ON THE SPECIFICITY OF ASSAYS TO DETECT CIRCULATING IMMUNOGLOBULIN-A FIBRONECTIN COMPLEXES - IMPLICATIONS FOR THE STUDY OF SEROLOGIC PHENOMENA IN PATIENTS WITH IMMUNOGLOBULIN-A NEPHROPATHY

Immunoglobulin A (IgA)-fibronectin complexes have been proposed as spe cific serologic markers of IgA nephropathy. They have been detected by the use of ELISA composed of an immobilized antifibronectin antibody (or albumin as a negative control) and an enzyme-conjugated anti-IgA a ntibody (antifibronectin capture assay). By the use of this type of as say, plasma samples from 32 normal controls, 38 IgA nephropathy patien ts, and 81 patients with other types of glomerulonephritis were analyz ed. Extinction values in IgA nephropathy patients were higher (P = 0.0 6) than in patients with other glomerulonephritis types and significan tly higher than in normals. Markedly lower values were obtained when t he plates were coated with albumin. However, when the antifibronectin antibody was replaced by normal IgG or F(ab')(2) fragments, almost ide ntical extinctions were measured, The use of different antifibronectin antibodies, IgG, ELISA plates, or blocking regimens did not modify th ese results. Extinction values could not be suppressed by the addition of exogenous fibronectin. Similar extinctions were observed when plas ma samples were replaced by physiologic concentrations of fibronectin- free IgA. Extinction values measured in the plasma samples correlated significantly with IgA concentrations in plasma as analyzed by nephelo metry. A collagen binding assay, a second type of assay used to measur e IgA-fibronectin complexes, also allowed the detection of fibronectin -free IgA, and again, extinctions measured in plasma could not be supp ressed by exogenous fibronectin. In conclusion, both antifibronectin c apture ELISA and collagen binding assays do not specifically detect on ly IgA-fibronectin complexes, but also total plasma IgA, which is freq uently, but nonspecifically, elevated in IgA nephropathy. These data a lso show that plasma IgA may interact with immobilized IgG, which may affect various assays used to identify (auto)immune phenomena in patie nts with IgA nephropathy.