Evidence for a specific IgA receptor in rat and human mesangial cells

Previous works have demonstrated that mesangial cells (MC) possess Fc receptors for IgG. We have investigated whether serum IgA could bind to MC in culture through a specific receptor. The binding of 125I-IgA (4 degrees C, 60 min) to MC was dose dependent and saturable, and the Scatchard analysis revealed a population of 1 x 10(5) binding sites per cell with a Ka of 7.8 x 10(8) M-1. This receptor was specific for IgA because unlabeled IgA and its Fc fragment were able to inhibit the binding, whereas IgG, IgM, and the IgA F(ab) fragment were not. Parallel experiments showed that human IgA bound in a similar manner both in a homologous system (human MC) and in heterologous systems (rat MC and peritoneal macrophages). In saturation studies using neuraminidase-desialylated IgA (desIgA), which has more carbohydrate residues exposed to binding, similar results to those of untreated IgA were obtained. Incubation with several carbohydrates decreased the IgA and desIgA binding to MC, obtaining maximal inhibition with simultaneous addition of galactose and N-acetyl-galactosamine. The possible IgA receptor (IgA-R) was purified from MC by affinity chromatography on an IgA-Sepharose column and appeared on SDS-PAGE gels as a 60-kDa band. The expression of IgA-R molecules on MC was enhanced by the presence of IgA in the culture medium. Finally, a band comparable with that of U937 cells was shown by Northern blot assay hybridizing mRNA from rat and human MC with a cDNA probe for the Fc alpha R. Taken together, these results strongly suggest the existence of a 60-kDa protein on the surface of MC that is able to bind IgA with high affinity.