in innate immunity: dual functions of host soluble beta-galactoside-binding lectins as damage-associated molecular patterns (DAMPs) and as receptors for pathogen-associated molecular patterns (PAMPs)

The glycocalyx is a glycan layer found on the surfaces of host cells as well as microorganisms and enveloped virus. Its thickness may easily exceed 50 nm. The glycocalyx does not only serve as a physical protective barrier but also contains various structurally different glycans, which provide cell- or microorganism-specific 'glycoinformation'. This information is decoded by host glycan-binding proteins, lectins. The roles of lectins in innate immunity are well established, as exemplified by collectins, dectin-1, and dendritic cell (DC)-specific intracellular adhesion molecule-3-grabbing non-integrin (DC-SIGN). These mammalian lectins are synthesized in the secretory pathway and presented on the cell surface to bind to specific glycan 'epitopes'. As they recognize non-self glycans presented by microorganisms, they can be considered as receptors for pathogen-associated molecular patterns (PAMPs), i.e. pattern recognition receptors (PRRs). One notable exception is the galectin family. Galectins are synthesized and stored in the cytoplasm, but upon infection-initiated tissue damage and/or following prolonged infection, cytosolic galectins are either passively released by dying cells or actively secreted by inflammatory activated cells through a non-classical pathway, the 'leaderless' secretory pathway. Once exported, galectins act as PRR, as well as immunomodulators (or cytokine-like modulators) in the innate response to some infectious diseases. As galectins are dominantly found in the lesions where pathogen-initiated tissue damage signals appear, this lectin family is also considered as potential damage-associated molecular pattern (DAMP) candidates that orchestrate innate immune responses alongside the PAMP system.

Host responses to a versatile commensal: PAMPs and PRRs interplay leading to tolerance or infection by Candida albicans

The molecular interactions between commensal microorganisms and their host are basically different from those triggered by pathogens since they involve tolerance. When the commensal is genetically equipped to become an opportunistic pathogen, as is the case with Candida albicans, the picture becomes more complex. In this case, the balance between protection and invasion depends on host reactivity to altered microbial expression of ligands interacting with innate immune sensors. Based on experimental evidence obtained with C. albicans, we discuss the different molecular processes involved in the sensing of this important opportunistic human pathogen by a panel of pattern recognition receptors (PRRs) according to the numerous pathogen-associated molecular patterns (PAMPs) that can be exposed at its surface. Beneficial or deleterious immune responses that either maintain a commensal state or favour damage by the yeast result from this dynamic interplay.

Candida albicans–macrophage interactions: genomic and proteomic insights

Candida albicans infection is a significant cause of morbidity and mortality in immunocompromised patients. In vivo and in vitro models have been developed to study both the fungal and the mammalian immune responses. Phagocytic cells (i.e., macrophages) play a key role in innate immunity against C. albicans by capturing, killing and processing the pathogen for presentation to T cells. The use of microarray technology to study global fungal transcriptional changes after interaction with different host cells has revealed how C. albicans adapts to its environment. Proteomic tools complement molecular approaches and computational methods enable the formulation of relevant biological hypotheses. Therefore, the combination of genomics, proteomics and bioinformatics tools (i.e., network analyses) is a powerful strategy to better understand the biological situation of the fungus inside macrophages; part of the fungal population is killed while a significantly high percentage survives.

Actin and phosphoinositide recruitment to fully formed Candida albicans phagosomes in mouse macrophages

Candida albicans is a dimorphic yeast that enters macrophages (Mphi) via the beta-glucan receptor dectin-1. Phagocytosis of C. albicans is characterized by actin polymerization, Syk kinase activation and rapid acquisition of phagolysosomal markers. In mice, C. albicans are able to resist the harsh environment of the phagosome and form pseudohyphae inside the phagolysosomal compartment, eventually extending from the Mphi. In this study, we investigated these unique C. albicans phagosomes and found that actin localized dynamically around the phagosomes, before disintegrating. Membrane phosphoinositides, PI(4,5)P(2), PI(3,4,5)P(3), PI(3,4)P(2), and PI(3)P also localized to the phagosomes. Localization was not related to actin polymerization, and inhibitor studies showed that polymerization of actin on the C. albicans phagosome was independent of PI3K. The ability of mature C. albicans phagosomes to stimulate actin polymerization could facilitate the escape of the growing yeast from the Mphi.

Specific recognition of Candida albicans by macrophages requires galectin-3 to discriminate Saccharomyces cerevisiae and needs association with TLR2 for signaling

Stimulation of cells of the macrophage lineage is a crucial step in the sensing of yeasts by the immune system. Glycans present in both Candida albicans and Saccharomyces cerevisiae cell walls have been shown to act as ligands for different receptors leading to different stimulating pathways, some of which need receptor co-involvement. However, among these ligand-receptor couples, none has been shown to discriminate the pathogenic yeast C. albicans. We explored the role of galectin-3, which binds C. albicans beta-1,2 mannosides. These glycans are specifically and prominently expressed at the surface of C. albicans but not on S. cerevisiae. Using a mouse cell line and galectin-3-deleted cells from knockout mice, we demonstrated a specific enhancement of the cellular response to C. albicans compared with S. cerevisiae, which depended on galectin-3 expression. However, galectin-3 was not required for recognition and endocytosis of yeasts. In contrast, using PMA-induced differentiated THP-1, we observed that the presence of TLR2 was required for efficient uptake and endocytosis of both C. albicans and S. cerevisiae. TLR2 and galectin-3, which are expressed at the level of phagosomes containing C. albicans, were shown to be associated in differentiated macrophages after incubation with this sole species. These data suggest that macrophages differently sense C. albicans and S. cerevisiae through a mechanism involving TLR2 and galectin-3, which probably associate for binding of ligands expressing beta-1,2 mannosides specific to the C. albicans cell wall surface.

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".

Antigenicity of Cell Wall Mannans of Candida albicans and Candida stellatoidea Cultured at High Temperatures in BACTEC Medium

The study of the antigenicity of pathogenic Candida albicans and Candida stellatoidea cells grown in BACTEC fungal medium (BFM) is useful for clinical analysis so as accurately to diagnose candidiasis. When C. albicans NIH A-207 was grown in BFM and fetal bovine serum-added BFM at the high temperatures of 36 and 40 degrees C, the cell density increased, with a mixture of yeast cells, pseudohyphae, and hyphae and with full hyphal development in the cultures compared with cultivation (mostly cells in yeast form) at 27 degrees C in both media. The mannans produced when cells were grown at these high temperatures were less reactive by enzyme-linked immunosorbent assay with factor sera 4, 5, and 6 in the commercially available kit 'Candida Check' than were the mannans obtained following growth at 27 degrees C. Based on 1H-nuclear magnetic resonance analysis, the mannans from cells grown at high temperatures had lost a phosphate group and a beta-1,2-linked mannopyranose unit, and had increased the number of non-reducing terminal alpha-1,3-linked mannopyranose units. We obtained similar results for mannans produced by C. albicans J-1012, C. albicans NIH B-792, C. albicans JCM 9061, C. stellatoidea ATCC 20408, and C. stellatoidea ATCC 36232 strains cultivated in BFM at 36 degrees C. These results suggest that both C. albicans and C. stellatoidea cells cultured at high temperatures, irrespective of the medium and shape of the cells, alter their antigenicity and chemical structure of cell wall mannans.

Dectin-1 mediates macrophage recognition of Candida albicans yeast but not filaments

The ability of Candida albicans to rapidly and reversibly switch between yeast and filamentous morphologies is crucial to pathogenicity, and it is thought that the filamentous morphology provides some advantage during interaction with the mammalian immune system. Dectin-1 is a receptor that binds beta-glucans and is important for macrophage phagocytosis of fungi. The receptor also collaborates with Toll-like receptors for inflammatory activation of phagocytes by fungi. We show that yeast cell wall beta-glucan is largely shielded from Dectin-1 by outer wall components. However, the normal mechanisms of yeast budding and cell separation create permanent scars which expose sufficient beta-glucan to trigger antimicrobial responses through Dectin-1, including phagocytosis and activation of reactive oxygen production. During filamentous growth, no cell separation or subsequent beta-glucan exposure occurs, and the pathogen fails to activate Dectin-1. The data demonstrate a mechanism by which C. albicans shape alone directly contributes to the method by which phagocytes recognize the fungus.

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.

Inactivation of CaMIT1 inhibits Candida albicans phospholipomannan beta-mannosylation, reduces virulence, and alters cell wall protein beta-mannosylation

Studies on Candida albicans phospholipomannan have suggested a novel biosynthetic pathway for yeast glycosphingolipids. This pathway is thought to diverge from the usual pathway at the mannose-inositol-phospho-ceramide (MIPC) step. To confirm this hypothesis, a C. albicans gene homologue for the Saccharomyces cerevisiae SUR1 gene was identified and named MIT1 as it coded for GDP-mannose:inositol-phospho-ceramide mannose transferase. Two copies of this gene were disrupted. Western blots of cell extracts revealed that strain mit1Delta contained no PLM. Thin layer chromatography and mass spectrometry confirmed that mit1Delta did not synthesize MIPC, demonstrating a role of MIT1 in the mannosylation of C. albicans IPCs. As MIT1 disruption prevented downstream beta-1,2 mannosylation, mit1Delta represents a new C. albicans mutant affected in the expression of these specific virulence attributes, which act as adhesins/immunomodulators. mit1Delta was less virulent during both the acute and chronic phases of systemic infection in mice (75 and 50% reduction in mortality, respectively). In vitro, mit1Delta was not able to escape macrophage lysis through down-regulation of the ERK1/2 phosphorylation pathway previously shown to be triggered by PLM. Phenotypic analysis also revealed pleiotropic effects of MIT1 disruption. The most striking observation was a reduced beta-mannosylation of phosphopeptidomannan. Increased beta-mannosylation of mannoproteins was observed under growth conditions that prevented the association of beta-oligomannosides with phosphopeptidomannan, but not with PLM. This suggests that C. albicans has strong regulatory mechanisms associating beta-oligomannoses with different cell wall carrier molecules. These mechanisms and the impact of the different presentations of beta-oligomannoses on the host response need to be defined.

Immune-deficient Drosophila melanogaster: a model for the innate immune response to human fungal pathogens

We explored the host-pathogen interactions of the human opportunistic fungus Candida albicans using Drosophila melanogaster. We established that a Drosophila strain devoid of functional Toll receptor is highly susceptible to the human pathogen C. albicans. Using this sensitive strain, we have been able to show that a set of specific C. albicans mutants of different virulence in mammalian infection models are also impaired in virulence in Drosophila and remarkably display the same rank order of virulence. This immunodeficient insect model also revealed virulence properties undetected in an immunocompetent murine model of infection. The genetic systems available in both host and pathogen will enable the identification of host-specific components and C. albicans genes involved in the host-fungal interplay.

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.

Beta-1,2- and alpha-1,2-linked oligomannosides mediate adherence of Candida albicans blastospores to human enterocytes in vitro

Candida albicans is a commensal dimorphic yeast of the digestive tract that causes hematogenously disseminated infections in immunocompromised individuals. Endogenous invasive candidiasis develops from C. albicans adhering to the intestinal epithelium. Adherence is mediated by the cell wall surface, a domain composed essentially of mannopyranosyl residues bound to proteins, the N-linked moiety of which comprises sequences of alpha-1,2- and beta-1,2-linked mannose residues. Beta-1,2-linked mannosides are also associated with a glycolipid, phospholipomannan, at the C. albicans surface. In order to determine the roles of beta-1,2 and alpha-1,2 oligomannosides in the C. albicans-enterocyte interaction, we developed a model of adhesion of C. albicans VW32 blastospores to the apical regions of differentiated Caco-2 cells. Preincubation of yeasts with monoclonal antibodies (MAbs) specific for alpha-1,2 and beta-1,2 mannan epitopes resulted in a dose-dependent decrease in adhesion (50% of the control with a 60- micro g/ml MAb concentration). In competitive assays beta-1,2 and alpha-1,2 tetramannosides were the most potent carbohydrate inhibitors, with 50% inhibitory concentrations of 2.58 and 6.99 mM, respectively. Immunolocalization on infected monolayers with MAbs specific for alpha-1,2 and beta-1,2 oligomannosides showed that these epitopes were shed from the yeast to the enterocyte surface. Taken together, our data indicate that alpha-1,2 and beta-1,2 oligomannosides are involved in the C. albicans-enterocyte interaction and participate in the adhesion of the yeasts to the mucosal surface.

Differential role of MyD88 in macrophage-mediated responses to opportunistic fungal pathogens

Toll-like receptors mediate macrophage recognition of microbial ligands, inducing expression of microbicidal molecules and cytokines via the adapter protein MyD88. We investigated the role of MyD88 in regulating murine macrophage responses to a pathogenic yeast (Candida albicans) and mold (Aspergillus fumigatus). Macrophages derived from bone marrow of MyD88-deficient mice (MyD88(-/-)) demonstrated impaired phagocytosis and intracellular killing of C. albicans compared to wild-type (MyD88(+/+)) macrophages. In contrast, ingestion and killing of A. fumigatus conidia was MyD88 independent. Cytokine production by MyD88(-/-) macrophages in response to C. albicans yeasts and hyphae was substantially decreased, but responses to A. fumigatus hyphae were preserved. These results provide evidence that MyD88 signaling is involved in phagocytosis and killing of live C. albicans, but not A. fumigatus. The differential role of MyD88 may represent one mechanism by which macrophages regulate innate responses specific to different pathogenic fungi.

Mannose receptor contribution to Candida albicans phagocytosis by murine E-clone J774 macrophages

Mannoproteins, as the main constituents of the outer layer of yeast cell walls, are able to interact with phagocytic cells in an opsonin-independent manner through the mannose receptor (MR) and to induce yeast ingestion by the professional phagocytes. Moreover, the MR also mediates endocytosis of soluble ligands through clathrin-coated pits. Here, we studied some aspects of the interaction between the MR and Candida albicans using murine E-clone macrophages and the consequences on MR trafficking. Using a pull-down assay involving mixture E-clone macrophage detergent lysate with mannosylated Sepharose beads and glutaraldehyde-fixed, heat-killed (HK) C. albicans, we found that binding of solubilized MR to mannosylated particles occurred with characteristics similar to the receptor's cell-surface mannose-binding activity. We then demonstrated that MR expressed on E-clone macrophages contributed to phagocytosis of unopsonized, HK C. albicans and that yeast phagocytosis induced a decrease in MR endocytic activity without concomitant degradation of the receptor in the time lapse studied.

Candida albicans phospholipomannan promotes survival of phagocytosed yeasts through modulation of bad phosphorylation and macrophage apoptosis

The surface of the pathogenic yeast Candida albicans is coated with phospholipomannan (PLM), a phylogenetically unique glycolipid composed of beta-1,2-oligomannosides and phytoceramide. This study compared the specific contribution of PLM to the modulation of signaling pathways linked to the survival of C. albicans in macrophages in contrast to Saccharomyces cerevisiae. C. albicans endocytosis by J774 and disregulation of the ERK1/2 signal transduction pathway was associated downstream with a reduction in Bad Ser-112 phosphorylation and disappearance of free Bcl-2. This suggested an apoptotic effect, which was confirmed by staining of phosphatidylserine in the macrophage outer membrane. The addition of PLM to macrophages incubated with S. cerevisiae mimicked each of the disregulation steps observed with C. albicans and promoted the survival of S. cerevisiae. Externalization of membranous phosphatidylserine, loss of mitochondrial integrity, and DNA fragmentation induced by PLM showed that this molecule promoted yeast survival by inducing host cell death. These findings suggest strongly that PLM is a virulence attribute of C. albicans and that elucidation of the relationship between structure and apoptotic activity is an innovative field of research.

Synthetic analogues of beta-1,2 oligomannosides prevent intestinal colonization by the pathogenic yeast Candida albicans

The pathogenic yeast Candida albicans displays at its cell surface beta-1,2 oligomannosides (beta-1,2-Mans). In contrast to the ubiquitous alpha-Mans, beta-1,2-Mans bind to galectin-3, a major endogenous lectin expressed on epithelial cells. The specific role of beta-1,2-Mans in colonization of the gut by C. albicans was assessed in a mouse model. A selected virulent strain of C. albicans (expressing more beta-1,2-Man epitopes) induced more intense and sustained colonization than an avirulent strain (expressing less beta-1,2-Man epitopes). Synthetic (Sigma) beta-and alpha-linked tetramannosides with antigenicities that mimicked the antigenicities of C. albicans-derived oligomannosides were then constructed. Oral administration of Sigmabeta-1,2-Man (30 mg/kg of body weight) prior to inoculation with the virulent strain resulted in almost complete eradication of yeasts from stool samples, whereas administration of Sigmaalpha-Man at the same dose did not. As most cases of human systemic candidiasis are endogenous in origin, this first demonstration that a synthetic analogue of a yeast adhesin can prevent yeast colonization in the gut opens the possibility of new prophylactic strategies.

Beta-1,2-mannosylation of Candida albicans mannoproteins and glycolipids differs with growth temperature and serotype

Increasing the growth temperature from 28 to 37 degrees C reduced the expression of beta-1,2-oligomannoside epitopes on mannoproteins of Candida albicans serotypes A and B. In contrast, beta-1,2-mannosylation of phospholipomannan (PLM) remained constant despite a slight decrease in the relative molecular weight (M(r)) of this compound. At all growth temperatures investigated, serotype A PLM displayed an M(r) and an antigenicity different from those of serotype B PLM when they were tested with a panel of monoclonal antibodies.

Contribution of phospholipomannan to the surface expression of beta-1,2-oligomannosides in Candida albicans and its presence in cell wall extracts

beta-1,2-Oligomannosides (beta-1,2-Man) derived from Candida albicans mannan have been shown to act as adhesins and to induce protective antibodies. We used monoclonal antibodies specific for beta-1,2-Man in electron, confocal, and fluorescence microscopy to study the surface expression of beta-1,2-Man epitopes. These monoclonal antibodies were also used for Western blotting of cell surface extracts to study the nature of the molecules expressing the beta-Man epitopes. Evidence was obtained for the contribution of a glycolipid, phospholipomannan (PLM), to the complex expression of beta-1,2-Man epitopes at the cell wall surfaces of yeasts grown on solid media. PLM was present in intercellular matrixes of colonies grown on agar and was detected as a contaminant in mannan batches prepared by conventional methods.

Synthesis of di- to hexasaccharide 1,2-linked beta-mannopyranan oligomers, a terminal S-linked tetrasaccharide congener and the corresponding BSA glycoconjugates

Homo oligomers of (1-->2)-beta-D-mannopyranosyl residues have been synthesized in order to study the unique immunological properties of the cell wall mannan of C. albicans. p-Chlorobenzyl-protected ulosyl bromide (2) in combination with the sterically hindered, participating solvent, pivaloyl nitrile, facilitated a new approach for the synthesis of these unique homooligomers ranging from disaccharide up to hexasaccharide. The glycosyl donor 2 demonstrates high diastereoselectivity over both the glycosylation and subsequent reduction step and minimizes the number of protecting group manipulations necessary for the synthesis. Congeners of the (1-->2)-beta-D-mannotetraose were synthesized containing a terminal S-linked (1-->2)-beta-D-mannopyranosyl residue. Deprotection of these compounds afforded the propyl glycosides as well as oligomers with amino terminated aglyconic tethers. The tethers were generated from the oligosaccharide allyl glycosides by photoaddition with 2-aminoethanethiol. The functionalized haptens were coupled to BSA via squarate conjugation, and the degree of incorporation was established by TOF mass spectrometry.

Peptides that mimic Candida albicans-derived beta-1,2-linked mannosides

Beta-1,2-linked mannosides from Candida albicans phosphopeptidomannan (PPM) bind to macrophages through a receptor independent from the macrophage alpha-linked mannose receptor and stimulate these cells to secrete immune mediators. Anti-beta-1,2-linked mannoside but not anti-alpha-linked mannoside antibodies produced after immunization with neoglycoproteins protect animals from disseminated candidiasis. In this study, peptides that mimic beta-1,2-linked mannosides were isolated using phage display methodology. A phage library expressing random peptides was panned with an anti-beta-1,2-linked mannoside monoclonal antibody (mAb). After three rounds of biopanning, the isolated phages were able to inhibit recognition of C. albicans by the mAb. Sixty percent of the phages had an identical DNA insert corresponding to the peptide sequence FHENWPS that was recognized specifically by the mAb. Injection of KLH-coupled peptide into mice generated high titers of polyclonal antibodies against C. albicans yeast cell walls. The anti-FHENWPS antibodies bound to C. albicans PPM and were inhibited by soluble beta-1,2-mannotetraose. Together, these data provide evidence for mimotopic activity of the peptide selected by biopanning with the anti-beta-1,2-oligomannoside mAb.

Comparison of the hydrophobic properties of Candida albicans and Candida dubliniensis

Although Candida dubliniensis is a close genetic relative of Candida albicans, it colonizes and infects fewer sites. Nearly all instances of candidiasis caused by C. dubliniensis are restricted to the oral cavity. As cell surface hydrophobicity (CSH) influences virulence of C. albicans, CSH properties of C. dubliniensis were investigated and compared to C. albicans. Growth temperature is one factor which affects the CSH status of stationary-phase C. albicans. However, C. dubliniensis, similar to other pathogenic non-albicans species of Candida, was hydrophobic regardless of growth temperature. For all Candida species tested in this study (C. albicans, C. dubliniensis, C. glabrata, C. krusei, C. parapsilosis, and C. tropicalis), CSH status correlated with coaggregation with the anaerobic oral bacterium Fusobacterium nucleatum. Previous studies have shown that CSH status of C. albicans involves multiple surface proteins and surface protein N-glycans. The hydrophobic surface glycoprotein CAgp38 appears to be expressed by C. albicans constitutively regardless of growth temperature and medium. C. dubliniensis expresses a 38-kDa protein that cross-reacts with the anti-CAgp38 monoclonal antibody; however, expression of the protein was growth medium and growth temperature dependent. The anti-CAgp38 monoclonal antibody has been shown to inhibit adhesion of C. albicans to extracellular matrix proteins and to vascular endothelial cells. Since protein glycosylation influences the CSH status of C. albicans, we compared the cell wall mannoprotein content and composition between C. albicans and C. dubliniensis. Similar bulk compositional levels of hexose, phosphate, and protein in their N-glycans were determined. However, a component of the C. albicans N-glycan, acid-labile phosphooligomannoside, is expressed much less or negligibly by C. dubliniensis, and when present, the oligomannosides are predominantly less than five mannose residues in length. In addition, the acid-labile phosphooligomannoside profiles varied among the three strains of C. dubliniensis we tested, indicating the N-glycan of C. dubliniensis differs from C. albicans. For C. albicans, the acid-labile phosphooligomannoside influences virulence and surface fibrillar conformation, which affects exposure of hydrophobic surface proteins. Given the combined role in C. albicans of expression of specific surface hydrophobic proteins in pathogenesis and of surface protein glycosylation on exposure of the proteins, the lack of these virulence-associated CSH entities in C. dubliniensis could contribute to its limited ability to cause disseminated infections.