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

Applications of high content screening in life science research

Over the last decade, imaging as a detection mode for cell based assays has opened a new world of opportunities to measure "phenotypic endpoints" in both current and developing biological models. These "high content" methods combine multiple measurements of cell physiology, whether it comes from sub-cellular compartments, multicellular structures, or model organisms. The resulting multifaceted data can be used to derive new insights into complex phenomena from cell differentiation to compound pharmacology and toxicity. Exploring the major application areas through review of the growing compendium of literature provides evidence that this technology is having a tangible impact on drug discovery and the life sciences.

Beta-glucan recognition by the innate immune system

Beta-glucans are recognized by the innate immune system. This recognition plays important roles in host defense and presents specific opportunities for clinical modulation of the host immune response. Neutrophils, macrophages, and dendritic cells among others express several receptors capable of recognizing beta-glucan in its various forms. This review explores what is currently known about beta-glucan recognition and how this recognition stimulates immune responses. Special emphasis is placed on Dectin-1, as we know the most about how this key beta-glucan receptor translates recognition into intracellular signaling, stimulates cellular responses, and participates in orchestrating the adaptive immune response.

Candida albicans Cas5, a regulator of cell wall integrity, is required for virulence in murine and toll mutant fly models

Candida albicans is the most common human fungal pathogen, yet the pathogenesis of C. albicans infection remains incompletely understood. We hypothesized that C. albicans has developed evolutionarily conserved mechanisms to invade disparate hosts and tested whether Toll mutant flies could serve as a model host for high-throughput screening of C. albicans virulence genes. We screened 34 C. albicans mutants defective in putative transcription factor genes (see http://www.tigr.org/tigr-scripts/e2k1/qzhao/page.cgi?num=1 ) by means of a previously established model of invasive candidiasis in Toll mutant flies. C. albicans mutants that displayed attenuated virulence in flies were subsequently tested for virulence in a mouse model of hematogenous candidiasis. Of the 34 C. albicans mutants tested, only the prototrophic cas5Delta/Delta mutant (strain VIC1186) exhibited attenuated virulence in Toll mutant flies that was restored in the complemented strain (VIC1190). Similarly, BALB/c mice infected intravenously with the cas5Delta/Delta mutant had significantly better survival and a lower fungal burden in kidneys and spleen than did those infected with the isogenic wild-type strain DAY185. CAS5 encodes a key transcriptional regulator of genes involved in cell wall integrity and lacks an orthologue in Saccharomyces cerevisiae. Our findings support the notion that Drosophila melanogaster is a promising model for large-scale studies of genes involved in the pathogenesis of C. albicans infection in mammals.

BAR domain proteins Rvs161 and Rvs167 contribute to Candida albicans endocytosis, morphogenesis, and virulence

The Candida albicans plasma membrane plays critical roles in growth and virulence and as a target for antifungal drugs. Three C. albicans genes that encode Bin-Amphiphysin-Rvs homology domain proteins were mutated to define their roles in plasma membrane function. The deletion of RVS161 and RVS167, but not RVS162, caused strong defects. The rvs161Delta mutant was more defective in endocytosis and morphogenesis than rvs167Delta, but both were strongly defective in polarizing actin patches. Other plasma membrane constituents were still properly localized, including a filipin-stained domain at the hyphal tips. An analysis of growth under different in vitro conditions showed that the rvs161Delta and rvs167Delta mutants grew less invasively in agar and also suggested that they have defects in cell wall synthesis and Rim101 pathway signaling. These mutants were also more resistant to the antimicrobial peptide histatin 5 but showed essentially normal responses to the drugs caspofungin and amphotericin. Surprisingly, the rvs161Delta mutant was more sensitive to fluconazole, whereas the rvs167Delta mutant was more resistant, indicating that these mutations cause overlapping but distinct effects on cells. The rvs161Delta and rvs167Delta mutants both showed greatly reduced virulence in mice. However, the mutants were capable of growing to high levels in kidneys. Histological analyses of infected kidneys revealed that these rvsDelta mutants grew in a large fungal mass that was walled off by leukocytes, rather than forming disseminated microabscesses as seen for the wild type. The diminished virulence is likely due to a combination of the morphogenesis defects that reduce invasive growth and altered cell wall construction that exposes proinflammatory components to the host immune system.

Dynamics of cell wall components of Magnaporthe grisea during infectious structure development

Oligosaccharides derived from cell wall of fungal pathogens induce host primary immune responses. To understand fungal strategies circumventing the host plant immune responses, cell wall polysaccharide localization was investigated using fluorescent labels during infectious structure differentiation in the rice blast fungus Magnaporthe grisea. alpha-1,3-glucan was labelled only on appressoria developing on plastic surfaces, whereas it was detected on both germ tubes and appressoria on plant surfaces. Chitin, chitosan and beta-1,3-glucan were detected on germ tubes and appressoria regardless of the substrate. Major polysaccharides labelled at accessible surface of infectious hyphae were alpha-1,3-glucan and chitosan, but after enzymatic digestion of alpha-1,3-glucan, beta-1,3-glucan and chitin became detectable. Immunoelectron microscopic analysis showed alpha-1,3-glucan and beta-1,3-glucan intermixed in the cell wall of infectious hyphae; however, alpha-1,3-glucan tended to be distributed farther from the fungal cell membrane. The fungal cell wall became more tolerant to chitinase digestion upon accumulation of alpha-1,3-glucan. Accumulation of alpha-1,3-glucan was dependent on the Mps1 MAP kinase pathway, which was activated by a plant wax derivative, 1,16-hexadecanediol. Taken together, alpha-1,3-glucan spatially and functionally masks beta-1,3-glucan and chitin in the cell wall of infectious hyphae. Thus, a dynamic change of composition of cell wall polysaccharides occurs during plant infection in M. grisea.

Defining responses to therapy and study outcomes in clinical trials of invasive fungal diseases: Mycoses Study Group and European Organization for Research and Treatment of Cancer consensus criteria

Invasive fungal diseases (IFDs) have become major causes of morbidity and mortality among highly immunocompromised patients. Authoritative consensus criteria to diagnose IFD have been useful in establishing eligibility criteria for antifungal trials. There is an important need for generation of consensus definitions of outcomes of IFD that will form a standard for evaluating treatment success and failure in clinical trials. Therefore, an expert international panel consisting of the Mycoses Study Group and the European Organization for Research and Treatment of Cancer was convened to propose guidelines for assessing treatment responses in clinical trials of IFDs and for defining study outcomes. Major fungal diseases that are discussed include invasive disease due to Candida species, Aspergillus species and other molds, Cryptococcus neoformans, Histoplasma capsulatum, and Coccidioides immitis. We also discuss potential pitfalls in assessing outcome, such as conflicting clinical, radiological, and/or mycological data and gaps in knowledge.

Protection by anti-beta-glucan antibodies is associated with restricted beta-1,3 glucan binding specificity and inhibition of fungal growth and adherence

Anti-β-glucan antibodies elicited by a laminarin-conjugate vaccine confer cross-protection to mice challenged with major fungal pathogens such as Candida albicans, Aspergillus fumigatus and Cryptococcus neoformans. To gain insights into protective β-glucan epitope(s) and protection mechanisms, we studied two anti-β-glucan monoclonal antibodies (mAb) with identical complementarity-determining regions but different isotypes (mAb 2G8, IgG2b and mAb 1E12, IgM). C. albicans, the most relevant fungal pathogen for humans, was used as a model.

Both mAbs bound to fungal cell surface and to the β1,3-β1,6 glucan of the fungal cell wall skeleton, as shown by immunofluorescence, electron-microscopy and ELISA. They were also equally unable to opsonize fungal cells in a J774 macrophage phagocytosis and killing assay. However, only the IgG2b conferred substantial protection against mucosal and systemic candidiasis in passive vaccination experiments in rodents. Competition ELISA and microarray analyses using sequence-defined glucan oligosaccharides showed that the protective IgG2b selectively bound to β1,3-linked (laminarin-like) glucose sequences whereas the non-protective IgM bound to β1,6- and β1,4-linked glucose sequences in addition to β1,3-linked ones. Only the protective IgG2b recognized heterogeneous, polydisperse high molecular weight cell wall and secretory components of the fungus, two of which were identified as the GPI-anchored cell wall proteins Als3 and Hyr1. In addition, only the IgG2b inhibited in vitro two critical virulence attributes of the fungus, hyphal growth and adherence to human epithelial cells.

Our study demonstrates that the isotype of anti-β-glucan antibodies may affect details of the β-glucan epitopes recognized, and this may be associated with a differing ability to inhibit virulence attributes of the fungus and confer protection in vivo. Our data also suggest that the anti-virulence properties of the IgG2b mAb may be linked to its capacity to recognize β-glucan epitope(s) on some cell wall components that exert critical functions in fungal cell wall structure and adherence to host cells.

Anidulafungin is fungicidal and exerts a variety of postantifungal effects against Candida albicans, C. glabrata, C. parapsilosis, and C. krusei isolates

Anidulafungin targets the cell walls of Candida species by inhibiting beta-1,3-glucan synthase, thereby killing isolates and exerting prolonged postantifungal effects (PAFEs). We performed time-kill and PAFE experiments on Candida albicans (n = 4), C. glabrata (n = 3), C. parapsilosis (n = 3), and C. krusei (n = 2) isolates and characterized the PAFEs in greater detail. MICs were 0.008 to 0.125 microg/ml against C. albicans, C. glabrata, and C. krusei and 1.0 to 2.0 microg/ml against C. parapsilosis. During time-kill experiments, anidulafungin caused significant kills at 16x MIC (range, log 2.68 to 3.89) and 4x MIC (log 1.87 to 3.19), achieving fungicidal levels (>or=log 3) against nine isolates. A 1-hour drug exposure during PAFE experiments resulted in kills ranging from log 1.55 to 3.47 and log 1.18 to 2.89 (16x and 4x MIC, respectively), achieving fungicidal levels against four isolates. Regrowth of all 12 isolates was inhibited for >or=12 h after drug washout. Isolates of each species collected 8 h after a 1-hour exposure to anidulafungin (16x and 4x MIC) were hypersusceptible to sodium dodecyl sulfate (0.01 to 0.04%) and calcofluor white (40 microg/ml). Moreover, PAFEs were associated with major cell wall disturbances, as evident in electron micrographs of viable cells, and significant reductions in adherence to buccal epithelial cells (P <or= 0.01). Finally, three of four PAFE isolates tested were hypersusceptible to killing by J774 macrophages (P <or= 0.007). Our data suggest that the efficacy of anidulafungin in the treatment of candidiasis might stem from both direct fungicidal activity and indirect PAFEs that lessen the ability of Candida cells to establish invasive disease and to persist within infected hosts.

Inducing efficient cross-priming using antigen-coated yeast particles

Saccharomyces cerevisiae stimulates dendritic cells (DCs) and represents a promising candidate for cancer vaccine development. Effective cross-presentation of antigen delivered to DCs is necessary for successful induction of cellular immunity. Here, we present a yeast-based vaccine approach that is independent of yeast's ability to express the chosen antigen, which is instead produced separately and conjugated to the yeast cell wall. The conjugation method is site-specific (based on the SNAP-tag) and designed to facilitate antigen release in the DC phagosome and subsequent translocation for cross-presentation. We demonstrate that nonsite-specific chemical conjugation of the same protein hinders cross-presentation. Phagosomal antigen release was further expedited through the insertion of the invariant chain ectodomain as a linker, which is rapidly cleaved by Cathepsin S. The dose of delivered antigen was increased in several ways: by using yeast strains with higher surface amine densities, by using yeast hulls (cell wall fragments) instead of whole cells, and by conjugating multiple layers of antigen. The novel multilayer conjugation scheme takes advantage of Sfp phosphopantetheinyl transferase and remains site-specific; it enables the antigen dose to grow linearly with the number of layers. We show that whole yeast cells coated with 1 layer of the cancer-testis antigen NY-ESO-1 and yeast hulls bearing 3 layers were able to cross-prime naive CD8 T cells in vitro, with the latter resulting in higher frequencies of antigen-specific cells after 10 days. This cross-presentation-efficient antigen conjugation scheme is not limited to yeast and can readily be applied toward the development of other particulate vaccines.

Cell mediated immunity to fungi: a reassessment

Protective immunity against fungal pathogens is achieved by the integration of two distinct arms of the immune system, the innate and adaptive responses. Innate and adaptive immune responses are intimately linked and controlled by sets of molecules and receptors that act to generate the most effective form of immunity for protection against fungal pathogens. The decision of how to respond will still be primarily determined by interactions between pathogens and cells of the innate immune system, but the actions of T cells will feed back into this dynamic equilibrium to regulate the balance between tolerogenic and inflammatory responses. In the last two decades, the immunopathogenesis of fungal infections and fungal diseases was explained primarily in terms of Th1/Th2 balance. Although Th1 responses driven by the IL-12/IFN-gamma axis are central to protection against fungi, other cytokines and T cell-dependent pathways have come of age. The newly described Th17 developmental pathway may play an inflammatory role previously attributed to uncontrolled Th1 responses and serves to accommodate the seemingly paradoxical association of chronic inflammatory responses with fungal persistence in the face of an ongoing inflammation. Regulatory T cells in their capacity to inhibit aspects of innate and adaptive antifungal immunity have become an integral component of immune resistance to fungi, and provide the host with immune defense mechanisms adequate for protection, without necessarily eliminating fungal pathogens which would impair immune memory--or causing an unacceptable level of tissue damage. The enzyme indoleamine 2,3-dioxygenase and tryptophan metabolites contribute to immune homeostasis by inducing Tregs and taming overzealous or heightened inflammatory responses.

Pattern recognition: recent insights from Dectin-1

The β-glucan receptor Dectin-1 is an archetypical non-toll-like pattern recognition receptor expressed predominantly by myeloid cells, which can induce its own intracellular signalling and can mediate a variety of cellular responses, such as cytokine production. Recent identification of the components of these signalling pathways, such as Syk kinase, CARD9 and Raf-1, has provided novel insights into the molecular mechanisms underlying Dectin-1 function. Furthermore, a broader appreciation of the cellular responses mediated by this receptor and the effects of interactions with other receptors, including the TLRs, have greatly furthered our understanding of innate immunity and how this drives the development of adaptive immunity, particularly Th17 responses. Recent studies have highlighted the importance of Dectin-1 in anti-fungal immunity, in both mice and humans, and have suggested a possible involvement of this receptor in the control of mycobacterial infections.

Dynamic, morphotype-specific Candida albicans beta-glucan exposure during infection and drug treatment

Candida albicans, a clinically important dimorphic fungal pathogen that can evade immune attack by masking its cell wall β-glucan from immune recognition, mutes protective host responses mediated by the Dectin-1 β-glucan receptor on innate immune cells. Although the ability of C. albicans to switch between a yeast- or hyphal-form is a key virulence determinant, the role of each morphotype in β-glucan masking during infection and treatment has not been addressed. Here, we show that during infection of mice, the C. albicans β-glucan is masked initially but becomes exposed later in several organs. At all measured stages of infection, there is no difference in β-glucan exposure between yeast-form and hyphal cells. We have previously shown that sub-inhibitory doses of the anti-fungal drug caspofungin can expose β-glucan in vitro, suggesting that the drug may enhance immune activity during therapy. This report shows that caspofungin also mediates β-glucan unmasking in vivo. Surprisingly, caspofungin preferentially unmasks filamentous cells, as opposed to yeast form cells, both in vivo and in vitro. The fungicidal activity of caspofungin in vitro is also filament-biased, as corroborated using yeast-locked and hyphal-locked mutants. The uncloaking of filaments is not a general effect of anti-fungal drugs, as another anti-fungal agent does not have this effect. These results highlight the advantage of studying host–pathogen interaction in vivo and suggest new avenues for drug development.

Candida is a common human commensal but disseminated candidiasis is a serious clinical problem, especially among immunocompromised patients. The innate immune system controls Candida infection, in part through the germline-encoded β-glucan receptor Dectin-1. However, during in vitro growth, Candida albicans mutes Dectin-1 recognition by cloaking its β-glucan underneath a layer of mannan. Bridging these two seemingly contradictory observations, we demonstrate that C. albicans masks β-glucan early during infection, but it becomes exposed later, allowing Dectin-1 to recognize the fungi and mediate immunity. Remarkably, treatment of mice with sub-therapeutic doses of the antifungal drug caspofungin causes exposure of β-glucan on C. albicans even when it would not be exposed naturally. We introduce a new technique for monitoring of epitope exposure during infection, which can be used to monitor the availability of any epitope for immune recognition. This technique allowed us to show that natural unmasking of β-glucan is not morphotype-specific, but drug-mediated unmasking is biased towards the invasive filamentous form of C. albicans. These results highlight the unexplored area of dynamic epitope exposure during infection and therapy, which might be targetable to enhance immune recognition and fungal clearance.

Candida albicans–macrophage interactions: genomic and proteomic insights

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

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.

Live Candida albicans suppresses production of reactive oxygen species in phagocytes

Production of reactive oxygen species (ROS) is an important aspect of phagocyte-mediated host responses. Since phagocytes play a crucial role in the host response to Candida albicans, we examined the ability of Candida to modulate phagocyte ROS production. ROS production was measured in the murine macrophage cell line J774 and in primary phagocytes using luminol-enhanced chemiluminescence. J774 cells, murine polymorphonuclear leukocytes (PMN), human monocytes, and human PMN treated with live C. albicans produced significantly less ROS than phagocytes treated with heat-killed C. albicans. Live C. albicans also suppressed ROS production in murine bone marrow-derived macrophages from C57BL/6 mice, but not from BALB/c mice. Live C. albicans also suppressed ROS in response to external stimuli. C. albicans and Candida glabrata suppressed ROS production by phagocytes, whereas Saccharomyces cerevisiae stimulated ROS production. The cell wall is the initial point of contact between Candida and phagocytes, but isolated cell walls from both heat-killed and live C. albicans stimulated ROS production. Heat-killed C. albicans has increased surface exposure of 1,3-beta-glucan, a cell wall component that can stimulate phagocytes. To determine whether surface 1,3-beta-glucan exposure accounted for the difference in ROS production, live C. albicans cells were treated with a sublethal dose of caspofungin to increase surface 1,3-beta-glucan exposure. Caspofungin-treated C. albicans was fully able to suppress ROS production, indicating that suppression of ROS overrides stimulatory signals from 1,3-beta-glucan. These studies indicate that live C. albicans actively suppresses ROS production in phagocytes in vitro, which may represent an important immune evasion mechanism.

The cell wall of the human pathogen Candida glabrata: differential incorporation of novel adhesin-like wall proteins

The cell wall of the human pathogen Candida glabrata governs initial host-pathogen interactions that underlie the establishment of fungal infections. With the aim of identifying species-specific features that may directly relate to its virulence, we have investigated the cell wall of C. glabrata using a multidisciplinary approach that combines microscopy imaging, biochemical studies, bioinformatics, and tandem mass spectrometry. Electron microscopy revealed a bilayered wall structure in which the outer layer is packed with mannoproteins. Biochemical studies showed that C. glabrata walls incorporate 50% more protein than Saccharomyces cerevisiae walls and, consistent with this, have a higher mannose/glucose ratio. Evidence is presented that C. glabrata walls contain glycosylphosphatidylinositol (GPI) proteins, covalently bound to the wall 1,6-beta-glucan, as well as proteins linked through a mild-alkali-sensitive linkage to 1,3-beta-glucan. A comprehensive genome-wide in silico inspection showed that in comparison to other fungi, C. glabrata contains an exceptionally large number, 67, of genes encoding adhesin-like GPI proteins. Phylogenetically these adhesin-like proteins form different clusters, one of which is the lectin-like EPA family. Mass spectrometric analysis identified 23 cell wall proteins, including 4 novel adhesin-like proteins, Awp1/2/3/4, and Epa6, which is involved in adherence to human epithelia and biofilm formation. Importantly, the presence of adhesin-like proteins in the wall depended on the growth stage and on the genetic background used, and this was reflected in alterations in adhesion capacity and cell surface hydrophobicity. We propose that the large repertoire of adhesin(-like) genes of C. glabrata contributes to its adaptability and virulence.

Caspofungin modulates inflammatory responses to Aspergillus fumigatus through stage-specific effects on fungal beta-glucan exposure

Echinocandins target fungal beta-1,3 glucan synthesis and are used clinically to treat invasive aspergillosis. Although echinocandins do not completely inhibit in vitro growth of Aspergillus fumigatus, they do induce morphological changes in fungal hyphae. Because beta-1,3 glucans activate host antifungal pathways via the Dectin-1 receptor, we investigated the effect of echinocandins on inflammatory responses to A. fumigatus. Caspofungin- or micafungin-treated conidia and germlings induced less secretion of tumor necrosis factor (TNF) and CXCL2 by macrophages than did their untreated counterparts. Diminished secretion of TNF and CXCL2 correlated with diminished beta-glucan exposure on echinocandin-treated germ tubes. In contrast to treated conidia and germlings, echinocandin-treated hyphae stimulated increased release of TNF and CXCL2 by macrophages and demonstrated intense staining with a beta-glucan-specific antibody, particularly at hyphal tips. Our experiments demonstrate that echinocandin-induced morphological changes in A. fumigatus hyphae are accompanied by increased beta-glucan exposure, with consequent increases in Dectin-1-mediated inflammatory responses by macrophages.

Caspofungin-mediated beta-glucan unmasking and enhancement of human polymorphonuclear neutrophil activity against Aspergillus and non-Aspergillus hyphae

BACKGROUND

We investigated whether caspofungin and other echinocandins have immune-enhancing properties that influence human polymorphonuclear neutrophil (PMN)-mediated mold hyphal damage.

MATERIALS AND METHODS

Using aniline blue staining, we compared patterns of beta-glucan exposure in Aspergillus fumigatus, Aspergillus terreus, Rhizopus oryzae, Fusarium solani, Fusarium oxysporum, Scedosporium prolificans, and Scedosporium apiospermum hyphae after caspofungin exposure. We also determined PMN-mediated hyphal damage occurring with or without preexposure to caspofungin or with preexposure to the combination of caspofungin and anti-beta-glucan monoclonal antibody, using 2,3-bis (2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino) carbonyl]-sH-tetrazolium hydroxide (XTT) assay.

RESULTS

Preincubation with caspofungin (32 microg/mL for R. oryzae; 0.0625 microg/mL for other isolates) increased exposure to beta-glucan. PMN-induced damage increased after caspofungin exposure and was further augmented by the addition of anti-beta-glucan antibody. Preincubation with micafungin or anidulafungin had similar effects on PMN-induced damage of A. fumigatus hyphae. Finally, preexposure of A. fumigatus, but not S. prolificans, to caspofungin induced expression of Dectin-1 by PMN.

CONCLUSIONS

The results of the present study suggest inducement of beta-glucan unmasking by echinocandins and enhancement of PMN activity against mold hyphae, thereby supporting the immunopharmacologic mode of action of echinocandins.

Candida albicans transcription factor Rim101 mediates pathogenic interactions through cell wall functions

pH-responsive transcription factors of the Rim101/PacC family govern virulence in many fungal pathogens. These family members control expression of target genes with diverse functions in growth, morphology and environmental adaptation, so the mechanistic relationship between Rim101/PacC and infection is unclear. We have focused on Rim101 from Candida albicans, which we find to be required for virulence in an oropharyngeal candidiasis model. Rim101 affects the yeast-hypha morphological transition, a major virulence requirement in disseminated infection models. However, virulence in the oropharyngeal candidiasis model is independent of the yeast-hypha transition because it is unaffected by an nrg1 mutation, which prevents formation of yeast cells. Here we have identified Rim101 target genes in an nrg1Delta/Delta mutant background and surveyed function using an overexpression-rescue approach. Increased expression of Rim101 target genes ALS3, CHT2, PGA7/RBT6, SKN1 or ZRT1 can partially restore pathogenic interaction of a rim101Delta/Delta mutant with oral epithelial cells. Four of these five genes govern cell wall structure. Our results indicate that Rim101-dependent cell wall alteration contributes to C. albicans pathogenic interactions with oral epithelial cells, independently of cell morphology.

IL-17 and therapeutic kynurenines in pathogenic inflammation to fungi

Largely viewed as proinflammatory, innate responses combine with adaptive immunity to generate the most effective form of antifungal resistance, and T cells exercise feedback control over diverse effects of inflammation on infection. Some degree of inflammation is required for protection, particularly in mucosal tissues, during the transitional response occurring between the rapid innate and slower adaptive response. However, progressive inflammation worsens disease and ultimately prevents pathogen eradication. IDO, tryptophan catabolites ("kynurenines"), and regulatory T cells help to tame overzealous and exaggerated inflammatory responses. In this context, IL-23 and the Th17 pathway, which down-regulate tryptophan catabolism, may instead favor pathology and serve to accommodate the seemingly paradoxical association of chronic inflammation with fungal persistence. Recent data support a view in which IL-23/IL-17 antagonistic strategies, including the administration of synthetic kynurenines, could represent a new means of harnessing progressive or potentially harmful inflammation.

Novel immune regulatory pathways and their role in immune reconstitution syndrome in organ transplant recipients with invasive mycoses

Immune regulatory pathways involving the newly discovered T regulatory (Treg) and Th17 cells are amongst the principal targets of immunosuppressive agents employed in transplant recipients and key mediators of host inflammatory responses in fungal infections. These novel signaling pathways, in concert with or independent of Th1/Th2 responses, have potentially important implications for yielding valuable insights into the pathogenesis of immune reconstitution syndrome (IRS) in transplant recipients, for aiding the diagnosis of this entity, and for achieving a balance of immune responses that enhance host immunity while curbing unfettered inflammation in IRS.

Superantigen-like effects of a Candida albicans polypeptide

The amino terminal sequence of the Candida albicans cell wall protein Int1 exhibited partial identity with the major histocompatibility complex (MHC) class II binding site of the Mycoplasma arthritidis superantigen MAM. Int1-positive C. albicans blastospores activated human T lymphocytes and expanded Vbeta subsets 2, 3, and/or 14; Int1-negative strains were inactive. Release of interferon-gamma (IFN-gamma) but not of tumor necrosis factor-alpha or interleukin-6 was Int1 dependent; interleukin-4 and interleukin-10 were not detected. T lymphocyte activation, Vbeta expansion, and IFN-gamma release were associated with a soluble polypeptide that encompassed the first 263 amino acids of Int1 (Pep(263)). Monoclonal antibody 163.5, which recognizes an Int1 epitope that overlaps the region of identity with MAM, significantly inhibited these activities when triggered by Int1-positive blastospores or Pep(263) but not by staphylococcal enterotoxin B. Histidine(263) was required. Pep(263) bound to T lymphocytes and MHC class II and was detected in the urine of a patient with C. albicans fungemia. These studies identify a candidal protein that displays superantigen-like activities.

Stage-specific innate immune recognition of Aspergillus fumigatus and modulation by echinocandin drugs

The pulmonary innate immune system clears inhaled Aspergillus fumigatus conidia (spores) from terminal airways. Failure to control conidial germination in immune compromised hosts can result in hyphal tissue invasion and fatal disease. Insight into the molecular recognition of A. fumigatus by host leukocytes indicates that the innate immune system exploits obligate changes in fungal cell wall composition that occur at the first stage of germination, conidial swelling. Germinating spores activate at least two host signal transduction pathways. Surface exposure of fungal beta-(1,3) glucan, a polysaccharide constituent of the fungal cell wall, triggers dectin-1 signaling by host phagocytes. Spore germination leads to the induction of Toll-like receptor (TLR) signaling as well. This stage-specific recognition mechanism focuses host antifungal responses on cells with the potential for tissue invasion and may serve to limit potentially deleterious effects of inflammation in space and time. Fungal beta-(1,3) glucan not only activates host innate immune responses but also represents the target of echinocandin drugs. The activity of echinocandin drugs has largely been understood on the basis of pharmacologic growth inhibition of yeast and moulds, resulting in lysis of yeast cells and stunting of dysmorphic hyphae. The recognition that fungal beta-1,3 glucan activates dectin-1 signaling suggests that echinocandin drugs may exert immune modulatory effects by altering innate immune responses to drug-treated fungal cells, a view supported by recent data from studies on C. albicans, A. fumigatus, and non-Aspergillus moulds.

Regulation of the Candida albicans cell wall damage response by transcription factor Sko1 and PAS kinase Psk1

The environmental niche of each fungus places distinct functional demands on the cell wall. Hence cell wall regulatory pathways may be highly divergent. We have pursued this hypothesis through analysis of Candida albicans transcription factor mutants that are hypersensitive to caspofungin, an inhibitor of beta-1,3-glucan synthase. We report here that mutations in SKO1 cause this phenotype. C. albicans Sko1 undergoes Hog1-dependent phosphorylation after osmotic stress, like its Saccharomyces cerevisiae orthologues, thus arguing that this Hog1-Sko1 relationship is conserved. However, Sko1 has a distinct role in the response to cell wall inhibition because 1) sko1 mutants are much more sensitive to caspofungin than hog1 mutants; 2) Sko1 does not undergo detectable phosphorylation in response to caspofungin; 3) SKO1 transcript levels are induced by caspofungin in both wild-type and hog1 mutant strains; and 4) sko1 mutants are defective in expression of caspofungin-inducible genes that are not induced by osmotic stress. Upstream Sko1 regulators were identified from a panel of caspofungin-hypersensitive protein kinase-defective mutants. Our results show that protein kinase Psk1 is required for expression of SKO1 and of Sko1-dependent genes in response to caspofungin. Thus Psk1 and Sko1 lie in a newly described signal transduction pathway.

Depressive symptoms and proinflammatory cytokine levels in chronic renal failure patients

BACKGROUND/AIMS

Cytokine secretion is known to play an important role in the pathophysiology of depression, and levels of proinflammatory cytokines are increased in chronic renal failure (CRF) patients. The objective of this study was to examine the correlation between levels of proinflammatory cytokines in CRF patients and degree of depression.

METHODS

31 patients on hemodialysis, 31 patients on continuous ambulatory peritoneal dialysis, and 31 conservatively managed chronic kidney disease (CKD) patients were enrolled in this study. Depressive symptoms were measured with the Beck Depression Inventory (BDI), and 'elevated symptoms of depression' were defined as a BDI score > or =17. IL-6 and TNFalpha cytokine levels were measured by ELISA.

RESULTS

'Elevated symptoms of depression' occurred in 37 of 93 patients (40%). IL-6 and TNFalpha levels were not significantly different among CRF patients with and without elevated depressive symptoms (p = 0.937 and p = 0.414, respectively). When analyzed by treatment subgroup, proinflammatory cytokine levels were not significantly different in patients with and without elevated symptoms of depression.

CONCLUSION

In patients with CRF, elevated symptoms of depression were not associated with increased cytokine levels.

Rho1 has distinct functions in morphogenesis, cell wall biosynthesis and virulence of Fusarium oxysporum

Rho-type GTPases regulate polarized growth in yeast by reorganization of the actin cytoskeleton and through signalling pathways that control the expression of cell wall biosynthetic genes. We report the cloning and functional analysis of rho1 from Fusarium oxysporum, a soilborne fungal pathogen causing vascular wilt on plants and opportunistic infections in humans. F. oxysporum strains carrying either a Deltarho1 loss-of-function mutation or a rho1(G14V) gain-of-function allele were viable, but displayed a severely restricted colony phenotype which was partially relieved by the osmotic stabilizer sorbitol, indicating structural alterations in the cell wall. Consistent with this hypothesis, Deltarho1 strains showed increased resistance to cell wall-degrading enzymes and staining with Calcofluor white, as well as changes in chitin and glucan synthase gene expression and enzymatic activity. Re-introduction of a functional rho1 allele into the Deltarho1 mutant fully restored the wild-type phenotype. The Deltarho1 strain had dramatically reduced virulence on tomato plants, but was as virulent as the wild type on immunodepressed mice. Thus, Rho1 plays a key role during fungal infection of plants, but not of mammalian hosts.

The fungal pattern recognition receptor, Dectin-1, and the associated cluster of C-type lectin-like receptors

The mammalian natural killer gene complex (NKC) contains several families of type II transmembrane C-type lectin-like receptors (CLRs) that are best known for their involvement in the detection of virally infected or transformed cells, through the recognition of endogenous (or self) proteinacious ligands. However, certain CLR families within the NKC, particularly those expressed by myeloid cells, recognize structurally diverse ligands and perform a variety of other immune and homoeostatic functions. One such family is the 'Dectin-1 cluster' of CLRs, which includes MICL, CLEC-2, CLEC12B, CLEC9A, CLEC-1, Dectin-1 and LOX-1. Here, we review each of these CLRs, exploring our current understanding of their ligands and functions and highlighting where they have provided new insights into the underlying mechanisms of immunity and homeostasis.

Phagocytosis by human neutrophils is stimulated by a unique fungal cell wall component

Innate immunity depends upon recognition of surface features common to broad groups of pathogens. The glucose polymer beta-glucan has been implicated in fungal immune recognition. Fungal walls have two kinds of beta-glucan: beta-1,3-glucan and beta-1,6-glucan. Predominance of beta-1,3-glucan has led to the presumption that it is the key immunological determinant for neutrophils. Examining various beta-glucans for their ability to stimulate human neutrophils, we find that the minor cell wall component beta-1,6-glucan mediates neutrophil activity more efficiently than beta-1,3-glucan, as measured by engulfment, production of reactive oxygen species, and expression of heat shock proteins. Neutrophils rapidly ingest beads coated with beta-1,6-glucan while ignoring those coated with beta-1,3-glucan. Complement factors C3b/C3d are deposited on beta-1,6-glucan more readily than on beta-1,3-glucan. Beta-1,6-glucan is also important for efficient engulfment of the human pathogen Candida albicans. These unique stimulatory effects offer potential for directed stimulation of neutrophils in a therapeutic context.

Invasive Candida species disease in infants and children: occurrence, risk factors, management, and innate host defense mechanisms

PURPOSE OF REVIEW

Invasive infections by opportunistic Candida species significantly impact morbidity and mortality. This review provides an update of the incidence, risk factors, and management of invasive candidal disease in infants and children, focusing on very-low-birth-weight neonates, and highlights recent advances in understanding candidal virulence factors and innate anti-Candida species host defense mechanisms.

RECENT FINDINGS

Invasive infections with Candida species are the most common cause of late-onset, blood culture-proven nosocomial sepsis in very-low-birth-weight neonates. Risk factors include colonization, long stay in neonatal intensive care units, and use of broad-spectrum antibiotics, central venous catheters, parenteral nutrition, and mechanical ventilation. These risks are compounded by increasing resistance of Candida species to standard antifungal agents. Recent data suggest that, in addition to the macrophage mannose receptor, beta-glucan receptors, Toll-like receptors, and galectin-3 play an important role in host recognition of Candida species.

SUMMARY

Reduction of proven risk factors, more aggressive eradication of colonizing fungi by anticandidal agents, and possibly Candida species vaccines may reduce Candida species-associated morbidity and mortality. Accumulating data of molecular mechanisms that underlie innate immune functions against Candida species may provide a basis to prevent and treat candidal infections more efficiently.

Immune recognition of Candida albicans beta-glucan by dectin-1

Beta (1,3)-glucans represent 40% of the cell wall of the yeast Candida albicans. The dectin-1 lectin-like receptor has shown to recognize fungal beta (1,3)-glucans and induce innate immune responses. The importance of beta-glucan-dectin-1 pathways for the recognition of C. albicans by human primary blood cells has not been firmly established. In this study we demonstrate that cytokine production by both human peripheral blood mononuclear cells and murine macrophages is dependent on the recognition of beta-glucans by dectin-1. Heat killing of C. albicans resulted in exposure of beta-glucans on the surface of the cell wall and subsequent recognition by dectin-1, whereas live yeasts stimulated monocytes mainly via recognition of cell-surface mannans. Dectin-1 induced cytokine production through the following 2 pathways: Syk-dependent production of the T-helper (Th) 2-type anti-inflammatory cytokine interleukin-10 and Toll-like receptor-Myd88-dependent stimulation of monocyte-derived proinflammatory cytokines, such as tumor necrosis factor-alpha . In contrast, stimulation of Th1-type cytokines, such as interferon-gamma , by C. albicans was independent of the recognition of beta-glucans by dectin-1. In conclusion, C. albicans induces production of monocyte-derived and T cell-derived cytokines through distinct pathways dependent on or independent of dectin-1.

Induction of a high affinity fibronectin receptor in Candida albicans by caspofungin: requirements for beta (1,6) glucans and the developmental regulator Hbr1p

Candida albicans expresses at least two biochemically distinct fibronectin receptors. Hemoglobin induces expression of a low affinity receptor recognizing the fibronectin cell-binding domain, whereas growth in complex media induces a high affinity receptor recognizing the collagen-binding domain. We now show that sub-inhibitory concentrations of caspofungin and nikkomycin Z, but not fluconazole, induce the high affinity fibronectin receptor in a dose-dependent manner. Macromolecular complexes mechanically sheared from caspofungin-treated cells retained high affinity fibronectin binding that was sensitive to protease, disulfide reduction, and beta (1,3) glucanase digestion. The high affinity fibronectin receptor was not inducible in a Kre9 mutant strain of C. albicans deficient in beta (1,6) glucans. Conversely, a mutant strain lacking the fibronectin binding protein Als5p showed no defects in induction of high or low affinity fibronectin receptors. Heterozygous mutants of a regulator of white-opaque phenotypic switching, HBR1, lacked any detectable high affinity fibronectin receptor expression in response to caspofungin, and re-introduction of the gene restored activity. Therefore, sub-inhibitory dosages of caspofungin induce a high affinity fibronectin receptor that is distinct from the known receptor Als5p and is dependent on beta (1,6) glucans and HBR1.

Genomics and the development of new diagnostics and anti-Candida drugs

Pathogenic Candida species remain a significant medical problem despite the availability of antifungal therapies. Two key issues must be addressed to improve the treatment of life-threatening systemic Candida infections. First, advanced diagnostic tools are required to facilitate the early identification of these infections, when therapeutic intervention is more likely to be effective. Second, improved antifungal therapies are needed. These therapies, which might include combinations of antifungals, need to be less toxic to the patient and more potent in killing a broader range of Candida species. Recent advances in unravelling the genomics of these species should facilitate efforts to achieve these goals. We discuss the contribution of genomics to the development of novel antifungals and new diagnostic tools.

Immune reconstitution syndrome associated with opportunistic mycoses

Host immunity is essential in facilitating the eradication of infection. However, immunological recovery and an imbalance characterised by either suboptimum or excessive expression of immune responses can also be harmful to the host. Inflammatory responses triggered by rapid resolution of immunosuppression can lead to a series of localised and systemic reactions, termed immune reconstitution syndrome (IRS), that are often misconstrued as failure of specific antifungal therapy to eliminate the offending fungal pathogen. Recognition of IRS has become increasingly relevant in the context of our current use of potent immunosuppressive agents and immunostimulators that allow rapid manipulation of the immune system. Whereas the conceptual principles of IRS underscore the adverse effects of an overzealous and dysregulated immune response, they also support a role of immunotherapies to augment immunity if induction of endogenous responses is inadequate for the control of infection.

MAP kinase pathways as regulators of fungal virulence

MAP kinases are dual phosphorylated protein kinases, present in eukaryotes, which mediate differentiation programs and immune responses in mammalian cells. In pathogenic fungi, MAP kinases are key elements that control adaptation to environmental stress. Recent studies have shown that these pathways have an essential role in the control of essential virulence factors such as capsule biogenesis in Cryptococcus neoformans or morphogenesis, invasion and oxidative stress in Candida albicans. Although MAP kinases sense different activating signals, there is a considerable degree of crosstalk and/or overlap, which enables them to integrate, amplify and modulate the appropriate protective and adaptive response. MAP kinases behave as a 'functional nervous system' that controls virulence and influences the progression of the disease.