0021-972X/91/7201-0188$02.00/0 Journal of Clinical Endocrinology and Metabolism Copyright © 1991 by The Endocrine Society

Vol. 72, No. 1 Printed in U.S.A.

Heterogeneity of Autoantibodies against Thyroid Peroxidase in Autoimmune Thyroid Disease: Evidence against Antibodies Directly Inhibiting Peroxidase Activity as Regulatory Factors in Thyroid Hormone Metabolism BERNHARD SALLER*, RUDOLF HORMANN, AND KLAUS MANN Department of Internal Medicine II, Klinikum Grosshadem, University of Munich,

8000 Munich 70, Germany

ABSTRACT. The close relationship between thyroid microsomal antigen and thyroid peroxidase (TPO) is now well established. The present study evaluates the significance of autoantibodies against TPO (anti-TPO-Abs) in the various forms and stages of autoimmune thyroid disease with respect to a possible heterogeneous nature and particularly to their influence on TPO activity. When measured by a RIA using purified human TPO, anti-TPO-Abs were highly correlated with microsomal antibodies determined by enyzme-linked immunosorbant assay (r = 0.96; P < 0.0001) and with the results of a TPO immunoprecipitation assay using crude microsomal preparations (r = 0.76; P < 0.001). Relating the results of these assays to the reactivities of patients' sera with thyroid microsomes in immunoblot under nonreducing and reducing conditions, discordant results could be observed in a few cases. Further analysis of these data indicate a heterogeneous nature of anti-TPO-Abs, which react with at least two antigenic domains of the TPO molecule. The comparative analysis of patients with hyperthyroid Graves' disease,

patients with Graves' disease in clinical remission, and patients with hypothyroid Hashimoto's thyroiditis revealed no significant differences in the antibody spectrum. When evaluating the direct influence of anti-TPO-Abs on the activity of TPO under a rigorous methodological approach, we found no significant inhibition of the enzymatic activity by any of the sera investigated from patients with autoimmune thyroid disease compared to that in sera from normal controls. In conclusion, the data indicate a heterogeneous nature of anti-TPO-Abs. The spectrum of antigenic epitopes recognized by anti-TPO-Abs seems not to be significantly different in the various forms and stages of autoimmune thyroid disease. The lack of an inhibitory effect of autoantibodies on TPO activity argues against direct binding of autoantibodies to the enzymatic sites of TPO and indicates that they are not important factors in producing thyroid dysfunction in autoimmune thyroid disease. (J Clin Endocrinol Metab 72: 188-195, 1991)

I

N PATIENTS with autoimmune thyroid disease, the presence of autoantibodies against several distinct antigens, including the TSH receptor, thyroglobulin, and the microsomal antigen, is well established (1). The microsomal antigen is a glycoprotein with an apparent mol wt of about 100 kDa (2-4), and its major antigenic component is thyroid peroxidase (TPO) (5-9). The recently described biochemical nature of the microsomal antigen gives the basis for a more distinct investigation of microsomal antibodies, particularly antibodies against TPO (anti-TPO-Abs), using different newly described methodological approaches. Thus, it allows a more precise view of the nature and pathophysiological significance of these autoantibodies. Recent studies suggested a heterogeneous nature of anti-TPO-

Abs (10) and reported that those were not only capable of binding to TPO, but also of inhibiting its enzymatic activity (11-14). However, the important question of whether the antigenic determinants of TPO include its catalytic sites is presently controversial, since another study failed to find any inhibition of TPO activity by anti-TPO-Abs (5). The present studies were undertaken to investigate anti-TPO-Abs in different types and stages of autoimmune thyroid disease, i.e. untreated hyperthyroid Graves' disease (GD), GD in clinical remission, and hypothyroid Hashimoto's thyroiditis, with respect to a possible heterogeneous nature. Thereby, it was of particular interest to evaluate by a rigorous methodological approach whether anti-TPO-Abs may directly inhibit TPO activity and whether such an inhibitory effect would be related to the clinical stage or presentation of the underlying autoimmune process.

Received May 16, 1990. * To whom requests for reprints should be addressed.

188

The Endocrine Society. Downloaded from press.endocrine.org by [${individualUser.displayName}] on 08 December 2016. at 04:38 For personal use only. No other uses without permission. . All rights reserved.

ANTI-TPO-Abs IN AUTOIMMUNE THYROID DISEASE

Materials and Methods Patient serum samples Serum samples were obtained from 37 patients with autoimmune thyroid disease selected on the basis of positive titers of microsomal antibodies in a routine hemagglutination test (Thymune-M-HAT, Deutsche Wellcome, Grossburgwedel, Germany). In all patients, total serum T4 and total serum T 3 were determined by enzyme immunoassays [normal ranges: T4 (Syva, Darmstadt, Germany), 58-135 nmol/L; and T 3 (Boehringer Mannheim, Germany), 1.5-3.2 nmol/L]. TSH before and after TRH application was measured by immunoradiometric assay (IRMAclon, Henning, Berlin, Germany). TSH receptor antibodies (TRAb) were determined by RRA (TRAK-Assay, Henning; normal range,

Heterogeneity of autoantibodies against thyroid peroxidase in autoimmune thyroid disease: evidence against antibodies directly inhibiting peroxidase activity as regulatory factors in thyroid hormone metabolism.

The close relationship between thyroid microsomal antigen and thyroid peroxidase (TPO) is now well established. The present study evaluates the signif...
954KB Sizes 0 Downloads 0 Views