Protective immunity in coccidioidomycosis

Genetic Vaccination as a Flexible Tool to Overcome the Immunological Complexity of Invasive Fungal Infections

The COVID-19 pandemic has highlighted genetic vaccination as a powerful and cost-effective tool to counteract infectious diseases. Invasive fungal infections (IFI) remain a major challenge among immune compromised patients, particularly those undergoing allogeneic hematopoietic bone marrow transplantation (HSCT) or solid organ transplant (SOT) both presenting high morbidity and mortality rates. Candidiasis and Aspergillosis are the major fungal infections among these patients and the failure of current antifungal therapies call for new therapeutic aids. Vaccination represents a valid alternative, and proof of concept of the efficacy of this approach has been provided at clinical level. This review will analyze current understanding of antifungal immunology, with a particular focus on genetic vaccination as a suitable strategy to counteract these diseases.

Interleukin-8 Receptor 2 (IL-8R2)-Deficient Mice Are More Resistant to Pulmonary Coccidioidomycosis than Control Mice

The pathology of human coccidioidomycosis is granulomatous inflammation with many neutrophils surrounding ruptured spherules, but the chemotactic pathways that draw neutrophils into the infected tissues are not known. We previously showed that formalin-killed spherules (FKS) stimulate mouse macrophages to secret macrophage inflammatory protein 2 (MIP-2), which suggested that CXC ELR+ chemokines might be involved in neutrophil recruitment in vivo To test that hypothesis, we intranasally infected interleukin-8R2 (IL-8R2) (Cxcr2)-deficient mice on a BALB/c background with Coccidioides immitis RS. IL-8R2-deficient mice had fewer neutrophils in infected lungs than controls, but unexpectedly the IL-8R2-deficient mice had fewer organisms in their lungs than the control mice. Infected IL-8R2-deficient mouse lungs had higher expression of genes associated with lymphocyte activation, including the Th1 and Th17-related cytokines Ifnγ and Il17a and the transcription factors Stat1 and Rorc Additionally, bronchial alveolar lavage fluid from infected IL-8R2-deficient mice contained more IL-17A and interferon-γ (IFN-γ). We postulate that neutrophils in the lung directly or indirectly interfere with the development of a protective Th1/Th17 immune response to C. immitis at the site of infection.

Novel canine anti-Coccidioides immunoglobulin G enzyme immunoassay aids in diagnosis of coccidioidomycosis in dogs

The diagnosis of coccidioidomycosis (CM) in dogs is typically based on clinical presentation, serology, and (less frequently) spherule identification. Agar gel immunodiffusion (AGID) is the most commonly employed serological method, but AGID is slow (requiring up to a week for titer). A Coccidioides antigen enzyme immunoassay (EIA) is also available; however, sensitivity is low in CM dogs. An antibody EIA was developed to detect canine immunoglobulin G (IgG) reacting to Coccidioides antigens. Serum was evaluated from dogs with pathology proven CM and/or AGID positive CM, as well as dogs with histoplasmosis, blastomycosis, non-fungal infections, or healthy dogs. A standard curve was used to convert optical density (OD) values into EIA units (EU). Serum and urine samples from CM dogs were also tested in the antigen EIA. Sensitivity and specificity for IgG were 89.2% and 97.2%, respectively, upon evaluation of dogs with proven or probable CM and control dogs. Cross-reactivity was observed in 7.7% and in 6.4% of dogs with histoplasmosis or blastomycosis, respectively. The antigen EIA alone was insensitive (33.8%). Combined IgG and antigen testing increased sensitivity to 93.2%, as three dogs were IgG-negative but had detectable serum or urine antigen. In 22 dogs with proven CM, sensitivity was statistically similar for antibody EIA and AGID (86% and 73%; P = .487). The MiraVista® canine Coccidioides antibody IgG EIA may aid in the diagnosis of CM by improving turnaround time with comparable sensitivity to AGID. Serial or concurrent testing by antibody and antigen EIAs may be beneficial when screening dogs for CM.

Erythema Sweetobullosum: A Reactive Cutaneous Manifestation of Coccidioidomycosis

Reactive cutaneous coccidioidal skin manifestations are commonly noticed during the early stage of coccidioidomycosis. These skin lesions are devoid of any active coccidioidal organism, and the immune trigger mechanisms are not elucidated. We describe 6 cases of unusual reactive cutaneous coccidioidal manifestation, characterized by painful vesiculobullous lesions known as erythema sweetobullosum. The biopsy of the lesions revealed neutrophilic dermatosis with inflammatory cells resulting in a cleft and elevation of the most superficial layer of the skin forming a bulla. The reactive cutaneous lesion is self-limited and requires no specific therapy.

Immune Response to Coccidioidomycosis and the Development of a Vaccine

Coccidioidomycosis is a fungal infection caused by Coccidioides posadasii and Coccidioides immitis. It is estimated that 150,000 new infections occur in the United States each year. The incidence of this infection continues to rise in endemic regions. There is an urgent need for the development of better therapeutic drugs and a vaccine against coccidioidomycosis. This review discusses the features of host innate and adaptive immune responses to Coccidioides infection. The focus is on the recent advances in the immune response and host-pathogen interactions, including the recognition of spherules by the host and defining the signal pathways that guide the development of the adaptive T-cell response to Coccidioides infection. Also discussed is an update on progress in developing a vaccine against these fungal pathogens.

Rational Design of T Lymphocyte Epitope-Based Vaccines Against Coccidioides Infection

Coccidioidomycosis is a potentially life-threatening mycosis endemic to the Southwestern USA and some arid regions of Central and South America. A vaccine against Coccidioides infection would benefit over 30-million people who reside in or visit the endemic regions. Vaccine candidates against systemic fungal infections come in many forms. Live attenuated vaccines are derived from disease-causing pathogens and generally stimulate excellent protective immunity. Since attenuated vaccines contain living microbes, there is a degree of unpredictability raising concerns regarding safety and stability. Generation of a subunit vaccine has initiated efforts to design a safe reagent suitable for administration to humans at risk of coccidioidomycosis. Epitope-based vaccines allow for eliciting specific protective immune responses and removal of potentially detrimental sequences to improve safety. This chapter describes methods for the identification of T cell epitopes derived from Coccidioides antigens, design, and production of a recombinant vaccine containing multiple T cell epitopes, and evaluation of its protective efficacy and vaccine immunity against pulmonary Coccidioides infection using a strain of transgenic mice that express a human MHC II molecule.

Flow Cytometric Analysis of Protective T-Cell Response Against Pulmonary Coccidioides Infection

The incidence of systemic fungal infections has increased throughout the world, spurring much interest in developing effective vaccines. Coccidioidomycosis, also known as San Joaquin Valley fever, is a potentially life-threatening respiratory mycosis. A vaccine against Coccidioides infection would contribute significantly to the well-being of the approx. 30 million residents in the Southwestern USA as well as the multitude of travelers who annually visit the endemic regions. We have applied a live, attenuated vaccine (∆T) to explore the nature of vaccine immunity in mice after intranasal challenge with a potentially lethal dose of Coccidioides spores. Coccidioides spores are airborne and highly infectious for mammalian hosts and classified as a biosafety level 3 agent. T cells are critical in the development of protective immunity against a variety of microorganisms as well as the development of autoimmune disease and allergic responses. Profiles of cytokines detected in lung homogenates of ∆T-vaccinated mice were indicative of a mixed Th1, Th2, and Th17 immune response. We have developed an intracellular cytokine staining and flow cytometric (ICS) technique to measure activated CD4(+) and CD8(+) T cells and IFN-γ-, IL-4-, IL-5-, and IL-17A-producing T cells in the lungs of mice that are challenged with a potentially lethal dose of Coccidioides spores. The numbers of pulmonary Th1 and Th17 cells during the first 2 weeks post-challenge showed a progressive increase in vaccinated mice and corresponded with reduction of fungal burden. In this protocol, we describe the methodology for culture and isolation of the live, attenuated ΔT spores of Coccidioides used to vaccinate mice, preparation of pulmonary cells, and staining protocol for cell surface markers and intracellular cytokines. This is the most reliable and robust procedure to measure frequencies and numbers of each selected T-cell subsets in lungs of vaccinated versus control mice and can be readily applied to evaluate T-cell response against other microbial infections.

Dectin-1 is required for resistance to coccidioidomycosis in mice

We assessed the role of Dectin-1 in the immune response to the pathogenic fungus Coccidioides, both in vitro and in vivo, using mice with a targeted mutation in Clec7a. Elicited peritoneal macrophages responded to formalin-killed spherules (FKS) and alkali-treated FKS by secreting proinflammatory cytokines in a Dectin-1- and β-glucan-dependent manner. The responses of bone marrow-derived dendritic cells (BMDC) to the same stimulants were more complex; interleukin 1β (IL-1β) and tumor necrosis factor alpha (TNF-α) secretion was independent of Dectin-1, while IL-6, IL-10, and granulocyte-macrophage colony-stimulating factor (GM-CSF) were largely but not entirely dependent on Dectin-1. After intranasal infection, Dectin-1(-/-) mice had lower concentrations of IL-12p70, gamma interferon (IFN-γ), IL-1β, and the Th17 cytokines IL-22, IL-23, and 17A in the lung lavage fluid, which may explain why they were significantly more susceptible to pulmonary coccidioidomycosis two weeks after infection. The Dectin-1 mutation was even more deleterious in (B6 × DBA/2)F(2) mice, which are more resistant to coccidioidomycosis than B6 mice by virtue of protective genes from DBA/2, a genetically resistant strain. We also found that two susceptible strains of mice (B6 and BALB/c) expressed much less Dectin-1 in their lungs than did resistant DBA/2 mice. We conclude that Dectin-1 is necessary for resistance to Coccidioides immitis, that Dectin-1 promotes both Th1 and Th17 protective immune responses to this infection, and that there is a correlation between expression of Dectin-1 by the inflammatory infiltrate and resistance to coccidioidomycosis. IMPORTANCE Coccidioidomycosis is a fungal infection endemic in the southwestern United States and neighboring Mexico, causing ~150,000 lung infections in people and resulting in ~17,000 hospitalizations annually in California alone. Very little is known about innate immunity to this fungus. This paper shows that Dectin-1, the primary β-glucan receptor on myeloid cells, is required for resistance to this pathogen. Dectin-1 is part of the innate immune system, and it is needed to direct the acquired immune response toward into a pathway that will lead to macrophage activation. Lungs from infected mice lacking Dectin-1 had lower concentrations of Th1 and Th17 cytokines, two cytokine pathways that are very important for acquired T cell immunity to Coccidioides spp. This is the first demonstration that Dectin-1 is required for host resistance to a dimorphic, primary pathogenic fungus.

A genetically engineered live attenuated vaccine of Coccidioides posadasii protects BALB/c mice against coccidioidomycosis

Coccidioidomycosis (also known as San Joaquin Valley fever) is an occupational disease. Workers exposed to outdoor dust which contains spores of the soil-inhabiting fungus have a significantly increased risk of respiratory infection. In addition, people with compromised T-cell immunity, the elderly, and certain racial groups, particularly African-Americans and Filipinos, who live in regions of endemicity in the southwestern United States have an elevated incidence of symptomatic infection caused by inhalation of spores of Coccidioides posadasii or Coccidioides immitis. Recurring epidemics and escalation of medical costs have helped to motivate production of a vaccine against valley fever. The major focus has been the development of a defined, T-cell-reactive, recombinant protein vaccine. However, none of the products described to date have provided full protection to coccidioidal disease-susceptible BALB/c mice. Here we describe the first genetically engineered, live, attenuated vaccine that protects both BALB/c and C57BL/6 mice against coccidioidomycosis. Two chitinase genes (CTS2 and CTS3) were disrupted to yield the attenuated strain, which was unable to endosporulate and was no longer infectious. Vaccinated survivors mounted an immune response characterized by production of both T-helper-1- and T-helper-2-type cytokines. Histology revealed well-formed granulomas and markedly diminished inflammation. Significantly fewer organisms were observed in the lungs of survivors than in those of nonvaccinated mice. Additional investigations are required to further define the nature of the live, attenuated vaccine-induced immunity against Coccidioides infection.

Coccidioidomycosis in dogs and cats: a review

The dimorphic fungi Coccidioides immitis and Coccidioides posadasii are the causative agents of coccidioidomycosis. Dogs and cats residing in and visiting endemic areas are at risk of exposure to infectious arthrospores. The primary infection is pulmonary and frequently results in chronic cough. Disseminated disease is common and causes cutaneous, osseous, cardiac, ocular, nervous system, or other organ disease. Radiographic changes include a variable degree of interstitial pulmonary infiltration, hilar lymphadenopathy, and osseous lesions. Serological titers support the diagnosis, but definitive diagnosis relies on identification of Coccidioides in cytological or tissue samples. Coccidioidomycosis should be considered in any dog or cat that has been potentially exposed during the previous 3 years and is presented with chronic illness, respiratory signs, lameness, lymphadenopathy, nonhealing cutaneous lesions, or neurological, ocular, or cardiac abnormalities.

Virulence mechanisms of coccidioides

Coccidioides is a fungal respiratory pathogen of humans that can cause disease in both immunosuppressed and immunocompetent individuals. We describe here three mechanisms by which the pathogen survives in the hostile host environment: production of a dominant spherule outer wall glycoprotein (SOWgp) that modulates host immune response and results in compromised cell-mediated immunity to coccidioidal infection, depletion of SOWgp presentation on the surface of endospores, which prevents host recognition of the pathogen when the fungal cells are most vulnerable to phagocytic defenses, and induction of elevated production of host arginase I and coccidioidal urease, which contribute to tissue damage at sites of infection. Arginase I competes with inducible nitric oxide synthase (iNOS) in macrophages for the common substrate, L-arginine, and thereby reduces nitric oxide (NO) production and increases the synthesis of host orinithine and urea. Host-derived L-ornithine may promote pathogen growth and proliferation by providing a pool of the monoamine, which could be taken up and used for synthesis of polyamines via metabolic pathways of the parasitic cells. We have shown that high concentrations of Coccidioides- and host-derived urea at infection sites in the presence of urease produced and released by the pathogen, results in secretion of ammonia and contributes to alkalinization of the microenvironment. We propose that ammonia and enzymatically active urease released from spherules during the parasitic cycle of Coccidioides exacerbate the severity of coccidioidal infection by contributing to a compromised immune response to infection and damage of host tissue at foci of infection.

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.

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.

High levels of interleukin-10 impair resistance to pulmonary coccidioidomycosis in mice in part through control of nitric oxide synthase 2 expression

We have shown previously that there is a direct correlation between IL-10 levels and susceptibility to Coccidioides immitis peritonitis in C57BL/6 (B6), DBA/2, and BXD recombinant inbred mice. We now show that B6 mice are also more susceptible to C. immitis pneumonia and that interleukin-10 (IL-10)-deficient (IL-10-/-) B6 mice are more resistant to C. immitis pneumonia. In addition, we established that high levels of IL-10 are sufficient to make genetically resistant mice susceptible to both C. immitis peritonitis and pneumonia by infecting h.IL-10 transgenic mice. Infected h.IL-10 transgenic mice express lower levels of gamma interferon, IL-12 p40, and inducible nitric oxide synthetase 2 (NOS2) mRNA in their lungs, implicating inducible NOS as a defense mechanism in this disease. We treated DBA/2 mice with aminoguanidine, and they became more susceptible to C. immitis peritonitis and pneumonia. We conclude that high levels of IL-10 are both necessary and sufficient to make mice susceptible to C. immitis, regardless of the genetic background of the mice, and that IL-10 impairs resistance to C. immitis in part by suppressing NO synthesis.

A recombinant aspartyl protease of Coccidioides posadasii induces protection against pulmonary coccidioidomycosis in mice

Coccidioidomycosis is a respiratory disease of humans caused by the desert soil-borne fungal pathogens Coccidioides spp. Recurrent epidemics of this mycosis in the southwestern United States have contributed significantly to escalated health care costs. Clinical and experimental studies indicate that prior symptomatic coccidioidomycosis induces immunity against subsequent infection, and activation of T cells is essential for containment of the pathogen and its clearance from host tissue. Development of a human vaccine against coccidioidomycosis has focused on recombinant T-cell-reactive antigens which elicit a durable protective immune response against pulmonary infection in mice. In this study we fractionated a protective multicomponent parasitic cell wall extract in an attempt to identify T-cell antigens. Immunoblots of electrophoretic separations of this extract identified patient seroreactive proteins which were subsequently excised from two-dimensional polyacrylamide gel electrophoresis gels, trypsin digested, and sequenced by tandem mass spectrometry. The full-length gene which encodes a dominant protein in the immunoblot was identified using established methods of bioinformatics. The gene was cloned and expressed, and the recombinant protein was shown to stimulate immune T cells in vitro. The deduced protein was predicted to contain epitopes that bind to human major histocompatibility complex class II molecules using a TEPITOPE-based algorithm. Synthetic peptides corresponding to the predicted T-cell epitopes induced gamma interferon production by immune T lymphocytes. The T-cell-reactive antigen, which is homologous to secreted fungal aspartyl proteases, protected mice against pulmonary infection with Coccidioides posadasii. We argue that this immunoproteomic/bioinformatic approach to the identification of candidate vaccines against coccidioidomycosis is both efficient and productive.

Immune response of vaccinated and non-vaccinated mice to Coccidioides posadasii infection

An immunogenic, recombinant protein of the fungal respiratory pathogen, Coccidioides posadasii, was previously identified as a beta-1,3-glucanosyltransferase homolog (Gel1) and shown to confer protection to C57BL/6 mice against coccidioidomycosis. However, little is known about the nature of the humoral and cellular immune responses of these vaccinated mice to intranasal infection with a lethal inoculum of C. posadasii spores compared to non-immune control animals. Our studies showed that protective immunity in mice vaccinated with two 1 microg doses of the recombinant Gel1 (rGel1) plus adjuvant was characterized by high titers of antigen-specific IgG2c and elevated levels of interleukin (IL)-12 and interferon-gamma (IFN-gamma) production at 7-14 days post-challenge compared to significantly lower levels of the respective antibody and cytokines in non-immune, infected mice. Mice immunized with either 0.2 or 5 microg doses of rGel1 plus adjuvant were less well protected and showed evidence of a marked decrease in the level of T helper-type 1 (T(H)1) immune response. Early T(H)1 immune regulation is essential for protection against pulmonary infection with Coccidioides, and the dose of the rGel1 vaccine narrowly defines the nature of immune response in the lungs of infected mice.

Innate immunity to the pathogenic fungus Coccidioides posadasii is dependent on Toll-like receptor 2 and Dectin-1

Coccidioides posadasii is a pathogenic fungus that causes endemic and epidemic coccidioidomycosis in the deserts of North, Central, and South America. How the innate immune system responds to the organism is not well understood. Here we show that elicited mouse peritoneal macrophages respond to spherules (the tissue form of the fungus) by producing proinflammatory cytokines as measured by quantitative PCR of cellular transcripts and by enzyme-linked immunosorbent assay (ELISA) assays for secreted protein. We examined the contribution of Toll-like receptors (TLR) and MyD88 in macrophage responses to formalin-killed spherules (FKS) by comparing cytokine responses of elicited macrophages from different knockout mice. FKS were added to elicited mouse peritoneal macrophages from wild-type, TLR2-/-, and MyD88-/- cells, and wild-type cells made more tumor necrosis factor alpha, MIP-2, and interleukin 6 than did the mutant macrophages. In contrast, the C3H/HeJ mice, which have a point mutation in TLR4, and TLR4-/- B6 mice exhibited no defect in cytokine production compared to the control mice. We also investigated the role of the macrophage beta-glucan receptor, Dectin-1. RAW 264.7 macrophages overexpressing Dectin-1 produced more cytokines in respond to FKS, live spherules, and purified beta-glucan than did control RAW cells. Blockage of Dectin-1 with antibodies inhibited cytokine production in elicited mouse peritoneal macrophages. Taken together, these results show that cytokine responses in mouse peritoneal macrophages to C. posadasii spherules are dependent on TLR2, MyD88, and Dectin-1.

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.

Innate and adaptive determinants of host susceptibility to medically important fungi

The host response is the outcome of an interplay between innate immunity, adaptive immunity (Th1, Th2, T regulatory cells, B cells and antibodies) and fungal virulence factors. Dendritic cells are the gatekeepers between innate and adaptive immunity and have been the intense focus of recent studies on innate immunity to fungi because of their ability to distinguish between different forms of a fungal species, to drive Th1 versus Th2 versus T regulatory responses, and potentially be modulated by fungal products. New mechanisms have been described by which anti-fungal antibodies can modulate infection and augment T cell immunity. Th1 responses are central to limiting infection with many fungi; thus, a great deal of attention has been focused recently on the antigen(s) that trigger such a response.

Role of signal sequence in vaccine-induced protection against experimental coccidioidomycosis

The vaccine efficacy of the gene sequence encoding the signal peptide of the antigen known as antigen 2 or proline-rich antigen (Ag2/PRA), an immunodominant antigen present in the cell wall of the fungal pathogen Coccidioides immitis, was investigated in a murine model of coccidioidomycosis. Expression plasmids for Ag2/PRA(1-18) DNA (signal sequence), Ag2/PRA(19-194) DNA (lacking the signal sequence), and Ag2/PRA(1-194) DNA (full length) were inserted in the pVR1012 vector, and the constructs were used to vaccinate the highly susceptible BALB/c mouse strain. Immunization with the signal gene sequence significantly reduced the fungal burden in the lungs and spleens of mice 12 days after intraperitoneal challenge with a lethal dose of 2,500 C. immitis arthroconidia, to a level comparable to the protection induced in mice immunized with the full-length Ag2/PRA(1-194) DNA. The Ag2/PRA(19-194) gene protected mice but to a significantly lower level than the signal sequence or the full-length Ag2 gene. The immunizing capacity of Ag2/PRA(1-18) was not attributable to a nonspecific immunostimulatory effect of DNA, as evidenced by the fact that mice immunized with a frameshift mutation of Ag2/PRA(1-18) were not protected against challenge. Furthermore, a synthetic peptide corresponding to the translated sequence of Ag2/PRA(1-18) DNA protected mice, albeit at a lower level than the Ag2/PRA(1-18) DNA vaccine. The protection induced with the signal gene vaccine correlated with the production of gamma interferon when splenocytes from Ag2/PRA(1-18)-immunized mice were stimulated with recombinant full-length Ag2 and was not associated with the production of anti-Coccidioides immunoglobulin G antibody. This is the first study to establish that a signal peptide sequence alone, administered as a gene vaccine or synthetic peptide, can induce protective immunity against a microbial pathogen.

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.

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.

Coadministration of interleukin 12 expression vector with antigen 2 cDNA enhances induction of protective immunity against Coccidioides immitis

Interleukin 12 (IL-12) plays an important role in the induction of protective immunity against cancer and infectious diseases. In this study we asked whether IL-12 cDNA could increase the protective capacity of the antigen 2 (Ag2) gene vaccine in experimental coccidioidomycosis. Coimmunization of BALB/c mice with a single-chain IL-12 cDNA (p40-L-p35) and Ag2 cDNA, both subcloned into the pVR1012 plasmid, significantly enhanced protection against systemic challenge with 2,500 arthroconidia, as evidenced by a greater-than-1.3-log-unit reduction in the fungal load in the lungs and spleens compared to mice receiving the pVR1012 vector alone, Ag2 cDNA alone, or IL-12 cDNA alone. The enhanced protection was associated with increased gamma interferon secretion; production of immunoglobulin G2a (IgG2a), IgG2b, and IgG3 antibodies to Coccidioides immitis antigen; and the influx of CD4(+) and CD8(+) T cells in lungs and spleens. When challenged by the pulmonary route, mice covaccinated with Ag2 cDNA and IL-12 cDNA were not protected at the lung level but did show a significant reduction in the fungal load in their livers and spleens compared to mice vaccinated with Ag2 cDNA or IL-12 cDNA alone. These results suggest that IL-12 acts as a therapeutic adjuvant to enhance Ag2 cDNA-induced protective immunity against experimental coccidioidomycosis through the induction of Th1-associated immune responses.

Construction of a single-chain interleukin-12-expressing retroviral vector and its application in cytokine gene therapy against experimental coccidioidomycosis

T-cell-mediated immunity is an important determinant in protection against primary infection with Coccidioides immitis, a dimorphic fungal pathogen that causes the disease coccidioidomycosis. To determine if interleukin-12 (IL-12) gene therapy could potentiate host response against C. immitis, we constructed a single-chain cDNA encoding the p40 and p35 subunits linked by a polylinker and, using a retroviral vector, transfected J774 macrophages with the construct. The transduced J774 cells expressed IL-12 in vitro, with a mean concentration of 28,440 pg from 10(6) cells in 48 h as measured by an IL-12 (p75)-specific enzyme-linked immunosorbent assay. The secreted IL-12 was biologically active, as judged by its ability to induce the production of gamma interferon (IFN-gamma) by spleen cells from BALB/c mice. Treatment of the highly susceptible BALB/c mouse strain with the IL-12-transduced J774 cells inhibited C. immitis growth in tissues from mice challenged by a pulmonary route, as evidenced by 1.37-, 2.59-, and 1.22-log reductions in the number of CFU in the lungs, spleens, and livers, respectively, compared to the fungal load in mice given vector-transduced J774 cells. The protective effect of IL-12 gene therapy was accompanied by increased levels of IFN-gamma in the lungs and sera of mice treated with IL-12-transduced J774 cells and the constitutive production of IFN-gamma by their spleen cells cultured in vitro. These results suggest that IL-12 gene therapy could be used as adjunct therapy for 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.