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

Dectin-1 Fc targeting of aspergillus fumigatus beta-glucans augments innate defense against invasive pulmonary aspergillosis

Invasive pulmonary aspergillosis (IPA) has significantly increased over the last decade. Here, a fusion protein consisting of the Dectin-1 extracellular domain linked to the Fc portion of murine immunoglobulin G1 augmented alveolar macrophage killing of Aspergillus fumigatus and shifted mortality associated with IPA via attenuation of A. fumigatus growth in the lung.

Collaboration between the innate immune receptors dectin-1, TLRs, and Nods

Microbes are complex and present a wide variety of structures that phagocytes may recognize using innate immune receptors. Recognition triggers anti-microbial killing mechanisms and production of inflammatory cytokines and chemokines that orchestrate host defense. As a general rule, no single receptor is likely to be the sole mediator of activation of protective immune responses. Recent studies highlight the importance of collaboration between Toll-like receptors, the nucleotide oligomerization domain (Nod) proteins, and dectin-1 in regulating inflammatory responses. Studies on the molecular mechanisms of cross-talk and synergy between these receptors provide a framework in which to understand the importance of having multiple receptors recognize individual microbes.

IL-23 and IL-17 have a multi-faceted largely negative role in fungal infection

The role of IL-23 and IL-17 in the response to fungal infection has been the focus of recent reports. In this issue of the European Journal of Immunology there is an article that reports an important role for IL-23 and IL-17 in limiting fungal control, promoting neutrophillic inflammation and regulating the killing activity of neutrophils. In the fungal model it appears that IL-23 and IL-17 are counter-productive for protection.

MyD88 signaling contributes to early pulmonary responses to Aspergillus fumigatus

Toll-like receptors and the beta-glucan receptor, dectin-1, mediate macrophage inflammatory responses to Aspergillus fumigatus through MyD88-dependent and -independent signaling mechanisms; however, pulmonary inflammatory responses in MyD88-deficient mice challenged with A. fumigatus are poorly defined. The role of MyD88 signaling in early pulmonary inflammation and fungal clearance was evaluated in C57BL/6J wild-type (WT) and MyD88-deficient (MyD88-/-) mice. Early (<48 h) after infection, MyD88-/- mice had higher fungal burdens than those of WT mice, although fungal burdens rapidly declined (>72 h) in both. MyD88-/- mice had less consolidated inflammation, with fewer NK cells, in lung tissue early (24 h) after infection than did WT mice. At the latter time point, MyD88-/- mouse lungs were characterized by a large amount of necrotic cellular debris and fibrin, while WT lungs had organized inflammation. Although there were equivalent numbers of macrophages in WT and MyD88-/- mouse lung tissues, MyD88-/- cells demonstrated delayed uptake of green fluorescent protein-expressing A. fumigatus (GFP-Af293); histologically, MyD88-/- mouse lungs had more hyphal invasion of terminal airways and vessels, the appearance of bronchiolar epithelial cell necrosis, and necrotizing vasculitis. MyD88-/- lung homogenates contained comparatively decreased amounts of interleukin-1beta (IL-1beta), IL-6, KC, and gamma interferon and paradoxically increased amounts of tumor necrosis factor alpha and macrophage inflammatory protein 1alpha. These data indicate that the MyD88-dependent pathway mediates acute pulmonary fungal clearance, inflammation, and tissue injury very early after infection. Resolution of abnormalities within a 3-day window demonstrates the importance of redundant signaling pathways in mediating pulmonary inflammatory responses to fungi.

Activator protein 1 is triggered by Aspergillus fumigatus beta-glucans surface-exposed during specific growth stages

Aspergillus fumigatus has been shown to trigger the activation of nuclear factor-kappaB (NF-kappaB), but it remains unclear whether other transcription factors are also induced following infection by this organism. In this study, we demonstrated that A. fumigatus also triggers activator protein 1 (AP-1), a transcription factor that plays an important role during the production of cytokines and chemokines. Swollen conidia strongly induce the activation of AP-1, and more than 80% of these were stained positively with anti-beta-glucan antibodies by fluorescence microscopy. Hyphae were also stained with anti-beta-glucan antibodies, albeit significantly weaker compared with swollen conidia. Furthermore, our present findings also showed that A. fumigatus triggers the activation of AP-1 in a dectin-1 (receptor for beta-glucan)-dependent manner. These data thus suggest that AP-1 is triggered by beta-glucan recognition on the surface of A. fumigatus. We also showed that Syk tyrosine kinase is required for AP-1 induction in this pathway. We therefore speculate that the dectin-1/Syk/AP-1 signaling pathway plays an important role in the host defense response to fungal infection.

Role of Toll-like receptors in lung innate defense against invasive aspergillosis. Distinct impact in immunocompetent and immunocompromized hosts

Toll-like receptors are key to pathogen recognition by a host and to the subsequent triggering of an innate immune response. Experimental and clinical evidence shows that defects in Toll-like receptors or in signaling pathways downstream from these receptors render hosts susceptible to various types of infection, including aspergillosis. Patients receiving an immunosuppressive regimen, including corticosteroid therapy or cytotoxic chemotherapy, are also susceptible to infections. Aspergillus fumigatus is an opportunistic pathogen that infects the lungs of immunosuppressed hosts. Here, we review the evidence that experimental inactivation of various Toll-like receptors and of their signaling pathways may worsen cases of invasive pulmonary aspergillosis. Moreover, the literature clearly indicates that the type of immunosuppression is very important, as it influences whether or not Toll-like receptors contribute to infection. The involvement of Toll-like receptors, based on the immunological status of the patient, should be considered if an immunosuppressive treatment must be administered.

The lung responds to zymosan in a unique manner independent of toll-like receptors, complement, and dectin-1

In vitro studies indicate that the inflammatory response to zymosan, a fungal wall preparation, is dependent on Toll-like receptor (TLR) 2, and that this response is enhanced by the dectin-1 receptor. Complement may also play an important role in this inflammatory response. However, the relevance of these molecules within the in vivo pulmonary environment remains unknown. To examine pulmonary in vivo inflammatory responses of the lung to zymosan, zymosan was administered by intratracheal aerosolization to C57BL/6, TLR2- TLR4-, MyD88-, and complement-deficient mice. Outcomes included bronchoalveolar fluid cell counts. We next examined effects of dectin-1 inhibition on response to zymosan in alveolar macrophages in vitro and in lungs of C57BL/6, TLR2-, and complement-deficient mice. Finally, the effect of alveolar macrophage depletion on in vivo pulmonary responses was assessed. Marked zymosan-induced neutrophil responses were unaltered in TLR2-deficient mice despite a TLR2-dependent response seen with synthetic TLR2 agonists. TLR4, MyD88, and complement activation were not required for the inflammatory response to zymosan. Although dectin-1 receptor inhibition blocked the inflammatory response of alveolar macrophages to zymosan in vitro, in vivo pulmonary leukocyte recruitment was not altered even in the absence of TLR2 or complement. Depletion of alveolar macrophages did not affect the response to zymosan. Neither complement, macrophages, nor TLR2, TLR4, MyD88, and/or dectin-1 receptors were involved in the pulmonary in vivo inflammatory response to zymosan.

Production of IL-6, in contrast to other cytokines and chemokines, in macrophage innate immune responses: effect of serum and fungal (Blastomyces) challenge

Murine peritoneal macrophages, after adherence and establishment in culture in vitro in the presence of medium containing fetal bovine serum (FBS) for 20 h, then cultured for 20 h, produced several cytokines. If, in the second 20 h period, a fungus (heat-killed Blastomyces, HK-Bd) was introduced, a more complex pattern of cytokine (particularly TNF) and chemokine production ensued. The cytokine production, assayed by antibody array and also quantitation in supernatants, was depressed (particularly TNF) by the addition of mouse serum to these cultures, with the exception of IL-6. Macrophages could be cultured in the presence or absence of serum during the initial 20 h adherence and establishment period, enabling study of the effect of serum factors. In the absence of serum, with or without fungal stimulation, cytokine and chemokine production was more restricted, largely to TNF and IL-6. The addition of mouse serum [corrected] resulted in marked depression of TNF and enhancement of IL-6. The combination of HK-Bd and mouse serum resulted in more IL-6 production than either component alone. The enhancement of IL-6 by mouse serum was concentration-dependent and maximal at 8 h. The effects of fungus or serum on macrophage production of cytokines were similar in an outbred and an inbred mouse strain. The larger repertoire of cytokine production in the macrophages that had been cultured longer (20 h+20 h) in serum may be related to maturation of cell receptors. IL-6 production in vivo in response to fungal-serum complexes could affect pathogenesis by opposing the host defense modulation by proinflammatory cytokines or by modulating the destructive effects of inflammation on host tissues.

Human polymorphonuclear leukocytes inhibit Aspergillus fumigatus conidial growth by lactoferrin-mediated iron depletion

Aspergillus fumigatus, a common mold, rarely infects humans, except during prolonged neutropenia or in cases of chronic granulomatous disease (CGD), a primary immunodeficiency caused by mutations in the NADPH oxidase that normally produces fungicidal reactive oxygen species. Filamentous hyphae of Aspergillus are killed by normal, but not CGD polymorphonuclear leukocytes (PMN); however, the few studies on PMN-mediated host defenses against infectious conidia (spores) of this organism have yielded conflicting results, some showing that PMN do not inhibit conidial growth, with others showing that they do, most likely using reactive oxygen species. Given that CGD patients are exposed daily to hundreds of viable A. fumigatus conidia, yet considerable numbers of them survive years without infection, we reasoned that PMN use ROS-independent mechanisms to combat Aspergillus. We show that human PMN from both normal controls and CGD patients are equipotent at arresting the growth of Aspergillus conidia in vitro, indicating the presence of a reactive oxygen species-independent factor(s). Cell-free supernatants of degranulated normal and CGD neutrophils both suppressed fungal growth and were found to be rich in lactoferrin, an abundant PMN secondary granule protein. Purified iron-poor lactoferrin at concentrations occurring in PMN supernatants (and reported in human mucosal secretions in vivo) decreased fungal growth, whereas saturation of lactoferrin or PMN supernatants with iron, or testing in the presence of excess iron in the form of ferritin, completely abolished activity against conidia. These results demonstrate that PMN lactoferrin sequestration of iron is important for host defense against Aspergillus.

Enhanced defense against Pneumocystis carinii mediated by a novel dectin-1 receptor Fc fusion protein

Pneumocystis carinii (PC) pneumonia is a leading opportunistic infection found among HIV-infected individuals worldwide. Although CD4(+) T cell deficiency clearly correlates with susceptibility to PC pneumonia, murine models of disease indicate that PC-directed Abs may prevent infection and/or inhibit growth of existing PC within the lungs. Recognition of PC by alveolar macrophages involves the beta-glucan receptor Dectin-1 and macrophage effector function against PC is enhanced by Abs derived from PC-vaccinated hosts. We developed a fusion protein consisting of the extracellular domain of Dectin-1 linked to the Fc portion of murine IgG1, which we hypothesized would enhance host recognition and opsonic phagocytosis of PC. The recombinant protein, Dectin-Fc, is dimeric and the Ag recognition site identifies beta-1,3 glucan linkages specifically and with high affinity (K(D) = 2.03 x 10(-7) M). Dectin-Fc enhances RAW264.7 macrophage recognition of the beta-glucan containing particulate zymosan in an FcgammaRII- and FcgammaRIII-dependent manner and preopsonization of PC organisms with Dectin-Fc increased alveolar and peritoneal macrophage-dependent killing of PC. SCID mice treated with a replication incompetent adenoviral vector expressing Dectin-Fc had attenuated growth of PC within the lungs, overall decreased PC lung burden, and diminished correlates of PC-related lung damage relative to SCID mice receiving a control vector. These findings demonstrate that targeting PC beta-glucan with Dectin-Fc enhances host recognition and clearance of PC in the absence of B and T cells, and suggest that FcgammaR-based targeting of PC, via cell wall carbohydrate recognition, may promote resistance against PC pneumonia in the immunodeficient host.

Chronic intranasal administration of mould spores or extracts to unsensitized mice leads to lung allergic inflammation, hyper-reactivity and remodelling

Allergic asthma is a serious multifaceted disease characterized by eosinophil-rich airway inflammation, airway hyperreactivity and airway wall modifications known as remodelling. We previously demonstrated that the spores of two allergenic moulds, Alternaria alternata and Cladosporium herbarum, were potent inducers of immunoglobulin E (IgE) production. Moreover, mice sensitized by two intraperitoneal injections before intranasal challenge with A. alternata or C. herbarum spores developed an allergic lung inflammation and hyperreactivity. Here we report on the effect of chronic intranasal administration of C. herbarum spores or A. alternata extracts to unsensitized BALB/c mice. Our results demonstrate that this chronic treatment led to an increase of total serum IgE and the appearance of specific IgE and IgG1. Total cell number in bronchoalveolar lavage fluid from treated mice was highly increased compared to phosphate-buffered-saline-treated mice because of the accumulation of macrophages, neutrophils, lymphocytes and eosinophils. Airway hyperreactivity appeared after 3 weeks (extract) and 7 weeks (spores) and was maintained during the whole treatment. Increased interleukin-13 mRNA expression in the lungs and T helper type 2 cytokines (interleukin-4, -5, -6 and -13) and transforming growth factor-beta secretion in bronchoalveolar lavage fluid were also observed. Lung hydroxyproline and fibronectin contents indicated increased fibrosis in mice treated with mould allergen. These observations were confirmed by histological analysis demonstrating airway wall remodelling and strong mucus production. These observations show that this model, using chronic intranasal administration of relevant particulate allergens, is an interesting tool for the study of mechanisms leading to allergic pulmonary diseases and lung remodelling.

The role of the β-glucan receptor Dectin-1 in control of fungal infection

During fungal infection, a variety of receptors initiates immune responses, including TLR and the beta-glucan receptor Dectin-1. TLR recognition of fungal ligands and subsequent signaling through the MyD88 pathway were thought to be the most important interactions required for the control of fungal infection. However, recent papers have challenged this view, highlighting the role of Dectin-1 in induction of cytokine responses and the respiratory burst. Two papers, using independently derived, Dectin-1-deficient mice, address the role of Dectin-1 in control of fungal infection. Saijo et al. [1] argue that Dectin-1 plays a minor role in control of Pneumocystis carinii by direct killing and that TLR-mediated cytokine production controls P. carinii and Candida albicans. By contrast, Taylor et al. [2] argue that Dectin-1-mediated cytokine and chemokine production, leading to efficient recruitment of inflammatory cells, is required for control of fungal infection. In this review, we argue that collaborative responses induced during infection may partially explain these apparently contradictory results. We propose that Dectin-1 is the first of many pattern recognition receptors that can mediate their own signaling, as well as synergize with TLR to initiate specific responses to infectious agents.

The role of CC chemokine receptor 6 in host defense in a model of invasive pulmonary aspergillosis

RATIONALE

Invasive aspergillosis is a severe fungal infection afflicting immunocompromised patients, particularly patients with neutrophil defects. CCR6, a beta-chemokine receptor, mediates migration of dendritic cells (DCs) and several lymphocyte subsets to sites of epithelial inflammation, but its role in infections has not been examined extensively.

OBJECTIVES

To test the hypothesis that CCR6-mediated leukocyte recruitment is necessary for effective host defense in neutropenic hosts with invasive pulmonary aspergillosis.

METHODS

Neutropenic wild-type mice and mice with targeted deletion of CCR6 were infected with Aspergillus fumigatus. The host responses to the infection were compared in vivo and leukocyte responses to the fungus were examined in vitro.

MEASUREMENTS AND MAIN RESULTS

In the context of infection, immature myeloid DCs were the major population of CCR6-expressing cells in the lungs. As compared with wild-type animals, CCR6-deficient mice developed a more severe infection when challenged with A. fumigatus conidia, as documented by a higher mortality rate and greater lung fungal burden. This was associated with reduced accumulation of DCs in the lungs. CCR6-deficient and wild-type DCs did not differ in their phagocytosis of conidia, cytokine response, or maturation in vitro. In adoptive transfer experiments, however, DCs from CCR6-deficient donors showed lesser accumulation in the lungs of infected mice as compared with wild-type cells, and transfer of wild-type, but not CCR6-deficient, DCs resulted in attenuated severity of infection in CCR6-deficient recipients.

CONCLUSIONS

Taken together, these results implicate CCR6-mediated DC influx into the lung in the initial host defense in invasive aspergillosis.

Dectin-1 Stimulation by Candida albicans Yeast or Zymosan Triggers NFAT Activation in Macrophages and Dendritic Cells

Innate immune pattern recognition receptors play critical roles in pathogen detection and initiation of antimicrobial responses. We and others have previously demonstrated the importance of the beta-glucan receptor Dectin-1 in the recognition of pathogenic fungi by macrophages and dendritic cells and have elucidated some of the mechanisms by which Dectin-1 signals to coordinate the antifungal response. While Dectin-1 signals alone are sufficient to trigger phagocytosis and Src-Syk-mediated induction of antimicrobial reactive oxygen species, collaboration with TLR2 signaling enhances NF-kappaB activation and regulates cytokine production. In this study we demonstrate that Dectin-1 signaling can also directly modulate gene expression via activation of NFAT. Dectin-1 ligation by zymosan particles or live Candida albicans yeast triggers NFAT activation in macrophages and dendritic cells. Dectin-1-triggered NFAT activation plays a role in the induction of early growth response 2 and early growth response 3 transcription factors, and cyclooxygenase-2. Furthermore, we show that NFAT activation regulates IL-2, IL-10 and IL-12 p70 production by zymosan-stimulated dendritic cells. These data establish NFAT activation in myeloid cells as a novel mechanism of regulation of the innate antimicrobial response.

Aspergillosis: spectrum of disease, diagnosis, and treatment

Infections by Aspergillus species present a particular challenge. The organism, which is ubiquitous in the environment, causes allergic disease in otherwise healthy individuals and devastating disease in the immunosuppressed. This article examines the range of infections caused by Aspergillus species, the challenges of diagnosis, and current treatment options.

Histoplasma capsulatum alpha-(1,3)-glucan blocks innate immune recognition by the beta-glucan receptor

Successful infection by fungal pathogens depends on subversion of host immune mechanisms that detect conserved cell wall components such as beta-glucans. A less common polysaccharide, alpha-(1,3)-glucan, is a cell wall constituent of most fungal respiratory pathogens and has been correlated with pathogenicity or linked directly to virulence. However, the precise mechanism by which alpha-(1,3)-glucan promotes fungal virulence is unknown. Here, we show that alpha-(1,3)-glucan is present in the outermost layer of the Histoplasma capsulatum yeast cell wall and contributes to pathogenesis by concealing immunostimulatory beta-glucans from detection by host phagocytic cells. Production of proinflammatory TNFalpha by phagocytes was suppressed either by the presence of the alpha-(1,3)-glucan layer on yeast cells or by RNA interference based depletion of the host beta-glucan receptor dectin-1. Thus, we have functionally defined key molecular components influencing the initial host-pathogen interaction in histoplasmosis and have revealed an important mechanism by which H. capsulatum thwarts the host immune system. Furthermore, we propose that the degree of this evasion contributes to the difference in pathogenic potential between dimorphic fungal pathogens and opportunistic fungi.

Dectin-1 is required for beta-glucan recognition and control of fungal infection

Beta-glucan is one of the most abundant polysaccharides in fungal pathogens, yet its importance in antifungal immunity is unclear. Here we show that deficiency of dectin-1, the myeloid receptor for beta-glucan, rendered mice susceptible to infection with Candida albicans. Dectin-1-deficient leukocytes demonstrated significantly impaired responses to fungi even in the presence of opsonins. Impaired leukocyte responses were manifested in vivo by reduced inflammatory cell recruitment after fungal infection, resulting in substantially increased fungal burdens and enhanced fungal dissemination. Our results establish a fundamental function for beta-glucan recognition by dectin-1 in antifungal immunity and demonstrate a signaling non-Toll-like pattern-recognition receptor required for the induction of protective immune responses.

Myeloid C-type lectins in innate immunity

C-type lectins expressed on myeloid cells comprise a family of proteins that share a common structural motif, and some act as receptors in pathogen recognition. But just as the presence of leucine-rich repeats alone is not sufficient to define a Toll-like receptor, the characterization of C-type lectin receptors in innate immunity requires the identification of accompanying signaling motifs. Here we focus on the known signaling pathways of myeloid C-type lectins and on their possible functions as autonomous activating or inhibitory receptors involved in innate responses to pathogens or self.

Innate Immune Activation and CD4+ T Cell Priming during Respiratory Fungal Infection

Aspergillus fumigatus is a mold that causes a spectrum of diseases, including lethal lung infections in immunocompromised humans and allergic asthma in atopic individuals. T helper 1 (Th1) CD4(+) T cells protect against invasive A. fumigatus infections whereas Th2 CD4(+) T cells exacerbate asthma upon inhalation of A. fumigatus spores. Herein, we demonstrate that A. fumigatus-specific T cells were rapidly primed in lymph nodes draining the lung and fully differentiated into interferon-gamma (IFN-gamma)-producing Th1 CD4(+) T cells upon arrival in the airways. T-bet induction in A. fumigatus-specific CD4(+) T cells was enhanced by MyD88-mediated signals in draining lymph nodes, but T cell proliferation, trafficking, and Th1 differentiation in the airways were Toll-like receptor (TLR) and MyD88 independent. Our studies demonstrate that CD4(+) T cell differentiation during respiratory fungal infection occurs incrementally, with TLR-mediated signals in the lymph node enhancing the potential for IFN-gamma production whereas MyD88-independent signals promote Th1 differentiation in the lung.

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.

Immunity to fungi

Innate and adaptive immune responses target pathogenic fungi and provide defense against fungal infections. Recent studies demonstrate that specific host receptors recognize ligands that are unique to fungi and activate signaling cascades that lead to phagocytosis of fungi, generation of pro-inflammatory mediators, formation of reactive oxygen species, trafficking of inflammatory cells to sites of infection, and initiation of adaptive immune responses. Greater understanding of the molecular mechanisms that underlie antifungal defense has provided a framework for the investigation of protective vaccines and strategies for therapeutic adoptive cell transfer.

Soluble Dectin-1 as a tool to detect beta-glucans

Beta-glucans are structural components of fungal cell walls which are involved in the immune recognition of fungal pathogens and possess beneficial immunomodulatory activities in isolated form. Here we have developed a soluble chimeric form of the major mammalian beta-glucan receptor, Dectin-1, and demonstrate its application for the detection and characterisation of soluble and insoluble beta-glucans, including fungal particles, using ELISA, flow cytometric and fluorescence-based microscopy assays.

Dectin-1 and TLRs permit macrophages to distinguish between different Aspergillus fumigatus cellular states

Aspergillus fumigatus is a common cause of invasive and allergic pulmonary disease. Resting conidia of the filamentous fungus are constantly inhaled, but cause infection only after initiating hyphal growth. In this study, we have explored whether macrophages can distinguish between resting spores and the maturing, potentially invasive form of the fungus. Although macrophages bind and ingest A. fumigatus resting conidia efficiently, there is little inflammatory response; NF-kappabeta is not activated, inflammatory cytokines are not induced, and reactive oxygen species are not produced. However, maturing A. fumigatus conidia and germ tubes stimulate NF-kappabeta, secretion of proinflammatory cytokines and production of reactive oxygen by human monocyte-derived macrophages and murine macrophages from multiple anatomical sites. These responses are in part mediated by dectin-1, which binds cell wall beta-glucan that is not present on the surface of dormant conidia, but is present after cellular swelling and loss of the hydrophobic proteinaceous cell wall. Dectin-1 binding to germ tubes augments, but is not required for, TLR2-mediated inflammatory cytokine secretion. Dectin-1 recognition of germ tubes also stimulates TNF-alpha production in the absence of both TLR2 and MyD88 signaling. These data demonstrate one mechanism by which the pulmonary inflammatory response is tailored toward metabolically active cells, thereby avoiding unnecessary tissue damage with frequent inhalation of ubiquitous spores.

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.

Genetic and respiratory tract risk factors for aspergillosis: ABPA and asthma with fungal sensitization

Allergic bronchopulmonary aspergillosis (ABPA) is a Th2 allergic hypersensitivity lung disease due to bronchial colonization of Aspergillus fumigatus that affects 1-2% of asthmatic and 7-9% of cystic fibrosis (CF) patients. We hypothesize that genetic risk factors predispose these patients to develop ABPA. We previously reported HLA-DR2 and DR5 restriction as a risk factor for the development of ABPA. We further propose that HLA-DR restriction is necessary but not sufficient for the development of ABPA. Recently, we reported that IL-4Rα single nucleotide polymorphisms (SNP) and in particular the ile75val SNP in the IL-4 binding region is another risk factor and is associated with increased sensitivity to IL-4 stimulation. It has been reported that the combination of IL-4Rα and IL-13 SNP, ile75val/arg110gln, is associated with more severe asthma. In preliminary studies, we have observed increased frequency of this combination in ABPA asthmatic and CF patients. Another genetic risk factor reported by Brouard et al. is the -1082 GG genotype in the IL-10 promoter in CF patients for the colonization of A. fumigatus and development of ABPA. This genotype was associated with increased plasma IL-10 levels, and perhaps may be associated with increased skewing of Th2 Aspergillus responses rather than down-regulation of inflammatory responses. We hypothesize that increased sensitivity of IL-4 mediated activities secondary to polymorphisms IL-4R in conjunction of other polymorphisms such as IL-13 and IL-10 in conjunction with HLA-DR2/DR5 restriction to Aspergillus antigens in ABPA patients result in increased B-cell activity, monocyte/dendritic cell phenotype that skews Th2 responses, and skewing of Aspergillus-specific Th2 cells. This model system may be applicable to other fungi such as Alternaria and Cladosporium which is associated with increased asthma severity.