Cryptococcus neoformans resides in an acidic phagolysosome of human macrophages

Glucosylceramide synthase is an essential regulator of pathogenicity of Cryptococcus neoformans

The pathogenic fungus Cryptococcus neoformans infects humans upon inhalation and causes the most common fungal meningoencephalitis in immunocompromised subjects worldwide. In the host, C. neoformans is found both intracellularly and extracellularly, but how these two components contribute to the development of the disease is largely unknown. Here we show that the glycosphingolipid glucosylceramide (GlcCer), which is present in C. neoformans, was essential for fungal growth in host extracellular environments, such as in alveolar spaces and in the bloodstream, which are characterized by a neutral/alkaline pH, but not in the host intracellular environment, such as in the phagolysosome of macrophages, which is characteristically acidic. Indeed, a C. neoformans mutant strain lacking GlcCer did not grow in vitro at a neutral/alkaline pH, yet it had no growth defect at an acidic pH. The mechanism by which GlcCer regulates alkali tolerance was by allowing the transition of C. neoformans through the cell cycle. This study establishes C. neoformans GlcCer as a key virulence factor of cryptococcal pathogenicity, with important implications for future development of new antifungal strategies.

Human macrophages do not require phagosome acidification to mediate fungistatic/fungicidal activity against Histoplasma capsulatum

Histoplasma capsulatum (Hc) is a facultative intracellular fungus that modulates the intraphagosomal environment to survive within macrophages (Mphi). In the present study, we sought to quantify the intraphagosomal pH under conditions in which Hc yeasts replicated or were killed. Human Mphi that had ingested both viable and heat-killed or fixed yeasts maintained an intraphagosomal pH of approximately 6.4-6.5 over a period of several hours. These results were obtained using a fluorescent ratio technique and by electron microscopy using the 3-(2,4-dinitroanilo)-3'-amino-N-methyldipropylamine reagent. Mphi that had ingested Saccharomyces cerevisae, a nonpathogenic yeast that is rapidly killed and degraded by Mphi, also maintained an intraphagosomal pH of approximately 6.5 over a period of several hours. Stimulation of human Mphi fungicidal activity by coculture with chloroquine or by adherence to type 1 collagen matrices was not reversed by bafilomycin, an inhibitor of the vacuolar ATPase. Human Mphi cultured in the presence of bafilomycin also completely degraded heat-killed Hc yeasts, whereas mouse peritoneal Mphi digestion of yeasts was completely reversed in the presence of bafilomycin. However, bafilomycin did not inhibit mouse Mphi fungistatic activity induced by IFN-gamma. Thus, human Mphi do not require phagosomal acidification to kill and degrade Hc yeasts, whereas mouse Mphi do require acidification for fungicidal but not fungistatic activity.

Phenotypic switching in a Cryptococcus neoformans variety gattii strain is associated with changes in virulence and promotes dissemination to the central nervous system

This is the first report of a Cryptococcus neoformans var. gattii strain (serotype B) that switches reversibly between its parent mucoid (NP1-MC) colony morphology and a smooth (NP1-SM) colony morphology. Similar to C. neoformans var. grubii and C. neoformans var. neoformans strains, the switch is associated with changes in the polysaccharide capsule and virulence in animal models. In murine infection models, NP1-MC is significantly more virulent than NP1-SM (P < 0.021). In contrast to the serotype A and D strains, the serotype B strain switches in vivo reversibly between both colony morphologies. The polysaccharide of NP1-MC exhibits a thicker capsule, and thus NP1-MC exhibits enhanced intracellular survival in macrophages. Consistent with this finding, switching to the mucoid variant is observed in pulmonary infection with NP1-SM. In contrast, the thin polysaccharide capsule of NP1-SM permits better crossing of the blood-brain barrier. In this regard, only smooth colonies were grown from brain homogenates of NP1-MC-infected mice. Our findings have important implications for the pathogenesis of cryptococcosis and suggest that phenotypic switching affects host-pathogen interactions in the local microenvironment. This altered interaction then selects for specific colony variants to arise in a pathogen population.

Phenotypic switching in Cryptococcus neoformans

Phenotypic switching has been described in serotype A and D strains of Cryptococcus neoformans. It occurs in vivo during chronic infection and is associated with differential gene expression and changes in virulence. The switch involves changes in the polysaccharide capsule and cell wall that affect the yeast's ability to resist phagocytosis. In addition, the phenotypic switch variants elicit qualitatively different inflammatory responses in the host. In animal models of chronic cryptococosis, the immune response of the host ultimately determines which of the switch variants are selected and maintained. The importance of phenotypic switching is further underscored by several findings that are relevant in the setting of human disease. These include the ability of the mucoid colony variant of RC-2 (RC-2 MC) but not the smooth variant (RC-2 SM) to promote increased intracerebral pressure in a rat model of cryptococcal meningitis. Furthermore, chemotherapeutic and immunological antifungal interventions can promote the selection of the RC-2 MC variant during chronic murine infection.

The Aspergillus pH-responsive transcription factor PacC regulates virulence

The ability of a pathogen to adapt to the host environment is usually required for the initiation of disease. Here we have investigated the importance of the Aspergillus nidulans PacC-mediated pH response in the pathogenesis of pulmonary aspergillosis. Using mutational analysis, we demonstrate that, in neutropenic mice, elimination of the A. nidulans pH-responsive transcription factor PacC, blocking the ambient pH signal transduction pathway or prevention of PacC proteolytic processing acutely attenuates virulence. Infections caused by these alkali-sensitive mutants are characterized by limited growth in vivo and a reduction of inflammatory cell infiltration. In stark contrast, constitutive activation of PacC causes increased mortality marked by extensive fungal invasive growth. PacC action is therefore required for, and able to enhance virulence, demonstrating that the A. nidulans pH-responsive transcription factor PacC plays a pivotal role in pulmonary pathogenesis.

Prophylactic role of liposomized chloroquine against murine cryptococcosis less susceptible to fluconazole

PURPOSE

The prophylactic role of liposomized chloroquine (lip-CQ) has been assessed against less susceptible Cryptococcus neoformans infection in murine model.

METHODS

In the current study, we investigated the antifungal activity of lip-CQ against C. neoformans in macrophages cell line (J 774) and murine model. Mice were pretreated with free as well as liposomized formulations of CQ at various doses. The anticryptococcal activity of fluconazole was compared in mice with or without CQ pretreatment. The efficacy of CQ prophylaxis was assessed by survival as well as colony forming units (cfu) in brain and lungs of treated mice.

RESULTS

Fluconazole alone was not found significantly effective against C. neoformans in both in vitro and in vivo studies. However, the antifungal activity of fluconazole increases in chloroquine-pretreated mice. Lip-CQ was found to be more effective in comparison to the same dose of free chloroquine in reducing fungal burden from macrophages in vitro and lungs and brain of C. neoformans infected mice.

CONCLUSIONS

The enhanced prophylactic activity of lip-CQ seems due to rapid uptake of drug-containing liposomes by macrophages. The liposome-mediated accumulation of CQ in macrophages makes the environment unfavorable (alkaline) for the intracellular multiplication of C. neoformans. Moreover, the increased incidence of multi-drug resistance and diversity of pathogenic microorganisms inhibited or killed by CQ makes it the drug of choice for prophylactic therapy.

Mannoproteins from Cryptococcus neoformans promote dendritic cell maturation and activation

Our previous data show that mannoproteins (MPs) from Cryptococcus neoformans are able to induce protective responses against both C. neoformans and Candida albicans. Here we provide evidence that MPs foster maturation and activation of human dendritic cells (DCs). Maturation was evaluated by the ability of MPs to facilitate expression of costimulatory molecules such as CD40, CD86, CD83, and major histocompatibility complex classes I and II and to inhibit receptors such as CD14, CD16, and CD32. Activation of DCs was measured by the capacity of MPs to promote interleukin-12 and tumor necrosis factor alpha secretion. DC-induced maturation and interleukin-12 induction are largely mediated by engagement of mannose receptors and presume MP internalization and degradation. DC activation leads to IkappaBalpha phosphorylation, which is necessary for nuclear factor kappaB transmigration into the nucleus. MP-loaded DCs are efficient stimulators of T cells and show a remarkable capacity to promote CD4 and CD8 proliferation. In conclusion, we have evidenced a novel regulatory role of MPs that promotes their candidacy as a vaccine against fungi.

Forced retraction of spinal root injury enhances activation of p38 MAPK cascade in infiltrating macrophages

Root-rupture injury is a type of preganglionic brachial plexus injury resulting from traction force, where a small section of the spinal root is usually left behind. We have established experimental models of both root-rupture injury with traction force and rhizotomy without traction force in rats and we examined the activation of microglia/ macrophages in both conditions. LGP107 and LGP96, which are rat homologs of lysosome-associated membrane proteins, were most useful as immunohistochemical markers of mononuclear phagocytes. The metabolic activation of macrophages was analyzed by immunohistochemistry with a series of antibodies against tumor necrosis factor-alpha (TNF-alpha), cathepsin B, p38 mitogen-activated protein kinase (MAPK), and mitogen-activated kinase kinase 3 (MKK3). Both root-rupture injury and rhizotomy rapidly induced the aggregation of numerous macrophages from the injured dorsal root to the dorsal funiculus and TNF-alpha was highly expressed by the macrophages in the injured dorsal root at 48 h. Activation of p38 MAPK was preferentially observed in the macrophages at the ruptured dorsal root; however, only slight activation of p38 MAPK was observed at the rhizotomized dorsal root. These findings suggest that traction injury of the spinal root might induce activation of the p38 MAPK cascade and production of TNF-alpha in the infiltrating macrophages, both of which might participate in aggravation of the root injury.

The Nramp orthologue of Cryptococcus neoformans is a pH-dependent transporter of manganese, iron, cobalt and nickel

Cryptococcus neoformans is an important human opportunistic pathogen and a facultative intracellular parasite, particularly in HIV-infected individuals. Little is known about metal ion transport in this organism. C. neoformans encodes a single member of the Nramp (natural resistance-associated macrophage protein) family of bivalent cation transporters, known as Cramp, which we have cloned and expressed in Xenopus laevis oocytes and Spodoptera frugiperda Sf 21 insect cells. Cramp induces saturable transport of a broad range of bivalent transition series cations, including Mn2+, Fe2+, Co2+ and Ni2+. Maximal cation transport occurs at pH 5.5-6.0, consistent with the proton gradient-based energetics of other Nramp orthologues. Mn2+ transport is diminished in the presence of 140 mM Na+, compatible with a Na+ slippage mechanism proposed for the Saccharomyces cerevisiae Nramp orthologue Smf1p. Cramp resembles Smf1p with respect to predicted membrane topology, substrate specificity and pH dependence, but differs in terms of its apparent affinity for Mn2+ and negligible inhibition by Zn2+. Cramp is the first Nramp orthologue from a fungal pathogen to be functionally characterized. Insights afforded by these findings will allow the formulation of new hypotheses regarding the role of metal ions in the pathophysiology of cryptococcosis.

Pharmacokinetics/pharmacodynamics of echinocandins

The novel class of echinocandins represents a milestone in antifungal drug research that has further expanded our therapeutic options. The favorable pharmacokinetic profile of the echinocandins has been elucidated in animal and human studies. The echinocandins are targeted for once-daily dosing and are not metabolized through the cytochrome P450 enzyme system, and they are generally well tolerated due to lack of mechanism-based toxicity. Little is known, however, about the disposition of these compounds in tissues and body fluids and the relationships between dosage, concentrations in the body, and antifungal efficacy in vivo. Many unanswered questions remain, including the importance of the high protein binding and the concentrations of free antifungal agents at target sites. Although recent attempts have been made to ensure the reproducibility of in vitro tests, the clinical usefulness of these tests is still unreliable and their relevance remains controversial. In vitro activity must be correlated with achievable concentrations at the site of infection. As little is known about the relationship between the pharmacokinetics and the pharmacodynamics of the echinocandins, increased incorporation of these principles in experimental and clinical studies is an important objective that will benefit the treatment and prophylaxis of life-threatening invasive fungal infections in immunocompromised patients.

Cryptococcus neoformans var. gattii can exploit Acanthamoeba castellanii for growth

It has recently been proposed that the origin and maintenance of virulence in certain environmental fungi is influenced by their interactions with non-vertebrate hosts such as amoebae and nematodes. In prior studies we have shown that the interactions of the soil amoebae Acanthamoeba castellanii with Cryptococcus neoformans varieties neoformans and grubii resemble those with macrophages. Here we extend those studies to C. neoformans variety gattii and describe quantitative differences in the type and outcome of the interactions observed relative to the other varieties. C. neoformans var. gattii proliferated in the presence of A. castellanii but the interaction was primarily extracellular with a paucity of phagocytic events. Experiments with acapsular cells coated with polysaccharide suggest that differences in the capsule structure may be responsible for the different interactions between cells of varieties neoformans, grubii, and gattii with amoebae. The ability of C. neoformans var. gattii to exploit amoebae indicates that despite major biological differences between C. neoformans varieties, all retain the ability to be pathogenic for A. castellanii.

Cryptococcal meningitis

Cryptococcal meningitis is a common opportunistic infection in AIDS patients, particularly in Southeast Asia and Africa. Cases also occur in patients with other forms of immunosupression and in apparently immunocompetent individuals. Mortality from HIV-associated cryptococcal meningitis remains high (10-30%), even in developed countries, because of the inadequacy of current antifungal drugs and the complication of raised intracranial pressure. In cohorts of HIV-infected patients from sub-Saharan Africa, cryptococcosis has accounted for 13-44% of all deaths. Optimal current therapy is with amphotericin B 0.7-1 mg/kg/day plus flucytosine 100 mg/kg/day for 2 weeks, followed by fluconazole 400 mg/day for 8 weeks and 200 mg/day thereafter. Saline loading reduces amphotericin B nephrotoxicity. If there is no contraindication on CT head scan, repeat lumbar puncture with drainage of cerebrospinal fluid (CSF) is recommended for patients with very raised CSF opening pressure. Expansion of antiretroviral programmes raises the prospect of transforming the long-term prognosis of these patients, provided that they survive the acute phase of the illness. Studies are needed to define more fungicidal drug regimens and to improve the treatment of raised intracranial pressure.

Identification of App1 as a regulator of phagocytosis and virulence of Cryptococcus neoformans

Cryptococcus neoformans is a fungal pathogen that, after inhalation, can disseminate to the brain. Host alveolar macrophages (AMs) represent the first defense against the fungus. Once phagocytosed by AMs, fungal cells are killed by a concerted mechanism, involving the host-cellular response. If the cellular response is impaired, phagocytosis of the fungus may be detrimental for the host, since C. neoformans can grow within macrophages. Here, we identified a novel cryptococcal gene encoding antiphagocytic protein 1 (App1). App1 is a cryptococcal cytoplasmic protein that is secreted extracellularly and found in the serum of infected patients. App1 does not affect melanin production, capsule formation, or growth of C. neoformans. Treatment with recombinant App1 inhibited phagocytosis of fungal cells through a complement-mediated mechanism, and Deltaapp1 mutant is readily phagocytosed by AMs. Interestingly, the Deltaapp1 mutant strain showed a decreased virulence in mice deficient for complement C5 (A/Jcr), but it was hypervirulent in mice deficient for T and NK cells (Tgepsilon26). This study identifies App1 as a novel regulator of virulence for C. neoformans, and it highlights that internalization of fungal cells by AMs increases the dissemination of C. neoformans when the host cellular response is impaired.

Phagocytosis and intracellular fate of Aspergillus fumigatus conidia in alveolar macrophages

Aspergillus fumigatus is the most prevalent airborne fungal pathogen responsible for fatal invasive aspergillosis in immunocompromised patients. Upon arrival in the lung alveolus, conidia of A. fumigatus are phagocytosed by alveolar macrophages, the major phagocytic cells of the lung. Engulfment and intracellular trafficking of A. fumigatus conidia in alveolar macrophages of two different origins, the murine cell line MH-S and human pulmonary alveolar macrophages, were analyzed by electron microscopy and immunofluorescence. Phagocytosis of A. fumigatus conidia required actin polymerization and phosphatidylinositol 3-kinase activity. Fusion of A. fumigatus phagosomes with early and late endosomes was shown by immunolabeling with specific markers for the transferrin receptor, early endosome antigen, and Rab7. Maturation of A. fumigatus phagolysosomes was monitored by using a fixable acidotropic probe, LysoTracker Red DND-99, and an anti-cathepsin D antibody. Bafilomycin A-induced inhibition of lysosomal acidification abolished the conidial killing by the macrophages. These data suggest that the maturation of A. fumigatus phagosomes results from fusion with the compartments of the endocytic pathway and that the killing of conidia depends on phagolysosome acidification. A model for the phagocytosis of A. fumigatus conidia by alveolar macrophages is proposed on the basis of these results.

PKSP-dependent reduction of phagolysosome fusion and intracellular kill of Aspergillus fumigatus conidia by human monocyte-derived macrophages

Previously, we described the isolation of an Aspergillus fumigatus mutant producing non-pigmented conidia, as a result of a defective polyketide synthase gene, pksP (polyketide synthase involved in pigment biosynthesis). The virulence of the pksP mutant was attenuated in a murine animal infection model and its conidia showed enhanced susceptibility towards damage by monocytes in vitro. Because macrophage-mediated killing is critical for host resistance to aspergillosis, the interaction of both grey-green wild-type conidia and white pksP mutant conidia with human monocyte-derived macrophages (MDM) was studied with respect to intracellular processing of ingested conidia. After phagocytosis, the percentage of wild-type conidia residing in an acidic environment was approximately fivefold lower than that observed for non-pigmented pksP mutant conidia. The phagolysosome formation, as assessed by co-localization of LAMP-1 and cathepsin D with ingested conidia, was significantly lower for wild-type conidia compared with pksP mutant conidia. Furthermore, the intracellular kill of pksP mutant conidia was significantly higher than of wild-type conidia, which was markedly increased by chloroquine, a known enhancer of phagolysosome fusion. Taken together, these findings suggest that the presence of a functional pksP gene in A. fumigatus conidia is associated with an inhibition of phagolysosome fusion in human MDM. These data show for the first time that a fungus has the capability to inhibit the fusion of the phagosome with the lysosome. This finding might help explain the attenuated virulence of pksP mutant strains in a murine animal model and provides a conceptual frame to understand the virulence of A. fumigatus.

Phenotypic switching in Cryptococcus neoformans

Cryptococcus neoformans strains exhibit considerable phenotype variability with regards to the capsular polysaccharide, sterol composition of the cell wall, and cell and colony morphology. Phenotypic changes can occur spontaneously during in vitro passage of strains or during chronic infection in vivo and may be associated with differences in virulence. Studies from our laboratory have demonstrated that phenotype variability can be the result of phenotypic switching. Phenotypic switching is defined as a reversible change of an observable colony phenotype that occurs at a frequency above the expected frequency for somatic mutations. This implies that phenotypic switching represents controlled and programmed changes in this pathogenic yeast rather than random mutations. We have shown that a phenotypic switch from a smooth colony phenotype to a mucoid colony phenotype occurs in vitro and in vivo during chronic infection of mice. More importantly we have now demonstrated that the switch is associated with an increase in virulence and a change in the host immune response. Implications of these findings for the pathogenesis of cryptococcosis are discussed.

Inhibition of mitogen-activated protein kinase signaling by chloroquine

Previously, we demonstrated that the anti-inflammatory drug chloroquine (CQ) inhibited LPS-induced TNF-alpha transcription. To define further the mechanism of CQ, we studied the effect of this drug on mitogen-activated protein kinase signaling pathways involved in regulation of TNF production. CQ interfered with phosphorylation of extracellular signal-regulated kinases (ERK)1/2 and the ERK-activating kinases mitogen-activating protein/ERK kinase (MEK)1/2. Both CQ and PD98059, a MEK1 inhibitor, reduced luciferase reporter activity driven by human TNF promoter sequences. However, CQ appeared to mediate these effects by deactivating Raf, the upstream activator of MEK. These findings were supported by functional data demonstrating that CQ and PD98059 interfered with TNF expression in several human and murine cell types while neither inhibitor blocked TNF production in murine RAW264.7 macrophages, a cell line that does not require MEK-ERK signaling for TNF production. Finally, we evaluated whether CQ could sensitize HeLa cells to undergo anti-Fas-mediated apoptosis, an effect observed when ERK activation is interrupted in this cell line. CQ rendered HeLa cells sensitive to anti-Fas treatment in a manner similar to PD98059. Taken together, these data argue that therapeutic concentrations of CQ interfere with ERK activation by a novel mechanism, an effect that could be responsible, at least in part, for the potent anti-inflammatory effects of this drug.

Replication of Cryptococcus neoformans in macrophages is accompanied by phagosomal permeabilization and accumulation of vesicles containing polysaccharide in the cytoplasm

Cryptococcus neoformans (CN), an encapsulated, ubiquitous environmental yeast, is pathogenic for humans, primarily those with compromised immune function. CN is believed to be a facultative intracellular pathogen. Time-lapsed video microscopy revealed that yeast began to replicate and divide 2 hours after ingestion by J774.16 macrophage cells, with the average cell hosting 10-40 organisms of varying morphologies before ultimately lysing and releasing organisms, either singly or in clumps. Intracellular growth was accompanied by the accumulation of polysaccharide-filled vesicles in the macrophage. Studies with fluorescently labeled dextran revealed that the phagolysosomal compartment became leaky during the course of intracellular infection. Consistent with this observation, phagosomes containing CN had an increased pH relative to similar phagosomes containing inert magnetic beads, as indicated by a colorimetric change in the pH-sensitive Lysosensor dye. Immunocytochemistry revealed differences in the reactivity of polysaccharide elaborated by CN inside macrophages relative to that expressed in vitro. Taken together these results are suggestive of a novel mechanism of intracellular survival by an encapsulated organism, whereby ingestion is followed by damage to the phagosomal membrane resulting in continuity with the cytoplasm, accumulation of polysaccharide-containing vesicles, and possibly, production of a structurally different polysaccharide.

Cryptococcus neoformans interactions with amoebae suggest an explanation for its virulence and intracellular pathogenic strategy in macrophages

Cryptococcus neoformans (Cn) is a soil fungus that causes life-threatening meningitis in immunocompromised patients and is a facultative intracellular pathogen capable of replication inside macrophages. The mechanism by which environmental fungi acquire and maintain virulence for mammalian hosts is unknown. We hypothesized that the survival strategies for Cn after ingestion by macrophages and amoebae were similar. Microscopy, fungal and amoebae killing assays, and phagocytosis assays revealed that Cn is phagocytosed by and replicates in Acanthamoeba castellanii, which leads to death of amoebae. An acapsular strain of Cn did not survive when incubated with amoebae, but melanization protected these cells against killing by amoebae. A phospholipase mutant had a decreased replication rate in amoebae compared with isogenic strains. These observations suggest that cryptococcal characteristics that contribute to mammalian virulence also promote fungal survival in amoebae. Intracellular replication was accompanied by the accumulation of polysaccharide containing vesicles similar to those described in Cn-infected macrophages. The results suggest that the virulence of Cn for mammalian cells is a consequence of adaptations that have evolved for protection against environmental predators such as amoebae and provide an explanation for the broad host range of this pathogenic fungus.

Laccase of Cryptococcus neoformans is a cell wall-associated virulence factor

Virulence is the outcome of an interaction between the host and a microbe and is characterized by a large array of opposing reactions operating at the host-pathogen interface. Cryptococcus neoformans is an important opportunistic pathogen in immunocompromised patients, including those with human immunodeficiency virus, and expresses a virulence-associated laccase which is believed to oxidize brain catecholamines and iron as a defense against host immune cells. In the present report, we investigated the cellular location of laccase to understand more fully how it contributes to cryptococcal virulence. A monoclonal antibody to the C. neoformans laccase was generated and used to show localization in the cell walls of representative serotype A (H99) and serotype D (B-3501) strains by immunoelectron microscopy. In addition, confocal microscopy was used to show a peripheral location of green fluorescent protein-tagged laccase expressed in live H99 cells. Biochemical studies showed that laccase could be released from intact cells or cell wall fractions with glucanase enzymes but was retained in the cell wall after sequential extraction with 1 M NaCl, 6 M urea, and 1% sodium dodecyl sulfate. The presence of a hydrolyzable bond linking laccase to the cell wall was suggested by removal of laccase from cell wall preparations after they were boiled in 1% sodium dodecyl sulfate, as was the presence of a disulfide or thioester bond by removal with dithiothreitol or beta-mercaptoethanol. These data show that laccase is present as a tightly associated cell wall enzyme that is readily accessible for interactions with host immune cells.

Intracellular parasitism of macrophages by Cryptococcus neoformans

Cryptococcus neoformans, an encapsulated fungal pathogen, causes meningoencephalitis in immunocompromised patients. Recent in vivo studies have demonstrated that C. neoformans is a facultative intracellular pathogen, as was previously suggested by in vitro studies. For survival in macrophages, C. neoformans utilizes a novel strategy for intracellular parasitism that includes the accumulation of intracellular polysaccharide in cytoplasmic vesicles. Confirmation of the fact that C. neoformans is a facultative intracellular pathogen could provide new insights into several poorly understood areas of cryptococcal pathogenesis, including mechanisms for latency and persistence and the lack of efficacy of humoral immunity. The finding that C. neoformans replicates inside macrophages in vitro in a manner similar to that observed in vivo provides an excellent system to dissect the molecular mechanisms responsible for this unique pathogenic strategy.

Roles for inositol-phosphoryl ceramide synthase 1 (IPC1) in pathogenesis of C. neoformans

Cryptococcus neoformans is a leading cause of life-threatening fungal infection in immunocompromised patients. Inositol-phosphoryl ceramide synthase 1 (Ipc1) is a fungus-specific enzyme, encoded by the essential IPC1 gene, that catalyzes the formation of complex sphingolipids and may also regulate the levels of phytoceramide and diacylglycerol. Here, we investigated the functions of this essential gene by modulating its expression in C. neoformans using a galactose-inducible promoter. Down-regulation of IPC1 significantly lowers the expression of certain virulence traits such as melanin pigmentation and, remarkably, impairs pathogenicity of C. neoformans in an established rabbit model. Interestingly, we found that IPC1 down-regulation significantly decreases the intracellular growth of C. neoformans in the J774.16 murine macrophage-like cells. Finally, we studied the effect of IPC1 expression under different stress conditions and found that down-regulation of IPC1 confers a defect on in vitro growth at low pH. Because this environment is similar to that in the phagolysosome of J774.16 macrophage-like cells, our findings indicate that down-regulation of IPC1 confers a growth defect in vivo through a pH-dependent mechanism. In conclusion, our study is the first to define a novel and crucial function of Ipc1 in fungal pathogenesis.

Immunoregulation by capsular components of Cryptococcus neoformans

Cryptococcus neoformans is an encapsulated yeast that is pathogenic for humans. The capsule is a major virulence factor composed mainly of glucuronoxylomannan (GXM) and two minor constituents, galactoxylomannan, and mannoprotein (MP). A hallmark of disseminated cryptococcosis is the presence of high concentrations of GXM in body fluids of infected hosts. GXM provides a critical negative signal for T cell activation and neutrophil migration at the site of the inflammatory process. There is also strong evidence that MP promotes critical events associated with protective responses such as delayed type hypersensitivity and presumably a T helper type 1 response. The contrasting roles of GXM and MP in regulation of the immune response to C. neoformans offer a promising template for a successful approach to intervention, by scavenging GXM to attenuate its negative signals, while preserving the positive effects of MP.

Human neutrophil-mediated nonoxidative antifungal activity against Cryptococcus neoformans

It has long been appreciated that polymorphonuclear leukocytes (PMN) kill Cryptococcus neoformans, at least in part via generation of fungicidal oxidants. The aim of this study was to examine the contribution of nonoxidative mechanisms to the inhibition and killing of C. neoformans. Treatment of human PMN with inhibitors and scavengers of respiratory burst oxidants only partially reversed anticryptococcal activity, suggesting that both oxidative and nonoxidative mechanisms were operative. To define the mediators of nonoxidative anticryptococcal activity, PMN were fractionated into cytoplasmic, primary (azurophil) granule, and secondary (specific) granule fractions. Incubation of C. neoformans with these fractions for 18 h resulted in percent inhibition of growth of 67.4 +/- 3.4, 84.6 +/- 4.4, and 29.2 +/- 10.5 (mean +/- standard error, n = 3), respectively. Anticryptococcal activity of the cytoplasmic fraction was abrogated by zinc and depletion of calprotectin. Antifungal activity of the primary granules was significantly reduced by pronase treatment, boiling, high ionic strength, and magnesium but not calcium. Fractionation of the primary granules by reverse phase high-pressure liquid chromatography on a C(4) column over an acetonitrile gradient revealed multiple peaks with anticryptococcal activity. Of these, peaks 1 and 6 had substantial fungistatic and fungicidal activity. Peak 1 was identified by acid-urea polyacrylamide gel electrophoresis (PAGE) and mass spectroscopy as human neutrophil proteins (defensins) 1 to 3. Analysis of peak 6 by sodium dodecyl sulfate-PAGE revealed multiple bands. Thus, human PMN have nonoxidative anticryptococcal activity residing principally in their cytoplasmic and primary granule fractions. Calprotectin mediates the cytoplasmic activity, whereas multiple proteins, including defensins, are responsible for activity of the primary granules.

Chloroquine and the fungal phagosome

The antimalarial drug chloroquine accumulates inside the macrophage phagolysosome by ion trapping where it exerts potent antifungal activity against Histoplasma capsulatum and Cryptococcus neoformans by distinct mechanisms. Chloroquine inhibits growth of H. capsulatum by pH-dependent iron deprivation, whereas it is directly toxic to C. neoformans. Clearly, clinical studies are required to document the potential therapeutic efficacy of chloroquine or related congeners as adjuvant therapy in fungal disease. Moreover, the diversity of pathogenic microorganisms inhibited and/or killed by chloroquine makes this drug an attractive candidate for prophylactic therapy.

Cryptococcus neoformans is a facultative intracellular pathogen in murine pulmonary infection

To produce chronic infection, microbial pathogens must escape host immune defenses. Infection with the human pathogenic fungus Cryptococcus neoformans is typically chronic. To understand the mechanism by which C. neoformans survives in tissue after the infection of immunocompetent hosts, we systematically studied the course of pulmonary infection in mice by electron microscopy. The macrophage was the primary phagocytic cell at all times of infection, but neutrophils also ingested yeast. Alveolar macrophages rapidly internalized yeast cells after intratracheal infection, and intracellular yeast cells were noted at all times of infection from 2 h through 28 days. However, the proportion of yeast cells in the intracellular and extracellular spaces varied with the time of infection. Early in infection, yeast cells were found predominantly in the intracellular compartment. A shift toward extracellular predominance occurred by 24 h that was accompanied by macrophage cytotoxicity and disruption. Later in infection, intracellular persistence in vivo was associated with replication, residence in a membrane-bound phagosome, polysaccharide accumulation inside cells, and cytotoxicity to macrophages, despite phagolysosomal fusion. Many phagocytic vacuoles with intracellular yeast had discontinuous membranes. Macrophage infection resulted in cells with a distinctive appearance characterized by large numbers of vacuoles filled with polysaccharide antigen. Similar results were observed in vitro using a macrophage-like cell line. Our results show that C. neoformans is a facultative intracellular pathogen in vivo. Furthermore, our observations suggest that C. neoformans occupies a unique niche among the intracellular pathogens whereby survival in phagocytic cells is accompanied by intracellular polysaccharide production.

Purification and characterization of secretory phospholipase B, lysophospholipase and lysophospholipase/transacylase from a virulent strain of the pathogenic fungus Cryptococcus neoformans

Infection caused by the fungus Cryptococcus neoformans is potentially fatal. A highly active extracellular phospholipase, demonstrating phospholipase B (PLB), lysophospholipase (LPL) and lysophospholipase/transacylase (LPTA) activities, was purified to homogeneity from C. neoformans using (NH(4))(2)SO(4) fractionation, and hydrophobic-interaction, anion-exchange and gel-filtration chromatography. All three enzyme activities co-purified as a single protein with an apparent molecular mass of 70-90 kDa by SDS/PAGE and 160-180 kDa by gel filtration. The ratio of the three activities remained constant after each purification step. The amino acid composition, as well as the sequences of the N-terminus and of five internal peptide fragments were novel. The protein was an acidic glycoprotein containing N-linked carbohydrate moieties, with pI values of 5.5 and 3.5. The apparent V(max) values for PLB and LPL activities were 12.3 and 870 micromol/min per mg of protein respectively; the corresponding K(m) values were approx. 185.3 and 92.2 microM. The enzyme was active only at acidic pH (pH optimum of 4.0 for PLB and 4.0-5.0 for LPL and LPTA). Enzyme activity did not require added cations, but was inhibited by Fe(3+). LPL and LPTA activities were decreased by 0.1% (v/v) Triton X-100 to 50% of the control value. Palmitoylcarnitine (0.5 mM) inhibited PLB (97% inhibition) and LPL and LPTA activities (35% inhibition) competitively. All phospholipids except phosphatidic acid were degraded by PLB, but dipalmitoyl phosphatidylcholine and dioleoyl phosphatidylcholine were the preferred substrates. This is the first complete description of the purification and properties of a phospholipase, which may be involved in virulence, from a pathogenic fungus.

Urease as a virulence factor in experimental cryptococcosis

Urease catalyzes the hydrolysis of urea to ammonia and carbamate and has been found to be an important pathogenic factor for certain bacteria. Cryptococcus neoformans is a significant human pathogenic fungus that produces large amounts of urease; thus we wanted to investigate the importance of urease in the pathogenesis of cryptococcosis. We cloned and sequenced the genomic locus containing the single-copy C. neoformans urease gene (URE1) and used this to disrupt the native URE1 in the serotype A strain H99. The ure1 mutant strains were found to have in vitro growth characteristics, phenoloxidase activity, and capsule size similar to those of the wild type. Comparison of a ure1 mutant with H99 after intracisternal inoculation into corticosteroid-treated rabbits revealed no significant differences in colony counts recovered from the cerebrospinal fluid. However, when these two strains were compared in both the murine intravenous and inhalational infection models, there were significant differences in survival. Mice infected with a ure1 strain lived longer than mice infected with H99 in both models. The ure1 strain was restored to urease positivity by complementation with URE1, and two resulting transformants were significantly more pathogenic than the ure1 strain. Our results suggest that urease activity is involved in the pathogenesis of cryptococcosis but that the importance may be species and/or infection site specific.

Characterization and intracellular trafficking pattern of vacuoles containing Chlamydia pneumoniae in human epithelial cells

Chlamydiae are obligate intracellular pathogens that reside within a membrane-bound vacuole throughout their developmental cycle. In this study, the intraphagosomal pH of Chlamydia pneumoniae (Cpn) was qualitatively assessed, and the intracellular fate of the pathogen-containing vacuole and its interaction with endocytic organelles in human epithelial cells were analysed using conventional immunofluorescence and confocal microscopy. The pH-sensitive probes acridine orange (AO), LysoTracker (LyT) and DAMP did not accumulate in the bacterial inclusion. In addition, exposure of cells to bafilomycin A1(BafA1), a potent acidification inhibitor, did not inhibit or delay chlamydial growth. The chlamydial compartment was not accessible to the fluid-phase tracer Texas Red (TR)-dextran and did not exhibit any level of staining for the late endosomal marker cation-independent mannose-6-phosphate receptor (Ci-M6PR) or for the lysosomal-associated membrane proteins (LAMP-1 and -2) and CD63. In addition, transferrin receptor (TfR)-enriched vesicles were observed close to Cpn vacuoles, potentially indicating a specific translocation of these organelles through the cytoplasm to the vicinity of the vacuole. We conclude that Cpn, like other chlamydial spp., circumvents the host endocytic pathway and inhabits a non-acidic vacuole, which is dissociated from late endosomes and lysosomes, but selectively accumulates early endosomes.