The hsp60 gene of the human pathogenic fungus Coccidioides immitis encodes a T-cell reactive protein

Biochemical characterization of an in-house Coccidioides antigen: perspectives for the immunodiagnosis of coccidioidomycosis

The objective of this study was to evaluate the reactivity of an in-house antigen, extracted from a strain of C. posadasii isolated in northeastern Brazil, by radial immunodiffusion and Western blotting, as well as to establish its biochemical characterization. The protein antigen was initially extracted with the use of solid ammonium sulfate and characterized by 1-D electrophoresis. Subsequently, it was tested by means of double radial immunodiffusion and Western blotting. A positive reaction was observed against the antigen by both immunodiagnostic techniques tested on sera from patients suffering from coccidioidomycosis. Besides this, two immunoreactive protein bands were observed and were revealed to be a β-glucosidase and a glutamine synthetase after sequencing of the respective N-terminal regions. Our in-house Coccidioides antigen can be promising as a quick and low-cost diagnostic tool without the risk of direct manipulation of the microorganism.

Vaccination with heat shock protein 60 induces a protective immune response against experimental Paracoccidioides brasiliensis pulmonary infection

Paracoccidioides brasiliensis causes a chronic granulomatous mycosis prevalent in Latin America. The successful resolution of infection with this fungus is dependent on the activation of cellular immunity. We previously identified heat shock protein 60 (HSP60) as a target of the humoral response in paracoccidioidomycosis. Herein we expressed the gene encoding HSP60 in Escherichia coli and analyzed the immunological activity of this recombinant antigen. The immunization of BALB/c mice with recombinant protein emulsified in adjuvant stimulated a cellular immune response. Splenocytes from immunized mice proliferated in response to antigen and released interleukin-12 and gamma interferon (IFN-gamma). Vaccination with HSP60 reduced the fungal burden in mice given 10(6) or 10(7) yeasts and protected mice from a lethal challenge. The efficacy of the vaccination was blunted by the neutralization of IFN-gamma. CD4(+) cells were necessary for the efficacy of the vaccination in both the afferent and efferent phases. Thus, we have demonstrated that this immunodominant antigen is a candidate for the development of a vaccine against this fungus.

Molecular Cloning and Expression of a cDNA Encoding a Coccidioides posadasii Cu,Zn Superoxide Dismutase Identified by Proteomic Analysis of the Coccidioidal T27K Vaccine

Previous studies have demonstrated that the coccidioidal T27K vaccine preparation is protective in mice against respiratory challenge using Coccidioides posadasii (C. posadasii) arthroconidia. Proteomic methods have been employed to define the molecular components within the vaccine. This method has led to the identification of novel and previously uncharacterized coccidioidal proteins including a Cu,Zn superoxide dismutase. A two-dimensional gel of the T27K vaccine was run and spots were excised for mass spectrometric analysis. One peptide was obtained from the T27K gel that matched a TIGR C. posadasii 2.0 gene index tentative consensus sequence, TC1072, which is similar to fungal Cu,Zn superoxide dismutase. Activity assays performed with native PAGE gels of the T27K vaccine showed that the vaccine contains superoxide dismutase. The cDNA encoding the enzyme has been cloned and sequenced and expressed as a recombinant protein.

Fungal heat-shock proteins in human disease

Heat-shock proteins (hsps) have been identified as molecular chaperones conserved between microbes and man and grouped by their molecular mass and high degree of amino acid homology. This article reviews the major hsps of Saccharomyces cerevisiae, their interactions with trehalose, the effect of fermentation and the role of the heat-shock factor. Information derived from this model, as well as from Neurospora crassa and Achlya ambisexualis, helps in understanding the importance of hsps in the pathogenic fungi, Candida albicans, Cryptococcus neoformans, Aspergillus spp., Histoplasma capsulatum, Paracoccidioides brasiliensis, Trichophyton rubrum, Phycomyces blakesleeanus, Fusarium oxysporum, Coccidioides immitis and Pneumocystis jiroveci. This has been matched with proteomic and genomic information examining hsp expression in response to noxious stimuli. Fungal hsp90 has been identified as a target for immunotherapy by a genetically recombinant antibody. The concept of combining this antibody fragment with an antifungal drug for treating life-threatening fungal infection and the potential interactions with human and microbial hsp90 and nitric oxide is discussed.

Characterization of a serodiagnostic complement fixation antigen of Coccidioides posadasii expressed in the nonpathogenic Fungus Uncinocarpus reesii

Coccidioides spp. (immitis and posadasii) are the causative agents of human coccidioidomycosis. In this study, we developed a novel system to overexpress coccidioidal proteins in a nonpathogenic fungus, Uncinocarpus reesii, which is closely related to Coccidioides. A promoter derived from the heat shock protein gene (HSP60) of Coccidioides posadasii was used to control the transcription of the inserted gene in the constructed coccidioidal protein expression vector (pCE). The chitinase gene (CTS1) of C. posadasii, which encodes the complement fixation antigen, was expressed using this system. The recombinant Cts1 protein (rCts1(Ur)) was induced in pCE-CTS1-transformed U. reesii by elevating the cultivation temperature. The isolated rCts1(Ur) showed chitinolytic activity that was identical to that of the native protein and had serodiagnostic efficacy comparable to those of the commercially available antigens in immunodiffusion-complement fixation tests. Using the purified rCts1(Ur), 74 out of the 77 coccidioidomycosis patients examined (96.1%) were positively identified by enzyme-linked immunosorbent assay. The rCts1(Ur) protein showed higher chitinolytic activity and slightly greater seroreactivity than the bacterially expressed recombinant Cts1. These data suggest that this novel expression system is a useful tool to produce coccidioidal antigens for use as diagnostic antigens.

A metalloproteinase of Coccidioides posadasii contributes to evasion of host detection

Coccidioides posadasii is a fungal respiratory pathogen of humans that can cause disease in immunocompetent individuals. Coccidioidomycosis ranges from a mild to a severe infection. It is frequently characterized either as a persistent disease that requires months to resolve or as an essentially asymptomatic infection that can reactivate several years after the original insult. In this report we describe a mechanism by which the pathogen evades host detection during the pivotal reproductive (endosporulation) phase of the parasitic cycle. A metalloproteinase (Mep1) secreted during endospore differentiation digests an immunodominant cell surface antigen (SOWgp) and prevents host recognition of endospores during the phase of development when these fungal cells are most vulnerable to phagocytic cell defenses. C57BL/6 mice were immunized with recombinant SOWgp and then challenged with a mutant strain of C. posadasii in which the MEP1 gene was disrupted. The animals showed a significant increase in percent survival compared to SOWgp-immune mice challenged with the parental strain. To explain these results, we proposed that retention of SOWgp on the surfaces of endospores of the mutant strain in the presence of high titers of antibody to the immunodominant antigen contributes to opsonization, increased phagocytosis, and killing of the fungal cells. In vitro studies of the interaction between a murine alveolar macrophage cell line and parasitic cells coated with SOWgp showed that the addition of anti-SOWgp antibody could enhance phagocytosis and killing of Coccidioides. We suggest that Mep1 plays a pivotal role as a pathogenicity determinant during coccidioidal infections and contributes to the ability of the pathogen to persist within the mammalian host.

A vaccine against coccidioidomycosis is justified and attainable

Coccidioides is a fungal pathogen of humans which can cause a life-threatening respiratory disease in immunocompetent individuals. Recurrent epidemics of coccidioidal infections in Southwestern United States has raised the specter of awareness of this soil-borne microbe, particularly among residents of Arizona and Southern California, and has galvanized research efforts to develop a human vaccine against coccidioidomycosis. In this review, we discuss the rationale for such a vaccine, examine the features of host innate and acquired immune response to Coccidioides infection, describe strategies used to identify and evaluate vaccine candidates, and provide an update on progress toward development of a vaccine against this endemic pathogen.

Coccidioidomycosis: host response and vaccine development

Coccidioidomycosis is caused by the dimorphic fungi in the genus Coccidioides. These fungi live as mycelia in the soil of desert areas of the American Southwest, and when the infectious spores, the arthroconidia, are inhaled, they convert into the parasitic spherule/endospore phase. Most infections are mild, but these organisms are frank pathogens and can cause severe lethal disease in fully immunocompetent individuals. While there is increased risk of disseminated disease in certain racial groups and immunocompromised persons, the fact that there are hosts who contain the initial infection and exhibit long-term immunity to reinfection supports the hypothesis that a vaccine against these pathogens is feasible. Multiple studies have shown that protective immunity against primary disease is associated with T-helper 1 (Th-1)-associated immune responses. The single best vaccine in animal models, formalin-killed spherules (FKS), was tested in a human trial but was not found to be significantly protective. This result has prompted studies to better define immunodominant Coccidioides antigen with the thought that a subunit vaccine would be protective. These efforts have defined multiple candidates, but the single best individual immunogen is the protein termed antigen 2/proline-rich antigen (Ag2/PRA). Studies in multiple laboratories have shown that Ag2/PRA as both protein and genetic vaccines provides significant protection against mice challenged systemically with Coccidioides. Unfortunately, compared to the FKS vaccine, it is significantly less protective as measured by both assays of reduction in fungal CFU and assays of survival. The capacity of Ag2/PRA to induce only partial protection was emphasized when animals were challenged intranasally. Thus, there is a need to define new candidates to create a multivalent vaccine to increase the effectiveness of Ag2/PRA. Efforts of genomic screening using expression library immunization or bioinformatic approaches to identify new candidates have revealed at least two new protective proteins, expression library immunization antigen 1 (ELI-Ag1) and a beta-1,3-glucanosyltransferase (GEL-1). In addition, previously discovered antigens such as Coccidioides-specific antigen (CSA) should be evaluated in assays of protection. While studies have yet to be completed with combinations of the current candidates, the hypothesis is that with increased numbers of candidates in a multivalent vaccine, there will be increased protection. As the genome sequences of the two Coccidioides strains which are under way are completed and annotated, the effort to find new candidates can increase to provide a complete genomic scan for immunodominant proteins. Thus, much progress has been made in the discovery of subunit vaccine candidates against Coccidioides and there are several candidates showing modest levels of protection, but for complete protection against pulmonary challenge we need to continue the search for additional candidates.

Cryptococcus neoformans gene expression during experimental cryptococcal meningitis

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

Identification of heat shock protein 60 as the ligand on Histoplasma capsulatum that mediates binding to CD18 receptors on human macrophages

Histoplasma capsulatum (Hc), is a facultative intracellular fungus that binds to CD11/CD18 receptors on macrophages (Mphi). To identify the ligand(s) on Hc yeasts that is recognized by Mphi, purified human complement receptor type 3 (CR3, CD11b/CD18) was used to probe a Far Western blot of a detergent extract of Hc cell wall and cell membrane. CR3 recognized a single 60-kDa protein, which was identified as heat shock protein 60 (hsp60). Biotinylation of viable yeasts, followed by precipitation with streptavidin-coated beads, and Western blotting with anti-hsp60 demonstrated that hsp60 was on the surface of Hc yeasts. Electron and confocal microscopy revealed that hsp60 resided on the yeast cell wall in discrete clusters. Recombinant hsp60 (rhsp60) inhibited attachment of Hc yeasts to Mphi. Recombinant hsp60 and Abs to CD11b and CD18 inhibited binding of yeasts to Chinese hamster ovary cells transfected with CR3 (CHO3). Polystyrene beads coated with rhsp60 bound to Mphi, and attachment was inhibited by Abs to CD11 and CD18. Freeze/thaw extract (F/TE), a preparation of Hc yeast surface proteins that contained hsp60, inhibited the attachment of Hc yeasts to Mphi. Depletion of hsp60 from F/TE removed the capacity of F/TE to block binding of Hc to Mphi. Interestingly, rhsp60 did not inhibit binding of Hc yeasts to dendritic cells (DC), which recognize Hc via very late Ag 5. Moreover, F/TE inhibited attachment of Hc to DC even when depleted of hsp60. Thus, Hc hsp60 appears to be a major ligand that mediates attachment of Hc to Mphi CD11/CD18, whereas DC recognize Hc via a different ligand(s).

Molecular approaches to the study of Coccidioides immitis

The study of the molecular biology of Coccidioides sp. is only just beginning. As the importance of coccidioidomycosis grows as a public health problem, our need for understanding of pathogenesis, immune responses, and improved antifungal therapy also increases in proportion. Tools have now become available to study gene manipulation in this pathogen and this will allow molecular approaches to be used. Genetic experiments will also be accelerated by the availability of the whole coccidioidal genome, expected to be made public in the spring of 2003 (see http://www.tigr.org/tdb/tgi/cigi/GenInfo.html). Thus, there seems to be several reasons to expect considerable progress in the coming years.

Heterologous expression, purification, and immunological reactivity of a recombinant HSP60 from Paracoccidioides brasiliensis

The complete coding cDNA of HSP60 from Paracoccidioides brasiliensis was overexpressed in an Escherichia coli host to produce high levels of recombinant protein. The protein was purified by affinity chromatography. A total of 169 human serum samples were tested for reactivity by Western blot analysis with the purified HSP60 recombinant protein. Immunoblots indicated that the recombinant P. brasiliensis HSP60 was recognized by antibodies in 72 of 75 sera from paracoccidioidomycosis patients. No cross-reactivity was detected with individual sera from patients with aspergillosis, sporotrichosis, cryptococcosis, and tuberculosis. Reactivity to HSP60 was observed in sera from 9.52% of control healthy individuals and 11.5% of patients with histoplasmosis. The high sensitivity and specificity (97.3 and 92.5%, respectively) for HSP60 suggested that the recombinant protein can be used singly or in association with other recombinant antigens to detect antibody responses in P. brasiliensis-infected patients.

Molecular cloning, characterization and expression of the heat shock protein 60 gene from the human pathogenic fungus Paracoccidioides brasiliensis

A gene encoding the heat shock protein (HSP) 60 from Paracoccidioides brasiliensis (Pb) was cloned and characterized. The hsp60 gene is composed of three exons divided by two introns. Structural analysis of the promoter detected canonical sequences characteristic of regulatory regions from eukaryotic genes. The deduced amino acid sequence of the Pb hsp60 gene and the respective cloned cDNA consists of 592 residues highly homologous to other fungal HSP60 proteins. The hsp60 gene is present as a single copy in the genome, as shown by Southern blot analysis. The HSP60 protein was isolated from Pb yeast cellular extracts. N-terminal amino acid sequencing of HSP60 confirmed that the cloned hsp60 gene correlated to the predicted protein in Pb. HSP60 expression appeared to be regulated during form transition in Pb, as different levels of expression were detected in in vitro labeling of cells and northern blot analysis. The complete coding region of Pb hsp60 was fused with plasmid pGEX-4T-3 and expressed in Escherichia coli as a glutathione S-transferase-tagged recombinant protein. The protein reacted with a mouse monoclonal antibody raised to a human recombinant HSP60. Western immunoblot experiments demonstrated that the recombinant protein and the native HSP60 were recognized by sera from humans with paracoccidioidomycosis (PCM).

Recombinant urease and urease DNA of Coccidioides immitis elicit an immunoprotective response against coccidioidomycosis in mice

Coccidioides immitis antigens which stimulate a T helper cell 1 (Th1) pathway of host immune response are considered to be essential components of a vaccine against coccidioidomycosis. Recombinant urease (rURE) and recombinant heat shock protein 60 (rHSP60) of C. immitis were expressed in Escherichia coli and tested as vaccine candidates in BALB/c mice. A synthetic oligodeoxynucleotide which contained unmethylated CpG dinucleotides and was previously shown to enhance a murine Th1 response was used as an immunoadjuvant. T cells isolated from the spleens and lymph nodes of the rURE- and rHSP60-immune mice showed in vitro proliferative responses to the respective recombinant protein, but only those T lymphocytes from rURE-immunized mice revealed markedly elevated levels of expression of selected Th1-type cytokine genes. BALB/c mice immunized subcutaneously with rURE and subsequently challenged by the intraperitoneal (i.p.) route with a lethal inoculum of C. immitis arthroconidia demonstrated a significant reduction in the level of C. immitis infection compared to control animals. rHSP60 was much less effective as a protective antigen. Evaluation of cytokine gene expression in lung tissue and levels of recombinant urease-specific immunoglobulins (immunoglobulin G1 [IgG1] versus IgG2a) in murine sera at 12 days after challenge provided additional evidence that immunization with rURE stimulated a Th1 response to the pathogen. Urease was further evaluated by expression of the URE gene in a mammalian plasmid vector (pSecTag2A.URE) which was used to immunize mice by the intradermal route. In this case, 82% of the vector construct-immunized animals survived more than 40 days after i.p. infection, compared to only 10% of the mice immunized with the vector alone. In addition, 87% of the pSecTag2A.URE-immunized survivors had sterile lungs and spleens. These data support the need for further evaluation of the C. immitis urease as a candidate vaccine against coccidioidomycosis.

Cloning and expression of the gene which encodes a tube precipitin antigen and wall-associated beta-glucosidase of Coccidioides immitis

We report the structure and expression of the Coccidioides immitis BGL2 gene which encodes a previously characterized 120-kDa glycoprotein of this fungal respiratory pathogen. The glycoprotein is recognized by immunoglobulin M tube precipitin (TP) antibody present in sera of patients with coccidioidomycosis, a reaction which has been used for serodiagnosis of early coccidioidal infection. The deduced amino acid sequence of BGL2 shows 12 potential N glycosylation sites and numerous serine-threonine-rich regions which could function as sites for O glycosylation. In addition, the protein sequence includes a domain which is characteristic of family 3 glycosyl hydrolases. Earlier biochemical studies of the purified 120-kDa TP antigen revealed that it functions as a beta-glucosidase (EC 3.2.1.21). Its amino acid sequence shows high homology to several other reported fungal beta-glucosidases which are members of the family 3 glycosyl hydrolases. Results of previous studies have also suggested that the 120-kDa beta-glucosidase participates in wall modification during differentiation of the parasitic cells (spherules) of C. immitis. In this study we showed that expression of the BGL2 gene is elevated during isotropic growth of spherules and the peak of wall-associated BGL2 enzyme activity correlates with this same phase of parasitic cell differentiation. These data support our hypothesis that the 120-kDa beta-glucosidase plays a morphogenetic role in the parasitic cycle of C. immitis.

Seeking a vaccine against Coccidioides immitis and serologic studies: expectations and realities

The studies reported in this review indicate that, whereas the expectations from the molecular approach help to excite and enlighten us, the realities suggest that even some less modern approaches may provide the necessary practical solutions to problems of serology and vaccination against coccidioidomycosis. Concurrent conduct of the two approaches should maximize the chances for success.

Sequence, expression and functional analysis of the Coccidioides immitis ODC (ornithine decarboxylase) gene

The ornithine decarboxylase (ODC) gene of the human respiratory fungal pathogen, Coccidioides immitis (Ci) was cloned, sequenced, chromosome-mapped, and expressed in Escherichia coli (Ec). The genomic, cDNA and translated sequences are presented. Transformation of an ODC null mutant strain of Ec (EWH 319) with the Ci ODC gene was conducted to confirm function of the protein encoded by the fungal gene. Activity of the enzyme by the bacterial transformant was inhibited by 1, 4-diamino-2-butanone (DAB), a known inhibitor of eukaryotic ODC. Temporal expression of the Ci ODC gene during the parasitic cell cycle is constitutive, based on results of RT PCR. However, results of enzyme activity assays of cell homogenates obtained at different stages of parasitic cell development in vitro showed that the functional protein is present only during periods of isotropic growth and segmentation, and these morphogenetic events can be arrested by the addition of DAB. The observed absence of a difference in steady-state mRNA transcript amounts, and the developmentally correlated variation in levels of enzyme activity, suggest a translational or post-translational mechanism of ODC regulation. Since no PEST sequence was detected in the Ci ODC, enzyme regulation by programmed protein degradation as reported for many other eukaryotic ODCs may not occur in this case. ODC activity appears to play a key role in the morphogenesis of Ci, and the enzyme could be a rational target for therapy of disseminated coccidioidomycosis.

Genetic vaccination against Coccidioides immitis: comparison of vaccine efficacy of recombinant antigen 2 and antigen 2 cDNA

Previous studies from our laboratory established that C-ASWS, an alkali-soluble, water-soluble extract from cell walls of Coccidioides immitis, protects mice against lethal challenge with this fungus. The C-ASWS extract contains a glycosylated protein, designated antigen 2 (Ag2), and a polysaccharide antigen. We recently cloned Ag2 cDNA and showed that the recombinant fusion protein elicited strong delayed-type hypersensitivity responses in immunized mice. This investigation was undertaken to determine if the recombinant Ag2 protein, expressed as an Ag2-glutathione S-transferase (GST) fusion protein, or Ag2 cDNA would protect mice against lethal challenge with C. immitis. The recombinant Ag2-GST protein protected BALB/c mice against intraperitoneal challenge with 250 arthroconidia, as assessed by a decrease in fungal CFU in tissues. The Ag2-GST-immunized mice did not show, however, an increased survival during a 30-day period postinfection. By contrast, immunization of mice with Ag2 cDNA ligated into the pVR1012 plasmid engendered protection against intraperitoneal challenge with 2,500 arthroconidia and against pulmonary challenge with 50 arthroconidia. Vaccine efficacy paralleled the development of delayed-type hypersensitivity responses to C. immitis antigen. Whereas mice vaccinated with the recombinant Ag2-GST protein did not mount footpad hypersensitivity to C-ASWS or the recombinant Ag2-GST protein, mice vaccinated with the pVR1012-Ag2 construct mounted a strong footpad hypersensitivity and their spleen cells secreted gamma interferon upon in vitro stimulation with the Ag2-containing C-ASWS extract. This is the first investigation to show that genetic immunization can protect against lethal challenge with C. immitis.

Isolation and confirmation of function of the Coccidioides immitis URA5 (orotate phosphoribosyl transferase) gene

The OPRTase (URA5) gene of the human pathogenic fungus, Coccidioides immitis (Ci), was cloned, sequenced, chromosome-mapped and expressed both by transformation of Escherichia coli and by complementation of wdura5Delta, an auxotrophic strain of Wangiella dermatitidis (Wd) with a disrupted URA5 gene. A functional assay of the recombinant URA5 expressed by E. coli was conducted to ensure that the isolated Ci gene encodes the appropriate enzyme. In the absence of a transformation system for Ci, we also used a reported method of introduction of heterologous DNA into cells of the phylogenetically related fungus, Wangiella dermatitidis, to confirm the function of the Ci URA5 gene. Both the genomic and cDNA sequences of the Ci URA5 gene are presented. The transcription start point and two poly(A) addition sites were confirmed. The gene contains a 714-bp ORF that translates a 238-amino-acid (aa) protein of 25.5kDa and pI of 6.5. No introns are present. The translated protein contains a single, putative N-glycosylation site. The deduced Ci protein showed 55-63% aa sequence similarity to reported fungal OPRTases. The URA5 gene was mapped to chromosome IV of Ci, and was shown to be a single copy gene by Southern and Northern hybridizations. Transformation of the wdura5Delta mutant to prototrophy was accomplished by electroporation of Wd yeast cells with the Ci URA5 gene. Cellular uptake of the heterologous DNA was confirmed by Southern hybridization. The stable transformants were unable to grow on a medium containing 5-FOA. Expression of the Ci URA5 gene can be used as a selectable marker for a transformation system, and the latter is essential for molecular studies of this pathogenic fungus.