A drug-sensitive genetic network masks fungi from the immune system

Transcription factor Efg1 shows a haploinsufficiency phenotype in modulating the cell wall architecture and immunogenicity of Candida albicans

The Candida albicans transcription factor Efg1 is known to be involved in many different cellular processes, including morphogenesis, general metabolism, and virulence. Here we show that besides its manifold roles, Efg1 also has a prominent effect on cell wall structure and composition, strongly affecting the structural glucan part. Deletion of only one allele of EFG1 already results in severe phenotypes for cell wall biogenesis, comparable to those with deletion of both alleles, indicative of a severe haploinsufficiency for EFG1. The observed defects in structural setup of the cell wall, together with previously reported alterations in expression of cell surface proteins, result in altered immunogenic properties of strains with compromised Efg1 function. This is shown by interaction studies with macrophages and primary dendritic cells. The structural changes in the cell wall carbohydrate meshwork presented here, together with the manifold changes in cell wall protein composition and metabolism reported in other studies, contribute to the altered immune response mounted by innate immune cells and to the altered virulence phenotypes observed for strains lacking EFG1.

Organic modification and subsequent biofunctionalization of porous anodic alumina using terminal alkynes

Porous anodic alumina (PAA) is a well-defined material that has found many applications. The range of applications toward sensing and recognition can be greatly expanded if the alumina surface is covalently modified with an organic monolayer. Here, we present a new method for the organic modification of PAA based on the reaction of terminal alkynes with the alumina surface. The reaction results in the the formation of a monolayer within several hours at 80 °C and is dependent on both oxygen and light. Characterization with X-ray photoelectron spectroscopy and infrared spectroscopy indicates formation of a well-defined monolayer in which the adsorbed species is an oxidation product of the 1-alkyne, namely, its α-hydroxy carboxylate. The obtained monolayers are fairly stable in water and at elevated temperatures, as was shown by monitoring the water contact angle. Modification with 1,15-hexadecadiyne resulted in a surface that has alkyne end groups available for further reaction, as was demonstrated by the subsequent reaction of N-(11-azido-3,6,9-trioxaundecyl)trifluoroacetamide with the modified surface. Biofunctionalization was explored by coupling 11-azidoundecyl lactoside to the surface and studying the subsequent adsorption of the lectin peanut agglutinin (PNA) and the yeast Candida albicans, respectively. Selective and reversible binding of PNA to the lactosylated surfaces was demonstrated. Moreover, PNA adsorption was higher on surfaces that exposed the β-lactoside than on those that displayed the α anomer, which was attributed to surface-associated steric hindrance. Likewise, the lactosylated surfaces showed increased colonization of C. albicans compared to unmodified surfaces, presumably due to interactions involving the cell wall β-glucan. Thus, this study provides a new modification method for PAA surfaces and shows that it can be used to induce selective adsorption of proteins and microorganisms.

Candida biofilms and the host: models and new concepts for eradication

Biofilms define mono- or multispecies communities embedded in a self-produced protective matrix, which is strongly attached to surfaces. They often are considered a general threat not only in industry but also in medicine. They constitute a permanent source of contamination, and they can disturb the proper usage of the material onto which they develop. This paper relates to some of the most recent approaches that have been elaborated to eradicate Candida biofilms, based on the vast effort put in ever-improving models of biofilm formation in vitro and in vivo, including novel flow systems, high-throughput techniques and mucosal models. Mixed biofilms, sustaining antagonist or beneficial cooperation between species, and their interplay with the host immune system are also prevalent topics. Alternative strategies against biofilms include the lock therapy and immunotherapy approaches, and material coating and improvements. The host-biofilm interactions are also discussed, together with their potential applications in Candida biofilm elimination.

Phospholipase Cγ2 (PLCγ2) is key component in Dectin-2 signaling pathway, mediating anti-fungal innate immune responses

C-type lectin receptors (CLRs) such as Dectin-2 function as pattern recognition receptors to sense fungal infection. However, the signaling pathways induced by these receptors remain largely unknown. Previous studies suggest that the CLR-induced signaling pathway may utilize similar signaling components as the B cell receptor-induced signaling pathway. Phospholipase Cγ2 (PLCγ2) is a key component in B cell receptor signaling, but its role in other signaling pathways has not been fully characterized. Here, we show that PLCγ2 functions downstream of Dectin-2 in response to the stimulation by the hyphal form of Candida albicans, an opportunistic pathogenic fungus. Using PLCγ2- and PLCγ1-deficient macrophages, we found that the lack of PLCγ2, but not PLCγ1, impairs cytokine production in response to infection with C. albicans. PLCγ2 deficiency results in the defective activation of NF-κB and MAPK and a significantly reduced production of reactive oxygen species following fungal challenge. In addition, PLCγ2-deficient mice are defective in clearing C. albicans infection in vivo. Together, these findings demonstrate that PLCγ2 plays a critical role in CLR-induced signaling pathways, governing antifungal innate immune responses.

Elevated cell wall chitin in Candida albicans confers echinocandin resistance in vivo

Candida albicans cells with increased cell wall chitin have reduced echinocandin susceptibility in vitro. The aim of this study was to investigate whether C. albicans cells with elevated chitin levels have reduced echinocandin susceptibility in vivo. BALB/c mice were infected with C. albicans cells with normal chitin levels and compared to mice infected with high-chitin cells. Caspofungin therapy was initiated at 24 h postinfection. Mice infected with chitin-normal cells were successfully treated with caspofungin, as indicated by reduced kidney fungal burdens, reduced weight loss, and decreased C. albicans density in kidney lesions. In contrast, mice infected with high-chitin C. albicans cells were less susceptible to caspofungin, as they had higher kidney fungal burdens and greater weight loss during early infection. Cells recovered from mouse kidneys at 24 h postinfection with high-chitin cells had 1.6-fold higher chitin levels than cells from mice infected with chitin-normal cells and maintained a significantly reduced susceptibility to caspofungin when tested in vitro. At 48 h postinfection, caspofungin treatment induced a further increase in chitin content of C. albicans cells harvested from kidneys compared to saline treatment. Some of the recovered clones had acquired, at a low frequency, a point mutation in FKS1 resulting in a S645Y amino acid substitution, a mutation known to confer echinocandin resistance. This occurred even in cells that had not been exposed to caspofungin. Our results suggest that the efficacy of caspofungin against C. albicans was reduced in vivo due to either elevation of chitin levels in the cell wall or acquisition of FKS1 point mutations.

Current perspectives on echinocandin class drugs

It has been nearly a decade since caspofungin was approved for clinical use as the first echinocandin class antifungal agent, followed by micafungin and anidulafungin. The echinocandin drugs target the fungal cell wall by inhibiting the synthesis of β-1,3-D-glucan, a critical cell wall component of many pathogenic fungi. They are fungicidal for Candida spp. and fungistatic for moulds, such as Aspergillus fumigatus, where they induce abnormal morphology and growth properties. The echinocandins have a limited antifungal spectrum but are highly active against most Candida spp., including azole-resistant strains and biofilms. As they target glucan synthase, an enzyme absent in mammalian cells, the echinocandins have a favorable safety profile. They show potent MIC and epidemiological cutoff values against susceptible Candida and Aspergillus isolates, and the frequency of resistance is low. When clinical breakthrough occurs, it is associated with high MIC values and mutations in Fks subunits of glucan synthase, which can reduce the sensitivity of the enzyme to the drug by several thousand-fold. Such strains were not adequately captured by an early clinical breakpoint for susceptibility prompting a revised lower value, which addresses the FKS resistance mechanism and new pharmacokinetic/pharmacodynamic studies. Elevated MIC values unlinked to therapeutic failure can occur and result from adaptive cell behavior, which is FKS-independent and involves the molecular chaperone Hsp90 and the calcineurin pathway. Mutations in FKS1 and/or FKS2 alter the kinetic properties of glucan synthase, which reduces the relative fitness of mutant strains causing them to be less pathogenic. The echinocandin drugs also modify the cell wall architecture exposing buried glucans, which in turn induce a variety of important host immune responses. Finally, the future for glucan synthase inhibitors looks bright with the development of new orally active compounds.

Fitness and virulence costs of Candida albicans FKS1 hot spot mutations associated with echinocandin resistance

The identification of FKS1 mutations in Candida albicans associated with echinocandin resistance has raised concerns over the spread of drug-resistant strains. We studied the impact of fks1 mutations on C. albicans virulence and fitness. Compared with wild-type strains for FKS1, echinocandin-resistant C. albicans strains with homozygous fks1 hot-spot mutations had reduced maximum catalytic capacity of their glucan synthase complexes and thicker cell walls attributable to increased cell wall chitin content. The fks1 mutants with the highest chitin contents had reduced growth rates and impaired filamentation capacities. Fks1 mutants were hypovirulent in fly and mouse models of candidiasis, and this phenotype correlated with the cell wall chitin content. In addition, we observed reduced fitness of echinocandin-resistant C. albicans in competitive mixed infection models. We conclude that fks1 mutations that confer echinocandin resistance come at fitness and virulence costs, which may limit their epidemiological and clinical impact.

Fungal recognition is mediated by the association of dectin-1 and galectin-3 in macrophages

Dectin-1, the major β-glucan receptor in leukocytes, triggers an effective immune response upon fungal recognition. Here we use sortase-mediated transpeptidation, a technique that allows placement of a variety of probes on a polypeptide backbone, to monitor the behavior of labeled functional dectin-1 in live cells with and without fungal challenge. Installation of probes on dectin-1 by sortagging permitted highly specific visualization of functional protein on the cell surface and its subsequent internalization upon ligand presentation. Retrieval of sortagged dectin-1 expressed in macrophages uncovered a unique interaction between dectin-1 and galectin-3 that functions in the proinflammatory response of macrophages to pathogenic fungi. When macrophages expressing dectin-1 are exposed to Candida albicans mutants with increased exposure of β-glucan, the loss of galectin-3 dramatically accentuates the failure to trigger an appropriate TNF-α response.

Hyphal induction in the human fungal pathogen Candida albicans reveals a characteristic wall protein profile

The ability of Candida albicans to switch from yeast to hyphal growth is essential for its virulence. The walls and especially the covalently attached wall proteins are involved in the primary host–pathogen interactions. Three hyphal induction methods were compared, based on fetal calf serum, the amino sugar N-acetylglucosamine (GlcNAc) and the mammalian cell culture medium Iscove’s modified Dulbecco’s medium (IMDM). GlcNAc and IMDM were preferred, allowing stable hyphal growth over a prolonged period without significant reversion to yeast growth and with high biomass yields. We employed Fourier transform-MS combined with a 15N-metabolically labelled reference culture as internal standard for relative quantification of changes in the wall proteome upon hyphal induction. A total of 21 wall proteins were quantified. Our induction methods triggered a similar response characterized by (i) a category of wall proteins showing strongly increased incorporation levels (Als3, Hwp2, Hyr1, Plb5 and Sod5), (ii) another category with strongly decreased levels (Rhd3, Sod4 and Ywp1) and (iii) a third one enriched for carbohydrate-active enzymes (including Cht2, Crh11, Mp65, Pga4, Phr1, Phr2 and Utr2) and showing only a limited response. This is, to our knowledge, the first systematic, quantitative analysis of the changes in the wall proteome of C. albicans upon hyphal induction. Finally, we propose new wall-protein-derived candidates for vaccine development.

Candida albicans infection of Caenorhabditis elegans induces antifungal immune defenses

Candida albicans yeast cells are found in the intestine of most humans, yet this opportunist can invade host tissues and cause life-threatening infections in susceptible individuals. To better understand the host factors that underlie susceptibility to candidiasis, we developed a new model to study antifungal innate immunity. We demonstrate that the yeast form of C. albicans establishes an intestinal infection in Caenorhabditis elegans, whereas heat-killed yeast are avirulent. Genome-wide, transcription-profiling analysis of C. elegans infected with C. albicans yeast showed that exposure to C. albicans stimulated a rapid host response involving 313 genes (124 upregulated and 189 downregulated, ∼1.6% of the genome) many of which encode antimicrobial, secreted or detoxification proteins. Interestingly, the host genes affected by C. albicans exposure overlapped only to a small extent with the distinct transcriptional responses to the pathogenic bacteria Pseudomonas aeruginosa or Staphylococcus aureus, indicating that there is a high degree of immune specificity toward different bacterial species and C. albicans. Furthermore, genes induced by P. aeruginosa and S. aureus were strongly over-represented among the genes downregulated during C. albicans infection, suggesting that in response to fungal pathogens, nematodes selectively repress the transcription of antibacterial immune effectors. A similar phenomenon is well known in the plant immune response, but has not been described previously in metazoans. Finally, 56% of the genes induced by live C. albicans were also upregulated by heat-killed yeast. These data suggest that a large part of the transcriptional response to C. albicans is mediated through “pattern recognition,” an ancient immune surveillance mechanism able to detect conserved microbial molecules (so-called pathogen-associated molecular patterns or PAMPs). This study provides new information on the evolution and regulation of the innate immune response to divergent pathogens and demonstrates that nematodes selectively mount specific antifungal defenses at the expense of antibacterial responses.

Despite being a part of the normal flora of healthy individuals, Candida albicans is the most common fungal pathogen of humans and can cause infections that are associated with staggeringly high mortality rates. Here we devise a model for the study of the host immune response to C. albicans infection using the nematode C. elegans. We found that infection with the yeast form of C. albicans induces rapid and robust transcriptional changes in C. elegans. Analyses of these differentially regulated genes indicate that the nematode mounts antifungal defenses that are remarkably distinct from the host responses to pathogenic bacteria and that the nematode recognizes components possessed by heat-killed C. albicans to initiate this response. Interestingly, during infection with a pathogenic fungus, the nematode downregulates antibacterial immune response genes, which may reflect an evolutionary tradeoff between bacterial and fungal defense.

Regulatory circuitry governing fungal development, drug resistance, and disease

Pathogenic fungi have become a leading cause of human mortality due to the increasing frequency of fungal infections in immunocompromised populations and the limited armamentarium of clinically useful antifungal drugs. Candida albicans, Cryptococcus neoformans, and Aspergillus fumigatus are the leading causes of opportunistic fungal infections. In these diverse pathogenic fungi, complex signal transduction cascades are critical for sensing environmental changes and mediating appropriate cellular responses. For C. albicans, several environmental cues regulate a morphogenetic switch from yeast to filamentous growth, a reversible transition important for virulence. Many of the signaling cascades regulating morphogenesis are also required for cells to adapt and survive the cellular stresses imposed by antifungal drugs. Many of these signaling networks are conserved in C. neoformans and A. fumigatus, which undergo distinct morphogenetic programs during specific phases of their life cycles. Furthermore, the key mechanisms of fungal drug resistance, including alterations of the drug target, overexpression of drug efflux transporters, and alteration of cellular stress responses, are conserved between these species. This review focuses on the circuitry regulating fungal morphogenesis and drug resistance and the impact of these pathways on virulence. Although the three human-pathogenic fungi highlighted in this review are those most frequently encountered in the clinic, they represent a minute fraction of fungal diversity. Exploration of the conservation and divergence of core signal transduction pathways across C. albicans, C. neoformans, and A. fumigatus provides a foundation for the study of a broader diversity of pathogenic fungi and a platform for the development of new therapeutic strategies for fungal disease.

Cryptococcus neoformans mediator protein Ssn8 negatively regulates diverse physiological processes and is required for virulence

Cryptococcus neoformans is a ubiquitously distributed human pathogen. It is also a model system for studying fungal virulence, physiology and differentiation. Light is known to inhibit sexual development via the evolutionarily conserved white collar proteins in C. neoformans. To dissect molecular mechanisms regulating this process, we have identified the SSN8 gene whose mutation suppresses the light-dependent CWC1 overexpression phenotype. Characterization of sex-related phenotypes revealed that Ssn8 functions as a negative regulator in both heterothallic a-α mating and same-sex mating processes. In addition, Ssn8 is involved in the suppression of other physiological processes including invasive growth, and production of capsule and melanin. Interestingly, Ssn8 is also required for the maintenance of cell wall integrity and virulence. Our gene expression studies confirmed that deletion of SSN8 results in de-repression of genes involved in sexual development and melanization. Epistatic and yeast two hybrid studies suggest that C. neoformans Ssn8 plays critical roles downstream of the Cpk1 MAPK cascade and Ste12 and possibly resides at one of the major branches downstream of the Cwc complex in the light-mediated sexual development pathway. Taken together, our studies demonstrate that the conserved Mediator protein Ssn8 functions as a global regulator which negatively regulates diverse physiological and developmental processes and is required for virulence in C. neoformans.

The dectin-1/inflammasome pathway is responsible for the induction of protective T-helper 17 responses that discriminate between yeasts and hyphae of Candida albicans

In the mucosa, the immune pathways discriminating between colonizing and invasive Candida, thus inducing tolerance or inflammation, are poorly understood. Th17 responses induced by Candida albicans hyphae are central for the activation of mucosal antifungal immunity. An essential step for the discrimination between yeasts and hyphae and induction of Th17 responses is the activation of the inflammasome by C. albicans hyphae and the subsequent release of active IL-1β in macrophages. Inflammasome activation in macrophages results from differences in cell-wall architecture between yeasts and hyphae and is partly mediated by the dectin-1/Syk pathway. These results define the dectin-1/inflammasome pathway as the mechanism that enables the host immune system to mount a protective Th17 response and distinguish between colonization and tissue invasion by C. albicans.

Caspofungin dose escalation for invasive candidiasis due to resistant Candida albicans

Previous in vivo studies have reported caspofungin dose escalation to be effective against Candida glabrata with reduced susceptibility. We hypothesized that higher doses of caspofungin would be effective against invasive candidiasis caused by the more virulent species Candida albicans, including isolates resistant to this echinocandin. Immunocompetent mice were inoculated with one of three C. albicans isolates, including one susceptible and two resistant isolates with different FKS1 hot spot 1 point mutations. Mice received daily caspofungin treatment for 7 days and were then followed off therapy for 2 weeks to assess survival. Kidney tissue and blood were collected, and fungal burden and serum (1 → 3)-β-D-glucan were measured. Significant differences in virulence were observed among the three C. albicans isolates, which translated into differences in responses to caspofungin. The most virulent of the resistant isolates studied (isolate 43001; Fks1p F641S) did not respond to caspofungin doses of up to 10 mg/kg of body weight, as there were no differences in survival (survival range, 0 to 12% with treatment), tissue burden, or (1 → 3)-β-D-glucan concentration compared to those for untreated controls. Higher doses of caspofungin did improve survival against the second resistant isolate (53264; Fks1p S645P) that demonstrated reduced virulence (5 and 10 mg/kg; 80% survival). In contrast, caspofungin doses as low as 1 mg/kg improved survival (85 to 95%) and reduced tissue burden and (1 → 3)-β-D-glucan concentration against the susceptible isolate (ATCC 90028). These data suggest that caspofungin dose escalation for invasive candidiasis may not be consistently effective against resistant C. albicans isolates, and this may be associated with the virulence of the strain.

Comparative in vivo dose-dependent activity of caspofungin and anidulafungin against echinocandin-susceptible and -resistant Aspergillus fumigatus

BACKGROUND

Echinocandin resistance in Aspergillus species is rare. We examined if mutations in FKS1 would result in a complete loss of echinocandin activity in vivo in an experimental model of aspergillosis.

METHODS

Neutropenic mice were infected with either an echinocandin-susceptible Aspergillus fumigatus (AF 293) or an echinocandin-resistant A. fumigatus laboratory strain harbouring 'hot-spot' substitution in Fks1p (AF Ser678Pro). Mice then received daily treatment with either anidulafungin or caspofungin at varying dosages (0.25-16 mg/kg/day) for 5 days and Aspergillus lung fungal burden was assessed by quantitative real-time PCR.

RESULTS

Both strains produced histological evidence of progressive invasive pulmonary aspergillosis, but AF Ser678Pro was less virulent than AF 293, as evidenced by lower lung fungal burden and longer median survival time. At > 0.5 mg/kg/day, both anidulafungin and caspofungin reduced the lung fungal burden in neutropenic animals infected with AF 293, but had mixed efficacy against the resistant AF Ser678Pro strain. For caspofungin, the fungal burden was reduced only at doses <1 mg/kg/day. Anidulafungin also modestly reduced AF Ser678Pro lung fungal burden, but only at > 4 mg/kg/day.

CONCLUSIONS

Despite a lack of appreciable antifungal activity in vitro, both anidulafungin and caspofungin were still modestly effective in vivo against a laboratory-generated A. fumigatus mutant harbouring the Ser678Pro mutation in Fks1p. This persistent activity, combined with impaired fitness of the isolate in vivo, could partially explain why microbiologically documented echinocandin-resistance in Aspergillus species remains a rare clinical occurrence.

Antifungal prophylaxis and therapy in patients with hematological malignancies and hematopoietic stem cell transplant recipients

Patients with acute leukemia and hematopoietic stem cell transplant recipients are at risk of a spectrum of invasive fungal diseases corresponding to the type and intensity of immunosuppression. The development of newer antifungal agents has broadened therapeutic options. In the 1990s, lipid formulations of amphotericin B became widely used as safer alternatives to amphotericin B deoxycholate. In addition, fluconazole was shown to be beneficial as a yeast-active prophylaxis in hematopoietic stem cell transplant recipients. In the past decade, the antifungal armamentarium was further enhanced with the availability of extended-spectrum azoles and echinocandins. The development of effective broad-spectrum antifungal agents has led to their use as prophylaxis rather than delaying treatment until clinical signs of infection manifest. Antigen-based and PCR-based diagnostic adjuncts facilitate earlier detection of invasive fungal diseases compared with conventional culture, and have been incorporated into strategies in which initiation or modification of an antifungal regimen is targeted to patients with the highest likelihood of having fungal disease. Here, we review the pharmacological data and major clinical trials that guide the use of antifungals, as well as areas of uncertainty and future perspectives.

Human antimicrobial peptide LL-37 inhibits adhesion of Candida albicans by interacting with yeast cell-wall carbohydrates

Candida albicans is the major fungal pathogen of humans. Fungal adhesion to host cells is the first step of mucosal infiltration. Antimicrobial peptides play important roles in the initial mucosal defense against C. albicans infection. LL-37 is the only member of the human cathelicidin family of antimicrobial peptides and is commonly expressed in various tissues and cells, including epithelial cells of both the oral cavity and urogenital tract. We found that, at sufficiently low concentrations that do not kill the fungus, LL-37 was still able to reduce C. albicans infectivity by inhibiting C. albicans adhesion to plastic surfaces, oral epidermoid OECM-1 cells, and urinary bladders of female BALB/c mice. Moreover, LL-37-treated C. albicans floating cells that did not adhere to the underlying substratum aggregated as a consequence of LL-37 bound to the cell surfaces. According to the results of a competition assay, the inhibitory effects of LL-37 on cell adhesion and aggregation were mediated by its preferential binding to mannan, the main component of the C. albicans cell wall, and partially by its ability to bind chitin or glucan, which underlie the mannan layer. Therefore, targeting of cell-wall carbohydrates by LL-37 provides a new strategy to prevent C. albicans infection, and LL-37 is a useful, new tool to screen for other C. albicans components involved in adhesion.

Recognition and blocking of innate immunity cells by Candida albicans chitin

Chitin is a skeletal cell wall polysaccharide of the inner cell wall of fungal pathogens. As yet, little about its role during fungus-host immune cell interactions is known. We show here that ultrapurified chitin from Candida albicans cell walls did not stimulate cytokine production directly but blocked the recognition of C. albicans by human peripheral blood mononuclear cells (PBMCs) and murine macrophages, leading to significant reductions in cytokine production. Chitin did not affect the induction of cytokines stimulated by bacterial cells or lipopolysaccharide (LPS), indicating that blocking was not due to steric masking of specific receptors. Toll-like receptor 2 (TLR2), TLR4, and Mincle (the macrophage-inducible C-type lectin) were not required for interactions with chitin. Dectin-1 was required for immune blocking but did not bind chitin directly. Cytokine stimulation was significantly reduced upon stimulation of PBMCs with heat-killed chitin-deficient C. albicans cells but not with live cells. Therefore, chitin is normally not exposed to cells of the innate immune system but is capable of influencing immune recognition by blocking dectin-1-mediated engagement with fungal cell walls.

The tetraspanin CD82 is specifically recruited to fungal and bacterial phagosomes prior to acidification

CD82 is a member of the tetraspanin superfamily, whose physiological role is best described in the context of cancer metastasis. However, CD82 also associates with components of the class II major histocompatibility complex (MHC) antigen presentation pathway, including class II MHC molecules and the peptide-loading machinery, as well as CD63, another tetraspanin, suggesting a role for CD82 in antigen presentation. Here, we observe the dynamic rearrangement of CD82 after pathogen uptake by imaging CD82-mRFP1 expressed in primary living dendritic cells. CD82 showed rapid and specific recruitment to Cryptococcus neoformans-containing phagosomes compared to polystyrene-containing phagosomes, similar to CD63. CD82 was also actively recruited to phagosomes containing other pathogenic fungi, including Candida albicans and Aspergillus fumigatus. Recruitment of CD82 to fungal phagosomes occurred independently of Toll-like receptor (TLR) signaling. Recruitment was not limited to fungi, as bacterial organisms, including Escherichia coli and Staphylococcus aureus, also induced CD82 recruitment to the phagosome. CD82 intersected the endocytic pathway used by lipopolysaccharide (LPS), implicating CD82 in trafficking of small, pathogen-associated molecules. Despite its partial overlap with lysosomal compartments, CD82 recruitment to C. neoformans-containing phagosomes occurred independently of phagosome acidification. Kinetic analysis of fluorescence imaging revealed that CD82 and class II MHC simultaneously appear in the phagosome, indicating that the two proteins may be associated. Together, these data show that the CD82 tetraspanin is specifically recruited to pathogen-containing phagosomes prior to fusion with lysosomes.

Proteolytic cleavage of covalently linked cell wall proteins by Candida albicans Sap9 and Sap10

The cell wall of the human-pathogenic fungus Candida albicans is a robust but also dynamic structure which mediates adaptation to changing environmental conditions during infection. Sap9 and Sap10 are cell surface-associated proteases which function in C. albicans cell wall integrity and interaction with human epithelial cells and neutrophils. In this study, we have analyzed the enzymatic properties of Sap9 and Sap10 and investigated whether these proteases cleave proteins on the fungal cell surface. We show that Sap9 and Sap10, in contrast to other aspartic proteases, exhibit a near-neutral pH optimum of proteolytic activity and prefer the processing of peptides containing basic or dibasic residues. However, both proteases also cleaved at nonbasic sites, and not all tested peptides with dibasic residues were processed. By digesting isolated cell walls with Sap9 or Sap10, we identified the covalently linked cell wall proteins (CWPs) Cht2, Ywp1, Als2, Rhd3, Rbt5, Ecm33, and Pga4 as in vitro protease substrates. Proteolytic cleavage of the chitinase Cht2 and the glucan-cross-linking protein Pir1 by Sap9 was verified using hemagglutinin (HA) epitope-tagged versions of both proteins. Deletion of the SAP9 and SAP10 genes resulted in a reduction of cell-associated chitinase activity similar to that upon deletion of CHT2, suggesting a direct influence of Sap9 and Sap10 on Cht2 function. In contrast, cell surface changes elicited by SAP9 and SAP10 deletion had no major impact on the phagocytosis and killing of C. albicans by human macrophages. We propose that Sap9 and Sap10 influence distinct cell wall functions by proteolytic cleavage of covalently linked cell wall proteins.

The yeast-phase virulence requirement for α-glucan synthase differs among Histoplasma capsulatum chemotypes

Histoplasma capsulatum strains can be classified into two chemotypes based on cell wall composition. The cell wall of chemotype II yeast contains a layer of α-(1,3)-glucan that masks immunostimulatory β-(1,3)-glucans from detection by the Dectin-1 receptor on host phagocytes. This α-(1,3)-glucan cell wall component is essential for chemotype II Histoplasma virulence. In contrast, chemotype I yeast cells lack α-(1,3)-glucan in vitro, yet they remain fully virulent in vivo. Analysis of the chemotype I α-glucan synthase (AGS1) locus revealed a 2.7-kb insertion in the promoter region that diminishes AGS1 expression. Nonetheless, AGS1 mRNA can be detected during respiratory infection with chemotype I yeast, suggesting that α-(1,3)-glucan could be produced during in vivo growth despite its absence in vitro. To directly test whether AGS1 contributes to chemotype I strain virulence, we prevented AGS1 function by RNA interference and by insertional mutation. Loss of AGS1 function in chemotype I does not impair the cytotoxicity of ags1(-) mutant yeast to cultured macrophages, nor does it affect the intracellular growth of yeast. In a murine model of histoplasmosis, the ags1(-) chemotype I mutant strains show no defect in lung infection or in extrapulmonary dissemination. Together, these studies demonstrate that AGS1 expression is dispensable for chemotype I yeast virulence, in contrast to the case for chemotype II yeast. Despite the absence of cell wall α-(1,3)-glucan, chemotype I yeast can avoid detection by Dectin-1 in a growth stage-dependent manner. This suggests the production of a unique Histoplasma chemotype I factor that, at least partially, circumvents the α-(1,3)-glucan requirement for yeast virulence.

Absence of TLR2 influences survival of neutrophils after infection with Candida albicans

Candida albicans is an opportunistic pathogen, which causes local and/or disseminated diseases in immunosuppressed humans. Phagocytic cells play a critical role in the immune response against C. albicans. Toll like receptors (TLR) are important in the identification of invading microorganisms and in the regulation of neutrophil survival. TLR2 has been shown to participate in the response against pathogenic yeasts and to increase the functional life span of neutrophils. In view of these observations, we studied the involvement of TLR2 in neutrophil function after C. albicans infection. The absence of TLR2 resulted in lower chemotaxis of neutrophils to the site of infection. This in turn was associated with lower levels of chemokines from neutrophils, facilitating the dissemination of the pathogen to the lymph nodes and spleen. A high frequency of apoptotic neutrophils and macrophages in the inflammatory exudates from TLR2(-/-) mice was found. In addition, the phagocytic activity of neutrophils and macrophages, nitric oxide production and myeloperoxidase activity were diminished in cells from TLR2(-/-) mice. Together, these data demonstrate the importance of TLR2 signals for neutrophils activation and survival after C. albicans infection.

Dectin-1: a role in antifungal defense and consequences of genetic polymorphisms in humans

The clinical relevance of fungal infections has increased dramatically in recent decades as a consequence of the rise of immunocompromised populations, and efforts to understand the underlying mechanisms of protective immunity have attracted renewed interest. Here we review Dectin-1, a pattern recognition receptor involved in antifungal immunity, and discuss recent discoveries of polymorphisms in the gene encoding this receptor which result in human disease.

Deletion of the Candida albicans histidine kinase gene CHK1 improves recognition by phagocytes through an increased exposure of cell wall beta-1,3-glucans

The pathogenic fungus Candida albicans is able to cover its most potent proinflammatory cell wall molecules, the β-glucans, underneath a dense mannan layer, so that the pathogen becomes partly invisible for immune cells such as phagocytes. As the C. albicans histidine kinases Chk1p, Cos1p and CaSln1p had been reported to be involved in virulence and cell wall biosynthesis, we investigated whether deletion of the respective genes influences the activity of phagocytes against C. albicans. We found that among all histidine kinase genes, CHK1 plays a prominent role in phagocyte activation. Uptake of the deletion mutant Δchk1 as well as the acidification of Δchk1-carrying phagosomes was significantly increased compared with the parental strain. These improved activities could be correlated with an enhanced accessibility of the mutant β-1,3-glucans for immunolabelling. In addition, any inhibition of β-1,3-glucan-mediated phagocytosis resulted in a reduced uptake of Δchk1, while ingestion of the parental strain was hardly affected. Moreover, deletion of CHK1 caused an enhanced release of interleukins 6 and 10, indicating a stronger activation of the β-1,3-glucan receptor dectin-1. In conclusion, the Chk1p protein is likely to be involved in masking β-1,3-glucans from immune recognition. As there are no homologues of fungal histidine kinases in mammals, Chk1p has to be considered as a promising target for new antifungal agents.

PHR1, a pH-regulated gene of Candida albicans encoding a glucan-remodelling enzyme, is required for adhesion and invasion

The fungal cell wall plays a crucial role in host–pathogen interactions. Its formation is the result of the coordinated activity of several extracellular enzymes, which assemble the constituents, and remodel and hydrolyse them in the extracellular space. Candida albicans Phr1 and Phr2 proteins belong to family GH72 of the β-(1,3)-glucanosyltransferases and play a crucial role in cell wall assembly. PHR1 and PHR2, homologues of Saccharomyces cerevisiae GAS1, are differently regulated by extracellular pH. PHR1 is expressed when ambient pH is 5.5 or higher, whereas PHR2 has the reverse expression pattern. Their deletion causes a pH-conditional defect in morphogenesis and virulence. In this work we explored whether PHR1 deletion affects the ability of C. albicans to adhere to and invade human epithelia. PHR1 null mutants exhibited a marked reduction in adhesion to both abiotic surfaces and epithelial cell monolayers. In addition, the mutant was unable to penetrate and invade reconstituted human epithelia. Transcription profiling of selected hyphal-specific and adhesin-encoding genes indicated that in the PHR1 null mutant, HWP1 and ECE1 transcript levels were similarly reduced in both adhesion and suspension conditions. These results, combined with microscopy analysis of the septum position, suggest that PHR1 is not required for the induction of hyphal development but plays a key role in the maintenance of hyphal growth. Thus, the β-(1,3)-glucan processing catalysed by Phr1p is of fundamental importance in the maintenance of the morphological state on which the adhesive and invasive properties of C. albicans greatly depend.

Yeast flocculation and its biotechnological relevance

Adhesion properties of microorganisms are crucial for many essential biological processes such as sexual reproduction, tissue or substrate invasion, biofilm formation and others. Most, if not all microbial adhesion phenotypes are controlled by factors such as nutrient availability or the presence of pheromones. One particular form of controlled cellular adhesion that occurs in liquid environments is a process of asexual aggregation of cells which is also referred to as flocculation. This process has been the subject of significant scientific and biotechnological interest because of its relevance for many industrial fermentation processes. Specifically adjusted flocculation properties of industrial microorganisms could indeed lead to significant improvements in the processing of biotechnological fermentation products such as foods, biofuels and industrially produced peptides. This review briefly summarises our current scientific knowledge on the regulation of flocculation-related phenotypes, their importance for different biotechnological industries, and possible future applications for microorganisms with improved flocculation properties.

Synergy of caspofungin with human polymorphonuclear granulocytes for killing Candida albicans

The influence of caspofungin on polymorphonuclear leukocyte (PMN) phagocytosis and intracellular killing of Candida albicans was investigated. Caspofungin, at all of the concentrations tested (2, 3.2, and 8 microg/ml), significantly increased intracellular killing by PMNs through its direct action on both yeast cells and PMNs, indicating the potential ability of caspofungin to synergize with phagocytes for candidal killing. Caspofungin may therefore constitute an effective therapeutic option for the treatment of invasive fungal infections, including those refractory to conventional treatment with azole agents.

CARD9 mediates dectin-2-induced IkappaBalpha kinase ubiquitination leading to activation of NF-kappaB in response to stimulation by the hyphal form of Candida albicans

The scaffold protein CARD9 plays an essential role in anti-fungus immunity and is implicated in mediating Dectin-1/Syk-induced NF-kappaB activation in response to Candida albicans infection. However, the molecular mechanism by which CARD9 mediates C. albicans-induced NF-kappaB activation is not fully characterized. Here we demonstrate that CARD9 is involved in mediating NF-kappaB activation induced by the hyphal form of C. albicans hyphae (Hyphae) but not by its heat-inactivated unicellular form. Our data show that inhibiting Dectin-2 expression selectively blocked Hyphae-induced NF-kappaB, whereas inhibiting Dectin-1 mainly suppressed zymosan-induced NF-kappaB, indicating that Hyphae-induced NF-kappaB activation is mainly through Dectin-2 and not Dectin-1. Consistently, we find that the hyphae stimulation induces CARD9 association with Bcl10, an adaptor protein that functions downstream of CARD9 and is also involved in C. albicans-induced NF-kappaB activation. This association is dependent on Dectin-2 but not Dectin-1 following the hyphae stimulation. Finally, we find that although both CARD9 and Syk are required for Hyphae-induced NF-kappaB activation, they regulate different signaling events in which CARD9 mediates IkappaBalpha kinase ubiquitination, whereas Syk regulates IkappaBalpha kinase phosphorylation. Together, our data demonstrated that CARD9 is selectively involved in Dectin-2-induced NF-kappaB activation in response to C. albicans hyphae challenging.

Interactions between human phagocytes and Candida albicans biofilms alone and in combination with antifungal agents

BACKGROUND

Biofilm formation is an important component of vascular catheter infections caused by Candida albicans. Little is known about the interactions between human phagocytes, antifungal agents, and Candida biofilms.

METHODS

The interactions between C. albicans biofilms and human phagocytes alone and in combination with anidulafungin or voriconazole were investigated and compared with their corresponding planktonic counterparts by means of an in vitro biofilm model with clinical intravascular and green fluorescent protein (GFP)-expressing strains. Phagocyte-mediated and antifungal agent-mediated damages were determined by 2,3-bis[ 2- methoxy-4-nitro-5-sulfophenyl]2H-tetrazolium-5-carboxanilide assay, and structural effects were visualized by confocal microscopy. Oxidative burst was evaluated by flow cytometric measurement of dihydrorhodamine 123 oxidation, and cytokine release was measured by enzyme-linked immunosorbent assay.

RESULTS

Phagocytes alone and in combination with antifungal agents induced less damage against biofilms compared with planktonic cells. However, additive effects occurred between phagocytes and anidulafungin against Candida biofilms. Confocal microscopy demonstrated the absence of phagocytosis within biofilms but marked destruction caused by anidulafungin and phagocytes. Anidulafungin but not voriconazole elicited tumor necrosis factor alpha release from phagocytes compared with that from untreated biofilms.

CONCLUSIONS

C. albicans within biofilms are more resistant to phagocytic host defenses but are susceptible to additive effects between phagocytes and an echinocandin.

Echinocandin pharmacodynamics: review and clinical implications

Echinocandins have made a significant impact in the treatment of select invasive fungal infections, most notably invasive candidiasis and aspergillosis. However, treatment outcomes for such infections are still less than optimal, prompting an examination of dosing and administration techniques in an attempt to exploit known pharmacodynamic properties and improve outcomes. Echinocandins generally exhibit concentration-dependent, fungicidal activity against Candida spp. and fungistatic activity against Aspergillus spp. However, increasing drug concentrations of echinocandins above the organism's MIC may result in a paradoxical increase in fungal growth as demonstrated in some in vitro and in vivo models (known most commonly as the 'Eagle effect'). Therefore, the potential impact of dose escalations on improving the clinical efficacy of echinocandins based on in vitro and animal models are uncertain and are still being evaluated. In addition, such strategies have to consider the potential for increased treatment-related toxicities and costs. To date, published clinical studies (both superiority and non-inferiority) demonstrating the potential for dose-related improvements in treatment outcomes have been limited to mucocutaneous and oesophageal candidiasis. Further research is needed to determine if a role exists for optimizing echinocandin pharmacodynamics in various clinical settings.

Fungal toxins and multiple sclerosis: a compelling connection

Multiple sclerosis occurs as a consequence of central nervous system neuronal demyelination. Decades of research suggest that the primary suspects (e.g., viruses, genes, immune system) are associative rather than causative agents, but a surprisingly coherent relationship can be made between multiple sclerosis and fungal toxins. Specifically, certain pathogenic fungi sequester in non-neuronal tissue and release toxins that target and destroy CNS astrocytes and oligodendrocytes. Without these glial support cells, myelin degrades triggering the onset of multiple sclerosis and its associated symptoms. We propose here that fungal toxins are the underlying cause of multiple sclerosis and thus may offer an avenue towards an effective cure.

An extensive circuitry for cell wall regulation in Candida albicans

Protein kinases play key roles in signaling and response to changes in the external environment. The ability of Candida albicans to quickly sense and respond to changes in its environment is key to its survival in the human host. Our guiding hypothesis was that creating and screening a set of protein kinase mutant strains would reveal signaling pathways that mediate stress response in C. albicans. A library of protein kinase mutant strains was created and screened for sensitivity to a variety of stresses. For the majority of stresses tested, stress response was largely conserved between C. albicans, Saccharomyces cerevisiae, and Schizosaccharomyces pombe. However, we identified eight protein kinases whose roles in cell wall regulation (CWR) were not expected from functions of their orthologs in the model fungi Saccharomyces cerevisiae and Schizosaccharomyces pombe. Analysis of the conserved roles of these protein kinases indicates that establishment of cell polarity is critical for CWR. In addition, we found that septins, crucial to budding, are both important for surviving and are mislocalized by cell wall stress. Our study shows an expanded role for protein kinase signaling in C. albicans cell wall integrity. Our studies suggest that in some cases, this expansion represents a greater importance for certain pathways in cell wall biogenesis. In other cases, it appears that signaling pathways have been rewired for a cell wall integrity response.

Candida albicans is the major fungal commensal and pathogen of humans. Like most microorganisms, C. albicans is surrounded and protected by a cell wall. This cell wall has two purposes: to act as a rigid “exoskeleton” to prevent cells from bursting, and to provide a surface where a cell can interact with the outside environment while protecting the cell itself from this environment. Maintenance of this structure has been well studied in the model fungus, Saccharomyces cerevisiae, but previous evidence suggested that C. albicans might have additional inputs to this process. By creating and testing a library of mutant strains for sensitivity to cell wall stress, we were able to identify a number of conserved genes with roles in this process not shared by their S. cerevisiae counterparts. Although some of these genes had previously been linked to cell wall integrity, it appears that they have increased impact on this process in C. albicans. For other genes, their role in cell wall integrity may represent a novel connection. Our findings provide insight into novel aspects of cell wall integrity in this pathogen and lay a foundation for its more detailed analysis.

Phosphatidylserine synthase and phosphatidylserine decarboxylase are essential for cell wall integrity and virulence in Candida albicans

Phospholipid biosynthetic pathways play crucial roles in the virulence of several pathogens; however, little is known about how phospholipid synthesis affects pathogenesis in fungi such as Candida albicans. A C. albicans phosphatidylserine (PS) synthase mutant, cho1 Delta/Delta, lacks PS, has decreased phosphatidylethanolamine (PE), and is avirulent in a mouse model of systemic candidiasis. The cho1 Delta/Delta mutant exhibits defects in cell wall integrity, mitochondrial function, filamentous growth, and is auxotrophic for ethanolamine. PS is a precursor for de novo PE biosynthesis. A psd1 Delta/Delta psd2 Delta/Delta double mutant, which lacks the PS decarboxylase enzymes that convert PS to PE in the de novo pathway, has diminished PE levels like those of the cho1 Delta/Delta mutant. The psd1 Delta/Delta psd2 Delta/Delta mutant exhibits phenotypes similar to those of the cho1 Delta/Delta mutant; however, it is slightly more virulent and has less of a cell wall defect. The virulence losses exhibited by the cho1 Delta/Delta and psd1 Delta/Delta psd2 Delta/Delta mutants appear to be related to their cell wall defects which are due to loss of de novo PE biosynthesis, but are exacerbated by loss of PS itself. Cho1p is conserved in fungi, but not mammals, so fungal PS synthase is a potential novel antifungal drug target.

Fungal echinocandin resistance

The echinocandins are the newest class of antifungal agents in the clinical armory. These secondary metabolites are non-competitive inhibitors of the synthesis of beta-(1,3)-glucan, a major structural component of the fungal cell wall. Recent work has shown that spontaneous mutations can arise in two hot spot regions of Fks1 the target protein of echinocandins that reduce the enzyme's sensitivity to the drug. However, other strains have been isolated in which the sequence of FKS1 is unaltered yet the fungus has decreased sensitivity to echinocandins. In addition it has been shown that echinocandin-treatment can induce cell wall salvage mechanisms that result in the compensatory upregulation of chitin synthesis in the cell wall. This salvage mechanism strengthens cell walls damaged by exposure to echinocandins. Therefore, fungal resistance to echinocandins can arise due to the selection of either stable mutational or reversible physiological alterations that decrease susceptibility to these antifungal agents.

Contribution of Candida albicans cell wall components to recognition by and escape from murine macrophages

The pathogenicity of the opportunistic human fungal pathogen Candida albicans depends on its ability to escape destruction by the host immune system. Using mutant strains that are defective in cell surface glycosylation, cell wall protein synthesis, and yeast-hypha morphogenesis, we have investigated three important aspects of C. albicans innate immune interactions: phagocytosis by primary macrophages and macrophage cell lines, hyphal formation within macrophage phagosomes, and the ability to escape from and kill macrophages. We show that cell wall glycosylation is critically important for the recognition and ingestion of C. albicans by macrophages. Phagocytosis was significantly reduced for mutants deficient in phosphomannan biosynthesis (mmn4Delta, pmr1Delta, and mnt3 mnt5Delta), whereas O- and N-linked mannan defects (mnt1Delta mnt2Delta and mns1Delta) were associated with increased ingestion, compared to the parent wild-type strains and genetically complemented controls. In contrast, macrophage uptake of mutants deficient in cell wall proteins such as adhesins (ece1Delta, hwp1Delta, and als3Delta) and yeast-locked mutants (clb2Delta, hgc1Delta, cph1Delta, efg1Delta, and efg1Delta cph1Delta), was similar to that observed for wild-type C. albicans. Killing of macrophages was abrogated in hypha-deficient strains, significantly reduced in all glycosylation mutants, and comparable to wild type in cell wall protein mutants. The diminished ability of glycosylation mutants to kill macrophages was not a consequence of impaired hyphal formation within macrophage phagosomes. Therefore, cell wall composition and the ability to undergo yeast-hypha morphogenesis are critical determinants of the macrophage's ability to ingest and process C. albicans.

Echinocandin treatment of pneumocystis pneumonia in rodent models depletes cysts leaving trophic burdens that cannot transmit the infection

Fungi in the genus Pneumocystis cause pneumonia (PCP) in hosts with debilitated immune systems and are emerging as co-morbidity factors associated with chronic diseases such as COPD. Limited therapeutic choices and poor understanding of the life cycle are a result of the inability of these fungi to grow outside the mammalian lung. Within the alveolar lumen, Pneumocystis spp., appear to have a bi-phasic life cycle consisting of an asexual phase characterized by binary fission of trophic forms and a sexual cycle resulting in formation of cysts, but the life cycle stage that transmits the infection is not known. The cysts, but not the trophic forms, express β -1,3-D-glucan synthetase and contain abundant β -1,3-D-glucan. Here we show that therapeutic and prophylactic treatment of PCP with echinocandins, compounds which inhibit the synthesis of β -1,3-D-glucan, depleted cysts in rodent models of PCP, while sparing the trophic forms which remained in significant numbers. Survival was enhanced in the echincandin treated mice, likely due to the decreased β -1,3-D-glucan content in the lungs of treated mice and rats which coincided with reductions of cyst numbers, and dramatic remodeling of organism morphology. Strong evidence for the cyst as the agent of transmission was provided by the failure of anidulafungin-treated mice to transmit the infection. We show for the first time that withdrawal of anidulafungin treatment with continued immunosuppression permitted the repopulation of cyst forms. Treatment of PCP with an echinocandin alone will not likely result in eradication of infection and cessation of echinocandin treatment while the patient remains immunosuppressed could result in relapse. Importantly, the echinocandins provide novel and powerful chemical tools to probe the still poorly understood bi-phasic life cycle of this genus of fungal pathogens.

Protein glycosylation in Candida

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

Candida albicans beta-glucan exposure is controlled by the fungal CEK1-mediated mitogen-activated protein kinase pathway that modulates immune responses triggered through dectin-1

Innate immunity to Candida albicans depends upon the recognition of molecular patterns on the fungal cell wall. However, the masking of major components such as beta-glucan seems to be a mechanism that fungi have evolved to avoid immune cell recognition through the dectin-1 receptor. Although the role of C. albicans mitogen-activated protein kinase (MAPK) pathways as virulence determinants has been established previously with animal models, the mechanism involved in this behavior is largely unknown. In this study we demonstrate that a disruption of the C. albicans extracellular signal-regulated kinase (ERK)-like 1 (CEK1)-mediated MAPK pathway causes enhanced cell wall beta-glucan exposure, triggering immune responses more efficiently than the wild type, as measured by dectin-1-mediated specific binding and human dendritic cell (hDC)- and macrophage-mediated phagocytosis, killing, and activation of intracellular signaling pathways. At the molecular level, the disruption of CEK1 resulted in altered spleen tyrosine kinase (Syk), Raf-1, and ERK1/2 activations together with IkappaB degradation on hDCs and increased dectin-1-dependent activator protein 1 (AP-1) activation on transfected cells. In addition, concurring with these altered pathways, we detected increased reactive oxygen species production and cytokine secretion. In conclusion, the CEK1-mediated MAPK pathway is involved in beta-glucan exposure in a fungal pathogen, hence influencing dectin-1-dependent immune cell recognition, thus establishing this fungal intracellular signaling route as a promising novel therapeutic target.

Influence of mannan and glucan on complement activation and C3 binding by Candida albicans

The complement system is important for host resistance to hematogenously disseminated candidiasis. However, modulation of complement activation by cell wall components of Candida albicans has not been characterized. Although intact yeast display mannan on the surface, glucan, typically located in the interior, becomes exposed during C. albicans infection. We show here the distinct effects of mannan and glucan on complement activation and opsonophagocytosis. Previous studies showed that intact cells are resistant to initiation of complement activation through the alternative pathway, and antimannan antibody reverses this resistance via an Fc-independent mechanism. The present study shows that this mannan-dependent resistance can be overcome by periodate-borohydride conversion of mannose polysaccharides to polyalcohols; cells treated with periodate-borohydride initiate the alternative pathway without the need for antibody. These observations identify an inhibitory role for intact mannan in complement activation. Next, removal of the surface-displayed mannan by acid treatment of periodate-borohydride cells exposes glucan. Glucan-displaying cells or purified beta-glucan initiate the alternative pathway when incubated with the purified proteins of the alternative pathway alone, suggesting that C. albicans glucan is a natural activator of the alternative pathway. Finally, ingestion of mannan-displaying cells by human neutrophils requires anti-mannan antibody, whereas ingestion of glucan-displaying cells requires complement. These results demonstrate a contrasting requirement of natural antibody and complement for opsonophagocytosis of C. albicans cells displaying mannan or glucan. Thus, differential surface expression of mannan and glucan may influence recognition of C. albicans by the complement system.

Treatment of invasive candidiasis with echinocandins

Blood stream infections by Candida spp. represent the majority of invasive fungal infections in intensive care patients. The high crude mortality of invasive candidiasis remained essentially unchanged during the last two decades despite new treatment options that became available. The echinocandins, the latest class of antifungals introduced since 2001, exhibit potent activity against clinically relevant fungi including most Candida spp. In several randomised multicentre phase III trials, anidulafungin, caspofungin and micafungin showed convincing efficacy when compared with standard treatment regimens. In all trials, echinocandins were at least non-inferior to standard treatments. Anidulafungin was shown to be superior to fluconazole. Echinocandins have a favourable tolerability profile and exhibit a minimal potential for drug interactions since their pharmacokinetics is independent of renal and--largely--hepatic function. As a result of these properties, echinocandins are appropriate drugs of choice for invasive candidiasis in intensive care where many patients experience organ failure and receive multiple drugs with complex interactions.

Characterization of mucosal Candida albicans biofilms

C. albicans triggers recurrent infections of the alimentary tract mucosa that result from biofilm growth. Although the ability of C. albicans to form a biofilm on abiotic surfaces has been well documented in recent years, no information exists on biofilms that form directly on mucosal surfaces. The objectives of this study were to characterize the structure and composition of Candida biofilms forming on the oral mucosa. We found that oral Candida biofilms consist of yeast, hyphae, and commensal bacteria, with keratin dispersed in the intercellular spaces. Neutrophils migrate through the oral mucosa and form nests within the biofilm mass. The cell wall polysaccharide β-glucan is exposed during mucosal biofilm growth and is more uniformly present on the surface of biofilm organisms invading the oral mucosa. We conclude that C. albicans forms complex mucosal biofilms consisting of both commensal bacterial flora and host components. These discoveries are important since they can prompt a shift of focus for current research in investigating the role of Candida-bacterial interactions in the pathogenesis of mucosal infections as well as the role of β-glucan mediated signaling in the host response.