Estimation of polymeric IgA in human serum: an assay based on binding of radiolabeled human secretory component with applications in the study of IgA nephropathy, IgA monoclonal gammopathy, and liver disease.

Binding of 125I-human secretory component (SC) to human polymeric immunoglobulin A (pIgA) was employed to measure quantitatively the pIgA present in human sera. Interference by IgM in some sera was prevented by removal of IgM with glutaraldehyde polymerized anti-IgM antibodies. 125I-SC complexed to pIgA was measured by precipitation with anti-IgA antibodies and the quantity of pIgA in human serum was estimated by comparing the quantity of 125I-SC bound by several dilutions of human serum to that bound by standard quantities of human monoclonal pIgA proteins. The assay was specific for pIgA because heat-aggregated monomeric IgA or hypogammaglobulinemic serum did not bind 125I-SC greater than a precipitate formed with human monoclonal IgG and anti-IgG. Moreover, analysis of a series of IgA myeloma sera indicated no correlation between the IgA content of the serum and the quantity of pIgA measured. The quantity of pIgA found in 30 normal human sera was 0.13 +/- 0.08 mg/ml (1S.D.), which consisted of 11.3 +/- 5.3% (1 SD) of the total IgA. Patients with IgA monoclonal gammopathy were most often found to have predominantly monomeric IgA. Patients with IgA nephropathy also showed an elevation of pIgA, but this appeared to be a consequence of an overt IgA elevation. IgA nephropathy patients with elevated serum IgA in fact showed a significant elevation of monomeric IgA. Selective elevation of pIgA was observed in patients with primary biliary cirrhosis and alcoholic liver disease. A comparison of this assay with other assays to measure pIgA is discussed.