Are Th17 Cells Playing a Role in Immunity to Dermatophytosis?

Expression of fungal and host markers in models of dermatophytosis on mice and human epidermis

Increasing resistance of dermatophytes against antifungals creates global public health problems, rendering essential a better understanding of virulence mechanisms and factors determining host-specificity of dermatophytes. Since dermatophytes switch from a saprophytic to a parasitic lifestyle by reprogramming gene expression, reliable experimental models are needed to investigate the pathogenesis of dermatophytosis. Here, a relevant mouse model of Trichophyton benhamiae dermatophytosis was assessed, together with a model based on reconstructed human epidermis (RHE), allowing their respective validation regarding fungal gene expressed during infection. The use of a standardized inoculum induced a natural-like superficial infection in mice. The severity and persistence of lesions enabled the assessment of infection markers, including mouse-specific pro-inflammatory molecules and fungal genes previously reported as potential virulence factors. Upregulated expression of fungal genes, including those encoding subtilisins, in infected RHE revealed that dermatophytes deploy similar processes as those observed during in vivo infection. The RHE model was then used to compare infections by anthropophilic Trichophyton rubrum and zoophilic T. benhamiae. Therefore, these two models represent complementary analytical tools to study the pathogenesis of acute dermatophytoses. In addition, we have identified certain fungal markers of infection and highlighted the existence of different mechanisms deployed by zoophilic versus anthropophilic dermatophytes.

Host-Pathogen Interaction and Resistance Mechanisms in Dermatophytes

Dermatophytes are widely distributed in the environment, with an estimated prevalence of 20-25% of the the global population yearly. These fungi are keratinophilic and keratinolytic and cause the infection of keratin-rich structures such as skin, hair, and nails. The pattern of this infectious disease covers a wide spectrum from exposed individuals without symptoms to those with acutely inflammatory or non-inflammatory, chronic to invasive, and even life-threatening symptoms. This review summarizes current information on the pathogenicity, virulence factors, and drug resistance mechanisms associated with dermatophytes. A greater number of virulence factors of these fungi are important for the occurrence of infection and the changes that occur, including those regarding adhesins, the sulfite efflux pump, and proteolytic enzymes. Other virulence factors include mechanisms of evading the host defense, while the development of resistance to antifungal drugs is increasing, resulting in treatment failure. The investigation of host-pathogen interactions is essential for developing a more complete understanding of the mechanisms underlying dermatophyte pathogenesis and host response to inform the use of diagnostics methods and antifungal therapeutics to minimize the high fungal burden caused by dermatophytes and to control the spread of resistance.

Microsporum gypseum Isolated from Ailuropoda melanoleuca Provokes Inflammation and Triggers Th17 Adaptive Immunity Response

Microsporum gypseum causes dermatomycoses in giant pandas (Ailuropoda melanoleuca). This study aimed to investigate the immune response of M. gypseum following deep infection. The degree of damage to the heart, liver, spleen, lungs, and kidneys was evaluated using tissue fungal load, organ index, and histopathological methods. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) detected the mRNA expression of receptors and cytokines in the lung, and immunofluorescence staining and flow cytometry, were used to assess immune cells in the lung. The results indicated that conidia mainly colonized the lungs and caused serious injury with M. gypseum infection. Furthermore, dectin-1, TLR-2, and TLR-4 played a role in recognizing M. gypseum cells. Numerous inflammatory cells, mainly macrophages, dendritic cells, polymorphonuclear neutrophils, and inflammatory cytokines (TGF-β, TNF-α, IL-1β, IL-6, IL-10, IL-12, and IL-23), were activated in the early stages of infection. With the high expression of IL-22, IL-17A, and IL-17F, the Th17 pathway exerted an adaptive immune response to M. gypseum infection. These results can potentially aid in the diagnosis and treatment of diseases caused by M. gypseum in giant pandas.

Circulating redox status in sheep naturally infected with Trichophyton verrucosum

Trichophyton verrucosum is a zoophilic dermatophyte that causes skin inflammation. The present study aimed to evaluate the redox status in the blood of sheep clinically infected with T. verrucosum. According to clinical and mycological investigations, 48 juvenile male Balady sheep were selected in their natural habitat and divided into four groups depending on the lesion size: mild (MID), moderate (MOD), severe (SEV) T. verrucosum infection, and healthy control groups. Compared to the controls, plasma superoxide anion increased (P < 0.05) in both MOD and SEV but total peroxides (TPx) gradually increased (P < 0.05) in MID followed by MOD and SEV. Superoxide dismutase and total antioxidant capacity (TAC) were higher (P < 0.05) in MID and lower (P < 0.05) in MOD and SEV than in controls, but SEV showed lower TAC than MOD. Malondialdehyde (MDA, a lipid peroxide marker) increased (P < 0.05) in SEV than in controls, but protein carbonyl (PC, a protein peroxidation marker) was augmented (P < 0.05) as lesions progressed from mild to severe. The oxidative stress index (TPx/TAC ratio) progressively increased (P < 0.05) in MOD and SEV. The correlation of PC was positive with TPx and negative with TAC (P < 0.01). In conclusion, sheep infection with T. verrucosum is characterized by increased TPx and decreased TAC in plasma depending on the lesion area. The redox status is shifted towards the oxidizing state, particularly in MOD and SEV cases. This results in a condition of OS, which may contribute to the pathogenesis of the disease.

Trends in the epidemiology of dermatophytosis in the Middle East and North Africa region

Dermatophytosis corresponds to a broad series of infections, mostly superficial, caused by a group of keratinophilic and keratinolytic filamentous fungi called dermatophytes. These mycoses are currently considered to be a major public health concern worldwide, particularly in developing countries such as those in the Middle East and North Africa (MENA) region. Here we compiled and discussed existing epidemiologic data on these infections in the MENA region. Most of the available studies were based on conventional diagnostic strategies and were published before the last taxonomic revision of dermatophytes. This has led to misidentifications, which might have resulted in the underestimation of the real burden of these infections in the MENA countries. Our analysis of the available literature highlights an urgent need for further studies based on reliable diagnostic tools and standard susceptibility testing methods for dermatophytosis, which represents a major challenge for these countries. This is crucial for guiding appropriate interventions and activating antifungal stewardship programs in the future.

Immunopathogenesis of Dermatophytoses and Factors Leading to Recalcitrant Infections

The pathogenesis of dermatophytic infections involves the interplay of three major factors: the dermatophyte, the inherent host defense, and the adaptive host immune response. The fungal virulence factors determine the adhesion and invasion of the skin while the immune response depends on an interaction of the pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMP) with pattern recognition receptors (PRRs) of the host, which lead to a differential Th (T helper) 1, Th2, Th17, and Treg response. While anthropophilic dermatophytes Trichophyton rubrum and now increasingly by T. interdigitale subvert the immune response via mannans, zoophilic species are eliminated due to a brisk immune response. Notably, delayed-type hypersensitivity (Th1) response of T lymphocytes causes the elimination of fungal infection, while chronic disease caused by anthropophilic species corresponds to toll-like receptor 2 mediated IL (interleukin)-10 release and generation of T-regulatory cells with immunosuppressive potential. Major steps that determine the ultimate clinical course and chronicity include genetic susceptibility factors, impaired epidermal and immunological barriers, variations in the composition of sebum and sweat, carbon dioxide tension, skin pH, and topical steroid abuse. It is important to understand these multifarious aspects to surmount the problem of recalcitrant dermatophytosis when the disorder fails conventional therapeutic agents.

Host immune responses in dermatophytes infection

Dermatophytosis is a skin infection caused by keratinophilic, filamentous fungi. These are highly prevalent, common mycoses, affecting approximately 20% of the population. These fungi invade the stratum corneum, and other keratinised tissues, like nails and hair, where they grow by secreting enzymes and degrading keratin to obtain nutrients. Clinical presentation is variable and may depend on many factors, such as the infection site, the host's immunity and the dermatophyte's virulence. Generally, patients with acute superficial dermatophytosis mount cell-mediated immune responses. However, those suffering from chronic or recurrent infections are unable to develop this response, for reasons yet unknown. Several reports have described severe and occasionally life-threatening invasive diseases (deep dermatophytosis) associated with genetic mutations in the innate immunity-associated molecule CARD9, displaying the need to better understand its immune response. These dermatoses have substantial clinical consequences, producing chronic and difficult to treat skin lesions. They also lead to a decline in the patient's quality of life and impact their self-esteem. This review summarises findings on the immune response against dermatophytes.

Human primary epidermal organoids enable modeling of dermatophyte infections

Technology of generating human epidermal derivatives with physiological relevance to in vivo epidermis is continuously investigated for improving their effects on modeling of human natural dermatological status in basic and clinical studies. Here, we report a method of robust establishment and expansion of human primary epidermal organoids (hPEOs) under a chemically defined condition. hPEOs reconstruct morphological, molecular, and functional features of human epidermis and can expand for 6 weeks. Remarkably, hPEOs are permissive for dermatophyte infections caused by Trichophyton Rubrum (T. rubrum). The T. rubrum infections on hPEOs reflect many aspects of known clinical pathological reactions and reveal that the repression on IL-1 signaling may contribute to chronic and recurrent infections with the slight inflammation caused by T. rubrum in human skin. Thus, our present study provides a new insight into the pathogenesis of T. rubrum infections and indicates that hPEOs are a potential ex vivo model for both basic studies of skin diseases and clinical studies of testing potential antifungal drugs.

RNA-Seq analysis of the guppy immune response against Gyrodactylus bullatarudis infection

Gyrodactylids are ubiquitous ectoparasites of teleost fish, but our understanding of the host immune response against them is fragmentary. Here, we used RNA-Seq to investigate genes involved in the primary response to infection with Gyrodactylus bullatarudis on the skin of guppies, Poecilia reticulata, an important evolutionary model, but also one of the most common fish in the global ornamental trade. Analysis of differentially expressed genes identified several immune-related categories, including IL-17 signalling pathway and Th17 cell differentiation, cytokine-cytokine receptor interaction, chemokine signalling pathway, NOD-like receptor signalling pathway, natural killer cell-mediated cytotoxicity and pathways involved in antigen recognition, processing and presentation. Components of both the innate and the adaptive immune responses play a role in response to gyrodactylid infection. Genes involved in IL-17/Th17 response were particularly enriched among differentially expressed genes, suggesting a significant role for this pathway in fish responses to ectoparasites. Our results revealed a sizable list of genes potentially involved in the teleost-gyrodactylid immune response.

The interplay among Th17 and T regulatory cells in the immune dysregulation of chronic dermatophytic infection

The delineation of the pathogenic interaction between the host skin immune responses and dermatophytes has remained indigent. The obscure enigma in host-dermatophyte immunopathogenic interactions is the T regulatory (Treg) and T-helper (Th) 17  cell role in maintaining immune homeostasis. We attempted to understand the regulation and recognition of lineage-specific response in chronic dermatophytic skin infection patients. The percentages of Th17 (CD4⁺CD161⁺IL23R⁺) and Treg (CD4⁺CD25⁺FoxP3⁺) cell subpopulations in the peripheral circulation of thirty chronic dermatophytic skin infection patients and twenty healthy individuals was determined. The serum cytokine levels were estimated for disease correlation. The mean duration of the disease was 10.68 ± 8.72 months, with Trichophyton mentagrophytes complex as the major pathogen. Total serum IgE level of patients was significantly higher compared to healthy controls (305 ± 117 vs 98.53 ± 54.55 IU/ml; p < 0.01). Expression of Th17 and Treg cell markers on CD4⁺ T cells was significantly elevated in patients than controls (p < 0.05). Comparatively, serum interleukin (IL)-4 and interferon (IFN)-γ levels were increased, with low IL-10 levels in patients. Our data envisages a complex immune dysfunction in chronic dermatophytosis, arising either as a result of dermatophyte exposure or paradoxical precedence of disease establishment. Designing new treatment strategies and preventing recurrences are challenges for future research.

In situ immune response in human dermatophytosis: possible role of Langerhans cells (CD1a+) as a risk factor for dermatophyte infection

Dermatophytosis is a cutaneous mycosis caused by a plethora of keratinophilic fungi, but Trichophyton rubrum is the most common etiological agent. Despite its high prevalence worldwide, little is known about the host defense mechanisms in this infection, particularly the in situ immune response. Using an immunohistochemistry approach, we investigated the density of CD1a+, factor XIIIa+ and CD68+ cells in the skin of dermatophytosis patients. Langerhans cells (CD1a+ cells) were significantly decreased in the epidermis of patients, both in affected and unaffected areas. In the dermis, however, no differences in the density of macrophages (CD68+ cells) and dermal dendrocytes (factor XIIIa+ cells) were observed. These results suggest that the decreased number of Langerhans cells may be a risk factor for development of dermatophytosis.

Host- and pathogen-dependent susceptibility and predisposition to dermatophytosis

Dermatophytes are a highly specialized group of keratinophilic and keratinolytic filamentous fungi causing a ringworm disease called dermatophytosis or superficial mycoses. Although dermatophyte infections do not threaten the host's life, they lower its quality in humans by causing discomfort related to cosmetic problems and through their epidemiological significance, whereas in farm animals they are responsible for economic losses and constitute a source of the spread of spores. Evidence from countless observational studies that have been conducted over the last 90 years indicates that dermatophytes infect humans of every age, race, gender and socioeconomic status with strikingly high rates, as well as both farmed and wild animals in various health conditions and with various epidemiological statuses. However, the prevalence of superficial fungal infections is highly variable, since it depends on several parameters associated with the infected individual and the dermatophyte, their mutual interactions, and epidemiological and geographical factors. The curious disparity in dermatophyte infection patterns has prompted many investigators to search for a link between the host, the host's predispositions and susceptibility to the disease, and the dermatophyte species and virulence. Thus, the question arises as to whether, in addition to the generally recognized factors predisposing hosts to diseases, there are some other predispositions to dermatophyte infections in a species-specific host. In this review, we describe recent findings about the mechanism of dermatophyte infections, focusing on the adaptation of the fungi to the host and conditions predisposing each side to the disease.

New insights in dermatophyte research

Dermatophyte research has renewed interest because of changing human floras with changing socioeconomic conditions, and because of severe chronic infections in patients with congenital immune disorders. Main taxonomic traits at the generic level have changed considerably, and now fine-tuning at the species level with state-of-the-art technology has become urgent. Research on virulence factors focuses on secreted proteases now has support in genome data. It is speculated that most protease families are used for degrading hard keratin during nitrogen recycling in the environment, while others, such as Sub6 may have emerged as a result of ancestral gene duplication, and are likely to have specific roles during infection. Virulence may differ between mating partners of the same species and concepts of zoo- and anthropophily may require revision in some recently redefined species. Many of these questions benefit from international cooperation and exchange of materials. The aim of the ISHAM Working Group Dermatophytes aims to stimulate and coordinate international networking on these fungi.

Dermatophyte Resistance to Antifungal Drugs: Mechanisms and Prospectus

Dermatophytes comprise pathogenic fungi that have a high affinity for the keratinized structures present in nails, skin, and hair, causing superficial infections known as dermatophytosis. A reasonable number of antifungal drugs currently exist on the pharmaceutical market to control mycoses; however, their cellular targets are restricted, and fungi may exhibit tolerance or resistance to these agents. For example, the stress caused by antifungal and cytotoxic drugs in sub-inhibitory concentrations promotes compensatory stress responses, with the over-expression of genes involved in cellular detoxification, drug efflux, and signaling pathways being among the various mechanisms that may contribute to drug tolerance. In addition, the ATP-binding cassette transporters in dermatophytes that are responsible for cellular efflux can act synergistically, allowing one to compensate for the absence of the other, revealing the complexity of drug tolerance phenomena. Moreover, mutations in genes coding for target enzymes could lead to substitutions in amino acids involved in the binding of antifungal agents, hindering their performance and leading to treatment failure. The relevance of each one of these mechanisms of resistance to fungal survival is hard to define, mainly because they can act simultaneously in the cell. However, an understanding of the molecular mechanisms involved in the resistance/tolerance processes, the identification of new antifungal targets, as well as the prospective of new antifungal compounds among natural or synthetic products, are expected to bring advances and new insights that facilitate the improvement or development of novel strategies for antifungal therapy.

IL-17-Mediated Immunity Controls Skin Infection and T Helper 1 Response during Experimental Microsporum canis Dermatophytosis

Despite worldwide prevalence of superficial mycoses, the immune response in dermatophytosis has scarcely been investigated. In this study, we developed a model of superficial skin infection in C57BL/6 mice with Microsporum canis, a highly prevalent human pathogen. This model mimics mild inflammatory human dermatophytosis, characterized by neutrophil recruitment and fungal invasion limited to the epidermis and exhibits the establishment of a specific T helper type 17 immune response during infection. By using IL-17RA- or IL-17A/F-deficient mice we showed that, in the absence of a functional IL-17 pathway, M. canis extensively colonizes the epidermis and promotes an exaggerated skin inflammation and a shift to an IFN-γ-mediated (T helper type 1) response. IL-17 signaling was not involved in neutrophil influx to skin or fungal invasion to deeper tissues. Finally, this study shows that skin langerin-expressing cells contribute to the antifungal T helper type 17 response in vivo. In conclusion, these data directly show a dual function of IL-17 cytokines in dermatophytosis by controlling superficial infection and down-modulating a T helper type 1 antifungal response.