Sporothrix globosa melanin inhibits antigenpresentation by macrophages and enhances deep organ dissemination

Current situation of sporotrichosis in China

Sporotrichosis, a mycosis resulting from cutaneous or subcutaneous infection with the dimorphic fungus Sporothrix, has been reported in China, particularly in the northeast region. In this review, we conducted a thorough examination of the recent advancements in sporotrichosis in China, encompassing aspects such as etiology, epidemiology, pathogenesis, clinical manifestations, diagnosis and treatment strategies. Within the Chinese context, fixed cutaneous sporotrichosis represents the prevailing clinical manifestation. Fungal culture stands as the gold standard for diagnosing sporotrichosis, while polymerase chain reaction techniques can enhance both the specificity and sensitivity of diagnosis. Besides conventional systemic antifungal agents, alternative modalities such as Chinese herbal medicines, photodynamic therapy and laser therapy show potential efficacy against sporotrichosis.

Low toxicity contributes to Sporothrix globosa invade the skin of patients in low-epidemic areas of China

OBJECTIVE

This study aims to assess the clinical characteristics of sporotrichosis in low-endemic areas of China, including the prevalence geography, genotypic traits of patients, clinical manifestations, and strain virulence and drug sensitivities. The objective is to improve the currently used clinical management strategies for sporotrichosis.

METHODS

Retrospective data were collected from patients diagnosed with sporotrichosis through fungal culture identification. The isolates from purified cultures underwent identification using CAL (Calmodulin) gene sequencing. Virulence of each strain was assessed using a Galleria mellonella (G. mellonella) larvae infection model. In vitro susceptibility testing against commonly used clinical antifungal agents for sporotrichosis was conducted following CLSI criteria.

RESULTS

In our low-endemic region for sporotrichosis, the majority of cases (23) were observed in middle-aged and elderly women with a history of trauma, with a higher incidence during winter and spring. All clinical isolates were identified as Sporothrix globosa (S. globosa). The G. mellonella larvae infection model indicated independent and dose-dependent virulence among strains, with varying toxicity levels demonstrated by the degree of melanization of the G. mellonella. Surprisingly, lymphocutaneous types caused by S. globosa exhibited lower in vitro virulence but were more common in affected skin. In addition, all S.globosa strains displayed high resistances to fluconazole, while remaining highly susceptible to terbinafine, itraconazole and amphotericin B.

CONCLUSION

Given the predominance of elderly women engaged in agricultural labour in our region, which is a low-epidemic areas, they should be considered as crucial targets for sporotrichosis monitoring. S. globosa appears to be the sole causative agent locally. However, varying degrees of melanization in larvae were observed among these isolates, indicating a divergence in their virulence. Itraconazole, terbinafine and amphotericin B remain viable first-line antifungal options for treating S.globosa infection.

CRISPR-based tools for targeted genetic manipulation in pathogenic Sporothrix species

Sporothrix brasiliensis is an emerging fungal pathogen frequently associated with zoonotic transmission of sporotrichosis by contaminated cats. Within 25 years, the disease has spread not only throughout Brazil but now to neighboring countries in Latin America. Thermo-dimorphism, melanin, glycans, adhesins, and secreted vesicles have been associated with the ability of Sporothrix species to cause disease in the mammalian host. Although certain virulence factors have been proposed as potential determinants for sporotrichosis, the scarcity of molecular tools for performing reverse genetics in Sporothrix has significantly impeded the dissection of mechanisms underlying the disease. Here, we demonstrate that PEG-mediated protoplast transformation is a powerful method for heterologous gene expression in S. brasiliensis, S. schenckii, and S. chilensis. Combined with CRISPR/Cas9 gene editing, this transformation protocol enabled the deletion of the putative DHN-melanin synthase gene pks1, which is a proposed virulence factor of Sporothrix species. To improve in locus integration of deletion constructs, we deleted the KU80 homolog that is critical for non-homologous end-joining DNA repair. The use of Δku80 strains from S. brasiliensis enhanced homologous-directed repair during transformation resulting in increased targeted gene deletion in combination with CRISPR/Cas9. In conclusion, our CRISPR/Cas9-based transformation protocol provides an efficient tool for targeted gene manipulation in Sporothrix species. IMPORTANCE Sporotrichosis caused by Sporothrix brasiliensis is a disease that requires long periods of treatment and is rapidly spreading across Latin America. The virulence of this fungus and the surge of atypical and more severe presentations of the disease raise the need for an understanding of the molecular mechanisms underlying sporotrichosis, as well as the development of better diagnostics and antifungal therapies. By developing molecular tools for accurate genetic manipulation in Sporothrix, this study addresses the paucity of reliable and reproducible tools for stable genetic engineering of Sporothrix species, which has represented a major obstacle for studying the virulence determinants and their roles in the establishment of sporotrichosis.

Differential Recognition of Sporothrix schenckii, Sporothrix brasiliensis, and Sporothrix globosa by Human Monocyte-Derived Macrophages and Dendritic Cells

Background

Sporotrichosis is a mycosis frequently caused by Sporothrix schenckii, Sporothrix brasiliensis, and Sporothrix globosa. The cell wall is a species-specific fungal structure with a direct role in activating the host's immune response. The current knowledge about anti-Sporothrix immunity comes from studies using S. schenckii or S. brasiliensis and murine cells. Macrophages and dendritic cells detect and eliminate pathogens, and although the function of these cells links innate with adaptive immunity, little is known about their interaction with Sporothrix spp.

Methods

S. schenckii, S. brasiliensis, and S. globosa conidia or yeast-like cells were co-incubated with human monocyte-derived macrophages or dendritic cells, and the phagocytosis and cytokine stimulation were assessed. These interactions were also performed in the presence of specific blocking agents of immune receptors or fungal cells with altered walls to analyze the contribution of these molecules to the immune cell-fungus interaction.

Results

Both types of immune cells phagocytosed S. globosa conidia and yeast-like cells to a greater extent, followed by S. brasiliensis and S. schenckii. Furthermore, when the wall internal components were exposed, the phagocytosis level increased for S. schenckii and S. brasiliensis, in contrast to S. globosa. Thus, the cell wall components have different functions during the interaction with macrophages and dendritic cells. S. globosa stimulated an increased proinflammatory response when compared to the other species. In macrophages, this was a dectin-1-, mannose receptor-, and TLR2-dependent response, but dectin-1- and TLR2-dependent stimulation in dendritic cells. For S. schenckii and S. brasiliensis, cytokine production was dependent on the activation of TLR4, CR3, and DC-SIGN.

Conclusion

The results of this study indicate that these species are recognized by immune cells differently and that this may depend on both the structure and cell wall organization of the different morphologies.

Differential Recognition of Clinically Relevant Sporothrix Species by Human Mononuclear Cells

Sporotrichosis is a human and animal fungal infection distributed worldwide that is caused by the thermodimorphic species of the Sporothrix pathogenic clade, which includes Sporothrix brasiliensis, Sporothrix schenckii, and Sporothrix globosa. The cell wall composition and the immune response against the Sporothrix species have been studied mainly in S. brasiliensis and S. schenckii, whilst little is known about the S. globosa cell wall and the immune response that its components trigger. Therefore, in this study, we aimed to analyze the cell wall composition of S. globosa in three morphologies (germlings, conidia, and yeast-like cells) and the differences in cytokine production when human peripheral blood mononuclear cells (PBMCs) interact with these morphotypes, using S. schenckii and S. brasiliensis as a comparison. We found that S. globosa conidia and yeast-like cells have a higher cell wall chitin content, while all three morphologies have a higher β-1,3-glucan content, which was found most exposed at the cell surface when compared to S. schenckii and S. brasiliensis. In addition, S. globosa has lower levels of mannose- and rhamnose-based glycoconjugates, as well as of N- and O-linked glycans, indicating that this fungal cell wall has species-specific proportions and organization of its components. When interacting with PBMCs, S. brasiliensis and S. globosa showed a similar cytokine stimulation profile, but with a higher stimulation of IL-10 by S. globosa. Additionally, when the inner cell wall components of S. globosa were exposed at the surface or N- and O-glycans were removed, the cytokine production profile of this species in its three morphotypes did not significantly change, contrasting with the S. schenckii and S. brasiliensis species that showed different cytokine profiles depending on the treatment applied to the walls. In addition, it was found that the anti-inflammatory response stimulated by S. globosa was dependent on the activation of dectin-1, mannose receptor, and TLR2, but not TLR4. All of these results indicate that the cell wall composition and structure of the three Sporothrix species in the three morphologies are different, affecting their interaction with human PBMCs and generating species-specific cytokine profiles.

Innate Immune Responses to Sporothrix schenckii: Recognition and Elimination

Sporothrix schenckii (S. schenckii), a ubiquitous thermally dimorphic fungus, is the etiological agent of sporotrichosis, affecting immunocompromised and immunocompetent individuals. Despite current antifungal regimens, sporotrichosis results in prolonged treatment and significant mortality rates in the immunosuppressed population. The innate immune system forms the host's first and primary line of defense against S. schenckii, which has a bi-layered cell wall structure. Many components act as pathogen-associated molecular patterns (PAMPs) in pathogen-host interactions. PAMPs are recognized by pattern recognition receptors (PRRs) such as toll-like receptors, C-type lectin receptors, and complement receptors, triggering innate immune cells such as neutrophils, macrophages, and dendritic cells to phagocytize or produce mediators, contributing to S. schenckii elimination. The ultrastructure of S. schenckii and pathogen-host interactions, including PRRs and innate immune cells, are summarized in this review, promoting a better understanding of the innate immune response to S. schenckii and aiding in the development of protective and therapeutic strategies to combat sporotrichosis.

Biological and Clinical Attributes of Sporothrix globosa, a Causative Agent of Sporotrichosis

Sporotrichosis is an important subcutaneous mycosis with high prevalence and threat to human and animal health worldwide. Sporothrix schenckii, Sporothrix brasiliensis, and Sporothrix globosa are the main etiological agents of this disease; and even though many efforts have been made recently to understand the Sporothrix-host interaction, little is known about S. globosa, an underestimated species. This organism shows the lowest virulence among the members of the Sporothrix pathogenic clade and represents an important pathogenic agent due to its global distribution. Here, we offer a review with all the known information about S. globosa, including its genome and proteomic information, and compare it with S. schenckii and S. brasiliensis, to explain the differences observed among these species, in terms of virulence, the host immune response, and the antifungal sensitivity. Also, we provide the gene prediction of some S. globosa putative virulence factors.

Current Models to Study the Sporothrix-Host Interaction

Sporotrichosis is a worldwide distributed subcutaneous mycosis that affects mammals, including human beings. The infection is caused by members of the Sporothrix pathogenic clade, which includes Sporothrix schenckii, Sporothrix brasiliensis, and Sporothrix globosa. The fungus can be acquired through traumatic inoculation of conidia growing in vegetal debris or by zoonotic transmission from sick animals. Although is not considered a life-threatening disease, it is an emergent health problem that affects mostly immunocompromised patients. The sporotrichosis causative agents differ in their virulence, host range, and sensitivity to antifungal drugs; therefore, it is relevant to understand the molecular bases of their pathogenesis, interaction with immune effectors, and mechanisms to acquired resistance to antifungal compounds. Murine models are considered the gold standard to address these questions; however, some alternative hosts offer numerous advantages over mammalian models, such as invertebrates like Galleria mellonella and Tenebrio molitor, or ex vivo models, which are useful tools to approach questions beyond virulence, without the ethical or budgetary features associated with the use of animal models. In this review, we analyze the different models currently used to study the host-Sporothrix interaction.

Melanin of Sporothrix globosa affects the function of THP-1 macrophages and modulates the expression of TLR2 and TLR4

BACKGROUND

Melanin is an important virulence factor for Sporothrix globosa, the causative agent of sporotrichosis, a subcutaneous mycosis that occurs worldwide. Although previous research suggests that melanin is involved in the pathogenesis of sporotrichosis, little is known about its influence on the macrophages that represent the frontline components of innate immunity.

OBJECTIVES

To evaluate the effects of melanin on phagocytic activity and the expression of Toll-like receptor (TLR)2 and TLR4 during S. globosa infection of macrophages in vitro.

METHODS

To compare phagocytic activity and survival rates, THP-1 macrophages and primary mouse peritoneal macrophages were co-cultured with a wild-type S. globosa strain (Mel+), an albino mutant strain (Mel-), a tricyclazole-treated Mel + strain (TCZ-Mel+), or melanin ghosts extracted from S. globosa conidia. Reactive oxygen species (ROS), nitric oxide (NO) generation, tumor necrosis factor (TNF)-α and interleukin (IL)-6 were assayed in THP-1 cells infected with S. globosa conidia. Quantitative PCR and western blotting were used to observe the effect of melanin on TLR2 and TLR4 expression. Knockdown of TLR2/4 expression with small interfering RNA was performed to further verify the role of these receptors during infection.

RESULTS

Macrophages infected with Mel + conidia showed a lower phagocytosis index and a higher survival rate than TCZ-Mel+ and Mel- in vitro. After incubation with S. globosa, the release of ROS, NO, TNF-α and IL-6 by THP-1 were decreased in the presence of melanin. Increased mRNA and protein expression of TLR2 and TLR4 occurred upon S. globosa infection in THP-1, whereas the presence of melanin suppressed TLR2 and TLR4. Moreover, TLR2 or TLR4 knockdown showed a trend toward reducing the pernicious effect of S. globosa conidia on THP-1 cells in vitro.

CONCLUSIONS

Collectively, our results indicated that melanin inhibits the phagocytosis of S. globosa and guards against macrophage attack by providing protection from oxygen- and nitrogen-derived radicals, as well as suppressing the host pro-inflammatory cytokine response (TNF-α and IL-6). Melanin was also involved in modulating TLR2 and TLR4 receptor expression, weakening the killing efficiency of S. globosa.

Fungal Melanin and the Mammalian Immune System

Melanins are ubiquitous complex polymers that are commonly known in humans to cause pigmentation of our skin. Melanins are also present in bacteria, fungi, and helminths. In this review, we will describe the diverse interactions of fungal melanin with the mammalian immune system. We will particularly focus on Cryptococcus neoformans and also discuss other major melanotic pathogenic fungi. Melanin interacts with the immune system through diverse pathways, reducing the effectiveness of phagocytic cells, binding effector molecules and antifungals, and modifying complement and antibody responses.

5-Aminolevulinic acid-mediated photodynamic therapy has effective antifungal activity against Sporothrix globosa in vitro

BACKGROUND

An alternative therapy for sporotrichosis is necessary to reduce the treatment time and raise clinical efficacy. The 5-Aminolevulinic acid-mediated photodynamic therapy (ALA-PDT) is a promising platform with which to treat mycoses. However, despite the worldwide prevalence of Sporothrix globosa, a causative agent of sporotrichosis, the effect of ALA-PDT on this pathogen has not been validated.

OBJECTIVES

The aim of this study was to evaluate the effect of ALA-PDT on S globosa and the protection of melanin through an in vitro study. The mechanisms involved were also investigated.

METHODS

To estimate the survival rate of S globosa treated with ALA-PDT and the protection offered by melanin, the conidia and yeast cells of wild-type S globosa (Mel+), other clinical strains, tricyclazole-treated Mel+ and an albino mutant strain (Mel-) were incubated with and without ALA or irradiation. Reactive oxygen species generation by Mel+ conidia induced by ALA-PDT was assayed. SEM and TEM were conducted to obverse ultrastructural changes in the conidia. A comet assay was performed to evaluate DNA damage.

RESULTS

The survival rate of S globosa conidia and yeast cells significantly decreased following incubation with 1.19M ALA and 162 J/cm² irradiation in vitro. Melanin was not only capable of protecting the conidia against ALA-PDT, but also against ALA or irradiation alone. After induction by ALA-PDT, alterations in reactive oxygen species generation, DNA damage and ultrastructural changes were observed.

CONCLUSIONS

ALA-PDT inhibits the survival of S globosa conidia in vitro and therefore has potential for the treatment of sporotrichosis.