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

Dual-species transcriptional profiling during systemic candidiasis reveals organ-specific host-pathogen interactions

Candida albicans is a common cause of life-threatening fungal bloodstream infections. In the murine model of systemic candidiasis, the kidney is the primary target organ while the fungal load declines over time in liver and spleen. To better understand these organ-specific differences in host-pathogen interaction, we performed gene expression profiling of murine kidney, liver and spleen and determined the fungal transcriptome in liver and kidney. We observed a delayed transcriptional immune response accompanied by late induction of fungal stress response genes in the kidneys. In contrast, early upregulation of the proinflammatory response in the liver was associated with a fungal transcriptome resembling response to phagocytosis, suggesting that phagocytes contribute significantly to fungal control in the liver. Notably, C. albicans hypha-associated genes were upregulated in the absence of visible filamentation in the liver, indicating an uncoupling of gene expression and morphology and a morphology-independent effect by hypha-associated genes in this organ. Consistently, integration of host and pathogen transcriptional data in an inter-species gene regulatory network indicated connections of C. albicans cell wall remodelling and metabolism to the organ-specific immune responses.

Transcriptional Control of Drug Resistance, Virulence and Immune System Evasion in Pathogenic Fungi: A Cross-Species Comparison

Transcription factors are key players in the control of the activation or repression of gene expression programs in response to environmental stimuli. The study of regulatory networks taking place in fungal pathogens is a promising research topic that can help in the fight against these pathogens by targeting specific fungal pathways as a whole, instead of targeting more specific effectors of virulence or drug resistance. This review is focused on the analysis of regulatory networks playing a central role in the referred mechanisms in the human fungal pathogens Aspergillus fumigatus, Cryptococcus neoformans, Candida albicans, Candida glabrata, Candida parapsilosis, and Candida tropicalis. Current knowledge on the activity of the transcription factors characterized in each of these pathogenic fungal species will be addressed. Particular focus is given to their mechanisms of activation, regulatory targets and phenotypic outcome. The review further provides an evaluation on the conservation of transcriptional circuits among different fungal pathogens, highlighting the pathways that translate common or divergent traits among these species in what concerns their drug resistance, virulence and host immune evasion features. It becomes evident that the regulation of transcriptional networks is complex and presents significant variations among different fungal pathogens. Only the oxidative stress regulators Yap1 and Skn7 are conserved among all studied species; while some transcription factors, involved in nutrient homeostasis, pH adaptation, drug resistance and morphological switching are present in several, though not all species. Interestingly, in some cases not very homologous transcription factors display orthologous functions, whereas some homologous proteins have diverged in terms of their function in different species. A few cases of species specific transcription factors are also observed.

Pho4 mediates phosphate acquisition in Candida albicans and is vital for stress resistance and metal homeostasis

This study provides the first evidence that the phosphate-responsive transcription factor Pho4 is vital for survival of Candida albicans to diverse and physiologically relevant stresses. Pho4 is important for C. albicans pathogenesis, and thus these findings illustrate how metabolic adaptation promotes C. albicans survival in the host.

During interactions with its mammalian host, the pathogenic yeast Candida albicans is exposed to a range of stresses such as superoxide radicals and cationic fluxes. Unexpectedly, a nonbiased screen of transcription factor deletion mutants revealed that the phosphate-responsive transcription factor Pho4 is vital for the resistance of C. albicans to these diverse stresses. RNA-Seq analysis indicated that Pho4 does not induce stress-protective genes directly. Instead, we show that loss of Pho4 affects metal cation toxicity, accumulation, and bioavailability. We demonstrate that pho4Δ cells are sensitive to metal and nonmetal cations and that Pho4-mediated polyphosphate synthesis mediates manganese resistance. Significantly, we show that Pho4 is important for mediating copper bioavailability to support the activity of the copper/zinc superoxide dismutase Sod1 and that loss of Sod1 activity contributes to the superoxide sensitivity of pho4Δ cells. Consistent with the key role of fungal stress responses in countering host phagocytic defenses, we also report that C. albicans pho4Δ cells are acutely sensitive to macrophage-mediated killing and display attenuated virulence in animal infection models. The novel connections between phosphate metabolism, metal homeostasis, and superoxide stress resistance presented in this study highlight the importance of metabolic adaptation in promoting C. albicans survival in the host.

LC3-associated phagocytosis: a crucial mechanism for antifungal host defence against Aspergillus fumigatus

LC3-associated phagocytosis (LAP) is a non-canonical autophagy pathway involved in the maturation of single-membrane phagosomes and subsequent killing of ingested pathogens by phagocytes. This pathway is initiated following recognition of pathogens by pattern recognition receptors and leads to the recruitment of LC3 into the phagosomal membrane. This form of phagocytosis is utilized for the antifungal host defence and is required for an efficient fungal killing. Here, we provide an overview of the LAP pathway and review the role of LAP in anti-Aspergillus host defence, as well as mechanisms induced by Aspergillus that modulate LAP to promote its survival in the host.

Dectin-1 is required for miR155 upregulation in murine macrophages in response to Candida albicans

The commensal fungal pathogen Candida albicans is a leading cause of lethal systemic infections in immunocompromised patients. One of the main mechanisms of host immune evasion and virulence by this pathogen is the switch from yeast form to hyphal growth morphologies. Micro RNAs (miRNAs), a small regulatory non-coding RNA, has been identified as an important part of the immune response to a wide variety of pathogens. In general, miRNAs act by modulating the intensity of inflammatory responses. miRNAs act by base-paring binding to specific sequences of target mRNAs, generally causing their silencing through mRNA degradation or translational repression. To study the impact of C. albicans cell morphology upon host miRNA expression, we investigated the differential modulation of 9 different immune response-related miRNAs in primary murine bone marrow-derived macrophages (BMDMs) exposed to either yeasts or hyphal forms of Candida albicans. Here, we show that the different growth morphologies induce distinct miRNA expression patterns in BMDMs. Interestingly, our data suggest that the C-Type lectin receptor Dectin-1 is a major PRR that orchestrates miR155 upregulation in a Syk-dependent manner. Our results suggest that PRR-mediating signaling events are key drivers of miRNA-mediated gene regulation during fungal pathogenesis.

IL-27 Induced by Select Candida spp. via TLR7/NOD2 Signaling and IFN-β Production Inhibits Fungal Clearance

Candida spp. elicit cytokine production downstream of various pathogen recognition receptors, including C-type lectin-like receptors, TLRs, and nucleotide oligomerization domain (NOD)–like receptors. IL-12 family members IL-12p70 and IL-23 are important for host immunity against Candida spp. In this article, we show that IL-27, another IL-12 family member, is produced by myeloid cells in response to selected Candida spp. We demonstrate a novel mechanism for Candida parapsilosis–mediated induction of IL-27 in a TLR7-, MyD88-, and NOD2-dependent manner. Our data revealed that IFN-β is induced by C. parapsilosis, which in turn signals through the IFN-α/β receptor and STAT1/2 to induce IL-27. Moreover, IL-27R (WSX-1)–deficient mice systemically infected with C. parapsilosis displayed enhanced pathogen clearance compared with wild-type mice. This was associated with increased levels of proinflammatory cytokines in the serum and increased IFN-γ and IL-17 responses in the spleens of IL-27R–deficient mice. Thus, our data define a novel link between C. parapsilosis, TLR7, NOD2, IFN-β, and IL-27, and we have identified an important role for IL-27 in the immune response against C. parapsilosis. Overall, these findings demonstrate an important mechanism for the suppression of protective immune responses during infection with C. parapsilosis, which has potential relevance for infections with other fungal pathogens.

The SPS amino acid sensor mediates nutrient acquisition and immune evasion in Candida albicans

Candida albicans is well adapted to its host and is able to sense and respond to the nutrients available within. We have shown that C. albicans avidly utilizes amino acids as a carbon source, which allows this opportunistic pathogen to neutralize acidic environments, including the macrophage phagosome. The transcription factor Stp2 is a key regulator of this phenomenon, and we sought to understand the mechanism of activation of Stp2, focusing on the SPS sensor system previously characterized for its role in nitrogen acquisition. We generated deletion mutants of the three components, SSY1, PTR3 and SSY5 and demonstrated that these strains utilize amino acids poorly as carbon source, cannot neutralize the medium in response to these nutrients, and have reduced ammonia release. Exogenous amino acids rapidly induce proteolytic processing of Stp2 and nuclear translocation in an SPS-dependent manner. A truncated version of Stp2, lacking the amino terminal nuclear exclusion domain, could suppress the growth and pH neutralization defects of the SPS mutants. We showed that the SPS system is required for normal resistance of C. albicans to macrophages and that mutants defective in this system reside in more acidic phagosomes compared with wild type cells; however, a more equivocal contribution was observed in the murine model of disseminated candidiasis. Taken together, these results indicate that the SPS system is activated under carbon starvation conditions resembling host environments, regulating Stp2 functions necessary for amino acid catabolism and normal interactions with innate immune cells.

SOCS3 is a modulator of human macrophage phagocytosis

Suppressor of cytokine signaling (SOCS) proteins are recognized as key feedback inhibitors modulating the inflammatory activities of macrophages, but comparatively little is known about whether and how they affect phagocytosis. Here, we evaluated the role of SOCS3 in driving the inflammatory phenotype and phagocytic uptake of apoptotic cells by human macrophages and the signaling pathways that are necessary for efficient phagocytosis. In M1-activated human monocyte-derived macrophages, SOCS3 silencing, using short interfering RNA technology, resulted in a decreased expression of proinflammatory markers and an increased expression of M2 macrophage markers. Strikingly, we demonstrated for the first time that SOCS3 knockdown significantly enhances the phagocytic capacity of M1 macrophages for carboxylate-modified beads and apoptotic neutrophils. With the use of live-cell video microscopy, we showed that SOCS3 knockdown radically affects the temporal dynamics of particle engulfment, enabling more rapid uptake of a second target and delaying postengulfment processing, as evidenced by deferred acquisition of phagosome maturation markers. SOCS3 knockdown impacts on phagocytosis through increased PI3K and Ras-related C3 botulinum toxin substrate 1 (Rac1) activity, pathways essential for engulfment and clearance of apoptotic cells. Enhanced phagocytosis in SOCS3-silenced cells was reversed by pharmacological PI3K inhibition. Furthermore, we revealed that actin polymerization, downstream of PI3K/Rac1 activation, was significantly altered in SOCS3-silenced cells, providing a mechanism for their greater phagocytic activity. The findings support a new model, whereby SOCS3 not only plays an important role in driving macrophage inflammatory responses but modulates key signaling pathways organizing the actin cytoskeleton to regulate the efficiency of phagocytic processes.

Fungal morphogenetic changes inside the mammalian host

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

Immune Interactions with Pathogenic and Commensal Fungi: A Two-Way Street

We are exposed to a wide spectrum of fungi including innocuous environmental organisms, opportunistic pathogens, commensal organisms, and fungi that can actively and explicitly cause disease. Much less is understood about effective host immunity to fungi than is generally known about immunity to bacterial and viral pathogens. Innate and adaptive arms of the immune system are required for effective host defense against Candida, Aspergillus, Cryptococcus, and others, with specific elements of the host response regulating specific types of fungal infections (e.g., mucocutaneous versus systemic). Here we will review themes and controversies that are currently shaping investigation of antifungal immunity (primarily to Candida and Aspergillus) and will also examine the emerging field of the role of fungi in the gut microbiome.

Interactions of fungal pathogens with phagocytes

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

Different Candida parapsilosis clinical isolates and lipase deficient strain trigger an altered cellular immune response

Numerous human diseases can be associated with fungal infections either as potential causative agents or as a result of changed immune status due to a primary disease. Fungal infections caused by Candida species can vary from mild to severe dependent upon the site of infection, length of exposure, and past medical history. Patients with impaired immune status are at increased risk for chronic fungal infections. Recent epidemiologic studies have revealed the increasing incidence of candidiasis caused by non-albicans species such as Candida parapsilosis. Due to its increasing relevance we chose two distinct C. parapsilosis strains, to describe the cellular innate immune response toward this species. In the first section of our study we compared the interaction of CLIB 214 and GA1 cells with murine and human macrophages. Both strains are commonly used to investigate C. parapsilosis virulence properties. CLIB 214 is a rapidly pseudohyphae-forming strain and GA1 is an isolate that mainly exists in a yeast form. Our results showed, that the phagocyte response was similar in terms of overall uptake, however differences were observed in macrophage migration and engulfment of fungal cells. As C. parapsilosis releases extracellular lipases in order to promote host invasion we further investigated the role of these secreted components during the distinct stages of the phagocytic process. Using a secreted lipase deficient mutant strain and the parental strain GA1 individually and simultaneously, we confirmed that fungal secreted lipases influence the fungi's virulence by detecting altered innate cellular responses. In this study we report that two isolates of a single species can trigger markedly distinct host responses and that lipase secretion plays a role on the cellular level of host–pathogen interactions.

Systems Level Dissection of Candida Recognition by Dectins: A Matter of Fungal Morphology and Site of Infection

Candida albicans is an ubiquitous fungal commensal of human skin and mucosal surfaces, and at the same time a major life-threatening human fungal pathogen in immunocompromised individuals. Host defense mechanisms rely on the capacity of professional phagocytes to recognize Candida cell wall antigens. During the past decade, the host immune response to Candida was dissected in depth, highlighting the essential role of C-type lectin receptors, especially regarding the power of the Dectins’ family in discriminating between the tolerated yeast-like form of Candida and its invading counterpart, the hyphae. This review focuses on the immuno-modulatory properties of the Candida morphologies and their specific interactions with the host innate immune system in different body surfaces.

Intracellular survival of Candida glabrata in macrophages: immune evasion and persistence

Candida glabrata is a successful human opportunistic pathogen which causes superficial but also life-threatening systemic infections. During infection, C. glabrata has to cope with cells of the innate immune system such as macrophages, which belong to the first line of defense against invading pathogens. Candida glabrata is able to survive and even replicate inside macrophages while causing surprisingly low damage and cytokine release. Here, we present an overview of recent studies dealing with the interaction of C. glabrata with macrophages, from phagocytosis to intracellular growth and escape. We review the strategies of C. glabrata that permit intracellular survival and replication, including poor host cell activation, modification of phagosome maturation and phagosome pH, adaptation to antimicrobial activities, and mechanisms to overcome the nutrient limitations within the phagosome. In summary, these studies suggest that survival within macrophages may be an immune evasion and persistence strategy of C. glabrata during infection.

Dressed to impress: impact of environmental adaptation on the Candida albicans cell wall

Candida albicans is an opportunistic fungal pathogen of humans causing superficial mucosal infections and life‐threatening systemic disease. The fungal cell wall is the first point of contact between the invading pathogen and the host innate immune system. As a result, the polysaccharides that comprise the cell wall act as pathogen associated molecular patterns, which govern the host–pathogen interaction. The cell wall is dynamic and responsive to changes in the external environment. Therefore, the host environment plays a critical role in regulating the host–pathogen interaction through modulation of the fungal cell wall. This review focuses on how environmental adaptation modulates the cell wall structure and composition, and the subsequent impact this has on the innate immune recognition of C. albicans.

O-glycosylation in cell wall proteins in Scedosporium prolificans is critical for phagocytosis and inflammatory cytokines production by macrophages

In this study, we analyze the importance of O-linked oligosaccharides present in peptidorhamnomannan (PRM) from the cell wall of the fungus Scedosporium prolificans for recognition and phagocytosis of conidia by macrophages. Adding PRM led to a dose-dependent inhibition of conidia phagocytosis, whereas de-O-glycosylated PRM did not show any effect. PRM induced the release of macrophage-derived antimicrobial compounds. However, O-linked oligosaccharides do not appear to be required for such induction. The effect of PRM on conidia-induced macrophage killing was examined using latex beads coated with PRM or de-O-glycosylated PRM. A decrease in macrophage viability similar to that caused by conidia was detected. However, macrophage killing was unaffected when beads coated with de-O-glycosylated PRM were used, indicating the toxic effect of O-linked oligosaccharides on macrophages. In addition, PRM triggered TNF-α release by macrophages. Chemical removal of O-linked oligosaccharides from PRM abolished cytokine induction, suggesting that the O-linked oligosaccharidic chains are important moieties involved in inflammatory responses through the induction of TNF-α secretion. In summary, we show that O-glycosylation plays a role in the recognition and uptake of S. prolificans by macrophages, killing of macrophages and production of pro- inflammatory cytokines.

Global analysis of fungal morphology exposes mechanisms of host cell escape

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

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

Rab14 regulates maturation of macrophage phagosomes containing the fungal pathogen Candida albicans and outcome of the host-pathogen interaction

Avoidance of innate immune defense is an important mechanism contributing to the pathogenicity of microorganisms. The fungal pathogen Candida albicans undergoes morphogenetic switching from the yeast to the filamentous hyphal form following phagocytosis by macrophages, facilitating its escape from the phagosome, which can result in host cell lysis. We show that the intracellular host trafficking GTPase Rab14 plays an important role in protecting macrophages from lysis mediated by C. albicans hyphae. Live-cell imaging of macrophages expressing green fluorescent protein (GFP)-tagged Rab14 or dominant negative Rab14, or with small interfering RNA (siRNA)-mediated knockdown of Rab14, revealed the temporal dynamics of this protein and its influence on the maturation of macrophage phagosomes following the engulfment of C. albicans cells. Phagosomes containing live C. albicans cells became transiently Rab14 positive within 2 min following engulfment. The duration of Rab14 retention on phagosomes was prolonged for hyphal cargo and was directly proportional to hyphal length. Interference with endogenous Rab14 did not affect the migration of macrophages toward C. albicans cells, the rate of engulfment, the overall uptake of fungal cells, or early phagosome processing. However, Rab14 depletion delayed the acquisition of the late phagosome maturation markers LAMP1 and lysosomal cathepsin, indicating delayed formation of a fully bioactive lysosome. This was associated with a significant increase in the level of macrophage killing by C. albicans. Therefore, Rab14 activity promotes phagosome maturation during C. albicans infection but is dysregulated on the phagosome in the presence of the invasive hyphal form, which favors fungal survival and escape.