Specific binding of human fibrinogen fragment D to Aspergillus fumigatus conidia

Adhesins in the virulence of opportunistic fungal pathogens of human

Aspergillosis, candidiasis, and cryptococcosis are the most common cause of mycoses-related disease and death among immune-compromised patients. Adhesins are cell-surface exposed proteins or glycoproteins of pathogens that bind to the extracellular matrix (ECM) constituents or mucosal epithelial surfaces of the host cells. The forces of interaction between fungal adhesins and host tissues are accompanied by ligand binding, hydrophobic interactions and protein-protein aggregation. Adherence is the primary and critical step involved in the pathogenesis; however, there is limited information on fungal adhesins compared to that on the bacterial adhesins. Except a few studies based on screening of proteome for adhesin identification, majority are based on characterization of individual adhesins. Recently, based on their characteristic signatures, many putative novel fungal adhesins have been predicted using bioinformatics algorithms. Some of these novel adhesin candidates have been validated by in-vitro studies; though, most of them are yet to be characterised experimentally. Morphotype specific adhesin expression as well as tissue tropism are the crucial determinants for a successful adhesion process. This review presents a comprehensive overview of various studies on fungal adhesins and discusses the targetability of the adhesins and adherence phenomenon, for combating the fungal infection in a preventive or therapeutic mode.

Unraveling the nanoscale surface properties of chitin synthase mutants of Aspergillus fumigatus and their biological implications

Understanding the surface properties of the human opportunistic pathogen Aspergillus fumigatus conidia is essential given the important role they play during the fungal interactions with the human host. Although chitin synthases with myosin motor-like domain (CSM) play a major role in cell wall biosynthesis, the extent to which deletion of the CSM genes alter the surface structural and biophysical-biological properties of conidia is not fully characterized. We used three complementary atomic force microscopy techniques-i.e., structural imaging, chemical force microscopy with hydrophobic tips, and single-molecule force spectroscopy with lectin tips-to gain detailed insights into the nanoscale surface properties (ultrastructure, hydrophobicity) and polysaccharide composition of the wild-type and the chitin synthase mutant (ΔcsmA, ΔcsmB, and ΔcsmA/csmB) conidia of A. fumigatus. Wild-type conidia were covered with a highly hydrophobic layer of rodlet nanostructures. By contrast, the surface of the ΔcsmA mutant was almost completely devoid of rodlets, leading to loss of hydrophobicity and exposure of mannan and chitin polysaccharides. The ΔcsmB and ΔcsmA/csmB mutants showed a different behavior, i.e., the surfaces featured poorly organized rodlet layers, yet with a low hydrophobicity and substantial amounts of exposed mannan and chitin at the surface. As the rodlet layer is important for masking recognition of immunogenic fungal cell wall components by innate immune cells, disappearance of rodlet layers in all three chitin synthase mutant conidia was associated with an activation of human dendritic cells. These nanoscale analyses emphasize the important and distinct roles that the CSMA and CSMB genes play in modulating the surface properties and immune interactions of A. fumigatus and demonstrate the power of atomic force microscopy in fungal genetic studies for assessing the phenotypic characteristics of mutants altered in cell surface organization.

Molecular characterization of AfuFleA, an L-fucose-specific lectin from Aspergillus fumigatus

Aspergillus fumigatus causes life-threatening infections in immunocompromised patients. We have found that extracts of mycelial mats of A. fumigatus contain a potent hemagglutinin. To clarify the characteristics of this factor, the hemagglutinin was purified from late-stage cultures and characterized at the molecular level. The hemagglutinin is a 32-kilodalton protein that shows activity as an L-fucose lectin. The gene encoding this protein, AfufleA, was identified from a genomic DNA library utilizing consensus primers designed for amino acid sequences obtained from peptides following limited trypsin proteolysis. An open reading frame was found that consists of 942 nucleotides encoding 314 amino acids with a deduced molecular mass of 34,498 and contains all seven of trypsin-digested peptide sequences; four short introns, 49-63 bp, were also identified. AfufleA shares homology with a fucose-specific lectin produced by the orange peel mushroom, Aleuria aurantia. The role of AfufleA fucose-specific lectin is not clear, but this lectin may enhance attachment of fungal spores to mammalian cell membranes and contribute to the pathogenicity of A. fumigatus.

Pathogenesis of Aspergillus fumigatus in Invasive Aspergillosis

Aspergillus species are globally ubiquitous saprophytes found in a variety of ecological niches. Almost 200 species of aspergilli have been identified, less than 20 of which are known to cause human disease. Among them, Aspergillus fumigatus is the most prevalent and is largely responsible for the increased incidence of invasive aspergillosis (IA) in the immunocompromised patient population. IA is a devastating illness, with mortality rates in some patient groups reaching as high as 90%. Studies identifying and assessing the roles of specific factors of A. fumigatus that contribute to the pathogenesis of IA have traditionally focused on single-gene deletion and mutant characterization. In combination with recent large-scale approaches analyzing global fungal responses to distinct environmental or host conditions, these studies have identified many factors that contribute to the overall pathogenic potential of A. fumigatus. Here, we provide an overview of the significant findings regarding A. fumigatus pathogenesis as it pertains to invasive disease.

Identification and characterization of a laminin-binding protein of Aspergillus fumigatus: extracellular thaumatin domain protein (AfCalAp)

Aspergillus fumigatus, an opportunistic fungal pathogen, infects the human host via inhalation of airborne conidia. Adhesion of fungal conidia, to host cells and extracellular matrix (ECM) components associated with host tissue surfaces, is thought to be the primary step in the pathogenesis and dissemination of infection. To identify novel adhesion proteins (adhesins) of A. fumigatus, we screened its proteome in silico using spaan (software program for prediction of adhesins and adhesin-like proteins using neural networks). One of the predicted adhesin-encoding genes with a P ad (probability of being adhesin) value >0.9, the gene encoding extracellular thaumatin domain protein (AfCalA), was cloned and expressed in Escherichia coli. Recombinant AfCalAp showed significant binding with laminin and murine lung cells. Anti-AfCalAp antibodies inhibited the binding of AfCalAp to laminin in a dose-dependent manner. Significant binding of anti-AfCalAp antibodies to 2 h swollen conidia suggests the presence of AfCalAp on the conidial surface. AfCalA transcript was not detectable in resting conidia but was detected in conidia incubated with RPMI 1640 medium in the presence and absence of lung epithelial cell line (A539)-derived ECM. Elevated levels of IgE antibodies specific to AfCalAp were observed in the sera of two out of seven patients with allergic bronchopulmonary aspergillosis. The study confirms the relevance of the bioinformatic approach for predicting fungal adhesins and establishes AfCalAp as a novel laminin-binding protein of A. fumigatus.

High-resolution cell surface dynamics of germinating Aspergillus fumigatus conidia

We used real-time atomic force microscopy with a temperature-controlled stage (37 degrees C) to probe the structural and physicochemical dynamics of single Aspergillus fumigatus conidia during germination. Nanoscale topographic images of dormant spores revealed the presence of a layer of rodlets made of hydrophobins, in agreement with earlier electron microscopy observations. Within the 3-h germination period, progressive disruption of the rodlet layer was observed, revealing hydrophilic inner cell wall structures. Using adhesion force mapping with hydrophobic tips, these ultrastructural changes were shown to correlate with major differences in cell surface hydrophobicity. That is, the rodlet surface was uniformly hydrophobic due to the presence of hydrophobins, whereas the cell wall material appearing upon germination was purely hydrophilic. This study illustrates the potential of real-time atomic force microscopy imaging and force spectroscopy for tracking cell-surface dynamics.

Purification and partial characterization of a Paracoccidioides brasiliensis protein with capacity to bind to extracellular matrix proteins

Microorganisms adhere to extracellular matrix proteins by means of their own surface molecules. Paracoccidioides brasiliensis conidia have been shown to be capable of interacting with extracellular matrix proteins. We aimed at determining the presence of fungal proteins that could interact with extracellular matrix protein and, if found, attempt their purification and characterization. Various extracts were prepared from P. brasiliensis mycelial and yeast cultures (total homogenates, beta-mercaptoethanol, and sodium dodecyl sulfate [SDS] extracts) and analyzed by ligand affinity assays with fibronectin, fibrinogen and laminin. Two polypeptides were detected in both fungal forms. SDS extracts that interacted with all the extracellular matrix protein were tested; their molecular masses were 19 and 32 kDa. Analysis of the N-terminal amino acid sequence of the purified 32-kDa mycelial protein showed substantial homology with P. brasiliensis, Histoplasma capsulatum, and Neurospora crassa hypothetical proteins. Additionally, a monoclonal antibody (MAb) produced against this protein recognized the 32-kDa protein in the SDS extracts of both fungal forms for immunoblot. Immunofluorescence analysis revealed that this MAb reacted not only with mycelia and yeast cells, but also with conidia, indicating that this protein was shared by the three fungal propagules. By immunoelectron microscopy, this protein was detected in the cell walls and in the cytoplasm. Both the 32-kDa purified protein and MAb inhibited the adherence of conidia to the three extracellular matrix proteins in a dose-dependent manner. These findings demonstrate the presence of two polypeptides capable of interacting with extracellular matrix proteins on the surface of P. brasiliensis propagules, indicating that there may be common receptors for laminin, fibronectin, and fibrinogen. These proteins would be crucial for initial conidial adherence and perhaps also in dissemination of paracoccidioidomycosis.

Recognition of extracellular matrix proteins byParacoccidioides brasiliensisyeast cells

The adhesion of microorganism to host cells or extracellular matrix (ECM) proteins is the first step in the establishment of an infectious process. Interaction between Paracoccidioides brasiliensis yeast cells and ECM proteins has been previously noted. In vivo, in the chronic phase of experimental paracoccidioidomycosis (PCM), laminin and fibronectin have been detected on the surface of yeast cells located inside granulomatous lesions. The aim of the present study was to examine the ability of P. brasiliensis yeast cells to interact with extracellular matrix proteins (laminin, fibrinogen and fibronectin) and to establish which molecules were involved in this interaction. Immunofluorescence microscopy and flow cytometry demonstrated that all three ECM proteins tested were able to bind to the surface of P. brasiliensis yeast cells. Treatment with trypsin, chymotrypsin, chitinase, proteinase K or different sugars resulted in no change in laminin binding. In addition, ligand affinity assays were performed using different yeast extracts (total homogenates, beta-mercaptoethanol, SDS extracts). These assays demonstrated the presence of 19 and 32-kDa proteins in the cell wall with the ability to bind to laminin, fibrinogen and fibronectin. This interaction could be important in mediating attachment of the fungus to host tissues and may consequently play a role in the pathogenesis of PCM.

Neonatal primary cutaneous aspergillosis: case report and review of the literature

Neonatal primary cutaneous aspergillosis usually presents as an erythematous plaque with pustules that evolve into an eschar. Immunocompromised patients and premature neonates are at risk of developing this disease. Early diagnosis and treatment are critical in preventing progression to a systemic illness. We report a preterm neonate with primary cutaneous aspergillosis and discuss the etiologies, presentations, and treatments for this illness.

Corneal ulceration in pediatric patients: a brief overview of progress in topical treatment

Pediatric microbial keratitis is a rare but potentially devastating condition. The condition is similar to adult microbial keratitis, but is often characterized by a more severe inflammatory response. The micro-organisms that cause microbial keratitis in children are similar to the causative agents in adults, with herpes simplex and bacteria being the predominant causative agents, and fungi being less frequent. Of the bacterial pathogens, Pseudomonas aeruginosa, Staphylococcus aureus and alpha-hemolytic streptococci are common. The risk factors for pediatric keratitis include colonization of the eyes during birth and trauma to the cornea. Certain microbial factors involved in microbial keratitis are common to all micro-organisms, including adhesion to the cornea, penetration into the cornea, destruction of the corneal stroma (usually by microbial and/or host proteases), and recruitment of white blood cells to help defend the eye. Specific inflammatory responses that occur during pediatric microbial keratitis are not known in detail, but it is likely that cytokines and polymorphonuclear leucocytes are major factors, as they are in adult microbial keratitis. Treatment for pediatric microbial keratitis is usually the same as treatment for adult microbial keratitis; topical application of antimicrobial agents initially, followed by application of anti-inflammatory agents. With pediatric microbial keratitis, extra care must be taken to ensure nontoxicity due to blood adsorption. New microbial keratitis treatments are being developed and these mainly focus on new antimicrobials, antivirulence agents (such as vaccination against microbial toxins) or specific anti-inflammatory agents. There remains a clear need for increased research into the specific responses during microbial keratitis in children which will help progress new therapies as well as the development of new antimicrobials, especially new antifungal therapies.

Role of early diagnosis and aggressive surgery in the management of invasive pulmonary aspergillosis in neutropenic patients

Invasive pulmonary aspergillosis (IPA) occurs mostly in immunocompromised hosts and especially in neutropenic patients. Improved prognosis for IPA requires early diagnosis. We report our experience in the management of IPA in patients with hematological malignancies. In prolonged neutropenia (> 10 days), thoracic CT scanning seems to be the best choice for the diagnosis of IPA (with CT halo or air-crescent signs). Its systematic use allows a dramatic reduction in the time to achieve the diagnosis, if there is evidence of a halo sign. The systematic screening for the detection of Aspergillus antigenemia with an ELISA test is helpful for early diagnosis. The detection of Aspergillus antigen (with the less sensitive latex agglutination test) on bronchoalveolar lavage (BAL) fluid may also be as useful. The treatment of IPA relies on amphotericin B (or its lipid formulations) or on azole antifungal agents. Pulmonary surgical resection should be considered either as an emergency procedure (despite persistent neutropenia) to avoid massive hemoptysis, or as an elective or diagnostic procedure. This global strategy for the management of IPA is associated with a 75-80% success rate in hematological patients. Nevertheless, the control of underlying malignancy remains a major prognostic factor.

Aspects of fungal pathogenesis in humans

Fungal diseases have become increasingly important in the past few years. Because few fungi are professional pathogens, fungal pathogenic mechanisms tend to be highly complex, arising in large part from adaptations of preexisting characteristics of the organisms' nonparasitic lifestyles. In the past few years, genetic approaches have elucidated many fungal virulence factors, and increasing knowledge of host reactions has also clarified much about fungal diseases. The literature on fungal pathogenesis has grown correspondingly; this review, therefore, will not attempt to provide comprehensive coverage of fungal disease but focuses on properties of the infecting fungus and interactions with the host. These topics have been chosen to make the review most useful to two kinds of readers: fungal geneticists and molecular biologists who are interested in learning about the biological problems posed by infectious diseases, and physicians who want to know the kinds of basic approaches available to study fungal virulence.

Putative virulence factors of Aspergillus fumigatus

Various putative virulence factors of Aspergillus fumigatus have been studied over the past decades. A. fumigatus gliotoxin is a potent inhibitor of the mucociliary system. Several fungal metabolites interfere with phagocytosis and opsonization including toxins, 'conidial inhibitory factor', 'A. fumigatus diffusible product' and 'complement inhibitory factor'. A. fumigatus can bind specifically to different host tissues components, whereas toxins give a general and significant immunosuppressive effect on host defences. Circumstantial evidence links the production of elastinolytic proteases with the ability to cause disease. However, none of the reports demonstrates conclusively a decisive role for any of the virulence factors described thus far. It is conceivable that proteolytic enzyme activities such as those expressed by AFAlp are one of a number of factors, each with a minor effect, that combine to facilitate disease progression.

Aspergillus fumigatus and aspergillosis

Aspergillus fumigatus is one of the most ubiquitous of the airborne saprophytic fungi. Humans and animals constantly inhale numerous conidia of this fungus. The conidia are normally eliminated in the immunocompetent host by innate immune mechanisms, and aspergilloma and allergic bronchopulmonary aspergillosis, uncommon clinical syndromes, are the only infections observed in such hosts. Thus, A. fumigatus was considered for years to be a weak pathogen. With increases in the number of immunosuppressed patients, however, there has been a dramatic increase in severe and usually fatal invasive aspergillosis, now the most common mold infection worldwide. In this review, the focus is on the biology of A. fumigatus and the diseases it causes. Included are discussions of (i) genomic and molecular characterization of the organism, (ii) clinical and laboratory methods available for the diagnosis of aspergillosis in immunocompetent and immunocompromised hosts, (iii) identification of host and fungal factors that play a role in the establishment of the fungus in vivo, and (iv) problems associated with antifungal therapy.

Endocarditis due to Aspergillus flavus

Fungal endocarditis has emerged as an important complication of patients undergoing cardiovascular surgery. Our patient had no past history of cardiac surgery, intravenous drug abuse or immunosuppressive therapy. He had received broad-spectrum antibiotics for varying periods, which might have predisposed him to this infection. The diagnosis was based on the demonstration of hyaline, septate branched fungal elements in the infected valvular tissue and isolation of Aspergillus flavus in culture. The delay in establishing the ante-mortem diagnosis because of repeatedly negative blood cultures, presence of disseminated intravascular coagulopathy and rapidly deteriorating kidney function were the major factors contributing to his poor prognosis and death, despite surgical removal of infected valves and antifungal therapy. This is the first report of endocarditis due to A. flavus from the Middle East.

Sialic acid-dependent recognition of laminin and fibrinogen by Aspergillus fumigatus conidia

In an attempt to define the molecular basis of the adherence of Aspergillus fumigatus conidia to the host tissues, a step which might be mediated by the recognition of basement membrane laminin or fibrinogen, we analyzed the binding of these glycoproteins by flow cytometry and a microtiter plate adherence assay. Flow cytometry revealed that the binding of fluorescein isothiocyanate-labeled laminin to conidia was saturable and specific. Moreover, the ability of conidia to bind laminin increased with their maturation. Competition experiments showed a cross-reactivity between laminin and fibrinogen binding and a lack of interactions with glycosaminoglycans. In addition, the binding of laminin was not inhibited by the different adhesive synthetic peptides tested. Furthermore, the microtiter plate assay of adherence to chymotrypsin degradation products of laminin or fibrinogen purified by gel filtration suggested a unique binding site common to sequential degradation fragments or the presence of multiple binding sites on the two ligands. Therefore, the role of carbohydrates in the recognition process was investigated. Among the carbohydrates tested, constitutive of the conidial wall or of the oligosaccharide side chains of laminin and fibrinogen, only N-acetylneuraminic acid and sialyllactose inhibited the binding of these glycoproteins to conidia. In conclusion, these results strengthen the idea that the laminin and fibrinogen receptors in A. fumigatus are identical and suggest an interaction mediated by a sialic acid-specific lectin of the conidial wall.

Expression and identification of a laminin-binding protein in Aspergillus fumigatus conidia

Adhesion of Aspergillus fumigatus, the causative agent of human aspergillosis, to the extracellular matrix protein laminin has been previously demonstrated. This study investigated the expression of laminin receptors during swelling of conidia, a step leading to germination and subsequent colonization of tissues. Scanning electron microscopy showed that the laminin binding sites were distributed over the external rodlet layer of resting conidia. During swelling, the characteristic rodlet layer progressively disintegrated and conidia surrounded by a smooth cell wall layer appeared. Flow cytometry using fluorescein isothiocyanate-conjugated laminin demonstrated that expression of laminin receptors at the surface of conidia was swelling dependent. Resting conidia expressed high levels of laminin receptors on their surface. A gradual decrease of laminin binding was then observed as swelling occurred, reaching a minimum for 4-h-swollen conidia. This correlated with a loss of adherence of swollen conidia to laminin immobilized on microtiter plates. Trypsin pretreatment of conidia reduced laminin binding. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and ligand blotting with laminin identified in a cell wall extract a major 72-kDa cell wall glycoprotein which binds laminin. Thus, one of the initial events in the host colonization may be the recognition of basement membrane laminin by this 72-kDa cell wall surface component.

Binding of extracellular matrix proteins to Aspergillus fumigatus conidia

As detected by confocal immunofluorescence microscopy, binding of fibronectin and laminin appeared to be associated with the protrusions present on the outer cell wall layer of resting Aspergillus fumigatus conidia. Flow cytometry confirmed that binding of laminin to conidia was dose dependent and saturable. Laminin binding was virtually eliminated in trypsin-treated organisms, thus suggesting the protein nature of the binding site. Conidia were also able to specifically adhere to laminin immobilized on microtiter plates. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting (immunoblotting) with laminin and antilaminin antibody of whole conidial homogenates allowed identification, among the complex array of protein and glycoprotein species, of one polypeptide with an apparent molecular mass of 37 kDa which specifically interacts with laminin. The fact that binding of conidia to soluble or immobilized laminin or fibronectin was inhibited by fibronectin or laminin, respectively, suggests the existence of common binding sites for both ligands on the surface of conidia. Intact conidia were also able to adhere to type I and IV collagen immobilized on microtiter plates; adhesion was found to be dose dependent and saturable. Adhesion to immobilized type I and IV collagen was markedly inhibited by laminin and weakly inhibited by fibronectin. Coincubation of conidia with Arg-Gly-Asp (RGD) peptides caused a dose-dependent decrease in binding of cells to immobilized or soluble fibronectin, yet interaction of cells with soluble or immobilized laminin and type I and IV collagen remained unaffected. Interactions described here could be important in mediating attachment of the fungus to host tissues, thus playing a role in the establishment of the disease.

Binding of Aspergillus fumigatus spores to lung epithelial cells and basement membrane proteins: relevance to the asthmatic lung

BACKGROUND

Aspergillus fumigatus is an opportunistic pathogen to which asthmatic subjects are particularly susceptible. The ability of spores of A fumigatus to bind to pulmonary cells and basement membrane proteins was investigated to determine the mechanisms involved in this susceptibility.

METHODS

Cells of the A549 pulmonary epithelial cell line or purified basement membrane proteins were immobilised on the wells of microtitre plates. They were then exposed to spores of A fumigatus in suspension, with or without various pretreatments of the spores, cells, and proteins. Adherent spores were counted by light microscopy.

RESULTS

Spores of A fumigatus bound in a concentration dependent manner to A549 epithelial cells and pretreatment of cells with interferon gamma (2500 units/ml) caused a significant doubling of spore binding. Binding of spores to A549 cells was inhibited by about a third by pre-incubation of the spores with fibrinogen (100 micrograms/ml). Spores bound specifically to extracellular matrix (ECM) components laid down by A549 cells, and pretreatment of the ECM components with hydrogen peroxide (25-80 microM) enhanced spore binding by approximately one third. They also bound specifically and in a saturable manner to purified fibrinogen, fibronectin, laminin, type I collagen, and type IV collagen. Pre-incubation of spores with Arg-Gly-Asp tripeptide (RGD; 50-200 micrograms/ ml) inhibited binding to fibronectin and type I collagen by 50%.

CONCLUSIONS

This study suggests that the presence of activated epithelial cells and the exposure of basement membrane that occurs in asthma, together with oxidant stress, may facilitate the colonisation of the asthmatic lung by A fumigatus. The RGD sequence may be involved in spore binding to some ECM proteins. Free fibrinogen may protect against binding of A fumigatus spores to the pulmonary epithelium.

Virulence factors of medically important fungi

Human fungal pathogens have become an increasingly important medical problem with the explosion in the number of immunocompromised patients as a result of cancer, steroid therapy, chemotherapy, and AIDS. Additionally, the globalization of travel and expansion of humankind into previously undisturbed habitats have led to the reemergence of old fungi and new exposure to previously undescribed fungi. Until recently, relatively little was known about virulence factors for the medically important fungi. With the advent of molecular genetics, rapid progress has now been made in understanding the basis of pathogenicity for organisms such as Aspergillus species and Cryptococcus neoformans. The twin technologies of genetic transformation and "knockout" deletion construction allowed for genetic tests of virulence factors in these organisms. Such knowledge will prove invaluable for the rational design of antifungal therapies. Putative virulence factors and attributes are reviewed for Aspergillus species, C. neoformans, the dimorphic fungal pathogens, and others, with a focus upon a molecular genetic approach. Candida species are excluded from coverage, having been the subject of numerous recent reviews. This growing body of knowledge about fungal pathogens and their virulence factors will significantly aid efforts to treat the serious diseases they cause.

Binding of human fibronectin to Aspergillus fumigatus conidia

Aspergillus fumigatus conidia exhibited the ability to bind purified human fibronectin, whereas mycelial forms did not bind the ligand, as detected by an indirect immunofluorescence assay with an antifibronectin polyclonal antibody after incubation of the cells with fibronectin. Flow cytometry confirmed that binding of the ligand to conidia was dose dependent and saturable. Pretreatment of the cells with trypsin markedly reduced binding, which suggested a protein nature for the binding sites present at the surface of conidia. Intact conidia were also able to adhere to fibronectin or antifibronectin antibodies, a significant reduction (from 88 to 92%) in the binding of conidia was noticed, thus suggesting that adhesion to the immobilized ligand was specific. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western immunoblotting with fibronectin and antifibronectin antibody of whole conidial homogenates and 2-mercaptoethanol extracts from isolated conidial cell walls allowed identification, among the complex array of protein and glycoprotein species present in both cell-free preparations, of two polypeptides with apparent molecular masses of 23 and 30 kDa which specifically interact with fibronectin.

Cell surface properties of Aspergillus fumigatus conidia: correlation between adherence, agglutination, and rearrangements of the cell wall

Culture conditions that lead to swelling and germination dramatically influence cell surface characteristics and properties of Aspergillus fumigatus conidia. Conidial adherence to polystyrene and agglutination markedly increased during swelling, in a time-dependent manner. Agglutination appeared to be sensitive to cycloheximide and calcium. Removal of cell wall polysaccharides by lyticase or sodium metaperiodate suppressed agglutination of conidia. Proteinase K weakly decreased it whereas dithiothreitol strongly dispersed the cells. These observations suggest that both cell surface carbohydrates and proteins are involved in the agglutination process. Electron microscopic observations demonstrated that the cell wall of conidia was subject to some rearrangements during swelling, involving degradation and loss of the external convoluted layer, and subsequent exposure of underlying ligands. This was confirmed using lectins labelled with gold or fluorescein isothiocyanate, which showed that some carbohydrates, particularly those acting as ligands for peanut agglutinin, are largely exposed during the process. Finally, SDS-PAGE revealed major protein changes between resting and swollen conidia. We conclude that the ability of A. fumigatus conidia to aggregate correlates with an increase in adherence and biochemical reorganization of the cell wall.

The search for virulence determinants in Aspergillus fumigatus

The development of aspergillosis in an immunodeficient host depends on interactions between fungal and host components. The recognition by Aspergillus fumigatus of fibrinogen and laminin, and the secretion of extracellular proteinases and ribonucleotoxin have been suggested to mediate adherence to mucosal surfaces and subsequently to bring about host-tissue invasion.

Microbial adhesins recognizing extracellular matrix macromolecules

Microorganisms express a family of cell-surface adhesins that specifically recognize and bind components of the extracellular matrix. Adhesion of microorganisms to host tissues represents a critical phase in the development of many types of infections. Recent studies have focused on the mechanisms of microbial attachment at a molecular level, including the identification of ligand-binding domains in several cell-surface adhesins from Gram-positive bacteria and the construction of adhesin-deficient isogenic mutants.

rodletless mutants of Aspergillus fumigatus

Conidia of Aspergillus fumigatus adhere in vitro to host proteins and cells via the outer cell wall layer. The rodA gene of A. fumigatus was cloned by homology with the rodA gene of Aspergillus nidulans, which is involved in the structure of the rodlets characteristic of the surface layer. The A. fumigatus RODA protein sequence has 85% similarity to that of A. nidulans RODA; the sequence codes for a hydrophobin, a low-molecular-weight protein moderately hydrophobic and rich in cysteines. The gene was disrupted with the hygromycin B resistance gene. By transformation of protoplasts with the disrupted gene, RodA- mutants were generated. These mutants are deficient in the ability to disperse their conidia; their conidia lack the rodlet layer and are hydrophilic. The adhesion of the rodletless conidia to collagen and bovine serum albumin was lower than that of the wild type; in contrast, there was no difference between RodA- and RodA+ conidia in adhesion to pneumocytes, fibrinogen, and laminin, suggesting that RODA is not the receptor for these cells and proteins. RodA- conidia were pathogenic for mice.

Specific interaction of Aspergillus fumigatus with fibrinogen and its role in cell adhesion

Interaction between Aspergillus fumigatus conidia and different proteins known to mediate the attachment of malignant tumor cells or microorganisms to the host tissues was studied in vitro. Flow cytometry using fluorescein isothiocyanate-conjugated fibrinogen confirmed that binding of human fibrinogen to the conidia was dose dependent and specific. Binding was inhibited by unlabeled fibrinogen and by basement membrane laminin. Moreover, the expression of fibrinogen receptors at the surfaces of conidia seemed to be related to the maturation of the conidia. Binding sites appeared to be located in the D domains of the fibrinogen molecule. However, the peptide sequence recognized by the fungus could not be identified but was different from the classical adhesive recognition sequences, RGDS and fibrinogen gamma-chain dodecapeptide. In addition, an assay of adherence to proteins immobilized onto microtiter plates allowed us to establish the role of these interactions in fungal adhesion. Conidia strongly adhered to human fibrinogen and to laminin but not to fibronectin. Adhesion to fibrinogen substrates was specific, since it was inhibited by soluble fibrinogen and by specific antibodies, and seemed to be mediated by the D domains of the molecule. Study of the adhesion of numerous strains or clinical isolates to various mammalian fibrinogens did not reveal any particular affinity of strains for some animal species. Finally, by cultivation of the fungus in the presence of 125I-human fibrinogen and analysis of the radiolabeled material bound to the surface of the fungus, we were able to specify the sequence of events allowing its installation within the host. The interactions identified here may play an important role in governing fungal adherence and host tissue invasion.

Extracellular fibrinogenolytic enzyme of Aspergillus fumigatus: substrate-dependent variations in the proteinase synthesis and characterization of the enzyme

To get a better understanding of the role of the previously reported fibrinogenolytic enzyme of Aspergillus fumigatus, we investigated the in vitro conditions of enzyme synthesis and attempted to characterize it. Modification of the nitrogen source did not influence the extracellular serine-proteinase profile, but resulted in important quantitative differences in the yields in batch cultures. The enzyme synthesis appeared to be an inducible phenomenon in A. fumigatus since it was initiated exclusively in the presence of proteins or protein hydrolysate. Free amino acids or inorganic nitrogen compounds could not promote significant enzyme production. Moreover, peptone at a concentration of 0.1% appeared to be the best inducer of enzyme synthesis. Conversely, modification of the carbon source did not affect fungal growth or enzyme synthesis. However, the production of chymotrypsin was highly sensitive to the carbohydrate level in the culture medium and, with peptone as nitrogen source, highest yields were obtained in the presence of 0.3 or 0.5% glucose. Culture filtrates of A. fumigatus CBS 113.26 grown with peptone or nitrate as nitrogen source were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Comparison of the protein patterns suggested for the proteinase a molecular mass of 33 kDa which was confirmed by chromatographic purification of the enzyme through (N alpha-CBZ)-D-phenylalanine agarose.

Purification and characterization of a fibrinogenolytic serine proteinase from Aspergillus fumigatus culture filtrate

A fibrinogenolytic proteinase has been isolated from Aspergillus fumigatus culture filtrate by ammonium sulfate precipitation followed by successive chromatographies on Sephadex G-75 and immobilized phenylalanine. The purified proteinase exhibited a molecular weight of about 33 kDa. When analysed by SDS-polyacrylamide gels containing co-polymerized fibrinogen, the proteinase appeared as a broad band at the top of the gels, which could correspond to polymerization of the enzyme, as suggested by SDS-PAGE analysis of the unboiled eluate. The isoelectric point was 8.75 and the enzyme was not glycosylated. Proteinase activity was optimum at pH 9 and between 37 and 42 degrees C, although a decrease in activity was observed above 37 degrees C. PMSF and chymostatin markedly inhibited the proteinase activity, and good kinetic constants were obtained for the synthetic substrate, N-Suc-Ala-Ala-Pro-Phe-pNA. These results provide direct evidence that this enzyme belongs to the chymotrypsin-like serine proteinase group.