BTK drives neutrophil activation for sterilizing antifungal immunity

We describe a previously-unappreciated role for Bruton's tyrosine kinase (BTK) in fungal immune surveillance against aspergillosis, an unforeseen complication of BTK inhibitors (BTKi) used for treating B-cell lymphoid malignancies. We studied BTK-dependent fungal responses in neutrophils from diverse populations, including healthy donors, BTKi-treated patients, and X-linked agammaglobulinemia patients. Upon fungal exposure, BTK was activated in human neutrophils in a TLR2-, Dectin-1-, and FcγR-dependent manner, triggering the oxidative burst. BTK inhibition selectively impeded neutrophil-mediated damage to Aspergillus hyphae, primary granule release, and the fungus-induced oxidative burst by abrogating NADPH oxidase subunit p40phox and GTPase RAC2 activation. Moreover, neutrophil-specific Btk deletion in mice enhanced aspergillosis susceptibility by impairing neutrophil function, not recruitment or lifespan. Conversely, GM-CSF partially mitigated these deficits by enhancing p47phox activation. Our findings underline the crucial role of BTK signaling in neutrophils for antifungal immunity and provide a rationale for GM-CSF use to offset these deficits in susceptible patients.

Intestinal Bacteroides modulates inflammation, systemic cytokines, and microbial ecology via propionate in a mouse model of cystic fibrosis

Persons with cystic fibrosis (CF), starting in early life, show intestinal microbiome dysbiosis characterized in part by a decreased relative abundance of the genus Bacteroides. Bacteroides is a major producer of the intestinal short chain fatty acid propionate. We demonstrate here that cystic fibrosis transmembrane conductance regulator-defective (CFTR-/-) Caco-2 intestinal epithelial cells are responsive to the anti-inflammatory effects of propionate. Furthermore, Bacteroides isolates inhibit the IL-1β-induced inflammatory response of CFTR-/- Caco-2 intestinal epithelial cells and do so in a propionate-dependent manner. The introduction of Bacteroides-supplemented stool from infants with cystic fibrosis into the gut of CftrF508del mice results in higher propionate in the stool as well as the reduction in several systemic pro-inflammatory cytokines. Bacteroides supplementation also reduced the fecal relative abundance of Escherichia coli, indicating a potential interaction between these two microbes, consistent with previous clinical studies. For a Bacteroides propionate mutant in the mouse model, pro-inflammatory cytokine KC is higher in the airway and serum compared with the wild-type (WT) strain, with no significant difference in the absolute abundance of these two strains. Taken together, our data indicate the potential multiple roles of Bacteroides-derived propionate in the modulation of systemic and airway inflammation and mediating the intestinal ecology of infants and children with CF. The roles of Bacteroides and the propionate it produces may help explain the observed gut-lung axis in CF and could guide the development of probiotics to mitigate systemic and airway inflammation for persons with CF.IMPORTANCEThe composition of the gut microbiome in persons with CF is correlated with lung health outcomes, a phenomenon referred to as the gut-lung axis. Here, we demonstrate that the intestinal microbe Bacteroides decreases inflammation through the production of the short-chain fatty acid propionate. Supplementing the levels of Bacteroides in an animal model of CF is associated with reduced systemic inflammation and reduction in the relative abundance of the opportunistically pathogenic group Escherichia/Shigella in the gut. Taken together, these data demonstrate a key role for Bacteroides and microbially produced propionate in modulating inflammation, gut microbial ecology, and the gut-lung axis in cystic fibrosis. These data support the role of Bacteroides as a potential probiotic in CF.

Aspergillus fumigatus cytochrome c impacts conidial survival during sterilizing immunity

IMPORTANCE

Aspergillus fumigatus can cause a life-threatening infection known as invasive pulmonary aspergillosis (IPA), which is marked by fungus-attributable mortality rates of 20%-30%. Individuals at risk for IPA harbor genetic mutations or incur pharmacologic defects that impair myeloid cell numbers and/or function, exemplified by bone marrow transplant recipients, patients that receive corticosteroid therapy, or patients with chronic granulomatous disease (CGD). However, treatments for Aspergillus infections remain limited, and resistance to the few existing drug classes is emerging. Recently, the World Health Organization classified A. fumigatus as a critical priority fungal pathogen. Our cell death research identifies an important aspect of fungal biology that impacts susceptibility to leukocyte killing. Furthering our understanding of mechanisms that mediate the outcome of fungal-leukocyte interactions will increase our understanding of both the underlying fungal biology governing cell death and innate immune evasion strategies utilized during mammalian infection pathogenesis. Consequently, our studies are a critical step toward leveraging these mechanisms for novel therapeutic advances.

The cystic fibrosis treatment Trikafta affects the growth, viability, and cell wall of Aspergillus fumigatus biofilms

IMPORTANCE

PwCF commonly test positive for pathogenic fungi, and more than 90% of the cystic fibrosis patient population is approved for the modulator treatment, Trikafta. Therefore, it is critical to understand how fungal communities, specifically A. fumigatus, respond to Trikafta exposure. Therefore, we sought to determine whether Trikafta impacted the biology of A. fumigatus biofilms. Our data demonstrate that Trikafta reduces biomass in several laboratory strains as well as clinical strains isolated from the expectorated sputum of pwCF. Furthermore, Trikafta reduces fungal viability and the capacity of biofilms to recover following treatment. Of particular importance, Trikafta affects how A. fumigatus biofilms respond to cell wall stressors, suggesting that Trikafta modulates components of the cell wall. Since the cell wall directly affects how a host immune system will respond to and effectively neutralize pathogens, our work, demonstrating that Trikafta impacts the A. fumigatus cell wall, is potentially highly relevant to fungal-induced disease pathogenesis.

Recent Advances in Understanding the Human Fungal Pathogen Hypoxia Response in Disease Progression

Fungal-mediated disease progression and antifungal drug efficacy are significantly impacted by the dynamic infection microenvironment. At the site of infection, oxygen often becomes limiting and induces a hypoxia response in both the fungal pathogen and host cells. The fungal hypoxia response impacts several important aspects of fungal biology that contribute to pathogenesis, virulence, antifungal drug susceptibility, and ultimately infection outcomes. In this review, we summarize recent advances in understanding the molecular mechanisms of the hypoxia response in the most common human fungal pathogens, discuss potential therapeutic opportunities, and highlight important areas for future research.

Aspergillus fumigatus cytochrome c impacts conidial survival during sterilizing immunity

Invasive pulmonary aspergillosis (IPA) is a life-threatening infection caused by species in the ubiquitous fungal genus Aspergillus . While leukocyte-generated reactive oxygen species (ROS) are critical for the clearance of fungal conidia from the lung and resistance to IPA, the processes that govern ROS-dependent fungal cell death remain poorly defined. Using a flow cytometric approach that monitors two independent cell death markers, an endogenous histone H2A:mRFP nuclear integrity reporter and Sytox Blue cell impermeable (live/dead) stain, we observed that loss of A. fumigatus cytochrome c ( cycA ) results in reduced susceptibility to cell death from hydrogen peroxide (H 2 O 2 ) treatment. Consistent with these observations in vitro , loss of cycA confers resistance to both NADPH-oxidase -dependent and -independent killing by host leukocytes. Fungal ROS resistance is partly mediated in part by Bir1, a homolog to survivin in humans, as Bir1 overexpression results in decreased ROS-induced conidial cell death and reduced killing by innate immune cells in vivo . We further report that overexpression of the Bir1 N-terminal BIR domain in A. fumigatus conidia results in altered expression of metabolic genes that functionally converge on mitochondrial function and cytochrome c ( cycA ) activity. Together, these studies demonstrate that cycA in A. fumigatus contributes to cell death responses that are induced by exogenous H 2 O 2 and by host leukocytes.

Importance

Aspergillus fumigatus can cause a life-threatening infection known as invasive pulmonary aspergillosis (IPA), which is marked by fungus-attributable mortality rates of 20%-30%. Individuals at risk of IPA harbor genetic mutations or incur pharmacologic defects that impair myeloid cell numbers and/or function, exemplified by bone marrow transplant recipients, patients that receive corticosteroid therapy, or patients with Chronic Granulomatous Disease (CGD). However, treatments for Aspergillus infections remains limited, and resistance to the few existing drug classes is emerging. Recently, the World Health Organization (WHO) classified A. fumigatus as a critical priority fungal pathogen. Our research identifies an important aspect of fungal biology that impacts susceptibility to leukocyte killing. Furthering our understanding of mechanisms that mediate the outcome of fungal-leukocyte interactions will increase our understanding of both the underlying fungal biology governing cell death and innate immune evasion strategies utilized during mammalian infection pathogenesis. Consequently, our studies are a critical step toward leveraging these mechanisms for novel therapeutic advances.

Luciferase-Based High-Throughput Screen with Aspergillus fumigatus to Identify Antifungal Small Molecules

Only three classes of contemporary antifungal drugs are routinely utilized in the clinic against filamentous fungal pathogens such as Aspergillus fumigatus. High-throughput phenotypic screens to identify small molecules with activity against filamentous fungi remain challenging due to the hyphal, biofilm-like growth morphology of these important organisms. In this chapter, we describe a protocol for utilizing a bioluminescent A. fumigatus strain for identifying small molecules that potentiate the activity of the triazole antifungal drug fluconazole. The assay holds great promise for identifying small molecules with activity against filamentous fungal pathogens.

Postinfluenza Environment Reduces Aspergillus fumigatus Conidium Clearance and Facilitates Invasive Aspergillosis In Vivo

Aspergillus fumigatus is a human fungal pathogen that is most often avirulent in immunecompetent individuals because the innate immune system is efficient at eliminating fungal conidia. However, recent clinical observations have shown that severe influenza A virus (IAV) infection can lead to secondary A. fumigatus infections with high mortality. Little is currently known about how IAV infection alters the innate antifungal immune response. Here, we established a murine model of IAV-induced A. fumigatus (IAV-Af) superinfection by inoculating mice with IAV followed 6 days later by A. fumigatus conidia challenge. We observed increased mortality in the IAV-Af-superinfected mice compared to mice challenged with either IAV or A. fumigatus alone. A. fumigatus conidia were able to germinate and establish a biofilm in the lungs of the IAV-Af superinfection group, which was not seen following fungal challenge alone. While we did not observe any differences in inflammatory cell recruitment in the IAV-Af superinfection group compared to single-infection controls, we observed defects in Aspergillus conidial uptake and killing by both neutrophils and monocytes after IAV infection. pHrodo Green zymosan bioparticle (pHrodo-zymosan) and CM-H2DCFDA [5-(and-6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate] staining, indicators of phagolysosome maturation and reactive oxygen species (ROS) production, respectively, revealed that the fungal killing defect was due in part to reduced phagolysosome maturation. Collectively, our data demonstrate that the ability of neutrophils and monocytes to kill and clear Aspergillus conidia is strongly reduced in the pulmonary environment of an IAV-infected lung, which leads to invasive pulmonary aspergillosis and increased overall mortality in our mouse model, recapitulating what is observed clinically in humans. IMPORTANCE Influenza A virus (IAV) is a common respiratory virus that causes seasonal illness in humans, but can cause pandemics and severe infection in certain patients. Since the emergence of the 2009 H1N1 pandemic strains, there has been an increase in clinical reports of IAV-infected patients in the intensive care unit (ICU) developing secondary pulmonary aspergillosis. These cases of flu-Aspergillus superinfections are associated with worse clinical outcomes than secondary bacterial infections in the setting of IAV. To date, we have a limited understanding of the cause(s) of secondary fungal infections in immunocompetent hosts. IAV-induced modulation of cytokine production and innate immune cellular function generates a unique immune environment in the lung, which could make the host vulnerable to a secondary fungal infection. Our work shows that defects in phagolysosome maturation in neutrophils and monocytes after IAV infection impair the ability of these cells to kill A. fumigatus, thus leading to increased fungal germination and growth and subsequent invasive aspergillosis. Our work lays a foundation for future mechanistic studies examining the exact immune modulatory events occurring in the respiratory tract after viral infection leading to secondary fungal infections.

The future of fungi: threats and opportunities

The fungal kingdom represents an extraordinary diversity of organisms with profound impacts across animal, plant, and ecosystem health. Fungi simultaneously support life, by forming beneficial symbioses with plants and producing life-saving medicines, and bring death, by causing devastating diseases in humans, plants, and animals. With climate change, increased antimicrobial resistance, global trade, environmental degradation, and novel viruses altering the impact of fungi on health and disease, developing new approaches is now more crucial than ever to combat the threats posed by fungi and to harness their extraordinary potential for applications in human health, food supply, and environmental remediation. To address this aim, the Canadian Institute for Advanced Research (CIFAR) and the Burroughs Wellcome Fund convened a workshop to unite leading experts on fungal biology from academia and industry to strategize innovative solutions to global challenges and fungal threats. This report provides recommendations to accelerate fungal research and highlights the major research advances and ideas discussed at the meeting pertaining to 5 major topics: (1) Connections between fungi and climate change and ways to avert climate catastrophe; (2) Fungal threats to humans and ways to mitigate them; (3) Fungal threats to agriculture and food security and approaches to ensure a robust global food supply; (4) Fungal threats to animals and approaches to avoid species collapse and extinction; and (5) Opportunities presented by the fungal kingdom, including novel medicines and enzymes.

Influence of management practice on the microbiota of a critically endangered species: a longitudinal study of kākāpō chick faeces and associated nest litter

BACKGROUND

The critically endangered kākāpō is a flightless, nocturnal parrot endemic to Aotearoa New Zealand. Recent efforts to describe the gastrointestinal microbial community of this threatened herbivore revealed a low-diversity microbiota that is often dominated by Escherichia-Shigella bacteria. Given the importance of associated microbial communities to animal health, and increasing appreciation of their potential relevance to threatened species conservation, we sought to better understand the development of this unusual gut microbiota profile. To this end, we conducted a longitudinal analysis of faecal material collected from kākāpō chicks during the 2019 breeding season, in addition to associated nest litter material.

RESULTS

Using an experimental approach rarely seen in studies of threatened species microbiota, we evaluated the impact of a regular conservation practice on the developing kākāpō microbiota, namely the removal of faecal material from nests. Artificially removing chick faeces from nests had negligible impact on bacterial community diversity for either chicks or nests (p > 0.05). However, the gut microbiota did change significantly over time as chick age increased (p < 0.01), with an increasing relative abundance of Escherichia-Shigella coli over the study period and similar observations for the associated nest litter microbiota (p < 0.01). Supplementary feeding substantially altered gut bacterial diversity of kākāpō chicks (p < 0.01), characterised by a significant increase in Lactobacillus bacteria.

CONCLUSIONS

Overall, chick age and hand rearing conditions had the most marked impact on faecal bacterial communities. Similarly, the surrounding nest litter microbiota changed significantly over time since a kākāpō chick was first placed in the nest, though we found no evidence that removal of faecal material influenced the bacterial communities of either litter or faecal samples. Taken together, these observations will inform ongoing conservation and management of this most enigmatic of bird species.

CF-Seq, an accessible web application for rapid re-analysis of cystic fibrosis pathogen RNA sequencing studies

Researchers studying cystic fibrosis (CF) pathogens have produced numerous RNA-seq datasets which are available in the gene expression omnibus (GEO). Although these studies are publicly available, substantial computational expertise and manual effort are required to compare similar studies, visualize gene expression patterns within studies, and use published data to generate new experimental hypotheses. Furthermore, it is difficult to filter available studies by domain-relevant attributes such as strain, treatment, or media, or for a researcher to assess how a specific gene responds to various experimental conditions across studies. To reduce these barriers to data re-analysis, we have developed an R Shiny application called CF-Seq, which works with a compendium of 128 studies and 1,322 individual samples from 13 clinically relevant CF pathogens. The application allows users to filter studies by experimental factors and to view complex differential gene expression analyses at the click of a button. Here we present a series of use cases that demonstrate the application is a useful and efficient tool for new hypothesis generation. (CF-Seq: http://scangeo.dartmouth.edu/CFSeq/ ).

Avian-associated Aspergillus fumigatus displays broad phylogenetic distribution, no evidence for host specificity, and multiple genotypes within epizootic events

Birds are highly susceptible to aspergillosis, which can manifest as a primary infection in both domestic and wild birds. Aspergillosis in wild birds causes mortalities ranging in scale from single animals to large-scale epizootic events. However, pathogenicity factors associated with aspergillosis in wild birds have not been examined. Specifically, it is unknown whether wild bird-infecting strains are host-adapted (i.e. phylogenetically related). Similarly, it is unknown whether epizootics are driven by contact with clonal strains that possess unique pathogenic or virulence properties, or by distinct and equally pathogenic strains. Here, we use a diverse collection of Aspergillus fumigatus isolates taken from aspergillosis-associated avian carcasses, representing 24 bird species from a wide geographic range, and representing individual bird mortalities as well as epizootic events. These isolates were sequenced and analyzed along with 130 phylogenetically diverse human clinical isolates to investigate the genetic diversity and phylogenetic placement of avian-associated A. fumigatus, the geographic and host distribution of avian isolates, evidence for clonal outbreaks among wild birds, and the frequency of azole resistance in avian isolates. We found that avian isolates were phylogenetically diverse, with no clear distinction from human clinical isolates, and no sign of host or geographic specificity. Avian isolates from the same epizootic events were diverse and phylogenetically distant, suggesting that avian aspergillosis is not contagious among wild birds and that outbreaks are likely driven by environmental spore loads or host comorbidities. Finally, all avian isolates were susceptible to Voriconazole and none contained the canonical azole resistance gene variants.

An Alanine Aminotransferase Is Required for Biofilm-Specific Resistance of Aspergillus fumigatus to Echinocandin Treatment

Alanine metabolism has been suggested as an adaptation strategy to oxygen limitation in organisms ranging from plants to mammals. Within the pulmonary infection microenvironment, Aspergillus fumigatus forms biofilms with steep oxygen gradients defined by regions of oxygen limitation. An alanine aminotransferase, AlaA, was observed to function in alanine catabolism and is required for several aspects of A. fumigatus biofilm physiology. Loss of alaA, or its catalytic activity, results in decreased adherence of biofilms through a defect in the maturation of the extracellular matrix polysaccharide galactosaminogalactan (GAG). Additionally, exposure of cell wall polysaccharides is also impacted by loss of alaA, and loss of AlaA catalytic activity confers increased biofilm susceptibility to echinocandin treatment, which is correlated with enhanced fungicidal activity. The increase in echinocandin susceptibility is specific to biofilms, and chemical inhibition of alaA by the alanine aminotransferase inhibitor β-chloro-l-alanine is sufficient to sensitize A. fumigatus biofilms to echinocandin treatment. Finally, loss of alaA increases susceptibility of A. fumigatus to in vivo echinocandin treatment in a murine model of invasive pulmonary aspergillosis. Our results provide insight into the interplay of metabolism, biofilm formation, and antifungal drug resistance in A. fumigatus and describe a mechanism of increasing susceptibility of A. fumigatus biofilms to the echinocandin class of antifungal drugs. IMPORTANCE Aspergillus fumigatus is a ubiquitous filamentous fungus that causes an array of diseases depending on the immune status of an individual, collectively termed aspergillosis. Antifungal therapy for invasive pulmonary aspergillosis (IPA) or chronic pulmonary aspergillosis (CPA) is limited and too often ineffective. This is in part due to A. fumigatus biofilm formation within the infection environment and the resulting emergent properties, particularly increased antifungal resistance. Thus, insights into biofilm formation and mechanisms driving increased antifungal drug resistance are critical for improving existing therapeutic strategies and development of novel antifungals. In this work, we describe an unexpected observation where alanine metabolism, via the alanine aminotransferase AlaA, is required for several aspects of A. fumigatus biofilm physiology, including resistance of A. fumigatus biofilms to the echinocandin class of antifungal drugs. Importantly, we observed that chemical inhibition of alanine aminotransferases is sufficient to increase echinocandin susceptibility and that loss of alaA increases susceptibility to echinocandin treatment in a murine model of IPA. AlaA is the first gene discovered in A. fumigatus that confers resistance to an antifungal drug specifically in a biofilm context.

Aspergillus fumigatus In-Host HOG Pathway Mutation for Cystic Fibrosis Lung Microenvironment Persistence

The prevalence of Aspergillus fumigatus colonization in individuals with cystic fibrosis (CF) and subsequent fungal persistence in the lung is increasingly recognized. However, there is no consensus for clinical management of A. fumigatus in CF individuals, due largely to uncertainty surrounding A. fumigatus CF pathogenesis and virulence mechanisms. To address this gap in knowledge, a longitudinal series of A. fumigatus isolates from an individual with CF were collected over 4.5 years. Isolate genotypes were defined with whole-genome sequencing that revealed both transitory and persistent A. fumigatus in the lung. Persistent lineage isolates grew most readily in a low-oxygen culture environment, and conidia were more sensitive to oxidative stress-inducing conditions than those from nonpersistent isolates. Closely related persistent isolates harbored a unique allele of the high-osmolarity glycerol (HOG) pathway mitogen-activated protein kinase kinase, Pbs2 (pbs2C2). Data suggest this novel pbs2C2 allele arose in vivo and is necessary for the fungal response to osmotic stress in a low-oxygen environment through hyperactivation of the HOG (SakA) signaling pathway. Hyperactivation of the HOG pathway through pbs2C2 comes at the cost of decreased conidial stress resistance in the presence of atmospheric oxygen levels. These novel findings shed light on pathoadaptive mechanisms of A. fumigatus in CF, lay the foundation for identifying persistent A. fumigatus isolates that may require antifungal therapy, and highlight considerations for successful culture of persistent Aspergillus CF isolates. IMPORTANCE Aspergillus fumigatus infection causes a spectrum of clinical manifestations. For individuals with cystic fibrosis (CF), allergic bronchopulmonary aspergillosis (ABPA) is an established complication, but there is a growing appreciation for A. fumigatus airway persistence in CF disease progression. There currently is little consensus for clinical management of A. fumigatus long-term culture positivity in CF. A better understanding of A. fumigatus pathogenesis mechanisms in CF is expected to yield insights into when antifungal therapies are warranted. Here, a 4.5-year longitudinal collection of A. fumigatus isolates from a patient with CF identified a persistent lineage that harbors a unique allele of the Pbs2 mitogen-activated protein kinase kinase (MAPKK) necessary for unique CF-relevant stress phenotypes. Importantly for A. fumigatus CF patient diagnostics, this allele provides increased fitness under CF lung-like conditions at a cost of reduced in vitro growth under standard laboratory conditions. These data illustrate a molecular mechanism for A. fumigatus CF lung persistence with implications for diagnostics and antifungal therapy.

Aspergillus fumigatus biofilms: Toward understanding how growth as a multicellular network increases antifungal resistance and disease progression

Aspergillus fumigatus is a saprophytic, filamentous fungus found in soils and compost and the causative agent of several pulmonary diseases in humans, birds, and other mammals. A. fumigatus and other filamentous fungi grow as networks of filamentous hyphae that have characteristics of a classic microbial biofilm. These characteristics include production of an extracellular matrix (ECM), surface adhesion, multicellularity, and increased antimicrobial drug resistance. A. fumigatus biofilm growth occurs in vivo at sites of infection, highlighting the importance of defining mechanisms underlying biofilm development and associated emergent properties. We propose that there are 3 distinct phases in the development of A. fumigatus biofilms: biofilm initiation, immature biofilm, and mature biofilm. These stages are defined both temporally and by unique genetic and structural changes over the course of development. Here, we review known mechanisms within each of these stages that contribute to biofilm structure, ECM production, and increased resistance to contemporary antifungal drugs. We highlight gaps in our understanding of biofilm development and function that when addressed are expected to aid in the development of novel antifungal therapies capable of killing filamentous fungal biofilms.

A Heterogeneously Expressed Gene Family Modulates the Biofilm Architecture and Hypoxic Growth of Aspergillus fumigatus

The genus Aspergillus encompasses human pathogens such as Aspergillus fumigatus and industrial powerhouses such as Aspergillus niger In both cases, Aspergillus biofilms have consequences for infection outcomes and yields of economically important products. However, the molecular components influencing filamentous fungal biofilm development, structure, and function remain ill defined. Macroscopic colony morphology is an indicator of underlying biofilm architecture and fungal physiology. A hypoxia-locked colony morphotype of A. fumigatus has abundant colony furrows that coincide with a reduction in vertically oriented hyphae within biofilms and increased low oxygen growth and virulence. Investigation of this morphotype has led to the identification of the causative gene, biofilm architecture factor A (bafA), a small cryptic open reading frame within a subtelomeric gene cluster. BafA is sufficient to induce the hypoxia-locked colony morphology and biofilm architecture in A. fumigatus Analysis across a large population of A. fumigatus isolates identified a larger family of baf genes, all of which have the capacity to modulate hyphal architecture, biofilm development, and hypoxic growth. Furthermore, introduction of A. fumigatusbafA into A. niger is sufficient to generate the hypoxia-locked colony morphology, biofilm architecture, and increased hypoxic growth. Together, these data indicate the potential broad impacts of this previously uncharacterized family of small genes to modulate biofilm architecture and function in clinical and industrial settings.IMPORTANCE The manipulation of microbial biofilms in industrial and clinical applications remains a difficult task. The problem is particularly acute with regard to filamentous fungal biofilms for which molecular mechanisms of biofilm formation, maintenance, and function are only just being elucidated. Here, we describe a family of small genes heterogeneously expressed across Aspergillus fumigatus strains that are capable of modifying colony biofilm morphology and microscopic hyphal architecture. Specifically, these genes are implicated in the formation of a hypoxia-locked colony morphotype that is associated with increased virulence of A. fumigatus Synthetic introduction of these gene family members, here referred to as biofilm architecture factors, in both A. fumigatus and A. niger additionally modulates low oxygen growth and surface adherence. Thus, these genes are candidates for genetic manipulation of biofilm development in aspergilli.

Differential effects of the cystic fibrosis lung inflammatory environment on mesenchymal stromal cells

Growing evidence demonstrates that human mesenchymal stromal cells (MSCs) modify their in vivo anti-inflammatory actions depending on the specific inflammatory environment encountered. Understanding this better is crucial to refine MSC-based cell therapies for lung and other diseases. Using acute exacerbations of cystic fibrosis (CF) lung disease as a model, the effects of ex vivo MSC exposure to clinical bronchoalveolar lavage fluid (BALF) samples, as a surrogate for the in vivo clinical lung environment, on MSC viability, gene expression, secreted cytokines, and mitochondrial function were compared with effects of BALF collected from healthy volunteers. CF BALF samples that cultured positive for Aspergillus sp. (Asp) induced rapid MSC death, usually within several hours of exposure. Further analyses suggested the fungal toxin gliotoxin as a potential mediator contributing to CF BALF-induced MSC death. RNA sequencing analyses of MSCs exposed to either Asp+ or Asp- CF BALF samples identified a number of differentially expressed transcripts, including those involved in interferon signaling, antimicrobial gene expression, and cell death. Toxicity did not correlate with bacterial lung infections. These results suggest that the potential use of MSC-based cell therapies for CF or other lung diseases may not be warranted in the presence of Aspergillus.

MDA5 Is an Essential Sensor of a Pathogen-Associated Molecular Pattern Associated with Vitality That Is Necessary for Host Resistance against Aspergillus fumigatus

RIG-I-like receptors (RLR) are cytosolic RNA sensors that signal through the MAVS adaptor to activate IFN responses against viruses. Whether the RLR family has broader effects on host immunity against other pathogen families remains to be fully explored. In this study, we demonstrate that MDA5/MAVS signaling was essential for host resistance against pulmonary Aspergillus fumigatus challenge through the regulation of antifungal leukocyte responses in mice. Activation of MDA5/MAVS signaling was driven by dsRNA from live A. fumigatus serving as a key vitality-sensing pattern recognition receptor. Interestingly, induction of type I IFNs after A. fumigatus challenge was only partially dependent on MDA5/MAVS signaling, whereas type III IFN expression was entirely dependent on MDA5/MAVS signaling. Ultimately, type I and III IFN signaling drove the expression of CXCL10. Furthermore, the MDA5/MAVS-dependent IFN response was critical for the induction of optimal antifungal neutrophil killing of A. fumigatus spores. In conclusion, our data broaden the role of the RLR family to include a role in regulating antifungal immunity against A. fumigatus.

Aspergillus fumigatus Hsp90 interacts with the main components of the cell wall integrity pathway and cooperates in heat shock and cell wall stress adaptation

The initiation of Aspergillus fumigatus infection occurs via dormant conidia deposition into the airways. Therefore, conidial germination and subsequent hyphal extension and growth occur in a sustained heat shock (HS) environment promoted by the host. The cell wall integrity pathway (CWIP) and the essential eukaryotic chaperone Hsp90 are critical for fungi to survive HS. Although A. fumigatus is a thermophilic fungus, the mechanisms underpinning the HS response are not thoroughly described and important to define its role in pathogenesis, virulence and antifungal drug responses. Here, we investigate the contribution of the CWIP in A. fumigatus thermotolerance. We observed that the CWIP components PkcA, MpkA and RlmA are Hsp90 clients and that a PkcA^G579R mutation abolishes this interaction. PkcA^G579R also abolishes MpkA activation in the short-term response to HS. Biochemical and biophysical analyses indicated that Hsp90 is a dimeric functional ATPase, which has a higher affinity for ADP than ATP and prevents MpkA aggregation in vitro. Our data suggest that the CWIP is constitutively required for A. fumigatus to cope with the temperature increase found in the mammalian lung environment, emphasising the importance of this pathway in supporting thermotolerance and cell wall integrity.

Platelets are critical for survival and tissue integrity during murine pulmonary Aspergillus fumigatus infection

Beyond their canonical roles in hemostasis and thrombosis, platelets function in the innate immune response by interacting directly with pathogens and by regulating the recruitment and activation of immune effector cells. Thrombocytopenia often coincides with neutropenia in patients with hematologic malignancies and in allogeneic hematopoietic cell transplant recipients, patient groups at high risk for invasive fungal infections. While neutropenia is well established as a major clinical risk factor for invasive fungal infections, the role of platelets in host defense against human fungal pathogens remains understudied. Here, we examined the role of platelets in murine Aspergillus fumigatus infection using two complementary approaches to induce thrombocytopenia without concurrent neutropenia. Thrombocytopenic mice were highly susceptible to A. fumigatus challenge and rapidly succumbed to infection. Although platelets regulated early conidial phagocytosis by neutrophils in a spleen tyrosine kinase (Syk)-dependent manner, platelet-regulated conidial phagocytosis was dispensable for host survival. Instead, our data indicated that platelets primarily function to maintain hemostasis and lung integrity in response to exposed fungal antigens, since thrombocytopenic mice exhibited severe hemorrhage into the airways in response to fungal challenge in the absence of overt angioinvasion. Challenge with swollen, heat-killed, conidia was lethal in thrombocytopenic hosts and could be reversed by platelet transfusion, consistent with the model that fungus-induced inflammation in platelet-depleted mice was sufficient to induce lethal hemorrhage. These data provide new insights into the role of platelets in the anti-Aspergillus host response and expand their role to host defense against filamentous molds.

Aspergillus fumigatus is a ubiquitous environmental mold that forms airborne spores, termed conidia. When inhaled by immune compromised individuals, A. fumigatus conidia can germinate into tissue-invasive hyphae and cause invasive aspergillosis, a major cause of infectious morbidity and mortality in patients with leukemia and in bone marrow transplant recipients. Although a low platelet count has been identified as a risk factor for clinical outcomes in patients with invasive aspergillosis, the precise role of platelets in the anti-fungal host response remains poorly understood. Here, we report an essential requirement for platelets in anti-Aspergillus host defence in a mouse model of fungal pneumonia. Although platelets play a role in activating the innate immune system after infection, they are critical for preventing lethal hemorrhage after A. fumigatus challenge. Our findings raise the question as to whether platelets can be used as a basis for therapeutic strategies in vulnerable patient populations.

Mda5/MAVS are essential for host resistance against Aspergillus fumigatus

Type I and III interferons act as important activators of antifungal neutrophil response in the lungs. RIG-I like receptors (RLR) are cytosolic RNA sensors that signal through the MAVS adaptor in order to activate interferon responses against viruses. Whether this pattern-recognition receptor family has broader effects on host immunity against other pathogen families remains to be fully explored. Herein we demonstrate that Mda5/MAVS signaling was essential for host resistance against pulmonary Aspergillus fumigatus challenge through the regulation of antifungal leukocyte responses. Interesting, induction of type I interferons after A. fumigatus challenge was only partially dependent on Mda5/MAVS signaling, while type III interferon expression was entirely dependent on Mda5/MAVS signaling. Ultimately, type I and III interferon signaling drove the expression of CXCL10, which is critical in resistance against invasive aspergillosis in transplant patients. Importantly, we found that polymorphisms in Ifih1 and Mavs were associated with the incidence of invasive pulmonary aspergillosis in a human HSCT cohort. Moreover, polymorphisms in the Ifih1 gene alter the inflammatory response induced in patients with invasive pulmonary aspergillosis, which include interferon-responsive chemokines. In conclusion, our data broaden the role of the RLR family in the regulating innate immunity during infection to include a role in immunity against Aspergillus fumigatus in both mice and humans.

Experimental evolution identifies an IL-1α dependent host response against Aspergillus fumigatus regulated by the fungal SakA-HOG1 pathway

Fungal infections are emerging global health threats, costing the United States $7.2 billion annually in health care costs, with Aspergillus fumigatus accounting for $1.2 billion of those costs, the highest for a single fungal species and tied with Candida spp. infections. A. fumigatus has an inexcusably high mortality rate of 40–90%, disproportionately affecting those with compromised immune systems or impaired pulmonary clearance disorders such as CF and COPD. It is critical that the mechanisms behind A. fumigatus pathogenesis is elucidated. In this study, we explore A. fumigatus divergent pathologies amongst multiple strains which induce distinctly different host immune responses based on germination rate. In vivo and in vitro germination of fungal conidia segregate into two distinct groups of “fast” germinators, which elicit an IL-1α dependent host response, and “slow” germinators, which invoke a type I and III interferon dominant response. Here we use experimental evolution to find that the SakA-HOG1 stress response pathway regulates fungal growth in the lung, and serves as a switch for the induction of IL-1α dominant defense. This study demonstrates a need for revision of clinical protocols for treatment of A. fumigatus based on the strain and type of inflammation on-going in patients with A. fumigatus infections as health care standards move toward individualized patient care and antifungal stewardship.

The negative cofactor 2 complex is a key regulator of drug resistance in Aspergillus fumigatus

The frequency of antifungal resistance, particularly to the azole class of ergosterol biosynthetic inhibitors, is a growing global health problem. Survival rates for those infected with resistant isolates are exceptionally low. Beyond modification of the drug target, our understanding of the molecular basis of azole resistance in the fungal pathogen Aspergillus fumigatus is limited. We reasoned that clinically relevant antifungal resistance could derive from transcriptional rewiring, promoting drug resistance without concomitant reductions in pathogenicity. Here we report a genome-wide annotation of transcriptional regulators in A. fumigatus and construction of a library of 484 transcription factor null mutants. We identify 12 regulators that have a demonstrable role in itraconazole susceptibility and show that loss of the negative cofactor 2 complex leads to resistance, not only to the azoles but also the salvage therapeutics amphotericin B and terbinafine without significantly affecting pathogenicity.

Resistance to primary treatments of invasive aspergillosis is growing. Here, the authors generate a knockout library for 484 transcription factors in Aspergillus fumigatus, and show that loss of the NCT complex leads to cross-resistance to all primary and some salvage therapeutics without affecting pathogenicity.

Fungal biofilm morphology impacts hypoxia fitness and disease progression

Microbial populations form intricate macroscopic colonies with diverse morphologies whose functions remain to be fully understood. Despite fungal colonies isolated from environmental and clinical samples revealing abundant intraspecies morphological diversity, it is unclear how this diversity affects fungal fitness and disease progression. Here we observe a notable effect of oxygen tension on the macroscopic and biofilm morphotypes of the human fungal pathogen Aspergillus fumigatus. A hypoxia-typic morphotype is generated through the expression of a subtelomeric gene cluster containing genes that alter the hyphal surface and perturb interhyphal interactions to disrupt in vivo biofilm and infection site morphologies. Consequently, this morphotype leads to increased host inflammation, rapid disease progression and mortality in a murine model of invasive aspergillosis. Taken together, these data suggest that filamentous fungal biofilm morphology affects fungal-host interactions and should be taken into consideration when assessing virulence and host disease progression of an isolated strain.

Natamycin and Voriconazole Exhibit Synergistic Interactions with Nonantifungal Ophthalmic Agents against Fusarium Species Ocular Isolates

The in vitro activities of two antifungal drugs in combination with four nonantifungal ophthalmic agents were evaluated using a broth microdilution method and a collection of eight Fusarium ocular isolates that exhibited resistance to both natamycin (MICs, 14 to 32 μg/ml) and voriconazole (MICs, 4 to >128 μg/ml). Synergistic and indifferent interactions were observed for natamycin and 5-fluorouracil and natamycin with timolol dependent on the Fusarium isolate tested. Isolate-dependent synergistic and indifferent interactions were also observed for natamycin with EDTA and natamycin with dorzolamide. Synergistic or indifferent interactions were observed for voriconazole with timolol and voriconazole with 5-fluorouracil depending on Fusarium isolate. Taken together, these data suggest that commonly used ophthalmic agents enhance the in vitro activity of antifungal drugs against drug-recalcitrant ocular fungal pathogens.

An Ssd1 Homolog Impacts Trehalose and Chitin Biosynthesis and Contributes to Virulence in Aspergillus fumigatus

Regulation of fungal cell wall biosynthesis is critical to maintain cell wall integrity in dynamic fungal infection microenvironments. Genes involved in this response that impact fungal fitness and host immune responses remain to be fully defined. In this study, we observed that a yeast ssd1 homolog, ssdA, in the filamentous fungus Aspergillus fumigatus is involved in trehalose and cell wall homeostasis. An ssdA null mutant strain exhibited an increase in trehalose levels and a reduction in fungal colony growth rate. In contrast, overexpression of ssdA perturbed trehalose biosynthesis and reduced germination of conidia. The ssdA null mutant strain was more resistant to cell wall-perturbing agents, while overexpression of ssdA increased sensitivity. Overexpression of ssdA significantly increased chitin levels, and both loss and overexpression of ssdA altered subcellular localization of the class V chitin synthase CsmA. Strikingly, overexpression of ssdA abolished adherence to abiotic surfaces and severely attenuated the virulence of A. fumigatus in a murine model of invasive pulmonary aspergillosis. Despite the severe in vitro fitness defects observed upon loss of ssdA, neither surface adherence nor murine survival was impacted. In conclusion, A. fumigatus SsdA plays a critical role in cell wall homeostasis impacting A. fumigatus-host interactions.IMPORTANCE The incidence of life-threatening infections caused by the filamentous fungus Aspergillus fumigatus is increasing along with an increase in the number of fungal strains resistant to contemporary antifungal therapies. The fungal cell wall and the associated carbohydrates required for its synthesis and maintenance are attractive drug targets given that many genes encoding proteins involved in cell wall biosynthesis and integrity are absent in humans. Importantly, genes and associated cell wall biosynthesis and homeostasis regulatory pathways remain to be fully defined in A. fumigatus In this report, we identify SsdA as an important component of trehalose and fungal cell wall biosynthesis in A. fumigatus that consequently impacts the host immune response and fungal virulence in animal models of infection.

HIF-1α is essential for T cell suppression by murine LP-BM5 retrovirus infection-augmented Monocytic Myeloid Derived Suppressor Cells

Compared to naïve mice, monocytic myeloid-derived suppressor cells (M-MDSCs) increase in number, and suppressive efficiency, after infection with the immunodeficiency causing mouse retrovirus, LP-BM5. In ex vivo suppression assays, M-MDSCs, enriched from spleens of LP-BM5 infected mice inhibited both T- and as a novel finding, B-cell responsiveness. For M-MDSC suppression of T-cell proliferation and IFN-gamma production, we previously reported the mechanism to be almost completely iNOS/NO dependent. In contrast, the suppression of B-cell proliferation and IL-10 secretion by predominantly T2-B cells was only ~50% iNOS/NO dependent, with V-domain Ig suppressor of T cell activation (VISTA), superoxide and peroxynitrite also playing substantial effector roles. In light of recent reports on the Hypoxia-inducible transcription factor 1, α subunit (HIF-1α) regulating the function and differentiation of MDSCs, and reports of its role as a principal regulator of NO production, we describe here the characterization of M-MDSCs from myeloid-specific HIF-1α deficient, vs. w.t., B6 mice. As early as 5 weeks post infection, enriched M-MDSC preparations from HIF-1α deficient mice were significantly impaired functionally, in some cases with a total inability, to suppress in vitro T cell responsiveness. However, these same M-MDSC preparations suppressed B-cell responsiveness at a level very similar to that of control w.t. M-MDSCs. Experiments are in progress to further understand the role HIF-1α plays in the impairment of M-MDSC ex vivo suppression of T cell proliferation and function, including a possibility that HIF-1α regulates iNOS/NO production in M-MDSCs only when in the process of suppressing T- but not B-cell targets.

Characterizing the Pathogenic, Genomic, and Chemical Traits of Aspergillus fischeri, a Close Relative of the Major Human Fungal Pathogen Aspergillus fumigatus

Aspergillus fischeri is closely related to Aspergillus fumigatus, the major cause of invasive mold infections. Even though A. fischeri is commonly found in diverse environments, including hospitals, it rarely causes invasive disease. Why A. fischeri causes less human disease than A. fumigatus is unclear. A comparison of A. fischeri and A. fumigatus for pathogenic, genomic, and secondary metabolic traits revealed multiple differences in pathogenesis-related phenotypes. We observed that A. fischeri NRRL 181 is less virulent than A. fumigatus strain CEA10 in multiple animal models of disease, grows slower in low-oxygen environments, and is more sensitive to oxidative stress. Strikingly, the observed differences for some traits are of the same order of magnitude as those previously reported between A. fumigatus strains. In contrast, similar to what has previously been reported, the two species exhibit high genomic similarity; ∼90% of the A. fumigatus proteome is conserved in A. fischeri, including 48/49 genes known to be involved in A. fumigatus virulence. However, only 10/33 A. fumigatus biosynthetic gene clusters (BGCs) likely involved in secondary metabolite production are conserved in A. fischeri and only 13/48 A. fischeri BGCs are conserved in A. fumigatus Detailed chemical characterization of A. fischeri cultures grown on multiple substrates identified multiple secondary metabolites, including two new compounds and one never before isolated as a natural product. Additionally, an A. fischeri deletion mutant of laeA, a master regulator of secondary metabolism, produced fewer secondary metabolites and in lower quantities, suggesting that regulation of secondary metabolism is at least partially conserved. These results suggest that the nonpathogenic A. fischeri possesses many of the genes important for A. fumigatus pathogenicity but is divergent with respect to its ability to thrive under host-relevant conditions and its secondary metabolism.IMPORTANCEAspergillus fumigatus is the primary cause of aspergillosis, a devastating ensemble of diseases associated with severe morbidity and mortality worldwide. A. fischeri is a close relative of A. fumigatus but is not generally observed to cause human disease. To gain insights into the underlying causes of this remarkable difference in pathogenicity, we compared two representative strains (one from each species) for a range of pathogenesis-relevant biological and chemical characteristics. We found that disease progression in multiple A. fischeri mouse models was slower and caused less mortality than A. fumigatus Remarkably, the observed differences between A. fischeri and A. fumigatus strains examined here closely resembled those previously described for two commonly studied A. fumigatus strains, AF293 and CEA10. A. fischeri and A. fumigatus exhibited different growth profiles when placed in a range of stress-inducing conditions encountered during infection, such as low levels of oxygen and the presence of chemicals that induce the production of reactive oxygen species. We also found that the vast majority of A. fumigatus genes known to be involved in virulence are conserved in A. fischeri, whereas the two species differ significantly in their secondary metabolic pathways. These similarities and differences that we report here are the first step toward understanding the evolutionary origin of a major fungal pathogen.

Protein Kinase A and High-Osmolarity Glycerol Response Pathways Cooperatively Control Cell Wall Carbohydrate Mobilization in Aspergillus fumigatus

Aspergillus fumigatus mitogen-activated protein kinases (MAPKs) are involved in maintaining the normal morphology of the cell wall and providing resistance against cell wall-damaging agents. Upon cell wall stress, cell wall-related sugars need to be synthesized from carbohydrate storage compounds. Here we show that this process is dependent on cAMP-dependent protein kinase A (PKA) activity and regulated by the high-osmolarity glycerol response (HOG) MAPKs SakA and MpkC. These protein kinases are necessary for normal accumulation/degradation of trehalose and glycogen, and the lack of these genes reduces glucose uptake and glycogen synthesis. Alterations in glycogen synthesis were observed for the sakA and mpkC deletion mutants, which also displayed alterations in carbohydrate exposure on the cell wall. Carbohydrate mobilization is controlled by SakA interaction with PkaC1 and PkaR, suggesting a putative mechanism where the PkaR regulatory subunit leaves the complex and releases the SakA-PkaC1 complex for activation of enzymes involved in carbohydrate mobilization. This work reveals the communication between the HOG and PKA pathways for carbohydrate mobilization for cell wall construction.IMPORTANCEAspergillus fumigatus is an opportunistic human pathogen causing allergic reactions or systemic infections such as invasive pulmonary aspergillosis, especially in immunocompromised patients. The fungal cell wall is the main component responsible for recognition by the immune system, due to the specific composition of polysaccharide carbohydrates exposed on the surface of the fungal cell wall called pathogen-associated molecular patterns (PAMPs). Key enzymes in the fungal cell wall biosynthesis are a good target for fungal drug development. This report elucidates the cooperation between the HOG and PKA pathways in the mobilization of carbohydrates for fungal cell wall biosynthesis. We suggest that the reduced mobilization of simple sugars causes defects in the structure of the fungal cell wall. In summary, we propose that SakA is important for PKA activity, therefore regulating the availability and mobilization of monosaccharides for fungal cell wall biosynthesis during cell wall damage and the osmotic stress response.

Beta-glucan-induced inflammatory monocytes mediate antitumor efficacy in the murine lung

β-Glucan is a naturally occurring glucose polysaccharide with immunostimulatory activity in both infection and malignancy. β-Glucan's antitumor effects have been attributed to the enhancement of complement receptor 3-dependent cellular cytotoxicity, as well as modulation of suppressive and stimulatory myeloid subsets, which in turn enhances antitumor T cell immunity. In the present study, we demonstrate antitumor efficacy of yeast-derived β-glucan particles (YGP) in a model of metastatic-like melanoma in the lung, through a mechanism that is independent of previously reported β-glucan-mediated antitumor pathways. Notably, efficacy is independent of adaptive immunity, but requires inflammatory monocytes. YGP-activated monocytes mediated direct cytotoxicity against tumor cells in vitro, and systemic YGP treatment upregulated inflammatory mediators, including TNFα, M-CSF, and CCL2, in the lungs. Collectively, these studies identify a novel role for inflammatory monocytes in β-glucan-mediated antitumor efficacy, and expand the understanding of how this immunomodulator can be used to generate beneficial immune responses against metastatic disease.

Response to Comment on "Sterilizing immunity in the lung relies on targeting fungal apoptosis-like programmed cell death"

Aouacheria et al question the interpretation of contemporary assays to monitor programmed cell death with apoptosis-like features (A-PCD) in Aspergillus fumigatus Although our study focuses on fungal A-PCD for host immune surveillance and infectious outcomes, the experimental approach incorporates multiple independent A-PCD markers and genetic manipulations based on fungal rather than mammalian orthologs to circumvent the limitations associated with any single approach.

Unique metabolic activation of adipose tissue macrophages in obesity promotes inflammatory responses

AIMS/HYPOTHESIS

Recent studies have identified intracellular metabolism as a fundamental determinant of macrophage function. In obesity, proinflammatory macrophages accumulate in adipose tissue and trigger chronic low-grade inflammation, that promotes the development of systemic insulin resistance, yet changes in their intracellular energy metabolism are currently unknown. We therefore set out to study metabolic signatures of adipose tissue macrophages (ATMs) in lean and obese conditions.

METHODS

F4/80-positive ATMs were isolated from obese vs lean mice. High-fat feeding of wild-type mice and myeloid-specific Hif1α^-/- mice was used to examine the role of hypoxia-inducible factor-1α (HIF-1α) in ATMs part of obese adipose tissue. In vitro, bone marrow-derived macrophages were co-cultured with adipose tissue explants to examine adipose tissue-induced changes in macrophage phenotypes. Transcriptome analysis, real-time flux measurements, ELISA and several other approaches were used to determine the metabolic signatures and inflammatory status of macrophages. In addition, various metabolic routes were inhibited to determine their relevance for cytokine production.

RESULTS

Transcriptome analysis and extracellular flux measurements of mouse ATMs revealed unique metabolic rewiring in obesity characterised by both increased glycolysis and oxidative phosphorylation. Similar metabolic activation of CD14⁺ cells in obese individuals was associated with diabetes outcome. These changes were not observed in peritoneal macrophages from obese vs lean mice and did not resemble metabolic rewiring in M1-primed macrophages. Instead, metabolic activation of macrophages was dose-dependently induced by a set of adipose tissue-derived factors that could not be reduced to leptin or lactate. Using metabolic inhibitors, we identified various metabolic routes, including fatty acid oxidation, glycolysis and glutaminolysis, that contributed to cytokine release by ATMs in lean adipose tissue. Glycolysis appeared to be the main contributor to the proinflammatory trait of macrophages in obese adipose tissue. HIF-1α, a key regulator of glycolysis, nonetheless appeared to play no critical role in proinflammatory activation of ATMs during early stages of obesity.

CONCLUSIONS/INTERPRETATION

Our results reveal unique metabolic activation of ATMs in obesity that promotes inflammatory cytokine release. Further understanding of metabolic programming in ATMs will most likely lead to novel therapeutic targets to curtail inflammatory responses in obesity.

DATA AVAILABILITY

Microarray data of ATMs isolated from obese or lean mice have been submitted to the Gene Expression Omnibus (accession no. GSE84000).

Sterilizing immunity in the lung relies on targeting fungal apoptosis-like programmed cell death

Humans inhale mold conidia daily and typically experience lifelong asymptomatic clearance. Conidial germination into tissue-invasive hyphae can occur in individuals with defects in myeloid function, although the mechanism of myeloid cell-mediated immune surveillance remains unclear. By monitoring fungal physiology in vivo, we demonstrate that lung neutrophils trigger programmed cell death with apoptosis-like features in Aspergillus fumigatus conidia, the most prevalent human mold pathogen. An antiapoptotic protein, AfBIR1, opposes this process by inhibiting fungal caspase activation and DNA fragmentation in the murine lung. Genetic and pharmacologic studies indicate that AfBIR1 expression and activity underlie conidial susceptibility to NADPH (reduced form of nicotinamide adenine dinucleotide phosphate) oxidase-dependent killing and, in turn, host susceptibility to invasive aspergillosis. Immune surveillance exploits a fungal apoptosis-like programmed cell death pathway to maintain sterilizing immunity in the lung.

Host-Derived Leukotriene B4 Is Critical for Resistance against Invasive Pulmonary Aspergillosis

Aspergillus fumigatus is a mold that causes severe pulmonary infections. Our knowledge of how immune competent hosts maintain control of fungal infections while constantly being exposed to fungi is rapidly emerging. It is known that timely neutrophil recruitment to and activation in the lungs is critical to the host defense against development of invasive pulmonary aspergillosis, but the inflammatory sequelae necessary remains to be fully defined. Here, we show that 5-Lipoxygenase (5-LO) and Leukotriene B₄ (LTB₄) are critical for leukocyte recruitment and resistance to pulmonary A. fumigatus challenge in a fungal-strain-dependent manner. 5-LO activity was needed in radiosensitive cells for an optimal anti-fungal response and in vivo LTB₄ production was at least partially dependent on myeloid-derived hypoxia inducible factor-1α. Overall, this study reveals a role for host-derived leukotriene synthesis in innate immunity to A. fumigatus.

Overview of carbon and nitrogen catabolite metabolism in the virulence of human pathogenic fungi

It is estimated that fungal infections, caused most commonly by Candida albicans, Aspergillus fumigatus and Cryptococcus neoformans, result in more deaths annually than malaria or tuberculosis. It has long been hypothesized the fungal metabolism plays a critical role in virulence though specific nutrient sources utilized by human pathogenic fungi in vivo has remained enigmatic. However, the metabolic utilisation of preferred carbon and nitrogen sources, encountered in a host niche-dependent manner, is known as carbon catabolite and nitrogen catabolite repression (CCR, NCR), and has been shown to be important for virulence. Several sensory and uptake systems exist, including carbon and nitrogen source-specific sensors and transporters, that allow scavenging of preferred nutrient sources. Subsequent metabolic utilisation is governed by transcription factors, whose functions and essentiality differ between fungal species. Furthermore, additional factors exist that contribute to the implementation of CCR and NCR. The role of the CCR and NCR-related factors in virulence varies greatly between fungal species and a substantial gap in knowledge exists regarding specific pathways. Further elucidation of carbon and nitrogen metabolism mechanisms is therefore required in a fungal species- and animal model-specific manner in order to screen for targets that are potential candidates for anti-fungal drug development.

Modulation of Immune Signaling and Metabolism Highlights Host and Fungal Transcriptional Responses in Mouse Models of Invasive Pulmonary Aspergillosis

Incidences of invasive pulmonary aspergillosis, an infection caused predominantly by Aspergillus fumigatus, have increased due to the growing number of immunocompromised individuals. While A. fumigatus is reliant upon deficiencies in the host to facilitate invasive disease, the distinct mechanisms that govern the host-pathogen interaction remain enigmatic, particularly in the context of distinct immune modulating therapies. To gain insights into these mechanisms, RNA-Seq technology was utilized to sequence RNA derived from lungs of 2 clinically relevant, but immunologically distinct murine models of IPA on days 2 and 3 post inoculation when infection is established and active disease present. Our findings identify notable differences in host gene expression between the chemotherapeutic and steroid models at the interface of immunity and metabolism. RT-qPCR verified model specific and nonspecific expression of 23 immune-associated genes. Deep sequencing facilitated identification of highly expressed fungal genes. We utilized sequence similarity and gene expression to categorize the A. fumigatus putative in vivo secretome. RT-qPCR suggests model specific gene expression for nine putative fungal secreted proteins. Our analysis identifies contrasting responses by the host and fungus from day 2 to 3 between the two models. These differences may help tailor the identification, development, and deployment of host- and/or fungal-targeted therapeutics.

Interleukin 1α Is Critical for Resistance against Highly Virulent Aspergillus fumigatus Isolates

Heterogeneity among Aspergillus fumigatus isolates results in unique virulence potential and inflammatory responses. How these isolates drive specific immune responses and how this affects fungally induced lung damage and disease outcome are unresolved. We demonstrate that the highly virulent CEA10 strain is able to rapidly germinate within the immunocompetent lung environment, inducing greater lung damage, vascular leakage, and interleukin 1α (IL-1α) release than the low-virulence Af293 strain, which germinates with a lower frequency in this environment. Importantly, the clearance of CEA10 was consequently dependent on IL-1α, in contrast to Af293. The release of IL-1α occurred by a caspase 1/11- and P2XR7-independent mechanism but was dependent on calpain activity. Our finding that early fungal conidium germination drives greater lung damage and IL-1α-dependent inflammation is supported by three independent experimental lines. First, pregermination of Af293 prior to in vivo challenge drives greater lung damage and an IL-1α-dependent neutrophil response. Second, the more virulent EVOL20 strain, derived from Af293, is able to germinate in the airways, leading to enhanced lung damage and IL-1α-dependent inflammation and fungal clearance. Third, primary environmental A. fumigatus isolates that rapidly germinate under airway conditions follow the same trend toward IL-1α dependency. Our data support the hypothesis that A. fumigatus phenotypic variation significantly contributes to disease outcomes.

Sniffing out the hypoxia volatile metabolic signature of Aspergillus fumigatus

Invasive aspergillosis (IA) is a life-threatening infectious disease caused by fungi from the genus Aspergillus, with an associated mortality as high as 90% in certain populations. IA-associated pulmonary lesions are characteristically depleted in oxygen relative to normal lung tissue, and it has been shown that the most common causal agent of IA, Aspergillus fumigatus, must respond to low-oxygen environments for pathogenesis and disease progression. Previous studies have demonstrated marked alterations to the Aspergillus fumigatus transcriptome in response to low-oxygen environments that induce a 'hypoxia response'. Consequently, we hypothesized that these transcriptomic changes would alter the volatile metabolome and generate a volatile hypoxia signature. In the present study, we analyzed the volatile molecules produced by A. fumigatus in both oxygen replete (normoxia) and depleted (hypoxia) environments via headspace solid-phase micro-extraction coupled to two-dimensional gas chromatography-time-of-flight mass spectrometry. Using the machine learning algorithm random forest, we identified 19 volatile molecules that were discriminatory between the four growth conditions assessed in this study (i.e., early hypoxia (1 h), late hypoxia (8 h), early normoxia (1 h), and late normoxia (8 h)), as well as a set of 19 that were discriminatory between late hypoxia cultures and all other growth conditions in aggregate. Nine molecules were common to both comparisons, while the remaining 20 were specific to only one of two. We assigned putative identifications to 13 molecules, of which six were most highly abundant in late hypoxia cultures. Previously acquired transcriptomic data identified putative biochemical pathways induced in hypoxia conditions that plausibly account for the production of a subset of these molecules, including 2,3-butanedione and 3-hydroxy-2-butanone. These two molecules may represent a novel hypoxia fitness pathway in A. fumigatus, and could be useful in the detection of hypoxia-associated A. fumigatus lesions that develop in established IA infections.

Filamentous fungal carbon catabolite repression supports metabolic plasticity and stress responses essential for disease progression

Aspergillus fumigatus is responsible for a disproportionate number of invasive mycosis cases relative to other common filamentous fungi. While many fungal factors critical for infection establishment are known, genes essential for disease persistence and progression are ill defined. We propose that fungal factors that promote navigation of the rapidly changing nutrient and structural landscape characteristic of disease progression represent untapped clinically relevant therapeutic targets. To this end, we find that A. fumigatus requires a carbon catabolite repression (CCR) mediated genetic network to support in vivo fungal fitness and disease progression. While CCR as mediated by the transcriptional repressor CreA is not required for pulmonary infection establishment, loss of CCR inhibits fungal metabolic plasticity and the ability to thrive in the dynamic infection microenvironment. Our results suggest a model whereby CCR in an environmental filamentous fungus is dispensable for initiation of pulmonary infection but essential for infection maintenance and disease progression. Conceptually, we argue these data provide a foundation for additional studies on fungal factors required to support fungal fitness and disease progression and term such genes and factors, DPFs (disease progression factors).

Regulation of Sterol Biosynthesis in the Human Fungal Pathogen Aspergillus fumigatus: Opportunities for Therapeutic Development

Sterols are a major component of eukaryotic cell membranes. For human fungal infections caused by the filamentous fungus Aspergillus fumigatus, antifungal drugs that target sterol biosynthesis and/or function remain the standard of care. Yet, an understanding of A. fumigatus sterol biosynthesis regulatory mechanisms remains an under developed therapeutic target. The critical role of sterol biosynthesis regulation and its interactions with clinically relevant azole drugs is highlighted by the basic helix loop helix (bHLH) class of transcription factors known as Sterol Regulatory Element Binding Proteins (SREBPs). SREBPs regulate transcription of key ergosterol biosynthesis genes in fungi including A. fumigatus. In addition, other emerging regulatory pathways and target genes involved in sterol biosynthesis and drug interactions provide additional opportunities including the unfolded protein response, iron responsive transcriptional networks, and chaperone proteins such as Hsp90. Thus, targeting molecular pathways critical for sterol biosynthesis regulation presents an opportunity to improve therapeutic options for the collection of diseases termed aspergillosis. This mini-review summarizes our current understanding of sterol biosynthesis regulation with a focus on mechanisms of transcriptional regulation by the SREBP family of transcription factors.

Functional and Genomic Architecture of Borrelia burgdorferi-Induced Cytokine Responses in Humans

Despite the importance of immune variation for the symptoms and outcome of Lyme disease, the factors influencing cytokine production during infection with the causal pathogen Borrelia burgdorferi remain poorly understood. Borrelia infection-induced monocyte- and T cell-derived cytokines were profiled in peripheral blood from two healthy human cohorts of Western Europeans from the Human Functional Genomics Project. Both non-genetic and genetic host factors were found to influence Borrelia-induced cytokine responses. Age strongly impaired IL-22 responses, and genetic studies identified several independent QTLs that impact Borrelia-induced cytokine production. Genetic, transcriptomic, and functional validation studies revealed an important role for HIF-1α-mediated glycolysis in the cytokine response to Borrelia. HIF-1α pathway activation and increase in glycolysis-derived lactate was confirmed in Lyme disease patients. In conclusion, functional genomics approaches reveal the architecture of cytokine production induced by Borrelia infection of human primary leukocytes and suggest a connection between cellular glucose metabolism and Borrelia-induced cytokine production.

Aspergillus fumigatus virulence through the lens of transcription factors

Invasive aspergillosis (IA), most commonly caused by the filamentous fungus Aspergillus fumigatus, occurs in immune compromised individuals. The ability of A. fumigatus to proliferate in a multitude of environments is hypothesized to contribute to its pathogenicity and virulence. Transcription factors (TF) have long been recognized as critical proteins for fungal pathogenicity, as many are known to play important roles in the transcriptional regulation of pathways implicated in virulence. Such pathways include regulation of conidiation and development, adhesion, nutrient acquisition, adaptation to environmental stress, and interactions with the host immune system among others. In both murine and insect models of IA, TF loss of function in A. fumigatus results in cases of hyper- and hypovirulence as determined through host survival, fungal burden, and immune response analyses. Consequently, the study of specific TFs in A. fumigatus has revealed important insights into mechanisms of pathogenicity and virulence. Although in vitro studies have identified virulence-related functions of specific TFs, the full picture of their in vivo functions remain largely enigmatic and an exciting area of current research. Moreover, the vast majority of TFs remain to be characterized and studied in this important human pathogen. Here in this mini-review we provide an overview of selected TFs in A. fumigatus and their contribution to our understanding of this important human pathogen's pathogenicity and virulence.

Aspergillus fumigatus Photobiology Illuminates the Marked Heterogeneity between Isolates

The given strain of Aspergillus fumigatus under study varies across laboratories, ranging from a few widely used "standards," e.g., Af293 or CEA10, to locally acquired isolates that may be unique to one investigator. Since experiments concerning physiology or gene function are seldom replicated by others, i.e., in a different A. fumigatus background, the extent to which behavioral heterogeneity exists within the species is poorly understood. As a proxy for assessing such intraspecies variability, we analyzed the light response of 15 A. fumigatus isolates and observed striking quantitative and qualitative heterogeneity among them. The majority of the isolates fell into one of two seemingly mutually exclusive groups: (i) "photopigmenters" that robustly accumulate hyphal melanin in the light and (ii) "photoconidiators" that induce sporulation in the light. These two distinct responses were both governed by the same upstream blue light receptor, LreA, indicating that a specific protein's contribution can vary in a strain-dependent manner. Indeed, while LreA played no apparent role in regulating cell wall homeostasis in strain Af293, it was essential in that regard in strain CEA10. The manifest heterogeneity in the photoresponses led us to compare the virulence levels of selected isolates in a murine model; remarkably, the virulence did vary greatly, although not in a manner that correlated with their overt light response. Taken together, these data highlight the extent to which isolates of A. fumigatus can vary, with respect to both broad physiological characteristics (e.g., virulence and photoresponse) and specific protein functionality (e.g., LreA-dependent phenotypes).

IMPORTANCE

The current picture of Aspergillus fumigatus biology is akin to a collage, patched together from data obtained from disparate "wild-type" strains. In a systematic assessment of 15 A. fumigatus isolates, we show that the species is highly heterogeneous with respect to its light response and virulence. Whereas some isolates accumulate pigments in light as previously reported with strain Af293, most induce sporulation which had not been previously observed. Other photoresponsive behaviors are also nonuniform, and phenotypes of identical gene deletants vary in a background-dependent manner. Moreover, the virulence of several selected isolates is highly variable in a mouse model and apparently does not track with any observed light response. Cumulatively, this work illuminates the fact that data obtained with a single A. fumigatus isolate are not necessarily predictive of the species as whole. Accordingly, researchers should be vigilant when making conclusions about their own work or when interpreting data from the literature.

New advances in invasive aspergillosis immunobiology leading the way towards personalized therapeutic approaches

Invasive aspergillosis (IA) remains a devastating disease in immune compromised patients despite significant advances in our understanding of fungal virulence and host defense mechanisms. In this review, we summarize important research advances in the fight against IA with particular focus on early events in the interactions between Aspergillus fumigatus and the host that occur in the respiratory tract. Advances in understanding mechanisms of immune effector cell recruitment, antifungal effector mechanisms, and how the dynamic host-fungal interaction alters the local microenvironment to effect outcomes are highlighted. These advances illustrate exciting new therapeutic opportunities, but also emphasize the importance of understanding each unique fungus-host interaction for improving patient outcomes.

Beta-glucan-induced trained innate immunity mediates antitumor efficacy in the mouse lung

The innate arm of the vertebrate immune system has classically been regarded as nonspecific and lacking the capacity for adaptive responses, in contrast to the adaptive arm defined by its robust memory responses. Recent high impact work has demonstrated enhanced non-specific responses of innate immune cells to pathogens and malignancy after priming with the fungal cell wall component beta-glucan, a phenomenon termed trained innate immunity (TII). In a pathogen model, the training effect was mediated by epigenetic modifications underlying a metabolic shift to mTOR- and HIF1α-mediated aerobic glycolysis. To investigate whether these same pathways play a role in non-specific enhancement of antitumor responses, we assessed beta-glucan-mediated anti-tumor efficacy in a mouse model of metastatic melanoma. We observed that systemic pretreatment with a particulate beta-glucan significantly diminished the growth of metastatic-like B16 melanoma in the lungs, but not initial tumor cell engraftment. Further, lungs in beta-glucan treated animals had a robust myeloid immune infiltrate, particularly neutrophils and monocytes. Interestingly, initial studies demonstrate that tumor suppressed HIF1α expression, and beta-glucan pretreatment prevented this suppression. These results are consistent with beta-glucan induction of TII, leading to suppression of cancer growth. By extension, TII may modulate the immunogenic nature of the tumor microenvironment, thereby acting as an adjuvant to enhance the efficacy of existing therapies.

Aspergillus fumigatus strain-specific host response is regulated by glucose sensing

Our knowledge of how Aspergillus fumigatus growth is controlled within the respiratory tract is developing, but still enigmatic. Recruitment of neutrophils and inflammatory monocytes is critical to control fungal germination. We have recently found that the CEA10 and Af293 strains of A. fumigatus induce unique inflammatory leukocyte responses. Here, we aim to understand why these A. fumigatus strains induce different immune responses and how this affects fungal-induced immunopathology and disease outcome. We used an immunocompetent murine model of invasive pulmonary aspergillosis to evaluate the immune response induced by CEA10 and Af293. We observed that CEA10 undergoes significantly greater germination than Af293, which was regulated by its ability to sense glucose through CreA-mediated signaling. Moreover, the CEA10 strain induced greater lung damage, vascular leakage, and inflammation. These findings suggest the inflammatory response to A. fumigatus might be regulated in a stepwise manner in response to the threat posed by the specific A. fumigatus strain. Specifically, the Af293 strain, which swells but fails to germinate, activates the inflammasome leading to IL-1beta-dependent neutrophil recruitment. In contrast, the CEA10 strain is able to germinate efficiently within the lung, leading to necrotic host cell death, IL-1alpha release, and subsequent IL-1alpha dependent neutrophil recruitment. Clinically, our data support the idea that A. fumigatus strain phenotypic variation may significantly contribute to disease outcomes. Understanding why different A. fumigatus strains induce distinct immune pathology can reveal novel immunotherapeutic targets for the treatment of invasive aspergillosis.

Role of Granulocyte-Macrophage Colony-Stimulating Factor Signaling in Regulating Neutrophil Antifungal Activity and the Oxidative Burst During Respiratory Fungal Challenge

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a pleiotropic cytokine that plays a critical role in regulating myeloid cell host defense. In this study, we demonstrated that GM-CSF signaling plays an essential role in antifungal defense against Aspergillus fumigatus. Mice that lack the GM-CSF receptor β chain (GM-CSFRβ) developed invasive hyphal growth and exhibited impaired survival after pulmonary challenge with A. fumigatus conidia. GM-CSFRβ signaling regulated the recruitment of inflammatory monocytes to infected lungs, but not the recruitment of effector neutrophils. Cell-intrinsic GM-CSFRβ signaling mediated neutrophil and inflammatory monocyte antifungal activity, because lung GM-CSFRβ(-/-) leukocytes exhibited impaired conidial killing compared with GM-CSFRβ(+/+) counterparts in mixed bone marrow chimeric mice. GM-CSFRβ(-/-) neutrophils exhibited reduced (hydrogenated) nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity in vivo. Conversely, administration of recombinant GM-CSF enhanced neutrophil NADPH oxidase function, conidiacidal activity, and lung fungal clearance in A. fumigatus-challenged mice. Thus, our study illustrates the functional role of GM-CSFRβ signaling on lung myeloid cell responses against inhaled A. fumigatus conidia and demonstrates a benefit for systemic GM-CSF administration.