Antibody response in human nocardiosis: identification of two immunodominant culture-filtrate antigens derived from Nocardia asteroides.

An ammonium sulfate precipitate derived from a culture filtrate of Nocardia asteroides contained 20-30 protein subunits, as identified by gel electrophoresis. Western blot analysis of these proteins (obtained from serum samples from patients with systemic nocardiosis) revealed that all of the 17 samples reacted with a subunit of a protein with an apparent molecular weight of 55,000 and that 11 of the 17 samples reacted with a subunit with a molecular weight of 31,000. In contrast, only two of 25 serum samples obtained from healthy controls or patients hospitalized without nocardial or mycobacterial disease reacted with these subunits. Sera from 21 patients infected with Mycobacterium tuberculosis, a historically troublesome cross-reactive group, failed to react with either the 55,000 or the 31,000 molecular weight subunit. The presence of only two protein subunits that reacted with sera from humans with nocardiosis suggests that these molecules may have use as diagnostic reagents or probes of the immunologic reaction to Nocardia.