Glucuronoxylomannan exhibits potent immunosuppressive properties

Glucuronoxylomannan (GXM) modulates macrophage proliferation and apoptosis through the STAT1 signaling pathway

cryptococcus neoformans (C. neoformans) is a crucial opportunistic fungus that possesses an encapsulated fungal pathogen. The cryptococcal capsule is mainly composed of the polysaccharide glucuronoxylomannan (GXM). Macrophages form the first-line innate defense against cryptococcosis; however, the underlying mechanism remains unclear. In this study, GXM-treated RAW264.7 macrophages showed a notably reduced survival rate and increased apoptosis, accompanied by the promoted inducible nitric oxide synthase (iNOS) expression and NO production. Signal transducer and activator of transcription 1 (STAT1) expression was also found to be directly proportional to GXM concentration; STAT1 knockdown could alleviate GXM-induced proliferation decrease and apoptosis increase of macrophages, as well as reduce M1 polarization, iNOS expression and NO release. In conclusion, this study concluded that GXM was the main virulence factor of C. neoformans, which is critical in determining the mechanism of GXM-mediated protective immune response postinfection. The STAT1 signal pathway mediates the effect of GXM stimulation on macrophages, potentially providing a reference for further understanding the biological role of GXM.

In Vitro Interaction Between Yeast Extracellular Vesicles and Human Monocyte-Derived Dendritic Cells

Extracellular vesicles (EVs) are lipid-bound particles produced by a wide variety of cells from different biological species. EVs can carry molecules, such as nucleic acids and metabolites, and are involved in cell functioning, communication, and signaling. Recent literature reported that pathogenic or commensal yeast strains can produce EVs targeting the host's immune system and exerting immunomodulatory actions. In humans, yeast EVs can be endocytosed by dendritic cells (DCs), characterized by phagocyting and migrating capabilities with the role of capturing antigens to present to T lymphocytes, triggering the immune response. Physiological or disease-associated immunosenescence impairs both DC functionality and gut microbiota; thus investigating the interaction between commensal microorganisms and the host's immune system would help elucidate the impact of aging on the immune system-microbiota interplay. We hereby present a protocol for the incubation of in vitro-generated human monocyte-derived DCs with EVs purified from different yeast strains isolated from fermented milk. The protocol includes flow cytometry analysis on DC activation markers and endocytosis assay.

Immunomodulatory Potential of Fungal Extracellular Vesicles: Insights for Therapeutic Applications

Extracellular vesicles (EVs) are membranous vesicular organelles that perform a variety of biological functions including cell communication across different biological kingdoms. EVs of mammals and, to a lesser extent, bacteria have been deeply studied over the years, whereas investigations of fungal EVs are still in their infancy. Fungi, encompassing both yeast and filamentous forms, are increasingly recognized for their production of extracellular vesicles (EVs) containing a wealth of proteins, lipids, and nucleic acids. These EVs play pivotal roles in orchestrating fungal communities, bolstering pathogenicity, and mediating interactions with the environment. Fungal EVs have emerged as promising candidates for innovative applications, not only in the management of mycoses but also as carriers for therapeutic molecules. Yet, numerous questions persist regarding fungal EVs, including their mechanisms of generation, release, cargo regulation, and discharge. This comprehensive review delves into the present state of knowledge regarding fungal EVs and provides fresh insights into the most recent hypotheses on the mechanisms driving their immunomodulatory properties. Furthermore, we explore the considerable potential of fungal EVs in the realms of medicine and biotechnology. In the foreseeable future, engineered fungal cells may serve as vehicles for tailoring cargo- and antigen-specific EVs, positioning them as invaluable biotechnological tools for diverse medical applications, such as vaccines and drug delivery.

Characteristics and potential clinical applications of the extracellular vesicles of human pathogenic Fungi

Extracellular vesicles (EVs) are a heterogeneous group of lipid membrane-enclosed compartments that contain different biomolecules and are released by almost all living cells, including fungal genera. Fungal EVs contain multiple bioactive components that perform various biological functions, such as stimulation of the host immune system, transport of virulence factors, induction of biofilm formation, and mediation of host-pathogen interactions. In this review, we summarize the current knowledge on EVs of human pathogenic fungi, mainly focusing on their biogenesis, composition, and biological effects. We also discuss the potential markers and therapeutic applications of fungal EVs.

Inhibition of myeloid-derived suppressor cell arginase-1 production enhances T-cell-based immunotherapy against Cryptococcus neoformans infection

Cryptococcosis is a potentially lethal disease that is primarily caused by the fungus Cryptococcus neoformans, treatment options for cryptococcosis are limited. Here, we show glucuronoxylomannan, the major polysaccharide component of C. neoformans, induces the recruitment of neutrophilic myeloid-derived suppressor cells in mice and patients with cryptococcosis. Depletion of neutrophilic myeloid-derived suppressor cells enhances host defense against C. neoformans infection. We identify C-type lectin receptor-2d recognizes glucuronoxylomannan to potentiate the immunosuppressive activity of neutrophilic myeloid-derived suppressor cells by initiating p38-mediated production of the enzyme arginase-1, which inhibits T-cell mediated antifungal responses. Notably, pharmacological inhibition of arginase-1 expression by a specific inhibitor of p38, SB202190, or an orally available receptor tyrosine kinase inhibitor, vandetanib, significantly enhances T-cell mediated antifungal responses against cryptococcosis. These data reveal a crucial suppressive role of neutrophilic myeloid-derived suppressor cells during cryptococcosis and highlight a promising immunotherapeutic application by inhibiting arginase-1 production to combat infectious diseases.

Cryptococcus neoformans causes opportunistic infection and potentially lethal immunopathology but therapeutic options are limited. Here the authors implicate myeloid derived suppressor cells during C. neoformans infection and suggest targeting arginase-1 production as a potential therapeutic strategy.

Adjuvant Curdlan Contributes to Immunization against Cryptococcus gattii Infection in a Mouse Strain-Specific Manner

The low efficacy and side effects associated with antifungal agents have highlighted the importance of developing immunotherapeutic approaches to treat Cryptococcus gattii infection. We developed an immunization strategy that uses selective Dectin-1 agonist as an adjuvant. BALB/c or C57BL/6 mice received curdlan or β-glucan peptide (BGP) before immunization with heat-killed C. gattii, and the mice were infected with viable C. gattii on day 14 post immunization and euthanized 14 days after infection. Adjuvant curdlan restored pulmonary tumor necrosis factor- α (TNF-α) levels, as induced by immunization with heat-killed C. gattii. The average area and relative frequency of C. gattii titan cells in the lungs of curdlan-treated BALB/c mice were reduced. However, this did not reduce the pulmonary fungal burden or decrease the i0,nflammatory infiltrate in the pulmonary parenchyma of BALB/c mice. Conversely, adjuvant curdlan induced high levels of interferon-γ (IFN-γ) and interleukin (IL)-10 and decreased the C. gattii burden in the lungs of C57BL/6 mice, which was not replicated in β-glucan peptide-treated mice. The adjuvant curdlan favors the control of C. gattii infection depending on the immune response profile of the mouse strain. This study will have implications for developing new immunotherapeutic approaches to treat C. gattii infection.

Communication is key: Extracellular vesicles as mediators of infection and defence during host-microbe interactions in animals and plants

Extracellular vesicles (EVs) are now understood to be ubiquitous mediators of cellular communication. In this review, we suggest that EVs have evolved into a highly regulated system of communication with complex functions including export of wastes, toxins and nutrients, targeted delivery of immune effectors and vectors of RNA silencing. Eukaryotic EVs come in different shapes and sizes and have been classified according to their biogenesis and size distributions. Small EVs (or exosomes) are released through fusion of endosome-derived multivesicular bodies with the plasma membrane. Medium EVs (or microvesicles) bud off the plasma membrane as a form of exocytosis. Finally, large EVs (or apoptotic bodies) are produced as a result of the apoptotic process. This review considers EV secretion and uptake in four eukaryotic kingdoms, three of which produce cell walls. The impacts cell walls have on EVs in plants and fungi are discussed, as are roles of fungal EVs in virulence. Contributions of plant EVs to development and innate immunity are presented. Compelling cases are sporophytic self-incompatibility and cellular invasion by haustorium-forming filamentous pathogens. The involvement of EVs in all of these eukaryotic processes is reconciled considering their evolutionary history.

Anti-glycan antibodies: roles in human disease

Carbohydrate-binding antibodies play diverse and critical roles in human health. Endogenous carbohydrate-binding antibodies that recognize bacterial, fungal, and other microbial carbohydrates prevent systemic infections and help maintain microbiome homeostasis. Anti-glycan antibodies can have both beneficial and detrimental effects. For example, alloantibodies to ABO blood group carbohydrates can help reduce the spread of some infectious diseases, but they also impose limitations for blood transfusions. Antibodies that recognize self-glycans can contribute to autoimmune diseases, such as Guillain-Barre syndrome. In addition to endogenous antibodies that arise through natural processes, a variety of vaccines induce anti-glycan antibodies as a primary mechanism of protection. Some examples of approved carbohydrate-based vaccines that have had a major impact on human health are against pneumococcus, Haemophilus influeanza type b, and Neisseria meningitidis. Monoclonal antibodies specifically targeting pathogen associated or tumor associated carbohydrate antigens (TACAs) are used clinically for both diagnostic and therapeutic purposes. This review aims to highlight some of the well-studied and critically important applications of anti-carbohydrate antibodies.

Cryptococcus neoformans-Infected Macrophages Release Proinflammatory Extracellular Vesicles: Insight into Their Components by Multi-omics

Cryptococcus neoformans causes cryptococcal meningitis, which is frequent in patients with HIV/AIDS, especially in less-developed countries. The incidence of cryptococcal meningitis is close to 1 million each year globally.

Cryptococcus neoformans causes deadly mycosis in immunocompromised individuals. Macrophages are key cells fighting against microbes. Extracellular vesicles (EVs) are cell-to-cell communication mediators. The roles of EVs from infected host cells in the interaction with Cryptococcus remain uninvestigated. Here, EVs from viable C. neoformans-infected macrophages reduced fungal burdens but led to shorter survival of infected mice. In vitro, EVs induced naive macrophages to an inflammatory phenotype. Transcriptome analysis showed that EVs from viable C. neoformans-infected macrophages activated immune-related pathways, including p53 in naive human and murine macrophages. Conserved analysis demonstrated that basic cell biological processes, including cell cycle and division, were activated by infection-derived EVs from both murine and human infected macrophages. Combined proteomics, lipidomics, and metabolomics of EVs from infected macrophages showed regulation of pathways such as extracellular matrix (ECM) receptors and phosphatidylcholine. This form of intermacrophage communication could serve to prepare cells at more distant sites of infection to resist C. neoformans infection.

The Transcription Factor Pdr802 Regulates Titan Cell Formation and Pathogenicity of Cryptococcus neoformans

The pathogenic yeast Cryptococcus neoformans presents a worldwide threat to human health, especially in the context of immunocompromise, and current antifungal therapy is hindered by cost, limited availability, and inadequate efficacy. After the infectious particle is inhaled, C. neoformans initiates a complex transcriptional program that integrates cellular responses and enables adaptation to the host lung environment.

Cryptococcus neoformans is a ubiquitous, opportunistic fungal pathogen that kills almost 200,000 people worldwide each year. It is acquired when mammalian hosts inhale the infectious propagules; these are deposited in the lung and, in the context of immunocompromise, may disseminate to the brain and cause lethal meningoencephalitis. Once inside the host, C. neoformans undergoes a variety of adaptive processes, including secretion of virulence factors, expansion of a polysaccharide capsule that impedes phagocytosis, and the production of giant (Titan) cells. The transcription factor Pdr802 is one regulator of these responses to the host environment. Expression of the corresponding gene is highly induced under host-like conditions in vitro and is critical for C. neoformans dissemination and virulence in a mouse model of infection. Direct targets of Pdr802 include the quorum sensing proteins Pqp1, Opt1, and Liv3; the transcription factors Stb4, Zfc3, and Bzp4, which regulate cryptococcal brain infectivity and capsule thickness; the calcineurin targets Had1 and Crz1, important for cell wall remodeling and C. neoformans virulence; and additional genes related to resistance to host temperature and oxidative stress, and to urease activity. Notably, cryptococci engineered to lack Pdr802 showed a dramatic increase in Titan cells, which are not phagocytosed and have diminished ability to directly cross biological barriers. This explains the limited dissemination of pdr802 mutant cells to the central nervous system and the consequently reduced virulence of this strain. The role of Pdr802 as a negative regulator of Titan cell formation is thus critical for cryptococcal pathogenicity.

Cryptococcus neoformans Secretes Small Molecules That Inhibit IL-1β Inflammasome-Dependent Secretion

Cryptococcus neoformans is an encapsulated yeast that causes disease mainly in immunosuppressed hosts. It is considered a facultative intracellular pathogen because of its capacity to survive and replicate inside phagocytes, especially macrophages. This ability is heavily dependent on various virulence factors, particularly the glucuronoxylomannan (GXM) component of the polysaccharide capsule. Inflammasome activation in phagocytes is usually protective against fungal infections, including cryptococcosis. Nevertheless, recognition of C. neoformans by inflammasome receptors requires specific changes in morphology or the opsonization of the yeast, impairing proper inflammasome function. In this context, we analyzed the impact of molecules secreted by C. neoformans B3501 strain and its acapsular mutant Δcap67 in inflammasome activation in an in vitro model. Our results showed that conditioned media derived from B3501 was capable of inhibiting inflammasome-dependent events (i.e., IL-1β secretion and LDH release via pyroptosis) more strongly than conditioned media from Δcap67, regardless of GXM presence. We also demonstrated that macrophages treated with conditioned media were less responsive against infection with the virulent strain H99, exhibiting lower rates of phagocytosis, increased fungal burdens, and enhanced vomocytosis. Moreover, we showed that the aromatic metabolite DL-Indole-3-lactic acid (ILA) and DL-p-Hydroxyphenyllactic acid (HPLA) were present in B3501's conditioned media and that ILA alone or with HPLA is involved in the regulation of inflammasome activation by C. neoformans. These results were confirmed by in vivo experiments, where exposure to conditioned media led to higher fungal burdens in Acanthamoeba castellanii culture as well as in higher fungal loads in the lungs of infected mice. Overall, the results presented show that conditioned media from a wild-type strain can inhibit a vital recognition pathway and subsequent fungicidal functions of macrophages, contributing to fungal survival in vitro and in vivo and suggesting that secretion of aromatic metabolites, such as ILA, during cryptococcal infections fundamentally impacts pathogenesis.

Animal Models of Cryptococcus neoformans in Identifying Immune Parameters Associated With Primary Infection and Reactivation of Latent Infection

Cryptococcus species are environmental fungal pathogens and the causative agents of cryptococcosis. Infection occurs upon inhalation of infectious particles, which proliferate in the lung causing a primary infection. From this primary lung infection, fungal cells can eventually disseminate to other organs, particularly the brain, causing lethal meningoencephalitis. However, in most cases, the primary infection resolves with the formation of a lung granuloma. Upon severe immunodeficiency, dormant cryptococcal cells will start proliferating in the lung granuloma and eventually will disseminate to the brain. Many investigators have sought to study the protective host immune response to this pathogen in search of host parameters that keep the proliferation of cryptococcal cells under control. The majority of the work assimilates research carried out using the primary infection animal model, mainly because a reactivation model has been available only very recently. This review will focus on anti-cryptococcal immunity in both the primary and reactivation models. An understanding of the differences in host immunity between the primary and reactivation models will help to define the key host parameters that control the infections and are important for the research and development of new therapeutic and vaccine strategies against cryptococcosis.

Fungal Extracellular Vesicles as Potential Targets for Immune Interventions

The release of extracellular vesicles (EVs) by fungi is a fundamental cellular process. EVs carry several biomolecules, including pigments, proteins, enzymes, lipids, nucleic acids, and carbohydrates, and are involved in physiological and pathological processes. EVs may play a pivotal role in the establishment of fungal infections, as they can interact with the host immune system to elicit multiple outcomes.

The release of extracellular vesicles (EVs) by fungi is a fundamental cellular process. EVs carry several biomolecules, including pigments, proteins, enzymes, lipids, nucleic acids, and carbohydrates, and are involved in physiological and pathological processes. EVs may play a pivotal role in the establishment of fungal infections, as they can interact with the host immune system to elicit multiple outcomes. It has been observed that, depending on the fungal pathogen, EVs can exacerbate or attenuate fungal infections. The study of the interaction between fungal EVs and the host immune system and understanding of the mechanisms that regulate those interactions might be useful for the development of new adjuvants as well as the improvement of protective immune responses against infectious or noninfectious diseases. In this review, we describe the immunomodulatory properties of EVs produced by pathogenic fungi and discuss their potential as adjuvants for prophylactic or therapeutic strategies.

Cryptococcus neoformans Glucuronoxylomannan and Sterylglucoside Are Required for Host Protection in an Animal Vaccination Model

The number of deaths from cryptococcal meningitis is around 180,000 per year. The disease is the second leading cause of mortality among individuals with AIDS. Antifungal treatment is costly and associated with adverse effects and resistance, evidencing the urgency of development of both therapeutic and prophylactic tools. Here we demonstrate the key roles of polysaccharide- and glycolipid-containing structures in a vaccination model to prevent cryptococcosis.

Cryptococcus neoformans is an encapsulated fungal pathogen that causes meningoencephalitis. There are no prophylactic tools for cryptococcosis. Previously, our group showed that a C. neoformans mutant lacking the gene encoding sterylglucosidase (Δsgl1) induced protection in both immunocompetent and immunocompromised murine models of cryptococcosis. Since sterylglucosidase catalyzes degradation of sterylglucosides (SGs), accumulation of this glycolipid could be responsible for protective immunity. In this study, we analyzed whether the activity of SGs is sufficient for the protective effect induced by the Δsgl1 strain. We observed that the accumulation of SGs impacted several properties of the main polysaccharide that composes the fungal capsule, glucuronoxylomannan (GXM). We therefore used genetic manipulation to delete the SGL1 gene in the acapsular mutant Δcap59 to generate a double mutant (strain Δcap59/Δsgl1) that was shown to be nonpathogenic and cleared from the lung of mice within 7 days post-intranasal infection. The inflammatory immune response triggered by the Δcap59/Δsgl1 mutant in the lung differed from the response seen with the other strains. The double mutant did not induce protection in a vaccination model, suggesting that SG-related protection requires the main capsular polysaccharide. Finally, GXM-containing extracellular vesicles (EVs) enriched in SGs delayed the acute lethality of Galleria mellonella against C. neoformans infection. These studies highlighted a key role for GXM and SGs in inducing protection against a secondary cryptococcal infection, and, since EVs notoriously contain GXM, these results suggest the potential use of Δsgl1 EVs as a vaccination strategy for cryptococcosis.

Involvement of the capsular GalXM-induced IL-17 cytokine in the control of Cryptococcus neoformans infection

Cryptococcus neoformans is an opportunistic fungus that can cause lethal brain infections in immunosuppressed individuals. Infection usually occurs via the inhalation of a spore or desiccated yeast which can then disseminate from the lung to the brain and other tissues. Dissemination and disease is largely influence by the production of copious amounts of cryptococcal polysaccharides, both which are secreted to the extracellular environment or assembled into a thick capsule surrounding the cell body. There are two important polysaccharides: glucuronoxylomannan (GXM) and galactoxylomannan, also called as glucuronoxylomanogalactan (GXMGal or GalXM). Although GXM is more abundant, GalXM has a more potent modulatory effect. In the present study, we show that GalXM is a potent activator of murine dendritic cells, and when co-cultured with T cells, induces a Th17 cytokine response. We also demonstrated that treating mice with GalXM prior to infection with C. neoformans protects from infection, and this phenomenon is dependent on IL-6 and IL-17. These findings help us understand the immune biology of capsular polysaccharides in fungal pathogenesis.

A Predicted Mannoprotein Participates in Cryptococcus gattii Capsular Structure

The yeast-like pathogen Cryptococcus gattii is an etiological agent of cryptococcosis. The major cryptococcal virulence factor is the polysaccharide capsule, which is composed of glucuronoxylomannan (GXM), galactoxylomannan (GalXM), and mannoproteins (MPs). The GXM and GalXM polysaccharides have been extensively characterized; however, there is little information about the role of mannoproteins in capsule assembly and their participation in yeast pathogenicity. The present study characterized the function of a predicted mannoprotein from C. gattii, designated Krp1. Loss-of-function and gain-of-function mutants were generated, and phenotypes associated with the capsular architecture were evaluated. The null mutant cells were more sensitive to a cell wall stressor that disrupts beta-glucan synthesis. Also, these cells displayed increased GXM release to the culture supernatant than the wild-type strain did. The loss of Krp1 influenced cell-associated cryptococcal polysaccharide thickness and phagocytosis by J774.A1 macrophages in the early hours of interaction, but no difference in virulence in a murine model of cryptococcosis was observed. In addition, recombinant Krp1 was antigenic and differentially recognized by serum from an individual with cryptococcosis, but not with serum from an individual with candidiasis. Taken together, these results indicate that C. gattii Krp1 is important for the cell wall structure, thereby influencing capsule assembly, but is not essential for virulence in vivoIMPORTANCECryptococcus gattii has the ability to escape from the host's immune system through poorly understood mechanisms and can lead to the death of healthy individuals. The role of mannoproteins in C. gattii pathogenicity is not completely understood. The present work characterized a protein, Kpr1, that is essential for the maintenance of C. gattii main virulence factor, the polysaccharide capsule. Our data contribute to the understanding of the role of Kpr1 in capsule structuring, mainly by modulating the distribution of glucans in C. gattii cell wall.

Cryptococcal pathogenic mechanisms: a dangerous trip from the environment to the brain

Cryptococcus neoformans is an opportunistic pathogenic yeast that causes serious infections, most commonly of the central nervous system (CNS). C. neoformans is mainly found in the environment and acquired by inhalation. It could be metaphorically imagined that cryptococcal disease is a "journey" for the microorganism that starts in the environment, where this yeast loads its suitcase with virulence traits. C. neoformans first encounters the infected mammalian host in the lungs, a site in which it must choose the right elements from its "virulence suitcase" to survive the pulmonary immune response. However, the lung is often only the first stop in this journey, and in some individuals the fungal trip continues to the brain. To enter the brain, C. neoformans must "open" the main barrier that protects this organ, the blood brain barrier (BBB). Once in the brain, C. neoformans expresses a distinct set of protective attributes that confers a strong neurotropism and the ability to cause brain colonisation. In summary, C. neoformans is a unique fungal pathogen as shown in its ability to survive in the face of multiple stress factors and to express virulence factors that contribute to the development of disease.

Fungal Strategies to Evade the Host Immune Recognition

The recognition of fungal cells by the host immune system is key during the establishment of a protective anti-fungal response. Even though the immune system has evolved a vast number of processes to control these organisms, they have developed strategies to fight back, avoiding the proper recognition by immune components and thus interfering with the host protective mechanisms. Therefore, the strategies to evade the immune system are as important as the virulence factors and attributes that damage the host tissues and cells. Here, we performed a thorough revision of the main fungal tactics to escape from the host immunosurveillance processes. These include the composition and organization of the cell wall, the fungal capsule, the formation of titan cells, biofilms, and asteroid bodies; the ability to undergo dimorphism; and the escape from nutritional immunity, extracellular traps, phagocytosis, and the action of humoral immune effectors.

Immune Recognition of Fungal Polysaccharides

The incidence of fungal infections has dramatically increased in recent years, in large part due to increased use of immunosuppressive medications, as well as aggressive medical and surgical interventions that compromise natural skin and mucosal barriers. There are relatively few currently licensed antifungal drugs, and rising resistance to these agents has led to interest in the development of novel preventative and therapeutic strategies targeting these devastating infections. One approach to combat fungal infections is to augment the host immune response towards these organisms. The polysaccharide-rich cell wall is the initial point of contact between fungi and the host immune system, and therefore, represents an important target for immunotherapeutic approaches. This review highlights the advances made in our understanding of the mechanisms by which the immune system recognizes and interacts with exopolysaccharides produced by four of the most common fungal pathogens: Aspergillus fumigatus, Candida albicans, Cryptococcus neoformans, and Histoplasma capsulatum. Work to date suggests that inner cell wall polysaccharides that play an important structural role are the most conserved across diverse members of the fungal kingdom, and elicit the strongest innate immune responses. The immune system senses these carbohydrates through receptors, such as lectins and complement proteins. In contrast, a greater diversity of polysaccharides is found within the outer cell walls of pathogenic fungi. These glycans play an important role in immune evasion, and can even induce anti-inflammatory host responses. Further study of the complex interactions between the host immune system and the fungal polysaccharides will be necessary to develop more effective therapeutic strategies, as well as to explore the use of immunosuppressive polysaccharides as therapeutic agents to modulate inflammation.

The Membrane Phospholipid Binding Protein Annexin A2 Promotes Phagocytosis and Nonlytic Exocytosis of Cryptococcus neoformans and Impacts Survival in Fungal Infection

Cryptococcus neoformans is a fungal pathogen with a unique intracellular pathogenic strategy that includes nonlytic exocytosis, a phenomenon whereby fungal cells are expunged from macrophages without lysing the host cell. The exact mechanism and specific proteins involved in this process have yet to be completely defined. Using murine macrophages deficient in the membrane phospholipid binding protein, annexin A2 (ANXA2), we observed a significant decrease in both phagocytosis of yeast cells and the frequency of nonlytic exocytosis. Cryptococcal cells isolated from Anxa2-deficient (Anxa2(-/-)) bone marrow-derived macrophages and lung parenchyma displayed significantly larger capsules than those isolated from wild-type macrophages and tissues. Concomitantly, we observed significant differences in the amount of reactive oxygen species produced between Anxa2(-/-) and Anxa2(+/+) macrophages. Despite comparable fungal burden, Anxa2(-/-) mice died more rapidly than wild-type mice when infected with C. neoformans, and Anxa2(-/-) mice exhibited enhanced inflammatory responses, suggesting that the reduced survival reflected greater immune-mediated damage. Together, these findings suggest a role for ANXA2 in the control of cryptococcal infection, macrophage function, and fungal morphology.

Interferon-γ promotes phagocytosis of Cryptococcus neoformans but not Cryptococcus gattii by murine macrophages

Among invasive fungal infections, cryptococcosis caused by inhalation of Cryptococcus neoformans or Cryptococcus gattii is particularly dangerous because it can disseminate to the central nervous system and cause life-threatening meningitis or meningoencephalitis. Previous reports described significant differences in the histopathological features of C. neoformans and C. gattii infection, such as greater pathogen proliferation and a limited macrophage response in mouse lung infected by C. gattii. To elucidate the difference in pathogenicity of these two Cryptococcus species, we investigated the interaction of C. neoformans and C. gattii with murine macrophages, the first line of host defense, by confocal laser microscopy. Only thin-capsulated, and not thick-capsulated C. neoformans and C. gattii were phagocytosed by macrophages. Preactivation with interferon-γ increased the phagocytic rate of thin-capsulated C. neoformans up to two-fold, but did not promote phagocytosis of thin-capsulated C. gattii. Lipopolysaccharide preactivation or Aspergillus fumigatus conidia co-incubation had no effect on internalization of thin-capsulated C. neoformans or C. gattii by macrophages. Phagocytosis of live thin-capsulated C. neoformans, but not that of live thin-capsulated C. gattii, induced interleukin-12 release from macrophages. However, phagocytosis of heat-killed or paraformaldehyde-fixed thin-capsulated C. neoformans did not increase IL-12 release, showing that the internalization of live yeast is important for initiating the immune response during C. neoformans-macrophage interactions. Our data suggest that macrophage response to C. gattii is limited compared with that to C. neoformans and that these results may partially explain the limited immune response and the greater pathogenicity of C. gattii.

Elucidating the immunological function of the Cryptococcus neoformans capsule

The encapsulated fungal pathogen Cryptococcus neoformans represents a significant agent of life-threatening infections in immunocompromised subjects. A unique characteristic of Cryptococcus species is the presence of a polysaccharide capsule, which is essential for virulence and endows Cryptococcus with potent immunoregulatory properties. This review provides an overview of the immunological properties of the principal components of C. neoformans capsule.

Glucuronoxylomannan facilitates generation of chondrocytes from bone marrow-derived mononuclear cells

Chondrocyte generation is an important process in cartilage and bone repair. Factors facilitating the induction of chondrocyte need to be further investigated. The present study aims to investigate the role of glucuronoxylomannan (Gxm) in the generation of chondrocytes. Human bone marrow cells were prepared using to generate the chondrocytes in the presence of Gxm. The results showed that Gxm has the capacity to facilitate the generation of chondrocytes in which transforming growth factor (TGF)-β played a critical role. The Bmmc-derived CD14+ cells were the major source of TGF-β in the culture. The present study indicates that Gxm is a potential drug candidate to be used in the generation of chondrocyte.

Candida albicans: A Model Organism for Studying Fungal Pathogens

Candida albicans is an opportunistic human fungal pathogen that causes candidiasis. As healthcare has been improved worldwide, the number of immunocompromised patients has been increased to a greater extent and they are highly susceptible to various pathogenic microbes and C. albicans has been prominent among the fungal pathogens. The complete genome sequence of this pathogen is now available and has been extremely useful for the identification of repertoire of genes present in this pathogen. The major challenge is now to assign the functions to these genes of which 13% are specific to C. albicans. Due to its close relationship with yeast Saccharomyces cerevisiae, an edge over other fungal pathogens because most of the technologies can be directly transferred to C. albicans from S. cerevisiae and it is amenable to mutation, gene disruption, and transformation. The last two decades have witnessed enormous amount of research activities on this pathogen that leads to the understanding of host-parasite interaction, infections, and disease propagation. Clearly, C. albicans has emerged as a model organism for studying fungal pathogens along with other two fungi Aspergillus fumigatus and Cryptococcus neoformans. Understanding its complete life style of C. albicans will undoubtedly be useful for developing potential antifungal drugs and tackling Candida infections. This will also shed light on the functioning of other fungal pathogens.

Chitin-like molecules associate with Cryptococcus neoformans glucuronoxylomannan to form a glycan complex with previously unknown properties

In prior studies, we demonstrated that glucuronoxylomannan (GXM), the major capsular polysaccharide of the fungal pathogen Cryptococcus neoformans, interacts with chitin oligomers at the cell wall-capsule interface. The structural determinants regulating these carbohydrate-carbohydrate interactions, as well as the functions of these structures, have remained unknown. In this study, we demonstrate that glycan complexes composed of chitooligomers and GXM are formed during fungal growth and macrophage infection by C. neoformans. To investigate the required determinants for the assembly of chitin-GXM complexes, we developed a quantitative scanning electron microscopy-based method using different polysaccharide samples as inhibitors of the interaction of chitin with GXM. This assay revealed that chitin-GXM association involves noncovalent bonds and large GXM fibers and depends on the N-acetyl amino group of chitin. Carboxyl and O-acetyl groups of GXM are not required for polysaccharide-polysaccharide interactions. Glycan complex structures composed of cryptococcal GXM and chitin-derived oligomers were tested for their ability to induce pulmonary cytokines in mice. They were significantly more efficient than either GXM or chitin oligomers alone in inducing the production of lung interleukin 10 (IL-10), IL-17, and tumor necrosis factor alpha (TNF-α). These results indicate that association of chitin-derived structures with GXM through their N-acetyl amino groups generates glycan complexes with previously unknown properties.

Capsular Material of Cryptococcus neoformans: Virulence and Much More

The capsule is generally considered one of the more powerful virulence factors of microorganisms, driving research in the field of microbial pathogenesis and in the development of vaccines. Cryptococcus neoformans is unique among the most common human fungal pathogens in that it possesses a complex polysaccharide capsule. This review focuses on the Cryptococcus neoformans capsule from the viewpoint of fungal pathogenesis, and the effective immune response target of the capsule's main component, glucuronoxylomannan.

The presence of 3-hydroxy oxylipins in pathogenic microbes

There is a sufficient body of work documenting the distribution of 3-hydroxy oxylipins in microbes. However, there is limited information on the role of these compounds in microbial pathogenesis. When derived from mammalian cells, these compounds regulate patho-biological processes, thus an understanding of 3-hydroxy oxylipin function and metabolism could prove important in shedding light on how these compounds mediate cellular pathology and physiology. This could present 3-hydroxy oxylipin biosynthetic pathways as targets for drug development. In this minireview, we interrogate the relevant yeast and bacterial 3-hydroxy oxylipin literature in order to appreciate how these compounds may influence the inflammatory response leading to disease development.

Cryptococcus neoformans suppresses the activation of bone marrow-derived dendritic cells stimulated with its own DNA, but not with DNA from other fungi

DNA from Cryptococcus neoformans activates bone marrow-derived dendritic cells (BM-DCs) in a TLR9-dependent manner. In this study, we examined the effect of the culture supernatants of C. neoformans on the activation of BM-DCs caused by its own DNA. C. neoformans supernatants suppressed IL-12p40, IL-6 production and CD40 expression by BM-DCs stimulated with its own DNA, but not with CpG-ODN and DNA from Candida albicans, Saccharomyces cerevisiae or Escherichia coli. In a confocal microscopic analysis, C. neoformans DNA was colocalized with LAMP-1, a late endosomal marker, and TLR9. The culture supernatants did not show any apparent suppression of these responses. In a luciferase reporter assay, C. neoformans supernatants inhibited NFκB activation caused by its own DNA. These inhibitory activities were attenuated by treatment with heat or trypsin. These results indicate that C. neoformans secrete certain proteinous molecules that suppress the activation of BM-DCs caused by its own DNA.

Cryptococcus neoformans capsular enlargement and cellular gigantism during Galleria mellonella infection

We have studied infection of Cryptococcus neoformans in the non-vertebrate host Galleria mellonella with particular interest in the morphological response of the yeast. Inoculation of C. neoformans in caterpillars induced a capsule-independent increase in haemocyte density 2 h after infection. C. neoformans manifested a significant increase in capsule size after inoculation into the caterpillar. The magnitude of capsule increase depended on the temperature, being more pronounced at 37°C than at 30°C, which correlated with an increased virulence of the fungus and reduced phagocytosis at 37°C. Capsule enlargement impaired phagocytosis by haemocytes. Incubation of the yeast in G. mellonella extracts also resulted in capsule enlargement, with the polar lipidic fraction having a prominent role in this effect. During infection, the capsule decreased in permeability. A low proportion of the cells (<5%) recovered from caterpillars measured more than 30 µm and were considered giant cells. Giant cells recovered from mice were able to kill the caterpillars in a manner similar to regular cells obtained from in vivo or grown in vitro, establishing their capacity to cause disease. Our results indicate that the morphological transitions exhibited by C. neoformans in mammals also occur in a non-vertebrate host system. The similarities in morphological transitions observed in different animal hosts and in their triggers are consistent with the hypothesis that the cell body and capsular responses represent an adaptation of environmental survival strategies to pathogenesis.

Multiple Disguises for the Same Party: The Concepts of Morphogenesis and Phenotypic Variations in Cryptococcus neoformans

Although morphological transitions (such as hyphae and pseudohyphae formation) are a common feature among fungi, the encapsulated pathogenic yeast Cryptococcus neoformans is found during infection as blastoconidia. However, this fungus exhibits striking variations in cellular structure and size, which have important consequences during infection. This review will summarize the main aspects related with phenotypic and morphological variations in C. neoformans, which can be divided in three classes. Two of them are related to changes in the capsule, while the third one involves changes in the whole cell. The three morphological and phenotypic variations in C. neoformans can be classified as: (1) changes in capsule structure, (2) changes in capsule size, and (3) changes in the total size of the cell, which can be achieved by the formation of cryptococcal giant/titan cells or microforms. These changes have profound consequences on the interaction with the host, involving survival, phagocytosis escape and immune evasion and dissemination. This article will summarize the main features of these changes, and highlight their importance during the interaction with the host and how they contribute to the development of the disease.

Fungal polysaccharides: biological activity beyond the usual structural properties

Studies on structure and function of polysaccharides in biological systems classically involve sequence and compositional analyses, anomeric configuration, type of glycosidic linkage, and presence of substituents. Recent studies, however, indicates that other structural parameters, so far little explored, can directly influence the biological activity of microbial polysaccharides. Among these parameters, we highlight the molecular dimensions of Cryptococcus neoformans polysaccharides, which appear to be inversely correlated with their immunobiological activity. These recent observations raise new concepts about the structure and function of polysaccharides, which stimulates the design of new experimental approaches and suggests previously unknown applications.

Cryptococcus neoformans galactoxylomannan is a potent negative immunomodulator, inspiring new approaches in anti-inflammatory immunotherapy

Cryptococcus neoformans is an opportunistic fungal pathogen responsible for life-threatening infections in immunocompromised individuals and occasionally in those with no known immune impairment. The fungus is endowed with several virulence factors, including capsular polysaccharides that play a key role in virulence. The capsule is composed of 90–95% glucuronoxylomannan (GXM), 5–8% galactoxylomannan (GalXM) and <1% mannoproteins. Capsular polysaccharides are shed into tissue where they produce many deleterious effects. Since GalXM has a smaller molecular mass, the molar concentration of GalXM in polysaccharide that is shed could exceed that of GXM in C. neoformans exopolysaccharides. Moreover, GalXM exhibits a number of unusual biologic properties both in vitro and in vivo. Here, we summarize the principal immunomodulatory effects of GalXM described during the last 20 years, particularly the mechanisms leading to induction of apoptosis in T lymphocytes, B lymphocytes and macrophages. Since the capacity of GalXM to induce widespread immune suppression is believed to contribute to the virulence of C. neoformans, this property might be exploited therapeutically to dampen the aberrant activation of immune cells during autoimmune disorders.

A critical role for FcgammaRIIB in up-regulation of Fas ligand induced by a microbial polysaccharide

The microbial capsular polysaccharide glucuronoxylomannan (GXM) from the opportunistic fungus Cryptoccocus neoformans is able to alter the innate and adaptive immune response through multi-faceted mechanisms of immunosuppression. The ability of GXM to dampen the immune response involves the induction of T cell apoptosis, which is dependent on GXM-induced up-regulation of Fas ligand (FasL) on antigen-presenting cells. In this study we elucidate the mechanism exploited by GXM to induce up-regulation of FasL. We demonstrate that (i) the activation of FasL is dependent on GXM interaction with FcgammaRIIB (FcγRIIB); (ii) GXM induces activation of c-Jun NH(2) -terminal kinase (JNK) and p38 signal transduction pathways via FcγRIIB; (iii) this leads to downstream activation of c-Jun; (iv) JNK and p38 are simultaneously, but independently, activated; (v) FasL up-regulation occurs via JNK and p38 activation; and (vi) apoptosis occurs via FcγRIIB engagement with consequent JNK and p38 activation. Our results highlight a fast track to FasL up-regulation via FcγRIIB, and assign to this receptor a novel anti-inflammatory role that also accounts for induced peripheral tolerance. These results contribute to our understanding of the mechanism of immunosuppression that accompanies cryptococcosis.

A major role for capsule-independent phagocytosis-inhibitory mechanisms in mammalian infection by Cryptococcus neoformans

The antiphagocytic polysaccharide capsule of the human fungal pathogen Cryptococcus neoformans is a major virulence attribute. However, previous studies of the pleiotropic virulence determinant Gat201, a GATA family transcription factor, suggested that capsule-independent antiphagocytic mechanisms exist. We have determined that Gat201 controls the mRNA levels of ∼1100 genes (16% of the genome) and binds the upstream regions of ∼130 genes. Seven Gat201-bound genes encode for putative and known transcription factors--including two previously implicated in virulence--suggesting an extensive regulatory network. Systematic analysis pinpointed two critical Gat201-bound genes, GAT204 (a transcription factor) and BLP1, which account for much of the capsule-independent antiphagocytic function of Gat201. A strong correlation was observed between the quantitative effects of single and double mutants on phagocytosis in vitro and on host colonization in vivo. This genetic dissection provides evidence that capsule-independent antiphagocytic mechanisms are pivotal for successful mammalian infection by C. neoformans.

Glucuronoxylomannan promotes the generation of antigen-specific T regulatory cell that suppresses the antigen-specific Th2 response upon activation

T regulatory cells (Treg) have the capability to suppress the skewed immune response, but the generation of antigen (Ag)-specific Treg for therapeutic purpose is a challenge; the mechanism of Ag-specific Treg activation remains obscure. Here, we report that glucuronoxylomannan (GXM) is capable of promoting the development of human tolerogenic dendritic cells (DC). GXM-pulsed DCs increased the expression of forkhead box P3 (Foxp3) in naïve human CD4(+)CD25(-) T cells via activating Fc gamma receptor IIb and activator protein-1 and promoting the expression of transforming growth factor beta in dendritic cells. Furthermore, the conjugated complex of house dust mite Ag, Dermatophagoides pteronyssinus (Der p) 1, and GXM-pulsed DCs to drive the naïve human CD4(+)CD25(-) T cells to develop into the Der p 1-specific Tregs, which efficiently suppressed the Ag-specific Th2 responses. We conclude that GXM-conjugated specific Ag have the capacity to up-regulate the tolerogenic property of DCs and promote the generation of Ag-specific Tregs; the latter can be activated upon the re-exposure to specific Ag and suppress the skewed Ag-specific T helper (Th)2 responses.

Immunomodulatory effects of serotype B glucuronoxylomannan from Cryptococcus gattii correlate with polysaccharide diameter

Glucuronoxylomannan (GXM), the major capsular component in the Cryptococcus complex, interacts with the immune system in multiple ways, which include the activation of Toll-like receptors (TLRs) and the modulation of nitric oxide (NO) production by phagocytes. In this study, we analyzed several structural parameters of GXM samples from Cryptococcus neoformans (serotypes A and D) and Cryptococcus gattii (serotypes B and C) and correlated them with the production of NO by phagocytes and the activation of TLRs. GXM fractions were differentially recognized by TLR2/TLR1 (TLR2/1) and TLR2/6 heterodimers expressed on TLR-transfected HEK293A cells. Higher NF-kappaB luciferase reporter activity induced by GXM was observed in cells expressing TLR2/1 than in cells transfected with TLR2/6 constructs. A serotype B GXM from C. gattii was the most effective polysaccharide fraction activating the TLR-mediated response. This serotype B polysaccharide, which was also highly efficient at eliciting the production of NO by macrophages, was similar to the other GXM samples in monosaccharide composition, zeta potential, and electrophoretic mobility. However, immunofluorescence with four different monoclonal antibodies and dynamic light-scattering analysis revealed that the serotype B GXM showed particularities in serological reactivity and had the smallest effective diameter among the GXM samples analyzed in this study. Fractionation of additional serotype B GXMs, followed by exposure of these fractions to macrophages, revealed a correlation between NO production and reduced effective diameters. Our results demonstrate a great functional diversity in GXM samples from different isolates and establish their abilities to differentially activate cellular responses. We propose that serological properties as well as physical chemical parameters, such as the diameter of polysaccharide molecules, may potentially influence the inflammatory response against Cryptococcus spp. and may contribute to the differences in granulomatous inflammation between cryptococcal species.

Role for chitin and chitooligomers in the capsular architecture of Cryptococcus neoformans

Molecules composed of beta-1,4-linked N-acetylglucosamine (GlcNAc) and deacetylated glucosamine units play key roles as surface constituents of the human pathogenic fungus Cryptococcus neoformans. GlcNAc is the monomeric unit of chitin and chitooligomers, which participate in the connection of capsular polysaccharides to the cryptococcal cell wall. In the present study, we evaluated the role of GlcNAc-containing structures in the assembly of the cryptococcal capsule. The in vivo expression of chitooligomers in C. neoformans varied depending on the infected tissue, as inferred from the differential reactivity of yeast forms to the wheat germ agglutinin (WGA) in infected brain and lungs of rats. Chromatographic and dynamic light-scattering analyses demonstrated that glucuronoxylomannan (GXM), the major cryptococcal capsular component, interacts with chitin and chitooligomers. When added to C. neoformans cultures, chitooligomers formed soluble complexes with GXM and interfered in capsular assembly, as manifested by aberrant capsules with defective connections with the cell wall and no reactivity with a monoclonal antibody to GXM. Cultivation of C. neoformans in the presence of an inhibitor of glucosamine 6-phosphate synthase resulted in altered expression of cell wall chitin. These cells formed capsules that were loosely connected to the cryptococcal wall and contained fibers with decreased diameters and altered monosaccharide composition. These results contribute to our understanding of the role played by chitin and chitooligosaccharides on the cryptococcal capsular structure, broadening the functional activities attributed to GlcNAc-containing structures in this biological system.

A microbial polysaccharide reduces the severity of rheumatoid arthritis by influencing Th17 differentiation and proinflammatory cytokines production

Rheumatoid arthritis (RA) is a chronic and debilitating autoimmune disease characterized by chronic joint inflammation with subsequent cartilage and bone destruction. RA is emerging as a model of IL-17-driven autoimmune inflammatory disease. IL-17 is a marker for Th17 cells, with its master regulator being the retinoic acid receptor-related orphan receptor (RORgammat) regulated by STAT3 signaling. Glucuronoxylomannan (GXM), a polysaccharide representing the main component of the capsular material of the opportunistic yeast Cryptococcus neoformans, exhibits potent immunosuppressive properties both in vitro and in vivo. The present study investigates the effects of GXM treatment on the progression of collagen-induced arthritis. GXM suppressed clinical signs of collagen-induced arthritis and blocked joint erosion progression. This effect was mediated by down-regulation of key cytokines involved in the pathogenesis of RA such as TNF-alpha and IL-1beta, and up-regulation of the inhibitory cytokine IL-10. Moreover, a reduction of IL-6 and TGF-beta, which inhibit Th17 differentiation with consequent decreased IL-17 production at the local and systemic level, was observed. The effect of GXM on Th17 differentiation mirrored the reduction in STAT3 activation and inhibition of RORgammat synthesis. Consequently, this work highlights the beneficial properties of an efficacious compound that could eventually be destined to the clinic.

Structural and functional properties of the Trichosporon asahii glucuronoxylomannan

The virulence attributes of Trichosporon asahii are virtually unknown, despite its growing relevance as causative agent of superficial and invasive diseases in humans. Glucuronoxylomannan (GXM) is a well described virulence factor of pathogenic species in the Cryptococcus genus. GXM is also produced by species of the Trichosporon genus, and both polysaccharides share antigenic determinants, but unlike cryptococcal GXM, relatively little work has been done on trichosporal GXMs. In this study, we analyzed structural and functional aspects of GXM produced by T. asahii and compared them to the properties of the cryptococcal polysaccharide. Trichosporal and cryptococcal GXM shared antigenic reactivity, but the former polysaccharide had smaller effective diameter and negative charge. GXM anchoring to the cell wall was perturbed by dimethylsulfoxide and required interactions of chitin-derived oligomers with the polysaccharide. GXM from T. asahii supernatants are incorporated by acapsular mutants of Cryptococcus neoformans, which renders these cells more resistant to phagocytosis by mouse macrophages. In summary, our results establish that despite similarities in cell wall anchoring, antigenic and antiphagocytic properties, trichosporal and cryptococcal GXMs manifest major structural differences that may directly affect polysaccharide assembly at the fungal surface.

How sweet it is! Cell wall biogenesis and polysaccharide capsule formation in Cryptococcus neoformans

Cryptococcus neoformans is a pathogenic fungus responsible for severe opportunistic infections. The most prominent feature of this yeast is its elaborate polysaccharide capsule, a complex structure that is required for virulence. The capsule is intimately associated with the cell wall, which underlies the capsule and offers the organism strength and flexibility in potentially hostile environments. Both structures are primarily composed of polysaccharides, offering a glimpse of the tremendous variation inherent in natural carbohydrate structures and their multiple biological functions. The steps in cell wall and capsule biosynthesis and assembly pose fascinating questions of metabolism, enzymology, cell biology, and regulation; the answers have potential application to treatment of a deadly infection. This article reviews current knowledge of cryptococcal cell wall and capsule biosynthesis and outstanding questions for the future.

The capsule of the fungal pathogen Cryptococcus neoformans

The capsule of the fungal pathogen Cryptococcus neoformans has been studied extensively in recent decades and a large body of information is now available to the scientific community. Well-known aspects of the capsule include its structure, antigenic properties and its function as a virulence factor. The capsule is composed primarily of two polysaccharides, glucuronoxylomannan (GXM) and galactoxylomannan (GalXM), in addition to a smaller proportion of mannoproteins (MPs). Most of the studies on the composition of the capsule have focused on GXM, which comprises more than 90% of the capsule's polysaccharide mass. It is GalXM, however, that is of particular scientific interest because of its immunological properties. The molecular structure of these polysaccharides is very complex and has not yet been fully elucidated. Both GXM and GalXM are high molecular mass polymers with the mass of GXM equaling roughly 10 times that of GalXM. Recent findings suggest, however, that the actual molecular weight might be different to what it has traditionally been thought to be. In addition to their structural roles in the polysaccharide capsule, these molecules have been associated with many deleterious effects on the immune response. Capsular components are therefore considered key virulence determinants in C. neoformans, which has motivated their use in vaccines and made them targets for monoclonal antibody treatments. In this review, we will provide an update on the current knowledge of the C. neoformans capsule, covering aspects related to its structure, synthesis and particularly, its role as a virulence factor.

Systematic genetic analysis of virulence in the human fungal pathogen Cryptococcus neoformans

The fungus Cryptococcus neoformans is a leading cause of mortality and morbidity among HIV-infected individuals. We utilized the completed genome sequence and optimized methods for homologous DNA replacement using high-velocity particle bombardment to engineer 1201 gene knockout mutants. We screened this resource in vivo for proliferation in murine lung tissue and in vitro for three well-recognized virulence attributes-polysaccharide capsule formation, melanization, and growth at body temperature. We identified dozens of previously uncharacterized genes that affect these known attributes as well as 40 infectivity mutants without obvious defects in these traits. The latter mutants affect predicted regulatory factors, secreted proteins, and immune-related factors, and represent powerful tools for elucidating novel virulence mechanisms. In particular, we describe a GATA family transcription factor that inhibits phagocytosis by murine macrophages independently of the capsule, indicating a previously unknown mechanism of innate immune modulation.

Cryptococcus neoformans inhibits nitric oxide synthesis caused by CpG-oligodeoxynucleotide-stimulated macrophages in a fashion independent of capsular polysaccharides

Cryptococcus neoformans is eradicated by macrophages via production of NO. Unmethylated CpG-ODN protect mice from infection with this fungal pathogen by inducing IFN-gamma. The present study was designed to elucidate the effect of C. neoformans on the synthesis of NO by alveolar macrophages. For this purpose, MH-S, an alveolar macrophage cell line, was stimulated with CpG-ODN in the presence of IFN-gamma. A highly virulent strain of C. neoformans with thick capsule suppressed the production of NO. Capsular polysaccharides were not essential for this suppression, because there was no difference between acapsular mutant (Cap67) and its parent strain. Physical or close interaction of Cap67 with MH-S was necessary, as shown by the loss of such effect when direct contact was interfered by nitrocellulose membrane. Similar effects were observed by disrupted as well as intact Cap67. Whereas the inhibitory effect of intact Cap67 was completely abrogated by heat treatment, disrupted Cap67 did not receive such influence. Finally, disrupted Cap67 did not show any inhibitory effect on the TLR9-mediated activation of NF-kappaB in a luciferase reporter assay with HEK293T cells, although the TLR4-mediated activation was suppressed. These results revealed that C. neoformans suppressed the synthesis of NO by CpG-ODN and IFN-gamma-stimulated macrophages in a fashion independent of capsular polysaccharides, although the precise mechanism remains to be elucidated.

Phenotypic switching in Cryptococcus neoformans contributes to virulence by changing the immunological host response

Cryptococcus neoformans is an encapsulated opportunistic organism that can undergo phenotypic switching. In this process, the parent smooth colony (SM) switches to a more virulent mucoid colony (MC) variant. The host responses mounted against the SM and MC variants differ, and lower tissue interleukin 10 (IL-10) levels are consistently observed in lungs of MC-infected C57BL/6 and BALB/c mice. This suggested different roles of this cytokine in SM and MC infections. The objective of this study was to compare survival rates and characterize the host responses of SM- and MC-infected IL-10-depleted (IL-10(-/-)) mice, which exhibit a Th1-polarized immune response and are considered resistant hosts. As expected, SM-infected IL-10(-/-) mice survived longer than wild-type mice, whereas MC-infected IL-10(-/-) mice did not exhibit a survival benefit. Consistent with this observation, we demonstrated marked differences in the inflammatory responses of SM- and MC-infected IL-10(-/-) and wild-type mice. This included a more Th1-polarized inflammatory response with enhanced recruitment of macrophages and natural killer and CD8 cells in MC- than in SM-infected IL-10(-/-) and wild-type mice. In contrast, both SM-infected IL-10(-/-) and wild-type mice exhibited higher recruitment of CD4 cells, consistent with enhanced survival and differences in recruitment and Th1/Th2 polarization. Lung tissue levels of IL-21, IL-6, IL-4, transforming growth factor beta, IL-12, and gamma interferon were higher in MC-infected IL-10(-/-) and wild-type mice than in SM-infected mice, whereas tumor necrosis factor alpha levels were higher in SM-infected IL-10(-/-) mice. In conclusion, the MC variant elicits an excessive inflammatory response in a Th1-polarized host environment, and therefore, the outcome is negatively affected by the absence of IL-10.

Deoxynucleic acids from Cryptococcus neoformans activate myeloid dendritic cells via a TLR9-dependent pathway

The mechanism of host cell recognition of Cryptococcus neoformans, an opportunistic fungal pathogen in immunocompromised patients, remains poorly understood. In the present study, we asked whether the DNA of this yeast activates mouse bone marrow-derived myeloid dendritic cells (BM-DCs). BM-DCs released IL-12p40 and expressed CD40 upon stimulation with cryptococcal DNA, and the response was abolished by treatment with DNase, but not with RNase. IL-12p40 production and CD40 expression were attenuated by chloroquine, bafilomycin A, and inhibitory oligodeoxynucleotides (ODN) that suppressed the responses caused by CpG-ODN. Activation of BM-DCs by cryptococcal DNA was almost completely abrogated in TLR9 gene-disrupted (TLR9(-/-)) mice and MyD88(-/-) mice, similar to that by CpG-ODN. In addition, upon stimulation with whole yeast cells of acapsular C. neoformans, TLR9(-/-) BM-DCs produced a lower amount of IL-12p40 than those from wild-type mice, and TLR9(-/-) mice were more susceptible to pulmonary infection with this fungal pathogen than wild-type mice, as shown by increased number of live colonies in lungs. Treatment of cryptococcal DNA with methylase resulted in reduced IL-12p40 synthesis by BM-DCs. Furthermore, using a luciferase reporter assay, cryptococcal DNA activated NF-kappaB in HEK293 cells transfected with the TLR9 gene. Finally, confocal microscopy showed colocalization of fluorescence-labeled cryptococcal DNA with CpG-ODN and the findings merged in part with the distribution of TLR9 in BM-DCs. Our results demonstrate that cryptococcal DNA causes activation of BM-DCs in a TLR9-dependent manner and suggest that the CpG motif-containing DNA may contribute to the development of inflammatory responses after infection with C. neoformans.

Spleen deposition of Cryptococcus neoformans capsular glucuronoxylomannan in rodents occurs in red pulp macrophages and not marginal zone macrophages expressing the C-type lectin SIGN-R1

The fate of microbial polysaccharides in host tissues is an important consideration because these compounds are often immune modulators. Splenic marginal zone macrophages that express the C-type lectin receptor SIGN-R1, take up neutral polysaccharides such as dextran and the capsular polysaccharide of Streptococcus pneumoniae. Given that the major component of Cryptococcus neoformans capsular polysaccharide, glucuronoxylomannan (GXM), localizes in the spleen when injected intravenously, we investigated whether GXM uptake was mediated by splenic macrophages expressing the SIGN-R1 receptor in mice. No significant differences in the amount and location of GXM deposition were detected in the spleens of mice treated with a SIGN-R1 blocking antibody when compared to controls. Similarly, a blocking antibody to Dectin-1, a co-receptor of -SIGN-R1, had no effects on GXM distribution within the spleen. Histological examination of spleens from mice and rats injected with FITC-Dextran and GXM revealed no significant co-localization, with Dextran and GXM being found in marginal and red pulp macrophages, respectively. Hence we conclude that GXM was not deposited in marginal zone macrophages. However, GXM deposition was found in the red pulp. These results indicate that there is a selective localization of these polysaccharides to different receptors such as SIGN-R1 for FITC dextran in marginal zone and a to-be-identified receptor selectively expressed by red pulp macrophages for GXM.