We studied serum IgA as antibodies to dietary antigens (Ag), as lectin-binding molecules, and as conglutininbinding immune complexes (IgAIC) in people from geographical areas in which IgA nephropathy (IgAGN) is particularly frequent. Sera from 63 Italian, 21 Australian, and 25 Japanese patients affected by IgAGN and 24 Italian, 20 Australian, and 40 Japanese healthy controls were studied. Increased values of IgAIC were detected in 42.8% of Italian patients, while only in 23.8% and 8% of Australian and Japanese patients, respectively. Mean values were significantly increased only in Italian patients (P< 0.0001). Positive values of IgA antibodies against dietary Ag had variable prevalences, but again Italian patients showed the highest frequency, from 19% to 28.5% versus 0 to 38% in Australians and 0 to 16% in Japanese. Mean values of these antibodies were not significantly increased in any patient groups in comparison to the corresponding healthy populations. However, patients with elevated values of IgAIC had significantly higher serum concentrations of antibodies to alimentary components and a linear correlation was found between IgAIC and some IgA antibodies to food components. The relationship between these two series of data was particularly evident for Italian and Australian IgAGN patients. Moreover, the patients with positive data tended to have a cluster of increased levels of IgA antibodies against several alimentary Ag at the same time. A linear correlation was evident between values of IgA antibodies to gluten fractions and to heterologous albumins. None of these correlations was evident among healthy controls. Besides the role of the antibodies, some IgA in sera from the patients investigated were able to bind several lectins by a nonimmune bond, which was inhibited by competitive sugars. Comparing patients' data with the corresponding controls, we found increased lectin-binding IgA activity in up to 72% of Japanese patients, in up to 56% of Australians, and in 6% to 33% of Italians. Only in Japanese patients' sera were mean values of lectin-binding IgA activity significantly increased, but without correlation with IgAIC levels. These data suggest that serum IgA in IgAGN patients may participate in immune complex formation with IgA binding to alimentary antigens and also forming nonimmune complexes by IgA-lectin interactions. © 1991, National Kidney Foundation, Inc.. All rights reserved.
