Megalin (gp330) is a multiligand receptor found on the apical surface of selected epithelial cells, including thyroid cells. We recently showed that megalin is a high-affinity receptor for thyroglobulin. Megalin is capable of inducing autoantibodies, as shown in the rat model, Heymann nephritis. Based on this consideration and on the knowledge that autoantibodies against several thyroid antigens develop in patients with autoimmune thyroid diseases, we searched for antimegalin antibodies in 78 patients with autoimmune and nonautoimmune thyroid diseases. We developed an assay, based on flow cytometry, to measure binding of serum IgGs to L2 cells, a rat carcinoma cell line that expresses abundant megalin. After incubation of L2 cells with serum samples and then with fluorescein isothiocynate-conjugated antihuman IgG Fc-specific antibody, the mean fluorescence intensity (MFI) was determined. Using results obtained in sera from 32 normal subjects, we established a cutoff value for MFI (50.62), above which, tests were considered positive. Significantly elevated values were found in 18 patients, including 13 of 26 patients with autoimmune thyroiditis (50.0%) and in 2 of 19 patients with Graves' disease (10.5%). Furthermore, 2 of 19 patients with nontoxic goiter (10.5%) and 1 of 14 patients with differentiated thyroid cancer (7.14%) had MFI values greater than 50.62, associated with the presence of circulating antithyroid autoantibodies. As a control cell line, we used Chinese hamster ovary cells, which do not express megalin. We found that, among the 18 patients with positive tests for binding to L2 cells, only 1 patient with nontoxic goiter had significant binding of serum IgGs to Chinese hamster ovary cells. Binding of serum IgGs to L2 cells was significantly reduced by coincubation with purified megalin in 15 of 18 positive patients (83.33%) and by a rabbit antimegalin antibody in 11 patients (61.11%). Further and more conclusive evidence that positive tests (MFI >50.62) for binding to L2 cells were attributable to serum antimegalin antibodies was demonstrated by immunoprecipitation experiments. After incubation of serum samples with L2 cell extracts, incubation with antihuman IgG Fc-specific agarose beads resulted in immunoprecipitation of megalin in all the 18 positive patients, but not in normal subjects, as assessed by Western blotting using a monoclonal antibody against megalin. Furthermore, the intensity of the band corresponding to megalin precipitated by serum IgGs in the above 18 patients was significantly correlated with the L2 binding MFI. This is the first clear-cut demonstration of antibodies against megalin in humans. Further studies are needed to determine whether antimegalin antibodies have pathogenic significance or diagnostic value in autoimmune thyroid diseases.
