Phylogeographic Analysis of Blastomyces dermatitidis and Blastomyces gilchristii Reveals an Association with North American Freshwater Drainage Basins

Blastomyces dermatitidis and Blastomyces gilchristii are dimorphic fungal pathogens that cause serious pulmonary and systemic infections in humans. Although their natural habitat is in the environment, little is known about their specific ecologic niche(s). Here, we analyzed 25 microsatellite loci from 169 strains collected from various regions throughout their known endemic range in North America, representing the largest and most geographically diverse collection of isolates studied to date. Genetic analysis of multilocus microsatellite data divided the strains into four populations of B. dermatitidis and four populations of B. gilchristii. B. dermatitidis isolates were recovered from areas throughout North America, while the B. gilchristii strains were restricted to Canada and some northern US states. Furthermore, the populations of both species were associated with major freshwater drainage basins. The four B. dermatitidis populations were partitioned among (1) the Nelson River drainage basin, (2) the St. Lawrence River and northeast Atlantic Ocean Seaboard drainage basins, (3) the Mississippi River System drainage basin, and (4) the Gulf of Mexico Seaboard and southeast Atlantic Ocean Seaboard drainage basins. A similar partitioning of the B. gilchristii populations was observed among the more northerly drainage basins only. These associations suggest that the ecologic niche where the sexual reproduction, growth, and dispersal of B. dermatitidis and B. gilchristii occur is intimately linked to freshwater systems. For most populations, sexual reproduction was rare enough to produce significant linkage disequilibrium among loci but frequent enough that mating-type idiomorphic ratios were not skewed from 1:1. Furthermore, the evolutionary divergence of B. dermatitidis and B. gilchristii was estimated at 1.9 MYA during the Pleistocene epoch. We suggest that repeated glaciations during the Pleistocene period and resulting biotic refugia may have provided the impetus for speciation as theorized for other species associated with temperate freshwater systems.

Systemic Blastomycosis in a Horse

Progressive multisystemic disease caused by Blastomyces dermatitidis was diagnosed in a 17-year-old Quarter horse broodmare. The mare had been treated unsuccessfully with antibiotics for mastitis 3 months postpartum. The disease progressed to exudative cutaneous lesions affecting the ventrum, pectoral region, and limbs accompanied by weight loss across several months. Yeast bodies were observed in swabs of the cutaneous exudate, suggesting a clinical diagnosis of blastomycosis. Following referral, pleural effusion, cavitated lung lesions, and hyperproteinemia were identified, and the mare was euthanized because of poor prognosis. Necropsy revealed extensive pyogranulomas in the mammary gland, skin, subcutaneous tissues, and lungs, accompanied by thrombi in major blood vessels of the lungs and hind limbs. Histologically, pyogranulomatous inflammation was evident in many tissues, and fungal organisms were seen in sections of mammary gland, skin, subcutis, pericardium, and lung. Blastomyces dermatitidis was cultured from mammary tissue, lungs, lymph node, and an inguinal abscess. Although blastomycosis is endemic in the area of origin of the mare in northwestern Wisconsin, the disease is extremely rare in horses and hence easily misdiagnosed. Unique features of this case included the extent of mammary gland involvement and the presence of thrombi in multiple sites.

The Dynamic Genome and Transcriptome of the Human Fungal Pathogen Blastomyces and Close Relative Emmonsia

Three closely related thermally dimorphic pathogens are causal agents of major fungal diseases affecting humans in the Americas: blastomycosis, histoplasmosis and paracoccidioidomycosis. Here we report the genome sequence and analysis of four strains of the etiological agent of blastomycosis, Blastomyces, and two species of the related genus Emmonsia, typically pathogens of small mammals. Compared to related species, Blastomyces genomes are highly expanded, with long, often sharply demarcated tracts of low GC-content sequence. These GC-poor isochore-like regions are enriched for gypsy elements, are variable in total size between isolates, and are least expanded in the avirulent B. dermatitidis strain ER-3 as compared with the virulent B. gilchristii strain SLH14081. The lack of similar regions in related species suggests these isochore-like regions originated recently in the ancestor of the Blastomyces lineage. While gene content is highly conserved between Blastomyces and related fungi, we identified changes in copy number of genes potentially involved in host interaction, including proteases and characterized antigens. In addition, we studied gene expression changes of B. dermatitidis during the interaction of the infectious yeast form with macrophages and in a mouse model. Both experiments highlight a strong antioxidant defense response in Blastomyces, and upregulation of dioxygenases in vivo suggests that dioxide produced by antioxidants may be further utilized for amino acid metabolism. We identify a number of functional categories upregulated exclusively in vivo, such as secreted proteins, zinc acquisition proteins, and cysteine and tryptophan metabolism, which may include critical virulence factors missed before in in vitro studies. Across the dimorphic fungi, loss of certain zinc acquisition genes and differences in amino acid metabolism suggest unique adaptations of Blastomyces to its host environment. These results reveal the dynamics of genome evolution and of factors contributing to virulence in Blastomyces.

Dimorphic fungal pathogens including Blastomyces are the cause of major fungal diseases in North and South America. The genus Emmonsia includes species infecting small mammals as well as a newly emerging pathogenic species recently reported in HIV-positive patients in South Africa. Here, we synthesize both genome sequencing of four isolates of Blastomyces and two species of Emmonsia as well as deep sequencing of Blastomyces RNA to draw major new insights into the evolution of this group and the pathogen response to infection. We investigate the trajectory of genome evolution of this group, characterizing the phylogenetic relationships of these species, a remarkable genome expansion that formed large isochore-like regions of low GC content in Blastomyces, and variation of gene content, related to host interaction, among the dimorphic fungal pathogens. Using RNA-Seq, we profile the response of Blastomyces to macrophage and mouse pulmonary infection, identifying key pathways and novel virulence factors. The identification of key fungal genes involved in adaptation to the host suggests targets for further study and therapeutic intervention in Blastomyces and related dimorphic fungal pathogens.

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.

Structure and function of a fungal adhesin that binds heparin and mimics thrombospondin-1 by blocking T cell activation and effector function

Blastomyces adhesin-1 (BAD-1) is a 120-kD surface protein on B. dermatitidis yeast. We show here that BAD-1 contains 41 tandem repeats and that deleting even half of them impairs fungal pathogenicity. According to NMR, the repeats form tightly folded 17-amino acid loops constrained by a disulfide bond linking conserved cysteines. Each loop contains a highly conserved WxxWxxW motif found in thrombospondin-1 (TSP-1) type 1 heparin-binding repeats. BAD-1 binds heparin specifically and saturably, and is competitively inhibited by soluble heparin, but not related glycosaminoglycans. According to SPR analysis, the affinity of BAD-1 for heparin is 33 nM±14 nM. Putative heparin-binding motifs are found both at the N-terminus and within each tandem repeat loop. Like TSP-1, BAD-1 blocks activation of T cells in a manner requiring the heparan sulfate-modified surface molecule CD47, and impairs effector functions. The tandem repeats of BAD-1 thus confer pathogenicity, harbor motifs that bind heparin, and suppress T-cell activation via a CD47-dependent mechanism, mimicking mammalian TSP-1.

Discordant influence of Blastomyces dermatitidis yeast-phase-specific gene BYS1 on morphogenesis and virulence

Blastomyces dermatitidis is a thermally induced dimorphic fungus capable of causing lung and systemic infections in immunocompetent animal hosts. With the publication of genomic sequences from three different strains of B. dermatitidis and the development of RNA interference as a gene-silencing tool, it has become possible to easily ascertain the virulence and morphological effects of knocking down the expression of candidate genes of interest. BYS1 (Blastomyces yeast-phase-specific 1), first identified by Burg and Smith, is expressed at high levels in yeast cells and is undetectable in mold. The deduced protein sequence of BYS1 has a putative signal sequence at its N terminus, opening the possibility that the BYS1-encoded protein is associated with the yeast cell wall. Herein, strains of B. dermatitidis with silenced expression of BYS1 were engineered and tested for morphology and virulence. The silenced strains produced rough-surfaced cultures on agar medium and demonstrated a propensity to form pseudohyphal cells on prolonged culture in vitro and in vivo, as measured in the mouse lung. Tests using a mouse model of blastomycosis with either yeast or spore inocula showed that the bys1-silenced strains were as virulent as control strains. Thus, although silencing of BYS1 alters morphology at 37 degrees C, it does not appear to impair the pathogenicity of B. dermatitidis.

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.

Analysis of non-adhesive pathogenic mechanisms of BAD1 on Blastomyces dermatitidis

The adhesin BAD1 is required for virulence of Blastomyces dermatitidis in a pulmonary model of infection. Herein, we explored mechanisms by which BAD1 enhances pathogenicity of the fungus. Isogenic strains with and without BAD1 exhibited similar phenotypic differences in virulence by pulmonary and intravenous routes of infection, indicating that BAD1 may exert virulence beyond adherence to respiratory lining cells. Non-adhesive mechanisms including maintenance of intrinsic resistance of yeast against phagocyte responses and products were excluded. A shift in the balance of type 1 and 2 cytokines and in the cellular profile of the inflammatory response after the first week of pulmonary infection was associated with BAD1. By the second week of infection, infection with wild-type yeast was associated with less IL-12 and IFN-gamma, and more IL-10, and an influx of inflammatory cells rich in neutrophils and poor in T-cells, when compared to infection with the BAD1 null strain. Taken together with previously reported BAD1 perturbations of TNF-gamma and TGF-beta, these data suggest that BAD1 contributes significantly to the pathogenicity of B. dermatitidis by also deviating host adaptive immunity, and leukocyte responses.

Effect of lung surfactant collectins on bronchoalveolar macrophage interaction with Blastomyces dermatitidis: inhibition of tumor necrosis factor alpha production by surfactant protein D

Alveolar surfactant modulates the antimicrobial function of bronchoalveolar macrophages (BAM). Little is known about the effect of surfactant-associated proteins in bronchoalveolar lavage fluid (BALF) on the interaction of BAM and Blastomyces dermatitidis. We investigated BALF enhancement or inhibition of TNF-alpha production by BAM stimulated by B. dermatitidis. BAM from CD-1 mice were stimulated with B. dermatitidis without or with normal BALF, surfactant protein A-deficient (SP-A-/-) or surfactant protein D-deficient (SP-D-/-) BALF, or a mixture of SP-A-/- and SP-D-/- BALF. An enzyme-linked immunosorbent assay was used to measure tumor necrosis factor alpha (TNF-alpha) in culture supernatants. BALFs were standardized in protein concentration. BAM plus B. dermatitidis (BAM-B. dermatitidis) TNF-alpha production was inhibited > or = 47% by BALF or SP-A-/- BALF (at 290 or 580 microg of protein/ml, P < 0.05 to 0.01); in contrast, SP-D-/- BALF did not significantly inhibit TNF-alpha production. If SP-A-/- BALF was mixed in equal amounts with SP-D-/- BALF, TNF-alpha production by BAM-B. dermatitidis was inhibited (P < 0.01). Finally, pure SP-D added to SP-D-/- BALF inhibited TNF-alpha production by BAM-B. dermatitidis (P < 0.01). B. dermatitidis incubated with BALF and washed, plus BAM, stimulated 63% less production of TNF-alpha than did unwashed B. dermatitidis (P < 0.05). SP-D was detected by anti-SP-D antibody on BALF-treated unwashed B. dermatitidis in an immunofluorescence assay (IFA). The BALF depleted by a coating of B. dermatitidis lost the ability to inhibit TNF-alpha production (P < 0.05). 1,3-beta-Glucan was a good stimulator of BAM for TNF-alpha production and was detected on B. dermatitidis by IFA. beta-Glucan incubated with BALF inhibited the binding of SP-D in BALF to B. dermatitidis as demonstrated by IFA. Our data suggest that SP-D in BALF binds beta-glucan on B. dermatitidis, blocking BAM access to beta-glucan, thereby inhibiting TNF-alpha production. Thus, whereas BALF constituents commonly mediate antimicrobial activity, B. dermatitidis may utilize BALF constituents, such as SP-D, to blunt the host defensive reaction; this effect could reduce inflammation and tissue destruction but could also promote disease.

Clinical aspects of natural infection withBlastomyces dermatitidisin cats: 8 cases (1991–2005)

OBJECTIVE

To evaluate clinical and laboratory findings, treatment, and clinical outcome in cats with blastomycosis.

DESIGN

Retrospective case series.

ANIMALS

8 cats with naturally occurring blastomycosis.

PROCEDURES

Medical records of the University of Illinois Veterinary Teaching Hospital were searched for cases of blastomycosis in cats diagnosed via cytologic or histopathologic findings. Clinical and laboratory findings, treatment, and clinical outcome were determined. Radiographs were reviewed for the 8 cases.

RESULTS

All cats were systemically ill. Respiratory tract signs and dermal lesions were most commonly observed. All cats had radiographic evidence of respiratory tract disease. Seven of the 8 cats had ill-defined soft-tissue opacities (nodules or masses) or alveolar consolidation of the lungs. Antemortem diagnosis was achieved cytologically in 6 of the 8 cats, and 3 were successfully treated and survived.

CONCLUSIONS AND CLINICAL RELEVANCE

In contrast to previous reports, diagnosis was achieved antemortem in most of the cats (all by cytologic identification of the organism). Clinical signs, laboratory findings, and outcome were similar to previous descriptions of this rare disease in cats.

Global control of dimorphism and virulence in fungi

Microbial pathogens that normally inhabit our environment can adapt to thrive inside mammalian hosts. There are six dimorphic fungi that cause disease worldwide, which switch from nonpathogenic molds in soil to pathogenic yeast after spores are inhaled and exposed to elevated temperature. Mechanisms that regulate this switch remain obscure. We show that a hybrid histidine kinase senses host signals and triggers the transition from mold to yeast. The kinase also regulates cell-wall integrity, sporulation, and expression of virulence genes in vivo. This global regulator shapes how dimorphic fungal pathogens adapt to the mammalian host, which has broad implications for treating and preventing systemic fungal disease.

Progress in vaccination for histoplasmosis and blastomycosis: coping with cellular immunity

Human infection with Histoplasma capsulatum or Blastomyces dermatitidis is sufficiently frequent to warrant exploring the development of vaccines. This review examines the advancements that have been accomplished over the last few years. The availability of molecular tools to create recombinant antigens or mutant strains has produced a small number of useful vaccine candidates. More importantly, the studies summarized herein demonstrate that understanding the host response to a protein or mutant fungus is critical to creating a vaccine that may be useful for the immunocompromised patient.

Calcium binding by the essential virulence factor BAD-1 of Blastomyces dermatitidis

BAD-1 (Blastomyces adhesin 1), a 120-kDa protein of Blastomyces dermatitidis, functions as an adhesin, immune modulator, and essential virulence factor. Structurally, BAD-1 is composed of a short N-terminal region, a core of 30 tandem repeats critical for virulence, and a C-terminal epidermal growth factor domain that binds the protein to yeast cell surface chitin. Each of the 30 acidic residue-rich tandem repeats contains a sequence that resembles the calcium-binding loop of the EF-hand domain found in many calcium-binding proteins. Here, we investigated the binding of calcium by BAD-1 and its biological significance. Yeast washed with double distilled H2O released surface-bound BAD-1, but EGTA washes were an order of magnitude more efficient, suggesting an interaction between BAD-1 and calcium. Immobilized BAD-1 was stained with ruthenium red dye, an indicator of calcium-binding proteins. In equilibrium dialysis, BAD-1 bound 45Ca2+ with an affinity of 0.41 x 10(-5) m and a capacity of 27 calcium/mol. Mass spectrometry confirmed this capacity. Elevated [Ca2+] diminished BAD-1 solubility. Upon deletion of its C-terminal epidermal growth factor-like domain, BAD-1 resisted aggregation by elevated [Ca2+] but retained its affinity and capacity for calcium. Removing 20 copies of the tandem repeat, however, sharply reduced the capacity of BAD-1 for calcium. Growth of the bad-1 null yeast was inhibited by 5 mm EGTA, and re-expression of BAD-1 in trans or the addition of exogenous purified BAD-1 restored growth. Thus, BAD-1 is a high capacity calcium-binding protein. This property contributes to the structure and function of BAD-1, as well as to B. dermatitidis acquisition of calcium from the environment.

Exploiting type 3 complement receptor for TNF-alpha suppression, immune evasion, and progressive pulmonary fungal infection

TNF-alpha is crucial in defense against intracellular microbes. Host immune cells use type 3 complement receptors (CR3) to regulate excess TNF-alpha production during physiological clearance of apoptotic cells. BAD1, a virulence factor of Blastomyces dermatitidis, is displayed on yeast and released during infection. BAD1 binds yeast to macrophages (Mphi) via CR3 and CD14 and suppresses TNF-alpha, which is required for resistance. We investigated whether blastomyces adhesin 1 (BAD1) exploits host receptors for immune deviation and pathogen survival. Soluble BAD1 rapidly entered Mphi, accumulated intracellularly by 10 min after introduction to cells, and trafficked to early and late endosomes. Inhibition of receptor recycling by monodansyl cadaverine blocked association of BAD1 with Mphi and reversed TNF-alpha suppression in vitro. Inhibition of BAD1 uptake with cytochalasin D and FcR-redirected delivery of soluble BAD1 as Ag-Ab complexes or of wild-type yeast opsonized with IgG similarly reversed TNF-alpha suppression. Hence, receptor-mediated entry of BAD1 is requisite in TNF-alpha suppression, and the route of entry is critical. Binding of soluble BAD1 to Mphi of CR3(-/-) and CD14(-/-) mice was reduced to 50 and 33%, respectively, of that in wild-type mice. Mphi of CR3(-/-) and CD14(-/-) mice resisted soluble BAD1 TNF-alpha suppression in vitro, but, in contrast to CR3(-/-) cells, CD14(-/-) cells were still subject to suppression mediated by surface BAD1 on wild-type yeast. CR3(-/-) mice resisted both infection and TNF-alpha suppression in vivo, in contrast to wild-type and CD14(-/-) mice. BAD1 of B. dermatitidis thus co-opts normal host cell physiology by exploiting CR3 to subdue TNF-alpha production and foster pathogen survival.

Blastomyces dermatitidis produces melanin in vitro and during infection

Melanin is made by several important pathogenic fungi and is implicated in the pathogenesis of a number of mycoses. This study investigates whether the thermally dimorphic fungal pathogen Blastomyces dermatitidis produces melanin. Using techniques developed to study melanization in other fungi, we demonstrate that B. dermatitidis conidia and yeast produce melanin in vitro and that yeast cells synthesize melanin or melanin-like pigment in vivo. Melanization reduced susceptibility to amphotericin B, but not to itraconazole or voriconazole. Since melanin is an important virulence factor in other pathogenic fungi, this pigment may affect the pathogenesis of blastomycosis.

Interaction of Blastomyces dermatitidis, Sporothrix schenckii, and Histoplasma capsulatum with Acanthamoeba castellanii

Several dimorphic fungi are important human pathogens, but the origin and maintenance of virulence in these organisms is enigmatic, since an interaction with a mammalian host is not a requisite for fungal survival. Recently, Cryptococcus neoformans was shown to interact with macrophages, slime molds, and amoebae in a similar manner, suggesting that fungal pathogenic strategies may arise from environmental interactions with phagocytic microorganisms. In this study, we examined the interactions of three dimorphic fungi with the soil amoeba Acanthameobae castellanii. Yeast forms of Blastomyces dermatitidis, Sporothrix schenckii, and Histoplasma capsulatum were each ingested by amoebae and macrophages, and phagocytosis of yeast cells resulted in amoeba death and fungal growth. H. capsulatum conidia were also cytotoxic to amoebae. For each fungal species, exposure of yeast cells to amoebae resulted in an increase in hyphal cells. Exposure of an avirulent laboratory strain of H. capsulatum to A. castellanii selected for, or induced, a phenotype of H. capsulatum that caused a persistent murine lung infection. These results are consistent with the view that soil amoebae may contribute to the selection and maintenance of certain traits in pathogenic dimorphic fungi that confer on these microbes the capacity for virulence in mammals.

Blastomycosis: a case study of a dimorphic fungal disease

A 17-year-old male presented to a local hospital with symptoms of pneumonia and a hacking cough productive of yellow sputum. Direct examination of a bronchial lavage sample revealed organisms morphologically consistent with Blastomyces dermatitidis, which was confirmed by culture. The patient was placed on intravenous antifungal therapy and his condition improved dramatically.

OBJECTIVES

to describe the gross and microscopic morphologies of both yeast and mold forms of B. dermatitidis; to identify B. dermatitidis given patient history, and microscopic and colony morphology; to describe the symptoms of primary pulmonary infections caused by B. dermatitidis, and to name additional tissues typically affected by the systemic pathogen.

Vaccine immunity to pathogenic fungi overcomes the requirement for CD4 help in exogenous antigen presentation to CD8+ T cells: implications for vaccine development in immune-deficient hosts

Systemic fungal infections with primary and opportunistic pathogens have become increasingly common and represent a growing health menace in patients with AIDS and other immune deficiencies. T lymphocyte immunity, in particular the CD4+ Th 1 cells, is considered the main defense against these pathogens, and their absence is associated with increased susceptibility. It would seem illogical then to propose vaccinating these vulnerable patients against fungal infections. We report here that CD4+ T cells are dispensable for vaccine-induced resistance against experimental fungal pulmonary infections with two agents, Blastomyces dermatitidis an extracellular pathogen, and Histoplasma capsulatum a facultative intracellular pathogen. In the absence of T helper cells, exogenous fungal antigens activated memory CD8+ cells in a major histocompatibility complex class I-restricted manner and CD8+ T cell-derived cytokines tumor necrosis factor alpha, interferon gamma, and granulocyte/macrophage colony-stimulating factor-mediated durable vaccine immunity. CD8+ T cells could also rely on alternate mechanisms for robust vaccine immunity, in the absence of some of these factors. Our results demonstrate an unexpected plasticity of immunity in compromised hosts at both the cellular and molecular level and point to the feasibility of developing vaccines against invasive fungal infections in patients with severe immune deficiencies, including those with few or no CD4+ T cells.

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.

Blastomycosis

Blastomycosis is an endemic mycoses in the central United States caused by a dimorphic fungus, Blastomyces dermatitidis, that exists in nature in mycelial phase and converts to yeast phase at body temperature. The organism may produce epidemics of infection following a point source of infection or sporadic endemic infection. Blastomycosis can be a subclinical illness with subsequent protection against progressive infection afforded by cellular immune mechanisms, but it may present with progressive disease with either pulmonary or extrapulmonary disease or both. Itraconazole has been shown to be the drug of choice for both infections, except in cases of life-threatening infection when amphotericin B should be used.

Molecular genetic analysis of Blastomyces dermatitidis reveals new insights about pathogenic mechanisms

Fungal pathogens have emerged as a public health menace owing to the expanding population of vulnerable patients and to a heightened exposure to fungi in our environment, particularly for the systemic dimorphic fungi that inhabit soil worldwide. A better understanding of these microbes and their pathogenic mechanisms is badly needed to further research into therapeutic options. Advances in the molecular tools for genetic manipulation of Blastomyces dermatitidis have enhanced our ability to study this poorly understood dimorphic fungal pathogen. Recent refinements in gene-transfer technique, new selection markers, reliable reporter fusions and successes in gene targeting have shed light upon the importance of the mycelium-to-yeast transition and the crucial and complex role the BAD1 adhesin plays in pathogenesis.

BAD1, an essential virulence factor of Blastomyces dermatitidis, suppresses host TNF-alpha production through TGF-beta-dependent and -independent mechanisms

We investigated how BAD1, an adhesin and virulence factor of Blastomyces dermatitidis, suppresses phagocyte proinflammatory responses. Wild-type yeast cocultured with murine neutrophils or macrophages prompted release of a soluble factor into conditioned supernatant that abolished TNF-alpha production in response to the fungus; isogenic, attenuated BAD1 knockout yeast did not have this effect. Phagocytes released 4- to 5-fold more TGF-beta in vitro in response to wild-type yeast vs BAD1 knockout yeast. Treatment of inhibitory, conditioned supernatant with anti-TGF-beta mAb neutralized detectable TGF-beta and restored phagocyte TNF-alpha production. Similarly, addition of anti-TGF-beta mAb into cultures of phagocytes and wild-type yeast reversed BAD1 inhibition of TNF-alpha production. Conversely, TGF-beta treatment of phagocytes cultured with knockout yeast suppressed TNF-alpha production. Hence, TGF-beta mediates BAD1 suppression of TNF-alpha by wild-type B. dermatitidis cultured in vitro with phagocytes. In contrast to these findings, neutralization of elevated TGF-beta levels during experimental pulmonary blastomycosis did not restore BAD1-suppressed TNF-alpha levels in the lung or ameliorate disease. Soluble BAD1 was found to accumulate in the alveoli of infected mice at levels that suppressed TNF-alpha production by phagocytes. However, in contrast to yeast cell surface BAD1, which induced TGF-beta, soluble BAD1 failed to do so and TNF-alpha suppression mediated by soluble BAD1 was unaffected by neutralization of TGF-beta. Thus, BAD1 of B. dermatitidis induces suppression of TNF-alpha and progressive infection by both TGF-beta-dependent and -independent mechanisms.

Using new genetic tools to study the pathogenesis of Blastomyces dermatitidis

Fungal pathogens have emerged as a public health menace owing to the expanding population of vulnerable patients and a heightened exposure to fungi in our environment, particularly for the systemic dimorphic fungi that inhabit soil worldwide. A better understanding of these invaders and their pathogenic mechanisms is badly needed to further research into therapeutic options. Advances in the molecular tools available for genetic manipulation of Blastomyces dermatitidis have enhanced our ability to study this poorly understood dimorphic fungal pathogen. Recent refinements in gene-transfer techniques, new selection markers, reliable reporter fusions and successes in gene targeting have shed light upon the importance of the mycelium-to-yeast transition and the crucial and complex role the BAD1 adhesin plays in pathogenesis.

The WI-1 adhesin blocks phagocyte TNF-alpha production, imparting pathogenicity on Blastomyces dermatitidis

The WI-1 adhesin is indispensable for pathogenicity of Blastomyces dermatitidis and is thought to promote pulmonary infection by fixing yeast to lung tissue and cells. Recent findings suggest that WI-1 confers pathogenicity by mechanisms in addition to adherence. Here, we investigated whether WI-1 modulates host immunity by altering production of pro-inflammatory cytokines. Production of TNF-alpha in lung alveolar fluids of mice infected with B. dermatitidis was severalfold higher for WI-1 knockout yeast compared with wild-type yeast, and in vitro coculture of unseparated lung cells with these isogenic yeast disclosed similar differences. Upon coculture with purified macrophages and neutrophils, wild-type yeast blocked TNF-alpha production, yet WI-1 knockout yeast stimulated production. Coating knockout yeast with purified WI-1 converted them from stimulating TNF-alpha production to inhibiting production. Addition of purified WI-1 into stimulated phagocyte cultures led to concentration-dependent inhibition of TNF-alpha production. Neutralization of TNF-alpha in vivo exacerbated experimental pulmonary infection, particularly for the nonpathogenic WI-1 knockout yeast. Inducing increased TNF-alpha levels in the lung by adenovirus-vectored gene therapy controlled infection with wild-type yeast. Thus, the WI-1 adhesin on yeast modulates host immunity through blocking TNF-alpha production by phagocytes, which fosters progression of pulmonary infection.

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.

Molecular basis of pathogenicity in Blastomyces dermatitidis: the importance of adhesion

An understanding of the molecular bases of pathogenicity in Blastomyces dermatitidis and related systemic dimorphic fungi has been limited until recent years. Yeast cells of B. dermatitidis display an adhesion promoting protein termed WI-1. Recent studies entailing homologous gene targeting and mutation of WI-1 have provided null mutants at this locus and demonstrated the crucial role of the WI-1 adhesin in pathogenesis of blastomycosis. Ongoing studies are pointing to a link between phase-specific expression of WI-1 and the observation that transition to yeast cells is essential for the acquisition of pathogenicity by B. dermatitidis. Recombinant attenuated yeast that lack WI-1 are serving as invaluable tools for induction of vaccine resistance and are pointing to new insights about adaptive immunity to B. dermatitidis.

Murine models of blastomycosis, coccidioidomycosis, and histoplasmosis

Animal models have contributed much to the knowledge of fungal infections and their corresponding therapeutic treatments. This is true for animal models of the primary fungal pathogens, Blastomyces dermatitidis, Coccidioides immitis, and Histoplasma capsulatum. This review gives a brief background of human diseases associated with these organisms and describes the development, details, and utility of murine models of blastomycosis, as well as coccidioidomycosis and histoplasmosis.

Cell wall biogenesis of Blastomyces dermatitidis. Evidence for a novel mechanism of cell surface localization of a virulence-associated adhesin via extracellular release and reassociation with cell wall chitin

Pathogenic yeast of Blastomyces dermatitidis express a surface protein adhesin, WI-1. Due to the crucial role of WI-1 in adherence and disease pathogenesis, we investigated how the protein localizes to the surface of B. dermatitidis. WI-1 released extracellularly by wild-type yeast coated the surfaces of co-cultured knockout yeast within 3 h of incubation, implying that secreted WI-1 provides a pathway for loading the protein onto the yeast cell wall. In radioligand binding assays, purified WI-1 bound saturably, specifically, and with high affinity (K(d) = 8.3 x 10(-9)) to the cell surface of knockout yeast devoid of WI-1. WI-1 added exogenously, in vitro, to knockout yeast was indistinguishable from native cell surface WI-1 by fluorescence staining and restored adhesivity to the knockout yeast in macrophage binding and phagocytosis assays. Analysis of interactions between WI-1 and elements of the yeast cell wall identified chitin as the anchor point for WI-1. This interaction was shown to hinge on the 24-amino acid tandem repeat sequence of WI-1. Efforts to extract surface WI-1 from the yeast demonstrated that it is fastened to the wall by non-covalent interactions and covalent links between cysteine residues. We conclude that the yeast cell surface adhesin WI-1 localizes to the cell wall, in part, through extracellular release followed by high affinity binding back onto exposed chitin fibrils. These findings point to a novel pathway of cell wall biogenesis in yeast and an unanticipated role for chitin in anchoring and displaying a surface adhesin and virulence determinant.

Disseminated blastomycosis in a rhesus monkey (Macaca mulatta)

An 8-year-old male rhesus monkey (Macaca mulatta) died following a 6-day illness consisting of progressive depression, anorexia, labored abdominal breathing, coughing, and tachypnea. Gross necropsy findings included severe multifocal (miliary) granulomatous pneumonia, granulomatous splenitis, and multifocal cerebral abscesses. Histologic examination revealed 10-15-microm broad-based budding organisms within pyogranulomatous inflammatory lesions in the lung, tracheobronchial lymph node, brain, spleen, and liver. The distribution of extrapulmonary lesions was intermediate between that described for dogs and that described for humans. These findings were consistent with blastomycosis, which is previously unreported in nonhuman primates.

Targeted gene disruption reveals an adhesin indispensable for pathogenicity of Blastomyces dermatitidis

Systemic fungal infections are becoming more common and difficult to treat, yet the pathogenesis of these infectious diseases remains poorly understood. In many cases, pathogenicity can be attributed to the ability of the fungi to adhere to target tissues, but the lack of tractable genetic systems has limited progress in understanding and interfering with the offending fungal products. In Blastomyces dermatitidis, the agent of blastomycosis, a respiratory and disseminated mycosis of people and animals worldwide, expression of the putative adhesin encoded by the WI-1 gene was investigated as a possible virulence factor. DNA-mediated gene transfer was used to disrupt the WI-1 locus by allelic replacement, resulting in impaired binding and entry of yeasts into macrophages, loss of adherence to lung tissue, and abolishment of virulence in mice; each of these properties was fully restored after reconstitution of WI-1 by means of gene transfer. These findings establish the pivotal role of WI-1 in adherence and virulence of B. dermatitidis yeasts. To our knowledge, they offer the first example of a genetically proven virulence determinant among systemic dimorphic fungi, and underscore the value of reverse genetics for studies of pathogenesis in these organisms.

[Etiology of the first African case of systemic blastomycosis]

Several authors have suggested that african and north american forms of Blastomyces dermatitidis appeared separate taxa. Morphologic and pathogenic differences not only prevail, but also the failing to mate between them. African forms lose their mycelium-yeast conversion capability in old cultures. So I propose to create the complementary appellation Blastomyces dermatitidis tunisiens nov.subsp. from the african forms, in memory of the first case of systemic blastomycosis in this continent, discovered in Tunisia.

Virulent isolates and mutants of Blastomyces in mice: a legacy for studies of pathogenesis

This article provides information on the history of Blastomyces dermatitidis isolates and spontaneous mutants, the relationship with maintenance of cultures, and their virulence as quantified in murine models. Virulent isolates have been obtained from soil or from patients. Regardless of origin, mutants attenuated in virulence have sometimes arisen from storage as frozen yeast, frozen mycelia, or refrigerated yeast or serial passage in vitro at 35 degrees to 37 degrees C. Once virulence is lost, reversion to the virulent phenotype (even after serial passage in animals) has not been seen. These data, methods, and isolates provide a basis for future studies of virulence factors.

Animal models of blastomycosis

Mice offer many advantages as a model system for the study of experimental blastomycosis. The murine model has allowed quantitative assessments of disease initiation and progression. These methods of assessment have enabled studies of histopathology, and of the effect of age, host genetic background, fungal isolate virulence, and different routes of challenge on the outcome of infection. The model has been a particularly fertile soil for researching dissection of the immune response to fungal infections and the study of new agents (particularly polyenes and azoles) for the treatment of the mycoses.

Role of cell surface molecules of Blastomyces dermatitidis in the pathogenesis and immunobiology of blastomycosis

The cell surface of pathogenic microbes is critical in directing their interactions with the host. The discovery of a 120 kd antigen/adhesin WI-1 on the yeast or parasitic form of Blastomyces dermatitidis has elucidated the molecular basis of host/pathogen interactions in blastomycosis. WI-1 has three structural domains: (1) an N-terminal hydrophobic domain that spans the cell membrane, (2) a C-terminal epidermal growth factor-like domain that may bind extracellular matrix, and (3) a central domain of many 24- or 25 amino-acid repeats arrayed in tandem. The repeat is homologous to invasin, a Yersinia adhesin, and binds CD11b/CD18 (CR3) and CD14 receptors on host cells. WI-1 expression is altered on genetically related strains of B dermatitidis differing in virulence and modulates how hypovirulent mutants interact with macrophages. WI-1 also evokes humoral and cell-mediated immune responses in acquired resistance to B dermatitidis that may help clear the fungus in the host. These observations on WI-1 provide new insight into a key pathogenic factor and antigen of the fungus and may ultimately help in designing new ways to diagnose, treat, and prevent blastomycosis.

Transforming DNA integrates at multiple sites in the dimorphic fungal pathogen Blastomyces dermatitidis

Blastomyces dermatitidis is a primary fungal pathogen of man and other mammals, but like many other human fungal pathogens, relatively little is known about the factors that account for its virulence and pathogenicity. We developed a transformation system to facilitate molecular genetic studies of putative virulence factors from B. dermatitidis. Transformation of the multinucleate yeasts was achieved by electroporation of DNAs containing a dominant selectable marker, hygromycin B (HygB) resistance. Southern analysis showed that transforming DNA invariably integrated ectopically into the chromosome. No evidence was found for extrachromosomal DNA. The HygB resistance could be expressed by either a 375-bp promoter fragment of the B. dermatitidis WI-1 gene encoding adhesin or an Aspergillus gpdA promoter placed 5' of the E. coli hph gene. Primer extension analysis showed that for plasmids containing the WI-1 promoter, transcription of the hph gene initiated within the 375-bp WI-1 promoter fragment. The combination of gene transfer and two promoters capable of independent transcription will allow us to restore or augment gene expression in appropriate strains and test an influence on virulence. Molecular genetic manipulation of B. dermatitidis represents a major advance in our ability to investigate the pathogenesis of blastomycosis and other similar fungal diseases.

Virulence factors of medically important fungi

Human fungal pathogens have become an increasingly important medical problem with the explosion in the number of immunocompromised patients as a result of cancer, steroid therapy, chemotherapy, and AIDS. Additionally, the globalization of travel and expansion of humankind into previously undisturbed habitats have led to the reemergence of old fungi and new exposure to previously undescribed fungi. Until recently, relatively little was known about virulence factors for the medically important fungi. With the advent of molecular genetics, rapid progress has now been made in understanding the basis of pathogenicity for organisms such as Aspergillus species and Cryptococcus neoformans. The twin technologies of genetic transformation and "knockout" deletion construction allowed for genetic tests of virulence factors in these organisms. Such knowledge will prove invaluable for the rational design of antifungal therapies. Putative virulence factors and attributes are reviewed for Aspergillus species, C. neoformans, the dimorphic fungal pathogens, and others, with a focus upon a molecular genetic approach. Candida species are excluded from coverage, having been the subject of numerous recent reviews. This growing body of knowledge about fungal pathogens and their virulence factors will significantly aid efforts to treat the serious diseases they cause.

Role of cell-surface molecules of Blastomyces dermatitidis in host-pathogen interactions

The fungal pathogen Blastomyces dermatitidis produces an adhesin (WI-1) in yeast stages, which contains repetitive regions that bind host-cell receptors. Adhesin and glucan may modulate fungal interactions with macrophages; their level of expression is altered in hypovirulent mutants. Adhesin is also involved in immune responses, and may be important in eliciting the clearance of the fungus.

Double-stranded RNA virus in the human pathogenic fungus Blastomyces dermatitidis

Double-stranded RNA viruses were detected in a strain of Blastomyces dermatitidis isolated from a patient in Uganda. The viral particles are spherical (mostly 44 to 50 nm in diameter) and consist of about 25% double-stranded RNA (5 kb) and 75% protein (90 kDa). The virus contains transcriptional RNA polymerase activity; it synthesized single-stranded RNA in vitro in a conservative manner. The newly synthesized single-stranded RNA was a full-length strand, and the rate of chain elongation was approximately 170 nucleotides per min. The virus-containing strain shows no morphological difference from virus-free strains in the mycelial phase. Although the association with the presence of the virus is unclear, the virus-infected strain converts to the yeast form at 37 degrees C, but the yeast cells fail to multiply at that temperature.

Altered expression of surface protein WI-1 in genetically related strains of Blastomyces dermatitidis that differ in virulence regulates recognition of yeasts by human macrophages

The molecular basis for pathogenicity and virulence of the dimorphic fungus Blastomyces dermatitidis remains unknown. WI-1 is a major cell wall protein of B. dermatitidis yeasts and is a recognition target of both humoral and cell-mediated immunity. As an initial study to determine if WI-1 might be linked to virulence of B. dermatitidis, we quantified WI-1 expression on three genetically related strains that differ in their virulence for mice: wild-type virulent ATCC strain 26199, mutant ATCC strain 60915 (which is 10,000-fold reduced in virulence), and mutant ATCC strain 60916 (which is avirulent). Two principal alterations in WI-1 expression were observed in the mutants. First, the mutants express more WI-1 on their surface, as quantified by flow cytometry with monoclonal antibody to WI-1 and by radioimmunoassay, but the WI-1 on their cell wall is less extractable than that on the wild-type strain. Second, the mutants shed less WI-1 during culture and demonstrate impaired processing of shed WI-1. Surface alterations in WI-1 were accompanied by significant differences in the binding of the virulent and mutant strains to human monocyte-derived macrophages. Attachment of yeasts to macrophages paralleled and was proportional to the expression of WI-1. Compared with wild-type yeasts, both mutants bound to macrophages more rapidly and in two- to threefold-greater magnitude. Furthermore, about 75% of yeast binding to macrophages was inhibited by a Fab anti-WI-1 monoclonal antibody. These results suggest that altered WI-1 expression on attenuated and avirulent mutant B. dermatitidis yeasts greatly facilitates macrophage recognition and binding of yeasts and, in turn, may contribute to more rapid ingestion and killing in the host.

Altered expression of surface alpha-1,3-glucan in genetically related strains of Blastomyces dermatitidis that differ in virulence

Recent studies of the dimorphic fungal pathogens Histoplasma capsulatum and Paracoccidioides brasiliensis have suggested a role in virulence for the cell surface carbohydrate alpha-(1,3)-glucan. To investigate a possible basis for alpha-(1,3)-glucan in the pathogenicity and virulence of the dimorphic fungus Blastomyces dermatitidis, we examined three genetically related strains of B. dermatitidis that differ in their virulence for mice: wild-type virulent strain ATCC 26199; mutant strain ATCC 60915, which is 10,000-fold reduced in virulence; and mutant strain ATCC 60916, which is avirulent. Immunologic quantitation of cell wall alpha-(1,3)-glucan revealed that the mutant yeasts were almost devoid of this sugar moiety, in contrast to the high concentration of alpha-(1,3)-glucan on the cell wall of the wild-type yeasts. These differences are discussed in relation to previous studies of yeast surface expression of the WI-1 antigen and recognition and binding of the related strains by human monocyte-derived macrophages.

Development of pulmonary infection in mice inoculated with Blastomyces dermatitidis conidia

Intratracheal injection of Balb/cByJ mice with 10(4) Blastomyces dermatitidis conida produces chronic pulmonary and disseminated blastomycosis characterized by pyogranulomatous inflammation. To study the evolution of the pulmonary infection, mice were killed at varying intervals after inoculation, their lungs cultured and examined histologically. Nodular intraalveolar infiltrates of macrophages (M phi) were seen on Day 1 with occasional admixed polymorphonuclear leukocytes (PMN). Phagocytized yeast forms within M phi were evident by Day 5. By Day 28 pyogranulomas, which developed first as central microabscesses associated with a peripheral zone of M phi and giant cells containing internalized yeast, were a prominent feature of the infection. Lymphocytic and plasmacytic infiltrates, accumulating next to granulomas, formed the major peripheral component of the granuloma by Day 35. Formation of pyogranulomas was coincident with the host's failure to contain fungal growth measured by the sharp rise in colony-forming units recovered from lungs. Antibody against B. dermatitidis was first detected at Day 35 by enzyme immunoassay, but not until Day 63 by double immunodiffusion. During the 4 wk after inoculation, pulmonary lavage fluid contained > 90% M phi and < 3% PMN. On day 28, PMN rose to 17%, reaching 40% on Day 42. These data contribute to our knowledge of this model and help form the basis for investigations into the roles of fungal pathogenic and host defense mechanisms in blastomycosis.

Blastomycotic cranial osteomyelitis

This is the second case report of a temporal bone osteomyelitis caused by Blastomyces dermatitidis, which presented as a chronic serous otitis media. The presenting serous otitis media was refractory to conventional medical and surgical management and progressed to a temporal bone osteomyelitis prior to diagnosis. B. dermatitidis is a rare fungal pathogen that causes a systemic pyogranulomatous disease that primarily manifests itself in the skin, bones, pulmonary, and genitourinary systems. If left untreated it is associated with a high rate of mortality. The otologic presentation of this rare disease is emphasized, while the clinical and therapeutic features are reviewed.

WI-1, a novel 120-kilodalton surface protein on Blastomyces dermatitidis yeast cells, is a target antigen of cell-mediated immunity in human blastomycosis

A large body of experimental data has demonstrated the central role of T cells in acquired resistance to the dimorphic fungus Blastomyces dermatitidis. We examined the human T-cell response to WI-1, a 120-kDa B. dermatitidis yeast cell surface protein recently shown to be an immunodominant antigen of the B-cell response in infected humans. Peripheral blood lymphocytes from 10 blastomycosis patients studied proliferated in response to WI-1 (mean, 19,431 cpm) and to the standard, crude cell wall antigen, Blastomyces alkali- and water-soluble antigen (B-ASWS) (mean, 19,131 cpm); lymphocytes from 10 histoplasmosis patients and 10 normal control subjects did not respond to WI-1. WI-1 stimulation of patient lymphocytes and rechallenge with WI-1 or B-ASWS showed that the antigens share immunodominant epitopes. Of 100 WI-1-responsive T-cell clones derived from peripheral blood, 10 were studied in detail to assess the phenotype, function, and ligands recognized. The clones exhibit the CD3+ CD4+ phenotype of helper T cells; 2 of 10 clones (and 21% of antigen-stimulated peripheral blood lymphocytes) use the V beta 8 T-cell receptor gene element to respond to WI-1. All the clones proliferate in response to both WI-1 and B-ASWS but not other fungal antigens, and some mediate potent cytolytic effects on WI-1- and B-ASWS-labeled targets. WI-1 recognition requires antigen processing and presentation of epitopes in association with HLA-DR (to noncytolytic clones) and HLA-DP (to cytolytic clones). From these findings, we conclude that CD4+ T cells with regulatory and cytolytic properties are involved in the development of acquired resistance of B. dermatitidis, that the cells are directed against WI-1, and that the manner of display of WI-1 peptide epitopes in conjunction with major histocompatibility complex class II may influence the profile of the immune response.

Molecular evolution of the fungi: human pathogens

The morphological, ecological, and clinical diversity among ascomycete fungi that are pathogenic to humans suggest that the potential for pathogenicity may have arisen multiple times within these higher fungi. We have obtained 18S ribosomal DNA sequences from a diverse group of human pathogenic fungi in order to determine their evolutionary origins. The fungi studied include a skin pathogen that is confined to humans (Trichophyton rubrum) and three systemic, facultative parasites that cause histoplasmosis (Histoplasma capsulatum), blastomycosis (Blastomyces dermatitidis) and coccidioidomycosis (Coccidioides immitis) in humans and other higher animals. Also included in our analysis are representatives of non-pathogenic fungi, as well as two opportunistic pathogens, Pneumocystis carinii and Candida albicans, that cause severe disease in immunocompromised individuals, especially those with AIDS. Two of the fungi we sequenced, T. rubrum and C. immitis, are limited to asexual modes of reproduction and therefore lack the sexual structures that are most useful for evolutionary comparison as well as being essential for classification among the higher fungi. Coccidioides immitis is particularly problematic owing to its contradictory and confusing asexual morphologies, which have caused it to be placed in three fungal classes and the protista. Our analysis shows that the specialized, superficial parasite and the systemic, facultative parasites, including C. immitis, are closely related ascomycetes, which clearly demonstrates the power of molecular characters to compensate for missing or confusing reproductive morphology. Analysis also shows that the opportunistic pathogens are more distantly related, with the likely explanation that pathogenicity has arisen more than once within the Ascomycetes.