Involvement of toll-like receptor 2 in experimental invasive pulmonary aspergillosis

Experimental Models of Infectious Pulmonary Complications Following Hematopoietic Cell Transplantation

Pulmonary infections remain a major cause of morbidity and mortality in hematopoietic cell transplantation (HCT) recipients. The prevalence and type of infection changes over time and is influenced by the course of immune reconstitution post-transplant. The interaction between pathogens and host immune responses is complex in HCT settings, since the conditioning regimens create periods of neutropenia and immunosuppressive drugs are often needed to prevent graft rejection and limit graft-versus-host disease (GVHD). Experimental murine models of transplantation are valuable tools for dissecting the procedure-related alterations to innate and adaptive immunity. Here we review mouse models of post-HCT infectious pulmonary complications, primarily focused on three groups of pathogens that frequently infect HCT recipients: bacteria (often P. aeruginosa), fungus (primarily Aspergillus fumigatus), and viruses (primarily herpesviruses). These mouse models have advanced our knowledge regarding how the conditioning and HCT process negatively impacts innate immunity and have provided new potential strategies of managing the infections. Studies using mouse models have also validated clinical observations suggesting that prior or occult infections are a potential etiology of noninfectious pulmonary complications post-HCT as well.

TLR7 Expression Aggravates Invasive Pulmonary Aspergillosis by Suppressing Anti-Aspergillus Immunity of Macrophages

Toll-like receptors (TLRs) play a critical role in early immune recognition of Aspergillus, which can regulate host defense during invasive pulmonary Aspergillosis (IPA). However, the role of TLR7 in the pathogenesis of IPA remains unknown. In this study, an in vivo model of IPA was established to investigate the contribution of TLR7 to host anti-Aspergillus immunity upon invasive pulmonary Aspergillus fumigatus infection. The effects of TLR7 on phagocytosis and killing capacities of A. fumigatus by macrophages and neutrophils were investigated in vitro We found that TLR7 knockout mice exhibited lower lung inflammatory response and tissue injury, higher fungal clearance, and greater survival in an in vivo model of IPA compared with wild-type mice. TLR7 activation by R837 ligand led to wild-type mice being more susceptible to invasive pulmonary Aspergillus fumigatus infection. Macrophages, but not neutrophils, were required for the protection against IPA observed in TLR7 knockout mice. Mechanistically, TLR7 impaired phagocytosis and killing of A. fumigatus by macrophages but not neutrophils. Together, these data identify TLR7 as an important negative regulator of anti-Aspergillus innate immunity in IPA, and we propose that targeting TLR7 will be beneficial in the treatment of IPA.

The Association of TLR2, TLR3, and TLR9 Gene Polymorphisms With Susceptibility to Talaromycosis Among Han Chinese AIDS Patients in Guangdong

Background

Talaromycosis (TM) caused by Talaromyces marneffei (T. marneffei) is a growing public health concern. Although Toll-like receptor (TLR) genes play a critical role in the host defense against fungal infection, the influence of polymorphisms in these genes on the susceptibility of acquired immune deficiency syndrome (AIDS) patients to TM remains unknown. This study aims to uncover the associations of single nucleotide polymorphisms (SNPs) in TLR genes with TM susceptibility among patients with AIDS.

Methods

Altogether 200 AIDS patients complicated with TM, 200 matched AIDS patients without TM, and 76 healthy controls (HCs) were enrolled in this case-control study. In total, 23 SNPs in the TLR2, TLR4, and TLR9 genes, which may influence the susceptibility of AIDS patients to TM, were checked by the time of flight mass spectrometry (TOF/MS) method among these Han Chinese subjects.

Results

No significant differences in genotype or allele frequencies of selected SNPs were found among the TM group, Non-TM group, and HC group. Haplotype analysis also demonstrated no correlation of these SNPs with TM. However, subgroup analysis showed that the genotype TT and the T allele in TLR2 SNP rs1339 were more frequent in typical TM cases than controls (50.0 vs. 35.8%, 70.5 vs. 59.7%); the frequency of the GT genotype in TLR2 SNP rs7656411 was markedly higher in severe TM cases compared to controls (57.8 vs. 34.4%).

Conclusion

Our results demonstrate a genetic connection of TLR2 SNPs rs1339 and rs7656411 with an increased susceptibility and severity of TM among Han Chinese populations.

Aspergillus fumigatus Recognition by Dendritic Cells Negatively Regulates Allergic Lung Inflammation through a TLR2/MyD88 Pathway

Aspergillus fumigatus is an opportunistic fungal pathogen responsible for a spectrum of clinical manifestations. Dendritic cells recognize pathogen-associated molecular patterns of Aspergillus via two main receptor families, Toll-like receptors (TLRs) and C-type lectin receptors (CLR). Here, the importance of TLR and CLR signaling in the regulation of T-helper cell type 2 (Th2) responses was analyzed using a mouse model based on the transfer of bone marrow-derived dendritic cells (BMDCs) pulsed with A. fumigatus conidia. BMDCs were generated from mice deficient in either MyD88 or MALT1 (mucosa-associated lymphoid tissue lymphoma translocation protein 1). Both the MyD88 and MALT1 signaling pathway in BMDCs contributed to the production of inflammatory cytokines induced by A. fumigatus conidia. Mice sensitized with MyD88^-/- BMDCs pulsed in vitro with A. fumigatus conidia showed an exacerbated allergic inflammation, with stronger eosinophil recruitment in the BAL and higher Th2 cytokine production compared with mice sensitized with wild-type or MALT1^-/- BMDCs. This exacerbation was not observed when MyD88^-/- BMDCs were pulsed with Cladosporium sphaerospermum, a nonpathogenic mold. A lack of TLR2 signaling recapitulated the exacerbation of the A. fumigatus Th2 response observed in the absence of MyD88 signaling, whereas TLR2 agonist dampened the response induced with A. fumigatus and C. sphaerospermum conidia. IL-10 production by BMDCs in response to A. fumigatus was dependent on the expression of TLR2 and MyD88. IL-10^-/- BMDCs exacerbated, whereas MyD88^-/- BMDCs supplemented with exogenous IL-10 decreased the allergic pulmonary inflammation. These results indicate that TLR2/MyD88-specific recognition of PAMPs from A. fumigatus conidia can upregulate IL-10 production and downregulate lung eosinophilia and the development of a Th2 response.

The Host Immune Response to Scedosporium/Lomentospora

Infections caused by the opportunistic pathogens Scedosporium/Lomentospora are on the rise. This causes problems in the clinic due to the difficulty in diagnosing and treating them. This review collates information published on immune response against these fungi, since an understanding of the mechanisms involved is of great interest in developing more effective strategies against them. Scedosporium/Lomentospora cell wall components, including peptidorhamnomannans (PRMs), α-glucans and glucosylceramides, are important immune response activators following their recognition by TLR2, TLR4 and Dectin-1 and through receptors that are yet unknown. After recognition, cytokine synthesis and antifungal activity of different phagocytes and epithelial cells is species-specific, highlighting the poor response by microglial cells against L. prolificans. Moreover, a great number of Scedosporium/Lomentospora antigens have been identified, most notably catalase, PRM and Hsp70 for their potential medical applicability. Against host immune response, these fungi contain evasion mechanisms, inducing host non-protective response, masking fungal molecular patterns, destructing host defense proteins and decreasing oxidative killing. In conclusion, although many advances have been made, many aspects remain to be elucidated and more research is necessary to shed light on the immune response to Scedosporium/Lomentospora.

Galactomannan Produced by Aspergillus fumigatus: An Update on the Structure, Biosynthesis and Biological Functions of an Emblematic Fungal Biomarker

The galactomannan (GM) that is produced by the human fungal pathogen Aspergillus fumigatus is an emblematic biomarker in medical mycology. The GM is composed of two monosaccharides: mannose and galactofuranose. The furanic configuration of galactose residues, absent in mammals, is responsible for the antigenicity of the GM and has favoured the development of ELISA tests to diagnose aspergillosis in immunocompromised patients. The GM that is produced by A. fumigatus is a unique fungal polysaccharide containing a tetramannoside repeat unit and having three different forms: (i) membrane bound through a glycosylphosphatidylinositol (GPI)-anchor, (ii) covalently linked to β-1,3-glucans in the cell wall, or (iii) released in the culture medium as a free polymer. Recent studies have revealed the crucial role of the GM during vegetative and polarized fungal growth. This review highlights these recent data on its biosynthetic pathway and its biological functions during the saprophytic and pathogenic life of this opportunistic human fungal pathogen.

Bronchial Epithelial Cells on the Front Line to Fight Lung Infection-Causing Aspergillus fumigatus

Aspergillus fumigatus is an environmental filamentous fungus that can be pathogenic for humans, wherein it is responsible for a large variety of clinical forms ranging from allergic diseases to life-threatening disseminated infections. The contamination occurs by inhalation of conidia present in the air, and the first encounter of this fungus in the human host is most likely with the bronchial epithelial cells. Although alveolar macrophages have been widely studied in the Aspergillus–lung interaction, increasing evidence suggests that bronchial epithelium plays a key role in responding to the fungus. This review focuses on the innate immune response of the bronchial epithelial cells against A. fumigatus, the predominant pathogenic species. We have also detailed the molecular interactants and the effects of the different modes of interaction between these cells and the fungus.

PAMPs of the Fungal Cell Wall and Mammalian PRRs

Fungi are opportunistic pathogens that infect immunocompromised patients and are responsible for an estimated 1.5 million deaths every year. The antifungal innate immune response is mediated through the recognition of pathogen-associated molecular patterns (PAMPs) by the host's pattern recognition receptors (PRRs). PRRs are immune receptors that ensure the internalisation and the killing of fungal pathogens. They also mount the inflammatory response, which contributes to initiate and polarise the adaptive response, controlled by lymphocytes. Both the innate and adaptive immune responses are required to control fungal infections. The immune recognition of fungal pathogen primarily occurs at the interface between the membrane of innate immune cells and the fungal cell wall, which contains a number of PAMPs. This chapter will focus on describing the main mammalian PRRs that have been shown to bind to PAMPs from the fungal cell wall of the four main fungal pathogens: Candida albicans, Aspergillus fumigatus, Cryptococcus neoformans and Pneumocystis jirovecii. We will describe these receptors, their functions and ligands to provide the reader with an overview of how the immune system recognises fungal pathogens and responds to them.

Pathobiology of Aspergillus Fumigatus Endophthalmitis in Immunocompetent and Immunocompromised Mice

Despite Aspergillus being the leading cause of exogenous fungal endophthalmitis following traumatic injury to the eye, its pathogenesis is not fully understood. In the current study, we developed a murine model of Aspergillus fumigatus (AF) endophthalmitis and investigated the disease pathobiology. Endophthalmitis was induced by intravitreal injection of Aspergillus spores in immunocompetent and immunocompromised (neutropenic) C57BL/6 mice, and disease severity was assessed by eye exam, fungal burden estimation, and histological examination. Our data showed that AF infection caused a time-dependent increase in corneal haze, opacity, and hypopyon beginning at two days post-infection (DPI). The fungal burden in infected eyes of immunocompetent mice peaked at 2 DPI and declined over 9 DPI. AF-infected neuroretina exhibited induction of innate immune response via upregulation of Toll-like receptors (TLRs) and inflammatory mediators (TNFα, IL-1β, and IL6), and increased polymorphonuclear neutrophil (PMN) infiltration. Histological analysis revealed heavy cellular infiltrates in the vitreous cavity as well as disruption of normal retinal architecture and increased retinal cell death. Neutropenic mice exhibited severe disease pathology with the prolonged fungal burden and increased inflammatory mediators. Our study described the first immunocompetent murine model of exogenous AF endophthalmitis and demonstrated an important role of neutrophils in innate defense against fungal endophthalmitis.

Pathogenic Fungal Infection in the Lung

Respiratory fungal infection is a severe clinical problem, especially in patients with compromised immune functions. Aspergillus, Cryptococcus, Pneumocystis, and endemic fungi are major pulmonary fungal pathogens that are able to result in life-threatening invasive diseases. Growing data being reported have indicated that multiple cells and molecules orchestrate the host's response to a fungal infection in the lung. Upon fungal challenge, innate myeloid cells including macrophages, dendritic cells (DC), and recruited neutrophils establish the first line of defense through the phagocytosis and secretion of cytokines. Natural killer cells control the fungal expansion in the lung via the direct and indirect killing of invading organisms. Adaptive immune cells including Th1 and Th17 cells confer anti-fungal activity by producing their signature cytokines, interferon-γ, and IL-17. In addition, lung epithelial cells (LEC) also participate in the resistance against fungal infection by internalization, inflammatory cytokine production, or antimicrobial peptide secretion. In the host cells mentioned above, various molecules with distinct functions modulate the immune defense signaling: Pattern recognition receptors (PRRs) such as dectin-1 expressed on the cell surface are involved in fungal recognition; adaptor proteins such as MyD88 and TRAF6 are required for transduction of signals to the nucleus for transcriptional regulation; inflammasomes also play crucial roles in the host's defense against a fungal infection in the lung. Furthermore, transcriptional factors modulate the transcriptions of a series of genes, especially those encoding cytokines and chemokines, which are predominant regulators in the infectious microenvironment, mediating the cellular and molecular immune responses against a fungal infection in the lung.

Antifungal Drugs Influence Neutrophil Effector Functions

There is a growing body of evidence for immunomodulatory side effects of antifungal agents on different immune cells, e.g., T cells. Therefore, the aim of our study was to clarify these interactions with regard to the effector functions of polymorphonuclear neutrophils (PMN). Human PMN were preincubated with fluconazole (FLC), voriconazole (VRC), posaconazole (POS), isavuconazole (ISA), caspofungin (CAS), micafungin (MFG), conventional amphotericin B (AMB), and liposomal amphotericin B (LAMB). PMN then were analyzed by flow cytometry for activation, degranulation, and phagocytosis and by dichlorofluorescein assay to detect reactive oxygen species (ROS). Additionally, interleukin-8 (IL-8) release was measured by enzyme-linked immunosorbent assay. POS led to enhanced activation, degranulation, and generation of ROS, whereas IL-8 release was reduced. In contrast, ISA-pretreated PMN showed decreased activation signaling, impaired degranulation, and lower generation of ROS. MFG caused enhanced expression of activation markers but impaired degranulation, phagocytosis, generation of ROS, and IL-8 release. CAS showed increased phagocytosis, whereas degranulation and generation of ROS were reduced. AMB led to activation of almost all effector functions besides impaired phagocytosis, whereas LAMB did not alter any effector functions. Independent from class, antifungal agents show variable influence on neutrophil effector functions in vitro Whether this is clinically relevant needs to be clarified.

Menacing Mold: Recent Advances in Aspergillus Pathogenesis and Host Defense

The genus Aspergillus is ubiquitous in the environment and contains a number of species, primarily A. fumigatus, that cause mold-associated disease in humans. Humans inhale several hundred to several thousand Aspergillus conidia (i.e., vegetative spores) daily and typically clear these in an asymptomatic manner. In immunocompromised individuals, Aspergillus conidia can germinate into tissue-invasive hyphae, disseminate, and cause invasive aspergillosis. In this review, we first discuss novel concepts in host defense against Aspergillus infections and emphasize new insights in fungal recognition and signaling, innate immune activation, and fungal killing. Second, the review focuses on novel concepts of Aspergillus pathogenesis and highlights emerging knowledge regarding fungal strain heterogeneity, stress responses, and metabolic adaptations on infectious outcomes. Mechanistic insight into the host-pathogen interplay is thus critical to define novel druggable fungal targets and to exploit novel immune-based strategies to improve clinical outcomes associated with aspergillosis in vulnerable patient populations.

Fungal ligands released by innate immune effectors promote inflammasome activation during Aspergillus fumigatus infection

Invasive pulmonary aspergillosis causes substantial mortality in immunocompromised individuals. Recognition of Aspergillus fumigatus by the host immune system leads to activation of the inflammasome, which provides protection against infection. However, regulation of inflammasome activation at the molecular level is poorly understood. Here, we describe two distinct pathways that coordinately control inflammasome activation during A. fumigatus infection. The C-type lectin receptor pathway activates both MAPK and NF-κB signalling, which leads to induction of downstream mediators, such as the transcription factor IRF1, and also primes the inflammasomes. Toll-like receptor signalling through the adaptor molecules MyD88 and TRIF in turn mediates efficient activation of IRF1, which induces IRGB10 expression. IRGB10 targets the fungal cell wall, and the antifungal activity of IRGB10 causes hyphae damage, modifies the A. fumigatus surface and inhibits fungal growth. We also demonstrate that one of the major fungal pathogen-associated molecular patterns, β-glucan, directly triggers inflammasome assembly. Thus, the concerted activation of both Toll-like receptors and C-type lectin receptors is required for IRF1-mediated IRGB10 regulation, which is a key event governing ligand release and inflammasome activation upon A. fumigatus infection.

Human Bronchial Epithelial Cells Inhibit Aspergillus fumigatus Germination of Extracellular Conidia via FleA Recognition

Aspergillus fumigatus is an environmental filamentous fungus that may act as an opportunistic pathogen causing a variety of diseases, including asthma or allergic bronchopulmonary aspergillosis, and infection, ranging from asymptomatic colonization to invasive pulmonary form, especially in immunocompromised patients. This fungus is characterized by different morphotypes including conidia which are the infective propagules able to germinate into hyphae. Due to their small size (2–3 µm), conidia released in the air can reach the lower respiratory tract. The objective of this study was to characterize the interactions between conidia and bronchial epithelial cells. To this end, we studied the role of bronchial epithelial cells, i.e., the BEAS-2B cell line and human primary cells, in conidial germination of a laboratory strain and three clinical strains of A. fumigatus. Microscopic observations and galactomannan measurements demonstrated that contact between epithelial cells and conidia leads to the inhibition of conidia germination. We demonstrated that this fungistatic process is not associated with the release of any soluble components nor internalization by the epithelial cells. We highlight that this antifungal process involves the phosphoinositide 3-kinase pathway on the host cellular side and the lectin FleA on the fungal side. Collectively, our results show that bronchial epithelial cells attenuate fungal virulence by inhibiting germination of extracellular conidia, thus preventing the morphological change from conidia to filaments, which is responsible for tissue invasion.

FIBCD1 Binds Aspergillus fumigatus and Regulates Lung Epithelial Response to Cell Wall Components

Aspergillus fumigatus (A. fumigatus) is a ubiquitous fungus of clinical importance associated with development of various pulmonary diseases and allergic hypersensitivity reactions. It is protected against environmental stress by a cell wall that contains polysaccharides such as chitin. We previously demonstrated that fibrinogen C domain-containing protein 1 (FIBCD1) is a membrane-bound protein that binds chitin through a conserved S1 binding site and is expressed in intestinal epithelium and salivary glands. Here, we further localized FIBCD1 protein expression at the surface of bronchial and alveolar human lung epithelium, observed recognition of A. fumigatus cell wall with S1 site-independent recognition. We observed FIBCD1-mediated suppression of IL-8 secretion, mucin production, and transcription of genes associated with airway inflammation and homeostasis in FIBCD1-transfected lung epithelial cells. These modulations were generally enforced by stimulation with A. fumigatus cell wall polysaccharides. In parallel, we demonstrated a FIBCD1-mediated modulation of IL-8 secretion induced by TLR2,−4, and −5. Collectively, our findings support FIBCD1 as a human lung epithelial pattern recognition receptor that recognizes the complex A. fumigatus cell wall polysaccharides and modulates the lung epithelial inflammatory response by suppressing inflammatory mediators and mucins.

Animal Models of Aspergillosis

Aspergillosis is an airborne fungal disease caused by Aspergillus spp., a group of ubiquitous molds. This disease causes high morbidity and mortality in both humans and animals. The growing importance of this infection over recent decades has created a need for practical and reproducible models of aspergillosis. The use of laboratory animals provides a platform to understand fungal virulence and pathophysiology, assess diagnostic tools, and evaluate new antifungal drugs. In this review, we describe the fungus, various Aspergillus-related diseases in humans and animals and various experimental animal models. Overall, we highlight the advantages and limitations of the animal models, the experimental variables that can affect the course of the disease and the reproducibility of infection, and the critical need for standardization of the species, immunosuppressive drugs, route of infection, and diagnostic criteria to use.

Aspergillus fumigatus Cell Wall α-(1,3)-Glucan Stimulates Regulatory T-Cell Polarization by Inducing PD-L1 Expression on Human Dendritic Cells

Background

Human dendritic cell (DC) response to α-(1,3)-glucan polysaccharide of Aspergillus fumigatus and ensuing CD4+ T-cell polarization are poorly characterized.

Methods

α-(1,3)-Glucan was isolated from A. fumigatus conidia and mycelia cell wall. For the analysis of polarization, DCs and autologous naive CD4+ T cells were cocultured. Phenotype of immune cells was analyzed by flow cytometry, and cytokines by enzyme-linked immunosorbent assay (ELISA). Blocking antibodies were used to dissect the role of Toll-like receptor 2 (TLR2) and programmed death-ligand 1 (PD-L1) in regulating α-(1,3)-glucan-mediated DC activation and T-cell responses. DCs from TLR2-deficient mice were additionally used to consolidate the findings.

Results

α-(1,3)-Glucan induced the maturation of DCs and was dependent in part on TLR2. "α-(1,3)-Glucan-educated" DCs stimulated the activation of naive T cells and polarized a subset of these cells into CD4+CD25+FoxP3+ regulatory T cells (Tregs). Mechanistically, Treg stimulation by α-(1,3)-glucan was dependent on the PD-L1 pathway that negatively regulated interferon-gamma (IFN-γ) secretion. Short α-(1,3)-oligosaccharides lacked the capacity to induce maturation of DCs but significantly blocked α-(1,3)-glucan-induced Treg polarization.

Conclusions

PD-L1 dictates the balance between Treg and IFN-γ responses induced by α-(1,3)-glucan. Our data provide a rationale for the exploitation of immunotherapeutic approaches that target PD-1-PD-L1 to enhance protective immune responses to A. fumigatus infections.

Epidemiological features of invasive mold infections among solid organ transplant recipients: PATH Alliance® registry analysis

Epidemiological characteristics of 333 proven and probable invasive mould infections (IMIs) among solid organ transplant recipients (SOTRs) identified between 2004 and 2008 from the Prospective Antifungal Therapy Alliance (PATH) registry are presented. Liver transplant recipients (LTRs) had the lowest median time to IMIs (109 days; interquartile range [IQR] 24-611 days), the highest rate of disseminated disease (n/N = 18/33; 55%), and highest mortality (n/N = 21/33; 64%). Lung transplant recipients had highest median time to IMIs (486 days; IQR 117-1358 days) and lowest mortality (n/N = 31/184; 17%). Complete or partial response at week 12 in patients with invasive aspergillosis (IA) was 67% (n/N = 189/281), and 41% (n/N = 9/22) in mucormycosis patients. In the composite outcome of death or no response to therapy, LTRs had the worst outcome. Higher suspicion of mold infection and institution of appropriate antifungal prophylactic strategies are warranted, especially in high risk LTRs.

Aspergillus fumigatus morphology and dynamic host interactions

Aspergillus fumigatus is an environmental filamentous fungus that can cause life-threatening disease in immunocompromised individuals. The interactions between A. fumigatus and the host environment are dynamic and complex. The host immune system needs to recognize the distinct morphological forms of A. fumigatus to control fungal growth and prevent tissue invasion, whereas the fungus requires nutrients and needs to adapt to the hostile environment by escaping immune recognition and counteracting host responses. Understanding these highly dynamic interactions is necessary to fully understand the pathogenesis of aspergillosis and to facilitate the design of new therapeutics to overcome the morbidity and mortality caused by A. fumigatus. In this Review, we describe how A. fumigatus adapts to environmental change, the mechanisms of host defence, and our current knowledge of the interplay between the host immune response and the fungus.

Toll-like receptor 2 induced cytotoxic T-lymphocyte-associated protein 4 regulates Aspergillus-induced regulatory T-cells with pro-inflammatory characteristics

Patients with cystic fibrosis, chronic obstructive pulmonary disease, severe asthma, pre-existing pulmonary lesions, and severely immunocompromised patients are susceptible to develop infections with the opportunistic pathogenic fungus Aspergillus fumigatus, called aspergillosis. Infections in these patients are associated with persistent pro-inflammatory T-helper (T_H)2 and T_H17 responses. Regulatory T-cells, natural suppressor cells of the immune system, control pro-inflammatory T-cell responses, but can also contribute to disease by shifting to a pro-inflammatory T_H17-like phenotype. Such a shift could play an important role in the detrimental immunopathology that is seen in aspergillosis. Our study demonstrates that Aspergillus fumigatus induces regulatory T-cells with a T_H17-like phenotype. We also demonstrate that these regulatory T-cells with a pro-inflammatory T_H17-like phenotype can be reprogrammed to their “classical” anti-inflammatory phenotype by activating Toll-like receptor 2 (TLR2), which regulates the induction of cytotoxic T-lymphocyte-associated protein 4 (CTLA4). Similarly, soluble CTLA4 could reverse the pro-inflammatory phenotype of Aspergillus-induced regulatory T-cells. In conclusion, our results suggest a role for regulatory T-cells with a pro-inflammatory T_H17-like phenotype in Aspergillus-associated immunopathology, and identifies key players, i.e. TLR2 and CTLA4, involved in this mechanism.

Toll-like receptor 2 and dectin-1 function as promising biomarker for Aspergillus fumigatus infection

In recent years, along with the wide application of organ transplantation and immunosuppressive agents, as well as the abuse of broad spectrum antibiotics, the incidence of invasive fungal infections has been increasing gradually. The present study aimed to identify novel biomarkers in cells infected with Aspergillus fumigatus. Human umbilical vein endothelial cells (HUVECs) were infected with Aspergillus fumigatus and then harvested at different time-points (0, 1, 2, 4 and 6 h). The expression Toll-like receptor 2 (TLR2) and dectin-1 expression were examined using flow cytometry and western blotting, and fluorescence-based microscopy was used to evaluate their distribution. The results indicated that TLR2 and dectin-1 protein levels were localized on the surface of HUVECs, and that dectin-1 was distributed on HUVEC membranes as observed under confocal microscope. Immunofluorescence assay result revealed that the optical intensity of dectin-1 in the Aspergillus fumigatus-infected group was significantly increased at 0, 1 and 2 h compared with the control group (P<0.05). However, the optical intensity of TLR2 in the Aspergillus fumigatus-infected group was markedly decreased between 0 and 6 h, as compared with the control group (P<0.05). Western blot analysis indicated that dectin-1 expression was significantly increased and TLR2 expression was significantly decreased at 0, 1 and 2 h post infection in the Aspergillus fumigatus-infected group compared with the control group. Furthermore, the expression of TLR2 was also negatively correlated with the concentration of Aspergillus fumigatus. In conclusion, upon infection of cells with Aspergillus fumigatus, TLR2 and dectin-1 expression levels were significantly altered. Therefore, TLR2 and dectin-1 levels may function as promising biomarkers for the treatment or diagnosis of Aspergillus fumigatus infection.

Rodent Models of Invasive Aspergillosis due to Aspergillus fumigatus: Still a Long Path toward Standardization

Invasive aspergillosis has been studied in laboratory by the means of plethora of distinct animal models. They were developed to address pathophysiology, therapy, diagnosis, or miscellaneous other concerns associated. However, there are great discrepancies regarding all the experimental variables of animal models, and a thorough focus on them is needed. This systematic review completed a comprehensive bibliographic analysis specifically-based on the technical features of rodent models infected with Aspergillus fumigatus. Out the 800 articles reviewed, it was shown that mice remained the preferred model (85.8% of the referenced reports), above rats (10.8%), and guinea pigs (3.8%). Three quarters of the models involved immunocompromised status, mainly by steroids (44.4%) and/or alkylating drugs (42.9%), but only 27.7% were reported to receive antibiotic prophylaxis to prevent from bacterial infection. Injection of spores (30.0%) and inhalation/deposition into respiratory airways (66.9%) were the most used routes for experimental inoculation. Overall, more than 230 distinct A. fumigatus strains were used in models. Of all the published studies, 18.4% did not mention usage of any diagnostic tool, like histopathology or mycological culture, to control correct implementation of the disease and to measure outcome. In light of these findings, a consensus discussion should be engaged to establish a minimum standardization, although this may not be consistently suitable for addressing all the specific aspects of invasive aspergillosis.

Aspergillosis and stem cell transplantation: An overview of experimental pathogenesis studies

Invasive aspergillosis is a life-threatening infection caused by the opportunistic filamentous fungus Aspergillus fumigatus. Patients undergoing haematopoietic stem cell transplant (HSCT) for the treatment of hematological malignancy are at particularly high risk of developing this fatal infection. The susceptibility of HSCT patients to infection with A. fumigatus is a consequence of a complex interplay of both fungal and host factors. Here we review our understanding of the host-pathogen interactions underlying the susceptibility of the immunocompromised host to infection with A. fumigatus with a focus on the experimental validation of fungal and host factors relevant to HSCT patients. These include fungal factors such as secondary metabolites, cell wall constituents, and metabolic adaptations that facilitate immune evasion and survival within the host microenvironment, as well as the innate and adaptive immune responses involved in host defense against A. fumigatus.

Suppression of epithelial signal transducer and activator of transcription 1 activation by extracts of Aspergillus fumigatus

Aspergillus fumigatus (AF) is often pathogenic in immune-deficient individuals and can cause life-threatening infections such as invasive aspergillosis. The pulmonary epithelial response to AF infection and the signaling pathways associated with it have not been completely studied. BEAS-2B cells or primary human bronchial epithelial cells were exposed to extracts of AF and challenged with IFN-β or the Toll-like receptor 3 agonist double-stranded RNA (dsRNA). Cytokine release (B-cell activating factor of the TNF family [BAFF], IFN-γ-induced protein-10 [IP-10], etc.) was assessed. AF extract was separated into low-molecular-weight (LMW) and high-molecular-weight (HMW) fractions using ultra 4 centrifugal force filters to characterize the activity. Real-time PCR was performed with a TaqMan method, and protein estimation was performed using ELISA techniques. Western blot was performed to assess phosphorylation of signal transducer and activator of transcription 1 (STAT1). IFN-β and dsRNA induced messenger RNA (mRNA) expression of BAFF (350- and 452-fold, respectively [n = 3]) and IP-10 (1,081- and 3,044-fold, respectively [n = 3]) in BEAS-2B cells. When cells were pretreated with AF extract for 1 hour and then stimulated with IFN-β or dsRNA for 6 hours, induction of BAFF and IP-10 mRNA was strongly suppressed relative to levels produced by IFN-β and dsRNA alone. When compared with control, soluble BAFF and IP-10 protein levels were maximally suppressed in dsRNA-stimulated wells treated with 1:320 wt/vol AF extract (P < 0.005). Upon molecular size fractionation, a LMW fraction of AF extract had no measurable suppressive effect on IP-10 mRNA expression. However, a HMW fraction of the AF extract significantly suppressed IP-10 expression in BEAS-2B cells that were stimulated with dsRNA or IFN-β. When BEAS-2B cells were pretreated with AF extract and then stimulated with IFN-β, reduced levels of pSTAT1 were observed, with maximum suppression at 4 and 6 hours. Our results show that AF extracts suppressed expression of inflammatory cytokines in association with inhibition of the IFN-β signaling pathway and suppression of the formation of pSTAT1.

Pattern recognition receptors and their role in invasive aspergillosis

Pattern recognition receptors (PRRs) are germline receptors that recognize conserved structures on microorganisms. Several PRRs have been identified in the recent years that are involved in the immune response against Aspergillus fumigatus. The role of PRRs in invasive pulmonary aspergillosis becomes especially apparent in the setting of an immunocompromised status of the host because of the redundancy of many PRRs in the host defense against A. fumigatus. Studies that investigated the PRRs and their effector pathways in invasive aspergillosis have led to a better understanding of the pathogenesis of invasive aspergillosis in immunocompromised patients. This knowledge may pave the way for novel diagnostic and immunomodulatory treatment strategies that are needed to overcome the high mortality associated with invasive A. fumigatus infection in immunocompromised patients.

Fungal pathogens-a sweet and sour treat for toll-like receptors

Hundred-thousands of fungal species are present in our environment, including normal colonizers that constitute part of the human microbiota. The homeostasis of host-fungus interactions encompasses efficient fungal sensing, tolerance at mucosal surfaces, as well as antifungal defenses. Decrease in host immune fitness or increase in fungal burden may favor pathologies, ranging from superficial mucocutaneous diseases to invasive life-threatening fungal infections. Toll-like receptors (TLRs) are essential players in this balance, due to their ability to control both inflammatory and anti-inflammatory processes upon recognition of fungal-specific pathogen-associated molecular patterns (PAMPs). Certain members of the TLR family participate to the initial recognition of fungal PAMPs on the cell surface, as well as inside phagosomes of innate immune cells. Active signaling cascades in phagocytes ultimately enable fungus clearance and the release of cytokines that shape and instruct other innate immune cells and the adaptive immune system. Some TLRs cooperate with other pattern recognition receptors (PRRs) (e.g., C-type lectins and Galectins), thus allowing for a tailored immune response. The spatio-temporal and physiological contributions of individual TLRs in fungal infections remains ill-defined, although in humans, TLR gene polymorphisms have been linked to increased susceptibility to fungal infections. This review focuses entirely on the role of TLRs that control the host susceptibility to environmental fungi (e.g., Aspergillus, Cryptoccocus, and Coccidoides), as well as to the most frequent human fungal pathogens represented by the commensal Candida species. The emerging roles of TLRs in modulating host tolerance to fungi, and the strategies that evolved in some of these fungi to evade or use TLR recognition to their advantage will also be discussed, as well as their potential suitability as targets in vaccine therapies.

Role and relevance of mast cells in fungal infections

In addition to their detrimental role in allergic diseases, mast cells (MCs) are well known to be important cells of the innate immune system. In the last decade, they have been shown to contribute significantly to optimal host defense against numerous pathogens including parasites, bacteria, and viruses. The contribution of MCs to the immune responses in fungal infections, however, is largely unknown. In this review, we first discuss key features of mast cell responses to pathogens in general and then summarize the current knowledge on the function of MCs in the defense against fungal pathogens. We especially focus on the potential and proven mechanisms by which MCs can detect fungal infections and on possible MC effector mechanisms in protecting from fungal infections.

Innate Immunity and the Role of Epithelial Barrier During Aspergillus fumigatus Infection

Fungi are the most important eukaryotic infective agents in Europe which largely overpass parasite infections. Total number of people dying of fungal infection is increasing and this trend is likely to continue due to the increase in immunosuppressive treatments. The opportunistic pathogen Aspergillus fumigatus (Af) is a saprophytic filamentous fungus that can cause invasive pulmonary diseases in immuno-compromised hosts. In veterinary medicine aspergillosis is also a recurrent problem since it infects various species, birds are particularly susceptible. It propagates through airborne conidia (spores), which are inhaled into the small airways where they may germinate and initiate an infection. The host epithelium has permanent contact with the environment and a multitude of diverse microorganisms, resulting in a network of the host’s defense mechanisms. Pathogens use various strategies to invade epithelial barriers, to exploit eukaryotic host function to their own benefit and disseminate throughout the host using the epithelium as a reservoir. The current revue will discuss the ways how epithelial and innate immunity cells can contlol Af infection. We will focus on Af strategies for the host’s invasion, antifungal innate immune response and antimicrobial activities of the respiratory epithelial cells.

Conidial Dihydroxynaphthalene Melanin of the Human Pathogenic Fungus Aspergillus fumigatus Interferes with the Host Endocytosis Pathway

Aspergillus fumigatus is the most important air-borne fungal pathogen of humans. The interaction of the pathogen with the host's immune system represents a key process to understand pathogenicity. For elimination of invading microorganisms, they need to be efficiently phagocytosed and located in acidified phagolysosomes. However, as shown previously, A. fumigatus is able to manipulate the formation of functional phagolysosomes. Here, we demonstrate that in contrast to pigmentless pksP mutant conidia of A. fumigatus, the gray-green wild-type conidia inhibit the acidification of phagolysosomes of alveolar macrophages, monocyte-derived macrophages, and human neutrophil granulocytes. Therefore, this inhibition is independent of the cell type and applies to the major immune effector cells required for defense against A. fumigatus. Studies with melanin ghosts indicate that the inhibitory effect of wild-type conidia is due to their dihydroxynaphthalene (DHN)-melanin covering the conidia, whereas the hydrophobin RodA rodlet layer plays no role in this process. This is also supported by the observation that pksP conidia still exhibit the RodA hydrophobin layer, as shown by scanning electron microscopy. Mutants defective in different steps of the DHN-melanin biosynthesis showed stronger inhibition than pksP mutant conidia but lower inhibition than wild-type conidia. Moreover, A. fumigatus and A. flavus led to a stronger inhibition of phagolysosomal acidification than A. nidulans and A. terreus. These data indicate that a certain type of DHN-melanin that is different in the various Aspergillus species, is required for maximal inhibition of phagolysosomal acidification. Finally, we identified the vacuolar ATPase (vATPase) as potential target for A. fumigatus based on the finding that addition of bafilomycin which inhibits vATPase, led to complete inhibition of the acidification whereas the fusion of phagosomes containing wild-type conidia and lysosomes was not affected.

Cracking the Toll-like receptor code in fungal infections

Innate control of fungal infection requires the specific recognition of invariant fungal molecular structures by a variety of innate immune receptors, including Toll-like receptors. In addition to the role in inducing protective immune responses, Toll-like receptor engagement may paradoxically favor fungal infections, by inducing inflammatory pathology and impairing antifungal immunity. Although the dissection of complex genetic traits modulating susceptibility to fungal infections is complex, the contribution of host genetics may hold the key to elucidating new risk factors for these severe, often fatal diseases. Understanding host-pathogen interactions at the innate immune interface will eventually lead to the development of new therapeutics and genetic markers in fungal infections.

TLR4 recognizes Pseudallescheria boydii conidia and purified rhamnomannans

Pseudallescheria boydii (Scedosporium apiospermum) is a saprophytic fungus widespread in the environment, and has recently emerged as an agent of localized as well as disseminated infections, particularly mycetoma, in immunocompromised and immunocompetent hosts. We have previously shown that highly purified α-glucan from P. boydii activates macrophages through Toll-like receptor TLR2, however, the mechanism of P. boydii recognition by macrophage is largely unknown. In this work, we investigated the role of innate immune receptors in the recognition of P. boydii. Macrophages responded to P. boydii conidia and hyphae with secretion of proinflammatory cytokines. The activation of macrophages by P. boydii conidia required functional MyD88, TLR4, and CD14, whereas stimulation by hyphae was independent of TLR4 and TLR2 signaling. Removal of peptidorhamnomannans from P. boydii conidia abolished induction of cytokines by macrophages. A fraction highly enriched in rhamnomannans was obtained and characterized by NMR, high performance TLC, and GC-MS. Preparation of rhamnomannans derived from P. boydii triggered cytokine release by macrophages, as well as MAPKs phosphorylation and IκBα degradation. Cytokine release induced by P. boydii-derived rhamnomannans was dependent on TLR4 recognition and required the presence of non-reducing end units of rhamnose of the rhamnomannan, but not O-linked oligosaccharides from the peptidorhamnomannan. These results imply that TLR4 recognizes P. boydii conidia and this recognition is at least in part due to rhamnomannans expressed on the surface of P. boydii.

Influenza A virus protein PB1-F2 exacerbates IFN-beta expression of human respiratory epithelial cells

The PB1-F2 protein of the influenza A virus (IAV) contributes to viral pathogenesis by a mechanism that is not well understood. PB1-F2 was shown to modulate apoptosis and to be targeted by the CD8(+) T cell response. In this study, we examined the downstream effects of PB1-F2 protein during IAV infection by measuring expression of the cellular genes in response to infection with wild-type WSN/33 and PB1-F2 knockout viruses in human lung epithelial cells. Wild-type virus infection resulted in a significant induction of genes involved in innate immunity. Knocking out the PB1-F2 gene strongly decreased the magnitude of expression of cellular genes implicated in antiviral response and MHC class I Ag presentation, suggesting that PB1-F2 exacerbates innate immune response. Biological network analysis revealed the IFN pathway as a link between PB1-F2 and deregulated genes. Using quantitative RT-PCR and IFN-β gene reporter assay, we determined that PB1-F2 mediates an upregulation of IFN-β expression that is dependent on NF-κB but not on AP-1 and IFN regulatory factor-3 transcription factors. Recombinant viruses knocked out for the PB1-F2 and/or the nonstructural viral protein 1 (the viral antagonist of the IFN response) genes provide further evidence that PB1-F2 increases IFN-β expression and that nonstructural viral protein 1 strongly antagonizes the effect of PB1-F2 on the innate response. Finally, we compared the effect of PB1-F2 variants taken from several IAV strains on IFN-β expression and found that PB1-F2-mediated IFN-β induction is significantly influenced by its amino acid sequence, demonstrating its importance in the host cell response triggered by IAV infection.

Immunogenetics of invasive aspergillosis

Invasive aspergillosis is one of the most important infections in hematopoietic stem cell transplant recipients, with an incidence rate of 5-15% and an associated mortality of 30-60%. It remains unclear why certain patients develop invasive aspergillosis while others, undergoing identical transplant regimen and similar post transplant immunosuppression, do not. Over the last decade, pattern recognition receptors such as Toll-like receptors (TLRs) and the C-type lectin receptors (CLRs) have emerged as critical components of the innate immune system. By detecting specific molecular patterns from invading microbes and initiating inflammatory and subsequent adaptive immune responses, pattern recognition receptors are strategically located at the molecular interface of hosts and pathogens. Polymorphisms in pattern recognition receptors and downstream signaling molecules have been associated with increased or decreased susceptibility to infections, suggesting that their detection may have an increasing impact on the treatment and prevention of infectious diseases in the coming years. Infectious risk stratification may be particularly relevant for patients with hematologic malignancies, because of the high prevalence and severity of infections in this population. This review summarizes the innate immune mechanisms involved in Aspergillus fumigatus detection and the role of host genetic polymorphisms in susceptibility to invasive aspergillosis.

Distinct roles for Dectin-1 and TLR4 in the pathogenesis of Aspergillus fumigatus keratitis

Aspergillus species are a major worldwide cause of corneal ulcers, resulting in visual impairment and blindness in immunocompetent individuals. To enhance our understanding of the pathogenesis of Aspergillus keratitis, we developed a murine model in which red fluorescent protein (RFP)-expressing A. fumigatus (Af293.1RFP) conidia are injected into the corneal stroma, and disease progression and fungal survival are tracked over time. Using Mafia mice in which c-fms expressing macrophages and dendritic cells can be induced to undergo apoptosis, we demonstrated that the presence of resident corneal macrophages is essential for production of IL-1beta and CXCL1/KC, and for recruitment of neutrophils and mononuclear cells into the corneal stroma. We found that beta-glucan was highly expressed on germinating conidia and hyphae in the cornea stroma, and that both Dectin-1 and phospho-Syk were up-regulated in infected corneas. Additionally, we show that infected Dectin-1(-/-) corneas have impaired IL-1beta and CXCL1/KC production, resulting in diminished cellular infiltration and fungal clearance compared with control mice, especially during infection with clinical isolates expressing high beta-glucan. In contrast to Dectin 1(-/-) mice, cellular infiltration into infected TLR2(-/-), TLR4(-/-), and MD-2(-/-) mice corneas was unimpaired, indicating no role for these receptors in cell recruitment; however, fungal killing was significantly reduced in TLR4(-/-) mice, but not TLR2(-/-) or MD-2(-/-) mice. We also found that TRIF(-/-) and TIRAP(-/-) mice exhibited no fungal-killing defects, but that MyD88(-/-) and IL-1R1(-/-) mice were unable to regulate fungal growth. In conclusion, these data are consistent with a model in which beta-glucan on A.fumigatus germinating conidia activates Dectin-1 on corneal macrophages to produce IL-1beta, and CXCL1, which together with IL-1R1/MyD88-dependent activation, results in recruitment of neutrophils to the corneal stroma and TLR4-dependent fungal killing.

Innate immunity to Aspergillus species

All humans are continuously exposed to inhaled Aspergillus conidia, yet healthy hosts clear the organism without developing disease and without the development of antibody- or cell-mediated acquired immunity to this organism. This suggests that for most healthy humans, innate immunity is sufficient to clear the organism. A failure of these defenses results in a uniquely diverse set of illnesses caused by Aspergillus species, which includes diseases caused by the colonization of the respiratory tract, invasive infection, and hypersensitivity. A key concept in immune responses to Aspergillus species is that the susceptibilities of the host determine the morphological form, antigenic structure, and physical location of the fungus. In this review, we summarize the current literature on the multiple layers of innate defenses against Aspergillus species that dictate the outcome of this host-microbe interaction.

In vivo biofilm composition of Aspergillus fumigatus

The in vivo composition of the mycelial extracellular matrix (ECM) of Aspergillus fumigatus during host invasion is reported here for the first time. A new galactosaminogalactan and the galactomannan were the major polysaccharides of the in vivo ECM. The composition of the ECM in vivo varied with the aspergillosis pathologies.

Impaired TLR-2 expression and TLR-2-mediated cytokine secretion in macrophages from patients with atopic dermatitis

BACKGROUND

In many patients with atopic dermatitis (AD), the disease is complicated by their enhanced susceptibility to bacterial skin infections, especially with Staphylococcus aureus. The pattern recognition receptor toll-like receptor (TLR)-2 recognizes components of S. aureus, for example, lipoteichoic acid (LTA) and peptidoglycan (PGN) and, therefore, might be crucial in the pathogenesis and flare-ups of AD.

OBJECTIVE

To investigate TLR-2 expression and cytokine secretion in macrophages from patients with AD compared to healthy controls upon TLR-2 stimulation with PGN, LTA and Pam3Cys.

METHODS

Macrophages were cultivated from highly purified peripheral blood monocytes of AD patients and nonatopic healthy controls and stimulated with PGN, LTA and Pam3Cys in a time and dose-dependent manner. Afterwards, TLR-2 expression and cytokine secretion were measured on protein and mRNA level. TLR-1 and TLR-6 expression were investigated on the mRNA level. Immunohistochemical stainings from punch biopsies were performed to investigate TLR-2 expression in skin macrophages.

RESULTS

We could clearly show that macrophages from patients with AD expressed significantly less TLR-2, whereas the expression pattern of TLR-1 and TLR-6 were not altered. Macrophages had a reduced capacity to produce pro-inflammatory cytokines such as IL-6, IL-8 and IL-1beta after stimulation with TLR-2 ligands.

CONCLUSION

Our findings clearly show an impaired TLR-2 expression and functional differences of TLR-2-mediated effects on macrophages of AD patients compared to healthy controls which might contribute to the enhanced susceptibility to skin infections with S. aureus in AD.

Pseudomonas aeruginosa LPS or flagellin are sufficient to activate TLR-dependent signaling in murine alveolar macrophages and airway epithelial cells

BACKGROUND

The human lung is exposed to a large number of airborne pathogens as a result of the daily inhalation of 10,000 liters of air. Innate immunity is thus essential to defend the lungs against these pathogens. This defense is mediated in part through the recognition of specific microbial ligands by Toll-like receptors (TLR) of which there are at least 10 in humans. Pseudomonas aeruginosa is the main pathogen that infects the lungs of cystic fibrosis patients. Based on whole animal experiments, using TLR knockout mice, the control of this bacterium is believed to occur by the recognition of LPS and flagellin by TLRs 2,4 and 5, respectively.

METHODOLOGY/PRINCIPAL FINDINGS

In the present study, we investigated in vitro the role of these same TLR and ligands, in alveolar macrophage (AM) and epithelial cell (EC) activation. Cellular responses to P. aeruginosa was evaluated by measuring KC, TNF-alpha, IL-6 and G-CSF secretion, four different markers of the innate immune response. AM and EC from WT and TLR2, 4, 5 and MyD88 knockout mice for were stimulated with the wild-type P. aeruginosa or with a mutant devoid of flagellin production.

CONCLUSIONS/SIGNIFICANCE

The results clearly demonstrate that only two ligand/receptor pairs are necessary for the induction of KC, TNF-alpha, and IL-6 synthesis by P. aeruginosa-activated cells, i.e. TLR2,4/LPS and TLR5/flagellin. Either ligand/receptor pair is sufficient to sense the bacterium and to trigger cell activation, and when both are missing lung EC and AM are unable to produce such a response as were cells from MyD88(-/-) mice.

Pathogenesis of Aspergillus fumigatus in Invasive Aspergillosis

Aspergillus species are globally ubiquitous saprophytes found in a variety of ecological niches. Almost 200 species of aspergilli have been identified, less than 20 of which are known to cause human disease. Among them, Aspergillus fumigatus is the most prevalent and is largely responsible for the increased incidence of invasive aspergillosis (IA) in the immunocompromised patient population. IA is a devastating illness, with mortality rates in some patient groups reaching as high as 90%. Studies identifying and assessing the roles of specific factors of A. fumigatus that contribute to the pathogenesis of IA have traditionally focused on single-gene deletion and mutant characterization. In combination with recent large-scale approaches analyzing global fungal responses to distinct environmental or host conditions, these studies have identified many factors that contribute to the overall pathogenic potential of A. fumigatus. Here, we provide an overview of the significant findings regarding A. fumigatus pathogenesis as it pertains to invasive disease.

Innate immunity and inflammation--two facets of the same anti-infectious reaction

Innate immunity is the host's first line of defence against infection. In this review, we present the innate immune response implicated in three examples of pulmonary infection of viral, fungal and bacterial origin. We show that this defence against infection can be a double-edged sword. Thus, the same cells, molecules and mechanisms involved in this protective process can also be involved in deleterious inflammation. A delicate balance between immunity and inflammation is therefore required, making it possible to fight pathogens effectively while limiting inflammation that might be damaging to the host.

Modulation of Toll-like receptor 2 (TLR2) and TLR4 responses by Aspergillus fumigatus

Toll-like receptor (TLR)-based signaling pathways in the host may be modulated by pathogens during the course of infection. We describe a novel immunomodulatory mechanism in which Aspergillus fumigatus conidia induce attenuation of TLR2- and TLR4-mediated interleukin (IL)-6 and IL-1beta proinflammatory responses in human mononuclear cells with suppression of IL-1beta mRNA transcription. Background TLR2 and TLR4 mRNA transcription was not influenced. A. fumigatus conidia induced TLR2 internalization and uptake into the phagosome with a resultant decrease in surface receptor expression. A. fumigatus hyphae, on the other hand, selectively downregulated the TLR4-mediated response. These novel immunosuppressive effects may facilitate the invasiveness of A. fumigatus.

Anthrax lethal toxin impairs IL-8 expression in epithelial cells through inhibition of histone H3 modification

Lethal toxin (LT) is a critical virulence factor of Bacillus anthracis, the etiological agent of anthrax, whose pulmonary form is fatal in the absence of treatment. Inflammatory response is a key process of host defense against invading pathogens. We report here that intranasal instillation of a B. anthracis strain bearing inactive LT stimulates cytokine production and polymorphonuclear (PMN) neutrophils recruitment in lungs. These responses are repressed by a prior instillation of an LT preparation. In contrast, instillation of a B. anthracis strain expressing active LT represses lung inflammation. The inhibitory effects of LT on cytokine production are also observed in vitro using mouse and human pulmonary epithelial cells. These effects are associated with an alteration of ERK and p38-MAPK phosphorylation, but not JNK phosphorylation. We demonstrate that although NF-κB is essential for IL-8 expression, LT downregulates this expression without interfering with NF-κB activation in epithelial cells. Histone modifications are known to induce chromatin remodelling, thereby enhancing NF-κB binding on promoters of a subset of genes involved in immune response. We show that LT selectively prevents histone H3 phosphorylation at Ser 10 and recruitment of the p65 subunit of NF-κB at the IL-8 and KC promoters. Our results suggest that B. anthracis represses the immune response, in part by altering chromatin accessibility of IL-8 promoter to NF-κB in epithelial cells. This epigenetic reprogramming, in addition to previously reported effects of LT, may represent an efficient strategy used by B. anthracis for invading the host.

Bacillus anthracis, the etiological agent of anthrax, can infect mammals either accidentally or as a potential consequence of a terrorism threat. Pulmonary infection is a life-threatening form of the disease, causing a near 100% mortality rate in the absence of appropriate therapy. Thus, it is important to understand the mechanisms of host defense against B. anthracis. We examined the effects of various B. anthracis strains on lung inflammation in a mouse model of pulmonary anthrax and on human lung epithelial cells, the first barrier of lung against invading pathogens. We showed that a B. anthracis strain expressing lethal toxin inhibits inflammation. In contrast, a strain in which this toxin has been inactivated induces lung inflammation. We next examined the mechanisms involved in the inhibitory effect of lethal toxin. We showed that B. anthracis injects lethal toxin into epithelial cells, blocks the molecules associated on the chromosome, and thus represses production of mediators involved in inflammation. As the latter is a key process in host defense, its alteration by lethal toxin predisposes the host to infection by B. anthracis. This effect on the chromosomal machinery may represent an efficient strategy used by B. anthracis for invading the host.

Herpes simplex virus type 1 dysregulates anti-fungal defenses preventing monocyte activation and downregulating toll-like receptor-2

We investigated the interplay occurring between pathogens in the course of dual infections, using an in vitro model in which the THP-1 monocytic cell line is first infected with HSV-1 and then exposed to Ca or Cn. These three pathogens share some pathogenic features: they cause opportunistic infections, target macrophages and are neurotropic. Here, we show that HSV-1-infected THP-1 cells exhibited augmented phagocytosis against the two opportunistic fungi but reduced capability to counteract fungal infection: the better ingestion by monocytes was followed by facilitated fungal survival and replication. Reduced IL-12 production was also observed. Cytofluorimetric analysis showed that HSV-1-infected monocytes exhibit: (i) downregulated TLR-2 and TLR-4, critical structures in fungal recognition; (ii) reduced expression of CD38 and CD69, known to be important markers of monocyte activation; and (iii) enhanced expression of apoptosis and necrosis markers, in the absence of altered cell proliferation. Overall, these findings imply that HSV-1 infection prevents monocyte activation, thus leading to a significant dysfunction of the monocyte-mediated anti-Candida response; HSV-1 induced apoptosis and necrosis of monocytes further contribute to this impairment.

Importance of TLR2 in early innate immune response to acute pulmonary infection with Porphyromonas gingivalis in mice

The periodontal pathogen Porphyromonas gingivalis is implicated in certain systemic diseases including atherosclerosis and aspiration pneumonia. This organism induces innate responses predominantly through TLR2, which also mediates its ability to induce experimental periodontitis and accelerate atherosclerosis. Using a validated mouse model of intratracheal challenge, we investigated the role of TLR2 in the control of P. gingivalis acute pulmonary infection. TLR2-deficient mice elicited reduced proinflammatory or antimicrobial responses (KC, MIP-1alpha, TNF-alpha, IL-6, IL-12p70, and NO) in the lung and exhibited impaired clearance of P. gingivalis compared with normal controls. However, the influx of polymorphonuclear leukocytes into the lung and the numbers of resident alveolar macrophages (AM) were comparable between the two groups. TLR2 signaling was important for in vitro killing of P. gingivalis by polymorphonuclear leukocytes or AM and, moreover, the AM bactericidal activity required NO production. Strikingly, AM were more potent than peritoneal or splenic macrophages in P. gingivalis killing, attributed to diminished AM expression of complement receptor-3 (CR3), which is exploited by P. gingivalis to promote its survival. The selective expression of CR3 by tissue macrophages and the requirement of TLR2 inside-out signaling for CR3 exploitation by P. gingivalis suggest that the role of TLR2 in host protection may be contextual. Thus, although TLR2 may mediate destructive effects, as seen in models of experimental periodontitis and atherosclerosis, we have now shown that the same receptor confers protection against P. gingivalis in acute lung infection.