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

Exploring Virulence Determinants of Filamentous Fungal Pathogens through Interactions with Soil Amoebae

Infections with filamentous fungi are common to all animals, but attention is rising especially due to the increasing incidence and high mortality rates observed in immunocompromised human individuals. Here, Aspergillus fumigatus and other members of its genus are the leading causative agents. Attributes like their saprophytic life-style in various ecological niches coupled with nutritional flexibility and a broad host range have fostered the hypothesis that environmental predators could have been the actual target for some of their virulence determinants. In this mini review, we have merged the recent findings focused on the potential dual-use of fungal defense strategies against innate immune cells and soil amoebae as natural phagocytes. Well-established virulence attributes like the melanized surface of fungal conidia or their capacity to produce toxic secondary metabolites have also been found to be protective against the model amoeba Dictyostelium discoideum. Some of the recent advances during interaction studies with human cells have further promoted the adaptation of other amoeba infection models, including the wide-spread generalist Acanthamoeba castellanii, or less prominent representatives like Vermamoeba vermiformis. We further highlight prospects and limits of these natural phagocyte models with regard to the infection biology of filamentous fungi and in comparison to the phagocytes of the innate immune system.

NOX2 complex-derived ROS as immune regulators

Reactive oxygen species (ROS) are a heterogeneous group of highly reactive molecules that oxidize targets in a biologic system. During steady-state conditions, ROS are constantly produced in the electron-transport chain during cellular respiration and by various constitutively active oxidases. ROS production can also be induced by activation of the phagocyte NADPH oxidase 2 (NOX2) complex in a process generally referred to as an oxidative burst. The induced ROS have long been considered proinflammatory, causing cell and tissue destruction. Recent findings have challenged this inflammatory role of ROS, and today, ROS are also known to regulate immune responses and cell proliferation and to determine T-cell autoreactivity. NOX2-derived ROS have been shown to suppress antigen-dependent T-cell reactivity and remarkably to reduce the severity of experimental arthritis in both rats and mice. In this review, we discuss the role of ROS and the NOX2 complex as suppressors of autoimmunity, inflammation, and arthritis.

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.

Effect of aeration on gliotoxin production by Aspergillus fumigatus in its culture filtrate

Gliotoxin, one of the mycotoxins produced by Aspergillus fumigatus, has various, potent bioactivities. However, it has not been considered to be a toxic (or virulence) factor because of its slow production. The aim of the present study was to investigate the effects of aeration on the cytotoxicity of A. fumigatus culture filtrate, and to determine the optimal condition for the rapid production of gliotoxin from this fungus. Fungal culture filtrates were made in three different containers under various conditions of aeration and O2 concentration. These filtrates were compared in terms of their cytotoxicity on murine macrophages and analyzed by gas chromatography. The culture filtrate showed high cytotoxicity when it was made under highly aerated conditions, but it was significantly less cytotoxic when prepared under non-aerated conditions. The cytotoxic activity became evident within 15 h of culture at 20% O2, when the fungus had already started producing gliotoxin. The culture filtrates also contained some other as yet unidentified substances that might also to some extent contribute to the cytotoxicity. In light of these results, the authors propose that a highly aerated condition is responsible for the rapid production of gliotoxin, and that gliotoxin might play an important role in the respiratory infection by A. fumigatus, with other toxic substances acting additively or synergistically.

Immunosuppressive substances in Aspergillus fumigatus culture filtrate

Invasive aspergillosis has become a serious problem in clinical practice, but the actual factor that confers virulence on the fungus has not been thoroughly elucidated. To identify and isolate the immunosuppressive substances produced by the fungus, the bioactivity of culture filtrates was assessed, and analyses of the culture filtrates were carried out. Culture filtrates from different strains of Aspergillus fumigatus were assessed for their effect on human polymorphonuclear leukocytes and murine macrophages. To assess their activities in vivo, their effect on the survival of mice infected by the fungus was also studied. Subsequently, the composition of the culture filtrates was analyzed by gas chromatography-mass spectrometry. The analyses revealed that the culture filtrates contained gliotoxin at concentrations of 3 to 4 microgram/ml, and some other unidentified compounds. The bioactivities of the culture filtrates were similar to those of gliotoxin. The fungal culture filtrate reduced the survival of infected mice, but the filtrate itself did not cause the death of mice. However, all the bioactivities could not be accounted for by gliotoxin itself. These results indicate that gliotoxin in the culture filtrates may be responsible for part of the immunosuppressive activity, but some other components produced by A. fumigatus contribute, in an additive or synergistic manner, to the virulence of the fungus.

Exhaled nitric oxide in cystic fibrosis patients with allergic bronchopulmonary aspergillosis

Exhaled nitric oxide (NO) is thought to be a marker of asthmatic inflammation. Levels in cystic fibrosis (CF) are generally low. This study aimed to measure exhaled NO in CF patients at high risk of developing ABPA and patients at low risk. We studied nine patients at high risk of developing ABPA and 36 at low risk. The two groups were similar in age and spirometry. All patients in the high-risk group were taking oral or inhaled glucocorticoids, compared to 56% in the low-risk group (P=0.02). The exhaled NO levels were lower in the high-risk group than in the low-risk group (2.0 vs. 3.6 ppb), mean difference (95% CI) 1.6 (-3.6 to 0.4) ppb, P=0.001. On subgroup analysis of patients on oral glucocorticoids, the exhaled NO levels were significantly lower in patients with a high risk of developing ABPA (n=7) than patients with a low risk (n=8) (P=0.011). The number of patients who were on inhaled, but not oral glucocorticoids was too small to analyse usefully. Exhaled NO levels were lower in CF patients with a high risk of developing ABPA and on glucocorticoids. This may be because oral glucocorticoids exert a greater effect on exhaled NO than inhaled glucocorticoids. Alternatively, inducible nitric oxide synthase may be down-regulated by Aspergillus toxin.

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.

Human alveolar macrophage responses to air pollution particulates are associated with insoluble components of coarse material, including particulate endotoxin

Inhalation of particulate matter in the ambient air has been shown to cause pulmonary morbidity and exacerbate asthma. Alveolar macrophage (AM) are essential for effective removal of inhaled particles and microbes in the lower airways. While some particles minimally effect AM function others inhibit antimicrobial activity or cause cytokine and growth factor production leading to inflammation and tissue remodeling. This study has investigated the effects of water soluble (s) and insoluble (is) components of Chapel Hill, North Carolina ambient particulate matter in the size ranges 0.1-2.5 microm (PM2.5) and 2.5-10 microm (PM10) diameter, on human AM IL-6, TNFalpha, and MCP-1 cytokine production and host defense mechanisms including phagocytosis and oxidant production. Cytokines were found to be induced by isPM10 to a much higher extent (>50-fold) than sPM10, which in turn stimulated production better than isPM2.5, while sPM2.5 was inactive. Previous studies have indicated that endotoxin (ETOX) is a component of sPM10 responsible for cytokine production. Here, it is shown that inhibition of isPM10-induced cytokine production was partially achieved with polymyxin B and LPS-binding protein (LBP), but not with a metal chelator, implicating ETOX as a cytokine-inducing moiety also in isPM10. In addition to inducing cytokines, exposure to isPM10, but not the other PM fractions, also inhibited phagocytosis and oxidant generation in response to yeast. This inhibition was ETOX independent. The decrease in host defenses may be the result of apoptosis in the AM population, which was also found to be specifically caused by isPM10. These results show that the functional capacity of AM is selectively modulated by insoluble components of coarse PM, including the biocontaminant ETOX.

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.

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.

Effects of Aspergillus fumigatus culture filtrate on antifungal activity of human phagocytes in vitro

BACKGROUND

Aspergillus fumigatus can colonise the airways and the lungs with localised underlying conditions and occasionally invade the surrounding lung tissues even in subjects without systemic predisposing factors, presumably by escaping the local host defences. The aim of this study was to investigate the effects of A fumigatus culture filtrate (ACF) on the activities of human phagocytes--inhibition of germination of A fumigatus spores by alveolar macrophages (AMs) and hyphal damage by polymorphonuclear leucocytes (PMNs)--which are the critical host defences against A fumigatus.

METHODS

Spores were incubated with AMs at a ratio of 1:1 in a medium containing different concentrations of ACF for 10 hours at 37 degrees C. Spore germination was visualised with light microscopy and the inhibition rate was calculated. The percentage of hyphal damage caused by PMNs pretreated with various concentrations of ACF was measured by a colorimetric tetrazolium metabolic assay.

RESULTS

The inhibition rate of spore germination by AMs cultured with medium alone (control) was 90 (0.8)% whereas that by AMs cultured with the medium containing 10% ACF was significantly (p < 0.05) reduced to 41.7 (4.6)%. ACF suppressed the inhibition of spore germination in a dose dependent manner without altering the phagocytosing activity against the spores. The percentage of hyphal damage caused by PMNs pretreated with medium-199 (control) was 78.1 (2.3)% compared with 65.3 (2.8)% when PMNs were pretreated with 50% ACF (p < 0.05).

CONCLUSIONS

A fumigatus releases biologically active substance(s) which suppress the inhibition of spore germination by AMs and also suppress PMN mediated hyphal damage, and thus may contribute to the pathogenicity of this fungus.

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.

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.