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

Neutron Scattering Analysis of Cryptococcus neoformans Polysaccharide Reveals Solution Rigidity and Repeating Fractal-like Structural Patterns

Cryptococcus neoformans is a fungal pathogen that can cause life-threatening brain infections in immunocompromised individuals. Unlike other fungal pathogens, it possesses a protective polysaccharide capsule that is crucial for its virulence. During infections, Cryptococcus cells release copious amounts of extracellular polysaccharides (exo-PS) that interfere with host immune responses. Both exo-PS and capsular-PS play pivotal roles in Cryptococcus infections and serve as essential targets for disease diagnosis and vaccine development strategies. However, understanding their structure is complicated by their polydispersity, complexity, sensitivity to sample isolation and processing, and scarcity of methods capable of isolating and analyzing them while preserving their native structure. In this study, we employ small-angle neutron scattering (SANS) and ultra-small-angle neutron scattering (USANS) for the first time to investigate both fungal cell suspensions and extracellular polysaccharides in solution. Our data suggests that exo-PS in solution exhibits collapsed chain-like behavior and demonstrates mass fractal properties that indicate a relatively condensed pore structure in aqueous environments. This observation is also supported by scanning electron microscopy (SEM). The local structure of the polysaccharide is characterized as a rigid rod, with a length scale corresponding to 3-4 repeating units. This research not only unveils insights into exo-PS and capsular-PS structures but also demonstrates the potential of USANS for studying changes in cell dimensions and the promise of contrast variation in future neutron scattering studies.

Neutron Scattering Analysis of Cryptococcus neoformans Polysaccharide Reveals Solution Rigidity and Repeating Fractal-like Structural Patterns

Cryptococcus neoformans is a fungal pathogen that can cause life-threatening brain infections in immunocompromised individuals. Unlike other fungal pathogens, it possesses a protective polysaccharide capsule that is crucial for its virulence. During infections, Cryptococcus cells release copious amounts of extracellular polysaccharides (exo-PS) that interfere with host immune responses. Both exo-PS and capsular-PS play pivotal roles in Cryptococcus infections and serve as essential targets for disease diagnosis and vaccine development strategies. However, understanding their structure is complicated by their polydispersity, complexity, sensitivity to sample isolation and processing, and scarcity of methods capable of isolating and analyzing them while preserving their native structure. In this study, we employ small-angle neutron scattering (SANS) and ultra-small angle neutron scattering (USANS) for the first time to investigate both fungal cell suspensions and extracellular polysaccharides in solution. Our data suggests that exo-PS in solution exhibits collapsed chain-like behavior and demonstrates mass fractal properties that indicate a relatively condensed pore structure in aqueous environments. This observation is also supported by scanning electron microscopy (SEM). The local structure of the polysaccharide is characterized as a rigid rod, with a length-scale corresponding to 3 to 4 repeating units. This research not only unveils insights into exo-PS and capsular-PS structures but also demonstrates the potential of USANS for studying changes in cell dimensions and the promise of contrast variation in future neutron scattering studies.

Role of toll-like receptors and nod-like receptors in acute lung infection

The respiratory system exposed to microorganisms continuously, and the pathogenicity of these microbes not only contingent on their virulence factors, but also the host's immunity. A multifaceted innate immune mechanism exists in the respiratory tract to cope with microbial infections and to decrease tissue damage. The key cell types of the innate immune response are macrophages, neutrophils, dendritic cells, epithelial cells, and endothelial cells. Both the myeloid and structural cells of the respiratory system sense invading microorganisms through binding or activation of pathogen-associated molecular patterns (PAMPs) to pattern recognition receptors (PRRs), including Toll-like receptors (TLRs) and NOD-like receptors (NLRs). The recognition of microbes and subsequent activation of PRRs triggers a signaling cascade that leads to the activation of transcription factors, induction of cytokines/5chemokines, upregulation of cell adhesion molecules, recruitment of immune cells, and subsequent microbe clearance. Since numerous microbes resist antimicrobial agents and escape innate immune defenses, in the future, a comprehensive strategy consisting of newer vaccines and novel antimicrobials will be required to control microbial infections. This review summarizes key findings in the area of innate immune defense in response to acute microbial infections in the lung. Understanding the innate immune mechanisms is critical to design host-targeted immunotherapies to mitigate excessive inflammation while controlling microbial burden in tissues following lung infection.

Adjuvant Pam3CSk4 does not improve the immunization against Cryptococcus gattii infection in C57BL/6 mice

Background

Cryptococcosis is a relevant invasive fungal infection that affects immunocompromised and immunocompetent individuals when caused by Cryptococcus gattii. Host innate and adaptive immune responses can be subverted by C. gattii, that blocks the differentiation of T helper (Th) 1 and Th17 cells, which are involved in the protection against cryptococcosis. Moreover, the macrophage polarization is modulated by C. gattii infection that requires a balance in the macrophage subsets to control the C. gattii infection. Toll-like receptor (TLR) 2 agonists are important immunomodulators favoring a pro-inflammatory response with potential fungicidal activity, and TLR2 agonists have been used as adjuvants in vaccines against infections caused by bacteria or viruses. Therefore, this work aimed to evaluate the immunomodulatory effect of the tripalmitoyl lipopeptide S-glycerol cysteine (Pam3CSK4 or P3C4), a TLR2 agonist, as an adjuvant in the vaccination against C. gattii infection.

Methods and Results

C57BL/6 mice were immunized with 2 × 10⁷ inactivated yeasts of C. gattii via intranasal route on day 1, 14 and 28 (Immunized group). Immunization was associated with 1µg or 10µg of adjuvant P3C4 (Immunized+P3C4-1µg or Immunized+P3C4-10 µg), followed by C. gattii infection on day 42 after the immunization protocol. Immunized+P3C4-1 µg group had reduced levels of IgG1, IgG2a and IgA and no significant difference in the IgG and IgM anti-GXM antibody titer was detected, compared to the Immunized group. High levels of IL-17 and IL-1β in lung tissue of mice from the Immunized+P3C4-1µg group did not promote a predominance of Th17 cells, in contrast, the frequency of TLR2⁺ cells was increased in immunized mice that received 1 µg of P3C4. The reduction in the relative expression of T-bet and high levels of Foxp3 detected in the lungs of the Immunized+P3C4-1µg group suggest a prevalence of regulatory T cells in the tissue, which did not contribute to the control of C. gattii infection. The immunization protocol associated with 10 µg of adjuvant P3C4 induced high levels of IL-17 in the lung tissue, whereas the levels of pro-inflammatory cytokines were downregulated. To evaluate the effect of adjuvant P3C4 in the control of C. gattii infection, quantification of the fungal burden in the lungs was performed by the CFU assay, and the groups with adjuvant P3C4 showed a pulmonary C. gattii burden that was not significantly altered when compared with the immunized group. The mice that received 1 µg of adjuvant P3C4 had a lower percentage of inflammatory infiltrate in the lungs.

Conclusion

The immunomodulatory effect of P3C4, associated with the immunization protocol, plays an imbalance between pro- and anti-inflammatory response in the lungs that did not favor a protection against C. gattii infection, which is related to the immune response characterized by a suppressive/regulatory profile in the pulmonary microenvironment after C. gattii infection.

The Cryptococcus gattii species complex: Unique pathogenic yeasts with understudied virulence mechanisms

Members of Cryptococcus gattii/neoformans species complex are the etiological agents of the potentially fatal human fungal infection cryptococcosis. C. gattii and its sister species cause disease in both immunocompetent and immunocompromised hosts, while the closely related species C. neoformans and C. deneoformans predominantly infect immunocompromised hosts. To date, most studies have focused on similarities in pathogenesis between these two groups, but over recent years, important differences have become apparent. In this review paper, we highlight some of the major phenotypic differences between the C. gattii and neoformans species complexes and justify the need to study the virulence and pathogenicity of the C. gattii species complex as a distinct cryptococcal group.

Host immune responses in the central nervous system during fungal infections

Fungal infections in the central nervous system (CNS) cause high morbidity and mortality. The frequency of CNS mycosis has increased over the last two decades as more individuals go through immunocompromised conditions for various reasons. Nevertheless, options for clinical interventions for CNS mycoses are still limited. Thus, there is an urgent need to understand the host-pathogen interaction mechanisms in CNS mycoses for developing novel treatments. Although the CNS has been regarded as an immune-privileged site, recent studies demonstrate the critical involvement of immune responses elicited by CNS-resident and CNS-infiltrated cells during fungal infections. In this review, we discuss mechanisms of fungal invasion in the CNS, fungal pathogen detection by CNS-resident cells (microglia, astrocytes, oligodendrocytes, neurons), roles of CNS-infiltrated leukocytes, and host immune responses. We consider that understanding host immune responses in the CNS is crucial for endeavors to develop treatments for CNS mycosis.

Bioinformatics strategies for studying the molecular mechanisms of fungal extracellular vesicles with a focus on infection and immune responses

Fungal extracellular vesicles (EVs) are released during pathogenesis and are found to be an opportunistic infection in most cases. EVs are immunocompetent with their host and have paved the way for new biomedical approaches to drug delivery and the treatment of complex diseases including cancer. With computing and processing advancements, the rise of bioinformatics tools for the evaluation of various parameters involved in fungal EVs has blossomed. In this review, we have complied and explored the bioinformatics tools to analyze the host-pathogen interaction, toxicity, omics and pathogenesis with an array of specific tools that have depicted the ability of EVs as vector/carrier for therapeutic agents and as a potential theme for immunotherapy. We have also discussed the generation and pathways involved in the production, transport, pathogenic action and immunological interactions of EVs in the host system. The incorporation of network pharmacology approaches has been discussed regarding fungal pathogens and their significance in drug discovery. To represent the overview, we have presented and demonstrated an in silico study model to portray the human Cryptococcal interactions.

The Sweet Side of Fungal Infections: Structural Glycan Diversity and Its Importance for Pathogenic Adaptation

Fungal infections are the most common secondary infections in debilitated individuals in a state of chronic disease or immunosuppression. Despite this, most fungal infections are neglected, mainly due to the lower frequency of their more severe clinical forms in immunocompetent individuals with a healthy background. However, over the past few years, several cases of severe fungal infections in healthy individuals have provoked a change in the epidemiological dynamics of fungal infections around the world, both due to recurrent outbreaks in previously infrequent regions and the greater emergence of more pathogenic fungal variants affecting healthy individuals, such as in the Cryptococcus genus. Therefore, before the arrival of a scenario of prevalent severe fungal infections, it is necessary to assess more carefully what are the real reasons for the increased incidence of fungal infection globally. What are the factors that are currently contributing to this new possible epidemiological dynamic? Could these be of a structural nature? Herein, we propose a discussion based on the importance of the virulence factors of glycoconjugate composition in the adaptation of pathogenic fungal species into the current scenario of increasing severity of these infections.

Cryptococcus spp. and Cryptococcosis: focusing on the infection in Brazil

Cryptococcosis is a global fungal infection caused by the Cryptococcus neoformans/Cryptococcus gattii yeast complex. This infection is acquired by inhalation of propagules such as basidiospores or dry yeast, initially causing lung infections with the possibility of progressing to the meninges. This infection mainly affects immunocompromised HIV and transplant patients; however, immunocompetent patients can also be affected. This review proposes to evaluate cryptococcosis focusing on studies of this mycosis in Brazilian territory; moreover, recent advances in the understanding of its virulence mechanism, animal models in research are also assessed. For this, literature review as realized in PubMed, Scielo, and Brazilian legislation. In Brazil, cryptococcosis has been identified as one of the most lethal fungal infections among HIV patients and C. neoformans VNI and C. gattii VGII are the most prevalent genotypes. Moreover, different clinical settings published in Brazil were described. As in other countries, cryptococcosis is difficult to treat due to a limited therapeutic arsenal, which is highly toxic and costly. The presence of a polysaccharide capsule, thermo-tolerance, production of melanin, biofilm formation, mechanisms for iron use, and morphological alterations is an important virulence mechanism of these yeasts. The introduction of cryptococcosis as a compulsory notification disease could improve data regarding incidence and help in the management of these infections.

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

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

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

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

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

The Host Immune Response to Scedosporium/Lomentospora

Infections caused by the opportunistic pathogens Scedosporium/Lomentospora are on the rise. This causes problems in the clinic due to the difficulty in diagnosing and treating them. This review collates information published on immune response against these fungi, since an understanding of the mechanisms involved is of great interest in developing more effective strategies against them. Scedosporium/Lomentospora cell wall components, including peptidorhamnomannans (PRMs), α-glucans and glucosylceramides, are important immune response activators following their recognition by TLR2, TLR4 and Dectin-1 and through receptors that are yet unknown. After recognition, cytokine synthesis and antifungal activity of different phagocytes and epithelial cells is species-specific, highlighting the poor response by microglial cells against L. prolificans. Moreover, a great number of Scedosporium/Lomentospora antigens have been identified, most notably catalase, PRM and Hsp70 for their potential medical applicability. Against host immune response, these fungi contain evasion mechanisms, inducing host non-protective response, masking fungal molecular patterns, destructing host defense proteins and decreasing oxidative killing. In conclusion, although many advances have been made, many aspects remain to be elucidated and more research is necessary to shed light on the immune response to Scedosporium/Lomentospora.

Phagocytosis enhancement, endotoxin tolerance, and signal mechanisms of immunologically active glucuronoxylomannan from Auricularia auricula-judae

Glucuronoxylomannan (AAPS) from the edible wood ear mushroom Auricularia auricula-judae has been demonstrated to exhibit immunostimulatory properties through its binding to TLR4. However, the mechanisms of immune modulation by AAPS in mammalian cells remains unclear. In the present study, we demonstrated that AAPS induced immunostimulatory effects were regulated by reactive oxygen species, mitogen-activated protein kinases, protein kinase C-α and NF-κB. AAPS remarkably increased the phagocytosis and bactericidal activity of macrophages. In lipopolysaccharide-activated macrophages, AAPS induced endotoxin tolerance like effect characterized by the downregulation of nitric oxide, interleukin-6 and TNF-α via the downregulation of NF-κB activation. Our findings provide firm scientific evidences for the immunoenhancing properties of wood ear mushroom, and the potential of AAPS to be strong candidates for the development of new carbohydrate-based nutraceutical supplements in the management of immunity related disorders in the future.

Pro-inflammatory response ensured by LPS and Pam3CSK4 in RAW 264.7 cells did not improve a fungistatic effect on Cryptococcus gattii infection

Background

The macrophage lineage is characterized by plasticity due to the acquisition of distinct functional phenotypes, and two major subsets are evaluated; classical M1 activation (strong microbicidal activity) and alternative M2 activation (immunoregulatory functions). The M1 subset expresses inducible nitric oxide synthase (iNOS), which is a primary marker to identify these cells, whereas M2 macrophages are characterized by expression of Arginase-1, found in inflammatory zone 1 (Fizz1), chitinase-like molecule (Ym-1), and CD206. The micro-environmental stimuli and signals in tissues are critical in the macrophage polarization. Toll-like receptors (TLR) ligands, such as lipopolysaccharide (LPS), palmitoyl-3-cysteine-serine-lysine-4 (Pam3CSK4), and ArtinM (mannose-binding lectin) are inductors of M1 subset. The impact of TLR2 and TLR4 signals to fight against Cryptococcus gattii infection is unknown, which is a fungal pathogen that preferentially infects the lung of immunocompetent individuals. The macrophages initiate an immune response to combat the C. gattii, then we evaluated in RAW 264.7 cell the effect of TLR2 and TLR4 agonists on the macrophage polarization dynamic and the impact on the growth of C. gattii.

Methods and Results

We demonstrated that P3C4, LPS, and ArtinM induced an increase in the levels of iNOS transcripts in RAW 264.7 cells, whereas the relative expression of arginase-1, Ym-1, and Fizz1 was significantly increased in the presence of IL-4 alone. The effects of TLR2 and TLR4 agonists on repolarization from the M2 to M1 subset was evaluated, and the first stimulus was composed of IL-4 and, after 24 h of incubation, the cells were submitted to a second stimulus of P3C4, LPS, ArtinM, or Medium. These TLR agonists induced the production of TNF-α in polarized RAW 264.7 cells to the M2 subset, moreover the measurement of M1/M2 markers using qRT-PCR demonstrated that a second stimulus with LPS for 24 h induced a significant augmentation of levels of iNOS mRNA. This impact of TLR2 and TLR4 agonists in the activation of the RAW 264.7 macrophage was assayed in the presence of C. gattii, the macrophages stimulated with TLR2 and TLR4 agonists for 24 h and co-cultured with C. gattii, as a second stimulus, reached high levels of TNF-α even after incubation with different concentrations of C. gattii. The activation of RAW 264.7 cells induced by TLR2 and TLR4 agonists favored the phagocytosis of C. gattii and inhibited the growth of yeast in the early period of infection. However, RAW 264.7 cells incubated with C. gattii in the presence of TLR2 and TLR4 agonists did not result a significant difference in the colony forming unit (CFU) assay in the early period of C. gattii infection, compared to negative control.

Conclusion

Polarized RAW 264.7 cells to the M1 subset with TLR2 and TLR4 agonists did not inhibit the growth of C. gattii, whereas robust immunity was identified that could dysregulate host tolerance to this pathogen.

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

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

Intracellular Cryptococcus neoformans disrupts the transcriptome profile of M1- and M2-polarized host macrophages

Macrophages serve as a first line of defense against infection with the facultative intracellular pathogen, Cryptococcus neoformans (Cn). However, the ability of these innate phagocytic cells to destroy ingested Cn is strongly influenced by polarization state with classically (M1) activated macrophages better able to control cryptococcal infections than alternatively (M2) activated cells. While earlier studies have demonstrated that intracellular Cn minimally affects the expression of M1 and M2 markers, the impact on the broader transcriptome associated with these states remains unclear. To investigate this, an in vitro cell culture model of intracellular infection together with RNA sequencing-based transcriptome profiling was used to measure the impact of Cn infection on gene expression in both polarization states. The gene expression profile of both M1 and M2 cells was extensively altered to become more like naive (M0) macrophages. Gene ontology analysis suggested that this involved changes in the activity of the Janus kinase-signal transducers and activators of transcription (JAK-STAT), p53, and nuclear factor-κB (NF-κB) pathways. Analyses of the principle polarization markers at the protein-level also revealed discrepancies between the RNA- and protein-level responses. In contrast to earlier studies, intracellular Cn was found to increase protein levels of the M1 marker iNos. In addition, common gene expression changes were identified that occurred post-Cn infection, independent of polarization state. This included upregulation of the transcriptional co-regulator Cited1, which was also apparent at the protein level in M1-polarized macrophages. These changes constitute a transcriptional signature of macrophage Cn infection and provide new insights into how Cn impacts gene expression and the phenotype of host phagocytes.

PAMPs of the Fungal Cell Wall and Mammalian PRRs

Fungi are opportunistic pathogens that infect immunocompromised patients and are responsible for an estimated 1.5 million deaths every year. The antifungal innate immune response is mediated through the recognition of pathogen-associated molecular patterns (PAMPs) by the host's pattern recognition receptors (PRRs). PRRs are immune receptors that ensure the internalisation and the killing of fungal pathogens. They also mount the inflammatory response, which contributes to initiate and polarise the adaptive response, controlled by lymphocytes. Both the innate and adaptive immune responses are required to control fungal infections. The immune recognition of fungal pathogen primarily occurs at the interface between the membrane of innate immune cells and the fungal cell wall, which contains a number of PAMPs. This chapter will focus on describing the main mammalian PRRs that have been shown to bind to PAMPs from the fungal cell wall of the four main fungal pathogens: Candida albicans, Aspergillus fumigatus, Cryptococcus neoformans and Pneumocystis jirovecii. We will describe these receptors, their functions and ligands to provide the reader with an overview of how the immune system recognises fungal pathogens and responds to them.

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.

A glucuronoxylomannan-like glycan produced by Trichosporon mucoides

Trichosporon asahii shares with Cryptococcus species the ability to produce glucuronoxylomannan (GXM), an immunomodulatory fungal polysaccharide. The ability of other opportunistic species of Trichosporon to produce GXM-like polysaccharides is unknown. In this study, we observed that T. mucoides was less pathogenic than T. asahii in an infection model of Galleria mellonella and asked whether this difference was related to the characteristics of GXM-like molecules. Compositional analysis of samples obtained from both pathogens indicated that the components of GXM (mannose, xylose and glucuronic acid) were, in fact, detected in T. mucoides and T. asahii glycans. The identification of the T. mucoides glycan as a GXM-like molecule was confirmed by its reactivity with a monoclonal antibody raised to cryptococcal GXM and incorporation of the glycan into the cell surface of an acapsular mutant of C. neoformans. T. mucoides and T. asahii glycans differed in molecular dimensions. The antibody to cryptococcal GXM recognized T. mucoides yeast forms less efficiently than T. asahii cells. Experiments with animal cells revealed that the T. mucoides glycan manifested antiphagocytic properties. Comparative phagocytosis assays revealed that T. mucoides and T. asahii were similarly recognized by macrophages. However, fungal association with the phagocytes did not depend on the typical receptors of cryptococcal GXM, as concluded from assays using macrophages obtained from Tlr2-/- and Cd14-/- knockout mice. These results add T. mucoides to the list of fungal pathogens producing GXM-like glycans, but also indicate a high functional diversity of this major fungal immunogen.

Dectin-3 Recognizes Glucuronoxylomannan of Cryptococcus neoformans Serotype AD and Cryptococcus gattii Serotype B to Initiate Host Defense Against Cryptococcosis

Cryptococcus neoformans and Cryptococcus gattii cause life-threatening meningoencephalitis or lung diseases in immunocompetent individuals or immunocompromised ones. C. neoformans and C. gattii are subdivided into five serotypes based on their capsular glucuronoxylomannan (GXM). C. neoformans consists of serotypes A, D, and AD hybrid, and C. gattii consists of serotypes B and C. Given structural differences of GXM between C. neoformans and C. gattii, it remains unclear that how innate immune system recognizes GXM. Here, we report that C-type lectin receptor Dectin-3 (MCL encoded by Clec4d) is a direct receptor for GXMs from C. neoformans serotype AD (C.n-AD) and C. gattii serotype B (C.g-B). GXMs from C.n-AD and C.g-B activated NF-κB and ERK pathways to induce pro-inflammatory cytokine production, whereas it was completely abolished due to deficiency of Dectin-3 or caspase recruitment domain family member 9 (CARD9). Upon pulmonary C.n-AD and C.g-B infection, Dectin-3- and CARD9-deficient mice were highly susceptible and showed augmented lung injury due to impairment of alveolar macrophage accumulation and killing activities. Our study provides the first biological and genetic evidence demonstrating that Dectin-3 recognizes GXM of C.n-AD and C.g-B to initiate host defense against cryptococcosis.

A Predicted Mannoprotein Participates in Cryptococcus gattii Capsular Structure

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

Eosinophils in fungal diseases: An overview

Eosinophils are the prominent cells in asthma, allergic bronchopulmonary mycosis (ABPMs), and fungal-sensitization-associated asthma, but their roles in the immunopathology of these disorders are not well understood. Moreover, the immunological mechanisms underlying the molecular direct effector interactions between fungi and eosinophils are rare and not fully known. Here, we provide an overview of eosinophil contributions to allergic asthma and ABPMs. We also revise the major general mechanisms of fungal recognition by eosinophils and consider past and recent advances in our understanding of the molecular mechanisms associated with eosinophil innate effector responses to different fungal species relevant to ABPMs (Alternaria alternata, Candida albicans, and Aspergillus fumigatus). We further examine and speculate about the therapeutic relevance of these findings in fungus-associated allergic pulmonary diseases.

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.

Genotypic and Phenotypic Diversity of Cryptococcus gattii VGII Clinical Isolates and Its Impact on Virulence

The Cryptococcus gattii species complex harbors the main etiological agents of cryptococcosis in immunocompetent patients. C. gattii molecular type VGII predominates in the north and northeastern regions of Brazil, leading to high morbidity and mortality rates. C. gattii VGII isolates have a strong clinical relevance and phenotypic variations. These phenotypic variations among C. gattii species complex isolates suggest that some strains are more virulent than others, but little information is available related to the pathogenic properties of those strains. In this study, we analyzed some virulence determinants of C. gattii VGII strains (CG01, CG02, and CG03) isolated from patients in the state of Piauí, Brazil. The C. gattii R265 VGIIa strain, which was isolated from the Vancouver outbreak, differed from C. gattii CG01, CG02 and CG03 isolates (also classified as VGII) when analyzed the capsular dimensions, melanin production, urease activity, as well as the glucuronoxylomannan (GXM) secretion. Those differences directly reflected in their virulence potential. In addition, CG02 displayed higher virulence compared to R265 (VGIIa) strain in a cryptococcal murine model of infection. Lastly, we examined the genotypic diversity of these strains through Multilocus Sequence Type (MLST) and one new subtype was described for the CG02 isolate. This study confirms the presence and the phenotypic and genotypic diversity of highly virulent strains in the Northeast region of Brazil.

The Overlooked Glycan Components of the Cryptococcus Capsule

Pathogenic species of Cryptococcus kill approximately 200,000 people each year. The most important virulence mechanism of C. neoformans and C. gattii, the causative agents of human and animal cryptococcosis, is the ability to form a polysaccharide capsule. Acapsular mutants of C. neoformans are avirulent in mice models of infection, and extracellularly released capsular polysaccharides are deleterious to the immune system. The principal capsular component in the Cryptococcus genus is a complex mannan substituted with xylosyl and glucuronyl units, namely glucuronoxylomannan (GXM). The second most abundant component of the cryptococcal capsule is a galactan with multiple glucuronyl, xylosyl, and mannosyl substitutions, namely glucuronoxylomannogalactan (GXMGal). The literature about the structure and functions of these two polysaccharides is rich, and a number of comprehensive reviews on this topic are available. Here, we focus our discussion on the less explored glycan components associated with the cryptococcal capsule, including mannoproteins and chitin-derived molecules. These glycans were selected for discussion on the basis that i) they have been consistently detected not only in the cell wall but also within the cryptococcal capsular network and ii) they have functions that impact immunological and/or pathogenic mechanisms in the Cryptococcus genus. The reported functions of these molecules strongly indicate that the biological roles of the cryptococcal capsule go far beyond the well-known properties of GXM and GXMGal.

Atorvastatin as a promising anticryptococcal agent

Cryptococcosis caused by Cryptococcus gattii leads to pneumonia and meningoencephalitis, and has a high mortality rate worldwide due to the inadequacy of available therapy and increasing drug resistance. There is a need to develop effective treatments, and drug repositioning is an interesting alternative to achieve new strategies to treat cryptococcosis. Atorvastatin (ATO), a statin currently used to treat hypercholesterolaemia, was tested in this study as an adjuvant to control infections caused by C. gattii. Several aspects of the effect of ATO on the host and the yeast were evaluated, with particular focus on the association of ATO with fluconazole (FLC), which (i) reduced ergosterol content in the cell membrane and altered properties of the polysaccharide capsule of C. gattii; (ii) increased the production of reactive oxygen species by macrophages; and (iii) reduced yeast phagocytosis and the intracellular proliferation rate. In an animal model, infected mice treated with ATO + FLC showed increased survival, improved clinical condition, and reduced fungal burden in the lungs and brain. This study is the first to perform in vivo tests with ATO + FLC for the treatment of cryptococcosis. The results suggest that ATO may be an important adjuvant for the treatment of cryptococcosis.

Analysis of multiple components involved in the interaction between Cryptococcus neoformans and Acanthamoeba castellanii

Cryptococcus neoformans is an environmental fungus that can cause lethal meningoencephalitis in immunocompromised individuals. The mechanisms by which environmental microbes become pathogenic to mammals are still obscure, but different studies suggest that fungal virulence evolved from selection imposed by environmental predators. The soil-living Acanthamoeba castellanii is a well-known predator of C. neoformans. In this work, we evaluated the participation of C. neoformans virulence-associated structures in the interaction of fungal cells with A. castellanii. Fungal extracellular vesicles (EVs) and the polysaccharide glucuronoxylomannan (GXM) were internalized by A. castellanii with no impact on the viability of amoebal cells. EVs, but not free GXM, modulated antifungal properties of A. castellanii by inducing enhanced yeast survival. Phagocytosis of C. neoformans by amoebal cells and the pathogenic potential in a Galleria mellonella model were not affected by EVs, but previous interactions with A. castellanii rendered fungal cells more efficient in killing this invertebrate host. This observation was apparently associated with marked amoeba-induced changes in surface architecture and increased resistance to both oxygen- and nitrogen-derived molecular species. Our results indicate that multiple components with the potential to impact pathogenesis are involved in C. neoformans environmental interactions.

Anti-Immune Strategies of Pathogenic Fungi

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

Immune Modulation Capability of Exopolysaccharides Synthesised by Lactic Acid Bacteria and Bifidobacteria

During recent years, the exopolysaccharides (EPS) produced by some strains of lactic acid bacteria and bifidobacteria have attracted the attention of researchers, mainly due to their potential technological applications. However, more recently, it has been observed that some of these EPS present immunomodulatory properties, which suggest a potential effect on human health. Whereas EPS from lactic acid bacteria have been studied in some detail, those of bifidobacteria largely remain uncharacterized in spite of the ubiquity of EPS genes in Bifidobacterium genomes. In this review, we have analysed the data collected in the literature about the potential immune-modulating capability of EPS produced by lactic acid bacteria and bifidobacteria. From this data analysis, as well as from results obtained in our group, a hypothesis relating the physicochemical characteristics of EPS with their immune modulation capability was highlighted. We propose that EPS having negative charge and/or small size (molecular weight) are able to act as mild stimulators of immune cells, whereas those polymers non-charged and with a large size present a suppressive profile.

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.

What makes Cryptococcus gattii a pathogen?

Cryptococcosis is an invasive fungal infection of humans and other animals, typically caused by the species Cryptococcus neoformans in patients with impaired immunity. However, there is growing recognition of the importance of the related species C. gattii in causing infections in apparently immunocompetent individuals. In particular, an ongoing outbreak of cryptococcal disease in the Pacific Northwest region, which started in 1999, has driven an intense research effort into this previously neglected pathogen. Here, we discuss some of the recent discoveries in this organism from the Pacific Northwest region and highlight areas for future investigation.

Extracellular Vesicles: Role in Inflammatory Responses and Potential Uses in Vaccination in Cancer and Infectious Diseases

Almost all cells and organisms release membrane structures containing proteins, lipids, and nucleic acids called extracellular vesicles (EVs), which have a wide range of functions concerning intercellular communication and signaling events. Recently, the characterization and understanding of their biological role have become a main research area due to their potential role in vaccination, as biomarkers antigens, early diagnostic tools, and therapeutic applications. Here, we will overview the recent advances and studies of Evs shed by tumor cells, bacteria, parasites, and fungi, focusing on their inflammatory role and their potential use in vaccination and diagnostic of cancer and infectious diseases.

Thiazole compounds with activity against Cryptococcus gattii and Cryptococcus neoformans in vitro

Human cryptococcosis can occur as a primary or opportunistic infection and develop as an acute, subacute, or chronic, systemic infection involving different host organs. We evaluated the antifungal activity of thirteen compounds against Cryptococcus gattii and Cryptococcus neoformans in vitro, by assessing the toxicity of the compounds showing the greatest antifungal activity in VERO cells and murine macrophages. From these results, four compounds were considered promising for further studies because they displayed low cytotoxicity and significant antifungal activity. The heterocyclic compounds 1b, 1c, 1d, and 1m have antifungal activity levels between that of amphotericin B and fluconazole in vitro. The death curve of Cryptococcus spp. treated with these four compounds was similar to the curve obtained for amphotericin B, in that we observed a significant reduction in cell viability within the first 24 h of treatment. Additionally, we found that there was no effect when these compounds were combined with amphotericin and fluconazole, except for 1c, which antagonized the effect of amphotericin B against C. gattii, also reflected in the reduction of the post-antifungal effect (PAFE); however, this interaction did not alter the ergosterol content. The results shown in this paper reveal the discovery of novel thiazole compounds, which are easy to synthesize, and with potentially exhibit antifungal activity, and display low cytotoxicity in normal mammalian cells. These compounds can be used as prototypes for the design of new antifungal drugs against C. gattii and C. neoformans.

Cryptococcus neoformans glucuronoxylomannan fractions of different molecular masses are functionally distinct

AIMS

Glucuronoxylomannan (GXM) is the major polysaccharide component of Cryptococcus neoformans. We evaluated in this study whether GXM fractions of different molecular masses were functionally distinct.

MATERIALS & METHODS

GXM samples isolated from C. neoformans cultures were fractionated to generate polysaccharide preparations differing in molecular mass. These fractions were used in experiments focused on the association of GXM with cell wall components of C. neoformans, as well as on the interaction of the polysaccharide with host cells.

RESULTS & CONCLUSION

GXM fractions of variable molecular masses bound to the surface of a C. neoformans acapsular mutant in a punctate pattern that is in contrast to the usual annular pattern of surface coating observed when GXM samples containing the full molecular mass range were used. The polysaccharide samples were also significantly different in their ability to stimulate cytokine production by host cells. Our findings indicate that GXM fractions are functionally distinct depending on their mass.

Fungal glycans and the innate immune recognition

Polysaccharides such as α- and β-glucans, chitin, and glycoproteins extensively modified with both N- and O-linked carbohydrates are the major components of fungal surfaces. The fungal cell wall is an excellent target for the action of antifungal agents, since most of its components are absent from mammalian cells. Recognition of these carbohydrate-containing molecules by the innate immune system triggers inflammatory responses and activation of microbicidal mechanisms by leukocytes. This review will discuss the structure of surface fungal glycoconjugates and polysaccharides and their recognition by innate immune receptors.

Two fundamentals of mammalian defense in fungal infections: endothermy and innate antifungal immunity

The environment of animals is inhabited by enormous fungal species, but only a few hundreds are pathogenic for mammals. Most of potentially pathogenic fungal species, excluding dermatophytes, seldom cause the disease in immunocompetent hosts. Data from literature indicate, that an immune system and endothermy are foundations for this mammalian relative resistance to fungal systemic infections. Stable and high temperature of the body restricts invasion and growth of potentially pathogenic fungi. Together with elevated metabolism it supports the effectiveness of mammalian immunity. The innate immunity is assigned to prevent the invasion of various microbes (including fungi) to the hosts' organism. It consists of cellular receptors and several humoral factors as the Antimicrobial Peptides. If the physical barriers fail in stopping the invader, it is recognized as "alien" by multiple Pattern Recognition Receptors (PRRs) like Toll Like Receptors (TLRs) expressed by cells of innate immunity and/or C-type lectins. At the same time innate inflammation begins and the complement cascade is activated. These mechanisms are able to stop and clear some fungal infections. During existing infection the adaptive immunity is induced. This review aims to show the role of mammalian endothermy and to point the most important elements of innate antifungal immunity.

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.

Allergen1 regulates polysaccharide structure in Cryptococcus neoformans

Cryptococcus neoformans is an important human, fungal pathogen that sheds polysaccharide (exo-PS) into host tissues. While shed exo-PS mediates numerous untoward effects (including promoting increased intracranial pressure), little is known about the regulation of this phenomenon. Since downregulation of the Allergen 1 (ALL1) gene is associated with high ICP, we investigated the relationship between ALL1 expression and exo-PS structure using a variety of biophysical techniques. The Δall1 mutants of two serotypes produced a shorter exo-PS with less branching and structural complexity than the parental strains. Consistent with lower branching, these exo-PSs manifested higher intrinsic viscosity than the parental strains. The Δall1 mutant strains manifested differences in epitope expression and significant resistance to phagocytosis. Exo-PS of Δall1 mutant exhibited anti-phagocytic properties. Comparative transcriptome analysis of mutant and parental strain under iron-deprived conditions indicated a role of ALL1 in iron homeostasis, characterized by differential regulation of genes that mediate iron reduction and transport. Together, our results demonstrate a role of ALL1 in regulating conformational aspects of PS structure and iron homeostasis. These findings provide a mechanism to explain how changes in ALL1 expression influence virulence of switch variants and suggest that structural changes and polymer length are epigenetically regulated.

Fungal pathogens-a sweet and sour treat for toll-like receptors

Hundred-thousands of fungal species are present in our environment, including normal colonizers that constitute part of the human microbiota. The homeostasis of host-fungus interactions encompasses efficient fungal sensing, tolerance at mucosal surfaces, as well as antifungal defenses. Decrease in host immune fitness or increase in fungal burden may favor pathologies, ranging from superficial mucocutaneous diseases to invasive life-threatening fungal infections. Toll-like receptors (TLRs) are essential players in this balance, due to their ability to control both inflammatory and anti-inflammatory processes upon recognition of fungal-specific pathogen-associated molecular patterns (PAMPs). Certain members of the TLR family participate to the initial recognition of fungal PAMPs on the cell surface, as well as inside phagosomes of innate immune cells. Active signaling cascades in phagocytes ultimately enable fungus clearance and the release of cytokines that shape and instruct other innate immune cells and the adaptive immune system. Some TLRs cooperate with other pattern recognition receptors (PRRs) (e.g., C-type lectins and Galectins), thus allowing for a tailored immune response. The spatio-temporal and physiological contributions of individual TLRs in fungal infections remains ill-defined, although in humans, TLR gene polymorphisms have been linked to increased susceptibility to fungal infections. This review focuses entirely on the role of TLRs that control the host susceptibility to environmental fungi (e.g., Aspergillus, Cryptoccocus, and Coccidoides), as well as to the most frequent human fungal pathogens represented by the commensal Candida species. The emerging roles of TLRs in modulating host tolerance to fungi, and the strategies that evolved in some of these fungi to evade or use TLR recognition to their advantage will also be discussed, as well as their potential suitability as targets in vaccine therapies.

A Paracoccidioides brasiliensis glycan shares serologic and functional properties with cryptococcal glucuronoxylomannan

The cell wall of the yeast form of the dimorphic fungus Paracoccidioides brasiliensis is enriched with α1,3-glucans. In Cryptococcus neoformans, α1,3-glucans interact with glucuronoxylomannan (GXM), a heteropolysaccharide that is essential for fungal virulence. In this study, we investigated the occurrence of P. brasiliensis glycans sharing properties with cryptococcal GXM. Protein database searches in P. brasiliensis revealed the presence of sequences homologous to those coding for enzymes involved in the synthesis of GXM and capsular architecture in C. neoformans. In addition, monoclonal antibodies (mAbs) raised to cryptococcal GXM bound to P. brasiliensis cells. Using protocols that were previously established for extraction and analysis of C. neoformans GXM, we recovered a P. brasiliensis glycan fraction composed of mannose and galactose, in addition to small amounts of glucose, xylose and rhamnose. In comparison with the C. neoformans GXM, the P. brasiliensis glycan fraction components had smaller molecular dimensions. The P. brasiliensis components, nevertheless, reacted with different GXM-binding mAbs. Extracellular vesicle fractions of P. brasiliensis also reacted with a GXM-binding mAb, suggesting that the polysaccharide-like molecule is exported to the extracellular space in secretory vesicles. An acapsular mutant of C. neoformans incorporated molecules from the P. brasiliensis extract onto the cell wall, resulting in the formation of surface networks that resembled the cryptococcal capsule. Coating the C. neoformans acapsular mutant with the P. brasiliensis glycan fraction resulted in protection against phagocytosis by murine macrophages. These results suggest that P. brasiliensis and C. neoformans share metabolic pathways required for the synthesis of similar polysaccharides and that P. brasiliensis yeast cell walls have molecules that mimic certain aspects of C. neoformans GXM. These findings are important because they provide additional evidence for the sharing of antigenically similar components across phylogenetically distant fungal species. Since GXM has been shown to be important for the pathogenesis of C. neoformans and to elicit protective antibodies, the finding of similar molecules in P. brasiliensis raises the possibility that these glycans play similar functions in paracoccidiomycosis.

Virulence factors identified by Cryptococcus neoformans mutant screen differentially modulate lung immune responses and brain dissemination

Deletions of cryptococcal PIK1, RUB1, and ENA1 genes independently rendered defects in yeast survival in human CSF and within macrophages. We evaluated virulence potential of these genes by comparing wild-type Cryptococcus neoformans strain H99 with deletant and complement strains in a BALB/c mouse model of pulmonary infection. Survival of infected mice; pulmonary cryptococcal growth and pathology; immunological parameters; dissemination kinetics; and CNS pathology were examined. Deletion of each PIK1, RUB1, and ENA1 differentially reduced pulmonary growth and dissemination rates of C. neoformans and extended mice survival. Furthermore, pik1Δ induced similar pathologies to H99, however, with significantly delayed onset; rub1Δ was more efficiently contained within pulmonary macrophages and was further delayed in causing CNS dissemination/pathology; whereas ena1Δ was progressively eliminated from the lungs and did not induce pathological lesions or disseminate into the CNS. The diminished virulence of mutant strains was associated with differential modulation of pulmonary immune responses, including changes in leukocyte subsets, cytokine responses, and macrophage activation status. Compared to H99 infection, mutants induced more hallmarks of a protective Th1 immune response, rather than Th2, and more classical, rather than alternative, macrophage activation. The magnitude of immunological effects precisely corresponded to the level of virulence displayed by each strain. Thus, cryptococcal PIK1, RUB1, and ENA1 differentially contribute to cryptococcal virulence, in correlation with their differential capacity to modulate immune responses.

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

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

The Role of TLR2 in Infection and Immunity

Toll-like receptors (TLRs) are recognition molecules for multiple pathogens, including bacteria, viruses, fungi, and parasites. TLR2 forms heterodimers with TLR1 and TLR6, which is the initial step in a cascade of events leading to significant innate immune responses, development of adaptive immunity to pathogens and protection from immune sequelae related to infection with these pathogens. This review will discuss the current status of TLR2 mediated immune responses by recognition of pathogen-associated molecular patterns (PAMPS) on these organisms. We will emphasize both canonical and non-canonical responses to TLR2 ligands with emphasis on whether the inflammation induced by these responses contributes to the disease state or to protection from diseases.

Toll-like receptors (TLR2 and TLR4) recognize polysaccharides of Pseudallescheria boydii cell wall

Pseudallescheria boydii is an opportunistic fungus widespread in the environment, and has recently emerged as an agent of localized as well as disseminated infections in both immunocompromised and immunocompetent hosts. The host response to fungi is in part dependent on the activation of evolutionary conserved receptors including Toll-like receptors and phagocytic receptors. This review will discuss the isolation and structural characterization of α-glucans and rhamnomannans from P. boydii cell wall and their roles in the induction of innate immune response.

Capsules from pathogenic and non-pathogenic Cryptococcus spp. manifest significant differences in structure and ability to protect against phagocytic cells

Capsule production is common among bacterial species, but relatively rare in eukaryotic microorganisms. Members of the fungal Cryptococcus genus are known to produce capsules, which are major determinants of virulence in the highly pathogenic species Cryptococcus neoformans and Cryptococcus gattii. Although the lack of virulence of many species of the Cryptococcus genus can be explained solely by the lack of mammalian thermotolerance, it is uncertain whether the capsules from these organisms are comparable to those of the pathogenic cryptococci. In this study, we compared the characteristic of the capsule from the non-pathogenic environmental yeast Cryptococcus liquefaciens with that of C. neoformans. Microscopic observations revealed that C. liquefaciens has a capsule visible in India ink preparations that was also efficiently labeled by three antibodies generated to specific C. neoformans capsular antigens. Capsular polysaccharides of C. liquefaciens were incorporated onto the cell surface of acapsular C. neoformans mutant cells. Polysaccharide composition determinations in combination with confocal microscopy revealed that C. liquefaciens capsule consisted of mannose, xylose, glucose, glucuronic acid, galactose and N-acetylglucosamine. Physical chemical analysis of the C. liquefaciens polysaccharides in comparison with C. neoformans samples revealed significant differences in viscosity, elastic properties and macromolecular structure parameters of polysaccharide solutions such as rigidity, effective diameter, zeta potential and molecular mass, which nevertheless appeared to be characteristics of linear polysaccharides that also comprise capsular polysaccharide of C. neoformans. The environmental yeast, however, showed enhanced susceptibility to the antimicrobial activity of the environmental phagocytes, suggesting that the C. liquefaciens capsular components are insufficient in protecting yeast cells against killing by amoeba. These results suggest that capsular structures in pathogenic Cryptococcus species and environmental species share similar features, but also manifest significant difference that could influence their potential to virulence.

Fungal surface and innate immune recognition of filamentous fungi

The innate immune system performs specific detection of molecules from infectious agents through pattern recognition receptors. This recognition triggers inflammatory responses and activation of microbicidal mechanisms by leukocytes. Infections caused by filamentous fungi have increased in incidence and represent an important cause of mortality and morbidity especially in individuals with immunosuppression. This review will discuss the innate immune recognition of filamentous fungi molecules and its importance to infection control and disease.