Isolation of Coccidioides immitis F antigen by immunoaffinity chromatography with monospecific antiserum

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.

Recombinant Coccidioides immitis complement-fixing antigen: detection of an epitope shared by C. immitis, Histoplasma capsulatum, and Blastomyces dermatitidis

We undertook an investigation to assess the utility of a recombinant Coccidioides immitis complement-fixing (CF) antigen for detecting CF antibody in sera from patients with coccidioidomycosis. Enzyme-linked immunosorbent assays established that recombinant CF antigen and, for comparison, a commercially available coccidioidin were reactive with 19 of 19 sera from patients with active coccidioidomycosis. The recombinant antigen was significantly more sensitive than coccidioidin. The median titer obtained when patients' sera were assayed against recombinant CF antigen was 1:51,200 compared to 1:25,600 with coccidioidin (P < 0.027). The recombinant antigen was also more effective in distinguishing the antibody levels in sera from patients with pulmonary coccidioidomycosis than in sera from those with disseminated disease. Whereas patients with pulmonary disease showed a median antibody titer of 1:25,600, those with multifocal disease showed a median titer of 1:102,400 (P < 0.028). The recombinant CF antigen was found, however, to express an epitope(s) that reacted with sera from 6 of 12 patients with histoplasmosis and 2 of 12 patients with blastomycosis.

Molecular cloning and characterization of the Coccidioides immitis complement fixation/chitinase antigen

Detection of anti-Coccidioides complement-fixing (CF) antibody is a valuable diagnostic and prognostic aid in coccidioidomycosis. The CF antibody response is directed against a heat-labile antigen that has chitinase activity, hereafter referred to as the CF/chitinase protein. To identify and clone this immunoreactive enzyme, we constructed a Coccidioides immitis cDNA lambda ZAP expression library from spherule RNA and detected fusion peptides expressing CF epitopes by immunoscreening. A cDNA clone consisting of 1,623 bp was identified, sequenced, and found to contain a single open reading frame that encodes a protein of 47 kDa with 427 amino acids. Deduced amino acid sequence analyses showed that the cloned CF/chitinase cDNA contains a 35-amino-acid region, beginning at Ser-18 and ending at and ending at Arg-52 which has 92% homology with the reported N-terminal amino acid sequence of authentic CF/chitinase protein. The first 17 amino acids in the deduced sequence of the cloned cDNA are not present on the mature CF/chitinase protein, suggesting that it may be a signal peptide. Expression of the CF/chitinase cDNA insert by using the pGEX-4T-3 vector yields a fusion peptide that bears CF-specific epitopes and shows chitinase activity. The CF/chitinase clone will enable large-scale production of the recombinant CF antigen for use in immunoassays and facilitate studies on the role of chitinase in the morphogenesis of C. immitis.

An arthroconidial-spherule antigen of Coccidioides immitis: differential expression during in vitro fungal development and evidence for humoral response in humans after infection or vaccination

A 33-kDa protein antigen purified from spherules of Coccidioides immitis was analyzed for ultrastructural localization and for binding to serum antibodies from infected or immunized humans. By using colloidal gold detection of affinity-purified anti-33-kDa protein antibodies, electron photomicrographs showed binding to the inner cell wall of arthroconidia and spherules and to the septa and glycocalyx surrounding endospores. Enzyme immunoassay measurements also demonstrated that the antigen was most abundant in mature spherules. Of 37 patients with coccidioidomycosis but without concurrent human immunodeficiency virus infections, all but 2 demonstrated immunoglobulin M (IgM) (usually with early infection) or IgG antibodies for the 33-kDa antigen. In contrast, only one of four HIV-infected patients with active coccidioidal infections demonstrated antibody. On the other hand, 107 of 108 patients without evident coccidioidomycosis and 15 of 16 patients with histoplasmosis did not have similar antibodies, indicating a high degree of specificity. Immunization of humans with a spherule vaccine produced IgM responses to this antigen that were not evident in placebo recipients.

The coccidioidal complement fixation and immunodiffusion-complement fixation antigen is a chitinase

Culture filtrates and autolysates of Coccidioides immitis have provided suitable crude antigens for the serodiagnosis and prognosis of coccidioidomycosis. One of these, a heat-labile antigen which participates in the immunodiffusion reaction corresponding to the complement fixation reaction (IDCF), has been characterized as a 110-kDa native protein that, when subjected to reducing conditions and heat, yields a 48-kDa component. The present report provides serologic and biochemical evidence that this antigen is a chitinase. This chitinase, isolated from 48-h culture filtrate of the spherule-endospore-phase C. immitis by affinity adsorption to chitin, formed a line of identity with the IDCF reference antigen and participated in the complement fixation reaction with human serum. It lost its enzymatic as well as antigenic activity when heated, but when not heated it retained its enzymatic activity even when precipitated with coccidiodal antibody present in human serum. This chitinase represents a significant serodiagnostic substance and may be important in the morphogenesis of C. immitis.

An immunoreactive apoglycoprotein purified from Coccidioides immitis

Deglycosylation of glycoproteins in a lysate of spherules of Coccidioides immitis has permitted purification and partial characterization of a proline-rich pronase-sensitive antigen. Moreover, soluble antigen specifically stimulated lymphocytes from persons with dermal delayed-type hypersensitivity to coccidioidal antigens. When related to reference coccidioidin by tandem two-dimensional immunoelectrophoresis, the antigen fused in the anodal region with a specific reference antigen (antigen 2). It did not show identity with coccidioidal antigens used in conventional serologic assays. Although immunoblots of the purified protein with monospecific rabbit antiserum showed a single antigen at 33 kDa, the parent spherule lysate bound the same antibody in a broad band between 70 and greater than 200 kDa, which could be explained by microheterogeneity of glycosylation. Immunoelectron microscopy using affinity-purified human antibodies localized the antigen to the cell wall and internal septa of spherules. These findings suggest that the apoglycoprotein may be important in human immune responses to coccidioidal infection.

Production and characterization of a monoclonal antibody to the complement fixation antigen of Coccidioides immitis

Detection of complement-fixing antibody to coccidioidin by using the complement fixation test or an immunodiffusion assay for complement-fixing antibody (IDCF) is widely viewed as the most useful immunodiagnostic test for coccidioidomycosis. In this investigation, we report the production of an immunoglobulin G subclass 1 (IgG1) monoclonal antibody (MAb) to the IDCF antigen for use as a biospecific ligand for purifying the IDCF antigen on solid-phase immunosorbents and for use as a reagent for screening genomic or cDNA expression libraries from Coccidioides immitis. BALB/c mice were immunized by intramuscular injections of coccidioidin in adjuvant, followed by an intrasplenic booster injection of coccidioidin in saline. The spleen cells were fused with SP2/0 Ag14 myeloma cells, and the fusion products were screened for IgG antibody to coccidiodin by using an enzyme-linked immunosorbent assay. Positive hybridomas were cloned and evaluated for reactivity to the IDCF antigen by two-dimensional immunoelectrophoresis and by immunoblotting. An IgG1 Mab was produced that was specific for the IDCF antigen when evaluated by two-dimensional immunoelectrophoresis and immunoblotting. The epitope recognized by the MAb was heat labile (60 degrees C, 30 min) and susceptible to enzymatic digestion with pronase but was resistant to treatment with lipase, alpha-mannosidase, glucose oxidase, and endoglycosidase H. This heat-labile peptide epitope appears to be specific to C. immitis, as judged by the fact that the MAb was not reactive in immunoblots or enzyme-linked immunosorbent assays of histoplasmin or blastomycin.

Serology of coccidioidomycosis

Serologic tests have assisted in the diagnosis and prognosis of coccidioidomycosis for a half-century. The causative agent, Coccidioides immitis, is a dimorphic fungus existing in a hyphal form with arthroconidia in nature and in the usual culture. The arthroconidia represent the inhaled infective forms which in vivo and under special laboratory conditions form spherules which endosporulate. The culture filtrate/autolysate (coccidioidin) from the hyphal phase has provided antigens of suitable reliability for currently used serologic tests. These tests are primarily to determine the two major antibody responses: the early immunoglobulin M (IgM) response is useful in the diagnosis of acute primary coccidioidomycosis. Later, IgG is produced and usually outlasts the IgM, persisting in chronic coccidioidomycosis. The IgM is detectable by tube precipitin, a corresponding immunodiffusion, or latex particle agglutination tests. The pertinent antigen(s) is heat stable and pronase resistant and appears to be largely carbohydrate, mainly mannose with some 3-O-methyl mannose. The IgG detectable in the serum and other body fluids by complement fixation and a corresponding immuno-diffusion is useful in diagnosis, and its quantitation provides an indicator of progression of disease (increasing titer) or regression (decreasing titer). The pertinent antigen appears to be a heat-labile, pronase-sensitive protein which in an unreduced form has a molecular weight of 110,000. A third very useful serologic procedure is the exoantigen test for identification of putative cultures of C. immitis.

Purification and amino-terminal sequence analysis of the complement-fixing and precipitin antigens from Coccidioides immitis

Two proteins (21 and 48 kilodaltons) purified from endospore-spherule culture filtrates of Coccidioides immitis are identified as precipitin and complement-fixing antigens, respectively. To allow specific structural comparison to antigens identified by other investigators and as a first step to eventual serodiagnostic antigen production by recombinant DNA technology, amino-terminal amino acid sequences were determined for these antigens.

Development and characterization of a monoclonal antibody against the tube precipitin antigen of Coccidioides immitis

Primary infection with Coccidioides immitis is commonly accompanied by the production of an immunoglobulin M precipitin antibody which is detected by the tube precipitin (TP) assay or by the immunodiffusion assay for TP antibody (IDTP assay). In the present investigation, spleen cells from spherulin-immunized BALB/c mice were fused with SP2/O Ag14 myeloma cells, and the resulting hybridomas were screened for antibody to the IDTP antigen by using an enzyme-linked immunosorbent assay. Positive hybridomas were cloned by limiting dilution and injected into pristane-primed mice for ascites production. Characterization of antibody reactivity was accomplished with the IDTP assay, two-dimensional immunoelectrophoresis, and immunoblotting. An immunoglobulin G1 monoclonal antibody which reacts with the IDTP antigen of C. immitis is described. The epitope that is recognized by the monoclonal antibody is also present, but to a lesser extent, on a second coccidioidal antigen which has been designated antigen 2. The monoclonal antibody was not reactive in immunoblots of histoplasmin or blastomycin, indicating that the epitope recognized by this antibody may be specific for C. immitis.

Serological techniques for diagnosis of fungal infection

This review summarizes recent developments in the serodiagnosis of candidiasis, aspergillosis, cryptococcosis, histoplasmosis, blastomycosis, coccidioidomycosis, mucormycosis and sporotrichosis. A number of studies have substantiated the presence of circulating antigens in invasive candidiasis, invasive aspergillosis, disseminated histoplasmosis and coccidioidomycosis, and immunoassays for antigen detection provide moderate sensitivity but high specificity for disease. Improved detection may result mainly from repeated serum or concentrated urine samplings rather than from the development of more sensitive immunoassays. Immunoblot analysis of the serological response is a useful tool for the identification of immunogenic fungal components that elicit a specific antibody response in invasive disease. This method, and others, have been successfully applied to the study of the immune response to several fungi, including Candida, Aspergillus and Rhizopus.

Characterization of a soluble protein of Coccidiodes immitis with activity as an immunodiffusion-complement fixation antigen

A 48-kilodalton (kDa) electrophoretically distinct antigen from Coccidioides immitis mycelial- and spherule-endospore-phase filtrates was previously associated by immunoblotting with the immunodiffusion band that corresponds to complement-fixing activity (ID-CF). To characterize this antigen and its precursor, both mycelial- and spherule-endospore-phase filtrates were fractionated by size exclusion chromatography, lectin affinity chromatography, and nondenaturing electrophoresis. By size exclusion chromatography, most of the protein and carbohydrate of the crude filtrates eluted in a peak of average molecular size less than 30 kDa, although other components were detected. ID-CF activity was associated with the component at a relative mobility of 110 kDa. Fractions containing the ID band that corresponded to tube precipitin activity occurred from 200 to 40 kDa. The appearance of the 48-kDa band in denaturing sodium dodecyl sulfate-polyacrylamide gel electrophoresis (PAGE) specifically coincided with the fractions containing ID-CF activity. Nondenaturing PAGE of filtrates showed silver-stainable and immunoblot-reactive bands in the region of 110 kDa. Prior treatment with pronase destroyed this electrophoretically separable antigen, whereas periodate had no effect. Trypsin did not affect the 110-kDa band in unheated or unreduced antigen. Mycelial filtrates were chromatographed on lentil lectin or concanavalin A-Sepharose 4B to deplete them of glucose- or mannose-containing carbohydrate. The effluent fraction contained ID-CF activity and, upon denaturing electrophoresis, the 48-kDa antigen. The 110-kDa protein represents the ID-CF antigen which is heat labile and denatured to a 48-kDa band by sodium dodecyl sulfate-PAGE.

Antigenic identity of biologically active antigens in coccidioidin and spherulin

We recently reported the isolation of three clinically relevant antigens from coccidioidin; viz., the antigen that is reactive in the immunodiffusion (ID) assay for detecting tube precipitin (TP) antibody (designated IDTP); the antigen that is reactive in the ID assay for detecting complement-fixing antibody (designated IDCF); and the heat-stable (HS) antigen which, when demonstrated in soluble extracts of fungal cultures by using the IDHS assay, establishes the mycologic identification of Coccidioides immitis. This investigation was undertaken to determine whether the IDTP, IDCF, and IDHS antigens isolated from coccidioidin are of antigenic identity with components in spherulin. By employing the purified coccidioidin antigens in line-immunoelectrophoresis with spherulin and by assaying coccidioidin and spherulin by tandem two-dimensional immunoelectrophoresis against antisera produced to the purified antigens, we report that these three coccidioidin-derived antigens are antigenically identical to precipitinogens in spherulin.