Capsule structure changes associated with Cryptococcus neoformans crossing of the blood-brain barrier

Survival in macrophages induces enhanced virulence in Cryptococcus

Cryptococcus is a ubiquitous environmental fungus and frequent colonizer of human lungs. Colonization can lead to diverse outcomes, from clearance to long-term colonization to life-threatening meningoencephalitis. Regardless of the outcome, the process starts with an encounter with phagocytes. Using the zebrafish model of this infection, we have noted that cryptococcal cells first spend time inside macrophages before they become capable of pathogenic replication and dissemination. What "licensing" process takes place during this initial encounter, and how are licensed cryptococcal cells different? To address this, we isolated cryptococcal cells after phagocytosis by cultured macrophages and found these macrophage-experienced cells to be markedly more virulent in both zebrafish and mouse models. Despite producing a thick polysaccharide capsule, they were still subject to phagocytosis by macrophages in the zebrafish. Analysis of antigenic cell wall components in these licensed cells demonstrated that components of mannose and chitin are more available for staining than they are in culture-grown cells or cells with capsule production induced in vitro. Cryptococcus is capable of exiting or transferring between macrophages in vitro, raising the likelihood that this fungus alternates between intracellular and extracellular life during growth in the lungs. Our results raise the possibility that intracellular life has its advantages over time, and phagocytosis-induced alteration in mannose and chitin exposure is one way that makes subsequent rounds of phagocytosis more beneficial to the fungus.IMPORTANCECryptococcosis begins in the lungs and can ultimately travel through the bloodstream to cause devastating infection in the central nervous system. In the zebrafish model, small amounts of cryptococcus inoculated into the bloodstream are initially phagocytosed and become far more capable of dissemination after they exit macrophages. Similarly, survival in the mouse lung produces cryptococcal cell types with enhanced dissemination. In this study, we have evaluated how phagocytosis changes the properties of Cryptococcus during pathogenesis. Macrophage-experienced cells (MECs) become "licensed" for enhanced virulence. They out-disseminate culture-grown cells in the fish and out-compete non-MECs in the mouse lung. Analysis of their cell surface demonstrates that MECs have increased availability of cell wall components mannose and chitin substances involved in provoking phagocytosis. These findings suggest how Cryptococcus might tune its cell surface to induce but survive repeated phagocytosis during early pathogenesis in the lung.

Similar evolutionary trajectories in an environmental Cryptococcus neoformans isolate after human and murine infection

A pet cockatoo was the suspected source of Cryptococcus neoformans recovered from an immunocompromised patient with cryptococcosis based on molecular analyses available in 2000. Here, we report whole genome sequence analysis of the clinical and cockatoo strains. Both are closely related MATα strains belonging to the VNII lineage, confirming that the human infection likely originated from pet bird exposure. The two strains differ by 61 single nucleotide polymorphisms, including eight nonsynonymous changes involving seven genes. To ascertain whether changes in these genes are selected for during mammalian infection, we passaged the cockatoo strain in mice. Remarkably, isolates obtained from mouse tissue possess a frameshift mutation in one of the seven genes altered in the human sample (LQVO5_000317), a gene predicted to encode an SWI-SNF chromatin-remodeling complex protein. In addition, both cockatoo and patient strains as well as mouse-passaged isolates obtained from brain tissue had a premature stop codon in a homologue of ZFC3 (LQVO5_004463), a predicted single-zinc finger containing protein, which is associated with larger capsules when deleted and reverted to a full-length protein in the mouse-passaged isolates obtained from lung tissue. The patient strain and mouse-passaged isolates show variability in virulence factors, with differences in capsule size, melanization, rates of nonlytic expulsion from macrophages, and amoeba predation resistance. Our results establish that environmental strains undergo genomic and phenotypic changes during mammalian passage, suggesting that animal virulence can be a mechanism for genetic change and that the genomes of clinical isolates may provide a readout of mutations acquired during infection.

RTA1 Is Involved in Resistance to 7-Aminocholesterol and Secretion of Fungal Proteins in Cryptococcus neoformans

Cryptococcus neoformans (Cn) is a pathogenic yeast that is the leading cause of fungal meningitis in immunocompromised patients. Various Cn virulence factors, such as the enzyme laccase and its product melanin, phospholipase, and capsular polysaccharide have been identified. During a screen of knockout mutants, the gene resistance to aminocholesterol 1 (RTA1) was identified, the function of which is currently unknown in Cn. Rta1 homologs in S. cerevisiae belong to a lipid-translocating exporter family of fungal proteins with transmembrane regions and confer resistance to the antimicrobial agent 7-aminocholesterol when overexpressed. To determine the role of RTA1 in Cn, the knock-out (rta1Δ) and reconstituted (rta1Δ+RTA1) strains were created and phenotypically tested. RTA1 was involved in resistance to 7-aminocholesterol, and also in exocyst complex component 3 (Sec6)-mediated secretion of urease, laccase, and the major capsule component, glucuronoxylomannan (GXM), which coincided with significantly smaller capsules in the rta1Δ and rta1Δ+RTA1 strains compared to the wild-type H99 strain. Furthermore, RTA1 expression was reduced in a secretory 14 mutant (sec14Δ) and increased in an RNAi Sec6 mutant. Transmission electron microscopy demonstrated vesicle accumulation inside the rta1Δ strain, predominantly near the cell membrane. Given that Rta1 is likely to be a transmembrane protein located at the plasma membrane, these data suggest that Rta1 may be involved in both secretion of various fungal virulence factors and resistance to 7-aminocholesterol in Cn.

A dissemination-prone morphotype enhances extrapulmonary organ entry by Cryptococcus neoformans

Environmental pathogens move from ecological niches to mammalian hosts, requiring adaptation to dramatically different environments. Microbes that disseminate farther, including the fungal meningitis pathogen Cryptococcus neoformans, require additional adaptation to diverse tissues. We demonstrate that the formation of a small C. neoformans morphotype-called "seed" cells due to their colonizing ability-is critical for extrapulmonary organ entry. Seed cells exhibit changes in fungal cell size and surface expression that result in an enhanced macrophage update. Seed cell formation is triggered by environmental factors, including C. neoformans' environmental niche, and pigeon guano with phosphate plays a central role. Seed cells show the enhanced expression of phosphate acquisition genes, and mutants unable to acquire phosphate fail to adopt the seed cell morphotype. Additionally, phosphate can be released by tissue damage, potentially establishing a feed-forward loop of seed cell formation and dissemination. Thus, C. neoformans' size variation represent inducible morphotypes that change host interactions to facilitate microbe spread.

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

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

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.

IL-9 plays a protective role on host defense against the infection of Cryptococcus Neoformans

Cryptococcus neoformans is an opportunistic fungal pathogen that causes neurological disease in immunocompromised patients. Preliminary experiments showed that cryptococcal strains could induce the expression of interleukin-9 (IL-9). The use of a neutralizing antibody against IL-9 decreased the survival rates of mice in a murine model. In this study, we found that in vitro, IL-9 could enhance the phagocytic function of M1 macrophages and promote the killing of extracellular pathogens by had no effect on the killing of invading pathogens. IL-9 could also promote the expression of IL-6 while suppressing the expression of TNF-α in M1 macrophages. In vivo, IL-9 reduced the colony-forming units (CFUs) in the brain and liver, but there were no differences in the lung. Furthermore, the weight of mice in the IL-9 group decreased slower than that of mice in the phosphate-buffered saline (PBS) group after infection. Moreover, IL-9 could enhance the survival rate at 21 days. The results also showed that IL-9 could promote the secretion of IL-17 while blocking the secretion of IL-4. Therefore, we concluded that IL-9 plays a protective role in C. neoformans infection.

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.

The Combination of Iron and Copper Increases Pathogenicity and Induces Proteins Related to the Main Virulence Factors in Clinical Isolates of Cryptococcus neoformans var. grubii

In fungi, metals are associated with the expression of virulence factors. However, it is unclear whether the uptake of metals affects their pathogenicity. This study aimed to evaluate the effect of iron/copper in modulating pathogenicity and proteomic response in two clinical isolates of C. neoformans with high and low pathogenicity. Methods: In both isolates, the effect of 50 µM iron and 500 µM copper on pathogenicity, capsule induction, and melanin production was evaluated. We then performed a quantitative proteomic analysis of cytoplasmic extracts exposed to that combination. Finally, the effect on pathogenicity by iron and copper was evaluated in eight additional isolates. Results: In both isolates, the combination of iron and copper increased pathogenicity, capsule size, and melanin production. Regarding proteomic data, proteins with increased levels after iron and copper exposure were related to biological processes such as cell stress, vesicular traffic (Ap1, Vps35), cell wall structure (Och1, Ccr4, Gsk3), melanin biosynthesis (Hem15, Mln2), DNA repair (Chk1), protein transport (Mms2), SUMOylation (Uba2), and mitochondrial transport (Atm1). Increased pathogenicity by exposure to metal combination was also confirmed in 90% of the eight isolates. Conclusions: The combination of these metals enhances pathogenicity and increases the abundance of proteins related to the main virulence factors.

Reimagining the future of African brain health: Perspectives for basic research on the pathogenesis of cryptococcal meningitis

Cryptococcal meningitis is a fatal opportunistic infection of the brain and a leading cause of neurological damage and death in immunocompromised individuals. This neglected fungal disease of the brain is a huge burden on the health systems of developing countries, especially in Sub-Saharan Africa, where up to 25% of people living with HIV/AIDS succumb to it. Cryptococcal fungal cells have a predilection for the brain and they are capable of traversing the blood brain barrier and invade the brain where they cause infection, inflammation and a disruption of normal brain function. A robust host neuroimmune response is critical for pathogen clearance and survival, and a good understanding of the mechanisms underlying its development in the host is critical for the development of effective treatments. However, past basic research studies have been focussed on the characteristics of the fungus and its effect on the peripheral immune system; with little attention paid to how it interacts with brain immune cells. This mini review briefly discusses the paucity of basic research data on the neuroimmune response to cryptococcal infection, raises pertinent questions on how the brain cells respond to the fungal infection, and thereafter discusses models, techniques and advanced technologies that could be useful for carrying out high-throughput research on the pathogenesis of cryptococcal meningitis.

Phenotypic Plasticity in the Productions of Virulence Factors Within and Among Serotypes in the Cryptococcus neoformans Species Complex

The Cryptococcus neoformans species complex (CNSC) is a common opportunistic human fungal pathogen and the most frequent cause of fungal meningitis. There are three major serotypes in CNSC: A, D, and their hybrids AD, and they have different geographic distributions and medical significance. Melanin pigment and a polysaccharide capsule are the two major virulence factors in CNSC. However, the relationships between serotype and virulence factor production and how environmental factors might impact their relationships are not known. This study investigated the expressions of melanin and capsular polysaccharide in a genetically diverse group of CNSC strains and how their phenotypic expressions were influenced by oxidative and nitrosative stress levels. We found significant differences in melanin and capsular polysaccharide productions among serotypes and across stress conditions. Under oxidative stress, the laboratory hybrids exhibited the highest phenotypic plasticity for melanin production while serotype A showed the highest for capsular polysaccharide production. In contrast, serotype D exhibited the highest phenotypic plasticity for capsular polysaccharide production and clinical serotype AD the highest phenotypic plasticity for melanin production under nitrosative stress. These results demonstrated that different serotypes have different environmental condition-specific mechanisms to modulate the expression of virulence factors.

A mutation in C. neoformans mitochondrial NADH dehydrogenase results in increased virulence in mice

Cryptococcus neoformans: (H99W) was serially passaged in the invertebrate wax moth Galleria mellonella fifteen times to study how fungal virulence evolves under selection and whether those adaptations affect virulence. The G. mellonella passaged strain (P15) and the pre-passage H99W strains were used to infect three different host models of C. neoformans: C. elegans, G. mellonella, and Balb/c mice. While there was no difference in survival in the invertebrate models, P15 killed mice faster than H99W through both intratracheal and intravenous routes of infection and mice infected intravenously with P15 showed higher fungal burden in the brain. Characterization of the major virulence factors of C. neoformans found that P15 had increased capsule size, GXM release, and melanization. Whole genome sequencing of P15 and H99W revealed two mutations in P15, an insertion in the promoter region of NADH dehydrogenase (CNAG_09000) and an insertion in the LMP1 gene (CNAG_06765). Both ATP production and metabolic rate were higher in P15 compared to H99W. Quantitative RT-PCR suggested that the increased ATP was due to increased RNA levels of NADH dehydrogenase. Thus, adaptation to growth in hemocytes resulted in increased production of ATP, increased metabolic rate, and increased virulence in mice. This was likely due to differential expression of virulence factors, which skewed the host immune response to a less efficient Th2 response, with higher levels of IL-4, IL-10, and TNF-α in the brain. Overall, serial passage experiments have increased our understanding of how this yeast evolves under innate immune selection pressure.

Vph1 is associated with the copper homeostasis of Cryptococcus neoformans serotype D

We clarified the roles of VPH1 in Cryptococcus neoformans serotype D by examining the detailed phenotypes of VPH1-deficient cells (Δvph1) in terms of their capability to grow in acidic and alkaline pH, at a high temperature, and under high osmotic conditions, in addition to the involvement of VPH1 in copper (Cu) homeostasis and the expression of some C. neoformans virulence factors. Δvph1 could grow well on minimal medium (YNB) but exhibited hypersensitivity to 20 μM Cu due to the failure to induce Cu-detoxifying metallothionein genes (CMT1 and CMT2). In contrast, Δvph1 exhibited defective growth on rich medium (YPD), and the induction of Cu transporter genes (CTR1 and CTR4) did not occur in this medium, implying that this strain was incapable of the uptake of Cu ions for growth. However, the addition of excess Cu promoted CTR gene expression and supported Δvph1 growth. These results suggested that the lack of the VPH1 gene disturbed Cu homeostasis in C. neoformans. Moreover, the loss of Vph1 function influenced the urease activity of C. neoformans.

The Environmental Effects on Virulence Factors and the Antifungal Susceptibility of Cryptococcus neoformans

Cryptococcus neoformans is a facultative intracellular pathogen responsible for fungal meningoencephalitis primarily in immunocompromised individuals. It has become evident the pathogenicity of C. neoformans is dependent on the fungal cell’s environment. The differential expression of virulence factors, based on the cell’s environmental conditions, is one mechanism allowing for the environmental control of the pathogenic ability of C. neoformans. Here, we discuss how these virulence factors (including melanin, the polysaccharide capsule, and Antiphagocytic protein 1) have been shown to be differentially expressed dependent on the cell’s environment. The genetics and signaling pathways leading to the environmental-dependent regulation of virulence factors will also be examined. Susceptibility to antifungal therapeutics is also regulated by the environment, and thus affects the pathogenic abilities of C. neoformans and disease outcomes. This review will also examine the role of the C. neoformans’s environment on antifungal susceptibilities, and the genetics and signaling pathways responsible for these susceptibility alterations. By examining the complex interplay between the environment and the pathogenicity of C. neoformans, we have a better understanding of the intricacies of the pathogen–environment interaction and how to exploit this interaction to develop the most effective treatment protocols.

Quantitative analysis reveals internalisation of Cryptococcus neoformans by brain endothelial cells in vivo

Migration of Cryptococcus neoformans from the blood to the brain parenchyma is crucial to cause fatal meningoencephalitis. Although mechanisms involved in brain migration of C. neoformans have been widely studied in vitro, less is known about how the fungus crosses the blood-brain barrier (BBB) in vivo. This is in part because of the lack of an approach to quantitatively analyse the dynamics of fungal transmigration into the brain across the BBB in vivo. In this study, we report a novel approach to quantitatively analyse the interactions between C. neoformans and brain endothelial cells in a mouse model using flow cytometry. Using this system, we show that C. neoformans was internalised by brain endothelial cells in vivo and that mice infected with acapsular or heat-killed C. neoformans yeast cells displayed a lower frequency of brain endothelial cells containing the yeast cell compared to mice infected with wild-type or viable yeast cells, respectively. We further demonstrate that brain endothelial cells were invaded by serotype A strain (H99 strain) at a higher rate compared to serotype D strain (52D strain). Our experiments established that internalisation of C. neoformans by brain endothelial cells occurred in vivo and offered a powerful approach to quantitatively analyse fungal migration into the brain.

The interplay of phenotype and genotype in Cryptococcus neoformans disease

Cryptococcus neoformans is an opportunistic fungal pathogen that causes life-threatening meningitis primarily in immunocompromised individuals. In order to survive and proliferate during infection, C. neoformans must adapt to a variety of stresses it encounters within the host. Patient outcome depends on the interaction between the pathogen and the host. Understanding the mechanisms that C. neoformans uses to facilitate adaptation to the host and promote pathogenesis is necessary to better predict disease severity and establish proper treatment. Several virulence phenotypes have been characterized in C. neoformans, but the field still lacks a complete understanding of how genotype and phenotype contribute to clinical outcome. Furthermore, while it is known that C. neoformans genotype impacts patient outcome, the mechanisms remain unknown. This lack of understanding may be due to the genetic heterogeneity of C. neoformans and the extensive phenotypic variation observed between and within isolates during infection. In this review, we summarize the current understanding of how the various genotypes and phenotypes observed in C. neoformans correlate with human disease progression in the context of patient outcome and recurrence. We also postulate the mechanisms underlying the genetic and phenotypic changes that occur in vivo to promote rapid adaptation in the host.

Serial Passage of Cryptococcus neoformans in Galleria mellonella Results in Increased Capsule and Intracellular Replication in Hemocytes, but Not Increased Resistance to Hydrogen Peroxide

To gain insight into how pathogens adapt to new hosts, Cryptococcus neoformans (H99W) was serially passaged in Galleria mellonella. The phenotypic characteristics of the passaged strain (P15) and H99W were evaluated. P15 grew faster in hemolymph than H99W, in vitro and in vivo, suggesting that adaptation had occurred. However, P15 was more susceptible to hydrogen peroxide in vitro, killed fewer mouse macrophages, and had less fungal burden in human ex vivo macrophages than H99W. Analysis of gene expression changes during Galleria infection showed only a few different genes involved in the reactive oxygen species response. As P15 sheds more GXM than H99W, P15 may have adapted by downregulating hemocyte hydrogen peroxide production, possibly through increased capsular glucuronoxylomannan (GXM) shedding. Hemocytes infected with P15 produced less hydrogen peroxide, and hydrogen peroxide production in response to GXM-shedding mutants was correlated with shed GXM. Histopathological examination of infected larvae showed increased numbers and sizes of immune nodules for P15 compared to H99W, suggesting an enhanced, but functionally defective, response to P15. These results could explain why this infection model does not always correlate with murine models. Overall, C. neoformans’ serial passage in G. mellonella resulted in a better understanding of how this yeast evolves under selection.

The Role of Testosterone and Gibberellic Acid in the Melanization of Cryptococcus neoformans

Cryptococcus neoformans, a spore-producing pathogenic yeast, affects immunocompromised individuals causing meningoencephalitis. Once C. neoformans is introduced via the respiratory tract, it is engulfed by macrophages and other phagocytes. One of C. neoformans's primary virulence factors is the pigment melanin, which is formed in the cell wall and protects the yeast against UV radiation and oxidizing agents produced by macrophages during phagocytosis. To better understand the observed sex bias (3:1; male:female) in C. neoformans infections, the phenotype of various virulence factors was determined in the presence of exogenous sex hormones. C. neoformans melanized faster in the presence of testosterone than it did in the presence of estradiol. Using a combination of RNA sequencing analysis and ELISA results, we identified a growth hormone, gibberellic acid (GA), produced in C. neoformans that was highly upregulated in the presence of testosterone. A variety of knockout strains of genes involved in the GA biosynthesis pathway showed significantly reduced melanization in the presence of testosterone. Additionally, inhibitors of GA also reduced melanization in the presence of testosterone. Thus, these data suggest that the gibberellic biosynthesis pathway is involved in melanization in C. neoformans, and the melanization difference observed in the presence of testosterone may be due to increased production of GA, which may partly explain the sex bias observed in C. neoformans infections.

A novel method for identifying and distinguishing Cryptococcus neoformans and Cryptococcus gattii by surface-enhanced Raman scattering using positively charged silver nanoparticles

There are approximately 1 million cryptococcal infections per year among HIV+ individuals, resulting in nearly 625,000 deaths. Cryptococcus neoformans and Cryptococcus gattii are the two most common species that cause human cryptococcosis. These two species of Cryptococcus have differences in pathogenicity, diagnosis, and treatment. Cryptococcal infections are usually difficult to identify because of their slow growth in vitro. In addition, the long detection cycle of Cryptococcus in clinical specimens makes the diagnosis of Cryptococcal infections difficult. Here, we used positively charged silver nanoparticles (AgNPs⁺) as a substrate to distinguish between C. neoformans and C. gattii in clinical specimens directly via surface-enhanced Raman scattering (SERS) and spectral analysis. The AgNPs⁺ self-assembled on the surface of the fungal cell wall via electrostatic aggregation, leading to enhanced SERS signals that were better than the standard substrate negatively charged silver nanoparticles (AgNPs). The SERS spectra could also be used as a sample database in the multivariate analysis via orthogonal partial least-squares discriminant analysis. This novel SERS detection method can clearly distinguish between the two Cryptococcus species using principal component analysis. The accuracy of the training data and test data was 100% after a tenfold crossover validation.

An Antivirulence Approach for Preventing Cryptococcus neoformans from Crossing the Blood-Brain Barrier via Novel Natural Product Inhibitors of a Fungal Metalloprotease

Fungal infections like cryptococcal meningitis are difficult to resolve because of the limited therapies available. The small arsenal of antifungal drugs reflect the difficulty in finding available targets in fungi because like mammalian cells, fungi are eukaryotes. The limited efficacy, toxicity, and rising resistance of antifungals contribute to the high morbidity and mortality of fungal infections and further underscore the dire but unmet need for new antifungal drugs. The traditional approach in antifungal drug development has been to target fungal growth, but an attractive alternative is to target mechanisms of pathogenesis. An important attribute of Cryptococcus neoformans (Cn) pathogenesis is its ability to enter the central nervous system. Here, we describe a large-scale screen that identified three natural products that prevented Cn from crossing the blood-brain barrier by inhibiting the virulence factor Mpr1 without affecting the growth of Cn. We propose that compounds identified here could be further developed as antivirulence therapy that would be administered preemptively or serve as a prophylactic in patients at high risk for developing cryptococcal meningitis.

Cryptococcus neoformans (Cn) is the leading cause of fungal meningitis, a deadly disease with limited therapeutic options. Dissemination to the central nervous system hinges on the ability of Cn to breach the blood-brain barrier (BBB) and is considered an attribute of Cn virulence. Targeting virulence instead of growth for antifungal drug development has not been fully exploited despite the benefits of this approach. Mpr1 is a secreted fungal metalloprotease not required for fungal growth, but rather, it functions as a virulence factor by facilitating Cn migration across the BBB. This central role for Mpr1, its extracellular location, and lack of expression in mammalian cells make Mpr1 a high-value target for an antivirulence approach aimed at developing therapeutics for cryptococcal meningitis. To test this notion, we devised a large-scale screen to identify compounds that prohibited Cn from crossing the BBB by selectively blocking Mpr1 proteolytic activity, without inhibiting the growth of Cn. A phytochemical natural product-derived library was screened to identify new molecular scaffolds of prototypes unique to a Cn microecosystem. Of the 240 pure natural products examined, 3 lead compounds, abietic acid, diosgenin, and lupinine inhibited Mpr1 proteolytic activity with 50% inhibitory concentration (IC₅₀) values of <10 μM, displayed little to no mammalian cell toxicity, and did not affect Cn growth. Notably, the lead compounds blocked Cn from crossing the BBB, without damaging the barrier integrity, suggesting the bioactive molecules had no off-target effects. We propose that these new drug scaffolds are promising candidates for the development of antivirulence therapy against cryptococcal meningitis.

Choroid plexus and the blood-cerebrospinal fluid barrier in disease

The choroid plexus (CP) forming the blood-cerebrospinal fluid (B-CSF) barrier is among the least studied structures of the central nervous system (CNS) despite its clinical importance. The CP is an epithelio-endothelial convolute comprising a highly vascularized stroma with fenestrated capillaries and a continuous lining of epithelial cells joined by apical tight junctions (TJs) that are crucial in forming the B-CSF barrier. Integrity of the CP is critical for maintaining brain homeostasis and B-CSF barrier permeability. Recent experimental and clinical research has uncovered the significance of the CP in the pathophysiology of various diseases affecting the CNS. The CP is involved in penetration of various pathogens into the CNS, as well as the development of neurodegenerative (e.g., Alzheimer´s disease) and autoimmune diseases (e.g., multiple sclerosis). Moreover, the CP was shown to be important for restoring brain homeostasis following stroke and trauma. In addition, new diagnostic methods and treatment of CP papilloma and carcinoma have recently been developed. This review describes and summarizes the current state of knowledge with regard to the roles of the CP and B-CSF barrier in the pathophysiology of various types of CNS diseases and sets up the foundation for further avenues of research.

Variation in Cell Surface Hydrophobicity among Cryptococcus neoformans Strains Influences Interactions with Amoebas

Cryptococcus neoformans and Cryptococcus gattii are pathogenic fungi that cause significant morbidity and mortality. Cell surface hydrophobicity (CSH) is a biophysical parameter that influences the adhesion of fungal cells or spores to biotic and abiotic surfaces. C. neoformans is encased by polysaccharide capsule that is highly hydrophilic and is a critical determinant of virulence. In this study, we report large differences in the CSH of some C. neoformans and C. gattii strains. The capsular polysaccharides of C. neoformans strains differ in repeating motifs and therefore vary in the number of hydroxyl groups, which, along with higher-order structure of the capsule, may contribute to the variation in hydrophobicity that we observed. We found that cell wall composition, in the context of chitin-chitosan content, does not influence CSH. For C. neoformans, CSH correlated with phagocytosis by natural soil predator Acanthamoeba castellanii Furthermore, capsular binding of the protective antibody (18B7), but not the nonprotective antibody (13F1), altered the CSH of C. neoformans strains. Variability in CSH could be an important characteristic in comparing the biological properties of cryptococcal strains.IMPORTANCE The interaction of a microbial cell with its environment is influenced by the biophysical properties of a cell. The affinity of the cell surface for water, defined by the cell surface hydrophobicity (CSH), is a biophysical parameter that varies among different strains of Cryptococcus neoformans The CSH influences the phagocytosis of the yeast by its natural predator in the soil, the amoeba. Studying variation in biophysical properties like CSH gives us insight into the dynamic host-predator interaction and host-pathogen interaction in a damage-response framework.

Basic principles of the virulence of Cryptococcus

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

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

Vomocytosis: Too Much Booze, Base, or Calcium?

Macrophages are well known for their phagocytic activity and their role in innate immune responses. Macrophages eat non-self particles, via a variety of mechanisms, and typically break down internalized cargo into small macromolecules. However, some pathogenic agents have the ability to evade this endosomal degradation through a nonlytic exocytosis process termed vomocytosis.

Macrophages are well known for their phagocytic activity and their role in innate immune responses. Macrophages eat non-self particles, via a variety of mechanisms, and typically break down internalized cargo into small macromolecules. However, some pathogenic agents have the ability to evade this endosomal degradation through a nonlytic exocytosis process termed vomocytosis. This phenomenon has been most often studied for Cryptococcus neoformans, a yeast that causes roughly 180,000 deaths per year, primarily in immunocompromised (e.g., human immunodeficiency virus [HIV]) patients. Existing dogma purports that vomocytosis involves distinctive cellular pathways and intracellular physicochemical cues in the host cell during phagosomal maturation. Moreover, it has been observed that the immunological state of the individual and macrophage phenotype affect vomocytosis outcomes. Here we compile the current knowledge on the factors (with respect to the phagocytic cell) that promote vomocytosis of C. neoformans from macrophages.

New Insights Into Cryptococcus Spp. Biology and Cryptococcal Meningitis

PURPOSE OF REVIEW

Defective cell-mediated immunity is a major risk factor for cryptococcosis, a fatal disease if untreated. Cryptococcal meningitis (CM), the main presentation of disseminated disease, occurs through hematogenous spread to the brain from primary pulmonary foci, facilitated by yeast virulence factors. We revisit remarkable recent improvements in the prevention, diagnosis and management of CM.

RECENT FINDINGS

Cryptococcal antigen (CrAg), main capsular polysaccharide of Cryptococcus spp. is detectable in blood and cerebrospinal fluid of infected patients with point of care lateral flow assays. Recent World Health Organization guidelines recommend 7-day amphotericin B plus flucytosine, then 7-day high dose (1200 mg/day) fluconazole for induction treatment of HIV-associated CM. Management of raised intracranial pressure, a consequence of CM, should rely mainly on daily therapeutic lumbar punctures until normalisation. In HIV-associated CM, following introduction of antifungal therapy, (re)initiation of antiretroviral therapy should be delayed by 4-6 weeks to prevent immune reconstitution inflammatory syndrome, common in CM. CM is a fatal disease whose diagnosis has recently been simplified. Treatment should always include antifungal combination therapy and management of raised intracranial pressure. Screening for immune deficiency should be mandatory in all patients with cryptococcosis.

Nonlytic exocytosis of Cryptococcus neoformans from neutrophils in the brain vasculature

Background

Cryptococcus neoformans (C. neoformans) is an encapsulated budding yeast that causes life-threatening meningoencephalitis in immunocompromised individuals, especially those with acquired immunodeficiency syndrome (AIDS). To cause meningoencephalitis, C. neoformans circulating in the bloodstream must first be arrested in the brain microvasculature. Neutrophils, the most abundant phagocytes in the bloodstream and the first leukocytes to be recruited to an infection site, can ingest C. neoformans. Little is known about how neutrophils interact with arrested fungal cells in the brain microvasculature.

Methods

A blood-brain barrier (BBB) in vitro model was established. The interactions between neutrophils adhering to brain endothelial cells and fungi were observed under a live cell imaging microscope. A flow cytometry assay was developed to explore the mechanisms. Immunofluorescence staining of brain tissues was utilized to validate the in vitro phenomena.

Results

Using real-time imaging, we observed that neutrophils adhered to a monolayer of mouse brain endothelial cells could expel ingested C. neoformans without lysis of the neutrophils or fungi in vitro, demonstrating nonlytic exocytosis of fungal cells from neutrophils. Furthermore, nonlytic exocytosis of C. neoformans from neutrophils was influenced by either the fungus (capsule and viability) or the neutrophil (phagosomal pH and actin polymerization). Moreover, nonlytic exocytosis of C. neoformans from neutrophils was recorded in brain tissue.

Conclusion

These results highlight a novel function by which neutrophils extrude C. neoformans in the brain vasculature.

Graphical abstract

Electronic supplementary material

The online version of this article (10.1186/s12964-019-0429-0) contains supplementary material, which is available to authorized users.

Rheological properties of cryptococcal polysaccharide change with fiber size, antibody binding and temperature

Aim: Cryptococcus neoformans is the major agent of cryptococcosis. The main virulence factor is the polysaccharide (PS) capsule. Changes in cryptococcal PS properties have been poorly elucidated. Materials & methods: We analyzed the mechanical properties of secreted PS and intact capsules, using dynamic light scattering and optical tweezers. Results: Storage and loss moduli showed that secreted PS behaves as a viscoelastic liquid, while capsular PS behaves as a viscoelastic solid. The secreted PS remains as a viscoelastic fluid at different temperatures with thermal hysteresis after 85°C. Antibody binding altered the viscoelastic behavior of both secreted and capsular PS. Conclusion: Deciphering the mechanical aspects of these structures could reveal features that may have consequences in novel therapies against cryptococcosis.

Toxoflavin Produced by Burkholderia gladioli from Lycoris aurea Is a New Broad-Spectrum Fungicide

Fungal infections not only cause extensive agricultural damage but also result in serious diseases in the immunodeficient populations of human beings. Moreover, the increasing emergence of drug resistance has led to a decrease in the efficacy of current antifungals. Thus, screening of new antifungal agents is imperative in the fight against antifungal drug resistance. In this study, we show that an endophytic bacterium, Burkholderia gladioli HDXY-02, isolated from the medicinal plant Lycoris aurea, showed broad-spectrum antifungal activity against plant and human fungal pathogens. An antifungal ability assay indicated that the bioactive component was produced from strain HDXY-02 having an extracellular secreted component with a molecular weight lower than 1,000 Da. In addition, we found that this new antifungal could be produced effectively by liquid fermentation of HDXY-02. Furthermore, the purified component contributing to the antifungal activity was identified to be toxoflavin, a yellow compound possessing a pyrimido[5,4-e][1,2,4]triazine ring. In vitro bioactivity studies demonstrated that purified toxoflavin from B. gladioli HDXY-02 cultures had a significant antifungal activity against the human fungal pathogen Aspergillus fumigatus, resulting in abolished germination of conidia. More importantly, the growth inhibition by toxoflavin was observed in both wild-type and drug-resistant mutants (cyp51A and non-cyp51A) of A. fumigatus Finally, an optimized protocol for the large-scale production of toxoflavin (1,533 mg/liter) has been developed. Taken together, our findings provide a promising biosynthetic resource for producing a new antifungal reagent, toxoflavin, from isolates of the endophytic bacterium B. gladioli IMPORTANCE Human fungal infections are a growing problem associated with increased morbidity and mortality. Moreover, a growing number of antifungal-resistant fungal isolates have been reported over the past decade. Thus, the need for novel antifungal agents is imperative. In this study, we show that an endophytic bacterium, Burkholderia gladioli, isolated from the medicinal plant Lycoris aurea, is able to abundantly secrete a compound, toxoflavin, which has a strong fungicidal activity not only against plant fungal pathogens but also against human fungal pathogens Aspergillus fumigatus and Candida albicans, Cryptococcus neoformans, and the model filamentous fungus Aspergillus nidulans More importantly, toxoflavin also displays an efficacious inhibitory effect against azole antifungal-resistant mutants of A. fumigatus Consequently, our findings provide a promising approach to abundantly produce toxoflavin, which has novel broad-spectrum antifungal activity, especially against those currently problematic drug-resistant isolates.

A Glucuronoxylomannan Epitope Exhibits Serotype-Specific Accessibility and Redistributes towards the Capsule Surface during Titanization of the Fungal Pathogen Cryptococcus neoformans

Disseminated infections with the fungal species Cryptococcus neoformans or, less frequently, Cryptococcus gattii are an important cause of mortality in immunocompromised individuals. Central to the virulence of both species is an elaborate polysaccharide capsule that consists predominantly of glucuronoxylomannan (GXM). Due to its abundance, GXM is an ideal target for host antibodies, and several monoclonal antibodies (mAbs) have previously been derived using purified GXM or whole capsular preparations as antigens. In addition to their application in the diagnosis of cryptococcosis, anti-GXM mAbs are invaluable tools for studying capsule structure. In this study, we report the production and characterization of a novel anti-GXM mAb, Crp127, that unexpectedly reveals a role for GXM remodeling during the process of fungal titanization. We show that Crp127 recognizes a GXM epitope in an O-acetylation-dependent, but xylosylation-independent, manner. The epitope is differentially expressed by the four main serotypes of Cryptococcus neoformans and C. gattii, is heterogeneously expressed within clonal populations of C. gattii serotype B strains, and is typically confined to the central region of the enlarged capsule. Uniquely, however, this epitope redistributes to the capsular surface in titan cells, a recently characterized morphotype where haploid 5-μm cells convert to highly polyploid cells of >10 μm with distinct but poorly understood capsular characteristics. Titan cells are produced in the host lung and critical for successful infection. Crp127 therefore advances our understanding of cryptococcal morphological change and may hold significant potential as a tool to differentially identify cryptococcal strains and subtypes.

The Buoyancy of Cryptococcus neoformans Is Affected by Capsule Size

Cryptococcus neoformans is an environmental pathogenic fungus with a worldwide geographical distribution that is responsible for hundreds of thousands of human cryptococcosis cases each year. During infection, the yeast undergoes a morphological transformation involving capsular enlargement that increases microbial volume. To understand the factors that play a role in environmental dispersal of C. neoformans and C. gattii, we evaluated the cell density of Cryptococcus using Percoll isopycnic gradients. We found differences in the cell densities of strains belonging to C. neoformans and C. gattii species complexes. The buoyancy of C. neoformans strains varied depending on growth medium. In minimal medium, the cryptococcal capsule made a major contribution to the cell density such that cells with larger capsules had lower density than those with smaller capsules. Removing the capsule, by chemical or mechanical methods, increased the C. neoformans cell density and reduced buoyancy. Melanization of the C. neoformans cell wall, which also contributes to virulence, produced a small but consistent increase in cell density. Encapsulated C. neoformans sedimented much more slowly in seawater as its density approached the density of water. Our results suggest a new function for the capsule whereby it can function as a flotation device to facilitate transport and dispersion in aqueous fluids.IMPORTANCE The buoyancy of a microbial cell is an important physical characteristic that may affect its transportability in fluids and interactions with tissues during infection. The polysaccharide capsule surrounding C. neoformans is required for infection and dissemination in the host. Our results indicate that the capsule has a significant effect on reducing cryptococcal cell density, altering its sedimentation in seawater. Modulation of microbial cell density via encapsulation may facilitate dispersal for other important encapsulated pathogens.

A chemical genetic screen reveals a role for proteostasis in capsule and biofilm formation by Cryptococcus neoformans

Pathogenic microorganisms employ specialized virulence factors to cause disease. Biofilm formation and the production of a polysaccharide capsule are two important virulence factors in Cryptococcus neoformans, the fungal pathogen that causes meningoencephalitis. Here, we show that the bipolar disorder drug lithium inhibits formation of both virulence factors by a mechanism involving dysregulation of the ubiquitin/proteasome system. By using a chemical genetics approach and bioinformatic analyses, we describe the cellular landscape affected by lithium treatment. We demonstrate that lithium affects many different pathways in C. neoformans, including the cAMP/protein kinase A, inositol biosynthesis, and ubiquitin/proteasome pathways. By analyzing mutants with defects in the ubiquitin/proteasome system, we uncover a role for proteostasis in both capsule and biofilm formation. Moreover, we demonstrate an additive influence of lithium and the proteasome inhibitor bortezomib in inhibiting capsule production, thus establishing a link between lithium activity and the proteasome system. Finally, we show that the lithium-mimetic drug ebselen potently blocks capsule and biofilm formation, and has additive activity with lithium or bortezomib. In summary, our results illuminate the impact of lithium on C. neoformans, and link dysregulation of the proteasome to capsule and biofilm inhibition in this important fungal pathogen.

Hybrids and hybridization in the Cryptococcus neoformans and Cryptococcus gattii species complexes

The basidiomycetous yeasts of the Cryptococcus neoformans and Cryptococcus gattii species complexes (CNSC and CGSC respectively) are the causative agents of cryptococcosis, a set of life-threatening diseases affecting the central nervous system, lungs, skin, and other body sites of humans and other mammals. Both the CNSC and CGSC can be subdivided into varieties, serotypes, molecular types, and lineages based on structural variations, molecular characteristics and genetic sequences. Hybridization between the haploid lineages within and between the two species complexes is known to occur in natural and clinical settings, giving rise to intraspecific and interspecific diploid/aneuploid hybrid strains. Since their initial discovery in 1977, cryptococcal hybrids have been increasingly discovered in both clinical and environmental settings with over 30% of all cryptococcal infections in some regions of Europe being caused by hybrid strains. This review summarizes the major findings to date on cryptococcal hybrids, including their possible origins, prevalence, genomic profiles and phenotypic characteristics. Our analyses suggest that CNSC and CGSC can be an excellent model system for studying fungal hybridization.

The structure-function analysis of the Mpr1 metalloprotease determinants of activity during migration of fungal cells across the blood-brain barrier

Cryptococcal meningoencephalitis, the most common form of cryptococcosis, is caused by the opportunistic fungal pathogen, Cryptococcus neoformans. Molecular strategies used by C. neoformans to invade the central nervous system (CNS) have been the focus of several studies. Recently, the role of a novel secreted metalloprotease (Mpr1) in the pathogenicity of C. neoformans was confirmed by studies demonstrating that Mpr1 mediated the migration of fungal cells into the CNS. Given this central function, the aim here was to identify the molecular determinants of Mpr1 activity and resolve their role in the migration of cryptococci across the blood-brain barrier (BBB). The Mpr1 protein belongs to an understudied group of metalloproteases of the M36 class of fungalysins unique to fungi. They are generally synthesized as propeptides with fairly long prodomains and highly conserved regions within their catalytic core. Through structure-function analysis of Mpr1, our study identified the prodomain cleavage sites of Mpr1 and demonstrated that when mutated, the prodomain appears to remain attached to the catalytic C-terminus of Mpr1 rendering a nonfunctional Mpr1 protein and an inability for cryptococci to cross the BBB. We found that proteolytic activity of Mpr1 was dependent on the coordination of zinc with two histidine residues in the active site of Mpr1, since amino acid substitutions in the HExxH motif abolished Mpr1 proteolytic activity and prevented the migration of cryptococci across the BBB. A phylogenetic analysis of Mpr1 revealed a distinct pattern likely reflecting the neurotropic nature of C. neoformans and the specific function of Mpr1 in breaching the BBB. This study contributes to a deeper understanding of the molecular regulation of Mpr1 activity and may lead to the development of specific inhibitors that could be used to restrict fungal penetration of the CNS and thus prevent cryptococcal meningoencephalitis-related deaths.

The capsule of Cryptococcus neoformans

The capsule of Cryptococcus neoformans is its dominant virulence factor and plays a key role in the biology of this fungus. In this essay, we focus on the capsule as a cellular structure and note the limitations inherent in the current methodologies available for its study. Given that no single method can provide the structure of the capsule, our notions of what is the cryptococcal capsule must be arrived at by synthesizing information gathered from very different methodological approaches including microscopy, polysaccharide chemistry and physical chemistry of macromolecules. The emerging picture is one of a carefully regulated dynamic structure that is constantly rearranged as a response to environmental stimulation and cellular replication. In the environment, the capsule protects the fungus against desiccation and phagocytic predators. In animal hosts the capsule functions in both offensive and defensive modes, such that it interferes with immune responses while providing the fungal cell with a defensive shield that is both antiphagocytic and capable of absorbing microbicidal oxidative bursts from phagocytic cells. Finally, we delineate a set of unsolved problems in the cryptococcal capsule field that could provide fertile ground for future investigations.

First report of urease activity in the novel systemic fungal pathogen Emergomyces africanus: a comparison with the neurotrope Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic pathogen responsible for the AIDS-defining illness, cryptococcal meningitis. During the disease process, entry of cryptococcal cells into the brain is facilitated by virulence factors that include urease enzyme activity. A novel species of an Emmonsia-like fungus, recently named Emergomyces africanus, was identified as a cause of disseminated mycosis in HIV-infected persons in South Africa. However, in contrast to C. neoformans, the enzymes produced by this fungus, some of which may be involved in pathogenesis, have not been described. Using a clinical isolate of C. neoformans as a reference, the study aim was to confirm, characterise and quantify urease activity in E. africanus clinical isolates. Urease activity was tested using Christensen's urea agar, after which the presence of a urease gene in the genome of E. africanus was confirmed using gene sequence analysis. Subsequent evaluation of colorimetric enzyme assay data, using Michaelis-Menten enzyme kinetics, revealed similarities between the substrate affinity of the urease enzyme produced by E. africanus (Km ca. 26.0 mM) and that of C. neoformans (Km ca. 20.6 mM). However, the addition of 2.5 g/l urea to the culture medium stimulated urease activity of E. africanus, whereas nutrient limitation notably increased cryptococcal urease activity.

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

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

Crossing the Blood-Brain Barrier by Neuroinvasive Pathogens

The penetration of the blood-brain barrier (BBB) and invasion of the central nervous system (CNS) are important steps for all neuroinvasive pathogens. All of the ways of pathogens passing through the BBB are still unclear. Among known pathways, pathogen traversal can occur paracellularly, transcellularly or using a “Trojan horse” mechanism. The first step of translocation across the BBB is the interactions of the pathogen’s ligands with the receptors of the host brain cells. Lyme disease, the most common vector-borne disease in the temperate zones of Europe and North America, are caused by Borreliella species (former Borrelia burgdorferi sensu lato) that affects the peripheral and the CNS. In this review, we have presented various pathogen interactions with endothelial cells, which allow the disruption of the BBB so that the pathogens can pass across the BBB.

Titan cell production in Cryptococcus neoformans reshapes the cell wall and capsule composition during infection

Cryptococcus neoformans is a human fungal pathogen that often causes infections in immunocompromised individuals. Upon inhalation into the lungs C. neoformans differentiates into cells with altered size and morphology, including production of large titan cells. Titan cells possess thickened cell wall and dense, cross-linked capsule when compared to in vitro grown cells. In addition, titan cells have increased cell wall chitin that is associated with a detrimental anti-inflammatory immune response. Here we examined the cell wall and capsule composition of in vitro, in vivo typical-sized and in vivo titan cells using High Performance Liquid Chromatography (HPLC). The monomer composition of cell wall polysaccharides showed that in vivo C. neoformans cells contained more glucosamine and less glucose than in vitro cells, suggesting alteration in abundance of both chitin and glucans, respectively. Low levels of galactosamine were also detected in carbohydrates from both in vivo and vitro cells. Within the in vivo cell population, differences in the proportions of cell wall and capsule monomers between typical and titan cells were also observed. Taken together, these results demonstrate that C. neoformans reshapes its cell wall and capsule composition during infection. These cell wall and capsule alterations likely help C. neoformans escape recognition by, and allow modulation of, the host immune system.

Mechanisms of Pulmonary Escape and Dissemination by Cryptococcus neoformans

Cryptococcus neoformans is a common environmental saprophyte and human fungal pathogen that primarily causes disease in immunocompromised individuals. Similar to many environmentally acquired human fungal pathogens, C. neoformans initiates infection in the lungs. However, the main driver of mortality is invasive cryptococcosis leading to fungal meningitis. After C. neoformans gains a foothold in the lungs, a critical early step in invasion is transversal of the respiratory epithelium. In this review, we summarize current knowledge relating to pulmonary escape. We focus on fungal factors that allow C. neoformans to disseminate from the lungs via intracellular and extracellular routes.

The blood-brain barrier internalises Cryptococcus neoformans via the EphA2-tyrosine kinase receptor

Cryptococcus neoformans is an opportunistic fungal pathogen that causes life-threatening meningitis most commonly in populations with impaired immunity. Here, we resolved the transcriptome of the human brain endothelium challenged with C. neoformans to establish whether C. neoformans invades the CNS by co-opting particular signalling pathways as a means to promote its own entry. Among the 5 major pathways targeted by C. neoformans, the EPH-EphrinA1 (EphA2) tyrosine kinase receptor-signalling pathway was examined further. Silencing the EphA2 receptor transcript in a human brain endothelial cell line or blocking EphA2 activity with an antibody or chemical inhibitor prevented transmigration of C. neoformans in an in vitro model of the blood-brain barrier (BBB). In contrast, treating brain endothelial cells with an EphA2 chemical agonist or an EphA2 ligand promoted greater migration of fungal cells across the BBB. C. neoformans activated the EPH-tyrosine kinase pathway through a CD44-dependent phosphorylation of EphA2, promoting clustering and internalisation of EphA2 receptors. Moreover, HEK293T cells expressing EphA2 revealed an association between EphA2 and C. neoformans that boosted internalisation of C. neoformans. Collectively, the results suggest that C. neoformans promotes EphA2 activity via CD44, and this in turn creates a permeable barrier that facilitates the migration of C. neoformans across the BBB.

Nutritional Requirements and Their Importance for Virulence of Pathogenic Cryptococcus Species

Cryptococcus sp. are basidiomycete yeasts which can be found widely, free-living in the environment. Interactions with natural predators, such as amoebae in the soil, are thought to have promoted the development of adaptations enabling the organism to survive inside human macrophages. Infection with Cryptococcus in humans occurs following inhalation of desiccated yeast cells or spore particles and may result in fatal meningoencephalitis. Human disease is caused almost exclusively by the Cryptococcus neoformans species complex, which predominantly infects immunocompromised patients, and the Cryptococcus gattii species complex, which is capable of infecting immunocompetent individuals. The nutritional requirements of Cryptococcus are critical for its virulence in animals. Cryptococcus has evolved a broad range of nutrient acquisition strategies, many if not most of which also appear to contribute to its virulence, enabling infection of animal hosts. In this review, we summarise the current understanding of nutritional requirements and acquisition in Cryptococcus and offer perspectives to its evolution as a significant pathogen of humans.

Innate immune evasion strategies against Cryptococcal meningitis caused by Cryptococcus neoformans

As an infectious fungus that affects the respiratory tract, Cryptococcus neoformans (C. neoformans) commonly causes asymptomatic pulmonary infection. C. neoformans may target the brain instead of the lungs and cross the blood-brain barrier (BBB) in the early phase of infection; however, this is dependent on successful evasion of the host innate immune system. During the initial stage of fungal infection, a complex network of innate immune factors are activated. C. neoformans utilizes a number of strategies to overcome the anti-fungal mechanisms of the host innate immune system and cross the BBB. In the present review, the defensive mechanisms of C. neoformans against the innate immune system and its ability to cross the BBB were discussed, with an emphasis on recent insights into the activities of anti-phagocytotic and anti-oxidative factors in C. neoformans.

Trojan Horse Transit Contributes to Blood-Brain Barrier Crossing of a Eukaryotic Pathogen

The blood-brain barrier (BBB) protects the central nervous system (CNS) by restricting the passage of molecules and microorganisms. Despite this barrier, however, the fungal pathogen Cryptococcus neoformans invades the brain, causing a meningoencephalitis that is estimated to kill over 600,000 people annually. Cryptococcal infection begins in the lung, and experimental evidence suggests that host phagocytes play a role in subsequent dissemination, although this role remains ill defined. Additionally, the disparate experimental approaches that have been used to probe various potential routes of BBB transit make it impossible to assess their relative contributions, confounding any integrated understanding of cryptococcal brain entry. Here we used an in vitro model BBB to show that a “Trojan horse” mechanism contributes significantly to fungal barrier crossing and that host factors regulate this process independently of free fungal transit. We also, for the first time, directly imaged C. neoformans-containing phagocytes crossing the BBB, showing that they do so via transendothelial pores. Finally, we found that Trojan horse crossing enables CNS entry of fungal mutants that cannot otherwise traverse the BBB, and we demonstrate additional intercellular interactions that may contribute to brain entry. Our work elucidates the mechanism of cryptococcal brain invasion and offers approaches to study other neuropathogens.

The fungal pathogen Cryptococcus neoformans invades the brain, causing a meningoencephalitis that kills hundreds of thousands of people each year. One route that has been proposed for this brain entry is a Trojan horse mechanism, whereby the fungus crosses the blood-brain barrier (BBB) as a passenger inside host phagocytes. Although indirect experimental evidence supports this intriguing mechanism, it has never been directly visualized. Here we directly image Trojan horse transit and show that it is regulated independently of free fungal entry, contributes to cryptococcal BBB crossing, and allows mutant fungi that cannot enter alone to invade the brain.

Complex Roles of Annexin A2 in Host Blood-Brain Barrier Invasion by Cryptococcus neoformans

INTRODUCTION

Fungal transversal across the brain microvascular endothelial cells (BMECs) is the essential step for the development of cryptococcal meningoencephalitis. Annexin A2 (AnxA2) is an important signaling protein involved in several intracellular processes such as membrane trafficking, endocytosis, and exocytosis.

AIM

To investigate the roles and mechanism of AnxA2 during cryptococcal transversal of BMECs.

RESULTS

Cryptococcus neoformans infection initiated upregulation of AnxA2 in mouse BMECs. Blockade with anti-AnxA2 antibody led to a reduction in fungal transcytosis activity but no change in its adhesion efficiency. Intriguingly, AnxA2 depletion caused a significant increase in fungal association activity but had no effect on their transcytosis. AnxA2 suppression resulted in marked reduction in its partner protein S100A10, and S100A10 suppression in BMECs significantly reduced the cryptococcal transcytosis efficiency. Furthermore, AnxA2 dephosphorylation at Tyr23 and dephosphorylation of downstream cofilin were required for cryptococcal transversal of BMECs, both of which might be primarily involved in the association of C. neoformans with host cells.

CONCLUSIONS

Our work indicated that AnxA2 played complex roles in traversal of C. neoformans across host BMECs, which might be dependent on downstream cofilin to inhibit fungal adhesion but rely on its partner S100A10 to promote cryptococcal transcytosis.