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

An Indian lineage of Histoplasma with strong signatures of differentiation and selection

Histoplasma, a genus of dimorphic fungi, is the etiological agent of histoplasmosis, a pulmonary disease widespread across the globe. Whole genome sequencing has revealed that the genus harbors a previously unrecognized diversity of cryptic species. To date, studies have focused on Histoplasma isolates collected in the Americas with little knowledge of the genomic variation from other localities. In this report, we report the existence of a well-differentiated lineage of Histoplasma occurring in the Indian subcontinent. The group is differentiated enough to satisfy the requirements of a phylogenetic species, as it shows extensive genetic differentiation along the whole genome and has little evidence of gene exchange with other Histoplasma species. Next, we leverage this genetic differentiation to identify genetic changes that are unique to this group and that have putatively evolved through rapid positive selection. We found that none of the previously known virulence factors have evolved rapidly in the Indian lineage but find evidence of strong signatures of selection on other alleles potentially involved in clinically-important phenotypes. Our work serves as an example of the importance of correctly identifying species boundaries to understand the extent of selection in the evolution of pathogenic lineages. IMPORTANCE: Whole genome sequencing has revolutionized our understanding of microbial diversity, including human pathogens. In the case of fungal pathogens, a limiting factor in understanding the extent of their genetic diversity has been the lack of systematic sampling. In this piece, we show the results of a collection in the Indian subcontinent of the pathogenic fungus Histoplasma, the causal agent of a systemic mycosis. We find that Indian samples of Histoplasma form a distinct clade which is highly differentiated from other Histoplasma species. We also show that the genome of this lineage shows unique signals of natural selection. This work exemplifies how the combination of a robust sampling along with population genetics, and phylogenetics can reveal the precise genetic changes that differentiate lineages of fungal pathogens.

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.

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.

Fantastic yeasts and where to find them: the hidden diversity of dimorphic fungal pathogens

Dimorphic fungal pathogens are a significant cause of human disease worldwide. Notably, the dimorphic fungal pathogens within the order Onygenales are considered primary pathogens, causing disease in healthy hosts. Current changes in taxonomy are underway due to advances in molecular phylogenetics, population genetics, and new emerging dimorphic fungal pathogens causing human disease. In this review, we highlight evolutionary relationships of dimorphic fungal pathogens that cause human disease within the order Onygenales and provide rationale to support increased investment in studies understanding the evolutionary relationships of these pathogens to improve rapid diagnostics, help identify mechanisms of antifungal resistance, understand adaptation to human host, and factors associated with virulence.

Adhesion of Histoplasma capsulatum to pneumocytes and biofilm formation on an abiotic surface

The pathogenic fungus, Histoplasma capsulatum, causes the respiratory and systemic disease 'histoplasmosis'. This disease is primarily acquired via inhalation of aerosolized microconidia or hyphal fragments of H. capsulatum. Evolution of this respiratory disease depends on the ability of H. capsulatum yeasts to survive and replicate within alveolar macrophages. It is known that adhesion to host cells is the first step in colonization and biofilm formation. Some microorganisms become attached to biological and non-biological surfaces due to the formation of biofilms. Based on the importance of biofilms and their persistence on host tissues and cell surfaces, the present study was designed to investigate biofilm formation by H. capsulatum yeasts, as well as their ability to adhere to pneumocyte cells. H. capsulatum biofilm assays were performed in vitro using two different clinical strains of the fungus and biofilms were characterized using scanning electron microscopy. The biofilms were measured using a 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino)carbonyl]-2H-tetrazolium-hydroxide (XTT) reduction assay. The results showed that both the H. capsulatum strains tested were very efficient at adhering to host cells and forming biofilm. Therefore, this is a possible survival strategy adopted by this fungus.

Histoplasma virulence and host responses

Histoplasma capsulatum is the most prevalent cause of fungal respiratory disease. The disease extent and outcomes are the result of the complex interaction between the pathogen and a host's immune system. The focus of our paper consists in presenting the current knowledge regarding the multiple facets of the dynamic host-pathogen relationship in the context of the virulence arsenal displayed by the fungus and the innate and adaptive immune responses of the host.

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.

Molecular phylogenetics of ascomycotal adhesins--a novel family of putative cell-surface adhesive proteins in fission yeasts

In this work, we identify a family of putative adhesins in the fission yeasts Schizosaccharomyces pombe and Schizosaccharomyces japonicus. The members of this family share a conserved tandem repeat related to those found in the Candida albicans Als family of adhesins. Unlike previously characterised adhesins that possess conserved ligand-binding domains at the N-terminus, this group of proteins carry ligand-binding domains at their C-termini. We demonstrate that one such domain--the uncharacterised GLEYA domain, is related to the lectin-like ligand-binding domain found in the Saccharomyces cerevisiae Flo proteins. Unlike the Flo and Als proteins, the fission yeast adhesins do not contain detectable glycosyl phosphatidyl inositol (GPI) membrane anchor signals to mediate their attachment to the cell wall, which may suggest a novel cell wall attachment mechanism. Further sequence analysis identified several putative adhesins in the sub-phylum of Pezizomycotina, where only a few adhesins have been described to date.

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.

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.

A C-terminal EGF-like domain governs BAD1 localization to the yeast surface and fungal adherence to phagocytes, but is dispensable in immune modulation and pathogenicity of Blastomyces dermatitidis

BAD1, an adhesin and immune modulator of Blastomyces dermatitidis, is an essential virulence factor that is released extracellularly before association with the yeast surface. Here, deletion of the C-terminal EGF-like domain profoundly affected BAD1 function, leading to non-association with yeast, extracellular accumulation and impaired yeast adherence to macrophages. In equilibrium binding assays, DeltaC-term BAD1, lacking an EGF-like domain, bound poorly to BAD1 null yeast, yielding a low affinity (Kd, 3 x 10(-7) M versus 5 x 10(-8) M) and Bmax (1.9 x 10(5) versus 7.9 x 10(5)) compared with BAD1. Similar protein binding profiles were observed using chitin particles, reinforcing the notion that chitin fibrils are a receptor for BAD1, and that the EGF-like domain is critical for BAD1 interactions with chitin on yeast. DeltaC-term strains bound poorly to macrophages, compared with parental or BAD1-reconstituted null strains. However, DeltaC-term strains and the purified protein itself sharply suppressed tumour necrosis factor (TNF)-alpha release by phagocytes in vitro and in lung in vivo, and the strains retained pathogenicity in a murine model of blastomycosis. Our results illustrate the previously undefined role of the EGF-like domain for BAD1 localization to yeast surfaces during cell wall biogenesis. They also demonstrate that the requirements for host cell binding and immune modulation by BAD1 can be dissociated from one another, and that the former is unexpectedly dispensable in the requisite role of BAD1 in pathogenesis.

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.

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 epidemiology, pathogenesis, and genetics of the dimorphic fungus Histoplasma capsulatum

Histoplasma capsulatum, the causative agent of the most common systemic fungal infection, histoplasmosis, has become subject to increasing study in parallel with rising prevalence of human immunodeficiency. This review presents a summary of the advances made in the investigation of H. capsulatum genomics, molecular epidemiology, pathogenesis, and molecular genetics.