Iodination of human thyroglobulin (Tg) alters its immunoreactivity. II. Fine specificity of a monoclonal antibody that recognizes iodinated Tg.

In a previous investigation, we found that murine MoAb 42C3, raised against human Tg, recognized Tg differently depending upon its level of iodination of Tg. A possible explanation for this finding is that iodine is directly involved with the specific epitope recognized by MoAb 42C3. In the present study, we report that the binding of MoAb 42C3 to iodinated Tg is inhibited by T4, T3, reverse T3 (rT3), triiodothyroacetic acid (triac), diiodothyronine (T2), diiodotyrosine (DIT), but not by thyronine (TO) or tyrosine. The order of inhibition of these iodinated compounds is T4 > T3 > rT3 > triac > T2 > DIT. The MoAb 42C3 does not have the same specificity as the T3, T4-receptor since the order of binding of these iodinated compounds on the receptor differed from the order of their inhibition of this MoAb. Monoclonal antibody 42C3 also recognized non-iodinated Tg that was subsequently iodinated in vitro. It failed to recognize another protein, bovine serum albumin, that was iodinated in vitro by the same method. These results suggest that iodinated tyrosines and thyronines determine the binding specificity of MoAb 42C3. The inhibitory effects of these compounds on MoAb 42C3 depend on their iodine content as well as location of iodine in the aromatic ring.

A murine monoclonal antibody derived from the immunization of human thyroglobulin reacts equally with L-thyroxine and reverse triiodo-L-thyronine and has a unique idiotype.

Murine monoclonal antibody (mAb 16.3.2) to human thyroglobulin which bound equally to various thyroglobulins derived from nine species was obtained from the fusion of C3H/He spleen cells sensitized with normal human thyroglobulin. Characterization of mAb 16.3.2 revealed that both L-thyroxine (T4) and reverse triiodo-L-thyroxine (rT3) were very efficient in the competitive binding inhibition test and that a molar ratio between T4 and rT3 needed for 50% inhibition of binding to immunized thyroglobulin was about 1:1. Further studies on the idiotype of mAb 16.3.2 using both binding and competitive binding inhibition tests showed that mAb 16.3.2 had a unique idiotype not cross-reacting with other monoclonal antibodies to thyroglobulins. Therefore, a possible explanation offered was that mAb 16.3.2 was endowed with a unique idiotype to be regulated by a distinct idiotype network from those of other mAbs.

[Studies on anti-T3 and anti-T4 autoantibodies found in 2 sisters with juvenile hypothyroidism due to Hashimoto's thyroiditis. III. Anti-rT3 autoantibodies].

We previously reported that two sisters with juvenile hypothyroidism due to Hashimoto's thyroiditis (case 1: 13 years old, case 2: 10 years old) had antibodies against T3 and T4, and that the titers of these antibodies decreased but remained above normal levels even in the euthyroid state during L-T4 treatment. In our further investigations we found the binding of 125I-rT3 to serum gamma-globulin in both cases in the pre-treatment period. This binding was completely inhibited by the addition of unlabelled rT3. In addition to these findings, we also found the presence of anti-rT3 antibodies in two rabbits (TG-1, TG-2) immunized with human thyroglobulin. Since it has been suggested that autoantibodies against thyroglobulin cross-react with T3 and T4, we examined the specificities of anti-rT3 antibodies which were found in both cases and in the two rabbits in order to clarify the role of thyroglobulin in rT3 antibody production. The association constants and binding capacities of rT3 antibodies in cases 1 and 2 were 2.9 X 10(8) M -1 and 50 ng/ml serum, and 2.2 X 10(9) M -1 and 1.5 ng/ml serum, respectively. Cross reactivities of these anti-rT3 antibodies with T3 and T4 were 1.8% and 276% in case 1, and 0.24% and 38% in case 2, respectively. These results suggest that anti-rT3 antibodies in case 1 are anti-T4 antibodies interacting with rT3, and those in case 2 are antibodies against rT3 which have a high cross reactivity with T4. In both cases, however, cross reactivities with T3 were very small. Cross reactivities of anti-rT3 antibodies with T3 and T4 in TG-1 and TG-2 were 4.0% and 53%, and 53% and 182%, respectively. These data were compatible with those obtained from both cases. The observation that anti-rT3 antibodies found in two sisters and two rabbits immunized with human thyroglobulin had similar characteristics in terms of cross reactivities with T3 and T4 suggests the role of thyroglobulin as an antigen in rT3 antibody production in both patients.

Effects and clinical significance of exogenous thyroxine therapy in patients with circulating thyroid hormone autoantibodies.

The effects of exogenous L-thyroxine therapy (0.2-0.3 mg/day) on spontaneously occurring thyroid hormone autoantibodies (THAA) and on serum levels of thyroxine (T4), triiodothyronine (T3) and reverse triiodothyronine (rT3) are described in a boy with lymphocytic thyroiditis. Prior to T4 medication, THAA bound virtually all thyroid hormones as noted by serum gamma globulin binding of tracer hormones. During T4 therapy, however, gamma globulin binding of tracer thyroid hormones decreased markedly. T4 and T3 levels as determined by radioimmunoassay (RIA) were grossly distorted prior to thyroxine therapy and bore no relation to clinical status, the single antibody technics yielding falsely low values whereas the double antibody procedures spuriously exaggerating serum hormone concentration. That spontaneously occurring THAA interfered in RIA hormone measurements was clearly shown by the lack of inconsistencies in laboratory and clinical evaluation when hormone determinations were performed in protein free serum extracts (to remove circulating THAA), or when RIAs were carried out in sera during thyroxine therapy when THAA were saturated by exogenous hormones. The thyroid stimulating hormone (TSH) level prior to therapy was 300 microunits/ml which decreases to less than 3 microunits/ml during T4 medication (0.3 mg/day). There was a gross inverse correlation between TSH secretion and T4 antibody saturation. The marked inhibition of antibody binding of tracer thyroid hormones observed during therapy suggested possible masking of thyroid hormone antibody activity during T4 treatment; experiments involving addition of tracer thyroid hormones to hormone stripped serums from thyroxine treated patients confirmed the hypothesis in two cases of hypothyroidism--one of which was diagnosed in the boy with chronic thyroiditis and the other a woman with postradioactive iodine-induced hypothyroidism. The pathophysiologic significance of thyroid hormone autoantibodies in the transport of thyroid hormones is briefly discussed.

Autoregulation of 3, 3',5'-triiodothyronine production by rat liver microsomes.

Conversion of thyroxine (T4) to 3,3',5'-triiodothyronine (rT3) was studied in rat liver microsomes. Addition of rT3 at a physiological concentration to the incubation medium inhibited the deiodination of thyroxine to rT3. With a concentration of rT3 greater than 37.6 nM no net rT3 production at pH 8.0 was observed. Further increases in rT3 concentration resulted only in degradation of added rT3 and no net synthesis of rT3 from T4 could be detected. The inhibitory effect of rT3 upon its own production from T4 was pH dependent, 5 fold lower amounts of hormone being required to inhibit completely rT3 production at pH 7.4 than at pH 8.0. With the same experimental conditions no significant effect of rT3 on the conversion of T4 to 3,5,3'-triiodothyronine (T3) could be observed at pH 8.0 with all concentrations of added iodothyronine. A linear production of 3,3'-T2 from added rT3 was determined over the whole range of rT3 concentration, suggesting a lack of saturation of deiodinating enzyme. Binding of rT3 by anti-rT3 antibody added to the incubation mixture enhanced rT3 production from T4 by protecting rT3 from being degraded and/or diminishing the inhibitory effect of this iodothyronine on its own production. It was concluded that rT3 influenced its own production and that this effect may represent an important autoregulatory process in the iodothyronine metabolism.

Autoantibodies to thyroglobulin cross reacting with iodothyronines.

Serum thyroxine was consistently unmeasurable by radioimmunoassay in an elderly patient with myxoedema after successful treatment with oral thyroxine. Abnormal binding of thyroxine was suspected and shown to be due to the presence in serum of antibodies of the IgG variety. The characteristics of these antibodies with respect to their binding of thyroxine (T4), triiodothyronine (T3), reverse triiodothyronine (rT3) and human thyroglobulin (Tg) were systematically studied. Three preparations of Tg, and t4, T3 and rT3 were examined for their ability to compete with 125I-Tg, 125I-T4, 125I-T3 and 125I-rT3 for binding to the antibodies. For each tracer used the order of competitive efficiency was Tg greater than T4 greater than T3 greater than rT3. This provides for the first time direct evidence that iodothyronine reacting antibodies occurring in man are generated against Tg. All three iodothyronines were able to inhibit tracer binding of labelled iodothyronines completely, the order of effectiveness being T4 greater than T3 greater than rT3, suggesting antibodies with one type of binding site and that these were probably raised against a Tg sequence incorporating T4, although there was some evidence for the existence of a minor subpopulation of antibodies with higher specificity for T3. Complete displacement of labelled Tg by cold iodothyronines, however, was not possible. The experimental evidence suggests two classes of Tg antibodies, 70% of which were directed towards the T4 containing region, and 30% directed against other part(s) of the Tg molecule. Despite the presence of such Tg antibodies conventional haemagglutination tests of the patient's serum for Tg antibodies were negative.

International survey of the radioimmunological measurement of serum reverse triiodothyronine.

An international survey was conducted to study the performance of the radioimmunoassay of serum reverse triiodothyronine (rT3). Eight laboratories were provided with 10 serum samples containing several concentrations of rT3 and thyroxine (T4). Each laboratory also obtained the same batch of pure rT3. T4 and rT3 were measured by all participants in these samples. Half of the investigators routinely correct their data for T4 cross-reactivity. The results demonstrate that the performance of the rT3 radioimmunoassay is, on the average, satisfactory as judged from recovery experiments. Serum T4 influences the rT3 measurement to a lesser extent in the laboratories where corrections are performed compared with the others. Simple correction methods appear to fail, however, as the effect of a certain T4 concentration was found to vary with the rT3 level in the sample.

Correction for the presence of cross-reactants in saturation assays: application to thyroxine cross-reactivity in 3,3',5'-(reverse)-triiodothyronine radioimmunoassay.

Cross-reaction of anti-3,3',5'-triiodothyronine (rT3) antisera with thyroxine has proved problematical in the development of radioimmunoassays for rT3. Results of experimental work with two antisera with differing specificities are presented which illustrate certain aspects of cross-reacting assay systems. The mathematical theory of a single binding-site, two ligand assay is discussed and extended by use of a multiple binding-site computer model to a two binding-site, two ligand system. It is suggested that for the practical evaluation of the nature and extent of cross-reaction, a family of response curves for the hormone should be drawn, each curve representing the addition of a fixed mass of the cross-reactant to a set of standard incubation mixtures. Such curves will reveal whether the antiserum is (a) specific, implying that assay results require no correction, (b) behaves as a single binding site system, in which case measurement of the relative potency of the two ligands at the point on the response curve generated by the serum sample will enable an algebraic correction to be made, given that the T4 concentration is known or (c) behaves as a multiple binding-site system where correction necessitates the use of a nomogram.

Radioimmunoassay of reverse triiodothyronine.

rT3 RIA standard curves were generated using rT3 standards from various sources. The rT3 antiserum and the method of free and bound hormone separation used were the same in all assays. The slopes of rT3 RIA curves differed considerably; standard curves generated with Henning (Berlin) rT3 standard had the steepest slope. Because of the differences in slopes of standard curves, bound to free estimates, when extrapolated from standard curves generated with various rT3 standards, resulted in 2- to 3-fold differences in rT3 assay values in the same sera. Circulating rT3 concentration in human sera was 20 ng/100 ml when Henning rT3 standard was used in the assay, whereas with Jorgensen's rT3 standard, a value of 42 ng/100 ml was noted. As the rT3 antiserum and assay conditions used were identical in all comparisons (except rT3 standards), the 2- to 3-fold differences in rT3 values noted in the same sera suggested variability in potency of rT3 standards. The differences in rT3 values in the same sera (resulting from extrapolation from standard curves using different rT3 standards) were of the same magnitude as that noted in normal human sera in various studies, suggesting that the reported rT3 variations in sera of euthyroid individuals may also be due to differences in rT3 standards employed in the assay.