Candida albicans hypha formation and mannan masking of β-glucan inhibit macrophage phagosome maturation

Alpha1-antitrypsin impacts innate host-pathogen interactions with Candida albicans by stimulating fungal filamentation

Our immune system possesses sophisticated mechanisms to cope with invading microorganisms, while pathogens evolve strategies to deal with threats imposed by host immunity. Human plasma protein α1-antitrypsin (AAT) exhibits pleiotropic immune-modulating properties by both preventing immunopathology and improving antimicrobial host defence. Genetic associations suggested a role for AAT in candidemia, the most frequent fungal blood stream infection in intensive care units, yet little is known about how AAT influences interactions between Candida albicans and the immune system. Here, we show that AAT differentially impacts fungal killing by innate phagocytes. We observed that AAT induces fungal transcriptional reprogramming, associated with cell wall remodelling and downregulation of filamentation repressors. At low concentrations, the cell-wall remodelling induced by AAT increased immunogenic β-glucan exposure and consequently improved fungal clearance by monocytes. Contrastingly, higher AAT concentrations led to excessive C. albicans filamentation and thus promoted fungal immune escape from monocytes and macrophages. This underscores that fungal adaptations to the host protein AAT can differentially define the outcome of encounters with innate immune cells, either contributing to improved immune recognition or fungal immune escape.

H3K56 acetylation affects Candida albicans morphology and secreted soluble factors interacting with the host

In recent years, epigenetics has been revealed as a mechanism able to modulate the expression of virulence traits in diverse pathogens, including Candida albicans. Indeed, epigenetic regulation can sense environmental changes, leading to the rapid and reversible modulation of gene expression with consequent adaptation to novel environments. How epigenetic changes can impact expression and signalling output, including events associated with mechanisms of morphological transition and virulence, is still poorly studied. Here, using nicotinamide as a sirtuin inhibitor, we explored how the accumulation of the H3K56 acetylation, the most prominent histone acetylation in C. albicans, might affect its interaction with the host. Our experiments demonstrate that H3K56 acetylation profoundly affects the production and/or secretion of soluble factors compromising actin remodelling and cytokine production. ChIP- and RNA-seq analyses highlighted a direct impact of H3K56 acetylation on genes related to phenotypic switching, biofilm formation and cell aggregation. Direct and indirect regulation also involves genes related to cell wall protein biosynthesis, β-glucan and mannan exposure, and hydrolytic secreted enzymes, supporting the hypothesis that the fluctuations of H3K56 acetylation in C. albicans might impair the macrophage response to the yeast and thus promote the host-immune escaping.

Cortactin: A major cellular target of viral, protozoal, and fungal pathogens

Many viral, protozoal, and fungal pathogens represent major human and animal health problems due to their great potential of causing infectious diseases. Research on these pathogens has contributed substantially to our current understanding of both microbial virulence determinants and host key factors during infection. Countless studies have also shed light on the molecular mechanisms of host-pathogen interactions that are employed by these microbes. For example, actin cytoskeletal dynamics play critical roles in effective adhesion, host cell entry, and intracellular movements of intruding pathogens. Cortactin is an eminent host cell protein that stimulates actin polymerization and signal transduction, and recently emerged as fundamental player during host-pathogen crosstalk. Here we review the important role of cortactin as major target for various prominent viral, protozoal and fungal pathogens in humans, and its role in human disease development and cancer progression. Most if not all of these important classes of pathogens have been reported to hijack cortactin during infection through mediating up- or downregulation of cortactin mRNA and protein expression as well as signaling. In particular, pathogen-induced changes in tyrosine and serine phosphorylation status of cortactin at its major phospho-sites (Y-421, Y-470, Y-486, S-113, S-298, S-405, and S-418) are addressed. As has been reported for various Gram-negative and Gram-positive bacteria, many pathogenic viruses, protozoa, and fungi also control these regulatory phospho-sites, for example, by activating kinases such as Src, PAK, ERK1/2, and PKD, which are known to phosphorylate cortactin. In addition, the recruitment of cortactin and its interaction partners, like the Arp2/3 complex and F-actin, to the contact sites between pathogens and host cells is highlighted, as this plays an important role in the infection process and internalization of several pathogens. However, there are also other ways in which the pathogens can exploit the function of cortactin for their needs, as the cortactin-mediated regulation of cellular processes is complex and involves numerous different interaction partners. Here, the current state of knowledge is summarized.

Candida albicans and Candida glabrata: global priority pathogens

SUMMARYA significant increase in the incidence of Candida-mediated infections has been observed in the last decade, mainly due to rising numbers of susceptible individuals. Recently, the World Health Organization published its first fungal pathogen priority list, with Candida species listed in medium, high, and critical priority categories. This review is a synthesis of information and recent advances in our understanding of two of these species-Candida albicans and Candida glabrata. Of these, C. albicans is the most common cause of candidemia around the world and is categorized as a critical priority pathogen. C. glabrata is considered a high-priority pathogen and has become an increasingly important cause of candidemia in recent years. It is now the second most common causative agent of candidemia in many geographical regions. Despite their differences and phylogenetic divergence, they are successful as pathogens and commensals of humans. Both species can cause a broad variety of infections, ranging from superficial to potentially lethal systemic infections. While they share similarities in certain infection strategies, including tissue adhesion and invasion, they differ significantly in key aspects of their biology, interaction with immune cells, host damage strategies, and metabolic adaptations. Here we provide insights on key aspects of their biology, epidemiology, commensal and pathogenic lifestyles, interactions with the immune system, and antifungal resistance.

Functionality of macrophages encapsulated in porcine decellularized adipose matrix hydrogels and interaction with Candida albicans

Extracellular matrix hydrogels are considered one of the most suitable biomaterials for tissue regeneration due to their similarity with the extracellular microenvironment of the native tissue. Their properties are dependent on their composition, material concentration, fiber density and the fabrication approaches, among other factors. The encapsulation of immune cells in this kind of hydrogels, both in absence or presence of a pathogen, represents a promising strategy for the development of platforms that mimic healthy and infected tissues, respectively. In this work, we have encapsulated macrophages in 3D hydrogels of porcine decellularized adipose matrices (pDAMs) without and with the Candida albicans fungus, as 3D experimental models to study the macrophage immunocompetence in a closer situation to the physiological conditions and to mimic an infection scenario. Our results indicate that encapsulated macrophages preserve their functionality within these pDAM hydrogels and phagocytose live pathogens. In addition, their behavior is influenced by the hydrogel pore size, inversely related to the hydrogel concentration. Thus, larger pore size promotes the polarization of macrophages towards M2 phenotype along the time and enhances their phagocytosis capability. It is important to point out that encapsulated macrophages in absence of pathogen showed an M2 phenotype, but macrophages coencapsulated with C. albicans can switch towards an M1 inflammatory phenotype to resolve the infection, depending on the fungus quantity. The present study reveals that pDAM hydrogels preserve the macrophage plasticity, demonstrating their relevance as new models for macrophage-pathogen interaction studies that mimic an infection scenario with application in regenerative medicine research.

Recognition of yeast β-glucan particles triggers immunometabolic signaling required for trained immunity

Fungal β-glucans are major drivers of trained immunity which increases long-term protection against secondary infections. Heterogeneity in β-glucan source, structure, and solubility alters interaction with the phagocytic receptor Dectin-1 and could impact strategies to improve trained immunity in humans. Using a panel of diverse β-glucans, we describe the ability of a specific yeast-derived whole-glucan particle (WGP) to reprogram metabolism and thereby drive trained immunity in human monocyte-derived macrophages in vitro and mice bone marrow in vivo. Presentation of pure, non-soluble, non-aggregated WGPs led to the formation of the Dectin-1 phagocytic synapse with subsequent lysosomal mTOR activation, metabolic reprogramming, and epigenetic rewiring. Intraperitoneal or oral administration of WGP drove bone marrow myelopoiesis and improved mature macrophage responses, pointing to therapeutic and food-based strategies to drive trained immunity. Thus, the investment of a cell in a trained response relies on specific recognition of β-glucans presented on intact microbial particles through stimulation of the Dectin-1 phagocytic response.

Understanding yeast shells: structure, properties and applications

Background and purpose

The employment of yeasts for biomedical purposes has become increasingly frequent for the delivery of prophylactic and therapeutic products. Its structural components, such as β-glucans, mannan, and chitin, can be explored as immunostimulators that show safety and low toxicity. Besides, this system minimizes antigen degradation after administration, facilitating the delivery to the target cells.

Review approach

This review sought to present molecules derived from yeast, called yeast shells (YS), and their applications as carrier vehicles for drugs, proteins, and nucleic acids for immunotherapy purposes. Furthermore, due to the diversity of information regarding the production and immunostimulation of these compounds, a survey of the protocols and immune response profiles generated was presented.

Key results

The use of YS has allowed the development of strategies that combine efficiency and effectiveness in antigen delivery. The capsular structure can be recognized and phagocytized by dendritic cells and macrophages. In addition, the combination with different molecules, such as nanoparticles or even additional adjuvants, improves the cargo loading, enhancing the system. Activation by specific immune pathways can also be achieved by different administration routes.

Conclusion

Yeast derivatives combined in different ways can increase immunostimulation, enhancing the delivery of medicines and vaccine antigens. These aspects, combined with the simplicity of the production steps, make these strategies more accessible to be applied in the prevention and treatment of various diseases.

Identification of 10 genes on Candida albicans chromosome 5 that control surface exposure of the immunogenic cell wall epitope β-glucan and cell wall remodeling in caspofungin-adapted mutants

IMPORTANCE

Candida infections are often fatal in immuno-compromised individuals, resulting in many thousands of deaths per year. Caspofungin has proven to be an excellent anti-Candida drug and is now the frontline treatment for infections. However, as expected, the number of resistant cases is increasing; therefore, new treatment modalities are needed. We are determining metabolic pathways leading to decreased drug susceptibility in order to identify mechanisms facilitating evolution of clinical resistance. This study expands the understanding of genes that modulate drug susceptibility and reveals new targets for the development of novel antifungal drugs.

Helium Cold Atmospheric Plasma Causes Morphological and Biochemical Alterations in Candida albicans Cells

(1) Background: Previous studies reported the promising inhibitory effect of cold atmospheric plasma (CAP) on Candida albicans. However, the exact mechanisms of CAP's action on the fungal cell are still poorly understood. This study aims to elucidate the CAP effect on C. albicans cell wall, by evaluating the alterations on its structure and biochemical composition; (2) Methods: C. albicans cells treated with Helium-CAP were analyzed by atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR) in order to detect morphological, topographic and biochemical changes in the fungal cell wall. Cells treated with caspofungin were also analyzed for comparative purposes; (3) Results: Expressive morphological and topographic changes, such as increased roughness and shape modification, were observed in the cells after CAP exposure. The alterations detected were similar to those observed after the treatment with caspofungin. The main biochemical changes occurred in polysaccharides content, and an overall decrease in glucans and an increase in chitin synthesis were detected; (4) Conclusions: Helium-CAP caused morphological and topographic alterations in C. albicans cells and affected the cell wall polysaccharide content.

Polyphyllin I, a strong antifungal compound against Candida albicans

This study was performed to explore the antifungal and antibiofilm effects of polyphyllin I (PPI) on Candida albicans. Microdilution assay was performed to determine the minimal inhibitory concentrations (MIC) of PPI against Candida species. Adhesion assay, hyphal growth assay, biofilm formation, and development were used to test the impacts of PPI on C. albicans virulence factors. Propidium iodide staining was performed to test whether the permeability of cell membrane was influenced by PPI. PPI showed significant antifungal activities against several Candida species, with MIC below or equal to 6.25 μM. PPI also inhibited the adhesion to polystyrene surfaces, hyphal growth, and biofilm formation. PPI significantly increased the permeability of C. albicans cell membrane. In sum, PPI can suppress the planktonic growth and biofilm of C. albicans and its mechanism involves the increased permeability of cell membrane.

Metabolic degradation of polysaccharides from Lentinus edodes by Kupffer cells via the Dectin-1/Syk signaling pathway

It had been shown that lentinan (LNT) was mainly distributed in the liver after intravenous administration. The study aimed to investigate the integrated metabolic processes and mechanisms of LNT in the liver, as these have not been thoroughly explored. In current work, 5-([4,6-dichlorotriazin-2-yl] amino) fluorescein and cyanine 7 were used to label LNT for tracking its metabolic behavior and mechanisms. Near-infrared imaging demonstrated that LNT was captured mainly by the liver. Kupffer cell (KC) depletion reduced LNT liver localization and degradation in BALB/c mice. Moreover, experiments with Dectin-1 siRNA and Dectin-1/Syk signaling pathway inhibitors showed that LNT was mainly taken up by KCs via the Dectin-1/Syk pathway and promoted lysosomal maturation in KCs via this same pathway, which in turn promoted LNT degradation. These empirical findings offer novel insights into the metabolism of LNT in vivo and in vitro, which will facilitate the further application of LNT and other β-glucans.

Fungal mechanisms of intracellular survival: what can we learn from bacterial pathogens?

Fungal infections represent a major, albeit neglected, public health threat with serious medical and economic burdens globally. With unacceptably high mortality rates, invasive fungal pathogens are responsible for millions of deaths each year, with a steadily increasing incidence primarily in immunocompromised individuals. The poor therapeutic options and rise of antifungal drug resistance pose further challenges in controlling these infections. These fungal pathogens have adapted to survive within mammalian hosts and can establish intracellular niches to promote survival within host immune cells. To do that, they have developed diverse methods to circumvent the innate immune system attack. This includes strategies such as altering their morphology, counteracting macrophage antimicrobial action, and metabolic adaptation. This is reminiscent of how bacterial pathogens have adapted to survive within host cells and cause disease. However, relative to the great deal of information available concerning intracellular bacterial pathogenesis, less is known about the mechanisms fungal pathogens employ. Therefore, here we review our current knowledge and recent advances in our understanding of how fungi can evade and persist within host immune cells. This review will focus on the major fungal pathogens, including Cryptococcus neoformans, Candida albicans, and Aspergillus fumigatus, among others. As we discover and understand the strategies used by these fungi, similarities with their bacterial counterparts are becoming apparent, hence we can use the abundant information from bacteria to guide our studies in fungi. By understanding these strategies, new lines of research will open that can improve the treatments of these devastating fungal diseases.

Development of Echinocandin Resistance in Candida haemulonii: An Emergent, Widespread, and Opportunistic Fungal Pathogen

Echinocandins, used for the prevention and treatment of invasive fungal infections, have led to a rise in breakthrough infections caused by resistant Candida species. Among these species, those belonging to the Candida haemulonii complex are rare multidrug-resistant (MDR) yeasts that are frequently misidentified but have emerged as significant healthcare-associated pathogens causing invasive infections. The objectives of this study were to investigate the evolutionary pathways of echinocandin resistance in C. haemulonii by identifying mutations in the FKS1 gene and evaluating the impact of resistance on fitness. After subjecting a MDR clinical isolate of C. haemulonii (named Ch4) to direct selection using increasing caspofungin concentrations, we successfully obtained an isolate (designated Ch4'r) that exhibited a high level of resistance, with MIC values exceeding 16 mg/L for all tested echinocandin drugs (caspofungin, micafungin, and anidulafungin). Sequence analysis revealed a specific mutation in the resistant Ch4'r strain, leading to an arginine-histidine amino acid substitution (R1354H), occurring at the G4061A position of the HS2 region of the FKS1 gene. Compared to the wild-type strain, Ch4'r exhibited significantly reduced growth proliferation, biofilm formation capability, and phagocytosis ratio, indicating a decrease in fitness. Transmission electron microscopy analysis revealed alterations in cell wall components, with a notable increase in cell wall thickness. The resistant strain also exhibited higher amounts (2.5-fold) of chitin, a cell wall-located molecule, compared to the wild-type strain. Furthermore, the resistant strain demonstrated attenuated virulence in the Galleria mellonella larval model. The evolved strain Ch4'r maintained its resistance profile in vivo since the treatment with either caspofungin or micafungin did not improve larval survival or reduce the fungal load. Taken together, our findings suggest that the acquisition of pan-echinocandin resistance occurred rapidly after drug exposure and was associated with a significant fitness cost in C. haemulonii. This is particularly concerning as echinocandins are often the first-line treatment option for MDR Candida species.

Release of neutrophil extracellular traps in response to Candida albicans yeast, as a secondary defense mechanism activated by phagocytosis

Candida albicans is one of the main pathogens responsible for the development of difficult-to-fight fungal infections called candidiasis. Neutrophils are the major effector cells involved in the eradication of fungal pathogens. This group of immune cells uses several mechanisms that enable the rapid neutralization of pathogens. The most frequently identified mechanisms are phagocytosis and the release of neutrophil extracellular traps (NETs). The mechanism for selecting the type of neutrophil immune response is still unknown. In our study, we analyzed the relationship between the activation of phagocytosis and netosis. We detected the presence of two neutrophil populations characterized by different response patterns to contact with C. albicans blastospores. The first neutrophil population showed an increased ability to rapidly release NETs without prior internalization of the pathogen. In the second population, the netosis process was inherently associated with phagocytosis. Differences between populations also referred to the production of reactive oxygen species. Our results suggest that neutrophils use different strategies to fight C. albicans and, contrary to previous reports, these mechanisms are not mutually exclusive.

The secretory Candida effector Sce1 licenses fungal virulence by masking the immunogenic β-1,3-glucan and promoting apoptosis of the host cells

Candida albicans deploys a variety of mechanisms such as morphological switch and elicitor release to promote virulence. However, the intricate interactions between the fungus and the host remain poorly understood, and a comprehensive inventory of fungal virulence factors has yet to be established. In this study, we identified a C. albicans secretory effector protein Sce1, whose induction and secretion are associated with vagina-simulative conditions and chlamydospore formation. Sequence alignment showed that Sce1 belongs to a Pir family in C. albicans, which is conserved across several fungi and primarily characterized as a β-glucan binding protein in the Saccharomyces cerevisiae. Mechanically, Sce1 is primarily localized to the cell wall in a cleaved form as an alkali-labile β-1,3-glucan binding protein and plays a role in masking β-glucan in acidic environments and chlamydospores, a feature that might underline C. albicans' ability to evade host immunity. Further, a cleaved short form of Sce1 protein could be released into extracellular compartments and presented in bone marrow-derived macrophages infected with chlamydospores. This cleaved short form of Sce1 also demonstrated a unique ability to trigger the caspases-8/9-dependent apoptosis in various host cells. Correspondingly, genetic deletion of SCE1 led to dampened vaginal colonization of C. albicans and diminished fungal virulence during systemic infection. The discovery of Sce1 as a versatile virulence effector that executes at various compartments sheds light on the fungus-host interactions and C. albicans pathogenesis.

DectiSomes: C-type lectin receptor-targeted liposomes as pan-antifungal drugs

Combatting the ever-increasing threat from invasive fungal pathogens faces numerous fundamental challenges, including constant human exposure to large reservoirs of species in the environment, the increasing population of immunocompromised or immunosuppressed individuals, the unsatisfactory efficacy of current antifungal drugs and their associated toxicity, and the scientific and economic barriers limiting a new antifungal pipeline. DectiSomes represent a new drug delivery platform that enhances antifungal efficacy for diverse fungal pathogens and reduces host toxicity for current and future antifungals. DectiSomes employ pathogen receptor proteins - C-type lectins - to target drug-loaded liposomes to conserved fungal cognate ligands and away from host cells. DectiSomes represent one leap forward for urgently needed effective pan-antifungal therapy. Herein, we discuss the problems of battling fungal diseases and the state of DectiSome development.

Quantifying the Mechanical Properties of Yeast Candida albicans Using Atomic Force Microscopy-based Force Spectroscopy

Fungi can adapt to a wide range of environmental stresses in the wild and host milieu by employing their plastic genome and great diversity in morphology. Among different adaptive strategies, mechanical stimuli, such as changes in osmotic pressure, surface remodeling, hyphal formation, and cell divisions, could guide the physical cues into physiological responses through a complex signaling network. While fungal pathogens require a pressure-driven force to expand and penetrate host tissues, quantitatively studying the biophysical properties at the host-fungal interface is critical to understand the development of fungal diseases. Microscopy-based techniques have enabled researchers to monitor the dynamic mechanics on fungal cell surface in responses to the host stress and antifungal drugs. Here, we describe a label-free, high-resolution method based on atomic force microscopy, with a step-by-step protocol to measure the physical properties in human fungal pathogen Candida albicans.

Nature of β-1,3-Glucan-Exposing Features on Candida albicans Cell Wall and Their Modulation

Candida albicans exists as a commensal of mucosal surfaces and the gastrointestinal tract without causing pathology. However, this fungus is also a common cause of mucosal and systemic infections when antifungal immune defenses become compromised. The activation of antifungal host defenses depends on the recognition of fungal pathogen-associated molecular patterns (PAMPs), such as β-1,3-glucan. In C. albicans, most β-1,3-glucan is present in the inner cell wall, concealed by the outer mannan layer, but some β-1,3-glucan becomes exposed at the cell surface. In response to host signals, such as lactate, C. albicans induces the Xog1 exoglucanase, which shaves exposed β-1,3-glucan from the cell surface, thereby reducing phagocytic recognition. We show here that β-1,3-glucan is exposed at bud scars and punctate foci on the lateral wall of yeast cells, that this exposed β-1,3-glucan is targeted during phagocytic attack, and that lactate-induced masking reduces β-1,3-glucan exposure at bud scars and at punctate foci. β-1,3-Glucan masking depends upon protein kinase A (PKA) signaling. We reveal that inactivating PKA, or its conserved downstream effectors, Sin3 and Mig1/Mig2, affects the amounts of the Xog1 and Eng1 glucanases in the C. albicans secretome and modulates β-1,3-glucan exposure. Furthermore, perturbing PKA, Sin3, or Mig1/Mig2 attenuates the virulence of lactate-exposed C. albicans cells in Galleria. Taken together, the data are consistent with the idea that β-1,3-glucan masking contributes to Candida pathogenicity. IMPORTANCE Microbes that coexist with humans have evolved ways of avoiding or evading our immunological defenses. These include the masking by these microbes of their "pathogen-associated molecular patterns" (PAMPs), which are recognized as "foreign" and used to activate protective immunity. The commensal fungus Candida albicans masks the proinflammatory PAMP β-1,3-glucan, which is an essential component of its cell wall. Most of this β-1,3-glucan is hidden beneath an outer layer of the cell wall on these microbes, but some can become exposed at the fungal cell surface. Using high-resolution confocal microscopy, we examine the nature of the exposed β-1,3-glucan at C. albicans bud scars and at punctate foci on the lateral cell wall, and we show that these features are targeted by innate immune cells. We also reveal that downstream effectors of protein kinase A (Mig1/Mig2, Sin3) regulate the secretion of major glucanases, modulate the levels of β-1,3-glucan exposure, and influence the virulence of C. albicans in an invertebrate model of systemic infection. Our data support the view that β-1,3-glucan masking contributes to immune evasion and the virulence of a major fungal pathogen of humans.

β-1,3-Glucan/chitin unmasking in the Saccharomyces cerevisiae mutant, Δmnn9, promotes immune response and resistance of the Pacific oyster (Crassostrea gigas) to Vibrio coralliilyticus infection

Yeast cells can play a crucial role in immune activation in fish and shellfish predominantly due to the cell wall component β-1,3-glucan, providing protection against bacterial or viral infections. However, the immunostimulatory capacity of dietary yeast cells remains poorly studied in bivalves. To understand the role of yeast cell wall components (mannan, β-glucan and chitin) as immune activators, this study characterized the surface carbohydrate exposure of the wild-type baker's yeast Saccharomyces cerevisiae (WT) and its Δmnn9 mutant, which presents a defective mannan structure, and compared these profiles with that of β-glucan particles, using fluorescein isothiocyanate (FITC)-labeled lectin binding analysis. Then, a first trial evaluated the immunological response in Crassostrea gigas juveniles after being fed for 24 h with an algae-based diet (100A) and its 50% substituted version (based on dry weight) with WT (50A50WT) and Δmnn9 (50A50Y), and the posterior resistance of the juveniles against Vibrio coralliilyticus infection (trial 1). The mRNA expression was measured for β-glucan-binding protein (CgβGBP), Toll-like receptor 4 (CgTLR4), C-type lectin receptor 3 (CgCLec-3), myeloid differentiation factor 88 (CgMyD88), nuclear factor-kappa B (CgNFκB), lysozyme (CgLys), interleukin 17-5 (CgIL17-5), and superoxide dismutase (CgSOD), in oysters, before and 24 h after the bacterial inoculation. A second trial tested the effect of incorporating Δmnn9 into the 100A diet for 24 h at different substitution levels: 0, 5, 10, 25, and 50% (100A, 95A5Y, 90A10Y, 75A25Y, and 50A50Y), followed by the bacterial challenge with V. coralliilyticus (trial 2). Our findings showed that the outer cell wall surface of WT is largely composed of mannan, while Δmnn9 presents high exposure of β-glucan and chitin, exhibiting similar FITC-lectin binding profiles (fluorescence intensity) to β-glucan particles. A significantly higher survival after the bacterial challenge was observed in oysters fed on 50A50Y compared to those fed 50A50WT and 100A in trial 1. This better performance of 50A50Y was supported by significantly higher gene expressions of CgLys, CgSOD, CgMyD88, and CgβGBP compared to 100A, and CgSOD and CgNFκB in relation to those fed on 50A50WT, prior to the bacterial inoculation. Furthermore, improved survival was observed in oysters fed 50A50Y compared to those offered lower Δmnn9 levels and 100A in trial 2. The superior performance of Δmnn9-fed oysters is mostly associated with the elevated presence of unmasked β-glucans on Δmnn9 cell wall surface, facilitating their interactions with oyster hemocytes. Further studies are needed to evaluate administration dose and frequency of Δmnn9 to develop strategies for long-term feeding.

The Role of Sfp1 in Candida albicans Cell Wall Maintenance

The cell wall is the first interface for Candida albicans interaction with the surrounding environment and the host cells. Therefore, maintenance of cell wall integrity (CWI) is crucial for C. albicans survival and host-pathogen interaction. In response to environmental stresses, C. albicans undergoes cell wall remodeling controlled by multiple signaling pathways and transcription regulators. Here, we explored the role of the transcription factor Sfp1 in CWI. A deletion of the SFP1 gene not only caused changes in cell wall properties, cell wall composition and structure but also modulated expression of cell wall biosynthesis and remodeling genes. In addition, Cas5 is a known transcription regulator for C. albicans CWI and cell wall stress response. Interestingly, our results indicated that Sfp1 negatively controls the CAS5 gene expression by binding to its promoter element. Together, this study provides new insights into the regulation of C. albicans CWI and stress response.

Antifungal activity of vitamin D3 against Candida albicans in vitro and in vivo

The incidence of intra-abdominal candidiasis (IAC), characterized by high morbidity and mortality, has become a serious concern. The limitations of current antifungal drugs on the market underscores the importance of the development of novel antifungal agents. In the present study, the antifungal activity of vitamin D₃ (VD₃) against various Candida species was investigated. In vitro, the broth microdilution method and solid plate assay confirmed that VD₃ inhibited the growth of Candida spp. in a broad-spectrum, dose-dependent manner. VD₃ also had a significant antifungal effect on the initiation, development, and maturation phases of biofilm formation in Candida albicans. The mechanism of VD₃ action was explored by transcriptomics and reverse transcription quantitative PCR (RT-qPCR) analysis, and showed that VD₃ affects ribosome biogenesis, coenzyme metabolism, and carbon metabolism. These results suggested that VD₃ may have multitarget effects against C. albicans. In the murine IAC model, VD₃ reduced the fungal burden in the liver, kidneys, and small intestine. Further histopathological analysis and quantification of plasma cytokine levels confirmed that VD₃ treatment significantly decreased the infiltration of inflammatory cells and the levels of plasma interferon (IFN)-γ and tumor necrosis factor (TNF)-α. Taken together, these findings suggest a new antifungal mechanism for VD₃ and indicate that VD₃ could be an effective therapeutic agent for use in IAC treatment.

Optotracing for live selective fluorescence-based detection of Candida albicans biofilms

Candida albicans is the most common fungal pathogen in humans, implicated in hospital-acquired infections, secondary infections in human immunodeficiency virus (HIV) patients, and is a significant contributor to the global antimicrobial resistance (AMR) burden. Early detection of this pathogen is needed to guide preventative strategies and the selection and development of therapeutic treatments. Fungal biofilms are a unique heterogeneous mix of cell types, extracellular carbohydrates and amyloid aggregates. Perhaps due to the dominance of carbohydrates in fungi, to date, few specific methods are available for the detection of fungal biofilms. Here we present a new optotracing-based method for the detection and analysis of yeast and biofilms based on C. albicans SC5314 as a model. Using commercial extracts of cell wall carbohydrates, we showed the capability of the optotracer EbbaBiolight 680 for detecting chitin and β-glucans. The sensitivity of this tracer to these carbohydrates in their native environment within fungal cells enabled the visualization of both yeast and hyphal forms of the microbe. Analysis of optotracer fluorescence by confocal laser scanning microscopy revealed extensive staining of fungi cell walls as well as the presence of intracellular amyloid aggregates within a subpopulation of cells within the biofilm. Further analysis of the photophysical properties of bound tracers by spectroscopy and spectral imaging revealed polymorphisms between amyloid aggregates within yeast and hyphal cells and enabled their differentiation. With exceptional spatial and temporal resolution, this assay adds a new technique that facilitates future understanding of fungal biofilms and their formation, and enables direct, unbiased diagnostics of these medically relevant biofilms, as well as the development of antifungal strategies.

β-Glucan-Functionalized Nanoparticles Down-Modulate the Proinflammatory Response of Mononuclear Phagocytes Challenged with Candida albicans

Systemic fungal infections are associated with significant morbidity and mortality, and Candida albicans is the most common causative agent. Recognition of yeast cells by immune cell surface receptors can trigger phagocytosis of fungal pathogens and a pro-inflammatory response that may contribute to fungal elimination. Nevertheless, the elicited inflammatory response may be deleterious to the host by causing excessive tissue damage. We developed a nanoparticle-based approach to modulate the host deleterious inflammatory consequences of fungal infection by using β1,3-glucan-functionalized polystyrene (β-Glc-PS) nanoparticles. β-Glc-PS nanoparticles decreased the levels of the proinflammatory cytokines TNF-α, IL-6, IL-1β and IL-12p40 detected in in vitro culture supernatants of bone marrow-derived dendritic cells and macrophage challenged with C. albicans cells. Moreover, β-Glc-PS nanoparticles impaired the production of reactive oxygen species by bone marrow-derived dendritic cells incubated with C. albicans. This immunomodulatory effect was dependent on the nanoparticle size. Overall, β-Glc-PS nanoparticles reduced the proinflammatory response elicited by fungal cells in mononuclear phagocytes, setting the basis for a targeted therapy aimed at protecting the host by lowering the inflammatory cost of infection.

Neutrophil Extracellular Traps in Candida albicans Infection

Candida albicans is the most common pathogen causing clinical Candida infections. Neutrophils are a key member of the host innate immunity that plays an essential role in clearing invading C. albicans. In addition to the well-known defensive approaches such as phagocytosis, degranulation, and reactive oxygen species production, the formation of neutrophil extracellular traps (NETs) has also become an important way for neutrophils to defend against various pathogens. C. albicans has been reported to be capable of activating neutrophils to release NETs that subsequently kill fungi. The induction of NETs is affected by both the morphology and virulence factors of C. albicans, which also develops specific strategies to respond to the attack by NETs. Our review specifically focuses on the mechanisms by which C. albicans triggers NET formation and their subsequent interactions, which might provide meaningful insight into the innate immunity against C. albicans infection.

Candida albicans oscillating UME6 expression during intestinal colonization primes systemic Th17 protective immunity

Systemic immunity is stringently regulated by commensal intestinal microbes, including the pathobiont Candida albicans. This fungus utilizes various transcriptional and morphological programs for host adaptation, but how this heterogeneity affects immunogenicity remains uncertain. We show that UME6, a transcriptional regulator of filamentation, is essential for intestinal C. albicans-primed systemic Th17 immunity. UME6 deletion and constitutive overexpression strains are non-immunogenic during commensal colonization, whereas immunogenicity is restored by C. albicans undergoing oscillating UME6 expression linked with β-glucan and mannan production. In turn, intestinal reconstitution with these fungal cell wall components restores protective Th17 immunity to mice colonized with UME6-locked variants. These fungal cell wall ligands and commensal C. albicans stimulate Th17 immunity through multiple host pattern recognition receptors, including Toll-like receptor 2 (TLR2), TLR4, Dectin-1, and Dectin-2, which work synergistically for colonization-induced protection. Thus, dynamic gene expression fluctuations by C. albicans during symbiotic colonization are essential for priming host immunity against disseminated infection.

Genetic Screening of Candida albicans Inactivation Mutants Identifies New Genes Involved in Macrophage-Fungal Cell Interactions

Candida albicans, an important fungal pathogen of humans, displays different morphologies, such as yeast, pseudo-hyphae and hyphae, which are recognized unequally by phagocytic cells of the innate immune response. Once C. albicans cells invade host tissues, immune cells such as macrophages are attracted to the site of infection and activated to recognize, engulf and kill the pathogen. We have investigated this fungal cell-macrophage interface by using high-throughput screening of the C. albicans GRACE library to identify genes that can influence this interaction and modify the kinetics of engulfment. Compared with the wild-type (WT) strain, we identified generally faster rates of engulfment for those fungal strains with constitutive pseudo-hyphal and hyphal phenotypes, whereas yeast-form-locked strains showed a reduced and delayed recognition and internalization by macrophages. We identified a number of GRACE strains that showed normal morphological development but exhibited different recognition and engulfment kinetics by cultured macrophages and characterized two mutants that modified interactions with the murine and human-derived macrophages. One mutant inactivated an uncharacterized C. albicans open reading frame that is the ortholog of S. cerevisiae OPY1, the other inactivated CaKRE1. The modified interaction was monitored during a 4 h co-culture. Early in the interaction, both opy1 and kre1 mutant strains showed reduced recognition and engulfment rates by macrophages when compared with WT cells. At fungal germ tube initiation, the engulfment kinetics increased for both mutants and WT cells, however the WT cells still showed a higher internalization by macrophages up to 2 h of interaction. Subsequently, between 2 and 4 h of the interaction, when most macrophages contain engulfed fungal cells, the engulfment kinetics increased for the opy1 mutant and further decreased for the kre1 mutant compared with Ca-WT. It appears that fungal morphology influences macrophage association with C. albicans cells and that both OPY1 and KRE1 play roles in the interaction of the fungal cells with phagocytes.

DectiSomes: Glycan Targeting of Liposomal Drugs Improves the Treatment of Disseminated Candidiasis

Candida albicans causes life-threatening disseminated candidiasis. Individuals at greatest risk have weakened immune systems. An outer cell wall, exopolysaccharide matrix, and biofilm rich in oligoglucans and oligomannans help Candida spp. evade host defenses. Even after antifungal treatment, the 1-year mortality rate exceeds 25%. Undoubtedly, there is room to improve drug performance. The mammalian C-type lectin pathogen receptors Dectin-1 and Dectin-2 bind to fungal oligoglucans and oligomannans, respectively. We previously coated amphotericin B-loaded liposomes, AmB-LLs, pegylated analogs of AmBisome, with the ligand binding domains of these two Dectins. DectiSomes, DEC1-AmB-LLs and DEC2-AmB-LLs, showed two distinct patterns of binding to the exopolysaccharide matrix surrounding C. albicans hyphae grown in vitro. Here we showed that DectiSomes were preferentially associated with fungal colonies in the kidneys. In a neutropenic mouse model of candidiasis, DEC1-AmB-LLs and DEC2-AmB-LLs delivering only one dose of 0.2 mg/kg AmB reduced the kidney fungal burden several fold relative to AmB-LLs. DEC1-AmB-LLs and DEC2-AmB-LLs increased the percent of surviving mice 2.5-fold and 8.3-fold, respectively, relative to AmB-LLs. Dectin-2 targeting of anidulafungin loaded liposomes, DEC2-AFG-LLs, and of commercial AmBisome, DEC2-AmBisome, reduced fungal burden in the kidneys several fold over their untargeted counterparts. The data herein suggest that targeting of a variety of antifungal drugs to fungal glycans may achieve lower safer effective doses and improve drug efficacy against a variety of invasive fungal infections.

A dynamically evolving war between autophagy and pathogenic microorganisms

Autophagy is an intracellular degradation process that maintains cellular homeostasis. It is essential for protecting organisms from environmental stress. Autophagy can help the host to eliminate invading pathogens, including bacteria, viruses, fungi, and parasites. However, pathogens have evolved multiple strategies to interfere with autophagic signaling pathways or inhibit the fusion of autophagosomes with lysosomes to form autolysosomes. Moreover, host cell matrix degradation by different types of autophagy can be used for the proliferation and reproduction of pathogens. Thus, determining the roles and mechanisms of autophagy during pathogen infections will promote understanding of the mechanisms of pathogen‍‒‍host interactions and provide new strategies for the treatment of infectious diseases.

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.

The macrophage-derived protein PTMA induces filamentation of the human fungal pathogen Candida albicans

Evasion of killing by immune cells is crucial for fungal survival in the host. For the human fungal pathogen Candida albicans, internalization by macrophages induces a transition from yeast to filaments that promotes macrophage death and fungal escape. Nutrient deprivation, alkaline pH, and oxidative stress have been implicated as triggers of intraphagosomal filamentation; however, the impact of other host-derived factors remained unknown. Here, we show that lysates prepared from macrophage-like cell lines and primary macrophages robustly induce C. albicans filamentation. Enzymatic treatment of lysate implicates a phosphorylated protein, and bioactivity-guided fractionation coupled to mass spectrometry identifies the immunomodulatory phosphoprotein PTMA as a candidate trigger of C. albicans filamentation. Immunoneutralization of PTMA within lysate abolishes its activity, strongly supporting PTMA as a filament-inducing component of macrophage lysate. Adding to the known repertoire of physical factors, this work implicates a host protein in the induction of C. albicans filamentation within immune cells.

The human fungal pathogen Candida albicans filaments within host macrophages, enabling its escape. Case et al. demonstrate that lysates prepared from macrophage-like cell lines and primary macrophages induce C. albicans filamentation and implicate the immunomodulatory protein prothymosin alpha (PTMA) as a trigger of filamentation produced by host immune cells.

Revisiting the Vital Drivers and Mechanisms of β-Glucan Masking in Human Fungal Pathogen, Candida albicans

Among the several human fungal pathogens, Candida genus represents one of the most implicated in the clinical scenario. There exist several distinctive features that govern the establishment of Candida infections in addition to their capacity to adapt to multiple stress conditions inside humans which also include evasion of host immune responses. The complex fungal cell wall of the prevalent pathogen, Candida albicans, is one of the main targets of antifungal drugs and recognized by host immune cells. The wall consists of tiered arrangement of an outer thin but dense covering of mannan and inner buried layers of β-glucan and chitin. However, the pathogenic fungi adopt strategies to evade immune recognition by masking these molecules. This capacity to camouflage the immunogenic polysaccharide β-glucan from the host is a key virulence factor of C. albicans. The present review is an attempt to collate various underlying factors and mechanisms involved in Candida β-glucan masking from the available pool of knowledge and provide a comprehensive understanding. This will further improve therapeutic approaches to candidiasis by identifying new antifungal targets that blocks fungal immune evasion.

Intervening in Symbiotic Cross-Kingdom Biofilm Interactions: a Binding Mechanism-Based Nonmicrobicidal Approach

Early childhood caries is a severe oral disease that results in aggressive tooth decay. Particularly, a synergistic association between a fungus, Candida albicans, and a cariogenic bacterium, Streptococcus mutans, promotes the development of hard-to-remove and highly acidic biofilms, exacerbating the virulent damage. These interactions are largely mediated via glucosyltransferases (GtfB) binding to mannans on the cell wall of C. albicans. Here, we present an enzymatic approach to target GtfB-mannan interactions in this cross-kingdom consortium using mannan-degrading exo- and endo-enzymes. These exo- and endo-enzymes are highly effective in reducing biofilm biomass without killing microorganisms, as well as alleviating the production of an acidic pH environment conducive to tooth decay. To corroborate these results, we present biophysical evidence using single-molecule atomic force microscopy, biofilm shearing, and enamel surface topography analyses. Data show a drastic decrease in binding forces of GtfB to C. albicans (∼15-fold reduction) following enzyme treatment. Furthermore, enzymatic activity disrupted biofilm mechanical stability and significantly reduced human tooth enamel demineralization without cytotoxic effects on gingival keratinocytes. Our results represent significant progress toward a novel nonbiocidal therapeutic intervention against pathogenic bacterial-fungal biofilms by targeting the interkingdom receptor-ligand binding interactions.

The impact of the Fungus-Host-Microbiota interplay upon Candida albicans infections: current knowledge and new perspectives

Candida albicans is a major fungal pathogen of humans. It exists as a commensal in the oral cavity, gut or genital tract of most individuals, constrained by the local microbiota, epithelial barriers and immune defences. Their perturbation can lead to fungal outgrowth and the development of mucosal infections such as oropharyngeal or vulvovaginal candidiasis, and patients with compromised immunity are susceptible to life-threatening systemic infections. The importance of the interplay between fungus, host and microbiota in driving the transition from C. albicans commensalism to pathogenicity is widely appreciated. However, the complexity of these interactions, and the significant impact of fungal, host and microbiota variability upon disease severity and outcome, are less well understood. Therefore, we summarise the features of the fungus that promote infection, and how genetic variation between clinical isolates influences pathogenicity. We discuss antifungal immunity, how this differs between mucosae, and how individual variation influences a person's susceptibility to infection. Also, we describe factors that influence the composition of gut, oral and vaginal microbiotas, and how these affect fungal colonisation and antifungal immunity. We argue that a detailed understanding of these variables, which underlie fungal-host-microbiota interactions, will present opportunities for directed antifungal therapies that benefit vulnerable patients.

Immune cells fold and damage fungal hyphae

Innate immunity provides essential protection against life-threatening fungal infections. However, the outcomes of individual skirmishes between immune cells and fungal pathogens are not a foregone conclusion because some pathogens have evolved mechanisms to evade phagocytic recognition, engulfment, and killing. For example, Candida albicans can escape phagocytosis by activating cellular morphogenesis to form lengthy hyphae that are challenging to engulf. Through live imaging of C. albicans-macrophage interactions, we discovered that macrophages can counteract this by folding fungal hyphae. The folding of fungal hyphae is promoted by Dectin-1, β2-integrin, VASP, actin-myosin polymerization, and cell motility. Folding facilitates the complete engulfment of long hyphae in some cases and it inhibits hyphal growth, presumably tipping the balance toward successful fungal clearance.

Fungal immunity and pathogenesis in mammals versus the invertebrate model organism Galleria mellonella

In recent decades, Galleria mellonella (Lepidoptera: Pyralidae) have emerged as a model system to explore experimental aspects of fungal pathogenesis. The benefits of the G. mellonella model include being faster, cheaper, higher throughput and easier compared with vertebrate models. Additionally, as invertebrates, their use is subject to fewer ethical and regulatory issues. However, for G. mellonella models to provide meaningful insight into fungal pathogenesis, the G. mellonella-fungal interactions must be comparable to mammalian-fungal interactions. Indeed, as discussed in the review, studies suggest that G. mellonella and mammalian immune systems share many similarities, and fungal virulence factors show conserved functions in both hosts. While the moth model has opened novel research areas, many comparisons are superficial and leave large gaps of knowledge that need to be addressed concerning specific mechanisms underlying G. mellonella-fungal interactions. Closing these gaps in understanding will strengthen G. mellonella as a model for fungal virulence in the upcoming years. In this review, we provide comprehensive comparisons between fungal pathogenesis in mammals and G. mellonella from immunological and virulence perspectives. When information on an antifungal immune component is unknown in G. mellonella, we include findings from other well-studied Lepidoptera. We hope that by outlining this information available in related species, we highlight areas of needed research and provide a framework for understanding G. mellonella immunity and fungal interactions.

Modulation of Immune Responses by Particle Size and Shape

The immune system has to cope with a wide range of irregularly shaped pathogens that can actively move (e.g., by flagella) and also dynamically remodel their shape (e.g., transition from yeast-shaped to hyphal fungi). The goal of this review is to draw general conclusions of how the size and geometry of a pathogen affect its uptake and processing by phagocytes of the immune system. We compared both theoretical and experimental studies with different cells, model particles, and pathogenic microbes (particularly fungi) showing that particle size, shape, rigidity, and surface roughness are important parameters for cellular uptake and subsequent immune responses, particularly inflammasome activation and T cell activation. Understanding how the physical properties of particles affect immune responses can aid the design of better vaccines.

Split Enzyme-Based Biosensors for Structural Characterization of Soluble and Insoluble β-Glucans

β-Glucan is widely distributed in various plants and microorganisms and is composed of β-1,3-linked d-glucose units. It may have a branched short or long side chain of glucose units with β-1,6- or β-1,4-linkage. Numerous studies have investigated different β-glucans and revealed their bioactivities. To understand the structure-function relationship of β-glucan, we constructed a split-luciferase complementation assay for the structural analysis of long-chain β-1,6-branched β-1,3-glucan. The N- and C-terminal fragments of luciferase from deep-sea shrimp were fused to insect-derived β-1,3-glucan recognition protein and fungal endo-β-1,6-glucanase (Neg1)-derived β-1,6-glucan recognition protein, respectively. In this approach, two β-glucan recognition proteins bound to β-glucan molecules come into close proximity, resulting in the assembly of the full-length reporter enzyme and induction of transient luciferase activity, indicative of the structure of β-glucan. To test the applicability of this assay, β-glucan and two β-glucan recognition proteins were mixed, resulting in an increase in the luminescence intensity in a β-1,3-glucan with a long polymer of β-1,6-glucan in a dose-dependent manner. This simple test also allows the monitoring of real-time changes in the side chain structure and serves as a convenient method to distinguish between β-1,3-glucan and long-chain β-1,6-branched β-1,3-glucan in various soluble and insoluble β-glucans.

In vitro infection models to study fungal-host interactions

Fungal infections (mycoses) affect over a billion people per year. Approximately, two million of these infections are life-threatening, especially for patients with a compromised immune system. Fungi of the genera Aspergillus, Candida, Histoplasma and Cryptococcus are opportunistic pathogens that contribute to a substantial number of mycoses. To optimize the diagnosis and treatment of mycoses, we need to understand the complex fungal–host interplay during pathogenesis, the fungal attributes causing virulence and how the host resists infection via immunological defenses. In vitro models can be used to mimic fungal infections of various tissues and organs and the corresponding immune responses at near-physiological conditions. Furthermore, models can include fungal interactions with the host–microbiota to mimic the in vivo situation on skin and mucosal surfaces. This article reviews currently used in vitro models of fungal infections ranging from cell monolayers to microfluidic 3D organ-on-chip (OOC) platforms. We also discuss how OOC models can expand the toolbox for investigating interactions of fungi and their human hosts in the future.

Involvement of amyloid proteins in the formation of biofilms in the pathogenic yeast Candida albicans

Candida species represent a major fungal threat for human health. Within the Candida genus, the yeast Candida albicans is the most frequently incriminated species during episodes of candidiasis or candidemia. Biofilm formation is used by C. albicans to produce a microbial community that is important in an infectious context. The cell wall, the most superficial cellular compartment, is of paramount importance regarding the establishment of biofilms. C. albicans cell wall contains proteins with amyloid properties that are necessary for biofilm formation due to their adhesion properties. This review focuses on these amyloid proteins during biofilm formation in the yeast C. albicans.

Interaction Between Dendritic Cells and Candida krusei β-Glucan Partially Depends on Dectin-1 and It Promotes High IL-10 Production by T Cells

Host-Candida interaction has been broadly studied during Candida albicans infection, with a progressive shift in focus toward non-albicans Candida species. C. krusei is an emerging multidrug resistant pathogen causing rising morbidity and mortality worldwide. Therefore, understanding the interplay between the host immune system and C. krusei is critically important. Candia cell wall β-glucans play significant roles in the induction of host protective immune responses. However, it remains unclear how C. krusei β-glucan impacts dendritic cell (DC) responses. In this study, we investigated DC maturation and function in response to β-glucans isolated from the cell walls of C. albicans, C. tropicalis, and C. krusei. These three distinct Candida β-glucans had differential effects on expression of the DC marker, CD11c, and on DC maturation. Furthermore, bone-marrow derived DCs (BMDCs) showed enhanced cytokine responses characterized by substantial interleukin (IL)-10 production following C. krusei β-glucan stimulation. BMDCs stimulated with C. krusei β-glucan augmented IL-10 production by T cells in tandem with increased IL-10 production by BMDCs. Inhibition of dectin-1 ligation demonstrated that the interactions between dectin-1 on DCs and cell wall β-glucans varied depending on the Candida species. The effects of C. krusei β-glucan were partially dependent on dectin-1, and this dependence, in part, led to distinct DC responses. Our study provides new insights into immune regulation by C. krusei cell wall components. These data may be of use in the development of new clinical approaches for treatment of patients with C. krusei infection.

Anti-inflammatory potential of myristic acid and palmitic acid synergism against systemic candidiasis in Danio rerio (Zebrafish)

Nosocomial Candida colonization causes Systemic candidiasis in human with invasive infections in immunocompromised patients. Of all Candida spp., C. albicans is dominant in morbidity of all systemic candidiasis but C. tropicalis is phenomenal in mortality, virulence aspects and resistance development against antifungal drugs. The present study investigated the synergistic anti-virulent activity of myristic acid (MA) and palmitic acid (PA) against insidious dimorphic Candida spp. (C. albicans and C. tropicalis). In vitro and qPCR results revealed the mechanisms of MA-PA combination effectively inhibiting various virulence aspects such as biofilm, hyphal formation, secreted aspartyl proteases, lipases, ergosterol biosynthesis and drug effluxes. Further, in Danio rerio (Zebrafish), the MA-PA treatment increased the survival of animals and also the treated groups showed decreased level of fungal burden compared to the infected controls, after 3^rd day of post infection. Histopathology of vital organs and SEM analysis of skin revealed a drastic recovery and reduced the inflammation of both Candida spp. infections in MA-PA treated animals. In addition, MA-PA treatment reduced the haemolysin and increased the susceptibility of Candida spp. in human blood model. Hence, this study suggested the therapeutic utilization of MA-PA as synergistic combination for their anti-inflammatory potency against systemic candidiasis and candidemia.

Virulence Factors in Candida species

Fungal diseases are severe and have very high morbidity as well as up to 60% mortality for patients diagnosed with invasive fungal infection. In this review, in vitro and in vivo studies provided us with the insight into the role of Candida virulence factors that mediate their success as pathogens, such as: membrane and cell wall (CW) barriers, dimorphism, biofilm formation, signal transduction pathway, proteins related to stress tolerance, hydrolytic enzymes (e.g. proteases, lipases, haemolysins), and toxin production. The review characterized the virulence of clinically important C. albicans, C. parapsilosis, C. tropicalis, C. glabrata and C. krusei. Due to the white-opaque transition in the mating-type locus MTL-homozygous cells, C. albicans demonstrates an advantage over other less related species of Candida as a human commensal and pathogen. It was reviewed that Candida ergosterol biosynthesis genes play a role in cellular stress and are essential for Candida pathogenesis both in invasive and superficial infections. Hydrolases associated with CW are involved in the host-pathogen interactions. Adhesins are crucial in colonization and biofilm formation, an important virulence factor for candidiasis. Calcineurin is involved in membrane and CW stress as well as virulence. The hyphae-specific toxin, named candidalysin, invades mucosal cells facilitating fungal invasion into deeper tissues. Expression of this protein promotes resistance to neutrophil killing in candidiasis. The virulence factors provide immunostimulatory factors, activating dendric cells and promoting T cell infiltration and activation. Targeting virulence factors, can reduce the risk of resistance development in Candida infections.

Comparative host transcriptome in response to pathogenic fungi identifies common and species-specific transcriptional antifungal host response pathways

Candidiasis, aspergillosis, and mucormycosis cause the majority of nosocomial fungal infections in immunocompromised patients. Using an unbiased transcriptional profiling in PBMCs exposed to the fungal species causing these infections, we found a core host response in healthy individuals that may govern effective fungal clearance: it consists of 156 transcripts, involving canonical and non-canonical immune pathways. Systematic investigation of key steps in antifungal host defense revealed fungal-specific signatures. As previously demonstrated, Candida albicans induced type I and Type II interferon-related pathways. In contrast, central pattern recognition receptor, reactive oxygen species production, and host glycolytic pathways were down-regulated in response to Rhizopus oryzae, which was associated with an ER-stress response. TLR5 was identified to be uniquely regulated by Aspergillus fumigatus and to control cytokine release in response to this fungus. In conclusion, our data reveals the transcriptional profiles induced by C. albicans, A. fumigatus, and R. oryzae, and describes both the common and specific antifungal host responses that could be exploited for novel therapeutic strategies.

Recognition of Candida albicans and Role of Innate Type 17 Immunity in Oral Candidiasis

Candida albicans is an opportunistic pathogenic fungus considered to be a common member of the human microflora. Similar to some other opportunistic microbes, C. albicans can invade and benefit from its host when the immune status of that host is weakened. Most often this happens to immunocompromised individuals, leading to the infection of oral and vaginal mucosae or the systemic spread of the pathogen throughout the entire body. Oropharyngeal candidiasis (OPC) occurs in up to 90 percent of patients with acquired immunodeficiency syndrome (AIDS), making it the most frequent opportunistic infection for this group. Upon first signs of fungal invasion, a range of host signaling activates in order to eliminate the threat. Epithelial and myeloid type cells detect C. albicans mainly through receptor tyrosine kinases and pattern-recognition receptors. This review provides an overview of downstream signaling resulting in an adequate immune response through the activation of various transcription factors. The study discusses recent advances in research of the interleukin-17 (IL-17) producing innate cells, including natural T helper 17 (nTh17) cells, γδ T cells, invariant natural killer T (iNKT) cells and type 3 innate lymphoid cells (ILC3) that are involved in response to oral C. albicans infections.

Clinical Candida albicans Vaginal Isolates and a Laboratory Strain Show Divergent Behaviors during Macrophage Interactions

Vulvovaginal candidiasis is one of the most common fungal infections in humans with Candida albicans as the major causative agent. This study is the first to compare clinical vaginal isolates of defined patient groups in their interaction with macrophages, highlighting the vastly different outcomes in comparison to a laboratory strain using commonly applied virulence-determining assays.

Typically, established lab strains are widely used to study host-pathogen interactions. However, to better reflect the infection process, the experimental use of clinical isolates has come more into focus. Here, we analyzed the interaction of multiple vaginal isolates of the opportunistic fungal pathogen Candida albicans, the most common cause of vulvovaginal candidiasis in women, with key players of the host immune system: macrophages. We tested several strains isolated from asymptomatic or symptomatic women with acute and recurrent infections. While all clinical strains showed a response similar to the commonly used lab strain SC5314 in various in vitro assays, they displayed remarkable differences during interaction with macrophages. This coincided with significantly reduced β-glucan exposure on the cell surface, which appeared to be a shared property among the tested vaginal strains for yeast extract/peptone/dextrose-grown cells, which is partly lost when the isolates faced vaginal niche-like nutrient conditions. However, macrophage damage, survival of phagocytosis, and filamentation capacities were highly strain-specific. These results highlight the high heterogeneity of C. albicans strains in host-pathogen interactions, which have to be taken into account to bridge the gap between laboratory-gained data and disease-related outcomes in an actual patient.

IMPORTANCE Vulvovaginal candidiasis is one of the most common fungal infections in humans with Candida albicans as the major causative agent. This study is the first to compare clinical vaginal isolates of defined patient groups in their interaction with macrophages, highlighting the vastly different outcomes in comparison to a laboratory strain using commonly applied virulence-determining assays.

Differences in fungal immune recognition by monocytes and macrophages: N-mannan can be a shield or activator of immune recognition

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Cytokine response to N-mannan mutants was dependent on the immune cell type used.

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N-mannan mutants stimulated less cytokines from monocytes but more from macrophages.

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N-mannan can therefore act as both an immune agonist or an immune shield.

We designed experiments to assess whether fungal cell wall mannans function as an immune shield or an immune agonist. Fungal cell wall β-(1,3)-glucan normally plays a major and dominant role in immune activation. The outer mannan layer has been variously described as an immune shield, because it has the potential to mask the underlying β-(1,3)-glucan, or an immune activator, as it also has the potential to engage with a wide range of mannose detecting PRRs. To resolve this conundrum we examined species-specific differences in host immune recognition in the och1Δ N-mannosylation-deficient mutant background in four species of yeast-like fungi. Irrespective of the fungal species, the cytokine response (TNFα and IL-6) induced by the och1Δ mutants in human monocytes was reduced compared to that of the wild type. In contrast, TNFα production induced by och1Δ was increased, relative to wild type, due to increased β-glucan exposure, when mouse or human macrophages were used. These observations suggest that N-mannan is not a major PAMP for macrophages and that in these cells mannan does shield the fungus from recognition of the inner cell wall β-glucan. However, N-mannan is a significant inducer of cytokine for monocytes. Therefore the metaphor of the fungal “mannan shield” can only be applied to some, but not all, myeloid cells used in immune profiling experiments of fungal species.