Structural characterization of the galactoxylomannan of Cryptococcus neoformans Cap67

Identification of a putative α-galactoside β-(1 → 3)-galactosyltransferase involved in the biosynthesis of galactomannan side chain of glucuronoxylomannogalactan in Cryptococcus neoformans

The cell surface of Cryptococcus neoformans is covered by a thick capsular polysaccharide. The capsule is the most important virulence factor of C. neoformans; however, the complete mechanism of its biosynthesis is unknown. The capsule is composed of glucuronoxylomannan (GXM) and glucuronoxylomannogalactan (GXMGal). As GXM is the most abundant component of the capsule, many studies have focused on GXM biosynthesis. However, although GXMGal has an important role in virulence, studies on its biosynthesis are scarce. Herein, we have identified a GT31 family β-(1 → 3)-galactosyltransferase Ggt2, which is involved in the biosynthesis of the galactomannan side chain of GXMGal. Comparative analysis of GXMGal produced by a ggt2 disruption strain revealed that Ggt2 is a glycosyltransferase that catalyzes the initial reaction in the synthesis of the galactomannan side chain of GXMGal. The ggt2 disruption strain showed a temperature-sensitive phenotype at 37°C, indicating that the galactomannan side chain of GXMGal is important for high-temperature stress tolerance in C. neoformans. Our findings provide insights into complex capsule biosynthesis in C. neoformans.

Cryptococcus: History, Epidemiology and Immune Evasion

Cryptococcosis is a disease caused by the pathogenic fungi Cryptococcus neoformans and Cryptococcus gattii, both environmental fungi that cause severe pneumonia and may even lead to cryptococcal meningoencephalitis. Although C. neoformans affects more fragile individuals, such as immunocompromised hosts through opportunistic infections, C. gattii causes a serious indiscriminate primary infection in immunocompetent individuals. Typically seen in tropical and subtropical environments, C. gattii has increased its endemic area over recent years, largely due to climatic factors that favor contagion in warmer climates. It is important to point out that not only C. gattii, but the Cryptococcus species complex produces a polysaccharidic capsule with immunomodulatory properties, enabling the pathogenic species of Cryptococccus to subvert the host immune response during the establishment of cryptococcosis, facilitating its dissemination in the infected organism. C. gattii causes a more severe and difficult-to-treat infection, with few antifungals eliciting an effective response during chronic treatment. Much of the immunopathology of this cryptococcosis is still poorly understood, with most studies focusing on cryptococcosis caused by the species C. neoformans. C. gattii became more important in the epidemiological scenario with the outbreaks in the Pacific Northwest of the United States, which resulted in phylogenetic studies of the virulent variant responsible for the severe infection in the region. Since then, the study of cryptococcosis caused by C. gattii has helped researchers understand the immunopathological aspects of different variants of this pathogen.

Role of carbohydrate antigens in antifungal glycoconjugate vaccines and immunotherapy

The emergence of fungal infection is a growing public health concern that in the latest years is becoming a serious threat to humans, particularly for immunocompromised individuals. Invasive fungal infections (IFIs), which are associated with significant morbidity and mortality, are on the rise due to the availability of only a few old antifungal agents. In addition to this, the growing use of antibiotics makes the population increasingly susceptible to these infections. Since carbohydrates are the main component of the fungal cell wall, the study of fungal glycans as potential targets for the fight against IFIs has aroused much interest in recent decades. In most fungal species the saccharides of the core are made up of chitin and β-glucans, while the outer layer carbohydrates vary according to the fungal species, such as mannans for Candida albicans, galactomannans for Aspergillus fumigatus hyphae, α-glucans for Aspergillus fumigatus and Cryptococcus neoformans, glucuronoxylomannans (GXM) and galactoxylomannans (GalXM) for Criptococcus neoformans. Being surface antigens, fungal carbohydrates are a logical target for the development of antifungal glycoconjugate vaccines and for immunotherapy with monoclonal antibodies. This review summarizes recent findings on active and passive immunization strategies based on fungal carbohydrates explored preclinically for three of the major fungal pathogens: Candida albicans, Cryptococcus neoformans and Aspergillus fumigatus.

The Diverse Roles of Monocytes in Cryptococcosis

Monocytes are considered to play a central role in the pathogenesis of Cryptococcus neoformans infection. Monocytes and monocyte-derived macrophages and dendritic cells are key components for the control of infection, but paradoxically they can also contribute to detrimental host responses and may even support fungal proliferation and dissemination. Simultaneously, the C. neoformans polysaccharide capsule can impair the functions of monocytes. Although monocytes are often seen as simple precursor cells, they also function as independent immune effector cells. In this review, we summarize these monocyte-specific functions during cryptococcal infection and the influence of C. neoformans on monocyte responses. We also cover the most recent findings on the functional and phenotypic heterogeneity of monocytes and discuss how new advanced technologies provide a platform to address outstanding questions in the field.

Variation in Cell Surface Hydrophobicity among Cryptococcus neoformans Strains Influences Interactions with Amoebas

Cryptococcus neoformans and Cryptococcus gattii are pathogenic fungi that cause significant morbidity and mortality. Cell surface hydrophobicity (CSH) is a biophysical parameter that influences the adhesion of fungal cells or spores to biotic and abiotic surfaces. C. neoformans is encased by polysaccharide capsule that is highly hydrophilic and is a critical determinant of virulence. In this study, we report large differences in the CSH of some C. neoformans and C. gattii strains. The capsular polysaccharides of C. neoformans strains differ in repeating motifs and therefore vary in the number of hydroxyl groups, which, along with higher-order structure of the capsule, may contribute to the variation in hydrophobicity that we observed. We found that cell wall composition, in the context of chitin-chitosan content, does not influence CSH. For C. neoformans, CSH correlated with phagocytosis by natural soil predator Acanthamoeba castellanii Furthermore, capsular binding of the protective antibody (18B7), but not the nonprotective antibody (13F1), altered the CSH of C. neoformans strains. Variability in CSH could be an important characteristic in comparing the biological properties of cryptococcal strains.IMPORTANCE The interaction of a microbial cell with its environment is influenced by the biophysical properties of a cell. The affinity of the cell surface for water, defined by the cell surface hydrophobicity (CSH), is a biophysical parameter that varies among different strains of Cryptococcus neoformans The CSH influences the phagocytosis of the yeast by its natural predator in the soil, the amoeba. Studying variation in biophysical properties like CSH gives us insight into the dynamic host-predator interaction and host-pathogen interaction in a damage-response framework.

Dectin-2-mediated signaling triggered by the cell wall polysaccharides of Cryptococcus neoformans

Cryptococcus neoformans is rich in polysaccharides of the cell wall and capsule. Dectin-2 recognizes high-mannose polysaccharides and plays a central role in the immune response to fungal pathogens. Previously, we demonstrated Dectin-2 was involved in the activation of dendritic cells upon stimulation with C. neoformans, suggesting the existence of a ligand recognized by Dectin-2. In the present study, we examined the cell wall structures of C. neoformans contributing to the Dectin-2-mediated activation of immune cells. In a NFAT-GFP reporter assay of the reported cells expressing Dectin-2, the lysates, but not the whole yeast cells, of an acapsular strain of C. neoformans (Cap67) delivered Dectin-2-mediated signaling. This activity was detected in the supernatant of β-glucanase-treated Cap67 and more strongly in the semi-purified polysaccharides of this supernatant using ConA-affinity chromatography (ConA-bound fraction), in which a large amount of saccharides, but not protein, were detected. Treatment of this supernatant with periodic acid and the addition of excessive mannose, but not glucose or galactose, strongly inhibited this activity. The ConA-bound fraction of the β-glucanase-treated Cap67 supernatant was bound to Dectin-2-Fc fusion protein in a dose-dependent manner and strongly induced the production of interleukin-12p40 and tumour necrosis factor-α by dendritic cells; this was abrogated under the Dectin-2-deficient condition. Finally, 98 kDa mannoprotein (MP98) derived from C. neoformans showed activation of the reporter cells expressing Dectin-2. These results suggested that a ligand with mannose moieties may exist in the cell walls and play a critical role in the activation of dendritic cells during infection with C. neoformans.

Dectin-2-Targeted Antifungal Liposomes Exhibit Enhanced Efficacy

Invasive fungal diseases caused by Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus have mortality rates ranging from 10 to 95%. Individual patient costs may exceed $100,000 in the United States. All antifungals in current use have serious limitations due to host toxicity and/or insufficient fungal cell killing that results in recurrent infections. Few new antifungal drugs have been introduced in the last 2 decades. Hence, there is a critical need for improved antifungal therapeutics. By targeting antifungal-loaded liposomes to α-mannans in the extracellular matrices secreted by these fungi, we dramatically reduced the effective dose of drug. Dectin-2-coated liposomes loaded with amphotericin B bound 50- to 150-fold more strongly to C. albicans, C. neoformans, and A. fumigatus than untargeted liposomes and killed these fungi more than an order of magnitude more efficiently. Targeting drug-loaded liposomes specifically to fungal cells has the potential to greatly enhance the efficacy of most antifungal drugs.

Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus cause life-threatening candidiasis, cryptococcosis, and aspergillosis, resulting in several hundred thousand deaths annually. The patients at the greatest risk of developing these life-threatening invasive fungal infections have weakened immune systems. The vulnerable population is increasing due to rising numbers of immunocompromised individuals as a result of HIV infection or immunosuppressed individuals receiving anticancer therapies and/or stem cell or organ transplants. While patients are treated with antifungals such as amphotericin B, all antifungals have serious limitations due to lack of sufficient fungicidal effect and/or host toxicity. Even with treatment, 1-year survival rates are low. We explored methods of increasing drug effectiveness by designing fungicide-loaded liposomes specifically targeted to fungal cells. Most pathogenic fungi are encased in cell walls and exopolysaccharide matrices rich in mannans. Dectin-2 is a mammalian innate immune membrane receptor that binds as a dimer to mannans and signals fungal infection. We coated amphotericin-loaded liposomes with monomers of Dectin-2’s mannan-binding domain, sDectin-2. sDectin monomers were free to float in the lipid membrane and form dimers that bind mannan substrates. sDectin-2-coated liposomes bound orders of magnitude more efficiently to the extracellular matrices of several developmental stages of C. albicans, C. neoformans, and A. fumigatus than untargeted control liposomes. Dectin-2-coated amphotericin B-loaded liposomes reduced the growth and viability of all three species more than an order of magnitude more efficiently than untargeted control liposomes and dramatically decreased the effective dose. Future efforts focus on examining pan-antifungal targeted liposomal drugs in animal models of fungal diseases.

IMPORTANCE Invasive fungal diseases caused by Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus have mortality rates ranging from 10 to 95%. Individual patient costs may exceed $100,000 in the United States. All antifungals in current use have serious limitations due to host toxicity and/or insufficient fungal cell killing that results in recurrent infections. Few new antifungal drugs have been introduced in the last 2 decades. Hence, there is a critical need for improved antifungal therapeutics. By targeting antifungal-loaded liposomes to α-mannans in the extracellular matrices secreted by these fungi, we dramatically reduced the effective dose of drug. Dectin-2-coated liposomes loaded with amphotericin B bound 50- to 150-fold more strongly to C. albicans, C. neoformans, and A. fumigatus than untargeted liposomes and killed these fungi more than an order of magnitude more efficiently. Targeting drug-loaded liposomes specifically to fungal cells has the potential to greatly enhance the efficacy of most antifungal drugs.

Characterization of the 6-O-acetylated lipoglucuronomannogalactan a novel Cryptococcus neoformans cell wall polysaccharide

Glucuronoxylomannogalactans (GXMGals) are characteristic capsular polysaccharides produced by the opportunistic fungus C. neoformans, which are implicated in cryptococcal virulence, via impairment of the host immune response. We determined for the first time the structure of a lipoglucuronomannogalactan (LGMGal), isolated from the surface of a mutant C. neoformans carrying a deletion in the UDP-GlcA decarboxylase gene. Monosaccharide composition and methylation analyses, as well as nuclear magnetic resonance spectroscopy were employed in discerning the structure. Our results show that the polysaccharide structure of the LGMGal differs from GXMGal by the absence of xylose and 2-O-acetylated mannose residues. LGMGal consists of a galactan main chain -[-6-α-Gal-]-, where every second Gal residue is substituted at O-3 with an oligosaccharide α-Man6OAc-3-α-Man-4-(β-GlcA-3)-β-Gal-; components in italic being non-stoichiometric. The substitution rate of β-Galp units by GlcpA is 35%. Additionally, we determined that the glycolipid anchor of the LGMGal is based on an myo-inositol phosphoceramide composed of C₁₈-phytosphingosine and monohydroxylated lignoceric acid (2OHC_24:0 fatty acid).

Unraveling synthesis of the cryptococcal cell wall and capsule

Fungal pathogens cause devastating infections in millions of individuals each year, representing a huge but underappreciated burden on human health. One of these, the opportunistic fungus Cryptococcus neoformans, kills hundreds of thousands of patients annually, disproportionately affecting people in resource-limited areas. This yeast is distinguished from other pathogenic fungi by a polysaccharide capsule that is displayed on the cell surface. The capsule consists of two complex polysaccharide polymers: a mannan substituted with xylose and glucuronic acid, and a galactan with galactomannan side chains that bear variable amounts of glucuronic acid and xylose. The cell wall, with which the capsule is associated, is a matrix of alpha and beta glucans, chitin, chitosan, and mannoproteins. In this review, we focus on synthesis of the wall and capsule, both of which are critical for the ability of this microbe to cause disease and are distinct from structures found in either model yeasts or the mammals afflicted by this infection. Significant research effort over the last few decades has been applied to defining the synthetic machinery of these two structures, including nucleotide sugar metabolism and transport, glycosyltransferase activities, polysaccharide export, and assembly and association of structural elements. Discoveries in this area have elucidated fundamental biology and may lead to novel targets for antifungal therapy. In this review, we summarize the progress made in this challenging and fascinating area, and outline future research questions.

Involvement of the capsular GalXM-induced IL-17 cytokine in the control of Cryptococcus neoformans infection

Cryptococcus neoformans is an opportunistic fungus that can cause lethal brain infections in immunosuppressed individuals. Infection usually occurs via the inhalation of a spore or desiccated yeast which can then disseminate from the lung to the brain and other tissues. Dissemination and disease is largely influence by the production of copious amounts of cryptococcal polysaccharides, both which are secreted to the extracellular environment or assembled into a thick capsule surrounding the cell body. There are two important polysaccharides: glucuronoxylomannan (GXM) and galactoxylomannan, also called as glucuronoxylomanogalactan (GXMGal or GalXM). Although GXM is more abundant, GalXM has a more potent modulatory effect. In the present study, we show that GalXM is a potent activator of murine dendritic cells, and when co-cultured with T cells, induces a Th17 cytokine response. We also demonstrated that treating mice with GalXM prior to infection with C. neoformans protects from infection, and this phenomenon is dependent on IL-6 and IL-17. These findings help us understand the immune biology of capsular polysaccharides in fungal pathogenesis.

The capsule of Cryptococcus neoformans

The capsule of Cryptococcus neoformans is its dominant virulence factor and plays a key role in the biology of this fungus. In this essay, we focus on the capsule as a cellular structure and note the limitations inherent in the current methodologies available for its study. Given that no single method can provide the structure of the capsule, our notions of what is the cryptococcal capsule must be arrived at by synthesizing information gathered from very different methodological approaches including microscopy, polysaccharide chemistry and physical chemistry of macromolecules. The emerging picture is one of a carefully regulated dynamic structure that is constantly rearranged as a response to environmental stimulation and cellular replication. In the environment, the capsule protects the fungus against desiccation and phagocytic predators. In animal hosts the capsule functions in both offensive and defensive modes, such that it interferes with immune responses while providing the fungal cell with a defensive shield that is both antiphagocytic and capable of absorbing microbicidal oxidative bursts from phagocytic cells. Finally, we delineate a set of unsolved problems in the cryptococcal capsule field that could provide fertile ground for future investigations.

Distribution of the O-acetyl groups and β-galactofuranose units in galactoxylomannans of the opportunistic fungus Cryptococcus neoformans

Galactoxylomannans (GalXMs) are a mixture of neutral and acidic capsular polysaccharides produced by the opportunistic fungus Cryptococcus neoformans that exhibit potent suppressive effects on the host immune system. Previous studies describing the chemical structure of C. neoformans GalXMs have reported species without O-acetyl substituents. Herein we describe that C. neoformans grown in capsule-inducing medium produces highly O-acetylated GalXMs. The location of the O-acetyl groups was determined by nuclear magnetic resonance (NMR) spectroscopy. In the neutral GalXM (NGalXM), 80% of 3-linked mannose (α-Manp) residues present in side chains are acetylated at the O-2 position. In the acidic GalXM also termed glucuronoxylomannogalactan (GXMGal), 85% of the 3-linked α-Manp residues are acetylated either in the O-2 (75%) or in the O-6 (25%) position, but O-acetyl groups are not present at both positions simultaneously. In addition, NMR spectroscopy and methylation analysis showed that β-galactofuranose (β-Galf) units are linked to O-2 and O-3 positions of nonbranched α-galactopyranose (α-Galp) units present in the GalXMs backbone chain. These findings highlight new structural features of C. neoformans GalXMs. Among these features, the high degree of O-acetylation is of particular interest, since O-acetyl group-containing polysaccharides are known to possess a range of immunobiological activities.

Titan cell production in Cryptococcus neoformans reshapes the cell wall and capsule composition during infection

Cryptococcus neoformans is a human fungal pathogen that often causes infections in immunocompromised individuals. Upon inhalation into the lungs C. neoformans differentiates into cells with altered size and morphology, including production of large titan cells. Titan cells possess thickened cell wall and dense, cross-linked capsule when compared to in vitro grown cells. In addition, titan cells have increased cell wall chitin that is associated with a detrimental anti-inflammatory immune response. Here we examined the cell wall and capsule composition of in vitro, in vivo typical-sized and in vivo titan cells using High Performance Liquid Chromatography (HPLC). The monomer composition of cell wall polysaccharides showed that in vivo C. neoformans cells contained more glucosamine and less glucose than in vitro cells, suggesting alteration in abundance of both chitin and glucans, respectively. Low levels of galactosamine were also detected in carbohydrates from both in vivo and vitro cells. Within the in vivo cell population, differences in the proportions of cell wall and capsule monomers between typical and titan cells were also observed. Taken together, these results demonstrate that C. neoformans reshapes its cell wall and capsule composition during infection. These cell wall and capsule alterations likely help C. neoformans escape recognition by, and allow modulation of, the host immune system.

Nutritional Requirements and Their Importance for Virulence of Pathogenic Cryptococcus Species

Cryptococcus sp. are basidiomycete yeasts which can be found widely, free-living in the environment. Interactions with natural predators, such as amoebae in the soil, are thought to have promoted the development of adaptations enabling the organism to survive inside human macrophages. Infection with Cryptococcus in humans occurs following inhalation of desiccated yeast cells or spore particles and may result in fatal meningoencephalitis. Human disease is caused almost exclusively by the Cryptococcus neoformans species complex, which predominantly infects immunocompromised patients, and the Cryptococcus gattii species complex, which is capable of infecting immunocompetent individuals. The nutritional requirements of Cryptococcus are critical for its virulence in animals. Cryptococcus has evolved a broad range of nutrient acquisition strategies, many if not most of which also appear to contribute to its virulence, enabling infection of animal hosts. In this review, we summarise the current understanding of nutritional requirements and acquisition in Cryptococcus and offer perspectives to its evolution as a significant pathogen of humans.

Galactofuranose antigens, a target for diagnosis of fungal infections in humans

The use of biomarkers for the detection of fungal infections is of interest to complement histopathological and culture methods. Since the production of antibodies in immunocompromised patients is scarce, detection of a specific antigen could be effective for early diagnosis. D-Galactofuranose (Galf) is the antigenic epitope in glycoconjugates of several pathogenic fungi. Since Galf is not biosynthesized by mammals, it is an attractive candidate for diagnosis of infection. A monoclonal antibody that recognizes Galf is commercialized for detection of aspergillosis. The linkage of Galf in the natural glycans and the chemical structures of the synthesized Galf-containing oligosaccharides are described in this paper. The oligosaccharides could be used for the synthesis of artificial carbohydrate-based antigens, not enough exploited for diagnosis.

D-Galactofuranose (Galf) is the unit in polysaccharides and glycoconjugates of several pathogenic fungi that is recognized by the immune system. Since Galf is not synthesized by mammals, it is an attractive candidate for diagnosis of infection. Since the production of antibodies in immunocompromised patients is scarce, detection of a specific antigen could be effective for early diagnosis. An antibody that recognizes Galf is commercialized for the detection of aspergillosis. Chemically synthesized Galf-containing oligosaccharides, reviewed in this paper, could therefore be used for the synthesis of artificial carbohydrate-based antigens and in diagnosis.

Masking the Pathogen: Evolutionary Strategies of Fungi and Their Bacterial Counterparts

Pathogens reduce immune recognition of their cell surfaces using a variety of inert structural polysaccharides. For example, capsular polysaccharides play critical roles in microbial survival strategies. Capsules are widely distributed among bacterial species, but relatively rare in eukaryotic microorganisms, where they have evolved considerable complexity in structure and regulation and are exemplified by that of the HIV/AIDS-related fungus Cryptococcus neoformans. Endemic fungi that affect normal hosts such as Histoplasma capsulatum and Blastomyces dermatitidis have also evolved protective polysaccharide coverings in the form of immunologically inert α-(1,3)-glucan polysaccharides to protect their more immunogenic β-(1,3)-glucan-containing cell walls. In this review we provide a comparative update on bacterial and fungal capsular structures and immunogenic properties as well as the polysaccharide masking strategies of endemic fungal pathogens.

Cryptococcus neoformans capsular polysaccharides form branched and complex filamentous networks viewed by high-resolution microscopy

Cryptococcus neoformans is a fungal pathogen that causes life-threatening infections in immunocompromised individuals. Its main virulence factor is an extracellular polysaccharide capsule whose structure, assembly and dynamics remain poorly understood. In this study, we apply improved protocols for sample preparation and recently-developed scanning microscopy techniques to visualize the ultrastructure of the C. neoformans capsule at high-resolution (up to 1 nm) and improved structural preservation. Although most capsule structures in nature consist of linear polymers, we show here that the C. neoformans capsule is a 'microgel-like' structure composed of branched polysaccharides. Moreover, we imaged the capsule-to-cell wall link, which is formed by thin fibers that branch out of thicker capsule filaments, and have one end firmly embedded in the cell wall structure. Together, our findings provide compelling ultrastructural evidence for a branched and complex capsule conformation, which may have important implications for the biological activity of the capsule as a virulence factor.

Capsular polysaccharides from Cryptococcus neoformans modulate production of neutrophil extracellular traps (NETs) by human neutrophils

In the present study, we characterized the in vitro modulation of NETs (neutrophil extracellular traps) induced in human neutrophils by the opportunistic fungus Cryptococcus neoformans, evaluating the participation of capsular polysaccharides glucuronoxylomanan (GXM) and glucuronoxylomannogalactan (GXMGal) in this phenomenon. The mutant acapsular strain CAP67 and the capsular polysaccharide GXMGal induced NET production. In contrast, the wild-type strain and the major polysaccharide GXM did not induce NET release. In addition, C. neoformans and the capsular polysaccharide GXM inhibited PMA-induced NET release. Additionally, we observed that the NET-enriched supernatants induced through CAP67 yeasts showed fungicidal activity on the capsular strain, and neutrophil elastase, myeloperoxidase, collagenase and histones were the key components for the induction of NET fungicidal activity. The signaling pathways associated with NET induction through the CAP67 strain were dependent on reactive oxygen species (ROS) and peptidylarginine deiminase-4 (PAD-4). Neither polysaccharide induced ROS production however both molecules blocked the production of ROS through PMA-activated neutrophils. Taken together, the results demonstrate that C. neoformans and the capsular component GXM inhibit the production of NETs in human neutrophils. This mechanism indicates a potentially new and important modulation factor for this fungal pathogen.

Cryptococcus neoformans and Cryptococcus gattii, the etiologic agents of cryptococcosis

Cryptococcus neoformans and Cryptococcus gattii are the two etiologic agents of cryptococcosis. They belong to the phylum Basidiomycota and can be readily distinguished from other pathogenic yeasts such as Candida by the presence of a polysaccharide capsule, formation of melanin, and urease activity, which all function as virulence determinants. Infection proceeds via inhalation and subsequent dissemination to the central nervous system to cause meningoencephalitis. The most common risk for cryptococcosis caused by C. neoformans is AIDS, whereas infections caused by C. gattii are more often reported in immunocompetent patients with undefined risk than in the immunocompromised. There have been many chapters, reviews, and books written on C. neoformans. The topics we focus on in this article include species description, pathogenesis, life cycle, capsule, and stress response, which serve to highlight the specializations in virulence that have occurred in this unique encapsulated melanin-forming yeast that causes global deaths estimated at more than 600,000 annually.

Cryptococcus neoformans dual GDP-mannose transporters and their role in biology and virulence

Cryptococcus neoformans is an opportunistic yeast responsible for lethal meningoencephalitis in humans. This pathogen elaborates a polysaccharide capsule, which is its major virulence factor. Mannose constitutes over one-half of the capsule mass and is also extensively utilized in cell wall synthesis and in glycosylation of proteins and lipids. The activated mannose donor for most biosynthetic reactions, GDP-mannose, is made in the cytosol, although it is primarily consumed in secretory organelles. This compartmentalization necessitates specific transmembrane transporters to make the donor available for glycan synthesis. We previously identified two cryptococcal GDP-mannose transporters, Gmt1 and Gmt2. Biochemical studies of each protein expressed in Saccharomyces cerevisiae showed that both are functional, with similar kinetics and substrate specificities in vitro. We have now examined these proteins in vivo and demonstrate that cells lacking Gmt1 show significant phenotypic differences from those lacking Gmt2 in terms of growth, colony morphology, protein glycosylation, and capsule phenotypes. Some of these observations may be explained by differential expression of the two genes, but others suggest that the two proteins play overlapping but nonidentical roles in cryptococcal biology. Furthermore, gmt1 gmt2 double mutant cells, which are unexpectedly viable, exhibit severe defects in capsule synthesis and protein glycosylation and are avirulent in mouse models of cryptococcosis.

Pbx proteins in Cryptococcus neoformans cell wall remodeling and capsule assembly

The cryptococcal capsule is a critical virulence factor of an important pathogen, but little is known about how it is associated with the cell or released into the environment. Two mutants lacking PBX1 and PBX2 were found to shed reduced amounts of the capsule polysaccharide glucuronoxylomannan (GXM). Nuclear magnetic resonance, composition, and physical analyses showed that the shed material was of normal mass but was slightly enriched in xylose. In contrast to previous reports, this material contained no glucose. Notably, the capsule fibers of pbxΔ mutant cells grown under capsule-inducing conditions were present at a lower than usual density and were loosely attached to the cell wall. Mutant cell walls were also defective, as indicated by phenotypes including abnormal cell morphology, reduced mating filamentation, and altered cell integrity. All observed phenotypes were shared between the two mutants and exacerbated in a double mutant. Consistent with a role in surface glycan synthesis, the Pbx proteins localized to detergent-resistant membrane domains. These results, together with the sequence motifs in the Pbx proteins, suggest that Pbx1 and Pbx2 are redundant proteins that act in remodeling the cell wall to maintain normal cell morphology and precursor availability for other glycan synthetic processes. Their absence results in aberrant cell wall growth and metabolic imbalance, which together impact cell wall and capsule synthesis, cell morphology, and capsule association. The surface changes also lead to increased engulfment by host phagocytes, consistent with the lack of virulence of pbx mutants in animal models.

Unusual galactofuranose modification of a capsule polysaccharide in the pathogenic yeast Cryptococcus neoformans

Galactofuranose (Galf) is the five-membered ring form of galactose. Although it is absent from mammalian glycans, it occurs as a structural and antigenic component of important cell surface molecules in a variety of microbes, ranging from bacteria to parasites and fungi. One such organism is Cryptococcus neoformans, a pathogenic yeast that causes lethal meningoencephalitis in immunocompromised individuals, particularly AIDS patients. C. neoformans is unique among fungal pathogens in bearing a complex polysaccharide capsule, a critical virulence factor reported to include Galf. Notably, how Galf modification contributes to the structure and function of the cryptococcal capsule is not known. We have determined that Galf is β1,2-linked to an unusual tetrasubstituted galactopyranose of the glucuronoxylomannogalactan (GXMGal) capsule polysaccharide. This discovery fills a longstanding gap in our understanding of a major polymer of the cryptococcal capsule. We also engineered a C. neoformans strain that lacks UDP-galactopyranose mutase; this enzyme forms UDP-Galf, the nucleotide sugar donor required for Galf addition. Mutase activity was required for the incorporation of Galf into glucuronoxylomannogalactan but was dispensable for vegetative growth, cell integrity, and virulence in a mouse model.

A Paracoccidioides brasiliensis glycan shares serologic and functional properties with cryptococcal glucuronoxylomannan

The cell wall of the yeast form of the dimorphic fungus Paracoccidioides brasiliensis is enriched with α1,3-glucans. In Cryptococcus neoformans, α1,3-glucans interact with glucuronoxylomannan (GXM), a heteropolysaccharide that is essential for fungal virulence. In this study, we investigated the occurrence of P. brasiliensis glycans sharing properties with cryptococcal GXM. Protein database searches in P. brasiliensis revealed the presence of sequences homologous to those coding for enzymes involved in the synthesis of GXM and capsular architecture in C. neoformans. In addition, monoclonal antibodies (mAbs) raised to cryptococcal GXM bound to P. brasiliensis cells. Using protocols that were previously established for extraction and analysis of C. neoformans GXM, we recovered a P. brasiliensis glycan fraction composed of mannose and galactose, in addition to small amounts of glucose, xylose and rhamnose. In comparison with the C. neoformans GXM, the P. brasiliensis glycan fraction components had smaller molecular dimensions. The P. brasiliensis components, nevertheless, reacted with different GXM-binding mAbs. Extracellular vesicle fractions of P. brasiliensis also reacted with a GXM-binding mAb, suggesting that the polysaccharide-like molecule is exported to the extracellular space in secretory vesicles. An acapsular mutant of C. neoformans incorporated molecules from the P. brasiliensis extract onto the cell wall, resulting in the formation of surface networks that resembled the cryptococcal capsule. Coating the C. neoformans acapsular mutant with the P. brasiliensis glycan fraction resulted in protection against phagocytosis by murine macrophages. These results suggest that P. brasiliensis and C. neoformans share metabolic pathways required for the synthesis of similar polysaccharides and that P. brasiliensis yeast cell walls have molecules that mimic certain aspects of C. neoformans GXM. These findings are important because they provide additional evidence for the sharing of antigenically similar components across phylogenetically distant fungal species. Since GXM has been shown to be important for the pathogenesis of C. neoformans and to elicit protective antibodies, the finding of similar molecules in P. brasiliensis raises the possibility that these glycans play similar functions in paracoccidiomycosis.

Emerging themes in cryptococcal capsule synthesis

Cryptococcus neoformans, a basidiomycete yeast and opportunistic pathogen, expends significant biosynthetic effort on construction of a polysaccharide capsule with a radius that may be many times that of the cell. Beyond posing a stimulating challenge in terms of defining biosynthetic pathways, the capsule is required for this yeast to cause fatal disease. This combination has focused the attention of researchers on this system. Here we briefly review two aspects of the rapidly advancing field of capsule synthesis: the extensive variation that occurs in capsule polymers and the regulation of capsule biosynthesis.

Cryptococcus neoformans galactoxylomannan is a potent negative immunomodulator, inspiring new approaches in anti-inflammatory immunotherapy

Cryptococcus neoformans is an opportunistic fungal pathogen responsible for life-threatening infections in immunocompromised individuals and occasionally in those with no known immune impairment. The fungus is endowed with several virulence factors, including capsular polysaccharides that play a key role in virulence. The capsule is composed of 90–95% glucuronoxylomannan (GXM), 5–8% galactoxylomannan (GalXM) and <1% mannoproteins. Capsular polysaccharides are shed into tissue where they produce many deleterious effects. Since GalXM has a smaller molecular mass, the molar concentration of GalXM in polysaccharide that is shed could exceed that of GXM in C. neoformans exopolysaccharides. Moreover, GalXM exhibits a number of unusual biologic properties both in vitro and in vivo. Here, we summarize the principal immunomodulatory effects of GalXM described during the last 20 years, particularly the mechanisms leading to induction of apoptosis in T lymphocytes, B lymphocytes and macrophages. Since the capacity of GalXM to induce widespread immune suppression is believed to contribute to the virulence of C. neoformans, this property might be exploited therapeutically to dampen the aberrant activation of immune cells during autoimmune disorders.

Biosynthesis of galactofuranose in kinetoplastids: novel therapeutic targets for treating leishmaniasis and chagas' disease

Cell surface proteins of parasites play a role in pathogenesis by modulating mammalian cell recognition and cell adhesion during infection. β-Galactofuranose (Galf) is an important component of glycoproteins and glycolipids found on the cell surface of Leishmania spp. and Trypanosoma cruzi. β-Galf-containing glycans have been shown to be important in parasite-cell interaction and protection against oxidative stress. Here, we discuss the role of β-Galf in pathogenesis and recent studies on the Galf-biosynthetic enzymes: UDP-galactose 4′ epimerase (GalE), UDP-galactopyranose mutase (UGM), and UDP-galactofuranosyl transferase (GalfT). The central role in Galf formation, its unique chemical mechanism, and the absence of a homologous enzyme in humans identify UGM as the most attractive drug target in the β-Galf-biosynthetic pathway in protozoan parasites.

A xylosylphosphotransferase of Cryptococcus neoformans acts in protein O-glycan synthesis

Cryptococcal meningoencephalitis is an AIDS-defining illness caused by the opportunistic pathogen Cryptococcus neoformans. This organism possesses an elaborate polysaccharide capsule that is unique among pathogenic fungi, and the glycobiology of C. neoformans has been a focus of research in the field. The capsule and other cellular glycans and glycoconjugates have been described, but the machinery responsible for their synthesis remains largely unexplored. We recently discovered Xpt1p, an enzyme with the unexpected activity of generating a xylose-phosphate-mannose linkage. We now demonstrate that this novel activity is conserved throughout the C. neoformans species complex, localized to the Golgi apparatus, and functions in the O-glycosylation of proteins. We also present the first survey of O-glycans from C. neoformans.

Evaluation of Cryptococcus neoformans galactoxylomannan-protein conjugate as vaccine candidate against murine cryptococcosis

Galactoxylomannan (GalXM) is a complex polysaccharide produced by the human pathogenic fungus Cryptococcus neoformans that mediates profound immunological derangements in murine models. GalXM is essentially non-immunogenic and produces immune paralysis in mice. Previous studies have attempted to enhance immunogenicity by conjugating GalXM to a protein carrier, but only transient antibody responses were elicited. Here we report the generation of two GalXM conjugates with bovine serum albumin (BSA) and protective antigen (PA) of Bacillus anthracis, respectively, using 1-cyano-4-dimethylaminopyridinium tetrafluoroborate (CDAP) as the cyanylating reagent. Both conjugates induced potent and sustained antibody responses as detected by both cross antigen-based and CovaLink direct ELISAs. We confirmed the specificity of the response to GalXM by inhibition ELISA and immunofluorescence. The isotype composition analysis revealed that IgG and IgM were abundant in the immune sera against GalXM, consistent with the induction of a T cell-dependent response. IgG1 was the predominant IgG subclass against GalXM, while immunization with Quil A as adjuvant elicited a significantly higher production of IgG2a than with Freund's adjuvant. Immune sera were not opsonic for C. neoformans and there was no survival difference between immune and non-immune mice challenged with C. neoformans. These results demonstrated the effectiveness of the GalXM-protein conjugate to induce robust immune responses although no evidence was obtained that such responses contributed to host defense.

Evidence for branching in cryptococcal capsular polysaccharides and consequences on its biological activity

The encapsulated fungus Cryptococcus neoformans is a common cause of life-threatening disease in immunocompromised individuals. Its major virulence determinant is the polysaccharide (PS) capsule. An unsolved problem in cryptococcal biology is whether the PSs composing the capsule are linear or complex branched polymers, as well as the implications of this structural composition in pathogenesis. In this study we approached the problem by combining static and dynamic light scattering, viscosity analysis, and high-resolution microscopy and correlated the findings with biological properties. Analysis of the dependence of capsular PS molecular mass and the radius of gyration provided strong evidence against a simple linear PS configuration. Shape factors calculated from light scattering measurements in solution revealed values consistent with polymer branching. Furthermore, viscosity measurements provided complementary evidence for structural branching. Electron microscopy showed PS spherical-like structures similar to other branched PS. Finally, we show that the capacity of capsular PS to interfere in complement-mediated phagocytosis, inhibit nitric oxide production by macrophage-like cells, protect against reactive oxygen species, antibody reactivity and half-life in serum were influenced by the degree of branching, providing evidence for the notion that PS branching is an important parameter in determining the biological activity of C. neoformans PS.

Glucuronoxylomannan, galactoxylomannan, and mannoprotein occupy spatially separate and discrete regions in the capsule of Cryptococcus neoformans

The capsular polysaccharides of Cryptococcus neoformans have historically been divided into three components namely, glucuronoxylomannan (GXM), galactoxylomannan (GalXM), and mannoprotein (MP) but their relative spatial-geographical relationship in the capsule is unknown.  To explore this problem would require the capacity for visualizing these components in the capsule.  Prior studies have reported serological reagents to GXM and GalXM but no antibodies are available against MPs.  Consequently, we immunized Balb/c mice with C. neoformans recombinant mannoprotein 98 and recovered twelve monoclonal antibodies (mAbs) of which one, an IgG2a designated 18F2, bound to intact cells by immunofluorescence. mAb 18F2 bound to the cell wall surface in acapsular and encapsulated cells.  Using mAb 18F2 and previously generated antibodies to GXM and GalXM we have established the localization of three capsular components GXM, GalXM and one type of mannoprotein, MP98 on the C. neoformans cell.  The results show that MP98, like GalXM, is found near the cell wall and this information allows us to begin to discern the geography of the cryptococcal capsule.

Role of CD45 signaling pathway in galactoxylomannan-induced T cell damage

Previously, we reported that Galactoxylomannan (GalXM) activates the extrinsic and intrinsic apoptotic pathways through an interaction with the glycoreceptors on T cells. In this study we establish the role of the glycoreceptor CD45 in GalXM-induced T cell apoptosis, using CD45(+/+) and CD45(-/-) cell lines, derived from BW5147 murine T cell lymphoma. Our results show that whereas CD45 expression is not required for GalXM association by the cells, it is essential for apoptosis induction. In CD45(+/+) cells, CD45 triggering by GalXM reduces the activation of Lck, ZAP70 and Erk1/2. Conversely, in CD45(-/-) cells, Lck was hyperphosphorylated and did not show any modulation after GalXM stimulation. On the whole, our findings provide evidence that the negative regulation of Lck activation occurs via CD45 engagement. This appears to be related to the capacity of GalXM to antagonize T cell activation and induce T cell death. Overall this mechanism may be responsible for the immune paralysis that follows GalXM administration and could explain the powerful immunosuppression that accompanies cryptococcosis.

Biosynthesis of a new UDP-sugar, UDP-2-acetamido-2-deoxyxylose, in the human pathogen Bacillus cereus subspecies cytotoxis NVH 391-98

We have identified an operon and characterized the functions of two genes from the severe food-poisoning bacterium, Bacillus cereus subsp. cytotoxis NVH 391-98, that are involved in the synthesis of a unique UDP-sugar, UDP-2-acetamido-2-deoxyxylose (UDP-N-acetyl-xylosamine, UDP-XylNAc). UGlcNAcDH encodes a UDP-N-acetyl-glucosamine 6-dehydrogenase, converting UDP-N-acetylglucosamine (UDP-GlcNAc) to UDP-N-acetyl-glucosaminuronic acid (UDP-GlcNAcA). The second gene in the operon, UXNAcS, encodes a distinct decarboxylase not previously described in the literature, which catalyzes the formation of UDP-XylNAc from UDP-GlcNAcA in the presence of exogenous NAD(+). UXNAcS is specific and cannot utilize UDP-glucuronic acid and UDP-galacturonic acid as substrates. UXNAcS is active as a dimer with catalytic efficiency of 7 mM(-1) s(-1). The activity of UXNAcS is completely abolished by NADH but unaffected by UDP-xylose. A real-time NMR-based assay showed unambiguously the dual enzymatic conversions of UDP-GlcNAc to UDP-GlcNAcA and subsequently to UDP-XylNAc. From the analyses of all publicly available sequenced genomes, it appears that UXNAcS is restricted to pathogenic Bacillus species, including Bacillus anthracis and Bacillus thuringiensis. The identification of UXNAcS provides insight into the formation of UDP-XylNAc. Understanding the metabolic pathways involved in the utilization of this amino-sugar may allow the development of drugs to combat and eradicate the disease.

Identification of a bifunctional UDP-4-keto-pentose/UDP-xylose synthase in the plant pathogenic bacterium Ralstonia solanacearum strain GMI1000, a distinct member of the 4,6-dehydratase and decarboxylase family

The UDP-sugar interconverting enzymes involved in UDP-GlcA metabolism are well described in eukaryotes but less is known in prokaryotes. Here we identify and characterize a gene (RsU4kpxs) from Ralstonia solanacearum str. GMI1000, which encodes a dual function enzyme not previously described. One activity is to decarboxylate UDP-glucuronic acid to UDP-beta-l-threo-pentopyranosyl-4''-ulose in the presence of NAD(+). The second activity converts UDP-beta-l-threo-pentopyranosyl-4''-ulose and NADH to UDP-xylose and NAD(+), albeit at a lower rate. Our data also suggest that following decarboxylation, there is stereospecific protonation at the C5 pro-R position. The identification of the R. solanacearum enzyme enables us to propose that the ancestral enzyme of UDP-xylose synthase and UDP-apiose/UDP-xylose synthase was diverged to two distinct enzymatic activities in early bacteria. This separation gave rise to the current UDP-xylose synthase in animal, fungus, and plant as well as to the plant Uaxs and bacterial ArnA and U4kpxs homologs.

Galactoxylomannans from Cryptococcus neoformans varieties neoformans and grubii are structurally and antigenically variable

Prior studies have established that the Cryptococcus neoformans capsular polysaccharide component galactoxylomannan (GalXM) manifests serotype-related structural differences that translate into antigenic differences. We analyzed GalXM from acapsular serotype A and D strains by carbohydrate analysis and static and dynamic light scattering to determine mass, effective diameter, polydispersity, and diffusion coefficients. Multiangle laser light scattering showed that GalXM from C. neoformans var. grubii strain cap59 (serotype A) had larger molecular mass (4.21 x 10(6) +/- 0.95 x 10(6) g/mol) and radius of gyration (207 +/- 27 nm) than GalXM from C. neoformans var. neoformans cap67 (serotype D). cap67 GalXM had corresponding values of 0.70 x 10(6) +/- 0.05 x 10(6) g/mol and 120 +/- 22 nm, respectively. The effective diameter for GalXM and polydispersity from the two strains varied depending on temperature and medium growth conditions, indicating that GalXM structure can vary within a strain, depending on its environment. Zeta potential determinations were negative for GalXM from both strains under all conditions, consistent with the recently reported presence of glucuronic acid. These results imply that C. neoformans GalXM, like glucuronoxylomannan, can manifest variety- and growth condition-related variations. Analysis of 16 C. neoformans and 7 Cryptococcus gattii strains with polyclonal antibody to a GalXM strain revealed antigenic similarities among the C. neoformans variety neoformans and grubii strains and no reactivity with C. gattii. As a result of the deleterious effects of GalXM on immune function, structural and antigenic variability between serotypes may translate into differences in immunomodulatory effects.

How sweet it is! Cell wall biogenesis and polysaccharide capsule formation in Cryptococcus neoformans

Cryptococcus neoformans is a pathogenic fungus responsible for severe opportunistic infections. The most prominent feature of this yeast is its elaborate polysaccharide capsule, a complex structure that is required for virulence. The capsule is intimately associated with the cell wall, which underlies the capsule and offers the organism strength and flexibility in potentially hostile environments. Both structures are primarily composed of polysaccharides, offering a glimpse of the tremendous variation inherent in natural carbohydrate structures and their multiple biological functions. The steps in cell wall and capsule biosynthesis and assembly pose fascinating questions of metabolism, enzymology, cell biology, and regulation; the answers have potential application to treatment of a deadly infection. This article reviews current knowledge of cryptococcal cell wall and capsule biosynthesis and outstanding questions for the future.

A novel xylosylphosphotransferase activity discovered in Cryptococcus neoformans

Cryptococcus neoformans is a fungal pathogen that causes serious disease in immunocompromised individuals. The organism produces a distinctive polysaccharide capsule that is necessary for its virulence, a predominantly polysaccharide cell wall, and a variety of protein- and lipid-linked glycans. The glycan synthetic pathways of this pathogen are of great interest. Here we report the detection of a novel glycosylphosphotransferase activity in C. neoformans, identification of the corresponding gene, and characterization of the encoded protein. The observed activity is specific for UDP-xylose as a donor and for mannose acceptors and forms a xylose-alpha-1-phosphate-6-mannose linkage. This is the first report of a xylosylphosphotransferase activity in any system.

Galactoxylomannan-mediated immunological paralysis results from specific B cell depletion in the context of widespread immune system damage

The mechanisms responsible for polysaccharide-induced immunological paralysis have remained unexplained almost a century after this phenomenon was first described. Cryptococcus neoformans capsular polysaccharides glucuronoxylomannan and galactoxylomannan (GalXM) elicit little or no Ab responses. This study investigates the immunological and biological effects of GalXM in mice. GalXM immunization elicits a state of immunological paralysis in mice characterized by the disappearance of Ab-producing cells in the spleen. Immunological paralysis and lack of immunogenicity could not be overcome by immunization with GalXM conjugated to a protein carrier, Bacillus anthracis protective Ag. Additionally, immunization with GalXM in either complete or IFA was associated with spleen enlargement in BALB/c mice. TUNEL and flow cytometry revealed widespread apoptosis in the spleen after GalXM administration. Administration of a cocktail of caspase-3 inhibitor Z-DEVD-FMK and general caspase inhibitor Z-VAD-FMK or Fas-deficient mice abrogated the complete disappearance of Ab-producing cells. Analysis of spleen cytokine expression in response to GalXM systemic injection revealed that GalXM down-regulated the production of inflammatory cytokines. Hence, we conclude that GalXM-induced immune paralysis is a result of specific B cell depletion mediated by its proapoptotic properties in the context of widespread dysregulation of immune function.

The capsule of the fungal pathogen Cryptococcus neoformans

The capsule of the fungal pathogen Cryptococcus neoformans has been studied extensively in recent decades and a large body of information is now available to the scientific community. Well-known aspects of the capsule include its structure, antigenic properties and its function as a virulence factor. The capsule is composed primarily of two polysaccharides, glucuronoxylomannan (GXM) and galactoxylomannan (GalXM), in addition to a smaller proportion of mannoproteins (MPs). Most of the studies on the composition of the capsule have focused on GXM, which comprises more than 90% of the capsule's polysaccharide mass. It is GalXM, however, that is of particular scientific interest because of its immunological properties. The molecular structure of these polysaccharides is very complex and has not yet been fully elucidated. Both GXM and GalXM are high molecular mass polymers with the mass of GXM equaling roughly 10 times that of GalXM. Recent findings suggest, however, that the actual molecular weight might be different to what it has traditionally been thought to be. In addition to their structural roles in the polysaccharide capsule, these molecules have been associated with many deleterious effects on the immune response. Capsular components are therefore considered key virulence determinants in C. neoformans, which has motivated their use in vaccines and made them targets for monoclonal antibody treatments. In this review, we will provide an update on the current knowledge of the C. neoformans capsule, covering aspects related to its structure, synthesis and particularly, its role as a virulence factor.

Production of extracellular polysaccharides by CAP mutants of Cryptococcus neoformans

The human pathogen Cryptococcus neoformans causes meningoencephalitis. The polysaccharide capsule is one of the main virulence factors and consists of two distinct polysaccharides, glucuronoxylomannan (GXM) and galactoxylomannan (GalXM). How capsular polysaccharides are synthesized, transported, and assembled is largely unknown. Previously, it was shown that mutations in the CAP10, CAP59, CAP60, and CAP64 genes result in an acapsular phenotype. Here, it is shown that these acapsular mutants do secrete GalXM and GXM-like polymers. GXM and GalXM antibodies specifically reacted with whole cells and the growth medium of the wild type and CAP mutants, indicating that the capsule polysaccharides adhere to the cell wall and are shed into the environment. These polysaccharides were purified from the medium, either with or without anion-exchange chromatography. Monosaccharide analysis of polysaccharide fractions by gas-liquid chromatography/mass spectrometry showed that wild-type cells secrete both GalXM and GXM. The CAP mutants, on the other hand, were shown to secrete GalXM and GXM-like polymers. Notably, the GalXM polymers were shown to contain glucuronic acid. One-dimensional (1)H nuclear magnetic resonance confirmed that the CAP mutants secrete GalXM and also showed the presence of O-acetylated polymers. This is the first time it is shown that CAP mutants secrete GXM-like polymers in addition to GalXM. The small amount of this GXM-like polymer, 1 to 5% of the total amount of secreted polysaccharides, may explain the acapsular phenotype.

Involvement of glycoreceptors in galactoxylomannan-induced T cell death

The major virulence factor of Cryptococcus neoformans is its capsular polysaccharide, which is also released into tissues. The shed polysaccharide is composed of glucuronoxylomannan, galactoxylomannan (GalXM), and mannoproteins. In a previous study, we demonstrated a direct interaction of purified soluble GalXM with T cells that induced their apoptosis. In this study, we focus on the mechanisms involved in the apoptotic effect of GalXM. In our experimental system, we analyzed the effect of GalXM on purified human T cells and Jurkat cells, a T cell line routinely used for apoptotic studies. Our results reveal that GalXM activates the extrinsic and intrinsic apoptotic pathways through the cleavage and recruitment of caspase-8. Caspase-8 elicits the downstream executioner caspase-3, caspase-6, and caspase-7 both directly and indirectly, via Bid cleavage and caspase-9 activation. These effects appeared to be primarily mediated by the interaction of GalXM with the glycoreceptors, which differed in human T and Jurkat cells. CD45 was primarily involved in Jurkat cells apoptosis while CD7 and CD43 mediated human T cell apoptosis. Our results highlight a new mechanism by which a microbial product can contribute to virulence through direct interaction with T cell glycoreceptors, thereby triggering lymphocyte apoptosis.

Virulence in Cryptococcus species

Cryptococcus neoformans and Cryptococcus gattii are the cause of life-threatening meningoencephalitis in immunocompromised and immunocompetent individuals respectively. The increasing incidence of cryptococcal infection as a result of the AIDS epidemic, the recent emergence of a hypervirulent cryptococcal strain in Canada and the fact that mortality from cryptococcal disease remains high have stimulated intensive research into this organism. Here we outline recent advances in our understanding of C. neoformans and C. gattii, including intraspecific complexity, virulence factors, and key signaling pathways. We discuss the molecular basis of cryptococcal virulence and the interaction between these pathogens and the host immune system. Finally, we discuss future challenges in the study and treatment of cryptococcosis.

The structure of Cryptococcus neoformans galactoxylomannan contains beta-D-glucuronic acid

The structure of galactoxylomannan, a capsular polysaccharide from the opportunistic yeast Cryptococcus neoformans, was re-examined by NMR spectroscopy and GC-MS. The residue that is 3-linked to the side chain galactose and was previously assigned as beta-D-xylose [Vaishnav, V. V.; Bacon, B. E.; O'Neill, M.; Cherniak, R. Carbohydr. Res.1998, 306, 315-330] was determined to be beta-D-glucuronic acid. A revised structure for this polymer is presented, along with a proposal that this compound be termed glucuronoxylomannogalactan (GXMGal).

The role of the cell wall in fungal pathogenesis

Fungal infections are a serious health problem. In recent years, basic research is focusing on the identification of fungal virulence factors as promising targets for the development of novel antifungals. The wall, as the most external cellular component, plays a crucial role in the interaction with host cells mediating processes such as adhesion or phagocytosis that are essential during infection. Specific components of the cell wall (called PAMPs) interact with specific receptors in the immune cell (called PRRs), triggering responses whose molecular mechanisms are being elucidated. We review here the main structural carbohydrate components of the fungal wall (glucan, mannan and chitin), how their biogenesis takes place in fungi and the specific receptors that they interact with. Different model fungal pathogens are chosen to illustrate the functional consequences of this interaction. Finally, the identification of the key components will have important consequences in the future and will allow better approaches to treat fungal infections.

An extracellular galactoxylomannan of acapsular Cryptococcus laurentii mutant

An extracellular galactoxylomannan (GalXM) composed of D-Gal (34.0%), D-Xyl (26.6%) and D-Man (31.0%), and a small amount of L-Ara (4.0%) and D-Glc (4.4%) has been isolated from culture medium of acapsulated mutant of Cryptococcus laurentii by ethanol precipitation and gel filtration. Phosphorylated polymer of Mw approximately 75,000 contained 90% carbohydrates, 3.9% phosphorus and 5.3% proteins. Results of chemical and spectroscopic studies showed a highly branched structure of GalXM with a 1,6-linked mannopyranosyl-galactopyranosyl backbone (approximately 44%) branched predominantly at C-2 and C-3 of mannosyl, and C-3 of galactosyl residues by side chains terminated mainly by xylosyl and mannosyl residues, and to a less extent by arabinosyl and glucosyl ones.