Variable expression of a yeast-phase-specific gene in Histoplasma capsulatum strains differing in thermotolerance and virulence

Genotypic diversity, virulence, and molecular genetic tools in Histoplasma

SUMMARYHistoplasmosis is arguably the most common fungal respiratory infection worldwide, with hundreds of thousands of new infections occurring annually in the United States alone. The infection can progress in the lung or disseminate to visceral organs and can be difficult to treat with antifungal drugs. Histoplasma, the causative agent of the disease, is a pathogenic fungus that causes life-threatening lung infections and is globally distributed. The fungus has the ability to germinate from conidia into either hyphal (mold) or yeast form, depending on the environmental temperature. This transition also regulates virulence. Histoplasma and histoplasmosis have been classified as being of emergent importance, and in 2022, the World Health Organization included Histoplasma as 1 of the 19 most concerning human fungal pathogens. In this review, we synthesize the current understanding of the ecological niche, evolutionary history, and virulence strategies of Histoplasma. We also describe general patterns of the symptomatology and epidemiology of histoplasmosis. We underscore areas where research is sorely needed and highlight research avenues that have been productive.

Immunoproteomics Reveals Pathogen's Antigens Involved in Homo sapiens-Histoplasma capsulatum Interaction and Specific Linear B-Cell Epitopes in Histoplasmosis

Histoplasmosis is one of the most frequent systemic mycosis in HIV patients. In these patients, histoplasmosis has high rates of morbidity/mortality if diagnosis and treatment are delayed. Despite its relevance, there is a paucity of information concerning the interaction between Histoplasma capsulatum and the human host, especially regarding the B-cell response, which has a direct impact on the diagnosis. Culture-based “gold-standard” methods have limitations, making immunodiagnostic tests an attractive option for clinical decisions. Despite the continuous development of those tests, improving serological parameters is necessary to make these methods efficient tools for definitive diagnosis of histoplasmosis. This includes the determination of more specific and immunogenic antigens to improve specificity and sensitivity of assays. In this study, we performed a co-immunoprecipitation assay between a protein extract from the yeast form of H. capsulatum and pooled sera from patients with proven histoplasmosis, followed by shotgun mass spectrometry identification of antigenic targets. Sera from patients with other pulmonary infections or from healthy individuals living in endemic areas of histoplasmosis were also assayed to determine potentially cross-reactive proteins. The primary structures of H. capsulatum immunoprecipitated proteins were evaluated using the DNAStar Protean 7.0 software. In parallel, the online epitope prediction server, BCPREDS, was used to complement the B-epitope prediction analysis. Our approach detected 132 reactive proteins to antibodies present in histoplasmosis patients’ sera. Among these antigens, 127 were recognized also by antibodies in heterologous patients’ and/or normal healthy donors’ sera. Therefore, the only three antigens specifically recognized by antibodies of histoplasmosis patients were mapped as potential antigenic targets: the M antigen, previously demonstrated in the diagnosis of histoplasmosis, and the catalase P and YPS-3 proteins, characterized as virulence factors of H. capsulatum, with antigenic properties still unclear. The other two proteins were fragments of the YPS-3 and M antigen. Overlapping results obtained from the two aforementioned bioinformatic tools, 16 regions from these three proteins are proposed as putative B-cell epitopes exclusive to H. capsulatum. These data reveal a new role for these proteins on H. capsulatum interactions with the immune system and indicate their possible use in new methods for the diagnosis of histoplasmosis.

Sensing the heat and the host: Virulence determinants of Histoplasma capsulatum

Histoplasma capsulatum is a member of a group of fungal pathogens called thermally dimorphic fungi, all of which respond to mammalian body temperature by converting from an environmental mold form into a parasitic host form that causes disease. Histoplasma is a primary fungal pathogen, meaning it is able to cause disease in healthy individuals. We are beginning to understand how host temperature is utilized as a key signal to facilitate growth in the parasitic yeast form and promote production of virulence factors. In recent years, multiple regulators of morphology and virulence have been identified in Histoplasma. Mutations in these regulators render the pathogen unable to convert to the parasitic yeast form. Additionally, several virulence factors have been characterized for their importance in in vivo survival and pathogenesis. These virulence factors and regulators can serve as molecular handles for the development of effective drugs and therapeutics to counter Histoplasma infection.

Dimorphism and Dissemination of Histoplasma capsulatum in the Upper Respiratory Tract after Intranasal Infection of Bats and Mice with Mycelial Propagules

This article describes, for the first time, the role of the nasal mucosa (NM) as the initial site for the Histoplasma capsulatum mycelial-to-yeast transition. The results highlight that yeasts may arrive to the cervical lymph nodes (CLN) via phagocytes. Bats and mice were intranasally infected with H. capsulatum mycelial propagules and they were killed 10, 20, and 40 minutes and 1, 2, and 3 hours after infection. The NM and the CLN were monitored for fungal presence. Yeasts compatible with H. capsulatum were detected within the NM and the CLN dendritic cells (DCs) 2–3 hours postinfection, using immunohistochemistry. Histoplasma capsulatum was re-isolated by culturing at 28°C from the CLN of both mammalian hosts 2–3 hours postinfection. Reverse transcription-polymerase chain reaction assays were designed to identify fungal dimorphism, using mycelial-specific (MS8) and yeast-specific (YPS3) gene expression. This strategy supported fast fungal dimorphism in vivo, which began in the NM 1 hour postinfection (a time point when MS8 and YPS3 genes were expressed) and it was completed at 3 hours (a time point when only the YPS3 transcripts were detected) in both bats and mice. The presence of intracellular yeasts in the nasal-associated lymphoid tissue (NALT), in the NM nonassociated with the NALT, and within the interdigitating DCs of the CLN suggests early fungal dissemination via the lymph vessels.

Thermotolerance of Histoplasma capsulatum at 40 °C predominates among clinical isolates from different Latin American regions

The yeast phase of 22 Histoplasma capsulatum clinical isolates from Mexico, Argentina, Colombia, and Guatemala and three reference strains, one from Panama and two from the United States of America (USA), were screened for thermosensitivity characteristics using different analyses. Growth curves at 0, 3, 6, 12, 24, and 30 h of incubation at 37 and 40 °C, the growth inhibition percentage at 40 °C, and the doubling time at 37 and 40 °C were determined for all yeasts studied. Most of the isolates examined exhibited thermotolerant phenotypes at 40 °C, whereas a thermosensitive phenotype at 40 °C was only detected in the Downs reference strain from the USA. Growth inhibition values lower than 33.8% supported the predominance of the thermotolerant phenotype at 40 °C. The doubling time means found for the different isolates were 5.14 h ± 1.47 h at 37 °C and 5.55 h ± 1.87 h at 40 °C. This is the first report to underscore the predominance of thermotolerant and delayed doubling time phenotypes in H. capsulatum clinical isolates from different regions of Latin America.

Molecular regulation of Histoplasma dimorphism

Temperature serves as a fundamental signal in biological systems. In some microbial pathogens of humans, mammalian body temperature triggers establishment and maintenance of a developmental program that allows the microbe to survive and thrive in the host. Histoplasma capsulatum is one of a group of fungal pathogens called thermally dimorphic fungi, all of which respond to mammalian body temperature by converting from an environmental mold form that inhabits the soil into a parasitic form that causes disease in the host. It has been known for decades that temperature is a key signal that is sufficient to trigger the switch from the soil to host form (and vice versa) in the laboratory. Recent molecular studies have identified a number of key regulators that are required to specify each of the developmental forms in response to temperature. Here we review the regulatory circuits that govern temperature-dependent dimorphism in Histoplasma.

Genome Sequences Reveal Cryptic Speciation in the Human Pathogen Histoplasma capsulatum

Histoplasma capsulatum is a pathogenic fungus that causes life-threatening lung infections. About 500,000 people are exposed to H. capsulatum each year in the United States, and over 60% of the U.S. population has been exposed to the fungus at some point in their life. We performed genome-wide population genetics and phylogenetic analyses with 30 Histoplasma isolates representing four recognized areas where histoplasmosis is endemic and show that the Histoplasma genus is composed of at least four species that are genetically isolated and rarely interbreed. Therefore, we propose a taxonomic rearrangement of the genus.IMPORTANCE The evolutionary processes that give rise to new pathogen lineages are critical to our understanding of how they adapt to new environments and how frequently they exchange genes with each other. The fungal pathogen Histoplasma capsulatum provides opportunities to precisely test hypotheses about the origin of new genetic variation. We find that H. capsulatum is composed of at least four different cryptic species that differ genetically and also in virulence. These results have implications for the epidemiology of histoplasmosis because not all Histoplasma species are equivalent in their geographic range and ability to cause disease.

The Mould-specific M46 gene is not essential for yeast-mould dimorphism in the pathogenic fungus Histoplasma capsulatum

Histoplasma capsulatum (Hc) is the causative agent for the respiratory infection histoplasmosis. The fungus exists in the environment as a saprophytic multi-cellular mould. Spores are inhaled by mammals whereupon the organism will convert into the single-celled yeast morphotype resulting in infection. The shift to the yeast morphotype is required for pathogenesis. Most studies on dimorphism have examined yeast-phase-specific genes and few mould-phase-specific genes have been investigated. It is likely, that some mould-phase-specific genes must be downregulated for the yeast to form or upregulated for the mould to form. We isolated a strongly expressed mould-specific gene, M46, from an expression library enriched for mould upregulated genes in Hc strain G186AS. To determine if M46 is involved in dimorphism, M46 was ectopically expressed in yeast phase growing temperature, and an m46 knockout strain was created via allelic replacement. Ectopically expressing M46 in yeast, did not induce filamentous growth. Genomic disruption of M46 by allelic replacement did not alter the morphology of the mould as seen in bright field microscopy, scanning electron microscopy, and transmission electron microscopy. A growth curve study, revealed that M46 is not involved in maintaining the growth rate of cells. These findings indicate that the mould specific M46 gene is not necessary nor essential for dimorphism, maintaining the normal mould morphology, and growth rate of Histoplasma capsulatum.

Histoplasma capsulatum surmounts obstacles to intracellular pathogenesis

The fungal pathogen Histoplasma capsulatum causes respiratory and disseminated disease, even in immunocompetent hosts. In contrast to opportunistic pathogens, which are readily controlled by phagocytic cells, H. capsulatum yeasts are able to infect macrophages, survive antimicrobial defenses, and proliferate as an intracellular pathogen. In this review, we discuss some of the molecular mechanisms that enable H. capsulatum yeasts to overcome obstacles to intracellular pathogenesis. H. capsulatum yeasts gain refuge from extracellular obstacles such as antimicrobial lung surfactant proteins by engaging the β-integrin family of phagocytic receptors to promote entry into macrophages. In addition, H. capsulatum yeasts conceal immunostimulatory β-glucans to avoid triggering signaling receptors such as the β-glucan receptor Dectin-1. H. capsulatum yeasts counteract phagocyte-produced reactive oxygen species by expression of oxidative stress defense enzymes including an extracellular superoxide dismutase and an extracellular catalase. Within the phagosome, H. capsulatum yeasts block phagosome acidification, acquire essential metals such as iron and zinc, and utilize de novo biosynthesis pathways to overcome nutritional limitations. These mechanisms explain how H. capsulatum yeasts avoid and negate macrophage defense strategies and establish a hospitable intracellular niche, making H. capsulatum a successful intracellular pathogen of macrophages.

Comparison of phylogenetically distinct Histoplasma strains reveals evolutionarily divergent virulence strategies

Infection with the dimorphic fungus Histoplasma capsulatum results from the inhalation of contaminated soil. Disease outcome is variable and depends on the immune status of the host, number of organisms inhaled, and the H. capsulatum strain. H. capsulatum is divided into seven distinct clades based on phylogenetic analyses, and strains from two separate clades have been identified in North America (denoted as NAm strains). We characterized an H. capsulatum isolate (WU24) from the NAm 1 lineage in relation to two other well-characterized Histoplasma isolates, the Panamanian strain G186A and the NAm 2 strain G217B. We determined that WU24 is a chemotype II strain and requires cell wall α-(1,3)-glucan for successful in vitro infection of macrophages. In a mouse model of histoplasmosis, WU24 exhibited a disease profile that was very similar to that of strain G186A at a high sublethal dose; however, at this dose G217B had markedly different kinetics. Surprisingly, infection with a lower dose mitigated many of the differences during the course of infection. The observed differences in fungal burden, disease kinetics, symptomology, and cytokine responses all indicate that there is a sophisticated relationship between host and fungus that drives the development and progression of histoplasmosis. Importance: Histoplasmosis has a wide range of clinical manifestations, presenting as mild respiratory distress, acute respiratory infection, or a life-threatening disseminated disease most often seen in immunocompromised patients. Additionally, the outcome appears to be dependent on the amount and strain of fungus inhaled. In this study, we characterized a recent clinical H. capsulatum isolate that was collected from an HIV(+) individual in North America. In contrast to other isolates from the same lineage, this strain, WU24, infected both macrophages and wild-type mice. We determined that in contrast to many other North American strains, WU24 infection of macrophages is dependent on the presence of cell wall α-(1,3)-glucan. Surprisingly, comparison of WU24 with two previously characterized isolates revealed that many conclusions regarding relative strain virulence and certain hallmarks of histoplasmosis are dependent on the inoculum size.

Histoplasma yeast and mycelial transcriptomes reveal pathogenic-phase and lineage-specific gene expression profiles

Background

The dimorphic fungus Histoplasma capsulatum causes respiratory and systemic disease in mammalian hosts by expression of factors that enable survival within phagocytic cells of the immune system. Histoplasma’s dimorphism is distinguished by growth either as avirulent mycelia or as pathogenic yeast. Geographically distinct strains of Histoplasma differ in their relative virulence in mammalian hosts and in production of and requirement for specific virulence factors. The close similarity in the genome sequences of these diverse strains suggests that phenotypic variations result from differences in gene expression rather than gene content. To provide insight into how the transcriptional program translates into morphological variation and the pathogenic lifestyle, we compared the transcriptional profile of the pathogenic yeast phase and the non-pathogenic mycelial phase of two clinical isolates of Histoplasma.

Results

To overcome inaccuracies in ab initio genome annotation of the Histoplasma genome, we used RNA-seq methodology to generate gene structure models based on experimental evidence. Quantitative analyses of the sequencing reads revealed 6% to 9% of genes are differentially regulated between the two phases. RNA-seq-based mRNA quantitation was strongly correlated with gene expression levels determined by quantitative RT-PCR. Comparison of the yeast-phase transcriptomes between strains showed 7.6% of all genes have lineage-specific expression differences including genes contributing, or potentially related, to pathogenesis. GFP-transcriptional fusions and their introduction into both strain backgrounds revealed that the difference in transcriptional activity of individual genes reflects both variations in the cis- and trans-acting factors between Histoplasma strains.

Conclusions

Comparison of the yeast and mycelial transcriptomes highlights genes encoding virulence factors as well as those involved in protein glycosylation, alternative metabolism, lipid remodeling, and cell wall glycanases that may contribute to Histoplasma pathogenesis. These studies lay an essential foundation for understanding how gene expression variations contribute to the strain- and phase-specific virulence differences of Histoplasma.

N-acetylglucosamine (GlcNAc) triggers a rapid, temperature-responsive morphogenetic program in thermally dimorphic fungi

The monosaccharide N-acetylglucosamine (GlcNAc) is a major component of microbial cell walls and is ubiquitous in the environment. GlcNAc stimulates developmental pathways in the fungal pathogen Candida albicans, which is a commensal organism that colonizes the mammalian gut and causes disease in the setting of host immunodeficiency. Here we investigate GlcNAc signaling in thermally dimorphic human fungal pathogens, a group of fungi that are highly evolutionarily diverged from C. albicans and cause disease even in healthy individuals. These soil organisms grow as polarized, multicellular hyphal filaments that transition into a unicellular, pathogenic yeast form when inhaled by a human host. Temperature is the primary environmental cue that promotes reversible cellular differentiation into either yeast or filaments; however, a shift to a lower temperature in vitro induces filamentous growth in an inefficient and asynchronous manner. We found GlcNAc to be a potent and specific inducer of the yeast-to-filament transition in two thermally dimorphic fungi, Histoplasma capsulatum and Blastomyces dermatitidis. In addition to increasing the rate of filamentous growth, micromolar concentrations of GlcNAc induced a robust morphological transition of H. capsulatum after temperature shift that was independent of GlcNAc catabolism, indicating that fungal cells sense GlcNAc to promote filamentation. Whole-genome expression profiling to identify candidate genes involved in establishing the filamentous growth program uncovered two genes encoding GlcNAc transporters, NGT1 and NGT2, that were necessary for H. capsulatum cells to robustly filament in response to GlcNAc. Unexpectedly, NGT1 and NGT2 were important for efficient H. capsulatum yeast-to-filament conversion in standard glucose medium, suggesting that Ngt1 and Ngt2 monitor endogenous levels of GlcNAc to control multicellular filamentous growth in response to temperature. Overall, our work indicates that GlcNAc functions as a highly conserved cue of morphogenesis in fungi, which further enhances the significance of this ubiquitous sugar in cellular signaling in eukaryotes.

Comparative transcriptomics of infectious spores from the fungal pathogen Histoplasma capsulatum reveals a core set of transcripts that specify infectious and pathogenic states

Histoplasma capsulatum is a fungal pathogen that infects both healthy and immunocompromised hosts. In regions where it is endemic, H. capsulatum grows in the soil and causes respiratory and systemic disease when inhaled by humans. An interesting aspect of H. capsulatum biology is that it adopts specialized developmental programs in response to its environment. In the soil, it grows as filamentous chains of cells (mycelia) that produce asexual spores (conidia). When the soil is disrupted, conidia aerosolize and are inhaled by mammalian hosts. Inside a host, conidia germinate into yeast-form cells that colonize immune cells and cause disease. Despite the ability of conidia to initiate infection and disease, they have not been explored on a molecular level. We developed methods to purify H. capsulatum conidia, and we show here that these cells germinate into filaments at room temperature and into yeast-form cells at 37°C. Conidia internalized by macrophages germinate into the yeast form and proliferate within macrophages, ultimately lysing the host cells. Similarly, infection of mice with purified conidia is sufficient to establish infection and yield viable yeast-form cells in vivo. To characterize conidia on a molecular level, we performed whole-genome expression profiling of conidia, yeast, and mycelia from two highly divergent H. capsulatum strains. In parallel, we used homology and protein domain analysis to manually annotate the predicted genes of both strains. Analyses of the resultant data defined sets of transcripts that reflect the unique molecular states of H. capsulatum conidia, yeast, and mycelia.

Histoplasma mechanisms of pathogenesis--one portfolio doesn't fit all

Histoplasma capsulatum is the leading cause of endemic mycosis in the world. Analyses of clinical isolates from different endemic regions show important diversity within the species. Recent molecular studies of two isolates, the Chemotype I NAm2 strain G217B and the Chemotype II Panamanian strain G186A, reveal significant genetic, structural, and molecular differences between these representative Histoplasma strains. Some of these variations have functional consequences, representing distinct molecular mechanisms that facilitate Histoplasma pathogenesis. The realization of Histoplasma strain diversity highlights the importance of characterizing Histoplasma virulence factors in the context of specific clinical strain isolates.

Phenotypic characteristics of isolates of Aspergillus section Fumigati from different geographic origins and their relationships with genotypic characteristics

BACKGROUND

Epidemiological studies worldwide have shown that A. fumigatus exhibits important phenotypic and genotypic diversity, and these findings have been of great importance in improving the diagnosis and treatment of diseases caused by this fungus. However, few studies have been carried out related to the epidemiology of this fungus in Latin America. This study's aim is to report on the epidemiology of the fungus by analyzing the phenotypic variability of Aspergillus section Fumigati isolates from different Latin American countries and the relationship between this variability, the geographical origin and genotypic characteristics.

METHODS

We analyzed the phenotypic characteristics (macro- and micromorphology, conidial size, vesicles size, antifungal susceptibility and thermotolerance at 28, 37 and 48°C) of A. section Fumigati isolates from Mexico (MX), Argentina (AR), Peru (PE) and France (FR). The results were analyzed using analysis of variance (ANOVA) and Tukey's multiple comparison test to detect significant differences. Two dendrograms among isolates were obtained with UPGMA using the Euclidean distance index. One was drawn for phenotypic data, and the other for phenotypic and genotypic data. A PCoA was done for shown isolates in a space of reduced dimensionality. In order to determine the degree of association between the phenotypic and genotypic characteristics AFLP, we calculated the correlation between parwise Euclidean distance matrices of both data sets with the nonparametric Mantel test.

RESULTS

No variability was found in the macromorphology of the studied isolates; however, the micromorphology and growth rate showed that the PE isolates grew at a faster rate and exhibited the widest vesicles in comparison to the isolates from MX, AR and FR. The dendrogram constructed with phenotypic data showed three distinct groups. The group I and II were formed with isolates from PE and FR, respectively, while group III was formed with isolates from MX and AR. The dendrogram with phenotypic and genotypic data showed the same cluster, except for an isolate from FR that formed a separate cluster. This cluster was confirmed using PCoA. The correlation between the phenotypic and genotypic data of the isolates revealed a statistically significant association between these characteristics.

CONCLUSIONS

The PE isolates showed specific phenotypic characteristics that clearly differentiate them from the rest of the isolates, which matches the genotypic data. The correlation between the phenotypic and genotypic characteristics showed a statistically significant association. In conclusion, phenotypic and genotypic methods together increase the power of correlation between isolates.

Definition of the extracellular proteome of pathogenic-phase Histoplasma capsulatum

The dimorphic fungal pathogen Histoplasma capsulatum causes respiratory and systemic disease. Within the mammalian host, pathogenic Histoplasma yeast infect, replicate within, and ultimately kill host phagocytes. Surprisingly, few factors have been identified that contribute to Histoplasma virulence. To address this deficiency, we have defined the constituents of the extracellular proteome using LC-MS/MS analysis of the proteins in pathogenic-phase culture filtrates of Histoplasma. In addition to secreted Cbp1, the extracellular proteome of pathogenic Histoplasma yeast consists of 33 deduced proteins. The proteins include glycanases, extracellular enzymes related to oxidative stress defense, dehydrogenase enzymes, chaperone-like factors, and five novel culture filtrate proteins (Cfp's). For independent verification of proteomics-derived identities, we employed RNA interference (RNAi)-based depletion of candidate factors and showed loss of specific proteins from the cell-free culture filtrate. Quantitative RT-PCR revealed the expression of 10 of the extracellular factors was particularly enriched in pathogenic yeast cells as compared to nonpathogenic Histoplasma mycelia, suggesting that these proteins are linked to Histoplasma pathogenesis. In addition, Histoplasma yeast express these factors within macrophages and during infection of murine lungs. As extracellular proteins are positioned at the interface between host and pathogen, the definition of the pathogenic-phase extracellular proteome provides a foundation for the molecular dissection of how Histoplasma alters the host-pathogen interaction to its advantage.

Histoplasma capsulatum heat-shock 60 orchestrates the adaptation of the fungus to temperature stress

Heat shock proteins (Hsps) are among the most widely distributed and evolutionary conserved proteins. Hsps are essential regulators of diverse constitutive metabolic processes and are markedly upregulated during stress. A 62 kDa Hsp (Hsp60) of Histoplasma capsulatum (Hc) is an immunodominant antigen and the major surface ligand to CR3 receptors on macrophages. However little is known about the function of this protein within the fungus. We characterized Hc Hsp60-protein interactions under different temperature to gain insights of its additional functions oncell wall dynamism, heat stress and pathogenesis. We conducted co-immunoprecipitations with antibodies to Hc Hsp60 using cytoplasmic and cell wall extracts. Interacting proteins were identified by shotgun proteomics. For the cell wall, 84 common interactions were identified among the 3 growth conditions, including proteins involved in heat-shock response, sugar and amino acid/protein metabolism and cell signaling. Unique interactions were found at each temperature [30°C (81 proteins), 37°C (14) and 37/40°C (47)]. There were fewer unique interactions in cytoplasm [30°C (6), 37°C (25) and 37/40°C (39)] and four common interactions, including additional Hsps and other known virulence factors. These results show the complexity of Hsp60 function and provide insights into Hc biology, which may lead to new avenues for the management of histoplasmosis.

Comparison of different DNA-based methods for molecular typing of Histoplasma capsulatum

Histoplasma capsulatum is very prevalent in the environment and is one of the most common causes of mycoses in humans and diverse animals in Brazil. Multiple typing methods have been developed to study H. capsulatum epidemiology; however, there is limited information concerning comparisons of results obtained with different methods using the same set of isolates. To explore the diversity of H. capsulatum in Brazil and to determine correlations between the results of three different molecular typing techniques, we examined 51 environmental, animal, and human isolates by M13 PCR fingerprinting, PCR-restriction fragment length polymorphism (RFLP) analysis of the internal transcribed region 1 (ITS1)-5.8S-ITS2 region of the rDNA locus, and DNA sequencing and phylogenetic analysis of parts of four protein-encoding genes, the Arf (ADP ribosylation factor), H-anti (H antigen precursor), Ole (delta-9 fatty acid desaturase), and Tub1 (alpha-tubulin) genes. Each method identified three major genetic clusters, and there was a high level of concordance between the results of the typing techniques. The M13 PCR fingerprinting and PCR-RFLP analyses produced very similar results and separated the H. capsulatum isolates included in this study into three major groups. An additional approach used was comparison of our Brazilian ITS1-5.8S-ITS2 sequences with the sequences deposited previously in NCBI data banks. Our analyses suggest that H. capsulatum can be divided into different molecular types that are dispersed around the world. Our results indicate that the three methods used in this study are reliable and reproducible and that they have similar sensitivities. However, M13 PCR fingerprinting has some advantages over the other two methods as it is faster, cheaper, and more user friendly, which especially increases its utility for molecular typing of Histoplasma in situations where laboratory facilities are relatively limited.

Generating cell surface diversity in Candida albicans and other fungal pathogens

The fungal cell surface contributes to pathogenesis by mediating interactions with host cells and eliciting host immune responses. This review focuses on the cell wall proteome of the major fungal pathogen Candida albicans and discusses how diversity at the cell surface can be introduced by altering the expression and structure of cell wall proteins. Remodelling the cell wall architecture is critical to maintain cellular integrity in response to different environments and stresses including challenge with antifungal drugs. In addition, the dynamic nature of the cell surface alters the physical properties of the fungal interface with host cells and thereby influences adhesion to the host and recognition by components of the host's immune system. Examples of the role of cell surface diversity in the pathogenesis of a number of microorganisms are described.

Histoplasma capsulatum yeast phase-specific protein Yps3p induces Toll-like receptor 2 signaling

Histoplasma capsulatum is a common cause of fungal infection in certain geographic areas, and although most infections are asymptomatic, it is capable of causing histoplasmosis, a disseminated, life-threatening disease, especially in immunocompromised individuals. A deeper understanding of this host-pathogen interaction is needed to develop novel therapeutic strategies to counter lethal infection. Although several lines of evidence suggest that this fungus is neurotropic in HIV patients, little is known about the immunobiology of Histoplasma infection in the central nervous system [CNS]. The goal of the present study was to understand the innate neuroimmune mechanisms that recognize H. capsulatum during the initial stages of infection. Using a 293T stable cell line expressing murine Toll-like receptor 2 [TLR2], we show here that TLR2 recognizes H. capsulatum cell wall protein Yps3p and induces the activation of NF-κB. In further experiments, we tested the ability of Yps3p to induce signaling from TLR2 in primary microglial cells, the resident brain macrophages of the CNS. Our data show that H. capsulatum Yps3p induced TLR2 signaling in wild-type microglia, but not in microglia isolated from TLR2 KO mice, confirming that Yps3p is a ligand for TLR2. Furthermore, Yps3p-induced TLR2 signaling was suppressed by vaccinia virus-encoded TLR inhibitors. This is the first demonstration of a fungal protein serving as a TLR ligand and mediating signaling in primary brain cells.

Histoplasma capsulatum pathogenesis: making a lifestyle switch

The dimorphism of Histoplasma reflects a developmental switch in morphology and lifestyle that is necessary for virulence. The dimorphism regulating kinase DRK1 and the Histoplasma WOR1 homolog RYP1 mediate the thermally induced transition to the pathogenic yeast-phase program. The genes expressed as part of this regulon influence the host-pathogen interaction to favor Histoplasma virulence. While surface localized HSP60 supports yeast attachment to host macrophages, yeast alpha-glucan polysaccharides conceal immunostimulatory cell wall beta-glucans from detection by macrophage receptors. Intramacrophage growth of yeast cells is facilitated by CBP a secreted, protease-resistant calcium-binding protein tailored to function within the phagolysosomal environment. In some Histoplasma strains, YPS3 promotes dissemination of yeast from pulmonary infection sites. The Histoplasma yeast-phase program includes additional cell surface and extracellular molecules that potentially function in further aspects of Histoplasma virulence.

Detection and measurement of two-component systems that control dimorphism and virulence in fungi

Systemic dimorphic fungi include six phylogenetically related ascomycetes. These organisms grow in a mold form in the soil on most continents around the world. After the mold spores, which are the infectious particles, are inhaled into the lung of a susceptible mammalian host, they undergo a morphological change into a pathogenic yeast form. The ability to convert to the yeast form is essential for this class of fungal agents to be pathogenic and produce disease. Temperature change is one key stimulus that triggers the phase transition from mold (25 degrees ) to yeast (37 degrees ). Genes that are expressed only in the pathogenic yeast form of these fungi have been identified to help explain how and why this phase transition is required for virulence. However, the regulators of yeast-phase specific genes, especially of phase transition from mold to yeast, have remained poorly understood. We used Agrobacterium-mediated gene transfer for insertional mutagenesis to create mutants that are defective in the phase transition and to identify genes that regulate this critical event. We discovered that a hybrid histidine kinase senses environmental signals such as temperature and regulates phase transition, dimorphism, and virulence in members of this fungal family. This chapter describes our approach to the identification and analysis of this global regulator.

Dimorphism and virulence in fungi

The signature feature of systemic dimorphic fungi - a family of six primary fungal pathogens of humans - is a temperature-induced phase transition. These fungi grow as a mold in soil at ambient temperature and convert to yeast after infectious spores are inhaled into the lungs of a mammalian host. Seminal work 20 years ago established that a temperature-induced phase transition from mold to yeast is required for virulence. Several yeast-phase specific genes, identified one-by-one and studied by reverse genetics, have revealed mechanisms by which the phase transition promotes disease pathogenesis. Transcriptional profiling of microarrays built with genomic elements of Histoplasma capsulatum and ESTs of Paracoccidioides brasiliensis that represent partial genomes has identified 500 genes and 328 genes, respectively, that are differentially expressed upon the phase transition. The genomes of most of the dimorphic fungi are now in varying stages of being sequenced. The creation of additional microarrays and the application of new reverse genetic tools promise fresh insight into genes and mechanisms that regulate pathogenesis and morphogenesis. The use of insertional mutagenesis by Agrobacterium has uncovered a hybrid histidine kinase that regulates dimorphism and pathogenicity in Blastomyces dermatitidis and H. capsulatum. Two-component signaling appears to be a common strategy for model and pathogenic fungi to sense and respond to environmental stresses.

Detection of constitutive molecules onHistoplasma capsulatumyeasts through single chain variable antibody fragments displayed in M13 phages

A nonimmune library, containing single chain variable fragments (scFv) of immunoglobulin human genes displayed on the surface of M13 filamentous phages, was used to recognize molecules exposed on Histoplasma capsulatum yeasts' surface, during their growth in synthetic medium. The scFv clones were checked in their consistency by Dot-ELISA using HRP/anti-M13 conjugate, and they were tested to recognize molecules on H. Capsulatum yeasts' surface by ELISA in plates. Three out of 80 scFv cones (C2, C6, and C52) reacted consistently with H. capsulatum molecules, and they recognized molecules from both H. capsulatum morphologic phases. However, C6 and C52 clones reacted better with molecules on the surface of whole yeasts, with molecules from the yeasts' cell-wall extract, and with molecules released to the supernatant of the yeast culture. Mycelial supernatants from other fungi, as well as from a Mycobacterium filtrate, were not recognized by scFv phage monoclones. Monoclones C2, C6, and C52 recognized yeast molecules irrespective of the H. capsulatum strains used; the C6 clone revealed a specific immunohistochemistry reaction when tested against homologous and heterologous fungal infected tissues. The scFv clones isolated will be a useful toll to define the role of their target molecules in the host-parasite relationship of histoplasmosis.

RNA interference-mediated silencing of the YPS3 gene of Histoplasma capsulatum reveals virulence defects

The YPS3 gene of Histoplasma capsulatum encodes a protein that is both surface localized in the cell wall of H. capsulatum and released into the culture medium. This protein is produced only during the pathogenic yeast phase of infection and is also expressed differentially in H. capsulatum strains of different virulence levels. In this study, we silenced the YPS3 transcript by using an interfering-RNA strategy and examined the silenced mutants for phenotypic differences in vitro and during infection. The mutants showed no growth defect during in vitro culture in a defined medium at 37 degrees C and appeared to have normal virulence in a RAW 264.7 murine macrophage-like cell line. In a C57BL/6 mouse model of infection, however, the mutants caused significantly decreased fungal burdens, particularly in the peripheral phagocyte-rich tissues of livers and spleens. This defect in organ colonization was evident within 3 days of infection; however, it appeared to be exacerbated at later time points.

Expression and interstrain variability of the YPS3 gene of Histoplasma capsulatum

The YPS3 locus of the dimorphic fungus Histoplasma capsulatum encodes a secreted and surface-localized protein specific to the pathogenic yeast phase. In this study we examined this locus in 32 H. capsulatum strains and variants. Although protein production is limited to a select group of strains, the North American restriction fragment length polymorphism class 2/NAm 2 isolates, the locus was present in all the strains we examined. The YPS3 gene is well conserved in its 5' and 3' regions but displays an intragenic hypervariable region of tandem repeats that fluctuates in size between strains. This feature is similar to that seen with genes encoding several cell surface proteins in other fungi.

Defining Virulence Genes in the Dimorphic Fungi

Most dimorphic fungal pathogens cause respiratory disease in mammals and must therefore possess virulence mechanisms to combat and overcome host pulmonary defenses. Over the past decade, advances in genetic tools have made it possible to investigate the basis of dimorphic fungal pathogenesis at the molecular level. Gene disruptions and RNA interference have now formally demonstrated the involvement of six virulence factors: CBP, alpha-(1,3)-glucan, BAD1, SOWgp, Mep1, and urease. Additional candidate virulence-associated genes have been identified on the premise that factors necessary for pathogenicity are associated specifically with the parasitic form. This principle continues to form the foundation for genomics-based analyses to further augment the list. Thus, the stage is set and the tools are in place for the next phase of medical mycology research: defining the virulence-associated factors underlying the success of dimorphic fungal pathogens.

Surface localization of the Yps3p protein of Histoplasma capsulatum

The YPS3 gene of Histoplasma capsulatum encodes a protein that is both resident in the cell wall and also released into the culture medium. This protein is produced only during the pathogenic yeast phase of infection and is also expressed differently in H. capsulatum strains that differ in virulence. We investigated the cellular localization of Yps3p. We demonstrated that the cell wall fraction of Yps3p was surface localized in restriction fragment length polymorphism class 2 strains. We also established that Yps3p released into the G217B culture supernatant binds to the surface of strains that do not naturally express the protein. This binding was saturable and occurred within 5 min of exposure and occurred similarly with live and heat-killed H. capsulatum. Flow cytometric analysis of H. capsulatum after enzymatic treatments was consistent with Yps3p binding to chitin, a carbohydrate polymer that is a component of fungal cell walls. Polysaccharide binding assays demonstrated that chitin but not cellulose binds to and extracts Yps3p from culture supernatants.

RNA interference in Histoplasma capsulatum demonstrates a role for alpha-(1,3)-glucan in virulence

Histoplasma capsulatum is a fungal pathogen that causes respiratory and systemic disease by proliferating within macrophages. While much is known about histoplasmosis, only a single virulence factor has been defined, in part because of the inefficiency of Histoplasma reverse genetics. As an alternative to allelic replacement, we have developed a telomeric plasmid-based system for silencing gene expression in Histoplasma by RNA interference (RNAi). Episomal expression of long RNAs that form stem-loop structures triggered gene silencing. To test the effectiveness of RNAi in Histoplasma, we depleted expression of a gfp transgene as well as two endogenous genes, ADE2 and URA5, and showed significant reductions in corresponding gene function. Silencing was target gene specific, stable during macrophage infection and reversible. We used RNAi targeting AGS1 (encoding alpha-(1,3)-glucan synthase) to deplete levels of alpha-(1,3)-glucan, a cell wall polysaccharide. Loss of alpha-(1,3)-glucan by RNAi yielded phenotypes indistinguishable from an AGS1 deletion: attenuation of the ability to kill macrophages and colonize murine lungs. This demonstrates for the first time that alpha-(1,3)-glucan is an important contributor to Histoplasma virulence.

Identifying phase-specific genes in the fungal pathogen Histoplasma capsulatum using a genomic shotgun microarray

A fundamental feature of the fungal pathogen Histoplasma capsulatum is its ability to shift from a mycelial phase in the soil to a yeast phase in its human host. Each form plays a critical role in infection and disease, but little is understood about how these two morphologic phases are established and maintained. To identify phase-regulated genes of H. capsulatum, we carried out expression analyses by using a genomic shotgun microarray representing approximately one-third of the genome, and identified 500 clones that were differentially expressed. Genes induced in the mycelial phase included several involved in conidiation, cell polarity, and melanin production in other organisms. Genes induced in the yeast phase included several involved in sulfur metabolism, extending previous observations that sulfur metabolism influences morphology in H. capsulatum. Other genes with increased expression in the yeast phase were implicated in nutrient acquisition and cell cycle regulation. Unexpectedly, differential regulation of the site of transcript initiation was also observed in the two phases. These findings identify genes that may determine some of the major characteristics of the mycelial and yeast phases.

Molecular cell biology and molecular genetics of Histoplasma capsulatum

Histoplasma capsulatum is a dimorphic ascomycete which is capable of producing a broad spectrum of disease ranging from mild asymptomatic, pulmonary illness to severe, life-threatening systemic mycosis. Regulatory mechanisms that use temperature and other environmental cues are paramount to the successful adaptation of the organism as an effective intracellular pathogenic yeast. Although the biochemistry and phenomenology of reversible morphogenesis have been well examined in Histoplasma, the identification and functional characterization of genes and their products that are required for early establishment or maintenance of the parasitic yeast phase in intracellular host compartments have only recently been fruitful. Advances in the molecular biology of Histoplasma, including approaches to introduce telomeric plasmids, reporter fusion constructs, and gene disruption cassettes into the fungus are poised to solidify the pre-eminence of this fungus as a model system which can be applied to other dimorphic fungal pathogens that exhibit similar cellular and immunological complexities. This review centers on recent developments in the molecular cell biology and molecular genetics of Histoplasma capsulatum that provide important new avenues for examining the mold-to-yeast phase transition beyond the historical, binary view of dimorphism and the implications that these successful approaches may have on seminal issues in fungal pathogenesis.

Gel shift assay of nuclear extracts from Histoplasma capsulatum demonstrates the presence of several DNA binding proteins

A gel shift assay was optimized to detect several general DNA binding proteins from Histoplasma capsulatum strain G217B. The electrophoretic mobility shift assay (EMSA) technique also detected protein(s) recognizing a pyrimidine-rich motif found in several Histoplasma promoters. Establishment of EMSA conditions provides an important framework to evaluate regulation of homeostatic or phase-specific genes that may influence virulence in Histoplasma and other dimorphic fungal pathogens.

The mold-specific MS8 gene is required for normal hypha formation in the dimorphic pathogenic fungus Histoplasma capsulatum

The dimorphic fungus Histoplasma capsulatum is the etiologic agent of one of the most common systemic mycoses of humans, histoplasmosis. In the environment, H. capsulatum grows in a differentiated mold form and shifts to an undifferentiated yeast form after mold fragments or spores are inhaled. This mold-to-yeast shift is required for disease. Little is known about the molecular biology of dimorphism in Histoplasma, and most studies have been directed toward yeast-specific genes. While it is important to examine the role of genes upregulated in the yeast morphotype, genes which are silenced in the yeast (i.e., mold-specific genes) may also play a critical role in dimorphism. To begin to examine this hypothesis, we report here the first misexpression and knockout analysis of a mold-specific gene in Histoplasma. The strongly expressed MS8 gene encodes a predicted 21-kDa protein extremely rich in glycine and glutamine. Forced expression of MS8 driven by the TEF1 promoter in yeast did not alter the yeast morphology at 37 degrees C or mold formation at 25 degrees C. Yeast expressing MS8 did exhibit clumping in liquid medium and formed "sticky" colonies on agar plates. Allelic replacement of MS8 was accomplished by a positive-negative selection procedure. ms8 knockout mutants formed apparently normal yeast at 37 degrees C but gave rise to aberrant mycelia at 25 degrees C. The mold colonies of the knockouts were less than half as large as normal, had a granular surface, produced a dark-red pigment, and formed short hyphae which were 40% wider with a distinctive twisted "zig-zag" shape.

Linking fungal morphogenesis with virulence

Pathogenic fungi have become an increasingly common cause of systemic disease in healthy people and those with impaired immune systems. Although a vast number of fungal species inhabit our planet, just a small number are pathogens, and one feature that links many of them is the ability to differentiate morphologically from mould to yeast, or yeast to mould. Morphological differentiation between yeast and mould forms has commanded attention for its putative impact on the pathogenesis of invasive fungal infections. This review explores the current body of evidence linking fungal morphogenesis and virulence. The topics addressed cover work on phase-locked fungal cells, expression of phase-specific virulence traits and modulation of host responses by fungal morphotypes. The effect of morphological differentiation on fungal interaction with host cells, immune modulation and the net consequence on pathogenesis of disease in animal model systems are considered. The evidence argues strongly that morphological differentiation plays a vital role in the pathogenesis of fungal infection, suggesting that factors associated with this conversion process represent promising therapeutic targets.

Histoplasma capsulatum molecular genetics, pathogenesis, and responsiveness to its environment

Histoplasma capsulatum is a thermally dimorphic ascomycete that is a significant cause of respiratory and systemic disease in mammals including humans, especially immunocompromised individuals such as AIDS patients. As an environmental mold found in the soil, it is a successful member of a competitive polymicrobial ecosystem. Its host-adapted yeast form is a facultative intracellular pathogen of mammalian macrophages. H. capsulatum faces a variety of environmental changes during the course of infection and must survive under harsh conditions or modulate its microenvironment to achieve success as a pathogen. Histoplasmosis may be considered the fungal homolog of the bacterial infection tuberculosis, since both H. capsulatum and Mycobacterium tuberculosis exploit the macrophage as a host cell and can cause acute or persistent pulmonary and disseminated infection and reactivation disease. The identification and functional analysis of biologically or pathogenically important H. capsulatum genes have been greatly facilitated by the development of molecular genetic experimental capabilities in this organism. This review focuses on responsiveness of this fungus to its environment, including differential expression of genes and adaptive phenotypic traits.

Molecular mycology: a genetic toolbox for Histoplasma capsulatum

Research in medical mycology has traditionally been a mix of exciting biology and frustrating genetics, although the excitement has been steadily increasing as genetic obstacles have been successfully overcome. Now, a variety of fungal pathogens can be studied using molecular techniques derived from classical bacterial and yeast genetics, but with selective and strategic adaptations. Histoplasma capsulatum is the best-studied of the primary pathogens known as 'dimorphic' fungi, and tailored molecular genetic strategies are beginning to reveal a repertoire of genes and gene products intimately associated with pathogenesis.

Cloning and analysis of mold-specific genes in the dimorphic fungus Histoplasma capsulatum

A critical feature in the pathogenesis of the respiratory pathogen Histoplasma capsulatum is the conversion from the mold form (found in soil) to the yeast form in the lungs of the host. Little is known about the molecular biology of Histoplasma dimorphism. In particular, the possible roles of genes which are transcriptionally silent in yeast (i.e. mold-specific) have not been studied. We have produced a cDNA library highly enriched for mold-upregulated clones by fragmenting cDNA and removing yeast-specific and common sequences with a highly efficient enzyme degrading subtraction method. Screening of randomly selected clones identified cDNA fragments representing 16 different mold-upregulated genes. Because multiple cDNA fragments can be treated as alleles in a genetic screen, we were able to apply probability analysis to estimate the total number of mold-upregulated genes. We estimate that there are 27 upregulated genes; cDNA fragments of 16 have been isolated. Here we report the first isolation and analysis of cDNA from two mold-specific genes, MS8 (GenBank AF292398) and MS88 (GenBank AF357882). The MS8 transcript was very strongly expressed in mold but not detected on Northern blots with yeast RNA. The putative MS8 protein was predicted to be 21.3 kDa (203 aa), very rich in glutamine and glycine and had a calculated pI of 6.76. The MS88 transcript was weakly expressed in mold and not detected in yeast. The putative MS88 protein was predicted to be 22.5 kDa (219 aa) with a pI of 4.46. GenBank similarity searches revealed that the putative MS8 protein was similar to a glutamine-rich protein, of unknown function, from the fungus Colletotrichum gloeosporioides (GenBank U94186). No significant matches were found for the putative MS88 protein.

Selective expression of the virulence factor BAD1 upon morphogenesis to the pathogenic yeast form of Blastomyces dermatitidis: evidence for transcriptional regulation by a conserved mechanism

Most dimorphic fungal pathogens grow as non-pathogenic moulds in soil and convert to pathogenic yeast in the host, suggesting that virulence factors are upregulated during phase transition. Such factors have been difficult to identify. We analysed BAD1 (formerly WI-1), a virulence factor in the dimorphic fungus Blastomyces dermatitidis, for expression in yeast and mycelial morphotypes. BAD1 was expressed in yeast but not in mycelia of North American strains of B. dermatitidis, and this expression pattern was confirmed for BAD1 transcript. BAD1 under the control of its promoter was transferred into African B. dermatitidis lacking a native BAD1 locus, and phase-specific expression was conserved. Sequence similarity was identified between the BAD1 promoter and the promoters of two yeast phase-specific genes in Histoplasma capsulatum. In H. capsulatum BAD1 transformants, yeast phase-specific expression of BAD1 was conserved, and no transcript was detected in mycelia. BAD1 beta-galactosidase reporter fusions analysed in B. dermatitidis and H. capsulatum confirmed that BAD1 is transcriptionally regulated in both fungi. BAD1 promoter activity and surface BAD1 expression were detected 6 h after shifting mycelia to 37 degrees C. Thus, BAD1 is expressed after transition to the pathogenic yeast morphotype and is regulated by a mechanism for phase-specific gene expression that appears to be conserved.

Microbiology. Turning up the heat on Histoplasma capsulatum

Many fungal pathogens are opportunistic, that is, they infect individuals who have a compromised immune system. Histoplasma capsulatum is a common pathogenic fungus that lives happily inside the phagosomes of macrophages. As Klein explains in his Perspective, an important H. capsulatum virulence factor, CBP1, has been found, which mops up free calcium ions within the phagosome, enabling the yeast to live under calcium-poor conditions (Sebhgati et al.). Chelating calcium ions may also have the added benefit that when the phagosome fuses with the lysosome, destructive lysosomal enzymes that require calcium ions for activity remain inactive.

The ins and outs of DNA fingerprinting the infectious fungi

DNA fingerprinting methods have evolved as major tools in fungal epidemiology. However, no single method has emerged as the method of choice, and some methods perform better than others at different levels of resolution. In this review, requirements for an effective DNA fingerprinting method are proposed and procedures are described for testing the efficacy of a method. In light of the proposed requirements, the most common methods now being used to DNA fingerprint the infectious fungi are described and assessed. These methods include restriction fragment length polymorphisms (RFLP), RFLP with hybridization probes, randomly amplified polymorphic DNA and other PCR-based methods, electrophoretic karyotyping, and sequencing-based methods. Procedures for computing similarity coefficients, generating phylogenetic trees, and testing the stability of clusters are then described. To facilitate the analysis of DNA fingerprinting data, computer-assisted methods are described. Finally, the problems inherent in the collection of test and control isolates are considered, and DNA fingerprinting studies of strain maintenance during persistent or recurrent infections, microevolution in infecting strains, and the origin of nosocomial infections are assessed in light of the preceding discussion of the ins and outs of DNA fingerprinting. The intent of this review is to generate an awareness of the need to verify the efficacy of each DNA fingerprinting method for the level of genetic relatedness necessary to answer the epidemiological question posed, to use quantitative methods to analyze DNA fingerprint data, to use computer-assisted DNA fingerprint analysis systems to analyze data, and to file data in a form that can be used in the future for retrospective and comparative studies.

The ins and outs of DNA fingerprinting the infectious fungi

DNA fingerprinting methods have evolved as major tools in fungal epidemiology. However, no single method has emerged as the method of choice, and some methods perform better than others at different levels of resolution. In this review, requirements for an effective DNA fingerprinting method are proposed and procedures are described for testing the efficacy of a method. In light of the proposed requirements, the most common methods now being used to DNA fingerprint the infectious fungi are described and assessed. These methods include restriction fragment length polymorphisms (RFLP), RFLP with hybridization probes, randomly amplified polymorphic DNA and other PCR-based methods, electrophoretic karyotyping, and sequencing-based methods. Procedures for computing similarity coefficients, generating phylogenetic trees, and testing the stability of clusters are then described. To facilitate the analysis of DNA fingerprinting data, computer-assisted methods are described. Finally, the problems inherent in the collection of test and control isolates are considered, and DNA fingerprinting studies of strain maintenance during persistent or recurrent infections, microevolution in infecting strains, and the origin of nosocomial infections are assessed in light of the preceding discussion of the ins and outs of DNA fingerprinting. The intent of this review is to generate an awareness of the need to verify the efficacy of each DNA fingerprinting method for the level of genetic relatedness necessary to answer the epidemiological question posed, to use quantitative methods to analyze DNA fingerprint data, to use computer-assisted DNA fingerprint analysis systems to analyze data, and to file data in a form that can be used in the future for retrospective and comparative studies.

Identification and characterization of a phase-specific, nuclear DNA binding protein from the dimorphic pathogenic fungus Histoplasma capsulatum

Genes expressed in the parasitic yeast (Y) phase of the dimorphic fungal pathogen Histoplasma capsulatum which are transcriptionally silent in the mycelial (M) phase have recently been cloned and analyzed. To understand the molecular regulation of genes involved in the transition to and maintenance of the Y phase, the presumptive 5' regulatory regions of two Y phase-specific genes (yps-3 and yps 21:E-9) were PCR amplified as labelled probes to identify nuclear DNA binding proteins which may influence phase-specific gene transcription. Protein-DNA interactions were assessed by Southwestern blot analysis in which sodium dodecyl sulfate-polyacrylamide gel electrophoresis-separated protein extracts from Y and M phases of the virulent G217B strain of H. capsulatum were visualized by their capability for in situ binding to the labelled 517-bp (G217B yps-3) or the 395-bp (G217B yps 21:E-9) putative 5' regulatory regions. A 30-kDa nuclear protein unique to the M-phase extracts of the highly virulent G217B strain, but absent in the Y phase of the same organism, was identified. In contrast, the low-virulence, thermal-sensitive Downs strain of H. capsulatum lacked detectable p30 binding activity in either yeast- or mycelial phase extracts, regardless of the source of labelled probe (395-bp G217B yps 21:E-9 probe or 512-bp HindIII-EcoRI-labelled Downs yps21:E-9). A decanucleotide motif, TCCTTTTTTT, was identified in the upstream regulatory regions of these yps genes, as well as in the putative alpha-tubulin promoter, and was conserved with 70 to 100% homology. This recognition sequence was sufficient for p30M binding with 32P-labelled ligated oligonucleotides when used in the Southwestern assay. These findings describe the first nuclear DNA binding factor identified in H. capsulatum which binds to target sequences in a phase-specific manner, suggesting that p30M may govern aspects of gene transcription in this pathogenic fungus, in which a temperature-sensitive switch influences morphology and virulence.

Probing the yeast phase-specific expression of the CBP1 gene in Histoplasma capsulatum

Histoplasma capsulatum is a pathogenic fungus that exists in two distinct forms. The saprophytic mycelial phase inhabits moist soil environments; once inhaled, hyphae and conidia convert to a unicellular yeast phase that is capable of parasitizing macrophage phagolysosomes. Yeasts cultures, but not mycelial cultures, release large quantities of a calcium-binding protein (CBP) which may be important in calcium acquisition during intracellular parasitism. In this study, we show that the gene encoding CBP (CBP1) is transcriptionally regulated. To identify promoter sequences that are important for yeast phase-specific activity, we created a series of fusions between successively truncated CBP1 5' untranslated regulatory sequences and the Escherichia coli lacZ gene. The fusions were constructed on a telomeric shuttle plasmid that can replicate autonomously in the fungus. By assaying for beta-galactosidase activity from H. capsulatum transformants, we identified a 102-bp region that mediates promoter activation and yeast phase promoter activity. Base pair substitution analysis suggests that the sequences between 839 and 877 bp upstream of the start codon are the most important for this positive regulation.

Molecular cloning and characterization of a recombinant Histoplasma capsulatum antigen for antibody-based diagnosis of human histoplasmosis

Immunological cross-reactivity among fungi has hampered the development of specific serodiagnostic assays for histoplasmosis. We report the molecular cloning and characterization of a Histoplasma capsulatum cDNA (GH17) that encodes an antigen with immunodiagnostic potential. GH17 is an 810-bp cDNA which encodes a protein of 211 amino acid residues. The GH17 sequence has almost no significant homology with other sequences in GenBank. Southern blot analysis suggests that GH17 is confined to a single location in the genomic DNA of H. capsulatum. Immunoblots indicated that the protein product of GH17 (expressed as a 140-kDa beta-galactosidase fusion protein) was recognized by antibodies in 18 of 18 sera from histoplasmosis patients, but not by antibodies in sera from patients or animals infected with other fungi. GH17 was expressed in a prokaryotic expression vector, pPROEX-1, and recombinant protein was purified by preparative electrophoresis. Antibodies raised to this protein bound to a 60-kDa native antigen in immunoblots of H. capsulatum yeast antigen extract. These results suggest that GH17 encodes an H. capsulatum antigen that may be useful for the diagnosis of histoplasmosis in humans.

Localization of a yeast-phase-specific gene product to the cell wall in Histoplasma capsulatum

A yeast-phase-specific gene, yps-3, has been identified in the virulent Histoplasma capsulatum strain, G217B. Although DNA sequencing of the genomic yps-3 gene from G217B failed to detect homologies with known proteins, the 5' end of a yps-3 cDNA contained a consensus signal sequence. A 519-bp fragment of the cDNA containing the translational stop codon was linker modified and inserted into the bacterial expression vector, pATH 1. Escherichia coli extracts containing the pATH 1 vector alone expressed a major 34-kDa TrpE polypeptide following induction with indoleacrylic acid, while the pATH 1/yps-3 construct produced a predominant 54-kDa TrpE/yps-3 fusion protein. Polyclonal rabbit sera directed against G217B reacted exclusively with the 54-kDa fusion protein in Western blots (immunoblots); serum samples from three patients with acute pulmonary or disseminated histoplasmosis were also positive. To localize the yps-3 protein within G217B, a monoclonal antibody (MAb 7.1) which recognized the yps-3 portion of the fusion protein was generated. A 17.4-kDa protein was detected with MAb 7.1 in Western blots prepared from cell wall fractions of G217B; cytoplasmic fractions were unreactive. No yps-3 antigen was detected in either fraction of the Downs strain, which fails to express the yps-3 gene. MAb 7.1 also detected a 17.4-kDa antigen in ethanol-precipitated culture supernatants derived from G217B. These findings localize the yps-3 gene product to the cell wall and culture supernatants, where the protein may influence the phase transition or the maintenance of the yeast state.

Fungal virulence genes as targets for antifungal chemotherapy

Fungal virulence genes have now met the age of molecular pathogenesis. The definition of virulence genes needs to be broad so that it encompasses the focus on molecular antifungal targets and vaccine epitopes. However, in the broad but simple definition of a virulence gene, there will be many complex genetic and host interactions which investigators will need to carefully define. Nevertheless, with the increasing numbers of serious fungal infections produced by old and newly reported organisms, the paucity of present antifungal drugs, and the likelihood of increasing drug resistance, the need for investigations into understanding fungal virulence at the molecular level has never been more important.

Heat stress alters the virulence of a rifampin-resistant mutant of Francisella tularensis LVS

We have studied the stress response of a rifampin-resistant mutant of Francisella tularensis LVS. This mutant, Rif 7, was avirulent with an intraperitoneally administered 50% lethal dose greater than 10(7) CFU in a murine model of infection. Exposure of Rif 7 to heat stress for 5 h in vitro resulted in a 2-log decrease in its 50% lethal dose (P < 0.02). The increase in virulence was dependent on the time of exposure to high temperature and was maximal at 5 h. Envelope preparations from heat-stressed cells showed increased levels of several proteins. Notable among these were polypeptides with approximate molecular masses of 16, 60, and 75 kDa. Increases in both virulence and envelope protein levels were reversed when heat-treated cells were subsequently grown at 37 degrees C. Inhibition of protein synthesis by actinomycin D during heat stress blocked the increase in virulence of Rif 7. Cell-free media from the heat-stressed Rif 7 reacted with the whole spectrum of bacterial proteins were not toxic to mice. Hyperimmune serum against Rif 7 reacted with the whole spectrum of bacterial proteins in Western blots (immunoblots), although its reaction with 34- and 45-kDa proteins and two 60- and 75-kDa proteins upregulated during heat stress was weak. Other stress conditions, low iron and low pH, caused similar increases in the virulence of Rif 7. However, examination of the protein profile did not reveal any major common polypeptides induced by different stresses. Heat-treated Rif 7 bacteria were fully able to replicate in macrophages in vitro and in the host tissues, even though heat treatment only partially restored virulence.

Cloning and characterization of bys1, a temperature-dependent cDNA specific to the yeast phase of the pathogenic dimorphic fungus Blastomyces dermatitidis

The pathogenic dimorphic fungal organism Blastomyces dermatitidis exists as a budding yeast at 37 degrees C and as a mycelium at 25 degrees C. While the conversion of one morphological phase of B. dermatitidis to another has long been known to be a thermally dependent process, little of the accompanying biochemical or genetic events controlling the phase transition has been elucidated. Using differential cDNA library screening, we have identified one transcript, bys1, in B. dermatitidis that is expressed at very high levels in the yeast phase but whose levels diminish rapidly when yeast cells are transferred to 25 degrees C to promote conversion to the mycelial phase. Although the 0.95-kb bys1 transcript is absent in B. dermatitidis mycelia maintained at 25 degrees C, transfer of mycelial cultures to 37 degrees C results in the reappearance of bys1 within 12 h. bys1 codes for a protein of 18.6 kDa that contains multiple putative phosphorylation sites, a hydrophobic N terminus, and two 34-amino-acid domains with similarly spaced nine-amino-acid degenerative repeating motifs. Although the nature of the thermal dependency of bys1 expression and the function of the bys1 protein are unknown, the strong expression of this transcript specifically in the yeast phase of B. dermatitidis may prove to be very useful in the development of more specific and sensitive diagnostic methods for blastomycosis.