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

Integration of Fungus-Specific CandA-C1 into a Trimeric CandA Complex Allowed Splitting of the Gene for the Conserved Receptor Exchange Factor of CullinA E3 Ubiquitin Ligases in Aspergilli

Aspergillus species are important for biotechnological applications, like the production of citric acid or antibacterial agents. Aspergilli can cause food contamination or invasive aspergillosis to immunocompromised humans or animals. Specific treatment is difficult due to limited drug targets and emerging resistances. The CandA complex regulates, as a receptor exchange factor, the activity and substrate variability of the ubiquitin labeling machinery for 26S proteasome-mediated protein degradation. Only Aspergillus species encode at least two proteins that form a CandA complex. This study shows that Aspergillus species had to integrate a third component into the CandA receptor exchange factor complex that is unique to aspergilli and required for vegetative growth, sexual reproduction, and activation of the ubiquitin labeling machinery. These features have interesting implications for the evolution of protein complexes and could make CandA-C1 an interesting candidate for target-specific drug design to control fungal growth without affecting the human ubiquitin-proteasome system.

E3 cullin-RING ubiquitin ligase (CRL) complexes recognize specific substrates and are activated by covalent modification with ubiquitin-like Nedd8. Deneddylation inactivates CRLs and allows Cand1/A to bind and exchange substrate recognition subunits. Human as well as most fungi possess a single gene for the receptor exchange factor Cand1, which is split and rearranged in aspergilli into two genes for separate proteins. Aspergillus nidulans CandA-N blocks the neddylation site, and CandA-C inhibits the interaction to the adaptor/substrate receptor subunits similar to the respective N-terminal and C-terminal parts of single Cand1. The pathogen Aspergillus fumigatus and related species express a CandA-C with a 190-amino-acid N-terminal extension domain encoded by an additional exon. This extension corresponds in most aspergilli, including A. nidulans, to a gene directly upstream of candA-C encoding a 20-kDa protein without human counterpart. This protein was named CandA-C1, because it is also required for the cellular deneddylation/neddylation cycle and can form a trimeric nuclear complex with CandA-C and CandA-N. CandA-C and CandA-N are required for asexual and sexual development and control a distinct secondary metabolism. CandA-C1 and the corresponding domain of A. fumigatus control spore germination, vegetative growth, and the repression of additional secondary metabolites. This suggests that the dissection of the conserved Cand1-encoding gene within the genome of aspergilli was possible because it allowed the integration of a fungus-specific protein required for growth into the CandA complex in two different gene set versions, which might provide an advantage in evolution.

The inhibitor of apoptosis protein MoBir1 is involved in the suppression of hydrogen peroxide-induced fungal cell death, reactive oxygen species generation, and pathogenicity of rice blast fungus

The inhibitor of apoptosis protein (IAP) family has been identified in a variety of organisms. All IAPs contain one to three baculoviral IAP repeat (BIR) domains, which are required for anti-apoptotic activity. Here, we identified a type II BIR domain-containing protein, MoBir1, in the rice blast fungus Magnaporthe oryzae. Expression of the MoBIR1 gene in Saccharomyces cerevisiae suppressed hydrogen peroxide-induced cell death and delayed yeast cell chronological aging. Delayed aging was found to require the carboxyl terminus of MoBir1. M. oryzae transformants overexpressing the MoBIR1 gene demonstrated increased growth rate and biomass, delayed mycelial aging, and enhanced resistance to hydrogen peroxide but reduced reactive oxygen species generation and virulence. Moreover, MoBIR1-overexpressing transformants exhibited anti-apoptotic activity. However, MoBIR1 silencing resulted in no obvious phenotypic changes, compared with the wild-type M. oryzae strain Guy11. Our findings broaden the knowledge on fungal type II BIR domain-containing proteins.

Host defense mechanisms against Aspergillus fumigatus lung colonization and invasion

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

Menacing Mold: Recent Advances in Aspergillus Pathogenesis and Host Defense

The genus Aspergillus is ubiquitous in the environment and contains a number of species, primarily A. fumigatus, that cause mold-associated disease in humans. Humans inhale several hundred to several thousand Aspergillus conidia (i.e., vegetative spores) daily and typically clear these in an asymptomatic manner. In immunocompromised individuals, Aspergillus conidia can germinate into tissue-invasive hyphae, disseminate, and cause invasive aspergillosis. In this review, we first discuss novel concepts in host defense against Aspergillus infections and emphasize new insights in fungal recognition and signaling, innate immune activation, and fungal killing. Second, the review focuses on novel concepts of Aspergillus pathogenesis and highlights emerging knowledge regarding fungal strain heterogeneity, stress responses, and metabolic adaptations on infectious outcomes. Mechanistic insight into the host-pathogen interplay is thus critical to define novel druggable fungal targets and to exploit novel immune-based strategies to improve clinical outcomes associated with aspergillosis in vulnerable patient populations.

Mucor circinelloides Thrives inside the Phagosome through an Atf-Mediated Germination Pathway

Mucormycosis is an emerging fungal infection that is often lethal due to the ineffectiveness of current therapies. Here, we have studied the first stage of this infection-the germination of Mucor circinelloides spores inside phagocytic cells-from an integrated transcriptomic and functional perspective. A relevant fungal gene network is remodeled in response to phagocytosis, being enriched in crucial functions to survive and germinate inside the phagosome, such as nutritional adaptation and response to oxidative stress. Correspondingly, the phagocytic cells induced a specific proinflammatory and apoptotic response to the pathogenic strain. Deletion of fungal genes encoding putative transcription factors (atf1, atf2, and gcn4), extracellular proteins (chi1 and pps1), and an aquaporin (aqp1) revealed that these genes perform important roles in survival following phagocytosis, germination inside the phagosome, and virulence in mice. atf1 and atf2 play a major role in these pathogenic processes, since their mutants showed the strongest phenotypes and both genes control a complex gene network of secondarily regulated genes, including chi1 and aqp1 These new insights into the initial phase of mucormycosis define genetic regulators and molecular processes that could serve as pharmacological targets.IMPORTANCE Mucorales are a group of ancient saprophytic fungi that cause neglected infectious diseases collectively known as mucormycoses. The molecular processes underlying the establishment and progression of this disease are largely unknown. Our work presents a transcriptomic study to unveil the Mucor circinelloides genetic network triggered in fungal spores in response to phagocytosis by macrophages and the transcriptional response of the host cells. Functional characterization of differentially expressed fungal genes revealed three transcription factors and three extracellular proteins essential for the fungus to survive and germinate inside the phagosome and to cause disease in mice. Two of the transcription factors, highly similar to activating transcription factors (ATFs), coordinate a complex secondary gene response involved in pathogenesis. The significance of our research is in characterizing the initial stages that lead to evasion of the host innate immune response and, in consequence, the dissemination of the infection. This genetic study offers possible targets for novel antifungal drugs against these opportunistic human pathogens.

CD11b Regulates Fungal Outgrowth but Not Neutrophil Recruitment in a Mouse Model of Invasive Pulmonary Aspergillosis

ß2 integrin receptors consist of an alpha subunit (CD11a-CD11d) and CD18 as the common beta subunit, and are differentially expressed by leukocytes. ß2 integrins are required for cell-cell interaction, transendothelial migration, uptake of opsonized pathogens, and cell signaling processes. Functional loss of CD18—termed leukocyte-adhesion deficiency type 1 (LAD1)—results in an immunocompromised state characterized by frequent occurrence of severe infections. In immunosuppressed individuals Aspergillus fumigatus is a frequent cause of invasive pulmonary fungal infection, and often occurs in patients suffering from LAD1. Here, we asked for the importance of CD11b/CD18 also termed MAC-1 which is required for phagocytosis of opsonized A. fumigatus conidia by polymorphonuclear neutrophils (PMN) for control of pulmonary A. fumigatus infection. We show that CD11b^−/− mice infected with A. fumigatus were unaffected in long term survival, similar to wild type (WT) mice. However, bronchoalveolar lavage (BAL) performed 1 day after infection revealed a higher lung infiltration of PMN in case of infected CD11b^−/− mice than observed for WT mice. BAL derived from infected CD11b^−/− mice also contained a higher amount of leukocyte-attracting CCL5 chemokine, but lower amounts of proinflammatory innate cytokines. In accordance, lung tissue of A. fumigatus infected CD11b^−/− mice was characterized by lower cellular inflammation, and a higher fungal burden. In agreement, CD11b^−/−PMN exerted lower phagocytic activity on serum-opsonized A. fumigatus conidia than WT PMN in vitro. Our study shows that MAC-1 is required for effective clearance of A. fumigatus by infiltrating PMN, and the establishment of an inflammatory microenvironment in infected lung. Enhanced infiltration of CD11b^−/− PMN may serve to compensate impaired PMN function.

Targeting intrinsic cell death pathways to control fungal pathogens

Fungal pathogens pose an increasing threat to public health. Limited clinical drug regimens and emerging drug-resistant isolates challenge infection control. The global burden of human fungal pathogens is estimated at 1 billion infections and 1.5 million deaths annually. In addition, plant fungal pathogens increasingly threaten global food resources. Novel strategies are needed to combat emerging fungal diseases and pan-resistant fungi. An untapped mechanistically novel approach is to pharmacologically activate the intrinsic cell death pathways encoded by pathogenic fungi. This strategy is analogous to new anti-cancer therapeutics now entering the clinic. Here we summarize the best understood examples of cell death mechanisms encoded by pathogenic fungi, contrast these to mammalian cell death pathways, and highlight the gaps in knowledge towards identifying potential death effectors as druggable targets.

Expression of Bax/Bcl-2, TGF-β1, and Type III Collagen Fiber in Congenital Muscular Torticollis

BACKGROUND This study investigated the expression of Bax/Bcl-2, TGF-β1 and type III collagen fiber in sternocleidomastoid of congenital muscular torticollis (CMT), and explored the possible mechanisms of fibrosis in sternocleidomastoid of CMT. MATERIAL AND METHODS The localization and expression of Bax, Bcl-2, TGF-β1, and type III collagen were detected in the control group and experimental group by using immunohistochemical staining method. The RT-PCR assay was used to measure the expression of TGF-β1 in the control group and experimental group. RESULTS HE staining results showed that the collagen fiber in the experimental group had more abundant hyperplasia compared to the control group (p<0.05). Immunohistochemical staining results showed that the expression of Bax, Bax/Bcl-2, TGF-β1, and type III collagen in the experimental group was significantly increased compared to the control group (p<0.01). There were positive correlations between expression of Bax/Bcl-2 and TGF-b1, and between expression of TGF-β1 and type III collagen fiber (p<0.05, r=0.32 and 0.83, respectively). The RT-PCR results showed that the expression of TGF-β1 mRNA was also significantly elevated in the experimental group compared to the control group (p<0.05). CONCLUSIONS Increased muscular apoptosis may aggravate the formation of muscular fibrosis, which may be involved in the pathogenesis of sternocleidomastoid of CMT.

Programmed cell death in human pathogenic fungi - a possible therapeutic target

INTRODUCTION

Diseases caused by pathogenic fungi are increasing because of antibiotic overuse, the rise of immunosuppressive therapies, and climate change. The limited variety of antimycotics and the rapid adaptation of pathogenic fungi to antifungal agents serve to exacerbate this issue. Unfortunately, about 1.6 million people are killed by fungal infections annually. Areas covered: The discovery of the small antimicrobial proteins produced by microorganisms, animals, humans, and plants will hopefully overcome challenges in the treatment of fungal infections. These small proteins are highly stable and any resistance to them rarely evolves; therefore, they are potentially good candidates for the treatment and prevention of infections caused by pathogenic fungi. Some of these proteins target the programmed cell death machinery of pathogenic fungi; this is potentially a novel approach in antimycotic therapies. In this review, we highlight the elements of apoptosis in human pathogenic fungi and related model organisms and discuss the possible therapeutic potential of the apoptosis-inducing, small, antifungal proteins. Expert opinion: Small antimicrobial proteins may establish a new class of antimycotics in the future. The rarity of resistance and their synergistic effects with other frequently used antifungal agents may help pave the way for their use in the clinic.

Do Fungi Undergo Apoptosis-Like Programmed Cell Death?

This question of whether fungi undergo apoptosis-like programmed cell death can be separated into two questions. One question is about applying the term "apoptosis" to fungi, and the other is a more challenging question of whether fungi have evolved mechanisms that inflict self-injury. The answers to both questions depend on the definitions applied to "apoptosis" and "programmed cell death." Considering how these and other cell death terms originated and are currently defined for animals, some confusion arises when the terms are applied to fungi. While it is difficult to defend the concept of fungal apoptosis, the more interesting issue is whether fungi will eventually be found to encode programmed or extemporaneous self-destructive processes, as suggested by intriguing new findings.

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

Shlezinger et al (Reports, 8 September 2017, p. 1037) report that the common fungus Aspergillus fumigatus, a cause of aspergillosis, undergoes caspase-dependent apoptosis-like cell death triggered by lung neutrophils. However, the technologies they used do not provide reliable evidence that fungal cells die via a protease signaling cascade thwarted by a fungal caspase inhibitor homologous to human survivin.

The role of neutrophils in host defense against invasive fungal infections

Purpose of Review

Invasive fungal infections caused by the commensal yeast Candida and the ubiquitous, inhaled mold Aspergillus have emerged as major causes of morbidity and mortality in critically ill and immunosuppressed patient populations. Here, we review how neutrophils contribute to effective immunity against these infections.

Recent Findings

Studies in mouse models of invasive candidiasis and aspergillosis, and observations in hematological patients with chemotherapy-induced neutropenia and in patients with primary immunodeficiency disorders that manifest with these infections have highlighted the critical role of neutrophils and have identified key immune factors that promote neutrophil-mediated effective host defense against invasive fungal disease.

Summary

Neutrophils are crucial in host protection against invasive candidiasis and aspergillosis. Recent advances in our understanding of the molecular cues that mediate protective neutrophil recruitment and effector function against these infections hold promise for developing immune-based strategies to improve the outcomes of affected patients.

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

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

Antifungal immune responses: emerging host-pathogen interactions and translational implications

Understanding the complex and highly dynamic interactions between fungi and host cells in a tissue-specific manner is crucial to facilitate the development of new therapeutic approaches to infections. Here, we discuss recent studies that are revealing the mechanisms underlying this context-dependent interplay.

Antifungal Innate Immunity: A Perspective from the Last 10 Years

Fungal pathogens can rarely cause diseases in immunocompetent individuals. However, commensal and normally nonpathogenic environmental fungi can cause life-threatening infections in immunocompromised individuals. Over the last few decades, there has been a huge increase in the incidence of invasive opportunistic fungal infections along with a worrying increase in antifungal drug resistance. As a consequence, research focused on understanding the molecular and cellular basis of antifungal immunity has expanded tremendously in the last few years. This review will provide an overview of the most exciting recent advances in innate antifungal immunity, discoveries that are helping to pave the way for the development of new strategies that are desperately needed to combat these devastating diseases.

The Emerging Roles for Telomerase in the Central Nervous System

Telomerase, a specialized ribonucleoprotein enzyme complex, maintains telomere length at the 3′ end of chromosomes, and functions importantly in stem cells, cancer and aging. Telomerase exists in neural stem cells (NSCs) and neural progenitor cells (NPCs), at a high level in the developing and adult brains of humans and rodents. Increasing studies have demonstrated that telomerase in NSCs/NPCs plays important roles in cell proliferation, neuronal differentiation, neuronal survival and neuritogenesis. In addition, recent works have shown that telomerase reverse transcriptase (TERT) can protect newborn neurons from apoptosis and excitotoxicity. However, to date, the link between telomerase and diseases in the central nervous system (CNS) is not well reviewed. Here, we analyze the evidence and summarize the important roles of telomerase in the CNS. Understanding the roles of telomerase in the nervous system is not only important to gain further insight into the process of the neural cell life cycle but would also provide novel therapeutic applications in CNS diseases such as neurodegenerative condition, mood disorders, aging and other ailments.

Mitochondrial Fragmentation in Aspergillus fumigatus as Early Marker of Granulocyte Killing Activity

The host's defense against invasive mold infections relies on diverse antimicrobial activities of innate immune cells. However, studying these mechanisms in vitro is complicated by the filamentous nature of such pathogens that typically form long, branched, multinucleated and compartmentalized hyphae. Here we describe a novel method that allows for the visualization and quantification of the antifungal killing activity exerted by human granulocytes against hyphae of the opportunistic pathogen Aspergillus fumigatus. The approach relies on the distinct impact of fungal cell death on the morphology of mitochondria that were visualized with green fluorescent protein (GFP). We show that oxidative stress induces complete fragmentation of the tubular mitochondrial network which correlates with cell death of affected hyphae. Live cell microscopy revealed a similar and non-reversible disruption of the mitochondrial morphology followed by fading of fluorescence in Aspergillus hyphae that were killed by human granulocytes. Quantitative microscopic analysis of fixed samples was subsequently used to estimate the antifungal activity. By utilizing this assay, we demonstrate that lipopolysaccharides as well as human serum significantly increase the killing efficacy of the granulocytes. Our results demonstrate that evaluation of the mitochondrial morphology can be utilized to assess the fungicidal activity of granulocytes against A. fumigatus hyphae.

Heavy Metal-Induced Expression of PcaA Provides Cadmium Tolerance to Aspergillus fumigatus and Supports Its Virulence in the Galleria mellonella Model

Most of the metal transporters in Aspergillus fumigatus are yet uncharacterized. Their role in fungal metabolism and virulence remains unclear. This paper describes the novel PIB-type cation ATPase PcaA, which links metal homeostasis and heavy metal tolerance in the opportunistic human pathogen A. fumigatus. The protein possesses conserved ATPase motif and shares 51% amino acid sequence identity with the Saccharomyces cerevisiae cadmium exporter Pca1p. A pcaA deletion, an overexpression and a gfp-pcaA complementation strain of A. fumigatus were constructed and their heavy metal susceptibilities were studied. The pcaA knock out strain showed drastically decreased cadmium tolerance, however, its growth was not affected by the exposure to high concentrations of copper, iron, zinc, or silver ions. Although the lack of PcaA had no effect on copper adaption, we demonstrated that not only cadmium but also copper ions are able to induce the transcription of pcaA in A. fumigatus wild type Af293. Similarly, cadmium and copper ions could induce the copper exporting ATPase crpA. These data imply a general response on the transcriptomic level to heavy metals in A. fumigatus through the induction of detoxification systems. Confocal microscopy of the gfp-pcaA complementation strain expressing functional GFP-PcaA supports the predicted membrane localization of PcaA. The GFP-PcaA fusion protein is located in the plasma membrane of A. fumigatus in the presence of cadmium ions. Virulence assays support a function of PcaA for virulence of A. fumigatus in the Galleria mellonella wax moth larvae model, which might be linked to the elimination of reactive oxygen species.

Neutrophil extracellular traps in fungal infection

Fungal infections are a continuously increasing problem in modern health care. Understanding the complex biology of the emerging pathogens and unraveling the mechanisms of host defense may form the basis for the development of more efficient diagnostic and therapeutic tools. Neutrophils play a pivotal role in the defense against fungal pathogens. These phagocytic hunters migrate towards invading fungal microorganisms and eradicate them by phagocytosis, oxidative burst and release of neutrophil extracellular traps (NETs). In the last decade, the process of NET formation has received unparalleled attention, with numerous studies revealing the relevance of this neutrophil function for control of various mycoses. Here, we describe NET formation and summarize its role as part of the innate immune defense against fungal pathogens. We highlight factors influencing the formation of these structures and molecular mechanisms employed by fungi to impair the formation of NETs or subvert their antifungal effects.

Oxidative responses and fungal infection biology

The balance between reactive oxygen species and reactive nitrogen species production by the host and stress response by fungi is a key axis of the host-pathogen interaction. This review will describe emerging themes in fungal pathogenesis underpinning this axis.

Size matters - how the immune system deals with fungal hyphae

Fungal hyphae constitute a special challenge for the immune system, since they are too large to be phagocytosed. This review summarizes our current knowledge on those immune cells that are able to attack and eliminate hyphae and we discuss the different means that are employed by these cells in order to kill hyphae.

Host Control of Fungal Infections: Lessons from Basic Studies and Human Cohorts

In the last few decades, the AIDS pandemic and the significant advances in the medical management of individuals with neoplastic and inflammatory conditions have resulted in a dramatic increase in the population of immunosuppressed patients with opportunistic, life-threatening fungal infections. The parallel development of clinically relevant mouse models of fungal disease and the discovery and characterization of several inborn errors of immune-related genes that underlie inherited human susceptibility to opportunistic mycoses have significantly expanded our understanding of the innate and adaptive immune mechanisms that protect against ubiquitous fungal exposures. This review synthesizes immunological knowledge derived from basic mouse studies and from human cohorts and provides an overview of mammalian antifungal host defenses that show promise for informing therapeutic and vaccination strategies for vulnerable patients.

Human and Murine Innate Immune Cell Populations Display Common and Distinct Response Patterns during Their In Vitro Interaction with the Pathogenic Mold Aspergillus fumigatus

Aspergillus fumigatus is the main cause of invasive fungal infections occurring almost exclusively in immunocompromised patients. An improved understanding of the initial innate immune response is key to the development of better diagnostic tools and new treatment options. Mice are commonly used to study immune defense mechanisms during the infection of the mammalian host with A. fumigatus. However, little is known about functional differences between the human and murine immune response against this fungal pathogen. Thus, we performed a comparative functional analysis of human and murine dendritic cells (DCs), macrophages, and polymorphonuclear cells (PMNs) using standardized and reproducible working conditions, laboratory protocols, and readout assays. A. fumigatus did not provoke identical responses in murine and human immune cells but rather initiated relatively specific responses. While human DCs showed a significantly stronger upregulation of their maturation markers and major histocompatibility complex molecules and phagocytosed A. fumigatus more efficiently compared to their murine counterparts, murine PMNs and macrophages exhibited a significantly stronger release of reactive oxygen species after exposure to A. fumigatus. For all studied cell types, human and murine samples differed in their cytokine response to conidia or germ tubes of A. fumigatus. Furthermore, Dectin-1 showed inverse expression patterns on human and murine DCs after fungal stimulation. These specific differences should be carefully considered and highlight potential limitations in the transferability of murine host–pathogen interaction studies.

Microbial volatile communication in human organotypic lung models

We inhale respiratory pathogens continuously, and the subsequent signaling events between host and microbe are complex, ultimately resulting in clearance of the microbe, stable colonization of the host, or active disease. Traditional in vitro methods are ill-equipped to study these critical events in the context of the lung microenvironment. Here we introduce a microscale organotypic model of the human bronchiole for studying pulmonary infection. By leveraging microscale techniques, the model is designed to approximate the structure of the human bronchiole, containing airway, vascular, and extracellular matrix compartments. To complement direct infection of the organotypic bronchiole, we present a clickable extension that facilitates volatile compound communication between microbial populations and the host model. Using Aspergillus fumigatus, a respiratory pathogen, we characterize the inflammatory response of the organotypic bronchiole to infection. Finally, we demonstrate multikingdom, volatile-mediated communication between the organotypic bronchiole and cultures of Aspergillus fumigatus and Pseudomonas aeruginosa.

Papers of note in Science (6355)

This week’s articles describe an alternative mechanism of secretion that is deployed by intestinal cells in the presence of pathogens; how the immune system protects the lungs from inhaled fungal spores; and metabolic reprogramming of tissue-resident antigen-presenting cells in the lung.

The balance between immunity and inflammation

Immunity in the lung is calibrated to protect against inhaled pathogens while avoiding damaging inflammation.