Caspofungin Increases Fungal Chitin and Eosinophil and γδ T Cell-Dependent Pathology in Invasive Aspergillosis

Adiponectin pathway activation dampens inflammation and enhances alveolar macrophage fungal killing via LC3-associated phagocytosis

Although innate immunity is critical for antifungal host defense against the human opportunistic fungal pathogen Aspergillus fumigatus, potentially damaging inflammation must be controlled. Adiponectin (APN) is an adipokine produced mainly in adipose tissue that exerts anti-inflammatory effects in adipose-distal tissues such as the lung. We observed 100% mortality and increased fungal burden and inflammation in neutropenic mice with invasive aspergillosis (IA) that lack APN or the APN receptors AdipoR1 or AdipoR2. Alveolar macrophages (AMs), early immune sentinels that detect and respond to lung infection, express both receptors, and APN-/- AMs exhibited an inflammatory/M1 phenotype that was associated with decreased fungal killing. Pharmacological stimulation of AMs with AdipoR agonist AdipoRon partially rescued deficient killing in APN-/- AMs that was dependent on both receptors. Finally, APN-enhanced fungal killing was associated with increased activation of the non-canonical LC3 pathway of autophagy. Thus, our study identifies a novel role for APN in LC3-mediated killing of A.fumigatus.

Survival Factor A (SvfA) Contributes to Aspergillus nidulans Pathogenicity

Survival factor A (SvfA) in Aspergillus nidulans plays multiple roles in growth and developmental processes. It is a candidate for a novel VeA-dependent protein involved in sexual development. VeA is a key developmental regulator in Aspergillus species that can interact with other velvet-family proteins and enter into the nucleus to function as a transcription factor. In yeast and fungi, SvfA-homologous proteins are required for survival under oxidative and cold-stress conditions. To assess the role of SvfA in virulence in A. nidulans, cell wall components, biofilm formation, and protease activity were evaluated in a svfA-gene-deletion or an AfsvfA-overexpressing strain. The svfA-deletion strain showed decreased production of β-1,3-glucan in conidia, a cell wall pathogen-associated molecular pattern, with a decrease in gene expression for chitin synthases and β-1,3-glucan synthase. The ability to form biofilms and produce proteases was reduced in the svfA-deletion strain. We hypothesized that the svfA-deletion strain was less virulent than the wild-type strain; therefore, we performed in vitro phagocytosis assays using alveolar macrophages and analyzed in vivo survival using two vertebrate animal models. While phagocytosis was reduced in mouse alveolar macrophages challenged with conidia from the svfA-deletion strain, the killing rate showed a significant increase with increased extracellular signal-regulated kinase ERK activation. The svfA-deletion conidia infection reduced host mortality in both T-cell-deficient zebrafish and chronic granulomatous disease mouse models. Taken together, these results indicate that SvfA plays a significant role in the pathogenicity of A. nidulans.

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.

Living without eosinophils: evidence from mouse and man

The enduring view of eosinophils, as immune effector cells whose primary function is host defence against infection by helminths and other microbial pathogens, sets the stage for a fundamental question regarding the safety of therapeutic eosinophil depletion. If eosinophils are significantly reduced or altogether depleted in an effort to alleviate the negative effects of tissue eosinophilia and eosinophilic inflammation in conditions such as asthma, COPD, chronic rhinosinusitis with nasal polyps, eosinophilic granulomatosis with polyangiitis and hypereosinophilic syndrome, would these patients become susceptible to infection or another illness? Development of mouse models in which the eosinophil lineage has been ablated, observations in patients naturally lacking eosinophils and data from studies of eosinophil-depleting medical therapies indicate that the absence of eosinophils is not detrimental to health. The evidence available to date, as presented in this review, supports the conclusion that even if certain homeostatic roles for the eosinophil may be demonstrable in controlled animal models and human in vitro settings, the evolution of the human species appears to have provided sufficient immune redundancy such that one may be hale and hearty without eosinophils.

Revisiting the Vital Drivers and Mechanisms of β-Glucan Masking in Human Fungal Pathogen, Candida albicans

Among the several human fungal pathogens, Candida genus represents one of the most implicated in the clinical scenario. There exist several distinctive features that govern the establishment of Candida infections in addition to their capacity to adapt to multiple stress conditions inside humans which also include evasion of host immune responses. The complex fungal cell wall of the prevalent pathogen, Candida albicans, is one of the main targets of antifungal drugs and recognized by host immune cells. The wall consists of tiered arrangement of an outer thin but dense covering of mannan and inner buried layers of β-glucan and chitin. However, the pathogenic fungi adopt strategies to evade immune recognition by masking these molecules. This capacity to camouflage the immunogenic polysaccharide β-glucan from the host is a key virulence factor of C. albicans. The present review is an attempt to collate various underlying factors and mechanisms involved in Candida β-glucan masking from the available pool of knowledge and provide a comprehensive understanding. This will further improve therapeutic approaches to candidiasis by identifying new antifungal targets that blocks fungal immune evasion.

Evolved to protect, designed to destroy: IL-17-producing γδ T cells in infection, inflammation, and cancer

T cells of the gamma delta (γδ) lineage are evolutionary conserved from jawless to cartilaginous and bony fish to mammals and represent the "swiss army knife" of the immune system capable of antigen-dependent or independent responses, memory, antigen presentation, regulation of other lymphocytes, tissue homeostasis, and mucosal barrier maintenance, to list a few. Over the last 10 years, γδ T cells that produce the cytokine IL-17 (γδT17) have taken a leading position in our understanding of how our immune system battles infection, inflicts tissue damage during inflammation, and gets rewired by the tumor microenvironment. A lot of what we know about γδT17 cells stems from mouse models, however, increasing evidence implicates these cells in numerous human diseases. Herein, we aim to give an overview of the most common mouse models that have been used to study the role of γδT17 cells in infection, inflammation, and cancer, while at the same time we will evaluate evidence for their importance in humans. We hope and believe that in the next 10 years, means to take advantage of the protective and destructive properties of γδ T and in particular γδT17 cells will be part of our standard immunotherapy toolkit.

Mannan Molecular Substructures Control Nanoscale Glucan Exposure in Candida

Cell wall mannans of Candida albicans mask β-(1,3)-glucan from recognition by Dectin-1, contributing to innate immune evasion. Glucan exposures are predominantly single receptor-ligand interaction sites of nanoscale dimensions. Candida species vary in basal glucan exposure and molecular complexity of mannans. We used super-resolution fluorescence imaging and a series of protein mannosylation mutants in C. albicans and C. glabrata to investigate the role of specific N-mannan features in regulating the nanoscale geometry of glucan exposure. Decreasing acid labile mannan abundance and α-(1,6)-mannan backbone length correlated most strongly with increased density and nanoscopic size of glucan exposures in C. albicans and C. glabrata, respectively. Additionally, a C. albicans clinical isolate with high glucan exposure produced similarly perturbed N-mannan structures and elevated glucan exposure geometry. Thus, acid labile mannan structure influences the nanoscale features of glucan exposure, impacting the nature of the pathogenic surface that triggers immunoreceptor engagement, aggregation, and signaling.

Schizophyllum commune induces IL-17-mediated neutrophilic airway inflammation in OVA-induced asthma model mice

Schizophyllum commune is a ubiquitous basidiomycetous fungus typically found across the world, which has been detected in indoor and outdoor air. Some studies indicated that sensitization to S. commune is correlated with asthma severity in patients. Patients with chronic severe or acute fatal asthma have neutrophil-dominant airway inflammation. We hypothesized that S. commune can exacerbate asthma. To test this hypothesis, we evaluated the direct immunomodulatory activities of S. commune in allergic airway inflammation induced by non-fungal sensitization. Ovalbumin (OVA)-induced asthma model mice were generated using wild-type (WT) and Il-17a^-/-Il-17f^-/- mice that were intratracheally exposed to S. commune, then immune responses in the lungs were assessed after 24 h. Intratracheal administration of S. commune in OVA-induced asthma model mice enhanced neutrophilic airway inflammation, increased the mRNA expression of CXCL1 and CXCL2 in the lungs, and provoked IL-17A, and IL-17F production in BAL fluid. In addition, neutrophilic airway inflammation was significantly inhibited in Il-17a^-/-Il-17f^-/- mice compared with those found in WT mice. We demonstrated that S. commune induces neutrophilic airway inflammation in OVA-induced asthma model mice, and IL-17A and IL-17F had central roles in this activity. As S. commune inhabits the general environment, including indoor and outdoor air, our results suggested that S. commune is a causative agent of asthma exacerbation. This study has provided clues regarding the mechanisms behind fungi and asthma exacerbation.

The Metabolic Cytokine Adiponectin Inhibits Inflammatory Lung Pathology in Invasive Aspergillosis

Systemic immunity and metabolism are coregulated by soluble factors, including the insulin-regulating adipose tissue cytokine adiponectin. How these factors impact detrimental inflammatory responses during fungal infection remains unknown. In this study, we observed that mortality, fungal burden, and tissue histopathology were increased in adiponectin-deficient mice in a neutropenic model of invasive aspergillosis. Lung RNA sequencing, quantitative RT-PCR, and subsequent pathway analysis demonstrated activation of inflammatory cytokine pathways with upstream regulation by IL-1 and TNF in adiponectin-deficient mice with decreased/inhibited anti-inflammatory genes/pathways, suggesting broad cytokine-mediated pathology along with ineffective fungal clearance. Quantitative RT-PCR analysis confirmed increased transcription of IL-1a, IL-6, IL-12b, IL-17A/F, and TNF in adiponectin-deficient mice at early time points postinfection, with a specific increase in intracellular TNF in alveolar macrophages. Although eosinophil recruitment and activation were increased in adiponectin-deficient mice, mortality was delayed, but not decreased, in mice deficient in both adiponectin and eosinophils. Interestingly, neutrophil depletion was required for increased inflammation in adiponectin-deficient mice in response to swollen/fixed conidia, suggesting that immune suppression enhances detrimental inflammation, whereas invasive fungal growth is dispensable. Our results suggest that adiponectin inhibits excessive lung inflammation in invasive aspergillosis. Our study has therefore identified the adiponectin pathway as a potential source for novel therapeutics in immune-compromised patients with detrimental immunity to invasive fungal infection.

Reciprocal Inhibition of Adiponectin and Innate Lung Immune Responses to Chitin and Aspergillus fumigatus

Chitin is a structural biopolymer found in numerous organisms, including pathogenic fungi, and recognized as an immune-stimulating pathogen associated molecular pattern by pattern recognition molecules of the host immune system. However, programming and regulation of lung innate immunity to chitin inhalation in the context of inhalation of fungal pathogens such as Aspergillus fumigatus is complex and our understanding incomplete. Here we report that the systemic metabolism-regulating cytokine adiponectin is decreased in the lungs and serum of mice after chitin inhalation, with a concomitant decrease in surface expression of the adiponectin receptor AdipoR1 on lung leukocytes. Constitutive lung expression of acidic mammalian chitinase resulted in decreased inflammatory cytokine gene expression and neutrophil recruitment, but did not significantly affect lung adiponectin transcription. Exogenous recombinant adiponectin specifically dampened airway chitin-mediated eosinophil recruitment, while adiponectin deficiency resulted in increased airway eosinophils. The presence of adiponectin also resulted in decreased CCL11-mediated migration of bone marrow-derived eosinophils. In contrast to purified chitin, aspiration of viable conidia from the high chitin-expressing A. fumigatus isolate Af5517 resulted in increased neutrophil recruitment and inflammatory cytokine gene expression in adiponectin-deficient mice, while no significant changes were observed in response to the isolate Af293. Our results identify a novel role for the adiponectin pathway in inhibition of lung inflammatory responses to chitin and A. fumigatus inhalation.

Host defense mechanisms against Aspergillus fumigatus lung colonization and invasion

The human lung is continually exposed to airborne conidia of the fungus Aspergillus fumigatus (AF) and related species. The innate immune system efficiently eliminates inhaled AF conidia from the lung in normal individuals, but immunocompromised patients are at risk for highly lethal invasive aspergillosis (IA). Some individuals not at risk for IA may still suffer from failed clearance of AF in the form of noninvasive colonization associated with conditions such as allergic bronchopulmonary aspergillosis. Understanding of normal innate immune function against AF as well as failures of these functions will enable better treatment of these patient groups. In this review, we will focus on recent research that elucidates mechanisms of host defense and their failures resulting in colonization as well as tissue invasion.

Characterisation of novel-cell-wall LysM-domain proteins LdpA and LdpB from the human pathogenic fungus Aspergillus fumigatus

Aspergillus fumigatus, a filamentous fungus that is ubiquitous in the environment, causes several human pulmonary disorders, including chronic and acute invasive infections and allergic diseases. Lysin motif (LysM) is a small protein domain that binds chitin, a major component of fungal cell wall polysaccharides. Several secreted LysM-domain proteins without catalytic function (LysM effectors) have been identified. They act as virulence factors in plant pathogenic fungi by preventing the immune response induced by chitin; however, LysM proteins in mammalian pathogenic fungi remain largely unexplored. We describe two novel LysM-domain proteins, LdpA and LdpB, in A. fumigatus. Functional analyses of single and double knockouts revealed no significant effects on cell wall chitin content, cell wall integrity, fungal morphology and fungal growth. Fluorescent signals from LdpA-green fluorescent protein (GFP) and LdpB-GFP were observed in cell wall and extracellular matrix. In a mouse model of invasive pulmonary aspergillosis, survival did not differ between ΔldpA/B and wild-type infection; however, further studies are required to reveal their functions in fungal−host interactions.

Eosinophil-Associated Innate IL-17 Response Promotes Aspergillus fumigatus Lung Pathology

Aspergillus fumigatus is a common widespread microorganism with environmental, biological and clinical relevance. After inhalation, swollen conidia can germinate, colonize and invade pulmonary tissues. Eosinophils have been described as key cells in A. fumigatus lung infection. However, their specific role in protecting or damaging lung tissue as well as their relatioship among different A. fumigatus strains is poorly understood. Previously, it has been reported that eosinophils are able to produce IL-17 and mediate an innate response that protected mice from infection using Af293 and CEA10 strains. Here, we have developed a set of new experiments with the CEA17-derived A1163 strain of A. fumigatus. Using ΔdblGATA1 mice, we demonstrate that eosinophils produce IL-17 and are involved in control of neutrophil, macrophage and lymphocyte recruitment. We found that eosinophils also induce high levels of cytokines and chemokines, generating an intense inflammatory process. Eosinophils are responsible for increased pulmonary dysfunction and elevated lethality rates in mice. Curiously, fungal burden was not affected. To address the role of IL-17 signaling, pharmacological inhibition of this mediator in the airways with anti-IL-17 antibody was able to reduce inflammation in the airways and protect infected mice. In conclusion, our results demonstrate that eosinophils control IL-17-mediated response and contribute to lung pathology after A. fumigatus infection. Therefore, eosinophils may represent a potential target for controlling exacerbated inflammation and prevent tissue damage during this fungal infection.

Acidic Mammalian Chitinase Negatively Affects Immune Responses during Acute and Chronic Aspergillus fumigatus Exposure

Chitin is a polysaccharide that provides structure and rigidity to the cell walls of fungi and insects. Mammals possess multiple chitinases, which function to degrade chitin, thereby supporting a role for chitinases in immune defense. However, chitin degradation has been implicated in the pathogenesis of asthma. Here, we determined the impact of acidic mammalian chitinase (AMCase) (Chia) deficiency on host defense during acute exposure to the fungal pathogen Aspergillus fumigatus as well as its contribution to A. fumigatus-associated allergic asthma. We demonstrate that chitin in the fungal cell wall was detected at low levels in A. fumigatus conidia, which emerged at the highest level during hyphal transition. In response to acute A. fumigatus challenge, Chia^-/- mice unexpectedly demonstrated lower A. fumigatus lung burdens at 2 days postchallenge. The lower fungal burden correlated with decreased lung interleukin-33 (IL-33) levels yet increased IL-1β and prostaglandin E₂ (PGE₂) production, a phenotype that we reported previously to promote the induction of IL-17A and IL-22. During chronic A. fumigatus exposure, AMCase deficiency resulted in lower dynamic and airway lung resistance than in wild-type mice. Improved lung physiology correlated with attenuated levels of the proallergic chemokines CCL17 and CCL22. Surprisingly, examination of inflammatory responses during chronic exposure revealed attenuated IL-17A and IL-22 responses, but not type 2 responses, in the absence of AMCase. Collectively, these data suggest that AMCase functions as a negative regulator of immune responses during acute fungal exposure and is a contributor to fungal asthma severity, putatively via the induction of proinflammatory responses.

Dynamic Fungal Cell Wall Architecture in Stress Adaptation and Immune Evasion

Deadly infections from opportunistic fungi have risen in frequency, largely because of the at-risk immunocompromised population created by advances in modern medicine and the HIV/AIDS pandemic. This review focuses on dynamics of the fungal polysaccharide cell wall, which plays an outsized role in fungal pathogenesis and therapy because it acts as both an environmental barrier and as the major interface with the host immune system. Human fungal pathogens use architectural strategies to mask epitopes from the host and prevent immune surveillance, and recent work elucidates how biotic and abiotic stresses present during infection can either block or enhance masking. The signaling components implicated in regulating fungal immune recognition can teach us how cell wall dynamics are controlled, and represent potential targets for interventions designed to boost or dampen immunity.

Histological Quantification to Determine Lung Fungal Burden in Experimental Aspergillosis

The quantification of lung fungal burden is critical for the determination of the relative levels of immune protection and fungal virulence in mouse models of pulmonary fungal infection. Although multiple methods are used to assess fungal burden, quantitative polymerase chain reaction (qPCR) of fungal DNA has emerged as a technique with several advantages over previous culture-based methods. Currently, a comprehensive assessment of lung pathology, leukocyte recruitment, fungal burden, and gene expression in mice with invasive aspergillosis (IA) necessitates the use of a significant number of experimental and control animals. Here the quantification of lung histological staining to determine fungal burden using a reduced number of animals was examined in detail. Lung sections were stained to identify fungal structures with Gomori's modified methanamine silver (GMS) staining. Images were taken from the GMS-stained sections from 4 discrete fields of each formalin-fixed paraffin-embedded lung. The GMS stained areas within each image were quantified using an image analysis program, and from this quantification, the mean percentage of stained area was determined for each sample. Using this strategy, eosinophil-deficient mice exhibited decreased fungal burden and disease with caspofungin therapy, while wild-type mice with IA did not improve with caspofungin. Similarly, fungal burden in mice lacking γδ T cells were also improved by caspofungin, as measured by qPCR and GMS quantification. GMS quantification is therefore introduced as a method for the determination of relative lung fungal burden that may ultimately reduce the quantity of experimental animals required for comprehensive studies of invasive aspergillosis.