Early events in macrophage killing of Aspergillus fumigatus conidia: new flow cytometric viability assay

Pathogenicity and virulence of Aspergillus fumigatus

Pulmonary infections caused by the mould pathogen Aspergillus fumigatus are a major cause of morbidity and mortality globally. Compromised lung defences arising from immunosuppression, chronic respiratory conditions or more recently, concomitant viral or bacterial pulmonary infections are recognised risks factors for the development of pulmonary aspergillosis. In this review, we will summarise our current knowledge of the mechanistic basis of pulmonary aspergillosis with a focus on emerging at-risk populations.

Extracellular vesicles derived from Talaromyces marneffei contain immunogenic compounds and modulate THP-1 macrophage responses

Pathogenic eukaryotes including fungi release extracellular vesicles (EVs) which are composed of a variety of bioactive components, including peptides, nucleic acids, polysaccharides, and membrane lipids. EVs contain virulence-associated molecules suggesting a crucial role of these structures in disease pathogenesis. EVs derived from the pathogenic yeast phase of Talaromyces (Penicillium) marneffei, a causative agent of systemic opportunistic mycoses "talaromycosis," were studied for their immunogenic components and immunomodulatory properties. Some important virulence factors in EVs including fungal melanin and yeast phase specific mannoprotein were determined by immunoblotting. Furthermore, fluorescence microscopy revealed that T. marneffei EVs were internalized by THP-1 human macrophages. Co-incubation of T. marneffei EVs with THP-1 human macrophages resulted in increased levels of supernatant interleukin (IL)-1β, IL-6 and IL-10. The expression of THP-1 macrophage surface CD86 was significantly increased after exposed to T. marneffei EVs. These findings support the hypothesis that fungal EVs play an important role in macrophage "classical" M1 polarization. T. marneffei EVs preparations also increased phagocytosis, suggesting that EV components stimulate THP-1 macrophages to produce effective antimicrobial compounds. In addition, T. marneffei EVs stimulated THP-1 macrophages were more effective at killing T. marneffei conidia. These results indicate that T. marneffei EVs can potently modulate macrophage functions, resulting in the activation of these innate immune cells to enhance their antimicrobial activity.

Computational Modeling of Macrophage Iron Sequestration during Host Defense against Aspergillus

Iron is essential to the virulence of Aspergillus species, and restricting iron availability is a critical mechanism of antimicrobial host defense. Macrophages recruited to the site of infection are at the crux of this process, employing multiple intersecting mechanisms to orchestrate iron sequestration from pathogens. To gain an integrated understanding of how this is achieved in aspergillosis, we generated a transcriptomic time series of the response of human monocyte-derived macrophages to Aspergillus and used this and the available literature to construct a mechanistic computational model of iron handling of macrophages during this infection. We found an overwhelming macrophage response beginning 2 to 4 h after exposure to the fungus, which included upregulated transcription of iron import proteins transferrin receptor-1, divalent metal transporter-1, and ZIP family transporters, and downregulated transcription of the iron exporter ferroportin. The computational model, based on a discrete dynamical systems framework, consisted of 21 3-state nodes, and was validated with additional experimental data that were not used in model generation. The model accurately captures the steady state and the trajectories of most of the quantitatively measured nodes. In the experimental data, we surprisingly found that transferrin receptor-1 upregulation preceded the induction of inflammatory cytokines, a feature that deviated from model predictions. Model simulations suggested that direct induction of transferrin receptor-1 (TfR1) after fungal recognition, independent of the iron regulatory protein-labile iron pool (IRP-LIP) system, explains this finding. We anticipate that this model will contribute to a quantitative understanding of iron regulation as a fundamental host defense mechanism during aspergillosis.

IMPORTANCE Invasive pulmonary aspergillosis is a major cause of death among immunosuppressed individuals despite the best available therapy. Depriving the pathogen of iron is an essential component of host defense in this infection, but the mechanisms by which the host achieves this are complex. To understand how recruited macrophages mediate iron deprivation during the infection, we developed and validated a mechanistic computational model that integrates the available information in the field. The insights provided by this approach can help in designing iron modulation therapies as anti-fungal treatments.

Deficiency of GPI Glycan Modification by Ethanolamine Phosphate Results in Increased Adhesion and Immune Resistance of Aspergillus fumigatus

Glycosylphosphatidylinositol (GPI)-anchored proteins play important roles in maintaining the function of the cell wall and participating in pathogenic processes. The addition and removal of phosphoethanolamine (EtN-P) on the second mannose residue in the GPI anchor are vital for maturation and sorting of GPI-anchored proteins. Previously, we have shown that deletion of the gpi7, the gene that encodes an EtN-P transferase responsible for the addition of EtN-P to the second mannose residue of the GPI anchor, leads to the mislocalization of GPI-anchored proteins, abnormal polarity, reduced conidiation, and fast germination in Aspergillus fumigatus. In this report, the adherence and virulence of the A. fumigatus gpi7 deletion mutant were further investigated. The germinating conidia of the mutant exhibited an increased adhesion and a higher exposure of cell wall polysaccharides. Although the virulence was not affected, an increased adherence and a stronger inflammation response of the mutant were documented in an immunocompromised mouse model. An in vitro assay confirmed that the Δgpi7 mutant induced a stronger immune response and was more resistant to killing. Our findings, for the first time, demonstrate that in A. fumigatus, GPI anchoring is required for proper organization of the conidial cell wall. The lack of Gpi7 leads to fast germination, stronger immune response, and resistance to macrophage killing.

The Glycosylphosphatidylinositol-Anchored Superoxide Dismutase of Scedosporium apiospermum Protects the Conidia from Oxidative Stress

Scedosporium species are common fungal pathogens in patients with cystic fibrosis (CF). To colonize the CF lungs, fungi must cope with the host immune response, especially the reactive oxygen species (ROS) released by phagocytic cells. To this aim, pathogens have developed various antioxidant systems, including superoxide dismutases (SODs) which constitute the first-line protection against oxidative stress. Interestingly, one of the S. apiospermum SOD-encoding genes (SODD gene) exhibits a glycosylphosphatidylinositol (GPI) anchor-binding site and encodes a conidial-specific surface SOD. In this study, a SODDΔ mutant was engineered from a non-homologous end joining-deficient strain (KU70Δ) of S. apiospermum. Compared to its parent strain, the double mutant KU70Δ/SODDΔ exhibited increased susceptibility to various oxidizing agents and triazole antifungals. In addition, the loss of SodD resulted in an increased intracellular killing of the conidia by M1 macrophages derived from human blood monocytes, suggesting the involvement of this superoxide dismutase in the evasion to the host defenses. Nevertheless, one cannot disregard an indirect role of the enzyme in the synthesis or assembly of the cell wall components since transmission electron microscopic analysis revealed a thickening of the inner cell wall layer of the conidia. Further studies are needed to confirm the role of this enzyme in the pathogenesis of Scedosporium infections, including the production of a recombinant protein and study of its protective effect against the infection in a mouse model of scedosporiosis.

Phenomic profiling of a novel sibling species within the Scedosporium complex in Thailand

Background

Scedosporium species are a group of pathogenic fungi, which can be found worldwide around high human-impacted areas. Infections of Scedosporium have been reported in several immunocompromised and immunocompetent patients with a high mortality rate. Recently, we have isolated and identified several Scedosporium strains during an environmental survey in Thailand.

Results

We describe the isolate, TMMI-012, possibly a new species isolated from soils in the Chatuchak public park, Bangkok, Thailand. TMMI-012 is phylogenetically related to the Scedosporium genus and is a sibling to S. boydii but shows distinct morphological and pathological characteristics. It is fast growing and highly resistant to antifungal drugs and abiotic stresses. Pathological studies of in vitro and in vivo models confirm its high virulence and pathogenicity.

Conclusion

TMMI-012 is considered a putative novel Scedosporium species. The high antifungal resistance of TMMI-012 compared with its sibling, Scedosporium species is likely related to its clinical impact on human health.

Tissue-Resident Macrophages in Fungal Infections

Invasive fungal infections result in high morbidity and mortality. Host organs targeted by fungal pathogens vary depending on the route of infection and fungal species encountered. Cryptococcus neoformans infects the respiratory tract and disseminates throughout the central nervous system. Candida albicans infects mucosal tissues and the skin, and systemic Candida infection in rodents has a tropism to the kidney. Aspergillus fumigatus reaches distal areas of the lung once inhaled by the host. Across different tissues in naïve hosts, tissue-resident macrophages (TRMs) are one of the most populous cells of the innate immune system. Although they function to maintain homeostasis in a tissue-specific manner during steady state, TRMs may function as the first line of defense against invading pathogens and may regulate host immune responses. Thus, in any organs, TRMs are uniquely positioned and specifically programmed to function. This article reviews the current understanding of the roles of TRMs during major fungal infections.

Interaction of THP-1 Monocytes with Conidia and Hyphae of Different Curvularia Strains

Interaction of the human monocytic cell line, THP-1 with clinical isolates of three Curvularia species were examined. Members of this filamentous fungal genus can cause deep mycoses emerging in both immunocompromised and immunocompetent patients. It was found that monocytes reacted only to the hyphal form of Curvularia lunata. Cells attached to the germ tubes and hyphae and production of elevated levels of interleukin (IL)-8 and IL-10 and a low level of TNF-α were measured. At the same time, monocytes failed to produce IL-6. This monocytic response, especially with the induction of the anti-inflammatory IL-10, correlates well to the observation that C. lunata frequently cause chronic infections even in immunocompetent persons. Despite the attachment to the hyphae, monocytes could not reduce the viability of the fungus and the significant decrease in the relative transcript level of HLA-DRA assumes the lack of antigen presentation of the fungus by this cell type. C. spicifera and C. hawaiiensis failed to induce the gathering of the cells or the production of any analyzed cytokines. Monocytes did not recognize conidia of Curvularia species, even when melanin was lacking in their cell wall.

Flow Cytometry Is a Powerful Tool for Assessment of the Viability of Fungal Conidia in Metalworking Fluids

Fungal contamination of metalworking fluids (MWF) is a dual problem in automated processing plants because resulting fungal biofilms obstruct cutting, drilling, and polishing machines. Moreover, some fungal species of MWF comprise pathogens such as Fusarium solani Therefore, the development of an accurate analytical tool to evaluate conidial viability in MWF is important. We developed a flow cytometric method to measure fungal viability in MWF using F. solani as the model organism. To validate this method, viable and dead conidia were mixed in several proportions and flow was cytometrically analyzed. Subsequently, we assessed the fungicidal activity of two commercial MWF using flow cytometry (FCM) and compared it with microscopic analyses and plating experiments. We evaluated the fungal growth in both MWF after 7 days using quantitative PCR (qPCR) to assess the predictive value of FCM. Our results showed that FCM distinguishes live from dead conidia as early as 5 h after exposure to MWF, whereas the microscopic germination approach detected conidial viability much later and less accurately. At 24 h, microscopic analyses of germinating conidia and live/dead analyses by FCM correlated well, although the former consistently underestimated the proportion of viable conidia. In addition, the reproducibility and sensitivity of the flow cytometric method were high and allowed assessment of the fungicidal properties of two commercial MWF. Importantly, the obtained flow cytometric results on viability of F. solani conidia at both early time points (5 h and 24 h) correlated well with fungal biomass measurements assessed via a qPCR methodology 7 days after the start of the experiment.IMPORTANCE This result shows the predictive power of flow cytometry (FCM) in assessing the fungicidal capacity of MWF formulations. It also implies that FCM can be implemented as a rapid detection tool to estimate the viable fungal load in an industrial processing matrix (MWF).

Pseudomonas aeruginosa Inhibits the Growth of Scedosporium and Lomentospora In Vitro

In vitro bacterial-fungal interaction studies in cystic fibrosis (CF) have mainly focused on interactions between bacteria and Candida. Here we investigated the effect of Pseudomonas aeruginosa on the growth of Scedosporium/Lomentospora spp. Standard suspensions of P. aeruginosa (16 non-mucoid and nine mucoid isolates) were dropped onto paper disks, placed on lawns of Lomentospora prolificans (formerly Scedosporium prolificans) strain WM 14.140 or Scedosporium aurantiacum strain WM 11.78 on solid agar. The median inhibitory activity (mIz) was calculated for each fungal-bacterial combination. As a group, mIz values for non-mucoid phenotype P. aeruginosa strains were significantly lower than those for mucoid strains (P < 0.001); 14/16 (87.5%) non-mucoid strains had mIz <1.0 against both fungi versus just 3/9 mucoid strains (33.4%) (P = 0.01). One non-mucoid (PA14) and one mucoid (CIDMLS-PA-28) P. aeruginosa strain effecting inhibition were selected for further studies. Inhibition of both L. prolificans and S. aurantiacum by these strains was confirmed using the XTT (2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino)carbonyl]-2H-tetrazolium hydroxide) reduction assay. Following incubation with XTT, inhibition of fungal growth was determined as the ratio of absorbance in liquid culture with Pseudomonas to that in control fungal cultures. An absorbance ratio of <1.0 consistent with bacterial inhibition of fungal growth was observed for all four P. aeruginosa-fungal combinations (P < 0.05). Fluorescence microscopy, subsequent to co-culture of either fungal isolate with P. aeruginosa strain PA14 or CIDMLS-PA-28 revealed poorly formed hyphae, compared with control fungal cultures. P. aeruginosa inhibits growth of L. prolificans and S. aurantiacum in vitro, with non-mucoid strains more commonly having an inhibitory effect. As P. aeruginosa undergoes phenotype transitions from non-mucoid to the mucoid form with progression of CF lung disease, this balance may influence the appearance of Scedosporium fungi in the airways.

The innate immune response to Aspergillus fumigatus at the alveolar surface

Aspergillus fumigatus is an ubiquitous, saprophytic mould that forms and releases airborne conidia which are inhaled by humans on a daily basis. When the immune system is compromised (e.g. immunosuppressive therapy prior to organ transplantation) or there is pre-existing pulmonary malfunction (e.g. asthma, cystic fibrosis, TB lesions), A. fumigatus exploits weaknesses in the host defenses which can result in the development of saphrophytic, allergic or invasive aspergillosis. If not effectively eliminated by the innate immune response, conidia germinate and form invasive hyphae which can penetrate pulmonary tissues. The innate immune response to A. fumigatus is stage-specific and various components of the host's defenses are recruited to challenge the different cellular forms of the pathogen. In immunocompetent hosts, anatomical barriers (e.g. the mucociliary elevator) and professional phagocytes such as alveolar macrophages (AM) and neutrophils prevent the development of aspergillosis by inhibiting the growth of conidia and hyphae. The recognition of inhaled conidia by AM leads to the intracellular degradation of the spores and the secretion of proinflammatory mediators which recruit neutrophils to assist in fungal clearance. During the later stages of infection, dendritic cells activate a protective A. fumigatus-specific adaptive immune response which is driven by Th1 CD4(+) T cells.

Characterization of sakA gene from pathogenic dimorphic fungus Penicillium marneffei

Eukaryotes utilize stress activated protein kinase (SAPK) pathways to adapt to environmental stress, including heat, osmotic, oxidative or nutrient stresses. Penicillium marneffei (Talaromyces marneffei), the dimorphic pathogenic fungus that can cause disseminated mycosis in HIV-infected patients, has to encounter various types of stresses both outside and inside host cells. However, the strategies used by this fungus in response to these stresses are still unclear. In this report, the stress-activated kinase (sakA) gene of P. marneffei was characterized and the roles of this gene on various stress conditions were studied. The sakA gene deletion mutant was constructed using the split marker method. The phenotypes and sensitivities to varieties of stresses, including osmotic, oxidative, heat and cell wall stresses of the deletion mutant were compared with the wild type and the sakA complemented strains. Results demonstrated that the P. marneffei sakA gene encoded a putative protein containing TXY phosphorylation lip found in the stress high osmolarity glycerol 1 (Hog1)/Spc1/p38 MAPK family, and that this gene was involved not only in tolerance against oxidative and heat stresses, but also played a role in asexual development, chitin deposition, yeast cell generation in vitro and survival inside mouse and human macrophages.

What is the impact of hypogammaglobulinemia on the rate of infections and survival in solid organ transplantation? A meta-analysis

Hypogammaglobulinemia has been described after solid organ transplantation and has been associated with increased risk of infections. The aim of the study was to evaluate the rate of severe hypogammaglobulinemia and its relationship with the risk of infections during the first year posttransplantation. Eighteen studies (1756 patients) that evaluated hypogammaglobulinemia and posttransplant infections were included. The data were pooled using the DerSimonian and Laird random-effects model. Q statistic method was used to assess statistical heterogeneity. Within the first year posttransplantation, the rate of hypogammaglobulinemia (IgG < 700 mg/dL) was 45% (95% CI: 0.34-0.55; Q = 330.1, p < 0.0001), the rate of mild hypogammaglobulinemia (IgG = 400-700 mg/dL) was 39% (95% CI: 0.22-0.56; Q = 210.09, p < 0.0001) and the rate of severe hypogammaglobulinemia (IgG < 400 mg/dL) was 15% (95% CI: 0.08-0.22; Q = 50.15, p < 0.0001). The rate of hypogammaglobulinemia by allograft type: heart 49% (21%-78%; Q = 131.16, p < 0.0001); kidney 40% (30%-49%; Q = 24.55, p = 0.0002); liver 16% (0.001%-35%; Q = 14.31, p = 0.0002) and lung 63% (53%-74%; Q = 6.85, p = 0.08). The odds of respiratory infection (OR = 4.83; 95% CI: 1.66-14.05; p = 0.004; I(2) = 0%), CMV (OR = 2.40; 95% CI: 1.16-4.96; p = 0.02; I(2) = 26.66%), Aspergillus (OR = 8.19; 95% CI: 2.38-28.21; p = 0.0009; I(2) = 17.02%) and other fungal infections (OR = 3.69; 95% CI: 1.11-12.33; p = 0.03; I(2) = 0%) for patients with IgG < 400 mg/dL were higher than the odds for patients with IgG > 400 mg/dL. The odds for 1-year all-cause mortality for severe hypogammaglobulinemia group was 21.91 times higher than those for IgG > 400 mg/dL group (95% CI: 2.49-192.55; p = 0.005; I(2) = 0%). Severe hypogammaglobulinemia during the first year posttransplantation significantly increased the risk of CMV, fungal and respiratory infections, and was associated with higher 1-year all-cause mortality.

The immune interplay between the host and the pathogen in Aspergillus fumigatus lung infection

The interplay between Aspergillus fumigatus and the host immune response in lung infection has been subject of studies over the last years due to its importance in immunocompromised patients. The multifactorial virulence factors of A. fumigatus are related to the fungus biological characteristics, for example, structure, ability to grow and adapt to high temperatures and stress conditions, besides capability of evading the immune system and causing damage to the host. In this context, the fungus recognition by the host innate immunity occurs when the pathogen disrupts the natural and chemical barriers followed by the activation of acquired immunity. It seems clear that a Th1 response has a protective role, whereas Th2 reactions are often associated with higher fungal burden, and Th17 response is still controversial. Furthermore, a fine regulation of the effector immunity is required to avoid excessive tissue damage associated with fungal clearance, and this role could be attributed to regulatory T cells. Finally, in this work we reviewed the aspects involved in the complex interplay between the host immune response and the pathogen virulence factors, highlighting the immunological issues and the importance of its better understanding to the development of novel therapeutic approaches for invasive lung aspergillosis.

Corticosteroids block autophagy protein recruitment in Aspergillus fumigatus phagosomes via targeting dectin-1/Syk kinase signaling

Aspergillus fumigatus is the predominant airborne fungal pathogen in immunocompromised patients. Genetic defects in NADPH oxidase (chronic granulomatous disease [CGD]) and corticosteroid-induced immunosupression lead to impaired killing of A. fumigatus and unique susceptibility to invasive aspergillosis via incompletely characterized mechanisms. Recent studies link TLR activation with phagosome maturation via the engagement of autophagy proteins. In this study, we found that infection of human monocytes with A. fumigatus spores triggered selective recruitment of the autophagy protein LC3 II in phagosomes upon fungal cell wall swelling. This response was induced by surface exposure of immunostimulatory β-glucans and was mediated by activation of the Dectin-1 receptor. LC3 II recruitment in A. fumigatus phagosomes required spleen tyrosine kinase (Syk) kinase-dependent production of reactive oxygen species and was nearly absent in monocytes of patients with CGD. This pathway was important for control of intracellular fungal growth, as silencing of Atg5 resulted in impaired phagosome maturation and killing of A. fumigatus. In vivo and ex vivo administration of corticosteroids blocked LC3 II recruitment in A. fumigatus phagosomes via rapid inhibition of phosphorylation of Src and Syk kinases and downstream production of reactive oxygen species. Our studies link Dectin-1/Syk kinase signaling with autophagy-dependent maturation of A. fumigatus phagosomes and uncover a potential mechanism for development of invasive aspergillosis in the setting of CGD and corticosteroid-induced immunosupression.

Impaired bactericidal but not fungicidal activity of polymorphonuclear neutrophils in patients with chronic lymphocytic leukemia

We examined the qualitative polymorphonuclear neutrophil (PMN)-associated immune impairment in patients with chronic lymphocytic leukemia (CLL) by characterizing phagocytic killing of key non-opsonized bacterial (Staphylococcus aureus and Pseudomonas aeruginosa) and fungal (Candida albicans and Aspergillus fumigatus) pathogens. Neutrophils were collected from 47 non-neutropenic patients with CLL (PMN count > 1000/mm(3)) and age-matched and young healthy controls (five each). A subset of patients (13%) had prior or subsequent infections. We found that the patients with CLL had diminished PMN microbicidal response against bacteria but not against fungi compared with the controls. Compared to patients with effective PMN responses, we did not identify differences of basal PMN pathogen-associated molecular pattern receptor gene expression, soluble pathogen-associated molecular pattern gene expression or inflammatory cytokine signatures in patients with impaired PMN responses when PMNs were analyzed in multiplex real-time polymerase chain reaction assays. However, differences in PMN microbicidal response against A. fumigatus in patients with CLL were associated with the degree of hypogammaglobulinemia.

Phagocytic receptors on macrophages distinguish between different Sporothrix schenckii morphotypes

Sporothrix schenckii is a human pathogen that causes sporotrichosis, a cutaneous subacute or chronic mycosis. Little is known about the innate immune response and the receptors involved in host recognition and phagocytosis of S. schenckii. Here, we demonstrate that optimal phagocytosis of conidia and yeast is dependent on preimmune human serum opsonisation. THP-1 macrophages efficiently ingested opsonised conidia. Competition with D-mannose, methyl α-D-mannopyranoside, D-fucose, and N-acetyl glucosamine blocked this process, suggesting the involvement of the mannose receptor in binding and phagocytosis of opsonised conidia. Release of TNF-α was not stimulated by opsonised or non-opsonised conidia, although reactive oxygen species (ROS) were produced, resulting in the killing of conidia by THP-1 macrophages. Heat inactivation of the serum did not affect conidia internalization, which was markedly decreased for yeast cells, suggesting the role of complement components in yeast uptake. Conversely, release of TNF-α and production of ROS were induced by opsonised and non-opsonised yeast. These data demonstrate that THP-1 macrophages respond to opsonised conidia and yeast through different phagocytic receptors, inducing a differential cellular response. Conidia induces a poor pro-inflammatory response and lower rate of ROS-induced cell death, thereby enhancing the pathogen's survival.

Germination of Aspergillus fumigatus inside avian respiratory macrophages is associated with cytotoxicity

Although aspergillosis is one of the most common diseases in captive birds, the pathogenesis of avian aspergillosis is poorly known. We studied the role of avian respiratory macrophages as a first line of defense against avian aspergillosis. The phagocytic and killing capacities of avian respiratory macrophages were evaluated using pigeon respiratory macrophages that were inoculated with Aspergillus fumigatus conidia. On average, 25% of macrophage-associated conidia were phagocytosed after one hour. Sixteen percents of these cell-associated conidia were killed after 4 h and conidial germination was inhibited in more than 95% of the conidia. A. fumigatus conidia were shown to be cytotoxic to the macrophages. Intracellularly germinating conidia were located free in the cytoplasm of necrotic cells, as shown using transmission electron microscopy. These results suggest that avian respiratory macrophages may prevent early establishment of infection, unless the number of A. fumigatus conidia exceeds the macrophage killing capacity, leading to intracellular germination and colonization of the respiratory tract.

Development of an in vitro model for the multi-parametric quantification of the cellular interactions between Candida yeasts and phagocytes

We developed a new in vitro model for a multi-parameter characterization of the time course interaction of Candida fungal cells with J774 murine macrophages and human neutrophils, based on the use of combined microscopy, fluorometry, flow cytometry and viability assays. Using fluorochromes specific to phagocytes and yeasts, we could accurately quantify various parameters simultaneously in a single infection experiment: at the individual cell level, we measured the association of phagocytes to fungal cells and phagocyte survival, and monitored in parallel the overall phagocytosis process by measuring the part of ingested fungal cells among the total fungal biomass that changed over time. Candida albicans, C. glabrata, and C. lusitaniae were used as a proof of concept: they exhibited species-specific differences in their association rate with phagocytes. The fungal biomass uptaken by the phagocytes differed significantly according to the Candida species. The measure of the survival of fungal and immune cells during the interaction showed that C. albicans was the more aggressive yeast in vitro, destroying the vast majority of the phagocytes within five hours. All three species of Candida were able to survive and to escape macrophage phagocytosis either by the intraphagocytic yeast-to-hyphae transition (C. albicans) and the fungal cell multiplication until phagocytes burst (C. glabrata, C. lusitaniae), or by the avoidance of phagocytosis (C. lusitaniae). We demonstrated that our model was sensitive enough to quantify small variations of the parameters of the interaction. The method has been conceived to be amenable to the high-throughput screening of mutants in order to unravel the molecular mechanisms involved in the interaction between yeasts and host phagocytes.

Persistence versus escape: Aspergillus terreus and Aspergillus fumigatus employ different strategies during interactions with macrophages

Invasive bronchopulmonary aspergillosis (IBPA) is a life-threatening disease in immunocompromised patients. Although Aspergillus terreus is frequently found in the environment, A. fumigatus is by far the main cause of IBPA. However, once A. terreus establishes infection in the host, disease is as fatal as A. fumigatus infections. Thus, we hypothesized that the initial steps of disease establishment might be fundamentally different between these two species. Since alveolar macrophages represent one of the first phagocytes facing inhaled conidia, we compared the interaction of A. terreus and A. fumigatus conidia with alveolar macrophages. A. terreus conidia were phagocytosed more rapidly than A. fumigatus conidia, possibly due to higher exposure of β-1,3-glucan and galactomannan on the surface. In agreement, blocking of dectin-1 and mannose receptors significantly reduced phagocytosis of A. terreus, but had only a moderate effect on phagocytosis of A. fumigatus. Once phagocytosed, and in contrast to A. fumigatus, A. terreus did not inhibit acidification of phagolysosomes, but remained viable without signs of germination both in vitro and in immunocompetent mice. The inability of A. terreus to germinate and pierce macrophages resulted in significantly lower cytotoxicity compared to A. fumigatus. Blocking phagolysosome acidification by the v-ATPase inhibitor bafilomycin increased A. terreus germination rates and cytotoxicity. Recombinant expression of the A. nidulans wA naphthopyrone synthase, a homologue of A. fumigatus PksP, inhibited phagolysosome acidification and resulted in increased germination, macrophage damage and virulence in corticosteroid-treated mice. In summary, we show that A. terreus and A. fumigatus have evolved significantly different strategies to survive the attack of host immune cells. While A. fumigatus prevents phagocytosis and phagolysosome acidification and escapes from macrophages by germination, A. terreus is rapidly phagocytosed, but conidia show long-term persistence in macrophages even in immunocompetent hosts.

Cystic fibrosis transmembrane conductance regulator regulates epithelial cell response to Aspergillus and resultant pulmonary inflammation

RATIONALE

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) alter epithelial cell (EC) interactions with multiple microbes, such that dysregulated inflammation and injury occur with airway colonization in people with cystic fibrosis (CF). Aspergillus fumigatus frequently colonizes CF airways, but it has been assumed to be an innocent saprophyte; its potential role as a cause of lung disease is controversial.

OBJECTIVES

To study the interactions between Aspergillus and EC, and the role of the fungus in evoking inflammatory responses.

METHODS

A. fumigatus expressing green fluorescent protein was developed for in vitro and in vivo models, which used cell lines and mouse tracheal EC.

MEASUREMENTS AND MAIN RESULTS

Fungal spores (conidia) are rapidly ingested by ECs derived from bronchial cell lines and murine tracheas, supporting a role for EC in early airway clearance. Bronchial ECs harboring CFTR mutations (ΔF508) or deletion demonstrate impaired uptake and killing of conidia, and ECs with CFTR mutation undergo more conidial-induced apoptosis. Germinated (hyphal) forms of the fungus evoke secretion of inflammatory mediators, with CFTR mutation resulting in increased airway levels of macrophage inflammatory protein 2 and KC, and higher lung monocyte chemotactic protein-1. After A. fumigatus inhalation, CFTR(-/-) mice develop exaggerated lymphocytic inflammation, mucin accumulation, and lung injury.

CONCLUSIONS

Data demonstrate a critical role for CFTR in mediating EC responses to A. fumigatus. Results suggest that the fungus elicits aberrant pulmonary inflammation in the setting of CFTR mutation, supporting the potential role of antifungals to halt progressive CF lung disease.

Toll like receptor 5 (TLR5) may be involved in the immunological response to Aspergillus fumigatus in vitro

Toll-like receptors (TLRs) are involved in the host defense against Aspergillus fumigatus infections, and some TLRs may even be exploited by the mould to escape immune mechanisms. We have previously shown that conidia from A. fumigatus increase expression of TLR5 in human monocytes. When further investigating a possible role of TLR5 in A. fumigatus infections, we observed a decrease in conidial viability after culturing with TLR5-knockdown THP-1 monocytes. Secondly, our experiments showed an increase in conidial viability when THP-1 monocytes, together with flagellin, are cultured with conidia. Thirdly, we found that treatment of THP-1 monocytes with a monoclonal antibody against TLR5 resulted in increased conidial viability after culturing. Experiments with a HEK-293 cell line only expressing TLR5 did not indicate that conidia directly interact with TLR5. Further studies of the intracellular molecular mechanisms activated concomitant with activation of TLR5 that have an enhancing effect on the viability of conidia may shed new light on the defense against conidia in monocytic cells, and possibly also on the function of the TLR5 system.

A sterol-regulatory element binding protein is required for cell polarity, hypoxia adaptation, azole drug resistance, and virulence in Aspergillus fumigatus

At the site of microbial infections, the significant influx of immune effector cells and the necrosis of tissue by the invading pathogen generate hypoxic microenvironments in which both the pathogen and host cells must survive. Currently, whether hypoxia adaptation is an important virulence attribute of opportunistic pathogenic molds is unknown. Here we report the characterization of a sterol-regulatory element binding protein, SrbA, in the opportunistic pathogenic mold, Aspergillus fumigatus. Loss of SrbA results in a mutant strain of the fungus that is incapable of growth in a hypoxic environment and consequently incapable of causing disease in two distinct murine models of invasive pulmonary aspergillosis (IPA). Transcriptional profiling revealed 87 genes that are affected by loss of SrbA function. Annotation of these genes implicated SrbA in maintaining sterol biosynthesis and hyphal morphology. Further examination of the SrbA null mutant consequently revealed that SrbA plays a critical role in ergosterol biosynthesis, resistance to the azole class of antifungal drugs, and in maintenance of cell polarity in A. fumigatus. Significantly, the SrbA null mutant was highly susceptible to fluconazole and voriconazole. Thus, these findings present a new function of SREBP proteins in filamentous fungi, and demonstrate for the first time that hypoxia adaptation is likely an important virulence attribute of pathogenic molds.

The incidence of potentially lethal infections caused by normally benign molds has increased tremendously over the last two decades. One disease in particular, invasive pulmonary aspergillosis (IPA), caused by the common mold Aspergillus fumigatus, has become the leading cause of death due to invasive mycoses. Currently, we have a limited understanding of how this opportunistic pathogen causes disease in immunocompromised patients. In this study, we discover a previously unexplored mechanism required by this mold to cause disease, hypoxia (low oxygen) adaptation. We report that hypoxia adaptation in A. fumigatus is mediated in part by a highly conserved transcription factor, SrbA, a protein in the sterol regulatory element binding protein family. A null mutant of SrbA was unable to grow in hypoxia, displayed increased susceptibility to the azole class of antifungal drugs, and was avirulent in two distinct murine models of IPA. Importantly, we report the discovery of a novel function of SrbA in molds related to maintenance of cell polarity. The finding that SrbA regulates resistance to the azole class of antifungal drugs presents an opportunity to uncover new mechanisms of antifungal drug resistance in A. fumigatus.

Saccharomyces cerevisiae samples stained with FUN-1 dye can be stored at ?20�C for later observation

FUN-1, a fluorescent vital dye, has been observed to form cylindrical intravacuolar structures within the vacuoles of metabolically active yeast cells. FUN-1 staining, which begins as a diffuse pool of fluorescent cytoplasmic stain, uses an unknown endogenous biochemical processing mechanism to compact and form orange-red cylindrical intravacuolar structures within the cell vacuole. In the clinical setting, FUN-1 is primarily used for identification of fungal infection. FUN-1 is utilized in the laboratory to distinguish between metabolically active and dead fungal cells. Although this stain is useful for distinguishing between live and dead fungal dead cells, few studies have utilized this chemical. This lack of use in the scientific community may be due to the requirement that cells are visualized directly after staining. Thus, it would be of interest to be able to stain cells and store them for later use. Our lab examined the longevity of cylindrical intravacuolar structures in two strains of Saccharomyces cerevisiae stained with FUN-1 and stored at -20 degrees C. We found that cylindrical intravacuolar structures could be reliably observed and imaged utilizing differential interference contrast microscopy and fluorescence microscopy for 21 days. We also observed that cells stained with FUN-1 would resume propagation on yeast extract, peptone, dextrose (YPD) plates after being frozen at -20 degrees C for 21 days. These modifications to the published procedure for FUN-1 dye staining should allow for a more prevalent and less time sensitive use of this important biological tool.

Effects of tricyclazole (5-methyl-1,2,4-triazol[3,4] benzothiazole), a specific DHN-melanin inhibitor, on the morphology of Fonsecaea pedrosoi conidia and sclerotic cells

The influence of tricyclazole (5-methyl-1,2,4-triazol[3,4]benzothiazole), a specific DHN-melanin inhibitor, on the cell walls and intracellular structures of Fonsecaea pedrosoi conidia and sclerotic cells was analyzed by transmission electron microscopy (TEM), deep-etching, and field emission scanning electron microscopy. The treatment of the fungus with 16 microg mL(-1) of tricyclazole (TC) did not significantly affect fungal viability, but electron microscopy observations showed several important morphological differences between TC-treated and non-TC treated cells. Control sclerotic cells presented patched granules, with an average diameter of 47 nm, on the cell surface, which were absent in TC-treated cells. Also, TC-treated sclerotic cells showed an undulated relief. TC treatment leads to an accumulation of electron lucent vacuoles in the fungal cytoplasm of both conidia and sclerotic cells, and treated conidia observed by deep etching showed a relevant thickening of the fungal cell wall. Together, these observations support the previous data of our group that F. pedrosoi synthesizes melanin in intracellular organelles. In addition, we suggest that melanin is not only an extracellular constituent but could also be dispersing all over the cell walls and could have an effective role in cross-linking different cell wall compounds that help maintain the regular shape of the cell wall.

Immunostimulatory effects of the anionic alkali mineral complex Barodon on equine lymphocytes

Previous studies have shown that the anionic alkali mineral complex BARODON has an immunoenhancing effect on pigs as an adjuvant and as a nonspecific immunostimulant. Likewise, the equine immune system has been defined with various monoclonal antibodies specific to equine leukocyte differentiation antigens to determine the possibility of enhancing equine resistance to respiratory diseases and promoting other immunostimulatory effects with the application of BARODON. Compared with the control group, after 3 weeks of treatment, BARODON-treated groups showed higher proportions of cells (P < 0.05) expressing major histocompatibility complex class II and CD2, CD4(+), CD4(+) CD25(+), CD8(+), and CD8(+) CD25(+) T lymphocytes, dendritic cells, and surface immunoglobulin M(+) B lymphocytes in peripheral blood, as well as enhanced cell proliferative responses with phytohemagglutinin and increased phagocytic activity against Streptococcus equi and Staphylococcus aureus strains with high antibiotic resistance, the bacteria frequently identified as etiologic agents of equine respiratory diseases at the Seoul Race Park in Seoul, Korea. This study shows that BARODON may act as an immunostimulator and can be an effective alternative to antimicrobial feed additives for nonspecific improvements in equine immune responses, particularly against respiratory diseases.

Phylogenomic analysis of non-ribosomal peptide synthetases in the genus Aspergillus

Fungi from the genus Aspergillus are important saprophytes and opportunistic human fungal pathogens that contribute in these and other diverse ways to human well-being. Part of their impact on human well-being stems from the production of small molecular weight secondary metabolites, which may contribute to the ability of these fungi to cause invasive fungal infections and allergic diseases. In this study, we identified one group of enzymes responsible for secondary metabolite production in five Aspergillus species, the non-ribosomal peptide synthetases (NRPS). Hidden Markov models were used to search the genome databases of A. fumigatus, A. flavus, A. terreus, A. nidulans, and A. oryzae for domains conserved in NRPS proteins. A genealogy of adenylation domains was utilized to identify orthologous and unique NRPS among the Aspergillus species examined, as well as gain an understanding of the potential evolution of Aspergillus NRPS. mRNA abundance of the 14 NRPS identified in the A. fumigatus genome was analyzed using real-time reverse transcriptase PCR in different environmental conditions to gain a preliminary understanding of the possible functions of the NRPSs' peptide products. Our results suggest that Aspergillus species contain conserved and unique NRPS genes with a complex evolutionary history. This result suggests that the genus Aspergillus produces a substantial diversity of non-ribosomally synthesized peptides. Further analysis of these genes and their peptide products may identify important roles for secondary metabolites produced by NRPS in Aspergillus physiology, ecology, and fungal pathogenicity.

Comparison of three methodologies for the determination of pulmonary fungal burden in experimental murine aspergillosis

Quantitative culture, quantitative PCR and the galactomannan enzyme immunoassay (EIA) were compared for their ability to determine the pulmonary fungal burden in a murine model of invasive aspergillosis. Quantitative culture of specimens containing hyphae under-represented the absolute fungal burden in established infection when compared with the two other methods. The best correlation was observed between the two non-culture methods. Higher variability was observed with the galactomannan EIA when compared with quantitative PCR. Collectively, these data suggest that quantitative PCR is the preferred method for determination of the pulmonary fungal burden in experimental aspergillosis.

LaeA, a regulator of morphogenetic fungal virulence factors

Opportunistic animal and plant pathogens, well represented by the genus Aspergillus, have evolved unique mechanisms to adapt to and avoid host defenses. Aspergillus fumigatus, an increasingly serious pathogen owing to expanding numbers of immunocompromised patients, causes the majority of human infections; however, an inability to identify bona fide virulence factors has impeded therapeutic advances. We show that an A. fumigatus mutation in a developmentally expressed transcriptional regulator (deltalaeA) coordinating morphological and chemical differentiation reduces virulence in a murine model; impaired virulence is associated with decreased levels of pulmonary gliotoxin and multiple changes in conidial and hyphal susceptibility to host phagocytes ex vivo. LaeA, a conserved protein in filamentous fungi, is a developmental regulator of virulence genes and, possibly, the first antimicrobial target specific to filamentous fungi that are pathogenic to plants and animals.

The immune response to fungal infections

During the past two decades, invasive fungal infections have emerged as a major threat to immunocompromised hosts. Patients with neoplastic diseases are at significant risk for such infections as a result of their underlying illness and its therapy. Aspergillus, Candida, Cryptococcus and emerging pathogens, such as the zygomycetes, dark walled fungi, Trichosporon and Fusarium, are largely opportunists, causing infection when host defences are breached. The immune response varies with respect to the fungal species and morphotype encountered. The risk for particular infections differs, depending upon which aspect of immunity is impaired. This article reviews the current understanding of the role and relative importance of innate and adaptive immunity to common and emerging fungal pathogens. An understanding of the host response to these organisms is important in decisions regarding use of currently available antifungal therapies and in the design of new therapeutic modalities.

Inhibition of melanin synthesis pathway by tricyclazole increases susceptibility ofFonsecaea pedrosoi against mouse macrophages

Fonsecaea pedrosoi produces melanin, a pigment related to virulence in pathogenic fungi. To understand the involvement of melanin in the protection of fungi, the authors used tricyclazole to inhibit the melanin pathway in F. pedrosoi. Experiments of pigmentation suggested that F. pedrosoi uniquely produces dihydroxynaphthalene-melanin. Pigments produced on cultures modified or not with tricyclazole were extracted by an alkali-acid method and submitted to infrared and ion exchange chromatography analysis; also cytochemistry analysis for cationized ferritin of whole cells was carried out. This group of experiments showed that the tricyclazole treatment on F. pedrosoi produced a melanin-like pigment, but less negatively charged and with less affinity for iron ions than that without the tricyclazole treatment, and this in turn lead to a less negatively charge cell wall surface. Scanning electron microscopy of such pigments showed that the melanin from control cultures maintained their hyphae-like structures, which have been described as "melanin-ghosts," whereas the tricyclazole pigment showed an amorphous surface. Interaction of conidia from cultures of F. pedrosoi, modified by tricyclazole or not, with peritoneal activated macrophages suggested that tricyclazole causes higher association of fungus with macrophages, weakens the fungus capacity to destroy the macrophages, and diminishes the resistance to dry fracture procedures on samples prepared for high resolution scanning electron microscopy.

Adaptation of FUN-1 and Calcofluor white stains to assess the ability of viable and nonviable yeast to adhere to and be internalized by cultured mammalian cells

The FUN-1 and Calcofluor white stains can be used in concert to assess the ability of viable and nonviable yeast to adhere to, and be internalized by, host mammalian cells in vitro. With this method, only extracellular yeast stain with Calcofluor, dead yeast cells have diffuse cytoplasmic yellow-green fluorescence, and live yeast have cytoplasmic orange-red or yellow-orange fluorescent intravacuolar structures.

Flow cytometry antifungal susceptibility testing of Aspergillus fumigatus and comparison of mode of action of voriconazole vis-à-vis amphotericin B and itraconazole

Aspergillus fumigatus isolates were tested with three antifungals by flow cytometry (FC) and fluorescence-activated cell sorting. FC results after 4 h correlated well with MICs obtained by the NCCLS M38-A method; voriconazole exhibited fungicidal activity, albeit to a lesser extent than amphotericin B, but to a greater extent than itraconazole.

Differential role of MyD88 in macrophage-mediated responses to opportunistic fungal pathogens

Toll-like receptors mediate macrophage recognition of microbial ligands, inducing expression of microbicidal molecules and cytokines via the adapter protein MyD88. We investigated the role of MyD88 in regulating murine macrophage responses to a pathogenic yeast (Candida albicans) and mold (Aspergillus fumigatus). Macrophages derived from bone marrow of MyD88-deficient mice (MyD88(-/-)) demonstrated impaired phagocytosis and intracellular killing of C. albicans compared to wild-type (MyD88(+/+)) macrophages. In contrast, ingestion and killing of A. fumigatus conidia was MyD88 independent. Cytokine production by MyD88(-/-) macrophages in response to C. albicans yeasts and hyphae was substantially decreased, but responses to A. fumigatus hyphae were preserved. These results provide evidence that MyD88 signaling is involved in phagocytosis and killing of live C. albicans, but not A. fumigatus. The differential role of MyD88 may represent one mechanism by which macrophages regulate innate responses specific to different pathogenic fungi.

Killing of Aspergillus fumigatus by alveolar macrophages is mediated by reactive oxidant intermediates

Phagocytosis and mechanisms of killing of Aspergillus fumigatus conidia by murine alveolar macrophages (AM), which are the main phagocytic cells of the innate immunity of the lung, were investigated. Engulfment of conidia by murine AM lasts 2 h. Killing of A. fumigatus conidia by AM begins after 6 h of phagocytosis. Swelling of the conidia inside the AM is a prerequisite for killing of conidia. The contributions of NADPH oxidase and inducible nitric oxide synthase to the conidicidal activity of AM were studied using AM from OF1, wild-type and congenic p47phox(-/-) 129Sv, and wild-type and congenic iNOS(-/-) C57BL/6 mice. AM from p47phox(-/-) mice were unable to kill A. fumigatus conidia. Inhibitors of NADPH oxidase that decreased the production of reactive oxidant intermediates inhibited the killing of A. fumigatus without altering the phagocytosis rate. In contrast to NADPH oxidase, nitric oxide synthase does not play a role in killing of conidia. Corticosteroids did not alter the internalization of conidia by AM but did inhibit the production of reactive oxidant intermediates and the killing of A. fumigatus conidia by AM. Impairment of production of reactive oxidant intermediates by corticosteroids is responsible for the development of invasive aspergillosis in immunosuppressed mice.

The antifungal echinocandin caspofungin acetate kills growing cells of Aspergillus fumigatus in vitro

Caspofungin acetate is an antifungal antibiotic that inhibits synthesis of 1,3-beta-D-glucan, an essential component of the fungal cell wall. While caspofungin causes cell death in yeasts and dimorphic fungi such as Candida albicans, its effect on Aspergillus fumigatus is less well understood. We used the fluorescent dyes 5,(6)-carboxyfluorescein diacetate (CFDA) and bis-(1,3-dibutylbarbituric acid) trimethine oxonol (DiBAC), which stain live and dead cells, respectively, to further characterize the antifungal activity of caspofungin. For comparison, compounds whose mode of action was either fungistatic (fluconazole, itraconazole) or fungicidal (amphotericin B) were also evaluated. A correlation between caspofungin-induced loss of viability, decreased CFDA staining, and increased DiBAC staining was established first with C. albicans. For A. fumigatus, caspofungin caused similar dye-staining changes, which were quantified by fluorimetric analysis of stained hyphae grown in a medium that promoted dispersed growth. The minimum concentration of caspofungin required to produce these changes also decreased the level of growth-dependent reduction of the indicator dye Alamar Blue. We observed a differential effect of caspofungin as a function of cell position: 88% of apical cells and 61% of subapical branching cells failed to stain with the viable dye CFDA, but only 24% of subapical cells were unstained. Complementary results were seen with germlings from DiBAC-stained, caspofungin-treated cultures. Extended incubation of A. fumigatus with a single dose of caspofungin affected the same proportion of apical and subapical branching cells for up to 72 h. The dye-staining patterns illustrate that the cells at the active centers for new cell wall synthesis within A. fumigatus hyphae are killed when they are exposed to caspofungin.

Conidial viability assay for rapid susceptibility testing of Aspergillus species

Antifungal susceptibility testing of filamentous fungi has become more important given the recognition of drug-resistant organisms and the availability of therapies other than amphotericin B (AMB). As current microdilution and E-test methods are limited by a 2 to 3 day incubation time required to obtain results, a more rapid method for susceptibility testing of fungi is needed. We report here a flow cytometric assay that relies on conidial metabolism of the viability dye FUN-1. Conidia are incubated in media containing increasing concentrations of AMB for 3 h, exposed to FUN-1, and then analyzed by flow cytometry. Relative susceptibility to AMB can be measured both by forward and side scatter characteristics of the conidial population and by mean fluorescence intensity (MFI) of the dye. MIC, calculated as the concentration of AMB to yield 90% reduction in MFI relative to growth controls, was determined for 27 clinical isolates Aspergillus species and correlated well with the standard (i.e., NCCLS) method. The results of these studies illustrate a method by which AMB susceptibility can be rapidly and reproducibly determined by measuring conidial viability.