Candida albicans morphogenesis is not required for macrophage interleukin 1β production

Non-canonical activation of IRE1α during Candida albicans infection enhances macrophage fungicidal activity

While the canonical function of IRE1α is to detect misfolded proteins and activate the unfolded protein response (UPR) to maintain cellular homeostasis, microbial pathogens can also activate IRE1α, which modulates innate immunity and infection outcomes. However, how infection activates IRE1α and its associated inflammatory functions have not been fully elucidated. Recognition of microbe-associated molecular patterns can activate IRE1α, but it is unclear whether this depends on protein misfolding. Here, we report that a common and deadly fungal pathogen, Candida albicans, activates macrophage IRE1α through C-type lectin receptor signaling, reinforcing a role for IRE1α as a central regulator of host responses to infection by a broad range of pathogens. This activation did not depend on protein misfolding in response to C. albicans infection. Moreover, lipopolysaccharide treatment was also able to activate IRE1α prior to protein misfolding, suggesting that pathogen-mediated activation of IRE1α occurs through non-canonical mechanisms. During C. albicans infection, we observed that IRE1α activity promotes phagolysosomal fusion that supports the fungicidal activity of macrophages. Consequently, macrophages lacking IRE1α activity displayed inefficient phagosome maturation, enabling C. albicans to lyse the phagosome, evade fungal killing, and drive aberrant inflammatory cytokine production. Mechanistically, we show that IRE1α activity supports phagosomal calcium flux after phagocytosis of C. albicans, which is crucial for phagosome maturation. Importantly, deletion of IRE1α activity decreased the fungicidal activity of phagocytes in vivo during systemic C. albicans infection. Together, these data provide mechanistic insight for the non-canonical activation of IRE1α during infection, and reveal central roles for IRE1α in macrophage antifungal responses.

Targeting Microglia/Macrophages Notch1 Protects Neurons from Pyroptosis in Ischemic Stroke

BACKGROUND AND AIMS

The immune-inflammatory cascade and pyroptosis play an important role in the pathogenesis of cerebral ischemia-reperfusion injury (CIRI). The maintenance of immune homeostasis is inextricably linked to the Notch signaling pathway, but whether myeloid Notch1 affects microglia polarization as well as neuronal pyroptosis in CIRI is not fully understood. This study was designed to clarify the role of myeloid Notch1 in CIRI, providing new therapeutic strategies for ischemic stroke.

METHODS AND RESULTS

Myeloid-specific Notch1 knockout (Notch1M-KO) mice and the floxed Notch1 (Notch1FL/FL) mice were subjected to middle cerebral artery occlusion (MCAO). After 3 days of CIRI, we evaluated the neurological deficit score and cerebral infarction volume. Immunofluorescence staining was used to detect the expression of Notch1 and microglial subtype markers. Cerebral infiltrating macrophages were detected by flow cytometry. RT-qPCR was used to detect pro-inflammatory cytokines. Western blot was used to detect the expression of pyroptosis related proteins. The Notch1-siRNA transfected BV2 cells were co-cultured with HT22 cells to investigate the potential mechanisms by which microglial Notch1 affects neuronal pyroptosis induced by anoxia/reoxygenation in vitro. We found that Notch1 was activated in cerebral microglia/macrophages after CIRI. Myeloid Notch1 deficiency decreased the cerebral infarct volume (24.17 ± 3.29 vs. 36.17 ± 2.27, p < 0.001), neurological function scores (2.33 ± 0.47 vs. 3.17 ± 0.37, p < 0.001) and the infiltration of peripheral monocytes/macrophages (3.26 ± 0.53 vs. 5.67 ± 0.57, p < 0.01). Strikingly, myeloid-specific Notch1 knockout alleviated pyroptosis. Compared with microglia M1, increased microglia M2 were detected in the ischemic penumbra. In parallel in vitro co-culture experiments, we found that Notch1 knockdown in microglial BV2 cells inhibited anoxia/reoxygenation-induced JAK2/STAT3 activation and pyroptosis in hippocampal neuron HT22 cells.

CONCLUSIONS

Our findings elucidate the underlying mechanism of the myeloid Notch1 signaling pathway in regulating neuronal pyroptosis in CIRI, suggesting that targeting myeloid-specific Notch1 is an effective strategy for the treatment of ischemic stroke.

Imaging-Based Screening Identifies Modulators of the eIF3 Translation Initiation Factor Complex in Candida albicans

Fungal pathogens like Candida albicans can cause devastating human disease. Treatment of candidemia is complicated by the high rate of resistance to common antifungal therapies. Additionally, there is host toxicity associated with many antifungal compounds due to the conservation between essential mammalian and fungal proteins. An attractive new approach for antimicrobial development is to target virulence factors: non-essential processes that are required for the organism to cause disease in human hosts. This approach expands the potential target space while reducing the selective pressure toward resistance, as these targets are not essential for viability. In C. albicans, a key virulence factor is the ability to transition to hyphal morphology. We developed a high-throughput image analysis pipeline to distinguish between yeast and filamentous growth in C. albicans at the single cell level. Based on this phenotypic assay, we screened the FDA drug repurposing library of 2,017 compounds for their ability to inhibit filamentation and identified 33 compounds that block the hyphal transition in C. albicans with IC50 values ranging from 0.2 to 150 μM. Multiple compounds showed a phenyl sulfone chemotype, prompting further analysis. Of these phenyl sulfones, NSC 697923 displayed the most efficacy, and by selecting for resistant mutants, we identified eIF3 as the target of NSC 697923 in C. albicans.

Imaging-based screening identifies modulators of the eIF3 translation initiation factor complex in Candida albicans

Fungal pathogens like Candida albicans can cause devastating human disease. Treatment of candidemia is complicated by the high rate of resistance to common antifungal therapies. Additionally, there is host toxicity associated with many antifungal compounds due to the conservation between essential mammalian and fungal proteins. An attractive new approach for antimicrobial development is to target virulence factors: non-essential processes that are required for the organism to cause disease in human hosts. This approach expands the potential target space while reducing the selective pressure towards resistance, as these targets are not essential for viability. In C. albicans, a key virulence factor is the ability to transition to hyphal morphology. We developed a high-throughput image analysis pipeline to distinguish between yeast and filamentous growth in C. albicans at the single cell level. Based on this phenotypic assay, we screened the FDA drug repurposing library of 2,017 compounds for their ability to inhibit filamentation and identified 33 compounds that block the hyphal transition in C. albicans with IC 50 values ranging from 0.2 to 150 µM. Multiple compounds showed a phenyl vinyl sulfone chemotype, prompting further analysis. Of these phenyl vinyl sulfones, NSC 697923 displayed the most efficacy, and by selecting for resistant mutants, we identified eIF3 as the target of NSC 697923 in C. albicans .

Application of the Mutant Libraries for Candida albicans Functional Genomics

Candida albicans is a typical opportunistic pathogen in humans that causes serious health risks in clinical fungal infections. The construction of mutant libraries has made remarkable developments in the study of C. albicans molecular and cellular biology with the ongoing advancements of gene editing, which include the application of CRISPR-Cas9 and novel high-efficient transposon. Large-scale genetic screens and genome-wide functional analysis accelerated the investigation of new genetic regulatory mechanisms associated with the pathogenicity and resistance to environmental stress in C. albicans. More importantly, sensitivity screening based on C. albicans mutant libraries is critical for the target identification of novel antifungal compounds, which leads to the discovery of Sec7p, Tfp1p, Gwt1p, Gln4p, and Erg11p. This review summarizes the main types of C. albicans mutant libraries and interprets their applications in morphogenesis, biofilm formation, fungus-host interactions, antifungal drug resistance, and target identification.

Vulvovaginal Candida albicans Clinical Isolates’ Resistance to Phagocytosis In-Vitro

Previous studies have revealed that Candida albicans isolates involved in chronic vulvovaginal candidosis (cVVC) phenotypically express less virulent traits than clinical isolates involved in sporadic infections. In this study, we aimed to further explore this finding by studying the behaviour of those same clinical isolates in in-vitro models of infection. Eighteen clinical Candida albicans isolates were collected from women suffering sporadic (eight isolates) or chronic infections (ten isolates). Adhesion to HeLa cells (human cervical cancer epithelial cell line) and resistance to phagocytosis by RAW 264.7 cells (murine macrophages cell line) were tested in-vitro. In addition, phenotypic expression of virulence factors related with either adhesion or resistance to phagocytosis was tested in-vitro. Results indicated that yeast isolates involved in sporadic infection adhered in a higher proportion of HeLa cells than those of chronic infections, which was related with their ability to produce biofilm (p < 0.05). The ability to evade phagocytosis was related to an elevated production of proteases (p < 0.05) by chronic isolates, while sporadic isolates’ resistance to phagocytosis was related to a higher hydrophobicity of cell walls (p < 0.05). We conclude that the evasion of macrophage-mediated phagocytosis related to the production of proteases might be an important factor involved in the recurrence of vulvovaginal candidosis infection.

EFG1, Everyone’s Favorite Gene in Candida albicans: A Comprehensive Literature Review

Candida sp. are among the most common fungal commensals found in the human microbiome. Although Candida can be found residing harmlessly on the surface of the skin and mucosal membranes, these opportunistic fungi have the potential to cause superficial skin, nail, and mucus membrane infections as well as life threatening systemic infections. Severity of infection is dependent on both fungal and host factors including the immune status of the host. Virulence factors associated with Candida sp. pathogenicity include adhesin proteins, degradative enzymes, phenotypic switching, and morphogenesis. A central transcriptional regulator of morphogenesis, the transcription factor Efg1 was first characterized in Candida albicans in 1997. Since then, EFG1 has been referenced in the Candida literature over three thousand times, with the number of citations growing daily. Arguably one of the most well studied genes in Candida albicans, EFG1 has been referenced in nearly all contexts of Candida biology from the development of novel therapeutics to white opaque switching, hyphae morphology to immunology. In the review that follows we will synthesize the research that has been performed on this extensively studied transcription factor and highlight several important unanswered questions.

Interactions of Both Pathogenic and Nonpathogenic CUG Clade Candida Species with Macrophages Share a Conserved Transcriptional Landscape

Candida species are a leading cause of opportunistic, hospital-associated bloodstream infections with high mortality rates, typically in immunocompromised patients. Several species, including Candida albicans, the most prevalent cause of infection, belong to the monophyletic CUG clade of yeasts. Innate immune cells such as macrophages are crucial for controlling infection, and C. albicans responds to phagocytosis by a coordinated induction of pathways involved in catabolism of nonglucose carbon sources, termed alternative carbon metabolism, which together are essential for virulence. However, the interactions of other CUG clade species with macrophages have not been characterized. Here, we analyzed transcriptional responses to macrophage phagocytosis by six Candida species across a range of virulence and clinical importance. We define a core induced response common to pathogenic and nonpathogenic species alike, heavily weighted to alternative carbon metabolism. One prominent pathogen, Candida parapsilosis, showed species-specific expansion of phagocytosis-responsive genes, particularly metabolite transporters. C. albicans and Candida tropicalis, the other prominent pathogens, also had species-specific responses, but these were largely comprised of functionally uncharacterized genes. Transcriptional analysis of macrophages also demonstrated highly correlated proinflammatory transcriptional responses to different Candida species that were largely independent of fungal viability, suggesting that this response is driven by recognition of conserved cell wall components. This study significantly broadens our understanding of host interactions in CUG clade species, demonstrating that although metabolic plasticity is crucial for virulence in Candida, it alone is not sufficient to confer pathogenicity. Instead, we identify sets of mostly uncharacterized genes that may explain the evolution of pathogenicity.

Systems biology of host-Candida interactions: understanding how we shape each other

Candida albicans is both a member of the human mucosal microbiota and a common agent of invasive fungal disease. Systems biology approaches allow for analysis of the interactions between this fungus and its mammalian host. Framing these studies by considering how C. albicans and its host construct the niche the other occupies provides insight into how these interactions shape the ecosystems, behavior, and evolution of each organism. Here, we discuss recent work on multiscale systems biology approaches for examining C. albicans in relation to the host ecosystem to identify the emergent properties of the interactions and new variables that can be targeted for development of therapeutic strategies.

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

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

Metabolic competition between host and pathogen dictates inflammasome responses to fungal infection

The NLRP3 inflammasome has emerged as a central immune regulator that senses virulence factors expressed by microbial pathogens for triggering inflammation. Inflammation can be harmful and therefore this response must be tightly controlled. The mechanisms by which immune cells, such as macrophages, discriminate benign from pathogenic microbes to control the NLRP3 inflammasome remain poorly defined. Here we used live cell imaging coupled with a compendium of diverse clinical isolates to define how macrophages respond and activate NLRP3 when faced with the human yeast commensal and pathogen Candida albicans. We show that metabolic competition by C. albicans, rather than virulence traits such as hyphal formation, activates NLRP3 in macrophages. Inflammasome activation is triggered by glucose starvation in macrophages, which occurs when fungal load increases sufficiently to outcompete macrophages for glucose. Consistently, reducing Candida’s ability to compete for glucose and increasing glucose availability for macrophages tames inflammatory responses. We define the mechanistic requirements for glucose starvation-dependent inflammasome activation by Candida and show that it leads to inflammatory cytokine production, but it does not trigger pyroptotic macrophage death. Pyroptosis occurs only with some Candida isolates and only under specific experimental conditions, whereas inflammasome activation by glucose starvation is broadly relevant. In conclusion, macrophages use their metabolic status, specifically glucose metabolism, to sense fungal metabolic activity and activate NLRP3 when microbial load increases. Therefore, a major consequence of Candida-induced glucose starvation in macrophages is activation of inflammatory responses, with implications for understanding how metabolism modulates inflammation in fungal infections.

Activation of the immune regulator NLRP3 inflammasome by microbial pathogens has been shown to play both protective and destructive roles in infection, underscoring the importance of tight control over NLRP3-driven inflammation to ensure host health. A key microbe recognised by NLRP3 is the human yeast commensal and pathogen Candida albicans, which is responsible for mucosal and invasive infections. We demonstrate that innate immune cells sense their metabolic status to trigger NLRP3 activation only when microbial numbers have reached dangerous levels. This regulation is a consequence of metabolic competition between C. albicans and macrophages for an essential nutrient–glucose. The NLRP3 inflammasome is activated when increased fungal load in the infection microenvironment drives down glucose levels, thereby causing glucose starvation in macrophages. Restoring glucose homeostasis in macrophages reduced NLRP3 activation and production of the proinflammatory cytokine IL-1β, suggesting that metabolism regulates NLRP3 inflammasome activity in fungal infections.

The Dual Function of the Fungal Toxin Candidalysin during Candida albicans-Macrophage Interaction and Virulence

The dimorphic fungus Candida albicans is both a harmless commensal organism on mucosal surfaces and an opportunistic pathogen. Under certain predisposing conditions, the fungus can overgrow the mucosal microbiome and cause both superficial and life-threatening systemic infections after gaining access to the bloodstream. As the first line of defense of the innate immune response, infecting C. albicans cells face macrophages, which mediate the clearance of invading fungi by intracellular killing. However, the fungus has evolved sophisticated strategies to counteract macrophage antimicrobial activities and thus evade immune surveillance. The cytolytic peptide toxin, candidalysin, contributes to this fungal defense machinery by damaging immune cell membranes, providing an escape route from the hostile phagosome environment. Nevertheless, candidalysin also induces NLRP3 inflammasome activation, leading to an increased host-protective pro-inflammatory response in mononuclear phagocytes. Therefore, candidalysin facilitates immune evasion by acting as a classical virulence factor but also contributes to an antifungal immune response, serving as an avirulence factor. In this review, we discuss the role of candidalysin during C. albicans infections, focusing on its implications during C. albicans-macrophage interactions.

Fungal Pathogens: Shape-Shifting Invaders

Fungal infections are on the rise due to new medical procedures that have increased the number of immune compromised patients, antibacterial antibiotics that disrupt the microbiome, and increased use of indwelling medical devices that provide sites for biofilm formation. Key to understanding the mechanisms of pathogenesis is to determine how fungal morphology impacts virulence strategies. For example, small budding cells use very different strategies to disseminate compared with long hyphal filaments. Furthermore, cell morphology must be monitored in the host, as many fungal pathogens change their shape to disseminate into new areas, acquire nutrients, and avoid attack by the immune system. This review describes the shape-shifting alterations in morphogenesis of human fungal pathogens and how they influence virulence strategies.

Ahr1 and Tup1 Contribute to the Transcriptional Control of Virulence-Associated Genes in Candida albicans

Candida albicans is a major human fungal pathogen and the leading cause of systemic Candida infections. In recent years, Als3 and Ece1 were identified as important factors for fungal virulence. Transcription of both corresponding genes is closely associated with hyphal growth. Here, we describe how Tup1, normally a global repressor of gene expression as well as of filamentation, and the transcription factor Ahr1 contribute to full expression of ALS3 and ECE1 in C. albicans hyphae. Both regulators are required for high mRNA amounts of the two genes to ensure functional relevant protein synthesis and localization. These observations identified a new aspect of regulation in the complex transcriptional control of virulence-associated genes in C. albicans.

The capacity of Candida albicans to reversibly change its morphology between yeast and filamentous stages is crucial for its virulence. Formation of hyphae correlates with the upregulation of genes ALS3 and ECE1, which are involved in pathogenicity processes such as invasion, iron acquisition, and host cell damage. The global repressor Tup1 and its cofactor Nrg1 are considered to be the main antagonists of hyphal development in C. albicans. However, our experiments revealed that Tup1, but not Nrg1, was required for full expression of ALS3 and ECE1. In contrast to NRG1, overexpression of TUP1 was found to inhibit neither filamentous growth nor transcription of ALS3 and ECE1. In addition, we identified the transcription factor Ahr1 as being required for full expression of both genes. A hyperactive version of Ahr1 bound directly to the promoters of ALS3 and ECE1 and induced their transcription even in the absence of environmental stimuli. This regulation worked even in the absence of the crucial hyphal growth regulators Cph1 and Efg1 but was dependent on the presence of Tup1. Overall, our results show that Ahr1 and Tup1 are key contributors in the complex regulation of virulence-associated genes in the different C. albicans morphologies.

Multiple Alternative Carbon Pathways Combine To Promote Candida albicans Stress Resistance, Immune Interactions, and Virulence

The phagocytic cells of the innate immune system are an essential first line of antimicrobial defense, and yet Candida albicans, one of the most problematic fungal pathogens, is capable of resisting the stresses imposed by the macrophage phagosome, eventually resulting in the destruction of the phagocyte. C. albicans rapidly adapts to the phagosome by upregulating multiple alternative carbon utilization pathways, particularly those for amino acids, carboxylic acids, and N-acetylglucosamine (GlcNAc). Here, we report that C. albicans recognizes these carbon sources both as crucial nutrients and as independent signals in its environment. Even in the presence of glucose, each carbon source promotes increased resistance to a unique profile of stressors; lactate promotes increased resistance to osmotic and cell wall stresses, amino acids increased resistance to oxidative and nitrosative stresses, and GlcNAc increased resistance to oxidative stress and caspofungin, while all three alternative carbon sources have been shown to induce resistance to fluconazole. Moreover, we show mutants incapable of utilizing these carbon sources, in particular, strains engineered to be defective in all three pathways, are significantly attenuated in both macrophage and mouse models, with additive effects observed as multiple carbon pathways are eliminated, suggesting that C. albicans simultaneously utilizes multiple carbon sources within the macrophage phagosome and during disseminated candidiasis. Taking the data together, we propose that, in addition to providing energy to the pathogen within host environments, alternative carbon sources serve as niche-specific priming signals that allow C. albicans to recognize microenvironments within the host and to prepare for stresses associated with that niche, thus promoting host adaptation and virulence.IMPORTANCECandida albicans is a fungal pathogen and a significant cause of morbidity and mortality, particularly in people with defects, sometimes minor ones, in innate immunity. The phagocytes of the innate immune system, particularly macrophages and neutrophils, generally restrict this organism to its normal commensal niches, but C. albicans shows a robust and multifaceted response to these cell types. Inside macrophages, a key component of this response is the activation of multiple pathways for the utilization of alternative carbon sources, particularly amino acids, carboxylic acids, and N-acetylglucosamine. These carbon sources are key sources of energy and biomass but also independently promote stress resistance, induce cell wall alterations, and affect C. albicans interactions with macrophages. Engineered strains incapable of utilizing these alternative carbon pathways are attenuated in infection models. These data suggest that C. albicans recognizes nutrient composition as an indicator of specific host environments and tailors its responses accordingly.

Candidalysin: discovery and function in Candida albicans infections

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Candidalysin is the first peptide toxin identified in any human fungal pathogen.

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Candidalysin is critical for Candida albicans mucosal and systemic infections.

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Candidalysin activates danger-response and damage-protection pathways in host cells.

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Candidalysin activates the epidermal growth factor receptor in epithelial cells and the NLRP3 inflammasome in macrophages.

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Candidalysin drives neutrophil recruitment and Type 17 immunity.

Candidalysin is a cytolytic peptide toxin secreted by the invasive form of the human pathogenic fungus, Candida albicans. Candidalysin is critical for mucosal and systemic infections and is a key driver of host cell activation, neutrophil recruitment and Type 17 immunity. Candidalysin is regarded as the first true classical virulence factor of C. albicans but also triggers protective immune responses. This review will discuss how candidalysin was discovered, the mechanisms by which this peptide toxin contributes to C. albicans infections, and how its discovery has advanced our understanding of fungal pathogenesis and disease.

Inhibition of Caspase-1-dependent pyroptosis attenuates copper-induced apoptosis in chicken hepatocytes

The purpose of this study was to investigate the effects of copper (Cu) on hepatocyte pyroptosis and the relationship between pyroptosis and apoptosis in the mechanisms of Cu toxicity. Primary chicken hepatocytes were cultured in different concentrations of Cu sulfate (CuSO₄) (0, 10, 50, and 100 μM), N-acetylcysteine (NAC) (1 mM), and Z-YVAD-fluoromethylketone (Z-YVAD-FMK) (10 μM) for 24 h, and the combination of Cu and NAC or Z-YVAD-FMK for 24 h. Cellular morphology and function, cell viability, mitochondria membrane potential (MMP), apoptosis rate, mRNA expression of pyroptosis-related and apoptosis-related genes, and Caspase-1, Caspase-3 proteins expression were determined. These results indicated that Cu markedly induced the mRNA expression of pyroptosis-related genes (Caspase-1, IL-1β, IL-18, and NLRP3) and Caspase-1 protein expression. Furthermore, contents of Caspase-1, IL-1β, and IL-18 in the supernatant fluid of culture hepatocytes were significantly increased in hepatocytes. NAC relieved excess Cu-caused the changes of above genes and proteins. Additionally, Z-YVAD-FMK, caspase-1 inhibitor, which attenuated Cu-induced the increased lactic dehydrogenase (LDH), aspartate amino transferase (AST), alanine aminotransferase (ALT) activities. Furthermore, treatment with Cu and Z-YVAD-FMK could down-regulate the mRNA levels of Caspase-3, Bak1, Bax, and CytC and Caspase-3 protein expression, up-regulate the mRNA expression of Bcl2, increase the MMP and reduce cell apoptosis compared to treatment with Cu in hepatocytes. Collectively, these finding evidenced that excess Cu induced pyroptosis by generating ROS in hepatocytes, and the inhibition of Caspase-1-dependent pyroptosis might attenuate Cu-induced apoptosis.

Enrichment of ATP Binding Proteins Unveils Proteomic Alterations in Human Macrophage Cell Death, Inflammatory Response, and Protein Synthesis after Interaction with Candida albicans

Macrophages are involved in the primary human response to Candida albicans. After pathogen recognition, signaling pathways are activated, leading to the production of cytokines, chemokines, and antimicrobial peptides. ATP binding proteins are crucial for this regulation. Here, a quantitative proteomic and phosphoproteomic approach was carried out for the study of human macrophage ATP-binding proteins after interaction with C. albicans. From a total of 547 nonredundant quantified proteins, 137 were ATP binding proteins and 59 were detected as differentially abundant. From the differentially abundant ATP-binding proteins, 6 were kinases (MAP2K2, SYK, STK3, MAP3K2, NDKA, and SRPK1), most of them involved in signaling pathways. Furthermore, 85 phosphopeptides were quantified. Macrophage proteomic alterations including an increase of protein synthesis with a consistent decrease in proteolysis were observed. Besides, macrophages showed changes in proteins of endosomal trafficking together with mitochondrial proteins, including some involved in the response to oxidative stress. Regarding cell death mechanisms, an increase of antiapoptotic over pro-apoptotic signals is suggested. Furthermore, a high pro-inflammatory response was detected, together with no upregulation of key mi-RNAs involved in the negative feedback of this response. These findings illustrate a strategy to deepen the knowledge of the complex interactions between the host and the clinically important pathogen C. albicans.

Candidalysin Crucially Contributes to Nlrp3 Inflammasome Activation by Candida albicans Hyphae

Candidiasis is a potentially lethal condition that is caused by systemic dissemination of Candida albicans, a common fungal commensal residing mostly on mucosal surfaces. The transition of C. albicans from an innocuous commensal to an opportunistic pathogen goes hand in hand with its morphological transformation from a fungus to a hyphal appearance. On the one hand, the latter manifestation enables C. albicans to penetrate tissues, while on the other hand, the expression of many hypha-specific genes also endows it with the capacity to trigger particular cytokine responses. The Nlrp3 inflammasome is a crucial component of the innate immune system that provokes release of the IL-1β cytokine from myeloid cells upon encountering C. albicans hyphae. Our study reveals the peptide candidalysin as one of the hypha-derived drivers of Nlrp3 inflammasome responses in primary macrophages and, thus, contributes to better understanding the fungal mechanisms that determine the pathogenicity of C. albicans.

Candida albicans is an opportunistic fungal pathogen that can cause life-threatening infections, particularly in immunocompromised patients. C. albicans induced activation of the Nlrp3 inflammasome, leading to secretion of bioactive interleukin 1β (IL-1β) is a crucial myeloid cell immune response needed for antifungal host defense. Being a pleiomorphic fungus, C. albicans can provoke Nlrp3 inflammasome responses only upon morphological transformation to its hyphal appearance. However, the specific hyphal factors that enable C. albicans to activate the Nlrp3 inflammasome in primary macrophages remain to be revealed. Here, we identify candidalysin, a peptide derived from the hypha-specific ECE1 gene, as a fungal trigger for Nlrp3 inflammasome-mediated maturation and secretion of IL-1β from primary macrophages. Direct peptide administration experiments showed that candidalysin was sufficient for inducing secretion of mature IL-1β from macrophages in an Nlrp3 inflammasome-dependent manner. Conversely, infection experiments using candidalysin-deficient C. albicans showed that candidalysin crucially contributed to the capacity of this fungus to induce maturation and secretion of IL-1β from primary macrophages. These complementary observations identify the expression of candidalysin as one of the molecular mechanisms by which hyphal transformation equips C. albicans with its proinflammatory capacity to elicit the release of bioactive IL-1β from macrophages.

In Fungal Intracellular Pathogenesis, Form Determines Fate

For pathogenic microbes to survive ingestion by macrophages, they must subvert powerful microbicidal mechanisms within the phagolysosome. After ingestion, Candida albicans undergoes a morphological transition producing hyphae, while the surrounding phagosome exhibits a loss of phagosomal acidity. However, how these two events are related has remained enigmatic. Now Westman et al. (mBio 9:e01226-18, 2018, https://doi.org/10.1128/mBio.01226-18) report that phagosomal neutralization results from disruption of phagosomal membrane integrity by the enlarging hyphae, directly implicating the morphological transition in physical damage that promotes intracellular survival. The C. albicans intracellular strategy shows parallels with another fungal pathogen, Cryptococcus neoformans, where a morphological changed involving capsular enlargement intracellularly is associated with loss of membrane integrity and death of the host cell. These similarities among distantly related pathogenic fungi suggest that morphological transitions that are common in fungi directly affect the outcome of the fungal cell-macrophage interaction. For this class of organisms, form determines fate in the intracellular environment.

The fungal peptide toxin Candidalysin activates the NLRP3 inflammasome and causes cytolysis in mononuclear phagocytes

Clearance of invading microbes requires phagocytes of the innate immune system. However, successful pathogens have evolved sophisticated strategies to evade immune killing. The opportunistic human fungal pathogen Candida albicans is efficiently phagocytosed by macrophages, but causes inflammasome activation, host cytolysis, and escapes after hypha formation. Previous studies suggest that macrophage lysis by C. albicans results from early inflammasome-dependent cell death (pyroptosis), late damage due to glucose depletion and membrane piercing by growing hyphae. Here we show that Candidalysin, a cytolytic peptide toxin encoded by the hypha-associated gene ECE1, is both a central trigger for NLRP3 inflammasome-dependent caspase-1 activation via potassium efflux and a key driver of inflammasome-independent cytolysis of macrophages and dendritic cells upon infection with C. albicans. This suggests that Candidalysin-induced cell damage is a third mechanism of C. albicans-mediated mononuclear phagocyte cell death in addition to damage caused by pyroptosis and the growth of glucose-consuming hyphae.

High-Throughput Screening Identifies Genes Required for Candida albicans Induction of Macrophage Pyroptosis

The innate immune system is the first line of defense against invasive fungal infections. As a consequence, many successful fungal pathogens have evolved elegant strategies to interact with host immune cells. For example, Candida albicans undergoes a morphogenetic switch coupled to cell wall remodeling upon phagocytosis by macrophages and then induces macrophage pyroptosis, an inflammatory cell death program. To elucidate the genetic circuitry through which C. albicans orchestrates this host response, we performed the first large-scale analysis of C. albicans interactions with mammalian immune cells. We identified 98 C. albicans genes that enable macrophage pyroptosis without influencing fungal cell morphology in the macrophage, including specific determinants of cell wall biogenesis and the Hog1 signaling cascade. Using these mutated genes, we discovered that defects in the activation of pyroptosis affect immune cell recruitment during infection. Examining host circuitry required for pyroptosis in response to C. albicans infection, we discovered that inflammasome priming and activation can be decoupled. Finally, we observed that apoptosis-associated speck-like protein containing a CARD (ASC) oligomerization can occur prior to phagolysosomal rupture by C. albicans hyphae, demonstrating that phagolysosomal rupture is not the inflammasome activating signal. Taking the data together, this work defines genes that enable fungal cell wall remodeling and activation of macrophage pyroptosis independently of effects on morphogenesis and identifies macrophage signaling components that are required for pyroptosis in response to C. albicans infection.

Candida albicans is a natural member of the human mucosal microbiota that can also cause superficial infections and life-threatening systemic infections, both of which are characterized by inflammation. Host defense relies mainly on the ingestion and destruction of C. albicans by innate immune cells, such as macrophages and neutrophils. Although some C. albicans cells are killed by macrophages, most undergo a morphological change and escape by inducing macrophage pyroptosis. Here, we investigated the C. albicans genes and host factors that promote macrophage pyroptosis in response to intracellular fungi. This work provides a foundation for understanding how host immune cells interact with C. albicans and may lead to effective strategies to modulate inflammation induced by fungal infections.

A Genome-Wide Screen of Deletion Mutants in the Filamentous Saccharomyces cerevisiae Background Identifies Ergosterol as a Direct Trigger of Macrophage Pyroptosis

Phagocytic cells such as macrophages play an important role in the host defense mechanisms mounted in response to the common human fungal pathogen Candida albicans. In vitro, C. albicans triggers macrophage NLRP3-Casp1/11-mediated pyroptosis, an inflammatory programmed cell death pathway. Here, we provide evidence that Casp1/11-dependent pyroptosis occurs in the kidney of infected mice during the early stages of infection. We have also used a genome-wide screen of nonessential Σ1278b Saccharomyces cerevisiae genes to identify genes required for yeast-triggered macrophage pyroptosis. The set of genes identified by this screen was enriched for those with functions in lipid and sterol homeostasis and trafficking. These observations led us to discover that cell surface localization and/or total levels of ergosterol correlate with the ability of S. cerevisiae, C. albicans, and Cryptococcus neoformans to trigger pyroptosis. Since the mammalian sterol cholesterol triggers NLRP3-mediated pyroptosis, we hypothesized that ergosterol may also do so. Consistent with that hypothesis, ergosterol-containing liposomes but not ergosterol-free liposomes induce pyroptosis. Cell wall mannoproteins directly bind ergosterol, and we found that Dan1, an ergosterol receptor mannoprotein, as well as specific mannosyltransferases, is required for pyroptosis, suggesting that cell wall-associated ergosterol may mediate the process. Taken together, these data indicate that ergosterol, like mammalian cholesterol, plays a direct role in yeast-mediated pyroptosis.

Innate immune cells such as macrophages are key components of the host response to the human fungal pathogen Candida albicans. Macrophages undergo pyroptosis, an inflammatory, programmed cell death, in response to some species of pathogenic yeast. Prior to the work described in this report, yeast-triggered pyroptosis has been observed only in vitro; here, we show that pyroptosis occurs in the initial stages of murine kidney infection, suggesting that it plays an important role in the initial response of the innate immune system to invasive yeast infection. We also show that a key component of the fungal plasma membrane, ergosterol, directly triggers pyroptosis. Ergosterol is also present in the fungal cell wall, most likely associated with mannoproteins, and is increased in hyphal cells compared to yeast cells. Our data indicate that specific mannoproteins are required for pyroptosis. This is consistent with a potential mechanism whereby ergosterol present in the outer mannoprotein layer of the cell wall is accessible to the macrophage-mediated process. Taken together, our data provide the first evidence that ergosterol plays a direct role in the host-pathogen interactions of fungi.

Differences in interleukin-1β release-inducing activity of Candida albicans toward dendritic cells and macrophages

OBJECTIVE

The purpose of this study is to elucidate differences in the mechanism of the IL-1β release-inducing activity of Candida albicans toward dendritic cells and macrophages because IL-1β is one of the proinflammatory cytokines which is crucial in host defense against candidiasis.

DESIGN

Two C. albicans strains were used in this study. One strain is uridine-auxotrophic (CAI4) that needs uridine to grow and form hyphae, and another is a strain without any specific auxotrophy (pACT1-GFP), which forms hyphae naturally by culturing with serum components. Murine macrophage and dendritic cell lines were primed with LPS and then stimulated with C. albicans CAI4 or pACT1-GFP.

RESULTS

Both strains of C. albicans induced IL-1β release from dendritic cells, and C. albicans pACT1-GFP induced IL-1β release but CAI4 induced little amounts in macrophages. These differences were suggested to be due to the difference in the amount of extracellular ATP released in the cell culture supernatants induced by C. albicans CAI4 or pACT1-GFP. For induction of IL-1β release from both macrophages and dendritic cells by C. albicans, direct contacts of the microbes with cells were required. In addition, macrophages required morphological change of C. albicans from yeast to hyphae for induction of IL-1β release, whereas dendritic cells did not require it. Dead C. albicans could induce IL-1β release from dendritic cells, but could not from macrophages.

CONCLUSIONS

There are different mechanisms by which C. albicans induces IL-1β release from dendritic cells and macrophages.

Antifungal Compounds against Candida Infections from Traditional Chinese Medicine

Infections caused by Candida albicans, often refractory and with high morbidity and mortality, cause a heavy burden on the public health while the current antifungal drugs are limited and are associated with toxicity and resistance. Many plant-derived molecules including compounds isolated from traditional Chinese medicine (TCM) are reported to have antifungal activity through different targets such as cell membrane, cell wall, mitochondria, and virulence factors. Here, we review the recent progress in the anti-Candida compounds from TCM, as well as their antifungal mechanisms. Considering the diverse targets and structures, compounds from TCM might be a potential library for antifungal drug development.

Candida albicans-epithelial interactions and induction of mucosal innate immunity

Candida albicans is a human fungal pathogen that causes millions of mucosal and life-threatening infections annually. C. albicans initially interacts with epithelial cells, resulting in fungal recognition and the formation of hyphae. Hypha formation is critical for host cell damage and immune activation, which are both driven by the secretion of Candidalysin, a recently discovered peptide toxin. Epithelial activation leads to the production of inflammatory mediators that recruit innate immune cells including neutrophils, macrophages and innate Type 17 cells, which together work with epithelial cells to clear the fungal infection. This review will focus on the recent discoveries that have advanced our understanding of C. albicans-epithelial interactions and the induction of mucosal innate immunity.

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

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

The Major Chromoblastomycosis Etiologic Agent Fonsecaea pedrosoi Activates the NLRP3 Inflammasome

Fonsecaea pedrosoi is the main etiologic agent of chromoblastomycosis (CBM), one of the most prevalent subcutaneous mycosis in tropical and subtropical countries. CBM is a poorly characterized chronic infection that commonly starts after transcutaneous inoculation of conidia and saprophytic hyphae of F. pedrosoi. Recently, we have shown that unlike conidia, hyphae and muriform cells (the parasitic morphotype) of F. pedrosoi promotes an intense inflammatory response pattern in vivo, which comprises the production of an inflammasome-derived cytokine, IL-1β. Nonetheless, the mechanisms underlying IL-1β production and maturation upon F. pedrosoi infection and its functional output in the course of CBM remains unknown. We show here that F. pedrosoi hyphae, differently from conidia, induce IL-1β secretion in both bone marrow-derived dendritic cells and macrophages. Using inhibitors and knockout cells, we demonstrated that the mechanisms underlying IL-1β production by hyphae-infected macrophages were dependent on dectin-1, -2, and -3 receptors and the Syk-NF-kB signaling pathway. Furthermore, F. pedrosoi promoted a NLRP3-dependent inflammasome activation, which required potassium efflux, reactive oxygen species production, phagolysosomal acidification, and cathepsin B release as triggers. IL-1β processing and release was mediated primarily by caspase-1 and, to a lesser extent, by caspase-8-dependent cleavage. Finally, we showed using a murine CBM model that F. pedrosoi elicits a NLRP3-regulated IL-1β and interleukin-18 release in vivo, but without NLRP3 inflammasome activation interfering in the course of the experimental infection.

Genome-wide functional analysis in Candida albicans

Candida albicans is an important etiological agent of superficial and life-threatening infections in individuals with compromised immune systems. To date, we know of several overlapping genetic networks that govern virulence attributes in this fungal pathogen. Classical use of deletion mutants has led to the discovery of numerous virulence factors over the years, and genome-wide functional analysis has propelled gene discovery at an even faster pace. Indeed, a number of recent studies using large-scale genetic screens followed by genome-wide functional analysis has allowed for the unbiased discovery of many new genes involved in C. albicans biology. Here we share our perspectives on the role of these studies in analyzing fundamental aspects of C. albicans virulence properties.

Specific pathways mediating inflammasome activation by Candida parapsilosis

Candida albicans and C. parapsilosis are human pathogens causing severe infections. The NLRP3 inflammasome plays a crucial role in host defence against C. albicans, but it has been previously unknown whether C. parapsilosis activates this complex. Here we show that C. parapsilosis induces caspase-1 activation and interleukin-1β (IL-1β) secretion in THP-1, as well as primary, human macrophages. IL-1β secretion was dependent on NLRP3, K+-efflux, TLR4, IRAK, Syk, caspase-1, caspase-8 and NADPH-oxidase. Importantly, while C. albicans induced robust IL-1β release after 4 h, C. parapsilosis was not able to stimulate the production of IL-1β after this short incubation period. We also found that C. parapsilosis was phagocytosed to a lesser extent, and induced significantly lower ROS production and lysosomal cathepsin B release compared to C. albicans, suggesting that the low extent of inflammasome activation by C. parapsilosis may result from a delay in the so-called "signal 2". In conclusion, this is the first study to examine the molecular pathways responsible for the IL-1β production in response to a non-albicans Candida species, and these results enhance our understanding about the immune response against C. parapsilosis.

Phagosomal Neutralization by the Fungal Pathogen Candida albicans Induces Macrophage Pyroptosis

The interaction of Candida albicans with the innate immune system is the key determinant of the pathogen/commensal balance and has selected for adaptations that facilitate the utilization of nutrients commonly found within the host, including proteins and amino acids; many of the catabolic pathways needed to assimilate these compounds are required for persistence in the host. We have shown that C. albicans co-opts amino acid catabolism to generate and excrete ammonia, which raises the extracellular pH, both in vitro and in vivo and induces hyphal morphogenesis. Mutants defective in the uptake or utilization of amino acids, such as those lacking STP2, a transcription factor that regulates the expression of amino acid permeases, are impaired in multiple aspects of fungus-macrophage interactions resulting from an inability to neutralize the phagosome. Here we identified a novel role in amino acid utilization for Ahr1p, a transcription factor previously implicated in regulation of adherence and hyphal morphogenesis. Mutants lacking AHR1 were defective in growth, alkalinization, and ammonia release on amino acid-rich media, similar to stp2Δ and ahr1Δ stp2Δ cells, and occupied more acidic phagosomes. Notably, ahr1Δ and stp2Δ strains did not induce pyroptosis, as measured by caspase-1-dependent interleukin-1β release, though this phenotype could be suppressed by pharmacological neutralization of the phagosome. Altogether, we show that C. albicans-driven neutralization of the phagosome promotes hyphal morphogenesis, sufficient for induction of caspase-1-mediated macrophage lysis.

Inflammasomes and Their Role in Innate Immunity of Sexually Transmitted Infections

Inflammasomes are multiprotein complexes present in the cytosol as pattern recognition receptors or as sensors of damage-associated molecular patterns. After recognition of microbe-associated molecular patterns or host-derived danger signals, nucleotide oligomerization domain-like receptors oligomerize to form inflammasomes. The activation of inflammasomes results in an alarm, which is raised to alert adjacent cells through the processing and release of a number of other substrates present in the cytosol. A wide array of inflammasomes and their adapter molecules have been identified in the host’s innate immune system in response to various pathogens. Components of specific pathogens activate different inflammasomes, which once activated in response to pathogen-induced infection, induce the activation of caspases, and the release of mature forms of interleukin-1β (IL-1β) and IL-18. Identifying the mechanisms underlying pathogen-induced inflammasome activation is important if we are to develop novel therapeutic strategies to target sexually transmitted infections (STIs) related pathogens. This information is currently lacking in literature. In this review, we have discussed the role of various inflammasomes in sensing different STIs, as well as the beneficial or detrimental effects of inflammasome signaling in host resistance. Additionally, we have discussed both canonical and non-canonical processing of IL-1β induced with respect to particular infections. Overall, these findings transform our understanding of both the basic biology and clinical relevance of inflammasomes.

Anti-Immune Strategies of Pathogenic Fungi

Pathogenic fungi have developed many strategies to evade the host immune system. Multiple escape mechanisms appear to function together to inhibit attack by the various stages of both the adaptive and the innate immune response. Thus, after entering the host, such pathogens fight to overcome the immune system to allow their survival, colonization and spread to different sites of infection. Consequently, the establishment of a successful infectious process is closely related to the ability of the pathogen to modulate attack by the immune system. Most strategies employed to subvert or exploit the immune system are shared among different species of fungi. In this review, we summarize the main strategies employed for immune evasion by some of the major pathogenic fungi.

Alarmin(g) the innate immune system to invasive fungal infections

Fungi encounter numerous stresses in a mammalian host, including the immune system, which they must adapt to in order to grow and cause disease. The host immune system tunes its response to the threat level posed by the invading pathogen. We discuss recent findings on how interleukin (IL)-1 signaling is central to tuning the immune response to the virulence potential of invasive fungi, as well as other pathogens. Moreover, we discuss fungal factors that may drive tissue invasion and destruction that regulate IL-1 cytokine release. Moving forward understanding the mechanisms of fungal adaption to the host, together with understanding how the host innate immune system recognizes invading fungal pathogens will increase our therapeutic options for treatment of invasive fungal infections.

The Endoplasmic Reticulum-Mitochondrion Tether ERMES Orchestrates Fungal Immune Evasion, Illuminating Inflammasome Responses to Hyphal Signals

The pathogenic yeast Candida albicans escapes macrophages by triggering NLRP3 inflammasome-dependent host cell death (pyroptosis). Pyroptosis is inflammatory and must be tightly regulated by host and microbe, but the mechanism is incompletely defined. We characterized the C. albicans endoplasmic reticulum (ER)-mitochondrion tether ERMES and show that the ERMES mmm1 mutant is severely crippled in killing macrophages despite hyphal formation and normal phagocytosis and survival. To understand dynamic inflammasome responses to Candida with high spatiotemporal resolution, we established live-cell imaging for parallel detection of inflammasome activation and pyroptosis at the single-cell level. This showed that the inflammasome response to mmm1 mutant hyphae is delayed by 10 h, after which an exacerbated activation occurs. The NLRP3 inhibitor MCC950 inhibited inflammasome activation and pyroptosis by C. albicans, including exacerbated inflammasome activation by the mmm1 mutant. At the cell biology level, inactivation of ERMES led to a rapid collapse of mitochondrial tubular morphology, slow growth and hyphal elongation at host temperature, and reduced exposed 1,3-β-glucan in hyphal populations. Our data suggest that inflammasome activation by C. albicans requires a signal threshold dependent on hyphal elongation and cell wall remodeling, which could fine-tune the response relative to the level of danger posed by C. albicans. The phenotypes of the ERMES mutant and the lack of conservation in animals suggest that ERMES is a promising antifungal drug target. Our data further indicate that NLRP3 inhibition by MCC950 could modulate C. albicans-induced inflammation. IMPORTANCE The yeast Candida albicans causes human infections that have mortality rates approaching 50%. The key to developing improved therapeutics is to understand the host-pathogen interface. A critical interaction is that with macrophages: intracellular Candida triggers the NLRP3/caspase-1 inflammasome for escape through lytic host cell death, but this also activates antifungal responses. To better understand how the inflammasome response to Candida is fine-tuned, we established live-cell imaging of inflammasome activation at single-cell resolution, coupled with analysis of the fungal ERMES complex, a mitochondrial regulator that lacks human homologs. We show that ERMES mediates Candida escape via inflammasome-dependent processes, and our data suggest that inflammasome activation is controlled by the level of hyphal growth and exposure of cell wall components as a proxy for severity of danger. Our study provides the most detailed dynamic analysis of inflammasome responses to a fungal pathogen so far and establishes promising pathogen- and host-derived therapeutic strategies.

Fungal morphogenetic changes inside the mammalian host

One of the main features of the majority of pathogenic fungi is the ability to switch between different types of morphological forms. These changes include the transition between cells of different shapes (such as the formation of pseudohyphae and hyphae), or the massive growth of the blastoconidia and formation of titan cells. Morphological changes occur during infection, and there is extensive evidence that they play a key role in processes required for disease, such as adhesion, invasion and dissemination, immune recognition evasion, and phagocytosis avoidance. In the present review, we will provide an overview of how morphological transitions contribute to the development of fungal disease, with special emphasis in two cases: Candida albicans as an example of yeast that switches between blastoconidia and filaments, and Cryptococcus neoformans as an example of a fungus that changes the size without modifying the shape of the cell.

Interactions of fungal pathogens with phagocytes

The surveillance and elimination of fungal pathogens rely heavily on the sentinel behaviour of phagocytic cells of the innate immune system, especially macrophages and neutrophils. The efficiency by which these cells recognize, uptake and kill fungal pathogens depends on the size, shape and composition of the fungal cells and the success or failure of various fungal mechanisms of immune evasion. In this Review, we describe how fungi, particularly Candida albicans, interact with phagocytic cells and discuss the many factors that contribute to fungal immune evasion and prevent host elimination of these pathogenic microorganisms.

The interplay between inflammasome activation and antifungal host defense

Fungal infections cause significant morbidity and mortality in humans, and they are a growing problem due to the increased usage of broad-spectrum antibiotics and immunosuppressive therapies. The equilibrium between the commensal microbial flora and the immune system that protects the host against invasive fungal infection is disturbed during disease, and understanding this disturbed balance is important to develop new therapeutic interventions for the treatment of fungal infection. In the context of tolerating fungi during colonization and eliciting a vigorous immune response to eliminate invading fungal pathogens when needed, the inflammasome has been identified as an integral component of antifungal host defense. It contributes to mucosal host defense by regulating T-helper 17 (Th17) cell responses, and contributes to protective responses such as neutrophil influx during fungal sepsis. Several aspects are important for understanding the role of the inflammasome for antifungal host defense, such as the role of fungal cell wall morphology and its components in triggering the inflammasome, the pattern recognition pathways and downstream signaling cascades involved in the activation of the inflammasome, and the effects of inflammasome activation during fungal infection. The future perspectives of inflammasome research in fungal immunology, with emphasis on targeting the inflammasome for the design of future immunotherapies, is also discussed.

Pathogenic NLRP3 Inflammasome Activity during Candida Infection Is Negatively Regulated by IL-22 via Activation of NLRC4 and IL-1Ra

Candida albicans is a well-tolerated resident of human mucosal tissues. This implies that host defense mechanisms cooperate to limit inflammation while controlling fungal burden. The cytokine IL-22 and inflammasomes are essential components of the mucosal responses to C. albicans. How these components cooperate to mediate the balance of inflammation and host defense is not explored. We find that NLRP3 inflammasome activation promotes neutrophil recruitment and inflammation during infection and that this activity is counteracted by IL-22. Mechanistically, IL-22 activated NLRC4 for sustained production of the IL-1 receptor antagonist IL-1Ra, which restrained NLRP3 activity. Symptomatic infection in mice and humans occurred under conditions of IL-1Ra deficiency and was rescued in mice by replacement therapy with the recombinant IL-1Ra anakinra. Thus, pathogenic inflammasome activity during Candida infection is negatively regulated by the IL-22/NLRC4/IL-1Ra axis. Our findings offer insights into the pathogenesis of C. albicans and suggest therapeutic avenues for candidiasis.

Evaluation of the relationship between fungal infection, neutrophil leukocytes and macrophages in cervicovaginal smears: Light microscopic examination

Background:

Right after opportunistic fungi become pathogenic, they face immune system cells including macrophages and neutrophil leukocytes. Although the relationship between fungi and immune cells are being widely studied by using animal models and culture techniques, cervicovaginal smears have not been used to evaluate this interaction yet.

Aim:

The aim of this study was to investigate the interactions between fungal infection, macrophages and neutrophil leukocytes in cervicovaginal smear.

Materials and Methods:

Papanicolaou-stained cervicovaginal smears from 2307 women, aged between 18 and 73 years, were examined by light microscopy. Periodic acid–Schiff stain was also used to confirm the presence of fungal cell walls.

Results:

Fungal infections were detected in 239 of 2307 patients (10.4%), and these cases were taken as the study group. Cases without any infectious agents (n = 1800, 78%) were considered as the control group. When the study and control groups were statistically compared in view of macrophages and neutrophil leukocytes, a significant relationship between presence of fungal infection, macrophages and neutrophil leukocytes was detected (P < 0.05). Furthermore, macrophages and neutrophil leukocytes were found to work against the fungal infection together (P < 0.05). Additionally, when the relationship between the existence of yeast or filamentous forms and these immune cells were evaluated, a significant correlation was not found (P > 0.05).

Conclusions:

Our findings indicate that macrophages and neutrophils may play a determining role in host defense against fungal infection together, but neither yeast nor filamentous forms affect the presence of neutrophil leukocytes and macrophages. As a result of this, both yeast and filamentous forms may have pathogenic effects.

Global analysis of fungal morphology exposes mechanisms of host cell escape

Developmental transitions between single-cell yeast and multicellular filaments underpin virulence of diverse fungal pathogens. For the leading human fungal pathogen Candida albicans, filamentation is thought to be required for immune cell escape via induction of an inflammatory programmed cell death. Here we perform a genome-scale analysis of C. albicans morphogenesis and identify 102 negative morphogenetic regulators and 872 positive regulators, highlighting key roles for ergosterol biosynthesis and N-linked glycosylation. We demonstrate that C. albicans filamentation is not required for escape from host immune cells; instead, macrophage pyroptosis is driven by fungal cell-wall remodelling and exposure of glycosylated proteins in response to the macrophage phagosome. The capacity of killed, previously phagocytized cells to drive macrophage lysis is also observed with the distantly related fungal pathogen Cryptococcus neoformans. This study provides a global view of morphogenetic circuitry governing a key virulence trait, and illuminates a new mechanism by which fungi trigger host cell death.

Several pathogenic fungi such as Candida albicans undergo transitions between single-celled forms and multicellular filaments. Here the authors perform a genome-scale analysis of C. albicans and show that, contrary to common belief, filamentation is not required for escape from host immune cells.

Differential Proteinase Patterns among Candida albicans Strains Isolated from Root Canal and Lingual Dorsum: Possible Roles in Periapical Disease

INTRODUCTION

Proteinases play pivotal roles in Candida albicans infections. Although the yeast can colonize the pulpal environment, there is no information about the enzymatic profile of this organism. This in vitro study aimed to determine the proteolysis levels and to investigate differences in the expression of aspartyl proteinase genes (Sap 1, Sap 2, and Sap 4) among various root canal strains and clinical isolates from the lingual dorsum.

METHODS

The extracellular proteinase activity of 104 C. albicans samples isolated from the lingual dorsum and from necrotic root canals was measured with respect to bovine serum albumin degradation after 5 days of incubation at 37°C. We used reverse-transcription polymerase chain reaction, a highly sensitive method, to detect messenger RNA transcripts of aspartyl proteinase genes (Sap 1, Sap 2, and Sap 4). The C. albicans strain SC 5314 was used as a positive control for both experiments because it is recognized as being highly proteolytic. All tests were performed in triplicate.

RESULTS

Regardless of the isolation site, all C. albicans strains produced an opaque precipitation halo around the colonies, indicating some proteinase activity. However, the production of proteinase on the plates was significantly greater (P < .05) by the endodontic samples. Sap 2 was the most commonly expressed gene in all samples. Among the root canal samples, the detection of Sap 1 transcripts was always associated with the expression of Sap 2 and Sap 4. Sap 4 gene expression was detected in all root canal samples. The simultaneous expression of the 3 investigated Sap genes (Sap 1, Sap 2, and Sap 4) was more common in strains isolated from the lingual dorsum (50%) than in those isolated from root canals (29.4%).

CONCLUSIONS

The increased proteolytic activity as well as the distinct pattern of Sap expression observed among the root canal samples may suggest a pathogenic role for C. albicans in endodontic infections.

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.

Kinetic studies of Candida parapsilosis phagocytosis by macrophages and detection of intracellular survival mechanisms

Even though the number of Candida infections due to non-albicans species like C. parapsilosis has been increasing, little is known about their pathomechanisms. Certain aspects of C. parapsilosis and host interactions have already been investigated; however we lack information about the innate cellular responses toward this species. The aim of our project was to dissect and compare the phagocytosis of C. parapsilosis to C. albicans and to another Candida species C. glabrata by murine and human macrophages by live cell video microscopy. We broke down the phagocytic process into three stages: macrophage migration, engulfment of fungal cells and host cell killing after the uptake. Our results showed increased macrophage migration toward C. parapsilosis and we observed differences during the engulfment processes when comparing the three species. The engulfment time of C. parapsilosis was comparable to that of C. albicans regardless of the pseudohypha length and spatial orientation relative to phagocytes, while the rate of host cell killing and the overall uptake regarding C. parapsilosis showed similarities mainly with C. glabrata. Furthermore, we observed difference between human and murine phagocytes in the uptake of C. parapsilosis. UV-treatment of fungal cells had varied effects on phagocytosis dependent upon which Candida strain was used. Besides statistical analysis, live cell imaging videos showed that this species similarly to the other two also has the ability to survive in host cells via the following mechanisms: yeast replication, and pseudohypha growth inside of phagocytes, exocytosis of fungal cells and also abortion of host cell mitosis following the uptake. According to our knowledge this is the first study that provides a thorough examination of C. parapsilosis phagocytosis and reports intracellular survival mechanisms associated with this species.

Fungal Pathogens: Survival and Replication within Macrophages

The innate immune system is a critical line of defense against pathogenic fungi. Macrophages act at an early stage of infection, detecting and phagocytizing infectious propagules. To avoid killing at this stage, fungal pathogens use diverse strategies ranging from evasion of uptake to intracellular parasitism. This article will discuss five of the most important human fungal pathogens (Candida albicans, Aspergillus fumigatus, Cryptococcus neoformans, Coccidiodes immitis, and Histoplasma capsulatum) and consider the strategies and virulence factors adopted by each to survive and replicate within macrophages.

The complex roles of NADPH oxidases in fungal infection

NADPH oxidases play key roles in immunity and inflammation that go beyond the production of microbicidal reactive oxygen species (ROS). The past decade has brought a new appreciation for the diversity of roles played by ROS in signalling associated with inflammation and immunity. NADPH oxidase activity affects disease outcome during infections by human pathogenic fungi, an important group of emerging and opportunistic pathogens that includes Candida, Aspergillus and Cryptococcus species. Here we review how alternative roles of NADPH oxidase activity impact fungal infection and how ROS signalling affects fungal physiology. Particular attention is paid to roles for NADPH oxidase in immune migration, immunoregulation in pulmonary infection, neutrophil extracellular trap formation, autophagy and inflammasome activity. These recent advances highlight the power and versatility of spatiotemporally controlled redox regulation in the context of infection, and point to a need to understand the molecular consequences of NADPH oxidase activity in the cell.