Tryptic peptides of human thyroglobulin: II. Immunoreactivity with sera from patients with thyroid diseases

Characterization of a new highly sensitive immunometric assay for thyroglobulin with reduced interference from autoantibodies

Measurements of serum thyroglobulin (Tg) with sensitive immunoassays are of great importance for the management of patients with differentiated thyroid carcinomas. However, interference of circulating autoantibodies to Tg (hTgAb) hampers the usefulness of most assays. We have produced a panel of monoclonal antibodies (mAbs) selected to bind Tg in the presence of Tg autoantibodies and developed a sensitive immunoassay for Tg with minor interference by hTgAbs. The antibodies were characterized by cross-inhibition and immunoassay combination studies, as well as affinity estimation. The within-run and total imprecision of the assay were determined with 2664 samples in 60 separate runs. The most sensitive assay combination with superior protection against autoantibodies consisted of two solid phase mAbs and two tracer mAbs with distinct binding sites. The assay was linear and displayed a wide dynamic range up to 1342 μg/l with a functional sensitivity of 0.1 μg/l and a total imprecision of less than 10 %. There was good agreement between the new high sensitive immunofluorometric assay (IFMA) and two well-established Tg assays from Brahms Kryptor and Roche Diagnostics. Mean difference between the new IFMA and the Kryptor assay was 0.059 μg/l with a 95 % confidence interval of −0.032 to 0.151 μg/l, whereas the mean difference between the new IFMA and the Roche assay was −0.80 μg/l with a 95 % confidence interval of −1.24 to −0.35 μg/l.

Avidity of thyroglobulin antibody in sera from patients with Hashimoto's thyroiditis with different thyroid functional status

The mechanism of disease progression in Hashimoto's thyroiditis (HT) is still unclear. Thyroglobulin antibody (TgAb) is a diagnostic hallmark of HT. The aim of our study was to evaluate the avidity of TgAb in sera from HT patients with different thyroid functional status. Sera from 50 patients with newly diagnosed HT were collected and divided into three groups according to thyroid function: patients with hypothyroidism (H, n=18), subclinical hypothyroidism (sH, n=18) and euthyroidism (Eu, n=14). Titres and avidity of TgAb were determined by enzyme-linked immunosorbent assays (ELISAs). Avidity constant (aK) was determined as the reciprocal value of the thyroglobulin molar concentration in the liquid phase resulting in 50% inhibition of TgAb binding to thyroglobulin in solid-phase ELISAs. The titres and aK of TgAb were performed using log-transformation, and expressed as lgT and lgaK, respectively. Mean lgT of TgAb in sera was 4.19+/-0.60 in H, 3.77+/-0.63 in sH, and 3.29+/-0.64 in Eu, respectively. The median avidity of TgAb was 2.30x10(9) in H, 8.80x10(8) in sH, 2.00x10(7) in Eu, respectively. lgT and lgaK of TgAb were at significantly lower levels in Eu than in sH and H (P<0.05). Correlation was found between lgT and lgaK (r=0.594, P<0.05). lgaK was also related to TSH (r=0.308, P<0.05). Our study indicated that patients with high-avidity TgAb might be at high risk of developing subclinical, even to overt, hypothyroidism.

Cloning and characterization of canine thyroglobulin complementary DNA

Canine thyroglobulin (cTg) is one of the thyroid autoantigens associated with hypothyroidism caused by autoimmune thyroiditis in dog. To identify canine-specific areas in cTg, we cloned, by reverse transcriptase PCR, and sequenced the complete cDNA of cTg. It revealed an open reading frame of 8289 nucleotides, which encode a polypeptide of 2762 amino acids that is 78.9 and 78.1% identical to bovine and human thyroglobulin, respectively. This complete cTg sequence may be useful to promote the understanding of the primary structure of cTg and, it will be informative data in the further search about antigenic epitopes associated with autoimmune thyroiditis and pathogenesis of cTg-associated thyroid diseases in dog.

Iodination of murine thyroglobulin enhances autoimmune reactivity in the NOD.H2 mouse

Autoimmune thyroiditis in humans has been linked to excess iodine intake. A causative relationship between dietary iodine and thyroiditis has been clearly established in animal models of thyroiditis, including the NOD.H2(h4) mouse strain, which develops enhanced thyroiditis spontaneously after supplementation of drinking water with sodium iodide. To assess the mechanisms by which iodine may contribute to disease pathogenesis, we have purified hypoiodinated thyroglobulin (Lo-I Tg) from the thyroids of mice fed methimazole and potassium perchlorate. This preparation contained only a trace of iodine and was poorly reactive to monoclonal antibody 42C3, which has been shown previously to distinguish hypoiodinated from normal Tg. A cloned T cell line 2D11 from a diseased NOD.H2(h4) mouse proliferated in response to normal Tg, but not to Lo-I Tg. Serum antibodies from NOD.H2(h4) mice with thyroiditis were poorly reactive to Lo-I Tg. To determine that these changes were due specifically to iodine content, Lo-I Tg was reiodinated in vitro. Reiodination of Lo-I Tg partially re-established the reactivity of NOD.H2(h4) serum antibodies. The data demonstrate that the reactivity of thyroglobulin-specific antibodies and certain T cells are dependent on the iodine content of thyroglobulin. These findings suggest that iodine contributes to autoimmune thyroiditis in the NOD.H2(h4) mouse by directly enhancing the antigenicity of thyroglobulin.

Tryptic peptides of canine thyroglobulin reactive with sera of patients with canine hypothyroidism caused by autoimmune thyroiditis

Canine thyroglobulin (cTg) was treated with trypsin at a ratio of trypsin to cTg of 1:100 (w/w). Tryptic peptides of cTg were analysed by Western immunoblotting for their reactivity to serum thyroglobulin autoantibodies (TgAA) from patients with TgAA-positive hypothyroidism and normal individuals. The sera of patients with TgAA-positive hypothyroidism reacted with several peptides: 43, 32.5 and 31 kDa; the sera of normal individuals did not bind these tryptic peptides. Some of the TgAA-positive sera of patients reacted with 25 kDa peptide in addition to three tryptic peptides above. This experiment was the first report about antigenic epitopes of cTg. These small tryptic peptides recognized by TgAA may be related with the induction of TgAA and may be useful as markers for autoimmune thyroid diseases in dog.

Why measure thyroglobulin autoantibodies rather than thyroid peroxidase autoantibodies?

Autoantibodies to thyroglobulin (TgAb) and thyroid peroxidase (TPOAb) are of immunoglobulin G (IgG) class and have high affinities for their respective autoantigens. Both autoantibodies are markers of thyroid autoimmunity and they can be measured by a variety of assays. From the clinical perspective, TgAb are less prevalent than TPOAb and less useful than TPOAb for prediction of thyroid dysfunction. Moreover, TgAb interfere with Tg measurements to monitor metastases in thyroid cancer. However, increasing evidence suggests that these TgAb provide a surrogate for Tg. In terms of disease pathogenesis, Tg has been suggested to play a role in Graves' ophthalmopathy. Pending further studies, TgAb epitopes could distinguish between individuals who are euthyroid or who have clinical disease. A final, intriguing reason for measuring and characterizing TgAb is the interest these autoantibodies have rekindled in their autoantigen. It is conceivable that Tg polymorphisms, combined with the explosive mix of iodine, TPO and H2O2 necessary for thyroid hormone synthesis, inadvertently provide the trigger for the autoimmune thyroid response.

Thyroglobulin as an autoantigen: what can we learn about immunopathogenicity from the correlation of antigenic properties with protein structure?

Autoantibodies against human thyroglobulin are a hallmark of autoimmune thyroid disease in humans, and are often found in normal subjects. Their pathogenic significance is debated. Several B-cell epitope-bearing peptides have been identified in thyroglobulin. They are generally located away from the cysteine-rich regions of tandem sequence repetition. It is possible that our current epitopic map is incomplete because of the difficulty that proteolytic and recombinant approaches have in restituting conformational epitopes based upon proper pairing between numerous cysteinyl residues. Furthermore, the homology of cysteine-rich repeats with a motif occurring in several proteins, endowed with antiprotease activity, suggests that these regions may normally escape processing and presentation to the immune system, and brings attention to the mechanisms, such as oxidative cleavage, by which such cryptic epitopes may be exposed. A number of T-cell epitope-bearing peptides, endowed with thyroiditogenic power in susceptible mice, were also identified. None of them was dominant, as none was able to prime in vivo lymph node cells that would proliferate or transfer autoimmune thyroiditis to syngeneic hosts, upon stimulation with intact thyroglobulin in vitro. More than half of them are located within the acetylcholinesterase-homologous domain of thyroglobulin, and overlap B-cell epitopes associated with autoimmune thyroid disease, while the others are located within cysteine-rich repeats. The immunopathogenic, non-dominant character of these epitopes also favours the view that the development of autoimmune thyroid disease may involve the unmasking of cryptic epitopes, whose exposure may cause the breaking of peripheral tolerance to thyroglobulin. Further research in this direction seems warranted.

Recognition Pattern of Thyroglobulin Autoantibody from Hypothyroid Dogs to Tryptic Peptides of Canine Thyroglobulin

Circulating thyroglobulin autoantibody (TgAA) was analyzed using the Western immunoblot for determination of the dominant epitopes recognized by TgAA on tryptic peptides of canine thyroglobulin (cTg) in hypothyroid dogs. TgAA was measured in hypothyroid dogs, non-hypothyroid dogs with skin diseases and clinically normal dogs. Five of the 7 hypothyroid dogs, 1 of the 8 dogs with skin diseases and 1 of the 4 normal dogs were positive for TgAA. Four of the 5 TgAA-positive hypothyroid dogs were Golden Retrievers, and 3 of them showed high antibody titers. The sera of TgAA positive-dogs reacted to several peptides, and their patterns varied from sample to sample. Sera from 3 dogs with high titers of TgAA reacted broadly to high molecular weight peptides ranging from 45 to 90 kDa. These Western immunoblot patterns of the sera were disappeared after pretreatment with sufficient amount of intact cTg. All serum samples of both TgAA positive dogs and negative controls reacted to low molecular weight peptides ranging from 15 to 20 kDa. These immunoblot patterns of the sera were not disappeared even after pretreatment with sufficient amount of intact cTg. These findings show the possibility that the epitopes recognized by TgAA depend upon individual dogs with hypothyroidism and these autoantibodies recognize conformational epitopes on the cTg molecule.

Serum thyroglobulin measurement. Utility in clinical practice

Serum thyroglobulin measurement has greatly facilitated the clinical management of patients with differentiated thyroid cancer and a variety of other thyroid disorders. Thyroglobulin autoantibodies remain a significant obstacle to the clinical use of thyroglobulin measurement. The interpretation of any given thyroglobulin value requires the careful synthesis of all pertinent clinical and laboratory data available to the clinician. The diagnostic use of rhTSH-stimulated thyroglobulin levels has greatly facilitated the follow-up of low-risk patients with thyroid cancer. Although the measurement of thyroglobulin mRNA from peripheral blood is likely to affect the future management of these patients, it is expected that serum thyroglobulin measurement will continue to have a principal role in the care of patients with differentiated thyroid cancer.

Peptides of Human Thyroglobulin Reactive with Sera of Patients with Autoimmune Thyroid Disease

Autoantibodies to thyroglobulin (Tg) are a prominent feature of the two autoimmune thyroid diseases, chronic lymphocytic (Hashimoto’s) thyroiditis and Graves’ disease. Similar autoantibodies are found in the serum of many normal individuals without evidence of thyroid disease. Previous studies have indicated that patients with autoimmune thyroid disease recognize epitopes of Tg which are not usually recognized by normal individuals. The goal of this investigation was to identify peptide fragments of Tg bearing these disease-associated epitopes. For this purpose, we utilized a panel of mAbs that bind to different epitopes of the Tg molecule. One of these mAbs (137C1) reacted with an epitope that was also recognized by the sera of patients with autoimmune thyroiditis. In the present study, we show that two peptides (15 and 23 kDa) that reacted with mAb 137C1 are located in different parts of the Tg molecule. Each peptide inhibited the binding of mAb 137C1 to the other peptide and to the intact Tg, indicating that the same epitope was represented on the two peptides. Loops and helices of the secondary structure of the two peptides might be involved in the conformational epitope recognized by mAb 137C1. A striking finding of this study is that two apparently unrelated fragments of the Tg molecule bind to the same mAb. These findings may have important ramifications with regard to epitope spread and the progression of the autoimmune response to disease.

Analysis of autoantibody epitopes on human thyroid peroxidase

A number of studies have indicated that the major autoantibody epitopes on human thyroid peroxidase (TPO) are conformational and are formed by two overlapping immunodominant regions on the TPO molecule. In order to investigate further autoantibody reactivity with TPO, we have studied the TPO binding characteristics of sera from patients with autoimmune thyroid disease (n = 20), autoimmune adrenal disease (Addison's disease; n = 8) and apparently healthy blood donors (n = 9) using recombinant TPO expressed with a series of truncations and internal deletions. This material was obtained using an in vitro transcription/translation system in the presence of 35S-methionine and the reactivity of TPO autoantibodies tested in an immunoprecipitation assay. In addition, we have studied the effects of denaturing purified recombinant TPO by reduction and/or sodium dodecyl sulphate on its reactivity with TPO autoantibodies by Western blotting analysis. These studies show that TPO autoantibodies can recognise TPO in Western blotting analysis when large amounts of purified TPO are run on the gels and the blotted proteins renatured prior to addition of antibody. Under these conditions TPO autoantibodies in all 20 Graves' or Hashimoto's sera tested reacted strongly with blots of non-reduced TPO but reduction of TPO had a marked effect on the ability of autoantibodies to recognise it in Western blotting analysis. Analysis of TPO autoantibody binding to 35S-labelled TPO proteins containing N-terminal, central or C-terminal deletions indicated that all modifications studied caused a statistically significant lowering of binding. In the case of some modifications, there were differences in the reactivity of TPO autoantibodies in sera from patients with Addison's disease compared to TPO autoantibodies in autoimmune thyroid disease and/or healthy blood donor sera. Overall, our results of analysis of T PO autoantibody binding in Western blotting and with modified TPO proteins in immunoprecipitation assays suggest that the main autoantibody binding sites on the TPO molecule involve extensive amino acid sequences. Our studies also suggest that TPO autoantibodies from patients with autoimmune thyroid disease, Addison's disease and apparently healthy blood donors show some differences in epitope recognition on TPO and this approach may allow differentiation between disease related and unrelated TPO autoantibodies.

A study of the epitopes on steroid 21-hydroxylase recognized by autoantibodies in patients with or without Addison's disease

Steroid 21-hydroxylase (21-OH) autoantibodies are found in patients with autoimmune Addison's disease (AAD), either isolated or associated with autoimmune polyglandular syndrome (APS) type I and II and in adrenal-cortex autoantibody (ACA)-positive patients without AAD. In order to assess any differences in the 21-OH autoantibodies in these different patient groups, we have studied their reactivity with different epitopes on 21-OH using full length and modified 35S-labelled 21-OH proteins produced in an in vitro transcription/translation system. There were no major differences in the pattern of autoantibody reactivity with the different modified 21-OH proteins in patients with isolated AAD or with APS types I and II, and in 21-OH autoantibody-positive patients with clinical AAD, subclinical AAD and those maintaining a normal adrenal function. Our studies also indicate that the main epitopes for 21-OH autoantibodies in patients with different forms of autoimmune adrenal disease are located in the C-terminal end and in a central region of 21-OH.

Molecular analysis of the antibody response to thyroglobulin and thyroid peroxidase

In this review, we discuss the latest results concerning the molecular analysis of antibodies (Ab) directed toward thyroid autoantigens. In particular, we attempt to define patterns within the Ab repertoire that correlate best to their activities. Whilst a considerable amount is now known concerning the Ab response to thyroid peroxidase (TPO), there is still much we do not understand. We review evidence for the site of interaction of TPO-reactive Ab with native TPO. The Ab responses to thyroglobulin (Tg) and, in particular, the thyroid-stimulating hormone receptor (TSH-R), are much less well characterised. In this review, we focus on the molecular analysis of the Ab response to Tg and TPO, assessing the repertoire as it is currently known. In addition, we have tried to link this information with the analysis of the epitopes recognised by the various Ab. Finally, we discuss one of the more unusual features of the thyroid Ab repertoire, the use of D-D fusion at heavy chain junctions, and questions raised by our current state of knowledge, such as the role of Ab using germline V regions in antigen recognition.

Amino acid sequence of a tryptic peptide of human thyroglobulin reactive with sera of patients with thyroid diseases

Autoantibodies to human thyroglobulin (hTg) are found in the sera of many patients with thyroid diseases. To localize epitopes recognized by these autoantibodies, hTg was incubated with tryspin for 4 hours at 37 degrees C under non-reducing conditions. Releasing peptides from hTg in their natural conformation. These peptides were then analyzed by western immunoblot using either autoantibodies from patients with autoimmune thyroiditis or murine monoclonal antibodies (mAb) produced against hTg. The autoantibodies reacted primarily with two low molecular weight peptides with apparent molecular weights (MWap) of 15 and 20 kDa. The pattern of tryptic peptides recognized by these autoantibodies resembled that of one of the mAbs (137C1), as shown by immunoblots in either one or two dimensional SDS-PAGE. To characterize these peptides further, they were separated by a high performance liquid chromatography (HPLC). The column separated the 4-hour tryptic digest of hTg into multiple peptide peaks. Further analysis by SDS-PAGE showed that one of these peaks contained the 15 kDa peptide. The 15 amino acid sequence at the amino-terminus of this peptide was determined. This amino acid sequence (KVPTFATPWPDFVPR) corresponds to a unique sequence near the carboxyl-terminal end of hTg, starting with amino acid 2657. The size of the peptide indicates that it extends to the carboxyl-terminal end of hTg. This fragment contains one of the antigenic sites of hTg that binds autoantibodies from patients with autoimmune thyroid disease.