Aspergillus fumigatus cell wall components differentially modulate host TLR2 and TLR4 responses

Susceptibility from the immunological perspective of COVID-19-associated pulmonary aspergillosis: A literature review

The incidence rate of COVID-19-associated pulmonary aspergillosis (CAPA) is rising. However, the pathogenesis of CAPA remains unclear. Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infection disrupts pathways related to type I interferon and Toll-like receptors, key components in innate immunity, thereby elevating the incidence of CAPA. Additionally, SARS-CoV-2 infection results in T and B cell functional deficiencies or exhaustion within adaptive immunity, weakening the defense against invasive Aspergillus. Furthermore, SARS-CoV-2 infection enhances the replication of cytomegalovirus and alters the gut microbiota, factors that may aid in diagnosing CAPA. Immunosuppressive therapy in COVID-19 patients is also believed to heighten the risk of invasive aspergillosis. Therefore, this review, examines the immune response to SARS-CoV-2 infection combined with invasive aspergillosis, and explores the pathogenesis and susceptibility factors of CAPA. We propose that variations in an individual's immune response significantly determine susceptibility to CAPA. The aim of this paper is to deepen clinical understanding of CAPA's pathogenesis, thereby aiding in mitigating susceptibility risk and advancing novel treatment approaches.

Precision Detection of Fungal Co-Infections for Enhanced COVID-19 Treatment Strategies Using FESEM Imaging

The treatment of fungal infections presents significant challenges due to the lack of standardized diagnostic procedures, a restricted range of antifungal treatments, and the risk of harmful interactions between antifungal medications and the immunosuppressive drugs used in anti-inflammatory treatment for critically ill patients with COVID-19. Mucormycosis and aspergillosis are the primary invasive fungal infections in patients with severe COVID-19, occurring singly or in combination. Histopathological examination is a vital diagnostic technique that details the presence and invasion of fungi within tissues and blood vessels, and the body's response to the infection. However, the pathology report omits information on the most common fungi associated with the observed morphology, as well as other potential fungi and parasites that ought to be included in the differential diagnosis. This research marks significance in diagnosing fungal infections, such as mucormycosis and aspergillosis associated to COVID-19 by field emission scanning electron microscopy (FESEM) imaging to examine unstained histopathology slides, allowing for a detailed morphological analysis of the fungus. FESEM provides an unprecedented resolution and detail, surpassing traditional Hematoxylin & Eosin (H&E) and Grocott's Methenamine Silver (GMS) staining methods in identifying and differentiating dual fungal infections and diverse fungal species. The findings underscore the critical need for individualized treatment plans for patients severely affected by COVID-19 and compounded by secondary fungal infections. The high-magnification micrographs reveal specific fungal morphology and reproductive patterns. Current treatment protocols largely depend on broad-spectrum antifungal therapies. However this FESEM guided diagnostic approach can help in targeted patient specific anti fungal therapies. Such precision could lead to more effective early interventions, addressing the complex management required for severe COVID-19 cases with coexisting fungal infections. This approach significantly advances disease management and patient recovery, advocating for personalized, precision medicine in tackling this multifaceted clinical challenge.

Graphical Abstract

Interplay of Cytokines and Chemokines in Aspergillosis

Aspergillosis is a fungal infection caused by various species of Aspergillus, most notably A. fumigatus. This fungus causes a spectrum of diseases, including allergic bronchopulmonary aspergillosis, aspergilloma, chronic pulmonary aspergillosis, and invasive aspergillosis. The clinical manifestations and severity of aspergillosis can vary depending on individual immune status and the specific species of Aspergillus involved. The recognition of Aspergillus involves pathogen-associated molecular patterns (PAMPs) such as glucan, galactomannan, mannose, and conidial surface proteins. These are recognized by the pathogen recognition receptors present on immune cells such as Toll-like receptors (TLR-1,2,3,4, etc.) and C-type lectins (Dectin-1 and Dectin-2). We discuss the roles of cytokines and pathogen recognition in aspergillosis from both the perspective of human and experimental infection. Several cytokines and chemokines have been implicated in the immune response to Aspergillus infection, including interferon-γ (IFN-γ), tumor necrosis factor-α (TNF-α), CCR4, CCR17, and other interleukins. For example, allergic bronchopulmonary aspergillosis (ABPA) is characterized by Th2 and Th9 cell-type immunity and involves interleukin (IL)-4, IL-5, IL-13, and IL-10. In contrast, it has been observed that invasive aspergillosis involves Th1 and Th17 cell-type immunity via IFN-γ, IL-1, IL-6, and IL-17. These cytokines activate various immune cells and stimulate the production of other immune molecules, such as antimicrobial peptides and reactive oxygen species, which aid in the clearance of the fungal pathogen. Moreover, they help to initiate and coordinate the immune response, recruit immune cells to the site of infection, and promote clearance of the fungus. Insight into the host response from both human and animal studies may aid in understanding the immune response in aspergillosis, possibly leading to harnessing the power of cytokines or cytokine (receptor) antagonists and transforming them into precise immunotherapeutic strategies. This could advance personalized medicine.

Chick heterophils release DNA extracellular traps (DETs) in vitro and in vivo upon Aspergillus fumigatus conidia exposure

Aspergillosis is a common fungal disease in avian species, causing high mortality in young chicks in agricultural farms and yards. It is caused by fungi belonging to the genus Aspergillus. Aspergillosis occurs by inhalation of fungal conidia, and in chickens, effective infection control relies on a rapid and large influx of heterophils to the lungs. Heterophils, upon different stimuli, release to the extracellular milieu their chromatin associated with several proteins that ensnare and kill different pathogens similarly to neutrophil extracellular traps. Here, we showed that Aspergillus fumigatus conidia and the peptidogalactomannan (PGM), isolated from the fungus cell wall, induce the release of DNA extracellular traps (DETs) in chicks' blood and lung heterophils. We demonstrated that reactive oxygen species, elastase and peptidyl arginine deiminase (PAD) were involved in DETs extrusion, the occurrence of DETs in the lungs of A. fumigatus-exposed chicks in vivo, and its role in chick survival. These results may contribute to developing more efficient tools for the therapeutic and diagnosis of aspergillosis.

Commander-in-chief: monocytes rally the troops for defense against aspergillosis

The detrimental impact of fungal infections to human health has steadily increased over the past decades. In October of 2022, the World Health Organization published the first ever fungal-pathogen priority list highlighting increased awareness of this problem, and the need for more research in this area. There were four distinct fungal pathogens identified as critical priority groups with Aspergillus fumigatus (Af) being the only mold. Af is a common environmental fungus responsible for over 90% of invasive aspergillosis cases worldwide. Pulmonary protection against Af is critically dependent on innate effector cells with essential roles played by neutrophils and monocytes. In this review, we will summarize our current understanding of how monocytes help orchestrate antifungal defense against Af.

The C-Type Lectin Receptor Dectin-2 Is a Receptor for Aspergillus fumigatus Galactomannan

Aspergillus fumigatus is a ubiquitous environmental mold that causes significant mortality particularly among immunocompromised patients. The detection of the Aspergillus-derived carbohydrate galactomannan in patient serum and bronchoalveolar lavage fluid is the major biomarker used to detect A. fumigatus infection in clinical medicine. Despite the clinical relevance of this carbohydrate, we lack a fundamental understanding of how galactomannan is recognized by the immune system and its consequences. Galactomannan is composed of a linear mannan backbone with galactofuranose sidechains and is found both attached to the cell surface of Aspergillus and as a soluble carbohydrate in the extracellular milieu. In this study, we utilized fungal-like particles composed of highly purified Aspergillus galactomannan to identify a C-type lectin host receptor for this fungal carbohydrate. We identified a novel and specific interaction between Aspergillus galactomannan and the C-type lectin receptor Dectin-2. We demonstrate that galactomannan bound to Dectin-2 and induced Dectin-2-dependent signaling, including activation of spleen tyrosine kinase, gene transcription, and tumor necrosis factor alpha (TNF-α) production. Deficiency of Dectin-2 increased immune cell recruitment to the lungs but was dispensable for survival in a mouse model of pulmonary aspergillosis. Our results identify a novel interaction between galactomannan and Dectin-2 and demonstrate that Dectin-2 is a receptor for galactomannan, which leads to a proinflammatory immune response in the lung. IMPORTANCE Aspergillus fumigatus is a fungal pathogen that causes serious and often fatal disease in humans. The surface of Aspergillus is composed of complex sugar molecules. Recognition of these carbohydrates by immune cells by carbohydrate lectin receptors can lead to clearance of the infection or, in some cases, benefit the fungus by dampening the host response. Galactomannan is a carbohydrate that is part of the cell surface of Aspergillus but is also released during infection and is found in patient lungs as well as their bloodstreams. The significance of our research is that we have identified Dectin-2 as a mammalian immune cell receptor that recognizes, binds, and signals in response to galactomannan. These results enhance our understanding of how this carbohydrate interacts with the immune system at the site of infection and will lead to broader understanding of how release of galactomannan by Aspergillus effects the immune response in infected patients.

The Lipid Raft-Associated Protein Stomatin Is Required for Accumulation of Dectin-1 in the Phagosomal Membrane and for Full Activity of Macrophages against Aspergillus fumigatus

Alveolar macrophages belong to the first line of defense against inhaled conidia of the human-pathogenic fungus Aspergillus fumigatus. In lung alveoli, they contribute to phagocytosis and elimination of conidia. As a counterdefense, conidia have a gray-green pigment that enables them to survive in phagosomes of macrophages for some time. Previously, we showed that this conidial pigment interferes with the formation of flotillin-dependent lipid raft microdomains in the phagosomal membrane, thereby preventing the formation of functional phagolysosomes. Besides flotillins, stomatin is a major component of lipid rafts and can be targeted to the membrane. However, only limited information on stomatin is available, in particular on its role in defense against pathogens. To determine the function of this integral membrane protein, a stomatin-deficient macrophage line was generated by CRISPR/Cas9 gene editing. Immunofluorescence microscopy and flow cytometry revealed that stomatin contributes to the phagocytosis of conidia and is important for recruitment of the β-glucan receptor dectin-1 to both the cytoplasmic membrane and phagosomal membrane. In stomatin knockout cells, fusion of phagosomes and lysosomes, recruitment of the vATPase to phagosomes, and tumor necrosis factor alpha (TNF-α) levels were reduced when cells were infected with pigmentless conidia. Thus, our data suggest that stomatin is involved in maturation of phagosomes via fostering fusion of phagosomes with lysosomes. IMPORTANCE Stomatin is an integral membrane protein that contributes to the uptake of microbes, e.g., spores of the human-pathogenic fungus Aspergillus fumigatus. By generation of a stomatin-deficient macrophage line by advanced genetic engineering, we found that stomatin is involved in the recruitment of the β-glucan receptor dectin-1 to the phagosomal membrane of macrophages. Furthermore, stomatin is involved in maturation of phagosomes via fostering fusion of phagosomes with lysosomes. The data provide new insights on the important role of stomatin in the immune response against human-pathogenic fungi.

Thymol Ameliorates Aspergillus fumigatus Keratitis by Downregulating the TLR4/ MyD88/ NF-kB/ IL-1β Signal Expression and Reducing Necroptosis and Pyroptosis

Fungal keratitis is a refractory kind of keratopathy. We attempted to investigate the anti-inflammatory role of thymol on Aspergillus fumigatus (A. fumigatus) keratitis. Wound healing and fluorescein staining of the cornea were applied to verify thymol's safety. Mice models of A. fumigatus keratitis underwent subconjunctival injection of thymol. The anti-inflammatory roles of thymol were verified by hematoxylin-eosin (HE) staining, slit lamp observation, quantitative real-time polymerase chain reaction (qRT-PCR), and Western blotting. In contrast with the DMSO group, more transparent corneas and less inflammatory cells infiltration were detected in mice treated with 50 μg/ml thymol. Thymol downregulated the synthesis of TLR4, MyD88, NF-kB, IL-1β, NLRP3, caspase 1, caspase 8, GSDMD, RIPK3 and MLKL. In summary, we proved that thymol played a protective part in A. fumigatus keratitis by cutting down inflammatory cells aggregation, downregulating the TLR4/ MyD88/ NF-kB/ IL-1β signal expression and reducing necroptosis and pyroptosis.

Chitin Biosynthesis in Aspergillus Species

The fungal cell wall (FCW) is a dynamic structure responsible for the maintenance of cellular homeostasis, and is essential for modulating the interaction of the fungus with its environment. It is composed of proteins, lipids, pigments and polysaccharides, including chitin. Chitin synthesis is catalyzed by chitin synthases (CS), and up to eight CS-encoding genes can be found in Aspergillus species. This review discusses in detail the chitin synthesis and regulation in Aspergillus species, and how manipulation of chitin synthesis pathways can modulate fungal growth, enzyme production, virulence and susceptibility to antifungal agents. More specifically, the metabolic steps involved in chitin biosynthesis are described with an emphasis on how the initiation of chitin biosynthesis remains unknown. A description of the classification, localization and transport of CS was also made. Chitin biosynthesis is shown to underlie a complex regulatory network, with extensive cross-talks existing between the different signaling pathways. Furthermore, pathways and recently identified regulators of chitin biosynthesis during the caspofungin paradoxical effect (CPE) are described. The effect of a chitin on the mammalian immune system is also discussed. Lastly, interference with chitin biosynthesis may also be beneficial for biotechnological applications. Even after more than 30 years of research, chitin biosynthesis remains a topic of current interest in mycology.

COVID-19 associated mold infections: Review of COVID-19 associated pulmonary aspergillosis and mucormycosis

COVID-19-associated mold infection (CAMI) is defined as development of mold infections in COVID-19 patients. Co-pathogenesis of viral and fungal infections include the disruption of tissue barrier following SARS CoV-2 infection with the damage in the alveolar space, respiratory epithelium and endothelium injury and overwhelming inflammation and immune dysregulation during severe COVID-19. Other predisposing risk factors permissive to fungal infections during COVID-19 include the administration of immune modulators such as corticosteroids and IL-6 antagonist. COVID-19-associated pulmonary aspergillosis (CAPA) and COVID-19-associated mucormycosis (CAM) is increasingly reported during the COVID-19 pandemic. CAPA usually developed within the first month of COVID infection, and CAM frequently arose 10-15 days post diagnosis of COVID-19. Diagnosis is challenging and often indistinguishable during the cytokine storm in COVID-19, and several diagnostic criteria have been proposed. Development of CAPA and CAM is associated with a high mortality despiteappropriate anti-mold therapy. Both isavuconazole and amphotericin B can be used for treatment of CAPA and CAM; voriconazole is the primary agent for CAPA and posaconazole is an alternative for CAM. Aggressive surgery is recommended for CAM to improve patient survival. A high index of suspicion and timely and appropriate treatment is crucial to improve patient outcome.

Dietary yeast culture alleviates intestinal-hepatic damage related to TLR2-MyD88-NF-κB signaling pathway and antioxidant capability in Pseudobagrus ussuriensis

Yeast culture (YC), as a member of probiotics family is a natural product produced from yeast fermentation, affects of improving immunity. However, the intestine and liver injury and immunosuppression mechanism caused by SBA in fish are unclear and more functions of YC supplement in the diet need to be developed. Soybean agglutinin (SBA) is an anti-nutritional factor in soybean and leads to growth-inhibitory effect in feeding of the high proportion of soybean meal replacing fish meal (FM). Therefore, one hundred and thirty-five Pseudobagrus ussuriensis (6.5 ± 0.27 g) were randomly selected and divided into three groups (Control, SBA and YC+SBA groups). For the model, fish were fed with 2% YC for 8 weeks and then given intragastric administration of 0.2-mL SBA solution for 20 days. The results showed that SBA damaged the immune and antioxidant capacity, causing an inflammatory reaction, leading to abnormal expression of cytokines in the intestine and liver of Pseudobagrus ussuriensis. YC could effectively attenuate intestinal and liver damage and downregulate the TLR2/MyD88/NF-κB signaling pathway and suppress oxidative stress in Pseudobagrus ussuriensis. Besides, YC had obvious immune advantage, which could improve the immune ability. In summary, these results showed that YC could reduce immunosuppression and intestinal-liver injury by inhibiting the TLR2/MyD88/NF-κB signal pathway and oxidative stress induced by SBA. This study provided some explanations for the problems of fish diet caused by anti-nutritional factors from soybean meal and provided a theoretical basis for the function development of YC in aquaculture.

Comparative Host-Pathogen Interaction Analyses of SARS-CoV2 and Aspergillus fumigatus, and Pathogenesis of COVID-19-Associated Aspergillosis

COVID-19 caused a global catastrophe with a large number of cases making it one of the major pandemics of the human history. The clinical presentations of the disease are continuously challenging healthcare workers with the variation of pandemic waves and viral variants. Recently, SARS-CoV2 patients have shown increased occurrence of invasive pulmonary aspergillosis infection even in the absence of traditional risk factors. The mechanism of COVID-19-associated aspergillosis is not completely understood and therefore, we performed this system biological study in order to identify mechanistic implications of aspergillosis susceptibility in COVID-19 patients and the important targets associated with this disease. We performed host-pathogen interaction (HPI) analysis of SARS-CoV2, and most common COVID-19-associated aspergillosis pathogen, Aspergillus fumigatus, using in silico approaches. The known host-pathogen interactions data of SARS-CoV2 was obtained from BIOGRID database. In addition, A. fumigatus host-pathogen interactions were predicted through homology modeling. The human targets interacting with both pathogens were separately analyzed for their involvement in aspergillosis. The aspergillosis human targets were screened from DisGeNet and GeneCards. The aspergillosis targets involved in both HPI were further analyzed for functional overrepresentation analysis using PANTHER. The results indicate that both pathogens interact with a number of aspergillosis targets and altogether they recruit more aspergillosis targets in host-pathogen interaction than alone. Common aspergillosis targets involved in HPI with both SARS-CoV2 and A. fumigatus can indicate strategies for the management of both conditions by modulating these common disease targets.

LYSMD3: A mammalian pattern recognition receptor for chitin

Chitin, a major component of fungal cell walls, has been associated with allergic disorders such as asthma. However, it is unclear how mammals recognize chitin and the principal receptor(s) on epithelial cells that sense chitin remain to be determined. In this study, we show that LYSMD3 is expressed on the surface of human airway epithelial cells and demonstrate that LYSMD3 is able to bind chitin, as well as β-glucan, on the cell walls of fungi. Knockdown or knockout of LYSMD3 also sharply blunts the production of inflammatory cytokines by epithelial cells in response to chitin and fungal spores. Competitive inhibition of the LYSMD3 ectodomain by soluble LYSMD3 protein, multiple ligands, or antibody against LYSMD3 also blocks chitin signaling. Our study reveals LYSMD3 as a mammalian pattern recognition receptor (PRR) for chitin and establishes its role in epithelial cell inflammatory responses to chitin and fungi.

MiR-146a Negatively Regulates Aspergillus fumigatus-Induced TNF-α and IL-6 Secretion in THP-1 Macrophages

Aspergillus fumigatu (A. fumigatus) is one of the most common important fungal pathogens that cause life-threatening infectious disease in immunocompromised individuals. However, the host immune response against this pathogenic mold is not fully understood. MicroRNAs (miRNAs) play essential roles in regulating innate immunity. Thus, we investigated the function of miR-146a in inflammatory responses in macrophages after A. fumigatus stimulation in this study. We found that TNF-α and IL-6 were increased in THP-1 macrophage-like cells treated with A. fumigatus at both the mRNA and protein levels. The interaction between THP-1 macrophage-like cells and A. fumigatus resulted in a long-lasting increase in miR-146a expression dependent on p38 MAPK and NF-κB signaling. In A. fumigatus-challenged THP-1 macrophage-like cells, overexpression of miR-146a by miR-146a mimics decreased TNF-α and IL-6 production, whereas downregulation of miR-146a by anti-miR-146a significantly enhanced the level of TNF-α and IL-6. Our study demonstrates that the crosstalk between miR-146a and the inflammation-regulating p38 MAPK and NF-κB pathways might be a fine-tuning mechanism in the modulation of the inflammatory response in macrophages infected with A. fumigatus. Our findings illuminate the crucial role of miR-146a in the pathogenesis of human diseases associated with A. fumigatus infection.

Fungal immunity and pathogenesis in mammals versus the invertebrate model organism Galleria mellonella

In recent decades, Galleria mellonella (Lepidoptera: Pyralidae) have emerged as a model system to explore experimental aspects of fungal pathogenesis. The benefits of the G. mellonella model include being faster, cheaper, higher throughput and easier compared with vertebrate models. Additionally, as invertebrates, their use is subject to fewer ethical and regulatory issues. However, for G. mellonella models to provide meaningful insight into fungal pathogenesis, the G. mellonella-fungal interactions must be comparable to mammalian-fungal interactions. Indeed, as discussed in the review, studies suggest that G. mellonella and mammalian immune systems share many similarities, and fungal virulence factors show conserved functions in both hosts. While the moth model has opened novel research areas, many comparisons are superficial and leave large gaps of knowledge that need to be addressed concerning specific mechanisms underlying G. mellonella-fungal interactions. Closing these gaps in understanding will strengthen G. mellonella as a model for fungal virulence in the upcoming years. In this review, we provide comprehensive comparisons between fungal pathogenesis in mammals and G. mellonella from immunological and virulence perspectives. When information on an antifungal immune component is unknown in G. mellonella, we include findings from other well-studied Lepidoptera. We hope that by outlining this information available in related species, we highlight areas of needed research and provide a framework for understanding G. mellonella immunity and fungal interactions.

Recognition of Fungal Components by the Host Immune System

By being the first point of contact of the fungus with the host, the cell wall plays an important role in the pathogenesis, having many molecules that participate as antigens that are recognized by immune cells, and also that help the fungus to establish infection. The main molecules reported to trigger an immune response are chitin, glucans, oligosaccharides, proteins, melanin, phospholipids, and others, being present in the principal pathogenic fungi with clinical importance worldwide, such as Histoplasma capsulatum, Paracoccidioides brasiliensis, Aspergillus fumigatus, Candida albicans, Cryptococcus neoformans, Blastomyces dermatitidis, and Sporothrix schenckii. Knowledge and understanding of how the immune system recognizes and responds to fungal antigens are relevant for the future research and development of new diagnostic tools and treatments for the control of mycosis caused by these fungi.

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.

Using Expanded Natural Killer Cells as Therapy for Invasive Aspergillosis

Invasive aspergillosis (IA) is a major opportunistic fungal infection in patients with haematological malignancies. Morbidity and mortality rates are high despite anti-fungal treatment, as the compromised status of immune system prevents the host from responding optimally to conventional therapy. This raises the consideration for immunotherapy as an adjunctive treatment. In this study, we evaluated the utility of expanded human NK cells as treatment against Aspergillus fumigatus infection in vitro and in vivo. The NK cells were expanded and activated by K562 cells genetically modified to express 4-1BB ligand and membrane-bound interleukin-15 (K562-41BBL-mbIL-15) as feeders. The efficacy of these cells was investigated in A. fumigatus killing assays in vitro and as adoptive cellular therapy in vivo. The expanded NK cells possessed potent killing activity at low effector-to-target ratio of 2:1. Fungicidal activity was morphotypal-dependent and most efficacious against A. fumigatus conidia. Fungicidal activity was mediated by dectin-1 receptors on the expanded NK cells leading to augmented release of perforin, resulting in enhanced direct cytolysis. In an immunocompromised mice pulmonary aspergillosis model, we showed that NK cell treatment significantly reduced fungal burden, hence demonstrating the translational potential of expanded NK cells as adjunctive therapy against IA in immunocompromised patients.

Nitric Oxide Regulates Macrophage Fungicidal Activity via S-nitrosylation of Dectin-1

Introduction: Recognition of fungal surface β-glucan by pattern recognition receptor Dectin-1 is a critical process for fungal clearance in the lung. In humans, persistent fungal infection is observed in individuals with particular Dectin-1 polymorphism. We have identified that nitric oxide (NO) modifies critical cysteines in pattern recognition molecules to disassemble and alter protein function. There is a hydrophobic S-nitrosylation motif present in surfactant protein-D (SP-D) that is also present in Dectin-1. We hypothesized that Dectin-1 can be modified by nitrosative stress potentially leading to impairment of fungal clearance. Materials and Methods: Recombinant Dectin-1 was incubated with l-nitrosocysteine (L-SNOC) and S-nitrosylated Dectin-1 was detected by Biotin-switch assay. Cells of a murine macrophage line (Raw 264.7) were incubated with S-nitroso-glutathione (GSNO) and Dectin-1 shedding from the cell surface was determined by Western blot. Dectin-1 quaternary structure was determined by native gel electrophoresis. Dectin-1 function was assayed by NF-κB activity and IL-6 mRNA real-time polymerase chain reaction (PCR). Phagocytic activity was measured by fluorescence labeled zymosan beads. Results: Dectin-1 was S-nitrosylated by l-nitrosocysteine (L-SNOC) in vitro, as determined by Biotin-switch assay, resulting in structural disruption. We used Western blotting and flow cytometry to demonstrate that incubation of a murine macrophage cell line (Raw 264.7 cells) with GSNO reduced the surface Dectin-1 expression as a result of shedding to the media. The shedding of Dectin-1 is due to formation of S-nitrosothiol (SNO)-Dectin-1 and disruption of the Dectin-1 oligomeric complex. GSNO also induces Dectin-1 shedding from the cell surface. The functional significance of GSNO treatment of macrophages is shown by reduced β-glucan-mediated signaling in terms of NF-κB function and IL-6 expression. Finally, it was demonstrated that GSNO treatment reduces the capability of macrophages to phagocytose zymosan. Conclusions: These data provide mechanistic data to support the role of Dectin-1 nitrosylation as a mediator of reduced fungal clearance in the face of increased NO exposure.

Microglial Response to Aspergillus flavus and Candida albicans: Implications in Endophthalmitis

Aspergillus flavus is the most common etiology of fungal endophthalmitis in India, while Candida albicans is the causative agent in the West. In this study, we determined the role of microglial cells in evoking an inflammatory response following an infection with A. flavus and C. albicans strains isolated from patients with endophthalmitis. Microglia (CHME-3) cells were infected with A. flavus and C. albicans and the expression of Toll-Like Receptors (TLRs), cytokines and Matrix metalloproteinases (MMPs) were assessed at various time intervals. A. flavus infected cells induced higher expressions of TLR-1, -2, -5, -6, -7 and -9 and cytokines such as IL-1α, IL-6, IL-8, IL-10 and IL-17. In contrast, C. albicans infected microglia induced only TLR-2 along with the downregulation of IL-10 and IL-17. The expression of MMP-9 (Matrix metalloproteinase-9) was however upregulated in both A. flavus and C. albicans infected microglia. These results indicate that microglial cells have the ability to incite an innate response towards endophthalmitis causing fungal pathogens via TLRs and inflammatory mediators. Moreover, our study highlights the differential responses of microglia towards yeast vs. filamentous fungi.

How Fungal Glycans Modulate Platelet Activation via Toll-Like Receptors Contributing to the Escape of Candida albicans from the Immune Response

Platelets are essential for vascular repair and for the maintenance of blood homeostasis. They contribute to the immune defence of the host against many infections caused by bacteria, viruses and fungi. Following infection, platelet function is modified, and these cells form aggregates with microorganisms leading, to a decrease in the level of circulating platelets. During candidaemia, mannans, β-glucans and chitin, exposed on the cell wall of Candida albicans, an opportunistic pathogenic yeast of humans, play an important role in modulation of the host response. These fungal polysaccharides are released into the circulation during infection and their detection allows the early diagnosis of invasive fungal infections. However, their role in the modulation of the immune response and, in particular, that of platelets, is not well understood. The structure and solubility of glycans play an important role in the orientation of the immune response of the host. This short review focuses on the effect of fungal β-glucans and chitin on platelet activation and how these glycans modulate platelet activity via Toll-like receptors, contributing to the escape of C. albicans from the immune response.

Pathogenicity and Immune Responses of Aspergillus fumigatus Infection in Chickens

Aspergillus fumigatus is a ubiquitous pathogen in poultry farms, causing aspergillosis in chickens. To study the pathogenicity of A. fumigatus, 14-days-old chickens were infected with fungal conidia (2 × 10⁷ CFU/mL) via thoracic intra-air sacs inoculation. The clinical symptoms, gross and histopathological lesions, and fungal load in the lungs were examined. Additionally, the mRNAs of Toll like receptors (TLR) and pro-inflammatory cytokines were evaluated by quantitative PCR to explore the immune responses induced by A. fumigatus. The results showed that overt depression, ruffled feathers, and dyspnea were observed in the infected chickens as early as 3 days post infection (dpi). Eleven out of 25 infected chickens died from 5 to 9 dpi, and A. fumigatus could also be reisolated from the infected lung. Histopathological examination revealed obvious airsacculitis and pneumonia, characterized by inflammatory cell infiltration (heterophils and macrophages), and granulomatous lesions in the lung. The mRNA expressions of TLR1 and TLR2 were upregulated in the lung and spleen, and most pro-inflammatory cytokines including IL-1β, Cxcl-8, TNF-α, IL-12, and IFN-γ were increased in both the lung and spleen during the tested period, suggesting that the innate immune responses were triggered by A. fumigatus infection, and these cytokines participated in the inflammatory responses against A. fumigatus. These results indicate that A. fumigatus infection by thoracic intra-air sacs inoculation can cause severe respiratory damage in chickens, activate TLR1 and TLR2 mediated immune responses, and elicit large expression of pro-inflammatory cytokines such as IL-1β, Cxcl-8, and IFN-γ. These data will help further understanding of the pathogenesis and immune responses of A. fumigatus infection in the chicken.

The complement system in Aspergillus fumigatus infections and its crosstalk with pentraxins

Aspergillosis is a life-threatening infection mostly affecting immunocompromised individuals and primarily caused by the saprophytic fungus Aspergillus fumigatus. At the host-pathogen interface, both cellular and humoral components of the innate immune system are increasingly acknowledged as essential players in the recognition and disposal of this opportunistic mold. Fundamental hereof is the contribution of the complement system, which deploys all three activation pathways in the battle against A. fumigatus, and functionally cooperates with other soluble pattern recognition molecules, including pentraxins. In particular, preclinical and clinical observations point to the long pentraxin PTX3 as a nonredundant and complement-dependent effector with protective functions against A. fumigatus. Based on past and current literature, here we discuss how the complement participates in the immune response to this fungal pathogen, and illustrate its crosstalk with the pentraxins, with a focus on PTX3. Emphasis is placed on the molecular mechanisms underlying such processes, the genetic evidence from human epidemiology, and the translational potential of the currently available knowledge.

Galactomannan as a Potential Modulator of Intestinal Ischemia-Reperfusion Injury

BACKGROUND

Galactomannan (GAL), a polysaccharide present on the cell wall of several fungi, has shown an ability to modulate inflammatory responses through the dectin-1 receptor in human macrophages. However, studies evaluating the modulatory properties of this polysaccharide in in vivo inflammatory scenarios are scarce. We hypothesized that GAL pretreatment would modulate local and remote damage related to intestinal reperfusion after an ischemic insult.

MATERIALS AND METHODS

Adult male Balb/c mice were subjected to intestinal ischemia-reperfusion injury by reversible occlusion of the superior mesenteric artery, consisting of 45 min of ischemia followed by 3 or 24 h of reperfusion. Intragastric GAL (70 mg/kg) was administered 12 h before ischemia, and saline solution was used in the control animals. Jejunum, lung, and blood samples were taken for the analysis of histology, gene expression, plasma cytokine levels, and nitrosative stress.

RESULTS

Intestinal and lung histologic alterations were attenuated by GAL pretreatment, showing significant differences compared with nontreated animals. Interleukin 1β, monocyte chemoattractant protein 1, and IL-6 messenger RNA expression were considerably downregulated in the small intestine of the GAL group. In addition, GAL treatment significantly prevented plasma interleukin 6 and monocyte chemoattractant protein 1 upregulation and diminished nitrate and nitrite levels after 3 h of intestinal reperfusion.

CONCLUSIONS

GAL pretreatment constitutes a novel and promising therapy to reduce local and remote damage triggered by intestinal ischemia-reperfusion injury. Further in vivo and in vitro studies to understand GAL's modulatory effects are warranted.

Trichophyton rubrum Elicits Phagocytic and Pro-inflammatory Responses in Human Monocytes Through Toll-Like Receptor 2

Dermatophytosis is a superficial fungal infection mostly restricted to keratinized tissues such as skin, hair, and nails but with potential to cause invasive or even systemic disease in immunocompromised patients. Trichophyton rubrum is the main etiologic agent, accounting for approximately 80% of the cases. Mononuclear phagocytes respond to pathogens through phagocytosis followed by production of several antimicrobial molecules, such as reactive oxygen and nitrogen species, and failure in doing so may contribute to development of chronic fungal infections. Toll-like receptors (TLRs) located on the surface of phagocytic cells bind either directly to target particles or through opsonizing ligands and trigger an actin-mediated ingestion. Even though the mechanisms involved in TLR-mediated cytokine responses are well established, the contribution of TLR in the recognition of T. rubrum by adherent monocytes remains unclear. Here, we report that phagocytosis of T. rubrum conidia by adherent monocytes is mediated by TLR2. Blockade of TLR2 by neutralizing antibodies impaired the fungicidal activity of monocytes as well their secretion of tumor necrosis factor (TNF)-α, but neither nitric oxide (NO) production nor interleukin (IL)-10 secretion was disturbed. So far, our data suggest that TLR2 is required for efficient conidial phagocytosis, and the absence of TLR2 signaling in human monocytes may impair the subsequent inflammatory response. These findings expand our understanding of phagocyte modulation by this important fungal pathogen and may represent a potential target for interventions aiming at enhancing antifungal immune responses.

Bromodomain inhibitor I-BET151 suppresses immune responses during fungal-immune interaction

Changes in the epigenetic landscape of immune cells are a crucial component of gene activation during the induction of inflammatory responses, therefore it has been hypothesized that epigenetic modulation could be employed to restore homeostasis in inflammatory scenarios. Fungal pathogens cause a large burden of morbidity and even mortality due to the hyperinflammatory processes that induce mucosal, allergic or systemic infections. Bromodomain and extraterminal domain (BET) proteins are considered as one as the most tantalizing pharmacological targets for the modulation of inflammatory responses at the epigenetic level. Nothing is known of the role of BET inhibitors on the inflammation induced by fungal pathogens. In the present study, we assessed the in vitro efficacy of the small molecular histone mimic BET inhibitor I-BET151 to modulate innate immune responses during fungal-immune interaction with the clinically relevant fungal pathogens Candida albicans and Aspergillus fumigatus. Our results prove that BET inhibitors (I-BETs) represent an important modulator of inflammation induced by fungal pathogens: both direct production of proinflammatory cytokines and the induction of trained immunity were inhibited by I-BET151. These modulatory effects are likely to have important potential implications in clinically relevant situations.

Nutritional Heterogeneity Among Aspergillus fumigatus Strains Has Consequences for Virulence in a Strain- and Host-Dependent Manner

Acquisition and subsequent metabolism of different carbon and nitrogen sources have been shown to play an important role in virulence attributes of the fungal pathogen Aspergillus fumigatus, such as the secretion of host tissue-damaging proteases and fungal cell wall integrity. We examined the relationship between the metabolic processes of carbon catabolite repression (CCR), nitrogen catabolite repression (NCR) and virulence in a variety of A. fumigatus clinical isolates. A considerable amount of heterogeneity with respect to the degree of CCR and NCR was observed and a positive correlation between NCR and virulence in a neutropenic mouse model of pulmonary aspergillosis (PA) was found. Isolate Afs35 was selected for further analysis and compared to the reference strain A1163, with both strains presenting the same degree of virulence in a neutropenic mouse model of PA. Afs35 metabolome analysis in physiological-relevant carbon sources indicated an accumulation of intracellular sugars that also serve as cell wall polysaccharide precursors. Genome analysis showed an accumulation of missense substitutions in the regulator of protease secretion and in genes encoding enzymes required for cell wall sugar metabolism. Based on these results, the virulence of strains Afs35 and A1163 was assessed in a triamcinolone murine model of PA and found to be significantly different, confirming the known importance of using different mouse models to assess strain-specific pathogenicity. These results highlight the importance of nitrogen metabolism for virulence and provide a detailed example of the heterogeneity that exists between A. fumigatus isolates with consequences for virulence in a strain-specific and host-dependent manner.

Two KTR Mannosyltransferases Are Responsible for the Biosynthesis of Cell Wall Mannans and Control Polarized Growth in Aspergillus fumigatus

The fungal cell wall is a complex and dynamic entity essential for the development of fungi. It allows fungal pathogens to survive environmental challenge posed by nutrient stress and host defenses, and it also is central to polarized growth. The cell wall is mainly composed of polysaccharides organized in a three-dimensional network. Aspergillus fumigatus produces a cell wall galactomannan whose biosynthetic pathway and biological functions remain poorly defined. Here, we described two new mannosyltransferases essential to the synthesis of the cell wall galactomannan. Their absence leads to a growth defect with misregulation of polarization and altered conidiation, with conidia which are bigger and more permeable than the conidia of the parental strain. This study showed that in spite of its low concentration in the cell wall, this polysaccharide is absolutely required for cell wall stability, for apical growth, and for the full virulence of A. fumigatus.

Fungal cell wall mannans are complex carbohydrate polysaccharides with different structures in yeasts and molds. In contrast to yeasts, their biosynthetic pathway has been poorly investigated in filamentous fungi. In Aspergillus fumigatus, the major mannan structure is a galactomannan that is cross-linked to the β-1,3-glucan-chitin cell wall core. This polymer is composed of a linear mannan with a repeating unit composed of four α1,6-linked and α1,2-linked mannoses with side chains of galactofuran. Despite its use as a biomarker to diagnose invasive aspergillosis, its biosynthesis and biological function were unknown. Here, we have investigated the function of three members of the Ktr (also named Kre2/Mnt1) family (Ktr1, Ktr4, and Ktr7) in A. fumigatus and show that two of them are required for the biosynthesis of galactomannan. In particular, we describe a newly discovered form of α-1,2-mannosyltransferase activity encoded by the KTR4 gene. Biochemical analyses showed that deletion of the KTR4 gene or the KTR7 gene leads to the absence of cell wall galactomannan. In comparison to parental strains, the Δktr4 and Δktr7 mutants showed a severe growth phenotype with defects in polarized growth and in conidiation, marked alteration of the conidial viability, and reduced virulence in a mouse model of invasive aspergillosis. In yeast, the KTR proteins are involved in protein 0- and N-glycosylation. This study provided another confirmation that orthologous genes can code for proteins that have very different biological functions in yeasts and filamentous fungi. Moreover, in A. fumigatus, cell wall mannans are as important structurally as β-glucans and chitin.

Chitin: A "Hidden Figure" in the Fungal Cell Wall

Chitin and chitosan are two related polysaccharides that provide important structural stability to fungal cell walls. Often embedded deeply within the cell wall structure, these molecules anchor other components at the cell surface. Chitin-directed organization of the cell wall layers allows the fungal cell to effectively monitor and interact with the external environment. For fungal pathogens, this interaction includes maintaining cellular strategies to avoid excessive detection by the host innate immune system. In turn, mammalian and plant hosts have developed their own strategies to process fungal chitin, resulting in chitin fragments of varying molecular size. The size-dependent differences in the immune activation behaviors of variably sized chitin molecules help to explain how chitin and related chitooligomers can both inhibit and activate host immunity. Moreover, chitin and chitosan have recently been exploited for many biomedical applications, including targeted drug delivery and vaccine development.

A 3-O-methylated heterogalactan from Pleurotus eryngii activates macrophages

Mushroom-derived polysaccharides exhibit various biological activities owing to their diverse structural features. Here, we purified a 3-O-methylated heterogalactan (WPEP-N-b, Mw 21.4 kDa) from the fruiting bodies of Pleurotus eryngii. WPEP-N-b is composed primarily of galactose (43.8%), mannose (39.3%), methyl-galactose (11.7%) and glucose (9.2%) residues, with the main chain being composed of α-1,6-linked D-Galp and 3-O-Me-D-Galp, branched at O-2 with single t-β-D-Manp as major the side chain. β-1,6-D-Glcp residues are present as minor components either in side-chains or backbone. WPEP-N-b increases macrophage phagocytosis and secretion of NO, TNF-α, IL-6 and IL-1β. Mechanistic studies demonstrate that WPEP-N-b promotes the degradation of IκB-α, and enhances phosphorylation of MAPKs and the NF-κB p65 subunit. Our results also indicate that this polysaccharide activates RAW264.7 cells via MAPK and NF-κB signaling pathways and the Toll-like receptor 2(TLR2). These results increase our understanding as to how mushroom-derived polysaccharides modulate the immunologic process.

Effects of dietary yeast culture on growth performance, immune response and disease resistance of gibel carp (Carassius auratus gibelio CAS Ⅲ)

A 50-day feeding trial was carried out to evaluate the partial replacement of fishmeal by yeast culture (YC) on growth performance, immune response and resistance against Aeromonas hydrophila in gibel carp CAS Ⅲ (Carassius auratus gibelio). Four isonitrogenous and isoenergetic practical diets including a basal diet (the control diet containing 10% fish meal, D0) and three yeast culture diets (substituting 20%, 40%, 60% of the fishmeal in the basal diet, D20, D40 and D60, respectively) were formulated. Each diet was randomly allocated to quadruplicate fish groups (average initial body weight: 28.70 ± 0.03 g) reared in a recirculating system. After the growth trial, bacterial challenge test was conducted. The results showed that no noteworthy variations in feed intake, growth performance and morphology indices were found among groups (P > 0.05). YC Supplemented diet exerted little significant influence on plasma parameters including triglyceride, glucose, creatinine, total protein and urea nitrogen compared with the control group (P > 0.05). No obvious variations were found in activities of plasma lysozyme, IgM, MPO and SOD before challenge test among dietary treatments (P > 0.05), whereas considerable higher value of the foresaid indicators was discovered in D40 after bacteria challenge (P < 0.05). Transcriptional levels of Toll like receptor 2 (TLR2), myeloid differentiation factor 88 (MyD88), Toll/IL-1 receptor domain-containing adaptor protein (TIRAP) and interleukin-1β (IL-1β) in spleen after challenge were significantly up-regulated in D40 compared with D0 (P < 0.05). Cumulative survival rate in D40 and D60 were significantly higher than those in D0 and D20 (P < 0.05). Taken together, yeast culture could be a suitable fishmeal alternative in diets of gibel carp and dietary inclusion of 4 g YC per 100 g diet enhanced the immunity and disease resistance of gibel carp partly via TLR2 pathway.

β-1,3-glucan-lacking Aspergillus fumigatus mediates an efficient antifungal immune response by activating complement and dendritic cells

Complement system and dendritic cells (DCs) form - beside neutrophils and macrophages - the first line of defense to combat fungal infections. Therefore, we here studied interactions of these first immune elements with Aspergillus fumigatus lacking ß-1,3-glucans (fks1_tetOn^rep under repressed conditions) to mechanistically explain the mode of action of echinocandins in more detail. Echinocandins are cell wall active agents blocking β-glucan synthase, making the A. fumigatus fks1_tetOn mutant a good model to study immune-modulatory actions of these drugs. We now demonstrate herein, that complement was activated to significantly higher levels by the fks1-deficient strain compared to its respective wild type. This enhanced covalent linking of complement fragments to the A. fumigatus fks1_tetOn^rep mutant further resulted in enhanced DC binding and internalization of the fungus. Additionally, we found that fks1_tetOn^rep induced a Th1-/Th17-polarizing cytokine profile program in DCs. The effect was essentially dependent on massive galactomannan shedding, since blocking of DC-SIGN significantly reduced the fks1_tetOn^rep-mediated induction of an inflammatory cytokine profile.Our data demonstrate that lack of ß-1,3-glucan, also found under echinocandin therapy, results in improved recognition of Aspergillus fumigatus by complement and DCs and therefore not only directly affects the fungus by its fungistatic actions, but also is likely to exert indirect antifungal mechanisms by strengthening innate host immune mechanisms.Abbreviations: C: complement; CR:complement receptor; DC: dendritic cell; iDC: immature dendritic cell; DC-SIGN: Dendritic Cell-Specific Intercellular adhesion molecule-3-Grabbing Non-integrin; ERK: extracellular signal-regulated kinases; JNK : c-Jun N-terminal kinases; MAPK: mitogen-activated protein kinase; NHS: normal human serum; PRR: pattern recognition receptor; Th :T helper; TLR :Toll-like receptor; WT: wild type.

A synthetic glycosaminoglycan reduces sinonasal inflammation in a murine model of chronic rhinosinusitis

Chronic rhinosinusitis (CRS) is characterized by sustained mucosal inflammation, impaired mucociliary clearance, loss of cilia and epithelial barrier breakdown, and tissue remodeling. Certain glycosaminoglycans inhibit various inflammatory mediators, suppress bacterial growth, and provide important functions in mucosal tissue repair and mucociliary clearance. Herein, we evaluated the effects of a synthetic glycosaminoglycan, GM-1111, on the clinical signs and inflammatory tissue changes associated with CRS in mice. CRS was generated by repeated intranasal applications of Aspergillus fumigatus (A. fumigatus) extracts over 4 weeks. Mice were then intranasally administered GM-1111 (600 μg per dose, 5 times a week) or vehicle (phosphate buffered saline, PBS) for an additional 4 weeks while still being given A. fumigatus extracts to maintain a chronic inflammatory environment with acute exacerbations. Clinical signs indicative of sinonasal inflammation were recorded throughout the study. After 9 weeks, whole blood and sinonasal tissues were harvested for hematological, histological, and biochemical examination. The clinical signs, white blood cell counts, tissue markers of sinonasal inflammation, and histological changes caused by A. fumigatus extract administration were compared to the healthy (PBS vehicle) and GM-1111-treated groups (n = 12 per treatment group). Compared to vehicle-treated animals, animals treated with GM-1111 demonstrated significant reductions in clinical signs (p<0.05), degenerative tissue changes, goblet cell hyperplasia, inflammatory cell infiltration (p<0.01), innate immunity- (tlr2, tlr4, myd88, il1b, tnfa, il6, and il12) and adaptive immunity-associated (ccl11, ccl24, ccl5, il4, il5, and il13) cytokine gene expression (p<0.05 to p<0.0001) in sinonasal tissues, and serum IgE levels (p<0.01). Our data suggest that GM-1111 significantly reduces local and systemic effects of CRS-associated sinonasal inflammation.

Microbiological and immunological characteristics of a lethal pulmonary Aspergillus niger infection in a non-neutropenic patient

Invasive pulmonary aspergillosis is increasingly described in non-neutropenic patients, such as patients with COPD receiving corticosteroids and the critically ill. Here, we present a case of a lethal pulmonary Aspergillus niger infection in a COPD patient. Immunological tests showed an impaired innate and adaptive immune response to Aspergillus. A history of COPD, unresponsiveness to antibiotics and especially a suggestive CT-scan should trigger the clinician to consider diseases caused by Aspergillus.

Oxidative stress and cellular pathways of asthma and inflammation: Therapeutic strategies and pharmacological targets

Asthma is a complex inflammatory disease characterized by airway inflammation and hyperresponsiveness. The mechanisms associated with the development and progression of asthma have been widely studied in multiple populations and animal models, and these have revealed involvement of various cell types and activation of intracellular signaling pathways that result in activation of inflammatory genes. Significant contributions of Toll-like-receptors (TLRs) and transcription factors such as NF-кB, have been reported as major contributors to inflammatory pathways. These have also recently been associated with mechanisms of oxidative biology. This is of important clinical significance as the observed inefficacy of current available treatments for severe asthma is widely attributed to oxidative stress. Therefore, targeting oxidizing molecules in conjunction with inflammatory mediators and transcription factors may present a novel therapeutic strategy for asthma. In this review, we summarize TLRs and NF-кB pathways in the context of exacerbation of asthma pathogenesis and oxidative biology, and we discuss the potential use of polyphenolic flavonoid compounds, known to target these pathways and possess antioxidant activity, as potential therapeutic agents for asthma.

Levels of interleukin (IL)-6 and IL-8 are elevated in serum and bronchoalveolar lavage fluid of haematological patients with invasive pulmonary aspergillosis

Aspergillus spp. have been shown to induce T-helper cell (Th) 1 and Th17 subsets resulting in elevated levels of several cytokines. The objective of this study was to analyse a bundle of cytokines in serum and bronchoalveolar lavage fluid (BALF) in patients with and without invasive pulmonary aspergillosis (IPA). This nested case-control analysis included 10 patients with probable/proven IPA and 20 matched controls without evidence of IPA, out of a pool of prospectively enrolled (2014-2017) adult cases with underlying haematological malignancies and suspected pulmonary infection. Serum samples were collected within 24 hours of BALF sampling. All samples were stored at -70°C for retrospective determination of cytokines. IL-6 and IL-8 were significantly associated with IPA in both serum (P = .011 and P = .028) and BALF (P = .006 and P = .012, respectively), and a trend was observed for serum IL-10 (P = .059). In multivariate conditional logistic regression analysis, IL-10 remained a significant predictor of IPA in serum and IL-8 among BALF cytokines. In conclusion, levels of IL-6 and IL-8 were significantly associated with probable/proven IPA, and a similar trend was observed for serum IL-10. Future cohort studies should determine the diagnostic potential of these cytokines for IPA, and evaluate combinations with other IPA biomarkers/diagnostic tests.

Immune Recognition of Fungal Polysaccharides

The incidence of fungal infections has dramatically increased in recent years, in large part due to increased use of immunosuppressive medications, as well as aggressive medical and surgical interventions that compromise natural skin and mucosal barriers. There are relatively few currently licensed antifungal drugs, and rising resistance to these agents has led to interest in the development of novel preventative and therapeutic strategies targeting these devastating infections. One approach to combat fungal infections is to augment the host immune response towards these organisms. The polysaccharide-rich cell wall is the initial point of contact between fungi and the host immune system, and therefore, represents an important target for immunotherapeutic approaches. This review highlights the advances made in our understanding of the mechanisms by which the immune system recognizes and interacts with exopolysaccharides produced by four of the most common fungal pathogens: Aspergillus fumigatus, Candida albicans, Cryptococcus neoformans, and Histoplasma capsulatum. Work to date suggests that inner cell wall polysaccharides that play an important structural role are the most conserved across diverse members of the fungal kingdom, and elicit the strongest innate immune responses. The immune system senses these carbohydrates through receptors, such as lectins and complement proteins. In contrast, a greater diversity of polysaccharides is found within the outer cell walls of pathogenic fungi. These glycans play an important role in immune evasion, and can even induce anti-inflammatory host responses. Further study of the complex interactions between the host immune system and the fungal polysaccharides will be necessary to develop more effective therapeutic strategies, as well as to explore the use of immunosuppressive polysaccharides as therapeutic agents to modulate inflammation.

Fungal Pathogens in CF Airways: Leave or Treat?

Chronic airway infection plays an essential role in the progress of cystic fibrosis (CF) lung disease. In the past decades, mainly bacterial pathogens, such as Pseudomonas aeruginosa, have been the focus of researchers and clinicians. However, fungi are frequently detected in CF airways and there is an increasing body of evidence that fungal pathogens might play a role in CF lung disease. Several studies have shown an association of fungi, particularly Aspergillus fumigatus and Candida albicans, with the course of lung disease in CF patients. Mechanistically, in vitro and in vivo studies suggest that an impaired immune response to fungal pathogens in CF airways renders them more susceptible to fungi. However, it remains elusive whether fungi are actively involved in CF lung disease pathologies or whether they rather reflect a dysregulated airway colonization and act as microbial bystanders. A key issue for dissecting the role of fungi in CF lung disease is the distinction of dynamic fungal-host interaction entities, namely colonization, sensitization or infection. This review summarizes key findings on pathophysiological mechanisms and the clinical impact of fungi in CF lung disease.

NOD-Like Receptor P3 Inflammasome Controls Protective Th1/Th17 Immunity against Pulmonary Paracoccidioidomycosis

The NOD-like receptor P3 (NLRP3) inflammasome is an intracellular multimeric complex that triggers the activation of inflammatory caspases and the maturation of IL-1β and IL-18, important cytokines for the innate immune response against pathogens. The functional NLRP3 inflammasome complex consists of NLRP3, the adaptor protein apoptosis-associated speck-like protein, and caspase-1. Various molecular mechanisms were associated with NLRP3 activation including the presence of extracellular ATP, recognized by the cell surface P2X7 receptor (P2X7R). Several pattern recognition receptors on innate immune cells recognize Paracoccidioides brasiliensis components resulting in diverse responses that influence adaptive immunity and disease outcome. However, the role of NLRP3 inflammasome was scantily investigated in pulmonary paracoccidioidomycosis (PCM), leading us to use an intratracheal (i.t.) model of infection to study the influence of this receptor in anti-fungal immunity and severity of infection. For in vivo studies, C57BL/6 mice deficient for several NLRP3 inflammasome components (Nlrp3^−/−, Casp1/11^−/−, Asc^−/−) as well as deficient for ATP receptor (P2x7r^−/−) were infected via i.t. with P. brasiliensis and several parameters of immunity and disease severity analyzed at the acute and chronic periods of infection. Pulmonary PCM was more severe in Nlrp3^−/−, Casp1/11^−/−, Asc^−/−, and P2x7r^−/− mice as demonstrated by the increased fungal burdens, mortality rates and tissue pathology developed. The more severe disease developed by NLRP3, ASC, and Caspase-1/11 deficient mice was associated with decreased production of IL-1β and IL-18 and reduced inflammatory reactions mediated by PMN leukocytes and activated CD4⁺ and CD8⁺ T cells. The decreased T cell immunity was concomitant with increased expansion of CD4⁺CD25⁺Foxp3 regulatory T (Treg) cells. Characterization of intracellular cytokines showed a persistent reduction of CD4⁺ and CD8⁺ T cells expressing IFN-γ and IL-17 whereas those producing IL-4 and TGF-β appeared in increased frequencies. Histopathological studies showed that all deficient mouse strains developed more severe lesions containing elevated numbers of budding yeast cells resulting in increased mortality rates. Altogether, these findings led us to conclude that the activation of the NLRP3 inflammasome has a crucial role in the immunoprotection against pulmonary PCM by promoting the expansion of Th1/Th17 immunity and reducing the suppressive control mediated by Treg cells.

Immunological Outcomes of Antibody Binding to Glycans Shared between Microorganisms and Mammals

Glycans constitute basic cellular components of living organisms across biological kingdoms, and glycan-binding Abs participate in many cellular interactions during immune defense against pathogenic organisms. Glycan epitopes are expressed as carbohydrate-only entities or as oligomers or polymers on proteins and lipids. Such epitopes on glycoproteins may be formed by posttranslational modifications or neoepitopes resulting from metabolic-catabolic processes and can be altered during inflammation. Pathogenic organisms can display host-like glycans to evade the host immune response. However, Abs to glycans, shared between microorganisms and the host, exist naturally. These Abs are able to not only protect against infectious disease, but also are involved in host housekeeping functions and can suppress allergic disease. Despite the reactivity of these Abs to glycans shared between microorganisms and host, diverse tolerance-inducing mechanisms permit the B cell precursors of these Ab-secreting cells to exist within the normal B cell repertoire.

Role of microglia in fungal infections of the central nervous system

Most fungi are capable of disseminating into the central nervous system (CNS) commonly being observed in immunocompromised hosts. Microglia play a critical role in responding to these infections regulating inflammatory processes proficient at controlling CNS colonization by these eukaryotic microorganisms. Nonetheless, it is this inflammatory state that paradoxically yields cerebral mycotic meningoencephalitis and abscess formation. As peripheral macrophages and fungi have been investigated aiding our understanding of peripheral disease, ascertaining the key interactions between fungi and microglia may uncover greater abilities to treat invasive fungal infections of the brain. Here, we present the current knowledge of microglial physiology. Due to the existing literature, we have described to greater extent the opportunistic mycotic interactions with these surveillance cells of the CNS, highlighting the need for greater efforts to study other cerebral fungal infections such as those caused by geographically restricted dimorphic and rare fungi.

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.

Krüppel-like Factor 4 modulates interleukin-6 release in human dendritic cells after in vitro stimulation with Aspergillus fumigatus and Candida albicans

Invasive fungal infections are associated with high mortality rates and are mostly caused by the opportunistic fungi Aspergillus fumigatus and Candida albicans. Immune responses against these fungi are still not fully understood. Dendritic cells (DCs) are crucial players in initiating innate and adaptive immune responses against fungal infections. The immunomodulatory effects of fungi were compared to the bacterial stimulus LPS to determine key players in the immune response to fungal infections. A genome wide study of the gene regulation of human monocyte-derived dendritic cells (DCs) confronted with A. fumigatus, C. albicans or LPS was performed and Krüppel-like factor 4 (KLF4) was identified as the only transcription factor that was down-regulated in DCs by both fungi but induced by stimulation with LPS. Downstream analysis demonstrated the influence of KLF4 on the interleukine-6 expression in human DCs. Furthermore, KLF4 regulation was shown to be dependent on pattern recognition receptor ligation. Therefore KLF4 was identified as a controlling element in the IL-6 immune response with a unique expression pattern comparing fungal and LPS stimulation.

Short fungal fractions of β-1,3 glucans affect platelet activation

Platelets are capable of binding, aggregating, and internalizing microorganisms, which enhances the elimination of pathogens from the blood. The yeast Candida albicans is a pathobiont causing life-threatening invasive infections. Its cell wall contains β-1,3 glucans that are known to trigger a wide range of host cell activities and to circulate during infection. We studied the effect of β-1,3 glucan fractions (BGFs) consisting of diglucosides (Glc2), tetraglucosides (Glc4), and pentaglucosides (Glc5) on human platelets, their mechanisms of action, and their possible impact on host defenses. The effect of BGFs on the coagulation process was determined by measuring thrombin generation. Platelets pretreated with BGFs were analyzed in terms of activation, receptor expression, aggregation, and adhesion to neutrophils and to C. albicans The results show that BGFs affected the endogenous thrombin potential in a concentration-dependent manner. For platelet activation, BGFs at a low concentration (2 μmol/l) reduced ATP release and prevented the phosphorylation of protein kinase C. BGFs diminished the expression of P-selectin and the activation of αIIbβ3 BGFs decreased platelet aggregation and the interaction between thrombin-stimulated platelets and neutrophils, fibrinogen, and C. albicans GLc5 decreased ATP release and TGF-β1 production in response to TLR4 upregulation in thrombin-stimulated platelets, but TLR4 blockage abolished the effect of BGFs on platelets. This study provides evidence that fungal pentaglucosides modulate platelet activity mediated via TLR4 stimulation and reduce platelet-neutrophil interaction.

Toll-like receptor 4-related immunostimulatory polysaccharides: Primary structure, activity relationships, and possible interaction models

Toll-like receptor (TLR) 4 is an important polysaccharide receptor; however, the relationships between the structures and biological activities of TLR4 and polysaccharides remain unknown. Many recent findings have revealed the primary structure of TLR4/MD-2-related polysaccharides, and several three-dimensional structure models of polysaccharide-binding proteins have been reported; and these models provide insights into the mechanisms through which polysaccharides interact with TLR4. In this review, we first discuss the origins of polysaccharides related to TLR4, including polysaccharides from higher plants, fungi, bacteria, algae, and animals. We then briefly describe the glucosidic bond types of TLR4-related heteroglycans and homoglycans and describe the typical molecular weights of TLR4-related polysaccharides. The primary structures and activity relationships of polysaccharides with TLR4/MD-2 are also discussed. Finally, based on the existing interaction models of LPS with TLR4/MD-2 and linear polysaccharides with proteins, we provide insights into the possible interaction models of polysaccharide ligands with TLR4/MD-2. To our knowledge, this review is the first to summarize the primary structures and activity relationships of TLR4-related polysaccharides and the possible mechanisms of interaction for TLR4 and TLR4-related polysaccharides.

Interactions of Aspergillus fumigatus Conidia with Airway Epithelial Cells: A Critical Review

Aspergillus fumigatus is an environmental filamentous fungus that also acts as an opportunistic pathogen able to cause a variety of symptoms, from an allergic response to a life-threatening disseminated fungal infection. The infectious agents are inhaled conidia whose first point of contact is most likely to be an airway epithelial cell (AEC). The interaction between epithelial cells and conidia is multifaceted and complex, and has implications for later steps in pathogenesis. Increasing evidence has demonstrated a key role for the airway epithelium in the response to respiratory pathogens, particularly at early stages of infection; therefore, elucidating the early stages of interaction of conidia with AECs is essential to understand the establishment of infection in cohorts of at-risk patients. Here, we present a comprehensive review of the early interactions between A. fumigatus and AECs, including bronchial and alveolar epithelial cells. We describe mechanisms of adhesion, internalization of conidia by AECs, the immune response of AECs, as well as the role of fungal virulence factors, and patterns of fungal gene expression characteristic of early infection. A clear understanding of the mechanisms involved in the early establishment of infection by A. fumigatus could point to novel targets for therapy and prophylaxis.

Challenges and Strategies for Proteome Analysis of the Interaction of Human Pathogenic Fungi with Host Immune Cells

Opportunistic human pathogenic fungi including the saprotrophic mold Aspergillus fumigatus and the human commensal Candida albicans can cause severe fungal infections in immunocompromised or critically ill patients. The first line of defense against opportunistic fungal pathogens is the innate immune system. Phagocytes such as macrophages, neutrophils and dendritic cells are an important pillar of the innate immune response and have evolved versatile defense strategies against microbial pathogens. On the other hand, human-pathogenic fungi have sophisticated virulence strategies to counteract the innate immune defense. In this context, proteomic approaches can provide deeper insights into the molecular mechanisms of the interaction of host immune cells with fungal pathogens. This is crucial for the identification of both diagnostic biomarkers for fungal infections and therapeutic targets. Studying host-fungal interactions at the protein level is a challenging endeavor, yet there are few studies that have been undertaken. This review draws attention to proteomic techniques and their application to fungal pathogens and to challenges, difficulties, and limitations that may arise in the course of simultaneous dual proteome analysis of host immune cells interacting with diverse morphotypes of fungal pathogens. On this basis, we discuss strategies to overcome these multifaceted experimental and analytical challenges including the viability of immune cells during co-cultivation, the increased and heterogeneous protein complexity of the host proteome dynamically interacting with the fungal proteome, and the demands on normalization strategies in terms of relative quantitative proteome analysis.

Galactomannan Downregulates the Inflammation Responses in Human Macrophages via NFκB2/p100

We show that galactomannan, a polysaccharide consisting of a mannose backbone with galactose side groups present on the cell wall of several fungi, induces a reprogramming of the inflammatory response in human macrophages through dectin-1 receptor. The nuclear factor kappa-light-chain-enhancer of activated B cells 2 (NFκB2)/p100 was overexpressed after galactomannan challenge. Knocking down NFκB2/p100 using small interfering RNA (siRNA) indicated that NFκB2/p100 expression is a crucial factor in the progression of the galactomannan-induced refractoriness. The data presented in this study could be used as a modulator of inflammatory response in clinical situations where refractory state is required.

Identification of the antiphagocytic trypacidin gene cluster in the human-pathogenic fungus Aspergillus fumigatus

The opportunistic human pathogen Aspergillus fumigatus produces numerous different natural products. The genetic basis for the biosynthesis of a number of known metabolites has remained unknown. The gene cluster encoding for the biosynthesis of the conidia-bound metabolite trypacidin is of particular interest because of its antiprotozoal activity and possible role in the infection process. Here, we show that the genes encoding the biosynthesis enzymes of trypacidin reside within an orphan gene cluster in A. fumigatus. Genome mining identified tynC as an uncharacterized polyketide synthase with high similarity to known enzymes, whose products are structurally related to trypacidin including endocrocin and fumicycline. Gene deletion of tynC resulted in the complete absence of trypacidin production, which was fully restored when the mutant strain was complemented with the wild-type gene. When confronted with macrophages, the tynC deletion mutant conidia were more frequently phagocytosed than those of the parental wild-type strain. This was also found for phagocytic amoebae of the species Dictyostelium discoideum, which showed increased phagocytosis of ΔtynC conidia. Both macrophages and amoebae were also sensitive to trypacidin. Therefore, our results suggest that the conidium-bound trypacidin could have a protective function against phagocytes both in the environment and during the infection process.