A drug-sensitive genetic network masks fungi from the immune system

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.

The atypical antipsychotic aripiprazole alters the outcome of disseminated Candida albicans infections

Invasive fungal infections impose an enormous clinical, social, and economic burden on humankind. One of the most common species responsible for invasive fungal infections is Candida albicans. More than 30% of patients with disseminated candidiasis fail therapy with existing antifungal drugs, including the widely used azole class. We previously identified a collection of 13 medications that antagonize the activity of the azoles on C. albicans. Although gain-of-function mutations responsible for antifungal resistance are often associated with reduced fitness and virulence, it is currently unknown how exposure to azole antagonistic drugs impacts C. albicans physiology, fitness, or virulence. In this study, we examined how exposure to seven azole antagonists affects C. albicans phenotype and capacity to cause disease. Most of the azole antagonists appear to have little impact on fungal growth, morphology, stress tolerance, or gene transcription. However, aripiprazole had a modest impact on C. albicans hyphal growth and increased cell wall chitin content. It also aggravated the disseminated C. albicans infections in mice. This effect was abrogated in immunosuppressed mice, indicating that it is at least in part dependent upon host immune responses. Collectively, these data provide proof of principle that unanticipated drug-fungus interactions have the potential to influence the incidence and outcomes of invasive fungal disease.

A correlative study of the genomic underpinning of virulence traits and drug tolerance of Candida auris

Candida auris is an opportunistic fungal pathogen with high mortality rates which presents a clear threat to public health. The risk of C. auris infection is high because it can colonize the body, resist antifungal treatment, and evade the immune system. The genetic mechanisms for these traits are not well known. Identifying them could lead to new targets for new treatments. To this end, we present an analysis of the genetics and gene expression patterns of C. auris carbon metabolism, drug resistance, and macrophage interaction. We chose to study two C. auris isolates simultaneously, one drug sensitive (B11220 from Clade II) and one drug resistant (B11221 from Clade III). Comparing the genomes, we confirm the previously reported finding that B11220 was missing a 12.8 kb region on chromosome VI. This region contains a gene cluster encoding proteins related to alternative sugar utilization. We show that B11221, which has the gene cluster, readily assimilates and utilizes D-galactose and L-rhamnose as compared to B11220, which harbors the deletion. B11221 exhibits increased adherence and drug resistance compared to B11220 when grown in these sugars. Transcriptomic analysis of both isolates grown on glucose or galactose showed that the gene cluster was upregulated when grown on D-galactose. These findings reinforce growing evidence of a link between metabolism and drug tolerance. B11221 resists phagocytosis by macrophages and exhibits decreased β-1,3-glucan exposure, a key determinant that allows Candida to evade the host immune system, as compared to B11220. In a transcriptomic analysis of both isolates co-cultured with macrophages, we find upregulation of genes associated with transport and transcription factors in B11221. Our studies show a positive correlation between membrane composition and immune evasion, alternate sugar utilization, and drug tolerance in C. auris.

Impact of secreted glucanases upon the cell surface and fitness of Candida albicans during colonisation and infection

Host recognition of the pathogen-associated molecular pattern (PAMP), β-1,3-glucan, plays a major role in antifungal immunity. β-1,3-glucan is an essential component of the inner cell wall of the opportunistic pathogen Candida albicans. Most β-1,3-glucan is shielded by the outer cell wall layer of mannan fibrils, but some can become exposed at the cell surface. In response to host signals such as lactate, C. albicans shaves the exposed β-1,3-glucan from its cell surface, thereby reducing the ability of innate immune cells to recognise and kill the fungus. We have used sets of barcoded xog1 and eng1 mutants to compare the impacts of the secreted β-glucanases Xog1 and Eng1 upon C. albicans in vitro and in vivo. Flow cytometry of Fc-dectin-1-stained strains revealed that Eng1 plays the greater role in lactate-induced β-1,3-glucan masking. Transmission electron microscopy and stress assays showed that neither Eng1 nor Xog1 are essential for cell wall maintenance, but the inactivation of either enzyme compromised fungal adhesion to gut and vaginal epithelial cells. Competitive barcode sequencing suggested that neither Eng1 nor Xog1 strongly influence C. albicans fitness during systemic infection or vaginal colonisation in mice. However, the deletion of XOG1 enhanced C. albicans fitness during gut colonisation. We conclude that both Eng1 and Xog1 exert subtle effects on the C. albicans cell surface that influence fungal adhesion to host cells and that affect fungal colonisation in certain host niches.

A CO2 sensing module modulates β-1,3-glucan exposure in Candida albicans

Microbial species capable of co-existing with healthy individuals, such as the commensal fungus Candida albicans, exploit multifarious strategies to evade our immune defenses. These strategies include the masking of immunoinflammatory pathogen-associated molecular patterns (PAMPs) at their cell surface. We reported previously that C. albicans actively reduces the exposure of the proinflammatory PAMP, β-1,3-glucan, at its cell surface in response to host-related signals such as lactate and hypoxia. Here, we show that clinical isolates of C. albicans display phenotypic variability with respect to their lactate- and hypoxia-induced β-1,3-glucan masking. We have exploited this variability to identify responsive and non-responsive clinical isolates. We then performed RNA sequencing on these isolates to reveal genes whose expression patterns suggested potential association with lactate- or hypoxia-induced β-1,3-glucan masking. The deletion of two such genes attenuated masking: PHO84 and NCE103. We examined NCE103-related signaling further because NCE103 has been shown previously to encode carbonic anhydrase, which promotes adenylyl cyclase-protein kinase A (PKA) signaling at low CO2 levels. We show that while CO2 does not trigger β-1,3-glucan masking in C. albicans, the Sch9-Rca1-Nce103 signaling module strongly influences β-1,3-glucan exposure in response to hypoxia and lactate. In addition to identifying a new regulatory module that controls PAMP exposure in C. albicans, our data imply that this module is important for PKA signaling in response to environmental inputs other than CO2.IMPORTANCEOur innate immune defenses have evolved to protect us against microbial infection in part via receptor-mediated detection of "pathogen-associated molecular patterns" (PAMPs) expressed by invading microbes, which then triggers their immune clearance. Despite this surveillance, many microbial species are able to colonize healthy, immune-competent individuals, without causing infection. To do so, these microbes must evade immunity. The commensal fungus Candida albicans exploits a variety of strategies to evade immunity, one of which involves reducing the exposure of a proinflammatory PAMP (β-1,3-glucan) at its cell surface. Most of the β-1,3-glucan is located in the inner layer of the C. albicans cell wall, hidden by an outer layer of mannan fibrils. Nevertheless, some β-1,3-glucan can become exposed at the fungal cell surface. However, in response to certain specific host signals, such as lactate or hypoxia, C. albicans activates an anticipatory protective response that decreases β-1,3-glucan exposure, thereby reducing the susceptibility of the fungus to impending innate immune attack. Here, we exploited the natural phenotypic variability of C. albicans clinical isolates to identify strains that do not display the response to β-1,3-glucan masking signals observed for the reference isolate, SC5314. Then, using genome-wide transcriptional profiling, we compared these non-responsive isolates with responsive controls to identify genes potentially involved in β-1,3-glucan masking. Mutational analysis of these genes revealed that a sensing module that was previously associated with CO2 sensing also modulates β-1,3-glucan exposure in response to hypoxia and lactate in this major fungal pathogen of humans.

The atypical antipsychotic aripiprazole alters the outcome of disseminated Candida albicans infections

Invasive fungal infections (IFIs) impose an enormous clinical, social, and economic burden on humankind. For many IFIs, ≥ 30% of patients fail therapy with existing antifungal drugs, including the widely used azole class. We previously identified a collection of 13 approved medications that antagonize azole activity. While gain-of-function mutants resulting in antifungal resistance are often associated with reduced fitness and virulence, it is currently unknown how exposure to azole antagonistic drugs impact C. albicans physiology, fitness, or virulence. In this study, we examined how exposure to azole antagonists affected C. albicans phenotype and capacity to cause disease. We discovered that most of the azole antagonists had little impact on fungal growth, morphology, stress tolerance, or gene transcription. However, aripiprazole had a modest impact on C. albicans hyphal growth and increased cell wall chitin content. It also worsened the outcome of disseminated infections in mice at human equivalent concentrations. This effect was abrogated in immunosuppressed mice, indicating an additional impact of aripiprazole on host immunity. Collectively, these data provide proof-of-principle that unanticipated drug-fungus interactions have the potential to influence the incidence and outcomes of invasive fungal disease.

Impact of micafungin on Candida auris β-glucan masking and neutrophil interactions

Invasive fungal pathogen Candida auris has become a public health threat causing outbreaks of high mortality infections. Drug resistance often limits treatment options. For Candida albicans, subinhibitory concentrations of echinocandins unmask immunostimulatory β-glucan, augmenting immunity. Here we analyze the impact of echinocandin treatment of C. auris on β-glucan exposure and human neutrophil interactions. We show subinhibitory concentrations lead to minimal glucan unmasking and only subtle influences on neutrophil functions for the isolates belonging to circulating clades. The data suggest that echinocandin treatment will not largely alter phagocytic responses. Glucan masking pathways appear to differ between C. auris and C. albicans.

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.

Dectin-1 multimerization and signaling depends on fungal β-glucan structure and exposure

Dectin-1A is a C-type lectin innate immunoreceptor that recognizes β-(1,3;1,6)-glucan, a structural component of Candida species cell walls. β-Glucans can adopt solution structures ranging from random coil to insoluble fiber due to tertiary (helical) and quaternary structure. Fungal β-glucans of medium and high molecular weight are highly structured, but low molecular weight glucan is much less structured. Despite similar affinity for Dectin-1, the ability of glucans to induce Dectin-1A-mediated signaling correlates with degree of structure. Glucan denaturation experiments showed that glucan structure determines agonistic potential, but not receptor binding affinity. We explored the impact of glucan structure on molecular aggregation of Dectin-1A. Stimulation with glucan signaling decreased Dectin-1A diffusion coefficient. Fluorescence measurements provided direct evidence of ligation-induced Dectin-1A aggregation, which positively correlated with increasing glucan structure content. In contrast, Dectin-1A is predominantly in a low aggregation state in resting cells. Molecular aggregates formed during interaction with highly structured, agonistic glucans did not exceed relatively small (<15 nm) clusters of a few engaged receptors. Finally, we observed increased molecular aggregation of Dectin-1A at fungal particle contact sites in a manner that positively correlated with the degree of exposed glucan on the particle surface. These results indicate that Dectin-1A senses the solution conformation of β-glucans through their varying ability to drive receptor dimer/oligomer formation and activation of membrane proximal signaling events.

Caspofungin-induced β(1,3)-glucan exposure in Candida albicans is driven by increased chitin levels

To successfully induce disease, Candida albicans must effectively evade the host immune system. One mechanism used by C. albicans to achieve this is to mask immunogenic β(1,3)-glucan epitopes within its cell wall under an outer layer of mannosylated glycoproteins. Consequently, induction of β(1,3)-glucan exposure (unmasking) via genetic or chemical manipulation increases fungal recognition by host immune cells in vitro and attenuates disease during systemic infection in mice. Treatment with the echinocandin caspofungin is one of the most potent drivers of β(1,3)-glucan exposure. Several reports using murine infection models suggest a role for the immune system, and specifically host β(1,3)-glucan receptors, in mediating the efficacy of echinocandin treatment in vivo. However, the mechanism by which caspofungin-induced unmasking occurs is not well understood. In this report, we show that foci of unmasking co-localize with areas of increased chitin within the yeast cell wall in response to caspofungin, and that inhibition of chitin synthesis via nikkomycin Z attenuates caspofungin-induced β(1,3)-glucan exposure. Furthermore, we find that both the calcineurin and Mkc1 mitogen-activated protein kinase pathways work synergistically to regulate β(1,3)-glucan exposure and chitin synthesis in response to drug treatment. When either of these pathways are interrupted, it results in a bimodal population of cells containing either high or low chitin content. Importantly, increased unmasking correlates with increased chitin content within these cells. Microscopy further indicates that caspofungin-induced unmasking correlates with actively growing cells. Collectively, our work presents a model in which chitin synthesis induces unmasking within the cell wall in response to caspofungin in growing cells. IMPORTANCE Systemic candidiasis has reported mortality rates ranging from 20% to 40%. The echinocandins, including caspofungin, are first-line antifungals used to treat systemic candidiasis. However, studies in mice have shown that echinocandin efficacy relies on both its cidal impacts on Candida albicans, as well as a functional immune system to successfully clear invading fungi. In addition to direct C. albicans killing, caspofungin increases exposure (unmasking) of immunogenic β(1,3)-glucan moieties. To evade immune detection, β(1,3)-glucan is normally masked within the C. albicans cell wall. Consequently, unmasked β(1,3)-glucan renders these cells more visible to the host immune system and attenuates disease progression. Therefore, discovery of how caspofungin-induced unmasking occurs is needed to elucidate how the drug facilitates host immune system-mediated clearance in vivo. We report a strong and consistent correlation between chitin deposition and unmasking in response to caspofungin and propose a model in which altered chitin synthesis drives increased unmasking during drug exposure.

Identification of Botanicals that Unmask β-Glucan from the Cell Surface of an Opportunistic Fungal Pathogen

Dectin-1 expressed on host immune cells recognizes β-glucans within the cell walls of fungal pathogens and plays an important role in the clearance of fungal infections. However, because β-glucan is masked by an outer layer of mannoproteins, fungal pathogens can evade detection by host immune cells. In this study, a microplate-based screen was developed to identify β-glucan unmasking activity exhibited by botanicals. This screen measures the activity of a reporter gene in response to the transcriptional activation of NF-κB due to the interaction between β-glucan on the fungal cell surface and Dectin-1 present on host immune cells. In this proof-of-concept study, we screened a collection of botanicals (10 plants and some of their reported pure compound actives) used in traditional medicine for their antifungal properties. Several hits were identified in samples that unmasked β-glucan at sub-inhibitory concentrations. The hit samples were confirmed by fluorescent staining with a β-glucan antibody, verifying that the samples identified in the screen did indeed unmask β-glucan. These results indicate that the purported antifungal activities attributed to some botanicals may be due, at least in part, to the presence of compounds that exhibit β-glucan unmasking activity. Enhanced exposure of cell wall β-glucans would allow the host to build resilience against fungal infections by helping the immune system to detect the pathogen and mount a more effective clearance mechanism. This screen, together with direct killing/growth inhibition assays, may therefore serve as a valuable tool for substantiating the use of botanicals in preventing and/or treating fungal infections.

Architecture of the dynamic fungal cell wall

The fungal cell wall is essential for growth and survival, and is a key target for antifungal drugs and the immune system. The cell wall must be robust but flexible, protective and shielding yet porous to nutrients and membrane vesicles and receptive to exogenous signals. Most fungi have a common inner wall skeleton of chitin and β-glucans that functions as a flexible viscoelastic frame to which a more diverse set of outer cell wall polymers and glycosylated proteins are attached. Whereas the inner wall largely determines shape and strength, the outer wall confers properties of hydrophobicity, adhesiveness, and chemical and immunological heterogeneity. The spatial organization and dynamic regulation of the wall in response to prevailing growth conditions enable fungi to thrive within changing, diverse and often hostile environments. Understanding this architecture provides opportunities to develop diagnostics and drugs to combat life-threatening fungal infections.

Targeting β-glucans, vital components of the Pneumocystis cell wall

β-glucan is the most abundant polysaccharide in the cell wall of Pneumocystis jirovecii, which has attracted extensive attention because of its unique immunobiological characteristics. β-glucan binds to various cell surface receptors, which produces an inflammatory response and accounts for its immune effects. A deeper comprehension of the processes by Pneumocystis β-glucan recognizes its receptors, activates related signaling pathways, and regulates immunity as required. Such understanding will provide a basis for developing new therapies against Pneumocystis. Herein, we briefly review the structural composition of β-glucans as a vital component of the Pneumocystis cell wall, the host immunity mediated by β-glucans after their recognition, and discuss opportunities for the development of new strategies to combat Pneumocystis.

Human intestinal epithelial cells can internalize luminal fungi via LC3-associated phagocytosis

Background

Intestinal epithelial cells (IECs) are the first to encounter luminal microorganisms and actively participate in intestinal immunity. We reported that IECs express the β-glucan receptor Dectin-1, and respond to commensal fungi and β-glucans. In phagocytes, Dectin-1 mediates LC3-associated phagocytosis (LAP) utilizing autophagy components to process extracellular cargo. Dectin-1 can mediate phagocytosis of β-glucan-containing particles by non-phagocytic cells. We aimed to determine whether human IECs phagocytose β-glucan-containing fungal particles via LAP.

Methods

Colonic (n=18) and ileal (n=4) organoids from individuals undergoing bowel resection were grown as monolayers. Fluorescent-dye conjugated zymosan (β-glucan particle), heat-killed- and UV inactivated C. albicans were applied to differentiated organoids and to human IEC lines. Confocal microscopy was used for live imaging and immuno-fluorescence. Quantification of phagocytosis was carried out with a fluorescence plate-reader.

Results

zymosan and C. albicans particles were phagocytosed by monolayers of human colonic and ileal organoids and IEC lines. LAP was identified by LC3 and Rubicon recruitment to phagosomes and lysosomal processing of internalized particles was demonstrated by co-localization with lysosomal dyes and LAMP2. Phagocytosis was significantly diminished by blockade of Dectin-1, actin polymerization and NAPDH oxidases.

Conclusions

Our results show that human IECs sense luminal fungal particles and internalize them via LAP. This novel mechanism of luminal sampling suggests that IECs may contribute to the maintenance of mucosal tolerance towards commensal fungi.

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.

Multiple Genes of Candida albicans Influencing Echinocandin Susceptibility in Caspofungin-Adapted Mutants

Candida albicans is an opportunistic human fungal pathogen that causes invasive infections in immunocompromised individuals. Despite the high anticandidal activity among the echinocandins (ECNs), a first-line therapy, resistance remains an issue. Furthermore, many clinical isolates display decreased ECN susceptibility, a physiological state which is thought to lead to resistance. Determining the factors that can decrease susceptibility is of high importance. We searched for such factors genome-wide by comparing the transcriptional profiles of five mutants that acquired decreased caspofungin susceptibility in vitro in the absence of canonical FKS1 resistance mutations. The mutants were derived from two genetic backgrounds and arose due to independent mutational events, some with monosomic chromosome 5 (Ch5). We found that the mutants exhibit common transcriptional changes. In particular, all mutants upregulate five genes from Ch2 in concert. Knockout experiments show that all five genes positively influence caspofungin and anidulafungin susceptibility and play a role in regulating the cell wall mannan and glucan contents. The functions of three of these genes, orf19.1766, orf19.6867, and orf19.5833, were previously unknown, and our work expands the known functions of LEU42 and PR26. Importantly, orf19.1766 and LEU42 have no human orthologues. Our results provide important clues as to basic mechanisms of survival in the presence of ECNs while identifying new genes controlling ECN susceptibility and revealing new targets for the development of novel antifungal drugs.

Mucosal Infection with Unmasked Candida albicans Cells Impacts Disease Progression in a Host Niche-Specific Manner

Shielding the immunogenic cell wall epitope β(1, 3)-glucan under an outer layer of mannosylated glycoproteins is an essential virulence factor deployed by Candida albicans during systemic infection. Accordingly, mutants with increased β(1, 3)-glucan exposure (unmasking) display increased immunostimulatory capabilities in vitro and attenuated virulence during systemic infection in mice. However, little work has been done to assess the impact of increased unmasking during the two most common manifestations of candidiasis, namely, oropharyngeal candidiasis (OPC) and vulvovaginal candidiasis (VVC). We have shown previously that the expression of a single hyperactive allele of the MAP3K STE11^ΔN467 induces unmasking via the Cek1 MAPK pathway, attenuates fungal burden, and prolongs survival during systemic infection in mice. Here, we expand on these findings and show that infection with an unmasked STE11^ΔN467 mutant also impacts disease progression during OPC and VVC murine infection models. Male mice sublingually infected with the STE11^ΔN467 mutant showed a significant reduction in tongue fungal burden at 2 days postinfection and a modest reduction at 5 days postinfection. However, we find that selection for STE11^ΔN467 suppressor mutants that no longer display increased unmasking occurs within the oral cavity and is likely responsible for the restoration of fungal burden trends to wild-type levels later in the infection. In the VVC infection model, no attenuation in fungal burden was observed. However, polymorphonuclear cell recruitment and interleukin-1β (IL-1β) levels within the vaginal lumen, markers of immunopathogenesis, were increased in mice infected with unmasked STE11^ΔN467 cells. Thus, our data suggest a niche-specific impact for unmasking on disease progression.

Calcineurin Inhibitors Synergize with Manogepix to Kill Diverse Human Fungal Pathogens

Invasive fungal infections have mortality rates of 30–90%, depending on patient co-morbidities and the causative pathogen. The frequent emergence of drug resistance reduces the efficacy of currently approved treatment options, highlighting an urgent need for antifungals with new modes of action. Addressing this need, fosmanogepix (N-phosphonooxymethylene prodrug of manogepix; MGX) is the first in a new class of gepix drugs, and acts as a broad-spectrum, orally bioavailable inhibitor of the essential fungal glycosylphosphatidylinositol (GPI) acyltransferase Gwt1. MGX inhibits the growth of diverse fungal pathogens and causes accumulation of immature GPI-anchored proteins in the fungal endoplasmic reticulum. Relevant to the ongoing clinical development of fosmanogepix, we report a synergistic, fungicidal interaction between MGX and inhibitors of the protein phosphatase calcineurin against important human fungal pathogens. To investigate this synergy further, we evaluated a library of 124 conditional expression mutants covering 95% of the genes encoding proteins involved in GPI-anchor biosynthesis or proteins predicted to be GPI-anchored. Strong negative chemical-genetic interactions between the calcineurin inhibitor FK506 and eleven GPI-anchor biosynthesis genes were identified, indicating that calcineurin signalling is required for fungal tolerance to not only MGX, but to inhibition of the GPI-anchor biosynthesis pathway more broadly. Depletion of these GPI-anchor biosynthesis genes, like MGX treatment, also exposed fungal cell wall (1→3)-β-D-glucans. Taken together, these findings suggest the increased risk of invasive fungal infections associated with use of calcineurin inhibitors as immunosuppressants may be mitigated by their synergistic fungicidal interaction with (fos)manogepix and its ability to enhance exposure of immunostimulatory glucans.

Impact of changes at the Candida albicans cell surface upon immunogenicity and colonisation in the gastrointestinal tract

The immunogenicity of Candida albicans cells is influenced by changes in the exposure of microbe-associated molecular patterns (MAMPs) on the fungal cell surface. Previously, the degree of exposure on the C. albicans cell surface of the immunoinflammatory MAMP β-(1,3)-glucan was shown to correlate inversely with colonisation levels in the gastrointestinal (GI) tract. This is important because life-threatening systemic candidiasis in critically ill patients often arises from translocation of C. albicans strains present in the patient's GI tract. Therefore, using a murine model, we have examined the impact of gut-related factors upon β-glucan exposure and colonisation levels in the GI tract. The degree of β-glucan exposure was examined by imaging flow cytometry of C. albicans cells taken directly from GI compartments, and compared with colonisation levels. Fungal β-glucan exposure was lower in the cecum than the small intestine, and fungal burdens were correspondingly higher in the cecum. This inverse correlation did not hold for the large intestine. The gut fermentation acid, lactate, triggers β-glucan masking in vitro, leading to attenuated anti-Candida immune responses. Additional fermentation acids are present in the GI tract, including acetate, propionate, and butyrate. We show that these acids also influence β-glucan exposure on C. albicans cells in vitro and, like lactate, they influence β-glucan exposure via Gpr1/Gpa2-mediated signalling. Significantly, C. albicans gpr1Δ gpa2Δ cells displayed elevated β-glucan exposure in the large intestine and a corresponding decrease in fungal burden, consistent with the idea that Gpr1/Gpa2-mediated β-glucan masking influences colonisation of this GI compartment. Finally, extracts from the murine gut and culture supernatants from the mannan grazing gut anaerobe Bacteroides thetaiotaomicron promote β-glucan exposure at the C. albicans cell surface. Therefore, the local microbiota influences β-glucan exposure levels directly (via mannan grazing) and indirectly (via fermentation acids), whilst β-glucan masking appears to promote C. albicans colonisation of the murine large intestine.

The IL-20RB receptor and the IL-20 signaling pathway in regulating host defense in oral mucosal candidiasis

Pseudomembranous candidiasis (thrush), erythematous candidiasis, and fungal esophagitis are infections of the barrier mucosa of the upper gastrointestinal tract. The majority of these infections are caused by Candida albicans, an opportunistic fungal pathogen that frequently exists as a harmless commensal on mucosal surfaces lining the gastrointestinal tract. Oral infections are initiated in the superficial stratified squamous epithelium, in which keratinocytes are the most abundant host cells and are the initial points of contact with C. albicans present in saliva. Intrinsic features of oral keratinocytes are likely to play important roles in host defense and tissue homeostasis in oral candidiasis. One understudied pathway that may be important for modulating oral candidiasis is the IL-20 cytokine signaling pathway that employs keratinocyte IL-20RB receptors as ligands for IL-19, IL-20, and IL-24. We report that production of human oral keratinocyte il24 mRNA and protein are stimulated during co-culture with C. albicans. To test the role of the IL-20 family signaling pathway in oral candidiasis, Il20rb^-/- mice (lacking the IL-20RB receptor) were compared to wild-type mice in a murine model of oropharyngeal candidiasis. Fungal burdens and percent loss in body weight were determined. Despite comparable fungal burdens, the Il20rb^-/- mice exhibited less weight loss over the course of their infection compared to the B6 mice, suggestive of reduced overall disease consequences in the mutant mice. Interference with IL-20 family cytokine signaling may be useful for augmenting the ability of the host to defend itself against pathogens.

Cek1 regulates ß(1,3)-glucan exposure through calcineurin effectors in Candida albicans

In order to successfully induce disease, the fungal pathogen Candida albicans regulates exposure of antigens like the cell wall polysaccharide ß(1,3)-glucan to the host immune system. C. albicans covers (masks) ß(1,3)-glucan with a layer of mannosylated glycoproteins, which aids in immune system evasion by acting as a barrier to recognition by host pattern recognition receptors. Consequently, enhanced ß(1,3)-glucan exposure (unmasking) makes fungal cells more visible to host immune cells and facilitates more robust fungal clearance. However, an understanding of how C. albicans regulates its exposure levels of ß(1,3)-glucan is needed to leverage this phenotype. Signal transduction pathways and their corresponding effector genes mediating these changes are only beginning to be defined. Here, we report that the phosphatase calcineurin mediates unmasking of ß(1,3)-glucan in response to inputs from the Cek1 MAPK pathway and in response to caspofungin exposure. In contrast, calcineurin reduces ß-glucan exposure in response to high levels of extracellular calcium. Thus, depending on the input, calcineurin acts as a switchboard to regulate ß(1,3)-glucan exposure levels. By leveraging these differential ß(1,3)-glucan exposure phenotypes, we identified two novel effector genes in the calcineurin regulon, FGR41 and C1_11990W_A, that encode putative cell wall proteins and mediate masking/unmasking. Loss of either effector caused unmasking and attenuated virulence during systemic infection in mice. Furthermore, immunosuppression restored the colonization decrease seen in mice infected with the fgr41Δ/Δ mutant to wild-type levels, demonstrating a reliance on the host immune system for virulence attenuation. Thus, calcineurin and its downstream regulon are general regulators of unmasking.

Monoclonal Antibodies Targeting Surface-Exposed Epitopes of Candida albicans Cell Wall Proteins Confer In Vivo Protection in an Infection Model

Monoclonal antibody (mAb)-based immunotherapies targeting systemic and deep-seated fungal infections are still in their early stages of development, with no licensed antifungal mAbs currently being available for patients at risk. The cell wall glycoproteins of Candida albicans are of particular interest as potential targets for therapeutic antibody generation due to their extracellular location and key involvement in fungal pathogenesis. Here, we describe the generation of recombinant human antibodies specifically targeting two key cell wall proteins (CWPs) in C. albicans: Utr2 and Pga31. These antibodies were isolated from a phage display antibody library using peptide antigens representing the surface-exposed regions of CWPs expressed at elevated levels during in vivo infection. Reformatted human-mouse chimeric mAbs preferentially recognized C. albicans hyphal forms compared to yeast cells, and increased binding was observed when the cells were grown in the presence of the antifungal agent caspofungin. In J774.1 macrophage interaction assays, mAb pretreatment resulted in the faster engulfment of C. albicans cells, suggesting a role of the CWP antibodies as opsonizing agents during phagocyte recruitment. Finally, in a series of clinically predictive mouse models of systemic candidiasis, our lead mAb achieved improved survival (83%) and a several-log reduction of the fungal burden in the kidneys, similar to the levels achieved for the fungicidal drug caspofungin and superior to the therapeutic efficacy of any anti-Candida mAb reported to date.

Caspofungin resistance in Candida albicans: genetic factors and synergistic compounds for combination therapies

Caspofungin and other echinocandins have been used for the treatment of human infections by the opportunistic yeast pathogen, Candida albicans. There has been an increase in infections by non-albicans Candida species such as Candida glabrata, Candida parapsilosis, Candida tropicalis, Candida krusei, and Candida auris in clinical or hospital settings. This is problematic to public health due to the increasing prevalence of echinocandin resistant species/strains. This review will present a summary on various studies that investigated the inhibitory action of caspofungin on 1,3-β-D-glucan synthesis, on cell wall structure, and biofilm formation of C. albicans. It will highlight some of the issues linked to caspofungin resistance or reduced caspofungin sensitivity in various Candida species and the potential benefits of antimicrobial peptides and other compounds in synergy with caspofungin.

When Is It Appropriate to Take Off the Mask? Signaling Pathways That Regulate ß(1,3)-Glucan Exposure in Candida albicans

Candida spp. are an important source of systemic and mucosal infections in immune compromised populations. However, drug resistance or toxicity has put limits on the efficacy of current antifungals. The C. albicans cell wall is considered a good therapeutic target due to its roles in viability and fungal pathogenicity. One potential method for improving antifungal strategies could be to enhance the detection of fungal cell wall antigens by host immune cells. ß(1,3)-glucan, which is an important component of fungal cell walls, is a highly immunogenic epitope. Consequently, multiple host pattern recognition receptors, such as dectin-1, complement receptor 3 (CR3), and the ephrin type A receptor A (EphA2) are capable of recognizing exposed (unmasked) ß(1,3)-glucan moieties on the cell surface to initiate an anti-fungal immune response. However, ß(1,3)-glucan is normally covered (masked) by a layer of glycosylated proteins on the outer surface of the cell wall, hiding it from immune detection. In order to better understand possible mechanisms of unmasking ß(1,3)-glucan, we must develop a deeper comprehension of the pathways driving this phenotype. In this review, we describe the medical importance of ß(1,3)-glucan exposure in anti-fungal immunity, and highlight environmental stimuli and stressors encountered within the host that are capable of inducing changes in the levels of surface exposed ß(1,3)-glucan. Furthermore, particular focus is placed on how signal transduction cascades regulate changes in ß(1,3)-glucan exposure, as understanding the role that these pathways have in mediating this phenotype will be critical for future therapeutic development.

Sodium houttuyfonate enhances the mono-therapy of fluconazole on oropharyngeal candidiasis (OPC) through HIF-1α/IL-17 axis by inhibiting cAMP mediated filamentation in Candida albicans-Candida glabrata dual biofilms

Candida albicans and Candida glabrata are two common opportunistic fungi that can be co-isolated in oropharyngeal candidiasis (OPC). Hypha is a hallmark of the biofilm formation of C. albicans, indispensable for the attachment of C. glabrata, which is seldom in mycelial morphology. Increasing evidence reveals a hypoxic microenvironment in interior fungal biofilms, reminding of a fact that inflammation is usually accompanied by oxygen deprivation. As a result, it is assumed that the disaggregation of hypha-mediated hypoxia of biofilms might be a solution to alleviate OPC. Based on this hypothesis, sodium houttuyfonate (SH), a well-identified traditional herbal compound with antifungal activity, is used in combination with fluconazole (FLU), a well-informed synthesized antimycotics, to investigate their impact on filamentation in C. albicans and C. glabrata dual biofilms and the underlying mechanism of their combined treatment on OPC. The results show that compared with the single therapy, SH plus FLU can inhibit the hyphal growth in the mixed biofilms in vitro, decrease the fungal burden of oral tissues and internal organs, restore mucosal epithelial integrity and function, and reduce hypoxic microenvironment and inflammation in a mice OPC model. The possible mechanism of the combined therapy of SH plus FLU can be attributed to the regulation of HIF-1α/IL-17A axis through direct abrogation of the dual Candida biofilm formation. This study highlights the role of HIF-1α/IL-17A axis and the promising application of SH as a sensitizer of conventional antifungals in the treatment of OPC.

Estrogen promotes innate immune evasion of Candida albicans through inactivation of the alternative complement system

Candida albicans is a commensal of the urogenital tract and the predominant cause of vulvovaginal candidiasis (VVC). Factors that increase circulatory estrogen levels such as pregnancy, the use of oral contraceptives, and hormone replacement therapy predispose women to VVC, but the reasons for this are largely unknown. Here, we investigate how adaptation of C. albicans to estrogen impacts the fungal host-pathogen interaction. Estrogen promotes fungal virulence by enabling C. albicans to avoid the actions of the innate immune system. Estrogen-induced innate immune evasion is mediated via inhibition of opsonophagocytosis through enhanced acquisition of the human complement regulatory protein, Factor H, on the fungal cell surface. Estrogen-induced accumulation of Factor H is dependent on the fungal cell surface protein Gpd2. The discovery of this hormone-sensing pathway might pave the way in explaining gender biases associated with fungal infections and may provide an alternative approach to improving women's health.

Pathogenesis and virulence of Candida albicans

Candida albicans is a commensal yeast fungus of the human oral, gastrointestinal, and genital mucosal surfaces, and skin. Antibiotic-induced dysbiosis, iatrogenic immunosuppression, and/or medical interventions that impair the integrity of the mucocutaneous barrier and/or perturb protective host defense mechanisms enable C. albicans to become an opportunistic pathogen and cause debilitating mucocutaneous disease and/or life-threatening systemic infections. In this review, we synthesize our current knowledge of the tissue-specific determinants of C. albicans pathogenicity and host immune defense mechanisms.

CRISPR-Based Genetic Manipulation of Candida Species: Historical Perspectives and Current Approaches

The Candida genus encompasses a diverse group of ascomycete fungi that have captured the attention of the scientific community, due to both their role in pathogenesis and emerging applications in biotechnology; the development of gene editing tools such as CRISPR, to analyze fungal genetics and perform functional genomic studies in these organisms, is essential to fully understand and exploit this genus, to further advance antifungal drug discovery and industrial value. However, genetic manipulation of Candida species has been met with several distinctive barriers to progress, such as unconventional codon usage in some species, as well as the absence of a complete sexual cycle in its diploid members. Despite these challenges, the last few decades have witnessed an expansion of the Candida genetic toolbox, allowing for diverse genome editing applications that range from introducing a single point mutation to generating large-scale mutant libraries for functional genomic studies. Clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 technology is among the most recent of these advancements, bringing unparalleled versatility and precision to genetic manipulation of Candida species. Since its initial applications in Candida albicans, CRISPR-Cas9 platforms are rapidly evolving to permit efficient gene editing in other members of the genus. The technology has proven useful in elucidating the pathogenesis and host-pathogen interactions of medically relevant Candida species, and has led to novel insights on antifungal drug susceptibility and resistance, as well as innovative treatment strategies. CRISPR-Cas9 tools have also been exploited to uncover potential applications of Candida species in industrial contexts. This review is intended to provide a historical overview of genetic approaches used to study the Candida genus and to discuss the state of the art of CRISPR-based genetic manipulation of Candida species, highlighting its contributions to deciphering the biology of this genus, as well as providing perspectives for the future of Candida genetics.

Candida albicans-induced leukotriene biosynthesis in neutrophils is restricted to the hyphal morphology

Neutrophils are the most abundant leukocytes in circulation playing a key role in acute inflammation during microbial infections. Phagocytosis, one of the crucial defence mechanisms of neutrophils against pathogens, is amplified by chemotactic leukotriene (LT)B₄ , which is biosynthesized via 5-lipoxygenase (5-LOX). However, extensive liberation of LTB₄ can be destructive by over-intensifying the inflammatory process. While enzymatic biosynthesis of LTB₄ is well characterized, less is known about molecular mechanisms that activate 5-LOX and lead to LTB₄ formation during host-pathogen interactions. Here, we investigated the ability of the common opportunistic fungal pathogen Candida albicans to induce LTB₄ formation in neutrophils, and elucidated pathogen-mediated drivers and cellular processes that activate this pathway. We revealed that C. albicans-induced LTB₄ biosynthesis requires both the morphological transition from yeast cells to hyphae and the expression of hyphae-associated genes, as exclusively viable hyphae or yeast-locked mutant cells expressing hyphae-associated genes stimulated 5-LOX by [Ca²⁺ ]_i mobilization and p38 MAPK activation. LTB₄ biosynthesis was orchestrated by synergistic activation of dectin-1 and Toll-like receptor 2, and corresponding signaling via SYK and MYD88, respectively. Conclusively, we report hyphae-specific induction of LTB₄ biosynthesis in human neutrophils. This highlights an expanding role of neutrophils during inflammatory processes in the response to C. albicans infections.

Activation of Cph1 causes ß(1,3)-glucan unmasking in Candida albicans and attenuates virulence in mice in a neutrophil-dependent manner

Masking the immunogenic cell wall epitope ß(1,3)-glucan under an outer layer of mannosylated glycoproteins is an important virulence factor deployed by Candida albicans during infection. Consequently, increased ß(1,3)-glucan exposure (unmasking) reveals C. albicans to the host's immune system and attenuates its virulence. We have previously shown that activation of the Cek1 MAPK pathway via expression of a hyperactive allele of an upstream kinase (STE11ΔN467) induced unmasking. It also increased survival of mice in a murine disseminated candidiasis model and attenuated kidney fungal burden by ≥33 fold. In this communication, we utilized cyclophosphamide-induced immunosuppression to test if the clearance of the unmasked STE11ΔN467 mutant was dependent on the host immune system. Suppression of the immune response by cyclophosphamide reduced the attenuation in fungal burden caused by the STE11ΔN467 allele. Moreover, specific depletion of neutrophils via 1A8 antibody treatment also reduced STE11ΔN467-dependent fungal burden attenuation, but to a lesser extent than cyclophosphamide, demonstrating an important role for neutrophils in mediating fungal clearance of unmasked STE11ΔN467 cells. In an effort to understand the mechanism by which Ste11ΔN467 causes unmasking, transcriptomics were used to reveal that several components in the Cek1 MAPK pathway were upregulated, including the transcription factor CPH1 and the cell wall sensor DFI1. In this report we show that a cph1ΔΔ mutation restored ß(1,3)-glucan exposure to wild-type levels in the STE11ΔN467 strain, confirming that Cph1 is the transcription factor mediating Ste11ΔN467-induced unmasking. Furthermore, Cph1 is shown to induce a positive feedback loop that increases Cek1 activation. In addition, full unmasking by STE11ΔN467 is dependent on the upstream cell wall sensor DFI1. However, while deletion of DFI1 significantly reduced Ste11ΔN467-induced unmasking, it did not impact activation of the downstream kinase Cek1. Thus, it appears that once stimulated by Ste11ΔN467, Dfi1 activates a parallel signaling pathway that is involved in Ste11ΔN467-induced unmasking.

Insights on the Functional Role of Beta-Glucans in Fungal Immunity Using Receptor-Deficient Mouse Models

Understanding the host anti-fungal immunity induced by beta-glucan has been one of the most challenging conundrums in the field of biomedical research. During the last couple of decades, insights on the role of beta-glucan in fungal disease progression, susceptibility, and resistance have been greatly augmented through the utility of various beta-glucan cognate receptor-deficient mouse models. Analysis of dectin-1 knockout mice has clarified the downstream signaling pathways and adaptive effector responses triggered by beta-glucan in anti-fungal immunity. On the other hand, assessment of CR3-deficient mice has elucidated the compelling action of beta-glucans in neutrophil-mediated fungal clearance, and the investigation of EphA2-deficient mice has highlighted its novel involvement in host sensing and defense to oral mucosal fungal infection. Based on these accounts, this review focuses on the recent discoveries made by these gene-targeted mice in beta-glucan research with particular emphasis on the multifaceted aspects of fungal immunity.

Sodium houttuyfonate attenuates dextran sulfate sodium associated colitis precolonized with Candida albicans through inducing β-glucan exposure

Inflammatory bowel disease (IBD) including Crohn's disease and ulcerative colitis is a chronic intestinal disease most likely associated with gut dysbiosis. Candida related mycobiota has been demonstrated to play a role in IBD progression. Traditional Chinese herbal medicines (TCHMs) with antifungal activity have a potential in prevention and treatment of fungi-related IBD. Sodium houttuyfonate (SH) is a promising anti-Candida TCHMs. In this study, a dextran sulfate sodium induced colitis model with Candida albicans precolonization is established. SH gavage can significantly decrease the fungal burdens in feces and colon tissues, reduce disease activity index score, elongate colon length, and attenuate colonic damages. Moreover, SH markedly inhibits the levels of anti-Saccharomyces cerevisiae antibodies, β-glucan, and proinflammatory cytokine (IL-1β, IL-6, IL-8, TNF-α), and increases anti-inflammatory factor IL-10 level in serum and colon tissue. Further experiments demonstrate that SH could induce β-glucan exposure, priming intestinal macrophages to get rid of colonized C. albicans through the collaboration of Dectin-1 and TLR2/4. With the decreased fungal burden, the protein levels of Dectin-1, TLR2, TLR4, and NF-κBp65 are fallen back, indicating the primed macrophages calm down and the colitis is alleviated. Collectively, these results manifest that SH can attenuate C. albicans associated colitis via β-glucan exposure, deepening our understanding of TCHMs in the prevention and treatment of fungi associated IBD.

Commensal fungi and their cell-wall β-glucans direct differential responses in human intestinal epithelial cells

Intestinal epithelial cells (IECs) are the first to encounter luminal antigens and play an active role in intestinal immune responses. We previously reported that β-glucans, major fungal cell-wall glycans, induced chemokine secretion by IEC lines in a Dectin-1- and Syk-dependent manner. Here, we show that in contrast to β-glucans, stimulation of IEC lines with Candida albicans and Saccharomyces cerevisiae did not induce secretion of any of the proinflammatory cytokines IL-8, CCL2, CXCL1, and GM-CSF. Commensal fungi and β-glucans activated Syk and ERK in IEC lines. However, only β-glucans activated p38, JNK, and the transcription factors NF-κB p65 and c-JUN, which were necessary for cytokine secretion. Furthermore, costimulation of IEC lines with β-glucans and C. albicans yielded decreased cytokine secretion compared to stimulation with β-glucans alone. Finally, ex vivo stimulation of human colonic mucosal explants with zymosan and C. albicans, leads to epithelial Syk and ERK phosphorylation, implying recognition of fungi and similar initial signaling pathways as in IEC lines. Lack of cytokine secretion in response to commensal fungi may reflect IECs' response to fungal glycans, other than β-glucans, that contribute to mucosal tolerance. Skewed epithelial response to commensal fungi may impair homeostasis and contribute to intestinal inflammation.

Iron Assimilation during Emerging Infections Caused by Opportunistic Fungi with emphasis on Mucorales and the Development of Antifungal Resistance

Iron is a key transition metal required by most microorganisms and is prominently utilised in the transfer of electrons during metabolic reactions. The acquisition of iron is essential and becomes a crucial pathogenic event for opportunistic fungi. Iron is not readily available in the natural environment as it exists in its insoluble ferric form, i.e., in oxides and hydroxides. During infection, the host iron is bound to proteins such as transferrin, ferritin, and haemoglobin. As such, access to iron is one of the major hurdles that fungal pathogens must overcome in an immunocompromised host. Thus, these opportunistic fungi utilise three major iron acquisition systems to overcome this limiting factor for growth and proliferation. To date, numerous iron acquisition pathways have been fully characterised, with key components of these systems having major roles in virulence. Most recently, proteins involved in these pathways have been linked to the development of antifungal resistance. Here, we provide a detailed review of our current knowledge of iron acquisition in opportunistic fungi, and the role iron may have on the development of resistance to antifungals with emphasis on species of the fungal basal lineage order Mucorales, the causative agents of mucormycosis.

Intracellular PRRs Activation in Targeting the Immune Response Against Fungal Infections

The immune response against fungal infections is complex and exhibits several factors involving innate elements that participate in the interaction with the fungus. The innate immune system developed pattern recognition receptors that recognize different pathogen-associated molecular patterns present both on the surface of the fungi cell wall and on their genetic material. These receptors have the function of activating the innate immune response and regulating a subsequent adaptive immune response. Among pattern recognition receptors, the family of Toll-like receptors and C-type lectin receptors are the best described and characterized, they act directly in the recognition of pathogen-associated molecular patterns expressed on the wall of the fungus and consequently in directing the immune response. In recent years, the role of intracellular pattern recognition receptors (TLR3, TLR7, TLR8, and TLR9) has become increasingly important in the pathophysiology of some mycoses, as paracoccidioidomycosis, cryptococcosis, aspergillosis, and candidiasis. The recognition of nucleic acids performed by these receptors can be essential for the control of some fungal infections, as they can be harmful to others. Therefore, this review focuses on highlighting the role played by intracellular pattern recognition receptors both in controlling the infection and in the host's susceptibility against the main fungi of medical relevance.

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.

Epitope Shaving Promotes Fungal Immune Evasion

The immune system plays a critical role in protecting us against potentially fatal fungal infections. However, some fungal pathogens have evolved evasion strategies that reduce the efficacy of our immune defenses. Previously, we reported that the fungal pathogen Candida albicans exploits specific host-derived signals (such as lactate and hypoxia) to trigger an immune evasion strategy that involves reducing the exposure of β-glucan at its cell surface. Here, we show that this phenomenon is mediated by the induction of a major secreted exoglucanase (Xog1) by the fungus in response to these host signals. Inactivating XOG1-mediated “shaving” of cell surface-exposed β-glucan enhances immune responses against the fungus. Furthermore, inhibiting exoglucanase activity pharmacologically attenuates C. albicans virulence. In addition to revealing the mechanism underlying a key immune evasion strategy in a major fungal pathogen of humans, our work highlights the potential therapeutic value of drugs that block fungal immune evasion.

The cell wall provides a major physical interface between fungal pathogens and their mammalian host. This extracellular armor is critical for fungal cell homeostasis and survival. Fungus-specific cell wall moieties, such as β-1,3-glucan, are recognized as pathogen-associated molecular patterns (PAMPs) that activate immune-mediated clearance mechanisms. We have reported that the opportunistic human fungal pathogen Candida albicans masks β-1,3-glucan following exposure to lactate, hypoxia, or iron depletion. However, the precise mechanism(s) by which C. albicans masks β-1,3-glucan has remained obscure. Here, we identify a secreted exoglucanase, Xog1, that is induced in response to lactate or hypoxia. Xog1 functions downstream of the lactate-induced β-glucan “masking” pathway to promote β-1,3-glucan “shaving.” Inactivation of XOG1 blocks most but not all β-1,3-glucan masking in response to lactate, suggesting that other activities contribute to this phenomenon. Nevertheless, XOG1 deletion attenuates the lactate-induced reductions in phagocytosis and cytokine stimulation normally observed for wild-type cells. We also demonstrate that the pharmacological inhibition of exoglucanases undermines β-glucan shaving, enhances the immune visibility of the fungus, and attenuates its virulence. Our study establishes a new mechanism underlying environmentally induced PAMP remodeling that can be manipulated pharmacologically to influence immune recognition and infection outcomes.

11g, a Potent Antifungal Candidate, Enhances Candida albicans Immunogenicity by Unmasking β-Glucan in Fungal Cell Wall

In the course of optimizing GPI biosynthesis inhibitors, we designed and synthetized a 2-aminonicotinamide derivative named 11g. After evaluating the antifungal activity of compound 11g in vitro, we investigated the influences of 11g on fungi immunogenicity. In addition, we also took advantage of murine systemic candidiasis model to investigate the protective effects of 11g in vivo. Results show that 11g exhibited potent antifungal activity both in vitro and in vivo. Further study shows that 11g caused the unmasking of fungi β-glucan layer, leading to stronger immune responses in macrophages through Dectin-1. These results suggest that 11g is a very promising antifungal candidate, which assists in eliciting stronger immune responses to help host immune system disposing pathogens. The discovery of 11g might expand the toolbox of fungal infection treatment.

Paeonol alleviates dextran sodium sulfate induced colitis involving Candida albicans-associated dysbiosis

Inflammatory bowel disease (IBD), which consists of ulcerative colitis (UC) and Crohn's disease (CD), is a chronic inflammatory disorder of the gastrointestinal tract. Occurrence and development of UC have been associated with multiple potential causative factors, which include fungal dysbiosis. Growing evidence reveals that Candida albicans-associated dysbiosis is correlated with clinical deterioration in UC. Paeonol (PAE) is a commonly used traditional medicine with multiple reported properties including effective alleviation of UC. In this study, a murine UC model was established by colonizing mice with additional C. albicans via gavage prior to dextran sodium sulfate (DSS) administration. Effects of PAE treatment were also assessed at initiation and in preestablished C. albicans-associated colitis. The results showed that C. albicans supplementation could aggravate disease activity index (DAI), compromise mucosal integrity, exacerbate fecal and tissue fungal burdens, increase serum β-glucan and anti-Saccharomyces cerevisiae antibody (ASCA) levels, promote serum and colonic tissue pro-inflammatory cytokine secretion (tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6, and IL-8) and decrease the anti-inflammatory cytokine IL-10 level. It also stimulated Dectin-1, TLR2 and TLR4 as well as expression of their downstream effector NF-κB in colonic tissue. After PAE treatment, the adverse impacts of C. albicans on colitis were relieved, via decreased receptor-associated local and systemic inflammation. Our study suggests that PAE should be a candidate for treatment of fungal dysbiosis-associated UC and may act through the Dectin-1/NF-κB pathway in collaboration with TLR2 and TLR4.

LAY SUMMARY

Candida albicans is believed to be an important stimulator in ulcerative colitice (UC) development. Suppressing the growth of intestinal C. albicans can be contributory to the amelioration of UC. Paeonol (PAE) is a commonly used traditional medicine with multiple biological functions. In this study, we observed that PAE could alleviate symptoms in mice UC model accompanying with burden reduction of C. albicans. Therefore, we suppose that PAE can be a candidate in the treatment of C. albicans-associated UC.

Caspofungin Induced Cell Wall Changes of Candida Species Influences Macrophage Interactions

Candida species are known to differ in their ability to cause infection and have been shown to display varied susceptibilities to antifungal drugs. Treatment with the echinocandin, caspofungin, leads to compensatory alterations in the fungal cell wall. This study was performed to compare the structure and composition of the cell walls of different Candida species alone and in response to caspofungin treatment, and to evaluate how changes at the fungal cell surface affects interactions with macrophages. We demonstrated that the length of the outer fibrillar layer varied between Candida species and that, in most cases, reduced fibril length correlated with increased exposure of β-1,3-glucan on the cell surface. Candida glabrata and Candida guilliermondii, which had naturally more β-1,3-glucan exposed on the cell surface, were phagocytosed significantly more efficiently by J774 macrophages. Treatment with caspofungin resulted in increased exposure of chitin and β-1,3-glucan on the surface of the majority of Candida species isolates that were tested, with the exception of C. glabrata and Candida parapsilosis isolates. This increase in exposure of the inner cell wall polysaccharides, in most cases, correlated with reduced uptake by macrophages and in turn, a decrease in production of TNFα. Here we show that differences in the exposure of cell wall carbohydrates and variations in the repertoire of covalently attached surface proteins of different Candida species contributes to their recognition by immune cells.

Loss of Kex2 Affects the Candida albicans Cell Wall and Interaction with Innate Immune Cells

The secretory pathway in Candida albicans involves the protein translocation into the lumen of the endoplasmic reticulum and transport to the Golgi complex, where proteins undergo posttranslational modifications, including glycosylation and proteolysis. The Golgi-resident Kex2 protease is involved in such processing and disruption of its encoding gene affected virulence and dimorphism. These previous studies were performed using cells without URA3 or with URA3 ectopically placed into the KEX2 locus. Since these conditions are known to affect the cellular fitness and the host-fungus interaction, here we generated a kex2Δ null mutant strain with URA3 placed into the neutral locus RPS1. The characterization of this strain showed defects in the cell wall composition, with a reduction in the N-linked mannan content, and the increment in the levels of O-linked mannans, chitin, and β-glucans. The defects in the mannan content are likely linked to changes in Golgi-resident enzymes, as the α-1,2-mannosyltransferase and α-1,6-mannosyltransferase activities were incremented and reduced, respectively. The mutant cells also showed reduced ability to stimulate cytokine production and phagocytosis by human mononuclear cells and macrophages, respectively. Collectively, these data showed that loss of Kex2 affected the cell wall composition, the protein glycosylation pathways, and interaction with innate immune cells.

Inhibition of Respiration of Candida albicans by Small Molecules Increases Phagocytosis Efficacy by Macrophages

Candida albicans adapts to various conditions in different body niches by regulating gene expression, protein synthesis, and metabolic pathways. These adaptive reactions not only allow survival but also influence the interaction with host cells, which is governed by the composition and structure of the fungal cell wall. Numerous studies had shown linkages between mitochondrial functionality, cell wall integrity and structure, and pathogenicity. Thus, we decided to inhibit single complexes of the respiratory chain of C. albicans and to analyze the resultant interaction with macrophages via their phagocytic activity. Remarkably, inhibition of the fungal bc₁ complex by antimycin A increased phagocytosis, which correlated with an increased accessibility of β-glucans. To contribute to mechanistic insights, we performed metabolic studies, which highlighted significant changes in the abundance of constituents of the plasma membrane. Collectively, our results reinforce the strong linkage between fungal energy metabolism and other components of fungal physiology, which also determine the vulnerability to immune defense reactions.IMPORTANCE The yeast Candida albicans is one of the major fungal human pathogens, for which new therapeutic approaches are required. We aimed at enhancements of the phagocytosis efficacy of macrophages by targeting the cell wall structure of C. albicans, as the coverage of the β-glucan layer by mannans is one of the immune escape mechanisms of the fungus. We unambiguously show that inhibition of the fungal bc₁ complex correlates with increased accessibilities of β-glucans and improved phagocytosis efficiency. Metabolic studies proved not only the known direct effects on reactive oxygen species (ROS) production and fermentative pathways but also the clear downregulation of the ergosterol pathway and upregulation of unsaturated fatty acids. The changed composition of the plasma membrane could also influence the interaction with the overlying cell wall. Thus, our work highlights the far-reaching relevance of energy metabolism, indirectly also for host-pathogen interactions, without affecting viability.

Fluconazole and Lipopeptide Surfactin Interplay During Candida albicans Plasma Membrane and Cell Wall Remodeling Increases Fungal Immune System Exposure

Recognizing the β-glucan component of the Candida albicans cell wall is a necessary step involved in host immune system recognition. Compounds that result in exposed β-glucan recognizable to the immune system could be valuable antifungal drugs. Antifungal development is especially important because fungi are becoming increasingly drug resistant. This study demonstrates that lipopeptide, surfactin, unmasks β-glucan when the C. albicans cells lack ergosterol. This observation also holds when ergosterol is depleted by fluconazole. Surfactin does not enhance the effects of local chitin accumulation in the presence of fluconazole. Expression of the CHS3 gene, encoding a gene product resulting in 80% of cellular chitin, is downregulated. C. albicans exposure to fluconazole changes the composition and structure of the fungal plasma membrane. At the same time, the fungal cell wall is altered and remodeled in a way that makes the fungi susceptible to surfactin. In silico studies show that surfactin can form a complex with β-glucan. Surfactin forms a less stable complex with chitin, which in combination with lowering chitin synthesis, could be a second anti-fungal mechanism of action of this lipopeptide.

Antifungal evaluation of traditional herbal monomers and their potential for inducing cell wall remodeling in Candida albicans and Candida auris

Traditional herbal monomers (THMs) are widely distributed in many traditional Chinese formulas (TCFs) and decoctions (TCDs) and are frequently used for the prevention and treatment of fungal infections. The antifungal activities of five common THMs, including sodium houttuyfonate (SH), berberine (BER), palmatine (PAL), jatrorrhizine (JAT) and cinnamaldehyde (CIN), and their potential for inducing cell wall remodeling (CWR), were evaluated against Candida albicans SC5314 and Candida auris 12372. SH/CIN plus BER/PAL/JAT showed synergistic antifungal activity against both Candida isolates. Furthermore, SH-associated combinations (SH plus BER/PAL/JAT) induced stronger exposure of β-glucan and chitin than their counterparts, while CIN triggered more marked exposure compared with CIN-associated combinations (CIN plus BER/PAL/JAT). Collectively, this study demonstrated the anti-Candida effect and the CWR induction potential of the five THMs and their associated combinations, providing a possibility of their in vivo application against fungal-associated infections.

Pathways That Synthesize Phosphatidylethanolamine Impact Candida albicans Hyphal Length and Cell Wall Composition through Transcriptional and Posttranscriptional Mechanisms

Candida albicans is a leading cause of systemic bloodstream infections, and synthesis of the phospholipid phosphatidylethanolamine (PE) is required for virulence. The psd1Δ/Δ psd2Δ/Δ mutant, which cannot synthesize PE by the cytidine diphosphate diacylglycerol (CDP-DAG) pathway, is avirulent in the mouse model of systemic candidiasis. Similarly, an ept1Δ/Δ mutant, which cannot produce PE by the Kennedy pathway, exhibits decreased kidney fungal burden in systemically infected mice. Conversely, overexpression of EPT1 results in a hypervirulent phenotype in this model. Thus, mutations that increase PE synthesis increase virulence, and mutations that decrease PE synthesis decrease virulence. However, the mechanism by which virulence is regulated by PE synthesis is only partially understood. RNA sequencing was performed on strains with deficient or excessive PE biosynthesis to elucidate the mechanism. Decreased PE synthesis from loss of EPT1 or PSD1 and PSD2 leads to downregulation of genes that impact mitochondrial function. Losses of PSD1 and PSD2, but not EPT1, cause significant increases in transcription of glycosylation genes, which may reflect the substantial cell wall defects in the psd1Δ/Δ psd2Δ/Δ mutant. These accumulated defects could contribute to the decreased virulence observed for mutants with deficient PE synthesis. In contrast to mutants with decreased PE synthesis, there were no transcriptional differences between the EPT1 overexpression strain and the wild type, indicating that the hypervirulent phenotype is a consequence of posttranscriptional changes. It was found that overexpression of EPT1 causes increased chitin content and increased hyphal length. These phenotypes may help to explain the previously observed hypervirulence in the EPT1 overexpressor.

Mannan Molecular Substructures Control Nanoscale Glucan Exposure in Candida

Cell wall mannans of Candida albicans mask β-(1,3)-glucan from recognition by Dectin-1, contributing to innate immune evasion. Glucan exposures are predominantly single receptor-ligand interaction sites of nanoscale dimensions. Candida species vary in basal glucan exposure and molecular complexity of mannans. We used super-resolution fluorescence imaging and a series of protein mannosylation mutants in C. albicans and C. glabrata to investigate the role of specific N-mannan features in regulating the nanoscale geometry of glucan exposure. Decreasing acid labile mannan abundance and α-(1,6)-mannan backbone length correlated most strongly with increased density and nanoscopic size of glucan exposures in C. albicans and C. glabrata, respectively. Additionally, a C. albicans clinical isolate with high glucan exposure produced similarly perturbed N-mannan structures and elevated glucan exposure geometry. Thus, acid labile mannan structure influences the nanoscale features of glucan exposure, impacting the nature of the pathogenic surface that triggers immunoreceptor engagement, aggregation, and signaling.

Cell Wall-Modifying Antifungal Drugs

Antifungal therapy is a critical component of patient management for invasive fungal diseases. Yet, therapeutic choices are limited as only a few drug classes are available to treat systemic disease, and some infecting strains are resistant to one or more drug classes. The ideal antifungal inhibits a fungal-specific essential target not present in human cells to avoid off-target toxicities. The fungal cell wall is an ideal drug target because its integrity is critical to cell survival and a majority of biosynthetic enzymes and wall components is unique to fungi. Among currently approved antifungal agents and those in clinical development, drugs targeting biosynthetic enzymes of the cell wall show safe and efficacious antifungal properties, which validates the cell wall as a target. The echinocandins, which inhibit β-1,3-glucan synthase, are recommended as first-line therapy for Candida infections. Newer cell wall-active drugs in clinical development encompass next-generation glucan synthase inhibitors including a novel echinocandin and an enfumafungin, an inhibitor of Gwt1, a key component of GPI anchor protein biosynthesis, and a classic inhibitor of chitin biosynthesis. As the cell wall is rich in potential drug discovery targets, it is primed to help deliver the next generation of antifungal drugs.

Role of Protein Mannosylation in the Candida tropicalis-Host Interaction

Mannans are components of the fungal wall attached to proteins via N- or O-linkages. In Candida albicans, Och1 is an α1,6-mannosyltransferase that adds the first mannose unit to the N-linked mannan outer chain; whereas Pmr1 is an ion pump that imports Mn²⁺ into the Golgi lumen. This cation is the cofactor of Golgi-resident mannosyltransferases, and thus Pmr1 is involved in the synthesis of both N- and O-linked mannans. Since we currently have limited information about the genetic network behind the Candida tropicalis protein mannosylation machinery, we disrupted OCH1 and PMR1 in this organism. The C. tropicalis pmr1Δ and och1Δ mutants showed increased doubling times, aberrant colony and cellular morphology, reduction in the wall mannan content, and increased susceptibility to wall perturbing agents. These changes were accompanied by increased exposure of both β1,3-glucan and chitin at the wall surface of both mutant strains. Our results showed that O-linked mannans are dispensable for cytokine production by human mononuclear cells, but N-linked mannans and β1,3-glucan are key ligands to trigger cytokine production in a co-stimulatory pathway involving dectin-1 and mannose receptor. Moreover, we found that the N-linked mannan core found on the surface of C. tropicalis och1Δ null mutant was capable of inducing cytokine production; and that a mannan-independent pathway for IL-10 production is present in the C. tropicalis-mononuclear cell interaction. Both mutant strains showed virulence attenuation in the Galleria mellonella and the mouse model of systemic candidiasis. Therefore, mannans are relevant for cell wall composition and organization, and for the C. tropicalis-host interaction.

Non-canonical signalling mediates changes in fungal cell wall PAMPs that drive immune evasion

To colonise their host, pathogens must counter local environmental and immunological challenges. Here, we reveal that the fungal pathogen Candida albicans exploits diverse host-associated signals to promote immune evasion by masking of a major pathogen-associated molecular pattern (PAMP), β-glucan. Certain nutrients, stresses and antifungal drugs trigger β-glucan masking, whereas other inputs, such as nitrogen sources and quorum sensing molecules, exert limited effects on this PAMP. In particular, iron limitation triggers substantial changes in the cell wall that reduce β-glucan exposure. This correlates with reduced phagocytosis by macrophages and attenuated cytokine responses by peripheral blood mononuclear cells. Iron limitation-induced β-glucan masking depends on parallel signalling via the iron transceptor Ftr1 and the iron-responsive transcription factor Sef1, and the protein kinase A pathway. Our data reveal that C. albicans exploits a diverse range of specific host signals to trigger protective anticipatory responses against impending phagocytic attack and promote host colonisation.

The authors show that the fungal pathogen Candida albicans exploits diverse host-associated signals, including specific nutrients and stresses, to promote immune evasion by masking cell wall β-glucan, a major pathogen-associated molecular pattern.