Encapsulation of Cryptococcus neoformans with glucuronoxylomannan inhibits the antigen-presenting capacity of monocytes

Recruitment of Polymorphonuclear Myeloid-Derived Suppressor Cells During Cryptococcus neoformans Infection

Cryptococcosis is a disease originating in the lungs, often seen in immunosuppressed patients. In severe cases, it can lead to meningoencephalitis and can be fatal. Biochemical studies have shown that the capsule of Cryptococcus neoformans is predominantly composed of glucuronoxylomannan (GXM), with glucuronoxylomannogalactan (GXMGal) present in smaller amounts. These polysaccharides have different effects on the immune system, with GXM mainly having anti-inflammatory properties, while GXMGal is more pro-inflammatory. Myeloid-derived suppressor cells (MDSCs) are a diverse group of immature myeloid cells, including progenitor cells and precursors of macrophages, granulocytes, and dendritic cells at different stages of development. MDSCs are known to suppress immune responses in various diseases, including bacterial and fungal infections, through mechanisms such as the inhibition of T cell proliferation. In this study, we show that infection with either B3501 or CAP67 strains results in the accumulation of granulocytic MDSC precursors in bronchoalveolar cavities. The MDSCs recruited by the B3501 strain suppress T cell proliferation, while those recruited by the CAP67 strain do not. Furthermore, we observed the expression of PD-L1 on these MDSCs, suggesting a potential mechanism of immunosuppression during infection. These findings reveal how the polysaccharides of C. neoformans might weaken the host's immune defense.

Elements of dissemination in cryptococcosis

As healthcare improves and our ability to support patients with compromised immune systems increases, such patients become more vulnerable to microbes in the environment. These include fungal pathogens such as Cryptococcus neoformans, the primary cause of fungal meningitis and a top priority pathogen on the World Health Organization fungal pathogen list. Like many other environmental pathogens, C. neoformans must adapt to and thrive in diverse environments in order to cause disease: (i) the environmental niche, (ii) the lungs following inhalation of infectious particles, (iii) the bloodstream and/or lymphatic system during dissemination, and (iv) the central nervous system (CNS), where it causes a deadly cryptococcal meningoencephalitis. Because CNS infection is the driver of mortality and the presenting illness, understanding the dissemination process from both host and fungal perspectives is important for treating these infections. In this review, we discuss the different stages of dissemination, how fungal cells interact with host cells during disease, and the ability to adapt to different environments within hosts.

Cryptococcosis Associated With Biologic Therapy: A Narrative Review

Cryptococcus is an opportunistic fungal pathogen that can cause disseminated infection with predominant central nervous system involvement in patients with compromised immunity. Biologics are increasingly used in the treatment of neoplasms and autoimmune/inflammatory conditions and the prevention of transplant rejection, which may affect human defense mechanisms against cryptococcosis. In this review, we comprehensively investigate the association between cryptococcosis and various biologics, highlighting their risks of infection, clinical manifestations, and clinical outcomes. Clinicians should remain vigilant for the risk of cryptococcosis in patients receiving biologics that affect the Th1/macrophage activation pathways, such as tumor necrosis factor α antagonists, Bruton tyrosine kinase inhibitors, fingolimod, JAK/STAT inhibitors (Janus kinase/signal transducer and activator of transcription), and monoclonal antibody against CD52. Other risk factors-such as age, underlying condition, and concurrent immunosuppressants, especially corticosteroids-should also be taken into account during risk stratification.

Inhibition of Microbicidal Activity of Canine Macrophages DH82 Cell Line by Capsular Polysaccharides from Cryptococcus neoformans

Cryptococcus neoformans is a lethal fungus that primarily affects the respiratory system and the central nervous system. One of the main virulence factors is the capsule, constituted by the polysaccharides glucuronoxylomannan (GXM) and glucuronoxylomanogalactan (GXMGal). Polysaccharides are immunomodulators. One of the target cell populations for modulation are macrophages, which are part of the first line of defense and important for innate and adaptive immunity. It has been reported that macrophages can be modulated to act as a "Trojan horse," taking phagocytosed yeasts to strategic sites or having their machinery activation compromised. The scarcity of information on canine cryptococcosis led us to assess whether the purified capsular polysaccharides from C. neoformans would be able to modulate the microbicidal action of macrophages. In the present study, we observed that the capsular polysaccharides, GXM, GXMGal, or capsule total did not induce apoptosis in the DH82 macrophage cell line. However, it was possible to demonstrate that the phagocytic activity was decreased after treatment with polysaccharides. In addition, recovered yeasts from macrophages treated with polysaccharides after phagocytosis could be cultured, showing that their viability was not altered. The polysaccharides led to a reduction in ROS production and the mRNA expression of IL-12 and IL-6. We observed that GXMGal inhibits MHC class II expression and GXM reduces ERK phosphorylation. In contrast, GXMGal and GXM were able to increase the PPAR-γ expression. Furthermore, our data suggest that capsular polysaccharides can reduce the microbicidal activity of canine macrophages DH82.

Fungal mechanisms of intracellular survival: what can we learn from bacterial pathogens?

Fungal infections represent a major, albeit neglected, public health threat with serious medical and economic burdens globally. With unacceptably high mortality rates, invasive fungal pathogens are responsible for millions of deaths each year, with a steadily increasing incidence primarily in immunocompromised individuals. The poor therapeutic options and rise of antifungal drug resistance pose further challenges in controlling these infections. These fungal pathogens have adapted to survive within mammalian hosts and can establish intracellular niches to promote survival within host immune cells. To do that, they have developed diverse methods to circumvent the innate immune system attack. This includes strategies such as altering their morphology, counteracting macrophage antimicrobial action, and metabolic adaptation. This is reminiscent of how bacterial pathogens have adapted to survive within host cells and cause disease. However, relative to the great deal of information available concerning intracellular bacterial pathogenesis, less is known about the mechanisms fungal pathogens employ. Therefore, here we review our current knowledge and recent advances in our understanding of how fungi can evade and persist within host immune cells. This review will focus on the major fungal pathogens, including Cryptococcus neoformans, Candida albicans, and Aspergillus fumigatus, among others. As we discover and understand the strategies used by these fungi, similarities with their bacterial counterparts are becoming apparent, hence we can use the abundant information from bacteria to guide our studies in fungi. By understanding these strategies, new lines of research will open that can improve the treatments of these devastating fungal diseases.

Non-self glycan structures as possible modulators of cancer progression: would polysaccharides from Cryptococcus spp. impact this phenomenon?

Invasive fungal infections (IFI) are responsible for a large number of annual deaths. Most cases are closely related to patients in a state of immunosuppression, as is the case of patients undergoing chemotherapy. Cancer patients are severely affected by the worrisome proportions that an IFI can take during cancer progression, especially in an already immunologically and metabolically impaired patient. There is scarce knowledge about strategies to mitigate cancer progression in these cases, beyond conventional treatment with antifungal drugs with a narrow therapeutic range. However, in recent years, ample evidence has surfaced describing the possible interferences that IFI may have both on the progression of pre-existing cancers and in the induction of newly transformed cells. The leading gambit for modulation of tumor progression comes from the ability of fungal virulence factors to modulate the host's immune system, since they are found in considerable concentrations in the tumor microenvironment during infection. In this context, cryptococcosis is of particular concern, since the main virulence factor of the pathogenic yeast is its polysaccharide capsule, which carries constituents with high immunomodulatory properties and cytotoxic potential. Therefore, we open a discussion on what has already been described regarding the progression of cryptococcosis in the context of cancer progression, and the possible implications that fungal glycan structures may take in both cancer development and progression.

Antifungal Peptide SP1 Damages Polysaccharide Capsule of Cryptococcus neoformans and Enhances Phagocytosis of Macrophages

Cryptococcus neoformans is a fungal pathogen which causes nearly half a million deaths worldwide each year. Under host-relevant conditions, it produces a characteristic polysaccharide capsule. The polysaccharide capsule is one of the main virulence factors of C. neoformans, which involves antiphagocytosis and immune responses of the host to cause a lack of an immune. Meanwhile, the polysaccharide capsule is a promising drug target because of the absence of analogs in the host. Here, we demonstrate that antifungal peptide SP1, which is derived from the N terminus of Saccharomyces cerevisiae GAPDH (glyceraldehyde-3-phosphate dehydrogenase), disrupts the polysaccharide capsule of C. neoformans H99. The mechanism is possibly due to the interaction of SP1 with glucuronoxylomannan (GXM). Disruption of the polysaccharide capsule enhances the adhesion and phagocytosis of C. neoformans H99 by macrophages and reduces the replication of C. neoformans H99 within macrophages. Additionally, SP1 exhibits antifungal activity against cryptococcal biofilms associated with the capsular polysaccharides. These findings suggest the potential of SP1 as a drug candidate for the treatment of cryptococcosis. IMPORTANCE C. neoformans is an opportunistic pathogen that causes invasive infections with a high mortality rate. Currently, the clinical drugs available for the treatment of cryptococcosis are limited to amphotericin B, azoles, and flucytosine. Amphotericin is nephrotoxic, and the widespread use of azoles and 5-flucytosine has led to a rapid development of drug resistance in C. neoformans. There is an urgent need to develop new and effective anticryptococcal drugs. Targeting virulence factors is a novel strategy for developing antifungal drugs. The antifungal peptide SP1 is capable of disrupting the polysaccharide capsule, which is a principal virulence factor of C. neoformans. Studying the mechanism by which SP1 damages the polysaccharide capsule and investigating the potential benefits of SP1 in removing C. neoformans from the host provides baseline data to develop a therapeutic strategy against refractory cryptococcal infections. This strategy would involve both inhibiting virulence factors and directly killing C. neoformans cells.

Cryptococcus neoformans Infection in the Central Nervous System: The Battle between Host and Pathogen

Cryptococcus neoformans (C. neoformans) is a pathogenic fungus with a global distribution. Humans become infected by inhaling the fungus from the environment, and the fungus initially colonizes the lungs. If the immune system fails to contain C. neoformans in the lungs, the fungus can disseminate to the blood and invade the central nervous system, resulting in fatal meningoencephalitis particularly in immunocompromised individuals including HIV/AIDS patients. Following brain invasion, C. neoformans will encounter host defenses involving resident as well as recruited immune cells in the brain. To overcome host defenses, C. neoformans possesses multiple virulence factors capable of modulating immune responses. The outcome of the interactions between the host and C. neoformans will determine the disease progression. In this review, we describe the current understanding of how C. neoformans migrates to the brain across the blood–brain barrier, and how the host immune system responds to the invading organism in the brain. We will also discuss the virulence factors that C. neoformans uses to modulate host immune responses.

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.

Cryptococcus: History, Epidemiology and Immune Evasion

Cryptococcosis is a disease caused by the pathogenic fungi Cryptococcus neoformans and Cryptococcus gattii, both environmental fungi that cause severe pneumonia and may even lead to cryptococcal meningoencephalitis. Although C. neoformans affects more fragile individuals, such as immunocompromised hosts through opportunistic infections, C. gattii causes a serious indiscriminate primary infection in immunocompetent individuals. Typically seen in tropical and subtropical environments, C. gattii has increased its endemic area over recent years, largely due to climatic factors that favor contagion in warmer climates. It is important to point out that not only C. gattii, but the Cryptococcus species complex produces a polysaccharidic capsule with immunomodulatory properties, enabling the pathogenic species of Cryptococccus to subvert the host immune response during the establishment of cryptococcosis, facilitating its dissemination in the infected organism. C. gattii causes a more severe and difficult-to-treat infection, with few antifungals eliciting an effective response during chronic treatment. Much of the immunopathology of this cryptococcosis is still poorly understood, with most studies focusing on cryptococcosis caused by the species C. neoformans. C. gattii became more important in the epidemiological scenario with the outbreaks in the Pacific Northwest of the United States, which resulted in phylogenetic studies of the virulent variant responsible for the severe infection in the region. Since then, the study of cryptococcosis caused by C. gattii has helped researchers understand the immunopathological aspects of different variants of this pathogen.

The utility of cerebrospinal fluid white cell count during the prognostic assessment for cryptococcal meningitis patients: a retrospective study

Background

The incidence of cryptococcal meningitis (CM) has gradually increased in recent years. Cerebrospinal fluid (CSF) cytology and cell count are very important for CM on etiology diagnosis and assessment of disease status and therapeutic response. However, the clinical significance of CSF white cell count (WCC) in CM patients is not fully understood. Using longitudinal data of CSF WCC and its relationship with clinical outcomes in CM patients, we aimed to elucidate the clinical significance of this test.

Methods

We retrospectively analyzed the medical records of 150 CM patients admitted to our hospital between January 2008 and December 2018.

Results

CM patients with lower baseline CSF WCC, CSF protein concentration or CD4/CD8 ratio, and those with altered mentation or HIV coinfection were more likely to have poor clinical outcome (P<0.05). CM patients with triple therapy during the induction period presented with a better clinical outcome (P<0.05). Baseline CSF WCC had a moderate positive correlation with peripheral CD4+ T lymphocyte count (r = 0.738, P < 0.001) and CD4+ T lymphocyte percentage (r = 0.616, P < 0.001). The best cut-off value to predict a poor clinical outcome was 40 cells/μL during baseline CSF WCC. The predictive model incorporating longitudinal data of CSF WCC had better sensitivity, specificity, and accuracy than a model incorporating only baseline CSF WCC data.

Conclusions

Our results indicated that baseline CSF WCC and changes in CSF WCC over time could be used to assess the prognosis of CM patients.

Biology and function of exo-polysaccharides from human fungal pathogens

Purpose of review

Environmental fungi such as Cryptococcus neoformans and Aspergillus fumigatus must survive many different and changing environments as they transition from their environmental niches to human lungs and other organs. Fungi alter their cell surfaces and secreted macromolecules to respond to and manipulate their surroundings.

Recent findings

This review focuses on exo-polysaccharides, chains of sugars that transported out of the cell and spread to the local environment. Major exo-polysaccharides for C. neoformans and A. fumigatus are glucuronylxylomannan (GXM) and galactosaminogalactan (GAG), respectively, which accumulate at high concentrations in growth medium and infected patients.

Summary

Here we discuss GXM and GAG synthesis and export, their immunomodulatory properties, and their roles in biofilm formation. We also propose areas of future research to address outstanding questions in the field that could facilitate development of new disease treatments.

Longitudinal Changes in Cd4+, Cd8+ T Cell Phenotype and Activation Marker Expression Following Antiretroviral Therapy Initiation among Patients with Cryptococcal Meningitis

Despite improvement in the prognosis of HIV/AIDS (human immunodeficiency virus/acquired immune deficiency syndrome) patients on antiretroviral therapy (ART), cryptococcal meningitis (CM) still causes 10–15% mortality among HIV-infected patients. The immunological impact of ART on the CD4⁺ and CD8⁺ T cell repertoire during cryptococcal co-infection is unclear. We determined longitudinal phenotypic changes in T cell subsets among patients with CM after they initiated ART. We hypothesized that ART alters the clonotypic phenotype and structural composition of CD4⁺ and CD8⁺ T cells during CM co-infection. For this substudy, peripheral blood mononuclear cells (PBMC) were isolated at four time points from CM patients following ART initiation during the parent study (ClinicalTrials.gov number, NCT01075152). Phenotypic characterization of CD4⁺ and CD8⁺ T cells was done using T cell surface marker monoclonal antibodies by flow cytometry. There was variation in the expression of immunophenotypic markers defining central memory (CD27⁺CD45R0⁺), effector memory (CD45R0⁺CD27^–), immune activation (CD38⁺ and Human Leucocyte Antigen DR (HLA-DR⁺), and exhaustion (Programmed cell death protein one (PD-1) in the CD4⁺ T cell subset. In comparison to the CD4⁺ T cell population, the CD8⁺ central memory subset declined gradually with minimal increase in the effector memory subset. Both CD4⁺ and CD8⁺ T cell immune exhaustion and activation markers remained elevated over 12 weeks. The relative surge and decline in the expression of T cell surface markers outlines a variation in the differentiation of CD4⁺ T cells during ART treatment during CM co-infection.

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.

Variability in innate host immune responses to cryptococcosis

Cryptococcosis is an invasive fungal disease caused by Cryptococcus neoformans and the closely related species C. gattii. The severe form of the disease, cryptococcal meningitis (CM), is rapidly fatal without treatment. Although typically a disease of immunocompromised (especially HIV-positive) individuals, there is growing awareness of cryptococcal disease amongst non-immunocompromised patients. Whilst substantial progress has been made in understanding the pathogenicity of C. neoformans in HIV patients, prospective data on cryptococcosis outside the context of HIV remains lacking. Below we review how innate immune responses vary between hosts depending on immunological status, and discuss risk factors and predictors of disease outcome in different groups.

Cryptococcus neoformans infective endocarditis of native valves in an immunocompetent host

With the emergence of Human immunodeficiency virus (HIV) and the resulting immunocompromised state, Cryptococcus neoformans infections have gained more importance in clinical practice. Cryptococcal infections in immunocompetent hosts continue to be uncommon. We present a rare case of Cryptococcus neoformans infective endocarditis (IE) in a young immunocompetent male. As per our literature review, this is the first reported case of native valve Cryptococcus neoformans endocarditis in an immunocompetent host. All cases till date have been reported in patients with underlying immunocompromised state or prosthetic valve.

Regulated Release of Cryptococcal Polysaccharide Drives Virulence and Suppresses Immune Cell Infiltration into the Central Nervous System

Cryptococcus neoformans is a common environmental yeast and opportunistic pathogen responsible for 15% of AIDS-related deaths worldwide. Mortality primarily results from meningoencephalitis, which occurs when fungal cells disseminate to the brain from the initial pulmonary infection site. A key C. neoformans virulence trait is the polysaccharide capsule. Capsule shields C. neoformans from immune-mediated recognition and destruction. The main capsule component, glucuronoxylomannan (GXM), is found both attached to the cell surface and free in the extracellular space (as exo-GXM). Exo-GXM accumulates in patient serum and cerebrospinal fluid at microgram/milliliter concentrations, has well-documented immunosuppressive properties, and correlates with poor patient outcomes. However, it is poorly understood whether exo-GXM release is regulated or the result of shedding during normal capsule turnover. We demonstrate that exo-GXM release is regulated by environmental cues and inversely correlates with surface capsule levels. We identified genes specifically involved in exo-GXM release that do not alter surface capsule thickness. The first mutant, the liv7Δ strain, released less GXM than wild-type cells when capsule was not induced. The second mutant, the cnag_00658Δ strain, released more exo-GXM under capsule-inducing conditions. Exo-GXM release observed in vitro correlated with polystyrene adherence, virulence, and fungal burden during murine infection. Additionally, we found that exo-GXM reduced cell size and capsule thickness under capsule-inducing conditions, potentially influencing dissemination. Finally, we demonstrated that exo-GXM prevents immune cell infiltration into the brain during disseminated infection and highly inflammatory intracranial infection. Our data suggest that exo-GXM performs a distinct role from capsule GXM during infection, altering cell size and suppressing inflammation.

The CSF Immune Response in HIV-1-Associated Cryptococcal Meningitis: Macrophage Activation, Correlates of Disease Severity, and Effect of Antiretroviral Therapy

Supplemental Digital Content is Available in the Text.

Background:

Immune modulation may improve outcome in HIV-associated cryptococcal meningitis. Animal studies suggest alternatively activated macrophages are detrimental but human studies are limited. We performed a detailed assessment of the cerebrospinal fluid (CSF) immune response and examined immune correlates of disease severity and poor outcome, and the effects of antiretroviral therapy (ART).

Methodology:

We enrolled persons ≥18 years with first episode of HIV-associated cryptococcal meningitis. CSF immune response was assessed using flow cytometry and multiplex cytokine analysis. Principal component analysis was used to examine relationships between immune response, fungal burden, intracranial pressure and mortality, and the effects of recent ART initiation (<12 weeks).

Findings:

CSF was available from 57 persons (median CD4 34/μL). CD206 (alternatively activated macrophage marker) was expressed on 54% CD14⁺ and 35% CD14⁻ monocyte-macrophages. High fungal burden was not associated with CD206 expression but with a paucity of CD4⁺, CD8⁺_, and CD4⁻CD8⁻ T cells and lower interleukin-6, G-CSF, and interleukin-5 concentrations. High intracranial pressure (≥30 cm H₂O) was associated with fewer T cells, a higher fungal burden, and larger Cryptococcus organisms. Mortality was associated with reduced interferon-gamma concentrations and CD4⁻CD8⁻ T cells but lost statistical significance when adjusted for multiple comparisons. Recent ART was associated with increased CSF CD4/CD8 ratio and a significantly increased macrophage expression of CD206.

Conclusions:

Paucity of CSF T cell infiltrate rather than alternative macrophage activation was associated with severe disease in HIV-associated cryptococcosis. ART had a pronounced effect on the immune response at the site of disease.

The environmental yeast Cryptococcus liquefaciens produces capsular and secreted polysaccharides with similar pathogenic properties to those of C. neoformans

Invasive fungal infections, including cryptococcosis, are a growing threat to immunocompromised patients. Although Cryptococcus neoformans and Cryptococcus gattii are the main agents of human cryptococcosis, opportunistic infections by environmental species, such as C. liquefaciens, have been observed recently. The main Cryptococcus virulence factor is the production and secretion of polysaccharides (PS). Previously, we showed that both species produce PS of similar composition. Here, we examined the ultrastructure and biological activity of capsular and secreted PS from C. liquefaciens, and yeast pathogenicity to an invertebrate host, in comparison with C. neoformans. Ultrastructural analysis by high-resolution microscopy showed that both species produce large and complex capsules. PS from both species had indistinguishable effects on phagocytosis levels, NO production and the secretion of a variety of immune mediators. Challenge with C. liquefaciens or C. neoformans led to complete lethality of G. mellonella larvae. Treatment with C. liquefaciens PS could not protect mice against infection with C. neoformans. We conclude that polysaccharides of the environmental yeast C. liquefaciens have strikingly similar ultrastructural and biological properties to those of C. neoformans, highlighting the importance of monitoring the emergence of new fungal pathogens for which thermotolerance may be an important transitional step towards pathogenesis in humans.

Rim Pathway-Mediated Alterations in the Fungal Cell Wall Influence Immune Recognition and Inflammation

Compared to other fungal pathogens, Cryptococcus neoformans is particularly adept at avoiding detection by innate immune cells. To explore fungal cellular features involved in immune avoidance, we characterized cell surface changes of the C. neoformans rim101Δ mutant, a strain that fails to organize and shield immunogenic epitopes from host detection. These cell surface changes are associated with an exaggerated, detrimental inflammatory response in mouse models of infection. We determined that the disorganized strain rim101Δ cell wall increases macrophage detection in a contact-dependent manner. Using biochemical and microscopy methods, we demonstrated that the rim101Δ strain shows a modest increase in the levels of both cell wall chitin and chitosan but that it shows a more dramatic increase in chito-oligomer exposure, as measured by wheat germ agglutinin staining. We also created a series of mutants with various levels of cell wall wheat germ agglutinin staining, and we demonstrated that the staining intensity correlates with the degree of macrophage activation in response to each strain. To explore the host receptors responsible for recognizing the rim101Δ mutant, we determined that both the MyD88 and CARD9 innate immune signaling proteins are involved. Finally, we characterized the immune response to the rim101Δ mutant in vivo, documenting a dramatic and sustained increase in Th1 and Th17 cytokine responses. These results suggest that the Rim101 transcription factor actively regulates the C. neoformans cell wall to prevent the exposure of immune stimulatory molecules within the host. These studies further explored the ways in which immune cells detect C. neoformans and other fungal pathogens by mechanisms that include sensing N-acetylglucosamine-containing structures, such as chitin and chitosan.

Infectious microorganisms have developed many ways to avoid recognition by the host immune system. For example, pathogenic fungi alter their cell surfaces to mask immunogenic epitopes. We have created a fungal strain with a targeted mutation in a pH response pathway that is unable to properly organize its cell wall, resulting in a dramatic immune reaction during infection. This mutant cell wall is defective in hiding important cell wall components, such as the chito-oligomers chitin and chitosan. By creating a series of cell wall mutants, we demonstrated that the degree of chito-oligomer exposure correlates with the intensity of innate immune cell activation. This activation requires a combination of host receptors to recognize and respond to these infecting microorganisms. Therefore, these experiments explored host-pathogen interactions that determine the degree of the subsequent inflammatory response and the likely outcome of infection.

A Glucuronoxylomannan-Associated Immune Signature, Characterized by Monocyte Deactivation and an Increased Interleukin 10 Level, Is a Predictor of Death in Cryptococcal Meningitis

Background. Cryptococcal meningitis remains a significant cause of death among human immunodeficiency virus type 1 (HIV)–infected persons in Africa. We aimed to better understand the pathogenesis and identify immune correlates of mortality, particularly the role of monocyte activation.

Methods. A prospective cohort study was conducted in Cape Town, South Africa. Patients with a first episode of cryptococcal meningitis were enrolled, and their immune responses were assessed in unstimulated and stimulated blood specimens, using flow cytometry and cytokine analysis.

Results. Sixty participants were enrolled (median CD4⁺ T-cell count, 34 cells/µL). Mortality was 23% (14 of 60 participants) at 14 days and 39% (22 of 57) at 12 weeks. Nonsurvivors were more likely to have an altered consciousness and higher cerebrospinal fluid fungal burden at presentation. Principal component analysis identified an immune signature associated with early mortality, characterized by monocyte deactivation (reduced HLA-DR expression and tumor necrosis factor α response to lipopolysaccharide); increased serum interleukin 6, CXCL10, and interleukin 10 levels; increased neutrophil counts; and decreased T-helper cell type 1 responses. This immune signature remained an independent predictor of early mortality after adjustment for consciousness level and fungal burden and was associated with higher serum titers of cryptococcal glucuronoxylomannan.

Conclusions. Cryptococcal-related mortality is associated with monocyte deactivation and an antiinflammatory blood immune signature, possibly due to Cryptococcus modulation of the host immune response. Validation in other cohorts is required.

Cryptococcus neoformans capsular polysaccharides form branched and complex filamentous networks viewed by high-resolution microscopy

Cryptococcus neoformans is a fungal pathogen that causes life-threatening infections in immunocompromised individuals. Its main virulence factor is an extracellular polysaccharide capsule whose structure, assembly and dynamics remain poorly understood. In this study, we apply improved protocols for sample preparation and recently-developed scanning microscopy techniques to visualize the ultrastructure of the C. neoformans capsule at high-resolution (up to 1 nm) and improved structural preservation. Although most capsule structures in nature consist of linear polymers, we show here that the C. neoformans capsule is a 'microgel-like' structure composed of branched polysaccharides. Moreover, we imaged the capsule-to-cell wall link, which is formed by thin fibers that branch out of thicker capsule filaments, and have one end firmly embedded in the cell wall structure. Together, our findings provide compelling ultrastructural evidence for a branched and complex capsule conformation, which may have important implications for the biological activity of the capsule as a virulence factor.

Macrophage mitochondrial and stress response to ingestion of Cryptococcus neoformans

Human infection with Cryptococcus neoformans, a common fungal pathogen, follows deposition of yeast spores in the lung alveoli. The subsequent host-pathogen interaction can result in eradication, latency, or extrapulmonary dissemination. Successful control of C. neoformans infection is dependent on host macrophages, but macrophages display little ability to kill C. neoformans in vitro. Recently, we reported that ingestion of C. neoformans by mouse macrophages induces early cell cycle progression followed by mitotic arrest, an event that almost certainly reflects host cell damage. The goal of the present work was to understand macrophage pathways affected by C. neoformans toxicity. Infection of macrophages by C. neoformans was associated with alterations in protein translation rate and activation of several stress pathways, such as hypoxia-inducing factor-1-α, receptor-interacting protein 1, and apoptosis-inducing factor. Concomitantly we observed mitochondrial depolarization in infected macrophages, an observation that was replicated in vivo. We also observed differences in the stress pathways activated, depending on macrophage cell type, consistent with the nonspecific nature of C. neoformans virulence known to infect phylogenetically distant hosts. Our results indicate that C. neoformans infection impairs multiple host cellular functions and undermines the health of these critical phagocytic cells, which can potentially interfere with their ability to clear this fungal pathogen.

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.

Pro- and Anti-Inflammatory Cytokines Produced by Human Monocytes ChallengedIn VitrowithParacoccidioides brasiliensis

Monocytes and macrophages play a central role in innate and adaptive immune response against systemic fungal infections. Imbalances in suppressor or stimulatory cytokine secretion caused by these cells may influence disease development, microorganism death, and the nature of the adaptive immune response. This study analyzed the monocyte cytokine profiles of healthy individuals challenged with high and low virulent strains of P. brasiliensis and mRNA cytokine expression kinetics by reverse transcription polymerase chain reaction (RT-PCR). Peripheral blood monocytes from healthy volunteers were cultured in vitro with and without virulent (Pb18) or low virulence (Pb265) strains from P. brasiliensis viable yeast cells. Interleukin-1 beta (IL-1beta), IL-6, IL-8, IL-10, tumor necrosis factor-alpha (TNF-alpha), and transforming growth factor-beta (TGF-beta1) were measured in culture supernatants by enzyme immunoassay (ELISA), and mRNA cytokine expression was determined by RT-PCR at 0, 4, 8, 12, 18 and 48 hr. Both P. brasiliensis strains induced monocyte production of IL-1beta, IL-6, IL-10 and TNF-alpha. Pb18 induced higher levels of IL-1beta, IL-6, and IL-10 than Pb265. IL-8 and TGF-beta1 levels were not significantly different from those cultured without stimulus. The mRNA cytokine expression was similar to supernatant cytokines measured by ELISA. In vitro monocyte challenge with virulent P. brasiliensis strain induces earlier and higher levels of pro- and anti-inflammatory cytokines than low virulence strain.

The role of host gender in the pathogenesis of Cryptococcus neoformans infections

Cryptococcus neoformans (Cn) is a pathogenic yeast and the cause of cryptococcal meningitis. Prevalence of disease between males and females is skewed, with males having an increased incidence of disease. Based on the reported gender susceptibility differences to Cn in the literature, we used clinical isolates from Botswanan HIV-infected patients to test the hypothesis that different gender environments exerted different selective pressures on Cn. When we examined this data set, we found that men had significantly higher risk of death despite having significantly higher CD4⁺ T lymphocyte counts upon admittance to the hospital. These observations suggested that Cn strains are uniquely adapted to different host gender environments and that the male immune response may be less efficient in controlling Cn infection. To discriminate between these possibilities, we tested whether there were phenotypic differences between strains isolated from males and females and whether there was an interaction between Cn and the host immune response. Virulence phenotypes showed that Cn isolates from females had longer doubling times and released more capsular glucoronoxylomannan (GXM). The presence of testosterone but not 17-β estradiol was associated with higher levels of GXM release for a laboratory strain and 28 clinical isolates. We also measured phagocytic efficiency, survival of Cn, and amount of killing of human macrophages by Cn after incubation with four isolates. While macrophages from females phagocytosed more Cn than macrophages from males, male macrophages had a higher fungal burden and showed increased killing by Cn. These data are consistent with the hypothesis that differential interaction between Cn and macrophages within different gender environments contribute to the increased prevalence of cryptococcosis in males. This could be related to differential expression of cryptococcal virulence genes and capsule metabolism, changes in Cn phagocytosis and increased death of Cn-infected macrophages.

The Cryptococcus neoformans capsule: a sword and a shield

The human fungal pathogen Cryptococcus neoformans is characterized by its ability to induce a distinct polysaccharide capsule in response to a number of host-specific environmental stimuli. The induction of capsule is a complex biological process encompassing regulation at multiple steps, including the biosynthesis, transport, and maintenance of the polysaccharide at the cell surface. By precisely regulating the composition of its cell surface and secreted polysaccharides, C. neoformans has developed intricate ways to establish chronic infection and dormancy in the human host. The plasticity of the capsule structure in response to various host conditions also underscores the complex relationship between host and parasite. Much of this precise regulation of capsule is achieved through the transcriptional responses of multiple conserved signaling pathways that have been coopted to regulate this C. neoformans-specific virulence-associated phenotype. This review focuses on specific host stimuli that trigger the activation of the signal transduction cascades and on the downstream transcriptional responses that are required for robust encapsulation around the cell.

Cryptococcal genotype influences immunologic response and human clinical outcome after meningitis

In sub-Saharan Africa, cryptococcal meningitis (CM) continues to be a predominant cause of AIDS-related mortality. Understanding virulence and improving clinical treatments remain important. To characterize the role of the fungal strain genotype in clinical disease, we analyzed 140 Cryptococcus isolates from 111 Ugandans with AIDS and CM. Isolates consisted of 107 nonredundant Cryptococcus neoformans var. grubii strains and 8 C. neoformans var. grubii/neoformans hybrid strains. Multilocus sequence typing (MLST) was used to characterize genotypes, yielding 15 sequence types and 4 clonal clusters. The largest clonal cluster consisted of 74 isolates. The results of Burst and phylogenetic analysis suggested that the C. neoformans var. grubii strains could be separated into three nonredundant evolutionary groups (Burst group 1 to group 3). Patient mortality was differentially associated with the different evolutionary groups (P = 0.04), with the highest mortality observed among Burst group 1, Burst group 2, and hybrid strains. Compared to Burst group 3 strains, Burst group 1 strains were associated with higher mortality (P = 0.02), exhibited increased capsule shedding (P = 0.02), and elicited a more pronounced Th(2) response during ex vivo cytokine release assays with strain-specific capsule stimulation (P = 0.02). The results of these analyses suggest that cryptococcal strain variation can be an important determinant of human immune responses and mortality.

IMPORTANCE

Cryptococcus neoformans is a common life-threatening human fungal pathogen that is responsible for an estimated 1 million cases of meningitis in HIV-infected patients annually. Virulence factors that are important in human disease have been identified, yet the impacts of the fungal strain genotype on virulence and outcomes of human infection remain poorly understood. Using an analysis of strain variation based on in vitro assays and clinical data from Ugandans living with AIDS and cryptococcal infection, we report that strain genotype predicts the type of immune response and mortality risk. These studies suggest that knowledge of the strain genotype during human infections could be used to predict disease outcomes and lead to improved treatment approaches aimed at targeting the specific combination of pathogen virulence and host response.

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.

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.

Evidence for branching in cryptococcal capsular polysaccharides and consequences on its biological activity

The encapsulated fungus Cryptococcus neoformans is a common cause of life-threatening disease in immunocompromised individuals. Its major virulence determinant is the polysaccharide (PS) capsule. An unsolved problem in cryptococcal biology is whether the PSs composing the capsule are linear or complex branched polymers, as well as the implications of this structural composition in pathogenesis. In this study we approached the problem by combining static and dynamic light scattering, viscosity analysis, and high-resolution microscopy and correlated the findings with biological properties. Analysis of the dependence of capsular PS molecular mass and the radius of gyration provided strong evidence against a simple linear PS configuration. Shape factors calculated from light scattering measurements in solution revealed values consistent with polymer branching. Furthermore, viscosity measurements provided complementary evidence for structural branching. Electron microscopy showed PS spherical-like structures similar to other branched PS. Finally, we show that the capacity of capsular PS to interfere in complement-mediated phagocytosis, inhibit nitric oxide production by macrophage-like cells, protect against reactive oxygen species, antibody reactivity and half-life in serum were influenced by the degree of branching, providing evidence for the notion that PS branching is an important parameter in determining the biological activity of C. neoformans PS.

Rat eosinophils stimulate the expansion of Cryptococcus neoformans-specific CD4(+) and CD8(+) T cells with a T-helper 1 profile

Experimental Cryptococcus neoformans infection in rats has been shown to have similarities with human cryptococcosis, revealing a strong granulomatous response and a low susceptibility to dissemination. Moreover, it has been shown that eosinophils are components of the inflammatory response to C. neoformans infections. In this in vitro study, we demonstrated that rat peritoneal eosinophils phagocytose opsonized live yeasts of C. neoformans, and that the phenomenon involves the engagement of FcγRII and CD18. Moreover, our results showed that the phagocytosis of opsonized C. neoformans triggers eosinophil activation, as indicated by (i) the up-regulation of major histocompatibility complex (MHC) class I, MHC class II and costimulatory molecules, and (ii) an increase in interleukin (IL)-12, tumour necrosis factor-α (TNF-α) and interferon-γ (IFN-γ) production. However, nitric oxide (NO) and hydrogen peroxide (H(2) O(2) ) synthesis by eosinophils was down-regulated after interaction with C. neoformans. Furthermore, this work demonstrated that CD4(+) and CD8(+) T lymphocytes isolated from spleens of infected rats and cultured with C. neoformans-pulsed eosinophils proliferate in an MHC class II- and class I-dependent manner, respectively, and produce important amounts of T-helper 1 (Th1) type cytokines, such as TNF-α and IFN-γ, in the absence of T-helper 2 (Th2) cytokine synthesis. In summary, the present study demonstrates that eosinophils act as fungal antigen-presenting cells and suggests that C. neoformans-loaded eosinophils might participate in the adaptive immune response.

Cryptococcus neoformans histone acetyltransferase Gcn5 regulates fungal adaptation to the host

Cryptococcus neoformans is an environmental fungus and an opportunistic human pathogen. Previous studies have demonstrated major alterations in its transcriptional profile as this microorganism enters the hostile environment of the human host. To assess the role of chromatin remodeling in host-induced transcriptional responses, we identified the C. neoformans Gcn5 histone acetyltransferase and demonstrated its function by complementation studies of Saccharomyces cerevisiae. The C. neoformans gcn5Delta mutant strain has defects in high-temperature growth and capsule attachment to the cell surface, in addition to increased sensitivity to FK506 and oxidative stress. Treatment of wild-type cells with the histone acetyltransferase inhibitor garcinol mimics cellular effects of the gcn5Delta mutation. Gcn5 regulates the expression of many genes that are important in responding to the specific environmental conditions encountered by C. neoformans inside the host. Accordingly, the gcn5Delta mutant is avirulent in animal models of cryptococcosis. Our study demonstrates the importance of chromatin remodeling by the conserved histone acetyltransferase Gcn5 in regulating the expression of specific genes that allow C. neoformans to respond appropriately to the human host.

Cryptococcus neoformans variants generated by phenotypic switching differ in virulence through effects on macrophage activation

Macrophages have a central role in the pathogenesis of cryptococcosis since they are an important line of defense, serve as a site for fungal replication, and also can contribute to tissue damage. The objective of this study was to investigate the interaction of macrophages with cells from smooth-colony variants (SM) and mucoid-colony variants (MC) arising from phenotypic switching of Cryptococcus neoformans. Alveolar macrophages (AMs) isolated from SM- and MC-infected mice exhibited differences in gene and surface expression of PD-L1, PD-L2, and major histocompatibility class II (MHC-II). PD-L1 and PD-L2 are the ligands for PD1 and are differentially regulated in Th1- and Th2-type cells. In addition, macrophage activation in SM- and MC-infected mice was characterized as alternatively activated. Flow cytometric and cytokine analysis demonstrated that MC infection was associated with the emergence of Th17 cells and higher levels of interleukin-17 (IL-17) in lung tissue, which were reduced by AM depletion. In conclusion, our results indicate that macrophages play a significant role in maintaining damage-promoting inflammation in the lung during MC infection, which ultimately results in death.

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.

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.

Glycoconjugates and polysaccharides of fungal cell wall and activation of immune system

Glycoproteins, glycosphingolipids and polysaccharides exposed at the most external layers of the wall are involved in several types of interactions of fungal cells with the exocellular environment. These molecules are fundamental building blocks of organisms, contributing to the structure, integrity, cell growth, differentiation and signaling. Several of them are immunologically active compounds with potential as regulators of pathogenesis and the immune response of the host. Some of these structures can be specifically recognized by antibodies from patients’ sera, suggesting that they can be also useful in the diagnosis of fungal infections.

Role of the mannose receptor in a murine model of Cryptococcus neoformans infection

Cryptococcus neoformans is an encapsulated fungal pathogen with a predilection to infect persons with suppressed T-cell function. Cryptococcal mannoproteins (MP) are highly mannosylated antigens which elicit T-cell responses in infected mice and in convalescent patients. Key to the immunogenicity of MP is its capacity to bind to the conserved mannose receptor (MR), CD206, on dendritic cells (DCs). To test the role of the MR in the immune response to C. neoformans, wild-type and MR knockout (MR KO) mice were compared by using in vivo and ex vivo models of cryptococcosis. Following a pulmonary challenge with C. neoformans, MR KO mice died significantly faster than wild-type mice and had higher lung fungal burdens after 4 weeks of infection. Uptake of MP was similar when DCs obtained from wild-type and MR KO mice were compared. Additionally, MP did not upregulate the maturation markers major histocompatibility complex class II, CD86, and CD40 in either wild-type or MR KO DCs. However, MP stimulated lymphoproliferation in CD4(+) T cells obtained from the peripheral lymph nodes of infected wild-type but not MR KO mice. These studies demonstrate a nonredundant role for the MR in the development of CD4(+) T-cell responses to MP and protection from C. neoformans.

Immune regulation during allergic bronchopulmonary mycosis: lessons taught by two fungi

Allergic bronchopulmonary mycosis (ABPM) is a devastating pulmonary disease that results from an aggressive allergic response to fungal colonization in the airways. Animal models using either fungal antigen or live infection reproduce most of the clinical features seen during ABPM in humans. Results from these studies have facilitated a detailed analysis of the key factors involved in the afferent as well as efferent phase of the disease. This review focuses on allergic bronchopulmonary disease caused by two different fungi (Aspergillus fumigatus and Cryptococcus neoformans): allergic bronchopulmonary aspergillosis and allergic bronchopulmonary cryptococcosis. Observations from both models underline the importance of initial innate immune responses and their translation into appropriate adaptive responses. In addition, data derived from knockout studies give emphasis to targeting cytokines and chemokines as a therapeutic strategy in the treatment of ABPM.

Heat killed cells of Cryptococcus neoformans var. grubii induces protective immunity in rats: immunological and histopathological parameters

Different clinical parameters which included cell-mediated immune (CMI) response, were evaluated in a model of disseminated cryptococcosis in rats. The experimental animals were pretreated four days prior to their exposure to Cryptococcus neoformans var. grubii with either heat killed cells of this yeastlike pathogen (HKC) or capsular polysaccharide (CPS) emulsified in complete Freund adjuvant (CFA). Rats treated with HKC-CFA and intraperitoneally infected with C. neoformans var. grubii had significantly better clearance of yeasts from tissues, a lower concentration of the cryptococcal capsular polysaccharide, glucuronoxylomannan (GXM), in serum and tissues, and better histopathological parameters compared to unpretreated infected rats. In contrast, rats treated with CPS-CFA presented an exacerbation of infection with a significantly higher fungal burden in tissues, a higher concentration of GXM in serum, and worse histopathological parameters compared to similar unpretreated infected rats. In addition, HKC-CFA treatment produced a T helper 1 (Th1) profile with improvements in the spleen cell proliferative response, in the level of INFgamma production by CD4 T cells, and in the nitric oxide (NO) production by peritoneal cells. On the other hand, rats treated with CPS-CFA showed an increased level of the immunoregulatory cytokine IL10 production by CD4 T cells, but no modification in the NO production by peritoneal cells.

Radial mass density, charge, and epitope distribution in the Cryptococcus neoformans capsule

Exposure of Cryptococcus neoformans cells to gamma radiation results in a gradual release of capsular polysaccharide, in a dose-dependent manner. This method allows the systematic exploration of different capsular regions. Using this methodology, capsule density was determined to change according to the radial distribution of glucuronoxylomannan and total polysaccharide, becoming denser at the inner regions of the capsule. Scanning electron microscopy of cells following gamma radiation treatment confirmed this finding. The zeta potential of the capsule also increased as the capsule size decreased. However, neither charge nor density differences were correlated with any change in sugar composition (xylose, mannose, and glucuronic acid) in the different capsular regions, since the proportions of these sugars remained constant throughout the capsule. Analysis of the capsular antigenic properties by monoclonal antibody binding and Scatchard analysis revealed fluctuations in the binding affinity within the capsule but not in the number of antibody binding sites, suggesting that the spatial organization of high- and low-affinity epitopes within the capsule changed according to radial position. Finally, evidence is presented that the structure of the capsule changes with capsule age, since the capsule of older cells became more resistant to gamma radiation-induced ablation. In summary, the capsule of C. neoformans is heterogeneous in its spatial distribution and changes with age. Furthermore, our results suggest several mechanisms by which the capsule may protect the fungal cell against exogenous environmental factors.

Microbial immune suppression mediated by direct engagement of inhibitory Fc receptor

A microbial polysaccharide (glucuronoxylomannan (GXM)) exerts potent immunosuppression by direct engagement to immunoinhibitory receptor FcgammaRIIB. Activation of FcgammaRIIB by GXM leads to the recruitment and phosphorylation of SHIP that prevents IkappaBalpha activation. The FcgammaRIIB blockade inhibits GXM-induced IL-10 production and induces TNF-alpha secretion. GXM quenches LPS-induced TNF-alpha release via FcgammaRIIB. The addition of mAb to GXM reverses GXM-induced immunosuppression by shifting recognition from FcgammaRIIB to FcgammaRIIA. These findings indicate a novel mechanism by which microbial products can impair immune function through direct stimulation of an inhibitory receptor. Furthermore, our observations provide a new mechanism for the ability of specific Ab to reverse the immune inhibitory effects of certain microbial products.

Cryptococcus neoformans capsular glucuronoxylomannan induces expression of fas ligand in macrophages

The major component of capsular material of Cryptococcus neoformans is glucuronoxylomannnan (GXM), a polysaccharide that exhibits potent immunosuppressive properties in vitro and in vivo. The results reported here show that 1) soluble purified GXM induces a prompt, long-lasting, and potent up-regulation of Fas ligand (FasL) on macrophages, 2) the up-regulation of FasL is related to induced synthesis and increased mobilization to the cellular surface, 3) this effect is largely mediated by interaction between GXM and TLR4, 4) FasL up-regulation occurs exclusively in GXM-loaded macrophages, 5) macrophages that show up-regulation of FasL induce apoptosis of activated T cells expressing Fas and Jurkat cells that constitutively express Fas, and 6) anti-Fas Abs rescue T cells from apoptosis induced by GXM. Collectively our results reveal novel aspects of the immunoregulatory properties of GXM and suggest that this nontoxic soluble compound could be used to dampen the immune response, to promote or accelerate the death receptor, and to fix FasL expression in a TLR/ligand-dependent manner. In the present study, we delineate potential new therapeutic applications for GXM that exploit death receptors as key molecular targets in regulating cell-mediated cytotoxicity, immune homeostasis, and the immunopathology of diseases.

Immunosuppression, interleukin-10 synthesis and apoptosis are induced in rats inoculated with Cryptococcus neoformans glucuronoxylomannan

Glucuronoxylomannan (GXM) is the major Cryptococcus neoformans capsular polysaccharide and represents the main virulence factor of this fungus. In in vitro studies we have demonstrated previously that this acidic and high-molecular-weight polysaccharide suppresses lymphoproliferation, modulates cytokine production and promotes apoptosis in spleen mononuclear (Spm) cells from rats. In this study we demonstrate that these phenomena also occur in vivo after the intracardiac inoculation of GXM into normal Wistar rats. The results of this study show suppression of the proliferative response Spm cells to concanavalin A (Con A) or heat-killed C. neoformans (HKCn) in the first 2 weeks after polysaccharide administration. In addition, increased levels of interleukin (IL)-10 were produced by Con A-stimulated Spm cells, coinciding with immunohistochemical GXM detection in the white pulp of spleen. In particular, high production of IL-10 with diminution of IL-2, interferon (IFN)-gamma and tumour necrosis factor (TNF)-alpha synthesis were detected 14 days after GXM administration. In situ cell death detection by TdT-mediated biotin-dUTP nick-end labelling (TUNEL) reaction in sections of spleen, lung and liver demonstrates apoptosis in tissues with deposits of GXM. These data demonstrate the in vivo ability of GXM to modify cytokine synthesis by Spm cells and to promote host cell apoptosis.

Tolerability and anti-inflammatory effects of glucuronoxylomannan in collagen-induced arthritis

This investigation was planned to assess the therapeutic efficacy of glucuronoxylomannan (GXM) in collagen-induced arthritis (CIA). GXM was isolated from culture filtrate of Cryptococcus neoformans var. gattii, serotype C. CIA was induced by the immunization of Dark Agouti rats with bovine type II collagen in incomplete Freund's adjuvant. GXM solution at two doses, 25 and 50 mg/kg, was administered intraperitoneally. Onset of i.p. injections of GXM to prevention and treatment groups was days 0 and 10 postimmunization, respectively. The WEHI-164 cell line was used for assaying tolerability, matrix metalloproteinase type 2 (MMP-2) activity and apoptosis. MMP-2 activity was assessed using zymography. For assessment of apoptosis, the terminal deoxyribonucleotidyl transferase-mediated dUTP nick-end labelling method was used. The results of this experiment showed that the treatment of CIA with GXM at a dose of 50 mg/kg could suppress disease development both prophylactically and therapeutically. This beneficial effect of GXM was associated with a significant decrease in the anti-CII antibody response compared with untreated rats. Moreover, GXM therapy could diminish MMP-2 activity, but it had no notable effect on apoptosis. GXM also showed a high tolerability compared with certain steroidal and non-steroidal anti-inflammatory drugs. We conclude that GXM suppresses the development of disease in CIA and it could be recommended as a new immunosuppressive and anti-inflammatory agent for further investigations.

A synthetic peptide as a novel anticryptococcal agent

An engineered, killer decapeptide (KP) has been synthesized based on the sequence of a recombinant, single-chain anti-idiotypic antibody (KT-scFv) acting as a functional internal image of a yeast killer toxin. Killer decapeptide exerted a strong fungicidal activity against Candida albicans, which was attributed to peptide interaction with beta-glucan. As this polysaccharide is also a critical component of the cryptococcal cell wall, we wondered whether KP was also active against Cryptococcus neoformans, a human pathogen of increasing medical importance. We found that KP was able to kill both capsular and acapsular C. neoformans cells in vitro. Furthermore, KP impaired the production of specific C. neoformans virulence factors including protease and urease activity and capsule formation, rendering the fungus more susceptible to natural effector cells. In vivo treatment with KP significantly reduced fungal burden in mice with cryptococcosis and, importantly, protected the majority of immunosuppressed animals from an otherwise lethal infection. Given the relevance of cryptococcosis in immunocompromised individuals and the inability of conventional drugs to completely resolve the infection, the results of the present study indicate KP as an ideal candidate for further studies on novel anticryptococcal agents.