Previous studies carried out by screening a lambda gt11 human thyroid cDNA library with serum samples from selected patients with Hashimoto's thyroiditis and a polyclonal antibody to porcine thyroid peroxidase (TPO) confirmed, at the molecular level, that TPO is a major component of the thyroid microsomal antigen (M). That investigation led to the isolation of a clone (C2) which encodes an 85-amino acid segment of TPO and harbors a major epitope recognized by serum from several patients with autoimmune thyroid disease that contained anti-M autoantibodies (MAb). In this study, C2 antigen that was produced as a beta-galactosidase fusion protein was used to establish an enzyme-linked immunoabsorbent assay for the detection of anti-C2 autoantibodies (C2Ab). C2Ab then were assayed in 191 patients with different autoimmune and nonautoimmune thyroid disorders, and 50 patients with nonthyroidal autoimmune diseases. The results were compared with the titers of anti-TPO antibodies (TPOAb; as detected by monoclonal antibody-assisted RIA) and MAb (as detected by passive hemagglutination). Positive C2Ab was found in the serum of 85 of 136 (63%) patients whose serum contained TPOAb and/or MAb. A significant positive correlation was found between the levels of C2Ab and those of TPOAb (r = 0.76; P less than 0.001) or MAb (r = 0.69; P less than 0.001), which was independent of the type of underlying autoimmune thyroid disorder. Low levels of C2Ab also were found in 10 of 105 (9%) serum samples that did not contain TPOAb. Western blot analysis carried out on the latter samples showed that in 2 samples the apparent C2Ab reactivity was due to the presence of antibodies reacting with beta-galactosidase. In conclusion, we confirmed the validity of screening lambda gt11 cDNA human thyroid libraries to better characterize thyroid autoantigens and demonstrated the feasibility of using recombinant proteins to establish diagnostic assays for autoantibodies. ; Journal Article ; Research Support, Non-U.S. Gov't ; info:eu-repo/semantics/published