Detection of early phase specific surface appendages during germination of Aspergillus fumigatus conidia

The N-terminus of the Aspergillus fumigatus group III hybrid histidine kinase TcsC is essential for its physiological activity and targets the protein to the nucleus

Group III hybrid histidine kinases are fungal-specific proteins and part of the multistep phosphorelay, representing the initial part of the high osmolarity glycerol (HOG) pathway. TcsC, the corresponding kinase in Aspergillus fumigatus, was expected to be a cytosolic protein but is targeted to the nucleus. Activation of TcsC by the antifungal fludioxonil has lethal consequences for the fungus. The agent triggers a fast and TcsC-dependent activation of SakA and later on a redistribution of TcsC to the cytoplasm. High osmolarity also activates TcsC, which then exits the nucleus or concentrates in spot-like, intra-nuclear structures. The sequence corresponding to the N-terminal 208 amino acids of TcsC lacks detectable domains. Its loss renders TcsC cytosolic and non-responsive to hyperosmotic stress, but it has no impact on the antifungal activity of fludioxonil. A point mutation in one of the three putative nuclear localization sequences, which are present in the N-terminus, prevents the nuclear localization of TcsC, but not its ability to respond to hyperosmotic stress. Hence, this striking intracellular localization is no prerequisite for the role of TcsC in the adaptive response to hyperosmotic stress, instead, TcsC proteins that are present in the nuclei seem to modulate the cell wall composition of hyphae, which takes place in the absence of stress. The results of the present study underline that the spatiotemporal dynamics of the individual components of the multistep phosphorelay is a crucial feature of this unique signaling hub.

IMPORTANCE

Signaling pathways enable pathogens, such as Aspergillus fumigatus, to respond to a changing environment. The TcsC protein is the major sensor of the high osmolarity glycerol (HOG) pathway of A. fumigatus and it is also the target of certain antifungals. Insights in its function are therefore relevant for the pathogenicity and new therapeutic treatment options. TcsC was expected to be cytoplasmic, but we detected it in the nucleus and showed that it translocates to the cytoplasm upon activation. We have identified the motif that guides TcsC to the nucleus. An exchange of a single amino acid in this motif prevents a nuclear localization, but this nuclear targeting is no prerequisite for the TcsC-mediated stress response. Loss of the N-terminal 208 amino acids prevents the nuclear localization and renders TcsC unable to respond to hyperosmotic stress demonstrating that this part of the protein is of crucial importance.

Characterization of Aspergillus terreus Accessory Conidia and Their Interactions With Murine Macrophages

All Aspergillus species form phialidic conidia (PC) when the mycelium is in contact with the air. These small, asexual spores are ideally suited for an airborne dissemination in the environment. Aspergillus terreus and a few closely related species from section Terrei can additionally generate accessory conidia (AC) that directly emerge from the hyphal surface. In this study, we have identified galactomannan as a major surface antigen on AC that is largely absent from the surface of PC. Galactomannan is homogeneously distributed over the entire surface of AC and even detectable on nascent AC present on the hyphal surface. In contrast, β-glucans are only accessible in distinct structures that occur after separation of the conidia from the hyphal surface. During germination, AC show a very limited isotropic growth that has no detectable impact on the distribution of galactomannan. The AC of the strain used in this study germinate much faster than the corresponding PC, and they are more sensitive to desiccation than PC. During infection of murine J774 macrophages, AC are readily engulfed and trigger a strong tumor necrosis factor-alpha (TNFα) response. Both processes are not hampered by the presence of laminarin, which indicates that β-glucans only play a minor role in these interactions. In the phagosome, we observed that galactomannan, but not β-glucan, is released from the conidial surface and translocates to the host cell cytoplasm. AC persist in phagolysosomes, and many of them initiate germination within 24 h. In conclusion, we have identified galactomannan as a novel and major antigen on AC that clearly distinguishes them from PC. The role of this fungal-specific carbohydrate in the interactions with the immune system remains an open issue that needs to be addressed in future research.

The response regulator Skn7 of Aspergillus fumigatus is essential for the antifungal effect of fludioxonil

Aspergillus fumigatus is an important fungal pathogen that represents a major threat for severely immunocompromised patients. Cases of invasive aspergillosis are associated with a high mortality rate, which reflects the limited treatment options that are currently available. The development of novel therapeutic approaches is therefore an urgent task. An interesting compound is fludioxonil, a derivative of the bacterial secondary metabolite pyrrolnitrin. Both agents possess potent antimicrobial activity against A. fumigatus and trigger a lethal activation of the group III hybrid histidine kinase TcsC, the major sensor kinase of the High Osmolarity Glycerol (HOG) pathway in A. fumigatus. In the current study, we have characterized proteins that operate downstream of TcsC and analyzed their roles in the antifungal activity of fludioxonil and in other stress situations. We found that the SskA-SakA axis of the HOG pathway and Skn7 can independently induce an increase of the internal glycerol concentration, but each of these individual responses amounts for only half of the level found in the wild type. The lethal fludioxonil-induced ballooning occurs in the sskA and the sakA mutant, but not in the skn7-deficient strain, although all three strains show comparable glycerol responses. This indicates that an elevated osmotic pressure is necessary, but not sufficient and that a second, decisive and Skn7-dependent mechanism mediates the antifungal activity. We assume that fludioxonil triggers a reorganization in the fungal cell wall that reduces its rigidity, which in combination with the elevated osmotic pressure executes the lethal expansion of the fungal cells. Two findings link Skn7 to the cell wall of A. fumigatus: (1) the fludioxonil-induced massive increase in the chitin content depends on Skn7 and (2) the skn7 mutant is more resistant to the cell wall stressor Calcofluor white. In conclusion, our data suggest that the antifungal activity of fludioxonil in A. fumigatus relies on two distinct and synergistic processes: A high internal osmotic pressure and a weakened cell wall. The involvement of Skn7 in both processes most likely accounts for its particular importance in the antifungal activity of fludioxonil.

Functional comparison of the group III hybrid histidine kinases TcsC of Aspergillus fumigatus and NikA of Aspergillus nidulans

In filamentous fungi, group III hybrid histidine kinases (HHKs) are major and nonredundant sensing proteins of the high osmolarity glycerol pathway. In this study, we have compared the biological functions of the two homologous group III HHKs TcsC of Aspergillus fumigatus and NikA of A. nidulans. As expected from previous studies, the corresponding mutants are severely impaired in their ability to adapt to hyperosmotic stress and are both resistant to the antifungal agent fludioxonil. However, our data also reveal novel phenotypes and differences between these mutants. Both TcsC and NikA are required for wild-type-like growth on Czapek-Dox medium and a normal resistance to certain oxidative stressors, whereas an increased resistance to the cell wall disturbing agents Congo red and Calcofluor white was found for the ΔtcsC but not for the ΔnikA mutant. With respect to the cell wall reorganizations that are triggered by fludioxonil in a TcsC/NikA-dependent manner, we observed similarities but also striking differences. Strains from seven Aspergillus species, including A. fumigatus and A. nidulans incorporated more chitin into their cell walls in response to fludioxonil. In contrast, fludioxonil treatment resulted in a shedding of surface accessible galactomannan and β-1,3-glucan in all Aspergillus strains tested except A. nidulans. Hence, the fludioxonil-induced activation of NikA results in a distinct and apparently A. nidulans-specific pattern of cell wall reorganizations that is not due to NikA itself, but its integration into the A. nidulans signaling network.

Scedosporium apiospermum, Scedosporium aurantiacum, Scedosporium minutisporum and Lomentospora prolificans: a comparative study of surface molecules produced by conidial and germinated conidial cells

BACKGROUND

Scedosporium/Lomentospora species are opportunistic mould pathogens, presenting notable antifungal resistance.

OBJECTIVES/METHODS

We analysed the conidia and germinated conidia of S. apiospermum (Sap), S. aurantiacum (Sau), S. minutisporum (Smi) and L. prolificans (Lpr) by scanning electron microscopy and exposition of surface molecules by fluorescence microscopy.

FINDINGS

Conidia of Sap, Smi and Sau had oval, ellipsoidal and cylindrical shape, respectively, with several irregularities surrounding all surface areas, whereas Lpr conidia were rounded with a smooth surface. The germination of Sap occurred at the conidial bottom, while Smi and Sau germination primarily occurred at the centre of the conidial cell, and Lpr germination initiated at any part of the conidial surface. The staining of N-acetylglucosamine-containing molecules by fluorescein-labelled WGA primarily occurred during the germination of all studied fungi and in the conidial scars, which is the primary location of germination. Calcofluor white, which recognises the polysaccharide chitin, strongly stained the conidial cells and, to a lesser extent, the germination. Both mannose-rich glycoconjugates (evidenced by fluoresceinated-ConA) and cell wall externally located polypeptides presented distinct surface locations and expression according to both morphotypes and fungal species. In contrast, sialic acid and galactose-containing structures were not detected at fungal surfaces.

MAIN CONCLUSIONS

The present study demonstrated the differential production/exposition of surface molecules on distinct morphotypes of Scedosporium/Lomentospora species.

Lah is a transmembrane protein and requires Spa10 for stable positioning of Woronin bodies at the septal pore of Aspergillus fumigatus

Woronin bodies are specialized, fungal-specific organelles that enable an immediate closure of septal pores after injury to protect hyphae from excessive cytoplasmic bleeding. In most Ascomycetes, Woronin bodies are tethered at the septal pore by so-called Lah proteins. Using the pathogenic mold Aspergillus fumigatus as a model organism, we show that the C-terminal 288 amino acids of Lah (LahC₂₈₈) bind to the rim of the septal pore. LahC₂₈₈ essentially consists of a membrane spanning region and a putative extracellular domain, which are both required for the targeting to the septum. In an A. fumigatus rho4 deletion mutant that has a severe defect in septum formation, LahC₂₈₈ is recruited to spot-like structures in or at the lateral membrane. This suggests that LahC is recruited before Rho4 starts to govern the septation process. Accordingly, we found that in wild type hyphae Lah is bound before a cross-wall emerges and thus enables a tethering of Woronin bodies at the site of the newly formed septum. Finally, we identified Spa10, a member of a recently described family of septal pore-associated proteins, as a first protein that directly or indirectly interacts with LahC to allow a stable positioning of Woronin bodies at the mature septum.

Immunoproteome of Aspergillus fumigatus using sera of patients with invasive aspergillosis

Invasive aspergillosis is a life-threatening lung or systemic infection caused by the opportunistic mold Aspergillus fumigatus. The disease affects mainly immunocompromised hosts, and patients with hematological malignances or who have been submitted to stem cell transplantation are at high risk. Despite the current use of Platelia™ Aspergillus as a diagnostic test, the early diagnosis of invasive aspergillosis remains a major challenge in improving the prognosis of the disease. In this study, we used an immunoproteomic approach to identify proteins that could be putative candidates for the early diagnosis of invasive aspergillosis. Antigenic proteins expressed in the first steps of A. fumigatus germination occurring in a human host were revealed using 2-D Western immunoblots with the serum of patients who had previously been classified as probable and proven for invasive aspergillosis. Forty antigenic proteins were identified using mass spectrometry (MS/MS). A BLAST analysis revealed that two of these proteins showed low homology with proteins of either the human host or etiological agents of other invasive fungal infections. To our knowledge, this is the first report describing specific antigenic proteins of A. fumigatus germlings that are recognized by sera of patients with confirmed invasive aspergillosis who were from two separate hospital units.

Differential proteomic analysis of Aspergillus fumigatus morphotypes reveals putative drug targets

Aspergillus fumigatus is the main etiological agent of invasive aspergillosis, an important opportunistic infection for neutropenic patients. The main risk groups are patients with acute leukemia and bone marrow transplantation recipients. The lack of an early diagnostic test together with the limited spectrum of antifungal drugs remains a setback to the successful treatment of this disease. During invasive infection the inhaled fungal conidia enter the morphogenic cycle leading to angioinvasive hyphae. This work aimed to study differentially expressed proteins of A. fumigatus during morphogenesis. To achieve this goal, a 2D-DIGE approach was applied to study surface proteins extractable by reducing agents of two A. fumigatus morphotypes: germlings and hyphae. Sixty-three differentially expressed proteins were identified by MALDI-ToF/MS. We observed that proteins associated with biosynthetic pathways and proteins with multiple functions (miscellaneous) were over-expressed in the early stages of germination, while in hyphae, the most abundant proteins detected were related to metabolic processes or have unknown functions. Among the most interesting proteins regulated during morphogenesis, two putative drug targets were identified, the translational factor, eEF3 and the CipC-like protein. Neither of these proteins are present in mammalian cells.

Nucleotide oligomerization domain 2 contributes to the innate immune response in THCE cells stimulated by Aspergillus fumigatus conidia

AIM

To investigate the expression of nucleotide oligomerization domain 2 (NOD2) in the immortalized human corneal epithelial cell line (THCE), and its role in the innate immune response triggered by inactive Aspergillus fumigatus (Af) conidia.

METHODS

The normal THCE cells were investigated as controls. After incubation with inactive Af conidia for 0.5, 2, 4, 6, and 8 hours, THCE cells were harvested, mRNA expression of NOD2 and receptor interacting protein 2 (RIP2) was detected by RT-PCR. Intracellular proteins including NOD2, NF-κB and proinflammatory cytokines such as TNF-α, IL-8, IL-6 in the cell supernatant were analyzed by ELISA.

RESULTS

Our data indicate that NOD2 expressed in the normal THCE cells. After triggered by the inactive Af conidia, the expression of NOD2, RIP2 mRNA and the secretion of NOD2, NF-κB, TNF-α, IL-8, IL-6 both increased in a time-depended manner, and reached the peak point at 4, 6, 6, 4, 6, 6, 4 hours, respectively. And after pretreated with NOD2 neutralizing antibody, the expression of RIP2, NF-κB, TNF-α, IL-8 both decreased dramatically at the peak point, while the secretion of IL-6 changed little.

CONCLUSION

The results of this study suggest that NOD2 exists and expresses in the THCE cells, and contributes to the innate immune responses triggered by inactive Af conidia by induction of proinflammatory cytokines such as TNF-α and IL-8 through the NF-κB pathway.

Studies on galactofuranose-containing glycostructures of the pathogenic mold Aspergillus fumigatus

Galactofuranose is a hexose that is exclusively found in microbes and in particular in certain pathogenic species. In the mold Aspergillus fumigatus, it is the characteristic constituent of the cell wall component galactomannan. Detection of this carbohydrate is currently a widespread method used for diagnosis of systemic A. fumigatus infections. In this study, we raised and characterized 2 monoclonal antibodies that specifically react with galactofuranose-containing glycostructures. We investigated the distribution of surface-accessible galactomannan on different A. fumigatus morphotypes. We provide evidence that the antibodies recognize distinct antigens and are suitable to detect A. fumigatus hyphae in immunohistology. A mutant that is impaired in synthesis of galactofuranose stimulated a normal cytokine response in murine macrophages, which argues against galactomannan being a relevant PAMP, at least in mice. Purified galactomannan-specific monoclonal IgM L10-1 failed to inhibit the hyphal growth under in vitro conditions, but L10-1 binding to hyphae led to an enhanced deposition of the complement protein C1q. However, administration of purified L10-1 antibodies prior to infection was not able to protect mice. In conclusion, we have found no evidence for galactomannan being a relevant A. fumigatus PAMP and describe 2 novel galactomannan antibodies that might be valuable tools for the diagnosis of A. fumigatus infections and further analysis of the biological significance of galactomannan.

Cell wall alpha1-3glucans induce the aggregation of germinating conidia of Aspergillus fumigatus

The germination of Aspergillus fumigatus conidia can be divided into four stages: breaking of dormancy, isotropic swelling, establishment of cell polarity, and formation of a germ tube. Swelling of conidia is associated in liquid medium with a multi-cellular aggregation that produced large clumps of conidia. Conidial aggregation can be specifically prevented by the addition of alpha1-3glucanase. Swollen conidia specifically adhere to insoluble alpha1-3glucan chains. Electron microscopy studies showed that cell wall alpha1-3glucan chains became exposed at the cell surface during the swelling. These results demonstrate that cell wall alpha1-3glucans play an essential role in the aggregation between swollen conidia. Experiments with alpha1-3glucan coated latex beads show that alpha1-3glucan chains interacted between them without the requirement of any other cell wall component suggesting that biophysical properties of alpha1-3glucans are solely responsible for conidial aggregation.

Activation of Toll-like receptors 2 and 4 in Aspergillus fumigatus keratitis

Toll-like receptor (TLR) 2 and TLR4 are major receptors of Aspergillus fumigatus. Aspergillus fumigatus signaling in cornea induces the production of many pro-inflammatory molecules. In this study, we have shown that exposure of telomerase-immortalized human corneal epithelial cells (HCECs) to A. fumigatus antigens resulted in up-regulation of TLR2 and TLR4, and release of IL-1beta and IL-10 in HCECs, effects that could be inhibited by treatment with TLR2, and TLR4 antibodies. In addition, the A. fumigatus antigens-induced production of IL-1beta and IL-10 in supernatants of corneal epithelial cells was also attenuated by NF-kappaB inhibitor. Aspergillus fumigatus keratitis developed in Wistar rats, as evidenced by high SLE scores, influx of polymorphonuclear leukocytes (PMNs), activation of TLR2 and TLR4, and production of IL-1beta and IL-10 over controls. These findings indicate that the cornea has functional TLR2 and TLR4, and activation of TLR2 and TLR4 through NF-kappaB may contribute to pathogenesis of keratomycosis.

Characterisation of the CipC-like protein AFUA_5G09330 of the opportunistic human pathogenic mould Aspergillus fumigatus

Aspergillus fumigatus is currently the major airborne fungal pathogen that menaces immunocompromised individuals. Germination of inhaled conidia is a hallmark of the early infection process, but little is known about the underlying mechanisms. The intention of our ongoing studies is the identification of A. fumigatus proteins that are differentially expressed during germination and may provide insights in the germination process. Using a proteomic approach, we identified AFUA_5G09330 as a major hyphal-specific protein. This result was confirmed using monoclonal antibodies generated in this study. AFUA_5G09330 belongs to a fungal-specific protein family. The eponymous CipC protein of A. nidulans has been shown to be induced by concanamycin A, and transcriptional data from Cryptococcus neoformans demonstrate a strong up-regulation of the expression of a homologous gene during infection. Our data provide evidence that AFUA_5G09330 is a monomeric, cytoplasmic protein. We found no evidence for an overexpression of AFUA_5G09330 induced by concanamycin A or other stress conditions. AFUA_5G09330 is exclusively found in the hyphal morphotype that enables an invasive growth of A. fumigatus during infection. Further studies are required to define the biological function of this hyphae-specific protein and its potential relevance for the pathogenicity of A. fumigatus.

Aspergillus fumigatus antigens activate immortalized human corneal epithelial cells via toll-like receptors 2 and 4

PURPOSE

To evaluate the role of toll-like receptors (TLR) 2 and 4 in host responses to Aspergillus fumigatus by use of cultured telomerase-immortalized human corneal epithelial cells (HCECs).

METHODS

HCECs were stimulated with inactive antigens from A. fumigatus. The expression of TLR2 and TLR4, phosphorylation of Ikappa B-alpha (pIkappa B-alpha), and release of interleukin (IL)-1beta and IL-6 was measured with and without inhibitors to TLR2 and TLR4.

RESULTS

Exposure of HCECs to A. fumigatus antigens resulted in up-regulation of TLR2 and TLR4, activation of pIkappa B, and release of IL-1beta and IL-6 in HCECs, effects that could be inhibited by treatment with TLR2 and TLR4 antibodies.

CONCLUSIONS

TLR2 and TLR4-nuclear factor-kappa B signaling pathways in corneal epithelium play important roles in inflammatory responses against A. fumigatus antigens.

Role of respiration in the germination process of the pathogenic mold Aspergillus fumigatus

Inhalation of resting conidia is usually the first step of a systemic infection caused by the opportunistic fungal pathogen Aspergillus fumigatus. In the lung, the inhaled spores encounter an environment that permits germination. However, the relative importance of certain environmental conditions for conidial activation and subsequent hyphae formation has so far not been analyzed in detail. In this study, we studied the role of oxygen during germination. We found that inhibitors of the respiratory chain were nearly as efficient in blocking germination as cycloheximide, an inhibitor of protein synthesis, which is already known to prevent germination of Aspergillus nidulans. We also found that A. fumigatus is unable to grow or germinate under anaerobic conditions, and using the fluorescent mitotracker dye we detected active mitochondria already at the stage of swollen conidia, which indicates that respiration is an early event during germination. In line with these data, we found that significant oxygen consumption was detectable early during germination, whereas no oxygen consumption was measurable in suspensions of resting conidia. In summary, the present study provides evidence that respiration is absolutely required for the germination of A. fumigatus conidia.

Phagocytosis of Aspergillus fumigatus conidia by murine macrophages involves recognition by the dectin-1 beta-glucan receptor and Toll-like receptor 2

Aspergillus fumigatus is a fungal pathogen causing severe infections in immunocompromised patients. For clearance of inhaled conidia, an efficient response of the innate immune system is required. Macrophages represent the first line of defence and ingest and kill conidia. C-type lectins represent a family of receptors, which recognize pathogen-specific carbohydrates. One of them is beta1-3 glucan, a major component of the fungal cell wall. Here we provide evidence that beta1-3 glucan plays an important role for the elimination of A. fumigatus conidia. Laminarin, a soluble beta1-3 glucan and antibodies to dectin-1, a well known beta1-3 glucan receptor, significantly inhibited conidial phagocytosis. On resting conidia low amounts of surface accessible beta1-3 glucan were detected, whereas high amounts were found on small spores that appear early during germination and infection as well as on resting conidia of a pksP mutant strain. Swollen conidia also display larger quantities of beta1-3 glucan, although in an irregular spotted pattern. Resting pksP mutant conidia and swollen wild-type conidia are phagocytosed with high efficiency thereby confirming the relevance of beta1-3 glucans for conidial phagocytosis. Additionally we found that TLR2 and the adaptor protein MyD88 are required for efficient conidial phagocytosis, suggesting a link between the TLR2-mediated recognition of A. fumigatus and the phagocytic response.

Quantification of phagocytosis of Aspergillus conidia by macrophages using a novel antibody-independent assay

The pathogenic mould Aspergillus fumigatus can cause severe infections in immunocompromised patients. Phagocytosis of inhaled conidia is an early and crucial event in the defense of A. fumigatus infections. Here we describe a novel antibody-independent assay for quantification of phagocytosis, that in this study has been applied to different Aspergillus species, but that is in principle suitable for many fungi.

Diagnosis of aspergilloma in a pleural cavity (persistent pneumothorax) using classic imaging methods

The diagnosis of pulmonary aspergillosis is based on serum-analysis, as well as histological and microbiological analysis of bronchial lavage and transbronchial biopsies. When Aspergillus develops within a preformed cavity, however, these tests are likely to be negative. In this situation, classic imaging techniques such as chest X-ray and high resolution-computed tomography (HR-CT) can be of great diagnostic use. We here describe the case of a 62-year-old woman with a history of breast cancer and subsequent ablation of the left breast and radiotherapy. The case demonstrates an example of a pleuropulmonary aspergilloma, in which sero- and micro-biological detection failed. Thorax HR-CT exhibited the cavity, a small persistent pneumothorax, partially filled by an oval density. This density clearly dislocated according to gravity following a positional change of the patient from supine to prone. The density thus revealed mobility which was typical of aspergilloma. Following excision, this diagnosis was confirmed. A density within a cavity may be differentiated by its mobility from differential diagnoses such as lung cancer which would not be expected to exhibit mobility.

Toll-like receptors: Recent advances, open questions and implications for aspergillosis control

Aspergillus fumigatus is a pathogenic mould that can cause severe and life-threatening infections in immunocompromised patients. Apart from novel and improved antifungals, additional strategies are required to protect patients at risk from developing invasive aspergillosis. Given the problems in diagnosis of this disease, important perspectives lie in attempts to elicit and strengthen a protective immunity. The innate immune system is the first line of defence against A. fumigatus. Phagocytes engulf and kill inhaled conidia, but also closely communicate with the adaptive immune system. Recognition of invading microbes is mediated by pattern recognition receptors (PRRs), and Toll-like receptors (TLR) 2 and TLR4 have been implicated in the immune response to A. fumigatus. The analysis of this process is hampered by the fact that A. fumigatus infections are inevitably coupled to germination resulting in the appearance of different fungal morphotypes, like conidia and hyphae. While conflicting data still exist on the relative importance of TLR2 and 4 in recognition of distinct A. fumigatus morphotypes, recent evidence suggests that certain TLR agonists can be used to divert the immune response towards an optimal fungicidal activity in the absence of detrimental inflammatory consequences.

The beta-glucan receptor dectin-1 recognizes specific morphologies of Aspergillus fumigatus

Alveolar macrophages represent a first-line innate host defense mechanism for clearing inhaled Aspergillus fumigatus from the lungs, yet contradictory data exist as to which alveolar macrophage recognition receptor is critical for innate immunity to A. fumigatus. Acknowledging that the A. fumigatus cell wall contains a high beta-1,3–glucan content, we questioned whether the beta-glucan receptor dectin-1 played a role in this recognition process. Monoclonal antibody, soluble receptor, and competitive carbohydrate blockage indicated that the alveolar macrophage inflammatory response, specifically the production of tumor necrosis factor-α (TNF-α), interleukin-1α (IL-1α), IL-1β, IL-6, CXCL2/macrophage inflammatory protein-2 (MIP-2), CCL3/macrophage inflammatory protein-1α (MIP-1α), granulocyte-colony stimulating factor (G-CSF), and granulocyte monocyte–CSF (GM-CSF), to live A. fumigatus was dependent on recognition via the beta-glucan receptor dectin-1. The inflammatory response was triggered at the highest level by A. fumigatus swollen conidia and early germlings and correlated to the levels of surface-exposed beta glucans, indicating that dectin-1 preferentially recognizes specific morphological forms of A. fumigatus. Intratracheal administration of A. fumigatus conidia to mice in the presence of a soluble dectin-Fc fusion protein reduced both lung proinflammatory cytokine/chemokine levels and cellular recruitment while modestly increasing the A. fumigatus fungal burden, illustrating the importance of beta-glucan–initiated dectin-1 signaling in defense against this pathogen. Collectively, these data show that dectin-1 is centrally required for the generation of alveolar macrophage proinflammatory responses to A. fumigatus and to our knowledge provides the first in vivo evidence for the role of dectin-1 in fungal innate defense.

Individuals with defective immune systems are highly susceptible to infection by parasites, bacteria, viruses, and fungi. Infection by the opportunistic fungal organism Aspergillus fumigatus can be particularly severe in this population. Because many pathogenic microorganisms, including A. fumigatus, enter the body through the lung, it is important to understand the function of its immune system. The alveolar macrophage is one of the first cell types to come in contact with inhaled pathogens. An intense area of research is how lung immune cells—i.e., alveolar macrophages—recognize inhaled pathogens and respond to them. Steele et al. recently discovered that alveolar macrophages express a receptor on their surface, dectin-1, that is essential in recognizing and responding to inhaled fungal pathogens. They now have investigated the interaction between dectin-1 and A. fumigatus to determine how the dectin-1 receptor orchestrates the alveolar macrophage response. They found that alveolar macrophages respond poorly to A. fumigatus when the dectin-1 receptor is blocked. Also, in animal experiments, blocking dectin-1 renders the animals more susceptible to infection with A. fumigatus. This study may lay the foundation for developing new and novel strategies to combat infections caused by A. fumigatus.

Liver cells respond to Aspergillus fumigatus with an increase in C3 secretion and C3 gene expression as well as an expression increase in TLR2 and TLR4

Fungal infections by molds like Aspergillus fumigatus are an increasing health problem which can be fatal in immuno-compromised patients. In healthy individuals, these infections are easily eliminated by the innate and acquired immune system. Complement factor 3 (C3) has a key place within the complement cascade and C3 RNA expression can therefore be used to monitor an impending immune response. Employing a liver cell line (HepG2) as a model system, we have examined their responses to A. fumigatus or beta-glucan, a major component of the fungal wall. C3 RNA expression was increased after stimulation with both LPS and A. fumigatus as well as after incubation with beta-glucan, although with different kinetics. C3 protein release into the supernatant followed an inverse bell-shaped curve when cells were incubated with A. fumigatus or beta-glucan while during LPS stimulation, the release was more stable. HepG2 cells also express Toll-like receptors (TLRs) and both for TLR2 and TLR4, an expression increase was found. These data demonstrate that liver cells are able to react specifically to a fungal pathogen without the help of Kupffer cells.