Resistance of spores of Aspergillus fumigatus to ingestion by phagocytic cells

Dual effect of blue light on Fusariumsolani clinical corneal isolates in vitro

The purpose was to investigate the effect of daylight-intensity blue light on F. solani isolated from the cornea of patients with fungal keratitis. Spore suspensions of 5 F. solani strains (one standard strain and 4 clinical corneal isolates) were prepared in 6-well plates. Blue light groups were irradiated by a light-emitting diode (LED) device with a peak wavelength of 454 nm at 0.5 mW/cm² for 0 to 48 h, while the controls were maintained in darkness. Hyphal morphology in the 6-well plates was recorded at 0, 12, 24, 36, 48 h. One hundred microliters of spore suspensions of each strain at these five time points was transferred to SGA plates and cultured for 36 h at 29 °C; the number of colonies formed was counted as a measure of conidia quality and viability. Blue light has dual effects on F. solani. The hyphal length of F. solani exposed to blue light was significantly shorter than that of the control (P < 0.01), indicating that fungal growth was inhibited. Meanwhile, instead of reducing the viability of spores, blue light significantly enhanced the conidia quality and viability after at least 24 h irradiation. Daylight-intensity blue light exposure will inhibit the hyphal growth of F. solani but promote conidiation, which would be more harmful to fungal keratitis. Eliminating the influence of blue light for these patients should be taken into account.

Antifungal Treatments Delineate a Correlation between Cathepsins and Cytokines in Murine Model of Invasive Aspergillosis

In the pathogenesis of invasive pulmonary aspergillosis both fungal and host factors play roles. Though cytokines and phagocyte, as host factors, have been shown to participate in defence against Aspergillus species yet the role of cysteine proteases, that is cathepsins, a lysosomal enzymes of phagocytes, remains unknown in fungal infection. Studies are available which shows that cytokines regulate the cysteine proteases processed immune molecules for their further action but their relationship with each other under fungal infection is not clear. Therefore, in this study, we demonstrate the substantial role of cathepsins and cytokines in aspergillosis. In the present murine model of invasive pulmonary aspergillosis, on seventh day of Aspergillus fumigatus infection, both kidney and liver showed significant (P<0.05) fungal burdens, which was also confirmed by histological analyses. The activity profiles of four cathepsins in the kidney and liver tissue were analysed and correlated with blood cytokines level in the presence and absence of antifungal compounds (amphotericin B, a standard drug and 2-(3,4-dimethyl-2,5-dihydro-1H-pyrrole-2-yl)-1-methylethyl pentanoate, isolated in our laboratory from natural source) treatment. The data illustrate that the reduction in fungal load in both organs probably results in a decreased local inflammatory response, as measured by decreased levels of interleukin-4 and interleukin-10 and increased level of interferon gamma in the antifungal compounds treated mice. Interestingly, this altered level of cytokines relates well with the activity level of cathepsins, that is decreased in interleukines (interleukinL-4/interleukin-10) and cathepsins (cathepsin B, cathepsin C and cathepsin L); and increase in interferon gamma and cathepsin H levels in the mice treated with antifungal compounds were observed. These observations support not only the negative (cathepsin B, cathepsin C and cathepsin L) and positive (cathepsin H) role of cathepsins in aspergillosis but also prove the role of cytokines in remodelling of immune response. Overall, the study reveals a correlation between cathepsins and cytokines and their regulatory role in fungal mediated infection.

Phagocyte responses towards Aspergillus fumigatus

The saprophytic fungus Aspergillus fumigatus is a mold which is ubiquitously present in the environment. It produces large numbers of spores, called conidia that we constantly inhale with the breathing air. Healthy individuals normally do not suffer from true fungal infections with this pathogen. A normally robust resistance against Aspergillus is based on the presence of a very effective immunological defense system in the vertebrate body. Inhaled conidia are first encountered by lung-resident alveolar macrophages and then by neutrophil granulocytes. Both cell types are able to effectively ingest and destroy the fungus. Although some responses of the adaptive immune system develop, the key protection is mediated by innate immunity. The importance of phagocytes for defense against aspergillosis is also supported by large numbers of animal studies. Despite the production of aggressive chemicals that can extracellularly destroy fungal pathogens, the main effector mechanism of the innate immune system is phagocytosis. Very recently, the production of extracellular neutrophil extracellular traps (NETs) consisting of nuclear DNA has been added to the armamentarium that innate immune cells use against infection with Aspergillus. Phagocyte responses to Aspergillus are very broad, and a number of new observations have added to this complexity in recent years. To summarize established and newer findings, we will give an overview on current knowledge of the phagocyte system for the protection against Aspergillus.

Fungal spores: a critical review of the toxicological and epidemiological evidence as a basis for occupational exposure limit setting

Fungal spores are ubiquitous in the environment. However, exposure levels in workplaces where mouldy materials are handled are much higher than in common indoor and outdoor environments. Spores of all tested species induced inflammation in experimental studies. The response to mycotoxin-producing and pathogenic species was much stronger. In animal studies, nonallergic responses dominated after a single dose. Allergic responses also occurred, especially to mycotoxin-producing and pathogenic species, and after repeated exposures. Inhalation of a single spore dose by subjects with sick building syndrome indicated no observed effect levels of 4 x 10(3) Trichoderma harzianum spores/m(3) and 8 x 10(3) Penicillium chrysogenum spores/m(3) for lung function, respiratory symptoms, and inflammatory cells in the blood. In asthmatic patients allergic to Penicillium sp. or Alternaria alternata, lowest observed effect levels (LOELs) for reduced airway conductance were 1 x 10(4) and 2 x 10(4) spores/m(3), respectively. In epidemiological studies of highly exposed working populations lung function decline, respiratory symptoms and airway inflammation began to appear at exposure levels of 10(5) spores/m(3). Thus, human challenge and epidemiological studies support fairly consistent LOELs of approximately 10(5) spores/m(3) for diverse fungal species in nonsensitised populations. Mycotoxin-producing and pathogenic species have to be detected specifically, however, because of their higher toxicity.

Noninvasive pulmonary Aspergillus infections

Aspergillus can cause several forms of pulmonary disease ranging from colonization to invasive aspergillosis and largely depends on the underlying lung and immune function of the host. This article reviews the clinical presentation, diagnosis, pathogenesis, and treatment of noninvasive forms of Aspergillus infection, including allergic bronchopulmonary aspergillosis (ABPA), aspergilloma, and chronic pulmonary aspergillosis (CPA). ABPA is caused by a hypersensitivity reaction to Aspergillus species and is most commonly seen in patients who have asthma or cystic fibrosis. Aspergillomas, or fungus balls, can develop in previous areas of cavitary lung disease, most commonly from tuberculosis. CPA has also been termed semi-invasive aspergillosis and usually occurs in patients who have underlying lung disease or mild immunosuppression.

Pathogenesis of allergic bronchopulmonary aspergillosis and an evidence-based review of azoles in treatment

BACKGROUND

Allergic bronchopulmonary aspergillosis (ABPA) is a complex condition that affects people with asthma and cystic fibrosis (CF). It results from exposure to the fungus Aspergillus fumigatus, which leads to worsening airway inflammation and progressive damage to the lungs. The aim of this review is to outline the pathogenesis of the disorder, diagnostic criteria and to discuss the use of anti-fungal agents in its treatment.

METHODS

The Cochrane library of systematic reviews and the Cochrane database of controlled trials were searched for controlled trials on ABPA and its treatment in both asthma and CF. In addition, articles included within the reviews were examined separately, and a separate search carried out using Medline.

RESULTS

A systematic review for the use of azole anti-fungal agents in ABPA was identified for their use in both CF and non-CF-related disease. The review of ABPA alone identified two randomized-controlled trials of itraconazole in chronic disease. These trials demonstrated improvements in symptoms and immune activation, but were short-term trials and failed to show a significant change in lung function. No trials were identified in CF.

CONCLUSIONS

The use of anti-fungal agents in ABPA seems to be a rational one, with short-term efficacy demonstrated for the use of itraconazole. Further investigations are required to identify individuals who will benefit most from treatment and to establish the correct dose and means of delivering treatment in ABPA. Longer-term studies are required to demonstrate that treatment modifies the progressive decline in lung function seen with the disease.

Mild, moderate, and severe forms of allergic bronchopulmonary aspergillosis: a clinical and serologic evaluation

BACKGROUND

Allergic bronchopulmonary aspergillosis (ABPA) is a hypersensitivity disorder induced by Aspergillus species colonizing the bronchial tree. There are patients with asthma who fulfill the diagnostic criteria of ABPA by serologic evaluation (specific IgE/IgG to Aspergillus fumigatus), bronchography, CT, and or conventional linear tomography.

OBJECTIVE

To identify different forms of ABPA based on various diagnostic criteria.

METHODS

Eighteen patients with asthma fulfilling the criteria of ABPA were evaluated in the present study. Six patients each received a diagnosis of ABPA serologic positive (ABPA-S), ABPA with central bronchiectasis (ABPA-CB), and ABPA with central bronchiectasis and other radiologic features (ABPA-CB-ORF).

RESULTS

The spirometric changes in the ABPA-S group (group 1) were mild, in the ABPA-CB group (group 2) were moderate, and in the ABPA-CB-ORF group (group 3) were severe. Absolute eosinophil count was raised in each group but was maximum (1,233 micro L) in severe form of disease (group 3). Specific IgE against A fumigatus was raised in each group, and the maximum was 47.91 IU/mL in ABPA-CB-ORF. CT scan findings of the ABPA-S group were normal without central bronchiectasis. The exacerbation in symptoms was maximum in group 3 compared to other groups.

CONCLUSION

The present observations suggest that ABPA includes mild (ABPA-S), moderate (ABPA-CB), and severe (ABPA-CB-ORF) forms of disease. It is recommended, therefore, that the disease should be diagnosed early, treated at the mild form of disease (ABPA-S), and prevented from leading to ABPA-CB or ABPA-CB-ORF.

Evaluation of allergic bronchopulmonary aspergillosis in patients with and without central bronchiectasis

Allergic bronchopulmonary aspergillosis (ABPA) is a hypersensitivity disorder induced by Aspergillus species colonizing the bronchial tree. Thirty one patients fulfilling criteria of ABPA were evaluated in the present study. Eleven patients were diagnosed as ABPA-S (serological positive) and 20 patients as ABPA-CB (with central bronchiectasis). The two groups of patients were compared on the basis of clinical, serological, and radiographic observations. Serum anti Aspergillus fumigatus IgG was positive in 72% of cases of ABPA-S and 85% of ABPA-CB patients at the time of presentation. Specific IgE against A. fumigatus and total IgE were significantly lower in ABPA-S (specific IgE= 7.42 IU and total IgE= 1127 ng/mL) as compared to ABPA-CB (specific IgE = 44 IU and total IgE = 2874 ng/mL). The spirometric changes in ABPA-S (normal 80%, mild obstruction 10%, and severe obstruction 10%) were milder than in ABPA-CB (normal 40%, mild obstruction 10%, moderate obstruction 20%, and severe obstruction 30%). These patients were monitored closely for seasonal exacerbation with new pulmonary infiltrates which gave lower recordings in ABPA-S patients. No patient in the ABPA-S group progressed to end-stage lung disease. This may be due to early recognition and treatment. The present data suggest that ABPA-S represents the early stage of an apparently less aggressive form of ABPA than ABPA-CB.

Spore diffusate isolated from some strains of Aspergillus fumigatus inhibits phagocytosis by murine alveolar macrophages

Aspergillus fumigatus is a ubiquitous fungus that grows in decaying organic matter. It can cause disease in both immunodeficient and immunocompetent patients by using virulence factors to escape the host defenses. Some of these factors, such as a diffusate, released from the spores of A. fumigatus, have previously been described. This diffusate was demonstrated to inhibit oxidative burst and phagocytosis of coated red blood cells. The present study has shown that this substance can inhibit the phagocytosis of A. fumigatus spores by murine alveolar macrophages (MALU) and evaluated the action of this substance. We quantified phagocytosis by MALU cells with and without diffusate and evaluated the inhibition of phagocytosis by testing diffusates from different strains. We conclude that the spore diffusate of some strains of A. fumigatus can reversibly decrease the ability of alveolar macrophages to ingest A. fumigatus spores.

Allergic bronchopulmonary aspergillosis: new concepts of pathogenesis and treatment

Allergic bronchopulmonary aspergillosis (ABPA) is a condition that results from a hypersensitivity reaction to the fungus Aspergillus fumigatus. The purpose of the present review is to examine the pathogenesis of this condition and the evidence for treatments available. Allergic bronchopulmonary aspergillosis is characterized by an intense airway inflammation with eosinophils and the formation of mucus plugs. Clinically, there are periods of exacerbation and remission that may lead to proximal bronchiectasis and fibrotic lung disease. New evidence confirms the role of intense airway inflammation with eosinophils, but also suggests a role for interleukin (IL)-8/neutrophil-mediated inflammation in this process, and the potential deficiency of anti-inflammatory cytokines such as reduced IL-10. Treatment for ABPA has so far focused on corticosteroids to suppress eosinophilic airway inflammation. An expanding knowledge of the pathology of ABPA also suggests other therapies may be of potential benefit, particularly the use of azole antifungal agents. Allergic bronchopulmonary aspergillosis is itself an important complication of asthma and cystic fibrosis. A greater understanding of the condition is required to improve management and well-designed clinical trials need to be carried out to critically assess new and current treatments. In addition, the information gained from the studies of its pathogenesis has the potential to benefit our understanding of the disease processes in asthma and bronchiectasis.

Enhanced binding of Aspergillus fumigatus spores to A549 epithelial cells and extracellular matrix proteins by a component from the spore surface and inhibition by rat lung lavage fluid

BACKGROUND

Aspergillus fumigatus is a pathogenic fungus which causes a range of diseases, particularly in the human lung. The pathological mechanism is unknown but may involve a complex mixture of biomolecules which can diffuse from the spore surface. This material is known as A fumigatus diffusate (AfD) and has previously been shown to have a range of immunosuppressive functions. It is hypothesised that AfD may influence the binding of spores to extracellular matrix (ECM) proteins and lung epithelial cells, thereby affecting the ability of the fungus to cause infection.

METHODS

The binding of spores to ECM proteins and to epithelial cells was carried out using a direct binding assay in microtitre plates and spores were counted by phase contrast microscopy. Rat bronchoalveolar lavage (BAL) fluid was enriched for surfactant protein D (SP-D) using maltose agarose affinity chromatography. The effects of AfD and the SP-D enriched BAL fluid were assessed by pre-incubation with ECM proteins or epithelial cells in the direct binding assay.

RESULTS

AfD enhanced the binding of spores to laminin by 137% and to A549 epithelial cells by 250%. SP-D enriched BAL fluid inhibited spore binding to ECM proteins and epithelial cells. Pre-incubation of ECM proteins and epithelial cells with SP-D enriched BAL fluid prevented the enhancement of spore binding by AfD, and pre-incubation of ECM proteins and epithelial cells with AfD prevented the inhibition of spore binding by SP-D enriched BAL fluid. This pretreatment did not prevent the enhancement of spore binding, giving an increase of 95% for collagen I, 80% for fibronectin, 75% for laminin, and 150% for A549 cells.

CONCLUSIONS

The hypothesis that AfD would affect spore binding to ECM proteins and epithelial cells was confirmed. Rat BAL fluid, with SP-D as the possible bioactive agent, prevented this enhancement. The in vivo significance is unclear but the enhanced binding of spores may increase the chance of fungal infection in the lung which could be prevented by the protective effects of lung surfactant components (possibly SP-D). The results suggest that there may be competition between AfD and a BAL fluid component (possibly SP-D) for the same or similar binding sites on ECM proteins and epithelial cells. Whether this competition occurs in vivo requires further investigation.

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.

The effects of diffusates from the spores of Aspergillus fumigatus and A. terreus on human neutrophils, Naegleria gruberi and Acanthamoeba castellanii

Diffusates from dormant spores of Aspergillus fumigatus are inhibitory to certain functions of immune phagocytic cells and soil protozoa. An assay of human neutrophil phagocytosis and an in vitro method of measuring inhibition of the growth of free living amoebae are described. The anti-phagocytic and anti-amoebal effects of diffusates from clinical and environmental isolates of A. fumigatus and of A. terreus were measured using these methods. Spore diffusates from all isolates of A. fumigatus and A. terreus tested significantly inhibited neutrophil phagocytosis. Spore diffusates from A. fumigatus, but not A. terreus, significantly inhibited the growth of Naegleria gruberi. Spore diffusate from A. fumigatus did not inhibit the growth of Acanthamoeba castellanii. The relevance of these findings to the pathogenicity of A. fumigatus and A. terreus is discussed.

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.

Diffusible component from the spore surface of the fungus Aspergillus fumigatus which inhibits the macrophage oxidative burst is distinct from gliotoxin and other hyphal toxins

BACKGROUND

The fungus Aspergillus fumigatus, whose spores are present ubiquitously in the air, causes a range of diseases in the human lung. A small molecular weight (< 10 kD) heat stable toxin released from the spores of clinical and environmental isolates of A fumigatus within minutes of deposition in aqueous solution has previously been described. A key effect of the toxin was to inhibit the oxidative burst of macrophages as measured by superoxide anion release. It was hypothesised that the toxin was one of the commonly found A fumigatus hyphal toxins such as gliotoxin. This inhibitor may be an important factor which allows the fungus to colonise the lung.

METHODS

The spore derived inhibitor was shown to inhibit the respiratory burst of rat alveolar macrophages, as measured by the generation of superoxide anion. Samples of the spore diffusate were subject to reversed phase high performance liquid chromatography (HPLC), thin layer chromatography (TLC), high performance thin layer chromatography (HPTLC), or organic extraction followed by TLC or HPLC to identify the presence of gliotoxin, fumagillin, helvolic acid, fumigaclavine-C, and aurasperone-C. Commercially obtained preparations of the toxins gliotoxin, fumagillin and helvolic acid and extracts enriched for fumigaclavine-C and aurasperone-C were used as internal and external standards and in the respiratory burst measurements.

RESULTS

Gliotoxin, fumagillin, helvolic acid, fumigaclavine-C, and aurasperone-C were not detected in spore derived diffusate using PHLC or TLC. Using extraction procedures with solvents known to extract gliotoxin from A fumigatus culture supernatants, no gliotoxin was detected in the spore derived diffusate. Commercial gliotoxin, fumagillin, and helvolic acid or extracts enriched for fumigaclavine-C and aurasperone-C did not inhibit the oxidative burst of macrophages.

CONCLUSIONS

The hypothesis that the spore derived toxin is one of the toxins derived from hyphae such as gliotoxin, helvolic acid, fumagillin, fumigaclavine-C, or aurasperone-C is not proved. The spore toxin may exert its effect through its ability to diffuse rapidly into the lung lining fluid, diminish the macrophage oxidative burst, and play a part in allowing A fumigatus to persist in the lung and manifest its well known pathogenic effects.

Absence of respiratory burst in X-linked chronic granulomatous disease mice leads to abnormalities in both host defense and inflammatory response to Aspergillus fumigatus

Mice with X-linked chronic granulomatous disease (CGD) generated by targeted disruption of the gp91phox subunit of the NADPH-oxidase complex (X-CGD mice) were examined for their response to respiratory challenge with Aspergillus fumigatus. This opportunistic fungal pathogen causes infection in CGD patients due to the deficient generation of neutrophil respiratory burst oxidants important for damaging A. fumigatus hyphae. Alveolar macrophages from X-CGD mice were found to kill A. fumigatus conidia in vitro as effectively as alveolar macrophages from wild-type mice. Pulmonary disease in X-CGD mice was observed after administration of doses ranging from 10(5) to 48 spores, none of which produced disease in wild-type mice. Higher doses produced a rapidly fatal bronchopneumonia in X-CGD mice, whereas progression of disease was slower at lower doses, with development of chronic inflammatory lesions. Marked differences were also observed in the response of X-CGD mice to the administration of sterilized Aspergillus hyphae into the lung. Within 24 hours of administration, X-CGD mice had significantly higher numbers of alveolar neutrophils and increased expression of the proinflammatory cytokines IL-1 beta and TNF-alpha relative to the responses seen in wild-type mice. By one week after administration, pulmonary inflammation was resolving in wild-type mice, whereas X-CGD mice developed chronic granulomatous lesions that persisted for at least six weeks. This is the first experimental evidence that chronic inflammation in CGD does not always result from persistent infection, and suggests that the clinical manifestations of this disorder reflect both impaired microbial killing as well as other abnormalities in the inflammatory response in the absence of a respiratory burst.

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.

Inhibition of the alveolar macrophage oxidative burst by a diffusible component from the surface of the spores of the fungus Aspergillus fumigatus

BACKGROUND

Aspergillus fumigatus is a fungus that grows on dead and decaying organic matter in the environment and whose spores are present ubiquitously in the air. The fungus causes a range of diseases in the human lung. A study was undertaken to demonstrate and partially characterise an inhibitor of the macrophage respiratory burst from the surface of A fumigatus spores that could be an important factor in allowing the fungus to colonise the lung.

METHODS

The spore-derived inhibitor of the respiratory burst of rat alveolar macrophages, as measured by generation of superoxide anion, was demonstrated in Hank's balanced salt solution extracts of four clinical isolates and an environmental isolate of A fumigatus. The time course of the release of the inhibitor into aqueous solution was assessed and the cytotoxic potential of the spore-derived inhibitor towards macrophages was tested using the propidium iodide method. An oxygen electrode was used to confirm the superoxide anion measurements. Molecular weight cutoff filters were used to determine the size of the inhibitor as assessed in the respiratory burst assay and also by its ability to inhibit macrophage spreading on glass. The crude diffusate from the spore surface was fractionated by reversed phase high pressure liquid chromatography (HPLC) and the fractions analysed for inhibitory activity, protein, and carbohydrate content.

RESULTS

A small molecular weight (< 10 kD) heat stable toxin was released from the spores of clinical and environmental isolates of A fumigatus within minutes of deposition in aqueous solution. The key effect of the toxin demonstrated here was its ability to inhibit the oxidative burst of macrophages as measured by superoxide anion release. The inhibition was not due to cell death or detectable loss of membrane integrity as measured by permeability to propidium iodide. The toxin was not a scavenger of superoxide anion. Oxygen electrode studies suggested indirectly that the inhibitor acted to inhibit the assembly of the macrophage NADPH-oxidase complex. Fractions of < 10 kD also inhibited the spreading of alveolar macrophages, confirming that the toxin had an additional effect on macrophages that leads to loss of adherence or impairment of cytoskeletal function. In reversed phase HPLC fractions the inhibitory activity eluted with an associated carbohydrate, although the exact chemical nature of the toxin remains to be elucidated.

CONCLUSIONS

This spore toxin may, through its ability to diffuse rapidly into lung lining fluid, diminish the macrophage respiratory burst and play a part in allowing A fumigatus to persist in the lung and manifest its well known pathogenic effects. Future research will be focused on further molecular characterisation of the toxin and elaboration of the effect of the toxin on intracellular signalling pathways involved in the activation of alveolar macrophages.

Intratracheal exposure of rats to Aspergillus fumigatus spores isolated from sawmills in Sweden

Five strains of Aspergillus fumigatus (A, B, D, H, and K) isolated from sawmills were used to expose groups of three rats by intratracheal intubation. The dose was 10(9) spores per rat. At 48 h after administration, two rats from the D group and all rats from the K group died with symptoms of strong dyspnea and tachypnea. At 72 h postadministration and after, some animals showed mild to moderate dyspnea and tachypnea. Autopsies of all animals were performed, including a histopathological examination of the lungs. At 72 h after administration, two distinct morphological groups were identified histopathologically. Severe necrotizing pneumonia characterized by the presence of abundant fungal hyphae was seen in animals that died spontaneously within 48 h postadministration and rats with bronchopneumonia and was characterized by the presence of numerous fungal spores. There was an obvious difference in pathogenicity among the strains of A. fumigatus. Strains D and K were more pathogenic, and only the rats exposed to these strains showed the presence of fungal hyphae in the lungs. The mycotoxin gliotoxin that is produced by A. fumigatus and has antiphagocytic activity was not detected in the spores from any of the A. fumigatus strains.

Killing of Aspergillus fumigatus spores by human lung macrophages: a paradoxical effect of heat-labile serum components

Spores of Aspergillus fumigatus can interfere with certain aspects of phagocytic cell function. In this study we have looked at the ability of human pulmonary macrophages to bind and kill spores of A. fumigatus which have been opsonized in untreated and heat-treated serum. For comparison, the non-pathogenic fungus Penicillium ochrochloron has been used. More than 60% of spores of both fungal species became cell-associated with the macrophages following incubation at 37 degrees C for 1 h. Spores of A. fumigatus opsonized in 5% pooled normal serum were significantly more resistant to killing by pulmonary macrophages than similarly opsonized spores of P. ochrochloron (p less than 0.02). However, serum which had been heated to 56 degrees C for 30 min prior to opsonization significantly increased (by approximately 80%) the ability of pulmonary macrophages to kill spores of A. fumigatus when compared with untreated sera (p less than 0.001). No such difference occurred with spores of P. ochrochloron. These unexpected observations fit with the known propensity of A. fumigatus to colonize the airways of patients with asthma and cystic fibrosis, conditions in which a protein-rich bronchial exudate characteristically occurs. Moreover, the presence of such a protective mechanism in a soil organism strongly suggests that complement-like substances may also play a role in protozoal phagocytosis. This is an area of research that does not appear to have been investigated.

Inhibition of phagocyte migration and spreading by spore diffusates of Aspergillus fumigatus

Previous studies have shown that spores of Aspergillus fumigatus inhibit phagocytosis and killing by macrophages and polymorphonuclear leucocytes. In order to identify the mechanisms of this interference with host defences, we have examined the effects of A. fumigatus spore diffusates on phagocytic cell function. For comparison, we have used spore diffusates of the non-pathogenic fungus Penicillium ochrochloron. The diffusates of A. fumigatus reduced the number of human polymorphonuclear leukocytes migrating towards a known chemoattractant by approximately 50% (p less than 0.001). In addition spore diffusates of A. fumigatus significantly decreased (p less than 0.001) the capacity of primed mouse peritoneal exudate cells to spread on glass. Spore diffusates of P. ochrochloron showed no comparable inhibitory effects. These studies have shown that spore diffusates of A. fumigatus inhibit the movement of the phagocytic cell membrane and are thus able to interfere with a primary function of phagocytic cells.