Existence of branched side chains in the cell wall mannan of pathogenic yeast, Candida albicans. Structure-antigenicity relationship between the cell wall mannans of Candida albicans and Candida parapsilosis

A novel mannan-specific chimeric antigen receptor M-CAR redirects T cells to interact with Candida spp. hyphae and Rhizopus oryzae spores

Invasive fungal infections (IFIs) are responsible for elevated rates of morbidity and mortality, causing around of 1.5 million deaths annually worldwide. One of the main causative agents of IFIs is Candida albicans, and non-albicans Candida species have emerged as a spreading global public health concernment. Furthermore, COVID-19 has contributed to a boost in the incidence of IFIs, such as mucormycosis, in which Rhizopus oryzae is the most prevalent causative agent. The effector host immune response against IFIs depends on the activity of T cells, which are susceptible to the regulatory effects triggered by fungal virulence factors. The fungal cell wall plays a crucial role as a virulence factor, and its remodeling compromises the development of a specific T-cell response. The redirection of Jurkat T cells to target Candida spp. by recognizing targets expressed on the fungal cell wall can be facilitated using chimeric antigen receptor (CAR) technology. This study generated an M-CAR that contains an scFv with specificity to α-1,6 mannose backbone of fungal mannan, and the expression of M-CAR on the surface of modified Jurkat cells triggered a strong activation against Candida albicans (hyphae form), Candida tropicalis (hyphae form), Candida parapsilosis (pseudohyphal form), and Candida glabrata (yeast form). Moreover, M-CAR Jurkat cells recognized Rhizopus oryzae spores, which induced high expression of cell activation markers. Thus, a novel Mannan-specific CAR enabled strong signal transduction in modified Jurkat cells in the presence of Candida spp. or R. oryzae.

An immune-adrenergic pathway induces lethal levels of platelet-activating factor in mice

Acute immune responses with excess production of cytokines, lipid/chemical mediators, or coagulation factors, often result in lethal damage. In addition, the innate immune system utilizes multiple types of receptors that recognize neurotransmitters as well as pathogen-associated molecular patterns, making immune responses complex and clinically unpredictable. We here report an innate immune and adrenergic link inducing lethal levels of platelet-activating factor. Injecting mice with toll-like receptor (TLR) 4 ligand lipopolysaccharide (LPS), cell wall N-glycans of Candida albicans, and the α2-adrenergic receptor (α2-AR) agonist medetomidine induces lethal damage. Knocking out the C-type lectin Dectin-2 prevents the lethal damage. In spleen, large amounts of platelet-activating factor (PAF) are detected, and knocking out lysophospholipid acyltransferase 9 (LPLAT9/LPCAT2), which encodes an enzyme that converts inactive lyso-PAF to active PAF, protects mice from the lethal damage. These results reveal a linkage/crosstalk between the nervous and the immune system, possibly inducing lethal levels of PAF.

The Emerging Pathogen Candida metapsilosis: Biological Aspects, Virulence Factors, Diagnosis, and Treatment

Fungal infections represent a constant and growing menace to public health. This concern is due to the emergence of new fungal species and the increase in antifungal drug resistance. Mycoses caused by Candida species are among the most common nosocomial infections and are associated with high mortality rates when the infection affects deep-seated organs. Candida metapsilosis is part of the Candida parapsilosis complex and has been described as part of the oral microbiota of healthy individuals. Within the complex, this species is considered the least virulent; however, the prevalence has been increasing in recent years, as well as an increment in the resistance to some antifungal drugs. One of the main concerns of candidiasis caused by this species is the wide range of clinical manifestations, ranging from tissue colonization to superficial infections, and in more severe cases it can spread, which makes diagnosis and treatment difficult. The study of virulence factors of this species is limited, however, proteomic comparisons between species indicate that virulence factors in this species could be similar to those already described for C. albicans. However, differences may exist, taking into account changes in the lifestyle of the species. Here, we provide a detailed review of the current literature about this organism, the caused disease, and some sharing aspects with other members of the complex, focusing on its biology, virulence factors, the host-fungus interaction, the identification, diagnosis, and treatment of infection.

Identification of a new DC-SIGN binding pentamannoside epitope within the complex structure of Candida albicans mannan

The dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN) is an innate immune C-type lectin receptor that recognizes carbohydrate-based pathogen associated with molecular patterns of various bacteria, fungi, viruses and protozoa. Although a range of highly mannosylated glycoproteins have been shown to induce signaling via DC-SIGN, precise structure of the recognized oligosaccharide epitope is still unclear. Using the array of oligosaccharides related to selected fragments of main fungal antigenic polysaccharides we revealed a highly specific pentamannoside ligand of DC-SIGN, consisting of α-(1 → 2)-linked mannose chains with one inner α-(1 → 3)-linked unit. This structural motif is present in Candida albicans cell wall mannan and corresponds to its antigenic factors 4 and 13b. This epitope is not ubiquitous in other yeast species and may account for the species-specific nature of fungal recognition via DC-SIGN. The discovered highly specific oligosaccharide ligands of DC-SIGN are tractable tools for interdisciplinary investigations of mechanisms of fungal innate immunity and anti-Candida defense. Ligand- and receptor-based NMR data demonstrated the pentasaccharide-to-DC-SIGN interaction in solution and enabled the deciphering of the interaction topology.

Chemical characterization and biological effect of exopolysaccharides synthesized by Antarctic yeasts Cystobasidium ongulense AL101 and Leucosporidium yakuticum AL102 on murine innate immune cells

The current study aimed to investigate exopolysaccharides (EPSs) produced by two Antarctic yeasts isolated from soil and penguin feathers samples collected on Livingston Island (Antarctica). The strains were identified as belonging to the species Leucosporidium yakuticum (LY) and Cystobasidium ongulense (CO) based on molecular genetic analysis. The EPS production was investigated using submerged cultivation. Different chemical, chromatographic, and spectral analyses were employed to characterize EPSs. LY accumulated 5.5 g/L biomass and 4.0 g/L EPS after 120 h of cultivation, while CO synthesized 2.1 g/L EPS at the end of cultivation, and the biomass amount reached 5.5 g/L. LY-EPS was characterized by a higher total carbohydrate content (80%) and a lower protein content (18%) by comparison with CO-EPS (62%, 30%). The LY-EPS mainly consisted of mannose (90 mol%), whereas CO-EPS had also glucose, galactose, and small amounts of uronic acids (8-5 mol%). Spectral analyses (FT-IR and 1D, 2D NMR) revealed that LY-EPS comprised a typical β-(1 → 4)-mannan. Branched (hetero)mannan, together with β/α-glucans constituted the majority of CO-EPS. Unlike LY-EPS, which had a high percentage of high molecular weight populations, CO-EPS displayed a large quantity of lower molecular weight fractions and a higher degree of heterogeneity. LY-EPS (100 ng/mL) elevated significantly interferon gamma (IFN-γ) production in splenic murine macrophages and natural killer (NK) cells. The results indicated that newly identified EPSs might affect IFN-γ signaling and in turn, might enhance anti-infectious responses. The data obtained also revealed the potential of EPSs and yeasts for practical application in biochemical engineering and biotechnology.

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.

Mannose-binding analysis and biological application of pradimicins

Pradimicins (PRMs) are an exceptional family of natural products that specifically bind d-mannose (Man). In the past decade, their scientific significance has increased greatly, with the emergence of biological roles of Man-containing glycans. However, research into the use of PRMs has been severely limited by their inherent tendency to form water-insoluble aggregates. Recently, we have established a derivatization strategy to suppress PRM aggregation, providing an opportunity for practical application of PRMs in glycobiological research. This article first outlines the challenges in studying Man-binding mechanisms and structural modifications of PRMs, and then describes our approach to address them. We also present our recent attempts toward the development of PRM-based research tools.

Isolation, Physicochemical Characterization, Labeling, and Biological Evaluation of Mannans and Glucans

The cell wall contains mannans and glucans that are recognized by the host immune system. In this chapter, we will describe the methods to isolate mannans and glucans from the C. albicans cell wall. In addition, we describe how to determine purity, molecular size, and structure of the mannans and glucans. We also detail how to prepare the carbohydrates for in vitro, ex vivo, or in vivo use by describing endotoxin removal (depyrogenation), derivatization, and labeling and evaluation of bioactivity.

Novel Antifungal Activity of Q-Griffithsin, a Broad-Spectrum Antiviral Lectin

There is a rising global incidence of Candida strains with high levels of resistance to fluconazole and other antifungal drugs, hence the need for novel antifungal treatment strategies. Here, we describe the first evidence of antifungal activity of Q-Griffithsin (Q-GRFT), a recombinant oxidation-resistant variant of Griffithsin, a marine red algal lectin with broad-spectrum antiviral activity. We demonstrated that Q-GRFT binds to α-mannan in the Candida albicans cell wall. We also observed that Q-GRFT binding disrupted cell wall integrity and induced reactive oxidative species (ROS) formation, resulting in cell death. Furthermore, we showed that Q-GRFT inhibited the growth of other Candida species C. glabrata, C. parapsilosis, and C. krusei and had modest activity against some strains of multi- and pandrug-resistant C. auris. We found that Q-GRFT induced differential expression of numerous genes involved in response to cell stress, including those responsible for neutralizing ROS production and cell cycle regulation. In conclusion, this novel antifungal activity suggests that Q-GRFT is potentially an ideal drug candidate and represents an alternative strategy for the prevention and treatment of candidiasis. IMPORTANCE Fungal infections contribute to morbidity and mortality annually, and the number of organisms that are nonresponsive to the current available drug regimens are on the rise. There is a need to develop new agents to counter these infections and to add to the limited arsenal available to treat fungal infections. Our study has identified Q-GRFT, a broad-spectrum antiviral protein that harbors growth-inhibitory activity against several Candida strains, as a potential candidate for the prevention and treatment of fungal infections.

Fabrication of Silver Nanoparticles Against Fungal Pathogens

The use of silver nanoparticles (AgNPs) against various pathogens is now being well recognized in the agriculture and health sector. Nanoparticles have been shown to exhibit various novel properties and these properties, on other hand, rely upon the size, shape, and morphology of these particles. Moreover, these physical characteristics enable them to interact with microbes, plants, and animals. Smaller-sized particles have shown more toxicity than larger-sized nanoparticles. AgNPs have shown growth inhibition of many fungi like Aspergillus fumigates, A. niger, A. flavus, Trichophyton rubrum, Candida albicans, and Penicillium species. According to the current hypothesis, AgNPs act by producing reactive oxygen species and free radicals, which cause protein denaturation, nucleic acid and proton pump damage, lipid peroxidation, and cell wall damage. Therefore, they alter the cell membrane permeability, causing cell death. This mini-review summarizes the use of silver nanoparticles against fungal pathogens and fungal biofilm in the agricultural sector.

Anti-glycan antibodies: roles in human disease

Carbohydrate-binding antibodies play diverse and critical roles in human health. Endogenous carbohydrate-binding antibodies that recognize bacterial, fungal, and other microbial carbohydrates prevent systemic infections and help maintain microbiome homeostasis. Anti-glycan antibodies can have both beneficial and detrimental effects. For example, alloantibodies to ABO blood group carbohydrates can help reduce the spread of some infectious diseases, but they also impose limitations for blood transfusions. Antibodies that recognize self-glycans can contribute to autoimmune diseases, such as Guillain-Barre syndrome. In addition to endogenous antibodies that arise through natural processes, a variety of vaccines induce anti-glycan antibodies as a primary mechanism of protection. Some examples of approved carbohydrate-based vaccines that have had a major impact on human health are against pneumococcus, Haemophilus influeanza type b, and Neisseria meningitidis. Monoclonal antibodies specifically targeting pathogen associated or tumor associated carbohydrate antigens (TACAs) are used clinically for both diagnostic and therapeutic purposes. This review aims to highlight some of the well-studied and critically important applications of anti-carbohydrate antibodies.

Cell wall N-glycan of Candida albicans ameliorates early hyper- and late hypo-immunoreactivity in sepsis

Severe infection often causes a septic cytokine storm followed by immune exhaustion/paralysis. Not surprisingly, many pathogens are equipped with various anti-inflammatory mechanisms. Such mechanisms might be leveraged clinically to control septic cytokine storms. Here we show that N-glycan from pathogenic C. albicans ameliorates mouse sepsis through immunosuppressive cytokine IL-10. In a sepsis model using lipopolysaccharide (LPS), injection of the N-glycan upregulated serum IL-10, and suppressed pro-inflammatory IL-1β, TNF-α and IFN-γ. The N-glycan also improved the survival of mice challenged by LPS. Analyses of structurally defined N-glycans from several yeast strains revealed that the mannose core is key to the upregulation of IL-10. Knocking out the C-type lectin Dectin-2 abrogated the N-glycan-mediated IL-10 augmentation. Furthermore, C. albicans N-glycan ameliorated immune exhaustion/immune paralysis after acute inflammation. Our results suggest a strategy where the immunosuppressive mechanism of one pathogen can be applied to attenuate a severe inflammation/cytokine storm caused by another pathogen.

Structural analysis of carbohydrate binding by the macrophage mannose receptor CD206

The human mannose receptor expressed on macrophages and hepatic endothelial cells scavenges released lysosomal enzymes, glycopeptide fragments of collagen, and pathogenic microorganisms and thus reduces damage following tissue injury. The receptor binds mannose, fucose, or N-acetylglucosamine (GlcNAc) residues on these targets. C-type carbohydrate-recognition domain 4 (CRD4) of the receptor contains the site for Ca2+-dependent interaction with sugars. To investigate the details of CRD4 binding, glycan array screening was used to identify oligosaccharide ligands. The strongest signals were for glycans that contain either Manα1-2Man constituents or fucose in various linkages. The mechanisms of binding to monosaccharides and oligosaccharide substructures present in many of these ligands were examined in multiple crystal structures of CRD4. Binding of mannose residues to CRD4 results primarily from interaction of the equatorial 3- and 4-OH groups with a conserved principal Ca2+ common to almost all sugar-binding C-type CRDs. In the Manα1-2Man complex, supplementary interactions with the reducing mannose residue explain the enhanced affinity for this disaccharide. Bound GlcNAc also interacts with the principal Ca2+ through equatorial 3- and 4-OH groups, whereas fucose residues can bind in several orientations, through either the 2- and 3-OH groups or the 3- and 4-OH groups. Secondary contacts with additional sugars in fucose-containing oligosaccharides, such as the Lewis-a trisaccharide, provide enhanced affinity for these glycans. These results explain many of the biologically important interactions of the mannose receptor with both mammalian glycoproteins and microbes such as yeast and suggest additional classes of ligands that have not been previously identified.

Unique Cell Surface Mannan of Yeast Pathogen Candida auris with Selective Binding to IgG

The emerging, multidrug-resistant yeast pathogen Candida auris is responsible for healthcare-associated outbreaks across the globe with high mortality. The rapid spread of C. auris is linked to its successful colonization of human skin, followed by bloodstream infections. We compared glycomics and proteomics of C. auris to closely and distantly related human pathogenic yeasts, C. haemulonii and C. albicans, with the aim to understand the role of cell surface molecules in skin colonization and immune system interactions. Candida auris mannan is distinct from other pathogenic Candida species, as it is highly enriched in β-1,2-linkages. The experimental data showed that C. auris surface mannan β-1,2-linkages were important for the interactions with the immune protein IgG, found in blood and in sweat glands, and with the mannose binding lectin, found in the blood. Candida auris mannan binding to IgG was from 12- to 20-fold stronger than mannan from the more common pathogen C. albicans. The findings suggest unique C. auris mannan could be crucial for the biology and pathogenesis of this emerging pathogen.

Fungal Cell Wall: Emerging Antifungals and Drug Resistance

The cell wall is an essential component in fungal homeostasis. The lack of a covering wall in human cells makes this component an attractive target for antifungal development. The host environment and antifungal stress can lead to cell wall modifications related to drug resistance. Antifungals targeting the cell wall including the new β-D-glucan synthase inhibitor ibrexafungerp and glycosyl-phosphatidyl Inositol (GPI) anchor pathway inhibitor fosmanogepix are promising weapons against antifungal resistance. The fosmanogepix shows strong in vitro activity against the multidrug-resistant species Candida auris, Fusarium solani, and Lomentospora prolificans. The alternative carbon sources in the infection site change the cell wall β-D-glucan and chitin composition, leading to echinocandin and amphotericin resistance. Candida populations that survive echinocandin exposure develop tolerance and show high chitin content in the cell wall, while fungal species such as Aspergillus flavus with a higher β-D-glucan content may show amphotericin resistance. Therefore understanding fungal cell dynamics has become important not only for host-fungal interactions, but also treatment of fungal infections. This review summarizes recent findings regarding antifungal therapy and development of resistance related to the fungal cell wall of the most relevant human pathogenic species.

Gut mycobiota under scrutiny: fungal symbionts or environmental transients?

The human gastrointestinal tract is home to a thriving community of microbes including the fungal 'mycobiota'. Although sequencing methodology has enumerated diverse fungal genera within this niche, discerning persistent symbiotic residents from contaminants and purely environmental transients remains a challenge. Recent advances in culturomics and sequencing employing metagenomics, metatranscriptomics and longitudinal studies have begun to reveal a human symbiont 'core mycobiome' that may contribute to human health and disease. Trans-kingdom interactions between the bacterial microbiota and evolution within the niche have defined C. albicans as a true symbiont, setting a bar for defining other fungi. Additionally, elegant investigations of mammalian antifungal immunity have examined mononuclear phagocytes, neutrophils, antigen-specific recognition by T cells and other mechanisms important for local and systemic effects on the host, providing further evidence supporting gut persistence. In this review we discuss current research aimed at investigating the symbiotic mycobiota and propose four criteria aiding in the differentiation of fungal symbionts from environmental transients.

Importance of Candida Antigenic Factors: Structure-Driven Immunomodulation Properties of Synthetically Prepared Mannooligosaccharides in RAW264.7 Macrophages

The incidence and prevalence of serious fungal infections is rising, especially in immunosuppressed individuals. Moreover, co-administration of antibiotics and immunosuppressants has driven the emergence of new multidrug-resistant pathogens. The significant increase of multidrug-resistant pathogens, together with their ability to form biofilms, is associated with morbidity and mortality. Research on novel synthetically prepared immunomodulators as potential antifungal immunotherapeutics is of serious interest. Our study demonstrated the immunobiological activity of synthetically prepared biotinylated mannooligosaccharides mimicking Candida antigenic factors using RAW264.7 macrophages. Macrophage exposure to the set of eight structurally different mannooligosaccharides induced a release of Th1, Th2, Th17, and Treg cytokine signature patterns. The observed immune responses were tightly associated with structure, dose, exposure time, and selected signature cytokines. The viability/cytotoxicity of the mannooligosaccharide formulas was assessed based on cell proliferation. The structure-based immunomodulatory activity of the formulas was evaluated with respect to the length, branching and conformation of the various formulas. Glycoconjugate formulas with terminal β-mannosyl-units tended to be more potent in terms of Candida relevant cytokines IL-12 p70, IL-17, GM-CSF, IL-6, and TNFα induction and cell proliferation, and this tendency was associated with structural differences between the studied glycoconjugate formulas. The eight tested mannooligosaccharide conjugates can be considered potential in vitro immunomodulative agents suitable for in vitro Candida diagnostics or prospectively for subcellular anti-Candida vaccine design.

Targeting the fungal cell wall: current therapies and implications for development of alternative antifungal agents

Fungal infections are a worldwide problem associated with high morbidity and mortality. There are relatively few antifungal agents, and resistance has emerged within these pathogens for the newest antifungal drugs. As the fungal cell wall is critical for growth and development, it is one of the most important targets for drug development. In this review, the currently available cell wall inhibitors and suitable drug candidates for the treatment of fungal infections are explored. Future studies of the fungal cell wall and compounds that have detrimental effects on this important outer structural layer could aid in antifungal drug discovery and lead to the development of alternative cell wall inhibitors to fill gaps in clinical therapies for difficult-to-treat fungal infections.

Candida parapsilosis: from Genes to the Bedside

Patients with suppressed immunity are at the highest risk for hospital-acquired infections. Among these, invasive candidiasis is the most prevalent systemic fungal nosocomial infection. Over recent decades, the combined prevalence of non-albicans Candida species outranked Candida albicans infections in several geographical regions worldwide, highlighting the need to understand their pathobiology in order to develop effective treatment and to prevent future outbreaks. Candida parapsilosis is the second or third most frequently isolated Candida species from patients. Besides being highly prevalent, its biology differs markedly from that of C. albicans, which may be associated with C. parapsilosis' increased incidence. Differences in virulence, regulatory and antifungal drug resistance mechanisms, and the patient groups at risk indicate that conclusions drawn from C. albicans pathobiology cannot be simply extrapolated to C. parapsilosis Such species-specific characteristics may also influence their recognition and elimination by the host and the efficacy of antifungal drugs. Due to the availability of high-throughput, state-of-the-art experimental tools and molecular genetic methods adapted to C. parapsilosis, genome and transcriptome studies are now available that greatly contribute to our understanding of what makes this species a threat. In this review, we summarize 10 years of findings on C. parapsilosis pathogenesis, including the species' genetic properties, transcriptome studies, host responses, and molecular mechanisms of virulence. Antifungal susceptibility studies and clinician perspectives are discussed. We also present regional incidence reports in order to provide an updated worldwide epidemiology summary.

NMR comparison of hyphal and yeast Candida albicans serotype B mannans

A change from a globular to a filamentous hyphal form is an important feature in the pathogenicity of yeasts. Such a dimorphism while infecting a host organism is thought to be also accompanied in the case of Candida albicans spp. by a structural rearrangement of surface mannan antigen. The presented work brings new insights into the molecular structural changes of mannan C. albicans serotype B based on NMR experimental data. ¹H and ¹³C signal identification of the anomeric region and the assignment of their linkage type is presented here. 2D deconvolution of the HSQC spectra facilitated accurate integration of all anomeric cross-peaks. Analysis of the differences in the integrals led to the proposal that C. albicans serotype B hyphal mannan side chains have the shortened structural moieties: Manα1-2Manα1- and Manα1-3 [Manα1-6] Manα1-2Manα1-. These represent the dominant structures important for construction of a saccharide-based prospective anti-candida vaccine.

Accessibility and contribution to glucan masking of natural and genetically tagged versions of yeast wall protein 1 of Candida albicans

Yeast wall protein 1 (Ywp1) is an abundant glycoprotein of the cell wall of the yeast form of Candida albicans, the most prevalent fungal pathogen of humans. Antibodies that bind to the polypeptide backbone of isolated Ywp1 show little binding to intact yeast cells, presumably because the Ywp1 epitopes are masked by the polysaccharides of the mannoproteins that form the outer layer of the cell wall. Rare cells do exhibit much greater anti-Ywp1 binding, however, and one of these was isolated and characterized. No differences were seen in its Ywp1, but it exhibited greater adhesiveness, sensitivity to wall perturbing agents, and exposure of its underlying β-1,3-glucan layer to external antibodies. The molecular basis for this greater epitope accessibility has not been determined, but has facilitated exploration of how these properties change as a function of cell growth and morphology. In addition, previously engineered strains with reduced quantities of Ywp1 in their cell walls were also found to have greater β-1,3-glucan exposure, indicating that Ywp1 itself contributes to the masking of wall epitopes, which may be important for understanding the anti-adhesive effect of Ywp1. Ectopic production of Ywp1 by hyphae, which reduces the adhesivity of these filamentous forms of C. albicans, was similarly found to reduce exposure of the β-1,3-glucan in their walls. To monitor Ywp1 in the cell wall irrespective of its accessibility, green fluorescent protein (Gfp) was genetically inserted into wall-anchored Ywp1 using a bifunctional cassette that also allowed production from a single transfection of a soluble, anchor-free version. The wall-anchored Ywp1-Gfp-Ywp1 accumulated in the wall of the yeast forms but not hyphae, and appeared to have properties similar to native Ywp1, including its adhesion-inhibiting effect. Some pseudohyphal walls also detectably accumulated this probe. Strains of C. albicans with tandem hemagglutinin (HA) epitopes inserted into wall-anchored Ywp1 were previously created by others, and were further explored here. As above, rare cells with much greater accessibility of the HA epitopes were isolated, and also found to exhibit greater exposure of Ywp1 and β-1,3-glucan. The placement of the HA cassette inhibited the normal N-glycosylation and propeptide cleavage of Ywp1, but the wall-anchored Ywp1-HA-Ywp1 still accumulated in the cell wall of yeast forms. Bifunctional transformation cassettes were used to additionally tag these molecules with Gfp, generating soluble Ywp1-HA-Gfp and wall-anchored Ywp1-HA-Gfp-Ywp1 molecules. The former revealed unexpected electrophoretic properties caused by the HA insertion, while the latter further highlighted differences between the presence of a tagged Ywp1 molecule (as revealed by Gfp fluorescence) and its accessibility in the cell wall to externally applied antibodies specific for HA, Gfp and Ywp1, with accessibility being greatest in the rapidly expanding walls of budding daughter cells. These strains and results increase our understanding of cell wall properties and how C. albicans masks itself from recognition by the human immune system.

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).

Chitosan Mediates Germling Adhesion in Magnaporthe oryzae and Is Required for Surface Sensing and Germling Morphogenesis

The fungal cell wall not only plays a critical role in maintaining cellular integrity, but also forms the interface between fungi and their environment. The composition of the cell wall can therefore influence the interactions of fungi with their physical and biological environments. Chitin, one of the main polysaccharide components of the wall, can be chemically modified by deacetylation. This reaction is catalyzed by a family of enzymes known as chitin deacetylases (CDAs), and results in the formation of chitosan, a polymer of β1,4-glucosamine. Chitosan has previously been shown to accumulate in the cell wall of infection structures in phytopathogenic fungi. Here, it has long been hypothesized to act as a 'stealth' molecule, necessary for full pathogenesis. In this study, we used the crop pathogen and model organism Magnaporthe oryzae to test this hypothesis. We first confirmed that chitosan localizes to the germ tube and appressorium, then deleted CDA genes on the basis of their elevated transcript levels during appressorium differentiation. Germlings of the deletion strains showed loss of chitin deacetylation, and were compromised in their ability to adhere and form appressoria on artificial hydrophobic surfaces. Surprisingly, the addition of exogenous chitosan fully restored germling adhesion and appressorium development. Despite the lack of appressorium development on artificial surfaces, pathogenicity was unaffected in the mutant strains. Further analyses demonstrated that cuticular waxes are sufficient to over-ride the requirement for chitosan during appressorium development on the plant surface. Thus, chitosan does not have a role as a 'stealth' molecule, but instead mediates the adhesion of germlings to surfaces, thereby allowing the perception of the physical stimuli necessary to promote appressorium development. This study thus reveals a novel role for chitosan in phytopathogenic fungi, and gives further insight into the mechanisms governing appressorium development in M.oryzae.

Magnaporthe oryzae is a filamentous fungal pathogen which causes devastating crop losses in rice. Successful invasion of the host is dependent upon the ability of the fungus to remain undetected by the innate immune system of the plant, which recognizes conserved components of the fungal cell wall, such as chitin. Previous studies have demonstrated that infection-related changes in cell wall composition are necessary to allow the fungus to remain undetected during infection. One such change that has long been hypothesized to have a role as a 'stealth mechanism' is the deacetylation of the polysaccharide chitin by enzymes known as chitin deacetylases. The deacetylation of chitin produces a polysaccharide known as chitosan, which has previously been shown to accumulate specifically on infection structures in plant pathogenic fungi. However, in this study, we show that germling-localized chitosan is not required for pathogenicity, arguing against a role as a 'stealth mechanism' at this stage. Instead, chitosan is required for the development of the appressorium, a critical fungal infection structure required for the penetration of plant cells. This requirement can be attributed to chitosan mediating the adhesion of germlings to surfaces, which is required for the perception of physical stimuli.

Emerging roles of protein mannosylation in inflammation and infection

Proteins are frequently modified by complex carbohydrates (glycans) that play central roles in maintaining the structural and functional integrity of cells and tissues in humans and lower organisms. Mannose forms an essential building block of protein glycosylation, and its functional involvement as components of larger and diverse α-mannosidic glycoepitopes in important intra- and intercellular glycoimmunological processes is gaining recognition. With a focus on the mannose-rich asparagine (N-linked) glycosylation type, this review summarises the increasing volume of literature covering human and non-human protein mannosylation, including their structures, biosynthesis and spatiotemporal expression. The review also covers their known interactions with specialised host and microbial mannose-recognising C-type lectin receptors (mrCLRs) and antibodies (mrAbs) during inflammation and pathogen infection. Advances in molecular mapping technologies have recently revealed novel immuno-centric mannose-terminating truncated N-glycans, termed paucimannosylation, on human proteins. The cellular presentation of α-mannosidic glycoepitopes on N-glycoproteins appears tightly regulated; α-mannose determinants are relative rare glycoepitopes in physiological extracellular environments, but may be actively secreted or leaked from cells to transmit potent signals when required. Simultaneously, our understanding of the molecular basis on the recognition of mannosidic epitopes by mrCLRs including DC-SIGN, mannose receptor, mannose binding lectin and mrAb is rapidly advancing, together with the functional implications of these interactions in facilitating an effective immune response during physiological and pathophysiological conditions. Ultimately, deciphering these complex mannose-based receptor-ligand interactions at the detailed molecular level will significantly advance our understanding of immunological disorders and infectious diseases, promoting the development of future therapeutics to improve patient clinical outcomes.

Role of Protein Glycosylation in Candida parapsilosis Cell Wall Integrity and Host Interaction

Candida parapsilosis is an important, emerging opportunistic fungal pathogen. Highly mannosylated fungal cell wall proteins are initial contact points with host immune systems. In Candida albicans, Och1 is a Golgi α1,6-mannosyltransferase that plays a key role in the elaboration of the N-linked mannan outer chain. Here, we disrupted C. parapsilosis OCH1 to gain insights into the contribution of N-linked mannosylation to cell fitness and to interactions with immune cells. Loss of Och1 in C. parapsilosis resulted in cellular aggregation, failure of morphogenesis, enhanced susceptibility to cell wall perturbing agents and defects in wall composition. We removed the cell wall O-linked mannans by β-elimination, and assessed the relevance of mannans during interaction with human monocytes. Results indicated that O-linked mannans are important for IL-1β stimulation in a dectin-1 and TLR4-dependent pathway; whereas both, N- and O-linked mannans are equally important ligands for TNFα and IL-6 stimulation, but neither is involved in IL-10 production. Furthermore, mice infected with C. parapsilosis och1Δ null mutant cells had significantly lower fungal burdens compared to wild-type (WT)-challenged counterparts. Therefore, our data are the first to demonstrate that C. parapsilosis N- and O-linked mannans have different roles in host interactions than those reported for C. albicans.

Members of the Candida parapsilosis Complex and Candida albicans are Differentially Recognized by Human Peripheral Blood Mononuclear Cells

The systemic infections caused by members of the Candida parapsilosis complex are currently associated to high morbility and mortality rates, and are considered as relevant as those caused by Candida albicans. Since the fungal cell wall is the first point of contact with the host cells, here we performed a comparison of this organelle in members of the C. parapsilosis complex, and its relevance during interaction with human peripheral blood mononuclear cells (PBMCs). We found that the wall of the C. parapsilosis complex members is similar in composition, but differs to that from C. albicans, with less mannan content and more β-glucan and porosity levels. Furthermore, lectin-based analysis showed increased chitin and β1,3-glucan exposure at the surface of C. parapsilosis sensu lato when compared to C. albicans. Yeast cells of members of the C. parapsilosis complex stimulated more cytokine production by human PBMCs than C. albicans cells; and this significantly changed upon removal of O-linked mannans, indicating this wall component plays a significant role in cytokine stimulation by C. parapsilosis sensu lato. When inner wall components were exposed on the wall surface, C. parapsilosis sensu stricto and C. metapsilosis, but not C. orthopsilosis, stimulated higher cytokine production. Moreover, we found a strong dependency on β1,3-glucan recognition for the members of the C. parapsilosis complex, but not for live C. albicans cells; whereas TLR4 was required for TNFα production by the three members of the complex, and stimulation of IL-6 by C. orthopsilosis. Mannose receptor had a significant role during TNFα and IL-1β stimulation by members of the complex. Finally, we demonstrated that purified N- and O-mannans from either C. parapsilosis sensu lato or C. albicans are capable to block the recognition of these pathogens by human PBMCs. Together; our results suggest that the innate immune recognition of the members of the C. parapsilosis complex is differential of that reported for C. albicans. In addition, we propose that purified cell wall mannans can be used as antagonist to block specific receptors on innate immune cells.

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.

A new tool to quantify receptor recruitment to cell contact sites during host-pathogen interaction

To understand the process of innate immune fungal recognition, we developed computational tools for the rigorous quantification and comparison of receptor recruitment and distribution at cell-cell contact sites. We used these tools to quantify pattern recognition receptor spatiotemporal distributions in contacts between primary human dendritic cells and the fungal pathogens C. albicans, C. parapsilosis and the environmental yeast S. cerevisiae, imaged using 3D multichannel laser scanning confocal microscopy. The detailed quantitative analysis of contact sites shows that, despite considerable biochemical similarity in the composition and structure of these species' cell walls, the receptor spatiotemporal distribution in host-microbe contact sites varies significantly between these yeasts. Our findings suggest a model where innate immune cells discriminate fungal microorganisms based on differential mobilization and coordination of receptor networks. Our analysis methods are also broadly applicable to a range of cell-cell interactions central to many biological problems.

Characterization of the recombinant Candida albicans β-1,2-mannosyltransferase that initiates the β-mannosylation of cell wall phosphopeptidomannan

The presence of β-mannosides in their cell walls confers specific features on the pathogenic yeasts Candida albicans and Candida glabrata compared with non-pathogenic yeasts. In the present study, we investigated the enzymatic properties of Bmt1 (β-mannosyltransferase 1), a member of the recently identified β-mannosyltransferase family, from C. albicans. A recombinant soluble enzyme lacking the N-terminal region was expressed as a secreted protein from the methylotrophic yeast Pichia pastoris. In parallel, functionalized natural oligosaccharides isolated from Saccharomyces cerevisiae and a C. albicans mutant strain, as well as synthetic α-oligomannosides, were prepared and used as potential acceptor substrates. Bmt1p preferentially utilizes substrates containing linear chains of α-1,2-linked mannotriose or mannotetraose. The recombinant enzyme consecuti-vely transfers two mannosyl units on to these acceptors, leading to the production of α-mannosidase-resistant oligomannosides. NMR experiments further confirmed the presence of a terminal βMan (β-1,2-linked mannose) unit in the first enzyme product. In the future, a better understanding of specific β-1,2-mannosyltransferase molecular requirements will help the design of new potential antifungal drugs.

Immune cell response to Candida cell wall mannan derived branched α-oligomannoside conjugates in mice

BACKGROUND

Constructs composed of cell wall mannan-derived moieties conjugated to immunogenic proteins could be promising agents for induction of protective anti-Candida immune responses.

METHODS

This report is focused on the cellular immune response differences induced by BSA-based conjugates bearing synthetic α-1,6-branched oligomannosides. For monitoring of the immune responses following active immunization we evaluated changes in the frequencies of T and B lymphocytes and their activation status in the blood and spleen. We compared the immunization-induced changes of co-stimulatory molecules CD80 and CD86 expression on blood neutrophils and Th1/Th2 polarization of the immune response based on IFN-γ, TNF-α (pro-Th1), IL-4, and IL-10 (pro-Th2) cytokines levels and induction of IL-17.

RESULTS

The results pointed out a comparable effect of the conjugates on the modulation of T and B lymphocytes frequencies in blood and spleen. Both conjugates induced upregulation of CD25 surface antigen on CD4(+) T lymphocytes, independently on the structural differences of oligosaccharides. The differences in structure of oligomannoside antigens or conjugate constructs were reflected in the increase of co-stimulatory molecules CD80 and CD86 expression on neutrophils, and in induced cytokine response. M5-BSA conjugate induced only a slight increase in CD80 expression but a significant increase in IFN-γ, TNF-α, and IL-10. M6-BSA conjugate induced a significant increase of CD80 expression and increase of TNF-α, IL-4, and IL-10.

CONCLUSION

Obtained data demonstrate the importance of cellular immune response analysis for investigation of immunomodulatory properties of oligomannoside-protein conjugates.

Yeast Mnn9 is both a priming glycosyltransferase and an allosteric activator of mannan biosynthesis

The fungal cell possesses an essential carbohydrate cell wall. The outer layer, mannan, is formed by mannoproteins carrying highly mannosylated O- and N-linked glycans. Yeast mannan biosynthesis is initiated by a Golgi-located complex (M-Pol I) of two GT-62 mannosyltransferases, Mnn9p and Van1p, that are conserved in fungal pathogens. Saccharomyces cerevisiae and Candida albicans mnn9 knockouts show an aberrant cell wall and increased antibiotic sensitivity, suggesting the enzyme is a potential drug target. Here, we present the structure of ScMnn9 in complex with GDP and Mn²⁺, defining the fold and catalytic machinery of the GT-62 family. Compared with distantly related GT-78/GT-15 enzymes, ScMnn9 carries an unusual extension. Using a novel enzyme assay and site-directed mutagenesis, we identify conserved amino acids essential for ScMnn9 ‘priming’ α-1,6-mannosyltransferase activity. Strikingly, both the presence of the ScMnn9 protein and its product, but not ScMnn9 catalytic activity, are required to activate subsequent ScVan1 processive α-1,6-mannosyltransferase activity in the M-Pol I complex. These results reveal the molecular basis of mannan synthesis and will aid development of inhibitors targeting this process.

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.

Significant differences in the cell-wall mannans from three Candida glabrata strains correlate with antifungal drug sensitivity

Candida glabrata is often the second or third most common cause of candidiasis after Candida albicans. C. glabrata infections are difficult to treat, often resistant to many azole antifungal agents and are associated with a high mortality rate in compromised patients. We determined the antigenic structure of the cell-wall mannoproteins from three C. glabrata strains, NBRC 0005, NBRC 0622 and NBRC 103857. (1)H NMR and methylation analyses of the acetolysis products of these mannoproteins showed a significant difference in the amount of the β-1,2-linked mannose residue and side-chain structure. The C. glabrata NBRC 103857 strain contained up to the triose side chains and the nonreducing terminal of the triose was predominantly the β-1,2-linked mannose residue. By contrast, the mannans of the two former strains possessed up to the tetraose side chains and the amount of the β-1,2-linked mannose residue was very low. Larger oligosaccharides than tetraose in the acetolysis products of these mannans were identified as incomplete cleavage fragments by analyzing methylation, (1)H NMR spectra and the α1-2,3 mannosidase degradation reaction. Resistance to the antifungal drugs itraconazole and micafungin was significantly different in these strains. Interestingly, the NBRC 103857 strain, which involved a large amount of the β-1,2-linked mannose residues, exhibited significant sensitivity to these antifungal drugs.

Difference in fine specificity to polysaccharides of Candida albicans mannoprotein between mouse SIGNR1 and human DC-SIGN

C-type lectin SIGNR1 directly recognizes Candida albicans and zymosan and has been considered to share properties of polysaccharide recognition with human DC-SIGN (hDC-SIGN). However, the precise specificity of SIGNR1 and the difference from that of hDC-SIGN remain to be elucidated. We prepared soluble forms of SIGNR1 and hDC-SIGN and conducted experiments to examine their respective specificities. Soluble SIGNR1 (sSIGNR1) bound several types of live C. albicans clinical isolate strains in an EDTA-sensitive manner. Inhibition analyses of sSIGNR1 binding by glycans from various yeast strains demonstrated that SIGNR1 preferentially recognizes N-glycan α-mannose side chains in Candida mannoproteins, as reported in hDC-SIGN. Unlike shDC-SIGN, however, sSIGNR1 recognized not only Saccharomyces cerevisiae, but also C. albicans J-1012 glycan, even after α-mannosidase treatment that leaves only β1,2-mannose-capped α-mannose side chains. In addition, glycomicroarray analyses showed that sSIGNR1 binds mannans from C. albicans and S. cerevisiae but does not recognize Lewis(a/b/x/y) antigen polysaccharides as in shDC-SIGN. Consistent with these results, RAW264.7 cells expressing hDC-SIGN in which the carbohydrate recognition domain (CRD) was replaced with that of SIGNR1 (RAW-chimera) produced comparable amounts of interleukin 10 (IL-10) in response to glycans from C. albicans and S. cerevisiae, but those expressing hDC-SIGN produced less IL-10 in response to S. cerevisiae than C. albicans. Furthermore, RAW-hDC-SIGN cells remarkably reduced IL-10 production after α-mannosidase treatment compared with RAW-chimera cells. These results indicate that SIGNR1 recognizes C. albicans/yeast through a specificity partly distinct from that of its homologue hDC-SIGN.

Mannan structural complexity is decreased when Candida albicans is cultivated in blood or serum at physiological temperature

The Candida albicans cell wall provides an architecture that allows for the organism to survive environmental stress as well as interaction with host tissues. Previous work has focused on growing C. albicans on media such as Sabouraud or YPD at 30°C. Because C. albicans normally colonizes a host, we hypothesized that cultivation on blood or serum at 37°C would result in structural changes in cell wall mannan. C. albicans SC5314 was inoculated onto YPD, 5% blood, or 5% serum agar media three successive times at 30°C and 37°C, then cultivated overnight at 30°C in YPD. The mannan was extracted and characterized using 1D and 2D (1)H NMR techniques. At 30°C cells grown in blood and serum contain less acid-stable terminal β-(1→2)-linked d-mannose and α-(1→2)-linked d-mannose-containing side chains, while the acid-labile side chains of mannan grown in blood and serum contain fewer β-Man-(1→2)-α-Man-(1→ side chains. The decrement in acid-stable mannan side chains is greater at 37°C than at 30°C. Cells grown on blood at 37°C show fewer →6)-α-Man-(1→ structural motifs in the acid-stable polymer backbone. The data indicate that C. albicans, grown on media containing host-derived components, produces less complex mannan. This is accentuated when the cells are cultured at 37°C. This study demonstrates that the C. albicans cell wall is a dynamic and adaptive organelle, which alters its structural phenotype in response to growth in host-derived media at physiological temperature.

Vasculitis and anaphylactoid shock in mice induced by the polysaccharide fraction secreted into culture supernatants by the fungus Candida metapsilosis

The biological effects of Candida metapsilosis water-soluble fraction (CMWS), prepared using a completely synthesized medium, were examined to determine whether CMWS induces vasculitis similar to that seen in Kawasaki disease, and anaphylactoid shock, in mice. It was found that intraperitoneal injection of CMWS induces coronary arteritis and i.v. injection induces acute anaphylactoid shock in mice, similar to Candida albicans water-soluble fraction (CAWS)-induced arteritis and anaphylactoid shock. The mannan structure of the polysaccharide fraction was then analyzed by performing antiserum reactivity tests and nuclear magnetic resonance spectroscopy. The mannan structure was investigated because the present authors have recently found that the mannan moiety within the polysaccharide fraction might be responsible for these pathogenic activities. The structural analysis showed that the mannan structure within CMWS expresses α-mannan residues, but not β-mannan. In addition, the mannan structure of CMWS is quite similar to that of CAWS. The present findings indicate that the polysaccharide fraction from C. metapsilosis, which is mainly composed of mannan, contributes to coronary arteritis and acute shock, and that the mannan structure could be responsible for this pathogenicity.

Enzymatic synthesis of new oligosaccharides using mannosyltransferases from Candida species and their NMR assignments

The outer layer of the cell wall of pathogenic fungi, Candida species, consists of mannan, which plays an important role in infection. In this study, we synthesized several oligosaccharides using mannosyltransferases obtained from Candida parapsilosis and Candida albicans. Namely, we synthesized mannotetraoses [Manalpha1--> 2Manalpha1-->3Manalpha1-->2Man and Manalpha1-->3(Manalpha1-->6)Manalpha1-->2Man] from mannotriose, Manalpha1-->3Manalpha1--> 2Man, and mannohexaoses [Manalpha1-->2Manalpha1-->3Manalpha1-->3Manalpha1-->2Manalpha1-->2Man and Manalpha1-->3(Manalpha1-->6)Manalpha1-->3Manalpha1-->2Manalpha1-->2Man] from mannopentaose, Manalpha1-->3Manalpha1-->3Manalpha1-->2Manalpha1-->2Man. The linkage sequence of these oligosaccharides was identified by a sequential (1)H-NMR assignment method combined with rotating frame nuclear Overhauser enhancement spectroscopy and relayed coherence transfer spectroscopy. The steric effect by the alpha-1,6-linked branching mannose residue to the H-1 proton chemical shift of the neighboring 3-O-substituted mannose residue was different from that of the 2-O-substituted mannose residue. These oligosaccharides having novel structures seem to be useful as the substrate or ligand for glycomics.

Recognition of yeast by murine macrophages requires mannan but not glucan

The first barrier against infection by Candida albicans involves fungal recognition and destruction by phagocytic cells of the innate immune system. It is well established that interactions between different phagocyte receptors and components of the fungal cell wall trigger phagocytosis and subsequent immune responses, but the fungal ligands mediating the initial stage of recognition have not been identified. Here, we describe a novel assay for fungal recognition and uptake by macrophages which monitors this early recognition step independently of other downstream events of phagocytosis. To analyze infection in live macrophages, we validated the neutrality of a codon-optimized red fluorescent protein (yEmRFP) biomarker in C. albicans; growth, hyphal formation, and virulence in infected mice and macrophages were unaffected by yEmRFP production. This permitted a new approach for studying phagocytosis by carrying out competition assays between red and green fluorescent yeast cells to measure the efficiency of yeast uptake by murine macrophages as a function of dimorphism or cell wall defects. These competition experiments demonstrate that, given a choice, macrophages display strong preferences for phagocytosis based on genus, species, and morphology. Candida glabrata and Saccharomyces cerevisiae are taken up by J774 macrophage cells more rapidly than C. albicans, and C. albicans yeast cells are favored over hyphal cells. Significantly, these preferences are mannan dependent. Mutations that affect mannan, but not those that affect glucan or chitin, reduce the uptake of yeast challenged with wild-type competitors by both J774 and primary murine macrophages. These results suggest that mannose side chains or mannosylated proteins are the ligands recognized by murine macrophages prior to fungal uptake.