Fungal and host protein persulfidation are functionally correlated and modulate both virulence and antifungal response

Aspergillus fumigatus is a human fungal pathogen that can cause devastating pulmonary infections, termed “aspergilloses,” in individuals suffering immune imbalances or underlying lung conditions. As rapid adaptation to stress is crucial for the outcome of the host–pathogen interplay, here we investigated the role of the versatile posttranslational modification (PTM) persulfidation for both fungal virulence and antifungal host defense. We show that an A. fumigatus mutant with low persulfidation levels is more susceptible to host-mediated killing and displays reduced virulence in murine models of infection. Additionally, we found that a single nucleotide polymorphism (SNP) in the human gene encoding cystathionine γ-lyase (CTH) causes a reduction in cellular persulfidation and correlates with a predisposition of hematopoietic stem cell transplant recipients to invasive pulmonary aspergillosis (IPA), as correct levels of persulfidation are required for optimal antifungal activity of recipients’ lung resident host cells. Importantly, the levels of host persulfidation determine the levels of fungal persulfidation, ultimately reflecting a host–pathogen functional correlation and highlighting a potential new therapeutic target for the treatment of aspergillosis.

This study reveals that the post-translational modification persulfidation is important for both fungal virulence and the host antifungal response. The level of persulfidation in the host, which correlates with its antifungal potency, impacts the level required in the fungus to counteract host attack, reflecting a functional correlation. Thus modulating persulfidation may be a promising strategy to target both pathogens and immune responses.     

Antimicrobial Phytoprotectants and Fungal Pathogens: A Commentary

Many plants produce antifungal secondary metabolites. These may be preformed compounds which are found in healthy plants and which may represent in-built chemical barriers to infection by potential pathogens (preformed antimicrobial compounds or phytoanticipins). Alternatively they may be synthesized in response to pathogen attack as part of the plant defence response (phytoalexins). If these molecules do play a role in protecting plants against pathogen attack, then successful pathogens are presumably able to circumvent or tolerate these defences. Strategies may include avoidance, enzymatic degradation, and/or nondegradative mechanisms. This review outlines the different ways in which fungal pathogens may counter the antifungal compounds produced by their host plants and summarizes the evidence for and against these compounds as antimicrobial phytoprotectants.     

The β-glucan receptor Dectin-1 activates the integrin Mac-1 in neutrophils via Vav protein signaling to promote Candida albicans clearance

Resistance to fungal infections is attributed to engagement of host pattern-recognition receptors, notably the β-glucan receptor Dectin-1 and the integrin Mac-1, which induce phagocytosis and antifungal immunity. However, the mechanisms by which these receptors coordinate fungal clearance are unknown. We show that upon ligand binding, Dectin-1 activates Mac-1 to also recognize fungal components, and this stepwise process is critical for neutrophil cytotoxic responses. Both Mac-1 activation and Dectin-1- and Mac-1-induced neutrophil effector functions require Vav1 and Vav3, exchange factors for RhoGTPases. Mac-1- or Vav1,3-deficient mice have increased susceptibility to systemic candidiasis that is not due to impaired neutrophil recruitment but defective intracellular killing of C.?albicans yeast forms, and Mac-1 or Vav1,3 reconstitution in hematopoietic cells restores resistance. Our results demonstrate that antifungal immunity depends on Dectin-1-induced activation of Mac-1 functions that is coordinated by Vav proteins, a pathway that may localize cytotoxic responses of circulating neutrophils to infected tissues.     

Antifungals: mechanism of action and resistance, established and novel drugs

Serious fungal infections, caused mostly by opportunistic species, are increasingly common in immunocompromised and other vulnerable patients. The use of antifungal drugs, primarily azoles and polyenes, has increased in parallel. Yet, established agents do not satisfy the medical need completely: azoles are fungistatic and vulnerable to resistance, whereas polyenes cause serious host toxicity. Drugs in clinical development include echinocandins, pneumocandins, and improved azoles. Promising novel agents in preclinical development include several inhibitors of fungal protein, lipid and cell wall syntheses. Recent advances in fungal genomics, combinatorial chemistry, and high-throughput screening may accelerate the antifungal discovery process.     

The fungal cell wall as a target for the development of new antifungal therapies

In the past three decades invasive mycoses have globally emerged as a persistent source of healthcare-associated infections. The cell wall surrounding the fungal cell opposes the turgor pressure that otherwise could produce cell lysis. Thus, the cell wall is essential for maintaining fungal cell shape and integrity. Given that this structure is absent in host mammalian cells, it stands as an important target when developing selective compounds for the treatment of fungal infections. Consequently, treatment with echinocandins, a family of antifungal agents that specifically inhibits the biosynthesis of cell wall (1-3)β-D-glucan, has been established as an alternative and effective antifungal therapy. However, the existence of many pathogenic fungi resistant to single or multiple antifungal families, together with the limited arsenal of available antifungal compounds, critically affects the effectiveness of treatments against these life-threatening infections. Thus, new antifungal therapies are required. Here we review the fungal cell wall and its relevance in biotechnology as a target for the development of new antifungal compounds, disclosing the most promising cell wall inhibitors that are currently in experimental or clinical development for the treatment of some invasive mycoses.     

The impact of polyphenolic compounds on the in vitro growth of oak-associated foliar endophytic and saprotrophic fungi

Foliar fungal endophytes are horizontally transmitted symbionts that inhabit healthy, photosynthetic tissues of all lineages of land plants where they influence plant health and productivity. Endophyte communities often are more similar among closely related hosts, potentially as a result of a preference for particular morphological, ecophysiological, or chemical host traits. However, the various ecological and evolutionary factors that drive community assembly often are difficult to disentangle. Here, we examined the impact of six polyphenolic compounds on the growth of 15 phylogenetically diverse Quercus (oak)-associated fungal species and assessed whether tolerance to phenolics is associated with their degree of specialization to oaks in nature. Despite frequently reported antifungal properties of phenolics, we found that oak-associated fungi grew the same or better than positive controls in 78% of trials with all compounds, although fungal sensitivity differed as a function of compound type and concentration. Overall, species with high specificity to Quercus had the greatest tolerance to phenolics, whereas generalists were more sensitive. Differences between generalists and specialists suggest that variation in phenolic abundance and composition among oaks may act as a selective filter that influences endophyte host associations in nature.     

中药成分抗真菌机制研究进展

目前,寻找新型抗真菌药物迫在眉睫。已有研究表明,有效中药成分具有良好的抗真菌作用,可能具有应用前景。本文综述了抗真菌中药及其抗真菌的主要机制,涉及干扰物质运输、酵母-菌丝相转化、宿主免疫和氧化还原等方面,为中药及其提取物在抗真菌中的应用提供了一定的指导。     

The role of chitin detection in plant--pathogen interactions

Despite the deployment of antifungal defence strategies, fungal diseases occur in all types of multicellular organisms. In plants, the role of fungal chitin as pathogen-associated molecular pattern that activates host defence is well established. Interestingly, plants employ homologs of the chitin immune receptors to initiate microbial symbiosis. Accumulating evidence shows that fungal pathogens developed secreted effectors to disarm chitin-triggered host immunity.     

Interacting Symbionts and Immunity in the Amphibian Skin Mucosome Predict Disease Risk and Probiotic Effectiveness

Pathogenesis is strongly dependent on microbial context, but development of probiotic therapies has neglected the impact of ecological interactions. Dynamics among microbial communities, host immune responses, and environmental conditions may alter the effect of probiotics in human and veterinary medicine, agriculture and aquaculture, and the proposed treatment of emerging wildlife and zoonotic diseases such as those occurring on amphibians or vectored by mosquitoes. Here we use a holistic measure of amphibian mucosal defenses to test the effects of probiotic treatments and to assess disease risk under different ecological contexts. We developed a non-invasive assay for antifungal function of the skin mucosal ecosystem (mucosome function) integrating host immune factors and the microbial community as an alternative to pathogen exposure experiments. From approximately 8500 amphibians sampled across Europe, we compared field infection prevalence with mucosome function against the emerging fungal pathogen Batrachochytrium dendrobatidis. Four species were tested with laboratory exposure experiments, and a highly susceptible species, Alytes obstetricans, was treated with a variety of temperature and microbial conditions to test the effects of probiotic therapies and environmental conditions on mucosome function. We found that antifungal function of the amphibian skin mucosome predicts the prevalence of infection with the fungal pathogen in natural populations, and is linked to survival in laboratory exposure experiments. When altered by probiotic therapy, the mucosome increased antifungal capacity, while previous exposure to the pathogen was suppressive. In culture, antifungal properties of probiotics depended strongly on immunological and environmental context including temperature, competition, and pathogen presence. Functional changes in microbiota with shifts in temperature provide an alternative mechanistic explanation for patterns of disease susceptibility related to climate beyond direct impact on host or pathogen. This nonlethal management tool can be used to optimize and quickly assess the relative benefits of probiotic therapies under different climatic, microbial, or host conditions.     

T-cell Subsets and Antifungal Host Defenses

It has been long appreciated that protective immunity against fungal pathogens is dependent on activation of cellular adaptive immune responses represented by T lymphocytes. The T-helper (Th)1/Th2 paradigm has proven to be essential for the understanding of protective adaptive host responses. Studies that have examined the significance of regulatory T cells in fungal infection, and the recent discovery of a new T-helper subset called Th17 have provided crucial information for understanding the complementary roles played by the various T-helper lymphocytes in systemic versus mucosal antifungal host defense. This review provides an overview of the role of the various T-cell subsets during fungal infections and the reciprocal regulation between the T-cell subsets contributing to the tailored host response against fungal pathogens.     

涉及临床各科的侵袭性真菌感染

真菌数量大、种类多、分布广。真菌感染因菌种的毒力、感染途径和宿主免疫状态等差异而出现不同的临床表现,临床各科医生都会遇到真菌病患者。真菌病的诊断取决于在病变组织内发现病原真菌成分如菌丝和(或)孢子。从病变组织中分离出真菌是菌种鉴定前提,也是抗真菌治疗的依据。“重视临床、强调菌学”是医学真菌研究的基本思路。真菌学家应该在联系临床与实验室中起关键作用。     

Infection and disintegration of vascular tissues of non-suberized roots of spruce byHeterobasidion annosum and use of antibodies for characterizing infection

Vascular disintegration mainly of medulla rays of spruce roots is of major significance in root rot disease of spruce caused byH. annosum. Using seedling roots as an experimental model, the possible routes and initial host reactions preceding invasion of vascular tissues was investigated. Transmission electron microscopy showed that penetration through the endodermis was an obvious route but not without host resistance. Using antibodies againstH. annosum hyphal materials, some labelling of vascular tissues remote from sites of fungal colonization suggest the release of fungal secretory products partly active in tissue disintegration. Similarly, intense labelling was also observed in severely colonized host tissues at late stages of infection. Strong labelling recorded at 3 d p.i. mainly on fungal hyphae and scant gold particles on invaded host tissues could imply that induction of host antifungal metabolites may have been a late event. A correlation was found between total antigenic material in root homogenates measured by ELISA, density of tissue labelling by immunocytochemistry and severity of disease symptoms. The importance of this in relation to diagnosis of biotic root rot diseases in the field is discussed.     

Copper at the Fungal Pathogen-Host Axis

Fungal infections are responsible for millions of human deaths annually. Copper, an essential but toxic trace element, plays an important role at the host-pathogen axis during infection. In this review, we describe how the host uses either Cu compartmentalization within innate immune cells or Cu sequestration in other infected host niches such as in the brain to combat fungal infections. We explore Cu toxicity mechanisms and the Cu homeostasis machinery that fungal pathogens bring into play to succeed in establishing an infection. Finally, we address recent approaches that manipulate Cu-dependent processes at the host-pathogen axis for antifungal drug development.     

The role of adjuvant agents in treating fungal diseases

Invasive fungal infections (IFIs) remain a significant cause of morbidity and mortality despite the recent introduction of new antifungal medications. In this review, the available data on the use of adjuvant agents for the treatment of IFIs are discussed. Cytokines such as interferon-γ, colony-stimulating factors, granulocyte transfusions, and the monoclonal antibody efungumab may have in a role in the management of IFIs through augmentation of the host immune response, whereas pathogen-specific vaccines may help prevent infection. Pentraxin 3, an acute phase protein, may assist in the prevention and treatment of aspergillosis. Deferasirox, an iron chelator, is being investigated as an adjunctive therapy for the treatment of zygomycosis. Lactoferrin, an ironbinding protein, appears to have activity in Candida and Aspergillus infections, and omiganan may help prevent fungal catheter-related infections. Although none of these agents are currently approved for the treatment of IFIs, they may be involved in current and/or future treatment options when used in combination with antifungal drugs.     

Ploidy variation as an adaptive mechanism in human pathogenic fungi

Changes in ploidy have a profound and usually negative influence on cellular viability and proliferation, yet the vast majority of cancers and tumours exhibit an aneuploid karyotype. Whether this genomic plasticity is a cause or consequence of malignant transformation remains uncertain. Systemic fungal pathogens regularly develop aneuploidies in a similar manner during human infection, often far in excess of the natural rate of chromosome nondisjunction. As both processes fundamentally represent cells evolving under selective pressures, this suggests that changes in chromosome number may be a concerted mechanism to adapt to the hostile host environment. Here, we examine the mechanisms by which aneuploidy and polyploidy are generated in the fungal pathogens Candida albicans and Cryptococcus neoformans and investigate whether these represent an adaptive strategy under severe stress through the rapid generation of large-scale mutations. Insights into fungal ploidy changes, strategies for tolerating aneuploidies and proliferation during infection may yield novel targets for both antifungal and anticancer therapies.     

On the Roles of Calcineurin in Fungal Growth and Pathogenesis

Calcineurin is a calcium-activated phosphatase that controls morphogenesis and stress responses in eukaryotes. Fungal pathogens have adopted the calcineurin pathway to survive and effectively propagate within the host. The difficulty in treating fungal infections stems from similarities between pathogen and host eukaryotic cells. Using calcineurin inhibitors such as cyclosporin A or tacrolimus (FK506) in combination with antifungal drugs, including azoles or echinocandins, renders these drugs fungicidal, even towards drug-resistant species or strains, making calcineurin a promising drug target. This article summarizes the current understanding of the calcineurin pathway and its roles in governing the growth and virulence of pathogenic fungi, and compares and contrasts the roles of calcineurin in fungal pathogens that infect humans (Candida albicans and Cryptococcus neoformans) or plants (Magnaporthe oryzae and Ustilago maydis). Further investigation of calcineurin biology will advance opportunities to develop novel antifungal therapeutic approaches and provide insight into the evolution of virulence.     

Piperazinyl quinolines as chemosensitizers to increase fluconazole susceptibility of Candida albicans clinical isolates

The effectiveness of the potent antifungal drug fluconazole is being compromised by the rise of drug-resistant fungal pathogens. While inhibition of Hsp90 or calcineurin can reverse drug resistance in Candida, such inhibitors also impair the homologous human host protein and fungal-selective chemosensitizers remain rare. The MLPCN library was screened to identify compounds that selectively reverse fluconazole resistance in a Candida albicans clinical isolate, while having no antifungal activity when administered as a single agent. A piperazinyl quinoline was identified as a new small-molecule probe (ML189) satisfying these criteria.     

A nonredundant role for plasmacytoid dendritic cells in host defense against the human fungal pathogen Aspergillus fumigatus

While plasmacytoid dendritic cells (pDCs), a natural type I interferon (IFN)-producing cell type, are regarded as critical for innate immunity to viruses, their role in defense against fungal infections remains unknown. We examined the interactions of pDCs with hyphae of the invasive human fungal pathogen Aspergillus fumigatus. Human pDCs spread over hyphae and inhibited their growth. Antifungal activity was retained in pDC lysates, did not require direct fungal contact, and was partially reversed by zinc. Incubation with hyphae resulted in pDC cytotoxicity, partly due to fungal gliotoxin secretion. Following hyphal stimulation, pDCs released proinflammatory cytokines via a TLR9-independent mechanism. Pulmonary challenge of mice with A. fumigatus resulted in a substantial influx of pDCs into lungs, and pDC-depleted mice were hypersusceptible to invasive aspergillosis. These data demonstrate the antifungal activity of pDCs against A. fumigatus and establish their nonredundant role in host defenses against invasive aspergillosis in vivo.     

The importance of fungal pathogens and antifungal coatings in medical device infections

In recent years, increasing evidence has been collated on the contributions of fungal species, particularly Candida, to medical device infections. Fungal species can form biofilms by themselves or by participating in polymicrobial biofilms with bacteria. Thus, there is a clear need for effective preventative measures, such as thin coatings that can be applied onto medical devices to stop the attachment, proliferation, and formation of device-associated biofilms.However, fungi being eukaryotes, the challenge is greater than for bacterial infections because antifungal agents are often toxic towards eukaryotic host cells. Whilst there is extensive literature on antibacterial coatings, a far lesser body of literature exists on surfaces or coatings that prevent attachment and biofilm formation on medical devices by fungal pathogens. Here we review strategies for the design and fabrication of medical devices with antifungal surfaces. We also survey the microbiology literature on fundamental mechanisms by which fungi attach and spread on natural and synthetic surfaces. Research in this field requires close collaboration between biomaterials scientists, microbiologists and clinicians; we consider progress in the molecular understanding of fungal recognition of, and attachment to, suitable surfaces, and of ensuing metabolic changes, to be essential for designing rational approaches towards effective antifungal coatings, rather than empirical trial of coatings.