Cryptococcal glucuronoxylomannan interferes with neutrophil rolling on the endothelium

The Hidden Fortress: A Comprehensive Review of Fungal Biofilms with Emphasis on Cryptococcus neoformans

Biofilms are structurally organized communities of microorganisms that adhere to a variety of surfaces. These communities produce protective matrices consisting of polymeric polysaccharides, proteins, nucleic acids, and/or lipids that promote shared resistance to various environmental threats, including chemical, antibiotic, and immune insults. While algal and bacterial biofilms are more apparent in the scientific zeitgeist, many fungal pathogens also form biofilms. These surprisingly common biofilms are morphologically distinct from the multicellular molds and mushrooms normally associated with fungi and are instead an assemblage of single-celled organisms. As a collection of yeast and filamentous cells cloaked in an extracellular matrix, fungal biofilms are an extreme threat to public health, especially in conjunction with surgical implants. The encapsulated yeast, Cryptococcus neoformans, is an opportunistic pathogen that causes both pulmonary and disseminated infections, particularly in immunocompromised individuals. However, there is an emerging trend of cryptococcosis among otherwise healthy individuals. C. neoformans forms biofilms in diverse environments, including within human hosts. Notably, biofilm association correlates with increased expression of multiple virulence factors and increased resistance to both host defenses and antifungal treatments. Thus, it is crucial to develop novel strategies to combat fungal biofilms. In this review, we discuss the development and treatment of fungal biofilms, with a particular focus on C. neoformans.

MRI changes in cryptococcal meningoencephalitis exacerbated by antifungal treatment due to post-infectious inflammatory syndrome: A case report

Cryptococcal meningitis is one of the most common fungal meningitis in adults and causes disabling morbidity and mortality worldwide. The occurrence of postinfectious inflammatory response syndrome during cryptococcal meningitis treatment presents a diagnostic challenge. This time course seems paradoxical because patients show worsening symptoms and imaging findings. However, laboratory data improve with antifungal treatments. Herein, we present a case of an older woman diagnosed with cryptococcal meningitis who later developed postinfectious inflammatory response syndrome. Despite the initial antifungal treatment and improvements in cerebrospinal fluid analysis results, the patient's neurological condition deteriorated; imaging findings worsened. Magnetic resonance imaging at the time of postinfectious inflammatory response syndrome showed more prominent meningeal enhancement and brain edema, consistent with postinfectious inflammatory response syndrome, combined with negative repeat cerebrospinal fluid cultures for cryptococcal species. This case highlights the importance of considering postinfectious inflammatory response syndrome when patients with cryptococcal meningitis show clinical worsening during treatment. Prompt corticosteroid therapy significantly improves patient outcomes. Radiologists and clinicians should be aware of postinfectious inflammatory response syndrome to provide appropriate therapeutic options and improve prognosis in patients with cryptococcal meningitis.

Inhibition of RhoA Prevents Cryptococcus neoformans Capsule Glucuronoxylomannan-Stimulated Brain Endothelial Barrier Disruption

Cryptococcus neoformans (Cn) is an opportunistic fungus that causes severe central nervous system (CNS) disease in immunocompromised individuals. Brain parenchyma invasion requires fungal traversal of the blood-brain barrier. In this study, we describe that Cn alters the brain endothelium by activating small GTPase RhoA, causing reorganization of the actin cytoskeleton and tight junction modulation to regulate endothelial barrier permeability. We confirm that the main fungal capsule polysaccharide glucuronoxylomannan is responsible for these alterations. We reveal a therapeutic benefit of RhoA inhibition by CCG-1423 in vivo. RhoA inhibition prolonged survival and reduced fungal burden in a murine model of disseminated cryptococcosis, supporting the therapeutic potential of targeting RhoA in the context of cryptococcal infection. We examine the complex virulence of Cn in establishing CNS disease, describing cellular components of the brain endothelium that may serve as molecular targets for future antifungal therapies to alleviate the burden of life-threatening cryptococcal CNS infection.

Glucuronoxylomannan intranasal challenge prior to Cryptococcus neoformans pulmonary infection enhances cerebral cryptococcosis in rodents

The encapsulated fungus Cryptococcus neoformans is the most common cause of fungal meningitis, with the highest rate of disease in patients with AIDS or immunosuppression. This microbe enters the human body via inhalation of infectious particles. C. neoformans capsular polysaccharide, in which the major component is glucuronoxylomannan (GXM), extensively accumulates in tissues and compromises host immune responses. C. neoformans travels from the lungs to the bloodstream and crosses to the brain via transcytosis, paracytosis, or inside of phagocytes using a "Trojan horse" mechanism. The fungus causes life-threatening meningoencephalitis with high mortality rates. Hence, we investigated the impact of intranasal exogenous GXM administration on C. neoformans infection in C57BL/6 mice. GXM enhances cryptococcal pulmonary infection and facilitates fungal systemic dissemination and brain invasion. Pre-challenge of GXM results in detection of the polysaccharide in lungs, serum, and surprisingly brain, the latter likely reached through the nasal cavity. GXM significantly alters endothelial cell tight junction protein expression in vivo, suggesting significant implications for the C. neoformans mechanisms of brain invasion. Using a microtiter transwell system, we showed that GXM disrupts the trans-endothelial electrical resistance, weakening human brain endothelial cell monolayers co-cultured with pericytes, supportive cells of blood vessels/capillaries found in the blood-brain barrier (BBB) to promote C. neoformans BBB penetration. Our findings should be considered in the development of therapeutics to combat the devastating complications of cryptococcosis that results in an estimated ~200,000 deaths worldwide each year.

Clinical and pathological characterization of Central Nervous System cryptococcosis in an experimental mouse model of stereotaxic intracerebral infection

Infection of the Central Nervous System (CNS) by the encapsulated fungus Cryptococcus neoformans can lead to high mortality meningitis, most commonly in immunocompromised patients. While the mechanisms by which the fungus crosses the blood-brain barrier to initiate infection in the CNS are well recognized, there are still substantial unanswered questions about the disease progression once the fungus is established in the brain. C. neoformans is characterized by a glucuronoxylomannan (GXM)-rich polysaccharide capsule which has been implicated in immune evasion, but its role during the host CNS infection needs further elucidation. Therefore, the present study aims to examine these key questions about the mechanisms underlying cryptococcal meningitis progression and the impact of fungal GXM release by using an intracerebral rodent infection model via stereotaxic surgery. After developing brain infection, we analyzed distinct brain regions and found that while fungal load and brain weight were comparable one-week post-infection, there were region-specific histopathological (with and without brain parenchyma involvement) and disease manifestations. Moreover, we also observed a region-specific correlation between GXM accumulation and glial cell recruitment. Furthermore, mortality was associated with the presence of subarachnoid hemorrhaging and GXM deposition in the meningeal blood vessels and meninges in all regions infected. Our results show that using the present infection model can facilitate clinical and neuropathological observations during the progression of neurocryptococcosis. Importantly, this mouse model can be used to further investigate disease progression as it develops in humans.

Basic principles of the virulence of Cryptococcus

Among fungal pathogens, Cryptococcus neoformans has gained great importance among the scientific community of several reasons. This fungus is the causative agent of cryptococcosis, a disease mainly associated to HIV immunosuppression and characterized by the appearance of meningoencephalitis. Cryptococcal meningitis is responsible for hundreds of thousands of deaths every year. Research of the pathogenesis and virulence mechanisms of this pathogen has focused on three main different areas: Adaptation to the host environment (nutrients, pH, and free radicals), mechanism of immune evasion (which include phenotypic variations and the ability to behave as a facultative intracellular pathogen), and production of virulence factors. Cryptococcus neoformans has two phenotypic characteristics, the capsule and synthesis of melanin that have a profound effect in the virulence of the yeast because they both have protective effects and induce host damage as virulence factors. Finally, the mechanisms that result in dissemination and brain invasion are also of key importance to understand cryptococcal disease. In this review, I will provide a brief overview of the main mechanisms that makes C. neoformans a pathogen in susceptible patients.

Abbreviations: RNS: reactive nitrogen species; BBB: brain blood barrier; GXM: glucuronoxylomannan; GXMGal: glucuronoxylomannogalactan

[Cryptococcal meningoencephalitis in an immunocompetent patient caused by late onset exacerbation]

The case was a 29-year-old male with no previous history of serious disease. He developed headache and fever, which then worsened and he was admitted to our hospital. His temperature was 38.3°C and he had a stiff neck. In cerebrospinal fluid (CSF) tests, the opening pressure was high, the cell count was increased, and the CSF/serum glucose ratio was decreased. In addition, he was positive for cryptococcal antigen. According to these findings, he was diagnosed with cryptococcal meningoencephalitis and antifungal treatment was initiated. His symptoms then improved, but on day 18 after admission, he developed convulsions, and on day 28, right visual field defects appeared. Brain MRI showed disseminated lesions in the bilateral cerebral cortex. Despite a decrease of the cryptococcal antigenic value in the CSF, the IgG index was elevated. IL-6, 8 and 10 in CSF were high levels on Day 1, then gradually reduced as the symptoms improved. But on Day 28, worsening of symptoms, IL-10 was significantly increased dispite IL-6 and 8 reducing. Therefore, the exacerbation of his symptoms and expansion of the lesions were not caused by the Cryptococcus itself, and it was considered that they were due to the late deterioration of cryptococcosis, which responded to steroid treatment.

Phenotypic Variability Correlates with Clinical Outcome in Cryptococcus Isolates Obtained from Botswanan HIV/AIDS Patients

Pathogenic species of Cryptococcus cause hundreds of thousands of deaths annually. Considerable phenotypic variation is exhibited during infection, including increased capsule size, capsule shedding, giant cells (≥15 μm), and micro cells (≤1 μm). We examined 70 clinical isolates of Cryptococcus neoformans and Cryptococcus tetragattii from HIV/AIDS patients in Botswana to determine whether the capacity to produce morphological variants was associated with clinical parameters. Isolates were cultured under conditions designed to simulate in vivo stresses. Substantial variation was seen across morphological and clinical data. Giant cells were more common in C. tetragattii, while micro cells and shed capsule occurred in C. neoformans only. Phenotypic variables fell into two groups associated with differing symptoms. The production of "large" phenotypes (greater cell and capsule size and giant cells) was associated with higher CD4 count and was negatively correlated with intracranial pressure indicators, suggesting that these are induced in early stage infection. "Small" phenotypes (micro cells and shed capsule) were associated with lower CD4 counts, negatively correlated with meningeal inflammation indicators, and positively correlated with intracranial pressure indicators, suggesting that they are produced later during infection and may contribute to immune suppression and promote proliferation and dissemination. These trends persisted at the species level, indicating that they were not driven by association with particular Cryptococcus species. Isolates possessing giant cells, micro cells, and shed capsule were rare, but strikingly, they were associated with patient death (P = 0.0165). Our data indicate that pleomorphism is an important driver in Cryptococcus infection.IMPORTANCE Cryptococcosis results in hundreds of thousands of deaths annually, predominantly in sub-Saharan Africa. Cryptococcus is an encapsulated yeast, and during infection, cells have the capacity for substantial morphological changes, including capsule enlargement and shedding and variations in cell shape and size. In this study, we examined 70 Cryptococcus isolates causing meningitis in HIV/AIDS patients in Botswana in order to look for associations between phenotypic variation and clinical symptoms. Four variant phenotypes were seen across strains: giant cells of ≥15 µm, micro cells of ≤1 µm, shed extracellular capsule, and irregularly shaped cells. We found that "large" and "small" phenotypes were associated with differing disease symptoms, indicating that their production may be important during the disease process. Overall, our study indicates that Cryptococcus strains that can switch on cell types under different situations may be more able to sustain infection and resist the host response.

Innate Immunity against Cryptococcus, from Recognition to Elimination

Cryptococcus species, the etiological agents of cryptococcosis, are encapsulated fungal yeasts that predominantly cause disease in immunocompromised individuals, and are responsible for 15% of AIDS-related deaths worldwide. Exposure follows the inhalation of the yeast into the lung alveoli, making it incumbent upon the pattern recognition receptors (PRRs) of pulmonary phagocytes to recognize highly conserved pathogen-associated molecular patterns (PAMPS) of fungi. The main challenges impeding the ability of pulmonary phagocytes to effectively recognize Cryptococcus include the presence of the yeast's large polysaccharide capsule, as well as other cryptococcal virulence factors that mask fungal PAMPs and help Cryptococcus evade detection and subsequent activation of the immune system. This review will highlight key phagocyte cell populations and the arsenal of PRRs present on these cells, such as the Toll-like receptors (TLRs), C-type lectin receptors, NOD-like receptors (NLRs), and soluble receptors. Additionally, we will highlight critical cryptococcal PAMPs involved in the recognition of Cryptococcus. The question remains as to which PRR-ligand interaction is necessary for the recognition, phagocytosis, and subsequent killing of Cryptococcus.

Antimicrobial Octapeptin C4 Analogues Active against Cryptococcus Species

Resistance to antimicrobials is a growing problem in both developed and developing countries. In nations where AIDS is most prevalent, the human fungal pathogen Cryptococcus neoformans is a significant contributor to mortality, and its growing resistance to current antifungals is an ever-expanding threat. We investigated octapeptin C4, from the cationic cyclic lipopeptide class of antimicrobials, as a potential new antifungal. Octapeptin C4 was a potent, selective inhibitor of this fungal pathogen with an MIC of 1.56 μg/ml. Further testing of octapeptin C4 against 40 clinical isolates of C. neoformans var. grubii or neoformans showed an MIC of 1.56 to 3.13 μg/ml, while 20 clinical isolates of C. neoformans var. gattii had an MIC of 0.78 to 12.5 μg/ml. In each case, the MIC values for octapeptin C4 were equivalent to, or better than, current antifungal drugs fluconazole and amphotericin B. The negatively charged polysaccharide capsule of C. neoformans influences the pathogen's sensitivity to octapeptin C4, whereas the degree of melanization had little effect. Testing synthetic octapeptin C4 derivatives provided insight into the structure activity relationships, revealing that the lipophilic amino acid moieties are more important to the activity than the cationic diaminobutyric acid groups. Octapeptins have promising potential for development as anticryptococcal therapeutic agents.

Cryptococcal Disease in HIV-Infected Children

Cryptococcus neoformans is an encapsulated fungal pathogen that is remarkable for its tendency to cause meningoencephalitis, especially in patients with AIDS. While disease is less common in children than adults, it remains an important cause of morbidity and mortality among HIV-infected children without access to anti-retroviral therapy. This review highlights recent insights into both the biology and treatment of cryptococcosis with a special emphasis on the pediatric literature.

Toll-like receptor 4-related immunostimulatory polysaccharides: Primary structure, activity relationships, and possible interaction models

Toll-like receptor (TLR) 4 is an important polysaccharide receptor; however, the relationships between the structures and biological activities of TLR4 and polysaccharides remain unknown. Many recent findings have revealed the primary structure of TLR4/MD-2-related polysaccharides, and several three-dimensional structure models of polysaccharide-binding proteins have been reported; and these models provide insights into the mechanisms through which polysaccharides interact with TLR4. In this review, we first discuss the origins of polysaccharides related to TLR4, including polysaccharides from higher plants, fungi, bacteria, algae, and animals. We then briefly describe the glucosidic bond types of TLR4-related heteroglycans and homoglycans and describe the typical molecular weights of TLR4-related polysaccharides. The primary structures and activity relationships of polysaccharides with TLR4/MD-2 are also discussed. Finally, based on the existing interaction models of LPS with TLR4/MD-2 and linear polysaccharides with proteins, we provide insights into the possible interaction models of polysaccharide ligands with TLR4/MD-2. To our knowledge, this review is the first to summarize the primary structures and activity relationships of TLR4-related polysaccharides and the possible mechanisms of interaction for TLR4 and TLR4-related polysaccharides.

Dancing cheek to cheek: Cryptococcus neoformans and phagocytes

Meningoencephalitis caused by Cryptococcus neoformans (Cn) has become one of the leading causes of mortality in AIDS patients. Understanding the interactions between Cn and phagocytes is fundamental in exploring the pathogenicity of cryptococcal meningoencephalitis. Cn may be extracellular or contained in the monocytes, macrophages, neutrophils, dendritic cells and even endothelial cells. The internalized Cn may proliferate inside the host cells, or cause the lysis of host cells, or leave the host cells via non-lytic exocytosis, or even hijack the host cells (Trojan horse) for the brain dissemination, which are regulated by microbe factors and also immune molecules. Coexistence of protective and deleterious roles of phagocytes in the progression of cryptococcosis warrant further investigation.

The tools for virulence of Cryptococcus neoformans

Cryptococcus neoformans is a fungal pathogen that causes almost half a million deaths each year. It is believed that most humans are infected with C. neoformans, possibly in a form that survives through latency in the lung and can reactivate to cause disease if the host becomes immunosuppressed. C. neoformans has a remarkably sophisticated intracellular survival capacities yet it is a free-living fungus with no requirement for mammalian virulence whatsoever. In this review, we discuss the tools that C. neoformans possesses to achieve survival, latency and virulence within its host. Some of these tools are mechanisms to withstand starvation and others aim to protect against microbicidal molecules produced by the immune system. Furthermore, we discuss how these tools were acquired through evolutionary pressures and perhaps accidental stochastic events, all of which combined to produce an organism with an unusual and unique intracellular pathogenic strategy.

Mechanisms of cryptococcal virulence and persistence

Cryptococcus neoformans is an environmental yeast that is a leading cause of fatal mycosis in AIDS patients and a major cause of meningoencephalitis and CNS-related mortality around the globe. Although C. neoformans infection is mostly a manifestation of immune deficiency, up to 25% of cases reported in the USA occur in patients without recognizable immune defects, indicating that C. neoformans can develop mechanisms that allow it to evade immune defenses and persist in noncompromised hosts. This article discusses mechanisms and routes of infection and the most important elements of host response as well as the mechanisms that promote cryptococcal survival within the host. Metabolic adaptation to physiological host conditions and the mechanisms limiting immune recognition, interfering with phagocytosis and extending intracellular survival of C. neoformans are highlighted. We describe the mechanisms by which C. neoformans can alter adaptive host responses, especially cell-mediated immunity, which is required for clearance of this microbe. We also review cryptococcal strategies of survival in the CNS and briefly discuss adaptations developing in response to medical treatment.

Extracellular vesicles from Cryptococcus neoformans modulate macrophage functions

Cryptococcus neoformans and distantly related fungal species release extracellular vesicles that traverse the cell wall and contain a varied assortment of components, some of which have been associated with virulence. Previous studies have suggested that these extracellular vesicles are produced in vitro and during animal infection, but the role of vesicular secretion during the interaction of fungi with host cells remains unknown. In this report, we demonstrate by fluorescence microscopy that mammalian macrophages can incorporate extracellular vesicles produced by C. neoformans. Incubation of cryptococcal vesicles with murine macrophages resulted in increased levels of extracellular tumor necrosis factor alpha (TNF-alpha), interleukin-10 (IL-10), and transforming growth factor beta (TGF-beta). Vesicle preparations also resulted in a dose-dependent stimulation of nitric oxide production by phagocytes, suggesting that vesicle components stimulate macrophages to produce antimicrobial compounds. Treated macrophages were more effective at killing C. neoformans yeast. Our results indicate that the extracellular vesicles of C. neoformans can stimulate macrophage function, apparently activating these phagocytic cells to enhance their antimicrobial activity. These results establish that cryptococcal vesicles are biologically active.

The regulation of leucocyte transendothelial migration by endothelial signalling events

Leucocytes use sophisticated mechanisms to cross the endothelium lining the vasculature. This is initiated by chemokine- and adhesion molecule-induced intracellular signalling that controls adhesion, spreading, and motility. At the same time, adherent leucocytes trigger the endothelium, manipulating the barrier to promote their transmigration into the underlying tissues. Over the past years, our insights in the associated signalling events within the endothelium have increased considerably, albeit the order of events, their crosstalk, and the consequences for endothelial cells and leucocytes are only partially resolved. Here, we briefly review endothelial signalling that is initiated at the apical endothelial membrane, where the first contact with the leucocytes takes place and signal transduction is induced. In addition, we discuss subsequent events at endothelial cell-cell junctions insofar as they have been linked to transendothelial migration. Finally, we briefly touch upon the modulation of endothelial signalling by infectious pathogens, since these have developed additional, elegant ways to manipulate the endothelium and transendothelial migration that may provide new, relevant insights into this process.

Mucin AgC10 from Trypanosoma cruzi Interferes with L-selectin-mediated monocyte adhesion

The protozoan parasite Trypanosoma cruzi has evolved sophisticated systems to evade the immune response. An important requirement for a productive immune response is recruitment of the appropriate immune cells from the bloodstream to the sites of infection. Here, we show that a mucin expressed and secreted by the metacyclic infective form of T. cruzi, AgC10, is able to interfere with L-selectin-mediated monocyte adhesion. Thus, incubation of U937 monocytic cells stably expressing L-selectin (U937LAM) with AgC10 strongly reduced their adhesion on P-selectin under flow, which is dependent on L-selectin. This treatment also results in a significant inhibition by AgC10 of U937LAM and human primary monocyte adhesion to activated vascular endothelium. This effect was specific for L-selectin, because vascular cell adhesion molecule 1 (VCAM-1)-mediated adhesion was not affected by AgC10 pretreatment. This effect of AgC10 is likely due to its ability to induce L-selectin shedding from the monocyte membrane, since pharmacologic blocking of this shedding prevents AgC10 activity. This is the first description of a mechanism that prevents leukocyte adhesion to the endothelium by a parasite and represents a new potential countermeasure to evade the generation of a correct immune response.

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.

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.

How do microbes evade neutrophil killing?

Many microbial pathogens evolved to circumvent the attack of neutrophils, which are essential effector cells of the innate immune system. Here we review six major strategies that pathogenic bacteria and fungi use to evade neutrophil defences: (i) turning on survival and stress responses, (ii) avoiding contact, (iii) preventing phagocytosis, (iv) surviving intracellularly, (v) inducing cell death and (vi) evading killing by neutrophil extracellular traps. For each category we give examples and further focus on one particular pathogenic microbe in more detail. Pathogens include Candida albicans, Cryptococcus neoformans, Yersinia ssp., Helicobacter pylori, Staphylococcus aureus, Streptococcus pyogenes and Streptococcus pneumoniae.

Toll-like receptors as key mediators in innate antifungal immunity

The Toll protein of Drosophila is a transmembrane receptor involved in dorsoventral polarization during embryonic development and recognition of infection. In mammals, Toll-like receptors (TLRs) constitute a novel protein family involved in innate immunity and respond to a wide spectrum of microorganisms, including fungi, bacteria, viruses, and protozoa. Specific agonists for nine of the ten members of the human TLR family have been described to date. TLRs as well as the TLR-associated adaptor molecule MyD88 have been implicated in the recognition of the fungal pathogens Candida albicans, Aspergillus fumigatus, Cryptococcus neoformans and Pneumocystis carinii. Moreover, several pathogen associated molecular patterns (PAMPs) located in the cell wall or cell surface of fungi have been identified as potential ligands. Yeast zymosan activates TLR2/ TLR6 heterodimers, whereas Saccharomyces cerevisiae- and C. albicans-derived mannan seems to be detected by TLR4. Phospholipomannan, present in the cell surface of C. albicans has been shown to be recognized by TLR2, while TLR4 mainly interacts with glucuronoxylomannan, the major capsular polysaccharide of C. neoformans. MyD88 has been implicated in TLR signalling of linear (1 --> 3)-beta-D-glucan, and of beta-glucan from P. carinii. These data point towards the ability of the innate immune system to utilize TLRs that are specific to different types and components of pathogenic fungi. Recent evidence further suggests that TLRs cooperate with other immune receptors involved in fungal recognition and that the selective induction of adaptor proteins finally leads to distinct signalling events upon fungal challenge.

The immune response to fungal infections

During the past two decades, invasive fungal infections have emerged as a major threat to immunocompromised hosts. Patients with neoplastic diseases are at significant risk for such infections as a result of their underlying illness and its therapy. Aspergillus, Candida, Cryptococcus and emerging pathogens, such as the zygomycetes, dark walled fungi, Trichosporon and Fusarium, are largely opportunists, causing infection when host defences are breached. The immune response varies with respect to the fungal species and morphotype encountered. The risk for particular infections differs, depending upon which aspect of immunity is impaired. This article reviews the current understanding of the role and relative importance of innate and adaptive immunity to common and emerging fungal pathogens. An understanding of the host response to these organisms is important in decisions regarding use of currently available antifungal therapies and in the design of new therapeutic modalities.

O-acetylation of cryptococcal capsular glucuronoxylomannan is essential for interference with neutrophil migration

The capsular polysaccharide glucuronoxylomannan (GXM) of Cryptococcus neoformans has been shown to interfere with neutrophil migration. Although several receptors have been implied to mediate this process, the structural perspectives are unknown. Here, we assess the contribution of 6-O-acetylation and xylose substitution of the (1-->3)-alpha-d-mannan backbone of GXM, the variable structural features of GXM, to the interference with neutrophil migration. We compare chemically deacetylated GXM and acetyl- or xylose-deficient GXM from genetically modified strains with wild-type GXM in their ability to inhibit the different phases of neutrophil migration. Additionally, we verify the effects of de-O-acetylation on neutrophil migration in vivo. De-O-acetylation caused a dramatic reduction of the inhibitory capacity of GXM in the in vitro assays for neutrophil chemokinesis, rolling on E-selectin and firm adhesion to endothelium. Genetic removal of xylose only marginally reduced the ability of GXM to reduce firm adhesion. In vivo, chemical deacetylation of GXM significantly reduced its ability to interfere with neutrophil recruitment in a model of myocardial ischemia (65% reduction vs a nonsignificant reduction in tissue myeloperoxidase, respectively). Our findings indicate that 6-O-acetylated mannose of GXM is a crucial motive for the inhibition of neutrophil recruitment.

B cell response during infection with the MAT a and MAT alpha mating types of Cryptococcus neoformans

In the present study, we compared the B cell response of BALB/c and C57Bl/6 mice during Cryptococcus neoformans infection. This response was investigated using virulent serotype D forms of mating types alpha and a (MAT alpha and MAT a). C57Bl/6 mice showed massive (mainly cerebral) infection by both types, while BALB/c were resistant to infection. Some resistance of C57Bl/6 mice was induced by previous immunization with the capsular polysaccharide from MAT alpha. Passive immunization of C57Bl/6 mice with purified antibody (Ab) obtained from capsular polysaccharide-immunized mice also increased resistance to infection. Both mouse strains showed comparable low IgM response to the capsular polysaccharide from MAT alpha, and only C57Bl/6 mice produced IgM to the polysaccharide of MAT a. Comparable levels of different immunoglobulin (Ig) isotypes against capsular components of MAT alpha and MAT a were detected, and the response of C57Bl/6 mice was higher when compared to that of BALB/c mice. FACS analysis indicated an increase in the percentage of a high-granulosity (side-scatter) splenic subpopulation and in the percentage of splenic Gr-1+ cells in infected C57Bl/6 mice. In addition, the percentage of follicular splenic B cells was decreased after C. neoformans infection of C57Bl/6 mice. This response was more pronounced when we investigated infection induced by the MAT a mating type. Taken together, our results indicate that capsular polysaccharide derived from MAT alpha and MAT a types of C. neoformans have a stimulatory effect upon B cells but that there is no correlation between resistance of BALB/c mice and Ab production. However, the increase in resistance of C57Bl/6 mice parallels the production of Abs and a major change in splenic cell populations.

Interactions of Toll-like receptors with fungi

The role of Toll-like receptors (TLRs) in signaling immune responses to fungal pathogens is reviewed. TLR2 and TLR4, acting via the adapter protein MyD88, signal responses to Cryptococcus neoformans, Aspergillus fumigatus and Candida albicans in vitro, although the relative significance of these TLRs to host defenses in vivo is unclear.

Cryptococcal capsular glucuronoxylomannan reduces ischaemia-related neutrophil influx

BACKGROUND

The capsular polysaccharide glucuronoxylomannan (GXM) of Cryptococcus neoformans interferes with the chemotaxis and transendothelial migration of neutrophils. Intravenous administration of purified GXM has been shown to reduce the influx of inflammatory cells in an animal model of bacterial infection. Here we show that isolated GXM can also interfere with neutrophil migration in a model of inflammation not related to infection. We assessed the effects of intravenous GXM on neutrophil infiltration in a rat model of myocardial ischaemia, where neutrophil infiltration has been shown to contribute to postischaemic reperfusion injury.

MATERIALS AND METHODS

Rats were subjected to coronary artery ligation followed by a 3-h reperfusion period. Myeloperoxidase-activity was measured in the ischaemic tissues as a marker of neutrophil infiltration.

RESULTS

Intravenous administration of GXM markedly reduced the influx of neutrophils in the ischaemic myocardium as measured by a 65% reduction of tissue MPO activity. This reduction of MPO activity was clearly correlated to the serum concentration of GXM. As complement activation by GXM was minimal at the doses applied in vivo, it is unlikely that generation of chemotactic C5a in the circulation by GXM caused the observed reduction in leucocyte migration.

CONCLUSION

Purified cryptococcal GXM has the ability to reduce neutrophil influx even outside the scope of infection.