Protection against lethal murine coccidioidomycosis by a soluble vaccine from spherules

Vaccine development for pathogenic fungi: current status and future directions

INTRODUCTION

Fungal infections are caused by a broad range of pathogenic fungi that are found worldwide with different geographic distributions, incidences, and mortality rates. Considering that there are relatively few approved medications available for combating fungal diseases and no vaccine formulation commercially available, multiple groups are searching for new antifungal drugs, examining drugs for repurposing and developing antifungal vaccines, in order to control deaths, sequels, and the spread of these complex infections.

AREAS COVERED

This review provides a summary of advances in fungal vaccine studies and the different approaches under development, such as subunit vaccines, whole organism vaccines, and DNA vaccines, as well as studies that optimize the use of adjuvants. We conducted a literature search of the PubMed with terms: fungal vaccines and genus of fungal pathogens (Cryptococcus spp. Candida spp. Coccidioides spp. Aspergillus spp. Sporothrix spp. Histoplasma spp. Paracoccidioides spp. Pneumocystis spp. and the Mucorales order), a total of 177 articles were collected from database.

EXPERT OPINION

Problems regarding the immune response development in an immunocompromised organism, the similarity between fungal and mammalian cells, and the lack of attention by health organizations to fungal infections are closely related to the fact that, at present, there are no fungal vaccines available for clinical use.

A chromosomal-level reference genome of the widely utilized Coccidioides posadasii laboratory strain "Silveira"

Coccidioidomycosis is a common fungal disease that is endemic to arid and semi-arid regions of both American continents. Coccidioides immitis and Coccidioides posadasii are the etiological agents of the disease, also known as Valley Fever. For several decades, the C. posadasii strain Silveira has been used widely in vaccine studies, is the source strain for production of diagnostic antigens, and is a widely used experimental strain for functional studies. In 2009, the genome was sequenced using Sanger sequencing technology, and a draft assembly and annotation were made available. In this study, the genome of the Silveira strain was sequenced using single molecule real-time sequencing PacBio technology, assembled into chromosomal-level contigs, genotyped, and the genome was reannotated using sophisticated and curated in silico tools. This high-quality genome sequencing effort has improved our understanding of chromosomal structure, gene set annotation, and lays the groundwork for identification of structural variants (e.g. transversions, translocations, and copy number variants), assessment of gene gain and loss, and comparison of transposable elements in future phylogenetic and population genomics studies.

Coccidioidomycosis: Epidemiology, Fungal Pathogenesis, and Therapeutic Development

Purpose of Review

Coccidioidomycosis can result from the inhalation of infectious spores of Coccidioides species (spp.) immitis or posadasii. Clinical manifestations range from mild flu-like disease to severe disseminated infection that can require life-long therapy. Burden of this mycosis is high in the southwest region of the USA where it is well characterized, and in many areas of Mexico and Latin America where it is inadequately characterized. Here, we provide historical data and current knowledge on Coccidioides spp. pathogenesis as well as recent progress in therapeutic and vaccine development against coccidioidomycosis.

Recent Findings

The virulence mechanisms of Coccidioides spp. are largely unknown; however, production and regulation of a spherule glycoprotein, ammonium production, and melanization have all been proposed as integral factors in Coccidioides spp.' pathogenesis. Therapeutic options are limited and not 100% effective, but individualized treatment with triazoles or amphotericin B over the course of pulmonary or disseminated infection can be effective in resolution of coccidioidomycosis. Human immunization has not been achieved but efforts are ongoing.

Summary

Advances in therapeutic and vaccine development are imperative for the prevention and treatment of coccidioidomycosis, especially for those individuals at risk either living or traveling to or from endemic areas.

The Quest for a Vaccine Against Coccidioidomycosis: A Neglected Disease of the Americas

Coccidioidomycosis (Valley Fever) is a disease caused by inhalation of Coccidioides spp. This neglected disease has substantial public health impact despite its geographic restriction to desert areas of the southwestern U.S., Mexico, Central and South America. The incidence of this infection in California and Arizona has been increasing over the past fifteen years. Several large cities are within the endemic region in the U.S. Coccidioidomycosis accounts for 25,000 hospital admissions per year in California. While most cases of coccidioidomycosis resolve spontaneously, up to 40% are severe enough to require anti-fungal treatment, and a significant number disseminate beyond the lungs. Disseminated infection involving the meninges is fatal without appropriate treatment. Infection with Coccidioides spp. is protective against a second infection, so vaccination seems biologically plausible. This review of efforts to develop a vaccine against coccidioidomycosis focuses on vaccine approaches and the difficulties in identifying protein antigen/adjuvant combinations that protect in experimental mouse models. Although the quest for a vaccine is still in the early stage, scientific efforts for vaccine development may pave the way for future success.

Preclinical identification of vaccine induced protective correlates in human leukocyte antigen expressing transgenic mice infected with Coccidioides posadasii

There is an emerging interest to develop human vaccines against medically-important fungal pathogens and a need for a preclinical animal model to assess vaccine efficacies and protective correlates. HLA-DR4 (DRB1∗0401 allele) transgenic mice express a human major histocompatibility complex class II (MHC II) receptor in such a way that CD4⁺ T-cell response is solely restricted by this human molecule. In this study HLA-DR4 transgenic mice were immunized with a live-attenuated vaccine (ΔT) and challenged by the intranasal route with 50-70 Coccidioides posadasii spores, a potentially lethal dose. The same vaccination regimen offers 100% survival for C57BL/6 mice. Conversely, ΔT-vaccinated HLA-DR4 mice displayed 3 distinct manifestations of Coccidioides infection including 40% fatal acute (FAD), 30% disseminated (DD) and 30% pulmonary disease (PD). The latter 2 groups of mice had reduced loss of body weight and survived to at least 50days postchallenge (dpc). These results suggest that ΔT vaccinated HLA-DR4 mice activated heterogeneous immunity against pulmonary Coccidioides infection. Vaccinated HLA-DR4 mice displayed early expansion of Th1 and Th17 cells and recruitment of inflammatory innate cells into Coccidioides-infected lungs during the first 9dpc. While contraction rates of Th cells and the inflammatory response during 14-35dpc significantly differed among the 3 groups of vaccinated HLA-DR4 mice. The FAD group displayed a sharply reduced Th1 and Th17 response, while overwhelmingly recruiting neutrophils into lungs during 9-14days. The FAD group approached moribund by 14dpc. In contrast, vaccinated HLA-DR4 survivors gradually contracted Th cells and inflammatory response with the greatest rate in the PD group. While vaccinated HLA-DR4 mice are susceptible to Coccidioides infection, they are useful for evaluation of vaccine efficacy and identification of immunological correlates against this mycosis.

Progress Toward a Human Vaccine Against Coccidioidomycosis

Coccidioidomycosis (San Joaquin Valley fever) is a human respiratory disease caused by a soil-borne mold, and is recognized as an intransigent microbial infection by physicians who treat patients with the potentially life-threatening, disseminated form of this mycosis. Epidemiological studies based on surveys of skin-test reactivity of people who reside in the endemic regions of the Southwestern US have shown that at least 150,000 new infections occur annually. The clinical spectrum of coccidioidomycosis ranges from an asymptomatic insult to a severe pulmonary disease in which the pathogen may spread from the lungs to the skin, bones, brain and other body organs. Escalation of symptomatic infections and increased cost of long-term antifungal treatment warrant a concerted effort to develop a vaccine against coccidioidomycosis. This review examines recently reported strategies used to generate such a vaccine and summarizes current understanding of the nature of protective immunity to this formidable disease.

An agonist of human complement fragment C5a enhances vaccine immunity against Coccidioides infection

Coccidioides is a fungal pathogen and causative agent of a human respiratory disease against which no clinical vaccine exists. In this study we evaluated a novel vaccine adjuvant referred to as EP67, which is a peptide agonist of the biologically active C-terminal region of human complement component C5a. The EP67 peptide was conjugated to live spores of an attenuated vaccine strain (ΔT) of Coccidioides posadasii. The non-conjugated ΔT vaccine provided partial protection to BALB/c mice against coccidioidomycosis. In this report we compared the protective efficacy of the ΔT-EP67 conjugate to the ΔT vaccine in BALB/c mice. Animals immunized subcutaneously with the ΔT-EP67 vaccine showed significant increase in survival and decrease in fungal burden over 75 days postchallenge. Increased pulmonary infiltration of dendritic cells and macrophages was observed on day 7 postchallenge but marked decrease in neutrophil numbers had occurred by 11 days. The reduced influx of neutrophils may have contributed to the observed reduction of inflammatory pathology. Mice immunized with the ΔT-EP67 vaccine also revealed enhanced expression of MHC II molecules on the surface of antigen presenting cells, and in vitro recall assays of immune splenocytes showed elevated Th1- and Th17-type cytokine production. The latter correlated with a marked increase in lung infiltration of IFN-γ- and IL-17-producing CD4(+) T cells. Elevated expression of T-bet and RORc transcription factors in ΔT-EP67-vaccinated mice indicated the promotion of Th1 and Th17 cell differentiation. Higher titers of Coccidioides antigen-specific IgG1 and IgG2a were detected in mice immunized with the EP67-conjugated versus the non-conjugated vaccine. These combined results suggest that the EP67 adjuvant enhances protective efficacy of the live vaccine by augmentation of T-cell immunity, especially through Th1- and Th17-mediated responses to Coccidioides infection.

Vaccine immunity to coccidioidomycosis occurs by early activation of three signal pathways of T helper cell response (Th1, Th2, and Th17)

We have previously reported that C57BL/6 mice vaccinated with a live, attenuated mutant of Coccidioides posadasii, referred to as the ΔT vaccine, are fully protected against pulmonary coccidioidomycosis. This model was used here to explore the nature of vaccine immunity during the initial 2-week period after intranasal challenge. Elevated neutrophil and eosinophil infiltration into the lungs of nonvaccinated mice contrasted with markedly reduced recruitment of these cells in vaccinated animals. The numbers of lung-infiltrated macrophages and dendritic cells showed a progressive increase in vaccinated mice and corresponded with reduction of the lung infection. Concentrations of selected inflammatory cytokines and chemokines were initially higher in lung homogenates of vaccinated mice but then generally decreased at 14 days postchallenge in correlation with containment of the organism and apparent dampening of the inflammation of host tissue. Profiles of cytokines detected in lung homogenates of ΔT-vaccinated mice were indicative of a mixed T helper 1 (Th1)-, Th2-, and Th17-type immune response, a conclusion which was supported by detection of lung infiltration of activated T cells with the respective CD4(+) gamma interferon (IFN-γ)(+), CD4(+) interleukin-5 (IL-5)(+), and CD4(+) IL-17A(+) phenotypes. While Th1 and Th2 immunity was separately dispensed of by genetic manipulation without loss of ΔT vaccine-mediated protection, loss of functional Th17 cells resulted in increased susceptibility to infection in immunized mice. Characterization of the early events of protective immunity to Coccidioides infection in vaccinated mice contributes to the identification of surrogates of immune defense and provides potential insights into the design of immunotherapeutic protocols for treatment of coccidioidomycosis.

Vaccine-induced protection against 3 systemic mycoses endemic to North America requires Th17 cells in mice

Worldwide rates of systemic fungal infections, including three of the major pathogens responsible for such infections in North America (Coccidioides posadasii, Histoplasma capsulatum, and Blastomyces dermatitidis), have soared recently, spurring interest in developing vaccines. The development of Th1 cells is believed to be crucial for protective immunity against pathogenic fungi, whereas the role of Th17 cells is vigorously debated. In models of primary fungal infection, some studies have shown that Th17 cells mediate resistance, while others have shown that they promote disease pathology. Here, we have shown that Th1 immunity is dispensable and that fungus-specific Th17 cells are sufficient for vaccine-induced protection against lethal pulmonary infection with B. dermatitidis in mice. Further, vaccine-induced Th17 cells were necessary and sufficient to protect against the three major systemic mycoses in North America. Mechanistically, Th17 cells engendered protection by recruiting and activating neutrophils and macrophages to the alveolar space, while the induction of Th17 cells and acquisition of vaccine immunity unexpectedly required the adapter molecule Myd88 but not the fungal pathogen recognition receptor Dectin-1. These data suggest that human vaccines against systemic fungal infections should be designed to induce Th17 cells if they are to be effective.

Polyfunctional T lymphocytes are in the peripheral blood of donors naturally immune to coccidioidomycosis and are not induced by dendritic cells

Coccidioidomycosis is a fungal infection endemic in the southwestern United States that is increasing in incidence. While cellular immunity correlates with protection from clinical illness, the precise elements of that response are undefined. Using the coccidioidal antigen preparation T27K and multiparametric flow cytometry, the in vitro frequency of polyfunctional T lymphocytes in the peripheral blood of naturally immune healthy donors and those who were nonimmune was determined. Polyfunctional CD4 lymphocytes, defined as producing intracellular interleukin 2 (IL-2), gamma interferon (IFN-gamma), and tumor necrosis factor alpha simultaneously, had a frequency of 137 per 400,000 events among peripheral blood mononuclear cells (PBMC) of immune donors compared to 11 per 400,000 PBMC from nonimmune donors (P = 0.03). When monocyte-derived mature dendritic cells pulsed with T27K (mDC(T27K)) were used for antigen presentation, the frequency of polyfunctional CD4 T lymphocytes did not significantly increase for either group, although mDC(T27K) did significantly increase the concentrations of IL-2 and IFN-gamma released by PBMC from nonimmune donors (P = 0.02). After in vitro stimulation with T27K, polyfunctional CD4 and CD8 lymphocytes of PBMC from immune donors had a mixture of low- and high-expression CCR7 cells, suggesting both effector and central memory, compared with predominantly high-expression CCR7 cells when PBMC were incubated with the mitogen phytohemagglutinin (P = 0.03). These data demonstrate the presence of polyfunctional T lymphocytes in the peripheral blood of individuals with coccidioidal immunity and suggest a model for the in vitro testing of vaccine candidates for coccidioidomycosis.

Saccharomyces cerevisiae as a vaccine against coccidioidomycosis

Disseminated coccidioidomycosis is a life-threatening infection. In these studies, we examined protection against systemic murine coccidioidomycosis by vaccination with heat-killed Saccharomyces cerevisiae (HKY). CD-1 mice received HKY subcutaneously or by oral gavage with or without adjuvants once weekly beginning 3 or 4 weeks prior to infection; oral live Saccharomyces was also studied. All HKY sc regimens were equivalent, prolonging survival (P<or=0.005) and reducing fungal burden versus controls. Oral live Saccharomyces, but not HKY, prolonged survival (P=0.03), but did not reduce fungal burden. Survival of mice given HKY was equivalent to vaccination with formalin-killed spherules, but inferior in reduction of fungal burden. HKY was superior to a successful recombinant vaccine, PRA plus adjuvant. This novel heterologous protection afforded by HKY vaccination offers a new approach to a vaccine against coccidioidomycosis.

Vaccine-induced cellular immune responses differ from innate responses in susceptible and resistant strains of mice infected with Coccidioides posadasii

Susceptibility to Coccidioides spp. varies widely in humans and other mammals and also among individuals within a species. Among strains of mice with various susceptibilities, immunohistopathology revealed that C57BL/6 mice were highly susceptible to the disease following intranasal infection, DBA/2n mice were intermediate, and Swiss-Webster mice were innately resistant. Resistant Swiss-Webster mice developed prominent perivascular/peribronchiolar lymphocytic cuffing and well-formed granulomas with few fungal elements and debris in the necrotic center, surrounded by a mantle of macrophages, lymphocytes, and fibrocytes. Susceptible C57BL/6 mice became moribund between 14 and 18 days postinfection, with overwhelming numbers of neutrophils and spherules and very few T cells, the drastic reduction of which was associated with failure and death, while intermediate DBA/2n mice controlled the fungal burden but demonstrated progressive lung inflammation with prominent suppuration, and they deteriorated clinically. Vaccinated C57BL/6 mice had an early and robust lymphocyte response, which included significantly higher Mac2(+), CD3(+), and CD4(+) cell scores on day 18 than those of innately resistant SW mice and DBA/2n mice; they also had prominent perivascular/peribronchiolar lymphocytic infiltrates not present in their unvaccinated counterparts, and they appeared to be resolving lesions by day 56 compared to the other two strains, based on significantly lower disease scores and observably smaller and fewer lesions with few spherules and neutrophils.

Molecular cloning and expression of a cDNA encoding a Coccidioides posadasii 1,2-alpha-mannosidase identified in the coccidioidal T27K vaccine by immunoproteomic methods

The coccidioidal T27K vaccine is protective in mice against respiratory challenge with Coccidioides posadasii (C. posadasii) arthroconidia. The vaccine is a subcellular multicomponent preparation that has not been fully characterized. To identify potential protective antigens in the heterogeneous mixture, the vaccine has been separated by two-dimensional gel electrophoresis and then analyzed for seroreactive proteins using immunoblot analysis with pooled sera from patients with coccidioidomycosis. Two seroreactive spots of identical apparent molecular weight were identified and sequenced using tandem mass spectrometry. Three peptides were generated, two of which matched a tentative consensus sequence in the TIGR C. posadasii 2.0 gene index database that is similar to fungal 1,2-alpha-mannosidases. The 5' and 3' ends of the mannosidase cDNA were mapped using rapid amplification of cDNA ends (RACE) polymerase chain reaction (PCR), and a full-length cDNA was then obtained using reverse-transcription (RT) PCR. The cDNA was cloned and sequenced and expressed as a recombinant protein. The predicted protein consists of 519 amino acids, has a theoretical molecular weight and pI of 56,918 Da and 4.84, respectively, and is very similar (>60%) to other fungal 1,2-alpha-mannosidases. Class I 1,2-alpha-mannosidase enzyme activity was also detected in the T27K vaccine using the substrate, Man-alpha-1,2-Man-alpha-OCH(3) in a spectrophotometric assay.

The complex immunology of human coccidioidomycosis

The human immune response during coccidioidomycosis is intimately involved with the development of delayed-type hypersensitivity and cellular immunity. Sixty percent of those infected have no symptoms and benign outcome is generally associated with a specific cellular immune response to coccidioidal antigens. We have recently teased out the human pulmonary granulomatous response during coccidioidomycosis and noted that there are perigranulomatous clusters of lymphocytes consisting predominantly of B lymphocytes and CD4(+) T lymphocytes. In other work, we have found that the mannose receptor as well as the toll-like receptors TLR2 and TLR4 may have a role in recognizing glycosylated coccidioidal antigens. In addition, the IL-12 receptor axis appears to be operative during antigen recognition and IL-12p40 may be the active moiety. Finally, peripheral blood mononuclear cells from persons with disseminated coccidioidomycosis are able to respond to coccidioidal antigen when it is presented by a mature monocyte-derived IL-4-generated dendritic cell (DC). These observations could be useful in the development of a human vaccine against coccidiodomycosis.

Characteristics of the protective subcellular coccidioidal T27K vaccine

While the whole killed spherule vaccine, protective in mice and monkeys, did not prevent coccidioidal disease in humans, the 27K vaccine, a soluble derivative, retains protective activity in mice with little irritant action. Gel filtration and anion exchange fractions of thimerosal-inactivated spherules (T27K), when administered with alum adjuvant, also protect mice against lethal respiratory coccidioidal challenge. However, the superb protection afforded by T27K antigens is maintained for some 3 months, but may then diminish. This appears unrelated to the aging of the mice. Prolongation of the protective action may require addition of a different adjuvant or administration of booster doses of vaccine.

Experimental animal models of coccidioidomycosis

Experimental models of coccidioidomycosis performed using various laboratory animals have been, and remain, a critical component of elucidation and understanding of the pathogenesis and host resistance to infection with Coccidioides spp., as well as to development of more efficacious antifungal therapies. The general availability of genetically defined strains, immunological reagents, ease of handling, and costs all contribute to the use of mice as the primary laboratory animal species for models of this disease. Five types of murine models are studied and include primary pulmonary disease, intraperitoneal with dissemination, intravenous infection emulating systemic disease, and intracranial or intrathecal infection emulating meningeal disease. Each of these models has been used to examine various aspects of host resistance, pathogenesis, or antifungal therapy. Other rodent species, such as rat, have been used much less frequently. A rabbit model of meningeal disease, established by intracisternal infection, has proven to model human meningitis well. This model is useful in studies of host response, as well as in therapy studies. A variety of other animal species including dogs, primates, and guinea pigs have been used to study host response and vaccine efficacy. However, cost and increased needs of animal care and husbandry are limitations that influence the use of the larger animal species.

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.

Coccidioidomycosis: a review and update

Coccidioidomycosis occurs in arid and semi-arid regions of the New World from the western United States to Argentina. Highly endemic areas are present in the southwest United States. Coccidioides species live in the soil and produce pulmonary infection via airborne arthroconidia. The skin may be involved by dissemination of the infection, or by reactive eruptions, such as a generalized exanthem or erythema nodosum. Interstitial granulomatous dermatitis and Sweet's syndrome have recently been recognized as additional reactive signs of the infection. Coccidioidomycosis is a "great imitator" with protean manifestations. Cutaneous findings may be helpful clues in the diagnosis of this increasingly important disease.

Protein expression profiling of Coccidioides posadasii by two-dimensional differential in-gel electrophoresis and evaluation of a newly recognized peroxisomal matrix protein as a recombinant vaccine candidate

Coccidioides posadasii and Coccidioides immitis are dimorphic, soil-dwelling pathogenic ascomycetes endemic to the southwestern United States. Infection can result from inhalation of a very few arthroconidia, but following natural infection, long-lived immunity is the norm. Previous work in the field has shown that spherule-derived vaccines afford more protection than those from mycelia. We have used two-dimensional differential in-gel electrophoresis coupled with nano-high-performance liquid chromatography-tandem mass spectrometry to directly assess both absolute abundance and differential expression of proteins in the spherule and the mycelial phases of C. posadasii with the intent to identify potential vaccine candidates. Peptides derived from 40 protein spots were analyzed and a probable identity was assigned to each. One spherule-abundant protein, identified as Pmp1, showed homology to allergens from Aspergillus fumigatus and other fungi, all of which exhibit similarity to yeast thiol peroxidases. Recombinant Pmp1 was reactive with serum from individuals with both acute and protracted disease, and evoked protection in two murine models of infection with C. posadasii. These results demonstrate the utility of proteomic analysis as a point of discovery for protective antigens for possible inclusion in a vaccine candidate to prevent coccidioidomycosis.

Spherules derived from Coccidioides posadasii promote human dendritic cell maturation and activation

Previous studies have shown that dendritic cells (DC) pulsed with T27K, an antigenic preparation derived from spherules (of Coccidioides posadasii), activate peripheral blood mononuclear cells (PBMC) from nonimmune subjects as well as from patients with disseminated coccidioidomycosis. In this study, we have assessed the interaction between human DC and C. posadasii spherules in order to better understand the initial response between Coccidioides and the human host. Whole autoclaved spherules induced lymphocyte transformation in PBMC obtained from immune but not from nonimmune donors. Immature DC (iDC) bound fluorescein isothiocyanate-labeled spherules in a time- and temperature-dependent manner. This binding was blocked by the addition of mannan, suggesting mannose receptor involvement in the DC-Coccidioides interaction. Binding was subsequently associated with ingestion and intracellular processing of spherules. Coculturing of spherules with iDC was associated with the development of mature DC that were morphologically, phenotypically, and functionally similar to those induced by tumor necrosis factor alpha and prostaglandin E2. Finally, spherules incubated with iDC induced activation of PBMC from nonimmune donors. These data indicate that human DC are capable of binding, internalizing, and presenting antigens from Coccidioides spherules and suggest that DC may play a critical early role in the formation of a cellular immune response in human coccidioidomycosis.

Improved protection of mice against lethal respiratory infection with Coccidioides posadasii using two recombinant antigens expressed as a single protein

Two recombinant antigens which individually protect mice from lethal intranasal infection were studied in combination, either as a mixture of two separately expressed proteins or as a single chimeric expression product. Mice vaccinated with either combination survived longer than mice given single antigens. Immunized mice also exhibited specific IgG immunoglobulins and yielded splenocytes which produced interferon-gamma in response to either antigen. The chimeric antigen has the practical advantage of offering enhanced protection from multiple components without increasing production costs.

Preliminary evaluation of whole-blood gamma interferon release for clinical assessment of cellular immunity in patients with active coccidioidomycosis

Assessment of the cellular immune response in coccidioidomycosis has epidemiologic and prognostic importance. Measurement of delayed-type hypersensitivity to skin testing has been used in the past to determine cellular immunity in coccidioidomycosis. However, no skin tests are currently available in the United States. Assay of gamma interferon (IFN-gamma) release in whole blood in response to incubation with antigen has been used to assess cellular immunity in tuberculosis. We used a similar assay using the coccidioidal antigen preparation T27K to measure the in vitro cellular immune responses among a cohort of 69 subjects with active coccidioidomycosis. IFN-gamma release was bimodal, with concentrations above and below 5 IU/ml. Using multivariate logistic regression, underlying disease and disseminated or chronic pulmonary coccidioidomycosis was significantly associated with the release of IFN-gamma at a concentration of <5 IU/ml (P = 0.02 or 0.05, respectively). In addition, the release IFN-gamma concentration was <5 IU/ml in all subjects with a clinical severity score of > or =6 (P = 0.02). The release IFN-gamma concentration correlated with expression of CD69 on T lymphocytes in an in vitro assay using T27K as the antigen (Spearman's rho = 0.59; P < 0.01). These results suggest that the IFN-gamma release assay with T27K as the antigen may be a useful clinical test for assessing cellular immunity in patients with active coccidioidomycosis.

An Archived Lot of Coccidioidin Induces Specific Coccidioidal Delayed-type Hypersensitivity and Correlates with in vitro Assays of Coccidioidal Cellular Immune Response

No test for assessing cellular immune response in coccidioidomycosis is currently available in the United States. In the present study, we tested 49 healthy subjects living in the coccidioidal endemic region with a 1:55.8 dilution of a single lot of coccidioidin archived since the 1970s. In this group, 23 evaluable subjects demonstrated >/=5 mm of induration at 24, 48 or 72 h, with a mean+/-SEM maximum induration of 18.4+/-4.0 mm. The induration results among 14 subjects reactive at 24 h were compared to those from 179 individuals in an historical cohort studied in the 1980s using a reference lot of coccidioidin. Results were within 5% and not significantly different (P=0.924). The maximum induration response of all evaluable subjects correlated significantly with the results of in vitro tests of coccidioidal cellular immunity using supernatant interferon-gamma concentration and CD69 expression on T cells (Spearman rank correlation coefficients 0.69 and 0.68, respectively; P<0.01 for both). These data suggest that archived coccidioidin retains its potency and specificity and that in vitro test of coccidioidal immunity may have utility in the measurement of coccidioidal cellular immunity.

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.

Vaccinate against aspergillosis! A call to arms of the immune system

Invasive aspergillosis is a devastating and increasingly common disease, seen almost exclusively in immunosuppressed patients. Immunizing an immunocompromised host would seem to be a formidable task; however, virulence factors and immunogens of the pathogen have now been identified and could be targeted, mapping of the genome sequence of the pathogen will soon be completed, and the protective host immune responses and cytokine networking are better understood. These facts, together with recent advances in vaccine science, make consideration of such an approach now possible. Some populations that are at risk for aspergillosis might be likely candidates for receiving the first vaccinations against aspergillosis, or vaccination of a stem cell donor might be considered in some circumstances. Successful immunizations have been demonstrated in turkeys and mice.

In vitro modulation of cytokine production by lymphocytes in human coccidioidomycosis

The modulation of the cytokine response to coccidioidal antigen by lymphocytes from donors with coccidioidomycosis was examined. In initial experiments, samples from 13 healthy immune donors and seven donors with active coccidioidomycosis anergic to the coccidioidal antigen T27K were assessed for CD3 lymphocyte expression of intracellular IFN-gamma using whole blood analysis. Addition of 10 ng/ml of recombinant IL-12 significantly increased response to T27K among immune and anergic subjects (p<0.05), but the percent of cells expressing IFN-gamma was still significantly greater for immune subjects. Among immune donors, the percentage of CD3 lymphocytes expressing IFN-gamma was significantly reduced with the addition of 10 ng/ml of recombinant IL-4, IL-10, TGF-beta, or their combination (for all, p<0.05). Among anergic donors, addition of 10 ng/ml of anti-IL-10 significantly increased IFN-gamma production (p<0.05), but addition of anti-IL-4 or anti-TGF-beta did not. Among immune donors, the percent of both CD3 lymphocytes and NK cells expressing IFN-gamma after 24h of T27K was increased above control (p<0.05), while the percent of NK cells producing TNF-alpha in response to T27K was not greater than control. Depletion of NK cells from peripheral blood mononuclear cells resulted in significant increases in TNF-alpha and IL-10 (for both, p<0.05) but resulted in no significant decrease in IFN-gamma or IL-2. These data demonstrate a differential response to stimulation with the coccidioidal antigen T27K among donors with coccidioidomycosis that can be manipulated by cell type and cytokine environment.

In vitro whole-blood analysis of cellular immunity in patients with active coccidioidomycosis by using the antigen preparation T27K

Measurement of cellular immunity in human coccidioidomycosis has important diagnostic and prognostic implications. The coccidioidin skin test has been the standard for the measurement of this, but it is not available in the United States. We examined the utility of measuring surface expression of CD69 on T lymphocytes in whole blood incubated with the coccidioidal antigen preparation T27K as an alternative to the skin test. Seventy donors with active coccidioidomycosis were studied. The mean fluorescent intensity (MFI) of CD69 expression on CD3 lymphocytes in response to T27K was 28.61 +/- 1.77, significantly greater than the control response of 11.45 +/- 0.78 (P < 0.001). The MFI CD69 response to T27K above that for the control (MFI CD69 above control) was 6.35 +/- 2.18 for seven subjects with disseminated coccidioidomycosis who were studied within 5 months of diagnosis. This was significantly below the value of 20.17 +/- 3.17 for 18 subjects with pulmonary coccidioidomycosis studied within 5 months of diagnosis and the value of 19.58 +/- 2.91 for 27 subjects with disseminated coccidioidomycosis studied after 5 months of diagnosis (for both, P < 0.05). There was an inverse correlation between coccidioidal clinical score and MFI CD69 above control for all 34 subjects with disseminated coccidioidomycosis (r = 0.362; P = 0.036) but not for the 36 subjects with pulmonary disease (r < 0.001; P = 0.993). Among 30 subjects for whom data were available, there was a highly significant association between the MFI CD69 above control and the supernatant concentrations of gamma interferon, interleukin-2 (IL-2), and tumor necrosis factor alpha (for all, P < 0.001), but not for IL-4, IL-5, or IL-10. These data indicate that in vitro assessment of CD69 expression on T lymphocytes by using T27K may be a useful measure of cellular immune response among subjects with active coccidioidomycosis.

Reversal of coccidioidal anergy in vitro by dendritic cells from patients with disseminated coccidioidomycosis

Coccidioides immitis is a pathogenic, dimorphic fungus found in the southwestern United States and is the causative agent of coccidioidomycosis. Extrathoracic dissemination of coccidioidomycosis is associated with a lack of cellular immunity. Dendritic cells (DCs) have been shown to initiate and modulate cellular immune responses. To determine whether DCs could modulate or initiate the immune response in this disease, monocyte-derived DCs were generated from coccidioidal Ag nonresponsive patients with disseminated coccidioidomycosis and healthy nonimmune individuals. DCs generated from both groups demonstrated phenotypes characteristic of DCs and stimulated strong allogeneic MLR. DCs from patients and healthy nonimmune individuals pulsed with the coccidioidal Ag preparation T27K induced lymphocyte proliferation. Mature DCs were much more efficient than immature DCs in these stimulations. Furthermore, restimulation of T27K-primed PBMC with Ag-pulsed DCs generated a C. immitis-specific cellular immune response in PBMC from patients with disseminated coccidioidomycosis as well as healthy nonimmune individuals. These results show that 1) DCs have the capacity to stimulate specific cellular immune responses from patients with disseminated coccidioidomycosis who are nonresponsive to coccidioidal Ag and healthy nonimmune individuals in vitro; 2) DCs can be used to screen coccidioidal Ags as candidates for human vaccine development; and 3) DC therapy may be useful in the treatment of disseminated coccidioidomycosis.

A parasitic phase-specific adhesin of Coccidioides immitis contributes to the virulence of this respiratory Fungal pathogen

We report the isolation of a Coccidioides immitis gene (SOWgp) which encodes an immunodominant, spherule outer wall glycoprotein that is presented as a component of a parasitic phase-specific, membranous layer at the cell surface. The open reading frame of the gene from C. immitis isolate C735 translates a 422-amino-acid (aa) polypeptide that contains 6 copies of a 41- to 47-residue tandem repeat enriched in proline (20.4 mol%) and aspartate (19.7%). Two additional isolates of C. immitis produce SOWgps of different molecular sizes (328 and 375 aa) and show a corresponding difference in the number of tandem repeats (four and five, respectively). The accurate molecular sizes of these proline-rich antigens, as determined by surface-enhanced laser desorption/ionization mass spectrometry, are comparable to the predicted sizes from the translated protein sequences rather than the estimated sizes based on gel-electrophoretic separation. The results of Northern hybridization confirmed that SOWgp expression is parasitic phase specific, and immunoblot studies showed that elevated levels of production of this antigen occurred during early spherule development. The recombinant polypeptide (rSOWp) was shown to bind to mammalian extracellular matrix (ECM) proteins in an in vitro assay (laminin > fibronectin > collagen type IV), suggesting that the parasitic cell surface antigen may function as an adhesin. Deletion of the SOWgp gene by using a targeted gene replacement strategy resulted in partial loss of the ability of intact spherules to bind to ECM proteins and a significant reduction in virulence of the mutant strain. The wild-type gene was restored in the mutant by homologous recombination, and the revertant strain was shown to be as virulent as the parental isolate in our murine model of coccidioidomycosis. The parasitic cell surface glycoprotein encoded by the SOWgp gene appears to function as an adhesin and contributes to the virulence of C. immitis.

Protection of mice against Coccidioides immitis intranasal infection by vaccination with recombinant antigen 2/PRA

Subcutaneous vaccination with recombinant antigen 2/PRA (rAg2/PRA) protected BALB/c mice against intranasal infection with Coccidioides immitis. Subcutaneously vaccinated C57BL/6 mice and intranasally vaccinated BALB/c mice were protected against larger numbers of infecting spores. Weight loss correlated with lethality, but histologic appearance did not. These studies support rAg2/PRA vaccination to prevent coccidioidomycosis.

Assessment of the human cellular immune response to T27K, a coccidioidal antigen preparation, by flow cytometry of whole blood

Whole blood flow cytometry was performed among donors with various clinical forms of coccidioidomycosis using T27K, a coccidioidal antigen preparation protective in mice but not previously studied in humans. The median percent of CD3+ lymphocytes (CD3+) producing intracellular interferon-gamma (IFN-gamma) among healthy immune donors was 0.43%, significantly above that for non-immune donors (0.01%) and greater than that for subjects with other forms of coccidioidomycosis, including chronic pulmonary (0.11%), disseminated (0.09%) and concomitant human immunodeficiency virus (HIV) infection (0.07%) (P < or =0.002 for all). No increase in intracellular interleukin (IL)-10 production or apoptosis was noted in samples incubated with T27K. Among 14 HIV-infected patients with concomitant coccidioidomycosis, seven of eight patients whose peripheral blood CD4 concentration was > 200 cells microl(-1) had > 0.06% of CD3+ produce intracellular IFN-gamma, compared to none of six whose peripheral blood CD4+ lymphocyte concentration was < or =200 cells microl(-1) (P = 0.005). These data indicate that there is a specific human cellular immune response to T27K as a coccidioidal antigen and that this response can be categorized based on the clinical status of the coccidioidally infected patient.

Coccidioidomycosis Pulmonary Infection

Disease caused by Coccidioides immitis has increased in frequency in recent years because of marked population shifts into highly endemic areas, as well as perturbations of the environment caused by construction, agricultural, and natural activities. Because disease may occur in persons in locations outside of the endemic regions, practitioners must be aware of the possibility of infection in those who may have traveled or resided in areas of risk. Recently, advances in laboratory methods have facilitated diagnosis of coccidioidomycosis, and information about therapies that are easier to administer has become available. Challenges in the management of this infection still remain, particularly with regard to more reliable antifungal drugs and protective vaccines.

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.

Coccidioidomycosis as imported atypical pneumonia in Sweden

Coccidioidomycosis, an endemic fungal infection of the western hemisphere causes serious disease in immunocompromised individuals. In immunocompetent patients, a moderate flu-like disease may develop. We report here an imported Scandinavian case of a culture-proven coccidioidomycosis, initially presenting as an atypical pneumonia. Pleuritic symptoms, positive epidemiology and eosinophilia led to suspicion of the diagnosis, which was further supported by serology.

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.

Identification and cloning of an aspartyl proteinase from Coccidioides immitis

A 45 kDa protein was isolated from a soluble vaccine prepared from formaldehyde-killed spherules of Coccidioides immitis. From the N-terminal amino acid sequence, the protein yielded a 17-amino-acid peptide that was homologous to sequences of other fungal aspartyl proteinases. The coccidioidal cDNA encoding the proteinase was amplified using oligonucleotide primers designed from the 45 kDa N-terminal amino acid sequence and a fungal aspartyl proteinase consensus amino acid sequence. The PCR product was cloned and sequenced, and the remaining 5' upstream and 3' downstream cDNA was amplified, cloned, and sequenced. The cDNA encoding the coccidioidal aspartyl proteinase open reading frame was cloned and the fusion protein containing a C-terminal His-tag expressed in E. coli. The recombinant aspartyl proteinase was purified by immobilized metal affinity chromatography. This recombinant protein will be used for further studies to evaluate its antigenicity, including protective immunogenicity.

Influence of host genetics on the severity of coccidioidomycosis

Coccidioidomycosis, a mild flulike illness in approximately 40% of infected persons, progresses to severe pulmonary or disseminated disease in 1% to 10% of symptomatic cases. We examined host genetic influences on disease severity among class II HLA loci and the ABO blood group. Participants included African-American, Caucasian, and Hispanic persons with mild or severe disseminated coccidioidomycosis from Kern County, California. Among Hispanics, predisposition to symptomatic disease and severe disseminated disease is associated with blood types A and B, respectively. The HLA class II DRB1*1301 allele marks a pre-disposition to severe disseminated disease in each of the three groups. Reduced risk for severe disease is associated with DRB1*0301-DQB1*0201 among Caucasians and Hispanics and with DRB1*1501-DQB1*0602 among African-Americans. These data support the hypothesis that host genes, in particular HLA class II and the ABO blood group, influence susceptibility to severe coccidioidomycosis.

Resistance to Coccidioides immitis in mice after immunization with recombinant protein or a DNA vaccine of a proline-rich antigen

Two inbred strains of mice (BALB/c and C57BL/6) were vaccinated with either recombinant expression protein of a Coccidioides immitis spherule-derived proline-rich antigen (rPRA) in monophosphoryl lipid A-oil emulsion adjuvant or a DNA vaccine based on the same antigen. Four weeks after vaccination, mice were infected intraperitoneally with arthroconidia. By 2 weeks, groups of mice receiving saline or plasmids with no PRA insert exhibited significant weight loss, and quantitative CFUs in the lungs ranged from 5.9 to 6.4 log10. In contrast, groups of mice immunized with either rPRA or DNA vaccine had significantly smaller pulmonary fungal burdens, ranging from 3.0 to 4.5 log10 fewer CFUs. In vitro immunologic markers of lymphocyte proliferation and gamma interferon (IFN-gamma) release after splenocytes were stimulated with rPRA correlated with protection. Also, plasma concentrations of rPRA-specific total immunoglobulin G (IgG), IgG1, and IgG2a showed increases in vaccinated mice. These studies expand earlier work by demonstrating protection in mice which differ in H-2 background, by using an adjuvant that is potentially applicable to human use, and by achieving comparable protections with a DNA-based vaccine. Our in vitro results substantiate a Th1 response as evidenced by IFN-gamma release and increased IgG2a. However, IgG1 was also stimulated, suggesting some Th2 response as well. PRA is a promising vaccine candidate for prevention of coccidioidomycosis and warrants further investigation.

Fungal Infections of the Lung

Fungi, both endemic and opportunistic, continue to be recognized as increasingly frequent pulmonary pathogens. Better appreciation of their epidemiology and clinical course, as well as clarification of the roles of the newer triazoles and lipid formulations of amphotericin B in treatment, have occurred within the past few years. Both endemic and opportunistic fungal pulmonary pathogens are reviewed, with emphasis on recent therapeutic advances.

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.