Purification and characterization of a second immunoreactive mannoprotein from Cryptococcus neoformans that stimulates T-Cell responses

Identification of a protective antigen reveals the trade-off between iron acquisition and antigen exposure in a global fungal pathogen

Systemic infections caused by Cryptococcus claim over 161,000 lives annually, with global mortality rate close to 70% despite antifungal therapies. Currently, no vaccine is available. To develop an effective multivalent vaccine against this free-living opportunistic eukaryotic pathogen, it is critical to identify protective antigens. We previously discovered ZNF2oe strains elicit protective host immune responses and increase the abundance of antigens present in the capsule, which is required for its immunoprotection. Capsule is a defining feature of Cryptococcus species and composed of polysaccharides and mannoproteins. Here, we found increased levels of exposed mannoproteins in ZNF2oe cells. As mannoproteins are the primary components recognized by anticryptococcal cell-mediated immune responses and few have been characterized, we systemically screened all 49 predicted GPI-mannoproteins in Cryptococcus neoformans for enhanced host recognition. We identified those highly present in ZNF2oe cells and found Cig1 to be a protective antigen against cryptococcosis either as a recombinant protein vaccine or an mRNA vaccine. Cig1 is induced by iron limitation and is highly expressed by this fungus in infected mice and in patients with cryptococcal meningitis. Remarkably, iron restriction by the host induces cryptococcal cells to express iron-uptake proteins including Cig1, which act as cryptococcal antigens and in turn enhance host detection. Our results highlight an arms race between the pathogen and the host centered on iron competition, and the trade-off between cryptococcal iron acquisition and antigen exposure. These findings demonstrate the potential of leveraging this host-pathogen interaction for vaccine development.

Design of an epitope-based peptide vaccine against Cryptococcus neoformans

Cryptococcus neoformans is the highest-ranked fungal pathogen in the Fungal Priority Pathogens List (FPPL) released by the World Health Organization (WHO). In this study, through in silico simulations, a multi-epitope vaccine against Cryptococcus neoformans was developed using the mannoprotein antigen (MP88) as a vaccine candidate. Following the retrieval of the MP88 protein sequences, these were used to predict antigenic B-cell and T-cell epitopes via the bepipred tool and the artificial neural network, respectively. Conserved B-cell epitopes AYSTPA, AYSTPAS, PASSNCK, and DSAYPP were identified as the most promising B-cell epitopes. While YMAADQFCL, VSYEEWMNY, and FQQRYTGTF were identified as the best candidates for CD8+ T-cell epitopes; and YARLLSLNA, ISYGTAMAV, and INQTSYARL were identified as the most promising CD4+ T-cell epitopes. The vaccine construct was modeled along with adjuvant and peptide linkers and the expasy protparam tool was used to predict the physiochemical properties. According to this, the construct vaccine was predicted to be antigenic, nontoxic, nonallergenic, soluble, stable, hydrophilic, and thermostable. Furthermore, the three-dimensional structure was also used in docking analyses with Toll-like receptor (TLR4). Finally, the cDNA of vaccine was successfully cloned into the E. coli pET-28a (+) expression vector. The results presented here could contribute towards the design of an effective vaccine against Cryptococcus neoformans.

Extension of O-Linked Mannosylation in the Golgi Apparatus Is Critical for Cell Wall Integrity Signaling and Interaction with Host Cells in Cryptococcus neoformans Pathogenesis

The human-pathogenic yeast Cryptococcus neoformans assembles two types of O-linked glycans on its proteins. In this study, we identified and functionally characterized the C. neoformans CAP6 gene, encoding an α1,3-mannosyltransferase responsible for the second mannose addition to minor O-glycans containing xylose in the Golgi apparatus. Two cell surface sensor proteins, Wml1 (WSC/Mid2-like) and Wml2, were found to be independent substrates of Cap6-mediated minor or Ktr3-mediated major O-mannosylation, respectively. The double deletion of KTR3 and CAP6 (ktr3Δ cap6Δ) completely blocked the mannose addition at the second position of O-glycans, resulting in the accumulation of proteins with O-glycans carrying only a single mannose. Tunicamycin (TM)-induced phosphorylation of the Mpk1 mitogen-activated protein kinase (MAPK) was greatly decreased in both ktr3Δ cap6Δ and wml1Δ wml2Δ strains. Transcriptome profiling of the ktr3Δ cap6Δ strain upon TM treatment revealed decreased expression of genes involved in the Mpk1-dependent cell wall integrity (CWI) pathway. Consistent with its defective growth under several stress conditions, the ktr3Δ cap6Δ strain was avirulent in a mouse model of cryptococcosis. Associated with this virulence defect, the ktr3Δ cap6Δ strain showed decreased adhesion to lung epithelial cells, decreased proliferation within macrophages, and reduced transcytosis of the blood-brain barrier (BBB). Notably, the ktr3Δ cap6Δ strain showed reduced induction of the host immune response and defective trafficking of ergosterol, an immunoreactive fungal molecule. In conclusion, O-glycan extension in the Golgi apparatus plays critical roles in various pathobiological processes, such as CWI signaling and stress resistance and interaction with host cells in C. neoformans. IMPORTANCE Cryptococcus neoformans assembles two types of O-linked glycans on its surface proteins, the more abundant major O-glycans that do not contain xylose residues and minor O-glycans containing xylose. Here, we demonstrate the role of the Cap6 α1,3-mannosyltransferase in the synthesis of minor O-glycans. Previously proposed to be involved in capsule biosynthesis, Cap6 works with the related Ktr3 α1,2-mannosyltransferase to synthesize O-glycans on their target proteins. We also identified two novel C. neoformans stress sensors that require Ktr3- and Cap6-mediated posttranslational modification for full function. Accordingly, the ktr3Δ cap6Δ double O-glycan mutant strain displays defects in stress signaling pathways, CWI, and ergosterol trafficking. Furthermore, the ktr3Δ cap6Δ strain is completely avirulent in a mouse infection model. Together, these results demonstrate critical roles for O-glycosylation in fungal pathogenesis. As there are no human homologs for Cap6 or Ktr3, these fungus-specific mannosyltransferases are novel targets for antifungal therapy.

Harnessing the Immune Response to Fungal Pathogens for Vaccine Development

Invasive fungal infections are emerging diseases that kill over 1.5 million people per year worldwide. With the increase of immunocompromised populations, the incidence of invasive fungal infections is expected to continue to rise. Vaccines for viral and bacterial infectious diseases have had a transformative impact on human health worldwide. However, no fungal vaccines are currently in clinical use. Recently, interest in fungal vaccines has grown significantly. One Candida vaccine has completed phase 2 clinical trials, and research on vaccines against coccidioidomycosis continues to advance. Additionally, multiple groups have discovered various Cryptococcus mutant strains that promote protective responses to subsequent challenge in mouse models. There has also been progress in antibody-mediated fungal vaccines. In this review, we highlight recent fungal vaccine research progress, outline the wealth of data generated, and summarize current research for both fungal biology and immunology studies relevant to fungal vaccine development. We also review technological advancements in vaccine development and highlight the future prospects of a human vaccine against invasive fungal infections.

Cryptococcus neoformans Genotypic Diversity and Disease Outcome among HIV Patients in Africa

Cryptococcal meningoencephalitis, a disease with poor patient outcomes, remains the most prevalent invasive fungal infection worldwide, accounting for approximately 180,000 deaths each year. In several areas of sub-Saharan Africa with the highest HIV prevalence, cryptococcal meningitis is the leading cause of community-acquired meningitis, with a high mortality among HIV-infected individuals. Recent studies show that patient disease outcomes are impacted by the genetics of the infecting isolate. Yet, there is still limited knowledge of how these genotypic variations contribute to clinical disease outcome. Further, it is unclear how the genetic heterogeneity of C. neoformans and the extensive phenotypic variation observed between and within isolates affects infection and disease. In this review, we discuss current knowledge of how various genotypes impact disease progression and patient outcome in HIV-positive populations in sub-Saharan African, a setting with a high burden of cryptococcosis.

Macrophage Mediated Immunomodulation During Cryptococcus Pulmonary Infection

Macrophages are key cellular components of innate immunity, acting as the first line of defense against pathogens to modulate homeostatic and inflammatory responses. They help clear pathogens and shape the T-cell response through the production of cytokines and chemokines. The facultative intracellular fungal pathogen Cryptococcus neoformans has developed a unique ability to interact with and manipulate host macrophages. These interactions dictate how Cryptococcus infection can remain latent or how dissemination within the host is achieved. In addition, differences in the activities of macrophages have been correlated with differential susceptibilities of hosts to Cryptococcus infection, highlighting the importance of macrophages in determining disease outcomes. There is now abundant information on the interaction between Cryptococcus and macrophages. In this review we discuss recent advances regarding macrophage origin, polarization, activation, and effector functions during Cryptococcus infection. The importance of these strategies in pathogenesis and the potential of immunotherapy for cryptococcosis treatment is also discussed.

Influence of Pathogen Carbon Metabolism on Interactions With Host Immunity

Cryptococcus neoformans is a ubiquitous opportunistic fungal pathogen typically causing disease in immunocompromised individuals and is globally responsible for about 15% of AIDS-related deaths annually. C. neoformans first causes pulmonary infection in the host and then disseminates to the brain, causing meningoencephalitis. The yeast must obtain and metabolize carbon within the host in order to survive in the central nervous system and cause disease. Communication between pathogen and host involves recognition of multiple carbon-containing compounds on the yeast surface: polysaccharide capsule, fungal cell wall, and glycosylated proteins comprising the major immune modulators. The structure and function of polysaccharide capsule has been studied for the past 70 years, emphasizing its role in virulence. While protected by the capsule, fungal cell wall has likewise been a focus of study for several decades for its role in cell integrity and host recognition. Associated with both of these major structures are glycosylated proteins, which exhibit known immunomodulatory effects. While many studies have investigated the role of carbon metabolism on virulence and survival within the host, the precise mechanism(s) affecting host-pathogen communication remain ill-defined. This review summarizes the current knowledge on mutants in carbon metabolism and their effect on the host immune response that leads to changes in pathogen recognition and virulence. Understanding these critical interactions will provide fresh perspectives on potential treatments and the natural history of cryptococcal disease.

Deciphering the Association among Pathogenicity, Production and Polymorphisms of Capsule/Melanin in Clinical Isolates of Cryptococcus neoformans var. grubii VNI

BACKGROUND

Cryptococcus neoformans is an opportunistic fungal pathogen that can cause meningitis in immunocompromised individuals. The objective of this work was to study the relationship between the phenotypes and genotypes of isolates of clinical origin from different cities in Colombia.

METHODS

Genome classification of 29 clinical isolates of C. neoformans var. grubii was performed using multilocus sequence typing (MLST), and genomic sequencing was used to genotype protein-coding genes. Pathogenicity was assessed in a larval model, and melanin production and capsule size were evaluated in vitro and in vivo.

RESULTS

Eleven MLST sequence types (STs) were found, the most frequent being ST69 (n = 9), ST2, ST93, and ST377 (each with n = 4). In the 29 isolates, different levels of pigmentation, capsule size and pathogenicity were observed. Isolates classified as highly pathogenic showed a tendency to exhibit larger increases in capsule size. In the analysis of polymorphisms, 48 non-synonymous variants located in the predicted functional domains of 39 genes were found to be associated with capsule size change, melanin, or pathogenicity.

CONCLUSIONS

No clear patterns were found in the analysis of the phenotype and genotype of Cryptococcus. However, the data suggest that the increase in capsule size is a key variable for the differentiation of pathogenic isolates, regardless of the method used for its induction.

Evaluation of IFN-γ secretion after stimulation with C. neoformans and C. gattii antigens in individuals with frequent exposure to the fungus

In this study we produced antigenic extracts from prototypical strains of C. neoformans (VNI-VNIV) and C. gattii (VGI-VGIV) and tested IFN-γ secretion by Elispot. Antigens from the eight Cryptococcus molecular types (VNI -VNIV and VGI - VGIV) were obtained after capsule reduction. IFN-γ secretion by Elispot method were stimulated with C. neoformans and C. gattii antigens. Peripheral blood mononuclear cells of fourteen healthy control subjects, being: five ecotourists, two mycologists, three poultry keepers, and four individuals without reports of exposure to the fungus. We observed a significant increase in IFN-γ secretion in the group of ecotourists, mycologists and bird keepers in relation to the group of individuals without reports of occupational exposures to these agents. Our results suggest the significant increase in IFN-γ secretion may be related to the continuous exposure of these groups of individuals to the fungus, as well as to the specific antigen memory immune response developed during exposure to Cryptococcus.

Cryptococcus extracellular vesicles properties and their use as vaccine platforms

Whereas extracellular vesicle (EV) research has become commonplace in different biomedical fields, this field of research is still in its infancy in mycology. Here we provide a robust set of data regarding the structural and compositional aspects of EVs isolated from the fungal pathogenic species Cryptococcus neoformans, C. deneoformans and C. deuterogattii. Using cutting-edge methodological approaches including cryogenic electron microscopy and cryogenic electron tomography, proteomics, and flow cytometry, we revisited cryptococcal EV features and suggest a new EV structural model, in which the vesicular lipid bilayer is covered by mannoprotein-based fibrillar decoration, bearing the capsule polysaccharide as its outer layer. About 10% of the EV population is devoid of fibrillar decoration, adding another aspect to EV diversity. By analysing EV protein cargo from the three species, we characterized the typical Cryptococcus EV proteome. It contains several membrane-bound protein families, including some Tsh proteins bearing a SUR7/PalI motif. The presence of known protective antigens on the surface of Cryptococcus EVs, resembling the morphology of encapsulated virus structures, suggested their potential as a vaccine. Indeed, mice immunized with EVs obtained from an acapsular C. neoformans mutant strain rendered a strong antibody response in mice and significantly prolonged their survival upon C. neoformans infection.

Characterization of an immunogenic cellulase secreted by Cryptococcus pathogens

Members of the C. neoformans/C. gattiii species complex are an important cause of serious humans infections, including meningoencephalitis. We describe here a 45 kDa extracellular cellulase purified from culture supernatants of C. neoformans var. neoformans. The N-terminal sequence obtained from the purified protein was used to isolate a clone containing the full-length coding sequence from a C. neoformans var. neoformans (strain B-3501A) cDNA library. Bioinformatics analysis indicated that this gene is present, with variable homology, in all sequenced genomes of the C. neoformans/C. gattii species complex. The cDNA clone was used to produce a recombinant 45 kDa protein in E. coli that displayed the ability to convert carboxymethyl cellulose and was therefore designated as NG-Case (standing for Neoformans Gattii Cellulase). To explore its potential use as a vaccine candidate, the recombinant protein was used to immunize mice and was found capable of inducing T helper type 1 responses and delayed-type hypersensitivity reactions, but not immune protection against a highly virulent C. neoformans var grubii strain. These data may be useful to better understand the mechanisms underlying the ability C. neoformans/C. gattii to colonize plant habitats and to interact with the human host during infection.

Structural basis of Blastomyces Endoglucanase-2 adjuvancy in anti-fungal and -viral immunity

The development of safe subunit vaccines requires adjuvants that augment immunogenicity of non-replicating protein-based antigens. Current vaccines against infectious diseases preferentially induce protective antibodies driven by adjuvants such as alum. However, the contribution of antibody to host defense is limited for certain classes of infectious diseases such as fungi, whereas animal studies and clinical observations implicate cellular immunity as an essential component of the resolution of fungal pathogens. Here, we decipher the structural bases of a newly identified glycoprotein ligand of Dectin-2 with potent adjuvancy, Blastomyces endoglucanase-2 (Bl-Eng2). We also pinpoint the developmental steps of antigen-specific CD4⁺ and CD8⁺ T responses augmented by Bl-Eng2 including expansion, differentiation and tissue residency. Dectin-2 ligation led to successful systemic and mucosal vaccination against invasive fungal infection and Influenza A infection, respectively. O-linked glycans on Bl-Eng2 applied at the skin and respiratory mucosa greatly augment vaccine subunit- induced protective immunity against lethal influenza and fungal pulmonary challenge.

Fungal disease remains a challenging clinical and public health problem in part because there is no commercial vaccine available. The lack of suitable adjuvants is a critical barrier to developing safe and effective vaccines against fungal pathogens. Current adjuvants such as alum preferentially induce antibody responses which may be limited in mediating protection against fungi. Clinical observations and animal studies implicate cellular immunity as the essential component for the resolution of fungal infections. We have recently discovered an adjuvant that augments cell mediated immune responses and vaccine induced protection against fungi. Here, we identified the structural and mechanistic requirements by which this newly discovered adjuvant induces cell mediated immunity against fungi. As a proof of principle we also demonstrate that the adjuvant drives cellular immune responses against viruses such as influenza. We anticipate that our adjuvant can be used for vaccination with safe subunit vaccines against many microbial pathogens including viruses, intracellular bacteria, fungi and parasites that require cell mediated immune responses.

A Predicted Mannoprotein Cmp1 Regulates Fungal Virulence in Cryptococcus neoformans

The capsule of the fungal pathogen Cryptococcus neoformans consists of glucuronoxylomannan (GXM), glucuronoxylomannogalactan (GXMGal), and mannoproteins (MPs). MPs are a kind of glycoproteins with low content but high immunogenicity, which can stimulate the immune protection of the host. However, there is not much information about the role of mannoproteins in virulence of the human fungal pathogen C. neoformans. In this study, we reported the identification and functional analysis of a predicted mannoprotein Cmp1 that regulates fungal virulence in C. neoformans. Gene expression pattern analysis indicates that the CMP1 gene was ubiquitously expressed at all stages of cryptococcal development. Subcellular localization analysis indicated that Cmp1 was localized in the cytoplasm of cryptococcal cells. Disruption or overexpression of CMP1 results in impairing capsule formation in Cryptococcus, but it does not affect the melanin production and sensitivity under various stress conditions, nor does it affect the sexual reproduction process of Cryptococcus. Survival assay showed that the pathogenicity of the cmp1Δ mutant or the CMP1 overexpression strain was significantly attenuated in a murine inhalation model of cryptococcosis. In conclusion, our findings implied that the mannoprotein Cmp1 is required for the virulence of C. neoformans.

Protection of mice against experimental cryptococcosis using glucan particle-based vaccines containing novel recombinant antigens

Meningitis due to Cryptococcus neoformans is responsible for upwards of 180,000 deaths worldwide annually, mostly in immunocompromised individuals. Currently there are no licensed fungal vaccines, and even with anti-fungal drug treatment, cryptococcal meningitis is often fatal. Our lab previously demonstrated vaccination with recombinant cryptococcal proteins delivered in glucan particles (GPs) protects mice against an otherwise lethal infection. The aim of the present study was to discover additional cryptococcal antigens affording vaccine-mediated protection. Sixteen proteins, each with evidence of extracellularity, were selected for in vivo testing based on their abundance in protective alkaline extracts of an acapsular C. neoformans strain, their known immunogenicity, and/or their high transcript level during human infection. Candidate antigens were recombinantly expressed in E. coli, purified and loaded into GPs. BALB/c and C57BL/6 mice received three subcutaneous injections of GP-based vaccine, and survival was assessed for 84 days following a lethal orotracheal challenge with strain KN99. As with our six published GP-vaccines, we saw differences in overall protection between mouse strains such that BALB/c mice typically demonstrated better survival than C57BL/6 mice. From these studies, we identified seven new proteins which, when administered as GP-vaccines, protect BALB/c and/or C57BL/6 mice against cryptococcal infection. With these results, we expand the pool of novel protective antigens to eleven proteins and demonstrate the potential for selection of highly transcribed extracellular proteins as vaccine targets. These screens highlight the efficacy of GP-subunit vaccines and identify promising antigens for further testing in anti-cryptococcal, multi-epitope vaccine formulations.

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.

Chitin: A "Hidden Figure" in the Fungal Cell Wall

Chitin and chitosan are two related polysaccharides that provide important structural stability to fungal cell walls. Often embedded deeply within the cell wall structure, these molecules anchor other components at the cell surface. Chitin-directed organization of the cell wall layers allows the fungal cell to effectively monitor and interact with the external environment. For fungal pathogens, this interaction includes maintaining cellular strategies to avoid excessive detection by the host innate immune system. In turn, mammalian and plant hosts have developed their own strategies to process fungal chitin, resulting in chitin fragments of varying molecular size. The size-dependent differences in the immune activation behaviors of variably sized chitin molecules help to explain how chitin and related chitooligomers can both inhibit and activate host immunity. Moreover, chitin and chitosan have recently been exploited for many biomedical applications, including targeted drug delivery and vaccine development.

IFN-γ immune priming of macrophages in vivo induces prolonged STAT1 binding and protection against Cryptococcus neoformans

Development of vaccines against opportunistic infections is difficult as patients most at risk of developing disease are deficient in aspects of the adaptive immune system. Here, we utilized an experimental immunization strategy to induce innate memory in macrophages in vivo. Unlike current trained immunity models, we present an innate memory-like phenotype in macrophages that is maintained for at least 70 days post-immunization and results in complete protection against secondary challenge in the absence of adaptive immune cells. RNA-seq analysis of in vivo IFN-γ primed macrophages revealed a rapid up-regulation of IFN-γ and STAT1 signaling pathways following secondary challenge. The enhanced cytokine recall responses appeared to be pathogen-specific, dependent on changes in histone methylation and acetylation, and correlated with increased STAT1 binding to promoter regions of genes associated with protective anti-fungal immunity. Thus, we demonstrate an alternative mechanism to induce macrophage innate memory in vivo that facilitates pathogen-specific vaccine-mediated immune responses.

Fungal infections are a significant global health problem that can affect anyone, however, individuals with a weakened immune system are most at risk. Cryptococcus neoformans infections can progress to meningitis in immune compromised individuals accounting for nearly 220,000 new cases annually, resulting in 181,000 deaths. Vaccine strategies tend to target CD4⁺ T cells for the generation of protective memory responses. However, immune compromised individuals have decreased numbers of these adaptive cells, providing a challenge for anti-fungal vaccine design. Here, we define a cellular mechanism by which macrophages, an innate cell population, generate protective immune responses against C. neoformans following initial exposure to a C. neoformans strain that secretes IFN-γ. We determined that the macrophages primed in vivo have heightened proinflammatory cytokine responses upon secondary exposure to C. neoformans in a manner that is mTOR-independent, yet dependent on histone modification dynamics. We show that IFN-γ primed macrophages can maintain STAT1 binding to the promoter regions of key proinflammatory genes long after the initial exposure. Remarkably, our studies show long-lived, cryptococcal-specific protective immunity in vivo. The results presented herein demonstrate that innate cell populations, namely macrophages, can be utilized as vaccine targets to protect against cryptococcal infections in immune compromised populations.

Design of an epitope-based peptide vaccine againstCryptococcus neoformans.. [DOI: 10.1101/434779] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Introduction

This study aimed to design an immunogenic epitope for Cryptococcus neoformans the etiological agent of cryptococcosis using in silico simulations, for epitope prediction, we selected the mannoprotein antigen MP88 which it’s known to induce protective immunity.

Material & method

A total of 39 sequences of MP88 protein with length 378 amino acids were retrieved from the National Center for Biotechnology Information database (NCBI) in the FASTA format were used to predict antigenic B-cell and T cell epitopes via different bioinformatics tools at Immune Epitope Database and Analysis Resource (IEDB). The tertiary structure prediction of MP88 was created in RaptorX, and visualized by UCSF Chimera software.

Result

A Conserved B-cell epitopesAYSTPA, AYSTPAS, PASSNCK, and DSAYPPhave displayed the most promising B cell epitopes. While theYMAADQFCL, VSYEEWMNYandFQQRYTGTFthey represent the best candidates T-cell conserved epitopes, the 9-mer epitopeYMAADQFCLdisplay the greater interact with 9 MHC-I alleles and HLA-A*02:01 alleles have the best interaction with an epitope. TheVSYEEWMNYandFQQRYTGTFthey are non-allergen whileYMAADQFCLwas an allergen. For MHC class II peptide binding prediction, theYARLLSLNA, ISYGTAMAVandINQTSYARLrepresent the most Three highly binding affinity core epitopes. The core epitopeINQTSYARLwas found to interact with 14 MHC-II. The allergenicity prediction revealsISYGTAMAV, INQTSYARLwere non-allergen andYARLLSLNAwas an allergen. Regarding population coverage theYMAADQFCLexhibit, a higher percentage among the world (69.75%) and the average population coverage was93.01%.In MHC-II,ISYGTAMAVepitope reveal a higher percentage (74.39%) and the average population coverage was (81.94%). This successfully designed a peptide vaccine against Cryptococcus neoformans open up a new horizon in Cryptococcus neoformans research; the results require validation by in vitro and in vivo experiments.

Autocrine IL-10 Signaling Promotes Dendritic Cell Type-2 Activation and Persistence of Murine Cryptococcal Lung Infection

The substantial morbidity and mortality caused by invasive fungal pathogens, including Cryptococcus neoformans, necessitates increased understanding of protective immune responses against these infections. Our previous work using murine models of cryptococcal lung infection demonstrated that dendritic cells (DCs) orchestrate critical transitions from innate to adaptive immunity and that IL-10 signaling blockade improves fungal clearance. To further understand interrelationships among IL-10 production, fungal clearance, and the effect of IL-10 on lung DCs, we performed a comparative temporal analysis of cryptococcal lung infection in wild type C57BL/6J mice (designated IL-10^+/+) and IL-10^-/- mice inoculated intratracheally with C. neoformans (strain 52D). Early and sustained IL-10 production by lung leukocytes was associated with persistent infection in IL-10^+/+ mice, whereas fungal clearance was improved in IL-10^-/- mice during the late adaptive phase of infection. Numbers of monocyte-derived DCs, T cells, and alveolar and exudate macrophages were increased in lungs of IL-10^-/- versus IL-10^+/+ mice concurrent with evidence of enhanced DC type-1, Th1/Th17 CD4 cell, and classical macrophage activation. Bone marrow-derived DCs stimulated with cryptococcal mannoproteins, a component of the fungal capsule, upregulated expression of IL-10 and IL-10R, which promoted DC type-2 activation in an autocrine manner. Thus, our findings implicate fungus-triggered autocrine IL-10 signaling and DC type-2 activation as important contributors to the development of nonprotective immune effector responses, which characterize persistent cryptococcal lung infection. Collectively, this study informs and strengthens the rationale for IL-10 signaling blockade as a novel treatment for fungal infections.

Peeling the onion: the outer layers of Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen that is ubiquitous in the environment. It causes a deadly meningitis that is responsible for over 180,000 deaths worldwide each year, including 15% of all AIDS-related deaths. The high mortality rates for this infection, even with treatment, suggest a need for improved therapy. Unique characteristics of C. neoformans may suggest directions for drug discovery. These include features of three structures that surround the cell: the plasma membrane, the cell wall around it, and the outermost polysaccharide capsule. We review current knowledge of the fundamental biology of these fascinating structures and highlight open questions in the field, with the goal of stimulating further investigation that will advance basic knowledge and human health.

A Call to Arms: Quest for a Cryptococcal Vaccine

Cryptococcosis remains a significant cause of morbidity and mortality world-wide, particularly among AIDS patients. Yet, to date, there are no licensed vaccines clinically available to treat or prevent cryptococcosis. In this review, we provide a rationale to support continued investment in Cryptococcus vaccine research, potential challenges that must be overcome along the way, and a literature review of the current progress underway towards developing a vaccine to prevent cryptococcosis.

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.

Vaccination with Recombinant Cryptococcus Proteins in Glucan Particles Protects Mice against Cryptococcosis in a Manner Dependent upon Mouse Strain and Cryptococcal Species

Development of a vaccine to protect against cryptococcosis is a priority given the enormous global burden of disease in at-risk individuals. Using glucan particles (GPs) as a delivery system, we previously demonstrated that mice vaccinated with crude Cryptococcus-derived alkaline extracts were protected against lethal challenge with Cryptococcus neoformans and Cryptococcus gattii The goal of the present study was to identify protective protein antigens that could be used in a subunit vaccine. Using biased and unbiased approaches, six candidate antigens (Cda1, Cda2, Cda3, Fpd1, MP88, and Sod1) were selected, recombinantly expressed in Escherichia coli, purified, and loaded into GPs. Three mouse strains (C57BL/6, BALB/c, and DR4) were then vaccinated with the antigen-laden GPs, following which they received a pulmonary challenge with virulent C. neoformans and C. gattii strains. Four candidate vaccines (GP-Cda1, GP-Cda2, GP-Cda3, and GP-Sod1) afforded a significant survival advantage in at least one mouse model; some vaccine combinations provided added protection over that seen with either antigen alone. Vaccine-mediated protection against C. neoformans did not necessarily predict protection against C. gattii Vaccinated mice developed pulmonary inflammatory responses that effectively contained the infection; many surviving mice developed sterilizing immunity. Predicted T helper cell epitopes differed between mouse strains and in the degree to which they matched epitopes predicted in humans. Thus, we have discovered cryptococcal proteins that make promising candidate vaccine antigens. Protection varied depending on the mouse strain and cryptococcal species, suggesting that a successful human subunit vaccine will need to contain multiple antigens, including ones that are species specific.IMPORTANCE The encapsulated fungi Cryptococcus neoformans and Cryptococcus gattii are responsible for nearly 200,000 deaths annually, mostly in immunocompromised individuals. An effective vaccine could substantially reduce the burden of cryptococcosis. However, a major gap in cryptococcal vaccine development has been the discovery of protective antigens to use in vaccines. Here, six cryptococcal proteins with potential as vaccine antigens were expressed recombinantly and purified. Mice were then vaccinated with glucan particle preparations containing each antigen. Of the six candidate vaccines, four protected mice from a lethal cryptococcal challenge. However, the degree of protection varied as a function of mouse strain and cryptococcal species. These preclinical studies identify cryptococcal proteins that could serve as candidate vaccine antigens and provide a proof of principle regarding the feasibility of protein antigen-based vaccines to protect against cryptococcosis.

Inositol Polyphosphate Kinases, Fungal Virulence and Drug Discovery

Opportunistic fungi are a major cause of morbidity and mortality world-wide, particularly in immunocompromised individuals. Developing new treatments to combat invasive fungal disease is challenging given that fungal and mammalian host cells are eukaryotic, with similar organization and physiology. Even therapies targeting unique fungal cell features have limitations and drug resistance is emerging. New approaches to the development of antifungal drugs are therefore needed urgently. Cryptococcus neoformans, the commonest cause of fungal meningitis worldwide, is an accepted model for studying fungal pathogenicity and driving drug discovery. We recently characterized a phospholipase C (Plc1)-dependent pathway in C. neoformans comprising of sequentially-acting inositol polyphosphate kinases (IPK), which are involved in synthesizing inositol polyphosphates (IP). We also showed that the pathway is essential for fungal cellular function and pathogenicity. The IP products of the pathway are structurally diverse, each consisting of an inositol ring, with phosphate (P) and pyrophosphate (PP) groups covalently attached at different positions. This review focuses on (1) the characterization of the Plc1/IPK pathway in C. neoformans; (2) the identification of PP-IP₅ (IP₇) as the most crucial IP species for fungal fitness and virulence in a mouse model of fungal infection; and (3) why IPK enzymes represent suitable candidates for drug development.

Protection against Experimental Cryptococcosis following Vaccination with Glucan Particles Containing Cryptococcus Alkaline Extracts

A vaccine capable of protecting at-risk persons against infections due to Cryptococcus neoformans and Cryptococcus gattii could reduce the substantial global burden of human cryptococcosis. Vaccine development has been hampered though, by lack of knowledge as to which antigens are immunoprotective and the need for an effective vaccine delivery system. We made alkaline extracts from mutant cryptococcal strains that lacked capsule or chitosan. The extracts were then packaged into glucan particles (GPs), which are purified Saccharomyces cerevisiae cell walls composed primarily of β-1,3-glucans. Subcutaneous vaccination with the GP-based vaccines provided significant protection against subsequent pulmonary infection with highly virulent strains of C. neoformans and C. gattii. The alkaline extract derived from the acapsular strain was analyzed by liquid chromatography tandem mass spectrometry (LC-MS/MS), and the most abundant proteins were identified. Separation of the alkaline extract by size exclusion chromatography revealed fractions that conferred protection when loaded in GP-based vaccines. Robust Th1- and Th17-biased CD4⁺ T cell recall responses were observed in the lungs of vaccinated and infected mice. Thus, our preclinical studies have indicated promising cryptococcal vaccine candidates in alkaline extracts delivered in GPs. Ongoing studies are directed at identifying the individual components of the extracts that confer protection and thus would be promising candidates for a human vaccine.

The encapsulated yeast Cryptococcus neoformans and its closely related sister species, Cryptococcus gattii, are major causes of morbidity and mortality, particularly in immunocompromised persons. This study reports on the preclinical development of vaccines to protect at-risk populations from cryptococcosis. Antigens were extracted from Cryptococcus by treatment with an alkaline solution. The extracted antigens were then packaged into glucan particles, which are hollow yeast cell walls composed mainly of β-glucans. The glucan particle-based vaccines elicited robust T cell immune responses and protected mice from otherwise-lethal challenge with virulent strains of C. neoformans and C. gattii. The technology used for antigen extraction and subsequent loading into the glucan particle delivery system is relatively simple and can be applied to vaccine development against other pathogens.

Fungal Inositol Pyrophosphate IP7 Is Crucial for Metabolic Adaptation to the Host Environment and Pathogenicity

Inositol pyrophosphates (PP-IPs) comprising inositol, phosphate, and pyrophosphate (PP) are essential for multiple functions in eukaryotes. Their role in fungal pathogens has never been addressed. Cryptococcus neoformans is a model pathogenic fungus causing life-threatening meningoencephalitis. We investigate the cryptococcal kinases responsible for the production of PP-IPs (IP₇/IP₈) and the hierarchy of PP-IP importance in pathogenicity. Using gene deletion and inositol polyphosphate profiling, we identified Kcs1 as the major IP₆ kinase (producing IP₇) and Asp1 as an IP₇ kinase (producing IP₈). We show that Kcs1-derived IP₇ is the most crucial PP-IP for cryptococcal drug susceptibility and the production of virulence determinants. In particular, Kcs1 kinase activity is essential for cryptococcal infection of mouse lungs, as reduced fungal burdens were observed in the absence of Kcs1 or when Kcs1 was catalytically inactive. Transcriptome and carbon source utilization analysis suggested that compromised growth of the KCS1 deletion strain (Δkcs1 mutant) in the low-glucose environment of the host lung is due to its inability to utilize alternative carbon sources. Despite this metabolic defect, the Δkcs1 mutant established persistent, low-level asymptomatic pulmonary infection but failed to elicit a strong immune response in vivo and in vitro and was not readily phagocytosed by primary or immortalized monocytes. Reduced recognition of the Δkcs1 cells by monocytes correlated with reduced exposure of mannoproteins on the Δkcs1 mutant cell surface. We conclude that IP₇ is essential for fungal metabolic adaptation to the host environment, immune recognition, and pathogenicity.

Cryptococcus neoformans is responsible for 1 million cases of AIDS-associated meningitis and ~600,000 deaths annually. Understanding cellular pathways responsible for pathogenicity might have an impact on new drug development. We characterized the inositol polyphosphate kinases Kcs1 and Asp1, which are predicted to catalyze the production of inositol pyrophosphates containing one or two diphosphate moieties (PP-IPs). Using gene deletion analysis and inositol polyphosphate profiling, we confirmed that Kcs1 and Asp1 are major IP₆ and IP₇ kinases, respectively. Kcs1-derived IP₇, but not Asp1-derived IP₈, is crucial for pathogenicity. Global expression profiling and carbon source utilization testing suggest that IP₇-deficient cryptococci cannot adapt their metabolism to allow growth in the glucose-poor environment of the host lung, and consequently, fungal burdens are significantly reduced. Persistent asymptomatic Δkcs1 mutant infection correlated with decreased mannoprotein exposure on the Δkcs1 mutant surface and reduced phagocytosis. We conclude that IP₇ is crucial for the metabolic adaptation of C. neoformans to the host environment and for pathogenicity.

Exploiting fungal cell wall components in vaccines

Innate recognition of fungi leads to strong adaptive immunity. Investigators are trying to exploit this observation in vaccine development by combining antigens with evolutionarily conserved fungal cell wall carbohydrates to induce protective responses. Best studied is β-1,3-glucan, a glycan that activates complement and is recognized by dectin-1. Administration of antigens in association with β-1,3-glucan, either by direct conjugation or complexed in glucan particles, results in robust humoral and cellular immune responses. While the host has a host of mannose receptors, responses to fungal mannoproteins generally are amplified if cells are cooperatively stimulated with an additional danger signal such as a toll-like receptor agonist. Chitosan, a polycationic homopolymer of glucosamine manufactured by the deacetylation of chitin, is being studied as an adjuvant in DNA and protein-based vaccines. It appears particularly promising in mucosal vaccines. Finally, universal and organism-specific fungal vaccines have been formulated by conjugating fungal cell wall glycans to carrier proteins. A major challenge will be to advance these experimental findings so that at risk patients can be protected.

Mannoprotein MP84 mediates the adhesion of Cryptococcus neoformans to epithelial lung cells

The capsule is the most important virulence factor of the fungal pathogen Cryptococcus neoformans. This structure consists of highly hydrated polysaccharides, including glucuronoxylomannan (GXM), and galactoxylomannan (GalXM). It is also composed of mannoproteins (MPs) which corresponds to less than 1% of the capsular weight. Despite MPs being the minority and least studied components, four of these molecules with molecular masses of 115, 98, 88, and 84 kDa were identified and characterized as C. neoformans immunoreactive antigens involved in the pathogenesis, and are potential cryptococcosis vaccine candidates. With the aim to describe the adhesive property of MPs, we cloned and expressed the MP84, a mannoprotein with molecular weight of 84 kDa, on Pichia pastoris yeast, and performed interaction assays of C. neoformans with epithelial lung cells, in the presence or absence of capsule components. Two fungal strains, the wild type, NE-241, and a mutant, CAP67, deficient in GXM production, were used throughout this study. The adhesion assays were completed using epithelial lung cells, A549, and human prostate cancer cells, PC3, as a control. We observed that capsulated wild type (NE-241), and acapsular (CAP67) strains adhered significantly to A549 cells, compared with PC3 cells (p < 0.05). GXM inhibits the NE-241 adhesion, but not the CAP67. In contrast, CAP67 adhesion was only inhibited in the presence of MP84. These results demonstrate the involvement of MP in the adhesion of C. neoformans to epithelial lung cells. We conclude that this interaction possibly involves an adhesion-like interaction between MP on the fungal surface and the complementary receptor molecules on the epithelial cells.

Vaccine-mediated immune responses to experimental pulmonary Cryptococcus gattii infection in mice

Cryptococcus gattii is a fungal pathogen that can cause life-threatening respiratory and disseminated infections in immune-competent and immune-suppressed individuals. Currently, there are no standardized vaccines against cryptococcosis in humans, underlying an urgent need for effective therapies and/or vaccines. In this study, we evaluated the efficacy of intranasal immunization with C. gattii cell wall associated (CW) and/or cytoplasmic (CP) protein preparations to induce protection against experimental pulmonary C. gattii infection in mice. BALB/c mice immunized with C. gattii CW and/or CP protein preparations exhibited a significant reduction in pulmonary fungal burden and prolonged survival following pulmonary challenge with C. gattii. Protection was associated with significantly increased pro-inflammatory and Th1-type cytokine recall responses, in vitro and increased C. gattii-specific antibody production in immunized mice challenged with C. gattii. A number of immunodominant proteins were identified following immunoblot analysis of C. gattii CW and CP protein preparations using sera from immunized mice. Immunization with a combined CW and CP protein preparation resulted in an early increase in pulmonary T cell infiltrates following challenge with C. gattii. Overall, our studies show that C. gattii CW and CP protein preparations contain antigens that may be included in a subunit vaccine to induce prolonged protection against pulmonary C. gattii infection.

Cryptococcus antigens and immune responses: implications for a vaccine

Cryptococcosis is a fungal disease primarily occurring in immunocompromised individuals, such as AIDS patients, and is associated with high morbidity and mortality. However, cryptococcosis can occur within immunocompetent populations as observed during an outbreak in Vancouver Island, British Columbia, Canada, the Pacific Northwest and other regions of the USA and in Mediterranean Europe. Mortality rates due to cryptococcosis have significantly declined in economically developed countries since the widespread implementation of highly active antiretroviral therapy. However, the incidence and mortality of this disease remains high in economically undeveloped areas in Africa and Asia where HIV infections are high and availability of HAART is limited. The continuing AIDS epidemic coupled with the increased usage of immunosuppressive drugs to prevent organ transplant rejection or to treat autoimmune diseases has resulted in an increase in individuals at risk for developing cryptococcosis. The purpose of this review is to discuss the need, challenges and potential for developing vaccines against cryptococcosis.

Cryptococcus neoformans Rim101 is associated with cell wall remodeling and evasion of the host immune responses

Infectious microorganisms often play a role in modulating the immune responses of their infected hosts. We demonstrate that Cryptococcus neoformans signals through the Rim101 transcription factor to regulate cell wall composition and the host-pathogen interface. In the absence of Rim101, C. neoformans exhibits an altered cell surface in response to host signals, generating an excessive and ineffective immune response that results in accelerated host death. This host immune response to the rim101Δ mutant strain is characterized by increased neutrophil influx into the infected lungs and an altered pattern of host cytokine expression compared to the response to wild-type cryptococcal infection. To identify genes associated with the observed phenotypes, we performed whole-genome RNA sequencing experiments under capsule-inducing conditions. We defined the downstream regulon of the Rim101 transcription factor and determined potential cell wall processes involved in the capsule attachment defects and altered mechanisms of virulence in the rim101Δ mutant. The cell wall generates structural stability for the cell and allows the attachment of surface molecules such as capsule polysaccharides. In turn, the capsule provides an effective mask for the immunogenic cell wall, shielding it from recognition by the host immune system.

Cryptococcus neoformans is an opportunistic human pathogen that is a significant cause of death in immunocompromised individuals. There are two major causes of death due to this pathogen: meningitis due to uncontrolled fungal proliferation in the brain in the face of a weakened immune system and immune reconstitution inflammatory syndrome characterized by an overactive immune response to subclinical levels of the pathogen. In this study, we examined how C. neoformans uses the conserved Rim101 transcription factor to specifically remodel the host-pathogen interface, thus regulating the host immune response. These studies explored the complex ways in which successful microbial pathogens induce phenotypes that ensure their own survival while simultaneously controlling the nature and degree of the associated host response.

The Cryptococcus neoformans capsule: a sword and a shield

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

Vaccine and immunotherapeutic approaches for the prevention of cryptococcosis: lessons learned from animal models

Cryptococcus neoformans and C. gattii, the predominant etiological agents of cryptococcosis, can cause life-threatening infections of the central nervous system in immunocompromised and immunocompetent individuals. Cryptococcal meningoencephalitis is the most common disseminated fungal infection in AIDS patients, and C. neoformans remains the third most common invasive fungal infection among organ transplant recipients. Current anti-fungal drug therapies are oftentimes rendered ineffective due to drug toxicity, the emergence of drug resistant organisms, and/or the inability of the host's immune defenses to assist in eradication of the yeast. Therefore, there remains an urgent need for the development of immune-based therapies and/or vaccines to combat cryptococcosis. Studies in animal models have demonstrated the efficacy of various vaccination strategies and immune therapies to induce protection against cryptococcosis. This review will summarize the lessons learned from animal models supporting the feasibility of developing immunotherapeutics and vaccines to prevent cryptococcosis.

Vaccine potential of cytosolic proteins loaded fibrin microspheres of Cryptococcus neoformans in BALB/c mice

Cryptococcosis is a leading mycological cause of mortality among immunologically compromised individuals. In order to develop an effective vaccine against Cryptococcus neoformans, the cytosolic proteins (Cp) of the pathogen have been used as an antigen in combination with different formulations. In the present study, we have demonstrated that Cp encapsulated poly-lactide co-glycolide (PLGA) microsphere further co-encapsulated into the biocompatible fibrin cross-linked plasma beads (Fib-PLGA-Cp) mediated cytosolic delivery elicited strong immune response in the BALB/c mice. In contrast, other formulations of Cp failed to impart significant level of protection. The immune response, involved with Fib-PLGA-Cp protection, appear to interact with the target cells by both endocytosis as well as membrane fusion mode, thus helping in the activation of both CD4(+) and CD8(+) T-cells. Analysis of cytokine profiles in immunized animals revealed that the protective response was associated with the Th1/Th2 polarization in favor of type-1 cytokine [interferons (IFN)-γ and interleukin (IL)-2] cells. Furthermore, vaccination with Fib-PLGA-Cp elicited high immunoglobulin (Ig) G(l) and IgG(2a) isotype response; successfully cleared fungal burden in vital organs and also increased the survival rate of immunized animals. Altogether the present study is a clear indicative of the possible use of fibrin microsphere-based targeted delivery of cytosolic proteins to induce protective immune responses against experimental murine cryptococcosis.

Differential PbP27 expression in the yeast and mycelial forms of the Paracoccidioides brasiliensis species complex

p27 is an antigenic protein produced by Paracoccidioides brasiliensis, the etiologic agent of paracoccidioidomycosis (PCM). Despite its unknown function, it has been suggested as a putative virulence factor, proposed as a suitable target for the design of diagnostic tools and vaccines, and considered as an enhancer in antifungal treatment of PCM. We evaluated sequence polymorphisms of PbP27 gene sequence among isolates, finding some polymorphisms associated with the isolates' phylogenetic origin. In order to determine if there was a differential expression pattern between morphological states and among isolates, we also evaluated PbP27 expression, at transcriptional and translational levels, in mycelia and yeast cultures in 14 isolates belonging to the P. brasiliensis species complex (S1, PS2, PS3, and "Pb01-like", proposed to be named Paracoccidioides lutzii) by two techniques, real time RT-PCR (RT-qPCR) and protein dot blot. For the latter, four protein extracts from different cell localizations (SDS or β-mercaptoethanol, cytoplasmic and extracellular proteins) were analyzed for each isolate. p27 was present in the four extracts evaluated, mainly in the SDS extract, corresponding to an extract containing proteins loosely attached to the cell wall. This information correlates with immunohistochemical analysis, where positive staining of the yeasts' cell wall was observed. We found that p27 was present in all isolates, mainly in the yeast form. This pattern was corroborated by RT-qPCR results, with higher expression levels found in the yeast form for most of the isolates. The results provide new insights into the expression patterns of this protein, and further characterize it in view of potential uses as a diagnostic and/or therapeutic tool.

Searching for Moniliophthora perniciosa pathogenicity genes

The basidiomycete Moniliophthora perniciosa is the causal agent of witches' broom disease of Theobroma cacao (cacao). Pathogenesis mechanisms of this hemibiotrophic fungus are largely unknown. An approach to identify putative pathogenicity genes is searching for sequences induced in mycelia grown under in vitro conditions. Using this approach, genes from M. perniciosa induced under limiting nitrogen and light were identified from a cDNA library enriched by suppression subtractive hybridization as potential putative pathogenicity genes. From the 159 identified unique sequences, 59 were annotated and classified by gene ontology. Two sequences were categorized as "Defence genes, Virulence, and Cell response" presumably coding for allergenic proteins, whose homologues from other fungi are inducers of animal or plant defences. Differential gene expression was evaluated by quantitative amplification of reversed transcripts (RT-qPCR) of the putative identified genes coding for the two allergenic proteins (Aspf13 and 88KD), and for the enzymes Arylsulfatase (AS); Aryl-Alcohol Oxidase; Aldo-Keto Reductase (AK); Cytochrome P450 (P450); Phenylalanine Ammonia-Lyase; and Peroxidase from mycelia grown under contrasting N concentrations. All genes were validated for differential expression, except for the putative Peroxidase. The same eight genes were analysed for expression in susceptible plants inoculated with M. perniciosa, and six were induced during the early asymptomatic stage of the disease. In infected host tissues, transcripts of 88KD and AS were found more abundant at the biotrophic phase, while those from Aspf13, AK, PAL, and P450 accumulated at the necrotrophic phase, enabling to suggest that mycelia transition from biotrophic to necrotrophic might occur earlier than currently considered. These sequences appeared to be virulence life-style genes, which encode factors or enzymes that enable invasion, colonization or intracellular survival, or manipulate host factors to benefit the pathogen's own survival in the hostile environment.

Interaction of Cryptococcus neoformans Rim101 and protein kinase A regulates capsule

Cryptococcus neoformans is a prevalent human fungal pathogen that must survive within various tissues in order to establish a human infection. We have identified the C. neoformans Rim101 transcription factor, a highly conserved pH-response regulator in many fungal species. The rim101Δ mutant strain displays growth defects similar to other fungal species in the presence of alkaline pH, increased salt concentrations, and iron limitation. However, the rim101Δ strain is also characterized by a striking defect in capsule, an important virulence-associated phenotype. This capsular defect is likely due to alterations in polysaccharide attachment to the cell surface, not in polysaccharide biosynthesis. In contrast to many other C. neoformans capsule-defective strains, the rim101Δ mutant is hypervirulent in animal models of cryptococcosis. Whereas Rim101 activation in other fungal species occurs through the conserved Rim pathway, we demonstrate that C. neoformans Rim101 is also activated by the cAMP/PKA pathway. We report here that C. neoformans uses PKA and the Rim pathway to regulate the localization, activation, and processing of the Rim101 transcription factor. We also demonstrate specific host-relevant activating conditions for Rim101 cleavage, showing that C. neoformans has co-opted conserved signaling pathways to respond to the specific niche within the infected host. These results establish a novel mechanism for Rim101 activation and the integration of two conserved signaling cascades in response to host environmental conditions.

Cryptococcus neoformans is an environmental fungus and an opportunistic human pathogen. Survival of this fungus within a human host depends on its ability to sense the host environment and respond with protective cellular changes. It is known that the cAMP/PKA signal transduction cascade is important for sensing host-specific environments and regulating the cellular adaptations, such as capsule and increased iron uptake, that are necessary for growth inside the infected host. Here we document that, unlike what has been described in other fungal species, a C. neoformans Rim101 homologue is directly regulated by PKA. The Rim101 signaling pathway is also involved in capsule regulation and virulence. Our study demonstrates that Rim101 integrates two conserved signal transduction cascades, and it is important in regulating microbial pathogenesis.

Insights into the mechanisms of protective immunity against Cryptococcus neoformans infection using a mouse model of pulmonary cryptococcosis

Cryptococcus neoformans is an opportunistic fungal pathogen that causes life-threatening pneumonia and meningoencephalitis in immune compromised individuals. Previous studies have shown that immunization of BALB/c mice with an IFN-gamma-producing C. neoformans strain, H99gamma, results in complete protection against a second pulmonary challenge with an otherwise lethal cryptococcal strain. The current study evaluated local anamnestic cell-mediated immune responses against pulmonary cryptococcosis in mice immunized with C. neoformans strain H99gamma compared to mice immunized with heat-killed C. neoformans (HKC.n.). Mice immunized with C. neoformans strain H99gamma had significantly reduced pulmonary fungal burden post-secondary challenge compared to mice immunized with HKC.n. Protection against pulmonary cryptococcosis was associated with increased pulmonary granulomatous formation and leukocyte infiltration followed by a rapid resolution of pulmonary inflammation, which protected the lungs from severe allergic bronchopulmonary mycosis (ABPM)-pathology that developed in the lungs of mice immunized with HKC.n. Pulmonary challenge of interleukin (IL)-4 receptor, IL-12p40, IL-12p35, IFN-gamma, T cell and B cell deficient mice with C. neoformans strain H99gamma demonstrated a requirement for Th1-type T cell-mediated immunity, but not B cell-mediated immunity, for the induction of H99gamma-mediated protective immune responses against pulmonary C. neoformans infection. CD4(+) T cells, CD11c(+) cells, and Gr-1(+) cells were increased in both proportion and absolute number in protected mice. In addition, significantly increased production of Th1-type/pro-inflammatory cytokines and chemokines, and conversely, reduced Th2-type cytokine production was observed in the lungs of protected mice. Interestingly, protection was not associated with increased production of cytokines IFN-gamma or TNF-alpha in lungs of protected mice. In conclusion, immunization with C. neoformans strain H99gamma results in the development of protective anti-cryptococcal immune responses that may be measured and subsequently used in the development of immune-based therapies to combat pulmonary cryptococcosis.

A proteomic-based approach for the identification of immunodominant Cryptococcus neoformans proteins

Cryptococcus neoformans is an opportunistic fungal pathogen that can cause life-threatening meningoencephalitis in immune compromised patients. Previous, studies in our laboratory have shown that prior exposure to an IFN-gamma-producing C. neoformans strain (H99gamma) elicits protective immunity against a second pulmonary C. neoformans challenge. Here, we characterized the antibody response produced in mice protected against experimental pulmonary C. neoformans infection compared to nonprotected mice. Moreover, we evaluated the efficacy of using serum antibody from protected mice to detect immunodominant C. neoformans proteins. Protected mice were shown to produce significantly more C. neoformans-specific antibodies following a second experimental pulmonary cryptococcal challenge compared to nonprotected mice. Immunoblot analysis of C. neoformans proteins resolved by 2-DE using serum from nonprotected mice failed to show any reactivity. In contrast, serum from protected mice was reactive with several cryptococcal protein spots. Analysis of these spots by capillary HPLC-ESI-MS/MS identified several cryptococcal proteins shown to be associated with the pathogenesis of cryptococcosis. Our studies demonstrate that mice immunized with C. neoformans strain H99gamma produce antibodies that are immune reactive against specific cryptococcal proteins that may provide a basis for the development of immune based therapies that induce protective anticryptococcal immune responses.

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.

Surface-associated plasminogen binding of Cryptococcus neoformans promotes extracellular matrix invasion

Background

The fungal pathogen Cryptococcus neoformans is a leading cause of illness and death in persons with predisposing factors, including: malignancies, solid organ transplants, and corticosteroid use. C. neoformans is ubiquitous in the environment and enters into the lungs via inhalation, where it can disseminate through the bloodstream and penetrate the central nervous system (CNS), resulting in a difficult to treat and often-fatal infection of the brain, called meningoencephalitis. Plasminogen is a highly abundant protein found in the plasma component of blood and is necessary for the degradation of fibrin, collagen, and other structural components of tissues. This fibrinolytic system is utilized by cancer cells during metastasis and several pathogenic species of bacteria have been found to manipulate the host plasminogen system to facilitate invasion of tissues during infection by modifying the activation of this process through the binding of plasminogen at their surface.

Methodology

The invasion of the brain and the central nervous system by penetration of the protective blood-brain barrier is a prerequisite to the establishment of meningoencephalitis by the opportunistic fungal pathogen C. neoformans. In this study, we examined the ability of C. neoformans to subvert the host plasminogen system to facilitate tissue barrier invasion. Through a combination of biochemical, cell biology, and proteomic approaches, we have shown that C. neoformans utilizes the host plasminogen system to cross tissue barriers, providing support for the hypothesis that plasminogen-binding may contribute to the invasion of the blood-brain barrier by penetration of the brain endothelial cells and underlying matrix. In addition, we have identified the cell wall-associated proteins that serve as plasminogen receptors and characterized both the plasminogen-binding and plasmin-activation potential for this significant human pathogen.

Conclusions

The results of this study provide evidence for the cooperative role of multiple virulence determinants in C. neoformans pathogenesis and suggest new avenues for the development of anti-infective agents in the prevention of fungal tissue invasion.

Virulence in Cryptococcus species

Cryptococcus neoformans and Cryptococcus gattii are the cause of life-threatening meningoencephalitis in immunocompromised and immunocompetent individuals respectively. The increasing incidence of cryptococcal infection as a result of the AIDS epidemic, the recent emergence of a hypervirulent cryptococcal strain in Canada and the fact that mortality from cryptococcal disease remains high have stimulated intensive research into this organism. Here we outline recent advances in our understanding of C. neoformans and C. gattii, including intraspecific complexity, virulence factors, and key signaling pathways. We discuss the molecular basis of cryptococcal virulence and the interaction between these pathogens and the host immune system. Finally, we discuss future challenges in the study and treatment of cryptococcosis.

Isolation and purification of antigenic components of Cryptococcus

The encapsulated fungal pathogens Cryptococcus neoformans and Cryptococcus gattii are significant agents of life-threatening infections, particularly in persons with suppressed cell-mediated immunity. This chapter provides detailed methodology for the purification of two of the major antigen fractions of C. neoformans: glucuronoxylomannan (GXM) and mannoprotein (MP). GXM is the primary component of the polysaccharide capsule, which is the major cryptococcal virulence factor. In contrast, MPs have been identified as key antigens that stimulate T-cell responses. Purification of GXM and MP should assist investigators studying the antigenic, biochemical, and virulence properties of Cryptococcus species.

Loss of allergen 1 confers a hypervirulent phenotype that resembles mucoid switch variants of Cryptococcus neoformans

Microbial survival in a host is usually dependent on the ability of a pathogen to undergo changes that promote escape from host defense mechanisms. The human-pathogenic fungus Cryptococcus neoformans undergoes phenotypic switching in vivo that promotes persistence in tissue. By microarray and real-time PCR analyses, the allergen 1 gene (ALL1) was found to be downregulated in the hypervirulent mucoid switch variant, both during logarithmic growth and during intracellular growth in macrophages. The ALL1 gene encodes a small cytoplasmic protein that is involved in capsule formation. Growth of an all1Delta gene deletion mutant was normal. Similar to cells of the mucoid switch variant, all1Delta cells produced a larger polysaccharide capsule than cells of the smooth parent and the complemented strain produced, and the enlarged capsule inhibited macrophage phagocytosis. The mutant exhibited a modest defect in capsule induction compared to all of the other variants. In animal models the phenotype of the all1Delta mutant mimicked the hypervirulent phenotype of the mucoid switch variant, which is characterized by decreased host survival and elevated intracranial pressure. Decreased survival is likely the result of both an ineffective cell-mediated immune response and impaired phagocytosis by macrophages. Consequently, we concluded that, unlike loss of most virulence-associated genes, where loss of gene function results in attenuated virulence, loss of the ALL1 gene enhances virulence by altering the host-pathogen interaction and thereby impairing clearance. Our data identified the first cryptococcal gene associated with elevated intracranial pressure and support the hypothesis that an environmental opportunistic pathogen has modified its virulence in vivo by epigenetic downregulation of gene function.

Transcriptional regulation by protein kinase A in Cryptococcus neoformans

A defect in the PKA1 gene encoding the catalytic subunit of cyclic adenosine 5'-monophosphate (cAMP)-dependent protein kinase A (PKA) is known to reduce capsule size and attenuate virulence in the fungal pathogen Cryptococcus neoformans. Conversely, loss of the PKA regulatory subunit encoded by pkr1 results in overproduction of capsule and hypervirulence. We compared the transcriptomes between the pka1 and pkr1 mutants and a wild-type strain, and found that PKA influences transcript levels for genes involved in cell wall synthesis, transport functions such as iron uptake, the tricarboxylic acid cycle, and glycolysis. Among the myriad of transcriptional changes in the mutants, we also identified differential expression of ribosomal protein genes, genes encoding stress and chaperone functions, and genes for secretory pathway components and phospholipid synthesis. The transcriptional influence of PKA on these functions was reminiscent of the linkage between transcription, endoplasmic reticulum stress, and the unfolded protein response in Saccharomyces cerevisiae. Functional analyses confirmed that the PKA mutants have a differential response to temperature stress, caffeine, and lithium, and that secretion inhibitors block capsule production. Importantly, we also found that lithium treatment limits capsule size, thus reinforcing potential connections between this virulence trait and inositol and phospholipid metabolism. In addition, deletion of a PKA-regulated gene, OVA1, revealed an epistatic relationship with pka1 in the control of capsule size and melanin formation. OVA1 encodes a putative phosphatidylethanolamine-binding protein that appears to negatively influence capsule production and melanin accumulation. Overall, these findings support a role for PKA in regulating the delivery of virulence factors such as the capsular polysaccharide to the cell surface and serve to highlight the importance of secretion and phospholipid metabolism as potential targets for anti-cryptococcal therapy.