A ketogenic diet enhances fluconazole efficacy in murine models of systemic fungal infection

Invasive fungal infections are a significant public health concern, with mortality rates ranging from 20% to 85% despite current treatments. Therefore, we examined whether a ketogenic diet could serve as a successful treatment intervention in murine models of Cryptococcus neoformans and Candida albicans infection in combination with fluconazole-a low-cost, readily available antifungal therapy. The ketogenic diet is a high-fat, low-carbohydrate diet that promotes fatty acid oxidation as an alternative to glycolysis through the production of ketone bodies. In this series of experiments, mice fed a ketogenic diet prior to infection with C. neoformans and treated with fluconazole had a significant decrease in fungal burden in both the brain (mean 2.66 ± 0.289 log10 reduction) and lung (mean 1.72 ± 0.399 log10 reduction) compared to fluconazole treatment on a conventional diet. During C. albicans infection, kidney fungal burden of mice in the keto-fluconazole combination group was significantly decreased compared to fluconazole alone (2.37 ± 0.770 log10-reduction). Along with higher concentrations of fluconazole in the plasma and brain tissue, fluconazole efficacy was maximized at a significantly lower concentration on a keto diet compared to a conventional diet, indicating a dramatic effect on fluconazole pharmacodynamics. Our findings indicate that a ketogenic diet potentiates the effect of fluconazole at multiple body sites during both C. neoformans and C. albicans infection and could have practical and promising treatment implications.IMPORTANCEInvasive fungal infections cause over 2.5 million deaths per year around the world. Treatments for fungal infections are limited, and there is a significant need to develop strategies to enhance antifungal efficacy, combat antifungal resistance, and mitigate treatment side effects. We determined that a high-fat, low-carbohydrate ketogenic diet significantly potentiated the therapeutic effect of fluconazole, which resulted in a substantial decrease in tissue fungal burden of both C. neoformans and C. albicans in experimental animal models. We believe this work is the first of its kind to demonstrate that diet can dramatically influence the treatment of fungal infections. These results highlight a novel strategy of antifungal drug enhancement and emphasize the need for future investigation into dietary effects on antifungal drug activity.

Biolistic Transformation of Cryptococcus neoformans

Biolistic transformation of Cryptococcus neoformans is used as a molecular tool to genetically alter or delete targeted genes. The DNA is introduced into the yeast on DNA-coated gold beads by a helium shock wave produced using a biolistic particle system. The procedure often involves insertion of a dominant selectable marker into the desired site by homologous recombination. To increase the likelihood of homologous recombination, large fragments of overlapping DNA are used. The two most used dominant selectable markers are nourseothricin and Geneticin. With the need to generate multiple gene deletions in the same strain, there are recyclable marker systems, such as the bacteriophage P1 Cre-loxP system or CRISPR that provide additional useful molecular tools. While newer strategies exist to generate deletions and introduce markers and other gene modifications, biolistic transformation has remained a viable tool to facilitate the construction of genetically modified yeast strains. This chapter provides a working protocol on how to delete and restore a gene in C. neoformans.

Genetic diversity and microevolution in clinical Cryptococcus isolates from Cameroon

Cryptococcal meningitis is the second most common cause of death in people living with HIV/AIDS, yet we have a limited understanding of how cryptococcal isolates change over the course of infection. Cryptococcal infections are environmentally acquired, and the genetic diversity of these infecting isolates can also be geographically linked. Here, we employ whole genome sequences for 372 clinical Cryptococcus isolates from 341 patients with HIV-associated cryptococcal meningitis obtained via a large clinical trial, across both Malawi and Cameroon, to enable population genetic comparisons of isolates between countries. We see that isolates from Cameroon are highly clonal, when compared to those from Malawi, with differential rates of disruptive variants in genes with roles in DNA binding and energy use. For a subset of patients (22) from Cameroon, we leverage longitudinal sampling, with samples taken at days 7 and 14 post-enrollment, to interrogate the genetic changes that arise over the course of infection, and the genetic diversity of isolates within patients. We see disruptive variants arising over the course of infection in several genes, including the phagocytosis-regulating transcription factor GAT204. In addition, in 13% of patients sampled longitudinally, we see evidence for mixed infections. This approach identifies geographically linked genetic variation, signatures of microevolution, and evidence for mixed infections across a clinical cohort of patients affected by cryptococcal meningitis in Central Africa.

Pharmacodynamics of ATI-2307 in a rabbit model of cryptococcal meningoencephalitis

Cryptococcal meningoencephalitis (CM) is a devastating fungal disease with high morbidity and mortality. The current regimen that is standard-of-care involves a combination of three different drugs administered for up to one year. There is a critical need for new therapies due to both toxicity and inadequate fungicidal activity of the currently available antifungal drugs. ATI-2307 is a novel aryl amidine that disrupts the mitochondrial membrane potential and inhibits the respiratory chain complexes of fungi-it thus represents a new mechanism for direct antifungal action. Furthermore, ATI-2307 selectively targets fungal mitochondria via a fungal-specific transporter that is not present in mammalian cells. It has very potent in vitro anticryptococcal activity. In this study, the efficacy of ATI-2307 was tested in a rabbit model of CM. ATI-2307 demonstrated significant fungicidal activity at dosages between 1 and 2 mg/kg/d, and these results were superior to fluconazole and similar to amphotericin B treatment. When ATI-2307 was combined with fluconazole, the antifungal effect was greater than either therapy alone. While ATI-2307 has potent anticryptococcal activity in the subarachnoid space, its ability to reduce yeasts in the brain parenchyma was relatively less over the same study period. This new drug, with its unique mechanism of fungicidal action and ability to positively interact with an azole, has demonstrated sufficient anticryptococcal potential in this experimental setting to be further evaluated in clinical studies.

Susceptibility to Cryptococcus neoformans Infection with Bruton's Tyrosine Kinase Inhibition

Patients receiving the Bruton's tyrosine kinase (BTK) inhibitor ibrutinib have an increased likelihood of fungal infections. The objectives of this study were to determine if Cryptococcus neoformans infection severity was isolate dependent with BTK inhibition and whether blocking BTK impacted infection severity in a mouse model. We compared four clinical isolates from patients on ibrutinib to virulent (H99) and avirulent (A1-35-8) reference strains. BTK knockout (KO) and wild-type (WT) C57 mice and WT CD1 mice were infected by intranasal (i.n.), oropharyngeal aspiration (OPA), and intravenous (i.v.) routes. Infection severity was assessed by survival and fungal burden (CFU per gram of tissue). Ibrutinib (25 mg/kg) or vehicle was administered daily through intraperitoneal injections. In the BTK KO model, no isolate-dependent effect on fungal burden was observed, and infection severity was not significantly different from that of the WT with i.n., OPA, and i.v. routes. Ibrutinib treatment did not impact infection severity. However, when the four clinical isolates were compared to H99, two of these isolates were less virulent, with significantly longer survival and reduced rates of brain infection. In conclusion, C. neoformans infection severity in the BTK KO model does not appear to be isolate dependent. BTK KO and ibrutinib treatment did not result in significantly different infection severities. However, based on repeated clinical observations of increased susceptibility to fungal infections with BTK inhibitor therapy, further work is needed to optimize a mouse model with BTK inhibition to better understand the role that this pathway plays in susceptibility to C. neoformans infection.

Efficacy of APX2039 in a Rabbit Model of Cryptococcal Meningitis

Cryptococcal Meningitis (CM) is uniformly fatal if not treated, and treatment options are limited. We previously reported on the activity of APX2096, the prodrug of the novel Gwt1 inhibitor APX2039, in a mouse model of CM. Here, we investigated the efficacy of APX2039 in mouse and rabbit models of CM. In the mouse model, the controls had a mean lung fungal burden of 5.95 log10 CFU/g, whereas those in the fluconazole-, amphotericin B-, and APX2039-treated mice were 3.56, 4.59, and 1.50 log10 CFU/g, respectively. In the brain, the control mean fungal burden was 7.97 log10 CFU/g, while the burdens were 4.64, 7.16, and 1.44 log10 CFU/g for treatment with fluconazole, amphotericin B, and APX2039, respectively. In the rabbit model of CM, the oral administration of APX2039 at 50 mg/kg of body weight twice a day (BID) resulted in a rapid decrease in the cerebrospinal fluid (CSF) fungal burden, and the burden was below the limit of detection by day 10 postinfection. The effective fungicidal activity (EFA) was -0.66 log10 CFU/mL/day, decreasing from an average of 4.75 log10 CFU/mL to 0 CFU/mL, over 8 days of therapy, comparing favorably with good clinical outcomes in humans associated with reductions of the CSF fungal burden of -0.4 log10 CFU/mL/day, and, remarkably, 2-fold the EFA of amphotericin B deoxycholate in this model (-0.33 log10 CFU/mL/day). A total drug exposure of the area under the concentration-time curve from 0 to 24 h (AUC0-24) of 25 to 50 mg · h/L of APX2039 resulted in near-maximal antifungal activity. These data support the further preclinical and clinical evaluation of APX2039 as a new oral fungicidal monotherapy for the treatment of CM. IMPORTANCE Cryptococcal meningitis (CM) is a fungal disease with significant global morbidity and mortality. The gepix Gwt1 inhibitors are a new class of antifungal drugs. Here, we demonstrated the efficacy of APX2039, the second member of the gepix class, in rabbit and mouse models of cryptococcal meningitis. We also analyzed the drug levels in the blood and cerebrospinal fluid in the highly predictive rabbit model and built a mathematical model to describe the behavior of the drug with respect to the elimination of the fungal pathogen. We demonstrated that the oral administration of APX2039 resulted in a rapid decrease in the CSF fungal burden, with an effective fungicidal activity of -0.66 log10 CFU/mL/day, comparing favorably with good clinical outcomes in humans associated with reductions of -0.4 log10 CFU/mL/day. The drug APX2039 had good penetration of the central nervous system and is an excellent candidate for future clinical testing in humans for the treatment of CM.

Genomic Variation across a Clinical Cryptococcus Population Linked to Disease Outcome

Cryptococcus neoformans is the causative agent of cryptococcosis, a disease with poor patient outcomes that accounts for approximately 180,000 deaths each year. Patient outcomes may be impacted by the underlying genetics of the infecting isolate; however, our current understanding of how genetic diversity contributes to clinical outcomes is limited. Here, we leverage clinical, in vitro growth and genomic data for 284 C. neoformans isolates to identify clinically relevant pathogen variants within a population of clinical isolates from patients with human immunodeficiency virus (HIV)-associated cryptococcosis in Malawi. Through a genome-wide association study (GWAS) approach, we identify variants associated with the fungal burden and the growth rate. We also find both small and large-scale variation, including aneuploidy, associated with alternate growth phenotypes, which may impact the course of infection. Genes impacted by these variants are involved in transcriptional regulation, signal transduction, glycosylation, sugar transport, and glycolysis. We show that growth within the central nervous system (CNS) is reliant upon glycolysis in an animal model and likely impacts patient mortality, as the CNS yeast burden likely modulates patient outcome. Additionally, we find that genes with roles in sugar transport are enriched in regions under selection in specific lineages of this clinical population. Further, we demonstrate that genomic variants in two genes identified by GWAS impact virulence in animal models. Our approach identifies links between the genetic variation in C. neoformans and clinically relevant phenotypes and animal model pathogenesis, thereby shedding light on specific survival mechanisms within the CNS and identifying the pathways involved in yeast persistence. IMPORTANCE Infection outcomes for cryptococcosis, most commonly caused by C. neoformans, are influenced by host immune responses as well as by host and pathogen genetics. Infecting yeast isolates are genetically diverse; however, we lack a deep understanding of how this diversity impacts patient outcomes. To better understand both clinical isolate diversity and how diversity contributes to infection outcomes, we utilize a large collection of clinical C. neoformans samples that were isolated from patients enrolled in a clinical trial across 3 hospitals in Malawi. By combining whole-genome sequence data, clinical data, and in vitro growth data, we utilize genome-wide association approaches to examine the genetic basis of virulence. Genes with significant associations display virulence attributes in both murine and rabbit models, demonstrating that our approach can identify potential links between genetic variants and patho-biologically significant phenotypes.

Transcriptional Profiles Elucidate Differential Host Responses to Infection with Cryptococcus neoformans and Cryptococcus gattii

Many aspects of the host response to invasive cryptococcal infections remain poorly understood. In order to explore the pathobiology of infection with common clinical strains, we infected BALB/cJ mice with Cryptococcus neoformans, Cryptococcus gattii, or sham control, and assayed host transcriptomic responses in peripheral blood. Infection with C. neoformans resulted in markedly greater fungal burden in the CNS than C. gattii, as well as slightly higher fungal burden in the lungs. A total of 389 genes were significantly differentially expressed in response to C. neoformans infection, which mainly clustered into pathways driving immune function, including complement activation and TH2-skewed immune responses. C. neoformans infection demonstrated dramatic up-regulation of complement-driven genes and greater up-regulation of alternatively activated macrophage activity than seen with C gattii. A 27-gene classifier was built, capable of distinguishing cryptococcal infection from animals with bacterial infection due to Staphylococcus aureus with 94% sensitivity and 89% specificity. Top genes from the murine classifiers were also differentially expressed in human PBMCs following infection, suggesting cross-species relevance of these findings. The host response, as manifested in transcriptional profiles, informs our understanding of the pathophysiology of cryptococcal infection and demonstrates promise for contributing to development of novel diagnostic approaches.

Comparison of Cryptococcus gattii/neoformans Species Complex to Related Genera (Papiliotrema and Naganishia) Reveal Variances in Virulence Associated Factors and Antifungal Susceptibility

Cryptococcosis is an infectious disease of worldwide distribution, caused by encapsulated yeasts belonging to the phylum Basidiomycota. The genus Cryptococcus includes several species distributed around the world. The C. gattii/neoformans species complex is largely responsible for most cases of cryptococcosis. However, clinical series have been published of infections caused by Papiliotrema (Cryptococcus) laurentii and Naganishia albida (Cryptococcus albidus), among other related genera. Here, we examined the pathogenic potential and antifungal susceptibility of C. gattii/neoformans species complex (clades I and II) and related genera (Papiliotrema and Naganishia) isolated from environmental and clinical samples. P. laurentii (clade III), N. liquefasciens/N. albidosimilis (clade IV); and N. adeliensis/N. albida (clade V) strains produced higher levels of phospholipase and hemolysins, whereas the C. gattii/neoformans species complex strains (clades I and II) had markedly thicker capsules, produced more biofilm biomass and melanin, which are known virulence attributes. Interestingly, 40% of C. neoformans strains (clade II) had MICs above the ECV established for this species to amphotericin B. Several non-C. gattii/neoformans species complex (clades III to V) had MICs equal to or above the ECVs established for C. deuterogattii and C. neoformans for all the three antifungal drugs tested. Finally, all the non-C. gattii/neoformans clinical isolates (clades III to V) produced more melanin than the environmental isolates might reflect their particularly enhanced need for melanin during in vivo protection. It is very clear that C. gattii/neoformans species complex (clades I and II) strains, in general, show more similar virulence phenotypes between each other when compared to non-C. gattii/neoformans species complex (clades III to V) isolates. These observations together with the fact that P. laurentii and Naganishia spp. (clades III to V) strains were collected from the outside of a University Hospital, identify features of these yeasts important for environmental and patient colonization and furthermore, define mechanisms for infections with these uncommon pathogens.

Assessing the virulence of Cryptococcus neoformans causing meningitis in HIV infected and uninfected patients in Vietnam

We previously observed a substantial burden of cryptococcal meningitis in Vietnam atypically arising in individuals who are uninfected with human immunodeficiency virus (HIV). This disease was associated with a single genotype of Cryptococcus neoformans (sequence type [ST]5), which was significantly less common in HIV-infected individuals. Aiming to compare the phenotypic characteristics of ST5 and non-ST5 C. neoformans, we selected 30 representative Vietnamese isolates and compared their in vitro pathogenic potential and in vivo virulence. ST5 and non-ST5 organisms exhibited comparable characteristics with respect to in vitro virulence markers including melanin production, replication at 37°C, and growth in cerebrospinal fluid. However, the ST5 isolates had significantly increased variability in cellular and capsular sizing compared with non-ST5 organisms (P < .001). Counterintuitively, mice infected with ST5 isolates had significantly longer survival with lower fungal burdens at day 7 than non-ST5 isolates. Notably, ST5 isolates induced significantly greater initial inflammatory responses than non-ST5 strains, measured by TNF-α concentrations (P < .001). Despite being generally less virulent in the mouse model, we hypothesize that the significant within strain variation seen in ST5 isolates in the tested phenotypes may represent an evolutionary advantage enabling adaptation to novel niches including apparently immunocompetent human hosts.

Landscape of gene expression variation of natural isolates of Cryptococcus neoformans in response to biologically relevant stresses

Cryptococcus neoformans is an opportunistic fungal pathogen that at its peak epidemic levels caused an estimated million cases of cryptococcal meningitis per year worldwide. This species can grow in diverse environmental (trees, soil and bird excreta) and host niches (intracellular microenvironments of phagocytes and free-living in host tissues). The genetic basic for adaptation to these different conditions is not well characterized, as most experimental work has relied on a single reference strain of C. neoformans. To identify genes important for yeast infection and disease progression, we profiled the gene expression of seven C. neoformans isolates grown in five representative in vitro environmental and in vivo conditions. We characterized gene expression differences using RNA-Seq (RNA sequencing), comparing clinical and environmental isolates from two of the major lineages of this species, VNI and VNBI. These comparisons highlighted genes showing lineage-specific expression that are enriched in subtelomeric regions and in lineage-specific gene clusters. By contrast, we find few expression differences between clinical and environmental isolates from the same lineage. Gene expression specific to in vivo stages reflects available nutrients and stresses, with an increase in fungal metabolism within macrophages, and an induction of ribosomal and heat-shock gene expression within the subarachnoid space. This study provides the widest view to date of the transcriptome variation of C. neoformans across natural isolates, and provides insights into genes important for in vitro and in vivo growth stages.

Genotypic diversity and clinical outcome of cryptococcosis in renal transplant recipients in Brazil

Genotypic diversity and fluconazole susceptibility of 82 Cryptococcus neoformans and Cryptococcus gattii isolates from 60 renal transplant recipients in Brazil were characterized. Clinical characteristics of the patients and prognostic factors were analysed. Seventy-two (87.8%) isolates were C. neoformans and 10 (12.2%) were C. gattii. VNI was the most common molecular type (40 cases; 66.7%), followed by VNII (9 cases; 15%), VGII (6 cases; 10%), VNB (4 cases; 6.7%) and VNI/II (1 case; 1.7%). The isolates showed a high genetic diversity in the haplotype network and six new sequence types were described, most of them for VNB. There was a bias towards skin involvement in the non-VNI population (P = .012). VGII isolates exhibited higher fluconazole minimum inhibitory concentrations compared to C. neoformans isolates (P = 0.008). The 30-day mortality rate was 38.3%, and it was significantly associated with fungemia and absence of headache. Patients infected with VGII had a high mortality rate at 90 days (66.7%). A variety of molecular types produce disease in renal transplant recipients in Brazil and highlighted by VGII and VNB. We report the clinical appearance and impact of the molecular type, fluconazole susceptibility of the isolates, and clinical characteristics on patient outcome in this population.

Brain inositol is a novel stimulator for promoting Cryptococcus penetration of the blood-brain barrier

Cryptococcus neoformans is the most common cause of fungal meningitis, with high mortality and morbidity. The reason for the frequent occurrence of Cryptococcus infection in the central nervous system (CNS) is poorly understood. The facts that human and animal brains contain abundant inositol and that Cryptococcus has a sophisticated system for the acquisition of inositol from the environment suggests that host inositol utilization may contribute to the development of cryptococcal meningitis. In this study, we found that inositol plays an important role in Cryptococcus traversal across the blood-brain barrier (BBB) both in an in vitro human BBB model and in in vivo animal models. The capacity of inositol to stimulate BBB crossing was dependent upon fungal inositol transporters, indicated by a 70% reduction in transmigration efficiency in mutant strains lacking two major inositol transporters, Itr1a and Itr3c. Upregulation of genes involved in the inositol catabolic pathway was evident in a microarray analysis following inositol treatment. In addition, inositol increased the production of hyaluronic acid in Cryptococcus cells, which is a ligand known to binding host CD44 receptor for their invasion. These studies suggest an inositol-dependent Cryptococcus traversal of the BBB, and support our hypothesis that utilization of host-derived inositol by Cryptococcus contributes to CNS infection.

Cryptococcus neoformans is an AIDS-associated human fungal pathogen that annually causes over 1 million cases of meningitis world-wide, and more than 600,000 attributable deaths. Cryptococcus often causes lung and brain infection and is the leading cause of fungal meningitis in immunosuppressed patients. Why Cryptococcus frequently infects the central nervous system to cause fatal meningitis is an unanswered critical question. Our previous studies revealed a sophisticated inositol acquisition system in Cryptococcus that plays a central role in utilizing environmental inositol to complete its sexual cycle. Here we further demonstrate that inositol acquisition is also important for fungal infection in the brain, where abundant inositol is available. We found that inositol promotes the traversal of Cryptococcus across the blood-brain barrier (BBB), and such stimulation is fungal inositol transporter dependent. We also identified the effects of host inositol on fungal cellular functions that contribute to the stimulation of fungal penetration of the BBB. We propose that inositol utilization is a novel virulence factor for CNS cryptococcosis. Our work lays an important foundation for understanding how fungi respond to available host inositol and indicates the impact of host inositol acquisition on the development of cryptococcal meningitis.

Identification of genes from the fungal pathogen Cryptococcus neoformans related to transmigration into the central nervous system

Background

A mouse brain transmigration assessment (MBTA) was created to investigate the central nervous system (CNS) pathogenesis of cryptococcal meningoencephalitis.

Methodology/Principal Findings

Two cryptococcal mutants were identified from a pool of 109 pre-selected mutants that were signature-tagged with the nourseothricin acetyltransferase (NAT) resistance cassette. These two mutants displayed abnormal transmigration into the central nervous system. One mutant displaying decreased transmigration contains a null mutation in the putative FNX1 gene, whereas the other mutant possessing a null mutation in the putative RUB1 gene exhibited increased transmigration into the brain. Two macrophage adhesion-defective mutants in the pool, 12F1 and 3C9, showed reduced phagocytosis by macrophages, but displayed no defects in CNS entry suggesting that transit within macrophages (the “Trojan horse” model of CNS entry) is not the primary mechanism for C. neoformans migration into the CNS in this MBTA.

Conclusions/Significance

This research design provides a new strategy for genetic impact studies on how Cryptococcus passes through the blood-brain barrier (BBB), and the specific isolated mutants in this assay support a transcellular mechanism of CNS entry.

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

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

Pleiotropic effects of deubiquitinating enzyme Ubp5 on growth and pathogenesis of Cryptococcus neoformans

Ubiquitination is a reversible protein modification that influences various cellular processes in eukaryotic cells. Deubiquitinating enzymes remove ubiquitin, maintain ubiquitin homeostasis and regulate protein degradation via the ubiquitination pathway. Cryptococcus neoformans is an important basidiomycete pathogen that causes life-threatening meningoencephalitis primarily in the immunocompromised population. In order to understand the possible influence deubiquitinases have on growth and virulence of the model pathogenic yeast Cryptococcus neoformans, we generated deletion mutants of seven putative deubiquitinase genes. Compared to other deubiquitinating enzyme mutants, a ubp5Δ mutant exhibited severely attenuated virulence and many distinct phenotypes, including decreased capsule formation, hypomelanization, defective sporulation, and elevated sensitivity to several external stressors (such as high temperature, oxidative and nitrosative stresses, high salts, and antifungal agents). Ubp5 is likely the major deubiquitinating enzyme for stress responses in C. neoformans, which further delineates the evolutionary divergence of Cryptococcus from the model yeast S. cerevisiae, and provides an important paradigm for understanding the potential role of deubiquitination in virulence by other pathogenic fungi. Other putative deubiquitinase mutants (doa4Δ and ubp13Δ) share some phenotypes with the ubp5Δ mutant, illustrating functional overlap among deubiquitinating enzymes in C. neoformans. Therefore, deubiquitinating enzymes (especially Ubp5) are essential for the virulence composite of C. neoformans and provide an additional yeast survival and propagation advantage in the host.

Fatty acid synthesis is essential for survival of Cryptococcus neoformans and a potential fungicidal target

Fatty acid synthase in the yeast Cryptococcus neoformans is composed of two subunits encoded by FAS1 and FAS2 genes. We inserted a copper-regulated promoter (P(CTR4-2)) to regulate FAS1 and FAS2 expression in Cryptococcus neoformans (strains P(CTR4-2)/FAS1 and P(CTR4-2)/FAS2, respectively). Both mutants showed growth rates similar to those of the wild type in a low-copper medium in which FAS1 and FAS2 were expressed, but even in the presence of exogenous fatty acids, strains were suppressed in growth under high-copper conditions. The treatment of C. neoformans with fluconazole was shown to have an increased inhibitory activity and even became fungicidal when either FAS1 or FAS2 expression was suppressed. Furthermore, a subinhibitory dose of fluconazole showed anticryptococcal activity in vitro in the presence of cerulenin, a fatty acid synthase inhibitor. In a murine model of pulmonary cryptococcosis, a tissue census of yeast cells in P(CTR4-2)/FAS2 strain at day 7 of infection was significantly lower than that in mice treated with tetrathiomolybdate, a copper chelator (P < 0.05), and a yeast census of P(CTR4-2)/FAS1 strain at day 14 of infection in the brain was lower in the presence of more copper. In fact, no positive cultures from the brain were detected in mice (with or without tetrathiomolybdate treatment) infected with the P(CTR4-2)/FAS2 strain, which implies that this mutant did not reach the brain in mice. We conclude that both FAS1 and FAS2 in C. neoformans are essential for in vitro and in vivo growth in conditions with and without exogenous fatty acids and that FAS1 and FAS2 can potentially be fungicidal targets for C. neoformans with a potential for synergistic behavior with azoles.

Cryptococcus neoformans mitochondrial genomes from serotype A and D strains do not influence virulence

Cryptococcus neoformans is an encapsulated pathogenic yeast producing meningoencephalitis. Two primary strains in genetic studies, serotype A H99 and serotype D JEC21, possess dramatic differences in virulence. Since it has been shown that mitochondrial gene expression is prominent at the site of the infection and there are significant differences between mitochondrial gene structure and regulation between the serotype A and D strains, this study used AD hybrids to move serotype A and D mitochondria under different genomic influences. When the serotype D MATa strain is involved in the mating crosses, there is uniparental transmission of mitochondrial DNA, but with the serotype A MATa strain, mitochondrial DNA can be inherited from either parent and recombination in the mitochondrial genome may also occur. In virulence studies between serotype A and D strains, it was found that the primary genetic control of the virulence composite for growth in the central nervous system is encoded in the nuclear DNA and not through mitochondrial DNA.

Cryptococcus neoformans gene expression during experimental cryptococcal meningitis

Cryptococcus neoformans, an encapsulated basidiomycete fungus of medical importance, is capable of crossing the blood-brain barrier and causing meningitis in both immunocompetent and immunocompromised individuals. To gain insight into the adaptation of the fungus to the host central nervous system (CNS), serial analysis of gene expression (SAGE) was used to characterize the gene expression profile of C. neoformans cells recovered from the CNS of infected rabbits. A SAGE library was constructed, and 49,048 tags were sequenced; 16,207 of these tags were found to represent unique sequences or tag families. Of the 304 most-abundant tags, 164 were assigned to a putative gene for subsequent functional grouping. The results (as determined according to the number of tags that identified genes encoding proteins required for these functions) indicated that the C. neoformans cells were actively engaged in protein synthesis, protein degradation, stress response, small-molecule transport, and signaling. In addition, a high level of energy requirement of the fungal cells was suggested by a large number of tags that matched putative genes for energy production. Taken together, these findings provide the first insight into the transcriptional adaptation of C. neoformans to the host environment and identify the set of fungal genes most highly expressed during cerebrospinal fluid infection.

In vitro and in vivo efficacies of the new triazole albaconazole against Cryptococcus neoformans

The activity of albaconazole (UR-9825; J. Uriach & Cía. S.A., Barcelona, Spain) was compared to that of fluconazole against 12 isolates of Cryptococcus neoformans in vitro and against 1 isolate in vivo in a rabbit model of cryptococcal meningitis. Albaconazole was 100-fold more potent in vitro than fluconazole on a per-weight basis and was fungicidal at potentially relevant concentrations for two isolates. MICs ranged from </=0.0012 to 1.25 micro g/ml, with the MICs for most isolates being between 0.039 and 0.156 micro g/ml. Isolates were from human immunodeficiency virus (HIV)-infected and non-HIV-infected patients and were of serotypes A, B, and C; and the fluconazole MICs for some of the isolates were elevated. Infected rabbits were treated with either fluconazole or albaconazole at dosages ranging from 5 to 80 mg/kg of body weight/day. The peak concentrations of albaconazole in serum and cerebrospinal fluid (CSF) averaged 4.14 and 0.62 micro g/ml, respectively, in animals receiving 80 mg/kg/day. Comparison of the concentrations in serum and CSF suggested a level of CSF penetration of approximately 15%. Despite limited penetration into the subarachnoid space, at all three doses tested albaconazole was as effective as fluconazole for the treatment of cryptococcal meningitis in rabbits.

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.

Sexual cycle of Cryptococcus neoformans var. grubii and virulence of congenic a and alpha isolates

Cryptococcus neoformans is a human-pathogenic fungus that has evolved into three distinct varieties that infect most prominently the central nervous system. A sexual cycle involving haploid cells of a and alpha mating types has been reported for two varieties (C. neoformans var. neoformans, serotype D, and C. neoformans var. gattii, serotypes B and C), yet the vast majority of infections involve a distinct variety (C. neoformans var. grubii, serotype A) that has been thought to be clonal and restricted to the alpha mating type. We recently identified the first serotype A isolate of the a mating type which had been thought to be extinct (strain 125.91). Here we report that this unusual strain can mate with a subset of pathogenic serotype A strains to produce a filamentous dikaryon with fused clamp connections, basidia, and viable recombinant basidiospores. One meiotic segregant mated poorly with the serotype A reference strain H99 but robustly with a crg1 mutant that lacks a regulator of G protein signaling and is hyperresponsive to mating pheromone. This meiotic segregant was used to create congenic a and alpha mating type serotype A strains. Virulence tests with rabbit and murine models of cryptococcal meningitis showed that the serotype A congenic a and alpha mating type strains had equivalent virulence in animal models, in contrast to previous studies linking the alpha mating type to increased virulence in congenic serotype D strains. Our studies highlight a role for sexual recombination in the evolution of a human fungal pathogen and provide a robust genetic platform to establish the molecular determinants of virulence.

Regulation of cytochrome c oxidase subunit 1 (COX1) expression in Cryptococcus neoformans by temperature and host environment

In the study of differential gene expression of Cryptococcus neoformans, a transcript of COX1 (cytochrome oxidase c subunit 1) was identified in a serotype A strain. The transcript was upregulated at 37 degrees C compared to 30 degrees C and expressed by yeasts infecting the central nervous system. Northern analysis of COX1 from the serotype A strain revealed two polycistronic transcripts, a temperature-upregulated 2.3 kb transcript and a 1.9 kb transcript that was not affected by temperature. In contrast, COX1 in a serotype D strain showed only a 1.9 kb polycistronic transcript plus a 1.6 kb monocistronic message, and temperature had no effect on the transcripts. The sequence of COX1 revealed similar coding regions between the two strains, but the serotype D strain had five introns whereas no introns were found in the serotype A strain. The serotype D strain had reduced growth rates compared to the serotype A strain at 37 degrees C, but in an AD hybrid strain the serotype D COX1 gene could support efficient high temperature growth. These studies have revealed mitochondrial molecular differences between serotype A and D strains which show evolutionary divergence. It will be important to determine whether differences in mitochondrial structure and function can influence cryptococcosis.

Relationship of the glyoxylate pathway to the pathogenesis of Cryptococcus neoformans

Functional genomics has become a major focus in the study of microbial pathogenesis. This study used a functional genomic tool, differential display reverse transcription-PCR, to identify a transcriptional profile of Cryptococcus neoformans cells as they produced meningitis in an immunosuppressed host. This serial global gene expression during infection allowed for the identification of up- and down-regulated genes during infection. During this profiling, a single gene for the enzyme isocitrate lyase (ICL1) was found to be up regulated at 1 week of infection in a rabbit meningitis model and during a time of maximum host cellular response. The finding suggested that this enzyme and the glyoxylate shunt pathway are important to this yeast's energy production during infection. However, site-directed icl1 mutants had no apparent virulence defect in two animal models and no growth defect within macrophages. These observations suggest that although the yeast responded to a certain environmental cue(s) by an increase in ICL1 expression during infection, this gene was not necessary for progression of a C. neoformans infection. Compounds that specifically target only ICL1 are unlikely to cripple C. neoformans growth in vivo.

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.

Topoisomerase I is essential in Cryptococcus neoformans: role In pathobiology and as an antifungal target

Topisomerase I is the target of several toxins and chemotherapy agents, and the enzyme is essential for viability in some organisms, including mice and drosophila. We have cloned the TOP1 gene encoding topoisomerase I from the opportunistic fungal pathogen Cryptococcus neoformans. The C. neoformans topoisomerase I contains a fungal insert also found in topoisomerase I from Candida albicans and Saccharomyces cerevisiae that is not present in the mammalian enzyme. We were unable to disrupt the topoisomerase I gene in this haploid organism by homologous recombination in over 8000 transformants analyzed. When a second functional copy of the TOP1 gene was introduced into the genome, the topoisomerase I gene could be readily disrupted by homologous recombination (at 7% efficiency). Thus, topoisomerase I is essential in C. neoformans. This new molecular strategy with C. neoformans may also be useful in identifying essential genes in other pathogenic fungi. To address the physiological and pathobiological functions of the enzyme, the TOP1 gene was fused to the GAL7 gene promoter. The resulting GAL7::TOP1 fusion gene was modestly regulated by carbon source in a serotype A strain of C. neoformans. Modest overexpression of topoisomerase I conferred sensitivity to heat shock, gamma-rays, and camptothecin. In contrast, alterations in topoisomerase I levels had no effect on the toxicity of a novel class of antifungal agents, the dicationic aromatic compounds (DACs), indicating that topoisomerase I is not the target of DACs. In an animal model of cryptococcal meningitis, topoisomerase I regulation was not critically important to established infection, but may impact on the initial stress response to infection. In summary, our studies reveal that topoisomerase I is essential in the human pathogen C. neoformans and represents a novel target for antifungal agents.

Cryptococcus neoformans differential gene expression detected in vitro and in vivo with green fluorescent protein

Synthetic green fluorescent protein (GFP) was used as a reporter to detect differential gene expression in the pathogenic fungus Cryptococcus neoformans. Promoters from the C. neoformans actin, GAL7, or mating-type alpha pheromone (MFalpha1) genes were fused to GFP, and the resulting reporter genes were used to assess gene expression in serotype A C. neoformans. Yeast cells containing an integrated pACT::GFP construct demonstrated that the actin promoter was expressed during vegetative growth on yeast extract-peptone-dextrose medium. In contrast, yeast cells containing the inducible GAL7::GFP or MFalpha1::GFP reporter genes expressed significant GFP activity only during growth on galactose medium or V-8 agar, respectively. These findings demonstrated that the GAL7 and MFalpha1 promoters from a serotype D C. neoformans strain function when introduced into a serotype A strain. Because the MFalpha1 promoter is induced by nutrient deprivation and the MATalpha locus containing the MFalpha1 gene has been linked with virulence, yeast cells containing the pMFalpha1::GFP reporter gene were analyzed for GFP expression in the central nervous system (CNS) of immunosuppressed rabbits. In fact, significant GFP expression from the MFalpha1::GFP reporter gene was detected after the first week of a CNS infection. These findings suggest that there are temporal, host-specific cues that regulate gene expression during infection and that the MFalpha1 gene is induced during the proliferative stage of a CNS infection. In conclusion, GFP can be used as an effective and sensitive reporter to monitor specific C. neoformans gene expression in vitro, and GFP reporter constructs can be used as an approach to identify a novel gene(s) or to characterize known genes whose expression is regulated during infection.

A glucan synthase FKS1 homolog in cryptococcus neoformans is single copy and encodes an essential function

Cryptococcal meningitis is a fungal infection, caused by Cryptococcus neoformans, which is prevalent in immunocompromised patient populations. Treatment failures of this disease are emerging in the clinic, usually associated with long-term treatment with existing antifungal agents. The fungal cell wall is an attractive target for drug therapy because the syntheses of cell wall glucan and chitin are processes that are absent in mammalian cells. Echinocandins comprise a class of lipopeptide compounds known to inhibit 1,3-beta-glucan synthesis, and at least two compounds belonging to this class are currently in clinical trials as therapy for life-threatening fungal infections. Studies of Saccharomyces cerevisiae and Candida albicans mutants identify the membrane-spanning subunit of glucan synthase, encoded by the FKS genes, as the molecular target of echinocandins. In vitro, the echinocandins show potent antifungal activity against Candida and Aspergillus species but are much less potent against C. neoformans. In order to examine why C. neoformans cells are less susceptible to echinocandin treatment, we have cloned a homolog of S. cerevisiae FKS1 from C. neoformans. We have developed a generalized method to evaluate the essentiality of genes in Cryptococcus and applied it to the FKS1 gene. The method relies on homologous integrative transformation with a plasmid that can integrate in two orientations, only one of which will disrupt the target gene function. The results of this analysis suggest that the C. neoformans FKS1 gene is essential for viability. The C. neoformans FKS1 sequence is closely related to the FKS1 sequences from other fungal species and appears to be single copy in C. neoformans. Furthermore, amino acid residues known to be critical for echinocandin susceptibility in Saccharomyces are conserved in the C. neoformans FKS1 sequence.

Biochemical role of the Cryptococcus neoformans ADE2 protein in fungal de novo purine biosynthesis

Comparative studies of 5-aminoimidazole ribonucleotide (AIR) carboxylases from Escherichia coli and Gallus gallus have identified this central step in de novo purine biosynthesis as a case for unusual divergence in primary metabolism. Recent discoveries establish the fungal AIR carboxylase, encoded by the ADE2 gene, as essential for virulence in certain pathogenic organisms. This investigation is a biochemical analysis that links the fungal ADE2 protein to the function of the E. coli AIR carboxylase system. A cDNA clone of ADE2 from Cryptococcus neoformans was isolated by genetic complementation of a purE-deficient strain of E. coli. High-level expression of the C. neoformans ADE2 was achieved, which enabled the production and purification of AIR carboxylase. Amino acid sequence alignments, C-terminal deletion mutants, and biochemical assays indicate that the ADE2 enzyme is a two-domain, bifunctional protein. The N-terminal domain is related to E. coli PurK and a series of kinetic experiments show that the ADE2-PurK activity uses AIR, ATP, and HCO3- as substrates. The biosynthetic product of the ADE2-PurK reaction was identified as N5-carboxyaminoimidazole ribonucleotide (N5-CAIR) by 1H NMR, thus confirming that the C-terminal domain contains a catalytic activity similar to that of the E. coli PurE. By using an in situ system for substrate production, the steady-state kinetic constants for turnover of N5-CAIR by ADE2 were determined and together with stoichiometry measurements, these data indicate that ADE2 has a balance in the respective catalytic turnovers to ensure efficient flux. Distinctive features of the PurE active site were probed using 4-nitro-5-aminoimidazole ribonucleotide (NAIR), an analog of the product 4-carboxy-5-aminoimidazole ribonucleotide (CAIR). NAIR was shown to be a selective inhibitor of the ADE2-PurE activity (K1 = 2.4 microM), whereas it is a slow-binding inhibitor of the G. gallus enzyme which further distinguishes the fungal ADE2 from the G. gallus AIR carboxylase. As such, this enzyme represents a novel intracellular target for the discovery of antifungal agents.

Study of Cryptococcus neoformans actin gene regulation with a beta-galactosidase-actin fusion

An expression plasmid carrying a heterologous gene fusion between the Cryptococcus neoformans actin promoter and the Escherichia coli reporter gene, LACZ, was constructed to study actin regulation in C. neoformans. Two randomly stable transformants, designated 20.6 and 20.9, were selected for further examination. Both ectopic and homologous recombination with vector insertion in tandem repeats occurred in these transformants. Transformant 20.9 carried more copies of ACTp::LACZ in its genome than 20.6 and this was reflected in expressing higher levels of beta-galactosidase activity. In vitro, these transformants showed higher levels of beta-galactosidase activity expressed when the transformants were propagated at higher temperatures (37 degrees C vs 30 degrees C). However, beta-galactosidase expression in the transformants was variable during logarithmic and stationary growth phases and this differential expression was temperature dependent. This report shows that the constitutive actin gene in C. neoformans is regulated by temperature and growth and this fact should be taken into consideration when actin expression is used as a standard to compare the expression of other regulated genes. Also, a more sensitive reporter construct will be needed for in vivo gene analysis of regulation.

Calcineurin is required for virulence of Cryptococcus neoformans

Cyclosporin A (CsA) and FK506 are antimicrobial, immunosuppressive natural products that inhibit signal transduction. In T cells and Saccharomyces cerevisiae, CsA and FK506 bind to the immunophilins cyclophilin A and FKBP12 and the resulting complexes inhibit the Ca2+-regulated protein phosphatase calcineurin. We find that growth of the opportunistic fungal pathogen Cryptococcus neoformans is sensitive to CsA and FK506 at 37 degrees C but not at 24 degrees C, suggesting that CsA and FK506 inhibit a protein required for C. neoformans growth at elevated temperature. Genetic evidence supports a model in which immunophilin-drug complexes inhibit calcineurin to prevent growth at 37 degrees C. The gene encoding the C. neoformans calcineurin A catalytic subunit was cloned and disrupted by homologous recombination. Calcineurin mutant strains are viable but do not survive in vitro conditions that mimic the host environment (elevated temperature, 5% CO2 or alkaline pH) and are no longer pathogenic in an animal model of cryptococcal meningitis. Introduction of the wild-type calcineurin A gene complemented these growth defects and restored virulence. Our findings demonstrate that calcineurin is required for C. neoformans virulence and may define signal transduction elements required for fungal pathogenesis that could be targets for therapeutic intervention.

Dominant selection system for use in Cryptococcus neoformans

Present transformation systems for Cryptococcus neoformans depend on complementation of auxotrophic mutants. We have developed a dominant selection system for transformation of wild-type strains of cryptococci in which resistance to the antibiotic hygromycin B is used as the selectable marker. A heterologous fusion gene construct was created by attaching the putative promoter sequence and start site from a cryptococcal actin gene to a truncated hygromycin B phosphotransferase gene from E. coli. Biolistic transformation with this construct resulted in cryptococci resistant to hygromycin B, and transformation efficiencies approached approximately 500 transformants per microgram DNA. The construct was found to exist in transformants as both extrachromosomal and integrative forms. The transformants with integrated constructs were stable both in vitro and in vivo, and constructs were recoverable from most transformed cells using a plasmid rescue technique. This is the first dominant selection system for use in C. neoformans, and it should prove useful for molecular studies with this important pathogenic yeast.

Targeted gene replacement demonstrates that myristoyl-CoA: protein N-myristoyltransferase is essential for viability of Cryptococcus neoformans

Cryptococcus neoformans is a major cause of systemic fungal infection in immunocompromised patients. Myristoyl-CoA:protein N-myristoyltransferase (Nmt) catalyzes the transfer of myristate (C14:0) from myristoyl-CoA to the N-terminal glycine of a subset of cellular proteins produced during vegetative growth of C. neoformans. A Gly487-->Asp mutation was introduced into C. neoformans NMT by targeted gene replacement. The resulting strains are temperature-sensitive myristic acid auxotrophs. They are killed at 37 degrees C when placed in medium lacking myristate and, in an immunosuppressed animal model of cryptococcal meningitis, are completely eliminated from the subarachnoid space within 12 days of initial infection. C. neoformans and human Nmts exhibit differences in their peptide substrate specificities. These differences can be exploited to develop a new class of fungicidal drugs.

The gene encoding phosphoribosylaminoimidazole carboxylase (ADE2) is essential for growth of Cryptococcus neoformans in cerebrospinal fluid

A cryptococcal meningitis model in corticosteroid-treated rabbits was used to assess the requirement for the phosphoribosylaminoimidazole gene (ADE2) for virulence of Cryptococcus neoformans. A wild-type strain (H99), an ade2 auxotroph of H99 (M001), and a randomly selected prototrophic transformant of M001 (M001.1c) which had received the cloned ADE2 cDNA copy were inoculated intrathecally into immunosuppressed rabbits. While M001 was avirulent in the central nervous system model, virulence was completely restored to wild-type pathogenicity in the prototrophic transformant. This study identifies the pathogenic importance of an endogenous adenine pathway in this yeast and confirms that purine biosynthesis is a potential target for antifungal therapy. It also demonstrates that the virulence of C. neoformans can be molecularly changed and detected within a clinically relevant animal model.

Gene transfer in Cryptococcus neoformans by use of biolistic delivery of DNA

A transformation scheme for Cryptococcus neoformans to yield high-frequency, integrative events was developed. Adenine auxotrophs from a clinical isolate of C. neoformans serotype A were complemented by the cryptococcal phosphoribosylaminoimidazole carboxylase gene (ade2) with a biolistic DNA delivery system. Comparison of two DNA delivery systems (electroporation versus a biolistic system) showed notable differences. The biolistic system did not require linear vectors and transformed each auxotrophic strain at similar frequencies. Examination of randomly selected transformants by biolistics showed that 15 to 40% were stable, depending on the recipient auxotroph, with integrative events identified in all stable transformants by DNA analysis. Although the ade2 cDNA copy transformed at a low frequency, DNA analysis found homologous recombination in each of these transformants. DNA analysis of stable transformants receiving genomic ade2 revealed ectopic integration in a majority of cases, but approximately a quarter of the transformants showed homologous recombination with vector integration or gene replacement. This system has the potential for targeted gene disruption, and its efficiency will also allow for screening of DNA libraries within C. neoformans. Further molecular strategies to study the pathobiology of this pathogenic yeast are now possible with this transformation system.

Efficacy of immune therapy in early experimental Naegleria fowleri meningitis

Naegleria fowleri meningoencephalitis is usually fatal in humans despite treatment. As a new approach, we tested intracisternal passive immune therapy in rabbits with amebic meningoencephalitis by using antinaegleria immune serum, an immunoglobulin G fraction, and a newly developed monoclonal antibody to N. fowleri. Both the immune serum and an immunoglobulin G fraction isolated from it by affinity chromatography provided a consistent, although temporary, protective effect, shown by prolongation of survival (P = 0.001). Multiple doses of immune serum further prolonged survival (P = 0.005). The protective effect of serum was retained after heating to 56 degrees C. We then developed a monoclonal antibody to N. fowleri which provided similar protection. Passive intracisternal antibody therapy might serve as an adjunctive component in the treatment of amebic meningoencephalitis.

Augmentation of syngeneic tumor-specific immunity by semiallogeneic cell hybrids

Hybrid cell lines were established from fusions between lipopolysaccharide- (LPS) stimulated C57BL/6J spleen cells and MPC-11 tumor cells (45.6TG1.7, abbreviated M45), and were tested for their ability to immunize semiallogeneic mice against a parental tumor challenge. These hybrids were tumorigenic in syngeneic (BALB/c X C57BL/6J) F1 (CB6F1) mice but did not grow in semiallogeneic (BALB/c X A/J) F1 (CAF1) mice. All hybrids express both parental major histocompatibility antigens (H-2b and H-2d) as detected by indirect immunofluorescence and by their ability to function as either stimulators or targets for allogeneic cytotoxic lymphocytes (CTL). M45 tumor-associated antigens (TAA) were expressed on the hybrid surface as shown by their ability to act as either stimulators or targets for syngeneic CTL specific for M45 TAA. Immunization of semiallogeneic CAF1 mice with the hybrids i.p. followed by a challenge with M45 tumor cells resulted in extended survival when compared to untreated mice or animals immunized i.p. with M45 tumor cells. This immunity was specific and was not due to an allogeneic effect; immunization with an unrelated H-2bd tumor, 70Z/3, or H-2bd B6D2F1 spleen cells or with semiallogeneic spleen cells plus M45 did not protect mice from M45 challenge. Interestingly, prophylactic priming with semiallogeneic hybrid tumor cells or parental myeloma cells led to M45-specific CTL and "help" for an in vitro CTL response; however, the degree of CTL priming by hybrid tumors was not augmented when compared to the level of CTL achieved with parental tumor alone. Hence, stimulation of CTL activity per se by hybrid tumor cells cannot explain the protective effect of hybrid tumor immunization. These studies nevertheless confirm that semiallogeneic hybrids, which we show express TAA and alloantigens, can be used to immunize mice against a lethal syngeneic myeloma tumor challenge.

Augmentation of syngeneic tumor-specific immunity by semiallogeneic cell hybrids

Hybrid cell lines were established from fusions between lipopolysaccharide- (LPS) stimulated C57BL/6J spleen cells and MPC-11 tumor cells (45.6TG1.7, abbreviated M45), and were tested for their ability to immunize semiallogeneic mice against a parental tumor challenge. These hybrids were tumorigenic in syngeneic (BALB/c X C57BL/6J) F1 (CB6F1) mice but did not grow in semiallogeneic (BALB/c X A/J) F1 (CAF1) mice. All hybrids express both parental major histocompatibility antigens (H-2b and H-2d) as detected by indirect immunofluorescence and by their ability to function as either stimulators or targets for allogeneic cytotoxic lymphocytes (CTL). M45 tumor-associated antigens (TAA) were expressed on the hybrid surface as shown by their ability to act as either stimulators or targets for syngeneic CTL specific for M45 TAA. Immunization of semiallogeneic CAF1 mice with the hybrids i.p. followed by a challenge with M45 tumor cells resulted in extended survival when compared to untreated mice or animals immunized i.p. with M45 tumor cells. This immunity was specific and was not due to an allogeneic effect; immunization with an unrelated H-2bd tumor, 70Z/3, or H-2bd B6D2F1 spleen cells or with semiallogeneic spleen cells plus M45 did not protect mice from M45 challenge. Interestingly, prophylactic priming with semiallogeneic hybrid tumor cells or parental myeloma cells led to M45-specific CTL and "help" for an in vitro CTL response; however, the degree of CTL priming by hybrid tumors was not augmented when compared to the level of CTL achieved with parental tumor alone. Hence, stimulation of CTL activity per se by hybrid tumor cells cannot explain the protective effect of hybrid tumor immunization. These studies nevertheless confirm that semiallogeneic hybrids, which we show express TAA and alloantigens, can be used to immunize mice against a lethal syngeneic myeloma tumor challenge.