Role of Paracoccidioides brasiliensis cell wall fraction containing beta-glucan in tumor necrosis factor-alpha production by human monocytes: correlation with fungicidal activity

A Dectin-1-Caspase-8 Pathway Licenses Canonical Caspase-1 Inflammasome Activation and Interleukin-1β Release in Response to a Pathogenic Fungus

Background

Paracoccidioides brasiliensis is equipped with an arsenal of virulence factors that are crucial for causing infection. Our group previously defined the NLRP3 inflammasome as a mediator of P brasiliensis-induced cell damage recognition and induction of effective Th1 immune responses. However, deficiency of caspase-1 only partially reduced interleukin (IL)-1β levels.

Methods

In this study, using chemical inhibitors as well as genetically modified mice, we identify an additional pathway for IL-1β production in response to P brasiliensis infection.

Results

Paracoccidioides brasiliensis initiated caspase-8-mediated IL-1β production, an event that was necessary for transcriptional priming and posttranslational processing of pro-IL-1β. Caspase-8 synergizes with the canonical NLRP3 inflammasome pathway to control caspase-1 processing and IL-1β maturation, providing a regulatory role for caspase-8 in host resistance to in vivo P brasiliensis infection.

Conclusions

Taken together, these findings revealed an important role for caspase-8 in the innate immune response of host cells to P brasiliensis infection, demonstrating a connected network between noncanonical and canonical inflammasomes to coordinate IL-1β production during fungal challenge.

Anti-Immune Strategies of Pathogenic Fungi

Pathogenic fungi have developed many strategies to evade the host immune system. Multiple escape mechanisms appear to function together to inhibit attack by the various stages of both the adaptive and the innate immune response. Thus, after entering the host, such pathogens fight to overcome the immune system to allow their survival, colonization and spread to different sites of infection. Consequently, the establishment of a successful infectious process is closely related to the ability of the pathogen to modulate attack by the immune system. Most strategies employed to subvert or exploit the immune system are shared among different species of fungi. In this review, we summarize the main strategies employed for immune evasion by some of the major pathogenic fungi.

Paracoccidioides brasiliensis interferes on dendritic cells maturation by inhibiting PGE2 production

Paracoccidioidomycosis (PCM) is a systemic mycosis, endemic in most Latin American countries, especially in Brazil, whose etiologic agent is the thermodimorphic fungus of the genus Paracoccidioides, comprising cryptic species of Paracoccidioides brasiliensis, S1, PS2, PS3 and Paracoccidioides lutzii. The mechanisms involved in the initial interaction of the fungus with cells of the innate immune response, as dendritic cells (DCs), deserve to be studied. Prostaglandins (PGs) are eicosanoids that play an important role in modulating functions of immune cells including DCs. Here we found that human immature DCs derived from the differentiation of monocytes cultured with GM-CSF and IL-4 release substantial concentrations of PGE₂, which, however, were significantly inhibited after challenge with P. brasiliensis. In vitro blocking of pattern recognition receptors (PRRs) by monoclonal antibodies showed the involvement of mannose receptor (MR) in PGE₂ inhibition by the fungus. In addition, phenotyping assays showed that after challenge with the fungus, DCs do not change their phenotype of immature cells to mature ones, as well as do not produce IL-12 p70 or adequate concentrations of TNF-α. Assays using exogenous PGE₂ confirmed an association between PGE₂ inhibition and failure of cells to phenotypically mature in response to P. brasiliensis. We conclude that a P. brasiliensis evasion mechanism exists associated to a dysregulation on DC maturation. These findings may provide novel information for the understanding of the complex interplay between the host and this fungus.

The paracoccidioides cell wall: past and present layers toward understanding interaction with the host

The cell wall of pathogenic fungi plays import roles in the interaction with the host, so that its composition and structure may determine the course of infection. Here we present an overview of the current and past knowledge on the cell wall constituents of Paracoccidioides brasiliensis and P. lutzii. These are temperature-dependent dimorphic fungi that cause paracoccidioidomycosis, a systemic granulomatous, and debilitating disease. Focus is given on cell wall carbohydrate and protein contents, their immune-stimulatory features, adhesion properties, drug target characteristics, and morphological phase specificity. We offer a journey toward the future understanding of the dynamic nature of the cell wall and of the changes that may occur when the fungus infects the human host.

Surface architecture of histoplasma capsulatum

The dimorphic fungal pathogen Histoplasma capsulatum is the most frequent cause of clinically significant fungal pneumonia in humans. H. capsulatum virulence is achieved, in part, through diverse and dynamic alterations to the fungal cell surface. Surface components associated with H. capsulatum pathogenicity include carbohydrates, lipids, proteins, and melanins. Here, we describe the various structures comprising the cell surface of H. capsulatum that have been associated with virulence and discuss their involvement in the pathobiology of disease.

Surface glycans of Candida albicans and other pathogenic fungi: physiological roles, clinical uses, and experimental challenges

Although fungi have always been with us as commensals and pathogens, fungal infections have been increasing in frequency over the past few decades. There is a growing body of literature describing the involvement of carbohydrate groups in various aspects of fungal disease. Carbohydrates comprising the cell wall or capsule, or as a component of glycoproteins, are the fungal cell surface entities most likely to be exposed to the surrounding environment. Thus, the fungus-host interaction is likely to involve carbohydrates before DNA, RNA, or even protein. The interaction between fungal and host cells is also complex, and early studies using whole cells or crude cell fractions often produced seemingly conflicting results. What was needed, and what has been developing, is the ability to identify specific glycan structures and determine how they interact with immune system components. Carbohydrate analysis is complicated by the complexity of glycan structures and by the challenges of separating and detecting carbohydrates experimentally. Advances in carbohydrate chemistry have enabled us to move from the foundation of composition analysis to more rapid characterization of specific structures. This, in turn, will lead to a greater understanding of how fungi coexist with their hosts as commensals or exist in conflict as pathogens.

PTX3 function as an opsonin for the dectin-1-dependent internalization of zymosan by macrophages

Pentraxin 3 (PTX3) is a tumor necrosis factor and interleukin-1beta-stimulated gene that encodes a long PTX with proinflammatory activity. Here, we show that peritoneal macrophages derived from PTX3 transgenic (Tg) mice express higher levels of PTX3 mRNA than macrophages from wild-type (WT) mice, at basal level as well as upon stimulation with zymosan (Zy). Macrophages from Tg mice also showed improved opsonin-independent phagocytosis of Zy particles and the yeast form of the fungus Paracoccidioides brasiliensis. In the case of P. brasiliensis, an enhanced microbicidal activity accompanied by higher production of nitric oxide was also observed in macrophages from Tg mice. Using fluorescein-activated cell sorter analysis and reverse transcriptase-polymerase chain reaction, we demonstrated that basal level of Toll-like receptor-6 and Zy-induced dectin-1 expression was slightly but consistently higher in macrophages from Tg mice than in macrophages from WT mice. Recombinant (r)PTX3 protein binds to Zy particles as well as to yeast cells of P. brasiliensis and addition of rPTX3, to a culture of WT-derived macrophages containing Zy leads to an increase in the phagocytic index, which parallels that of Tg-derived macrophages, demonstrating the opsonin-like activity of PTX3. It is important that blockade of dectin-1 receptor inhibited the phagocytosis of Zy particles by WT and PTX3 Tg macrophages, pointing out the relevant role of dectin-1 as the main receptor involved in Zy uptake. Our results provide evidence for a role of PTX3 as an important component of the innate-immune response and as part of the host mechanisms that control fungal recognition and phagocytosis.