Isolation and identification of an exoantigen specific for Coccidioides immitis

Results of previous studies have established that mycelial-phase cells of Coccidioides immitis produce a heat-stable (HS) exoantigen that is specific for this fungus. In the present study, the HS exoantigen was isolated from a heterogeneous culture filtrate of C. immitis mycelia by using a combination of physicochemical procedures. Affinity chromatography of the culture filtrate on concanavalin A yielded two fractions: an effluent fraction that did not bind to the lectin and an eluate fraction that eluted in alpha-2-methylmannoside. Antigenic analyses by two-dimensional immunoelectrophoresis established that of the seven antigens detected in the unfractionated culture filtrate preparation, only two were present in the column effluent, and of these, only one was stable to heating at 56 degrees C for 30 min. Reactivity in the immunodiffusion assay for the HS exoantigen was demonstrable with the column effluent fraction and not the column eluate. The detection of only one precipitinogen in two-dimensional immunoelectrophoresis of the heat-treated concanavalin A effluent fraction, coupled with the reactivity of this fraction in the immunodiffusion assay for the HS antigen, provides strong, if not definitive, evidence that this antigen is the HS exoantigen. Purification of the HS antigen and the production of monospecific antiserum will provide the necessary reagents for the development of a sensitive and specific immunoassay for detecting the HS antigen in C. immitis cultures.