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.

Characterization of the Paracoccidioides Hypoxia Response Reveals New Insights into Pathogenesis Mechanisms of This Important Human Pathogenic Fungus

BACKGROUND

Hypoxic microenvironments are generated during fungal infection. It has been described that to survive in the human host, fungi must also tolerate and overcome in vivo microenvironmental stress conditions including low oxygen tension; however nothing is known how Paracoccidioides species respond to hypoxia. The genus Paracoccidioides comprises human thermal dimorphic fungi and are causative agents of paracoccidioidomycosis (PCM), an important mycosis in Latin America.

METHODOLOGY/PRINCIPAL FINDINGS

In this work, a detailed hypoxia characterization was performed in Paracoccidioides. Using NanoUPLC-MSE proteomic approach, we obtained a total of 288 proteins differentially regulated in 12 and 24 h of hypoxia, providing a global view of metabolic changes during this stress. In addition, a functional characterization of the homologue to the most important molecule involved in hypoxia responses in other fungi, the SREBP (sterol regulatory element binding protein) was performed. We observed that Paracoccidioides species have a functional homologue of SREBP, named here as SrbA, detected by using a heterologous genetic approach in the srbA null mutant in Aspergillus fumigatus. Paracoccidioides srbA (PbsrbA), in addition to involvement in hypoxia, is probable involved in iron adaptation and azole drug resistance responses.

CONCLUSIONS/SIGNIFICANCE

In this study, the hypoxia was characterized in Paracoccidioides. The first results can be important for a better understanding of the fungal adaptation to the host and improve the arsenal of molecules for the development of alternative treatment options in future, since molecules related to fungal adaptation to low oxygen levels are important to virulence and pathogenesis in human pathogenic fungi.

IL-1α signaling is critical for leukocyte recruitment after pulmonary Aspergillus fumigatus challenge

Aspergillus fumigatus is a mold that causes severe pulmonary infections. Our knowledge of how A. fumigatus growth is controlled in the respiratory tract is developing, but still limited. Alveolar macrophages, lung resident macrophages, and airway epithelial cells constitute the first lines of defense against inhaled A. fumigatus conidia. Subsequently, neutrophils and inflammatory CCR2+ monocytes are recruited to the respiratory tract to prevent fungal growth. However, the mechanism of neutrophil and macrophage recruitment to the respiratory tract after A. fumigatus exposure remains an area of ongoing investigation. Here we show that A. fumigatus pulmonary challenge induces expression of the inflammasome-dependent cytokines IL-1β and IL-18 within the first 12 hours, while IL-1α expression continually increases over at least the first 48 hours. Strikingly, Il1r1-deficient mice are highly susceptible to pulmonary A. fumigatus challenge exemplified by robust fungal proliferation in the lung parenchyma. Enhanced susceptibility of Il1r1-deficient mice correlated with defects in leukocyte recruitment and anti-fungal activity. Importantly, IL-1α rather than IL-1β was crucial for optimal leukocyte recruitment. IL-1α signaling enhanced the production of CXCL1. Moreover, CCR2+ monocytes are required for optimal early IL-1α and CXCL1 expression in the lungs, as selective depletion of these cells resulted in their diminished expression, which in turn regulated the early accumulation of neutrophils in the lung after A. fumigatus challenge. Enhancement of pulmonary neutrophil recruitment and anti-fungal activity by CXCL1 treatment could limit fungal growth in the absence of IL-1α signaling. In contrast to the role of IL-1α in neutrophil recruitment, the inflammasome and IL-1β were only essential for optimal activation of anti-fungal activity of macrophages. As such, Pycard-deficient mice are mildly susceptible to A. fumigatus infection. Taken together, our data reveal central, non-redundant roles for IL-1α and IL-1β in controlling A. fumigatus infection in the murine lung.

Aspergillus spp. are ubiquitous in the environment, and even though individuals are regularly exposed to fungal spores clinical invasive disease is a rare manifestation. In contrast, individuals with weakened immune systems develop severe disease, such as invasive pulmonary aspergillosis (IPA). IPA is associated with extremely poor prognoses and unacceptably high mortality rates. Knowledge gained from understanding how immunocompetent mammals control Aspergillus challenge will help develop new immunomodulatory strategies aimed at improving patient outcomes. It is well known that neutrophils and monocytes are crucial immune cells that act to limit fungal growth. Our work demonstrates a central role for the cytokine IL-1α in orchestrating the optimal recruitment of neutrophils and monocytes, whereas IL-1β and the inflammasome are more important in activation of anti-fungal activity of the monocytes. Moreover, our studies indicate that CCR2⁺ monocytes are required for optimal production of IL-1α in the lungs of A. fumigatus challenged mice. Thus, our data highlight a crucial role of the IL-1 cytokine in mediating anti-fungal immunity which might be harnessed to treat clinical cases of IPA.

Compartment-specific and sequential role of MyD88 and CARD9 in chemokine induction and innate defense during respiratory fungal infection

Aspergillus fumigatus forms ubiquitous airborne conidia that humans inhale on a daily basis. Although respiratory fungal infection activates the adaptor proteins CARD9 and MyD88 via C-type lectin, Toll-like, and interleukin-1 family receptor signals, defining the temporal and spatial pattern of MyD88- and CARD9-coupled signals in immune activation and fungal clearance has been difficult to achieve. Herein, we demonstrate that MyD88 and CARD9 act in two discrete phases and in two cellular compartments to direct chemokine- and neutrophil-dependent host defense. The first phase depends on MyD88 signaling because genetic deletion of MyD88 leads to delayed induction of the neutrophil chemokines CXCL1 and CXCL5, delayed neutrophil lung trafficking, and fatal pulmonary damage at the onset of respiratory fungal infection. MyD88 expression in lung epithelial cells restores rapid chemokine induction and neutrophil recruitment via interleukin-1 receptor signaling. Exogenous CXCL1 administration reverses murine mortality in MyD88-deficient mice. The second phase depends predominately on CARD9 signaling because genetic deletion of CARD9 in radiosensitive hematopoietic cells interrupts CXCL1 and CXCL2 production and lung neutrophil recruitment beyond the initial MyD88-dependent phase. Using a CXCL2 reporter mouse, we show that lung-infiltrating neutrophils represent the major cellular source of CXCL2 during CARD9-dependent recruitment. Although neutrophil-intrinsic MyD88 and CARD9 function are dispensable for neutrophil conidial uptake and killing in the lung, global deletion of both adaptor proteins triggers rapidly progressive invasive disease when mice are challenged with an inoculum that is sub-lethal for single adapter protein knockout mice. Our findings demonstrate that distinct signal transduction pathways in the respiratory epithelium and hematopoietic compartment partially overlap to ensure optimal chemokine induction, neutrophil recruitment, and fungal clearance within the respiratory tract.

Our understanding of how epithelial and hematopoietic cells in the lung coordinate immunity against inhaled fungal conidia (spores) remains limited. The mold Aspergillus fumigatus is a major cause of infectious mortality in immune compromised patients. Host defense against A. fumigatus involves the activation of two host signal transducers, MyD88 and CARD9, leading to neutrophil recruitment to the infection site. In this study, we define how MyD88- and CARD9-coupled signals operate in epithelial and hematopoietic compartments to regulate neutrophil-mediated defense against A. fumigatus. Our studies support a two-stage model in which MyD88 activation in epithelial cells, via the interleukin-1 receptor, supports the rapid induction of neutrophil-recruiting chemokines. This process is essential for the first phase of neutrophil recruitment. Mortality observed in MyD88-deficient mice can be significantly reversed by administration of a chemokine termed CXCL1 to infected airways. The second phase of neutrophil recruitment is initiated by CARD9 signaling in hematopoietic cells. Loss of both phases of chemokine induction and neutrophil recruitment dramatically increases murine susceptibility to tissue-invasive disease. In sum, our study defines a temporal sequence of events, initiated by interleukin-1 receptor/MyD88 signaling in the pulmonary epithelium and propagated by CARD9 signaling in hematopoietic cells, that induces protective immunity against inhaled fungal conidia.

mTOR- and HIF-1α-mediated aerobic glycolysis as metabolic basis for trained immunity

Epigenetic reprogramming of myeloid cells, also known as trained immunity, confers nonspecific protection from secondary infections. Using histone modification profiles of human monocytes trained with the Candida albicans cell wall constituent β-glucan, together with a genome-wide transcriptome, we identified the induced expression of genes involved in glucose metabolism. Trained monocytes display high glucose consumption, high lactate production, and a high ratio of nicotinamide adenine dinucleotide (NAD(+)) to its reduced form (NADH), reflecting a shift in metabolism with an increase in glycolysis dependent on the activation of mammalian target of rapamycin (mTOR) through a dectin-1-Akt-HIF-1α (hypoxia-inducible factor-1α) pathway. Inhibition of Akt, mTOR, or HIF-1α blocked monocyte induction of trained immunity, whereas the adenosine monophosphate-activated protein kinase activator metformin inhibited the innate immune response to fungal infection. Mice with a myeloid cell-specific defect in HIF-1α were unable to mount trained immunity against bacterial sepsis. Our results indicate that induction of aerobic glycolysis through an Akt-mTOR-HIF-1α pathway represents the metabolic basis of trained immunity.

Myeloid derived hypoxia inducible factor 1-alpha is required for protection against pulmonary Aspergillus fumigatus infection

Hypoxia inducible factor 1α (HIF1α) is the mammalian transcriptional factor that controls metabolism, survival, and innate immunity in response to inflammation and low oxygen. Previous work established that generation of hypoxic microenvironments occurs within the lung during infection with the human fungal pathogen Aspergillus fumigatus. Here we demonstrate that A. fumigatus stabilizes HIF1α protein early after pulmonary challenge that is inhibited by treatment of mice with the steroid triamcinolone. Utilizing myeloid deficient HIF1α mice, we observed that HIF1α is required for survival and fungal clearance early following pulmonary challenge with A. fumigatus. Unlike previously reported research with bacterial pathogens, HIF1α deficient neutrophils and macrophages were surprisingly not defective in fungal conidial killing. The increase in susceptibility of the myeloid deficient HIF1α mice to A. fumigatus was in part due to decreased early production of the chemokine CXCL1 (KC) and increased neutrophil apoptosis at the site of infection, resulting in decreased neutrophil numbers in the lung. Addition of recombinant CXCL1 restored neutrophil survival and numbers, murine survival, and fungal clearance. These results suggest that there are unique HIF1α mediated mechanisms employed by the host for protection and defense against fungal pathogen growth and invasion in the lung. Additionally, this work supports the strategy of exploring HIF1α as a therapeutic target in specific immunosuppressed populations with fungal infections.

Two C4-sterol methyl oxidases (Erg25) catalyse ergosterol intermediate demethylation and impact environmental stress adaptation in Aspergillus fumigatus

The human pathogen Aspergillus fumigatus adapts to stress encountered in the mammalian host as part of its ability to cause disease. The transcription factor SrbA plays a significant role in this process by regulating genes involved in hypoxia and low-iron adaptation, antifungal drug responses and virulence. SrbA is a direct transcriptional regulator of genes encoding key enzymes in the ergosterol biosynthesis pathway, including erg25A and erg25B, and ΔsrbA accumulates C4-methyl sterols, suggesting a loss of Erg25 activity [C4-sterol methyl oxidase (SMO)]. Characterization of the two genes encoding SMOs in Aspergillus fumigatus revealed that both serve as functional C4-demethylases, with Erg25A serving in a primary role, as Δerg25A accumulates more C4-methyl sterol intermediates than Δerg25B. Single deletion of these SMOs revealed alterations in canonical ergosterol biosynthesis, indicating that ergosterol may be produced in an alternative fashion in the absence of SMO activity. A Δerg25A strain displayed moderate susceptibility to hypoxia and the endoplasmic reticulum stress-inducing agent DTT, but was not required for virulence in murine or insect models of invasive aspergillosis. Inducing expression of erg25A partially restored the hypoxia growth defect of ΔsrbA. These findings implicated Aspergillus fumigatus SMOs in the maintenance of canonical ergosterol biosynthesis and indicated an overall involvement in the fungal stress response.

The Janus transcription factor HapX controls fungal adaptation to both iron starvation and iron excess

Balance of physiological levels of iron is essential for every organism. In Aspergillus fumigatus and other fungal pathogens, the transcription factor HapX mediates adaptation to iron limitation and consequently virulence by repressing iron consumption and activating iron uptake. Here, we demonstrate that HapX is also essential for iron resistance via activating vacuolar iron storage. We identified HapX protein domains that are essential for HapX functions during either iron starvation or high-iron conditions. The evolutionary conservation of these domains indicates their wide-spread role in iron sensing. We further demonstrate that a HapX homodimer and the CCAAT-binding complex (CBC) cooperatively bind an evolutionary conserved DNA motif in a target promoter. The latter reveals the mode of discrimination between general CBC and specific HapX/CBC target genes. Collectively, our study uncovers a novel regulatory mechanism mediating both iron resistance and adaptation to iron starvation by the same transcription factor complex with activating and repressing functions depending on ambient iron availability.

Endoplasmic reticulum localized PerA is required for cell wall integrity, azole drug resistance, and virulence in Aspergillus fumigatus

GPI-anchoring is a universal and critical post-translational protein modification in eukaryotes. In fungi, many cell wall proteins are GPI-anchored, and disruption of GPI-anchored proteins impairs cell wall integrity. After being synthesized and attached to target proteins, GPI anchors undergo modification on lipid moieties. In spite of its importance for GPI-anchored protein functions, our current knowledge of GPI lipid remodelling in pathogenic fungi is limited. In this study, we characterized the role of a putative GPI lipid remodelling protein, designated PerA, in the human pathogenic fungus Aspergillus fumigatus. PerA localizes to the endoplasmic reticulum and loss of PerA leads to striking defects in cell wall integrity. A perA null mutant has decreased conidia production, increased susceptibility to triazole antifungal drugs, and is avirulent in a murine model of invasive pulmonary aspergillosis. Interestingly, loss of PerA increases exposure of β-glucan and chitin content on the hyphal cell surface, but diminished TNF production by bone marrow-derived macrophages relative to wild type. Given the structural specificity of fungal GPI-anchors, which is different from humans, understanding GPI lipid remodelling and PerA function in A. fumigatus is a promising research direction to uncover a new fungal specific antifungal drug target.

Differential adaptation of Candida albicans in vivo modulates immune recognition by dectin-1

The β-glucan receptor Dectin-1 is a member of the C-type lectin family and functions as an innate pattern recognition receptor in antifungal immunity. In both mouse and man, Dectin-1 has been found to play an essential role in controlling infections with Candida albicans, a normally commensal fungus in man which can cause superficial mucocutaneous infections as well as life-threatening invasive diseases. Here, using in vivo models of infection, we show that the requirement for Dectin-1 in the control of systemic Candida albicans infections is fungal strain-specific; a phenotype that only becomes apparent during infection and cannot be recapitulated in vitro. Transcript analysis revealed that this differential requirement for Dectin-1 is due to variable adaptation of C. albicans strains in vivo, and that this results in substantial differences in the composition and nature of their cell walls. In particular, we established that differences in the levels of cell-wall chitin influence the role of Dectin-1, and that these effects can be modulated by antifungal drug treatment. Our results therefore provide substantial new insights into the interaction between C. albicans and the immune system and have significant implications for our understanding of susceptibility and treatment of human infections with this pathogen.

Fungal cell wall dynamics and infection site microenvironments: signal integration and infection outcome

Upon entrance into the host, fungi encounter a myriad of host effector products and microenvironments that they sense and adapt to for survival. Alterations of the structure and composition of the cell wall is a major fungal adaptation mechanism to evade these environments. Here we discuss recent findings of host-microenvironmental induced fungal cell wall changes, including structure, composition, and protein content, and their effects on host immune responses. A take home message from these recent studies is an emerging understanding of how integration of multiple signals, of both fungal and host responses to dynamic infection site microenvironments, determines outcomes of infection. A challenge moving forward is to further understand these mechanisms and harness them for therapeutic benefit.

Hypoxia enhances innate immune activation to Aspergillus fumigatus through cell wall modulation

Infection by the human fungal pathogen Aspergillus fumigatus induces hypoxic microenvironments within the lung that can alter the course of fungal pathogenesis. How hypoxic microenvironments shape the composition and immune activating potential of the fungal cell wall remains undefined. Herein we demonstrate that hypoxic conditions increase the hyphal cell wall thickness and alter its composition particularly by augmenting total and surface-exposed β-glucan content. In addition, hypoxia-induced cell wall alterations increase macrophage and neutrophil responsiveness and antifungal activity as judged by inflammatory cytokine production and ability to induce hyphal damage. We observe that these effects are largely dependent on the mammalian β-glucan receptor dectin-1. In a corticosteroid model of invasive pulmonary aspergillosis, A. fumigatus β-glucan exposure correlates with the presence of hypoxia in situ. Our data suggest that hypoxia-induced fungal cell wall changes influence the activation of innate effector cells at sites of hyphal tissue invasion, which has potential implications for therapeutic outcomes of invasive pulmonary aspergillosis.

Coordination of hypoxia adaptation and iron homeostasis in human pathogenic fungi

In mammals, hypoxia causes facilitated erythropoiesis that requires increased iron availability with established links between oxygen and iron in regulation of the transcription factor hypoxia-inducible factor. Therefore, cellular responses to hypoxia and iron starvation are linked in mammals and are host conditions that pathogens encounter during infection. In human pathogenic fungi, molecular mechanisms underlying hypoxia adaptation and iron homeostasis have been investigated. However, the interconnected regulation of hypoxia adaptation and iron homeostasis remains to be fully elucidated. This review discusses the potential transcriptional regulatory links between hypoxia adaptation and iron homeostasis in human pathogenic fungi. Transcriptome analyses demonstrate that core regulators of hypoxia adaptation and iron homeostasis are involved in regulation of several common genes responsible for iron acquisition and ergosterol biosynthesis. Importantly, iron starvation increases susceptibility of fungal cells to antifungal drugs and decreased levels of ergosterol, while key hypoxia regulators are also involved in responses to antifungal drugs and mediating ergosterol levels. We suggest that pathogenic fungi have developed a coordinated regulatory system in response to hypoxia and iron starvation through (i) regulation of expression of hypoxia-responsive and iron-responsive genes via cross-linked key regulators, and/or (ii) regulation of factors involved in ergosterol biosynthesis. Thus, both oxygen and iron availability are intimately tied with fungal virulence and responses to existing therapeutics and further elucidation of their interrelationship should have significant clinical implications.

Aspergillus fumigatus mitochondrial electron transport chain mediates oxidative stress homeostasis, hypoxia responses and fungal pathogenesis

We previously observed that hypoxia is an important component of host microenvironments during pulmonary fungal infections. However, mechanisms of fungal growth in these in vivo hypoxic conditions are poorly understood. Here, we report that mitochondrial respiration is active in hypoxia (1% oxygen) and critical for fungal pathogenesis. We generated Aspergillus fumigatus alternative oxidase (aoxA) and cytochrome C (cycA) null mutants and assessed their ability to tolerate hypoxia, macrophage killing and virulence. In contrast to ΔaoxA, ΔcycA was found to be significantly impaired in conidia germination, growth in normoxia and hypoxia, and displayed attenuated virulence. Intriguingly, loss of cycA results in increased levels of AoxA activity, which results in increased resistance to oxidative stress, macrophage killing and long-term persistence in murine lungs. Thus, our results demonstrate a previously unidentified role for fungal mitochondrial respiration in the pathogenesis of aspergillosis, and lay the foundation for future research into its role in hypoxia signalling and adaptation.

Transcriptomic and proteomic analyses of the Aspergillus fumigatus hypoxia response using an oxygen-controlled fermenter

BACKGROUND

Aspergillus fumigatus is a mold responsible for the majority of cases of aspergillosis in humans. To survive in the human body, A. fumigatus must adapt to microenvironments that are often characterized by low nutrient and oxygen availability. Recent research suggests that the ability of A. fumigatus and other pathogenic fungi to adapt to hypoxia contributes to their virulence. However, molecular mechanisms of A. fumigatus hypoxia adaptation are poorly understood. Thus, to better understand how A. fumigatus adapts to hypoxic microenvironments found in vivo during human fungal pathogenesis, the dynamic changes of the fungal transcriptome and proteome in hypoxia were investigated over a period of 24 hours utilizing an oxygen-controlled fermenter system.

RESULTS

Significant increases in transcripts associated with iron and sterol metabolism, the cell wall, the GABA shunt, and transcriptional regulators were observed in response to hypoxia. A concomitant reduction in transcripts was observed with ribosome and terpenoid backbone biosynthesis, TCA cycle, amino acid metabolism and RNA degradation. Analysis of changes in transcription factor mRNA abundance shows that hypoxia induces significant positive and negative changes that may be important for regulating the hypoxia response in this pathogenic mold. Growth in hypoxia resulted in changes in the protein levels of several glycolytic enzymes, but these changes were not always reflected by the corresponding transcriptional profiling data. However, a good correlation overall (R(2) = 0.2, p < 0.05) existed between the transcriptomic and proteomics datasets for all time points. The lack of correlation between some transcript levels and their subsequent protein levels suggests another regulatory layer of the hypoxia response in A. fumigatus.

CONCLUSIONS

Taken together, our data suggest a robust cellular response that is likely regulated both at the transcriptional and post-transcriptional level in response to hypoxia by the human pathogenic mold A. fumigatus. As with other pathogenic fungi, the induction of glycolysis and transcriptional down-regulation of the TCA cycle and oxidative phosphorylation appear to major components of the hypoxia response in this pathogenic mold. In addition, a significant induction of the transcripts involved in ergosterol biosynthesis is consistent with previous observations in the pathogenic yeasts Candida albicans and Cryptococcus neoformans indicating conservation of this response to hypoxia in pathogenic fungi. Because ergosterol biosynthesis enzymes also require iron as a co-factor, the increase in iron uptake transcripts is consistent with an increased need for iron under hypoxia. However, unlike C. albicans and C. neoformans, the GABA shunt appears to play an important role in reducing NADH levels in response to hypoxia in A. fumigatus and it will be intriguing to determine whether this is critical for fungal virulence. Overall, regulatory mechanisms of the A. fumigatus hypoxia response appear to involve both transcriptional and post-transcriptional control of transcript and protein levels and thus provide candidate genes for future analysis of their role in hypoxia adaptation and fungal virulence.

HacA-independent functions of the ER stress sensor IreA synergize with the canonical UPR to influence virulence traits in Aspergillus fumigatus

Endoplasmic reticulum (ER) stress is a condition in which the protein folding capacity of the ER becomes overwhelmed by an increased demand for secretion or by exposure to compounds that disrupt ER homeostasis. In yeast and other fungi, the accumulation of unfolded proteins is detected by the ER-transmembrane sensor IreA/Ire1, which responds by cleaving an intron from the downstream cytoplasmic mRNA HacA/Hac1, allowing for the translation of a transcription factor that coordinates a series of adaptive responses that are collectively known as the unfolded protein response (UPR). Here, we examined the contribution of IreA to growth and virulence in the human fungal pathogen Aspergillus fumigatus. Gene expression profiling revealed that A. fumigatus IreA signals predominantly through the canonical IreA-HacA pathway under conditions of severe ER stress. However, in the absence of ER stress IreA controls dual signaling circuits that are both HacA-dependent and HacA-independent. We found that a ΔireA mutant was avirulent in a mouse model of invasive aspergillosis, which contrasts the partial virulence of a ΔhacA mutant, suggesting that IreA contributes to pathogenesis independently of HacA. In support of this conclusion, we found that the ΔireA mutant had more severe defects in the expression of multiple virulence-related traits relative to ΔhacA, including reduced thermotolerance, decreased nutritional versatility, impaired growth under hypoxia, altered cell wall and membrane composition, and increased susceptibility to azole antifungals. In addition, full or partial virulence could be restored to the ΔireA mutant by complementation with either the induced form of the hacA mRNA, hacAⁱ, or an ireA deletion mutant that was incapable of processing the hacA mRNA, ireA^Δ10. Together, these findings demonstrate that IreA has both HacA-dependent and HacA-independent functions that contribute to the expression of traits that are essential for virulence in A. fumigatus.

Aspergillus fumigatus is the predominant mold pathogen of humans, responsible for life-threatening infections in patients with depressed immunity. The fungus is highly adapted for secretion, a feature that it uses to extract nutrients from the host environment. High rates of protein secretion can overwhelm the protein folding capacity of the endoplasmic reticulum (ER). The resulting ER stress is alleviated by the unfolded protein response (UPR), a signaling pathway that is triggered by the ER-membrane sensor IreA and executed by the downstream transcription factor HacA. This paper uncovers a novel role for IreA in the expression of multiple adaptive traits that allow the fungus to cope with stress conditions that are encountered during infection. Gene expression profiling of ΔireA and ΔhacA mutants revealed that IreA signals predominantly through the canonical IreA-HacA UPR pathway under extreme conditions of ER stress, but has unexpected HacA-dependent and HacA-independent functions even in the absence of ER stress. These findings establish IreA as an important regulator of A. fumigatus pathogenicity and suggest that therapeutic targeting of the dual functions of this protein could be an effective antifungal strategy.

In vivo hypoxia and a fungal alcohol dehydrogenase influence the pathogenesis of invasive pulmonary aspergillosis

Currently, our knowledge of how pathogenic fungi grow in mammalian host environments is limited. Using a chemotherapeutic murine model of invasive pulmonary aspergillosis (IPA) and (1)H-NMR metabolomics, we detected ethanol in the lungs of mice infected with Aspergillus fumigatus. This result suggests that A. fumigatus is exposed to oxygen depleted microenvironments during infection. To test this hypothesis, we utilized a chemical hypoxia detection agent, pimonidazole hydrochloride, in three immunologically distinct murine models of IPA (chemotherapeutic, X-CGD, and corticosteroid). In all three IPA murine models, hypoxia was observed during the course of infection. We next tested the hypothesis that production of ethanol in vivo by the fungus is involved in hypoxia adaptation and fungal pathogenesis. Ethanol deficient A. fumigatus strains showed no growth defects in hypoxia and were able to cause wild type levels of mortality in all 3 murine models. However, lung immunohistopathology and flow cytometry analyses revealed an increase in the inflammatory response in mice infected with an alcohol dehydrogenase null mutant strain that corresponded with a reduction in fungal burden. Consequently, in this study we present the first in vivo observations that hypoxic microenvironments occur during a pulmonary invasive fungal infection and observe that a fungal alcohol dehydrogenase influences fungal pathogenesis in the lung. Thus, environmental conditions encountered by invading pathogenic fungi may result in substantial fungal metabolism changes that influence subsequent host immune responses.

Iridovirus and microsporidian linked to honey bee colony decline

Background

In 2010 Colony Collapse Disorder (CCD), again devastated honey bee colonies in the USA, indicating that the problem is neither diminishing nor has it been resolved. Many CCD investigations, using sensitive genome-based methods, have found small RNA bee viruses and the microsporidia, Nosema apis and N. ceranae in healthy and collapsing colonies alike with no single pathogen firmly linked to honey bee losses.

Methodology/Principal Findings

We used Mass spectrometry-based proteomics (MSP) to identify and quantify thousands of proteins from healthy and collapsing bee colonies. MSP revealed two unreported RNA viruses in North American honey bees, Varroa destructor-1 virus and Kakugo virus, and identified an invertebrate iridescent virus (IIV) (Iridoviridae) associated with CCD colonies. Prevalence of IIV significantly discriminated among strong, failing, and collapsed colonies. In addition, bees in failing colonies contained not only IIV, but also Nosema. Co-occurrence of these microbes consistently marked CCD in (1) bees from commercial apiaries sampled across the U.S. in 2006–2007, (2) bees sequentially sampled as the disorder progressed in an observation hive colony in 2008, and (3) bees from a recurrence of CCD in Florida in 2009. The pathogen pairing was not observed in samples from colonies with no history of CCD, namely bees from Australia and a large, non-migratory beekeeping business in Montana. Laboratory cage trials with a strain of IIV type 6 and Nosema ceranae confirmed that co-infection with these two pathogens was more lethal to bees than either pathogen alone.

Conclusions/Significance

These findings implicate co-infection by IIV and Nosema with honey bee colony decline, giving credence to older research pointing to IIV, interacting with Nosema and mites, as probable cause of bee losses in the USA, Europe, and Asia. We next need to characterize the IIV and Nosema that we detected and develop management practices to reduce honey bee losses.

Regulation of hypoxia adaptation: an overlooked virulence attribute of pathogenic fungi?

Over the past two decades, the incidence of fungal infections has dramatically increased. This is primarily due to increases in the population of immunocompromised individuals attributed to the HIV/AIDS pandemic and immunosuppression therapies associated with organ transplantation, cancer, and other diseases where new immunomodulatory therapies are utilized. Significant advances have been made in understanding how fungi cause disease, but clearly much remains to be learned about the pathophysiology of these often lethal infections. Fungal pathogens face numerous environmental challenges as they colonize and infect mammalian hosts. Regardless of a pathogen's complexity, its ability to adapt to environmental changes is critical for its survival and ability to cause disease. For example, at sites of fungal infections, the significant influx of immune effector cells and the necrosis of tissue by the invading pathogen generate hypoxic microenvironments to which both the pathogen and host cells must adapt in order to survive. However, our current knowledge of how pathogenic fungi adapt to and survive in hypoxic conditions during fungal pathogenesis is limited. Recent studies have begun to observe that the ability to adapt to various levels of hypoxia is an important component of the virulence arsenal of pathogenic fungi. In this review, we focus on known oxygen sensing mechanisms that non-pathogenic and pathogenic fungi utilize to adapt to hypoxic microenvironments and their possible relation to fungal virulence.

Implications of hypoxic microenvironments during invasive aspergillosis

In order to cause disease, all pathogens must tolerate microenvironmental stresses encountered in vivo during infection. One microenvironmental stress that is known to occur at sites of tissue damage is hypoxia. Yet, the occurrence and impact of hypoxic microenvironments during invasive aspergillosis, caused by the mold Aspergillus fumigatus, are essentially unknown. Here, we briefly review the potential implications of hypoxic microenvironments on the Aspergillus-host interaction. We focus on three areas where hypoxia may play a role in determining the outcome of infection: fungal virulence, host immune responses, and efficacy of current antifungal drug treatments.