Cryptococcus neoformans infections: aspartyl protease potential to improve outcome in susceptible hosts

Though a confined or a broad population is exposed respectively to endemic or pandemic infections, in the same environment, some individuals resist the development of infections. The attributed reason is the inheritance of a set of immune system genes that can efficiently deal with the pathogens. In this study, we show how outbred mice differentially respond to Cryptococcus neoformans, a fungal pathogen, and the mechanism through which the surviving mice mount a protective immune defense. We identified that those mice developing antibodies specifically against Pep1p, an aspartic protease secreted by C. neoformans, had significantly improved survival. Vaccination (either prophylactic or therapeutic) with a recombinant Pep1p significantly increased the survival of the mice by decreasing the fungal load and stimulating a protective immune response. Passive immunization of C. neoformans-infected mice with monoclonal antibodies developed against Pep1p also improves the survival of the mice by increasing phagocytosis of C. neoformans and decreasing the multiplication of this fungus. Together, these data demonstrate the prophylactic and therapeutic potentials of the C. neoformans antigenic protein Pep1p or Pep1p-specific antibodies against this fungal infection. Also, this study suggests that the immunological interaction and thereby the responses developed against a pathogen guide the hosts to behave differentially against microbial pathogenicity.

IMPORTANCE

Vaccination and immunotherapies against fungal pathogens still remain a challenge. Here, we show using an in vivo model based on outbred mice that development of antibodies against Pep1p, an antigenic protein of the fungal pathogen Cryptococcus neoformans, confers resistance to this fungal infection. In support of this observation, prophylactic or therapeutic immunization of the mice with recombinant Pep1p could improve their survival when infected with a lethal dose of C. neoformans. Moreover, passive therapy with monoclonal anti-Pep1p antibodies also enhanced survival of the mice from C. neoformans infection. The associated antifungal mechanisms were mounting of a protective immune response and the development of fungal specific antibodies that decrease the fungal burden due to an increase in their phagocytosis and/or inhibit the fungal multiplication. Together, our study demonstrates (a) the mode of host-fungal interaction and the immune response developed thereby play a crucial role in developing resistance against C. neoformans; (b) Pep1p, an aspartic protease as well as an antigenic protein secreted by C. neoformans, can be exploited for vaccination (both prophylactic and therapeutic) or immunotherapy to improve the host defense during this fungal infection.

Skin innate immune response against fungal infections and the potential role of trained immunity

Fungal skin infections are distributed worldwide and can be associated with economic and social traits. The immune response related to skin cells is complex and its understanding is essential to the comprehension of each cell's role and the discovery of treatment alternatives. The first studies of trained immunity (TI) described the ability of monocytes, macrophages and natural killer (NK) cells to develop a memory-like response. However, the duration of TI does not reflect the shorter lifespan of these cells. These conclusions supported later studies showing that TI can be observed in stem and haematopoietic cells and, more recently, also in non-immune skin cells such as fibroblasts, highlighting the importance of resident cells in response to skin disorders. Besides, the participation of less studied proinflammatory cytokines in the skin immune response, such as IL-36γ, shed light into a new possibility of inflammatory pathway blockade by drugs. In this review, we will discuss the skin immune response associated with fungal infections, the role of TI in skin and clinical evidence supporting opportunities and challenges of TI and other inflammatory responses in the pathogenesis of fungal skin infections.

Correlation between immune-related Tryptophan-Kynurenine pathway and severity of severe pneumonia and inflammation-related polyunsaturated fatty acids

BACKGROUND

Immune dysfunction and oxidative stress caused by severe pneumonia can lead to multiple organ dysfunction and even death, causing a significant impact on health and the economy. Currently, great progress has been made in the diagnosis and treatment of this disease, but the mortality rate remains high (approximately 50%). Therefore, there is still potential for further exploration of the immune response mechanisms against severe pneumonia.

OBJECTIVE

This study analyzed the difference in serum metabolic profiles between patients with severe pneumonia and health individuals through metabolomics, aiming to uncover the correlation between the Tryptophan-Kynurenine pathway and the severity of severe pneumonia, as well as N-3/N-6 polyunsaturated fatty acids (PUFAs).

METHODS

In this study, 44 patients with severe pneumonia and 37 health controls were selected. According to the changes in the disease symptoms within the 7 days of admission, the patients were divided into aggravation (n = 22) and remission (n = 22) groups. Targeted metabolomics techniques were performed to quantify serum metabolites and analyze changes between groups.

RESULTS

Metabolomics analysis showed that serum kynurenine and kynurenine/tryptophan (K/T) were significantly increased and tryptophan was significantly decreased in patients with severe pneumonia; HETE and HEPE in lipids increased significantly, while eicosapentaenoic acid (EPA), docosapentaenoic acid (DPA), docosahexaenoic acid (DHA), α-linolenic acid (linolenic acid, α-LNA), arachidonic acid (ARA), Dihomo-γ-linolenic acid (DGLA), and 13(s)-hydroperoxylinoleic acid (HPODE) decreased significantly. Additionally, the longitudinal comparison revealed that Linolenic acid, DPA, and Tryptophan increased significantly in the remission group, while and kynurenine and K/T decreased significantly. In the aggravation group, Kynurenine and K/T increased significantly, while ARA, 8(S)-hydroxyeicosatetraenoic acid (HETE), 11(S)-HETE, and Tryptophan decreased significantly. The correlation analysis matrix demonstrated that Tryptophan was positively correlated with DGLA, 12(S)-hydroxyeicosapentaenoic acid (HEPE), ARA, EPA, α-LNA, DHA, and DPA. Kynurenine was positively correlated with 8(S)-HETE and negatively correlated with DHA. Additionally, K/T was negatively correlated with DGLA, ARA, EPA, α-LNA, DHA, and DPA.

CONCLUSION

This study revealed that during severe pneumonia, the Tryptophan-Kynurenine pathway was activated and was positively correlated with the disease progression. On the other hand, the activation of the Tryptophan-Kynurenine pathway was negatively correlated with N-3/N-6 PUFAs.

Pseudolaric Acid B Ameliorates Fungal Keratitis Progression by Suppressing Inflammation and Reducing Fungal Load

This study aimed to determine the mechanisms of antifungal and anti-inflammation effects of pseudolaric acid B (PAB) on Aspergillus fumigatus (A. fumigatus) keratitis. In vitro MIC assay and crystal violet staining were conducted to evaluate the efficacy of PAB against A. fumigatus. PAB inhibited A. fumigatus growth and inhibited the formation of fungal biofilms in a dose-dependent manner. Molecular docking analysis revealed that PAB possesses strong binding properties with Rho1 of A. fumigatus, which is devoted to encoding (1,3)-β-d-glucan of A. fumigatus. RT-PCR results also showed that Rho1 was inhibited by PAB. In vivo, PAB treatment reduced clinical scores, fungal load, and macrophage infiltration, which were increased by A. fumigatus in mice corneas. In addition, PAB treatment suppressed the expression of Mincle, p-Syk, and cytokines (TNF-α, MIP2, iNOS, and CCL2) in infected corneas and in RAW264.7 cells, which were tested by RT-PCR, Western blot, and enzyme-linked Immunosorbent Assay. Notably, trehalose-6,6-dibehenate, an agonist of Mincle, pretreatment reversed the regulatory function of PAB in RAW 264.7 cells. Moreover, flow cytometry showed that PAB upregulated the ratio of M2/M1 macrophages in A. fumigatus-infected corneas and RAW264.7 cells. In conclusion, PAB produced antifungal activities against A. fumigatus and decreased the inflammatory response in mouse A. fumigatus keratitis.

The network interplay of interferon and Toll-like receptor signaling pathways in the anti-Candida immune response

Fungal infections represent a major global health problem affecting over a billion people that kills more than 1.5 million annually. In this study, we employed an integrative approach to reveal the landscape of the human immune responses to Candida spp. through meta-analysis of microarray, bulk, and single-cell RNA sequencing (scRNA-seq) data for the blood transcriptome. We identified across these different studies a consistent interconnected network interplay of signaling molecules involved in both Toll-like receptor (TLR) and interferon (IFN) signaling cascades that is activated in response to different Candida species (C. albicans, C. auris, C. glabrata, C. parapsilosis, and C. tropicalis). Among these molecules are several types I IFN, indicating an overlap with antiviral immune responses. scRNA-seq data confirmed that genes commonly identified by the three transcriptomic methods show cell type-specific expression patterns in various innate and adaptive immune cells. These findings shed new light on the anti-Candida immune response, providing putative molecular pathways for therapeutic intervention.

Therapeutic Effect of an Antibody-Derived Peptide in a Galleria mellonella Model of Systemic Candidiasis

The synthetic peptide T11F (TCRVDHRGLTF), with sequence identical to a fragment of the constant region of human IgM, and most of its alanine-substituted derivatives proved to possess a significant candidacidal activity in vitro. In this study, the therapeutic efficacy of T11F, D5A, the derivative most active in vitro, and F11A, characterized by a different conformation, was investigated in Galleria mellonella larvae infected with Candida albicans. A single injection of F11A and D5A derivatives, in contrast with T11F, led to a significant increase in survival of larvae injected with a lethal inoculum of C. albicans cells, in comparison with infected animals treated with saline. Peptide modulation of host immunity upon C. albicans infection was determined by hemocyte analysis and larval histology, highlighting a different immune stimulation by the studied peptides. F11A, particularly, was the most active in eliciting nodule formation, melanization and fat body activation, leading to a better control of yeast infection. Overall, the obtained data suggest a double role for F11A, able to simultaneously target the fungus and the host immune system, resulting in a more efficient pathogen clearance.

Radionuclide Imaging of Fungal Infections and Correlation with the Host Defense Response

The human response to invading fungi includes a series of events that detect, kill, or clear the fungi. If the metabolic host response is unable to eliminate the fungi, an infection ensues. Some of the host response’s metabolic events to fungi can be imaged with molecules labelled with radionuclides. Several important clinical applications have been found with radiolabelled biomolecules of inflammation. ¹⁸F-fluorodeoxyglucose is the tracer that has been most widely investigated in the host defence of fungi. This tracer has added value in the early detection of infection, in staging and visualising dissemination of infection, and in monitoring antifungal treatment. Radiolabelled antimicrobial peptides showed promising results, but large prospective studies in fungal infection are lacking. Other tracers have also been used in imaging events of the host response, such as the migration of white blood cells at sites of infection, nutritional immunity in iron metabolism, and radiolabelled monoclonal antibodies. Many tracers are still at the preclinical stage. Some tracers require further studies before translation into clinical use. The application of therapeutic radionuclides offers a very promising clinical application of these tracers in managing drug-resistant fungi.

Gut mycobiomes are altered in people with type 2 Diabetes Mellitus and Diabetic Retinopathy

Studies have documented dysbiosis in the gut mycobiome in people with Type 2 diabetes mellitus (T2DM). However, it is not known whether dysbiosis in the gut mycobiome of T2DM patients would be reflected in people with diabetic retinopathy (DR) and if so, is the observed mycobiome dysbiosis similar in people with T2DM and DR. Gut mycobiomes were generated from healthy controls (HC), people with T2DM and people with DR through Illumina sequencing of ITS2 region. Data were analysed using QIIME and R software. Dysbiotic changes were observed in people with T2DM and DR compared to HC at the phyla and genera level. Mycobiomes of HC, T2DM and DR could be discriminated by heat map analysis, Beta diversity analysis and LEfSE analysis. Spearman correlation of fungal genera indicated more negative correlation in HC compared to T2DM and DR mycobiomes. This study demonstrates dysbiosis in the gut mycobiomes in people with T2DM and DR compared to HC. These differences were significant both at the phyla and genera level between people with T2DM and DR as well. Such studies on mycobiomes may provide new insights and directions to identification of specific fungi associated with T2DM and DR and help developing novel therapies for Diabetes Mellitus and DR.

Immunotherapy as an Antifungal Strategy in Immune Compromised Hosts

Purpose of Review

IFIs cause high morbidity and mortality in the immunocompromised host worldwide. Although highly effective, conventional antifungal chemotherapy faces new challenges due to late diagnosis and increasing numbers of drug-resistant fungal strains. Thus, antifungal immunotherapy represents a viable treatment option, and recent advances in the field are summarized in this review.

Recent Findings

Antifungal immunotherapies include application of immune cells as well as the administration of cytokines, growth factors, and antibodies. Novel strategies to treat IFIs in the immunocompromised host target intracellular signaling pathways using SMTs such as checkpoint inhibitors.

Summary

Studies using cytokines or chemokines exerted a potential adjuvant role to conventional antifungal therapy, but issues on toxicity for some agents have to be resolved. Cell-based immunotherapies are very labor-intense and costly, but NK cell transfer and CAR T cell therapy provide exciting strategies to combat IFIs. Antibody-mediated protection and checkpoint inhibition are additional novel immunotherapeutic approaches.

In vitro immune responses of human PBMCs against Candida albicans reveals fungal and leucocyte phenotypes associated with fungal persistence

Although there is a growing understanding of immunity against Candida albicans, efforts need to be pursued in order to decipher the cellular mechanisms leading to an uncontrolled immune response that eventually oppose disease eradication. We describe here significant intra- and inter-subject variations in immune response patterns of major human leucocyte subsets following an in vitro challenge with C. albicans clinical isolates. We also observed that there are Candida isolate-dependent changes in leucocyte phenotypes. Through a combination of multiple fungal growth and flow cytometric measurements, coupled to the tSNE algorithm, we showed that significant proliferation differences exist among C. albicans isolates, leading to the calculation of a strain specific persistent index. Despite substantial inter-subject differences in T cells and stability of myeloid cells at baseline, our experimental approach highlights substantial immune cell composition changes and cytokine secretion profiles after C. albicans challenge. The significant secretion of IL-17 by CD66+ cells, IFN-γ and IL-10 by CD4+ T cells 2 days after C. albicans challenge was associated with fungal control. Fungal persistence was associated with delayed secretion of IFN-γ, IL-17, IL-4, TNF-α and IL-10 by myeloid cells and IL-4 and TNF-α secretion by CD4+ and CD8+ T cells. Overall, this experimental and analytical approach is available for the monitoring of such fungal and human immune responses.

The IDO-AhR Axis Controls Th17/Treg Immunity in a Pulmonary Model of Fungal Infection

In infectious diseases, the enzyme indoleamine 2,3 dioxygenase-1 (IDO1) that catalyzes the tryptophan (Trp) degradation along the kynurenines (Kyn) pathway has two main functions, the control of pathogen growth by reducing available Trp and immune regulation mediated by the Kyn-mediated expansion of regulatory T (Treg) cells via aryl hydrocarbon receptor (AhR). In pulmonary paracoccidioidomycosis (PCM) caused by the dimorphic fungus Paracoccidioides brasiliensis, IDO1 was shown to control the disease severity of both resistant and susceptible mice to the infection; however, only in resistant mice, IDO1 is induced by TGF-β signaling that confers a stable tolerogenic phenotype to dendritic cells (DCs). In addition, in pulmonary PCM, the tolerogenic function of plasmacytoid dendritic cells was linked to the IDO1 activity. To further evaluate the function of IDO1 in pulmonary PCM, IDO1-deficient (IDO1^-/-) C57BL/6 mice were intratracheally infected with P. brasiliensis yeasts and the infection analyzed at three postinfection periods regarding several parameters of disease severity and immune response. The fungal loads and tissue pathology of IDO1^-/- mice were higher than their wild-type controls resulting in increased mortality rates. The evaluation of innate lymphoid cells showed an upregulated differentiation of the innate lymphoid cell 3 phenotype accompanied by a decreased expansion of ILC1 and NK cells in the lungs of infected IDO1^-/- mice. DCs from these mice expressed elevated levels of costimulatory molecules and cytokine IL-6 associated with reduced production of IL-12, TNF-α, IL-1β, TGF-β, and IL-10. This response was concomitant with a marked reduction in AhR production. The absence of IDO1 expression caused an increased influx of activated Th17 cells to the lungs with a simultaneous reduction in Th1 and Treg cells. Accordingly, the suppressive cytokines IL-10, TGF-β, IL-27, and IL-35 appeared in reduced levels in the lungs of IDO1^-/- mice. In conclusion, the immunological balance mediated by the axis IDO/AhR is fundamental to determine the balance between Th17/Treg cells and control the severity of pulmonary PCM.

Agaricus brasiliensis polysaccharides stimulate human monocytes to capture Candida albicans, express toll-like receptors 2 and 4, and produce pro-inflammatory cytokines

Background

Agaricus brasiliensis is a medicinal mushroom with immunomodulatory and antitumor activities attributed to the β-glucans presented in the polysaccharide fraction of its fruiting body. Since β-glucans enhance cellular immunoresponsiveness, in this study we aimed to evaluate the effect of an acid-treated polysaccharide-rich fraction (ATF) of A. brasiliensis on the ability of human monocytes to adhere/phagocyte C. albicans yeast cells, their expression of pattern recognition receptors and their ability to produce cytokines.

Methods

Adhesion/phagocytosis of FITC-labeled C. albicans was evaluated by flow cytometry. Cells were incubated with specific fluorochrome-labeled antibodies for TLR2 and 4, βGR and MR and also evaluated by flow cytometry. Monocytes were cultured with ATF, and culture supernatants were collected for analysis of in vitro cytokine production by ELISA (TNF-α, IL-1β, IL-12 and IL-10).

Results

ATF significantly increased the adherence/phagocytosis of C. albicans by monocytes and this was associated with enhanced expression of TLR2 and TLR4, while no effect was observed on βGR or MR. Moreover, expression of TLR4 and TLR2 was associated with higher levels of in vitro production of TNF-α and IL-1, respectively. Production of IL-10 was also increased by ATF treatment, but we found no association between its production and the expression of Toll-like receptors.

Conclusion

Our results provided us with evidence that A. brasiliensis polysaccharides affect human monocytes probably through the modulation of Toll-like receptors.

Therapeutic targeting of inflammation and tryptophan metabolism in colon and gastrointestinal cancer

Colorectal cancer (CRC) is the third most common cancer worldwide and the second leading cause of cancer death in the United States. Cytotoxic therapies cause significant adverse effects for most patients and do not offer cure in many advanced cases of CRC. Immunotherapy is a promising new approach to harness the body's own immune system and inflammatory response to attack and clear the cancer. Tryptophan metabolism along the kynurenine pathway (KP) is a particularly promising target for immunotherapy. Indoleamine 2,3-dioxygenase 1 (IDO1) is the most well studied of the enzymes that initiate this pathway and it is commonly overexpressed in CRC. Herein, we provide an in-depth review of how tryptophan metabolism and KP metabolites shape factors important to CRC pathogenesis including the host mucosal immune system, pivotal transcriptional pathways of neoplastic growth, and luminal microbiota. This pathway's role in other gastrointestinal (GI) malignancies such as gastric, pancreatic, esophageal, and GI stromal tumors is also discussed. Finally, we highlight how currently available small molecule inhibitors and emerging methods for therapeutic targeting of IDO1 might be applied to colon, rectal, and colitis-associated cancer.

Inflammation in aspergillosis: the good, the bad, and the therapeutic

Aspergillosis includes a spectrum of diseases caused by different Aspergillus spp. New insights into the cellular and molecular mechanisms of resistance and immune tolerance to the fungus in infection and allergy have been obtained in experimental settings. The fact that virulence factors, traditionally viewed as fungal attributes, are contingent upon microbial adaptation to various environmental stresses encountered in the human host implies that the host and fungus are jointly responsible for pathogenicity. Ultimately, despite the occurrence of severe aspergillosis in immunocompromised patients, clinical evidence indicates that aspergillosis also occurs in the setting of a heightened inflammatory response, in which immunity occurs at the expense of host damage and pathogen eradication. Thus, targeting pathogenicity rather than microbial growth, tolerance rather than resistance mechanisms of defense may pave the way to targeted anti-inflammatory strategies in difficult-to-treat patients. The challenge now is to translate promising results from experimental models to the clinic.

Signaling pathways bridging microbial-triggered inflammation and cancer

Microbial-triggered inflammation protects against pathogens and yet can paradoxically cause considerable secondary damage to host tissues that can result in tissue fibrosis and carcinogenesis, if persistent. In addition to classical pathogens, gut microbiota bacteria, i.e. a group of mutualistic microorganisms permanently inhabiting the gastrointestinal tract and which plays a key role in digestion, immunity, and cancer prevention, can induce inflammation-associated cancer following the alterations of their microenvironment. Emerging experimental evidence indicates that microbiota members like Escherichia coli and several other genotoxic and mutagenic pathogens can cause DNA damage in various cell types. In addition, the inflammatory response induced by chronic infections with pathogens like the microbiota members Helicobacter spp., which have been associated with liver, colorectal, cervical cancers and lymphoma, for instance, can also trigger carcinogenic processes. A microenvironment including active immune cells releasing high amounts of inflammatory signaling molecules can favor the carcinogenic transformation of host cells. Pivotal molecules released during immune response such as the macrophage migration inhibitory factor (MMIF) and the reactive oxygen and nitrogen species' products superoxide and peroxynitrite, can further damage DNA and cause the accumulation of oncogenic mutations, whereas pro-inflammatory cytokines, adhesion molecules, and growth factors may create a microenvironment promoting neoplastic cell survival and proliferation. Recent findings on the implication of inflammatory signaling pathways in microbial-triggered carcinogenesis as well as the possible role of microbiota modulation in cancer prevention are herein summarized and discussed.

DAMP signaling in fungal infections and diseases

Fungal infections and diseases predominantly affect patients with deregulated immunity. Compelling experimental and clinical evidence indicate that severe fungal diseases belong to the spectrum of fungus-related inflammatory diseases. Some degree of inflammation is required for protection during the transitional response occurring temporally between the rapid innate and slower adaptive response. However, progressive inflammation worsens disease and ultimately prevents pathogen eradication. The challenge now is to elucidate cellular and molecular pathways distinguishing protective vs. pathogenic inflammation to fungi. In addition to fungal ligands of pattern recognition receptors (pathogen-associated molecular patterns, PAMPs), several host-encoded proteins, the damage-associated molecular patterns (DAMPs), are released during tissue injury and activate innate recognition receptors. DAMPs have been shown to regulate inflammation in fungal diseases. The DAMP/receptor for advanced glycation end-products axis integrated with the PAMP/Toll-like receptors axis in the generation of the inflammatory response in experimental and clinical fungal pneumonia. These emerging themes better accommodate fungal pathogenesis in the face of high-level inflammation seen in several clinical settings and point to DAMP targeting as a novel immunomodulatory strategy in fungal diseases.

Immunity and tolerance to fungi in hematopoietic transplantation: principles and perspectives

Resistance and tolerance are two complementary host defense mechanisms that increase fitness in response to low-virulence fungi. Resistance is meant to reduce pathogen burden during infection through innate and adaptive immune mechanisms, whereas tolerance mitigates the substantial cost of resistance to host fitness through a multitude of anti-inflammatory mechanisms, including immunological tolerance. In experimental fungal infections, both defense mechanisms are activated through the delicate equilibrium between Th1/Th17 cells, which provide antifungal resistance, and regulatory T cells limiting the consequences of the ensuing inflammatory pathology. Indoleamine 2,3-dioxygenase (IDO), a rate-limiting enzyme in the tryptophan catabolism, plays a key role in induction of tolerance against fungi. Both hematopoietic and non-hematopoietic compartments contribute to the resistance/tolerance balance against Aspergillus fumigatus via the involvement of selected innate receptors converging on IDO. Several genetic polymorphisms in pattern recognition receptors influence resistance and tolerance to fungal infections in human hematopoietic transplantation. Thus, tolerance mechanisms may be exploited for novel diagnostics and therapeutics against fungal infections and diseases.

Immunity and tolerance to infections in experimental hematopoietic transplantation

Resistance and tolerance are two types of host defense mechanisms that increase fitness in response to fungi. Several genetic polymorphisms in pattern recognition receptors, most remarkably Toll-like receptors (TLRs), have been described to influence resistance and tolerance to aspergillosis in distinct clinical settings. TLRs on dendritic cells pivotally contribute in determining the balance between immunopathology and protective immunity to the fungus. Epithelial cells also contribute to this balance via selected TLRs converging on indoleamine-2,3-dioxygenase (IDO). Studies in experimental hematopoietic transplantation confirmed the dichotomy of pathways leading to resistance and tolerance to the fungus providing new insights on the relative contribution of the hematopoietic/nonhematopoietic compartments.

Immunotherapy against invasive fungal diseases in stem cell transplant recipients

Despite the availability of new antifungal compounds, morbidity and mortality of invasive fungal disease in allogeneic hematopoietic stem cell recipients are still unacceptably high. Over the past decade, one could witness an exciting improvement of the understanding of the molecular pathogenesis and of the complexity of host antifungal immune responses. This, in turn, provides critical information to augment host immunity against fungal pathogens. Strategies for enhancing the immune system include the administration of effector and regulatory cells (e.g., granulocytes, antigen-specific T cells, dendritic cells) as well as the administration of recombinant cytokines, interferons and growth factors (e.g., interferon-γ, keratinocyte growth factor, granulocyte- and granulocyte-macrophage colony stimulating factor). One has to recognize at the same time, however, that data of in vitro assays and animal models cannot necessarily be transferred into the clinical setting. In addition, meaningful clinical trials in allogeneic stem cell recipients suffering from invasive fungal disease require sufficiently large and homogenous cohorts of patients and can only be performed in international collaboration, but may ultimately improve the outcome of allogeneic transplant recipients with invasive fungal disease.

Cracking the Toll-like receptor code in fungal infections

Innate control of fungal infection requires the specific recognition of invariant fungal molecular structures by a variety of innate immune receptors, including Toll-like receptors. In addition to the role in inducing protective immune responses, Toll-like receptor engagement may paradoxically favor fungal infections, by inducing inflammatory pathology and impairing antifungal immunity. Although the dissection of complex genetic traits modulating susceptibility to fungal infections is complex, the contribution of host genetics may hold the key to elucidating new risk factors for these severe, often fatal diseases. Understanding host-pathogen interactions at the innate immune interface will eventually lead to the development of new therapeutics and genetic markers in fungal infections.

Genetic susceptibility to aspergillosis in allogeneic stem-cell transplantation

Invasive aspergillosis (IA) is a major threat to positive outcomes for allogeneic stem-cell transplantation (allo-SCT) patients. Despite presenting similar degrees of immunosuppression, not all individuals at-risk ultimately develop infection. Therefore, the traditional view of neutropenia as a key risk factor for aspergillosis needs to be accommodated within new conceptual advances on host immunity and its relationship to infection. Polymorphisms in innate immune genes, such as those encoding TLRs, cytokines and cytokine receptors, have recently been associated with susceptibility to IA in allo-SCT recipients. This suggests that understanding host-pathogen interactions at the level of host genetic susceptibility will allow the formulation of new targeted and patient-tailored antifungal therapeutics, including improved donor screening.

Variability of phenotypic traits in Cryptococcus varieties and species and the resulting implications for pathogenesis

Variability of phenotypic characteristics in Cryptococcus neoformans var. grubii and var. neoformans as well as Cryptococcus gattii can have diverse effects on the virulence of these fungi and are thus important for pathogenesis. This article summarizes the diverse phenotypic changes that these fungi can manifest. We divide changes into those that affect the entire fungal population and are predominantly induced by environmental signals, and those that involve subpopulations of the fungal population and have to be selected. Last, the article summarizes the experimental evidence that epitopes on the polysaccharide capsule also vary, which may have implications for the pathogenesis as these findings would further diversify the fungal population.

A systems biology approach to the mutual interaction between yeast and the immune system

Dendritic cells (DCs) are capable of sensing fungi and then to initiate an appropriate defense against the invading microbe. We studied interactions between host and microorganism by analyzing the transcriptional response of DCs stimulated by the harmless Saccharomyces cerevisiae and of this phagocytosed fungus. Pathway analyses provided insight into the mutual interactions. Of particular interest was the responses elicited by the DC in the fungus, including downregulation of the carbon-compound metabolism, and upregulation of lipid, fatty acid, glyoxylate and tricarboxylic acid cycles. This indicates that the yeast shifts to a starvation mode and induces morphogenetic and autophagic pathways as well as those associated with reshaping cell wall composition, to resist the immune clearance. This yeast response is independent of the presence of virulence traits as the same transcriptional cell reprogramming has also been observed in potentially pathogenic C. albicans hyphae phagocytosed by macrophages. When comparing our results with the previous findings, it appears that the yeast dimorphic switch is only one of the components of the evolutionarily conserved panels of survival strategies elicited by phagocytosis. In conclusion, a systems biology approach, which combines genomics and pathway analyses, provides a powerful strategy to further our understanding of complex host-pathogen interactions and may ultimately define the distinguishing features of pathogenicity and commensalism.

Cellular and cytokine changes in the alveolar environment among immunocompromised patients during Pneumocystis jirovecii infection

Limited data exist on the cytokine and cellular changes in the alveolar environment in immunocompromised patients during Pneumocystis jirovecii infection. A cellular and a cytokine analysis were performed on bronchoalveolar lavage (BAL) samples from three groups of patients, i.e., an initial study group of 64 immunocompromised P. jirovecii-positive individuals and two control groups of P. jirovecii-negative patients who had been or not immunosuppressed (65 patients). The results were related to alveolar dilution as determined by urea measurement. Compared with non-infected groups, P. jirovecii-infected patients had a lower level of alveolar macrophages (AM), particularly those with high burdens of P. jirovecii. Alveolar macrophages over-expressed the Dectin-1 receptor, which was largely implicated in P. jirovecii clearance. The alveolar CD8+T and CD4+T lymphocyte counts were increased and an inverse correlation was observed between the alveolar CD4+ cell count and the P. jirovecii burden. Although the alveolar IL-6 level was considerably increased, alveolar IL-17, IL-10, TNF-α, TGF-β concentrations of P. jirovecii patients were not different from the control groups. Changes in the pulmonary environment were also highlighted during P. jirovecii colonization. Our study suggests that there is a correlation between the P. jirovecii burden in the alveolus (from colonization to a high P. jirovecii burden), and the degree of impairment of the alveolar immune response.

Genetic variability of innate immunity impacts human susceptibility to fungal diseases

Fungi are a major threat in immunocompromised patients. Despite presenting similar degrees of immunosuppression, not all individuals at-risk ultimately develop fungal diseases. The traditional view of immune suppression as a key risk factor for susceptibility to fungal infections needs to be accommodated within new conceptual advances on host immunity and its relationship to fungal disease. The critical role of the immune system emphasizes the contribution of host genetic polymorphisms to fungal disease susceptibility. This review highlights the present knowledge on innate immunity genetics that associates with susceptibility to fungal diseases.

Early-expressed chemokines predict kidney immunopathology in experimental disseminated Candida albicans infections

BACKGROUND

The mouse intravenous challenge model of Candida albicans infection is widely used to determine aspects of host-fungus interaction. We investigated the production of cytokines in the kidneys and spleen of animals up to 48 h after challenge with virulent and attenuated isolates and related these responses to semi-quantitative estimations of histopathological changes in the kidney.

METHODOLOGY/PRINCIPAL FINDINGS

Progression of Candida albicans infection of the kidney in response to highly virulent fungal strains was characterized by higher levels of host cellular infiltrate, higher lesion densities and greater quantities of fungal elements at 24 and 48 h, and by higher kidney concentrations of IL-1beta, MCP-1, KC, IL-6, G-CSF, TNF, MIP-2 and MIP-1beta, among the immune effectors measured. Levels of the chemokine KC as early as 12 h after challenge correlated significantly with all later measurements of lesion severity. Early renal IL-6 and MIP-1beta concentrations also correlated with subsequent damage levels, but less significantly than for KC. All chemokines tested appeared in kidney homogenates, while most of the cytokines were undetectable in kidney and spleen homogenates. GM-CSF and IL-10 showed inverse correlations with measures of lesion severity, suggesting these alone may have exerted a defensive role. Spleen levels of KC at all times showed significant associations with kidney lesion measurements.

CONCLUSIONS/SIGNIFICANCE

Elevated chemokine levels, including KC, represent the earliest responses to C. albicans infection in the mouse kidney. Fungal strains of low mouse virulence stimulate a lower innate response and less host infiltrate than more virulent strains. These findings are consistent with immunopathological damage to kidneys in the mouse C. albicans infection model and with growing evidence implicating some TLR pathways as the main point of interaction between fungal surface polysaccharides and leukocytes.

Polymorphisms in Toll-like receptor genes and susceptibility to infections in allogeneic stem cell transplantation

OBJECTIVE

Discovery of genetic variations in the genes encoding for Toll-like receptors (TLRs) has highlighted a potential link between genomic variation of the host and susceptibility to infections.

MATERIALS AND METHODS

We investigated the association between polymorphisms in the TLR2, TLR4, and TLR9 genes in recipients of allogeneic hematopoietic stem cell transplant and susceptibility to infections caused by cytomegalovirus and filamentous fungi.

RESULTS

A significant association was observed between the presence of the T-1237C polymorphism (TLR9) and susceptibility to viral pneumonia (p=0.04; odds ratio [OR]: 1.73). For fungi, a significant association was observed between the presence of the cosegregating Asp299Gly/Thr399Ile polymorphisms (TLR4) and fungal colonization (p=0.003; OR: 10.6). However, susceptibility to fungal infections, predominantly fungal pneumonia, was instead significantly decreased in the presence of the same polymorphisms (p=0.03; OR: 0.23).

CONCLUSION

Thus, fungal colonization may not predict susceptibility to infection in the presence of these single nucleotide polymorphisms. The finding that defective viral but not fungal sensing may predict susceptibility to infection highlights the divergent function of TLRs in the pathogenesis of opportunistic infections.

Tuning inflammation in tuberculosis: the role of decoy receptors

Decoy receptors are "silent scavengers" of CC chemokines and cytokines, which play a key role in damping inflammation and tissue damage. In this review we discuss on recent findings demonstrating that these receptors set the balance between antimicrobial resistance, immune activation and inflammatory response in Mycobacterium tuberculosis infection.

Cell mediated immunity to fungi: a reassessment

Protective immunity against fungal pathogens is achieved by the integration of two distinct arms of the immune system, the innate and adaptive responses. Innate and adaptive immune responses are intimately linked and controlled by sets of molecules and receptors that act to generate the most effective form of immunity for protection against fungal pathogens. The decision of how to respond will still be primarily determined by interactions between pathogens and cells of the innate immune system, but the actions of T cells will feed back into this dynamic equilibrium to regulate the balance between tolerogenic and inflammatory responses. In the last two decades, the immunopathogenesis of fungal infections and fungal diseases was explained primarily in terms of Th1/Th2 balance. Although Th1 responses driven by the IL-12/IFN-gamma axis are central to protection against fungi, other cytokines and T cell-dependent pathways have come of age. The newly described Th17 developmental pathway may play an inflammatory role previously attributed to uncontrolled Th1 responses and serves to accommodate the seemingly paradoxical association of chronic inflammatory responses with fungal persistence in the face of an ongoing inflammation. Regulatory T cells in their capacity to inhibit aspects of innate and adaptive antifungal immunity have become an integral component of immune resistance to fungi, and provide the host with immune defense mechanisms adequate for protection, without necessarily eliminating fungal pathogens which would impair immune memory--or causing an unacceptable level of tissue damage. The enzyme indoleamine 2,3-dioxygenase and tryptophan metabolites contribute to immune homeostasis by inducing Tregs and taming overzealous or heightened inflammatory responses.

Candida albicans–macrophage interactions: genomic and proteomic insights

Candida albicans infection is a significant cause of morbidity and mortality in immunocompromised patients. In vivo and in vitro models have been developed to study both the fungal and the mammalian immune responses. Phagocytic cells (i.e., macrophages) play a key role in innate immunity against C. albicans by capturing, killing and processing the pathogen for presentation to T cells. The use of microarray technology to study global fungal transcriptional changes after interaction with different host cells has revealed how C. albicans adapts to its environment. Proteomic tools complement molecular approaches and computational methods enable the formulation of relevant biological hypotheses. Therefore, the combination of genomics, proteomics and bioinformatics tools (i.e., network analyses) is a powerful strategy to better understand the biological situation of the fungus inside macrophages; part of the fungal population is killed while a significantly high percentage survives.

Toll-like receptor 9 modulates immune responses to Aspergillus fumigatus conidia in immunodeficient and allergic mice

The role of Toll-like receptor 9 (TLR9) in antifungal responses in the immunodeficient and allergic host is unclear. We investigated the role of TLR9 in murine models of invasive aspergillosis and fungal asthma. Neutrophil-depleted TLR9 wild-type (TLR9(+/+)) and TLR9-deficient (TLR9(-/-)) mice were challenged with resting or swollen Aspergillus fumigatus conidia and monitored for survival and lung inflammatory responses. The absence of TLR9 delayed, but did not prevent, mortality in immunodeficient mice challenged with resting or swollen conidia compared to TLR9(+/+) mice. In a fungal asthma model, TLR9(+/+) and TLR9(-/-) mice were sensitized to soluble A. fumigatus antigens and challenged with resting or swollen A. fumigatus conidia, and both groups of mice were analyzed prior to and at days 7, 14, and 28 after the conidium challenge. When challenged with resting conidia, TLR9(-/-) mice exhibited significantly lower airway hyper-responsiveness compared to the TLR9(+/+) groups. In contrast, A. fumigatus-sensitized TLR9(-/-) mice exhibited pulmonary fungal growth at days 14 and 28 after challenge with swollen conidia, a finding never observed in their allergic wild-type counterparts. Increased fungal growth in allergic TLR9(-/-) mice correlated with markedly decreased dectin-1 expression in whole lung samples and isolated dendritic cell populations. Further, whole lung levels of interleukin-17 were lower in allergic TLR9(-/-) mice compared to similar TLR9(+/+) mice. Together, these data suggest that TLR9 modulates pulmonary antifungal immune responses to swollen conidia, possibly through the regulation of dectin-1 expression.

Parasites and autoimmunity: the case of fungi

Infectious agents can induce autoimmune diseases in several experimental settings, some of which have clinical counterparts. A variety of mechanisms have been invoked to explain these observations, including molecular mimicry and an increase in the immunogenicity of autoantigens caused by inflammation in the target organ. Paradoxically, infectious agents can also suppress allergic and autoimmune disorders. A central question is to determine whether immune dysregulation precedes, if not promotes, infection or alternatively, but not mutually exclusive, the extent to which microbial exposure/colonization contributes to the burst of pathogenic autoimmunity. Here we discussed recent evidence with fungi that help to accommodate microbes, either commensals or ubiquitous, within the immune homeostasis and its dysregulation.

IL-17 and therapeutic kynurenines in pathogenic inflammation to fungi

Largely viewed as proinflammatory, innate responses combine with adaptive immunity to generate the most effective form of antifungal resistance, and T cells exercise feedback control over diverse effects of inflammation on infection. Some degree of inflammation is required for protection, particularly in mucosal tissues, during the transitional response occurring between the rapid innate and slower adaptive response. However, progressive inflammation worsens disease and ultimately prevents pathogen eradication. IDO, tryptophan catabolites ("kynurenines"), and regulatory T cells help to tame overzealous and exaggerated inflammatory responses. In this context, IL-23 and the Th17 pathway, which down-regulate tryptophan catabolism, may instead favor pathology and serve to accommodate the seemingly paradoxical association of chronic inflammation with fungal persistence. Recent data support a view in which IL-23/IL-17 antagonistic strategies, including the administration of synthetic kynurenines, could represent a new means of harnessing progressive or potentially harmful inflammation.

Lack of Toll IL-1R8 exacerbates Th17 cell responses in fungal infection

TLRs contribute to the inflammatory response in fungal infections. Although inflammation is an essential component of the protective response to fungi, its dysregulation may significantly worsen fungal diseases. In this study, we tested the hypothesis that Toll IL-1R8 (TIR8)/single Ig IL-1-related receptor, a member of the IL-1R family acting as a negative regulator of TLR/IL-1R signaling, affects TLR responses in fungal infections. Genetically engineered Tir8(-/-) mice were assessed for inflammatory and adaptive Th cell responses to Candida albicans and Aspergillus fumigatus. Inflammatory pathology and susceptibility to infection were higher in Tir8(-/-) mice and were causally linked to the activation of the Th17 pathway. IL-1R signaling was involved in Th17 cell activation by IL-6 and TGF-beta in that limited inflammatory pathology and relative absence of Th17 cell activation were observed in IL-1RI(-/-) mice. These data demonstrate that TIR8 is required for host resistance to fungal infections and that it functions to negatively regulate IL-1-dependent activation of inflammatory Th17 responses. TIR8 may contribute toward fine-tuning the balance between protective immunity and immunopathology in infection.

Novel immune regulatory pathways and their role in immune reconstitution syndrome in organ transplant recipients with invasive mycoses

Immune regulatory pathways involving the newly discovered T regulatory (Treg) and Th17 cells are amongst the principal targets of immunosuppressive agents employed in transplant recipients and key mediators of host inflammatory responses in fungal infections. These novel signaling pathways, in concert with or independent of Th1/Th2 responses, have potentially important implications for yielding valuable insights into the pathogenesis of immune reconstitution syndrome (IRS) in transplant recipients, for aiding the diagnosis of this entity, and for achieving a balance of immune responses that enhance host immunity while curbing unfettered inflammation in IRS.

Influence of aging on murine neutrophil and macrophage function against Candida albicans

Previous work by our group showed that aged C57BL/6 mice develop an altered innate and adaptive immune response to Candida albicans and are more susceptible to systemic primary candidiasis. In this work, we used young (2-3 months old) and aged (18-20 months old) C57BL/6 mice to study in vitro the influence of aging on (1) the fungicidal activity of neutrophils and macrophages, (2) the production of cytokines by resident peritoneal macrophages in response to C. albicans, and (3) cell surface Toll-like receptor (TLR) 2 expression on resident peritoneal macrophages. Our results indicate that murine phagocytes have a fungicidal activity well preserved with aging. In vitro production of proinflammatory cytokines (IL-6, IL-1beta, and tumor necrosis factor-alpha and chemokines (MIP-2) by purified (CD11b(+)) peritoneal macrophages in response to yeasts and hyphae of C. albicans was significantly lower in aged mice as compared with young mice. However, the production of IL-10 by macrophages, in response to C. albicans, was similar in both young and aged animals. Moreover, baseline TLR2 surface expression level was lower on aged macrophages than on control macrophages. Taken together, these data indicate that the increased susceptibility to C. albicans disseminated infections in aged mice is correlated with defects in TLR2 expression and in cytokine production, but not with an impaired fungicidal activity.