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

Aspergillus fumigatus biology, immunopathogenicity and drug resistance

Aspergillus fumigatus is a saprophytic fungus prevalent in the environment and capable of causing severe invasive infection in humans. This organism can use strategies such as molecule masking, immune response manipulation and gene expression alteration to evade host defences. Understanding these mechanisms is essential for developing effective diagnostics and therapies to improve patient outcomes in Aspergillus-related diseases. In this Review, we explore the biology and pathogenesis of A. fumigatus in the context of host biology and disease, highlighting virus-associated pulmonary aspergillosis, a newly identified condition that arises in patients with severe pulmonary viral infections. In the post-pandemic landscape, in which immunotherapy is gaining attention for managing severe infections, we examine the host immune responses that are critical for controlling invasive aspergillosis and how A. fumigatus circumvents these defences. Additionally, we address the emerging issue of azole resistance in A. fumigatus, emphasizing the urgent need for greater understanding in an era marked by increasing antimicrobial resistance. This Review provides timely insights necessary for developing new immunotherapeutic strategies against invasive aspergillosis.

Environmental sensitivity of amyloidogenic motifs in fungal NOD-like receptor-mediated immunity: Molecular and structural insights into amyloid assembly

This study investigates the aggregation behavior of amyloidogenic motifs associated with fungal NOD-like receptor (NLR) proteins, focusing on their sensitivity to various environmental conditions. We aimed to develop a minimal model that explains amyloid aggregation, aligning with in vivo observations and the expected role of these motifs in amyloid-based signaling. The purpose was to understand how changes in physicochemical conditions influence amyloid formation, which is crucial for fungal immune responses and has potential applications in controlling fungal infections. To achieve this, two amyloidogenic motifs, PUASM_N and PUASM_C, derived from the fungus Colletotrichum gloeosporioides, were synthesized and subjected to different conditions that simulate their natural environment. These conditions included varying pH levels, peptide concentrations, and surface adsorption properties. The aggregation kinetics, morphology, and secondary structures of the peptides were analyzed using Thioflavin T (ThT) fluorescence assay, transmission electron microscopy (TEM), and Fourier transform infrared micro-spectroscopy (micro-FTIR). The results showed that PUASM_N aggregates rapidly without a lag phase, forming long, structured fibers. In contrast, PUASM_C aggregates more slowly, with a significant lag phase, forming shorter, irregular fibers. The aggregation of PUASM_C was highly sensitive to environmental factors, such as alkaline pH and surface hydrophobicity, which accelerated its aggregation. PUASM_N, however, displayed consistent aggregation behavior under different conditions. Our findings suggest that minor environmental changes can modulate the functional roles of PUASM peptides, potentially aiding Colletotrichum gloeosporioides in regulating its antipathogenic activity in response to environmental challenges.

The GPCR antagonist PPTN synergizes with caspofungin providing increased fungicidal activity against Aspergillus fumigatus

Fungal pathogens pose a serious threat to human health, with Candida and Aspergillus spp. representing some of the most significant opportunistic invaders. Aspergillus fumigatus causes aspergillosis, one of the most prevalent fungal diseases of humans. There is a limited number of drugs available to combat these infections, and antifungal drug resistance is on the rise. In this manuscript, we show 4-[4-(4-Piperidinyl) phenyl]-7-[4-(-(trifluoromethyl) phenyl]-2-naphthalenecarboxylic acid (PPTN), a highly specific antagonist of the human P2Y14 receptor, is a promising antifungal adjuvant against diverse fungal pathogens. PPTN interacts with caspofungin (CAS), ibrexafungerp, voriconazole (VOR), and amphotericin against A. fumigatus CAS- and VOR-resistant clinical isolates, and also CAS against Candida spp and Cryptococcus neoformans. The combination of PPTN and CAS increases cell death in A. fumigatus. In the model yeast Saccharomyces cerevisiae, heterozygous deletion of genes involved in chromatin remodeling results in PPTN hypersensitivity, and in A. fumigatus, PPTN can have increased fungicidal activity when combined with the histone deacetylase inhibitor trichostatin A and the DNA methyltransferase inhibitor 5-azacytidine. Finally, PPTN has reduced toxicity to human immortalized cell lineages and partially clears A. fumigatus conidia infection in A549 pulmonary epithelial cells. Our results indicate that PPTN is a novel adjuvant antifungal drug against fungal diseases caused by A. fumigatus and Candida spp.

IMPORTANCE

Invasive fungal infections have a high mortality rate, causing more deaths annually than tuberculosis or malaria. Aspergillus fumigatus is the main etiological agent of aspergillosis, one of the most prevalent and deadly fungal diseases. There are few therapeutic options for treating this disease, and treatment commonly fails due to host complications or the emergence of antifungal resistance. Drug repurposing, where existing drugs are deployed for other clinical indications, has increasingly been used in the process of drug discovery. Here, we show that 4-[4-(4-Piperidinyl) phenyl]-7-[4-(-(trifluoromethyl) phenyl]-2-naphthalenecarboxylic acid (PPTN), a highly specific antagonist of the human P2Y14 receptor, when combined with caspofungin (CAS), ibrexafungerp, voriconazole (VOR), and amphotericin can increase the fungicidal activity against not only A. fumigatus CAS- and VOR-resistant clinical isolates but also CAS against Candida spp.

Deletion of core septin gene aspB in Aspergillus fumigatus results in fungicidal activity of caspofungin

Septins are a family of GTP-binding proteins found in many eukaryotic lineages. Although highly conserved throughout many eukaryotes, their functions vary across species. In Aspergillus fumigatus, the etiological agent of invasive aspergillosis, septins participate in a variety of processes such as cell wall organization of conidia, septation, and response to cell wall stress. Previous studies determined that the ΔaspB strain had a greater sensitivity to anti-cell wall drugs, especially the echinocandin caspofungin, yet mechanisms behind this augmented sensitivity are unknown. We performed cell viability staining of the deletion strains post-caspofungin exposure and found that the ΔaspA, ΔaspB, and ΔaspC strains have significantly lower cell viability. Concomitant with the reduced viability, deletion strains are more susceptible to caspofungin on solid media. These results indicate that the septin cytoskeleton is important for A. fumigatus survival in the presence of caspofungin. Due to the potential of improved therapeutic outcome, we followed up using a neutropenic murine model of invasive aspergillosis. Animals infected with the ΔaspB strain and treated with caspofungin showed improved survival compared to the animals infected with akuB KU80 wild-type or complemented strains. Additionally, histological analysis showed reduced fungal burden and inflammation in the ΔaspB infected, caspofungin-treated group. Affinity purification coupled with quantitative proteomics identified proteins involved in the septin-dependent response to caspofungin, includng four candidate interactors involved in cell wall stress response. Deletion of these candidate genes resulted in increased susceptibility to caspofungin and moderately reduced viability post-drug exposure. Taken together, these data suggest that septin AspB contributes to the fungistatic response to caspofungin.

An IFN-STAT1-CYBB Axis Defines Protective Plasmacytoid DC to Neutrophil Crosstalk During Aspergillus fumigatus Infection

Aspergillus fumigatus is the most common cause of invasive aspergillosis (IA), a devastating infection in immunocompromised patients. Plasmacytoid dendritic cells (pDCs) regulate host defense against IA by enhancing neutrophil antifungal properties in the lung. Here, we define the pDC activation trajectory during A. fumigatus infection and the molecular events that underlie the protective pDC - neutrophil crosstalk. Fungus-induced pDC activation begins after bone marrow egress and results in pDC-dependent regulation of lung type I and type III IFN levels. These pDC-derived products act on type I and type III IFN receptor-expressing neutrophils and control neutrophil fungicidal activity and reactive oxygen species production via STAT1 signaling in a cell-intrinsic manner. Mechanistically, neutrophil STAT1 signaling regulates the transcription and expression of Cybb, which encodes one of five NADPH oxidase subunits. Thus, pDCs regulate neutrophil-dependent immunity against inhaled molds by controlling the local expression of a subunit required for NADPH oxidase assembly and activity in the lung.

Fungal Apoptosis-Related Proteins

Programmed cell death (PCD) plays a crucial role in the development and homeostasis maintenance of multicellular organisms. Apoptosis is a form of PCD that prevents pathological development by eliminating damaged or useless cells. Despite the complexity of fungal apoptosis mechanisms being similar to those of plants and metazoans, fungal apoptosis lacks the core regulatory elements of animal apoptosis. Apoptosis-like PCD in fungi can be triggered by a variety of internal and external factors, participating in biological processes such as growth, development, and stress response. Although the core regulatory elements are not fully understood, apoptosis-inducing factor and metacaspase have been found to be involved. This article summarizes various proteins closely related to fungal apoptosis, such as apoptosis-inducing factor, metacaspase, and inhibitors of apoptosis proteins, as well as their structures and functions. This research provides new strategies and ideas for the development of natural drugs targeting fungal apoptosis and the control of fungal diseases.

The pathobiology of human fungal infections

Human fungal infections are a historically neglected area of disease research, yet they cause more than 1.5 million deaths every year. Our understanding of the pathophysiology of these infections has increased considerably over the past decade, through major insights into both the host and pathogen factors that contribute to the phenotype and severity of these diseases. Recent studies are revealing multiple mechanisms by which fungi modify and manipulate the host, escape immune surveillance and generate complex comorbidities. Although the emergence of fungal strains that are less susceptible to antifungal drugs or that rapidly evolve drug resistance is posing new threats, greater understanding of immune mechanisms and host susceptibility factors is beginning to offer novel immunotherapeutic options for the future. In this Review, we provide a broad and comprehensive overview of the pathobiology of human fungal infections, focusing specifically on pathogens that can cause invasive life-threatening infections, highlighting recent discoveries from the pathogen, host and clinical perspectives. We conclude by discussing key future challenges including antifungal drug resistance, the emergence of new pathogens and new developments in modern medicine that are promoting susceptibility to infection.

Pulmonary Aspergillosis in People with Cystic Fibrosis

In the last decade, fungal respiratory diseases have been increasingly investigated for their impact on the clinical course of people with cystic fibrosis (CF), with a particular focus on infections caused by Aspergillus spp. The most common organisms from this genus detected from respiratory cultures are Aspergillus fumigatus and Aspergillus terreus, followed by Aspergillus flavus, Aspergillus niger, and Aspergillus nidulans. These species have been identified to be both chronic colonizers and sources of active infection and may negatively impact lung function in people with CF. This review article discusses definitions of aspergillosis, challenges in clinical practice, and current literature available for laboratory findings, clinical diagnosis, and treatment options for pulmonary diseases caused by Aspergillus spp. in people with 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.

Pathogenicity and virulence of Aspergillus fumigatus

Pulmonary infections caused by the mould pathogen Aspergillus fumigatus are a major cause of morbidity and mortality globally. Compromised lung defences arising from immunosuppression, chronic respiratory conditions or more recently, concomitant viral or bacterial pulmonary infections are recognised risks factors for the development of pulmonary aspergillosis. In this review, we will summarise our current knowledge of the mechanistic basis of pulmonary aspergillosis with a focus on emerging at-risk populations.

Dectin-1/CARD9 induction of the TFEB and TFE3 gene network is dispensable for phagocyte anti-Aspergillus activity in the lung

Myeloid phagocytes of the respiratory immune system, such as neutrophils, monocytes, and alveolar macrophages, are essential for immunity to Aspergillus fumigatus, the most common etiologic agent of mold pneumonia worldwide. Following the engulfment of A. fumigatus conidia, fusion of the phagosome with the lysosome is a critical process for killing conidia. TFEB and TFE3 are transcription factors that regulate lysosomal biogenesis under stress and are activated by inflammatory stimuli in macrophages, but it is unknown whether TFEB and TFE3 contribute to anti-Aspergillus immunity during infection. We found that lung neutrophils express TFEB and TFE3, and their target genes were upregulated during A. fumigatus lung infection. In addition, A. fumigatus infection induced nuclear accumulation of TFEB and TFE3 in macrophages in a process regulated by Dectin-1 and CARD9. Genetic deletion of Tfeb and Tfe3 impaired macrophage killing of A. fumigatus conidia. However, in a murine immune-competent Aspergillus infection model with genetic deficiency of Tfeb and Tfe3 in hematopoietic cells, we surprisingly found that lung myeloid phagocytes had no defects in conidial phagocytosis or killing. Loss of TFEB and TFE3 did not impact murine survival or clearance of A. fumigatus from the lungs. Our findings indicate that myeloid phagocytes activate TFEB and TFE3 in response to A. fumigatus, and while this pathway promotes macrophage fungicidal activity in vitro, genetic loss can be functionally compensated in the lung, resulting in no measurable defect in fungal control and host survival.

Immune responses to human fungal pathogens and therapeutic prospects

Pathogenic fungi have emerged as significant causes of infectious morbidity and death in patients with acquired immunodeficiency conditions such as HIV/AIDS and following receipt of chemotherapy, immunosuppressive agents or targeted biologics for neoplastic or autoimmune diseases, or transplants for end organ failure. Furthermore, in recent years, the spread of multidrug-resistant Candida auris has caused life-threatening outbreaks in health-care facilities worldwide and raised serious concerns for global public health. Rapid progress in the discovery and functional characterization of inborn errors of immunity that predispose to fungal disease and the development of clinically relevant animal models have enhanced our understanding of fungal recognition and effector pathways and adaptive immune responses. In this Review, we synthesize our current understanding of the cellular and molecular determinants of mammalian antifungal immunity, focusing on observations that show promise for informing risk stratification, prognosis, prophylaxis and therapies to combat life-threatening fungal infections in vulnerable patient populations.

Dectin-1/CARD9-induction of the TFEB and TFE3 gene network is dispensable for phagocyte anti-Aspergillus activity in the lung

Myeloid phagocytes of the respiratory immune system, such as neutrophils, monocytes, and alveolar macrophages, are essential for immunity to Aspergillus fumigatus, the most common etiologic agent of mold pneumonia worldwide. Following engulfment of A. fumigatus conidia, fusion of the phagosome with the lysosome, is a critical process for killing conidia. TFEB and TFE3 are transcription factors that regulate lysosomal biogenesis under stress and are activated by inflammatory stimuli in macrophages, but it is unknown whether TFEB and TFE3 contribute to anti-Aspergillus immunity during infection. We found that lung neutrophils express TFEB and TFE3, and their target genes were upregulated during A. fumigatus lung infection. Additionally, A. fumigatus infection induced nuclear accumulation of TFEB and TFE3 in macrophages in a process regulated by Dectin-1 and CARD9 signaling. Genetic deletion of Tfeb and Tfe3 impaired macrophage killing of A. fumigatus conidia. However, in a murine immune competent Aspergillus infection model with genetic deficiency of Tfeb and Tfe3 in hematopoietic cells, we surprisingly found that lung myeloid phagocytes had no defects in conidial phagocytosis or killing. Loss of TFEB and TFE3 did not impact murine survival or clearance of A. fumigatus from the lungs. Our findings indicate that myeloid phagocytes activate TFEB and TFE3 in response to A. fumigatus, and while this pathway promotes macrophage fungicidal activity in vitro, genetic loss can be functionally compensated at the portal of infection in the lung, resulting in no measurable defect in fungal control and host survival.

Immunity to pathogenic fungi in the eye

Fusarium, Aspergillus and Candida are important fungal pathogens that cause visual impairment and blindness in the USA and worldwide. This review will summarize the epidemiology and clinical features of corneal infections and discuss the immune and inflammatory responses that play an important role in clinical disease. In addition, we describe fungal virulence factors that are required for survival in infected corneas, and the activities of neutrophils in fungal killing, tissue damage and cytokine production.

Virus-associated fungal infections and lost immune resistance

Invasive fungal infections are an increasing threat to human health. Of recent concern is the emergence of influenza- or SARS-CoV-2-virus-associated invasive fungal infections. Understanding acquired susceptibilities to fungi requires consideration of the collective and newly explored roles of adaptive, innate, and natural immunity. Neutrophils are known to provide host resistance, but new concepts are emerging that implicate innate antibodies, the actions of specialized B1 B cell subsets, and B cell-neutrophil crosstalk in mediating antifungal host resistance. Based on emerging evidence, we propose that virus infections impact on neutrophil and innate B cell resistance against fungi, leading to invasive infections. These concepts provide novel approaches to developing candidate therapeutics with the aim of restoring natural and humoral immunity and boosting neutrophil resistance against fungi.

Immunity to fungi in the lung

The respiratory tree maintains sterilizing immunity against human fungal pathogens. Humans inhale ubiquitous filamentous molds and geographically restricted dimorphic fungal pathogens that form small airborne conidia. In addition, pathogenic yeasts, exemplified by encapsulated Cryptococcus species, and Pneumocystis pose significant fungal threats to the lung. Classically, fungal pneumonia occurs in immune compromised individuals, specifically in patients with HIV/AIDS, in patients with hematologic malignancies, in organ transplant recipients, and in patients treated with corticosteroids and targeted biologics that impair fungal immune surveillance in the lung. The emergence of fungal co-infections during severe influenza and COVID-19 underscores the impairment of fungus-specific host defense pathways in the lung by respiratory viruses and by medical therapies to treat viral infections. Beyond life-threatening invasive syndromes, fungal antigen exposure can exacerbate allergenic disease in the lung. In this review, we discuss emerging principles of lung-specific antifungal immunity, integrate the contributions and cooperation of lung epithelial, innate immune, and adaptive immune cells to mucosal barrier immunity, and highlight the pathogenesis of fungal-associated allergenic disease. Improved understanding of fungus-specific immunity in the respiratory tree has paved the way to develop improved diagnostic, pre-emptive, therapeutic, and vaccine approaches for fungal diseases of the lung.

Neutrophil and Macrophage NADPH Oxidase 2 Differentially Control Responses to Inflammation and to Aspergillus fumigatus in Mice

Aspergillus fumigatus is an important opportunistic fungal pathogen and causes invasive pulmonary aspergillosis in conditions with compromised innate antifungal immunity, including chronic granulomatous disease, which results from inherited deficiency of the superoxide-generating leukocyte NADPH oxidase 2 (NOX2). Derivative oxidants have both antimicrobial and immunoregulatory activity and, in the context of A. fumigatus, contribute to both fungal killing and dampening inflammation induced by fungal cell walls. As the relative roles of macrophage versus neutrophil NOX2 in the host response to A. fumigatus are incompletely understood, we studied mice with conditional deletion of NOX2. When NOX2 was absent in alveolar macrophages as a result of LysM-Cre-mediated deletion, germination of inhaled A. fumigatus conidia was increased. Reducing NOX2 activity specifically in neutrophils via S100a8 (MRP8)-Cre also increased fungal burden, which was inversely proportional to the level of neutrophil NOX2 activity. Moreover, diminished NOX2 in neutrophils synergized with corticosteroid immunosuppression to impair lung clearance of A. fumigatus. Neutrophil-specific reduction in NOX2 activity also enhanced acute inflammation induced by inhaled sterile fungal cell walls. These results advance understanding into cell-specific roles of NOX2 in the host response to A. fumigatus. We show that alveolar macrophage NOX2 is a nonredundant effector that limits germination of inhaled A. fumigatus conidia. In contrast, reducing NOX2 activity only in neutrophils is sufficient to enhance inflammation to fungal cell walls as well as to promote invasive A. fumigatus. This may be relevant in clinical settings with acquired defects in NOX2 activity due to underlying conditions, which overlap risk factors for invasive aspergillosis.

The pan-genome of Aspergillus fumigatus provides a high-resolution view of its population structure revealing high levels of lineage-specific diversity driven by recombination

Aspergillus fumigatus is a deadly agent of human fungal disease where virulence heterogeneity is thought to be at least partially structured by genetic variation between strains. While population genomic analyses based on reference genome alignments offer valuable insights into how gene variants are distributed across populations, these approaches fail to capture intraspecific variation in genes absent from the reference genome. Pan-genomic analyses based on de novo assemblies offer a promising alternative to reference-based genomics with the potential to address the full genetic repertoire of a species. Here, we evaluate 260 genome sequences of A. fumigatus including 62 newly sequenced strains, using a combination of population genomics, phylogenomics, and pan-genomics. Our results offer a high-resolution assessment of population structure and recombination frequency, phylogenetically structured gene presence-absence variation, evidence for metabolic specificity, and the distribution of putative antifungal resistance genes. Although A. fumigatus disperses primarily via asexual conidia, we identified extraordinarily high levels of recombination with the lowest linkage disequilibrium decay value reported for any fungal species to date. We provide evidence for 3 primary populations of A. fumigatus, with recombination occurring only rarely between populations and often within them. These 3 populations are structured by both gene variation and distinct patterns of gene presence-absence with unique suites of accessory genes present exclusively in each clade. Accessory genes displayed functional enrichment for nitrogen and carbohydrate metabolism suggesting that populations may be stratified by environmental niche specialization. Similarly, the distribution of antifungal resistance genes and resistance alleles were often structured by phylogeny. Altogether, the pan-genome of A. fumigatus represents one of the largest fungal pan-genomes reported to date including many genes unrepresented in the Af293 reference genome. These results highlight the inadequacy of relying on a single-reference genome-based approach for evaluating intraspecific variation and the power of combined genomic approaches to elucidate population structure, genetic diversity, and putative ecological drivers of clinically relevant fungi.

Host immune responses in the central nervous system during fungal infections

Fungal infections in the central nervous system (CNS) cause high morbidity and mortality. The frequency of CNS mycosis has increased over the last two decades as more individuals go through immunocompromised conditions for various reasons. Nevertheless, options for clinical interventions for CNS mycoses are still limited. Thus, there is an urgent need to understand the host-pathogen interaction mechanisms in CNS mycoses for developing novel treatments. Although the CNS has been regarded as an immune-privileged site, recent studies demonstrate the critical involvement of immune responses elicited by CNS-resident and CNS-infiltrated cells during fungal infections. In this review, we discuss mechanisms of fungal invasion in the CNS, fungal pathogen detection by CNS-resident cells (microglia, astrocytes, oligodendrocytes, neurons), roles of CNS-infiltrated leukocytes, and host immune responses. We consider that understanding host immune responses in the CNS is crucial for endeavors to develop treatments for CNS mycosis.

The conservation of IAP-like proteins in fungi, and their potential role in fungal programmed cell death

Programmed cell death (PCD) is a tightly regulated process which is required for survival and proper development of all cellular life. Despite this ubiquity, the precise molecular underpinnings of PCD have been primarily characterized in animals. Attempts to expand our understanding of this process in fungi have proven difficult as core regulators of animal PCD are apparently absent in fungal genomes, with the notable exception of a class of proteins referred to as inhibitors of apoptosis proteins (IAPs). These proteins are characterized by the conservation of a distinct Baculovirus IAP Repeat (BIR) domain and animal IAPs are known to regulate a number of processes, including cellular death, development, organogenesis, immune system maturation, host-pathogen interactions and more. IAP homologs are broadly conserved throughout the fungal kingdom, but our understanding of both their mechanism and role in fungal development/virulence is still unclear. In this review, we provide a broad and comparative overview of IAP function across taxa, with a particular focus on fungal processes regulated by IAPs. Furthermore, their putative modes of action in the absence of canonical interactors will be discussed.

Neutrophil-Specific Knockdown of β2 Integrins Impairs Antifungal Effector Functions and Aggravates the Course of Invasive Pulmonal Aspergillosis

β2-integrins are heterodimeric surface receptors that are expressed specifically by leukocytes and consist of a variable α (CD11a-d) and a common β-subunit (CD18). Functional impairment of CD18, which causes leukocyte adhesion deficiency type-1 results in an immunocompromised state characterized by severe infections, such as invasive pulmonary aspergillosis (IPA). The underlying immune defects have largely been attributed to an impaired migratory and phagocytic activity of polymorphonuclear granulocytes (PMN). However, the exact contribution of β2-integrins for PMN functions in-vivo has not been elucidated yet, since the mouse models available so far display a constitutive CD18 knockout (CD18^-/- or CD18^hypo). To determine the PMN-specific role of β2-integrins for innate effector functions and pathogen control, we generated a mouse line with a Ly6G-specific knockdown of the common β-subunit (CD18^Ly6G cKO). We characterized CD18^Ly6G cKO mice in-vitro to confirm the PMN-specific knockdown of β2-integrins. Next, we investigated the clinical course of IPA in A. fumigatus infected CD18^Ly6G cKO mice with regard to the fungal burden, pulmonary inflammation and PMN response towards A. fumigatus. Our results revealed that the β2-integrin knockdown was restricted to PMN and that CD18^Ly6G cKO mice showed an aggravated course of IPA. In accordance, we observed a higher fungal burden and lower levels of proinflammatory innate cytokines, such as TNF-α, in lungs of IPA-infected CD18^Ly6G cKO mice. Bronchoalveolar lavage revealed higher levels of CXCL1, a stronger PMN-infiltration, but concomitantly elevated apoptosis of PMN in lungs of CD18^Ly6G cKO mice. Ex-vivo analysis further unveiled a strong impairment of PMN effector function, as reflected by an attenuated phagocytic activity, and a diminished generation of reactive oxygen species (ROS) and neutrophil-extracellular traps (NET) in CD18-deficient PMN. Overall, our study demonstrates that β2-integrins are required specifically for PMN effector functions and contribute to the clearance of A. fumigatus by infiltrating PMN, and the establishment of an inflammatory microenvironment in infected lungs.

Cell death induction in Aspergillus fumigatus: accentuating drug toxicity through inhibition of the unfolded protein response (UPR)

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The UPR is an adaptive stress response network that is tightly linked to the ability of Aspergillus fumigatus, and other pathogenic fungi, to sustain viability in the presence of adverse environmental conditions, including the stress of infection. In this review, we summarize the evidence that supports the concept of targeting the A. fumigatus UPR as a strategy to reduce the ability of the fungus to withstand stress.

One of the most potent opportunistic fungal pathogens of humans is Aspergillus fumigatus, an environmental mold that causes a life-threatening pneumonia with a high rate of morbidity and mortality. Despite advances in therapy, issues of drug toxicity and antifungal resistance remain an obstacle to effective therapy. This underscores the need for more information on fungal pathways that could be pharmacologically manipulated to either reduce the viability of the fungus during infection, or to unleash the fungicidal potential of current antifungal drugs. In this review, we summarize the emerging evidence that the ability of A. fumigatus to sustain viability during stress relies heavily on an adaptive signaling pathway known as the unfolded protein response (UPR), thereby exposing a vulnerability in this fungus that has strong potential for future therapeutic intervention.

Pleiotropic Effects of the P5-Type ATPase SpfA on Stress Response Networks Contribute to Virulence in the Pathogenic Mold Aspergillus fumigatus

Aspergillus fumigatus is a human-pathogenic mold that extracts nutrients from the environment or from host tissues by secreting hydrolytic enzymes. The ability of A. fumigatus to adjust secretion levels in proportion to demand relies on the assistance of the unfolded protein response (UPR), an adaptive stress response pathway that regulates the unique protein-folding environment of the endoplasmic reticulum (ER). The P5-type ATPase Spf1 has recently been implicated in a novel mechanism of ER homeostasis that involves correcting errors in ER-membrane protein targeting. However, the contribution of this protein to the biology of A. fumigatus is unknown. Here, we employed a gene knockout and RNA sequencing strategy to determine the functional role of the A. fumigatus gene coding for the orthologous P5 ATPase SpfA. The data reveal that the spfA gene is induced by ER stress in a UPR-dependent manner. In the absence of spfA, the A. fumigatus transcriptome shifts toward a profile of altered redox and lipid balance, in addition to a signature of ER stress that includes srcA, encoding a second P-type ATPase in the ER. A ΔspfA deletion mutant showed increased sensitivity to ER stress, oxidative stress, and antifungal drugs that target the cell wall or plasma membrane. The combined loss of spfA and srcA exacerbated these phenotypes and attenuated virulence in two animal infection models. These findings demonstrate that the ER-resident ATPases SpfA and SrcA act jointly to support diverse adaptive functions of the ER that are necessary for fitness in the host environment. IMPORTANCE The fungal UPR is an adaptive signaling pathway in the ER that buffers fluctuations in ER stress but also serves as a virulence regulatory hub in species of pathogenic fungi that rely on secretory pathway homeostasis for pathogenicity. This study demonstrates that the gene encoding the ER-localized P5-type ATPase SpfA is a downstream target of the UPR in the pathogenic mold A. fumigatus and that it works together with a second ER-localized P-type ATPase, SrcA, to support ER homeostasis, oxidative stress resistance, susceptibility to antifungal drugs, and virulence of A. fumigatus.

The fungivorous amoeba Protostelium aurantium targets redox homeostasis and cell wall integrity during intracellular killing of Candida parapsilosis

Predatory interactions among microbes are major evolutionary driving forces for biodiversity. The fungivorous amoeba Protostelium aurantium has a wide fungal food spectrum including foremost pathogenic members of the genus Candida. Here we show that upon phagocytic ingestion by the amoeba, Candida parapsilosis is confronted with an oxidative burst and undergoes lysis within minutes of processing in acidified phagolysosomes. On the fungal side, a functional genomic approach identified copper and redox homeostasis as primary targets of amoeba predation, with the highly expressed copper exporter gene CRP1 and the peroxiredoxin gene PRX1 contributing to survival when encountered with P. aurantium. The fungicidal activity was largely retained in intracellular vesicles of the amoebae. Following their isolation, the content of these vesicles induced immediate killing and lysis of C. parapsilosis in vitro. Proteomic analysis identified 56 vesicular proteins from P. aurantium. Although completely unknown proteins were dominant, many of them could be categorised as hydrolytic enzymes targeting the fungal cell wall, indicating that fungal cell wall structures are under selection pressure by predatory phagocytes in natural environments. TAKE AWAY: The amoeba Protostelium aurantium feeds on fungi, such as Candida parapsilosis. Ingested yeast cells are exposed to reactive oxygen species. A copper exporter and a peroxiredoxin contribute to fungal defence. Yeast cells undergo intracellular lysis. Lysis occurs via a cocktail of hydrolytic enzymes from intracellular vesicles.

CaBir1 functions as an inhibitor-of-apoptosis and affects caspase-like activitiy in Candida albicans

CaMca1 is the only metacaspase in Candida albicans, which shows structural homology to the mammalian caspases. CaMca1 consists of the caspase domain, the P20 and P10 regions, and the conserved catalytic histidine-cysteine dyad that is required for executing apoptosis in C. albicans. However, little is known about the proteolytic processing of CaMca1 or its activation under apoptosis-inducing conditions. To understand the regulation of this process, we characterized CaBir1 which is the single IAP (inhibitor-of-apoptosis protein) in C. albicans. IAPs are a family of proteins whose members all harbor a BIR (baculovirus IAP repeat) domain and negatively regulate apoptosis by inhibiting caspases. We found that the Cabir1/Cabir1 deletion mutant exhibited increased apoptotic phenotypes, such as ROS accumulation, nuclear segmentation, and cell survival, under apoptosis-inducing conditions. Examination of CaMca1 cleavage patterns in response to various apoptotic stresses revealed that these cleavages were stress-specific and dependent on the catalytic histidine-cysteine residues of CaMca1. The Cabir1/Cabir1 mutation was not associated with altered CaMca1 processing with or without apoptotic stimuli, but the Cabir1/Cabir1 mutant exhibited significantly increased caspase-like activities. These results suggest that CaBir1 acts as an apoptosis inhibitor by regulating caspase-like activity, but not CaMca1 processing.

Pseudomonas Strains Induce Transcriptional and Morphological Changes and Reduce Root Colonization of Verticillium spp

Phytopathogenic Verticillia cause Verticillium wilt on numerous economically important crops. Plant infection begins at the roots, where the fungus is confronted with rhizosphere inhabiting bacteria. The effects of different fluorescent pseudomonads, including some known biocontrol agents of other plant pathogens, on fungal growth of the haploid Verticillium dahliae and/or the amphidiploid Verticillium longisporum were compared on pectin-rich medium, in microfluidic interaction channels, allowing visualization of single hyphae, or on Arabidopsis thaliana roots. We found that the potential for formation of bacterial lipopeptide syringomycin resulted in stronger growth reduction effects on saprophytic Aspergillus nidulans compared to Verticillium spp. A more detailed analyses on bacterial-fungal co-cultivation in narrow interaction channels of microfluidic devices revealed that the strongest inhibitory potential was found for Pseudomonas protegens CHA0, with its inhibitory potential depending on the presence of the GacS/GacA system controlling several bacterial metabolites. Hyphal tip polarity was altered when V. longisporum was confronted with pseudomonads in narrow interaction channels, resulting in a curly morphology instead of straight hyphal tip growth. These results support the hypothesis that the fungus attempts to evade the bacterial confrontation. Alterations due to co-cultivation with bacteria could not only be observed in fungal morphology but also in fungal transcriptome. P. protegens CHA0 alters transcriptional profiles of V. longisporum during 2 h liquid media co-cultivation in pectin-rich medium. Genes required for degradation of and growth on the carbon source pectin were down-regulated, whereas transcripts involved in redox processes were up-regulated. Thus, the secondary metabolite mediated effect of Pseudomonas isolates on Verticillium species results in a complex transcriptional response, leading to decreased growth with precautions for self-protection combined with the initiation of a change in fungal growth direction. This interplay of bacterial effects on the pathogen can be beneficial to protect plants from infection, as shown with A. thaliana root experiments. Treatment of the roots with bacteria prior to infection with V. dahliae resulted in a significant reduction of fungal root colonization. Taken together we demonstrate how pseudomonads interfere with the growth of Verticillium spp. and show that these bacteria could serve in plant protection.

Mitochondrial Reactive Oxygen Species Enhance Alveolar Macrophage Activity against Aspergillus fumigatus but Are Dispensable for Host Protection

Aspergillus fumigatus is the most common cause of mold pneumonia worldwide, and a significant cause of infectious morbidity and mortality in immunocompromised individuals. The oxidative burst, which generates reactive oxidative species (ROS), plays a pivotal role in host defense against aspergillosis and induces regulated cell death in Aspergillus conidia, the infectious propagules. Beyond the well-established role of NADP (NADPH) oxidase in ROS generation by neutrophils and other innate effector cells, mitochondria represent a major ROS production site in many cell types, though it is unclear whether mitochondrial ROS (mtROS) contribute to antifungal activity in the lung. Following A. fumigatus infection, we observed that innate effector cells, including alveolar macrophages (AMs), monocyte-derived dendritic cells (Mo-DCS), and neutrophils, generated mtROS, primarily in fungus-infected cells. To examine the functional role of mtROS, specifically the H₂O₂ component, in pulmonary host defense against A. fumigatus, we infected transgenic mice that expressed a mitochondrion-targeted catalase. Using a reporter of fungal viability during interactions with leukocytes, mitochondrial H₂O₂ (mtH₂O₂) was essential for optimal AM, but not for neutrophil phagocytic and conidiacidal activity in the lung. Catalase-mediated mtH₂O₂ neutralization did not lead to invasive aspergillosis in otherwise immunocompetent mice and did not shorten survival in mice that lack NADPH oxidase function. Collectively, these studies indicate that mtROS-associated defects in AM antifungal activity can be functionally compensated by the action of NADPH oxidase and by nonoxidative effector mechanisms during murine A. fumigatus lung infection. IMPORTANCE Aspergillus fumigatus is a fungal pathogen that causes invasive disease in humans with defects in immune function. Airborne conidia, the infectious propagules, are ubiquitous and inhaled on a daily basis. In the respiratory tree, conidia are killed by the coordinated actions of phagocytes, including alveolar macrophages, neutrophils, and monocyte-derived dendritic cells. The oxidative burst represents a central killing mechanism and relies on the assembly of the NADPH oxidase complex on the phagosomal membrane. However, NADPH oxidase-deficient leukocytes have significant residual fungicidal activity in vivo, indicating the presence of alternative effector mechanisms. Here, we report that murine innate immune cells produce mitochondrial reactive oxygen species (mtROS) in response to fungal interactions. Neutralizing the mtROS constituent hydrogen peroxide (H₂O₂) via a catalase expressed in mitochondria of innate immune cells substantially diminished fungicidal properties of alveolar macrophages, but not of other innate immune cells. These data indicate that mtH₂O₂ represent a novel AM killing mechanism against Aspergillus conidia. mtH₂O₂ neutralization is compensated by other killing mechanisms in the lung, demonstrating functional redundancy at the level of host defense in the respiratory tree. These findings have important implications for the development of host-directed therapies against invasive aspergillosis in susceptible patient populations.

Naringin-generated ROS promotes mitochondria-mediated apoptosis in Candida albicans

Naringin is a flavonoid which has a therapeutic effect. However, the details of its antifungal mechanism have not yet been fully elucidated. This study focused on clarifying the relationship between naringin and Candida albicans, to understand its mode of antifungal action. In general, naringin is an antioxidant, but our results indicated that 1 mM naringin generates intracellular superoxide (O₂ ^- ) and hydroxyl radicals (OH^- ). Reactive oxygen species (ROS) have a serious impact on Ca²⁺ signaling and the production of mitochondrial ROS. After exposure to enhanced O₂ ^- and OH^- , mitochondrial Ca²⁺ overload and mitochondrial O₂ ^- generation were confirmed in C. albicans. It was verified that mitochondrial O₂ ^- transforms mitochondrial glutathione (GSH) to oxidized GSH (GSSG), leading to extreme oxidative stress in mitochondria. The previously observed Ca²⁺ accumulation and oxidative stress resulted in mitochondrial membrane potential (MMP) alteration and increased mitochondrial mass. In succession, cytochrome c release from the mitochondria to the cytosol was detected due to MMP loss. Cytochrome c promotes the initiation of apoptosis, and further experiments were performed to assess the apoptotic hallmarks. Metacaspases activation, chromosomal condensation, DNA fragmentation, and phosphatidylserine exposure were observed, indicating that naringin induces apoptosis in C. albicans. In conclusion, our findings manifested that naringin-generated O₂ ^- and OH^- damage the mitochondria and that mitochondrial dysfunction-mediated apoptosis is novel antifungal mechanism of naringin.

The Peroxiredoxin Asp f3 Acts as Redox Sensor in Aspergillus fumigatus

The human pathogenic fungus Aspergillus fumigatus is readily eradicated by the innate immunity of immunocompetent human hosts, but can cause severe infections, such as invasive aspergillosis (IA), in immunocompromised individuals. During infection, the fungal redox homeostasis can be challenged by reactive oxygen species (ROS), either derived from the oxidative burst of innate immune cells or the action of antifungal drugs. The peroxiredoxin Asp f3 was found to be essential to cause IA in mice, but how Asp f3 integrates with fungal redox homeostasis remains unknown. Here, we show that in vivo, Asp f3 acts as a sensor for ROS. While global transcription in fungal hyphae under minimal growth conditions was fully independent of Asp f3, a robust induction of the oxidative stress response required the presence of the peroxiredoxin. Hyphae devoid of Asp f3 failed to activate several redox active genes, like members of the gliotoxin biosynthesis gene cluster and integral members of the Afyap1 regulon, the central activator of the ROS defense machinery in fungi. Upon deletion of the asp f3 gene Afyap1 displayed significantly reduced nuclear localization during ROS exposure, indicating that Asp f3 can act as an intracellular redox sensor for several target proteins.

Mitochondrial Reactive Oxygen Species Regulate Immune Responses of Macrophages to Aspergillus fumigatus

Reactive Oxygen Species (ROS) are highly reactive molecules that can induce oxidative stress. For instance, the oxidative burst of immune cells is well known for its ability to inhibit the growth of invading pathogens. However, ROS also mediate redox signalling, which is important for the regulation of antimicrobial immunity. Here, we report a crucial role of mitochondrial ROS (mitoROS) in antifungal responses of macrophages. We show that mitoROS production rises in murine macrophages exposed to swollen conidia of the fungal pathogen Aspergillus fumigatus compared to untreated macrophages, or those treated with resting conidia. Furthermore, the exposure of macrophages to swollen conidia increases the activity of complex II of the respiratory chain and raises mitochondrial membrane potential. These alterations in mitochondria of infected macrophages suggest that mitoROS are produced via reverse electron transport (RET). Significantly, preventing mitoROS generation via RET by treatment with rotenone, or a suppressor of site IQ electron leak, S1QEL1.1, lowers the production of pro-inflammatory cytokines TNF-α and IL-1β in macrophages exposed to swollen conidia of A. fumigatus. Rotenone and S1QEL1.1 also reduces the fungicidal activity of macrophages against swollen conidia. Moreover, we have established that elevated recruitment of NADPH oxidase 2 (NOX2, also called gp91phox) to the phagosomal membrane occurs prior to the increase in mitoROS generation. Using macrophages from gp91phox-/- mice, we have further demonstrated that NOX2 is required to regulate cytokine secretion by RET-associated mitoROS in response to infection with swollen conidia. Taken together, these observations demonstrate the importance of RET-mediated mitoROS production in macrophages infected with A. fumigatus.

Fungal-Induced Programmed Cell Death

Fungal infections are a cause of morbidity in humans, and despite the availability of a range of antifungal treatments, the mortality rate remains unacceptably high. Although our knowledge of the interactions between pathogenic fungi and the host continues to grow, further research is still required to fully understand the mechanism underpinning fungal pathogenicity, which may provide new insights for the treatment of fungal disease. There is great interest regarding how microbes induce programmed cell death and what this means in terms of the immune response and resolution of infection as well as microbe-specific mechanisms that influence cell death pathways to aid in their survival and continued infection. Here, we discuss how programmed cell death is induced by fungi that commonly cause opportunistic infections, including Candida albicans, Aspergillus fumigatus, and Cryptococcus neoformans, the role of programmed cell death in fungal immunity, and how fungi manipulate these pathways.

Aspergillus fumigatus Strain-Specific Conidia Lung Persistence Causes an Allergic Broncho-Pulmonary Aspergillosis-Like Disease Phenotype

Aspergillus fumigatus is a filamentous fungus which can cause multiple diseases in humans. Allergic broncho-pulmonary aspergillosis (ABPA) is a disease diagnosed primarily in cystic fibrosis patients caused by a severe allergic response often to long-term A. fumigatus colonization in the lungs. Mice develop an allergic response to repeated inhalation of A. fumigatus spores; however, no strains have been identified that can survive long-term in the mouse lung and cause ABPA-like disease. We characterized A. fumigatus strain W72310, which was isolated from the expectorated sputum of an ABPA patient, by whole-genome sequencing and in vitro and in vivo viability assays in comparison to a common reference strain, CEA10. W72310 was resistant to leukocyte-mediated killing and persisted in the mouse lung longer than CEA10, a phenotype that correlated with greater resistance to oxidative stressors, hydrogen peroxide, and menadione, in vitro In animals both sensitized and challenged with W72310, conidia, but not hyphae, were viable in the lungs for up to 21 days in association with eosinophilic airway inflammation, airway leakage, serum IgE, and mucus production. W72310-sensitized mice that were recall challenged with conidia had increased inflammation, Th1 and Th2 cytokines, and airway leakage compared to controls. Collectively, our studies demonstrate that a unique strain of A. fumigatus resistant to leukocyte killing can persist in the mouse lung in conidial form and elicit features of ABPA-like disease.IMPORTANCE Allergic broncho-pulmonary aspergillosis (ABPA) patients often present with long-term colonization of Aspergillus fumigatus Current understanding of ABPA pathogenesis has been complicated by a lack of long-term in vivo fungal persistence models. We have identified a clinical isolate of A. fumigatus, W72310, which persists in the murine lung and causes an ABPA-like disease phenotype. Surprisingly, while viable, W72310 showed little to no growth beyond the conidial stage in the lung. This indicates that it is possible that A. fumigatus can cause allergic disease in the lung without any significant hyphal growth. The identification of this strain of A. fumigatus can be used not only to better understand disease pathogenesis of ABPA and potential antifungal treatments but also to identify features of fungal strains that drive long-term fungal persistence in the lung. Consequently, these observations are a step toward helping resolve the long-standing question of when to utilize antifungal therapies in patients with ABPA and fungal allergic-type diseases.

Electron donor cytochrome b5 is required for hyphal tip accumulation of sterol-rich plasma membrane domains and membrane fluidity in Aspergillus fumigatus

The electron donor cytochrome b5 (CybE/Cyb5) fuels the activity of the ergosterol biosynthesis-related P450 enzymes/P450s by providing electrons to P450s to promote ergosterol biosynthesis. Previous studies reported that lack of Aspergillus fumigatus (A. fumigatus) CybE reduces the proportion of ergosterol in total sterols and induces severe growth defects. However, the molecular characteristics of CybE and the underlying mechanism for CybE maintaining A. fumigatus growth remain poorly understood. Here, we found that CybE locates at the endoplasmic reticulum by its C-terminus with two transmembrane regions. Therefore, lack of the C-terminus of CybE is able to phenocopy a cybE deletion. Notably, cybE deletion reduced the accumulation of the sterol-rich plasma membrane domains (SRDs, the assembly platform of polarity factors/cell end markers and growth machinery) in hyphal tips and decreased membrane fluidity, which correspond to tardiness of hyphal extension and hypersensitivity to low temperature in cybE deletion mutant. Additionally, overexpressing another electron donor-heme-independent P450 reductase (CPR) significantly rescued growth defects and recovered SRD accumulation in deletion of cybE almost to the wild-type level, suggesting CybE maintaining the growth and deposition of SRDs in hyphal tips attributes to its nature as an electron donor. Protein pull-down assays revealed that CybE probably participates in metabolism and transfer of lipids, construction of cytoskeleton and mitochondria-associated energy metabolism to maintain the SRD accumulation in hyphal tips, membrane fluidity and hyphal extension. Findings in this study give a hint that inhibition of CybE may be an effective strategy for resisting the infection of the human pathogen A. fumigatus Importance Investigating the knowledge of the growth regulation in the human opportunistic pathogen A. fumigatus is conducive to design new antifungal approach. The electron donor cytochrome b5 (CybE) plays a crucial role in maintaining the normal growth of A. fumigatus, however, the potential mechanism remains elusive. Herein, we characterized the molecular features of CybE and found the C-terminus with two transmembrane domains are required for its ER localization and functions. In addition, we demonstrated that CprA, an electron donor-heme-independent P450 reductase, provides a reciprocal function for the missing cytochrome b5 protein-CybE in A. fumigatus CybE maintains the normal growth probably via supporting two crucial physiological processes, the SRD accumulation in hyphal tips and membrane fluidity. Therefore, our finding reveals the mechanisms underlying the regulatory effect of CybE on A. fumigatus growth and indicates that inhibition of CybE might be an effective approach for alleviating A. fumigatus infection.

Analysis of Apoptosis-Related Genes Reveals that Apoptosis Functions in Conidiation and Pathogenesis of Fusarium pseudograminearum

Apoptosis, a type of programmed cell death, plays crucial roles in various physiological processes, from development to adaptive responses. Key features of apoptosis have been verified in various fungal microbes but not yet in Fusarium species. Here, we identified 19 apoptosis-related genes in Fusarium pseudograminearum using a genome-wide survey. Expression profile analysis revealed that several apoptosis-related genes were significantly increased during conidiation and infection stages. Among these is FpBIR1, with two BIR (baculovirus inhibitor-of-apoptosis protein repeat) domains at the N-terminal end of the protein, a homolog of Saccharomyces cerevisiae BIR1, which is a unique apoptosis inhibitor. FpNUC1 is the ortholog of S. cerevisiae NUC1, which triggers AIF1- or YCA1-independent apoptosis. The functions of these two proteins were assessed by creating Δfpbir1 and Δfpnuc1 mutants via targeted gene deletion. The Δfpbir1 mutant had more cells with nuclear fragmentation and exhibited reduced conidiation, conidial formation, and infectivity. Correspondingly, the Δfpnuc1 mutant contained multiple nuclei, produced thicker and more branched hyphae, was reduced in conidiation, and exhibited faster conidial formation and higher infection rates. Taken together, our results indicate that the apoptosis-related genes FpBIR1 and FpNUC1 function in conidiation, conidial germination, and infection by F. pseudograminearumIMPORTANCE The plant-pathogenic fungus F. pseudograminearum is the causal agent of Fusarium crown rot (FCR) in wheat and barley, resulting in substantial yield losses worldwide. Particularly, in the Huanghuai wheat-growing region of China, F. pseudograminearum was reported as the dominant Fusarium species in FCR infections. Apoptosis is an evolutionarily conserved mechanism in eukaryotes, playing crucial roles in development and cell responses to biotic and abiotic stresses. However, few reports on apoptosis in plant fungal pathogens have been published. In this study, we identified 19 conserved apoptosis-related genes in F. pseudograminearum, several of which were significantly increased during conidiation and infection stages. Potential apoptosis functions were assessed by deletion of the putative apoptosis inhibitor gene FpBIR1 and apoptosis trigger gene FpNUC1 in F. pseudograminearum The FpBIR1 deletion mutant exhibited defects in conidial germination and pathogenicity, whereas the FpNUC1 deletion mutant experienced faster conidial formation and higher infection rates. Apoptosis appears to negatively regulate the conidial germination and pathogenicity of F. pseudograminearum To our knowledge, this study is the first report of apoptosis contributing to infection-related morphogenesis and pathogenesis in F. pseudograminearum.

Fungal Bioreporters to Monitor Outcomes of Aspergillus: Host-Cell Interactions

Fluorescence-based techniques enable researchers to monitor physiologic processes, specifically fungal cell viability and death, during cellular encounters with the mammalian immune system with single event resolution. By incorporating two independent fluorescent probes in fungal organisms either prior to, or ensuing experimental infection in mice or in cultured leukocytes, it is possible to distinguish and quantify live and killed fungal cells to interrogate genetic, pharmacologic, and cellular determinants that shape host-fungal cell outcomes. This chapter reviews the techniques and applications of fluorescent fungal reporters of viability, with emphasis on the filamentous mold Aspergillus fumigatus.

Botrytis cinerea methyl isocitrate lyase mediates oxidative stress tolerance and programmed cell death by modulating cellular succinate levels

Fungi lack the entire animal core apoptotic machinery. Nevertheless, regulated cell death with apoptotic markers occurs in multicellular as well as in unicellular fungi and is essential for proper fungal development and stress adaptation. The discrepancy between appearance of an apoptotic-like regulated cell death (RCD) in the absence of core apoptotic machinery is further complicated by the fact that heterologous expression of animal apoptotic genes in fungi affects fungal RCD. Here we describe the role of BcMcl, a methyl isocitrate lyase from the plant pathogenic fungus Botrytis cinerea, in succinate metabolism, and the connection of succinate with stress responses and cell death. Over expression of bcmcl resulted in elevated tolerance to oxidative stress and reduced levels of RCD, which were associated with accumulation of elevated levels of succinate. Deletion of bcmcl had almost no effect on fungal development or stress sensitivity, and succinate levels were unchanged in the deletion strain. Gene expression experiments showed co-regulation of bcmcl and bcicl (isocitrate lyase); expression of the bcicl gene was enhanced in bcmcl deletion and suppressed in bcmcl over expression strains. External addition of succinate reproduced the phenotypes of the bcmcl over expression strains, including developmental defects, reduced virulence, and improved oxidative stress tolerance. Collectively, our results implicate mitochondria metabolic pathways, and in particular succinate metabolism, in regulation of fungal stress tolerance, and highlight the role of this onco-metabolite as potential mediator of fungal RCD.

The first line of defense: effector pathways of anti-fungal innate immunity

The innate immune system is critical to proper host defense against fungal pathogens, which is highlighted by increased susceptibility to invasive disease in immunocompromised patients. Innate cells (e.g. macrophages, neutrophils, dendritic cells, eosinophils) are equipped with intricate cell machinery to detect invading fungi and facilitate fungal killing, recruit additional immune cells, and direct the adaptive immune system responses. Understanding the mechanisms that govern a protective response will enable the development of novel treatment strategies. This review focuses on recent insights of signaling and regulation of C-type lectin receptors and their effector mechanisms enabling an effective host antifungal immunity.

Precise Expression of Afmed15 Is Crucial for Asexual Development, Virulence, and Survival of Aspergillus fumigatus

The identification and characterization of regulators essential for virulence or development constitute one approach for antifungal drug development. In this study, we screened and functionally characterized Afmed15, a novel developmental regulator in A. fumigatus. We demonstrate that the precise transcriptional expression of Afmed15 is crucial for fungal asexual development, virulence, and survival. Downregulating the expression of Afmed15 abolished the conidiation and decreased the fungal virulence in an insect model. In contrast, the overexpression of Afmed15 caused fungal death accompanied by intensive autophagy. Our study provides a foundation for further studies to identify compounds perturbing the expression of Afmed15 that may be used for the prevention of invasive A. fumigatus infections.

The rise of drug resistance in fungal pathogens is becoming a serious problem owing to the limited number of antifungal drugs available. Identifying and targeting factors essential for virulence or development unique to fungal pathogens is one approach to develop novel treatments for fungal infections. In this study, we present the identification and functional characterization of a novel developmental regulator in Aspergillus fumigatus, AfMed15, which contained a conserved Med15_fungal domain, as determined by screening of a mutant library that contained more than 2,000 hygromycin-resistant A. fumigatus transformants. Downregulating the expression of Afmed15 abolished the conidiation and decreased the fungal virulence in an insect model. Strikingly, the overexpression of Afmed15 caused fungal death accompanied by intensive autophagy. RNA sequencing of an Afmed15 overexpression strain revealed that altered gene expression patterns were associated with carbon metabolism, energy metabolism, and translation. Interestingly, the addition of metal ions could partially rescue fungal death caused by the overexpression of Afmed15, indicating that disordered ion homeostasis is a potential reason for the fungal death caused by the overexpression of Afmed15. Considering that the precise expression of Afmed15 is crucial for fungal development, virulence, and survival and that no ortholog was found in humans, Afmed15 is an ideal target for antifungal-drug development.

IMPORTANCE The identification and characterization of regulators essential for virulence or development constitute one approach for antifungal drug development. In this study, we screened and functionally characterized Afmed15, a novel developmental regulator in A. fumigatus. We demonstrate that the precise transcriptional expression of Afmed15 is crucial for fungal asexual development, virulence, and survival. Downregulating the expression of Afmed15 abolished the conidiation and decreased the fungal virulence in an insect model. In contrast, the overexpression of Afmed15 caused fungal death accompanied by intensive autophagy. Our study provides a foundation for further studies to identify compounds perturbing the expression of Afmed15 that may be used for the prevention of invasive A. fumigatus infections.

Identification of Essential Genes and Fluconazole Susceptibility Genes in Candida glabrata by Profiling Hermes Transposon Insertions

Within the budding yeasts, the opportunistic pathogen Candida glabrata and other members of the Nakaseomyces clade have developed virulence traits independently from C. albicans and C. auris. To begin exploring the genetic basis of C. glabrata virulence and its innate resistance to antifungals, we launched the Hermes transposon from a plasmid and sequenced more than 500,000 different semi-random insertions throughout the genome. With machine learning, we identified 1278 protein-encoding genes (25% of total) that could not tolerate transposon insertions and are likely essential for C. glabrata fitness in vitro. Interestingly, genes involved in mRNA splicing were less likely to be essential in C. glabrata than their orthologs in S. cerevisiae, whereas the opposite is true for genes involved in kinetochore function and chromosome segregation. When a pool of insertion mutants was challenged with the first-line antifungal fluconazole, insertions in several known resistance genes (e.g., PDR1, CDR1, PDR16, PDR17, UPC2A, DAP1, STV1) and 15 additional genes (including KGD1, KGD2, YHR045W) became hypersensitive to fluconazole. Insertions in 200 other genes conferred significant resistance to fluconazole, two-thirds of which function in mitochondria and likely down-regulate Pdr1 expression or function. Knockout mutants of KGD2 and IDH2, which consume and generate alpha-ketoglutarate in mitochondria, exhibited increased and decreased resistance to fluconazole through a process that depended on Pdr1. These findings establish the utility of transposon insertion profiling in forward genetic investigations of this important pathogen of humans.

Natural polymorphism of Ym1 regulates pneumonitis through alternative activation of macrophages

We have positionally cloned the Ym1 gene, with a duplication and a promoter polymorphism, as a major regulator of inflammation. Mice with the RIIIS/J haplotype, with the absence of Ym1 expression, showed reduced susceptibility to mannan-enhanced collagen antibody-induced arthritis and to chronic arthritis induced by intranasal exposure of mannan. Depletion of lung macrophages alleviated arthritis, whereas intranasal supplement of Ym1 protein to Ym1-deficient mice reversed the disease, suggesting a key role of Ym1 for inflammatory activity by lung macrophages. Ym1-deficient mice with pneumonitis had less eosinophil infiltration, reduced production of type II cytokines and IgG1, and skewing of macrophages toward alternative activation due to enhanced STAT6 activation. Proteomics analysis connected Ym1 polymorphism with changed lipid metabolism. Induced PPAR-γ and lipid metabolism in Ym1-deficient macrophages contributed to cellular polarization. In conclusion, the natural polymorphism of Ym1 regulates alternative activation of macrophages associated with pulmonary inflammation.

Listening In: Plasmacytoid DC, Monocyte-Derived DC, and Neutrophil Crosstalk in Antifungal Defense

Plasmacytoid DCs (pDCs) are typically thought to be key in antiviral defense. In this issue of Cell Host & Microbe, Guo, Kasahara et al. (2020) reveal a critical role for pDCs in antifungal immunity. Aspergillus-infected monocyte-derived DCs and neutrophils recruit pDCs, which promote neutrophil fungicidal activity.

During Aspergillus Infection, Monocyte-Derived DCs, Neutrophils, and Plasmacytoid DCs Enhance Innate Immune Defense through CXCR3-Dependent Crosstalk

Aspergillus fumigatus, a ubiquitous mold, is a common cause of invasive aspergillosis (IA) in immunocompromised patients. Host defense against IA relies on lung-infiltrating neutrophils and monocyte-derived dendritic cells (Mo-DCs). Here, we demonstrate that plasmacytoid dendritic cells (pDCs), which are prototypically antiviral cells, participate in innate immune crosstalk underlying mucosal antifungal immunity. Aspergillus-infected murine Mo-DCs and neutrophils recruited pDCs to the lung by releasing the CXCR3 ligands, CXCL9 and CXCL10, in a Dectin-1 and Card9- and type I and III interferon signaling-dependent manner, respectively. During aspergillosis, circulating pDCs entered the lung in response to CXCR3-dependent signals. Via targeted pDC ablation, we found that pDCs were essential for host defense in the presence of normal neutrophil and Mo-DC numbers. Although interactions between pDC and fungal cells were not detected, pDCs regulated neutrophil NADPH oxidase activity and conidial killing. Thus, pDCs act as positive feedback amplifiers of neutrophil effector activity against inhaled mold conidia.

The Essential Thioredoxin Reductase of the Human Pathogenic Mold Aspergillus fumigatus Is a Promising Antifungal Target

The identification of cellular targets for antifungal compounds is a cornerstone for the development of novel antimycotics, for which a significant need exists due to increasing numbers of susceptible patients, emerging pathogens, and evolving resistance. For the human pathogenic mold Aspergillus fumigatus, the causative agent of the opportunistic disease aspergillosis, only a limited number of established targets and corresponding drugs are available. Among several targets that were postulated from a variety of experimental approaches, the conserved thioredoxin reductase (TrxR) activity encoded by the trxR gene was assessed in this study. Its essentiality could be confirmed following a conditional TetOFF promoter replacement strategy. Relevance of the trxR gene product for oxidative stress resistance was revealed and, most importantly, its requirement for full virulence of A. fumigatus in two different models of infection resembling invasive aspergillosis. Our findings complement the idea of targeting the reductase component of the fungal thioredoxin system for antifungal therapy.

Human Neutrophils Produce Antifungal Extracellular Vesicles against Aspergillus fumigatus

Polymorphonuclear granulocytes (PMNs) are indispensable for controlling life-threatening fungal infections. In addition to various effector mechanisms, PMNs also produce extracellular vesicles (EVs). Their contribution to antifungal defense has remained unexplored. We reveal that the clinically important human-pathogenic fungus Aspergillus fumigatus triggers PMNs to release a distinct set of antifungal EVs (afEVs). Proteome analyses indicated that afEVs are enriched in antimicrobial proteins. The cargo and the release kinetics of EVs are modulated by the fungal strain confronted. Tracking of afEVs indicated that they associated with fungal cells and even entered fungal hyphae, resulting in alterations in the morphology of the fungal cell wall and dose-dependent antifungal effects. To assess as a proof of concept whether the antimicrobial proteins found in afEVs might contribute to growth inhibition of hyphae when present in the fungal cytoplasm, two human proteins enriched in afEVs, cathepsin G and azurocidin, were heterologously expressed in fungal hyphae. This led to reduced fungal growth relative to that of a control strain producing the human retinol binding protein 7. In conclusion, extracellular vesicles produced by neutrophils in response to A. fumigatus infection are able to associate with the fungus, limit growth, and elicit cell damage by delivering antifungal cargo. This finding offers an intriguing, previously overlooked mechanism of antifungal defense against A. fumigatus IMPORTANCE Invasive fungal infections caused by the mold Aspergillus fumigatus are a growing concern in the clinic due to the increasing use of immunosuppressive therapies and increasing antifungal drug resistance. These infections result in high rates of mortality, as treatment and diagnostic options remain limited. In healthy individuals, neutrophilic granulocytes are critical for elimination of A. fumigatus from the host; however, the exact extracellular mechanism of neutrophil-mediated antifungal activity remains unresolved. Here, we present a mode of antifungal defense employed by human neutrophils against A. fumigatus not previously described. We found that extracellular vesicles produced by neutrophils in response to A. fumigatus infection are able to associate with the fungus, limit growth, and elicit cell damage by delivering antifungal cargo. In the end, antifungal extracellular vesicle biology provides a significant step forward in our understanding of A. fumigatus host pathogenesis and opens up novel diagnostic and therapeutic possibilities.

Aspergillus fumigatus and Aspergillosis in 2019

Aspergillus fumigatus is a saprotrophic fungus; its primary habitat is the soil. In its ecological niche, the fungus has learned how to adapt and proliferate in hostile environments. This capacity has helped the fungus to resist and survive against human host defenses and, further, to be responsible for one of the most devastating lung infections in terms of morbidity and mortality. In this review, we will provide (i) a description of the biological cycle of A. fumigatus; (ii) a historical perspective of the spectrum of aspergillus disease and the current epidemiological status of these infections; (iii) an analysis of the modes of immune response against Aspergillus in immunocompetent and immunocompromised patients; (iv) an understanding of the pathways responsible for fungal virulence and their host molecular targets, with a specific focus on the cell wall; (v) the current status of the diagnosis of different clinical syndromes; and (vi) an overview of the available antifungal armamentarium and the therapeutic strategies in the clinical context. In addition, the emergence of new concepts, such as nutritional immunity and the integration and rewiring of multiple fungal metabolic activities occurring during lung invasion, has helped us to redefine the opportunistic pathogenesis of A. fumigatus.

Programmed Cell Death in Neurospora crassa Is Controlled by the Allorecognition Determinant rcd-1

Nonself recognition following cell fusion between genetically distinct individuals of the same species in filamentous fungi often results in a programmed cell death (PCD) reaction, where the heterokaryotic fusion cell is compartmentalized and rapidly killed. The allorecognition process plays a key role as a defense mechanism that restricts genome exploitation, resource plundering, and the spread of deleterious senescence plasmids and mycoviruses. Although a number of incompatibility systems have been described that function in mature hyphae, less is known about the PCD pathways in asexual spores, which represent the main infectious unit in various human and plant fungal pathogens. Here, we report the identification of regulator of cell death-1 (rcd-1), a novel allorecognition gene, controlling PCD in germinating asexual spores of Neurospora crassa; rcd-1 is one of the most polymorphic genes in the genomes of wild N. crassa isolates. The coexpression of two antagonistic rcd-1-1 and rcd-1-2 alleles was necessary and sufficient to trigger cell death in fused germlings and in hyphae. Based on analysis of wild populations of N. crassa and N. discreta, rcd-1 alleles appeared to be under balancing selection and associated with trans-species polymorphisms. We shed light on genomic rearrangements that could have led to the emergence of the incompatibility system in Neurospora and show that rcd-1 belongs to a much larger gene family in fungi. Overall, our work contributes toward a better understanding of allorecognition and PCD in an underexplored developmental stage of filamentous fungi.

The AP-1-like transcription factor ChAP1 balances tolerance and cell death in the response of the maize pathogen Cochliobolus heterostrophus to a plant phenolic

Fungal pathogens need to contend with stresses including oxidants and antimicrobial chemicals resulting from host defenses. ChAP1 of Cochliobolus heterostrophus, agent of Southern corn leaf blight, encodes an ortholog of yeast YAP1. ChAP1 is retained in the nucleus in response to plant-derived phenolic acids, in addition to its well-studied activation by oxidants. Here, we used transcriptome profiling to ask which genes are regulated in response to ChAP1 activation by ferulic acid (FA), a phenolic abundant in the maize host. Nuclearization of ChAP1 in response to phenolics is not followed by strong expression of genes needed for oxidative stress tolerance. We, therefore, compared the transcriptomes of the wild-type pathogen and a ChAP1 deletion mutant, to study the function of ChAP1 in response to FA. We hypothesized that if ChAP1 is retained in the nucleus under plant-related stress conditions yet in the absence of obvious oxidant stress, it should have additional regulatory functions. The transcriptional signature in response to FA in the wild type compared to the mutant sheds light on the signaling mechanisms and response pathways by which ChAP1 can mediate tolerance to ferulic acid, distinct from its previously known role in the antioxidant response. The ChAP1-dependent FA regulon consists mainly of two large clusters. The enrichment of transport and metabolism-related genes in cluster 1 indicates that C. heterostrophus degrades FA and removes it from the cell. When this fails at increasing stress levels, FA provides a signal for cell death, indicated by the enrichment of cell death-related genes in cluster 2. By quantitation of survival and by TUNEL assays, we show that ChAP1 promotes survival and mitigates cell death. Growth rate data show a time window in which the mutant colony expands faster than the wild type. The results delineate a transcriptional regulatory pattern in which ChAP1 helps balance a survival response for tolerance to FA, against a pathway promoting cell death in the pathogen. A general model for the transition from a phase where the return to homeostasis dominates to a phase leading to the onset of cell death provides a context for understanding these findings.