Characterization of a non-A, non-B hepatitis associated substance in stools

Using the 125J-labeled isolated IgG-fraction of reconvalescent sera from patients with NANB-hepatitis a radioimmunoassay for the detection of the NANB-hepatitis associated antigen in stool was established (RS). To characterize the antigen, stool suspensions of healthy persons and NANB-patients were analyzed by sedimentation on sucrose gradient (3-35%) before and after incubation of 125J-labeled normal IgG or IgG from reconvalescent patients. Stool was spiked with normal sera and with serum completely free from immunoglobulins. The antigen could be recovered in a fraction with similar sedimentation rate as IgG. Preincubation of Ag-positive stool with 125J-labeled IgG of reconvalescent serum resulted in a significantly higher sedimentation rate of the labeled IgG. The spiking of stool suspensions with serum of healthy donors and serum free of immunoglobulins resulted in a negative RIA-Test. The antigen could be absorbed on polystyrol beads preincubated with human serum of healthy donors. A positive test was achieved, however, only with the labeled IgG-fraction of reconvalescent serum and not with that of healthy donors. The results permit the following preliminary conclusions: The NANB associated antigen in stools which was detected by isolated IgG from reconvalescent sera shows no cross reactions with IgG of healthy donors, which excludes nonspecific binding to immunoglobulins. The sedimentation coefficient is in the range of 7s. Normal human sera may contain an antigen-masking substance, which inhibits its accessibility for specific antibodies. This may explain the well-known difficulties in establishing a valid radioimmunoassay for the detection of NANB-antigen or corresponding antibodies in the patient's sera.