Characterization and evaluation of a soluble antigen complex prepared from the yeast phase of Histoplasma capsulatum

Soluble yeast-phase (YPS) antigen preparations (Reeves et al., 1972) were obtained from three strains of Histoplasma capsulatum. These were analyzed by agar-gel diffusion and complement fixation tests with human sera from known cases of histoplasmosis. Two of the preparations contained the H and M antigens normally found in histoplasmin, whereas the third preparation contained the H but not the M antigen. In addition, a group of unknown antigens, designated as the Y (yeast) antigens, were demonstrated in all three YPS preparations. On the basis of Sephadex gel filtration data, the molecular weight of the YPS M antigen was estimated to range from 117,000 to greater than 200,000; that of the YPS H antigen was estimated to range from about 60,000 to greater than 200,000; and that of the Y antigens was estimated to range from less than 10,000 to 100,000. Complement fixation tests with human homologous and heterologous sera showed that the Y antigens were specific for H. capsulatum; Y antigens were not detected in the single lot of histoplasmin used in this study. When 60 human histoplasmosis sera and serum samples from nine animals experimentally infected with H. capsulatum were analyzed, it was found that antibodies to the Y antigens occurred with about the same frequency as antibodies to the H antigen but with less frequency than that of antibodies to the M antigen. When used in rabbits as immunogens for the preparation of specific antisera to H. capsulatum, the components of the YPS preparations caused the formation of numerous cross-reacting antibodies. The data from this study show that the value of the YPS preparations for the serological diagnosis of histoplasmosis rests on the specificity of the H, M, and Y antigens and on the fact that the primary production of antibodies is restricted to these antigens in the course of natural infections. The YPS preparations were found to be stable for a period of at least 11 months under a variety of storage conditions and temperatures. Data obtained with various killing agents and metabolic inhibitors suggest an improved method for preparing the YPS antigens by using a suitable strain and killing the cells with iodoacetate.