Use of a recombinant Coccidioides immitis complement fixation antigen-chitinase in conventional serological assays

Clinical Laboratory Utility of a Humanized Antibody in Commercially Available Enzyme Immunoassays for Coccidioidomycosis

Coccidioidomycosis, also called valley fever (VF), is a fungal infection with endemicity in desert regions of the western United States as well as certain arid regions of Central and South America. Laboratory-based diagnosis of VF often relies on the composite results from three serologic-based diagnostics, complement fixation, immunodiffusion, and enzyme immunoassay (EIA). EIA is commonly performed in clinical laboratories because results can be obtained in a few hours. Two commercially available EIAs, IMMY clarus Coccidioides antibody and Meridian Premier Coccidioides, look for the presence of anticoccidioidal IgG and IgM in patient sera that are diluted 1:441. Per regulatory requirements, this dilution step must be verified with a dilution step control despite not being provided as a reagent in either FDA-approved EIA kit. Therefore, clinical laboratories collect and reuse patient sera in subsequent tests that had a positive result in a previous test. This is a nonstandard process, reinforcing the need for a consistent and reliable dilution control. Here, we evaluate the performance of a humanized IgG and IgM antibody as a dilution control in both EIA kits. Both humanized IgG and IgM work well in each EIA and meet the appropriate threshold for positivity. IMPORTANCE In southwestern and western regions of the United States, at least half a million diagnostic tests for coccidioidomycosis (valley fever) are run annually. Enzyme immunoassays (EIAs) are blood tests which require precise dilution of patient serum prior to testing. To ensure patient serum is properly diluted, there is a regulatory requirement to ensure the dilution step is accurate. Two FDA-approved EIAs used to aid in the diagnosis of coccidioidomycosis do not contain controls for this dilution step, leaving clinical laboratories with the only option of using previously positive patient sera, which may not react in a reliable or predictable manner. Here, we evaluate a humanized monoclonal antibody against a coccidioidal antigen and its utility as a dilution control in both available commercial EIAs. The use of a humanized monoclonal antibody provides a standardized and well-characterized dilution control for use in serological assays that aid in diagnosis of coccidioidomycosis.

Development of a Quantitative Antigen Assay to Detect Coccidioidal Chitinase-1 (CTS1) in Human Serum

Background

Coccidioidomycosis is often diagnosed with a collection of tests that rely on the patient's ability to mount an immune response to the fungus (antibody-based diagnostics), making diagnosis of this infection challenging. Here we present an antigen-based assay that detects and quantifies coccidioidal chitinase-1 (CTS1) in human serum.

Methods

An inhibition-based enzyme-linked immunoassay (ELISA) was developed that utilizes a monoclonal antibody specific for coccidioidal CTS1. CTS1 was quantified in commercial antigen preparations using recombinant CTS1 as a standard. Sera from 192 individuals from an endemic area were tested, which included 78 patients (40.6%) with proven or probable coccidioidomycosis.

Results

The quantity of CTS1 in diagnostic commercial antigen preparations from different suppliers varied. CTS1 antigenemia was detected in 87.2% of patients with proven or probable coccidioidomycosis. Specificity was determined to be 96.94% using serum from individuals who reside in the Phoenix, Arizona area who did not have coccidioidomycosis. Levels of CTS1 correlated with low- and high-titer serology from patients with a coccidioidomycosis diagnosis.

Conclusions

Since the CTS1 inhibition ELISA described in this report does not depend on the host immune response, it is a promising diagnostic tool to aid in diagnosis and disease monitoring of coccidioidomycosis.

Comparative Study of Newer and Established Methods of Diagnosing Coccidioidal Meningitis

Meningitis is the most devastating form of coccidioidomycosis. A convenient, rapid diagnostic method could result in early treatment and avoid many meningitis complications. We studied cerebrospinal fluid (CSF) samples in patients with documented coccidioidal meningitis, and controls, with complement fixation (CF), immunodiffusion (ID) (the “classical” assays), lateral flow assays (LFA; one-strip and two-strip), and two enzyme immunoassays (EIA). The two-strip LFA and EIAs not only enabled separate testing for IgG and IgM antibodies separately, but also could aggregate results for each method. CF with ID or the aggregate use of IgG and IgM tests were considered optimal test uses. LFAs and EIAs were evaluated at 1:21 and 1:441 dilutions of specimens. All assays were compared to true patient status. With 49 patient specimens and 40 controls, this is the largest comparative study of CSF coccidioidal diagnostics. Sensitivity of these tests ranged from 71–95% and specificity 90–100%. IgM assays were less sensitive. Assays at 1:441 were similarly specific but less sensitive, suggesting that serial dilutions of samples could result in assays yielding titers. Agreement of positive results on cases was 87–100%. When kits are available, hospital laboratories in endemic areas can perform testing. LFA assays do not require a laboratory, are simple to use, and give rapid results, potentially even at the bedside.

Selection of Specific Peptides for Coccidioides spp. Obtained from Antigenic Fractions through SDS-PAGE and Western Blot Methods by the Recognition of Sera from Patients with Coccidioidomycosis

Antigenic fractions of 100, 50, 37, and 28 kDa obtained through the SDS-PAGE method that were more frequently recognized by anti-Coccidioides antibodies in the sera of coccidioidomycosis patients were selected using western blotting. Subsequently, these bands were sequenced, and the obtained proteins were analysed by BLAST to choose peptides specific for Coccidioides spp. from among the shared aligned sequences of related fungi. A peptide specific for C. immitis was selected from the "GPI anchored serine-threonine rich protein OS C. immitis", while from the "uncharacterized protein of C. immitis", we selected a peptide for C. immitis and C. posadasii. These proteins arose from the 100 kDa antigenic fraction. From the protein "fatty acid amide hydrolase 1 of C. posadasii" that was identified from the 50 kDa antigenic fraction, a peptide was selected that recognized C. immitis and C. posadasii. In addition, the analysis of all the peptides (353) of each of the assembled proteins showed that only 35 had 100% identity with proteins of C. immitis and C. posadasii, one had 100% identity with only C. immitis, and one had 100% identity with only C. posadasii. These peptides can be used as diagnostic reagents, vaccines, and antifungals.

Serodiagnosis of mycoses using recombinant antigens

The early diagnosis of mycoses is important for the institution of an effective antifungal therapy. Detection of antibodies against crude antigenic extracts is one of the standard techniques for the diagnosis of most mycoses. However, while these crude antigenic extracts are relatively easy to obtain, they usually show low reproducibility and are not very specific, since antibodies from patients with different mycoses may show cross-reactivity. The application of molecular biology techniques to the study of fungal antigens has allowed the production of recombinant antigens that may help to solve these problems. The purpose of this review is to discuss the use of recombinant fungal antigens in the diagnosis of mycoses.

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

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

Molecular characterization of a cell wall-associated beta(1-3)endoglucanase of Aspergillus fumigatus

A 74 kDa beta(1-3)endoglucanase of Aspergillus fumigatus was recently isolated from a cell wall autolysate and biochemically characterized. In this study, we report the cloning and the disruption of the ENGL1 gene encoding this beta(1-3)endoglucanase. ENGL1 contains an open reading frame of 2181 bp encoding a polypeptide of 727 amino acids. Sequence analysis showed that ENGL1 is the first characterized member of a new family of beta(1-3)glucanases. Disruption of ENGL1, however, did not lead to a phenotype distinct from the parental strain, indicating that this cell wall-associated beta(1-3)endoglucanase does not play an essential role in constitutive cell growth.

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.

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.

Mapping of a Coccidioides immitis-specific epitope that reacts with complement-fixing antibody

We have previously cloned the cDNA fragment that encodes the complement fixation antigen of Coccidioides immitis. The recombinant protein was highly sensitive in detecting CF antibody in sera from patients with coccidioidomycosis but was not specific to C. immitis, as evidenced by its reactivity with sera from patients with histoplasmosis and, to lesser extent, blastomycosis. We undertook this study to determine if the epitope(s) that reacts with CF antibody is the same or differs from the epitopes that are shared with Histoplasma capsulatum and Blastomyces dermatitidis. PCR-generated CF/chitinase cDNA fragments were cloned and examined for their reactivity in enzyme-linked immunosorbent assays using sera from patients with coccidioidomycosis, histoplasmosis, or blastomycosis. A peptide domain comprised of amino acid residues 20 through 310 was shown to express an epitope(s) that is specific to anti-Coccidioides CF antibody. The peptide detected serum antibody in 21 (95%) of 22 patients with active coccidioidomycosis and was without reactivity with sera from 20 patients with histoplasmosis, 15 patients with blastomycosis, and 14 healthy subjects. Antibody titers to the recombinant peptide directly correlated with CF antibody titers (P < 0.01), and preadsorption of reference CF antiserum with the peptide ablated the reactivity of the antiserum in the immunodiffusion assay for CF antibody. The delineation of a recombinant peptide that has both sensitivity and specificity will provide a valuable tool for detecting CF antibody and for evaluating the role of CF antibody in the host response to C. immitis.