Balancing inflammation and tolerance in vivo through dendritic cells by the commensal Candida albicans

Immunity to fungi and vaccine considerations

Fungal disease poses a growing threat to public health that our current antifungal therapies are not well equipped to meet. As the population of immunocompromised hosts expands, and ecological changes favor the emergence of fungal pathogens, the development of new antifungal agents, including vaccines, becomes a global priority. Here, we summarize recent advancements in the understanding of fungal pathogenesis, key features of the host antifungal immune response, and how these findings could be leveraged to design novel approaches to deadly fungal disease.

Profiling inflammatory outcomes of Candida albicans colonization and food allergy induction in the murine glandular stomach

We investigated the effects of Candida albicans colonization on inflammatory responses in the murine glandular stomach, which is similar to the glandular mucosa of the human stomach. We also explored whether the presence of a food allergy could exacerbate C. albicans-induced inflammation or if C. albicans would amplify allergic inflammation in the glandular stomach. C. albicans successfully colonized the stomach of amoxicillin-pre-treated BALB/c mice and induced gastritis in the limiting ridge with minimal inflammation in the glandular stomach. There was significant upregulation of Il18, calprotectin (S100a8 and S100a9), and several antimicrobial peptides, but minimal induction of type 1, 2, or 3 responses in the glandular stomach. A robust type 2 response, inflammatory cell recruitment, and tissue remodeling occurred in the glandular stomach following oral ovalbumin challenges in sensitized mice. The type 2 response was not augmented by C. albicans colonization, but there was significant upregulation of Il1b, Il12a, Tnf, and Il17a in C. albicans-colonized food allergic mice. The presence of C. albicans did not affect the expression of genes involved in barrier integrity and signaling, many of which were upregulated during food allergy. Overall, our data indicate that C. albicans colonization induces minimal inflammation in the glandular stomach but augments antimicrobial peptide expression. Induction of a food allergy results in robust type 2 inflammation in the glandular stomach, and while C. albicans colonization does not exacerbate type 2 inflammation, it does activate a number of innate and type 3 immune responses amid the backdrop of allergic inflammation.

IMPORTANCE

Food allergy continues to be a growing public health concern, affecting at least 1 in 10 individuals in the United States alone. However, little is known about the involvement of the gastric mucosa in food allergy. Gastrointestinal Candida albicans colonization has been reported to promote gastrointestinal inflammation in a number of chronic diseases. Using a mouse model of food allergy to egg white protein, we demonstrate regionalization of the inflammatory response to C. albicans colonization, induction of robust type 2 (allergic) inflammation in the stomach, and augmentation of innate and type 3 responses by C. albicans colonization during food allergy.

Therapy of traditional Chinese medicine in Candida spp. and Candida associated infections: A comprehensive review

Candida spp. are commonly a group of opportunistic dimorphic fungi, frequently causing diverse fungal infections in immunocompromised or immunosuppressant patients from mucosal disturbs (oropharyngeal candidiasis and vulvovaginal candidiasis) to disseminated infections (systemic candidiasis) with high morbidity and mortality. Importantly, several Candida species can be isolated from diseased individuals with digestive, neuropathic, respiratory, metabolic and autoimmune diseases. Due to increased resistance to conventional antifungal agents, the arsenal for antifungal purpose is in urgent need. Traditional Chinese Medicines (TCMs) are a huge treasury that can be used as promising candidates for antimycotic applications. In this review, we make a short survey of microbiological (morphology and virulence) and pathological (candidiasis and Candida related infections) features of and host immune response (innate and adaptive immunity) to Candida spp.. Based on the chemical structures and well-studied antifungal mechanisms, the monomers, extracts, decoctions, essential oils and other preparations of TCMs that are reported to have fair antifungal activities or immunomodulatory effects for anticandidal purpose are comprehensively reviewed. We also emphasize the importance of combination and drug pair of TCMs as useful anticandidal strategies, as well as network pharmacology and molecular docking as beneficial complements to current experimental approaches. This review construct a therapeutic module that can be helpful to guide in-future experimental and preclinical studies in the combat against fungal threats aroused by C. albicans and non-albicans Candida species.

The IL-12 family of heterodimeric cytokines in polycystic ovarian syndrome: biological role in induction, regulation, and treatment

Polycystic ovary syndrome (PCOS) is a diverse endocrine disorder widely recognized as the prevailing metabolic condition among women in their reproductive years. The precise pathophysiological mechanisms underlying PCOS remain incompletely understood. However, existing evidence suggests that the development of PCOS may be linked to factors such as abdominal obesity, hyperandrogenism, and insulin resistance (IR). Excessive central adiposity in women with PCOS may lead to the development of a chronic, low-grade inflammation characterized by the activation of proinflammatory cytokines. The cytokines that belong to the IL-12 family are a collection of distinct heterodimeric cytokines that include IL-12, IL-23, IL-27, and IL-35. Recent research has provided further evidence regarding the significance of IL-12 cytokines in influencing both innate and adaptive immune responses in different diseases. Additionally, these studies have discovered diverse roles for certain members of the IL-12 family, encompassing multiple immunological functions that can either act as effectors or regulators. In this discourse, we examine the distinctive and atypical structural and functional attributes of this particular cytokine family. This study aims to offer a comprehensive overview of the pathophysiological significance of the IL-12 family cytokines in PCOS patients. Additionally, the therapeutic potential of the cytokines as novel approaches for PCOS treatment will be proposed.

Dynamics of the Gut Mycobiome in Patients With Ulcerative Colitis

BACKGROUND & AIMS

Intestinal fungi have been implicated in the pathogenesis of ulcerative colitis (UC). However, it remains unclear if fungal composition is altered during active versus quiescent disease.

METHODS

We analyzed clinical and metagenomic data from the Study of a Prospective Adult Research Cohort with Inflammatory Bowel Disease (SPARC IBD), available via the IBD Plexus Program of the Crohn's & Colitis Foundation. We evaluated the fungal composition of fecal samples from 421 patients with UC during clinical activity and remission. Within a longitudinal subcohort (n = 52), we assessed for dynamic taxonomic changes across alterations in clinical activity over time. We examined if fungal amplicon sequence variants and fungal-bacterial relationships were altered during activity versus remission. Finally, we classified activity in UC using a supervised machine learning random forest model trained on fungal abundance data.

RESULTS

During clinical activity, the relative abundance of genus Candida was increased 3.5-fold (P-adj < 1 × 10-4) compared with during remission. Patients with longitudinal reductions in clinical activity demonstrated parallel reductions in Candida relative abundance (P < .05). Candida relative abundance correlated with Parabacteroides diastonis, Faecalibacterium prausnitzii, and Bacteroides dorei relative abundance (P < .05) during remission; however, these correlations were disrupted during activity. Fungal abundance data successfully classified patients with active or quiescent UC (area under the curve ∼0.80), with Candida relative abundance critical to the success of the model.

CONCLUSIONS

Clinical activity in UC is associated with an increased relative abundance of Candida, cross-sectionally and dynamically over time. The role of fecal Candida as a target for therapeutics in UC should be evaluated.

The mycobiome in atopic diseases: Inducers and triggers

Atopic diseases are characterized by type 2 inflammation, with high levels of allergen-specific TH2 cell immune responses and elevated production of IgE. These common disorders have increased in incidence around the world, which is partly explained by detrimental disturbances to the early-life intestinal microbiome. Although most studies have focused exclusively on bacterial members of the microbiome, intestinal fungi have started to be recognized for their impact on host immune development and atopy pathogenesis. From this perspective, we review recent findings demonstrating the strong interactions between members of the mycobiome and the host immune system early in life, leading to immune tolerance during eubiosis or inducing sensitization and overt TH2 cell responses during dysbiosis. Current evidence places intestinal fungi as central players in the development of allergic diseases and potential targets for atopy prevention and treatments.

In vitro differentiated human CD4+ T cells produce hepatocyte growth factor

Differentiation of naive CD4⁺ T cells into effector T cells is a dynamic process in which the cells are polarized into T helper (Th) subsets. The subsets largely consist of four fundamental categories: Th1, Th2, Th17, and regulatory T cells. We show that human memory CD4⁺ T cells can produce hepatocyte growth factor (HGF), a pleiotropic cytokine which can affect several tissue types through signaling by its receptor, c-Met. In vitro differentiation of T cells into Th-like subsets revealed that HGF producing T cells increase under Th1 conditions. Enrichment of HGF producing cells was possible by targeting cells with surface CD30 expression, a marker discovered through single-cell RNA-sequencing. Furthermore, pharmacological inhibition of PI3K or mTOR was found to inhibit HGF mRNA and protein, while an Akt inhibitor was found to increase these levels. The findings suggest that HGF producing T cells could play a role in disease where Th1 are present.

Immunomodulatory role of oral microbiota in inflammatory diseases and allergic conditions

In recent years, the interplay between oral microbiota and systemic disease has gained attention as poor oral health is associated with several pathologies. The oral microbiota plays a role in the maintenance of overall health, and its dysbiosis influences chronic inflammation and the pathogenesis of gum diseases. Periodontitis has also been associated with other diseases and health complications such as cancer, neurogenerative and autoimmune disorders, chronic kidney disease, cardiovascular diseases, rheumatic arthritis, respiratory health, and adverse pregnancy outcomes. The host microbiota can influence immune cell development and immune responses, and recent evidence suggests that changes in oral microbiota composition may also contribute to sensitization and the development of allergic reactions, including asthma and peanut allergies. Conversely, there is also evidence that allergic reactions within the gut may contribute to alterations in oral microbiota composition. Here we review the current evidence of the role of the oral microbiota in inflammatory diseases and health complications, as well as its future relevance in improving health and ameliorating allergic disease.

Neglected gut microbiome: interactions of the non-bacterial gut microbiota with enteric pathogens

A diverse array of commensal microorganisms inhabits the human intestinal tract. The most abundant and most studied members of this microbial community are undoubtedly bacteria. Their important role in gut physiology, defense against pathogens, and immune system education has been well documented over the last decades. However, the gut microbiome is not restricted to bacteria. It encompasses the entire breadth of microbial life: viruses, archaea, fungi, protists, and parasitic worms can also be found in the gut. While less studied than bacteria, their divergent but important roles during health and disease have become increasingly more appreciated. This review focuses on these understudied members of the gut microbiome. We will detail the composition and development of these microbial communities and will specifically highlight their functional interactions with enteric pathogens, such as species of the family Enterobacteriaceae. The interactions can be direct through physical interactions, or indirect through secreted metabolites or modulation of the immune response. We will present general concepts and specific examples of how non-bacterial gut communities modulate bacterial pathogenesis and present an outlook for future gut microbiome research that includes these communities.

They shall not grow mold: Soldiers of innate and adaptive immunity to fungi

Fungi are ubiquitous commensals, seasoned predators, and important agents of emerging infectious diseases [1 ]. The immune system assumes the essential responsibility for responding intelligently to the presence of known and novel fungi to maintain host health. In this Review, we describe the immune responses to pathogenic fungi and the varied array of fungal agents confronting the vertebrate host within the broader context of fungal and animal evolution. We provide an overview of the mechanistic details of innate and adaptive antifungal immune responses, as well as ways in which these basic mechanisms support the development of vaccines and immunotherapies.

B7-H4 reduction induced by Toxoplasma gondii infection results in dysfunction of decidual dendritic cells by regulating the JAK2/STAT3 pathway

Background

Primary infection of Toxoplasma gondii can cause serious abnormal pregnancy outcomes such as miscarriage and stillbirth. Inhibitory molecule B7-H4 is abundantly expressed in dendritic cells (DCs) and plays an important role in maintaining immune tolerance. However, the role of B7-H4 in decidual DCs (dDCs) in T. gondii-induced abnormal pregnancy outcomes is not clear.

Methods

We established T. gondii-infected abnormal pregnancy model in wild-type (WT) and B7-H4 knockout (B7-H4^−/−) pregnant mice in vivo and cultured primary human dDCs in vitro. The abnormal pregnancy outcomes were observed and the expression of B7-H4, functional molecules (CD80, CD86, and MHC-II or HLA-DR), indoleamine 2,3-dioxygenase (IDO), cytokines (IL-10 and IL-12), and signaling molecules JAK2/STAT3 in dDCs was detected by flow cytometry and Western blot.

Results

Our results showed that T. gondii infection significantly decreased B7-H4 expression in dDCs. In addition, B7-H4^−/− infected pregnant mice showed much more severe abnormal pregnancy outcomes than their counterparts. Importantly, B7-H4^−/− infection further regulated the expression of molecules (CD80, CD86, and MHC-II or HLA-DR), enzyme IDO, and cytokines (IL-10 and IL-12) in dDCs. We further discovered that B7-H4^−/− infection impairs the JAK2/STAT3 pathway, contributing to dDC dysfunction.

Conclusions

Taken together, the results show that reduction of B7-H4 by T. gondii infection significantly modulates the decrease in cytokine IL-10 and enzyme IDO and the increase in cytokine IL-12, contributing to dDC dysfunction. Moreover, the JAK2/STAT3 pathway is involved in the regulation of B7-H4 by T. gondii infection and in the subsequent IDO and cytokine production, which ultimately contributes to abnormal pregnancy outcomes.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05263-1.

"Molding" immunity-modulation of mucosal and systemic immunity by the intestinal mycobiome in health and disease

Fungi are important yet understudied contributors to the microbial communities of the gastrointestinal tract. Starting at birth, the intestinal mycobiome undergoes a period of dynamic maturation under the influence of microbial, host, and extrinsic influences, with profound functional implications for immune development in early life, and regulation of immune homeostasis throughout life. Candida albicans serves as a model organism for understanding the cross-talk between fungal colonization dynamics and immunity, and exemplifies unique mechanisms of fungal-immune interactions, including fungal dimorphism, though our understanding of other intestinal fungi is growing. Given the prominent role of the gut mycobiome in promoting immune homeostasis, emerging evidence points to fungal dysbiosis as an influential contributor to immune dysregulation in a variety of inflammatory and infectious diseases. Here we review current knowledge on the factors that govern host-fungi interactions in the intestinal tract and immunological outcomes in both mucosal and systemic compartments.

Amelioration of Graft-versus-Host Disease by Exopolysaccharide from a Commensal Bacterium

Acute graft-versus-host disease (aGvHD) is a severe, often lethal, complication of hematopoietic stem cell transplantation, and although prophylactic regimens are given as standard pretransplantation therapy, up to 60% of these patients develop aGvHD, and require additional immunosuppressive intervention. We treated mice with a purified probiotic molecule, exopolysaccharide (EPS) from Bacillus subtilis, shortly before and after induction of aGvHD and found that, whereas only 10% of control mice survived to day 80, 70% of EPS-treated mice survived to 80 d. EPS treatment of donor-only mice resulted in ∼60% survival. Using a biosensor mouse model to assess inflammation in live mice during aGvHD, we found that EPS prevented the activation of alloreactive donor T cells. In vitro, EPS did not affect T cells directly but, instead, induced bone marrow-derived dendritic cells (BMDCs) that displayed characteristics of inhibitory dendritic cells (DCs). Development of these BMDCs required TLR4 signaling through both MyD88 and TRIF pathways. Using BMDCs derived from IDO knockout mice, we showed that T cell inhibition by EPS-treated BMDCs was mediated through the suppressive effects of IDO. These studies describe a bacterial molecule that modulates immune responses by inducing inhibitory DCs in a TLR4-dependent manner, and these cells have the capacity to inhibit T cell activation through IDO. We suggest that EPS or EPS-treated DCs can serve as novel agents for preventing aGvHD.

Decreased Ecological Resistance of the Gut Microbiota in Response to Clindamycin Challenge in Mice Colonized with the Fungus Candida albicans

The mammalian gut microbiota is a complex community of microorganisms which typically exhibits remarkable stability. As the gut microbiota has been shown to affect many aspects of host health, the molecular keys to developing and maintaining a "healthy" gut microbiota are highly sought after. Yet, the qualities that define a microbiota as healthy remain elusive. We used the ability to resist change in response to antibiotic disruption, a quality we refer to as ecological resistance, as a metric for the health of the bacterial microbiota. Using a mouse model, we found that colonization with the commensal fungus Candida albicans decreased the ecological resistance of the bacterial microbiota in response to the antibiotic clindamycin such that increased microbiota disruption was observed in C. albicans-colonized mice compared to that in uncolonized mice. C. albicans colonization resulted in decreased alpha diversity and small changes in abundance of bacterial genera prior to clindamycin challenge. Strikingly, co-occurrence network analysis demonstrated that C. albicans colonization resulted in sweeping changes to the co-occurrence network structure, including decreased modularity and centrality and increased density. Thus, C. albicans colonization resulted in changes to the bacterial microbiota community and reduced its ecological resistance.IMPORTANCECandida albicans is the most common fungal member of the human gut microbiota, yet its ability to interact with and affect the bacterial gut microbiota is largely uncharacterized. Previous reports showed limited changes in microbiota composition as defined by bacterial species abundance as a consequence of C. albicans colonization. We also observed only a few bacterial genera that were significantly altered in abundance in C. albicans-colonized mice; however, C. albicans colonization significantly changed the structure of the bacterial microbiota co-occurrence network. Additionally, C. albicans colonization changed the response of the bacterial microbiota ecosystem to a clinically relevant perturbation, challenge with the antibiotic clindamycin.

Exposome and Immunity Training: How Pathogen Exposure Order Influences Innate Immune Cell Lineage Commitment and Function

Immune memory is a defining characteristic of adaptive immunity, but recent work has shown that the activation of innate immunity can also improve responsiveness in subsequent exposures. This has been coined “trained immunity” and diverges with the perception that the innate immune system is primitive, non-specific, and reacts to novel and recurrent antigen exposures similarly. The “exposome” is the cumulative exposures (diet, exercise, environmental exposure, vaccination, genetics, etc.) an individual has experienced and provides a mechanism for the establishment of immune training or immunotolerance. It is becoming increasingly clear that trained immunity constitutes a delicate balance between the dose, duration, and order of exposures. Upon innate stimuli, trained immunity or tolerance is shaped by epigenetic and metabolic changes that alter hematopoietic stem cell lineage commitment and responses to infection. Due to the immunomodulatory role of the exposome, understanding innate immune training is critical for understanding why some individuals exhibit protective phenotypes while closely related individuals may experience immunotolerant effects (e.g., the order of exposure can result in completely divergent immune responses). Research on the exposome and trained immunity may be leveraged to identify key factors for improving vaccination development, altering inflammatory disease development, and introducing potential new prophylactic treatments, especially for diseases such as COVID-19, which is currently a major health issue for the world. Furthermore, continued exposome research may prevent many deleterious effects caused by immunotolerance that frequently result in host morbidity or mortality.

Emerging Complexity and the Need for Advanced Drug Delivery in Targeting Candida Species

Background:

Candida species are the important etiologic agents for candidiasis, the most prevalent cause of opportunistic fungal infections. Candida invasion results in mucosal to systemic infections through immune dysfunction and helps in further invasion and proliferation at several sites in the host. The host defence system utilizes a wide array of the cells, proteins and chemical signals that are distributed in blood and tissues which further constitute the innate and adaptive immune system. The lack of antifungal agents and their limited therapeutic effects have led to high mortality and morbidity related to such infections.

Methods:

The necessary information collated on this review has been gathered from various literature published from 1995 to 2019.

Results:

This article sheds light on novel drug delivery approaches to target the immunological axis for several Candida species (C. albicans, C. glabrata, C. parapsilosis, C. tropicalis, C. krusei, C. rugose, C. hemulonii, etc.).

Conclusion:

It is clear that the novel drug delivery approaches include vaccines, adoptive transfer of primed immune cells, recombinant cytokines, therapeutic antibodies, and nanoparticles, which have immunomodulatory effects. Such advancements in targeting various underpinning mechanisms using the concept of novel drug delivery will provide a new dimension to the fungal infection clinic particularly due to Candida species with improved patient compliance and lesser side effects. This advancement in knowledge can also be extended to target various other similar microbial species and infections.

IL-9 and Mast Cells Are Key Players of Candida albicans Commensalism and Pathogenesis in the Gut

Candida albicans is implicated in intestinal diseases. Identifying host signatures that discriminate between the pathogenic versus commensal nature of this human commensal is clinically relevant. In the present study, we identify IL-9 and mast cells (MCs) as key players of Candida commensalism and pathogenicity. By inducing TGF-β in stromal MCs, IL-9 pivotally contributes to mucosal immune tolerance via the indoleamine 2,3-dioxygenase enzyme. However, Candida-driven IL-9 and mucosal MCs also contribute to barrier function loss, dissemination, and inflammation in experimental leaky gut models and are upregulated in patients with celiac disease. Inflammatory dysbiosis occurs with IL-9 and MC deficiency, indicating that the activity of IL-9 and MCs may go beyond host immunity to include regulation of the microbiota. Thus, the output of the IL-9/MC axis is highly contextual during Candida colonization and reveals how host immunity and the microbiota finely tune Candida behavior in the gut.

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IL-9/IL-9R signaling affects MC function in mucosal candidiasis

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IL-9 and mucosal MCs contribute to barrier function loss in leaky gut models

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IL-9 and stromal MCs induce local protective tolerance in infection via IDO1

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IL-9 and mucosal MCs expand and IDO1 decreases in human celiac disease

Regulatory T cells in paracoccidioidomycosis

This review addresses the role of regulatory T cells (Tregs), which are essential for maintaining peripheral tolerance and controlling pathogen immunity, in the host response against Paracoccidioides brasiliensis, a primary fungal pathogen. A brief introduction on the general features of Treg cells summarizes their main functions, subpopulations, mechanisms of suppression and plasticity. The main aspects of immunity in the diverse forms of the P. brasiliensis infection are presented, as are the few extant studies on the relevance of Treg cells in the control of severity of the human disease. Finally, the influence of Toll-like receptors, Dectin-1, NOD-like receptor P3 (NLRP3), Myeloid differentiation factor-88 (MyD88), as well as the enzyme indoleamine 2,3 dioxygenase (IDO) on the expansion and function of Treg cells in a murine model of pulmonary paracoccidioidomycosis (PCM) is also discussed. It is demonstrated that some of these components are involved in the negative control of Treg cell expansion, whereas others positively trigger the proliferation and activity of these cells. Finally, the studies here summarized highlight the dual role of Treg cells in PCM, which can be protective by controlling excessive immunity and tissue pathology but also deleterious by inhibiting the anti-fungal immunity necessary to control fungal growth and dissemination.

A Stealthy Fungal Attack Requires an Equally Clandestine Approach to Onychomycosis Treatment

Onychomycosis is a chronic fungal infection of the nail that is recalcitrant to treatment. It is unclear why normally effective antifungal therapy results in low cure rates. Evidence suggests that there may be a plethora of reasons that include the limited immune presence in the nail, reduced circulation, presence of commensal microbes, and fungal influence on immune signaling. Therefore, treatment should be designed to address these possibilities and work synergistically with both the innate and adaptive immune responses.

STAT3 Activation in Combination with NF-KappaB Inhibition Induces Tolerogenic Dendritic Cells with High Therapeutic Potential to Attenuate Collagen-Induced Arthritis

Dendritic cells (DCs) have the ability to induce tolerance or inflammation in response to self-antigens, which makes them fundamental players in autoimmunity. In this regard, immunogenic DCs produce IL-12 and IL-23 favouring the acquisition of Th1 and Th17 inflammatory phenotypes, respectively, by autoreactive CD4⁺ T-cells, thus promoting autoimmunity. Conversely, tolerogenic DCs produce IL-10 and TGF-β, inducing the generation of CD4⁺ T-cells with suppressive activity (Treg), which promote tolerance to self-constituents. Previous studies have shown that STAT3 signalling in DCs attenuates the production of proinflammatory cytokines, whilst NF-κB activation promotes it. In this study, we aimed to generate DCs displaying strong and constitutive tolerogenic profile to be used as immunotherapy in autoimmunity. To this end, we transduced bone marrow-derived DCs with lentiviral particles codifying for a constitutively active version of STAT3 (constitutively active STAT3 (STAT3ca)) or with a constitutive repressor of NF-κB (IκBα superrepressor (IκBαSR)), and their therapeutic potential was evaluated in a mouse model of arthritis induced by collagen (CIA). Our results show that STAT3ca transduction favoured the production of the anti-inflammatory mediator IL-10, whereas IκBαSR transduction attenuated the expression of the proinflammatory cytokine IL-23 in DCs. Moreover, both STAT3ca-transduced and IκBαSR-transduced DCs separately exerted a mild but significant therapeutic effect reducing the severity of CIA development. Furthermore, when DCs were transduced with both STAT3ca and IκBαSR together, they reduced CIA manifestation significantly stronger than when transduced with only STAT3ca or IκBαSR separately. These results show STAT3 and NF-κB as two important and complementary regulators of the tolerogenic behaviour of DCs, which should be considered as molecular targets in the design of DC-based suppressive immunotherapies for the treatment of autoimmune disorders.

Candida albicans Elicits Pro-Inflammatory Differential Gene Expression in Intestinal Peyer's Patches

The details of how gut-associated lymphoid tissues such as Peyer's patches (PPs) in the small intestine play a role in immune surveillance, microbial differentiation and the mucosal barrier protection in response to fungal organisms such as Candida albicans are still unclear. We particularly focus on PPs as they are the immune sensors and inductive sites of the gut that influence inflammation and tolerance. We have previously demonstrated that CD11c⁺ phagocytes that include dendritic cells and macrophages are located in the sub-epithelial dome within PPs sample C. albicans. To gain insight on how specific cells within PPs sense and respond to the sampling of fungi, we gavaged naïve mice with C. albicans strains ATCC 18804 and SC5314 as well as Saccharomyces cerevisiae. We measured the differential gene expression of sorted CD45⁺ B220⁺ B-cells, CD3⁺ T-cells and CD11c⁺ DCs within the first 24 h post-gavage using nanostring nCounter^® technology. The results reveal that at 24 h, PP phagocytes were the cell type that displayed differential gene expression. These phagocytes were able to sample C. albicans and discriminate between strains. In particular, strain ATCC 18804 upregulated fungal-specific pro-inflammatory genes pertaining to innate and adaptive immune responses. Interestingly, PP CD11c⁺ phagocytes also differentially expressed genes in response to C. albicans that were important in the protection of the mucosal barrier. These results highlight that the mucosal barrier not only responds to C. albicans, but also aids in the protection of the host.

Metabolic Regulation of Dendritic Cell Differentiation

Dendritic cells (DCs) are important antigen-presenting cells (APCs) that play essential roles in bridging innate and adaptive immune responses. Differentiation stages of DC subsets from bone marrow progenitor cells have been well-defined during the past decades. Features that distinguish DC progenitor cells from each differentiation stages, related signaling pathways and transcription factors that are crucial for DC lineage commitment have been well-elucidated in numerous studies. Recently, growing evidence are showing that cellular metabolism, as one of the most fundamental process of cells, has essential role in the modulation of immune system. There have been multiple reports and reviews that focus on the metabolic modulations on DC functions, however little attention had been paid to the metabolic regulation of DC development and differentiation. In recent years, increasing evidence suggests that metabolic regulations also exert significant impact on DC differentiation, as well as on the homeostasis of tissue resident DCs. The focus of this review is to summarize the findings from recent studies on the metabolic regulation of DC differentiation and to discuss the impacts of the three major aspects of metabolism on the processes of DC development and differentiation, namely the changes in metabolic pathways, the molecular signaling pathways that modulate cell metabolism, and the effects of metabolites and nutrients. The aim of this review is to draw attentions to this important and exciting research field where the effects of metabolic process and their regulation in DC differentiation need to be further explored.

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

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

Staphylococcus aureus PSM Peptides Modulate Human Monocyte-Derived Dendritic Cells to Prime Regulatory T Cells

Staphylococcus aureus (Sa), as one of the major human pathogens, has very effective strategies to subvert the human immune system. Virulence of the emerging community-associated methicillin-resistant Sa (CA-MRSA) depends on the secretion of phenol-soluble modulin (PSM) peptide toxins e.g., by binding to and modulation of innate immune cells. Previously, by using mouse bone marrow-derived dendritic cells we demonstrated that PSMs in combination with various Toll-like receptor (TLR) ligands induce a tolerogenic DC phenotype (tDC) characterized by the production of IL-10 and impaired secretion of pro-inflammatory cytokines. Consequently, PSM-induced tDCs favored priming of CD4⁺CD25⁺FoxP3⁺ T_regs with suppressor function while impairing the Th1 response. However, the relevance of these findings for the human system remained elusive. Here, we analyzed the impact of PSMα3 on the maturation, cytokine production, antigen uptake, and T cell stimulatory capacity of human monocyte-derived DCs (moDCs) treated simultaneously with either LPS (TLR4 ligand) or Sa cell lysate (TLR2 ligand). Herein, we demonstrate that PSMs indeed modulate human moDCs upon treatment with TLR2/4 ligands via multiple mechanisms, such as transient pore formation, impaired DC maturation, inhibited pro- and anti-inflammatory cytokine secretion, as well as reduced antigen uptake. As a result, the adaptive immune response was altered shown by an increased differentiation of naïve and even CD4⁺ T cells from patients with Th1/Th17-induced diseases (spondyloarthritis and rheumatoid arthritis) into CD4⁺CD127⁻CD25^hiCD45RA⁻FoxP3^hi regulatory T cells (T_regs) with suppressor function. This T_reg induction was mediated most predominantly by direct DC-T-cell interaction. Thus, PSMs from highly virulent Sa strains affect DC functions not only in the mouse, but also in the human system, thereby modulating the adaptive immune response and probably increasing the tolerance toward the bacteria. Moreover, PSMα3 might be a novel peptide for tolerogenic DC induction that may be used for DC vaccination strategies.

Gut fungal dysbiosis correlates with reduced efficacy of fecal microbiota transplantation in Clostridium difficile infection

Fecal microbiota transplantation (FMT) is effective in treating recurrent Clostridium difficile infection (CDI). Bacterial colonization in recipients after FMT has been studied, but little is known about the role of the gut fungal community, or mycobiota. Here, we show evidence of gut fungal dysbiosis in CDI, and that donor-derived fungal colonization in recipients is associated with FMT response. CDI is accompanied by over-representation of Candida albicans and decreased fungal diversity, richness, and evenness. Cure after FMT is associated with increased colonization of donor-derived fungal taxa in recipients. Recipients of successful FMT (“responders”) display, after FMT, a high relative abundance of Saccharomyces and Aspergillus, whereas “nonresponders” and individuals treated with antibiotics display a dominant presence of Candida. High abundance of C. albicans in donor stool also correlates with reduced FMT efficacy. Furthermore, C. albicans reduces FMT efficacy in a mouse model of CDI, while antifungal treatment reestablishes its efficacy, supporting a potential causal relationship between gut fungal dysbiosis and FMT outcome.

Fecal microbiota transplantation (FMT) is effective in treating recurrent Clostridium difficile infection (CDI). Here, the authors show that the composition of the gut fungal microbiota of donors and recipients, and especially the abundance of Candida, correlates with FMT outcome in CDI patients.

Interleukin-33 deficiency exacerbated experimental autoimmune encephalomyelitis with an influence on immune cells and glia cells

Interleukin (IL)-33, a member of the IL-1 cytokine family, is highly expressed in central nervous system (CNS), suggesting its potential role in CNS. Although some studies have focused on the role of IL-33 in multiple sclerosis (MS) / experimental autoimmune encephalomyelitis (EAE), an autoimmune disease characterized by demyelination and axonal damage in CNS, the exact role of IL-33 in MS/EAE remains unclear and controversial. Here, we used IL-33 knockout mice to clarify the role of endogenous IL-33 in EAE by simultaneously eliminating its role as a nuclear transcription factor and an extracellular cytokine. We found that the clinical score in IL-33 knockout EAE mice was higher accompanied by more severe demyelination compared with the wild-type (WT) EAE mice. As for the main immune cells participating in EAE in IL-33 knockout mice, pathogenic effector T cells increased both in peripheral immune organs and CNS, while CD4⁺FOXP3⁺ regulatory T cells decreased in spleen and lymph nodes, Th2 cells and natural killer (NK) cells decreased in CNS. Additionally, the populations of microglia/macrophages and CD11C⁺CD11B⁺ dendritic cells (DCs) increased in CNS of IL-33 knockout mice with EAE, among which iNOS-producing microglia/macrophages increased. Moreover, resident astrocytes/microglia were more activated in IL-33 knockout mice with EAE. In vitro, after blocking the IL-33, the proliferation of primary astrocytes, the production of MCP-1/CCL2 and TNF-α by astrocytes, and the production of TNF-α by primary microglia stimulated by the homogenate of the peak stage of EAE were increased. Our results indicate that IL-33 plays a protective role in EAE and exerts extensive influences on multiple immune cells and neural cells involved in EAE.

Emerging IL-12 family cytokines in the fight against fungal infections

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IL-12 and IL-23 have established roles during anti-fungal immunity.

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IL-27 promotes regulatory effector responses during fungal infections.

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IL-35 drives T cell differentiation to produce anti-inflammatory responses.

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Increasing evidence for IL-12 family cytokines in maintaining anti-fungal immune homeostasis.

Invasive fungal infections cause approximately 1.5 million deaths per year worldwide and are a growing threat to human health. Current anti-fungal therapies are often insufficient, therefore studies into host-pathogen interactions are critical for the development of novel therapies to improve mortality rates. Myeloid cells, such as macrophages and dendritic cells, express pattern recognition receptor (PRRs), which are important for fungal recognition. Engagement of these PRRs by fungal pathogens induces multiple cytokines, which in turn activate T effector responses. Interleukin (IL)-12 family members (IL-12p70, IL-23, IL-27 and IL-35) link innate immunity with the development of adaptive immunity and are also important for regulating T cell responses. IL-12 and IL-23 have established roles during anti-fungal immunity, whereas emerging roles for IL-27 and IL-35 have recently been reported. Here, we discuss the IL-12 family, focusing on IL-27 and IL-35 during anti-fungal immune responses to pathogens such as Candida and Aspergillus.

The Elusive Anti-Candida Vaccine: Lessons From the Past and Opportunities for the Future

Candidemia is a bloodstream fungal infection caused by Candida species and is most commonly observed in hospitalized patients. Even with proper antifungal drug treatment, mortality rates remain high at 40–50%. Therefore, prophylactic or preemptive antifungal medications are currently recommended in order to prevent infections in high-risk patients. Moreover, the majority of women experience at least one episode of vulvovaginal candidiasis (VVC) throughout their lifetime and many of them suffer from recurrent VVC (RVVC) with frequent relapses for the rest of their lives. While there currently exists no definitive cure, the only available treatment for RVVC is again represented by antifungal drug therapy. However, due to the limited number of existing antifungal drugs, their associated side effects and the increasing occurrence of drug resistance, other approaches are greatly needed. An obvious prevention measure for candidemia or RVVC relapse would be to immunize at-risk patients with a vaccine effective against Candida infections. In spite of the advanced and proven techniques successfully applied to the development of antibacterial or antiviral vaccines, however, no antifungal vaccine is still available on the market. In this review, we first summarize various efforts to date in the development of anti-Candida vaccines, highlighting advantages and disadvantages of each strategy. We next unfold and discuss general hurdles encountered along these efforts, such as the existence of large genomic variation and phenotypic plasticity across Candida strains and species, and the difficulty in mounting protective immune responses in immunocompromised or immunosuppressed patients. Lastly, we review the concept of “trained immunity” and discuss how induction of this rapid and nonspecific immune response may potentially open new and alternative preventive strategies against opportunistic infections by Candida species and potentially other pathogens.

The contribution of mast cells to bacterial and fungal infection immunity

Mast cells are hematopoietic progenitor-derived, granule-containing immune cells that are widely distributed in tissues that interact with the external environment, such as the skin and mucosal tissues. It is well-known that mast cells are significantly involved in IgE-mediated allergic reactions, but because of their location, it has also been long hypothesized that mast cells can act as sentinel cells that sense pathogens and initiate protective immune responses. Using mast cell or mast cell protease-deficient murine models, recent studies by our groups and others indicate that mast cells have pleiotropic regulatory roles in immunological responses against pathogens. In this review, we discuss studies that demonstrate that mast cells can either promote host resistance to infections caused by bacteria and fungi or contribute to dysregulated immune responses that can increase host morbidity and mortality. Overall, these studies indicate that mast cells can influence innate immune responses against bacterial and fungal infections via multiple mechanisms. Importantly, the contribution of mast cells to infection outcomes depends in part on the infection model, including the genetic approach used to assess the influence of mast cells on host immunity, hence highlighting the complexity of mast cell biology in the context of innate immune responses.

A Multifaceted Role of Tryptophan Metabolism and Indoleamine 2,3-Dioxygenase Activity in Aspergillus fumigatus-Host Interactions

Aspergillus fumigatus is the most prevalent filamentous fungal pathogen of humans, causing either severe allergic bronchopulmonary aspergillosis or often fatal invasive pulmonary aspergillosis (IPA) in individuals with hyper- or hypo-immune deficiencies, respectively. Disease is primarily initiated upon the inhalation of the ubiquitous airborne conidia—the initial inoculum produced by A. fumigatus—which are complete developmental units with an ability to exploit diverse environments, ranging from agricultural composts to animal lungs. Upon infection, conidia initially rely on their own metabolic processes for survival in the host’s lungs, a nutritionally limiting environment. One such nutritional limitation is the availability of aromatic amino acids (AAAs) as animals lack the enzymes to synthesize tryptophan (Trp) and phenylalanine and only produce tyrosine from dietary phenylalanine. However, A. fumigatus produces all three AAAs through the shikimate–chorismate pathway, where they play a critical role in fungal growth and development and in yielding many downstream metabolites. The downstream metabolites of Trp in A. fumigatus include the immunomodulatory kynurenine derived from indoleamine 2,3-dioxygenase (IDO) and toxins such as fumiquinazolines, gliotoxin, and fumitremorgins. Host IDO activity and/or host/microbe-derived kynurenines are increasingly correlated with many Aspergillus diseases including IPA and infections of chronic granulomatous disease patients. In this review, we will describe the potential metabolic cross talk between the host and the pathogen, specifically focusing on Trp metabolism, the implications for therapeutics, and the recent studies on the coevolution of host and microbe IDO activation in regulating inflammation, while controlling infection.

Disease Tolerance Mediated by Phosphorylated Indoleamine-2,3 Dioxygenase Confers Resistance to a Primary Fungal Pathogen

Resistance to primary fungal pathogens is usually attributed to the proinflammatory mechanisms of immunity conferred by interferon-γ activation of phagocytes that control microbial growth, whereas susceptibility is attributed to anti-inflammatory responses that deactivate immunity. This study challenges this paradigm by demonstrating that resistance to a primary fungal pathogen such as Paracoccidiodes brasiliensis can be mediated by disease tolerance, a mechanism that preserves host fitness instead of pathogen clearance. Among the mechanisms of disease tolerance described, a crucial role has been ascribed to the enzyme indoleamine-2,3 dioxygenase (IDO) that concomitantly controls pathogen growth by limiting tryptophan availability and reduces tissue damage by decreasing the inflammatory process. Here, we demonstrated in a pulmonary model of paracoccidioidomycosis that IDO exerts a dual function depending on the resistant pattern of hosts. IDO activity is predominantly enzymatic and induced by IFN-γ signaling in the pulmonary dendritic cells (DCs) from infected susceptible (B10.A) mice, whereas phosphorylated IDO (pIDO) triggered by TGF-β activation of DCs functions as a signaling molecule in resistant mice. IFN-γ signaling activates the canonical pathway of NF-κB that promotes a proinflammatory phenotype in B10.A DCs that control fungal growth but ultimately suppress T cell responses. In contrast, in A/J DCs IDO promotes a tolerogenic phenotype that conditions a sustained synthesis of TGF-β and expansion of regulatory T cells that avoid excessive inflammation and tissue damage contributing to host fitness. Therefore, susceptibility is unexpectedly mediated by mechanisms of proinflammatory immunity that are usually associated with resistance, whereas genetic resistance is based on mechanisms of disease tolerance mediated by pIDO, a phenomenon never described in the protective immunity against primary fungal pathogens.

Mucorales spores induce a proinflammatory cytokine response in human mononuclear phagocytes and harbor no rodlet hydrophobins

Mucormycoses are life-threatening infections in immunocompromised patients. This study characterizes the response of human mononuclear cells to different Mucorales and Ascomycota. PBMC, monocytes, and monocyte derived dendritic cells (moDCs) from healthy donors were stimulated with resting and germinated stages of Mucorales and Ascomycota. Cytokine response and expression of activation markers were studied. Both inactivated germ tubes and resting spores of Rhizopus arrhizus and other human pathogenic Mucorales species significantly stimulated mRNA synthesis and secretion of proinflammatory cytokines. Moreover, R. arrhizus spores induced the upregulation of co-stimulatory molecules on moDCs and a specific T-helper cell response. Removal of rodlet hydrophobins by hydrofluoric acid treatment of A. fumigatus conidia resulted in enhanced immunogenicity, whereas the cytokine response of PBMCs to dormant R. arrhizus spores was not influenced by hydrofluoric acid. Scanning electron micrographs of Mucorales spores did not exhibit any morphological correlates of rodlet hydrophobins. Taken together, this study revealed striking differences in the response of human mononuclear cells to resting stages of Ascomycota and Mucorales, which may be explained by absence of an immunoprotective hydrophobin layer in Mucorales spores.

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.

Candida albicans infection and intestinal immunity

Fungal infections cause high rates of morbidity and mortality in intensive care and immunocompromised patients, and can represent a life-threatening disease. As a microorganism commonly found in the intestine, Candida albicans (C. albicans) can invade the gut epithelium barrier via microfold cells and enter the bloodstream. The defensive potential of the intestinal barrier against invasive C. albicans is dependent on innate and adaptive immune responses which enable the host to eliminate pathogenic fungi. The lamina propria layer of the intestine contains numerous immune cells capable of inducing an innate cellular immune response against invasive fungi. This review focuses on the immune response triggered by a C. albicans infection in the intestine.

Immunity to Commensal Fungi: Detente and Disease

Fungi are ubiquitous in our environment, and a healthy immune system is essential to maintain adequate protection from fungal infections. When this protection breaks down, superficial and invasive fungal infections cause diseases that range from irritating to life-threatening. Millions of people worldwide develop invasive infections during their lives, and mortality for these infections often exceeds 50%. Nevertheless, we are normally colonized with many of the same disease-causing fungi (e.g., on the skin or in the gut). Recent research is dramatically expanding our understanding of the mechanisms by which our immune systems interact with these organisms in health and disease. In this review, we discuss what is currently known about where and how the immune system interacts with common fungi.

Tolerogenic Plasmacytoid Dendritic Cells Control Paracoccidioides brasiliensis Infection by Inducting Regulatory T Cells in an IDO-Dependent Manner

Plasmacytoid dendritic cells (pDCs), considered critical for immunity against viruses, were recently associated with defense mechanisms against fungal infections. However, the immunomodulatory function of pDCs in pulmonary paracoccidiodomycosis (PCM), an endemic fungal infection of Latin America, has been poorly defined. Here, we investigated the role of pDCs in the pathogenesis of PCM caused by the infection of 129Sv mice with 1 x 10⁶P. brasiliensis-yeasts. In vitro experiments showed that P. brasiliensis infection induces the maturation of pDCs and elevated synthesis of TNF-α and IFN-β. The in vivo infection caused a significant influx of pDCs to the lungs and increased levels of pulmonary type I IFN. Depletion of pDCs by a specific monoclonal antibody resulted in a less severe infection, reduced tissue pathology and increased survival time of infected mice. An increased influx of macrophages and neutrophils and elevated presence of CD4⁺ and CD8⁺ T lymphocytes expressing IFN-γ and IL-17 in the lungs of pDC-depleted mice were also observed. These findings were concomitant with decreased frequency of Treg cells and reduced levels of immunoregulatory cytokines such as IL-10, TGF-β, IL-27 and IL-35. Importantly, P. brasilienis infection increased the numbers of pulmonary pDCs expressing indoleamine 2,3-dioxygenase-1 (IDO), an enzyme with immunoregulatory properties, that were reduced following pDC depletion. In agreement, an increased immunogenic activity of infected pDCs was observed when IDO-deficient or IDO-inhibited pDCs were employed in co-cultures with lymphocytes Altogether, our results suggest that in pulmonary PCM pDCs exert a tolerogenic function by an IDO-mediated mechanism that increases Treg activity.

The fungus Paracoccidioides brasiliensis causes paracoccidioidomycosis (PCM), the most relevant deep mycosis in Latin America. The plasmacytoid dendritic cells (pDCs) are important immune cells involved in protection against viral infections, but their role in fungal infections remains unclear. Here, we investigated the role of pDCs in the pathogenesis of pulmonary PCM using a monoclonal antibody to deplete this DC subset. pDCs depletion leads to a less severe PCM associated with increased T cell response mainly mediated by Th1 and Th17 cells. The lung homogenates of depleted mice showed diminished levels of type I IFN and anti-inflammatory cytokines. In addition, a reduced number of regulatory T cells (Treg) paralleled a diminished number pDCs expressing IDO, a potent immunoregulatory enzyme. In agreement, pDCs of IDO^-/- mice or IDO-inhibited pDCs stimulated by P. brasiliensis yeasts expanded elevated numbers of T cells concomitant with a reduced expansion of Treg cells. Taken together, our results demonstrate a tolerogenic activity of pDCs that enhances the severity of a pulmonary mycosis mediated by the concerted action of IDO and Treg cells. These results reveal a new function for pDCs in primary fungal infections and open new perspectives for immunotherapeutic procedures of PCM involving the control of IDO and Treg activity.

Duality at the gate: Skin dendritic cells as mediators of vaccine immunity and tolerance

Since Edward Jenner's discovery that intentional exposure to cowpox could provide lifelong protection from smallpox, vaccinations have been a major focus of medical research. However, while the protective benefits of many vaccines have been successfully translated into the clinic, the cellular and molecular mechanisms that differentiate effective vaccines from sub-optimal ones are not well understood. Dendritic cells (DCs) are the gatekeepers of the immune system, and are ultimately responsible for the generation of adaptive immunity and lifelong protective memory through interactions with T cells. In addition to lymph node and spleen resident DCs, a number of tissue resident DC populations have been identified at barrier tissues, such as the skin, which migrate to the local lymph node (migDC). These populations have unique characteristics, and play a key role in the function of cutaneous vaccinations by shuttling antigen from the vaccination site to the draining lymph node, rapidly capturing freely draining antigens in the lymph node, and providing key stimuli to T cells. However, while migDCs are responsible for the generation of immunity following exposure to certain pathogens and vaccines, recent work has identified a tolerogenic role for migDCs in the steady state as well as during protein immunization. Here, we examine the roles and functions of skin DC populations in the generation of protective immunity, as well as their role as regulators of the immune system.

Immunoregulation in Fungal Diseases

This review addresses specific regulatory mechanisms involved in the host immune response to fungal organisms. We focus on key cells and regulatory pathways involved in these responses, including a brief overview of their broader function preceding a discussion of their specific relevance to fungal disease. Important cell types discussed include dendritic cells and regulatory T cells, with a focus on specific studies relating to their effects on immune responses to fungi. We highlight the interleukin-10, programmed cell death 1, and cytotoxic T lymphocyte-associated protein 4 signaling pathways and emphasize interrelationships between these pathways and the regulatory functions of dendritic cells and regulatory T cells. Throughout our discussion, we identify selected studies best illustrating the role of these cells and pathways in response to specific fungal pathogens to provide a contextual understanding of the tightly-controlled network of regulatory mechanisms critical to determining the outcome of exposure to fungal pathogens. Lastly, we discuss two unique phenomena relating to immunoregulation, protective tolerance and immune reactivation inflammatory syndrome. These two clinically-relevant conditions provide perspective as to the range of immunoregulatory mechanisms active in response to fungi.

Autoimmune Regulator Expression in DC2.4 Cells Regulates the NF-κB Signaling and Cytokine Expression of the Toll-Like Receptor 3 Pathway

Autoimmune regulator (Aire) mutations result in autoimmune polyendocrinopathy candidiasis ectodermal dystrophy (APECED), which manifests as multi-organ autoimmunity and chronic mucocutaneous candidiasis (CMC). Indendritic cells (DCs), pattern recognition receptors (PRR), such as Toll-like receptors (TLRs), are closely involved in the recognition of various pathogens, activating the intercellular signaling pathway, followed by the activation of transcription factors and the expression of downstream genes, which take part in mediating the immune response and maintaining immune tolerance. In this study, we found that Aire up-regulated TLR3 expression and modulated the downstream cytokine expression and nuclear factor-κB (NF-κB) of the TLR3 signaling pathway.

Opportunities for the development of novel therapies based on host-microbial interactions

Immune responses are fundamental for protecting against most infectious agents. However, there is now much evidence to suggest that the pathogenesis and tissue damage after infection are not usually related to the direct action of the replication of microorganisms, but instead to altered immune responses triggered after the contact with the pathogen. This review article discusses several mechanisms necessary for the host to protect against microbial infection and focuses in aspects that cause altered inflammation and drive immunopathology. These basic findings can ultimately reveal pathways amenable to host-directed therapy in adjunct to antimicrobial therapy for future improved control measures for many infectious diseases. Therefore, modulating the effects of inflammatory pathways may represent a new therapy during infection outcome and disease.

Regulation of Th2 Cell Immunity by Dendritic Cells

Th2 cell immunity is required for host defense against helminths, but it is detrimental in allergic diseases in humans. Unlike Th1 cell and Th17 cell subsets, the mechanism by which dendritic cells modulate Th2 cell responses has been obscure, in part because of the inability of dendritic cells to provide IL-4, which is indispensable for Th2 cell lineage commitment. In this regard, immune cells other than dendritic cells, such as basophils and innate lymphoid cells, have been suggested as Th2 cell inducers. More recently, multiple independent researchers have shown that specialized subsets of dendritic cells mediate Th2 cell responses. This review will discuss the current understanding related to the regulation of Th2 cell responses by dendritic cells and other immune cells.

Fungal Chitin Induces Trained Immunity in Human Monocytes during Cross-talk of the Host with Saccharomyces cerevisiae

The immune system is essential to maintain the mutualistic homeostatic interaction between the host and its micro- and mycobiota. Living as a commensal,Saccharomyces cerevisiaecould potentially shape the immune response in a significant way. We observed thatS. cerevisiaecells induce trained immunity in monocytes in a strain-dependent manner through enhanced TNFα and IL-6 production upon secondary stimulation with TLR ligands, as well as bacterial and fungal commensals. Differential chitin content accounts for the differences in training properties observed among strains, driving induction of trained immunity by increasing cytokine production and direct antimicrobial activity bothin vitroandin vivo These chitin-induced protective properties are intimately associated with its internalization, identifying a critical role of phagosome acidification to facilitate microbial digestion. This study reveals how commensal and passenger microorganisms could be important in promoting health and preventing mucosal diseases by modulating host defense toward pathogens and thus influencing the host microbiota-immune system interactions.

Hypoxia-Induced Epithelial-to-Mesenchymal Transition in Hepatocellular Carcinoma Induces an Immunosuppressive Tumor Microenvironment to Promote Metastasis

Portal vein tumor thrombosis (PVTT) is a significant risk factor for metastasis in hepatocellular carcinoma (HCC) patients and is therefore associated with poor prognosis. The presence of PVTT frequently accompanies substantial hypoxia within the tumor microenvironment, which is suggested to accelerate tumor metastasis, but it is unclear how this occurs. Recent evidence has shown that the hypoxia-inducible factor HIF-1α induces epithelial-to-mesenchymal transition (EMT) in tumor cells to facilitate metastasis. In this study, we investigated whether hypoxia-induced EMT in cancer cells also affects immune cells in the tumor microenvironment to promote immunosuppression. We found that hypoxia-induced EMT increased the expression of the CCL20 cytokine in hepatoma cells. Furthermore, coculture of monocyte-derived macrophages with hypoxic hepatoma cells revealed that the expression of indoleamine 2, 3-dioxygenase (IDO) was induced in monocyte-derived macrophages in a CCL20-dependent manner. In turn, these IDO-expressing monocyte-derived macrophages suppressed T-cell proliferation and promoted the expansion of immunosuppressive regulatory T cells. Moreover, high CCL20 expression in HCC specimens was associated with PVTT and poor patient survival. Collectively, our findings suggest that the HIF-1α/CCL20/IDO axis in hepatocellular carcinoma is important for accelerating tumor metastasis through both the induction of EMT and the establishment of an immunosuppressive tumor microenvironment, warranting further investigation into the therapeutic effects of blocking specific nodes of this signaling network.

Tolerogenic IDO(+) Dendritic Cells Are Induced by PD-1-Expressing Mast Cells

Mast cells (MCs) are tissue resident cells, rich in inflammatory mediators, involved in allergic reactions, and with an increasingly recognized role in immunomodulation. Dendritic cells (DCs), on the other hand, are central to the determination of immune response patterns, being highly efficient antigen-presenting cells that respond promptly to changes in their microenvironment. Here, we show that direct cell contact between immature monocyte-derived DCs (iDCs) and MC bends DCs toward tolerance induction. DCs that had direct contact with MC (MC-iDC) decreased HLA-DR but increased PD-L1 expression and stimulated regulatory T lymphocytes, which expresses FoxP3(+), secrete TGF-β and IL-10, and suppress the proliferation of mitogen-stimulated naïve T lymphocytes. Furthermore, MC-iDC expressed higher levels of indoleamine-2,3-deoxigenase (IDO), a phenomenon that was blocked by treatment of MC with anti-PD-1 or by the treatment of DCs with anti-PD-L1 or anti-PD-L2, but not by blocking of H1 and H2 histamine receptors on DCs. Contact with MC also increased phosphorylated STAT-3 levels in iDCs. When a STAT-3 inhibitor, JSI-124, was added to the DCs before contact with MC, the MC-iDC recovered their ability to induce allogeneic T cell proliferation and did not increase their IDO expression.

Silenced suppressor of cytokine signaling 1 (SOCS1) enhances the maturation and antifungal immunity of dendritic cells in response to Candida albicans in vitro

Dendritic cells (DCs) are known to play an important role in initiating and orchestrating antimicrobial immunity. Given the fact that candidiasis appears often in immunocompromised patients, it seems plausible that DCs hold the key to new antifungal strategies. One possibility to enhance the potency of DC-based immunotherapy is to silence the negative immunoregulatory pathways through the ablation suppressor of cytokine signaling suppressor 1 (SOCS1). Here, we deliver small interfering RNA (siRNA) against SOCS1 into murine bone marrow DCs, and as a consequence, we investigate the maturation/action of DCs and the subsequent T cell response after exposure to C. albicans. Our results show that the maturation of DCs (i.e., expressions of CD80, CD40, CD86, and MHC II) are significantly increased in the silenced SOCS1 treatment group after exposure to C. albicans. As a result, suppression of the SOCS1 promotes the greatest expression of IFN-γ and IL-12, and reduces IL-4 secretions, which induce CD4⁺ cell Th1 differentiation but inactivate Th2 cell development. The responses of IL-6 and TNF-β consist of up-regulation in the presence of C. albicans, but this is not specific to SOCS1 silencing, suggesting that these cytokines are not regulated by the SOCS1 gene in fungal infections. We find Th17 differentiation is unchanged regardless of SOCS1 inhibition. The increase in phagocytosis and killing of C. albicans in SOCS1 gene-treated DCs indicate a role for this cytokine suppressor in innate immunity as well. In conclusion, our findings support the view that SOCS1 protein is a critical inhibitory molecule for controlling cytokine response and antigen presentation by DCs, thereby regulating the magnitude of innate and adaptive immunities by generating IFN-γ-production T cells (Th1)—but not Th17—from naïve CD4⁺ T cells. Our study demonstrates that SOCS1 siRNA can serve as a useful vehicle to modulate the function of DCs against C. albicans infection.

Sampling of Candida albicans and Candida tropicalis by Langerin-positive dendritic cells in mouse Peyer's patches

Members of the Candida genus, including C. albicans and C. tropicalis are opportunistic fungal pathogens that are increasingly associated with gastrointestinal infections and inflammatory bowel diseases. In healthy populations, however, C. albicans and C. tropicalis are considered benign members of the mycobiome, and are presumably kept in check by the mucosal immune system. In this study, we demonstrate in mice that C. albicans and C. tropicalis are sampled by Peyer's patch (PP) dendritic cells (DCs). Uptake into gut-associated lymphoid tissues occurred rapidly and was at least partly M cell-dependent. C. albicans and C. tropicalis preferentially localized in (and persisted within) a recently identified sub- population of Peyer's patch DCs distinguished by their expression of the C-type lectin receptor, Langerin. This study is the first to identify a subset of PP DCs capable of sampling Candida species.

Beyond Candida albicans: Mechanisms of immunity to non-albicans Candida species

The fungal genus Candida encompasses numerous species that inhabit a variety of hosts, either as commensal microbes and/or pathogens. Candida species are a major cause of fungal infections, yet to date there are no vaccines against Candida or indeed any other fungal pathogen. Our knowledge of immunity to Candida mainly comes from studies on Candida albicans, the most frequent species associated with disease. However, non-albicans Candida (NAC) species also cause disease and their prevalence is increasing. Although research into immunity to NAC species is still at an early stage, it is becoming apparent that immunity to C. albicans differs in important ways from non-albicans species, with important implications for treatment, therapy and predicted demographic susceptibility. This review will discuss the current understanding of immunity to NAC species in the context of immunity to C. albicans, and highlight as-yet unanswered questions.