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

Autophagy is redundant for the host defense against systemic Candida albicans infections

Autophagy has been demonstrated to play an important role in the immunity against intracellular pathogens, but very little is known about its role in the host defense against fungal pathogens such as Candida albicans. Therefore, the role of autophagy for the host defense against C. albicans was assessed by complementary approaches using mice defective in autophagy, as well as immunological and genetic studies in humans. Although C. albicans induced LC3-II formation in macrophages, myeloid cell-specific ATG7(-/-) mice with defects in autophagy did not display an increased susceptibility to disseminated candidiasis. In in vitro experiments in human blood mononuclear cells, blocking autophagy modulated cytokine production induced by lipopolysaccharide, but not by C. albicans. Furthermore, autophagy modulation in human monocytes did not influence the phagocytosis and killing of C. albicans. Finally, 18 single-nucleotide polymorphisms in 13 autophagy genes were not associated with susceptibility to candidemia or clinical outcome of disease in a large cohort of patients, and there was no correlation between these genetic variants and cytokine production in either candidemia patients or healthy controls. Based on these complementary in vitro and in vivo studies, it can be concluded that autophagy is redundant for the host response against systemic infections with C. albicans.

The intracellular life of Cryptococcus neoformans

Cryptococcus neoformans is a fungal pathogen with worldwide distribution. Serological studies of human populations show a high prevalence of human infection, which rarely progresses to disease in immunocompetent hosts. However, decreased host immunity places individuals at high risk for cryptococcal disease. The disease can result from acute infection or reactivation of latent infection, in which yeasts within granulomas and host macrophages emerge to cause disease. In this review, we summarize what is known about the cellular recognition, ingestion, and killing of C. neoformans and discuss the unique and remarkable features of its intracellular life, including the proposed mechanisms for fungal persistence and killing in phagocytic cells.

C-type lectin receptors orchestrate antifungal immunity

Fungal infections are an emerging threat for human health. A coordinated host immune response is fundamental for successful elimination of an invading fungal microbe. A panel of C-type lectin receptors expressed on antigen-presenting dendritic cells enable innate recognition of fungal cell wall carbohydrates and tailors adaptive responses via the instruction of CD4+ T helper cell fates. Well-balanced T helper cell type 1 and IL-17-producing T helper cell responses are crucial in antifungal immunity and facilitate phagocytic clearance of fungal encounters. Strikingly, different classes of fungi trigger distinct sets of C-type lectin receptors to evoke a pathogen-specific T helper response. In this review, we outline the key roles of several C-type lectin receptors during the generation of protective antifungal immunity, with particular emphasis on the distinct signaling pathways and transcriptional programs triggered by these receptors, which collaborate to orchestrate polarization of the T helper response.

Genetic susceptibility to Candida infections

Candida spp. are medically important fungi causing severe mucosal and life-threatening invasive infections, especially in immunocompromised hosts. However, not all individuals at risk develop Candida infections, and it is believed that genetic variation plays an important role in host susceptibility. On the one hand, severe fungal infections are associated with monogenic primary immunodeficiencies such as defects in STAT1, STAT3 or CARD9, recently discovered as novel clinical entities. On the other hand, more common polymorphisms in genes of the immune system have also been associated with fungal infections such as recurrent vulvovaginal candidiasis and candidemia. The discovery of the genetic susceptibility to Candida infections can lead to a better understanding of the pathogenesis of the disease, as well as to the design of novel immunotherapeutic strategies. This review is part of the review series on host-pathogen interactions. See more reviews from this series.

Dectin-1 activation controls maturation of β-1,3-glucan-containing phagosomes

Elimination of fungal pathogens by phagocytes requires phagosome maturation, a process that involves the recruitment and fusion of intracellular proteins. The role of Dectin-1, a β-1,3-glucan receptor, critical for fungal recognition and triggering of Th17 responses, to phagosomal maturation has not been defined. We show that GFP-Dectin-1 translocates to the fungal phagosome, but its signal decays after 2 h. Inhibition of acidification results in retention of GFP-Dectin-1 to phagosome membranes highlighting the requirement for an acidic pH. Following β-1,3-glucan recognition, GFP-Dectin-1 undergoes tyrosine phosphorylation by Src kinases with subsequent Syk activation. Our results demonstrate that Syk is activated independently of intraphagosomal pH. Inhibition of Src or Syk results in prolonged retention of GFP-Dectin-1 to the phagosome signifying a link between Syk and intraphagosomal pH. β-1,3-glucan phagosomes expressing a signaling incompetent Dectin-1 failed to mature as demonstrated by prolonged Dectin-1 retention, presence of Rab5B, failure to acquire LAMP-1 and inability to acidify. Phagosomes containing Candida albicans also require Dectin-1-dependent Syk activation for phagosomal maturation. Taken together, these results support a model where Dectin-1 not only controls internalization of β-1,3-glucan containing cargo and triggers proinflammatory cytokines, but also acts as a master regulator for subsequent phagolysosomal maturation through Syk activation.

Interleukin-4- and interleukin-13-mediated alternatively activated macrophages: roles in homeostasis and disease

The macrophage, a versatile cell type prominently involved in host defense and immunity, assumes a distinct state of alternative activation in the context of polarized type 2 immune responses such as allergic inflammation and helminth infection. This alternatively activated phenotype is induced by the canonical type 2 cytokines interleukin (IL)-4 and IL-13, which mediate expression of several characteristic markers along with a dramatic shift in macrophage metabolic pathways that influence surrounding cells and tissues. We discuss recent advances in the understanding of IL-4- and IL-13-mediated alternatively activated macrophages and type 2 immune responses; such advances have led to an expanded appreciation for functions of these cells beyond immunity, including maintenance of physiologic homeostasis and tissue repair.

Characterization of the cellular immunity in patients presenting extensive dermatophytoses due to Trichophyton rubrum

Dermatophytes invade the stratum corneum of the skin and other keratinized tissues such as hair and nails, and Trichophyton rubrum causes approximately 80% of cutaneous mycoses in humans. To evaluate the cellular immune response of patients with extensive dermatophytosis caused by T. rubrum, we evaluated lymphocyte populations, the lymphoproliferative response to: phytohaemagglutinin (PHA); anti-CD3 (OKT3); and pokeweed mitogen (PWM), Candida sp. (CMA), an extract of T. rubrum, and the main fungal epitope TriR2 (T). We also evaluated interleukin (IL)-4, IL-10, IL-12 and IFN-γ after stimulation by PHA, CMA and TriR2. The immunophenotyping showed no differences between patients and controls. The lymphoproliferation test showed significant differences between the groups stimulated by PWM and CMA, as well as against TriR2, being significantly higher for the control group. Conversely, there were similar results for the groups after stimulation by the extract. The cytokines' quantification showed a significant difference between the groups only for IFN-γ stimulated by PHA and TriR2. We can conclude that the fungal extract can stimulate lymphoproliferation by both groups' lymphocytes. However, the response to Tri r2 was more specific. We showed that some patients with extensive dermatophytosis have normal cellular response, recognising both the extract and TriR2.

Differential dependencies of monocytes and neutrophils on dectin-1, dectin-2 and complement for the recognition of fungal particles in inflammation

We have re-investigated the role of the complement system and the non-opsonic pattern recognition receptors dectin-1 and dectin-2 in the recognition of fungal particles by inflammatory neutrophils, monocytes and macrophages. We have used in vivo and ex vivo models to study the recognition and response of these cells: i) We confirm previous observations regarding the importance of complement to neutrophil but not monocytic responses; ii) We show that dectin-1 is important for driving inflammatory cell recruitment to fungal stimuli and that it biases the immediate inflammatory response to one that favors neutrophil over monocyte recruitment; iii) We show that dectin-2 contributes to the physical recognition of fungal particles by inflammatory monocytes/macrophages, but is also expressed on neutrophils, where we show it has the potential to contribute to cellular activation; iv) Additionally, we show that serum-opsonization has the potential to interfere with non-opsonic recognition of fungal particles by dectin-1 and dectin-2, presumably through masking of ligands. Collectively these roles are consistent with previously described roles of dectin-1 and dectin-2 in driving inflammatory and adaptive immune responses and complement in containing fungal burdens. This study emphasizes the importance of heterogeneity of receptor expression across myeloid cell subsets in protective immune responses.

The mannose receptor

The MR is a highly effective endocytic receptor with a broad binding specificity encompassing ligands of microbial and endogenous origin and a poorly characterized ability to modulate cellular activation. This review provides an update of the latest developments in the field. It discusses how MR biology might be affected by glycosylation and proteolytic processing, MR involvement in antigen delivery, and the potential contribution of MR to T cell differentiation and cellular activation. Further understanding of these areas will, no doubt, inform the design of novel, therapeutic tools for improved vaccination, control of inflammation, and tumor chemotherapy, which will benefit from exploiting MR-efficient internalization properties and unique pattern of expression.

Phagocytic receptors on macrophages distinguish between different Sporothrix schenckii morphotypes

Sporothrix schenckii is a human pathogen that causes sporotrichosis, a cutaneous subacute or chronic mycosis. Little is known about the innate immune response and the receptors involved in host recognition and phagocytosis of S. schenckii. Here, we demonstrate that optimal phagocytosis of conidia and yeast is dependent on preimmune human serum opsonisation. THP-1 macrophages efficiently ingested opsonised conidia. Competition with D-mannose, methyl α-D-mannopyranoside, D-fucose, and N-acetyl glucosamine blocked this process, suggesting the involvement of the mannose receptor in binding and phagocytosis of opsonised conidia. Release of TNF-α was not stimulated by opsonised or non-opsonised conidia, although reactive oxygen species (ROS) were produced, resulting in the killing of conidia by THP-1 macrophages. Heat inactivation of the serum did not affect conidia internalization, which was markedly decreased for yeast cells, suggesting the role of complement components in yeast uptake. Conversely, release of TNF-α and production of ROS were induced by opsonised and non-opsonised yeast. These data demonstrate that THP-1 macrophages respond to opsonised conidia and yeast through different phagocytic receptors, inducing a differential cellular response. Conidia induces a poor pro-inflammatory response and lower rate of ROS-induced cell death, thereby enhancing the pathogen's survival.

Role and relevance of mast cells in fungal infections

In addition to their detrimental role in allergic diseases, mast cells (MCs) are well known to be important cells of the innate immune system. In the last decade, they have been shown to contribute significantly to optimal host defense against numerous pathogens including parasites, bacteria, and viruses. The contribution of MCs to the immune responses in fungal infections, however, is largely unknown. In this review, we first discuss key features of mast cell responses to pathogens in general and then summarize the current knowledge on the function of MCs in the defense against fungal pathogens. We especially focus on the potential and proven mechanisms by which MCs can detect fungal infections and on possible MC effector mechanisms in protecting from fungal infections.

Human genetic susceptibility to Candida infections

Infections with Candida spp. have different manifestations in humans, ranging from mucosal to bloodstream and deep-seated disseminated infections. Immunocompromised patients have increased susceptibility to these types of infections, due to reduced capacity to elicit effective innate or adaptive immunity. In addition, rare and common genetic variants in the human genome have been identified that influence susceptibility to Candida infections. Genetic determinants of primary immunodeficiencies leading to chronic mucocutaneous candidiasis have been reported, and polymorphisms in genes that are known to be involved in anti-Candida host defense are associated with increased susceptibility to systemic infection. These findings have greatly increased our understanding of pathways important for anti-Candida defense in humans, and patterns of prevalence of Candida infections. In addition, these pathways may offer novel therapeutic targets for treatment. This review provides an overview of the current insights in genetic susceptibility to Candida infections and their consequences for the immune response against Candida.

Dectin-1 isoforms contribute to distinct Th1/Th17 cell activation in mucosal candidiasis

The recognition of β-glucans by dectin-1 has been shown to mediate cell activation, cytokine production and a variety of antifungal responses. Here, we report that the functional activity of dectin-1 in mucosal immunity to Candida albicans is influenced by the genetic background of the host. Dectin-1 was required for the proper control of gastrointestinal and vaginal candidiasis in C57BL/6, but not BALB/c mice; in fact, the latter showed increased resistance in the absence of dectin-1. The susceptibility of dectin-1-deficient C57BL/6 mice to infection was associated with defects in IL-17A and aryl hydrocarbon receptor-dependent IL-22 production and in adaptive Th1 responses. In contrast, the resistance of dectin-1-deficient BALB/c mice was associated with increased IL-17A and IL-22 production and the skewing towards Th1/Treg immune responses that provide immunological memory. Disparate canonical/noncanonical NF-κB signaling pathways downstream of dectin-1 were activated in the two different mouse strains. Thus, the net activity of dectin-1 in antifungal mucosal immunity is dependent on the host's genetic background, which affects both the innate cytokine production and the adaptive Th1/Th17 cell activation upon dectin-1 signaling.

Characterisation of innate fungal recognition in the lung

The innate recognition of fungi by leukocytes is mediated by pattern recognition receptors (PRR), such as Dectin-1, and is thought to occur at the cell surface triggering intracellular signalling cascades which lead to the induction of protective host responses. In the lung, this recognition is aided by surfactant which also serves to maintain the balance between inflammation and pulmonary function, although the underlying mechanisms are unknown. Here we have explored pulmonary innate recognition of a variety of fungal particles, including zymosan, Candida albicans and Aspergillus fumigatus, and demonstrate that opsonisation with surfactant components can limit inflammation by reducing host-cell fungal interactions. However, we found that this opsonisation does not contribute directly to innate fungal recognition and that this process is mediated through non-opsonic PRRs, including Dectin-1. Moreover, we found that pulmonary inflammatory responses to resting Aspergillus conidia were initiated by these PRRs in acidified phagolysosomes, following the uptake of fungal particles by leukocytes. Our data therefore provides crucial new insights into the mechanisms by which surfactant can maintain pulmonary function in the face of microbial challenge, and defines the phagolysosome as a novel intracellular compartment involved in the innate sensing of extracellular pathogens in the lung.

The Role of TLR2 in Infection and Immunity

Toll-like receptors (TLRs) are recognition molecules for multiple pathogens, including bacteria, viruses, fungi, and parasites. TLR2 forms heterodimers with TLR1 and TLR6, which is the initial step in a cascade of events leading to significant innate immune responses, development of adaptive immunity to pathogens and protection from immune sequelae related to infection with these pathogens. This review will discuss the current status of TLR2 mediated immune responses by recognition of pathogen-associated molecular patterns (PAMPS) on these organisms. We will emphasize both canonical and non-canonical responses to TLR2 ligands with emphasis on whether the inflammation induced by these responses contributes to the disease state or to protection from diseases.

Genetic deletion of dectin-1 does not affect the course of murine experimental colitis

Background

It is believed that inflammatory bowel diseases (IBD) result from an imbalance in the intestinal immune response towards the luminal microbiome. Dectin-1 is a widely expressed pattern recognition receptor that recognizes fungi and upon recognition it mediates cytokine responses and skewing of the adaptive immune system. Hence, dectin-1 may be involved in the pathogenesis of IBD.

Methods

We assessed the responses of dectin-1 deficient macrophages to the intestinal microbiota and determined the course of acute DSS and chronic Helicobacter hepaticus induced colitis in dectin-1 deficient mice.

Results

We show that the mouse intestinal microbiota contains fungi and the cytokine responses towards this microbiota were significantly reduced in dectin-1 deficient macrophages. However, in two different colitis models no significant differences in the course of inflammation were found in dectin-1 deficient mice compared to wild type mice.

Conclusions

Together our data suggest that, although at the immune cell level there is a difference in response towards the intestinal flora in dectin-1 deficient macrophages, during intestinal inflammation this response seems to be redundant since dectin-1 deficiency in mice does not affect intestinal inflammation in experimental colitis.

Candida albicans morphogenesis and host defence: discriminating invasion from colonization

Candida albicans is a common fungal pathogen of humans that colonizes the skin and mucosal surfaces of most healthy individuals. Until recently, little was known about the mechanisms by which mucosal antifungal defences tolerate colonizing C. albicans but react strongly when hyphae of the same microorganism attempt to invade tissue. In this Review, we describe the properties of yeast cells and hyphae that are relevant to their interaction with the host, and the immunological mechanisms that differentially recognize colonizing versus invading C. albicans.

Adjunctive immunotherapy with recombinant cytokines for the treatment of disseminated candidiasis

Despite the discovery in the last decade of azoles and echinocandins as novel and potent antimycotic drugs, systemic Candida infections are still accompanied by an unacceptably high burden of morbidity and mortality. A rational novel therapeutic approach would be the use of adjuvant immunotherapy, with the aim of improving host defence against Candida. Increases in our understanding of the mechanisms that underlie the pathogenesis of Candida infections, such as the role played by pattern recognition receptors and the induction of proinflammatory cytokines during the early phases of infection, have led to the hypothesis of a potential therapeutic role of recombinant cytokines in systemic candidiasis. In the present review, we give an update of both experimental data and proof-of-principle studies in humans that argue for the use of adjunctive immunotherapy with recombinant cytokines in invasive Candida infections. Sufficiently powered studies on the role of cytokine-based treatment regimens for invasive candidiasis are needed to fully demonstrate the feasibility of this immunotherapeutic approach to improve the prognosis of severe invasive Candida infections.

Efficacy of zosteric acid sodium salt on the yeast biofilm model Candida albicans

Candida albicans is the most notorious and the most widely studied yeast biofilm former. Design of experiments (DoE) showed that 10 mg/L zosteric acid sodium salt reduced C. albicans adhesion and the subsequent biofilm formation by at least 70%, on both hydrophilic and hydrophobic surfaces of 96-well plates. Indeed, biofilm imaging revealed the dramatic impact of zosteric acid sodium salt on biofilm thickness and morphology, due to the inability of the cells to form filamentous structures while remaining metabolically active. In the same way, 10 mg/L zosteric acid sodium salt inhibited C. albicans biofilm formation when added after the adhesion phase. Contrary to zosteric acid sodium salt, methyl zosterate did not affect yeast biofilm. In addition, zosteric acid sodium salt enhanced sensitivity to chlorhexidine, chlorine, hydrogen peroxide, and cis-2-decenoic acid, with a reduction of 0.5 to 8 log units. Preliminary in vitro studies using suitable primary cell based models revealed that zosteric acid sodium salt did not compromise the cellular activity, adhesion, proliferation or morphology of either the murine fibroblast line L929 or the human osteosarcoma line MG-63. Thus the use of zosteric acid sodium salt could provide a suitable, innovative, preventive, and integrative approach to preventing yeast biofilm formation.

DC-SIGN and influenza hemagglutinin dynamics in plasma membrane microdomains are markedly different

DC-SIGN, a Ca(2+)-dependent transmembrane lectin, is found assembled in microdomains on the plasma membranes of dendritic cells. These microdomains bind a large variety of pathogens and facilitate their uptake for subsequent antigen presentation. In this study, DC-SIGN dynamics in microdomains were explored with several fluorescence microscopy methods and compared with dynamics for influenza hemagglutinin (HA), which is also found in plasma membrane microdomains. Fluorescence imaging indicated that DC-SIGN microdomains may contain other C-type lectins and that the DC-SIGN cytoplasmic region is not required for microdomain formation. Fluorescence recovery after photobleaching measurements showed that neither full-length nor cytoplasmically truncated DC-SIGN in microdomains appreciably exchanged with like molecules in other microdomains and the membrane surround, whereas HA in microdomains exchanged almost completely. Line-scan fluorescence correlation spectroscopy indicated an essentially undetectable lateral mobility for DC-SIGN but an appreciable mobility for HA within their respective domains. Single-particle tracking with defined-valency quantum dots confirmed that HA has significant mobility within microdomains, whereas DC-SIGN does not. By contrast, fluorescence recovery after photobleaching indicated that inner leaflet lipids are able to move through DC-SIGN microdomains. The surprising stability of DC-SIGN microdomains may reflect structural features that enhance pathogen uptake either by providing high-avidity platforms and/or by protecting against rapid microdomain endocytosis.

Immunity to fungal infections

Fungal diseases represent an important paradigm in immunology, as they can result from either a lack of recognition by the immune system or overactivation of the inflammatory response. Research in this field is entering an exciting period of transition from studying the molecular and cellular bases of fungal virulence to determining the cellular and molecular mechanisms that maintain immune homeostasis with fungi. The fine line between these two research areas is central to our understanding of tissue homeostasis and its possible breakdown in fungal infections and diseases. Recent insights into immune responses to fungi suggest that functionally distinct mechanisms have evolved to achieve optimal host-fungus interactions in mammals.

Conventional dendritic cells mount a type I IFN response against Candida spp. requiring novel phagosomal TLR7-mediated IFN-β signaling

Human fungal pathogens such as the dimorphic Candida albicans or the yeast-like Candida glabrata can cause systemic candidiasis of high mortality in immunocompromised individuals. Innate immune cells such as dendritic cells and macrophages establish the first line of defense against microbial pathogens and largely determine the outcome of infections. Among other cytokines, they produce type I IFNs (IFNs-I), which are important modulators of the host immune response. Whereas an IFN-I response is a hallmark immune response to bacteria and viruses, a function in fungal pathogenesis has remained unknown. In this study, we demonstrate a novel mechanism mediating a strong IFN-β response in mouse conventional dendritic cells challenged by Candida spp., subsequently orchestrating IFN-α/β receptor 1-dependent intracellular STAT1 activation and IFN regulatory factor (IRF) 7 expression. Interestingly, the initial IFN-β release bypasses the TLR 4 and TLR2, the TLR adaptor Toll/IL-1R domain-containing adapter-inducing IFN-β and the β-glucan/phagocytic receptors dectin-1 and CD11b. Notably, Candida-induced IFN-β release is strongly impaired by Src and Syk family kinase inhibitors and strictly requires completion of phagocytosis as well as phagosomal maturation. Strikingly, TLR7, MyD88, and IRF1 are essential for IFN-β signaling. Furthermore, in a mouse model of disseminated candidiasis we show that IFN-I signaling promotes persistence of C. glabrata in the host. Our data uncover for the first time a pivotal role for endosomal TLR7 signaling in fungal pathogen recognition and highlight the importance of IFNs-I in modulating the host immune response to C. glabrata.

Pathogenesis of invasive candidiasis

PURPOSE OF REVIEW

Disseminated candidiasis remains a life-threatening disease in the ICU. The development of invasive disease with Candida albicans is dependent on multiple factors, such as colonization and efficient host defense at the mucosa. In the present review, we describe the host defense mechanisms against Candida that are responsible for counteracting mucosal invasion, and eliminating the pathogen once invasion has taken place.

RECENT FINDINGS

The newly described T-helper subset Th17 is critical for mucosal anti-Candida host defense and plays a major role in controlling C. albicans colonization, whereas the Th1 response and monocyte-dependent cytokines such as IL-1 and TNF are predominantly responsible for activation of neutrophils and macrophages during disseminated candidiasis.

SUMMARY

This knowledge provides the basis of exploring new treatment options in the fight against invasive candidiasis. Reports of beneficial effects of recombinant cytokine therapy in fungal infections, renders them prime candidates for adjuvant immunotherapy in Candida sepsis.

Fungal recognition enhances mannose receptor shedding through dectin-1 engagement

The mannose receptor (MR) is an endocytic type I membrane molecule with a broad ligand specificity that is involved in both hemostasis and pathogen recognition. Membrane-anchored MR is cleaved by a metalloproteinase into functional soluble MR (sMR) composed of the extracellular domains of intact MR. Although sMR production was initially considered a constitutive process, enhanced MR shedding has been observed in response to the fungal pathogen Pneumocystis carinii. In this work, we have investigated the mechanism mediating enhanced MR shedding in response to fungi. We show that other fungal species, including Candida albicans and Aspergillus fumigatus, together with zymosan, a preparation of the cell wall of Saccharomyces cerevisiae, mimic the effect of P. carinii on sMR production and that this effect takes place mainly through β-glucan recognition. Additionally, we demonstrate that MR cleavage in response to C. albicans and bioactive particulate β-glucan requires expression of dectin-1. Our data, obtained using specific inhibitors, are consistent with the canonical Syk-mediated pathway triggered by dectin-1 being mainly responsible for inducing MR shedding, with Raf-1 being partially involved. As in the case of steady-state conditions, MR shedding in response to C. albicans and β-glucan particles requires metalloprotease activity. The induction of MR shedding by dectin-1 has clear implications for the role of MR in fungal recognition, as sMR was previously shown to retain the ability to bind fungal pathogens and can interact with numerous host molecules, including lysosomal hydrolases. Thus, MR cleavage could also impact on the magnitude of inflammation during fungal infection.

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.

C-type lectins, fungi and Th17 responses

Th17 cells are a recently discovered subset of T helper cells characterised by the release of IL-17, and are thought to be important for mobilization of immune responses against microbial pathogens, but which also contribute to the development of autoimmune diseases. The identification of C-type lectin receptors which are capable of regulating the balance between Th1 and Th17 responses has been of particular recent interest, which they control, in part, though the release of Th17 inducing cytokines. Many of these receptors recognise fungi, and other pathogens, and play key roles in driving the development of protective anti-microbial immunity. Here we will review the C-type lectins that have been linked to Th17 type responses and will briefly examine the role of Th17 responses in murine and human anti-fungal immunity.

Pathways regulating cytosolic phospholipase A2 activation and eicosanoid production in macrophages by Candida albicans

Resident tissue macrophages are activated by the fungal pathogen Candida albicans to release eicosanoids, which are important modulators of inflammation and immune responses. Our objective was to identify the macrophage receptors engaged by C. albicans that mediate activation of group IVA cytosolic phospholipase A(2) (cPLA(2)α), a regulatory enzyme that releases arachidonic acid (AA) for production of prostaglandins and leukotrienes. A comparison of peritoneal macrophages from wild type and knock-out mice demonstrates that the β-glucan receptor Dectin-1 and MyD88 regulate early release of AA and eicosanoids in response to C. albicans. However, cyclooxygenase 2 (COX2) expression and later phase eicosanoid production are defective in MyD88(-/-) but not Dectin-1(-/-) macrophages. Furthermore, C. albicans-stimulated activation of MAPK and phosphorylation of cPLA(2)α on Ser-505 are regulated by MyD88 and not Dectin-1. In contrast, Dectin-1 mediates MAPK activation, cPLA(2)α phosphorylation, and COX2 expression in response to particulate β-glucan suggesting that other receptors engaged by C. albicans preferentially mediate these responses. Results also implicate the mannan-binding receptor Dectin-2 in regulating cPLA(2)α. C. albicans-stimulated MAPK activation and AA release are blocked by d-mannose and Dectin-2-specific antibody, and overexpression of Dectin-2 in RAW264.7 macrophages enhances C. albicans-stimulated MAPK activation, AA release, and COX2 expression. In addition, calcium mobilization is enhanced in RAW264.7 macrophages overexpressing Dectin-1 or -2. The results demonstrate that C. albicans engages both β-glucan and mannan-binding receptors on macrophages that act with MyD88 to regulate the activation of cPLA(2)α and eicosanoid production.

Distinct roles for Dectin-1 and TLR4 in the pathogenesis of Aspergillus fumigatus keratitis

Aspergillus species are a major worldwide cause of corneal ulcers, resulting in visual impairment and blindness in immunocompetent individuals. To enhance our understanding of the pathogenesis of Aspergillus keratitis, we developed a murine model in which red fluorescent protein (RFP)-expressing A. fumigatus (Af293.1RFP) conidia are injected into the corneal stroma, and disease progression and fungal survival are tracked over time. Using Mafia mice in which c-fms expressing macrophages and dendritic cells can be induced to undergo apoptosis, we demonstrated that the presence of resident corneal macrophages is essential for production of IL-1beta and CXCL1/KC, and for recruitment of neutrophils and mononuclear cells into the corneal stroma. We found that beta-glucan was highly expressed on germinating conidia and hyphae in the cornea stroma, and that both Dectin-1 and phospho-Syk were up-regulated in infected corneas. Additionally, we show that infected Dectin-1(-/-) corneas have impaired IL-1beta and CXCL1/KC production, resulting in diminished cellular infiltration and fungal clearance compared with control mice, especially during infection with clinical isolates expressing high beta-glucan. In contrast to Dectin 1(-/-) mice, cellular infiltration into infected TLR2(-/-), TLR4(-/-), and MD-2(-/-) mice corneas was unimpaired, indicating no role for these receptors in cell recruitment; however, fungal killing was significantly reduced in TLR4(-/-) mice, but not TLR2(-/-) or MD-2(-/-) mice. We also found that TRIF(-/-) and TIRAP(-/-) mice exhibited no fungal-killing defects, but that MyD88(-/-) and IL-1R1(-/-) mice were unable to regulate fungal growth. In conclusion, these data are consistent with a model in which beta-glucan on A.fumigatus germinating conidia activates Dectin-1 on corneal macrophages to produce IL-1beta, and CXCL1, which together with IL-1R1/MyD88-dependent activation, results in recruitment of neutrophils to the corneal stroma and TLR4-dependent fungal killing.

Phagocytosis and phagosome acidification are required for pathogen processing and MyD88-dependent responses to Staphylococcus aureus

Innate immunity is vital for protection from microbes and is mediated by humoral effectors, such as cytokines, and cellular immune defenses, including phagocytic cells (e.g., macrophages). After internalization by phagocytes, microbes are delivered into a phagosome, a complex intracellular organelle with a well-established and important role in microbial killing. However, the role of this organelle in cytokine responses and microbial sensing is less well defined. In this study, we assess the role of the phagosome in innate immune sensing and demonstrate the critical interdependence of phagocytosis and pattern recognition receptor signaling during response to the Gram-positive bacteria Staphylococcus aureus. We show that phagocytosis is essential to initiate an optimal MyD88-dependent response to Staphylococcus aureus. Prior to TLR-dependent cytokine production, bacteria must be engulfed and delivered into acidic phagosomes where acid-activated host enzymes digest the internalized bacteria to liberate otherwise cryptic bacterial-derived ligands that initiate responses from the vacuole. Importantly, in macrophages in which phagosome acidification is perturbed, the impaired response to S. aureus can be rescued by the addition of lysostaphin, a bacterial endopeptidase active at neutral pH that can substitute for the acid-activated host enzymes. Together, these observations delineate the interdependence of phagocytosis with pattern recognition receptor signaling and suggest that therapeutics to augment functions and signaling from the vacuole may be useful strategies to increase host responses to S. aureus.

Fungal attacks on mammalian hosts: pathogen elimination requires sensing and tasting

Recognition of Candida spp. by immune cells is mediated by dedicated pattern recognition receptors (PRRs), including Toll-like receptors (TLRs) and lectins expressed on innate immune cells (e.g., macrophages, neutrophils and dendritic cells (DCs)). PRRs recognize Candida-specific pathogen-associated molecular patterns (PAMPs). Binding of fungal PAMPs (e.g., cell wall sugar polymers and proteins, fungal nucleic acids) to PRRs triggers the activation of innate effector cells. Recent findings underscore the role of DCs in relaying PAMP information through their PRRs to stimulate the adaptive response. In agreement, deficiencies in certain PRRs strongly impair survival to Candida infections in mice and is associated with enhanced susceptibility to mucocutaneous fungal infections in humans. Understanding the complex signaling networks protecting the host against fungal pathogens remains a challenge in the field.

Distinct roles of complement receptor 3, Dectin-1, and sialic acids in murine macrophage interaction with Histoplasma yeast

The yeast cells of dimorphic fungal pathogen Histoplasma reside primarily within the macrophages of an infected host; the interaction between the yeast and macrophage has a profound impact on host defense against the fungus. We used blocking antibodies and saccharides to identify the receptors that participate in the phagocytosis of and the cytokine response to Histoplasma. The phagocytosis and cytokine response results show that sialic acids on the macrophages were involved in the interaction between macrophages and Histoplasma. CR3, although not the only receptor involved, was responsible for phagocytosis and cytokine response. It is unclear which receptors other than CR3 are responsible for phagocytosis, but we did rule out the participation of TLR2, TLR4, MR, DC-SIGN/SIGNR1, FcgammaR, VLA-5, and Dectin-1. Even though Dectin-1 did not participate in phagocytosis, it collaborated with CR3 in the cytokine response to Histoplasma, suggesting that in the presence of phagocytic receptors, Histoplasma triggers cytokine signals through Dectin-1. Moreover, macrophage phagocytosis of and cytokine response to Histoplasma are Syk kinase-dependent. Our study delineated the distinct roles of CR3, Dectin-1, and sialic acids in the interaction with Histoplasma and suggested that multiple receptor use might be important to host defense against Histoplasma.

A novel pseudopodial component of the dendritic cell anti-fungal response: the fungipod

Fungal pathologies are seen in immunocompromised and healthy humans. C-type lectins expressed on immature dendritic cells (DC) recognize fungi. We report a novel dorsal pseudopodial protrusion, the “fungipod”, formed by DC after contact with yeast cell walls. These structures have a convoluted cell-proximal end and a smooth distal end. They persist for hours, exhibit noticeable growth and total 13.7±5.6 µm long and 1.8±0.67 µm wide at the contact. Fungipods contain clathrin and an actin core surrounded by a sheath of cortactin. The actin cytoskeleton, but not microtubules, is required for fungipod integrity and growth. An apparent rearward flow (225±55 nm/second) exists from the zymosan contact site into the distal fungipod. The phagocytic receptor Dectin-1 is not required for fungipod formation, but CD206 (Mannose Receptor) is the generative receptor for these protrusions. The human pathogen Candida parapsilosis induces DC fungipod formation strongly, but the response is species specific since the related fungal pathogens Candida tropicalis and Candida albicans induce very few and no fungipods, respectively. Our findings show that fungipods are dynamic actin-driven cellular structures involved in fungal recognition by DC. They may promote yeast particle phagocytosis by DC and are a specific response to large (i.e., 5 µm) particulate ligands. Our work also highlights the importance of this novel protrusive structure to innate immune recognition of medically significant Candida yeasts in a species specific fashion.

Yeasts are normal microbial commensals of humans and a significant source of opportunistic infections, especially in immunocompromised individuals. We report a novel cellular protrusive structure, the fungipod, which participates in the host-microbe interaction between human immature dendritic cells (DC) and yeasts. The fungipod's structure is based on and propelled by a robust process of local actin cytoskeleton growth at the DC-yeast contact site, and this cytoskeletal remodeling results in a durable tubular structure over 10 µm long connecting the dorsal DC membrane and yeast. The fungal cell wall polysaccharides mannan and chitin trigger fungipod formation by stimulating the carbohydrate pattern recognition receptor CD206. Fungipods are part of a specific response to large particulate objects (i.e., yeast), and they may promote the human immature DC's relatively poor phagocytosis of yeast. The human fungal pathogen, Candida parapsilosis, induces a strong fungipod response from DC, and this response is highly species specific since the related pathogens Candida albicans and Candida tropicalis induce fungipods rarely. Our work highlights a novel cell biological element of fungal recognition by the innate immune system.

Candida albicans beta-glucan exposure is controlled by the fungal CEK1-mediated mitogen-activated protein kinase pathway that modulates immune responses triggered through dectin-1

Innate immunity to Candida albicans depends upon the recognition of molecular patterns on the fungal cell wall. However, the masking of major components such as beta-glucan seems to be a mechanism that fungi have evolved to avoid immune cell recognition through the dectin-1 receptor. Although the role of C. albicans mitogen-activated protein kinase (MAPK) pathways as virulence determinants has been established previously with animal models, the mechanism involved in this behavior is largely unknown. In this study we demonstrate that a disruption of the C. albicans extracellular signal-regulated kinase (ERK)-like 1 (CEK1)-mediated MAPK pathway causes enhanced cell wall beta-glucan exposure, triggering immune responses more efficiently than the wild type, as measured by dectin-1-mediated specific binding and human dendritic cell (hDC)- and macrophage-mediated phagocytosis, killing, and activation of intracellular signaling pathways. At the molecular level, the disruption of CEK1 resulted in altered spleen tyrosine kinase (Syk), Raf-1, and ERK1/2 activations together with IkappaB degradation on hDCs and increased dectin-1-dependent activator protein 1 (AP-1) activation on transfected cells. In addition, concurring with these altered pathways, we detected increased reactive oxygen species production and cytokine secretion. In conclusion, the CEK1-mediated MAPK pathway is involved in beta-glucan exposure in a fungal pathogen, hence influencing dectin-1-dependent immune cell recognition, thus establishing this fungal intracellular signaling route as a promising novel therapeutic target.

PPARγ controls Dectin-1 expression required for host antifungal defense against Candida albicans

We recently showed that IL-13 or peroxisome proliferator activated receptor gamma (PPARgamma) ligands attenuate Candida albicans colonization of the gastrointestinal tract. Here, using a macrophage-specific Dectin-1 deficient mice model, we demonstrate that Dectin-1 is essential to control fungal gastrointestinal infection by PPARgamma ligands. We also show that the phagocytosis of yeast and the release of reactive oxygen intermediates in response to Candida albicans challenge are impaired in macrophages from Dectin-1 deficient mice treated with PPARgamma ligands or IL-13. Although the Mannose Receptor is not sufficient to trigger antifungal functions during the alternative activation of macrophages, our data establish the involvement of the Mannose Receptor in the initial recognition of non-opsonized Candida albicans by macrophages. We also demonstrate for the first time that the modulation of Dectin-1 expression by IL-13 involves the PPARgamma signaling pathway. These findings are consistent with a crucial role for PPARgamma in the alternative activation of macrophages by Th2 cytokines. Altogether these data suggest that PPARgamma ligands may be of therapeutic value in esophageal and gastrointestinal candidiasis in patients severely immunocompromised or with metabolic diseases in whom the prevalence of candidiasis is considerable.

Alternatively activated macrophages in infection and autoimmunity

Macrophages are innate immune cells that play an important role in activation of the immune response and wound healing. Pathogens that require T helper-type 2 (Th2) responses for effective clearance, such as parasitic worms, are strong inducers of alternatively activated or M2 macrophages. However, infections such as bacteria and viruses that require Th1-type responses may induce M2 as a strategy to evade the immune system. M2 are particularly efficient at scavenging self tissues following injury through receptors like the mannose receptor and scavenger receptor-A. Thus, M2 may increase autoimmune disease by presenting self tissue to T cells. M2 may also exacerbate immune complex (IC)-mediated pathology and fibrosis, a hallmark of autoimmune disease in women, due to the release of profibrotic factors such as interleukin-1beta, transforming growth factor-beta, fibronectin and matrix metalloproteinases. We have found that M2 comprise anywhere from 30% to 70% of the infiltrate during acute viral or experimental autoimmune myocarditis, and shifts in M2 populations correlate with increased IC deposition, fibrosis and chronic autoimmune pathology. Thus, women may be at an increased risk of M2-mediated autoimmunity due to estrogen's ability to increase Th2 responses.

Cytosolic phospholipase a2 activation by Candida albicans in alveolar macrophages: role of dectin-1

Candida albicans is an increasingly important pulmonary fungal pathogen. Resident alveolar macrophages are important in host defense against opportunistic fungal infections. Activation of Group IVA cytosolic phospholipase A(2)alpha (cPLA(2)alpha) in macrophages initiates arachidonic acid (AA) release for production of eicosanoids, which regulate inflammation and immune responses. We investigated the ability of C. albicans to activate cPLA(2)alpha in unprimed alveolar macrophages and after priming with granulocyte macrophage colony-stimulating factor (GM-CSF), which regulates alveolar macrophage maturation. AA was released within minutes by GM-CSF-primed but not unprimed alveolar macrophages in response to C. albicans, and was blocked by soluble glucan phosphate (S-GP). The expression of the beta-glucan receptor dectin-1 was increased in GM-CSF-primed macrophages, and AA release from GM-CSF-primed dectin-1(-/-) alveolar macrophages was reduced to basal levels. The enhanced activation of extracellular signal-regulated kinases and phosphorylation of cPLA(2)alpha on Ser-505 that occurred in GM-CSF-primed macrophages were reduced by MEK1 and Syk inhibitors, which also suppressed AA release. At later times after C. albicans infection (6 h), unprimed and GM-CSF-primed macrophages released similar levels of AA. The expression of cyclooxygenase 2 and prostanoid production at 6 hours was higher in GM-CSF-primed macrophages, but the responses were not dependent on dectin-1. However, dectin-1 contributed to the C. albicans-stimulated increase in TNF-alpha production that occurred in GM-CSF-primed macrophages. The results demonstrate that dectin-1 mediates the acute activation of cPLA(2)alpha in GM-CSF-primed alveolar macrophages, but not in the more delayed phase of AA release and GM-CSF-dependent prostanoid production.

The macrophage mannose receptor induces IL-17 in response to Candida albicans

The cytokine IL-17 controls neutrophil-mediated inflammatory responses. The pattern recognition receptor(s) that induce Th17 responses during infection, in the absence of artificial mitogenic stimulation with anti-CD3/anti-CD28 antibodies, remain obscure. We investigated the innate immune receptors and pathogen-associated molecular patterns involved in triggering Th17 responses during pathogen-specific host defense. The prototypic fungal pathogen Candida albicans was found to induce IL-17 more potently than Gram-negative bacteria. Candida mannan, but not zymosan, beta-glucans, Toll-like receptor (TLR) agonists, or the NOD2 ligand MDP, induced IL-17 production in the absence of anti-CD3/anti-CD28 antibodies. Candida-induced IL-17 response was dependent on antigen-presenting cells and the macrophage mannose receptor (MR), demonstrating that Candida mannan is not simply a mitogenic stimulus. The TLR2/dectin-1 pathway, but not TLR4 or NOD2, amplified MR-induced IL-17 production. This study identifies the specific pattern recognition receptors that trigger the Th17 response induced by a human pathogen in the absence of mitogenic stimulation.

The role of macrophages in inflammatory bowel diseases

The small and large intestine contain the largest number of macrophages in the body and these cells are strategically located directly underneath the epithelial layer, enabling them to sample the lumen. Such intestinal macrophages have a different phenotype from other tissue macrophages in that they ingest and may kill microbes but they do not mediate strong pro-inflammatory responses upon microbial recognition. These properties are essential for maintaining a healthy intestine. It is generally accepted that tolerance to the intestinal flora is lost in inflammatory bowel diseases, and genes involved in microbial recognition, killing and macrophage activation have already been associated with these diseases. In this review, we shed light on the intestinal macrophage and how it influences intestinal immunity.

Pattern recognition: recent insights from Dectin-1

The beta-glucan receptor Dectin-1 is an archetypical non-toll-like pattern recognition receptor expressed predominantly by myeloid cells, which can induce its own intracellular signalling and can mediate a variety of cellular responses, such as cytokine production. Recent identification of the components of these signalling pathways, such as Syk kinase, CARD9 and Raf-1, has provided novel insights into the molecular mechanisms underlying Dectin-1 function. Furthermore, a broader appreciation of the cellular responses mediated by this receptor and the effects of interactions with other receptors, including the TLRs, have greatly furthered our understanding of innate immunity and how this drives the development of adaptive immunity, particularly Th17 responses. Recent studies have highlighted the importance of Dectin-1 in anti-fungal immunity, in both mice and humans, and have suggested a possible involvement of this receptor in the control of mycobacterial infections.