Macrophages in resistance to candidiasis

Abrogation of efflux pump activity, biofilm formation, and immune escape by candidacidal geraniol in emerging superbug, Candida auris

During the last decade, Candida auris emerged as a threatening human fungal pathogen that notably caused outbreaks around the globe with high mortality. Considering C. auris species as newly discovered fungi, the evolutionary features remain elusive. The antifungal resistance which is a norm in C. auris underlines the need for innovative therapeutic options. ATP Binding Cassette (ABC) superfamily efflux pumps overexpression and biofilms are known to be major contributors to multidrug resistance (MDR) in C. auris. Therefore, herein, we investigated the antifungal potential of geraniol (Ger) as a promising natural compound in the fight against MDR C. auris. Our experiments proved that Ger was fungicidal in nature and impaired rhodamine 6G (R6G) efflux, confirming the specific effect on ABC transporters. Kinetic studies unravelled the competitive mode of inhibition by Ger for R6G efflux since the apparent Km increased with no change in Vmax value. Mechanistic insights also revealed that Ger depleted ergosterol content in C. auris. Furthermore, Ger led to inhibition in biofilm formation as evident from crystal violet staining, biofilm metabolic and biomass measurements. Additionally, enhanced survival of Caenorhabditis elegans model after C. auris infection demonstrated the in vivo efficacy of Ger. Lastly, the in vivo efficacy was confirmed from a THP-1 cell line model which depicted enhanced macrophage-mediated killing in the presence of Ger. Modulation of C. auris efflux pump activity and biofilm formation by Ger represents a promising approach to combat MDR. Together, this study demonstrated the potential therapeutic insights of Ger as a promising addition to the antifungal armamentarium required to treat emerging and resistant C. auris.

Th17 Cells: Orchestrators of Mucosal Inflammation and Potential Therapeutic Targets

T helper 17 (Th17) cells represent a specialized subgroup of effector CD4+ T cells known for their role in provoking neutrophil-driven tissue inflammation, particularly within mucosal tissues. Although they are pivotal for defending the host against extracellular bacteria and fungi, they have also been associated with development of various T cell-mediated inflammatory conditions, autoimmune diseases, and even cancer. Notably, Th17 cells exhibit a dual nature, with different Th17 cell subtypes showcasing distinct effector functions and varying capacities to incite autoimmune tissue inflammation. Furthermore, Th17 cells exhibit significant plasticity, which carries important functional implications, both in terms of their expression of cytokines typically associated with other effector T cell subsets and in their interactions with regulatory CD4+ T cells. The intricate balance of Th17 cytokines can also be a double-edged sword in inflammation, autoimmunity, and cancer. Within this article, we delve into the mechanisms that govern the differentiation, function, and adaptability of Th17 cells. We culminate with an exploration of therapeutic potentials in harnessing the power of Th17 cells and their cytokines. Targeted interventions to modulate Th17 responses are emerging as promising strategies for autoimmunity, inflammation, and cancer treatment. By precisely fine-tuning Th17-related pathways, we may unlock new avenues for personalized therapeutic approaches, aiming to restore immune balance, alleviate the challenges of these disorders, and ultimately enhance the quality of life for individuals affected by them.

Histone Acetylation Regulator Gcn5 Mediates Drug Resistance and Virulence of Candida glabrata

Candida glabrata is poised to adapt to drug pressure rapidly and acquire antifungal resistance leading to therapeutic failure. Given the limited antifungal armamentarium, there is an unmet need to explore new targets or therapeutic strategies for antifungal treatment. The lysine acetyltransferase Gcn5 has been implicated in the pathogenesis of C. albicans. Yet how Gcn5 functions and impacts antifungal resistance in C. glabrata is unknown. Disrupting GCN5 rendered C. glabrata cells more sensitive to various stressors, partially reverted resistance in drug-resistant mutants, and attenuated the emergence of resistance compared to wild-type cells. RNA sequencing (RNA-seq) analysis revealed transcriptomic changes involving multiple biological processes and different transcriptional responses to antifungal drugs in gcn5Δ cells compared to wild-type cells. GCN5 deletion also resulted in reduced intracellular survival within THP-1 macrophages. In summary, Gcn5 plays a critical role in modulating the virulence of C. glabrata and regulating its response to antifungal pressure and host defense. IMPORTANCE As an important and successful human pathogen, Candida glabrata is known for its swift adaptation and rapid acquisition of resistance to the most commonly used antifungal agents, resulting in therapeutic failure in clinical settings. Here, we describe that the histone acetyltransferase Gcn5 is a key factor in adapting to antifungal pressure and developing resistance in C. glabrata. The results provide new insights into epigenetic control over the drug response in C. glabrata and may be useful for drug target discovery and the development of new therapeutic strategies to combat fungal infections.

Macrophage depletion alters bacterial gut microbiota partly through fungal overgrowth in feces that worsens cecal ligation and puncture sepsis mice

Because macrophage dysfunction from some emerging therapies might worsen gut-derived sepsis, cecal ligation and puncture (CLP) sepsis are performed in mice with clodronate-induced macrophage depletion. Macrophage depletion (non-sepsis) increased fecal Ascormycota, with a subtle change in bacterial microbiota, that possibly induced gut-barrier defect as Candida pintolopesii and Enterococcus faecalis were identified from blood. Sepsis in macrophage-depleted mice was more severe than sepsis control as indicated by mortality, cytokines, organ injury (liver, kidney, and spleen), gut-leakage (FITC-dextran), fecal Proteobacteria, and blood organisms (bacteria and fungi). Lysate of C. pintolopesii or purified (1 → 3)-β-d-glucan (BG; a major component of fungal cell wall) enhanced growth of Klebsiella pneumoniae and Escherichia coli that were isolated from the blood of macrophage-depleted CLP mice implying a direct enhancer to some bacterial species. Moreover, the synergy of LPS and BG on enterocytes (Caco-2) (Transepithelial electrical resistance) and neutrophils (cytokines) also supported an influence of gut fungi in worsening sepsis. In conclusion, macrophage depletion enhanced sepsis through the selectively facilitated growth of some bacteria (dysbiosis) from increased fecal fungi that worsened gut-leakage leading to the profound systemic responses against gut-translocated LPS and BG. Our data indicated a possible adverse effect of macrophage-depleted therapies on enhanced sepsis severity through spontaneous elevation of fecal fungi.

Functionality of the human antibody response to Candida albicans

Candida albicans is a common commensal on human mucosal surfaces, but can become pathogenic, e.g. if the host is immunocompromised. While neutrophils, macrophages and T cells are regarded as major players in the defense against pathogenic C. albicans, the role of B cells and the protective function of their antibodies are less well characterized. In this study, we show that human serum antibodies are able to enhance the association of human THP-1 monocyte-like cells with C. albicans cells. Human serum antibodies are also capable of inhibiting the adherence and damage dealt to epithelial cells. Furthermore, human serum antibodies impair C. albicans invasion of human oral epithelial cells by blocking induced endocytosis and consequently host cell damage. While aspartic proteases secreted by C. albicans are able to cleave human IgG, this process does not appear to affect the protective function of human antibodies. Thus, humans are equipped with a robust antibody response to C. albicans, which can enhance antifungal activities and prevent fungal-mediated epithelial damage.

Effects of two formulations containing Phyllanthus emblica and Tinospora cordifolia with and without Ocimum sanctum in immunocompromised mice

Background

Current pandemic has led us to explore the role of traditional system of medicine to look for formulations that enhance immunity.

Objective

The obejctive of this experimental study was to evaluate the immunomodulatory effects of two formulations, Tinospora cordifolia (Tc) and Phyllanthus emblica (Pe) with and without coating of Ocimum sanctum (Os).

Materials and methods

After obtaining Institutional Animal Ethics Committee approval, present experimental study was conducted to evaluate the immunomodulatory effects of plant drug formulations against infection induced in mice subjected to major surgical stress. Hemisplenectomy was selected to induce major stress and the procedure for hemisplenectomy was standardized. A model of secondary fungal infection after hemisplenectomy was established followed by the treatment of mice with plant drugs and controls. They were subjected to hemisplenectomy or sham operation and 10⁵ C. albicans were injected intravenously. The therapy continued for next 14 days. Kidneys were isolated to estimate fungal load. Fungal load of the kidneys was estimated on post-operative Day 15.

Results

The test formulations Tc + Pe and Tc + Pe + Os showed significant reduction in the fungal burden of kidneys as compared to hemisplenectomized control group. However, Tc alone exerted better degree of protection as compared to Tc + Pe and Tc + Pe + Os.

Conclusion

The formulations, Tc + Pe and Tc + Pe + Os that were developed on the basis of theoretical concepts were not found to be superior to Tc. Though the individual ingredients have been shown to possess immunnostimulant activities, in combination, Pe and Os blunted the effects of Tc. The basis for this drug interaction needs further exploration. Thus, the current experimental study validates immunomodulatory role of Tc. However, the addition of Pe with bhavana of Os does not lead to any augmentation of immunomodulatory activity of Tc. This study also underlines the need to generate data on Ayurveda formulations to understand the rationality of the multi-ingredient Ayurvedic formulations.

Nitric Oxide Inhibition of Rickettsia rickettsii

Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever, is an enzootic, obligate, intracellular bacterial pathogen. Nitric oxide (NO) synthesized by the inducible NO synthase (iNOS) is a potent antimicrobial component of innate immunity and has been implicated in the control of virulent Rickettsia spp. in diverse cell types. In this study, we examined the antibacterial role of NO on R. rickettsii. Our results indicate that NO challenge dramatically reduces R. rickettsii adhesion through the disruption of bacterial energetics. Additionally, NO-treated R. rickettsii cells were unable to synthesize protein or replicate in permissive cells. Activated, NO-producing macrophages restricted R. rickettsii infections, but inhibition of iNOS ablated the inhibition of bacterial growth. These data indicate that NO is a potent antirickettsial effector of innate immunity that targets energy generation in these pathogenic bacteria to prevent growth and subversion of infected host cells.

Severe pneumonia in adults caused by Tropheryma whipplei and Candida sp. infection: a 2019 case series

Background

Whipple’s disease is a chronic infectious disease caused by the Gram-positive bacterium Tropheryma whipplei (TW), which not only affects the gastrointestinal tract and causes malabsorption of nutrients, but several other systems, such as the cardiovascular system, central nervous system, the joints, and the vascular system, can also be simultaneously involved. The aim of this report was to be able to alert the clinician to severe pneumonia caused by TW combined with Candida sp.

Case presentation

The case study was conducted on patients in September and November 2019. After routine examination and treatment, the results were not satisfactory. A bronchoalveolar lavage (BAL) using metagenomics next-generation sequencing was conducted on two adults who presented with fever, cough, and progressive dyspnea and who had no history of gastrointestinal symptoms, immunodeficiency diseases, or use of immunosuppressive agents. TW and Candida sp. were detected in in BAL.

Conclusions

This is a report of life-threatening pneumonia caused by TW combined with Candida sp. in a Chinese population.

Protein Kinases at the Intersection of Translation and Virulence

As free living organisms, fungi are challenged with a variety of environmental insults that threaten their cellular processes. In some cases, these challenges mimic conditions present within mammals, resulting in the accidental selection of virulence factors over evolutionary time. Be it within a host or the soil, fungi must contend with environmental challenges through the production of stress effector proteins while maintaining factors required for viability in any condition. Initiation and upkeep of this balancing act is mainly under the control of kinases that affect the propensity and selectivity of protein translation. This review will focus on kinases in pathogenic fungi that facilitate a virulence phenotype through translational control.

Serial Systemic Candida albicans Infection Highlighted by Proteomics

Candida albicans is the major pathogen isolated from nosocomial bloodstream infections, leading to higher mortality rates. Thus, due to its clinical relevance, studies aiming to understand host–pathogen interactions in C. albicans infection are necessary. Therefore, we performed proteomic analysis using a murine model of serial systemic infection by C. albicans to evaluate possible changes in the protein profile of the pathogen over time. Firstly, we observed a reduction in the median survival time of infected animals with increasing passage number, suggesting a higher pathogenicity acquired during repeated infections. By LC-MS/MS, it was possible to obtain protein profiles from the wild-type strain (WT) and compare them to proteins extracted from Candida cells recovered from infected tissues during passages one, three, and four (P1, P3, and P4). We obtained 56, 29, and 97 proteins in P1, P3, P4, respectively, all varying in abundance. Regarding biological processes, the majority of proteins were related to carbohydrate metabolism, stress responses and amino acid metabolism. The proteins were also categorized according to their potential role in virulence traits, such as biofilm production, yeast-to-hyphae transition, phenotypic switching, proteins related to stress responses, and uncharacterized proteins. Therefore, serial infection in combination with proteomic approach enabled us to deepen the existing knowledge about host-pathogen interactions.

Flavonoid Composition of Salacia senegalensis (Lam.) DC. Leaves, Evaluation of Antidermatophytic Effects, and Potential Amelioration of the Associated Inflammatory Response

Predominantly spread in West Tropical Africa, the shrub Salacia senegalensis (Lam.) DC. is known because of its medicinal properties, the leaves being used in the treatment of skin diseases. Prompted by the ethnomedicinal use, a hydroethanolic extract obtained from the leaves of the plant was screened against a panel of microbial strains, the majority of which involved in superficial infections. The extract was found to be active against the dermatophytes Trichophyton rubrum and Epidermophyton floccosum. Notable results were also recorded regarding the attenuation of the inflammatory response, namely the inhibitory effects observed against soybean 5-lipoxygenase (IC₅₀ = 71.14 μg mL^-1), no interference being recorded in the cellular viability of RAW 264.7 macrophages and NO levels. Relevantly, the extract did not lead to detrimental effects against the keratinocyte cell line HaCaT, at concentrations displaying antidermatophytic and anti-inflammatory effects. Flavonoid profiling of S. senegalensis leaves was achieved for the first time, allowing the identification and quantitation of myricitrin, three 3-O-substituted quercetin derivatives, and three other flavonoid derivatives, which may contribute, at least partially, to the observed antidermatophytic and anti-inflammatory effects. In the current study, the plant S. senegalensis is assessed concerning its antidermatophytic and anti-inflammatory properties.

Intravital Imaging Reveals Divergent Cytokine and Cellular Immune Responses to Candida albicans and Candida parapsilosis

In modern medicine, physicians are frequently forced to balance immune suppression against immune stimulation to treat patients such as those undergoing transplants and chemotherapy. More-targeted therapies designed to preserve immunity and prevent opportunistic fungal infection in these patients could be informed by an understanding of how fungi interact with professional and nonprofessional immune cells in mucosal candidiasis. In this study, we intravitally imaged these host-pathogen dynamics during Candida infection in a transparent vertebrate model host, the zebrafish. Single-cell imaging revealed an unexpected partitioning of the inflammatory response between phagocytes and epithelial cells. Surprisingly, we found that in vivo cytokine profiles more closely match in vitro responses of epithelial cells rather than phagocytes. Furthermore, we identified a disconnect between canonical inflammatory cytokine production and phagocyte recruitment to the site of infection, implicating noncytokine chemoattractants. Our study contributes to a new appreciation for the specialization and cross talk among cell types during mucosal infection.

Candida yeasts are common commensals that can cause mucosal disease and life-threatening systemic infections. While many of the components required for defense against Candida albicans infection are well established, questions remain about how various host cells at mucosal sites assess threats and coordinate defenses to prevent normally commensal organisms from becoming pathogenic. Using two Candida species, C. albicans and C. parapsilosis, which differ in their abilities to damage epithelial tissues, we used traditional methods (pathogen CFU, host survival, and host cytokine expression) combined with high-resolution intravital imaging of transparent zebrafish larvae to illuminate host-pathogen interactions at the cellular level in the complex environment of a mucosal infection. In zebrafish, C. albicans grows as both yeast and epithelium-damaging filaments, activates the NF-κB pathway, evokes proinflammatory cytokines, and causes the recruitment of phagocytic immune cells. On the other hand, C. parapsilosis remains in yeast morphology and elicits the recruitment of phagocytes without inducing inflammation. High-resolution mapping of phagocyte-Candida interactions at the infection site revealed that neutrophils and macrophages attack both Candida species, regardless of the cytokine environment. Time-lapse monitoring of single-cell gene expression in transgenic reporter zebrafish revealed a partitioning of the immune response during C. albicans infection: the transcription factor NF-κB is activated largely in cells of the swimbladder epithelium, while the proinflammatory cytokine tumor necrosis factor alpha (TNF-α) is expressed in motile cells, mainly macrophages. Our results point to different host strategies for combatting pathogenic Candida species and separate signaling roles for host cell types.

Inflammatory Cell Recruitment in Candida glabrata Biofilm Cell-Infected Mice Receiving Antifungal Chemotherapy

(1) Background: Due to a high rate of antifungal resistance, Candida glabrata is one of the most prevalent Candida spp. linked to systemic candidiasis, which is particularly critical in catheterized patients. The goal of this work was to simulate a systemic infection exclusively derived from C. glabrata biofilm cells and to evaluate the effectiveness of the treatment of two echinocandins—caspofungin (Csf) and micafungin (Mcf). (2) Methods: CD1 mice were infected with 48 h-biofilm cells of C. glabrata and then treated with Csf or Mcf. After 72 h, the efficacy of each drug was evaluated to assess the organ fungal burden through colony forming units (CFU) counting. The immune cell recruitment into target organs was evaluated by flow cytometry or histopathology analysis. (3) Results: Fungal burden was found to be higher in the liver than in the kidneys. However, none of the drugs was effective in completely eradicating C. glabrata biofilm cells. At the evaluated time point, flow cytometry analysis showed a predominant mononuclear response in the spleen, which was also evident in the liver and kidneys of the infected mice, as observed by histopathology analysis. (4) Conclusions: Echinocandins do not have a significant impact on liver and kidney fungal burden, or recruited inflammatory infiltrate, when mice are intravenously (i.v.) infected with C. glabrata biofilm-grown cells.

Yeast and Filaments Have Specialized, Independent Activities in a Zebrafish Model of Candida albicans Infection

Candida albicans dimorphism is a crucial virulence factor during invasive candidiasis infections, which claim the lives of nearly one-half of those afflicted. It has long been believed that filaments drive tissue invasion and yeast mediates bloodstream dissemination, but observation of these activities during infection has been prevented by technical limitations. We used a transparent zebrafish infection model to analyze more comprehensively how C. albicans utilizes shape to disseminate and invade. This model facilitated the use of diverse, complementary strategies to manipulate shape, allowing us to monitor dissemination, invasion, and pathogenesis via intravital imaging of individual fungal cells throughout the host. To control fungal cell shape, we employed three different strategies: gene deletion (efg1Δ/Δ cph1Δ/Δ, eed1Δ/Δ), overexpression of master regulators (NRG1 or UME6), and modulation of the infection temperature (21°C, 28°C, or 33°C). The effects of these orthogonal manipulations were consistent, support the proposed specialized roles of yeast in dissemination and filaments in tissue invasion and pathogenesis, and indicate conserved mechanisms in zebrafish. To test if either morphotype changes the effectiveness of the other, we infected fish with a known mixture of shape-locked strains. Surprisingly, mixed-strain infections were associated with additive, but not synergistic, filament invasion and yeast dissemination. These findings provide the most complete view of morphotype-function relationships for C. albicans to date, revealing independent roles of yeast and filaments during disseminated candidiasis.

Aspartyl proteases in Candida glabrata are required for suppression of the host innate immune response

A family of 11 cell surface-associated aspartyl proteases (CgYps1-11), also referred as yapsins, is a key virulence factor in the pathogenic yeast Candida glabrata However, the mechanism by which CgYapsins modulate immune response and facilitate survival in the mammalian host remains to be identified. Here, using RNA-Seq analysis, we report that genes involved in cell wall metabolism are differentially regulated in the Cgyps1-11Δ mutant. Consistently, the mutant contained lower β-glucan and mannan levels and exhibited increased chitin content in the cell wall. As cell wall components are known to regulate the innate immune response, we next determined the macrophage transcriptional response to C. glabrata infection and observed differential expression of genes implicated in inflammation, chemotaxis, ion transport, and the tumor necrosis factor signaling cascade. Importantly, the Cgyps1-11Δ mutant evoked a different immune response, resulting in an enhanced release of the pro-inflammatory cytokine IL-1β in THP-1 macrophages. Further, Cgyps1-11Δ-induced IL-1β production adversely affected intracellular proliferation of co-infected WT cells and depended on activation of spleen tyrosine kinase (Syk) signaling in the host cells. Accordingly, the Syk inhibitor R406 augmented intracellular survival of the Cgyps1-11Δ mutant. Finally, we demonstrate that C. glabrata infection triggers elevated IL-1β production in mouse organs and that the CgYPS genes are required for organ colonization and dissemination in the murine model of systemic infection. Altogether, our results uncover the basis for macrophage-mediated killing of Cgyps1-11Δ cells and provide the first evidence that aspartyl proteases in C. glabrata are required for suppression of IL-1β production in macrophages.

Phagocytosis and nitric oxide production by peritoneal adherent cells in response to Candida albicans in aging: a collaboration to elucidate the pathogenesis of denture stomatitis

Elderly denture wearers are commonly affected by Candida-associated denture stomatitis (DS), an inflammatory process of the oral mucosa strongly associated with Candida spp and other microorganisms, as well as local and systemic factors. The impaired immune response against pathogens is among the inherent host factors that have been also associated with the pathogenesis of DS. Mononuclear phagocytes respond to the pathogens through phagocytosis followed by the production of several substances inside the phagosomes, among them are the reactive nitrogen species (RNS). A failure in these mechanisms may contribute to the DS development.

Control of Mucosal Candidiasis in the Zebrafish Swim Bladder Depends on Neutrophils That Block Filament Invasion and Drive Extracellular-Trap Production

Candida albicans is a ubiquitous mucosal commensal that is normally prevented from causing acute or chronic invasive disease. Neutrophils contribute to protection in oral infection but exacerbate vulvovaginal candidiasis. To dissect the role of neutrophils during mucosal candidiasis, we took advantage of a new, transparent zebrafish swim bladder infection model. Intravital microscopic tracking of individual animals revealed that the blocking of neutrophil recruitment leads to rapid mortality in this model through faster disease progression. Conversely, artificial recruitment of neutrophils during early infection reduces disease pressure. Noninvasive longitudinal tracking showed that mortality is a consequence of C. albicans breaching the epithelial barrier and invading surrounding tissues. Accordingly, we found that a hyperfilamentous C. albicans strain breaches the epithelial barrier more frequently and causes mortality in immunocompetent zebrafish. A lack of neutrophils at the infection site is associated with less fungus-associated extracellular DNA and less damage to fungal filaments, suggesting that neutrophil extracellular traps help to protect the epithelial barrier from C. albicans breach. We propose a homeostatic model where C. albicans disease pressure is balanced by neutrophil-mediated damage of fungi, maintaining this organism as a commensal while minimizing the risk of damage to host tissue. The unequaled ability to dissect infection dynamics at a high spatiotemporal resolution makes this zebrafish model a unique tool for understanding mucosal host-pathogen interactions.

In vitro lymphoproliferative response and cytokine production in mice with experimental disseminated candidiasis

Objective(s):

Systemic candidiasis is an infection of Candida albicans (C. albicans) causing disseminated disease and sepsis, invariably when host defenses are compromised. We investigated the histopathological changes as well as the lymphoproliferative responses and cytokine production of splenic cells after stimulation with Concanavalin A (Con A) and Pokeweed mitogen (PWM) in mice with disseminated candidiasis.

Materials and Methods:

Lymphoproliferative responses were stimulated in vitro with Con A (1 µg/ml) and PWM (1 µg/ml) mitogens in Roswell Park Memorial Institute (RPMI) 1640 media, and the production of interferon (IFN)-γ and interleukin-4 (IL-4) in the supernatants was measured by enzyme-linked immunosorbent assay (ELISA).

Results:

The results revealed that C. albicans organisms multiplied to a greater extent in the kidneys than in the liver and spleen of infected mice. The most predominant forms of C. albicans in different parts of the kidneys were yeast mixed with hyphal forms. Infected mice had a significantly increased proliferative response when splenocytes were stimulated with PWM (2.0±0.16) and Con A (1.9±0.19) (P<0.05). PWM and Con A-stimulated production of IFN-γ significantly tended to be higher in infected mice (PWM: 68.4±14.0 pg/ml; Con A: 53.7±17.3 pg/ml) when compared to controls (P<0.05). Stimulation with PWM and Con A showed no differences in IL-4 production between infected mice and controls.

Conclusion:

These findings demonstrated a significant increase in both cell proliferation and IFN-γ secretion in supernatants of PWM and Con A- stimulated splenocyte cultures obtained from mice with disseminated candidiasis.

The Candida albicans Pho4 Transcription Factor Mediates Susceptibility to Stress and Influences Fitness in a Mouse Commensalism Model

The Pho4 transcription factor is required for growth under low environmental phosphate concentrations in Saccharomyces cerevisiae. A characterization of Candida albicans pho4 mutants revealed that these cells are more susceptible to both osmotic and oxidative stress and that this effect is diminished in the presence of 5% CO₂ or anaerobiosis, reflecting the relevance of oxygen metabolism in the Pho4-mediated response. A pho4 mutant was as virulent as wild type strain when assayed in the Galleria mellonella infection model and was even more resistant to murine macrophages in ex vivo killing assays. The lack of Pho4 neither impairs the ability to colonize the murine gut nor alters the localization in the gastrointestinal tract. However, we found that Pho4 influenced the colonization of C. albicans in the mouse gut in competition assays; pho4 mutants were unable to attain high colonization levels when inoculated simultaneously with an isogenic wild type strain. Moreover, pho4 mutants displayed a reduced adherence to the intestinal mucosa in a competitive ex vivo assays with wild type cells. In vitro competitive assays also revealed defects in fitness for this mutant compared to the wild type strain. Thus, Pho4, a transcription factor involved in phosphate metabolism, is required for adaptation to stress and fitness in C. albicans.

Roles of RPS41 in Biofilm Formation, Virulence, and Hydrogen Peroxide Sensitivity in Candida albicans

In eukaryotes, loss of cytoplasmic ribosomal proteins (RPs) results in a reduced growth rate and other phenotypic defects. The ability to transition from a unicellular budding yeast to a filamentous form is very important for biofilm formation and virulence in Candida albicans. Our recent study found that loss of the RPS41 (C2_10620W_A) gene but not its paralog RPS42 (C1_01640W_A) resulted in altered growth and filamentation changes in C. albicans, so we hypothesized that the RPS41 gene should play important roles in virulence and biofilm formation in this pathogen. We found that both virulence and the ability to form biofilms were defective due to deletion of the RPS41 gene. We also found that loss of the RPS41 gene increased sensitivity to hydrogen peroxide, and that hydrogen peroxide induced the expression of the RPS41 gene in a wild-type strain. These results suggested that the RPS41 gene plays important roles in C. albicans biofilm formation, virulence, and susceptibility to hydrogen peroxide.

The role of pattern recognition receptors in the innate recognition of Candida albicans

Candida albicans is both a commensal microorganism in healthy individuals and a major fungal pathogen causing high mortality in immunocompromised patients. Yeast-hypha morphological transition is a well known virulence trait of C. albicans. Host innate immunity to C. albicans critically requires pattern recognition receptors (PRRs). In this review, we summarize the PRRs involved in the recognition of C. albicans in epithelial cells, endothelial cells, and phagocytic cells separately. We figure out the differential recognition of yeasts and hyphae, the findings on PRR-deficient mice, and the discoveries on human PRR-related single nucleotide polymorphisms (SNPs).

Description of the interaction between Candida albicans and macrophages by mixed and quantitative proteome analysis without isolation

Candida albicans is an opportunistic pathogen that causes fatal diseases in immunocompromised hosts. Host resistance against C. albicans relies on ingestion of the pathogen by macrophages. Analysis of the escaping behavior of C. albicans from macrophages is required to understand the onset of systemic candidiasis. In this study, native interactions of C. albicans with macrophages were investigated by proteome analysis using high efficiency of long monolithic silica capillary column. Using this system, we developed a method of “mixed and quantitative proteome analysis” in which C. albicans and macrophages were simultaneously analyzed by nanoLC–MS/MS without the need to isolate the two individual living cells. Two hundred twenty-seven proteins from C. albicans and five proteins from macrophages were identified as candidate interaction-specific molecules. C. albicans seemed to produce glucose through a β-oxidation pathway, a glyoxylate cycle, and gluconeogenesis for escape from macrophages. Up-regulation of stress-related and candidate pathogenic proteins in C. albicans indicated how C. albicans endured the harsh environment inside the macrophages. Down-regulation of apoptosis-associated protein NOA1- and chaperone HSPA1A-syntheses in macrophage indicated that C. albicans was able to escape from macrophages in part by suppressing the production of these macrophage proteins.

Characterization of proliferating cell nuclear antigen (PCNA) from pathogenic yeast Candida albicans and its functional analyses in S. cerevisiae

Background

Proliferating cell nuclear antigen (PCNA/POL30) an essential protein forms a homotrimeric ring encircling dsDNA and serves as a molecular scaffold to recruit various factors during DNA replication, repair and recombination. According to Candida Genome Database (CGD), orf19.4616 sequence is predicted to encode C. albicans PCNA (CaPCNA) that has not been characterized yet.

Results

Molecular modeling studies of orf19.4616 using S. cerevisiae PCNA sequence (ScPCNA) as a template, and its subsequent biochemical characterizations suggest that like other eukaryotic PCNAs, orf19.4616 encodes for a conventional homotrimeric sliding clamp. Further we showed by surface plasmon resonance that CaPCNA physically interacted with yeast DNA polymerase eta. Plasmid segregation in genomic knock out yeast strains showed that CaPCNA but not its G178S mutant complemented for cell survival. Unexpectedly, heterologous expression of CaPCNA in S. cerevisiae exhibited slow growth phenotypes, sensitivity to cold and elevated temperatures; and showed enhanced sensitivity to hydroxyurea and various DNA damaging agents in comparison to strain bearing ScPCNA. Interestingly, wild type strains of C. albicans showed remarkable tolerance to DNA damaging agents when compared with similarly treated yeast cells.

Conclusions

Despite structural and physiochemical similarities; we have demonstrated that there are distinct functional differences between ScPCNA and CaPCNA, and probably the ways both the strains maintain their genomic stability. We propose that the growth of pathogenic C. albicans which is evolved to tolerate DNA damages could be controlled effectively by targeting this unique fungal PCNA.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0582-6) contains supplementary material, which is available to authorized users.

Nanoparticles Containing Curcumin Useful for Suppressing Macrophages In Vivo in Mice

To explore a novel method using liposomes to suppress macrophages, we screened food constituents through cell culture assays. Curcumin was one of the strongest compounds exhibiting suppressive effects on macrophages. We subsequently tried various methods to prepare liposomal curcumin, and eventually succeeded in preparing liposomes with sufficient amounts of curcumin to suppress macrophages by incorporating a complex of curcumin and bovine serum albumin. The diameter of the resultant nanoparticles, the liposomes containing curcumin, ranged from 60 to 100 nm. Flow cytometric analyses revealed that after intraperitoneal administration of the liposomes containing curcumin into mice, these were incorporated mainly by macrophages positive for F4/80, CD36, and CD11b antigens. Peritoneal cells prepared from mice injected in vivo with the liposomes containing curcumin apparently decreased interleukin-6-producing activities. Major changes in body weight and survival rates in the mice were not observed after administrating the liposomes containing curcumin. These results indicate that the liposomes containing curcumin are safe and useful for the selective suppression of macrophages in vivo in mice.

Fungal Pathogens: Survival and Replication within Macrophages

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

Stress adaptation in a pathogenic fungus

Candida albicans is a major fungal pathogen of humans. This yeast is carried by many individuals as a harmless commensal, but when immune defences are perturbed it causes mucosal infections (thrush). Additionally, when the immune system becomes severely compromised, C. albicans often causes life-threatening systemic infections. A battery of virulence factors and fitness attributes promote the pathogenicity of C. albicans. Fitness attributes include robust responses to local environmental stresses, the inactivation of which attenuates virulence. Stress signalling pathways in C. albicans include evolutionarily conserved modules. However, there has been rewiring of some stress regulatory circuitry such that the roles of a number of regulators in C. albicans have diverged relative to the benign model yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. This reflects the specific evolution of C. albicans as an opportunistic pathogen obligately associated with warm-blooded animals, compared with other yeasts that are found across diverse environmental niches. Our understanding of C. albicans stress signalling is based primarily on the in vitro responses of glucose-grown cells to individual stresses. However, in vivo this pathogen occupies complex and dynamic host niches characterised by alternative carbon sources and simultaneous exposure to combinations of stresses (rather than individual stresses). It has become apparent that changes in carbon source strongly influence stress resistance, and that some combinatorial stresses exert non-additive effects upon C. albicans. These effects, which are relevant to fungus–host interactions during disease progression, are mediated by multiple mechanisms that include signalling and chemical crosstalk, stress pathway interference and a biological transistor.

Ybp1 and Gpx3 signaling in Candida albicans govern hydrogen peroxide-induced oxidation of the Cap1 transcription factor and macrophage escape

AIMS

As Candida albicans is the major fungal pathogen of humans, there is an urgent need to understand how this pathogen evades toxic reactive oxygen species (ROS) generated by the host immune system. A key regulator of antioxidant gene expression, and thus ROS resistance, in C. albicans is the AP-1-like transcription factor Cap1. Despite this, little is known regarding the intracellular signaling mechanisms that underlie the oxidation and activation of Cap1. Therefore, the aims of this study were; (i) to identify the regulatory proteins that govern Cap1 oxidation, and (ii) to investigate the importance of Cap1 oxidation in C. albicans pathogenesis.

RESULTS

In response to hydrogen peroxide (H2O2), but not glutathione-depleting/modifying oxidants, Cap1 oxidation, nuclear accumulation, phosphorylation, and Cap1-dependent gene expression, is mediated by a glutathione peroxidase-like enzyme, which we name Gpx3, and an orthologue of the Saccharomyces cerevisiae Yap1 binding protein, Ybp1. In addition, Ybp1 also functions to stabilise Cap1 and this novel function is conserved in S. cerevisiae. C. albicans cells lacking Cap1, Ybp1, or Gpx3, are unable to filament and thus, escape from murine macrophages after phagocytosis, and also display defective virulence in the Galleria mellonella infection model.

INNOVATION

Ybp1 is required to promote the stability of fungal AP-1-like transcription factors, and Ybp1 and Gpx3 mediated Cap1-dependent oxidative stress responses are essential for the effective killing of macrophages by C. albicans.

CONCLUSION

Activation of Cap1, specifically by H2O2, is a prerequisite for the subsequent filamentation and escape of this fungal pathogen from the macrophage.

The role of Candida albicans AP-1 protein against host derived ROS in in vivo models of infection

Candida albicans is a major fungal pathogen of humans, causing mucosal infections that are difficult to eliminate and systemic infections that are often lethal primarily due to defects in the host's innate status. Here we demonstrate the utility of Caenorhabditis elegans, a model host to study innate immunity, by exploring the role of reactive oxygen species (ROS) as a critical innate response against C. albicans infections. Much like a human host, the nematode's innate immune response is activated to produce ROS in response to fungal infection. We use the C. albicans cap1 mutant, which is susceptible to ROS, as a tool to dissect this physiological innate immune response and show that cap1 mutants fail to cause disease and death, except in bli-3 mutant worms that are unable to produce ROS because of a defective NADPH oxidase. We further validate the ROS-mediated host defense mechanism in mammalian phagocytes by demonstrating that chemical inhibition of the NADPH oxidase in cultured macrophages enables the otherwise susceptible cap1 mutant to resists ROS-mediated phagolysis. Loss of CAP1 confers minimal attenuation of virulence in a disseminated mouse model, suggesting that CAP1-independent mechanisms contribute to pathogen survival in vivo. Our findings underscore a central theme in the process of infection-the intricate balance between the virulence strategies employed by C. albicans and the host's innate immune system and validates C. elegans as a simple model host to dissect this balance at the molecular level.

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

The interaction of Candida albicans with macrophages induces the production of interleukin 1β (IL-1β) through inflammasome activation in a process that is required for host survival. C. albicans hypha formation has been linked to IL-1β production, but the question of whether hyphae are sufficient to trigger IL-1β production has not been examined directly. To address this question, a C. albicans library of 165 transcription factor deletion mutants was screened for strains with altered IL-1β production by lipopolysaccharide (LPS)-primed J774 cells, a murine macrophage-like cell line. Eight mutants with decreased and two mutants with increased IL-1β secretion were identified. In addition, 12 mutants with previously identified morphology deficits were found to induce IL-1β secretion to levels similar to those of the wild type. Examination of the morphology of both low and normal IL-1β-inducing mutants in macrophages revealed that two mutants (upc2Δ/upc2Δ and ahr1Δ/Δ mutants) were indistinguishable from the wild type with respect to morphology yet induced low levels of IL-1β; conversely, the ndt80Δ/Δ mutant was deficient for hypha formation but induced levels of IL-1β similar to those of the wild type. Transcription factor mutants deficient for IL-1β secretion also caused markedly lower levels of macrophage lysis. Similarly, the ability of a mutant to cause macrophage lysis was independent of its ability to form hyphae. Taken together, our observations indicate that the physical formation of hyphae is not sufficient to trigger IL-1β secretion or macrophage lysis and suggest that other mechanisms, such as pyroptosis, a caspase-1-dependent response to intracellular pathogens, may play a role in the interaction of macrophages with C. albicans.

The ability of Candida albicans to transition from yeast to filamentous cells plays an important and complex role in pathogenesis. Recent results from a number of investigators indicate that the host responds to yeast and hyphal C. albicans differently. For example, a C. albicans mutant unable to form hyphae also fails to induce interleukin 1β (IL-1β) secretion from macrophages. We have identified C. albicans transcription factor mutants that have decreased IL-1β secretion but retain the ability to form hyphae in response to macrophages. In addition, these mutants cause significantly less macrophage lysis. These observations indicate that the physical presence of the hyphal structure in the macrophage is not sufficient to trigger IL-1β secretion nor does it cause physical lysis of the cell. Our data indicate that characteristics of hyphae separate from its physical morphology are responsible for triggering the release of IL-1β release and causing macrophage lysis. Since these observations are inconsistent with some current models, alternative mechanisms for the interaction of C. albicans with macrophages must be considered.

Functional genomic analysis of Candida glabrata-macrophage interaction: role of chromatin remodeling in virulence

Fungal septicemia is an increasingly common complication of immunocompromised patients worldwide. Candida species are the leading cause of invasive mycoses with Candida glabrata being the second most frequently isolated Candida species from Intensive Care Unit patients. Despite its clinical importance, very little is known about the mechanisms that C. glabrata employs to survive the antimicrobial and immune response of the mammalian host. Here, to decipher the interaction of C. glabrata with the host immune cells, we have screened a library of 18,350 C. glabrata Tn7 insertion mutants for reduced survival in human THP-1 macrophages via signature-tagged mutagenesis approach. A total of 56 genes, belonging to diverse biological processes including chromatin organization and golgi vesicle transport, were identified which are required for survival and/or replication of C. glabrata in macrophages. We report for the first time that C. glabrata wild-type cells respond to the intracellular milieu of macrophage by modifying their chromatin structure and chromatin resistance to micrococcal nuclease digestion, altered epigenetic signature, decreased protein acetylation and increased cellular lysine deacetylase activity are the hall-marks of macrophage-internalized C. glabrata cells. Consistent with this, mutants defective in chromatin organization (Cgrsc3-aΔ, Cgrsc3-bΔ, Cgrsc3-aΔbΔ, Cgrtt109Δ) and DNA damage repair (Cgrtt107Δ, Cgsgs1Δ) showed attenuated virulence in the murine model of disseminated candidiasis. Further, genome-wide transcriptional profiling analysis on THP-1 macrophage-internalized yeasts revealed deregulation of energy metabolism in Cgrsc3-aΔ and Cgrtt109Δ mutants. Collectively, our findings establish chromatin remodeling as a central regulator of survival strategies which facilitates a reprogramming of cellular energy metabolism in macrophage-internalized C. glabrata cells and provide protection against DNA damage.

Hospital-acquired fungal infections pose a colossal health and economic challenge. Candida species are the leading cause of disseminated fungal infections and rank fourth among the most common nosocomial pathogens. C. glabrata, an emerging opportunistic fungal pathogen, is the second most frequently isolated Candida species after C. albicans from Intensive Care Unit patients world-wide. Limited information is available on the unique strategies that C. glabrata employs to evade and replicate in host phagocytic cells since it lacks the key virulence traits of C. albicans including hyphal formation and secreted proteolytic activity. In the current study, we have identified a total of 56 genes, via a functional genomics approach, which are required for survival and/or replication of C. glabrata in human macrophages. Our data demonstrates an essential role for chromatin remodeling in the intracellular survival of C. glabrata with ingested C. glabrata cells displaying transcriptionally active chromatin in early-phase, compact, closed chromatin in mid-stage, and open chromatin in the late-stage of macrophage internalization. Our findings identify novel fungal virulence determinants and potentially implicate epigenetic changes in the metabolic adaptation of fungal cells to the nutrient-poor host environment and the survival against oxidative stress-induced DNA damage.

Chromatin-mediated Candida albicans virulence

Candida albicans is the most prevalent human fungal pathogen. To successfully propagate an infection, this organism relies on the ability to change morphology, express virulence-associated genes and resist DNA damage caused by the host immune system. Many of these events involve chromatin alterations that are crucial for virulence. This review will focus on the studies that have been conducted on how chromatin function affects pathogenicity of C. albicans and other fungi. This article is part of a Special Issue entitled: Histone chaperones and Chromatin assembly.

Scavenger receptors and β-glucan receptors participate in the recognition of yeasts by murine macrophages

Objectives

Numerous receptors have been implicated in recognition of pathogenic fungi by macrophages, including the β-glucan receptor dectin-1. The role of scavenger receptors (SRs) in anti-fungal immunity is not well characterized.

Methods

We studied uptake of unopsonized Saccharomycetes cerevisiae (zymosan) and live Candida albicans yeasts as well as zymosan-stimulated H₂O₂ production in J774 macrophage-like cells and peritoneal exudate macrophages (PEMs). The role of different receptors was assessed with the use of competitive ligands, transfected cells and receptor-deficient macrophages.

Results

The uptake of zymosan by untreated J774 cells was mediated approximately half by SRs and half by a β-glucan receptor which was distinct from dectin-1 and not linked to stimulation of H₂O₂ production. Ligands of β-glucan receptors and of SRs also inhibited uptake of C. albicans by macrophages (J774 cells and PEMs). In macrophages pretreated with a CpG motif-containing oligodeoxynucleotide (CpG-ODN) the relative contribution of SRs to yeast uptake increased and that of β-glucan receptors decreased. Whereas the class A SR MARCO participated in the uptake of both zymosan and C. albicans by CpG-ODN-pretreated, but not untreated macrophages, the related receptor SR-A/CD204 was involved in the uptake of zymosan, but not of C. albicans. The reduction of zymosan-stimulated H₂O₂ production observed in DS-pretreated J774 cells and in class A SRs-deficient PEMs suggest that class A SRs mediate part of this process.

Conclusions

Our results revealed that SRs belong to a redundant system of receptors for yeasts. Binding of yeasts to different receptors in resting versus CpG-ODN-pre-exposed macrophages may differentially affect polarization of adaptive immune responses.

Elevated cell wall chitin in Candida albicans confers echinocandin resistance in vivo

Candida albicans cells with increased cell wall chitin have reduced echinocandin susceptibility in vitro. The aim of this study was to investigate whether C. albicans cells with elevated chitin levels have reduced echinocandin susceptibility in vivo. BALB/c mice were infected with C. albicans cells with normal chitin levels and compared to mice infected with high-chitin cells. Caspofungin therapy was initiated at 24 h postinfection. Mice infected with chitin-normal cells were successfully treated with caspofungin, as indicated by reduced kidney fungal burdens, reduced weight loss, and decreased C. albicans density in kidney lesions. In contrast, mice infected with high-chitin C. albicans cells were less susceptible to caspofungin, as they had higher kidney fungal burdens and greater weight loss during early infection. Cells recovered from mouse kidneys at 24 h postinfection with high-chitin cells had 1.6-fold higher chitin levels than cells from mice infected with chitin-normal cells and maintained a significantly reduced susceptibility to caspofungin when tested in vitro. At 48 h postinfection, caspofungin treatment induced a further increase in chitin content of C. albicans cells harvested from kidneys compared to saline treatment. Some of the recovered clones had acquired, at a low frequency, a point mutation in FKS1 resulting in a S645Y amino acid substitution, a mutation known to confer echinocandin resistance. This occurred even in cells that had not been exposed to caspofungin. Our results suggest that the efficacy of caspofungin against C. albicans was reduced in vivo due to either elevation of chitin levels in the cell wall or acquisition of FKS1 point mutations.

Leukotrienes target F-actin/cofilin-1 to enhance alveolar macrophage anti-fungal activity

Candida albicans is the most common opportunistic fungal pathogen and causes local and systemic disease in immunocompromised patients. Alveolar macrophages (AMs) are pivotal for the clearance of C. albicans from the lung. Activated AMs secrete 5-lipoxygenase-derived leukotrienes (LTs), which in turn enhance phagocytosis and microbicidal activity against a diverse array of pathogens. Our aim was to investigate the role of LTB(4) and LTD(4) in AM antimicrobial functions against C. albicans and the signaling pathways involved. Pharmacologic and genetic inhibition of LT biosynthesis as well as receptor antagonism reduced phagocytosis of C. albicans when compared with untreated or WT controls. Conversely, exogenous LTs of both classes augmented base-line C. albicans phagocytosis by AMs. Although LTB(4) enhanced mainly mannose receptor-dependent fungal ingestion, LTD(4) enhanced mainly dectin-1 receptor-mediated phagocytosis. LT enhancement of yeast ingestion was dependent on protein kinase C-δ (PKCδ) and PI3K but not PKCα and MAPK activation. Both LTs reduced activation of cofilin-1, whereas they enhanced total cellular F-actin; however, LTB(4) accomplished this through the activation of LIM kinases (LIMKs) 1 and 2, whereas LTD(4) did so exclusively via LIMK-2. Finally, both exogenous LTB(4) and LTD(4) enhanced AM fungicidal activity in an NADPH oxidase-dependent manner. Our data identify LTB(4) and LTD(4) as key mediators of innate immunity against C. albicans, which act by both distinct and conserved signaling mechanisms to enhance multiple antimicrobial functions of AMs.

Innate antifungal immunity: the key role of phagocytes

Fungal diseases have emerged as significant causes of morbidity and mortality, particularly in immune-compromised individuals, prompting greater interest in understanding the mechanisms of host resistance to these pathogens. Consequently, the past few decades have seen a tremendous increase in our knowledge of the innate and adaptive components underlying the protective (and nonprotective) mechanisms of antifungal immunity. What has emerged from these studies is that phagocytic cells are essential for protection and that defects in these cells compromise the host's ability to resist fungal infection. This review covers the functions of phagocytes in innate antifungal immunity, along with selected examples of the strategies that are used by fungal pathogens to subvert these defenses.

Organ-specific innate immune responses in a mouse model of invasive candidiasis

In a fatal mouse model of invasive candidiasis (IC), fungal burden changes with variable dynamics in the kidney, brain, spleen, and liver and declines in all organs except for the kidney, which inexorably loses function. Since leukocytes are required to control Candida, we hypothesized that differential leukocyte infiltration determines organ-specific outcome of the infection. We defined leukocyte accumulation in the blood, kidney, brain, spleen, and liver after infection using fluorescent-activated cell sorting (FACS) and immunohistochemistry. Accumulation of Ly6c(int)CD11b(+) neutrophils predominated in all organs except the brain, where CD45(int)CD11b(+)CD11c(-) microglia were the major leukocytes detected, surrounding foci of invading Candida. Significantly more neutrophils accumulated in the spleen and liver than in the kidney during the first 24 h after infection, when neutrophil presence is critical for Candida control. Conversely, at later time points only the kidney continued to accumulate neutrophils, associated with immunopathology and organ failure. The distribution of neutrophils was completely different in each organ, with large abscesses exclusively forming in the kidney. Candida filamentation, an essential virulence factor, was seen in the kidney but not in the spleen or liver. IC induced Ly6c(hi)CD11b(+) inflammatory monocyte and NK1.1(+) cell expansion in the blood and all organs tested, and MHCII(+)F4/80(+)CD11c(-) macrophage accumulation, mainly in the spleen and liver. This study is the first detailed analysis of leukocyte subsets accumulating in different target organs during IC. The results delineate immune responses to the same pathogen that are highly idiosyncratic for each organ tested. The work provides novel insights into the balance between effective host defense and immunopathology in IC.

Thioredoxin regulates multiple hydrogen peroxide-induced signaling pathways in Candida albicans

The ability of the major systemic fungal pathogen of humans, Candida albicans, to sense and respond to reactive oxygen species (ROS), such as H(2)O(2) generated by the host immune system, is required for survival in the host. However, the intracellular signaling mechanisms underlying such responses are poorly understood. Here, we show that thioredoxin (Trx1), in addition to its antioxidant activity, plays a central role in coordinating the response of C. albicans to ROS by regulating multiple pathways. In particular, Trx1 function is important for H(2)O(2)-induced phosphorylation of the Hog1 stress-activated protein kinase and to reverse H(2)O(2)-induced oxidation and activation of the AP-1 like transcription factor Cap1. Furthermore, Trx1 regulates H(2)O(2)-induced hyperpolarized bud growth in a mechanism that involves activation of the Rad53 checkpoint kinase. Consistent with its key roles in responses to ROS, cells lacking Trx1 displayed significantly attenuated virulence in a murine model of C. albicans systemic infection. Collectively, our data indicate that Trx1 has a multifaceted role in H(2)O(2) signaling and promotes C. albicans survival in the host.

Limited role of secreted aspartyl proteinases Sap1 to Sap6 in Candida albicans virulence and host immune response in murine hematogenously disseminated candidiasis

Candida albicans secreted aspartyl proteinases (Saps) are considered virulence-associated factors. Several members of the Sap family were claimed to play a significant role in the progression of candidiasis established by the hematogenous route. This assumption was based on the observed attenuated virulence of sap-null mutant strains. However, the exclusive contribution of SAP genes to their attenuated phenotype was not unequivocally confirmed, as the Ura status of these mutant strains could also have contributed to the attenuation. In this study, we have reassessed the importance of SAP1 to SAP6 in a murine model of hematogenously disseminated candidiasis using sap-null mutant strains not affected in their URA3 gene expression and compared their virulence phenotypes with those of Ura-blaster sap mutants. The median survival time of BALB/c mice intravenously infected with a mutant strain lacking SAP1 to SAP3 was equivalent to that of mice infected with wild-type strain SC5314, while those infected with mutant strains lacking SAP5 showed slightly extended survival times. Nevertheless, no differences could be observed between the wild type and a Δsap456 mutant in their abilities to invade mouse kidneys. Likewise, a deficiency in SAP4 to SAP6 had no noticeable impact on the immune response elicited in the spleens and kidneys of C. albicans-infected mice. These results contrast with the behavior of equivalent Ura-blaster mutants, which presented a significant reduction in virulence. Our results suggest that Sap1 to Sap6 do not play a significant role in C. albicans virulence in a murine model of hematogenously disseminated candidiasis and that, in this model, Sap1 to Sap3 are not necessary for successful C. albicans infection.

Histone acetyltransferase Rtt109 is required for Candida albicans pathogenesis

Candida albicans is a ubiquitous opportunistic pathogen that is the most prevalent cause of hospital-acquired fungal infections. In mammalian hosts, C. albicans is engulfed by phagocytes that attack the pathogen with DNA-damaging reactive oxygen species (ROS). Acetylation of histone H3 lysine 56 (H3K56) by the fungal-specific histone acetyltransferase Rtt109 is important for yeast model organisms to survive DNA damage and maintain genome integrity. To assess the importance of Rtt109 for C. albicans pathogenicity, we deleted the predicted homolog of Rtt109 in the clinical C. albicans isolate, SC5314. C. albicans rtt109(-/-) mutant cells lack acetylated H3K56 (H3K56ac) and are hypersensitive to genotoxic agents. Additionally, rtt109(-/-) mutant cells constitutively display increased H2A S129 phosphorylation and elevated DNA repair gene expression, consistent with endogenous DNA damage. Importantly, C. albicans rtt109(-/-) cells are significantly less pathogenic in mice and more susceptible to killing by macrophages in vitro than are wild-type cells. Via pharmacological inhibition of the host NADPH oxidase enzyme, we show that the increased sensitivity of rtt109(-/-) cells to macrophages depends on the host's ability to generate ROS, providing a mechanistic link between the drug sensitivity, gene expression, and pathogenesis phenotypes. We conclude that Rtt109 is particularly important for fungal pathogenicity, suggesting a unique target for therapeutic antifungal compounds.

Glucose promotes stress resistance in the fungal pathogen Candida albicans

Metabolic adaptation, and in particular the modulation of carbon assimilatory pathways during disease progression, is thought to contribute to the pathogenicity of Candida albicans. Therefore, we have examined the global impact of glucose upon the C. albicans transcriptome, testing the sensitivity of this pathogen to wide-ranging glucose levels (0.01, 0.1, and 1.0%). We show that, like Saccharomyces cerevisiae, C. albicans is exquisitely sensitive to glucose, regulating central metabolic genes even in response to 0.01% glucose. This indicates that glucose concentrations in the bloodstream (approximate range 0.05-0.1%) have a significant impact upon C. albicans gene regulation. However, in contrast to S. cerevisiae where glucose down-regulates stress responses, some stress genes were induced by glucose in C. albicans. This was reflected in elevated resistance to oxidative and cationic stresses and resistance to an azole antifungal agent. Cap1 and Hog1 probably mediate glucose-enhanced resistance to oxidative stress, but neither is essential for this effect. However, Hog1 is phosphorylated in response to glucose and is essential for glucose-enhanced resistance to cationic stress. The data suggest that, upon entering the bloodstream, C. albicans cells respond to glucose increasing their resistance to the oxidative and cationic stresses central to the armory of immunoprotective phagocytic cells.

Nitrosative and oxidative stress responses in fungal pathogenicity

Fungal pathogenicity has arisen in polyphyletic manner during evolution, yielding fungal pathogens with diverse infection strategies and with differing degrees of evolutionary adaptation to their human host. Not surprisingly, these fungal pathogens display differing degrees of resistance to the reactive oxygen and nitrogen species used by human cells to counteract infection. Furthermore, whilst evolutionarily conserved regulators, such as Hog1, are central to such stress responses in many fungal pathogens, species-specific differences in their roles and regulation abound. In contrast, there is a high degree of commonality in the cellular responses to reactive oxygen and nitrogen species evoked in evolutionarily divergent fungal pathogens.

Macrophage-mediated responses to Candida albicans in mice expressing the human immunodeficiency virus type 1 transgene

The critical impairments of innate and adaptive immunity that cause susceptibility to mucosal candidiasis in human immunodeficiency virus (HIV) infection have not been fully determined. We therefore conducted an analysis of macrophage-mediated responses to Candida albicans in transgenic (Tg) mice expressing Nef, Env, and Rev of HIV type 1 (HIV-1) in CD4(+) T cells, dendritic cells, and macrophages and developing an AIDS-like disease (CD4C/HIV(MutA) Tg mice). Macrophages were successfully recruited to the oral and gastric mucosae of these Tg mice in response to chronic carriage of C. albicans and displayed polarization toward an alternatively activated phenotype. Functionally, peritoneal macrophages from uninfected Tg mice exhibited increased phagocytosis of C. albicans and enhanced production of interleukin 6 and monocyte chemoattractant protein 1, demonstrating that the HIV-1 transgene independently activates selected macrophage functions. Production of H(2)O(2) by macrophages from Tg mice primed with gamma interferon and treated with phorbol 12-myristate 13-acetate or C. albicans was moderately reduced, but expression of the HIV-1 transgene did not alter production of nitric oxide or reduce killing of C. albicans. A knockout of the inducible nitric oxide synthase (NOS2) gene in these Tg mice did not augment oral or gastrointestinal burdens during chronic carriage of C. albicans or cause systemic dissemination, likely due to a redundancy provided by partially preserved production of H(2)O(2) and oxygen-independent candidacidal mechanisms. Thus, the macrophage response to C. albicans is largely preserved in these Tg mice, and no functional macrophage defect appears to primarily determine the susceptibility to mucosal candidiasis.

Msb2 signaling mucin controls activation of Cek1 mitogen-activated protein kinase in Candida albicans

We have characterized the role that the Msb2 protein plays in the fungal pathogen Candida albicans by the use of mutants defective in the putative upstream components of the HOG pathway. Msb2, in cooperation with Sho1, controls the activation of the Cek1 mitogen-activated protein kinase under conditions that damage the cell wall, thus defining Msb2 as a signaling element of this pathway in the fungus. msb2 mutants display altered sensitivity to Congo red, caspofungin, zymolyase, or tunicamycin, indicating that this protein is involved in cell wall biogenesis. Msb2 (as well as Sho1 and Hst7) is involved in the transmission of the signal toward Cek1 mediated by the Cdc42 GTPase, as revealed by the use of activated alleles (Cdc42(G12V)) of this protein. msb2 mutants have a stronger defective invasion phenotype than sho1 mutants when tested on certain solid media that use mannitol or sucrose as a carbon source or under hypoxia. Interestingly, Msb2 contributes to growth under conditions of high osmolarity when both branches of the HOG pathway are altered, as triple ssk1 msb2 sho1 mutants (but not any single or double mutant) are osmosensitive. However, this phenomenon is independent of the presence of Hog1, as Hog1 phosphorylation, Hog1 translocation to the nucleus, and glycerol accumulation are not affected in this mutant following an osmotic shock. These results reveal essential functions in morphogenesis, invasion, cell wall biogenesis, and growth under conditions of high osmolarity for Msb2 in C. albicans and suggest the divergence and specialization of this signaling pathway in filamentous fungi.

Group V secretory phospholipase A2 modulates phagosome maturation and regulates the innate immune response against Candida albicans

Phospholipase A(2) (PLA(2)) hydrolyzes the sn-2 position of cell membrane phospholipids to release fatty acids and lysophospholipids. We have previously reported that group V secretory PLA(2) (sPLA(2)) translocates from the Golgi and recycling endosomes of mouse peritoneal macrophages to newly formed phagosomes and regulates the phagocytosis of zymosan, suggesting a role in innate immunity. Here we report that in macrophages lacking group V sPLA(2), phagosome maturation was reduced 50-60% at early time points while the binding of zymosan was unimpaired. The ability of group V sPLA(2) to regulate phagocytosis extended to phagocytosis of IgG- and complement-opsonized sheep RBC. Moreover, macrophages lacking group V sPLA(2) had delays in phagocytosis, phagosome maturation, and killing of Candida albicans. Cytokine production and eicosanoid generation were not impaired by the lack of group V sPLA(2). Furthermore, in a model of systemic candidiasis, mice lacking group V sPLA(2) had an increased fungal burden in the kidney, liver, and spleen at day 7 postinfection and increased mortality. Thus, group V sPLA(2) regulates phagocytosis through major phagocytic receptors and contributes to the innate immune response against C. albicans by regulating phagocytosis and killing through a mechanism that is likely dependent on phagolysosome fusion.

Stage-specific sampling by pattern recognition receptors during Candida albicans phagocytosis

Candida albicans is a medically important pathogen, and recognition by innate immune cells is critical for its clearance. Although a number of pattern recognition receptors have been shown to be involved in recognition and phagocytosis of this fungus, the relative role of these receptors has not been formally examined. In this paper, we have investigated the contribution of the mannose receptor, Dectin-1, and complement receptor 3; and we have demonstrated that Dectin-1 is the main non-opsonic receptor involved in fungal uptake. However, both Dectin-1 and complement receptor 3 were found to accumulate at the site of uptake, while mannose receptor accumulated on C. albicans phagosomes at later stages. These results suggest a potential role for MR in phagosome sampling; and, accordingly, MR deficiency led to a reduction in TNF-α and MCP-1 production in response to C. albicans uptake. Our data suggest that pattern recognition receptors sample the fungal phagosome in a sequential fashion.

Infection by Candida albicans has increased as a result of immunosuppression associated with AIDS and organ transplantation. We assessed the role of three pattern recognition receptors, namely Dectin-1 (a beta glucan receptor), the type 3 complement receptor (CR3), and the mannose receptor, in mediating uptake of this fungus. These receptors are known to recognize structures on the C. albicans cell wall, but their exact contribution to binding and uptake is still unclear. We show that only Dectin-1 plays a major role in binding and uptake of C. albicans. Furthermore, we are the first to find that these receptors sample the internalized particle in a sequential manner; intracellular mannose receptor is recruited later and is involved in secretion of immune modulators. These findings provide a better understanding of the innate immune mechanisms involved in protection against this medically important fungal pathogen.

Live Candida albicans suppresses production of reactive oxygen species in phagocytes

Production of reactive oxygen species (ROS) is an important aspect of phagocyte-mediated host responses. Since phagocytes play a crucial role in the host response to Candida albicans, we examined the ability of Candida to modulate phagocyte ROS production. ROS production was measured in the murine macrophage cell line J774 and in primary phagocytes using luminol-enhanced chemiluminescence. J774 cells, murine polymorphonuclear leukocytes (PMN), human monocytes, and human PMN treated with live C. albicans produced significantly less ROS than phagocytes treated with heat-killed C. albicans. Live C. albicans also suppressed ROS production in murine bone marrow-derived macrophages from C57BL/6 mice, but not from BALB/c mice. Live C. albicans also suppressed ROS in response to external stimuli. C. albicans and Candida glabrata suppressed ROS production by phagocytes, whereas Saccharomyces cerevisiae stimulated ROS production. The cell wall is the initial point of contact between Candida and phagocytes, but isolated cell walls from both heat-killed and live C. albicans stimulated ROS production. Heat-killed C. albicans has increased surface exposure of 1,3-beta-glucan, a cell wall component that can stimulate phagocytes. To determine whether surface 1,3-beta-glucan exposure accounted for the difference in ROS production, live C. albicans cells were treated with a sublethal dose of caspofungin to increase surface 1,3-beta-glucan exposure. Caspofungin-treated C. albicans was fully able to suppress ROS production, indicating that suppression of ROS overrides stimulatory signals from 1,3-beta-glucan. These studies indicate that live C. albicans actively suppresses ROS production in phagocytes in vitro, which may represent an important immune evasion mechanism.

The stress responsive and morphologically regulated hsp90 gene from Paracoccidioides brasiliensis is essential to cell viability

Background

Paracoccidioides brasiliensis is a dimorphic fungus that causes the most prevalent systemic mycosis in Latin America. The response to heat shock is involved in pathogenesis, as this pathogen switches from mycelium to yeast forms in a temperature dependent fashion that is essential to establish infection. HSP90 is a molecular chaperone that helps in the folding and stabilization of selected polypeptides. HSP90 family members have been shown to present important roles in fungi, especially in the pathogenic species, as an immunodominant antigen and also as a potential antifungal therapeutic target.

Results

In this work, we decided to further study the Pbhsp90 gene, its expression and role in cell viability because it plays important roles in fungal physiology and pathogenesis. Thus, we have sequenced a Pbhsp90 cDNA and shown that this gene is present on the genome as a single copy. We have also confirmed its preferential expression in the yeast phase and its overexpression during dimorphic transition and oxidative stress. Treatment of the yeast with the specific HSP90 inhibitors geldanamycin and radicicol inhibited growth at 2 and 10 μM, respectively.

Conclusion

The data confirm that the Pbhsp90 gene encodes a morphologically regulated and stress-responsive protein whose function is essential to cell viability of this pathogen. This work also enforces the potential of HSP90 as a target for antifungal therapies, since the use of HSP90 inhibitors is lethal to the P. brasiliensis yeast cells in a dose-responsive manner.

Genome-wide transcriptional profiling of the cyclic AMP-dependent signaling pathway during morphogenic transitions of Candida albicans

Candida albicans is an opportunistic human fungal pathogen that causes systemic candidiasis as well as superficial mucosal candidiasis. In response to the host environment, C. albicans transitions between yeast and hyphal forms. In particular, hyphal growth is important in facilitating adhesion and invasion of host tissues, concomitant with the expression of various hypha-specific virulence factors. In previous work, we showed that the cyclic AMP (cAMP) signaling pathway plays a crucial role in morphogenic transitions and virulence of C. albicans by studying genes encoding adenylate cyclase-associated protein (CAP1) and high-affinity phosphodiesterase (PDE2) (Y. S. Bahn, J. Staab, and P. Sundstrom, Mol. Microbiol. 50:391-409, 2003; and Y. S. Bahn and P. Sundstrom, J. Bacteriol. 183:3211-3223, 2001). However, little is known about the downstream targets of the cAMP signaling pathway that are responsible for morphological transitions and the expression of virulence factors. Here, microarrays were probed with RNA from strains with hypoactive (cap1/cap1 null mutant), hyperactive (pde2/pde2 null mutant), and wild-type cAMP signaling pathways to provide insight into the molecular mechanisms of virulence that are regulated by cAMP and that are related to the morphogenesis of C. albicans. Genes controlling metabolic specialization, cell wall structure, ergosterol/lipid biosynthesis, and stress responses were modulated by cAMP during hypha formation. Phenotypic traits predicted to be regulated by cAMP from the profiling results correlated with the relative strengths of the mutants when tested for resistance to azoles and subjected to heat shock stress and oxidative/nitrosative stress. The results from this study provide important insights into the role of the cAMP signaling pathway not only in morphogenic transitions of C. albicans but also for adaptation to stress and for survival during host infections.

Environmental sensing and signal transduction pathways regulating morphopathogenic determinants of Candida albicans

Candida albicans is an opportunistic fungal pathogen that is found in the normal gastrointestinal flora of most healthy humans. However, under certain environmental conditions, it can become a life-threatening pathogen. The shift from commensal organism to pathogen is often correlated with the capacity to undergo morphogenesis. Indeed, under certain conditions, including growth at ambient temperature, the presence of serum or N-acetylglucosamine, neutral pH, and nutrient starvation, C. albicans can undergo reversible transitions from the yeast form to the mycelial form. This morphological plasticity reflects the interplay of various signal transduction pathways, either stimulating or repressing hyphal formation. In this review, we provide an overview of the different sensing and signaling pathways involved in the morphogenesis and pathogenesis of C. albicans. Where appropriate, we compare the analogous pathways/genes in Saccharomyces cerevisiae in an attempt to highlight the evolution of the different components of the two organisms. The downstream components of these pathways, some of which may be interesting antifungal targets, are also discussed.

Interaction of Candida albicans with adherent human peripheral blood mononuclear cells increases C. albicans biofilm formation and results in differential expression of pro- and anti-inflammatory cytokines

Monocytes and macrophages are the cell types most commonly associated with the innate immune response against Candida albicans infection. Interactions between the host immune system and Candida organisms have been investigated for planktonic Candida cells, but no studies have addressed these interactions in a biofilm environment. In this study, for the first time, we evaluated the ability of C. albicans to form biofilms in the presence or absence of adherent peripheral blood mononuclear cells (PBMCs; enriched for monocytes and macrophages by adherence). Our analyses using scanning electron and confocal scanning laser microscopy showed that the presence of PBMCs enhanced the ability of C. albicans to form biofilms and that the majority of PBMCs were localized to the basal and middle layers of the biofilm. In contrast to the interactions of PBMCs with planktonic C. albicans, where PBMCs phagocytose fungal cells, PBMCs did not appear to phagocytose fungal cells in biofilms. Furthermore, time-lapse laser microscopy revealed dynamic interactions between C. albicans and PBMCs in a biofilm. Additionally, we found that (i) only viable PBMCs influence Candida biofilm formation, (ii) cell surface components of PBMCs did not contribute to the enhancement of C. albicans biofilm, (iii) the biofilm-enhancing effect of PBMCs is mediated by a soluble factor released into the coculture medium of PBMCs with C. albicans, and (iv) supernatant collected from this coculture contained differential levels of pro- and anti-inflammatory cytokines. Our studies provide new insight into the interaction between Candida biofilm and host immune cells and demonstrate that immunocytes may influence the ability of C. albicans to form biofilms.