Apoptosis induction by glucuronoxylomannan of Cryptococcus neoformans

Accumulation of endogenous free radicals is required to induce titan-like cell formation in Cryptococcus neoformans

IMPORTANCE

Cryptococcus neoformans is an excellent model to investigate fungal pathogenesis. This yeast can produce "titan cells," which are cells of an abnormally larger size that contribute to the persistence of the yeast in the host. In this work, we have used a new approach to characterize them by identifying drugs that inhibit this process. We have used a repurposing off-patent drug library, combined with an automatic method to image and analyze fungal cell size. In this way, we have identified many compounds that inhibit this transition. Interestingly, several compounds were antioxidants, allowing us to confirm that endogenous ROS and mitochondrial changes are important for titan cell formation. This work provides new evidence of the mechanisms required for titanization. Furthermore, the future characterization of the inhibitory mechanisms of the identified compounds by the scientific community will contribute to better understand the role of titan cells in virulence.

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.

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.

Immunomodulatory Role of Capsular Polysaccharides Constituents of Cryptococcus neoformans

Cryptococcosis is a systemic fungal infection caused by Cryptococcus neoformans. In immunocompetent patients, cryptococcal infection is often confined to the lungs. In immunocompromised individuals, C. neoformans may cause life-threatening illness, either from novel exposure or through reactivation of a previously acquired latent infection. For example, cryptococcal meningitis is a severe clinical disease that can manifest in people that are immunocompromised due to AIDS. The major constituents of the Cryptococcus polysaccharide capsule, glucuronoxylomannan (GXM), and galactoxylomannan (GalXM), also known as glucuronoxylomanogalactan (GXMGal), are considered the primary virulence factors of Cryptococcus. Despite the predominance of GXM in the polysaccharide capsule, GalXM has more robust immunomodulatory effects on host cellular immunity. This review summarizes current knowledge regarding host-Crytococcus neoformans interactions and the role of capsular polysaccharides in host immunomodulation. Future studies will likely facilitate a better understanding of the mechanisms involved in antigenic recognition and host immune response to C. neoformans and lead to the development of new therapeutic pathways for cryptococcal infection.

Characterization of the 6-O-acetylated lipoglucuronomannogalactan a novel Cryptococcus neoformans cell wall polysaccharide

Glucuronoxylomannogalactans (GXMGals) are characteristic capsular polysaccharides produced by the opportunistic fungus C. neoformans, which are implicated in cryptococcal virulence, via impairment of the host immune response. We determined for the first time the structure of a lipoglucuronomannogalactan (LGMGal), isolated from the surface of a mutant C. neoformans carrying a deletion in the UDP-GlcA decarboxylase gene. Monosaccharide composition and methylation analyses, as well as nuclear magnetic resonance spectroscopy were employed in discerning the structure. Our results show that the polysaccharide structure of the LGMGal differs from GXMGal by the absence of xylose and 2-O-acetylated mannose residues. LGMGal consists of a galactan main chain -[-6-α-Gal-]-, where every second Gal residue is substituted at O-3 with an oligosaccharide α-Man6OAc-3-α-Man-4-(β-GlcA-3)-β-Gal-; components in italic being non-stoichiometric. The substitution rate of β-Galp units by GlcpA is 35%. Additionally, we determined that the glycolipid anchor of the LGMGal is based on an myo-inositol phosphoceramide composed of C₁₈-phytosphingosine and monohydroxylated lignoceric acid (2OHC_24:0 fatty acid).

Mucor circinelloides Thrives inside the Phagosome through an Atf-Mediated Germination Pathway

Mucormycosis is an emerging fungal infection that is often lethal due to the ineffectiveness of current therapies. Here, we have studied the first stage of this infection-the germination of Mucor circinelloides spores inside phagocytic cells-from an integrated transcriptomic and functional perspective. A relevant fungal gene network is remodeled in response to phagocytosis, being enriched in crucial functions to survive and germinate inside the phagosome, such as nutritional adaptation and response to oxidative stress. Correspondingly, the phagocytic cells induced a specific proinflammatory and apoptotic response to the pathogenic strain. Deletion of fungal genes encoding putative transcription factors (atf1, atf2, and gcn4), extracellular proteins (chi1 and pps1), and an aquaporin (aqp1) revealed that these genes perform important roles in survival following phagocytosis, germination inside the phagosome, and virulence in mice. atf1 and atf2 play a major role in these pathogenic processes, since their mutants showed the strongest phenotypes and both genes control a complex gene network of secondarily regulated genes, including chi1 and aqp1 These new insights into the initial phase of mucormycosis define genetic regulators and molecular processes that could serve as pharmacological targets.IMPORTANCE Mucorales are a group of ancient saprophytic fungi that cause neglected infectious diseases collectively known as mucormycoses. The molecular processes underlying the establishment and progression of this disease are largely unknown. Our work presents a transcriptomic study to unveil the Mucor circinelloides genetic network triggered in fungal spores in response to phagocytosis by macrophages and the transcriptional response of the host cells. Functional characterization of differentially expressed fungal genes revealed three transcription factors and three extracellular proteins essential for the fungus to survive and germinate inside the phagosome and to cause disease in mice. Two of the transcription factors, highly similar to activating transcription factors (ATFs), coordinate a complex secondary gene response involved in pathogenesis. The significance of our research is in characterizing the initial stages that lead to evasion of the host innate immune response and, in consequence, the dissemination of the infection. This genetic study offers possible targets for novel antifungal drugs against these opportunistic human pathogens.

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.

Distribution of the O-acetyl groups and β-galactofuranose units in galactoxylomannans of the opportunistic fungus Cryptococcus neoformans

Galactoxylomannans (GalXMs) are a mixture of neutral and acidic capsular polysaccharides produced by the opportunistic fungus Cryptococcus neoformans that exhibit potent suppressive effects on the host immune system. Previous studies describing the chemical structure of C. neoformans GalXMs have reported species without O-acetyl substituents. Herein we describe that C. neoformans grown in capsule-inducing medium produces highly O-acetylated GalXMs. The location of the O-acetyl groups was determined by nuclear magnetic resonance (NMR) spectroscopy. In the neutral GalXM (NGalXM), 80% of 3-linked mannose (α-Manp) residues present in side chains are acetylated at the O-2 position. In the acidic GalXM also termed glucuronoxylomannogalactan (GXMGal), 85% of the 3-linked α-Manp residues are acetylated either in the O-2 (75%) or in the O-6 (25%) position, but O-acetyl groups are not present at both positions simultaneously. In addition, NMR spectroscopy and methylation analysis showed that β-galactofuranose (β-Galf) units are linked to O-2 and O-3 positions of nonbranched α-galactopyranose (α-Galp) units present in the GalXMs backbone chain. These findings highlight new structural features of C. neoformans GalXMs. Among these features, the high degree of O-acetylation is of particular interest, since O-acetyl group-containing polysaccharides are known to possess a range of immunobiological activities.

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

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

Fungal Strategies to Evade the Host Immune Recognition

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

Triclosan Demonstrates Synergic Effect with Amphotericin B and Fluconazole and Induces Apoptosis-Like Cell Death in Cryptococcus neoformans

Objectives:Cryptococcus neoformans is an opportunistic fungus that causes fatal meningoencephalitis especially in AIDS patients. There is an increasing need for discovery of new anti-cryptococcal drugs due to emergence of resistance cases in recent years. In this study, we aim to elucidate the antifungal effect of triclosan against C. neoformans.

Methods: Minimal inhibitory concentration (MIC) of triclosan in different C. neoformans strains was first examined. The in vitro interactions between triclosan and two standard anti-fungal drugs (amphotericin B and fluconazole) were further evaluated by microdilution checkerboard assay. Mechanism of triclosan fungicidal activity was then investigated by viewing the cell morphology under transmission electron microscope.

Results: We reported that triclosan potently inhibited the growth of C. neoformans. A combination of triclosan with amphotericin B or with fluconazole enhanced their fungicidal effects. Triclosan-treated C. neoformans displayed characteristics such as nuclear chromatin condensation, extensive intracellular vacuolation and mitochondrial swelling, indicating that triclosan triggered apoptosis-like cell death.

Conclusion: In summary, our report suggests triclosan as an independent drug or synergent for C. neoformans treatment.

The formation of titan cells in Cryptococcus neoformans depends on the mouse strain and correlates with induction of Th2-type responses

Cryptococcus neoformans is a pathogenic yeast that can form titan cells in the lungs, which are fungal cells of abnormal enlarged size. Little is known about the factors that trigger titan cells. In particular, it is not known how the host environment influences this transition. In this work, we describe the formation of titan cells in two mouse strains, CD1 and C57BL/6J. We found that the proportion of C. neoformans titan cells was significantly higher in C57BL/6J mice than in CD1. This higher proportion of titan cells was associated with a higher dissemination of the yeasts to the brain. Histology sections demonstrated eosinophilia in infected animals, although it was significantly lower in the CD1 mice which presented infiltration of lymphocytes. Both mouse strains presented infiltration of granulocytes, but the amount of eosinophils was higher in C57BL/6J. CD1 mice showed a significant accumulation of IFN-γ, TNF-α and IL17, while C57BL/BL mice had an increase in the anti-inflammatory cytokine IL-4. IgM antibodies to the polysaccharide capsule and total IgE were more abundant in the sera from C57BL/6J, confirming that these animals present a Th2-type response. We conclude that titan cell formation in C. neoformans depends, not only on microbe factors, but also on the host environment.

Cryptococcus neoformans induces antimicrobial responses and behaves as a facultative intracellular pathogen in the non mammalian model Galleria mellonella

Cryptococcus neoformans is an encapsulated opportunistic fungal pathogen that is found in multiple niches in the environment and that can cause fatal meningoencephalitis in susceptible patients, mainly HIV+ individuals. Cryptococcus also infects environmental hosts such as nematodes, insects and plants. In particular, C. neoformans can kill the lepidopteran Galleria mellonella, which offers a useful tool to study microbial virulence and drug efficacy. Galleria mellonella immunity relies on innate responses based on melanization, accumulation of antimicrobial peptides, and cellular responses as phagocytosis or multicellular encapsulation. In this work we have investigated the immune response of G. mellonella during cryptococcal infection. We found that G. mellonella infected with C. neoformans had a high lytic activity in their hemolymph. This response was temperature- and capsule-dependent. During interaction with phagocytic cells, C. neoformans behaved as an intracellular pathogen since it could replicate within hemocytes. Non-lytic events were also observed. In contrast to Candida species, C. neoformans did not induce melanization of G. mellonella after infection. Finally, passage of C. neoformans through G. mellonella resulted in changes in capsule structure as it has been also reported during infection in mammals. Our results highlight that G. mellonella is an optimal model to investigate innate immune responses against C. neoformans.

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.

Deletion of Cryptococcus neoformans AIF ortholog promotes chromosome aneuploidy and fluconazole-resistance in a metacaspase-independent manner

Apoptosis is a form of programmed cell death critical for development and homeostasis in multicellular organisms. Apoptosis-like cell death (ALCD) has been described in several fungi, including the opportunistic human pathogen Cryptococcus neoformans. In addition, capsular polysaccharides of C. neoformans are known to induce apoptosis in host immune cells, thereby contributing to its virulence. Our goals were to characterize the apoptotic signaling cascade in C. neoformans as well as its unique features compared to the host machinery to exploit the endogenous fungal apoptotic pathways as a novel antifungal strategy in the future. The dissection of apoptotic pathways revealed that apoptosis-inducing factor (Aif1) and metacaspases (Mca1 and Mca2) are independently required for ALCD in C. neoformans. We show that the apoptotic pathways are required for cell fusion and sporulation during mating, indicating that apoptosis may occur during sexual development. Previous studies showed that antifungal drugs induce ALCD in fungi and that C. neoformans adapts to high concentrations of the antifungal fluconazole (FLC) by acquisition of aneuploidy, especially duplication of chromosome 1 (Chr1). Disruption of aif1, but not the metacaspases, stimulates the emergence of aneuploid subpopulations with Chr1 disomy that are resistant to fluconazole (FLC^R) in vitro and in vivo. FLC^R isolates in the aif1 background are stable in the absence of the drug, while those in the wild-type background readily revert to FLC sensitivity. We propose that apoptosis orchestrated by Aif1 might eliminate aneuploid cells from the population and defects in this pathway contribute to the selection of aneuploid FLC^R subpopulations during treatment. Aneuploid clinical isolates with disomies for chromosomes other than Chr1 exhibit reduced AIF1 expression, suggesting that inactivation of Aif1 might be a novel aneuploidy-tolerating mechanism in fungi that facilitates the selection of antifungal drug resistance.

Fungal pathogens can cause life-threatening diseases, and the infections that they cause are notoriously difficult to treat. Despite the availability of antifungal drugs, most inhibit fungal growth but do not consistently or efficiently eliminate the pathogen. In addition, fungal cells are very similar to human cells, and therefore, many of the available antifungal agents have toxic side effects. Thus, more efficient drugs with less adverse effects are clearly needed. We investigated apoptosis, a process in which cells become programmed to commit suicide, in the pathogenic fungus Cryptococcus neoformans. We studied genes that regulate apoptosis in C. neoformans and, after inactivating three genes involved in this pathway, we observed defects in sexual reproduction. Such mating defects decrease the production of spores, which are inhaled and cause cryptococcal disease. We also showed that the absence of one investigated apoptotic gene, aif1, resulted in the selection of antifungal-resistant pathogens (when the fungal cells no longer respond to the drug), which makes treatment of the disease more difficult. The discovery of drugs that kill fungal cells specifically without affecting the cells of the patient being treated holds great potential. Therefore, triggering apoptosis should be further investigated as a new approach to treat fungal pathogens.

Eosinophils elicit proliferation of naive and fungal-specific cells in vivo so enhancing a T helper type 1 cytokine profile in favour of a protective immune response against Cryptococcus neoformans infection

Experimental Cryptococcus neoformans infection in rats has been shown to have similarities with human cryptococcosis, because as in healthy humans, rats can effectively contain cryptococcal infection. Moreover, it has been shown that eosinophils are components of the immune response to C. neoformans infections. In a previous in vitro study, we demonstrated that rat peritoneal eosinophils phagocytose opsonized live yeasts of C. neoformans, thereby triggering their activation, as indicated by the up-regulation of MHC and co-stimulatory molecules and the increase in interleukin-12, tumour necrosis factor-α and interferon-γ production. Furthermore, this work demonstrated that C. neoformans-specific CD4(+) and CD8(+) T lymphocytes cultured with these activated C. neoformans-pulsed eosinophils proliferated, and produced important amounts of T helper type 1 (Th1) cytokines in the absence of Th2 cytokine synthesis. In the present in vivo study, we have shown that C. neoformans-pulsed eosinophils are also able to migrate into lymphoid organs to present C. neoformans antigens, thereby priming naive and re-stimulating infected rats to induce T-cell and B-cell responses against infection with the fungus. Furthermore, the antigen-specific immune response induced by C. neoformans-pulsed eosinophils, which is characterized by the development of a Th1 microenvironment with increased levels of NO synthesis and C. neoformans-specific immunoglobulin production, was demonstrated to be able to protect rats against subsequent infection with fungus. In summary, the present work demonstrates that eosinophils act as antigen-presenting cells for the fungal antigen, hence initiating and modulating a C. neoformans-specific immune response. Finally, we suggest that C. neoformans-loaded eosinophils might participate in the protective immune response against these fungi.

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.

Response of macrophage Toll-like receptor 4 to a Sporothrix schenckii lipid extract during experimental sporotrichosis

Toll-like receptors have been implicated in the recognition of various pathogens, including bacteria, viruses, protozoa and fungi. However, no information is available about Toll-like receptor 4 (TLR4) participation in Sporothrix schenckii recognition and the consequent triggering of the immune response to this fungal pathogen. Following activation of TLRs by ligands of microbial origin, several responses are provoked, including reactions in immune cells that may lead them to produce signalling factors that trigger inflammation. The present study was designed to elucidate the role of TLR4 during the host response to S. schenckii. TLR4-deficient (C3H/HeJ) and control mice (C3H/HePas) were infected with S. schenckii yeast cells and immune response was assessed over 10 weeks by assaying production of pro-inflammatory mediator (nitric oxide and tumour necrosis factor-alpha) and anti-inflammatory cytokine (interleukin-10) by peritoneal macrophages and their correlation with apoptosis in peritoneal exudate cells. We found that both pro-inflammatory and anti-inflammatory mediators are reduced in TLR4-deficient mice, suggesting the involvement of this receptor in the recognition of this infectious agent. Translocation into the nucleus of nuclear transcription factor, nuclear factor-kappaB, was also evaluated and showed higher levels in TLR-4 normal mice, consistent with the results found for cytokine production. We are showing here, for the first time, the involvement of TLR4 in S. schenckii recognition. Taken together, our results demonstrate that the activation of peritoneal macrophages in response to S. schenckii lipid extracts has different responses in these two mouse strains which differ in TLR4 expression, suggesting an important role for TLR4 in governing the functions of macrophages in this fungal infection.

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.

Evaluation of phenotypic and genotypic alterations induced by long periods of subculturing of Cryptococcus neoformans strains

Cryptococcus neoformans is an encapsulated fungal organism that can cause disease in apparently immunocompetent, as well as immunocompromised, hosts. Since 1930, successive subculture has been used to preserve C. neoformans isolates in our Fungus Collection. In the 1970s, some of these Fungus Collection samples were selected to be subjected to a different methods of maintenance--that of lyophilized. Our objective was to analyze C. neoformans isolates in order to make a comparative evaluation between these two methods of preservation. The overall aim of this study was to qualify the preservation technique used in our mycology laboratory since the technique used might affect the survival, stability and purity of the primary isolates in culture. The samples were analyzed using classical mycology methods and using the randomly amplified polymorphic DNA technique In the analysis of phenotypes and genotypes, the typical characteristics of C. neoformans were found to differ in relation to the different methods of preservation employed. The aim of this study was to demonstrate the importance of selecting the appropriate method of preservation for fungus collections. This selection can affect the survival and purity of the cultures, and preserve the stability of their physiological, biochemical, and genetic characteristics.

Direct inhibition of T-cell responses by the Cryptococcus capsular polysaccharide glucuronoxylomannan

The major virulence factor of the pathogenic fungi Cryptococcus neoformans and C. gattii is the capsule. Glucuronoxylomannan (GXM), the major component of the capsule, is a high-molecular-weight polysaccharide that is shed during cryptococcosis and can persist in patients after successful antifungal therapy. Due to the importance of T cells in the anticryptococcal response, we studied the effect of GXM on the ability of dendritic cells (DCs) to initiate a T-cell response. GXM inhibited the activation of cryptococcal mannoprotein–specific hybridoma T cells and the proliferation of OVA-specific OT-II T cells when murine bone marrow–derived DCs were used as antigen-presenting cells. Inhibition of OT-II T-cell proliferation was observed when either OVA protein or OVA_323–339 peptide was used as antigen, indicating GXM did not merely prevent antigen uptake or processing. We found that DCs internalize GXM progressively over time; however, the suppressive effect did not require DCs, as GXM directly inhibited T-cell proliferation induced by anti-CD3 antibody, concanavalin A, or phorbol-12-myristate-13-acetate/ionomycin. Analysis of T-cell viability revealed that the reduced proliferation in the presence of GXM was not the result of increased cell death. GXM isolated from each of the four major cryptococcal serotypes inhibited the proliferation of human peripheral blood mononuclear cells stimulated with tetanus toxoid. Thus, we have defined a new mechanism by which GXM can impart virulence: direct inhibition of T-cell proliferation. In patients with cryptococcosis, this could impair optimal cell-mediated immune responses, thereby contributing to the persistence of cryptococcal infections.

Infections due to the pathogenic yeast Cryptococcus are a significant cause of morbidity and mortality in persons with impaired T-cell functions, particularly those with AIDS. The major virulence factor of Cryptococcus is its capsule, which is composed primarily of the polysaccharide glucuronoxylomannan (GXM). The capsule not only surrounds the organism but also is shed during cryptococcosis. GXM is taken up by macrophages in vitro and in vivo; however, little is known about the interaction between GXM and dendritic cells, which are the most potent cells capable of activating T cells. Because of the importance of T cells in the anticryptococcal response, the authors investigated the effect of GXM on the ability of dendritic cells to initiate a T-cell response. They found the polysaccharide was internalized by dendritic cells and inhibited antigen-specific T-cell responses. Furthermore, GXM had a direct, inhibitory effect on T-cell proliferation, independent of the effect on dendritic cells. These findings may help explain the persistence of cryptococcal infections and suggest that GXM could be therapeutic in situations where suppression of T-cell responses is desired.

Aspergillus fumigatus conidia inhibit tumour necrosis factor- or staurosporine-induced apoptosis in epithelial cells

A major innate immune response to inhaled conidia of the opportunistic pathogen Aspergillus fumigatus (Af) is the synthesis of pro-inflammatory cytokines, which include tumour necrosis factor (TNF)-alpha, a known inducer of apoptosis. Modulation of host cell apoptosis has been reported to be one of the mechanisms whereby pathogens overcome host cell defences. Our study was designed to investigate whether or not Af conidia could modulate apoptosis induced by TNF-alpha or staurosporine (STS). Exposure of epithelial cells treated by these inducers and exposed to Af conidia decreased the number of apoptotic cells detected by Annexin V staining, analysis of nuclear morphology, terminal deoxynucleotidyl transferase-mediated fluorescein-dUTP nick end-labelling reaction and immunoblotting. Inhibition of apoptosis by Af conidia was seen in cells of the A549 pneumocyte II line, human tracheal epithelial 16HBE and primary human respiratory cells. Inhibition of apoptosis by Af conidia was also observed when apoptosis was induced by co-cultivating A549 cells with activated human alveolar macrophages. Unlike Af conidia, conidia of Cladosporium cladosporioides as well as latex beads or killed Af conidia have no inhibitory effect on TNF-alpha or STS-induced apoptosis. For TNF-induced apoptosis, the observed anti-apoptotic effect of Af conidia was found to be associated with a significant reduction of caspase-3.

Relationship of virulence factor expression to evolved virulence in mouse-passaged Cryptococcus neoformans lines

Serial passage of Cryptococcus neoformans in mice increases virulence relative to the nonpassaged line. Postpassaged lines showed no difference in the expression of most known virulence factors, with the exception that the more virulent lines had smaller capsules in vitro. These data imply that other mechanisms of virulence remain to be discovered.

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.

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.

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.