Chemical and immunological characterization of the extracellular galactomannan of Aspergillus fumigatus

Galactomannan detection for invasive aspergillosis in immunocompromized patients

BACKGROUND

Invasive aspergillosis (IA) is the most common life-threatening opportunistic invasive mycosis in immunocompromized patients. A test for IA needs to be not too invasive and not too big a burden for the already weakened patient. The serum galactomannan ELISA seems to have potential for both requirements.

OBJECTIVES

To obtain summary estimates of the diagnostic accuracy of galactomannan detection in serum for the diagnosis of IA.

SEARCH STRATEGY

We searched MEDLINE, EMBASE and Web of Science with both Medical Headings and text words for both aspergillosis and the sandwich ELISA. We checked reference lists of included studies and review articles for additional studies.

SELECTION CRITERIA

Cross-sectional studies, case-control designs and consecutive series of patients assessing the diagnostic accuracy of galactomannan detection for the diagnosis of IA in patients with neutropenia or patients whose neutrophils are functionally compromised were included. The reference standard was composed of the criteria given by the European Organization for Research and Treatment of Cancer (EORTC) and the Mycoses Study Group (MSG).

DATA COLLECTION AND ANALYSIS

Two review authors independently assessed quality and extracted data

MAIN RESULTS

Thirty studies were included in the meta-analyses, with a median prevalence of IA (proven or probable) of 7.7%. Seven of these (901 patients) reported results for an Optical Density Index (ODI) of 0.5 as cut-off value. The overall sensitivity in these studies was 78% (61% to 89%) and overall specificity was 81% (72% to 88%). Twelve studies (1744 patients) reported the results for cut-off value of 1.0 ODI, overall sensitivity was 75% (59% to 86%) and mean specificity 91% (84% to 95%). Seventeen studies (2600 patients) reported the results for cut-off value 1.5 ODI, sensitivity was 64% (50% to 77%) and mean specificity 95% (91% to 97%).

AUTHORS' CONCLUSIONS

At a cut-off value 0.5 ODI in a population of 100 patients with a disease prevalence of 8% (overall median prevalence), 2 patients who have IA, will be missed (sensitivity 78%, 22% false negatives), and 17 patients will be treated or further referred unnecessarily (specificity of 81%, 19% false negatives). If we use the test at cut-off value 1.5 in the same population, that will mean that 3 IA patients will be missed (sensitivity 64%, 36% false negatives) and 5 patients will be treated or referred unnecessarily (specificity of 95%, 5% false negatives). These numbers should however be interpreted with caution, because the results were very heterogeneous.

Contribution of galactofuranose to the virulence of the opportunistic pathogen Aspergillus fumigatus

The filamentous fungus Aspergillus fumigatus is responsible for a lethal disease called invasive aspergillosis that affects immunocompromised patients. This disease, like other human fungal diseases, is generally treated by compounds targeting the primary fungal cell membrane sterol. Recently, glucan synthesis inhibitors were added to the limited antifungal arsenal and encouraged the search for novel targets in cell wall biosynthesis. Although galactomannan is a major component of the A. fumigatus cell wall and extracellular matrix, the biosynthesis and role of galactomannan are currently unknown. By a targeted gene deletion approach, we demonstrate that UDP-galactopyranose mutase, a key enzyme of galactofuranose metabolism, controls the biosynthesis of galactomannan and galactofuranose containing glycoconjugates. The glfA deletion mutant generated in this study is devoid of galactofuranose and displays attenuated virulence in a low-dose mouse model of invasive aspergillosis that likely reflects the impaired growth of the mutant at mammalian body temperature. Furthermore, the absence of galactofuranose results in a thinner cell wall that correlates with an increased susceptibility to several antifungal agents. The UDP-galactopyranose mutase thus appears to be an appealing adjunct therapeutic target in combination with other drugs against A. fumigatus. Its absence from mammalian cells indeed offers a considerable advantage to achieve therapeutic selectivity.

Glycoconjugates and polysaccharides of fungal cell wall and activation of immune system

Glycoproteins, glycosphingolipids and polysaccharides exposed at the most external layers of the wall are involved in several types of interactions of fungal cells with the exocellular environment. These molecules are fundamental building blocks of organisms, contributing to the structure, integrity, cell growth, differentiation and signaling. Several of them are immunologically active compounds with potential as regulators of pathogenesis and the immune response of the host. Some of these structures can be specifically recognized by antibodies from patients’ sera, suggesting that they can be also useful in the diagnosis of fungal infections.

IL1 gene cluster polymorphisms and its haplotypes may predict the risk to develop invasive pulmonary aspergillosis and modulate C-reactive protein level

OBJECTIVE

The aim of this study was to determine whether interleukin-1 alpha (IL1alpha), interleukin-1 beta (IL1beta), and IL1 receptor antagonist (IL1Ra) polymorphisms are implicated in invasive pulmonary aspergillosis (IPA) pathogenesis.

MATERIALS AND METHODS

Subjects comprised 110 hematological patients and 148 healthy controls. Genotypic and allelic frequencies were similar between hematological patients and controls. IPA was diagnosed in 59 of the 110 patients according to consensus criteria published by the European Organization for Research and Treatment of Cancer/Invasive Fungal Infections Cooperative Group (EORTC/IFICG).

RESULTS AND DISCUSSIONS

Individual locus analysis showed that IL1alpha and IL1Ra polymorphisms were not associated with the presence of IPA (p = 0.560 and p = 0.680, respectively). However, a trend towards a higher presence of IL1beta( - ) (511TT) genotype (or IL1beta(-511T) allele) in the IPA group than in the non-IPA patient group (p = 0.092 and p = 0.095, respectively) was found. Haplotype analysis revealed that VNTR2/-889C/-511T haplotype was strongly associated with susceptibility to develop IPA infection (p = 0.020). Haplotype analysis also showed an association between VNTR2/-889C/-511C haplotype and resistance to IPA infection (p = 0.028). Furthermore, patients with IL1Ra VNTR2/2 and IL1beta(-511)T/T genotypes had a higher positive serum galactomannan percentage versus patients with other genotypes. Finally, C-reactive protein (CRP) production was significantly associated with IL1 gene cluster polymorphisms, although CRP values were similar between IPA and non-IPA groups.

CONCLUSION

These findings indicate a critical role of IL1 gene cluster polymorphisms in the susceptibility to IPA infection and CRP production.

Development of an immunochromatographic lateral-flow device for rapid serodiagnosis of invasive aspergillosis

Aspergillus fumigatus is a cosmopolitan saprotrophic fungus that is second only to Candida species as a cause of invasive fungal infections in immunocompromised humans. Current immunodiagnostic tests for invasive aspergillosis (IA) are based on the detection of circulating galactomannan (GM) in a patient's serum by using a rat monoclonal antibody (MAb), EB-A2, that binds to tetra (1-->5)-beta-D-galactofuranoside, the immunodominant epitope in GM. The potential cross-reactivity of MAb EB-A2 with non-Aspergillus fungi, with contaminating GM in beta-lactam antibiotics and foodstuffs, and with bacterial lipoteichoic acids has prompted efforts to discover non-GM antigens that can act as surrogate markers for the diagnosis of IA. This paper describes the development of a mouse MAb, JF5, that binds to a protein epitope present on an extracellular glycoprotein antigen secreted constitutively during the active growth of A. fumigatus. The MAb was used to develop an immunochromatographic lateral-flow device (LFD) for the rapid (15-min) detection of Aspergillus antigens in human serum. The test is highly specific, reacting with antigens from Aspergillus species but not with antigens from a large number of clinically important fungi, including Candida species, Cryptococcus neoformans, Fusarium solani, Penicillium marneffei, Pseudallescheria boydii, and Rhizopus oryzae. The LFD was able to detect circulating antigen in serum samples from patients suspected of having or shown to have IA on the basis of their clinical symptoms and results from tests for GM and fungal (1-->3)-beta-D-glucan. The ease of use of the LFD provides a diagnostic platform for the routine testing of vulnerable patients who have an elevated risk of IA.

Improved detection of circulating Aspergillus antigen by use of a modified pretreatment procedure

Detection of circulating galactofuranose (galf) antigens, including galactomannan (GM), by the Platelia Aspergillus (PA) enzyme-linked immunosorbent assay (ELISA) is an important tool in the early diagnosis of invasive aspergillosis (IA). We used a modified pretreatment technique (MT) on consecutive negative PA ELISA plasma samples from IA patients in order to improve the detection of the fungal components present. Plasma samples (52) were collected from healthy donors, and 174 plasma samples with a galactomannan index (GMI) below 0.5 were collected from 25 unclassifiable and 23 IA patients. The PA ELISA reactivity of pretreated samples was determined before (conventional technique [CT]) and after (MT) filtration using a Microcon filter with a 50-kDa cutoff (Millipore). For the MT, the sensitivity of the PA ELISA increased from 42.9% (CT) to 78.6% (MT) using a cutoff for the GMI of 1.5 in the probable and proven group, whereas specificity slightly decreased from 98.7% to 96.1% in the control group. The 10-fold concentration step increased the GMI as high as 121-fold. The MT resulted not only in positive reactivity in samples that tested negative with the CT but also in the earlier detection of antigen by 2 to 17 days.

Well-characterized monoclonal antibodies against cell wall antigen of Aspergillus species improve immunoassay specificity and sensitivity

The diagnosis of invasive aspergillosis (IA) based on the detection of Aspergillus galactomannan (GM) is complicated by the presence of cross-reactive GM epitopes in patient specimens. We have developed a novel and specific Aspergillus antigen-capture enzyme-linked immunosorbent assay (ELISA) by the selection of two well-characterized monoclonal antibodies from 17 candidate antibodies. The epitopes recognized by the monoclonal antibodies were present on the cell walls of the hyphae and the conidia of Aspergillus species, which were circulating or excreted as immunodominant antigens during the acute phase of IA established in the animal models. The detection of experimental Aspergillus-mediated antigenemia was suitably sensitive, and the sensitivity was comparable to that of a commercial GM detection ELISA kit (the Platelia Aspergillus assay). Moreover, the specificity of this assay was 100% when it was used to test 382 serum specimens and 120 urine specimens from healthy individuals. Cross-reactivity with other common opportunistic fungi, such as Penicillium and Candida species, and with purified GM protein derived from Aspergillus was not evident. Therefore, the chemical nature of the epitopes captured in this assay is most likely not associated with the GM structure, indicating that this newly developed Aspergillus antigen-capture ELISA is a promising tool for the diagnosis of IA without the risk of the false-positive results that are problematic with current GM antigen assays.

Outcomes of Three Patients With Intracranially Invasive Sino-orbital Aspergillosis

PURPOSE

Although rare, invasive aspergillosis with intracranial spread usually is fatal and necessitates prompt diagnosis and treatment. We describe the prognoses for survival and vision in 3 consecutive patients with vision-threatening invasive sino-orbital aspergillosis.

METHODS

A case series of 3 patients, with review of treatments and outcomes.

RESULTS

Three diabetic patients presented with significant eye pain and headache, followed by progressive ophthalmic symptoms including vision loss and limitation of ocular motility. Histologic examination showed numerous hyphae of Aspergillus fumigatus. Following early diagnosis and surgical debridement, 1 patient (case 1) showed vision improvement, from counting fingers to 20/20. The other 2 patients, who were initially treated with corticosteroids after presumptive diagnoses of nonspecific orbital inflammation, showed no recovery of vision from no light perception, and 1 (case 3) died of massive intracranial spread and side effects of antifungal agents caused by delayed diagnosis and long-term use of corticosteroids.

CONCLUSIONS

Invasive sino-orbital aspergillosis with intracranial invasion often may resemble inflammatory conditions. Early diagnosis and surgical intervention are required to improve vision and survival. Repeated biopsies often are necessary to rule out fungal sinus infection before considering steroid use, especially in diabetic patients.

Fungal infections in solid organ transplantation

Renal, liver, heart and lung transplantation are now considered to be the standard therapeutic interventions in patients with end-stage organ failure. Infectious complications following transplantation are relatively common due to the transplant recipients overall immunosuppressed status. The incidence of invasive mycoses following solid organ transplant ranges from 5 to 42% depending on the organ transplanted. These mycoses are associated with high overall mortality rates. Candida and Aspergillus spp. produce most of these infections. This article will review the risk factors, clinical presentation and treatment of invasive fungal infections in solid organ transplant patients, and evaluate the role of prophylactic therapy in this group of patients.

Detection of circulating galactomannan in serum samples for diagnosis of Penicillium marneffei infection and cryptococcosis among patients infected with human immunodeficiency virus

Galactomannan (GM) is a heteropolysaccharide in the cell walls of most Aspergillus and Penicillium species. Cross-reactivity of Cryptococcus neoformans galactoxylomannan in an Aspergillus GM test has also been reported. In this study, we used a Platelia Aspergillus enzyme immunoassay kit (Bio-Rad) to test serum samples obtained from 48 human immunodeficiency virus (HIV)-infected patients (15 with penicilliosis [7 with fungemia alone, 4 with cavitary lung lesions alone, 3 with both fungemia and cavitary lung lesions, and 1 with disseminated disease], 22 with cryptococcosis [11 with fungemia alone, 5 with cavitary lung lesions, 3 with both, and 3 with meningitis alone], and 11 without any invasive fungal infection [control]) for GM levels. None of the patients had aspergillosis or concurrent use of piperacillin-tazobactam or amoxicillin-clavulanate. The median time between diagnosis of fungal infection and collection of serum samples was 0 days for penicilliosis and 1.5 days for cryptococcosis. Of patients with penicilliosis, cryptococcosis, and controls, 73.3%, 13.6%, and 9%, respectively, had GM optical density (OD) indices of >0.5 (P = 0.0001). GM OD indices were higher for penicilliosis (median OD index, 4.419; range, 0.158 to >20) than for cryptococcosis (median, 0.247; range, 0.112 to 3.849) cases (P < 0.001). Patients with fungemic penicilliosis had higher OD indices (median, 10.628; range, 0.401 to >20) than patients with nonfungemic penicilliosis (median, 0.378; range, 0.158 to 4.419) and patients with cryptococcemia (median, 0.231; range, 0.112 to 1.168) (P < 0.001). Of the 15 patients with cavitary lung lesions, those with penicilliosis had higher antigen levels (median OD index, 1.641; range, 0.247 to >20) than those with cryptococcosis (median, 0.227; range, 0.112 to 3.849) (P = 0.011). This study showed that the GM OD index was significantly elevated for HIV patients with penicilliosis. The use of the GM antigen assay may facilitate earlier diagnosis of Penicillium marneffei infection for HIV-infected patients in areas of endemicity.

Advances in the serological diagnosis of invasive Aspergillus infections in patients with haematological disorders

A reliable diagnosis of invasive aspergillosis in patients with haematological malignancies is seldom achieved antemortem. Conventional laboratory diagnostic methods are insensitive and time-consuming, resulting in late diagnosis and treatment and contributing to unacceptably high mortality. As a result, routine antifungal prophylaxis and early empirical treatment have been recommended. However, overtreatment associated with these strategies results in increased toxicity and cost. The use of sensitive and rapid non-culture-based diagnostic assays, such as detection of Aspergillus antigens (galactomannan, beta-D-glucan) or detection of genomic DNA sequences may allow a shift in emphasis from empirical to pre-emptive therapy, especially when substantiated by suggestive radiological findings. These new tools may be used to confirm a presumed diagnosis of invasive aspergillosis, or, when used to screen high-risk patients, may identify an infection at the early stage of disease. The excellent negative predictive value of these assays should convince clinicians to withhold antifungal therapy in persistently febrile neutropenic patients with no other signs of fungal infection. On the other hand, consecutive positive results in a high-risk population should at least trigger a complete diagnostic work-up. This review will focus on the diagnostic utility as well as on the pitfalls of serial screening for the presence of circulating fungal antigens in haematology patients.

Update on the contribution of galactomannan for the diagnosis of invasive aspergillosis

The diagnosis of invasive fungal infections (IFI) remains a challenge, particularly for diseases caused by filamentous fungi such as Aspergillus species. Unfortunately, many patients affected by these conditions are not identified before autopsy. Therefore, there is a need for new diagnostic methods for IFI. Galactomannan is a soluble antigen released during hyphal growth in tissues. A commercially available sandwich ELISA assay that detects galactomannan has been used in Europe for many years and is now approved for use in the USA. The test has an excellent negative predictive value in the detection of invasive aspergillosis (IA) in high-risk patients. In addition, it is more sensitive than culture and allows IA to be diagnosed before clinical manifestations occur. However, false-negative and false-positive results in certain populations are the main limitations to its use. The purpose of this review is to summarize the current knowledge about galactomannan testing in patients at risk for IA.

Nippostrongylus brasiliensis: identification of intelectin-1 and -2 as Stat6-dependent genes expressed in lung and intestine during infection

Elimination of the helminth parasite Nippostrongylus brasiliensis from infected mice is mediated by IL-4 or IL-13 and dependent on the IL-4Ralpha chain and the transcription factor Stat6 in non-hematopoietic cells. However, it is not clear which Stat6-dependent effector molecules mediate worm expulsion. We identified intelectin-1 and -2 as Stat6-dependent genes that are induced during infection. Intelectins can bind galactofuranose, a sugar present only in microorganisms and might therefore serve as microbial pattern element. To analyze whether constitutive expression of intelectin-1 or -2 leads to accelerated pathogen clearance, transgenic mice were generated which express high levels of these genes selectively in the lung. Infection with N. brasiliensis or Mycobacterium tuberculosis did not result in accelerated pathogen clearance in transgenic as compared to wild-type mice. Further, no significant modulation of the immune response in lung or lymph nodes was observed. Thus, under these conditions, intelectins did not enhance pathogen clearance.

Chemical probes of UDP-galactopyranose mutase

Many pathogenic prokaryotes and eukaryotes possess the machinery required to assemble galactofuranose (Galf)-containing glycoconjugates; these glycoconjugates can be critical for virulence or viability. Accordingly, compounds that block Galf incorporation may serve as therapeutic leads or as probes of the function of Galf-containing glycoconjugates. The enzyme UDP-galactopyranose mutase (UGM) is the only known generator of UDP-galactofuranose, the precursor to Galf residues. We previously employed a high-throughput fluorescence polarization assay to investigate the Klebsiella pneumoniae UGM. We demonstrate the generality of this assay by extending it to UGM from Mycobacterium tuberculosis. To identify factors influencing binding, we synthesized a directed library containing a 5-arylidene-2-thioxo-4-thiazolidinone core, a structure possessing features common to ligands for both homologs. Our studies offer a blueprint for identifying inhibitors of the growing family of UGM homologs and provide insight into UGM inhibition.

The structure and synthesis of the fungal cell wall

The fungal cell wall is a dynamic structure that protects the cell from changes in osmotic pressure and other environmental stresses, while allowing the fungal cell to interact with its environment. The structure and biosynthesis of a fungal cell wall is unique to the fungi, and is therefore an excellent target for the development of anti-fungal drugs. The structure of the fungal cell wall and the drugs that target its biosynthesis are reviewed. Based on studies in a number of fungi, the cell wall has been shown to be primarily composed of chitin, glucans, mannans and glycoproteins. The biosynthesis of the various components of the fungal cell wall and the importance of the components in the formation of a functional cell wall, as revealed through mutational analyses, are discussed. There is strong evidence that the chitin, glucans and glycoproteins are covalently cross-linked together and that the cross-linking is a dynamic process that occurs extracellularly.

Structural characterisation of novel lichen heteroglycans by NMR spectroscopy and methylation analysis

Two galactofuranomannans, Ths-4 and Ths-5, were isolated from the lichen, Thamnolia vermicularis var. subuliformis, using ethanol fractionation and anion-exchange and size-exclusion chromatography. The average molecular weights of Ths-4 and Ths-5 were estimated to be 19 and 200 kDa, respectively. Structural characterisation of Ths-4, Ths-5 and their partially hydrolysed derivatives was performed by methanolysis and methylation analysis. The intact and partially hydrolysed Ths-4 was further analysed using NMR spectroscopy (1D, COSY, NOESY, TOCSY, HSQC and HMBC). According to the data obtained, the heteroglycans Ths-4 and Ths-5 have similar structures, but have large differences in molecular weight. The structure is composed of 3-O-linked and 5-O-linked galactofuranosyl chains linked to a mannan core. The mannan core consists of a main chain of alpha-(1-->6)-linked mannopyranosyl residues, substituted at O-2 with either a single alpha-mannopyranosyl unit or an alpha-Manp-(1-->2)-alpha-Manp-(1-->2)-alpha-Manp group in the ratio of approximately 1:3, respectively. The polysaccharides have idealised repeating blocks as is shown.

Expression and initial characterization of WbbI, a putative D-Galf:alpha-D-Glc beta-1,6-galactofuranosyltransferase from Escherichia coli K-12

Cloning of E. coli K-12 orf8 (wbbI) and over-expression of the corresponding enzyme as a maltose-binding fusion protein provided recombinant WbbI beta-1,6-galactofuranosyltransferase activity. Challenged with synthetic acceptor analogues in the presence of UDP-galactofuranose as a donor, WbbI showed a modest preference for pyranoside acceptor substrates of the alpha-D-gluco-configuration but it also possessed the ability to turn-over acceptor analogues.

In vitro release by Aspergillus fumigatus of galactofuranose antigens, 1,3-beta-D-glucan, and DNA, surrogate markers used for diagnosis of invasive aspergillosis

Aspergillus markers are becoming increasingly important for the early diagnosis of invasive aspergillosis. The kinetics of release of these surrogate markers, however, is largely unknown. We investigated the release of beta-(1-5)-galactofuranosyl (galf) antigens (Platelia Aspergillus), 1,3-beta-D-glucan (BG) (Fungitell), and DNA (PCR) in an in vitro model of Aspergillus fumigatus. The results showed that release is correlated to the growth phase of the fungus, which depends on available nutrients. Whereas galf antigens and BG are released during logarithmic growth, DNA is released only after mycelium breakdown. During early logarithmic growth, galf antigens seem to be released somewhat earlier than BG. Furthermore, galf antigen concentrations of more than 120,000 times the serum cutoff value (0.5 ng/ml) can be measured, while BG concentrations reach a value only 978 times the serum cutoff value (60 pg/ml). During lytical growth, release of galf antigens further increased to a maximum level, which depended on pH. After that, the concentration of galf antigens stayed high (pH 7.4) or decreased to zero within 4 days (pH 5.0). In contrast to galf antigens, BG concentration decreased after 1 day of growth. The decrease of galf components seems to be due to the enzyme beta-galactofuranosidase, which is able to destroy galf epitopes and whose activity fluctuates in the culture filtrates in parallel with galf antigen concentration. Fungal DNA seems to be released only due to autolysis caused by nutrient limitation. In conclusion, several factors clearly influence the release of surrogate markers in vitro. These same factors might also play a role at the infection site of Aspergillus disease in humans.

Recombinant antigens as diagnostic markers for aspergillosis

Eight recombinant proteins and purified galactomannan of Aspergillus fumigatus were tested by enzyme-linked immunosorbent assay to quantify the anti-Aspergillus antibodies in sera of patients with aspergilloma, allergic bronchopulmonary aspergillosis (ABPA), and invasive aspergillosis (IA). In spite of the variability observed in the immune responses of individual patients, quantification of the antibody titers against the 18-kDa ribonuclease (RNU), the 360-kDa catalase (CAT), and the 88-kDa dipeptidylpeptidase V (DPPV) was useful for the diagnosis of aspergilloma and ABPA. Differential diagnosis of ABPA was even possible among cystic fibrosis as well as noncystic fibrosis patients. In the group of immunocompromised patients with IA, no antibody response was mounted in response to the Aspergillus infection in any of the patients. Interestingly, about half of the patients with proven IA came to the hospital with high titers of anti-Aspergillus antibodies, suggesting that they were infected upon entry to the hospital. These results suggest that recombinant RNU, CAT, and DPPV have a great potential in the serodiagnosis of all forms of aspergillosis in the immunocompromised and immunocompetent patient.

Occurrence and kinetics of false-positive Aspergillus galactomannan test results following treatment with beta-lactam antibiotics in patients with hematological disorders

Several reports have described a high rate of false-positive Aspergillus galactomannan (GM) test results for patients treated with piperacillin-tazobactam. In this retrospective study, we first examined the relationships between intravenous administration of three beta-lactam antibiotics and the occurrence of false-positive GM test results in hematology patients. We then estimated the kinetics of clearance of GM after the cessation of treatment. Sequential serum samples from 69 patients that had received beta-lactams were analyzed by using a Platelia Aspergillus test. A significant association was found between GM positivity (>/=0.5) and the administration of beta-lactams (P < 0.0001). The direct role of beta-lactams in patients' serum positivity was assessed by testing 39 batches of beta-lactams, of which 27 were positive for GM. None of the latter were positive according to a fungus- and Aspergillus-specific PCR. The kinetics of the decrease of GM was analyzed on sequential serum samples obtained after treatment. By use of a nonlinear regression model, the average time to negative antigen was assessed to be 5.5 days (95% confidence interval [CI], 4.1 to [7.0]), with a half-life of elimination of GM of 2.4 days (95% CI, 1.8 to 3.0). This study confirms that the administration of beta-lactams containing GM is responsible for false-positive diagnostic results, even up to 5 days after the cessation of treatment.

Identification and partial characterization of two eukaryotic UDP-galactopyranose mutases

Galactofuranose metabolism is valued as an important target for the development of new antituberculosis drugs. UDP-galactopyranose mutase, a central enzyme in galactofuranose biosynthesis, is essential for the growth and viability of mycobacteria. This enzyme catalyzes the conversion of UDP-galactopyranose into UDP-galactofuranose, the donor used by various galacto-furanosyltransferases. While D-galactofuranose residues are often found in important surface glycoconjugates of pathogenic bacteria, fungi and protozoan parasites, they are absent in the mammalian host, and thus their biosynthesis is an attractive target for the development of novel therapeutic strategies. In contrast to mycobacteria, the importance of galactofuranose for eukaryotic pathogens has not been ascertained because the enzymes involved in galactofuranose metabolism are unknown. Here, we report the identification and characterization of the first eukaryotic UDP-galactopyranose mutases. The genes encoding the enzymes were cloned from two different human pathogens: the parasite Leishmania major and the opportunistic fungus Aspergillus fumigatus. The newly identified eukaryotic enzymes exhibit 51% sequence identity, but are less than 20% identical to the prokaryotic counterparts. The sequence identity between pro- and eukaryotic enzymes is concentrated at amino acid residues that are involved in substrate and cofactor binding. Therefore, an inhibitor of UDP-galactopyranose mutase might be effective against a wide range of pathogenic organisms.

Glycosylphosphatidylinositol-anchored fungal polysaccharide in Aspergillus fumigatus

Galactomannan is a characteristic polysaccharide of the human filamentous fungal pathogen Aspergillus fumigatus that can be used to diagnose invasive aspergillosis. In this study, we report the isolation of a galactomannan fraction associated to membrane preparations from A. fumigatus mycelium by a lipid anchor. Specific chemical and enzymatic degradations and mass spectrometry analysis showed that the lipid anchor is a glycosylphosphatidylinositol (GPI). The lipid part is an inositol phosphoceramide containing mainly C18-phytosphingosine and monohydroxylated lignoceric acid (2OH-C(24:0) fatty acid). GPI glycan is a tetramannose structure linked to a glucosamine residue: Manalpha1-2Manalpha1-2Manalpha1-6Manalpha1-4GlcN. The galactomannan polymer is linked to the GPI structure through the mannan chain. The GPI structure is a type 1, closely related to the one previously described for the GPI-anchored proteins of A. fumigatus. This is the first time that a fungal polysaccharide is shown to be GPI-anchored.

Laboratory diagnosis of invasive aspergillosis

Invasive aspergillosis occurs in a wide range of clinical scenarios, is protean in its manifestations, and is still associated with an unacceptably high mortality rate. Early diagnosis is critical to a favourable outcome, but is difficult to achieve with current methods. Deep tissue diagnostic specimens are often difficult to obtain from critically ill patients. Newer antifungal agents exhibit differential mould activity, thus increasing the importance of establishing a specific diagnosis of invasive aspergillosis. For these reasons, a range of alternate diagnostic strategies have been investigated. Most investigative efforts have focused on molecular and serological diagnostic techniques. The detection of metabolites produced by Aspergillus spp and a range of aspergillus-specific antibodies represent additional, but relatively underused, diagnostic avenues. The detection of galactomannan has been incorporated into diagnostic criteria for invasive aspergillosis, reflecting an increased understanding of the performance, utility, and limitations of this technique. Measurement of (1,3)-beta-D glucan in blood may be useful as a preliminary screening tool for invasive aspergillosis, despite the fact that this antigen can be detected in a number of other fungi. There have been extensive efforts directed toward the detection of Aspergillus spp DNA, but a lack of technical standardisation and relatively poor understanding of DNA release and kinetics continues to hamper the broad applicability of this technique. This review considers the application, utility, and limitations of the currently available and investigational diagnostic modalities for invasive aspergillosis.

Eukaryotic UDP-galactopyranose mutase (GLF gene) in microbial and metazoal pathogens

Galactofuranose (Gal(f)) is a novel sugar absent in mammals but present in a variety of pathogenic microbes, often within glycoconjugates that play critical roles in cell surface formation and the infectious cycle. In prokaryotes, Gal(f) is synthesized as the nucleotide sugar UDP-Gal(f) by UDP-galactopyranose mutase (UGM) (gene GLF). Here we used a combinatorial bioinformatics screen to identify a family of candidate eukaryotic GLFs that had previously escaped detection. GLFs from three pathogens, two protozoa (Leishmania major and Trypanosoma cruzi) and one fungus (Cryptococcus neoformans), had UGM activity when expressed in Escherichia coli and assayed in vivo and/or in vitro. Eukaryotic GLFs are closely related to each other but distantly related to prokaryotic GLFs, showing limited conservation of core residues around the substrate-binding site and flavin adenine dinucleotide binding domain. Several eukaryotes not previously investigated for Gal(f) synthesis also showed strong GLF homologs with conservation of key residues. These included other fungi, the alga Chlamydomonas and the algal phleovirus Feldmannia irregularis, parasitic nematodes (Brugia, Onchocerca, and Strongyloides) and Caenorhabditis elegans, and the urochordates Halocynthia and Cionia. The C. elegans open reading frame was shown to encode UGM activity. The GLF phylogenetic distribution suggests that Gal(f) synthesis may occur more broadly in eukaryotes than previously supposed. Overall, GLF/Gal(f) synthesis in eukaryotes appears to occur with a disjunct distribution and often in pathogenic species, similar to what is seen in prokaryotes. Thus, UGM inhibition may provide an attractive drug target in those eukaryotes where Gal(f) plays critical roles in cellular viability and virulence.

Bifidobacterial lipoglycan as a new cause for false-positive platelia Aspergillus enzyme-linked immunosorbent assay reactivity

We previously hypothesized that a lipoglycan of Bifidobacterium bifidum subsp. pennsylvanicum cross-reacts with the Platelia Aspergillus (PA) enzyme-linked immunosorbent assay (ELISA) based on the presence of galactofuranosyl epitopes in the cell wall (M. A. S. H. Mennink-Kersten, R. R. Klont, A. Warris, H. J. M. Op den Camp, and P. E. Verweij, Lancet 363:325-327, 2004). We tested this hypothesis by testing bacterial suspensions of different bifidobacterial species and other gram-positive and -negative bacteria with the PA ELISA, which is used to detect circulating galactomannan for the serodiagnosis of invasive aspergillosis. Furthermore, neonatal fecal samples were enumerated for bifidobacteria by fluorescence in situ hybridization (FISH) and tested for PA ELISA reactivity. All bifidobacteria, except B. infantis and B. adolescentis, showed reactivity 6- to 600-fold higher compared to the controls (i.e., Micrococcus luteus and Propionibacterium freudenreichii, which contain a cell wall lipomannan). Eggerthella lenta showed a 25-fold-higher reactivity. ELISA reactivity was clearly shown to be associated with bacterial lipoglycans containing a beta-1,5-galactofuranosyl chain. All neonatal feces showed PA ELISA reactivity and associated numbers of bifidobacteria. Since high concentrations of bifidobacteria are present in the human gut, these bacteria or excreted lipoglycan may cause false serum PA ELISA reactivity in selected patient groups, especially neonates.

Cryptococcus neoformans Galactoxylomannan contains an epitope(s) that is cross-reactive with Aspergillus Galactomannan

We report a case of cryptococcosis in which a serum enzyme-linked immunosorbent assay (ELISA) for Aspergillus galactomannan was positive, with no evidence of aspergillosis. Soluble antigens from 19 Cryptococcus neoformans strains and purified carbohydrates of C. neoformans capsule were thus assayed in the Aspergillus galactomannan ELISA. Antigens from all C. neoformans strains, and purified galactoxylomannan, gave a positive reaction, suggesting that C. neoformans galactoxylomannan contains an epitope(s) that is cross-reactive with Aspergillus galactomannan.

Galactomannoproteins of Aspergillus fumigatus

Galactofuranose-containing molecules have been repeatedly shown to be important antigens among human fungal pathogens, including Aspergillus fumigatus. Immunogenic galactofuran determinants have been poorly characterized chemically, however. We reported here the characterization of two glycoproteins of A. fumigatus with an N-glycan containing galactofuranose. These proteins are a phospholipase C and a phytase. Chemical characterization of the N-glycan indicates that it is a mixture of Hex(5-13)HexNAc(2) oligosaccharides, the major molecular species corresponding to Hex(6-8)HexNAc(2). The N-glycan contained one galactofuranose unit that was in a terminal nonreducing position attached to the 2 position of Man. This single terminal nonreducing galactofuranose is essential for the immunoreactivity of the N-glycans assessed either with a monoclonal antibody that recognizes a tetra-beta-1,5-galactofuran chain of galactomannan or with Aspergillus-infected patient sera.

Two alpha(1-3) glucan synthases with different functions in Aspergillus fumigatus

Alpha(1-3) glucan is a main component of the Aspergillus fumigatus cell wall. In spite of its importance, synthesis of this amorphous polymer has not been investigated to date. Two genes in A. fumigatus, AGS1 and AGS2, are highly homologous to the AGS genes of Schizosaccharomyces pombe, which encode putative alpha(1-3) glucan synthases. The predicted Ags proteins of A. fumigatus have an estimated molecular mass of 270 kDa. AGS1 and AGS2 were disrupted in A. fumigatus. Both Deltaags mutants have similar altered hyphal morphologies and reduced conidiation levels. Only Deltaags1 presented a reduction in the alpha(1-3) glucan content of the cell wall. These results showed that Ags1p and Ags2p were functionally different. The cellular localization of the two proteins was in agreement with their different functions: Ags1p was localized at the periphery of the cell in connection with the cell wall, whereas Ags2p was intracellularly located. An original experimental model of invasive aspergillosis based on mixed infection and quantitative PCR was developed to analyze the virulence of A. fumigatus mutant and wild-type strains. Using this model, it was shown that the cell wall and morphogenesis defects of Deltaags1 and Deltaags2 were not associated with a reduction in virulence in either mutant. This result showed that a 50% reduction in the content of the cell wall alpha(1-3) glucan does not play a significant role in A. fumigatus pathogenicity.

Aspergillus infections in transplant recipients

Aspergillus infections are occurring with an increasing frequency in transplant recipients. Notable changes in the epidemiologic characteristics of this infection have occurred; these include a change in risk factors and later onset of infection. Management of invasive aspergillosis continues to be challenging, and the mortality rate, despite the use of newer antifungal agents, remains unacceptably high. Performing molecular studies to discern new targets for antifungal activity, identifying signaling pathways that may be amenable to immunologic interventions, assessing combination regimens of antifungal agents or combining antifungal agents with modulation of the host defense mechanisms, and devising diagnostic assays that can rapidly and reliably diagnose infections represent areas for future investigations that may lead to further improvement in outcomes.

Specific molecular features in the organization and biosynthesis of the cell wall ofAspergillus fumigatus

The cell wall of Aspergillus fumigatus is composed of a branched beta1,3 glucan covalently bound to chitin, beta1,3, beta1,4 glucans, and galactomannan, that is embedded in an amorphous cement composed of alpha1,3 glucan, galactomannan and polygalactosamin. The mycelial cell wall of A. fumigatus is very different from the yeast Saccharomyces cerevisiae cell wall, and in particular lacks beta1,6 glucans and proteins covalently bound to cell wall polysaccharides. The differences in cell wall composition between the mould A. fumigatus and the yeast S. cerevisiae are also reflected at the genomic level where unique features have been identified in A. fumigatus. A single gene codes for the glucan synthase catalytic subumit; this finding has lead to the development of a RNAi methodology for the disruption of essential genes in A. fumigatus. In contrast to the glucan synthase, multiple genes have been found in the chitin synthase and the alpha glucan synthase families; in spite of homologous sequences, each gene in each family have very different function. Similarly homologous mannosyltransferase genes are found in yeast and moulds but they lead to the synthesis of very different N-mannan structures. This chemo-genomic comparative analysis has also suggested that GPI-anchored proteins do not have a role of linker in the three dimensional organization of the fungal cell wall.

First synthesis of the immunodominant β-galactofuranose-containing tetrasaccharide present in the cell wall of Aspergillus fumigatus

beta-Galf-(1-->5)-beta-Galf-(1-->6)-alpha-Manp-(1-->6)-alpha-Manp, the immunodominant epitope in the cell-wall galactomannan of Aspergillus fumigatus, was synthesized for the first time as its allyl glycoside. The key disaccharide glycosyl donor, 2,3,5,6-tetra-O-benzoyl-beta-D-galactofuranosyl-(1-->5)-2-O-acetyl-3,6-di-O-benzoyl-beta-D-galactofuranosyl trichloroacetimidate (10), was constructed by 5-O-glycosylation of 1,2-O-isopropylidene-3,6-di-O-benzoyl-alpha-D-galactofuranose (4) with 2,3,5,6-tetra-O-benzoyl-beta-D-galactofuranosyl trichloroacetimidate (5), followed by 1,2-O-deacetonation, acetylation, selective 1-O-deacetylation, and trichloroacetimidation. The target tetrasaccharide 16 was obtained by the condensation of allyl 2,3,4-tri-O-benzoyl-alpha-D-mannopyranosyl-(1-->6)-2,3,4-tri-O-benzoyl-alpha-D-mannopyranoside (14) as glycosyl acceptor with the disaccharide glycosyl donor 10, followed by deprotection.

Detection of Aspergillus galactomannan by enzyme immunoabsorbent assay in recipients of allogeneic hematopoietic stem cell transplantation: a prospective study

BACKGROUND

Invasive aspergillosis (IA) has become the leading infectious cause of death after allogeneic hematopoietic stem cell transplantation (allo-HSCT). This is partially because of the lack of a sensitive, specific, and noninvasive diagnostic test. New diagnostic tests for IA, such as the detection of Aspergillus galactomannan antigen (AGA) by sandwich enzyme-linked immunoabsorbent assay (ELISA), have recently been described. This study validates the usefulness of this diagnostic tool in the allo-HSCT setting.

METHODS

From January 1999 to January 2001, all consecutive adult patients undergoing allo-HSCT were prospectively studied with a galactomannan antigenemia assay (ELISA test) twice weekly from admission until death or discharge, and weekly afterward if the patient received immunosuppressive therapy. Proven, probable, and possible IA were defined according to the European Organization for Research and Treatment of Cancer/Mycoses Study Group criteria.

RESULTS

During the 2 years of study, 74 patients underwent an allo-HSCT. A total of 832 serum samples were collected. According to the European Organization for Research and Treatment of Cancer/Mycoses Study Group criteria, it was ascertained that 66 patients did not fulfill any criteria of IA, 2 patients were classified with possible IA, 5 patients were classified with probable IA, and 1 patient was classified with proven IA. Fourteen samples were positive for AGA, all from patients with IA. The sensitivity and specificity of the test were 75% and 100%, respectively. The positive predictive and negative predictive values were 100% and 97%, respectively.

CONCLUSIONS

In this study, AGA detection was clearly related to IA. Although the ELISA test did not have any role in the anticipation of the diagnosis, it clarifies the diagnosis of IA in allo-HSCT.

Detection of circulating galactomannan for the diagnosis and management of invasive aspergillosis

The availability of the Platelia Aspergillus, a sandwich ELISA kit that detects circulating galactomannan, has been a major advance for managing patients at risk for invasive aspergillosis because of the early detection of the antigen. The assay is now widely used throughout the world, including the USA. Although initial studies that assessed the performance characteristics of this assay reported high sensitivity and specificity, more recent studies show significant variation in performance. The causes of this variability are multifactorial and, in large part, cannot be explained because there is insufficient understanding of the kinetics of galactomannan in vivo. We explored some of the factors that affect the release of the aspergillus antigen that bears the epitope that reacts with the monoclonal antibody used in the ELISA, its leakage from the site of infection into the blood, and its binding to substances present in the blood. Factors that affect the detection of antigen in blood are also discussed, most notably the pretreatment procedure aimed at liberating the antigen from immune complexes. Understanding the biology of galactomannan release by aspergillus will greatly enhance our understanding of the kinetics of this and other surrogate markers and allow their optimum use in the management of invasive aspergillosis.

Reactivity of platelia Aspergillus galactomannan antigen with piperacillin-tazobactam: clinical implications based on achievable concentrations in serum

The possible reactivities of commonly used antibiotics of fungal, nonfungal, and nonmicrobial or synthetic sources with the Platelia Aspergillus galactomannan assay were assessed. For drugs that tested positive, the minimal concentration of the antibiotic in serum that yielded a positive test (index, >0.5) was determined. At undiluted concentrations, piperacillin and multiple lots of piperacillin-tazobactam tested positive, whereas amoxicillin, ampicillin-sulbactam, nafcillin, cefazolin, ceftazidime, erythromycin, gentamicin, and levofloxacin tested negative. All three lots of piperacillin-tazobactam and all bags within each lot tested positive, with a mean index value of 5.168. At achievable concentrations in serum, however, only one of three lots of piperacillin-tazobactam yielded a positive test. Concentrations of 75, 150, and 300 microg/ml of serum tested positive with the Platelia Aspergillus enzyme immunoassay, whereas lower concentrations, mimicking the trough levels, tested negative. Thus, while achievable serum piperacillin-tazobactam concentrations may potentially result in a positive test for galactomannan, the timing of the collection of serum samples from patients may influence the test results, with reactivity being less likely in samples collected at trough levels or prior to the administration of a dose of the antibiotic.

Bifidobacterium lipoteichoic acid and false ELISA reactivity in aspergillus antigen detection

A major difficulty with the detection of circulating galactomannan, a cell-wall polysaccharide released by Aspergillus sp during growth, in the serodiagnosis of invasive aspergillosis is the occurrence of false-positive ELISA results, especially in neonates and infants. On the basis of molecule similarity, we postulate that a lipoteichoic acid of Bifidobacterium sp can act as epitope for the monoclonal antibody used in the ELISA. The neonatal gut is heavily colonised with Bifidobacterium sp and these bacteria or their lipoteichoic acid might cause ELISA reactivity with serum after translocation because of immaturity of the intestinal mucosa. If our hypothesis is correct, we might find a method to discriminate between false-positive and true-positive ELISA results and thereby prevent unnecessary pre-emptive treatment of patients.

Characterization of a cell-wall acid phosphatase (PhoAp) in Aspergillus fumigatus

In the filamentous fungus Aspergillus fumigatus, the vast majority of the cell-wall-associated proteins are secreted proteins that are in transit in the cell wall. These proteins can be solubilized by detergents and reducing agents. Incubation of a SDS/beta-mercaptoethanol-treated cell-wall extract with various recombinant enzymes that hydrolyse cell-wall polysaccharides resulted in the release of a unique protein in minute amounts only after incubation of the cell wall in the presence of 1,3-beta-glucanase. Sequence analysis and biochemical studies showed that this glycoprotein, with an apparent molecular mass of 80 kDa, was an acid phosphatase (PhoAp) that was active on both phosphate monoesters and phosphate diesters. PhoAp is a glycosylphosphatidylinositol-anchored protein that was recovered in the culture filtrate and cell-wall fraction of A. fumigatus after cleavage of its anchor. It is also a phosphate-repressible acid phosphatase. The absence of PhoAp from a phosphate-rich medium was not associated with a reduction in fungal growth, indicating that this cell-wall-associated protein does not play a role in the morphogenesis of A. fumigatus.

Aspergillus galactomannan antigen in the cerebrospinal fluid of bone marrow transplant recipients with probable cerebral aspergillosis

The Aspergillus galactomannan test was performed on cerebrospinal fluid and serum samples from 5 patients with probable cerebral aspergillosis and from 16 control patients. Cerebrospinal fluid galactomannan levels were significantly higher in aspergillosis patients, and most galactomannan was produced intrathecally. Comparison of serum galactomannan values in pulmonary and cerebral aspergillosis patients showed significant overlapping. Detection of Aspergillus galactomannan in cerebrospinal fluid may be diagnostic of cerebral aspergillosis.

Dendritic cells transport conidia and hyphae of Aspergillus fumigatus from the airways to the draining lymph nodes and initiate disparate Th responses to the fungus

Aspergilli are respiratory pathogens and pulmonary infections are usually acquired through the inhalation of conidia, able to reach small airways and the alveolar space where the impaired host defense mechanisms allow hyphal germination and subsequent tissue invasion. The invasive pulmonary aspergillosis is the most common manifestation of Aspergillus fumigatus infection in immunocompromised patients and is characterized by hyphal invasion and destruction of pulmonary tissue. A Th1/Th2 dysregulation and a switch to a Th2 immune response may contribute to the development and unfavorable outcome of invasive pulmonary aspergillosis. Dendritic cells (DC) have a primary role in surveillance for pathogens at the mucosal surfaces and are recognized as the initiators of immune responses to them. In the present study, we assessed the functional activity of pulmonary DC in response to A. fumigatus conidia and hyphae, both in vitro and in vivo. We analyzed mechanisms and receptors for phagocytosis by DC as well as DC migration, maturation, and Th priming in vivo upon exposure to either form of the fungus. We found a remarkable functional plasticity of DC in response to the different forms of the fungus, as pulmonary DC were able to: 1) internalize conidia and hyphae of A. fumigatus through distinct phagocytic mechanisms and recognition receptors; 2) discriminate between the different forms in terms of cytokine production; 3) undergo functional maturation upon migration to the draining lymph nodes and spleens; and 4) instruct local and peripheral Th cell reactivity to the fungus.

Characterization of AFMP1: a novel target for serodiagnosis of aspergillosis

We cloned the AFMP1 gene, which encodes the first antigenic cell wall galactomannoprotein in Aspergillus fumigatus. AFMP1 codes for a protein, Afmp1p, of 284 amino acid residues, with a few sequence features that are present in Mp1p, the antigenic cell wall mannoprotein in Penicillium marneffei that we described previously, as well as several other cell wall proteins of Saccharomyces cerevisiae and Candida albicans. It contains a serine- and threonine-rich region for O glycosylation, a signal peptide, and a putative glycosylphosphatidyl inositol attachment signal sequence. Specific anti-Afmp1p antibody was generated with recombinant Afmp1p protein purified from Escherichia coli to allow further characterization of Afmp1p. Afmp1p has a high affinity for Galanthus nivalis agglutinin, a characteristic indicative of a mannoprotein. Furthermore, it was recognized by a rat monoclonal antibody against the galactofuran side chain of galactomannan, indicating that it is a galactomannoprotein. Ultrastructural analysis by immunogold staining indicated that Afmp1p is present in the cell walls of the hyphae and conidia of A. fumigatus. Finally, it was observed that patients with aspergilloma and invasive aspergillosis due to A. fumigatus develop a specific antibody response against Afmp1p. This suggested that the recombinant protein and its antibody may be useful for serodiagnosis in patients with aspergilloma or invasive aspergillosis, and the protein may represent a good cell surface target for host humoral immunity.

The effect of pH on glucoamylase production, glycosylation and chemostat evolution of Aspergillus niger

The effect of ambient pH on production and glycosylation of glucoamylase (GAM) and on the generation of a morphological mutant produced by Aspergillus niger strain B1 (a transformant containing an additional 20 copies of the homologous GAM glaA gene) was studied. We have shown that a change in the pH from 4 to 5.4 during continuous cultivation of the A. niger B1 strain instigates or accelerates the spontaneous generation of a morphological mutant (LB). This mutant strain produced approx. 50% less extracellular protein and GAM during both chemostat and batch cultivation compared to another strain with parental-type morphology (PS). The intracellular levels of GAM were also lower in the LB strain. In addition, cultivation of the original parent B1 strain in a batch-pulse bioreactor at pH 5.5 resulted in a 9-fold drop in GAM production and a 5-fold drop in extracellular protein compared to that obtained at pH 4. Glycosylation analysis of the glucoamylases purified from shake-flask cultivation showed that both principal forms of GAM secreted by the LB strain possessed enhanced galactosylation (2-fold), compared to those of the PS. Four diagnostic methods (immunostaining, mild methanolysis, mild acid hydrolysis and beta-galactofuranosidase digestion) provided evidence that the majority of this galactose was of the furanoic conformation. The GAMs produced during batch-pulse cultivation at pH 5.5 similarly showed an approx. 2-fold increase in galactofuranosylation compared to pH 4. Interestingly, in both cases the increased galactofuranosylation appears primarily restricted to the O-linked glycan component. Ambient pH therefore regulates both GAM production and influences its glycosylation.

Galactofuranoic-oligomannose N-linked glycans of alpha-galactosidase A from Aspergillus niger

Extracellular alpha-galactosidase A was purified from the culture filtrate of an over-producing strain of Aspergillus niger containing multiple copies of the encoding aglA gene under the control of the glucoamylase (glaA) promoter. Endoglycosidase digestion followed by SDS/PAGE, lectin and immunoblotting suggested that glycosylation accounted for approximately 25% of the molecular size of the purified protein. Monosaccharide analysis showed that this was composed of N-acetyl glucosamine, mannose and galactose. Mild acid hydrolysis, mild methanolysis, immunoblotting and exoglycosidase digestion indicated that the majority of the galactosyl component was in the furanoic conformation (beta-D-galactofuranose, Galf). At least 20 different N-linked oligosaccharides were fractionated by high-pH anion-exchange chromatography following release from the polypeptide by peptide-N-glycosidase F. The structures of these were subsequently determined by fast atom bombardment mass spectrometry to be a linear series of Hex(7-26)HexHA(c2). Indicating that oligosaccharides from GlcNA(c2)Man(7), increasing in molecular size up to GlcNA(c2)Man(24) were present. Each of these were additionally substituted with up to three beta-Galf residues. Linkage analysis confirmed the presence of mild acid labile terminal hexofuranose residues. These results show that filamentous fungi are capable of producing a heterogeneous mixture of high molecular-size N-linked glycans substituted with galactofuranoic residues, on a secreted glycoprotein.

Beta-galactofuranoside glycoconjugates on conidia and conidiophores of Aspergillus niger

Galactose in the furanoic conformation appears to be limited to bacteria and lower eukaryotes. Galactofuranoic (Galf)-containing glycoconjugates that occur in organisms pathogenic or allergenic to man are frequently antigenic and immunodominant. We have used an immunochemical approach, employing a monoclonal antibody that recognises Galf epitopes, to investigate the presence of Galf-containing glycoconjugates within conidia and conidiophores of Aspergillus niger. ELISA and immunofluorescence microscopy indicated that specific and saturable binding sites were found on both. Inhibition studies confirmed that this binding was to Galf-containing glycoconjugates. Interestingly, the conidiophore heads were particularly rich in these glycoconjugates. Western blotting identified a Galf glycoprotein of 150-200 kDa from disrupted conidia.

Use of recombinant mitogillin for improved serodiagnosis of Aspergillus fumigatus-associated diseases

During human infection, Aspergillus fumigatus secretes a 18-kDa protein that can be detected as an immunodominant antigen in the urine of infected patients. Recently, this protein was shown to be mitogillin, a ribotoxin that cleaves a single phosphodiester bond of the 29S rRNA of eukaryotic ribosomes. We proved the immunogenic capacity of mitogillin in a rabbit animal model, indicating its usefulness as an antigen for serological diagnosis of invasive aspergillosis. The mitogillin gene from A. fumigatus was transferred from plasmid pMIT+ to expression vector pQE30 and expressed in Escherichia coli as a fusion protein. Purified recombinant mitogillin was recognized by serum immunoglobulin G (IgG) of polyclonal rabbit sera that were obtained by immunization with purified native mitogillin. Consequently, we developed an enzyme-linked immunosorbent assay for detection of IgG, IgM, and IgA antibodies to recombinant mitogillin. In serum samples of patients suffering from aspergilloma (AO; n = 32), invasive pulmonary aspergillosis (IPA; n = 42), or invasive disseminated aspergillosis (IDA; n = 40), a good correlation of production of IgG antibody against mitogillin and clinical disease was observed (for patients with AO, 100% [32 of 32] were positive; for patients with IPA, 64% [31 of 42] were positive; for patients with IDA, 60% [24 of 40] were positive). In contrast, positive titers for serum IgG and IgM antibodies against mitogillin were found in only 1.3% of the serum samples of healthy volunteers and positive titers for IgA antibody were found in only 1.0% of the serum samples of healthy volunteers (n = 307; specificity = 95.4%). These results indicate that recombinant mitogillin expressed in E. coli can be used for improvement of the serodiagnosis of A. fumigatus-associated diseases.

An extracellular beta-galactofuranosidase from Aspergillus niger and its use as a tool for glycoconjugate analysis

Aspergillus niger produces an extracellular beta-galactofuranosidase, which can specifically hydrolyse beta-D-galactofuranose (Galf) from glycoconjugates. The production of this enzyme can be induced by the addition of a Galf-containing A. niger mycelial wall extract. However, on other carbon sources accumulation occurred only during the starvation conditions of the late stationary phase. Extracellular glucoamylases from this stage of cultivation possessed significantly lower levels of Galf than those from the earlier exponential growth phase when beta-galactofuranosidase is absent, suggesting in situ beta-galactofuranosidic hydrolysis. The beta-galactofuranosidase responsible was subsequently purified to homogeneity and characterised. It is a glycoprotein of 90 kDa (determined by SDS-PAGE) with activity against beta-linked Galf residues, with a Km of 4 mM against p-nitrophenyl-beta-D-galactofuranoside and a pH optimum of 3-4. The preparation did not contain other contaminating glycosidase activities; p-nitrophenyl-beta-D- and -alpha-D-galactopyranose, and alpha-D-methyl-Galf were not hydrolysed. Results are presented to show that this enzyme could be employed as a useful tool for the analysis of glycoconjugates containing biologically important Galf components.