Coccidioidomycosis: Host Response and Vaccine Development

Abstract

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 β-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

Coccidioidomycosis in African Americans

“…The spherules increase in size and produce hundreds of endospores, which are subsequently released into the tissue and form new spherules, thereby continuing the life cycle ( Figure 2). 3 When the host inhales Coccidioides arthroconidia, the normal immune response occurs in 2 phases. 3 The early immune response consists of an influx of neutrophils, eosinophils, monocytes, and natural killer cells.…”

“…3 When the host inhales Coccidioides arthroconidia, the normal immune response occurs in 2 phases. 3 The early immune response consists of an influx of neutrophils, eosinophils, monocytes, and natural killer cells. After the arthroconidia become spherules, the early innate response is ineffective, and the cell-mediated immune response begins.…”

“…3,6 Two recognized species cause indistinguishable infection in humans: Coccidioides immitis and Coccidioides posadasii. 1,3 Coccidioides species grow as branching septate hyphae when the soil is moist.…”

“…3,6 Two recognized species cause indistinguishable infection in humans: Coccidioides immitis and Coccidioides posadasii. 1,3 Coccidioides species grow as branching septate hyphae when the soil is moist. As the soil dries, thick-walled arthroconidia form 6 ; when the soil and hyphae are disrupted, the disarticulated arthroconidia (spores) may become aerosolized.…”

“…3 Risk factors for severe or disseminated infection include racial heritage (such as African or Filipino) and medical conditions such as pregnancy or cell-mediated immunodeficiency (eg, use of immunosuppressant medications, infection with human immunodeficiency virus [HIV], or receipt of a transplanted organ). 1 Because of increasing populations and thriving tourism in the endemic area, coccidioidomycosis is a disease of interest not only to physicians in the Southwest but also to practitioners anywhere whose patients have resided in or traveled to areas where the disease is endemic.…”